[3H]bumetanide binding to mouse kidney membranes: identification of corresponding membrane proteins

Thick ascending limb: the Na(+):K (+):2Cl (-) co-transporter, NKCC2, and the calcium-sensing receptor, CaSR

The thick ascending limb of Henle's loop is a nephron segment that is vital to the formation of dilute and concentrated urine. This ability is accomplished by a consortium of functionally coupled proteins consisting of the apical Na(+):K(+):2Cl(-) co-transporter, the K(+) channel, and basolateral Cl(-) channel that mediate electroneutral salt absorption. In thick ascending limbs, salt absorption is importantly regulated by the calcium-sensing receptor. Genetic or pharmacological disruption impairing the function of any of these proteins results in Bartter syndrome. The thick ascending limb is also an important site of Ca(2+) and Mg(2+) absorption. Calcium-sensing receptor activation inhibits cellular Ca(2+) absorption induced by parathyroid hormone, as well as passive paracellular Ca(2+) transport. The present review discusses these functions and their genetic and molecular regulation.

Alternatively spliced isoform of apical Na(+)-K(+)-Cl(-) cotransporter gene encodes a furosemide-sensitive Na(+)-Cl(-)cotransporter

In the absence of vasopressin, medullary thick ascending limb cells express a K(+)-independent, furosemide-sensitive Na(+)-Cl(-) cotransporter that is inhibited by hypertonicity. The murine renal specific Na(+)-K(+)-2 Cl(-) cotransporter gene (SLC12A1) gives rise to six alternatively spliced isoforms. Three feature a long COOH-terminal domain that encodes the butmetanide-sensitive Na(+)-K(+)-2 Cl(-) cotransporter (BSC1-9/NKCC2), and three with a short COOH-terminal domain, known as mBSC1-A4, B4, or F4 (19). Here we have determined the functional characteristics of mBSC1-A4, as expressed in Xenopus laevis oocytes. When incubated at normal oocyte osmolarity (approximately 200 mosmol/kgH(2)O), mBSC1-4-injected oocytes do not express significant Na(+) uptake over H(2)O-injected controls, and immunohistochemical analysis shows that the majority of mBSC1-4 protein is in the oocyte cytoplasm and not at the plasma membrane. In contrast, when mBSC1-4 oocytes are exposed to hypotonicity (approximately 100 mosmol/kgH(2)O), a significant increase in Na(+) uptake but not in (86)Rb(+) uptake is observed. The increased Na(+) uptake is Cl(-) dependent, furosemide sensitive, and cAMP sensitive but K(+) independent. Sodium uptake increases with decreasing osmolarity between 120 and 70 mosmol/kgH(2)O (r = 0.95, P < 0.01). Immunohistochemical analysis shows that in hypotonic conditions mBSC1-A4 protein is expressed in the plasma membrane. These studies indicate that the mBSC1-A4 isoform of the SLC12A1 gene encodes a hypotonically activated, cAMP- and furosemide-sensitive Na(+)-Cl(-) cotransporter. Thus it is possible that alternative splicing of the BSC1 gene could provide the molecular mechanism enabling the Na(+)-Cl(-)-to-Na(+)-K(+)-2Cl(-) switching in thick ascending limb cells.

Isoforms of the Na-K-2Cl cotransporter in murine TAL I. Molecular characterization and intrarenal localization

We have identified several alternatively spliced cDNAs encoding mBSC1, an apical bumetanide-sensitive Na+-K+-2Cl- cotransporter from mouse kidney. Two full-length clones were isolated, designated C4 and C9, predicting proteins of 770 and 1,095 amino acids, respectively. The C4 isoforms are generated by utilization of an alternative polyadenylation site located within the intron between exons 16 and 17 of the mBSC1 gene on chromosome 2; the resultant transcripts predict a truncated COOH terminus ending in a unique 55 amino acid sequence. The predicted C4 and C9 COOH termini differ in the distribution of putative phosphorylation sites for both protein kinase A and C. Independent splicing events involve three previously described cassette exons, which are predicted to encode most of the second transmembrane domain. A total of six different isoforms are expressed, generated by the combinatorial association of three cassette exons and two alternative 3' ends. C9-specific and C4-specific antibodies detect proteins of approximately 150 and 120 kDa, respectively, in mouse kidney. Immunofluorescence and immunohistochemistry indicate expression of both COOH-terminal isoforms within the thick ascending limb of the loop of Henle (TAL). However, staining with the C4 antibody is more heterogeneous, with a decreased proportion of positive cells in the cortical TAL. Functional expression in Xenopus oocytes indicates a dominant negative function for C4 isoforms [companion study, C. Plata, D. B. Mount, V. Rubio, S. C. Hebert, and G. Gamba. Am. J. Physiol. 276 (Renal Physiol. 45): F347-F358, 1999], and the differential expression of these isoforms may contribute to functional heterogeneity of Na+-K+-2Cl- cotransport in mouse TAL.

