Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney

Mice lacking NKCC1 are protected from development of bacteremia and hypothermic sepsis secondary to bacterial pneumonia

The contribution of the Na(+)-K(+)-Cl(-) transporter (NKCC1) to fluid in ion transport and fluid secretion in the lung and in other secretory epithelia has been well established. Far less is known concerning the role of this cotransporter in the physiological response of the pulmonary system during acute inflammation. Here we show that mice lacking this transporter are protected against hypothermic sepsis and bacteremia developing as a result of Klebsiella pneumoniae infection in the lung. In contrast, this protection was not observed in NKCC1(-/-) mice with K. pneumoniae-induced peritonitis. Although overall recruitment of cells to the lungs was not altered, the number of cells present in the airways was increased in the NKCC1(-/-) animals. Despite this robust inflammatory response, the increase in vascular permeability observed in this acute inflammatory model was attenuated in the NKCC1(-/-) animals. Our studies suggest that NKCC1 plays a unique and untoward unrecognized role in acute inflammatory responses in the lung and that specific inhibition of this NKCC isoform could be beneficial in treatment of sepsis.

Norepinephrine, via beta-adrenoceptors, regulates bumetanide-sensitive cotransporter type 1 expression in thick ascending limb cells

The sympathetic nervous system, via norepinephrine, regulates renal sodium transport, and chronic sympathetic activation causes sustained increases in blood pressure by reducing sodium excretion. Our previous studies show that chronic norepinephrine infusion increases the abundance of the bumetanide-sensitive cotransporter type 1, the apical sodium transporter of the thick ascending limb of Henle's loop. The present study was initiated to elucidate the mechanisms by which norepinephrine regulates the protein levels of this transporter in an immortalized thick ascending limb epithelial cell line. Treatment with norepinephrine, either alone or in the presence of actinomycin D or cycloheximide, had no effect on cotransporter mRNA levels. Treatment with norepinephrine, however, increased bumetanide-sensitive cotransporter type 1 protein levels (70% increase versus control; P=0.012), and pretreatment with cycloheximide blocked the effect of norepinephrine on bumetanide-sensitive cotransporter type 1 protein levels. To further elucidate the mechanism, thick ascending limb cells were treated with norepinephrine in the presence of phentolamine (alpha-adrenoceptor blocker), propranolol (beta-adrenoceptor blocker), SQ22536 (adenylyl cyclase inhibitor), PD098059 (mitogen-activated protein kinase pathway inhibitor), H-89 (protein kinase A inhibitor), or staurosporine (protein kinase C inhibitor). Treatment with propranolol, SQ22536, and H-89 abolished the effects of norepinephrine on bumetanide-sensitive cotransporter type 1 protein levels, whereas staurosporine had no effect. Treatment with PD098059 partially inhibited the effects of norepinephrine (40% decrease versus norepinephrine; P=0.03), and treatment with phentolamine potentiated the effects of norepinephrine (30% increase versus norepinephrine; P=0.02) on bumetanide-sensitive cotransporter type 1 protein levels. We conclude that regulation of bumetanide-sensitive cotransporter type 1 by norepinephrine proceeds via the beta-adrenoceptor receptor-cAMP-protein kinase A pathway that involves in part mitogen-activated protein kinases and that alpha-adrenoceptor activation negatively regulates bumetanide-sensitive cotransporter type 1 protein levels.

Identification and functional characterization of cation-chloride cotransporters in plants

Chloride (Cl(-)) is an essential nutrient and one of the most abundant inorganic anions in plant tissues. We have cloned an Arabidopsis thaliana cDNA encoding for a member of the cation-Cl(-) cotransporter (CCC) family. Deduced plant CCC proteins are highly conserved, and phylogenetic analyses revealed their relationships to the sub-family of animal K(+):Cl(-) cotransporters. In Xenopus laevis oocytes, the A. thaliana CCC protein (At CCC) catalysed the co-ordinated symport of K(+), Na(+) and Cl(-), and this transport activity was inhibited by the 'loop' diuretic bumetanide, a specific inhibitor of vertebrate Na(+):K(+):Cl(-) cotransporters, indicating that At CCC encodes for a bona fide Na(+):K(+):Cl(-) cotransporter. Analysis of At CCC promoter-beta-glucuronidase transgenic Arabidopsis plants revealed preferential expression in the root and shoot vasculature at the xylem/symplast boundary, root tips, trichomes, leaf hydathodes, leaf stipules and anthers. Plants homozygous for two independent T-DNA insertions in the CCC gene exhibited shorter organs such as inflorescence stems, roots, leaves and siliques. The elongation zone of the inflorescence stem of ccc plants often necrosed during bolt emergence, while seed production was strongly impaired. In addition, ccc plants exhibited defective Cl(-) homeostasis under high salinity, as they accumulated higher and lower Cl(-) amounts in shoots and roots, respectively, than the treated wild type, suggesting At CCC involvement in long-distance Cl(-) transport. Compelling evidence is provided on the occurrence of cation-chloride cotransporters in the plant kingdom and their significant role in major plant developmental processes and Cl(-) homeostasis.

WNK4 kinase is a negative regulator of K+-Cl- cotransporters

WNK kinases [with no lysine (K) kinase] are emerging as regulators of several membrane transport proteins in which WNKs act as molecular switches that coordinate the activity of several players. Members of the cation-coupled chloride cotransporters family (solute carrier family number 12) are one of the main targets. WNK3 activates the Na(+)-driven cotransporters NCC, NKCC1, and NKCC2 and inhibits the K(+)-driven cotransporters KCC1 to KCC4. WNK4 inhibits the activity of NCC and NKCC1, while in the presence of the STE20-related proline-alanine-rich kinase SPAK activates NKCC1. Nothing is known, however, regarding the effect of WNK4 on the K(+)-Cl(-) cotransporters. Using the heterologous expression system of Xenopus laevis oocytes, here we show that WNK4 inhibits the activity of the K(+)-Cl(-) cotransporters KCC1, KCC3, and KCC4 under cell swelling, a condition in which these cotransporters are maximally active. The effect of WNK4 requires its catalytic activity because it was lost by the substitution of aspartate 318 for alanine (WNK4-D318A) that renders WNK4 catalytically inactive. In contrast, three different WNK4 missense mutations that cause pseudohypoaldosteronism type II do not affect the WNK4-induced inhibition of KCC4. Finally, we observed that catalytically inactive WNK4-D318A is able to bypass the tonicity requirements for KCC2 and KCC3 activation in isotonic conditions. This effect is enhanced by the presence of catalytically inactive SPAK, was prevented by the presence of protein phosphatase inhibitors, and was not present in KCC1 and KCC4. Our results reveal that WNK4 regulates the activity of the K(+)-Cl(-) cotransporters expressed in the kidney.

Cation-chloride cotransporters and GABA-ergic innervation in the human epileptic hippocampus

Intracellular chloride concentration, [Cl(-)](i), determines the polarity of GABA(A)-induced neuronal Cl(-) currents. In neurons, [Cl(-)](i) is set by the activity of Na(+), K(+), 2Cl(-) cotransporters (NKCC) such as NKCC1, which physiologically accumulate Cl(-) in the cell, and Cl(-) extruding K(+), Cl(-) cotransporters like KCC2. Alterations in the balance of NKCC1 and KCC2 activity may determine the switch from hyperpolarizing to depolarizing effects of GABA, reported in the subiculum of epileptic patients with hippocampal sclerosis. We studied the expression of NKCC (putative NKCC1) and KCC2 in human normal temporal neocortex by Western blot analysis and in normal and epileptic regions of the subiculum and the hippocampus proper using immunocytochemistry. Western blot analysis revealed NKCC and KCC2 proteins in adult human neocortical membranes similar to those in rat neocortex. NKCC and KCC2 immunolabeling of pyramidal and nonpyramidal cells was found in normal and epileptic hippocampal formation. In the transition between the subiculum with sclerotic regions of CA1, known to exhibit epileptogenic activity, double immunolabeling of NKCC and KCC2 revealed that approximately 20% of the NKCC-immunoreactive neurons do not express KCC2. In these same areas some neurons were distinctly hyperinnervated by parvalbumin (PV) positive hypertrophic basket formations that innervated mostly neurons expressing NKCC (74%) and to a lesser extent NKCC-immunonegative neurons (26%). Hypertrophic basket formations also innervated KCC2-positive (76%) and -negative (24%) neurons. The data suggest that changes in the relative expression of NKCC1 and KCC2 in neurons having aberrant GABA-ergic hyperinnervation may contribute to epileptiform activity in the subicular regions adjacent to sclerotic areas of the hippocampus.

