Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

Sodium acetate ameliorates doxorubicin-induced cardiac injury via upregulation of Nrf2/HO-1 signaling and downregulation of NFkB-mediated apoptotic signaling in Wistar rats

Despite the effectiveness of doxorubicin (DOX) in the management of a wide range of cancers, a major challenge is its cardio-toxic effect. Oxidative stress, inflammation, and apoptosis are major pathways for the cardiotoxic effect of DOX. On the other hand, acetate reportedly exerts antioxidant, anti-inflammatory, and anti-apoptotic activities. This particular research assessed the impact of acetate on cardiotoxicity induced by DOX. Mechanistically, acetate dramatically inhibited DOX-induced upregulation of xanthine oxidase and uric acid pathway as well as downregulation of Nrf2/HO-1 signaling and its upstream proteins (reduced glutathione peroxidase, superoxide dismutase, glutathione-S-transferase, glutathione, and catalase, glutathione reductase). In addition, acetate markedly attenuated DOX-driven rise inTNF-α, NFkB IL-6 and IL-1β expression, and myeloperoxidase activity. Furthermore, acetate significantly ameliorated DOX-led suppression of Bcl-2 and Ca2+-ATPase activity and upregulation of Bax, caspase 3, and caspase 9 actions. Improved body weight, heart structural integrity, and cardiac function as depicted by cardiac injury markers convoyed these cascades of events. Summarily, the present study demonstrated that acetate protects against DOX-induced cardiotoxicity by upregulating Nrf2/HO-1 signaling and downregulating NFkB-mediated activation of Bax/Bcl-2 and caspase signaling.

Microproteins: Overlooked regulators of physiology and disease

Ongoing efforts to generate a complete and accurate annotation of the genome have revealed a significant blind spot for small proteins (<100 amino acids) originating from short open reading frames (sORFs). The recent discovery of numerous sORF-encoded proteins, termed microproteins, that play diverse roles in critical cellular processes has ignited the field of microprotein biology. Large-scale efforts are currently underway to identify sORF-encoded microproteins in diverse cell-types and tissues and specialized methods and tools have been developed to aid in their discovery, validation, and functional characterization. Microproteins that have been identified thus far play important roles in fundamental processes including ion transport, oxidative phosphorylation, and stress signaling. In this review, we discuss the optimized tools available for microprotein discovery and validation, summarize the biological functions of numerous microproteins, outline the promise for developing microproteins as therapeutic targets, and look forward to the future of the field of microprotein biology.

Knockdown of the Sodium/Potassium ATPase Subunit Beta 2 Reduces Egg Production in the Dengue Vector, Aedes aegypti

The Na+/K+ ATPase (NKA) is present in the cellular membrane of most eukaryotic cells. It utilizes energy released by ATP hydrolysis to pump sodium ions out of the cell and potassium ions into the cell, which establishes and controls ion gradients. Functional NKA pumps consist of three subunits, alpha, beta, and FXYD. The alpha subunit serves as the catalytic subunit while the beta and FXYD subunits regulate the proper folding and localization, and ion affinity of the alpha subunit, respectively. Here we demonstrate that knockdown of NKA beta subunit 2 mRNA (nkaβ2) reduces fecundity in female Ae. aegypti. We determined the expression pattern of nkaβ2 in several adult mosquito organs using qRT-PCR. We performed RNAi-mediated knockdown of nkaβ2 and assayed for lethality, and effects on female fecundity. Tissue expression levels of nkaβ2 mRNA were highest in the ovaries with the fat body, midgut and thorax having similar expression levels, while Malpighian tubules had significantly lower expression. Survival curves recorded post dsRNA injection showed a non-significant decrease in survival of nkaβ2 dsRNA-injected mosquitoes compared to GFP dsRNA-injected mosquitoes. We observed a significant reduction in the number of eggs laid by nkaβ2 dsRNA-injected mosquitoes compared to control mosquitoes. These results, coupled with the tissue expression profile of nkaβ2, indicate that this subunit plays a role in normal female Ae. aegypti fecundity. Additional research needs to be conducted to determine the exact role played by NKAβ2 in mosquito post-blood meal nutrient sensing, transport, yolk precursor protein (YPP) synthesis and yolk deposition.

