Unitary anion currents through phospholemman channel molecules

Greasing the wheels or a spanner in the works? Regulation of the cardiac sodium pump by palmitoylation

The ubiquitous sodium/potassium ATPase (Na pump) is the most abundant primary active transporter at the cell surface of multiple cell types, including ventricular myocytes in the heart. The activity of the Na pump establishes transmembrane ion gradients that control numerous events at the cell surface, positioning it as a key regulator of the contractile and metabolic state of the myocardium. Defects in Na pump activity and regulation elevate intracellular Na in cardiac muscle, playing a causal role in the development of cardiac hypertrophy, diastolic dysfunction, arrhythmias and heart failure. Palmitoylation is the reversible conjugation of the fatty acid palmitate to specific protein cysteine residues; all subunits of the cardiac Na pump are palmitoylated. Palmitoylation of the pump's accessory subunit phospholemman (PLM) by the cell surface palmitoyl acyl transferase DHHC5 leads to pump inhibition, possibly by altering the relationship between the pump catalytic α subunit and specifically bound membrane lipids. In this review, we discuss the functional impact of PLM palmitoylation on the cardiac Na pump and the molecular basis of recognition of PLM by its palmitoylating enzyme DHHC5, as well as effects of palmitoylation on Na pump cell surface abundance in the cardiac muscle. We also highlight the numerous unanswered questions regarding the cellular control of this fundamentally important regulatory process.

Transcriptome Display During Testicular Differentiation of Channel Catfish (Ictalurus punctatus) as Revealed by RNA-Seq Analysis

Channel catfish (Ictalurus punctatus) has been recognized as a dominant freshwater aquaculture species in the United States. It is also a suitable model for studying the mechanisms of sex determination and differentiation because of its sexual plasticity and exhibition of both genetic and environmental sex determination. The testicular differentiation in male channel catfish normally starts between 90 and 102 days postfertilization (dpf), while the ovarian differentiation starts early from 19 dpf. As such, efforts to better understand the postponed testicular development at the molecular level are needed. Toward that end, we conducted transcriptomic comparison of gene expression of male and female gonads at 90, 100, and 110 dpf using high-throughput RNA-Seq. Transcriptomic profiles of male gonads on 90 and 100 dpf exhibited high similarities except for a small number of significantly up-regulated genes that were involved in development of germ cell-supporting somatic cells, while drastic changes were observed during 100-110 dpf, with a group of highly up-regulated genes that were involved in germ cells development, including nanog and pou5f1 Transcriptomic comparison between testes and ovaries identified male-preferential genes, such as gsdf, cxcl12, as well as other cytokines mediated the development of the gonad into a testis. Co-expression analysis revealed highly correlated genes and potential pathways underlying germ cell differentiation and spermatogonia stem cell development. The candidate genes and pathways identified in this study set the foundation for further studies on sex determination and differentiation in catfish as well as other teleosts.

The identification of a volume-regulated anion channel: an amazing Odyssey

The volume-regulated anion channel (VRAC) plays a pivotal role in cell volume regulation in essentially all cell types studied. Additionally, VRAC appears to contribute importantly to a wide range of other cellular functions and pathological events, including cell motility, cell proliferation, apoptosis and excitotoxic glutamate release in stroke. Although biophysically, pharmacologically and functionally thoroughly described, VRAC has until very recently remained a genetic orphan. The search for the molecular identity of VRAC has been long and has yielded multiple potential candidates, all of which eventually turned out to have properties not fully compatible with those of VRAC. Recently, two groups have independently identified the protein leucine-rich repeats containing 8A (LRRC8A), belonging to family of proteins (LRRC8A-E) distantly related to pannexins, as the likely pore-forming subunit of VRAC. In this brief review, we summarize the history of the discovery of VRAC, outline its basic biophysical and pharmacological properties, link these to several cellular functions in which VRAC appears to play important roles, and sketch the amazing search for the molecular identity of this channel. Finally, we describe properties of the LRRC8 proteins, highlight some features of the LRRC8A knockout mouse and discuss the impact of the discovery of LRRC8 as VRAC on future research.

Substrate recognition by the cell surface palmitoyl transferase DHHC5

The cardiac phosphoprotein phospholemman (PLM) regulates the cardiac sodium pump, activating the pump when phosphorylated and inhibiting it when palmitoylated. Protein palmitoylation, the reversible attachment of a 16 carbon fatty acid to a cysteine thiol, is catalyzed by the Asp-His-His-Cys (DHHC) motif-containing palmitoyl acyltransferases. The cell surface palmitoyl acyltransferase DHHC5 regulates a growing number of cellular processes, but relatively few DHHC5 substrates have been identified to date. We examined the expression of DHHC isoforms in ventricular muscle and report that DHHC5 is among the most abundantly expressed DHHCs in the heart and localizes to caveolin-enriched cell surface microdomains. DHHC5 coimmunoprecipitates with PLM in ventricular myocytes and transiently transfected cells. Overexpression and silencing experiments indicate that DHHC5 palmitoylates PLM at two juxtamembrane cysteines, C40 and C42, although C40 is the principal palmitoylation site. PLM interaction with and palmitoylation by DHHC5 is independent of the DHHC5 PSD-95/Discs-large/ZO-1 homology (PDZ) binding motif, but requires a ∼ 120 amino acid region of the DHHC5 intracellular C-tail immediately after the fourth transmembrane domain. PLM C42A but not PLM C40A inhibits the Na pump, indicating PLM palmitoylation at C40 but not C42 is required for PLM-mediated inhibition of pump activity. In conclusion, we demonstrate an enzyme-substrate relationship for DHHC5 and PLM and describe a means of substrate recruitment not hitherto described for this acyltransferase. We propose that PLM palmitoylation by DHHC5 promotes phospholipid interactions that inhibit the Na pump.

Reliability of nine programs of topological predictions and their application to integral membrane channel and carrier proteins

We evaluated topological predictions for nine different programs, HMMTOP, TMHMM, SVMTOP, DAS, SOSUI, TOPCONS, PHOBIUS, MEMSAT-SVM (hereinafter referred to as MEMSAT), and SPOCTOPUS. These programs were first evaluated using four large topologically well-defined families of secondary transporters, and the three best programs were further evaluated using topologically more diverse families of channels and carriers. In the initial studies, the order of accuracy was: SPOCTOPUS > MEMSAT > HMMTOP > TOPCONS > PHOBIUS > TMHMM > SVMTOP > DAS > SOSUI. Some families, such as the Sugar Porter Family (2.A.1.1) of the Major Facilitator Superfamily (MFS; TC #2.A.1) and the Amino Acid/Polyamine/Organocation (APC) Family (TC #2.A.3), were correctly predicted with high accuracy while others, such as the Mitochondrial Carrier (MC) (TC #2.A.29) and the K(+) transporter (Trk) families (TC #2.A.38), were predicted with much lower accuracy. For small, topologically homogeneous families, SPOCTOPUS and MEMSAT were generally most reliable, while with large, more diverse superfamilies, HMMTOP often proved to have the greatest prediction accuracy. We next developed a novel program, TM-STATS, that tabulates HMMTOP, SPOCTOPUS or MEMSAT-based topological predictions for any subdivision (class, subclass, superfamily, family, subfamily, or any combination of these) of the Transporter Classification Database (TCDB; www.tcdb.org) and examined the following subclasses: α-type channel proteins (TC subclasses 1.A and 1.E), secreted pore-forming toxins (TC subclass 1.C) and secondary carriers (subclass 2.A). Histograms were generated for each of these subclasses, and the results were analyzed according to subclass, family and protein. The results provide an update of topological predictions for integral membrane transport proteins as well as guides for the development of more reliable topological prediction programs, taking family-specific characteristics into account.

