Phospholemman expression induces a hyperpolarization-activated chloride current in Xenopus oocytes

FXYD protein isoforms differentially modulate human Na/K pump function

Tight regulation of the Na/K pump is essential for cellular function because this heteromeric protein builds and maintains the electrochemical gradients for Na+ and K+ that energize electrical signaling and secondary active transport. We studied the regulation of the ubiquitous human α1β1 pump isoform by five human FXYD proteins normally located in muscle, kidney, and neurons. The function of Na/K pump α1β1 expressed in Xenopus oocytes with or without FXYD isoforms was evaluated using two-electrode voltage clamp and patch clamp. Through evaluation of the partial reactions in the absence of K+ but presence of Na+ in the external milieu, we demonstrate that each FXYD subunit alters the equilibrium between E1P(3Na) and E2P, the phosphorylated conformations with Na+ occluded and free from Na+, respectively, thereby altering the apparent affinity for Na+. This modification of Na+ interaction shapes the small effects of FXYD proteins on the apparent affinity for external K+ at physiological Na+. FXYD6 distinctively accelerated both the Na+-deocclusion and the pump-turnover rates. All FXYD isoforms altered the apparent affinity for intracellular Na+ in patches, an effect that was observed only in the presence of intracellular K+. Therefore, FXYD proteins alter the selectivity of the pump for intracellular ions, an effect that could be due to the altered equilibrium between E1 and E2, the two major pump conformations, and/or to small changes in ion affinities that are exacerbated when both ions are present. Lastly, we observed a drastic reduction of Na/K pump surface expression when it was coexpressed with FXYD1 or FXYD6, with the former being relieved by injection of PKA's catalytic subunit into the oocyte. Our results indicate that a prominent effect of FXYD1 and FXYD6, and plausibly other FXYDs, is the regulation of Na/K pump trafficking.

FXYD proteins and sodium pump regulatory mechanisms

The sodium/potassium-ATPase (NKA) is the enzyme that establishes gradients of sodium and potassium across the plasma membrane. NKA activity is tightly regulated for different physiological contexts through interactions with single-span transmembrane peptides, the FXYD proteins. This diverse family of regulators has in common a domain containing a Phe-X-Tyr-Asp (FXYD) motif, two conserved glycines, and one serine residue. In humans, there are seven tissue-specific FXYD proteins that differentially modulate NKA kinetics as appropriate for each system, providing dynamic responsiveness to changing physiological conditions. Our understanding of how FXYD proteins contribute to homeostasis has benefitted from recent advances described in this review: biochemical and biophysical studies have provided insight into regulatory mechanisms, genetic models have uncovered remarkable complexity of FXYD function in integrated physiological systems, new posttranslational modifications have been identified, high-resolution structural studies have revealed new details of the regulatory interaction with NKA, and new clinical correlations have been uncovered. In this review, we address the structural determinants of diverse FXYD functions and the special roles of FXYDs in various physiological systems. We also discuss the possible roles of FXYDs in protein trafficking and regulation of non-NKA targets.

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].

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.

Intracellular trafficking of FXYD1 (phospholemman) and FXYD7 proteins in Xenopus oocytes and mammalian cells

FXYD proteins are a group of short single-span transmembrane proteins that interact with the Na(+)/K(+) ATPase and modulate its kinetic properties. This study characterizes intracellular trafficking of two FXYD family members, FXYD1 (phospholemman (PLM)) and FXYD7. Surface expression of PLM in Xenopus oocytes requires coexpression with the Na(+)/K(+) ATPase. On the other hand, the Na(+)/Ca(2+) exchanger, another PLM-interacting protein could not drive it to the cell surface. The Na(+)/K(+) ATPase-dependent surface expression of PLM could be facilitated by either a phosphorylation-mimicking mutation at Thr-69 or a truncation of three terminal arginine residues. Unlike PLM, FXYD7 could translocate to the cell surface of Xenopus oocytes independently of the coexpression of α1β1 Na(+)/K(+) ATPase. The Na(+)/K(+) ATPase-independent membrane translocation of FXYD7 requires O-glycosylation of at least two of three conserved threonines in its ectodomain. Subsequent experiments in mammalian cells confirmed the role of conserved extracellular threonine residues and demonstrated that FXYD7 protein, in which these have been mutated to alanine, is trapped in the endoplasmic reticulum and Golgi apparatus.

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.

