Volume sensitivity of cation-Cl- cotransporters is modulated by the interaction of two kinases: Ste20-related proline-alanine-rich kinase and WNK4

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.

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

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

Impact of Solute Carrier Transporters in Glioma Pathology: A Comprehensive Review

Solute carriers (SLCs) are essential for brain physiology and homeostasis due to their role in transporting necessary substances across cell membranes. There is an increasing need to further unravel their pathophysiological implications since they have been proposed to play a pivotal role in brain tumor development, progression, and the formation of the tumor microenvironment (TME) through the upregulation and downregulation of various amino acid transporters. Due to their implication in malignancy and tumor progression, SLCs are currently positioned at the center of novel pharmacological targeting strategies and drug development. In this review, we discuss the key structural and functional characteristics of the main SLC family members involved in glioma pathogenesis, along with their potential targeting options to provide new opportunities for CNS drug design and more effective glioma management.

Intracellular Ion Control of WNK Signaling

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

Sevoflurane induces neurotoxic effects on developing neurons through the WNK1/NKCC1/Ca2+ /Drp-1 signalling pathway

Children repeatedly exposed to anaesthesia have a high risk of cognitive impairment, but the mechanism of its regulation in this context is unknown. The objective of this study was to investigate the possible toxic mechanism of sevoflurane through the WNK1/NKCC1/Ca²⁺ /Drp-1 signalling pathway. The hippocampal neuronal HT22 cell line was used in this study. The intervention group was treated with the WNK1 inhibitor WNK-463, CaN inhibitor FK506 and Drp-1 inhibitor Mdivi-1 respectively in the medium for 30 min before sevoflurane anaesthesia. The sevofluane group and all intervention group treated with 4.1% sevoflurane for 6 h. Compared with the control group, sevoflurane treatment decreased cell viability and increased cellular apoptosis. Our study found that WNK-463, FK506 and Mdivi-1 can all alleviate the sevoflurane-induced reduction in cell viability, decrease the cell apoptosis. In addition, WNK-463 pretreatment could inhibit the increase of WNK1 kinase and NKCC1 protein concentration caused by sevoflurane. Further, sevoflurane anaesthesia causes intracellular calcium overload, increases the expression of CaN and induces the dephosphorylation of Drp-1 protein at ser637, while CaN inhibitor FK506 pretreatment could reduce the dephosphorylation of Drp-1. Therefore, the WNK1/NKCC1/Ca²⁺ /Drp-1 signalling pathway plays an important role in sevoflurane-related neurotoxicity. Reducing intracellular calcium influx may be one of the important mechanism to ameliorate sevoflurane toxicity.

WNK1/HSN2 mediates neurite outgrowth and differentiation via a OSR1/GSK3β-LHX8 pathway

With no lysine kinase 1 (WNK1) phosphorylates and activates STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress responsive kinase 1 (OSR1) to regulate ion homeostasis in the kidney. Mutations in WNK1 result in dysregulation of the WNK1-SPAK/OSR1 pathway and cause pseudohypoaldosteronism type II (PHAII), a form of hypertension. WNK1 is also involved in the autosomal recessive neuropathy, hereditary sensory and autonomic neuropathy type II (HSANII). Mutations in a neural-specific splice variant of WNK1 (HSN2) cause HSANII. However, the mechanisms underlying HSN2 regulation in neurons and effects of HSN2 mutants remain unclear. Here, we found that HSN2 regulated neurite outgrowth through OSR1 activation and glycogen synthase kinase 3β (GSK3β). Moreover, HSN2-OSR1 and HSN2-GSK3β signalling induced expression of LIM homeobox 8 (Lhx8), which is a key regulator of cholinergic neural function. The HSN2-OSR1/GSK3β-LHX8 pathway is therefore important for neurite outgrowth. Consistently, HSN2 mutants reported in HSANII patients suppressed SPAK and OSR1 activation and LHX8 induction. Interestingly, HSN2 mutants also suppressed neurite outgrowth by preventing interaction of between wild-type HSN2 and GSK3β. These results indicate that HSN2 mutants cause dysregulation of neurite outgrowth via GSK3β in the HSN2 and/or WNK1 pathways.

THE WNK4/SPAK PATHWAY STIMULATES ALVEOLAR FLUID CLEARANCE BY UPREGULATION OF EPITHELIAL SODIUM CHANNEL IN MICE WITH LIPOPOLYSACCHARIDE-INDUCED ACUTE RESPIRATORY DISTRESS SYNDROME

ABSTRACT

With-No lysine Kinases (WNKs) have been newly implicated in alveolar fluid clearance (AFC). Epithelial sodium channels (ENaCs) serve a vital role in AFC. The potential protective effect of WNK4 in acute respiratory distress syndrome (ARDS), mediated by ENaC-associated AFC was investigated in the study. A model of lipopolysaccharide (LPS)-induced ARDS was established in C57BL/6 mice. WNK4, Sterile 20-related proline-alanine-rich kinase (SPAK), small interfering RNA (siRNA)-WNK4 or siRNA-SPAK were transfected into mouse lung or primary alveolar epithelial type II (ATII) cells. AFC, bronchoalveolar lavage fluid and lung histomorphology were determined. The expression of ENaC was determined to investigate the regulation of AFC by WNK4-SPAK signaling pathway. Activation of WNK4-SPAK signaling improved lung injury and survival rate, with enhanced AFC and reduced pulmonary edema via the upregulation of ENaC in ARDS. In primary rat ATII cells, gene-silencing by siRNA transfection reduced ENaC expression and the level of WNK4-associated SPAK phosphorylation. Immunoprecipitation revealed that the level of neural precursor cell-expressed developmentally downregulated gene 4 (Nedd4-2) binding to ENaC was decreased as a result of WNK4-SPAK signaling. The present study demonstrated that the WNK4/SPAK pathway improved AFC during LPS-induced ARDS, which is mainly dependent on the upregulation of ENaC with Nedd4-2-mediated ubiquitination.

