The Na-K-Cl cotransporters

Molecular cloning and functional expression of the K-Cl cotransporter from rabbit, rat, and human. A new member of the cation-chloride cotransporter family

We report the cloning, sequence analysis, tissue distribution, and functional expression of the K-Cl cotransport protein, KCC1. KCC1 was identified by searching the human expressed sequence tag data base, based on the expectation that it would be distantly related to the Na-K-Cl cotransporter. Rabbit KCC1 (rbKCC1) and rat KCC1 (rtKCC1) were cloned by screening rabbit kidney and rat brain cDNA libraries using homologous cDNA probes. Human KCC1 (hKCC1) was obtained from I.M.A.G.E. clones and in part by reverse transcription-polymerase chain reaction; it exhibits 97% identity with rbKCC1. KCC1 encodes a 1085-residue polypeptide with substantial sequence homology (24-25% identity) to the bumetanide-sensitive Na-K-Cl cotransporter (NKCC or BSC) and the thiazide-sensitive Na-Cl cotransporter (NCC or TSC). Hydropathy analysis of KCC1 indicates structural homology to NKCC, including 12 transmembrane domains, a large extracellular loop with potential N-linked glycosylation sites, and cytoplasmic N- and C-terminal regions. Northern blot analysis revealed a ubiquitously expressed 3. 8-kilobase transcript. Much of the genomic sequence of hKCC1 is in the data base, and the gene has been previously localized to 16q22.1 (Larsen, F., Solhein, J., Kristensen, T., Kolsto, A. B., and Prydz, H.(1993) Hum. Mol. Genet. 2, 1589-1595). Epitope-tagged rbKCC1 was stably expressed in human embryonic kidney (HEK 293) cells, resulting in production of a approximately150-kDa glycoprotein. The initial rate of 86Rb efflux from cells expressing rbKCC1 was more than 7 times greater than efflux from control cells and was inhibited by 2 mM furosemide; 86Rb efflux was stimulated by cell swelling. Uptake of 86Rb into rbKCC1 cells after a 15-min pretreatment with 1 mM N-ethylmaleimide was dependent on external chloride but not on external sodium, and was inhibited by furosemide with a Ki of approximately 40 microM and by bumetanide with a Ki of approximately 60 microM. These data demonstrate that the KCC1 cDNAs encode a widely expressed K-Cl cotransporter with the characteristics of the K-Cl transporter that has been characterized in red cells.

Molecular characterization of a putative K-Cl cotransporter in rat brain. A neuronal-specific isoform

Using a combination of data base searching, polymerase chain reaction, and library screening, we have identified a putative K-Cl cotransporter isoform (KCC2) in rat brain that is specifically localized in neurons. A cDNA of 5566 bases was obtained from overlapping clones and encoded a protein of 1116 amino acids with a deduced molecular mass of 123.6 kDa. Over its full length, the amino acid sequence of KCC2 is 67% identical to the widely distributed K-Cl cotransporter isoform (KCC1) identified in rat brain and rabbit kidney (Gillen, C., Brill, S., Payne, J.A., and Forbush, B., III(1996) J. Biol. Chem. 271, 16237-16244) but only approximately25% identical to other members of the cation-chloride cotransporter gene family, including "loop" diuretic-sensitive Na-K-Cl cotransport and thiazide-sensitive Na-Cl cotransport. Based on analysis of the primary structure as well as homology with other cation-chloride cotransporters, we predict 12 transmembrane segments bounded by N- and C-terminal cytoplasmic regions. Four sites for N-linked glycosylation are predicted on an extracellular intermembrane loop between putative transmembrane segments 5 and 6. Northern blot analysis using a KCC2-specific cDNA probe revealed a very highly expressed approximately5.6-kilobase transcript only in brain. Reverse transcriptase-polymerase chain reaction revealed that KCC1 was present in rat primary astrocytes and rat C6 glioma cells but that KCC2 was completely absent from these cells, suggesting KCC2 was not of glial cell origin. In situ hybridization studies demonstrated that the KCC2 transcript was expressed at high levels in neurons throughout the central nervous system, including CA1-CA4 pyramidal neurons of the hippocampus, granular cells and Purkinje neurons of the cerebellum, and many groups of neurons throughout the brainstem.

Inositol 3,4,5,6-tetrakisphosphate inhibits the calmodulin-dependent protein kinase II-activated chloride conductance in T84 colonic epithelial cells

