Ste20-type kinases: evolutionarily conserved regulators of ion transport and cell volume

TNIK depletion induces inflammation and apoptosis in injured renal proximal tubule epithelial cells

In the aftermath of acute kidney injury (AKI), surviving proximal tubule epithelia repopulate injured tubules to promote repair. However, a portion of cells fail to repair [termed failed-repair proximal tubule cells (FR-PTCs)] and exert ongoing proinflammatory and profibrotic effects. To better understand the molecular drivers of the FR-PTC state, we reanalyzed a mouse ischemia-reperfusion injury single-nucleus RNA-sequencing (snRNA-seq) atlas to identify Traf2 and Nck interacting kinase (Tnik) to be exclusively expressed in FR-PTCs but not in healthy or acutely injured proximal tubules after AKI (2 and 6 wk) in mice. We confirmed expression of Tnik protein in injured mouse and human tissues by immunofluorescence. Then, to determine the functional role of Tnik in FR-PTCs, we depleted TNIK with siRNA in two human renal proximal tubule epithelial cell lines (primary and immortalized hRPTECs) and analyzed each by bulk RNA-sequencing. Pathway analysis revealed significant upregulation of inflammatory signaling pathways, whereas pathways associated with differentiated proximal tubules such as organic acid transport were significantly downregulated. TNIK gene knockdown drove reduced cell viability and increased apoptosis, including differentially expressed poly(ADP-ribose) polymerase (PARP) family members, cleaved PARP-1 fragments, and increased annexin V binding to phosphatidylserine. Together, these results indicate that Tnik upregulation in FR-PTCs acts in a compensatory fashion to suppress inflammation and promote proximal tubule epithelial cell survival after injury. Modulating TNIK activity may represent a prorepair therapeutic strategy after AKI.NEW & NOTEWORTHY The molecular drivers of successful and failed repair in the proximal tubule after acute kidney injury (AKI) are incompletely understood. We identified Traf2 and Nck interacting kinase (Tnik) to be exclusively expressed in failed-repair proximal tubule cells after AKI. We tested the effect of siTNIK depletion in two proximal tubule cell lines followed by bulk RNA-sequencing analysis. Our results indicate that TNIK acts to suppress inflammatory signaling and apoptosis in injured renal proximal tubule epithelial cells to promote cell survival.

Chromatin balances cell redox and energy homeostasis

Chromatin plays a central role in the conversion of energy in cells: alteration of chromatin structure to make DNA accessible consumes energy, and compaction of chromatin preserves energy. Alteration of chromatin structure uses energy sources derived from carbon metabolism such as ATP and acetyl-CoA; conversely, chromatin compaction and epigenetic modification feedback to metabolism and energy homeostasis by controlling gene expression and storing metabolites. Coordination of these dual chromatin events must be flexibly modulated in response to environmental changes such as during development and exposure to stress. Aging also alters chromatin structure and the coordination of metabolism, chromatin dynamics, and other cell processes. Noncoding RNAs and other RNA species that associate directly with chromatin or with chromatin modifiers contribute to spatiotemporal control of transcription and energy conversion. The time required for generating the large amounts of RNAs and chromatin modifiers observed in super-enhancers may be critical for regulation of transcription and may be impacted by aging. Here, taking into account these factors, we review alterations of chromatin that are fundamental to cell responses to metabolic changes due to stress and aging to maintain redox and energy homeostasis. We discuss the relationship between spatiotemporal control of energy and chromatin function, as this emerging concept must be considered to understand how cell homeostasis is maintained.

PAK1 and Therapy Resistance in Melanoma

Malignant melanoma claims more lives than any other skin malignancy. While primary melanomas are usually cured via surgical excision, the metastatic form of the disease portents a poor prognosis. Decades of intense research has yielded an extensive armamentarium of anti-melanoma therapies, ranging from genotoxic chemo- and radiotherapies to targeted interventions in specific signaling pathways and immune functions. Unfortunately, even the most up-to-date embodiments of these therapies are not curative for the majority of metastatic melanoma patients, and the need to improve their efficacy is widely recognized. Here, we review the reports that implicate p21-regulated kinase 1 (PAK1) and PAK1-related pathways in the response of melanoma to various therapeutic modalities. Ample data suggest that PAK1 may decrease cell sensitivity to programmed cell death, provide additional stimulation to growth-promoting molecular pathways, and contribute to the creation of an immunosuppressive tumor microenvironment. Accordingly, there is mounting evidence that the concomitant inhibition of PAK1 enhances the potency of various anti-melanoma regimens. Overall, the available information suggests that a safe and effective inhibition of PAK1-dependent molecular processes would enhance the potency of the currently available anti-melanoma treatments, although considerable challenges in implementing such strategies still exist.

STE20 kinase TAOK3 regulates type 2 immunity and metabolism in obesity

Healthy adipose tissue (AT) contains ST2+ Tregs, ILC2s, and alternatively activated macrophages that are lost in mice or humans on high caloric diet. Understanding how this form of type 2 immunity is regulated could improve treatment of obesity. The STE20 kinase Thousand And One amino acid Kinase-3 (TAOK3) has been linked to obesity in mice and humans, but its precise function is unknown. We found that ST2+ Tregs are upregulated in visceral epididymal white AT (eWAT) of Taok3-/- mice, dependent on IL-33 and the kinase activity of TAOK3. Upon high fat diet feeding, metabolic dysfunction was attenuated in Taok3-/- mice. ST2+ Tregs disappeared from eWAT in obese wild-type mice, but this was not the case in Taok3-/- mice. Mechanistically, AT Taok3-/- Tregs were intrinsically more responsive to IL-33, through higher expression of ST2, and expressed more PPARγ and type 2 cytokines. Thus, TAOK3 inhibits adipose tissue Tregs and regulates immunometabolism under excessive caloric intake.

Integrated single-cell transcriptome analysis of CD34 + enriched leukemic stem cells revealed intra- and inter-patient transcriptional heterogeneity in pediatric acute myeloid leukemia

To gain insights into the idiosyncrasies of CD34 + enriched leukemic stem cells, we investigated the nature and extent of transcriptional heterogeneity by single-cell sequencing in pediatric AML. Whole transcriptome analysis of 28,029 AML single cells was performed using the nanowell cartridge-based barcoding technology. Integrated transcriptional analysis identified unique leukemic stem cell clusters of each patient and intra-patient heterogeneity was revealed by multiple LSC-enriched clusters differing in their cell cycle processes and BCL2 expression. All LSC-enriched clusters exhibited gene expression profile of dormancy and self-renewal. Upregulation of genes involved in non-coding RNA processing and ribonucleoprotein assembly were observed in LSC-enriched clusters relative to HSC. The genes involved in regulation of apoptotic processes, response to cytokine stimulus, and negative regulation of transcription were upregulated in LSC-enriched clusters as compared to the blasts. Validation of top altered genes in LSC-enriched clusters confirmed upregulation of TCF7L2, JUP, ARHGAP25, LPAR6, and PRDX1 genes, and serine/threonine kinases (STK24, STK26). Upregulation of LPAR6 showed trend towards MRD positive status (Odds ratio = 0.126; 95% CI = 0.0144-1.10; p = 0.067) and increased expression of STK26 significantly correlated with higher RFS (HR = 0.231; 95% CI = 0.0506-1.052; p = 0.04). Our findings addressed the inter- and intra-patient diversity within AML LSC and potential signaling and chemoresistance-associated targets that warrant investigation in larger cohort that may guide precision medicine in the near future.