Regulatory mechanism of "K+recycling" for Na +reabsorption in renal tubules

Hypertension is one of the predominant risk factors for the progression of renal impairment, and the most common disorder in industrialized societies. Because reduction of the systemic blood pressure in hypertension can halt the progression of renal impairment, it is imperative to appropriately control the systemic blood pressure. Recent genetic analysis has reconfirmed that renal maladjustment of Na(+)-homeostasis, which determines the extracellular fluid volume, is a key element in the pathogenesis of hypertension. The distal tubules adjust the net Na(+)-excretion according to Na(+)-ingestion and maintain the Na(+)-homeostasis. The distal convoluted tubules and the connecting tubules are the predominant sites for the adjustment in individuals with a modern lifestyle. In these tubules, Na(+)-reabsorption depends on "K(+)-recycling", which is conducted through K(+) channels. Because the functional expression of K(+) channels in these tubules is regulated by signal motifs for intracellular localization, the adjustment of "K(+)-recycling" through the modification of signal motifs could be a new target for the treatment of hypertension.

Transcriptional and functional analyses of SLC12A3 mutations: new clues for the pathogenesis of Gitelman syndrome

Gitelman syndrome (GS) is a recessive salt-losing tubulopathy that is caused by mutations in the SLC12A3 gene that encodes the sodium-chloride co-transporter (NCC). GS is characterized by significant inter- and intrafamilial phenotype variability, with early onset and/or severe clinical manifestations in some patients. No correlations between the disease variability and the position/nature of SLC12A3 mutations have been investigated thus far. In this study, extensive mutational analyses of SLC12A3 were performed in 27 patients with GS, including genomic DNA sequencing, multiplex ligation-dependent probe amplification, cDNA analysis, and quantification of allele-specific transcripts, in parallel with functional analyses in Xenopus laevis oocytes and detailed phenotyping. Twenty-six SLC12A3 mutations were identified in 25 patients with GS, including eight novel (detection rate 80%). Transcript analysis demonstrated that splicing mutations of SLC12A3 lead to frameshifted mRNA subject to degradation by nonsense-mediated decay. Heterologous expression documented a novel class of NCC mutants with defective intrinsic transport activity. A subgroup of patients presented with early onset, growth retardation, and/or detrimental manifestations, confirming the potential severity of GS. The mutations that were associated with a severe presentation were the combination at least for one allele of a missplicing resulting in a truncated transcript that was downregulated by nonsense-mediated decay or a nonfunctional, cell surface-absent mutant. The most recurrent mutation on the second allele was a newly described NCC mutant that affected the functional properties of the co-transporter. These data suggest that the nature/position of SLC12A3 mutation, combined with male gender, is a determinant factor in the severity of GS and provide new insights in the underlying pathogenic mechanisms of the disease.

Recent advances in renal tubular calcium reabsorption

PURPOSE OF REVIEW

Knowledge of renal Ca2+ reabsorption has evolved greatly in recent years. This review focuses on two recent discoveries concerning passive and active Ca2+ reabsorption.

RECENT FINDINGS

The thiazide diuretics are known for their hypocalciuric effect. Recently, it has been demonstrated that TRPV5-knockout mice, in which active Ca2+ reabsorption in the distal convoluted tubule is completely abolished, show the same sensitivity towards thiazides as wild-type mice. This indicates that thiazide affects Ca2+ reabsorption indirectly via contraction of the extracellular volume, independent of active Ca2+ reabsorption in the distal convoluted tubule, thereby increasing passive paracellular Ca2+ transport in the proximal tubule. Moreover, the antiaging hormone Klotho regulates Ca2+ reabsorption in the distal convoluted tubule via a novel molecular mechanism. Klotho stabilizes the TRPV5 Ca2+ channel in the plasma membrane by deglycosylation of the protein.

SUMMARY

By showing that thiazide-induced hypercalciuria is due to increased passive Ca2+ reabsorption in the proximal tubule, a long-standing issue has been solved, underlining the importance of proximal paracellular Ca2+ reabsorption. Moreover, the molecular mechanism by which the antiaging hormone Klotho regulates TRPV5 activity may prove to be generally applicable in Klotho-mediated prevention of aging.

NKCC1 activity modulates formation of functional inhibitory synapses in cultured neocortical neurons

Intracellular Cl(-) concentration ([Cl(-)](i)) in immature neurons is higher than that expected for a passive distribution, therefore the equilibrium potential for chloride is more positive than the resting membrane potential, and the resulting GABA renders immature neurons depolarization. The higher [Cl(-)](i) in immature neurons is thought to be attributed to the uptake of Cl(-) mediated by NKCC1 (Na(+), K(+)-2Cl(-) cotransporter). Thus, a dysfunction of this transporter could affect synaptic development through a GABA(A) receptor-mediated pathway. To test this possibility, we examined the effects of a Cl(-)-uptake inhibitor on the development of synaptic activities of rat neocortical neurons in culture. Chronic treatment with bumetanide at 10 microM during the culture diminished the amplitude of synaptically-driven rhythmic depolarizing potentials (RDPs) in neurons and also decreased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) but not of spontaneous excitatory postsynaptic currents (sEPSCs). Chronic treatment with bumetanide decreased vesicular GABA transporter (VGAT)-immunopositive particles without affecting paired-pulse ratio of evoked IPSCs (eIPSCs), indicating decrease in the number of functional GABAergic synapses. Acute treatment with bumetanide (10 microM) decreased neuronal [Cl(-)](i), the amplitude of RDPs, and neuronal excitability, while bumetanide had no effect on RDPs and neuronal excitability in the presence of bicuculline. These results suggest that the uptake of Cl(-) by NKCC1 affects the development of inhibitory synapses by promoting a depolarizing GABA-mediated response.

Six2 is required for suppression of nephrogenesis and progenitor renewal in the developing kidney

During kidney development and in response to inductive signals, the metanephric mesenchyme aggregates, becomes polarized, and generates much of the epithelia of the nephron. As such, the metanephric mesenchyme is a renal progenitor cell population that must be replenished as epithelial derivatives are continuously generated. The molecular mechanisms that maintain the undifferentiated state of the metanephric mesenchymal precursor cells have not yet been identified. In this paper, we report that functional inactivation of the homeobox gene Six2 results in premature and ectopic differentiation of mesenchymal cells into epithelia and depletion of the progenitor cell population within the metanephric mesenchyme. Failure to renew the mesenchymal cells results in severe renal hypoplasia. Gain of Six2 function in cortical metanephric mesenchymal cells was sufficient to prevent their epithelial differentiation in an organ culture assay. We propose that in the developing kidney, Six2 activity is required for maintaining the mesenchymal progenitor population in an undifferentiated state by opposing the inductive signals emanating from the ureteric bud.

Normotonic cell shrinkage induced by Na+ deprivation results in apoptotic cell death in human epithelial HeLa cells

Apoptosis is a major form of cell death that occurs in response to a variety of signals in both physiological and pathological situations. A hallmark of apoptosis is normotonic cell shrinkage, called apoptotic volume decrease (AVD), the process of which involves fluxes of K(+), Cl(-), and Na(+). Na(+) influx was suggested to be required in Fas-induced apoptosis in human Jurkat T cells, whereas Na(+) efflux was found to be associated with AVD and apoptosis in human HL-60 cells. Here we examined the effects of extracellular Na(+) deprivation on cell volume and viability in human epithelial HeLa cells. The incubation of HeLa cells in normotonic Na(+)-free Ringer solution resulted in persistent cell shrinkage after > or = 30 min and reduction in cell viability after > or = 1 h. After exposure to Na(+)-free solution for 5 h, a marked reduction in cell viability was found to be associated with an activation of caspase-3 without showing significant LDH release, indicating that the cells underwent apoptosis but not necrosis. Na(+) deprivation-induced cell shrinkage and apoptotic cell death were significantly inhibited by a blocker of Na(+)-K(+)-2Cl(-) cotransporter (NKCC) or of the reverse-mode operation of Na(+)/Ca(2+) exchanger (NCX), but not by a blocker of Na(+)/H(+) exchanger (NHE). Therefore it is concluded that Na(+) deprivation causes persistent cell shrinkage resulting from Na(+) efflux mainly via NKCC and NCX and thereafter leads to apoptotic death of HeLa cells. It is also suggested that normotonic cell shrinkage per se, if persistent, provides a sufficient condition for apoptosis induction.

