SPAK kinase is a substrate and target of PKCtheta in T-cell receptor-induced AP-1 activation pathway

Excess Salt Intake Activates IL-21-Dominant Autoimmune Diabetogenesis via a Salt-Regulated Ste20-Related Proline/Alanine-Rich Kinase in CD4 T Cells

The fundamental mechanisms by which a diet affects susceptibility to or modifies autoimmune diseases are poorly understood. Excess dietary salt intake acts as a risk factor for autoimmune diseases; however, little information exists on the impact of salt intake on type 1 diabetes. To elucidate the potential effect of high salt intake on autoimmune diabetes, nonobese diabetic (NOD) mice were fed a high-salt diet (HSD) or a normal-salt diet (NSD) from 6 to 12 weeks of age and monitored for diabetes development. Our results revealed that the HSD accelerated diabetes progression with more severe insulitis in NOD mice in a CD4+ T-cell-autonomous manner when compared with the NSD group. Moreover, expression of IL-21 and SPAK in splenic CD4+ T cells from HSD-fed mice was significantly upregulated. Accordingly, we generated T-cell-specific SPAK knockout (CKO) NOD mice and demonstrated that SPAK deficiency in T cells significantly attenuated diabetes development in NOD mice by downregulating IL-21 expression in CD4+ T cells. Furthermore, HSD-triggered diabetes acceleration was abolished in HSD-fed SPAK CKO mice when compared with HSD-fed NOD mice, suggesting an essential role of SPAK in salt-exacerbated T-cell pathogenicity. Finally, pharmacological inhibition of SPAK activity using a specific SPAK inhibitor (closantel) in NOD mice ameliorated diabetogenesis, further illuminating the potential of a SPAK-targeting immunotherapeutic approach for autoimmune diabetes. Here, we illustrate that a substantial association between salt sensitivity and the functional impact of SPAK on T-cell pathogenicity is a central player linking high-salt-intake influences to immunopathophysiology of diabetogenesis in NOD mice.

ARTICLE HIGHLIGHTS

WNK kinases sense molecular crowding and rescue cell volume via phase separation

When challenged by hypertonicity, dehydrated cells must recover their volume to survive. This process requires the phosphorylation-dependent regulation of SLC12 cation chloride transporters by WNK kinases, but how these kinases are activated by cell shrinkage remains unknown. Within seconds of cell exposure to hypertonicity, WNK1 concentrates into membraneless condensates, initiating a phosphorylation-dependent signal that drives net ion influx via the SLC12 cotransporters to restore cell volume. WNK1 condensate formation is driven by its intrinsically disordered C terminus, whose evolutionarily conserved signatures are necessary for efficient phase separation and volume recovery. This disorder-encoded phase behavior occurs within physiological constraints and is activated in vivo by molecular crowding rather than changes in cell size. This allows kinase activity despite an inhibitory ionic milieu and permits cell volume recovery through condensate-mediated signal amplification. Thus, WNK kinases are physiological crowding sensors that phase separate to coordinate a cell volume rescue response.

Association of genetic variants of oxidative stress responsive kinase 1 (OXSR1) with asthma exacerbations in non-smoking asthmatics

BACKGROUND

Asthma exacerbation threatens patient's life. Several genetic studies have been conducted to determine the risk factors for asthma exacerbation, but this information is still lacking. We aimed to determine whether genetic variants of Oxidative Stress Responsive Kinase 1 (OXSR1), a gene with functions of salt transport, immune response, and oxidative stress, are associated with exacerbation of asthma.

METHODS

Clinical data were obtained from 1454 asthmatics and single nucleotide polymorphisms (SNPs) of OXSR1 were genotyped. Genetic associations with annual exacerbation rate were analyzed depending on smoking status.

RESULTS

Eleven SNPs were selected using Asian data in the International HapMap database. The common allele of rs1384006 C > T of OXSR1 showed a significantly higher annual exacerbation rate than the rare allele in non-smoking asthmatics (CC vs. CT vs. TT: 0.43 ± 0.04 vs. 0.28 ± 0.03 vs. 0.31 ± 0.09, P = 0.004, Pcorr = 0.039). The frequent exacerbators had a significantly higher frequency of the common allele of rs1384006 C > T than did the infrequent exacerbators (74.4% vs. 55.2%, P = 0.004, Pcorr = 0.038).

CONCLUSION

The common allele of rs1384006 C > T of OXSR1 was associated with the asthma exacerbation rate and a higher risk of being a frequent exacerbator, indicating that non-smoking asthmatics who carry common alleles may be vulnerable to asthma exacerbations.

Protein kinase D1 variant associated with human epilepsy and peripheral nerve hypermyelination

We report the case of a patient with severe progressive epilepsy and peripheral neuropathy and a novel de novo inactivating variant (p.E79X) in Protein Kinase D1 (PKD1). Using CRISPR/Cas9, we engineered the homologous variant in mice and showed that in the homozygote mouse, it recapitulated the patient peripheral nerve hypermyelination pathology. The lethality of the homozygote mouse prevented us from performing an assessment of locomotor behavior. The mutant heterozygote mouse; however, exhibited a significant increase in kainate-induced seizure activity over wild-type mice, supporting the hypothesis that the PKD1 variant is a candidate for the cause of the patient epilepsy. Because PKD1 was previously identified in a kinomic screen as an interacting partner of the K-Cl cotransporter 3 (KCC3), and since KCC3 is involved in peripheral nerve disease and brain hyperexcitability, one possible mechanism of action of PKD1 in disease is through KCC3. We show that catalytically inactive PKD1 stimulates KCC3 activity, consistent with tonic relief of inhibitory phosphorylation. Our findings implicate a novel role for PKD1 in the human nervous system, and uncover a mechanism that could serve as a potential target to promote nervous system myelination.

T-cell receptor (TCR) signaling promotes the assembly of RanBP2/RanGAP1-SUMO1/Ubc9 nuclear pore subcomplex via PKC-θ-mediated phosphorylation of RanGAP1

The nuclear pore complex (NPC) is the sole and selective gateway for nuclear transport, and its dysfunction has been associated with many diseases. The metazoan NPC subcomplex RanBP2, which consists of RanBP2 (Nup358), RanGAP1-SUMO1, and Ubc9, regulates the assembly and function of the NPC. The roles of immune signaling in regulation of NPC remain poorly understood. Here, we show that in human and murine T cells, following T-cell receptor (TCR) stimulation, protein kinase C-θ (PKC-θ) directly phosphorylates RanGAP1 to facilitate RanBP2 subcomplex assembly and nuclear import and, thus, the nuclear translocation of AP-1 transcription factor. Mechanistically, TCR stimulation induces the translocation of activated PKC-θ to the NPC, where it interacts with and phosphorylates RanGAP1 on Ser⁵⁰⁴ and Ser⁵⁰⁶. RanGAP1 phosphorylation increases its binding affinity for Ubc9, thereby promoting sumoylation of RanGAP1 and, finally, assembly of the RanBP2 subcomplex. Our findings reveal an unexpected role of PKC-θ as a direct regulator of nuclear import and uncover a phosphorylation-dependent sumoylation of RanGAP1, delineating a novel link between TCR signaling and assembly of the RanBP2 NPC subcomplex.

