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

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

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.

Experience-dependent changes in intracellular Cl− regulation in developing auditory neurons

A developmental change in GABA and glycine responses, from a depolarization to a hyperpolarization, have been reported for a range of CNS neurons, and has been demonstrated to be due to a developmental decrease in the intracellular Cl- concentration ([Cl-](i)). We examined [Cl-](i) in isolated rat lateral superior olive (LSO) neurons using patch-clamp recordings of glycine gated Cl- currents and by measuring intracellular Cl- -fluorescence. In neurons from 14-16-day-old rats (P14-P16), which had previously received unilateral or bilateral cochlear ablations before the onset of hearing, there was no developmental decrease in [Cl-](i). No significant differences in [Cl-](i) were observed amongst rats with either ipsi- and contralateral ablations. Implanted strychnine pellets also prevented the decrease in [Cl-](i) in most neurons. In some of these neurons in which [Cl-](i) remained high, there was a lack of expression of the K+-Cl- cotransporter 2 (KCC2) mRNA. These results demonstrate that the developmental decrease in [Cl-](i) in LSO neurons is dependent on neuronal activity and that both GABAergic/glycinergic and glutamatergic afferent activity contribute to this maturation of the Cl- regulatory mechanisms.

Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family

The electroneutral cation-chloride-coupled cotransporter gene family ( SLC12) was identified initially at the molecular level in fish and then in mammals. This nine-member gene family encompasses two major branches, one including two bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporters and the thiazide-sensitive Na(+):Cl(-) cotransporter. Two of the genes in this branch ( SLC12A1 and SLC12A3), exhibit kidney-specific expression and function in renal salt reabsorption, whereas the third gene ( SLC12A2) is expressed ubiquitously and plays a key role in epithelial salt secretion and cell volume regulation. The functional characterization of both alternatively-spliced mammalian Na(+)-K(+)-2Cl(-) cotransporter isoforms and orthologs from distantly related species has generated important structure-function data. The second branch includes four genes ( SLC12A4- 7) encoding electroneutral K(+)-Cl(-) cotransporters. The relative expression level of the neuron-specific SLC12A5 and the Na(+)-K(+)-2Cl(-) cotransporter SLC12A2 appears to determine whether neurons respond to GABA with a depolarizing, excitatory response or with a hyperpolarizing, inhibitory response. The four K(+)-Cl(-) cotransporter genes are co-expressed to varying degrees in most tissues, with further roles in cell volume regulation, transepithelial salt transport, hearing, and function of the peripheral nervous system. The transported substrates of the remaining two SLC12 family members, SLC12A8 and SLC12A9, are as yet unknown. Inactivating mutations in three members of the SLC12 gene family result in Mendelian disease; Bartter syndrome type I in the case of SLC12A1, Gitelman syndrome for SLC12A3, and peripheral neuropathy in the case of SLC12A6. In addition, knockout mice for many members of this family have generated important new information regarding their respective physiological roles.

Loss of K-Cl co-transporter KCC3 causes deafness, neurodegeneration and reduced seizure threshold

K-Cl co-transporters are encoded by four homologous genes and may have roles in transepithelial transport and in the regulation of cell volume and cytoplasmic chloride. KCC3, an isoform mutated in the human Anderman syndrome, is expressed in brain, epithelia and other tissues. To investigate the physiological functions of KCC3, we disrupted its gene in mice. This severely impaired cell volume regulation as assessed in renal tubules and neurons, and moderately raised intraneuronal Cl(-) concentration. Kcc3(-/-) mice showed severe motor abnormalities correlating with a progressive neurodegeneration in the peripheral and CNS. Although no spontaneous seizures were observed, Kcc3(-/-) mice displayed reduced seizure threshold and spike-wave complexes on electrocorticograms. These resembled EEG abnormalities in patients with Anderman syndrome. Kcc3(-/-) mice also displayed arterial hypertension and a slowly progressive deafness. KCC3 was expressed in many, but not all cells of the inner ear K(+) recycling pathway. These cells slowly degenerated, as did sensory hair cells. The present mouse model has revealed important cellular and systemic functions of KCC3 and is highly relevant for Anderman syndrome.

Sickle cell disease: from membrane pathophysiology to novel therapies for prevention of erythrocyte dehydration

Sickle cell anemia is characterized by the presence of dense dehydrated erythrocytes that have lost most of their K content. Due to the unique dependence of Hb S polymerization on intracellular Hb S concentration, preventing this dehydration should markedly reduce polymerization. The erythrocyte intermediate conductance Ca-activated K channel (hSK4 or KCNN4), first described by Gardos, has been shown to be a major pathway for sickle cell dehydration. Studies with the imidazole antimycotic clotrimazole have shown reduction of sickle cell dehydration in vivo in a small number of patients with sickle cell disease; dose-limiting gastrointestinal and liver toxicities were observed. Based on the chemical structure of clotrimazole metabolites, a novel Gardos channel inhibitor, ICA-17043, has been developed. It has shown substantial activity both in vitro and in vivo in transgenic sickle mice. ICA-17043 is currently in phase 2 human trials. Another potential therapeutic target is the K-Cl cotransport. When sickle erythrocytes are exposed to relatively acidic conditions, they undergo cell shrinkage via activation of this pathway. K-Cl cotransport can be blocked by increasing the abnormally low erythrocyte Mg content of sickle erythrocytes. Oral Mg supplementation has been shown to reduce sickle cell dehydration in vivo in transgenic sickle mice and in patients in two separate clinical trials. Oral Mg pidolate is being tested in clinical trials in homozygous sickle cell disease and in Hb S/HbC (SC) disease, either as a single agent or in combination with hydroxyurea. The ongoing trials will determine the clinical effectiveness of therapies aimed at preventing sickle erythrocyte dehydration.

Hereditary disorders of potassium homeostasis

There has been a dramatic recent increase in the understanding of the renal epithelial transport systems with the identification, cloning and characterization of a large number of membrane transport proteins. The aim of this chapter is to integrate this body of knowledge with the understanding of the clinical disorders that accompany gain, loss or dysregulation of function of these transport systems. The specific focus is on the best-defined human clinical syndromes in which there are derangements in potassium (K(+)) homeostasis. The focus is on inherited syndromes, rather than on acquired syndromes due to tubular transport defects, and the therapeutic approaches address chronic derangements of K(+) homeostasis rather than acute interventions directed at life-threatening hyperkalaemia.

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.

Changes in chloride homeostasis-regulating gene expressions in the rat hippocampus following amygdala kindling

In a rat kindling model, we examined expression patterns of NKCC1, KCC1, KCC2, and CLC-2. In the dentate granule cell layer, there was an activity-dependent increase in NKCC1 mRNA but significant decreases in KCC1 and CLC-2 mRNAs. In addition, CLC-2 mRNA expression was markedly decreased in CA1 pyramidal layer. These results suggest that an increase in [Cl-]i and a resultant reduction in GABAergic inhibition may occur in hippocampus of epileptic rats.