Differential cellular expression of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in normal human tissues: a study using high density human tissue microarrays

FXYD proteins have been proposed to function as regulators of Na, K-ATPase function by lowering affinities of the system for potassium and sodium. However, their distribution in normal human tissues has not been studied. We have therefore used immunohistochemistry and semi-quantitative histomorphometric analysis to determine the relative expression at the protein level and distribution of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in human Tissue MicroArrays (TMAs). Expression of FXYD1 was abundant in heart, kidney, placenta, skeletal muscle, gastric and anal mucosa, small intestine and colon. Lower FXYD1 expression was detected in uterine, intestinal and bladder smooth muscle, choroid plexus, liver, gallbladder, spleen, breast, prostate and epididymis. The tissue distribution of FXYD2 was less extensive compared to that of FXYD1. There was an abundant expression in kidney and choroid plexus and moderate expression in placenta, amniotic membranes, breast epithelium, salivary glands, pancreas and uterine endometrium. Weaker FXYD2 expression was detected in the adrenal medulla, liver, gallbladder, bladder and pancreas. The common denominator in the distribution of FXYD1 and FXYD2 was expression in highly active transport epithelia of the kidney, choroid plexus, placenta and salivary glands. This study reveals, in human tissues, the specific expression of FXYD proteins, which may associate with Na, K-ATPase in selected cell types and modulate its catalytic properties.

Effect of colchicine on sensitivity of duck salt gland Na,K-ATPase to Na+

Low molecular mass proteins of the FXYD family that affect the sensitivity of Na,K-ATPase to Na+ and K+ are known to be present in Na,K-ATPases in various tissues. In particular, in Na,K-ATPase from kidney a gamma-subunit (with electrophoretic mobility corresponding to molecular mass of about 10 kD) is present, and Na,K-ATPase preparations from heart contain phospholemman (electrophoretic mobility of this protein corresponds to molecular mass of 13-14 kD), which provides for the interaction of heart Na,K-ATPase with cytoskeletal microtubules. Disruption of microtubules by colchicine removes phospholemman from heart Na,K-ATPase preparations. The goal of the present study was to reveal a low molecular mass protein (probably a member of FXYD family) in preparation of Na,K-ATPase from duck salt glands. Immunoprecipitation of solubilized duck salt gland Na,K-ATPase using antibodies against alpha1-subunit results in the coprecipitation of a 13 kD protein with the Na,K-ATPase complex. Treatment of homogenate from duck salt glands with colchicine removes this protein from the purified preparation of Na,K-ATPase. Simultaneously, we observed a decrease in the sensitivity of Na,K-ATPase to Na+ at pH 6.5. However, colchicine treatment of homogenate from rabbit kidney does not affect either the sensitivity of Na,K-ATPase obtained from this homogenate to Na+ or the content of 10 kD protein (presumably gamma-subunit). The data suggest that phospholemman (or a similar member of the FXYD family) tightly interacts with Na,K-ATPase from duck salt glands and binds it to microtubules, simultaneously participating in the regulation of the sensitivity of Na,K-ATPase to Na+.

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.

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.

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.

Structural and functional properties of two human FXYD3 (Mat-8) isoforms

Six of 7 FXYD proteins have been shown to be tissue-specific modulators of Na,K-ATPase. In this study, we have identified two splice variants of human FXYD3, or Mat-8, in CaCo-2 cells. Short human FXYD3 has 72% sequence identity with mouse FXYD3, whereas long human FXYD3 is identical to short human FXYD3 but has a 26-amino acid insertion after the transmembrane domain. Short and long human FXYD3 RNAs and proteins are differentially expressed during differentiation of CaCo-2 cells. Long human FXYD3 is mainly expressed in nondifferentiated cells and short human FXYD3 in differentiated cells and both FXYD3 variants can be co-immunoprecipitated with a Na,K-ATPase antibody. In contrast to mouse FXYD3, which has two transmembrane domains for lack of cleavage of the signal peptide, human FXYD3 has a cleavable signal peptide and adopts a type I topology. After co-expression in Xenopus oocytes, both human FXYD3 variants associate stably only with Na,K-ATPase isozymes but not with H,K-ATPase or Ca-ATPase. Similar to mouse FXYD3, short human FXYD3 decreases the apparent K(+) and Na(+) affinity of Na,K-ATPase over a large range of membrane potentials. On the other hand, long human FXYD3 decreases the apparent K(+) affinity only at slightly negative and positive membrane potentials and increases the apparent Na(+) affinity of Na,K-ATPase. Finally, both short and long human FXYD3 induce a hyperpolarization activated current, similar to that induced by mouse FXYD3. Thus, we have characterized two human FXYD3 isoforms that are differentially expressed in differentiated and non-differentiated cells and show different functional properties.

[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.

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.

Beta-subunit of cardiac Na+-K+-ATPase dictates the concentration of the functional enzyme in caveolae