Codeine-induced hepatic injury is via oxido-inflammatory damage and caspase-3-mediated apoptosis

Codeine (3-methylmorphine) is a known analgesic, antitussive, and antidiarrheal drug that is often abused for recreational purposes. It is metabolized in the liver via the cytochrome P450 system and thus hypothesized to induce hepatic injury especially when misused. Thus, the present study aimed at investigating changes in liver function, hepatic enzyme biomarker, proton pumps, antioxidant status, free radicals and TNF-α levels, as well as caspase 3 activities and hepatic DNA fragmentation after 6 weeks of oral codeine administration. Twenty-one male rabbits were randomized into 3 groups (n = 7). The control group had 1 ml of normal saline, while the low-dose and high-dose codeine groups received 4 and 10 mg/kg b.w of codeine respectively daily. The codeine-treated animals had significantly lower levels of serum proteins, increased activities of hepatic enzyme biomarkers and caspase 3, raised hepatic concentrations of free radicals and TNF-α, as well as increased hepatic DNA fragmentation. Codeine treatment also led to a significant decline in hepatic weight, activities of hepatic enzymatic antioxidant, Na⁺-K⁺-ATPase and Ca²⁺-ATPase. These alterations were more pronounced in high-dose codeine treated animals than in the low-dose group. Histopathological study showed moderate fatty degeneration of hepatic parenchyma, infiltration of the portal tract by inflammatory cells with dense collagen fibre deposition in codeine-treated animals. The present study revealed that codeine induced liver injury and hepatic DNA damage via caspase 3-dependent signaling by suppressing hepatic antioxidant status and enhancing free radical and TNF-α generation.

Codeine exerts cardiorenal injury via upregulation of adenine deaminase/xanthine oxidase and caspase 3 signaling

AIMS

Codeine treatment has been shown to be associated with glucolipid deregulation, though data reporting this are inconsistent and the mechanisms are not well understood. Perturbation of glutathione-dependent antioxidant defense and adenosine deaminase (ADA)/xanthine oxidase (XO) signaling has been implicated in the pathogenesis of cardiometabolic disorders. We thus, hypothesized that depletion of glutathione contents and upregulation of ADA/XO are involved in codeine-induced glucolipid deregulation. The present study also investigated whether or not codeine administration would induce genotoxicity and apoptosis in cardiac and renal tissues.

MATERIALS AND METHODS

Male New Zealand rabbits received per os distilled water or codeine, either in low dose (4 mg/kg) or high dose (10 mg/kg) for 6 weeks.

KEY FINDINGS

Codeine treatment led to reduced absolute and relative cardiac and renal mass independent of body weight change, increased blood glucose, total cholesterol (TC), triglycerides (TG), and low-density lipoprotein (LDL-C), as well as increased atherogenic indices and triglyceride-glucose index (TyG). Codeine administration significantly increased markers of cardiac and renal injury, as well as impaired cardiorenal functions. Codeine treatment also resulted in increased cardiac and renal malondialdehyde, Advanced Glycation Endproducts (AGE) and 8-hydroxydeoxyguanosine (8-OH-dG), and myeloperoxidase (MPO), ADA, XO, and caspase 3 activities. These observations were accompanied by impaired activities of cardiac and renal proton pumps.

SIGNIFICANCE

Findings of this study demonstrate that upregulation of ADA/XO and caspase 3 signaling are, at least partly, contributory to the glucolipid deregulation and cardiorenal injury induced by codeine.

A structural rearrangement of the Na+/K+-ATPase traps ouabain within the external ion permeation pathway

With the use of the energy of ATP hydrolysis, the Na+/K+-ATPase is able to transport across the cell membrane Na+ and K+ against their electrochemical gradients. The enzyme is strongly inhibited by ouabain and its derivatives, some that are therapeutically used for patients with heart failure (cardiotonic steroids). Using lanthanide resonance energy transfer, we trace here the conformational changes occurring on the external side of functional Na+/K+-ATPases induced by the binding of ouabain. Changes in donor/acceptor pair distances are mainly observed within the α subunit of the enzyme. To derive a structural model matching the experimental lanthanide resonance energy transfer distances measured with bound ouabain, we carried out molecular dynamics simulations with energy restraints applied simultaneously using a novel methodology with multiple non-interacting fragments. The restrained simulation, initiated from the X-ray structure of the E2(2K+) state, became strikingly similar to the X-ray structure of the sodium-bound state. The final model shows that ouabain is trapped within the external ion permeation pathway of the pump.

