Apoptosis-associated tyrosine kinase scaffolding of protein phosphatase 1 and SPAK reveals a novel pathway for Na-K-2C1 cotransporter regulation

Predictive and Experimental Motif Interaction Analysis Identifies Functions of the WNK-OSR1/SPAK Pathway

The WNK-OSR1/SPAK protein kinase signaling pathway regulates ion homeostasis and cell volume, but its other functions are poorly understood. To uncover undefined signaling functions of the pathway we analyzed the binding specificity of the conserved C-terminal (CCT) domains of OSR1 and SPAK to find all possible interaction motifs in human proteins. These kinases bind the core consensus sequences R-F-x-V/I and R-x-F-x-V/I. Motifs were ranked based on sequence, conservation, cellular localization, and solvent accessibility. Out of nearly 3,700 motifs identified, 90% of previously published motifs were within the top 2% of those predicted. Selected candidates (TSC22D1, CAVIN1, ATG9A, NOS3, ARHGEF5) were tested. Upstream kinases WNKs 1-4 and their close relatives, the pseudokinases NRBP1/2, contain CCT-like domains as well. We identified additional distinct motif variants lacking the conserved arginine previously thought to be required, and found that the NRBP1 CCT-like domain binds TSC22D1 via the same motif as OSR1 and SPAK. Our results further highlight the rich and diverse functionality of CCT and CCT-like domains in connecting WNK signaling to cellular processes.

A revised nomenclature for the lemur family of protein kinases

The lemur family of protein kinases has gained much interest in recent years as they are involved in a variety of cellular processes including regulation of axonal transport and endosomal trafficking, modulation of synaptic functions, memory and learning, and they are centrally placed in several intracellular signalling pathways. Numerous studies have also implicated role of the lemur kinases in the development and progression of a wide range of cancers, cystic fibrosis, and neurodegenerative diseases. However, parallel discoveries and inaccurate prediction of their kinase activity have resulted in a confusing and misleading nomenclature of these proteins. Herein, a group of international scientists with expertise in lemur family of protein kinases set forth a novel nomenclature to rectify this problem and ultimately help the scientific community by providing consistent information about these molecules.

Pharmacology of Compounds Targeting Cation-Chloride Cotransporter Physiology

Transporters of the solute carrier family 12 (SLC12) carry inorganic cations such as Na+ and/or K+ alongside Cl across the plasma membrane of cells. These tightly coupled, electroneutral, transporters are expressed in almost all tissues/organs in the body where they fulfil many critical functions. The family includes two key transporters participating in salt reabsorption in the kidney: the Na-K-2Cl cotransporter-2 (NKCC2), expressed in the loop of Henle, and the Na-Cl cotransporter (NCC), expressed in the distal convoluted tubule. NCC and NKCC2 are the targets of thiazides and "loop" diuretics, respectively, drugs that are widely used in clinical medicine to treat hypertension and edema. Bumetanide, in addition to its effect as a loop diuretic, has recently received increasing attention as a possible therapeutic agent for neurodevelopmental disorders. This chapter also describes how over the past two decades, the pharmacology of Na+ independent transporters has expanded significantly to provide novel tools for research. This work has indeed led to the identification of compounds that are 100-fold to 1000-fold more potent than furosemide, the first described inhibitor of K-Cl cotransport, and identified compounds that possibly directly stimulate the function of the K-Cl cotransporter. Finally, the recent cryo-electron microscopy revolution has begun providing answers as to where and how pharmacological agents bind to and affect the function of the transporters.

The serine-threonine protein phosphatases that regulate the thiazide-sensitive NaCl cotransporter

The activity of the Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT) is finely tuned by phosphorylation networks involving serine/threonine kinases and phosphatases. While much attention has been paid to the With-No-lysine (K) kinase (WNK)- STE20-related Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive kinase 1 (OSR1) signaling pathway, there remain many unanswered questions regarding phosphatase-mediated modulation of NCC and its interactors. The phosphatases shown to regulate NCC's activity, directly or indirectly, are protein phosphatase 1 (PP1), protein phosphatase 2A (PP2A), calcineurin (CN), and protein phosphatase 4 (PP4). PP1 has been suggested to directly dephosphorylate WNK4, SPAK, and NCC. This phosphatase increases its abundance and activity when extracellular K⁺ is increased, which leads to distinct inhibitory mechanisms towards NCC. Inhibitor-1 (I1), oppositely, inhibits PP1 when phosphorylated by protein kinase A (PKA). CN inhibitors, like tacrolimus and cyclosporin A, increase NCC phosphorylation, giving an explanation to the Familial Hyperkalemic Hypertension-like syndrome that affects some patients treated with these drugs. CN inhibitors can prevent high K⁺-induced dephosphorylation of NCC. CN can also dephosphorylate and activate Kelch-like protein 3 (KLHL3), thus decreasing WNK abundance. PP2A and PP4 have been shown in in vitro models to regulate NCC or its upstream activators. However, no studies in native kidneys or tubules have been performed to test their physiological role in NCC regulation. This review focuses on these dephosphorylation mediators and the transduction mechanisms possibly involved in physiological states that require of the modulation of the dephosphorylation rate of NCC.

Epigenetically silenced apoptosis-associated tyrosine kinase (AATK) facilitates a decreased expression of Cyclin D1 and WEE1, phosphorylates TP53 and reduces cell proliferation in a kinase-dependent manner

Silencing of the Apoptosis associated Tyrosine Kinase gene (AATK) has been described in cancer. In our study, we specifically investigated the epigenetic inactivation of AATK in pancreatic adenocarcinoma, lower grade glioma, lung, breast, head, and neck cancer. The resulting loss of AATK correlates with impaired patient survival. Inhibition of DNA methyltransferases (DNMTs) reactivated AATK in glioblastoma and pancreatic cancer. In contrast, epigenetic targeting via the CRISPR/dCas9 system with either EZH2 or DNMT3A inhibited the expression of AATK. Via large-scale kinomic profiling and kinase assays, we demonstrate that AATK acts a Ser/Thr kinase that phosphorylates TP53 at Ser366. Furthermore, whole transcriptome analyses and mass spectrometry associate AATK expression with the GO term 'regulation of cell proliferation'. The kinase activity of AATK in comparison to the kinase-dead mutant mediates a decreased expression of the key cell cycle regulators Cyclin D1 and WEE1. Moreover, growth suppression through AATK relies on its kinase activity. In conclusion, the Ser/Thr kinase AATK represses growth and phosphorylates TP53. Furthermore, expression of AATK was correlated with a better patient survival for different cancer entities. This data suggests that AATK acts as an epigenetically inactivated tumor suppressor gene.

