Differential regulation of renal Na,K-ATPase by splice variants of the gamma subunit

FXYD2 mRNA expression represents a new independent factor that affects survival of glioma patients and predicts chemosensitivity of patients to temozolomide

PURPOSE

Glioma is the most common primary intracranial tumor. Owing to the poor prognosis associated with high-grade gliomas, there is an urgent need to identify biomarkers related to prognosis and treatment sensitivity. Here, we analyze the expression of FXYD2 mRNA in gliomas, and explore its clinical prognostic value and significance in this disease.

METHODS

Clinical features, FXYD2 mRNA expression levels, and survival data were analyzed for 516 glioma patients from the Chinese Glioma Genome Map Project, 481 from the cancer genome map datbase and 268 from the molecular braintumor database. The expression patterns for FXYD2 mRNA were compared using the chi-square test, and overall survival (OS) of glioma patients was evaluated according to FXYD2 mRNA expression levels. The factors affecting glioma survival were evaluated by Cox univariate and multivariate regression analysis.

RESULTS

FXYD2 mRNA expression was related to the grade of gliomas. The higher the level, the lower the expression. Meanwhile related to the pathological classification of gliomas. Oligodendroglioma, IDH-mutant and 1p/19q-codeleted was higher than Astrocytoma, IDH-mutant, higher than Glioblastoma, IDH-wildtype. Moreover, temozolomide (TMZ) chemotherapy was found to be an independent factor affecting survival in patients with high FXYD2 mRNA expression, but not in patients with low expression.

CONCLUSION

FXYD2 mRNA expression represents a new independent factor affecting the survival of glioma patients and may serve as an independent prognostic indicator to predict the sensitivity of gliomas to TMZ.

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.

Hormonal regulation of Na+-K+-ATPase from the evolutionary perspective

Na⁺-K⁺-ATPase, an α/β heterodimer, is an ancient enzyme that maintains Na⁺ and K⁺ gradients, thus preserving cellular ion homeostasis. In multicellular organisms, this basic housekeeping function is integrated to fulfill the needs of specialized organs and preserve whole-body homeostasis. In vertebrates, Na⁺-K⁺-ATPase is essential for many fundamental physiological processes, such as nerve conduction, muscle contraction, nutrient absorption, and urine excretion. During vertebrate evolution, three key developments contributed to diversification and integration of Na⁺-K⁺-ATPase functions. Generation of novel α- and β-subunits led to formation of multiple Na⁺-K⁺-ATPase isoenyzmes with distinct functional characteristics. Development of a complex endocrine system enabled efficient coordination of diverse Na⁺-K⁺-ATPase functions. Emergence of FXYDs, small transmembrane proteins that regulate Na⁺-K⁺-ATPase, opened new ways to modulate its function. FXYDs are a vertebrate innovation and an important site of hormonal action, suggesting they played an especially prominent role in evolving interaction between Na⁺-K⁺-ATPase and the endocrine system in vertebrates.

Functional Studies of Na(+),K(+)-ATPase Using Transfected Cell Cultures

The properties of different combinations of Na,K-ATPase subunits or their mutations can be studied in stably transfected mammalian cells. As a specific example, the methods here are for transfection of a modulatory subunit into cells with endogenous α and β subunits. Renal Na,K-ATPase is tightly bound to a small single-span membrane protein, the γ subunit, or FXYD2. The protein co-localizes and co-immunoprecipitates with the α/β complex, however it is not required for basic enzyme properties. Functional consequences of association with FXYD2 were investigated in stably transfected cells. The outcome was that FXYD2 reduced activity of Na,K-ATPase at the level of apparent affinity for Na(+) and to a smaller extent for K(+). Moreover, expression of FXYD2 reduced cell growth. Here we describe the methodologies as well as potential pitfalls.

Colorimetric Assays of Na,K-ATPase

The Na,K-ATPase is a plasma membrane enzyme that catalyzes active ion transport by the hydrolysis of ATP. Its activity in vivo is determined by many factors, particularly the concentration of intracellular sodium ions. It is the target of the cardiac glycoside class of drugs and of endogenous regulators. Its assay is often an endpoint in the investigation of physiological processes, and it is a promising drug target. As described in this unit, its enzymatic activity can be determined in extracts from tissues by test tube assay using a spectrophotometer or (32)P-ATP. The protocols in this chapter measure inorganic phosphate as the end product of hydrolysis of ATP.

Beneficial Renal and Pancreatic Phenotypes in a Mouse Deficient in FXYD2 Regulatory Subunit of Na,K-ATPase

The fundamental role of Na,K-ATPase in eukaryotic cells calls for complex and efficient regulation of its activity. Besides alterations in gene expression and trafficking, kinetic properties of the pump are modulated by reversible association with single span membrane proteins, the FXYDs. Seven members of the family are expressed in a tissue-specific manner, affecting pump kinetics in all possible permutations. This mini-review focuses on functional properties of FXYD2 studied in transfected cells, and on noteworthy and unexpected phenotypes discovered in a Fxyd2^−∕− mouse. FXYD2, the gamma subunit, reduces activity of Na,K-ATPase either by decreasing affinity for Na⁺, or reducing V_max. FXYD2 mRNA splicing and editing provide another layer for regulation of Na,K-ATPase. In kidney of knockouts, there was elevated activity for Na,K-ATPase and for NCC and NKCC2 apical sodium transporters. That should lead to sodium retention and hypertension, however, the mice were in sodium balance and normotensive. Adult Fxyd2^−∕− mice also exhibited a mild pancreatic phenotype with enhanced glucose tolerance, elevation of circulating insulin, but no insulin resistance. There was an increase in beta cell proliferation and beta cell mass that correlated with activation of the PI3K-Akt pathway. The Fxyd2^−∕− mice are thus in a highly desirable state: the animals are resistant to Na⁺ retention, and showed improved glucose control, i.e., they display favorable metabolic adaptations to protect against development of salt-sensitive hypertension and diabetes. Investigation of the mechanisms of these adaptations in the mouse has the potential to unveil a novel therapeutic FXYD2-dependent strategy.

Structure and Organization of the Engraulidae Family U2 snRNA: An Evolutionary Model Gene?

