Isolation and characterization of a mutant Chinese hamster ovary cell line that is resistant to Chlamydia trachomatis infection at a novel step in the attachment process

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.

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.

Fascinating Molecular and Immune Escape Mechanisms in the Treatment of STIs (Syphilis, Gonorrhea, Chlamydia, and Herpes Simplex)

The incidence of syphilis, gonorrhea, chlamydia, and herpes simplex has increased over the last decade, despite the numerous prevention strategies. Worldwide scientists report a surge in drug-resistant infections, particularly in immunocompromised patients. Antigenic variations in syphilis enable long-term infection, but benzathine penicillin G maintains its efficiency, whereas macrolides should be recommended with caution. Mupirocin and zoliflodacin were recently introduced as therapies against ceftriaxone-resistant gonococcus, which poses a larger global threat. The gastrointestinal and prostatic potential reservoirs of Chlamydia trachomatis may represent the key towards complete eradication. Similar to syphilis, macrolides resistance has to be considered in genital chlamydiosis. Acyclovir-resistant HSV may respond to the novel helicase-primase inhibitors and topical imiquimod, particularly in HIV-positive patients. Novel drugs can overcome these challenges while nanocarriers enhance their potency, particularly in mucosal areas. This review summarizes the most recent and valuable discoveries regarding the immunopathogenic mechanisms of these sexually transmitted infections and discusses the challenges and opportunities of the novel molecules and nanomaterials.

Polyplex transfection from intracerebroventricular delivery is not significantly affected by traumatic brain injury

Traumatic brain injury (TBI) is largely non-preventable and often kills or permanently disables its victims. Because current treatments for TBI merely ameliorate secondary effects of the initial injury like swelling and hemorrhaging, strategies for the induction of neuronal regeneration are desperately needed. Recent discoveries regarding the TBI-responsive migratory behavior and differentiation potential of neural progenitor cells (NPCs) found in the subventricular zone (SVZ) have prompted strategies targeting gene therapies to these cells to enhance neurogenesis after TBI. We have previously shown that plasmid polyplexes can non-virally transfect SVZ NPCs when directly injected in the lateral ventricles of uninjured mice. We describe the first reported intracerebroventricular transfections mediated by polymeric gene carriers in a murine TBI model and investigate the anatomical parameters that dictate transfection through this route of administration. Using both luciferase and GFP plasmid transfections, we show that the time delay between injury and polyplex injection directly impacts the magnitude of transfection efficiency, but that overall trends in the location of transfection are not affected by injury. Confocal microscopy of quantum dot-labeled plasmid uptake in vivo reveals association between our polymers and negatively charged NG2 chondroitin sulfate proteoglycans of the SVZ extracellular matrix. We further validate that glycosaminoglycans but not sulfate groups are required for polyplex uptake and transfection in vitro. These studies demonstrate that non-viral gene delivery is impacted by proteoglycan interactions and suggest the need for improved polyplex targeting materials that penetrate brain extracellular matrix to increase transfection efficiency in vivo.

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.

Chlamydial Infection From Outside to Inside

Chlamydia are obligate intracellular bacteria, characterized by a unique biphasic developmental cycle. Specific interactions with the host cell are crucial for the bacteria’s survival and amplification because of the reduced chlamydial genome. At the start of infection, pathogen-host interactions are set in place in order for Chlamydia to enter the host cell and reach the nutrient-rich peri-Golgi region. Once intracellular localization is established, interactions with organelles and pathways of the host cell enable the necessary hijacking of host-derived nutrients. Detailed information on the aforementioned processes will increase our understanding on the intracellular pathogenesis of chlamydiae and hence might lead to new strategies to battle chlamydial infection. This review summarizes how chlamydiae generate their intracellular niche in the host cell, acquire host-derived nutrients in order to enable their growth and finally exit the host cell in order to infect new cells. Moreover, the evolution in the development of molecular genetic tools, necessary for studying the chlamydial infection biology in more depth, is discussed in great detail.

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.

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.

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.

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.

All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function

The bacterial pathogens Chlamydia trachomatis and C. pneumoniae are obligate intracellular parasites, cause a number of serious diseases, and can infect various cell types in humans. Chlamydial infections are probably initiated by binding of the bacterial outer membrane protein OmcB to host cell glycosaminoglycans (GAGs). Here, we show that all nine members of the polymorphic membrane protein (Pmp) family of C. trachomatis mediate adhesion to human epithelial and endothelial cells. Importantly, exposure of infectious particles to soluble recombinant Pmps blocks subsequent infection, thus implicating an important function of the entire protein family in the infection process. Analogous experiments with pairs of recombinant Pmps or a combination of Pmp and OmcB revealed that all Pmps probably act in an adhesion pathway that is distinct from the OmcB-GAG pathway. Finally, we provide evidence that the Pmps of C. trachomatis and C. pneumoniae exhibit species and tissue specificity. These findings argue for the involvement of C. trachomatis Pmps in the initial phase of infection and suggest that they may interact with a receptor other than the epidermal growth factor receptor recently identified for their counterparts in C. pneumoniae.