KCl cotransport is an important modulator of human cervical cancer growth and invasion

Cervical cancer is a major world health problem for women, but the pathophysiology of this disease has received scant attention. Here we show that the growth and invasion of cervical cancer cells are strongly linked the expression and activity of the KCl cotransporter (KCC), an important regulator of the ionic and cellular osmotic homeostasis. Functional assays of KCl cotransport activation by osmotic swelling, staurosporine, and N-ethylmaleimide indicate that removal of the N-terminal 117 amino acids from KCC1 produces a dominant-negative loss-of-function phenotype for KCl cotransport in human cervical cancer cells. The capability for regulatory volume decrease is much attenuated in the loss-of-function KCC mutant cervical cancer cells. The loss-of-function KCC mutant cervical cancer cells exhibit inhibited cell growth accompanied by decreased activity of the cell cycle gene products retinoblastoma and cdc2 kinase. Reduced cellular invasiveness is in parallel by reduced expression of alpha v beta 3 and alpha 6 beta 4 integrins, accompanied by decreased activity of matrix metalloproteinase 2 and 9. Inhibition of tumor growth in SCID mice confirms the crucial role of KCC in promoting cervical cancer growth and invasion. Thus, blockade of KCl cotransport may be a useful therapeutic adjunctive strategy to retard or prevent cervical cancer invasion.

Deafness disrupts chloride transporter function and inhibitory synaptic transmission

Loss of sensory function leads to atrophy or death within the developing CNS, yet little is known about the physiology of remaining synapses. After bilateral deafening, gramicidin-perforated-patch recordings were obtained from gerbil inferior colliculus neurons in a brain slice preparation. Afferent-evoked IPSPs had a diminished ability to block current-evoked action potentials in deafened neurons. This change could be attributed, in part, to a loss of potassium-dependent chloride transport function, with little change in K-Cl cotransporter expression. Treatments that suppressed chloride cotransport (bumetanide, cesium, and genistein) had little or no effect on neurons from deafened animals. These same treatments depolarized the E(IPSC) of control neurons. Semiquantitative RT-PCR and immunohistochemical staining indicated no change in the expression of chloride cotransporter mRNA or protein after deafness. Therefore, profound hearing loss leads rapidly to the disruption of chloride homeostasis, which is likely attributable to the dysfunction of the potassium-dependent chloride cotransport mechanism, rather than a downregulation of its expression. This results in inhibitory synapses that are less able to block excitatory events.

The structural unit of the thiazide-sensitive NaCl cotransporter is a homodimer

The thiazide-sensitive NaCl cotransporter (NCC) is responsible for the reabsorption of 5% of the filtered load of NaCl in the kidney. Mutations in NCC cause Gitelman syndrome. To gain insight into its regulation, detailed information on the structural composition of its functional unit is essential. Western blot analysis of total membranes of Xenopus laevis oocytes heterologously expressing FLAG-tagged NCC revealed the presence of both complex-(140-kDa) and core (100-kDa)-glycosylated monomers and a broad band of high molecular mass (250-350-kDa) complexes. Chemical cross-linking with dithiobispropionimidate eliminated the low molecular weight bands and increased the intensity of the high molecular weight bands, indicating that NCC is present in multimeric complexes. Co-expression of HA- and FLAG-tagged NCC followed by co-immunoprecipitation demonstrated that these multimers contained at least two complex-glycosylated NCC proteins. The dimeric nature of the multimers was further substantiated by sucrose gradient centrifugation yielding a peak of approximately 310 kDa. A concatameric construct of two NCC polyproteins exhibited significant 22Na+ uptake, indicating that the transporter is functional as a homodimer. A concatamer of partially retarded G980R- and wild type (wt)-NCC displayed normal Na+ transport. This demonstrates that G980R-NCC, provided that it reaches the surface, is fully active and that wt-NCC is dominant in its association with this mutant. Conversely a concatamer of fully retarded G741R- and wt-NCC did not reach the cell surface, showing that wt-NCC is recessive in its association with this mutant. Oocytes co-expressing G741R- and wt-NCC did not show G741R staining at the plasma membrane, whereas Na+ transport was normal, indicating that wt-NCC dimerizes preferentially with itself. The results are discussed in relation to the recessive nature of NCC mutants in Gitelman syndrome.

Cloning and localization of KCC4 in rabbit kidney: expression in distal convoluted tubule

Cl-dependent K secretion is a feature of renal distal tubules and collecting ducts. Recent cloning and identification of K-Cl cotransporter proteins led us to search for additional novel KCC isoforms expressed in the renal distal nephron. A human expressed sequence tag (EST) with high homology to KCC1 was identified. The rabbit isoform was cloned by homology using degenerate primers and rapid amplification of cDNA ends (RACE). Our isoform is the rabbit homologue of mouse and human KCC4 published previously. The 4.35-kb rabbit KCC4 cDNA encodes a protein of 1,106 amino acids. Antibodies were generated to both NH2-terminal and COOH-terminal fusion proteins. Northern and Western blot analyses showed widespread mRNA and protein expression in many rabbit organs, in renal cortex, outer medulla, and inner medulla but not in skeletal muscle. Immunohistochemical localization of KCC4 showed expression exclusively along the basolateral membrane in many nephron segments. The distal convoluted tubule and connecting tubule exhibited the highest level of KCC4 immunoreactivity, followed by the medullary thick ascending limb. A low level of immunoreactivity was detected in the proximal tubule and collecting ducts. We postulate that KCC4 mediates potassium and chloride exit from the cell and may play an important role in salt absorption by the distal convoluted tubule.

Ammonium transport and pH regulation by K(+)-Cl(-) cotransporters

The Na(+)-K(+)-Cl(-) cotransporters (NKCCs), which belong to the cation-Cl(-) cotransporter (CCC) family, are able to translocate NH4(+) across cell membranes. In this study, we have used the oocyte expression system to determine whether the K(+)-Cl(-) cotransporters (KCCs) can also transport NH4(+) and whether they play a role in pH regulation. Our results demonstrate that all of the CCCs examined (NKCC1, NKCC2, KCC1, KCC3, and KCC4) can promote NH4(+) translocation, presumably through binding of the ion at the K(+) site. Moreover, kinetic studies for both NKCCs and KCCs suggest that NH4(+) is an excellent surrogate of Rb(+) or K(+) and that NH4(+) transport and cellular acidification resulting from CCC activity are relevant physiologically. In this study, we have also found that CCCs are strongly and differentially affected by changes in intracellular pH (independently of intracellular [NH4(+)]). Indeed, NKCC2, KCC1, KCC2, and KCC3 are inhibited at intracellular pH <7.5, whereas KCC4 is activated. These results indicate that certain CCC isoforms may be specialized to operate in acidic environments. CCC-mediated NH4(+) transport could bear great physiological implication given the ubiquitous distribution of these carriers.