The U2 snRNA multigene family has been analyzed in four species of the Engraulidae family--Engraulis encrasicolus, Engraulis mordax, Engraulis ringens, and Engraulis japonicas--with the object of understanding more about the structure of this multigene family in these pelagic species and studying their phylogenetic relationships. The results showed that the cluster of this gene family in the Engraulis genus is formed by the U2-U5 snRNA with highly conserved sequences of mini- and micro-satellites, such as (CTGT)n, embedded downstream of the transcription unit; findings indicate that this gene family evolved following the concerted model. The phylogenetic analysis of the non-transcribed spacer of cluster U2-U5 snDNA in the 4 species showed that the sequences of the species E. encrasicolus and E. japonicus are closely related; these two are genetically close to E. mordax and slightly more distant from E. ringens. The data obtained by molecular analysis of U2-U5 snDNA and their secondary structure, with the presence of the micro-satellite (CTGT)n and mini-satellites, show clearly that the species E. encrasicolus and E. japonicus are closely related and would be older than E. mordax and E. ringens.

Paradoxical activation of the sodium chloride cotransporter (NCC) without hypertension in kidney deficient in a regulatory subunit of Na,K-ATPase, FXYD2

Na,K‐ATPase generates the driving force for sodium reabsorption in the kidney. Na,K‐ATPase functional properties are regulated by small proteins belonging to the FXYD family. In kidney FXYD2 is the most abundant: it is an inhibitory subunit expressed in almost every nephron segment. Its absence should increase sodium pump activity and promote Na⁺ retention, however, no obvious renal phenotype was detected in mice with global deletion of FXYD2 (Arystarkhova et al. 2013). Here, increased total cortical Na,K‐ATPase activity was documented in the Fxyd2^−/− mouse, without increased α1β1 subunit expression. We tested the hypothesis that adaptations occur in distal convoluted tubule (DCT), a major site of sodium adjustments. Na,K‐ATPase immunoreactivity in DCT was unchanged, and there was no DCT hypoplasia. There was a marked activation of thiazide‐sensitive sodium chloride cotransporter (NCC; Slc12a3) in DCT, predicted to increase Na⁺ reabsorption in this segment. Specifically, NCC total increased 30% and NCC phosphorylated at T53 and S71, associated with activation, increased 4‐6 fold. The phosphorylation of the closely related thick ascending limb (TAL) apical NKCC2 (Slc12a1) increased at least twofold. Abundance of the total and cleaved (activated) forms of ENaC α‐subunit was not different between genotypes. Nonetheless, no elevation of blood pressure was evident despite the fact that NCC and NKCC2 are in states permissive for Na⁺ retention. Activation of NCC and NKCC2 may reflect an intracellular linkage to elevated Na,K‐ATPase activity or a compensatory response to Na⁺ loss proximal to the TAL and DCT.

We discovered a substantial activation of renal NCC cotransporter in mice genetically depleted for the regulatory inhibitory subunit of Na,K‐ATPase, FXYD2. Surprisingly, no significant changes in urine output as well as elevation of blood pressure were detected suggesting compensatory adaptation elsewhere in nephron

Structure of the Na,K-ATPase regulatory protein FXYD2b in micelles: implications for membrane-water interfacial arginines

FXYD2 is a membrane protein responsible for regulating the function of the Na,K-ATPase in mammalian kidney epithelial cells. Here we report the structure of FXYD2b, one of two splice variants of the protein, determined by NMR spectroscopy in detergent micelles. Solid-state NMR characterization of the protein embedded in phospholipid bilayers indicates that several arginine side chains may be involved in hydrogen bond interactions with the phospholipid polar head groups. The structure and the NMR data suggest that FXYD2b could regulate the Na,K-ATPase by modulating the effective membrane surface electrostatics near the ion binding sites of the pump.

Regulation of the cardiac Na(+) pump by palmitoylation of its catalytic and regulatory subunits

The Na+/K+-ATPase (Na+ pump) is the principal consumer of ATP in multicellular organisms. In the heart, the Na+ gradient established by the pump is essential for all aspects of cardiac function, and appropriate regulation of the cardiac Na+ pump is therefore crucial to match cardiac output to the physiological requirements of an organism. The cardiac pump is a multi-subunit enzyme, consisting of a catalytic α-subunit and regulatory β- and FXYD subunits. All three subunits may become palmitoylated, although the functional outcome of these palmitoylation events is incompletely characterized to date. Interestingly, both β- and FXYD subunits may be palmitoylated or glutathionylated at the same cysteine residues. These competing chemically distinct post-translational modifications may mediate functionally different effects on the cardiac pump. In the present article, we review the cellular events that control the balance between these modifications, and discuss the likely functional effects of pump subunit palmitoylation.

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].

Hyperplasia of pancreatic beta cells and improved glucose tolerance in mice deficient in the FXYD2 subunit of Na,K-ATPase

Restoration of the functional potency of pancreatic islets either through enhanced proliferation (hyperplasia) or increase in size (hypertrophy) of beta cells is a major objective for intervention in diabetes. We have obtained experimental evidence that global knock-out of a small, single-span regulatory subunit of Na,K-ATPase, FXYD2, alters glucose control. Adult Fxyd2(-/-) mice showed significantly lower blood glucose levels, no signs of peripheral insulin resistance, and improved glucose tolerance compared with their littermate controls. Strikingly, there was a substantial hyperplasia in pancreatic beta cells from the Fxyd2(-/-) mice compared with the wild type littermates, compatible with an observed increase in the level of circulating insulin. No changes were seen in the exocrine compartment of the pancreas, and the mice had only a mild, well-adapted renal phenotype. Morphometric analysis revealed an increase in beta cell mass in KO compared with WT mice. This appears to explain a phenotype of hyperinsulinemia. By RT-PCR, Western blot, and immunocytochemistry we showed the FXYD2b splice variant in pancreatic beta cells from wild type mice. Phosphorylation of Akt kinase was significantly higher under basal conditions in freshly isolated islets from Fxyd2(-/-) mice compared with their WT littermates. Inducible expression of FXYD2 in INS 832/13 cells produced a reduction in the phosphorylation level of Akt, and phosphorylation was restored in parallel with degradation of FXYD2. Thus we suggest that in pancreatic beta cells FXYD2 plays a role in Akt signaling pathways associated with cell growth and proliferation.

