Na+,K(+)-ATPase

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

Genetic probing of the stalk segments associated with M2 and M3 of the plasma membrane H+-ATPase from Saccharomyces cerevisiae

The stalk region of the H+-ATPase from Saccharomyces cerevisiae has been proposed to play a role in coupling ATP hydrolysis to proton transport. Genetic probing was used to examine the role of stalk segments S2 and S3, associated with M2 and M3, respectively. Saturation mutagenesis was used to explore the role of side group character at position Ile183 in S2, at which an alanine substitution was shown previously to partially uncouple the enzyme (Wang, G., Tamas, M. J., Hall, M. J., Pascual-Ahuir, A., and Perlin, D. S. (1996) J. Biol. Chem. 271, 25438-25445). Diverse side group substitutions were tolerated at this position, although three substitutions, I183N, I183R, and I183Y required second site mutations at the C terminus of the enzyme for stabilization. Substitution of glycine and proline at Ile183 resulted in lethal phenotypes, suggesting that the backbone may be more important than side group at this position. Proline/glycine mutagenesis was used to study additional sites in S2 and S3. The substitution of proline at Gly186 resulted in a lethal phenotype, whereas substitutions in S3 of proline or serine at Gly270 and proline or glycine at Thr287 resulted in viable mutants. Mutations G270P and T287P resulted in mutant enzymes that produced pronounced growth defects and ATP hydrolysis rates that were 35% and 60% lower than wild type enzyme, respectively. The mutant enzymes transported protons at rates consistent with their ATPase activity, suggesting that the growth defects observed were due to a reduced rate of ATP hydrolysis and not to uncoupling of proton transport. The prominent growth phenotypes produced by mutations G270P and T287P permitted the isolation of suppressor mutations, which restored wild type growth. Most of the suppressors either replaced the primary site mutation with alanine or restored the wild type residue by ectopic recombination with PMA2, both of which restore alpha-helical tendency. This study suggests that maintaining alpha-helical character is essential to S2 and may play an important role in S3.

Impaired Na+,K+ATPase activity in red blood cells in euthyroid women treated with levothyroxine after total thyroidectomy for Graves' disease

In patients suffering from hyperthyroidism dependent on Graves' disease, a reduction in Na+,K+ATPase activity has been demonstrated in red blood cells (RBCs), as well as an inverse correlation between this enzymatic action and free triiodothyronine (FT3) levels. The restoration of normal FT3 values also brings about a normalization of Na+,K+ATPase activity in erythrocytes. These results have made it possible to hypothesize that the thyroid hormones control Na+,K+ATPase activity and that this control is manifested by means of variations in the number of ouabain-binding sites. For this reason, the measurement of the activity of the Na/K pump can be considered as a further indicator of the peripheral effects of thyroid hormones. With a view to assess the relation between the course of treated hyperthyroidism and Na+,K+ATPase activity during antithyroid therapy and after surgical thyroidectomy followed by replacement therapy, we studied 24 patients affected by Graves' disease (group Graves [GG]). They were compared with 24 female Graves' patients who underwent total thyroidectomy for nontoxic and diffuse nodular goiter (NDNG) (group control [GC]) and with 24 normal healthy women (group normal [GN]). When Graves' hyperthyroidism was diagnosed, the Na+,K+ATPase activity in RBCs was impaired in all GG patients. Thionamide treatment restored the normal activity of the Na/K pump, accompanied by normalization of the number of ouabain-binding sites. One hundred eighty days after thyroidectomy, in conditions of clinical and biochemical euthyroidism due to replacement therapy with levothyroxine, the activity of Na+,K+ATPase in RBCs was once again reduced in GG, while appearing normal in GC and GN (1.77 +/- 0.16 mmol Pi h(-1) L(-1) RBCs v 2.09 +/- 0.26 v 2.09 +/- 0.24, P < .05). Different instrumental or biochemical parameters, such as glycemia, serum lipids, ions, serum alkaline phosphatase (AIPh), serum creatine phosphokinase (CPK), blood pressure, and heart rate, were evaluated and appeared normalized in GG and GC 180 days after surgery. We conclude that (1) in patients suffering from Graves' disease, subjected to total thyroidectomy followed by levothyroxine replacement therapy, there is a reduction in the activity of the Na+,K+ATPase on erythrocytes 6 months after the surgical approach; and (2) a similar alteration is not observed in patients subjected to thyroidectomy for NDNG. These findings allow the formulation of the hypothesis that (1) treatment with levothyroxine for 180 days after thyroidectomy in GG is not long enough to restore the normality of all the peripheral indicators of action of the thyroid hormones; and (2) levothyroxine replacement therapy is unable to guarantee euthyroidism in all the tissues in GG (eg, during hematopoiesis in the bone marrow).

Mutagenesis disrupts posttranslational processing of the Na,K-ATPase catalytic subunit

The first 5 amino acids of the catalytic alpha 1 isoform from Na,K-ATPase are cleaved enzymatically during or after translation. To evaluate the structural requirements for that cleavage, we constructed amino-terminal mutants of alpha 1 in which an epitope tag from the c-myc oncogene product was added. Immunoblots of isolated membranes from transfected monkey kidney cells revealed binding of an antibody specific for the first 9 residues of the alpha 1 nascent protein. Because this antibody does not recognize the shorter sequence corresponding to the processed polypeptide, these results indicate that the epitope tag prevented normal processing, a conclusion confirmed by the observed binding of an anti-myc antibody. In contrast, membranes from cells expressing deletion mutants that lack residues 10-24 and 10-31 of the nascent chain failed to bind the amino-terminal-directed antibody, suggesting that the mutants were cleaved normally and that amino acids downstream of the first 9 are not required for proteolysis. Amino-terminal mutants produced in other laboratories have shown an anomalous stimulation of ATPase activity by K+ when measured in low ATP concentrations. The myc-tagged and downstream deletion mutants were sensitive to K+ in the range from 0.05 to 5 mM, similar to wild-type enzyme, despite the differences in posttranslational processing. A mutant missing the first 40 residues of the nascent chain, however, displayed an activation by K+. These results suggest that amino-terminal processing of the alpha 1 isoform was prevented by mutation, yet that processing had little influence on the kinetic parameter most likely to be influenced by such changes.

Forskolin-induced down-regulation of Na+,K(+)-ATPase activity is not associated with internalization of the enzyme

Activation by protein kinase A by forskolin phosphorylates and inactivates Na+,K(+)-ATPase in COS-7 cells (Cheng et al. 1997b). In this study we show, using [3H]ouabain binding, that forskolin-induced inhibition of Na+,K(+)-ATPase activity is not because of internalization of the enzyme. The effect of forskolin on Na+,K(+)-ATPase activity was examined by two independent methods, ouabain-sensitive 86Rb+ uptake in intact cells and ATP hydrolysis in microsomal preparations from cells. The change in number of functional pumps on cell surface before and after protein kinase A activation was assessed by [3H]ouabain binding measured under equilibrium conditions. Cells, which had been ATP-depleted by antimycin A and 2-deoxyglucose treatment, served as a positive control for the internalization of Na+,K(+)-ATPase. Activation of protein kinase A with forskolin in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine, inhibited Na+,K(+)-ATPase activity, but this treatment had no effect on specific ouabain binding. No change in ouabain binding was found following activation of protein kinase C by phorbol ester or diacyl glycerol analogue treatment in cells. These data suggest that protein kinase A phosphorylation and inhibition of Na+,K(+)-ATPase activity does not lead to any internalization of the enzyme in COS-7 cells.

