The DUF582 Proteins of Chlamydia trachomatis Bind to Components of the ESCRT Machinery, Which Is Dispensable for Bacterial Growth In vitro

Identification of homologs of the Chlamydia trachomatis effector CteG reveals a family of Chlamydiaceae type III secreted proteins that can be delivered into host cells

Chlamydiae are a large group of obligate endosymbionts of eukaryotes that includes the Chlamydiaceae family, comprising several animal pathogens. Among Chlamydiaceae, Chlamydia trachomatis causes widespread ocular and urogenital infections in humans. Like many bacterial pathogens, all Chlamydiae manipulate host cells by injecting them with type III secretion effector proteins. We previously characterized the C. trachomatis effector CteG, which localizes at the host cell Golgi and plasma membrane during distinct phases of the chlamydial infectious cycle. Here, we show that CteG is a Chlamydiaceae-specific effector with over 60 homologs phylogenetically categorized into two distinct clades (CteG I and CteG II) and exhibiting several inparalogs and outparalogs. Notably, cteG I homologs are syntenic to C. trachomatis cteG, whereas cteG II homologs are syntenic among themselves but not with C. trachomatis cteG. This indicates a complex evolution of cteG homologs, which is unique among C. trachomatis effectors, marked by numerous events of gene duplication and loss. Despite relatively modest sequence conservation, nearly all tested CteG I and CteG II proteins were identified as type III secretion substrates using Yersinia as a heterologous bacterial host. Moreover, most of the type III secreted CteG I and CteG II homologs were delivered by C. trachomatis into host cells, where they localized at the Golgi region and cell periphery. Overall, this provided insights into the evolution of bacterial effectors and revealed a Chlamydiaceae family of type III secreted proteins that underwent substantial divergence during evolution while conserving the capacity to localize at specific host cell compartments.

The Fish Pathogen "Candidatus Clavichlamydia salmonicola"-A Missing Link in the Evolution of Chlamydial Pathogens of Humans

Chlamydiae like Chlamydia trachomatis and Chlamydia psittaci are well-known human and animal pathogens. Yet, the chlamydiae are a much larger group of evolutionary ancient obligate intracellular bacteria that includes predominantly symbionts of protists and diverse animals. This makes them ideal model organisms to study evolutionary transitions from symbionts in microbial eukaryotes to pathogens of humans. To this end, comparative genome analysis has served as an important tool. Genome sequence data for many chlamydial lineages are, however, still lacking, hampering our understanding of their evolutionary history. Here, we determined the first high-quality draft genome sequence of the fish pathogen "Candidatus Clavichlamydia salmonicola", representing a separate genus within the human and animal pathogenic Chlamydiaceae. The "Ca. Clavichlamydia salmonicola" genome harbors genes that so far have been exclusively found in Chlamydia species suggesting that basic mechanisms important for the interaction with chordate hosts have evolved stepwise in the history of chlamydiae. Thus, the genome sequence of "Ca. Clavichlamydia salmonicola" allows to constrain candidate genes to further understand the evolution of chlamydial virulence mechanisms required to infect mammals.

Extracellular Vesicles and Infection: From Hijacked Machinery to Therapeutic Tools

Extracellular vesicles (EVs) comprise a broad range of secreted cell-derived membrane vesicles. Beyond their more well-characterized role in cell communication, in recent years, EVs have also been shown to play important roles during infection. Viruses can hijack the biogenesis of exosomes (which are small EVs) to promote viral spreading. Additionally, these exosomes are also important mediators in inflammation and immune responses during both bacterial and viral infections. This review summarizes these mechanisms while also describing the impact of bacterial EVs in regulating immune responses. Finally, the review also focuses on the potential and challenges of using EVs, in particular, to tackle infectious diseases.

