Transmembrane topology of the secretory Na+-K+-2Cl- cotransporter NKCC1 studied by in vitro translation

Chloride-Dependent Cation Transport via SLC12 Carriers at Atomic Resolution

The SLC12 family of genes encodes electroneutral Cl--dependent cation transporters (i.e., Na-Cl, K-Cl, Na-K-2Cl cotransporters), which play significant roles in maintaining cell and body homeostasis. Recent resolution of their structures at the atomic level provides a new understanding how these transporters operate in health and disease and how they are targeted for therapeutic intervention. Overall, the SLC12 transporter cryo-EM structures confirm some key features established by traditional biochemical and molecular methods, such as the presence of 12 transmembrane domains and the formation of a functional dimer. Study of these structures also uncovers previously unknown features, such as the presence of strategic salt bridges that explain why transporters are stabilized in specific conformations. The cryo-EM structures show similarities with other transport protein structures, especially regarding the position of the cations. The structures also pose challenging questions regarding the number of ions bound and the strict electroneutrality that is conventional understanding.

The biogenesis of potassium transporters: implications of disease-associated mutations

The concentration of intracellular and extracellular potassium is tightly regulated due to the action of various ion transporters, channels, and pumps, which reside primarily in the kidney. Yet, potassium transporters and cotransporters play vital roles in all organs and cell types. Perhaps not surprisingly, defects in the biogenesis, function, and/or regulation of these proteins are linked to range of catastrophic human diseases, but to date, few drugs have been approved to treat these maladies. In this review, we discuss the structure, function, and activity of a group of potassium-chloride cotransporters, the KCCs, as well as the related sodium-potassium-chloride cotransporters, the NKCCs. Diseases associated with each of the four KCCs and two NKCCs are also discussed. Particular emphasis is placed on how these complex membrane proteins fold and mature in the endoplasmic reticulum, how non-native forms of the cotransporters are destroyed in the cell, and which cellular factors oversee their maturation and transport to the cell surface. When known, we also outline how the levels and activities of each cotransporter are regulated. Open questions in the field and avenues for future investigations are further outlined.

Both chloride-binding sites are required for KCC2-mediated transport

The K+-Cl- cotransporter 2 (KCC2) plays an important role in inhibitory neurotransmission, and its impairment is associated with neurological and psychiatric disorders, including epilepsy, schizophrenia, and autism. Although KCCs transport K+ and Cl- in a 1:1 stoichiometry, two Cl- coordination sites were indicated via cryo-EM. In a comprehensive analysis, we analyzed the consequences of point mutations of residues coordinating Cl- in Cl1 and Cl2. Individual mutations of residues in Cl1 and Cl2 reduce or abolish KCC2WT function, indicating a crucial role of both Cl- coordination sites for KCC2 function. Structural changes in the extracellular loop 2 by inserting a 3xHA tag switches the K+ coordination site to another position. To investigate, whether the extension of the extracellular loop 2 with the 3xHA tag also affects the coordination of the two Cl- coordination sites, we carried out the analogous experiments for both Cl- coordinating sites in the KCC2HA construct. These analyses showed that most of the individual mutation of residues in Cl1 and Cl2 in the KCC2HA construct reduces or abolishes KCC2 function, indicating that the coordination of Cl- remains at the same position. However, the coupling of K+ and Cl- in Cl1 is still apparent in the KCC2HA construct, indicating a mutual dependence of both ions. In addition, the coordination residue Tyr569 in Cl2 shifted in KCC2HA. Thus, conformational changes in the extracellular domain affect K+ and Cl--binding sites. However, the effect on the Cl--binding sites is subtler.

SLC12A cryo-EM: analysis of relevant ion binding sites, structural domains, and amino acids

The solute carrier family 12A (SLC12A) superfamily of membrane transporters modulates the movement of cations coupled with chloride across the membrane. In doing so, these cotransporters are involved in numerous aspects of human physiology: cell volume regulation, ion homeostasis, blood pressure regulation, and neurological action potential via intracellular chloride concentration modulation. Their physiological characterization has been largely studied; however, understanding the mechanics of their function and the relevance of structural domains or specific amino acids has been a pending task. In recent years, single-particle cryogenic electron microscopy (cryo-EM) has been successfully applied to members of the SLC12A family including all K+:Cl- cotransporters (KCCs), Na+:K+:2Cl- cotransporter NKCC1, and recently Na+:Cl- cotransporter (NCC); revealing structural elements that play key roles in their function. The present review analyzes the data provided by these cryo-EM reports focusing on structural domains and specific amino acids involved in ion binding, domain interactions, and other important SCL12A structural elements. A comparison of cryo-EM data from NKCC1 and KCCs is presented in the light of the two recent NCC cryo-EM studies, to propose insight into structural elements that might also be found in NCC and are necessary for its proper function. In the final sections, the importance of key coordination residues for substrate specificity and their implication on various pathophysiological conditions and genetic disorders is reviewed, as this could provide the basis to correlate structural elements with the development of novel and selective treatments, as well as mechanistic insight into the function and regulation of cation-coupled chloride cotransporters (CCCs).

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na+ -K+ -Cl- cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na+ , K+ , and Cl- ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na+ , Cl- , Ca2+ , Mg2+ , and HCO3- loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na+ -K+ -Cl- cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Structural changes in the extracellular loop 2 of the murine KCC2 potassium chloride cotransporter modulate ion transport

K+-Cl- cotransporters (KCCs) play important roles in physiological processes such as inhibitory neurotransmission and cell-volume regulation. KCCs exhibit significant variations in K+ affinities, yet recent atomic structures demonstrated that K+- and Cl--binding sites are highly conserved, raising the question of whether additional structural elements may contribute to ion coordination. The termini and the large extracellular domain (ECD) of KCCs exhibit only low sequence identity and were already discussed as modulators of transport activity. Here, we used the extracellular loop 2 (EL2) that links transmembrane helices (TMs) 3 and 4, as a mechanism to modulate ECD folding. We compared consequences of point mutations in the K+-binding site on the function of WT KCC2 and in a KCC2 variant, in which EL2 was structurally altered by insertion of a IFYPYDVPDYAGYPYDVPDYAGSYPYDVPDYAAHAAA (3xHA) tag (36 amino acids). In WT KCC2, individual mutations of five residues in the K+-binding site resulted in a 2- to 3-fold decreased transport rate. However, the same mutations in the KCC2 variant with EL2 structurally altered by insertion of a 3xHA tag had no effect on transport activity. Homology models of mouse KCC2 with the 3xHA tag inserted into EL2 using ab initio prediction were generated. The models suggest subtle conformational changes occur in the ECD upon EL2 modification. These data suggest that a conformational change in the ECD, for example, by interaction with EL2, might be an elegant way to modulate the K+ affinity of the different isoforms in the KCC subfamily.