Previous studies showed the presence of a significant fraction of Na(+)-K(+)-ATPase alpha-subunits in cardiac myocyte caveolae, suggesting the caveolar interactions of Na(+)-K(+)-ATPase with its signaling partners. Because both alpha- and beta-subunits are required for ATPase activity, to clarify the status of the pumping function of caveolar Na(+)-K(+)-ATPase, we have examined the relative distribution of two major subunit isoforms (alpha(1) and beta(1)) in caveolar and noncaveolar membranes of adult rat cardiac myocytes. When cell lysates treated with high salt (Na(2)CO(3) or KCl) concentrations were fractionated by a standard density gradient procedure, the resulting light caveolar membranes contained 30-40% of alpha(1)-subunits and 80-90% of beta(1)-subunits. Use of Na(2)CO(3) was shown to inactivate Na(+)-K(+)-ATPase; however, caveolar membranes obtained by the KCl procedure were not denatured and contained approximately 75% of total myocyte Na(+)-K(+)-ATPase activity. Sealed isolated caveolae exhibited active Na(+) transport. Confocal microscopy supported the presence of alpha,beta-subunits in caveolae, and immunoprecipitation showed the association of the subunits with caveolin oligomers. The findings indicate that cardiac caveolar inpocketings are the primary portals for active Na(+)-K(+) fluxes, and the sites where the pumping and signaling functions of Na(+)-K(+)-ATPase are integrated. Preferential concentration of beta(1)-subunit in caveolae was cell specific; it was also noted in neonatal cardiac myocytes but not in fibroblasts and A7r5 cells. Uneven distributions of alpha(1) and beta(1) in early and late endosomes of myocytes suggested different internalization routes of two subunits as a source of selective localization of active Na(+)-K(+)-ATPase in cardiac caveolae.

Functional interactions of phospholemman (PLM) (FXYD1) with Na+,K+-ATPase. Purification of alpha1/beta1/PLM complexes expressed in Pichia pastoris

Human FXYD1 (phospholemman, PLM) has been expressed in Pichia pastoris with porcine alpha1/His10-beta1 subunits of Na+,K+-ATPase or alone. Dodecyl-beta-maltoside-soluble complexes of alpha1/beta1/PLM have been purified by metal chelate chromatography, either from membranes co-expressing alpha1,His10-beta1, and PLM or by in vitro reconstitution of PLM with alpha1/His10-beta1 subunits. Comparison of functional properties of purified alpha1/His10-beta1 and alpha1/His10-beta1/PLM complexes show that PLM lowered K0.5 for Na+ ions moderately (approximately 30%) but did not affect the turnover rate or Km of ATP for activating Na+,K+-ATPase activity. PLM also stabilized the alpha1/His10-beta1 complex. In addition, PLM markedly (>3-fold) reduced the K0.5 of Na+ ions for activating Na+-ATPase activity. In membranes co-expressing alpha1/His10-beta1 with PLM the K0.5 of Na+ ions was also reduced, compared with the control, excluding the possibility that detergent or lipid in purified complexes compromise functional interactions. When expressed in HeLa cells with rat alpha1, rat PLM significantly raised the K0.5 of Na+ ions, whereas for a chimeric molecule consisting of transmembranes segments of PLM and extramembrane segments of FXYD4, the K0.5 of Na+ ions was significantly reduced, compared with the control. The opposite functional effects in P. pastoris and HeLa cells are correlated with endogenous phosphorylation of PLM at Ser68 or unphosphorylated PLM, respectively, as detected with antibodies, which recognize PLM phosphorylated at Ser68 (protein kinase A site) or unphosphorylated PLM. We hypothesize that PLM interacts with alpha1/His10-beta1 subunits at multiple locations, the different functional effects depending on the degree of phosphorylation at Ser68. We discuss the role of PLM in regulation of Na+,K+-ATPase in cardiac or skeletal muscle cells.

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.

FXYD Proteins: Tissue-Specific Regulators of the Na,K-ATPase

Work in several laboratories has led to the identification of a family of short single-span transmembrane proteins named after the invariant extracellular motif: FXYD. Four members of this group have been shown to interact with the Na,K-adenosine triphosphatase (ATPase) and alter the pump kinetics. Thus, it is assumed that FXYD proteins are tissue-specific regulatory subunits, which adjust the kinetic properties of the pump to the specific needs of the relevant tissue, cell type, or physiologic state, without affecting it elsewhere. A number of studies have provided evidence for additional and possibly unrelated functions of the FXYD proteins. This review summarizes current knowledge on the structure, function, and cellular distribution of FXYD proteins with special emphasis on their role in kidney electrolyte homeostasis.

Stable expression and visualization of Mat-8 (FXYD-3) tagged with a fluorescent protein in Chinese hamster ovary (CHO)-K1 cells

A type I transmembrane protein, Mat-8 (FXYD-3), was tagged with fluorescent protein, Discosoma red fluorescent protein, at the carboxyl terminal cytoplasmic tail, and stably expressed in Chinese Hamster Ovary (CHO)-K1 cells. The fluorescence signal was distributed in intracellular membranes, being not only detected around the nuclear envelope but also partly overlapping with an endoplasmic reticulum marker. Subcellular fractionation by density gradient centrifugation supported this partial overlapping. The spherical structures observed were not colocalized with markers for lysosomes, endosomes, and Golgi bodies, suggesting that Mat-8 is distributed in a distinct endoplasmic reticulum region and the nuclear envelope after synthesis on membrane-bound ribosomes.

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.

