© 1998 by European Society of Cardiology
Copyright © 1998, European Society of Cardiology
Modulation of contractility in human cardiac hypertrophy by myosin essential light chain isoforms
aInstitute of Pharmacology, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
bMax-Delbrück-Center for Molecular Medicine, D-13122 Berlin, Germany
* Corresponding author. Tel.: (+41-1) 635 5919; fax: (+41-1) 635 5708; e-mail: schaub@pharma.unizh.ch
Received 15 August 1997; accepted 14 October 1997
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Abstract |
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 Top Abstract 1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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Cardiac hypertrophy is an adaptive response that normalizes wall stress and compensates for increased workload. It is accompanied by distinct qualitative and quantitative changes in the expression of protein isoforms concerning contractility, intracellular Ca2+-homeostasis and metabolism. Changes in the myosin subunit isoform expression improves contractility by an increase in force generation at a given Ca2+-concentration (increased Ca2+-sensitivity) and by improving the economy of the chemo-mechanical transduction process per amount of utilised ATP (increased duty ratio). In the human atrium this is achieved by partial replacement of the endogenous fast myosin by the ventricular slow-type heavy and light chains. In the hypertrophic human ventricle the slow-type β-myosin heavy chains remain unchanged, but the ectopic expression of the atrial myosin essential light chain (ALC1) partially replaces the endogenous ventricular isoform (VLC1). The ventricular contractile apparatus with myosin containing ALC1 is characterised by faster cross-bridge kinetics, a higher Ca2+-sensitivity of force generation and an increased duty ratio. The mechanism for cross-bridge modulation relies on the extended Ala–Pro-rich N-terminus of the essential light chains of which the first eleven residues interact with the C-terminus of actin. A change in charge in this region between ALC1 and VLC1 explains their functional difference. The intracellular Ca2+-handling may be impaired in heart failure, resulting in either higher or lower cytosolic Ca2+-levels. Thus the state of the cardiomyocyte determines whether this hypertrophic adaptation remains beneficial or becomes detrimental during failure. Also discussed are the effects on contractility of long-term changes in isoform expression of other sarcomeric proteins. Positive and negative modulation of contractility by short-term phosphorylation reactions at multiple sites in the myosin regulatory light chain, troponin-I, troponin-T,
-tropomyosin and myosin binding protein-C are considered in detail.
KEYWORDS Cardiac hypertrophy; Heart failure; Cross-bridge kinetics; Contractility; Myosin heavy chain; Myosin light chain; Ca2+-sensitivity–tension relation
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1 Introduction |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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Heart muscle performs work with high contractile efficiency [1, 2]. Under increased workload it reacts with hypertrophy. This represents an adaptive response in order to normalise wall stress and compensate for the increased hemodynamic load [3]. When the load is chronically elevated, compensated hypertrophy may progress to pump failure. Except in the case of acute global ischemia which is accompanied by a collapse of energy production, it is not clear which process, or processes, are responsible for turning compensated hypertrophy into heart failure. Numerous physiological characteristics concerning metabolism, contractility and cellular structure are found to be impaired in heart failure. However, contractile function is often preserved or even improved in hypertrophy as well as in failure when studied at the subcellular level, i.e. on permeabilised muscle fibers or on isolated proteins in the in vitro motility assay and in solution studies.
The changes in the contractile apparatus (Fig. 1Fig. 2) result from long-lasting changes in the gene expression program (reviewed in [4, 5]). Because cardiomyocytes are no longer able to increase muscle mass by proliferation they resort to hypertrophy. Hypertrophy does not mean more of the same but involves specific qualitative alterations in the cell phenotype [5–7]. Reexpression of ‘fetal genes’ which are normally active in the fetal period coding for proteins such as β-myosin heavy chain (β-MHC) (see Table 1 for abbreviations), -skeletal and -smooth muscle actin or atrial natriuretic factor, may occur. The development of hypertrophy in humans is a slow and chronic process extending over years. However, the long-term changes in expression of contractile proteins is not simply a reactivation of the ‘fetal gene program’, but follows a pattern that seems to be determined by the physiological demands. These physiological demands are transmitted by a panoply of different stimuli, besides mechanical loading, involving hormones, catecholamines, growth factors, cytokines and vasoactive peptides (reviewed in [8]). Most of these hypertrophic stimuli use different intracellular signaling pathways which results in individual phenotypic responses as characterised by gene expression pattern, cell morphology and function. The effects of various hypertrophic stimuli are likely to be balanced in vivo, even during compensatory hypertrophy. However, the disturbance of any single factor may tip this balance for the worse. The precise knowledge of the function of the contractile apparatus including its regulatory mechanisms in the normal heart and during hypertrophy is indispensable when considering potential therapeutic strategies aimed at avoiding development of heart failure.
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The purpose of this review is to put together recently published results on the structure and function of the myosin light chain (MLC) isoforms whose expression changes during the development of cardiac hypertrophy. The discussion includes the position and relation of the MLC in the myosin head next to the motor domain, the variation in MHC and MLC isoforms and their possible hybrid molecular assemblies in atrial and ventricular muscles. The knowledge how calcium controls the myosin cross-bridge kinetics is instrumental in delineating the details of the mechanism by which the atrial MLC1 (ALC1) positively affects contractility of the ventricular muscle. The pathophysiological consequences of the expression of the ALC1 are discussed in view of the functional viability of the cardiomyocyte with regard to intracellular Ca2+-handling and energy supply during hypertrophy and in heart failure. In order to recognize the relevance of modulation of contractility by myosin essential light chain, we give an overview of other sarcomeric proteins associated with the contractile machinery and point out the role of protein phosphorylation in Ca2+-sensitivity and cross-bridge kinetics. Taken together, we provide evidence for the functional significance of the change in MLC isoform expression in the human heart ventricle where the MHC remain unaltered during hypertrophy. In view of space limitation, reviews which contain references to primary information are often cited.
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2 The myosin motor |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The myosin muscle motor (myosin type-II) is a hexameric protein (520 kDa) consisting of two heavy chains (MHC) of around 220 kDa (1930–1940 amino acid residues) and of two pairs of light chains (MLC) of around 20 kDa each. The MHC subunits, which provide both the motor and the filament forming properties, can each be divided into two functional domains: the globular N-terminal head domain (corresponding to the proteolytic subfragment-1 or S1 of 120 kDa) and the elongated
-helical domain that, together with the corresponding domain of the second MHC, forms the coiled coil C-terminal rod (Fig. 2). The S1 has an approximate length of 16.5 nm and the rod domain one of 140 nm. The S1 domain can be further subdivided into two parts, (i) the motor domain, running from the N-terminus through to residue 770 and containing the nucleotide binding pocket and the actin binding cleft, and (ii) the 8 nm long -helical light chain binding or regulatory domain (neck region) which extends from residue 771 to residue 843 and links the motor domain to the rod domain [9, 10] (Fig. 3). The -helix of this regulatory domain is stabilized by non-covalent association of one MLC of each type. Near its C-terminus the -helix bends sharply at the residues W829–P830–W831 and this last stretch of the -helix points along the myosin filament axis towards the middle of the sarcomere, before it joins the second MHC in the rod. The precise interactions of the MLC with the MHC in this region have been revealed by crystal structures of the chicken S1 [9] and of the regulatory domain of the molluscan scallop myosin [11]. The two MLC are located in series and are wrapped around the MHC in an anti-parallel orientation. In the chicken S1, MLC type-1 (essential light chain) binds to residues from L783 to M806 and MLC type-2 (regulatory light chain) to residues from E808 to L842 of MHC [9]. It has been agreed to give the position numbers for amino acids in the sequence of the chicken skeletal muscle MHC even when referring to mammalian skeletal or heart muscle myosins [12, 13].
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Myosin cross-bridges (myosin head portions protruding from the myosin filament) provoke movement during contraction by repetitive interactions with the actin filament in the sarcomere. Thereby the cross-bridges are thought to swing through a number of power strokes, each time displacing the two filaments relative to one another by a distance of 5 to 10 nm [14, 15]. Recent studies on time-resolved X-ray diffraction of live frog sartorius muscles [16] and fluorescence polarisation coupled with rapid length steps imposed on permeabilised rabbit psoas fibers (mechanically or chemically skinned fibers) [17], show attachment of cross-bridges with actin and synchronized head movements in the elementary force-generating process. However, the rowing cross-bridge model as accepted in the textbooks since 1971 [18] requires an update. The tip of the motor domain is now envisaged to bind actin with a more or less fixed geometry throughout the power stroke, while it is the distal C-terminal light chain binding region of the myosin molecule that actually moves [10, 17, 19]. The
-helix of this regulatory domain serves as a ‘lever arm’ to amplify relatively restricted rotational movements among different structural parts within the motor domain. It was shown that the length of the lever arm is directly proportional to the sliding velocity in an in vitro motility assay system [20]. For this purpose, the lever arm of the S1 domain of myosin-II from the slime mould Dictyostelium discoideum, a myosin that is homologous to vertebrate striated muscle myosin, was altered in length by addition or subtraction of light chain binding regions by genetic engineering. The advent of nano-technology has recently allowed to determine discrete stepwise movement averaging 11 nm under low load and single force transients averaging 3 to 4 pN under isometric conditions of rabbit fast skeletal muscle myosin [21]. In this case, myosin molecules were fixed on silica beads and one molecule was let to interact with an actin filament held at both ends by a feedback enhanced laser trap system. X-ray structural data of the individual proteins together with data obtained by fiber diffraction and electron microscopy have been used to build detailed models of the rigor interaction of myosin with the actin filament [22–24]. In the contracting muscle this state is thought to occur transiently at the end of the power stroke after release of the products Pi and ADP before a new substrate ATP molecule gets bound to myosin. In the rigor state one myosin head is suggested to interact with two adjacent actin monomers (Fig. 3). A number of hydrophobic residues on the opposing faces of actin and myosin contribute to the main contacts. These contacts are flanked by charged myosin surface loops which form predominantly ionic interactions with adjacent regions of actin. Two such loops represent the connector regions between the segments of the MHC of S1 that can be obtained by proteolysis with trypsin, i.e. the 25-kDa, 50-kDa and 20-kDa segments starting from the N-terminus [25]. These loops exhibit considerable sequence variability and it appears that their composition influences the kinetic behavior of myosin from different sources. Replacement of the 50–20-loop in Dictyostelium myosin with the equivalent myosin sequence from rabbit fast skeletal, chicken smooth or rat cardiac muscle leads to an actin activated ATPase activity that corresponds to the one of the respective parent myosin [25]. The 25–50-loop has been suggested to affect the release of ADP from the nucleotide binding pocket and thus may be responsible for the observed differences in motility between different myosins [26]. In addition, and as discussed in the section on MLC below, the N-terminus of the large isoform of the essential MLC1 has also been shown to interact with the C-terminus of an actin monomer (Fig. 3) and to affect the actin activated ATPase activity.
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3 Cardiac myosin heavy chains |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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At least eight different MHC types are expressed in mammalian striated muscles: two developmental isoforms (MHC-embryonic and MHC-perinatal), four fast isoforms (MHC-fast 2A, 2B, 2X/D and MHC-extraocular) as well as MHC-slow which is identical to the cardiac β-MHC. In addition, the heart also expresses the cardiac
-MHC isoform (for review see [27]). The human -MHC contains 1939 [28] and the β-MHC 1935 [29] amino acid residues. Among them they share 93.1% sequence identity. The rat -MHC contains 1938 and the β-MHC 1935 residues [30], and they share 93.2% sequence identity. A total of 131 residues differ between them and most of these differences are confined to regions of biological significance in the S1 subfragment such as the N-terminus, the ATP binding pocket, the actin binding cleft, the light chain binding domain and in the two hinge regions further down in the rod domain. In inter-species comparison between human and rat, -MHC shows over 97% sequence identity. The same holds for β-MHC.
During development and under pathophysiological conditions, the expression of the two cardiac MHC isoforms is regulated in a tissue-specific manner. Their expression is under additional hormonal control, notably by thyroid hormones [5, 31, 32]. In all mammalian species -MHC is expressed in the atria throughout life. In small mammals such as rat and mouse, -MHC is also the predominant isoform in the ventricles post partum and during adulthood. In addition, -MHC is expressed in extraocular muscles as well as in some mandibular muscles of carnivores. β-MHC is expressed in the embryonic/fetal ventricles of all mammals and disappears soon after birth in small animal species. In larger animals such as rabbit, dog, pig and human, β-MHC remains the predominant ventricular isoform throughout adulthood (Table 2).
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The fact that the β-MHC is also expressed in skeletal muscle fibers of the slow type-1 allows to perform functional studies on mutated β-MHC from patients with familial hypertrophic cardiomyopathy (FHC). In the in vitro motility assay of purified myosin from the soleus muscle of FHC patients with seven different point mutations in the β-MHC gene, the velocity of actin translocation was on average about half or less than that of myosin from healthy control persons (average control value 0.48 µm s–1) [33]. In addition, myosin prepared from interventricular septal sites as well as from the soleus muscle of FHC patients with the R403Q (position 405 in chicken) mutation exhibited the same low motility (0.11 and 0.13 µm s–1, respectively). Permeabilised slow fibers from the soleus muscle of FHC patients with either the R403Q or G741R (position 743 in chicken) mutations also showed impaired contractile functions, although the fraction of mutant MHC in these fibers was not known [34].
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4 Cardiac myosin light chains |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The myosin light chains (MLC) comprise two subfamilies, the essential light chains (ELC) and the regulatory light chains (RLC). Both MLC subfamilies belong, together with the calmodulin and the troponin-C subfamilies, to the superfamily of intracellular Ca2+-binding proteins that characteristically contain four EF-hand domains (helix–loop–helix motif) [35]. However, during evolution, all four EF-domains of vertebrate muscle myosin ELC have lost their ability to bind Ca2+, while in the case of the RLC only the first EF-domain can bind either Ca2+ or Mg2+ with high affinity in situ in the myosin molecule (binding affinities in the range of 107 M–1) [36]. The ELC is called ‘essential’ because it was originally thought to be essential for the hydrolytic activity of myosin; but later, this proved not to be the case [37, 38]. Alternative names are alkali MLC (since ELC can be removed from myosin by elevated pH) or MLC1. The RLC is termed ‘regulatory’ because in vertebrate smooth muscle and non-muscle cells, contractile activity is triggered by phosphorylation of this myosin light chain [39]. Alternative names are DTNB-MLC (since it can be removed from myosin by DTNB-treatment), PLC (it can be reversibly phosphorylated) or MLC2. The ATPase activity of isolated myosin does not seem to be affected by the ELC or RLC. It has however become increasingly evident, that within the complex sarcomeric structure, the MLC are involved in the fine-tuning of the contractile activity.
