Muhammet Fatih Gulen

Astrocyte-Restricted Ablation of Interleukin-17-Induced Act1-Mediated Signaling Ameliorates Autoimmune Encephalomyelitis

Interleukin-17 (IL-17) secreted by T helper 17 (Th17) cells is essential in the development of experimental autoimmune encephalomyelitis (EAE). However, it remains unclear how IL-17-mediated signaling in different cellular compartments participates in the central nervous system (CNS) inflammatory process. We examined CNS inflammation in mice with specific deletion of Act1, a critical component required for IL-17 signaling, in endothelial cells, macrophages and microglia, and neuroectoderm (neurons, astrocytes, and oligodendrocytes). In Act1-deficient mice, Th17 cells showed normal infiltration into the CNS but failed to recruit lymphocytes, neutrophils, and macrophages. Act1 deficiency in endothelial cells or in macrophages and microglia did not substantially impact the development of EAE. However, targeted Act1 deficiency in neuroectoderm-derived CNS-resident cells resulted in markedly reduced severity in EAE. Specifically, Act1-deficient astrocytes showed impaired IL-17-mediated inflammatory gene induction. Thus, astroctyes are critical in IL-17-Act1-mediated leukocyte recruitment during autoimmune-induced inflammation of the CNS.

The receptor SIGIRR suppresses Th17 cell proliferation via inhibition of the interleukin-1 receptor pathway and mTOR kinase activation

Interleukin-1 (IL-1)-mediated signaling in T cells is essential for T helper 17 (Th17) cell differentiation. We showed here that SIGIRR, a negative regulator of IL-1 receptor and Toll-like receptor signaling, was induced during Th17 cell lineage commitment and governed Th17 cell differentiation and expansion through its inhibitory effects on IL-1 signaling. The absence of SIGIRR in T cells resulted in increased Th17 cell polarization in vivo upon myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide immunization. Recombinant IL-1 promoted a marked increase in the proliferation of SIGIRR-deficient T cells under an in vitro Th17 cell-polarization condition. Importantly, we detected increased IL-1-induced phosphorylation of JNK and mTOR kinase in SIGIRR-deficient Th17 cells compared to wild-type Th17 cells. IL-1-induced proliferation was abolished in mTOR-deficient Th17 cells, indicating the essential role of mTOR activation. Our results demonstrate an important mechanism by which SIGIRR controls Th17 cell expansion and effector function through the IL-1-induced mTOR signaling pathway.

The inducible kinase IKKi is required for IL-17-dependent signaling associated with neutrophilia and pulmonary inflammation.

Interleukin 17 (IL-17) is critical in the pathogenesis of inflammatory and autoimmune diseases. Here we report that Act1, the key adaptor for the IL-17 receptor (IL-7R), formed a complex with the inducible kinase IKKi after stimulation with IL-17. Through the use of IKKi-deficient mice, we found that IKKi was required for IL-17-induced expression of genes encoding inflammatory molecules in primary airway epithelial cells, neutrophilia and pulmonary inflammation. IKKi deficiency abolished IL-17-induced formation of the complex of Act1 and the adaptors TRAF2 and TRAF5, activation of mitogen-activated protein kinases (MAPKs) and mRNA stability, whereas the Act1–TRAF6–transcription factor NF-κB axis was retained. IKKi was required for IL-17-induced phosphorylation of Act1 on Ser311, adjacent to a putative TRAF-binding motif. Substitution of the serine at position 311 with alanine impaired the IL-17-mediated Act1-TRAF2-TRAF5 interaction and gene expression. Thus, IKKi is a kinase newly identified as modulating IL-17 signaling through its effect on Act1 phosphorylation and consequent function.

The Essential Role of Single Ig IL-1 Receptor-Related Molecule/Toll IL-1R8 in Regulation of Th2 Immune Response

A novel cytokine IL-33, an IL-1 family member, signals via ST2 receptor and promotes Th2 responses, through the activation of NF-κB and MAP kinases. Previous studies reported that single Ig IL-1R-related molecule (SIGIRR)/Toll IL-1R8 acts as negative regulator for TLR-IL-1R-mediated signaling. We now found that SIGIRR formed a complex with ST2 upon IL-33 stimulation and specifically inhibited IL-33/ST2-mediated signaling in cell culture model. Furthermore, IL-33-induced Th2 response was enhanced in SIGIRR-deficient mice compared with that in wild-type control mice, suggesting a negative regulatory role of SIGIRR in IL-33/ST2 signaling in vivo. Similar to ST2, SIGIRR was highly expressed in in vitro polarized Th2 cells, but not Th1 cells. SIGIRR-deficient Th2 cells produce higher levels of Th2 cytokines, including IL-5, IL-4, and IL-13, than that in wild-type cells. Moreover, SIGIRR-deficient mice developed stronger Th2 immune response in OVA-challenged asthma model. Taken together, our results suggest that SIGIRR plays an important role in the regulation of Th2 response in vivo, possibly through its impact on IL-33-ST2-mediated signaling.