Aberrant chloride transport contributes to anoxic/ischemic white matter injury

Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl-. We used the in vitro rat optic nerve to study the role of aberrant Cl- transport in anoxia/ischemia. After 30 min of anoxia (NaN3, 2 mm), axonal membrane potential (V(m)) decreased to 42 +/- 11% of control and to 73 +/- 11% in the presence of tetrodotoxin (TTX) (1 microm). TTX + 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid disodium salt (500 microm), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 +/- 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 microm) together with TTX was also more effective than TTX alone (84 +/- 14%). The compound action potential (CAP) area recovered to 26 +/- 6% of control after 1 hr anoxia. KCC blockade (10 microm furosemide) improved outcome (40 +/- 4%), and TTX (100 nm) was even more effective (74 +/- 12%). In contrast, the Cl- channel blocker niflumic acid (50 microm) worsened injury (6 +/- 1%). Coapplication of TTX (100 nm) + furosemide (10 microm) was more effective than either agent alone (91 +/- 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl- gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl- fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.

Differential expression pattern of chloride transporters NCC, NKCC2, KCC1, KCC3, KCC4, and AE3 in the developing rat auditory brainstem

During development of inhibitory synapses, the action of the two neurotransmitters GABA and glycine shifts from depolarizing to hyperpolarizing. The shift is due to an age-dependent regulation of the intracellular free chloride concentration ([Cl(-)](i)) in postsynaptic neurons. A model system to study this maturation process is a glycinergic projection in the mammalian auditory brainstem. It is formed in the superior olivary complex (SOC) by neurons of the medial nucleus of the trapezoid body, whose axons terminate in the lateral superior olive (LSO). LSO neurons of perinatal rats and mice are depolarized upon glycine application, whereas older cells (>postnatal day (P) 8) are hyperpolarized. Here we examined the expression of six secondary active chloride transporter genes ( NCC, NKCC2, KCC1, KCC3, KCC4, and AE3) in the rat SOC to unravel the molecular mechanisms underlying this change. RT-PCR analysis demonstrated brainstem expression of KCC1, KCC3, KCC4, and AE3, but not of NCC and NKCC2. RNA in situ hybridization showed that only AE3 is highly expressed both at P3 (high [Cl(-)](i)) and P12 (low [Cl(-)](i)) in LSO neurons. KCC1 and KCC4 are weakly expressed in LSO neurons at P3 and P12, respectively. This study completes the expression analysis of all known chloride transporters sensitive to loop diuretic drugs in the SOC and demonstrates differences in the maturation between hippocampal and brainstem inhibitory synapses.

Cation-chloride co-transporters in neuronal communication, development and trauma

Electrical signaling in neurons is based on the operation of plasmalemmal ion pumps and carriers that establish transmembrane ion gradients, and on the operation of ion channels that generate current and voltage responses by dissipating these gradients. Although both voltage- and ligand-gated channels are being extensively studied, the central role of ion pumps and carriers is largely ignored in current neuroscience. Such an information gap is particularly evident with regard to neuronal Cl- regulation, despite its immense importance in the generation of inhibitory synaptic responses by GABA- and glycine-gated anion channels. The cation-chloride co-transporters (CCCs) have been identified as important regulators of neuronal Cl- concentration, and recent work indicates that CCCs play a key role in shaping GABA- and glycine-mediated signaling, influencing not only fast cell-to-cell communication but also various aspects of neuronal development, plasticity and trauma.

Effect of 1-chloro-2,4-dinitrobenzene on K+ transport in normal and sickle human red blood cells

1-Chloro-2,4-dinitrobenzene (CDNB), which causes oxidative stress through depletion of reduced glutathione (GSH), increases the passive K+ permeability of red cells. In this paper, we investigated the effects of CDNB (1 mM) on the activities of the K+-Cl- cotransporter (KCC; measured as Cl--dependent K+ influx) and the Gardos channel (taken as clotrimazole-sensitive K+ influx, 5 microM) in human red cells, using 86Rb+ as a K+ congener. 45Ca2+ was used to study passive Ca2+ entry and active Ca2+ efflux via the plasma membrane Ca2+ pump. Both the Gardos channel and KCC were stimulated in both normal and sickle red cells. In sickle cells, stimulation of KCC was similar in oxygenated and deoxygenated cells; that of the Gardos channel was greater in deoxygenated cells. In normal red cells, stimulation of both pathways was greater in oxygenated cells (by 4 +/- 1-fold; all means +/- S.E.M., n = 3). The effects on the Gardos channel were dependent on extracellular Ca2+ and were associated with inhibition of the plasma membrane Ca2+ pump (by 29 +/- 3 %, P < 0.01) and increased Ca2+ sensitivity of the channel (EC50 for [Ca2+]i reduced from 260 +/- 26 to 175 +/- 15 nM; P < 0.05). Cell volume, pHi, ATP levels and passive Ca2+ entry were not affected by CDNB. The effects on KCC were inhibited (93 +/- 6 %) by prior treatment with the protein phosphatase inhibitor calyculin A (100 nM) and were not additive with stimulation by N-ethylmaleimide (1 mM), regardless of the order of addition. These findings are therefore consistent with inhibition of a regulatory protein kinase, although stimulation of the conjugate protein phosphatase(s) may also occur. KCC stimulation was also Ca2+ dependent. These findings are important for understanding how GSH depletion alters membrane permeability and how to protect against red cell dehydration.

N-Glycosylation at two sites critically alters thiazide binding and activity of the rat thiazide-sensitive Na(+):Cl(-) cotransporter

The rat thiazide-sensitive Na-Cl cotransporter (rNCC) is expressed in the renal distal convoluted tubule and is the site of action of an important class of antihypertensive agents, the thiazide diuretics. The amino acid sequence contains two potential N-linked glycosylation consensus sites, N404 and N424. Either enzymatic deglycosylation or tunicamycin reduced the cotransporter to its core molecular weight (113 kD). Glycosylation site single mutants expressed in oocytes ran as thick bands at 115 kD, consistent with the high-mannose glycoprotein. The double mutant produced the single thin 113-kD band seen in the deglycosylated cotransporter. Functional expression of cotransporters in Xenopus laevis oocytes revealed that the mutants displayed drastically decreased thiazide-sensitive (22)Na(+) uptake compared with wild-type NCC. Analysis of enhanced green fluorescence protein (EGFP)-tagged cotransporters demonstrated that this decrease in function is predominantly secondary to decreased surface expression. The elimination of glycosylation in the double mutant increased thiazide sensitivity by more than two orders of magnitude and also increased Cl(-) affinity. Thus, we have demonstrated that rNCC is N-glycosylated in vivo at two sites, that glycosylation is essential for efficient function and surface expression of the cotransporter, and that the elimination of glycosylation allows much greater access of thiazide diuretics to their binding site.

Hemoglobinopathies

The outlook for patients with sickle cell disease has improved steadily during the last two decades. In spite of these improvements, curative therapies are currently available only to a small minority of patients. The main theme of this chapter is to describe new therapeutic options that are at different stages of development that might result in further improvements in the outlook for patients with these disorders.

Dr. Joseph DeSimone and his colleagues had previously made the important observation that the hypomethylating agent 5-azacytidine can reverse the switch from adult to fetal hemoglobin in adult baboons. Although similar activity was demonstrated in patients with sickle cell disease and β-thalassemia, concern about the toxicity of 5-azacytidine prevented its widespread use in these disorders. In Section I, Dr. DeSimone discusses the role of DNA methylation in globin gene regulation and describe recent clinical experience with decitabine (an analogue of 5-azacytidine) in patients with sickle cell disease. These encouraging studies demonstrate significant fetal hemoglobin inducing activity of decitabine in patients who fail to respond to hydroxyurea.