The Na+:Cl- cotransporter is activated and phosphorylated at the amino-terminal domain upon intracellular chloride depletion

The renal Na(+):Cl(-) cotransporter rNCC is mutated in human disease, is the therapeutic target of thiazide-type diuretics, and is clearly involved in arterial blood pressure regulation. rNCC belongs to an electroneutral cation-coupled chloride cotransporter family (SLC12A) that has two major branches with inverse physiological functions and regulation: sodium-driven cotransporters (NCC and NKCC1/2) that mediate cellular Cl(-) influx are activated by phosphorylation, whereas potassium-driven cotransporters (KCCs) that mediate cellular Cl(-) efflux are activated by dephosphorylation. A cluster of three threonine residues at the amino-terminal domain has been implicated in the regulation of NKCC1/2 by intracellular chloride, cell volume, vasopressin, and WNK/STE-20 kinases. Nothing is known, however, about rNCC regulatory mechanisms. By using rNCC heterologous expression in Xenopus laevis oocytes, here we show that two independent intracellular chloride-depleting strategies increased rNCC activity by 3-fold. The effect of both strategies was synergistic and dose-dependent. Confocal microscopy of enhanced green fluorescent protein-tagged rNCC showed no changes in rNCC cell surface expression, whereas immunoblot analysis, using the R5-anti-NKCC1-phosphoantibody, revealed increased phosphorylation of rNCC amino-terminal domain threonine residues Thr(53) and Thr(58). Elimination of these threonines together with serine residue Ser(71) completely prevented rNCC response to intracellular chloride depletion. We conclude that rNCC is activated by a mechanism that involves amino-terminal domain phosphorylation.

Expression of the basolateral Na-K-Cl cotransporter during mouse nephrogenesis and embryonic development

We examined the expression of Slc12a2 (NKCC1) transcripts in the developing mouse by Northern blot analysis and in situ hybridization (ISH) using riboprobes transcribed from a cDNA encoding the transmembrane domain of human Slc12a2. In developing kidney, the 7.5-kb Slc12a2 transcript was expressed at all stages examined (13.5 d.p.c. to adult) but was more abundant in immature metanephroi. ISH revealed that NKCC1 was expressed in both mesenchymal cells and early nephric structures, but not branching ureteric buds, of developing metanephroi. A marked increase in expression was observed in the endocapillary cells of capillary loop stage glomeruli, and high expression was observed in the glomeruli of more mature nephrons. This was in contrast to Slc12a1 (NKCC2), where expression was excluded from the glomerulus. Extra-renal expression of Slc12a2 was examined in 13.5, 15.5, and 16.5 d.p.c. mouse embryos. Slc12a2 was highly expressed in the developing lung, gut, submandibular gland, tooth bud, and nasal epithelium. Slc12a2 expression was also observed in the developing central and peripheral nervous systems, including choroid plexus and trigeminal and dorsal root ganglia.

Regulation of the expression of the Na/Cl cotransporter by WNK4 and WNK1: evidence that accelerated dynamin-dependent endocytosis is not involved

The novel serine/threonine kinases (with no lysine kinases or WNKs), WNK1 and WNK4, are encoded by the disease genes for Gordon syndrome (PRKWNK1 and PRKWNK4), a rare monogenic syndrome of hypertension and hyperkalemia. These proteins alter the expression of the thiazide-sensitive Na/Cl cotransporter (NCCT) in Xenopus laevis oocytes, although the details are controversial. We describe here our own experience and confirm that kinase-dead WNK4 (318D>A) is unable to affect Na+ fluxes through the thiazide-sensitive Na/Cl transporter (NCCT) or its membrane expression as an ECFP-NCCT fusion protein. However, the kinase domain is not sufficient for a functional WNK4 since deletion of the acidic motif (a motif unique to WNK family members) completely abolishes functional activity. Indeed, the NH2 terminal of WNK4 (1-620) containing the kinase domain and acidic motif retains full activity, but does not interact directly with NCCT in pull-down assays. Coexpression of WNK1 antagonizes the action of WNK4, and kinase-dead WNK1 (368D>A) or WNK1 carrying a WNK4 disease mutation (565Q>E) behaves in the same way as wild-type WNK1. This suggests kinase activity and charge conservation within the acidic motif are not essential for the WNK1-WNK4 interaction. We also report that WNK4 probably reduces surface expression largely through an effect on forward trafficking. Hence, the effect of WNK4 on NCCT expression is mimicked by dynamin, but the dominant-negative K44A dynamin mutant does not block the action of WNK4 itself. These results further highlight important differences in the mechanism by which WNK kinases affect expression of NCCT vs. other membrane proteins such as ROMK.

Affinity-defining Domains in the Na-Cl Cotransporter

The thiazide-sensitive Na+-Cl- cotransporter (NCC) is the major pathway for salt reabsorption in the distal convoluted tubule, serves as a receptor for thiazide-type diuretics, and is involved in inherited diseases associated with abnormal blood pressure. Little is known regarding the structure-function relationship in this cotransporter. Previous studies from our group reveal that mammalian NCC exhibits higher affinity for ions and thiazides than teleost NCC and suggest a role for glycosylation upon thiazide affinity. Here we have constructed a series of chimeric and mutant cDNAs between rat and flounder NCC to define the role of glycosylation status, the amino-terminal domain, the carboxyl-terminal domain, the extracellular glycosylated loop, and the transmembrane segments upon affinity for Na+, Cl-, and metolazone. Xenopus laevis oocytes were used as the heterologous expression system. We observed that elimination of glycosylation sites in flounder NCC did not affect the affinity of the cotransporter for metolazone. Also, swapping the amino-terminal domain, the carboxyl-terminal domain, the glycosylation sites, or the entire extracellular glycosylation loop between rat and flounder NCC had no effect upon ions or metolazone affinity. In contrast, interchanging transmembrane regions between rat and flounder NCC revealed that affinity-modifying residues for chloride are located within the transmembrane 1-7 region and for thiazides are located within the transmembrane 8-12 region, whereas both segments seem to be implicated in defining sodium affinity. These observations strongly suggest that binding sites for chloride and thiazide in NCC are different.

The Na+-K+-2Cl- cotransporter and the osmotic stress response in a model salt transport epithelium

Epithelia are physiologically exposed to osmotic stress resulting in alteration of cell volume in several aspects of their functioning; therefore, the activation of 'emergency' systems of rapid cell volume regulation is fundamental in their physiology. In this review, the physiological response to osmotic stress, particularly hypertonic stress, was described in a salt-transporting epithelium, the intestine of the euryhaline teleost European eel. This epithelium is physiologically exposed to changes in extracellular osmolarity and represents a good physiological model for functional studies on cellular volume regulation, permitting the study of volume regulated ion transport mechanisms in a native tissue. An absorptive form of the cotransporter, homologue of the renal NKCC2, localized on the apical membrane, was found in the intestine of the euryhaline teleost European eel. This cotransporter accounts for the luminal uptake of Cl-; it operates in series with a basolateral Cl- conductance and presumably a basolateral electroneutral KCl cotransport and in parallel with a luminal K+ conductance. The ion transport model described for eel intestine, based on the operation of an absorptive luminal Na+-K+-2Cl-, is basically the same as the model that has been proposed for the thick ascending limb (cTAL) of the mammalian renal cortex. This paper focuses on the role of Na+-K+-2Cl- cotransport in the responses to hypertonic stress in the eel intestine and the role of cytoskeleton (either actin-based or tubulin based) is discussed.

Residual urinary concentrating ability and AQP2 expression in a rat model for chronic renal failure

BACKGROUND

In chronic renal failure (CRF), a defect in urinary concentrating ability develops gradually as the renal failure progresses. Although several molecular mechanisms associated with renal urinary concentration are reported to be impaired in a rat model for renal failure, the mechanisms underlying residual urinary concentration ability in CRF remain to be elucidated.

METHODS

Rats that underwent an 8-week recovery period after 5/6 nephrectomy were used as the model for CRF. Urinary concentration was induced by 24-hour water restriction. Plasma osmolality and arginine vasopressin (AVP) were measured from blood sampled by inserting a catheter into the femoral artery before and after the water restriction. AQP2 mRNA expression in the inner medulla was examined by competitive PCR and in situ hybridization, and protein expression, by Western blotting. Rats that underwent sham operation were used as control.

RESULTS

Water restriction significantly reduced urine volume and increased urine osmolality in CRF rats, although such changes were much less than those in sham-operated rats. Plasma AVP was elevated at the basal condition, and further elevation was noted after water restriction. AQP2 mRNA signals were significantly intensified by water restriction even in CRF rats, although the increase was limited as in the case of urine osmolality. Western blotting also showed a small but significant enhancement of protein signals in response to water restriction in CRF rats.

