Solubilization and partial purification of the rabbit parotid Na/K/Cl-dependent bumetanide binding site

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Corneal endothelial NKCC: molecular identification, location, and contribution to fluid transport

Although Na(+)-K(+)-2Cl(-) cotransport has been demonstrated in cultured bovine corneal endothelial cells, its presence and role in the native tissue have been disputed. Using RT-PCR we have now identified a partial clone of the cotransporter protein in freshly dissected as well as in cultured corneal endothelial and epithelial cells. The deduced amino acid sequence of this protein segment is 99% identical to that of the bovine isoform (bNKCC1). [(3)H]bumetanide binding shows that the cotransporter sites are located in the basolateral membrane region at a density of 1.6 pmol/mg of protein, close to that in lung epithelium. Immunocytochemistry confirms the basolateral location of the cotransporter. We calculate the turnover rate of the cotransporter to be 83 s(-1). Transendothelial fluid transport, determined from deepithelialized rabbit corneal thickness measurements, is partially inhibited (30%) by bumetanide in a dose-dependent manner. Our results demonstrate that Na(+)-K(+)-2Cl(-) cotransporters are present in the basolateral domain of freshly dissected bovine corneal endothelial cells and contribute to fluid transport across corneal endothelial preparations.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Distribution and diversity of Na-K-Cl cotransport proteins: a study with monoclonal antibodies

The Na-K-Cl cotransporter (NKCC) is present in most animal cells where it functions in cell volume homeostasis and epithelial salt transport. We developed six monoclonal antibodies (designated T4, T8, T9, T10, T12, and T14) against a fusion protein fragment encompassing the carboxy-terminal 310 amino acids of the human colonic NKCC. These T antibodies selectively recognized putative NKCC proteins in a diverse variety of animal tissues. Western blot analysis of membranes isolated from 23 types of cells identified single bands of immunoreactive protein ranging in mass from 146 to 205 kDa. The amount of immunoreactive protein detected in these cells correlated with loop diuretic binding site density. Proteins identified previously as Na-K-Cl cotransporters by loop diuretic photoaffinity labeling were mutually recognized by multiple T antibodies. Most of the T antibodies effectively immunoprecipitated the denatured form of the NKCC protein. Immunocytochemical studies on the rabbit parotid gland demonstrated that NKCC is restricted to the basolateral margin of the acinar cells and absent from the ducts, in accord with the central role of Na-K-Cl cotransport in chloride secretion. In the rabbit kidney, NKCC was localized to the apical membrane of thick ascending limb cells, consistent with its role in chloride reabsorption.

Involvement of direct phosphorylation in the regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter

We identify a 175-kDa membrane phosphoprotein (pp175) in rat parotid acini whose properties correlate well with the Na(+)-K(+)-2Cl- cotransporter previously characterized functionally and biochemically in this tissue. pp175 was the only phosphoprotein immunoprecipitated by an anti-Na(+)-K(+)-2Cl- cotransporter antibody and the only membrane protein whose phosphorylation state was conspicuously altered after a brief (45-s) exposure of acini to the beta-adrenergic agonist isoproterenol. Phosphopeptide mapping provided evidence for three phosphorylation sites on pp175, only one of which was labeled in response to isoproterenol treatment. The half-maximal effect of isoproterenol on phosphorylation of pp175 (approximately 20 nM) was in excellent agreement with its previously demonstrated up-regulatory effect on cotransport activity. Increased phosphorylation of pp175 was also seen following acinar treatment with a permeant cAMP analogue and with forskolin, conditions that have likewise been shown to up-regulate the cotransporter. Combined with earlier results from our laboratory, these data provide strong evidence that the up-regulation of the cotransporter by these agents is due to direct phosphorylation mediated by protein kinase A. AlF(-)4 treatment, which results in an up-regulation of cotransport activity comparable with that observed with isoproterenol (approximately 6-fold), caused a similar increase in phosphorylation of pp175. However, hypertonic shrinkage and treatment with the protein phosphatase inhibitor calyculin A, which also up-regulate the cotransporter (approximately 3-fold and approximately 6-fold, respectively) caused no change in the phosphorylation level. Furthermore, although acinar treatment with the muscarinic agonist carbachol results in a dramatic up-regulation of cotransport activity and a concomitant phosphorylation of pp175, no phosphorylation of pp175 was seen with the Ca(2+)-mobilizing agent thapsigargin, which is able to fully mimic the up-regulatory effect of carbachol on transport activity. Taken together, these results indicate that direct phosphorylation is only one of the mechanisms involved in secretagogue-induced regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Identification, characterization and purification of a 160 kD bumetanide-binding glycoprotein from the rabbit parotid

