Molecular genetics of Na,K-ATPase

The epitope for the inhibitory antibody M7-PB-E9 contains Ser-646 and Asp-652 of the sheep Na+,K(+)-ATPase alpha-subunit

The binding of monoclonal antibody M7-PB-E9 to the alpha-subunit of Na+,K(+)-ATPase partially inhibits enzyme activity (35%) in competition with ATP, while in the presence of magnesium it stimulates the rate of ouabain binding severalfold [Ball, W. J. (1984) Biochemistry 23, 2275-2281]. These effects have been shown to result from an antibody-induced shifting of the enzyme's E1 <==> E2 conformational equilibrium to the right that affects all enzyme-ligand interactions except that with Mg2+ [Abbott, A.J., & Ball, W.J. (1992) Biochemistry 31, 11236-11243]. In order to identify the location of the M7-PB-E9 epitope, proteolytic fragments of the lamb kidney enzyme were generated and the immunoreactive alpha fragments were identified by Western blot analyses. These studies revealed a 47-kDa tryptic fragment, which bound both M7-PB-E9 and a -COOH terminus specific antisera and NH2-terminal sequencing showed to originate at Ala-590. Digestion with Staphylococcus aureus V8 protease produced a 36-kDa -COOH-terminus fragment which originated at Gly-697 and did not contain the antibody epitope. Thus the intracellular sequence region Ala-590 to Gly-697 was shown to contain the antibody epitope. When M7-PB-E9's ability to recognize the alpha subunits from various species and tissues was determined and correlated with available sequencing data, only Ser-646 was present in the highly reactive lamb, pig, and avian kidney alpha 1 proteins and altered (Asn) in the poorly recognized Xenopus and rat kidney and Torpedo electroplax organ enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional expression of the alpha 2 and alpha 3 isoforms of the Na,K-ATPase in baculovirus-infected insect cells

Multiple isoforms of both the alpha and beta subunits of Na,K-ATPase have been identified. Elucidating their roles has been complicated by the fact that most tissues express multiple isoforms and purification techniques specific for each isoform have not been achieved. The baculovirus expression system, which uses the baculovirus Autographica californica to infect insect cells, is an ideal system for studying the Na,K-ATPase isoforms since high amounts of foreign proteins can be produced and some insect cell lines have low levels of endogenous Na,K-ATPase. Recombinant baculoviruses containing the cDNAs for the alpha 2, alpha 3, and beta 1 isoforms of the rat Na,K-ATPase were prepared and used to infect Sf-9 cells, an insect cell line derived from the ovary of the fall armyworm Spodoptera frugiperda. By using this system, Na,K-ATPase alpha 2 and alpha 3 subunits that were antigenically and electrophoretically indistinguishable from the native subunits were produced. When each subunit is expressed independently in the Sf-9 cells, it is primarily delivered to the plasma membrane. Although the isolated expression of each Na,K-ATPase subunit did not render active Na,K-ATPase molecules, the coexpression of alpha 2 or alpha 3 with beta 1 resulted in catalytically active molecules. This activity could be measured as a ouabain-sensitive ATPase activity or directly demonstrated using either [gamma-32P]ATP or 32Pi to identify the phosphorylated intermediates of the alpha 2 and alpha 3 isoforms. [3H]Ouabain binding studies showed that both isoforms are capable of binding the cardiotonic steroid with high affinity, alpha 3 being more sensitive to ouabain. These results demonstrate that the baculovirus system is suitable for the expression of the Na,K-ATPase isoforms and should provide a useful method for the characterization of the enzymatic properties of each isoform.

Voltage-dependent inhibition of the sodium pump by external sodium: species differences and possible role of the N-terminus of the alpha-subunit