NKCC1, an Elusive Molecular Target in Brain Development: Making Sense of the Existing Data

Ionotropic GABA transmission is mediated by anion (mainly Cl⁻)-permeable GABA_A receptors (GABA_ARs). In immature neurons, GABA exerts depolarizing and sometimes functionally excitatory actions, based on active uptake of Cl⁻ by the Na-K-2Cl cotransporter NKCC1. While functional evidence firmly shows NKCC1-mediated ion transport in immature and diseased neurons, molecular detection of NKCC1 in the brain has turned out to be extremely difficult. In this review, we describe the highly inconsistent data that are available on the cell type-specific expression patterns of the NKCC1 mRNA and protein in the CNS. We discuss the major technical caveats, including a lack of knock-out-controlled immunohistochemistry in the forebrain, possible effects of alternative splicing on the binding of antibodies and RNA probes, and the wide expression of NKCC1 in different cell types, which make whole-tissue analyses of NKCC1 useless for studying its neuronal expression. We also review novel single-cell RNAseq data showing that most of the NKCC1 in the adult CNS may, in fact, be expressed in non-neuronal cells, especially in glia. As future directions, we suggest single-cell NKCC1 mRNA and protein analyses and the use of genetically tagged endogenous proteins or systematically designed novel antibodies, together with proper knock-out controls, for the visualization of endogenous NKCC1 in distinct brain cell types and their subcellular compartments.

Multiple Facets and Roles of Na+-K+-Cl−Cotransport: Mechanisms and Therapeutic Implications

The Na+-K+-Cl−cotransporters play key physiological and pathophysiological roles by regulating the membrane potential of many cell types and the movement of fluid across a variety of epithelial or endothelial structures. As such, they should soon become invaluable targets for the treatment of various disorders including pain, epilepsy, brain edema, and hypertension. This review highlights the nature of these roles, the mechanisms at play, and the unresolved issues in the field.

Oncotic Cell Death in Stroke

Oncotic cell death or oncosis represents a major mechanism of cell death in ischaemic stroke, occurring in many central nervous system (CNS) cell types including neurons, glia and vascular endothelial cells. In stroke, energy depletion causes ionic pump failure and disrupts ionic homeostasis. Imbalance between the influx of Na⁺ and Cl^- ions and the efflux of K⁺ ions through various channel proteins and transporters creates a transmembrane osmotic gradient, with ensuing movement of water into the cells, resulting in cell swelling and oncosis. Oncosis is a key mediator of cerebral oedema in ischaemic stroke, contributing directly through cytotoxic oedema, and indirectly through vasogenic oedema by causing vascular endothelial cell death and disruption of the blood-brain barrier (BBB). Hence, inhibition of uncontrolled ionic flux represents a novel and powerful strategy in achieving neuroprotection in stroke. In this review, we provide an overview of oncotic cell death in the pathology of stroke. Importantly, we summarised the therapeutically significant pathways of water, Na⁺, Cl^- and K⁺ movement across cell membranes in the CNS and their respective roles in the pathobiology of stroke.

Molecular insights into the normal operation, regulation, and multisystemic roles of K+-Cl- cotransporter 3 (KCC3)

Long before the molecular identity of the Na+-dependent K+-Cl- cotransporters was uncovered in the mid-nineties, a Na+-independent K+-Cl- cotransport system was also known to exist. It was initially observed in sheep and goat red blood cells where it was shown to be ouabain-insensitive and to increase in the presence of N-ethylmaleimide (NEM). After it was established between the early and mid-nineties, the expressed sequence tag (EST) databank was found to include a sequence that was highly homologous to those of the Na+-dependent K+-Cl- cotransporters. This sequence was eventually found to code for the Na+-independent K+-Cl- cotransport function that was described in red blood cells several years before. It was termed KCC1 and led to the discovery of three isoforms called KCC2, KCC3, and KCC4. Since then, it has become obvious that each one of these isoforms exhibits unique patterns of distribution and fulfills distinct physiological roles. Among them, KCC3 has been the subject of great attention in view of its important role in the nervous system and its association with a rare hereditary sensorimotor neuropathy (called Andermann syndrome) that affects many individuals in Quebec province (Canada). It was also found to play important roles in the cardiovascular system, the organ of Corti, and circulating blood cells. As will be seen in this review, however, there are still a number of uncertainties regarding the transport properties, structural organization, and regulation of KCC3. The same is true regarding the mechanisms by which KCC3 accomplishes its numerous functions in animal cells.

Molecular features and physiological roles of K+-Cl- cotransporter 4 (KCC4)

A K⁺-Cl^- cotransport system was documented for the first time during the mid-seventies in sheep and goat red blood cells. It was then described as a Na⁺-independent and ouabain-insensitive ion carrier that could be stimulated by cell swelling and N-ethylmaleimide (NEM), a thiol-reacting agent. Twenty years later, this system was found to be dispensed by four different isoforms in animal cells. The first one was identified in the expressed sequence tag (EST) database by Gillen et al. based on the assumption that it would be homologous to the Na⁺-dependent K⁺-Cl^- cotransport system for which the molecular identity had already been uncovered. Not long after, the three other isoforms were once again identified in the EST databank. Among those, KCC4 has generated much interest a few years ago when it was shown to sustain distal renal acidification and hearing development in mouse. As will be seen in this review, many additional roles were ascribed to this isoform, in keeping with its wide distribution in animal species. However, some of them have still not been confirmed through animal models of gene inactivation or overexpression. Along the same line, considerable knowledge has been acquired on the mechanisms by which KCC4 is regulated and the environmental cues to which it is sensitive. Yet, it is inferred to some extent from historical views and extrapolations.

Molecular cloning and biochemical characterization of two cation chloride cotransporter subfamily members of Hydra vulgaris

Cation Chloride Cotransporters (CCCs) comprise secondary active membrane proteins mainly mediating the symport of cations (Na⁺, K⁺) coupled with chloride (Cl⁻). They are divided into K⁺-Cl⁻ outward transporters (KCCs), the Na⁺-K⁺-Cl⁻ (NKCCs) and Na⁺-Cl⁻ (NCCs) inward transporters, the cation chloride cotransporter interacting protein CIP1, and the polyamine transporter CCC9. KCCs and N(K)CCs are established in the genome since eukaryotes and metazoans, respectively. Most of the physiological and functional data were obtained from vertebrate species. To get insights into the basal functional properties of KCCs and N(K)CCs in the metazoan lineage, we cloned and characterized KCC and N(K)CC from the cnidarian Hydra vulgaris. HvKCC is composed of 1,032 amino-acid residues. Functional analyses revealed that hvKCC mediates a Na⁺-independent, Cl⁻ and K⁺ (Tl⁺)-dependent cotransport. The classification of hvKCC as a functional K-Cl cotransporter is furthermore supported by phylogenetic analyses and a similar structural organization. Interestingly, recently obtained physiological analyses indicate a role of cnidarian KCCs in hyposmotic volume regulation of nematocytes. HvN(K)CC is composed of 965 amino-acid residues. Phylogenetic analyses and structural organization suggest that hvN(K)CC is a member of the N(K)CC subfamily. However, no inorganic ion cotransport function could be detected using different buffer conditions. Thus, hvN(K)CC is a N(K)CC subfamily member without a detectable inorganic ion cotransporter function. Taken together, the data identify two non-bilaterian solute carrier 12 (SLC12) gene family members, thereby paving the way for a better understanding of the evolutionary paths of this important cotransporter family.