Rare Genetic Variants of Large Effect Influence Risk of Type 1 Diabetes

Most replicated genetic determinants for type 1 diabetes are common (minor allele frequency [MAF] >5%). We aimed to identify novel rare or low-frequency (MAF <5%) single nucleotide polymorphisms with large effects on risk of type 1 diabetes. We undertook deep imputation of genotyped data followed by genome-wide association testing and meta-analysis of 9,358 type 1 diabetes case and 15,705 control subjects from 12 European cohorts. Candidate variants were replicated in a separate cohort of 4,329 case and 9,543 control subjects. Our meta-analysis identified 27 independent variants outside the MHC, among which 3 were novel and had MAF <5%. Three of these variants replicated with P _replication < 0.05 and P _combined < P _discovery In silico analysis prioritized a rare variant at 2q24.3 (rs60587303 [C], MAF 0.5%) within the first intron of STK39, with an effect size comparable with those of common variants in the INS and PTPN22 loci (combined [from the discovery and replication cohorts] estimate of odds ratio [OR_combined] 1.97, 95% CI 1.58-2.47, P _combined = 2.9 × 10^-9). Pharmacological inhibition of Stk39 activity in primary murine T cells augmented effector responses through enhancement of interleukin 2 signaling. These findings provide insight into the genetic architecture of type 1 diabetes and have identified rare variants having a large effect on disease risk.

Characterisation of amphioxus protein kinase C-δ/θ reveals a unique proto-V3 domain suggesting an evolutionary mechanism for PKC-θ unique V3

A primitive adaptive immune system has recently been suggested to be present in a basal chordate amphioxus (Branchiostoma belcheri, Bb), making it an ideal model for studying the origin of adaptive immune. The novel protein kinase C isoform PKC-θ, but not its closest isoform PKC-δ, plays a critical role for mammalian T-cell activation via translocation to immunological synapse (IS) mediated by a unique PKC-θ V3 domain containing one PxxP motif. To understand the evolution of this unique PKC-θ V3 domain and the primitive adaptive immune system in amphioxus, we comparatively studied the orthologs of PKC-δ and -θ from amphioxus and other species. Phylogenetic analysis showed BbPKC-δ/θ to be the common ancestor of vertebrate PKC-δ and PKC-θ, with a V3 domain containing two PxxP motifs. One motif is conserved in both zebrafish and mammalian PKC-θ but is absent in PKC-δ V3 domain of these species, and has already emerged in drosophila PKC-δ. The other non-conserved motif emerged in BbPKC-δ/θ, and only retained in Danio rerio PKC-δ (DrPKC-δ) but lost in mammalian PKC-δ and -θ. Comparative analyses of the sequence and function of BbPKC-δ/θ, DrPKC-δ, DrPKC-θ and Homo sapiens PKC-θ (HsPKC-θ) in IS translocation and T-cell receptor (TCR)-induced NF-κB activation revealed that retention of the conserved PxxP motif and loss of the non-conserved PxxP motif in mammalian PKC-θ and loss of both PxxP motifs in mammalian PKC-δ accomplish the unique function of PKC-θ in T cells. Together, this study suggests an evolutionary mechanism for PKC-θ unique V3 and reveals BbPKC-δ/θ is the common ancestor of PKC-δ and -θ with a functional proto-V3 domain, supplying new evidence for the existence of primitive adaptive immune system in amphioxus.

Paclitaxel Sensitivity of Ovarian Cancer Can be Enhanced by Knocking Down Pairs of Kinases that Regulate MAP4 Phosphorylation and Microtubule Stability

Purpose: Most patients with ovarian cancer receive paclitaxel chemotherapy, but less than half respond. Pre-treatment microtubule stability correlates with paclitaxel response in ovarian cancer cell lines. Microtubule stability can be increased by depletion of individual kinases. As microtubule stability can be regulated by phosphorylation of microtubule-associated proteins (MAPs), we reasoned that depletion of pairs of kinases that regulate phosphorylation of MAPs could induce microtubule stabilization and paclitaxel sensitization.Experimental Design: Fourteen kinases known to regulate paclitaxel sensitivity were depleted individually in 12 well-characterized ovarian cancer cell lines before measuring proliferation in the presence or absence of paclitaxel. Similar studies were performed by depleting all possible pairs of kinases in six ovarian cancer cell lines. Pairs that enhanced paclitaxel sensitivity across multiple cell lines were studied in depth in cell culture and in two xenograft models.Results: Transfection of siRNA against 10 of the 14 kinases enhanced paclitaxel sensitivity in at least six of 12 cell lines. Dual knockdown of IKBKB/STK39 or EDN2/TBK1 enhanced paclitaxel sensitivity more than silencing single kinases. Sequential knockdown was superior to concurrent knockdown. Dual silencing of IKBKB/STK39 or EDN2/TBK1 stabilized microtubules by inhibiting phosphorylation of p38 and MAP4, inducing apoptosis and blocking cell cycle more effectively than silencing individual kinases. Knockdown of IKBKB/STK39 or EDN2/TBK1 enhanced paclitaxel sensitivity in two ovarian xenograft models.Conclusions: Sequential knockdown of dual kinases increased microtubule stability by decreasing p38-mediated phosphorylation of MAP4 and enhanced response to paclitaxel in ovarian cancer cell lines and xenografts, suggesting a strategy to improve primary therapy. Clin Cancer Res; 24(20); 5072-84. ©2018 AACR.

The immunological synapse as a pharmacological target

The development of T cell mediated immunity relies on the assembly of a highly specialized interface between T cell and antigen presenting cell (APC), known as the immunological synapse (IS). IS assembly is triggered when the T cell receptor (TCR) binds to specific peptide antigen presented in association to the major histocompatibility complex (MHC) by the APC, and is followed by the spatiotemporal dynamic redistribution of TCR, integrins, co-stimulatory receptors and signaling molecules, allowing for the fine-tuning and integration of the signals that lead to T cell activation. The knowledge acquired to date about the mechanisms of IS assembly underscores this structure as a robust pharmacological target. The activity of molecules involved in IS assembly and function can be targeted by specific compounds to modulate the immune response in a number of disorders, including cancers and autoimmune diseases, or in transplanted patients. Here, we will review the state-of-the art of the current therapies which exploit the IS to modulate the immune response.

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.

Epstein-Barr Virus (EBV) Latent Protein EBNA3A Directly Targets and Silences the STK39 Gene in B Cells Infected by EBV

Epstein-Barr virus (EBV) establishes latent infection in human B cells and is associated with a wide range of cancers. The EBV nuclear antigen 3 (EBNA3) family proteins are critical for B cell transformation and function as transcriptional regulators. It is well established that EBNA3A and EBNA3C cooperate in the regulation of cellular genes. Here, we demonstrate that the gene STK39 is repressed only by EBNA3A. This is the first example of a gene regulated only by EBNA3A in EBV-transformed lymphoblastoid cell lines (LCLs) without the help of EBNA3C. This was demonstrated using a variety of LCLs carrying either knockout, revertant, or conditional EBNA3 recombinants. Investigating the kinetics of EBNA3A-mediated changes in STK39 expression showed that STK39 becomes derepressed quickly after EBNA3A inactivation. This derepression is reversible as EBNA3A reactivation represses STK39 in the same cells expressing a conditional EBNA3A. STK39 is silenced shortly after primary B cell infection by EBV, and no STK39-encoded protein (SPAK) is detected 3 weeks postinfection. Chromatin immunoprecipitation (ChIP) analysis indicates that EBNA3A directly binds to a regulatory region downstream of the STK39 transcription start site. For the first time, we demonstrated that the polycomb repressive complex 2 with the deposition of the repressive mark H3K27me3 is not only important for the maintenance of an EBNA3A target gene (STK39) but is also essential for the initial establishment of its silencing. Finally, we showed that DNA methyltransferases are involved in the EBNA3A-mediated repression of STK39.

