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

NKCC1: Newly Found as a Human Disease-Causing Ion Transporter

Among the electroneutral Na⁺-dependent chloride transporters, NKCC1 had until now evaded identification as a protein causing human diseases. The closely related SLC12A transporters, NKCC2 and NCC have been identified some 25 years ago as responsible for Bartter and Gitelman syndromes: two renal-dependent salt wasting disorders. Absence of disease was most surprising since the NKCC1 knockout mouse was shown in 1999 to be viable, albeit with a wide range of deleterious phenotypes. Here we summarize the work of the past 5 years that introduced us to clinical cases involving NKCC1. The most striking cases are of 3 children with inherited mutations, who have complete absence of NKCC1 expression. These cases establish that lack of NKCC1 causes deafness; CFTR-like secretory defects with mucus accumulation in lung and intestine; severe xerostomia, hypotonia, dysmorphic facial features, and severe neurodevelopmental disorder. Another intriguing case is of a patient with a dominant deleterious SLC12A2 allele. This de novo mutation introduced a premature stop codon leading to a truncated protein. This mutant transporter seems to exert dominant-negative effect on wild-type transporter only in epithelial cells. The patient who suffers from lung, bladder, intestine, pancreas, and multiple endocrine abnormalities has, however, normal hearing and cognition. Finally, new reports substantiate the haploinsufficiency prediction of the SLC12A2 gene. Cases with single allele mutations in SLC12A2 have been linked to hearing loss and neurodevelopmental disorders.

Detection of Cl- flux in the apical microenvironment of cultured foetal distal lung epithelial cells

A self-referencing Cl--selective microelectrode (Cl- SrE) was developed and used to detect changes in the direction and magnitude of the Cl- flux (J(Cl)) from the apical region of cultured foetal distal lung epithelial cells (FDLEs) as a function of external Cl- concentration ([Cl-]e) and in response to pharmacological challenges. The technique, which is similar to that developed for other ion-selective microelectrodes, centres on the oscillation of a Cl--selective microelectrode between known points, micrometres apart, orthogonal to the plasma membrane. Application of the Fick principle to the differential voltage obtained per excursion amplitude (the referenced signal) yields the Cl- flux (pmol × cm(−2) × s(−1)). A Cl- effusion gradient was used to confirm that empirical measurements of J(Cl) using the Cl- SrE were statistically similar to predicted flux values calculated from the fall in [Cl-] with distance from the tip of the effusion source. Apical J(Cl) was then measured as a function of [Cl-]e from polarised FDLE cultures grown on permeable supports. At [Cl-]e<50 mmol × l(−1), an apical-to-basolateral (inward) flux, maximal at 400 pmol × cm(−2) × s(−1), was observed; this reverted to a continuous basolateral-to-apical (outward) flux of 203 pmol × cm(−2) × s(−1) at [Cl]e>100 mmol × l(−1). At [Cl-]e>100 mmol × l(−1), isoproterenol (basolaterally applied, 10 micromol × l(−1)) activated a Cl- influx of 561 pmol × cm(−2) × s(−1), whereas UTP (apically applied, 100 micromol × l(−1)) stimulated a Cl- efflux of 300 pmol × cm(−2) × s(−1). In all cases, 50–70 % of J(Cl) was abolished by Cl- channel blockade using 10 micromol × l(−1) diphenylamine-2-carboxylic acid (DPC) or 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We conclude that the Cl- SrE resolves a Cl- gradient in the microenvironment of the apical region of lung epithelia that varies in both direction and magnitude as a function of external [Cl-]e and in response to Cl- channel blockade and to beta2 adrenoreceptor and P2Y receptor agonists.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Na(+)/H(+) exchangers (NHE1-3) have similar turnover numbers but different percentages on the cell surface

