TLR2 and TLR4 Mediate Differential Responses to Limb Ischemia through MyD88-Dependent and Independent Pathways

Introduction

The danger signal HMGB1 is released from ischemic myocytes, and mediates angiogenesis in the setting of hindlimb ischemia. HMGB1 is a ligand for innate immune receptors TLR2 and TLR4. While both TLR2 and TLR4 signal through myeloid differentiation factor 88 (MyD88), TLR4 also uniquely signals through TIR-domain-containing adapter-inducing interferon-β (TRIF). We hypothesize that TLR2 and TLR4 mediate ischemic myocyte regeneration and angiogenesis in a manner that is dependent on MyD88 signaling.

Methods

Mice deficient in TLR2, TLR4, MyD88 and TRIF underwent femoral artery ligation in the right hindlimb. Laser Doppler perfusion imaging was used to assess the initial degree of ischemia and the extent of perfusion recovery. Muscle regeneration, necrosis and fat replacement at 2 weeks post-ligation were assessed histologically and vascular density was quantified by immunostaining. In vitro, endothelial tube formation was evaluated in matrigel in the setting of TLR2 and TLR4 antagonism.

Results

While control and TLR4 KO mice demonstrated prominent muscle regeneration, both TLR2 KO and TRIF KO mice exhibited marked necrosis with significant inflammatory cell infiltrate. However, MyD88 KO mice had a minimal response to the ischemic insult with little evidence of injury. This observation could not be explained by differences in perfusion recovery which was similar at two weeks in all the strains of mice. TLR2 KO mice demonstrated abnormal vessel morphology compared to other strains and impaired tube formation in vitro.

Discussion

TLR2 and TRIF signaling are necessary for muscle regeneration after ischemia while MyD88 may instead mediate muscle injury. The absence of TLR4 did not affect muscle responses to ischemia. TLR4 may mediate inflammatory responses through MyD88 that are exaggerated in the absence of TLR2. Additionally, the actions of TLR4 through TRIF may promote regenerative responses that are required for recovery from muscle ischemia.     

Integrated Signaling Pathways Mediate Bordetella Immunomodulation,Persistence, and Transmission

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Mcl-1 Integrates the Opposing Actions of Signaling Pathways That Mediate Survival and Apoptosis

