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

Cotransporters, WNKs and hypertension: important leads from the study of monogenetic disorders of blood pressure regulation

Major advances are being made in identifying the structure and behaviour of regulatory cascades that control the activity of cation-Cl(-) cotransporters and certain Na(+), K(+) and Cl(-) channels. These transporters play key roles in regulating arterial blood pressure as they are not only responsible for NaCl reabsorption in the thick ascending limb and distal tubule of the kidney, but are also involved in regulating smooth muscle Ca(2+) levels. It is now apparent that defects in these transporters, and particularly in the regulatory cascades, cause some monogenetic forms of hypertension and may contribute to essential hypertension and problems with K(+) homoeostasis. Two families of kinases are prominent in these processes: the Ste-20-related kinases [OSR1 (oxidative stress-responsive kinase 1) and SPAK (Ste20/SPS1-related proline/alanine-rich kinase)] and the WNKs [with no lysine kinases]. These kinases affect the behaviour of their targets through both phosphorylation and by acting as scaffolding proteins, bringing together regulatory complexes. This review analyses how these kinases affect transport by activating or inhibiting individual transporters at the cell surface, or by changing the surface density of transporters by altering the rate of insertion or removal of transporters from the cell surface, and perhaps through controlling the rate of transporter degradation. This new knowledge should not only help us target antihypertensive therapy more appropriately, but could also provide the basis for developing new therapeutic approaches to essential hypertension.

Psoriasis: genetic associations and immune system changes

Psoriasis is a common inflammatory skin disease characterized by infiltration of inflammatory cells into the epidermis and altered keratinocyte differentiation. Psoriasis is currently thought of as a T-cell mediated 'Type-1' autoimmune disease. Gene expression changes in psoriasis lesions have been well documented, and strongly support an important role for tumor necrosis factor and interferon gamma signal pathways in its pathogenesis. The strongest genetic determinant of psoriasis identified to date lies within the class I region of the multiple histocompatibility locus antigen cluster, although its low penetrance implicates a requirement for other genetic risk factors. Multiple genome-wide linkage and an increasing number of association studies have been carried out, leading to multiple linkage peaks, and the identification of potential low risk variants. A number of these variants lie within genes encoding components of the immune system. However, the functional relationships between predisposing genetic variation is unclear, and presumably involves genetic susceptibility factors affecting both immune cell activation and keratinocyte differentiation. The interaction of environmental trigger factors with genetic effects is also not understood, but provide further evidence for the complex basis of this disease.

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

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

WNK protein kinases modulate cellular Cl- flux by altering the phosphorylation state of the Na-K-Cl and K-Cl cotransporters

Precise control of cellular Cl(-) transport is necessary for many fundamental physiological processes. For example, the intracellular concentration of Cl(-), fine-tuned through the coordinated action of cellular Cl(-) influx and efflux mechanisms, determines whether a neuron's response to GABA is excitatory or inhibitory. In epithelia, synchrony between apical and basolateral Cl(-) flux, and transcellular and paracellular Cl(-) transport, is necessary for efficient transepithelial Cl(-) reabsorption or secretion. In cells throughout the body, coordination of Cl(-) entry and exit mechanisms help defend against changes in cell volume. The Na-K-Cl and K-Cl cotransporters of the SLC12 gene family are important molecular determinants of Cl(-) entry and exit, respectively, in these systems. The WNK serine-threonine kinase family, members of which are mutated in an inherited form of human hypertension, are components of a signaling pathway that coordinates Cl(-) influx and efflux through SLC12 cotransporters to dynamically regulate intracellular Cl(-) activity.

Mutations in the K+/Cl- cotransporter gene kazachoc (kcc) increase seizure susceptibility in Drosophila

During a critical period in the developing mammalian brain, there is a major switch in the nature of GABAergic transmission from depolarizing and excitatory, the pattern of the neonatal brain, to hyperpolarizing and inhibitory, the pattern of the mature brain. This switch is believed to play a major role in determining neuronal connectivity via activity-dependent mechanisms. The GABAergic developmental switch may also be particularly vulnerable to dysfunction leading to seizure disorders. The developmental GABA switch is mediated primarily by KCC2, a neuronal K+/Cl- cotransporter that determines the intracellular concentration of Cl- and, hence, the reversal potential for GABA. Here, we report that kazachoc (kcc) mutations that reduce the level of the sole K+/Cl- cotransporter in the fruitfly Drosophila melanogaster render flies susceptible to epileptic-like seizures. Drosophila kcc protein is widely expressed in brain neuropil, and its level rises with developmental age. Young kcc mutant flies with low kcc levels display behavioral seizures and demonstrate a reduced threshold for seizures induced by electroconvulsive shock. The kcc mutation enhances a series of other Drosophila epilepsy mutations indicating functional interactions leading to seizure disorder. Both genetic and pharmacological experiments suggest that the increased seizure susceptibility of kcc flies occurs via excitatory GABAergic signaling. The kcc mutants provide an excellent model system in which to investigate how modulation of GABAergic signaling influences neuronal excitability and epileptogenesis.