Intrarenal and subcellular localization of rat CLC5

Dent's disease, an inherited disorder characterized by hypercalciuria, nephrolithiasis, nephrocalcinosis, rickets, low-molecular-weight proteinuria, Fanconi's syndrome, and renal failure, is caused by mutations in the renal chloride channel, CLC5. The normal role of CLC5 is unknown. We have investigated the intrarenal and subcellular localization of CLC5 in rat kidney by in situ hybridization and immunohistochemistry. By in situ hybridization, CLC5 mRNA was detected predominantly in cortical medullary ray and outer medullary tubule epithelial cells. Polyclonal antiserum was generated against a CLC5 fusion protein, affinity purified, and immunoadsorbed against CLC3 and CLC4 to yield a CLC5 isoform-specific antiserum. By immunohistochemistry, CLC5 protein was localized to the intracellular domain of tubular epithelial cells in the S3 segment of the proximal tubule and the medullary thick ascending limb. By subcellular membrane fractionation and flow cytometry, CLC5 expression was found in outer medullary endosomes. These findings are consistent with a model in which CLC5 encodes an endosomal chloride channel that facilitates acidification and trafficking of renal epithelial endosomes.

Hypertonicity enhances expression of functional Na+/K+/2Cl- cotransporters in Ehrlich ascites tumour cells

Ehrlich cells exposed to a hypertonic medium for five hours respond by an increased expression of Na+/K+/2Cl- cotransport proteins as estimated from immunoprecipitations using polyclonal anti-cotransporter antibodies. The 3.4-fold increase in cotransport expression is followed by a concomitant 2.6-fold increase in the maximal bumetanide-sensitive K+ influx during regulatory volume increase, indicating a 2.6-fold increase in the number of functional cotransporters in the plasma membrane.

Phosphorylation of the ATP-sensitive, inwardly rectifying K+ channel, ROMK, by cyclic AMP-dependent protein kinase

Activity of the recently cloned ATP-sensitive epithelial K+ channel, ROMK (Ho, K., Nichols, C. G., Lederer, W. J., Lytton, J., Vassilev, P. M., Kanazirska, M. V., and Hebert, S. C. (1993) Nature 362, 31-38), is regulated by phosphorylation-dephosphorylation processes with cAMP-dependent protein kinase (PKA)-dependent phosphorylation events being required for maintenance of channel activity in excised membrane patches (McNicholas, C. M., Wang, W., Ho, K., Hebert, S. C., and Giebisch, G. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 8077-8081; Kubokawa, M., McNicholas, C. M., Higgins, M. A., Wang, W., and Giebisch, G. (1995) Am. J. Physiol. 269, F355-F362). To determine whether this channel is a substrate for PKA, ROMK tagged with the hemagglutinin epitope was transiently transfected into HEK293 cells. In vitro labeling of immunoprecipitated proteins from transfected cells showed that ROMK could be phosphorylated by PKA. Metabolic labeling of ROMK resulted in a significantly increased phosphorylation upon pretreatment of the cells with forskolin, consistent with an action of cAMP-dependent protein kinase. Phosphoamino acid analyses of the ROMK phosphoproteins revealed that phosphate was attached exclusively to serine residues. Three putative PKA phosphorylation sites containing serine residues in the predicted ROMK proteins are shown directly to be substrates for PKA. Site-directed mutagenesis of each of these sites or double mutation of any two sites showed that ROMK proteins retained the ability to be phosphorylated by PKA both in vivo and in vitro to a variable extent, while triple mutation of all three PKA sites abolished the phosphorylation induced by cAMP agonists in transfected cells. Two-electrode voltage clamp experiments showed that PKA-dependent phosphorylation was required for ROMK channel activity and that at least two of the three sites were required for channel function when expressed in X. laevis oocytes. Taken together, these results provide strong evidence that direct phosphorylation of the channel polypeptide by PKA is involved in channel regulation and PKA-dependent phosphorylation is essential for ROMK channel activity.