Delineation of the key aspects in the regulation of epithelial monolayer formation

The formation, maintenance, and repair of epithelial barriers are of critical importance for whole-body homeostasis. However, the molecular events involved in epithelial tissue maturation are not fully established. To this end, we investigated the molecular processes involved in renal epithelial proximal-tubule monolayer maturation utilizing transcriptomic, metabolomic, and functional parameters. We uncovered profound dynamic alterations in transcriptional regulation, energy metabolism, and nutrient utilization over the maturation process. Proliferating cells exhibited high glycolytic rates and high transcript levels for fatty acid synthesis genes (FASN), whereas matured cells had low glycolytic rates, increased oxidative capacity, and preferentially expressed genes for beta oxidation. There were dynamic alterations in the expression and localization of several adherens (CDH1, -4, and -16) and tight junction (TJP3 and CLDN2 and -10) proteins. Genes involved in differentiated proximal-tubule function, cilium biogenesis (BBS1), and transport (ATP1A1 and ATP1B1) exhibited increased expression during epithelial maturation. Using TransAM transcription factor activity assays, we could demonstrate that p53 and FOXO1 were highly active in matured cells, whereas HIF1A and c-MYC were highly active in proliferating cells. The data presented here will be invaluable in the further delineation of the complex dynamic cellular processes involved in epithelial cell regulation.

A priori and a posteriori approaches for finding genes of evolutionary interest in non-model species: osmoregulatory genes in the kidney transcriptome of the desert rodent Dipodomys spectabilis (banner-tailed kangaroo rat)

One common goal in evolutionary biology is the identification of genes underlying adaptive traits of evolutionary interest. Recently next-generation sequencing techniques have greatly facilitated such evolutionary studies in species otherwise depauperate of genomic resources. Kangaroo rats (Dipodomys sp.) serve as exemplars of adaptation in that they inhabit extremely arid environments, yet require no drinking water because of ultra-efficient kidney function and osmoregulation. As a basis for identifying water conservation genes in kangaroo rats, we conducted a priori bioinformatics searches in model rodents (Mus musculus and Rattus norvegicus) to identify candidate genes with known or suspected osmoregulatory function. We then obtained 446,758 reads via 454 pyrosequencing to characterize genes expressed in the kidney of banner-tailed kangaroo rats (Dipodomys spectabilis). We also determined candidates a posteriori by identifying genes that were overexpressed in the kidney. The kangaroo rat sequences revealed nine different a priori candidate genes predicted from our Mus and Rattus searches, as well as 32 a posteriori candidate genes that were overexpressed in kidney. Mutations in two of these genes, Slc12a1 and Slc12a3, cause human renal diseases that result in the inability to concentrate urine. These genes are likely key determinants of physiological water conservation in desert rodents.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

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.

FXYD proteins stabilize Na,K-ATPase: amplification of specific phosphatidylserine-protein interactions

FXYD proteins are a family of seven small regulatory proteins, expressed in a tissue-specific manner, that associate with Na,K-ATPase as subsidiary subunits and modulate kinetic properties. This study describes an additional property of FXYD proteins as stabilizers of Na,K-ATPase. FXYD1 (phospholemman), FXYD2 (γ subunit), and FXYD4 (CHIF) have been expressed in Escherichia coli and purified. These FXYD proteins associate spontaneously in vitro with detergent-soluble purified recombinant human Na,K-ATPase (α1β1) to form α1β1FXYD complexes. Compared with the control (α1β1), all three FXYD proteins strongly protect Na,K-ATPase activity against inactivation by heating or excess detergent (C(12)E(8)), with effectiveness FXYD1 > FXYD2 ≥ FXYD4. Heating also inactivates E(1) ↔ E(2) conformational changes and cation occlusion, and FXYD1 protects strongly. Incubation of α1β1 or α1β1FXYD complexes with guanidinium chloride (up to 6 m) causes protein unfolding, detected by changes in protein fluorescence, but FXYD proteins do not protect. Thus, general protein denaturation is not the cause of thermally mediated or detergent-mediated inactivation. By contrast, the experiments show that displacement of specifically bound phosphatidylserine is the primary cause of thermally mediated or detergent-mediated inactivation, and FXYD proteins stabilize phosphatidylserine-Na,K-ATPase interactions. Phosphatidylserine probably binds near trans-membrane segments M9 of the α subunit and the FXYD protein, which are in proximity. FXYD1, FXYD2, and FXYD4 co-expressed in HeLa cells with rat α1 protect strongly against thermal inactivation. Stabilization of Na,K-ATPase by three FXYD proteins in a mammalian cell membrane, as well the purified recombinant Na,K-ATPase, suggests that stabilization is a general property of FXYD proteins, consistent with a significant biological function.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Na, K-ATPase: Ubiquitous Multifunctional Transmembrane Protein and its Relevance to Various Pathophysiological Conditions