GCKIII kinases in lipotoxicity: Roles in NAFLD and beyond

Nonalcoholic fatty liver disease (NAFLD) is defined by excessive accumulation of lipid droplets within hepatocytes. The STE20-type kinases comprising the germinal center kinase III (GCKIII) subfamily - MST3, MST4, and STK25 - decorate intrahepatocellular lipid droplets and have recently emerged as critical regulators of the initiation and progression of NAFLD. While significant advancement has been made toward deciphering the role of GCKIII kinases in hepatic fat accumulation (i.e., steatosis) as well as the aggravation of NAFLD into its severe form nonalcoholic steatohepatitis (NASH), much remains to be resolved. This review provides a brief overview of the recent studies in patient cohorts, cultured human cells, and mouse models, which have characterized the function of MST3, MST4, and STK25 in the regulation of hepatic lipid accretion, meta-inflammation, and associated cell damage in the context of NAFLD/NASH. We also highlight the conflicting data and emphasize future research directions that are needed to advance our understanding of GCKIII kinases as potential targets in the therapy of NAFLD and its comorbidities. Conclusions: Several lines of evidence suggest that GCKIII proteins govern the susceptibility to hepatic lipotoxicity and that pharmacological inhibition of these kinases could mitigate NAFLD development and aggravation. Comprehensive characterization of the molecular mode-of-action of MST3, MST4, and STK25 in hepatocytes as well as extrahepatic tissues is important, especially in relation to their impact on carcinogenesis, to fully understand the efficacy as well as safety of GCKIII antagonism.

ARG2, MAP4K5 and TSTA3 as Diagnostic Markers of Steroid-Induced Osteonecrosis of the Femoral Head and Their Correlation With Immune Infiltration

Background

The diagnosis for steroid-induced osteonecrosis of the femoral head (SONFH) is hard to achieve at the early stage, which results in patients receiving ineffective treatment options and a poor prognosis for most cases. The present study aimed to find potential diagnostic markers of SONFH and analyze the effect exerted by infiltration of immune cells in this pathology.

Materials and Methods

R software was adopted for identifying differentially expressed genes (DEGs) and conducting functional investigation based on the microarray dataset. Then we combined SVM-RFE, WGCNA, LASSO logistic regression, and random forest (RF) algorithms for screening the diagnostic markers of SONFH and further verification by qRT-PCR. The diagnostic values were assessed through receiver operating characteristic (ROC) curves. CIBERSORT was then adopted for assessing the infiltration of immune cells and the relationship of infiltration-related immune cells and diagnostic markers.

Results

We identified 383 DEGs overall. This study found ARG2, MAP4K5, and TSTA3 (AUC = 0.980) to be diagnostic markers of SONFH. The results of qRT-PCR showed a statistically significant difference in all markers. Analysis of infiltration of immune cells indicated that neutrophils, activated dendritic cells and memory B cells were likely to show the relationship with SONFH occurrence and progress. Additionally, all diagnostic markers had different degrees of correlation with T cell follicular helper, neutrophils, memory B cells, and activated dendritic cells.

Conclusion

ARG2, MAP4K5, and TSTA3 are potential diagnostic genes for SONFH, and infiltration of immune cells may critically impact SONFH occurrence and progression.

STE20-Type Protein Kinase MST4 Controls NAFLD Progression by Regulating Lipid Droplet Dynamics and Metabolic Stress in Hepatocytes

Nonalcoholic fatty liver disease (NAFLD) has emerged as a leading cause of chronic liver disease worldwide, primarily because of the massive global increase in obesity. Despite intense research efforts in this field, the factors that govern the initiation and subsequent progression of NAFLD are poorly understood, which hampers the development of diagnostic tools and effective therapies in this area of high unmet medical need. Here we describe a regulator in molecular pathogenesis of NAFLD: STE20-type protein kinase MST4. We found that MST4 expression in human liver biopsies was positively correlated with the key features of NAFLD (i.e., hepatic steatosis, lobular inflammation, and hepatocellular ballooning). Furthermore, the silencing of MST4 attenuated lipid accumulation in human hepatocytes by stimulating β-oxidation and triacylglycerol secretion, while inhibiting fatty acid influx and lipid synthesis. Conversely, overexpression of MST4 in human hepatocytes exacerbated fat deposition by suppressing mitochondrial fatty acid oxidation and triacylglycerol efflux, while enhancing lipogenesis. In parallel to these reciprocal alterations in lipid storage, we detected substantially decreased or aggravated oxidative/endoplasmic reticulum stress in human hepatocytes with reduced or increased MST4 levels, respectively. Interestingly, MST4 protein was predominantly associated with intracellular lipid droplets in both human and rodent hepatocytes. Conclusion: Together, our results suggest that hepatic lipid droplet-decorating protein MST4 is a critical regulatory node governing susceptibility to NAFLD and warrant future investigations to address the therapeutic potential of MST4 antagonism as a strategy to prevent or mitigate the development and aggravation of this disease.

Genome-Wide Transcriptional Responses of Marine Nematode Litoditis marina to Hyposaline and Hypersaline Stresses

Maintenance of osmotic homeostasis is essential for all organisms, especially for marine animals in the ocean with 3% salinity or higher. However, the underlying molecular mechanisms that how marine animals adapt to high salinity environment compared to their terrestrial relatives, remain elusive. Here, we investigated marine animal’s genome-wide transcriptional responses to salinity stresses using an emerging marine nematode model Litoditis marina. We found that the transthyretin-like family genes were significantly increased in both hyposaline and hypersaline conditions, while multiple neurotransmitter receptor and ion transporter genes were down-regulated in both conditions, suggesting the existence of conserved strategies for response to stressful salinity environments in L. marina. Unsaturated fatty acids biosynthesis related genes, neuronal related tubulins and intraflagellar transport genes were specifically up-regulated in hyposaline treated worms. By contrast, cuticle related collagen genes were enriched and up-regulated for hypersaline response. Given a wide range of salinity tolerance of the marine nematodes, this study and further genetic analysis of key gene(s) of osmoregulation in L. marina will likely provide important insights into biological evolution and environmental adaptation mechanisms in nematodes and other invertebrate animals in general.

TAO-kinase 3 governs the terminal differentiation of NOTCH2-dependent splenic conventional dendritic cells

Antigen-presenting conventional dendritic cells (cDCs) are broadly divided into type 1 and type 2 subsets that further adapt their phenotype and function to perform specialized tasks in the immune system. The precise signals controlling tissue-specific adaptation and differentiation of cDCs are currently poorly understood. We found that mice deficient in the Ste20 kinase Thousand and One Kinase 3 (TAOK3) lacked terminally differentiated ESAM+ CD4+ cDC2s in the spleen and failed to prime CD4+ T cells in response to allogeneic red-blood-cell transfusion. These NOTCH2- and ADAM10-dependent cDC2s were absent selectively in the spleen, but not in the intestine of Taok3-/- and CD11c-cre Taok3fl/fl mice. The loss of splenic ESAM+ cDC2s was cell-intrinsic and could be rescued by conditional overexpression of the constitutively active NOTCH intracellular domain in CD11c-expressing cells. Therefore, TAOK3 controls the terminal differentiation of NOTCH2-dependent splenic cDC2s.

CAB39 Promotes the Proliferation of Nasopharyngeal Carcinoma CNE-1 Cells via Up-Regulating p-JNK

Aim

To investigate the role of CAB39 in nasopharyngeal carcinoma (NPC) development and examine its expression level in NPC tumor samples.