Apical localization of the Na-K-Cl cotransporter, rBSC1, on rat thick ascending limbs

A bumetanide-sensitive Na-K-Cl cotransporter (rBSC1) was recently cloned from a rat renal outer medulla (OM) cDNA library and shown to be expressed predominantly in the kidney. The purpose of the present study was to examine the nephron distribution of cotransporter transcripts and protein in rat kidney. In situ hybridization showed an intense signal only in the outer medulla and extending along cortical medullary rays consistent with expression of rBSC1 transcripts in medullary (MTAL) and cortical (CTAL) thick ascending limbs. Polyclonal antibodies raised in rabbits against a unique 67 amino acid segment from the carboxyl terminus of rBSC1 identified a broad major band of 130 to 160 (midpoint of 150) kDa and at least two minor bands of 50 to 70 kD on Western blotting of homogenates from cortex (C) and outer medulla (OM), but not inner medulla (IM), of rat kidney. Thus the Na-K-Cl cotransporter protein detected by the polyclonal rBSC1 antibody in rat kidney was similar in size to the major approximately 150 kD bumetanide binding protein detected by others in mouse and dog kidneys. Immunofluorescence studies using the anti-rBSC1 polyclonal antibody on rat kidney sections showed an intense signal limited to apical surfaces of MTAL and CTAL segments. Colocalization with anti-Tamm-Horsfall antibody which is present in all TABA cells except macula densa cells confirmed the absence of anti-rBSC1 fluorescence in the macula densa cells. These results are consistent with rBSC1 encoding the, or the major isoform of the, apical Na-K-Cl cotransporter in the thick ascending limb. The Na-K-Cl cotransporter functionally detected in macula densa cells may be encoded by a different BSC isoform.

[Cl-]i-dependent phosphorylation of the Na-K-Cl cotransport protein of dog tracheal epithelial cells

Basolateral Na-K-Cl cotransport activity in primary cultures of dog tracheal epithelial cells is stimulated by beta-adrenergic agents, such as isoproterenol, and by apical UTP, which acts through an apical P2-purinergic receptor. While at least part of the stimulatory effect of isoproterenol appears to involve direct activation of the cotransporter via cAMP-dependent protein kinase, cotransport stimulation by apical UTP is entirely secondary to apical Cl- efflux and a resultant decrease in intracellular [Cl-] ([Cl-]i) and/or cell shrinkage (Haas, M., and McBrayer, D. G. (1994) Am. J. Physiol. 266, C1440-C1452). In the secretory epithelia of the shark rectal gland and avian salt gland, Na-K-Cl cotransport activation by both cAMP-dependent and cAMP-independent secretagogues has been shown to be accompanied by phosphorylation of the cotransport protein itself (Lytle, C., and Forbush, B., III (1992) J. Biol. Chem. 267, 25438-25443; Torchia, J., Lytle, C., Pon, D. J., Forbush, B., III, and Sen, A. K. (1992) J. Biol. Chem. 267, 25444-25450). In the present study, we immunoprecipitate the approximately 170-kDa Na-K-Cl cotransport protein of dog tracheal epithelial cells with a monoclonal antibody against the cotransporter of the intestinal cell line T84. Incubation of confluent primary cultures of tracheal cells with isoproterenol and apical UTP increases basolateral-to-apical 36Cl- flux 3.4- and 2.6-fold, respectively, and produces similar increases (3.2- and 2.8-fold, respectively) in 32P incorporation into the approximately 170-kDa cotransport protein. Decreasing [Cl-]i (without concomitant cell shrinkage) by incubating cultures with apical nystatin and reduced apical [Cl-] ([Cl-]alpha) likewise increases both cotransport activity and cotransport protein phosphorylation. These effects become more pronounced with greater reductions in [Cl-]alpha; after 20 min of incubation with nystatin and 32 mM [Cl-]alpha, cotransport activity and 32P incorporation into the cotransport protein are increased 2.8- and 2.7-fold, respectively, similar to increases seen with apical UTP. 2-3-fold increases in cotransporter activity and phosphorylation are also seen in nystatin-treated cells under hypertonic conditions (50 mM sucrose added apically and basolaterally). These findings suggest a close correlation between Na-K-Cl cotransport activity and phosphorylation of the approximately 170-kDa cotransport protein. The latter is phosphorylated in response to both reduced [Cl-]i and cell shrinkage, either or both of which are likely to be involved in secondary cotransport activation in response to apical UTP.