The Na(+), K(+)-ATPase (NKA) is an ubiquitous enzyme consisting of α, β and γ subunits, and is responsible for the creation and maintenance of the Na(+) and K(+) gradients across the cell membrane by transporting 3 Na(+) out and 2 K(+) into the cell. Sodium pump regulation is tissue as well as isoform specific. Intracellular messengers differentially regulate the activity of the individual NKA isozymes. Regulation of specific NKA isozymes gives cells the ability to precisely coordinate NKA activity to their physiological requirements. It is the only known receptor for the cardiac glycosides used to treat congestive heart failure and cardiac arrhythmias. Endogenous ligands structurally similar to cardiac glycosides may act as natural regulators of the sodium pump in heart and other tissues. Identification of naturally occurring regulators of  NKA could initiate the discovery of new hormone-like control systems involved in the etiology of selected disease processes, hence the importance of understanding the relation of the sodium pump and its ligands to disease. Diabetes has a marked effect on the metabolism of a variety of tissues and because the NKA is critical for the membrane potential and many transports, a change in its activity in diabetes would have profound consequence in these tissues. NKA is also involved in hypertension, salt balance, cardiovascular and renal disorders, sperm capacitation, cell volume regulation, apoptosis, rheumatoid arthritis, sepsis, neurological disorders, lung edema clearance and preeclampsia. NKA activity and expression in the collecting duct of kidney are modulated physiologically by hormones like aldosterone, vasopressin, and insulin. NKA enzyme activity and subunit levels are reduced in carcinoma, NKA-β levels were highly reduced in an invasive form of human renal clear cell carcinoma, androgen-dependent prostate cancer, in early stages of urothelial cancer, as well as in poorly differentiated, highly motile carcinoma cell lines obtained from various tissues suggesting a functional link between reduced NKA-β expression and cancer progression. It could be a target for the development of anticancer drugs as it serves as a signal transducer, it is a player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers.

KEYWORDS

Na(+), K(+)-ATPase (NKA); Cardiotonic steroids (CTS); Diabetes; Hypertension; Cardiovascular and renal disorders; Signal transducer; Anticancer drugs.

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.

Spontaneous calcium spike activity in embryonic spinal neurons is regulated by developmental expression of the Na+, K+-ATPase beta3 subunit

Different types and patterns of spontaneous electrical activity drive many aspects of neuronal differentiation. Neurons in the developing Xenopus spinal cord exhibit calcium spikes, which regulate gene transcription and neurotransmitter specification. The ionic currents necessary for spike production have been described. However, the mechanisms that generate the onset of this activity and the basis of its regulation remain unclear. Although signaling molecules appear to act on plasma membrane receptors to trigger calcium spike activity, other mechanisms for spontaneous calcium spike regulation may exist as well. Here, we analyze the developmental expression of the Na(+), K(+)-ATPase beta3 subunit in Xenopus tropicalis embryos and show that its levels are downregulated at a time during embryonic development that coincides with the onset of prominent calcium spike activity in spinal neurons. Inhibition of an earlier increase in beta3 expression leads to more depolarized resting membrane potentials and results in later reduction of spike activity. This suppression of beta3 levels also reduces expression of the store-operated calcium channel subunit, Orai1. These findings suggest that the Na(+), K(+)-ATPase plays a role in initiating calcium spike activity and regulating calcium homeostasis.

[Energetic applications: Na+/K+-ATPase and neuromuscular transmission]

Na/K-ATPase electrogenic activity and its indispensable role in maintaining gradients suggest that the modifications in isoform distribution and the functioning of the sodium pump have a major influence on both the neuronal functions, including excitability, and motor efficiency. This article proposes to clarify the involvement of Na/K-ATPase in the transmission of nerve influx within the peripheral nerve and then in the genesis, the maintenance, and the physiology of muscle contraction by comparing the data found in the literature with our work on neuron and muscle characterization of Na/K-ATPase activity and isoforms.

Structural arrangement and conformational dynamics of the gamma subunit of the Na+/K+-ATPase

The Na+/K+-ATPase couples the chemical energy in ATP to transport Na+ and K+ across the plasma membrane against a concentration gradient. The ion pump is composed of two mandatory subunits: the alpha subunit, which is the major catalytic subunit, and the beta subunit, which is required for proper trafficking of the complex to the plasma membrane. In some tissues, the ion pump also contains an optional third subunit, gamma, which modulates the pump activity. To examine the conformational dynamics of the gamma subunit during ion transport and its position in relation to the alpha and the beta subunits, we have used fluorescence resonance energy transfer under voltage clamp conditions. From these experiments, evidence is provided that the gamma subunit is located adjacent to the M2-M6-M9 pocket of the alpha subunit at the transmembrane-extracellular interface. We have also used fluorescence resonance energy transfer to investigate the relative movement of the three subunits as the ion pump shuttles between the two main conformational states, E1 and E2, as described by the Albers-Post scheme. The results from this study suggest that there is no relative change in distance between the alpha and gamma subunits but there is a relative change in distance between the beta and gamma subunits during the E2 to E1 transition. It was also observed that labeling the gamma subunit at specific residues with fluorophores induces a decrease in K+-induced stationary current. This result could be due to a perturbation in the K+ branch of the reaction cycle of the pump, representing a new way to inhibit the pump.

