Volume-sensitive K(+)/Cl(-) cotransport in rabbit erythrocytes. Analysis of the rate-limiting activation and inactivation events

The WNK-SPAK/OSR1 pathway: master regulator of cation-chloride cotransporters

The WNK-SPAK/OSR1 kinase complex is composed of the kinases WNK (with no lysine) and SPAK (SPS1-related proline/alanine-rich kinase) or the SPAK homolog OSR1 (oxidative stress-responsive kinase 1). The WNK family senses changes in intracellular Cl(-) concentration, extracellular osmolarity, and cell volume and transduces this information to sodium (Na(+)), potassium (K(+)), and chloride (Cl(-)) cotransporters [collectively referred to as CCCs (cation-chloride cotransporters)] and ion channels to maintain cellular and organismal homeostasis and affect cellular morphology and behavior. Several genes encoding proteins in this pathway are mutated in human disease, and the cotransporters are targets of commonly used drugs. WNKs stimulate the kinases SPAK and OSR1, which directly phosphorylate and stimulate Cl(-)-importing, Na(+)-driven CCCs or inhibit the Cl(-)-extruding, K(+)-driven CCCs. These coordinated and reciprocal actions on the CCCs are triggered by an interaction between RFXV/I motifs within the WNKs and CCCs and a conserved carboxyl-terminal docking domain in SPAK and OSR1. This interaction site represents a potentially druggable node that could be more effective than targeting the cotransporters directly. In the kidney, WNK-SPAK/OSR1 inhibition decreases epithelial NaCl reabsorption and K(+) secretion to lower blood pressure while maintaining serum K(+). In neurons, WNK-SPAK/OSR1 inhibition could facilitate Cl(-) extrusion and promote γ-aminobutyric acidergic (GABAergic) inhibition. Such drugs could have efficacy as K(+)-sparing blood pressure-lowering agents in essential hypertension, nonaddictive analgesics in neuropathic pain, and promoters of GABAergic inhibition in diseases associated with neuronal hyperactivity, such as epilepsy, spasticity, neuropathic pain, schizophrenia, and autism.

Volume regulation and KCl cotransport in reticulocyte populations of sickle and normal red blood cells

The potassium chloride co-transporter (KCC) is a member of the electroneutral cation chloride family of cotransporters found in multiple tissues that are involved in transepithelial ion transport and regulation of intracellular ion content and cell volume. We have shown previously that three of the four KCC genes - KCC1, KCC3, and KCC4 - are expressed in red blood cells (RBC) (Exp. Hem. 33:624, 2005). Functionally, the KCC mediates volume reduction of reticulocytes that establishes the higher cellular hemoglobin concentration (CHC) of mature RBC. KCC activity is higher in reticulocytes and diminishes with age. KCC activity in RBC containing sickle hemoglobin (SS RBC) is elevated compared to normal (AA RBC) in part due to reticulocytosis in SS blood. However, we have demonstrated that SS reticulocytes have abnormal regulation of KCC activity leading to increased CHC upon activation of KCC compared to AA reticulocytes (Blood 104:2954, 2004; Blood 109:1734, 2007). These findings implicate KCC as a factor in the dehydration of SS RBC, which leads to elevated Hb S concentration and enhances Hb S polymerization and hemolysis. Because KCC activity correlates with cell age, standard flux measurements on blood samples with different numbers of reticulocytes or young non-reticulocytes are not comparable. The Advia automated cell counter measures cell volume (MCV) and cellular hemoglobin concentration (CHC) in reticulocytes, an age-defined population of cells, and thus circumvents the problem of variable reticulocyte counts among SS and AA blood samples. In this study, reticulocyte CHC measurements on fresh blood demonstrated a clear difference between AA and SS cells, reflecting in vivo dehydration of SS reticulocytes, although there was significant inter-individual variation, and the CHC distributions of the two groups overlapped. After KCC activation in vitro by cell swelling using the nystatin method, the initial changes in reticulocyte MCV and CHC with time were used to estimate flux rates mediated by KCC, assuming that changes were associated with isotonic KCl movements. After 20-30min a final steady state MCV/CHC (set point) was achieved and maintained, reflecting inactivation of the transporter. CHC set points were 26.5-29g/dl in SS reticulocytes compared to 25-26.5g/dl in AA reticulocytes, reflecting abnormal regulation in SS cells. These results were reproducible in the same individual over time. KCC flux derived from CHC ranged from 5 to 10.3mmolK/kgHb/min in SS reticulocytes, compared to 2.9-7.2mmolK/kgHb/min in AA reticulocytes. Such measures of KCC activity in red cell populations controlled for cell age will facilitate further studies correlating KCC activity with phenotypic or genetic variability in sickle cell disease.