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.

Post-transcriptional control of Na,K-ATPase activity and cell growth by a splice variant of FXYD2 protein with modified mRNA

In kidney, FXYD proteins regulate Na,K-ATPase in a nephron segment-specific way. FXYD2 is the most abundant renal FXYD but is not expressed in most renal cell lines unless induced by hypertonicity. Expression by transfection of FXYD2a or FXYD2b splice variants in NRK-52E cells reduces the apparent Na(+) affinity of the Na,K-ATPase and slows the cell proliferation rate. Based on RT-PCR, mRNAs for both splice variants were expressed in wild type NRK-52E cells as low abundance species. DNA sequencing of the PCR products revealed a base alteration from C to T in FXYD2b but not FXYD2a from both untreated and hypertonicity-treated NRK-52E cells. The 172C→T sequence change exposed a cryptic KKXX endoplasmic reticulum retrieval signal via a premature stop codon. The truncation affected trafficking of FXYD2b and its association with Na,K-ATPase and blocked its effect on enzyme kinetics and cell growth. The data may be explained by altered splicing or selective RNA editing of FXYD2b, a supplementary process that would ensure that it was inactive even if transcribed and translated, in these cells that normally express only FXYD2a. 172C→T mutation was also identified after mutagenesis of FXYD2b by error-prone PCR coupled with a selection for cell proliferation. Furthermore, the error-prone PCR alone introduced the mutation with high frequency, implying a structural peculiarity. The data confirm truncation of FXYD2b as a potential mechanism to regulate the amount of FXYD2 at the cell surface to control activity of Na,K-ATPase and cell growth.

The role of hepatocyte nuclear factor-1beta in the pathogenesis of clear cell carcinoma of the ovary

PROBLEM

Clear cell carcinoma (CCC) of the ovary has a number of features distinguishing it from other epithelial ovarian carcinomas (EOC) because of its characteristic histology and biology, frequent concurrence with endometriotic lesion, and highly chemoresistant nature resulting in an extremely poor prognosis. The incidence of CCC has been steadily increasing in Japan. They comprise approximately 20% of all EOC. Understanding the mechanisms of CCC development and elucidating pathogenesis and pathophysiology are intrinsic to prevention and effective therapies for CCC.

METHOD OF STUDY

This article reviews the English language literature for biology, pathogenesis, and pathophysiological studies on endometriosis-associated EOC. Several data are discussed in the context of endometriosis and CCC biology.

RESULTS

Recent studies based on genome-wide expression analysis technology have noted specific expression of hepatocyte nuclear factor-1beta (HNF-1beta) in endometriosis and CCC, suggesting that early differentiation into the clear cell lineage takes place in the endometriosis. The HNF-1beta-dependent pathway of CCC will be discussed, which are providing new insights into regulation of apoptosis and glycogen synthesis and resistance of CCC to anticancer agents.

CONCLUSIONS

This review summarizes recent advances in the HNF-1beta and its target genes; the potential challenges to the understanding of carcinogenesis, pathogenesis, and pathophysiology of CCC; and a possible novel model is proposed.

Structural and functional interactions between FXYD5 and the Na+-K+-ATPase

FXYD5 is a member of a family of tissue-specific regulators of the Na(+)-K(+)-ATPase expressed in kidney tubules. Previously, we have shown that FXYD5 interacts with the alphabeta-subunits of the Na(+)-K(+)-ATPase and increases its V(max) (Lubarski I, Pihakaski-Maunsbach K, Karlish SJ, Maunsbach AB, Garty H. J Biol Chem 280: 37717-37724, 2005). The current study further characterizes structural interaction and structure-function relationships of FXYD5. FXYD5/FXYD4 chimeras expressed in Xenopus laevis oocytes have been used to demonstrate that both the high-affinity association with the pump and the increase in V(max) are mediated by the transmembrane domain of FXYD5. Several amino acids that participate in the high-affinity interaction between FXYD5 and the alpha-subunit of the Na(+)-K(+)-ATPase have been identified. The data suggest that different FXYD proteins interact similarly with the Na(+)-K(+)-ATPase and their transmembrane domains play a key role in both the structural interactions and functional effects. Other experiments have identified at least one splice variant of FXYD5 with 10 additional amino acids at the COOH terminus, suggesting the possibility of other functional effects not mediated by the transmembrane domain. FXYD5 could be specifically bound to wheat germ agglutinin beads, indicating that it is glycosylated. However, unlike previous findings in metastatic cells, such glycosylation does not evoke a large increase in the size of the protein expressed in native epithelia and X. laevis oocytes.

Regulation of Ca2+ and Na+ in normal and failing cardiac myocytes

Ca(2+) in cardiac myocytes regulates contractility and relaxation, and Ca(2+) and Na (+)regulation are linked via Na(+)/Ca(2+) exchange (NCX). Heart failure (HF) is accompanied by contractile dysfunction and arrhythmias, both of which may be due to altered cellular Ca(2+) handling. Smaller Ca(2+) transient and sarcoplasmic reticulum (SR) Ca(2+) content cause systolic dysfunction in HF. The reduced SR Ca(2+) content is due to: (a) reduced SR Ca(2+)-ATPase function (which also contributes to diastolic dysfunction), (b) increased expression and function of NCX (which competes with SR Ca(2+)-ATPase during relaxation, but preserves diastolic function), and (c) enhanced diastolic SR Ca(2+) leak. Relative contributions of these may vary with HF etiology and stage. Triggered arrhythmias (e.g., delayed afterdepolarizations [DADs]) are prominent in HF. DADs are due to spontaneous SR Ca(2+) release and consequent activation of transient inward NCX current, which in HF allows DADs to more readily trigger arrhythmogenic action potentials. Thus NCX and Na(+) are critical in systolic and diastolic function and arrhythmias. [Na(+)](i) is elevated in HF, which may limit SR unloading and provide some Ca(2+) influx during the HF action potential, thus limiting the depression of systolic function. High [Na(+)](i) in HF is due to enhanced Na(+) influx. Cellular Na(+)/K(+)-ATPase (NKA) function appears unaltered, despite reduced NKA expression. This dichotomy led us to test NKA regulation by phospholemman (PLM). We find that PLM regulates NKA in a manner analogous to phospholamban regulation of SR Ca(2+)-ATPase (i.e., inhibition that is relieved by PLM phosphorylation). We measured intermolecular FRET between PLM and NKA, which is reduced upon PLM phosphorylation. The lower expression level of more phosphorylated PLM in HF may explain the above dichotomy. Thus, altered Ca(2+) and Na(+) handling contributes to altered contractile function and arrhythmogenesis in HF.