Characterization of the interaction between the chlamydial adhesin OmcB and the human host cell

In a previous study, we reported that the OmcB protein from Chlamydia pneumoniae mediates adhesion of the infectious elementary body to human HEp-2 cells by interacting with heparin/heparan sulfate-like glycosaminoglycans (GAGs) via basic amino acids located in the first of a pair of XBBXBX heparin-binding motifs (K. Moelleken and J. H. Hegemann, Mol. Microbiol. 67:403-419, 2008). In the present study, we show that the basic amino acid at position 57 (arginine) in the first XBBXBX motif, the basic amino acid at position 61 (arginine) in the second motif, and another amino acid (lysine 69) C terminal to it play key roles in the interaction. In addition, we show that discrimination between heparin-dependent and -independent adhesion by C. trachomatis OmcBs is entirely dependent on three variable amino acids in the so-called variable domain C terminal to the conserved XBBXBX motif. Here, the predicted conformational change in the secondary structure induced by the proline at position 66 seems to be crucial for heparin recognition. Finally, we performed neutralization experiments using different anti-heparan sulfate antibodies to gain insight into the nature of the GAGs recognized by OmcB. The results suggest that C. trachomatis serovar L2 OmcB interacts with 6-O-sulfated domains of heparan sulfate, while C. pneumoniae OmcB apparently interacts with domains of heparan sulfate harboring a diverse subset of O-sulfations.

Endosulfatases SULF1 and SULF2 limit Chlamydia muridarum infection

The first step in attachment of Chlamydia to host cells is thought to involve reversible binding to host heparan sulfate proteoglycans (HSPGs), polymers of variably sulfated repeating disaccharide units coupled to diverse protein backbones. However, the key determinants of HSPG structure that are involved in Chlamydia binding are incompletely defined. A previous genome-wide Drosophila RNAi screen suggested that the level of HSPG 6-O sulfation rather than the identity of the proteoglycan backbone maybe a critical determinant for binding. Here, we tested in mammalian cells whether SULF1 or SULF2, human endosulfatases, which remove 6-O sulfates from HSPGs, modulate Chlamydia infection. Ectopic expression of SULF1 or SULF2 in HeLa cells, which decreases cell surface HSPG sulfation, diminished C. muridarum binding and decreased vacuole formation. ShRNA depletion of endogenous SULF2 in a cell line that primarily expresses SULF2 augmented binding and increased vacuole formation. C. muridarum infection of diverse cell lines resulted indownregulation of SULF2 mRNA. In a murine model of acute pneumonia, mice genetically deficient in both endosulfatases or in SULF2 alone demonstrated increased susceptibility to C. muridarum lung infection. Collectively, these studies demonstrate that the level of HSPG 6-O sulfation is a critical determinant of C. muridarum infection in vivo and that 6-O endosulfatases are previously unappreciated modulators of microbial pathogenesis.

Chlamydial biology and its associated virulence blockers

Chlamydiales are obligate intracellular parasites of eukaryotic cells. They can be distinguished from other Gram-negative bacteria through their characteristic developmental cycle, in addition to special biochemical and physical adaptations to subvert the eukaryotic host cell. The host spectrum includes humans and other mammals, fish, birds, reptiles, insects and even amoeba, causing a plethora of diseases. The first part of this review focuses on the specific chlamydial infection biology and metabolism. As resistance to classical antibiotics is emerging among Chlamydiae as well, the second part elaborates on specific compounds and tools to block chlamydial virulence traits, such as adhesion and internalization, Type III secretion and modulation of gene expression.

Attachment of Chlamydia trachomatis L2 to host cells requires sulfation

Chlamydia trachomatis is a pathogen responsible for a prevalent sexually transmitted disease. It is also the most common cause of infectious blindness in the developing world. We performed a loss-of-function genetic screen in human haploid cells to identify host factors important in C. trachomatis L2 infection. We identified and confirmed B3GAT3, B4GALT7, and SLC35B2, which encode glucuronosyltransferase I, galactosyltransferase I, and the 3'-phosphoadenosine 5'-phosphosulfate transporter 1, respectively, as important in facilitating Chlamydia infection. Knockout of any of these three genes inhibits Chlamydia attachment. In complementation studies, we found that the introduction of functional copies of these three genes into the null clones restored full susceptibility to Chlamydia infection. The degree of attachment of Chlamydia strongly correlates with the level of sulfation of the host cell, not simply with the amount of heparan sulfate. Thus, other, as-yet unidentified sulfated macromolecules must contribute to infection. These results demonstrate the utility of screens in haploid cells to study interactions of human cells with bacteria. Furthermore, the human null clones generated can be used to investigate the role of heparan sulfate and sulfation in other settings not limited to infectious disease.

