Co-evolution and exploitation of host cell signaling pathways by bacterial pathogens

Bacterial pathogens have evolved by combinations of gene acquisition, deletion, and modification, which increases their fitness. Additionally, bacteria are able to evolve in "quantum leaps" via the ability to promiscuously acquire new genes. Many bacterial pathogens - especially Gram-negative enteric pathogens - have evolved mechanisms by which to subvert signal transduction pathways of eukaryotic cells by expressing genes that mimic or regulate host protein factors involved in a variety of signaling cascades. This results in the ability to cause diseases ranging from tumor formation in plants to gastroenteritis and bubonic plague. Here, we present recent advances on mechanisms of bacterial pathogen evolution, including specific signaling cascades targeted by their virulence genes with an emphasis on the ubiquitin modification system, Rho GTPase regulators, cytoskeletal modulators, and host innate immunity. We also comment briefly on evolution of host defense mechanisms in place that limit disease caused by bacterial pathogens.     

Early signaling pathways activated by c-Kit in hematopoietic cells

c-Kit is a receptor tyrosine kinase that binds stem cell factor (SCF). Structurally, c-Kit contains five immunoglobulin-like domains extracellularly and a catalytic domain divided into two regions by a 77 amino acid insert intracellularly. Studies in white spotting and steel mice have shown that functional SCF and c-Kit are critical in the survival and development of stem cells involved in hematopoiesis, pigmentation and reproduction. Mutations in c-Kit are associated with a variety of human diseases. Interaction of SCF with c-Kit rapidly induces receptor dimerization and increases in autophosphorylation activity. Downstream of c-Kit, multiple signal transduction components are activated, including phosphatidylinositol-3-kinase, Src family members, the JAK/STAT pathway and the Ras-Raf-MAP kinase cascade. Structure-function studies have begun to address the role of these signaling components in SCF-mediated responses. This review will focus on the biochemical mechanism of action of SCF in hematopoietic cells.     

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

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Augmentation of innate host defenses against opportunistic fungal pathogens

Development of invasive fungal infection is the result of the complex interaction between fungal and host factors. The outcome of infection, once it has developed, depends upon appropriate use of antifungal therapy, surgical debridement as indicated, and improvement of host defenses. Thus, there have been major efforts for development of new strategies for immunomodulation and augmentation of host defenses in prevention and treatment of invasive mycoses. These modalities include granulocyte and granulocyte-macrophage colony-stimulating factors, interferon-γ, granulocyte transfusions, immunotherapy with infusion of dendritic cells and T cells, anti-heat shock protein 90 monoclonal antibodies, long pentraxin 3, mannose-binding lectin, and deferasirox. Although major strides in our understanding of augmentation of host response to invasive fungal infections are opening up novel avenues of therapy to harness patients’ innate immune systems against these frequently lethal pathogens, well-designed clinical trials are needed to demonstrate safety and efficacy of these new approaches.     

Control of pathogens and microbiota by innate lymphoid cells

Innate lymphoid cells (ILCs) are the innate counterpart of T cells. Upon infection or injury, ILCs react promptly to direct the developing immune response to the one most adapted to the threat facing the organism. Therefore, ILCs play an important role early in resistance to infection, but also to maintain homeostasis with the symbiotic microbiota following perturbations induced by diet and pathogens. Such roles of ILCs have been best characterized in the intestine and lung, mucosal sites that are exposed to the environment and are therefore colonized with diverse but specific types of microbes. Understanding the dialogue between pathogens, microbiota and ILCs may lead to new strategies to re-inforce immunity for prevention, vaccination and therapy.     

Mitotic lamin disassembly is triggered by lipid-mediated signaling

Disassembly of the nuclear lamina is a key step during open mitosis in higher eukaryotes. The activity of several kinases, including CDK1 (cyclin-dependent kinase 1) and protein kinase C (PKC), has been shown to trigger mitotic lamin disassembly, yet their precise contributions are unclear. In this study, we develop a quantitative imaging assay to study mitotic lamin B1 disassembly in living cells. We find that CDK1 and PKC act in concert to mediate phosphorylation-dependent lamin B1 disassembly during mitosis. Using ribonucleic acid interference (RNAi), we showed that diacylglycerol (DAG)-dependent PKCs triggered rate-limiting steps of lamin disassembly. RNAi-mediated depletion or chemical inhibition of lipins, enzymes that produce DAG, delayed lamin disassembly to a similar extent as does PKC inhibition/depletion. Furthermore, the delay of lamin B1 disassembly after lipin depletion could be rescued by the addition of DAG. These findings suggest that lipins activate a PKC-dependent pathway during mitotic lamin disassembly and provide evidence for a lipid-mediated mitotic signaling event.     