Effect of diaminopropionic acid (Dap) on the biophysical properties of a modified synthetic channel-forming peptide

Channel replacement therapy, based on synthetic channel-forming peptides (CFPs) with the ability to supersede defective endogenous ion channels, is a novel treatment modality that may augment existing interventions against multiple diseases. Previously, we derived CFPs from the second transmembrane segment of the α-subunit of the glycine receptor, M2GlyR, which forms chloride-selective channels in its native form. The best candidate, NK4-M2GlyR T19R, S22W (p22-T19R, S22W), was water-soluble, incorporated into cell membranes and was nonimmunogenic, but lacked the structural properties for high conductance and anion selectivity when assembled into a pore. Further studies suggested that the threonine residues at positions 13, 17, and 20 line the pore of assembled p22-T19R, S22W, and here we used 2,3-diaminopropionic acid (Dap) substitutions to introduce positive charges to the pore-lining interface of the predicted p22-T19R, S22W channel. Dap-substituted p22-T19R, S22W peptides retained the α-helical secondary structure characteristic of their parent peptide, and induced short-circuit transepithelial currents when exposed to the apical membrane of Madin-Darby canine kidney (MDCK) cells; the sequences containing multiple Dap-substituted residues induced larger currents than the peptides with single or no Dap substitutions. To gain further insights into the effects of Dap residues on the properties of the putative pore, we performed two-electrode voltage clamp electrophysiology on Xenopus oocytes exposed to p22-T19R, S22W or its Dap-modified analogues. We observed that Dap-substituted peptides also induced significantly larger voltage-dependent currents than the parent compound, but there was no apparent change in reversal potential upon replacement of external Na+, Cl- or K+, indicating that these currents remained nonselective. These results suggest that the introduction of positively charged side chains in predicted pore-lining residues does not improve anion-to-cation selectivity, but results in higher conductance, perhaps due to higher oligomerization numbers.

Novel regulation of cardiac Na pump via phospholemman

As the only quantitatively significant Na efflux pathway from cardiac cells, the Na/K ATPase (Na pump) is the primary regulator of intracellular Na. The transmembrane Na gradient it establishes is essential for normal electrical excitability, numerous coupled-transport processes and, as the driving force for Na/Ca exchange, thus setting cardiac Ca load and contractility. As Na influx varies with electrical excitation, heart rate and pathology, the dynamic regulation of Na efflux is essential. It is now widely recognized that phospholemman, a 72 amino acid accessory protein which forms part of the Na pump complex, is the key nexus linking cellular signaling to pump regulation. Phospholemman is the target of a variety of post-translational modifications (including phosphorylation, palmitoylation and glutathionation) and these can dynamically alter the activity of the Na pump. This review summarizes our current understanding of the multiple regulatory mechanisms that converge on phospholemman and govern NA pump activity in the heart. The corrected Fig. 4 is reproduced below. The publisher would like to apologize for any inconvenience caused. [corrected].

A separate pool of cardiac phospholemman that does not regulate or associate with the sodium pump: multimers of phospholemman in ventricular muscle

BACKGROUND

Phospholemman regulates the plasmalemmal sodium pump in excitable tissues.

RESULTS

In cardiac muscle, a subpopulation of phospholemman with a unique phosphorylation signature associates with other phospholemman molecules but not with the pump.

CONCLUSION

Phospholemman oligomers exist in cardiac muscle.

SIGNIFICANCE

Much like phospholamban regulation of SERCA, phospholemman exists as both a sodium pump inhibiting monomer and an unassociated oligomer. Phospholemman (PLM), the principal quantitative sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. Much like phospholamban, which regulates the related ATPase SERCA, PLM is reported to oligomerize. We investigated subpopulations of PLM in adult rat ventricular myocytes based on phosphorylation status. Co-immunoprecipitation identified two pools of PLM: one not associated with the sodium pump phosphorylated at Ser(63) and one associated with the pump, both phosphorylated at Ser(68) and unphosphorylated. Phosphorylation of PLM at Ser(63) following activation of PKC did not abrogate association of PLM with the pump, so its failure to associate with the pump was not due to phosphorylation at this site. All pools of PLM co-localized to cell surface caveolin-enriched microdomains with sodium pump α subunits, despite the lack of caveolin-binding motif in PLM. Mass spectrometry analysis of phosphospecific immunoprecipitation reactions revealed no unique protein interactions for Ser(63)-phosphorylated PLM, and cross-linking reagents also failed to identify any partner proteins for this pool. In lysates from hearts of heterozygous transgenic animals expressing wild type and unphosphorylatable PLM, Ser(63)-phosphorylated PLM co-immunoprecipitated unphosphorylatable PLM, confirming the existence of PLM multimers. Dephosphorylation of the PLM multimer does not change sodium pump activity. Hence like phospholamban, PLM exists as a pump-inhibiting monomer and an unassociated oligomer. The distribution of different PLM phosphorylation states to different pools may be explained by their differential proximity to protein phosphatases rather than a direct effect of phosphorylation on PLM association with the pump.

Coordinated regulation of cardiac Na(+)/Ca (2+) exchanger and Na (+)-K (+)-ATPase by phospholemman (FXYD1)

Phospholemman (PLM) is the founding member of the FXYD family of regulators of ion transport. PLM is a 72-amino acid protein consisting of the signature PFXYD motif in the extracellular N terminus, a single transmembrane (TM) domain, and a C-terminal cytoplasmic tail containing three phosphorylation sites. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. The TM domain of PLM interacts with TM9 of the α-subunit of Na(+)-K(+)-ATPase, while its cytoplasmic tail interacts with two small regions (spanning residues 248-252 and 300-304) of the proximal intracellular loop of Na(+)/Ca(2+) exchanger. Under stress, catecholamine stimulation phosphorylates PLM at serine(68), resulting in relief of inhibition of Na(+)-K(+)-ATPase by decreasing K(m) for Na(+) and increasing V(max), and simultaneous inhibition of Na(+)/Ca(2+) exchanger. Enhanced Na(+)-K(+)-ATPase activity lowers intracellular Na(+), thereby minimizing Ca(2+) overload and risks of arrhythmias. Inhibition of Na(+)/Ca(2+) exchanger reduces Ca(2+) efflux, thereby preserving contractility. Thus, the coordinated actions of PLM during stress serve to minimize arrhythmogenesis and maintain inotropy. In acute cardiac ischemia and chronic heart failure, either expression or phosphorylation of PLM or both are altered. PLM regulates important ion transporters in the heart and offers a tempting target for development of drugs to treat heart failure.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Phospholemman deficiency in postinfarct hearts: enhanced contractility but increased mortality

Phospholemman (PLM) regulates [Na(+) ](i), [Ca(2+)](i) and contractility through its interactions with Na(+)-K(+)-ATPase (NKA) and Na(+) /Ca(2+) exchanger (NCX1) in the heart. Both expression and phosphorylation of PLM are altered after myocardial infarction (MI) and heart failure. We tested the hypothesis that absence of PLM regulation of NKA and NCX1 in PLM-knockout (KO) mice is detrimental. Three weeks after MI, wild-type (WT) and PLM-KO hearts were similarly hypertrophied. PLM expression was lower but fractional phosphorylation was higher in WT-MI compared to WT-sham hearts. Left ventricular ejection fraction was severely depressed in WT-MI but significantly less depressed in PLM-KO-MI hearts despite similar infarct sizes. Compared with WT-sham myocytes, the abnormal [Ca(2+) ], transient and contraction amplitudes observed in WT-MI myocytes were ameliorated by genetic absence of PLM. In addition, NCX1 current was depressed in WT-MI but not in PLM-KO-MI myocytes. Despite improved myocardial and myocyte performance, PLM-KO mice demonstrated reduced survival after MI. Our findings indicate that alterations in PLM expression and phosphorylation are important adaptations post-MI, and that complete absence of PLM regulation of NKA and NCX1 is detrimental in post-MI animals.