Mcl-1 is a member of the Bcl2-related protein family that is a critical mediator of cell survival. Exposure of cells to stress causes inhibition of Mcl-1 mRNA translation and rapid destruction of Mcl-1 protein by proteasomal degradation mediated by a phosphodegron created by glycogen synthase kinase 3 (GSK3) phosphorylation of Mcl-1. Here we demonstrate that prior phosphorylation of Mcl-1 by the c-Jun N-terminal protein kinase (JNK) is essential for Mcl-1 phosphorylation by GSK3. Stress-induced Mcl-1 degradation therefore requires the coordinated activity of JNK and GSK3. Together, these data establish that Mcl-1 functions as a site of signal integration between the proapoptotic activity of JNK and the prosurvival activity of the AKT pathway that inhibits GSK3.Mcl-1 is an antiapoptotic member of the Bcl2 family. Gene knockout studies of mice demonstrate that Mcl-1 is essential for embryonic development and for the survival of hematopoietic cells (28-30). Studies of the stress response have demonstrated that Mcl-1 plays an important role in the sensitization of cells to apoptotic signals (1, 11, 25). Thus, exposure to UV radiation causes the rapid degradation of Mcl-1 and the release of proapoptotic partner proteins from Mcl-1 complexes (e.g., Bim). The mechanism of rapid Mcl-1 destruction is mediated by the combined actions of two different pathways. First, the exposure to stress causes phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2α) on the inhibitory site Ser-51 that prevents translation of Mcl-1 mRNA (1, 11, 25). Second, Mcl-1 is rapidly degraded by the ubiquitin-dependent proteasome pathway (27). Together, these pathways cause a rapid reduction in Mcl-1 expression. This loss of Mcl-1 may be a required initial response for the apoptosis of cells exposed to stress (25).The E3 ubiquitin protein ligase Mule/ARF-BP1 contains a BH3 domain that interacts with Mcl-1 and can initiate ubiquitin-dependent degradation of Mcl-1 (39). Recent studies have demonstrated that rapid stress-induced degradation of Mcl-1 is mediated by an alternative pathway involving the E3 ubiquitin protein ligase β-TrCP, which binds a stress-induced phosphodegron created by the phosphorylation of Mcl-1 by glycogen synthase kinase 3 (GSK3) (7, 21). How the exposure to stress causes GSK3-mediated phosphorylation of Mcl-1 is unclear, but GSK3 has been shown to directly phosphorylate Mcl-1 (7, 21). Mcl-1 phosphorylation and degradation may therefore be controlled by the prosurvival AKT pathway, which can negatively regulate GSK3 (7, 21).Mcl-1 is critically involved in the regulation of cell survival and is therefore subject to regulation by multiple mechanisms (26). Thus, Mcl-1 gene expression is regulated by many growth factors and cytokines (26), and Mcl-1 mRNA is regulated by microRNA pathways (24). The Mcl-1 protein is stabilized by binding TCTP (20) and the BH3-only protein Bim (4). In contrast, the BH3-only protein Noxa binds and destabilizes Mcl-1 (4, 36). Moreover, it is established that Mcl-1 is phosphorylated by several protein kinases on sites that may regulate Mcl-1 function. Phosphorylation of human Mcl-1 (hMcl-1) on Ser-64 (a site that is not conserved in other species) may enhance antiapoptotic activity by increasing the interaction of Mcl-1 with Bim, Noxa, and Bak (18). Phosphorylation on Ser-121 and Thr-163 may inhibit the antiapoptotic activity of hMcl-1 (15), and phosphorylation on Thr-163 may increase hMcl-1 protein stability (9). The conserved GSK3 phosphorylation site Ser-159 (and possibly Ser-155) can initiate rapid proteasomal degradation of hMcl-1 (7, 21). Together, these findings suggest that the function of Mcl-1 is very tightly regulated.The results of previous studies have implicated the c-Jun N-terminal protein kinase (JNK) in the regulation of Mcl-1 (15, 18). The purpose of this study was to test whether Mcl-1 is a target of signal transduction by JNK. We demonstrate that a key function of JNK is to prime Mcl-1 for phosphorylation by GSK3. JNK is required for GSK3-mediated degradation of Mcl-1 in response to stress. Coordinated regulation of the stress-activated JNK pathway and the AKT-inhibited GSK3 pathway is therefore required for stress-induced Mcl-1 degradation.     

NKG2D and its ligands

NKG2D is an activating immunoreceptor, first recognized on NK cells but subsequently found on γδ T cells, CD8⁺ αβ T cells and macrophages. In NK cells, inhibitory signals are generally dominate over activating signals. However, activating signals mediated through engagement of NKG2D by its ligands on target cells can bypass signals transmitted through inhibitory NK receptors, allowing NKG2D to function as a “master-switch” in determining the activation status of NK cells. NKG2D is important for T cell and NK cell-mediated immunity to viruses and tumours, and has roles in autoimmune disease, allogeneic transplantation, and xenotransplantation. Depending upon the situation, development of strategies to either block or to enhance the interactions between NKG2D and its ligands may have important implications for human health and disease.     

Engagement of TLR3, TLR7, and NKG2D regulate IFN-gamma secretion but not NKG2D-mediated cytotoxicity by human NK cells stimulated with suboptimal doses of IL-12