NHE1, NHE2, and NHE3 are well-characterized cloned members of the mammalian Na(+)/H(+) exchanger (NHE) gene family. Given the specialized function and regulation of NHE1, NHE2, and NHE3, we compared basal turnover numbers of NHE1, NHE2, and NHE3 measured in the same cell system: PS120 fibroblasts lacking endogenous NHEs. NHE1, NHE2, and NHE3 were epitope tagged with vesicular stomatitis virus glycoprotein (VSVG). The following characteristics were determined on the same passage of cells transfected with NHE1V, NHE2V, or NHE3V: 1) maximal reaction velocity (V(max)) by (22)Na(+) uptake and fluorometery, 2) total amount of NHE protein by quantitative Western analysis with internal standards of VSVG-tagged maltose-binding protein, and 3) cell surface expression by cell surface biotinylation. Cell surface expression (percentage of total NHE) was 88.8 +/- 3.5, 64.6 +/- 3.3, 20.0 +/- 2.6, and 14.0 +/- 1.3 for NHE1V, 85- and 75-kDa NHE2V, and NHE3V, respectively. Despite these divergent cell surface expression levels, turnover numbers for NHE1, NHE2, and NHE3 were similar (80.3 +/- 9.6, 92.1 +/- 8.6, and 99.2 +/- 9.1 s(-1), when V(max) was determined using (22)Na uptake at 22 degrees C and 742 +/- 47, 459 +/- 16, and 609 +/- 39 s(-1) when V(max) was determined using fluorometry at 37 degrees C). These data indicate that, in the same cell system, intrinsic properties that determine turnover number are conserved among NHE1, NHE2, and NHE3.

Molecular and functional evidence for Na(+)-K(+)-2Cl(-) cotransporter expression in rat skeletal muscle

Doubt has been raised about the expression of a functional Na(+)-K(+)-2Cl(-) cotransporter in rat skeletal muscle. In this study we present molecular and functional evidence for expression of a protein having the characteristics of a cotransporter. RT-PCR of RNA isolated from rat soleus muscle with primers to a conserved putative membrane-spanning domain resulted in a single product of predicted size. Sequencing of the product showed that it bears >90% homology with known rodent NKCC1 (BSC2) cotransporters. RNase protection assay of RNA isolated from the rat soleus muscle also identified this sequence. Immunologic detection of the cotransporter with two different antibodies indicated the presence of cotransporter protein, perhaps more than one, in blots of total muscle protein. Immunohistochemical detection by confocal microscopy localized the majority of expression of the protein to the muscle fibers. Functional studies of cotransport activity also indicate the appropriate sensitivity to inhibitors and ion dependence. Taken together, these data support the presence and function of Na(+)-K(+)-2Cl(-) cotransporter activity in the soleus muscle of the rat.

Na+,K+ pump and Na+-coupled ion carriers in isolated mammalian kidney epithelial cells: regulation by protein kinase C

This review updates our current knowledge on the regulation of Na+/H+ exchanger, Na+,K+,Cl- cotransporter, Na+,Pi cotransporter, and Na+,K+ pump in isolated epithelial cells from mammalian kidney by protein kinase C (PKC). In cells derived from different tubule segments, an activator of PKC, 4beta-phorbol 12-myristate 13-acetate (PMA), inhibits apical Na+/H+ exchanger (NHE3), Na+,Pi cotransport, and basolateral Na+,K+ cotransport (NKCC1) and augments Na+,K+ pump. In PMA-treated proximal tubules, activation of Na+,K+ pump probably plays a major role in increased reabsorption of salt and osmotically obliged water. In Madin-Darby canine kidney (MDCK) cells, which are highly abundant with intercalated cells from the collecting duct, PMA completely blocks Na+,K+,Cl- cotransport and decreases the activity of Na+,Pi cotransport by 30-40%. In these cells, agonists of P2 purinoceptors inhibit Na+,K+,Cl- and Na+,Pi cotransport by 50-70% via a PKC-independent pathway. In contrast with MDCK cells, in epithelial cells derived from proximal and distal tubules of the rabbit kidney, Na+,K+,Cl- cotransport is inhibited by PMA but is insensitive to P2 receptor activation. In proximal tubules, PKC-induced inhibition of NHE3 and Na+,Pi cotransporter can be triggered by parathyroid hormone. Both PKC and cAMP signaling contribute to dopaminergic inhibition of NHE3 and Na+,K+ pump. The receptors triggering PKC-mediated activation of Na+,K+ pump remain unknown. Recent data suggest that the PKC signaling system is involved in abnormalities of dopaminergic regulation of renal ion transport in hypertension and in the development of diabetic complications. The physiological and pathophysiological implications of PKC-independent regulation of renal ion transporters by P2 purinoceptors has not yet been examined.Key words: Na+/H+ exchanger, Na+,K+,Cl- and Na+,Pi cotransporters, Na+,K+ pump, protein kinase C, P2 purinoceptor.