Acid-base transport by the renal proximal tubule

One of the major tasks of the renal proximal tubule is to secrete acid into the tubule lumen, thereby reabsorbing approximately 80% of the filtered HCO3- as well as generating new HCO3- for regulating blood pH. This review summarizes the cellular and molecular events that underlie four major processes in HCO3- reabsorption. The first is CO2 entry across the apical membrane, which in large part occurs via a gas channel (aquaporin 1) and acidifies the cell. The second process is apical H+ secretion via Na-H exchange and H+ pumping, processes that can be studied using the NH4+ prepulse technique. The third process is the basolateral exit of HCO3- via the electrogenic Na/HCO3 co-transporter, which is the subject of at least 10 mutations that cause severe proximal renal tubule acidosis in humans. The final process is the regulation of overall HCO3- reabsorption by CO2 and HCO3- sensors at the basolateral membrane. Together, these processes ensure that the proximal tubule responds appropriately to acute acid-base disturbances and thereby contributes to the regulation of blood pH.

Understanding the pathogenesis of psoriasis, psoriatic arthritis, and autoimmunity via a fusion of molecular genetics and immunology

The goal of my laboratory is to understand the molecular genetics basis of the inflammatory skin disease psoriasis and associated psoriatic arthritis. In performing these studies my colleagues and I have begun to identify common pathways leading to autoimmunity as well, because some of the defective pathways leading to autoimmunity are the same in different autoimmune diseases. Some of these pathways are involved in determining the activation status of inflammatory cells in the resting state. Other pathways are likely to determine target organ specificity and will be unique to a particular disease. Our approaches rely on genetic studies with cases and families to identify the causative variants, and then functional studies to identify the role of these variants in the predisposition to psoriasis and autoimmunity. The advantage of genetics approaches to understanding diseases such as those of the immune system is that one can identify novel genes and pathways that were not previously suspected as being involved in either the immune system or tolerance.

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

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

WNK1 and OSR1 regulate the Na+, K+, 2Cl- cotransporter in HeLa cells

Oxidative stress-responsive kinase (OSR) 1 and sterile20-related, proline-, alanine-rich kinase (SPAK) are Ste20p-related protein kinases that bind to the sodium, potassium, two chloride cotransporter, NKCC. Here we present evidence that the protein kinase with no lysine [K] (WNK) 1 regulates OSR1, SPAK, and NKCC activities. OSR1 exists in a complex with WNK1 in cells, is activated by recombinant WNK1 in vitro, and is phosphorylated in a WNK1-dependent manner in cells. Depletion of WNK1 from HeLa cells by using small interfering RNA reduces OSR1 kinase activity. In addition, depletion of either WNK1 or OSR1 reduces NKCC activity, indicating that WNK1 and OSR1 are both required for NKCC function. OSR1 and SPAK are likely links between WNK1 and NKCC in a pathway that contributes to volume regulation and blood pressure homeostasis in mammals.

Identification of key functional domains in the C terminus of the K+-Cl- cotransporters

The K+-Cl- cotransporter (KCC) isoforms constitute a functionally heterogeneous group of ion carriers. Emerging evidence suggests that the C terminus (Ct) of these proteins is important in conveying isoform-specific traits and that it may harbor interacting sites for 4beta-phorbol 12-myristate 13-acetate (PMA)-induced effectors. In this study, we have generated KCC2-KCC4 chimeras to identify key functional domains in the Ct of these carriers and single point mutations to determine whether canonical protein kinase C sites underlie KCC2-specific behaviors. Functional characterization of wild-type (wt) and mutant carriers in Xenopus laevis oocytes showed for the first time that the KCCs do not exhibit similar sensitivities to changes in osmolality and that this distinguishing feature as well as differences in transport activity under both hypotonic and isotonic conditions are in part determined by the residue composition of the distal Ct. At the same time, several mutations in this domain and in the proximal Ct of the KCCs were found to generate allosteric-like effects, suggesting that the regions analyzed are important in defining conformational ensembles and that isoform-specific structural configurations could thus account for variant functional traits as well. Characterization of the other mutants in this work showed that KCC2 is not inhibited by PMA through phosphorylation of its canonical protein kinase C sites. Intriguingly, however, the substitutions N728S and S940A were seen to alter the PMA effect paradoxically, suggesting again that allosteric changes in the Ct are important determinants of transport activity and, furthermore, that the structural configuration of this domain can convey specific functional traits by defining the accessibility of cotransporter sites to regulatory intermediates such as PMA-induced effectors.

WNK3 bypasses the tonicity requirement for K-Cl cotransporter activation via a phosphatase-dependent pathway

