Rat brain has the alpha 3 form of the (Na+,K+)ATPase

Molecular cloning and characterization of porcine Na⁺/K⁺-ATPase isoforms α1, α2, α3 and the ATP1A3 promoter

Na⁺/K⁺-ATPase maintains electrochemical gradients of Na⁺ and K⁺ essential for a variety of cellular functions including neuronal activity. The α-subunit of the Na⁺/K⁺-ATPase exists in four different isoforms (α1-α4) encoded by different genes. With a view to future use of pig as an animal model in studies of human diseases caused by Na⁺/K⁺-ATPase mutations, we have determined the porcine coding sequences of the α1-α3 genes, ATP1A1, ATP1A2, and ATP1A3, their chromosomal localization, and expression patterns. Our ATP1A1 sequence accords with the sequences from several species at five positions where the amino acid residue of the previously published porcine ATP1A1 sequence differs. These corrections include replacement of glutamine 841 with arginine. Analysis of the functional consequences of substitution of the arginine revealed its importance for Na⁺ binding, which can be explained by interaction of the arginine with the C-terminus, stabilizing one of the Na⁺ sites. Quantitative real-time PCR expression analyses of porcine ATP1A1, ATP1A2, and ATP1A3 mRNA showed that all three transcripts are expressed in the embryonic brain as early as 60 days of gestation. Expression of α3 is confined to neuronal tissue. Generally, the expression patterns of ATP1A1, ATP1A2, and ATP1A3 transcripts were found similar to their human counterparts, except for lack of α3 expression in porcine heart. These expression patterns were confirmed at the protein level. We also report the sequence of the porcine ATP1A3 promoter, which was found to be closely homologous to its human counterpart. The function and specificity of the porcine ATP1A3 promoter was analyzed in transgenic zebrafish, demonstrating that it is active and drives expression in embryonic brain and spinal cord. The results of the present study provide a sound basis for employing the ATP1A3 promoter in attempts to generate transgenic porcine models of neurological diseases caused by ATP1A3 mutations.

Potassium recycling pathways in the human cochlea

OBJECTIVES/HYPOTHESIS

Potential pathways for recycling potassium (K+) used in the maintenance of inner ear electrochemical gradients have been elucidated in animal models. However, little is known about K+ transport in the human cochlea. This study was designed to characterize putative K+ recycling pathways in the human ear and to determine whether observations from animal models can be extrapolated to humans.

STUDY DESIGN

A prospective laboratory study using an immunohistochemical approach to analyze the distribution of key ion transport mediators in the human cochlea.

METHODS

Human temporal bones were fixed in situ within 1 to 6 hours of death and subsequently harvested at autopsy. Decalcification was accomplished with the aid of microwaving. Immunohistochemical staining was then performed to define the presence and cell type-specific distribution of Na,K-ATPase, sodium-potassium-chloride cotransporter (NKCC), and carbonic anhydrase (CA) in the inner ear.

RESULTS

Staining patterns visualized in the human cochlea closely paralleled those seen in other species. Anti-Na,K-ATPase stained strongly the basolateral plasma membrane of strial marginal cells and nerve endings underlying hair cells. This antibody also localized Na,K-ATPase to type II, type IV, and type V fibrocytes in the spiral ligament and in limbal fibrocytes. NKCC was present in the basolateral membrane of strial marginal cells as well as in type II, type V, and limbal fibrocytes. Immunoreactive carbonic anhydrase was present in type I and type III fibrocytes and in epithelial cells lining Reissner's membrane and the spiral prominence.

CONCLUSIONS

The distribution of several major ion transport proteins in the human cochlea is similar but not identical to that described in various rodent models. These results support the presence of a complex system for recycling and regulating K+ homeostasis in the human cochlea, similar to that described in other mammalian species.

Diet deficient in alpha-linolenic acid alters fatty acid composition and enzymatic properties of Na+, K+-ATPase isoenzymes of brain membranes in the adult rat

The effects of dietary (n-6)/(n-3) polyunsaturated fatty acid balance on fatty acid composition, ouabain inhibition, and Na(+) dependence of Na(+), K(+)-ATPase isoenzymes of whole brain membranes were studied in 60-day-old rats fed over two generations a diet either devoid of alpha-linolenic acid [18:3(n-3)] (sunflower oil diet) or rich in 18:3(n-3) (soybean oil diet). In the brain membranes, the sunflower oil diet led to a dramatic decrease in docosahexaenoic acid [22:6(n-3)] membrane content. The activities of Na(+), K(+)-ATPase isoenzymes were discriminated on the basis of their differential affinities for ouabain and their sensitivity to sodium concentration. The ouabain titration curve of Na(+), K(+)-ATPase activity displayed three inhibitory processes with markedly different affinity [i.e., low (alpha1), high (alpha2), and very high (alpha3)] for brain membranes of rats fed the sunflower oil diet, whereas the brain membranes of rats fed the soybean oil diet exhibited only two inhibitory processes, low (alpha1) and high (alpha2' = alpha2 + alpha3). Regardless of the diet, on the basis of the Na(+) dependence of Na(+), K(+)-ATPase activity, three isoenzymes were found: alpha1 form displaying an affinity 1.5- to 2-fold higher that of than alpha2 and 3-fold higher that of alpha3. In rats fed the sunflower oil diet, alpha2 isoenzyme exhibited higher affinity for sodium (Ka = 8.8 mmol/L) than that of rats fed the soybean oil diet (Ka = 11.7 mmol/L). These results suggest that the membrane lipid environment modulates the functional properties of Na(+), K(+)-ATPase isoenzymes of high ouabain affinity (alpha2).