Chloride co-transporters as possible therapeutic targets for stroke

Stroke is one of the major causes of death and disability worldwide. The major type of stroke is an ischaemic one, which is caused by a blockage that interrupts blood flow to the brain. There are currently very few pharmacological strategies to reduce the damage and social burden triggered by this pathology. The harm caused by the interruption of blood flow to the brain unfolds in the subsequent hours and days, so it is critical to identify new therapeutic targets that could reduce neuronal death associated with the spread of the damage. Here, we review some of the key molecular mechanisms involved in the progression of neuronal death, focusing on some new and promising studies. In particular, we focus on the potential of the chloride co-transporter (CCC) family of proteins, mediators of the GABAergic response, both during the early and later stages of stroke, to promote neuroprotection and recovery. Different studies of CCCs during the chronic and recovery phases post-stroke reveal the importance of timing when considering CCCs as potential neuroprotective and/or neuromodulator targets. The molecular regulatory mechanisms of the two main neuronal CCCs, NKCC1 and KCC2, are further discussed as an indirect approach for promoting neuroprotection and neurorehabilitation following an ischaemic insult. Finally, we mention the likely importance of combining different strategies in order to achieve more effective therapies.

Impact of Hybrid and Complex N-Glycans on Cell Surface Targeting of the Endogenous Chloride Cotransporter Slc12a2

The Na⁺K⁺2Cl⁻ cotransporter-1 (Slc12a2, NKCC1) is widely distributed and involved in cell volume/ion regulation. Functional NKCC1 locates in the plasma membrane of all cells studied, particularly in the basolateral membrane of most polarized cells. Although the mechanisms involved in plasma membrane sorting of NKCC1 are poorly understood, it is assumed that N-glycosylation is necessary. Here, we characterize expression, N-glycosylation, and distribution of NKCC1 in COS7 cells. We show that ~25% of NKCC1 is complex N-glycosylated whereas the rest of it corresponds to core/high-mannose and hybrid-type N-glycosylated forms. Further, ~10% of NKCC1 reaches the plasma membrane, mostly as core/high-mannose type, whereas ~90% of NKCC1 is distributed in defined intracellular compartments. In addition, inhibition of the first step of N-glycan biosynthesis with tunicamycin decreases total and plasma membrane located NKCC1 resulting in almost undetectable cotransport function. Moreover, inhibition of N-glycan maturation with swainsonine or kifunensine increased core/hybrid-type NKCC1 expression but eliminated plasma membrane complex N-glycosylated NKCC1 and transport function. Together, these results suggest that (i) NKCC1 is delivered to the plasma membrane of COS7 cells independently of its N-glycan nature, (ii) most of NKCC1 in the plasma membrane is core/hybrid-type N-glycosylated, and (iii) the minimal proportion of complex N-glycosylated NKCC1 is functionally active.

Molecular and evolutionary insights into the structural organization of cation chloride cotransporters

Cation chloride cotransporters (CCC) play an essential role for neuronal chloride homeostasis. K(+)-Cl(-) cotransporter (KCC2), is the principal Cl(-)-extruder, whereas Na(+)-K(+)-Cl(-) cotransporter (NKCC1), is the major Cl(-)-uptake mechanism in many neurons. As a consequence, the action of the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine strongly depend on the activity of these two transporters. Knowledge of the mechanisms involved in ion transport and regulation is thus of great importance to better understand normal and disturbed brain function. Although no overall 3-dimensional crystal structures are yet available, recent molecular and phylogenetic studies and modeling have provided new and exciting insights into structure-function relationships of CCC. Here, we will summarize our current knowledge of the gross structural organization of the proteins, their functional domains, ion binding and translocation sites, and the established role of individual amino acids (aa). A major focus will be laid on the delineation of shared and distinct organizational principles between KCC2 and NKCC1. Exploiting the richness of recently generated genome data across the tree of life, we will also explore the molecular evolution of these features.

Physiology and pathophysiology of SLC12A1/2 transporters

The electroneutral Na(+)-K(+)-Cl(-) cotransporters NKCC1 (encoded by the SLC12A2 gene) and NKCC2 (SLC12A1 gene) belong to the Na(+)-dependent subgroup of solute carrier 12 (SLC12) family of transporters. They mediate the electroneutral movement of Na(+) and K(+), tightly coupled to the movement of Cl(-) across cell membranes. As they use the energy of the ion gradients generated by the Na(+)/K(+)-ATPase to transport Na(+), K(+), and Cl(-) from the outside to the inside of a cell, they are considered secondary active transport mechanisms. NKCC-mediated transport occurs in a 1Na(+), 1K(+), and 2Cl(-) ratio, although NKCC1 has been shown to sometimes mediate partial reactions. Both transporters are blocked by bumetanide and furosemide, drugs which are commonly used in clinical medicine. NKCC2 is the molecular target of loop diuretics as it is expressed on the apical membrane of thick ascending limb of Henle epithelial cells, where it mediates NaCl reabsorption. NKCC1, in contrast, is found on the basolateral membrane of Cl(-) secretory epithelial cells, as well as in a variety of non-epithelial cells, where it mediates cell volume regulation and participates in Cl(-) homeostasis. Following their molecular identification two decades ago, much has been learned about their biophysical properties, their mode of operation, their regulation by kinases and phosphatases, and their physiological relevance. However, despite this tremendous amount of new information, there are still so many gaps in our knowledge. This review summarizes information that constitutes consensus in the field, but it also discusses current points of controversy and highlights many unanswered questions.

Vhc1, a novel transporter belonging to the family of electroneutral cation-Cl(-) cotransporters, participates in the regulation of cation content and morphology of Saccharomyces cerevisiae vacuoles

Cation-Cl(-) cotransporters (CCCs) are integral membrane proteins which catalyze the coordinated symport of Cl(-) with Na(+) and/or K(+) ions in plant and mammalian cells. Here we describe the first Saccharomyces cerevisiae CCC protein, encoded by the YBR235w open reading frame. Subcellular localization studies showed that this yeast CCC is targeted to the vacuolar membrane. Deletion of the YBR235w gene in a salt-sensitive strain (lacking the plasma-membrane cation exporters) resulted in an increased sensitivity to high KCl, altered vacuolar morphology control and decreased survival upon hyperosmotic shock. In addition, deletion of the YBR235w gene in a mutant strain deficient in K(+) uptake produced a significant growth advantage over the parental strain under K(+)-limiting conditions, and a hypersensitivity to the exogenous K(+)/H(+) exchanger nigericin. These results strongly suggest that we have identified a novel yeast vacuolar ion transporter mediating a K(+)-Cl(-) cotransport and playing a role in vacuolar osmoregulation. Considering its identified function, we propose to refer to the yeast YBR235w gene as VHC1 (vacuolar protein homologous to CCC family 1).