The Obligate Intracellular Bacterial Pathogen Anaplasma phagocytophilum Exploits Host Cell Multivesicular Body Biogenesis for Proliferation and Dissemination

Anaplasma phagocytophilum is the etiologic agent of the emerging infection, granulocytic anaplasmosis. This obligate intracellular bacterium lives in a host cell-derived vacuole that receives membrane traffic from multiple organelles to fuel its proliferation and from which it must ultimately exit to disseminate infection. Understanding of these essential pathogenic mechanisms has remained poor. Multivesicular bodies (MVBs) are late endosomal compartments that receive biomolecules from other organelles and encapsulate them into intralumenal vesicles (ILVs) using endosomal sorting complexes required for transport (ESCRT) machinery and ESCRT-independent machinery. Association of the ESCRT-independent protein, ALIX, directs MVBs to the plasma membrane where they release ILVs as exosomes. We report that the A. phagocytophilum vacuole (ApV) is acidified and enriched in lysobisphosphatidic acid, a lipid that is abundant in MVBs. ESCRT-0 and ESCRT-III components along with ALIX localize to the ApV membrane. siRNA-mediated inactivation of ESCRT-0 and ALIX together impairs A. phagocytophilum proliferation and infectious progeny production. RNA silencing of ESCRT-III, which regulates ILV scission, pronouncedly reduces ILV formation in ApVs and halts infection by arresting bacterial growth. Rab27a and its effector Munc13-4, which drive MVB trafficking to the plasma membrane and subsequent exosome release, localize to the ApV. Treatment with Nexinhib20, a small molecule inhibitor that specifically targets Rab27a to block MVB exocytosis, abrogates A. phagocytophilum infectious progeny release. Thus, A. phagocytophilum exploits MVB biogenesis and exosome release to benefit each major stage of its intracellular infection cycle: intravacuolar growth, conversion to the infectious form, and exit from the host cell. IMPORTANCE Anaplasma phagocytophilum causes granulocytic anaplasmosis, a globally emerging zoonosis that can be severe, even fatal, and for which antibiotic treatment options are limited. A. phagocytophilum lives in an endosomal-like compartment that interfaces with multiple organelles and from which it must ultimately exit to spread within the host. How the bacterium accomplishes these tasks is poorly understood. Multivesicular bodies (MVBs) are intermediates in the endolysosomal pathway that package biomolecular cargo from other organelles as intralumenal vesicles for release at the plasma membrane as exosomes. We discovered that A. phagocytophilum exploits MVB biogenesis and trafficking to benefit all aspects of its intracellular infection cycle: proliferation, conversion to its infectious form, and release of infectious progeny. The ability of a small molecule inhibitor of MVB exocytosis to impede A. phagocytophilum dissemination indicates the potential of this pathway as a novel host-directed therapeutic target for granulocytic anaplasmosis.

A literature review on correlation between HPV coinfection with C. trachomatis in cervical neoplasia - coinfection mediated cellular transformation

Cervical cancer is the fourth most common cause of mortality worldwide. Persistent infection with high-risk human papillomaviruses (hrHPV) is a known significant risk factor in cervical neoplasia development (CN). Though HPV contributes to carcinogenesis, other factors fabricate an ideal niche for HPV persistent especially, coinfection with Chlamydia trachomatis (CT) which has been linked to CN development. CT infection is associated with inflammation, cell proliferation, EMT transition and anti-apoptotic processes. To better understand the correlation between HPV-CT coinfection in CN development, a literature review was conducted on the prevalence of HPV-CT coinfection focusing on the role of infection-induced inflammation as HPV-CT coinfection creates an environment for cellular transformation, activates an innate immune response and triggers EMT transition. Moreover, inflammation plays a crucial role in developing neoplasia as there is a decrease in effector cells and a change in the expression of players like ROS and miRs. The CT infection induces chronic inflammation followed by cervical epithelial cell damage and increases susceptibility to HPV infection which may lead to cellular transformation. The literature search was performed based on a comprehensive investigation of publications in the PubMed journal database and Scopus development of CN. We have reviewed the prevalence of HPV-CT infection and the factors increasing the risk of developing CN.