Phospholemman (FXYD1) raises the affinity of the human α1β1 isoform of Na,K-ATPase for Na ions

The human α(1)/His(10)-β(1) isoform of the Na,K-ATPase has been expressed in Pichia pastoris, solubilized in n-dodecyl-β-maltoside, and purified by metal chelate chromatography. The α(1)β(1) complex spontaneously associates in vitro with the detergent-solubilized purified human FXYD1 (phospholemman) expressed in Escherichia coli. It has been confirmed that FXYD1 spontaneously associates in vitro with the α(1)/His(10)-β(1) complex and stabilizes it in an active mode. The functional properties of the α(1)/His(10)-β(1) and α(1)/His(10)-β(1)/FXYD1 complexes have been investigated by fluorescence methods. The electrochromic dye RH421 which monitors binding to and release of ions from the binding sites has been applied in equilibrium titration experiments to determine ion binding affinities and revealed that FXYD1 induces an ∼30% increase of the Na(+)-binding affinity in both the E(1) and P-E(2) conformations. By contrast, it does not affect the affinities for K(+) and Rb(+) ions. Phosphorylation induced partial reactions of the enzyme have been studied as backdoor phosphorylation by inorganic phosphate and in kinetic experiments with caged ATP in order to evaluate the ATP-binding affinity and the time constant of the conformational transition, Na(3)E(1)-P → P-E(2)Na(3). No significant differences with or without FXYD1 could be detected. Rate constants of the conformational transitions Rb(2)E(1) → E(2)(Rb(2)) and E(2)(Rb(2)) → Na(3)E(1), investigated with fluorescein-labeled Na,K-ATPase, showed only minor or no effects of FXYD1, respectively. The conclusion from all these experiments is that FXYD1 raises the binding affinity of α(1)β(1) for Na ions, presumably at the third Na-selective binding site. In whole cell expression studies FXYD1 reduces the apparent affinity for Na ions. Possible reasons for the difference from this study using the purified recombinant Na,K-ATPase are discussed.

Phospholemman: a novel cardiac stress protein

Phospholemman (PLM), a member of the FXYD family of regulators of ion transport, is a major sarcolemmal substrate for protein kinases A and C in cardiac and skeletal muscle. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. Functionally, when phosphorylated at serine(68), PLM stimulates Na(+)-K(+)-ATPase but inhibits Na(+)/Ca(2+) exchanger in cardiac myocytes. In heterologous expression systems, PLM modulates the gating of cardiac L-type Ca(2+) channel. Therefore, PLM occupies a key modulatory role in intracellular Na(+) and Ca(2+) homeostasis and is intimately involved in regulation of excitation-contraction (EC) coupling. Genetic ablation of PLM results in a slight increase in baseline cardiac contractility and prolongation of action potential duration. When hearts are subjected to catecholamine stress, PLM minimizes the risks of arrhythmogenesis by reducing Na(+) overload and simultaneously preserves inotropy by inhibiting Na(+)/Ca(2+) exchanger. In heart failure, both expression and phosphorylation state of PLM are altered and may partly account for abnormalities in EC coupling. The unique role of PLM in regulation of Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and potentially L-type Ca(2+) channel in the heart, together with the changes in its expression and phosphorylation in heart failure, make PLM a rational and novel target for development of drugs in our armamentarium against heart failure. Clin Trans Sci 2010; Volume 3: 189-196.

Na/K-ATPase--an integral player in the adrenergic fight-or-flight response

During activation of the sympathetic nervous system, cardiac performance is increased as part of the fight-or-flight stress response. The increase in contractility with sympathetic stimulation is an orchestrated combination of intrinsic inotropic, lusitropic, and chronotropic effects, mediated in part by activation of beta-adrenergic receptors and protein kinase A. This causes phosphorylation of several Ca cycling proteins in cardiac myocytes (increasing Ca entry via L-type Ca channels, sarcoplasmic reticulum Ca pumping, and the dissociation rate of Ca from the myofilaments). Here, we discuss how stimulation of the Na/K-ATPase, mediated by phosphorylation of phospholemman (a small sarcolemmal protein that associates with and modulates Na/K-ATPase), is an additional important player in the sympathetic fight-or-flight response. Enhancement of Na/K- ATPase activity limits the rise in [Na](i) caused by the higher level of Na influx and by doing so limits the rise in cellular and sarcoplasmic reticulum Ca load by favoring Ca extrusion via the Na/Ca exchanger. Thus, phospholemman-mediated activation of the Na/K-ATPase may prevent Ca overload and triggered arrhythmias during stress.

Na+ transport in cardiac myocytes; Implications for excitation-contraction coupling

Intracellular Na(+) concentration ([Na(+)](i)) is very important in modulating the contractile and electrical activity of the heart. Upon electrical excitation of the myocardium, voltage-dependent Na(+) channels open, triggering the upstroke of the action potential (AP). During the AP, Ca(2+) enters the myocytes via L-type Ca(2+) channels. This triggers Ca(2+) release from the sarcoplasmic reticulum (SR) and thus activates contraction. Relaxation occurs when cytosolic Ca(2+) declines, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion from the cell via the Na(+)/Ca(2+) exchanger (NCX). NCX extrudes one Ca(2+) ion in exchange for three Na(+) ions and its activity is critically regulated by [Na(+)](i). Thus, via NCX, [Na(+)](i) is centrally involved in the regulation of intracellular [Ca(2+)] and contractility. Na(+) brought in by Na(+) channels, NCX and other Na(+) entry pathways is extruded by the Na(+)/K(+) pump (NKA) to keep [Na(+)](i) low. NKA is regulated by phospholemman, a small sarcolemmal protein that associates with NKA. Unphosphorylated phospholemman inhibits NKA by decreasing the pump affinity for internal Na(+) and this inhibition is relieved upon phosphorylation. Here we discuss the main characteristics of the Na(+) transport pathways in cardiac myocytes and their physiological and pathophysiological relevance.

Isoform specificity of the Na/K-ATPase association and regulation by phospholemman

Phospholemman (PLM) phosphorylation mediates enhanced Na/K-ATPase (NKA) function during adrenergic stimulation of the heart. Multiple NKA isoforms exist, and their function/regulation may differ. We combined fluorescence resonance energy transfer (FRET) and functional measurements to investigate isoform specificity of the NKA-PLM interaction. FRET was measured as the increase in the donor fluorescence (CFP-NKA-alpha1 or CFP-NKA-alpha2) during progressive acceptor (PLM-YFP) photobleach in HEK-293 cells. Both pairs exhibited robust FRET (maximum of 23.6 +/- 3.4% for NKA-alpha1 and 27.5 +/- 2.5% for NKA-alpha2). Donor fluorescence depended linearly on acceptor fluorescence, indicating a 1:1 PLM:NKA stoichiometry for both isoforms. PLM phosphorylation induced by cAMP-dependent protein kinase and protein kinase C activation drastically reduced the FRET with both NKA isoforms. However, submaximal cAMP-dependent protein kinase activation had less effect on PLM-NKA-alpha2 versus PLM-NKA-alpha1. Surprisingly, ouabain virtually abolished NKA-PLM FRET but only partially reduced co-immunoprecipitation. PLM-CFP also showed FRET to PLM-YFP, but the relationship during progressive photobleach was highly nonlinear, indicating oligomers involving >or=3 monomers. Using cardiac myocytes from wild-type mice and mice where NKA-alpha1 is ouabain-sensitive and NKA-alpha2 is ouabain-resistant, we assessed the effects of PLM phosphorylation on NKA-alpha1 and NKA-alpha2 function. Isoproterenol enhanced internal Na(+) affinity of both isoforms (K((1/2)) decreased from 18.1 +/- 2.0 to 11.5 +/- 1.9 mm for NKA-alpha1 and from 16.4 +/- 2.5 to 10.4 +/- 1.5 mm for NKA-alpha2) without altering maximum transport rate (V(max)). Protein kinase C activation also decreased K((1/2)) for both NKA-alpha1 and NKA-alpha2 (to 9.4 +/- 1.0 and 9.1 +/- 1.1 mm, respectively) but increased V(max) only for NKA-alpha2 (1.9 +/- 0.4 versus 1.2 +/- 0.5 mm/min). In conclusion, PLM associates with and modulates both NKA-alpha1 and NKA-alpha2 in a comparable but not identical manner.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume control in phospholemman (PLM) knockout mice: do cardiac myocytes demonstrate a regulatory volume decrease and is this influenced by deletion of PLM?