SLC12A cation/Cl- cotransporters are mutated in human disease, are targets of diuretics, and are collectively involved in the regulation of cell volume, neuronal excitability, and blood pressure. This gene family has two major branches with different physiological functions and inverse regulation: K-Cl cotransporters (KCC1-KCC4) mediate cellular Cl- efflux, are inhibited by phosphorylation, and are activated by dephosphorylation; Na-(K)-Cl cotransporters (NCC and NKCC1/2) mediate cellular Cl- influx and are activated by phosphorylation. A single kinase/phosphatase pathway is thought to coordinate the activities of these cotransporters in a given cell; however, the mechanisms involved are as yet unknown. We previously demonstrated that WNK3, a paralog of serine-threonine kinases mutated in hereditary hypertension, is coexpressed with several cation/Cl- cotransporters and regulates their activity. Here, we show that WNK3 completely prevents the cell swelling-induced activation of KCC1-KCC4 in Xenopus oocytes. In contrast, catalytically inactive WNK3 abolishes the cell shrinkage-induced inhibition of KCC1-KCC4, resulting in a >100-fold stimulation of K-Cl cotransport during conditions in which transport is normally inactive. This activation is completely abolished by calyculin A and cyclosporine A, inhibitors of protein phosphatase 1 and 2B, respectively. Wild-type WNK3 activates Na-(K)-Cl cotransporters by increasing their phosphorylation, and catalytically inactive kinase inhibits Na-(K)-Cl cotransporters by decreasing their phosphorylation, such that our data suggest that WNK3 is a crucial component of the kinase/phosphatase signaling pathway that coordinately regulates the Cl- influx and efflux branches of the SLC12A cotransporter family.

A comprehensive description of the transcriptome of the hypothalamoneurohypophyseal system in euhydrated and dehydrated rats

The hypothalamoneurohypophyseal system (HNS) consists of the large peptidergic magnocellular neurons of the supraoptic hypo thalamic nucleus (SON) and the paraventricular hypothalamic nucleus (PVN), the axons of which course through the internal zone of the median eminence and terminate at blood capillaries of the posterior lobe of the pituitary gland. The HNS is a specialized brain neurosecretory apparatus responsible for the production of the antidiuretic peptide hormone vasopressin (VP). VP maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of VP from magnocellular neuron axon terminals in the posterior pituitary, which is accompanied by a plethora of changes in the morphology, electrophysiological properties, and biosynthetic and secretory activity of the HNS. We wish to understand this functional plasticity in terms of the differential expression of genes. We have therefore used microarrays to comprehensively catalog the genes expressed in the PVN, the SON and the neurointermediate lobe of the pituitary gland of control and dehydrated rats. Comparison of these gene lists has enabled us to identify transcripts that are regulated as a consequence of dehydration as well as RNAs that are enriched in the PVN or the SON. We suggest that these differentially expressed genes represent candidate regulators and effectors of HNS activity and remodeling.

A C-terminal domain in KCC2 confers constitutive K+-Cl- cotransport

The neuron-specific K(+)-Cl(-) cotransporter KCC2 plays a crucial role in determining intracellular chloride activity and thus the neuronal response to gamma-aminobutyric acid and glycine. Of the four KCCs, KCC2 is unique in mediating constitutive K(+)-Cl(-) cotransport under isotonic conditions; the other three KCCs are exclusively swelling-activated, with no isotonic activity. We have utilized a series of chimeric cDNAs to localize the determinant of isotonic transport in KCC2. Two generations of chimeric KCC4-KCC2 cDNAs initially localized this characteristic to within a KCC2-specific expansion of the cytoplasmic C terminus, between residues 929 and 1043. This region of KCC2 is rich in prolines, serines, and charged residues and encompasses two predicted PEST sequences. Substitution of this region in KCC2 with the equivalent sequence of KCC4 resulted in a chimeric KCC that was devoid of isotonic activity, with intact swelling-activated transport. A third generation of chimeras demonstrated that a domain just distal to the PEST sequences confers isotonic transport on KCC4. Mutagenesis of this region revealed that residues 1021-1035 of KCC2 are sufficient for isotonic transport. Swelling-activated K(+)-Cl(-) cotransport is abrogated by calyculin A, whereas isotonic transport mediated by KCC chimeras and KCC2 is completely resistant to this serine-threonine phosphatase inhibitor. In summary, a 15-residue C-terminal domain in KCC2 is both necessary and sufficient for constitutive K(+)-Cl(-) cotransport under isotonic conditions. Furthermore, unlike swelling-activated transport, constitutive K(+)-Cl(-) cotransport mediated by KCC2 is completely independent of serine-threonine phosphatase activity, suggesting that these two modes of transport are activated by distinct mechanisms.

The genetics of psoriasis and autoimmunity

Psoriasis is an inflammatory/autoimmune disease and, as with many autoimmune diseases, is associated with alleles from the major histocompatibility complex (MHC). With psoriasis and autoimmune disease, the penetrance of the MHC-associated alleles is never 100%, even for monozygotic twins. This may be because development requires additional environmental and/or genetic modifiers or requires specific T-cell receptor arrangements. Families segregating single or multilocus susceptibility alleles other than the MHC have also been reported. Overlapping genetic locations of loci for different autoimmune diseases have been known for several years and are starting to reveal common genes or genetic variants. These include genes normally involved in preventing spontaneous T-cell activation or proliferation, immune synapse formation, or cytokine production via pathways such as those mediated by NFkappaB and those involved in thymic selection. Autoimmunity may also involve dysregulation of genes or pathways regulated by the RUNX family of transcription factors. RUNX is involved in hematopoietic cell development, development of T cells in the thymus, chromatin remodeling, and gene silencing. Hence, its effect on cells of the immune system may be due to variable changes in gene expression and could account for variable body surface involvement and waxing and waning of disease.

