Both the N- and C- terminal regions of the Chlamydial inclusion protein D (IncD) are required for interaction with the pleckstrin homology domain of the ceramide transport protein CERT.

Chlamydia trachomatis is an obligate intracellular bacterium that replicates within a membranous compartment, the inclusion, in host cells. Its intracellular life cycle requires host sphingolipids, which are in part acquired through the ER-Golgi localized ceramide transport protein (CERT). The Chlamydia-encoded inclusion membrane protein IncD is composed of two closely linked long hydrophobic domains with their N- and C-termini exposed to the host cytosol. IncD binds directly to the pleckstrin homology (PH) domain of CERT, likely redirecting ceramide to the inclusion. The precise regions of IncD required for this interaction have not been delineated. Using co-transfection studies together with phylogenetic studies, we demonstrate that both the IncD N- and C-terminal regions are required for binding to the CERT PH domain and define key interaction residues. Native gel electrophoresis analysis demonstrates that the transmembrane region of IncD forms SDS-resistant but dithiothreitol-sensitive homodimers, which in turn can assemble to form higher order oligomers through additional N- and C-terminal domain contacts. IncD oligomerization may facilitate high affinity binding to CERT, allowing C. trachomatis to efficiently redirect host ceramide to the inclusion.

The Chlamydia effector IncE employs two short linear motifs to reprogram host vesicle trafficking.

Chlamydia trachomatis, a leading cause of bacteria sexually transmitted infections, creates a specialized intracellular replicative niche by translocation and insertion of a diverse array of effectors (Incs) into the inclusion membrane. Here, we characterize IncE, a multi-functional Inc that encodes two non-overlapping short linear motifs (SLiMs) within its short cytosolic C-terminus. The proximal SLiM mimics an R-SNARE motif to recruit syntaxin (STX) 7 and 12-containing vesicles to the inclusion. The distal SLiM mimics the Sorting Nexin (SNX) 5 and 6 cargo binding site to recruit SNX6-containing vesicles to the inclusion. By simultaneously binding to two distinct vesicle classes, IncE reprograms host cell trafficking to promote the formation of a class of hybrid vesicles at the inclusion that are required for C. trachomatis intracellular development. Our work suggests that Incs may have evolved SLiMs to facilitate rapid evolution in a limited protein space to disrupt host cell processes.

The Chlamydia effector IncE employs two short linear motifs to reprogram host vesicle trafficking.

Chlamydia trachomatis, a leading cause of bacterial sexually transmitted infections, creates a specialized intracellular replicative niche by translocation and insertion of a diverse array of effectors (Incs [inclusion membrane proteins]) into the inclusion membrane. Here, we characterize IncE, a multifunctional Inc that encodes two non-overlapping short linear motifs (SLiMs) within its short cytosolic C terminus. The proximal SLiM, by mimicking just a small portion of an R-N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE) motif, binds and recruits syntaxin (STX)7- and STX12-containing vesicles to the inclusion. The distal SLiM mimics the sorting nexin (SNX)5 and SNX6 cargo binding site to recruit SNX6-containing vesicles to the inclusion. By simultaneously binding two distinct vesicle classes, IncE brings these vesicles in close apposition with each other at the inclusion to facilitate C. trachomatis intracellular development. Our work suggests that Incs may have evolved SLiMs to enable rapid evolution in a limited protein space to disrupt host cell processes.

Global mapping of the Chlamydia trachomatis conventional secreted effector - host interactome reveals CebN interacts with nucleoporins and Rae1 to impede STAT1 nuclear translocation.

Chlamydia trachomatis (C.t.), the leading cause of bacterial sexually transmitted infections, employs a type III secretion system (T3SS) to translocate two classes of effectors, inclusion membrane proteins and conventional T3SS (cT3SS) effectors, into the host cell to counter host defense mechanisms. Here we employed three assays to directly evaluate secretion during infection, validating secretion for 23 cT3SS effectors. As bioinformatic analyses have been largely unrevealing, we conducted affinity purification-mass spectrometry to identify host targets and gain insights into the functions of these effectors, identifying high confidence interacting partners for 21 cT3SS effectors. We demonstrate that CebN localizes to the nuclear envelope in infected and bystander cells where it interacts with multiple nucleoporins and Rae1, blocking STAT1 nuclear import following IFN-γ stimulation. By building a cT3SS effector-host interactome, we have identified novel pathways that are targeted during bacterial infection and have begun to address how C.t. effectors combat cell autonomous immunity.

Chlamydia trachomatis co-opts the FGF2 signaling pathway to enhance infection.