Interleukin-1 (IL-1)-induced TAK1-dependent Versus MEKK3-dependent NFκB Activation Pathways Bifurcate at IL-1 Receptor-associated Kinase Modification

Interleukin-1 (IL-1) receptor-associated kinase (IRAK) is phosphorylated after it is recruited to the receptor, subsequently ubiquitinated, and eventually degraded upon IL-1 stimulation. Although a point mutation changing lysine 134 to arginine (K134R) in IRAK abolished IL-1-induced IRAK ubiquitination and degradation, mutations of serines and threonines adjacent to lysine 134 to alanines ((S/T)A (131–144)) reduced IL-1-induced IRAK phosphorylation and abolished IRAK ubiquitination. Through the study of these IRAK modification mutants, we uncovered two parallel IL-1-mediated signaling pathways for NFκB activation, TAK1-dependent and MEKK3-dependent, respectively. These two pathways bifurcate at the level of IRAK modification. The TAK1-dependent pathway leads to IKKα/β phosphorylation and IKKβ activation, resulting in classical NFκB activation through IκBα phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKγ phosphorylation and IKKα activation, resulting in NFκB activation through IκBα phosphorylation and subsequent dissociation from NFκB but without IκBα degradation. These results provide significant insight to our further understanding of NFκB activation pathways.

Epithelial Cell-Specific Act1 Adaptor Mediates Interleukin-25-Dependent Helminth Expulsion through Expansion of Lin−c-Kit+ Innate Cell Population

Interleukin-25 (IL-25 or IL-17E), a member of the structurally related IL-17 family, functions as an important mediator of T helper 2 cell-type (type 2) responses. We examined the cell type-specific role of IL-25-induced Act1-mediated signaling in protective immunity against helminth infection. Targeted Act1 deficiency in epithelial cells resulted in a marked delay in worm expulsion and abolished the expansion of the Lin(-)c-Kit(+) innate cell population in the mesenteric lymph node, lung, and liver. Th2 cell-inducing cytokine (IL-25 and IL-33) expression were reduced in the intestinal epithelial cells from the infected and IL-25-injected epithelial-specific Act1-deficient mice. Adoptive transfer of Lin(-)c-Kit(+) cells or combined injection of IL-25 and IL-33 restored the type 2 responses in these mice. Taken together, these results suggest that epithelial-specific Act1 mediates the expansion of the Lin(-)c-Kit(+) innate cell population through the positive-feedback loop of IL-25, initiating the type 2 immunity against helminth infection.

SIGIRR, a Negative Regulator of TLR/IL-1R Signalling Promotes Microbiota Dependent Resistance to Colonization by Enteric Bacterial Pathogens

Enteric bacterial pathogens such as enterohemorrhagic E. coli (EHEC) and Salmonella Typhimurium target the intestinal epithelial cells (IEC) lining the mammalian gastrointestinal tract. Despite expressing innate Toll-like receptors (TLRs), IEC are innately hypo-responsive to most bacterial products. This is thought to prevent maladaptive inflammatory responses against commensal bacteria, but it also limits antimicrobial responses by IEC to invading bacterial pathogens, potentially increasing host susceptibility to infection. One reason for the innate hypo-responsiveness of IEC is their expression of Single Ig IL-1 Related Receptor (SIGIRR), a negative regulator of interleukin (IL)-1 and TLR signaling. To address whether SIGIRR expression and the innate hypo-responsiveness of IEC impacts on enteric host defense, Sigirr deficient (−/−) mice were infected with the EHEC related pathogen Citrobacter rodentium. Sigirr −/− mice responded with accelerated IEC proliferation and strong pro-inflammatory and antimicrobial responses but surprisingly, Sigirr −/− mice proved dramatically more susceptible to infection than wildtype mice. Through haematopoietic transplantation studies, it was determined that SIGIRR expression by non-haematopoietic cells (putative IEC) regulated these responses. Moreover, the exaggerated responses were found to be primarily dependent on IL-1R signaling. Whilst exploring the basis for their susceptibility, Sigirr −/− mice were found to be unusually susceptible to intestinal Salmonella Typhimurium colonization, developing enterocolitis without the typical requirement for antibiotic based removal of competing commensal microbes. Strikingly, the exaggerated antimicrobial responses seen in Sigirr −/− mice were found to cause a rapid and dramatic loss of commensal microbes from the infected intestine. This depletion appears to reduce the ability of the microbiota to compete for space and nutrients (colonization resistance) with the invading pathogens, leaving the intestine highly susceptible to pathogen colonization. Thus, SIGIRR expression by IEC reflects a strategy that sacrifices maximal innate responsiveness by IEC in order to promote commensal microbe based colonization resistance against bacterial pathogens.