Acute and chronic regulation of Na+/K+-ATPase transport activity in the RN22 Schwann cell line in response to stimulation of cyclic AMP production

Na+/K+-ATPase-dependent Rb+ uptake of RN22 Schwann cells was stimulated by cholera toxin (0.25 microg/ml), forskolin (2 mM), or 8-bromo cAMP (1 mM). At 2 h Rb+ uptake was increased by 162+/-6% (cholera toxin), 151+/-14% (forskolin), and 207+/-15% (8-bromo cAMP). Cholera toxin or 8-bromo cAMP treatment for 12-24 h resulted in a second peak of Na+/K+-ATPase-dependent Rb+ transport activity of 186+/-12 and 265+/-9% of control, respectively. Cholera toxin also transiently stimulated the activity of the Na+, K+, 2Cl- -cotransporter with a peak at 2 h (179+/-9%), returning to basal levels by 24 h. Inhibition of the Na+,K+,2Cl- -cotransporter by bumetanide (0.1 mM) or by reduction of the Na+ gradient (10 mM veratridine treatment) prevented the early peak in ATPase activity but not the second peak. These results indicated that the early transient stimulation of Na+/K+ ATPase activity by cholera toxin was due to an increase in cellular Na+, secondary to stimulation of Na+,K+,2Cl -cotransport activity. Western blot analysis of cellular homogenates and purified membrane fractions showed that the second peak of Rb+ uptake activity was a result of translocation of transport protein from an intracellular microsomal pool to the plasma membrane. Rb+ uptake by dominant negative protein kinase A mutants of the RN22 cell was not stimulated by cholera toxin treatment (acute or chronic) confirming the cAMP/protein kinase A dependency of both acute and long-term regulation of transport activity. In the absence of a change in Michaelis constants or of an increase in total transport protein of cellular homogenates, neither a change in enzyme kinetics nor an increase in de novo synthesis of transport protein could account for the increase in transport activity.

Epitope and mimotope for an antibody to the Na, K-ATPase

The epitope of a monoclonal antibody specific for the alpha 2 isoform of the Na,K-ATPase was determined and its accessibility in native enzyme was examined. Protein fragmentation with N-chlorosuccinimide, formic acid, trypsin, and leucine aminopeptidase indicated binding near the Na,K-ATPase N-terminus but did not unambiguously delineate the extent of the epitope. The ability of the antibody to bind to denatured enzyme made it a good candidate for screening a random peptide library displayed on M13 phage, but the consensus sequence that emerged was not found in the Na,K-ATPase, Full-length cDNA for the Na,K-ATPase was randomly fragmented and cloned into beta-galactosidase to create a lambda gt11 expression library; screening with the antibody yielded a set of overlaps spanning 23 amino acids at the N-terminus. Chimeras of Na,K-ATPase alpha 1 and alpha 2 narrowed down the epitope to 14-19 amino acids. The antibody did not recognize fusion proteins constructed with shorter segments of this epitope. It did recognize a fusion protein containing the M13 library consensus sequence, however, indicating that this sequence, which is rich in proline and hydrophobic amino acids (FPPNFLFPPPP), was a mimotope. The natural epitope, unique to the Na,K-ATPase alpha 2 isoform, was GREYSPAATTAENG. Reconstitution of antibody binding in a foreign context such as M13 PIII protein or beta-galactosidase thus required a relatively large number of amino acids, indicating that antibody mapping approaches must allow for epitopes of significant size. The epitope was accessible in native enzyme and exposed on the cytoplasmic side, documenting the surface exposure of a stretch of amino acids at the N-terminus, where the Na,K-ATPase isoforms differ most.

Isoform-specific monoclonal antibodies to Na,K-ATPase alpha subunits. Evidence for a tissue-specific post-translational modification of the alpha subunit

Monoclonal antibodies to isoforms of the Na,K-ATPase have become important tools in the study of the enzyme's distribution, physiological roles, and gene regulation, and when their epitopes are defined, they are useful in the study of enzyme structure as well. Evidence is presented that the alpha3-specific antibody McBX3 recognizes an unusual epitope that is not present on alpha3 in the heart. The epitope, which is also found in kidney alpha1 from some species, was mapped to a site on the large intracellular loop near the ATP binding site. DNA sequencing of reverse transcribed-PCR products encompassing the corresponding regions from alpha3 from brain (where McBX3 recognizes alpha3) and heart demonstrated that the tissue difference in epitope is not due to alternative splicing of the mRNA. Instead, hydroxylamine sensitivity indicated that the antibody recognizes a post-translational modification. The epitope for a new antibody for alpha3, XVIF9-G10, was mapped to a site near the N terminus, a location analogous to the sites for the well-characterized antibodies McK1 (alpha1) and McB2 (alpha2). The antibody XVIF9-G10 reacted with the alpha3 of the heart as well as that of the brain; however, McBX3 and XVIF9-G10 both stained the same cellular structures in sections of the rat retina. A new alpha1-specific antibody, 6F, was characterized and mapped to another site near the N terminus; this antibody has broader species specificity than the other well-characterized alpha1 antibody, McK1.

Isolation of a monoclonal antibody reactive to brain microsome-associated antigen, lap-1, and its preferential localization in limbic system of rat brain

The distribution and subcellular localization of AH9 antigen, recognized by a monoclonal antibody AH9, were examined in rat brain. Highest expression was observed in the lamina lucidum of the dentate gyrus of the rat hippocampus. Synaptic subfields of other limbic areas also expressed AH9 antigen at a substantial level. The molecular size of the AH9 antigen is 15 kDa and it was found in the microsomal fraction of brain but not of heart or kidney. These results indicate that AH9 antigen is a novel synaptosomal protein that is relatively specific to the limbic system, at least in the rat brain. We designated AH9 antigen as a limbic system associated protein-1, lap-1.

Developmental changes of K(+)-dependent para-nitrophenylphosphatase (Na(+)-K(+)-ATPase) distribution in the synaptic regions in the cerebral cortex of rats

Using Mayahara's method, the distribution of K(+)-dependent p-nitrophenylphosphatase activity was examined electron microscopically in the synaptic regions of the cerebral cortex of 10, 15 and 60-day-old Wistar rats. The enzyme achieved gradually its characteristic localization and uniform distribution. The main developmental changes were associated with the establishment of the postsynaptic density's activity. The controls with ouabain revealed activity only on the postsynaptic densities.