NK cells express different TLRs, such as TLR3, TLR7, and TLR9, but little is known about their role in NK cell stimulation. In this study, we used specific agonists (poly(I:C), loxoribine, and synthetic oligonucleotides containing unmethylated CpG sequences to stimulate human NK cells without or with suboptimal doses of IL-12, IL-15, or IFN-alpha, and investigated the secretion of IFN-gamma, cytotoxicity, and expression of the activating receptor NKG2D. Poly(I:C) and loxoribine, in conjunction with IL-12, but not IL-15, triggered secretion of IFN-gamma. Inhibition of IFN-gamma secretion by chloroquine suggested that internalization of the TLR agonists was necessary. Also, secretion of IFN-gamma was dependent on MEK1/ERK, p38 MAPK, p70(S6) kinase, and NF-kappaB, but not on calcineurin. IFN-alpha induced a similar effect, but promoted lesser IFN-gamma secretion. However, cytotoxicity (51Cr release assays) against MHC class I-chain related A (MICA)- and MICA+ tumor targets remained unchanged, as well as the expression of the NKG2D receptor. Excitingly, IFN-gamma secretion was significantly increased when NK cells were stimulated with poly(I:C) or loxoribine and IL-12, and NKG2D engagement was induced by coculture with MICA+ tumor cells in a PI3K-dependent manner. We conclude that resting NK cells secrete high levels of IFN-gamma in response to agonists of TLR3 or TLR7 and IL-12, and this effect can be further enhanced by costimulation through NKG2D. Hence, integration of the signaling cascades that involve TLR3, TLR7, IL-12, and NKG2D emerges as a critical step to promote IFN-gamma-dependent NK cell-mediated effector functions, which could be a strategy to promote Th1-biased immune responses in pathological situations such as cancer.     

P2X7 Integrates PI3K/AKT and AMPK-PRAS40-mTOR Signaling Pathways to Mediate Tumor Cell Death

Background

Extracellular adenosine triphosphate (ATP) functions as a novel danger signal that boosts antitumor immunity and can also directly kill tumor cells. We have previously reported that chronic exposure of tumor cells to ATP provokes P2X7-mediated tumor cell death, by as yet incompletely defined molecular mechanisms.

Methodology/Principal Findings

Here, we show that acute exposure of tumor cells to ATP results in rapid cytotoxic effects impacting several aspects of cell growth/survival, leading to inhibition of tumor growth in vitro and in vivo. Using agonist and antagonist studies together with generation of P2X7 deficient tumor cell lines by lentiviral shRNA delivery system, we confirm P2X7 to be the central control node transmitting extracellular ATP signals. We identify that downstream intracellular signaling regulatory networks implicate two signaling pathways: the known P2X7-PI3K/AKT axis and remarkably a novel P2X7-AMPK-PRAS40-mTOR axis. When exposed to high levels of extracellular ATP, these two signaling axes perturb the balance between growth and autophagy, thereby promoting tumor cell death.

Conclusions

Our study defines novel molecular mechanisms underpinning the antitumor actions of P2X7 and provides a further rationale for purine-based drugs in targeted cancer therapy.     

MIG-10 Functions with ABI-1 to Mediate the UNC-6 and SLT-1 Axon Guidance Signaling Pathways

Extracellular guidance cues steer axons towards their targets by eliciting morphological changes in the growth cone. A key part of this process is the asymmetric recruitment of the cytoplasmic scaffolding protein MIG-10 (lamellipodin). MIG-10 is thought to asymmetrically promote outgrowth by inducing actin polymerization. However, the mechanism that links MIG-10 to actin polymerization is not known. We have identified the actin regulatory protein ABI-1 as a partner for MIG-10 that can mediate its outgrowth-promoting activity. The SH3 domain of ABI-1 binds to MIG-10, and loss of function of either of these proteins causes similar axon guidance defects. Like MIG-10, ABI-1 functions in both the attractive UNC-6 (netrin) pathway and the repulsive SLT-1 (slit) pathway. Dosage sensitive genetic interactions indicate that MIG-10 functions with ABI-1 and WVE-1 to mediate axon guidance. Epistasis analysis reveals that ABI-1 and WVE-1 function downstream of MIG-10 to mediate its outgrowth-promoting activity. Moreover, experiments with cultured mammalian cells suggest that the interaction between MIG-10 and ABI-1 mediates a conserved mechanism that promotes formation of lamellipodia. Together, these observations suggest that MIG-10 interacts with ABI-1 and WVE-1 to mediate the UNC-6 and SLT-1 guidance pathways.     