IMPORTANCE EBV is well known for its ability to transform B lymphocytes to continuously proliferating lymphoblastoid cell lines. This is achieved in part by the reprogramming of cellular gene transcription by EBV transcription factors, including the EBNA3 proteins that play a crucial role in this process. In the present study, we found that EBNA3A epigenetically silences STK39. This is the first gene where EBNA3A has been found to exert its repressive role by itself, without needing its coregulators EBNA3B and EBNA3C. Furthermore, we demonstrated that the polycomb repressor complex is essential for EBNA3A-mediated repression of STK39. Findings in this study provide new insights into the regulation of cellular genes by the transcription factor EBNA3A.

Affinity purification mass spectrometry analysis of PD-1 uncovers SAP as a new checkpoint inhibitor

Programmed cell death-1 (PD-1) is an essential inhibitory receptor in T cells. Antibodies targeting PD-1 elicit durable clinical responses in patients with multiple tumor indications. Nevertheless, a significant proportion of patients do not respond to anti-PD-1 treatment, and a better understanding of the signaling pathways downstream of PD-1 could provide biomarkers for those whose tumors respond and new therapeutic approaches for those whose tumors do not. We used affinity purification mass spectrometry to uncover multiple proteins associated with PD-1. Among these proteins, signaling lymphocytic activation molecule-associated protein (SAP) was functionally and mechanistically analyzed for its contribution to PD-1 inhibitory responses. Silencing of SAP augmented and overexpression blocked PD-1 function. T cells from patients with X-linked lymphoproliferative disease (XLP), who lack functional SAP, were hyperresponsive to PD-1 signaling, confirming its inhibitory role downstream of PD-1. Strikingly, signaling downstream of PD-1 in purified T cell subsets did not correlate with PD-1 surface expression but was inversely correlated with intracellular SAP levels. Mechanistically, SAP opposed PD-1 function by acting as a molecular shield of key tyrosine residues that are targets for the tyrosine phosphatase SHP2, which mediates PD-1 inhibitory properties. Our results identify SAP as an inhibitor of PD-1 function and SHP2 as a potential therapeutic target in patients with XLP.

MST-4 and TRAF-6 expression in the peripheral blood mononuclear cells of patients with Graves' disease and its significance

BACKGROUND

MST-4 and TRAF-6 are involved in the regulation of inflammatory and immune responses. However, whether they participate in the pathogenesis of Graves' disease (GD) has not yet been reported. Therefore, the purpose of this study was to investigate the expression of MST-4 and TRAF-6 in the peripheral blood of patients with GD to understand their role in the pathogenesis of GD.

METHODS

Thirty newly diagnosed GD patients, 24 GD patients in remission (eGD) and 30 normal controls (NC) were recruited. Thyroid function and autoantibody levels were determined using a chemiluminescence immunoassay. Peripheral blood mononuclear cells (PBMCs) were extracted, and MST-4 and TRAF-6 mRNA and protein levels were determined using real-time PCR and Western blotting, respectively.

RESULTS

1. Thyroid function in the GD group was significantly different from that in the eGD and NC groups (P < 0.05); however, there was no difference in thyroid function between the eGD group and the NC group (P > 0.05). The autoantibody levels in the NC group were significantly different from those in the GD and eGD groups (P < 0.05); however, the difference in the levels between the GD group and eGD group was not statistically significant (P > 0.05). 2. The MST-4 and TRAF-6 mRNA and protein levels in the GD group were significantly lower than those in the NC group (P < 0.05); however, there were no differences in mRNA and protein levels between the GD group and the eGD group or between the eGD group and the NC group (P > 0.05). 3. The correlation between the MST-4 and TRAF-6 mRNA and protein levels was not significant. However, there was a significant correlation between the TRAF-6 mRNA and TPO Ab levels in the eGD group and between the TRAF-6 mRNA and TR Ab levels in the NC group.

CONCLUSION

The MST-4 and TRAF-6 mRNA and protein levels were lower in the GD group than in the NC group, suggesting that MST-4 and TRAF-6 may be important in the pathogenesis of GD. Whether MST-4 influences the innate immune response through TRAF-6 and thus regulates the imbalance in downstream effector T cells requires further study. Investigating the expression of MST-4 and TRAF-6 in GD can provide a new perspective and targets for further study of the upstream mechanism responsible for effector T cell imbalance.

SPAK plays a pathogenic role in IgA nephropathy through the activation of NF-κB/MAPKs signaling pathway

Sterile 20/SPS1-related proline/alanine-rich kinase (SPAK) can stimulate production of proinflammatory cytokines and interact with inflammation-related molecules. However, it has yet to be determined whether SPAK plays a pathophysiological role in the complicated pathological mechanisms of IgA nephropathy (IgAN), which is mainly characterized by mesangial cell (MC) proliferation and is the most common form of glomerulonephritis. In the present study, we examined the pathophysiological role of SPAK in IgAN using a mouse model and cell models. Our results clearly showed that (1) SPAK deficiency prevents the development of IgAN and inhibits production of immune/inflammatory mediators and T cell activation and proliferation; and (2) when primed with IgA immune complexes (IgA IC), both peritoneal macrophages and primary MCs from SPAK knockout mice show markedly reduced production of proinflammatory cytokines and inhibition of NF-κB/MAPKs activation. We proposed that activation of SPAK and the NF-κB/MAPKs signaling pathway in MCs, macrophages and T cells of the glomerulus may be a mechanism underlying the pathogenesis of IgAN. The activation of SPAK in renal tubuloepithelial cells either directly by IgA IC or an indirect action of the activated MCs or infiltrating mononuclear leukocytes seen in the kidney may further aggravate the disease process of IgAN. Our results suggest that SPAK is a potential therapeutic target for the glomerular disorder.

Structural and biochemical insights into the activation mechanisms of germinal center kinase OSR1

The oxidative stress-responsive 1 (OSR1) kinase belongs to the mammalian STE20-like kinase family. OSR1 is activated by with no lysine [K] (WNKs) kinases, and then it phosphorylates cation-coupled Cl-cotransporters, regulating ion homeostasis and cell volume in mammalian cells. However, the specific mechanisms of OSR1 activation remains poorly defined, largely due to its extremely low basal activity. Here, we dissect in detail the regulatory mechanisms of OSR1 activation from the aspects of autoinhibition, upstream kinase WNK, and the newly identified master regulator mouse protein-25 (MO25). Based on our structural and biochemical studies, we propose a "double lock" model, accounting for the tight autoinhibition of OSR1, an effect that has to be removed by WNK before MO25 further activates OSR1. Particularly, the conserved C-terminal (CCT) domain and αAL helix act together to strongly suppress OSR1 basal activity. WNKs bind to the CCT and trigger its conformational rearrangement to release the kinase domain of OSR1, allowing for MO25 binding and full activation. Finally, the regulatory mechanisms of OSR1 activation were further corroborated by cellular studies of OSR1-regulated cell volume control through WNK-OSR1 signaling pathway. Collectively, these results provide insights into the OSR1 kinase activation to facilitate further functional study.

Physiology and pathophysiology of SLC12A1/2 transporters

The electroneutral Na(+)-K(+)-Cl(-) cotransporters NKCC1 (encoded by the SLC12A2 gene) and NKCC2 (SLC12A1 gene) belong to the Na(+)-dependent subgroup of solute carrier 12 (SLC12) family of transporters. They mediate the electroneutral movement of Na(+) and K(+), tightly coupled to the movement of Cl(-) across cell membranes. As they use the energy of the ion gradients generated by the Na(+)/K(+)-ATPase to transport Na(+), K(+), and Cl(-) from the outside to the inside of a cell, they are considered secondary active transport mechanisms. NKCC-mediated transport occurs in a 1Na(+), 1K(+), and 2Cl(-) ratio, although NKCC1 has been shown to sometimes mediate partial reactions. Both transporters are blocked by bumetanide and furosemide, drugs which are commonly used in clinical medicine. NKCC2 is the molecular target of loop diuretics as it is expressed on the apical membrane of thick ascending limb of Henle epithelial cells, where it mediates NaCl reabsorption. NKCC1, in contrast, is found on the basolateral membrane of Cl(-) secretory epithelial cells, as well as in a variety of non-epithelial cells, where it mediates cell volume regulation and participates in Cl(-) homeostasis. Following their molecular identification two decades ago, much has been learned about their biophysical properties, their mode of operation, their regulation by kinases and phosphatases, and their physiological relevance. However, despite this tremendous amount of new information, there are still so many gaps in our knowledge. This review summarizes information that constitutes consensus in the field, but it also discusses current points of controversy and highlights many unanswered questions.