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.

The role of Cl- and K+ efflux in NLRP3 inflammasome and innate immune response activation

Inflammation is part of innate immunity and is a natural response of the body to bacteria, virus, any other pathogen infections, or to damaged tissues. However, too much inflammation or chronic inflammation contributes to a wide variety of diseases such as inflammatory bowel disease, cancer, type 2 diabetes, heart disease, or autoimmune disease such as rheumatoid arthritis. Recent studies underscored the critical role of K⁺ and Cl^- efflux in the activation of the inflammasome. The NLRP3 inflammasome is a multiprotein complex that mediates the production of the proinflammatory cytokines IL-1β and IL-18 and initiates the inflammatory cell death or pyroptosis. The NLRP3 inflammasome can be activated by multiple stimuli such as extracellular ATP, microbial toxins, ROS, mitochondria DNA or particulate matter. Although the precise mechanisms of NLRP3 activation and regulation by these diverse agonists remain unclear, multiple reports indicate that all NLRP3 agonists ultimately lead to a drop in intracellular concentration of potassium (K⁺ efflux) and chloride (Cl^- efflux). The WNK-SPAK/OSR1-[N]KCC pathway plays a critical role maintaining K⁺ and Cl^- ions concentration in the cell. Recent advances indicate that the WNK-SPAK-[N]KCC pathway play a role in the activation of the innate immune response. This review highlights recent discoveries detailing how ion transport regulates innate immune cell response to inflammatory stimuli.

A five amino acids deletion in NKCC2 of C57BL/6 mice affects analysis of NKCC2 phosphorylation but does not impact kidney function

Aim

The phosphorylation level of the furosemide‐sensitive Na⁺‐K⁺‐2Cl⁻ cotransporter (NKCC2) in the thick ascending limb (TAL) is used as a surrogate marker for NKCC2 activation and TAL function. However, in mice, analyses of NKCC2 phosphorylation with antibodies against phosphorylated threonines 96 and 101 (anti‐pT96/pT101) give inconsistent results. We aimed (a) to elucidate these inconsistencies and (b) to develop a phosphoform‐specific antibody that ensures reliable detection of NKCC2 phosphorylation in mice.

Methods

Genetic information, molecular biology, biochemical techniques and mouse phenotyping was used to study NKCC2 and kidney function in two commonly used mouse strains (ie 129Sv and in C57BL/6 mice). Moreover, a new phosphoform‐specific mouse NKCC2 antibody was developed and characterized.

Results

Amino acids sequence alignment revealed that C57BL/6 mice have a strain‐specific five amino acids deletion (ΔF97‐T101) in NKCC2 that diminishes the detection of NKCC2 phosphorylation with previously developed pT96/pT101 NKCC2 antibodies. Instead, the antibodies cross‐react with the phosphorylated thiazide‐sensitive NaCl cotransporter (NCC), which can obscure interpretation of results. Interestingly, the deletion in NKCC2 does not impact on kidney function and/or expression of renal ion transport proteins as indicated by the analysis of the F2 generation of crossbred 129Sv and C57BL/6 mice. A newly developed pT96 NKCC2 antibody detects pNKCC2 in both mouse strains and shows no cross‐reactivity with phosphorylated NCC.

Conclusion

Our work reveals a hitherto unappreciated, but essential, strain difference in the amino acids sequence of mouse NKCC2 that needs to be considered when analysing NKCC2 phosphorylation in mice. The new pNKCC2 antibody circumvents this technical caveat.

Melatonin modulates nitric oxide-regulated WNK-SPAK/OSR1-NKCC1 signaling in dorsal raphe nucleus of rats

The sleep-wake cycle is regulated by the alternating activity of sleep- and wake-promoting neurons. The dorsal raphe nucleus (DRN) secretes 5-hydroxytryptamine (5-HT, serotonin), promoting wakefulness. Melatonin secreted from the pineal gland also promotes wakefulness in rats. Our laboratory recently demonstrated that daily changes in nitric oxide (NO) production regulates a signaling pathway involving with-no-lysine kinase (WNK), Ste20-related proline alanine rich kinase (SPAK)/oxidative stress response kinase 1 (OSR1), and cation-chloride co-transporters (CCC) in rat DRN serotonergic neurons. This study was designed to investigate the effect of melatonin on NO-regulated WNK-SPAK/OSR1-CCC signaling in wake-inducing DRN neurons to elucidate the mechanism underlying melatonin's wake-promoting actions in rats. Ex vivo treatment of DRN slices with melatonin suppressed neuronal nitric oxide synthase (nNOS) expression and increased WNK4 expression without altering WNK1, 2, or 3. Melatonin increased phosphorylation of OSR1 and the expression of sodium-potassium-chloride co-transporter 1 (NKCC1), while potassium-chloride cotransporter 2 (KCC2) remained unchanged. Melatonin increased the expression of tryptophan hydroxylase 2 (TPH2, serotonin-synthesizing enzyme). The present study suggests that melatonin may promote its wakefulness by modulating NO-regulated WNK-SPAK/OSR1-KNCC1 signaling in rat DRN serotonergic neurons.