Methods

Immunohistochemistry staining of NPC tissue microarray was conducted to detect the expression of CAB39 protein in NPC tissues, and the clinical significance of CAB39 was evaluated. Lentivirus-mediated over-expression of CAB39 was designed to increase CAB39 expression in CNE-1 cells. Cell colony formation, cell cycle and CCK-8 proliferation experiments were performed to compare the proliferation ability of CNE-1 cells with or without CAB39 over-expression. Western blotting was conducted to examine downstream targets of CAB39.

Results

CAB39 expression was higher in tumor samples compared to normal tissue and the higher CAB39 expression was positively correlated to higher TNM stage and distant metastasis rate and non-keratinized state. Further, CAB39 over-expression dramatically increased the proliferation and colony formation of CNE-1 cells. Finally, higher p-JNK protein level was found in CAB39 over-expressing cells.

Conclusion

CAB39 promotes the proliferation of CNE-1 cells via up-regulating p-JNK.

Protein kinase MST3 modulates lipid homeostasis in hepatocytes and correlates with nonalcoholic steatohepatitis in humans

Ectopic lipid storage in the liver is considered the main risk factor for nonalcoholic steatohepatitis (NASH). Understanding the molecular networks controlling hepatocellular lipid deposition is therefore essential for developing new strategies to effectively prevent and treat this complex disease. Here, we describe a new regulator of lipid partitioning in human hepatocytes: mammalian sterile 20-like (MST) 3. We found that MST3 protein coats lipid droplets in mouse and human liver cells. Knockdown of MST3 attenuated lipid accumulation in human hepatocytes by stimulating β-oxidation and triacylglycerol secretion while inhibiting fatty acid influx and lipid synthesis. We also observed that lipogenic gene expression and acetyl-coenzyme A carboxylase protein abundance were reduced in MST3-deficient hepatocytes, providing insight into the molecular mechanisms underlying the decreased lipid storage. Furthermore, MST3 expression was positively correlated with key features of NASH (i.e., hepatic lipid content, lobular inflammation, and hepatocellular ballooning) in human liver biopsies. In summary, our results reveal a role of MST3 in controlling the dynamic metabolic balance of liver lipid catabolism vs. lipid anabolism. Our findings highlight MST3 as a potential drug target for the prevention and treatment of NASH and related complex metabolic diseases.-Cansby, E., Kulkarni, N. M., Magnusson, E., Kurhe, Y., Amrutkar, M., Nerstedt, A., Ståhlman, M., Sihlbom, C., Marschall, H.-U., Borén, J., Blüher, M., Mahlapuu, M. Protein kinase MST3 modulates lipid homeostasis in hepatocytes and correlates with nonalcoholic steatohepatitis in humans.

MST3 is involved in ENaC-mediated hypertension

Liddle syndrome is an inherited form of human hypertension caused by increasing epithelial Na⁺ channel (ENaC) expression. Increased Na⁺ retention through ENaC with subsequent volume expansion causes hypertension. In addition to ENaC, the Na⁺-K⁺-Cl^- cotransporter (NKCC) and Na⁺-Cl^- symporter (NCC) are responsible for Na⁺ reabsorption in the kidneys. Several Na⁺ transporters are evolutionarily regulated by the Ste20 kinase family. Ste20-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 phosphorylate downstream NKCC2 and NCC to maintain Na⁺ and blood pressure (BP) homeostasis. Mammalian Ste20 kinase 3 (MST3) is another member of the Ste20 family. We previously reported that reduced MST3 levels were found in the kidneys in spontaneously hypertensive rats and that MST3 was involved in Na⁺ regulation. To determine whether MST3 is involved in BP stability through Na⁺ regulation, we generated a MST3 hypomorphic mutation and designated MST3^+/- and MST3^-/- mice to examine BP and serum Na⁺ and K⁺ concentrations. MST3^-/- mice exhibited hypernatremia, hypokalemia, and hypertension. The increased ENaC in the kidney played roles in hypernatremia. The reabsorption of more Na⁺ promoted more K⁺ secretion in the kidney and caused hypokalemia. The hypernatremia and hypokalemia in MST3^-/- mice were significantly reversed by the ENaC inhibitor amiloride, indicating that MST3^-/- mice reabsorbed more Na⁺ through ENaC. Furthermore, Madin-Darby canine kidney cells stably expressing kinase-dead MST3 displayed elevated ENaC currents. Both the in vivo and in vitro results indicated that MST3 maintained Na⁺ homeostasis through ENaC regulation. We are the first to report that MST3 maintains BP stability through ENaC regulation.

MST3 (mammalian Ste20-like protein kinase 3), a novel gene involved in ion homeostasis and renal regulation of blood pressure in spontaneous hypertensive rats

Defective renal salt and water excretion, together with increased salt intake, frequently contributes to hypertension. Recent studies indicate that Ste20 family kinases, such as proline-alanine-rich Ste20-related kinase (SPAK) and oxidative stress-response protein 1 (OSR1), are regulators of cell volume, ion transport, and hypertension. The aim of this study was to investigate whether mammalian sterile 20-like protein kinase 3 (MST3), which is also a stress-regulated kinase, is involved in the development of hypertension. MST3 expression was compared in Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) kidneys. MST3 expression was markedly reduced in principal cells of the collecting ducts from the renal inner medulla of SHR. The downregulation of MST3 expression was observed before and after the onset of hypertension in SHR. Mice fed high-salt diets (HS) exhibited a significant increase in MST3 protein level. This is the first study reporting that MST3, a Ste20-like kinase, exerts a conserved regulatory role in sodium homeostasis after high-salt diet and in the development of hypertension.

Mammalian Sterile20-like Kinases: Signalings and Roles in Central Nervous System

Mammalian Sterile20-like (MST) kinases are located upstream in the mitogen-activated protein kinase pathway, and play an important role in cell proliferation, differentiation, renewal, polarization and migration. Generally, five MST kinases exist in mammalian signal transduction pathways, including MST1, MST2, MST3, MST4 and YSK1. The central nervous system (CNS) is a sophisticated entity that takes charge of information reception, integration and response. Recently, accumulating evidence proposes that MST kinases are critical in the development of disease in different systems involving the CNS. In this review, we summarized the signal transduction pathways and interacting proteins of MST kinases. The potential biological function of each MST kinase and the commonly reported MST-related diseases in the neural system are also reviewed. Further investigation of MST kinases and their interaction with CNS diseases would provide the medical community with new therapeutic targets for human diseases.

MST kinases in innate immune signaling

The mammalian STE20-like (MST) protein kinases are composed of MST1, MST2, MST3, MST4 and YSK1. They play crucial roles in cell growth, migration, polarity and apoptosis. Dysfunction of these kinases often leads to diseases. MST kinases are extensively involved in development and function of immune system. Here, we review recent progresses on the regulatory function of MST kinases in innate immune signaling.

WNK signalling pathways in blood pressure regulation

Hypertension (high blood pressure) is a major public health problem affecting more than a billion people worldwide with complications, including stroke, heart failure and kidney failure. The regulation of blood pressure is multifactorial reflecting genetic susceptibility, in utero environment and external factors such as obesity and salt intake. In keeping with Arthur Guyton's hypothesis, the kidney plays a key role in blood pressure control and data from clinical studies; physiology and genetics have shown that hypertension is driven a failure of the kidney to excrete excess salt at normal levels of blood pressure. There is a number of rare Mendelian blood pressure syndromes, which have shed light on the molecular mechanisms involved in dysregulated ion transport in the distal kidney. One in particular is Familial hyperkalemic hypertension (FHHt), an autosomal dominant monogenic form of hypertension characterised by high blood pressure, hyperkalemia, hyperchloremic metabolic acidosis, and hypercalciuria. The clinical signs of FHHt are treated by low doses of thiazide diuretic, and it mirrors Gitelman syndrome which features the inverse phenotype of hypotension, hypokalemic metabolic alkalosis, and hypocalciuria. Gitelman syndrome is caused by loss of function mutations in the thiazide-sensitive Na/Cl cotransporter (NCC); however, FHHt patients do not have mutations in the SCL12A3 locus encoding NCC. Instead, mutations have been identified in genes that have revealed a key signalling pathway that regulates NCC and several other key transporters and ion channels in the kidney that are critical for BP regulation. This is the WNK kinase signalling pathway that is the subject of this review.

