Biological Cross-talk between WNK1 and the Transforming Growth Factor β-Smad Signaling Pathway

SMURF1/2 are novel regulators of WNK1 stability

Angiogenesis is essential for remodeling and repairing existing vessels, and this process requires signaling pathways including those controlled by transforming growth factor beta (TGF-β). We have previously reported crosstalk between TGF-β and the protein kinase With No lysine (K) 1 (WNK1). Homozygous disruption of the gene encoding WNK1 results in lethality in mice near embryonic day E12 due to impaired angiogenesis and this defect can be rescued by endothelial-specific expression of an activated form of the WNK1 substrate kinase Oxidative Stress-Responsive 1 (OSR1). However, molecular processes regulated via a collaboration between TGF-β and WNK1/OSR1 are not well understood. Here we show that WNK1 interacts with the E3 ubiquitin ligases SMURF1/2. In addition, we discovered that WNK1 regulates SMURF1/2 protein stability and vice versa. We also demonstrate that WNK1 activity regulates TGF-β receptor levels, in turn, controlling TGF-β signaling.

Inhibition of WNK Kinases in NK Cells Disrupts Cellular Osmoregulation and Control of Tumor Metastasis

INTRODUCTION

The serine/threonine with-no-lysine (WNK) kinase family function in blood pressure control, electrolyte homeostasis, and cellular osmoregulation. These kinases and their downstream effectors are considered promising therapeutic targets in hypertension and stroke. However, the role of WNK kinases in immune cells remains poorly understood.

METHODS

Using the small-molecule WNK kinase inhibitors WNK463 and WNK-IN-11, we investigated how WNK kinase inhibition affects natural killer (NK) cell physiology.

RESULTS

WNK kinase inhibition with WNK463 or WNK-IN-11 significantly decreased IL-2-activated NK cell volume, motility, and cytolytic activity. Treatment of NK cells with these inhibitors induced autophagy by activating AMPK and inhibiting mTOR signaling. Moreover, WNK kinase inhibition increased phosphorylation of Akt and c-Myc by misaligning activity of activating kinases and inhibitory phosphatases. Treatment of tumor-bearing mice with WNK463 impaired tumor metastasis control by adoptively transferred NK cells.

CONCLUSION

The catalytic activity of WNK kinases has a critical role of multiple aspects of NK cell physiology and their pharmacologic inhibition negatively impacts NK cell function.

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.

Receptor-activated transcription factors and beyond: multiple modes of Smad2/3-dependent transmission of TGF-β signaling

Transforming growth factor β (TGF-β) is a pleiotropic cytokine that is widely distributed throughout the body. Its receptor proteins, TGF-β type I and type II receptors, are also ubiquitously expressed. Therefore, the regulation of various signaling outputs in a context-dependent manner is a critical issue in this field. Smad proteins were originally identified as signal-activated transcription factors similar to signal transducer and activator of transcription proteins. Smads are activated by serine phosphorylation mediated by intrinsic receptor dual specificity kinases of the TGF-β family, indicating that Smads are receptor-restricted effector molecules downstream of ligands of the TGF-β family. Smad proteins have other functions in addition to transcriptional regulation, including post-transcriptional regulation of micro-RNA processing, pre-mRNA splicing, and m6A methylation. Recent technical advances have identified a novel landscape of Smad-dependent signal transduction, including regulation of mitochondrial function without involving regulation of gene expression. Therefore, Smad proteins are receptor-activated transcription factors and also act as intracellular signaling modulators with multiple modes of function. In this review, we discuss the role of Smad proteins as receptor-activated transcription factors and beyond. We also describe the functional differences between Smad2 and Smad3, two receptor-activated Smad proteins downstream of TGF-β, activin, myostatin, growth and differentiation factor (GDF) 11, and Nodal.

Neuronal K+-Cl- cotransporter KCC2 as a promising drug target for epilepsy treatment

Epilepsy is a prevalent neurological disorder characterized by unprovoked seizures. γ-Aminobutyric acid (GABA) serves as the primary fast inhibitory neurotransmitter in the brain, and GABA binding to the GABAA receptor (GABAAR) regulates Cl- and bicarbonate (HCO3-) influx or efflux through the channel pore, leading to GABAergic inhibition or excitation, respectively. The neuron-specific K+-Cl- cotransporter 2 (KCC2) is essential for maintaining a low intracellular Cl- concentration, ensuring GABAAR-mediated inhibition. Impaired KCC2 function results in GABAergic excitation associated with epileptic activity. Loss-of-function mutations and altered expression of KCC2 lead to elevated [Cl-]i and compromised synaptic inhibition, contributing to epilepsy pathogenesis in human patients. KCC2 antagonism studies demonstrate the necessity of limiting neuronal hyperexcitability within the brain, as reduced KCC2 functioning leads to seizure activity. Strategies focusing on direct (enhancing KCC2 activation) and indirect KCC2 modulation (altering KCC2 phosphorylation and transcription) have proven effective in attenuating seizure severity and exhibiting anti-convulsant properties. These findings highlight KCC2 as a promising therapeutic target for treating epilepsy. Recent advances in understanding KCC2 regulatory mechanisms, particularly via signaling pathways such as WNK, PKC, BDNF, and its receptor TrkB, have led to the discovery of novel small molecules that modulate KCC2. Inhibiting WNK kinase or utilizing newly discovered KCC2 agonists has demonstrated KCC2 activation and seizure attenuation in animal models. This review discusses the role of KCC2 in epilepsy and evaluates its potential as a drug target for epilepsy treatment by exploring various strategies to regulate KCC2 activity.

