The Na-K-Cl cotransport protein of shark rectal gland. II. Regulation by direct phosphorylation

Water and chloride as allosteric inhibitors in WNK kinase osmosensing

Osmotic stress and chloride regulate the autophosphorylation and activity of the WNK1 and WNK3 kinase domains. The kinase domain of unphosphorylated WNK1 (uWNK1) is an asymmetric dimer possessing water molecules conserved in multiple uWNK1 crystal structures. Conserved waters are present in two networks, referred to here as conserved water networks 1 and 2 (CWN1and CWN2). Here we show that PEG400 applied to crystals of dimeric uWNK1 induces de-dimerization. Both the WNK1 the water networks and the chloride binding site and are disrupted by PEG400. CWN1 is surrounded by a cluster of pan-WNK-conserved charged residues. Here we mutagenized these charges in WNK3, a highly active WNK isoform kinase domain, and WNK1, the isoform best studied crystallographically. Mutation of E314 in the Activation Loop of WNK3 (WNK3/E314Q and WNK3/E314A, and the homologous WNK1/E388A) enhanced the rate of autophosphorylation, and reduced chloride sensitivity. Other WNK3 Cluster mutants reduced the rate of autophosphorylation activity coupled with greater chloride sensitivity than wild-type. The water and chloride regulation thus appear linked. The lower activity of some mutants may reflect effects on catalysis. Crystallography showed that activating mutants introduced conformational changes in similar parts of the structure to those induced by PEG400. WNK activating mutations and crystallography support a role for CWN1 in WNK inhibition consistent with water functioning as an allosteric ligand.

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

Circadian rhythms in physiology and behavior sync organisms to external environmental cycles. Here, circadian oscillation in intracellular chloride in central pacemaker neurons of the fly, Drosophila melanogaster, is reviewed. Intracellular chloride links SLC12 cation-coupled chloride transporter function with kinase signaling and the regulation of inwardly rectifying potassium channels.

The Rectal Gland of the Shark: The Road to Understanding the Mechanism and Regulation of Transepithelial Chloride Transport

Pictured, described, and speculated on, for close to 400 years, the function of the rectal gland of elasmobranchs remained unknown. In the late 1950s, Burger discovered that the rectal gland of Squalus acanthias secreted an almost pure solution of sodium chloride, isosmotic with blood, which could be stimulated by volume expansion of the fish. Twenty five years later, Stoff discovered that the secretion of the gland was mediated by adenyl cyclase. Studies since then have shown that vasoactive intestinal peptide (VIP) is the neurotransmitter responsible for activating adenyl cyclase; however, the amount of circulating VIP does not change in response to volume expansion. The humoral factor involved in activating the secretion of the gland is C-type natriuretic peptide, secreted from the heart in response to volume expansion. C-type natriuretic peptide circulates to the gland where it stimulates the release of VIP from nerves within the gland, but it also has a direct effect, independent of VIP. Sodium, potassium, and chloride are required for the gland to secrete, and the secretion of the gland is inhibited by ouabain or furosemide. The current model for the secretion of chloride was developed from this information. Basolateral NaKATPase maintains a low intracellular concentration of sodium, which establishes the large electrochemical gradient for sodium directed into the cell. Sodium moves from the blood into the cell (together with potassium and chloride) down this electrochemical gradient, through a coupled sodium, potassium, and two chloride cotransporter (NKCC1). On activation, chloride moves from the cell into the gland lumen, down its electrical gradient through apical cystic fibrosis transmembrane regulator. The fall in intracellular chloride leads to the phosphorylation and activation of NKCC1 that allows more chloride into the cell. Transepithelial sodium secretion into the lumen is driven by an electrical gradient through a paracellular pathway. The aim of this review was to examine the history of the origin of this model for the transport of chloride and suggest that it is applicable to many epithelia that transport chloride, both in resorptive and secretory directions.

Intracellular Ion Control of WNK Signaling

The with no lysine (K) (WNK) kinases are an evolutionarily ancient group of kinases with atypical placement of the catalytic lysine and diverse physiological roles. Recent studies have shown that WNKs are directly regulated by chloride, potassium, and osmotic pressure. Here, we review the discovery of WNKs as chloride-sensitive kinases and discuss physiological contexts in which chloride regulation of WNKs has been demonstrated. These include the kidney, pancreatic duct, neurons, and inflammatory cells. We discuss the interdependent relationship of osmotic pressure and intracellular chloride in cell volume regulation. We review the recent demonstration of potassium regulation of WNKs and speculate on possible physiological roles. Finally, structural and mechanistic aspects of intracellular ion and osmotic pressure regulation of WNKs are discussed.

Arterial Blood Pressure, Neuronal Excitability, Mineral Metabolism and Cell Volume Regulation Mechanisms Revealed by Xenopus laevis oocytes

Xenopus laevis oocytes have been an invaluable tool to discover and explore the molecular mechanisms and characteristics of many proteins, in particular integral membrane proteins. The oocytes were fundamental in many projects designed to identify the cDNA encoding a diversity of membrane proteins including receptors, transporters, channels and pores. In addition to being a powerful tool for cloning, oocytes were later used to experiment with the functional characterization of many of the identified proteins. In this review I present an overview of my personal 30-year experience using Xenopus laevis oocytes and the impact this had on a variety of fields such as arterial blood pressure, neuronal excitability, mineral metabolism and cell volume regulation.

Post-translational regulation of the membrane transporters contributing to salt tolerance in plants

This review article summarises the role of membrane transporters and their regulatory kinases in minimising the toxicity of Na+ in the plant under salt stress. The salt-tolerant plants keep their cytosolic level of Na+ up to 10-50mM. The first line of action in this context is the generation of proton motive force by the plasma membrane H+-ATPase. The generated proton motive force repolarises the membrane that gets depolarised due to passive uptake of Na+ under salt stress. The proton motive force generated also drives the plasma membrane Na+/H+ antiporter, SOS1 that effluxes the cytosolic Na+ back into the environment. At the intracellular level, Na+ is sequestered by the vacuole. Vacuolar Na+ uptake is mediated by Na+/H+ antiporter, NHX, driven by the electrochemical gradient for H+, generated by tonoplast H+ pumps, both H+ATPase and PPase. However, it is the expression of the regulatory kinases that make these transporters active through post-translational modification enabling them to effectively manage the cytosolic level of Na+, which is essential for tolerance to salinity in plants. Yet our knowledge of the expression and functioning of the regulatory kinases in plant species differing in tolerance to salinity is scant. Bioinformatics-based identification of the kinases like OsCIPK24 in crop plants, which are mostly salt-sensitive, may enable biotechnological intervention in making the crop cultivar more salt-tolerant, and effectively increasing its annual yield.