The role of intracellular glutathione in the progression of Chlamydia trachomatis infection

The productive internalization in the host cell of Chlamydia trachomatis elementary bodies and their infectivity depends on the degree of reduction of disulfide bonds in the outer envelope of the elementary body. We have hypothesized that the reducing agent may be intracellular glutathione (GSH). Three approaches were used to modulate the intracellular GSH concentration: (1) treatment of cells with buthionine sulfoximine, which causes irreversible inhibition of GSH biosynthesis; (2) hydrogen peroxide-induced oxidation of GSH by intracellular glutathione peroxidases; and (3) treatment of cells with N-acetyl-l-cysteine (NAC), a precursor of glutathione. In the first two cases, we observed a four- to sixfold inhibition of C. trachomatis infection, whereas in NAC-treated cells we detected an increase in the size of chlamydial inclusions. Using a proteomics approach, we showed that the inhibition of chlamydial infection does not combine with alterations in protein expression patterns after cell treatment. These results suggest that GSH plays a key role in the reduction of disulfide bonds in the C. trachomatis outer envelope at an initial stage of the infection.

Uncivil engineers: Chlamydia, Salmonella and Shigella alter cytoskeleton architecture to invade epithelial cells

The obligate intracellular bacterial pathogen Chlamydia trachomatis is a major cause of blindness and sexually transmitted diseases. Like the enteric pathogens Salmonella and Shigella, Chlamydia injects effector proteins into epithelial cells to initiate extensive remodeling of the actin cytoskeleton at the bacterial attachment site, which culminates in the engulfment of the bacterium by plasma membrane extensions. Numerous Salmonella and Shigella effectors promote this remodeling by activating Rho GTPases and tyrosine kinase signaling cascades and by directly manipulating actin dynamics. Recent studies indicate that similar host-cell alterations occur during Chlamydia invasion, but few effectors are known. The identification of additional Chlamydia effectors and the elucidation of their modes of function are critical steps towards an understanding of how this clinically important pathogen breaches epithelial surfaces and causes infection.

Protein and genome evolution in Mammalian cells for biotechnology applications

Mutation and selection are the essential steps of evolution. Researchers have long used in vitro mutagenesis, expression, and selection techniques in laboratory bacteria and yeast cultures to evolve proteins with new properties, termed directed evolution. Unfortunately, the nature of mammalian cells makes applying these mutagenesis and whole-organism evolution techniques to mammalian protein expression systems laborious and time consuming. Mammalian evolution systems would be useful to test unique mammalian cell proteins and protein characteristics, such as complex glycosylation. Protein evolution in mammalian cells would allow for generation of novel diagnostic tools and designer polypeptides that can only be tested in a mammalian expression system. Recent advances have shown that mammalian cells of the immune system can be utilized to evolve transgenes during their natural mutagenesis processes, thus creating proteins with unique properties, such as fluorescence. On a more global level, researchers have shown that mutation systems that affect the entire genome of a mammalian cell can give rise to cells with unique phenotypes suitable for commercial processes. This review examines the advances in mammalian cell and protein evolution and the application of this work toward advances in commercial mammalian cell biotechnology.

Chlamydial entry involves TARP binding of guanine nucleotide exchange factors

Chlamydia trachomatis attachment to cells induces the secretion of the elementary body–associated protein TARP (Translocated Actin Recruiting Protein). TARP crosses the plasma membrane where it is immediately phosphorylated at tyrosine residues by unknown host kinases. The Rac GTPase is also activated, resulting in WAVE2 and Arp2/3-dependent recruitment of actin to the sites of chlamydia attachment. We show that TARP participates directly in chlamydial invasion activating the Rac-dependent signaling cascade to recruit actin. TARP functions by binding two distinct Rac guanine nucleotide exchange factors (GEFs), Sos1 and Vav2, in a phosphotyrosine-dependent manner. The tyrosine phosphorylation profile of the sequence YEPISTENIYESI within TARP, as well as the transient activation of the phosphatidylinositol 3-kinase (PI₃-K), appears to determine which GEF is utilized to activate Rac. The first and second tyrosine residues, when phosphorylated, are utilized by the Sos1/Abi1/Eps8 and Vav2, respectively, with the latter requiring the lipid phosphatidylinositol 3,4,5-triphosphate. Depletion of these critical signaling molecules by siRNA resulted in inhibition of chlamydial invasion to varying degrees, owing to a possible functional redundancy of the two pathways. Collectively, these data implicate TARP in signaling to the actin cytoskeleton remodeling machinery, demonstrating a mechanism by which C. trachomatis invades non-phagocytic cells.