Effects of protein kinase C modulators on Na+/K+ adenosine triphosphatase activity and phosphorylation in aortae from rats with cirrhosis

Protein kinase C (PKC) modulates the activity and phosphorylation of the catalytic alpha-subunit of sodium-potassium-adenosine triphosphatase (Na+/K+ ATPase) in normal arteries. Because PKC is altered in cirrhotic aortae, Na+/K+ ATPase may also be altered in these arteries. The aim of the present study was to investigate alpha-subunit activity and phosphorylation in aortae from normal and cirrhotic rats, under baseline conditions and during exposure to PKC modulators. Alpha-subunit activity was assessed by measuring the amount of 32P released by hydrolysis of [gamma-32P]ATP in freshly isolated cell membranes (in the absence of PKC modulators only) and membrane depolarization caused by ouabain-induced alpha-subunit inhibition in isolated aortae (in the absence and presence of PKC modulators). Alpha-subunit phosphorylation was assessed by incorporation of 32P into alpha-subunits. Staurosporine, a PKC inhibitor, and phorbol 12,13-dibutyrate (PDBU), a PKC activator, were used. In addition, alpha-subunit expression was studied by Western blot analysis. In the absence of PKC modulators, the amount of 32P released by hydrolysis of [gamma-32P]ATP and ouabain-induced membrane depolarization were significantly lower in cirrhotic than in normal aortae. Staurosporine suppressed ouabain-induced membrane depolarization in cirrhotic and normal arteries. Ouabain-induced membrane depolarization was similar in cirrhotic aortae exposed to PDBU and in normal arteries studied under baseline conditions. Alpha-subunit phosphorylation was significantly lower in cirrhotic than in normal aortae, in aortae under baseline conditions, and in arteries exposed to staurosporine. Phosphorylation of the alpha-subunit was similar in cirrhotic aortae exposed to PDBU and in normal arteries under baseline conditions. Western blot analysis showed that the amount of alpha-subunit did not significantly differ between cirrhotic and normal aortae. In conclusion, a decrease in baseline Na+/K+ ATPase alpha-subunit activity occurs in aortae from cirrhotic rats as a result of reduced basal PKC activity. This PKC-dependent decreased alpha-subunit activity may be caused by a reduction in PKC-induced alpha-subunit phosphorylation.

The cytoplasmic loop located between transmembrane segments 6 and 7 controls activation by Ca2+ of sarcoplasmic reticulum Ca2+-ATPase

During active cation transport, sarcoplasmic reticulum Ca2+-ATPase, like other P-type ATPases, undergoes major conformational changes, some of which are dependent on Ca2+ binding to high affinity transport sites. We here report that, in addition to previously described residues of the transmembrane region (Clarke, D. M., Loo, T. W., Inesi, G., and MacLennan, D. H. (1989) Nature 339, 476-478), the region located in the cytosolic L6-7 loop connecting transmembrane segments M6 and M7 has a definite influence on the sensitivity of the Ca2+-ATPase to Ca2+, i.e. on the affinity of the ATPase for Ca2+. Cluster mutation of aspartic residues in this loop results in a strong reduction of the affinity for Ca2+, as shown by the Ca2+ dependence of ATPase phosphorylation from either ATP or Pi. The reduction in Ca2+ affinity for phosphorylation from Pi is observed both at acidic and neutral pH, suggesting that these mutations interfere with binding of the first Ca2+, as proposed for some of the intramembranous residues essential for Ca2+ binding (Andersen, J. P. (1995) Biosci. Rep. 15, 243-261). Treatment of the mutated Ca2+-ATPase with proteinase K, in the absence or presence of various Ca2+ concentrations, leads to Ca2+-dependent changes in the proteolytic degradation pattern similar to those in the wild type but observed only at higher Ca2+ concentrations. This implies that these effects are not due to changes in the conformational state of Ca2+-free ATPase but that changes affecting the proteolytic digestion pattern require higher Ca2+ concentrations. We conclude that aspartic residues in the L6-7 loop might interact with Ca2+ during the initial steps of Ca2+ binding.

Mineralocorticoid receptor knockout mice: pathophysiology of Na+ metabolism

Mineralocorticoid receptor (MR)-deficient mice were generated by gene targeting. These animals had a normal prenatal development. During the first week of life, MR-deficient (-/-) mice developed symptoms of pseudohypoaldosteronism. They finally lost weight and eventually died at around day 10 after birth from dehydration by renal sodium and water loss. At day 8, -/- mice showed hyperkalemia, hyponatremia, and a strong increase in renin, angiotensin II, and aldosterone plasma concentrations. Methods were established to measure renal clearance and colonic transepithelial Na+ reabsorption in 8-day-old mice in vivo. The fractional renal Na+ excretion was elevated >8-fold. The glomerular filtration rate in -/- mice was not different from controls. The effect of amiloride on renal Na+ excretion and colonic transepithelial voltage reflects the function of amiloide-sensitive epithelial Na+ channels (ENaC). In -/- mice, it was reduced to 24% in the kidney and to 16% in the colon. There was, however, still significant residual ENaC-mediated Na+ reabsorption in both epithelia. RNase protection analysis of the subunits of ENaC and (Na++ K+)-ATPase did not reveal a decrease in -/- mice. The present data indicate that MR-deficient neonates die because they are not able to compensate renal Na+ loss. Regulation of Na+ reabsorption via MR is not achieved by transcriptional control of ENaC and (Na+ + K+)-ATPase in RNA abundance but by transcriptional control of other as yet unidentified genes. MR knockout mice will be a suitable tool for the search of these genes.

Effect of Na+ on Na+,K+-ATPase alpha-subunit expression and Na+-pump activity in aortic smooth muscle cells

In earlier studies we demonstrated that cyclical mechanical strain on vascular smooth muscle cells increases intracellular Na+ and upregulates the alpha-1 and alpha-2 isoform expression of Na+,K+-ATPase, and that the increase of intracellular Na+ and upregulation of the alpha-2 isoform expression are blocked by Gd3+, which blocks entry of ions (including Na+) through stretch-activated channels. The present study was designed to investigate the role of intracellular Na+ in Na+,K+-ATPase regulation by increasing intracellular Na+ with chronic ouabain treatment. In parallel experiments, we measured Na+,K+-ATPase alpha isoform expression, Na+-pump activity and intracellular Na+ in cultured aortic smooth muscle cells after treatment with two concentrations of ouabain for various time periods. Treatment with 100 nM ouabain resulted in a significant elevation in intracellular Na+ after 1 (21%) and 2 h (12%), but the value returned to baseline after 12 h. Both alpha-1 and alpha-2 subunits of Na+,K+-ATPase were significantly upregulated after 1 through 4 days. Na+-pump activity was also stimulated, and the time course of this effect closely followed protein expression. At 200 microM of ouabain, the effects on intracellular Na+, isoform expression and Na+-pump activity at earlier time points (1 h through 1 day) were similar to those with 100 nM treatment, but prolonged treatment (2 and 4 days) resulted in an accumulation of intracellular Na+ and inhibition of the isoform expression and Na+-pump activity, possibly due to general dysfunction of the cells as a result of chronic exposure to high concentrations of ouabain. We conclude that elevated intracellular Na+ can serve as a signal to mediate the alpha isoform upregulation and the regulatory process requires less than one day.

Multiple signal transduction pathways link Na+/K+-ATPase to growth-related genes in cardiac myocytes. The roles of Ras and mitogen-activated protein kinases

We showed before that in neonatal rat cardiac myocytes partial inhibition of Na+/K+-ATPase by nontoxic concentrations of ouabain causes hypertrophic growth and transcriptional regulations of genes that are markers of cardiac hypertrophy. In view of the suggested roles of Ras and p42/44 mitogen-activated protein kinases (MAPKs) as key mediators of cardiac hypertrophy, the aim of this work was to explore their roles in ouabain-initiated signal pathways regulating four growth-related genes of these myocytes, i.e. those for c-Fos, skeletal alpha-actin, atrial natriuretic factor, and the alpha3-subunit of Na+/K+-ATPase. Ouabain caused rapid activations of Ras and p42/44 MAPKs; the latter was sustained longer than 90 min. Using high efficiency adenoviral-mediated expression of a dominant-negative Ras mutant, and a specific inhibitor of MAPK kinase (MEK), activation of Ras-Raf-MEK-p42/44 MAPK cascade by ouabain was shown. The effects of the mutant Ras, an inhibitor of Ras farnesylation, and the MEK inhibitor on ouabain-induced changes in mRNAs of the four genes indicated that (a) skeletal alpha-actin induction was dependent on Ras but not on p42/44 MAPKs, (b) alpha3 repression was dependent on the Ras-p42/44 MAPK cascade, and (c) induction of c-fos or atrial natriuretic factor gene occurred partly through the Ras-p42/44 MAPK cascade, and partly through pathways independent of Ras and p42/44 MAPKs. All ouabain effects required extracellular Ca2+, and were attenuated by a Ca2+/calmodulin antagonist or a protein kinase C inhibitor. The findings show that (a) signal pathways linked to sarcolemmal Na+/K+-ATPase share early segments involving Ca2+ and protein kinase C, but diverge into multiple branches only some of which involve Ras, or p42/44 MAPKs, or both; and (b) there are significant differences between this network and the related gene regulatory pathways activated by other hypertrophic stimuli, including those whose responses involve increases in intracellular free Ca2+ through different mechanisms.