Transport and regulatory properties of the apical Na-K-2Cl cotransporter of macula densa cells

NH+4/NH3 fluxes were used to probe apical Na-K-2Cl transport activity of macula densa (MD) cells from rabbit kidney. In the presence of 25 mM NaCl and 5 mM Ba2+, addition of 20 mM NH+4 to the lumen produced a profound intracellular acidification, and approximately 80% of the initial acidification rate was bumetanide sensitive. The NH+4-induced acidification rate was dependent on luminal Cl- and Na+ with apparent affinities of 17 +/- 4 mM (Hill number 1.45) and 1.0 +/- 0.3 mM, respectively. In the presence of saturating luminal NaCl concentration ([NaCl]L), blockade of basolateral Cl- efflux with 10 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) reduced the NH+4-induced acidification rate by 51 +/- 6% (P > 0.01, n = 5). Under similar conditions, dibutyryl-cAMP (DBcAMP) + forskolin increased the NH+4-induced acidification rate by 27%, whereas it produced no detectable effect at low luminal NaCl concentration. Most of the observed DBcAMP + forskolin effect was probably due to the stimulation of the basolateral Cl- conductance, since, in the presence of basolateral NPPB, this activation was changed to a 17.1% and 16.6% inhibition of the NH+4-induced acidification rate observed at high or low [NaCl]L, respectively. We conclude that the cotransporter found in MD cells displays, with respect to other Na-K-2Cl cotransporters, a relatively high affinity for luminal Na+ and luminal Cl- and can be specifically inhibited by increases in intracellular Cl- and cAMP concentrations.

Comparison of Na-K-Cl cotransporters. NKCC1, NKCC2, and the HEK cell Na-L-Cl cotransporter

The Na-K-Cl cotransporter (NKCC) mediates the coupled movement of ions into most animal cells, playing important roles in maintenance of cell volume and in epithelial Cl transport. Two forms of NKCC have been described: NKCC1, the "housekeeping" isoform that is also responsible for Cl accumulation in secretory epithelial cells, and NKCC2, which mediates apical Na+K+Cl entry into renal epithelial cells. Here we examine the kinetic properties of NKCC1, NKCC2, and the endogenous HEK-293 cell cotransporter. Stable expression of rabbit NKCC2A was obtained in HEK-293 cells utilizing a chimera (h1r2A0.7) in which the 5'-untranslated region and cDNA encoding 104 amino acids of the N terminus are replaced by the corresponding sequence of NKCC1. h1r2A0.7 exhibits Na and Cl affinities near those of NKCC1, but it has a 4-fold lower Rb affinity, and a 3-fold higher affinity for the inhibitor bumetanide. The activity of h1r2A0.7 is increased on incubation in low [Cl] media as is NKCC1, but the resting level of activity is higher in h1r2A0.7 and activation is more rapid. h1r2A0.7 exhibits an appropriate volume response, unlike NKCC1 for which concomitant changes in [Cl]i appear to be the overriding factor. These results support a model in which apical NKCC2 activity is matched to basolateral Cl exit through changes in [Cl]i. Reverse transcriptase-polymerase chain reaction of HEK-293 cell mRNA is positive with NKCC1 primers and negative with NKCC2 primers. Surprisingly, we found that the behavior of the endogenous HEK cell Na-K-Cl cotransporter is unlike either of the two forms which have been described: compared with NKCC1, HEK cell cotransporter has a 2.5-fold lower Na affinity, an 8-fold lower Rb affinity, and a 4-fold higher bumetanide affinity. These results suggest the presence of a novel isoform of NKCC in HEK-293 cells.