Two varieties of ELC and RLC are expressed in the heart, characteristic for the atrial and the ventricular tissues. They are thus designated ALC1 and VLC1 for atrial and ventricular ELC, respectively, and ALC2 and VLC2 for the corresponding RLC species. ALC1 and VLC1 (human and rat) contain 190–200 (Table 3) and the VLC2 around 165 amino acid residues (no data available for ALC2) [35]. The molecular mass for the MLC1 and MLC2 types is around 22 and 19 kDa, respectively. However, the MLC1 migrates with a higher apparent mass of around 27 kDa in SDS gel-electrophoresis because of its particular N-terminus. The most striking difference between the two types of MLC concerns an additional peptide stretch of around 30 residues at the N-terminus of the ELC. An unusual accumulation of around 10 Ala and 10 Pro is found in this extra stretch. In addition, several positively and negatively charged amino acid residues are clustered near the N-terminus (Table 3). Such sequences rich in Ala and Pro form rigid extended structures [40]. It has been shown by biochemical in solution studies that the N-terminus of fast skeletal muscle ELC (ELC1fast) binds to the C-terminus of actin [41, 42]. This extended N-terminal structure of the ELC1fast cannot be seen in the crystal structure of the chicken S1 subfragment [9]. The last visible residue in the fast skeletal muscle ELC corresponds to positions 46–53 in the sequences of human and rat ALC1 and VLC1, and this residue lies roughly in the centre of the 3-dimensional structure of the ELC at a distance of 7–8 nm from the actin filament surface. Thus a stretch of around 50 residues is available for the N-terminus of ALC1 and VLC1 to bridge the gap of 7–8 nm and to contact the actin filament. In principle, only 24 residues are required to span a distance of 8 nm if present in an extended β-sheet conformation. Taken together, the odd 50 N-terminal residues in ALC1 and VLC1 are ample to run along the myosin head moiety and to interact with a proximate actin monomer (Fig. 3) [24]. It has recently been shown that the first 11 N-terminal residues of human ALC1 are responsible for binding to the actin C-terminus [43]. The interaction of the N-terminus of ELC has indeed been shown to affect contractility and actin activated ATPase activity in skeletal muscle protein systems (for reviews see [44–47]). It is this structural feature that seems to be of relevance for the pathophysiological aspects of cardiac hypertrophy discussed below. Interestingly, all ELC isoforms expressed in vertebrate striated muscles have such an extended N-terminus that is found neither in invertebrate ELC nor in the vertebrate smooth and non-muscle myosin ELC.
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The ALC1 is identical to the embryonic ELC1emb which is also transiently expressed in fetal skeletal muscles as well as in fetal ventricular heart tissue [48]. On the other hand, the VLC1 is identical to the skeletal muscle ELC1bslow which is expressed in adult slow skeletal muscles [49]. VLC2 is also expressed in adult slow skeletal muscles as RLC2slow. An additional ventricular RLC designated VLC2* that has a different isoelectric point but the same mass as VLC2 and can be resolved by 2-dimensional gel electrophoresis (Fig. 4), is found in ventricles of most mammals [50, 51]. Both VLC2 varieties have different amino acid sequences and each contains one phosphorylation site for a Ca2+-calmodulin dependent light chain kinase (MLCK) [52]. In contrast, the sole ALC2 variety in the human atrium can be phosphorylated at two sites. The significance of this feature for cardiac contractility is, however, not known [53].
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Several recently detected missense point mutations in either the ELC or RLC are associated with a rare variant of FHC and with skeletal muscle myopathy [54]. In contrast to most mutations in the MHC of FHC patients which are associated with impaired motility [33], myosin from a patient with the M149V mutation in the VLC1 displayed an actin filament translocation velocity that was increased by 41% as compared to control myosin. The two mutations M149V and R154H found in the VLC1 are localized in the region where it contacts the motor domain of the MHC. This putative interaction may correspond to the base around which the regulatory domain (lever arm) moves during contraction (Fig. 3) [10]. Furthermore, three mutations in the VLC2 (A13T, E22K and P94R) all seem to be localized near the phosphorylation site at S15 [54].
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5 Combinatorial associations of multiple myosin isoform subunits |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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It is known that the different isoforms of MHC as well as of MLC are responsible for distinct contractile properties. Therefore it is difficult to accept the idea that this isoform variety simply reflects an evolutionary relict of progredient accumulation of gene duplications, mutations and chromosomal rearrangements. The strict regulation during development and tissue-specific mode of expression seems more likely to favor the notion that the various isoforms present a store from which to choose in order to build molecular motors with functional isoform advantages. It might therefore be useful to consider the different heteromeric molecular assemblies that may possibly occur under normal and under pathological conditions.
Under normal conditions in adult human hearts the -MHC together with ALC1 and ALC2 are confined to the atrium, while the β-MHC together with VLC1, VLC2 and VLC2* are to be found in the ventricles. Some β-MHC seems to accumulate in atria with increasing age (Table 2), which in the atrial appendages may reach values up to 50% of total MHC [55–57]. However, pressure overload models in animals as well as hemodynamic overload in humans leads to characteristic changes in myosin composition in both atria and ventricles. The ventricular myosin subunit isoforms VLC1 and VLC2 become ectopically expressed in atria in small mammals as well as in humans [31, 58]. β-MHC increases in human atria up to 50% or more (Table 2) [56] and becomes reexpressed in ventricles of small animals which normally display predominantly -MHC. In normal human ventricles a small amount of -MHC, estimated to vary between low levels and 10%, can be observed beside the abundant β-MHC [59–62]. This residual pool of -MHC is absent in patients under hemodynamic overload. Electrophoretic resolution of native myosin isolated from ventricles of hemodynamically overloaded small animals with varying proportions of -MHC and β-MHC yields three species designated V1, V2 and V3 according to electrophoretic mobility. V1 and V3 consist of homodimeric and ββ combinations, respectively, and V2 represents the heterodimer β. Interestingly, myosin from overloaded atria resolves into two species only, namely A1 () and A2 (ββ). A1 and A2 do not co-migrate exactly with the corresponding ventricular species V1 and V3, because they contain different MLC complements [31].
In normal human atria, two MHC homodimers ( and ββ) together with ALC1 and ALC2 give rise to two myosin isoforms. In hemodynamically overloaded human atria, we are confronted with the co-existence of the two MHC homodimers with two types of MLC (ALC1 and ALC2 as well as VLC1 and VLC2). Both MHC homodimers may combine with each MLC type in three ways forming two homodimers and one heterodimer with regard to the MLC (ALC1/ALC1, VLC1/VLC1 and ALC1/VLC1 as well as ALC2/ALC2, VLC2/VLC2 and ALC2/VLC2). Combinatorially, this allows for 2x3x3=18 different myosin species. Since both MLC1 and MLC2 are present in two different isoforms, the sequential arrangement along one MHC may also play a role. Two sequential arrangements are possible in one myosin molecule: VLC1–VLC2/ALC1–ALC2 as well as VLC1–ALC2/ALC1–VLC2. Therefore, one additional species has to be added for each MHC homodimer ( and ββ) making a total of 20 instead of 18 isoforms. This number would increase even to 42 if the ventricular VLC2* were also considered. This isoform has been shown to be expressed in a proportion of VLC2* to VLC2 of around 0.4 in control as well as in overloaded human ventricles [63]. VLC2* has been shown to occur also in significant amounts in overloaded human atria [64]. In any case, multiple possible myosin isoforms could account for the altered contractility of diseased atrium [65, 66] when compared to normal atrium.
In normal human ventricle, mainly β-MHC and a small amount of -MHC beside the VLC1 and two types of MLC2 (VLC2 and VLC2*) are present. Ignoring the rare -MHC, this would give rise to three myosin isoforms containing VLC2/VLC2, VLC2/VLC2* or VLC2*/VLC2*. As mentioned above, most of the -MHC disappears under hemodynamic overload, leaving only the β-MHC species. However, we have reported that under such conditions the ALC1 becomes reexpressed in the ventricle varying in amounts up to 30% of total MLC1 (Fig. 4 and Table 2) [67–69]. Therefore, in diseased human ventricle the ββ-homodimer may combine with either VLC1 and/or ALC1 to form two homodimers (VLC1/VLC1 or ALC1/ALC1) and the heterodimer VLC1/ALC1. The same type of combinations are, of course, also possible with regard to the VLC2 and VLC2*. Thus, in the diseased human ventricle as many as 1x3x3=9 myosin isoforms may co-exist. The combination with four different MLC in one molecule allows also in this case two possible sequential alignments on the MHC. This brings the total number of possible isoforms in the overloaded ventricle to 10 instead of 9.
As discussed below, shifts in expression of cardiac MHC as well as MLC are known to affect the contractile properties. In order to form heterodimeric molecules, the corresponding isoform subunits must be expressed in the same cell. This seems to be the case for ventricular myocytes as probed with monoclonal antibodies against -MHC and β-MHC [62, 70]. Atrial myocytes have also been shown to co-express -MHC and β-MHC [55]; nevertheless, MHC heterodimers are not observed in atrial tissue. Whether or not all possible subunit associations discussed above are actually realized in vivo is not known. Functional myosin requires a strict stoichiometric relation of its subunits. The parallel occurrence of β-MHC and VLC2 in the atria of hypertensive baboons was taken to indicate a coordinated expression of myosin subunits [71]. However, cardiac MLC exhibit a slower turnover rate than MHC [72, 73]. Consequently there is a pool of unassembled MLC in the cytoplasm. In accordance, we found an excess of around 0.8 molar of VLC1 over MHC in human ventricle [69]. The additional expression of ALC1 in hypertrophic ventricles added up to an excess of 3.5 molar of total MLC1 (VLC1 plus ALC1) over MHC. Isolation and analysis of myosin from such tissue indicated that, compared to VLC1, the proportional contribution of ALC1 bound to ββ-myosin was roughly the same as its content in total tissue samples. In contrast, in transgenic mice ectopically expressing the skeletal myosin ELC2fast, incorporation into cardiac myosin was not proportional to the total amount actually expressed [74]. While in atria the foreign ELC2fast replaced the endogenous ALC2 almost entirely, this hardly occurred in the ventricles, despite the fact that in both tissues -myosin was the recipient protein. These cases illustrate that human ventricular ββ-myosin readily combines with ALC1 and that murine atrial -myosin accepts skeletal muscle ELC2fast while its ventricular counterpart (also -myosin) does not. It may thus be emphasised that correlated regulation of expression of MHC and MLC is not required for stoichiometric assembly. In general, synthesis of MLC may be in excess over that of MHC and a large portion of the unassembled MLC will probably be degraded. On the other hand, we observed a far larger amount of β-MHC in whole tissue samples of hemodynamically overloaded human atria than VLC1 (Table 2).
A further aspect may shed some light on the question of heteromeric myosin assemblies. During development the expression of MHC and MLC isoforms undergoes transitions in atria and ventricle which are temporally not closely linked [57]. This implies a lack of isotype-specific interactions between the MHC and MLC. Taken together, the available evidence suggests that different MLC can indeed combine with different MHC. Furthermore, the MHC and MLC may be regulated independently of one another and thus allow for a variety of mixed isotype assemblies to occur in vivo.
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6 Myosin cross-bridge kinetics |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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Cardiac contractility is directly related to the type of predominant myosin species. The motor protein defines the frame within which contractility may vary in terms of force production (unitary force per cross-bridge cycle) and velocity of displacement (kinetics). The structural and mechanical changes in the cross-bridge (a series of successive conformational changes in the myosin head domain) are coupled to a concomitant series of chemical reactions involving the hydrolysis of ATP to ADP and inorganic phosphate (confer [75, 76]). The simplified scheme in Fig. 5 combines the mechanical and kinetic cycles and delineates the main intermediate states in the actin–myosin cross-bridge cycle. The affinity of myosin (M) to actin (A) changes during the cross-bridge cycle. The species with bound ATP or its products resulting from hydrolysis (ADP and Pi) exhibit low affinity, do not produce force and oscillate rapidly (on a time scale of µs or less) between attached and detached states. Consequently, they do not present significant resistance to stretch of relaxed muscle. Force production is thought to be coupled to the release of Pi and comprises the transition from weakly to strongly binding states [77, 78]. These force generating cross-bridges exhibit a slower rate of oscillation between attachment and detachment bind to actin with positive cooperativity. This results in an increased resistance to passive stretch. The complete cross-bridge cycle is terminated in the nucleotide-free state after dissociation of ADP which resembles the state of rigor mortis. In vivo, upon binding of a new substrate ATP, the cross-bridge dissociates from actin and the cycle is ready to start again.
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Measurements of contractile parameters such as isometric tension, shortening velocity and force redevelopment following a brief period of shortening at zero load can provide information about the distribution of cross-bridges among weakly and strongly binding states and the kinetics of transitions between them [79]. The first quantitative model which related experimental parameters to the kinetics of the cross-bridge cycle considered two states: one attached and force-generating, and one detached non-force-generating [80]. These two states are related by two rate constants: the forward rate constant f determining the rate of attachment, and the forward rate constant g determining the rate of detachment. The same model is now applied to the reaction scheme with many intermediate cross-bridge states [76]. In analogy to the original model, the rate constant fapp now describes the transition from weakly binding, non-force generating states, to the strongly binding, force-generating states, and gapp describes that for the opposite transition via product release and rebinding of ATP (Fig. 5). Since weakly binding non-force-generating cross-bridges are still in rapid equilibrium with actin [81], the rate constants no longer describe attachment and detachment, respectively. The isometric force (F) of a muscle can thus be described as
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To illustrate the complexity of the interrelation between the mechanical and kinetic properties of contractility, let us consider a muscle fiber that is stimulated to contract. The amount of force produced depends on how it is free to move. The velocity at which the fiber shortens depends on how great a load it must bear. If no load is applied, the fiber shortens at the maximal speed (Vmax). If a force sufficient to prevent shortening is applied, the fiber develops force under isometric conditions. Whatever parameter one measures, and regardless of how it is measured, the results are not only determined by the intrinsic properties of the motor protein alone, but also by the continuously varying mechanical conditions imposed on the system by the reaction partner actin as well as by the regulatory proteins in the sarcomere. This complex ensemble is reflected in the force–velocity relation [32, 79]. In performing work against a load, muscle generates power, which is work per unit of time and is obtained by multiplying force with velocity. The maximal power output depends on Vmax, isometric tension (P0) and on the curvature of the force–velocity curve [84]. The shape of the curve is dependent on the force coefficient a, normalised to isometric force=a/P0. The power output is zero during shortening without load at Vmax as well as under isometric conditions. Since force and velocity are inversely related, their product will be greatest at an intermediate load. It reaches a maximum at around 20–40% of Vmax while at the same time producing around 30% of isometric tension.
P0 is determined by the number of attached cross-bridges per cross-sectional area at any given time, multiplied by the force development of a single cross-bridge. The actual number of attached cross-bridges can in turn be estimated by multiplying the total number of cross-bridges (n) by the fraction of time spent in the force-generating state (duty ratio or duty cycle). Vmax under zero load cannot be measured directly and has to be extrapolated from the force–velocity curve, or better, may be determined by Edman's slack-test [85]. Determination of isometric tension of permeabilised muscle fibers yields inconsistent results. This may be due, at least in part, to the experimental conditions (see ref. [32] for discussion). In general, the specific tension (force per cross-sectional area) does not vary much among different muscles or animal species [86, 87]. In vivo, movements are seldom performed at a speed close to Vmax. The range of speed (V/Vmax) at which muscles mostly work, is close to the one where the maximal power output is generated [88]. Since Vmax of fast skeletal muscle fibers is about 2–4 times higher than in slow fibers, the maximal power output is much lower in slow than in fast fibers [89]. It has been suggested that the maximal power output may be taken as a more reliable index of dynamic mechanical properties of a muscle than Vmax [86]. In view of the complex muscular mechanics, the diversity of myosin isoform compositions may be required to meet the physiological demands for a large number of optimal combinations between the power output and the corresponding shortening velocity in order to perform movement on an economic basis.