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

Phosphohippolin expression in the rat central nervous system

The FXYD family is a small single-span membrane protein family; recently, we have identified a novel member of this family from the cDNA library of the rat hippocampus and named phosphohippolin (Php) (Mol. Br. Res. vol. 86, 2001). The deduced amino acid sequence of this novel Php comprises 93 residues with a core motif of FXYD and a single transmembrane domain. This indicates that Php belongs to FXYD6 subfamily of the seven FXYD subfamilies (FXYD1-7). Php shows a 48.1% homology with rat phospholemman (FXYD1), a transmembrane family protein. In this study, polyclonal antibodies against the carboxyl-terminal sequence of rat Php were raised and purified. The spatial expression of the Php protein was in the neuronal fibers of the medial part of lateral habenula nucleus, thalamus, hypothalamus, stria terminalis, zona incerta, amygdaloid body and cingulum, olfactory bulb, hippocampus, cerebral cortex and cerebellum. A unique Php distribution was identified in the cerebellum, with a predominant expression pattern in the granule layer of lobules VI-IX of the posterior lobe. Developmental studies demonstrated that the highest level of Php expression was seen in the postnatal (PN) 3-week-old rat brain, and a significant amount of Php still existed in the adult brain. These findings suggest that Php may play an important role in the excitability of neurons in the central nervous system during postnatal development, as well as those in the adult brain.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Phospholemman expression in extraglomerular mesangium and afferent arteriole of the juxtaglomerular apparatus

The molecular mechanisms with which the juxtaglomerular apparatus accomplishes its twin functions, acute regulation of glomerular blood flow and secretion of renin, are still not clearly understood. Least understood is the role of the extraglomerular mesangial (EM) cells, also known as lacis or Goormaghtigh cells, which lie sandwiched between the macula densa and the afferent and efferent arterioles. Here, we report that immunoreactivity for phospholemman (FXYD1), a single-span membrane protein homologous to the gamma (gamma) sub-unit of the Na,K-ATPase, is found in the kidney in EM cells with the Na,K-ATPase beta2-subunit and in cortical blood vessels and the afferent arteriole with Na,K-ATPase alpha2 and beta2. Phospholemman's distribution in EM cells is distinct from that of the Na,K-ATPase gamma-subunit, which is found on the basolateral surface of macula densa cells with Na,K-ATPase alpha1 and beta1. Phospholemman is a major kinase target, and its location in the juxtaglomerular apparatus suggests that it is involved in tubuloglomerular feedback.

FXYD proteins: new tissue- and isoform-specific regulators of Na,K-ATPase

The recently defined FXYD protein family contains seven members that are small, single-span membrane proteins characterized by a signature sequence containing an FXYD motif and three other conserved amino acid residues. Until recently, the functional role of FXYD proteins was largely unknown, with the exception of the gamma subunit of Na,K-ATPase, which was shown to be a specific regulator of renal alpha1-beta1 isozymes. We have investigated whether other members of the FXYD family may have a similar role as the gamma subunit and have found that CHIF (corticosteroid hormone-induced factor, FXYD4), FXYD7, as well as phospholemman (FXYD1) specifically associate with Na,K-ATPase and preferentially with alpha1-beta isozymes in native tissues, and produce distinct effects on the transport properties of Na,K-ATPase that are adapted to the physiological demands of the tissues in which they are expressed. These results provide evidence for a unique and novel mode of regulation of Na,K-ATPase by FXYD proteins that involves a tissue-specific expression of an auxiliary subunit of distinct Na,K-ATPase isozymes.

FXYD7, the first brain- and isoform-specific regulator of Na,K-ATPase: biosynthesis and function of its posttranslational modifications

The FXYD protein family has recently been defined as a result of the search for homologues of the Na,K-ATPase gamma subunit, CHIF, and phospholemman in EST and gene data banks. FXYD7 has been seen to have a role as a brain- and isozyme-specific regulator of Na/K-ATPase. In this study, the biosynthesis, membrane topology, nature, and role of the processing of FXYD7 are investigated.

A specific functional interaction between CHIF and Na,K-ATPase: role of FXYD proteins in the cellular regulation of the pump

CHIF (corticosteroid hormone-induced factor) is a member of the FXYD family that shares approximately 50% homology with the gamma subunit of Na,K-ATPase. It is expressed in renal collecting duct and distal colon, and is upregulated by Na(+) deprivation and high K(+) diet. Both CHIF and gamma are coimmunoprecipitated by an anti-alpha subunit antibody, and alpha is immunoprecipitated by anti-gamma and anti-CHIF antibodies. (86)Rb(+) flux experiments in CHIF-transfected HeLa cells demonstrate that CHIF increases the affinity for cytoplasmic Na(+), but does not affect the affinity for extracellular K(Rb). A physiological role of CHIF in kidney function is further elucidated by the phenotypic analysis of CHIF knockout mice. Taken together with data by others, it appears that FXYD proteins are tissue-specific subunits or regulators of the Na,K-ATPase whose function is to adjust the pump kinetics to particular physiological needs.