Lemur tail kinase 1 (LMTK1) regulates the endosomal localization of β-secretase BACE1

Lemur tail kinase 1 (LMTK1), previously called apoptosis-associated tyrosine kinase (AATYK), is an endosomal Ser/Thr kinase. We recently reported that LMTK1 regulates axon outgrowth, dendrite arborization and spine formation via Rab11-mediated vesicle transport. Rab11, a small GTPase regulating recycling endosome trafficking, is shown to be associated with late-onset Alzheimer's disease (LOAD). In fact, genome-wide association studies identified many proteins regulating vesicle transport as risk factors for LOAD. Furthermore, LMTK1 has been reported to be a risk factor for frontotemporal dementia. Then, we hypothesized that LMTK1 contributes to AD development through vesicle transport and examined the effect of LMTK1 on the cellular localization of AD-related proteins, amyloid precursor protein (APP) and β-site APP cleaving enzyme 1 (BACE1). The β-cleavage of APP by BACE1 is the initial and rate-limiting step in Aβ generation. We found that LMTK1 accumulated BACE1, but not APP, to the perinuclear endosomal compartment, whereas the kinase-negative (kn) mutant of LMTK1A did not. The β-C-terminal fragment was prone to increase under overexpression of LMTK1A kn. Moreover, the expression level of LMTK1A was reduced in AD brains. These results suggest the possibility that LMTK1 is involved in AD development through the regulation of the proper endosomal localization of BACE1.

Roles of WNK4 and SPAK in K+-mediated dephosphorylation of the NaCl cotransporter

Phosphorylation of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule (DCT) is altered rapidly in response to changes in extracellular K+ concentration ([K+]). High extracellular [K+] is believed to activate specific phosphatases to dephosphorylate NCC, thereby reducing its activity. This process is defective in the human disease familial hyperkalemic hypertension, in which extracellular [K+] fails to dephosphorylate NCC, suggesting an interplay between NCC-activating and NCC-inactivating switches. Here, we explored the role of STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and intracellular Cl- concentration in the rapid effects of extracellular K+ on NCC phosphorylation. SPAK was found to be rapidly dephosphorylated in vitro in human embryonic kidney cells and ex vivo in kidney slices by high [K+]. Acute high-K+ challenge resulted in DCT1-specific SPAK dephosphorylation in vivo and dissolution of SPAK puncta. In line with the postulate of interplay between activating and inactivating switches, we found that the "on" switch, represented by with no lysine kinase 4 (WNK4)-SPAK, must be turned off for rapid NCC dephosphorylation by high [K+]. Longer-term WNK-SPAK-mediated stimulation, however, altered the sensitivity of the system, as it attenuated rapid NCC dephosphorylation due to acute K+ loading. Although blockade of protein phosphatase (PP)1 increased NCC phosphorylation at baseline, neither PP1 nor PP3, singly or in combination, was essential for NCC dephosphorylation. Overall, our data suggest that NCC phosphorylation is regulated by a dynamic equilibrium between activating kinases and inactivating phosphatases, with kinase inactivation playing a key role in the rapid NCC dephosphorylation by high extracellular K+.NEW & NOTEWORTHY Although a great deal is known about mechanisms by which thiazide-sensitive NaCl cotransporter is phosphorylated and activated, much less is known about dephosphorylation. Here, we show that rapid dephosphorylation by high K+ depends on the Cl- sensitivity of with no lysine kinase 4 and the rapid dephosphorylation of STE20/SPS1-related proline-alanine-rich protein kinase, primarily along the early distal convoluted tubule.

LMTK1, a Novel Modulator of Endosomal Trafficking in Neurons

Neurons extend long processes known as axons and dendrites, through which they communicate with each other. The neuronal circuits formed by the axons and dendrites are the structural basis of higher brain functions. The formation and maintenance of these processes are essential for physiological brain activities. Membrane components, both lipids, and proteins, that are required for process formation are supplied by vesicle transport. Intracellular membrane trafficking is regulated by a family of Rab small GTPases. A group of Rabs regulating endosomal trafficking has been studied mainly in nonpolarized culture cell lines, and little is known about their regulation in polarized neurons with long processes. As shown in our recent study, lemur tail (former tyrosine) kinase 1 (LMTK1), an as yet uncharacterized Ser/Thr kinase associated with Rab11-positive recycling endosomes, modulates the formation of axons, dendrites, and spines in cultured primary neurons. LMTK1 knockdown or knockout (KO) or the expression of a kinase-negative mutant stimulates the transport of endosomal vesicles in neurons, leading to the overgrowth of axons, dendrites, and spines. More recently, we found that LMTK1 regulates TBC1D9B Rab11 GAP and proposed the Cdk5/p35-LMTK1-TBC1D9B-Rab11 pathway as a signaling cascade that regulates endosomal trafficking. Here, we summarize the biochemical, cell biological, and physiological properties of LMTK1.

The LMTK1-TBC1D9B-Rab11A Cascade Regulates Dendritic Spine Formation via Endosome Trafficking

Dendritic spines are postsynaptic protrusions at excitatory synapses that are critical for proper neuronal synaptic transmission. While lipid and protein membrane components are necessary for spine formation, it is largely unknown how they are recruited to developing spines. Endosomal trafficking is one mechanism that may influence this development. We recently reported that Lemur kinase 1A (LMTK1A), a membrane-bound Ser/Thr kinase, regulates trafficking of endosomes in neurons. LMTK1 has been shown to be a p35 Cdk5 activator-binding protein and a substrate for Cdk5-p35; however, its neuronal function has not been sufficiently studied. Here, we investigate the role of LMTK1 in spine formation. Depletion of LMTK1 increases spine formation, maturation, and density in primary cultured neurons and in mouse brain of either sex. Additionally, expression of kinase-negative LMTK1 stimulates spine formation in primary neurons and in vivo LMTK1 controls spine formation through Rab11, a regulator of recycling endosome trafficking. We identify TBC1D9B, a Rab11A GTPase-activating protein (Rab11A GAP), as a LMTK1 binding protein, and find that TBC1D9B mediates LMTK1 activity on Rab11A. TBC1D9B inactivates Rab11A under the control of LMTK1A. Further, by analyzing the effect of decreased TBC1D9B expression in primary neurons, we demonstrate that TBC1D9B indeed regulates spine formation. This is the first demonstration of the biological function of TBC1D9B. Together, with the regulation of LMTK1 by Cdk5-p35, we propose the Cdk5-LMTK1-TBC1D9B-Rab11A cascade as a novel signaling mechanism regulating endosomal transport for synapse formation and function.SIGNIFICANCE STATEMENT Dendritic spines are postsynaptic specializations essential for synaptic transmission. However, it is not known how critical membrane components are recruited to spines for their formation. Endosomal trafficking is one such mechanism that may mediate this process. Here we investigate regulators of endosomal trafficking and their contribution to spine formation. We identify two novel factors, LMTK1 and TBC1D9B, which regulate spine formation upstream of Rab11A, a small GTPase. LMTK1 is a membrane bound Ser/Thr kinase regulated by Cdk5-p35, and TBC1D9B is a recently identified Rab11 GAP. LMTK1 controls the GAP activity of TBC1D9B on Rab11A, and TBC1D9B mediates the LMTK1 activity on Rab11A. We propose the Cdk5-LMTK1-TBC1D9B-Rab11A cascade as a novel mechanism controlling spine formation and function.