Altered Na+ and Li+ homeostasis in Saccharomyces cerevisiae cells expressing the bacterial cation antiporter NhaA

The bacterial Na+ (Li+)/H+ antiporter NhaA has been expressed in the yeast Saccharomyces cerevisiae. NhaA was present in both the plasma membrane and internal membranes, and it conferred lithium but not sodium tolerance. In cells containing the yeast Ena1-4 (Na+, Li+) extrusion ATPase, the extra lithium tolerance conferred by NhaA was dependent on a functional vacuolar H+ ATPase and correlated with an increase of lithium in an intracellular pool which exhibited slow efflux of cations. In yeast mutants without (Na+, Li+) ATPase, lithium tolerance conferred by NhaA was not dependent on a functional vacuolar H+ ATPase and correlated with a decrease of intracellular lithium. NhaA was able to confer sodium tolerance and to decrease intracellular sodium accumulation in a double mutant devoid of both plasma membrane (Na+, Li+) ATPase and vacuolar H+ ATPase. These results indicate that the bacterial antiporter NhaA expressed in yeast is functional at both the plasma membrane and the vacuolar membrane. The phenotypes conferred by its expression depend on the functionally of plasma membrane (Na+, Li+) ATPase and vacuolar H+ ATPase.

Identification of the site of inhibition by omeprazole of a alpha-beta fusion protein of the H,K-ATPase using site-directed mutagenesis

The alpha subunit of eukaryotic P-type ATPases has ten experimentally defined transmembrane or membrane inserted segments. The fifth and sixth of these are short, not predicted by hydropathy analysis, do not insert independently into microsomal membranes, and are readily removed after tryptic digestion and therefore may be membrane inserted sequences. Acid transport by the gastric H, K-ATPase is covalently inhibited by several substituted pyridyl methylsulfinyl benzimidazoles, such as omeprazole. These act as probes of accessible extracytoplasmic thiols because they are accumulated in the acid transporting gastric vesicles and then convert to thiol reactive, cationic tetracyclic sulfenamides. Inhibition is due mainly to disulfide formation with Cys813 or Cys822 in M5/6 and perhaps with a contribution from Cys892 in the loop between transmembrane segment (TM) 7 and TM8. Identification of the specific cysteine responsible for inhibition should be able to define the turn between M5 and M6. The gastric H,K-ATPase alpha-beta heterodimer was expressed as a fusion protein in HEK 293 cells. Transient transfection resulted in most of the protein being retained in the endoplasmic reticulum with only core glycosylation and minor activity of the ATPase evident. Stable transfection resulted in plasma membrane localization of the protein and complex glycosylation. The transfected but not the control cells displayed cation-stimulated, SCH 28080-inhibited ATPase activity and SCH 28080- and omeprazole-inhibited 86Rb uptake. The two cysteines in M5/6 and Cys892 in the TM7/8 loop were mutated to the amino acids found in the Na,K-ATPase in order to determine which of the three cysteine residues were important for benzimidazole inhibition. Mutation of one, two, or all three cysteines did not alter enzyme activity, 86Rb transport, or SCH 28080 inhibition. Only removal of Cys822 blocked omeprazole inhibition of 86Rb transport. These data suggest that Cys822 is present in a region of the enzyme most easily accessed by the cationic sulfenamide formed by omeprazole, presumably the turn between M5 and M6.

Selectivity of lipid-protein interactions with trypsinized Na, K-ATPase studied by spin-label EPR

The selectivity of the lipid-protein interactions in trypsinised Na, K-ATPase membranes from Squalus acanthias has been determined by using EPR spectroscopy with different lipid probes spin-labelled on the 14-C atom of the fatty acid chain. From measurements at low ionic strength and different pH values, the pattern of selectivity is: (stearic acid)->(phosphatidylserine)->(stearic acid)0>(phosphatidylcholine)+/-, where superscripts indicate the formal electrostatic charge on the lipid headgroup. This is in the same order as that determined with native Na,K-ATPase membranes [M. Esmann, D. Marsh, Biochemistry 24 (1985) 3572-3578]. The selectivity for phosphatidylserine is independent of pH, over the range pH 6.0-9. 0, as found also for native membranes. For membranes trypsinised in the presence of Rb+ ions, and in the presence of Na+ (which allows more extensive proteolysis), the relative association constants, Kr, of all lipids are the same as for control membranes, with the exception of ionised (stearic acid)- that shows the highest specificity. Therefore, both the stoichiometry and the principal determinants of the specificity of lipid-protein interaction are preserved on extensive trypsinisation of Na,K-ATPase membranes. This has implications for the location and arrangement of those amino acid side chains that determine the lipid selectivity of the native Na,K-ATPase.

Identification of a model cardiac glycoside receptor: comparisons with Na+,K+-ATPase

The availability of high-affinity anti-digoxin monoclonal antibodies (mAbs) offers the potential for their use as models for the characterization of the relationship between receptor structure and cardiac glycoside binding. We have characterized the binding of anthroylouabain (AO), a fluorescent derivative of the cardiac glycoside ouabain, to mAbs 26-10, 45-20, and 40-50 [Mudgett-Hunter, M., et al. (1995) Mol. Immunol. 22, 477] and lamb kidney Na+, K+-ATPase by monitoring the resultant AO fluorescence emission spectra, anisotropy, lifetime values, and Förster resonance energy transfer (FRET) from protein tryptophan(s) (Trp) to AO. These data suggest that the structural environment in the vicinity of the AO-binding site of Na+,K+-ATPase is similar to that of mAb 26-10 but not mAbs 45-20 and 40-50. A model of AO complexed to the antigen binding fragment (Fab) of mAb 26-10 which was generated using known X-ray crystal structural data [Jeffrey, P. D., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 10310] shows a heavy chain Trp residue (Trp-H100) that is close ( approximately 3 A) to the anthroyl moiety. This is consistent with the energy transfer seen upon AO binding to mAb 26-10 and suggests that Trp-H100, which is part of the antibody's cardiac glycoside binding site, is a major determinant of the fluorescence properties of bound AO. In contrast, the generated model of AO complexed to Fab 40-50 [Jeffrey, P. D., et al. (1995) J. Mol. Biol. 248, 344] shows a heavy chain Tyr residue (Tyr-H100) which is part of the cardiac glycoside binding site, located approximately 10 A from the anthroyl moiety. The closest Trp residues (H52 and L35) are located approximately 17 A from the anthroyl moiety, and no FRET is observed despite the fact that these Trp residues are close enough for significant FRET to occur. The energy transfer seen upon AO binding to Na+,K+-ATPase suggests the presence of one completely quenched or two highly quenched enzyme Trp residues approximately 10 and approximately 17 A, respectively, from the anthroyl moiety. These data suggest that the Na+,K+-ATPase Trp residue(s) involved in fluorescence energy transfer to AO is likely to be part of the cardiac glycoside binding site.

The M4M5 cytoplasmic loop of the Na,K-ATPase, overexpressed in Escherichia coli, binds nucleoside triphosphates with the same selectivity as the intact native protein

Escherichia coli was used to overexpress the large cytoplasmic loop of the rat Na,K-ATPase. A 1260-base DNA segment encoding Lys354-Lys774 of the rat alpha1-subunit was constructed via polymerase chain reaction. The polymerase chain reaction product was successfully subcloned into the expression vector pET-28 (Novagen), which produces an N-terminal 6-histidine-tagged fusion protein. The pET-28 vector containing rat alpha-loop, i.e. pAN, was used to transform calcium-competent E. coli BL21(DE3) cells, and positive clones were selected by kanamycin resistance. Bacterial cultures were grown, and protein synthesis was induced with isopropyl beta-D-thiogalactoside. Cells were harvested and lysed, revealing production of the His-tagged fusion protein ( approximately 46 kDa). The fusion protein was affinity-purified from other soluble cellular proteins via a Ni-NTA column, which routinely yielded approximately 20 mg of soluble His6-alpha-loop/L cell culture. The His6-alpha-loop retained significant native structure, as evidenced by the ability of ATP and ADP (but not AMP, CTP, GTP, or UTP) to protect against chemical modification by either fluorescein isothiocyanate or maleimidylanilinonapthalene sulfonic acid. More specifically, circular dichroism spectroscopy was used to estimate the secondary structure of the His6 loop, revealing an ordered folding composed of 23% alpha-helix, 23% antiparallel beta-sheet, 4% parallel beta-sheet, 19% beta-turn, and 32% random coil. The 6-histidine loop bound the fluorescent ATP analog trinitrophenyl-ATP with high affinity, as determined by measuring the fluorescence changes associated with binding. Affinities for ATP ( approximately 350 microM) and ADP ( approximately 550 microM) were determined by their ability to compete with and displace 2',3'-O-[2,4,6,-trinitrophenyl]-ATP. These nucleotide affinities are similar to those observed for the E2 conformation of the intact Na,K-ATPase.