Differences in the large extracellular loop between the K(+)-Cl(-) cotransporters KCC2 and KCC4

K(+)Cl(-) cotransporters (KCCs) play fundamental physiological roles in processes such as inhibitory neurotransmission and cell volume regulation. Mammalian genomes encode four distinct KCC paralogs, which share basic transport characteristics but differ significantly in ion affinity, pharmacology, and relative sensitivity to cell volume. Studies to identify divergence in functional characteristics have thus far focused on the cytoplasmic termini. Here, we investigated sequence requirements of the large extracellular loop (LEL) for function in KCC2 and KCC4. Mutation of all four evolutionarily conserved cysteines abolished KCC2 transport activity. This behavior differs from that of its closest relative, KCC4, which is insensitive to this mutation. Chimeras supported the differences in the LEL of the two cotransporters, because swapping wild-type LEL resulted in functional KCC2 but rendered KCC4 inactive. Insertion of the quadruple cysteine substitution mutant of the KCC4 loop, which was functional in the parental isoform, abolished transport activity in KCC2. Dose-response curves of wild-type and chimeric KCCs revealed that the LEL contributes to the different sensitivity to loop diuretics; a KCC2 chimera containing the KCC4 LEL displayed an IC(50) of 396.5 mum for furosemide, which was closer to KCC4 (548.8 mum) than to KCC2 (184.4 mum). Cell surface labeling and immunocytochemistry indicated that mutations do not affect trafficking to the plasma membrane. Taken together, our results show a dramatic and unexpected difference in the sequence requirements of the LEL between the closely related KCC2 and KCC4. Furthermore, they demonstrate that evolutionarily highly conserved amino acids can have different functions within KCC members.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

The regulation of salt transport and blood pressure by the WNK-SPAK/OSR1 signalling pathway

It has recently been shown that the WNK [with-no-K(Lys)] kinases (WNK1, WNK2, WNK3 and WNK4) have vital roles in the control of salt homeostasis and blood pressure. This Commentary focuses on recent findings that have uncovered the backbone of a novel signal-transduction network that is controlled by WNK kinases. Under hyperosmotic or hypotonic low-Cl– conditions, WNK isoforms are activated, and subsequently phosphorylate and activate the related protein kinases SPAK and OSR1. SPAK and OSR1 phosphorylate and activate ion co-transporters that include NCC, NKCC1 and NKCC2, which are targets for the commonly used blood-pressure-lowering thiazide-diuretic and loop-diuretic drugs. The finding that mutations in WNK1, WNK4, NCC and NKCC2 cause inherited blood-pressure syndromes in humans highlights the importance of these enzymes. We argue that these new findings indicate that SPAK and OSR1 are promising drug targets for the treatment of hypertension, because inhibiting these enzymes would reduce NCC and NKCC2 activity and thereby suppress renal salt re-absorption. We also discuss unresolved and controversial questions in this field of research.

Evidence for an apical Na-Cl cotransporter involved in ion uptake in a teleost fish

Cation-chloride cotransporters, such as the Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and Na(+)/Cl(-) cotransporter (NCC), are localized to the apical or basolateral plasma membranes of epithelial cells and are involved in active ion absorption or secretion. The objectives of this study were to clone and identify ;freshwater-type' and ;seawater-type' cation-chloride cotransporters of euryhaline Mozambique tilapia (Oreochromis mossambicus) and to determine their intracellular localization patterns within mitochondria-rich cells (MRCs). From tilapia gills, we cloned four full-length cDNAs homologous to human cation-chloride cotransporters and designated them as tilapia NKCC1a, NKCC1b, NKCC2 and NCC. Out of the four candidates, the mRNA encoding NKCC1a was highly expressed in the yolk-sac membrane and gills (sites of the MRC localization) of seawater-acclimatized fish, whereas the mRNA encoding NCC was exclusively expressed in the yolk-sac membrane and gills of freshwater-acclimatized fish. We then generated antibodies specific for tilapia NKCC1a and NCC and conducted whole-mount immunofluorescence staining for NKCC1a and NCC, together with Na(+)/K(+)-ATPase, cystic fibrosis transmembrane conductance regulator (CFTR) and Na(+)/H(+) exchanger 3 (NHE3), on the yolk-sac membrane of tilapia embryos acclimatized to freshwater or seawater. The simultaneous quintuple-color immunofluorescence staining allowed us to classify MRCs clearly into four types: types I, II, III and IV. The NKCC1a immunoreactivity was localized to the basolateral membrane of seawater-specific type-IV MRCs, whereas the NCC immunoreactivity was restricted to the apical membrane of freshwater-specific type-II MRCs. Taking account of these data at the level of both mRNA and protein, we deduce that NKCC1a is the seawater-type cotransporter involved in ion secretion by type-IV MRCs and that NCC is the freshwater-type cotransporter involved in ion absorption by type-II MRCs. We propose a novel ion-uptake model by MRCs in freshwater that incorporates apically located NCC. We also reevaluate a traditional ion-uptake model incorporating NHE3; the mRNA was highly expressed in freshwater, and the immunoreactivity was found at the apical membrane of other freshwater-specific MRCs.

Identification and functional characterization of cation-chloride cotransporters in plants

Chloride (Cl(-)) is an essential nutrient and one of the most abundant inorganic anions in plant tissues. We have cloned an Arabidopsis thaliana cDNA encoding for a member of the cation-Cl(-) cotransporter (CCC) family. Deduced plant CCC proteins are highly conserved, and phylogenetic analyses revealed their relationships to the sub-family of animal K(+):Cl(-) cotransporters. In Xenopus laevis oocytes, the A. thaliana CCC protein (At CCC) catalysed the co-ordinated symport of K(+), Na(+) and Cl(-), and this transport activity was inhibited by the 'loop' diuretic bumetanide, a specific inhibitor of vertebrate Na(+):K(+):Cl(-) cotransporters, indicating that At CCC encodes for a bona fide Na(+):K(+):Cl(-) cotransporter. Analysis of At CCC promoter-beta-glucuronidase transgenic Arabidopsis plants revealed preferential expression in the root and shoot vasculature at the xylem/symplast boundary, root tips, trichomes, leaf hydathodes, leaf stipules and anthers. Plants homozygous for two independent T-DNA insertions in the CCC gene exhibited shorter organs such as inflorescence stems, roots, leaves and siliques. The elongation zone of the inflorescence stem of ccc plants often necrosed during bolt emergence, while seed production was strongly impaired. In addition, ccc plants exhibited defective Cl(-) homeostasis under high salinity, as they accumulated higher and lower Cl(-) amounts in shoots and roots, respectively, than the treated wild type, suggesting At CCC involvement in long-distance Cl(-) transport. Compelling evidence is provided on the occurrence of cation-chloride cotransporters in the plant kingdom and their significant role in major plant developmental processes and Cl(-) homeostasis.