Coordinated control of volume regulatory Na+/H+ and K+/H+ exchange pathways in Amphiuma red blood cells

The Na(+)/H(+) and K(+)/H(+) exchange pathways of Amphiuma tridactylum red blood cells (RBCs) are quiescent at normal resting cell volume yet are selectively activated in response to cell shrinkage and swelling, respectively. These alkali metal/H(+) exchangers are activated by net kinase activity and deactivated by net phosphatase activity. We employed relaxation kinetic analyses to gain insight into the basis for coordinated control of these volume regulatory ion flux pathways. This approach enabled us to develop a model explaining how phosphorylation/dephosphorylation-dependent events control and coordinate the activity of the Na(+)/H(+) and K(+)/H(+) exchangers around the cell volume set point. We found that the transition between initial and final steady state for both activation and deactivation of the volume-induced Na(+)/H(+) and K(+)/H(+) exchange pathways in Amphiuma RBCs proceed as a single exponential function of time. The rate of Na(+)/H(+) exchange activation increases with cell shrinkage, whereas the rate of Na(+)/H(+) exchange deactivation increases as preshrunken cells are progressively swollen. Similarly, the rate of K(+)/H(+) exchange activation increases with cell swelling, whereas the rate of K(+)/H(+) exchange deactivation increases as preswollen cells are progressively shrunken. We propose a model in which the activities of the controlling kinases and phosphatases are volume sensitive and reciprocally regulated. Briefly, the activity of each kinase-phosphatase pair is reciprocally related, as a function of volume, and the volume sensitivities of kinases and phosphatases controlling K(+)/H(+) exchange are reciprocally related to those controlling Na(+)/H(+) exchange.

Chemical crosslinking studies with the mouse Kcc1 K-Cl cotransporter

Oligomerization, function, and regulation of unmodified mouse Kcc1 K-Cl cotransporter were studied by chemical crosslinking. Treatment of Xenopus oocytes and 293T cells expressing K-Cl cotransporter Kcc1 with several types of chemical cross-linkers shifted Kcc1 polypeptide to higher molecular weight forms. More extensive studies were performed with the amine-reactive disuccinyl suberate (DSS) and with the sulfhydryl-reactive bis-maleimidohexane (BMH). Kcc1 cross-linking was time-dependent in intact oocytes, and was independent of protein concentration in detergent lysates from oocytes or 293T cells. Kcc1 cross-linking by the cleavable cross-linker DTME was reversible. The N-terminal and C-terminal cytoplasmic tails of Kcc1 were not essential for Kcc1 crosslinking. PFO-PAGE and gel filtration revealed oligomeric states of uncrosslinked KCC1 corresponding in mobility to that of cross-linked protein. DSS and BMH each inhibited KCC1-mediated (86)Rb(+) uptake stimulated by hypotonicity or by N-ethylmaleimide (NEM) without reduction in nominal surface abundance of KCC1. These data add to evidence supporting the oligomeric state of KCC polypeptides.

Identification of regulatory phosphorylation sites in a cell volume- and Ste20 kinase-dependent ClC anion channel