In addition to modulatory actions on Na+-K+-ATPase, phospholemman (PLM) has been proposed to play a role in cell volume regulation. Overexpression of PLM induces ionic conductances, with 'PLM channels' exhibiting selectivity for taurine. Osmotic challenge of host cells overexpressing PLM increases taurine efflux and augments the cellular regulatory volume decrease (RVD) response, though a link between PLM and cell volume regulation has not been studied in the heart. We recently reported a depressed cardiac contractile function in PLM knockout mice in vivo, which was exacerbated in crystalloid-perfused isolated hearts, indicating that these hearts were osmotically challenged. To address this, the present study investigated the role of PLM in osmoregulation in the heart. Isolated PLM wild-type and knockout hearts were perfused with a crystalloid buffer supplemented with mannitol in a bid to prevent perfusate-induced cell swelling and maintain function. Accordingly, and in contrast to wild-type control hearts, contractile function was improved in PLM knockout hearts with 30 mM mannitol. To investigate further, isolated PLM wild-type and knockout cardiomyocytes were subjected to increasing hyposmotic challenges. Initial validation studies showed the IonOptix video edge-detection system to be a simple and accurate 'real-time' method for tracking cell width as a marker of cell size. Myocytes swelled equally in both genotypes, indicating that PLM, when expressed at physiological levels in cardiomyocytes, is not essential to limit water accumulation in response to a hyposmotic challenge. Interestingly, freshly isolated adult cardiomyocytes consistently failed to mount RVDs in response to cell swelling, adding to conflicting reports in the literature. A proposed perturbation of the RVD response as a result of the cell isolation process was not restored, however, with short-term culture in either adult or neonatal cardiomyocytes.

Exercise-induced regulation of phospholemman (FXYD1) in rat skeletal muscle: implications for Na+/K+-ATPase activity

BACKGROUND

Na(+)/K(+)-ATPase activity is upregulated during muscle exercise to maintain ionic homeostasis. One mechanism may involve movement of alpha-subunits to the outer membrane (translocation).

AIM

We investigated the existence of exercise-induced translocation and phosphorylation of phospholemman (PLM, FXYD1) protein in rat skeletal muscle and exercise-induced changes in V(max) and K(m) for Na(+) of the Na(+)/K(+)-ATPase.

METHODS

Two membrane fractionation methods and immunoprecipitation were used.

RESULTS

Both fractionation methods revealed a 200-350% increase in PLM in the sarcolemma after 30 min of treadmill running, while the phosphorylation of Ser-68 of PLM appeared to be unchanged. Exercise did not change V(max) or K(m) for Na(+) of the Na(+)/K(+)-ATPase in muscle homogenate, but induced a 67% increase in V(max) in the sarcolemmal giant vesicle preparation; K(m) for Na(+) remained constant. The main part of the increase in V(max) is related to a 36-53% increase in the level of alpha-subunits; the remainder may be related to increased PLM content. Similar results were obtained with another membrane purification method. In resting muscle, 29% and 32% of alpha(1)- and alpha(2)-subunits, respectively, were co-immunoprecipitated by PLM antibodies. In muscle homogenate prepared after exercise, immunoprecipitation of alpha(1)-subunits was increased to 227%, whereas the fraction of precipitated alpha(2) remained constant.

CONCLUSION

Exercise translocates PLM to the muscle outer membrane and increases its association with mainly the alpha(1)-subunit, which may contribute to the increased V(max) of the Na(+)/K(+)-ATPase.

Activation of Cl- channels by human chorionic gonadotropin in luteinized granulosa cells of the human ovary modulates progesterone biosynthesis

Chloride permeability pathways and progesterone (P4) secretion elicited by human chorionic gonadotropin (hCG) in human granulosa cells were studied by electrophysiological techniques and single-cell volume, membrane potential and Ca2+i measurements. Reduction in extracellular Cl(-) and equimolar substitution by the membrane-impermeant anions glutamate or gluconate significantly increased hCG-stimulated P4 accumulation. A similar result was achieved by exposing the cells to hCG in the presence of a hypotonic extracellular solution. Conversely, P4 accumulation was drastically reduced in cells challenged with hCG exposed to a hypertonic solution. Furthermore, conventional Cl(-) channel inhibitors abolished hCG-mediated P4 secretion. In contrast, 25-hydroxycholesterol-mediated P4 accumulation was unaffected by Cl(-) channel blockers. In human granulosa cells, hCG triggered the activation of a tamoxifen-sensitive outwardly rectifying Cl(-) current comparable to the volume-sensitive outwardly rectifying Cl(-) current. Exposure of human granulosa cells to hCG induced a rapid 4,4'-diisothiocyanatostilbene-2,2-disulphonic acid-sensitive cell membrane depolarization that was paralleled with an approximately 20% decrease in cell volume. Treatment with hCG evoked oscillatory and nonoscillatory intracellular Ca2+ signals in human granulosa cells. Extracellular Ca2+ removal and 4,4'-diisothiocyanatostilbene-2,2-disulphonic acid abolished the nonoscillatory component while leaving the Ca2+ oscillations unaffected. It is concluded that human granulosa cells express functional the volume-sensitive outwardly rectifying Cl(-) channels that are activated by hCG, which are critical for plasma membrane potential changes, Ca2+ influx, and P4 production.

Human FXYD2 G41R mutation responsible for renal hypomagnesemia behaves as an inward-rectifying cation channel

A mutation in the human FXYD2 polypeptide (Na-K-ATPase gamma subunit) that changes a conserved transmembrane glycine to arginine is linked to dominant renal hypomagnesemia. Xenopus laevis oocytes injected with wild-type FXYD2 or the mutant G41R cRNAs expressed large nonselective ion currents. However, in contrast to the wild-type FXYD2 currents, inward rectifying cation currents were induced by hyperpolarization pulses in oocytes expressing the G41R mutant. Injection of EDTA into the oocyte removed inward rectification in the oocytes expressing the mutant, but did not alter the nonlinear current-voltage relationship of the wild-type FXYD2 pseudo-steady-state currents. Extracellular divalent ions, Ca2+ and Ba2+, and trivalent cations, La3+, blocked both the wild-type and mutant FXYD2 currents. Site-directed mutagenesis of G41 demonstrated that a positive charge at this site is required for the inward rectification. When the wild-type FXYD2 was expressed in Madin-Darby canine kidney cells, the cells in the presence of a large apical-to-basolateral Mg2+ gradient and at negative potentials had an increase in transepithelial current compared with cells expressing the G41R mutant or control transfected cells. Moreover, this current was inhibited by extracellular Ba2+ at the basolateral surface. These results suggest that FXYD2 can mediate basolateral extrusion of magnesium from cultured renal epithelial cells and provide new insights into the understanding of the possible physiological roles of FXYD2 wild-type and mutant proteins.