Molecular Mechanism of Inhibiting WNK Binding to OSR1 by Targeting the Allosteric Pocket of the OSR1-CCT Domain with Potential Antihypertensive Inhibitors: An In Silico Study

The oxidative-stress-responsive kinase 1 (OSR1) and the STE20/SPS1-related proline-alanine-rich kinase (SPAK) are physiological substrates of the with-no-lysine (WNK) kinase. They are the master regulators of cation Cl^- cotransporters that could be targeted for discovering novel antihypertensive agents. Both kinases have a conserved carboxy-terminal (CCT) domain that recognizes a unique peptide motif (Arg-Phe-Xaa-Val) present in their upstream kinases and downstream substrates. Here, we have combined molecular docking with molecular dynamics simulations and free-energy calculations to identify potential inhibitors that can bind to the allosteric pocket of the OSR1-CCT domain and impede its interaction with the WNK peptide. Our study revealed that STOCK1S-14279 and Closantel bound strongly to the allosteric pocket of OSR1 and displaced the WNK peptide from the primary pocket of OSR1. We showed that primarily Arg1004 and Gln1006 of the WNK4-peptide motif were involved in strong H-bond interactions with Glu453 and Arg451 of OSR1. Besides, our study revealed that atoms of Arg1004 were solvent-exposed in cases of STOCK1S-14279 and Closantel, implying that the WNK4 peptide was moved out of the pocket. Overall, the predicted potential inhibitors altogether abolish the OSR1-WNK4-peptide interaction, suggesting their potency as a prospective allosteric inhibitor against OSR1.

Axon morphogenesis and maintenance require an evolutionary conserved safeguard function of Wnk kinases antagonizing Sarm and Axed

The molecular and cellular mechanisms underlying complex axon morphogenesis are still poorly understood. We report a novel, evolutionary conserved function for the Drosophila Wnk kinase (dWnk) and its mammalian orthologs, WNK1 and 2, in axon branching. We uncover that dWnk, together with the neuroprotective factor Nmnat, antagonizes the axon-destabilizing factors D-Sarm and Axundead (Axed) during axon branch growth, revealing a developmental function for these proteins. Overexpression of D-Sarm or Axed results in axon branching defects, which can be blocked by overexpression of dWnk or Nmnat. Surprisingly, Wnk kinases are also required for axon maintenance of adult Drosophila and mouse cortical pyramidal neurons. Requirement of Wnk for axon maintenance is independent of its developmental function. Inactivation of dWnk or mouse Wnk1/2 in mature neurons leads to axon degeneration in the adult brain. Therefore, Wnk kinases are novel signaling components that provide a safeguard function in both developing and adult axons.

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

Structures and an activation mechanism of human potassium-chloride cotransporters

Potassium-chloride cotransporters KCC1 to KCC4 mediate the coupled export of potassium and chloride across the plasma membrane and play important roles in cell volume regulation, auditory system function, and γ-aminobutyric acid (GABA) and glycine-mediated inhibitory neurotransmission. Here, we present 2.9- to 3.6-Å resolution structures of full-length human KCC2, KCC3, and KCC4. All three KCCs adopt a similar overall architecture, a domain-swap dimeric assembly, and an inward-facing conformation. The structural and functional studies reveal that one unexpected N-terminal peptide binds at the cytosolic facing cavity and locks KCC2 and KCC4 at an autoinhibition state. The C-terminal domain (CTD) directly interacts with the N-terminal inhibitory peptide, and the relative motions between the CTD and the transmembrane domain (TMD) suggest that CTD regulates KCCs' activities by adjusting the autoinhibitory effect. These structures provide the first glimpse of full-length structures of KCCs and an autoinhibition mechanism among the amino acid-polyamine-organocation transporter superfamily.

NKCC1: Newly Found as a Human Disease-Causing Ion Transporter

Among the electroneutral Na⁺-dependent chloride transporters, NKCC1 had until now evaded identification as a protein causing human diseases. The closely related SLC12A transporters, NKCC2 and NCC have been identified some 25 years ago as responsible for Bartter and Gitelman syndromes: two renal-dependent salt wasting disorders. Absence of disease was most surprising since the NKCC1 knockout mouse was shown in 1999 to be viable, albeit with a wide range of deleterious phenotypes. Here we summarize the work of the past 5 years that introduced us to clinical cases involving NKCC1. The most striking cases are of 3 children with inherited mutations, who have complete absence of NKCC1 expression. These cases establish that lack of NKCC1 causes deafness; CFTR-like secretory defects with mucus accumulation in lung and intestine; severe xerostomia, hypotonia, dysmorphic facial features, and severe neurodevelopmental disorder. Another intriguing case is of a patient with a dominant deleterious SLC12A2 allele. This de novo mutation introduced a premature stop codon leading to a truncated protein. This mutant transporter seems to exert dominant-negative effect on wild-type transporter only in epithelial cells. The patient who suffers from lung, bladder, intestine, pancreas, and multiple endocrine abnormalities has, however, normal hearing and cognition. Finally, new reports substantiate the haploinsufficiency prediction of the SLC12A2 gene. Cases with single allele mutations in SLC12A2 have been linked to hearing loss and neurodevelopmental disorders.

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

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

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

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

Component Identification and Functional Analysis of Outer Membrane Vesicles Released by Avibacterium paragallinarum

Avibacterium paragallinarum, the causative agent of infectious coryza, is known to release outer membrane vesicles (OMVs). In the present study, we investigated the composition, bioactivities, and functional properties of the OMVs of A. paragallinarum. Following extraction and purification, the OMVs were observed to be spherical in shape, with diameters ranging from 20 to 300 nm. The vesicles contained endotoxin as well as genomic DNA. The molecular weights of the OMV-contained protein fragments were mostly concentrated at 65 and 15 kDa. The components of the OMV proteins were mainly various functional enzymes (e.g., ATP-dependent RNA helicase), structural components (e.g., streptomycin B receptor and membrane protein), and some hypothetical proteins with unknown functions. The expression levels of inflammation-related factors, such as interleukin (IL)-2, IL-6, IL-1β, IL-10, and inducible nitric oxide synthase (iNOs), were significantly upregulated in chicken macrophage cells HD11 incubated with OMVs. Serum IgG antibodies were measured after two intramuscular injections of an OMV-based vaccine into specific pathogen-free (SPF) chickens. The vaccinated chickens were then challenged by A. paragallinarum of homologous and heterologous serovars. It was noted that the vaccinated chickens produced immunoglobulin G (IgG) antibodies against A. paragallinarum. The OMVs conferred an acceptable level of protection (70%), defined as an absence of colonization and of clinical signs, against the homologous strain (serovar A), while the cross-protection against heterologous challenge with serovars B and C was much weaker. However, the OMVS did provide significant protection against clinical signs for all three serovars. Overall, this study laid a foundation for further unraveling the functional roles of OMVs released by A. paragallinarum.