Alveolar epithelium and Na,K-ATPase in acute lung injury

Active transport of sodium across the alveolar epithelium, undertaken in part by the Na,K-adenosine triphosphatase (Na,K-ATPase), is critical for clearance of pulmonary edema fluid and thus the outcome of patients with acute lung injury. Acute lung injury results in disruption of the alveolar epithelial barrier and leads to impaired clearance of edema fluid and altered Na,K-ATPase function. There has been significant progress in the understanding of mechanisms regulating alveolar edema clearance and signaling pathways modulating Na,K-ATPase function during lung injury. The accompanying review by Morty et al. focuses on intact organ and animal models as well as clinical studies assessing alveolar fluid reabsorption in alveolar epithelial injury. Elucidation of the mechanisms underlying regulation of active Na⁺ transport, as well as the pathways by which the Na,K-ATPase regulates epithelial barrier function and edema clearance, are of significance to identify interventional targets to improve outcomes of patients with acute lung injury.

Regulation of cardiac Na+/Ca2+ exchanger by phospholemman

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

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.

[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.

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

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

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.

Altered contractility and [Ca2+]i homeostasis in phospholemman-deficient murine myocytes: role of Na+/Ca2+ exchange

Phospholemman (PLM) regulates contractility and Ca(2+) homeostasis in cardiac myocytes. We characterized excitation-contraction coupling in myocytes isolated from PLM-deficient mice backbred to a pure congenic C57BL/6 background. Cell length, cell width, and whole cell capacitance were not different between wild-type and PLM-null myocytes. Compared with wild-type myocytes, Western blots indicated total absence of PLM but no changes in Na(+)/Ca(2+) exchanger, sarcoplasmic reticulum (SR) Ca(2+)-ATPase, alpha(1)-subunit of Na(+)-K(+)-ATPase, and calsequestrin levels in PLM-null myocytes. At 5 mM extracellular Ca(2+) concentration ([Ca(2+)](o)), contraction and cytosolic [Ca(2+)] ([Ca(2+)](i)) transient amplitudes and SR Ca(2+) contents in PLM-null myocytes were significantly (P < 0.0004) higher than wild-type myocytes, whereas the converse was true at 0.6 mM [Ca(2+)](o). This pattern of contractile and [Ca(2+)](i) transient abnormalities in PLM-null myocytes mimics that observed in adult rat myocytes overexpressing the cardiac Na(+)/Ca(2+) exchanger. Indeed, we have previously reported that Na(+)/Ca(2+) exchange currents were higher in PLM-null myocytes. Activation of protein kinase A resulted in increased inotropy such that there were no longer any contractility differences between the stimulated wild-type and PLM-null myocytes. Protein kinase C stimulation resulted in decreased contractility in both wild-type and PLM-null myocytes. Resting membrane potential and action potential amplitudes were similar, but action potential duration was much prolonged (P < 0.04) in PLM-null myocytes. Whole cell Ca(2+) current densities were similar between wild-type and PLM-null myocytes, as were the fast- and slow-inactivation time constants. We conclude that a major function of PLM is regulation of cardiac contractility and Ca(2+) fluxes, likely by modulating Na(+)/Ca(2+) exchange activity.

The gamma subunit of Na+, K+-ATPase: role on ATPase activity and regulatory phosphorylation by PKA

In kidney, Na+, K+-ATPase is an oligomer (alphabeta gamma) with equimolar amounts of essential alpha and beta subunits and one small hydrophobic FXYD protein (gamma subunit). This report describes gamma subunit as an activator of pig kidney outer medulla Na+, K+-ATPase in aqueous medium. The effects of gamma subunit on Na+, K+-ATPase were dose-dependent and preincubation-dependent. Changes in alphabeta/gamma stoichiometry did not alter Km1 for ATP, and slightly increased Km2, but Vmax was increased at both catalytic and regulatory sites. Hydroxylamine treatment of enzyme phosphorylated by ATP (E-P), in the presence of additional gamma subunit, revealed that 52% of the E-P accumulation was not via acyl-phosphate formation. The gamma subunit was phosphorylated by endogenous kinases and by commercial catalytic subunit of protein kinase A (PKA). Additionally, we demonstrated that PKA phosphorylation of gamma subunit increased its capacity to stimulate ATP hydrolysis. These results suggest that gamma subunit can act as an intrinsic Na+, K+-ATPase regulator in kidney.