NH2-terminal heterogeneity in the KCC3 K+-Cl− cotransporter

The SLC12A6 gene encoding the K+-Cl− cotransporter KCC3 is expressed in multiple tissues, including kidney. Here, we report the molecular characterization of several NH2-terminal isoforms of human and mouse KCC3, along with intrarenal localization and functional characterization in Xenopus laevis oocytes. Two major isoforms, KCC3a and KCC3b, are generated by transcriptional initiation 5′ of two distinct first coding exons. Northern blot analysis of mouse tissues indicates that KCC3b expression is particularly robust in the kidney, which also expresses KCC3a. Western blotting of mouse tissue using an exon 3-specific antibody reveals that the kidney is also unique in expressing immunoreactive protein of a lower mass, suggestive evidence that the shorter KCC3b protein predominates in kidney. Immunofluorescence reveals basolateral expression of KCC3 protein along the entire length of the proximal tubule, in both the mouse and rat. Removal of the 15-residue exon 2 by alternative splicing generates the KCC3a-x2M and KCC3b-x2M isoforms; other splicing events at an alternative acceptor site within exon 1a generate the KCC3a-S isoform, which is 60 residues shorter than KCC3a. This variation in sequence of NH2-terminal cytoplasmic domains occurs proximal to a stretch of highly conserved residues and affects the content of putative phosphorylation sites. Kinetic characterization of KCC3a in X. laevis oocytes reveals apparent Kms for Rb+ and Cl− of 10.7 ± 2.5 and 7.3 ± 1.2 mM, respectively, with an anion selectivity of Br− > Cl− > PO4 = I− = SCN− = gluconate. All five NH2-terminal isoforms are activated by cell swelling (hypotonic conditions), with no activity under isotonic conditions. Although the isoforms do not differ in the osmotic set point of swelling activation, this activation is more rapid for the KCC3a-x2M and KCC3a-S proteins. In summary, there is significant NH2-terminal heterogeneity of KCC3, with particularly robust expression of KCC3b in the kidney. Basolateral swelling-activated K+-Cl− cotransport mediated by KCC3 likely functions in cell volume regulation during the transepithelial transport of both salt and solutes by the proximal tubule.

WNK1 Regulates Phosphorylation of Cation-Chloride-coupled Cotransporters via the STE20-related Kinases, SPAK and OSR1

The WNK1 and WNK4 genes have been found to be mutated in some patients with hyperkalemia and hypertension caused by pseudohypoaldosteronism type II. The clue to the pathophysiology of pseudohypoaldosteronism type II was its striking therapeutic response to thiazide diuretics, which are known to block the sodium chloride cotransporter (NCC). Although this suggests a role for WNK1 in hypertension, the precise molecular mechanisms are largely unknown. Here we have shown that WNK1 phosphorylates and regulates the STE20-related kinases, Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress response 1 (OSR1). WNK1 was observed to phosphorylate the evolutionary conserved serine residue located outside the kinase domains of SPAK and OSR1, and mutation of the OSR1 serine residue caused enhanced OSR1 kinase activity. In addition, hypotonic stress was shown to activate SPAK and OSR1 and induce phosphorylation of the conserved OSR1 serine residue, suggesting that WNK1 may be an activator of the SPAK and OSR1 kinases. Moreover, SPAK and OSR1 were found to directly phosphorylate the N-terminal regulatory regions of cation-chloride-coupled cotransporters including NKCC1, NKCC2, and NCC. Phosphorylation of NCC was induced by hypotonic stress in cells. These results suggested that WNK1 and SPAK/OSR1 mediate the hypotonic stress signaling pathway to the transporters and may provide insights into the mechanisms by which WNK1 regulates ion balance.

WNK3 modulates transport of Cl- in and out of cells: implications for control of cell volume and neuronal excitability

The regulation of Cl(-) transport into and out of cells plays a critical role in the maintenance of intracellular volume and the excitability of GABA responsive neurons. The molecular determinants of these seemingly diverse processes are related ion cotransporters: Cl(-) influx is mediated by the Na-K-2Cl cotransporter NKCC1 and Cl(-) efflux via K-Cl cotransporters, KCC1 or KCC2. A Cl(-)/volume-sensitive kinase has been proposed to coordinately regulate these activities via altered phosphorylation of the transporters; phosphorylation activates NKCC1 while inhibiting KCCs, and dephosphorylation has the opposite effects. We show that WNK3, a member of the WNK family of serine-threonine kinases, colocalizes with NKCC1 and KCC1/2 in diverse Cl(-)-transporting epithelia and in neurons expressing ionotropic GABA(A) receptors in the hippocampus, cerebellum, cerebral cortex, and reticular activating system. By expression studies in Xenopus oocytes, we show that kinase-active WNK3 increases Cl(-) influx via NKCC1, and that it inhibits Cl(-) exit through KCC1 and KCC2; kinase-inactive WNK3 has the opposite effects. WNK3's effects are imparted via altered phosphorylation and surface expression of its downstream targets and bypass the normal requirement of altered tonicity for activation of these transporters. Together, these data indicate that WNK3 can modulate the level of intracellular Cl(-) via opposing actions on entry and exit pathways. They suggest that WNK3 is part of the Cl(-)/volume-sensing mechanism necessary for the maintenance of cell volume during osmotic stress and the dynamic modulation of GABA neurotransmission.