Phospholemman-mediated activation of Na/K-ATPase limits [Na]i and inotropic state during beta-adrenergic stimulation in mouse ventricular myocytes

BACKGROUND

Cardiac Na/K-ATPase (NKA) regulates intracellular Na ([Na](i)), which in turn affects intracellular Ca and thus contractility via Na/Ca exchange. Recent evidence shows that phosphorylation of the NKA-associated small transmembrane protein phospholemman (PLM) mediates beta-adrenergic-induced NKA stimulation.

METHODS AND RESULTS

Here, we tested whether PLM phosphorylation during beta-adrenergic activation limits the rise in [Na](i), Ca transient amplitude, and triggered arrhythmias in mouse ventricular myocytes. In myocytes from wild-type (WT) mice, [Na](i) increased on field stimulation at 2 Hz from 11.1+/-1.8 mmol/L to a plateau of 15.2+/-1.5 mmol/L. Isoproterenol induced a decrease in [Na](i) to 12.0+/-1.2 mmol/L. In PLM knockout (PLM-KO) mice in which beta-adrenergic stimulation does not activate NKA, [Na](i) also increased at 2 Hz (from 10.4+/-1.2 to 17.0+/-1.5 mmol/L) but was unaltered by isoproterenol. The PLM-mediated decrease in [Na](i) in WT mice could limit the isoproterenol-induced inotropic state. Indeed, the isoproterenol-induced increase in the amplitude of Ca transients was significantly smaller in the WT mice (5.2+/-0.4- versus 7.1+/-0.5-fold in PLM-KO mice). This also was the case for the sarcoplasmic reticulum Ca content, which increased by 1.27+/-0.09-fold in WT mice versus 1.53+/-0.09-fold in PLM-KO mice. The higher sarcoplasmic reticulum Ca content in PLM-KO versus WT mice was associated with an increased propensity for spontaneous Ca transients and contractions in PLM-KO mice.

CONCLUSIONS

These data suggest that PLM phosphorylation and NKA stimulation are an integral part of the sympathetic fight-or-flight response, tempering the rise in [Na](i) and cellular Ca loading and perhaps limiting Ca overload-induced arrhythmias.

Swelling-induced taurine transport: relationship with chloride channels, anion-exchangers and other swelling-activated transport pathways

Cells have to regulate their volume in order to survive. Moreover, it is now evident that cell volume per se and the membrane transport processes which regulate it, comprise an important signalling unit. For example, macromolecular synthesis, apoptosis, cell growth and hormone secretion are all influenced by the cellular hydration state. Therefore, a thorough understanding of volume-activated transport processes could lead to new strategies being developed to control the function and growth of both normal and cancerous cells. Cell swelling stimulates the release of ions such as K(+) and Cl(-) together with organic osmolytes, especially the beta-amino acid taurine. Despite being the subject of intense research interest, the nature of the volume-activated taurine efflux pathway is still a matter of controversy. On the one hand it has been suggested that osmosensitive taurine efflux utilizes volume-sensitive anion channels whereas on the other it has been proposed that the band 3 anion-exchanger is a swelling-induced taurine efflux pathway. This article reviews the evidence for and against a role of anion channels and exchangers in osmosensitive taurine transport. Furthermore, the distinct possibility that neither pathway is involved in taurine transport is highlighted. The putative relationship between swelling-induced taurine transport and volume-activated anionic amino acid, alpha-neutral amino acid and K(+) transport is also examined.

Phosphorylation of phospholemman (FXYD1) by protein kinases A and C modulates distinct Na,K-ATPase isozymes

Phospholemman (FXYD1), mainly expressed in heart and skeletal muscle, is a member of the FXYD protein family, which has been shown to decrease the apparent K(+) and Na(+) affinity of Na,K-ATPase ( Crambert, G., Fuzesi, M., Garty, H., Karlish, S., and Geering, K. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 11476-11481 ). In this study, we use the Xenopus oocyte expression system to study the role of phospholemman phosphorylation by protein kinases A and C in the modulation of different Na,K-ATPase isozymes present in the heart. Phosphorylation of phospholemman by protein kinase A has no effect on the maximal transport activity or on the apparent K(+) affinity of Na,K-ATPase alpha1/beta1 and alpha2/beta1 isozymes but increases their apparent Na(+) affinity, dependent on phospholemman phosphorylation at Ser(68). Phosphorylation of phospholemman by protein kinase C affects neither the maximal transport activity of alpha1/beta1 isozymes nor the K(+) affinity of alpha1/beta1 and alpha2/beta1 isozymes. However, protein kinase C phosphorylation of phospholemman increases the maximal Na,K-pump current of alpha2/beta1 isozymes by an increase in their turnover number. Thus, our results indicate that protein kinase A phosphorylation of phospholemman has similar functional effects on Na,K-ATPase alpha1/beta and alpha2/beta isozymes and increases their apparent Na(+) affinity, whereas protein kinase C phosphorylation of phospholemman modulates the transport activity of Na,K-ATPase alpha2/beta but not of alpha1/beta isozymes. The complex and distinct regulation of Na,K-ATPase isozymes by phosphorylation of phospholemman may be important for the efficient control of heart contractility and excitability.

Phospholemman Transmembrane Structure Reveals Potential Interactions with Na+/K+-ATPase

Phospholemman (PLM) is a 72-residue bitopic cardiac transmembrane protein, which acts as a modulator of the Na(+)/K(+)-ATPase and the Na(+)/Ca(2+) exchanger and possibly forms taurine channels in nonheart tissue. This work presents a high resolution structural model obtained from a combination of site-specific infrared spectroscopy and experimentally constrained high throughput molecular dynamics (MD) simulations. Altogether, 37 experimental constraints, including nine long range orientational constraints, have been used during MD simulations in an explicit lipid bilayer/water system. The resulting tetrameric alpha-helical bundle has an average helix tilt of 7.3 degrees and a crossing angle close to 0 degrees . It does not reveal a hydrophilic pore, but instead strong interactions between various residues occlude any pore. The helix-helix packing is unusual, with Gly(19) and Gly(20) pointing to the outside of the helical bundle, facilitating potential interaction with other transmembrane proteins, thus providing a structural basis for the modulatory effect of PLM on the Na(+)/K(+)-ATPase. A two-stage model of interaction between PLM and the Na(+)/K(+)-ATPase is discussed involving PLM-ATPase interaction and subsequent formation of an unstable PLM trimer, which readily interacts with surrounding ATPase molecules. Further unconstrained MD simulations identified other packing models of PLM, one of which could potentially undergo a conformational transition to an open pore.

Structure of the Na,K-ATPase regulatory protein FXYD1 in micelles

FXYD1 is a major regulatory subunit of the Na,K-ATPase and the principal substrate of hormone-regulated phosphorylation by c-AMP dependent protein kinases A and C in heart and skeletal muscle sarcolemma. It is a member of an evolutionarily conserved family of membrane proteins that regulate the function of the enzyme complex in a tissue-specific and physiological-state-specific manner. Here, we present the three-dimensional structure of FXYD1 determined in micelles by NMR spectroscopy. Structure determination was made possible by measuring residual dipolar couplings in weakly oriented micelle samples of the protein. This allowed us to obtain the relative orientations of the helical segments and information about the protein dynamics. The structural analysis was further facilitated by the inclusion of distance restraints, obtained from paramagnetic spin label relaxation enhancements, and by refinement with a micelle depth restraint, derived from paramagnetic Mn line broadening effects. The structure of FXYD1 provides the foundation for understanding its intra-membrane association with the Na,K-ATPase alpha subunit and suggests a mechanism whereby the phosphorylation of conserved Ser residues, by protein kinases A and C, could induce a conformational change in the cytoplasmic domain of the protein to modulate its interaction with the alpha subunit.