Knockout of Ste20-like proline/alanine-rich kinase (SPAK) attenuates intestinal inflammation in mice

Inflammatory bowel diseases are characterized by epithelial barrier disruption and alterations in immune regulation. Ste20-like proline/alanine-rich kinase (SPAK) plays a role in intestinal inflammation, but the underlying mechanisms need to be defined. Herein, SPAK knockout (KO) C57BL/6 mice exhibited significant increases in intestinal transepithelial resistance, a marked decrease in paracellular permeability to fluorescence isothiocyanate-dextran, and altered apical side tight junction sodium ion selectivity, compared with wild-type mice. Furthermore, the expression of junction protein, claudin-2, decreased. In contrast, expressions of occludin, E-cadherin, β-catenin, and claudin-5 increased significantly, whereas no obvious change of claudin-1, claudin-4, zonula occludens protein 1, and zonula occludens protein 2 expressions was observed. In murine models of colitis induced by dextran sulfate sodium and trinitrobenzene sulfuric acid, KO mice were more tolerant than wild-type mice, as demonstrated by colonoscopy features, histological characteristics, and myeloperoxidase activities. Consistent with these findings, KO mice showed increased IL-10 levels and decreased proinflammatory cytokine secretion, ameliorated bacterial translocation on treatment with dextran sulfate sodium, and regulation of with no lysine (WNK) kinase activity. Together, these features may reduce epithelial permeability. In conclusion, SPAK deficiency increases intestinal innate immune homeostasis, which is important for control or attenuation of pathological responses in inflammatory bowel diseases.

PKCα and PKCβ cooperate functionally in CD3-induced de novo IL-2 mRNA transcription

The physiological functions of PKCα and PKCθ isotypes downstream of the antigen receptor have been defined in CD3(+) T cells. In contrast, no function of the second conventional PKC member, PKCβ, has been described yet in T cell antigen receptor signalling. To investigate the hypothesis that both conventional PKCα and PKCβ isotypes may have overlapping functions in T cell activation signalling, we generated mice that lacked the genes for both isotypes. We found that PKCα(-/-)/β(-/-) animals are viable, live normal life spans and display normal T cell development. However, these animals possess additive defects in T cell responses in comparison to animals that carry single mutations in these genes. Our studies demonstrate that the activities of PKCα and PKCβ converge to regulate IL-2 cytokine responses in anti-CD3 stimulated primary mouse T cells. Here, we present genetic evidence that PKCα and PKCβ cooperate in IL-2 transcriptional transactivation in primary mouse T cells independently of the actions of PKCθ.

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

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

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

Germinal center kinases in immune regulation

Germinal center kinases (GCKs) participate in a variety of signaling pathways needed to regulate cellular functions including apoptosis, cell proliferation, polarity and migration. Recent studies have shown that GCKs are participants in both adaptive and innate immune regulation. However, the differential activation and regulatory mechanisms of GCKs, as well as upstream and downstream signaling molecules, remain to be fully defined. It remains unresolved whether and how GCKs may cross-talk with existing signaling pathways. This review stresses the progresses in research of GCKs relevant to the immune system.

PKC-theta-mediated signal delivery from the TCR/CD28 surface receptors

Protein kinase C-theta (PKCθ) is a key enzyme in T lymphocytes, where it plays an important role in signal transduction downstream of the activated T cell antigen receptor (TCR) and the CD28 costimulatory receptor. Interest in PKCθ as a potential drug target has increased following recent findings that PKCθ is essential for harmful inflammatory responses mediated by Th2 (allergies) and Th17 (autoimmunity) cells as well as for graft-versus-host disease (GvHD) and allograft rejection, but is dispensable for beneficial responses such as antiviral immunity and graft-versus-leukemia (GvL) response. TCR/CD28 engagement triggers the translocation of the cytosolic PKCθ to the plasma membrane (PM), where it localizes at the center of the immunological synapse (IS), which forms at the contact site between an antigen-specific T cell and antigen-presenting cells (APC). However, the molecular basis for this unique localization, and whether it is required for its proper function have remained unresolved issues until recently. Our recent study resolved these questions by demonstrating that the unique V3 (hinge) domain of PKCθ and, more specifically, a proline-rich motif within this domain, is essential and sufficient for its localization at the IS, where it is anchored to the cytoplasmic tail of CD28 via an indirect mechanism involving Lck protein tyrosine kinase (PTK) as an intermediate. Importantly, the association of PKCθ with CD28 is essential not only for IS localization, but also for PKCθ-mediated activation of downstream signaling pathways, including the transcription factors NF-κB and NF-AT, which are essential for productive T cell activation. Hence, interference with formation of the PKCθ-Lck-CD28 complex provides a promising basis for the design of novel, clinically useful allosteric PKCθ inhibitors. An additional recent study demonstrated that TCR triggering activates the germinal center kinase (GSK)-like kinase (GLK) and induces its association with the SLP-76 adaptor at the IS, where GLK phosphorylates the activation loop of PKCθ, converting it into an active enzyme. This recent progress, coupled with the need to study the biology of PKCθ in human T cells, is likely to facilitate the development of PKCθ-based therapeutic modalities for T cell-mediated diseases.

Intervention of PKC-θ as an immunosuppressive regimen

PKC-θ is selectively enriched in T cells and specifically translocates to immunological synapse where it mediates critical T cell receptor signals required for T cell activation, differentiation, and survival. T cells deficient in PKC-θ are defective in their ability to differentiate into inflammatory effector cells that mediate actual immune responses whereas, their differentiation into regulatory T cells (Treg) that inhibits the inflammatory T cells is enhanced. Therefore, the manipulation of PKC-θ activity can shift the ratio between inflammatory effector T cells and inhibitory Tregs, to control T cell-mediated immune responses that are responsible for autoimmunity and allograft rejection. Indeed, PKC-θ-deficient mice are resistant to the development of several Th2 and Th17-dependent autoimmune diseases and are defective in mounting alloimmune responses required for rejection of transplanted allografts and graft-versus-host disease. Selective inhibition of PKC-θ is therefore considered as a potential treatment for prevention of autoimmune diseases and allograft rejection.

IκB kinase β is required for activation of NF-κB and AP-1 in CD3/CD28-stimulated primary CD4(+) T cells

Engagement of the TCR and CD28 coreceptor by their respective ligands activates signal transduction cascades that ultimately lead to the activation of the transcription factors NFAT, AP-1, and NF-κB, which are required for the expression of cytokines and T cell clonal expansion. Previous studies have demonstrated that in mature T cells, activation of AP-1 and NF-κB is dependent on protein kinase C θ, suggesting the existence of a common signaling pathway. In this study, we show that in human primary CD4(+) T cells, exposure to the cell-permeable IKKβ inhibitor PS-1145 or genetic ablation of IKKβ abrogates cell proliferation and impairs the activation of NF-κB and AP-1 transcription factors in response to engagement of CD3 and CD28 coreceptor. In addition, we show that stimulation of T cells in the absence of IKKβ activity promotes the time-dependent and cyclosporine-sensitive expression of negative regulators of T cell signaling leading to a hyporesponsive state of T cells.