Changes in phosphorylation regulate the activity of various ClC anion transport proteins. However, the physiological context under which such regulation occurs and the signaling cascades that mediate phosphorylation are poorly understood. We have exploited the genetic model organism Caenorhabditis elegans to characterize ClC regulatory mechanisms and signaling networks. CLH-3b is a ClC anion channel that is expressed in the worm oocyte and excretory cell. Channel activation occurs in response to oocyte meiotic maturation and swelling via serine/threonine dephosphorylation mediated by the type I phosphatases GLC-7alpha and GLC-7beta. A Ste20 kinase, germinal center kinase (GCK)-3, binds to the cytoplasmic C terminus of CLH-3b and inhibits channel activity in a phosphorylation-dependent manner. Analysis of hyperpolarization-induced activation kinetics suggests that phosphorylation may inhibit the ClC fast gating mechanism. GCK-3 is an ortholog of mammalian SPAK and OSR1, kinases that bind to, phosphorylate, and regulate the cell volume-dependent activity of mammalian cation-Cl(-) cotransporters. Using mass spectrometry and patch clamp electrophysiology, we demonstrate here that CLH-3b is a target of regulatory phosphorylation. Concomitant phosphorylation of S742 and S747, which are located 70 and 75 amino acids downstream from the GCK-3 binding site, are required for kinase-mediated channel inhibition. In contrast, swelling-induced channel activation occurs with dephosphorylation of S747 alone. Replacement of both S742 and S747 with glutamate gives rise to kinase- and swelling-insensitive channels that exhibit activity and biophysical properties similar to those of wild-type CLH-3b inhibited by GCK-3. Our studies provide novel insights into ClC regulation and mechanisms of cell volume signaling, and provide the foundation for studies aimed at defining how conformational changes in the cytoplasmic C terminus alter ClC gating and function in response to intracellular signaling events.

Activation of Na+/H+ and K+/H+ exchange by calyculin A in Amphiuma tridactylum red blood cells: implications for the control of volume-induced ion flux activity

Alteration in cell volume of vertebrates results in activation of volume-sensitive ion flux pathways. Fine control of the activity of these pathways enables cells to regulate volume following osmotic perturbation. Protein phosphorylation and dephosphorylation have been reported to play a crucial role in the control of volume-sensitive ion flux pathways. Exposing Amphiuma tridactylu red blood cells (RBCs) to phorbol esters in isotonic medium results in a simultaneous, dose-dependent activation of both Na(+)/H(+) and K(+)/H(+) exchangers. We tested the hypothesis that in Amphiuma RBCs, both shrinkage-induced Na(+)/H(+) exchange and swelling-induced K(+)/H(+) exchange are activated by phosphorylation-dependent reactions. To this end, we assessed the effect of calyculin A, a phosphatase inhibitor, on the activity of the aforementioned exchangers. We found that exposure of Amphiuma RBCs to calyculin-A in isotonic media results in simultaneous, 1-2 orders of magnitude increase in the activity of both K(+)/H(+) and Na(+)/H(+) exchangers. We also demonstrate that, in isotonic media, calyculin A-dependent increases in net Na(+) uptake and K(+) loss are a direct result of phosphatase inhibition and are not dependent on changes in cell volume. Whereas calyculin A exposure in the absence of volume changes results in stimulation of both the Na(+)/H(+) and K(+)/H(+) exchangers, superimposing cell swelling or shrinkage and calyculin A treatment results in selective activation of K(+)/H(+) or Na(+)/H(+) exchange, respectively. We conclude that kinase-dependent reactions are responsible for Na(+)/H(+) and K(+)/H(+) exchange activity, whereas undefined volume-dependent reactions confer specificity and coordinated control.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

K–Cl cotransport in red blood cells from patients with KCC3 isoform mutantsThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Red blood cells (RBCs) possess the K–Cl cotransport (KCC) isoforms 1, 3, and 4. Mutations within a given isoform may affect overall KCC activity. In a double-blind study, we analyzed, with Rb as a K congener, K fluxes (total flux, ouabain-sensitive Na+/K+ pump, and bumetanide-sensitive Na–K–2Cl cotransport, Cl-dependent, and ouabain- and bumetanide-insensitive KCC with or without stimulation by N-ethylmaleimide (NEM) and staurosporine or Mg removal, and basal channel-mediated fluxes, osmotic fragility, and ions and water in the RBCs of 8 controls, and of 8 patients with hereditary motor and sensory neuropathy with agenesis of corpus callosum (HMSN–ACC) with defined KCC3 mutations (813FsX813 and Phe529FsX532) involving the truncations of 338 and 619 C-terminal amino acids, respectively. Water and ion content and, with one exception, mean osmotic fragility, as well as K fluxes without stimulating agents, were similar in controls and HMSN–ACC RBCs. However, the NEM-stimulated KCC was reduced 5-fold (p < 0.0005) in HMSN–ACC vs control RBCs, as a result of a lower Vmax (p < 0.05) rather than a lower Km (p = 0.109), accompanied by corresponding differences in Cl activation. Low intracellular Mg activated KCC in 6 out of 7 controls vs 1 out of 6 HMSN–ACC RBCs, suggesting that regulation is compromised. The lack of differences in staurosporine-activated KCC indicates different action mechanisms. Thus, in HMSN–ACC patients with KCC3 mutants, RBC KCC activity, although indistinguishable from that of the control group, responded differently to biochemical stressors, such as thiol alkylation or Mg removal, thereby indirectly indicating an important contribution of KCC3 to overall KCC function and regulation.