Hypertonic activation of phospholemman in solitary rat hepatocytes in primary culture

Under hypertonic conditions, solitary rat hepatocytes in primary culture shrink and subsequently exhibit a distinct regulatory volume increase (RVI). Reverse-transcribed polymerase chain reaction and 5' and 3' RACE (rapid amplification of cDNA ends) techniques reveal that these cells express phospholemman (PLM). In whole-cell recordings, the hypertonic activation of a channel is observed that resembles PLM with respect to unitary conductance (600-700 pS), gating pattern, and non-selectivity for Na(+) over K(+). In Xenopus oocytes expressing hepatocyte PLM, hypertonic stress induces a non-selective cation conductance and noise analysis reveals the activation of a channel with a unitary conductance of approximately 700 pS. These results suggest a role of PLM in the RVI of rat hepatocytes.

FXYD proteins: new tissue-specific regulators of the ubiquitous Na,K-ATPase

Maintenance of the Na+ and K+ gradients between the intracellular and extracellular milieus of animal cells is a prerequisite for basic cellular homeostasis and for functions of specialized tissues. The Na,K-ATPase, an oligomeric P-type adenosine triphosphatase (ATPase), is composed of a catalytic alpha subunit and a regulatory beta subunit and is the main player that fulfils these tasks. A variety of regulatory mechanisms are necessary to guarantee appropriate Na,K-ATPase expression and activity adapted to changing physiological demands. Recently, a regulatory mechanism was defined that is mediated by interaction of Na,K-ATPase with small proteins of the FXYD family, which possess a single transmembrane domain and so far have been considered as channels or regulators of ion channels. The mammalian FXYD proteins FXYD1 through FXYD7 exhibit tissue-specific distribution. Phospholemman (FXYD1) in heart and skeletal muscle, the gamma subunit of Na,K-ATPase (FXYD2) and corticosteroid hormone-induced factor (FXYD4, also known as CHIF) in the kidney, and FXYD7 in the brain associate preferentially with the widely expressed Na,K-ATPase alpha1-beta1 isozyme and modulate its transport activity in a way that conforms to tissue-specific requirements. Thus, tissue- and isozyme-specific interaction of Na,K-ATPase with FXYD proteins contributes to proper handling of Na+ and K+ by the Na,K-ATPase, and ensures correct function in such processes as renal Na+-reabsorption, muscle contraction, and neuronal excitability.

Activity-independent homeostasis in rhythmically active neurons

The shal gene encodes the transient potassium current (I(A)) in neurons of the lobster stomatogastric ganglion. Overexpression of Shal by RNA injection into neurons produces a large increase in I(A), but surprisingly little change in the neuron's firing properties. Accompanying the increase in I(A) is a dramatic and linearly correlated increase in the hyperpolarization-activated inward current (I(h)). The enhanced I(h) electrophysiologically compensates for the enhanced I(A), since pharmacological blockade of I(h) uncovers the physiological effects of the increased I(A). Expression of a nonfunctional mutant Shal also induces a large increase in I(h), demonstrating a novel activity-independent coupling between the Shal protein and I(h) enhancement. Since I(A) and I(h) influence neuronal activity in opposite directions, our results suggest a selective coregulation of these channels as a mechanism for constraining cell activity within appropriate physiological parameters.

Challenging accepted ion channel biology: p64 and the CLIC family of putative intracellular anion channel proteins (Review)

Parchorin, p64 and the related chloride intracellular channel (CLIC) proteins are widely expressed in multicellular organisms and have emerged as candidates for novel, auto-inserting, self-assembling intracellular anion channels involved in a wide variety of fundamental cellular events including regulated secretion, cell division and apoptosis. Although the mammalian phosphoproteins p64 and parchorin (49 and 65K, respectively) have only been indirectly implicated in anion channel activity, two CLIC proteins (CLIC1 and CLIC4, 27 and 29K, respectively) appear to be essential molecular components of anion channels, and CLIC1 can form anion channels in planar lipid bilayers in the absence of other cellular proteins. However, these putative ion channel proteins are controversial because they exist in both soluble and membrane forms, with at least one transmembrane domain. Even more surprisingly, soluble CLICs share the same glutaredoxin fold as soluble omega class glutathione-S-transferases. Working out how these ubiquitous, soluble proteins unfold, insert into membranes and then refold to form integral membrane proteins, and how cells control this potentially dangerous process and make use of the associated ion channels, are challenging prospects. Critical to this future work is the need for better characterization of membrane topology, careful functional analysis of reconstituted and native channels, including their conductances and selectivities, and detailed structure/function studies including targeted mutagenesis to investigate the structure of the putative pore, the role of protein phosphorylation and the role of conserved cysteine residues.