An update regarding the role of WNK kinases in cancer

Mammalian WNK kinases (WNKs) are serine/threonine kinases that contain four members, WNK1-4. They function to maintain ion homeostasis and regulate blood pressure in mammals. Recent studies have revealed that the dysregulation of WNKs contributes to tumor growth, metastasis, and angiogenesis through complex mechanisms, especially through phosphorylating kinase substrates SPS1-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1). Here, we review and discuss the relationships between WNKs and several key factors/biological processes in cancer, including ion channels, cation chloride cotransporters, sodium bicarbonate cotransporters, signaling pathways, angiogenesis, autophagy, and non-coding RNAs. In addition, the potential drugs for targeting WNK-SPAK/OSR1 signaling have also been discussed. This review summarizes and discusses knowledge of the roles of WNKs in cancer, which provides a comprehensive reference for future studies.

WNK1 collaborates with TGF-β in endothelial cell junction turnover and angiogenesis

Angiogenesis is essential for growth of new blood vessels, remodeling existing vessels, and repair of damaged vessels, and these require reorganization of endothelial cell-cell junctions through a partial endothelial-mesenchymal transition. Homozygous disruption of the gene encoding the protein kinase WNK1 results in lethality in mice near embryonic day (E) 12 due to impaired angiogenesis. This angiogenesis defect can be rescued by endothelial-specific expression of an activated form of the WNK1 substrate kinase OSR1. We show that inhibition of WNK1 kinase activity not only prevents sprouting of endothelial cells from aortic slices but also vessel extension in inhibitor-treated embryos ex vivo. Mutations affecting TGF-β signaling also result in abnormal vascular development beginning by E10 and, ultimately, embryonic lethality. Previously, we demonstrated cross-talk of WNK1 with TGF-β-regulated SMAD signaling, and OSR1 was identified as a component of the TGF-β interactome. However, molecular events jointly regulated by TGF-β and WNK1/OSR1 have not been delineated. Here, we show that inhibition of WNK1 promotes TGF-β-dependent degradation of the tyrosine kinase receptor AXL, which is involved in TGF-β-mediated cell migration and angiogenesis. We also show that interaction between OSR1 and occludin, a protein associated with endothelial tight junctions, is an essential step to enable tight junction turnover. Furthermore, we show that these phenomena are WNK1 dependent, and sensitive to TGF-β. These findings demonstrate intimate connections between WNK1/OSR1 and multiple TGF-β-sensitive molecules controlling angiogenesis and suggest that WNK1 may modulate many TGF-β-regulated functions.

WNK1 in Malignant Behaviors: A Potential Target for Cancer?

Metastasis is the major cause of mortality in cancer patients. Analyses of mouse models and patient data have implicated the protein kinase WNK1 as one of a handful of genes uniquely linked to a subset of invasive cancers. WNK1 signaling pathways are widely implicated in the regulation of ion co-transporters and in controlling cell responses to osmotic stress. In this review we will discuss its actions in tumor malignancy in human cancers and present evidence for its function in invasion, migration, angiogenesis and mesenchymal transition.

WNK1 Enhances Migration and Invasion in Breast Cancer Models

Metastasis is the major cause of mortality in patients with breast cancer. Many signaling pathways have been linked to cancer invasiveness, but blockade of few protein components has succeeded in reducing metastasis. Thus, identification of proteins contributing to invasion that are manipulable by small molecules may be valuable in inhibiting spread of the disease. The protein kinase with no lysine (K) 1 (WNK1) has been suggested to induce migration of cells representing a range of cancer types. Analyses of mouse models and patient data have implicated WNK1 as one of a handful of genes uniquely linked to invasive breast cancer. Here, we present evidence that inhibition of WNK1 slows breast cancer metastasis. We show that depletion or inhibition of WNK1 reduces migration of several breast cancer cell lines in wound healing assays and decreases invasion in collagen matrices. Furthermore, WNK1 depletion suppresses expression of AXL, a tyrosine kinase implicated in metastasis. Finally, we demonstrate that WNK inhibition in mice attenuates tumor progression and metastatic burden. These data showing reduced migration, invasion, and metastasis upon WNK1 depletion in multiple breast cancer models suggest that WNK1 contributes to the metastatic phenotype, and that WNK1 inhibition may offer a therapeutic avenue for attenuating progression of invasive breast cancers.

WNK3 promotes the invasiveness of glioma cell lines under hypoxia by inducing the epithelial-to-mesenchymal transition

Background

The primary features of malignant glioma include high rates of mortality and recurrence, uncontrollable invasiveness, strong angiogenesis, and widespread hypoxia. The hypoxic microenvironment is an important factor affecting the malignant progression of glioma. However, the molecular mechanisms underlying glioma adaption in hypoxic microenvironments are poorly understood.

Objective

The work presented in this paper focuses on the role of WNK3 gene in glioma invasion under hypoxic conditions. Furthermore, we aim to explore its role in epithelial-to-mesenchymal transition (EMT).

Methods

ShRNA targeting WNK3 transfection was used to knockdown the WNK3 expression in U87 cells. We used western blot analysis to detect the relative expression of proteins in U87 cells. The effect of WNK3 on cell migration was explored using a transwell assay in the U87 cell line. We also evaluated WNK3 expression levels in glioma samples by immunohistochemistry analysis.

Results

WNK3 expression was significantly higher in high-grade (III and IV) gliomas than in low-grade (I and II) gliomas. WNK3 expression was up-regulated in U87 cells when cultured in a hypoxic environment in addition; WNK3 knockdown inhibited the invasion of U87 glioma cells by regulating the EMT, especially under hypoxic conditions.

Conclusion

These findings suggested that WNK3 plays an important role in the hypoxic microenvironment of glioma and might also be a candidate for therapeutic application in the treatment of glioma.