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

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

The Hippo/STE20 homolog SIK1 interacts with MOB1 to regulate cell proliferation and cell expansion in Arabidopsis

Multicellular organisms co-ordinate cell proliferation and cell expansion to maintain organ growth. In animals, the Hippo tumor suppressor pathway is a master regulator of organ size. Central to this pathway is a kinase cascade composed of Hippo and Warts, and their activating partners Salvador and Mob1/Mats. In plants, the Mob1/Mats homolog MOB1A has been characterized as a regulator of cell proliferation and sporogenesis. Nonetheless, no Hippo homologs have been identified. Here we show that the Arabidopsis serine/threonine kinase 1 (SIK1) is a Hippo homolog, and that it interacts with MOB1A to control organ size. SIK1 complements the function of yeast Ste20 in bud site selection and mitotic exit. The sik1 null mutant is dwarf with reduced cell numbers, endoreduplication, and cell expansion. A yeast two-hybrid screen identified Mob1/Mats homologs MOB1A and MOB1B as SIK1-interacting partners. The interaction between SIK1 and MOB1 was found to be mediated by an N-terminal domain of SIK1 and was further confirmed by bimolecular fluorescence complementation. Interestingly, sik1 mob1a is arrested at the seedling stage, and overexpression of neither SIK1 in mob1a nor MOB1A in sik1 can rescue the dwarf phenotypes, suggesting that SIK1 and MOB1 may be components of a larger protein complex. Our results pave the way for constructing a complete Hippo pathway that controls organ growth in higher plants.

Structure-Based Screen Identification of a Mammalian Ste20-like Kinase 4 (MST4) Inhibitor with Therapeutic Potential for Pituitary Tumors

Pituitary tumors of the gonadotrope lineage are often large and invasive, resulting in hypopituitarism. No medical treatments are currently available. Using a combined genetic and genomic screen of individual human gonadotrope pituitary tumor samples, we recently identified the mammalian sterile-20 like kinase 4 (MST4) as a protumorigenic effector, driving increased pituitary cell proliferation and survival in response to a hypoxic microenvironment. To identify novel inhibitors of the MST4 kinase for potential future clinical use, computational-based virtual library screening was used to dock the SelleckChem kinase inhibitor library into the ATP-binding site of the MST4 crystal structure. Several inhibitor candidates were identified with the potential to bind with high affinity. Using a TR-FRET in vitro recombinant kinase assay, hesperadin, initially described as an Aurora kinase inhibitor, exhibited potent inhibition of the MST4 kinase at nanomolar concentrations. The LβT2 gonadotrope pituitary cell hypoxic model was used to test the ability of this inhibitor to antagonize MST4 actions. Under short-term severe hypoxia (1% O2), MST4 protection from hypoxia-induced apoptosis was abrogated in the presence of hesperadin. Similarly, under chronic hypoxia (5%), hesperadin blocked the proliferative and colony-forming actions of MST4 as well as the ability to activate specific downstream signaling and hypoxia-inducible factor-1 effectors. Together, these data identify hesperadin as the first potent, selective inhibitor of the MST4 kinase with the capacity to block pituitary tumor cell growth in a hypoxic microenvironment.

K-Cl cotransporters, cell volume homeostasis, and neurological disease

K(+)-Cl(-) cotransporters (KCCs) were originally characterized as regulators of red blood cell (RBC) volume. Since then, four distinct KCCs have been cloned, and their importance for volume regulation has been demonstrated in other cell types. Genetic models of certain KCCs, such as KCC3, and their inhibitory WNK-STE20/SPS1-related proline/alanine-rich kinase (SPAK) serine-threonine kinases, have demonstrated the evolutionary necessity of these molecules for nervous system cell volume regulation, structure, and function, and their involvement in neurological disease. The recent characterization of a swelling-activated dephosphorylation mechanism that potently stimulates the KCCs has pinpointed a potentially druggable switch of KCC activity. An improved understanding of WNK/SPAK-mediated KCC cell volume regulation in the nervous system might reveal novel avenues for the treatment of multiple neurological diseases.

Regulated phosphorylation of the K-Cl cotransporter KCC3 is a molecular switch of intracellular potassium content and cell volume homeostasis

The defense of cell volume against excessive shrinkage or swelling is a requirement for cell function and organismal survival. Cell swelling triggers a coordinated homeostatic response termed regulatory volume decrease (RVD), resulting in K⁺ and Cl⁻ efflux via activation of K⁺ channels, volume-regulated anion channels (VRACs), and the K⁺-Cl⁻ cotransporters, including KCC3. Here, we show genetic alanine (Ala) substitution at threonines (Thr) 991 and 1048 in the KCC3a isoform carboxyl-terminus, preventing inhibitory phosphorylation at these sites, not only significantly up-regulates KCC3a activity up to 25-fold in normally inhibitory isotonic conditions, but is also accompanied by reversal of activity of the related bumetanide-sensitive Na⁺-K⁺-2Cl⁻ cotransporter isoform 1 (NKCC1). This results in a rapid (<10 min) and significant (>90%) reduction in intracellular K⁺ content (K_i) via both Cl-dependent (KCC3a + NKCC1) and Cl-independent [DCPIB (VRAC inhibitor)-sensitive] pathways, which collectively renders cells less prone to acute swelling in hypotonic osmotic stress. Together, these data demonstrate the phosphorylation state of Thr991/Thr1048 in KCC3a encodes a potent switch of transporter activity, K_i homeostasis, and cell volume regulation, and reveal novel observations into the functional interaction among ion transport molecules involved in RVD.

Mammalian Ste20-like kinase 4 promotes pituitary cell proliferation and survival under hypoxia

The genetic and molecular mechanisms that initiate and maintain pituitary tumorigenesis are poorly understood. Nonfunctioning tumors of the gonadotrope lineage represent 35% of all tumors; are usually macroadenomas, often resulting in hypopituitarism; and have no medical treatments. Using expression microarrays combined with whole-genome copy number screens on individual human tumors, we identified the mammalian sterile-20-like kinase (MST4) transcript, which was amplified within chromosome Xq26.2 in one tumor and up-regulated in all gonadotrope tumor samples. MST4 mRNA and protein were consistently overexpressed in human tumors compared with normal pituitaries. To mimic the pituitary tumor microenvironment, a hypoxia model using LβT2 murine gonadotrope cells was created to examine the functional role of the kinase. During long-term hypoxia, MST4 expression increased colony formation in a soft agar assay and rates of cell proliferation by activating p38 MAPK and AKT. Under short-term severe hypoxic stress, MST4 decreased the rates of apoptosis via p38 MAPK, AKT, hypoxia-inducible factor-1, and its cell-specific downstream targets. Analysis of MST4 mutants confirmed the importance of the kinase sequence but not the regulatory C terminus for its functional effects. Together these data identify the MST4 kinase as a novel candidate to mediate human pituitary tumorigenesis in a hypoxic environment and position it as a potential therapeutic target.