Osmosensing by WNK Kinases

With No Lysine (K) WNK kinases regulate electro-neutral cotransporters that are controlled by osmotic stress and chloride. We showed previously that autophosphorylation of WNK1 is inhibited by chloride, raising the possibility that WNKs are activated by osmotic stress. Here we demonstrate that unphosphorylated WNK isoforms 3 and 1 autophosphorylate in response to osmotic pressure in vitro, applied with the crowding agent polyethylene glycol (PEG)400 or osmolyte ethylene glycol (EG), and that this activation is opposed by chloride. Small angle x-ray scattering of WNK3 in the presence and absence of PEG400, static light scattering in EG, and crystallography of WNK1 were used to understand the mechanism. Osmosensing in WNK3 and WNK1 appears to occur through a conformational equilibrium between an inactive, unphosphorylated, chloride-binding dimer and an autophosphorylation-competent monomer. An improved structure of the inactive kinase domain of WNK1, and a comparison with the structure of a monophosphorylated form of WNK1, suggests that large cavities, greater hydration, and specific bound water may participate in the osmosensing mechanism. Our prior work showed that osmolytes have effects on the structure of phosphorylated WNK1, suggestive of multiple stages of osmotic regulation in WNKs.

High-Resolution Views and Transport Mechanisms of the NKCC1 and KCC Transporters

Cation-chloride cotransporters (CCCs) are responsible for the coupled co-transport of Cl- with K+ and/or Na+ in an electroneutral manner. They play important roles in myriad fundamental physiological processes--from cell volume regulation to transepithelial solute transport and intracellular ion homeostasis--and are targeted by medicines commonly prescribed to treat hypertension and edema. After several decades of studies into the functions and pharmacology of these transporters, there have been several breakthroughs in the structural determination of CCC transporters. The insights provided by these new structures for the Na+/K+/Cl- cotransporter NKCC1 and the K+/Cl- cotransporters KCC1, KCC2, KCC3 and KCC4 have deepened our understanding of their molecular basis and transport function. This focused review discusses recent advances in the structural and mechanistic understanding of CCC transporters, including architecture, dimerization, functional roles of regulatory domains, ion binding sites, and coupled ion transport.

LRRC8A-containing chloride channel is crucial for cell volume recovery and survival under hypertonic conditions

Rapid regulatory volume increase (RVI) is important for cell survival under hypertonic conditions. RVI is driven by Cl⁻ uptake via the Na–K–Cl cotransporter (NKCC), which is activated by WNK kinases following a reduction in intracellular [Cl⁻]. However, how intracellular [Cl⁻] is regulated to modulate the WNK–NKCC axis and engage a protective RVI remains unknown. Our work reveals that LRRC8A-containing chloride channel is a key protective factor against hypertonic shocks. Considering that LRRC8A (SWELL1) is typically activated by low ionic strength under hypotonic stress, our results posed another interesting question: what activates this chloride channel under hypertonic stress? We demonstrated that, upon hyperosmotic activation, the p38-MSK1 pathway gates LRRC8A-containing chloride channel to facilitate activation of WNK–NKCC and an effective RVI.

Regulation of cell volume is essential for tissue homeostasis and cell viability. In response to hypertonic stress, cells need rapid electrolyte influx to compensate water loss and to prevent cell death in a process known as regulatory volume increase (RVI). However, the molecular component able to trigger such a process was unknown to date. Using a genome-wide CRISPR/Cas9 screen, we identified LRRC8A, which encodes a chloride channel subunit, as the gene most associated with cell survival under hypertonic conditions. Hypertonicity activates the p38 stress-activated protein kinase pathway and its downstream MSK1 kinase, which phosphorylates and activates LRRC8A. LRRC8A-mediated Cl⁻ efflux facilitates activation of the with-no-lysine (WNK) kinase pathway, which in turn, promotes electrolyte influx via Na⁺/K⁺/2Cl⁻ cotransporter (NKCC) and RVI under hypertonic stress. LRRC8A-S217A mutation impairs channel activation by MSK1, resulting in reduced RVI and cell survival. In summary, LRRC8A is key to bidirectional osmotic stress responses and cell survival under hypertonic conditions.

The structural basis of function and regulation of neuronal cotransporters NKCC1 and KCC2

NKCC and KCC transporters mediate coupled transport of Na++K++Cl- and K++Cl- across the plasma membrane, thus regulating cell Cl- concentration and cell volume and playing critical roles in transepithelial salt and water transport and in neuronal excitability. The function of these transporters has been intensively studied, but a mechanistic understanding has awaited structural studies of the transporters. Here, we present the cryo-electron microscopy (cryo-EM) structures of the two neuronal cation-chloride cotransporters human NKCC1 (SLC12A2) and mouse KCC2 (SLC12A5), along with computational analysis and functional characterization. These structures highlight essential residues in ion transport and allow us to propose mechanisms by which phosphorylation regulates transport activity.

WNK4 kinase: from structure to physiology

With no lysine kinase-4 (WNK4) belongs to a serine-threonine kinase family characterized by the atypical positioning of its catalytic lysine. Despite the fact that WNK4 has been found in many tissues, the majority of its study has revolved around its function in the kidney, specifically as a positive regulator of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule of the nephron. This is explained by the description of gain-of-function mutations in the gene encoding WNK4 that causes familial hyperkalemic hypertension. This disease is mainly driven by increased downstream activation of the Ste20/SPS1-related proline-alanine-rich kinase/oxidative stress responsive kinase-1-NCC pathway, which increases salt reabsorption in the distal convoluted tubule and indirectly impairs renal K⁺ secretion. Here, we review the large volume of information that has accumulated about different aspects of WNK4 function. We first review the knowledge on WNK4 structure and enumerate the functional domains and motifs that have been characterized. Then, we discuss WNK4 physiological functions based on the information obtained from in vitro studies and from a diverse set of genetically modified mouse models with altered WNK4 function. We then review in vitro and in vivo evidence on the different levels of regulation of WNK4. Finally, we go through the evidence that has suggested how different physiological conditions act through WNK4 to modulate NCC activity.

Physiological Processes Modulated by the Chloride-Sensitive WNK-SPAK/OSR1 Kinase Signaling Pathway and the Cation-Coupled Chloride Cotransporters

The role of Cl- as an intracellular signaling ion has been increasingly recognized in recent years. One of the currently best described roles of Cl- in signaling is the modulation of the With-No-Lysine (K) (WNK) - STE20-Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive Kinase 1 (OSR1) - Cation-Coupled Cl- Cotransporters (CCCs) cascade. Binding of a Cl- anion to the active site of WNK kinases directly modulates their activity, promoting their inhibition. WNK activation due to Cl- release from the binding site leads to phosphorylation and activation of SPAK/OSR1, which in turn phosphorylate the CCCs. Phosphorylation by WNKs-SPAK/OSR1 of the Na+-driven CCCs (mediating ions influx) promote their activation, whereas that of the K+-driven CCCs (mediating ions efflux) promote their inhibition. This results in net Cl- influx and feedback inhibition of WNK kinases. A wide variety of alterations to this pathway have been recognized as the cause of several human diseases, with manifestations in different systems. The understanding of WNK kinases as Cl- sensitive proteins has allowed us to better understand the mechanistic details of regulatory processes involved in diverse physiological phenomena that are reviewed here. These include cell volume regulation, potassium sensing and intracellular signaling in the renal distal convoluted tubule, and regulation of the neuronal response to the neurotransmitter GABA.

