Control of IFN-gamma-mediated host resistance to intracellular pathogens by immunity-related GTPases (p47 GTPases)

IRG proteins (also known as p47 GTPases) are key mediators of interferon-gamma-induced resistance to pathogens. Absence of certain IRG proteins leads to profound susceptibility to protozoa and bacteria in mice. Underlying their roles in host resistance, IRG proteins regulate the processing of pathogen-containing vacuoles in host cells, and regulate hematopoiesis following infection.     

Control of IFN-γ-mediated host resistance to intracellular pathogens by immunity-related GTPases (p47 GTPases)

IRG proteins (also known as p47 GTPases) are key mediators of interferon-γ-induced resistance to pathogens. Absence of certain IRG proteins leads to profound susceptibility to protozoa and bacteria in mice. Underlying their roles in host resistance, IRG proteins regulate the processing of pathogen-containing vacuoles in host cells, and regulate hematopoiesis following infection.     

干扰素诱导的GTP酶及其在抗感染先天免疫系统中的作用

干扰素（interferon,IFN）介导脊椎动物对微生物天然免疫反应的重要免疫分子。一般认为其效应主要是通过诱导ISG（interferon-stimulated gene）转录、表达生成相应效应蛋白来实现。其中GTPase家族是IFN诱导的蛋白质中最丰富的成员之一。目前研究较多的几个GTP酶（GTPase）有Mx蛋白（myxovirus-resistant,抗黏液病毒蛋白）、p47GTPase（immunity-related GTPase,IRG）、GBP（guanine bindin gprotein）、VLIG（very large inducible GTPase）。不同的脊椎动物中,GTPase家族成员的基因分布、蛋白质结构和功能及其机制等存在较大的差异。     

p47 GTPases: regulators of immunity to intracellular pathogens

Activation of the innate immune system by interferon-gamma (IFN-gamma is crucial for host resistance to infection. IFN-gamma induces the expression of a wide range of mediators that undermine the ability of pathogens to survive in host cells, including a newly discovered family of 47-kDa GTPases. Elimination of different p47 GTPases in mice by gene targeting severely cripples IFN-gamma-regulated defence against Toxoplasma gondii, Listeria monocytogenes, Mycobacterium spp. and other pathogens. In this article, we review our understanding of the role of p47 GTPases in resistance to intracellular infection and discuss the present evidence concerning their mode of action.     

GTPases in intracellular trafficking: an overview

Small GTPases that belong to the ras sub-families of Rab, Arf, and Rho, and the large GTPase dynamin, regulate intracellular trafficking. This issue of Seminars of Cell and Developmental Biology highlights topics regarding mechanisms by which these GTPases regulate the different steps of vesicular transport: vesicle formation, scission, targeting and fusion. In addition, the emerging roles of GTPases in coordination of individual transport steps as well as coordination of intracellular trafficking with other cellular processes are reviewed. Finally, common structures and mechanisms underlying the function of the ras-like GTPases and the importance of their function to human health and disease are discussed.     

Herbivore-induced resistance against microbial pathogens in Arabidopsis

Caterpillars of the herbivore Pieris rapae stimulate the production of jasmonic acid (JA) and ethylene (ET) in Arabidopsis (Arabidopsis thaliana) and trigger a defense response that affects insect performance on systemic tissues. To investigate the spectrum of effectiveness of P. rapae-induced resistance, we examined the level of resistance against different pathogens. Although the necrotrophic fungus Alternaria brassicicola is sensitive to JA-dependent defenses, herbivore-induced resistance was not effective against this pathogen. By contrast, caterpillar feeding significantly reduced disease caused by the bacterial pathogens Pseudomonas syringae pv tomato and Xanthomonas campestris pv armoraciae. However, this effect was apparent only locally in caterpillar-damaged tissue. Arabidopsis mutants jar1, coi1, ein2, sid2, eds5, and npr1 showed wild-type levels of P. rapae-induced protection against P. syringae pv tomato, suggesting that this local, herbivore-induced defense response does not depend exclusively on either JA, ET, or salicylic acid (SA). Resistance against the biotroph Turnip crinkle virus (TCV) requires SA, but not JA and ET. Nevertheless, herbivore feeding strongly affected TCV multiplication and TCV lesion formation, also in systemic tissues. Wounding alone was not effective, but application of P. rapae regurgitate onto the wounds induced a similar level of protection. Analysis of SA-induced PATHOGENESIS RELATED-1 (PR-1) expression revealed that P. rapae grazing primed Arabidopsis leaves for augmented expression of SA-dependent defenses. Pharmacological experiments showed that ET acts synergistically on SA-induced PR-1, suggesting that the increased production of ET upon herbivore feeding sensitizes the tissue to respond faster to SA, thereby contributing to an enhanced defensive capacity toward pathogens, such as TCV, that trigger SA-dependent defenses upon infection.     