Phospholemman (FXYD1) associates with Na,K-ATPase and regulates its transport properties

A family of small, single-span membrane proteins (the FXYD family) has recently been defined based on their sequence and structural homology. Some members of this family have already been identified as tissue-specific regulators of Na,K-ATPase (NKA). In the present study, we demonstrate that phospholemman (PLM) (FXYD1), so far considered to be a heart- and muscle-specific channel or channel-regulating protein, associates specifically and stably with six different alpha-beta isozymes of NKA after coexpression in Xenopus oocytes, and with alpha1-beta, and less efficiently with alpha2-beta isozymes, in native cardiac and skeletal muscles. Stoichiometric association of PLM with NKA occurs posttranslationally either in the Golgi or the plasma membrane. Interaction of PLM with NKA induces a small decrease in the external K+ affinity of alpha1-beta1 and alpha2-beta1 isozymes and a nearly 2-fold decrease in the internal Na+ affinity. In conclusion, this study demonstrates that PLM is a tissue-specific regulator of NKA that may play an essential role in muscle contractility.

The gamma-Na+,K+-ATPase subunit assembles selectively with alpha1/beta1-Na+,K+-ATPase but not with the colonic H+,K+-ATPase

BACKGROUND

The ubiquitous Na+-pump (Na+,K+-ATPase) assembles as a heterodimer of composition alpha/beta in some nephron segments, while in other segments it may exist as a heterotrimer of composition alpha/beta/gamma. The gamma-subunit has been reported to increase the affinity of the Na+-pump for adenosine 5'-triphosphate (ATP), and decrease affinity for both Na+ and K+. The alpha-subunit of the colonic H+,K+-ATPase (cHK) shares 75% sequence similarity with alpha1-Na+,K+-ATPase (alpha1) and assembles with beta1-Na+,K+-ATPase (beta1) in distal colon and renal medulla. Differences in pharmacological properties have been ascribed to when heterologously expressed function has been compared to function in vitro. The purpose of this study was to determine if cHK might associate with the gamma-subunit of the Na+,K+-ATPase (gamma) as a possible explanation for these variations in function.

METHODS

An antibody specific for the gamma was used in coimmunoprecipitation experiments to determine if the gamma assembles stably in vitro with cHK and beta1 in rat renal medulla or distal colon.

RESULTS

Our results demonstrate that the gamma-subunit assembles specifically with the Na+-pump, but not with cHK. Furthermore, the gamma-subunit assembly was specific for rat kidney and was not observed in distal colon.

CONCLUSION

Since the gamma-subunit did not assemble with the cHK/beta1 complex, gamma-subunit assembly cannot explain those variations in ex vivo and in vitro pharmacologic properties ascribed to cHK.

Expression of a novel member of the ATP1G1/PLM/MAT8 family, phospholemman-like protein (PLP) gene, in the developmental process of mouse cerebellum

We have identified a new member of the ATP1G1/PLM/MAT8 family, named phospholemman-like protein (PLP), from a mouse cerebellum cDNA library. The family consists of small transmembrane proteins that modulate the activities of some ion channels. The deduced amino acid sequence of PLP consists of 93 residues that contain the ATP1G1/PLM/MAT8 motif and a single transmembrane domain, and is most similar to the sequence of mouse phospholemman. In situ hybridization analysis showed that the PLP gene is highly expressed in cerebellar granule cells. PLP expression is elevated in the postnatal developing cerebellum. Thus, it may be implicated in the proliferation, differentiation, and axon elongation of granule cells as they mature and migrate to the internal granule layer.

Heterologous expression of the Na(+),K(+)-ATPase gamma subunit in Xenopus oocytes induces an endogenous, voltage-gated large diameter pore

1. The gamma subunit is a specific component of the plasmalemmal Na(+),K(+)-ATPase. Like structurally related single-spanning membrane proteins such as cardiac phospholemman, Mat-8 and renal CHIF, large ion conductances are activated when gamma subunits are expressed in Xenopus oocytes. 2. Here we report critical properties of the gamma-activated conductance. The gamma-activated conductance showed non-selective cationic and anionic permeation, and extremely slow kinetics, with an activation time constant > 1 s following steps to -100 mV. 3. The gamma-activated conductance was inhibited by extracellular divalent ions including Ba(2+) (K(i) = 0.7 mM) and Ca(2+) (K(i) = 0.4 mM). 4. 2-Deoxyglucose (MW approximately 180), inulin (MW approximately 5000) and spermidine (MW approximately 148) efflux could occur through the gamma-activated conductance pathway, indicating a large pore diameter. In contrast, dextran-70 (MW approximately 70 000) did not pass through the gamma-activated channel, indicating an upper limit to the pore size of approximately 50 A (5 nm). 5. Similar conductances that are permeable to large molecules were activated by extreme hyperpolarization (> -150 mV) of uninjected oocytes. 6. We conclude that the Na(+),K(+)-ATPase gamma subunits activate Ca(2+)- and voltage-gated, non-selective, large diameter pores that are intrinsically present within the oocyte membrane.