E31-K352, the minimal cation binding moiety of Na+,K(+)-ATPase

Upon limited tryptic fragmentation of Na+,K(+)-ATPase a 35 kDa fragment (E31-K352) was formed that bound 204Tl+ on blot. Further fragmentation led to loss of binding, pointing to the conclusion that E31-K352 is the minimal cation binding unit in Na+,K(+)-ATPase.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Interactions between fragments of trypsinized Na,K-ATPase detected by thermal inactivation of Rb+ occlusion and dissociation of the M5/M6 fragment

This work provides evidence for interactions between fragments of "19-kDa membranes," a trypsinized preparation of Na,K-ATPase that retains cation occlusion and ouabain binding. Previously, we reported rapid thermal inactivation of Rb+ occlusion at 37 degreesC (Or, E., David, P., Shainskaya, A., Tal, D. M., and Karlish, S. J. D. (1993) J. Biol. Chem. 268, 16929-16937). We describe here the detailed kinetics of thermal inactivation. In the range 25-35 degreesC, a two-step model (N left and right arrow U --> I, where N is the native species, U is the reversibly unfolded intermediate, and I is the irreversibly denatured form) fits the data. Reversibility of inactivation has been observed at 25 degreesC, consistent with the model. At 37 degreesC and higher temperatures, the data can be fitted to the simple mechanism N --> I, i.e. U is not significant as an intermediate. Occluded cations (Na+, Rb+, K+, Tl+, NH4+, and Cs+) and ouabain protect strongly against thermal inactivation. Ca2+, Ba2+, and La3+ ions do not protect. Proteolysis experiments provide independent evidence that disorganization can occur in stages, first in transmembrane segments and then in extra-membrane segments of the fragments. Analysis of selective dissociation of the M5/M6 fragment at 37 degreesC (Lutsenko, S., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940), using a specific antibody, showed that inactivation of Rb+ occlusion precedes dissociation of the fragment, and only approximately 50% of the fragment is released when occlusion is fully inactivated. In the presence of Ca2+ ions, occlusion is inactivated, but the M5/M6 fragment is not released. The experiments demonstrate that occlusion is inactivated by disruption of interactions between fragments of 19-kDa membranes, and only then does the M5/M6 fragment dissociate. Interactions between the M5/M6 and M7/M10 fragments seem to be essential for maintenance of Rb+ occlusion.

Expression, distribution, and activity of Na+,K+-ATPase in normal and cholestatic rat liver

Hepatocellular Na+,K+-ATPase is an important driving force for bile secretion and has been localized to the basolateral plasma membrane domain. Cholestasis or impaired bile flow is known to modulate the expression, domain specificity, and activity of various transport systems involved in bile secretion. This study examined Na+, K+-ATPase after ethinylestradiol (EE) treatment and after bile duct ligation (BDL), two rat models of cholestasis. It applied quantitative immunoblotting, biochemical and cytochemical determination of enzyme activity, and immunocytochemistry to the same livers. The data showed a good correlation among the results of the different methods. Neither EE nor BDL induced alterations in the subcellular distribution of Na+,K+-ATPase, which was found in the basolateral but not in the canalicular (apical) plasma membrane domain. Protein expression and enzyme activity showed a small (approximately 10%) decrease after EE treatment and a similar increase after BDL. These modest changes could not be detected by immunofluorescence, immuno EM, or cytochemistry. The data, therefore, demonstrate that Na+,K+-ATPase is only slightly affected by EE and BDL. They suggest that other components of the bile secretory apparatus that take effect downstream of the primary basolateral driving force may play a more prominent role in the pathogenesis of cholestasis.

Measurements of (Na+,K+)ATPase after in vitro hypoxia and reoxygenation are affected by methods of membrane preparation

(Na+,K+ )ATPase activity was evaluated in membranes from rat hippocampal slices after in vitro hypoxia and reoxygenation. Membranes were prepared with two different methods, one using an isotonic medium and another using a hypotonic one. The changes that were found after hypoxia went into opposite directions in the two cases. Membranes prepared in a hypotonic medium are probably more suitable for these measurements. Using these membranes, hypoxia results in a slight decrease of (Na+,K+)ATPase activity and in a further decrease after reoxygenation. We also found that expressing (Na+,K+)ATPase activity as a percent of total ATPase activity is appropriate for membranes prepared under hypotonic conditions and can unveil (by reducing variability between experiments) significant changes that may be masked in small samples like ours.

Importance of intramembrane carboxylic acids for occlusion of K+ ions at equilibrium in renal Na,K-ATPase

Site-directed mutagenesis and assay of Rb+ and Tl+ occlusion in recombinant Na,K-ATPase from yeast were combined to establish structure-function relationships of amino acid side chains involved in high-affinity occlusion of K+ in the E2[2K] form. The wild-type yeast enzyme was capable of occluding 2 Rb+ or Tl+ ions/ouabain binding site or alpha 1 beta 1 unit with high apparent affinity (Kd(Tl+) = 7 +/- 2 microM), like the purified Na,K-ATPase from pig kidney. Mutations of Glu327(Gln,Asp), Asp804(Asn, Glu), Asp808(Asn, Glu) and Glu779(Asp) abolished high-affinity occlusion of Rb+ or Tl+ ions. The substitution of Glu779 for Gln reduced the occlusion capacity to 1 Tl+ ion/alpha 1 beta 1-unit with a 3-fold decrease of the apparent affinity for the ion (Kd(Tl+) = 24 +/- 8 microM). These effects on occlusion were closely correlated to effects of the mutations on K0.5(K+) for K+ displacement of ATP binding. Each of the four carboxylate residues Glu327, Glu779, and Asp804 or Asp808 in transmembrane segments 4, 5, and 6 is therefore essential for high-affinity occlusion of K+ in the E2[2K] form. These residues either may engage directly in cation coordination or they may be important for formation or stability of the occlusion cavity.

Evidence that human endothelial cells express different isoforms of Na,K-ATPase

BACKGROUND

The catalytic alpha and smaller beta subunits of the plasma membrane Na,K-ATPase occur in various molecular forms (alpha1, alpha2, alpha3, beta1 and beta2). The alpha isoforms of the enzyme have varying affinities for ouabain and exist in different tissues with particular distribution patterns.

OBJECTIVE

To document the existence of isoforms of the Na,K-ATPase in cultured human umbilical vein endothelial cells.

METHODS

Microsomal fractions were prepared by differential ultracentrifugation from primary cultures of human umbilical vein endothelial cells and from such cells obtained after three passages. Na,K-ATPase activity was assayed using the coupled assay method and sensitivity to ouabain was determined in the presence of varying concentrations of ouabain. Specific antibodies for the various Na,K-ATPase isoforms were used to label these different proteins by immunocytochemistry in endothelial cells and by Western blotting in isolated membranes.

RESULTS

In plotting the dose-response curves for Na,K-ATPase activity in response to ouabain we assumed the existence of two independent sites exhibiting different affinities for ouabain (in the micromol/l and the nmol/l ranges). The contribution of low-affinity sites was threefold that of high-affinity sites. After three passages in culture, a specific increase in Na,K-ATPase activity of the high-affinity sites was observed compared with that of cells from primary cultures. Confocal microscopy revealed the existence of beta1, beta2, and alpha1 subunit proteins in human umbilical endothelial cells. Staining for alpha3 isoform was less pronounced and no obvious alpha2 was detected.

CONCLUSION

These findings suggest that human umbilical vein endothelial cells contain beta1, beta2, a large amount of alpha1 isoform with an apparently low affinity for ouabain, and a lesser amount of high-affinity sites, which may correspond to the alpha3 protein.