Currents generated by the Na+/K+ ATPase were measured under voltage clamp in oocytes of Xenopus laevis. The dependence of pump current on external [Na+] was investigated for the endogenous Xenopus pump as well as for wild-type and mutated pumps of electroplax of Torpedo californica expressed in the oocytes. The mutants had alpha-subunits truncated before position Lys28 (alpha delta K28) or Thr29 (alpha delta T29) of the N-terminus. The currents generated by all variants of pump molecules in the presence of 5 mM K+ show voltage-dependent inhibition by external [Na+]. The apparent KI values increase with membrane depolarisation, and the potential dependence can be described by the movement of effective charges in the electrical potential gradient across the membrane. Taking into account Na(+)-K+ competition for external binding to the E2P form, apparent KI values and effective charges for the interaction of the Na+ ions with the E2P form can be estimated. For the Xenopus pump the effective charge amounts to 1.1 of an elementary charge and the KI value at 0 mV to 44 mM. For the wild-type Torpedo pump, the analysis yields values of 0.73 of an elementary charge and 133 mM, respectively. Truncation at the N-terminus removing a lysine-rich cluster of the alpha-subunit of the Torpedo pump leads to an increase of the effective charge and decrease of the KI value. For alpha delta K28, values of 0.83 of an elementary charge and 117 mM are obtained, respectively. If Lys28 is included in the truncation (alpha delta T29), the effective charge increases to 1.5 of an elementary charge and the apparent KI value is reduced to 107 mM. The KI values for pump inhibition by external Na+, calculated by taking into account Na(+)-K+ competition, are smaller than the K1/2 values determined in the presence of 5 mM [K+]. The difference is more pronounced for those pump variants that have higher Km values. The variations of the parameters describing inhibition by external [Na+] are qualitatively similar to those described for the stimulation of the pumps by external [K+] in the absence of extracellular [Na+]. The observations may be explained by an access channel within the membrane dielectric that has to be passed by the external Na+ and K+ ions to reach or leave their binding sites. The potential-dependent access and/or the interaction with the binding sites shows species differences and is affected by cytoplasmic lysine residues in the N-terminus.

Na,K-ATPase expression in C2C12 cells during myogenesis: minimal contribution of alpha 2 isoform to Na,K transport

Cells of the murine skeletal muscle line, C2C12, undergo differentiation from mononuclear myoblasts to multinuclear myotubes that express a number of proteins associated with striated muscle. We examined the relationship between the abundance of the mRNAs encoding the fast-twitch Ca-ATPase and the alpha isoforms of Na,K-ATPase and the subsequent expression of their respective polypeptides. Both the mRNA and protein levels of the alpha 1 isoform remained constant throughout differentiation. In contrast, the content of mRNAs encoding the alpha 2 isoform and fast-twitch Ca-ATPase increased coordinately with the abundance of their corresponding polypeptides during myotube development. Despite the dramatic increase in alpha 2 expression, estimates of in vitro Na,K-ATPase activity and assessments of in vivo transport activity suggest that alpha 2 contributes little to ionic homeostasis in C2C12 myotubes.

Cordil reversibly inhibits the Na,K-ATPase from outside of the cell membrane. Role of K-dependent dephosphorylation

Cordil-LND796 is a new cardiotonic glycoside under development. In rat brain microsomes where three isoforms of the Na,K-ATPase with differential affinities for cardiac glycosides have been identified, Cordil had higher affinity for the alpha 3 (IC50 = 0.02 microM) than for the alpha 2 (IC50 = 0.6 microM) and the alpha 1 (IC50 = 30 microM) isozymes. Cordil is potentially a selective inhibitor for both alpha 2 and alpha 3 Na,K-ATPase isoforms. Using inside out vesicles we have shown that Cordil binds to and inhibits Na,K-ATPase at an extracellular site. The dissociation kinetic rates (k-1) from the ATPase and the phosphatase activity (K-dependent dephosphorylation) of the Na,K-ATPase were similar for Cordil. Despite these similarities to ouabain comparison of the kinetics of the Na,K-ATPase inhibition by ouabain and Cordil revealed marked differences in their association rates (k+1 = 0.7 l mol-1 min-1 and k+1 = 6 x 10(-3) l mol-1 min-1 respectively) and their dissociation rates (k-1 = 1.3 +/- 0.2 x 10(-4) s-1 and k-1 = 69 +/- 7 x 10(-4) s-1 respectively). Both binding association and dissociation rates were enhanced for Cordil. These data are compatible with a stabilizing effect of Cordil on the E2P conformational state of Na,K-ATPase.

