Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

IgG exacerbates genital chlamydial pathology in females by enhancing pathogenic CD8+ T cell responses

Chlamydia trachomatis infections are an important sexually transmitted infection that can lead to inflammation, scarring and hydrosalpinx/infertility. However, infections are commonly clinically asymptomatic and do not receive treatment. The underlying cause of asymptomatic immunopathology remains unknown. Here, we demonstrate that IgG produced during male infection enhanced the incidence of immunopathology and infertility in females. Human endocervical cells expressing the neonatal Fc Receptor (FcRn) increased translocation of human IgG-opsonized C. trachomatis. Using total IgG purified from infected male mice, we opsonized C. muridarum and then infected female mice, mimicking sexual transmission. Following infection, IgG-opsonized Chlamydia was found to transcytose the epithelial barrier in the uterus, where it was phagocytosed by antigen-presenting cells (APCs) and trafficked to the draining lymph nodes. APCs then expanded both CD4+ and CD8+ T cell populations and caused significantly more infertility in female mice infected with non-opsonized Chlamydia. Enhanced phagocytosis of IgG-opsonized Chlamydia significantly increased pro-inflammatory signalling and T cell proliferation. As IgG is transcytosed by FcRn, we utilized FcRn-/- mice and observed that shedding kinetics of Chlamydia were only affected in FcRn-/- mice infected with IgG-opsonized Chlamydia. Depletion of CD8+ T cells in FcRn-/- mice lead to a significant reduction in the incidence of infertility. Taken together, these data demonstrate that IgG seroconversion during male infection can amplify female immunopathology, dependent on FcRn transcytosis, APC differentiation and enhanced CD8 T cell responses.

Chlamydial YAP activation in host endocervical epithelial cells mediates pro-fibrotic paracrine stimulation of fibroblasts

IMPORTANCE

Chronic or repeated infection of the female upper genital tract by C. trachomatis can lead to severe fibrotic sequelae, including tubal factor infertility and ectopic pregnancy. However, the molecular mechanisms underlying this effect are unclear. In this report, we define a transcriptional program specific to C. trachomatis infection of the upper genital tract, identifying tissue-specific induction of host YAP-a pro-fibrotic transcriptional cofactor-as a potential driver of infection-mediated fibrotic gene expression. Furthermore, we show that infected endocervical epithelial cells stimulate collagen production by fibroblasts and implicate chlamydial induction of YAP in this effect. Our results define a mechanism by which infection mediates tissue-level fibrotic pathology via paracrine signaling and identify YAP as a potential therapeutic target for the prevention of Chlamydia-associated scarring of the female genital tract.

Identification of the alternative sigma factor regulons of Chlamydia trachomatis using multiplexed CRISPR interference

Chlamydia trachomatis is a developmentally regulated, obligate intracellular bacterium that encodes three sigma factors: σ66, σ54, and σ28. σ66 is the major sigma factor controlling most transcription initiation during early- and mid-cycle development as the infectious elementary body (EB) transitions to the non-infectious reticulate body (RB) that replicates within an inclusion inside the cell. The roles of the minor sigma factors, σ54 and σ28, have not been well characterized to date; however, there are data to suggest each functions in late-stage development and secondary differentiation as RBs transition to EBs. As the process of secondary differentiation itself is poorly characterized, clarifying the function of these alternative sigma factors by identifying the genes regulated by them will further our understanding of chlamydial differentiation. We hypothesize that σ54 and σ28 have non-redundant and essential functions for initiating late gene transcription thus mediating secondary differentiation in Chlamydia. Here, we demonstrate the necessity of each minor sigma factor in successfully completing the developmental cycle. We have implemented and validated multiplexed Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) interference techniques, novel to the chlamydial field to examine the effects of knocking down each alternative sigma factor individually and simultaneously. In parallel, we also overexpressed each sigma factor. Altering transcript levels for either or both alternative sigma factors resulted in a severe defect in EB production as compared to controls. Furthermore, RNA sequencing identified differentially expressed genes during alternative sigma factor dysregulation, indicating the putative regulons of each. These data demonstrate that the levels of alternative sigma factors must be carefully regulated to facilitate chlamydial growth and differentiation. IMPORTANCE Chlamydia trachomatis is a significant human pathogen in both developed and developing nations. Due to the organism's unique developmental cycle and intracellular niche, basic research has been slow and arduous. However, recent advances in chlamydial genetics have allowed the field to make significant progress in experimentally interrogating the basic physiology of Chlamydia. Broadly speaking, the driving factors of chlamydial development are poorly understood, particularly regarding how the later stages of development are regulated. Here, we employ a novel genetic tool for use in Chlamydia while investigating the effects of dysregulating the two alternative sigma factors in the organism that help control transcription initiation. We provide further evidence for both sigma factors' essential roles in late-stage development and their potential regulons, laying the foundation for deeper experimentation to uncover the molecular pathways involved in chlamydial differentiation.

Roles of host SUMOylation in bacterial pathogenesis

Bacteria frequently interfere with the post-translational modifications of host cells to facilitate their survival and growth after invasion. SUMOylation, a reversible post-translational modification process, plays an important role in biological life activities. In addition to being critical to host cell metabolism and survival, SUMOylation also regulates gene expression and cell signal transmission. Moreover, SUMOylation in eukaryotic cells can be used by a variety of bacterial pathogens to advance bacterial invasion. In this minireview, we focused on the role and mechanism of host SUMOylation in the pathogenesis of six important clinical bacterial pathogens (Listeria monocytogenes, Shigella flexneri, Salmonella Typhimurium, Klebsiella pneumoniae, Staphylococcus aureus, and Escherichia coli). Taken together, this review provided new insights for understanding the unique pathogen-host interaction based on host SUMOylation and provided a novel perspective on the development of new strategies to combat bacterial infections in the future.

Dynamin-dependent entry of Chlamydia trachomatis is sequentially regulated by the effectors TarP and TmeA

Chlamydia invasion of epithelial cells is a pathogen-driven process involving two functionally distinct effectors - TarP and TmeA. They collaborate to promote robust actin dynamics at sites of entry. Here, we extend studies on the molecular mechanism of invasion by implicating the host GTPase dynamin 2 (Dyn2) in the completion of pathogen uptake. Importantly, Dyn2 function is modulated by TarP and TmeA at the levels of recruitment and activation through oligomerization, respectively. TarP-dependent recruitment requires phosphatidylinositol 3-kinase and the small GTPase Rac1, while TmeA has a post-recruitment role related to Dyn2 oligomerization. This is based on the rescue of invasion duration and efficiency in the absence of TmeA by the Dyn2 oligomer-stabilizing small molecule activator Ryngo 1-23. Notably, Dyn2 also regulated turnover of TarP- and TmeA-associated actin networks, with disrupted Dyn2 function resulting in aberrant turnover dynamics, thus establishing the interdependent functional relationship between Dyn2 and the effectors TarP and TmeA.

Type III Secretion in Chlamydia

Type III secretion systems (T3SSs) are utilized by Gram-negative pathogens to enhance their pathogenesis. This secretion system is associated with the delivery of effectors through a needle-like structure from the bacterial cytosol directly into a target eukaryotic cell. These effector proteins then manipulate specific eukaryotic cell functions to benefit pathogen survival within the host. The obligate intracellular pathogens of the family Chlamydiaceae have a highly evolutionarily conserved nonflagellar T3SS that is an absolute requirement for their survival and propagation within the host with about one-seventh of the genome dedicated to genes associated with the T3SS apparatus, chaperones, and effectors. Chlamydiae also have a unique biphasic developmental cycle where the organism alternates between an infectious elementary body (EB) and replicative reticulate body (RB). T3SS structures have been visualized on both EBs and RBs. And there are effector proteins that function at each stage of the chlamydial developmental cycle, including entry and egress. This review will discuss the history of the discovery of chlamydial T3SS and the biochemical characterization of components of the T3SS apparatus and associated chaperones in the absence of chlamydial genetic tools. These data will be contextualized into how the T3SS apparatus functions throughout the chlamydial developmental cycle and the utility of heterologous/surrogate models to study chlamydial T3SS. Finally, there will be a targeted discussion on the history of chlamydial effectors and recent advances in the field.