The human pathogen Chlamydia trachomatis is the causative agent of the most prevalent bacterial sexually transmitted disease in industrialized nations, and of the preventable blinding condition trachoma in developing countries. Survival and replication of chlamydial species occur exclusively inside a host cell, and thus, gaining access to the protective intracellular niche is an absolute requirement. This report describes how the chlamydia protein TARP, which is secreted at the base of the bacteria and across the host membrane, acts as a scaffold to which host signaling proteins bind. This assembly of the complex of signaling proteins, which include Sos1, Abi1, Eps8, and Vav2 results in the remodeling of the host cytoskeleton to facilitate engulfment of the infecting chlamydia. We conclude that these proteins have a role in chlamydia based on a number of observations including their interaction with the TARP protein, their ability to switch on known signaling participants in chlamydia invasion, their localization at the site of chlamydia entry, and the inhibition of chlamydia invasion in their absence. Altogether, the data functionally link TARP with signaling pathways that function in chlamydial invasion, demonstrating the direct involvement of TARP in the invasion of host cells by C. trachomatis.

RNA interference screen identifies Abl kinase and PDGFR signaling in Chlamydia trachomatis entry

To elucidate the mechanisms involved in early events in Chlamydia trachomatis infection, we conducted a large scale unbiased RNA interference screen in Drosophila melanogaster S2 cells. This allowed identification of candidate host factors in a simple non-redundant, genetically tractable system. From a library of 7,216 double stranded RNAs (dsRNA), we identified ∼226 host genes, including two tyrosine kinases, Abelson (Abl) kinase and PDGF- and VEGF-receptor related (Pvr), a homolog of the Platelet-derived growth factor receptor (PDGFR). We further examined the role of these two kinases in C. trachomatis binding and internalization into mammalian cells. Both kinases are phosphorylated upon infection and recruited to the site of bacterial attachment, but their roles in the infectious process are distinct. We provide evidence that PDGFRβ may function as a receptor, as inhibition of PDGFRβ by RNA interference or by PDGFRβ neutralizing antibodies significantly reduces bacterial binding, whereas depletion of Abl kinase has no effect on binding. Bacterial internalization can occur through activation of PDGFRβ or through independent activation of Abl kinase, culminating in phosphorylation of the Rac guanine nucleotide exchange factor (GEF), Vav2, and two actin nucleators, WAVE2 and Cortactin. Finally, we show that TARP, a bacterial type III secreted actin nucleator implicated in entry, is a target of Abl kinase. Together, our results demonstrate that PDGFRβ and Abl kinases function redundantly to promote efficient uptake of this obligate intracellular parasite.

Chlamydia trachomatis infections are a worldwide problem; they are the leading cause of preventable blindness in developing nations and the most common cause of sexually transmitted disease in the Western world. Binding and entry into host cells are critical steps to the pathogenesis of this obligate intracellular parasite; however little is known regarding the mechanism of these processes. In this work, we describe a large scale RNA interference screen to identify host factors essential for early steps in C. trachomatis infection. We discover that the Platelet Derived Growth Factor Receptor β (PDGFRβ) can function as a receptor for C. trachomatis, and that activation of both PDGFRβ and Abl kinase signaling pathways by C. trachomatis leads to phosphorylation of a Rac guanine nucleotide exchange factor, Vav2, and several actin nucleators, including WAVE2, Cortactin, and TARP, a Chlamydia type III secreted effector. Our work suggests a model of redundant activation of PDGFRβ and Abl kinase upon C. trachomatis binding that culminates in cytoskeletal rearrangements that modulate efficient uptake of this obligate intracellular parasite.

Selection of mutant CHO clones resistant to murine gammaherpesvirus 68 infection

Murine gammaherpesvirus 68 (MHV68) is used as a model to study gammaherpesvirus pathogenesis both in tissue culture systems and in vivo. We used a gene-trapping approach to get insight into cellular factors involved in MHV68 infection. By generating a library of gene-trapped CHO cells, we were able to isolate several clones that exhibited various degrees of resistance to MHV68-induced cytopathic effect. Clones that showed the highest degree of resistance were affected at the early stage of the viral cycle, with the vast majority of these clones being deficient for heparan sulfate (HS) expression at the cell surface. Heparan sulfate expression could be restored in all the HS-deficient clones by expression of EXT1, an enzyme that is essential for the biosynthesis of HS. Consistent with the role of HS in viral entry, HS-deficient CHO cells did not support viral internalization. Cell surface heparan sulfate proteoglycans (HSPG) are mostly composed of HS chains attached to two families of core proteins, the transmembrane syndecans and the GPI-anchored glypicans. Treatment of CHO cells with phosphatidylinositol-specific phospholipase C (PI-PLC) did not significantly affect the level of HS expression, indicating that the glypicans are not a major source of HSPG in CHO cells. By contrast, treatment of CHO cells with PMA, a drug known to accelerate syndecan shedding, resulted in a decrease in both HS expression and susceptibility to MHV68; these effects were abolished by TIMP-3, a specific inhibitor of syndecan shedding. All together, our results confirm the essential role of HS in MHV68 infection and identify the syndecans as a major source of HSPG used by the virus as coreceptors to infect CHO cells.