The LMTK-family of kinases: Emerging important players in cell physiology and pathogenesis

Lemur Tail (former tyrosine) Kinases (LMTKs) comprise a novel family of regulated serine/threonine specific kinases with three structurally and evolutionary related members. LMTKs exercise a confusing variety of cytosolic functions in cell signalling and membrane trafficking. Moreover, LMTK2 and LMTK3 also reside in the nucleus where they participate in gene transcription/regulation. As a consequence, LMTKs impact cell proliferation and apoptosis, cell growth and differentiation, as well as cell migration. All these fundamental cell behaviours can turn awry, most prominently during neuropathologies and tumour biogenesis. In cancer cells, LMTK levels are often correlated with poor overall prognosis and therapy outcome, not least owned to acquired drug resistance. In brain tissue, LMTKs are highly expressed and have been linked to neuronal and glia cell differentiation and cell homeostasis. For one member of the LMTK-family (LMTK2) a role in cystic fibrosis has been identified. Due to their role in fundamental cell processes, altered LMTK physiology may also warrant a hitherto unappreciated role in other diseases, and expose them as potential valuable drug targets. On the backdrop of a compendium of LMTK cell functions, we hypothesize that the primary role of LMTKs may dwell within the endocytic cargo recycling and/or nuclear receptor transport pathways.

LMTK3 is essential for oncogenic KIT expression in KIT-mutant GIST and melanoma

Certain cancers, including gastrointestinal stromal tumor (GIST) and subsets of melanoma, are caused by somatic KIT mutations that result in KIT receptor tyrosine kinase constitutive activity, which drives proliferation. The treatment of KIT-mutant GIST has been revolutionized with the advent of KIT-directed cancer therapies. KIT tyrosine kinase inhibitors (TKI) are superior to conventional chemotherapy in their ability to control advanced KIT-mutant disease. However, these therapies have a limited duration of activity due to drug-resistant secondary KIT mutations that arise (or that are selected for) during KIT TKI treatment. To overcome the problem of KIT TKI resistance, we sought to identify novel therapeutic targets in KIT-mutant GIST and melanoma cells using a human tyrosine kinome siRNA screen. From this screen, we identified lemur tyrosine kinase 3 (LMTK3) and herein describe its role as a novel KIT regulator in KIT-mutant GIST and melanoma cells. We find that LMTK3 regulated the translation rate of KIT, such that loss of LMTK3 reduced total KIT, and thus KIT downstream signaling in cancer cells. Silencing of LMTK3 decreased cell viability and increased cell death in KIT-dependent, but not KIT-independent GIST and melanoma cell lines. Notably, LMTK3 silencing reduced viability of all KIT-mutant cell lines tested, even those with drug-resistant KIT secondary mutations. Furthermore, targeting of LMTK3 with siRNA delayed KIT-dependent GIST growth in a xenograft model. Our data suggest the potential of LMTK3 as a target for treatment of patients with KIT-mutant cancer, particularly after failure of KIT TKIs.

Trafficking and regulation of the NKCC2 cotransporter in the thick ascending limb

PURPOSE OF REVIEW

The kidney Na-K-2Cl cotransporter (NKCC2) is essential for urinary concentration and renal electrolyte handling. Loss of function mutations in the NKCC2 gene cause urinary salt and potassium wasting, whereas excessive NKCC2 function has been linked to high blood pressure. Loop diuretics, targeting the transporter, are instrumental for relieving edema or hypertension. This review focuses on intrinsic mechanisms regulating NKCC2 activity at the posttranslational level, namely its trafficking and phosphorylation.

RECENT FINDINGS

Protein networks mediating cellular turnover of NKCC2 have recently received major attention. Several key components of its apical trafficking were identified, including respective chaperones, SNARE protein family members and raft-associated proteins. NKCC2 internalization has been characterized qualitatively and quantitatively. Kinase and phosphatase pathways regulating NKCC2 activity have been clarified and links between NKCC2 phosphorylation and trafficking proposed. Constitutive and inducible NKCC2 trafficking and phosphorylation mechanisms have been specified with focus on endocrine control of thick ascending limb (TAL) function by vasopressin.

SUMMARY

Proper NKCC2 trafficking and phosphorylation are critical to the TAL function in the physiological context of urinary concentration and extracellular volume regulation. Clarification of the underlying mechanisms and respective protein networks may open new therapeutic perspectives for better management of renal electrolyte disorders and blood pressure control.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

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.

Phosphoregulation of K+ -Cl- cotransporters during cell swelling: Novel insights

The K+ -Cl- cotransporters (KCCs) belong to the cation-Cl- cotransporter family and consist of four isoforms and many splice variants. Their main role is to promote electroneutral efflux of K+ and Cl- ions across the surface of many cell types and, thereby, to regulate intracellular ion concentration, cell volume, and epithelial salt movement. These transport systems are induced by an increase in cell volume and are less active at lower intracellular [Cl- ] (Cli ), but the mechanisms at play are still ill-defined. In this work, we have exploited the Xenopus laevis expression system to study the role of lysine-deficient protein kinases (WNKs), protein phosphatases 1 (PP1s), and SPS1-related proline/alanine-rich kinase (SPAK) in KCC4 regulation during cell swelling. We have found that WNK4 and PP1 regulate KCC4 activity as part of a common signaling module, but that they do not exert their effects through SPAK or carrier dephosphorylation. We have also found that the phosphatases at play include PP1α and PP1γ1, but that WNK4 acts directly on the PP1s instead of the opposite. Unexpectedly, however, both cell swelling and a T926A substitution in the C-terminus of full-length KCC4 led to higher levels of heterologous K+ -Cl- cotransport and overall carrier phosphorylation. These results imply that the response to cell swelling must also involve allosteric-sensitive kinase-dependent phosphoacceptor sites in KCC4. They are thus partially inconsistent with previous models of KCC regulation.

