Transport and pharmacological properties of nine different human Na, K-ATPase isozymes

Mechanistic studies of sodium pump

Sodium pump was the first ion pump discovered. A member of the family of active transporters that catalyze adenosine 5'-triphosphate hydrolysis by forming a phosphorylated enzyme intermediate, sodium pump couples the energy released to unequal countertransport of sodium and potassium ions. The ion gradient generated by the pump is important for a variety of secondary physiological processes ranging from metabolite transport to electrical excitation of nerve and muscle. Selected experiments relating structure to function are reviewed.

Hypothetical mechanism of sodium pump regulation by estradiol under primary hypertension

Causal relationship between sodium and hypertension has been proposed and various changes in Na+,K+-ATPase (sodium pump) activity have been described in established primary hypertension. A number of direct vascular effects of estradiol have been reported, including its impact on the regulation of sodium pump activity and vasomotor tone. The effects of estradiol involve the activation of multiple signaling cascades, including phosphatydil inositol-3 kinase (PI3K) and p42/44 mitogen-activated protein kinase (p42/44(MAPK)). In addition, some of the effects of estradiol have been linked to activity of cytosolic phospholipase A(2) (cPLA(2)). One possible cardioprotective mechanism of estradiol involves of the interaction between estradiol and the rennin-angiotensin system (RAS). Elevated circulating and tissue levels of angiotensin II (Ang II) have been implicated in the development of hypertension and heart failure. The aim of our investigation was to elucidate the signaling mechanisms employed by estradiol and Ang II in mediating sodium pump, in vascular smooth muscle cells (VSMC). The aim of our investigation was to elucidate the signaling mechanisms employed by estradiol and Ang II in mediating sodium pump activity/expression in VSMC, with particular emphasis on PI3K/cPLA(2)/p42/44(MAPK) signaling pathways. Our primary hypothesis is that estradiol stimulates sodium pump activity/expression in VSMC via PI3K/cPLA(2)/p42/44(MAPK) dependent mechanism and, that impaired estradiol-stimulated sodium pump activity/expression in hypertensive rodent models (i.e. SHR), Ang II-mediated vascular impairment of estradiol is related to a decrease ability of estradiol to stimulate the PI3K/cPLA(2)/p42/44(MAPK) signaling pathways. An important corollary to this hypothesis is that in hypertensive state (i.e. SHR rats) the decreasing in ACE enzyme activity and/or AT1 receptor expression caused by administration of estradiol is accompanying with abrogated ability of Ang II to decrease IRS-1/PI3K association, and consequent PI3K/cPLA(2)/p42/44(MAPK) activity and associated sodium pump activity/expression. A clear characterization of how Ang II attenuates estradiol signaling may lead to a better understanding of the molecular mechanism(s) underlying pathophysiological conditions such as hypertension and to understanding how certain pathophysiological situations affect sodium pump activity/expression in VSMC.

Generation of the endocochlear potential: a biophysical model

The generation and maintenance of the endocochlear potential (EP) by the stria vascularis is essential for proper function of the cochlea. We present a mathematical model that captures the critical biophysical interactions between the distinct cellular layers that generate the EP. By describing the relationship between the K(+) concentration in the intrastrial space and the intermediate cell transmembrane potential, we rationalize the presence of a large intermediate cell K(+) conductance and predict that the intrastrial [K(+)] is approximately 4 mM at steady state. The model also predicts that the stria vascularis is capable of buffering the EP against external perturbations in a manner modulated by changes in intrastrial [K(+)], thus facilitating hearing sensitivity across the broad dynamic range of the auditory system.

Inverse correlation between the extent of N-glycan branching and intercellular adhesion in epithelia. Contribution of the Na,K-ATPase beta1 subunit

The majority of cell adhesion molecules are N-glycosylated, but the role of N-glycans in intercellular adhesion in epithelia remains ill-defined. Reducing N-glycan branching of cellular glycoproteins by swainsonine, the inhibitor of N-glycan processing, tightens and stabilizes cell-cell junctions as detected by a 3-fold decrease in the paracellular permeability and a 2-3-fold increase in the resistance of the adherens junction proteins to extraction by non-ionic detergent. In addition, exposure of cells to swainsonine inhibits motility of MDCK cells. Mutagenic removal of N-glycosylation sites from the Na,K-ATPase beta(1) subunit impairs cell-cell adhesion and decreases the effect of swainsonine on the paracellular permeability of the cell monolayer and also on detergent resistance of adherens junction proteins, indicating that the extent of N-glycan branching of this subunit is important for intercellular adhesion. The N-glycans of the Na,K-ATPase beta(1) subunit and E-cadherin are less complex in tight renal epithelia than in the leakier intestinal epithelium. The complexity of the N-glycans linked to these proteins gradually decreases upon the formation of a tight monolayer from dispersed MDCK cells. This correlates with a cell-cell adhesion-induced increase in expression of GnT-III (stops N-glycan branching) and a decrease in expression of GnTs IVC and V (promote N-glycan branching) as detected by real-time quantitative PCR. Consistent with these results, partial silencing of the gene encoding GnT-III increases branching of N-glycans linked to the Na,K-ATPase beta(1) subunit and other glycoproteins and results in a 2-fold increase in the paracellular permeability of MDCK cell monolayers. These results suggest epithelial cells can regulate tightness of cell junctions via remodeling of N-glycans, including those linked to the Na,K-ATPase beta(1)-subunit.

Crystal structure of the sodium-potassium pump

The Na+,K+-ATPase generates electrochemical gradients for sodium and potassium that are vital to animal cells, exchanging three sodium ions for two potassium ions across the plasma membrane during each cycle of ATP hydrolysis. Here we present the X-ray crystal structure at 3.5 A resolution of the pig renal Na+,K+-ATPase with two rubidium ions bound (as potassium congeners) in an occluded state in the transmembrane part of the alpha-subunit. Several of the residues forming the cavity for rubidium/potassium occlusion in the Na+,K+-ATPase are homologous to those binding calcium in the Ca2+-ATPase of sarco(endo)plasmic reticulum. The beta- and gamma-subunits specific to the Na+,K+-ATPase are associated with transmembrane helices alphaM7/alphaM10 and alphaM9, respectively. The gamma-subunit corresponds to a fragment of the V-type ATPase c subunit. The carboxy terminus of the alpha-subunit is contained within a pocket between transmembrane helices and seems to be a novel regulatory element controlling sodium affinity, possibly influenced by the membrane potential.