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7 Ca2+-control of cross-bridge activity |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The twitch properties of fast and slow skeletal muscles depend not only on the contractile apparatus but also on the Ca2+-transients in the cytosol. This is even more so in the heart. The heart muscles do not relax fully during diastole, and neither do they contract maximally during systole [90, 91]. At submaximal Ca2+-concentrations, the rate of tension development shows pronounced Ca2+-sensitivity. The sensitivity of the myofibrillar apparatus to the activating Ca2+ is generally expressed by the pCa–tension relation as determined under isometric steady-state conditions on permeabilised fiber preparations. Some studies have also been carried out on intact fibers using intracellular Ca2+-indicators. pCa denotes the negative decadic logarithm of the free Ca2+-concentration. The normalized tension–pCa data can be fitted to the Hill equation [92]
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In general, permeabilised fibers from slow muscles exhibit a higher Ca2+-sensitivity than fibers from fast muscles [94]. For activation levels up to around 20% of maximal tension, increasing Ca2+ seems to increase ntot (recruitment of cross-bridges). At higher levels of activation, Ca2+ seems to increase tension by increasing fapp, thus affecting the cross-bridge turnover kinetics directly. These two modes of regulation do overlap, however, when going from rest to activation. The effect of Ca2+ on fapp is consistent with the observation that Ca2+ has only a small, if any, effect on Vmax, since this latter is mainly determined by gapp which is almost independent of Ca2+ [76]. Thus, factors which may affect cross-bridge kinetics via fapp or gapp can become important parameters for modulation of muscle activity.
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8 Effects of myosin subunit isoforms on contractility |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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In general, Vmax is roughly proportional to the ATPase activity and speed of contraction in heart as well as in skeletal muscles (for reviews see [32, 79]). The ATPase activity is highest in V1 (
), intermediate in the heterodimer V2 (β) and lowest in V3 (ββ) cardiac isomyosin of a given species [31]. In skeletal muscles the Ca2+-sensitivity is about 0.2–0.3 pCa units higher in slow than in fast fibers. On the other hand, fast fibers exhibit a much steeper tension–pCa curve with an H value of 4–6, while slow fibers have one of 2–3. Human cardiac fibers from atrium and ventricle have an equally low H value of 2–3 [51]. This may have to do with the fact that the cardiac TnC is also expressed in slow skeletal muscle fibers. This cardiac TnC isoform contains only one Ca2+-binding regulatory site instead of the two in the fast skeletal muscle TnCf [95]. In addition, the Ca2+-sensitivity is 0.17 units higher in ventricular than in atrial fibers concomitant with an unloaded Vmax (given as muscle length per second) of 2.6 for atrial and a lower one of 1.7 for ventricular fibers [51]. These characteristic differences seem to be mainly due to the atrial myosin containing mainly -MHC as opposed to the ventricular myosin which in man contains predominantly β-MHC. Measuring the ATPase activity during isometric tension generation at various Ca2+-concentrations allows to establish the ATPase-force relation for human atrial and ventricular muscle fibers [96]. Force at saturating Ca2+-concentrations was 14.0 for atrial and 21.1 kN/m2 for ventricular fibers. The amount of ATP used per force (tension cost) was independent of the Ca2+-concentration and just about three times higher for atrial than for ventricular fibers. The Ca2+-sensitivity was again 0.08 pCa units higher in ventricular fibers. While both Vmax and isometric tension generation differ only moderately between atrial and ventricular fibers, the difference in the rate of tension development is quite pronounced. The rate constant of tension development (ktd) after photolytic release of ATP from ‘caged ATP’ is seven-fold higher in atrial than in ventricular porcine muscle fibers [97].
As outlined above, the ATPase-force relation is proportional to gapp and the force of a muscle depends on its cross-bridge kinetics. Taken together, these results suggest that atrial myosin cross-bridges differ in their kinetics from the ventricular myosin by having a higher gapp, implying a faster transition rate from force-generating into non-force generating states. If the unitary force value (F') were different, one fiber type would still generate different absolute tension at a given Ca2+-concentration, but it should not change the normalized tension–pCa curve. A lower gapp in the kinetic of V3 myosin is consistent with a longer fraction of the cross-bridge cycle being spent in the force-generating states (increased duty ratio or duty cycle). This difference in kinetics between human atrial and ventricular myosin is thought to reside in the -MHC and β-MHC. It has to be born in mind, however, that these two myosins differ in their MLC complement.
V1 and V3 myosin isoforms with the same ventricular MLC complement (VLC1 and VLC2) can be generated in rabbits by making them hypothyroid (producing V3 myosin by feeding them propylthiouracil) or hyperthyroid (producing V1 myosin by administering levothyroxine) [98]. V1 myosin has twice the actin activated ATPase activity and three times the actin filament sliding velocity in an in vitro motility assay system, when compared to V3 myosin. Yet, V1 myosin produces only half the average cross-bridge force per cycle (0.15 pN for V1 and 0.30 pN for V3 myosin). These findings also point to a kinetic difference between the -MHC and the β-MHC in V1 and V3 myosin, respectively. Such a difference in kinetics may provide the molecular basis for the observation made earlier by myothermal techniques [99]. The economy of ATP utilisation for isometric force production (i.e. cross-bridge tension–time integral per ATP) by V3 was found to be twice that for V1 myosin. The force–time integral reflects the cross-bridge attachment time and is positively related to economy of contraction and negatively related to ATPase activity, shortening velocity and cross-bridge cycling rate. It should be noted that slower actin translocation velocities are not always associated with higher average cross-bridge force. Both V1 and V3 myosin have slower actin filament velocities than fast skeletal muscle myosin, but neither generates greater average force [100, 101].
Hyperthyroid rats supplied with triiodothyronine contain exclusively -MHC in the atria as well as in the ventricles [102]. There is no evidence that the tissue-specific MLC isoforms of atrium and ventricles undergo changes in relation to the thyroid state. The actin filament proteins do not seem to differ between these two tissues (reviewed in [102]). Thus, atrial and ventricular myocardium of hyperthyroid rats have identical composition of myofibrillar proteins except for the tissue-specific MLC. ATPase activities of myofibrils in solution (fully stimulated at pCa 4.4 and when inhibited at pCa>8) as well as of isolated myosin are the same in atrial and in ventricular preparations, yet the contractile properties differ markedly between permeabilised fibers from atrium and ventricle. Isometric tension is 9.7 for atrial and 22.6 kN/m2 for ventricular fibers (compare corresponding values given above for human). Maximal shortening velocity and maximal power output are almost twice as high in atrial fibers [102]. These findings suggest that in addition to the MHC also the MLC contribute to the contractile properties, while the myofibrillar ATPase activity in solution is determined entirely by the MHC species. The possible involvement of the MLC in modulation of contractility and ATPase activity has recently been suggested for skeletal muscle systems [32, 46, 103].
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9 Modulation of contractility in hypertrophy by the myosin essential light chain |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The relative composition of MHC and MLC1 isoforms in tissue samples from the free wall of all four chambers is summarised in Table 2 for six failing human hearts (five cases with dilated cardiomyopathy, DCM, and one with ischemic cardiomyopathy, ICM) obtained during transplantation and is compared to samples from five hemodynamically compensated subjects. The functional state of the patients was characterised by the New York Heart Association (NYHA) classification IV and by an ejection fraction (EF) of 16±2% (normal EF>60%). The mean pressure in the right and left atrium was 14±1 (normal<5) and 33±2 mmHg (normal<12), respectively.
-MHC and β-MHC can be quantified after resolution by SDS-PAGE while two-dimensional gel electrophoresis is required for quantification of the MLC isoforms [68, 69]. Both ALC1 and ALC2 display a higher apparent mass than the corresponding ventricular species [104]. For the ALC1 this applies only for human and pig (Fig. 4). In most other animals, including bovine, baboon and dogs, the ALC1 has a lower apparent mass than VLC1.
The change in myosin isoforms from V1 () to V3 (ββ) in the ventricle of small animals under pressure overload is associated with reduced ATPase activity and Vmax, concomitant with an increased economy of isometric force development (reviewed in [99]). The hemodynamically overloaded human atrium seems to follow a similar pattern. As mentioned earlier, the -MHC gets partially replaced by β-MHC in human atria under hemodynamic overload (Table 2). In accordance, permeabilised fibers from hypertrophic atria of patients with various valve diseases exhibit a Ca2+-sensitivity of tension 0.16 units higher and a Vmax 44% lower than in controls [65]. Because of the close relation between MHC isoform and myocardial function, energetics and biochemistry, it is assumed that the MHC is the main determinant of the contractile characteristics.
In human ventricles however, V3 (ββ) myosin with low ATPase activity is the predominant species under normal conditions (as also in large animals such as dog, pig, bovine) and does not significantly change under hemodynamic overload. The -MHC content of the right and left ventricles was below the biochemical detection limit both in the hemodynamically compensated and diseased hearts [67, 68] (Table 2). Nevertheless, isometric tension, tension–time integral and maximal rate of tension rise are 2–3 times higher in muscle fibers from hypertrophic left ventricles of patients with severe mitral regurgitation than in controls [99]. The Ca2+-sensitivity of tension was recently reported to be 0.28 units higher in left ventricular papillary muscle fibers of patients with DCM and heart failure (NYHA IV) than in samples of subjects without heart affection [105]. This shift in Ca2+-sensitivity of +0.28 units is even larger than those observed between normal and diseased atrium (+0.16) or between normal atrium and normal ventricle (+0.08 and +0.17). Taken together, these findings indicate that ventricular muscle fibers from patients in end-stage heart failure behave mechanically in a similar way as do fibers from overloaded atria where -MHC is partially replaced by β-MHC. If a change in expression of a myosin subunit isoform were responsible for the altered contractility of hypertrophic human ventricle it could only concern the reexpression of ALC1 (Table 2). The ALC2 has never been detected in normal ventricle during development, neither at high age [57], nor in diseased ventricles [68, 69].
At mid-gestation the ALC1 constitutes roughly half of the total MLC1 in the human ventricle [106, 107]. After birth it decreases rapidly to almost zero by the age of one year. The expression of the ALC1 persists, however, in patients with congenital heart diseases, causing an overload for the right ventricle. The persistence of ALC1 in ventricles of patients with congenital heart diseases, or its reexpression in patients with various types of heart affections, is consistently coupled with hemodynamic overload [63, 106, 107]. The amount of ALC1 in the ventricle may vary from a few up to over 30% of total MLC1. In 16% of over 100 patients with hemodynamic overload ALC1 was not detected in the ventricle.
The results of mechanical tests with permeabilised fibers from patients containing from zero up to 20% ALC1 are summarised in Fig. 7 (data taken from Refs. [63, 107]). Material without detectable ALC1 was included from subjects with normal hemodynamics. We showed for the first time, assuming other things being equal, a positive linear correlation between Vmax and Ca2+-sensitivity of isometric tension versus the ALC1 content. The correlation coefficient r for Vmax (12 cases) and Ca2+-sensitivity (8 cases) is 0.82 and 0.92, respectively (p<0.01 in both cases). Thus, both these two mechanical parameters are proportional to the ALC1 content. If taken together, the combined regression line in Fig. 7, would have a correlation coefficient of 0.86 with a significance level of p<0.001. An increase of the Ca2+-sensitivity by +0.36 units and an increase of Vmax by a factor of 1.7 can be derived from the regression for an increase of 20% ALC1 in the ventricular tissue. In addition to Vmax, the curvature of the force–velocity relation changes as indicated by a decrease of the normalised tension coefficient a/P0 from 0.21 in fibers lacking ALC1 to 0.15 in fibers containing 20% ALC1 [107]. The rate of tension development is almost twice as fast in fibers with high ALC1 when compared to fibers with low ALC1 content. Isometric tension at saturating Ca2+-concentrations was 352, 660 and 835 kN/m for fibers with zero, 3–5% or with 20% ALC1, respectively. All muscle specimens were treated with cardioplegic solution prior to chemical skinning and the MLC2 were thus fully dephosphorylated [63, 107].
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Taken together, these findings indicate that ALC1 modulates cross-bridge kinetics and thus contractility in human ventricular muscle. Increasing ALC1 increases Vmax which is proportional to gapp as well as the rate of tension development ktd which equals the sum of (fapp+gapp). ALC1 thus accelerates the cycling kinetics. This may be brought about by increasing either fapp or gapp. Which one occurs with ALC1 cannot be decided from the measurements of Vmax and ktd [107]. Either change would affect the fraction of force-generating cross-bridges, n=fapp/(fapp+gapp). The Ca2+-sensitivity of isometric tension would, however, be affected in opposite direction by an increase in either fapp (increasing it) or gapp (decreasing it) [76, 82]. In view of the positive correlation with an increase in Ca2+-sensitivity (Fig. 7), one may conclude that ALC1 increases cross-bridge kinetics by accelerating selectively fapp over gapp. Even if both rate constants were increased in the presence of ALC1, fapp should be accelerated significantly more than gapp. This results in a ββ-myosin cross-bridge with ALC1 having a longer duty cycle (fraction of time spent in force-generating states per ATP-cycle) in conjunction with faster kinetics than normal ventricular ββ-myosin with VLC1. These kinetic characteristics are compatible with the mechanics described for ventricular muscle fibers from patients with severe mitral regurgitation (mentioned above) for which the ALC1 content was not known [99]. Assuming constant unitary force F' of ββ-myosin with either MLC1, the species containing ALC1 with a longer duty cycle would represent an economic improvement over that with VLC1.
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10 Molecular mechanism |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The molecular mechanism for MLC1 to affect the cross-bridge kinetics seems to reside in its Ala–Pro-rich extended N-terminus which has been shown to interact with the C-terminus of actin [41, 42, 44]. The first hint that the MLC1 might affect the actin–myosin interaction and contractility came from experiments with skeletal muscles (reviewed in [45]). In vertebrate fast skeletal muscles, two isoforms of the MLC1 (ELC1fast and ELC2fast) are expressed which result from different transcription sites and differential splicing of the same gene [108]. The ELC2fast is missing the N-terminal 42 amino acid residues and does not bind to actin. Vmax is twice as high for rabbit fast psoas muscle fibers reconstituted with ELC2fast than with ELC1fast [45]. A similar difference in translocation velocity between myosin with either ELC2fast or ELC1fast is also observed in the in vitro motility assay system [109].