Ibuprofen disrupts a WNK1/GSK3β/SRPK1 protein complex required for expression of tumor-related splicing variant RAC1B in colorectal cells

A major risk factor promoting tumor development is chronic inflammation and the use of nonsteroidal anti-inflammatory drugs (NSAID), including ibuprofen, can decrease the risk of developing various types of cancer, including colorectal cancer (CRC). Although the molecular mechanism behind the antitumor properties of NSAIDs has been largely attributed to inhibition of cyclooxygenases (COXs), several studies have shown that the chemopreventive properties of ibuprofen also involve multiple COX-independent effects. One example is its ability to inhibit the alternative splicing event generating RAC1B, which is overexpressed in a specific subset of BRAF-mutated colorectal tumors and sustains cell survival. Here we describe the mechanism by which ibuprofen prevents RAC1B alternative splicing in a BRAF mutant CRC cell line: it leads to decreased translocation of SRPK1 and SRSF1 to the nucleus and is regulated by a WNK1/GSK3β/SRPK1 protein kinase complex. Surprisingly, we demonstrate that ibuprofen does not inhibit the activity of any of the involved kinases but rather promotes disassembly of this regulatory complex, exposing GSK3β serine 9 to inhibitory phosphorylation, namely by AKT, which results in nuclear exclusion of SRPK1 and SRSF1 hypophosphorylation. The data shed new light on the biochemical mechanisms behind ibuprofen’s action on alternative spliced RAC1B and may support its use in personalized approaches to CRC therapy or chemoprevention regimens.

Flaming the fight against cancer cells: the role of microRNA-93

There have been attempts to develop novel anti-tumor drugs in cancer therapy. Although satisfying results have been observed at a consequence of application of chemotherapeutic agents, the cancer cells are capable of making resistance into these agents. This has forced scientists into genetic manipulation as genetic alterations are responsible for generation of a high number of cancer cells. MicroRNAs (miRs) are endogenous, short non-coding RNAs that affect target genes at the post-transcriptional level. Increasing evidence reveals the potential role of miRs in regulation of biological processes including angiogenesis, metabolism, cell proliferation, cell division, and cell differentiation. Abnormal expression of miRs is associated with development of a number of pathologic events, particularly cancer. MiR-93 plays a significant role in both physiological and pathological mechanisms. At the present review, we show how this miR dually affects the proliferation and invasion of cancer cells. Besides, we elucidate the oncogenesis or oncosuppressor function of miR-93.

Role of TGF-β1 and C-Kit Mutations in the Development of Hepatocellular Carcinoma in Hepatitis C Virus-Infected Patients: in vitro Study

Transforming growth factor beta (TGF-β) acts as a tumor-suppressing cytokine in healthy tissues and non-malignant tumors. Yet, in malignancy, TGF-β can exert the opposite effects that can promote proliferation of cancer cells. C-Kit plays a prominent role in stem cell activation and liver regeneration after injury. However, little is known about the cross-talk between TGF-β and C-Kit and its role in the progression of hepatocellular carcinoma (HCC). Here, we studied the effect of increasing doses of TGF-β1 on CD44⁺CD90⁺ liver stem cells (LSCs) and C-Kit gene expression in malignant and adjacent non-malignant liver tissues excised from 32 HCC patients. The percentage of LSCs in malignant tumors was two times higher compared to their counterparts from the non-malignant tissues. When treated with increasing doses of TGF-β1, proliferation of both malignant and non-malignant LSCs was progressively suppressed, but low TGF-β1 dose failed to suppress the growth of malignant LSCs. Moreover, C-Kit exons 9 and 11 were expressed in malignant LSCs, but not in their non-malignant counterparts. Analysis of C-Kit detected mutations in exon 9 (but not in exon 11) in some malignant liver cells resulting in the changes in the amino acid sequence and dysregulation of protein structure and function. Interestingly, in malignant liver cells, mutations in exon 9 were associated with high-viremia hepatitis C virus (HCV), and expression of this exon was not suppressed by the TGF-β1 treatment at all doses. To our knowledge, this is the first report that mutations in the C-Kit gene in HCC patients are associated with high- viremia HCV. Our study emphasizes the need for investigation of the TGF-β1 level and C-Kit mutations in patients with chronic HCV for HCC prevention and better therapy management.

WNK pathways in cancer signaling networks

Background

The with no lysine [K] (WNK) pathway consists of the structurally unique WNK kinases, their downstream target kinases, oxidative stress responsive (OSR)1 and SPS/Ste20-related proline-alanine-rich kinase (SPAK), and a multitude of OSR1/SPAK substrates including cation chloride cotransporters.

Main body

While the best known functions of the WNK pathway is regulation of ion transport across cell membranes, WNK pathway components have been implicated in numerous human diseases. The goal of our review is to draw attention to how this pathway and its components exert influence on the progression of cancer, specifically by detailing WNK signaling intersections with major cell communication networks and processes.

Conclusion

Here we describe how WNKs and associated proteins interact with and influence PI3K-AKT, TGF-β, and NF-κB signaling, as well as its unanticipated role in the regulation of angiogenesis.

WNK lysine deficient protein kinase 1 regulates human endometrial stromal cell decidualization, proliferation, and migration in part through mitogen-activated protein kinase 7

The differentiation of endometrial stromal cells into decidual cells, termed decidualization, is an integral step in the establishment of pregnancy. The mitogen-activated protein kinase homolog, WNK lysine deficient protein kinase 1 (WNK1), is activated downstream of epidermal growth factor receptor during decidualization. Primary human endometrial stromal cells (HESCs) were subjected to small interfering RNA knockdown of WNK1 followed by in vitro decidualization. This abrogated expression of the decidual marker genes, insulin like growth factor binding protein 1 (IGFBP1) and prolactin (PRL), and prevented adoption of decidual cell morphology. Analysis of the WNK1-dependent transcriptome by RNA-Seq demonstrated that WNK1 regulates the expression of 1858 genes during decidualization. Gene ontology and upstream regulator pathway analysis showed that WNK1 regulates cell migration, differentiation, and proliferation. WNK1 was required for many of the gene expression changes that drive decidualization, including the induction of the inflammatory cytokines, C-C motif chemokine ligand 8 (CCL8), interleukin 1 beta (IL1B), and interleukin 15 (IL15), and the repression of transforming growth factor-beta (TGF-beta) pathway genes, including early growth response 2 (EGR2), SMAD family member 3 (SMAD3), integrin subunit alpha 2 (ITGA2), integrin subunit alpha 4 (ITGA4), and integrin subunit beta 3 (ITGB3). In addition to abrogating decidualization, WNK1 knockdown decreased the migration and proliferation of HESCs. Furthermore, mitogen-activated protein kinase 7 (MAPK7), a known downstream target of WNK1, was activated during decidualization in a WNK1-dependent manner. Small interfering RNA knockdown of MAPK7 demonstrated that MAPK7 regulates a subset of WNK1-regulated genes and controls the migration and proliferation of HESCs. These results indicate that WNK1 and MAPK7 promote migration and proliferation during decidualization and regulate the expression of inflammatory cytokines and TGF-beta pathway genes in HESCs.