The inhibition of fibroblast growth factor-2 export by cardenolides implies a novel function for the catalytic subunit of Na+,K+-ATPase

Basic fibroblast growth factor (FGF-2) is one of a select group of proteins that can exit cells through an alternate, endoplasmic reticulum/Golgi apparatus independent exocytic pathway. This alternate pathway has been termed protein export. In an attempt to better understand this process, we have identified a family of related compounds, "cardenolides," that inhibit FGF-2 export. The cardenolides inhibit FGF-2 export in a time and concentration dependent fashion. Inhibition of FGF-2 export is specific in that the cardenolides have no effect on conventional protein secretion as measured by their inability to block release of the secreted protein human chorionic gonadotropin-alpha. Because cardenolides are known to inhibit ion transport activity mediated by Na+,K+-ATPase, we investigated whether there are functional interactions between FGF-2 and their only known molecular target: the alpha-subunit of Na+, K+-ATPase. Export of FGF-2 from COS-1 cells is selectively inhibited when co-transfected with expression vectors encoding the alpha-subunit and FGF-2. Moreover, antibodies to the alpha-subunit specifically co-immunoprecipitate FGF-2 along with the alpha-subunit while conversely, antibodies to FGF-2 specifically co-immunoprecipitate the alpha-subunit along with FGF-2. Finally, the ion transporting activities of the Na+,K+-ATPase can be uncoupled from protein export. Varying the external concentration of K+ has little effect on export of FGF-2. Taken together, these data: 1) identify a novel activity for cardenolides; 2) suggest a previously unknown role for the alpha-subunit of Na+, K+-ATPase in FGF-2 export; and 3) raise the possibility that the alpha-subunit itself may be an integral component of this alternate exocytic pathway mediating translocation of cytosolic FGF-2 to the cell surface.

Role of Na+ and Ca2+ in stretch-induced Na(+)-K(+)-ATPase alpha-subunit regulation in aortic smooth muscle cells

This study was designed to test the role of Na+ and Ca2+ entry in the stretch-induced Na(+)-K(+)-ATPase alpha 1- and alpha 2-isoform upregulation observed in our previous studies. We measured intracellular Na+ in cyclically stretched rat aortic smooth muscle cells, with or without gadolinium treatment, for various durations and performed Western blotting to analyze the effects of stretch and the calcium channel blocker isradipine on the expression of alpha-isoforms. Intracellular Na+ was elevated significantly after 1- and 2-h stretch, but returned to baseline after 1-, 2-, and 4-day stretch. This increase in intracellular Na+ was blocked by gadolinium. Both alpha 1- and alpha 2-isoforms were upregulated after either 2 or 4 days of cyclical stretch. Isradipine had no apparent effect on stretch-induced upregulation on either alpha-isoform, thus suggesting that Ca2+ entry through L-type channels is not involved in the stretch-induced upregulation. We therefore conclude that a transient intracellular Na+ elevation during stretch may serve as a signal to mediate the alpha 1- and alpha 2-isoform upregulation.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

Na,K-ATPase mRNA levels and plaque load in Alzheimer's disease

The expression of Na,K-ATPase alpha 1- and alpha 3-mRNAs was analyzed by in situ hybridization in the superior frontal cortex and cerebellum of brains from five Alzheimer's disease (AD), five nondemented age-matched, and three young control subjects. Brains with well-preserved RNA, tested by Northern hybridization of immobilized RNA with [32P]-labeled human beta-actin riboprobe, were chosen for analysis. In situ hybridization was performed on formalin-fixed, 5 microns-thick Paraplast sections with [35S]-labeled riboprobes prepared by in vitro transcription of the respective linearized clones: a 537-bp EcoRI-PstI fragment of alpha 1-cDNA and a 342-bp PstI-EcoRI fragment of alpha 3-cDNA. In cortex, grains related to mRNA were measured by density per unit area in five cortical columns separated by 1.0-1.2 cm in each of two adjacent sections. Each cortical column of 180-micron width was divided into four depths orthogonal to the pial surface between the pia and the white matter. Amyloid plaques were counted in the same regions of adjacent sections. In addition, alpha 3-mRNA grain clusters over individual pyramidal neurons within depth 4 were analyzed. We found the following significant changes (p < 0.05): 1. Increases in total alpha 1-mRNA by 13-19% in AD compared to young and by 7-12% in AD compared to age-matched controls. 2. Decrease in total alpha 3-mRNA by 31-38% in AD compared to young and age-matched controls. 3. Decrease in alpha 3-mRNA content over individual pyramidal perikarya by 14% in normal aged brains without plaques compared to young controls, and by 44% in AD relative to young controls and by 35% compared to age-matched controls. No significant difference (p < 0.2) was found with respect to alpha 1- or alpha 3-mRNA in cerebellar cortex or individual Purkinje cells among any of the groups. In addition, there was a trend toward an inverse correlation between the levels of alpha 3-mRNA and of diffuse plaques, but not of neuritic plaques, in AD cases.

IN CONCLUSION

1. The increases in alpha 1-mRNA in AD may be related to an increased reactive gliosis. 2. The declines in alpha 3-mRNA per individual neuron found in normal aging occur prior to the formation of diffuse plaques and are greatly accelerated in AD. 3. The declines in alpha 3-mRNA per neuron found in normal aging may predispose to or potentiate AD pathogenesis.

Mutational analysis of Glu-327 of Na(+)-K(+)-ATPase reveals stimulation of 86Rb+ uptake by external K+

A competition assay of 86Rb+ uptake in HeLa cells transfected with ouabain-resistant Na(+)-K(+)-ATPase mutants revealed a stimulation of 86Rb+ uptake at low external concentrations (1 mM) of competitor (K+). Of the models that were tested, those that require that two K+ be bound before transport occurs gave the worst fits. Random and ordered binding schemes described the data equally well. General models in which both binding and transport were allowed to be cooperative yielded parameter errors larger than the parameters themselves and could not be utilized. Models that assumed noncooperative transport always showed positive cooperativity in binding. E327Q and E327L mutated forms of rat alpha 2 had lower apparent affinities for the first K+ bound than did wild-type rat alpha 2 modified to be ouabain resistant. The mutations did not affect the apparent affinity of the second K+ bound. Models that assumed noncooperativity in binding always showed positively cooperative transport, i.e., enzymes with two K+ bound had a higher flux than those with one K+ bound. Increases in external Na+ decreased the apparent affinity for K+ for all models and decreased the ratio of the apparent influx rate constants for E327L.

Cation and cardiac glycoside binding sites of the Na,K-ATPase

From the structural data obtained by systematically altering residues of the Na,K-ATPase, we are beginning to understand portions of how this active cation transporter couples hydrolysis of ATP with the vectorial movement of cations against their ionic gradients. In addition, the inhibitory action of cardiac glycosides and their interaction sites on the protein has focused our attentions on a catalytic core of the protein involving the H5-H6 transmembrane segment. In future investigations, both the ATP and the Na+ sites of the Na,K-ATPase must be uncovered to refine the structural picture of this complex transporter.

Leucine 332 at the boundary between the fourth transmembrane segment and the cytoplasmic domain of Na+,K+-ATPase plays a pivotal role in the ion translocating conformational changes

Mutants Gly330-->Ala, Leu332-->Ala, Leu332-->Pro, and Pro780-->Ala of the alpha1-isoform of rat kidney Na+,K+-ATPase were expressed in COS-1 cells to active site concentrations between 20 and 70 pmol per mg of membrane protein. The functional properties of the mutants were characterized by titrations of Na+-, K+-, and ATP-dependencies of Na+,K+-ATPase activity as well as by a series of assays measuring the K+-dependence of the steady-state phosphoenzyme level, the kinetics of dephosphorylation under a variety of conditions, and the ADP-ATP exchange activity. In mutants Gly330-->Ala, Leu332-->Ala, and Leu332-->Pro, the molecular turnover number was reduced relative to that of the wild-type Na+,K+-ATPase, and the steady-state phosphoenzyme level was high even in the presence of several millimolar K+. At a low Na+ concentration in the absence of K+, mutants Leu332-->Pro and Gly330-->Ala displayed high ADP-ATP exchange activity and formed a high level of the ADP-sensitive phosphoenzyme (E1P). The phosphoenzyme decayed slowly following a jump in salt concentration and chase with ATP and K+. Hence, the conversion of E1P to the K+-sensitive phosphoenzyme (E2P) was inhibited in mutants Leu332-->Pro and Gly330-->Ala. In the Leu332-->Ala mutant, a high level of E2P was accumulated in the absence of K+, and the ADP-ATP exchange activity was low at low Na+ concentration in the absence of K+, but rose sharply on addition of K+. Dephosphorylation experiments indicated that in the Leu332-->Ala mutant K+ induced reversal of the phosphoenzyme interconversion, forming E1P from E2P. Leu332 is therefore a pivotal residue in the conformational change. Mutants Gly330-->Ala and Pro780-->Ala displayed reduced K+ affinities relative to the wild type, determined in three independent assays.