Urea stimulation of KCl cotransport induces abnormal volume reduction in sickle reticulocytes

KCl cotransport (KCC) activity contributes to pathologic dehydration in sickle (SS) red blood cells (RBCs). KCC activation by urea was measured in SS and normal (AA) RBCs as Cl-dependent Rb influx. KCC-mediated volume reduction was assessed by measuring reticulocyte cellular hemoglobin concentration (CHC) cytometrically. Urea activated KCC fluxes in fresh RBCs to levels seen in swollen cells, although SS RBCs required lower urea concentrations than did normal (AA) RBCs. Little additional KCC stimulation by urea occurred in swollen AA or SS RBCs. The pH dependence of KCC in "euvolemic" SS RBCs treated with urea was similar to that in swollen cells. Urea triggered volume reduction in SS and AA reticulocytes, establishing a higher CHC. Volume reduction was Cl dependent and was limited by the KCC inhibitor, dihydro-indenyl-oxyalkanoic acid. Final CHC depended on urea concentration, but not on initial CHC. Under all activation conditions, volume reduction was exaggerated in SS reticulocytes and produced higher CHCs than in AA reticulocytes. The sulfhydryl-reducing agent, dithiothreitol, normalized the sensitivity of KCC activation to urea in SS RBCs and mitigated the urea-stimulated volume decrease in SS reticulocytes, suggesting that the dysfunctional activity of KCC in SS RBCs was due in part to reversible sulfhydryl oxidation.

Regulation of K-Cl cotransport by protein phosphatase 1alpha in mouse erythrocytes

The K-Cl cotransport (KCC) is an electroneutral-gradient-driven-membrane transport system, which is involved in regulation of red cell volume. Although the regulatory cascade of KCC is largely unknown, a signaling pathway involving phosphatases and kinases has been proposed. Here, we investigated the expression and the activity of protein phosphatase 1(PP-1) isoforms in mouse red cells, focusing on two models of abnormally activated KCC: mice genetically lacking the two Src-family tyrosine kinases, Hck and Fgr, (hck-/-fgr-/-) and the SAD transgenic sickle-cell-mice. The PP-1alpha, PP-1gamma, PP-1delta isoforms were expressed at similar levels in wild-type, hck-/-fgr-/- and SAD mouse erythrocytes and in each case were predominantly localized to cytoplasm. The PP-1alpha activity was significantly higher in both membrane and cytosol fractions of hck-/-fgr-/- and of SAD erythrocytes than in those of wild-type red cells, suggesting PP-1alpha as a target of the Hck and Fgr kinases. The PP2, a specific inhibitor of Src-family kinase, significantly increased KCC activity in wild-type mouse red cells, but failed to modify the already increased KCC activity in SAD erythrocytes. The lag-time for activation of KCC was considerably reduced in both hck-/-fgr-/- and SAD erythrocytes, suggesting that the rate limiting activation steps in both strains are freed from their tonic inhibition. Sulfhydryl reduction by dithiothreitol (DTT) lowered KCC activity only in SAD red cells, but did not affect the PP2-treated erythrocytes. These data suggest up-regulation of KCC in SAD red cells is mainly secondary to oxidative damage, which most likely reduces or removes the tonic KCC inhibition resulting from PP-1alpha activity controlled in turn by Src-family kinases.