Soluble bone-derived osteopontin promotes migration and stem-like behavior of breast cancer cells

Breast cancer is a leading cause of cancer death in women, with the majority of these deaths caused by metastasis to distant organs. The most common site of breast cancer metastasis is the bone, which has been shown to provide a rich microenvironment that supports the migration and growth of breast cancer cells. Additionally, growing evidence suggests that breast cancer cells that do successfully metastasize have a stem-like phenotype including high activity of aldehyde dehydrogenase (ALDH) and/or a CD44⁺CD24^- phenotype. In the current study, we tested the hypothesis that these ALDH^hiCD44⁺CD24^- breast cancer cells interact with factors in the bone secondary organ microenvironment to facilitate metastasis. Specifically, we focused on bone-derived osteopontin and its ability to promote the migration and stem-like phenotype of breast cancer cells. Our results indicate that bone-derived osteopontin promotes the migration, tumorsphere-forming ability and colony-forming ability of whole population and ALDH^hiCD44⁺CD24^- breast cancer cells in bone marrow-conditioned media (an ex vivo representation of the bone microenvironment) (p≤0.05). We also demonstrate that CD44 and RGD-dependent cell surface integrins facilitate this functional response to bone-derived osteopontin (p≤0.05), potentially through activation of WNK-1 and PRAS40-related pathways. Our findings suggest that soluble bone-derived osteopontin enhances the ability of breast cancer cells to migrate to the bone and maintain a stem-like phenotype within the bone microenvironment, and this may contribute to the establishment and growth of bone metastases.

KLHL3 Knockout Mice Reveal the Physiological Role of KLHL3 and the Pathophysiology of Pseudohypoaldosteronism Type II Caused by Mutant KLHL3

Mutations in the with-no-lysine kinase 1 (WNK1), WNK4, kelch-like 3 (KLHL3), and cullin3 (CUL3) genes are known to cause the hereditary disease pseudohypoaldosteronism type II (PHAII). It was recently demonstrated that this results from the defective degradation of WNK1 and WNK4 by the KLHL3/CUL3 ubiquitin ligase complex. However, the other physiological in vivo roles of KLHL3 remain unclear. Therefore, here we generated KLHL3-/- mice that expressed β-galactosidase (β-Gal) under the control of the endogenous KLHL3 promoter. Immunoblots of β-Gal and LacZ staining revealed that KLHL3 was expressed in some organs, such as brain. However, the expression levels of WNK kinases were not increased in any of these organs other than the kidney, where WNK1 and WNK4 increased in KLHL3-/- mice but not in KLHL3+/- mice. KLHL3-/- mice also showed PHAII-like phenotypes, whereas KLHL3+/- mice did not. This clearly demonstrates that the heterozygous deletion of KLHL3 was not sufficient to cause PHAII, indicating that autosomal dominant type PHAII is caused by the dominant negative effect of mutant KLHL3. We further demonstrated that the dimerization of KLHL3 can explain this dominant negative effect. These findings could help us to further understand the physiological roles of KLHL3 and the pathophysiology of PHAII caused by mutant KLHL3.

Posttranslational Regulation of Smads

Transforming growth factor β (TGF-β) family signaling dictates highly complex programs of gene expression responses, which are extensively regulated at multiple levels and vary depending on the physiological context. The formation, activation, and destruction of two major functional complexes in the TGF-β signaling pathway (i.e., the TGF-β receptor complexes and the Smad complexes that act as central mediators of TGF-β signaling) are direct targets for posttranslational regulation. Dysfunction of these complexes often leads or contributes to pathogenesis in cancer and fibrosis and in cardiovascular, and autoimmune diseases. Here we discuss recent insights into the roles of posttranslational modifications in the functions of the receptor-activated Smads in the common Smad4 and inhibitory Smads, and in the control of the physiological responses to TGF-β. It is now evident that these modifications act as decisive factors in defining the intensity and versatility of TGF-β responsiveness. Thus, the characterization of posttranslational modifications of Smads not only sheds light on how TGF-β controls physiological and pathological processes but may also guide us to manipulate the TGF-β responses for therapeutic benefits.

miR-93 inhibits the invasive potential of triple-negative breast cancer cells in vitro via protein kinase WNK1

Despite advances in treatment, the highly metastatic nature of breast tumors has given rise to the urgent need for development of novel therapeutic and prognostic markers. miR-93 is known to regulate the epithelial to mesenchymal transition process and to influence metastatic spread in breast carcinoma, although the exact mechanism(s)/genes involved remain unknown. In the present study, we examined the role of miR-93 in MDA-MB-231 breast cancer cells. Overexpression of mature miR-93-5p in MDA-MB-231 cells decreased cell migratory capability and invasive potential, as well as increased adhesion. In contrast, inhibition of miR-93 induced the opposite effects. miRNA-mRNA target prediction (TargetScan) identified WNK lysine deficient protein kinase 1 (WNK1), which is known to interact with diverse signaling pathways and regulate cell proliferation, survival, angiogenesis and metastasis, as one of the potential targets of miR-93. Furthermore, we showed by luciferase assay that WNK1 is a putative miR-93 target. siRNA mediated silencing of WNK1 also decreased the invasive ability of the cells, suggesting that the effects of miR-93 may be attributed at least in part to decreased WNK1 expression. Further in vivo studies are required to ascertain the miR-93-WNK1-metastasis cascade, that has potential implications in breast cancer therapy.