When Bacteria and Viruses Collide: A Tale of Chlamydia trachomatis and Sexually Transmitted Viruses

The global incidence of sexually transmitted infections (STIs) remains high, with the World Health Organization (WHO) estimating that over 1 million people acquire STIs daily. STIs can lead to infertility, pregnancy complications, and cancers. Co-infections with multiple pathogens are prevalent among individuals with an STI and can lead to heightened infectivity and more severe clinical manifestations. Chlamydia trachomatis (CT) is the most reported bacterial STI worldwide in both men and women, and several studies have demonstrated co-infection of CT with viral and other bacterial STIs. CT is a gram-negative bacterium with a unique biphasic developmental cycle including infectious extracellular elementary bodies (EBs) and metabolically active intracellular reticulate bodies (RBs). The intracellular form of this organism, RBs, has evolved mechanisms to persist for long periods within host epithelial cells in a viable but non-cultivable state. The co-infections of CT with the most frequently reported sexually transmitted viruses: human immunodeficiency virus (HIV), human papillomavirus (HPV), and herpes simplex virus (HSV) have been investigated through in vitro and in vivo studies. These research studies have made significant strides in unraveling the intricate interactions between CT, these viral STIs, and their eukaryotic host. In this review, we present an overview of the epidemiology of these co-infections, while specifically delineating the underlying mechanisms by which CT influences the transmission and infection dynamics of HIV and HSV. Furthermore, we explore the intricate relationship between CT and HPV infection, with a particular emphasis on the heightened risk of cervical cancer. By consolidating the current body of knowledge, we provide valuable insights into the complex dynamics and implications of co-infection involving CT and sexually transmitted viruses.

K. pneumoniae and M. smegmatis infect epithelial cells via different strategies

Background

As the first line of defense, epithelial cells play a vital role in the initiation and control of both innate and adaptive immunity, which participate in the development of disease. Despite its therapeutic significance, little is understood about the specific interaction between pathogenic microorganisms and lung epithelial cells.

Methods

In this study, we performed a head-to-head comparison of the virulence and infection mechanisms of Klebsiella pneumoniae (K. pneumoniae) and Mycobacterium smegmatis (M. smegmatis), which represent Gram-negative/positive respiratory pathogens, respectively, in lung epithelial cell models for the first time.

Results

Through scanning electron microscopy combined with bacterial infection experiments, we confirmed the ability of K. pneumoniae and M. smegmatis strains to form biofilm and cord factor out of the cell wall. M. smegmatis has stronger adhesion and intracellular retention ability, while K. pneumoniae is more likely to induce acute infection. These pathogens could stay and proliferate in lung epithelial cells and stimulate the secretion of specific cytokines and chemokines through a gene transcription regulator. M. smegmatis infection can promote crosstalk among epithelial cells and other immune cells in the lung from a very early stage by prompting the secretion of pro-inflammatory cytokines. Meanwhile, there were significant correlations between K. pneumonia infection and higher levels of interleukin-15 (IL-15), interleukin-1Rα (IL-1Rα), fibroblast growth factor (FGF) basic, and granulocyte colony-stimulating factor (G-CSF). At the same time, K. pneumonia infection also led to changes in the expression of cytoskeletal proteins in epithelial cells.

Conclusions

Our results emphasized the immunoprotection and immunomodulation of lung epithelial cells against exogenous pathogenic microorganisms, indicating that different pathogens damaged the host through different strategies and induced varying innate immune responses. At the same time, they provided important clues and key immune factors for dealing with complicated pulmonary infections.

Chlamydia trachomatis TmeB antagonizes actin polymerization via direct interference with Arp2/3 activity

Chlamydia trachomatis is an obligate intracellular pathogen that actively promotes invasion of epithelial cells. A virulence-associated type III secretion system contributes to chlamydial entry and at least four effectors have been described that are deployed during this time. Two of these invasion-related effectors, the translocated membrane-associated effectors A and B (TmeA and TmeB), are encoded in a bi-cistronic operon. TmeA directly activates host N-WASP to stimulate Arp2/3-dependent actin polymerization. According to current working models, TmeA-mediated N-WASP activation contributes to invasion. TmeB has not been functionally characterized. Unlike a tmeA null strain, loss of tmeB does not impact invasion efficiency of C. trachomatis. Using strains deficient for multiple genes, we provide evidence that TmeA is dispensable for invasion in the absence of TmeB. Our data indicate that overabundance of TmeB interferes with invasion and that this activity requires active Arp2/3 complex. We further show that TmeB is capable of interfering with Arp2/3-mediated actin polymerization. In aggregate, these data point to opposing functions for TmeA and TmeB that manifest during the invasion process. These studies raise intriguing questions regarding the dynamic interplay between TmeA, TmeB, and branched actin polymerization during chlamydial entry.

Chlamydial YAP activation in host endocervical epithelial cells mediates pro-fibrotic paracrine stimulation of fibroblasts

Infection of the female genital tract by Chlamydia trachomatis can produce severe fibrotic sequelae, including tubal factor infertility and ectopic pregnancy. While infection demonstrably mediates a pro-fibrotic response in host cells, it remains unclear if intrinsic properties of the upper genital tract exacerbate chlamydial fibrosis. The relatively sterile environment of the upper genital tract is primed for a pro-inflammatory response to infection, potentially enhancing fibrosis - however, subclinical C. trachomatis infections still develop fibrosis-related sequelae. Here, we compare infection-associated and steady-state gene expression of primary human cervical and vaginal epithelial cells. In the former, we observe enhanced baseline expression and infection-mediated induction of fibrosis-associated signal factors (e.g. TGFA , IL6 , IL8 , IL20 ), implying predisposition to Chlamydia -associated pro-fibrotic signaling. Transcription factor enrichment analysis identified regulatory targets of YAP, a transcriptional cofactor induced by infection of cervical epithelial cells, but not vaginal epithelial cells. YAP target genes induced by infection include secreted fibroblast-activating signal factors; therefore, we developed an in vitro model involving coculture of infected endocervical epithelial cells with uninfected fibroblasts. Coculture enhanced fibroblast expression of type I collagen, as well as prompting reproducible (albeit statistically insignificant) induction of α-smooth muscle actin. Fibroblast collagen induction was sensitive to siRNA-mediated YAP knockdown in infected epithelial cells, implicating chlamydial YAP activation in this effect. Collectively, our results present a novel mechanism of fibrosis initiated by Chlamydia, wherein infection-mediated induction of host YAP facilitates pro-fibrotic intercellular communication. Chlamydial YAP activation in cervical epithelial cells is thus a determinant of this tissue's susceptibility to fibrosis.

Importance

Chronic or repeated infection of the female upper genital tract by C. trachomatis can lead to severe fibrotic sequelae, including tubal factor infertility and ectopic pregnancy. However, the molecular mechanisms underlying this effect are unclear. In this report, we define a transcriptional program specific to C. trachomatis infection of the upper genital tract, identifying tissue-specific induction of host YAP - a pro-fibrotic transcriptional cofactor - as a potential driver of infection-mediated fibrotic gene expression. Further, we show that infected endocervical epithelial cells stimulate collagen production by fibroblasts, and implicate chlamydial induction of YAP in this effect. Our results define a mechanism by which infection mediates tissue-level fibrotic pathology via paracrine signaling, and identify YAP as a potential therapeutic target for prevention of Chlamydia -associated scarring of the female genital tract.

Distinct roles of the Chlamydia trachomatis effectors TarP and TmeA in the regulation of formin and Arp2/3 during entry

The obligate intracellular pathogen Chlamydia trachomatis manipulates the host actin cytoskeleton to assemble actin-rich structures that drive pathogen entry. The recent discovery of TmeA, which, like TarP, is an invasion-associated type III effector implicated in actin remodeling, raised questions regarding the nature of their functional interaction. Quantitative live-cell imaging of actin remodeling at invasion sites revealed differences in recruitment and turnover kinetics associated with the TarP and TmeA pathways, with the former accounting for most of the robust actin dynamics at invasion sites. TarP-mediated recruitment of actin nucleators, i.e. formins and the Arp2/3 complex, was crucial for rapid actin kinetics, generating a collaborative positive feedback loop that enhanced their respective actin-nucleating activities within invasion sites. In contrast, the formin Fmn1 was not recruited to invasion sites and did not collaborate with Arp2/3 within the context of TmeA-associated actin recruitment. Although the TarP-Fmn1-Arp2/3 signaling axis is responsible for the majority of actin dynamics, its inhibition had similar effects as the deletion of TmeA on invasion efficiency, consistent with the proposed model that TarP and TmeA act on different stages of the same invasion pathway.