Salicylic acid induced resistance in fruits to combat against postharvest pathogens: a review

Salicylic acid (SA), the plant hormone, is extensively involved in signalling pathway, primarily in defence to induced local and systemic resistances in fruits against postharvest pathogens. Exogenous application of SA on fruits at optimum or non-toxic concentration provided efficient control of decay caused by postharvest pathogens. SA has direct fungitoxic effect on pathogen growth. This review scrutinises the control of various postharvest diseases by the application of SA on different fruits. Furthermore, physiological mechanism involved in inducing resistance is also discussed. These findings are necessary for understanding the proper function of SA in harvested fruits. In future, there is a need to correlate the efficacy of SA with environmental conditions and should emphasise on molecular paths involved in signalling of induce resistance.     

The Chlamydia psittaci genome: a comparative analysis of intracellular pathogens

Background

Chlamydiaceae are a family of obligate intracellular pathogens causing a wide range of diseases in animals and humans, and facing unique evolutionary constraints not encountered by free-living prokaryotes. To investigate genomic aspects of infection, virulence and host preference we have sequenced Chlamydia psittaci, the pathogenic agent of ornithosis.

Results

A comparison of the genome of the avian Chlamydia psittaci isolate 6BC with the genomes of other chlamydial species, C. trachomatis, C. muridarum, C. pneumoniae, C. abortus, C. felis and C. caviae, revealed a high level of sequence conservation and synteny across taxa, with the major exception of the human pathogen C. trachomatis. Important differences manifest in the polymorphic membrane protein family specific for the Chlamydiae and in the highly variable chlamydial plasticity zone. We identified a number of psittaci-specific polymorphic membrane proteins of the G family that may be related to differences in host-range and/or virulence as compared to closely related Chlamydiaceae. We calculated non-synonymous to synonymous substitution rate ratios for pairs of orthologous genes to identify putative targets of adaptive evolution and predicted type III secreted effector proteins.

Conclusions

This study is the first detailed analysis of the Chlamydia psittaci genome sequence. It provides insights in the genome architecture of C. psittaci and proposes a number of novel candidate genes mostly of yet unknown function that may be important for pathogen-host interactions.     

A role for a menthone reductase in resistance against microbial pathogens in plants

Plants elaborate a vast array of enzymes that synthesize defensive secondary metabolites in response to pathogen attack. Here, we isolated the pathogen-responsive CaMNR1 [menthone: (+)-(3S)-neomenthol reductase] gene, a member of the short-chain dehydrogenase/reductase (SDR) superfamily, from pepper (Capsicum annuum) plants. Gas chromatography-mass spectrometry analysis revealed that purified CaMNR1 and its ortholog AtSDR1 from Arabidopsis (Arabidopsis thaliana) catalyze a menthone reduction with reduced nicotinamide adenine dinucleotide phosphate as a cofactor to produce neomenthol with antimicrobial activity. CaMNR1 and AtSDR1 also possess a significant catalytic activity for neomenthol oxidation. We examined the cellular function of the CaMNR1 gene by virus-induced gene silencing and ectopic overexpression in pepper and Arabidopsis plants, respectively. CaMNR1-silenced pepper plants were significantly more susceptible to Xanthomonas campestris pv vesicatoria and Colletotrichum coccodes infection and expressed lower levels of salicylic acid-responsive CaBPR1 and CaPR10 and jasmonic acid-responsive CaDEF1. CaMNR1-overexpressing Arabidopsis plants exhibited enhanced resistance to the hemibiotrophic pathogen Pseudomonas syringae pv tomato DC3000 and the biotrophic pathogen Hyaloperonospora parasitica isolate Noco2, accompanied by the induction of AtPR1 and AtPDF1.2. In contrast, mutation in the CaMNR1 ortholog AtSDR1 significantly enhanced susceptibility to both pathogens. Together, these results indicate that the novel menthone reductase gene CaMNR1 and its ortholog AtSDR1 positively regulate plant defenses against a broad spectrum of pathogens.     