We demonstrate the presence of a 160 kD protein in rabbit parotid basolateral membranes that can be labeled with the irreversible sulfhydryl reagent [14C]-N-ethylmaleimide in a bumetanide-protectable fashion. The specificity of this labeling, and our previous evidence for the existence of an essential sulfhydryl group closely associated with the bumetanide-binding site on the parotid Na(+)-K(+)-Cl-cotransporter (J. Membrane Biol. 112:51-58, 1989), provide strong evidence that this protein is a part or all of the parotid bumetanide-binding site. When this protein is treated with endoglycosidase F/N-glycosidase F to remove N-linked oligosaccharides, its apparent molecular weight decreases to 135 kD. The pI of this deglycosylated protein is approximately 6.4. The bumetanide-binding protein was purified using two preparative electrophoresis steps. First, a Triton X-100 extract enriched in this protein was run on preparative electrophoresis to obtain fractions containing proteins in the 160 kD range. These were then deglycosylated with endoglycosidase F/N-glycosidase F and selected fractions were pooled and rerun on preparative electrophoresis to obtain a final 135 kD fraction. The enrichment of the bumetanide-binding protein in this final 135 kD fraction estimated from [14C]-N-ethylmaleimide labeling was approximately 48 times relative to the starting membrane extract. Since the bumetanide-binding site represents approximately 2% of the total protein in this starting extract, this enrichment indicates a high degree of purity of this protein in the 135 kD fraction.

Inhibition of Na-K-2Cl cotransport and bumetanide binding by ethacrynic acid, its analogues, and adducts

The inhibitory effect of ethacrynic acid (EA) and a variety of its derivatives on Na-K-2Cl cotransport in avian erythrocytes was investigated. The most potent compound tested was the adduct of EA with L-cysteine, with an IC50 of 7.2 x 10(-7) M. EA itself, dihydro-EA, EA-D-cysteine, and adducts of EA with other sulfhydryl (-SH) compounds were much less potent. The mechanism of action of EA and EA-L-cysteine differed in several respects: 1) EA-L-cysteine acted more rapidly than EA (half times of < 1 and 4 min, respectively, at 37 degrees C); 2) the action of EA-L-cysteine was reversible by washing, whereas that of EA was not; and 3) the degree of inhibition by EA-L-cysteine varied with medium [K], whereas that of EA did not. The inhibitory effects of both EA-L-cysteine and EA were affected by medium [Na] and [Cl]. We conclude that EA-L-cysteine does not "deliver" EA to transport-related -SH residues or act as an alkylating agent but has some stereospecific effect on cotransport that is a property of the entire molecule. EA does appear to inhibit cotransport by alkylating -SH residues, as closely related compounds lacking the ability to covalently react with such groups were reversible, and other -SH reagents (e.g., N-ethylmaleimide) also inhibited cotransport. EA, EA-L-cysteine, and EA-D-cysteine all inhibited [3H]bumetanide binding to membranes from activated avian erythrocytes at concentrations similar to those that inhibited cotransport. It is possible that the EA and bumetanide types of diuretics interact with closely apposed sites on the Na-K-2Cl cotransporter.

