Enzymatic properties of human Na,K-ATPase alpha1beta3 isozyme

Na/K Pump Mutations Associated with Primary Hyperaldosteronism Cause Loss of Function

Primary hyperaldosteronism (Conn's syndrome), a common cause of secondary hypertension, is frequently produced by unilateral aldosterone-producing adenomas that carry mutations in ion-transporting genes, including ATP1A1, encoding the Na/K pump's α1 subunit. Whether Na/K pump mutant-mediated inward currents are required to depolarize the cell and increase aldosterone production remains unclear, as such currents were observed in four out of five mutants described so far. Here, we use electrophysiology and uptake of the K⁺ congener ⁸⁶Rb⁺, to characterize the effects of eight additional Na/K pump mutations in transmembrane segments TM1 (delM102-L103, delL103-L104, and delM102-I106), TM4 (delI322-I325 and I327S), and TM9 (delF956-E961, delF959-E961, and delE960-L964), expressed in Xenopus oocytes. All deletion mutants induced abnormal inward currents of different amplitudes at physiological voltages, while I327S lacked such currents. A detailed functional characterization revealed that I327S significantly reduces intracellular Na⁺ affinity without altering affinity for external K⁺. ⁸⁶Rb⁺-uptake experiments show that I327S dramatically impairs function under physiological concentrations of Na⁺ and K⁺. Since Na/K pumps in the adrenal cortex may be formed by association of α1 with β3 instead of β1 subunits, we evaluated whether G99R (another mutant without inward currents when associated with β1) would show inward currents when associated with β3. We found that the kinetic characteristics of either mutant or wild-type α1β3 pumps expressed in Xenopus oocytes to be indistinguishable from those of α1β1 pumps. The observed functional consequences of each hyperaldosteronism mutant point to the loss of Na/K pump function as the common feature of all mutants, which is sufficient to induce hyperaldosteronism.

Evaluating the involvement of cerebral microvascular endothelial Na+/K+-ATPase and Na+-K+-2Cl- co-transporter in electrolyte fluxes in an in vitro blood-brain barrier model of dehydration

The blood-brain barrier (BBB) is involved in brain water and salt homeostasis. Blood osmolarity increases during dehydration and water is osmotically extracted from the brain. The loss of water is less than expected from pure osmotic forces, due to brain electrolyte accumulation. Although the underlying molecular mechanisms are unresolved, the current model suggests the luminally expressed Na⁺-K⁺-2Cl^- co-transporter 1 (NKCC1) as a key component, while the role of the Na⁺/K⁺-ATPase remains uninvestigated. To test the involvement of these proteins in brain electrolyte flux under mimicked dehydration, we employed a tight in vitro co-culture BBB model with primary cultures of brain endothelial cells and astrocytes. The Na⁺/K⁺-ATPase and the NKCC1 were both functionally dominant in the abluminal membrane. Exposure of the in vitro BBB model to conditions mimicking systemic dehydration, i.e. hyperosmotic conditions, vasopressin, or increased [K⁺]_o illustrated that NKCC1 activity was unaffected by exposure to vasopressin and to hyperosmotic conditions. Hyperosmotic conditions and increased K⁺ concentrations enhanced the Na⁺/K⁺-ATPase activity, here determined to consist of the α1 β1 and α1 β3 isozymes. Abluminally expressed endothelial Na⁺/K⁺-ATPase, and not NKCC1, may therefore counteract osmotic brain water loss during systemic dehydration by promoting brain Na⁺ accumulation.