Ameliorated ConA-induced hepatitis in the absence of PKC-theta

Severe liver injury that occurs when immune cells mistakenly attack an individual's own liver cells leads to autoimmune hepatitis. In mice, acute hepatitis can be induced by concanavalin A (ConA) treatment, which causes rapid activation of CD1d-positive natural killer (NK) T cells. These activated NKT cells produce large amounts of cytokines, which induce strong inflammation that damages liver tissues. Here we show that PKC-θ^−/− mice were resistant to ConA-induced hepatitis due to essential function of PKC-θ in NKT cell development and activation. A dosage of ConA (25 mg/kg) that was lethal to wild-type (WT) mice failed to induce death resulting from liver injury in PKC-θ^−/− mice. Correspondingly, ConA-induced production of cytokines such as IFNγ, IL-6, and TNFα, which mediate the inflammation responsible for liver injury, were significantly lower in PKC-θ^−/− mice. Peripheral NKT cells had developmental defects at early stages in the thymus in PKC-θ^−/− mice, and as a result their frequency and number were greatly reduced. Furthermore, PKC-θ^−/− bone marrow adoptively transferred to WT mice displayed similar defects in NKT cell development, suggesting an intrinsic requirement for PKC-θ in NKT cell development. In addition, upon stimulation with NKT cell-specific lipid ligand, peripheral PKC-θ^−/− NKT cells produced lower levels of inflammatory cytokines than that of WT NKT cells, suggesting that activation of NKT cells also requires PKC-θ. Our results suggest PKC-θ is an essential molecule required for activation of NKT cell to induce hepatitis, and thus, is a potential drug target for prevention of autoimmune hepatitis.

Ischemia-induced stimulation of cerebral microvascular endothelial cell Na-K-Cl cotransport involves p38 and JNK MAP kinases

Previous studies have provided evidence that, in the early hours of ischemic stroke, a luminal membrane blood-brain barrier (BBB) Na-K-Cl cotransporter (NKCC) participates in ischemia-induced cerebral edema formation. Inhibition of BBB NKCC activity by intravenous bumetanide significantly reduces edema and infarct in the rat permanent middle cerebral artery occlusion model of ischemic stroke. We demonstrated previously that the BBB cotransporter is stimulated by hypoxia, aglycemia, and AVP, factors present during cerebral ischemia. However, the underlying mechanisms have not been known. Ischemic conditions have been shown to activate p38 and JNK MAP kinases (MAPKs) in brain, and the p38 and JNK inhibitors SB-239063 and SP-600125, respectively, have been found to reduce brain damage following middle cerebral artery occlusion and subarachnoid hemorrhage, respectively. The present study was conducted to determine whether one or both of these MAPKs participates in ischemic factor stimulation of BBB NKCC activity. Cultured cerebral microvascular endothelial cell NKCC activity was evaluated as bumetanide-sensitive (86)Rb influx. Activities of p38 and JNK were assessed by Western blot and immunofluorescence methods using antibodies that detect total vs. phosphorylated (activated) p38 or JNK. We report that p38 and JNK are present in cultured cerebral microvascular endothelial cells and in BBB endothelial cells in situ and that hypoxia (7% O(2) and 2% O(2)), aglycemia, AVP, and O(2)-glucose deprivation (5- to 120-min exposures) all rapidly activate p38 and JNK in the cells. We also provide evidence that SB-239063 and SP-600125 reduce or abolish ischemic factor stimulation of BBB NKCC activity. These findings support the hypothesis that ischemic factor stimulation of the BBB NKCC involves activation of p38 and JNK MAPKs.

The PKCθ pathway participates in the aberrant accumulation of Fra-1 protein in invasive ER-negative breast cancer cells

Fra-1 is aberrantly expressed in a large number of cancer cells and tissues, and emerging evidence suggests an important role for this Fos family protein in both oncogenesis and the progression or maintenance of many tumour types. Here, we show that the concentration of Fra-1 is high in invasive oestrogen receptor (ER)-negative (ER-) breast cancer cell lines, regardless of their Ras pathway status. All of the ER- cells express high levels of activated PKCθ, and the inhibition of PKCθ activity using RNA interference or the expression of a dominant-negative mutant results in a dramatic reduction in Fra-1 abundance. Conversely, the ectopic expression of constitutively active PKCθ leads to Fra-1 phosphorylation and accumulation in poorly invasive ER+ cells. This accumulation is due to the stabilisation of the Fra-1 protein through PKCθ signalling, whereas other members of the PKC family are ineffective. Both Ste20-related proline-alanine-rich kinase (SPAK) and ERK1/2, whose activities are upregulated by PKCθ, participate in PKCθ-driven Fra-1 stabilisation. Interestingly, their relative contributions appear to be different depending on the cell line studied. ERK1/2 signalling has a major role in ER- MDA-MB-231 cells, whereas Fra-1 accumulation occurs mainly through SPAK signalling in ER- BT549 cells. Fra-1 mutational analysis shows that the phosphorylation of S265, T223 and T230 is critical for PKCθ-driven Fra-1 stabilisation. Phosphorylation of the protein was confirmed using specific antisera against Fra-1 phosphorylated on T223 or S265. In addition, Fra-1 participates in PKCθ-induced cell invasion and is necessary for PKCθ-induced cell migration. In summary, we identified PKCθ signalling as an important regulator of Fra-1 accumulation in ER- breast cancer cells. Moreover, our results suggest that PKCθ could participate in progression of some breast cancers and could be a new therapeutic target.

Overexpression of Ste20-related proline/alanine-rich kinase exacerbates experimental colitis in mice

Inflammatory bowel disease, mainly Crohn's disease and ulcerative colitis, are characterized by epithelial barrier disruption and altered immune regulation. Colonic Ste20-like proline/alanine-rich kinase (SPAK) plays a role in intestinal inflammation, but its underlying mechanisms need to be defined. Both SPAK-transfected Caco2-BBE cells and villin-SPAK transgenic (TG) FVB/6 mice exhibited loss of intestinal barrier function. Further studies demonstrated that SPAK significantly increased paracellular intestinal permeability to FITC-dextran. In vivo studies using the mouse models of colitis induced by dextran sulfate sodium (DSS) and trinitrobenzene sulfonic acid showed that TG FVB/6 mice were more susceptible to DSS and trinitrobenzene sulfonic acid treatment than wild-type FVB/6 mice, as demonstrated by clinical and histological characteristics and enzymatic activities. Consistent with this notion, we found that SPAK increased intestinal epithelial permeability, which likely facilitated the production of inflammatory cytokines in vitro and in vivo, aggravated bacterial translocation in TG mice under DSS treatment, and consequently established a context favorable for the triggering of intestinal inflammation cascades. In conclusion, overexpression of SPAK inhibits maintenance of intestinal mucosal innate immune homeostasis, which makes regulation of SPAK important to attenuate pathological responses in inflammatory bowel disease.

A novel role for CD46 in wound repair

The intestinal epithelium not only provides a vital physical barrier between the host and environment but is also required for uptake of nutrients and the induction of tolerance against commensals. Deregulation of any of these functions leads to several disease states including chronic infection, inflammatory bowel disease, and cancer. Here, we describe a novel role for the complement regulator CD46 in the regulation of intestinal epithelial cell (IEC) barrier function. We found that CD46 directly interacts in IECs with the cytoplasmic kinase SPAK and with transmembrane E-cadherin, both proteins necessary for epithelial cell junction and barrier formation. Further, CD46 activation on Caco-2 cells induced rapid and significant decrease in transepithelial resistance with concomitant increase in paracellular permeability. Importantly, though CD46 activation of IEC layers allowed for increased transgression of pathogenic E. coli, it also increased epithelial cell proliferation and accelerated wound repair. These data suggest a previously unappreciated role for CD46 in the maintenance of epithelial cell barrier integrity as well as barrier repair. However, this role for CD46 as “gate keeper” of the epithelium could also provide reason as to why so many pathogens bind to CD46 as such event would facilitate infection.