In Section II, Dr. George Atweh continues the same theme by describing recent progress in the study of butyrate, another inducer of fetal hemoglobin, in patients with sickle cell disease and β-thalassemia. The main focus of his section is on the use of a combination of butyrate and hydroxyurea to achieve higher levels of fetal hemoglobin that might be necessary for complete amelioration of the clinical manifestations of these disorders. Dr. Atweh also describes novel laboratory studies that shed new light on the mechanisms of fetal hemoglobin induction by butyrate.

In Section III, Dr. Ronald Nagel discusses the different available transgenic sickle mice as experimental models for human sickle cell disease. These experimental models have already had a significant impact on our understanding of the pathophysiology of sickle cell disease. Dr. Nagel describes more recent studies in which transgenic sickle mice provide the first proof of principle that globin gene transfer into hematopoietic stem cells inhibits in vivo sickling and ameliorates the severity of the disease.

Although stroke in adult patients with sickle cell disease is not as common as in children, adult hematologists, like their pediatric colleagues, need to make management decisions in adult patients with a stroke or a history of stroke. Dr. Robert Adams has led several large clinical studies that investigated the role of transfusions in the prevention of stroke in children with sickle cell disease. Much less is known, however, about the prevention of first or subsequent strokes in adult patients with sickle cell disease. In Section IV, Dr. Adams provides some general guidelines for the management of adult patients with stroke while carefully distinguishing between recommendations that are evidence-based and those that are anecdotal in nature.

Molecular mechanisms and regulation of K+ transport in higher plants

Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light.

Cation chloride cotransporters interact with the stress-related kinases Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress response 1 (OSR1)

Cells respond to stress stimuli by mounting specific responses. During osmotic and oxidative stress, cation chloride cotransporters, e.g. Na-K-2Cl and K-Cl cotransporters, are activated to maintain fluid/ion homeostasis. Here we report the interaction of the stress-related serine-threonine kinases Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress response 1 (OSR1) with the cotransporters KCC3, NKCC1, and NKCC2 but not KCC1 and KCC4. The interaction was identified using yeast two-hybrid assays and confirmed via glutathione S-transferase pull-down experiments. Evidence for in vivo interaction was established by co-immunoprecipitation of SPAK from mouse brain with anti-NKCC1 antibody. The interacting region of both kinases comprises the last 100 amino acids of the protein. The SPAK/OSR1 binding motif on the cotransporters consists of nine residues, starting with an (R/K)FX(V/I) sequence followed by five additional residues that are essential for binding but for which no consensus was found. Immunohistochemical analysis of choroid plexus epithelium revealed co-expression of NKCC1 and SPAK on the apical membrane. In contrast, in choroid plexus epithelium from NKCC1 null mice, SPAK immunostaining was found in the cytoplasm. We conclude that several cation chloride co-transporters interact with SPAK and/or OSR1, and we hypothesize that this interaction might play a role during the initiation of the cellular stress response.

Allopregnanolone enhancement of GABAergic transmission in rat medial preoptic area neurons

Gamma-aminobutyric acid (GABA)-mediated transmission in the medial preoptic area (MPOA) of the hypothalamus plays an important role in functions such as sex steroid hormone dynamics and control of body temperature. The action of allopregnanolone, the primary metabolite of progesterone, on GABAergic transmission was investigated by employing patch clamp whole cell recording on acutely dissociated rat MPOA neurons with the functional connection of presynaptic terminals. Allopregnanolone enhanced spontaneous GABA release on the MPOA neurons and induced prolonged decay of miniature GABAergic-inhibitory postsynaptic currents (mIPSCs). The facilitation of GABA release from the presynaptic terminals by allopregnanolone disappeared in Ca2+-free extracellular solution. The presynaptic action of this neurosteroid was also blocked by bumetanide, a blocker of cation-Cl- cotransporters, and by removal of extracellular Na+. The results suggest that allopregnanolone enhances GABAergic transmission at the MPOA neurons by pre- and postsynaptic mechanisms. The enhancement of GABA release by allopregnanolone might require a high Cl- concentration in the presynaptic terminal maintained by Na+-dependent, bumetanide-sensitive mechanisms (e.g., Na+-K+-Cl- cotransporter) and might be mediated by Ca2+ influx into presynaptic terminal.

Patterns of cation-chloride cotransporter expression during embryonic rodent CNS development

Intracellular Cl- plays a key role in cellular volume regulation, cell cycle control and shaping the polarity of inhibitory postsynaptic responses mediated by anion-permeable GABA and glycine receptors. In this study, we have investigated the expression patterns of members of the cation-chloride cotransporters (CCCs), including the K-Cl cotransporters KCC1-4 and the Na-K-2 Cl cotranporter NKCC1 during rodent embryonic brain development. At the time of neurogenesis (embryonic days; E12.5-14.5), KCC1 was only detectable in the developing choroid plexus. KCC2 mRNA was detectable as early as E12.5 in the ventral part of the (cervical) spinal cord, and by E14.5, the expression had spread to TUJ1-positive differentiating regions of the rhombencephalon, diencephalon and olfactory bulb, in parallel with neuronal maturation. KCC3 mRNA was scarce in the cortical plate at E14.5, and slightly up-regulated at birth. In contrast, KCC4 mRNA was abundantly expressed in the ventricular zone and was down-regulated perinatally. At E14.5, NKCC1 was highly expressed in the vimentin-positive radial glia of the proliferative zone of the subcortical region. At later embryonic stages, during gliogenesis (E17-P0), there was a shift in NKCC1 expression to the neuron specific Class III beta-tubulin (betaIII) positive region of the cortical plate. These unique spatiotemporal expression patterns of distinct CCCs during embryonic development suggests that Cl- regulatory mechanisms are critically involved in the control of neuronal development.

Developmental changes in KCC1, KCC2, and NKCC1 mRNA expressions in the rat brain

We investigated the expressions of KCC1, KCC2 and NKCC1 mRNAs in the developing rat brain. The neuroepithelium showed abundant KCC1 and NKCC1 mRNA expressions, while KCC2 mRNA was not detected there. In contrast, KCC2 mRNA was preferentially expressed in postmitotic mature neurons. These results suggest that the appearance of KCC2 expression mainly depends on the maturation of individual neurons.

Functional and molecular characterization of the shark renal Na-K-Cl cotransporter: novel aspects

The Na-K-Cl cotransporter isoform 1 (NKCC1) has been isolated from several species, including Squalus acanthias. A second kidney-specific isoform (NKCC2) has been cloned mainly from higher vertebrates. Here, we have isolated the S. acanthias NKCC2 and found that it is produced in at least four spliced variants (saNKCC2A, saNKCC2F, saNKCC2AF, and saNKCC2AFno8) of approximately 1,090 residues. Expression of these transcripts in Xenopus laevis oocytes revealed that only the A and F variants are functional and that they are more active after incubation in low-Cl or hyperosmolar media. Rates of activation after exposure to these media were exceptionally rapid, demonstrating for the first time that the NKCC2 itself represents an important site of regulation by Cl and that extracellular domains are involved. Another remarkable finding in this study was the failure to identify NKCC2B, a variant found in the kidney of higher vertebrates and expressed specifically in macula densa cells. This result, in conjunction with the fact that the shark kidney lacks a well-developed juxtaglomerular apparatus, suggests that the B exon evolved as a result of selective pressure (presumably by exon duplication) and that a restricted relationship exists between NKCC2B and macula densa.