白细胞衍生趋化因子2及其受体介导的信号通路与疾病

白细胞衍生趋化因子2(leukocyte cell-derived chemotaxin-2,LECT2)属于肽酶M23家族,是具有趋化作用的蛋白质。LECT2能趋化免疫细胞吞噬病原微生物,抑制癌细胞的迁移,对多种疾病如肝癌、败血症、动脉粥样硬化均有重要作用。为了深入了解LECT2在疾病中的作用机制,本文对LECT2基因和蛋白质的结构、与间质表皮转化因子(mesenchymal epithelial transition factor, MET)、C型凝集素等受体的识别机制,在β-联蛋白、Wnt等信号通路中的调节作用,以及与多种疾病的关系进行综述。     

NKG2D免疫受体及其配体的作用

受到感染或损伤的细胞通过中间受体将信号传导至机体的免疫系统,NKG2D即是这样一种典型的具有较高免疫原性的免疫受体,它的主要作用是传导受损伤细胞产生的信号,诱导机体产生免疫应答。该文总结了近期关于NKG2D和其配体的多样性,以及NKG2D和其配体在信号传导,刺激免疫细胞产生,肿瘤细胞的监督和疾病预防方面的新发现。     

mTORC2调节细胞骨架的信号通路

近几年来,关于哺乳动物雷帕霉素靶（mammalian target of rapamycin,mTOR）在各种哺乳动物细胞中调节肌动蛋白微丝极化及肌球蛋白微丝网形成的研究一直在不断地取得新的进展。尽管到目前为止,包括mTORC2上游和下游在内的相关的调控路径还未明确,但是因为mTORC6,的物学多样性,使其成为了当今生物学研究的焦点之一。基于长久以来特别是近五年对mTORC2的研究,在涉及细胞运动迁移、增殖分化、蛋白质合成、凋亡及自噬等生物学功能的研究中,一些重要的下游相关调控分子和蛋白相继被发现,比如P—Rexl／2、Rho家族GTPases、PKC、cAMP、p27kip1等。该综述着重总结了mTORC2实现这些生物学功能所可能通过的四条路径。当然,仍然需要大量的实验数据和研究证据进一步地证实和完善这些已经发现的可能存在的路径。     

Promiscuity and the single receptor: NKG2D

NKG2D (natural-killer group 2, member D) is a powerful activating receptor expressed by natural killer (NK) cells and T cells that regulates immune responses during infection, cancer and autoimmunity. NKG2D ligands comprise a diverse array of MHC-class-I-related proteins that are upregulated by cellular stress. Why is it beneficial for the host to have so many ligands for the same receptor? In this Opinion article, we propose that although competition with viruses is the most likely evolutionary drive for this diversity, there might be other explanations.     

NKG2D及其配体研究进展

NKG2D在NK细胞以及T细胞参与的免疫过程中占有重要的地位。本文介绍了NKG2D受体复合体的组成、结构、功能及表达调控;同时介绍了NKG2D配体的分类,病原体感染对其表达的诱导作用以及异常NKG2D配体的表型及功能,最后简要分析了NKG2D免疫途径在肿瘤免疫和治疗方面的应用前景。     

Human tumor-derived exosomes down-modulate NKG2D expression

NKG2D is an activating receptor for NK, NKT, CD8(+), and gammadelta(+) T cells, whose aberrant loss in cancer is a key mechanism of immune evasion. Soluble NKG2D ligands and growth factors, such as TGFbeta1 emanating from tumors, are mechanisms for down-regulating NKG2D expression. Cancers thereby impair the capacity of lymphocytes to recognize and destroy them. In this study, we show that exosomes derived from cancer cells express ligands for NKG2D and express TGFbeta1, and we investigate the impact of such exosomes on CD8(+) T and NK cell NKG2D expression and on NKG2D-dependent functions. Exosomes produced by various cancer cell lines in vitro, or isolated from pleural effusions of mesothelioma patients triggered down-regulation of surface NKG2D expression by NK cells and CD8(+) T cells. This decrease was rapid, sustained, and resulted from direct interactions between exosomes and NK cells or CD8(+) T cells. Other markers (CD4, CD8, CD56, CD16, CD94, or CD69) remained unchanged, indicating the selectivity and nonactivatory nature of the response. Exosomal NKG2D ligands were partially responsible for this effect, as down-modulation of NKG2D was slightly attenuated in the presence of MICA-specific Ab. In contrast, TGFbeta1-neutralizing Ab strongly abrogated NKG2D down-modulation, suggesting exosomally expressed TGFbeta as the principal mechanism. Lymphocyte effector function was impaired by pretreatment with tumor exosomes, as these cells exhibited poor NKG2D-dependent production of IFN-gamma and poor NKG2D-dependent killing function. This hyporesponsiveness was evident even in the presence of IL-15, a strong inducer of NKG2D. Our data show that NKG2D is a likely physiological target for exosome-mediated immune evasion in cancer.     