Identification of the WNK-SPAK/OSR1 signaling pathway in rodent and human lenses

PURPOSE

To identify whether the kinases that regulate the activity of cation chloride cotransporters (CCC) in other tissues are also expressed in rat and human lenses.

METHODS

The expression of with-no-lysine kinase (WNK 1, 3, 4), oxidative stress response kinase 1 (OSR1), and Ste20-like proline alanine rich kinase (SPAK) were determined at either the transcript or protein levels in the rat and human lenses by reverse-transcriptase PCR and/or Western blotting, respectively. Selected kinases were regionally and subcellularly characterized in rat and human lenses. The transparency, wet weight, and tissue morphology of lenses extracted from SPAK knock-out animals was compared with wild-type lenses.

RESULTS

WNK 1, 3, 4, SPAK, and OSR1 were identified at the transcript level in rat lenses and WNK1, 4, SPAK, and OSR1 expression confirmed at the protein level in both rat and human lenses. SPAK and OSR1 were found to associate with membranes as peripheral proteins and exhibited distinct subcellular and region-specific expression profiles throughout the lens. No significant difference in the wet weight of SPAK knock-out lenses was detected relative to wild-type lenses. However, SPAK knock-out lenses showed an increased susceptibility to opacification.

CONCLUSIONS

Our results show that the WNK 1, 3, 4, OSR1, and SPAK signaling system known to play a role in regulating the phosphorylation status, and hence activity of the CCCs in other tissues, is also present in the rat and human lenses. The increased susceptibility of SPAK lenses to opacification suggests that disruption of this signaling pathway may compromise the ability of the lens to control its volume, and its ability to maintain its transparency.

The conserved misshapen-warts-Yorkie pathway acts in enteroblasts to regulate intestinal stem cells in Drosophila

Similar to the mammalian intestine, the Drosophila adult midgut has resident stem cells that support growth and regeneration. How the niche regulates intestinal stem cell activity in both mammals and flies is not well understood. Here, we show that the conserved germinal center protein kinase Misshapen restricts intestinal stem cell division by repressing the expression of the JAK-STAT pathway ligand Upd3 in differentiating enteroblasts. Misshapen, a distant relative to the prototypic Warts activating kinase Hippo, interacts with and activates Warts to negatively regulate the activity of Yorkie and the expression of Upd3. The mammalian Misshapen homolog MAP4K4 similarly interacts with LATS (Warts homolog) and promotes inhibition of YAP (Yorkie homolog). Together, this work reveals that the Misshapen-Warts-Yorkie pathway acts in enteroblasts to control niche signaling to intestinal stem cells. These findings also provide a model in which to study requirements for MAP4K4-related kinases in MST1/2-independent regulation of LATS and YAP.

The WNK-SPAK/OSR1 pathway: master regulator of cation-chloride cotransporters

The WNK-SPAK/OSR1 kinase complex is composed of the kinases WNK (with no lysine) and SPAK (SPS1-related proline/alanine-rich kinase) or the SPAK homolog OSR1 (oxidative stress-responsive kinase 1). The WNK family senses changes in intracellular Cl(-) concentration, extracellular osmolarity, and cell volume and transduces this information to sodium (Na(+)), potassium (K(+)), and chloride (Cl(-)) cotransporters [collectively referred to as CCCs (cation-chloride cotransporters)] and ion channels to maintain cellular and organismal homeostasis and affect cellular morphology and behavior. Several genes encoding proteins in this pathway are mutated in human disease, and the cotransporters are targets of commonly used drugs. WNKs stimulate the kinases SPAK and OSR1, which directly phosphorylate and stimulate Cl(-)-importing, Na(+)-driven CCCs or inhibit the Cl(-)-extruding, K(+)-driven CCCs. These coordinated and reciprocal actions on the CCCs are triggered by an interaction between RFXV/I motifs within the WNKs and CCCs and a conserved carboxyl-terminal docking domain in SPAK and OSR1. This interaction site represents a potentially druggable node that could be more effective than targeting the cotransporters directly. In the kidney, WNK-SPAK/OSR1 inhibition decreases epithelial NaCl reabsorption and K(+) secretion to lower blood pressure while maintaining serum K(+). In neurons, WNK-SPAK/OSR1 inhibition could facilitate Cl(-) extrusion and promote γ-aminobutyric acidergic (GABAergic) inhibition. Such drugs could have efficacy as K(+)-sparing blood pressure-lowering agents in essential hypertension, nonaddictive analgesics in neuropathic pain, and promoters of GABAergic inhibition in diseases associated with neuronal hyperactivity, such as epilepsy, spasticity, neuropathic pain, schizophrenia, and autism.

Identification and characterization of a Ste20-like kinase in Artemia and its role in the developmental regulation and resistance to environmental stress

Background

To adapt to extreme environments, the crustacean Artemia has evolved two alternative reproductive pathways. During ovoviviparous (direct) development, nauplius larvae are produced. In contrast, Artemia females release encysted diapause embryos (cysts) via the oviparous pathway. To date, the cellular mechanisms that regulate stress resistance of Artemia remain largely unknown. Ste20-like kinase (SLK) participates in multiple biological processes, including stress responses, apoptosis, and cell cycle progression.

Principal Finding

We isolated and characterized a member of the SLK superfamily termed ArSLK from Artemia parthenogenetica. The ArSLK gene is transcribed throughout both ovoviviparous and oviparous development; however, the protein is located mainly in the nuclei of stress-resistant diapause cysts, unlike the nauplii and nauplius-destined embryos where it is cytoplasmic. Interestingly, exposure of nauplii to heat shock, acidic pH, and UV irradiation induced the translocation of ArSLK from cytoplasm to nucleus. This translocation was reversed following stress removal. Moreover, under physiologically-stressful conditions, the nauplius larvae produced by adults after gene knockdown of endogenous ArSLK by RNAi, lost the ability of free-swimming much earlier than those of control larvae from females injected with GFP dsRNA.

Conclusions/Significance

Taken together, this study demonstrated that trafficking of ArSLK between the cytoplasm and the nucleus participates in regulating the stress resistance of Artemia. Our findings may provide significant insight into the functions of members of the SLK superfamily.

The WNK-regulated SPAK/OSR1 kinases directly phosphorylate and inhibit the K+-Cl- co-transporters

Precise homoeostasis of the intracellular concentration of Cl- is achieved via the co-ordinated activities of the Cl- influx and efflux. We demonstrate that the WNK (WNK lysine-deficient protein kinase)-activated SPAK (SPS1-related proline/alanine-rich kinase)/OSR1 (oxidative stress-responsive kinase 1) known to directly phosphorylate and stimulate the N[K]CCs (Na+-K+ ion co-transporters), also promote inhibition of the KCCs (K+-Cl- co-transporters) by directly phosphorylating a recently described C-terminal threonine residue conserved in all KCC isoforms [Site-2 (Thr1048)]. First, we demonstrate that SPAK and OSR1, in the presence of the MO25 regulatory subunit, robustly phosphorylates all KCC isoforms at Site-2 in vitro. Secondly, STOCK1S-50699, a WNK pathway inhibitor, suppresses SPAK/OSR1 activation and KCC3A Site-2 phosphorylation with similar efficiency. Thirdly, in ES (embryonic stem) cells lacking SPAK/OSR1 activity, endogenous phosphorylation of KCC isoforms at Site-2 is abolished and these cells display elevated basal activity of 86Rb+ uptake that was not markedly stimulated further by hypotonic high K+ conditions, consistent with KCC3A activation. Fourthly, a tight correlation exists between SPAK/OSR1 activity and the magnitude of KCC3A Site-2 phosphorylation. Lastly, a Site-2 alanine KCC3A mutant preventing SPAK/OSR1 phosphorylation exhibits increased activity. We also observe that KCCs are directly phosphorylated by SPAK/OSR1, at a novel Site-3 (Thr5 in KCC1/KCC3 and Thr6 in KCC2/KCC4), and a previously recognized KCC3-specific residue, Site-4 (Ser96). These data demonstrate that the WNK-regulated SPAK/OSR1 kinases directly phosphorylate the N[K]CCs and KCCs, promoting their stimulation and inhibition respectively. Given these reciprocal actions with anticipated net effects of increasing Cl- influx, we propose that the targeting of WNK-SPAK/OSR1 with kinase inhibitors might be a novel potent strategy to enhance cellular Cl- extrusion, with potential implications for the therapeutic modulation of epithelial and neuronal ion transport in human disease states.