Evidence for the regulation of Na+, K(+)-ATPase alpha 1 gene expression through the interaction of aldosterone and cAMP-inducible transcriptional factors

Mineralocorticoid hormones such as aldosterone modulate cellular ion homeostasis at least in part through the regulation of Na+, K(+)-ATPase (NAKA) gene expression. While aldosterone acts at the transcriptional level through its ligand-inducible mineralocorticoid receptor (MR), tissue specific and other transcriptional factors may interact with the MR to modulate this regulatory response. cAMP also regulates NAKA alpha 1 gene expression which at the transcriptional level is mediated, in part, through a cAMP response element (CRE) present on a highly conserved, 48 base pair enhancer region, the PUC-1 core, of the rat NAKA alpha 1 subunit gene promoter. We have tested the hypothesis that the MR interacts with cAMP induced transcriptional factors to modulate the NAKA alpha 1 gene expression. In transient transfection studies a PUC-1 core attached to an enhancerless SV40 promoter driven reporter gene (pB1CAT) was induced by 8-bromo-cAMP in HeLa cells. Co-transfected MR expression vector inhibited the 8-bromo-cAMP inducible activity of pB1CAT. DNA binding studies suggested that the PUC-1 core binds both CREB/ATF proteins as well as the glucocorticoid hormone class of steroid receptors. These results suggest that the MR suppresses cAMP-mediated activation of PUC-1 core driven CAT activity possibly through a direct interaction with CREB/ATF transcriptional factors. This in turn suggests that the interaction of two distinct signal transduction systems, aldosterone and cAMP, may define the mineralocorticoid responsiveness of the Na+, K(+)-ATPase alpha 1 gene.

Expression of alpha and beta subunit isoforms of Na,K-ATPase in the mouse inner ear and changes with mutations at the Wv or Sld loci

Mice homozygous for mutations at the viable dominant spotting (Wv) and Steel-dickie (Sld) loci exhibit a similar phenotype which includes deafness. The auditory dysfunction derives from failure of the stria vascularis to develop normally and to generate a high positive endocochlear potential (EP). Because strial function is driven by Na,K-ATPase its expression was investigated in inner ears of Wv/Wv and Sld/Sld mice and their wild-type littermates by immunostaining with antisera against four of the enzyme's subunit isoforms. Wild-type mice from two different genetic backgrounds showed an identical distribution of subunit isoforms among inner ear transport cells. Several epithelial cell types coexpressed the alpha 1 and beta 1 subunits. Vestibular dark cells showed no reactivity for beta 1 but expressed abundant beta 2, whereas, strial marginal cells stained strongly for both beta isoforms. The only qualitative difference between mutant and wild-type mice was the absence of beta 1 subunit in marginal cells of the mutant's stria. However, it is unlikely that this difference accounts for failure of mutants to generate a high EP because the beta 1 subunit is not present in the stria vascularis of either rats or gerbils with normal EP values. Strong immunostaining for Na,K-ATPase in lateral wall fibrocytes of normal mice along with diminished immunoreactivity in the mutants supports the concept that these strategically located transport fibrocytes actively resorb K+ leaked across Reissner's membrane into scala vestibuli or effluxed from hair cells and nerves into scala tympani. It is further speculated that the resorbed K+ normally is siphoned down its concentration gradient into the intrastrial space through gap junctions between fibrocytes and strial basal and intermediate cells where it is recycled back to endolymph via marginal cells. Thus, failure of mutants to generate a positive EP could be explained by the absence of intermediate cells which may form the final link in the conduit for moving K+ from perilymph to the intrastrial compartment.

Purification and immunochemical properties of human Na,K-ATPase alpha subunits and formic acid-derived polypeptide fragments

In this study, alpha (alpha) isoform proteins were purified from the partially purified Na,K-ATPase by SDS-PAGE and electroelution. Peptide mapping showed subtle biochemical differences between alpha subunit proteins of rat and human origin. The purified alpha proteins were treated with formic acid, the cleaved polypeptide fragments were separated by SDS-PAGE, the bands corresponding to 40, 50, and 60 kDa were excised, and the proteins were electroeluted. The purified 40, 50, and 60 kDa polypeptides were essentially homogeneous, and were used for preparation of polyclonal antibodies in rabbits. The antisera to alpha proteins (R alpha) and 60 & 40 kDa polypeptides (R60 & R40) were obtained and characterized by Western blotting. All three antisera were highly specific, since they cross-reacted with only the 100 kDa bands of the crude brainstem homogenates, of the axolemma, and of the cerebral cortex synaptosomes and microsomes. R alpha and R40 were successfully used for immunohistochemical staining of fibers in the white matter of the human brain frontal cortex. These antisera were not isoform-specific, they cross-reacted with 40, 50, and 60 kDa polypeptides as well as the three alpha bands.

Aluminum-induced alterations in [3H]ouabain binding and ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase

The (Na(+)+K+)-ATPase is responsible for maintenance of the ionic milieu of cells. The objective of this study is to investigate the effect of aluminum, an ion implicated in several neurological disorders, on ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase and on the binding of [3H]ouabain to this enzyme. AlCl3 (25-100 microM) inhibits the phosphatase activity of the (Na(+)+K+)-ATPase in a dose-dependent manner. AlCl3 appears to act as a reversible, noncompetitive inhibitor of (Na(+)+K+)-ATPase activity by decreasing the maximum velocity of the enzyme without significantly affecting the apparent dissociation constant with respect to ATP. AlCl3 may affect Mg2+ sites on the (Na(+)+K+)-ATPase but does not appear to interact with Na+ or K+ sites on the enzyme. In contrast to this inhibitory effect on the phosphatase function of the enzyme, AlCl3 (1-100 microM) stimulates the binding of [3H]ouabain to the (Na(+)+K+)-ATPase. This effect is due to an increase in the maximum [3H]ouabain binding capacity of the enzyme with no change in the [3H]ouabain binding affinity. These data support the hypothesis that AlCl3 may stabilize the phosphorylated form of the synaptosomal (Na(+)+K+)-ATPase which increases [3H]ouabain binding while inhibiting the phosphatase activity of the enzyme.