Purification of the human alpha2 Isoform of Na,K-ATPase expressed in Pichia pastoris. Stabilization by lipids and FXYD1

Human alpha1 and alpha2 isoforms of Na,K-ATPase have been expressed with porcine 10*Histidine-tagged beta1 subunit in Pichia pastoris. Methanol-induced expression of alpha2 is optimal at 20 degrees C, whereas at 25 degrees C, which is optimal for expression of alpha1, alpha2 is not expressed. Detergent-soluble alpha2beta1 and alpha1beta1 complexes have been purified in a stable and functional state. alpha2beta1 shows a somewhat lower Na,K-ATPase activity and higher K0.5K compared to alpha1beta1, while values of K0.5Na and KmATP are similar. Ouabain inhibits both alpha1beta1 (K0.5 24.6 +/- 6 nM) and alpha2beta1 (K0.5 102 +/- 14 nM) with high affinity. A striking difference between the isoforms is that alpha2beta1 is unstable. Both alpha1beta1 and alpha2beta1 complexes, prepared in C12E8 with an added phosphatidyl serine, are active, but alpha2beta1 is rapidly inactivated at 0 degrees C. Addition of low concentrations of cholesterol with 1-stearoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (SOPS) stabilizes strongly, maintaining alpha2beta1 active up to two weeks at 0 degrees C. By contrast, alpha1beta1 is stable at 0 degrees C without added cholesterol. Both alpha1beta1 and alpha2beta1 complexes are stabilized by cholesterol at 37 degrees C. Human FXYD1 spontaneously associates in vitro with either alpha1beta1 or alpha2beta1, to form alpha1beta1/FXYD1 and alpha2beta1/FXYD1 complexes. The reconstituted FXYD1 protects both alpha1beta1 and alpha2beta1 very strongly against thermal inactivation. Instability of alpha2 is attributable to suboptimal phophatidylserine-protein interactions. Residues within TM8, TM9 and TM10, near the alphabeta subunit interface, may play an important role in differential interactions of lipid with alpha1 and alpha2, and affect isoform stability. Possible physiological implications of isoform interactions with phospholipids and FXYD1 are discussed.

Differential expression of Na+/K+-ATPase alpha-subunits in mouse hippocampal interneurones and pyramidal cells

The sodium pump (Na+/K+-ATPase), maintains intracellular and extracellular concentrations of sodium and potassium by catalysing ATP. Three sodium pump alpha subunits, ATP1A1, ATP1A2 and ATP1A3, are expressed in brain. We compared their role in pyramidal cells and a subset of interneurones in the subiculum. Interneurones were identified by their expression of GFP under the GAD-65 promoter. We used the sensitivity to the cardiac glycoside, ouabain, to discriminate between different alpha subunit isoforms. GFP-positive interneurones were depolarized by nanomolar doses of ouabain, but higher concentrations were needed to depolarize pyramidal cells. Comparison of pump currents in these cells revealed a current sensitive to low doses of ouabain in interneurones, while micromolar doses of ouabain were needed to suppress the pump current in subicular pyramidal cells. As predicted, nanomolar doses of ouabain increased the frequency but not the amplitudes of IPSPs in pyramidal cells. Immunostaining confirmed a differential distribution of alpha-subunits of the Na+/K+-ATPase in subicular interneurones and pyramidal cells. In conclusion, these data suggest that while ATP1A3-isoforms regulate sodium and potassium homeostasis in subicular interneurones, ATP1A1-isoforms assume this function in pyramidal cells. This differential expression of sodium pump isoforms may contribute to differences in resting membrane potential of subicular interneurones and pyramidal cells.

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

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

Differential expression of gill Na+,K+-ATPase alpha- and beta-subunits, Na+,K+,2Cl- cotransporter and CFTR anion channel in juvenile anadromous and landlocked Atlantic salmon Salmo salar

This study examines changes in gill Na(+),K(+)-ATPase (NKA) alpha- and beta-subunit isoforms, Na(+),K(+),2Cl(-) cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR I and II) in anadromous and landlocked strains of Atlantic salmon during parr-smolt transformation, and after seawater (SW) transfer in May/June. Gill NKA activity increased from February through April, May and June among both strains in freshwater (FW), with peak enzyme activity in the landlocked salmon being 50% below that of the anadromous fish in May and June. Gill NKA-alpha1b, -alpha3, -beta(1) and NKCC mRNA levels in anadromous salmon increased transiently, reaching peak levels in smolts in April/May, whereas no similar smolt-related upregulation of these transcripts occurred in juvenile landlocked salmon. Gill NKA-alpha1a mRNA decreased significantly in anadromous salmon from February through June, whereas alpha1a levels in landlocked salmon, after an initial decrease in April, remained significantly higher than those of the anadromous smolts in May and June. Following SW transfer, gill NKA-alpha1b and NKCC mRNA increased in both strains, whereas NKA-alpha1a decreased. Both strains exhibited a transient increase in gill NKA alpha-protein abundance, with peak levels in May. Gill alpha-protein abundance was lower in SW than corresponding FW values in June. Gill NKCC protein abundance increased transiently in anadromous fish, with peak levels in May, whereas a slight increase was observed in landlocked salmon in May, increasing to peak levels in June. Gill CFTR I mRNA levels increased significantly from February to April in both strains, followed by a slight, though not significant increase in May and June. CFTR I mRNA levels were significantly lower in landlocked than anadromous salmon in April/June. Gill CFTR II mRNA levels did not change significantly in either strain. Our findings demonstrates that differential expression of gill NKA-alpha1a, -alpha1b and -alpha3 isoforms may be important for potential functional differences in NKA, both during preparatory development and during salinity adjustments in salmon. Furthermore, landlocked salmon have lost some of the unique preparatory upregulation of gill NKA, NKCC and, to some extent, CFTR anion channel associated with the development of hypo-osmoregulatory ability in anadromous salmon.

Diverse biological responses to different cardiotonic steroids

Cardiotonic steroids (CS) such as ouabain, digoxin and bufalin, are steroidal drugs prepared from the seeds and dried leaves of the genus Digitalis, and the skin and parotid gland of amphibians, are used as a cardiac stimulant. Steroids similar or identical to the cardiotonic steroids were identified in human tissues. The available literature unequivocally supports the notion that these endogenous CS function as hormones in mammals. Recent studies show that although similar in structure, the different CS exhibit diverse biological responses. This was shown at the molecular, cellular, tissue and whole animal levels. This review summarizes these diversities, raises a possible explanation for their presence and discusses their implication on the physiological role of the different steroids.

Salt-sensing mechanisms in blood pressure regulation and hypertension

High salt consumption contributes to the development of hypertension and is considered an independent risk factor for vascular remodeling, cardiac hypertrophy, and stroke incidence. In this review, we discuss the molecular origins of primary sensors involved in the phenomenon of salt sensitivity. Based on the analysis of literature data, we conclude that the kidneys and central nervous system (CNS) are two major sites for salt sensing via several distinct mechanisms: 1) [Cl(-)] sensing in renal tubular fluids, primarily by Na(+)-K(+)-Cl(-) cotransporter (NKCC) isoforms NKCC2B and NKCC2A, whose expression is mainly limited to macula densa cells; 2) [Na(+)] sensing in cerebrospinal fluid (CSF) by a novel isoform of Na(+) channels, Na(x), expressed in subfornical organs; 3) sensing of CSF osmolality by mechanosensitive, nonselective cation channels (transient receptor potential vanilloid type 1 channels), expressed in neuronal cells of supraoptic and paraventricular nuclei; and 4) osmolarity sensing by volume-regulated anion channels in glial cells of supraoptic and paraventricular nuclei. Such multiplicity of salt-sensing mechanisms likely explains the differential effects of Na(+) and Cl(-) loading on the long-term maintenance of elevated blood pressure that is documented in experimental models of salt-sensitive hypertension.