WNK Kinases in Development and Disease

WNK (With-No-Lysine (K)) kinases are serine-threonine kinases characterized by an atypical placement of a catalytic lysine within the kinase domain. Mutations in human WNK1 or WNK4 cause an autosomal dominant syndrome of hypertension and hyperkalemia, reflecting the fact that WNK kinases are critical regulators of renal ion transport processes. Here, the role of WNKs in the regulation of ion transport processes in vertebrate and invertebrate renal function, cellular and organismal osmoregulation, and cell migration and cerebral edema will be reviewed, along with emerging literature demonstrating roles for WNKs in cardiovascular and neural development, Wnt signaling, and cancer. Conserved roles for these kinases across phyla are emphasized.

NKCC1 up-regulation contributes to early post-traumatic seizures and increased post-traumatic seizure susceptibility

Traumatic brain injury (TBI) is not only a leading cause for morbidity and mortality in young adults (Bruns and Hauser, Epilepsia 44(Suppl 10):210, 2003), but also a leading cause of seizures. Understanding the seizure-inducing mechanisms of TBI is of the utmost importance, because these seizures are often resistant to traditional first- and second-line anti-seizure treatments. The early post-traumatic seizures, in turn, are a contributing factor to ongoing neuropathology, and it is critically important to control these seizures. Many of the available anti-seizure drugs target gamma-aminobutyric acid (GABA_A) receptors. The inhibitory activity of GABA_A receptor activation depends on low intracellular Cl⁻, which is achieved by the opposing regulation of Na⁺–K⁺–Cl⁻ cotransporter 1 (NKCC1) and K⁺–Cl⁻–cotransporter 2 (KCC2). Up-regulation of NKCC1 in neurons has been shown to be involved in neonatal seizures and in ammonia toxicity-induced seizures. Here, we report that TBI-induced up-regulation of NKCC1 and increased intracellular Cl⁻ concentration. Genetic deletion of NKCC1 or pharmacological inhibition of NKCC1 with bumetanide suppresses TBI-induced seizures. TGFβ expression was also increased after TBI and competitive antagonism of TGFβ reduced NKKC1 expression, ameliorated reactive astrocytosis, and inhibited seizures. Thus, TGFβ might be an important pathway involved in NKCC1 up-regulation after TBI. Our findings identify neuronal up-regulation of NKCC1 and its mediation by TGFβ, as a potential and important mechanism in the early post-traumatic seizures, and demonstrate the therapeutic potential of blocking this pathway.

Prostaglandin E2-Dependent Phosphorylation of RAS Inhibition 1 (RIN1) at Ser 291 and 292 Inhibits Transforming Growth Factor-β-Induced RAS Activation Pathway in Human Synovial Fibroblasts: Role in Cell Migration

Prostaglandin E2 (PGE2 )-stimulated G-protein-coupled receptor (GPCR) activation inhibits pro-fibrotic TGFβ-dependent stimulation of human fibroblast to myofibroblast transition (FMT), though the precise molecular mechanisms are not fully understood. In the present study, we describe the PGE2 -dependent suppression and reversal of TGFβ-induced events such as α-sma expression, stress fiber formation, and Ras/Raf/ERK/MAPK pathway-dependent activation of myofibroblast migration. In order to elucidate post-ligand-receptor signaling pathways, we identified a predominant PKA phosphorylation motif profile in human primary fibroblasts after treatment with exogenous PGE2 (EC50 30 nM, Vmax 100 nM), mimicked by the adenyl cyclase activator forskolin (EC50 5 μM, Vmax 10 μM). We used a global phosphoproteomic approach to identify a 2.5-fold difference in PGE2 -induced phosphorylation of proteins containing the PKA motif. Deducing the signaling pathway of our migration data, we identified Ras inhibitor 1 (RIN1) as a substrate, whereby PGE2 induced its phosphorylation at Ser291 and at Ser292 by a 5.4- and 4.8-fold increase, respectively. In a series of transient and stable over expression studies in HEK293T and HeLa cells using wild-type (wt) and mutant RIN1 (Ser291/292Ala) or Ras constructs and siRNA knock-down experiments, we showed that PGE2 -dependent phosphorylation of RIN1 resulted in the abrogation of TGFβ-induced Ras/Raf signaling activation and subsequent downstream blockade of cellular migration, emphasizing the importance of such phosphosites in PGE2 suppression of wound closure. Overexpression experiments in tandem with pull-down assays indicated that specific Ser291/292 phosphorylation of RIN1 favored binding to activated Ras. In principal, understanding PGE2 -GPCR activated signaling pathways mitigating TGFβ-induced fibrosis may lead to more evidence-based treatments against the disease. J. Cell. Physiol. 232: 202-215, 2017. © 2016 Wiley Periodicals, Inc.

(WNK)ing at death: With-no-lysine (Wnk) kinases in neuropathies and neuronal survival

Members of With-no-lysine (WNK) family of serine-threonine kinase are key regulators of chloride ion transport in diverse cell types, controlling the activity and the surface expression of cation-chloride (Na(+)/K(+)-Cl(-)) co-transporters. Mutations in WNK1 and WNK4 are linked to a hereditary form of hypertension, and WNKs have been extensively investigated pertaining to their roles in renal epithelial ion homeostasis. However, some members of the WNK family and their splice isoforms are also expressed in the mammalian brain, and have been implicated in aspects of hereditary neuropathy as well as neuronal and glial survival. WNK2, which is exclusively enriched in neurons, is well known as an anti-proliferative tumor suppressor. WNK3, on the other hand, appears to promote cell survival as its inhibition enhances neuronal apoptosis. However, loss of WNK3 has been recently shown to reduce ischemia-associated brain damage. In this review, I surveyed the potentially context-dependent roles of WNKs in neurological disorders and neuronal survival.