Functional identification of protein phosphatase 1-binding consensus residues in NBCe1-B

Protein phosphatase 1 (PP1) is involved in various signal transduction mechanisms as an extensive regulator. The PP1 catalytic subunit (PP1c) recognizes and binds to PP1-binding consensus residues (FxxR/KxR/K) in NBCe1-B. Consequently, we focused on identifying the function of the PP1-binding consensus residue, ⁹²²FMDRLK⁹²⁷, in NBCe1-B. Using site-directed mutagenesis and co-immunoprecipitation assays, we revealed that in cases where the residues were substituted (F922A, R925A, and K927A) or deleted (deletion of amino acids 922–927), NBCe1-B mutants inhibited PP1 binding to NBCe1-B. Additionally, by recording the intracellular pH, we found that PP1-binding consensus residues in NBCe1-B were not only critical for NBCe1-B activity, but also relevant to its surface expression level. Therefore, we reported that NBCe1-B, as a substrate of PP1, contains these residues in the C-terminal region and that the direct interaction between NBCe1-B and PP1 is functionally critical in controlling the regulation of the HCO₃⁻ transport. These results suggested that like IRBIT, PP1 was another novel regulator of HCO₃⁻ secretion in several types of epithelia.

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.

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.

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.

The power of the yeast two-hybrid system in the identification of novel drug targets: building and modulating PPP1 interactomes

Since the description of the yeast two-hybrid (Y2H) method, it has become more and more evident that it is the most commonly used method to identify protein-protein interactions (PPIs). The improvements in the original Y2H methodology in parallel with the idea that PPIs are promising drug targets, offer an excellent opportunity to apply the principles of this molecular biology technique to the pharmaceutical field. Additionally, the theoretical developments in the networks field make PPI networks very useful frameworks that facilitate many discoveries in biomedicine. This review highlights the relevance of Y2H in the determination of PPIs, specifically phosphoprotein phosphatase 1 interactions, and its possible outcomes in pharmaceutical research.

Calcineurin and Sorting-Related Receptor with A-Type Repeats Interact to Regulate the Renal Na⁺-K⁺-2Cl⁻ Cotransporter

The furosemide-sensitive Na(+)-K(+)-2Cl(-)-cotransporter (NKCC2) is crucial for NaCl reabsorption in kidney thick ascending limb (TAL) and drives the urine concentrating mechanism. NKCC2 activity is modulated by N-terminal phosphorylation and dephosphorylation. Serine-threonine kinases that activate NKCC2 have been identified, but less is known about phosphatases that deactivate NKCC2. Inhibition of calcineurin phosphatase has been shown to stimulate transport in the TAL and the distal convoluted tubule. Here, we identified NKCC2 as a target of the calcineurin Aβ isoform. Short-term cyclosporine administration in mice augmented the abundance of phospho-NKCC2, and treatment of isolated TAL with cyclosporine increased the chloride affinity and transport activity of NKCC2. Because sorting-related receptor with A-type repeats (SORLA) may affect NKCC2 phosphoregulation, we used SORLA-knockout mice to test whether SORLA is involved in calcineurin-dependent modulation of NKCC2. SORLA-deficient mice showed more calcineurin Aβ in the apical region of TAL cells and less NKCC2 phosphorylation and activity compared with littermate controls. In contrast, overexpression of SORLA in cultured cells reduced the abundance of endogenous calcineurin Aβ. Cyclosporine administration rapidly normalized the abundance of phospho-NKCC2 in SORLA-deficient mice, and a functional interaction between calcineurin Aβ and SORLA was further corroborated by binding assays in rat kidney extracts. In summary, we have shown that calcineurin Aβ and SORLA are key components in the phosphoregulation of NKCC2. These results may have clinical implications for immunosuppressive therapy using calcineurin inhibitors.

A novel Ste20-related proline/alanine-rich kinase (SPAK)-independent pathway involving calcium-binding protein 39 (Cab39) and serine threonine kinase with no lysine member 4 (WNK4) in the activation of Na-K-Cl cotransporters

Na(+)-dependent chloride cotransporters (NKCC1, NKCC2, and NCC) are activated by phosphorylation to play critical roles in diverse physiological responses, including renal salt balance, hearing, epithelial fluid secretion, and volume regulation. Serine threonine kinase WNK4 (With No K = lysine member 4) and members of the Ste20 kinase family, namely SPAK and OSR1 (Ste20-related proline/alanine-rich kinase, Oxidative stress-responsive kinase) govern phosphorylation. According to present understanding, WNK4 phosphorylates key residues within SPAK/OSR1 leading to kinase activation, allowing SPAK/OSR1 to bind to and phosphorylate NKCC1, NKCC2, and NCC. Recently, the calcium-binding protein 39 (Cab39) has emerged as a binding partner and enhancer of SPAK/OSR1 activity, facilitating kinase autoactivation and promoting phosphorylation of the cotransporters. In the present study, we provide evidence showing that Cab39 differentially interacts with WNK4 and SPAK/OSR1 to switch the classic two kinase cascade into a signal kinase transduction mechanism. We found that WNK4 in association with Cab39 activates NKCC1 in a SPAK/OSR1-independent manner. We discovered that WNK4 possesses a domain that bears close resemblance to the SPAK/OSR1 C-terminal CCT/PF2 domain, which is required for physical interaction between the Ste20 kinases and the Na(+)-driven chloride cotransporters. Modeling, yeast two-hybrid, and functional data reveal that this PF2-like domain located downstream of the catalytic domain in WNK4 promotes the direct interaction between the kinase and NKCC1. We conclude that in addition to SPAK and OSR1, WNK4 is able to anchor itself to the N-terminal domain of NKCC1 and to promote cotransporter activation.

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.

The WNK/SPAK and IRBIT/PP1 pathways in epithelial fluid and electrolyte transport

Fluid and electrolyte homeostasis is a fundamental physiological function required for survival and is associated with a plethora of diseases when aberrant. Systemic fluid and electrolyte composition is regulated by the kidney, and all secretory epithelia generate biological fluids with defined electrolyte composition by vectorial transport of ions and the obligatory water. A major regulatory pathway that immerged in the last several years is regulation of ion transporters by the WNK/SPAK kinases and IRBIT/PP1 pathways. The IRBIT/PP1 pathway functions to reverse the effects of the WNK/SPAK kinases pathway, as was demonstrated for NBCe1-B and CFTR. Since many transporters involved in fluid and electrolyte homeostasis are affected by PP1 and/or calcineurin, it is possible that WNK/SPAK and IRBIT/PP1 form a common regulatory pathway to tune the activity of fluid and electrolyte transport in response to physiological demands.