Characterization of the secondary structure and assembly of the transmembrane domains of trypsinized Na,K-ATPase by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy has been used to compare native Na,K-ATPase-containing membranes with those trypsinized in the presence of either Rb+ or Na+ ions to remove the extramembranous parts of the protein. The protein secondary structure content deduced from the amide I band is approximately 30-35% alpha-helix, 37-40% beta-structure, and 13-15% random coil for native membranes from shark rectal gland and from pig kidney, in both the Na- and K-forms. Trypsinization in either Rb+ (a K+ congener) or Na+ removes approximately 35% of the amide I band intensity of native membranes from shark rectal gland. The protein secondary structural content of the trypsinized membranes lies in the range of approximately 23-32% alpha-helix, 37-46% beta-structure, and 12-18% random coil for the shark and kidney enzymes. The distribution of intensity between the bands corresponding to protonated and deuterium-exchanged alpha-helices, and between the component bands attributed to beta-structure, changes considerably on trypsinization, in the direction of a greater proportion of protonated alpha-helix and a broader range of frequencies for beta-structure. The kinetics of deuteration of the slowly exchanging population of protein amide groups is also changed on trypsinization. The mean rate constant for deuteration of trypsinized membranes is approximately half that for native membranes, whereas the proportion of amides contributing to this population increases on trypsinization. The temperature dependence of the amide I band in the Fourier transform infrared spectra indicates that the onset of thermal denaturation occurs at 58 degrees C for native membranes (in either Na+ or K+) and for membranes trypsinized in Rb+, but the major denaturation event for membranes trypsinized in Na+ occurs at approximately 84 degrees C. These results correlate with the functional properties of the intramembranous section of the enzyme.

Evidence that Ser775 in the alpha subunit of the Na,K-ATPase is a residue in the cation binding pocket

Substitution of alanine for Ser775 in a ouabain-resistant alpha1 sheep isoform causes a 30-fold decrease in apparent affinity for K+ as an activator of the Na,K-ATPase, as well as an increase in apparent affinity for ATP (Arguello, J. M., and Lingrel, J. B (1995) J. Biol. Chem. 270, 22764-22771). This study was carried out to determine whether Ser775 is a direct cation-ligating residue or whether the change in apparent affinity for K+ is secondary to a conformational alteration as evidenced in the change in ATP affinity, with the following results. Kinetics of K+(Rb+) influx into intact cells show that the change is due to a change in K+ interaction at the extracellular surface. The K+ dependence of formation of K+-occluded enzyme (E2(K)) and of the rate of formation of deoccluded enzyme from E2(K) indicate that the Ser775 --> Ala mutation results in a marked increase (>/=30-fold) in rate of release of K+ from E2(K). The high affinity Na+-like competitive antagonist 1,3-dibromo2,4,6-tris-(methylisothiouronium)benzene (Br2TITU), which interacts with the E1 conformation and blocks cytoplasmic cation binding (Hoving, S., Bar-Shimon, M., Tijmes, J. J. , Tal, D. M., and Karlish, S. J. D. (1995) J. Biol. Chem. 270, 29788-29793), inhibits Na+-ATPase of the mutant less than the control enzyme. With intact cells, Br2TITU acts as a competitive inhibitor of extracellular K+ activation of both the mutant and control enzymes. In this case, the mutant was more sensitive to inhibition. With vanadate as a probe of conformation, a difference in conformational equilibrium between the mutant and control enzymes could not be detected under turnover conditions (Na+- ATPase) in the absence of K+. These results indicate that the increase in apparent affinity for ATP effected by the Ser775 --> Ala mutation is secondary to a change in intrinsic cation affinity/selectivity. The large change in affinity for extracellular K+ compared with cytoplasmic Na+ and to Br2TITU binding supports the conclusion that the serine hydroxyl is either part of the K+-gate structure or a direct cation-ligating residue that is shared by at least one Na+ ion, albeit with less consequence on rate constants for Na+ binding or release compared with K+.

Fluoresceinyl-ethylenediamine-ouabain detects an acidic environment in the cardiac glycoside binding site of Na+/K+-ATPase

To probe the pH value in the microenvironment of the cardiac glycoside-binding site of Na+/K+-ATPase, pH-sensitive fluorescent derivatives of ouabain were synthesized. The fluoresceinyl derivative of ethylenediamino-ouabain (FEDO) had a pKs of 6.0 and showed a H+-dependent fluorescence change, when its ratio of excitation at 490 nm/450 nm was recorded at 530 nm. Binding of FEDO inactivated Na+/K+-ATPase at 37 degrees C and pH 7.25 in a slow time-dependent process under the conditions of backdoor phosphorylation with k(on) of 891 s(-1) M(-1). The complex dissociated with k(on) of 0.35 x 10(-3) s(-1) resulting in a Kd value of 0.4 microM for the FEDO x enzyme complex. Binding of FEDO was associated with a decrease of the excitatory fluorescence ratio at 490 nm/450 nm which could be used to convert this change into a pH value. A pH value of 5.1 +/- 0.2 was calculated to exist in the microenvironment of the FEDO x enzyme complex. This pH value was independent of the pH of the incubation medium used to form the FEDO x enzyme complex. Analysis of the accessibility of the fluorophore in the FEDO x enzyme complex to the dynamic quencher potassium iodide detected a decrease of the Stern-Volmer constant from 6.2 mM(-1) (free FEDO) to 1.5 mM(-1) (FEDO x enzyme complex) indicating thereby a limited accessibility of the fluorophore to anions. Analysis of the microenvironment of the fluorescein residue of the FEDO x enzyme complex by measurements of the anisotropy and the fluorescence half-life time revealed that both processes differed significantly when H2O was replaced by D2O. We conclude, therefore, that a pH of 5.1 +/- 0.2 exists in the vicinity of ouabain that is hidden in the depth of the receptor site when the ouabain receptor complex has been formed.

Changes of intracellular Na(+) concentration in erythrocytes caused by pulsed electrical field

Changes of sodium ionic concentration of human erythrocytes applied to pulsed electrical field (PEF) were studied by using shift reagent and NMR spectroscopy. The results show that the concentration of intraceUular Na(+) increases with the increasing intensity of PEF when the erythrocytes are applied to PEF with higher intensities. The relationship between intracellular Na(+) concentrations and the intensities of PEF does not follow linear or exponential behavior. As the intensities increase, the intracellular Na(+) concentrations increase even faster by an exponential curve. However under effects of PEF at lower intensities, intracellular Na(+) concentration decreases. Ouabain can inhibit the decrease of intracellular Na(+) concentration, and the inhibition increases with the increasing concentration of ouabain, suggesting that Na(+), K(+)-ATPase on cell membrane can be activated by PEF at lower intensities. Direct measurement of activities of the enzyme by using Malachite green method has confirmed this observation. Cell permeabilities to ions, activation of enzymes by electrical fields and transmission of physical signals like PEF across cell membranes are discussed.

Role of serine/threonine protein phosphatases in insulin regulation of Na+/K+-ATPase activity in cultured rat skeletal muscle cells

In this study, we examined the potential role of serine/threonine protein phosphatase-1 (PP-1) and PP-2A in the mechanism of Na+/K+-ATPase activation by insulin in the rat skeletal muscle cell line L6. Incubation of L6 cells with insulin caused a time- and dose-dependent stimulation of ouabain-sensitive plasma membrane Na+/K+-ATPase activity. Pretreatment with okadaic acid (OA; 0.1-1 microM) or calyculin A (1 microM) blocked insulin's effect on Na+/K+-ATPase activation. Low concentrations of OA that specifically inhibit PP-2A were ineffective. Immunoprecipitation of the enzyme from 32P-labeled cells with an antibody directed against the alpha-1 subunit of the enzyme revealed a 60% decrease in 110-kDa protein phosphorylation in insulin-treated cells. The presence of calyculin A blocked insulin-mediated dephosphorylation of Na+/K+-ATPase, whereas low concentrations of OA were ineffective. To further confirm the role of PP-1, we used L6 cell lines that overexpress the glycogen/SR-associated regulatory subunit of PP-1, PP-1G. Overexpression of PP-1G resulted in a 3-fold increase in insulin-stimulated PP-1 catalytic activity. This was accompanied by a 30% increase in basal Na+/K+-ATPase activity and a >2-fold increase in insulin's effect on pump activity. Inhibition of phosphatidylinositol-3 kinase with wortmannin blocked insulin-stimulated PP-1 activation as well as the dephosphorylation and activation of Na+/K+-ATPase. We conclude that insulin regulates the activity of Na+/K+-ATPase by promoting dephosphorylation of the alpha subunit via an insulin-stimulated PP-1 and that phosphatidylinositol-3 kinase-generated signals may mediate insulin activation of PP-1 and Na+/K+-ATPase.