The sodium/proton exchanger NHA2 regulates blood pressure through a WNK4-NCC dependent pathway in the kidney

NHA2 is a sodium/proton exchanger associated with arterial hypertension in humans, but the role of NHA2 in kidney function and blood pressure homeostasis is currently unknown. Here we show that NHA2 localizes almost exclusively to distal convoluted tubules in the kidney. NHA2 knock-out mice displayed reduced blood pressure, normocalcemic hypocalciuria and an attenuated response to the thiazide diuretic hydrochlorothiazide. Phosphorylation of the thiazide-sensitive sodium/chloride cotransporter NCC and its upstream activating kinase Ste20/SPS1-related proline/alanine rich kinase (SPAK), as well as the abundance of with no lysine kinase 4 (WNK4), were significantly reduced in the kidneys of NHA2 knock-out mice. In vitro experiments recapitulated these findings and revealed increased WNK4 ubiquitylation and enhanced proteasomal WNK4 degradation upon loss of NHA2. The effect of NHA2 on WNK4 stability was dependent from the ubiquitylation pathway protein Kelch-like 3 (KLHL3). More specifically, loss of NHA2 selectively attenuated KLHL3 phosphorylation and blunted protein kinase A- and protein kinase C-mediated decrease of WNK4 degradation. Phenotype analysis of NHA2/NCC double knock-out mice supported the notion that NHA2 affects blood pressure homeostasis by a kidney-specific and NCC-dependent mechanism. Thus, our data show that NHA2 as a critical component of the WNK4-NCC pathway and is a novel regulator of blood pressure homeostasis in the kidney.

AMPK mediates inhibition of electrolyte transport and NKCC1 activity by reactive oxygen species

Reactive oxygen species such as H₂O₂ are believed to play a prominent role in the injury and loss of transport function that affect the intestinal epithelium in inflammatory conditions such as inflammatory bowel diseases. Defects in intestinal epithelial ion transport regulation contribute to dysbiosis and inflammatory phenotypes. We previously showed that H₂O₂ inhibits Ca²⁺-dependent Cl^- secretion across intestinal epithelial cells (IECs) via a phosphatidylinositol 3-kinase (PI3K)- and extracellular signal-regulated kinase (ERK)-dependent mechanism that occurs, at least in part, through inhibition of the basolateral Na⁺-K⁺-2Cl^- cotransporter NKCC1. NKCC1 governs Cl^- entry into crypt IECs and thus plays a critical role in maintaining the driving force for Cl^- secretion. Electrolyte transport consumes large amounts of cellular energy, and direct pharmacological activation of the cellular energy sensor AMP-activated protein kinase (AMPK) has been shown to inhibit a number of ion transport proteins. Here, we show that H₂O₂ activates AMPK in human IEC lines and ex vivo human colon. Moreover, we demonstrate that the inhibitory effect of H₂O₂ on Ca²⁺-dependent Cl^- secretion and NKCC1 activity is AMPK-dependent. This inhibitory effect is associated with a physical interaction between AMPK and NKCC1, as well as increased phosphorylation (Thr^212,217) of NKCC1, without causing NKCC1 internalization. These data identify a key role for AMPK-NKCC1 interaction as a point of convergence for suppression of colonic epithelial ion transport by inflammatory reactive oxygen species.NEW & NOTEWORTHY H₂O₂ inhibition of intestinal epithelial Ca²⁺-dependent Cl^- secretion involves recruitment of AMP-activated protein kinase (AMPK) downstream of ERK and phosphatidylinositol 3-kinase signaling pathways, physical interaction of AMPK with the Na⁺-K⁺-2Cl^- cotransporter NKCC1, and AMPK-dependent suppression of NKCC1-mediated electrolyte influx without causing NKCC1 internalization. It is intriguing that, in human intestinal epithelial cell lines and human colon, H₂O₂ activation of AMPK increased phosphorylation of NKCC1 residues required for promoting, not inhibiting, NKCC1 activity. These data identify an elevated complexity of AMPK regulation of NKCC1 in the setting of an inflammatory stimulus.

Endocytic recycling of Na+ -K+ -Cl- cotransporter type 2: importance of exon 4

KEY POINTS

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 27-exon membrane protein that is expressed in the thick ascending limb (TAL) of Henle where it is involved in reabsorption of the ultrafiltered NaCl load. It comes as three splice variants that are identical to each other except for the residue composition of exon 4 and that differ in their transport characteristics, functional roles and distributions along the TAL. In this report, it is shown that the variants also differ in their trafficking properties and that two residues in exon 4 play a key role in this regard. One of these residues was also shown to sustain carrier internalization. Through these results, a novel function for the alternatively spliced exon of NKCC2 has been identified and a domain that is involved in carrier trafficking has been uncovered for the first time in a cation-Cl^- cotransporter family member.

ABSTRACT

Na⁺ -K⁺ -Cl^- cotransporter type 2 (NKCC2) is a 12-transmembrane (TM) domain cell surface glycoprotein that is expressed in the thick ascending limb (TAL) of Henle and stimulated during cell shrinkage. It comes as three splice variants (A, B and F) that are identical to each other except for TM2 and the following connecting segment (CS2). Yet, these variants do not share the same localization, transport characteristics and physiological roles along the TAL. We have recently found that while cell shrinkage could exert its activating effect by increasing NKCC2 expression at the cell surface, the variants also responded differentially to this stimulus. In the current work, a mutagenic approach was exploited to determine whether CS2 could play a role in carrier trafficking and identify the residues potentially involved. We found that when the residue of position 238 in NKCC2A (F) and NKCC2B (Y) was replaced by the corresponding residue in NKCC2F (V), carrier activity increased by over 3-fold and endocytosis decreased concomitantly. We also found that when the residue of position 230 in NKCC2F (M) was replaced by the one in NKCC2B (T), carrier activity and affinity for ions both increased substantially whereas expression at the membrane decreased. Taken together, these results suggest that CS2 is involved in carrier trafficking and that two of its residues, those of positions 238 and 230, are part of an internalization motif. They also indicate that the divergent residue of position 230 plays the dual role of specifying ion affinity and sustaining carrier internalization.