Molecular physiology of SPAK and OSR1: two Ste20-related protein kinases regulating ion transport

SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress responsive kinase) are members of the germinal center kinase VI subfamily of the mammalian Ste20 (Sterile20)-related protein kinase family. Although there are 30 enzymes in this protein kinase family, their conservation across the fungi, plant, and animal kingdom confirms their evolutionary importance. Already, a large volume of work has accumulated on the tissue distribution, binding partners, signaling cascades, and physiological roles of mammalian SPAK and OSR1 in multiple organ systems. After reviewing this basic information, we will examine newer studies that demonstrate the pathophysiological consequences to SPAK and/or OSR1 disruption, discuss the development and analysis of genetically engineered mouse models, and address the possible role these serine/threonine kinases might have in cancer proliferation and migration.

Effects of ACE inhibition and ANG II stimulation on renal Na-Cl cotransporter distribution, phosphorylation, and membrane complex properties

The renal distal tubule Na-Cl cotransporter (NCC) reabsorbs <10% of the filtered Na(+) but is a key control point for blood pressure regulation by angiotensin II (ANG II), angiotensin-converting enzyme inhibitors (ACEI), and thiazide diuretics. This study aimed to determine whether NCC phosphorylation (NCCp) was regulated by acute (20-30 min) treatment with the ACEI captopril (12 μg/min × 20 min) or by a sub-pressor dose of ANG II (20 ng·kg(-1)·min(-1)) in Inactin-anesthetized rats. By immuno-EM, NCCp was detected exclusively in or adjacent to apical plama membranes (APM) in controls and after ACEI or ANG II treatment, while NCC total was detected in both APM and subapical cytoplasmic vesicles (SCV) in all conditions. In renal homogenates, neither ACEI nor ANG II treatment altered NCCp abundance, assayed by immunoblot. However, by density gradient fractionation we identified a pool of low-density APM in which NCCp decreased 50% in response to captopril and was restored during ANG II infusion, and another pool of higher-density APM that responded reciprocally, indicative of regulated redistribution between two APM pools. In both pools, NCCp was preferentially localized to Triton-soluble membranes. Blue Native gel electrophoresis established that APM NCCp localized to ~700 kDa complexes (containing γ-adducin) while unphosphorylated NCC in intracellular membranes primarily localized to ~400 kDa complexes: there was no evidence for native monomeric or dimeric NCC or NCCp. In summary, this study demonstrates that phosphorylated NCC, localized to multimeric complexes in the APM, redistributes in a regulated manner within the APM in response to ACEI and ANG II.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

Protein phosphatase 1α interacting proteins in the human brain

Protein Phosphatase 1 (PP1) is a major serine/threonine-phosphatase whose activity is dependent on its binding to regulatory subunits known as PP1 interacting proteins (PIPs), responsible for targeting PP1 to a specific cellular location, specifying its substrate or regulating its action. Today, more than 200 PIPs have been described involving PP1 in panoply of cellular mechanisms. Moreover, several PIPs have been identified that are tissue and event specific. In addition, the diversity of PP1/PIP complexes can further be achieved by the existence of several PP1 isoforms that can bind preferentially to a certain PIP. Thus, PP1/PIP complexes are highly specific for a particular function in the cell, and as such, they are excellent pharmacological targets. Hence, an in-depth survey was taken to identify specific PP1α PIPs in human brain by a high-throughput Yeast Two-Hybrid approach. Sixty-six proteins were recognized to bind PP1α, 39 being novel PIPs. A large protein interaction databases search was also performed to integrate with the results of the PP1α Human Brain Yeast Two-Hybrid and a total of 246 interactions were retrieved.

Increased aquaporin-1 and Na+ -K+ -2Cl- cotransporter 1 expression in choroid plexus leads to blood-cerebrospinal fluid barrier disruption and necrosis of hippocampal CA1 cells in acute rat models of hyponatremia

Hyponatremia is a metabolic disorder characterized by increased cerebrospinal fluid (CSF) volume and pressure, although the site of brain insult is unclear. Specifically, the hippocampus, which is in direct contact with expanding CSF ventricles, may be involved. The present study was undertaken to investigate the possible roles of choroid plexus aquaporin-1 (AQP1) and of cation chloride transporters (Na(+) -K(+) -2Cl(-) cotransporter 1 [NKCC1] and K(+) -Cl(-) cotransporter 4 [KCC4]) in the underlying hippocampal pathophysiology of hyponatremia in acute (6 and 12 hr duration) experimental models. It was found that the expressions of AQP1 and NKCC1 proteins in choroid plexus were significantly increased, whereas the expression of KCC4 protein was unchanged vs. control values after 6 and 12 hr of hyponatremia. Choroid plexuses with increased AQP1 and NKCC1 after 6 hr of hyponatremia showed caspase 3-dependent apoptosis and disruption of the blood-CSF barrier. Furthermore, necrotic changes in CA1 neuronal cells were observed after 6 and 12 hr of hyponatremia. Overall, these data suggest that increases in AQP1 and NKCC1 expression under hyposmotic stress may be one of the molecular mechanisms underlying the pathophysiology of acute hyponatremia, such as the necrotic cell death of hippocampal CA1 region by increasing water transport across the blood-CSF barrier. Furthermore, we suggest that opening of the blood-CSF barrier after acute hyponatremia may be triggered the secondary adverse conditions that are capable of enhancing selective necrosis in hippocampal CA1 cells.

SPAK and WNK kinases: a new target for blood pressure treatment?

PURPOSE OF REVIEW

The regulation of sodium reabsorption by the distal kidney is fundamental to blood pressure control. The clinical success of thiazide diuretics as antihypertensive drugs underscores the importance of its target, the thiazide-sensitive sodium/chloride cotransporter (NCC), in this process. However, thiazides are often ineffective as monotherapy and have significant side-effects. An understanding of NCC regulation at a molecular level may allow the design of better tolerated and more effective antihypertensive agents. The aim of this review is to provide an overview of the recent developments in the regulation of NCC, highlighting a potential new therapeutic target for the treatment of hypertension.

RECENT FINDINGS

It has been appreciated for several years that WNK kinases affect NCC expression, following the discovery that mutations in WNK genes cause Gordon syndrome (pseudohypoaldosteronism type II), although the precise molecular mechanisms were unclear. What has emerged from further in-vitro work is a WNK signalling cascade with the STE20 kinases SPAK and OSR1 as the 'missing' intermediate kinases that are activated by WNKs. Confirmation that this WNK-SPAK cascade operates in vivo comes from work on the phenotype of a kinase-dead SPAK knockin mouse. This mouse is markedly hypotensive, salt wasting, and almost all of its NCC protein in the distal kidney is dephosphorylated. Finally, a genome-wide association study has identified an intronic SPAK polymorphism that associates with human blood pressure.