The interaction of the HSV-1 tegument proteins pUL36 and pUL37 is essential for secondary envelopment during viral egress

The herpes simplex virus type 1 (HSV-1) tegument proteins pUL36 (VP1/2) and pUL37 are essential for viral egress. We previously defined a minimal domain in HSV-1 pUL36, residues 548-572, as important for binding pUL37. Here, we investigated the role of this region in binding to pUL37 and facilitating viral replication. We deleted residues 548-572 in frame in a virus containing a mRFP tag at the N-terminus of the capsid protein VP26 and an eGFP tag at the C-terminus of pUL37 (HSV-1pUL36∆548-572). This mutant virus was unable to generate plaques in Vero cells, indicating that deletion of this region of pUL36 blocks viral replication. Imaging of HSV-1pUL36∆548-572-infected Vero cells, in comparison to parental and resucant, revealed a block in secondary envelopment of cytoplasmic capsids. In addition, immunoblot analysis suggested that failure to bind pUL37 affected the stability of pUL36. This study provides further insight into the role of this essential interaction.

WNK4 inhibits Ca(2+)-activated big-conductance potassium channels (BK) via mitogen-activated protein kinase-dependent pathway

We used the perforated whole-cell recording technique to examine the effect of with-no-lysine kinase 4 (WNK4) on the Ca(2+) activated big-conductance K channels (BK) in HEK293T cells transfected with BK-α subunit (BK-α). Expression of WNK4 inhibited BK channels and decreased the outward K currents. Coexpression of SGK1 abolished the inhibitory effect of WNK4 on BK channels and restored the outward K currents. Expression of WNK4(S1169D//1196D), in which both SGK1-phosphorylation sites (serine 1169 and 1196) were mutated to aspartate, had no effect on BK channels. Moreover, coexpression of SGK1 had no additional effect on K currents in the cells transfected with BKα+WNK4(S1169D//1196D), suggesting that SGK1 reversed WNK4-induced inhibition of BK channels by stimulating WNK4 phosphorylation. Expression of WNK4 but not WNK4(S1169D//1196D) increased the phosphorylation of ERK and p38 mitogen-activated protein kinase (MAPK); an effect was abolished by coexpression of SGK1. The role of ERK and p38 MAPK in mediating the effect of WNK4 on BK channels was further suggested by the finding that the inhibition of ERK and P38 MAPK completely abolished the inhibitory effect of WNK4 on BK channels. In contrast, inhibition of MAPK failed to abolish the inhibitory effect of WNK4 on ROMK channels in both HEK cells and Xenopus oocytes. Expression of dominant negative dynaminK44A (Dyn(K44A)) or treatment of the cells with dynasore, a dynamin inhibitor, not only increased K currents but also largely abolished the inhibitory effect of WNK4 on BK channels. However, inhibition of MAPK still increased the outward K currents in the cells transfected with BKα+WNK4 and treated with dynasore. Similar results were obtained in experiments performed in the native tissue in which inhibition of ERK and p38 MAPK increased BK channel activity in the cortical collecting duct (CCD) treated with dynasore. We concluded that WNK4 inhibited BK channels by stimulating ERK and p38 MAPK and that activation of MAPK by WNK4 may inhibit BK channels partially via a mechanism other than stimulating endocytosis.

Expression patterns of As-ClC gene of Artemia sinica in early development and under salinity stress

As-ClC (chloride channels protein from Artemia sinica), a member from the chloride channels protein family, is a α-helical membrane protein predicted to traverse the cell membrane 11 times. It is important for several physiological functions such as cell volume regulation, cell proliferation, growth and differentiation. In this paper, the complete cDNA sequence of As-CIC was cloned from A. sinica for the first time using RACE technology. The expression pattern and location of the As-CIC gene was investigated in different stages of the embryonic development by means of quantitative real-time PCR and in situ hybridization (ISH) assay. As-CLC was distributed throughout the whole body in cells of different embryonic development of A. sinica as shown by ISH. There was a low expression level of the As-ClC gene after 0 h and a higher expression level after 15 and 40 h when the embryo entered the next growth period and the environmental salinity changed. At adult stage, the As-ClC maintained a high expression level. The results of the real-time PCR assay showed an increasing trend of As-ClC transcripts with increasing salinity. The expression of As-ClC was higher in the control group (28) than in the experimental group except at a salinity of 200 PSU. It indicated that As-ClC functions as salinity-stress-related gene, probably participated in cell volume regulation and osmotic regulation during the early embryonic development of A. sinica.

A molecular analysis of desiccation tolerance mechanisms in the anhydrobiotic nematode Panagrolaimus superbus using expressed sequenced tags

Background

Some organisms can survive extreme desiccation by entering into a state of suspended animation known as anhydrobiosis. Panagrolaimus superbus is a free-living anhydrobiotic nematode that can survive rapid environmental desiccation. The mechanisms that P. superbus uses to combat the potentially lethal effects of cellular dehydration may include the constitutive and inducible expression of protective molecules, along with behavioural and/or morphological adaptations that slow the rate of cellular water loss. In addition, inducible repair and revival programmes may also be required for successful rehydration and recovery from anhydrobiosis.

Results

To identify constitutively expressed candidate anhydrobiotic genes we obtained 9,216 ESTs from an unstressed mixed stage population of P. superbus. We derived 4,009 unigenes from these ESTs. These unigene annotations and sequences can be accessed at http://www.nematodes.org/nembase4/species_info.php?species=PSC. We manually annotated a set of 187 constitutively expressed candidate anhydrobiotic genes from P. superbus. Notable among those is a putative lineage expansion of the lea (late embryogenesis abundant) gene family. The most abundantly expressed sequence was a member of the nematode specific sxp/ral-2 family that is highly expressed in parasitic nematodes and secreted onto the surface of the nematodes' cuticles. There were 2,059 novel unigenes (51.7% of the total), 149 of which are predicted to encode intrinsically disordered proteins lacking a fixed tertiary structure. One unigene may encode an exo-β-1,3-glucanase (GHF5 family), most similar to a sequence from Phytophthora infestans. GHF5 enzymes have been reported from several species of plant parasitic nematodes, with horizontal gene transfer (HGT) from bacteria proposed to explain their evolutionary origin. This P. superbus sequence represents another possible HGT event within the Nematoda. The expression of five of the 19 putative stress response genes tested was upregulated in response to desiccation. These were the antioxidants glutathione peroxidase, dj-1 and 1-Cys peroxiredoxin, an shsp sequence and an lea gene.