A number of key experiments shed some light on the molecular mechanism how the MLC1 modulates contraction in cardiac muscle. Human ALC1 can be reconstituted to rabbit fast skeletal muscle myosin subfragment-1 (S1) for probing its effect on the actin activated ATPase activity. Incorporated into S1, the ALC1 can also be chemically crosslinked to actin [43, 47]. After removal of the first 11 residues by genetic engineering, the ALC1 can no longer be crosslinked to actin and the ATPase kinetics resemble those of the S1-actin complex with the short ELC2fast isoform. In conclusion, the first 11 residues at the N-terminus of the human ALC1 are sufficient to interact with actin and to modulate the ATPase kinetics. It is likely that this also applies to the human VLC1 as well as to the other MLC1 types. Indeed, removal of the first 13 residues from ELC1fast of chicken skeletal muscle myosin also converts the ATPase kinetics of the S1-actin complex with ELC1fast to those of S1 containing the short ELC2fast [110]. The 13-residue peptide 1-APKKDVKKPAAAA-13 was removed by proteolytic cleavage with papain and is identical in sequence to the corresponding N-terminus of the rat ELC1fast (Table 3). In particular, it contains the N-trimethylalanine at its N-terminus like all MLC1 beginning with Ala [111]. This 13-residue peptide can itself be crosslinked to actin and, when added to the S1-actin complex containing the endogenous intact ELC1fast, it increases the apparent Michaelis constant (Km) as well as the rate of ATPase activity. This was interpreted as indicating that the binding of the 13 residue peptide to actin prevents the normal interaction of the endogenous ELC1fast of S1 with actin and thus converts its ATPase kinetics to those of S1 with the short ELC2fast. Interestingly, a synthetic decapeptide 1-APKKDVKKPA-10 missing the N-terminal N-trimethyl group had hardly any effect on the ATPase kinetics [110].
All MLC1 given in Table 3 comprise four positive charges within the first nine residues distributed over two couples of Lys with one negative charge (Asp or Glu) in between. Removal of the positive charges in Lys3 and Lys4 by exchanging them for Ala by site directed mutagenesis in ALC1 also affects the ATPase kinetics approaching those of an S1-actin complex with the short ELC2fast [43]. Changing the first two Lys3 and Lys4 to Ala or all four, including also Lys7 and Lys8, leads to a gradual increase in Vmax of rabbit fast psoas muscle fibers after incorporation of the mutated rat ELC1fast species in comparison to fibers with its wild type ELC1fast [45]. This demonstrates that the four positive charges among the first nine amino acid residues of MLC1 are involved in long-range electrostatic interactions (salt bridges) with the C-terminus of actin. The C-terminus of actin does, indeed, contain three negative charges in the positions 12, 13, and 15 from the end (Table 3). Cross-bridges with MLC1 containing the characteristic N-terminal extension, display a relatively slow cycling rate in comparison to the short ELC2fast. Weakening these electrostatic interactions while leaving the lengths of the extended N-terminus of the MLC unchanged accelerates cross-bridge kinetics. Taken together, these findings indicate that all long MLC1 species attach with their extended N-terminus to actin and may exert a mechanical drag which slows down the kinetics of the cycling cross-bridge.
In contrast to ELC1fast, ALC1 contains one additional negative charge (Asp) following immediately after the second couple of Lys, while two negative charges (Glu) follow in the VLC1 (Table 3). The first nine residues in human ALC1 and VLC1 are identical in sequence. VLC1 contains Glu at position 9 instead of Asp representing a conservative exchange. An additional negative charge follows, however, in VLC1 with Glu-11. It was recently shown in an in vitro binding assay that a decapeptide corresponding to the residues 4–13 of human ALC1 binds with half the affinity to actin than the corresponding decapeptide from VLC1 [112]. The weaker binding of the ALC1 decapeptide is paralleled by faster cross-bridge kinetics in ventricular fibers with ALC1. These results imply that not only the positively charged residues of the MLC1 N-terminus interact with actin, but that also the additional negative charges in the cardiac MLC1 (not present in skeletal muscle ELC1fast) may differentially interact with the two positive charges near the end of the C-terminus of actin (Table 3).
The synthetic decapeptide 4-KPEPKKDDAK-13 of human VLC1 was used to test its effects on human cardiac muscle fibers [113]. It enhances Vmax, rate of tension development and rate of relaxation when diffused into intact atrial fibers. These effects are not associated with modulation of intracellular Ca2+-transients as monitored by the fluorescence of the Ca2+-indicator Fura-2. It further increases isometric tension at submaximal (pCa 5.5) and maximal (pCa 4.5) Ca2+-activation of permeabilised human ventricular papillary muscle fibers [113]. In addition, this human VLC1 decapeptide induces a supramaximal increase in the ATPase activity of rat ventricular myofibrils at submaximal Ca2+-levels with no effect at low (pCa below 7.0) and at maximal Ca2+-levels (pCa above 5.0) [114]. When troponin-I (TnI) and TnC were extracted from the myofibrils, the VLC1 decapeptide lost its ability to stimulate the ATPase rate. This effect was fully restored upon reconstitution with the TnI-TnC complex [114]. Thus, the activation of the myofibrillar ATPase activity by the VLC1 decapeptide requires a full complement of the actin filament regulatory proteins (troponin-I, -C, -T and tropomyosin). Interestingly, the stimulatory effect occurs at a ratio of 4 peptide molecules per one entire actin filament of roughly 1 µm length and containing approximately 400 actin monomers, suggesting that the peptide engages in a highly cooperative process. Based on these findings it was proposed that at a submaximal level of Ca2+ the peptide analogue allows bound cross-bridges to turn ‘on’ more actin monomers (or actin filament functional units) as opposed to without it [114]. The cooperative response may be the result of peptide-induced alterations in the interactions between the troponin complex, tropomyosin and the actin strands which can spread throughout most, or even all, of the actin filament.
It has been hypothesized that the primary function of the extended MLC1 N-terminus may be to provide a tether between the myosin and actin filaments. Such a tethering would serve to position the two filament systems for cross-bridge interaction [45]. This could be more important for sarcomere assembly during development than in mature striated muscles. This, in turn, may be reflected by the lack of expression of the short ELC2fast in early muscle development. The extended N-terminus is required to bridge the 8 nm gap between the bulk of the MLC1 and the actin surface. Due to the particular sequence rich in Ala and Pro the N-terminus is elongated and rigid prior to its binding to actin (reviewed in [47]), If it were flexible and would assume a globular structure in the detached state, actin binding would be accompanied by a transition from a folded to an extended form. Such a transition would be energetically unfavorable (breaking of intramolecular contacts and imposition of greater order to the system) [115]. The binding site at the end of an extended arm, as opposed to a flexible structure, may also enhance the association rate with actin, but leaving the dissociation rate unaffected [47].
Finally, myosin binds with its upper and lower portion of the 50-kDa segment to two adjacent monomers in the actin filament (Fig. 3) [23]. The MLC1 arm may then reach to the second actin which is also in contact with the lower portion of the 50-kDa, although it has not been demonstrated yet which of the actins interacts with the MLC1. There are then two pathways by which information could be transmitted to the active site which is about 8 nm distant from the actin filament [23, 44], one proceeding via the MHC in the head portion, and the other passing through the extended MLC1 arm and travelling backwards up the motor domain [47]. Taken together, these findings give new insight into the molecular complexity and subtlety of the actin–myosin interaction and its modulation of the cross-bridge kinetics by the MLC1 not anticipated before.
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11 Pathophysiological significance of myosin essential light chain expression in cardiac hypertrophy and failure |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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The human ventricular muscle contains ββ-myosin with low ATPase activity but which performs work at a higher economic level with regard to ATP utilisation in comparison to the faster contracting atrial
-myosin. Under hemodynamic overload, the type of MHC in the ventricle does not, in contrast to atrial muscle, change. However, the fetally expressed ALC1 becomes reexpressed in the ventricle. This ALC1 modulates ventricular contractility by accelerating the cross-bridge kinetics of ββ-myosin and at the same time, it improves the economy by prolonging the duty ratio or duty cycle. The most significant consequence may be the increase in Ca2+-sensitivity of the contractile apparatus containing ALC1. This is particularly important since myocardial contractility is governed by moderate fluctuations in cytosolic free Ca2+-concentrations between systole and diastole [90, 91]. Little or no tension is seen at around 0.1 µM free Ca2+ in the cytosol. Under rest the heart operates only at about 25% of maximal activation (reviewed in [116]). Thus force can increase considerably in response to increasing Ca2+-transients, even if the Ca2+-sensitivity of the myofilaments remains unchanged. The reexpression of ALC1 in the ventricle improves contractility and can be seen as part of the adaptive response during hypertrophy. The extent of ALC1 expression in patients with different types of cardiac hypertrophy varies considerably [68, 69, 117, 118]. The observation that the extent of ALC1 accumulation correlates with the degree of hemodynamic load the heart has to bear, but not with any other parameter such as muscle mass or non-muscle tissue content, points to its physiological relevance [68]. Consequently, the ALC1 content was indeed reduced after successful surgical interventions which lowered the hemodynamic overload in patients with aortic valve diseases [69]. Although muscle specimen are often obtained from patients with end-stage heart failure who underwent cardiac transplantation, the ALC1 content is by no means higher in the ventricles of these hearts than in patients in the compensated phase of hypertrophy (Table 2) [63, 68, 69, 105, 107]. The question arises, however, whether the increased contractility induced by ALC1 may turn into maladaptation in heart failure. Heart failure is characterised by decreased contractility and a low ejection fraction (around 30% or lower). A number of observations made recently, may be at the heart of the matter. Of foremost importance is the handling of intracellular Ca2+. A number of proteins involved in intracellular Ca2+-homeostasis have been shown to be lower in end-stage failing compared to stable cardiac hypertrophy: calsequestrin (Ca2+-binding protein of the sarcoplasmatic reticulum, SR), the Ryanodine receptor (Ca-release channel of SR), SERCA2 (SR-Ca-ATPase) and phospholamban (regulatory protein of SERCA2) [119]. In contrast, the Na–Ca-exchanger is increased [120]. Thus, more Ca2+ is extruded by the Na–Ca-exchanger instead of being sequestered by the intracellular SR, and as a sequel, the myocytes in the failing heart become impoverished on Ca2+. Consequently, much lower systolic Ca2+-transients are seen in cardiomyocytes from human hearts in end-stage failure [121]. This, in turn, results in blunting the normally observed frequency potentiation of contractile force [122–124]. The heart responds with enhanced frequency to the higher catecholamine levels during hypertrophy, but lacks the ability to increase contractile force. The cytosolic Ca2+-levels are only moderately increased during diastole in failing hearts [121] and hardly affect diastolic tension [125]. In addition, it was recently shown in the spontaneously hypertrophic rat model that a defect in excitation–contraction coupling (the Ca2+-induced Ca2+-release from SR) is compensated for by the increased level of circulating catecholamines [126]. Failing of this compensatory mechanism due to a down-regulation of β-adrenergic receptors [127] is thought to be causally linked to the onset of heart failure in this model. The presence of ALC1 then still represents an advantage even during heart failure, in that it allows for a faster and larger force development at decreased systolic Ca2+-levels in compliance with a high heart rate.
On the other hand, inadequate blood supply may often be associated with heart failure. Under such conditions, the energy demand of the hypoperfused myocardium exceeds the reduced energy supply. The myocardium reacts to this imbalance by reducing its contractile function. The mechanism responsible for the rapid reduction in contractile function during myocardial ischemia remains unclear [128, 129]. Teleologically speaking, it seems to represent an attempt to match energy demand with supply. This perfusion–contraction matching may be termed myocardial stunning or hibernation [116, 130]. The myocardium may recover without loss of function [131]. If it fails, however, to reach perfusion–contraction matching, myocardial infarction develops. Under normal conditions, 15 to 30% of the total energy expenditure is consumed by non-cross-bridge processes, mainly by ion pumps (sarcolemmal Na-K-ATPase and SERCA2) to maintain the Ca2+-homeostasis [1, 132]. This part of energy consumption becomes prominent in the hibernating myocardium. An insufficient energy supply may thus affect the membrane stability and lead to an uncontrolled increase in cytosolic Ca2+. Cytosolic Ca2+-levels of 2–75 µM have been shown to occur in ferret cardiomyocytes, at least transiently, during ischemia and post-ischemic reperfusion [133]. Any increase in cytosolic Ca2+ above threshold level of contractile activation during diastole causes diastolic dysfunction. This is characterised by a lack of sufficient relaxation and a build-up of the preload. Due to reduced chamber filling, the ejection fraction may diminish. If the myocardium contains ALC1, its increased Ca2+-sensitivity accentuates diastolic dysfunction and thus turns the originally beneficial adaptation into maladaptation.
Elevated cytosolic Ca2+-levels may, in addition, activate non-lysosomal neutral proteinases such as µ-calpain (Ca2+-dependent proteinase-I) which requires 1–100 µM Ca2+ for activation [134]. There are a number of sarcomeric proteins which are selectively susceptible to such neutral proteinases and which have been shown to be degraded in terminally failing hearts. The giant protein titin (3000 kDa) which spans the entire half sarcomere by connecting the myosin filament with the Z-line [135] is reduced in mRNA and protein content in failing human hearts [136]. This is accompanied by the occurrence of a titin with lower molecular mass, which probably represents a degradation product. At the same time, myofibrillar and cytoskeletal structures are impaired on the immunocytochemical and electron microscopic level [137–139].
The selective degradation of the VLC2 has been reported for patients with DCM undergoing heart transplantation [140]. Myosin isolated from these specimens displayed a lower actin activated ATPase activity and an impaired capability to assemble into synthetic myosin filaments in vitro. It was further shown that these hearts contained an increased proteolytic activity specific for VLC2 [140]. Removal of RLCfast has been shown in skeletal muscle fibers to slow down Vmax and to shift the pCa–tension curve to the left (increasing Ca2+-sensitivity) [141, 142]. It was concluded that removal of RLCfast slows down gapp and increases, therefore, the number of force generating cross-bridges. Whether degradation of VLC2 is typically associated with failing DCM hearts has not been established unequivocally. We did not observe a change in the stoichiometry of the MLC1 and MLC2 complement in terminally failing hearts of patients with DCM or ICM [63, 136].
It has recently been shown in the rat experimental model that TnI is selectively degraded in the stunned myocardium [143]. This degradation can be prevented when stunning is followed by low Ca2+-reperfusion. Addition of µ-calpain (calpain-I) to permeabilised myocardial fibers produces identical digestion patterns. It is hypothesized that the Ca2+-activated proteinase cleaves TnI in ischemic myocardium and that this may directly impair contractility. Ischemia and hibernation are, however, complex processes in which numerous factors contribute to the pathophysiology. Whether global or locally restricted, their repercussions will affect, and most probably, will precipitate the course of heart failure.
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12 Other mechanisms modulating contractility |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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Changes in isoform expression of sarcomeric proteins other than the MLC1 may also affect Ca2+-sensitivity of the contractile apparatus. The cardiac troponin (Tn) complex comprises the inhibitory (TnI), the Ca2+-binding (TnC) and the tropomyosin binding (TnT) subunits. Four of the 36 potential splice variants of TnT [144, 145] have been found in human hearts [146, 147]. Their appearance is developmentally regulated and TnT3c predominates in adult human hearts. Reexpression of the fetal TnT4c isoform has been observed on the mRNA and protein level in some few cases of end-stage heart failure [145–148]. The occurrence of TnT4c was associated with decreased myofibrillar ATPase activity in human hearts. The Ca2+-sensitivity of tension is also lower in permeabilised heart muscle fibers of rats and rabbits when TnT4c is present [149, 150]. Taken together, the fetal TnT4c isoform has the opposite effect on Ca-sensitivity than ALC1. It is not known whether the expression of TnT4c correlates to the hemodynamic load. If its expression were triggered by mechanical load as that of ALC1 seems to be, both TnT4c and ALC1 would increase concomitantly and antagonise each other's effect on contractility. Since mechanical data correlate linearly with the ALC1 content (Fig. 7) the possible negative effects of TnT4c, whatever its actual concentration, would be minor. It could be that a variable degree of expression of TnT4c contributes to the scatter of the data.