A regulatory locus of phosphorylation in the N terminus of the Na-K-Cl cotransporter, NKCC1

The secretory Na-K-Cl cotransporter NKCC1 is activated by secretagogues through a phosphorylation-dependent mechanism. We found a phosphorylation stoichiometry of 3.0 +/- 0.4 phosphorylated residues/NKCC1 protein harvested from shark rectal gland tubules maximally stimulated with forskolin and calyculin A, showing that at least three sites on the cotransporter are phosphorylated upon stimulation. Three phosphoacceptor sites were identified in the N-terminal domain of the protein (at Thr(184), Thr(189), and Thr(202)) using high pressure liquid chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry to analyze tryptic fragments of the radiolabeled cotransporter. None of these residues occurs in the context of strong consensus sites for known Ser/Thr kinases. The threonines and the surrounding amino acids are highly conserved between NKCC1 and NKCC2, and similarities are also present in the Na-Cl cotransporter NCC (or TSC). This strongly suggests that the phosphoregulatory mechanism is conserved among isoforms. Through expression of shark NKCC1 mutants in HEK-293 cells, Thr(189) was found to be necessary for activation of the protein, whereas phosphorylation at Thr(184) and Thr(202) was modulatory, but not required. In conjunction with the recent finding (Darmen, R. B., Flemmer, A., and Forbush, B. (2001) J. Biol. Chem. 276, 34359-34362) that protein phosphatase-1 binds to residues 107-112 in the shark NKCC1 sequence, these results demonstrate that the N terminus of NKCC1 constitutes a phosphoregulatory domain of the transporter.

Amygdala kindling induces upregulation of mRNA for NKCC1, a Na(+), K(+)-2Cl(-) cotransporter, in the rat piriform cortex

GABA, the main inhibitory neurotransmitter in the brain, elicits a hyperpolarizing response by activation of the GABA(A)-receptor/chloride-channel complex under conditions of normal Cl(-) homeostasis. Thus the pathogenesis of epilepsy could involve an impairment of GABA(A)-receptor-mediated inhibition due to a collapse of the Cl(-) gradient. We examined the expression patterns of Cl(-) transporters and a Cl(-) channel in a rat amygdala-kindling model. Activity-dependent increases were observed in the mRNA for NKCC1, an inwardly-directed Cl(-) transporter, in the piriform cortex. This suggests that an increase in [Cl(-)](i) and a resultant reduction in GABAergic inhibition may occur in the kindled piriform cortex.

An alternate pathway of cAMP-stimulated Cl secretion across the NKCC1-null murine duodenum

BACKGROUND & AIMS

Adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated anion secretion across the duodenal epithelium requires the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane and anion uptake proteins in the basolateral membrane. NKCC1, the epithelial Na(+)/K(+)/2Cl(-) cotransporter, is the major protein responsible for Cl(-) uptake. In this study, we evaluate the role of NKCC1 in determining the relative rates of transepithelial Cl(-) and HCO(3)(-) secretion during cAMP stimulation of the duodenum.

METHODS

Bicarbonate and chloride secretion across duodenal mucosa was measured in Ussing chambers by pH stat and (36)Cl flux methods using mice with either gene-targeted deletion of NKCC1 (NKCC1-/-) or bumetanide blockade of NKCC1.

RESULTS

Total anion secretion stimulated by forskolin treatment of NKCC1-null duodenum resulted from approximately equivalent rates of electrogenic chloride, electrogenic bicarbonate, and electroneutral bicarbonate secretion. Evaluation of the alternate chloride secretory pathway indicated chloride uptake by a basolateral membrane anion exchange process with characteristics consistent with the anion exchanger isoform AE2.

CONCLUSIONS

Chloride uptake by basolateral anion exchanger activity (AE2) supports intracellular cAMP-stimulated chloride secretion in the NKCC1-null duodenum. A model for the alternate chloride secretion pathway is proposed whereby chloride uptake via AE2 is coupled to basolateral NaHCO(3) cotransport to support CFTR-mediated chloride and bicarbonate secretion.

Developmental changes in KCC1, KCC2 and NKCC1 mRNAs in the rat cerebellum

Cation chloride cotransporters are considered to play pivotal roles in controlling the intracellular and extracellular ionic environments of neurons, hence controlling neuronal function. To establish how these cotransporters are involved in cerebellum development, we investigated the expression of KCC1, KCC2 and NKCC1 mRNAs in the developing rat cerebellum using in situ hybridization histochemistry. In the external germinal layer, where premature cells exist, we found substantial KCC1 and NKCC1 mRNA expression on P7 and P14, while KCC2 mRNA was not detected. In contrast, KCC2 mRNA was already expressed in Purkinje cells on P1. We also observed KCC2 mRNA expression in postmigratory granule cells after P7. The expression of KCC1, KCC2, and NKCC1 mRNAs reached adult patterns by P21. In the adult cerebellum, KCC2 mRNA was expressed in most neurons, including Purkinje cells, granule cells, and stella/basket cells, while KCC1 and NKCC1 mRNAs were only detected in granule cells and glial cells. These findings suggest that in the rat cerebellum KCC2 mRNA expression is induced when neurons arrive their final destinations.

Molecular, functional, and genomic characterization of human KCC2, the neuronal K-Cl cotransporter

The expression level of the neuronal-specific K-Cl cotransporter KCC2 (SLC12A5) is a major determinant of whether neurons will respond to GABA with a depolarizing, excitatory response or a hyperpolarizing, inhibitory response. In view of the potential role in human neuronal excitability we have characterized the hKCC2 cDNA and gene. The 5.9 kb hKCC2 transcript is specific to brain, and is induced during in vitro differentiation of NT2 teratocarcinoma cells into neuronal NT2-N cells. The 24-exon SLC12A5 gene is on human chromosome 20q13, and contains a polymorphic dinucleotide repeat within intron 1 near a potential binding site for neuron-restrictive silencing factor. Expression of hKCC2 cRNA in Xenopus laevis oocytes results in significant Cl(-)-dependent (86)Rb(+) uptake under isotonic conditions; cell swelling under hypotonic conditions causes a 20-fold activation, which is blocked by the protein phosphatase inhibitor calyculin-A. In contrast, oocytes expressing mouse KCC4 do not mediate isotonic K-Cl cotransport but express much higher absolute transport activity than KCC2 oocytes under hypotonic conditions. Initial and steady state kinetics of hKCC2-injected oocytes were performed in both isotonic and hypotonic conditions, revealing K(m)s for K(+) and Cl(-) of 9.3+/-1.8 mM and 6.8+/-0.9 mM, respectively; both affinities are significantly higher than KCC1 and KCC4. The K(m) for Cl(-) is close to the intracellular Cl(-) activity of mature neurons, as befits a neuronal efflux mechanism.