Regulation of cardiac Na+/Ca2+ exchanger by phospholemman

Phospholemman (PLM) is the first sequenced member of the FXYD family of regulators of ion transport. The mature protein has 72 amino acids and consists of an extracellular N terminus containing the signature FXYD motif, a single transmembrane (TM) domain, and a cytoplasmic C-terminal domain containing four potential sites for phosphorylation. PLM and other members of the FXYD family are known to regulate Na+-K+-ATPase. Using adenovirus-mediated gene transfer into adult rat cardiac myocytes, we showed that changes in contractility and intracellular Ca2+ homeostasis associated with PLM overexpression or downregulation are not consistent with the effects expected from inhibition of Na+-K+-ATPase by PLM. Additional studies with heterologous expression of PLM and cardiac Na+/Ca2+ exchanger 1 (NCX1) in HEK293 cells and cardiac myocytes isolated from PLM-deficient mice demonstrated by co-localization, co-immunoprecipitation, and electrophysiological and radioactive tracer uptake techniques that PLM associates with NCX1 in the sarcolemma and transverse tubules and that PLM inhibits NCX1, independent of its effects on Na+-K+-ATPase. Mutational analysis indicates that the cytoplasmic domain of PLM is required for its regulation of NCX1. In addition, experiments using phosphomimetic and phospho-deficient PLM mutants, as well as activators of protein kinases A and C, indicate that PLM phosphorylated at serine68 is the active form that inhibits NCX1. This is in sharp contrast to the finding that the unphosphorylated PLM form inhibits Na+-K+-ATPase. We conclude that PLM regulates cardiac contractility by modulating the activities of NCX and Na+-K+-ATPase.

The intracellular region of FXYD1 is sufficient to regulate cardiac Na/K ATPase

FXYD1 is a transmembrane protein predominantly expressed in excitable tissues that associates with and regulates Na/K ATPase. PKA phosphorylates FXYD1 at serine 68 (S68), however, the effects of phosphorylation on Na/K ATPase activity are not fully characterized. The objectives of this study were to characterize Na/K ATPase currents in FXYD1 wild-type (WT) and knockout (KO) adult mouse ventricular myocytes, and investigate the effects of FXYD1 on Na/K ATPase currents using the whole-cell patch-clamp technique. A peptide representing the 19 C-terminal residues of FXYD1 (FXYD1(54-72)) was introduced into the interior of FXYD1 KO and WT myocytes through the patch pipette. K-sensitive Na/K ATPase currents were higher in KO myocytes (2.9+/-0.1 pA/pF; n=4) compared with WT (1.9+/-0.1 pA/pF; n=4). Unphosphorylated FXYD1(54-72), at a concentration of 4 microM, reduced the currents in WT (from 2.1+/-0.1 to 1.3+/-0.1 pA/pF; P<0.05, n=7) and KO (from 2.9+/-0.1 to 1.7+/-0.1 pA/pF; P<0.05, n=5), whereas, 1 microM of FXYD1(54-72) phosphorylated at S68 increased currents in WT (from 1.91+/-0.09 to 3.1+/-0.5 pA/pF; P<0.05, n=6) and KO (from 2.7+/-0.11 to 3.8+/-0.2 pA/pF; P<0.05, n=6) myocytes. Coimmunoprecipitation studies demonstrated that S68 phosphorylated and unphosphorylated FXYD1(54-72) associates with Na/K ATPase alpha1 subunit. We conclude that unphosphorylated FXYD1 inhibits Na/K ATPase, whereas S68 phosphorylated FXYD1 stimulates Na/K ATPase to a level above that seen in the absence of FXYD1.

[FXYD proteins: novel regulators of Na,K-ATPase]

Members of the FXYD protein family are small membrane proteins which are characterized by an FXYD motif, two conserved glycines and a serine residue. FXYD proteins show a tissue-specific distribution. Recent evidence suggests that 6 out of 7 FXYD proteins, FXYD1 (phospholemman), FXYD2 (gamma subunit of Na,K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), FXYD5 (Ric) and FXYD7 associate with Na,K-ATPase and modulate its transport properties e.g. its Na+ and/or its K+ affinity in a distinct way. These results highlight the complex regulation of Na+ and K+ transport which is necessary to ensure proper tissue functions such as renal Na+-reabsorption, muscle contractility and neuronal excitability. Moreover, mutation of a conserved glycine residue into an arginine residue in FXYD2 has been linked to cases of human hypomagnesemia indicating that dysregulation of Na,K-ATPase by FXYD proteins may be implicated in pathophysiological states. A better characterization of this novel regulatory mechanism of Na,K-ATPase may help to better understand its role in physiological and pathophysiological conditions.

Phospholemman phosphorylation alters its fluorescence resonance energy transfer with the Na/K-ATPase pump

Phospholemman (PLM) or FXYD1 is a major cardiac myocyte phosphorylation target upon adrenergic stimulation. Prior immunoprecipitation and functional studies suggest that phospholemman associates with the Na/K-pump (NKA) and mediates adrenergic Na/K-pump regulation. Here, we tested whether the NKA-PLM interaction is close enough to allow fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent (CFP/YFP) fusion proteins of Na/K pump and phospholemman and whether phospholemman phosphorylation alters such FRET. Co-expressed NKA-CFP and PLM-YFP in HEK293 cells co-localized in the plasma membrane and exhibited robust FRET. Selective acceptor photobleach increased donor fluorescence (F(CFP)) by 21.5 +/- 4.1% (n = 13), an effect nearly abolished when co-expressing excess phospholemman lacking YFP. Activation of protein kinase C or A progressively and reversibly decreased FRET assessed by either the fluorescence ratio (F(YFP)/F(CFP)) or the enhancement of donor fluorescence after acceptor bleach. After protein kinase C activation, forskolin did not further reduce FRET, but after forskolin pretreatment, protein kinase C could still reduce FRET. This agreed with phospholemman phosphorylation measurements: by protein kinase C at both Ser-63 and Ser-68, but by protein kinase A only at Ser-68. Expression of PLM-YFP and PLM-CFP resulted in even stronger FRET than for NKA-PLM (F(CFP) increased by 37 +/- 1% upon YFP photobleach), and this FRET was enhanced by phospholemman phosphorylation, consistent with phospholemman multimerization. Co-expressed PLM-CFP and Na/Ca exchange-YFP were highly membrane co-localized, but FRET was undetectable. We conclude that phospholemman and Na/K-pump are in very close proximity (FRET occurs) and that phospholemman phosphorylation alters the interaction of Na/K-pump and phospholemman.