The mechanism by which inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5, 6)P4) regulates chloride (Cl-) secretion was evaluated in the colonic epithelial cell line T84 using whole cell voltage clamp techniques. Our studies focused on the calcium-dependent chloride conductance (gClCa) that was activated either by mobilizing intracellular calcium (Cai) stores with thapsigargin or by introduction of the autonomous, autophosphorylated calmodulin-dependent protein kinase II (CaMKII) into the cell via the patch pipette. Basal concentrations of Ins(3,4,5,6)P4 (1 microM) present in the pipette solution had no significant effect on Cl- current; however, as the concentration of the polyphosphate was increased there was a corresponding reduction in anion current, with near complete inhibition at 8-10 microM Ins(3,4,5,6)P4. Corresponding levels are found in cells after sustained receptor-dependent activation of phospholipase C. The Ins(3,4,5, 6)P4-induced inhibition of gClCa was isomer specific; neither Ins(1, 3,4,5)P4, Ins(1,3,4,6)P4, Ins(1,4,5,6)P4, nor Ins(1,3,4,5,6)P5 induced current inhibition at concentrations of up to 100 microM. Annexin IV also plays an inhibitory role in modulating gClCa in T84 cells. When 2 microM annexin IV was present in the pipette solution, a concentration that by itself has no effect on gClCa, the potency of Ins(3,4,5,6)P4 was approximately doubled. The combination of Ins(3,4,5,6)P4 and annexin IV did not alter the in vitro activity of CaMKII. These data demonstrate that Ins(3,4,5,6)P4 is an additional cellular signal that participates in the control of salt and fluid secretion, pH balance, osmoregulation, and other physiological activities that depend upon gClCa activation. Ins(3,4,5,6)P4 metabolism and action should also be taken into account when designing treatment strategies for cystic fibrosis.

Evidence against a contribution by Na(+)-Cl- cotransport to chloride accumulation in rat arterial smooth muscle

1. Chloride accumulation into rat saphenous arterial smooth muscle has been examined using chloride-sensitive microelectrodes, to assess the contribution of Na(+)-Cl-cotransport. 2. Bumetanide (10 microM) produced a fall in intracellular chloride ([Cl-]i), and a hyperpolarization of membrane potential (Em). However, [Cl-]i remained above the equilibrium level predicted from the membrane potential, indicating a residual accumulation. 3. Replacement of extracellular sodium with N-methyl-D-glucamine or choline caused a fall in [Cl-]i similar to that observed with bumetanide, but the hyperpolarization of Em was larger. In Na(+)-free media, bumetanide had no effect. [Cl-]i remained significantly above equilibrium. 4. In the presence of bumetanide, chlorothiazide produced a further dose-dependent fall in [Cl-]i, and hyperpolarization of Em. However, although [Cl-]i fell more than with bumetanide alone, it remained significantly above equilibrium. Metolazone was without effect at 100 microM. 5. In the presence of bumetanide, ethacrynic acid and N-ethyl maleimide caused a dose-dependent hyperpolarization of Em and a fall in [Cl-]i to equilibrium. 6. The third inward chloride pump in rat saphenous arterial smooth muscle appears not to be a form of Na(+)-Cl- cotransport. The potency series of thiazide diuretic action (acetazolamide > chlorothiazide > metolazone) differed significantly from that published for Na(+)-Cl- cotransport, and there is no sodium dependence.

Calcium dynamics and homeostasis in a mathematical model of the principal cell of the cortical collecting tubule

Calcium (Ca) dynamics are incorporated into a mathematical model of the principal cell in the cortical collecting tubule developed earlier in Strieter et al. (1992a. Am. J Physiol. 263:F1063-1075). The Ca components are modeled after the Othmer-Tang model for IP(3)-sensitive calcium channels (1993, in Experimental and Theoretical Advances in Biological Pattern Formation, 295-319). There are IP(3)-sensitive Ca channels and ATP-driven pumps on the membrane of the endoplasmic reticulum. Calcium enters the cell passively down its electrochemical gradient. A Ca pump and Na/Ca exchange in the basolateral membrane are responsible for the extrusion of cytoplasmic calcium. Na/Ca exchange can also operate in reverse mode to transport Ca into the cell. Regulatory effects of cytoplasmic Ca on the apical Na channels are modeled after experimental data that indicate apical Na permeability varies inversely with cytoplasmic Ca concentration. Numerical results on changes in intracellular Ca caused by decreasing NaCl in the bath and the lumen are similar to those from experiments in Bourdeau and Lau (1990. Am. J Physiol. 258:F1497-1503). This match of simulation and experiment requires the synergistic action of the Na/Ca exchanger and the Ca regulated apical Na permeability. In a homogeneous medium, cytoplasmic Ca becomes oscillatory when extracellular Na is severely decreased, as observed in experiments of cultured principal cells (Koster, H., C. van Os and R. Bindels. 1993. Kidney Int.43:828-836). This essentially pathological situation arises because the hyperpolarization of membrane potential caused by Na-free medium increases Ca influx into the cell, while the Na/Ca exchanger is inactivated by the low extracellular Na and can no longer move Ca out of the cell effectively. The raising of the total amount of intracellular Ca induces oscillatory Ca movement between the cytoplasm and the endoplasmic reticulum. Ca homeostasis is investigated under the condition of severe extracellular Ca variations. As extracellular Ca is decreased, Ca regulation is greatly impaired if Ca does not regulate apical ionic transport. The simulations indicate that the Na/Ca exchanger alone has only limited regulatory capacity. The Ca regulated apical sodium or potassium permeability are essential for regulation of cytoplasmic Ca in the principal cell of the cortical collecting tubule.