Evaluation of Selected Bacteria and Yeast for Probiotic Potential in Poultry Production

Performance and efficiency of feed utilization in poultry is highly influenced by gut health, which is dependent on intestinal microbial balance. Probiotics are live microbial feed supplements or viable microorganisms that beneficially affect the host animal by improving its gastrointestinal tract (GIT) microbial balance. However, their mode of action and suitable GIT environment favoring their colonization of the GIT is obscure. The probiotic properties of Lactobacillus plantarum, Bifidobacterium longum, and Saccharomyces boulardii were evaluated. These microbes were tested in vitro against gastrointestinal conditions for survivability and their ability to attach to the intestinal mucosa. The ability of the microbes to tolerate and survive varying pH levels and bile concentrations was assessed. The microbes were challenged with a pH of 2 to 7 for 5 h and bile concentrations of 1 to 3% for 6 hrs. The microbes were sampled hourly to evaluate growth or decline in colony-forming units (CFU). B. longum, L. Plantarum, and S. boulardii exhibited significantly higher CFU (p < 0.05) at a pH range of 5 to 7, 4 to 7, and 2 to 7, respectively, when compared with other pH levels. L. plantarum had much higher colony-forming units per mL within each pH level, except at pH 2 where S. boulardii was the only microbe to survive over time. While L. plantarum and S. boulardii were able to tolerate the various bile concentrations, B. longum and L. plantarum showed remarkable ability to attach to the intestinal mucosa and to inhibit pathogenic microbes.

Cross Talk between ARF1 and RhoA Coordinates the Formation of Cytoskeletal Scaffolds during Chlamydia Infection

Chlamydia trachomatis is an obligate intracellular bacterium that has developed sophisticated mechanisms to survive inside its infectious compartment, the inclusion. Notably, Chlamydia weaves an extensive network of microtubules (MTs) and actin filaments to enable interactions with host organelles and enhance its stability. Despite the global health and economic burden caused by this sexually transmitted pathogen, little is known about how actin and MT scaffolds are integrated into an increasingly complex virulence system. Previously, we established that the chlamydial effector InaC interacts with ARF1 to stabilize MTs. We now demonstrate that InaC regulates RhoA to control actin scaffolds. InaC relies on cross talk between ARF1 and RhoA to coordinate MTs and actin, where the presence of RhoA downregulates stable MT scaffolds and ARF1 activation inhibits actin scaffolds. Understanding how Chlamydia hijacks complex networks will help elucidate how this clinically significant pathogen parasitizes its host and reveal novel cellular signaling pathways.

Shifting proteomes: limitations in using the BioID proximity labeling system to study SNARE protein trafficking during infection with intracellular pathogens

We hypothesize that intracellular trafficking pathways are altered in chlamydial infected cells to maximize the ability of Chlamydia to scavenge nutrients while not overtly stressing the host cell. Previous data demonstrated the importance of two eukaryotic SNARE proteins, VAMP4 and syntaxin 10 (Stx10), in chlamydial growth and development. Although, the mechanism for these effects is still unknown. To interrogate whether chlamydial infection altered these proteins' networks, we created BirA*-VAMP4 and BirA*-Stx10 fusion constructs to use the BioID proximity labeling system. While we identified a novel eukaryotic protein-protein interaction between Stx10 and VAPB, we also identified caveats in using the BioID system to study the impact of infection by an obligate intracellular pathogen on SNARE protein networks. The addition of the BirA* altered the localization of VAMP4 and Stx10 during infection with Chlamydia trachomatis serovars L2 and D and Coxiella burnetii Nine Mile Phase II. We also discovered that BirA* traffics to and biotinylates Coxiella-containing vacuoles and, in general, has a propensity for labeling membrane or membrane-associated proteins. While the BioID system identified a novel association for Stx10, it is not a reliable methodology to examine intracellular trafficking pathway dynamics during infection with intracellular pathogens.

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.

Chlamydia trachomatis TmeA Directly Activates N-WASP To Promote Actin Polymerization and Functions Synergistically with TarP during Invasion

The increasing genetic tractability of Chlamydia trachomatis is accelerating the ability to characterize the unique infection biology of this obligate intracellular parasite. These efforts are leading to a greater understanding of the molecular events associated with key virulence requirements.

Chlamydia trachomatis is a medically significant human pathogen and is an epithelial-tropic obligate intracellular parasite. Invasion of nonprofessional phagocytes represents a crucial step in the infection process and has likely promoted the evolution of a redundant mechanism and routes of entry. Like many other viral and invasive bacterial pathogens, manipulation of the host cell cytoskeleton represents a focal point in Chlamydia entry. The advent of genetic techniques in C. trachomatis, such as creation of complete gene deletions via fluorescence-reported allelic exchange mutagenesis (FRAEM), is providing important tools to unravel the contributions of bacterial factors in these complex pathways. The type III secretion chaperone Slc1 directs delivery of at least four effectors during the invasion process. Two of these, TarP and TmeA, have been associated with manipulation of actin networks and are essential for normal levels of invasion. The functions of TarP are well established, whereas TmeA is less well characterized. We leverage chlamydial genetics and proximity labeling here to provide evidence that TmeA directly targets host N-WASP to promote Arp2/3-dependent actin polymerization. Our work also shows that TmeA and TarP influence separate, yet synergistic pathways to accomplish chlamydial entry. These data further support an appreciation that a pathogen, confined by a reductionist genome, retains the ability to commit considerable resources to accomplish bottle-neck steps during the infection process.

A glance on the role of actin in osteogenic and adipogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the capacity to differentiate into multiple lineages including osteogenic and adipogenic lineages. An increasing number of studies have indicated that lineage commitment by MSCs is influenced by actin remodeling. Moreover, actin has roles in determining cell shape, nuclear shape, cell spreading, and cell stiffness, which eventually affect cell differentiation. Osteogenic differentiation is promoted in MSCs that exhibit a large spreading area, increased matrix stiffness, higher levels of actin polymerization, and higher density of stress fibers, whereas adipogenic differentiation is prevalent in MSCs with disrupted actin networks. In addition, the mechanical properties of F-actin empower cells to sense and transduce mechanical stimuli, which are also reported to influence differentiation. Various biomaterials, mechanical, and chemical interventions along with pathogen-induced actin alteration in the form of polymerization and depolymerization in MSC differentiation were studied recently. This review will cover the role of actin and its modifications through the use of different methods in inducing osteogenic and adipogenic differentiation.

Fluorescence-Reported Allelic Exchange Mutagenesis-Mediated Gene Deletion Indicates a Requirement for Chlamydia trachomatis Tarp during In Vivo Infectivity and Reveals a Specific Role for the C Terminus during Cellular Invasion

The translocated actin recruiting phosphoprotein (Tarp) is a multidomain type III secreted effector used by Chlamydia trachomatis In aggregate, existing data suggest a role of this effector in initiating new infections. As new genetic tools began to emerge to study chlamydial genes in vivo, we speculated as to what degree Tarp function contributes to Chlamydia's ability to parasitize mammalian host cells. To address this question, we generated a complete tarP deletion mutant using the fluorescence-reported allelic exchange mutagenesis (FRAEM) technique and complemented the mutant in trans with wild-type tarP or mutant tarP alleles engineered to harbor in-frame domain deletions. We provide evidence for the significant role of Tarp in C. trachomatis invasion of host cells. Complementation studies indicate that the C-terminal filamentous actin (F-actin)-binding domains are responsible for Tarp-mediated invasion efficiency. Wild-type C. trachomatis entry into HeLa cells resulted in host cell shape changes, whereas the tarP mutant did not. Finally, using a novel cis complementation approach, C. trachomatis lacking tarP demonstrated significant attenuation in a murine genital tract infection model. Together, these data provide definitive genetic evidence for the critical role of the Tarp F-actin-binding domains in host cell invasion and for the Tarp effector as a bona fide C. trachomatis virulence factor.