CONCLUSIONS

We noted a weak but significant response of AQP2 expression to dehydration in CRF rats. This response in the collecting duct may be one of the factors contributing to residual urinary concentrating ability in CRF.

Increased expression of sodium transporters in rats chronically inhibited of nitric oxide synthesis

The present study was done to determine whether endogenous nitric oxide (NO) plays a role in the regulation of sodium transporters in the kidney. Male Sprague-Dawley rats were treated with NG-nitro-L-arginine methyl ester (L-NAME, 100 mg/L drinking water) for 4 weeks. Control rats were supplied with tap water without drugs. Expression of Na, K-ATPase, type 3 Na/H exchanger (NHE3), Na/K/2Cl cotransporter (BSC1), and thiazide-sensitive Na/Cl cotransporter (TSC) proteins was determined in the kidney by Western blot analysis. Catalytic activity of Na,K-ATPase was also determined. The treatment with L-NAME significantly and steadily increased the systemic blood pressure. Total and fractional excretion of urinary sodium decreased significantly, while creatinine clearance remained unaltered. Neither plasma renin activity nor aldosterone concentration was significantly altered. The alpha1 subunit expression and the catalytic activity of Na, K-ATPase were increased in the kidney. The expression of NHE3, BSC1 and TSC was also increased significantly. These results suggest that endogenously-derived NO exerts a tonic inhibitory effect on the expression of sodium transporters, including Na, K-ATPase, NHE3, BSC1, and TSC, in the kidney.

Vestibular malformation in mice lacking Na-K-2Cl cotransporter 1 and expression of Na-K-2Cl cotransporter 1 in human vestibular end organs

CONCLUSION

The Na-K-2Cl cotransporter-1 (NKCCl) may be essential for the maintenance and functioning of the vestibular morphology in mice and it is strongly expressed in human vestibular end organs.

OBJECTIVE

NKCCl is a member of the cation-coupled chloride transporter which participates in salt transport and cell volume regulation in diverse tissues. NKCCl-deficient mice exhibit deafness, and show structural alterations in the cochlea. In addition to hearing loss, NKCCl-deficient mice show a shaker-waltzer behavior, which suggests a vestibular system defect. In this study we investigated the morphology of the vestibular system of NKCCl-deficient mice and also evaluated whether NKCCl mRNA and its protein are expressed in human vestibular end organs.

MATERIAL AND METHODS

NKCCl-deficient and wild-type mice aged 4-5 weeks were sacrificed. Their heads were cut in the midsagittal plane, fixed and decalcified. For light microscopy, 5-microm sections were cut and stained with hematoxylin-eosin. Human vestibular end organs were harvested during acoustic tumor surgery via a translabyrinthine approach. Some of these end organs were used for total mRNA extraction and the remainder for immunostaining. Reverse transcriptase polymerase chain reaction and immunostaining were performed for NKCCl.

RESULTS

The scala media of the cochleae of the NKCCl-deficient mice had collapsed but the bony labyrinth appeared unaffected. However, the semicircular canals (SCCs) were much smaller than those in the wild-type mice. Furthermore, the SCCs were completely missing in some NKCCl-deficient mice. NKCCl mRNA was expressed in both the human macula and crista ampullaris, and its protein was expressed mainly in the transitional and dark cell areas of the human crista ampullaris.

Activity of the renal Na+-K+-2Cl- cotransporter is reduced by mutagenesis of N-glycosylation sites: role for protein surface charge in Cl- transport

The renal-specific Na(+)-K(+)-2Cl(-) cotransporter NKCC2 belongs to the SLC12 gene family; it is the target for loop diuretics and the cause of type I Bartter's syndrome. Because the NKCC2 sequence contains two putative N-linked glycosylation sites, one of which is conserved with the renal Na(+)-Cl(-) cotransporter in which glycosylation affects thiazide affinity, we assessed the role of glycosylation on NKCC2 functional properties. One (N442Q or N452Q) or both (N442,452Q) N-glycosylation sites were eliminated by site-directed mutagenesis. Wild-type NKCC2 and mutant clones were expressed in Xenopus laevis oocytes and analyzed by (86)Rb(+) influx, Western blotting, and confocal microscopy. Inhibition of glycosylation with tunicamycin in wild-type NKCC2-injected oocytes resulted in an 80% reduction of NKCC2 activity. Immunoblot of injected oocytes revealed that glycosylation of NKCC2 was completely prevented in N442,452Q-injected oocytes. Functional activity was reduced by 50% in N442Q- and N452Q-injected oocytes and by 80% in oocytes injected with N442,452Q, whereas confocal microscopy of oocytes injected with wild-type or mutant enhanced green fluorescent protein-tagged NKCC2 clones revealed that surface fluorescence intensity was reduced approximately 20% in single mutants and 50% in the double mutant. Ion transport kinetic analyses revealed no changes in cation affinity and a small increase in Cl(-) affinity by N442Q and N442,452Q. However, a slight decrease in bumetanide affinity was observed. Our data demonstrate that NKCC2 is glycosylated and suggest that prevention of glycosylation reduces its functional expression by affecting insertion into the plasma membrane and the intrinsic activity of the cotransporter.

cAMP increases surface expression of NKCC2 in rat thick ascending limbs: role of VAMP

NaCl absorption by the thick ascending limb of Henle's loop (TAL) is mediated by the apical Na-K-2Cl cotransporter NKCC2. cAMP increases NaCl absorption in the TAL by stimulating NKCC2. In oocytes, cAMP increases NKCC2 activity by regulating its trafficking. However, the mechanism by which cAMP stimulates NKCC2 in TALs is not clear. We hypothesized that cAMP increases surface expression of NKCC2 and NaCl absorption in TALs and that vesicle-associated membrane protein (VAMP) is involved in this mechanism. We used surface biotinylation of rat medullary TALs (mTAL) to examine surface and total NKCC2 levels. When mTAL suspensions were treated with dibutyryl cAMP (db-cAMP) or forskolin plus IBMX for 20 min, surface NKCC2 expression increased by 126 +/- 23 and 92 +/- 17% above basal, respectively (P < 0.03). No changes in total NKCC2 expression were observed, suggesting that cAMP increased translocation of NKCC2. We studied the role of VAMP in NKCC2 translocation and found that incubating mTALs with tetanus toxin (30 nM), which inhibits vesicle trafficking by inactivating VAMP-2 and -3, completely blocked the stimulatory effect of db-cAMP on surface NKCC2 expression (tetanus toxin = 100% vs. tetanus toxin + db-cAMP = 102 +/- 21% of control; not significant). We studied VAMP-2 and -3 expression and localization in isolated perfused TALs by confocal microscopy and found that both of them were located in the subapical space of the TAL. Finally, in isolated perfused mTALs, db-cAMP increased net Cl absorption by 95.0 +/- 34.8% (P < 0.03), and pretreatment of TALs with tetanus toxin blocked the stimulation of Cl absorption (from 110.9 +/- 15.9 to 109.7 +/- 15.6 pmol.min(-1).mm(-1); not significant). We concluded that cAMP increases NKCC2 surface expression by a mechanism involving VAMP and that NKCC2 trafficking to the apical membrane is involved in the stimulation of TAL NaCl absorption by cAMP.

Molecular mechanisms of cation transport by the renal Na+-K+-Cl- cotransporter: structural insight into the operating characteristics of the ion transport sites

Two variants of the renal Na(+)-K(+)-Cl(-) cotransporter (NKCC2), called NKCC2A and NKCC2F, display marked differences in Na(+), Rb(+), and Cl(-) affinities, yet are identical to one another except for a 23-residue membrane-associated domain that is derived from alternatively spliced exons. The proximal portion of these exons is predicted to encode the second transmembrane domain (tm2) in the form of an alpha-helix, and the distal portion, part of the following connecting segment (cs1a). In recent studies, we have taken advantage of the A-F differences in kinetic behavior to determine which regions in tm2-cs1a are involved in ion transport. Functional characterizations of chimeras in which tm2 or cs1a were interchanged between the variants showed that both regions are important in specifying ion affinities, but did not allow delineating the contribution of individual residues. Here, we have extended these structure-function analyses by studying additional mutants in which variant residues between A and F were interchanged individually in the tm2-cs1a region (amino acid number 216, 220, 223, 229, or 233 in NKCC2). None of the substitutions were found to affect K(m (C1-)), suggesting that the affinity difference for anion transport is conveyed by a combination of variant residues in this domain. However, 2 substitutions in the tm2 of F were found to affect cation constants specifically; interestingly, one of these mutations (residue 216) only affected K(m (Rb+)) while the other (residue 220) only affected K(m (Na+)). We have thus identified two novel residues in NKCC2 that play a key role in cation transport. Because such residues should be adjacent to one another on the vertical axis of the tm2 alpha-helix, our results imply, furthermore, that the ion transport sites in NKCC2 could be physically linked.