An integrated genomic approach identifies persistent tumor suppressive effects of transforming growth factor-β in human breast cancer

INTRODUCTION

Transforming growth factor-βs (TGF-βs) play a dual role in breast cancer, with context-dependent tumor-suppressive or pro-oncogenic effects. TGF-β antagonists are showing promise in early-phase clinical oncology trials to neutralize the pro-oncogenic effects. However, there is currently no way to determine whether the tumor-suppressive effects of TGF-β are still active in human breast tumors at the time of surgery and treatment, a situation that could lead to adverse therapeutic responses.

METHODS

Using a breast cancer progression model that exemplifies the dual role of TGF-β, promoter-wide chromatin immunoprecipitation and transcriptomic approaches were applied to identify a core set of TGF-β-regulated genes that specifically reflect only the tumor-suppressor arm of the pathway. The clinical significance of this signature and the underlying biology were investigated using bioinformatic analyses in clinical breast cancer datasets, and knockdown validation approaches in tumor xenografts.

RESULTS

TGF-β-driven tumor suppression was highly dependent on Smad3, and Smad3 target genes that were specifically enriched for involvement in tumor suppression were identified. Patterns of Smad3 binding reflected the preexisting active chromatin landscape, and target genes were frequently regulated in opposite directions in vitro and in vivo, highlighting the strong contextuality of TGF-β action. An in vivo-weighted TGF-β/Smad3 tumor-suppressor signature was associated with good outcome in estrogen receptor-positive breast cancer cohorts. TGF-β/Smad3 effects on cell proliferation, differentiation and ephrin signaling contributed to the observed tumor suppression.

CONCLUSIONS

Tumor-suppressive effects of TGF-β persist in some breast cancer patients at the time of surgery and affect clinical outcome. Carefully tailored in vitro/in vivo genomic approaches can identify such patients for exclusion from treatment with TGF-β antagonists.

Antibody-directed coupling of endoglin and MMP-14 is a key mechanism for endoglin shedding and deregulation of TGF-β signaling

Endoglin is a transforming growth factor β (TGF-β) coreceptor that serves as a prognostic, diagnostic and therapeutic vascular target in human cancer. A number of endoglin ectodomain-targeting antibodies (Abs) can effectively suppress both normal and tumor-associated angiogenesis, but their molecular actions remain poorly characterized. Here we define a key mechanism for TRACON105 (TRC105), a humanized monoclonal Ab in clinical trials for treatment of advanced or metastatic tumors. TRC105, along with several other endoglin Abs tested, enhance endoglin shedding through direct coupling of endoglin and the membrane-type 1 matrix metalloproteinase (MMP)-14 at the cell surface to release the antiangiogenic factor, soluble endoglin (sEng). In addition to this coupling process, endoglin shedding is further amplified by increased MMP-14 expression that requires TRC105 concentration-dependent c-Jun N-terminal kinase (JNK) activation. There were also notable counterbalancing effects on canonical Smad signaling in which TRC105 abrogated both the steady-state and TGF-β-induced Smad1/5/8 activation while augmenting Smad2/3 activation. Interestingly, TRC105-induced sEng and aberrant Smad signaling resulted in an excessive migratory response through enhanced stress fiber formation and disruption of endothelial cell–cell junctions. Collectively, our study defines endoglin shedding and deregulated TGF-β signaling during migration as major mechanisms by which TRC105 inhibits angiogenesis.

Oncogenic PAK4 regulates Smad2/3 axis involving gastric tumorigenesis

The alteration of p21-activated kinase 4 (PAK4) and transforming growth factor-beta (TGF-β) signaling effector Smad2/3 was detected in several types of tumors, which acts as oncogenic factor and tumor suppressor, but the relationship between these events has not been explored. Here, we demonstrate that PAK4 interacts with and modulates phosphorylation of Smad2/3 via both kinase-dependent and kinase-independent mechanisms, which attenuate Smad2/3 axis transactivation and TGF-β-mediated growth inhibition in gastric cancer cells. First, PAK4 interaction with Smad2/3, which is independent of PAK4 kinase activity, blocks TGF-β1-induced phosphorylation of Smad2 Ser465/467 or Smad3 Ser423/425 and the consequent activation. In addition, PAK4 phosphorylates Smad2 on Ser465, leading to the degradation of Smad2 through ubiquitin-proteasome-dependent pathway under hepatocyte growth factor (HGF) stimulation. Interestingly, PAK4 expression correlates negatively with phospho-Ser465/467 Smad2 but positively with phospho-Ser465 Smad2 in gastric cancer tissues. Furthermore, the expressions of HGF, phospho-Ser474 PAK4 and phospho-Ser465 Smad2 are markedly increased in gastric cancer tissues, and the expression of Smad2 is decreased in gastric cancer tissues. Our results document an oncogenic role of PAK4 in repression of Smad2/3 transactivation that involved in tumorigenesis, and suggest PAK4 as a potential therapeutic target for gastric cancer.

Wnk kinases are positive regulators of canonical Wnt/β-catenin signalling

Wnt/β-catenin signalling is central to development and its regulation is essential in preventing cancer. Using phosphorylation of Dishevelled as readout of pathway activation, we identified Drosophila Wnk kinase as a new regulator of canonical Wnt/β-catenin signalling. WNK kinases are known for regulating ion co-transporters associated with hypertension disorders. We demonstrate that wnk loss-of-function phenotypes resemble canonical Wnt pathway mutants, while Wnk overexpression causes gain-of-function canonical Wnt-signalling phenotypes. Importantly, knockdown of human WNK1 and WNK2 also results in decreased Wnt signalling in mammalian cell culture, suggesting that Wnk kinases have a conserved function in ensuring peak levels of canonical Wnt signalling.