Regions of the Catalytic α Subunit of Na,K-ATPase Important for Functional Interactions with FXYD 2

The gamma modulator (FXYD 2) is a member of the FXYD family of single transmembrane proteins that modulate the kinetic behavior of Na,K-ATPase. This study concerns the identification of regions in the alpha subunit that are important for its functional interaction with gamma. An important effect of gamma is to increase K+ antagonism of cytoplasmic Na+ activation apparent as an increase in KNa' at high [K+]. We show that although gamma associates with alpha1, alpha2, and alpha3 isoforms, it increases the KNa' of alpha1 and alpha3 but not alpha2. Accordingly, chimeras of alpha1 and alpha2 were used to identify regions of alpha critical for the increased KNa'. As with alpha1 and alpha2, all chimeras associate with gamma. Kinetic analysis of alpha2front/alpha1back chimeras indicate that the C-terminal (Lys907-Tyr1018) region of alpha1, which includes transmembrane (TM)9 close to gamma, is important for the increase in KNa'. However, similar experiments with alpha1front/alpha2back chimeras indicate a modulatory role of the loop between TMs 7 and 8. Thus, as long as the alpha1 L7/8 loop is present, replacement of TM9 of alpha1 with that of alpha2 does not abrogate the gamma effect on KNa'. In contrast, as long as TM9 is that of alpha1, replacement of L7/8 of alpha1 with that of alpha2 does not abolish the effect. It is suggested that structural association of the TM regions of alpha and FXYD 2 is not the sole determinant of this effect of FXYD on KNa' but is subject to long range modulation by the extramembranous L7/8 loop of alpha.

Physiological role of the α1- and α2-isoforms of the Na+-K+-ATPase and biological significance of their cardiac glycoside binding site

An interesting feature of Na+-K+-ATPase is that it contains four isoforms of the catalytic α-subunit, each with a tissue-specific distribution. Our laboratory has used gene targeting to define the functional role of the α1- and α2-isoforms. While knockout mice demonstrated the importance of the α1- and α2-isoforms for survival, the knockin mice, in which each isoform can be individually inhibited by ouabain and its function determined, demonstrated that both isoforms are regulators of cardiac muscle contractility. Another intriguing aspect of the Na+-K+-ATPase is that it contains a binding site for cardiac glycosides, such as digoxin. Conservation of this site suggests that it may have an in vivo role and that a natural ligand must exist to interact with this site. In fact, cardiac glycoside-like compounds have been observed in mammals. Our recent study demonstrates that the cardiac glycoside binding site of the Na+-K+-ATPase plays a role in the regulation of blood pressure and that it mediates both ouabain-induced and ACTH-induced hypertension in mice. Whereas chronic administration of ouabain or ACTH caused hypertension in wild-type mice, it had no effect on blood pressure in mice with a ouabain-resistant α2-isoform of Na+-K+-ATPase. Interestingly, animals with the ouabain-sensitive α1-isoform and a ouabain-resistant α2-isoform develop ACTH-induced hypertension to a greater extent than wild-type animals. Taken together, these results demonstrate that the cardiac glycoside binding of the Na+-K+-ATPase has a physiological role and suggests a function for a naturally occurring ligand that is stimulated by administration of ACTH.

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.

Structural interactions between FXYD proteins and Na+,K+-ATPase: alpha/beta/FXYD subunit stoichiometry and cross-linking

Interactions of rat FXYD4 (corticosteroid hormone-induced factor (CHIF)), FXYD2 (gamma), or FXYD1 (phospholemman (PLM)) proteins with rat alpha1 subunits of Na(+),K(+)-ATPase have been analyzed by co-immunoprecipitation and covalent cross-linking. In detergent-solubilized membranes from HeLa cells expressing both gamma and CHIF or CHIF and hemagglutinin A-tagged CHIF, mixed complexes of CHIF and gamma or CHIF and hemagglutinin A-tagged CHIF with alpha/beta subunits are undetectable. This implies that the alpha/beta/FXYD protomer is the major species in detergent solution. A lipid-soluble cysteine-cysteine bifunctional reagent, dibromobimane, cross-links CHIF to alpha in colonic membranes but not gamma or PLM to alpha in kidney or heart membranes, respectively. Sequence comparisons of the FXYD proteins suggested that Cys-49 in the trans-membrane segment of CHIF could be involved. In detergent-solubilized HeLa cell membranes, dibromobimane cross-links wild-type CHIF to alpha but not the C49F mutant, and also the corresponding F36C mutant but not wild-type gammab, and F48C but not wild-type PLM. C140S, C338A, C804A, and C966S mutants of the alpha subunit have been expressed. Only the C140S mutant prevents cross-linking with CHIF. The data demonstrated the proximity of trans-membrane segments of CHIF, gamma, and PLM to M2 of alpha. Molecular modeling is consistent with location of the trans-membrane segment of all FXYD proteins between M2, M6, and M9 and the proximity of Cys-49 of CHIF or Phe-36 of gamma with Cys-140 of M2. Cross-linking also demonstrated CHIF-alpha and CHIF-beta proximities in extra-membrane regions, similar to the evidence for gamma-alpha and gamma-beta cross-links.