Novel insights regarding the operational characteristics and teleological purpose of the renal Na+-K+-Cl2 cotransporter (NKCC2s) splice variants

The absorptive Na(+)-K(+)-Cl(-) cotransporter (NKCC2) is a polytopic protein that forms homooligomeric complexes in the apical membrane of the thick ascending loop of Henle (TAL). It occurs in at least four splice variants (called B, A, F, and AF) that are identical to one another except for a short region in the membrane-associated domain. Although each of these variants exhibits unique functional properties and distributions along the TAL, their teleological purpose and structural organization remain poorly defined. In the current work, we provide additional insight in these regards by showing in mouse that the administration of either furosemide or an H(2)O-rich diet, which are predicted to alter NKCC2 expression in the TAL, exerts differential effects on mRNA levels for the variants, increasing those of A (furosemide) but decreasing those of F and AF (furosemide or H(2)O). Based on a yeast two-hybrid mapping analysis, we also show that the formation of homooligomeric complexes is mediated by two self-interacting domains in the COOH terminus (residues 671 to 816 and 910 to 1098), and that these complexes could probably include more than one type of variant. Taken together, the data reported here suggest that A, F, and AF each play unique roles that are adapted to specific physiological needs, and that the accomplishment of such roles is coordinated through the splicing machinery as well as complex NKCC2-NKCC2 interactions.

A model of prediction system for adverse cardiovascular reactions by calcineurin inhibitors among patients with renal transplants using gene-based single-nucleotide polymorphisms

The application of pharmacogenomic information to diagnostic assays is expected to improve the prediction of drug efficacy and toxicity, leading to appropriate therapeutic regimens for individual patients. Cardiovascular events are common and severe adverse drug reactions (ADRs) among transplant patients treated with calcineurin inhibitors (CNIs). We conducted case-control association studies using 50,947 gene-based single-nucleotide polymorphisms (SNPs) to identify genetic variations that might be associated with cardiovascular risk factors in 72 renal transplant recipients with CNI therapy. The overall incidence of cardiovascular events was 13.9% (10/72) among patients receiving cyclosporine or tacrolimus; arrhythmias in six patients (8.3%), ischemic heart diseases in two patients (2.8%), and heart failure in two patients (2.8%). On the basis of results of the genome-wide association studies, we attempted to establish a scoring system to predict individual risks for cardiovascular toxicity of cyclosporine and tacrolimus. Estimation of the predictive performance was carried out by the use of internal leave-one-out cross-validation test. When we combined arrhythmia, ischemic heart disease and heart failure cases as subjects with a cardiotoxicity phenotype, nine of ten ADR patients and 50 of 62 non-ADR patients were correctly classified into the respective categories using the top eight SNPs. In addition, the proportion of individuals in the control population (n=246) with scores over the cut-off (11.0%) was close to the cardiovascular ADR frequency (8.3%) among renal transplant patients in the previous clinical study. Our results open the possibility that prediction of CNI-induced cardiovascular complications can lead to better prognosis and quality of life among kidney-transplant patients, and to improved immunosuppressive regimens.

FGF8 is required for cell survival at distinct stages of nephrogenesis and for regulation of gene expression in nascent nephrons

During kidney morphogenesis, the formation of nephrons begins when mesenchymal nephron progenitor cells aggregate and transform into epithelial vesicles that elongate and assume an S-shape. Cells in different regions of the S-shaped body subsequently differentiate into the morphologically and functionally distinct segments of the mature nephron. Here, we have used an allelic series of mutations to determine the role of the secreted signaling molecule FGF8 in nephrogenesis. In the absence of FGF8 signaling, nephron formation is initiated, but the nascent nephrons do not express Wnt4or Lim1, and nephrogenesis does not progress to the S-shaped body stage. Furthermore, the nephron progenitor cells that reside in the peripheral zone, the outermost region of the developing kidney, are progressively lost. When FGF8 signaling is severely reduced rather than eliminated, mesenchymal cells differentiate into S-shaped bodies. However, the cells within these structures that normally differentiate into the tubular segments of the mature nephron undergo apoptosis, resulting in the formation of kidneys with severely truncated nephrons consisting of renal corpuscles connected to collecting ducts by an abnormally short tubular segment. Thus, unlike other FGF family members, which regulate growth and branching morphogenesis of the collecting duct system, Fgf8 encodes a factor essential for gene regulation and cell survival at distinct steps in nephrogenesis.

Getting under the skin: the immunogenetics of psoriasis

Psoriasis is a chronic inflammatory disorder of the skin that is mediated by T cells, dendritic cells and inflammatory cytokines. We now understand many of the cellular alterations that underlie this disease, and genomic approaches have recently been used to assess the alterations of gene expression in psoriatic skin lesions. Genetic susceptibility factors that contribute to predisposition to psoriasis are now also being identified. It is hoped that we will soon be able to correlate the cellular pathogenesis that occurs in psoriasis with these genetic factors. In this Review article, we describe what is known about genes that confer increased susceptibility to psoriasis, and we integrate this with what is known about the molecular and cellular mechanisms that occur in other inflammatory and autoimmune disorders.