The molecular details of Chlamydia trachomatis binding, entry, and spread are incompletely understood, but heparan sulfate proteoglycans (HSPGs) play a role in the initial binding steps. As cell surface HSPGs facilitate the interactions of many growth factors with their receptors, we investigated the role of HSPG-dependent growth factors in C. trachomatis infection. Here, we report a novel finding that Fibroblast Growth Factor 2 (FGF2) is necessary and sufficient to enhance C. trachomatis binding to host cells in an HSPG-dependent manner. FGF2 binds directly to elementary bodies (EBs) where it may function as a bridging molecule to facilitate interactions of EBs with the FGF receptor (FGFR) on the cell surface. Upon EB binding, FGFR is activated locally and contributes to bacterial uptake into non-phagocytic cells. We further show that C. trachomatis infection stimulates fgf2 transcription and enhances production and release of FGF2 through a pathway that requires bacterial protein synthesis and activation of the Erk1/2 signaling pathway but that is independent of FGFR activation. Intracellular replication of the bacteria results in host proteosome-mediated degradation of the high molecular weight (HMW) isoforms of FGF2 and increased amounts of the low molecular weight (LMW) isoforms, which are released upon host cell death. Finally, we demonstrate the in vivo relevance of these findings by showing that conditioned medium from C. trachomatis infected cells is enriched for LMW FGF2, accounting for its ability to enhance C. trachomatis infectivity in additional rounds of infection. Together, these results demonstrate that C. trachomatis utilizes multiple mechanisms to co-opt the host cell FGF2 pathway to enhance bacterial infection and spread.

Chlamydia trachomatis co-opts GBF1 and CERT to acquire host sphingomyelin for distinct roles during intracellular development.

The strain designated Chlamydia trachomatis serovar that was used for experiments in this paper is Chlamydia muridarum, a species closely related to C. trachomatis (and formerly termed the Mouse Pneumonitis strain of C. trachomatis. [corrected]. The obligate intracellular pathogen Chlamydia trachomatis replicates within a membrane-bound inclusion that acquires host sphingomyelin (SM), a process that is essential for replication as well as inclusion biogenesis. Previous studies demonstrate that SM is acquired by a Brefeldin A (BFA)-sensitive vesicular trafficking pathway, although paradoxically, this pathway is dispensable for bacterial replication. This finding suggests that other lipid transport mechanisms are involved in the acquisition of host SM. In this work, we interrogated the role of specific components of BFA-sensitive and BFA-insensitive lipid trafficking pathways to define their contribution in SM acquisition during infection. We found that C. trachomatis hijacks components of both vesicular and non-vesicular lipid trafficking pathways for SM acquisition but that the SM obtained from these separate pathways is being utilized by the pathogen in different ways. We show that C. trachomatis selectively co-opts only one of the three known BFA targets, GBF1, a regulator of Arf1-dependent vesicular trafficking within the early secretory pathway for vesicle-mediated SM acquisition. The Arf1/GBF1-dependent pathway of SM acquisition is essential for inclusion membrane growth and stability but is not required for bacterial replication. In contrast, we show that C. trachomatis co-opts CERT, a lipid transfer protein that is a key component in non-vesicular ER to trans-Golgi trafficking of ceramide (the precursor for SM), for C. trachomatis replication. We demonstrate that C. trachomatis recruits CERT, its ER binding partner, VAP-A, and SM synthases, SMS1 and SMS2, to the inclusion and propose that these proteins establish an on-site SM biosynthetic factory at or near the inclusion. We hypothesize that SM acquired by CERT-dependent transport of ceramide and subsequent conversion to SM is necessary for C. trachomatis replication whereas SM acquired by the GBF1-dependent pathway is essential for inclusion growth and stability. Our results reveal a novel mechanism by which an intracellular pathogen redirects SM biosynthesis to its replicative niche.

Lipid acquisition by intracellular Chlamydiae.

Chlamydia species are obligate intracellular pathogens that are important causes of human genital tract, ocular and respiratory infections. The bacteria replicate within a specialized membrane-bound compartment termed the inclusion and require host-derived lipids for intracellular growth and development. Emerging evidence indicates that Chlamydia has evolved clever strategies to fulfil its lipid needs by interacting with multiple host cell compartments and redirecting trafficking pathways to its intracellular niche. In this review, we highlight recent findings that have significantly expanded our understanding of how Chlamydia exploit lipid trafficking pathways to ensure the survival of this important human pathogen.