Chlamydia trachomatis species-specific induction of ezrin tyrosine phosphorylation functions in pathogen entry

Chlamydia trachomatis is an obligate intracellular pathogen of humans that exhibits species-specific biological characteristics in its early interactions with host cells that are likely important to pathogenesis. One such characteristic is the tyrosine phosphorylation (Tyr-P) of an approximately 70-kDa polypeptide that occurs only after infection of mammalian cells by human strains. We sought to identify this protein because of its potential significance to the pathogenesis of human chlamydial infections. Using an immunoproteomic approach we identified the host protein ezrin, a member of the ezrin-radixin-moesin (ERM) protein family that serves as a physical link between host cell receptors and the actin cytoskeleton. Confocal microscopy studies showed colocalization of ezrin and actin at the tips and crypts of microvilli, the site of chlamydial attachment and entry, respectively. To demonstrate a functional role for ezrin we infected cells with a dominant-negative (DN) ezrin phenotype or treated cells with ezrin-specific small interfering RNA (siRNA). We found that both DN and siRNA-treated cells were significantly less susceptible to infection by human chlamydial strains. Moreover, we demonstrated that inhibition of infection in ezrin DN cells occurred at the stage of chlamydial entry. We hypothesize that the C. trachomatis-specific Tyr-P of ezrin might relate to an undefined species-specific mechanism of pathogen entry that involves chlamydial specific ligand(s) and host cell coreceptor usage.

Rac interacts with Abi-1 and WAVE2 to promote an Arp2/3-dependent actin recruitment during chlamydial invasion

Chlamydiae are Gram-negative obligate intracellular pathogens to which access to an intracellular environment is fundamental to their development. Chlamydial attachment to host cells induces the activation of the Rac GTPase, which is required for the localization of WAVE2 at the sites of chlamydial entry. Co-immunoprecipitation experiments demonstrated that Chlamydia trachomatis infection promoted the interaction of Rac with WAVE2 and Abi-1, but not with IRSp53. siRNA depletion of WAVE2 and Abi-1 abrogated chlamydia-induced actin recruitment and significantly reduced the uptake of the pathogen by the depleted cells. Chlamydia invasion also requires the Arp2/3 complex as demonstrated by its localization to the sites of chlamydial attachment and the reduced efficiency of chlamydial invasion in cells overexpressing the VCA domain of the neural Wiskott-Aldrich syndrome protein. Thus, C. trachomatis activates Rac and promotes its interaction with WAVE2 and Abi-1 to activate the Arp2/3 complex resulting in the induction of actin cytoskeletal rearrangements that are required for invasion.

Mechanisms of Chlamydia trachomatis entry into nonphagocytic cells

The mechanisms of entry for the obligate intracellular bacterium C. trachomatis were examined by functional disruption of proteins essential for various modes of entry. RNA interference was used to disrupt proteins with established roles in clathrin-mediated endocytosis (clathrin heavy chain, dynamin-2, heat shock 70-kDa protein 8, Arp2, cortactin, and calmodulin), caveola-mediated endocytosis (caveolin-1, dynamin-2, Arp2, NSF, and annexin II), phagocytosis (RhoA, dynamin-2, Rac1, and Arp2), and macropinocytosis (Pak1, Rac1, and Arp2). Comparative quantitative PCR analysis was performed on small interfering RNA-transfected HeLa cells to accurately determine the extent of C. trachomatis entry after these treatments. Key structural and regulatory factors associated with clathrin-mediated endocytosis were found to be involved in Chlamydia entry, whereas those for caveola-mediated endocytosis, phagocytosis, and macropinocytosis were not. Thus, clathrin and its coordinate accessory factors were required for entry of C. trachomatis, although additional, uncharacterized mechanisms are also utilized.

Epithelial membrane protein 2 modulates infectivity of Chlamydia muridarum (MoPn)

Chlamydiae are bacterial pathogens which have evolved efficient strategies to enter, replicate, and survive inside host epithelial cells, resulting in acute and chronic diseases in humans and other animals. Several candidate molecules in the host receptor complex have been identified, but the precise mechanisms of infection have not been elucidated. Epithelial membrane protein-2 (EMP2), a 4-transmembrane protein, is highly expressed in epithelial cells in sites of chlamydial infections. Here we show that infectivity of the Chlamydia muridarum (MoPn) is associated with host cellular expression of EMP2 in multiple cell lines. Recombinant knockdown of EMP2 impairs infectivity, whereas infectivity is augmented in cells recombinantly modified to over-express EMP2. An epithelial cell line without native expression of EMP2 is relatively resistant to MoPn infection, whereas infectivity is markedly increased by recombinant expression of EMP2 in that cell line. Blockade of surface EMP2 using a specific anti-EMP2 antibody significantly reduces chlamydial infection efficiency. In addition, MoPn infectivity as measured in the EMP2 overexpressing cell line is not heparin-dependent, suggesting a possible role for EMP2 in the non-reversible phase of early infection. These findings identify EMP2 as a candidate host protein involved in infection of C. muridarum (MoPn).