Regulation of potassium transport in human lens epithelial cells

The major K influx pathways and their response to thiol modification by N-ethylmaleimide (NEM) and protein kinase and phosphatase inhibitors were characterized in human lens epithelial B3 (HLE-B3) cells with Rb as K congener. Ouabain (0.1 mM) and bumetanide (5 microM) discriminated between the Na/K pump ( approximately 35% of total Rb influx) and Na-K-2Cl cotransport (NKCC) ( approximately 50%). Cl-replacement with nitrate or sulfamate revealed <10% residual [ouabain+bumetanide]-insensitive K-Cl cotransport (KCC). At 0.3-0.5 mM, NEM stimulated the Na/K pump by 2-fold independent of external Na, KCC between 2 and 4-fold, and abolished approximately 90% of NKCC. Calyculin-A, a serine/threonine protein phosphatase-1 inhibitor, did not affect NKCC but inhibited KCC, whereas 10 microM staurosporine, a serine/threonine kinase inhibitor, abolished NKCC, and stimulated KCC only when followed by NEM treatment. The tyrosine-kinase inhibitor genistein, at concentrations >100 microM, activated the Na/K pump and abolished NKCC but did not affect KCC. The data suggest at least partial inverse regulation of KCC and NKCC in HLE-B3 cells by signaling cascades involving serine, threonine and tyrosine phosphorylation/dephosphorylation equilibria.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Impaired Cl- extrusion in layer V pyramidal neurons of chronically injured epileptogenic neocortex

In the mature brain, the K(+)/Cl- cotransporter KCC2 is important in maintaining low [Cl-]i, resulting in hyperpolarizing GABA responses. Decreases in KCC2 after neuronal injuries result in increases in [Cl-]i and enhanced neuronal excitability due to depolarizing GABA responses. We used the gramicidin perforated-patch technique to measure E(Cl) ( approximately E(GABA)) in layer V pyramidal neurons in slices of partially isolated sensorimotor cortex of adult rats to explore the potential functional consequence of KCC2 downregulation in chronically injured cortex. E(GABA) was measured by recording currents evoked with brief GABA puffs at various membrane potentials. There was no significant difference in E(Cl) between neurons in control and undercut animals (-71.2 +/- 2.6 and -71.8 +/- 2.8 mV, respectively). However, when loaded with Cl- by applying muscimol puffs at 0.2 Hz for 60 s, neurons in the undercut cortex had a significantly shorter time constant for the positive shift in E(Cl) during the Cl- loading phase (4.3 +/- 0.5 s for control and 2.2 +/- 0.4 s for undercut, P < 0.01). The positive shift in E(Cl) 3 s after the beginning of Cl- loading was also significantly larger in the undercut group than in the control, indicating that neurons in undercut cortex were less effective in maintaining low [Cl-]i during repetitive activation of GABA(A) receptors. Application of furosemide eliminated the difference between the control and undercut groups for both of these measures of [Cl-]i regulation. The results suggest an impairment in Cl- extrusion resulting from decreased KCC2 expression that may reduce the strength of GABAergic inhibition and contribute to epileptogenesis.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

KCl cotransport mediates abnormal sulfhydryl-dependent volume regulation in sickle reticulocytes

KCl cotransport (KCC) activation by cell swelling and pH was compared in sickle (SS) and normal (AA) red blood cells (RBCs). KCC fluxes had the same relationship to mean corpuscular hemoglobin concentration (MCHC) in SS and AA RBCs when normalized to the maximal volume-stimulated (VSmax) flux (MCHC &lt; 270 g/L [27 g/dL]). Acid-stimulated (pH 6.9) KCC flux in SS RBCs was 60% to 70% of VSmax KCC versus 20% in AA RBCs. Density gradients were used to track changes in reticulocyte MCHC during KCC-mediated regulatory volume decrease (RVD). Swelling to MCHC of 260 g/L (26 g/dL) produced Cl-dependent RVD that resulted in higher MCHC in SS than AA reticulocytes. In acid pH, RVD was also greater in SS than AA reticulocytes. Sulfhydryl reduction by dithiothreitol (DTT) lowered VSmax KCC flux in AA and SS RBCs by one third but did not alter swelling-induced RVD. DTT lowered acid-activated KCC in SS RBCs by 50% and diminished acid-induced RVD in SS reticulocytes. Thus, swelling activation of KCC is normal in SS RBCs but KCC-mediated RVD produces higher MCHC in SS than AA reticulocytes. Acid activation of KCC is exaggerated in SS RBCs and causes dehydration in SS reticulocytes. KCC response to acid stimulation was mitigated by DTT, suggesting that it arises from sulfhydryl oxidation.