Molecular characterization of the epithelial Na-K-Cl cotransporter isoforms

Recent advances in the molecular characterization of specific isoforms of the Na-K-Cl cotransporter have allowed rapid progress in the study of the structure, function, and regulation of these members of a family of Cl-dependent cation cotransporters. Two distinct isoforms have been identified, one from Cl(-)-secretory epithelia and another found specifically in the diluting segment of the vertebrate kidney, a Cl(-)-absorptive epithelium. The discovery of three alternatively spliced variants of the absorptive isoform, which differ only by 31 amino acids and which appear to be differentially distributed within the mammalian thick ascending limb of the loop of Henle, highlight this spliced region as an important functional component of the protein.

Primary structure, functional expression, and chromosomal localization of the bumetanide-sensitive Na-K-Cl cotransporter in human colon

By moving chloride into epithelial cells, the Na-K-Cl cotransporter aids transcellular movement of chloride across both secretory and absorptive epithelia. Using cDNA probes from the recently identified elasmobranch secretory Na-K-Cl cotransporter (sNKCC1) (Xu, J. C., Lytle, C. Zhu, T. T., Payne, J. A., Benz, E., and Forbush, B., III (1994) Proc. Natl. Acad. Sci. 91, 2201-2205), we have identified the human homologue. By screening cDNA libraries of a human colonic carcinoma line, T84 cell, we identified a sequence of 4115 bases from overlapping clones. The deduced protein is 1212 amino acids in length, and analysis of the primary structure indicates 12 transmembrane segments. The primary structure is 74% identical to sNKCC1, 91% identical to a mouse Na-K-Cl cotransporter (mNKCC1), 58% identical to rabbit and rat renal Na-K-Cl cotransporters (NKCC2), and 43% identical to the thiazide-sensitive Na-Cl cotransporters from flounder urinary bladder and rat kidney. Similar to sNKCC1 and mNKCC1, the 5'-end of the human colonic cotransporter is rich in G + C content. Interestingly, a triple repeat (GCG)7 occurs within the 5'-coding region and contributes to a large alanine repeat (Ala15). Two sites for N-linked glycosylation are predicted on an extracellular loop between putative transmembrane segments 7 and 8. A single potential site for phosphorylation by protein kinase A is present in the predicted cytoplasmic C-terminal domain. Northern blot analysis revealed a 7.4-7.5-kilobase transcript in T84 cells and shark rectal gland and a approximately 7.2-kilobase transcript in mammalian colon, kidney, lung, and stomach. Metaphase spreads from lymphocytes were probed with biotin-labeled cDNA and avidin fluorescein (the cotransporter gene was localized to human chromosome 5 at position 5q23.3). Human embryonic kidney cells stably transfected with the full-length cDNA expressed a approximately 170-kDa protein recognized by anti-cotransporter antibodies. Following treatment with N-glycosidase F, the molecular mass of the expressed protein was similar to that predicted for the core protein from the cDNA sequence (132-kDa) and identical to that of deglycosylated T84 cotransporter (approximately 135-kDa). The stably transfected cells exhibited a approximately 15-fold greater bumetanide-sensitive 86Rb influx than control cells, and this flux required external sodium and chloride. Flux kinetics were consistent with an electroneutral cotransport of 1Na:1K:2Cl. Preincubation in chloride-free media was necessary to activate fully the expressed cotransporter, suggesting a [Cl]-dependent regulatory mechanism.