Impact of in ovo administered pioneer colonizers on intestinal proteome on day of hatch

Pioneer colonization of the gastrointestinal tract (GIT) by bacteria is thought to have major influence on neonatal tissue development. Previous studies have shown in ovo inoculation of embryos with saline (S), species of Citrobacter (C, C2), or lactic acid bacteria (L) resulted in an altered microbiome on day of the hatch (DOH). The present study investigated GIT proteomic changes at DOH in relation to different inoculations. Embryos were inoculated in ovo with S or ∼10² cfu of C, C2, or L at 18 embryonic days. On DOH, the GIT was collected, and tissue proteins were extracted for analysis via tandem mass spectrometry. A total of 493 proteins were identified for differential comparison with S at P ≤ 0.10. Different levels were noted in 107, 39, and 78 proteins in C, C2, and L groups, respectively, which were uploaded to Ingenuity Pathway Analysis to determine canonical pathways and biological functions related to these changes. Three members of the cytokine family (interleukin [IL]-1β, IL6, and Oncostatin M) were predicted to be activated in C2, indicated with Z-score ≥ 1.50, which suggested an overall proinflammatory GIT condition. This was consistent with the activation of the acute-phase response signaling pathway seen exclusively in C2 (Z-score = 2.00, P < 0.01). However, activation (Z-score = 2.00) of IL-13, upregulation of peroxiredoxin-1 and superoxide dismutase 1, in addition to activation of nitric oxide signaling in the cardiovascular system of the L treatment may predict a state of increased antioxidant capacity and decreased inflammatory status. The nuclear factor erythroid 2-related factor 2 (NRF2)-mediated oxidative stress response (Z-score = 2.00, P < 0.01) was predicted to be upregulated in C which suggested that chicks were in an inflammatory state and associated oxidative stress, but the impact of these pathways differed from that of C2. These changes in the proteome suggest that pioneer colonizing microbiota may have a strong impact on pathways associated with GIT immune and cellular development.

Pathogenic Puppetry: Manipulation of the Host Actin Cytoskeleton by Chlamydia trachomatis

The actin cytoskeleton is crucially important to maintenance of the cellular structure, cell motility, and endocytosis. Accordingly, bacterial pathogens often co-opt the actin-restructuring machinery of host cells to access or create a favorable environment for their own replication. The obligate intracellular organism Chlamydia trachomatis and related species exemplify this dynamic: by inducing actin polymerization at the site of pathogen-host attachment, Chlamydiae induce their own uptake by the typically non-phagocytic epithelium they infect. The interaction of chlamydial adhesins with host surface receptors has been implicated in this effect, as has the activity of the chlamydial effector TarP (translocated actin recruitment protein). Following invasion, C. trachomatis dynamically assembles and maintains an actin-rich cage around the pathogen’s membrane-bound replicative niche, known as the chlamydial inclusion. Through further induction of actin polymerization and modulation of the actin-crosslinking protein myosin II, C. trachomatis promotes egress from the host via extrusion of the inclusion. In this review, we present the experimental findings that can inform our understanding of actin-dependent chlamydial pathogenesis, discuss lingering questions, and identify potential avenues of future study.

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.

Alternatively Activated Macrophages Are Host Cells for Chlamydia trachomatis and Reverse Anti-chlamydial Classically Activated Macrophages

The obligate intracellular pathogen Chlamydia trachomatis (Ctr) is the causative agent of the most common form of sexually transmitted disease in the United States. Genital infections with C. trachomatis can lead to inflammatory tissue damage followed by scarring and tissue remodeling during wound healing. Extensive scarring can lead to ectopic pregnancy or infertility. Classically activated macrophages (CA mϕ), with their anti-microbial effector mechanisms, are known to be involved in acute inflammatory processes during the course of infection. In contrast, alternatively activated macrophages (AA mϕ) contribute to tissue repair at sites of wound healing, and have reduced bactericidal functions. They are present during infection, and thus potentially can provide a growth niche for C. trachomatis during a course of infection. To address this question, macrophages derived from CD14-positive monocytes magnetically isolated from peripheral blood mononuclear cells (PBMC) were treated with interferon-γ or interleukin-4 to produce CA mϕ or AA mϕ, respectively. Confocal microscopy of chlamydial inclusions and quantification of infectious yields revealed better pathogen growth and development in AA mϕ than CA mϕ, which correlated with the reduced expression of indoleamine 2,3-dioxygenase, a known anti-chlamydial effector of the host. Furthermore, AA mϕ stained strongly for transferrin receptor and secreted higher amounts of anti-inflammatory interleukin-10 compared to CA mϕ, characteristics that indicate its suitability as host to C. trachomatis. CA, AA, and resting mϕ were infected with Ctr serovar L2. The data suggest that IL-10 produced by infected AA mϕ attenuated the anti-chlamydial function of CA mϕ with growth recovery observed in infected CA mϕ in the presence of infected, but not mock-infected AA mϕ. This could be related to our observation that IL-10 treatment of infected CA mϕ promoted better chlamydial growth. Thus, in addition to serving as an additional niche, AA mϕ might also serve as a means to modulate the immediate environment by attenuating the anti-chlamydial functions of nearby CA mϕ in a manner that could involve IL-10 produced by infected AA mϕ.

CPn0572, the C. pneumoniae ortholog of TarP, reorganizes the actin cytoskeleton via a newly identified F-actin binding domain and recruitment of vinculin

Chlamydia pneumoniae is one of the two major species of the Chlamydiaceae family that have a profound effect on human health. C. pneumoniae is linked to a number of severe acute and chronic diseases of the upper and lower respiratory tract including pneumonia, asthma, bronchitis and infection by the pathogen might play a role in lung cancer. Following adhesion, Chlamydiae secrete effector proteins into the host cytoplasm that modulate the actin cytoskeleton facilitating internalization and infection. Members of the conserved TarP protein family comprise such effector proteins that polymerize actin, and in the case of the C. trachomatis TarP protein, has been shown to play a critical role in pathogenesis. In a previous study, we demonstrated that, upon bacterial invasion, the C. pneumoniae TarP family member CPn0572 is secreted into the host cytoplasm and recruits and associates with actin via an actin-binding domain conserved in TarP proteins. We have now extended our analysis of CPn0572 and found that the CPn0572 actin binding and modulating capability is more complex. With the help of the fission yeast system, a second actin modulating domain was identified independent of the actin binding domain. Microscopic analysis of HEp-2 cells expressing different CPn0572 deletion variants mapped this domain to the C-terminal part of the protein as CPn0572536-755 binds F-actin in vitro and colocalizes with aberrantly thickened actin cables in vivo. Finally, microscopic and bioinformatic analysis revealed the existence of a vinculin binding sequence in CPn0572. Our findings contribute to the understanding of the function of the TarP family and underscore the existence of several actin binding domains and a vinculin binding site for host actin modulation.

A FACS-Based Genome-wide CRISPR Screen Reveals a Requirement for COPI in Chlamydia trachomatis Invasion

The invasion of Chlamydia trachomatis, an obligate intracellular bacterium, into epithelial cells is driven by a complex interplay of host and bacterial factors. To comprehensively define the host genes required for pathogen invasion, we undertook a fluorescence-activated cell sorting (FACS)-based CRISPR screen in human cells. A genome-wide loss-of-function library was infected with fluorescent C. trachomatis and then sorted to enrich for invasion-deficient mutants. The screen identified heparan sulfate, a known pathogen receptor, as well as coatomer complex I (COPI). We found that COPI, through a previously unappreciated role, promotes heparan sulfate cell surface presentation, thereby facilitating C. trachomatis attachment. The heparan sulfate defect does not fully account for the resistance of COPI mutants. COPI also promotes the activity of the pathogen's type III secretion system. Together, our findings establish the requirement for COPI in C. trachomatis invasion and the utility of FACS-based CRISPR screening for the elucidation of host factors required for pathogen invasion.

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FACS-based CRISPR screen to identify host factors required for C. trachomatis invasion

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Candidate genes comprise heparan sulfate biosynthesis, actin remodeling, and COPI

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COPI regulates heparan sulfate cell surface presentation and C. trachomatis attachment

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COPI is also required for efficient C. trachomatis T3SS translocation

Capsid protein Vp1 from chlamydiaphage φCPG1 effectively alleviates cytotoxicity induced by Chlamydia trachomatis

Chlamydia trachomatis is the leading cause of sexually transmitted bacterial infections. C. trachomatis genital infection may lead to pelvic inflammatory disease, ectopic pregnancy and tubal infertility, which are major public health problems. However, the pathogenic mechanisms of this bacterium remain unclear, and the efficacy of clinical therapeutics is unsatisfactory. In the current study, whether Vp1 can alleviate the cytotoxicity induced by Chlamydia trachomatis infection was investigated. C. trachomatis was pre-treated with BSA or purified Vp1 protein and used to infect HeLa cells. It was observed that Vp1 significantly inhibited the infectivity of C. trachomatis in cell cultures. In addition, the Vp1 pretreatment reduced the chlamydial Hsp60 protein levels and decreased the C. trachomatis inclusion number. The Vp1 pretreatment also prevented C. trachomatis-induced cytotoxicity in host cells. Furthermore, the chlamydial suppression of host cell proapoptotic p53 protein and the induction of antiapoptotic cIAP-2 and Mcl-1 gene expression were reversed by the Vp1 pretreatment. These observations suggest that Vp1 has a clear inhibitory effect on C. trachomatis growth in vitro.