In both normal and diseased human hearts, we found only cardiac TnI [63]. Slow skeletal muscle TnI (TnIs) mRNA may be present in infants but not in adult human hearts [151]. TnIs has been shown to increase Ca2+-sensitivity of cardiac myofibrillar ATPase activity in comparison to cardiac TnI [152]. The two isoforms of TnC (TnCf in fast skeletal muscles and TnCs in heart and slow skeletal muscles) originate from two different genes [153]. Only TnCs is found in the heart. Replacement of TnCf by TnCs reduced the Ca2+-sensitivity of tension in fast skeletal muscle fibers in vitro [154]. This transition is accompanied by a decrease of the Hill coefficient from 7.7 to 2.4. In contrast, the Hill coefficient remains unchanged around 2 with increasing ALC1 content and increasing Ca2+-sensitivity in failing human hearts [63]. If sufficient TnCf was expressed in failing hearts to positively affect the Ca2+-sensitivity, the Hill-coefficient would be expected to rise concomitantly.
Improved contractility has been shown in BALB/c mice which express skeletal muscle -actin in their hearts in comparison to normal mice with mainly cordiac -actin [155]. In human hearts both cardiac (
40%) and skeletal (60%) -actin isoforms are expressed. However, this ratio does not change in patients with DCM or ICM as indicated by mRNA levels [156]. Ectopic expression of the β-tropomyosin (β-Tm) in the heart of transgenic mice produces an increase in Ca2+-sensitivity and a decrease in the rightward shift of the pCa–tension relation induced by cAMP-dependent phosphorylation of TnI (see below) as opposed to the -Tm isoform which is normally present [157]. In human myocardium we find almost exclusively the -Tm isoform and no change in patients with compensated hypertrophy [69] nor in end-stage heart failure [63].
Familial hypertrophic cardiomyopathy (FHC) constitutes a genetically heterogeneous disease. The hypertrophy seems to be a reactive adaptation to the underlying disturbance of the contractile apparatus which is caused mostly by missense or deletion mutations in any of the following sarcomeric proteins: MHC, VLC1, VLC2, -Tm, cardiac MyBP-C or cardiac TnT (reviewed in [5, 27, 54]). Most of these mutated proteins are functionally dominant-negative by reducing contractility and motility in in vitro assay systems.
A number of short-term modulatory mechanisms involving reversible phosphorylation (Table 4) affect cardiac contractility in addition to the long-term adaptive changes in gene expression. Human VLC2 (164 amino acids) contains near its N-terminus (1-APKKAKKRAGGANSNVF-17) one recognition site for phosphorylation of the Ser14 by either myosin light chain kinase (MLCK) or protein kinase-C (PKC) but not by protein kinase-A (PKA) [128, 158, 159]. The three basic residues (KKR) upstream of Ser14 and the highly conserved three residues following Ser14 (NVF) constitute the substrate specificity for MLCK. In addition, all vertebrate RLC contain the two adjacent Lys (K) near their N-terminus. Thus the clusters of positive charges are reminiscent of those in the MLC1 isoforms (compare Table 3). These positive charges near the N-terminus may in both cases facilitate to contact the binding site of the reaction partner: (i) the proximate actin monomer in the case of MLC1 (as discussed above) and (ii) MLCK in the case of MLC2. Phosphorylation of both VLC2 and ALC2 affects contractility by increasing the Ca2+-sensitivity of force development as well as ATPase activity [97, 160–163]. This seems to be achieved by an increase in the rate fapp of the transition from non-force to force-generating cross-bridges [97]. ALC2 can also be bis-phosphorylated, however, the physiological significance of this feature is not known [53]. Sequence data are not yet available for ALC2.
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Phosphorylation of TnI by PKA which results from β-adrenergic stimulation, leads to a decrease in the sensitivity of the contractile proteins to Ca2+ which is accompanied by an increase in cross-bridge cycling rate (reviewed in [128]). Cardiac TnI is thought to exert its effects by reducing the affinity of TnCs for Ca2+. This may function as a negative feed-back mechanism by accelerating the relaxation rate of the beating heart under the influence of β-adrenergic stimulation [164]. Human cardiac TnI contains 209 amino acids with a 31 residue extension at its N-terminus found neither in the fast nor in the slow skeletal isoforms [165]. Phosphorylation of the two adjacent Ser22/Ser23 is required for these effects (Table 4) [166]. This represents an interesting situation where the tissue-specific hormonal response is discriminated not at the receptor level at the cell surface, but inside the cell by a particular amino acid sequence in the target protein which is not present in the isoforms of other muscle tissues. The same principle applies also to the cardiac isoform of the myosin binding protein-C (see below). In addition, cardiac TnI as well as TnT can be phosphorylated at multiple sites by PKC isoforms (Table 4). In general, phosphorylation of either cardiac TnI or TnT by PKC isoforms reduces Ca2+-sensitivity and actin activated ATPase activity [167].
The -Tm can also be phosphorylated at its penultimate Ser283 which enhances the actin activated ATPase activity without changing the Ca2+-sensitivity [168]. This phosphorylation site lies in the region where two adjacent Tm molecules join head-to-tail together by overlapping with about 9 residues. The N-terminal portion of TnT which connects the Tn complex to Tm also makes contact in this region [168]. Responsible for -Tm phosphorylation is a Tm kinase which does not require Ca2+, calmodulin nor cAMP [169]. The myosin binding protein-C (MyBP-C, 137 kDa) (Figs. 1 and 2
) is the only protein in the myosin filament that can be phosphorylated by PKA [128, 170]. Human cardiac MyBP-C (1173 amino acids) contains a nine-residue specific loop 174-LAGGGRRIS-182 in the N-terminal region which can be reversibly phosphorylated upon β-adrenergic stimulation and which is not present in the skeletal muscle isoforms [171]. As a consequence, the cardiac MyBP-C can be phosphorylated at four sites in contrast to only one such site in the skeletal muscle isoforms (Table 4). One of the four sites in cardiac MyBP-C (Ser182 located in the cardiac-specific loop) needs to be phosphorylated first by a Ca2+-calmodulin dependent protein kinase (CAMK) before PKA is able to phosphorylate the remaining three sites (Ser173/Ser202/Thr261) [171]. MyBP-C can also be phosphorylated by PKC. Both PKA and PKC have been shown to phosphorylate the same sites on cardiac MyBP-C [172]. Phosphorylation of MyBP-C provokes the cross-bridges to extend from the backbone of the myosin filament [170]. This could facilitate cross-bridge attachment to the actin filament and thereby modify the kinetics of force generation in activated cardiac muscle. Its specific effect on contractility is not known at present, since no method exists for removal and reconstitution of MyBP-C in isolated contractile systems. In situ phosphorylation of MyBP-C cannot be controlled selectively without affecting other sarcomeric proteins as well.
Finally, cardiac contractility is also influenced by changes in sarcomere length and variations in intracellular metabolites. Lowering of intracellular pH and accumulation of inorganic phosphate Pi are known to lower Ca2+-sensitivity of force. It has recently been shown that the pH-dependent influence on the Ca2+-sensitivity is mediated by TnI which modulates the binding affinity of Ca2+ to cardiac TnC [173]. During hypoxia accumulation of intracellular Pi may reach levels higher than 5 mM [174]. This leads to reduced tension generation at unchanged or increased intracellular Ca2+-transients [175, 176]. In normal heart muscle as well as during hypertrophy the Ca2+-sensitivity significantly increases with stretch, i.e. with increasing sarcomere length. This phenomenon provides the basis for the Frank–Starling mechanism which is one of the most important regulatory devices in controlling cardiac contractility from beat-to-beat. The length-dependent change in Ca2+-sensitivity of permeabilised cardiac muscle fibers correlates more closely to the change in interfilament spacing than to the change in sarcomere length [177, 178]. The narrowing of the interfilament lattice spacing during lengthening seems to facilitate cross-bridge attachment to the actin filament. Replacement of endogenous cardiac TnC by the skeletal muscle isoform of TnC results in a reduction of the length dependence of cardiac fibers to the level of skeletal muscle fibers [179]. The lowered length dependence can be reversed to the original cardiac level by reconstitution with a TnC chimera containing the first 41 amino acid residues of cardiac TnC spliced onto the skeletal muscle isoform. Ectopic expression of skeletal muscle TnC in the hearts of transgenic mice did, however, not change the Ca2+-sensitivity of tension in these hearts [177]. There is some controversy in the literature whether or not this mechanism is operative in the failing heart. Left ventricular papillary muscle strips from patients with DCM and heart failure failed to increase force upon increasing preload when electrically driven at 1 Hz and 37°C, as did those from non-failing hearts [105]. Permeabilised muscle fibers from the same failing hearts displayed a higher Ca2+-sensitivity than controls, a sensitivity however, which did not further increase under pre-stretch. This is in contrast to a recent study in which an unchanged positive Frank–Starling mechanism was demonstrated in hearts of patients with end-stage failure suffering from DCM or ICM [180]. Mechanical tests were done on whole left ventricles, on isolated intact muscle strips and on permeabilised fibers. The reason for the different results is not clear. One wonders, whether such differences may be due to degradation processes either in the contractile or cytoskeletal structures stemming from in vivo stages or occurring during sample preparation.
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13 Conclusions and outlook |
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TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
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We have first reported that the reexpression of ALC1 in human ventricle during hypertrophy is related to the degree of hemodynamic load [68, 69]. We are now able to explain the underlaying molecular mechanism for the improved ventricular contractility by ALC1. ALC1 enhances cross-bridge kinetics in such a way that force generation is increased at a given cytosolic Ca2+-concentration. This functional effect can be traced to the particular primary structure of the ALC1 which modulates the interaction of myosin with the actin filament during the cross-bridge cycle.
In addition, covalent protein modification by reversible phosphorylation may transiently affect contractility, but it does so in a complex manner. Increased β-adrenergic stimulation is characteristic for hypertrophy as well as for heart failure. This stimulation mainly operates via the adenylate cyclase/cAMP/cAMP-dependent PKA system (reviewed in [8]). Increased PKA activity is known to induce phosphorylation of (i) Ca2+-channels in the sarcolemma, (ii) phospholamban (regulatory protein of SERCA2) and (iii) sarcomeric proteins TnI and MyBP-C (reviewed in [181]). This results in a higher intracellular Ca2+-level. The phosphorylated TnI attenuates contractility. The increased Ca2+ (together with calmodulin) stimulates the activity of MLCK and increases MLC2 phosphorylation. Phosphorylated MLC2 promotes contractility and thus counteracts the attenuating effect of phosphorylated TnI. Ca2+-calmodulin is also required for activation of CAMK which, in turn, allows for the sequential phosphorylation of MyBP-C by PKA which may also enhance contractility. On the other hand, a number of G-protein coupled receptor systems (including muscarinic cholinergic, angiotensin-II, endothelin-1 and -adrenergic) in the cardiomyocyte induce PKC activation [8, 172]. PKC phosphorylates TnT and TnI at various sites depending on the isoform species. PKC furthermore, competes with MLCK and PKA for phosphorylation of the same sites in MLC2 and MyBP-C, respectively (Table 4). The interconnection between different signaling pathways involving various kinds of protein kinases which may result in identical phosphorylation of several target proteins, seems at first sight to comprise some redundancy. However, in view of the multifactorial etiology that leads to hypertrophy and heart failure, the cardiomyocyte depends on a complex repertoire of mechanisms for modulating and fine-tuning its contractile responses to varying demands.
In conclusion, the short-term regulatory processes of cardiomyocytes may vary rapidly with time in response to global changes in the whole body or to local changes in the heart muscle. These rapid changes may enhance or restrict contractility in accordance with temporary demands and energy supply. The adaptive long-lasting changes in gene expression provide the basis the short-term modulatory processes may act upon. The particular state in which the myocyte finds itself, thus, determines whether the increased Ca2+-sensitivity of the contractile apparatus brought about by the expression of ALC1 in the hypertrophic ventricle may be beneficial or detrimental in the event of heart failure.
In the near future we will have to learn more about molecular mechanisms of normal and compromised heart function. This may be pursued by two main routes of research involving (i) transgenic animals with either ablation or overexpression of specific genes, and (ii) primary cultures of cardiomyocytes and other heart cells as an in vitro model system for the hypertrophy reaction. The first approach gives information on functional alterations in the intact organism with all its complex responses and compensatory mechanisms. The second approach allows to study the effects of single hypertrophic stimuli (such as hormones, growth factors, cytokines, vasoactive peptides as well as mechanical stress) under defined conditions on cell morphology as well as on gene expression [5, 7, 8]. Today, primary culture systems still have to be used as long as no viable established myocardial cell lines exist. This arises from the fact that the cell cycle of the terminally differentiated cardiomyocytes is blocked. A further goal in heart research will certainly be to overcome this block by interference with the cell cycle control mechanisms. This could eventually open the possibility to compensate cardiovascular overload by controlled hyperplasia (proliferation of cardiomyocytes) rather than hypertrophy alone. As long as this route is barred, it seems desirable to better control the hypertrophy reaction by furthering the positive and minimizing the negative aspects. To this end, the gene expression pattern needs changing. That may be achieved by either administering drugs such as cell mediators and anti-sense oligonucleotides, or by somatic gene transfer. Overexpression of several genes in the heart has been shown recently to augment cardiac performance in laboratory animal models [182]. If the latter approach were taken, it has to be born in mind, however, that overexpression of a particular gene should be driven by a promoter which is specific for human ventricular tissue. In summary, detailed knowledge of the molecular mechanisms connected with the hypertrophy reaction and its transition into heart failure is a prerequisite for the development of new potential therapeutic strategies as well as of more adequate animal models to test them in vivo.
Time for primary review 26 days.
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Acknowledgements |
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The work reported herein was supported by the Swiss National Science Foundation Grant 31-40694.94 and the Swiss Heart Foundation-1995 (M.C.S.) and the Deutsche Forschungsgemeinschaft Grant Mo-362/15-1 (I.M.).
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References |
|---|
|
TopAbstract1 Introduction2 The myosin motor3 Cardiac myosin heavy...4 Cardiac myosin light...5 Combinatorial associations of...6 Myosin cross-bridge kinetics7 Ca2+-control of cross-bridge...8 Effects of myosin...9 Modulation of contractility...10 Molecular mechanism11 Pathophysiological...12 Other mechanisms modulating...13 Conclusions and outlookReferences |
|---|
- Peterson J.N., Alpert N.R. Time course of mechanical efficiency during afterloaded contractions in isolated cardiac muscle. Am J Physiol (1991) 261(4 Suppl):27–29.[Web of Science][Medline]
- Gibbs C.L., Barclay C.J. Cardiac efficiency. Cardiovasc Res (1995) 30:627–634.