Stabilization of trypsin by association to plasma membranes: Implications for tryptic cleavage of membrane-bound Na,K-ATPase

Tryptic cleavage has been a potential method for studying the structure and mechanism of many membrane transport proteins. Here, we report tight association of trypsin to pig kidney plasma membranes enriched in Na,K-ATPase. Trypsin also associated with protein-free vesicles prepared from plasma membrane lipids. Membrane-associated trypsin was found to be highly resistant to autolysis and insensitive to inhibition by PMSF. Na,K-ATPase substrate ions differentially influenced the level of trypsin membrane association. Thus, NaCl significantly increased trypsin membrane association compared to KCl. The ions seem to exert direct effects on the membrane independent of their effects on protein conformation. Bicarbonate anions, which detach peripheral membrane proteins, efficiently released trypsin from the membrane. Trypsin membrane association was found to enhance the cleavage of the Na,K-ATPase gamma-subunit. Comparison between membranes from shark rectal gland and pig kidney showed that trypsin association was significantly higher in the former. This was found to be partly due to the presence of higher cholesterol levels in the membrane. In conclusion, the differential membrane association of trypsin may affect the outcome of proteolytic cleavage of membrane-bound proteins.

Ouabain induces an increase of retinal ganglion cell survival in vitro: the involvement of protein kinase C

Ouabain is a steroid derivative that can regulate many cellular events such as growth and proliferation. It modulates Na+,K+-ATPase activity leading to the activation of different intracellular pathways through protein-protein interactions that have been characterized during the last few years. The aim of this work was to study the role of ouabain in rat retinal ganglion cell survival after 48 h in culture. Our results demonstrated that ouabain significantly induced an increase in retinal ganglion cell survival. The effect was dose-dependent and was maximal with 3.0 nM. The blockade of protein kinase C activity by 1.25 microM chelerythrine chloride abolished the ouabain effect, indicating an involvement of this intracellular pathway. None of the protein kinase inhibitors usually employed in the study of ouabain-driven intracellular pathways (PD98059, Ly294002, herbimycin, and genistein) was able to influence neuronal survival induced by ouabain. The data presented suggest that ouabain may be the trigger of an intracellular pathway responsible for neuronal survival.

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

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

Functional Significance of the Shark Na,K-ATPase N-Terminal Domain. Is the Structurally Variable N-Terminus Involved in Tissue-Specific Regulation by FXYD Proteins?

The proteolytic profile after mild controlled trypsin cleavage of shark rectal gland Na,K-ATPase was characterized and compared to that of pig kidney Na,K-ATPase, and conditions for achieving N-terminal cleavage of the alpha-subunit at the T(2) trypsin cleavage site were established. Using such conditions, the shark enzyme N-terminus was much more susceptible to proteolysis than the pig enzyme. Nevertheless, the maximum hydrolytic activity was almost unaffected for the shark enzyme, whereas it was significantly decreased for the pig kidney enzyme. The apparent ATP affinity was unchanged for shark but increased for pig enzyme after N-terminal truncation. The main common effect following N-terminal truncation of shark and pig Na,K-ATPase is a shift in the E(1)-E(2) conformational equilibrium toward E(1). The phosphorylation and the main rate-limiting E(2) --> E(1) step are both accelerated after N-terminal truncation of the shark enzyme, but decreased significantly in the pig kidney enzyme. Some of the kinetic differences, like the acceleration of the phosphorylation reaction, following N-terminal truncation of the two preparations may be due to the fact that under the conditions used for N-terminal truncation, the C-terminal domain of the FXYD regulatory protein of the shark enzyme, PLMS or FXYD10, was also cleaved, whereas the gamma or FXYD2 of the pig enzyme was not. In the shark enzyme, N-terminal truncation of the alpha-subunit abolished association of exogenous PLMS with the alpha-subunit and the functional interactions were abrogated. Moreover, PKC phosphorylation of the preparation, which relieves PLMS inhibition of Na,K-ATPase activity, exposed the N-terminal trypsin cleavage site. It is suggested that PLMS interacts functionally with the N-terminus of the shark Na,K-ATPase to control the E(1)-E(2) conformational transition of the enzyme and that such interactions may be controlled by regulatory protein kinase phosphorylation of the N-terminus. Such interactions are likely in shark enzyme where PLMS has been demonstrated by cross-linking to associate with the Na,K-ATPase A-domain.