Chlamydial virulence factor TarP mimics talin to disrupt the talin-vinculin complex

Vinculin is a central component of mechanosensitive adhesive complexes that form between cells and the extracellular matrix. A myriad of infectious agents mimic vinculin binding sites (VBS), enabling them to hijack the adhesion machinery and facilitate cellular entry. Here, we report the structural and biochemical characterisation of VBS from the chlamydial virulence factor TarP. Whilst the affinities of isolated VBS peptides from TarP and talin for vinculin are similar, their behaviour in larger fragments is markedly different. In talin, VBS are cryptic and require mechanical activation to bind vinculin, whereas the TarP VBS are located in disordered regions, and so are constitutively active. We demonstrate that the TarP VBS can uncouple talin:vinculin complexes, which may lead to adhesion destabilisation.

Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry

Pathogens hijack host endocytic pathways to force their own entry into eukaryotic target cells. Many bacteria either exploit receptor-mediated zippering or inject virulence proteins directly to trigger membrane reorganisation and cytoskeletal rearrangements. By contrast, extracellular C. trachomatis elementary bodies (EBs) apparently employ facets of both the zipper and trigger mechanisms and are only ~400 nm in diameter. Our cryo-electron tomography of C. trachomatis entry revealed an unexpectedly diverse array of host structures in association with invading EBs, suggesting internalisation may progress by multiple, potentially redundant routes or several sequential events within a single pathway. Here we performed quantitative analysis of actin organisation at chlamydial entry foci, highlighting filopodial capture and phagocytic cups as dominant and conserved morphological structures early during internalisation. We applied inhibitor-based screening and employed reporters to systematically assay and visualise the spatio-temporal contribution of diverse endocytic signalling mediators to C. trachomatis entry. In addition to the recognised roles of the Rac1 GTPase and its associated nucleation-promoting factor (NPF) WAVE, our data revealed an additional unrecognised pathway sharing key hallmarks of macropinocytosis: i) amiloride sensitivity, ii) fluid-phase uptake, iii) recruitment and activity of the NPF N-WASP, and iv) the localised generation of phosphoinositide-3-phosphate (PI3P) species. Given their central role in macropinocytosis and affinity for PI3P, we assessed the role of SNX-PX-BAR family proteins. Strikingly, SNX9 was specifically and transiently enriched at C. trachomatis entry foci. SNX9^-/- cells exhibited a 20% defect in EB entry, which was enhanced to 60% when the cells were infected without sedimentation-induced EB adhesion, consistent with a defect in initial EB-host interaction. Correspondingly, filopodial capture of C. trachomatis EBs was specifically attenuated in SNX9^-/- cells, implicating SNX9 as a central host mediator of filopodial capture early during chlamydial entry. Our findings identify an unanticipated complexity of signalling underpinning cell entry by this major human pathogen, and suggest intriguing parallels with viral entry mechanisms.

Chlamydia trachomatis remains the leading bacterial agent of sexually transmitted disease worldwide and causes a form of blindness called trachoma in Developing nations, which is recognised by the World Health Organisation as a neglected tropical disease. Despite this burden, we know comparatively little about how it causes disease at a molecular level. Chlamydia must live inside human cells to survive, and here we study the mechanism of how it enters cells, which is critical to the lifecycle. We study how the bacterium exploits signalling pathways inside the cell to its own advantage to deform the cell membrane by reorganising the underlying cell skeleton, and identify new factors involved in this process. Our findings suggest intriguing similarities with how some viruses enter cells. A better understanding of these processes may help to develop future vaccines and new treatments.

Biophysical characterization of actin bundles generated by the Chlamydia trachomatis Tarp effector

Chlamydia trachomatis entry into host cells is mediated by pathogen-directed remodeling of the actin cytoskeleton. The chlamydial type III secreted effector, translocated actin recruiting phosphoprotein (Tarp), has been implicated in the recruitment of actin to the site of internalization. Tarp harbors G-actin binding and proline rich domains required for Tarp-mediated actin nucleation as well as unique F-actin binding domains implicated in the formation of actin bundles. Little is known about the mechanical properties of actin bundles generated by Tarp or the mechanism by which Tarp mediates actin bundle formation. In order to characterize the actin bundles and elucidate the role of different Tarp domains in the bundling process, purified Tarp effectors and Tarp truncation mutants were analyzed using Total Internal Reflection Fluorescence (TIRF) microscopy. Our data indicate that Tarp mediated actin bundling is independent of actin nucleation and the F-actin binding domains are sufficient to bundle actin filaments. Additionally, Tarp-mediated actin bundles demonstrate distinct bending stiffness compared to those crosslinked by the well characterized actin bundling proteins fascin and alpha-actinin, suggesting Tarp may employ a novel actin bundling strategy. The capacity of the Tarp effector to generate novel actin bundles likely contributes to chlamydia's efficient mechanism of entry into human cells.

The intrinsically disordered Tarp protein from chlamydia binds actin with a partially preformed helix

Tarp (translocated actin recruiting phosphoprotein) is an effector protein common to all chlamydial species that functions to remodel the host-actin cytoskeleton during the initial stage of infection. In C. trachomatis, direct binding to actin monomers has been broadly mapped to a 100-residue region (726-825) which is predicted to be predominantly disordered, with the exception of a ~10-residue α-helical patch homologous to other WH2 actin-binding motifs. Biophysical investigations demonstrate that a Tarp726-825 construct behaves as a typical intrinsically disordered protein; within it, NMR relaxation measurements and chemical shift analysis identify the ten residue WH2-homologous region to exhibit partial α-helix formation. Isothermal titration calorimetry experiments on the same construct in the presence of monomeric G-actin show a well defined binding event with a 1:1 stoichiometry and Kd of 102 nM, whilst synchrotron radiation circular dichroism spectroscopy suggests the binding is concomitant with an increase in helical secondary structure. Furthermore, NMR experiments in the presence of G-actin indicate this interaction affects the proposed WH2-like α-helical region, supporting results from in silico docking calculations which suggest that, when folded, this α-helix binds within the actin hydrophobic cleft as seen for other actin-associated proteins.

Fluorescence-Reported Allelic Exchange Mutagenesis Reveals a Role for Chlamydia trachomatis TmeA in Invasion That Is Independent of Host AHNAK

Development of approaches to genetically manipulate Chlamydia is fostering important advances in understanding pathogenesis. Fluorescence-reported allelic exchange mutagenesis (FRAEM) now enables the complete deletion of specific genes in C. trachomatis L2. We have leveraged this technology to delete the coding sequences for a known type III effector. The evidence provided here indicates that CT694/CTL0063 is a virulence protein involved in chlamydial invasion. Based on our findings, we designate the gene product corresponding to ct694-ctl0063translocated membrane-associated effector A (TmeA). Deletion of tmeA did not impact development of intracellular chlamydiae. However, the absence of TmeA manifested as a decrease in infectivity in both tissue culture and murine infection models. The in vitro defect was reflected by impaired invasion of host cells. TmeA binds human AHNAK, and we demonstrate here that AHNAK is transiently recruited by invading chlamydiae. TmeA, however, is not required for endogenous AHNAK recruitment. TmeA also impairs AHNAK-dependent actin bundling activity. This TmeA-mediated effect likely does not explain impaired invasion displayed by the tmeA strain of Chlamydia, since AHNAK-deficient cells revealed no invasion phenotype. Overall, our data indicate the efficacy of FRAEM and reveal a role of TmeA during chlamydial invasion that manifests independently of effects on AHNAK.

Taking control: reorganization of the host cytoskeleton by Chlamydia

Both actin and microtubules are major cytoskeletal elements in eukaryotic cells that participate in many cellular processes, including cell division and motility, vesicle and organelle movement, and the maintenance of cell shape. Inside its host cell, the human pathogen Chlamydia trachomatis manipulates the cytoskeleton to promote its survival and enhance its pathogenicity. In particular, Chlamydia induces the drastic rearrangement of both actin and microtubules, which is vital for its entry, inclusion structure and development, and host cell exit. As significant progress in Chlamydia genetics has greatly enhanced our understanding of how this pathogen co-opts the host cytoskeleton, we will discuss the machinery used by Chlamydia to coordinate the reorganization of actin and microtubules.