Ouabain binds with high affinity to the Na,K-ATPase in human polycystic kidney cells and induces extracellular signal-regulated kinase activation and cell proliferation

In autosomal dominant polycystic kidney disease (ADPKD), cyst formation and enlargement require proliferation of mural renal epithelial cells and the transepithelial secretion of fluid into the cyst cavity. Na,K-ATPase is essential for solute and water transport in ADPKD cells, and ouabain blocks fluid secretion in these cells. By binding to the Na,K-ATPase, ouabain also induces proliferation in some cell types. Surprisingly, it was found that nanomolar concentrations of ouabain, similar to those circulating in blood, induced ADPKD cell proliferation but had no statistically significant effect on normal human kidney (NHK) cells. Ouabain, acting from the basolateral side of the cells, also caused an increase in the level of phosphorylated extracellular signal-regulated kinases (ERK). Mitogen-activated protein kinase kinase (MEK) inhibitor U0126 blocked ouabain-induced ERK activation and cell proliferation, suggesting that ouabain effect is mediated through the MEK-ERK pathway. In contrast to NHK cells, the dose-response curve for ouabain inhibition of Na,K-ATPase activity indicated that approximately 20% of the enzyme in ADPKD cells exhibits a higher affinity for ouabain. The increased ouabain affinity of ADPKD cells was not due to differences in Na,K-ATPase isoform expression because these cells, like NHK cells, possess only the alpha1 and beta1 subunits. The gamma variants of the Na,K-ATPase also are expressed in the cells but are elevated in ADPKD cells. Currently, the basis for the differences in ouabain sensitivity of NHK and ADPKD cells is unknown. It is concluded that ouabain stimulates proliferation in ADPKD cells by binding to the Na,K-ATPase with high affinity and via activation of the MEK-ERK pathway.

Phospholemman phosphorylation mediates the protein kinase C-dependent effects on Na+/K+ pump function in cardiac myocytes

Because phospholemman (PLM) regulates the Na(+)/K(+) pump (NKA) and is a major cardiac phosphorylation target for both protein kinase A (at Ser68) and protein kinase C (PKC) (at both Ser63 and Ser68), we evaluated whether PLM mediates the PKC-dependent regulation of NKA function and protein kinase A/PKC crosstalk in ventricular myocytes. PKC was activated by PDBu (300 nmol/L), and we measured NKA-mediated [Na(+)](i) decline (fluorescence measurements) and current (I(pump)) (voltage clamp). In wild-type mouse myocytes, PDBu increased PLM phosphorylation at Ser63 and Ser68, I(pump) (both at 10 and 100 mmol/L Na(+) in the pipette solution) and maximal NKA-mediated Na(+) extrusion rate (V(max)) from 7.9+/-1.1 to 12.7+/-1.9 mmol.L(-1) per minute without altering NKA affinity for internal Na(+) (K(0.5)). In PLM knockout mice, PDBu had no effect on either V(max) or K(0.5). After pretreatment with isoproterenol (ISO) (1 mumol/L), PDBu still increased the NKA V(max) and PLM phosphorylation at Ser63 and Ser68. Conversely, after pretreatment with PDBu, ISO further increased the Na(+) affinity of NKA and phosphorylation at Ser68, as it did alone without PDBu. The final NKA activity was independent of the application sequence. The NKA activity in PLM knockout myocytes, after normalizing the protein level, was similar to that after PDBu and ISO treatment. We conclude that (1) PLM mediates the PKC-dependent activation of NKA function in cardiac myocytes, (2) PDBu and ISO effects are additive in the mouse (affecting mainly V(max) and K(0.5), respectively), and (3) PDBu and ISO combine to activate NKA in wild-type to the level found in the PLM knockout mouse.

Multiplicity of expression of FXYD proteins in mammalian cells: dynamic exchange of phospholemman and gamma-subunit in response to stress

Functional properties of Na-K-ATPase can be modified by association with FXYD proteins, expressed in a tissue-specific manner. Here we show that expression of FXYDs in cell lines does not necessarily parallel the expression pattern of FXYDs in the tissue(s) from which the cells originate. While being expressed only in lacis cells in the juxtaglomerular apparatus and in blood vessels in kidney, FXYD1 was abundant in renal cell lines of proximal tubule origin (NRK-52E, LLC-PK1, and OK cells). Authenticity of FXYD1 as a part of Na-K-ATPase in NRK-52E cells was demonstrated by co-purification, co-immunoprecipitation, and co-localization. Induction of FXYD2 by hypertonicity (500 mosmol/kgH(2)O with NaCl for 48 h or adaptation to 700 mosmol/kgH(2)O) correlated with downregulation of FXYD1 at mRNA and protein levels. The response to hypertonicity was influenced by serum factors and entailed, first, dephosphorylation of FXYD1 at Ser(68) (1-5 h) and, second, induction of FXYD2a and a decrease in FXYD1 with longer exposure. FXYD1 was completely replaced with FXYD2a in cells adapted to 700 mosmol/kgH(2)O and showed a significantly decreased sodium affinity. Thus dephosphorylation of FXYD1 followed by exchange of regulatory subunits is utilized to make a smooth transition of properties of Na-K-ATPase. We also observed expression of mRNA for multiple FXYDs in various cell lines. The expression was dynamic and responsive to physiological stimuli. Moreover, we demonstrated expression of FXYD5 protein in HEK-293 and HeLa cells. The data imply that FXYDs are obligatory rather than auxiliary components of Na-K-ATPase, and their interchangeability underlies responses of Na-K-ATPase to cellular stress.