Effect of fish oil diet on fatty acid composition of phospholipids of brain membranes and on kinetic properties of Na+,K(+)-ATPase isoenzymes of weaned and adult rats

The influence of dietary (n-3) fatty acids (such as eicosapentaenoic and docosahexaenoic acids) as found in fish oil on Na+ sensitivity and ouabain affinity of Na+,K(+)-ATPase isoenzymes (alpha 1, alpha 2, alpha 3) was studied in whole brain membranes from weaned and adult rats fed diets for two generations. The long chain (n-3) fatty acids supplied by fish oil decreased the fatty acids of the (n-6) series compared with the standard diet, resulting in a decrease in the (n-6)/(n-3) molar ratio in both 21- and 60-day-old rats. On the basis of ouabain titration, three inhibitory processes with markedly different affinities were associated with isoenzymes, i.e., low affinity (alpha 1), high affinity (alpha 2), and very high affinity (alpha 3). It appears that the fish oil diet, in part via the modification of membrane fatty acid composition, altered the proportion and ouabain affinity of isoenzymes. Na+ sensitivity is the best criterion of physiologic change induced by fish oil diet. We calculated the Na+ activation for each isoenzyme and found one Na+ sensitivity and two Na+ sensitivities per isoenzyme in weanling and adult rats fed different diets, respectively. In contrast to alpha 2 and alpha 3, alpha 1 appears insensitive to membrane change induced by fish oil diet. Fish oil diet, which is known to confer cardioprotection, induced significant modulation of Na+,K(+)-ATPase isoenzymes at the brain level.

Cell-type specific expression of Na+, K(+)-ATPase catalytic subunits in cultured neurons and glia: evidence for polarized distribution in neurons

Na+,K(+)-ATPase (the sodium pump) is a family of proteins consisting of catalytic (alpha) and glycoprotein (beta) subunit isoforms which are differentially expressed in excitable tissue. To gain insight into the cell-type distribution of sodium pump protein, we determined the expression pattern of fetal rat telencephalic cultures, of telencephalic cultures depleted of neurons, and of pure astrocyte cultures. Isoform-specific antibodies were used for immunoblotting and immunohistochemistry, with supplemental [3H]ouabain binding to assess levels of functional alpha 2/alpha 3 protein. The results show that neurons of mixed telencephalic cultures uniquely express alpha 3 and high levels of alpha 1. The marked similarity in the distribution of microtubule-associated protein-2 and alpha 1 immunocytochemical staining strongly suggests that alpha 1 subunits are enriched in dendrites. Further, highly correlative growth cone-associated protein-43 and alpha 3 staining is consistent with a preferential expression of alpha 3 subunits in axons, which are also characterized by low levels of alpha 1 and no alpha 2 immunoreactivity. Process-bearing glia are intimately associated with neuronal aggregates and express high levels of both alpha 1 and alpha 2 protein, as well as GFAP. Interestingly, polygonal, flat glia not within neuronal aggregates are weakly immunopositive only for alpha 1 and GFAP. Pure astrocytic cultures possess appreciable alpha 1 protein and GFAP, but lack both alpha 2 and alpha 3 immunoreactivity. As predicted by the immunohistochemical findings, [3H]ouabain binding was low in pure astrocytic cultures, and much higher in the neuron-enriched mixed cultures. These observations confirm that neurons express all three catalytic isoforms of the sodium pump. They also suggest that specific alpha-isoforms may be polarized to targeted membrane regions of neurons. Further, glia intimately associated with neurons express alpha 2, bind significant amounts of [3H]ouabain, and possess much higher levels of alpha 1 and GFAP compared to glia not near neurons. Thus, neurons may regulate glial sodium pump expression.

Control of the Na+,K(+)-ATPase under normal and pathological conditions

The Na+,K(+)-ATPase is an important enzyme in determining the ionic milieu of the cerebromicrovasculature and neurons. The effect of hypertension or aging on this enzyme, as well as its susceptibility to regulation by fatty acids or aluminum, is the focus of this study. A significant increase (34%) in the apparent affinity constant (KD) but no change in the maximum binding capacity (Bmax) for [3H]ouabain binding to the cerebromicrovascular Na+,K(+)-ATPase occurs after induction of acute hypertension. In addition, long chain unsaturated fatty acids stimulate the binding of [3H]ouabain to the enzyme in microvessels from normotensive and hypertensive rats. The synaptosomal Na+,K(+)-ATPase is sensitive to aluminum. AlCl3 (1-100 microM) inhibits the K(+)-dependent-p-nitrophenylphosphatase (K(+)-NPPase) activity of the Na+,K(+)-ATPase in a dose-dependent manner. AlCl3 (100 microM) decreases the Vmax by 14% but does not alter the KM, suggestive of non-competitive inhibition. The enzyme from aged brain displays a greater Vmax, but shows the same susceptibility to AlCl3 as the enzyme from younger brain. In summary, disruption of the Na+,K(+)-ATPase may underlie, at least in part, abnormalities of nerve and vascular cell function in disorders where elevated concentrations of fatty acids or metal ions are involved.

Effect of dietary alpha-linolenic acid on functional characteristic of Na+/K(+)-ATPase isoenzymes in whole brain membranes of weaned rats

The influence of dietary fatty acids on Na+ sensitivity and ouabain affinity of Na+/K(+)-ATPase isoenzymes of whole brain membranes were studied in weaned rats fed for two generations with diets either devoid of alpha-linolenic acid (sunflower oil diet) or rich in alpha-linolenic acid (soya oil diet). The (n--3) deficiency induced by the sunflower oil diet led to an increase in the (n--6)/(n--3) molar ratio in whole brain membranes. Na+/K(+)-ATPase isoenzymes were discriminated on the basis of their differential affinities for ouabain. In rats fed sunflower oil diet, the ouabain titration displayed three inhibitory processes with markedly different affinities: low affinity (alpha 1); high affinity (alpha 2); and very high affinity (alpha 3). Membranes of rats fed soya oil diet exhibited only two inhibitory processes, i.e., low affinity (likely alpha 1+ alpha 2) and high affinity (likely alpha 2+ alpha 3) with the low affinity form intermediate between the sunflower alpha 1 and alpha 2 forms, and the high affinity form intermediate between the sunflower alpha 2 and alpha 3 forms. In fact, the Na+ response shows that the three isoenzymes have different Na+ sensitivities. Regardless of the diet, alpha 1 has a similar Na+ sensitivity (less than 1 mM), whilst alpha 2 and alpha 3 are more sensitive in soya oil membranes compared to sunflower oil membranes (5.1 vs. 7.2 mM and about 11 vs. 22.5 mM, respectively). Thus, sodium appears to be a better criterion of heterogeneity than ouabain.