Stimulatory Role of SPAK Signaling in the Regulation of Large Conductance Ca2+-Activated Potassium (BK) Channel Protein Expression in Kidney

SPS1-related proline/alanine-rich kinase (SPAK) plays important roles in regulating the function of numerous ion channels and transporters. With-no-lysine (WNK) kinase phosphorylates SPAK kinase to active the SPAK signaling pathway. Our previous studies indicated that WNK kinases regulate the activity of the large-conductance Ca²⁺-activated K⁺ (BK) channel and its protein expression via the ERK1/2 signaling pathway. It remains largely unknown whether SPAK kinase directly modulates the BK protein expression in kidney. In this study, we investigated the effect of SPAK on renal BK protein expression in both HEK293 cells and mouse kidney. In HEK293 cells, siRNA-mediated knockdown of SPAK expression significantly reduced BK protein expression and increased ERK1/2 phosphorylation, whereas overexpression of SPAK significantly enhanced BK expression and decreased ERK1/2 phosphorylation in a dose-dependent manner. Knockdown of ERK1/2 prevented SPAK siRNA-mediated inhibition of BK expression. Similarly, pretreatment of HEK293 cells with either the lysosomal inhibitor bafilomycin A1 or the proteasomal inhibitor MG132 reversed the inhibitory effects of SPAK knockdown on BK expression. We also found that there is no BK channel activity in PCs of CCD in SPAK KO mice using the isolated split-open tubule single-cell patching. In addition, we found that BK protein abundance in the kidney of SPAK knockout mice was significantly decreased and ERK1/2 phosphorylation was significantly enhanced. A high-potassium diet significantly increased BK protein abundance and SPAK phosphorylation levels, while reducing ERK1/2 phosphorylation levels. These findings suggest that SPAK enhances BK protein expression by reducing ERK1/2 signaling-mediated lysosomal and proteasomal degradations of the BK channel.

Staurosporine and NEM mainly impair WNK-SPAK/OSR1 mediated phosphorylation of KCC2 and NKCC1

The pivotal role of KCC2 and NKCC1 in development and maintenance of fast inhibitory neurotransmission and their implication in severe human diseases arouse interest in posttranscriptional regulatory mechanisms such as (de)phosphorylation. Staurosporine (broad kinase inhibitor) and N-ethylmalemide (NEM) that modulate kinase and phosphatase activities enhance KCC2 and decrease NKCC1 activity. Here, we investigated the regulatory mechanism for this reciprocal regulation by mass spectrometry and immunoblot analyses using phospho-specific antibodies. Our analyses revealed that application of staurosporine or NEM dephosphorylates Thr1007 of KCC2, and Thr203, Thr207 and Thr212 of NKCC1. Dephosphorylation of Thr1007 of KCC2, and Thr207 and Thr212 of NKCC1 were previously demonstrated to activate KCC2 and to inactivate NKCC1. In addition, application of the two agents resulted in dephosphorylation of the T-loop and S-loop phosphorylation sites Thr233 and Ser373 of SPAK, a critical kinase in the WNK-SPAK/OSR1 signaling module mediating phosphorylation of KCC2 and NKCC1. Taken together, these results suggest that reciprocal regulation of KCC2 and NKCC1 via staurosporine and NEM is based on WNK-SPAK/OSR1 signaling. The key regulatory phospho-site Ser940 of KCC2 is not critically involved in the enhanced activation of KCC2 upon staurosporine and NEM treatment, as both agents have opposite effects on its phosphorylation status. Finally, NEM acts in a tissue-specific manner on Ser940, as shown by comparative analysis in HEK293 cells and immature cultured hippocampal neurons. In summary, our analyses identified phospho-sites that are responsive to staurosporine or NEM application. This provides important information towards a better understanding of the cooperative interactions of different phospho-sites.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

A mutation in the Na-K-2Cl cotransporter-1 leads to changes in cellular metabolism

The Na-K-Cl cotransporter-1 (NKCC1), by mediating the electroneutral transport of Na⁺ , K⁺ , and Cl^- plays an important role in cell volume regulation, epithelial transport, and the control of neuronal excitability. Recently, we reported the first known human mutation in SLC12A2, the gene encoding NKCC1. The 17-year old patient suffers from multiorgan failure. Laboratory tests conducted on muscle and liver biopsies of the patient showed abnormal increase in mitochondrial DNA copy number and increased glycogen levels, indicating the possibility that the transporter may play a role in energy metabolism. Here, we show that fibroblasts isolated from the patient demonstrate a significant increase in mitochondrial respiration, compared to fibroblasts isolated from healthy individuals. Similarly, Madin Darby canine kidney (MDCK) cells transfected with enhanced green fluorescent protein (EGFP)-tagged mutant NKCC1 DNA demonstrated increased mitochondrial respiration when compared to MDCK cells expressing EGFP-tagged wild-type (WT) cotransporter. Direct inhibition of the cotransporter through addition of bumetanide did not change the rate of basal respiration, but led to increased maximal mitochondrial respiration. Fibroblasts extracted from NKCC1^WT/DFX and NKCC1^DFX/DFX mice also demonstrated a significant elevation in mitochondrial respiration, compared to fibroblasts isolated from their WT littermates. Expression of the mutant protein was associated with an increase in hydrogen peroxide and peroxidase activity and a decrease in messenger RNA transcript levels for protein involved in the unfolded protein response. These data reveal that cells expressing the mutant cotransporter demonstrate increased mitochondrial respiration and behave like they are experiencing a state of starvation.