Chloride increases adrenergic receptor-mediated platelet and vascular responses

BACKGROUND

We postulated that increasing intracellular chloride concentration ([Cl-]i) in human platelets would potentiate alpha2 adrenergic receptor (A2AR)-mediated platelet aggregation, and that vascular reactivity would also be increased by raising [Cl-]i in blood vessels. We further hypothesized that ligands binding to the A2AR would increase [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Because diuretics are potent inhibitors of carbonic anhydrase, we speculated that these agents inhibit platelet aggregation and vascular contractility through inhibition of chloride influx by decreasing carbonic anhydrase activity, and subsequently, chloride/bicarbonate exchange. The aim of this study was to test these hypotheses.

METHODS

Platelet aggregation was measured by determining changes in optical density of platelet-rich plasma. Contractile responses to A2AR agonists were recorded in isolated vascular smooth muscle. The substances [Cl-]i and intracellular pH (pHi) were measured using microfluorometric methods. Carbonic anhydrase activity and chloride/bicarbonate exchange were determined by an in vitro assay based on the Stewart cycle.

RESULTS

Increasing [Cl-]i potentiated platelet aggregation and vascular contractility, and epinephrine raised [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Furthermore, diuretic-dependent inhibition of carbonic anhydrase activity decreased chloride/bicarbonate exchange.

CONCLUSIONS

Our data support the concept that diuretics inhibit carbonic anhydrase activity and chloride/bicarbonate exchange in platelets and vascular smooth muscle. The ensuing reduction in [Cl-]i that is induced by diuretics in these tissues could play a role in reducing the effect of catecholamines on precipitating thrombotic stroke or myocardial infarction.

Renal genetic disorders related to K+ and Mg2+

The recent knowledge of the renal epithelial transport systems has exploded with the identification, cloning, and characterization of a large number of membrane transport proteins. The fundamental aspects of these transporters are beginning to emerge at the molecular level and are summarized in the accompanying contributions in this volume of the Annual Review of Physiology. The aim of my review is to integrate this body of knowledge with the understanding of the clinical disorders of human mineral homeostasis that accompany gain, loss, or dysregulation of function of these transport systems. The specific focus is on the best defined human clinical syndromes in which there are derangements in K(+) and Mg(2+) homeostasis.

Functional differences between flounder and rat thiazide-sensitive Na-Cl cotransporter

The purpose of the present study was to determine the major functional, pharmacological, and regulatory properties of the flounder thiazide-sensitive Na-Cl cotransporter (flTSC) to make a direct comparison with our recent characterization of the rat TSC (rTSC; Monroy A, Plata C, Hebert SC, and Gamba G. Am J Physiol Renal Physiol 279: F161-F169, 2000). When expressed in Xenopus laevis oocytes, flTSC exhibits lower affinity for Na(+) than for Cl(-), with apparent Michaelis-Menten constant (K(m)) values of 58.2 +/- 7.1 and 22.1 +/- 4.2 mM, respectively. These K(m) values are significantly higher than those observed in rTSC. The Na(+) and Cl(-) affinities decreased when the concentration of the counterion was lowered, suggesting that the binding of one ion increases the affinity of the transporter for the other. The effect of several thiazides on flTSC function was biphasic. Low concentrations of thiazides (10(-9) to 10(-7) M) resulted in activation of the cotransporter, whereas higher concentrations (10(-6) to 10(-4) M) were inhibitory. In rTSC, this biphasic effect was observed only with chlorthalidone. The affinity for thiazides in flTSC was lower than in rTSC, but the affinity in flTSC was not affected by the Na(+) or the Cl(-) concentration in the uptake medium. In addition to thiazides, flTSC and rTSC were inhibited by Hg(2+), with an apparent higher affinity for rTSC. Finally, flTSC function was decreased by activation of protein kinase C with phorbol esters and by hypertonicity. In summary, we have found significant regulatory, kinetic, and pharmacological differences between flTSC and rTSC orthologues.

Interaction with grp58 increases activity of the thiazide-sensitive Na-Cl cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) is expressed by distal convoluted tubule cells of the mammalian kidney. We used yeast two-hybrid screening to identify that glucose-regulated protein 58 (grp58), a protein induced by glucose deprivation, binds to the COOH terminus of the NCC. Immunoprecipitation of rat kidney cortex homogenates using a guinea pig anti-NCC antibody confirmed that grp58 associates with the NCC in vivo. Northern blots indicated that grp58 is highly expressed in rat kidney cortex. Immunofluorescence showed that grp58 protein abundance in kidney is highest in epithelial cells of the distal nephron, where it colocalizes with NCC near the apical membrane. To determine whether this interaction has a functional significance, NCC and grp58 cRNA were coexpressed in Xenopus laevis oocytes. In oocytes overexpressing grp58, sodium uptake was increased compared with control. Because oocytes express endogenous grp58, antisense experiments were performed to evaluate whether endogenous grp58 affected NCC activity in oocytes. Sodium uptake was lower in oocytes injected with both antisense grp58 cRNA and sense NCC compared with sense NCC oocytes. Western blot analysis did not show any effect of grp58 expression on processing of the NCC. These data indicate a novel, functionally important interaction between grp58 and the NCC in rat kidney cortex.

Osmoregulation and cell volume regulation in the preimplantation embryo

The early preimplantation mammalian embryo possesses mechanisms that regulate intracellular osmolarity and cell volume. While transport of osmotically active inorganic ions might play a role in this process in embryos, the major mechanisms that have been identified and studied are those that employ organic osmolytes. Organic osmolytes provide a substantial portion of intracellular osmotic support in embryos and are required for their development under in vivo conditions. The main osmolytes that have been identified in cleavage stage embryos are accumulated via two transport systems of the neurotransmitter transporter family active in early preimplantation embryos--the glycine transport system (GLY) and the beta-amino acid transport system (system beta). While system beta has been established to have a similar role in many other cells, this is a novel function for the GLY transport system. The intracellular concentration of organic osmolytes such as glycine in early preimplantation embryos is regulated by tonicity, allowing the embryo to regulate its volume against shrinkage and to control its internal osmolarity. In addition, the cells of the embryo can regulate against an increase in volume via controlled release of osmolytes from the cytoplasm. This is mediated by a swelling-activated anion channel that is also highly permeable to a range of organic osmolytes, and which closely resembles similar channels found in many other cell types (VSOAC channels). Together, these mechanisms appear to regulate cell volume in the egg through the early cleavage stages of embryogenesis, after which there are indications that the mechanisms of osmoregulation change.