Heterogeneous Na+ sensitivity of Na+,K(+)-ATPase isoenzymes in whole brain membranes

The Na+ sensitivity of whole brain membrane Na+,K(+)-ATPase isoenzymes was studied using the differential inhibitory effect of ouabain (alpha 1, low affinity for ouabain; alpha 2, high affinity; and alpha 3, very high affinity). At 100 mM Na+, we found that the proportion of isoforms with low, high, and very high ouabain affinity was 21, 38, and 41%, respectively. Using two ouabain concentrations (10(-5) and 10(-7) M), we were able to discriminate Na+ sensitivity of Na+,K(+)-ATPase isoenzymes using nonlinear regression. The ouabain low-affinity isoform, alpha 1, exhibited high Na+ sensitivity [Ka of 3.88 +/- 0.25 mM Na+ and a Hill coefficient (n) of 1.98 +/- 0.13]; the ouabain high-affinity isoform, alpha 2, had two Na+ sensitivities, a high (Ka of 4.98 +/- 0.2 mM Na+ and n of 1.34 +/- 0.10) and a low (Ka of 28 +/- 0.5 mM Na+ and an n of 1.92 +/- 0.18) Na+ sensitivity activated above a threshold (22 +/- 0.3 mM Na+); and the ouabain very-high-affinity isoform, alpha 3, was resolved by two processes and appears to have two Na+ sensitivities (apparent Ka values of 3.5 and 20 mM Na+). We show that Na+ dependence in the absence of ouabain is the result of at least of five Na+ reactivities. This molecular functional characteristic of isoenzymes in membranes could explain the diversity of physiological roles attributed to isoenzymes.

Regulatory role of a neurotransmitter (5-HT) on glial Na+/K(+)-ATPase in the rat brain

In the present work we studied the effect of serotonin (5-HT) on the kinetics of Na+/K(+)-ATPase in subcellular preparations of the cerebral cortex from male Wistar rats using various concentrations of ATP and K+ with and without added 5-HT. Also we studied the effect of 5-HT on the enzyme in glial or neuronal preparations. The results indicated that there was a significant increase (P < 0.05) of the Vmax in the presence of 5-HT in the whole tissue preparation (homogenate) but not in the subcellular fractions, suggesting that the interaction could be preferentially with the glial pump. Further results supported that this was the case since activation by 5-HT was mainly in the glial preparations. Kinetic data and the binding of [3H]ouabain supported that the enzyme is activated by 5-HT through the exposure of more enzymatic active sites.

Loss and recovery of activities of alpha+ and alpha isozymes of (Na(+) + K+)-ATPase in cortical focal ischemia: GM1 ganglioside protects plasma membrane structure and function

Alterations in cellular membrane structure and the subsequent failure of its function after CNS ischemia were monitored by analyzing changes in the plasma membrane marker enzyme (Na(+) + K(+)-ATPase. The levels of two isozymes of (Na(+) + K(+)-ATPase, alpha+ and alpha, which have distinct cellular and anatomical distributions, were studied to determine if differential cellular damage occurs in primary and peri-ischemic injury areas. The efficacy of monosialoganglioside (GM1) treatment was assessed, since this glycosphingolipid has been shown to reduce ischemic injury by protecting cell membrane structure/function. Using a rat model of cortical focal ischemia, levels of both ATPase isozyme activities were assayed in total membrane fractions from primary ischemic tissue (parietal cortex) and three peri-ischemic tissue areas (frontal, occipital, and temporal cortex) at 1, 3, 5, 7, and 14 days after ischemia. No significant loss of either isozyme's activity occurred in any tissue area at 1 day after ischemia. At 5 days, in the primary ischemic area, both isozyme activity levels decreased by 70-75%. The alpha+ enzyme activity loss persisted up to 14 days, while a 17% recovery in alpha activity occurred. In the three peri-ischemic tissue areas, enzyme activity losses ranged from 42%-59% at 3 days after ischemia. A complete restoration of both isozyme activities was seen at 14 days. After three days of GM1 ganglioside treatment there was no loss of total (Na*+) + K(+)-ATPase activity in the three peri-ischemic areas, and a significantly reduced loss in the primary infarct tissue. An autoradiographic analysis of brain coronal sections using 3H-ouabain supports the enzymatic data and GM1 effects. Reductions in 3H-ouabain binding in all cortical layers at 3 days after ischemia were visualized. GM1 treatment significantly reduced these 3H-ouabain binding losses. In summary, time-dependent quantitative changes in activity levels of ATPase isozymes (alpha+ and alpha) reflect the different degree of membrane damage that occurs in primary vs. peri-ischemic tissues (e.g., irreversible vs. reversible membrane damage), and that ischemia affects cell membranes of all neural elements in a largely similar fashion. GM1 ganglioside was found to reduce plasma membrane damage in all CNS cell types.

Na,K-ATPase of cultured bovine lens epithelial cells: H2O2 effects

Na,K-ATPase function was studied in cultured bovine lens epithelial cells under confluent and non-confluent conditions. The affinity of the Na,K-ATPase for the cardiac glycoside, ouabain, differs between the confluent and non-confluent cultures. The confluent cells have a higher affinity for ouabain than do the non-confluent cells. The ouabain affinity of the confluent cells is similar to that for the Na,K-ATPase isolated from the bovine axolemma and the bovine lens cortex. The ouabain affinity of the non-confluent cells is similar to that for the Na,K-ATPase of the renal medulla and bovine lens epithelium. Similar results are not found with confluent and non-confluent MDCK cells. H2O2 treatment of confluent and non-confluent lens epithelial cell cultures has differing effects on the Na,K-ATPase function. In the confluent cell preparations, H2O2 affects K(+)-dependent dephosphorylation of the intermediate phosphoenzyme. In the non-confluent preparations. H2O2 appears to inhibit K(+)-occlusion.