Na+/K+/2Cl- cotransport in medullary thick ascending limb cells: kinetics and bumetanide binding

We examined the properties of Na+/K+/2Cl- cotransport in cultured mouse mTAL cells with respect to its kinetics, the contribution of K/K exchange to K fluxes mediated by the cotransporter, and [3H]bumetanide binding and turnover numbers in media with varying osmolality. The addition of bumetanide, the replacement of external Na+ or the replacement of external Cl- resulted in an almost identical (approx. 50%) decrease in K+ influx, suggesting that Na(+)-dependent, Cl(-)-dependent, BS K+ influx was a measure of Na+/K+/2Cl- cotransport. The kinetics of the BS K+ influx revealed a high affinity for external Na+ (apparent Km 7 mM) and external K+ (apparent Km 1.3 mM), but a very low affinity for external Cl- (apparent Km 67 mM with a two-site model). Of interest was the finding that none of the K+ (86Rb+) efflux was sensitive to bumetanide, suggesting the absence of cotransport mediated K/K exchange in this cell type. Specific [3H]bumetanide binding was a saturable function of free bumetanide concentration with a Kd of 0.20 microM and maximum binding (Bmax) of 0.63 pmol/mg, or about 53,000 sites per cell. Simultaneous transport and bumetanide binding assays yielded a turnover number of 255 min-1. The omission of external Na+, K+ or Cl- reduced specific [3H]bumetanide binding to values indistinguishable from zero. Changing medium osmolarity resulted in a co-ordinate change in BS K+ influx and bumetanide binding, with a monotonic increase in both transport and bumetanide binding with increase in osmolality from 200 to 400 mosmol/kg. About 85% of the cotransporter sites were located on the apical side, as in the intact mTAL tubule. The simultaneous measurement of BS ion transport and [3H]bumetanide binding in the mTAL cell may provide valuable insights into the regulation of Na+/K+/2Cl- cotransport in this nephron segment.

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.

Primary structure and functional expression of a cDNA encoding the thiazide-sensitive, electroneutral sodium-chloride cotransporter

Electroneutral Na+:Cl- cotransport systems are involved in a number of important physiological processes including salt absorption and secretion by epithelia and cell volume regulation. One group of Na+:Cl- cotransporters is specifically inhibited by the benzothiadiazine (thiazide) class of diuretic agents and can be distinguished from Na+:K+:2Cl- cotransporters based on a lack of K+ requirement and insensitivity to sulfamoylbenzoic acid diruetics like bumetanide. We report here the isolation of a cDNA encoding a thiazide-sensitive, electroneutral sodium-chloride cotransporter from the winter flounder urinary bladder using an expression cloning strategy. The pharmacological and kinetic characteristics of the cloned cotransporter are consistent with the properties of native thiazide-sensitive sodium-chloride cotransporters in teleost urinary bladder and mammalian renal distal tubule epithelia. The nucleotide sequence predicts a protein of 1023 amino acids (112 kDa) with 12 putative membrane-spanning regions, which is not related to other previously cloned sodium or chloride transporters. Northern hybridization shows two different gene products: a 3.7-kb mRNA localized only to the urinary bladder and a 3.0-kb mRNA present in several non-bladder/kidney tissues.

Transport defects of rabbit medullary thick ascending limb cells in obstructive nephropathy

To characterize the sodium transport defect responsible for salt wasting in obstructive nephropathy, the major sodium transporters in the medullary thick ascending limb (mTAL), the apical Na-K-2Cl cotransporter and the basolateral Na-K-ATPase, were studied in fresh suspensions of mTAL cells and outer medulla plasma membranes prepared from obstructed and untreated kidneys. Oxygen consumption (QO2) studies in intact cells revealed marked reductions in the inhibitory effects of both furosemide and ouabain on QO2 in cells from obstructed, as compared with control animals, indicating a reduction in activities of both the Na-K-2Cl cotransporter and the Na-K-ATPase. Saturable [3H]bumetanide binding was reduced in membranes isolated from obstructed kidneys, but the Kd for [3H]bumetanide was unchanged, indicating a decrease in the number of functional luminal Na-K-2Cl cotransporters in obstructed mTAL. Ouabain sensitive Na-K-ATPase activity in plasma membranes was also reduced, and immunoblots using specific monoclonal antibodies directed against the alpha and beta subunits of rabbit Na-K-ATPase showed decreased amounts of both subunits in outer medullas of obstructed kidney. A significant decrease in [3H]bumetanide binding was detected after 4 h of ureteral obstruction, whereas Na-K-ATPase activity at this time was still not different from control. We conclude that ureteral obstruction reduces the amounts of both luminal Na-K-2Cl cotransporter and basolateral Na-K-ATPase in mTAL of obstructed kidney and that these reductions contribute to the salt wasting observed after release of obstruction.