Cardiotonic steroids on the road to anti-cancer therapy

The sodium pump, Na(+)/K(+)-ATPase, could be an important target for the development of anti-cancer drugs as it serves as a versatile signal transducer, it is a key player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers. Cardiotonic steroids, known ligands of the sodium pump have been widely used for the treatment of heart failure. However, early epidemiological evaluations and subsequent demonstration of anti-cancer activity in vitro and in vivo have indicated the possibility of developing this class of compound as chemotherapeutic agents in oncology. Their development to date as anti-cancer agents has however been impaired by a narrow therapeutic margin resulting from their potential to induce cardiovascular side-effects. The review will thus discuss (i) sodium pump structure, function, expression in diverse cancers and its chemical targeting and that of its sub-units, (ii) reported in vitro and in vivo anti-cancer activity of cardiotonic steroids, (iii) managing the toxicity of these compounds and the limitations of existing preclinical models to adequately predict the cardiotoxic potential of new molecules in man and (iv) the potential of chemical modification to reduce the cardiovascular side-effects and improve the anti-cancer activity of new molecules.

A novel family of transmembrane proteins interacting with beta subunits of the Na,K-ATPase

We characterized a family consisting of four mammalian proteins of unknown function (NKAIN1, 2, 3 and 4) and a single Drosophila ortholog dNKAIN. Aside from highly conserved transmembrane domains, NKAIN proteins contain no characterized functional domains. Striking amino acid conservation in the first two transmembrane domains suggests that these proteins are likely to function within the membrane bilayer. NKAIN family members are neuronally expressed in multiple regions of the mouse brain, although their expression is not ubiquitous. We demonstrate that mouse NKAIN1 interacts with the beta1 subunit of the Na,K-ATPase, whereas Drosophila ortholog dNKAIN interacts with Nrv2.2, a Drosophila homolog of the Na,K-ATPase beta subunits. We also show that NKAIN1 can form a complex with another beta subunit-binding protein, MONaKA, when binding to the beta1 subunit of the Na,K-ATPase. Our results suggest that a complex between mammalian NKAIN1 and MONaKA is required for NKAIN function, which is carried out by a single protein, dNKAIN, in Drosophila. This hypothesis is supported by the fact that dNKAIN, but not NKAIN1, induces voltage-independent amiloride-insensitive Na(+)-specific conductance that can be blocked by lanthanum. Drosophila mutants with decreased dNKAIN expression due to a P-element insertion in the dNKAIN gene exhibit temperature-sensitive paralysis, a phenotype also caused by mutations in the Na,K-ATPase alpha subunit and several ion channels. The neuronal expression of NKAIN proteins, their membrane localization and the temperature-sensitive paralysis of NKAIN Drosophila mutants strongly suggest that this novel protein family may be critical for neuronal function.

Functional analysis of Na+/K+-ATPase isoform distribution in rat ventricular myocytes

The Na+/K+-ATPase (NKA) is the main route for Na+ extrusion from cardiac myocytes. Different NKA α-subunit isoforms are present in the heart. NKA-α1 is predominant, although there is a variable amount of NKA-α2 in adult ventricular myocytes of most species. It has been proposed that NKA-α2 is localized mainly in T-tubules (TT), where it could regulate local Na+/Ca2+ exchange and thus cardiac myocyte Ca2+. However, there is controversy as to where NKA-α1 vs. NKA-α2 are localized in ventricular myocytes. Here, we assess the TT vs. external sarcolemma (ESL) distribution functionally using formamide-induced detubulation of rat ventricular myocytes, NKA current (IPump) measurements and the different ouabain sensitivity of NKA-α1 (low) and NKA-α2 (high) in rat heart. Ouabain-dependent IPump inhibition in control myocytes indicates a high-affinity NKA isoform (NKA-α2, K1/2 = 0.38 ± 0.16 μM) that accounts for 29.5 ± 1.3% of IPump and a low-affinity isoform (NKA-α1, K1/2 = 141 ± 17 μM) that accounts for 70.5% of IPump. Detubulation decreased cell capacitance from 164 ± 6 to 120 ± 8 pF and reduced IPump density from 1.24 ± 0.05 to 1.02 ± 0.05 pA/pF, indicating that the functional density of NKA is significantly higher in TT vs. ESL. In detubulated myocytes, NKA-α2 accounted for only 18.2 ± 1.1% of IPump. Thus, ∼63% of IPump generated by NKA-α2 is from the TT (although TT are only 27% of the total sarcolemma), and the NKA-α2/NKA-α1 ratio in TT is significantly higher than in the ESL. The functional density of NKA-α2 is ∼4.5 times higher in the T-tubules vs. ESL, whereas NKA-α1 is almost uniformly distributed between the TT and ESL.

The alpha1 subunit of the sodium pump could represent a novel target to combat non-small cell lung cancers

With an overall 5 year survival rate as low as 15% for non-small cell lung cancer (NSCLC), even with surgical intervention and the use of newer molecules in adjuvant chemotherapy, there is an urgent need for new biological targets and associated novel anti-cancer agents. The present study was undertaken to evaluate the potential of the Na(+)/K(+)-ATPase alpha1 subunit as a novel target in NSCLC and revealed that alpha1 expression is markedly higher in a significant proportion of NSCLC clinical samples compared to normal lung tissue. Furthermore, reduction in alpha1 expression in A549 NSCLC cells by anti-alpha1 siRNA resulted in markedly impaired proliferation and migration of these cancer cells. Finally, of three cardenolides investigated, UNBS1450, which is known to bind to Na(+)/K(+)-ATPase and displays potent anti-tumour activity in vivo in experimental models of human NSCLCs, is the most potent inhibitor of Na(+)/K(+)-ATPase isozymes (alpha1beta1, alpha2beta1 and alpha3beta1), most strikingly of alpha1beta1. This was reflected in the compound's more potent anti-proliferative activity in all NSCLC cell lines evaluated (A549, Cal-12T, NCI-H727 and A427); the first three of which over-express alpha1. The marked impairment in A549 NSCLC cell proliferation and migration, and resulting similar morphology following anti-alpha1 siRNA or UNBS1450 treatment, was associated with features of abnormal cytokinesis, mediated in the case of UNBS1450 by disorganization of the actin cytoskeleton. Collectively these data strongly suggest that targeting the Na(+)/K(+)-ATPase alpha1 using specific cardenolides could represent a novel means to combat certain NSCLCs.

Selective basolateral localization of overexpressed Na-K-ATPase β1- and β2- subunits is disrupted by butryate treatment of MDCK cells

The exclusive basolateral localization of the Na-K-ATPase in kidney epithelium is a critical aspect of nephron function. It has been suggested that mislocalized delivery of the Na-K-ATPase to the apical surface in autosomal dominant polycystic kidney disease (ADPKD) is due to the inappropriate expression of an alternative isoform of the β-subunit, the β2-isoform. It has been reported that heterologous expression of this β2-isoform in Madin-Darby canine kidney (MDCK) cells results in apical delivery of the Na-K-ATPase. We created a MDCK cell line containing a tetracycline-inducible promoter and expressed either myc-tagged β2- or flag-tagged β1-subunits to study the surface localization of these β-subunit isoforms in polarized monolayers. We find that the β2-isoform is targeted to the basolateral surface of the plasma membrane in a polarization pattern indistinguishable from the β1-isoform. However, inclusion of butyrate in the growth medium leads to upregulation of overexpressed β1- or β2-subunits and to their appearance at the apical surface. The β2-isoform expressed in MDCK cells does not assemble into α1β2heterodimers with the endogenous α1. Our findings demonstrate that expression of the β2-isoform does not lead to apical localization of the Na-K-ATPase in MDCK cells and provides evidence for an unexpected effect of butyrate on the trafficking of Na pump subunits.