Cryo-EM structures of DrNKCC1 and hKCC1: a new milestone in the physiology of cation-chloride cotransporters

New milestones have been reached in the field of cation-Cl- cotransporters with the recently released cryo-electron microscopy (EM) structures of the Danio rerio (zebrafish) Na+-K+-2Cl- cotransporter (DrNKCC1) and the human K+-Cl- cotransporter (hKCC1). In this review we provide a brief timeline that identifies the multiple breakthroughs in the field of solute carrier 12 transporters that led to the structure resolution of two of its key members. While cation-Cl- cotransporters share the overall architecture of carriers belonging to the amino acid-polyamine-organocation (APC) superfamily and some of their substrate binding sites, several new insights are gained from the two individual structures. A first major feature relates to the largest extracellular domain between transmembrane domain (TMD) 5 and TMD6 of KCC1, which stabilizes the dimer and forms a cap that likely participates in extracellular gating. A second feature is the conservation of the K+ and Cl- binding sites in both structures and evidence of an unexpected second Cl- coordination site in the KCC1 structure. Structural data are discussed in the context of previously published studies that examined the basic and kinetics properties of these cotransport mechanisms. A third characteristic is the evidence of an extracellular gate formed by conserved salt bridges between charged residues located toward the end of TMD3 and TMD4 in both transporters and the existence of an additional neighboring bridge in the hKCC1 structure. A fourth feature of these newly solved structures relates to the multiple points of contacts between the monomer forming the cotransporter homodimer units. These involve the TMDs, the COOH-terminal domains, and the large extracellular loop for hKCC1.

Cryo-EM structures of the human cation-chloride cotransporter KCC1

Cation-chloride cotransporters (CCCs) mediate the coupled, electroneutral symport of cations with chloride across the plasma membrane and are vital for cell volume regulation, salt reabsorption in the kidney, and γ-aminobutyric acid (GABA)-mediated modulation in neurons. Here we present cryo-electron microscopy (cryo-EM) structures of human potassium-chloride cotransporter KCC1 in potassium chloride or sodium chloride at 2.9- to 3.5-angstrom resolution. KCC1 exists as a dimer, with both extracellular and transmembrane domains involved in dimerization. The structural and functional analyses, along with computational studies, reveal one potassium site and two chloride sites in KCC1, which are all required for the ion transport activity. KCC1 adopts an inward-facing conformation, with the extracellular gate occluded. The KCC1 structures allow us to model a potential ion transport mechanism in KCCs and provide a blueprint for drug design.

Intracellular chloride: a regulator of transepithelial transport in the distal nephron

PURPOSE OF REVIEW

This review focuses on the role of intracellular chloride in regulating transepithelial ion transport in the distal convoluted tubule (DCT) in response to perturbations in plasma potassium homeostasis.

RECENT FINDINGS

Low dietary potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the DCT, and vice versa, affecting sodium-dependent potassium secretion in the downstream aldosterone-sensitive distal nephron. In cells, NCC phosphorylation is increased by lowering of intracellular chloride, via activation of the chloride-sensitive with no lysine (WNK)-SPAK/OSR1 (Ste20-related proline/alanine-rich kinase/oxidative stress response) kinase cascade. In-vivo studies have demonstrated pathway activation in the kidney in response to low dietary potassium. A possible mechanism is lowering of DCT intracellular chloride in response to low potassium because of parallel basolateral potassium and chloride channels. Recent studies support a role for these channels in the response of NCC to varying potassium. Studies examining chloride-insensitive WNK mutants, in the Drosophila renal tubule and in the mouse, lend further support to a role for chloride in regulating WNK activity and transepithelial ion transport. Caveats, alternatives, and future directions are also discussed.

SUMMARY

Chloride sensing by WNK kinase provides a mechanism to allow coupling of extracellular potassium with NCC phosphorylation and activity to maintain potassium homeostasis.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

A role for KCC3 in maintaining cell volume of peripheral nerve fibers

The potassium chloride cotransporter, KCC3, is an electroneutral cotransporter expressed in the peripheral and central nervous system. KCC3 is responsible for the efflux of K+ and Cl- in neurons to help maintain cell volume and intracellular chloride levels. A loss-of-function (LOF) of KCC3 causes Hereditary Motor Sensory Neuropathy with Agenesis of the Corpus Callosum (HMSN/ACC) in a population of individuals in the Charlevoix/Lac-Saint-Jean region of Quebec, Canada. A variety of mouse models have been created to understand the physiological and deleterious effects of a KCC3 LOF. Though this KCC3 LOF in mouse models has recapitulated the peripheral neuropathy phenotype of HMSN/ACC, we still know little about the development of the disease pathophysiology. Interestingly, the most recent KCC3 mouse model that we created recapitulated a peripheral neuropathy-like phenotype originating from a KCC3 gain-of-function (GOF). Despite the past two decades of research in attempting to understand the role of KCC3 in disease, we still do not understand how dysfunction of this cotransporter can lead to the pathophysiology of peripheral neuropathy. This review focuses on the function of KCC3 in neurons and its role in human and health and disease.

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.