Cytoplasmic tail of phospholemman interacts with the intracellular loop of the cardiac Na+/Ca2+ exchanger

Phospholemman (PLM), a member of the FXYD family of small ion transport regulators, inhibits cardiac Na+/Ca2+ exchanger (NCX1). NCX1 is made up of N-terminal domain consisting of the first five transmembrane segments (residues 1-217), a large intracellular loop (residues 218-764), and a C-terminal domain comprising the last four transmembrane segments (residues 765-938). Using glutathione S-transferase (GST) pull-down assay, we demonstrated that the intracellular loop, but not the N- or C-terminal transmembrane domains of NCX1, was associated with PLM. Further analysis using protein constructs of GST fused to various segments of the intracellular loop of NCX1 suggest that PLM bound to residues 218-371 and 508-764 but not 371-508. Split Na+/Ca2+ exchangers consisting of N- or C-terminal domains with different lengths of the intracellular loop were co-expressed with PLM in HEK293 cells that are devoid of endogenous PLM and NCX1. Although expression of N-terminal but not C-terminal domain alone resulted in correct membrane targeting, co-expression of both N- and C-terminal domains was required for correct membrane targeting and functional exchange activity. NCX1 current measurements indicate that PLM decreased NCX1 current only when the split exchangers contained residues 218-358 of the intracellular loop. Co-immunoprecipitation experiments with PLM and split exchangers suggest that PLM associated with the N-terminal domain of NCX1 when it contained intracellular loop residues 218-358. TM43, a PLM mutant with its cytoplasmic tail truncated, did not co-immunoprecipitate with wild-type NCX1 when co-expressed in HEK293 cells, confirming little to no interaction between the transmembrane domains of PLM and NCX1. We conclude that PLM interacted with the intracellular loop of NCX1, most likely at residues 218-358.

Function of FXYD proteins, regulators of Na, K-ATPase

In this short review, we summarize our work on the role of members of the FXYD protein family as tissue-specific modulators of Na, K-ATPase. FXYD1 or phospholemman, mainly expressed in heart and skeletal muscle increases the apparent affinity for intracellular Na(+) of Na, K-ATPase and may thus be important for appropriate muscle contractility. FXYD2 or gamma subunit and FXYD4 or CHIF modulate the apparent affinity for Na(+) of Na, K-ATPase in an opposite way, adapted to the physiological needs of Na(+) reabsorption in different segments of the renal tubule. FXYD3 expressed in stomach, colon, and numerous tumors also modulates the transport properties of Na, K-ATPase but it has a lower specificity of association than other FXYD proteins and an unusual membrane topology. Finally, FXYD7 is exclusively expressed in the brain and decreases the apparent affinity for extracellular K(+), which may be essential for proper neuronal excitability.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

Secondary structure, orientation, and oligomerization of phospholemman, a cardiac transmembrane protein

Human phospholemman (PLM) is a 72-residue protein, which is expressed at high density in the cardiac plasma membrane and in various other tissues. It forms ion channels selective for K+, Cl-, and taurine in lipid bilayers and colocalizes with the Na+/K+-ATPase and the Na+/Ca2+-exchanger, which may suggest a role in the regulation of cell volume. Here we present the first structural data based on synthetic peptides representing the transmembrane domain of PLM. Perfluoro-octaneoate-PAGE of reconstituted proteoliposomes containing PLM reveals a tetrameric homo-oligomerization. Infrared spectroscopy of proteoliposomes shows that the PLM peptide is completely alpha-helical, even beyond the hydrophobic core residues. Hydrogen/deuterium exchange experiments reveal that a core of 20-22 residues is not accessible to water, thus embedded in the lipid membrane. The maximum helix tilt is 17 degrees +/- 2 degrees obtained by attenuated total reflection infrared spectroscopy. Thus, our data support the idea of ion channel formation by the PLM transmembrane domain.

FXYD proteins: new regulators of Na-K-ATPase

FXYD proteins belong to a family of small-membrane proteins. Recent experimental evidence suggests that at least five of the seven members of this family, FXYD1 (phospholemman), FXYD2 (gamma-subunit of Na-K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), and FXYD7, are auxiliary subunits of Na-K-ATPase and regulate Na-K-ATPase activity in a tissue- and isoform-specific way. These results highlight the complexity of the regulation of Na+ and K+ handling by Na-K-ATPase, which is necessary to ensure appropriate tissue functions such as renal Na+ reabsorption, muscle contractility, and neuronal excitability. Moreover, a mutation in FXYD2 has been linked to cases of human hypomagnesemia, indicating that perturbations in the regulation of Na-K-ATPase by FXYD proteins may be critically involved in pathophysiological states. A better understanding of this novel regulatory mechanism of Na-K-ATPase should help in learning more about its role in pathophysiological states. This review summarizes the present knowledge of the role of FXYD proteins in the modulation of Na-K-ATPase as well as of other proteins, their regulation, and their structure-function relationship.

Cytoplasmic targeting signals mediate delivery of phospholemman to the plasma membrane

The FXYD protein family consists of several small, single-span membrane proteins that exhibit a high degree of homology. The best-known members of the family include the gamma-subunit of the Na(+)-K(+)-ATPase and phospholemman (PLM), a phosphoprotein of cardiac sarcolemma. Other members of the family include corticosteroid hormone-induced factor (CHIF), mammary tumor protein of 8 kDa (Mat-8), and related to ion channels (RIC). The exact physiological roles of the FXYD proteins remain unknown. To better characterize the function of the members of the FXYD protein family, we expressed several members of the family in Madin-Darby canine kidney (MDCK) cells. All of the FXYD proteins, with the exception of PLM, were primarily found in the basolateral plasma membrane. Surprisingly, PLM, a previously characterized plasma membrane protein, was found to colocalize with the endoplasmic reticulum marker protein disulfide isomerase. Treatment of MDCK cells expressing PLM with an agonist of PKC caused some of the PLM to be redistributed to the plasma membrane. Site-directed mutagenesis of residues within the cytoplasmic domain of PLM indicated that a negative charge at Ser69 is necessary to shift the localization of PLM to the plasma membrane. In addition, other regions of PLM necessary for either its endoplasmic reticulum or plasma membrane localization have been elucidated. In contrast to PLM, the plasma membrane localization of CHIF and RIC was not altered by mutation of potential cytoplasmic phosphorylation sites. Overall, these results suggest that phosphorylation of specific residues of PLM may direct PLM from an intracellular compartment to the plasma membrane.

Correlation of gene and protein structures in the FXYD family proteins

The FXYD family proteins are auxiliary subunits of the Na,K-ATPase, expressed primarily in tissues that specialize in fluid or solute transport, or that are electrically excitable. These proteins range in size from about 60 to 160 amino acid residues, and share a core homology of 35 amino acid residues in and around a single transmembrane segment. Despite their relatively small sizes, they are all encoded by genes with six to nine small exons. We show that the helical secondary structures of three FXYD family members, FXYD1, FXYD3, and FXYD4, determined in micelles by NMR spectroscopy, reflect the structures of their corresponding genes. The coincidence of helical regions, and connecting segments, with the positions of intron-exon junctions in the genes, support the hypothesis that the FXYD proteins may have been assembled from discrete structural modules through exon shuffling.