Acquisition of Rab11 and Rab11-Fip2-A novel strategy for Chlamydia pneumoniae early survival

The initial steps in chlamydial infection involve adhesion and internalization into host cells and, most importantly, modification of the nascent inclusion to establish the intracellular niche. Here, we show that Chlamydia pneumoniae enters host cells via EGFR-dependent endocytosis into an early endosome with a phosphatidylinositol 3-phosphate (PI3P) membrane identity. Immediately after entry, the early chlamydial inclusion acquires early endosomal Rab GTPases including Rab4, Rab5, Rab7, as well as the two recycling-specific Rabs Rab11 and Rab14. While Rab5, Rab11 and Rab14 are retained in the vesicular membrane, Rab4 and Rab7 soon disappear. Loss of Rab7 enables the C. pneumoniae inclusion to escape delivery to, and degradation in lysosomes. Loss of Rab4 and retention of Rab11/ Rab14 designates the inclusion as a slowly recycling endosome—that is protected from degradation. Furthermore, we show that the Rab11/ Rab14 adaptor protein Rab11-Fip2 (Fip2) is recruited to the nascent inclusion upon internalization and retained in the membrane throughout infection. siRNA knockdown of Fip2 demonstrated that the protein is essential for internalization and infection, and expression of various deletion variants revealed that Fip2 regulates the intracellular positioning of the inclusion. Additionally, we show that binding to Rab11 and Fip2 recruits the unconventional actin motor protein myosin Vb to the early inclusion and that together they regulate the relocation of the nascent inclusion from the cell periphery to the perinuclear region, its final destination. Here, we characterize for the first time inclusion identity and inclusion-associated proteins to delineate how C. pneumoniae establishes the intracellular niche essential for its survival.

Here, we show for the first time how Chlamydia pneumoniae an obligate intracellular pathogen establishes its intracellular niche. After EGFR-dependent endocytosis into host cells, the nascent chlamydial inclusion acquires early endosomal membrane identity and the Rab GTPases Rab4, Rab5 and Rab7, as well as the recycling-specific Rab11 and Rab14. We show that Rab5, Rab11 and Rab14 are retained in the vesicular membrane, while Rab4 and Rab7 subsequently disappear. Thus, C. pneumoniae escapes lysosomal degradation by hiding in a recycling endosome vesicle. Furthermore, we show that the Rab11/Rab14 adaptor protein Rab11-Fip2 (Fip2), together with the unconventional actin motor protein myosin Vb, is recruited to the nascent inclusion. Both are essential for internalization and infection, as they regulate the intracellular positioning of the inclusion, which is essential for intracellular transport from the cell periphery to the perinuclear region. Here, we characterize for the first time inclusion identity and inclusion-associated proteins to understand how C. pneumoniae establishes the intracellular niche, which is essential for its survival.

The role of septins in infections with vacuole-dwelling intracellular bacteria

Septins are a relatively little understood group of GTPases that form large assemblies in cells from all eukaryotes other than plants. Septins were first identified in cell division but have also been implicated in microbial infections. Septins often associate with cytoskeletal proteins - most often described for filamentous (F-) actin - and are considered cytoskeletal components themselves. Septins have increasingly been found to partake in processes that are linked to intracellular membranes, from mitochondria to phagosomes, and evidence is accumulating that septins specifically bind to membranes. Since a number of microorganisms have specialized to live and grow inside membranous vacuoles in the cytosol of mammalian cells, this membrane-association of septins suggests that septins may also be involved in the membranous, vacuolar structures that develop around these microbes. However, data are limited on this issue: septins have been identified by proteome analysis on some microbe-bearing vacuoles, but more extensive experimental data are only available for infections with the obligate intracellular bacterium Chlamydia trachomatis. In this review article I will discuss the available data and speculate about the mechanisms of recruitment and potential functions of septins for vacuole-dwelling microorganisms, which may be peculiar to Chlamydia or may pertain more generally to this class of microbes.

Chlamydia Hijacks ARF GTPases To Coordinate Microtubule Posttranslational Modifications and Golgi Complex Positioning

The intracellular bacterium Chlamydia trachomatis develops in a parasitic compartment called the inclusion. Posttranslationally modified microtubules encase the inclusion, controlling the positioning of Golgi complex fragments around the inclusion. The molecular mechanisms by which Chlamydia coopts the host cytoskeleton and the Golgi complex to sustain its infectious compartment are unknown. Here, using a genetically modified Chlamydia strain, we discovered that both posttranslationally modified microtubules and Golgi complex positioning around the inclusion are controlled by the chlamydial inclusion protein CT813/CTL0184/InaC and host ARF GTPases. CT813 recruits ARF1 and ARF4 to the inclusion membrane, where they induce posttranslationally modified microtubules. Similarly, both ARF isoforms are required for the repositioning of Golgi complex fragments around the inclusion. We demonstrate that CT813 directly recruits ARF GTPases on the inclusion membrane and plays a pivotal role in their activation. Together, these results reveal that Chlamydia uses CT813 to hijack ARF GTPases to couple posttranslationally modified microtubules and Golgi complex repositioning at the inclusion.

Chlamydia trachomatis is an important cause of morbidity and a significant economic burden in the world. However, how Chlamydia develops its intracellular compartment, the so-called inclusion, is poorly understood. Using genetically engineered Chlamydia mutants, we discovered that the effector protein CT813 recruits and activates host ADP-ribosylation factor 1 (ARF1) and ARF4 to regulate microtubules. In this context, CT813 acts as a molecular platform that induces the posttranslational modification of microtubules around the inclusion. These cages are then used to reposition the Golgi complex during infection and promote the development of the inclusion. This study provides the first evidence that ARF1 and ARF4 play critical roles in controlling posttranslationally modified microtubules around the inclusion and that Chlamydia trachomatis hijacks this novel function of ARF to reposition the Golgi complex.

Hijacking Host Cell Highways: Manipulation of the Host Actin Cytoskeleton by Obligate Intracellular Bacterial Pathogens

Intracellular bacterial pathogens replicate within eukaryotic cells and display unique adaptations that support key infection events including invasion, replication, immune evasion, and dissemination. From invasion to dissemination, all stages of the intracellular bacterial life cycle share the same three-dimensional cytosolic space containing the host cytoskeleton. For successful infection and replication, many pathogens hijack the cytoskeleton using effector proteins introduced into the host cytosol by specialized secretion systems. A subset of effectors contains eukaryotic-like motifs that mimic host proteins to exploit signaling and modify specific cytoskeletal components such as actin and microtubules. Cytoskeletal rearrangement promotes numerous events that are beneficial to the pathogen, including internalization of bacteria, structural support for bacteria-containing vacuoles, altered vesicular trafficking, actin-dependent bacterial movement, and pathogen dissemination. This review highlights a diverse group of obligate intracellular bacterial pathogens that manipulate the host cytoskeleton to thrive within eukaryotic cells and discusses underlying molecular mechanisms that promote these dynamic host-pathogen interactions.

Targeted Disruption of Chlamydia trachomatis Invasion by in Trans Expression of Dominant Negative Tarp Effectors

Chlamydia trachomatis invasion of eukaryotic host cells is facilitated, in part, by the type III secreted effector protein, Tarp. The role of Tarp in chlamydiae entry of host cells is supported by molecular approaches that examined recombinant Tarp or Tarp effectors expressed within heterologous systems. A major limitation in the ability to study the contribution of Tarp to chlamydial invasion of host cells was the prior absence of genetic tools for chlamydiae. Based on our knowledge of Tarp domain structure and function along with the introduction of genetic approaches in C. trachomatis, we hypothesized that Tarp function could be disrupted in vivo by the introduction of dominant negative mutant alleles. We provide evidence that transformed C. trachomatis produced epitope tagged Tarp, which was secreted into the host cell during invasion. We examined the effects of domain specific Tarp mutations on chlamydial invasion and growth and demonstrate that C. trachomatis clones harboring engineered Tarp mutants lacking either the actin binding domain or the phosphorylation domain had reduced levels of invasion into host cells. These data provide the first in vivo evidence for the critical role of Tarp in C. trachomatis pathogenesis and indicate that chlamydial invasion of host cells can be attenuated via the introduction of engineered dominant negative type three effectors.