Robust Heat Shock Response in Chlamydia Lacking a Typical Heat Shock Sigma Factor

Cells reprogram their transcriptome in response to stress, such as heat shock. In free-living bacteria, the transcriptomic reprogramming is mediated by increased DNA-binding activity of heat shock sigma factors and activation of genes normally repressed by heat-induced transcription factors. In this study, we performed transcriptomic analyses to investigate heat shock response in the obligate intracellular bacterium Chlamydia trachomatis, whose genome encodes only three sigma factors and a single heat-induced transcription factor. Nearly one-third of C. trachomatis genes showed statistically significant (≥1.5-fold) expression changes 30 min after shifting from 37 to 45°C. Notably, chromosomal genes encoding chaperones, energy metabolism enzymes, type III secretion proteins, as well as most plasmid-encoded genes, were differentially upregulated. In contrast, genes with functions in protein synthesis were disproportionately downregulated. These findings suggest that facilitating protein folding, increasing energy production, manipulating host activities, upregulating plasmid-encoded gene expression, and decreasing general protein synthesis helps facilitate C. trachomatis survival under stress. In addition to relieving negative regulation by the heat-inducible transcriptional repressor HrcA, heat shock upregulated the chlamydial primary sigma factor σ⁶⁶ and an alternative sigma factor σ²⁸. Interestingly, we show for the first time that heat shock downregulates the other alternative sigma factor σ⁵⁴ in a bacterium. Downregulation of σ⁵⁴ was accompanied by increased expression of the σ⁵⁴ RNA polymerase activator AtoC, thus suggesting a unique regulatory mechanism for reestablishing normal expression of select σ⁵⁴ target genes. Taken together, our findings reveal that C. trachomatis utilizes multiple novel survival strategies to cope with environmental stress and even to replicate. Future strategies that can specifically target and disrupt Chlamydia’s heat shock response will likely be of therapeutic value.

Endosomal sorting complexes required for transport-0 (ESCRT-0) are essential for fungal development, pathogenicity, autophagy and ER-phagy in Magnaporthe oryzae

Magnaporthe oryzae is an important plant pathogen that causes rice blast. Hse1 and Vps27 are components of ESCRT-0 involved in the multivesicular body (MVB) sorting pathway and biogenesis. To date, the biological functions of ESCRT-0 in M. oryzae have not been determined. In this study, we identified and characterized Hse1 and Vps27 in M. oryzae. Disruption of MoHse1 and MoVps27 caused pleiotropic defects in growth, conidiation, sexual development and pathogenicity, thereby resulting in loss of virulence in rice and barley leaves. Disruption of MoHse1 and MoVps27 triggered increased lipidation of MoAtg8 and degradation of GFP-MoAtg8, indicating that ESCRT-0 is involved in the regulation of autophagy. ESCRT-0 was determined to interact with coat protein complex II (COPII), a regulator functioning in homeostasis of the endoplasmic reticulum (ER homeostasis), and disruption of MoHse1 and MoVps27 also blocked activation of the unfolded protein response (UPR) and autophagy of the endoplasmic reticulum (ER-phagy). Overall, our results indicate that ESCRT-0 plays critical roles in regulating fungal development, virulence, autophagy and ER-phagy in M. oryzae.

Got mutants? How advances in chlamydial genetics have furthered the study of effector proteins

Chlamydia trachomatis is the leading cause of infectious blindness and a sexually transmitted infection. All chlamydiae are obligate intracellular bacteria that replicate within a membrane-bound vacuole termed the inclusion. From the confines of the inclusion, the bacteria must interact with many host organelles to acquire key nutrients necessary for replication, all while promoting host cell viability and subverting host defense mechanisms. To achieve these feats, C. trachomatis delivers an arsenal of virulence factors into the eukaryotic cell via a type 3 secretion system (T3SS) that facilitates invasion, manipulation of host vesicular trafficking, subversion of host defense mechanisms and promotes bacteria egress at the conclusion of the developmental cycle. A subset of these proteins intercalate into the inclusion and are thus referred to as inclusion membrane proteins. Whereas others, referred to as conventional T3SS effectors, are released into the host cell where they localize to various eukaryotic organelles or remain in the cytosol. Here, we discuss the functions of T3SS effector proteins with a focus on how advances in chlamydial genetics have facilitated the identification and molecular characterization of these important factors.