Culturing induced expression of basolateral Na+-K+-2Cl- cotransporter BSC2 in proximal tubule, aortic endothelium, and vascular smooth muscle

So far, two isoforms of the neutral Na+K+-2Cl- cotransporter have been cloned in mammals. One isoform, BSC1, mediates apical ion entry in the renal thick ascending limb of Henle and a second, BSC2, appears to be an ubiquitously expressed Na+K+-2Cl- cotransporter. In primary cultures of rabbit proximal tubule, porcine aortic endothelial cells, and rat vascular smooth muscle cells, expression of the second isoform BSC2 was demonstrated by Northern blot analysis and bumetanide-sensitive 86Rb+ uptake studies. A surprising finding was the absence of BSC2 in fully differentiated freshly-isolated proximal tubule, porcine aortic endothelial cells, and rat vascular smooth muscle cells. Several studies have reported modulation of Na+K+-2Cl- cotransport activity by vasoactive substances and suggested a role for disturbed cotransport in, for example, the pathogenesis of essential hypertension. All these observations, however, were made in cultured cells which, in view of our findings, makes the physiological relevance questionable.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

Ammonia inhibits cAMP-regulated intestinal Cl- transport. Asymmetric effects of apical and basolateral exposure and implications for epithelial barrier function

The colon, unlike most organs, is normally exposed to high concentrations of ammonia, a weak base which exerts profound and diverse biological effects on mammalian cells. The impact of ammonia on intestinal cell function is largely unknown despite its concentration of 4-70 mM in the colonic lumen. The human intestinal epithelial cell line T84 was used to model electrogenic Cl- secretion, the transport event which hydrates mucosal surfaces and accounts for secretory diarrhea. Transepithelial transport and isotopic flux analysis indicated that physiologically-relevant concentrations of ammonia (as NH4Cl) markedly inhibit cyclic nucleotide-regulated Cl- secretion but not the response to the Ca2+ agonist carbachol. Inhibition by ammonia was 25-fold more potent with basolateral compared to apical exposure. Ion substitution indicated that the effect of NH4Cl was not due to altered cation composition or membrane potential. The site of action of ammonia is distal to cAMP generation and is not due simply to cytoplasmic alkalization. The results support a novel role for ammonia as an inhibitory modulator of intestinal epithelial Cl- secretion. Secretory responsiveness may be dampened in pathological conditions associated with increased mucosal permeability due to enhanced access of lumenal ammonia to the basolateral epithelial compartment.

Urea inhibits NaK2Cl cotransport in human erythrocytes

We examined the effect of urea on NaK2Cl cotransport in human erythrocytes. In erythrocytes from nine normal subjects, the addition of 45 mM urea, a concentration commonly encountered in uremic subjects, inhibited NaK2Cl cotransport by 33 +/- 7%. Urea inhibited NaK2Cl cotransport reversibly, and in a concentration-dependent fashion with half-maximal inhibition at 63 +/- 10 mM. Acute cell shrinkage increased, and acute cell swelling decreased NaK2Cl cotransport in human erythrocytes. Okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, increased NaK2Cl cotransport by nearly 80%, suggesting an important role for these phosphatases in the regulation of NaK2Cl cotransport. Urea inhibited bumetanide-sensitive K influx even when protein phosphatases were inhibited with OA, suggesting that urea acted by inhibiting a kinase. In cells subjected to shrinking and OA pretreatment, maneuvers expected to increase the net phosphorylation, urea inhibited cotransport only minimally, suggesting that urea acted by causing a net dephosphorylation of the cotransport protein, or some key regulatory protein. The finding that concentrations of urea found in uremic subjects inhibited NaK2Cl cotransport, a widespread transport pathway with important physiological functions, suggests that urea is not only a marker for accumulation of other uremic toxins, but may be a significant uremic toxin itself.

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

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

HCl-induced cell edema in rabbit esophageal epithelium: a bumetanide-sensitive process

BACKGROUND & AIMS

The morphology of acid damage to esophageal epithelium is characterized by marked cell (swelling) edema. This observation suggests that, in the process of acid damage, an increase in osmotic forces develops within the cell that accounts for the increase in cell water. The aim of this study was to document that esophageal cells swell at acidic pH and to explore the nature of the osmolytes and mechanisms responsible for it.

METHODS

Cell edema was assessed in sections of rabbit esophageal epithelium by correlating morphological change with change in tissue wet weight after immersion in acidic solutions for up to 4 hours.

RESULTS

At pH < or = 2 for 2 hours, tissues gained weight and showed cell edema on both light and electron microscopy. In addition to being time- and pH-dependent, cell edema was dependent on bathing solution osmolytes, specifically Na+, K+, and Cl-, and could be inhibited by tissue pretreatment with bumetanide (or ethacrynic acid).

CONCLUSIONS

HCl exposure can spontaneously produce cell edema in esophageal epithelium. The phenomenon is pH- and time-dependent and requires acid stimulation of osmolyte absorption through a bumetanide-sensitive process compatible with an NaK2Cl cotransporter in the epithelial cell membrane.

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

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

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.