Phosphoregulation of the intracellular termini of K+-Cl- cotransporter 2 (KCC2) enables flexible control of its activity

The pivotal role of K⁺-Cl^- cotransporter 2 (KCC2) in inhibitory neurotransmission and severe human diseases fosters interest in understanding posttranslational regulatory mechanisms such as (de)phosphorylation. Here, the regulatory role of the five bona fide phosphosites Ser³¹, Thr³⁴, Ser⁹³², Thr⁹⁹⁹, and Thr¹⁰⁰⁸ was investigated by the use of alanine and aspartate mutants. Tl⁺-based flux analyses in HEK-293 cells demonstrated increased transport activity for S932D (mimicking phosphorylation) and T1008A (mimicking dephosphorylation), albeit to a different extent. Increased activity was due to changes in intrinsic activity, as it was not caused by increased cell-surface abundance. Substitutions of Ser³¹, Thr³⁴, or Thr⁹⁹⁹ had no effect. Additionally, we show that the indirect actions of the known KCC2 activators staurosporine and N-ethylmaleimide (NEM) involved multiple phosphosites. S31D, T34A, S932A/D, T999A, or T1008A/D abrogated staurosporine mediated stimulation, and S31A, T34D, or S932D abolished NEM-mediated stimulation. This demonstrates for the first time differential effects of staurosporine and NEM on KCC2. In addition, the staurosporine-mediated effects involved both KCC2 phosphorylation and dephosphorylation with Ser⁹³² and Thr¹⁰⁰⁸ being bona fide target sites. In summary, our data reveal a complex phosphoregulation of KCC2 that provides the transporter with a toolbox for graded activity and integration of different signaling pathways.

Water Homeostasis and Cell Volume Maintenance and Regulation

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

WNK-SPAK/OSR1 signaling: lessons learned from an insect renal epithelium

WNK [with no lysine (K)] kinases regulate renal epithelial ion transport to maintain homeostasis of electrolyte concentrations, extracellular volume, and blood pressure. The SLC12 cation-chloride cotransporters, including the sodium-potassium-2-chloride (NKCC) and sodium chloride cotransporters (NCC), are targets of WNK regulation via the intermediary kinases SPAK (Ste20-related proline/alanine-rich kinase) and OSR1 (oxidative stress response). The pathway is activated by low dietary potassium intake, resulting in increased phosphorylation and activity of NCC. Chloride regulates WNK kinases in vitro by binding to the active site and inhibiting autophosphorylation and has been proposed to modulate WNK activity in the distal convoluted tubule in response to low dietary potassium. WNK-SPAK/OSR1 regulation of NKCC-dependent ion transport is evolutionarily ancient, and it occurs in the Drosophila Malpighian (renal) tubule. Here, we review recent studies from the Drosophila tubule demonstrating cooperative roles for chloride and the scaffold protein Mo25 (mouse protein-25, also known as calcium-binding protein-39) in the regulation of WNK-SPAK/OSR1 signaling in a transporting renal epithelium. Insights gained from this genetically manipulable and physiologically accessible epithelium shed light on molecular mechanisms of regulation of the WNK-SPAK/OSR1 pathway, which is important in human health and disease.

Mistargeting of a truncated Na-K-2Cl cotransporter in epithelial cells

We recently reported the case of a young patient with multisystem failure carrying a de novo mutation in SLC12A2, the gene encoding the Na-K-2Cl cotransporter-1 (NKCC1). Heterologous expression studies in nonepithelial cells failed to demonstrate dominant-negative effects. In this study, we examined expression of the mutant cotransporter in epithelial cells. Using Madin-Darby canine kidney (MDCK) cells grown on glass coverslips, permeabilized support, and Matrigel, we show that the fluorescently tagged mutant cotransporter is expressed in cytoplasm and at the apical membrane and affects epithelium integrity. Expression of the mutant transporter at the apical membrane also results in the mislocalization of some of the wild-type transporter to the apical membrane. This mistargeting is specific to NKCC1 as the Na⁺-K⁺-ATPase remains localized on the basolateral membrane. To assess transporter localization in vivo, we created a mouse model using CRISPR/cas9 that reproduces the 11 bp deletion in exon 22 of Slc12a2. Although the mice do not display an overt phenotype, we show that the colon and salivary gland expresses wild-type NKCC1 abundantly at the apical pole, confirming the data obtained in cultured epithelial cells. Enough cotransporter must remain, however, on the basolateral membrane to participate in saliva secretion, as no significant decrease in saliva production was observed in the mutant mice.

Expression of OsWNK9 in Arabidopsis conferred tolerance to salt and drought stress

With No Lysine (WNK) kinase belongs to ser/thr protein kinase group in which conserved catalytic lysine (K) residue of subdomain II is shifted to subdomain I. In this study, we cloned full-length coding region of WNK9 from Oryza sativa (OsWNK9) and performed in silico studies to confirm the presence of all kinase signature regulatory elements. The transcript analysis revealed that OsWNK9 was strongly down regulated under salinity, drought and ABA stress in shoots. Constitutive expression of OsWNK9 in Arabidopsis thaliana imparted increased tolerance to salt, drought, and ABA stress. Transgenic lines showed healthy phenotypes such as green leaves, achieved higher fresh weight and longer roots under salt, drought and ABA stress as compared to wild-type (WT). Transgenic plants showed better seed germination, higher chlorophyll retention and less water loss under salt and drought stress compared to WT. Promoter/gene expression studies revealed that OsWNK9 were expressed throughout plant tissues with higher expression in roots. Subcellular localization studies of OsWNK9 showed their presence in the nucleus. The transcript analysis of abiotic stress marker genes and ABA dependent genes showed they were highly expressed in transgenic lines compared to WT in response to salt and drought stress. The endogenous ABA level under salt and drought stress in transgenic lines was higher than WT. The results indicated that OsWNK9 may regulate salt and drought response in ABA dependent manner.

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.