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.

Locations, abundances, and possible functions of FXYD ion transport regulators in rat renal medulla

The gamma-subunit of Na-K-ATPase (FXYD2) and corticosteroid hormone-induced factor (CHIF; FXYD4) are considered pump regulators in kidney tubules. The aim of this study was to expand the information about their locations in the kidney medulla and to evaluate their importance for electrolyte excretion in an animal model. The cellular and subcellular locations and abundances of gamma and CHIF in the medulla of control and sodium-depleted rats were analyzed by immunofluorescence and immunoelectron microscopy and semiquantitative Western blotting. The results showed that antibodies against the gamma-subunit COOH terminus and splice variant gamma(a), but not splice variant gamma(b), labeled intercalated cells, but not principal cells, in the initial part of the inner medullary collecting duct (IMCD1). In subsequent segments (IMCD2 and IMCD3), all principal cells exhibited distinct basolateral labeling for both the gamma-subunit COOH terminus, splice variant gamma(a), and CHIF. Splice variant gamma(b) was abundant in the inner stripe of the outer medulla but absent in the inner medulla (IM). Double labeling by high-resolution immunoelectron microscopy showed close structural association between CHIF and the Na-K-ATPase alpha(1)-subunit in basolateral membranes. The present observations provide new information about the cellular and subcellular locations of gamma and CHIF in the renal medulla and show a new gamma variant in the IM. Extensive NaCl depletion did not induce significant changes in the locations or abundances of the gamma-subunit COOH terminus and CHIF in different kidney zones. We conclude that the unchanged levels of these two FXYD proteins suggest that they are not primary determinants for urine electrolyte composition during NaCl depletion.

NF-kappaB protects neurons from ischemic injury after middle cerebral artery occlusion in mice

Knowledge about the molecular mechanisms of neuronal survival following ischemia is crucial to the development of therapeutic interventions for victims of stroke. Previous research in our laboratory has implicated nuclear factor-kappaB (NF-kappaB) as contributing to neuronal survival in response to toxic or ischemic brain insult, with in vivo models having focused on the rat. To take advantage of genetic alterations available in the mouse, we utilized a murine transient endovascular middle cerebral artery occlusion (MCAO) model to examine the influence of NF-kappaB on neuronal survival. When brains were immunostained for the nuclear localization sequence (NLS) of the p50 subunit of NF-kappaB, a unilateral increase in immunoreactivity was seen, especially in pyramidal cell layers of the ipsilateral (stroked) hippocampus. When transgenic mice lacking p50 were compared with non-transgenic counterparts using Fluoro-Jade, a marker for neurodegeneration, both the hippocampus and striatum showed enhanced neurodegeneration at various survival times after 1 h of MCAO. In the hippocampus specifically, there was an eightfold increase in Fluoro-jade staining in the p50 knockout group vs. the non-transgenic group. Sections double stained for Fluoro-Jade and NF-kappaB activity (using a mouse engineered with a NF-kappaB responsive promoter driving a LacZ gene to produce beta galactosidase) demonstrated neuronal degeneration only in regions sparsely showing NF-kappaB activity, and those demonstrating NF-kappaB activity failed to degenerate. These data provide evidence that NF-kappaB participates in survival signaling following temporary focal ischemia, and thus may represent an attractive target for pharmacologic activation in the treatment of stroke.

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.

Splice Variants of the Gamma Subunit (FXYD2) and Their Significance in Regulation of the Na, K-ATPase in Kidney

The recent discovery of a family of single-span membrane proteins (the FXYD proteins) introduced a new direction to the rather complicated area of regulation of Na, K-ATPase. At least six members of the family have been shown to associate with the Na, K-ATPase in a cell- and tissue-specific manner, while four of them, namely the gamma subunit (FXYD2), CHIF (FXYD4), phospholemman (FXYD1), and dysadherin (FXYD5) have been identified in kidney. All four exhibited different effects on the properties of the pump in heterologous expression systems. Taken along with their non-overlapping expression patterns in the nephron, this provides a potential structural basis for the segment-specific properties of the Na, K-ATPase that had been reported in a number of papers on kidney physiology. This brief review summarizes our own contributions on structure/functional characterization of one of the family members, the gamma subunit (FXYD2). The focus is on splice variants of gamma, their structural similarity and yet distinct effects conferred to Na, K-ATPase.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

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.

Expression and phosphorylation of the na-pump regulatory subunit phospholemman in heart failure

Intracellular [Na] is approximately 3 mmol/L higher in heart failure (HF; in our arrhythmogenic rabbit model), and this can profoundly affect cardiac Ca and contractile function via Na/Ca exchange and Na/H exchange. Na/K-ATPase is the primary mechanism of Na extrusion. We examine here in HF rabbits (and human hearts) expression of Na/K-ATPase isoforms and phospholemman (PLM), a putative Na/K-ATPase regulatory subunit that inhibits pump function and is a major cardiac phosphorylation target. Na/K-ATPase alpha1- and alpha2-isoforms were reduced in HF in rabbit ventricular homogenates (by 24%) and isolated myocytes (by 30% and 17%), whereas alpha3 was increased (50%) in homogenates and decreased (52%) in myocytes (P<0.05). Homogenate Na/K-ATPase activity in left ventricle was also decreased in HF. However, we showed previously that Na/K-ATPase characteristics in intact ventricular myocytes were unaltered in HF. To reconcile these findings, we assessed PLM expression, phosphorylation, and association with Na/K-ATPase. PLM coimmunoprecipitated with Na/K-ATPase alpha1 and alpha2 in control and HF rabbit myocytes. PLM expression was reduced in HF by 42% in isolated rabbit left ventricular (LV) myocytes, by 48% in rabbit LV homogenates, and by 24% in human LV homogenate. The fraction of PLM phosphorylated at Ser-68 was increased dramatically in HF. Our results are consistent with a role for PLM analogous to that of phospholamban for SR Ca-ATPase (SERCA): inhibition of Na/K-ATPase function that is relieved on PLM phosphorylation. So reduced Na/K-ATPase expression in HF may be functionally offset by lower inhibition by PLM (because of reduced PLM expression and higher PLM phosphorylation).