SUMMARY

SPAK is a recently identified regulator of NCC and, hence, sodium reabsorption in the distal nephron. SPAK gene variants may also be important players in essential hypertension. If the phenotype of the kinase-dead SPAK mouse mimics pharmacological inhibition of this kinase, then SPAK is a potentially very interesting new antihypertensive drug target.

Thiazide-induced hyponatraemia: epidemiology and clues to pathogenesis

Thiazide diuretics are one of the most widely used and cost-effective classes of antihypertensive agents worldwide. Thiazides however have a significant side effect profile and are frequently insufficient to normalize blood pressure alone. Thiazide-induced hyponatraemia (TIH) is a major adverse effect, affecting up to one in seven patients receiving these drugs. TIH is more common in females, the elderly and those of low body weight and may cause symptoms such as confusion, falls and seizures. It is a common cause of hospital admission in the elderly. Although TIH occurs at least as frequently as hypokalaemia, much less is understood about the mechanism by which this occurs. Thiazides lower blood pressure by reducing the reabsorption of sodium from the distal nephron by inhibition of the NaCl cotransporter. The molecular mechanism by which this occurs together with the little known role of thiazides in regulating water reabsorbtion from the collecting ducts is discussed and the relevance to TIH evaluated. TIH is highly reproducible by thiazide rechallenge suggesting there may be a genetic predisposition. Both targeted resequencing of candidate genes and genome wide association techniques offer promising strategies by which such genetic contributions may be investigated. The rewards for uncovering the molecular mechanisms underlying TIH and the regulation of distal nephron sodium and water absorption are significant; not only could it inform the design of better tolerated, more efficacious thiazide-like antihypertensive agents but it may also facilitate the pharmacogenomic profiling of hypertensive patients to avoid thiazides in those likely to suffer adverse effects.

Phosphorylation and transport in the Na-K-2Cl cotransporters, NKCC1 and NKCC2A, compared in HEK-293 cells

Na-K-2Cl cotransporters help determine cell composition and volume. NKCC1 is widely distributed whilst NKCC2 is only found in the kidney where it plays a vital role reabsorbing 20% of filtered NaCl. NKCC2 regulation is poorly understood because of its restricted distribution and difficulties with its expression in mammalian cell cultures. Here we compare phosphorylation of the N-termini of the cotransporters, measured with phospho-specific antibodies, with bumetanide-sensitive transport of K⁺ (⁸⁶Rb⁺) (activity) in HEK-293 cells stably expressing fNKCC1 or fNKCC2A which were cloned from ferret kidney. Activities of transfected transporters were distinguished from those of endogenous ones by working at 37°C. fNKCC1 and fNKCC2A activities were highest after pre-incubation of cells in hypotonic low-[Cl⁻] media to reduce cell [Cl⁻] and volume during flux measurement. Phosphorylation of both transporters more than doubled. Pre-incubation with ouabain also strongly stimulated fNKCC1 and fNKCC2A and substantially increased phosphorylation, whereas pre-incubation in Na⁺-free media maximally stimulated fNKCC1 and doubled its phosphorylation, but inhibited fNKCC2A, with a small increase in its phosphorylation. Kinase inhibitors halved phosphorylation and activity of both transporters whereas inhibition of phosphatases with calyculin A strongly increased phosphorylation of both transporters but only slightly stimulated fNKCC1 and inhibited fNCCC2A. Thus kinase inhibition reduced phosphorylation and transport, and transport stimulation was only seen when phosphorylation increased, but transport did not always increase with phosphorylation. This suggests phosphorylation of the N-termini determines the transporters' potential capacity to move ions, but final activity also depends on other factors. Transport cannot be reliably inferred solely using phospho-specific antibodies on whole-cell lysates.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

Hypertension, dietary salt intake, and the role of the thiazide-sensitive sodium chloride transporter NCCT

A high salt intake in industrialized countries is an important cardiovascular risk factor. It remains at typically twice the maximum recommended levels of 5-6 g/day, and halving this would have enormous public health benefit in preventing stroke and cardiovascular disease. Salt homeostasis can also be affected pharmacologically by diuretic drugs that target mechanisms within the distal kidney nephron to cause salt wasting. Indeed, thiazide-type diuretics are among the most widely used agents in the management of hypertension and work by blocking NCCT, the NaCl-transporter in the distal nephron. The biology of this membrane transporter was not previously well understood until the discovery of the molecular basis of a rare familial form of hypertension called Gordon syndrome (pseudohypoaldosteronism type 2, PHAII). This has established that the NCCT transporter is dynamically regulated in the kidney by WNK kinases and a signaling cascade using a second kinase called SPAK. Common polymorphisms in the SPAK gene have recently been shown to affect blood pressure in human cohorts and removing its function lowers blood pressure in mice. The SPAK-deficient mouse actually has a phenotype reminiscent of Gitelman syndrome. This suggests that specific inhibitors of SPAK kinase may provide a novel class of diuretic drugs to lower blood pressure through salt wasting. The expectation is that SPAK inhibitors would mimic the on-target effects of thiazides but not their adverse off-target effects. An antihypertensive drug that could lower blood pressure with the efficacy of a thiazide without producing metabolic side effects such as hyperuricaemia or impaired glucose tolerance is therapeutically very attractive. It also exemplifies how data coming from the rare monogenic hypertension syndromes can together with genome-wide association studies in hypertension deliver novel druggable targets.

Progressive NKCC1-dependent neuronal chloride accumulation during neonatal seizures

Seizures induce excitatory shifts in the reversal potential for GABA(A)-receptor-mediated responses, which may contribute to the intractability of electro-encephalographic seizures and preclude the efficacy of widely used GABAergic anticonvulsants such as phenobarbital. We now report that, in intact hippocampi prepared from neonatal rats and transgenic mice expressing Clomeleon, recurrent seizures progressively increase the intracellular chloride concentration ([Cl(-)](i)) assayed by Clomeleon imaging and invert the net effect of GABA(A) receptor activation from inhibition to excitation assayed by the frequency of action potentials and intracellular Ca(2+) transients. These changes correlate with increasing frequency of seizure-like events and reduction in phenobarbital efficacy. The Na(+)-K(+)-2Cl(-) (NKCC1) cotransporter blocker bumetanide inhibited seizure-induced neuronal Cl(-) accumulation and the consequent facilitation of recurrent seizures. Our results demonstrate a novel mechanism by which seizure activity leads to [Cl(-)](i) accumulation, thereby increasing the probability of subsequent seizures. This provides a potential mechanism for the early crescendo phase of neonatal seizures.