Expression of the α3/β1 isoform of human Na,K-ATPase in the methylotrophic yeastPichia pastoris

Na,K-ATPase is a crucial enzyme for ion homeostasis in human tissues. Different isozymes are produced by assembly of four alpha- and three beta-subunits. The expression of the alpha3/beta1 isozyme is confined to brain and heart. Its heterologous production has so far never been attempted in a lower eukaryote. In this work we explored whether the methylotrophic yeast Pichia pastoris is capable of expressing the alpha3/beta1 isoform of human Na,K-ATPase. cDNAs encoding the alpha(3) and the beta(1)-subunits were cloned under the control of the inducible promoter of Pichia pastoris alcohol oxidase 1. Pichia pastoris could express the single alpha3- and beta1-subunits and even coexpress them after methanol induction. beta1-subunit was produced as a major 44-kDa glycosylated polypeptide and alpha3 as a 110-kDa unglycosylated polypeptide. Expression at the plasma membrane was limited in shaking flask cultures but by cultivating P. pastoris cells in a fermenter there was a 10-fold increase of the number of ouabain binding sites per cell. The exported enzyme was estimated to be about 0.230 mg L(-1) at the end of a bioreactor run. Na,K-ATPase proved active and the dissociation constant of the recombinant enzyme-ouabain interaction was determined.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

The digitalis-like steroid hormones: new mechanisms of action and biological significance

Digitalis-like compounds (DLC) are a family of steroid hormones synthesized in and released from the adrenal gland. DLC, the structure of which resembles that of plant cardiac glycosides, bind to and inhibit the activity of the ubiquitous cell surface enzyme Na(+), K(+)-ATPase. However, there is a large body of evidence suggesting that the regulation of ion transport by Na(+), K(+)-ATPase is not the only physiological role of DLC. The binding of DLC to Na(+), K(+)-ATPase induces the activation of various signal transduction cascades that activate changes in intracellular Ca(++) homeostasis, and in specific gene expression. These, in turn, stimulate endocytosis and affect cell growth and proliferation. At the systemic level, DLC were shown to be involved in the regulation of major physiological parameters including water and salt homeostasis, cardiac contractility and rhythm, systemic blood pressure and behavior. Furthermore, the DLC system has been implicated in several pathological conditions, including cardiac arrhythmias, hypertension, cancer and depressive disorders. This review evaluates the evidence for the different aspects of DLC action and delineates open questions in the field.

Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the alpha3 Na(+)/K(+)-ATPase

The excitability of CNS presynaptic terminals after a tetanic burst of action potentials is important for synaptic plasticity. The mechanisms that regulate excitability, however, are not well understood. Using direct recordings from the rat calyx of Held terminal, we found that a fast Na(+)/K(+)-ATPase (NKA)-mediated post-tetanic hyperpolarization (PTH) controls the probability and precision of subsequent firing. Notably, increasing the concentration of internal Ca(2+) buffers or decreasing Ca(2+) influx led to larger PTH amplitudes, indicating that an increase in [Ca(2+)](i) regulates PTH via inhibition of NKAs. The characterization for the first time of a presynaptic NKA pump current, combined with immunofluorescence staining, identified the alpha3-NKA isoform on calyx terminals. Accordingly, the increased ability of the calyx to faithfully fire during a high-frequency train as it matures is paralleled by a larger expression of alpha3-NKA during development. We propose that this newly discovered Ca(2+) dependence of PTH is important in the post-burst excitability of nerve terminals.

FXYD6 is a novel regulator of Na,K-ATPase expressed in the inner ear

The exquisite sensitivity of the cochlea, which mediates the transduction of sound waves into nerve impulses, depends on the endolymph ionic composition and the endocochlear potential. A key protein in the maintenance of the electrochemical composition of the endolymph is the Na,K-ATPase. In this study, we have looked for the presence in the rat inner ear of members of the FXYD protein family, recently identified as tissue-specific modulators of Na,K-ATPase. Only FXYD6 is detected at the protein level. FXYD6 is expressed in various epithelial cells bordering the endolymph space and in the auditory neurons. FXYD6 co-localizes with Na,K-ATPase in the stria vascularis and can be co-immunoprecipitated with Na,K-ATPase. After expression in Xenopus oocytes, FXYD6 associates with Na,K-ATPase alpha1-beta1 and alpha1-beta2 isozymes, which are preferentially expressed in different regions of the inner ear and also with gastric and non-gastric H,K-ATPases. The apparent K(+) and Na(+) affinities of alpha1-beta1 and alpha1-beta2 isozymes are different. Association of FXYD6 with Na,K-ATPase alpha1-beta1 isozymes slightly decreases their apparent K(+) affinity and significantly decreases their apparent Na(+) affinity. On the other hand, association with alpha1-beta2 isozymes increases their apparent K(+) and Na(+) affinity. The effects of FXYD6 on the apparent Na(+) affinity of Na,K-ATPase and the voltage dependence of its K(+) effect are distinct from other FXYD proteins. In conclusion, this study defines the last FXYD protein of unknown function as a modulator of Na,K-ATPase. Among FXYD protein, FXYD6 is unique in its expression in the inner ear, suggesting a role in endolymph composition.

Dynamic expression of FXYD6 in the inner ear suggests a role of the protein in endolymph homeostasis and neuronal activity

A key protein in the production and in the maintenance of the endocochlear potential is the Na,K-ATPase. Previously, we have shown that FXYD6 is a modulator of the Na,K-ATPase expressed in the inner ear (Delprat et al. [2007] J Biol Chem 282:7450-7456). To investigate the potential role of FXYD6 in inner ear function, we studied the developmental expression of FXYD6. Reverse transcriptase-polymerase chain reaction analysis demonstrates that FXYD6 is present as two splice variants. Both variants coimmunoprecipitate with Na,K-ATPase after expression in Xenopus oocytes. Immunohistochemistry of the cochlea (from birth to postnatal day 30) shows that FXYD6 is expressed in several epithelial cells important for endolymph homeostasis. Marked similarities were found in the developmental expression patterns of FXYD6 and Na,K-ATPase, suggesting functional cooperation between the two proteins in the generation and maintenance of the endocochlear potential and ion composition of the endolymph.

Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: different roles of the N-glycans of their beta subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic alpha-subunit and a glycosylated beta-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the beta-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective alpha subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled beta-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the beta-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase beta1-subunit is the major beta-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase beta1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the beta1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the alpha-subunit to the cytoskeleton. This intercellular beta1-beta1 interaction is also important for formation of cell-cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase beta2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the beta2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase beta-subunit determine apical sorting of this subunit.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Na, K ATPase ?3 subunit (CD298): association with ? subunit and expression on peripheral blood cells

Beta3 subunit is described as one of the Na, K ATPase subunits. Recently, we generated a monoclonal antibody (mAb), termed P-3E10. This mAb was shown to react with the Na, K ATPase beta3 subunit or CD298. By immunofluorescence analysis using mAb P-3E10, it was found that all peripheral blood leukocytes express Na, K ATPase beta3. The presence of beta3 subunit on leukocytes is not in a quantitative polymorphic manner. Upon phytohemagglutinin or phorbol myristate acetate activation, the expression level of the Na, K ATPase beta3 subunit on activated peripheral blood mononuclear cells was not altered in comparison with those of unstimulated cells. Red blood cells (RBCs) of healthy donors showed negative reactivity with mAb P-3E10. However, more than 80% of thalassemic RBCs showed positive reactivity. By immunoprecipitation, moreover, a protein band of 55-65 kDa was precipitated from normal RBC membrane using mAb P-3E10. These results evidenced that the beta3 subunit of Na, K ATPase is expressed on RBC membrane but the epitope recognized by mAb P-3E10 is hidden in normal RBCs. Furthermore, we showed the association of beta3 subunit and alpha subunit of Na, K ATPase. This information is important for further understanding of the functional roles of this molecule.

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

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

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.

[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.

Fiber type-specific immunostaining of the Na+,K+-ATPase subunit isoforms in skeletal muscle: age-associated differential changes

The expression of the Na(+),K(+)-ATPase alpha and beta subunit isoforms in rat skeletal muscle and its age-associated changes have been shown to be muscle-type dependent. The cellular basis underlying these findings is not completely understood. In this study, we examined the expression of Na(+),K(+)-ATPase isoforms in individual fiber types and tested the hypothesis that, with age, the changes in the expression of the isoforms differ among individual fibers. We utilized immunohistochemical techniques to examine the expression of the subunit isoforms at the individual fiber levels. Immunofluorescence staining of the subunit isoforms in both white gastrocnemius (GW) and red gastrocnemius (GR) revealed a predominance of staining on the sarcolemmal membrane. Compared to the skeletal muscle of 6-month-old rats, there were substantial increases in the levels of alpha1, beta1, and beta3 subunit isoforms, and decreases in the levels of alpha2 and beta2 in 30-month-old rats. In addition, we found distinct patterns of staining for the alpha1, alpha2, beta1, and beta2 isoforms in tissue sections from young and aged rats. Muscle fiber-typing was performed to correlate the pattern of staining with specific fiber types. Staining for alpha1 and alpha2 isoforms in the skeletal muscle of young rats was generally evenly distributed among the fibers of GW and GR, with the exception of higher alpha1 levels in slow-twitch oxidative Type I fibers of GR. By contrast, staining for the beta1 and beta2 isoforms in the mostly oxidative fibers and the mostly glycolytic fibers, respectively, was almost mutually exclusive. With age, there was a fiber-type selective qualitative decrease of alpha2 and beta2 in Type IIB fibers, and increase of beta1 in Type IIB fibers and beta2 in Type IID fibers of white gastrocnemius. These results provide, at the individual fiber level, a cellular basis for the differential expression of the Na(+),K(+)-ATPase subunit isoforms in the muscle groups. The data further indicate that the aged-associated changes in expression of the subunit isoforms occur in both a fiber-type specific as well as an across fiber-type manner. Because of the differing biochemical properties of the subunit isoforms, these changes add another layer of complexity in our understanding of the adaptation of the Na-pump in skeletal muscle with advancing age.

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

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

The Na,K-ATPase alpha4 isoform from humans has distinct enzymatic properties and is important for sperm motility

In the rat, the Na,K-ATPase alpha4 isoform exhibits unique enzymatic characteristics and is important for sperm motility. In this work, we studied expression, localization and function of alpha4 in human spermatozoa. We show two catalytically active Na,K-ATPase alpha polypeptides with different ouabain affinity and identified expression of alpha1, alpha4, beta1 and beta3 isoforms in the gametes. In addition, human sperm presented two Na,K-ATPases composed of alpha4, alpha4beta1 and alpha4beta3. Kinetic analysis of these isozymes produced in insect cells showed that, compared with human alpha1beta1, alpha4beta1 and alpha4beta3 exhibit higher Na(+) and lower K(+) affinity and higher sensitivity to ouabain. These particular enzymatic properties suggested a role for alpha4 in sperm function. Using computer-assisted sperm analysis (CASA), we found that ouabain inhibition of alpha4 significantly decreased percentage sperm motility. In contrast, ouabain did not affect linearity of forward progression, amplitude of lateral head displacement, beat cross frequency and sperm straight-line, curvilinear or average path velocities. This suggests a primary role of alpha4 in flagellar motility. Accordingly, we found alpha4 in the sperm tail, predominating in the mid-piece of the flagellum. Therefore, similar to the rat ortholog, human Na,K-ATPase alpha4 isoform has a distinct activity that is essential for sperm function.

Subunit composition and role of Na+,K+-ATPases in ventricular myocytes

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.

Separate Na,K-ATPase genes are required for otolith formation and semicircular canal development in zebrafish

We have investigated the role of Na,K-ATPase genes in zebrafish ear development. Six Na,K-ATPase genes are differentially expressed in the developing zebrafish inner ear. Antisense morpholino knockdown of Na,K-ATPase alpha1a.1 expression blocked formation of otoliths. This effect was phenocopied by treatment of embryos with ouabain, an inhibitor of Na,K-ATPase activity. The otolith defect produced by morpholinos was rescued by microinjection of zebrafish alpha1a.1 or rat alpha1 mRNA, while the ouabain-induced defect was rescued by expression of ouabain-resistant zebrafish alpha1a.1 or rat alpha1 mRNA. Knockdown of a second zebrafish alpha subunit, alpha1a.2, disrupted development of the semicircular canals. Knockdown of Na,K-ATPase beta2b expression also caused an otolith defect, suggesting that the beta2b subunit partners with the alpha1a.1 subunit to form a Na,K-ATPase required for otolith formation. These results reveal novel roles for Na,K-ATPase genes in vestibular system development and indicate that different isoforms play distinct functional roles in formation of inner ear structures. Our results highlight zebrafish gene knockdown-mRNA rescue as an approach that can be used to dissect the functional properties of zebrafish and mammalian Na,K-ATPase genes.

Differential expression proteomics of human colon cancer

The focus of this study was to use differential protein expression to investigate operative pathways in early stages of human colon cancer. Colorectal cancer represents an ideal model system to study the development and progression of human tumors, and the proteomic approach avoids overlooking posttranslational modifications not detected by microarray analyses and the limited correlation between transcript and protein levels. Colon cancer samples, confined to the intestinal wall, were analyzed by expression proteomics and compared with matched samples from normal colon tissue. Samples were processed by two-dimensional gel electrophoresis, and spots differentially expressed and consistent across all patients were identified by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analyses and by Western blot analyses. After differentially expressed proteins and their metabolic pathways were analyzed, the following main conclusions were achieved for tumor tissue: 1) a shift from beta-oxidation, as the main source of energy, to anaerobic glycolysis was observed owed to the alteration of nuclear- versus mitochondrial-encoded proteins and other proteins related to fatty acid and carbohydrate metabolism; 2) lower capacity for Na(+) and K(+) cycling; and 3) operativity of the apoptosis pathway, especially the mitochondrial one. This study of the human colon cancer proteome represents a step toward a better understanding of the metabolomics of colon cancer at early stages confined to the intestinal wall.

