The Human Centrosomal Protein CCDC146 Binds Chlamydia trachomatis Inclusion Membrane Protein CT288 and Is Recruited to the Periphery of the Chlamydia-Containing Vacuole

Hijacking host cell vesicular transport: New insights into the nutrient acquisition mechanism of Chlamydia

Chlamydia infection is an important cause of public health diseases, and no effective vaccine is currently available. Owing to its unique intracellular lifestyle, Chlamydia requires a variety of nutrients and substrates from host cells, particularly sphingomyelin, cholesterol, iron, amino acids, and the mannose-6-phosphate receptor, which are essential for inclusion development. Here, we summarize the recent advances in Chlamydia nutrient acquisition mechanism by hijacking host cell vesicular transport, which plays an important role in chlamydial growth and development. Chlamydia obtains the components necessary to complete its intracellular developmental cycle by recruiting Rab proteins (major vesicular trafficking regulators) and Rab effector proteins to the inclusion, interfering with Rab-mediated multivesicular trafficking, reorienting the nutrition of host cells, and reconstructing the intracellular niche environment. Consequently, exploring the role of vesicular transport in nutrient acquisition offers a novel perspective on new approaches for preventing and treating Chlamydia infection.

Lack of CCDC146, a ubiquitous centriole and microtubule-associated protein, leads to non-syndromic male infertility in human and mouse

From a cohort of 167 infertile patients suffering from multiple morphological abnormalities of the flagellum (MMAF), pathogenic bi-allelic mutations were identified in the CCDC146 gene. In somatic cells, CCDC146 is located at the centrosome and at multiple microtubule-related organelles during mitotic division, suggesting that it is a microtubule-associated protein (MAP). To decipher the molecular pathogenesis of infertility associated with CCDC146 mutations, a Ccdc146 knock-out (KO) mouse line was created. KO male mice were infertile, and sperm exhibited a phenotype identical to CCDC146 mutated patients. CCDC146 expression starts during late spermiogenesis. In the spermatozoon, the protein is conserved but is not localized to centrioles, unlike in somatic cells, rather it is present in the axoneme at the level of microtubule doublets. Expansion microscopy associated with the use of the detergent sarkosyl to solubilize microtubule doublets suggests that the protein may be a microtubule inner protein (MIP). At the subcellular level, the absence of CCDC146 impacted all microtubule-based organelles such as the manchette, the head-tail coupling apparatus (HTCA), and the axoneme. Through this study, a new genetic cause of infertility and a new factor in the formation and/or structure of the sperm axoneme were characterized.

The emerging complexity of Chlamydia trachomatis interactions with host cells as revealed by molecular genetic approaches

Chlamydia trachomatis (Ct) is an intracellular bacterial pathogen that relies on the activity of secreted proteins known as effectors to promote replication and avoidance of immune clearance. Understanding the contribution of Ct effectors to pathogenesis has proven to be challenging, given that these proteins often perform multiple functions during intracellular infection. Recent advances in molecular genetic analysis of Ct have provided valuable insights into the multifaceted nature of secreted effector proteins and their impact on the interaction between Ct and host cells and tissues. This review highlights significant findings from genetic analysis of Ct effector functions, shedding light on their diverse roles. We also discuss the challenges faced in this field of study and explore potential opportunities for further research.

Recent advances in genetic systems in obligate intracellular human-pathogenic bacteria

The ability to genetically manipulate a pathogen is fundamental to discovering factors governing host-pathogen interactions at the molecular level and is critical for devising treatment and prevention strategies. While the genetic "toolbox" for many important bacterial pathogens is extensive, approaches for modifying obligate intracellular bacterial pathogens were classically limited due in part to the uniqueness of their obligatory lifestyles. Many researchers have confronted these challenges over the past two and a half decades leading to the development of multiple approaches to construct plasmid-bearing recombinant strains and chromosomal gene inactivation and deletion mutants, along with gene-silencing methods enabling the study of essential genes. This review will highlight seminal genetic achievements and recent developments (past 5 years) for Anaplasma spp., Rickettsia spp., Chlamydia spp., and Coxiella burnetii including progress being made for the still intractable Orientia tsutsugamushi. Alongside commentary of the strengths and weaknesses of the various approaches, future research directions will be discussed to include methods for C. burnetii that should have utility in the other obligate intracellular bacteria. Collectively, the future appears bright for unraveling the molecular pathogenic mechanisms of these significant pathogens.