Highly ouabain-sensitive alpha 3 isoform of Na+,K(+)-ATPase in human brain

The Na+,K(+)-ATPase alpha 3 isoform has recently been demonstrated immunochemically in human brain. Conclusive biochemical evidence, however, is still lacking. In this study, a unique 50-kDa polypeptide, which is known to be specific to the rat alpha 3 isoform, has been found in human brainstem Na+,K(+)-ATPase following formic acid treatment of the purified alpha isoform proteins. Human alpha 3 Na+,K(+)-ATPase is also highly sensitive to ouabain inhibition, with a 50% ouabain inhibition value of 1.0 x 10(-7) M. These results provide clear and direct evidence for the existence of the alpha 3 isoform in human brain.

Molecular isoforms of Na+/K(+)-ATPase in the nervous system and kidney of the spontaneously diabetic BB/Wor rat

Na+/K(+)-ATPase was evaluated in the retina and kidney of the spontaneously diabetic BB/Wor rat after 1 and 4 months of insulin dependency. Retinal synthesis of the Na+/K(+)-ATPase was measured during a 2-h intravitreal pulse of [35S]methionine and analyzed by SDS-PAGE and scintillation counting. Synthesis of the alpha-1 and 'alpha(+)' (includes both alpha-2 and alpha-3) isoforms of the catalytic subunit was increased 123% and 69%, respectively at 4 months. Increases were also suggested at 1 month, but were not significant. The diabetes-dependent peak of synthesis in long-term diabetic rats turned over rapidly and by 3 days after intravitreal labeling, radioactively labeled enzyme was equal in both control and diabetic retinae. The amount of axonally transported, labeled enzyme recovered from endings of the optic nerve in the superior colliculus paralleled retinal labeling. Significant renal hypertrophy (48%) was noted at 4 months, but not at 1 month. The strophanthidin-inhibition constant for diabetes-induced renal enzyme was the same as for control enzyme (approx. 10(-4) M), indicating that diabetic renal hypertrophy does not induce a Na pump isozyme that is more sensitive to cardiotonic steroids. SDS-PAGE of the renal enzyme also failed to indicate more than one isoform of the alpha subunit.

Immunochemical demonstration of alpha 3 isozyme of Na,K-ATPase in human brain

The catalytic alpha (alpha) subunit of the human Na,K-ATPase is encoded by multiple genes, as revealed from molecular cloning studies, yet the presence of the alpha 3 isoform has still not been resolved. We investigated the human alpha 3 isoform by Western blotting employing polyclonal anti-rat brain alpha 3 fusion protein, and purified human and rat axolemma Na,K-ATPase preparations. The antibody to rat brain alpha 3 fusion protein cross-reacted with both human and rat antigens, and the stained band superimposed on the alpha 2 band. This result suggests that alpha 3 isoform also exists in human brain.

The alpha-2 isomer of the sodium pump is inhibited by calcium at physiological levels

The inhibition of the (Na,K)ATPase by calcium was investigated in plasma membrane preparations of rat axolemma, skeletal muscle and kidney outer medulla. Ouabain titration curves demonstrated that physiological calcium (0.08-5 microM) inhibited mainly the high affinity alpha 2 isomer. In axolemma all the (Na,K)ATPase had high ouabain affinity and calcium inhibited 40-50% of the activity with a Ki of 1.9 +/- 0.9 x 10(-7) M. In skeletal muscle high and low ouabain affinity components were present in equal amounts and calcium inhibited only the high affinity component with a Ki of 1.3 +/- 0.3 x 10(-7) M. Kidney enzyme had a low affinity for ouabain and showed very little sensitivity to calcium in the physiological range. It was demonstrated that high calcium levels inhibit the enzyme in a general sense, irrespective of the isomer, with a Ki of 6.5 +/- 6 x 10(-4) M for the kidney and 5.9 +/- 4 x 10(-4) M for the axolemma enzymes. In axolemma, enzyme activity was studied as a function of sodium concentration. Physiological calcium reduced Vmax while not significantly changing K 0.5 for sodium binding.

Anomalies in the electrophoretic resolution of Na+/K(+)-ATPase catalytic subunit isoforms reveal unusual protein--detergent interactions

Three different isozymes of the Na+/K(+)-ATPase have slightly different different electrophoretic mobilities in sodium dodecyl sulfate (SDS). Certain procedures (reduction and alkylation, heating, and the use of sodium tetradecyl sulfate) have been reported either to improve the electrophoretic separation of isoforms or to reveal the presence of new isoforms. The variables affecting gel electrophoretic mobility were investigated here. Reduction and alkylation decreased the mobility of all three isozymes, and slightly improved the separation of alpha 1 from alpha 2 and alpha 3 without causing a qualitative change in the alpha isoforms detected. Heating the enzyme in SDS caused splitting into two bands. Both bands were intact polypeptides but migrated differently in 5% and 15% polyacrylamide, disclosing an anomalous conformation in detergent. The use of sodium tetradecyl or decyl sulfate instead of dodecyl sulfate altered the relative mobilities of the isozymes, revealing differences in detergent affinity, but no new isoforms were found. In conclusion, Na+/K(+)-ATPase alpha-subunit mobility reflects complex detergent-protein interaction that can be affected by experimental conditions. The existence of more than one band on gels may reflect different conformations in detergent, but should not be accepted alone as evidence for subunit structural heterogeneity.

Detection of a highly ouabain sensitive isoform of rat brainstem Na,K-ATPase

The present work provides evidence for the existence in rat brainstem of a form of Na,K-ATPase catalytic subunit that displays a high affinity for ouabain (Kd about 10(-9) M). Its kinetic identification was made out from studies on dose response curves of ouabain inhibition of Na,K-ATPase activity, ouabain inhibition of Na(+)-dependent phosphorylation from ATP and ouabain stabilized phosphoenzyme formation from inorganic phosphate (Pi). In all these studies this isoform comprises around 11 percent of the total Na,K-ATPase enzyme. The PAGE electrophoretic mobility of its phosphoprotein obtained from Pi in the presence of ouabain is lower than that of the alpha-1 form but it cannot be distinguished from that of alpha-2. Whether this highly ouabain sensitive form corresponds to the alpha-3 isoenzyme or represents the translational product of one of the additional genes described for the large catalytic subunit remains at the moment an open question.