Toll-Like Receptor (TLR2 and TLR4) Polymorphisms and Chronic Obstructive Pulmonary Disease

Toll-like receptors (TLRs) participate in the defence against bacterial infections that are common in patients with Chronic Obstructive Pulmonary Disease (COPD). We studied all tagging SNPs in TLR2 and TLR4 and their associations with the level and change over time of both FEV(1) and sputum inflammatory cells in moderate-to-severe COPD. Nine TLR2 SNPs and 17 TLR4 SNPs were genotyped in 110 COPD patients. Associations of SNPs with lung function and inflammatory cells in induced sputum were analyzed cross-sectionally with linear regression and longitudinally with linear mixed-effect models. Two SNPs in TLR2 (rs1898830 and rs11938228) were associated with a lower level of FEV(1) and accelerated decline of FEV(1) and higher numbers of sputum inflammatory cells. None of the TLR4 SNPs was associated with FEV(1) level. Eleven out of 17 SNPs were associated with FEV(1) decline, including rs12377632 and rs10759931, which were additionally associated with higher numbers of sputum inflammatory cells at baseline and with increase over time. This is the first longitudinal study showing that tagging SNPs in TLR2 and TLR4 are associated with the level and decline of lung function as well as with inflammatory cell numbers in induced sputum in COPD patients, suggesting a role in the severity and progression of COPD.     

Roles of the NKG2D immunoreceptor and its ligands

According to present concepts, innate immunity is regulated by receptors that determine danger levels by responding to molecules that are associated with infection or cellular distress. NKG2D is, perhaps, the best characterized receptor that is associated with responses to cellular distress, defined as transformation, infection or cell stress. This review summarizes recent findings that concern NKG2D, its ligands, its signalling properties and its role in disease, and provides a framework for considering how the induction of immune responses can be regulated by cellular responses to injury.     

分选连接蛋白及其相关细胞信号通路

分选连接蛋白（sorting nexins,SNXs）是一类包含PX（phox homology）结构域的高度保守真核生物蛋白,其功能主要是参与负载蛋白的内吞、分选和降解过程,以维持细胞信号的稳态和平衡。SNXs参与调控与肿瘤等疾病相关的重要信号通路,如SNX3介导分泌型糖蛋白Wnt受体Wntless的胞内循环|SNX1、SNX5等众多SNXs介导表皮生长因子受体（epidermal growth factor receptor, EGFR）和转化因子β受体（TGF β）等的内吞、分选和降解等过程。其中,对EGFR降解的调控研究最多,尤其是在肿瘤方面的进展令人鼓舞,可也较为复杂,仍有许多未解之谜。随着SNXs的深入研究,将对疾病的发生机制产生新的认识。     

The NKG2D receptor: sensing stressed cells

The activating killer cell lectin-like receptor NKG2D plays a key role in the natural killer (NK) cell-mediated lysis of tumours and infected cells. Unlike other receptors, the ligands recognised by NKG2D are 'induced-self' ligands on stressed cells. This system requires precise regulation because inappropriate expression of NKG2D ligands might compromise NK cell activation. For therapeutic purposes it is essential to understand the mechanisms that regulate the expression and function of the NKG2D system. This review focuses on the importance of the signalling pathways involved in the regulation of the NKG2D receptor and its ligand expression in arming the immune response against infected or tumour cells and for the identification of new molecular targets and therapeutic strategies.