Chlamydia attachment to mammalian cells requires protein disulfide isomerase

For Chlamydia, an intracellular pathogen of humans, host cell invasion is obligatory for survival, growth and pathogenesis. At the molecular level, little is known about the binding and entry of Chlamydia into the mammalian host cell. Chlamydia are genetically intractable therefore experimental approaches targeting the host are often necessary. CHO6 is a mutagenized cell line resistant to attachment and infection by Chlamydia. In this study, CHO6 was shown using proteomic methods to have a defect in processing of the leader sequence for protein disulfide isomerase (PDI). Complementation by expression of full-length PDI restored C. trachomatis binding and infectivity in the CHO6 mutant cell line. The cell line was also resistant to diphtheria toxin and required complemented cell-surface PDI for toxin entry. These data demonstrate that native PDI at the cell surface is required for effective chlamydial attachment and infectivity.

Chlamydial TARP is a bacterial nucleator of actin

Chlamydia trachomatis entry into host cells results from a parasite-directed remodeling of the actin cytoskeleton. A type III secreted effector, TARP (translocated actin recruiting phosphoprotein), has been implicated in the recruitment of actin to the site of internalization. To elucidate the role of TARP in actin recruitment, we identified host cell proteins that associated with recombinant GST-TARP fusions. TARP directly associated with actin, and this interaction promoted actin nucleation as determined by in vitro polymerization assays. Domain analysis of TARP identified an actin-binding domain that bears structural and primary amino acid sequence similarity to WH2 domain family proteins. In addition, a proline-rich domain was found to promote TARP oligomerization and was required for TARP-dependent nucleation of new actin filaments. Our findings reveal a mechanism by which chlamydiae induce localized cytoskeletal changes by the translocated effector TARP during entry into host cells.

A plasmid-cured Chlamydia muridarum strain displays altered plaque morphology and reduced infectivity in cell culture

A highly conserved cryptic plasmid is present in Chlamydia trachomatis yet naturally occurring plasmid-deficient isolates are very rare. This paper describes the isolation and characterization of a plasmid-deficient strain of C. muridarum, using novobiocin as a curing agent. Plasmid-deficient derivatives of C. muridarum strain Nigg were generated at high efficiencies (4-30%). Phenotypic characterization revealed that the cured derivative was unable to accumulate glycogen within intracytoplasmic inclusions. In addition, this strain formed small plaques at a reduced efficiency compared to the wild-type parent.

Recent insights into the mechanisms of Chlamydia entry

Chlamydia are widespread bacteria that grow in human and animal cells. They enter their host cell, establish an intracellular environment favourable for their multiplication and finally exit the host cell. A combination of host cell factors and of bacterial proteins contribute to pathogen entry. Recent advances have shed new light on the entry mechanism, following attachment. Here we review recent data concerning endocytosis, host cell signalling, proteins secreted by the bacteria, the actin cytoskeleton in entry and the involvement of small GTPases.

Drosophila melanogaster S2 cells: a model system to study Chlamydia interaction with host cells

Chlamydia spp. are major causes of important human diseases, but dissecting the host-pathogen interactions has been hampered by the lack of bacterial genetics and the difficulty in carrying out forward genetic screens in mammalian hosts. RNA interference (RNAi)-based methodologies for gene inactivation can now be easily carried out in genetically tractable model hosts, such as Drosophila melanogaster, and offer a new approach to identifying host genes required for pathogenesis. We tested whether Chlamydia trachomatis infection of D. melanogaster S2 cells recapitulated critical aspects of mammalian cell infections. As in mammalian cells, C. trachomatis entry was greatly reduced by heparin and cytochalasin D. Inclusions were formed in S2 cells, acquired Golgi-derived sphingolipids, and avoided phagolysosomal fusion. Elementary body (EB) to reticulate body (RB) differentiation was observed, however, no RB to EB development or host cell killing was observed. RNAi-mediated inactivation of Rac, a Rho GTPase recently shown to be required for C. trachomatis entry in mammalian cells, inhibits C. trachomatis infection in S2 cells. We conclude that Drosophila S2 cells faithfully mimic early events in Chlamydia host cell interactions and provides a bona fide system to systematically dissect host functions important in the pathogenesis of obligate intracellular pathogens.