A new electrophoretic variant of alpha subunit of Na+/K(+)-ATPase from the submandibular gland of rats

The alpha catalytic subunits of Na+/K(+)-ATPase were isolated from the kidney and brain of rats (alpha 1 and alpha 2, respectively). The antisera raised against these subunits were used as probes to analyze the isoform of catalytic subunits of Na+/K(+)-ATPase in various tissues of rats. Of 27 rat tissues examined, most had a catalytic subunit identical to alpha 1 but some, such as the nervous and muscle tissues, had both alpha 1 and alpha 2 isoforms as judged by their reactivities to antisera and their electrophoretic mobility. We found that the submandibular gland contained a new electrophoretic variant of immunoreactive alpha subunit (designated alpha(S) in this report) in addition to alpha 1 identical to those found in kidney and brain. The new variant, alpha(S), strongly cross-reacted with anti-alpha 1 antiserum, but to a lesser extent with anti-alpha 2 antiserum. The alpha(S) had a molecular mass which was found to be slightly less (approx. 90 kDa) than brain and kidney alpha 1. We examined whether or not the alpha(S) is formed by proteolytic cleavage of alpha subunits during preparation and concluded that this is not the case. The alpha(S) reacted with [gamma-32P]ATP, resulting in the formation of radioactive alpha subunit which was stabilized by 2 mM ouabain but which was labile in the presence of 70 mM potassium chloride. Since N-terminal amino acid sequence of alpha(S) protein [G()DKY()PAAVS] corresponds exactly and uniquely with the sequence of the alpha 1 chain between residues 1 and 11, it is very probable that alpha(S) protein originated from alpha 1 protein following the post-translational processing.

Na(+)-K(+)-ATPase gene expression during in vitro development of rat fetal forebrain

The existence of at least three isoforms of Na(+)-K(+)-ATPase in adult brain tissues [alpha 1, kidney type; alpha 2 [or alpha(+)]; alpha 3] suggests that these genes might be regulated in a cell-specific and time-dependent manner during development. We have studied this question in serum-free aggregating cell cultures of mechanically dissociated rat fetal telencephalon. At the protein level, the relative rate of synthesis of the pool of alpha 1-, alpha 2-, and alpha 3-subunits increased approximately twofold over 15 days of culture, leading to a marked increase in the immunochemical pool of alpha-subunits as measured by a panspecific polyclonal antibody. Concomitantly, Na(+)-K(+)-ATPase enzyme-specific activity increased three- (lower forebrain) to sixfold (upper forebrain). The transcripts of all three alpha-isoforms and beta-subunit were detected in vitro in similar proportion to the level observed in vivo. alpha 3-mRNA (3.7 kb) was more abundant than alpha 1 (3.7 kb) or alpha 2 (5.3 and 3.4 kb). Cytosine arabinoside (0.4 microM) and cholera toxin (0.1 microM) were used to selectively eliminate glial cells or neurons, respectively. It was found that alpha 2-mRNA is predominantly transcribed in glial cell cultures, whereas alpha 3- and beta 1-mRNA (2.7, 2.3, and 1.8 kb) are predominant in neuronal cultures.

Sodium affinity of brain Na(+)-K(+)-ATPase is dependent on isozyme and environment of the pump

The sodium affinities for the two forms of the Na(+)-K(+)-ATPase in brain were characterized. To mimic physiological conditions, synaptosomes, which are pinched off presynaptic nerve termini, were used. Examination of the pump in vitro was performed by preparing synaptic plasma membranes (SPMs). It was first shown that synaptosomes contain the two forms of the Na(+)-K(+)-ATPase, alpha 1 and alpha 2, and that these forms have markedly different affinities for the inhibitory cardiac glycoside ouabain. The apparent dissociation constant (K0.5) of alpha 1 for sodium changed from 12 to 9 mM when going from synaptosomes to membranes. For alpha 2, however, a shift from 36 to 12.5 mM was evident. The conclusion is that in vivo alpha 2 exists as a low sodium affinity species but can be altered to a high-affinity form simply by vesicle disruption. By comparison, the Na(+)-K(+)-ATPase from the mouse fibroblast cell line, 3T3-F442A cells, expressed only the alpha 1-isozyme, as shown by immunoblotting and by measurement of its ouabain and sodium affinities. The physiological relevance of these observations is also presented.

Complex Expression Patterns for Na+,K+-ATPase Isoforms in Retina and Optic Nerve

Three genetically distinct isozymes of the catalytic subunit of the Na,K-ATPase have been detected and have been designated alpha1, alpha2, and alpha3. To determine whether their expression is restricted to identifiable neurons and glia, specific monoclonal antibodies were used for immunofluorescent localization in the rat retina and optic nerve. The patterns of staining were markedly different, suggesting differences in cellular localization. Photoreceptor inner segments and optic nerve fibers expressed predominantly alpha3. Müller glia in the retina and astrocytes in the optic nerve expressed alpha1 and alpha2. Isolated, dissociated bipolar, horizontal, and Müller cells expressed different isozymes separately or in combination. The complexity of staining of neurons and their axons and dendrites suggested that Na,K-ATPase isozyme expression is not stereotyped, but is tailored to the ion transport needs of individual cell types, and targeted to specified membrane domains.