Molecular basis for the insensitivity of the Monarch (Danaus plexippus) to cardiac glycosides

The Monarch (Danaus plexippus) sequesters cardiac glycosides for its chemical defence against predators. Larvae and adults of this butterfly are insensitive towards dietary cardiac glycosides, whereas other Lepidoptera, such as Manduca sexta and Creatonotos transiens are sensitive and intoxicated by ouabain. Ouabain inhibits the Na+,K(+)-ATPase by binding to its alpha-subunit. We have amplified and cloned the DNA sequence encoding the respective ouabain binding site. Instead of the amino acid asparagine at position 122 in ouabain-sensitive insects, the Monarch has a histidine in the putative ouabain binding site, which consists of about 12 amino acids. This change may explain the ouabain insensitivity.

Ouabain--a link in the genesis of high blood pressure?

Hypertension or high blood pressure is a risk factor that increases risk of myocardial infarction, renal failure or cerebral stroke. The pathogenesis of hypertension is due to a variety of causes, including inherited predisposition, dietary habits, especially salt intake, smoking, and also 'general lifestyle'. But for the scientist interested in the complex interplay of physiological and molecular factors, the actual causes of high blood pressure remain uninvestigated. The following article is concerned with new reports that ouabain, a plant derivative, occurs in human beings, in whom it appears to have a hormonal function; ouabain may even play a key role in the pathogenesis of hypertension. We are thus brought a step closer to the background of cardiovascular disease; we may also be afforded a lead to a new therapeutic principle.

Isoforms of the Na,K-ATPase are present in both axons and dendrites of hippocampal neurons in culture

The distributions of isoforms of the Na,K-ATPase alpha subunit were determined in mature cultured hippocampal neurons and in a polarized epithelial cell line. We find that hippocampal neurons express the alpha 1 and alpha 3 isoforms in the membranes of both axons and dendrites. In contrast the alpha 1 and alpha 3 proteins are exclusively basolateral when expressed endogenously or by stable transfection in renal epithelial cells. These data suggest that epithelial cells and hippocampal neurons localize these proteins by different mechanisms. These observations contrast with those made for the vesicular stomatitis virus and the influenza glycoproteins, which are polarized in both epithelial and neuronal cells.

Inhibition of α2/α3 sodium pump isoforms potentiates glutamate neurotoxicity

Excessive stimulation of neurons by glutamic acid initiates a destructive cascade of ion fluxes, cellular swelling, and death. Homeostatic mechanisms which rectify these disturbances depend largely upon transmembrane ion gradients maintained by Na+,K(+)-ATPase (NaP). We proposed that the neurotoxicity of glutamate is enhanced when the NaP capacity is exceeded, and therefore, that the degree of neuronal death varies inversely with endogenous NaP activity. To test this concept, we directly reduced NaP activity in cultured rat telencephalic cells using either the specific inhibitor ouabain, or dcAMP, and assessed whether these treatments increased glutamate-induced neuronal death. Since rodent NaP catalytic subunits possess both low (alpha 1) and high (alpha 2/alpha 3) affinity for ouabain, we were able to inhibit selectively the alpha 2 (principally glial) and alpha 3 (neuronal) catalytic subunits without affecting the alpha 1 isoform. Brief exposures (5-60 min) to high ouabain concentrations (1-10 mM), which blocks the activity of all three catalytic subunits, killed differentiated neurons but spared glia. In contrast, differential inhibition of the alpha 2/alpha 3 isoforms (by 1 microM ouabain) was not of itself toxic, but produced a supersensitivity to glutamate. [3H]Ouabain binding studies confirmed that the glutamate neurotoxicity observed varied inversely with the degree of NaP inhibition. Further, this relationship was not absolutely dependent upon ouabain, since reductions in alpha 2/alpha 3 pump activity induced by dcAMP also amplified glutamate toxicity. We conclude that inhibition of neuronal NaP with high affinity for ouabain is not lethal to unstimulated cells, but markedly increases susceptibility to glutamate excitotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Activity-dependent regulation of Na+, K(+)-ATPase alpha isoform mRNA expression in vivo

To investigate the functional role of the different Na+, K(+)-ATPase alpha (catalytic) subunit isoforms in neuronal cells, we used quantitative in situ hybridization with riboprobes specific for alpha 1, alpha 2, and alpha 3 isoforms to measure the level of alpha isoform-specific expression in the neuroendocrine cells of the supraoptic (SON) and paraventricular (PVN) nuclei of rat hypothalamus. A prolonged increase in electrical activity of these cells, achieved by 5 days of salt treatment, increased the amount of alpha 1 isoform mRNA in the SON and PVN by 50%. Levels of alpha 1 mRNA in other brain regions and levels of alpha 2 and alpha 3 mRNAs were not affected by salt treatment. We conclude that the alpha 1 isoform Na+, K(+)-ATPase may be specifically adapted to pump out Na+, which enters the cells through voltage-gated channels during neuronal depolarization.