The ubiquitous isoform of Na/K-ATPase (ATP1A1) regulates junctional proteins, connexin 43 and claudin 11 via Src-EGFR-ERK1/2-CREB pathway in rat Sertoli cells

Interaction of Na/K-ATPase with its ligand ouabain has been implicated in the regulation of various biological processes. The objective was to investigate roles of Na/K-ATPase isoforms in formation and function of junctional complexes in Sertoli cells. Primary cultures of Sertoli cells were obtained by enzymatic digestion of 20-day-old rat testes and grown on Matrigel-coated dishes for 7 days. Sertoli cells predominantly expressed the ubiquitous isoform of Na/K-ATPase (ATP1A1), confirmed by immunoblotting, PCR, immunofluorescence, and mass spectrometry. Treatment of Sertoli cells with 50 nM ouabain increased transepithelial electrical resistance (TER) and expression of claudin 11 (tight junctions) and connexin 43 (gap junctions), whereas 1 mM ouabain had opposite effects. Involvement of Src-EGFR-ERK1/2-CREB pathway in ouabain-mediated expression of claudin 11 and connexin 43 was evaluated. Incubation of Sertoli cells with 50 nM ouabain increased content of p-Src, p-EGFR, p-ERK1/2, and p-CREB; in contrast, 1 mM ouabain decreased phosphorylation of these signaling molecules. Preincubation of Sertoli cells with inhibitors of Src and MAPK pathways inhibited ouabain-induced effects on these signaling molecules, TER, and expression of claudin 11 and connexin 43. In conclusion, we inferred that ATP1A1 regulated Sertoli cell tight junctions and gap junctions through the Src-EGFR-ERK1/2-CREB pathway. Ouabain is an endogenous steroid; therefore, its interaction with ATP1A1 may be a critical signaling mechanism for the regulation of Sertoli cell function and male fertility.

Mineralocorticoids modulate the expression of the β-3 subunit of the Na+, K+-ATPase in the renal collecting duct

Renal sodium reabsorption depends on the activity of the Na⁺,K⁺-ATPase α/β heterodimer. Four α (α_1-4) and 3 β (β_1-3) subunit isoforms have been described. It is accepted that renal tubule cells express α₁/β₁ dimers. Aldosterone stimulates Na⁺,K⁺-ATPase activity and may modulate α₁/β₁ expression. However, some studies suggest the presence of β₃ in the kidney. We hypothesized that the β₃ isoform of the Na⁺,K⁺-ATPase is expressed in tubular cells of the distal nephron, and modulated by mineralocorticoids. We found that β₃ is highly expressed in collecting duct of rodents, and that mineralocorticoids decreased the expression of β₃. Thus, we describe a novel molecular mechanism of sodium pump modulation that may contribute to the effects of mineralocorticoids on sodium reabsorption.

Expression and distribution of Na, K-ATPase isoforms in the human uterus

Na, K-ATPase activity relies on the composition of its catalytic alpha, beta, and FXYD constituents, all of which are expressed as multiple isoforms (4alpha, 4beta, and 7 FXYD). We used reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry to study Na, K-ATPase expression in uterine samples from nonlaboring elective and laboring emergency caesarean sections (CSs). Transcripts of alpha1 to 3, beta1 to 3, and FXYD1 isoforms were detected in all samples, but FXYD2 was only present in hysterectomy samples. Abundant immunoreactivity of alpha1 and moderate alpha2 was localized in myometrial smooth muscle and secretory glands of all groups. Smooth muscle and gland epithelia showed diffuse cytoplasmic alpha3 immunoreactivity. beta isoforms were detected in all groups but beta3 showed much denser immunoreactivity in myometrial samples taken from women in labor. In pregnancy, there was a switch in isoform expression, resulting in increased beta3 and decreased FXYD2 at the protein and messenger RNA (mRNA) levels. Na, K-ATPase isoform alterations may modulate uterine contractility during labor.

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.

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.

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.

The human Na,K-ATPase alpha4 isoform is a ouabain-sensitive alpha isoform that is expressed in sperm