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

N-Ethylmaleimide increases KCC2 cotransporter activity by modulating transporter phosphorylation

K+/Cl- cotransporter 2 (KCC2) is selectively expressed in the adult nervous system and allows neurons to maintain low intracellular Cl- levels. Thus, KCC2 activity is an essential prerequisite for fast hyperpolarizing synaptic inhibition mediated by type A γ-aminobutyric acid (GABAA) receptors, which are Cl--permeable, ligand-gated ion channels. Consistent with this, deficits in the activity of KCC2 lead to epilepsy and are also implicated in neurodevelopmental disorders, neuropathic pain, and schizophrenia. Accordingly, there is significant interest in developing activators of KCC2 as therapeutic agents. To provide insights into the cellular processes that determine KCC2 activity, we have investigated the mechanism by which N-ethylmaleimide (NEM) enhances transporter activity using a combination of biochemical and electrophysiological approaches. Our results revealed that, within 15 min, NEM increased cell surface levels of KCC2 and modulated the phosphorylation of key regulatory residues within the large cytoplasmic domain of KCC2 in neurons. More specifically, NEM increased the phosphorylation of serine 940 (Ser-940), whereas it decreased phosphorylation of threonine 1007 (Thr-1007). NEM also reduced with no lysine (WNK) kinase phosphorylation of Ste20-related proline/alanine-rich kinase (SPAK), a kinase that directly phosphorylates KCC2 at residue Thr-1007. Mutational analysis revealed that Thr-1007 dephosphorylation mediated the effects of NEM on KCC2 activity. Collectively, our results suggest that compounds that either increase the surface stability of KCC2 or reduce Thr-1007 phosphorylation may be of use as enhancers of KCC2 activity.

The modulation of the phosphorylation status of NKCC1 in organ cultured bovine lenses: Implications for the regulation of fiber cell and overall lens volume

In previous work, we have shown the Sodium/Potassium/2 Chloride Cotransporter (NKCC1) to be a key effector of lens fiber cell volume regulation. Since others have shown that the activity of NKCC1 is regulated via its phosphorylation status, the purpose of this study was to investigate whether NKCC1 phosphorylation can be modulated in organ cultured bovine lenses, and to see how this relates to changes in lens wet weight. Western blotting was first used to confirm the expression of NKCC1, phosphorylated NKCC1 (NKCC1-P) and the regulatory kinases WNK/SPAK and phosphatases PP1/PP2A in bovine lenses at the protein level. Changes to NKCC1-P status were then assessed by organ culturing bovine lenses in either isotonic, hypertonic or hypotonic solutions in the presence or absence of the NKCC inhibitor, bumetanide, or phosphatase inhibitors okadaic acid and calyculin A. After 1-22 h of culturing, lenses were weighed, assessed for transparency and the cortical protein fractions analyzed by western blot using antibodies to detect total NKCC1 and NKCC1-P. NKCC1, NKCC1-P, SPAK, PP1 and PP2A were all detected in the membrane fraction of bovine lenses. Under hypertonic conditions, NKCC1 is phosphorylated and activated to mediate a regulatory volume increase. Finally, NKCC1-P signal increased in the presence of phosphatase inhibitors indicating that PP1/PP2A can dephosphorylate NKCC1. These results show that the phosphorylation status and hence activity of NKCC1 is dynamically regulated and that in response to hypertonic stress, NKCC1 activity is increased to effect a regulatory volume increase that limits cell shrinkage. These findings support the view that the lens dynamically regulates ion fluxes to maintain steady state lens volume, and suggest that dysfunction of this regulation maybe an initiating factor in the localized fiber cell swelling that is a characteristic of diabetic lens cataract.

WNK Kinase Signaling in Ion Homeostasis and Human Disease

WNK kinases, along with their upstream regulators (CUL3/KLHL3) and downstream targets (the SPAK/OSR1 kinases and the cation-Cl^- cotransporters [CCCs]), comprise a signaling cascade essential for ion homeostasis in the kidney and nervous system. Recent work has furthered our understanding of the WNKs in epithelial transport, cell volume homeostasis, and GABA signaling, and uncovered novel roles for this pathway in immune cell function and cell proliferation.

Functional kinomics establishes a critical node of volume-sensitive cation-Cl- cotransporter regulation in the mammalian brain

Cell volume homeostasis requires the dynamically regulated transport of ions across the plasmalemma. While the ensemble of ion transport proteins involved in cell volume regulation is well established, the molecular coordinators of their activities remain poorly characterized. We utilized a functional kinomics approach including a kinome-wide siRNA-phosphoproteomic screen, a high-content kinase inhibitor screen, and a kinase trapping-Orbitrap mass spectroscopy screen to systematically identify essential kinase regulators of KCC3 Thr991/Thr1048 phosphorylation - a key signaling event in cell swelling-induced regulatory volume decrease (RVD). In the mammalian brain, we found the Cl--sensitive WNK3-SPAK kinase complex, required for cell shrinkage-induced regulatory volume decrease (RVI) via the stimulatory phosphorylation of NKCC1 (Thr203/Thr207/Thr212), is also essential for the inhibitory phosphorylation of KCC3 (Thr991/Thr1048). This is mediated in vivo by an interaction between the CCT domain in SPAK and RFXV/I domains in WNK3 and NKCC1/KCC3. Accordingly, genetic or pharmacologic WNK3-SPAK inhibition prevents cell swelling in response to osmotic stress and ameliorates post-ischemic brain swelling through a simultaneous inhibition of NKCC1-mediated Cl- uptake and stimulation of KCC3-mediated Cl- extrusion. We conclude that WNK3-SPAK is an integral component of the long-sought "Cl-/volume-sensitive kinase" of the cation-Cl- cotransporters, and functions as a molecular rheostat of cell volume in the mammalian brain.

Kinase-KCC2 coupling: Cl- rheostasis, disease susceptibility, therapeutic target

The intracellular concentration of Cl(-) ([Cl(-)]i) in neurons is a highly regulated variable that is established and modulated by the finely tuned activity of the KCC2 cotransporter. Despite the importance of KCC2 for neurophysiology and its role in multiple neuropsychiatric diseases, our knowledge of the transporter's regulatory mechanisms is incomplete. Recent studies suggest that the phosphorylation state of KCC2 at specific residues in its cytoplasmic COOH terminus, such as Ser940 and Thr906/Thr1007, encodes discrete levels of transporter activity that elicit graded changes in neuronal Cl(-) extrusion to modulate the strength of synaptic inhibition via Cl(-)-permeable GABAA receptors. In this review, we propose that the functional and physical coupling of KCC2 to Cl(-)-sensitive kinase(s), such as the WNK1-SPAK kinase complex, constitutes a molecular "rheostat" that regulates [Cl(-)]i and thereby influences the functional plasticity of GABA. The rapid reversibility of (de)phosphorylation facilitates regulatory precision, and multisite phosphorylation allows for the control of KCC2 activity by different inputs via distinct or partially overlapping upstream signaling cascades that may become more or less important depending on the physiological context. While this adaptation mechanism is highly suited to maintaining homeostasis, its adjustable set points may render it vulnerable to perturbation and dysregulation. Finally, we suggest that pharmacological modulation of this kinase-KCC2 rheostat might be a particularly efficacious strategy to enhance Cl(-) extrusion and therapeutically restore GABA inhibition.