A new kindred with pseudohypoaldosteronism type II and a novel mutation (564D>H) in the acidic motif of the WNK4 gene

We identified a new kindred with the familial syndrome of hypertension and hyperkalemia (pseudohypoaldosteronism type II or Gordon's syndrome) containing an affected father and son. Mutation analysis confirmed a single heterozygous G to C substitution within exon 7 (1690G>C) that causes a missense mutation within the acidic motif of WNK4 (564D>H). We confirmed the function of this novel mutation by coexpressing it in Xenopus oocytes with either the NaCl cotransporter (NCCT) or the inwardly rectifying K-channel (ROMK). Wild-type WNK4 inhibits 22Na+ flux in Xenopus oocytes expressing NCCT by approximately 90% (P<0.001), whereas the 564D>H mutant had no significantly inhibitory effect on flux through NCCT. In oocytes expressing ROMK, wild-type WNK4 produced >50% inhibition of steady-state current through ROMK at a +20-mV holding potential (P<0.001). The 564D>H mutant produced further inhibition with steady-state currents to some 60% to 70% of those seen with the wild-type WNK4. Using fluorescent-tagged NCCT (enhanced cyan fluorescent protein-NCCT) and ROMK (enhanced green fluorescent protein-ROMK) to quantify the expression of the proteins in the oocyte membrane, it appears that the functional effects of the 564D>H mutation can be explained by alteration in the surface expression of NCCT and ROMK. Compared with wild-type WNK4, WNK4 564D>H causes increased cell surface expression of NCCT but reduced expression of ROMK. This work confirms that the novel missense mutation in WNK4, 564D>H, is functionally active and highlights further how switching charge on a single residue in the acid motif of WNK4 affects its interaction with the thiazide-sensitive target NCCT and the potassium channel ROMK.

Beyond the signal sequence: protein routing in health and disease

Receptors, hormones, enzymes, ion channels, and structural components of the cell are created by the act of protein synthesis. Synthesis alone is insufficient for proper function, of course; for a cell to operate effectively, its components must be correctly compartmentalized. The mechanism by which proteins maintain the fidelity of localization warrants attention in light of the large number of different molecules that must be routed to distinct subcellular loci, the potential for error, and resultant disease. This review summarizes diseases known to have etiologies based on defective protein folding or failure of the cell's quality control apparatus and presents approaches for therapeutic intervention.

Thiazide-sensitive NaCl-cotransporter in the Intestine

Thiazides, such as hydrochlorothiazide (HCTZ), are used to control blood pressure and to reduce renal calcium excretion. These effects are a result of interactions with the NaCl-cotransporter (NCC). This is demonstrated by the fact that mutations within the NCC protein lead to salt-resistant hypotension and hypocalciuria, paralleled by an increase in bone mineral density. These symptoms are also known as Gitelman syndrome. It has become increasingly evident that the effect of HCTZ on blood pressure and calcium homeostasis cannot be attributed exclusively to kidney functions, where the primary action of HCTZ on NCC is postulated to occur. We demonstrated the presence of the NCC transporter in the rat small intestine (ileum and jejunum) and human HT-29 cells, by using reverse transcription-PCR, Northern blot, Western blot, and immunofluorescence. Furthermore, we show that HCTZ modulates Ca(2+) uptake by intestinal cells, while affecting the electrical parameters of the cellular membrane, thus suggesting a functional interaction between NCC and the epithelial voltage-dependent calcium channel. The experiments presented here support the hypothesis of a direct involvement of the intestinal cells in the interaction between HCTZ and NaCl, as well as calcium homeostasis.

Mechanisms of WNK1 and WNK4 interaction in the regulation of thiazide-sensitive NaCl cotransport

With-no-lysine (WNK) kinases are highly expressed along the mammalian distal nephron. Mutations in either WNK1 or WNK4 cause familial hyperkalemic hypertension (FHHt), suggesting that the protein products converge on a final common pathway. We showed previously that WNK4 downregulates thiazide-sensitive NaCl cotransporter (NCC) activity, an effect suppressed by WNK1. Here we investigated the mechanisms by which WNK1 and WNK4 interact to regulate ion transport. We report that WNK1 suppresses the WNK4 effect on NCC activity and associates with WNK4 in a protein complex involving the kinase domains. Although a kinase-dead WNK1 also associates with WNK4, it fails to suppress WNK4-mediated NCC inhibition; the WNK1 kinase domain alone, however, is not sufficient to block the WNK4 effect. The carboxyterminal 222 amino acids of WNK4 are sufficient to inhibit NCC, but this fragment is not blocked by WNK1. Instead, WNK1 inhibition requires an intact WNK4 kinase domain, the region that binds to WNK1. In summary, these data show that: (a) the WNK4 carboxyl terminus mediates NCC suppression, (b) the WNK1 kinase domain interacts with the WNK4 kinase domain, and (c) WNK1 inhibition of WNK4 is dependent on WNK1 catalytic activity and an intact WNK1 protein. These findings provide insight into the complex interrelationships between WNK1 and WNK4 and provide a molecular basis for FHHt.

Salt handling in the distal nephron: lessons learned from inherited human disorders

The molecular basis of inherited salt-losing tubular disorders with secondary hypokalemia has become much clearer in the past two decades. Two distinct segments along the nephron turned out to be affected, the thick ascending limb of Henle's loop and the distal convoluted tubule, accounting for two major clinical phenotypes, hyperprostaglandin E syndrome and Bartter-Gitelman syndrome. To date, inactivating mutations have been detected in six different genes encoding for proteins involved in renal transepithelial salt transport. Careful examination of genetically defined patients (“human knockouts”) allowed us to determine the individual role of a specific protein and its contribution to the overall process of renal salt reabsorption. The recent generation of several genetically engineered mouse models that are deficient in orthologous genes further enabled us to compare the human phenotype with the animal models, revealing some unexpected interspecies differences. As the first line treatment in hyperprostaglandin E syndrome includes cyclooxygenase inhibitors, we propose some hypotheses about the mysterious role of PGE2in the etiology of renal salt-losing disorders.

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.

Differential functional expression of cation-Cl- cotransporter mRNAs (KCC1, KCC2, and NKCC1) in rat trigeminal nervous system

GABA is the main inhibitory neurotransmitter in the adult brain, which causes Cl- influx into the cell via GABAA receptors. The direction of Cl- inflow is dependent on the Cl- gradient across the membrane. Cation-Cl- cotransporters have been considered to play pivotal roles in controlling intracellular Cl- concentration ([Cl-]i) of neurons; hence, they modulate the GABAergic function. To elucidate how these cotransporters are distributed in the trigeminal nuclei, we investigated the expressions of K+-Cl- cotransporters (KCC1 and KCC2) and Na+-K+-2Cl- cotransporter (NKCC1) mRNAs by using in situ hybridization histochemistry. KCC2 mRNA was expressed in the motor trigeminal nucleus (Mo5), the principal trigeminal nucleus (Pr5), and the spinal trigeminal nucleus (Sp5), but not in the trigeminal ganglion (TG) and the mesencephalic trigeminal nucleus (Me5). On the other hand, KCC1 and NKCC1 mRNAs were expressed in all the trigeminal nuclei. The resting [Cl-]i of Me5 neurons was significantly higher than that of Mo5 neurons. Thus, in primary sensory neurons such as the TG and the Me5, [Cl-]i would be higher than those in the other trigeminal nuclei because of the lack of KCC2 mRNA expression. Since Me5 neurons, but not Mo5 neurons, responded to GABA by depolarization, GABA would have differential physiological functions among trigeminal nuclei and TG.