Conclusions

P. superbus appears to utilise a strategy of combined constitutive and inducible gene expression in preparation for entry into anhydrobiosis. The apparent lineage expansion of lea genes, together with their constitutive and inducible expression, suggests that LEA3 proteins are important components of the anhydrobiotic protection repertoire of P. superbus.

Identification of host cell proteins which interact with herpes simplex virus type 1 tegument protein pUL37

The herpes simplex virus type 1 (HSV-1) structural tegument protein pUL37, which is conserved across the Herpesviridae family, is known to be essential for secondary envelopment during the egress of viral particles. To shed light on additional roles of pUL37 during viral replication a yeast two-hybrid screen of a human brain cDNA library was undertaken. This screen identified ten host cell proteins as potential pUL37 interactors. One of the interactors, serine threonine kinase TAOK3, was subsequently confirmed to interact with pUL37 using an in vitro pulldown assay. Such host cell/pUL37 interactions provide further insights into the multifunctional role of this herpesviral tegument protein.

Functional insights into the activation mechanism of Ste20-related kinases

Mammalian Ste20-related kinases modulate salt transport and ion homeostasis through physical interaction and phosphorylation of cation-chloride cotransporters. Identification of a sea urchin (Strongylocentrotus purpuratus) ortholog of the mouse Oxidative Stress Response 1 (OSR1) kinase prompted the cloning and testing of the functional effect of a non-mammalian kinase on a mammalian cotransporter. Heterologous expression of sea urchin OSR1 (suOSR1) cRNA with mouse WNK4 cRNA and mouse NKCC1 cRNA in Xenopus laevisoocytes activated the cotransporter indicating evolutionary conservation of the WNK4-OSR1-NKCC signaling pathway. However, expression of a suOSR1 kinase mutated to confer constitutive activity did not result in stimulation of the cotransporter. Using a chimeric strategy, we determined that both the mutated catalytic and regulatory domains of the suOSR1 kinase were functional, suggesting that the tertiary structure of full-length mutated suOSR1 must somehow adopt an inactive conformation. In order to identify the regions or residues which lock the suOSR1 kinase in an inactive conformation, we created and tested several additional chimeras by replacing specific portions of the suOSR1 gene with complimentary mouse OSR1 sequences. Co-expression of these chimeras identified several regions in both the catalytic and regulatory domain of suOSR1 which possibly prevented the kinase from acquiring an active conformation. Interestingly, non-functional suOSR1 chimeras were able to activate mouse NKCC1 when a mouse scaffolding protein, Cab39, was co-expressed in frog oocytes. Sea urchin/mouse OSR1 chimeras and kinase stabilization with mouse Cab39 has provided some novel insights into the activation mechanism of the Ste20-related kinases.

C. elegans STK39/SPAK ortholog-mediated inhibition of ClC anion channel activity is regulated by WNK-independent ERK kinase signaling

Mammalian Ste20-like proline/alanine-rich kinase (SPAK) and oxidative stress-responsive 1 (OSR1) kinases phosphorylate and regulate cation-coupled Cl(-) cotransporter activity in response to cell volume changes. SPAK and OSR1 are activated via phosphorylation by upstream with-no-lysine (WNK) kinases. In Caenorhabditis elegans, the SPAK/OSR1 ortholog germinal center kinase (GCK)-3 binds to and regulates the activity of the cell volume- and meiotic cell cycle-dependent ClC anion channel CLH-3b. We tested the hypothesis that WNK kinases function in the GCK-3/CLH-3b signaling cascade. CLH-3b heterologously expressed in human embryonic kidney (HEK) cells was unaffected by coexpression with the single C. elegans WNK kinase, WNK-1, or kinase-dead WNK-1 dominant-negative mutants. RNA interference (RNAi) knockdown of the single Drosophila WNK kinase had no effect on the activity of CLH-3b expressed in Drosophila S2 cells. Similarly, RNAi silencing of C. elegans WNK-1 had no effect on basal or cell volume-sensitive activity of CLH-3b expressed endogenously in worm oocytes. Previous yeast 2-hybrid studies suggested that ERK kinases may function upstream of GCK-3. Pharmacological inhibition of ERK signaling disrupted CLH-3b activity in HEK cells in a GCK-3-dependent manner. RNAi silencing of the C. elegans ERK kinase MPK-1 or the ERK phosphorylating/activating kinase MEK-2 constitutively activated native CLH-3b. MEK-2 and MPK-1 play important roles in regulating the meiotic cell cycle in C. elegans oocytes. Cell cycle-dependent changes in MPK-1 correlate with the pattern of CLH-3b activation observed during oocyte meiotic maturation. We postulate that MEK-2/MPK-1 functions upstream from GCK-3 to regulate its activity during cell volume and meiotic cell cycle changes.

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Signal transduction protein array analysis links LRRK2 to Ste20 kinases and PKC zeta that modulate neuronal plasticity

Background

Dominant mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease, however, the underlying pathogenic mechanisms are poorly understood. Several in vitro studies have shown that the most frequent mutation, LRRK2(G2019S), increases kinase activity and impairs neuronal survival. LRRK2 has been linked to the mitogen-activated protein kinase kinase kinase family and the receptor-interacting protein kinases based on sequence similarity within the kinase domain and in vitro substrate phosphorylation.

Methodology/Principal Findings

We used an unbiased proteomic approach to identify the kinase signaling pathways wherein LRRK2 may be active. By incubation of protein microarrays containing 260 signal transduction proteins we detected four arrayed Ste20 serine/threonine kinase family members (TAOK3, STK3, STK24, STK25) as novel LRRK2 substrates and LRRK2 interacting proteins, respectively. Moreover, we found that protein kinase C (PKC) zeta binds and phosphorylates LRRK2 both in vitro and in vivo.

Conclusions/Significance

Ste20 kinases and PKC zeta contribute to neuronal Tau phosphorylation, neurite outgrowth and synaptic plasticity under physiological conditions. Our data suggest that these kinases may also be involved in synaptic dysfunction and neurite fragmentation in transgenic mice and in human PD patients carrying toxic gain-of-function LRRK2 mutations.

Structural comparison of human mammalian ste20-like kinases

BACKGROUND

The serine/threonine mammalian Ste-20 like kinases (MSTs) are key regulators of apoptosis, cellular proliferation as well as polarization. Deregulation of MSTs has been associated with disease progression in prostate and colorectal cancer. The four human MSTs are regulated differently by C-terminal regions flanking the catalytic domains.

PRINCIPAL FINDINGS

We have determined the crystal structure of kinase domain of MST4 in complex with an ATP-mimetic inhibitor. This is the first structure of an inactive conformation of a member of the MST kinase family. Comparison with active structures of MST3 and MST1 revealed a dimeric association of MST4 suggesting an activation loop exchanged mechanism of MST4 auto-activation. Together with a homology model of MST2 we provide a comparative analysis of the kinase domains for all four members of the human MST family.

SIGNIFICANCE

The comparative analysis identified new structural features in the MST ATP binding pocket and has also defined the mechanism for autophosphorylation. Both structural features may be further explored for inhibitors design.