Ionic homeostasis in brain conditioning

Most of the current focus on developing neuroprotective therapies is aimed at preventing neuronal death. However, these approaches have not been successful despite many years of clinical trials mainly because the numerous side effects observed in humans and absent in animals used at preclinical level. Recently, the research in this field aims to overcome this problem by developing strategies which induce, mimic, or boost endogenous protective responses and thus do not interfere with physiological neurotransmission. Preconditioning is a protective strategy in which a subliminal stimulus is applied before a subsequent harmful stimulus, thus inducing a state of tolerance in which the injury inflicted by the challenge is mitigated. Tolerance may be observed in ischemia, seizure, and infection. Since it requires protein synthesis, it confers delayed and temporary neuroprotection, taking hours to develop, with a pick at 1–3 days. A new promising approach for neuroprotection derives from post-conditioning, in which neuroprotection is achieved by a modified reperfusion subsequent to a prolonged ischemic episode. Many pathways have been proposed as plausible mechanisms to explain the neuroprotection offered by preconditioning and post-conditioning. Although the mechanisms through which these two endogenous protective strategies exert their effects are not yet fully understood, recent evidence highlights that the maintenance of ionic homeostasis plays a key role in propagating these neuroprotective phenomena. The present article will review the role of protein transporters and ionic channels involved in the control of ionic homeostasis in the neuroprotective effect of ischemic preconditioning and post-conditioning in adult brain, with particular regards to the Na⁺/Ca2⁺ exchangers (NCX), the plasma membrane Ca2⁺-ATPase (PMCA), the Na⁺/H⁺ exchange (NHE), the Na⁺/K⁺/2Cl⁻ cotransport (NKCC) and the acid-sensing cation channels (ASIC). Ischemic stroke is the third leading cause of death and disability. Up until now, all clinical trials testing potential stroke neuroprotectants failed. For this reason attention of researchers has been focusing on the identification of brain endogenous neuroprotective mechanisms activated after cerebral ischemia. In this context, ischemic preconditioning and ischemic post-conditioning represent two neuroprotecive strategies to investigate in order to identify new molecular target to reduce the ischemic damage.

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.

Ammonia exposure increases the expression of Na(+):K (+):2Cl (-) cotransporter 1a in the gills of the giant mudskipper, Periophthalmodon schlosseri

The giant mudskipper, Periophthalmodon schlosseri, is an obligate air-breathing teleost that can actively excrete ammonia against high concentrations of environmental ammonia. This study aimed to clone and sequence the Na (+) :K (+) :2Cl (-) cotransporter 1 (nkcc1) from the gills of P. schlosseri, and to determine the effects of ammonia exposure on its mRNA expression and protein abundance after pre-acclimation to slightly brackish water (salinity 3; SBW) for 2 weeks. The complete coding cDNA sequences of nkcc1a consisted of 3453 bp, coding for 1151 amino acid with an estimated molecular mass of 125.4 kDa. Exposure to 75 mmol l(-1) NH4Cl in SBW had no effect on the mRNA expression of nkcc1a. However, western blotting revealed a significant increase in the protein abundance of multiple T4-immunoreactive bands of molecular mass 170-250 kDa in the gills of P. schlosseri exposed to ammonia. Furthermore, immunofluorescence microscopy demonstrated the colocalization of the increased T4-immunoreactive protein with Na(+)/K(+)-ATPase (Nka) α-subunit to the basolateral membrane of certain ionocytes in the gills of the ammonia-exposed fish. As Nkcc1 is known to have a basolateral localization, it can be concluded that ammonia exposure led to an increase in the expression of glycosylated Nkcc1, the molecular masses of which were reduced upon enzymatic deglycosylation, in the gills of P. schlosseri. The dependency on post-transcriptional and post-translational regulation of branchial Nkcc1 in P. schlosseri would facilitate prompt responses to changes in environmental condition. As NH4 (+) can replace K(+), NH4 (+) could probably enter ionocytes through the basolateral Nkcc1a during active ammonia excretion, but increased influx of Na(+), NH4 (+) and 2Cl(-) would alter the transmembrane Na(+) gradient. Consequently, exposure of P. schlosseri to ammonia would also result in an increase in branchial activity of Nka with decreased NH4 (+) affinity so as to maintain intracellular Na(+) and K(+) homeostasis as reported elsewhere.

Ammonia, like K(+), stimulates the Na(+), K(+), 2 Cl(-) cotransporter NKCC1 and the Na(+),K(+)-ATPase and interacts with endogenous ouabain in astrocytes

Brain edema during hepatic encephalopathy or acute liver failure as well as following brain ischemia has a multifactorial etiology, but it is a dangerous and occasionally life-threatening complication because the brain is enclosed in the rigid skull. During ischemia the extracellular K(+) concentration increases to very high levels, which when energy becomes available during reperfusion stimulate NKCC1, a cotransporter driven by the transmembrane ion gradients established by the Na(+),K(+)-ATPase and accumulating Na(+), K(+) and 2 Cl(-) together with water. This induces pronounced astrocytic swelling under pathologic conditions, but NKCC1 is probably also activated, although to a lesser extent, during normal brain function. Redistribution of ions and water between extra- and intracellular phases does not create brain edema, which in addition requires uptake across the blood-brain barrier. During hepatic encephalopathy and acute liver failure a crucial factor is the close resemblance between K(+) and NH4(+) in their effects not only on NKCC1 and Na(+),K(+)-ATPase but also on Na(+),K(+)-ATPase-induced signaling by endogenous ouabains. These in turn activate production of ROS and nitrosactive agents which slowly sensitize NKCC1, explaining why cell swelling and brain edema generally are delayed under hyperammonemic conditions, although very high ammonia concentrations can cause immediate NKCC1 activation.

