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Cardiovascular Research 2005 65(1):13-15; doi:10.1016/j.cardiores.2004.10.015
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Copyright © 2004, European Society of Cardiology

Phospholemman, a chaperone of Na+,K+-ATPase?

Paul Fransen*

Laboratory of Physiopharmacology, Department of Pharmacology, University of Antwerp, CDE, Universiteitsplein 1, B-2610 Antwerp, Belgium

* Tel.: +32 3 820 25 87; fax: +32 3 820 25 67. Email address: paul.fransen{at}ua.ac.be

Received 1 October 2004; accepted 13 October 2004

See article by Silverman et al. [8] (pages 93–103) in this issue.

Control of intracellular Na+ homeostasis is of crucial importance for normal functioning of the heart. Na+ extrusion in cardiomyocytes occurs mainly via the sodium pump or Na+,K+-ATPase. This enzyme consists of a catalytic {alpha}-subunit with binding sites for ATP, Na+, K+ and cardiac glycosides, and a glycoprotein β-subunit. The main function of this pump in cardiac myocytes is to maintain and restore ion gradients for Na+ and K+ during electrical activity by mediating active transport of 3Na+ out and 2K+ into the cell. Of the four {alpha}-isoforms, {alpha}1, {alpha}2 and {alpha}3 have been described in cardiac tissue, where they have multiple functions and a different localization in the different cell types or even within a single cardiomyocyte. The isoforms display variable sensitivity to inhibition by cardiac glycosides; the {alpha}1 isoform, for example, is less sensitive to ouabain than the {alpha}2 or {alpha}3 isoform [1–5].

The activity of the Na+,K+-ATPase is sensitive to changing cellular conditions and to physiological stimuli. As a consequence, other Na+-dependent membrane transporters such as the Na+/Ca2+ exchanger, the Na+/H+ exchanger and the Na+/HCO3– transporter, and, hence, cellular Ca2+ and pH homeostasis, are dependent on Na+/K+-ATPase activity [6]. Most of the hormones that regulate Na+,K+-ATPase do so through signalling mechanisms that modulate the activities of a group of protein kinases, phospholipases and phosphatases. For example, sympathetic stimulation activates β- and {alpha}1-adrenergic receptors leading to protein kinase A (PKA)- and protein kinase C (PKC)-mediated phosphorylation of the different isoforms of the {alpha}-subunit of the Na+,K+-ATPase. Although phosphorylation of Na+,K+-ATPase mediated by PKA and PKC may be direct, other more complex phosphorylation mechanisms have been observed [2,6,7].

In the present issue of Cardiovascular Research, Silverman et al. [8] describe isoform ({alpha}1)-specific activation of the Na+,K+-ATPase via forskolin-cAMP-PKA independently of direct phosphorylation of the {alpha}1 subunit isoform at serine 938, which is assumed to be the PKA consensus phosphorylation site. Phosphorylation, however, occurred via an associated protein: the sarcolemmal protein phospholemman (PLM). PLM belongs to the recently defined FXYD domain-containing protein family, which contains seven members that are small, single-span, hydrophobic membrane proteins with, probably, tissue-specific distribution. This family includes phospholemman (FXYD1, heart, brain and skeletal muscle), the {gamma} subunit of Na+,K+-ATPase (FXYD2, kidney), mammary tumor marker (FXDY3 or Mat-8), channel-inducing factor (FXYD4 or CHIF, kidney), proteins related to ion channels (FXYD5 or RIC), FXYD6 and FXYD7 (brain) [9]. They are considered to be regulators of ion transporters and most, if not all, are physically and functionally associated with Na+,K+-ATPase. Silverman et al. [8] further demonstrated that PLM co-precipitated with {alpha}1, β1 and PKA, but not with the {alpha}2 subunit isoform. Co-localization of PLM with {alpha}1 in the sarcolemma, but not with {alpha}2, which was mainly targeted to the t-tubules, was also demonstrated by immunofluorescent staining of guinea-pig myocytes. PLM interacted with {alpha}1 only as it was the isoform in the same subcellular location. The possibility of PLM–{alpha}2 interaction could not be excluded.