[Free Full Text] - Grossman W. Cardiac hypertrophy: useful adaptation or pathologic process? Am J Med (1980) 69:576–584.[CrossRef][Web of Science][Medline]
- van Bilsen M., Chien K.R. Growth and hypertrophy of the heart: towards an understanding of cardiac specific and inducible gene expression. Cardiovasc Res (1993) 27:1140–1149.[Web of Science][Medline]
- Schaub MC, Hefti MA, Harder BA, Eppenberger HM. Various hypertrophic stimuli induce distinct phenotypes in cardiomyocytes. J Mol Med 1997; (in press).
- Boluyt M.O., O'Neill L., Meredith A.L., Bing O.H., Brooks W.W., Conrad C.H., Crow M.T., Lakatta E.G. Alterations in cardiac gene expression during the transition from stable hypertrophy to heart failure. Marked upregulation of genes encoding extracellular matrix components. Circ Res (1994) 75:23–32.
[Abstract/Free Full Text] - Glennon P.E., Sugden P.H., Poole-Wilson P.A. Cellular mechanisms of cardiac hypertrophy. Br Heart J (1995) 73:496–499.
[Free Full Text] - Hefti MA, Harder BA, Eppenberger HM, Schaub MC. Hypertrophy in cultured cardiomyocytes, signaling pathways and phenotypes. J Mol Cell Cardiol 1997; (in press).
- Rayment I., Rypniewski W.R., Schmidt-Base K., Smith R., Tomchick D.R., Benning M.M., Winkelmann D.A., Wesenberg G., Holden H.M. Three-dimensional structure of myosin subfragment-1: a molecular motor. Science (1993) 261:50–58.
[Abstract/Free Full Text] - Holmes K.C. The swinging lever-arm hypothesis of muscle contraction. Curr Biol (1997) 7:R112–118.[CrossRef][Web of Science][Medline]
- Xie X., Harrison D.H., Schlichting I., Sweet R.M., Kalabokis V.N., Szent-Gyorgyi A.G., Cohen C. Structure of the regulatory domain of scallop myosin at 2.8 Å resolution. Nature (1994) 368:306–312.[CrossRef][Medline]
- Rayment I., Holden H.M., Sellers J.R., Fananapazir L., Epstein N.D. Structural interpretation of the mutations in the beta-cardiac myosin that have been implicated in familial hypertrophic cardiomyopathy. Proc Natl Acad Sci USA (1995) 92:3864–3868.
[Abstract/Free Full Text] - Sellers J.R., Goodson H.V. Motor proteins 2: myosin. Protein Profile (1995) 2(12):1323–1423.[Web of Science][Medline]
- Reedy M.K., Holmes K.C., Tregear R.T. Induced changes in orientation of the cross-bridges of glycerinated insect flight muscle. Nature (1965) 207:1276–1280.[CrossRef][Medline]
- Huxley H.E. The mechanism of muscular contraction. Science (1969) 164:1356–1365.
[Free Full Text] - Bordas J., Diakun G.P., Diaz F.G., Harries J.E., Lewis R.A., Lowy J., Mant G.R., Martin-Fernandez M.L., Towns-Andrews E. Two-dimensional time-resolved X-ray diffraction studies of live isometrically contracting frog sartorius muscle. J Muscle Res Cell Motil (1993) 14:311–324.[CrossRef][Web of Science][Medline]
- Irving M., St. Claire Allen T., Sabido-David C., Craik J.S., Brandmeier B., Kendrick-Jones J., Corrie J.E., Trentham D.R., Goldman Y.E. Tilting of the light-chain region of myosin during step length changes and active force generation in skeletal muscle. Nature (1995) 375:688–691.[CrossRef][Medline]
- Huxley A.F., Simmons R.M. Proposed mechanism of force generation in striated muscle. Nature (1971) 233:533–538.[CrossRef][Medline]
- Cooke R. The mechanism of muscle contraction. CRC Crit Rev Biochem (1986) 21:53–118.[Web of Science][Medline]
- Uyeda T.Q., Abramson P.D., Spudich J.A. The neck region of the myosin motor domain acts as a lever arm to generate movement. Proc Natl Acad Sci USA (1996) 93:4459–4464.
[Abstract/Free Full Text] - Finer J.T., Simmons R.M., Spudich J.A. Single myosin molecule mechanics: piconewton forces and nanometre steps. Nature (1994) 368:113–119.[CrossRef][Medline]
- Holmes K.C., Popp D., Gebhard W., Kabsch W. Atomic model of the actin filament. Nature (1990) 347:44–49.[CrossRef][Medline]
- Rayment I., Holden H.M., Whittaker M., Yohn C.B., Lorenz M., Holmes K.C., Milligan R.A. Structure of the actin–myosin complex and its implications for muscle contraction. Science (1993) 261:58–65.
[Abstract/Free Full Text] - Milligan R.A. Protein–protein interactions in the rigor actomyosin complex. Proc Natl Acad Sci USA (1996) 93:21–26.
[Abstract/Free Full Text] - Uyeda T.Q., Ruppel K.M., Spudich J.A. Enzymatic activities correlate with chimaeric substitutions at the actin-binding face of myosin. Nature (1994) 368:567–569.[CrossRef][Medline]
- Spudich J.A. How molecular motors work. Nature (1994) 372:515–518.[CrossRef][Medline]
- Vikstrom K.L., Leinwand L.A. Contractile protein mutations and heart disease. Curr Opin Cell Biol (1996) 8:97–105.[CrossRef][Web of Science][Medline]
- Matsuoka R., Beisel K.W., Furutani M., Arai S., Takao A. Complete sequence of human cardiac alpha-myosin heavy chain gene and amino acid comparison to other myosins based on structural and functional differences. Am J Med Genet (1991) 41:537–547.[CrossRef][Web of Science][Medline]
- Jaenicke T., Diederich K.W., Haas W., Schleich J., Lichter P., Pfordt M., Bach A., Vosberg H.P. The complete sequence of the human beta-myosin heavy chain gene and a comparative analysis of its product. Genomics (1990) 8:194–206.[CrossRef][Web of Science][Medline]
- McNally E.M., Kraft R., Bravo-Zehnder M., Taylor D.A., Leinwand L.A. Full-length rat alpha and beta cardiac myosin heavy chain sequences. Comparisons suggest a molecular basis for functional differences. J Mol Biol (1989) 210:665–671.[CrossRef][Web of Science][Medline]
- Swynghedauw B. Developmental and functional adaptation of contractile proteins in cardiac and skeletal muscles. Physiol Rev (1986) 66:710–771.
[Abstract/Free Full Text] - Schiaffino S., Reggiani C. Molecular diversity of myofibrillar proteins: gene regulation and functional significance. Physiol Rev (1996) 76:371–423.
[Abstract/Free Full Text] - Cuda G., Fananapazir L., Epstein N.D., Sellers J.R. The in vitro motility activity of beta-cardiac myosin depends on the nature of the beta-myosin heavy chain gene mutation in hypertrophic cardiomyopathy. J Muscle Res Cell Motil (1997) 18:275–283.[CrossRef][Web of Science][Medline]
- Lankford E.B., Epstein N.D., Fananapazir L., Sweeney H.L. Abnormal contractile properties of muscle fibers expressing beta-myosin heavy chain gene mutations in patients with hypertrophic cardiomyopathy. J Clin Invest (1995) 95:1409–1414.[Web of Science][Medline]
- Kawasaki H., Kretsinger R.H. Calcium-binding proteins. 1: EF-hands. Protein Profile (1994) 1(4):343–517.[Web of Science][Medline]
- Watterson J.G., Kohler L., Schaub M.C. Evidence for two distinct affinities in the binding of divalent metal ions to myosin. J Biol Chem (1979) 254:6470–6477.
[Free Full Text] - Wagner P.D., Giniger E. Hydrolysis of ATP and reversible binding to F-actin by myosin heavy chains free of all light chains. Nature (1981) 292:560–562.[CrossRef][Medline]
- Sivaramakrishnan M., Burke M. The free heavy chain of vertebrate skeletal myosin subfragment 1 shows full enzymatic activity. J Biol Chem (1982) 257:1102–1105.
[Abstract/Free Full Text] - Adelstein R.S., Eisenberg E. Regulation and kinetics of the actin–myosin–ATP interaction. Annu Rev Biochem (1980) 49:921–956.[CrossRef][Web of Science][Medline]
- Di Blasio B., Pavone V., Saviano M., Lombardi A., Nastri F., Pedone C., Benedetti E., Crisma M., Anzolin M., Toniolo C. Structural characterization of the beta-bend ribbon. Spiral crystallographic analysis of two long L-Pro-Aib-N sequential peptides. J Am Chem Soc (1992) 114:6273–6278.[CrossRef][Web of Science]
- Sutoh K. Identification of myosin-binding sites on the actin sequence. Biochemistry (1982) 21:3654–3661.[CrossRef][Web of Science][Medline]
- Trayer I.P., Trayer H.R., Levine B.A. Evidence that the N-terminal region of A1-light chain of myosin interacts directly with the C-terminal region of actin. A proton magnetic resonance study. Eur J Biochem (1987) 164:259–266.[Web of Science][Medline]
- Timson D.J., Trayer I.P. The myosin essential light chain: how it fine tunes a protein machine. J Muscle Res Cell Motil (1997) 18:260. abstract.
- Milligan R.A., Whittaker M., Safer D. Molecular structure of F-actin and location of surface binding sites. Nature (1990) 348:217–221.[CrossRef][Medline]
- Sweeney H.L. Function of the N terminus of the myosin essential light chain of vertebrate striated muscle. Biophys J (1995) 68(4 Suppl):112S–118S. discussion 118S–119S.[Medline]
- Sweeney H.L., Holzbaur E.L. Mutational analysis of motor proteins. Annu Rev Physiol (1996) 58:751–792.[CrossRef][Web of Science][Medline]
- Timson D.J., Trayer I.P. The role of the proline-rich region in A1-type myosin essential light chains: implications for information transmission in the actomyosin complex. FEBS Lett (1997) 400:31–36.[CrossRef][Web of Science][Medline]
- Barton P.J., Robert B., Cohen A., Garner I., Sassoon D., Weydert A., Buckingham M.E. Structure and sequence of the myosin alkali light chain gene expressed in adult cardiac atria and fetal striated muscle. J Biol Chem (1988) 263:12669–12676.
[Abstract/Free Full Text] - Barton P.J., Cohen A., Robert B., Fiszman M.Y., Bonhomme F., Guenet J.L., Leader D.P., Buckingham M.E. The myosin alkali light chains of mouse ventricular and slow skeletal muscle are indistinguishable and are encoded by the same gene. J Biol Chem (1985) 260:8578–8584.
[Abstract/Free Full Text] - Price K.M., Littler W.A., Cummins P. Human atrial and ventricular myosin light-chains subunits in the adult and during development. Biochem J (1980) 191:571–580.[Web of Science][Medline]
- Morano I., Arndt H., Gartner C., Ruegg J.C. Skinned fibers of human atrium and ventricle: myosin isoenzymes and contractility. Circ Res (1988) 62:632–639.
[Abstract/Free Full Text] - Kamm K.E., Stull J.T. The function of myosin and myosin light chain kinase phosphorylation in smooth muscle. Annu Rev Pharmacol Toxicol (1985) 25:593–620.[CrossRef][Web of Science][Medline]
- Morano I., Wankerl M., Bohm M., Erdmann E., Ruegg J.C. Myosin P-light chain isoenzymes in the human heart: evidence for diphosphorylation of the atrial P-LC form. Basic Res Cardiol (1989) 84:298–305.[CrossRef][Web of Science][Medline]
- Poetter K., Jiang H., Hassanzadeh S., Master S.R., Chang A., Dalakas M.C., Rayment I., Sellers J.R., Fananapazir L., Epstein N.D. Mutations in either the essential or regulatory light chains of myosin are associated with a rare myopathy in human heart and skeletal muscle. Nature Gen (1996) 13:63–69.[CrossRef][Web of Science][Medline]
- Bouvagnet P., Leger J., Dechesne C.A., Dureau G., Anoal M., Leger J.J. Local changes in myosin types in diseased human atrial myocardium: a quantitative immunofluorescence study. Circulation (1985) 72:272–279.
[Abstract/Free Full Text] - Buttrick P.M., Malhotra A., Brodman R., McDermott L., Lam L. Myosin isoenzyme distribution in overloaded human atrial tissue. Circulation (1986) 74:477–483.
[Abstract/Free Full Text] - Cummins P., Lambert S.J. Myosin transitions in the bovine and human heart. A developmental and anatomical study of heavy and light chain subunits in the atrium and ventricle. Circ Res (1986) 58:846–858.
[Abstract/Free Full Text] - Cummins P. Contractile protein transitions in human cardiac overload: reality and limitations. Eur Heart J (1984) 5(Suppl_F):119–127.
[Abstract/Free Full Text] - Gorza L., Mercadier J.J., Schwartz K., Thornell L.E., Sartore S., Schiaffino S. Myosin types in the human heart. An immunofluorescence study of normal and hypertrophied atrial and ventricular myocardium. Circ Res (1984) 54:694–702.
[Abstract/Free Full Text] - Schwartz K., Apstein C., Mercadier J.J., Lecarpentier Y., de la Bastie D., Bouveret P., Wisnewsky C., Swynghedauw B. Left ventricular isomyosins in normal and hypertrophied rat and human hearts. Eur Heart J (1984) 5(Suppl F):77–83.
[Abstract/Free Full Text] - Yazaki Y., Tsuchimochi H., Kuro-o M., Kurabayashi M., Isobe M., Ueda S., Nagai R., Takaku F. Distribution of myosin isozymes in human atrial and ventricular myocardium: comparison in normal and overloaded heart. Eur Heart J (1984) 5(Suppl F):103–110.
[Abstract/Free Full Text] - Bouvagnet P., Mairhofer H., Leger J.O., Puech P., Leger J.J. Distribution pattern of alpha and beta myosin in normal and diseased human ventricular myocardium. Basic Res Cardiol (1989) 84:91–102.[CrossRef][Web of Science][Medline]
- Morano I., Hadicke K., Haase H., Bohm M., Erdmann E., Schaub M. Changes in essential myosin light chain isoform expression provide a molecular basis for isometric force regulation in the failling human heart. J Mol Cell Cardiol (1997) 29:1177–1187.[CrossRef][Web of Science][Medline]
- Cummins P. Transitions in human atrial and ventricular myosin light-chain isoenzymes in response to cardiac-pressure-overload-induced hypertrophy. Biochem J (1982) 205:195–204.[Web of Science][Medline]
- Arndt H., Bletz C., Katus H.A., Mall G., Ruegg J.C. Calcium sensitivity and unloaded shortening velocity of hypertrophied and non-hypertrophied skinned human atrial fibres. Pflugers Arch (1989) 415:209–213.[CrossRef][Web of Science][Medline]
- Wankerl M., Bohm M., Morano I., Ruegg J.C., Eichhorn M., Erdmann E. Calcium sensitivity and myosin light chain pattern of atrial and ventricular skinned cardiac fibers from patients with various kinds of cardiac disease. J Mol Cell Cardiol (1990) 22:1425–1438.[CrossRef][Web of Science][Medline]
- Schaub M.C., Tuchschmid C.R., Srihari T., Hirzel H.O. Myosin isoenzymes in human hypertrophic hearts. Shift in atrial myosin heavy chains and in ventricular myosin light chains. Eur Heart J (1984) 5(Suppl F):85–93.[Medline]
- Hirzel H.O., Tuchschmid C.R., Schneider J., Krayenbuehl H.P., Schaub M.C. Relationship between myosin isoenzyme composition, hemodynamics and myocardial structure in various forms of human cardiac hypertrophy. Circ Res (1985) 57:729–740.