Molecular motions involved in Na-K-Cl cotransporter-mediated ion transport and transporter activation revealed by internal cross-linking between transmembrane domains 10 and 11/12

We examined the relationship between transmembrane domain (TM) 10 and TM11/12 in NKCC1, testing homology models based on the structure of AdiC in the same transporter superfamily. We hypothesized that introduced cysteine pairs would be close enough for disulfide formation and would alter transport function: indeed, evidence for cross-link formation with low micromolar concentrations of copper phenanthroline or iodine was found in 3 of 8 initially tested pairs and in 1 of 26 additionally tested pairs. Inhibition of transport was observed with copper phenanthroline and iodine treatment of P676C/A734C and I677C/A734C, consistent with the proximity of these residues and with movement of TM10 during the occlusion step of ion transport. We also found Cu(2+) inhibition of the single-cysteine mutant A675C, suggesting that this residue and Met(382) of TM3 are involved in a Cu(2+)-binding site. Surprisingly, cross-linking of P676C/I730C was found to prevent rapid deactivation of the transporter while not affecting the dephosphorylation rate, thus uncoupling the phosphorylation and activation steps. Consistent with this, (a) cross-linking of P676C/I730C was dependent on activation state, and (b) mutants lacking the phosphoregulatory domain could still be activated by cross-linking. These results suggest a model of NKCC activation that involves movement of TM12 relative to TM10, which is likely tied to movement of the large C terminus, a process somehow triggered by phosphorylation of the regulatory domain in the N terminus.

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.

High brain ammonia tolerance and down-regulation of Na+:K+:2Cl(-) Cotransporter 1b mRNA and protein expression in the brain of the Swamp Eel, Monopterus albus, exposed to environmental ammonia or terrestrial conditions

Na(+):K(+):2Cl(-) cotransporter 1 (NKCC1) has been implicated in mediating ischemia-, trauma- or ammonia-induced astrocyte swelling/brain edema in mammals. This study aimed to determine the effects of ammonia or terrestrial exposure on ammonia concentrations in the plasma and brain, and the mRNA expression and protein abundance of nkcc/Nkcc in the brain, of the swamp eel Monopterusalbus. Ammonia exposure led to a greater increase in the ammonia concentration in the brain of M. albus than terrestrial exposure. The brain ammonia concentration of M. albus reached 4.5 µmol g(-1) and 2.7 µmol g(-1) after 6 days of exposure to 50 mmol l(-1) NH4Cl and terrestrial conditions, respectively. The full cDNA coding sequence of nkcc1b from M. albus brain comprised 3276 bp and coded for 1092 amino acids with an estimated molecular mass of 119.6 kDa. A molecular characterization indicated that it could be activated through phosphorylation and/or glycosylation by osmotic and/or oxidative stresses. Ammonia exposure for 1 day or 6 days led to significant decreases in the nkcc1b mRNA expression and Nkcc1b protein abundance in the brain of M. albus. In comparison, a significant decrease in nkcc1b mRNA expression was observed in the brain of M. albus only after 6 days of terrestrial exposure, but both 1 day and 6 days of terrestrial exposure resulted in significant decreases in the protein abundance of Nkcc1b. These results are novel because it has been established in mammals that ammonia up-regulates NKCC1 expression in astrocytes and NKCC1 plays an important role in ammonia-induced astrocyte swelling and brain edema. By contrast, our results indicate for the first time that M. albus is able to down-regulate the mRNA and protein expression of nkcc1b/Nkcc1b in the brain when confronted with ammonia toxicity, which could be one of the contributing factors to its extraordinarily high brain ammonia tolerance.

Deficiency of the calcium-sensing receptor in the kidney causes parathyroid hormone-independent hypocalciuria

Rare loss-of-function mutations in the calcium-sensing receptor (Casr) gene lead to decreased urinary calcium excretion in the context of parathyroid hormone (PTH)-dependent hypercalcemia, but the role of Casr in the kidney is unknown. Using animals expressing Cre recombinase driven by the Six2 promoter, we generated mice that appeared grossly normal but had undetectable levels of Casr mRNA and protein in the kidney. Baseline serum calcium, phosphorus, magnesium, and PTH levels were similar to control mice. When challenged with dietary calcium supplementation, however, these mice had significantly lower urinary calcium excretion than controls (urinary calcium to creatinine, 0.31±0.03 versus 0.63±0.14; P=0.001). Western blot analysis on whole-kidney lysates suggested an approximately four-fold increase in activated Na(+)-K(+)-2Cl(-) cotransporter (NKCC2). In addition, experimental animals exhibited significant downregulation of Claudin14, a negative regulator of paracellular cation permeability in the thick ascending limb, and small but significant upregulation of Claudin16, a positive regulator of paracellular cation permeability. Taken together, these data suggest that renal Casr regulates calcium reabsorption in the thick ascending limb, independent of any change in PTH, by increasing the lumen-positive driving force for paracellular Ca(2+) transport.

WNK3-SPAK interaction is required for the modulation of NCC and other members of the SLC12 family

The serine/threonine with no lysine kinase 3 (WNK3) modulates the activity of the electroneutral cation-coupled chloride cotransporters (CCC) to promote Cl(-) influx and prevent Cl(-) efflux, thus fitting the profile for a putative "Cl(-)-sensing kinase". The Ste20-type kinases, SPAK/OSR1, become phosphorylated in response to reduction in intracellular chloride concentration and regulate the activity of NKCC1. Several studies have now shown that WNKs function upstream of SPAK/OSR1. This study was designed to analyze the role of WNK3-SPAK interaction in the regulation of CCCs with particular emphasis on NCC. In this study we used the functional expression system of Xenopus laevis oocytes to show that different SPAK binding sites in WNK3 ((241, 872, 1336)RFxV) are required for the kinase to have effects on CCCs. WNK3-F1337A no longer activated NKCC2, but the effects on NCC, NKCC1, and KCC4 were preserved. In contrast, the effects of WNK3 on these cotransporters were prevented in WNK3-F242A. The elimination of F873 had no consequence on WNK3 effects. WNK3 promoted NCC phosphorylation at threonine 58, even in the absence of the unique SPAK binding site of NCC, but this effect was abolished in the mutant WNK3-F242A. Thus, our data support the hypothesis that the effects of WNK3 upon NCC and other CCCs require the interaction and activation of the SPAK kinase. The effect is dependent on one of the three binding sites for SPAK that are present in WNK3, but not on the SPAK binding sites on the CCCs, which suggests that WNK3 is capable of binding both SPAK and CCCs to promote their phosphorylation.