[Status of digitalis in therapy of acute and chronic heart failure]

Although supported by more than 200 years of experience and anecdotal clinical evidence, the efficacy of digitalis in the management of heart failure has been questioned until the past decade. The idea to improve contractility of the diseased myocardium with an inotropic agent is fundamental in the management of left ventricular dysfunction. The majority of clinical trials published since 1980, most of which examined patients with mild to moderate heart failure, indicate that digitalis alone or in combination with vasodilators may improve the clinical outcome particular in those patients with more advanced symptoms and poorer left ventricular function. Aside from its action as an inotropic drug the pharmacology and the mechanisms by which digitalis influence the diseased myocardium and peripheral circulation in heart failure has gained more complexity within the last years, raising the idea of other mechanisms that might be involved in its action. Particular for ACE inhibition multiple clinical trials have conclusively demonstrated its impact on survival and morbidity in congestive heart failure. Improvement of clinical outcome as measured in terms of fewer hospitalizations and improvement of symptoms in patients receiving digitalis seems to be comparable to patients receiving beta-blockers additional to diuretics and ACE inhibitors, an entirely different approach to the treatment of heart failure. Despite initial improvement of hemodynamics it now appears that there is no survival benefit found for digitalis in the management of heart failure.

Sites of reaction of the gastric H,K-ATPase with extracytoplasmic thiol reagents

The vesicular gastric H,K-ATPase catalyzes an electroneutral H for K exchange allowing acidification of the intravesicular space. There is a total of 28 cysteines present in the alpha subunit of the gastric H,K-ATPase, of which 10 are found in the predicted transmembrane segments and their connecting loop, and 9 are present in the beta subunit, of which 6 are disulfide-linked. To determine which of these was accessible to extracytoplasmic attack, the enzyme was inhibited by four different substituted 2-pyridylmethylsulfinyl benzimidazoles, 5-methoxy-2-[(4-methoxy-3, 5-dimethyl-2-pyridyl)methylsulfinyl]-1H-benzimidazole (omeprazole), 2-[(4-trifluoroethoxy-3-methyl-2-pyridyl)methylsulfinyl]-1H-ben zimida zole (lansoprazole), 5-difluoromethoxy-2-[3, 4-methoxy-2-pyridyl)methylsulfinyl]-1H-benzimidazole (pantoprazole), and 2-[(4-(3-methoxypropoxy)-3-methyl)-2-pyridyl)methylsulfinyl]-1H-++ +benzi midazole (rabeprazole), under acid transporting conditions. All of these compounds are weak bases that accumulate in the acidic space generated by the pump and undergo an acid catalyzed rearrangement to a cationic sulfenamide, which forms disulfides with accessible cysteines. The relative rates of acid activation of these compounds corresponded to the relative rates of inhibition of ATPase activity and acid transport. Fragmentation of the enzyme by trypsin followed by SDS-polyacrylamide gel electrophoresis showed that omeprazole bound covalently to one of the two cysteines in the domains containing the fifth and sixth transmembrane segments and their extracytoplasmic loop and to cysteine 892 in the loop between the seventh and eighth transmembrane segments, but inhibition correlated with the reaction with cysteines in the fifth and sixth domain. Lansoprazole bound to the cysteines in these two domains as well as to cysteine 321 toward the extracytoplasmic end of the third transmembrane segments. Pantoprazole bound only to either cysteine 813 or 822 in the fifth and sixth transmembrane region. The inhibition of Rabeprazole correlated also with its binding to this part of the protein, but this compound continued to bind after full inhibition, eventually binding also to cysteines 321 and 892. No binding was found to any of the cysteines in the seventh to tenth transmembrane segments. Thermolysin digestion of the isolated omeprazole-labeled fifth and sixth transmembrane pair showed that cysteine 813 was the site of labeling. It is concluded that binding of these sided reagents to cysteine 813 in the loop between transmembrane (TM)5 and TM6 is sufficient for inhibition of ATPase activity and acid transport by the gastric acid pump. Of the 10 cysteines present in the membrane and extracytoplasmic domain, only three are exposed sufficiently to allow reactivity with these cationic thiol reagents. The binding to cysteine 813 defines the location of the extracytoplasmic loop between TM5 and TM6 and places the carboxylic acids 820 and 824 conserved between the gastric H,K- and the Na,K-ATPases in TM6, consistent with their assumed role in cation binding.

Modulation of cytokine production and protection against lethal endotoxemia by the cardiac glycoside ouabain

BACKGROUND

Recent studies have shown that cytokines are capable of modulating cardiovascular function and that some drugs used in the treatment of heart failure variably modulate the production of cytokines. To examine whether cardiac glycosides also modulate cytokine production, we evaluated the effects of ouabain on the production of cytokines in vitro and in vivo.

METHODS AND RESULTS

Human peripheral blood mononuclear cells (PBMC) were obtained from healthy volunteers. PBMC were cultured with or without ouabain in the presence or absence of lipopolysaccharide (LPS). Ouabain induced the production of interleukin (IL)-1beta, IL-6, and tumor necrosis factor (TNF)-alpha in PBMC and induced mRNA of these cytokines, an induction apparently at the transcriptional level. Amiloride, staurosporin, and genistein inhibited cytokine production, and protein kinase C and tyrosine kinase appeared to be involved in the modulation of cytokine production induced by ouabain. However, when PBMC were stimulated with LPS, ouabain suppressed the production of IL-6 and TNF-alpha. To investigate whether ouabain modulates cytokine production in vivo, we evaluated the effects of ouabain in LPS-treated mice. Ouabain was found to protect against LPS-induced lethal toxicity in mice and decreased circulating IL-6 and TNF-alpha levels in vivo.

CONCLUSIONS

These previously unrecognized immunomodulating effects of a cardiac glycoside may explain either the beneficial or the detrimental effects of these drugs in heart failure patients.

Fe-catalyzed cleavage of the alpha subunit of Na/K-ATPase: evidence for conformation-sensitive interactions between cytoplasmic domains

Incubation of Na/K-ATPase with ascorbate plus H2O2 produces specific cleavage of the alpha subunit. Five fragments with intact C termini and complementary fragments with intact N termini were observed. The beta subunit is not cleaved. Cleavages depend on the presence of contaminant or added Fe2+ ions, as inferred by suppression of cleavages with nonspecific metal complexants (histidine, EDTA, phenanthroline) or the Fe3+-specific complexant desferrioxamine, or acceleration of cleavages by addition of low concentrations of Fe2+ but not of other heavy metal ions. Na/K-ATPase is inactivated in addition to cleavage, and both effects are insensitive to OH. radical scavengers. Cleavages are sensitive to conformation. In low ionic strength media (E2) or media containing Rb ions [E2(Rb)], cleavage is much faster than in high ionic strength media (E1) or media containing Na ions (E1Na). N-terminal fragments and two C-terminal fragments (N-terminals E214 and V712) have been identified by amino acid sequencing. Approximate positions of other cleavages were determined with specific antibodies. The results suggest that Fe2+ (or Fe3+) ions bind with high affinity at the cytoplasmic surface and catalyze cleavages of peptide bonds close to the Fe2+ (or Fe3+) ion. Thus, cleavage patterns can provide information on spatial organization of the polypeptide chain. We propose that highly conserved regions of the alpha subunit, within the minor and major cytoplasmic loops, interact in the E2 or E2(Rb) conformations but move apart in the E1 or E1Na conformations. We discuss implications of domain interactions for the energy transduction mechanism. Fe-catalyzed cleavages may be applicable to other P-type pumps or membrane proteins.