Mitochondria-derived reactive oxygen species negatively regulates immune innate signaling pathways triggered by a DNA virus, but not by an RNA virus

Reactive oxygen species (ROS) are crucial secondary messengers of signaling pathways. Redox-dependent signaling events have been previously described in the innate immune response. However, the mechanism by which ROS modulates anti-viral innate immune signaling is not fully clarified. Here, we report that mitochondria-derived ROS differentially regulate the innate response to DNA and RNA viruses (herpes simplex virus (HSV) and Sendai virus (SeV), respectively), with the cytokine response to HSV being negatively regulated by mitochondrial ROS. Importantly, specific activation of Toll-like receptors (TLRs) and DNA receptors (DNARs) but not retinoic acid inducible gene I (RIG-I)-like receptors (RLRs), led to signaling cascades that were inhibited by mitochondrial ROS production. Thus, localized mitochondrial ROS exerts negative modulation of innate immune responses to the DNA virus HSV-2 but not the RNA virus SeV.     

Functional comparison of innate immune signaling pathways in primates

Humans respond differently than other primates to a large number of infections. Differences in susceptibility to infectious agents between humans and other primates are probably due to inter-species differences in immune response to infection. Consistent with that notion, genes involved in immunity-related processes are strongly enriched among recent targets of positive selection in primates, suggesting that immune responses evolve rapidly, yet providing only indirect evidence for possible inter-species functional differences. To directly compare immune responses among primates, we stimulated primary monocytes from humans, chimpanzees, and rhesus macaques with lipopolysaccharide (LPS) and studied the ensuing time-course regulatory responses. We find that, while the universal Toll-like receptor response is mostly conserved across primates, the regulatory response associated with viral infections is often lineage-specific, probably reflecting rapid host-virus mutual adaptation cycles. Additionally, human-specific immune responses are enriched for genes involved in apoptosis, as well as for genes associated with cancer and with susceptibility to infectious diseases or immune-related disorders. Finally, we find that chimpanzee-specific immune signaling pathways are enriched for HIV-interacting genes. Put together, our observations lend strong support to the notion that lineage-specific immune responses may help explain known inter-species differences in susceptibility to infectious diseases.     

Autonomous rexinoid death signaling is suppressed by converging signaling pathways in immature leukemia cells.

On their own, retinoid X receptor (RXR)-selective ligands (rexinoids) are silent in retinoic acid receptor (RAR)-RXR heterodimers, and no selective rexinoid program has been described as yet in cellular systems. We report here on the rexinoid signaling capacity that triggers apoptosis of immature promyelocytic NB4 cells as a default pathway in the absence of survival factors. Rexinoid-induced apoptosis displays all features of bona fide programmed cell death and is inhibited by RXR, but not RAR antagonists. Several types of survival signals block rexinoid-induced apoptosis. RARalpha agonists switch the cellular response toward differentiation and induce the expression of antiapoptosis factors. Activation of the protein kinase A pathway in the presence of rexinoid agonists induces maturation and blocks immature cell apoptosis. Addition of nonretinoid serum factors also blocks cell death but does not induce cell differentiation. Rexinoid-induced apoptosis is linked to neither the presence nor stability of the promyelocytic leukemia-RARalpha fusion protein and operates also in non-acute promyelocytic leukemia cells. Together our results support a model according to which rexinoids activate in certain leukemia cells a default death pathway onto which several other signaling paradigms converge. This pathway is entirely distinct from that triggered by RAR agonists, which control cell maturation and postmaturation apoptosis.     

Caenorhabditis elegans innate immune response triggered by Salmonella enterica requires intact LPS and is mediated by a MAPK signaling pathway

Compared to mammals, insects, and plants, relatively little is known about innate immune responses in the nematode Caenorhabditis elegans. Previous work showed that Salmonella enterica serovars cause a persistent infection in the C. elegans intestine that triggers gonadal programmed cell death (PCD) and that C. elegans cell death (ced) mutants are more susceptible to Salmonella-mediated killing. To further dissect the role of PCD in C. elegans innate immunity, we identified both C. elegans and S. enterica factors that affect the elicitation of Salmonella-induced PCD. Salmonella-elicited PCD was shown to require the C. elegans homolog of the mammalian p38 mitogen-activated protein kinase (MAPK) encoded by the pmk-1 gene. Inactivation of pmk-1 by RNAi blocked Salmonella-elicited PCD, and epistasis analysis showed that CED-9 lies downstream of PMK-1. Wild-type Salmonella lipopolysaccharide (LPS) was also shown to be required for the elicitation of PCD, as well as for persistence of Salmonella in the C. elegans intestine. However, a presumptive C. elegans TOLL signaling pathway did not appear to be required for the PCD response to Salmonella. These results establish a PMK-1-dependant PCD pathway as a C. elegans innate immune response to Salmonella.     