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

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

Renal tubular transport and the genetic basis of hypertensive disease

Several monogenic hypertensive disorders are caused by genetic mutations leading to the deranged function and/or regulation of renal tubular NaCl transport, such as mutations of the renal epithelial Na+ channel (ENaC) in Liddle syndrome, of the kinase WNK1 (with no K) in Gordon syndrome, and of the mineralocorticoid receptor, or of 11beta-hydroxysteroid dehydrogenase. Moreover, excessive formation of aldosterone in glucocorticoid-remediable hypertension leads to severe hypertension. Conversely, impaired function of the Na+,K+,2Cl- cotransporter (NKCC2), the renal outer medullary K+ channel (ROMK1), and the renal epithelial Cl- channel ClCKb/Barttin causes Bartter syndrome and defective Na+,Cl+ cotransporter (NCCT) Gitelman syndrome, salt-wasting disorders with hypotension. These monogenic disorders are rare, but illustrate the significance of renal tubular transport in blood pressure regulation. There is little doubt, however, that deranged renal salt reabsorption significantly contributes to essential hypertension polymorphisms of several genes participating in the regulation of renal Na+ transport have been shown to be associated with blood pressure and prevalence of hypertension. Two common genes will be discussed in more detail. The first encodes the renal Cl- channel ClCKb. A gain-of-function mutation of ClCKb, increasing channel activity by 7- to 20-fold is found in approximately 20% of unselected Caucasians and 40% of an unselected African population. The second common gene variant (prevalence, 3%-5% in unselected Caucasians), to be discussed in more detail, affects the serum and glucocorticoid inducible kinase SGK1, a kinase upregulated by mineralocorticoids and enhancing the activity of ENaC, ROMK, and Na+/K+ATPase. Both gene variants are associated with slightly increased blood pressure. SGK1 further stimulates the glucose transporter SGLT1, and the SGK1 gene variant correlates, in addition, with increased body mass index.

Mechanisms of bicarbonate secretion in the pancreatic duct

In many species the pancreatic duct epithelium secretes HCO3- ions at a concentration of around 140 mM by a mechanism that is only partially understood. We know that HCO3- uptake at the basolateral membrane is achieved by Na+-HCO3- cotransport and also by a H+-ATPase and Na+/H+ exchanger operating together with carbonic anhydrase. At the apical membrane, the secretion of moderate concentrations of HCO3- can be explained by the parallel activity of a Cl-/HCO3- exchanger and a Cl- conductance, either the cystic fibrosis transmembrane conductance regulator (CFTR) or a Ca2+-activated Cl- channel (CaCC). However, the sustained secretion of HCO3- into a HCO- -rich luminal fluid cannot be explained by conventional Cl-/HCO3- exchange. HCO3- efflux across the apical membrane is an electrogenic process that is facilitated by the depletion of intracellular Cl-, but it remains to be seen whether it is mediated predominantly by CFTR or by an electrogenic SLC26 anion exchanger.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Molecular mechanisms of cerebrospinal fluid production

The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H(2)O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na(+)-K(+)ATPase, K(+) channels and Na(+)-2Cl(-)-K(+) cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl(-)- HCO(3) exchangers, a variety of Na(+) coupled HCO(3)(-) transporters and K(+)-Cl(-) cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K(+) is transported from the CSF to the blood.

Is myosin light-chain phosphorylation a regulatory signal for the osmotic activation of the Na+-K+-2Cl- cotransporter?

Myosin light-chain (MLC) kinase (MLCK)-dependent increase in MLC phosphorylation has been proposed to be a key mediator of the hyperosmotic activation of the Na+-K+-2Cl- cotransporter (NKCC). To address this hypothesis and to assess whether MLC phosphorylation plays a signaling or permissive role in NKCC regulation, we used pharmacological and genetic means to manipulate MLCK, MLC phosphorylation, or myosin ATPase activity and followed the impact of these alterations on the hypertonic stimulation of NKCC in porcine kidney tubular LLC-PK1 epithelial cells. We found that the MLCK inhibitor ML-7 suppressed NKCC activity independently of MLC phosphorylation. Notably, ML-7 reduced both basal and hypertonically stimulated NKCC activity without influencing MLC phosphorylation under these conditions, and it inhibited NKCC activation by Cl- depletion, a treatment that did not increase MLC phosphorylation. Furthermore, prevention of the osmotically induced increase in MLC phosphorylation by viral induction of cells with a nonphosphorylatable, dominant negative MLC mutant (AA-MLC) did not affect the hypertonic activation of NKCC. Conversely, a constitutively active MLC mutant (DD-MLC) that mimics the diphosphorylated form neither stimulated isotonic nor potentiated hypertonic NKCC activity. Furthermore, a depolarization-induced increase in endogenous MLC phosphorylation failed to activate NKCC. However, complete abolition of basal MLC phosphorylation by K252a or the inhibition of myosin ATPase by blebbistatin significantly reduced the osmotic stimulation of NKCC without suppressing its basal or Cl- depletion-triggered activity. These results indicate that an increase in MLC phosphorylation is neither a sufficient nor a necessary signal to stimulate NKCC in tubular cells. However, basal myosin activity plays a permissive role in the optimal osmotic responsiveness of NKCC.