Regulation of alpha 1-beta 3-NA(+)-K(+)-ATPase isozyme during meiotic maturation of Xenopus laevis oocytes

During progesterone-induced maturation of Xenopus oocytes, the transport and ouabain binding capacity of Na(+)-K(+)-ATPase at the plasma membrane is completely downregulated. To elucidate the mechanism and the physiological significance of this process, we have followed the fate of oocyte alpha-beta 3-Na(+)-K(+)-ATPase complexes during meiotic maturation and early embryonic development. An immunocytochemical follow-up of the catalytic alpha-subunit, ouabain binding studies, cell surface iodination, and oocyte cell fractionation combined with immunochemical subunit detection provides evidence that following progesterone treatment Na(+)-K(+)-ATPase molecules are retrieved from the oocyte plasma membrane. The enzyme complexes are recovered in an active form in an intracellular compartment in both in vitro and in vivo matured eggs. Exogenous Xenopus alpha 1- and beta 1-complexes expressed in the oocyte from injected cRNAs are regulated by progesterone similar to endogenous Na(+)-K(+)-ATPase complexes. Finally, active Na(+)-K+ pumps internalized during oocyte maturation appear to be redistributed to plasma membrane fractions during blastula formation in Xenopus embryos. In conclusion, our data suggest that endocytosis of alpha 1- and beta 3-complexes during meiotic maturation of Xenopus oocytes is responsible for downregulation of Na(+)-K(+)-ATPase activity and results in an intracellular pool of functional enzymes, which might be reexpressed during early development in response to physiological needs.

The A2 isoform of rat Na+,K(+)-adenosine triphosphatase is active and exhibits high ouabain affinity when expressed in transfected fibroblasts

The alpha isoforms of the Na+,K(+)-ATPase (Na+ pump) are expressed with developmental and tissue heterogeneity in rodents and possess different sensitivity to inhibition by ouabain. We directly characterized the ouabain sensitivity of the rat A2 (alpha 2) isoform by transfecting NIH 3T3 cells with rat A2. The treated cells exhibit high affinity (40 nM) ouabain binding with a density of 2 pmol/mg protein. 86Rb+ flux studies confirm that A2 is functional in this system and that A2 is inhibited by submicromolar concentrations of ouabain. These findings are consistent with measurements of ouabain affinity in tissues which express the A2 isoform.

Na,K-ATPase: isoform structure, function, and expression

An interesting feature of the Na,K-ATPase is the multiplicity of alpha and beta isoforms. Three isoforms exist for the alpha subunit, alpha 1, alpha 2, and alpha 3, as well for the beta subunit, beta 1, beta 2, and beta 3. The functional significance of these isoforms is unknown, but they are expressed in a tissue- and developmental-specific manner. For example, all three isoforms of the alpha subunit are present in the brain, while only alpha 1 is present in kidney and lung, and alpha 2 represents the major isoform in skeletal muscle. Therefore, it is possible that each of these isoforms confers different properties on the Na,K-ATPase which allows effective coupling to the physiological process for which it provides energy in the form of an ion gradient. It is also possible that the multiple isoforms are the result of gene triplication and that each isoform exhibits similar enzymatic properties. In this case, the expression of the triplicated genes would be individually regulated to provide the appropriate amount of Na,K-ATPase to the particular tissue and at specific times of development. While differences are observed in such parameters as Na+ affinity and sensitivity to cardiac glycosides, it is not known if these properties play a functional role within the cell. Site-directed mutagenesis has identified amino acid residues in the first extracellular region of the alpha subunit as major determinants in the differential sensitivity to cardiac glycosides. Similar studies have failed to identify residues in the second extracellular region involved in cardiac glycoside inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of ouabain high-affinity binding to rat cerebral cortex. Modulation by melatonin