[Abstract/Free Full Text] - Sutsch G., Brunner U.T., von Schulthess C., Hirzel H.O., Hess O.M., Turina M., Krayenbuehl H.P., Schaub M.C. Hemodynamic performance and myosin light chain-1 expression of the hypertrophied left ventricle in aortic valve disease before and after valve replacement. Circ Res (1992) 70:1035–1043.
[Abstract/Free Full Text] - Eppenberger M.E., Hauser I., Baechi T., Schaub M.C., Brunner U.T., Dechesne C.A., Eppenberger H.M. Immunocytochemical analysis of the regeneration of myofibrils in long-term cultures of adult cardiomyocytes of the rat. Dev Biol (1988) 130:1–15.[CrossRef][Web of Science][Medline]
- Henkel R.D., Kammerer C.M., Escobedo L.V., VandeBerg J.L., Walsh R.A. Correlated expression of atrial myosin heavy chain and regulatory light chain isoforms with pressure overload hypertrophy in the non-human primate. Cardiovasc Res (1993) 27:416–422.
[Abstract/Free Full Text] - Zak R. Metabolism of myofibrillar proteins in the normal and hypertrophic heart. Basic Res Cardiol (1977) 72:235–240.[CrossRef][Web of Science][Medline]
- Martin A.F. Turnover of cardiac troponin subunits. Kinetic evidence for a precursor pool of troponin-I. J Biol Chem (1981) 256:964–968.
[Abstract/Free Full Text] - Gulick J., Hewett T.E., Klevitsky R., Buck S.H., Moss R.L., Robbins J. Transgenic remodeling of the regulatory myosin light chains in the mammalian heart. Circ Res (1997) 80:655–664.
[Abstract/Free Full Text] - Eisenberg E., Hill T.L. Muscle contraction and free energy transduction in biological systems. Science (1985) 227:999–1006.
[Abstract/Free Full Text] - Brenner B. The cross-bridge cycle in muscle. Mechanical, biochemical and structural studies on single skinned rabbit psoas fibers to characterize cross-bridge kinetics in muscle for correlation with the actomyosin-ATPase in solution. Basic Res Cardiol (1986) 81(Suppl 1):1–15.[CrossRef][Web of Science][Medline]
- Dantzig J.A., Goldman Y.E., Millar N.C., Lacktis J., Homsher E. Reversal of the cross-bridge force-generating transition by photogeneration of phosphate in rabbit psoas muscle fibres. J Physiol (1992) 451:247–278.
[Abstract/Free Full Text] - Brenner B., Yu L.C. Structural changes in the actomyosin cross-bridges associated with force generation. Proc Natl Acad Sci USA (1993) 90:5252–5256.
[Abstract/Free Full Text] - Moss R.L., Diffee G.M., Greaser M.L. Contractile properties of skeletal muscle fibers in relation to myofibrillar protein isoforms. Rev Physiol Biochem Pharmacol (1995) 126:1–63.[Web of Science][Medline]
- Huxley A.F. Muscle structure and theories of cintraction. Prog Biophys Chem (1957) 7:255–318.
- Brenner B., Schoenberg M., Chalovich J.M., Greene L.E., Eisenberg E. Evidence for cross-bridge attachment in relaxed muscle at low ionic strength. Proc Natl Acad Sci USA (1982) 79:7288–7291.
[Abstract/Free Full Text] - Brenner B. Effect of Ca2+ on cross-bridge turnover kinetics in skinned single rabbit psoas fibers: implications for regulation of muscle contraction. Proc Natl Acad Sci USA (1988) 85:3265–3269.
[Abstract/Free Full Text] - Siemankowski R.F., Wiseman M.O., White H.D. ADP dissociation from actomyosin subfragment 1 is sufficiently slow to limit the unloaded shortening velocity in vertebrate muscle. Proc Natl Acad Sci USA (1985) 82:658–662.
[Abstract/Free Full Text] - Marechal G., Beckers-Bleukx G. Force–velocity relation and isomyosins in soleus muscles from two strains of mice (C57 and NMRI). Pflugers Arch (1993) 424:478–487.[CrossRef][Web of Science][Medline]
- Edman K.A.P. The velocity of unloaded shortening and its relation to sarcomere length and isometric force in vertebrate muscle fibres. J Physiol (1979) 291:143–159.
[Abstract/Free Full Text] - Seow C.Y., Ford L.E. Shortening velocity and power output of skinned muscle fibers from mammals having a 25,000-fold range of body mass. J Gen Physiol (1991) 97:541–560.
[Abstract/Free Full Text] - Rome L.C., Sosnicki A.A., Goble D.O. Maximum velocity of shortening of three fibre types from horse soleus muscle: implications for scaling with body size. J Physiol (1990) 431:173–185.
[Abstract/Free Full Text] - Rome L.C., Funke R.P., Alexander R.M., Lutz G., Aldridge H., Scott F., Freadman M. Why animals have different muscle fibre types. Nature (1988) 335:824–827.[CrossRef][Medline]
- Bottinelli R., Schiaffino S., Reggiani C. Force–velocity relations and myosin heavy chain isoform compositions of skinned fibres from rat skeletal muscle. J Physiol (1991) 437:655–672.
[Abstract/Free Full Text] - Barry W.H., Bridge J.H. Intracellular calcium homeostasis in cardiac myocytes. Circulation (1993) 87:1806–1815.
[Abstract/Free Full Text] - Gao W.D., Backx P.H., Azan-Backx M., Marban E. Myofilament Ca2+ sensitivity in intact versus skinned rat ventricular muscle. Circ Res (1994) 74:408–415.
[Abstract/Free Full Text] - Hill A.V. The possible effects of the aggregation of the molecules and haemoglobin on its dissociation curves. Nature (1910) 40:4–7.
- Tobacman L.S. Thin filament-mediated regulation of cardiac contraction. Annu Rev Physiol (1996) 58:447–481.[CrossRef][Web of Science][Medline]
- Ruegg JC. Calcium in Muscle Contraction, 2nd edn. Berlin: Springer Verlag 1992.
- Parmacek M.S., Leiden J.M. Structure, function and regulation of troponin C. Circulation (1991) 84:991–1003.
[Free Full Text] - Morano I., Bletz C., Wojciechowski R., Ruegg J.C. Modulation of crossbridge kinetics by myosin isoenzymes in skinned human heart fibers. Circ Res (1991) 68:614–618.
[Abstract/Free Full Text] - Morano I., Osterman A., Arner A. Rate of active tension development from rigor in skinned atrial and ventricular cardiac fibres from swine following photolytic release of ATP from caged ATP. Acta Physiol Scand (1995) 154:343–353.[Web of Science][Medline]
- VanBuren P., Harris D.E., Alpert N.R., Warshaw D.M. Cardiac V1 and V3 myosins differ in their hydrolytic and mechanical activities in vitro. Circ Res (1995) 77:439–444.
[Abstract/Free Full Text] - Hasenfuss G., Mulieri L.A., Blanchard E.M., Holubarsch C., Leavitt B.J., Ittleman F., Alpert N.R. Energetics of isometric force development in control and volume-overload human myocardium. Comparison with animal species. Circ Res (1991) 68:836–846.
[Abstract/Free Full Text] - Harris D.E., Work S.S., Wright R.K., Alpert N.R., Warshaw D.M. Smooth, cardiac and skeletal muscle myosin force and motion generation assessed by cross-bridge mechanical interactions in vitro. J Muscle Res Cell Motil (1994) 15:11–19.[CrossRef][Web of Science][Medline]
- VanBuren P., Work S.S., Warshaw D.M. Enhanced force generation by smooth muscle myosin in vitro. Proc Natl Acad Sci USA (1994) 91:202–205.
[Abstract/Free Full Text] - Bottinelli R., Canepari M., Cappelli V., Reggiani C. Maximum speed of shortening and ATPase activity in atrial and ventricular myocardia of hyperthyroid rats. Am J Physiol (1995) 269:C785–790.[Web of Science][Medline]
- Lowey S., Trybus K.M. Role of skeletal and smooth muscle myosin light chains. Biophys J (1995) 68(4 Suppl):120S–126S.[Medline]
- Cummins P., Russell G. A comparison of myosin light chain subunits in the atria and ventricles of mammals. Comp Biochem Physiol [B] (1986) 84:343–348.[CrossRef][Medline]
- Schwinger R.H., Bohm M., Koch A., Schmidt U., Morano I., Eissner H.J., Uberfuhr P., Reichart B., Erdmann E. The failing human heart is unable to use the Frank–Starling mechanism. Circ Res (1994) 74:959–969.
[Abstract/Free Full Text] - Auckland L.M., Lambert S.J., Cummins P. Cardiac myosin light and heavy chain isotypes in tetralogy of Fallot. Cardiovasc Res (1986) 20:828–836.
[Abstract/Free Full Text] - Morano M., Zacharzowski U., Maier M., Lange P.E., Alexi-Meskishvili V., Haase H., Morano I. Regulation of human heart contractility by essential myosin light chain isoforms. J Clin Invest (1996) 98:467–473.[Web of Science][Medline]
- Periasamy M., Strehler E.E., Garfinkel L.I., Gubits R.M., Ruiz-Opazo N., Nadal-Ginard B. Fast skeletal muscle myosin light chains 1 and 3 are produced from a single gene by a combined process of differential RNA transcription and splicing. J Biol Chem (1984) 259:13595–13604.
[Abstract/Free Full Text] - Lowey S., Waller G.S., Trybus K.M. Skeletal muscle myosin light chains are essential for physiological speeds of shortening. Nature (1993) 365:454–456.[CrossRef][Medline]
- Hayashibara T., Miyanishi T. Binding of the amino-terminal region of myosin alkali 1 light chain to actin and its effect on actin–myosin interaction. Biochemistry (1994) 33:12821–12827.[CrossRef][Web of Science][Medline]
- Henry G.D., Trayer I.P., Brewer S., Levine B.A. The widespread distribution of alpha-N-trimethylalanine as the N-terminal amino acid of light chains from vertebrate striated muscle myosins. Eur J Biochem (1985) 148:75–82.[Web of Science][Medline]
- Morano I., Haase H. Different actin affinities of human cardiac essential myosin light chain isoforms. FEBS Lett (1997) 408:71–74.[CrossRef][Web of Science][Medline]
- Morano I., Ritter O., Bonz A., Timek T., Vahl C.F., Michel G. Myosin light chain–actin interaction regulates cardiac contractility. Circ Res (1995) 76:720–725.
[Abstract/Free Full Text] - Rarick H.M., Opgenorth T.J., Vongeldern T.W., Wuwong J.S.R., Solaro R.J. An essential myosin light chain peptide induces supramaximal stimulation of cardiac myofibrillar atpase activity. J Biol Chem (1996) 271:27039–27043.
[Abstract/Free Full Text] - Williamson M.P. The structure and function of proline-rich regions in proteins. Biochem J (1994) 297:249–260.[Web of Science][Medline]
- Atar D., Gao W.D., Marban E. Alterations of excitation–contraction coupling in stunned myocardium and in failing myocardium. J Mol Cell Cardiol (1995) 27:783–791.[CrossRef][Web of Science][Medline]
- Nakao K., Yasue H., Fujimoto K., Jougasaki M., Yamamoto H., Hitoshi Y., Takatsu K., Miyamoto E. Increased expression and regional differences of atrial myosin light chain 1 in human ventricles with old myocardial infarction. Analyses using two monoclonal antibodies. Circulation (1992) 86:1727–1737.
[Abstract/Free Full Text] - Fujimoto K., Yasue H., Nakao K., Yamamoto H., Hitoshi Y., Jougasaki M., Okumura K., Ogawa H., Takatsu K., Miyamoto E. Novel monoclonal antibodies specific for human cardiac myosin light-chain 1: useful tools for analysis of normal and pathological hearts. J Histochem Cytochem (1993) 41:35–42.[Abstract]
- Meyer M., Schillinger W., Pieske B., Holubarsch C., Heilmann C., Posival H., Kuwajima G., Mikoshiba K., Just H., Hasenfuss G., et al. Alterations of sarcoplasmic reticulum proteins in failing human dilated cardiomyopathy. Circulation (1995) 92:778–784.
[Abstract/Free Full Text] - Studer R., Reinecke H., Bilger J., Eschenhagen T., Bohm M., Hasenfuss G., Just H., Holtz J., Drexler H. Gene expression of the cardiac Na(+)–Ca2+ exchanger in end-stage human heart failure. Circ Res (1994) 75:443–453.
[Abstract/Free Full Text] - Beuckelmann D.J., Nabauer M., Erdmann E. Intracellular calcium handling in isolated ventricular myocytes from patients with terminal heart failure. Circulation (1992) 85:1046–1055.
[Abstract/Free Full Text] - Mulieri L.A., Hasenfuss G., Leavitt B., Allen P.D., Alpert N.R. Altered myocardial force–frequency relation in human heart failure. Circulation (1992) 85:1743–1750.
[Abstract/Free Full Text] - Pieske B., Kretschmann B., Meyer M., Holubarsch C., Weirich J., Posival H., Minami K., Just H., Hasenfuss G. Alterations in intracellular calcium handling associated with the inverse force–frequency relation in human dilated cardiomyopathy. Circulation (1995) 92:1169–1178.
[Abstract/Free Full Text] - Pieske B., Sutterlin M., Schmidt-Schweda S., Minami K., Meyer M., Olschewski M., Holubarsch C., Just H., Hasenfuss G. Diminished post-rest potentiation of contractile force in human dilated cardiomyopathy. Functional evidence for alterations in intracellular Ca2+ handling. J Clin Invest (1996) 98:764–776.[Web of Science][Medline]
- Mulieri L.A., Leavitt B.J., Hasenfuss G., Allen P.D., Alpert N.R. Contraction frequency dependence of twitch and diastolic tension in human dilated cardiomyopathy (tension–frequency relation in cardiomyopathy). Basic Res Cardiol (1992) 87(Suppl 1):199–212.[Web of Science][Medline]
- Gomez A.M., Valdivia H.H., Cheng H., Lederer M.R., Santana L.F., Cannell M.B., McCune S.A., Altschuld R.A., Lederer W.J. Defective excitation–contraction coupling in experimental cardiac hypertrophy and heart failure. Science (1997) 276:800–806.