The Na,K-ATPase generates electrochemical gradients across the plasma membrane that are responsible for numerous cellular and physiological processes. The active Na,K-ATPase is minimally composed of an alpha and a beta subunit and families of isoforms for both subunits exist. Recent studies have identified a physiological role for the rat Na,K-ATPase alpha4 isoform in sperm motility. However, very little is known about the human Na,K-ATPase alpha4 isoform other than its genomic sequence and structure and its mRNA expression pattern. Here, the human alpha4 isoform of the Na,K-ATPase is cloned, expressed, and characterized. Full length cDNAs encoding the putative human alpha4 isoform of the Na,K-ATPase were identified from a number of ESTs and a protein product corresponding to this isoform was shown to be expressed from these cDNAs. The human Na,K-ATPase alpha4 isoform protein was found to be expressed in mature sperm in human testes sections and it is localized specifically to the principle piece of human sperm. In addition, the presence of the Na,K-ATPase alpha4 isoform is absent in immature testes however its expression appears coincident with sexual maturity. And finally, the human Na,K-ATPase alpha4 isoform was shown to be as sensitive to cardiac glycoside inhibition as the human Na,K-ATPase alpha1 isoform. Considering the important role of the rat Na,K-ATPase alpha4 isoform in rat sperm motility, the demonstration that the human alpha4 isoform is a sperm-specific protein localized to the flagellum suggests a role for the human Na,K-ATPase alpha4 isoform in human sperm physiology.

Accumulation of βm, a structural member of X,K-ATPase β-subunit family, in nuclear envelopes of perinatal myocytes

Recently discovered muscle-specific βmprotein is structurally closely related to the X,K-ATPase β-subunits. However, it has a number of unique properties such as predominant localization in intracellular stores and lack of association with known X,K-ATPase α-subunits on heterologous coexpression. In this study, the primary structure of mouse βmwas determined and developmental regulation of the gene (ATP1B4) was analyzed. The expression is first detected at day 14 of gestation, is sharply increased at day 16, and reaches its maximum at day 18. After birth, the expression quickly decreases and is hardly detectable in adult mice. A more detailed subcellular localization study was undertaken, and its results indicate that βmnot only is located in sarcoplasmic reticulum but is concentrated in nuclear envelopes of both prenatal and postnatal skeletal muscles. Immunohistochemical studies show that βmis specific to myocytes and, at the subcellular level, many nuclear envelopes are intensively labeled in both fetal and newborn skeletal muscles. Accordingly, βmis detected by immunoblotting in purified nuclei and nuclear membranes from neonatal skeletal muscles. On transfection of human rhabdomyosarcoma cell line RD, green fluorescent protein-tagged βmresides intracellularly with significant enrichment in nuclear envelopes, whereas βmwith transmembrane domain deleted localizes in both cytoplasm and nucleoplasm. Nuclear βmapparently is not in association with Na,K-ATPase because we never detected its α-subunit in myonuclear membranes. These results indicate that βmhas a specialized function in mammalian perinatal myocytes, different from functions of other X,K-ATPase β-subunits. The unique temporospatial distribution of βmprotein expression suggests its important role in development of growing skeletal muscle.

Isoform specificity of human Na(+), K(+)-ATPase localization and aldosterone regulation in mouse kidney cells

Short-term aldosterone coordinately regulates the cell-surface expression of luminal epithelial sodium channels (ENaC) and of basolateral Na(+) pumps (Na(+), K(+)-ATPase alpha1-beta1) in aldosterone-sensitive distal nephron (ASDN) cells. To address the question of whether the subcellular localization of the Na(+), K(+)-ATPase and its regulation by aldosterone depend on subunit isoform-specific structures, we expressed the cardiotonic steroid-sensitive human alpha isoforms 1-3 by retroviral transduction in mouse collecting duct mpkCCD(c14) cells. Each of the three exogenous human isoforms could be detected by Western blotting. Immunofluorescence indicated that the exogenous alpha1 subunit to a large extent localizes to the basolateral membrane or close to it, whereas much of the alpha2 subunit remains intracellular. An ouabain-sensitive current carried by exogenous pumps could be detected in apically amphotericin B-permeabilized epithelia expressing human alpha1 and alpha2 subunits, but not the alpha3 subunit. This current displayed a higher apparent Na(+) affinity in pumps containing human alpha2 subunits (10 mM) than in pumps containing human alpha1 (33.2 mM) or endogenous (cardiotonic steroid-resistant) mouse alpha1 subunits (mean: 16.3 mM). A very low mRNA level of the Na(+), K(+)-ATPase gamma subunit (FXYD2) in mpkCCD(c14) cells suggested that this ancillary gene product is not responsible for the relatively low apparent Na(+) affinity measured for a1 subunit-containing pumps. Aldosterone increased the pump current carried by endogenous pumps and by pumps containing the human alpha1 subunit. In contrast, the current carried by pumps with a human alpha2 subunit was not stimulated by the same treatment. In summary, quantitative basolateral localization of the Na(+), K(+)-ATPase and its responsiveness to aldosterone require alpha1 subunit-specific sequences that differentiate this isoform from the alpha2 and alpha3 subunit isoforms.