IKKα negatively regulates ASC-dependent inflammasome activation

The inflammasomes are multiprotein complexes that activate caspase-1 in response to infections and stress, resulting in the secretion of pro-inflammatory cytokines. Here we report that IKKα is a critical negative regulator of ASC-dependent inflammasomes. IKKα controls the inflammasome at the level of the adaptor ASC, which interacts with IKKα in the nucleus of resting macrophages in an IKKα kinase-dependent manner. Loss of IKKα kinase activity results in inflammasome hyperactivation. Mechanistically, the downstream nuclear effector IKKi facilitates translocation of ASC from the nucleus to the perinuclear area during inflammasome activation. ASC remains under the control of IKKα in the perinuclear area following translocation of the ASC/IKKα complex. Signal 2 of NLRP3 activation leads to inhibition of IKKα kinase activity through the recruitment of PP2A, allowing ASC to participate in NLRP3 inflammasome assembly. Taken together, these findings reveal a IKKi-IKKα-ASC axis that serves as a common regulatory mechanism for ASC-dependent inflammasomes.

Inactivation of the Enzyme GSK3α by the Kinase IKKi Promotes AKT-mTOR Signaling Pathway that Mediates Interleukin-1-Induced Th17 Cell Maintenance

Interleukin-1 (IL-1)-induced activation of the mTOR kinase pathway has major influences on Th17 cell survival, proliferation, and effector function. Via biochemical and genetic approaches, the kinases IKKi and GSK3α were identified as the critical intermediate signaling components for IL-1-induced AKT activation, which in turn activated mTOR. Although insulin-induced AKT activation is known to phosphorylate and inactivate GSK3α and GSK3β, we found that GSK3α but not GSK3β formed a constitutive complex to phosphorylate and suppress AKT activation, showing that a reverse action from GSK to AKT can take place. Upon IL-1 stimulation, IKKi was activated to mediate GSK3α phosphorylation at S21, thereby inactivating GSK3α to promote IL-1-induced AKT-mTOR activation. Thus, IKKi has a critical role in Th17 cell maintenance and/or proliferation through the GSK-AKT-mTOR pathway, implicating the potential of IKKi as a therapeutic target.

Loss of single immunoglobulin interlukin-1 receptor-related molecule leads to enhanced colonic polyposis in Apcmin mice

BACKGROUND & AIMS Commensal bacteria can activate signaling by the Toll-like and interleukin-1 receptors (TLR and IL-1R) to mediate pathogenesis of inflammatory bowel diseases and colitis-associated cancer. We investigated the role of the single immunoglobulin IL-1 receptor-related (SIGIRR) molecule, a negative regulator of TLR and IL-1R signaling, as a tumor suppressor to determine whether SIGIRR controls cell-cycle progression, genetic instability, and colon tumor initiation by modulating commensal TLR signaling in the gastrointestinal tract. METHODS We analyzed adenomatous polyposis coli (Apc)min/+/Sigirr-/- mice for polyps, microadenomas, and anaphase bridge index. Commensal bacteria were depleted from mice with antibiotics. Akt, mammalian target of rapamycin (mTOR), and beta-catenin pathways were examined by immunoblotting and immunohistochemistry. Loss of heterozygosity of Apc and expression of cytokines and proinflammatory mediators were measured by nonquantitative or quantitative polymerase chain reaction. RESULTS Apcmin/+/Sigirr-/- mice had increased loss of heterozygosity of Apc and microadenoma formation, resulting in spontaneous colonic polyposis, compared with Apcmin/+/Sigirr+/+ mice. The increased colonic tumorigenesis that occurred in the Apcmin/+/Sigirr-/- mice depended on the presence of commensal bacteria in the gastrointestinal tract. Cell proliferation and chromosomal instability increased in colon crypt cells of the Apcmin/+/Sigirr-/- mice. Akt, mTOR, and their substrates were hyperactivated in colon epithelium of Apcmin/+/Sigirr-/- mice in response to TLR or IL-1R ligands. Inhibition of the mTOR pathway by rapamycin reduced formation of microadenomas and polyps in the Apcmin/+/Sigirr-/- mice. CONCLUSIONS SIGIRR acts as a tumor suppressor in the colon by inhibiting TLR-induced, mTOR-mediated cell-cycle progression and genetic instability.