Both seawater acclimation and environmental ammonia exposure lead to increases in mRNA expression and protein abundance of Na⁺:K⁺:2Cl⁻ cotransporter in the gills of the climbing perch, Anabas testudineus

The freshwater climbing perch, Anabas testudineus, is an obligatory air-breathing teleost which can acclimate to seawater, survive long period of emersion, and actively excrete ammonia against high concentrations of environmental ammonia. This study aimed to clone and sequence the Na⁺:K⁺:2Cl⁻ cotransporter (nkcc) from the gills of A. testudineus, and to determine the effects of seawater acclimation or exposure to 100 mmol l⁻¹ NH₄Cl in freshwater on its branchial mRNA expression. The complete coding cDNA sequence of nkcc from the gills of A. testudineus consisted of 3,495 bp, which was translated into a protein with 1,165 amino acid residues and an estimated molecular mass of 127.4 kDa. A phylogenetic analysis revealed that the translated Nkcc of A. testudineus was closer to fish Nkcc1a than to fish Nkcc1b or Nkcc2. After a progressive increase in salinity, there were significant increases in the mRNA expression and protein abundance of nkcc1a in the gills of fish acclimated to seawater as compared with that of the freshwater control. Hence, it can be concluded that similar to marine teleosts, Cl⁻ excretion through basolateral Nkcc1 of mitochondrion-rich cells (MRCs) was essential to seawater acclimation in A. testudineus. Exposure of A. testudineus to 100 mmol l⁻¹ NH₄Cl for 1 or 6 days also resulted in significant increases in the mRNA expression of nkcc1a in the gills, indicating a functional role of Nkcc1a in active ammonia excretion. It is probable that NH₄⁺ enter MRCs through basolateral Nkcc1a before being actively transported across the apical membrane. Since the operation of Nkcc1a would lead to an increase in the intracellular Na⁺ concentration, it can be deduced that an upregulation of basolateral Na⁺/K⁺-ATPase (Nka) activity would be necessary to compensate for the increased influx of Na⁺ into MRCs during active NH₄⁺ excretion. This would imply that the main function of Nka in active NH₄⁺ excretion is to maintain intracellular Na⁺ and K⁺ homeostasis instead of transporting NH₄⁺ directly into MRCs as proposed previously. In conclusion, active salt secretion during seawater acclimation and active NH₄⁺ excretion during exposure to ammonia in freshwater could involve similar transport mechanisms in the gills of A. testudineus.

WNK2 kinase is a novel regulator of essential neuronal cation-chloride cotransporters

NKCC1 and KCC2, related cation-chloride cotransporters (CCC), regulate cell volume and γ-aminobutyric acid (GABA)-ergic neurotranmission by modulating the intracellular concentration of chloride [Cl(-)]. These CCCs are oppositely regulated by serine-threonine phosphorylation, which activates NKCC1 but inhibits KCC2. The kinase(s) that performs this function in the nervous system are not known with certainty. WNK1 and WNK4, members of the WNK (with no lysine [K]) kinase family, either directly or via the downstream SPAK/OSR1 Ste20-type kinases, regulate the furosemide-sensitive NKCC2 and the thiazide-sensitive NCC, kidney-specific CCCs. What role the novel WNK2 kinase plays in this regulatory cascade, if any, is unknown. Here, we show that WNK2, unlike other WNKs, is not expressed in kidney; rather, it is a neuron-enriched kinase primarily expressed in neocortical pyramidal cells, thalamic relay cells, and cerebellar granule and Purkinje cells in both the developing and adult brain. Bumetanide-sensitive and Cl(-)-dependent (86)Rb(+) uptake assays in Xenopus laevis oocytes revealed that WNK2 promotes Cl(-) accumulation by reciprocally activating NKCC1 and inhibiting KCC2 in a kinase-dependent manner, effectively bypassing normal tonicity requirements for cotransporter regulation. TiO(2) enrichment and tandem mass spectrometry studies demonstrate WNK2 forms a protein complex in the mammalian brain with SPAK, a known phosphoregulator of NKCC1. In this complex, SPAK is phosphorylated at Ser-383, a consensus WNK recognition site. These findings suggest a role for WNK2 in the regulation of CCCs in the mammalian brain, with implications for both cell volume regulation and/or GABAergic signaling.

Depolarising and hyperpolarising actions of GABA(A) receptor activation on gonadotrophin-releasing hormone neurones: towards an emerging consensus

The gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones of a complex neuronal network that controls fertility. It is now appreciated that GABAergic neurones within this network provide an important regulatory influence on GnRH neurones. However, the consequences of direct GABA(A) receptor activation on adult GnRH neurones have been controversial for nearly a decade now, with both hyperpolarising and depolarising effects being reported. This review provides: (i) an overview of GABA(A) receptor function and its investigation using electrophysiological approaches and (ii) re-examines the past and present results relating to GABAergic regulation of the GnRH neurone, with a focus on mouse brain slice data. Although it remains difficult to reconcile the results of the early studies, there is a growing consensus that GABA can act through the GABA(A) receptor to exert both depolarising and hyperpolarising effects on GnRH neurones. The most recent studies examining the effects of endogenous GABA release on GnRH neurones indicate that the predominant action is that of excitation. However, we are still far from a complete understanding of the effects of GABA(A) receptor activation upon GnRH neurones. We argue that this will require not only a better understanding of chloride ion homeostasis in individual GnRH neurones, and within subcellular compartments of the GnRH neurone, but also a more integrative view of how multiple neurotransmitters, neuromodulators and intrinsic conductances act together to regulate the activity of these important cells.

Cotransport of water by the Na+-K+-2Cl(-) cotransporter NKCC1 in mammalian epithelial cells

Water transport by the Na+-K+-2Cl(-) cotransporter (NKCC1) was studied in confluent cultures of pigmented epithelial (PE) cells from the ciliary body of the fetal human eye. Interdependence among water, Na+ and Cl(-) fluxes mediated by NKCC1 was inferred from changes in cell water volume, monitored by intracellular self-quenching of the fluorescent dye calcein. Isosmotic removal of external Cl(-) or Na+ caused a rapid efflux of water from the cells, which was inhibited by bumetanide (10 μm). When returned to the control solution there was a rapid water influx that required the simultaneous presence of external Na+ and Cl(-). The water influx could proceed uphill, against a transmembrane osmotic gradient, suggesting that energy contained in the ion fluxes can be transferred to the water flux. The influx of water induced by changes in external [Cl(-)] saturated in a sigmoidal fashion with a Km of 60 mm, while that induced by changes in external [Na+] followed first order kinetics with a Km of about 40 mm. These parameters are consistent with ion transport mediated by NKCC1. Our findings support a previous investigation, in which we showed water transport by NKCC1 to be a result of a balance between ionic and osmotic gradients. The coupling between salt and water transport in NKCC1 represents a novel aspect of cellular water homeostasis where cells can change their volume independently of the direction of an osmotic gradient across the membrane. This has relevance for both epithelial and symmetrical cells.