PKC-θ function at the immunological synapse: prospects for therapeutic targeting

Protein kinase C (PKC)-θ regulates conventional effector T (Teff) cell function. Since this initial finding, it has become clear that the role of PKC-θ in T cells is complex. PKC-θ plays a central role in Teff cell activation and survival, and negatively regulates stability of the immunological synapse (IS). Recent studies demonstrated that PKC-θ is required for the development of natural CD4(+)Foxp3(+) regulatory T (Treg) cells, and mediates negative regulation of Treg cell function. Here, we examine the role of PKC-θ in the IS, evidence for its distinct localization in Treg cells and the therapeutic implications of inhibiting PKC-θ in Teff cells, to reduce effector function, and in Treg cells, to increase suppressor function, for the prevention and treatment of autoimmune and alloimmune disease states.

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.

Complement regulator CD46 temporally regulates cytokine production by conventional and unconventional T cells

In this study we demonstrate a new form of immunoregulation: engagement on CD4(+) T cells of the complement regulator CD46 promoted the effector potential of T helper type 1 cells (T(H)1 cells), but as interleukin 2 (IL-2) accumulated, it switched cells toward a regulatory phenotype, attenuating IL-2 production via the transcriptional regulator ICER/CREM and upregulating IL-10 after interaction of the CD46 tail with the serine-threonine kinase SPAK. Activated CD4(+) T cells produced CD46 ligands, and blocking CD46 inhibited IL-10 production. Furthermore, CD4(+) T cells in rheumatoid arthritis failed to switch, consequently producing excessive interferon-gamma (IFN-gamma). Finally, gammadelta T cells, which rarely produce IL-10, expressed an alternative CD46 isoform and were unable to switch. Nonetheless, coengagement of T cell antigen receptor (TCR) gammadelta and CD46 suppressed effector cytokine production, establishing that CD46 uses distinct mechanisms to regulate different T cell subsets during an immune response.

Multigene kinase network, kidney transport, and salt in essential hypertension

Evidence is mounting that a multi-gene kinase network is central to the regulation of renal Na(+) and K(+) excretion and that aberrant signaling through the pathway can result in renal sodium retention and hypertension (HTN). The kinase network minimally includes the Ste20-related proline-alanine-rich kinase (SPAK), the with-no-lysine kinases (WNKs), WNK4 and WNK1, and their effectors, the thiazide-sensitive NaCl cotransporter and the potassium secretory channel, ROMK. Available evidence indicates that the kinase network normally functions as a switch to change the mineralocorticoid hormone response of the kidney to either conserve sodium or excrete potassium, depending on whether aldosterone is induced by a change in dietary sodium or potassium. Recently, common genetic variants in the SPAK gene have been identified as HTN susceptibility factors in the general population, suggesting that altered WNK-SPAK signaling plays an important role in essential HTN. Here, we highlight recent breakthroughs in this emerging field and discuss areas of consensus and uncertainty.

Kinase regulation of Na+-K+-2Cl- cotransport in primary afferent neurons

The Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is expressed in sensory neurons where it accumulates intracellular Cl(-) and facilitates primary afferent depolarization. Depolarization of primary afferent fibre terminals interferes with the gating of incoming sensory signals to the spinal cord. The cotransporter belongs to a family of ion transporters which are sensitive to changes in cell volume. Cell shrinkage, through mechanisms that are still unknown, leads to the phosphorylation and activation of NKCC1. Similarly, axotomy results in increased NKCC1 phosphorylation in dorsal root ganglion (DRG) neurons. This review summarizes the work on the kinases that directly mediate NKCC1 activation. These are the sterile-20-like kinases SPAK and OSR1. Upon their activation through phosphorylation by upstream kinases, SPAK and OSR1 bind to specific peptides located in the cytosolic N-terminal tail of NKCC1, phosphorylate, and stimulate cotransport activity. Expression of SPAK and OSR1 varies from tissue to tissue, but in DRG neurons and in spinal cord, SPAK and OSR1 expression levels are similar. In DRG neurons, both kinases participate in the modulation of NKCC1, as the knockdown of one kinase only results in a partial decrease of NKCC1 function, while the knockdown of both kinases is additive. The identity of the kinases (e.g. WNK kinases) that possibly act upstream of SPAK and OSR1 is also discussed.

On the substrate recognition and negative regulation of SPAK, a kinase modulating Na+-K+-2Cl- cotransport activity

Threonines targeted by Ste20-related proline-alanine-rich kinase (SPAK) for phosphorylation have been identified in Na+-K+-2Cl(-) cotransporter type 1 (NKCC1), NKCC2, and Na+-Cl(-) cotransporter (NCC). However, what constitutes the substrate recognition of the kinase is still unknown. Using site-directed mutagenesis and functional measurement of NKCC1 activity in Xenopus laevis oocytes, we determined that SPAK recognizes two threonine residues separated by four amino acids. Addition or removal of a single residue abrogated SPAK activation of NKCC1. Although both threonines are followed by hydrophobic residues, in vivo experiments have determined that SPAK activation of the cotransporter only requires a hydrophobic residue after the first threonine. Interestingly, downstream of the second threonine residue, we have identified a conserved aspartic acid residue which is critical for NKCC1 function. Mouse SPAK activity requires phosphorylation of two specific residues by WNK [with no lysine (K)] kinases: a threonine (T243) in the catalytic domain and a serine (S383) in the regulatory domain. We found that mutating the threonine residue into a glutamic acid (T243E) combined with mutation of the serine into an aspartic acid (S383D) rendered SPAK constitutively active. Surprisingly, alanine substitution of S383 or mutation of residues surrounding this residue also resulted in a constitutively active kinase. Interestingly, deletion of amino acids 356-398 identified another serine residue in the catalytic domain (S321) as another putative target of WNK phosphorylation. We found that WNK4 is capable of stimulating the deletion mutant when S321 is present, but not when S321 is mutated into an alanine.

The SWI/SNF chromatin-remodeling complex modulates peripheral T cell activation and proliferation by controlling AP-1 expression

The SWI/SNF chromatin-remodeling complex has been implicated in the activation and proliferation of T cells. After T cell receptor signaling, the SWI/SNF complex rapidly associates with chromatin and controls gene expression in T cells. However, the process by which the SWI/SNF complex regulates peripheral T cell activation has not been elucidated. In this study, we show that the SWI/SNF complex regulates cytokine production and proliferation of T cells. During T cell activation, the SWI/SNF complex is recruited to the promoter of the transcription factor AP-1, and it increases the expression of AP-1. Increased expression of the SWI/SNF complex resulted in enhanced AP-1 activity, cytokine production, and proliferation of peripheral T cells, whereas knockdown of the SWI/SNF complex expression impaired the AP-1 expression and reduced the activation and proliferation of T cells. Moreover, mice that constitutively expressed the SWI/SNF complex in T cells were much more susceptible to experimentally induced autoimmune encephalomyelitis than the normal mice were. These results suggest that the SWI/SNF complex plays a critical role during T cell activation and subsequent immune responses.