Quantitative contribution of NHE2 and NHE3 to rabbit ileal brush-border Na+/H+ exchange

Intestinal neutral NaCl absorption, which is made up of brush-border (BB) Na+/H+ exchange linked to BB Cl-/HCO3- exchange, is up- and downregulated as part of digestion and diarrheal diseases. Glucocorticoids stimulate ileal NaCl absorption and BB Na+/H+ exchange. Intestinal BB contains two Na+/H+ exchanger isoforms, NHE2 and NHE3, but their relative roles in rabbit ileal BB Na+/H+ exchange has not been determined. A technique to separate the contribution of NHE2 and NHE3 to ileal BB Na+/H+ exchange activity was standardized by using an amiloride-related compound, HOE-694. Under basal conditions, both NHE2 and NHE3 contribute approximately 50% to ileal Na+/H+ exchange. Glucocorticoids (methylprednisolone) increase BB Na+/H+ exchange (2.5 times) but increase only ileal NHE3 activity (4.1 times), without an effect on NHE2 activity. Thus ileal BB Na+/H+ exchange in animals treated with glucocorticoids is 69% via NHE3. A quantitative Western analysis for NHE3 was developed, using as an internal standard a fusion protein of the COOH-terminal 85 amino acids of NHE3 and maltose binding protein. Glucocorticoid treatment increased the amount of BB NHE3. The quantitative Western analysis showed that NHE3 makes up 0.018% of ileal BB protein in control rabbits and 0.042% (2.3 times as much) in methylprednisolone-treated rabbits. Methylprednisolone treatment did not alter the amount of ileal BB NHE2 protein. NHE3 turnover number was estimated to be 458 cycles/s under basal conditions and 708 cycles/s in glucocorticoid-treated ileum. Thus methylprednisolone stimulates ileal BB Na+/H+ exchange activity only by an effect on NHE3 and not on NHE2; it does so primarily by increasing the amount of BB NHE3, although it also increases the NHE3 turnover number.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the "loop" diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCI isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]i), with a reduction in [Cl]i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.

Urea inhibits Na-K-2Cl cotransport in medullary thick ascending limb cells

We examined the effect of physiological concentrations of urea (100-500 mM) on Na-K-2Cl cotransport in cultured cells from mouse medullary thick ascending limb (mTAL). Urea acutely inhibited bumetanide-sensitive K influx in mTAL cells in a concentration-dependent fashion, with a statistically significant inhibition (19%) at 100 mM and 86% inhibition at 500 mM. The effect of urea was entirely reversible and was blocked by prior treatment with okadaic acid, a phosphatase inhibitor, suggesting that urea exerts its action upstream of the phosphorylation-dephosphorylation step. Cell volume was unchanged in the presence of 500 mM urea. The number of [3H]bumetanide binding sites, a measure of the number of functioning cotransporter sites, was decreased in the presence of urea, and the decrease in bumetanide binding was proportional to the decrease in bumetanide-sensitive K influx. Urea also stimulated the Ba-sensitive swelling-activated K efflux from mTAL cells. Thus urea, in concentrations that prevail in the renal medulla, alters ion transport in mTAL cells.

Prostaglandin E2 inhibits Na-K-2Cl cotransport in medullary thick ascending limb cells

Prostaglandin E2 (PGE2) is known to inhibit transepithelial Cl transport in medullary thick ascending limb (mTAL), but the mechanism of inhibition or the transport pathway affected has not been identified. We undertook this study to examine the effect of PGE2 on Na-K-2Cl cotransport in mouse mTAL cells in culture. In nanomolar concentrations, PGE2 inhibited the Na- and Cl-dependent, bumetanide-sensitive K influx by 45%, and this inhibition was also observed in the presence of 3 mM ouabain. Although PGE2 also inhibited ouabain-sensitive K flux, that inhibition was abolished in the presence of apical nystatin, suggesting that the pump inhibition was secondary to diminished Na entry into the cells. The effect of PGE2 was concentration dependent. Inhibition was observed at a concentration of < 1 nM, and half-maximal effect was observed at 2.5 nM. The effect of PGE2 was not mediated by an action on cytosolic Ca because cytosolic Ca was unchanged after the addition of PGE2. PGE2 reduced the maximal velocity for the cotransporter but had no effect on the affinity of the cotransporter for external Na, K, or Cl. Specific [3H]bumetanide binding was reduced in the presence of PGE2, suggesting that PGE2 affected bumetanide-sensitive K influx by downregulating the number of functioning Na-K-2Cl cotransporters. These results suggest that Na-K-2Cl cotransport in the mTAL cells may be under tonic inhibitory control of PGE2.

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

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

The Na-K-Cl cotransporters

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