G2736A polymorphism of thiazide-sensitive Na-Cl cotransporter gene predisposes to hypertension in young women

OBJECTIVE

The thiazide-sensitive Na-Cl cotransporter (TSC) is located in the distal renal tubules. Several mutations of the TSC gene cause Gitelman's syndrome, which is an autosomal recessive disease characterized by low blood pressure and hypokalemia. Recently, an association between TSC gene polymorphisms (Arg904Gln, G2736A; Thr465Thr, C1420T; Gly264Ala, G816C) and essential hypertension has been reported in Sweden. We examined the genetic involvement of the TSC gene in essential hypertension in Japanese.

DESIGN

Participants were recruited from outpatients of Osaka University Hospital. We investigated 386 hypertensive and 371 normotensive subjects.

METHODS

Genotypes of TSC polymorphisms (G2736A, C1420T, G816C) were determined by the TaqMan polymerase chain reaction (PCR) method, and statistical significance was examined using JMP 5.0.1J (SAS Institute Inc., Cary, North Carolina, USA). The allele frequency of A2736 and T1420 was 6.0 and 3.0%, respectively, whereas we could not detect the G816C polymorphism in this study. Only the G2736A polymorphism was significantly associated with the prevalence of hypertension (P <0.04), and the estimated odds ratio was 1.8 (95% confidence interval, 1.1-3.0) in A2736 allele carriers. The odds ratio for hypertension in A2736 carriers was increased to 2.2 (1.1-4.9) in women (n=413), and further to 3.3 (1.4-8.0) in women with early onset of hypertension (< or = 50 years old). In addition, all subjects with the homozygous A2736 allele in this study (n=2) and the Swedish study (n=5) were hypertensive.

CONCLUSION

G2736A polymorphism of the TSC gene is a genetic predisposing factor for essential hypertension in Japanese women.

Role of cation-chloride-cotransporters (CCC) in pain and hyperalgesia

The importance of the GABAergic system in spinal nociceptive processing has long been appreciated but we have only recently begun to understand how this system is modulated by the regulation of anion gradients. In neuronal tissues, cation-chloride cotransporters regulate Cl- homeostasis and the activity and/or expression of these transporters has important implications for the direction and magnitude of anion flow through GABA-A channels. Here we review recent evidence that two cation-chloride cotransporters, NKCC1 and KCC2 are involved in pain and enhanced nociception. On the one hand, NKCC1 activity is upregulated in primary afferents following an inflammatory insult and this produces excessive GABAergic depolarization in primary afferents leading to cross excitation between low and high threshold afferents. On the other hand, KCC2 expression is reduced in dorsal horn neurons following peripheral nerve injury resulting in a loss of GABA-/glycinergic inhibitory tone and, in some cases, inverting its action into net excitation. Pharmacological targeting of these cation chloride cotransporters to restore normal GABA-/glycinergic transmission in the spinal cord represents an entirely novel approach to the development of analgesics.

Expression of K+-Cl- cotransporters in the alpha-cells of rat endocrine pancreas

The expression of K+-Cl- cotransporters (KCC) was examined in pancreatic islet cells. mRNA for KCC1, KCC3a, KCC3b and KCC4 were identified by RT-PCR in islets isolated from rat pancreas. In immunocytochemical studies, an antibody specific for KCC1 and KCC4 revealed the expression of KCC protein in alpha-cells, but not pancreatic beta-cells nor delta-cells. A second antibody which does not discriminate among KCC isoforms identified KCC expression in both alpha-cell and beta-cells. Exposure of isolated alpha-cells to hypotonic solutions caused cell swelling was followed by a regulatory volume decrease (RVD). The RVD was blocked by 10 microM [dihydroindenyl-oxy] alkanoic acid (DIOA; a KCC inhibitor). DIOA was without effect on the RVD in beta-cells. NEM (0.2 mM), a KCC activator, caused a significant decrease of alpha-cell volume, which was completely inhibited by DIOA. By contrast, NEM had no effects on beta-cell volume. In conclusion, KCCs are expressed in pancreatic alpha-cells and beta-cells. However, they make a significant contribution to volume homeostasis only in alpha-cells.

A Na+/Cl- -coupled GABA transporter, GAT-1, from Caenorhabditis elegans: structural and functional features, specific expression in GABA-ergic neurons, and involvement in muscle function