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

Na,K-ATPase plays a crucial role in cellular ion homeostasis and is the pharmacological receptor for digitalis in man. Nine different human Na,K-ATPase isozymes, composed of 3 alpha and beta isoforms, were expressed in Xenopus oocytes and were analyzed for their transport and pharmacological properties. According to ouabain binding and K(+)-activated pump current measurements, all human isozymes are functional but differ in their turnover rates depending on the alpha isoform. On the other hand, variations in external K(+) activation are determined by a cooperative interaction mechanism between alpha and beta isoforms with alpha2-beta2 complexes having the lowest apparent K(+) affinity. alpha Isoforms influence the apparent internal Na(+) affinity in the order alpha1 > alpha2 > alpha3 and the voltage dependence in the order alpha2 > alpha1 > alpha3. All human Na,K-ATPase isozymes have a similar, high affinity for ouabain. However, alpha2-beta isozymes exhibit more rapid ouabain association as well as dissociation rate constants than alpha1-beta and alpha3-beta isozymes. Finally, isoform-specific differences exist in the K(+)/ouabain antagonism which may protect alpha1 but not alpha2 or alpha3 from digitalis inhibition at physiological K(+) levels. In conclusion, our study reveals several new functional characteristics of human Na,K-ATPase isozymes which help to better understand their role in ion homeostasis in different tissues and in digitalis action and toxicity.

Cellular and subcellular specification of Na,K-ATPase alpha and beta isoforms in the postnatal development of mouse retina

The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha1 was found in Müller and horizontal cells, alpha2 in some Müller glia, and alpha3 in photoreceptors and all retinal neurons. beta1 was largely restricted to horizontal, amacrine, and ganglion cells; beta2 was largely restricted to photoreceptors, bipolar cells, and Müller glia; and beta3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha3 and beta2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta1 expression in horizontal cells preceded migration, whereas increases in beta2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.

Plasticity of Na,K-ATPase isoform expression in cultures of flat astrocytes: species differences in gene expression

The Na,K-ATPase plays an active role in glial physiology, contributing to K+ uptake as well as to the Na+ gradients used by other membrane carriers. There are multiple isoforms of Na,K-ATPase alpha and beta subunits, and different combinations result in different affinities for Na+ and K+. Isoform choice should thus influence K+ and Na+ homeostasis in astrocytes. Prior studies of astrocyte Na,K-ATPase subunit composition have produced apparently conflicting results, suggesting plasticity of gene expression. Purified flat astrocytes from the cerebral cortex and cerebellum of both mouse and rat were systematically investigated here. Using antibodies specific for the alpha1, alpha2, alpha3, beta1, beta2, and beta3 subunits, isoform level was assessed with Western blots, and cellular distribution was visualized with immunofluorescence. Although alpha1 was always expressed, differences were observed in the expression of alpha2 and beta2, subunits that can be expressed in astrocytes in vivo and in coculture with neurons. In addition, abundant alpha subunit was expressed in rat astrocytes and in mouse cerebellar astrocytes without an equivalent level of any of the known beta isoforms, suggesting that an additional beta subunit important for glia is yet to be discovered. Conditions that have been shown to increase Na,K-ATPase activity in astrocyte cultures, such as dibutyryl cAMP, high extracellular K+, and glutamate, did not specifically induce missing subunits, suggesting that cellular interactions are required to alter the ion transporter phenotype.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.