The membrane integration of a naturally occurring alpha-helical hairpin

Helical hairpins, two closely spaced helical membrane spanning segments separated by a short surface turn, are thought to be common in integral membrane proteins. Here, we study the membrane integration of a naturally occurring helical hairpin from the secretory Na(+)-K(+)-2Cl(-) cotransporter NKCC1. This sequence is only slightly longer and significantly less hydrophobic than a previously identified minimal poly-leucine model hairpin structure. Using site directed mutagenesis we document the importance of the turn propensity of the amino acids in the intervening surface turn but, somewhat surprisingly, our results indicate that the formation of this natural hairpin apparently does not depend on specific helix-helix interactions. Our results suggest that helical hairpins may be formed quite readily from even minimally hydrophobic sequences separated by a short, sufficiently strong, turn signal, and that current methods for predicting integral membrane protein topology may miss many similar short helical hairpin sequences. Thus the occurrence of these structures may be much more common than presently thought.

Topology of the Type IIa Na+/Pi Cotransporter

The type IIa Na(+)/P(i) cotransporter (NaPi-IIa) plays a key role in the reabsorption of inorganic phosphate (P(i)) in the renal proximal tubule. The rat NaPi-IIa isoform is a protein of 637 residues for which different algorithms predict 8-12 transmembrane domains (TMDs). Epitope tagging experiments demonstrated that both the N and the C termini of NaPi-IIa are located intracellularly. Site-directed mutagenesis revealed two N-glycosylation sites in a large putative extracellular loop. Results from structure-function studies suggested the assembly of two similar opposed regions that possibly constitute part of the substrate translocation pathway for one phosphate ion together with three sodium ions. Apart from these topological aspects, other structural features of NaPi-IIa are not known. In this study, we have addressed the topology of NaPi-IIa using in vitro transcription/translation of HK-M0 and HK-M1 fusion vectors designed to test membrane insertion properties of cDNA sequences encoding putative NaPi-IIa TMDs. Based on the results of in vitro transcription/translation analyses, we propose a model of NaPi-IIa comprising 12 TMDs, with both N and C termini orientated intracellularly and a large hydrophilic extracellular loop between the fifth and sixth TMDs. The proposed model is in good agreement with the prediction of the NaPi-IIa structure obtained by the hidden Markov algorithm HMMTOP.

Oligomerization of KCC2 correlates with development of inhibitory neurotransmission

The neuron-specific K+-Cl- cotransporter KCC2 extrudes Cl- and renders GABA and glycine action hyperpolarizing. Thus, it plays a pivotal role in neuronal inhibition. Development-dependent KCC2 activation is regulated at the transcriptional level and by unknown posttranslational mechanisms. Here, we analyzed KCC2 activation at the protein level in the developing rat lateral superior olive (LSO), a prominent auditory brainstem structure. Electrophysiology demonstrated ineffective KCC2-mediated Cl- extrusion in LSO neurons at postnatal day 3 (P3). Immunohistochemical analyses by confocal and electron microscopy revealed KCC2 signals at the plasma membrane in the somata and dendrites of both immature and mature neurons. Biochemical analysis demonstrated mature glycosylation pattern of KCC2 at both stages. Immunoblot analysis of the immature brainstem demonstrated mainly monomeric KCC2. In contrast, three KCC2 oligomers with molecular masses of approximately 270, approximately 400, and approximately 500 kDa were identified in the mature brainstem. These oligomers were sensitive to sulfhydryl-reducing agents and resistant to SDS, contrary to the situation seen in the related Na+-(K+)-Cl- cotransporter. In HEK-293 cells, coexpressed hemagglutinin-tagged KCC2 assembled with histidine-tagged KCC2, demonstrating formation of homomers. Based on these findings, we conclude that the oligomers represent KCC2 dimers, trimers, and tetramers. Finally, immunoblot analysis identified a development-dependent increase in the oligomer/monomer ratio from embryonic day 18 to P30 throughout the brain that correlates with KCC2 activation. Together, our data indicate that the developmental shift from depolarization to hyperpolarization can be determined by both increased gene expression and KCC2 oligomerization.

Biogenesis and Topology of the Secretory Na+−K+−2Cl-Cotransporter (NKCC1) Studied in Intact Mammalian Cells†

The "secretory" Na+-K+-2Cl- cotransporter (NKCC1) is a member of a small gene family with nine homologues in vertebrates. Of these, seven are known to be electroneutral chloride transporters. These transporters play a number of important physiological roles related to salt and water homeostasis and the control of intracellular chloride levels. Hydropathy analyses suggest that all of these transporters have a similar transmembrane topology consisting of relatively large intracellular N and C termini and a central hydrophobic domain containing 12 membrane-spanning segments (MSSs). In recent experiments from our laboratory [Gerelsaikhan, T., and Turner, R. J. (2000) J. Biol. Chem. 275, 40471-40477], we employed an in vitro translation system to confirm that each of the putative MSSs of NKCC1 was capable of membrane integration in a manner consistent with a 12 MSS model. Here, we extend that work to the study of the biogenesis of NKCC1 in intact cells. We employ a truncation mutant approach that allows us to monitor this process quantitatively as successive MSSs are synthesized. While the results presented here confirm the 12 MSS model, they also indicate that the integration of NKCC1 into the membrane does not occur via a simple cotranslational process. In particular, we demonstrate that two MSSs, the second and sixth, require the presence of downstream sequence to efficiently integrate into the membrane.

KCC3 and KCC4 expression in rat adult forebrain

Potassium chloride ion cotransporters (KCCs) are part of a family of transporters classically described as being involved in cell volume regulation. Recently, KCC2 has been shown to have a role in the development of the inhibitory actions of amine transmitters, whereas KCC3 also plays a fundamental role in the development and function of the central and peripheral nervous system. We have re-assessed the expression of each of the known KCCs in the rat forebrain using RT-PCR and in situ hybridisation histochemistry. As well as confirming the widespread expression of KCC1 and KCC2 throughout the brain, we now show a more restricted expression of KCC3a in the hippocampus, choroid plexus and piriform cortex, as well as KCC4 in the choroid plexus and the suprachiasmatic nucleus of the hypothalamus. The expression of KCC4 in the latter and KCC2 in the lateral hypothalamic and ventromedial hypothalamic nuclei suggests that these cotransporters may have selective roles in neuroendocrine or homeostatic functions. Finally, we demonstrate the existence of a truncated splice variation of KCC3a in the rat that appears to be expressed exclusively in neurons (as is KCC2), whereas the native form of KCC3a and KCC4 appears to be expressed in glial cells.