GABA functions as an inhibitory neurotransmitter in body muscles and as an excitatory neurotransmitter in enteric muscles in Caenorhabditis elegans. Whereas many of the components of the GABA-ergic neurotransmission in this organism have been identified at the molecular and functional levels, no transporter specific for this neurotransmitter has been identified to date. Here we report on the cloning and functional characterization of a GABA transporter from C. elegans (ceGAT-1) and on the functional relevance of the transporter to the biology of body muscles and enteric muscles. ceGAT-1 is coded by snf-11 gene, a member of the sodium-dependent neurotransmitter symporter gene family in C. elegans. The cloned ceGAT-1 functions as a Na(+)/Cl(-)-coupled high-affinity transporter selective for GABA with a K(t) of approximately 15 microm. The Na(+):Cl(-):GABA stoichiometry for ceGAT-1-mediated transport process is 2:1:1. The transport process is electrogenic as evidenced from GABA-induced inward currents in Xenopus laevis oocytes that express ceGAT-1 heterologously. The transporter is expressed exclusively in GABA-ergic neurons and in two other additional neurons. We also investigated the functional relevance of ceGAT-1 to the biology of body muscles and enteric muscles by ceGAT-1-specific RNA interference (RNAi) in rrf-3 mutant, a strain of C. elegans in which neurons are not refractory to RNAi as in the wild type strain. The down-regulation of ceGAT-1 by RNAi leads to an interesting phenotype associated with altered function of body muscles (as evident from changes in thrashing frequency) and enteric muscles (as evident from the rates of defecation failure) and also with altered sensitivity to aldicarb-induced paralysis. These findings provide unequivocal evidence for a modulatory role of GABA and ceGAT-1 in the biology of cholinergic neurons and in the function of body muscles and enteric muscles in this organism.

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

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

Identification of MEF2-regulated genes during muscle differentiation

Although a great deal has been elucidated concerning the mechanisms regulating muscle differentiation, little is known about transcription factor-specific gene regulation. Our understanding of the genetic mechanisms regulating cell differentiation is quite limited. Much of what has been defined centers on regulatory signaling cascades and transcription factors. Surprisingly few studies have investigated the association of genes with specific transcription factors. To address these issues, we have utilized a method coupling chromatin immunoprecipitation and CpG microarrays to characterize the genes associated with MEF2 in differentiating C(2)C(12) cells. Results demonstrated a defined binding pattern over the course of differentiation. Filtered data demonstrated 9 clones to be elevated at 0 h, 792 at 6 h, 163 by 1 day, and 316 at 3 days. Using unbiased selection parameters, we selected a subset of 291 prospective candidates. Clones were sequenced and filtered for removal of redundancy between clones and for the presence of repetitive elements. We were able to place 50 of these on the mouse genome, and 20 were found to be located near well-annotated genes. From this list, previously undefined associations with MEF2 were discovered. Many of these genes represent proteins involved in neurogenesis, neuromuscular junctions, signaling and metabolism. The remaining clones include many full-length cDNA and represent novel gene targets. The results of this study provides for the first time, a unique look at gene regulation at the level of transcription factor binding in differentiating muscle.

Self-interacting domains in the C terminus of a cation-Cl- cotransporter described for the first time

The first isoform of the Na+-K+-Cl- cotransporter (NKCC1), a widely distributed member of the cation-Cl- cotransporter superfamily, plays key roles in many physiological processes by regulating the ion and water content of animal cells and by sustaining electrolyte secretion across various epithelia. Indirect studies have led to the prediction that NKCC1 operates as a dimer assembled through binding domains that are distal to the amino portion of the carrier. In this study, evidence is presented that NKCC1 possesses self-interacting properties that result in the formation of a large complex between the proximal and the distal segment of the cytosolic C terminus. Elaborate mapping studies of these segments showed that the contact sites are dispersed along the entire C terminus, and they also led to the identification of a critical interacting residue that belongs to a putative forkhead-associated binding domain. In conjunction with previous findings, our results indicate that the uncovered interacting domains are probably a major determinant of the NKCC1 conformational landscape and assembly into a high order structure. A model is proposed in which the carrier could alternate between monomeric and homo-oligomeric units via chemical- or ligand-dependent changes in conformational dynamics.

Localization of the kcc4 potassium–chloride cotransporter in the nervous system

Potassium-chloride cotransporters (KCCs) collectively play a crucial role in the function and development of both the peripheral and central nervous systems. KCC4 is perhaps the least abundant KCC in the adult mammalian brain, where its localization is unknown. In the embryonic brain, KCC4 mRNA is found in the periventricular zone, cranial nerves and choroid plexus [Eur J Neurosci 16 (2002) 2358]. To investigate the distribution of KCC4 protein in the nervous system we developed a rabbit polyclonal antibody directed against a short N-terminal peptide. Western blot analysis of brain microsomal protein using purified antibody revealed the presence of a band at approximately 145 kDa, consistent with the size of a glycosylated K-Cl cotransporter. Western blot analysis of brain, spinal cord and peripheral nerves revealed high expression levels in peripheral nerves and spinal cord, with low levels in whole brain. Within the brain, the cerebral cortex, hippocampus, and cerebellum revealed minimal KCC4 expression, whereas midbrain and brainstem demonstrated higher levels. In the adult mouse brain, KCC4 staining was observed on the apical membrane of choroid plexus epithelial cells as well as in cranial nerves. All other brain structures, e.g. cortex, hippocampus, cerebellum showed no KCC4 immunoreactivity, suggesting very low or absent expression of the cotransporter in these regions. Co-staining of KCC4 with anti-MAP2, GFAP and CNPase revealed that KCC4 is expressed in peripheral neurons. Thus, KCC4 is expressed on the apical membrane of the choroid plexus, where it likely participates to K(+) reabsorption. KCC4 is also expressed in peripheral neurons, where its function remains to be determined.