The silk moth Bombyx mori U1 and U2 snRNA variants are differentially expressed

Five U1 and eight U2 isoforms of the silk moth Bombyx mori exhibiting internal nucleotide differences have been previously identified and characterized in various tissues and developmental stages. In this investigation, it is demonstrated that the levels of some snRNA variants differ in egg and silk gland tissue and change during development. Qualitative and quantitative differences in the U1 and U2 variant populations were observed at three developmental points (early, middle and late) of the silk gland (SG) during the fifth instar larval stage of the silk moth. Statistical analyses of the various isoform populations across the fifth instar larval and egg stages show significant differences for some of the U1 and U2 variants. The representation of variant sequences in expressed U1 and U2 sequences (RT-PCR libraries) and in a whole-genome shotgun (WGS) assembly database was confirmed. In addition, conserved elements in the promoter 5'-flanking region of the U1 and U2 variants were identified in the WGS.

Interaction of FXYD10 (PLMS) with Na,K-ATPase from shark rectal glands. Close proximity of Cys74 of FXYD10 to Cys254 in the a domain of the alpha-subunit revealed by intermolecular thiol cross-linking

FXYD domain-containing proteins are tissue-specific regulators of the Na,K-ATPase that have been shown to have significant physiological implications. Information about the sites of interaction between some FXYD proteins and subunits of the Na,K-ATPase is beginning to emerge. We previously identified an FXYD protein in plasma membranes from shark rectal gland cells and demonstrated that this protein (FXYD10) modulates shark Na,K-ATPase activity. The present study was undertaken to identify the location of the C-terminal domain of FXYD10 on the alpha-subunit of Na,K-ATPase, using covalent cross-linking combined with proteolytic cleavage. Treatment of Na,K-ATPase-enriched membranes with the homobifunctional thiol cross-linker 1,4-bismaleimidyl-2,3-dihydroxybutane resulted in cross-linking of FXYD10 to the alpha-subunit. Cross-linking was not affected by preincubation with sodium or potassium but was significantly reduced after pre-incubation with the non-hydrolyzable ATP analog beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP). A peptic assay was developed, in which pepsin treatment of Na,K-ATPase at low pH resulted in extensive cleavage of the alpha-subunit while FXYD10 was left intact. Proteolytic fragments of control and cross-linked preparations were isolated by immunoprecipitation and analyzed by gel electrophoresis. A proteolytic fragment containing FXYD10 cross-linked to a fragment from the alpha-subunit could be localized on SDS gels. Sequencing of this fragment showed the presence of FXYD10 as well as a fragment within the A domain of the alpha-subunit comprising 33 amino acids, including a single Cys residue, Cys254. Thus, regulation of Na,K-ATPase by FXYD10 occurs in part via cytoplasmic interaction of FXYD10 with the A domain of the shark alpha-subunit.

Covalent Cross-links between the γ Subunit (FXYD2) and α and β Subunits of Na,K-ATPase

This study describes specific intramolecular covalent cross-linking of the gamma to alpha and gamma to beta subunits of pig kidney Na,K-ATPase and rat gamma to alpha co-expressed in HeLa cells. For this purpose pig gammaa and gammab sequences were determined by cloning and mass spectrometry. Three bifunctional reagents were used: N-hydroxysuccinimidyl-4-azidosalicylic acid (NHS-ASA), disuccinimidyl tartrate (DST), and 1-ethyl-3-[3dimethylaminopropyl]carbodiimide (EDC). NHS-ASA induced alpha-gamma, DST induced alpha-gamma and beta-gamma, and EDC induced primarily beta-gamma cross-links. Specific proteolytic and Fe(2+)-catalyzed cleavages located NHS-ASA- and DST-induced alpha-gamma cross-links on the cytoplasmic surface of the alpha subunit, downstream of His(283) and upstream of Val(440). Additional considerations indicated that the DST-induced and NHS-ASA-induced cross-links involve either Lys(347) or Lys(352) in the S4 stalk segment. Mutational analysis of the rat gamma subunit expressed in HeLa cells showed that the DST-induced cross-link involves Lys(55) and Lys(56) in the cytoplasmic segment. DST and EDC induced two beta-gamma cross-links, a major one at the extracellular surface within the segment Gly(143)-Ser(302) of the beta subunit and another within Ala(1)-Arg(142). Based on the cross-linking and other data on alpha-gamma proximities, we modeled interactions of the transmembrane alpha-helix and an unstructured cytoplasmic segment SKRLRCGGKKHR of gamma with a homology model of the pig alpha1 subunit. According to the model, the transmembrane segment fits in a groove between M2, M6, and M9, and the cytoplasmic segment interacts with loops L6/7 and L8/9 and stalk S5.

Na,K-ATPase from mice lacking the gamma subunit (FXYD2) exhibits altered Na+ affinity and decreased thermal stability

The gamma subunit of the Na,K-ATPase, a 7-kDa single-span membrane protein, is a member of the FXYD gene family. Several FXYD proteins have been shown to bind to Na,K-ATPase and modulate its properties, and each FXYD protein appears to alter enzyme kinetics differently. Different results have sometimes been obtained with different experimental systems, however. To test for effects of gamma in a native tissue environment, mice lacking a functional gamma subunit gene (Fxyd2) were generated. These mice were viable and without observable pathology. Prior work in the mouse embryo showed that gamma is expressed at the blastocyst stage. However, there was no delay in blastocele formation, and the expected Mendelian ratios of offspring were obtained even with Fxyd2-/- dams. In adult Fxyd2-/- mouse kidney, splice variants of gamma that have different nephron segment-specific expression patterns were absent. Purified gamma-deficient renal Na,K-ATPase displayed higher apparent affinity for Na+ without significant change in apparent affinity for K+. Affinity for ATP, which was expected to be decreased, was instead slightly increased. The results suggest that regulation of Na+ sensitivity is a major functional role for this protein, whereas regulation of ATP affinity may be context-specific. Most importantly, this implies that gamma and other FXYD proteins have their effects by local and not global conformation change. Na,K-ATPase lacking the gamma subunit had increased thermal lability. Combined with other evidence that gamma participates in an early step of thermal denaturation, this indicates that FXYD proteins may play an important structural role in the enzyme complex.