High-affinity [3H]ouabain binding to membrane preparations of rat cerebral cortex was examined using a rapid filtration procedure. At 37 degrees C, binding reached equilibrium in about 60 min. Scatchard analyses of the data at equilibrium revealed a single population of binding sites with a dissociation constant of KD = 3.1 +/- 0.36 nM and a binding site concentration of Bmax = 246.4 +/- 18.4 fmol/mg protein. Kinetic analyses of the association and dissociation curves indicated a kinetic KD = 4.6 nM, which is in good agreement with the value obtained at equilibrium. When various digitalis compounds were tested for their ability to inhibit [3H]ouabain binding, the following Ki values (nM) were obtained: ouabain (3.9); digoxin (18); acetyl-digitoxin (66); k-strophanthin (95); digitoxin (236). When melatonin was added to the incubation medium, the ability of ouabain to inhibit [3H]ouabain binding increased in a dose-related manner to yield the following Ki values (nM): melatonin 10 nM (2); melatonin 20 nM (1.2); melatonin 40 nM (0.8). These data suggest the existence in the rat cerebral cortex of high-affinity ouabain binding sites which may be a locus for the molecular action of melatonin.

Role of renal Na+,K(+)-ATPase in the regulation of sodium excretion under normal conditions and in acute congestive heart failure

BACKGROUND

Cardiac glycosides have traditionally been used as inotropic agents in the treatment of congestive heart failure (CHF). The renal actions of cardiac glycosides independent of inotropic effects are not characterized. The presence of endogenous digitalis-like factors (EDLFs) with characteristic Na+,K(+)-ATPase activity has been postulated in volume-expanded states such as CHF, and recent studies have demonstrated that at least one EDLF shares structural homology with ouabain. This study was undertaken to evaluate the renal actions of ouabain in normal dogs and those with CHF.

METHODS AND RESULTS

After surgical preparation, normal dogs (n = 6) and dogs in pacing-induced CHF (n = 6) received intrarenal ouabain in sequential doses of 0.167 micrograms/kg/min, 0.334 micrograms/kg/min, and 0.668 micrograms/kg/min. Hemodynamics and renal function were evaluated during the infusion. There was no change in heart rate or mean arterial pressure during the infusion compared with baseline in both groups. Sodium excretion and urine volume significantly increased in both groups. Plasma renin activity, activated by the onset of pacing in the CHF group, was inhibited by the administration of intrarenal ouabain in this group only.

CONCLUSIONS

These studies demonstrate that ouabain has diuretic and natriuretic actions independent of cardiac hemodynamics that are preserved in CHF. Furthermore, intrarenal ouabain suppresses activation of renin in CHF.

Effects of ouabain on the rotational dynamics of renal Na,K-ATPase studied by saturation-transfer EPR

The interaction of a nitroxide spin-labeled derivative of ouabain with sheep kidney Na,K-ATPase and the motional behavior of the ouabain spin label-Na,K-ATPase complex have been studied by means of electron paramagnetic resonance (EPR) and saturation-transfer EPR (ST-EPR). Spin-labeled ouabain binds with high affinity to the Na,K-ATPase with concurrent inhibition of ATPase activity. Enzyme preparations retain 0.61 +/- 0.1 mol of bound ouabain spin label per mole of ATP-dependent phosphorylation sites, even after repeated centrifugation and resuspension of the purified ATPase-containing membrane fragments. The conventional EPR spectrum of the ouabain spin label bound to the ATPase consists almost entirely (greater than 99%) of a broad resonance at 0 degrees C, characteristic of a tightly bound spin label which is strongly immobilized by the protein backbone. Saturation-transfer EPR measurements of the spin-labeled ATPase preparations yield effective correlation times for the bound labels significantly longer than 100 microseconds at 0 degrees C. Since the conventional EPR measurements of the ouabain spin-labeled Na,K-ATPase indicated the label was strongly immobilized, these rotational correlation times most likely represent the motion of the protein itself rather than the independent motion of mobile spin probes relative to a slower moving protein. Additional ST-EPR measurements of ouabain spin-labeled Na,K-ATPase (a) cross-linked with glutaraldehyde and (b) crystallized in two-dimensional arrays indicated that the observed rotational correlation times predominantly represented the motion of large Na,K-ATPase-containing membrane fragments, as opposed to the motion of individual monomeric or dimeric polypeptides within the membrane fragment. The results suggest that the binding of spin-labeled ouabain to the ATPase induces the protein to form large aggregates, implying that cardiac glycoside induced enzyme aggregation may play a role in the mechanism of action of the cardiac glycosides in inhibiting the Na,K-ATPase.