Phospholemman overexpression inhibits Na+-K+-ATPase in adult rat cardiac myocytes: relevance to decreased Na+ pump activity in postinfarction myocytes

Messenger RNA levels of phospholemman (PLM), a member of the FXYD family of small single-span membrane proteins with putative ion-transport regulatory properties, were increased in postmyocardial infarction (MI) rat myocytes. We tested the hypothesis that the previously observed reduction in Na+-K+-ATPase activity in MI rat myocytes was due to PLM overexpression. In rat hearts harvested 3 and 7 days post-MI, PLM protein expression was increased by two- and fourfold, respectively. To simulate increased PLM expression post-MI, PLM was overexpressed in normal adult rat myocytes by adenovirus-mediated gene transfer. PLM overexpression did not affect the relative level of phosphorylation on serine68 of PLM. Na+-K+-ATPase activity was measured as ouabain-sensitive Na+-K+ pump current (Ip). Compared with control myocytes overexpressing green fluorescent protein alone, Ip measured in myocytes overexpressing PLM was significantly (P < 0.0001) lower at similar membrane voltages, pipette Na+ ([Na+]pip) and extracellular K+ ([K+]o) concentrations. From -70 to +60 mV, neither [Na+]pip nor [K+]o required to attain half-maximal Ip was significantly different between control and PLM myocytes. This phenotype of decreased V(max) without appreciable changes in K(m) for Na+ and K+ in PLM-overexpressed myocytes was similar to that observed in MI rat myocytes. Inhibition of Ip by PLM overexpression was not due to decreased Na+-K+-ATPase expression because there were no changes in either protein or messenger RNA levels of either alpha1- or alpha2-isoforms of Na+-K+-ATPase. In native rat cardiac myocytes, PLM coimmunoprecipitated with alpha-subunits of Na+-K+-ATPase. Inhibition of Na+-K+-ATPase by PLM overexpression, in addition to previously reported decrease in Na+-K+-ATPase expression, may explain altered V(max) but not K(m) of Na+-K+-ATPase in postinfarction rat myocytes.

Serine 68 phospholemman phosphorylation during forskolin-induced swine carotid artery relaxation

BACKGROUND

Phospholemman (PLM) is an abundant phosphoprotein in the plasma membrane of cardiac, skeletal and smooth muscle. It is a member of the FXYD family of proteins that bind to and regulate the Na,K-ATPase. Protein kinase A (PKA) is known to phosphorylate PLM on serine 68 (S68), although the functional effect of S68 PLM phosphorylation is unclear. We therefore evaluated S68 PLM phosphorylation in swine carotid arteries.

METHODS

Two anti-PLM antibodies, one to S68 phosphorylated PLM and one to unphosphorylated PLM, were made to PLM peptides in rabbits and tested with purified PLM and PKA-treated PLM. Swine carotid arteries were mounted isometrically, contracted, relaxed with forskolin and then homogenized. Proteins were separated on SDS gels and the intensity of immunoreactivity to the two PLM antibodies determined on immunoblots.

RESULTS

The antipeptide antibody 'C2' primarily reacted with unphosphorylated PLM, and the antipeptide antibody 'CP68' detected S68 PLM phosphorylation. Histamine stimulation of intact swine carotid artery induced a contraction, increased the CP68 PLM antibody signal and reduced the C2 PLM antibody signal. High extracellular [K(+)] depolarization induced a contraction without altering the C2 or CP68 PLM signal. Forskolin-induced relaxation of histamine or extracellular [K(+)] contracted arteries correlated with an increased CP68 signal. Nitroglycerin-induced relaxation was not associated with changes in the C2 or CP68 PLM signal.

CONCLUSIONS

These data suggest that a contractile agonist increased S68 PLM phosphorylation. Agents that increase [cAMP], but not agents that increase [cGMP], increased S68 PLM phosphorylation. S68 PLM phosphorylation may be involved in cAMP-dependent regulation of smooth muscle force.

Identification of an endogenous inhibitor of the cardiac Na+/Ca2+ exchanger, phospholemman

Rapid and precise control of Na(+)/Ca(2+) exchanger (NCX1) activity is essential in the maintenance of beat-to-beat Ca(2+) homeostasis in cardiac myocytes. Here, we show that phospholemman (PLM), a 15-kDa integral sarcolemmal phosphoprotein, is a novel endogenous protein inhibitor of cardiac NCX1. Using a heterologous expression system that is devoid of both endogenous PLM and NCX1, we first demonstrated by confocal immunofluorescence studies that both exogenous PLM and NCX1 co-localized at the plasma membrane. Reciprocal co-immunoprecipitation studies revealed specific protein-protein interaction between PLM and NCX1. The functional consequences of direct association of PLM with NCX1 was the inhibition of NCX1 activity, as demonstrated by whole-cell patch clamp studies to measure NCX1 current density and radiotracer flux assays to assess Na(+)-dependent (45)Ca(2+) uptake. Inhibition of NCX1 by PLM was specific, because a single mutation of serine 68 to alanine in PLM resulted in a complete loss of inhibition of NCX1 current, although association of the PLM mutant with NCX1 was unaltered. In native adult cardiac myocytes, PLM co-immunoprecipitated with NCX1. We conclude that PLM, a member of the FXYD family of small ion transport regulators known to modulate Na(+)-K(+)-ATPase, also regulates Na(+)/Ca(2+) exchange in the heart.

Hypertrophy, increased ejection fraction, and reduced Na-K-ATPase activity in phospholemman-deficient mice

Phospholemman (FXYD1), a 72-amino acid transmembrane protein abundantly expressed in the heart and skeletal muscle, is a major substrate for phosphorylation in the cardiomyocyte sarcolemma. Biochemical, cellular, and electrophysiological studies have suggested a number of possible roles for this protein, including ion channel modulator, taurine-release channel, Na(+)/Ca(2+) exchanger modulator, and Na-K-ATPase-associated subunit. We have generated a phospholemman-deficient mouse. The adult null mice exhibited increased cardiac mass, larger cardiomyocytes, and ejection fractions that were 9% higher by magnetic resonance imaging compared with wild-type animals. Notably, this occurred in the absence of hypertension. Total Na-K-ATPase activity was 50% lower in the phospholemman-deficient hearts. Expression (per unit of membrane protein) of total Na-K-ATPase was only slightly diminished, but expression of the minor alpha(2)-isoform, which has been specifically implicated in the control of contractility, was reduced by 60%. The absence of phospholemman thus results in a complex response, including a surprisingly large reduction in intrinsic Na-K-ATPase activity, changes in Na-K-ATPase isoform expression, increase in ejection fraction, and increase in cardiac mass. We hypothesize that a primary effect of phospholemman is to modulate the Na-K-ATPase and that its reduced activity initiates compensatory responses.

Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT

A large body of genetic, reverse genetic, and epidemiological data has linked chloroquine-resistant malaria to polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum. To investigate the biological function of the chloroquine resistance transporter, PfCRT, as well as its role in chloroquine resistance, we functionally expressed this protein in Xenopus laevis oocytes. Our data show that PfCRT-expressing oocytes exhibit a depolarized resting membrane potential and a higher intracellular pH compared with control oocytes. Pharmacological and electrophysiological studies link the higher intracellular pH to an enhanced amiloride-sensitive H(+) extrusion and the low membrane potential to an activated nonselective cation conductance. The finding that both properties are independent of each other, together with the fact that they are endogenously present in X. laevis oocytes, supports a model in which PfCRT activates transport systems. Our data suggest that PfCRT plays a role as a direct or indirect activator or modulator of other transporters.

Effects of phospholemman expression on swelling-activated ion currents and volume regulation in embryonic kidney cells

Phospholemman (PLM) is a 72-amino-acid phosphoprotein that is a major substrate for cAMP-dependent protein kinase, protein kinase C, and NIMA kinase. In lipid bilayers, PLM forms ion channels selective for Cl-, K+, and taurine. Effluxes of these abundant intracellular osmolytes play an important role in the control of dynamic cell volume changes in many cell types. We measured swelling-activated ion currents and regulatory volume decrease (RVD) in human embryonic kidney cells stably overexpressing canine cardiac PLM. In response to swelling, two clonal cell lines overexpressing PLM had increased swelling-activated ion current densities and faster and more extensive RVD. A third clonal cell line overexpressing mutant PLM showed reduced ion current densities and a diminished RVD response. These results suggest a role for PLM in the regulation of cell volume, perhaps as a modulator of an endogenous swelling-activated signal transduction pathway or possibly by participating directly in swelling-induced osmolyte efflux.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.