Human and murine phenotypes associated with defects in cation-chloride cotransport

The diuretic-sensitive cotransport of cations with chloride is mediated by the cation-chloride cotransporters, a large gene family encompassing a total of seven Na-Cl, Na-K-2Cl, and K-Cl cotransporters, in addition to two related transporters of unknown function. The cation-chloride cotransporters perform a wide variety of physiological roles and differ dramatically in patterns of tissue expression and cellular localization. The renal-specific Na-Cl cotransporter (NCC) and Na-K-2Cl cotransporter (NKCC2) are involved in Gitelman and Bartter syndrome, respectively, autosomal recessive forms of metabolic alkalosis. The associated phenotypes due to loss-of-function mutations in NCC and NKCC2 are consistent, in part, with their functional roles in the distal convoluted tubule and thick ascending limb, respectively. Other cation-chloride cotransporters are positional candidates for Mendelian human disorders, and the K-Cl cotransporter KCC3, in particular, may be involved in degenerative peripheral neuropathies linked to chromosome 15q14. The characterization of mice with both spontaneous and targeted mutations of several cation-chloride cotransporters has also yielded significant insight into the physiological and pathophysiological roles of several members of the gene family. These studies implicate the Na-K-2Cl cotransporter NKCC1 in hearing, salivation, pain perception, spermatogenesis, and the control of extracellular fluid volume. Targeted deletion of the neuronal-specific K-Cl cotransporter KCC2 generates mice with a profound seizure disorder and confirms the central role of this transporter in modulating neuronal excitability. Finally, the comparison of human and murine phenotypes associated with loss-of-function mutations in cation-chloride cotransporters indicates important differences in physiology of the two species and provides an important opportunity for detailed physiological and morphological analysis of the tissues involved.

The KCl cotransporter isoform KCC3 can play an important role in cell growth regulation

The KCl cotransporter (KCC) plays a significant role in the ionic and osmotic homeostasis of many cell types. Four KCC isoforms have been cloned. KCC1 and KCC4 activity is osmolality-sensitive and involved in volume regulation. KCC2, a neuronal-specific isoform, can lower intracellular Cl(-) and is critical for inhibitory GABA responses in the mature central nervous system. KCC3, initially cloned from vascular endothelial cells, is widely but not universally distributed and has an unknown physiological significance. Here we show a tight link between the expression and activity of KCC3 and cell growth by a NIH/3T3 fibroblast expression system. KCC3 activity is sensitive to [(dihydroindenyl)oxy] alkanoic acid (DIOA) and N-ethylmaleimide and is regulated by tyrosine phosphorylation. Osmotic swelling does not activate KCC3, and the process of regulatory volume decrease is refractory to DIOA, indicating that KCC3 is not involved in volume regulation. KCC3 expression enhances cell proliferation, and this growth advantage can be abolished by the inhibition of KCC3 by DIOA. Fluorescence-activated cell sorting measurements and Western blot analysis show DIOA caused a significant reduction of the cell fraction in proliferative phase and a change in phosphorylation of retinoblastoma protein (Rb) and cdc2, suggesting that KCC3 activity is important for cell cycle progression. Insulin-like growth factor-1 up-regulates KCC3 expression and stimulates cell growth. Tumor necrotic factor-alpha down-regulates KCC3 expression and causes growth arrest. These data indicate that KCC3 is an important KCC isoform that may be involved in cell proliferation.

GSH depletion, K-Cl cotransport, and regulatory volume decrease in high-K/high-GSH dog red blood cells

Thiol reagents activate K-Cl cotransport (K-Cl COT), the Cl-dependent and Na-independent ouabain-resistant K flux, in red blood cells (RBCs) of several species, upon depletion of cellular glutathione (GSH). K-Cl COT is physiologically active in high potassium (HK), high GSH (HG) dog RBCs. In this unique model, we studied whether the same inverse relationship exists between GSH levels and K-Cl COT activity found in other species. The effects of GSH depletion by three different chemical reactions [nitrite (NO(2))-mediated oxidation, diazene dicarboxylic acid bis-N,N-dimethylamide (diamide)-induced dithiol formation, and glutathione S-transferase (GST)-catalyzed conjugation of GSH with 1-chloro-2,4-dinitrobenzene (CDNB)] were tested on K-Cl COT and regulatory volume decrease (RVD). After 85% GSH depletion, all three interventions stimulated K-Cl COT half-maximally with the following order of potency: diamide > NO(2) > CDNB. Repletion of GSH reversed K-Cl COT stimulation by 50%. Cl-dependent RVD accompanied K-Cl COT activation by NO(2) and diamide. K-Cl COT activation at concentration ratios of oxidant/GSH greater than unity was irreversible, suggesting either nitrosothiolation, heterodithiol formation, or GST-mediated dinitrophenylation of protein thiols. The data support the hypothesis that an intact redox system, rather than the absolute GSH levels, protects K-Cl COT activity and cell volume regulation from thiol modification.

Mechanisms of cell volume regulation and possible nature of the cell volume sensor

In animal organisms, cell volume undergoes dynamic changes in many physiological and pathological processes. To protect themselves against lysis and apoptosis and to maintain an optimal concentration of intracellular enzymes and metabolites, most animal cells actively regulate their volume. In the present review, we shortly summarize the data on ion transport mechanisms involved in regulatory volume decrease (RVD) and regulatory volume increase (RVI) with an emphasis on unresolved aspects of this problem such as: (i) how cells sense their volume changes; (ii) what signals are generated upon cell volume alterations; and (iii) how these signals are transferred to the ion transport systems executing cell volume regulation.

Nitric oxide signaling pathway regulates potassium chloride cotransporter-1 mRNA expression in vascular smooth muscle cells

Rat vascular smooth muscle cells (VSMCs) express at least two mRNAs for K-Cl cotransporters (KCC): KCC1 and KCC3. cGMP-dependent protein kinase I regulates KCC3 mRNA expression in these cells. Here, we show evidence implicating the nitric oxide (NO)/cGMP signaling pathway in the expression of KCC1 mRNA, considered to be the major cell volume regulator. VSMCs, expressing soluble guanylyl cyclase (sGC) and PKG-I isoforms showed a time- and concentration-dependent increase in KCC1 mRNA levels after treatment with sodium nitroprusside as demonstrated by semiquantitative RT-PCR. sGC-dependent regulation of KCC1 mRNA expression was confirmed using YC-1, a NO-independent sGC stimulator. The sGC inhibitor LY83583 blocked the effects of sodium nitroprusside and YC-1. Moreover, 8-Br-cGMP increased KCC1 mRNA expression in a concentration- and time-dependent fashion. The 8-Br-cGMP effect was partially blocked by KT5823 but not by actinomycin D. However, actinomycin D and cycloheximide increased basal KCC1 mRNA in an additive manner, suggesting different mechanisms of action for both drugs. These findings suggest that in VSMCs, the NO/cGMP-signaling pathway participates in KCC1 mRNA regulation at the post-transcriptional level.