The Chlamydia trachomatis type III-secreted effector protein CteG induces centrosome amplification through interactions with centrin-2

The centrosome is the main microtubule organizing center of the cell and is crucial for mitotic spindle assembly, chromosome segregation, and cell division. Centrosome duplication is tightly controlled, yet several pathogens, most notably oncogenic viruses, perturb this process leading to increased centrosome numbers. Infection by the obligate intracellular bacterium Chlamydia trachomatis (C.t.) correlates with blocked cytokinesis, supernumerary centrosomes, and multipolar spindles; however, the mechanisms behind how C.t. induces these cellular abnormalities remain largely unknown. Here we show that the secreted effector protein, CteG, binds to centrin-2 (CETN2), a key structural component of centrosomes and regulator of centriole duplication. Our data indicate that both CteG and CETN2 are necessary for infection-induced centrosome amplification, in a manner that requires the C-terminus of CteG. Strikingly, CteG is important for in vivo infection and growth in primary cervical cells but is dispensable for growth in immortalized cells, highlighting the importance of this effector protein to chlamydial infection. These findings begin to provide mechanistic insight into how C.t. induces cellular abnormalities during infection, but also indicate that obligate intracellular bacteria may contribute to cellular transformation events. Centrosome amplification mediated by CteG-CETN2 interactions may explain why chlamydial infection leads to an increased risk of cervical or ovarian cancer.

The Chlamydia trachomatis IncM Protein Interferes with Host Cell Cytokinesis, Centrosome Positioning, and Golgi Distribution and Contributes to the Stability of the Pathogen-Containing Vacuole

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes ocular and urogenital infections in humans. The ability of C. trachomatis to grow intracellularly in a pathogen-containing vacuole (known as an inclusion) depends on chlamydial effector proteins transported into the host cell by a type III secretion system. Among these effectors, several inclusion membrane proteins (Incs) insert in the vacuolar membrane. Here, we show that human cell lines infected by a C. trachomatis strain deficient for Inc CT288/CTL0540 (renamed IncM) displayed less multinucleation than when infected by IncM-producing strains (wild type or complemented). This indicated that IncM is involved in the ability of Chlamydia to inhibit host cell cytokinesis. The capacity of IncM to induce multinucleation in infected cells was shown to be conserved among its chlamydial homologues and appeared to require its two larger regions predicted to be exposed to the host cell cytosol. C. trachomatis-infected cells also displayed IncM-dependent defects in centrosome positioning, Golgi distribution around the inclusion, and morphology and stability of the inclusion. The altered morphology of inclusions containing IncM-deficient C. trachomatis was further affected by depolymerization of host cell microtubules. This was not observed after depolymerization of microfilaments, and inclusions containing wild-type C. trachomatis did not alter their morphology upon depolymerization of microtubules. Overall, these findings suggest that IncM may exert its effector function by acting directly or indirectly on host cell microtubules.

In Search of a Mechanistic Link between Chlamydia trachomatis-Induced Cellular Pathophysiology and Oncogenesis

Centrosome duplication and cell cycle progression are essential cellular processes that must be tightly controlled to ensure cellular integrity. Despite their complex regulatory mechanisms, microbial pathogens have evolved sophisticated strategies to co-opt these processes to promote infection. While misregulation of these processes can greatly benefit the pathogen, the consequences to the host cell can be devastating. During infection, the obligate intracellular pathogen Chlamydia trachomatis induces gross cellular abnormalities, including supernumerary centrosomes, multipolar spindles, and defects in cytokinesis. While these observations were made over 15 years ago, identification of the bacterial factors responsible has been elusive due to the genetic intractability of Chlamydia. Recent advances in techniques of genetic manipulation now allows for the direct linking of bacterial virulence factors to manipulation of centrosome duplication and cell cycle progression. In this review, we discuss the impact, both immediate and downstream, of C. trachomatis infection on the host cell cycle regulatory apparatus and centrosome replication. We highlight links between C. trachomatis infection and cervical and ovarian cancers and speculate whether perturbations of the cell cycle and centrosome are sufficient to initiate cellular transformation. We also explore the biological mechanisms employed by Inc proteins and other secreted effector proteins implicated in the perturbation of these host cell pathways. Future work is needed to better understand the nuances of each effector's mechanism and their collective impact on Chlamydia's ability to induce host cellular abnormalities.