High salt differentially regulates surface NKCC2 expression in thick ascending limbs of Dahl salt-sensitive and salt-resistant rats

NaCl reabsorption by the thick ascending limb of the loop of Henle (THAL) occurs via the apical Na-K-2Cl cotransporter, NKCC2. Overall, NKCC2 activity and NaCl reabsorption are regulated by the amount of NKCC2 at the apical surface, and also by phosphorylation. Dahl salt-sensitive rats (SS) exhibit higher NaCl reabsorption by the THAL compared with Dahl salt-resistant rats (SR), and they become hypertensive during high-salt (HS) intake. However, the effect of HS on THAL transport, surface NKCC2 expression, and NKCC2 NH(2)-terminus phosphorylation has not been studied. We hypothesized that HS enhances surface NKCC2 and its phosphorylation in THALs from Dahl SS. THAL suspensions were obtained from a group of SS and SR rats on normal-salt (NS) or HS intake. In SR rats THAL NaCl transport measured as furosemide-sensitive oxygen consumption was decreased by HS (-34%, P < 0.05). In contrast, HS did not affect THAL transport in SS rats. As expected, HS increased systolic blood pressure only in SS rats (Δ 23 ± 2 mmHg, P < 0.002) but not in SR rats (Δ 5 ± 3 mmHg). We next tested the effect of HS intake on apical surface NKCC2 and its NH(2)-terminus threonine phosphorylation (P-NKCC2) in SS and SR rats. HS intake decreased surface NKCC2 by 15 ± 2% (P < 0.03) in THALs from SR without affecting total NKCC2 or NH(2)-terminus P-NKCC2. In contrast, in SS rats HS intake increased surface NKCC2 by 54 ± 6% (P < 0.01) without affecting total NKCC2 expression or P-NKCC2. We conclude that HS intake causes different effects on surface NKCC2 in SS and SR rats. Our data suggest that enhanced surface NKCC2 in SS rats might contribute to enhanced NaCl reabsorption in SS rats during HS intake.

The Na-K-Cl cotransporter in the brain edema of acute liver failure

BACKGROUND & AIMS

Astrocyte swelling and brain edema associated with increased intracranial pressure are major complications of acute liver failure (ALF). The mechanism for such astrocyte swelling/brain edema, however, is not well understood. We recently found that ammonia, a key etiological factor in ALF, caused the activation of the Na-K-Cl cotransporter-1 (NKCC1) in cultured astrocytes, and that inhibition of such activation led to a reduction in astrocyte swelling, suggesting that NKCC1 activation may be an important factor in the mechanism of brain edema in ALF. To determine whether NKCC activation is also involved in brain edema in vivo, we examined whether NKCC activation occurs in the thioacetamide (TAA) rat model of ALF and determined whether treatment with the NKCC inhibitor bumetanide reduces the severity of brain edema in TAA-treated rats.

METHODS

Brain water content was measured using the gravimetric method. NKCC1 phosphorylation and protein expression were measured by Western blots. NKCC activity was measured in brain cortical slices.

RESULTS

NKCC activity was elevated in brain cortical slices of TAA-treated rats as compared to sham animals. Western blot analysis showed significant increases in total as well as phosphorylated (activated) NKCC1 protein expression in the cortical tissue. These findings were associated with a significant increase in brain water content which was attenuated by treatment with the NKCC inhibitor bumetanide.

CONCLUSIONS

Our studies suggest the involvement of NKCC in the development of brain edema in experimental ALF, and that targeting NKCC may represent a useful therapeutic strategy in humans with ALF.

Phosphorylation state of the Na+-K+-Cl- cotransporter (NKCC1) in the gills of Atlantic killifish (Fundulus heteroclitus) during acclimation to water of varying salinity

Euryhaline teleosts such as Atlantic killifish (Fundulus heteroclitus) are able to acclimate to changing environmental salinity by tightly regulating NaCl absorption and secretion across their gills. Many studies have examined the mechanisms responsible for long-term (days) salinity acclimation; however, much remains unknown about the mechanisms of acute (hours) salinity acclimation. In this study, we tested the hypotheses that phosphorylation of the Na(+)-K(+)-Cl(-) cotransporter (NKCC1) located in the basolateral membrane of the gill plays a role in acute salinity acclimation and that changes in NKCC1 phosphorylation are mediated by a cAMP-protein kinase A (cAMP-PKA) pathway. Using a phospho-specific antibody, we determined the time course of changes in total and phosphorylated NKCC1 protein during acclimation to water of various salinities. Long-term (>or=14 days) acclimation of killifish to seawater (SW) and 2x SW resulted in 4- to 6-fold and 5- to 8-fold increases, respectively, in total gill NKCC1 protein relative to fish maintained in freshwater (FW). NKCC1 was found to be between 20% and 70% activated in fish, with lower average activation in fish acclimated to SW and 2x SW compared with FW fish. Increases and decreases in the fractional level of NKCC1 phosphorylation were seen within 1 h of transfer of fish to water of higher and lower salinity, respectively, consistent with a regulatory role of phosphorylation prior to an increase in the biosynthesis of NKCC1; large changes in protein expression of NKCC1 were observed over periods of hours to days. We found that NKCC1 phosphorylation is acutely regulated in the killifish gill in response to changing environmental salinity and that phosphorylation in excised gills increases in response to forskolin stimulation of the cAMP-PKA pathway. The role of phosphorylation is further underscored by the observation that mRNA expression of sterile 20 (Ste20)-related proline-alanine-rich kinase (SPAK) changes with salinity acclimation, being 2.7-fold greater in SW-acclimated killifish relative to FW fish. Overall, these results demonstrate an important role of NKCC1 phosphorylation in the gill of Atlantic killifish during acute salinity acclimation.

Molecular determinants of hyperosmotically activated NKCC1-mediated K+/K+ exchange

Na(+)-K(+)-2Cl(-) cotransport (NKCC) mediates the movement of two Cl(-) ions for one Na(+) and one K(+) ion. Under isosmotic conditions or with activation of the kinases SPAK/WNK4, the NKCC1-mediated Cl(-) uptake in Xenopus laevis oocytes, as measured using (36)Cl, is twice the value of K(+) uptake, as determined using (86)Rb. Under hyperosmotic conditions, there is a significant activation of the bumetanide-sensitive K(+) uptake with only a minimal increase in bumetanide-sensitive Cl(-) uptake. This suggests that when stimulated by hypertonicity, the cotransporter mediates K(+)/K(+) and Cl(-)/Cl(-) exchange. Although significant stimulation of K(+)/K(+) exchange was observed with NKCC1, a significantly smaller hyperosmotic stimulatory effect was observed with NKCC2. In order to identify the molecular determinant(s) of this NKCC1-specific activation, we created chimeras of the mouse NKCC1 and the rat NKCC2. Swapping the regulatory amino termini of the cotransporters neither conferred activation to NKCC2 nor prevented activation of NKCC1. Using unique restrictions sites, we created additional chimeric molecules and determined that the first intracellular loop between membrane-spanning domains one and two and the second extracellular loop between membrane-spanning domains three and four of NKCC1 are necessary components of the hyperosmotic stimulation of K(+)/K(+) exchange.