Na+/K+/Cl− cotransporter activates MAP-kinase cascade downstream to protein kinase C, and upstream to MEK

In this study, we demonstrated that the specific inhibitors of the Na+/K+/Cl- cotransporter (NKCC1), bumetanide and furosemide, inhibited extracellular regulated kinase (ERK) phosphorylation in Balb/c 3T3 fibroblasts, stimulated with a variety of mitogens. In addition to fibroblast growth factor (FGF) shown before, the various mitogens tested in the present study (endothelial growth factor (EGF), platelet-derived growth factor (PDGF), insulin, thrombin, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA)). Enter, the Ras/Raf/MEK/ERK cascade via different growth factors receptors and through one of the two main routes. The results of the present study provide evidence that have led us to conclude that the target protein which is controlled by the Na+/K+/Cl- cotransporter, is downstream of tyrosine kinase receptors, as well as of the G-protein-coupled receptor (GPCR). Several additional lines of evidence supported the above conclusion: (i) furosemide inhibits phosphorylation of MAPK kinase (MEK) induced by receptor tyrosine kinase (RTK) ligands, such as PDGF, FGF, and EGF. (ii) Furosemide also inhibited ERK phosphorylation, induced by thrombin, a GPCR. (iii) Furosemide inhibited MEK and ERK phosphorylation even when ERK phosphorylation was induced by direct activation of protein kinase C (PKC) by TPA, which bypasses early steps of the mitogenic cascade. In addition, we found that furosemide did not affect PKC phosphorylation induced directly by TPA. Taken together, the results of the present study indicate that the signal transduction protein, controlled by the Na+/K+/Cl- cotransporter, must be downstream of the PKC, and at/or upstream to MEK in the Ras/Raf/MEK/ERK cascade.

Cloning and expression of sheep renal K-CI cotransporter-1

Sheep K-Cl cotransporter-1(shKCC1) cDNA was cloned from kidney by RT-PCR with an open reading frame of 3258 base pairs exhibiting 92%, 90%, 88% and 87% identity with pig, rabbit and human, rat and mouse KCC1 cDNAs, respectively, encoding an approximately 122 kDa polypeptide of 1086-amino acids. Hydropathy analysis reveals the familiar KCC1 topology with 12 transmembrane domains (TMDs) and the hydrophilic NH2-terminal (NTD) and COOH-terminal (CTD) domains both at the cytoplasmic membrane face. However, shKCC1 has two rather than one large extracellular loops (ECL): ECL3 between TMDs 5 and 6, and ECL6, between TMDs 11 and 12. The translated shKCC1 protein differs in 12 amino acid residues from other KCC1s, mainly within the NTD, ECL3, ICL4, ECL6, and CTD. Notably, a tyrosine residue at position 996 replaces aspartic acid conserved in all other species. Human embryonic kidney (HEK293) cells and mouse NIH/3T3 fibroblasts, transiently transfected with shKCCI-cDNA, revealed the glycosylated approximately 150 kDa proteins by Western blots and positive immunofluorescence-staining with polyclonal rabbit anti-ratKCC1 antibodies. ShKCC1 was functionally expressed in NIH/3T3 cells by an elevated basal Cl-dependent K influx measured with Rb as K-congener that was stimulated three-fold by the KCC-activator N-ethylmaleimide.

Expression of human heme oxygenase-1 in the thick ascending limb attenuates angiotensin II-mediated increase in oxidative injury

BACKGROUND

Heme oxygenase-1 (HO-1) catalyzes the conversion of heme to bilirubin, carbon monoxide (CO), and free iron, thus controlling the level of cellular heme. The medullary thick ascending limb of the loop of Henle (TALH) is situated in a site of markedly diminished oxygen tension and, as such, is highly vulnerable to ischemic insult. We hypothesize that selective upregulation of HO-1 in TALH by gene transfer attenuates oxidative stress caused by angiotensin II (Ang II).

METHODS

An adenoviral vector expressing the human HO-1 under the control of the TALH-specific promoter [Na(+)-K(+)-Cl(-) cotransporter (NKCC2 promoter)] was constructed and the cell specific expression of the recombinant adenovirus was examined using several types of cells, including endothelial, vascular smooth muscle, and TALH cells. The effects of HO-1 transduction on HO-1 expression, HO activity and the response to Ang II with respect to cyclooxygenase-2 (COX-2) up-regulation and oxidative injury [growth-stimulating hormone (GSH) levels and cell death] were determined.

RESULTS

Western blot and reverse transcription-polymerase chain reaction (RT-PCR) revealed that human HO-1 was selectively expressed in primary cultured TALH cells following infection with Ad-NKCC2-HO-1. In TALH cells infected with Ad-NKCC2-HO-1, Ang II-stimulated prostaglandin E(2) (PGE(2)) levels were reduced by 40%. Ang II caused a marked decrease in GSH levels and this decrease was greatly attenuated in TALH cells transduced with Ad-NKCC2-HO-1. Moreover, Ang II-mediated DNA degradation was completely blocked by the site-specific expression of human HO-1 gene.

CONCLUSION

These results indicate that TALH cell survival after exposure to oxidative stress injury may be facilitated by selective upregulation of HO-1, thusly blocking inflammation and apoptosis.

Long-term adaptation of renal ion transporters to chronic diuretic treatment

Loop and thiazide diuretics are clinically useful to induce negative sodium balance. However, with chronic treatment, their effects tend to be blunted since the kidney adapts to diuretics. Molecular identification of the renal ion transporters has provided us with a new understanding of the mechanisms of intrarenal adaptation to diuretics at molecular levels. In the kidney, loop and thiazide diuretics are secreted from the proximal tubule via the organic anion transporter-1 (OAT1) and exert their diuretic action by binding to the Na-K-2Cl cotransporter type 2 (NKCC2) in the thick ascending limb and the Na-Cl cotransporter (NCC) in the distal convoluted tubule, respectively. Recent studies in animal models suggest that abundance of these ion transporters is affected by long-term diuretic administration. Downstream from the primary site of diuretic action, an increase in epithelial Na+ channel (ENaC) abundance is induced by chronic furosemide or hydrochlorothiazide treatment. This adaptation is consistent with previous reports showing cellular hypertrophy and increased Na+ absorption in distal tubular segments. The abundance of NKCC2 and NCC is increased by furosemide and hydrochlorothiazide, respectively. This compensatory upregulation suggests that either diuretic may activate the ion transporter within the primary site of action. In the proximal tubule, the abundance of OAT1 is increased by chronic treatment with furosemide or hydrochlorothiazide. This upregulation of OAT1 seems to be induced by substrate stimulation, lessening diuretic tolerance associated with long-term diuretic use.

Ion and diuretic specificity of chimeric proteins between apical Na+-K+-2Cl−and Na+-Cl−cotransporters

The mammalian kidney bumetanide-sensitive Na+-K+-2Cl−and thiazide-sensitive Na+-Cl−cotransporters are the major pathways for salt reabsorption in the thick ascending limb of Henle's loop and distal convoluted tubule, respectively. These cotransporters serve as receptors for the loop- and thiazide-type diuretics, and inactivating mutations of corresponding genes are associated with development of Bartter's syndrome type I and Gitleman's disease, respectively. Structural requirements for ion translocation and diuretic binding specificity are unknown. As an initial approach for analyzing structural determinants conferring ion or diuretic preferences in these cotransporters, we exploited functional differences and structural similarities between Na+-K+-2Cl−and Na+-Cl−cotransporters to design and study chimeric proteins in which the NH2-terminal and/or COOH-terminal domains were switched between each other. Thus six chimeric proteins were produced. Using the heterologous expression system of Xenopus laevis oocytes, we observed that four chimeras exhibited functional activity. Our results revealed that, in the Na+-K+-2Cl−cotransporter, ion translocation and diuretic binding specificity are determined by the central hydrophobic domain. Thus NH2-terminal and COOH-terminal domains do not play a role in defining these properties. A similar conclusion can be suggested for the Na+-Cl−cotransporter.

Proximo-distal specialization of epithelial transport processes within the Xenopus pronephric kidney tubules

The embryonic kidneys of larval aquatic vertebrates such as fish and frogs serve as excellent model systems for exploring the early development of nephric organs. These experimental systems can easily be manipulated by microsurgery, microinjection, genetics, or combinations of these approaches. However, little is known about how physiologically similar these simple kidneys are to the more complex mammalian adult kidneys. In addition, almost nothing is known about proximo-distal patterning of nephrons in any organism. In order begin to explore the physiological specialization of the pronephric tubules along the proximo-distal axis, a combination of uptake assays using fluorescently tagged proteins, LDL particles and dextrans, and an informatics-targeted in situ screen for transport proteins have been performed on embryos of the frog, Xenopus laevis. Genes identified to be expressed within unique subdomains of the pronephric tubules include an ABC transporter, two amino acid cotransporters, two sodium bicarbonate cotransporters, a novel sodium glucose cotransporter, a sodium potassium chloride cotransporter (NKCC2), a sodium chloride organic solute cotransporter (ROSIT), and a zinc transporter. A novel combination of colorimetric and fluorescent whole-mount in situ hybridization (FCIS) was used to precisely map the expression domain of each gene within the pronephros. These data indicate specialized physiological function and define multiple novel segments of the pronephric tubules, which contain at least six distinct transport domains. Uptake studies identified functional transport domains and also demonstrated that early glomeral leakage can allow visualization of protein movement into the pronephric tubules and thus establish a system for investigating experimentally induced proteinuria and glomerulonephritis.