Epigenetic silencing of Stk39 in B-cell lymphoma inhibits apoptosis from genotoxic stress

B-cell lymphomas, the most frequent human immune system malignancies, often contain dysregulated TCL1 oncogene expression. TCL1 transgenic (TCL1-tg) mice develop a spectrum of B-cell malignancies, supporting an oncogenic role for TCL1 in B cells. Our prior global survey of DNA methylation patterns in TCL1-tg B-cell lymphomas identified many lymphoma-specific candidate hypermethylated genes, including Stk39. The Stk39 encoded protein, sterile 20-like-related proline-alanine-rich kinase (SPAK), regulates cell stress responses, and microarray studies identified reduced SPAK expression in metastatic prostate and treatment-resistant breast cancers, suggesting that its loss may have a role in cancer progression. Here we identified DNA hypermethylation and SPAK silencing in TCL1-tg B-cell lymphomas and SPAK silencing without DNA methylation in multiple subtypes of human B-cell lymphomas. SPAK knockdown by shRNA protected B cells from caspase-dependent apoptosis induced by DNA double-strand breaks but not apoptosis in response to osmotic or oxidative cell stressors. Caspase 3 activation by cleavage was impaired with SPAK repression in DNA damaged B cells. Interestingly, c-Jun NH(2)-terminal kinase is potentially activated by SPAK and pharmacological inhibition of c-Jun NH(2)-terminal kinase in SPAK-expressing B cells recapitulated the cell-protective phenotype of SPAK knockdown. Taken together, these data indicate that SPAK loss in B-cell lymphomas promotes increased cell survival with DNA damage and provides a potential mechanism for increased resistance to genotoxic stress in cancer.

No specific subcellular localization of protein kinase C is required for cytotoxic T cell granule exocytosis

Cytotoxic T cells kill virus-infected cells and tumor cells by releasing lytic granules that contain cell-killing contents. Exocytosis requires calcium influx and protein kinase C (PKC) activation. Here, we extend our previous finding regarding the lack of isoform specificity of PKCs in the granule release step, showing that mutant constitutively active PKCdelta can substitute for phorbol esters and support exocytosis. PKCdelta, a novel PKC isoform, was recently shown to play a role in lytic granule reorientation. Surprisingly, however, our results suggested that mutant PKCdelta did not localize to the plasma membrane (PM). To test directly whether PKC has to be in the PM to drive exocytosis, we generated mutants of various PKC isoforms that were tethered either to the outer mitochondrial membrane or to the PM. Tethered mutant PKCdeltas were able to promote exocytosis as effectively as the untethered version. The substrates of PKCs involved in lytic granule exocytosis are currently unknown, but subcellular localization is believed to be a critical factor in determining PKC accessibility to substrates. That there is no requirement for specific PKC localization in lytic granule exocytosis may have important implications for the identity of PKC substrates.

Ste20-related proline/alanine-rich kinase (SPAK) regulated transcriptionally by hyperosmolarity is involved in intestinal barrier function

The Ste20-related protein proline/alanine-rich kinase (SPAK) plays important roles in cellular functions such as cell differentiation and regulation of chloride transport, but its roles in pathogenesis of intestinal inflammation remain largely unknown. Here we report significantly increased SPAK expression levels in hyperosmotic environments, such as mucosal biopsy samples from patients with Crohn's disease, as well as colon tissues of C57BL/6 mice and Caco2-BBE cells treated with hyperosmotic medium. NF-kappaB and Sp1-binding sites in the SPAK TATA-less promoter are essential for SPAK mRNA transcription. Hyperosmolarity increases the ability of NF-kappaB and Sp1 to bind to their binding sites. Knock-down of either NF-kappaB or Sp1 by siRNA reduces the hyperosmolarity-induced SPAK expression levels. Furthermore, expression of NF-kappaB, but not Sp1, was upregulated by hyperosmolarity in vivo and in vitro. Nuclear run-on assays showed that hyperosmolarity increases SPAK expression levels at the transcriptional level, without affecting SPAK mRNA stability. Knockdown of SPAK expression by siRNA or overexpression of SPAK in cells and transgenic mice shows that SPAK is involved in intestinal permeability in vitro and in vivo. Together, our data suggest that SPAK, the transcription of which is regulated by hyperosmolarity, plays an important role in epithelial barrier function.

Crystal structure of domain-swapped STE20 OSR1 kinase domain

OSR1 (oxidative stress-responsive-1) and SPAK (Ste20/Sps1-related proline/alanine-rich kinase) belong to the GCK-VI subfamily of Ste20 group kinases. OSR1 and SPAK are key regulators of NKCCs (Na(+)/K(+)/2Cl(-) cotransporters) and activated by WNK family members (with-no-lysine kinase), mutations of which are known to cause Gordon syndrome, an autosomal dominant form of inherited hypertension. The crystal structure of OSR1 kinase domain has been solved at 2.25 A. OSR1 forms a domain-swapped dimer in an inactive conformation, in which P+1 loop and alphaEF helix are swapped between dimer-related monomers. Structural alignment with nonswapped Ste20 TAO2 kinase indicates that the integrity of chemical interactions in the kinase domain is well preserved in the domain-swapped interfaces. The OSR1 kinase domain has now been added to a growing list of domain-swapped protein kinases recently reported, suggesting that the domain-swapping event provides an additional layer of complexity in regulating protein kinase activity.

BCL11B enhances TCR/CD28-triggered NF-kappaB activation through up-regulation of Cot kinase gene expression in T-lymphocytes

BCL11B is a transcriptional regulator with an important role in T-cell development and leukaemogenesis. We demonstrated recently that BCL11B controls expression from the IL (interleukin)-2 promoter through direct binding to the US1 (upstream site 1). In the present study, we provide evidence that BCL11B also participates in the activation of IL-2 gene expression by enhancing NF-kappaB (nuclear factor kappaB) activity in the context of TCR (T-cell receptor)/CD28-triggered T-cell activation. Enhanced NF-kappaB activation is not a consequence of BCL11B binding to the NF-kappaB response elements or association with the NF-kappaB-DNA complexes, but rather the result of higher translocation of NF-kappaB to the nucleus caused by enhanced degradation of IkappaB (inhibitor of NF-kappaB). The enhanced IkappaB degradation in cells with increased levels of BCL11B was specific for T-cells activated through the TCR, but not for cells activated through TNFalpha (tumour necrosis factor alpha) or UV light, and was caused by increased activity of IkappaB kinase, as indicated by its increase in phosphorylation. As BCL11B is a transcription factor, we investigated whether the expression of genes upstream of IkappaB kinase in the TCR/CD28 signalling pathway was affected by increased BCL11B expression, and found that Cot (cancer Osaka thyroid oncogene) kinase mRNA levels were elevated. Cot kinase is known to promote enhanced IkappaB kinase activity, which results in the phosphorylation and degradation of IkappaB and activation of NF-kappaB. The implied involvement of Cot kinase in BCL11B-mediated NF-kappaB activation in response to TCR activation is supported by the fact that a Cot kinase dominant-negative mutant or Cot kinase siRNA (small interfering RNA) knockdown blocked BCL11B-mediated NF-kappaB activation. In support of our observations, in the present study we report that BCL11B enhances the expression of several other NF-kappaB target genes, in addition to IL-2. In addition, we provide evidence that BCL11B associates with intron 2 of the Cot kinase gene to regulate its expression.

Differential requirement of PKC-theta in the development and function of natural regulatory T cells

CD4+CD25+ natural Treg cells, which are developed in the thymus, migrate to the periphery to actively maintain self-tolerance. Similar to conventional T cells, TCR signals are critical for the development and activation of Treg cell inhibitory function. While PKC-theta-mediated TCR signals are required for the activation of peripheral naïve T cells, they are dispensable for their thymic development. Here, we show that mice deficient in PKC-theta had a greatly reduced number of CD4+Foxp3+ Treg cells, which was independent of PKC-theta-regulated survival, as transgenic Bcl-x(L) could not restore the Treg cell population in PKC-theta(-/-) mice. Active and WT PKC-theta markedly stimulated, whereas inactive PKC-theta and dominant negative NFAT inhibited Foxp3 promoter activity. In addition, mice-deficient in calcineurin Abeta had a decreased Treg cell population, similar to that observed in PKC-theta deficient mice. It is likely that PKC-theta promoted the development of Treg cells by enhancing Foxp3 expression via activation of the calcineurin/NFAT pathway. Finally, Treg cells deficient in PKC-theta were as potent as WT Treg cells in inhibiting T cell activation, indicating that PKC-theta was not required for Treg cell-mediated inhibitory function. Our data highlight the contrasting roles PKC-theta plays in conventional T cell and natural Treg cell function.