PKA-mediated phosphorylation and inhibition of Na(+)-K(+)-ATPase in response to beta-adrenergic hormone

The activity of Na(+)-K(+)-ATPase can be regulated by hormones that activate adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA). Here, using a site-directed phosphorylation state-specific antibody, we show that hormonal regulation of Na(+)-K(+)-ATPase can occur via phosphorylation of Ser-943 on its alpha-subunit. cDNAs coding for wild-type rat Na(+)-K(+)-ATPase and Na(+)-K(+)-ATPase in which the PKA phosphorylation site Ser-943 was mutated to Ala were stably and transiently transfected into COS cells. In COS cells expressing wild-type Na(+)-K(+)-ATPase the beta-adrenergic agonist isoproterenol (1 microM) significantly increased the level of phosphorylation of the alpha-subunit. Phosphorylation was accompanied by a significant inhibition of the enzyme activity, as reflected by a decrease in ATP hydrolysis and 86Rb+ transport. The effect of isoproterenol was reproduced by the PKA activator forskolin used in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and was abolished by the specific PKA inhibitor H-89. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A, enhanced phosphorylation and inhibition of Na(+)-K(+)-ATPase induced by isoproterenol. The changes in activity of Na(+)-K(+)-ATPase linearly correlated with the extent of the alpha-subunit of Na(+)-K(+)-ATPase being phosphorylated. When Ser-943 was replaced by alanine, stimulation of the phosphorylation and inhibition of the activity of Na(+)-K(+)-ATPase induced by isoproterenol, alone or in combination with okadaic acid, were not observed. These results indicate that, in intact cells, modulation of the activity of Na(+)-K(+)-ATPase can be achieved by regulation of the state of phosphorylation of Ser-943. Moreover, they provide a biochemical mechanism by which beta-adrenergic agonists can regulate Na(+)-K(+)-ATPase activity.

Extensive random mutagenesis analysis of the Na+/K+-ATPase alpha subunit identifies known and previously unidentified amino acid residues that alter ouabain sensitivity--implications for ouabain binding

Random mutagenesis with ouabain selection has been used to comprehensively scan the extracellular and transmembrane domains of the alpha1 subunit of the sheep Na+/K+-ATPase for amino acid residues that alter ouabain sensitivity. The four random mutant libraries used in this study include all of the transmembrane and extracellular regions of the molecule as well as 75% of the cytoplasmic domains. Through an extensive number of HeLa cell transfections of these libraries and subsequent ouabain selection, 24 ouabain-resistant clones have been identified. All previously described amino acids that confer ouabain resistance were identified, confirming the completeness of this random mutagenesis screen. The amino acid substitutions that confer the greatest ouabain resistance, such as Gln111-->Arg, Asp121-->Gly, Asp121-->Glu, Asn122-->Asp, and Thr797-->Ala were identified more than once in this study. This extensive survey of the extracellular and transmembrane regions of the Na+/K+-ATPase molecule has identified two new regions of the molecule that affect ouabain sensitivity: the H4 and the H10 transmembrane regions. The new substitutions identified in this study are Leu330-->Gln, Ala331-->Gly, Thr338-->Ala, and Thr338-->Asn in the H4 transmembrane domain and Phe982-->Ser in the H10 transmembrane domain. These substitutions confer modest increases in the concentration of cardiac glycoside needed to produce 50% inhibition of activity (IC50 values), 3.1-7.9-fold difference. The results of this extensive screening of the Na+/K+-ATPase alpha1 subunit to identify amino acids residues that are important in ouabain sensitivity further supports our hypothesis that the H1-H2 and H4-H8 regions represent the major binding sites for the cardiac glycoside class of drugs.

Enzymatic properties of human Na,K-ATPase alpha1beta3 isozyme

Recent results of a wide-scale human cDNA sequencing project have identified a cDNA which encodes a hitherto unknown human protein sequence exhibiting structural similarities with beta-subunits of the Na,K- and H,K-ATPase family and with the amphibian Na,KATPase beta3-subunit, in particular. In this study the ability of the putative human beta3-subunit to assemble with the human alpha1-subunit in functionally active Na,KATPase was examined using the baculovirus expression system. The recombinant baculovirus simultaneously expressing both alpha1 and beta3 human proteins was produced using the dual-promoter transfer vector p2Bac. The expression of both human proteins in baculovirus-infected Sf-9 cell membranes detected with specific antibodies resulted in the formation of a catalytically competent alpha1beta3 ATPase complex. Characterization of the recombinant ATPase complex involved the analysis of Na+, K+, and ATP dependencies of enzyme activity and its sensitivity toward ouabain. Preparations of HeLa cell membranes containing alpha1beta1 isozyme of human Na,K-ATPase were used as control. The data obtained clearly demonstrated that alpha1beta3 ATPase exhibits enzymatic properties which are characteristic of Na, K-ATPase. The recombinant alpha1beta3 isozyme displayed significantly lower sensitivity to ouabain than native alpha1beta1. These findings indicate that the hitherto unknown alpha1beta3 isozyme of human Na,K-ATPase is likely to exist in vivo, thus suggesting further expansion of human Na,K-ATPase isozyme diversity. The present studies are the first in which heterologous expression has been used for the characterization of an isozyme of human Na, K-ATPase.

Mutation of the protein kinase C phosphorylation site on rat alpha1 Na+,K+-ATPase alters regulation of intracellular Na+ and pH and influences cell shape and adhesiveness

The enzyme Na+,K+-ATPase creates the transmembrane Na+ gradient that is of vital importance for functioning of all eukaryotic cells. Na+, K+-ATPase can be phosphorylated by protein kinase A (PKA) and protein kinase C (PKC), and these sites of phosphorylation have been identified. In the present study, we have examined the physiological significance of PKC phosphorylation of rat Na+,K+-ATPase. In COS cells transfected with wild type rat Na+,K+-ATPase alpha1, intracellular Na+ was higher and pH was lower than in cells transfected with rat Na+,K+-ATPase alpha1 in which the PKC phosphorylation site, Ser-23, had been mutated into alanine. Phorbol dibutyrate inhibited Na+,K+-ATPase-dependent ATP hydrolysis and Rb+ uptake in cells expressing wild type Na+,K+-ATPase but not in cells expressing S23A Na+,K+-ATPase. Cells expressing the S23A mutant had a more rounded appearance and attached less well to fibronectin than did untransfected cells or cells transfected with wild type rat Na+, K+-ATPase alpha1. These results indicate a functional role for PKC-mediated phosphorylation of rat Na+,K+-ATPase alpha1 and suggest a connection between this enzyme and cell adhesion.

Sodium kinetics of Na,K-ATPase alpha isoforms in intact transfected HeLa cells

By participating in the regulation of ion and voltage gradients, the Na-K pump (i.e., Na,K-ATPase) influences many aspects of cellular physiology. Of the four alpha isoforms of the pump, alpha1 is ubiquitous, alpha2 is predominant in skeletal muscle, and alpha3 is found in neurons and the cardiac conduction system. To determine whether the isoforms have different intracellular Na+ affinities, we used the Na+-sensitive dye sodium-binding benzofuran isophthalate (SBFI) to measure pump-mediated Na+ efflux as a function of [Na+]i in human HeLa cells stably transfected with rat Na-K pump isoforms. We Na+-loaded the cells, and then monitored the time course of the decrease in [Na+]i after removing external Na+. All transfected rat alpha subunits were highly ouabain resistant: the alpha1 isoform is naturally resistant, whereas the alpha2 and alpha3 isoforms had been mutagenized to render them resistant. Thus, the Na+ efflux mediated by endogenous and transfected pumps could be separated by studying the cells at low (1 microM) and high (4 mM) ouabain concentrations. We found that the apparent Km for Na+ efflux attributable to the native human alpha1 isoform was 12 mM, which was similar to the Km of rat alpha1. The alpha2 and alpha3 isoforms had apparent Km's of 22 and 33 mM, respectively. The cells expressing alpha3 had a high resting [Na+]i. The maximal activity of native alpha1 in the alpha3-transfected cells was only approximately 56% of native alpha1 activity in untransfected HeLa cells, suggesting that transfection with alpha3 led to a compensatory decrease in endogenous alpha1 pumps. We conclude that the apparent Km(Na+) for rat Na-K pump isoforms increases in the sequence alpha1 < alpha2 < alpha3. The alpha3 isoform may be suited for handling large Na+ loads in electrically active cells.

1,25-Dihydroxyvitamin D3 selectively induces increased expression of the Na,K-ATPase beta 1 subunit in avian myelomonocytic cells without a concomitant change in Na,K-ATPase activity

Treatment of avian myelomonocytic cells with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) results in an approximately two fold increase in levels of Na,K-ATPase beta 1 subunit mRNA and protein (both total and plasma membrane-associated). The changes in beta 1 subunit expression occur in the absence of a detectable increase in expression of any of the three alpha subunit isoforms or in Na,K-ATPase activity. The selective induction of the expression of the beta subunit in avian myelomonocytic cells by 1,25(OH)2D3 reveals a previously unobserved feature of the regulation of Na,K-ATPase expression, while the targeting of beta subunit polypeptides to the plasma membrane in the absence of a corresponding increase in active Na,K-ATPase suggests that, in these cells, transport of the beta subunit to the plasma membrane may be independent of its binding to the alpha subunit.