Dictyostelium discoideum: a new host model system for intracellular pathogens of the genus Legionella

The soil amoeba Dictyostelium discoideum is a haploid eukaryote that, upon starvation, aggregates and enters a developmental cycle to produce fruiting bodies. In this study, we infected single-cell stages of D. discoideum with different Legionella species. Intracellular growth of Legionella in this new host system was compared with their growth in the natural host Acanthamoeba castellanii . Transmission electron microscopy of infected D. discoideum cells revealed that legionellae reside within the phagosome. Using confocal microscopy, it was observed that replicating, intracellular, green fluorescent protein (GFP)-tagged legionellae rarely co-localized with fluorescent antibodies directed against the lysosomal protein DdLIMP of D. discoideum . This indicates that the bacteria inhibit the fusion of phagosomes and lysosomes in this particular host system. In addition, Legionella infection of D. discoideum inhibited the differentiation of the host into the multicellular fruiting stage. Co-culture studies with profilin-minus D. discoideum mutants and Legionella resulted in higher rates of infection when compared with infections of wild-type amoebae. Because the amoebae are amenable to genetic manipulation as a result of their haploid genome and because a number of cellular markers are available, we show for the first time that D. discoideum is a valuable model system for studying intracellular pathogenesis of microbial pathogens.     

The P2X7 receptor and intracellular pathogens: a continuing struggle

The purinergic receptor, P2X₇, has recently emerged as an important component of the innate immune response against microbial infections. Ligation of P2X₇ by ATP can stimulate inflammasome activation and secretion of proinflammatory cytokines, but it can also lead directly to killing of intracellular pathogens in infected macrophages and epithelial cells. Thus, while some intracellular pathogens evade host defense responses by modulating with membrane trafficking or cell signaling in the infected cells, the host cells have also developed mechanisms for inhibiting infection. This review will focus on the effects of P2X₇ on control of infection by intracellular pathogens, microbial virulence factors that interfere with P2X₇ activity, and recent evidence linking polymorphisms in human P2X₇ with susceptibility to infection.     

The silent treatment: RNAi as a defense against virus infection in mammals

RNA interference (RNAi) is a mechanism for sequence-specific gene silencing guided by double-stranded RNA. In plants and insects it is well established that RNAi is instrumental in the response to viral infections; whether RNAi has a similar function in mammals is under intense investigation. Recent studies to address this question have identified some unanticipated interactions between the RNAi machinery and mammalian viruses. Furthermore, introduction of virus-specific small interfering RNAs (siRNAs) into cells, thus programming the RNAi machinery to target viruses, is an effective therapeutic approach to inhibit virus replication in vitro and in animal models. Although several issues remain to be addressed, such as delivery and viral escape, these findings hold tremendous potential for the development of RNAi-based antiviral therapeutics.     

Mechanisms and functions of guanylate‐binding proteins and related interferon‐inducible GTPases: Roles in intracellular lysis of pathogens

Guanylate‐binding proteins (GBPs) are a group interferon‐inducible GTPases within the constellation of the dynamin GTPase superfamily. These proteins restrict the replication of intracellular pathogens in both immune and non‐immune cells. GBPs and their related family members immunity‐related GTPases target and lyse the membrane of the pathogen‐containing vacuole, destroying the residential niche of vacuolar protozoal and bacterial pathogens. They also prevent virion infectivity and target replication complexes of ribonucleic acid viruses. The exciting concept that GBPs and immunity‐related GTPases can directly target the membrane of bacteria and protozoa has emerged. Rupture and lysis of the pathogen membrane mediates liberation of concealed microbial ligands for activation of innate immune sensing pathways and the inflammasome. Further studies have demonstrated a capacity of GBPs to recruit additional antimicrobial factors, highlighting the complexity of the molecular mechanisms involved in pathogen killing. In this mini‐review, we discuss recent advances describing the localisation and functions of GBPs on the host and pathogen membrane. We also highlight unresolved questions related to the regulation of GBPs in cell‐autonomous immunity to intracellular pathogens.     

NLRC5 regulates MHC class I antigen presentation in host defense against intracellular pathogens

NOD-like receptors (NLRs) are a family of intracellular proteins that play critical roles in innate immunity against microbial infection. NLRC5, the largest member of the NLR family, has recently attracted much attention. However, in vitro studies have reported inconsistent results about the roles of NLRC5 in host defense and in regulating immune signaling pathways. The in vivo function of NLRC5 remains unknown. Here, we report that NLRC5 is a critical regulator of host defense against intracellular pathogens in vivo. NLRC5 was specifically required for the expression of genes involved in MHC class I antigen presentation. NLRC5-deficient mice showed a profound defect in the expression of MHC class I genes and a concomitant failure to activate L. monocytogenes-specific CD8⁺ T cell responses, including activation, proliferation and cytotoxicity, and the mutant mice were more susceptible to the pathogen infection. NLRP3-mediated inflammasome activation was also partially impaired in NLRC5-deficient mice. However, NLRC5 was dispensable for pathogen-induced expression of NF-κB-dependent pro-inflammatory genes as well as type I interferon genes. Thus, NLRC5 critically regulates MHC class I antigen presentation to control intracellular pathogen infection.