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.

Guanylin and uroguanylin induce natriuresis in mice lacking guanylyl cyclase-C receptor

BACKGROUND

Guanylin (GN) and uroguanylin (UGN) are intestinally derived peptide hormones that are similar in structure and activity to the diarrhea-causing Escherichia coli heat-stable enterotoxins (STa). These secretagogues have been shown to affect fluid, Na+, K+, and Cl- transport in both the intestine and kidney, presumably by intracellular cyclic guanosine monophosphate (cGMP)-dependent signal transduction. However, the in vivo consequences of GN, UGN, and STa on renal function and their mechanism of action have yet to be rigorously tested.

METHODS

We hypothesized that intravenous administration of GN, UGN, or STa would cause an increase in natriuresis in wild-type mice via cGMP and guanylyl cyclase-C (GC-C, Gucy2c), the only known receptor for these peptide-hormones, and that the peptide-induced natriuresis would be blunted in genetically altered mice devoid of GC-C receptors (GC-C(-/-) null).

RESULTS

In wild-type mice using a modified renal clearance model, GN, UGN, and STa elicited significant natriuresis, kaliuresis, and diuresis as well as increased urinary cGMP levels in a time- and dose-dependent fashion. Absolute and fractional urinary sodium excretion levels were greatest approximately 40 minutes following a bolus infusion with pharmacologic doses of these peptides. Unexpectedly, GC-C(-/-) null mice also responded to the GN peptides similarly to that observed in wild-type mice. Glomerular filtration rate (GFR), blood pressure, and plasma cGMP in the mice (wild-type or GC-C(-/-) null) did not significantly vary between the vehicle- and peptide-treatment groups. The effects of UGN may also influence long-term renal function due to down-regulation of the Na+/K+ ATPase gamma-subunit and the Cl- channel ClC-K2 by 60% and 75%, respectively, as assessed by differential display polymerase chain reaction (PCR) (DD-PCR) and Northern blot analysis of kidney mRNA from mice treated with UGN.

CONCLUSION

GN, UGN, and STa act on the mouse kidney, in part, through a cGMP-dependent, GC-C-independent mechanism, causing significant natriuresis by renal tubular processes. UGN may have further long-term effects on the kidney by altering the expression of such transport-associated proteins as Na+/K+ ATPase and ClC-K2.

Stress-induced expression of the gamma subunit (FXYD2) modulates Na,K-ATPase activity and cell growth

In kidney, the Na,K-ATPase is associated with a single span protein, the gamma subunit (FXYD2). Two splice variants are differentially expressed along the nephron and have a differential influence on Na,K-ATPase when stably expressed in mammalian cells in culture. Here we used a combination of gene induction and gene silencing techniques to test the functional impact of gamma by means other than transfection. NRK-52E cells (of proximal tubule origin) do not express gamma as a protein under regular tissue culture conditions. However, when they were exposed to hyperosmotic medium, induction of only the gammaa splice variant was observed, which was accompanied by a reduction in the rate of cell division. Kinetic analysis of stable enzyme properties from control (alpha1beta1) and hypertonicity-treated cultures (alpha1beta1gammaa) revealed a significant reduction (up to 60%) of Na,K-ATPase activity measured under V(max) conditions with little or no change in the amounts of alpha1beta1. This effect as well as the reduction in cell growth rate was practically abolished when gamma expression was knocked down using specific small interfering RNA duplexes. Surprisingly, a similar induction of endogenous gammaa because of hypertonicity was seen in rat cell lines of other than renal origin: C6 (glioma), PC12 (pheochromocytoma), and L6 (myoblasts). Furthermore, exposure of NRK-52E cells to other stress inducers such as heat shock, exogenous oxidation, and chemical stress also resulted in a selective induction of gammaa. Taken together, the data imply that induction of gammaa may have adaptive value by being a part of a general cellular response to genotoxic stress.

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

Gene expression profiles during human CD4+ T cell differentiation

To develop a comprehensive catalogue of phenotypic and functional parameters of human CD4(+) T cell differentiation stages, we have performed microarray gene expression profiling on subpopulations of human thymocytes and circulating naive CD4(+) T cells, including CD3(-)CD4(+)CD8(-) intrathymic T progenitor cells, CD3(int)CD4(+)CD8(+) 'double positive' thymocytes, CD3(high)CD4(+)CD8(-) 'single positive' thymocytes, CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from cord blood and CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from adult blood. These subpopulations were sort-purified to >98% purity and their expressed RNAs were analyzed on Affymetrix Human Genome U133 arrays. Comparison of gene expression signals between these subpopulations and with early passage fetal thymic stromal cultures identify: (i) transcripts that are preferentially expressed in human CD4(+) T cell subpopulations and not in thymic stromal cells; (ii) major shifts in gene expression as progenitor T cells mature into progeny; (iii) preferential expression of transcripts at the progenitor cell stage with plausible relevance to the regulation of expansion and differentiation of these cells; and (iv) preferential expression of potential markers of recent thymic emigrants in naive-phenotype CD4(+) T cells from cord blood. Further evaluation of these findings may lead to a better definition of human thymopoiesis as well as to improved approaches to monitor and to augment the function of this important organ of T cell production.

Regulation of Na,K-ATPase by PLMS, the phospholemman-like protein from shark: molecular cloning, sequence, expression, cellular distribution, and functional effects of PLMS

In Na,K-ATPase membrane preparations from shark rectal glands, we have previously identified an FXYD domain-containing protein, phospholemman-like protein from shark, PLMS. This protein was shown to associate and modulate shark Na,K-ATPase activity in vitro. Here we describe the complete coding sequence, expression, and cellular localization of PLMS in the rectal gland of the shark Squalus acanthias. The mature protein contained 74 amino acids, including the N-terminal FXYD motif and a C-terminal protein kinase multisite phosphorylation motif. The sequence is preceded by a 20 amino acid candidate cleavable signal sequence. Immunogold labeling of the Na,K-ATPase alpha-subunit and PLMS showed the presence of alpha and PLMS in the basolateral membranes of the rectal gland cells and suggested their partial colocalization. Furthermore, through controlled proteolysis, the C terminus of PLMS containing the protein kinase phosphorylation domain can be specifically cleaved. Removal of this domain resulted in stimulation of maximal Na,K-ATPase activity, as well as several partial reactions. Both the E1 approximately P --> E2-P reaction, which is partially rate-limiting in shark, and the K+ deocclusion reaction, E2(K) --> E1, are accelerated. The latter may explain the finding that the apparent Na+ affinity was increased by the specific C-terminal PLMS truncation. Thus, these data are consistent with a model where interaction of the phosphorylation domain of PLMS with the Na,K-ATPase alpha-subunit is important for the modulation of shark Na,K-ATPase activity.