Interaction of microbial pathogens with host exocytic pathways

Many microbial pathogens co‐opt or perturb host membrane trafficking pathways. This review covers recent examples in which microbes interact with host exocytosis, the fusion of intracellular vesicles with the plasma membrane. The bacterial pathogens Listeria monocytogenes and Staphylococcus aureus subvert recycling endosomal pathways of exocytosis in order to induce their entry into human cells. By contrast, entry of the protozoan pathogen Trypanosoma cruzi or the virus adenovirus into host cells involves exploitation of lysosomal exocytosis. Toxins produced by Bacillus anthracis or Vibrio cholerae interfere with exocytosis pathways mediated by the GTPase Rab11 and the exocyst complex. By doing so, anthrax or cholera toxins impair recycling of cadherins to cell–cell junctions and disrupt the barrier properties of endothelial cells or intestinal epithelial cells, respectively. Uropathogenic Escherichia coli (UPEC) is expelled from bladder epithelial cells through two different exocytic routes that involve sensing of bacteria in vacuoles by host Toll‐like receptor 4 (TLR4) or monitoring of the pH of lysosomes harbouring UPEC. The TLR4 pathway is mediated by multiple Rab GTPases and the exocyst, whereas the other pathway involves exocytosis of lysosomes. Expulsion of UPEC through these pathways is thought to benefit the host.     

造血干细胞发育过程中的信号通路调控

血液系统是维持机体生命活动最重要的系统之一,为机体提供所需的氧气和营养物质,通过物质交换维持内环境的稳态,同时为机体提供免疫防御与保护。血细胞是血液的重要组成成分,机体中成熟血细胞类型起源于具有自我更新及分化潜能的多能成体干细胞—造血干细胞(hematopoietic stem cells,HSCs)。造血干细胞及各类血细胞产生、发育及成熟的过程称为造血过程,该过程开始于胚胎发育早期并贯穿整个生命过程,任一阶段出现异常都可能导致血液疾病的发生。因此,深入探究造血发育过程及其调控机制对于认识并治疗血液疾病至关重要。近年来,以小鼠(Mus musculus)和斑马鱼(Danio rerio)作为动物模型来研究造血发育取得了一系列的进展。其中,BMP、Notch和Wnt等信号通路对造血干细胞的命运决定和产生发挥了重要作用。本文对这些信号通路在小鼠和斑马鱼造血过程中的调控作用进行系统总结,以期能够完善造血发育过程的调控网络并为临床应用提供指导。     

Exploitation of host signaling pathways by microbial quorum sensing signals

Environmental and commensal microbes that live within, on and around us have an enormous impact on human health. Recent progress in studies of prokaryotic interplay as well as host-bacteria interactions suggests that secreted microbial products, including quorum sensing signals (QSS), are important mediators of these intrakingdom and interkingdom relations. Reports have assigned QSS diverse and sometimes seemingly contradictory effects on mammalian cell physiology ranging from either blunting of the immune response or exerting pro-inflammatory activities to inducing cellular stress pathways and ultimately apoptosis. Thus, it is still unclear whether microbes utilize QSS to establish and maintain infections via modulation of host signaling pathways or if the eukaryotic host uses the conserved microbial QSS structures as molecular danger beacons to detect and fight infections. Along the same lines exactly how and under what circumstances QSS are detected by host cells remains a mystery, especially considering the distinct chemical properties of the QSS classes with some being small enough to passively diffuse across membranes while others most likely require extracellular recognition mechanisms.     

CRHSP-24 phosphorylation is regulated by multiple signaling pathways in pancreatic acinar cells