Gene structure and expression of phospholemman in mouse

Phospholemman (PLM) is a small transmembrane cardiac protein that is the major sarcolemmal substrate for phosphorylation in response to adrenergic stimulation. PLM likely plays a role in muscle contractility and cell volume regulation through its function as a channel or a channel regulator. We are the first to describe the structure of the PLM gene and to demonstrate PLM cDNA splice variants. We cloned the murine PLM cDNA and used it as a probe to isolate the gene from a 129/SvJ genomic library. The gene contains seven introns and eight exons. The coding sequence is interrupted by five introns; the 5' untranslated region by two. Using rapid amplification of 5' cDNA ends we identified transcription start sites and four splice variants of the 5' untranslated domain. There was no TATA box or CAAT box in the putative promoter regions. The gene has several stretches of dinucleotide repeats. The 3' untranslated domains of mouse PLM cDNA clones show sequence differences not accounted for by alternative splicing. Mouse PLM shares 93, 83 and 80% amino acid identity with rat, dog, and human PLMs, respectively. Tissue expression of murine PLM parallels that in other species, being highest in heart, skeletal muscle, and liver.

pH modulation of an inward rectifier chloride current in cultured rat cortical astrocytes

The effects of changes in extra- and intracellular pH in the pathophysiological range (6.0-8.0) on astroglial plasma membrane ionic currents were investigated with the whole-cell patch-clamp technique. In cultured rat neocortical type-1 astrocytes differentiated by a long-term treatment with dibutyryl cyclic-AMP, exposure to an extracellular pH of 6.4 induced, as compared with the control extracellular pH at 7.3, a sustained and reversible increase in the holding current at -60mV. The rise in current was accompanied by a decrease in the apparent input resistance. Ion substitution experiments indicated that extracellular pH 6.4 upregulated the resting Cl(-) conductance, whereas an opposite effect could be observed at extracellular pH 8.0. Recordings of isolated Cl(-) currents showed that this modulation occurred on the previously identified hyperpolarization-activated, inwardly rectifying Cl(-) current, I(Clh). Extracellular acidification to pH 6.4 shifted the voltage dependence of I(Clh) activation by approximately 20mV towards more positive potentials, whereas a approximately 20mV opposite shift was observed upon exposure to extracellular pH 8.0. These effects were paralleled by an increase (extracellular pH 6.4) or decrease (extracellular pH 8.0) in the maximal conductance. Decreasing (6.0) or increasing (8.0) the intracellular pH shifted the steady-state activation of I(Clh) towards more negative or positive potentials, respectively, leaving unchanged the current sensitivity to extracellular pH modifications. The modulation of the inward rectifier Cl(-) current expressed by differentiated cultured neocortical astrocytes indicates that extra- and intracellular changes in pH occurring in a pathophysiological range may contribute to regulating Cl(-) accumulation in astroglial cells.

Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase c via a novel member of the FXYDY family

The Na,K-ATPase provides the driving force for many ion transport processes through control of Na(+) and K(+) concentration gradients across the plasma membranes of animal cells. It is composed of two subunits, alpha and beta. In many tissues, predominantly in kidney, it is associated with a small ancillary component, the gamma-subunit that plays a modulatory role. A novel 15-kDa protein, sharing considerable homology to the gamma-subunit and to phospholemman (PLM) was identified in purified Na,K-ATPase preparations from rectal glands of the shark Squalus acanthias, but was absent in pig kidney preparations. This PLM-like protein from shark (PLMS) was found to be a substrate for both PKA and PKC. Antibodies to the Na, K-ATPase alpha-subunit coimmunoprecipitated PLMS. Purified PLMS also coimmunoprecipitated with the alpha-subunit of pig kidney Na, K-ATPase, indicating specific association with different alpha-isoforms. Finally, PLMS and the alpha-subunit were expressed in stoichiometric amounts in rectal gland membrane preparations. Incubation of membrane bound Na,K-ATPase with non-solubilizing concentrations of C(12)E(8) resulted in functional dissociation of PLMS from Na,K-ATPase and increased the hydrolytic activity. The same effects were observed after PKC phosphorylation of Na,K-ATPase membrane preparations. Thus, PLMS may function as a modulator of shark Na,K-ATPase in a way resembling the phospholamban regulation of the Ca-ATPase.

A hyperpolarization- and acid-activated nonselective cation current in Xenopus oocytes

Heterologous expression of a variety of membrane proteins in Xenopus oocytes sometimes results in the appearance of a hyperpolarization-activated inward current. The nature of this current remains incompletely understood. Some investigators have suggested that this current is a Cl current, whereas others have identified it as a nonselective cation current. The purpose of this investigation was to characterize this current in more detail. The hyperpolarization-activated inward current (I(IN)) present in native oocytes was composed of a current carried at least partly by Ca and Mg under physiological ionic conditions plus a Ca-activated Cl current. The Ca-activated Cl current was blocked by chelation of cytosolic Ca with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid. When Cl currents were blocked, the cation current could be carried by Ca, Mg, or Co, but not appreciably by Ba, Mn, or Cd. I(IN) was stimulated by intracellular acidification. The properties of I(IN) were quite different from those of the store-operated Ca current. Heterologous expression of transient receptor potential-like gene product (TRPL), one of the members of the transient receptor potential family of putative store-operated Ca channels, apparently resulted in alteration of the voltage sensitivity of the endogenous I(IN).