Inclusion Membrane Growth and Composition Are Altered by Overexpression of Specific Inclusion Membrane Proteins in Chlamydia trachomatis L2

Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections. This obligate intracellular bacterium develops within a membrane-bound vacuole called an inclusion, which sequesters the chlamydiae from the host cytoplasm. Host-pathogen interactions at this interface are mediated by chlamydial inclusion membrane proteins (Incs). However, the specific functions of most Incs are poorly characterized. Previous work from our laboratories indicated that expressing an IncF fusion protein at high levels in C. trachomatis L2 negatively impacted inclusion expansion and progeny production. We hypothesize that some Incs function in the structure and organization of the inclusion membrane and that overexpression of those Incs will alter the composition of endogenous Incs within the inclusion membrane. Consequently, inclusion biogenesis and chlamydial development are negatively impacted. To investigate this, C. trachomatis L2 was transformed with inducible expression plasmids encoding IncF-, CT813-, or CT226-FLAG. Overexpression of IncF-FLAG or CT813-FLAG, but not CT226-FLAG, altered chlamydial development, as demonstrated by smaller inclusions, fewer progeny, and increased plasmid loss. The overexpression of CT813-FLAG reduced the detectable levels of endogenous IncE and IncG in the inclusion membrane. Notably, recruitment of sorting nexin-6, a eukaryotic protein binding partner of IncE, was also reduced after CT813 overexpression. Gene expression studies and ultrastructural analysis of chlamydial organisms demonstrated that chlamydial development was altered when CT813-FLAG was overexpressed. Overall, these data indicate that disrupting the expression of specific Incs changed the composition of Incs within the inclusion membrane and the recruitment of associated host cell proteins, which negatively impacted C. trachomatis development.

An endometrial organoid model of interactions between Chlamydia and epithelial and immune cells

Our understanding of how the obligate intracellular bacterial pathogen Chlamydia trachomatis reprograms the function of infected cells in the upper genital tract is largely based on observations made in cell culture with transformed epithelial cell lines. Here, we describe a primary organoid system derived from endometrial tissue to recapitulate epithelial cell diversity, polarity and ensuing responses to Chlamydia infection. Using high-resolution and time-lapse microscopy, we catalog the infection process in organoids from invasion to egress, including the reorganization of the cytoskeleton and positioning of intracellular organelles. We show this model is amenable to screening C. trachomatis mutants for defects in the fusion of pathogenic vacuoles, the recruitment of intracellular organelles and inhibition of cell death. Moreover, we reconstructed a primary immune cell response by co-culturing infected organoids with neutrophils, and determined that effectors like CPAF (also known as CT858) and TepP (also known as CT875) limit the recruitment of neutrophils to infected organoids. Collectively, our model can be applied to study the cell biology of Chlamydia infections in three-dimensional structures that better reflect the diversity of cell types and polarity encountered by Chlamydia in their animal hosts.

The Hypothetical Inclusion Membrane Protein CPSIT_0846 Regulates Mitochondrial-Mediated Host Cell Apoptosis via the ERK/JNK Signaling Pathway

Chlamydia psittaci is an important zoonotic factor associated with human and animal atypical pneumonia. Resisting host cell apoptosis is central to sustaining Chlamydia infection in vivo. Chlamydia can secrete inclusion membrane proteins (Incs) that play important roles in their development cycle and pathogenesis. CPSIT_0846 is an Inc protein in C. psittaci identified by our team in previous work. In the current study, we investigated the regulatory role of CPSIT_0846 in HeLa cell apoptosis, and explored potential mechanisms. The results showed that HeLa cells treated with CPSIT_0846 contained fewer apoptotic bodies and exhibited a lower apoptotic rate than untreated cells either with Hoechst 33258 fluorescence staining or flow cytometry with or without induction by staurosporine (STS). CPSIT_0846 could increase the phosphorylation of the extracellular signal-regulated kinases 1/2 (ERK1/2) or stress-activated protein kinases/c-Jun amino-terminal kinases (SAPK/JNK) signaling pathways, and the Bcl-2 associated X protein (Bax)/B cell lymphoma 2 (Bcl-2) ratio, levels of cleaved caspase-3/9 and cleaved Poly-ADP-ribose polymerase (PARP) were significantly up-regulated following inhibition of ERK1/2 or SAPK/JNK pathways with U0126 or SP600125. After carbonyl cyanide 3-chlorophenylhydrazone (CCCP) treatment, the mitochondrial membrane potential (MMP) of cells was significantly decreased in control group, but stable in the CPSIT_0846 treated one, and less cytochrome c (Cyt.c) was released into the cytoplasm. Inhibition of the ERK1/2 or SAPK/JNK pathway significantly decreased the JC-1 red-green fluorescence signal, and promoted Cyt.c discharge into the cytoplasm in HeLa cells treated with CPSIT_0846. In conclusion, CPSIT_0846 can regulate mitochondrial pathway-mediated apoptosis in HeLa cells by activating the ERK/JNK signaling pathway.