Glycogen synthase kinase 3ß functions as a positive effector in the WNK signaling pathway

The with no lysine (WNK) protein kinase family is conserved among many species. Some mutations in human WNK gene are associated with pseudohypoaldosteronism type II, a form of hypertension, and hereditary sensory and autonomic neuropathy type 2A. In kidney, WNK regulates the activity of STE20/SPS1-related, proline alanine-rich kinase and/or oxidative-stress responsive 1, which in turn regulate ion co-transporters. The misregulation of this pathway is involved in the pathogenesis of pseudohypoaldosteronism type II. In the neural system, WNK is involved in the specification of the cholinergic neuron, but the pathogenesis of hereditary sensory and autonomic neuropathy type 2A is still unknown. To better understand the WNK pathway, we isolated WNK-associated genes using Drosophila. We identified Glycogen synthase kinase 3ß (GSK3ß)/Shaggy (Sgg) as a candidate gene that was shown to interact with the WNK signaling pathway in both Drosophila and mammalian cells. Furthermore, GSK3ß was involved in neural specification downstream of WNK. These results suggest that GSK3ß/Sgg functions as a positive effector in the WNK signaling pathway.

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.

Implications of the N-terminal heterogeneity for the neuronal K-Cl cotransporter KCC2 function

The neuron-specific K-Cl cotransporter KCC2 maintains the low intracellular chloride concentration required for the fast hyperpolarizing responses of the inhibitory neurotransmitters γ-aminobutyric acid (GABA) and glycine. The two KCC2 isoforms, KCC2a and KCC2b differ by their N-termini as a result of alternative promoter usage. Whereas the role of KCC2b in mediating the chloride transport is unequivocal, the physiological role of KCC2a in neurons has remained obscure. We show that KCC2a isoform can decrease the intracellular chloride concentration in cultured neurons and attenuate calcium responses evoked by application of the GABAA receptor agonist muscimol. While the biotinylation assay detected both KCC2 isoforms at the cell surface of cultured neurons, KCC2a was not detected at the plasma membrane in immunostainings, suggesting that the N-terminal KCC2a epitope is masked. Confirming this hypothesis, KCC2a surface expression was detected by the C-terminal KCC2 pan antibody but not by the N-terminal KCC2a antibody in KCC2b-deficient neurons. One possible cause for the epitope masking is the binding site of Ste20-related proline-alanine-rich kinase (SPAK) in the KCC2a N-terminus. SPAK, a known regulator of K-Cl cotransporters, was co-immunoprecipitated in a complex with KCC2a but not KCC2b isoform. Moreover, SPAK overexpression decreased the transport activity of KCC2a but not that of KCC2b, as revealed by rubidium flux assay in HEK293 cells. Thus, our data indicate that both KCC2 isoforms perform as chloride cotransporters in neuronal cells, while their N-terminal heterogeneity could play an important role in fine-tuning of the K-Cl transport activity.

Pharmacological targeting of SPAK kinase in disorders of impaired epithelial transport

The mammalian SPS1-related proline/alanine-rich serine-threonine kinase SPAK (STK39) modulates ion transport across and between epithelial cells in response to environmental stimuli such osmotic stress and inflammation. Research over the last decade has established a central role for SPAK in the regulation of ion and water transport in the distal nephron, colonic crypts, and pancreatic ducts, and has implicated deregulated SPAK signaling in NaCl-sensitive hypertension, ulcerative colitis and Crohn's disease, and cystic fibrosis. Areas covered: We review recent advances in our understanding of the role of SPAK kinase in the regulation of epithelial transport. We highlight how SPAK signaling - including its upstream Cl^- sensitive activators, the WNK kinases, and its downstream ion transport targets, the cation- Cl^- cotransporters contribute to human disease. We discuss prospects for the pharmacotherapeutic targeting of SPAK kinase in specific human disorders that feature impaired epithelial homeostasis. Expert opinion: The development of novel drugs that antagonize the SPAK-WNK interaction, inhibit SPAK kinase activity, or disrupt SPAK kinase activation by interfering with its binding to MO25α/β could be useful adjuncts in essential hypertension, inflammatory colitis, and cystic fibrosis.

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

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

Rafoxanide and Closantel Inhibit SPAK and OSR1 Kinases by Binding to a Highly Conserved Allosteric Site on Their C-terminal Domains

SPAK and OSR1 are two protein kinases that have emerged as attractive targets in the discovery of novel antihypertensive agents due to their role in regulating electrolyte balance in vivo. Herein we report the identification of an allosteric pocket on the highly conserved C-terminal domains of these two kinases, which influences their activity. We also show that some known WNK signaling inhibitors bind to this allosteric site. Using in silico screening, we identified the antiparasitic agent rafoxanide as a novel allosteric inhibitor of SPAK and OSR1. Collectively, this work will facilitate the rational design of novel SPAK and OSR1 kinase inhibitors that could be useful antihypertensive agents.

The Zn2+-sensing receptor, ZnR/GPR39, upregulates colonocytic Cl- absorption, via basolateral KCC1, and reduces fluid loss

Administration of zinc, as a complement to oral rehydration solutions, effectively diminishes duration and severity of diarrhea, but it is not known whether it merely fulfills a nutritional deficiency, or if zinc has a direct role of regulating solute absorption. We show that Zn2+ acts via a specific receptor, ZnR/GPR39, to reduce fluid loss. Intestinal fluid secretion triggered by cholera toxin (CTx) was lower in WT mice compared to ZnR/GPR39 KO. In the absence of dietary Zn2+ we observed similar fluid accumulation in WT and ZnR/GPR39 KO mice, indicating that Zn2+ and ZnR/GPR39 are both required for a beneficial effect of Zn2+ in diarrhea. In primary colonocytes and in Caco-2 colonocytic cells, activation of ZnR/GPR39 enhanced Cl- transport, a critical factor in diarrhea, by upregulating K+/Cl- cotransporter (KCC1) activity. Importantly, we show basolateral expression of KCC1 in mouse and human colonocytes, thus identifying a novel Cl- absorption pathway. Finally, inhibition of KCC-dependent Cl- transport enhanced CTx-induced fluid loss. Altogether, our data indicate that Zn2+ acting via ZnR/GPR39 has a direct role in controlling Cl- absorption via upregulation of basolateral KCC1 in the colon. Moreover, colonocytic ZnR/GPR39 and KCC1 reduce water loss during diarrhea and may therefore serve as effective drug targets.