Renal Sodium Handling for Body Fluid Maintenance and Blood Pressure Regulation

Renal sodium handling is an essential physiologic function in mammal for body fluid maintenance and blood pressure regulation. Recent advances in molecular biology have led to the identification of kidney-specific sodium transporters in the renal tubule, thereby supplying vast information for renal physiology as well as systemic physiology. Renal urinary concentration for body fluid maintenance is accomplished by counter current multiplication in the distal tubule. Sodium transport in the thick ascending limb of Henle (TAL) is the initial process of this system. We have demonstrated that renal urinary concentration is regulated in part by the expression of the Na(+)-K(+)-2Cl(-) co-transporter (BSC1) in TAL, by showing two mechanisms of BSC1 expression: pitressin vasopressin (AVP)-dependent and AVP-independent mechanisms. Two additional findings, namely, a lack of the ability to increase BSC1 expression leads to urinary concentrating defect and an enhanced BSC1 expression underlies the edema-forming condition, confirm the close association between sodium handling in TAL and body fluid accumulation. The lines of evidence from our genetic studies of the general Japanese population suggest the importance of mendelian hypertension genes in the genetic investigation of essential hypertension. Because those genes directly or indirectly regulate sodium transport by the Na-Cl co-transporter or the epithelial sodium channel in the distal convoluted tubule to the collecting duct (distal tubular segments after TAL), sodium handling in this part of the renal tubule may be, at least in part, involved in blood pressure regulation. The unveiling of such physiologic roles of sodium handling based on the sodium transporters or on the tubular segments may lead to a better understanding of systemic physiology as well as to the development of novel therapy for body fluid or blood pressure disorders.

Regulation of renal K transport by dietary K intake

Extracellular K must be kept within a narrow concentration range for the normal function of neurons, skeletal muscle, and cardiac myocytes. Maintenance of normal plasma K is achieved by a dual mechanism that includes extrarenal factors such as insulin and beta-adrenergic agonists, which stimulate the movement of K from extracellular to intracellular fluid and modulate renal K excretion. Dietary K intake is an important factor for the regulation of K secretion: An increase in K intake stimulates secretion, whereas a decrease inhibits K secretion and enhances absorption. This effect of changes in dietary K intake on tubule K transport is mediated by aldosterone-dependent and -independent mechanisms. Recently, it has been demonstrated that the protein tyrosine kinase (PTK)-dependent signal transduction pathway is an important aldosterone-independent regulatory mechanism that mediates the effect of altered K intake on K secretion. A low-K intake stimulates PTK activity, which leads to increase in phosphorylation of cloned inwardly rectifying renal K (ROMK) channels, whereas a high-K intake has the opposite effect. Stimulation of tyrosine phosphorylation also suppresses K secretion in principal cell by facilitating the internalization of apical K channels in the collecting duct.

Pathophysiology of functional mutations of the thiazide-sensitive Na-Cl cotransporter in Gitelman disease

Most of the missense mutations that have been described in the human SLC12A3 gene encoding the thiazide-sensitive Na(+)-Cl(-) cotransporter (TSC, NCC, or NCCT), as the cause of Gitelman disease, block TSC function by interfering with normal protein processing and glycosylation. However, some mutations exhibit considerable activity. To investigate the pathogenesis of Gitelman disease mediated by such mutations and to gain insights into structure-function relationships on the cotransporter, five functional disease mutations were introduced into mouse TSC cDNA, and their expression was determined in Xenopus laevis oocytes. Western blot analysis revealed immunoreactive bands in all mutant TSCs that were undistinguishable from wild-type TSC. The activity profile was: wild-type TSC (100%) > G627V (66%) > R935Q (36%) = V995M (32%) > G610S (12%) > A585V (6%). Ion transport kinetics in all mutant clones were similar to wild-type TSC, except in G627V, in which a small but significant increase in affinity for extracellular Cl(-) was observed. In addition, G627V and G610S exhibited a small increase in metolazone affinity. The surface expression of wild-type and mutant TSCs was performed by laser-scanning confocal microscopy. All mutants exhibited a significant reduction in surface expression compared with wild-type TSC, with a profile similar to that observed in functional expression analysis. Our data show that biochemical and functional properties of the mutant TSCs are similar to wild-type TSC but that the surface expression is reduced, suggesting that these mutations impair the insertion of a functional protein into the plasma membrane. The small increase in Cl(-) and thiazide affinity in G610S and G627V suggests that the beginning of the COOH-terminal domain could be implicated in defining kinetic properties.

A single nucleotide polymorphism alters the activity of the renal Na+:Cl- cotransporter and reveals a role for transmembrane segment 4 in chloride and thiazide affinity

The thiazide-sensitive Na+:Cl- cotransporter is the major salt transport pathway in the distal convoluted tubule of the kidney, and a role of this cotransporter in blood pressure homeostasis has been defined by physiological studies on pressure natriuresis and by its involvement in monogenic diseases that feature arterial hypotension or hypertension. Data base analysis revealed that 135 single nucleotide polymorphisms along the human SLC12A3 gene that encodes the Na+:Cl- cotransporter have been reported. Eight are located within the coding region, and one results in a single amino acid change; the residue glycine at the position 264 is changed to alanine (G264A). This residue is located within the fourth transmembrane domain of the predicted structure. Because Gly-264 is a highly conserved residue, we studied the functional properties of this polymorphism by using in vitro mutagenesis and the heterologous expression system in Xenopus laevis oocytes. G264A resulted in a significant and reproducible reduction ( approximately 50%) in (22)Na+ uptake when compared with the wild type cotransporter. The affinity for extracellular Cl- and for thiazide diuretics was increased in G264A. Western blot analysis showed similar immunoreactive bands between the wild type and the G264A cotransporters, and confocal images of oocytes injected with enhanced green fluorescent protein-tagged wild type and G264A cotransporter showed no differences in the protein surface expression level. These observations suggest that the G264A polymorphism is associated with reduction in the substrate translocation rate of the cotransporter, due to a decrease in the intrinsic activity. Our study also reveals a role of the transmembrane segment 4 in defining the affinity for extracellular Cl- and thiazide diuretics.

Molecular Mechanisms of Cl- Transport by the Renal Na+-K+-Cl- Cotransporter

The 2nd transmembrane domain (tm) of the secretory Na(+)-K(+)-Cl(-) cotransporter (NKCC1) and of the kidney-specific isoform (NKCC2) has been shown to play an important role in cation transport. For NKCC2, by way of illustration, alternative splicing of exon 4, a 96-bp sequence from which tm2 is derived, leads to the formation of the NKCC2A and F variants that both exhibit unique affinities for cations. Of interest, the NKCC2 variants also exhibit substantial differences in Cl- affinity as well as in the residue composition of the first intracellular connecting segment (cs1a), which immediately follows tm2 and which too is derived from exon 4. In this study, we have prepared chimeras of the shark NKCC2A and F (saA and saF) to determine whether cs1a could play a role in Cl- transport; here, tm2 or cs1a in saF was replaced by the corresponding domain from saA (generating saA/F or saF/A, respectively). Functional analyses of these chimeras have shown that cs1a-specific residues account for most of the A-F difference in Cl- affinity. For example, Km(Cl-)s were approximately 8 mm for saF/A and saA, and approximately 70 mm for saA/F and saF. Intriguingly, variant residues in cs1a also affected cation transport; here, Km(Na+)s for the chimeras and for saA were all approximately 20 mM, and Km(Rb+) all approximately 2 mM. Regarding tm2, our studies have confirmed its importance in cation transport and have also identified novel properties for this domain. Taken together, our results demonstrate for the first time that an intracellular loop in NKCC contributes to the transport process perhaps by forming a flexible structure that positions itself between membrane spanning domains.