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.

Functional modulation of the sodium pump: the regulatory proteins "Fixit"

Proteins of the FXYD family act as tissue-specific regulators of the Na-K-ATPase. They are small hydrophobic type I proteins with a single-transmembrane span containing an extracellular invariant FXYD sequence. FXYD proteins are not an integral part of the Na-K-ATPase but function to modulate its catalytic properties by molecular interactions with specific Na-K-ATPase domains.

Phospholemman, a single-span membrane protein, is an accessory protein of Na,K-ATPase in cerebellum and choroid plexus

Phospholemman (FXYD1) is a homolog of the Na,K-ATPase gamma subunit (FXYD2), a small accessory protein that modulates ATPase activity. Here we show that phospholemman is highly expressed in selected structures in the CNS. It is most abundant in cerebellum, where it was detected in the molecular layer, in Purkinje neurons, and in axons traversing the granule cell layer. Phospholemman was particularly enriched in choroid plexus, the organ that secretes CSF in the ventricles, where it colocalized with Na,K-ATPase in the apical membrane. It was also enriched, with Na,K-ATPase, in certain tanycytes or ependymal cells of the ventricle wall. Two different experimental approaches demonstrated that phospholemman physically associated with the Na,K-ATPase in cerebellum and choroid plexus: the proteins copurified after detergent treatment and co-immunoprecipitated from solubilized crude membranes using either anti-phospholemman or anti-Na,K-ATPase antibodies. Phospholemman antibodies precipitated all three Na,K-ATPase alpha subunit isoforms (alpha1-alpha3) from cerebellum, indicating that the interaction is not specific to a particular alpha isoform and consistent with the presence of phospholemman in both neurons and glia. Antibodies against the C-terminal domain of phospholemman reduced Na,K-ATPase activity in vitro without effect on Na+ affinity. At least two other FXYD family members have been detected in the CNS, suggesting that additional complexity of sodium pump regulation will be found.

FXYD proteins as regulators of the Na,K-ATPase in the kidney

The FXYD gene family has seven members in mammals and others in fish. Five of these (FXYD1, FXYD2, FXYD4, FXYD7, and PLMS from shark) have been shown to alter the activity of the Na,K-ATPase, as described by other papers in this volume. The gene structure of FXYD family members suggests assembly from protein domain modules and gene duplication. The gamma subunit is unique in the family for having alternative splice variants in the coding region and can be posttranslationally modified with different final consequences for enzyme properties. The nonoverlapping distribution of gamma and CHIF (FXYD4) in kidney helps to explain physiological differences in Na(+) affinity among nephron segments. We also detected phospholemman (FXYD1) in kidney. By immunofluorescence, it was found in extraglomerular mesangial cells (EM cells) of the juxtaglomerular apparatus and in the afferent arteriole. Contrary to many reports that only alpha1 and beta1 are expressed in the kidney, we found that alpha2 and beta2 are present, although not in any nephron segment. Both were detected in arterioles, and beta2 was found in the EM cells. In contrast, alpha1, beta1, and gamma were found in adjacent macula densa. Phospholemman, alpha2, and beta2 are proposed to have distinct roles in regulating the sodium pump in structures 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.

Gamma structural variants differentially regulate Na,K-ATPase properties

Renal Na,K-ATPase is tightly bound to a small regulatory protein, the gamma subunit (FXYD2). In rat, it occurs in two splice variants with different N-termini. Immunolocalization on kidney sections revealed distinct distribution of the gamma splice variants along the rat nephron. Where coexpressed, they coimmunoprecipitated with each other along with the alpha subunit, suggesting assembly in oligomeric complexes. Functional consequences of association with gamma were monitored in stably transfected NRK-52E cells. The outcome was that splice variants can differentially modulate the major intrinsic properties of the Na,K-ATPase under normal and stress-related conditions. The data imply an adaptive physiological mechanism of regulation of renal Na,K-ATPase through modulation of pump properties, gene expression, or both.

Dominant isolated renal magnesium loss is caused by misrouting of the Na+,K+-ATPase gamma-subunit

Hereditary primary hypomagnesemia comprises a clinically and genetically heterogeneous group of disorders in which hypomagnesemia is due to either renal or intestinal Mg(2+) wasting. These disorders share the general symptoms of hypomagnesemia, tetany and epileptiformic convulsions, and often include secondary or associated disturbances in calcium excretion. In a large Dutch family with autosomal dominant renal hypomagnesemia, associated with hypocalciuria, we mapped the disease locus to a 5.6-cM region on chromosome 11q23. After candidate screening, we identified a heterozygous mutation in the FXYD2 gene, encoding the Na(+),K(+)-ATPase gamma-subunit, cosegregating with the patients of this family, which was not found in 132 control chromosomes. The mutation leads to a G41R substitution, introducing a charged amino acid residue in the predicted transmembrane region of the gamma-subunit protein. Expression studies in insect Sf9 and COS-1 cells showed that the mutant gamma-subunit protein was incorrectly routed and accumulated in perinuclear structures. In addition to disturbed routing of the G41R mutant, Western blot analysis of Xenopus oocytes expressing wild-type or mutant gamma-subunit showed mutant gamma-subunit lacking a posttranslational modification. Finally, we investigated two individuals lacking one copy of the FXYD2 gene and found their serum Mg(2+) levels to be within the normal range. We conclude that the arrest of mutant gamma-subunit in distinct intracellular structures is associated with aberrant posttranslational processing and that the G41R mutation causes dominant renal hypomagnesemia associated with hypocalciuria through a dominant negative mechanism.