Although this novel and fascinating finding adds importantly to our understanding of Na+/K+-ATPase activity regulation, it remains to be determined whether FXYD-based regulation of sodium pump activity is a general event in different tissues and species and whether this regulation is isoform-specific. For example, when compared with guinea pig cardiomyocytes, rat myocytes have been described to display a reverse distribution of {alpha} isoforms in cardiomyocytes with {alpha}1 isoforms in the t-tubules and {alpha}2 isoforms uniformly distributed throughout the sarcolemma [10]. Nevertheless, in the rat heart, Fuller et al. [11] found a substantial, ischemia-induced, PKA-dependent activation of Na+,K+-ATPase activity that coincided with phosphorylation of phospholemman. Does PLM interact with the {alpha}2 isoform in the rat or does it have another subcellular localization? Recently, Fransen et al. [1] compared the distribution of {alpha}1 and {alpha}2 in rat and rabbit ventricular myocytes and found that the {alpha}1 isoform in both species preferentially localized at the plasmalemma, whereas the {alpha}2 isoform was uniquely present in the t-tubules. Similar observations have been made for guinea-pig/rabbit cardiomyocytes and rat arterial myocytes [2,12].

Nevertheless, the result of Silverman and coworkers in this issue shows the importance of investigating the subcellular distribution of {alpha}1, {alpha}2 or {alpha}3 isoforms and PLM to describe PKA-(and eventually PKC-) induced modulation of Na+,K+-ATPase activity. The situation becomes probably even more complex in the human heart, where the three {alpha} isoforms are present and where the subcellular localization of the isoforms has not been studied yet. The species-, tissue- or cell-dependent distribution of PLM or other FXYD domain-containing protein family members might somehow be linked to the species-, tissue- or cell-dependent distribution of Na+,K+-ATPase {alpha} subunit isoforms.

One can only speculate at present about the role of PLM-mediated phosphorylation of Na+,K+-ATPase isoforms in pathophysiological conditions. Following myocardial infarction in rat heart, expression of Na+,K+-ATPase {alpha}1 and {alpha}2 isoforms was decreased in the infarct scar versus noninfarcted, sham-operated controls, whereas, in viable left ventricle, {alpha}1 and {alpha}3 were significantly elevated and expression of {alpha}2 was decreased compared to controls [13]. The latter observations could be compatible with the observation that PLM expression increased in the rat after myocardial infarction [14]. Besides co-localization with the {alpha}1 subunit of Na+,K+-ATPase, PLM also seems to co-localize with the Na+,Ca2+-exchanger in adult rat cardiac myocytes in the plasma membrane, and t-tubules and PLM downregulation enhanced, while overexpression inhibited, Na+/Ca2+ exchange [15,16]. Is the Na+,K+-ATPase involved here? In rabbit cardiomyocytes, Schillinger et al. [17] demonstrated that inhibition of Na+,K+-ATPase by ouabain induced a combined diastolic and systolic dysfunction of myocytes overexpressing the Na+,Ca2+-exchanger, suggesting a functional link between the Na+,Ca2+-exchanger and Na+,K+-ATPase. Interaction of the Na+,K+-ATPase and Na+,Ca2+-exchange in a restricted space of guinea-pig ventricular cells has been described [18]. These studies seem to provide evidence for the existence of a fuzzy space near the sarcolemma (or near the t-tubules) where subsarcolemmal [Na+] variations exist with effects on local [Ca2+]. This could explain the positive inotropic effect of ouabain in a number of models where ouabain addition is not associated with significant changes in bulk intracellular [Na+] [12,19]. It could also explain the modulatory effect of internal Ca2+ on the PKA-mediated phosphorylation of Na+,K+-ATPase and the activity of the enzyme. It is expected that tissue- and isoform-specific interaction of Na+,K+-ATPase with FXYD proteins, which may be very localized, not only contributes to proper handling of intracellular Na+ and K+, but also of Ca2+ and ensures its correct function in muscle contraction.


   References
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