Affinity-defining Domains in the Na-Cl Cotransporter

The thiazide-sensitive Na+-Cl- cotransporter (NCC) is the major pathway for salt reabsorption in the distal convoluted tubule, serves as a receptor for thiazide-type diuretics, and is involved in inherited diseases associated with abnormal blood pressure. Little is known regarding the structure-function relationship in this cotransporter. Previous studies from our group reveal that mammalian NCC exhibits higher affinity for ions and thiazides than teleost NCC and suggest a role for glycosylation upon thiazide affinity. Here we have constructed a series of chimeric and mutant cDNAs between rat and flounder NCC to define the role of glycosylation status, the amino-terminal domain, the carboxyl-terminal domain, the extracellular glycosylated loop, and the transmembrane segments upon affinity for Na+, Cl-, and metolazone. Xenopus laevis oocytes were used as the heterologous expression system. We observed that elimination of glycosylation sites in flounder NCC did not affect the affinity of the cotransporter for metolazone. Also, swapping the amino-terminal domain, the carboxyl-terminal domain, the glycosylation sites, or the entire extracellular glycosylation loop between rat and flounder NCC had no effect upon ions or metolazone affinity. In contrast, interchanging transmembrane regions between rat and flounder NCC revealed that affinity-modifying residues for chloride are located within the transmembrane 1-7 region and for thiazides are located within the transmembrane 8-12 region, whereas both segments seem to be implicated in defining sodium affinity. These observations strongly suggest that binding sites for chloride and thiazide in NCC are different.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Topology of the C-terminal fragment of human presenilin 1

Mutations of human presenilin 1 (PS1) have been genetically linked to early-onset familial Alzheimer's disease. PS1 contains 10 hydrophobic regions (HRs) sufficiently long to be alpha-helical membrane spanning segments. Most previous topology studies agree that the N-terminus of PS1 is cytosolic and HRs 1-6 span the membrane but HR 7 does not. However, whether HRs 8 and 9 are membrane spanning segments remains controversial. Here we study the topology and biogenesis of this region of PS1 using a reporter gene fusion approach, where portions of the PS1 sequence containing possible membrane spanning segments were fused up- or downstream of a reporter sequence whose translocation into the endoplasmic reticulum could be monitored via its glycosylation. We provide strong evidence, supported by cysteine accessibility studies in full-length PS1, that HRs 8 and 9 are indeed membrane spanning and that the integration of HR 8 into the membrane is dependent on the presence of HR 9. We also explain how our results reconcile previous apparently divergent conclusions regarding the topology of HRs 8 and 9.

Membrane integration and topology of the first transmembrane segment in normal and Southeast Asian ovalocytosis human erythrocyte anion exchanger 1

Anion exchanger 1 (AE1, or Band 3) is an integral membrane glycoprotein found in erythrocytes, responsible for the electroneutral exchange of chloride and bicarbonate ions across the plasma membrane. Southeast Asian ovalocytosis (SAO) results from a nine-amino acid deletion in the first transmembrane segment (TM) of the AE1 protein that abolishes its transport function. The effects of the SAO deletion on: (1) the efficiency of integration of TM1 into the membrane, and (2) the precise positioning of TM1 relative to the membrane were investigated using scanning N-glycosylation mutagenesis in a cell-free transcription/translation system and in transfected HEK293 cells. AE1 or SAO constructs containing either the endogenous N-glycosylation site at Asn642 in extracellular loop 4 (EC4) or single N-glycosylation sites engineered into an expanded extracellular loop 1 (EC1) were used. N-glycosylation efficiency of EC1 in the SAO construct was significantly lower than that of the AE1 construct, indicating that the SAO deletion impairs membrane integration of TM1 and the translocation of EC1 across the membrane. Scanning N-glycosylation mapping of EC1 in the cell-free system and in transfected cells showed that the C-terminus of both AE1 and SAO TM1 were at the same position relative to the membrane. Thus, the SAO deletion is likely to cause a pulling-in of the polar amino acid sequence immediately N-terminal to the deletion into the lipid bilayer, allowing SAO TM1 that was inserted to assume a transmembrane disposition.

Activity of the renal Na+-K+-2Cl- cotransporter is reduced by mutagenesis of N-glycosylation sites: role for protein surface charge in Cl- transport

The renal-specific Na(+)-K(+)-2Cl(-) cotransporter NKCC2 belongs to the SLC12 gene family; it is the target for loop diuretics and the cause of type I Bartter's syndrome. Because the NKCC2 sequence contains two putative N-linked glycosylation sites, one of which is conserved with the renal Na(+)-Cl(-) cotransporter in which glycosylation affects thiazide affinity, we assessed the role of glycosylation on NKCC2 functional properties. One (N442Q or N452Q) or both (N442,452Q) N-glycosylation sites were eliminated by site-directed mutagenesis. Wild-type NKCC2 and mutant clones were expressed in Xenopus laevis oocytes and analyzed by (86)Rb(+) influx, Western blotting, and confocal microscopy. Inhibition of glycosylation with tunicamycin in wild-type NKCC2-injected oocytes resulted in an 80% reduction of NKCC2 activity. Immunoblot of injected oocytes revealed that glycosylation of NKCC2 was completely prevented in N442,452Q-injected oocytes. Functional activity was reduced by 50% in N442Q- and N452Q-injected oocytes and by 80% in oocytes injected with N442,452Q, whereas confocal microscopy of oocytes injected with wild-type or mutant enhanced green fluorescent protein-tagged NKCC2 clones revealed that surface fluorescence intensity was reduced approximately 20% in single mutants and 50% in the double mutant. Ion transport kinetic analyses revealed no changes in cation affinity and a small increase in Cl(-) affinity by N442Q and N442,452Q. However, a slight decrease in bumetanide affinity was observed. Our data demonstrate that NKCC2 is glycosylated and suggest that prevention of glycosylation reduces its functional expression by affecting insertion into the plasma membrane and the intrinsic activity of the cotransporter.

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.

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.

Cloning and expression of sheep renal K-CI cotransporter-1

Sheep K-Cl cotransporter-1(shKCC1) cDNA was cloned from kidney by RT-PCR with an open reading frame of 3258 base pairs exhibiting 92%, 90%, 88% and 87% identity with pig, rabbit and human, rat and mouse KCC1 cDNAs, respectively, encoding an approximately 122 kDa polypeptide of 1086-amino acids. Hydropathy analysis reveals the familiar KCC1 topology with 12 transmembrane domains (TMDs) and the hydrophilic NH2-terminal (NTD) and COOH-terminal (CTD) domains both at the cytoplasmic membrane face. However, shKCC1 has two rather than one large extracellular loops (ECL): ECL3 between TMDs 5 and 6, and ECL6, between TMDs 11 and 12. The translated shKCC1 protein differs in 12 amino acid residues from other KCC1s, mainly within the NTD, ECL3, ICL4, ECL6, and CTD. Notably, a tyrosine residue at position 996 replaces aspartic acid conserved in all other species. Human embryonic kidney (HEK293) cells and mouse NIH/3T3 fibroblasts, transiently transfected with shKCCI-cDNA, revealed the glycosylated approximately 150 kDa proteins by Western blots and positive immunofluorescence-staining with polyclonal rabbit anti-ratKCC1 antibodies. ShKCC1 was functionally expressed in NIH/3T3 cells by an elevated basal Cl-dependent K influx measured with Rb as K-congener that was stimulated three-fold by the KCC-activator N-ethylmaleimide.