Post-translational regulation of TGF-β receptor and Smad signaling

TGF-β family signaling through Smads is conceptually a simple and linear signaling pathway, driven by sequential phosphorylation, with type II receptors activating type I receptors, which in turn activate R-Smads. Nevertheless, TGF-β family proteins induce highly complex programs of gene expression responses that are extensively regulated, and depend on the physiological context of the cells. Regulation of TGF-β signaling occurs at multiple levels, including TGF-β activation, formation, activation and destruction of functional TGF-β receptor complexes, activation and degradation of Smads, and formation of Smad transcription complexes at regulatory gene sequences that cooperate with a diverse set of DNA binding transcription factors and coregulators. Here we discuss recent insights into the roles of post-translational modifications and molecular interaction networks in the functions of receptors and Smads in TGF-β signal responses. These layers of regulation demonstrate how a simple signaling system can be coopted to exert exquisitely regulated, complex responses.

Phosphatases in SMAD regulation

SMAD transcription factors are key mediators of the transforming growth factor-beta (TGFß) family of cytokines. Reversible phosphorylation of SMAD proteins plays a key role in regulating their function. Several phosphatases have been proposed to act on SMAD proteins to influence TGFß/BMP signalling. Here we provide an overview of the SMAD regulation by different protein phosphatases and review the evidence supporting each phosphatase as a candidate SMAD-phosphatase.

Similar effects of all WNK3 variants on SLC12 cotransporters

With-no-lysine kinase 3 (WNK3) is a member of a subfamily of serine/threonine kinases that modulate the activity of the electroneutral cation-coupled chloride cotransporters. WNK3 activates NKCC1/2 and NCC and inhibits the KCCs. Four splice variants are generated from the WNK3 gene. Our previous studies focused on the WNK3-18a variant. However, it has been suggested that other variants could have different effects on the cotransporters. Thus, the present study was designed to define the effects of all WNK3 variants on members of the SLC12 family. By RT-PCR from a fetal brain library, exons 18b and 22 were separately amplified and subcloned into the original WNK3-18a or catalytically inactive WNK3-D294A to obtain all four potential combinations with and without catalytic activity (18a, 18a+22, 18b, and 18b+22). The basal activity of the cotransporters and the effects of WNK3 isoforms were assessed in Xenopus laevis oocytes coinjected with each of the WNK3 variant cRNAs. In isotonic conditions, the basal activity of NCC and NKCC1/2 were increased by coinjection with any of the WNK3. The positive effects occurred even in hypotonic conditions, in which the basal activity of NKCC1 is completely prevented. Consistent with these observations, when expressed in hypotonicity, all KCCs were active, but in the presence of any of the WNK3 variants, KCC activity was completely reduced. That is, NKCC1/2 and NCC were inhibited, even in hypertonicity, while KCCs were activated, even in isotonic conditions. We conclude that the effects of all WNK3 variants toward SLC12 proteins are similar.

Role of Smads in TGFβ signaling

Transforming growth factor-β (TGFβ) is the prototype for a large family of pleiotropic factors that signal via heterotetrameric complexes of type I and type II serine/threonine kinase receptors. Important intracellular mediators of TGFβ signaling are members of the Smad family. Smad2 and 3 are activated by C-terminal receptor-mediated phosphorylation, whereafter they form complexes with Smad4 and are translocated to the nucleus where they, in cooperation with other transcription factors, co-activators and co-repressors, regulate the transcription of specific genes. Smads have key roles in exerting TGFβ-induced programs leading to cell growth arrest and epithelial-mesenchymal transition. The activity and stability of Smad molecules are carefully regulated by a plethora of post-translational modifications, including phosphorylation, ubiquitination, sumoylation, acetylation and poly(ADP)-ribosylation. The Smad function has been shown to be perturbed in certain diseases such as cancer.

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.

Protein kinase WNK1 promotes cell surface expression of glucose transporter GLUT1 by regulating a Tre-2/USP6-BUB2-Cdc16 domain family member 4 (TBC1D4)-Rab8A complex

One mechanism by which mammalian cells regulate the uptake of glucose is the number of glucose transporter proteins (GLUT) present at the plasma membrane. In insulin-responsive cells types, GLUT4 is released from intracellular stores through inactivation of the Rab GTPase activating protein Tre-2/USP6-BUB2-Cdc16 domain family member 4 (TBC1D4) (also known as AS160). Here we describe that TBC1D4 forms a protein complex with protein kinase WNK1 in human embryonic kidney (HEK293) cells. We show that WNK1 phosphorylates TBC1D4 in vitro and that the expression levels of WNK1 in these cells regulate surface expression of the constitutive glucose transporter GLUT1. WNK1 was found to increase the binding of TBC1D4 to regulatory 14-3-3 proteins while reducing its interaction with the exocytic small GTPase Rab8A. These effects were dependent on the catalytic activity because expression of a kinase-dead WNK1 mutant had no effect on binding of 14-3-3 and Rab8A, or on surface GLUT1 levels. Together, the data describe a pathway regulating constitutive glucose uptake via GLUT1, the expression level of which is related to several human diseases.

Serum and glucocorticoid-induced kinase (SGK) 1 and the epithelial sodium channel are regulated by multiple with no lysine (WNK) family members

The four WNK (with no lysine (K)) protein kinases affect ion balance and contain an unusual protein kinase domain due to the unique placement of the active site lysine. Mutations in two WNKs cause a heritable form of ion imbalance culminating in hypertension. WNK1 activates the serum- and glucocorticoid-induced protein kinase SGK1; the mechanism is noncatalytic. SGK1 increases membrane expression of the epithelial sodium channel (ENaC) and sodium reabsorption via phosphorylation and sequestering of the E3 ubiquitin ligase neural precursor cell expressed, developmentally down-regulated 4-2 (Nedd4-2), which otherwise promotes ENaC endocytosis. Questions remain about the intrinsic abilities of WNK family members to regulate this pathway. We find that expression of the N termini of all four WNKs results in modest to strong activation of SGK1. In reconstitution experiments in the same cell line all four WNKs also increase sodium current blocked by the ENaC inhibitor amiloride. The N termini of the WNKs also have the capacity to interact with SGK1. More detailed analysis of activation by WNK4 suggests mechanisms in common with WNK1. Further evidence for the importance of WNK1 in this process comes from the ability of Nedd4-2 to bind to WNK1 and the finding that endogenous SGK1 has reduced activity if WNK1 is knocked down by small interfering RNA.