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Phosphoregulation of the Na-K-2Cl and K-Cl cotransporters by the WNK kinases

Precise regulation of the intracellular concentration of chloride [Cl-]i is necessary for proper cell volume regulation, transepithelial transport, and GABA neurotransmission. The Na-K-2Cl (NKCCs) and K-Cl (KCCs) cotransporters, related SLC12A transporters mediating cellular chloride influx and efflux, respectively, are key determinants of [Cl-]i in numerous cell types, including red blood cells, epithelial cells, and neurons. A common "chloride/volume-sensitive kinase", or related system of kinases, has long been hypothesized to mediate the reciprocal but coordinated phosphoregulation of the NKCCs and the KCCs, but the identity of these kinase(s) has remained unknown. Recent evidence suggests that the WNK (with no lysine = K) serine-threonine kinases directly or indirectly via the downstream Ste20-type kinases SPAK/OSR1, are critical components of this signaling pathway. Hypertonic stress (cell shrinkage), and possibly decreased [Cl-]i, triggers the phosphorylation and activation of specific WNKs, promoting NKCC activation and KCC inhibition via net transporter phosphorylation. Silencing WNK kinase activity can promote NKCC inhibition and KCC activation via net transporter dephosphorylation, revealing a dynamic ability of the WNKs to modulate [Cl-]. This pathway is essential for the defense of cell volume during osmotic perturbation, coordination of epithelial transport, and gating of sensory information in the peripheral system. Commiserate with their importance in serving these critical roles in humans, mutations in WNKs underlie two different Mendelian diseases, pseudohypoaldosteronism type II (an inherited form of salt-sensitive hypertension), and hereditary sensory and autonomic neuropathy type 2. WNKs also regulate ion transport in lower multicellular organisms, including Caenorhabditis elegans, suggesting that their functions are evolutionarily-conserved. An increased understanding of how the WNKs regulate the Na-K-2Cl and K-Cl cotransporters may provide novel opportunities for the selective modulation of these transporters, with ramifications for common human diseases like hypertension, sickle cell disease, neuropathic pain, and epilepsy.

The Caenorhabditis elegans Ste20 kinase, GCK-3, is essential for postembryonic developmental timing and regulates meiotic chromosome segregation

Ste20 kinases constitute a large family of serine/threonine kinases with a plethora of biological functions. Members of the GCK-VI subfamily have been identified as important regulators of osmohomeostasis across species functioning upstream of ion channels. Although the expression of the two highly similar mammalian GCK-VI kinases is eminent in a wide variety of tissues, which includes also the testis, their potential roles in development remain elusive. Caenorhabditis elegans contains a single ancestral ortholog termed GCK-3. Here, we report a comprehensive analysis of gck-3 function and demonstrate its requirement for several developmental processes independent of ion homeostasis, i.e., larval progression, vulva, and germ line formation. Consistent with a wide range of gck-3 function we find that endogenous GCK-3 is expressed ubiquitously. The serine/threonine kinase activity of GCK-3, but not its presumed C-terminal substrate interaction domain, is essential for gck-3 gene function. Although expressed in female germ cells, we find GCK-3 progressively accumulating during spermatogenesis where it promotes the first meiotic cell division and facilitates faithful chromosome segregation. In particular, we find that different levels of gck-3 activity appear to be important for various aspects of germ line development. Taken together, our findings suggest that members of the GCK-VI kinase subfamily may act as key regulators of many developmental processes and that this newly described role in meiotic progression might be conserved and an important part of sexual reproduction.

Small-molecule p21-activated kinase inhibitor PF-3758309 is a potent inhibitor of oncogenic signaling and tumor growth

Despite abundant evidence that aberrant Rho-family GTPase activation contributes to most steps of cancer initiation and progression, there is a dearth of inhibitors of their effectors (e.g., p21-activated kinases). Through high-throughput screening and structure-based design, we identify PF-3758309, a potent (K(d) = 2.7 nM), ATP-competitive, pyrrolopyrazole inhibitor of PAK4. In cells, PF-3758309 inhibits phosphorylation of the PAK4 substrate GEF-H1 (IC(50) = 1.3 nM) and anchorage-independent growth of a panel of tumor cell lines (IC(50) = 4.7 +/- 3 nM). The molecular underpinnings of PF-3758309 biological effects were characterized using an integration of traditional and emerging technologies. Crystallographic characterization of the PF-3758309/PAK4 complex defined determinants of potency and kinase selectivity. Global high-content cellular analysis confirms that PF-3758309 modulates known PAK4-dependent signaling nodes and identifies unexpected links to additional pathways (e.g., p53). In tumor models, PF-3758309 inhibits PAK4-dependent pathways in proteomic studies and regulates functional activities related to cell proliferation and survival. PF-3758309 blocks the growth of multiple human tumor xenografts, with a plasma EC(50) value of 0.4 nM in the most sensitive model. This study defines PAK4-related pathways, provides additional support for PAK4 as a therapeutic target with a unique combination of functions (apoptotic, cytoskeletal, cell-cycle), and identifies a potent, orally available small-molecule PAK inhibitor with significant promise for the treatment of human cancers.

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.

MST kinases monitor actin cytoskeletal integrity and signal via c-Jun N-terminal kinase stress-activated kinase to regulate p21Waf1/Cip1 stability

As well as providing a structural framework, the actin cytoskeleton plays integral roles in cell death, survival, and proliferation. The disruption of the actin cytoskeleton results in the activation of the c-Jun N-terminal kinase (JNK) stress-activated protein kinase (SAPK) pathway; however, the sensor of actin integrity that couples to the JNK pathway has not been characterized in mammalian cells. We now report that the mammalian Ste20-like (MST) kinases mediate the activation of the JNK pathway in response to the disruption of the actin cytoskeleton. One consequence of actin disruption is the JNK-mediated stabilization of p21(Waf1/Cip1) (p21) via the phosphorylation of Thr57. The expression of MST1 or MST2 was sufficient to stabilize p21 in a JNK- and Thr57-dependent manner, while the stabilization of p21 by actin disruption required MST activity. These data indicate that, in addition to being components of the Salvador-Warts-Hippo tumor suppressor network and binding partners of c-Raf and the RASSF1A tumor suppressor, MST kinases serve to monitor cytoskeletal integrity and couple via the JNK SAPK pathway to the regulation of a key cell cycle regulatory protein.

Sites of regulated phosphorylation that control K-Cl cotransporter activity

Modulation of intracellular chloride concentration ([Cl(-)](i)) plays a fundamental role in cell volume regulation and neuronal response to GABA. Cl(-) exit via K-Cl cotransporters (KCCs) is a major determinant of [Cl(-)](I); however, mechanisms governing KCC activities are poorly understood. We identified two sites in KCC3 that are rapidly dephosphorylated in hypotonic conditions in cultured cells and human red blood cells in parallel with increased transport activity. Alanine substitutions at these sites result in constitutively active cotransport. These sites are highly phosphorylated in plasma membrane KCC3 in isotonic conditions, suggesting that dephosphorylation increases KCC3's intrinsic transport activity. Reduction of WNK1 expression via RNA interference reduces phosphorylation at these sites. Homologous sites are phosphorylated in all human KCCs. KCC2 is partially phosphorylated in neonatal mouse brain and dephosphorylated in parallel with KCC2 activation. These findings provide insight into regulation of [Cl(-)](i) and have implications for control of cell volume and neuronal function.

The mammalian family of sterile 20p-like protein kinases

Twenty-eight kinases found in mammalian genomes share similarity to the budding yeast kinase Ste20p. This review article examines the biological function of these mammalian Ste20 kinases. Some of them have conserved the Ste20p function of transducing extracellular signals to mitogen-activated kinases. Others affect ion transport, cell cycle, cytoskeleton organization, and program cell death. A number of molecular details involved in the activation of the kinases are discussed including autophosphorylation, substrate recognition, autoinhibition, dimerization, and substrate binding.