[Abstract/Free Full Text] - Bristow M.R., Feldman A.M. Changes in the receptor-G protein-adenylyl cyclase system in heart failure from various types of heart muscle disease. Basic Res Cardiol (1992) 87(Suppl 1):15–35.[Web of Science][Medline]
- Winegrad S. Endothelial cell regulation of contractility of the heart. Annu Rev Physiol (1997) 59:505–525.[CrossRef][Web of Science][Medline]
- Shah A.M., Mebazaa A., Yang Z.K., Cuda G., Lankford E.B., Pepper C.B., Sollott S.J., Sellers J.R., Robotham J.L., Lakatta E.G. Inhibition of myocardial crossbridge cycling by hypoxic endothelial cells — a potential mechanism for matching oxygen supply and demand. Circ Res (1997) 80:688–698.
[Abstract/Free Full Text] - Heusch G., Schulz R. Hibernating myocardium: a review. J Mol Cell Cardiol (1996) 28:2359–2372.[CrossRef][Web of Science][Medline]
- Shivalkar B., Maes A., Borgers M., Ausma J., Scheys I., Nuyts J., Mortelmans L., Flameng W. Only hibernating myocardium invariably shows early recovery after coronary revascularization. Circulation (1996) 94:308–315.
[Abstract/Free Full Text] - Schramm M., Klieber H.G., Daut J. The energy expenditure of actomyosin-ATPase, Ca(2+)-ATPase and Na+,K(+)-ATPase in guinea-pig cardiac ventricular muscle. J Physiol (Lond) (1994) 481:647–662.
[Abstract/Free Full Text] - Marban E., Kitakaze M., Koretsune Y., Yue D.T., Chacko V.P., Pike M.M. Quantification of [Ca2+]i in perfused hearts. Critical evaluation of the 5F-BAPTA and nuclear magnetic resonance method as applied to the study of ischemia and reperfusion. Circ Res (1990) 66:1255–1267.
[Abstract/Free Full Text] - Mellgren R.L. Proteolysis of nuclear proteins by mu-calpain and m-calpain. J Biol Chem (1991) 266:13920–13924.
[Abstract/Free Full Text] - Labeit S., Kolmerer B. Titins: giant proteins in charge of muscle ultrastructure and elasticity. Science (1995) 27:293–296.
- Morano I., Hadicke K., Grom S., Koch A., Schwinger R.H., Bohm M., Bartel S., Erdmann E., Krause E.G. Titin, myosin light chains and C-protein in the developing and failing human heart. J Mol Cell Cardiol (1994) 26:361–368.[CrossRef][Web of Science][Medline]
- Schaper J., Froede R., Hein S., Buck A., Hashizume H., Speiser B., Friedl A., Bleese N. Impairment of the myocardial ultrastructure and changes of the cytoskeleton in dilated cardiomyopathy. Circulation (1991) 83:504–514.
[Abstract/Free Full Text] - Hein S., Scholz D., Fujitani N., Rennollet H., Brand T., Friedl A., Schaper J. Altered expression of titin and contractile proteins in failing human myocardium. J Mol Cell Cardiol (1994) 26:1291–1306.[CrossRef][Web of Science][Medline]
- Ausma J., Furst D., Thone F., Shivalkar B., Flameng W., Weber K., Ramaekers F., Borgers M. Molecular changes of titin in left ventricular dysfunction as a result of chronic hibernation. J Mol Cell Cardiol (1995) 27:1203–1212.[CrossRef][Web of Science][Medline]
- Margossian S.S., White H.D., Caulfield J.B., Norton P., Taylor S., Slayter H.S. Light chain 2 profile and activity of human ventricular myosin during dilated cardiomyopathy. Identification of a causal agent for impaired myocardial function. Circulation (1992) 85:1720–1733.
[Abstract/Free Full Text] - Hofmann P.A., Metzger J.M., Greaser M.L., Moss R.L. Effects of partial extraction of light chain 2 on the Ca2+ sensitivities of isometric tension, stiffness and velocity of shortening in skinned skeletal muscle fibers. J Gen Physiol (1990) 95:477–498.
[Abstract/Free Full Text] - Szczesna D., Zhao J., Potter J.D. The regulatory light chains of myosin modulate cross-bridge cycling in skeletal muscle. J Biol Chem (1996) 271:5246–5250.
[Abstract/Free Full Text] - Gao W.D., Atar D., Liu Y., Perez N.G., Murphy A.M., Marban E. Role of troponin I proteolysis in the pathogenesis of stunned myocardium. Circ Res (1997) 80:393–399.[Web of Science][Medline]
- Cooper T.A., Ordahl C.P. A single cardiac troponin T gene generates embryonic and adult isoforms via developmentally regulated alternate splicing. J Biol Chem (1985) 260:11140–11148.
[Abstract/Free Full Text] - Mesnard L., Logeart D., Taviaux S., Diriong S., Mercadier J.J., Samson F. Human cardiac troponin T: cloning and expression of new isoforms in the normal and failing heart. Circ Res (1995) 76:687–692.
[Abstract/Free Full Text] - Anderson P.A., Greig A., Mark T.M., Malouf N.N., Oakeley A.E., Ungerleider R.M., Allen P.D., Kay B.K. Molecular basis of human cardiac troponin T isoforms expressed in the developing, adult, and failing heart. Circ Res (1995) 76:681–686.
[Abstract/Free Full Text] - Townsend P.J., Barton P.J., Yacoub M.H., Farza H. Molecular cloning of human cardiac troponin T isoforms: expression in developing and failing heart. J Mol Cell Cardiol (1995) 27:2223–2236.[CrossRef][Web of Science][Medline]
- Anderson P.A., Malouf N.N., Oakeley A.E., Pagani E.D., Allen P.D. Troponin T isoform expression in humans. A comparison among normal and failing adult heart, fetal heart and adult and fetal skeletal muscle. Circ Res (1991) 69:1226–1233.
[Abstract/Free Full Text] - Nassar R., Malouf N.N., Kelly M.B., Oakeley A.E., Anderson P.A. Force-pCa relation and troponin T isoforms of rabbit myocardium. Circ Res (1991) 69:1470–1475.
[Abstract/Free Full Text] - Akella A.B., Ding X.L., Cheng R., Gulati J. Diminished Ca2+ sensitivity of skinned cardiac muscle contractility coincident with troponin T-band shifts in the diabetic rat. Circ Res (1995) 76:600–606.
[Abstract/Free Full Text] - Hunkeler N.M., Kullman J., Murphy A.M. Troponin I isoform expression in human heart. Circ Res (1991) 69:1409–1414.
[Abstract/Free Full Text] - Martin A.F., Ball K., Gao L.Z., Kumar P., Solaro R.J. Identification and functional significance of troponin I isoforms in neonatal rat heart myofibrils. Circ Res (1991) 69:1244–1252.
[Abstract/Free Full Text] - Gahlmann R., Wade R., Gunning P., Kedes L. Differential expression of slow and fast skeletal muscle troponin C. Slow skeletal muscle troponin C is expressed in human fibroblasts. J Mol Biol (1988) 201:379–391.[CrossRef][Web of Science][Medline]
- Moss R.L., Lauer M.R., Giulian G.G., Greaser M.L. Altered Ca2+ dependence of tension development in skinned skeletal muscle fibers following modification of troponin by partial substitution with cardiac troponin C. J Biol Chem (1986) 261:6096–6099.
[Abstract/Free Full Text] - Hewett T.E., Grupp I.L., Grupp G., Robbins J. Alpha-skeletal actin is associated with increased contractility in the mouse heart. Circ Res (1994) 74:740–746.
[Abstract/Free Full Text] - Boheler K.R., Carrier L., de la Bastie D., Allen P.D., Komajda M., Mercadier J.J., Schwartz K. Skeletal actin mRNA increases in the human heart during ontogenic development and is the major isoform of control and failing adult hearts. J Clin Invest (1991) 88:323–330.[Web of Science][Medline]
- Palmiter K.A., Kitada Y., Muthuchamy M., Wieczorek D.F., Solaro R.J. Exchange of beta- for alpha-tropomyosin in hearts of transgenic mice induces changes in thin filament response to Ca2+, strong cross-bridge binding and protein phosphorylation. J Biol Chem (1996) 271:11611–11614.
[Abstract/Free Full Text] - Zhi G., Herring B.P., Stull J.T. Structural requirements for phosphorylation of myosin regulatory light chain from smooth muscle. J Biol Chem (1994) 269:24723–24727.
[Abstract/Free Full Text] - Noland T.A. Jr., Kuo J.F. Phosphorylation of cardiac myosin light chain 2 by protein kinase C and myosin light chain kinase increases Ca(2+)-stimulated actomyosin MgATPase activity. Biochem Biophys Res Commun (1993) 193:254–260.[CrossRef][Web of Science][Medline]
- Morano I., Hofmann F., Zimmer M., Ruegg J.C. The influence of P-light chain phosphorylation by myosin light chain kinase on the calcium sensitivity of chemically skinned heart fibres. FEBS Lett (1985) 189:221–224.[CrossRef][Web of Science][Medline]
- Morano I., Bachle-Stolz C., Katus A., Ruegg J.C. Increased calcium sensitivity of chemically skinned human atria by myosin light chain kinase. Basic Res Cardiol (1988) 83:350–359.[CrossRef][Web of Science][Medline]
- Morano I., Rosch J., Arner A., Ruegg J.C. Phosphorylation and thiophosphorylation by myosin light chain kinase: different effects on mechanical properties of chemically skinned ventricular fibers from the pig. J Mol Cell Cardiol (1990) 22:805–813.[CrossRef][Web of Science][Medline]
- Clement O., Puceat M., Walsh M.P., Vassort G. Protein kinase C enhances myosin light-chain kinase effects on force development and ATPase activity in rat single skinned cardiac cells. Biochem J (1992) 285:311–317.[Web of Science][Medline]
- Zhang R., Zhao J., Potter J.D. Phosphorylation of both serine residues in cardiac troponin I is required to decrease the Ca2+ affinity of cardiac troponin C. J Biol Chem (1995) 270:30773–30780.
[Abstract/Free Full Text] - Bhavsar P.K., Brand N.J., Yacoub M.H., Barton P.J.R. Isolation and characterization of the human cardiac troponin I gene (TNNI3). Genomics (1996) 35:11–23.[CrossRef][Web of Science][Medline]
- Reiffert S.U., Jaquet K., Heilmeyer L.M. Jr., Ritchie M.D., Geeves M.A. Bisphosphorylation of cardiac troponin I modulates the Ca(2+)-dependent binding of myosin subfragment S1 to reconstituted thin filaments. FEBS Lett (1996) 384:43–47.[CrossRef][Web of Science][Medline]
- Noland T.A. Jr., Raynor R.L., Jideama N.M., Guo X., Kazanietz M.G., Blumberg P.M., Solaro R.J., Kuo J.F. Differential regulation of cardiac actomyosin S-1 MgATPase by protein kinase C isozyme-specific phosphorylation of specific sites in cardiac troponin I and its phosphorylation site mutants. Biochemistry (1996) 35:14923–14931.[CrossRef][Web of Science][Medline]
- Heeley D.H. Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T. Eur J Biochem (1994) 221:129–137.[Web of Science][Medline]
- deBelle I., Mak A.S. Isolation and characterization of tropomyosin kinase from chicken embryo. Biochim Biophys Acta (1987) 925:17–26.[Medline]
- Weisberg A., Winegrad S. Alteration of myosin cross bridges by phosphorylation of myosin-binding protein C in cardiac muscle. Proc Natl Acad Sci USA (1996) 93:8999–9003.
[Abstract/Free Full Text] - Gautel M., Zuffardi O., Freiburg A., Labeit S. Phosphorylation switches specific for the cardiac isoform of myosin binding protein-C: a modulator of cardiac contraction? Embo J (1995) 14:1952–1960.[Web of Science][Medline]
- Venema R.C., Kuo J.F. Protein kinase C-mediated phosphorylation of troponin I and C-protein in isolated myocardial cells is associated with inhibition of myofibrillar actomyosin MgATPase. J Biol Chem (1993) 268:2705–2711.
[Abstract/Free Full Text] - Wattanapermpool J., Reiser P.J., Solaro R.J. Troponin I isoforms and differential effects of acidic pH on soleus and cardiac myofilaments. Am J Physiol (1995) 268:C323–330.[Web of Science][Medline]
- Allen D.G., Morris P.G., Orchard C.H., Pirolo J.S. A nuclear magnetic resonance study of metabolism in the ferret heart during hypoxia and inhibition of glycolysis. J Physiol (Lond) (1985) 361:185–204.
[Abstract/Free Full Text] - Allen D.G., Orchard C.H. Intracellular calcium concentration during hypoxia and metabolic inhibition in mammalian ventricular muscle. J Physiol (Lond) (1983) 339:107–122.
[Abstract/Free Full Text] - Kentish J.C. The effects of inorganic phosphate and creatine phosphate on force production in skinned muscles from rat ventricle. J Physiol (Lond) (1986) 370:585–604.
[Abstract/Free Full Text] - McDonald K.S., Field L.J., Parmacek M.S., Soonpaa M., Leiden J.M., Moss R.L. Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C. J Physiol (Lond) (1995) 483:131–139.
[Abstract/Free Full Text] - Fuchs F., Wang Y.P. Sarcomere length versus interfilament spacing as determinants of cardiac myofilament Ca2+ sensitivity and Ca2+ binding. J Mol Cell Cardiol (1996) 28:1375–1383.[CrossRef][Web of Science][Medline]
- Akella A.B., Su H., Sonnenblick E.H., Rao V.G., Gulati J. The cardiac troponin C isoform and the length dependence of Ca2+ sensitivity of tension in myocardium. J Mol Cell Cardiol (1997) 29:381–389.[CrossRef][Web of Science][Medline]
- Holubarsch C., Ruf T., Goldstein D.J., Ashton R.C., Nickl W., Pieske B., Pioch K., Ludemann J., Wiesner S., Hasenfuss G., Posival H., Just H., Burkhoff D. Existence of the Frank–Starling mechanism in the failing human heart. Investigations on the organ, tissue and sarcomere levels. Circulation (1996) 94:683–689.
[Abstract/Free Full Text] - Hofmann P.A., Lange J.H. Effects of phosphorylation of troponin I and C protein on isometric tension and velocity of unloaded shortening in skinned single cardiac myocytes from rats. Circ Res (1994) 74:718–726.
[Abstract/Free Full Text] - Peppel K., Koch W.J., Lefkowitz R.J. Gene transfer strategies for augmenting cardiac function. Trends Cardiovasc Med (1997) 7:145–150.[CrossRef][Web of Science]
- Carl S.L., Felix K., Caswell A.H., Brandt N.R., Ball W.J. Jr., Vaghy P.L., Meissner G., Ferguson D.G. Immunolocalization of sarcolemmal dihydropyridine receptor and sarcoplasmic reticular triadin and ryanodine receptor in rabbit ventricle and atrium. J Cell Biol (1995) 129:672–682.[Medline]
- Sheterline P., Sparrow J.C. Actin Protein Profile. (1994) 1(1):1–121.
- Jideama N.M., Noland T.A. Jr., Raynor R.L., Blobe G.C., Fabbro D., Kazanietz M.G., Blumberg P.M., Hannun Y.A., Kuo J.F. Phosphorylation specificities of protein kinase C isozymes for bovine cardiac troponin I and troponin T and sites within these proteins and regulation of myofilament properties. J Biol Chem (1996) 271:23277–23283.
[Abstract/Free Full Text]
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