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.

The expression of the human neuronal alpha3 Na+,K+-ATPase subunit gene is regulated by the activity of the Sp1 and NF-Y transcription factors

The Na+,K+-ATPase is a ubiquitous protein found in virtually all animal cells which is involved in maintaining the electrochemical gradient across the plasma membrane. It is a multimeric enzyme consisting of alpha, beta and gamma subunits that may be present as different isoforms, each of which has a tissue-specific expression profile. The expression of the Na+,K+-ATPase alpha3 subunit in humans is confined to developing and adult brain and heart, thus suggesting that its catalytic activity is strictly required in excitable tissues. In the present study, we used structural, biochemical and functional criteria to analyse the transcriptional mechanisms controlling the expression of the human gene in neurons, and identified a minimal promoter region of approx. 100 bp upstream of the major transcription start site which is capable of preferentially driving the expression of a reporter gene in human neuronal cell lines. This region contains the cognate DNA sites for the transcription factors Sp1/3/4 (transcription factors 1/3/4 purified from Sephacryl and phosphocellulose columns), NF-Y (nuclear factor-Y) and a half CRE (cAMP-response element)-like element that binds a still unknown protein. Although the expression of these factors is not tissue-specific, co-operative functional interactions among them are required to direct the activity of the promoter predominantly in neuronal cells.

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.

The Effect of the Gamma Modulator on Na/K Pump Activity of Intact Mammalian Cells

This study concerns the modulatory effects of the gamma modulator of the Na/K pump, in particular whether the effects seen in previous experiments with isolated membranes are relevant to Na/K pump behavior in intact mammalian cells. For this purpose, HeLa cells previously transfected with the rat Na/K catalytic subunit were used. The results show that both variants of the regulator, gammaa and gammab, decrease the apparent affinity of the pump for Na(+) and cause a modest increase in apparent ATP affinity as seen in measurements of ouabain-sensitive (86)Rb(K(+)) influx into cells in which ATP was varied using antimycin A and glucose. Equivalent results had been obtained previously in our analyses of Na,K-ATPase activity of membrane fragments, i.e., an increase in K(0.5(Na)) at high K(+) concentration and a decrease in K'(ATP). Comparison of clones of gamma-transfected and mock-transfected cells (with similar V(max) values) indicated that gamma causes a modest approximately 30% increase in the steady-state concentration of intracellular Na(+). Furthermore, for both gammaa and gammab, values of intracellular Na(+) were similar to those predicted from the kinetic constants, K(0.5(Na)) and V(max). Finally, there was a gamma-mediated increase in apparent affinity for extracellular K(+), which had not been detected in assays of permeabilized membranes.

Synthesis of the Na-K-ATPase γ-subunit is regulated at both the transcriptional and translational levels in IMCD3 cells

We previously reported that hypertonicity-mediated upregulation of the γ-subunit of Na-K-ATPase is dependent on both the JNK and the PI3 kinase pathways ( Proc Natl Acad Sci USA 98: 13414, 2001). The present experiments were undertaken to explore the mechanisms whereby these pathways regulate the expression of the γ-subunit in inner medullary collecting duct cells (IMCD3). Inhibition of JNK with SP-600125 (20 μM), a concentration that causes an ∼95% inhibition of hypertonicity-stimulated JNK activation, markedly decreased the amount of the γ-subunit in response to 550 mosmol/kgH2O for 48 h. This was accompanied by a parallel decrease in the γ-subunit mRNA. The rate at which the γ-subunit mRNA decreased was unaffected by actinomycin D. In contrast, inhibition of PI3 kinase with LY-294002 results in a marked decrease in the amount of γ-subunit protein but without alteration in γ-subunit message. The rate at which the γ-subunit protein decreased was unaffected by cyclohexamide. Transfection of IMCD3 cells with a γ-subunit construct results in the expression of both γ-subunit message and protein. However, in cortical collecting duct cells (M1 cells) such transfection resulted in expression of only the message and not the protein. We conclude that JNK regulates the γ-subunit at the transcriptional level while PI3 kinase regulates γ-subunit expression at the translational level. There is also posttranscriptional cell specificity in the expression of the γ -subunit of Na-K-ATPase.