Dimeric Architecture of the Human Bumetanide-Sensitive Na-K-Cl Co-transporter

ABSTRACT. The primary mediator of NaCl reabsorption in the renal distal tubule is the human bumetanide-sensitive Na+-K+-2Cl− co-transporter (hNKCC2), located at the apical membrane of the thick ascending limb of Henle’s loop. The physiologic importance of this transporter is emphasized by the tubular disorder Bartter syndrome type I, which arises from the functional impairment of hNKCC2 as a result of mutations in the SLC12A1 gene. The aim of the present study was to investigate the oligomeric state of hNKCC2 to understand further its operational mechanism. To this end, hNKCC2 was heterologously expressed in Xenopus laevis oocytes. Chemical cross-linking with dimethyl-3,3-dithio-bis-propionamidate indicated that hNKCC2 subunits can reversibly form high molecular weight complexes. Co-immunoprecipitation of tagged hNKCC2 subunits further substantiated a physical interaction between individual hNKCC2 subunits. The size of the hNKCC2 multimers was determined by sucrose gradient centrifugation, and a preference for dimeric complexes (approximately 320 kD) was demonstrated. Finally, concatemeric constructs consisting of two wild-type subunits or a wild-type and a functionally impaired hNKCC2 subunit (G319R) were expressed in oocytes. Subsequently, the concatemers were functionally characterized, resulting in a significant bumetanide-sensitive 22Na+ uptake of 2.5 ± 0.2 nmol/oocyte per 30 min for the wild-type–wild-type concatemer, which was reduced to 1.3 ± 0.1 nmol/oocyte per 30 min for the wild-type–G319R concatemer. In conclusion, this study suggests that hNKCC2 forms at least functional dimers when expressed in Xenopus laevis oocytes of which the individual subunits transport Na+ independently.

Phylogeny and cloning of ion transporters in mosquitoes

Membrane transport in insect epithelia appears to be energized through proton-motive force generated by the vacuolar type proton ATPase (V-ATPase). However, secondary transport mechanisms that are coupled to V-ATPase activity have not been fully elucidated. Following a blood meal, the female mosquito regulates fluid and ion homeostasis through a series of characteristic behaviors that require brain-derived factors to regulate ion secretion. Despite the knowledge on the behaviors of the mosquito, little is known of the targets of several factors that have been implicated in cellular changes following a blood meal. This review discusses current models of membrane transport in insects and specific data on mosquito ion regulation together with the molecular aspects of membrane transport systems that are potentially linked to V-ATPase activity, which collectively determine the functioning of mosquito midgut and Malpighian tubules. Ion transport mechanisms will be discussed from a comparative physiology perspective to gain appreciation of the exquisite mechanisms of mosquito ion regulation.

Functional Expression of the Human Thiazide-Sensitive NaCl Cotransporter in Madin-Darby Canine Kidney Cells

ABSTRACT. The thiazide-sensitive Na+-Cl−cotransporter (NCC), which is expressed on the apical membrane of epithelial cells lining the distal convoluted tubule, is responsible for the reabsorption of 5% to 10% of filtered Na+and Cl−. To date, functional studies on the structural and regulatory requirements for localized trafficking and ion-transporting activity of NCC have been hampered by lack of a suitable cell system expressing this cotransporter. Reported here is the functional expression of human NCC (hNCC) in a polarized mammalian cell of renal origin—that is, the high-resistance Madin-Darby canine kidney (MDCK) cell. Western blot testing revealed that the cells predominantly expressed the complex glycosylated (approximately 140 kD) form of hNCC. hNCC was present primarily in the apical part of the cell. The functionality of hNCC was demonstrated by the gain of thiazide-sensitive Na+uptake and transepithelial transport activity. Na+uptake was significantly increased after short-term (15 min) treatment with forskolin, whereas cyclic guanosine monophosphate, wortmannin, phorbol 12-myriatate 13-acetate, and staurosporine were without effect. This indicates that hNCC activity is regulated through cyclic adenosine monophosphate, rather than via cyclic guanosine monophosphate, phospho-inositide 3-kinases or protein kinase C. Aldosterone did not alter Na+uptake in the short term (15 min) but significantly increased the transport activity in the long term (16 h). The latter effect of aldosterone was due to an effect on the cytomegalovirus promoter/enhancer driving the expression of hNCC. hNCC-MDCK cells are a good model for the study of the regulation of apical trafficking and ion-transporting activity of hNCC. E-mail r.bindels@ncmls.kun.nl

Acid pH increases the stability of BSC1/NKCC2 mRNA in the medullary thick ascending limb

Chronic metabolic acidosis enhances the ability of the medullary thick ascending limb (MTAL) to absorb NH(4)(+) at least in part by stimulating the mRNA and protein expression of BSC1/NKCC2, the MTAL apical Na(+)-K(+)(NH(4)(+))-2Cl(-) co-transporter. For assessing the mechanism by which an acid pH enhances the BSC1 mRNA abundance, MTAL were harvested from adrenalectomized rats and incubated in control (pH 7.35) and acid (pH 7.10) 1:1 mixtures of Ham's nutrient mixture F-12 and DME. rBSC1 mRNA abundance and gene transcription rate were quantified by quantitative reverse transcription-PCR and run-off assay, respectively. Acid incubation enhanced mRNA abundance within 4 h in whole cell (P < 0.02) but not in nucleus. BSC1 gene transcription rate was not affected by acid incubation. In contrast, under conditions in which gene transcription was blocked, rBSC1 mRNA decreased within 6 h by 38 +/- 11% in control but only by 15 +/- 15% in acid medium (P < 0.02), which represented an increase in the BSC1 mRNA half-life from approximately 7 to approximately 17 h. Furthermore, in a mouse TAL cell line, acid incubation for 16 h significantly increased (P < 0.02) the amount of BSC1 mRNA in cells transfected with the full-length mBSC1 cDNA but not in cells transfected with a mBSC1 cDNA lacking the 3'-UTR. These results demonstrate that acid pH enhances the stability of BSC1 mRNA probably by activating pathways that act on the AU-rich 3'-UTR of BSC1 mRNA, which contributes to the renal response to metabolic acidosis.

Vasopressin-independent renal urinary concentration: increased rBSC1 and enhanced countercurrent multiplication

BACKGROUND

A close association between the expression of the sodium transporter, rat bumetanide sensitive cotransporter (rBSC), in thick ascending limb of Henle and urinary concentration has been reported. However, direct evidence for this association and the mechanism of rBSC1 expression are still to be elucidated.

METHODS

Brattleboro (BB) rats weighing approximately 200 g were dehydrated by water restriction for 4 hours, which induced around a 5% body weight reduction. Although plasma arginine vasopressin (AVP) was undetectable even after the water restriction, BB rats concentrated urine from 182 +/- 23 (mean +/- SD) at baseline to 404 +/- 65 mOsm/kg. H2O.

RESULTS

Urinary volume was reduced from 5.8 +/- 1.8 to 1.4 +/- 0.6 mL/h. This treatment significantly increased sodium and urea accumulation in the renal medulla and reduced urinary sodium excretion. rBSC1 signals for both mRNA and protein were increased in dehydrated rats, although aquaporin type 2 (AQP2) expression was not enhanced in dehydrated BB rats. Subcutaneous infusion of desmopressin acetate (DDAVP) intensified rBSC1 signals of BB rats more than those in dehydrated condition.

CONCLUSION

Dehydration increased rBSC1 expression and enhanced countercurrent multiplication even in AVP deficiency. These results supply strong evidence for the association between rBSC1 expression and urinary concentration, and indicate the presence of an AVP-independent mechanism for urine concentration.

Expression of an Aedes aegypti cation-chloride cotransporter and its Drosophila homologues

Insects maintain haemolymph homeostasis under different environmental conditions by modulating the concentrations of Na+, K+ and Cl- ions. One group of proteins involved in ion transport across cell membranes consists of cation-chloride cotransporters that form a family of structurally similar proteins. Although much is known about these proteins in mammalian systems, our understanding of them in insects is lacking. The recent sequencing of two insect genomes, Drosophila and Anopheles, enabled us to identify globally members of the family of cation chloride cotransporters in these insects. Using RT-PCR we monitored the transcription of members of this family in development and in several tissues. Our analyses showed that transcription of these genes differ considerably from the ubiquitously and highly expressed CG5594 gene to the almost silent gene CG31547. Comparison of Drosophila CG12773 and its Aedes homologue AaeCG12773 showed that they have similar transcript expression profiles. Immunohistochemical analysis of AaeCG1277 gene expression revealed that it is highly expressed in the gut of larvae and female adults but not in Malpighian tubules. A more detailed analysis showed that this protein is localized predominantly in the basolateral membrane of these tissues. This expression pattern confirmed the results of RT-PCR analysis. We also created a mutant for one of the genes, CG10413, in Drosophila using P-element excision. Analysis of this mutant showed this protein does not appear to be essential for development.