Regulation of endogenous and expressed Na+/K+ pumps in Xenopus oocytes by membrane potential and stimulation of protein kinases

Modulation of the current generated by the Na+/K+ pump by membrane potential and protein kinases was investigated in oocytes of Xenopus laevis. In addition to a positive slope region in the current-voltage (I-V) relationship of the Na+/K+ pump, a negative slope region has been described in these cells (Lafaire & Schwarz, 1986) and has been attributed to a voltage-dependent apparent Km value for pump stimulation by external [K+] (Rakowski et al., 1991). To study this feature in more detail, Xenopus oocytes were used for comparative analysis of the negative slope of the I-V relationship of the endogenous Na+/K+ pump and of the Na+/K+ pump of the electric organ of Torpedo californica expressed in the oocytes. The effects of stimulation of protein kinases A and C on the negative slope were also analyzed. To investigate the negative slope over a wide potential range, experiments were performed in Na(+)-free solution and in the presence of high concentrations of Ba2+ and tetraethylammonium, to block all nonpump related K(+)-sensitive currents. Pump currents and pump-mediated fluxes were determined as differences of currents or fluxes in solutions with and without extracellular K+. The voltage dependence of the Km value for stimulation of the Na+/K+ pump by external [K+] shows significant species differences. Over the entire voltage range from -140 to +20 mV, the Km value for the Na+/K+ pump of Torpedo electroplax is substantially higher than for the endogenous pump and exhibits more pronounced voltage dependence. For the Xenopus pump, the voltage dependence can be described by voltage-dependent stimulation by external [K+] and can be interpreted by voltage-dependent K+ binding, assuming that an effective charge between 0.37 and 0.56 of an elementary charge is moved in the electrical field. An analogous evaluation of the voltage dependence of the Torpedo pump requires the assumption of movement of two effective charges of 0.16 and 1.0 of an elementary charge. Application of 1,2-dioctanoyl-sn-glycerol (diC8, 10-50 microM), which is known to stimulate protein kinase C, reduces the maximum activity of the Xenopus pumps in the oocyte membrane by 40% and modulates the voltage dependence of K+ stimulation. For the endogenous Xenopus pump, the apparent effective charge increased from 0.37 to 0.51 of elementary charge and the apparent Km at 0 mV increased from 0.46 to 0.83 mM. For the Torpedo pump, one of the apparent effective charges increased from 1.0 to 2.5 of elementary charge.(ABSTRACT TRUNCATED AT 400 WORDS)

Na,K-ATPase expression in normal and failing human left ventricle

The expression of the Na,K-ATPase was studied in both normal and failing human myocardium which was collected within 5 min of cardiac explantation in preparation for orthotopic transplantation or at the time of organ harvest. Abundance of mRNA for all three catalytic alpha subunits of the Na,K-ATPase was analyzed in samples from patients with end-stage heart failure due to either ischemic or dilated cardiomyopathy, as well as from normal controls. Vanadate facilitated 3H-ouabain binding before and after a Digibind wash was analyzed on tissue from a subset of these patients. mRNA analysis demonstrated that all three catalytic Na,K-ATPase alpha subunits were expressed in human heart and that there was no evidence for change in relative expression or abundance induced by disease. The specific digitalis receptor concentration was 760 +/- 58 and 614 +/- 47 pmol/g wet weight in the samples from normal and failing hearts, respectively (p = NS). From these studies it can be concluded that, whereas there is a tendency for a decrease in the number of ouabain receptors in heart failure, there is no significant alteration in the expression of Na,K-ATPase message or protein caused by chronic heart failure.