One Face of Chlamydia trachomatis: The Infectious Elementary Body

The lifestyle of Chlamydiae is unique: the bacteria alternate between two morphologically distinct forms, an infectious non-replicative elementary body (EB), and a replicative, non-infectious reticulate body (RB). This review focuses on recent advances in understanding the structure and function of the infectious form of the best-studied member of the phylum, the human pathogen Chlamydia trachomatis. Once considered as an inert particle of little functional capacity, the EB is now perceived as a sophisticated entity that encounters at least three different environments during each infectious cycle. We review current knowledge on its composition and morphology, and emerging metabolic activities. These features confer resistance to the extracellular environment, the ability to penetrate a host cell and ultimately enable the EB to establish a niche enabling bacterial survival and growth. The bacterial and host molecules involved in these processes are beginning to emerge.

Vinculin Interacts with the Chlamydia Effector TarP Via a Tripartite Vinculin Binding Domain to Mediate Actin Recruitment and Assembly at the Plasma Membrane

The mammalian protein vinculin is often a target of bacterial pathogens to subvert locally host cell actin dynamics. In Chlamydia infection, vinculin has been implicated in RNA interference screens, but the molecular basis for vinculin requirement has not been characterized. In this report, we show that vinculin was involved in the actin recruitment and F-actin assembly at the plasma membrane to facilitate invasion. Vinculin was recruited to the plasma membrane via its interaction with a specific tripartite motif within TarP that resembles the vinculin-binding domain (VBD) found in the Shigella invasion factor IpaA. The TarP-mediated plasma membrane recruitment of vinculin resulted in the localized recruitment of actin. In vitro pulldown assays for protein-protein interaction and imaging-based evaluation of recruitment to the plasma membrane demonstrated the essential role of the vinculin-binding site 1 (VBS1), and the dispensability of VBS2 and VBS3. As further support for the functionality of VBD-vinculin interaction, VBD-mediated actin recruitment required vinculin. Interestingly, while both vinculin and the focal adhesion kinase (FAK) colocalized at the sites of adhesion, the recruitment of one was independent of the other; and the actin recruitment function of the VBD/vinculin signaling axis was independent of the LD/FAK pathway.

EtpE Binding to DNase X Induces Ehrlichial Entry via CD147 and hnRNP-K Recruitment, Followed by Mobilization of N-WASP and Actin

Obligate intracellular bacteria, such as Ehrlichia chaffeensis, perish unless they can enter eukaryotic cells. E. chaffeensis is the etiological agent of human monocytic ehrlichiosis, an emerging infectious disease. To infect cells, Ehrlichia uses the C terminus of the outer membrane invasin entry-triggering protein (EtpE) of Ehrlichia (EtpE-C), which directly binds the mammalian cell surface glycosylphosphatidyl inositol-anchored protein, DNase X. How this binding drives Ehrlichia entry is unknown. Here, using affinity pulldown of host cell lysates with recombinant EtpE-C (rEtpE-C), we identified two new human proteins that interact with EtpE-C: CD147 and heterogeneous nuclear ribonucleoprotein K (hnRNP-K). The interaction of CD147 with rEtpE-C was validated by far-Western blotting and coimmunoprecipitation of native EtpE with endogenous CD147. CD147 was ubiquitous on the cell surface and also present around foci of rEtpE-C-coated-bead entry. Functional neutralization of surface-exposed CD147 with a specific antibody inhibited Ehrlichia internalization and infection but not binding. Downregulation of CD147 by short hairpin RNA (shRNA) impaired E. chaffeensis infection. Functional ablation of cytoplasmic hnRNP-K by a nanoscale intracellular antibody markedly attenuated bacterial entry and infection but not binding. EtpE-C also interacted with neuronal Wiskott-Aldrich syndrome protein (N-WASP), which is activated by hnRNP-K. Wiskostatin, which inhibits N-WASP activation, and cytochalasin D, which inhibits actin polymerization, inhibited Ehrlichia entry. Upon incubation with host cell lysate, EtpE-C but not an EtpE N-terminal fragment stimulated in vitro actin polymerization in an N-WASP- and DNase X-dependent manner. Time-lapse video images revealed N-WASP recruitment at EtpE-C-coated bead entry foci. Thus, EtpE-C binding to DNase X drives Ehrlichia entry by engaging CD147 and hnRNP-K and activating N-WASP-dependent actin polymerization.

Ehrlichia chaffeensis, an obligate intracellular bacterium, causes a blood-borne disease called human monocytic ehrlichiosis, one of the most prevalent life-threatening emerging tick-transmitted infectious diseases in the United States. The survival of Ehrlichia bacteria, and hence, their ability to cause disease, depends on their specific mode of entry into eukaryotic host cells. Understanding the mechanism by which E. chaffeensis enters cells will create new opportunities for developing effective therapies to prevent bacterial entry and disease in humans. Our findings reveal a novel cellular signaling pathway triggered by an ehrlichial surface protein called EtpE to induce its infectious entry. The results are also important from the viewpoint of human cell physiology because three EtpE-interacting human proteins, DNase X, CD147, and hnRNP-K, are hitherto unknown partners that drive the uptake of small particles, including bacteria, into human cells.

Host-pathogen reorganisation during host cell entry by Chlamydia trachomatis

Chlamydia trachomatis is obligate intracellular bacterial pathogen that remains a significant public health burden worldwide. A critical early event during infection is chlamydial entry into non-phagocytic host epithelial cells. Like other Gram-negative bacteria, C. trachomatis uses a type III secretion system (T3SS) to deliver virulence effector proteins into host cells. These effectors trigger bacterial uptake and promote bacterial survival and replication within the host cell. In this review, we highlight recent cryo-electron tomography that has provided striking insights into the initial interactions between Chlamydia and its host. We describe the polarised structure of extracellular C. trachomatis elementary bodies (EBs), and the supramolecular organisation of T3SS complexes on the EB surface, in addition to the changes in host and pathogen architecture that accompany bacterial internalisation and EB encapsulation into early intracellular vacuoles. Finally, we consider the implications for further understanding the mechanism of C. trachomatis entry and how this might relate to those of other bacteria and viruses.

A working model for the type III secretion mechanism in Chlamydia

It has been appreciated for almost 20 years that members of the Chlamydiales possess a virulence-associated type III secretion mechanism. Given the obligate intracellular nature of these bacteria, defining exactly how type III secretion functions to promote pathogenesis has been challenging. We present a working model herein that is based on current evidence.

Giardia duodenalis Surface Cysteine Proteases Induce Cleavage of the Intestinal Epithelial Cytoskeletal Protein Villin via Myosin Light Chain Kinase

Giardia duodenalis infections are among the most common causes of waterborne diarrhoeal disease worldwide. At the height of infection, G. duodenalis trophozoites induce multiple pathophysiological processes within intestinal epithelial cells that contribute to the development of diarrhoeal disease. To date, our understanding of pathophysiological processes in giardiasis remains incompletely understood. The present study reveals a previously unappreciated role for G. duodenalis cathepsin cysteine proteases in intestinal epithelial pathophysiological processes that occur during giardiasis. Experiments first established that Giardia trophozoites indeed produce cathepsin B and L in strain-dependent fashion. Co-incubation of G. duodenalis with human enterocytes enhanced cathepsin production by Assemblage A (NF and S2 isolates) trophozoites, but not when epithelial cells were exposed to Assemblage B (GSM isolate) trophozoites. Direct contact between G. duodenalis parasites and human intestinal epithelial monolayers resulted in the degradation and redistribution of the intestinal epithelial cytoskeletal protein villin; these effects were abolished when parasite cathepsin cysteine proteases were inhibited. Interestingly, inhibition of parasite proteases did not prevent degradation of the intestinal tight junction-associated protein zonula occludens 1 (ZO-1), suggesting that G. duodenalis induces multiple pathophysiological processes within intestinal epithelial cells. Finally, this study demonstrates that G. duodenalis-mediated disruption of villin is, at least, in part dependent on activation of myosin light chain kinase (MLCK). Taken together, this study indicates a novel role for parasite cathepsin cysteine proteases in the pathophysiology of G. duodenalis infections.