An N-terminal Sequence Targets and Tethers Na+ Pump α2 Subunits to Specialized Plasma Membrane Microdomains

Sodium pumps (alphabeta dimers) with the alpha1 isoform of the catalytic (alpha) subunit are expressed in all cells. Additionally, most cells express Na+ pumps with a second alpha isoform. For example, astrocytes and arterial myocytes also express Na+ pumps with the alpha2 isoform. The alpha2 pumps localize to plasma membrane (PM) microdomains overlying "junctional" sarco-/endoplasmic reticulum (S/ER), but the alpha1 pumps are more uniformly distributed. To study alpha2 targeting, we expressed alpha1/alpha2 and alpha2/alpha1 chimeras and 1-90 and 1-120 amino acid N-terminal peptides in primary cultured mouse astrocytes. Immunocytochemistry revealed that alpha2/alpha1 (but not alpha1/alpha2) chimeras markedly reduced native alpha2 (i.e. were "dominant negatives"). N-terminal (1-120 and 1-90 amino acids) alpha2 (and alpha3), but not alpha1 peptides also targeted to the PM-S/ER junctions and were dominant negative for native alpha2 in astrocytes and arterial myocytes. Thus alpha2 and alpha3 have the same targeting sequence. Ca2+ (fura-2) signals in astrocytes expressing the 1-90 alpha2 peptide were comparable to signals in cells from alpha2 null mutants (i.e. functionally dominant negative): 1 microM ATP-evoked Ca2+ transients were augmented, and 100 nM ouabain-induced amplification was abolished. Amino acid substitutions in the 1-120 alpha1 and alpha2 constructs, and in full-length alpha1, revealed that Leu-27 and Ala-35 are essential for targeting/tethering the constructs to PM-S/ER junctions.

Loss of acidification of anterior prostate fluids in Atp12a-null mutant mice indicates that nongastric H-K-ATPase functions as proton pump in vivo

The physiological functions of nongastric (colonic) H-K-ATPase (gene symbol Atp12a), unlike those of Na-K-ATPase and gastric H-K-ATPase, are poorly understood. It has been suggested that it pumps Na+ more efficiently than H+; however, so far, there is no direct evidence that it pumps H+ in vivo. Previously, we found that the nongastric H-K-ATPase alpha-subunit is expressed in apical membranes of rodent anterior prostate epithelium, in a complex with the Na-K-ATPase beta1-subunit. Here we report the effects of Atp12a gene ablation on polarization of the beta1-subunit and secretory function of the anterior prostate. In nongastric H-K-ATPase-deficient prostate, the Na-K-ATPase alpha-subunit resided exclusively in basolateral membranes; however, the beta1-subunit disappeared from apical membranes, demonstrating that beta1 is an authentic subunit of nongastric H-K-ATPase in vivo and that apical localization of beta1 in the prostate is completely dependent on its association with the nongastric H-K-ATPase alpha-subunit. A remarkable reduction in acidification of anterior prostate fluids was observed: pH 6.38 +/- 0.14 for wild-type mice and 6.96 +/- 0.10 for homozygous mutants. These results show that nongastric H-K-ATPase is required for acidification of luminal prostate fluids, thereby providing a strong in vivo correlate of previous functional expression studies demonstrating that it operates as a proton pump.

Kinetics behaviors of Na,K-ATPase: comparison of solubilized and DPPC:DPPE-liposome reconstituted enzyme

We describe and compare the main kinetic characteristics of rabbit kidney Na,K-ATPase incorporated inside-out in DPPC:DPPE-liposomes with the C(12)E(8) solubilized and purified form. In proteoliposomes, we observed that the ATP hydrolysis of the enzyme is favored and also its affinity for Na(+)-binding sites increases, keeping the negative cooperativity with two classes of hydrolysis sites: one of high affinity (K(0.5)=6 microM and 4 microM for reconstituted enzyme and purified form, respectively) and another of low affinity (K(0.5)=0.4 mM and 1.4 mM for reconstituted enzyme and purified form, respectively). Our data showed a biphasic curve for ATP hydrolysis, suggesting the presence of (alphabeta)(2) oligomer in reconstituted Na,K-ATPase similar to the solubilized enzyme. The Mg(2+) concentration dependence in the proteoliposomes stimulated the Na,K-ATPase activity up to 476 U/mg with a K(0.5) value of 0.4 mM. The Na(+) ions also presented a single saturation curve with V(M)=551 U/mg and K(0.5)=0.2 mM with cooperative effects. The activity was also stimulated by K(+) ions through a single curve of saturation sites (K(0.5)=2.8 mM), with cooperative effects and V(M)=641 U/mg. The lipid microenvironment close to the proteic structure and the K(+) internal to the liposome has a key role in enzyme regulation, affecting its kinetic parameters while it can also modulate the enzyme's affinity for substrate and ions.

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.

Different expression and activity of the alpha1 and alpha4 isoforms of the Na,K-ATPase during rat male germ cell ontogeny

Two catalytic isoforms of the Na,K-ATPase, alpha1 and alpha4, are present in testis. While alpha1 is ubiquitously expressed in tissues, alpha4 predominates in male germ cells. Each isoform has distinct enzymatic properties and appears to play specific roles. To gain insight into the relevance of the Na,K-ATPase alpha isoforms in male germ cell biology, we have studied the expression and activity of alpha1 and alpha4 during spermatogenesis and epididymal maturation. This was explored in rat testes at different ages, in isolated spermatogenic cells and in spermatozoa from the caput and caudal regions of the epididymis. Our results show that alpha1 and alpha4 undergo differential regulation during development. Whereas alpha1 exhibits only modest changes, alpha4 increases with gamete differentiation. The most drastic changes for alpha4 take place in spermatocytes at the mRNA level, and with the transition of round spermatids into spermatozoa for expression and activity of the protein. No further changes are detected during transit of spermatozoa through the epididymis. In addition, the cellular distribution of alpha4 is modified with development, being diffusely expressed at the plasma membrane and intracellular compartments of immature cells, finally to localize to the midregion of the spermatozoon flagellum. In contrast, the alpha1 isoform is evenly present along the plasma membrane of the developing and mature gametes. In conclusion, the Na,K-ATPase alpha1 and alpha4 isoforms are functional in diploid, meiotic and haploid male germ cells, alpha4 being significantly upregulated during spermatogenesis. These results support the importance of alpha4 in male gamete differentiation and function.