Expression of multiple Na+,K(+)-ATPase genes reveals a gradient of isoforms along the nonpigmented ciliary epithelium: functional implications in aqueous humor secretion

The Na+,K(+)-ATPase alpha 1, alpha 2, and alpha 3 subunit isoforms have been shown to be differentially expressed in the nonpigmented (NPE) and pigmented (PE) cells of the ocular ciliary epithelium (CE) (Martin-Vasallo et al., J. Cell. Physiol., 141:243-252, 1989; Ghosh et al., J. Biol. Chem., 265:2935-2940, 1990). In this study we analyzed and compared the pattern of expression of the multiple Na+,K(+)-ATPase alpha (alpha 1, alpha 2, alpha 3) subunit genes with the pattern of expression of the Na+,K(+)-ATPase beta (beta 1, beta 2) subunit genes along the bovine CE. We have selected three regions in the CE, referred to as 1) the anterior region of the pars plicata, near the iris; 2) the middle region of the pars plicata; and 3) the posterior region of the pars plana, near the ora serrata. Using isoform-specific cDNA probes and antibodies for the Na+,K(+)-ATPase alpha 1, alpha 2, alpha 3, beta 1, and beta 2 subunits on Northern and Western blot analysis, we found that mRNA and polypeptides are expressed in all three CE regions with different abundance. The pattern of expression of alpha and beta isoforms detected along the NPE cell layers suggests a gradient of alpha 1, alpha 2, alpha 3, beta 1, and beta 2 mRNAs and polypeptides that correlates with decreasing Na+,K(+)-ATPase activity from the most anterior region at the pars plicata towards the posterior region at the ora serrata. We also found marked differences in the pattern of immunolocalization of Na+,K(+)-ATPase alpha 1, alpha 2, alpha 3, beta 1, and beta 2 subunit isoforms in different regions of the CE. In the anterior region, NPE cells stained intensely at the basal lateral membrane with specific monoclonal and polyclonal antibodies for each of the alpha (alpha 1, alpha 2, alpha 3) and beta (beta 1, beta 2) Na,K-ATPase isoforms. In the middle and posterior regions of the CE, NPE cells showed lower or absent levels of staining with alpha 1, alpha 2, alpha 3, and beta 1 antibodies, although staining with beta 2 was abundant. In contrast, PE cells throughout the CE were stained at the basal lateral membrane by antibodies to alpha 1 and beta 1, while no staining signals were detected with the rest of the antibodies (i.e. alpha 2, alpha 3, and beta 2). Our results support the conclusion that the three alpha and two beta isoforms of the Na+,K(+)-ATPase are differentially expressed in the two cell layers that make up the CE.(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of sodium pump activity in Xenopus laevis oocytes by expression of heterologous beta 1 subunits of the sodium pump

Recent evidence suggests that the beta subunit of the Na+ pump is essential for the alpha subunit to express catalytic activity and for assembly of the holoenzyme in the plasma membrane. We report here that injection into Xenopus laevis oocytes of cRNAs specific for beta 1 subunit isoforms of the Na+ pump of four species (Torpedo californica, chicken, mouse and rat) causes a time-dependent increase in the number of ouabain-binding sites, both in the plasma membrane and in internal membranes. Expression of the beta 1 subunit of the Na+ pump of mouse and rat in the oocytes could be substantiated by immunoprecipitation using a polyclonal antiserum against the mouse beta 1 subunit. Scatchard analysis in permeabilized cells disclosed that the affinity for ouabain is unchanged after expression of each of the beta 1 subunits. A proportional increase in ouabain-sensitive 86Rb+ uptake indicates that the additionally expressed ouabain-binding sites on the cell surface represent functional Na+ pumps. The findings support the concept of Geering. Theulaz, Verrey, Häuptle & Rossier [(1989) Am. J. Physiol. 257, C851-C858] that beta 1 subunits expressed in oocytes associate with an excess of endogenous alpha subunits of the Na+ pump to form a hybrid enzyme. In addition, all of the beta 1 isoforms investigated in the present study were also capable of combining with the co-expressed alpha 1 subunit of the Torpedo Na+ pump to produce a functional enzyme. Injection of cRNA encoding for the Torpedo alpha 1 subunit alone had no effect on the ouabain-binding capacity of the surface and intracellular membranes of the oocyte.

Expression of Na,K-ATPase isoforms in human heart

The expression pattern of the multiple isoforms of Na,K-ATPase was examined in the human heart. Isoform specific oligonucleotide probes for the alpha 1, alpha 2, alpha 3 and beta 1 subunits were used to probe Northern blots. The adult human ventricle expresses mRNAs for all three alpha subunit isoforms in addition to beta 1 subunit mRNA.