AATYK1A phosphorylation by Cdk5 regulates the recycling endosome pathway

Trafficking of recycling endosomes (REs) is regulated by the small GTPase, Rab11A; however, the regulatory mechanism remains elusive. Apoptosis-associated tyrosine kinase 1A (AATYK1A) is a Ser/Thr kinase expressed highly in brain. We have recently shown that AATYK1A localizes to Rab11A-positive RE and is phosphorylated at Ser34 by cyclin-dependent kinase 5 (Cdk5). Here, we have investigated a role of AATYK1A and its phosphorylation in recycling endosomal trafficking using Chinese hamster ovary-K1 (CHO-K1) cells. AATYK1A localizes predominantly to Rab11A-positive pericentrosomal endocytic recycling compartment (ERC). Phosphorylation at Ser34 of AATYK1A disrupts its accumulation in the pericentrosomal ERC. Consistently, phosphorylation-mimic mutant (AATYK1A-S34D) did not accumulate in the ERC and additionally attenuated ERC formation. ERC formation suppression can be reversed by constitutively active Rab11A-Q70L, suggesting a functional link between AATYK1A phosphorylation and Rab11A activity. Although no direct interaction between AATYK1A and Rab11A could be detected, the exchange of guanine nucleotides bound to Rab11A was significantly reduced in the presence of the phosphorylation-mimic AATYK1A-S34D. Together, our results reveal a regulatory role for AATYK1A in the formation of pericentrosomal ERC. They furthermore indicate that Cdk5 can disrupt ERC formation via Ser34 phosphorylation of AATYK1A. Finally, our data suggest a mechanism by which AATYK1A signaling couples Cdk5 to Rab11A activity.

Multiple pathways for protein phosphatase 1 (PP1) regulation of Na-K-2Cl cotransporter (NKCC1) function: the N-terminal tail of the Na-K-2Cl cotransporter serves as a regulatory scaffold for Ste20-related proline/alanine-rich kinase (SPAK) AND PP1

The Na-K-2Cl cotransporter (NKCC1) participates in epithelial transport and in cell volume maintenance by mediating the movement of ions and water across plasma membranes. Functional studies have previously demonstrated that NKCC1 activity is stimulated by protein phosphatase 1 (PP1) inhibitors. In this study, we utilized both in vivo (heterologous cRNA expression in Xenopus laevis oocytes) and in vitro ((32)P-phosphorylation assays with glutathione S-transferase fusion proteins) experiments to determine whether PP1 exerts its inhibitory effect directly on the cotransporter, or indirectly by affecting the activating kinase. We found that PP1 reduced NKCC1 activity in oocytes under both isotonic and hypertonic conditions to the same level as in water-injected controls. Interestingly, mutation of key residues in the PP1 binding motif located in the N-terminal tail of NKCC1 significantly reduced the inhibitory effect of PP1. In vitro experiments performed with recombinant PP1, SPAK (Ste20-related proline/alanine-rich kinase, which activates NKCC1), and the N terminus of NKCC1 fused to glutathione S-transferase demonstrated that PP1 dephosphorylated both the kinase and the cotransporter in a time-dependent manner. More importantly, PP1 dephosphorylation of SPAK was significantly greater when protein-protein interaction between the kinase and the N-terminal tail of NKCC1 was present in the reaction, indicating the necessity of scaffolding the phosphatase and kinase in proximity to one another. Taken together, our data are consistent with PP1 inhibiting NKCC1 activity directly by dephosphorylating the cotransporter and indirectly by dephosphorylating SPAK.

A rare Asian founder polymorphism of Raptor may explain the high prevalence of Moyamoya disease among East Asians and its low prevalence among Caucasians

BACKGROUND

In an earlier study, we identified a locus for Moyamoya disease (MMD) on 17q25.3.

METHODS

Linkage analysis and fine mapping were conducted for two new families in additional to the previously studied 15 families. Three genes, CARD14, Raptor, and AATK, were selected based on key words, namely, "inflammation", "apoptosis", "proliferation", and "vascular system", for further sequencing. A segregation analysis of 34 pedigrees was performed, followed by a case-control study in Japanese (90 cases vs. 384 controls), Korean (41 cases vs. 223 controls), Chinese (23 cases and 100 controls), and Caucasian (25 cases and 164 controls) populations.

RESULTS

Linkage analysis increased the LOD score from 8.07 to 9.67 on 17q25.3. Fine mapping narrowed the linkage signal to a 2.1-Mb region. Sequencing revealed that only one newly identified polymorphism, ss161110142, which was located at position -1480 from the transcription site of the Raptor gene, was common to all four unrelated sequenced familial affected individuals. ss161110142 was then shown to segregate in the 34 pedigrees studied, resulting in a two-point LOD score of 14.2 (P = 3.89 × 10(-8)). Its penetrance was estimated to be 74.0%. Among the Asian populations tested (Japanese, Korean, and Chinese), the rare allele was much more frequent in cases (26, 33, and 4%, respectively) than in controls (1, 1, and 0%, respectively) and was associated with an increased odds ratio of 52.2 (95% confidence interval 27.2-100.2) (P = 2.5 × 10(-49)). This allele was, however, not detected in the Caucasian samples. Its population attributable risk was estimated to be 49% in the Japanese population, 66% in the Korean population, and 9% in the Chinese population.

CONCLUSION

ss161110142 may confer susceptibility to MMD among East Asian populations.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12199-009-0116-7) contains supplementary material, which is available to authorized users.

Renal and brain isoforms of WNK3 have opposite effects on NCCT expression

Mutations in the WNK kinases WNK1 and WNK4 cause a rare familial form of hypertension (Gordon syndrome) by increasing expression of the thiazide-sensitive co-transporter NCCT in the kidney. Regulation of NCCT expression involves a scaffold of proteins composed of several kinases, including the third member of the WNK kinase family, WNK3. This protein, expressed in several tissues including kidney and brain, displays splice variation around exons 18 and 22. We expressed these proteins in Xenopus oocytes and found that the renal isoform of WNK3 increased but the brain isoform decreased NCCT expression and activity. Introduction of a kinase-inactivating mutation into renal WNK3 reversed its action on NCCT, and the same mutation in the brain isoforms led to loss of function. We also studied the effect of phosphorylation of a key NCCT threonine (T58) on the effects of WNK3/4 coexpression; NCCT mutants with a T58A or T58D substitution had the same surface expression as T58 but had significantly altered transporter activity; however, both isoforms of WNK3 as well as WNK4 still modulated expression of these NCCT mutants. Finally, experiments using kinase-dead STE20/SPS1-related proline/alanine-rich kinase (SPAK), a putative downstream target for WNKs, revealed that brain WNK3 acts in tandem with SPAK, whereas renal WNK3 seems to upregulate NCCT through a SPAK-independent pathway. Taken together, these results suggest that the C-terminal motifs contributed by exons 18 and 22 play an important role in the actions of WNK3 isoforms on NCCT.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

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.