Prolonged submaximal exercise induces isoform-specific Na+-K+-ATPase mRNA and protein responses in human skeletal muscle

This study investigated effects of prolonged submaximal exercise on Na+-K+-ATPase mRNA and protein expression, maximal activity, and content in human skeletal muscle. We also investigated the effects on mRNA expression of the transcription initiator gene, RNA polymerase II (RNAP II), and key genes involved in protein translation, eukaryotic initiation factor-4E (eIF-4E) and 4E-binding protein 1 (4E-BP1). Eleven subjects (6 men, 5 women) cycled at 75.5% (SD 4.8%) peak O2uptake and continued until fatigue. A vastus lateralis muscle biopsy was taken at rest, fatigue, and 3 and 24 h postexercise. We analyzed muscle for Na+-K+-ATPase α1, α2, α3, β1, β2, and β3, as well for RNAP II, eIF-4E, and 4E-BP1 mRNA expression by real-time RT-PCR and Na+-K+-ATPase isoform protein abundance using immunoblotting. Muscle homogenate maximal Na+-K+-ATPase activity was determined by 3 -O-methylfluorescein phosphatase activity and Na+-K+-ATPase content by [3H]ouabain binding. Cycling to fatigue [54.5 (SD 20.6) min] immediately increased α3( P = 0.044) and β2mRNA ( P = 0.042) by 2.2- and 1.9-fold, respectively, whereas α1mRNA was elevated by 2.0-fold at 24 h postexercise ( P = 0.036). A significant time main effect was found for α3protein abundance ( P = 0.046). Exercise transiently depressed maximal Na+-K+-ATPase activity ( P = 0.004), but Na+-K+-ATPase content was unaltered throughout recovery. Exercise immediately increased RNAP II mRNA by 2.6-fold ( P = 0.011) but had no effect on eIF-4E and 4E-BP1 mRNA. Thus a single bout of prolonged submaximal exercise induced isoform-specific Na+-K+-ATPase responses, increasing α1, α3, and β2mRNA but only α3protein expression. Exercise also increased mRNA expression of RNAP II, a gene initiating transcription, but not of eIF-4E and 4E-BP1, key genes initiating protein translation.

Digitalis: new actions for an old drug

The mechanisms by which digitalis causes its therapeutic and toxic actions have been studied for nearly a half century, revealing a great deal about cardiac cell regulation of intracellular ions via the Na-K-ATPase (NKA) and how it is altered by cardiac glycosides. However, recent observations suggest that digitalis may have additional effects on cardiac cell function in both the short and long term that include intracellular effects, interactions with specific NKA isoforms in different cellular locations, effects on intracellular (including nuclear) signaling, and long-term regulation of intracellular ionic balances through circulating ouabain-like compounds. The purpose of this review is to examine the current status of a number of the newest and most interesting developments in the study of digitalis with a particular focus on cardiac function, although we will also discuss some of the new advances in other relevant cardiovascular effects. This new information has important implications for both our understanding of ionic regulation in normal and diseased hearts as well as for potential avenues for the development of future therapeutic interventions for the treatment of heart failure.

Functional effects of Na+,K+-ATPase gene mutations linked to familial hemiplegic migraine

Familial hemiplegic migraine type 2, an autosomal dominant form of migraine with aura, has been associated with four distinct mutations in the alpha2-subunit of the Na+,K+-ATPase. We have introduced these mutations in the alpha2-subunit of the human Na+,K+-ATPase and the corresponding mutations in the Bufo marinus alpha1-subunit and studied these mutants by expression in Xenopus oocyte. Metabolic labeling studies showed that the mutants were synthesized and associated with the beta-subunit, except for the alpha2HW887R mutant, which was poorly synthesized, and the alpha1BW890R, which was partially retained in the endoplasmic reticulum. [3H]ouabain binding showed the presence of the alpha2HR689Q and alpha2HM731T at the membrane, whereas the alpha2HL764P and alpha2HW887R could not be detected. Functional studies with the mutants of the B. marinus Na+,K+-ATPase showed a reduced or abolished electrogenic activity and a low K+ affinity for the alpha1BW890R mutant. Through different mechanisms, all these mutations result in a strong decrease of the functional expression of the Na+,K+-pump. The decreased activity in alpha2 isoform of the Na+,K+-pump expressed in astrocytes seems an essential component of hemiplegic migraine pathogenesis and may be responsible for the cortical spreading depression, which is one of the first events in migraine attacks.

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.

Mechanisms underlying increases in rat soleus Na+-K+-ATPase activity by induced contractions

Acute regulation of the Na(+)-K(+)-ATPase activity in rat soleus muscle was investigated in response to 15 and 90 min of electrically induced contractile activity (500-ms trains at 30 Hz every 1.5 s). Kinetic measurements of Na(+)-K(+)-ATPase activity, assessed by the 3-O-methylfluorescein K(+)-stimulated phosphatase assay (3-O-MFP), were performed on crude homogenates (Hom) and on tissue separated into two membrane fractions, the sarcolemmal/particulate (SLP) and endosomal (En), in both stimulated (Stim) and contralateral control (Con) muscles. Maximal 3-O-MFP activity (V(max), nmol.mg protein(-1).h(-1)) was elevated (P < 0.05) in Stim by 40% and by 53% in Hom and by 37 and 40% in SLP at 15 and 90 min, respectively. The 38% increase (P < 0.05) in the alpha(2)-isoform subunit distribution in SLP at 15 min, as assessed by quantitative immunoblotting, persisted at 90 min, whereas for En a 42% decrease (P < 0.05) was observed only at 15 min. For the alpha(1)-subunit at 15 min, a 27% decrease (P < 0.05) was observed in En, whereas the 13% increase observed in SLP was not significant (P = 0.08). At 90 min, alpha(1) was increased (P < 0.05) by 14% in SLP and by 29% in En. No changes were observed in beta(1)-subunit distribution in En and SLP regardless of time of stimulation. Immunoprecipitation with antiphosphotyrosine antibody and quantitative immunoblotting with alpha(1)- and alpha(2)-antibodies indicated increases (P < 0.05) in tyrosine phosphorylation of 51% in alpha(2) at 15 min only. These results suggest that the increases in V(max) during contractile activity are mediated both by increased phosphorylation and by translocation of the enzyme to the plasma membrane.

Alterations in the alpha2 isoform of Na,K-ATPase associated with familial hemiplegic migraine type 2

A number of missense mutations in the Na,K-ATPase alpha2 catalytic subunit have been identified in familial hemiplegic migraine with aura. Two alleles (L764P and W887R) showed loss-of-function, whereas a third (T345A) is fully functional but with altered Na,K-ATPase kinetics. This study describes two additional mutants, R689Q and M731T, originally identified by Vanmolkot et al. [Vanmolkot, K. R., et al. (2003) Ann. Neurol. 54, 360-366], which we show here to also be functional and kinetically altered. Both mutants have reduced catalytic turnover and increased apparent affinity for extracellular K(+). For both R689Q and M731T, sensitivity to vanadate inhibition is decreased, suggesting that the steady-state E(1) <==> E(2) poise of the enzyme is shifted toward E(1). Whereas the K'(ATP) is not affected by the R689Q replacement, the M731T mutant has an increase in apparent affinity for ATP. Analysis of the structural changes effected by T345A, R689Q, and M731T mutations, based on homologous replacements in the known crystal structure of the sarcoplasmic reticulum Ca-ATPase, provides insights into the molecular bases for the kinetic alterations. It is suggested that the disease phenotype is the consequence of lowered molecular activity of the alpha2 pump isoform due to either decreased K(+) affinity (T345A) or catalytic turnover (R689Q and M731T), thus causing a delay in extracellular K(+) clearance and/or altered localized Ca(2+) handling/signaling secondary to reduced activity in colocalized Na(+)/Ca(2+) exchange.