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.

Biogenesis and Topology of the Transient Receptor Potential Ca2+ Channel TRPC1

The TRPC ion channels are candidates for the store-operated Ca(2+) entry pathway activated in response to depletion of intracellular Ca(2+) stores. Hydropathy analyses indicate that these proteins contain eight hydrophobic regions (HRs) that could potentially form alpha-helical membrane-spanning segments. Based on limited sequence similarities to other ion channels, it has been proposed that only six of the eight HRs actually span the membrane and that the last two membrane-spanning segments (HRs 6 and 8) border the ion-conducting pore of which HR 7 forms a part. Here we study the biogenesis and transmembrane topology of human TRPC1 to test this model. We have employed a truncation mutant approach combined with insertions of glycosylation sites into full-length TRPC1. In our truncation mutants, portions of the TRPC1 sequence containing one or more HRs were fused between the enhanced green fluorescent protein and a C-terminal glycosylation tag. These chimeras were transiently expressed in the human embryonic cell line HEK-293T. Glycosylation of the tag was used to monitor its location relative to the lumen of the endoplasmic reticulum and thereby HR orientation. Our data indicate that HRs 1, 4, and 6 cross the membrane from cytosol to the ER lumen, that HRs 2, 5, and 8 have the opposite orientation, and that HR 3 is left out of the membrane on the cytosolic side. Our results also show that the sequence downstream of HR 8 plays an important role in anchoring its C-terminal end on the cytosolic side of the membrane. This effect appears to prevent HR 7 from spanning the bilayer and to result in its forming a pore-like structure of the type previously envisioned for the TRPC channels. We speculate that a similar mechanism may be responsible for the formation of other ion channel pores.

Purinergic stimulation induces Ca2+-dependent activation of Na+-K+-2Cl- cotransporter in human nasal epithelia

Increasing evidence suggests that P2 receptors (P2Rs) in airway epithelial cells perform critical functions in auto- or paracrine regulation of fluid and mucus secretion. In the present study, we characterized the effects of P2R stimulation on Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity in normal human nasal epithelial (NHNE) cells. [Ca(2+)](i) and pH(i) were measured in primary cultures of NHNE cells using a double perfusion chamber, which enabled us to analyze membrane-specific transporter activities. NKCC activities were estimated by the pH(i) reduction due to Na(+)-dependent and bumetanide-sensitive intracellular uptake of NH(4)(+). NKCC activities were observed in the basolateral membrane, but not in the luminal membrane, of NHNE cells. Interestingly, P2Rs were expressed in both membranes, and the stimulation of either luminal or basolateral P2R increased NKCC activity. Blockades of luminal Cl(-) channels, basolateral K(+) channels, or protein kinase C did not affect the activation of NKCC by basolateral P2R stimulation. The effects of luminal P2R stimulation were partially reduced by Cl(-) channel blockers. However, chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) treatment completely blocked the stimulatory effects of luminal and basolateral P2Rs on NKCC. In addition, increasing [Ca(2+)](i) by treatment with ionomycin-stimulated NKCC activity. These results provide evidence that stimulation of P2Rs directly activates basolateral NKCC by Ca(2+)-dependent pathways in NHNE cells, which is an important aspect of the purinergic regulation of ion and fluid secretions in human airway epithelia under physiologic and pathologic conditions.

A single nucleotide polymorphism alters the activity of the renal Na+:Cl- cotransporter and reveals a role for transmembrane segment 4 in chloride and thiazide affinity

The thiazide-sensitive Na+:Cl- cotransporter is the major salt transport pathway in the distal convoluted tubule of the kidney, and a role of this cotransporter in blood pressure homeostasis has been defined by physiological studies on pressure natriuresis and by its involvement in monogenic diseases that feature arterial hypotension or hypertension. Data base analysis revealed that 135 single nucleotide polymorphisms along the human SLC12A3 gene that encodes the Na+:Cl- cotransporter have been reported. Eight are located within the coding region, and one results in a single amino acid change; the residue glycine at the position 264 is changed to alanine (G264A). This residue is located within the fourth transmembrane domain of the predicted structure. Because Gly-264 is a highly conserved residue, we studied the functional properties of this polymorphism by using in vitro mutagenesis and the heterologous expression system in Xenopus laevis oocytes. G264A resulted in a significant and reproducible reduction ( approximately 50%) in (22)Na+ uptake when compared with the wild type cotransporter. The affinity for extracellular Cl- and for thiazide diuretics was increased in G264A. Western blot analysis showed similar immunoreactive bands between the wild type and the G264A cotransporters, and confocal images of oocytes injected with enhanced green fluorescent protein-tagged wild type and G264A cotransporter showed no differences in the protein surface expression level. These observations suggest that the G264A polymorphism is associated with reduction in the substrate translocation rate of the cotransporter, due to a decrease in the intrinsic activity. Our study also reveals a role of the transmembrane segment 4 in defining the affinity for extracellular Cl- and thiazide diuretics.

Molecular Mechanisms of Cl- Transport by the Renal Na+-K+-Cl- Cotransporter

The 2nd transmembrane domain (tm) of the secretory Na(+)-K(+)-Cl(-) cotransporter (NKCC1) and of the kidney-specific isoform (NKCC2) has been shown to play an important role in cation transport. For NKCC2, by way of illustration, alternative splicing of exon 4, a 96-bp sequence from which tm2 is derived, leads to the formation of the NKCC2A and F variants that both exhibit unique affinities for cations. Of interest, the NKCC2 variants also exhibit substantial differences in Cl- affinity as well as in the residue composition of the first intracellular connecting segment (cs1a), which immediately follows tm2 and which too is derived from exon 4. In this study, we have prepared chimeras of the shark NKCC2A and F (saA and saF) to determine whether cs1a could play a role in Cl- transport; here, tm2 or cs1a in saF was replaced by the corresponding domain from saA (generating saA/F or saF/A, respectively). Functional analyses of these chimeras have shown that cs1a-specific residues account for most of the A-F difference in Cl- affinity. For example, Km(Cl-)s were approximately 8 mm for saF/A and saA, and approximately 70 mm for saA/F and saF. Intriguingly, variant residues in cs1a also affected cation transport; here, Km(Na+)s for the chimeras and for saA were all approximately 20 mM, and Km(Rb+) all approximately 2 mM. Regarding tm2, our studies have confirmed its importance in cation transport and have also identified novel properties for this domain. Taken together, our results demonstrate for the first time that an intracellular loop in NKCC contributes to the transport process perhaps by forming a flexible structure that positions itself between membrane spanning domains.