Sigma 54-Regulated Transcription Is Associated with Membrane Reorganization and Type III Secretion Effectors during Conversion to Infectious Forms of Chlamydia trachomatis

The factors that control the growth and infectious processes for Chlamydia are still poorly understood. This study used recently developed genetic tools to determine the regulon for one of the key transcription factors encoded by Chlamydia, sigma 54. Surrogate and computational analyses provide additional support for the hypothesis that sigma 54 plays a key role in controlling the expression of many components critical to converting and enabling the infectious capability of Chlamydia. These components include those that remodel the membrane for the extracellular environment and incorporation of an arsenal of type III secretion effectors in preparation for infecting new cells.

Chlamydia bacteria are obligate intracellular organisms with a phylum-defining biphasic developmental cycle that is intrinsically linked to its ability to cause disease. The progression of the chlamydial developmental cycle is regulated by the temporal expression of genes predominantly controlled by RNA polymerase sigma (σ) factors. Sigma 54 (σ⁵⁴) is one of three sigma factors encoded by Chlamydia for which the role and regulon are unknown. CtcC is part of a two-component signal transduction system that is requisite for σ⁵⁴ transcriptional activation. CtcC activation of σ⁵⁴ requires phosphorylation, which relieves inhibition by the CtcC regulatory domain and enables ATP hydrolysis by the ATPase domain. Prior studies with CtcC homologs in other organisms have shown that expression of the ATPase domain alone can activate σ⁵⁴ transcription. Biochemical analysis of CtcC ATPase domain supported the idea of ATP hydrolysis occurring in the absence of the regulatory domain, as well as the presence of an active-site residue essential for ATPase activity (E242). Using recently developed genetic approaches in Chlamydia to induce expression of the CtcC ATPase domain, a transcriptional profile was determined that is expected to reflect the σ⁵⁴ regulon. Computational evaluation revealed that the majority of the differentially expressed genes were preceded by highly conserved σ⁵⁴ promoter elements. Reporter gene analyses using these putative σ⁵⁴ promoters reinforced the accuracy of the model of the proposed regulon. Investigation of the gene products included in this regulon supports the idea that σ⁵⁴ controls expression of genes that are critical for conversion of Chlamydia from replicative reticulate bodies into infectious elementary bodies.

Role of bacterial infections in extracellular vesicles release and impact on immune response

Extracellular vesicle (EV) biology involves understanding the cellular and molecular mechanisms of cell communication. Studies conducted so far with various bacterial infection models demonstrate the release of various types of EVs that include exosomes and microvesicles. Depending upon the infection and cell type, EV cargo composition changes and ultimately might impact the host immune response and bacterial growth. The mechanisms behind the EVs release, cargo composition, and impact on the immune system have not been fully investigated. Future research needs to include in vivo models to understand the relevance of EVs in host immune function during bacterial infection, and to determine aspects that are shared or species-specific in the host. This would aid in the development of EVs as therapeutics or as markers of disease.

Chlamydia psittaci-Infected Dendritic Cells Communicate with NK Cells via Exosomes To Activate Antibacterial Immunity

Dendritic cells (DCs) and natural killer (NK) cells are critically involved in the early response against various bacterial microbes. Functional activation of infected DCs and NK cell-mediated gamma interferon (IFN-γ) secretion essentially contribute to the protective immunity against Chlamydia How DCs and NK cells cooperate during the antichlamydial response is not fully understood. Therefore, in the present study, we investigated the functional interplay between Chlamydia-infected DCs and NK cells. Our biochemical and cell biological experiments show that Chlamydia psittaci-infected DCs display enhanced exosome release. We find that such extracellular vesicles (referred to as dexosomes) do not contain infectious bacterial material but strongly induce IFN-γ production by NK cells. This directly affects C. psittaci growth in infected target cells. Furthermore, NK cell-released IFN-γ in cooperation with tumor necrosis factor alpha (TNF-α) and/or dexosomes augments apoptosis of both noninfected and infected epithelial cells. Thus, the combined effect of dexosomes and proinflammatory cytokines restricts C. psittaci growth and attenuates bacterial subversion of apoptotic host cell death. In conclusion, this provides new insights into the functional cooperation between DCs, dexosomes, and NK cells in the early steps of antichlamydial defense.