The K-Cl cotransporter in the lobster stretch receptor neurone--a kinetic analysis

Experiments were performed to define quantitatively the substrate (K(+) and Cl(-)) dependence of the transport function (production of equally large and oppositely directed K(+)and Cl(-) flows/currents) of an earlier (Theander et al., 1999) identified electroneutral K-Cl cotransporter in the slowly adapting stretch receptor neurone of the European lobster. The experiments were based on microelectrode techniques. This allowed us to perform steady-state measurements of the so-called "instantaneous" current-voltage relationships (around a holding voltage of -65 mV after a blockage of the cell's action potential and hyperpolarization-activated currents) and intracellular ion concentrations at various settings of the extracellular K(+) and Cl(-) concentrations. From the results, we could then define steady-state values of all of the cell's non-KCl cotransporter K(+) and Cl(-) currents. Finally, the negative sums of the inferred non-KCl cotransporter K(+) and Cl(-) currents could be taken as equivalents of the K-Cl cotransporter's K(+) and Cl(-) currents for the reason that, in steady state, all membrane currents add up to zero. For the cotransporter currents, thus inferred for a range from 2.5/410.5 to 40.0/448.0 mM external K(+)/Cl(-), we found that their absolute values increased in a nonlinear fashion from about 5 nA cell(-1) at the lowest, to about 20 nA cell(-1) at the highest external K(+)/Cl(-) concentrations. Formally, this relationship could be reproduced by a Hill function-based enzyme kinetic expression simulating inward and outward transmembrane electroneutral ion transports. Following insertion of this expression into a comprehensive model of electrical membrane functions and intracellular solute and solvent control in the lobster stretch receptor neurone, the model predictions suggested that the K-Cl cotransporter does play an important role in (a) keeping intracellular Cl(-) low for a proper function of the cell's inhibitory system, and (b) enabling rapid transmembrane K(+) shifts that provide for a stabilization of the cell's membrane voltage and membrane excitability in cases of varying extracellular K(+) concentrations. The model predictions gave, however, no clear evidence that the K-Cl cotransporter is critically involved in the cell's volume regulation in conditions of varying extracellular osmolalities.

Reduction of KCC2 expression and GABAA receptor-mediated excitation after in vivo axonal injury

After axotomy, application of muscimol, a GABA(A) receptor agonist, induced an increase in intracellular Ca(2+) ([Ca(2+)](i)) in dorsal motor neurons of the vagus (DMV neurons). Elevation of [Ca(2+)](i) by muscimol was blocked by bicuculline, tetrodotoxin, and Ni(2+). In axotomized DMV neurons measured with gramicidin perforated-patch recordings, reversal potentials of the GABA(A) receptor-mediated response, presumably equal to the equilibrium potential of Cl(-), were more depolarized than that in intact neurons. Thus, GABA(A) receptor-mediated excitation is suggested to be attributable to Cl(-) efflux out of the cell because of increased intracellular Cl(-) concentration ([Cl(-)](i)) in axotomized neurons. Regulation of [Cl(-)](i) in both control and injured neurons was disturbed by furosemide and bumetanide and by manipulating cation balance across the membrane, suggesting that functional alteration of furosemide-sensitive cation-Cl(-) cotransporters is responsible for the increase of [Cl(-)](i) after axotomy. In situ hybridization revealed that neuron-specific K(+)-Cl(-) cotransporter (KCC2) mRNA was significantly reduced in the DMV after axotomy compared with that in control neurons. Similar expression of Na(+), K(+)-Cl(-) cotransporter mRNA was observed between axotomized and control DMV neurons. Thus, axotomy led to disruption of [Cl(-)](i) regulation attributable to a decrease of KCC2 expression, elevation of intracellular Cl(-), and an excitatory response to GABA. A switch of GABA action from inhibitory to excitatory might be a mechanism contributing to excitotoxicity in injured neurons.