Cotransporters, WNKs and hypertension: an update

PURPOSE OF REVIEW

Studies of inherited conditions characterized by high or low blood pressure reveal the importance of a new signalling cascade, With no Lysine kinases (WNK) --> ste20/SPS1-related proline/alanine-rich kinase (SPAK)/oxidative stress-responsive kinase-1 (OSR1) --> Cation-Chloride Cotransporters (CCC), in regulating blood pressure and in the pathogenesis of essential hypertension. This review explores how these molecules interact to co-ordinate sodium homeostasis and how errors in these interactions may result in hypertension.

RECENT FINDINGS

Studies using transgenic animals and gene knockins have clarified the role of mutant WNK4 in hypertension, by revealing its main action to be increasing the expression and activity of sodium-chloride cotransporter (NCC) in the kidney. Functional studies show how phosphorylation of WNK1 regulates both its activity and ability to interact with SPAK/OSR1, and clearly place it upstream of SPAK/OSR1 in the cascade. The structural basis for the interactions between SPAK/OSR1 and targets has been identified.

SUMMARY

WNKs, activated by upstream kinases or autophosphorylation, bind and phosphorylate SPAK/OSR1, which in turn phosphorylate and activate NCCs and Na-K-Cl cotransporters (NKCCs). This increases sodium retention in the kidney (NKCC2, NCC) and vascular resistance (NKCC1), but decreases renin release (NKCC1). Hypertension-associated mutant WNKs increase surface expression and activation of renal tubular NKCC2 and NCC. Whether this adequately explains the hypertension awaits studies of these mutants in other tissues.

Molecular physiology of the WNK kinases

Mutations in the serine-threonine kinases WNK1 and WNK4 cause a Mendelian disease featuring hypertension and hyperkalemia. In vitro and in vivo studies have revealed that these proteins are molecular switches that have discrete functional states that impart different effects on downstream ion channels, transporters, and the paracellular pathway. These effects enable the distal nephron to allow either maximal NaCl reabsorption or maximal K+ secretion in response to hypovolemia or hyperkalemia, respectively. The related kinase WNK3 has reciprocal actions on the primary mediators of cellular Cl(-) influx and efflux, effects that can serve to regulate cell volume during growth and in response to osmotic stress as well as to modulate neuronal responses to GABA. These findings define a versatile new family of kinases that coordinate the activities of diverse ion transport pathways to achieve and maintain fluid and electrolyte homeostasis.

Activation of the thiazide-sensitive Na+-Cl- cotransporter by the WNK-regulated kinases SPAK and OSR1

Mutations increasing WNK1 kinase expression in humans cause the pseudohypoaldosteronism type II hypertension syndrome. This condition is treated effectively by thiazide diuretics, which exert their effects by inhibiting the Na+-Cl(-) cotransporter (NCC), suggesting a link between WNK1 and NCC. Here, we demonstrate that the SPAK and OSR1 kinases that are activated by WNK1 phosphorylate human NCC at three conserved residues (Thr46, Thr55 and Thr60). Activation of the WNK1-SPAK/OSR1 signalling pathway by treatment of HEK293 or mpkDCT kidney distal-convoluted-tubule-derived cells with hypotonic low-chloride conditions induced phosphorylation of NCC at residues phosphorylated by SPAK/OSR1. Efficient phosphorylation of NCC was dependent upon a docking interaction between an RFXI motif in NCC and SPAK/OSR1. Mutation of Thr60 to Ala in NCC markedly inhibited phosphorylation of Thr46 and Thr55 as well as NCC activation induced by hypotonic low-chloride treatment of HEK293 cells. Our results establish that the WNK1-SPAK/OSR1 signalling pathway plays a key role in controlling the phosphorylation and activity of NCC. They also suggest a mechanism by which increased WNK1 overexpression could lead to hypertension and that inhibitors of SPAK/OSR1 might be of use in reducing blood pressure by suppressing phosphorylation and hence activity of NCC.

SPAK and OSR1: STE20 kinases involved in the regulation of ion homoeostasis and volume control in mammalian cells

Since the discovery of an interaction between membrane transport proteins and the mammalian STE20 (sterile 20)-like kinases SPAK (STE20/SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase-1), a significant body of work has been performed probing the molecular physiology of these two kinases. To date, the function of SPAK and OSR1 is probably the best known of all mammalian kinases of the STE20 family. As they regulate by direct phosphorylation key ion transport mechanisms involved in fluid and ion homoeostasis, SPAK and OSR1 constitute key end-of-pathway effectors. Their significance in such fundamental functions as ion homoeostasis and cell volume control is evidenced by the evolutionary pressure that resulted in the duplication of the OSR1 gene in higher vertebrates. This review examines the distribution of these two kinases in the animal kingdom and tissue expression within a single organism. It also describes the main molecular features of these two kinases with emphasis on the interacting domain located at their extreme C-terminus. A large portion of the present review is devoted to the extensive biochemical and physiological studies that have resulted in our current understanding of SPAK/OSR1 function. Finally, as our understanding is a work in progress, we also identify unresolved questions and controversies that warrant further investigation.

Test of blockers of AQP1 water permeability by a high-resolution method: no effects of tetraethylammonium ions or acetazolamide

The effects of putative water channel blockers were tested on AQP1-expressing Xenopus laevis oocytes by a fast optical method with a time resolution of 1 s and a volume resolution of 20 pl. The oocytes were exposed to external hyposmolarity and the osmotic water permeability (Lp) derived from the initial 10 s of volume change. For longer durations, the effective osmotic gradient across the membrane was reduced significantly because of dilution of the intracellular contents and of ion transport across the membrane. The latter was monitored by voltage clamp of the oocytes. In contrast to previous reports based on slower and less sensitive assays, we found no effects of tetraethylammonium ions (TEA+) and acetazolamide on Lp. We have no single explanation for this, but several factors are considered: (a) If the osmotic gradient is assumed to be constant for periods longer than 10 s, the Lp will be underestimated. (b) Hyposmotic gradients implemented by dilution with water will entail changes in the ionic strength as well; this may enhance loss of salt from the oocyte. (c) By voltage clamping the AQP1-expressing oocytes during hyposmotic challenges, we found that TEA+-treated oocytes were more electrically leaky than untreated ones. This may obscure comparisons between the Lp of treated and untreated oocytes. (d) The nature of the ion transport mechanisms in the plasma membrane depends on how oocytes have been prepared for experiments and on their viability as indicated by the membrane potential. These parameters may vary between laboratories.