The multiple functions of the numerous Chlamydia trachomatis secreted proteins: the tip of the iceberg

Chlamydia trachomatis serovars are obligate intracellular bacterial pathogens mainly causing ocular and urogenital infections that affect millions of people worldwide and which can lead to blindness or sterility. They reside and multiply intracellularly within a membrane-bound vacuolar compartment, known as inclusion, and are characterized by a developmental cycle involving two morphologically and physiologically distinct chlamydial forms. Completion of the developmental cycle involves the secretion of > 70 C. trachomatis proteins that function in the host cell cytoplasm and nucleus, in the inclusion membrane and lumen, and in the extracellular milieu. These proteins can, for example, interfere with the host cell cytoskeleton, vesicular and non-vesicular transport, metabolism, and immune signalling. Generally, this promotes C. trachomatis invasion into, and escape from, host cells, the acquisition of nutrients by the chlamydiae, and evasion of cell-autonomous, humoral and cellular innate immunity. Here, we present an in-depth review on the current knowledge and outstanding questions about these C. trachomatis secreted proteins.

The Chlamydia trachomatis Extrusion Exit Mechanism Is Regulated by Host Abscission Proteins

The cellular exit strategies of intracellular pathogens have a direct impact on microbial dissemination, transmission, and engagement of immune responses of the host. Chlamydia exit their host via a budding mechanism called extrusion, which offers protective benefits to Chlamydia as they navigate their extracellular environment. Many intracellular pathogens co-opt cellular abscission machinery to facilitate cell exit, which is utilized to perform scission of two newly formed daughter cells following mitosis. Similar to viral budding exit strategies, we hypothesize that an abscission-like mechanism is required to physically sever the chlamydial extrusion from the host cell, co-opting the membrane fission activities of the endosomal sorting complex required for transport (ESCRT) family of proteins that are necessary for cellular scission events, including abscission. To test this, C. trachomatis L2-infected HeLa cells were depleted of key abscission machinery proteins charged multivesicle body protein 4b (CHMP4B), ALIX, centrosome protein 55 (CEP55), or vacuolar protein sorting-associated protein 4A (VPS4A), using RNA interference (RNAi). Over 50% reduction in extrusion formation was achieved by depletion of CHMP4B, VPS4A, and ALIX, but no effect on extrusion was observed with CEP55 depletion. These results demonstrate a role for abscission machinery in C. trachomatis extrusion from the host cell, with ALIX, VPS4A and CHMP4B playing key functional roles in optimal extrusion release.

CteG is a Chlamydia trachomatis effector protein that associates with the Golgi complex of infected host cells

Chlamydia trachomatis is a bacterial pathogen causing ocular and genital infections in humans. C. trachomatis multiplies exclusively inside host cells within a characteristic vacuole, from where it manipulates host cells by injecting them with type III secretion effector proteins. Here, we identified CteG as the first C. trachomatiseffector associated with the Golgi. For this, C. trachomatis strains expressing candidate effectors fused to a double hemagglutinin (2HA) tag were constructed. Then, among these strains, immunofluorescence microscopy revealed that CteG-2HA was delivered into the cytoplasm of infected cells. Between 16–20 h post-infection, CteG-2HA mostly associated with the Golgi; however, CteG-2HA also appeared at the host cell plasma membrane, and at 30 or 40 h post-infection this was its predominant localization. This change in the main localization of CteG-2HA was independent of intact microfilaments or microtubules. Ectopic expression of different regions of CteG (656 amino acid residues) in uninfected cells revealed that its first 100 residues contain a Golgi targeting region. Although a C. trachomatis cteG mutant did not display a defect in intracellular multiplication, CteG induced a vacuolar protein sorting defect when expressed in Saccharomyces cerevisiae. This suggested that CteG might function by subverting host cell vesicular transport.