Depressed Na+-K+-ATPase activity in skeletal muscle at fatigue is correlated with increased Na+-K+-ATPase mRNA expression following intense exercise

We investigated whether depressed muscle Na+-K+-ATPase activity with exercise reflected a loss of Na+-K+-ATPase units, the time course of its recovery postexercise, and whether this depressed activity was related to increased Na+-K+-ATPase isoform gene expression. Fifteen subjects performed fatiguing, knee extensor exercise at ∼40% maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue, 3 h, and 24 h postexercise and analyzed for maximal Na+-K+-ATPase activity via 3- O-methylfluorescein phosphatase (3- O-MFPase) activity, Na+-K+-ATPase content via [3H]ouabain binding sites, and Na+-K+-ATPase α1-, α2-, α3-, β1-, β2- and β3-isoform mRNA expression by real-time RT-PCR. Exercise [352 (SD 267) s] did not affect [3H]ouabain binding sites but decreased 3- O-MFPase activity by 10.7 (SD 8)% ( P < 0.05), which had recovered by 3 h postexercise, without further change at 24 h. Exercise elevated α1-isoform mRNA by 1.5-fold at fatigue ( P < 0.05). This increase was inversely correlated with the percent change in 3- O-MFPase activity from rest to fatigue (%Δ3- O-MFPaserest-fatigue) ( r = −0.60, P < 0.05). The average postexercise (fatigue, 3 h, 24 h) α1-isoform mRNA was increased 1.4-fold ( P < 0.05) and approached a significant inverse correlation with %Δ3- O-MFPaserest-fatigue( r = −0.56, P = 0.08). Exercise elevated α2-isoform mRNA at fatigue 2.5-fold ( P < 0.05), which was inversely correlated with %Δ3- O-MFPaserest-fatigue( r = −0.60, P = 0.05). The average postexercise α2-isoform mRNA was increased 2.2-fold ( P < 0.05) and was inversely correlated with the %Δ3- O-MFPaserest-fatigue( r = −0.68, P < 0.05). Nonsignificant correlations were found between %Δ3- O-MFPaserest-fatigueand other isoforms. Thus acute exercise transiently decreased Na+-K+-ATPase activity, which was correlated with increased Na+-K+-ATPase gene expression. This suggests a possible signal-transduction role for depressed muscle Na+-K+-ATPase activity with exercise.

Peroxynitrite induced decrease in Na+, K+-ATPase activity is restored by taurine

AIM: Peroxynitrite (ONOO^-) is a powerful oxidant shown to damage membranes. In the present study, the effect of taurine on changes of liver plasma membrane Na⁺, K⁺-ATPase induced by ONOO^- was investigated.

METHODS: Liver plasma membrane was exposed to ONOO^- with or without taurine. Na⁺, K⁺-ATPase activity and lipid peroxidation as thiobarbituric acid reactive substances (TBARS) levels were measured.

RESULTS: Different concentrations of ONOO^- (100, 200, 500, and 1000 μmol/L) were found to decrease liver plasma membrane Na⁺, K⁺-ATPase activity significantly. The depletion of enzyme activity was not concentration dependent. Effects of different concentrations of taurine on liver plasma membrane Na⁺, K⁺-ATPase activity were also measured. Taurine did not cause any increase in enzyme activity. When plasma membranes were treated with 200 μmol/L ONOO^- with different concentrations of taurine, a restoring effect of taurine on enzyme activity was observed. TBARS levels were also measured and taurine was found to decrease the elevated values.

CONCLUSION: Taurine is observed to act as an antioxidant of ONOO^- to decrease lipid peroxidation and thus affect liver plasma membrane Na⁺, K⁺-ATPase by restoring its activity.

Interleukin-2-dependent regulation of Na/K pump in human lymphocytes

The present study provides the first evidence that the abundance of catalytic alpha1-subunit of Na,K-ATPase increases in the course of T cell blast transformation. Immunodepressant cyclosporin A at anti-proliferative doses diminished the induction of alpha1 protein in activated lymphocytes. Furthermore, in competent T cells, IL-2 increases both the transport activity of Na/K pump and the content of Na,K-ATPase alpha1 protein in a time-dependent manner. A correlation was found between the long-term elevation in ouabain-sensitive Rb influxes and the increase in alpha1 protein content in late activated T cells. These results suggest that (1) the increased expression of Na,K-ATPase proteins underlie the cell cycle-dependent upregulation of ion pump during T cell transformation, and (2) IL-2 is involved in the regulated expression of Na,K-ATPase in human lymphocytes.

Na+-K+-ATPase properties in rat heart and skeletal muscle 3 mo after coronary artery ligation

This study was designed to determine whether chronic heart failure (CHF) results in changes in Na(+)-K(+)-ATPase properties in heart and skeletal muscles of different fiber-type composition. Adult rats were randomly assigned to a control (Con; n = 8) or CHF (n = 8) group. CHF was induced by ligation of the left main coronary artery. Examination of Na(+)-K(+)-ATPase activity (means +/- SE) 12 wk after the ligation measured, using the 3-O-methylfluorescein phosphatase assay (3-O-MFPase), indicated higher (P < 0.05) levels in soleus (Sol) (250 +/- 13 vs. 179 +/- 18 nmol.mg protein(-1).h(-1)) and lower (P < 0.05) levels in diaphragm (Dia) (200 +/- 12 vs. 272 +/- 27 nmol.mg protein(-1).h(-1)) and left ventricle (LV) (760 +/- 62 vs. 992 +/- 16 nmol.mg protein(-1).h(-1)) in CHF compared with Con, respectively. Na(+)-K(+)-ATPase protein content, measured by the [(3)H]ouabain binding technique, was higher (P < 0.05) in white gastrocnemius (WG) (166 +/- 12 vs. 135 +/- 7.6 pmol/g wet wt) and lower (P < 0.05) in Sol (193 +/- 20 vs. 260 +/- 8.6 pmol/g wet wt) and LV (159 +/- 10 vs. 221 +/- 10 pmol/g wet wt) in CHF compared with Con, respectively. Isoform content in CHF, measured by Western blot techniques, showed both increases (WG; P < 0.05) and decreases (Sol; P < 0.05) in alpha(1). For alpha(2), only increases [red gastrocnemius (RG), Sol, and Dia; P < 0.05] occurred. The beta(2)-isoform was decreased (LV, Sol, RG, and WG; P < 0.05) in CHF, whereas the beta(1) was both increased (WG and Dia; P < 0.05) and decreased (Sol and LV; P < 0.05). For beta(3), decreases (P < 0.05) in RG were observed in CHF, whereas no differences were found in Sol and WG between CHF and Con. It is concluded that CHF results in alterations in Na(+)-K(+)-ATPase that are muscle specific and property specific. Although decreases in Na(+)-K(+)-ATPase content would appear to explain the lower 3-O-MFPase in the LV, such does not appear to be the case in skeletal muscles where a dissociation between these properties was observed.