A dominant negative mutant of the KCC1 K-Cl cotransporter: both N- and C-terminal cytoplasmic domains are required for K-Cl cotransport activity

K-Cl cotransport regulates cell volume and chloride equilibrium potential. Inhibition of erythroid K-Cl cotransport has emerged as an important adjunct strategy for the treatment of sickle cell anemia. However, structure-function relationships among the polypeptide products of the four K-Cl cotransporter (KCC) genes are little understood. We have investigated the importance of the N- and C-terminal cytoplasmic domains of mouse KCC1 to its K-Cl cotransport function expressed in Xenopus oocytes. Truncation of as few as eight C-terminal amino acids (aa) abolished function despite continued polypeptide accumulation and surface expression. These C-terminal loss-of-function mutants lacked a dominant negative phenotype. Truncation of the N-terminal 46 aa diminished function. Removal of 89 or 117 aa (Delta(N)117) abolished function despite continued polypeptide accumulation and surface expression and exhibited dominant negative phenotypes that required the presence of the C-terminal cytoplasmic domain. The dominant negative loss-of-function mutant Delta(N)117 was co-immunoprecipitated with wild type KCC1 polypeptide, and its co-expression did not reduce wild type KCC1 at the oocyte surface. Delta(N)117 also exhibited dominant negative inhibition of human KCC1 and KCC3 and, with lower potency, mouse KCC4 and rat KCC2.

Alternative splicing and diversity of renal transporters

The growing molecular identification of renal transporter genes is revealing that alternative splicing is common among transporters. In this paper, I review the physiological consequences of alternative splicing in some genes encoding renal transporters in which spliced isoforms have recently been identified. In some cases, the spliced isoforms resulted in nonfunctional proteins, which, however, possess a dominant negative effect on the cotransporter function, suggesting that the presence of such isoforms can be important in the functional regulation of the transporter. In most transporter genes, however, the spliced isoforms have been shown to be functional, resulting in a variety of physiological consequences, including, for example, changes in the polarization of isoforms to the apical or basolateral membrane, changes in pharmacological or kinetic properties, and changes in tissue distribution or intrarenal localization. In some cases, although the spliced isoform is functional, the consequence of splicing is still unknown. Different regulation among isoforms is an interesting possibility. Thus the diversity of several renal transporters is enhanced by alternative splicing mechanisms.

K-Cl cotransport modulation by intracellular Mg in erythrocytes from mice bred for low and high Mg levels

Mg is an important determinant of erythrocyte cation transport system(s) activity. We investigated cation transport in erythrocytes from mice bred for high (MGH) and low (MGL) Mg levels in erythrocytes and plasma. We found that K-Cl cotransport activity was higher in MGL than in MGH erythrocytes, and this could explain their higher mean corpuscular hemoglobin concentration, median density, and reduced cell K content. Although mouse KCC1 protein abundance was comparable in MGL and MGH erythrocytes, activities of Src family tyrosine kinases were higher in MGH than in MGL erythrocytes. In contrast, protein phosphatase (PP) isoform 1 alpha (PP1 alpha) enzymatic activity, which has been suggested to play a positive regulatory role in K-Cl cotransport, was lower in MGH than in MGL erythrocytes. Additionally, we found that the Src family kinase c-Fgr tyrosine phosphorylates PP1 alpha in vitro. These findings suggest that in vivo downregulation of K-Cl cotransport activity by Mg is mediated by enhanced Src family kinase activity, leading to inhibition of the K-Cl cotransport stimulator PP1.

K-Cl co-transport: immunocytochemical and functional evidence for more than one KCC isoform in high K and low K sheep erythrocytes

K-Cl co-transport (COT) is significantly higher in low K (LK), L-antigen (L) positive, than in high K (HK), M-antigen (M) positive, sheep red blood cells (SRBCs) and is inhibited by sheep allo-anti-L1 antibody. To answer the question of whether this difference in K-Cl co-transport activity resides at the level of the transporter or its regulation, a combined immunocytochemical and functional approach was taken. At least four isoforms of K-Cl COT encoded by different KCC genes are known, with 12 transmembrane domains and cytoplasmic C- and N-terminal domains (Ctd and Ntd, respectively). Polyclonal anti-rat (rt)KCC1 antibodies against a fusion peptide with 77 amino acids from the Ctd of rtKCC1 and anti-human (h)KCC3 against an 18-aa peptide from the Ntd of hKCC3, were prepared in rabbits (rb). Two distinctly separate protein bands of 180 and 145 kDa molecular mass were detected in hemoglobin-free ghosts from RBCs of two LK (one homozygous LL and one heterozygous LM) and one HK (homozygous MM) sheep by Western blots with rb anti-rtKCC1 and rb anti-hKCC3. Confocal microscopy showed specific immunostaining of KCC1 with rb anti-rtKCC1, and of KCC3 with rb anti-hKCC3, in white ghosts from both LK and HK SRBCs. To test the functional heterogeneity of K-Cl COT, the effect of the anti-L1 antibody was assessed on K-Cl COT activated by the kinase inhibitor staurosporine. Incubation of LK SRBCs with anti-L1 serum inhibited by 30% staurosporine-stimulated K-Cl COT suggesting that approximately two-thirds of the transport activity is independent of the L1 antigen. That staurosporine altered the L1 antigen/antibody reaction is unlikely since the action of another antibody, anti-Lp, stimulating the Na/K pump flux, was not modified. The present results, in conjunction with earlier work, lead to the hypothesis that the partial anti-L1 inhibition of K-Cl COT may be related to the molecular KCC dimorphism, seen in these cells with anti-KCC1 and anti-KCC3 antibodies.

Ion transport and ligand binding by the Na-K-Cl cotransporter, structure-function studies

The cation-Cl cotransporters (CCCs) mediate the coupled movement of Na and/or K to that of Cl across the plasmalemma of animal cells. Eight CCCs have been identified to date: two Na-K-Cl cotransporters (NKCC), four K-Cl cotransporters (KCCs), one Na-Cl cotransporter (NCC) and one CCC interacting protein (CIP). All of the NKCCs and KCCs are inhibited by loop diuretics; mercury and other modifying agents are also known to block NKCC-mediated transport. In this work, we have utilized a mutational approach to study the interaction between different substrates and the NKCCs. We relied on the strategy of exchanging domains between functionally distinct carriers (the shark NKCCl and the human NKCCl) to identify residues or group of residues that are involved in the interaction with ions, loop diuretics and Hg. Our results show that the N- and C-termini have no role in determining the species differences in ion transport and bumetanide binding. On the other hand, the interaction between Hg and the NKCCs is found to partially involve the C-terminus through residues that contain available sulfhydryl groups. Within the transmembrane segments, variant residues in the 2nd, 4th and 7th predicted alpha-helices are shown to encode the differences in ion transport between the shark and the human cotransporters. For loop diuretic binding, several regions throughout the central domain appear to be involved. Interestingly, these regions are not the same as those involved in cation or anion transport, and in Hg binding.

Renal potassium-chloride cotransporters

The electroneutral cotransport of potassium and chloride is mediated by potassium-chloride transporters, which are encoded by members of the gene family of cation-chloride cotransporters. A significant body of evidence argues for swelling-activated, basolateral potassium-chloride transport in the proximal tubule and thick ascending limb, with a potential role in transepithelial salt transport. However, the lack of specific inhibitors has impeded progress in this area. The cloning of the four potassium-chloride cotransporter genes has sparked new interest in this transport pathway, and promises to yield novel insights into their roles in cellular and renal physiology.