Role of the WNK-activated SPAK kinase in regulating blood pressure

Mutations within the with-no-K(Lys) (WNK) kinases cause Gordon's syndrome characterized by hypertension and hyperkalaemia. WNK kinases phosphorylate and activate the STE20/SPS1-related proline/alanine-rich kinase (SPAK) protein kinase, which phosphorylates and stimulates the key Na⁺:Cl⁻ cotransporter (NCC) and Na⁺:K⁺:2Cl⁻ cotransporters (NKCC2) cotransporters that control salt reabsorption in the kidney. To define the importance of this pathway in regulating blood pressure, we generated knock-in mice in which SPAK cannot be activated by WNKs. The SPAK knock-in animals are viable, but display significantly reduced blood pressure that was salt-dependent. These animals also have markedly reduced phosphorylation of NCC and NKCC2 cotransporters at the residues phosphorylated by SPAK. This was also accompanied by a reduction in the expression of NCC and NKCC2 protein without changes in messenger RNA (mRNA) levels. On a normal Na⁺-diet, the SPAK knock-in mice were normokalaemic, but developed mild hypokalaemia when the renin–angiotensin system was activated by a low Na⁺-diet. These observations establish that SPAK plays an important role in controlling blood pressure in mammals. Our results imply that SPAK inhibitors would be effective at reducing blood pressure by lowering phosphorylation as well as expression of NCC and NKCC2. See accompanying Closeup by Maria Castañeda-Bueno and Gerald Gamba (DOI 10.1002/emmm.200900059).

Emerging roles for WNK kinases in cancer

The subfamily of WNK protein kinases is composed of four human genes and is characterised by a typical sequence variation within the conserved catalytic domain. Although most research has focussed on the role of WNK1, WNK3 and WNK4 in regulating different ion transporters in both the kidney and extrarenal tissues, there is growing evidence for additional roles of WNK kinases in various signalling cascades related to cancer. Here, we review the connection between WNK kinases and tumorigenesis and describe existing experimental evidence as well as potential new links to major aspects of tumour biology. In particular, we discuss their role in G1/S cell cycle progression, metabolic tumour cell adaptation, evasion of apoptosis and metastasis.

WNK kinases and blood pressure control

The WNK (With No K-Lysine) family of proteins is widely expressed and has been shown to promote blood pressure homeostasis through a variety of mechanisms. Members of this family have been reported to affect sodium/chloride cotransporters, sodium/potassium/chloride cotransporters, potassium/chloride cotransporters, the renal outer medullary potassium channel, and the epithelial sodium channel, directly and indirectly. Mutations in WNK1 and WNK4 were shown to cause pseudohypoaldosteronism type II, a Mendelian disorder characterized by hypertension, hyperkalemia, and acidosis. Because of the complexity of the renal system, it has been difficult to completely define the role of these kinases in kidney function. This article reviews current knowledge of the role of these proteins in ion homeostasis and volume control.

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.

WNK kinases, renal ion transport and hypertension

Two members of a recently discovered family of protein kinases are the cause of an inherited disease known as pseudohypoaldosteronism type II (PHAII). These patients exhibit arterial hypertension together with hyperkalemia and metabolic acidosis. This is a mirror image of Gitelman disease that is due to inactivating mutations of the SLC12A3 gene that encodes the thiazide-sensitive Na(+):Cl(-) cotransporter. The uncovered genes causing PHAII encode for serine/threonine kinases known as WNK1 and WNK4. Physiological and biochemical studies have revealed that WNK1 and WNK4 modulate the activity of several transport pathways of the aldosterone-sensitive distal nephron, thus increasing our understanding of how diverse renal ion transport proteins are coordinated to regulate normal blood pressure levels. Observations discussed in the present work place WNK1 and WNK4 as genes involved in the genesis of essential hypertension and as potential targets for the development of antihypertensive drugs.

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.

WNK1 is a novel regulator of Munc18c-syntaxin 4 complex formation in soluble NSF attachment protein receptor (SNARE)-mediated vesicle exocytosis

Defects in soluble NSF attachment protein receptor (SNARE)-mediated granule exocytosis occur in islet beta cells, adipocytes, and/or skeletal muscle cells correlate with increased susceptibility to insulin resistance and diabetes. The serine/threonine kinase WNK1 (with no K (lysine)) has recently been implicated in exocytosis and is expressed in all three of these cell types. To search for WNK1 substrates related to exocytosis, we conducted a WNK1 two-hybrid screen, which yielded Munc18c. Munc18c is known to be a key regulator of accessibility of the target membrane (t-SNARE) protein syntaxin 4 to participate in SNARE core complex assembly, although a paucity of Munc18c-binding factors has precluded discovery of its precise functions. To validate WNK1 as a new Munc18c-interacting partner, the direct interaction between WNK1 and Munc18c was confirmed using in vitro binding analysis, and endogenous WNK1-Munc18c complexes were detected in the cytosolic and plasma membrane compartments of the islet beta cell line MIN6. This binding interaction is mediated through the N-terminal 172 residues of Munc18c and the kinase domain residues of WNK1 (residues 159-491). Expression of either of these two minimal interaction domains resulted in inhibition of glucose-stimulated insulin secretion, consistent with a functional importance for the endogenous WNK1-Munc18c complex in exocytosis. Interestingly, Munc18c failed to serve as a WNK1 substrate in kinase activity assays, suggesting that WNK1 functions in SNARE complex assembly outside its role as a kinase. Taken together, these data support a novel role for WNK1 and a new mechanism for the regulation of SNARE complex assembly by WNK1-Munc18c complexes.