Make It a Sweet Home: Responses of Chlamydia trachomatis to the Challenges of an Intravacuolar Lifestyle

Intravacuolar development has been adopted by several bacteria that grow inside a host cell. Remaining in a vacuole, as opposed to breaching the cytosol, protects the bacteria from some aspects of the cytosolic innate host defense and allows them to build an environment perfectly adapted to their needs. However, this raises new challenges: the host resources are separated from the bacteria by a lipid bilayer that is nonpermeable to most nutrients. In addition, the area of this lipid bilayer needs to expand to accommodate bacterial multiplication. This requires building material and energy that are not directly invested in bacterial growth. This article describes the strategies acquired by the obligate intracellular pathogen Chlamydia trachomatis to circumvent the difficulties raised by an intravacuolar lifestyle. We start with an overview of the origin and composition of the vacuolar membrane. Acquisition of host resources is largely, although not exclusively, mediated by interactions with membranous compartments of the eukaryotic cell, and we describe how the inclusion modifies the architecture of the cell and distribution of the neighboring compartments. The second part of this review describes the four mechanisms characterized so far by which the bacteria acquire resources from the host: (i) transport/diffusion across the vacuole membrane, (ii) fusion of this membrane with host compartments, (iii) direct transfer of lipids at membrane contact sites, and (iv) engulfment by the vacuole membrane of large cytoplasmic entities.

Subversion of the Endocytic and Secretory Pathways by Bacterial Effector Proteins

Intracellular bacteria have developed numerous strategies to hijack host vesicular trafficking pathways to form their unique replicative niches. To promote intracellular replication, the bacteria must interact with host organelles and modulate host signaling pathways to acquire nutrients and membrane for the growing parasitophorous vacuole all while suppressing activation of the immune response. To facilitate host cell subversion, bacterial pathogens use specialized secretion systems to deliver bacterial virulence factors, termed effectors, into the host cell that mimic, agonize, and/or antagonize the function of host proteins. In this review we will discuss how bacterial effector proteins from Coxiella burnetii, Brucella abortus, Salmonella enterica serovar Typhimurium, Legionella pneumophila, Chlamydia trachomatis, and Orientia tsutsugamushi manipulate the endocytic and secretory pathways. Understanding how bacterial effector proteins manipulate host processes not only gives us keen insight into bacterial pathogenesis, but also enhances our understanding of how eukaryotic membrane trafficking is regulated.

The Chlamydia trachomatis type III secretion substrates CT142, CT143, and CT144 are secreted into the lumen of the inclusion

Chlamydia trachomatis is a human bacterial pathogen causing ocular and genital infections. It multiplies exclusively within an intracellular membrane-bound vacuole, the inclusion, and uses a type III secretion system to manipulate host cells by injecting them with bacterially-encoded effector proteins. In this work, we characterized the expression and subcellular localization in infected host cells of the C. trachomatis CT142, CT143, and CT144 proteins, which we previously showed to be type III secretion substrates. Transcriptional analyses in C. trachomatis confirmed the prediction that ct142, ct143 and ct144 are organized in an operon and revealed that their expression is likely driven by the main σ factor, σ⁶⁶. In host cells infected by C. trachomatis, production of CT142 and CT143 could be detected by immunoblotting from 20–26 h post-infection. Immunofluorescence microscopy of infected cells revealed that from 20 h post-infection CT143 appeared mostly as globular structures outside of the bacterial cells but within the lumen of the inclusion. Furthermore, immunofluorescence microscopy of cells infected by C. trachomatis strains carrying plasmids producing CT142, CT143, or CT144 under the control of the ct142 promoter and with a C-terminal double hemagglutinin (2HA) epitope tag revealed that CT142-2HA, CT143-2HA or CT144-2HA showed an identical localization to chromosomally-encoded CT143. Moreover, CT142-2HA or CT144-2HA and CT143 produced by the same bacteria co-localized in the lumen of the inclusion. Overall, these data suggest that the CT142, CT143, and CT144 type III secretion substrates are secreted into the lumen of the inclusion where they might form a protein complex.