Cerebrospinal fluid hypersecretion in pediatric hydrocephalus

Hydrocephalus, despite its heterogeneous causes, is ultimately a disease of disordered CSF homeostasis that results in pathological expansion of the cerebral ventricles. Our current understanding of the pathophysiology of hydrocephalus is inadequate but evolving. Over this past century, the majority of hydrocephalus cases has been explained by functional or anatomical obstructions to bulk CSF flow. More recently, hydrodynamic models of hydrocephalus have emphasized the role of abnormal intracranial pulsations in disease pathogenesis. Here, the authors review the molecular mechanisms of CSF secretion by the choroid plexus epithelium, the most efficient and actively secreting epithelium in the human body, and provide experimental and clinical evidence for the role of increased CSF production in hydrocephalus. Although the choroid plexus epithelium might have only an indirect influence on the pathogenesis of many types of pediatric hydrocephalus, the ability to modify CSF secretion with drugs newer than acetazolamide or furosemide would be an invaluable component of future therapies to alleviate permanent shunt dependence. Investigation into the human genetics of developmental hydrocephalus and choroid plexus hyperplasia, and the molecular physiology of the ion channels and transporters responsible for CSF secretion, might yield novel targets that could be exploited for pharmacotherapeutic intervention.

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.

Regulation of Renal Electrolyte Transport by WNK and SPAK-OSR1 Kinases

The discovery of four genes responsible for pseudohypoaldosteronism type II, or familial hyperkalemic hypertension, which features arterial hypertension with hyperkalemia and metabolic acidosis, unmasked a complex multiprotein system that regulates electrolyte transport in the distal nephron. Two of these genes encode the serine-threonine kinases WNK1 and WNK4. The other two genes [kelch-like 3 (KLHL3) and cullin 3 (CUL3)] form a RING-type E3-ubiquitin ligase complex that modulates WNK1 and WNK4 abundance. WNKs regulate the activity of the Na(+):Cl(-) cotransporter (NCC), the epithelial sodium channel (ENaC), the renal outer medullary potassium channel (ROMK), and other transport pathways. Interestingly, the modulation of NCC occurs via the phosphorylation by WNKs of other serine-threonine kinases known as SPAK-OSR1. In contrast, the process of regulating the channels is independent of SPAK-OSR1. We present a review of the remarkable advances in this area in the past 10 years.

Multistep regulation of autophagy by WNK1

The with-no-lysine (K) (WNK) kinases are an atypical family of protein kinases that regulate ion transport across cell membranes. Mutations that result in their overexpression cause hypertension-related disorders in humans. Of the four mammalian WNKs, only WNK1 is expressed throughout the body. We report that WNK1 inhibits autophagy, an intracellular degradation pathway implicated in several human diseases. Using small-interfering RNA-mediated WNK1 knockdown, we show autophagosome formation and autophagic flux are accelerated. In cells with reduced WNK1, basal and starvation-induced autophagy is increased. We also show that depletion of WNK1 stimulates focal class III phosphatidylinositol 3-kinase complex (PI3KC3) activity, which is required to induce autophagy. Depletion of WNK1 increases the expression of the PI3KC3 upstream regulator unc-51-like kinase 1 (ULK1), its phosphorylation, and activation of the kinase upstream of ULK1, the AMP-activated protein kinase. In addition, we show that the N-terminal region of WNK1 binds to the UV radiation resistance-associated gene (UVRAG) in vitro and WNK1 partially colocalizes with UVRAG, a component of a PI3KC3 complex. This colocalization decreases upon starvation of cells. Depletion of the SPS/STE20-related proline-alanine-rich kinase, a WNK1-activated enzyme, also induces autophagy in nutrient-replete or -starved conditions, but depletion of the related kinase and WNK1 substrate, oxidative stress responsive 1, does not. These results indicate that WNK1 inhibits autophagy by multiple mechanisms.

A patient with multisystem dysfunction carries a truncation mutation in human SLC12A2, the gene encoding the Na-K-2Cl cotransporter, NKCC1

This study describes a 13-yr-old girl with orthostatic intolerance, respiratory weakness, multiple endocrine abnormalities, pancreatic insufficiency, and multiorgan failure involving the gut and bladder. Exome sequencing revealed a de novo, loss-of-function allele in SLC12A2, the gene encoding the Na-K-2Cl cotransporter-1. The 11-bp deletion in exon 22 results in frameshift (p.Val1026Phefs*2) and truncation of the carboxy-terminal tail of the cotransporter. Preliminary studies in heterologous expression systems demonstrate that the mutation leads to a nonfunctional transporter, which is expressed and trafficked to the plasma membrane alongside wild-type NKCC1. The truncated protein, visible at higher molecular sizes, indicates either enhanced dimerization or misfolded aggregate. No significant dominant-negative effect was observed. K+ transport experiments performed in fibroblasts from the patient showed reduced total and NKCC1-mediated K+ influx. The absence of a bumetanide effect on K+ influx in patient fibroblasts only under hypertonic conditions suggests a deficit in NKCC1 regulation. We propose that disruption in NKCC1 function might affect sensory afferents and/or smooth muscle cells, as their functions depend on NKCC1 creating a Cl- gradient across the plasma membrane. This Cl- gradient allows the γ-aminobutyric acid (GABA) receptor or other Cl- channels to depolarize the membrane affecting processes such as neurotransmission or cell contraction. Under this hypothesis, disrupted sensory and smooth muscle function in a diverse set of tissues could explain the patient's phenotype.