Functional expression of N-terminal truncated alpha-subunits of Na,K-ATPase in Xenopus laevis oocytes

N-terminal deletion mutants of Na,K-ATPase alpha 1 isoforms initiating translation at Met34 (alpha 1T1) or at Met43 (alpha 1T2) were expressed in X. laevis oocytes. Compared to beta 3 cRNA injected controls, the co-expression of alpha 1wt, alpha 1T1, alpha 1T2 with beta 3 subunits results in a 2- to 3-fold increase of ouabain binding sites, parallelled by a concomitant increase in Na,K-pump current. The apparent K1/2 for potassium activation of the alpha 1T2/beta 3 Na,K-pumps is significantly higher than that of the alpha 1wt/beta 3 or alpha 1T1/beta 3 Na,K-pumps expressed at the cell surface. Total deletion of the lysine-rich N-terminal domain thus allows the expression of active Na,K-pump but with distinct cation transport properties.

The functional role of the beta-subunit in the maturation and intracellular transport of Na,K-ATPase

The minimal functional enzyme unit of Na,K-ATPase consists of an alpha-beta complex. The alpha-subunit bears all functional domains of the enzyme and so far a regulatory role for the beta-subunit in the catalytic cycle has not been established. On the other hand, increasing experimental evidence suggests that the beta-subunit is an indispensable element for the structural and functional maturation of the enzyme as well as its intracellular transport to the plasma membrane. This brief review summarizes the experimental data supporting the hypothesis that assembly of the beta-subunit is needed for the alpha-subunit to acquire the correct, stable configuration necessary for the acquisition of functional properties and its exit from the ER.

The family of human Na,K-ATPase genes. ATP1AL1 gene is transcriptionally competent and probably encodes the related ion transport ATPase

The multigene family of human Na,K-ATPase is composed of 5 alpha-subunit genes, 3 of which were shown to encode the functionally active alpha 1, alpha 2 and alpha 3 isoforms of the catalytic subunits. This report describes the isolation, mapping and partial sequencing of the fourth gene (ATP1AL1) that was demonstrated here to be functionally active and expressed in human brain and kidney. Limited DNA sequencing of the ATP1AL1 exons allowed one to suggest that the gene probably encodes a new ion transport ATPase rather than an isoform of the Na,K-ATPase or the closely related H,K-ATPase.

Structure of the human gastric H,K-ATPase gene and comparison of the 5'-flanking sequences of the human and rat genes

We have isolated and analyzed the genes encoding the human and rat gastric H,K-ATPase catalytic subunits. The complete sequence of the human gene, including 2.2 kb of 5'-flanking sequence, and the 5' end of the rat gene, including exons 1-4 and 2.5 kb of 5'-flanking sequence, have been determined. The human gene contains 22 exons. Its intron-exon organization is identical to that of the Na,K-ATPase gene, except that exon 6 corresponds to a fusion of exons 6 and 7 of the Na,K-ATPase gene. The transcription initiation sites of both the human and rat genes were determined by primer extension and S1 nuclease protection analyses. Comparison of the 5'-flanking regions of the human and rat genes revealed three extended regions of high sequence similarity, one of which includes a potential TATA box and other basic promoter elements beginning about 30 nucleotides upstream of the transcription start site. Other conserved sequences, including possible response elements for Ca2+ and cAMP, which are known intracellular mediators of acid secretion, are located up to 2 kb 5' to the transcription initiation site.

Characterization of the 5'-flanking region of the human and rat Na,K-ATPase alpha 3 gene

Genomic clones containing the 5'-flanking region and exon 1 of the human and rat Na,K-ATPase alpha 3 isoform gene have been isolated and characterized. The nucleotide sequences of 1.6 kb of the rat gene and 2.8 kb of the human gene in the 5'-flanking region were determined. Mapping of transcription initiation sites by primer extension and S1 nuclease protection analyses indicates that transcription is initiated in the same region in both genes although the rat gene has a greater number of initiation sites. Neither gene has a canonical TATA box, having instead a ATAT sequence preceding the transcription initiation sites. There is a perfect CCAAT sequence, in the reverse orientation, approximately 30 bp upstream of the potential TATA box in both genes. We have identified potential binding sites for transcription factors Sp-1, AP-1, AP-2, and AP-4, as well as for glucocorticoid and thyroid hormone receptors in the 5'-flanking regions. These are conserved in both human and rat alpha 3 isoform genes.