Trafficking of Estrella lausannensis in human macrophages

Estrella lausannensis is a new member of the Chlamydiales order. Like other Chlamydia-related bacteria, it is able to replicate in amoebae and in fish cell lines. A preliminary study investigating the pathogenic potential of Chlamydia-related bacteria found a correlation between antibody response to E. lausannensis and pneumonia in children. To further investigate the pathogenic potential of E. lausannensis, we determined its ability to grow in human macrophages and its intracellular trafficking. The replication in macrophages resulted in viable E. lausannensis; however, it caused a significant cytopathic effect. The intracellular trafficking of E. lausannensis was analyzed by determining the interaction of the Estrella-containing inclusions with various endocytic markers as well as host organelles. The E. lausannensis inclusion escaped the endocytic pathway rapidly avoiding maturation into phagolysosomes by preventing both EEA-1 and LAMP-1 accumulation. Compared to Waddlia chondrophila, another Chlamydia-related bacteria, the recruitment of mitochondria and endoplasmic reticulum was minimal for E. lausannensis inclusions. Estrella lausannensis appears to use a distinct source of nutrients and energy compared to other members of the Chlamydiales order. In conclusion, we hypothesize that E. lausannensis has a restricted growth in human macrophages, due to its reduced capacity to control programmed cell death.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

Chlamydia psittaci: update on an underestimated zoonotic agent

Chlamydia (C.) psittaci is an economically relevant pathogen in poultry and pet birds, where it causes psittacosis/ornithosis, and also a human pathogen causing atypical pneumonia after zoonotic transmission. Despite its well-documented prevalence, the agent has received less attention by researchers than other Chlamydia spp. in the last decades. In the present paper, we review recently published data on C. psittaci infection and attempt to single out characteristic features distinguishing it from related chlamydial agents. It is remarkable that C. psittaci is particularly efficient in disseminating in the host organism causing systemic disease, which occasionally can take a fulminant course. At the cellular level, the pathogen's broad host cell spectrum (from epithelial cells to macrophages), its rapid entry and fast replication, proficient use of intracellular transport routes to mitochondria and the Golgi apparatus, the pronounced physical association of chlamydial inclusions with energy-providing cell compartments, as well as the subversive regulation of host cell survival during productive and persistent states facilitate the characteristic efficient growth and successful host-to-host spread of C. psittaci. At the molecular level, the pathogen was shown to upregulate essential chlamydial genes when facing the host immune response. We hypothesize that this capacity, in concert with expression of specific effectors of the type III secretion system and efficient suppression of selected host defense signals, contributes to successful establishment of the infection in the host. Concerning the immunology of host-pathogen interactions, C. psittaci has been shown to distinguish itself by coping more efficiently than other chlamydiae with pro-inflammatory mediators during early host response, which can, to some extent, explain the effective evasion and adaptation strategies of this bacterium. We conclude that thorough analysis of the large number of whole-genome sequences already available will be essential to identify genetic markers of the species-specific features and trigger more in-depth studies in cellular and animal models to address such vital topics as treatment and vaccination.

Activation of epidermal growth factor receptor is required for Chlamydia trachomatis development

Background

Chlamydia trachomatis (C. trachomatis) is a clinically significant human pathogen and one of the leading causative agents of sexually transmitted diseases. As obligate intracellular bacteria, C. trachomatis has evolved strategies to redirect the host’s signaling and resources for its own survival and propagation. Despite the clinical notoriety of Chlamydia infections, the molecular interactions between C. trachomatis and its host cell proteins remain elusive.

Results

In this study, we focused on the involvement of the host cell epidermal growth factor receptor (EGFR) in C. trachomatis attachment and development. A combination of molecular approaches, pharmacological agents and cell lines were used to demonstrate distinct functional requirements of EGFR in C. trachomatis infection. We show that C. trachomatis increases the phosphorylation of EGFR and of its downstream effectors PLCγ1, Akt and STAT5. While both EGFR and platelet-derived growth factor receptor-β (PDGFRβ) are partially involved in bacterial attachment to the host cell surface, it is only the knockdown of EGFR and not PDGFRβ that affects the formation of C. trachomatis inclusions in the host cells. Inhibition of EGFR results in small immature inclusions, and prevents C. trachomatis-induced intracellular calcium mobilization and the assembly of the characteristic F-actin ring at the inclusion periphery. By using complementary approaches, we demonstrate that the coordinated regulation of both calcium mobilization and F-actin assembly by EGFR are necessary for maturation of chlamydial inclusion within the host cells. A particularly important finding of this study is the co-localization of EGFR with the F-actin at the periphery of C. trachomatis inclusion where it may function to nucleate the assembly of signaling protein complexes for cytoskeletal remodeling required for C. trachomatis development.

Conclusion

Cumulatively, the data reported here connect the function of EGFR to C. trachomatis attachment and development in the host cells, and this could lead to new venues for targeting C. trachomatis infections and associated diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0277-4) contains supplementary material, which is available to authorized users.

Host nectin-1 is required for efficient Chlamydia trachomatis serovar E development

Interaction of Herpes Simplex Virus (HSV) glycoprotein D (gD) with the host cell surface during Chlamydia trachomatis/HSV co-infection stimulates chlamydiae to become persistent. During viral entry, gD interacts with one of 4 host co-receptors: HVEM (herpes virus entry mediator), nectin-1, nectin-2 and 3-O-sulfated heparan sulfate. HVEM and nectin-1 are high-affinity entry receptors for both HSV-1 and HSV-2. Nectin-2 mediates HSV-2 entry but is inactive for HSV-1, while 3-O-sulfated heparan sulfate facilitates HSV-1, but not HSV-2, entry. Western blot and RT-PCR analyses demonstrate that HeLa and HEC-1B cells express nectin-1 and nectin-2, but not HVEM. Because both HSV-1 and HSV-2 trigger persistence, these data suggest that nectin-1 is the most likely co-receptor involved. Co-infections with nectin-1 specific HSV-1 mutants stimulate chlamydial persistence, as evidenced by aberrant body (AB) formation and decreased production of elementary bodies (EBs). These data indicate that nectin-1 is involved in viral-induced chlamydial persistence. However, inhibition of signal transduction molecules associated with HSV attachment and entry does not rescue EB production during C. trachomatis/HSV-2 co-infection. HSV attachment also does not activate Cdc42 in HeLa cells, as would be expected with viral stimulated activation of nectin-1 signaling. Additionally, immunofluorescence assays confirm that HSV infection decreases nectin-1 expression. Together, these observations suggest that gD binding-induced loss of nectin-1 signaling negatively influences chlamydial growth. Chlamydial infection studies in nectin-1 knockdown (NKD) HeLa cell lines support this hypothesis. In NKD cells, chlamydial inclusions are smaller in size, contain ABs, and produce significantly fewer infectious EBs compared to C. trachomatis infection in control HeLa cells. Overall, the current study indicates that the actions of host molecule, nectin-1, are required for successful C. trachomatis development.

Reconceptualizing the chlamydial inclusion as a pathogen-specified parasitic organelle: an expanded role for Inc proteins

Chlamydia is an obligate intracellular pathogen that develops in the host cell in a vacuole termed the chlamydial inclusion. The prevailing concept of the chlamydial inclusion is of a parasitophorous vacuole. Here, the inclusion is the recipient of one-way host-pathogen interactions thus draining nutrients from the cell and negatively impacting it. While Chlamydia orchestrates some aspects of cell function, recent data indicate host cells remain healthy up until, and even after, chlamydial egress. Thus, while Chlamydia relies on the host cell for necessary metabolites, the overall function of the host cell, during chlamydial growth and development, is not grossly disturbed. This is consistent with the obligate intracellular organism's interest to maintain viability of its host. To this end, Chlamydia expresses inclusion membrane proteins, Incs, which serve as molecular markers for the inclusion membrane. Incs also contribute to the physical structure of the inclusion membrane and facilitate host-pathogen interactions across it. Given the function of Incs and the dynamic interactions that occur at the inclusion membrane, we propose that the inclusion behaves similarly to an organelle-albeit one that benefits the pathogen. We present the hypothesis that the chlamydial inclusion acts as a pathogen-specified parasitic organelle. This representation integrates the inclusion within existing subcellular trafficking pathways to divert a subset of host-derived metabolites thus maintaining host cell homeostasis. We review the known interactions of the chlamydial inclusion with the host cell and discuss the role of Inc proteins in the context of this model and how this perspective can impact the study of these proteins. Lessons learnt from the chlamydial pathogen-specified parasitic organelle can be applied to other intracellular pathogens. This will increase our understanding of how intracellular pathogens engage the host cell to establish their unique developmental niches.