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.

Eukaryotic SNARE VAMP3 Dynamically Interacts with Multiple Chlamydial Inclusion Membrane Proteins

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs.

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs. As with all chlamydial proteins, Incs are temporally expressed, modifying the chlamydial inclusion during the early and mid-developmental cycle. VAMP3 and VAMP4 are eukaryotic SNARE proteins that mediate membrane fusion and are recruited to the inclusion to facilitate inclusion expansion. Their recruitment requires de novo chlamydial protein synthesis during the mid-developmental cycle. Thus, we hypothesize that VAMP3 and VAMP4 are recruited by Incs. In chlamydia-infected cells, identifying Inc binding partners for SNARE proteins specifically has been elusive. To date, most studies examining chlamydial Inc and eukaryotic proteins have benefitted from stable interacting partners or a robust interaction at a specific time postinfection. While these types of interactions are the predominant class that have been identified, they are likely the exception to chlamydia-host interactions. Therefore, we applied two separate but complementary experimental systems to identify candidate chlamydial Inc binding partners for VAMPs. Based on these results, we created transformed strains of C. trachomatis serovar L2 to inducibly express a candidate Inc-FLAG protein. In chlamydia-infected cells, we found that five Incs temporally and transiently interact with VAMP3. Further, loss of incA or ct813 expression altered VAMP3 localization to the inclusion. For the first time, our studies demonstrate the transient nature of certain host protein-Inc interactions that contribute to the chlamydial developmental cycle.

Host and Bacterial Glycolysis during Chlamydia trachomatis Infection

The obligate intracellular pathogen Chlamydia trachomatis is the leading cause of noncongenital blindness and causative agent of the most common sexually transmitted infection of bacterial origin. With a reduced genome, C. trachomatis is dependent on its host for survival, in part due to a need for the host cell to compensate for incomplete bacterial metabolic pathways. However, relatively little is known regarding how C. trachomatis is able to hijack host cell metabolism. In this study, we show that two host glycolytic enzymes, aldolase A and pyruvate kinase, as well as lactate dehydrogenase, are enriched at the C. trachomatis inclusion membrane during infection. Inclusion localization was not species specific, since a similar phenotype was observed with C. muridarum Time course experiments showed that the number of positive inclusions increased throughout the developmental cycle. In addition, these host enzymes colocalized to the same inclusion, and their localization did not appear to be dependent on sustained bacterial protein synthesis or on intact host actin, vesicular trafficking, or microtubules. Depletion of the host glycolytic enzyme aldolase A resulted in decreased inclusion size and infectious progeny production, indicating a role for host glycolysis in bacterial growth. Finally, quantitative PCR analysis showed that expression of C. trachomatis glycolytic enzymes inversely correlated with host enzyme localization at the inclusion. We discuss potential mechanisms leading to inclusion localization of host glycolytic enzymes and how it could benefit the bacteria. Altogether, our findings provide further insight into the intricate relationship between host and bacterial metabolism during Chlamydia infection.

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.

A predation assay using amoebae to screen for virulence factors unearthed the first W. chondrophila inclusion membrane protein

Waddlia chondrophila is an intracellular bacterium phylogenetically related to the well-studied human and animal pathogens of the Chlamydiaceae family. In the last decade, W. chondrophila was convincingly demonstrated to be associated with adverse pregnancy outcomes in humans and abortions in animals. All members of the phylum Chlamydiae possess a Type Three Secretion System that they use for delivering virulence proteins into the host cell cytosol to modulate their environment and create optimal conditions to complete their life cycle. To identify W. chondrophila virulence proteins, we used an original screening approach that combines a cosmid library with an assay monitoring resistance to predation by phagocytic amoebae. This technique combined with bioinformatic data allowed the identification of 28 candidate virulence proteins, including Wimp1, the first identified inclusion membrane protein of W. chondrophila.

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.

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.