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Phospholemman is a substrate for myotonic dystrophy protein kinase

The genetic abnormality in myotonic muscular dystrophy, multiple CTG repeats lie upstream of a gene that encodes a novel protein kinase, myotonic dystrophy protein kinase (DMPK). Phospholemman (PLM), a major membrane substrate for phosphorylation by protein kinases A and C, induces Cl currents (I(Cl(PLM))) when expressed in Xenopus oocytes. To test the idea that PLM is a substrate for DMPK, we measured in vitro phosphorylation of purified PLM by DMPK. To assess the functional effects of PLM phosphorylation we compared I(Cl(PLM)) in Xenopus oocytes expressing PLM alone to currents in oocytes co-expressing DMPK, and examined the effect of DMPK on oocyte membrane PLM expression. We found that PLM is indeed a good substrate for DMPK in vitro. Co-expression of DMPK with PLM in oocytes resulted in a reduction in I(Cl(PLM)). This was most likely a specific effect of phosphorylation of PLM by DMPK, as the effect was not present in oocytes expressing a phos(-) PLM mutant in which all potential phosphorylation had been disabled by Ser --> Ala substitution. The biophysical characteristics of I(Cl(PLM)) were not changed by DMPK or by the phos(-) mutation. Co-expression of DMPK reduced the expression of PLM in oocyte membranes, suggesting a possible mechanism for the observed reduction in I(Cl(PLM)) amplitude. These data show that PLM is a substrate for phosphorylation by DMPK and provide functional evidence for modulation of PLM function by phosphorylation.

The promoter for constitutive expression of the human ICln gene CLNS1A

The ICln protein is expressed ubiquitously in mammals. Experiments designed to knock down the ICln protein in NIH 3T3 fibroblasts as well as in epithelial cells led to the conclusion that this protein is crucially involved in volume regulation after cytoplasmic swelling. Reconstitution of the ICln protein in lipid bilayers revealed the ion channel nature of ICln. Here we describe a new human promoter sequence, composed of 89 nucleotides, which is responsible for a highly constitutive expression of the ICln protein. The promoter sequence lacks a TATA box, and the transcription can be effected at multiple sites. In addition to the starting sites, upstream sequence elements are mandatory for an efficient transcription of the ICln gene (CLNS1A). These new nucleotide elements were defined by site-directed mutagenesis.

Volume sensitive efflux of taurine in HEK293 cells overexpressing phospholemman

The role of the phospholemman (PLM) on the efflux of taurine and chloride induced by swelling was studied in HEK293 cells overexpressing stable transfected PLM. PLM, a substrate for protein kinases C and A, is a protein that induces an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. In PLM-overexpressing cells the process of RVD was more rapid and efficient (75%) than in control cells (44%). Also, [(3)H]taurine and (125)I efflux induced by hyposmolarity were markedly increased (30-100%) in two subclones of cells overexpressing PLM. This increased efflux was sensitive to the Cl channel blockers DDF, NPPB and DIDS. Acute treatment of control cells with isoproterenol and norepinephrine induced a significant potentiation (50-60%) of [(3)H]taurine release induced by hyposmolarity. In PLM-overexpressing cells the potentiation by these drugs was higher (100%). Insulin induced also an increase in [(3)H]taurine release, but only in PLM-overexpressing cells (50%). These results indicate that PLM may play a role in the RVD and that its phosphorylation may have a physiological significance during this process. The mechanisms involved in this process could include the activation of PLM itself as channel or the modulation of other preexisting channels.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Modulation of Xenopus oocyte-expressed phospholemman-induced ion currents by co-expression of protein kinases

Phospholemman (PLM), the major sarcolemmal substrate for phosphorylation by cAMP-dependent kinase (PKA) protein kinase C (PKC) and NIMA kinase in muscle, induces hyperpolarization-activated anion currents in Xenopus oocytes, most probably by enhancing endogenous oocyte currents. PLM peptides from the cytoplasmic tail are phosphorylated by PKA at S68, by NIMA kinase at S63, and by PKC at both S63 and S68. We have confirmed the phosphorylation sites in the intact protein, and we have investigated the role of phosphorylation in the regulatory activity of PLM using oocyte expression experiments. We found: (1) the cytoplasmic domain is not essential for inducing currents in oocytes; (2) co-expression of PKA increased the amplitude of oocyte currents and the amount of PLM in the oocyte membrane largely, but not exclusively, through phosphorylation of S68; (3) co-expression of PKA had no effect on a PLM mutant in which all putative phosphorylation sites had been inactivated by serine to alanine mutation (SSST 62, 63, 68, 69 AAAA); (4) co-expression of PKC had no effect in this system; (5) co-expression of NIMA kinase increased current amplitude and membrane protein level, but did not require PLM phosphorylation. These findings point to a role for phosphorylation in the function of PLM.