The chlamydial developmental cycle

Intracellular parasitism by bacterial pathogens is a complex, multi-factorial process that has been exploited successfully by a wide variety of organisms. Members of the Order Chlamydiales are obligate intracellular bacteria that are transmitted as metabolically inactive particles and must differentiate, replicate, and re-differentiate within the host cell to carry out their life cycle. Understanding the developmental cycle has been greatly advanced by the availability of complete genome sequences, DNA microarrays, and advanced cell biology techniques. Measuring transcriptional changes throughout the cycle has allowed investigators to determine the nature of the temporal gene expression changes required for bacterial growth and development.

Tyrosine phosphorylation of the chlamydial effector protein Tarp is species specific and not required for recruitment of actin

Chlamydiae are obligate intracellular pathogens that efficiently induce their endocytosis by susceptible eukaryotic host cells. Recently, a Chlamydia trachomatis type III secreted effector protein, Tarp, was found to be translocated and tyrosine phosphorylated at the site of entry and associated with the recruitment of actin that coincides with endocytosis. C. trachomatis Tarp possesses up to six direct repeats of approximately 50 amino acids each. The majority of the tyrosine residues are found within this repeat region. Here we have ectopically expressed distinct domains of Tarp in HeLa 229 cells and demonstrated that tyrosine phosphorylation occurs primarily within the repeat region, while recruitment of actin is mediated by the C-terminal domain of the protein. A comparison of other sequenced chlamydial genomes revealed that each contains an ortholog of Tarp, although Chlamydia muridarum, Chlamydophila caviae, and Chlamydophila pneumoniae Tarp lack the large repeat region. Immunofluorescence and immunoblotting using an antiphosphotyrosine antibody show no evidence of phosphotyrosine at the site of entry of C. muridarum, C. caviae, and C. pneumoniae, although each species similarly recruits actin. Ectopic expression of full-length C. trachomatis and C. caviae Tarp confirmed that both recruit actin but only C. trachomatis Tarp is tyrosine phosphorylated. The data indicate that the C-terminal domain of Tarp is essential for actin recruitment and that tyrosine phosphorylation may not be an absolute requirement for actin recruitment. The results further suggest the potential for additional, unknown signal transduction pathways associated specifically with C. trachomatis.

Chlamydial infection induces pathobiotype-specific protein tyrosine phosphorylation in epithelial cells

Members of the genus Chlamydia are strict obligate intracellular pathogens that exhibit marked differences in host range and tissue tropism despite sharing a remarkable level of genomic synteny. These pathobiotype differences among chlamydiae are also mirrored in their early interactions with cultured mammalian host cells. Chlamydial attachment and entry is known to trigger protein tyrosine phosphorylation. In this study, we examined the kinetics and pattern of protein tyrosine phosphorylation induced by infection with a comprehensive collection of chlamydial strains exhibiting diversity in host, tissue, and disease tropisms. We report new findings showing that protein tyrosine phosphorylation patterns induced by infection directly correlate with the pathobiotype of the infecting organism. Patterns of protein tyrosine phosphorylation were induced following early infection that unambiguously categorized chlamydial pathobiotypes into four distinct groups: (i) Chlamydia trachomatis trachoma biovars (serovars A to H), (ii) C. trachomatis lymphogranuloma venereum biovars (serovars L1 to L3), (iii) C. muridarum, and (iv) C. pneumoniae and C. caviae. Notably, chlamydia-infected murine and human epithelial cells exhibited the same protein tyrosine phosphorylation patterns; this is indirect evidence suggesting that the phosphorylated protein(s) is of chlamydial origin. If our hypothesis is correct, these heretofore-uncharacterized proteins may represent a novel class of bacterial molecules that influence pathogen-host range or tissue tropism.

Lipid raft-mediated entry is not required for Chlamydia trachomatis infection of cultured epithelial cells

Using pharmacologic and biochemical criteria, we evaluated whether uptake of four different Chlamydia trachomatis serovars, D, E, K, and L2, was dependent upon lipid rafts. Our data suggest that lipid raft-mediated entry is not required for C. trachomatis infection of cultured epithelial cells.

Chlamydia- Host Cell Interactions: Recent Advances on Bacterial Entry and Intracellular Development

Bacteria of the Chlamydiales order are very successful intracellular organisms that grow in human and animal cells, and even in amoebae. They fulfill several essential functions to enter their host cells, establish an intracellular environment favorable for their multiplication and exit the host cell. They multiply in a unique organelle called the inclusion, which is isolated from the endocytic but not the exocytic pathway. A combination of host cell factors and of proteins secreted by the bacteria, from within the inclusion, contribute to the establishment and development of this inclusion. Here we review recent data on the entry mechanisms and maturation of the inclusion.