Ste20-related proline/alanine-rich kinase: A novel regulator of intestinal inflammation

Recently, inflammatory bowel disease (IBD) has been the subject of considerable research, with increasing attention being paid to the loss of intestinal epithelial cell barrier function as a mechanism of pathogenesis. Ste20-related proline/alanine-rich kinase (SPAK) is involved in regulating barrier function. SPAK is known to interact with inflammation-related kinases (such as p38, JNK, NKCC1, PKCtheta;, WNK and MLCK), and with transcription factor AP-1, resulting in diverse biological phenomena, including cell differentiation, cell transformation and proliferation, cytoskeleton rearrangement, and regulation of chloride transport. This review examines the involvement of Ste20-like kinases and downstream mitogen-activated protein kinases (MAPKs) pathways in the pathogenesis and control of intestinal inflammation. The primary focus will be on the molecular features of intestinal inflammation, with an emphasis on the interaction between SPAK and other molecules, and the effect of these interactions on homeostatic maintenance, cell volume regulation and increased cell permeability in intestinal inflammation.

Nuclear factor-kappaB is a critical mediator of Ste20-like proline-/alanine-rich kinase regulation in intestinal inflammation

Inflammatory bowel disease (IBD) is thought to result from commensal flora, aberrant cellular stress, and genetic factors. Here we show that the expression of colonic Ste20-like proline-/alanine-rich kinase (SPAK) that lacks a PAPA box and an F-alpha helix loop is increased in patients with IBD. The same effects were observed in a mouse model of dextran sodium sulfate-induced colitis and in Caco2-BBE cells treated with the pro-inflammatory cytokine tumor necrosis factor (TNF)-alpha. The 5'-flanking region of the SPAK gene contains two transcriptional start sites, three transcription factor Sp1-binding sites, and one transcription factor nuclear factor (NF)-kappaB-binding site, but no TATA elements. The NF-kappaB-binding site was essential for stimulated SPAK promoter activity by TNF-alpha, whereas the Sp1-binding sites were important for basal promoter activity. siRNA-induced knockdown of NF-kappaB, but not of Sp1, reduced TNF-alpha-induced SPAK expression. Nuclear run-on and mRNA decay assays demonstrated that TNF-alpha directly increased SPAK mRNA transcription without affecting SPAK mRNA stability. Furthermore, up-regulation of NF-kappaB expression and demethylation of the CpG islands induced by TNF-alpha also played roles in the up-regulation of SPAK expression. In conclusion, our data indicate that during inflammatory conditions, TNF-alpha is a key regulator of SPAK expression. The development of compounds that can either modulate or disrupt the activity of SPAK-mediated pathways is therefore important for the control and attenuation of downstream pathological responses, particularly in IBD.

A critical role for protein kinase C-theta-mediated T cell survival in cardiac allograft rejection

Protein kinase C (PKC)-theta mediates the critical TCR signals required for T cell activation. Previously, we have shown that in response to TCR stimulation, PKC-theta-/- T cells undergo apoptosis due to greatly reduced levels of the anti-apoptotic molecule, Bcl-xL. In this study, we demonstrate that PKC-theta-regulated expression of Bcl-xL is essential for T cell-mediated cardiac allograft rejection. Rag1-/- mice reconstituted with wild-type T cells readily rejected fully mismatched cardiac allografts, whereas Rag1-/- mice reconstituted with PKC-theta-/- T cells failed to promote rejection. Transgenic expression of Bcl-xL in PKC-theta-/- T cells was sufficient to restore cardiac allograft rejection, suggesting that PKC-theta-regulated survival is required for T cell-mediated cardiac allograft rejection in this adoptive transfer model. In contrast to adoptive transfer experiments, intact PKC-theta-/- mice displayed delayed, but successful cardiac allograft rejection, suggesting the potential compensation for PKC-theta function. Finally, a subtherapeutic dose of anti-CD154 Ab or CTLA4-Ig, which was not sufficient to prevent cardiac allograft rejection in the wild-type mice, prevented heart rejection in the PKC-theta-/- mice. Thus, in combination with other treatments, inhibition of PKC-theta may facilitate achieving long-term survival of allografts.

PASK (proline-alanine-rich Ste20-related kinase) binds to tubulin and microtubules and is involved in microtubule stabilization

Proline-alanine-rich Ste20-related kinase (PASK, also referred to as SPAK) has been linked to ion transport regulation. Here, we report two novel activities of PASK: binding to tubulin and microtubules and the promotion of microtubule assembly. Tubulin binding assay showed that full-length PASK and its kinase domain bound to purified tubulin whereas the N-terminal or C-terminal non-catalytic domains of PASK did not. The full-length PASK and its kinase domain were sedimented with paclitaxel-stabilized microtubules by ultracentrifugation. These results indicate that the kinase domain of PASK can interact directly with both microtubules and soluble tubulin in vitro. Truncated PASK lacking the N-terminal non-catalytic domain promoted microtubule assembly at a subcritical concentration of purified tubulin. FLAG-PASK expressed in COS-7 cells translocated to the cytoskeleton when the cells were stimulated with hypertonic sodium chloride, and stabilized microtubules against depolymerization by nocodazole. Our findings suggest that PASK may regulate the cytoskeleton by modulating microtubule stability.

PKCdelta acts upstream of SPAK in the activation of NKCC1 by hyperosmotic stress in human airway epithelial cells

Airway epithelial Na-K-2Cl (NKCC1) cotransport is activated through hormonal stimulation and hyperosmotic stress via a protein kinase C (PKC) delta-mediated intracellular signaling pathway. Down-regulation of PKCdelta prevents activation of NKCC1 expressed in Calu-3 cells. Previous studies of this signaling pathway identified coimmunoprecipitation of PKCdelta with SPAK (Ste20-related proline alanine-rich kinase). We hypothesize that endogenous PKCdelta activates SPAK, which subsequently activates NKCC1 through phosphorylation. Double-stranded silencing RNA directed against SPAK reduced SPAK protein expression by 65.8% and prevented increased phosphorylation of NKCC1 and functional activation of NKCC1 during hyperosmotic stress, measured as bumetanide-sensitive basolateral to apical (86)Rb flux. Using recombinant proteins, we demonstrate direct binding of PKCdelta to SPAK, PKCdelta-mediated activation of SPAK, binding of SPAK to the amino terminus of NKCC1 (NT-NKCC1, amino acids 1-286), and competitive inhibition of SPAK-NKCC1 binding by a peptide encoding a SPAK binding site on NT-NKCC1. The carboxyl terminus of SPAK (amino acids 316-548) pulls down endogenous NKCC1 from Calu-3 total cell lysates and glutathione S-transferase-tagged NT-NKCC1 pulls down endogenous SPAK. In intact cells, hyperosmotic stress increased phosphorylated PKCdelta, indicating activation of PKCdelta, and activity of endogenous SPAK kinase. Inhibition of PKCdelta activity with rottlerin blocked the increase in SPAK kinase activity. The results indicate that PKCdelta acts upstream of SPAK to increase activity of NKCC1 during hyperosmotic stress.