Evidence supporting a role for KCl cotransporter in the thick ascending limb of Henle's loop

BACKGROUND

A basolateral Ba(2+)-sensitive KCl cotransporter has previously been proposed as participating in basolateral K+ recycling and transepithelial NaCl reabsorption in the thick ascending limb of Henle's loop (TAL). The aim of the present study was to answer the question as to whether this cotransporter plays a role in transepithelial K+ reabsorption and whether dietary Mg(2+) deficiency, known to regulate the KCl cotransporter in erythrocytes, also regulates KCl transport in the TAL.

METHODS

The effects of a low-Mg(2+) diet were investigated on urinary and plasma K+ concentration in control mice and Mg(2+)-deficient mice. Transepithelial Na+, Cl- and K+ net fluxes (J(Na), J(Cl), J(K)), determined in isolated perfused TALs with electron probe analysis or cation-exchange high-performance liquid chromatography (HPLC) and electrophysiological parameters (V(te), R(te)), were measured in both animal groups. Expression of transcripts for the KCl cotransporter and its possible regulation by low-Mg(2+) were studied by RT-PCR in microdissected mouse cortical TAL (CTAL) and medullary TAL (MTAL) segments.

RESULTS

In isolated perfused CTALs, basolateral Ba(2+) and amiloride induced a large K+ net secretion towards the tubular lumen, paralleled by a 50% decrease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg(2+) diet led to diminished urinary K+ excretion, lowered plasma K+ concentration and up-regulation of KCC1 transcripts in the TAL. For low-Mg(2+) diet, this upregulation was associated with increased transepithelial K+ reabsorption in the in vitro-perfused CTAL.

CONCLUSIONS

Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepithelial K+ reabsorption. Direct inhibition of this transporter by Ba(2+) and its indirect inhibition by amiloride lead to a strong transepithelial K+ secretion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mRNA by dietary Mg(2+) restriction is associated with an increased K+ reabsorption in the in vitro perfused CTAL.

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.

Direct estimate of 1:1 stoichiometry of K(+)-Cl(-) cotransport in rabbit erythrocytes

This work was undertaken to obtain a direct measure of the stoichiometry of Na(+)-independent K(+)-Cl(-) cotransport (KCC), with rabbit red blood cells as a model system. To determine whether (86)Rb(+) can be used quantitatively as a tracer for KCC, (86)Rb(+) and K(+) effluxes were measured in parallel after activation of KCC with N-ethylmaleimide (NEM). The rate constant for NEM-stimulated K(+) efflux into isosmotic NaCl was smaller than that for (86)Rb(+) by a factor of 0.68 +/- 0.11 (SD, n = 5). This correction factor was used in all other experiments to calculate the K(+) efflux from the measured (86)Rb(+) efflux. To minimize interference from the anion exchanger, extracellular Cl(-) was replaced with SO, and 4,4'-diisothiocyanothiocyanatodihydrostilbene-2,2'-disulfonic acid was present in the flux media. The membrane potential was clamped near 0 mV with the protonophore 2,4-dinitrophenol. The Cl(-) efflux at 25 degrees C under these conditions is approximately 100,000-fold smaller than the uninhibited Cl(-)/Cl(-) exchange flux and is stimulated approximately 2-fold by NEM. The NEM-stimulated (36)Cl(-) flux is inhibited by okadaic acid and calyculin A, as expected for KCC. The ratio of the NEM-stimulated K(+) to Cl(-) efflux is 1.12 +/- 0.26 (SD, n = 5). We conclude that K(+)-Cl(-) cotransport in rabbit red blood cells has a stoichiometry of 1:1.