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

The strain designated Chlamydia trachomatis serovar L2 that was used for experiments in this paper is Chlamydia muridarum, a species closely related to C. trachomatis (and formerly termed the Mouse Pneumonitis strain of C. trachomatis). This conclusion was verified by deep sequencing and by PCR using species-specific primers. All data presented in the results section that refer to C. trachomatis should be interpreted as referring to C. muridarum. Since C. muridarum TARP lacks the consensus tyrosine repeats present in C. trachomatis TARP, we cannot make any conclusions about the role of TARP phosphorylation and C. muridarum entry. However, the conclusion that C. trachomatis L2 TARP is a target of Abl kinase is still valid as these experiments were performed with C. trachomatis L2 TARP [corrected]. 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 approximately 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 PDGFRbeta may function as a receptor, as inhibition of PDGFRbeta by RNA interference or by PDGFRbeta neutralizing antibodies significantly reduces bacterial binding, whereas depletion of Abl kinase has no effect on binding. Bacterial internalization can occur through activation of PDGFRbeta 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 PDGFRbeta and Abl kinases function redundantly to promote efficient uptake of this obligate intracellular parasite.

RNA interference inhibition of Mus81 reduces mitotic recombination in human cells.

Mus81 is a highly conserved endonuclease with homology to the XPF subunit of the XPF-ERCC1 complex. In yeast Mus81 associates with a second subunit, Eme1 or Mms4, which is essential for endonuclease activity in vitro and for in vivo function. Human Mus81 binds to a homolog of fission yeast Eme1 in vitro and in vivo. We show that recombinant Mus81-Eme1 cleaves replication forks, 3' flap substrates, and Holliday junctions in vitro. By use of differentially tagged versions of Mus81 and Eme1, we find that Mus81 associates with Mus81 and that Eme1 associates with Eme1. Thus, complexes containing two or more Mus81-Eme1 units could function to coordinate substrate cleavage in vivo. Down-regulation of Mus81 by RNA interference reduces mitotic recombination in human somatic cells. The recombination defect is rescued by expression of a bacterial Holliday junction resolvase. These data provide direct evidence for a role of Mus81-Eme1 in mitotic recombination in higher eukaryotes and support the hypothesis that Mus81-Eme1 resolves Holliday junctions in vivo.

Chlamydia interfere with an interaction between the mannose-6-phosphate receptor and sorting nexins to counteract host restriction.

Chlamydia trachomatis is an obligate intracellular pathogen that resides in a membrane-bound compartment, the inclusion. The bacteria secrete a unique class of proteins, Incs, which insert into the inclusion membrane and modulate the host-bacterium interface. We previously reported that IncE binds specifically to the Sorting Nexin 5 Phox domain (SNX5-PX) and disrupts retromer trafficking. Here, we present the crystal structure of the SNX5-PX:IncE complex, showing IncE bound to a unique and highly conserved hydrophobic groove on SNX5. Mutagenesis of the SNX5-PX:IncE binding surface disrupts a previously unsuspected interaction between SNX5 and the cation-independent mannose-6-phosphate receptor (CI-MPR). Addition of IncE peptide inhibits the interaction of CI-MPR with SNX5. Finally, C. trachomatis infection interferes with the SNX5:CI-MPR interaction, suggesting that IncE and CI-MPR are dependent on the same binding surface on SNX5. Our results provide new insights into retromer assembly and underscore the power of using pathogens to discover disease-related cell biology.

Global Mapping of the Inc-Human Interactome Reveals that Retromer Restricts Chlamydia Infection.

Chlamydia trachomatis is a leading cause of genital and ocular infections for which no vaccine exists. Upon entry into host cells, C. trachomatis resides within a membrane-bound compartment­the inclusion­and secretes inclusion membrane proteins (Incs) that are thought to modulate the host-bacterium interface. To expand our understanding of Inc function(s), we subjected putative C. trachomatis Incs to affinity purification-mass spectroscopy (AP-MS). We identified Inc-human interactions for 38/58 Incs with enrichment in host processes consistent with Chlamydia's intracellular life cycle. There is significant overlap between Inc targets and viral proteins, suggesting common pathogenic mechanisms among obligate intracellular microbes. IncE binds to sorting nexins (SNXs) 5/6, components of the retromer, which relocalizes SNX5/6 to the inclusion membrane and augments inclusion membrane tubulation. Depletion of retromer components enhances progeny production, revealing that retromer restricts Chlamydia infection. This study demonstrates the value of proteomics in unveiling host-pathogen interactions in genetically challenging microbes.

Chlamydial intracellular survival strategies.

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen and the causative agent of blinding trachoma. Although Chlamydia is protected from humoral immune responses by residing within remodeled intracellular vacuoles, it still must contend with multilayered intracellular innate immune defenses deployed by its host while scavenging for nutrients. Here we provide an overview of Chlamydia biology and highlight recent findings detailing how this vacuole-bound pathogen manipulates host-cellular functions to invade host cells and maintain a replicative niche.

Species-specific interactions of Src family tyrosine kinases regulate Chlamydia intracellular growth and trafficking.

Src family kinases (SFKs) regulate key cellular processes and are emerging as important targets for intracellular pathogens. In this commentary, we briefly review the role of SFKs in bacterial pathogenesis and highlight new work on the role of SFKs during the intracellular cycle of Chlamydia species.