Two active Na+/K+-ATPases of high affinity for ouabain in adult rat brain membranes

The degree of heterogeneity of active Na+/K(+)-ATPases has been investigated in terms of ouabain sensitivity. A mathematical analysis of the dose-response curves (inhibition of Na+/K(+)-ATPase) at equilibrium is consistent with the putative existence of three inhibitory states for ouabain two of high (very high plus high) and one of low affinity. The computed IC50 values are: 23.0 +/- 0.15 nM, 460 +/- 4.0 nM and 320 +/- 4.6 microM, respectively. The relative abundance of the three inhibitory states was estimated as: 39%, 36% and 20%, respectively. Direct measurements of [3H]ouabain-binding at equilibrium carried out on membrane preparations with ATP, Mg2+ and Na+ also revealed two distinct high affinity-binding sites, the apparent Kd values of which were 17.0 +/- 0.2 nM (very high) and 80 +/- 1 nM (high), respectively. Dissociation processes were studied at different ouabain concentrations according to both reversal of enzyme inhibition and [3H]ouabain release. The reversal of enzyme inhibition occurred at three different rates, depending upon the ouabain doses used (10 nM, 2 and 100 microM). When the high-affinity sites were involved (ouabain doses lower than 2 microM) the dissociation process was biphasic. A similar biphasic pattern was also detected by [3H]ouabain-release. The time-course of [3H]ouabain dissociation (0.1 microM) was also biphasic. These data indicate that the three catalytic subunits of rat brain Na+/K(+)-ATPase alpha 1, alpha 2 and alpha 3 (Hsu, Y.-M. and Guidotti, G. (1989) Biochemistry 28, 569-573) are able to hydrolyse ATP and exhibit different affinities for cardiac glycosides.

The adhesion molecule on glia (AMOG) is a homologue of the beta subunit of the Na,K-ATPase

AMOG (adhesion molecule on glia) is a Ca2(+)-independent adhesion molecule which mediates selective neuron-astrocyte interaction in vitro (Antonicek, H., E. Persohn, and M. Schachner. 1987. J. Cell Biol. 104:1587-1595). Here we report the structure of AMOG and its association with the Na,K-ATPase. The complete cDNA sequence of mouse AMOG revealed 40% amino acid identity with the previously cloned beta subunit of rat brain Na,K-ATPase. Immunoaffinity-purified AMOG and the beta subunit of detergent-purified brain Na,K-ATPase had identical apparent molecular weights, and were immunologically cross-reactive. Immunoaffinity-purified AMOG was associated with a protein of 100,000 Mr. Monoclonal antibodies revealed that this associated protein comprised the alpha 2 (and possibly alpha 3) isoforms of the Na,K-ATPase catalytic subunit, but not alpha 1. The monoclonal AMOG antibody that blocks adhesion was shown to interact with Na,K-ATPase in intact cultured astrocytes by its ability to increase ouabain-inhibitable 86Rb+ uptake. AMOG-mediated adhesion occurred, however, both at 4 degrees C and in the presence of ouabain, an inhibitor of the Na,K-ATPase. Both AMOG and the beta subunit are predicted to be extracellularly exposed glycoproteins with single transmembrane segments, quite different in structure from the Na,K-ATPase alpha subunit or any other ion pump. We hypothesize that AMOG or variants of the beta subunit of the Na,K-ATPase, tightly associated with an alpha subunit, are recognition elements for adhesion that subsequently link cell adhesion with ion transport.

Expression of Na,K-ATPase alpha subunit isoforms in the human ciliary body and cultured ciliary epithelial cells

We have analyzed the expression of Na,K-ATPase alpha subunit isoforms in the transporting ciliary processes of the human eye and in cultured cells derived from non-pigmented (NPE) and pigmented (PE) ciliary epithelium. Northern hybridization analysis shows that the mRNAs encoding all the three distinct forms of Na,K-ATPase alpha subunit [alpha 1, alpha 2, and alpha 3] are expressed in the human ciliary processes in vivo. Immunohistochemical analysis using antibodies specific for each of the three alpha subunit isoforms confirms that these polypeptides are present in the microsomal fraction from the human ciliary processes. The monoclonal antibody McB2, which is specific to the Na,K-ATPase alpha 2 subunit isoform, has been found to decorate specifically the basolateral membrane domains of NPE cells but not of the PE cells, suggesting its expression in vivo only in the ocular NPE ciliary epithelium. However, cultured cells derived from the NPE and PE layers exhibit a different pattern of expression of mRNA and protein for the Na,K-ATPase alpha subunit isoforms when compared to the tissue. Both the NPE and PE cells express alpha 1 and alpha 3 mRNA and polypeptide, whereas alpha 2 mRNA and polypeptide are undetectable in these cells. The established cell lines derived from the NPE layer express comparable levels of the alpha 1 and alpha 3 isoforms of Na,K-ATPase as detected in the primary culture. However, the established NPE cell lines are also distinguishable from the normal PE cells when analyzed by Western blot analysis with A x 2 antibodies. The results presented here clearly show that the NPE and PE cells in the ciliary body have a distinct expression of Na,K-ATPase alpha subunit isoforms as compared to cultured cells.

Immunoaffinity isolation of Na,K-ATPase alpha 3 isoform from pig kidney

The Na,K-ATPase alpha 3 isoform of the catalytic subunit has been isolated from pig kidney microsomes. The procedure employs immunoaffinity chromatography on Sepharose 4B covalently coupled with monospecific antibodies a-II against the synthetic peptide including the putative alpha 3 N-terminus. The structural analysis provides unambiguous proof that the isolated protein corresponds to the third transcript for the alpha 3 isoform. The N-terminal amino acid sequence determined. Met-Gly-Asp-Lys-Lys-Asp-Asp, shows that unlike the alpha 1 and alpha 2 proteins, the mature Na,K-ATPase isoform lacks post-translational proteolytic processing.

Expression of active alpha-3 subunit of rat brain Na+,K+-ATPase from the messenger RNA injected into Xenopus oocytes

Cloned cDNA encoding the so-far uncharacterized alpha-3 subunit of rat brain Na+,K+-ATPase (Hara et al. (1987) J. Biochem. 102, 43-58, Shull et al. (1986) Biochemistry 25, 8125-8132) was incorporated into a vector carrying the SP6 promoter. The mRNA produced in vitro was injected into Xenopus oocytes with the mRNA encoding the Na+,K+-ATPase beta subunit of Torpedo electroplax. Increased Na+,K+-ATPase activity in the oocyte membrane was observed. This newly expressed activity was inhibited by ouabain (Ki = 1.5 x 10(-7) M), suggesting that the alpha-3 subunit of rat brain Na+,K+-ATPase is a highly ouabain-sensitive catalytic subunit.