Functional and molecular characterization of the K-Cl cotransporter of Xenopus laevis oocytes

The K-Cl cotransporters (KCCs) have a broad range of physiological roles, in a number of cells and species. We report here that Xenopus laevis oocytes express a K-Cl cotransporter with significant functional and molecular similarity to mammalian KCCs. Under isotonic conditions, defolliculated oocytes exhibit a Cl(-)-dependent (86)Rb(+) uptake mechanism after activation by the cysteine-reactive compounds N-ethylmaleimide (NEM) and mercuric chloride (HgCl(2)). The activation of this K-Cl cotransporter by cell swelling is prevented by inhibition of protein phosphatase-1 with calyculin A; NEM activation of the transporter was not blocked by phosphatase inhibition. Kinetic characterization reveals apparent values for the Michaelis-Menten constant of 27.7 +/- 3.0 and 15.4 +/- 4.7 mM for Rb(+) and Cl(-), respectively, with an anion selectivity for K(+) transport of Cl(-) = PO(4)(3-) = Br(-) > I(-) > SCN(-) > gluconate. The oocyte K-Cl cotransporter was sensitive to several inhibitors, including loop diuretics, with apparent half-maximal inhibition values of 200 and 500 microM for furosemide and bumetanide, respectively. A partial cDNA encoding the Xenopus K-Cl cotransporter was cloned from oocyte RNA; the corresponding transcript is widely expressed in Xenopus tissues. The predicted COOH-terminal protein fragment exhibited particular homology to the KCC1/KCC3 subgroup of the mammalian KCCs, and the functional characteristics are the most similar to those of KCC1 (Mercado A, Song L, Vazquez N, Mount DB, and Gamba G. J Biol Chem 275: 30326--30334, 2000).

Ictal epileptiform activity is facilitated by hippocampal GABAA receptor-mediated oscillations

The cellular and network mechanisms of the transition of brief interictal discharges to prolonged seizures are a crucial issue in epilepsy. Here we used hippocampal slices exposed to ACSF containing 0 Mg(2+) to explore mechanisms for the transition to prolonged (3-42 sec) seizure-like ("ictal") discharges. Epileptiform activity, evoked by Shaffer collateral stimulation, triggered prolonged bursts in CA1, in 50-60% of slices, from both adult and young (postnatal day 13-21) rats. In these cases the first component of the CA1 epileptiform burst was followed by a train of population spikes at frequencies in the gamma band and above (30-120 Hz, reminiscent of tetanically evoked gamma oscillations). The gamma burst in turn could be followed by slower repetitive "tertiary" bursts. Intracellular recordings from CA1 during the gamma phase revealed long depolarizations, action potentials rising from brief apparent hyperpolarizations, and a drop of input resistance. The CA1 gamma rhythm was completely blocked by bicuculline (10-50 microm), by ethoxyzolamide (100 microm), and strongly attenuated in hyperosmolar perfusate (50 mm sucrose). Subsequent tertiary bursts were also blocked by bicuculline, ethoxyzolamide, and in hyperosmolar perfusate. In all these cases intracellular recordings from CA3 revealed only short depolarizations. We conclude that under epileptogenic conditions, gamma band oscillations arise from GABA(A)ergic depolarizations and that this activity may lead to the generation of ictal discharges.

Ictal epileptiform activity is facilitated by hippocampal GABAA receptor-mediated oscillations

The cellular and network mechanisms of the transition of brief interictal discharges to prolonged seizures are a crucial issue in epilepsy. Here we used hippocampal slices exposed to ACSF containing 0 Mg(2+) to explore mechanisms for the transition to prolonged (3-42 sec) seizure-like ("ictal") discharges. Epileptiform activity, evoked by Shaffer collateral stimulation, triggered prolonged bursts in CA1, in 50-60% of slices, from both adult and young (postnatal day 13-21) rats. In these cases the first component of the CA1 epileptiform burst was followed by a train of population spikes at frequencies in the gamma band and above (30-120 Hz, reminiscent of tetanically evoked gamma oscillations). The gamma burst in turn could be followed by slower repetitive "tertiary" bursts. Intracellular recordings from CA1 during the gamma phase revealed long depolarizations, action potentials rising from brief apparent hyperpolarizations, and a drop of input resistance. The CA1 gamma rhythm was completely blocked by bicuculline (10-50 microm), by ethoxyzolamide (100 microm), and strongly attenuated in hyperosmolar perfusate (50 mm sucrose). Subsequent tertiary bursts were also blocked by bicuculline, ethoxyzolamide, and in hyperosmolar perfusate. In all these cases intracellular recordings from CA3 revealed only short depolarizations. We conclude that under epileptogenic conditions, gamma band oscillations arise from GABA(A)ergic depolarizations and that this activity may lead to the generation of ictal discharges.