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

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

Phylogeny and cloning of ion transporters in mosquitoes

Membrane transport in insect epithelia appears to be energized through proton-motive force generated by the vacuolar type proton ATPase (V-ATPase). However, secondary transport mechanisms that are coupled to V-ATPase activity have not been fully elucidated. Following a blood meal, the female mosquito regulates fluid and ion homeostasis through a series of characteristic behaviors that require brain-derived factors to regulate ion secretion. Despite the knowledge on the behaviors of the mosquito, little is known of the targets of several factors that have been implicated in cellular changes following a blood meal. This review discusses current models of membrane transport in insects and specific data on mosquito ion regulation together with the molecular aspects of membrane transport systems that are potentially linked to V-ATPase activity, which collectively determine the functioning of mosquito midgut and Malpighian tubules. Ion transport mechanisms will be discussed from a comparative physiology perspective to gain appreciation of the exquisite mechanisms of mosquito ion regulation.

Expression of an Aedes aegypti cation-chloride cotransporter and its Drosophila homologues

Insects maintain haemolymph homeostasis under different environmental conditions by modulating the concentrations of Na+, K+ and Cl- ions. One group of proteins involved in ion transport across cell membranes consists of cation-chloride cotransporters that form a family of structurally similar proteins. Although much is known about these proteins in mammalian systems, our understanding of them in insects is lacking. The recent sequencing of two insect genomes, Drosophila and Anopheles, enabled us to identify globally members of the family of cation chloride cotransporters in these insects. Using RT-PCR we monitored the transcription of members of this family in development and in several tissues. Our analyses showed that transcription of these genes differ considerably from the ubiquitously and highly expressed CG5594 gene to the almost silent gene CG31547. Comparison of Drosophila CG12773 and its Aedes homologue AaeCG12773 showed that they have similar transcript expression profiles. Immunohistochemical analysis of AaeCG1277 gene expression revealed that it is highly expressed in the gut of larvae and female adults but not in Malpighian tubules. A more detailed analysis showed that this protein is localized predominantly in the basolateral membrane of these tissues. This expression pattern confirmed the results of RT-PCR analysis. We also created a mutant for one of the genes, CG10413, in Drosophila using P-element excision. Analysis of this mutant showed this protein does not appear to be essential for development.

Identification of a functionally important conformation-sensitive region of the secretory Na+-K+-2Cl- cotransporter (NKCC1)

The secretory Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) is a member of a small gene family of electroneutral salt transporters that play essential roles in salt and water homeostasis in many mammalian tissues. We have identified a highly conserved residue (Ala-483) in the sixth membrane-spanning segment of rat NKCC1 that when mutated to cysteine renders the transporter sensitive to inhibition by the sulfhydryl reagents 2-aminoethyl methanethiosulfonate (MTSEA) and 2-(trimethylammonium)ethyl methanethiosulfonate (MTSET). The mutation of Ala-483 to cysteine (A483C) results in little or no change in the affinities of NKCC1 for substrate ions but produces a 6-fold increase in sensitivity to the inhibitor bumetanide, suggesting a specific modification of the bumetanide binding site. When residues surrounding Ala-483 were mutated to cysteine, only I484C was sensitive to inhibition by MTSEA and MTSET. Surprisingly I484C showed increased transport activity in the presence of low concentrations of mercury (1-10 microm), whereas A483C showed inhibition. The inhibition of A483C by MTSEA was unaffected by the presence or absence of sodium and potassium but required the presence of extracellular chloride. Taken together, our results indicate that Ala-483 lies at or near an important functional site of NKCC1 and that the exposure of this site to the extracellular medium is dependent on the conformation of the transporter. Specifically, our results indicate that the cysteine introduced at residue 483 is only available for interaction with MTSEA when chloride is bound to NKCC1 at the extracellular surface.

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

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

Regulation of Na-K-2Cl cotransport by phosphorylation and protein-protein interactions

The Na-K-2Cl cotransporter plays important roles in cell ion homeostasis and volume control and is particularly important in mediating the movement of ions and thus water across epithelia. In addition to being affected by the concentration of the transported ions, cotransport is affected by cell volume, hormones, growth factors, oxygen tension, and intracellular ionized Mg(2+) concentration. These probably influence transport through three main routes acting in parallel: cotransporter phosphorylation, protein-protein interactions and cell Cl(-) concentration. Many effects are mediated, at least in part, by changes in protein phosphorylation, and are disrupted by kinase and phosphatase inhibitors, and manoeuvres that reduce cell ATP content. In some cases, phosphorylation of the cotransporter itself on serine and threonine (but not tyrosine) is associated with changes in transport rate, in others, phosphorylation of associated proteins has more influence. Analysis of the stimulation of cotransport by calyculin A, arsenite and deoxygenation suggests that the cotransporter is phosphorylated by several kinases and dephosphorylated by several phosphatases. These kinases and phosphatases may themselves be regulated by phosphorylation of residues including tyrosine, with Src kinases possibly playing an important role. Protein-protein interactions also influence cotransport activity. Cotransporter molecules bind to each other to form high molecular weight complexes, they also bind to other members of the cation-chloride cotransport family, to a variety of cytoskeletal proteins, and to enzymes that are part of regulatory cascades. Many of these interactions affect transport and may override the effects of cotransporter phosphorylation. Cell Cl(-) may also directly affect the way the cotransporter functions independently of its role as substrate.

Evidence that the transmembrane biogenesis of aquaporin 1 is cotranslational in intact mammalian cells

Most polytopic membrane proteins are believed to integrate into the membrane of the endoplasmic reticulum (ER) cotranslationally. However, recent studies with Xenopus oocytes and dog pancreatic microsomes have suggested that this is not the case for human aquaporin 1 (AQP1). These experiments indicate that membrane-spanning segments (MSSs) 2 and 4 of AQP1 do not integrate into the membrane cotranslationally so that this protein initially adopts a four MSS topology. A later maturation event involving a 180-degree rotation of MSS 3 from an N(lum)/C(cyt) to an N(cyt)/C(lum) orientation and the concomitant integration of MSSs 2 and 4 into the membrane results in the final six MSS topology. Here we examine the biogenesis of AQP1 in the human embryonic kidney cell line HEK-293T. To do this, we constructed an expression vector for a fusion protein consisting of the enhanced green fluorescent protein followed by an insertion site for AQP1 sequences and a C-terminal glycosylation tag. We then transiently transfected HEK-293T cells with this vector containing the AQP1 sequence truncated after each MSS. Glycosylation of the C-terminal tag was used to monitor its location relative to the ER lumen and consequently the membrane integration and orientation of successive MSSs. In contrast to previous studies our results indicate that AQP1 integrates into the ER membrane cotranslationally in intact HEK-293T cells.

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

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