Ca(2+)-dependent heat production by rat skeletal muscle in hypertonic media depends on Na(+)-Cl- co-transport stimulation

1. The rate of energy dissipation (E) in isolated, superfused soleus muscles from young rats was continuously measured under normosmotic and 100-mosM hyperosmotic conditions. The substantial increase of E with respect to basal level in hyperosmolarity (excess E), which is entirely dependent on the presence of extracellular sodium, was largely prevented or inhibited by bumetanide, a potent inhibitor of Na(+)-Cl- co-transport system, or by the removal of chloride from the superfusate (isethionate substitution). Bumetanide or the removal of chloride also acutely decreased basal E, by about 7%. 2. Bumetanide almost entirely suppressed the major, Ca(2+)-dependent part of excess E in hyperosmolarity, as well as the concomitant increase of 45Ca2+ efflux and small increase in resting muscle tension; in contrast, the part of excess E associated with stimulation of Na(+)-H+ exchange in hyperosmolarity was left unmodified. 3. Reduction of 22Na+ influx by bumetanide was more marked in hyperosmolarity than under control conditions, although stimulation of total 22Na+ influx by a 100-mosM stress was not statistically significant. Inhibition of Ca2+ release into the sarcoplasm using dantrolene sodium did not prevent the stimulation of bumetanide-sensitive 22Na+ influx, but rather increased it about fourfold. 4. It is concluded that the largest part of excess E in hyperosmolarity, which is Ca(2+)-dependent energy expenditure, is suppressed when steady-state stimulation of a Na(+)-Cl- co-transport system is inhibited either directly by bumetanide or the removal of extracellular chloride, or indirectly by the blocking of active Na(+)-K+ transport. How the stimulation of Na(+)-Cl- co-transport, by as little as 1 nmol s-1 (g wet muscle weight)-1 during a 100-mosM stress, enhances Ca(2+)-dependent heat by as much as 2.5 mW (g wet muscle weight)-1 remains to be clarified.

Role of phospholipids in the binding of bumetanide to the rabbit parotid Na/K/Cl cotransporter

It was recently reported (Turner, R.J., George, J.N., 1990, J. Membrane Biol. 113:203-210) that the high affinity bumetanide binding site of the rabbit parotid Na/K/Cl cotransporter could be extracted from a basolateral membrane preparation from this gland using relatively low concentrations of the non-ionic detergent Triton X-100. At the detergent:protein ratios required for complete membrane solubilization bumetanide binding activity in this extract was lost but could be recovered by the addition of crude soybean lipids. In the present paper the ability of various purified lipids to restore high affinity bumetanide binding activity in detergent solubilized rabbit parotid basolateral membranes is studied. We show that the effect of exogenous lipid on the detergent-inactivated bumetanide binding site is to increase the affinity of binding without affecting the number of binding sites. Of the 11 lipid species tested, several relatively minor, negatively charged membrane phospholipids are the most effective in restoring binding activity (phosphatidylserine approximately phosphatidylglycerol greater than phosphatidylinositol greater than cardiolipin), while the major mammalian plasma membrane lipid components phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and cholesterol are without effect. In addition, we show that in the presence of these minor lipids the affinity of bumetanide binding is considerably increased over that observed in the native membrane (e.g., Kd approximately 0.06 microM in membranes extracted with 0.3% Triton and treated with 0.15% wt/vol phosphatidylserine, vs. Kd approximately 3 microM in native basolateral membranes). This dramatic dependence of bumetanide binding affinity on the presence of certain lipid species suggests that the properties of the bumetanide binding protein in situ may be quite dependent on the minor lipid content of the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Na-K-C1 cotransport in Ehrlich ascites cells by antiserum against purified proteins of the cotransporter

Two proteins were purified earlier from solubilized membranes of Ehrlich ascites cells by using a bumetanide-Sepharose affinity column. These proteins were proposed to be constituents of the Na-K-C1 cotransporter. However, the specificity of binding of bumetanide to the cotransporter was insufficient evidence for this proposal. We now have direct evidence that the purified protein contains components of the cotransporter. Antiserum raised against the bumetanide-binding proteins strongly inhibits Na-K-C1 cotransport measured by two independent methods. Cotransport was induced by hypertonic challenge and was measured as the bumetanide-sensitive portion of unidirectional C1 influx and as regulatory cell volume increase. In both assays, cotransport was strongly inhibited by the antiserum. Fab fragments of the antibodies inhibited cotransport to a similar extent.