Ca2+-regulated heat-stable protein of 24 kDa (CRHSP-24) is a serine phosphoprotein originally identified as a physiological substrate for the Ca2+-calmodulin regulated protein phosphatase calcineurin (PP2B). CRHSP-24 is a paralog of the brain-specific mRNA-binding protein PIPPin and was recently shown to interact with the STYX/dead phosphatase protein in developing spermatids (Wishart MJ and Dixon JE. Proc Natl Acad Sci USA 99: 2112-2117, 2002). Investigation of the effects of phorbol ester (12-o-tetradecanoylphorbol-13-acetate; TPA) and cAMP analogs in 32P-labeled pancreatic acini revealed that these agents acutely dephosphorylated CRHSP-24 by a Ca2+-independent mechanism. Indeed, cAMP- and TPA-mediated dephosphorylation of CRHSP-24 was fully inhibited by the PP1/PP2A inhibitor calyculin A, indicating that the protein is regulated by an additional phosphatase other than PP2B. Supporting this, CRHSP-24 dephosphorylation in response to the Ca2+-mobilizing hormone cholecystokinin was differentially inhibited by calyculin A and the PP2B-selective inhibitor cyclosporin A. Stimulation of acini with secretin, a secretagogue that signals through the cAMP pathway in acini, induced CRHSP-24 dephosphorylation in a concentration-dependent manner. Isoelectric focusing and immunoblotting indicated that elevated cellular Ca2+ dephosphorylated CRHSP-24 on at least three serine sites, whereas cAMP and TPA partially dephosphorylated the protein on at least two sites. The cAMP-mediated dephosphorylation of CRHSP-24 was inhibited by low concentrations of okadaic acid (10 nM) and fostriecin (1 microM), suggesting that CRHSP-24 is regulated by PP2A or PP4. Collectively, these data indicate that CRHSP-24 is regulated by diverse and physiologically relevant signaling pathways in acinar cells, including Ca2+, cAMP, and diacylglycerol.     

Extracellular vesicles secreted by Giardia duodenalis regulate host cell innate immunity via TLR2 and NLRP3 inflammasome signaling pathways

Giardia duodenalis, also known as G. intestinalis or G. lamblia, is the major cause of giardiasis leading to diarrheal disease with 280 million people infections annually worldwide. Extracellular vesicles (EVs) have emerged as a ubiquitous mechanism participating in cells communications. The aim of this study is to explore the roles of G. duodenalis EVs (GEVs) in host-pathogen interactions using primary mouse peritoneal macrophages as a model. Multiple methods of electron microscopy, nanoparticle tracking analysis, proteomic assays, flow cytometry, immunofluorescence, qPCR, western blot, ELISA, inhibition assays, were used to characterize GEVs, and explore its effects on the host cell innate immunity as well as the underlying mechanism using primary mouse peritoneal macrophages. Results showed that GEVs displayed typical cup-shaped structure with 150 nm in diameter. GEVs could be captured by macrophages and triggered immune response by increasing the production of inflammatory cytokines Il1β, Il6, Il10, Il12, Il17, Ifng, Tnf, Il18, Ccl20 and Cxcl2. Furthermore, activation of TLR2 and NLRP3 inflammasome signaling pathways involved in this process. In addition, CA-074 methyl ester (an inhibitor of cathepsin B) or zVAD-fmk (an inhibitor of pan-caspase) pretreatment entirely diminished these effects triggered by GEVs exposure. Taken together, these findings demonstrated that GEVs could be internalized into mouse peritoneal macrophages and regulate host cell innate immunity via TLR2 and NLRP3 inflammasome signaling pathways.     

Denitrosylation of S-nitrosylated OGT is triggered in LPS-stimulated innate immune response

Rad is a member of a subclass of small GTP-binding proteins, the RGK family. In the present study we investigated the role of Rad protein in regulating cardiomyocyte viability. DNA fragmentation and TUNEL assays demonstrated that Rad promoted rat neonatal cardiomyocyte apoptosis. Rad silencing fully blocked serum deprivation induced apoptosis, indicating Rad is necessary for trigger cardiomyocyte apoptosis. Rad overexpression caused a dramatic decrease of the anti-apoptotic molecule Bcl-x_L, whereas Bcl-x_L overexpression protected cardiomyocytes against Rad-induced apoptosis. Rad-triggered apoptosis was mediated by the activation of p38 MAPK. The p38 blocker SB203580 effectively protected cardiomyocytes against Rad-evoked apoptosis.     

Tumor necrosis factor receptor (TNFR)-associated factor 5 is a critical intermediate of costimulatory signaling pathways triggered by glucocorticoid-induced TNFR in T cells

Tumor necrosis factor receptor (TNFR) family members such as glucocorticoid-induced TNFR (GITR) control T cell activation, differentiation, and effector functions. Importantly, GITR functions as a pivotal regulator of physiologic and pathologic immune responses by abrogating the suppressive effects of T regulatory cells and costimulating T effector cells. However, the molecular mechanisms underlying GITR-triggered signal transduction pathways remain unclear. Interestingly, GITR-induced stimulation of TNFR-associated factor (TRAF) 5-deficient T cells resulted in decreased activation of nuclear factor kappaB as well as the mitogen-activated protein kinases p38 and extracellular signal-regulated protein kinase, whereas activation of c-Jun N-terminal kinase was less affected. Consistent with impaired signaling, costimulatory effects of GITR were diminished in TRAF5-/- T cells. In sum, our studies indicate that TRAF5 plays a crucial role in GITR-induced signaling pathways that augment T cell activation.