A chlamydial type III translocated protein is tyrosine-phosphorylated at the site of entry and associated with recruitment of actin

The obligate intracellular bacterium Chlamydia trachomatis rapidly induces its own entry into host cells. Initial attachment is mediated by electrostatic interactions to heparan sulfate moieties on the host cell, followed by irreversible binding to an unknown secondary receptor. This secondary binding leads to the recruitment of actin to the site of attachment, formation of an actin-rich, pedestal-like structure, and finally internalization of the bacteria. How chlamydiae induce this process is unknown. We have identified a high-molecular-mass tyrosine-phosphorylated protein that is rapidly phosphorylated on attachment to the host cell. Immunoelectron microscopy studies revealed that this tyrosine-phosphorylated protein is localized to the cytoplasmic face of the plasma membrane at the site of attachment of surface-associated chlamydiae. The phosphoprotein was isolated by immunoprecipitation with the antiphosphotyrosine antibody 4G10 and identified as the chlamydial protein CT456, a hypothetical protein with unknown function. The chlamydial protein (Tarp) appears to be translocated into the host cell by type III secretion because it is exported in a Yersinia heterologous expression assay. Phosphotyrosine signaling across the plasma membrane preceded the recruitment of actin to the site of chlamydial attachment and may represent the initial signal transduced from pathogen to the host cell. These results suggest that C. trachomatis internalization is mediated by a chlamydial type III-secreted effector protein.

Selection of mutant cell lines resistant to infection by Chlamydia spp [corrected]

The lytic outcome of natural infection by Chlamydia trachomatis was exploited to select CHO (Chinese hamster ovary) cells, following chemical mutagenesis, that were deficient in their ability to sustain productive chlamydial infection. Four distinct mutant cell phenotypes with defects in either attachment or postattachment mechanisms that are required for infection by C. trachomatis and Chlamydia pneumoniae were characterized.

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

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

Identification of MEK- and phosphoinositide 3-kinase-dependent signalling as essential events during Chlamydia pneumoniae invasion of HEp2 cells

The ability of Chlamydia pneumoniae to survive and cause disease is predicated on efficient invasion of cellular hosts. While it is recognized that chlamydial determinants are important for mediating attachment and uptake into non-phagocytic cells, little is known about the bacterial ligands and cellular receptors that facilitate invasion or host cell signal transduction pathways implicated in this process. We used transmission and scanning electron microscopy to demonstrate that attachment of bacteria to host cells induced the appearance of microvilli on host cell membranes. Invasion occurred 30-120 min after cell contact with the subsequent loss of membrane microvilli. Using an epithelial cell infection model, C. pneumoniae invasion caused a rapid and sustained increase in MEK-dependent phosphorylation and activation of ERK1/2, followed by PI 3-kinase-dependent phosphorylation and activation of Akt. Tyrosine phosphorylation of focal adhesion kinase (FAK) preceded its appearance in a complex with the p85 subunit of PI 3-kinase during chlamydial invasion and isoform-specific tyrosine phosphorylation of the docking protein Shc also occurred at the time of attachment and entry of bacteria. Chlamydia entry but not attachment could be abrogated with specific inhibitors of MEK, PI 3-kinase and actin polymerization, demonstrating the importance of these signalling pathways and an intact actin cytoskeleton for C. pneumoniae invasion. These results suggest that activation of specific cell signalling pathways is an essential strategy used by C. pneumoniae to invade epithelial cells.

Effects of chemically modified heparin on Chlamydia trachomatis serovar L2 infection of eukaryotic cells in culture

The mechanism and inhibitors of Chlamydia trachomatis serovar L2 infection of eukaryotic host cells were studied using a tissue culture model infection system. Potent inhibition of infectivity was observed when elementary bodies (EBs) were exposed to heparin or when HeLa 229 cells were treated with heparinase. No significant inhibition was seen the other way around. The same potent inhibition was observed when EBs were exposed to chemically 2-O-desulfated heparin (2-ODS heparin), which is composed of repeating disaccharide units of IdoA-GlcNS(6S), but not when exposed to chemically 6-ODS heparin or completely desulfated and N-resulfated heparin, which is composed of repeating disaccharide units of IdoA(2S)-GlcNS or IdoA-GlcNS, respectively. The inhibitory effects of 2-ODS heparin could be seen only with oligosaccharides longer than dodecasaccharides. The mutant Chinese hamster ovary (CHO) cell line 677, which is deficient in the biosynthesis of heparan sulfate, was less sensitive to C. trachomatis infection than were wild-type CHO cells. F-17 cells, deficient in 2-O-sulfation of heparan sulfate, had the same sensitivity to infection as wild-type CHO cells did. These data suggest that infection of host cells by EBS results from the specific binding of ligand molecules with affinity for heparin on the EB surface to heparan sulfate proteoglycans on the host cell surface. This binding may depend on host cell heparan sulfate chains that are 6-O-sulfated and longer than dodecasaccharides. The 2-ODS heparin oligosaccharides may be a potential agent for the prevention of C. trachomatis infection.