Disulfide bonding within components of the Chlamydia type III secretion apparatus correlates with development

Chlamydia spp. exhibit a unique biphasic developmental cycle whereby infectious elementary bodies (EBs) invade host epithelial cells and differentiate into noninfectious, metabolically active reticulate bodies (RBs). EBs posses a unique outer envelope where rigidity is achieved by disulfide bonding among cysteine-rich envelope-associated proteins. Conversely, these disulfide bonds become reduced in RBs to accommodate vegetative growth, thereby linking the redox status of cysteine-rich envelope proteins with progression of the developmental cycle. We investigated the potential role of disulfide bonding within the chlamydial type III secretion system (T3SS), since activity of this system is also closely linked to development. We focused on structural components of the T3S apparatus that contain an unusually high number of cysteine residues compared to orthologs in other secretion systems. Nonreducing SDS-PAGE revealed that EB-localized apparatus proteins such as CdsF, CdsD, and CdsC form higher-order complexes mediated by disulfide bonding. The most dramatic alterations were detected for the needle protein CdsF. Significantly, disulfide bonding patterns shifted during differentiation of developmental forms and were completely reduced in RBs. Furthermore, at later time points during infection following RB to EB conversion, we found that CdsF is reoxidized into higher-order complexes. Overall, we conclude that the redox status of specific T3SS apparatus proteins is intimately linked to the developmental cycle and constitutes a newly appreciated aspect of functionally significant alterations within proteins of the chlamydial envelope.     

Evidence that CT694 is a novel Chlamydia trachomatis T3S substrate capable of functioning during invasion or early cycle development

Chlamydia trachomatis is an obligate intracellular parasite, occupies a membrane-bound vacuole throughout development and is capable of manipulating the eukaryotic host by translocating effector molecules via a type III secretion system (T3SS). The infectious chlamydial elementary body (EB) is metabolically inactive yet possesses a functional T3S apparatus capable of translocating effector proteins into the host cell to facilitate invasion and other early cycle events. We present evidence here that the C. trachomatis protein CT694 represents an early cycle-associated effector protein. CT694 is secreted by the Yersinia T3SS and immunodetection studies of infected HeLa cultures indicate that CT694-specific signal accumulates directly adjacent to, but not completely overlapping with EBs during invasion. Yeast two-hybrid analyses revealed an interaction of CT694 with the repeat region and C-terminus of human AHNAK. Immunolocalization studies of CT694 ectopically expressed in HeLa cells were consistent with an interaction with endogenous AHNAK. Additionally, expression of CT694 in HeLa cells resulted in alterations in the detection of stress fibres that correlated with the ability of CT694 to interact with AHNAK. These data indicate that CT694 is a novel T3S-dependent substrate unique to C. trachomatis , and that its interaction with host proteins such as AHNAK may be important for aspects of invasion or development particular to this species.     

Chlamydia trachomatis type III secretion: evidence for a functional apparatus during early-cycle development

The obligate intracellular bacterium Chlamydia trachomatis occupies a parasitophorous vacuole termed an inclusion. During its intracellular developmental cycle, C. trachomatis maintains this intracellular niche, presumably by expressing a type III secretion system, which deploys a set of host cell-interactive proteins including inclusion membrane-localized proteins termed Incs. Some Incs are expressed and secreted by 2 h (early cycle) after infection, whereas the expression of type III-specific genes is not detectable until 6-12 h (mid-cycle). To resolve this paradox, we investigated the presence of a type III apparatus on elementary bodies (EBs) that might function early in infection. We demonstrate the existence of the type III secretory apparatus by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and immunoblot analyses of purified EB extracts. Immunoblots using polyclonal antibodies specific for the core apparatus component CdsJ identified this protein in both EB and reticulate body (RB) extracts. Furthermore, CdsJ-specific signals were detected by immunoblot of whole infected-culture extracts and by indirect immunofluorescence of infected monolayers at times before the detection of cdsJ-specific message. Finally, expression of IncC, expressed by 2 h after infection during C. trachomatis infections, in Yersinia pseudotuberculosis resulted in its secretion via the Yersinia type III apparatus. Based on these data, we propose a model in which type III secretion pores are present on EBs and mediate secretion of early Incs and possible additional effectors. Mid-cycle expression of type III genes would then replenish secretion apparatus on vegetative RBs and serve as a source of secretion pores for subsequently formed EBs.     

Development of a transformation system for Chlamydia trachomatis: restoration of glycogen biosynthesis by acquisition of a plasmid shuttle vector

Chlamydia trachomatis remains one of the few major human pathogens for which there is no transformation system. C. trachomatis has a unique obligate intracellular developmental cycle. The extracellular infectious elementary body (EB) is an infectious, electron-dense structure that, following host cell infection, differentiates into a non-infectious replicative form known as a reticulate body (RB). Host cells infected by C. trachomatis that are treated with penicillin are not lysed because this antibiotic prevents the maturation of RBs into EBs. Instead the RBs fail to divide although DNA replication continues. We have exploited these observations to develop a transformation protocol based on expression of β-lactamase that utilizes rescue from the penicillin-induced phenotype. We constructed a vector which carries both the chlamydial endogenous plasmid and an E.coli plasmid origin of replication so that it can shuttle between these two bacterial recipients. The vector, when introduced into C. trachomatis L2 under selection conditions, cures the endogenous chlamydial plasmid. We have shown that foreign promoters operate in vivo in C. trachomatis and that active β-lactamase and chloramphenicol acetyl transferase are expressed. To demonstrate the technology we have isolated chlamydial transformants that express the green fluorescent protein (GFP). As proof of principle, we have shown that manipulation of chlamydial biochemistry is possible by transformation of a plasmid-free C. trachomatis recipient strain. The acquisition of the plasmid restores the ability of the plasmid-free C. trachomatis to synthesise and accumulate glycogen within inclusions. These findings pave the way for a comprehensive genetic study on chlamydial gene function that has hitherto not been possible. Application of this technology avoids the use of therapeutic antibiotics and therefore the procedures do not require high level containment and will allow the analysis of genome function by complementation.     

An early event in the herpes simplex virus type-2 replication cycle is sufficient to induce Chlamydia trachomatis persistence

Epidemiological studies have demonstrated that co-infections of herpes simplex virus type 2 (HSV-2) and Chlamydia trachomatis occur in vivo. Data from a tissue culture model of C. trachomatis/HSV-2 co-infection indicate that viral co-infection stimulates the formation of persistent chlamydiae. Transmission electron microscopic (TEM) analyses demonstrated that in both HeLa and HEC-1B cells, co-infection caused developing chlamydiae to exhibit swollen, aberrantly shaped reticulate bodies (RBs), characteristically observed in persistence. Additionally, HSV-2 co-infection suppressed production of infectious chlamydial elementary bodies (EBs) in both host cell types. Co-infection with HSV type 1 (HSV-1) produced similar morphologic alterations and abrogated infectious EB production. These data indicate that virus-induced chlamydial persistence was neither host cell- nor virus strain-specific. Purification of crude HSV-2 stocks demonstrated that viral particles were required for coinfection-induced chlamydial persistence to occur. Finally, co-infection with either UV-inactivated, replication-incompetent virus or replication-competent HSV-2 in the presence of cyclohexamide reduced chlamydial infectivity without altering chlamydial genomic DNA accumulation. These data demonstrate that productive viral replication is not required for the induction of chlamydial persistence and suggest that HSV attachment and entry can provide the necessary stimulus to alter C. trachomatis development.     

Identification of Chlamydia trachomatis CT621, a protein delivered through the type III secretion system to the host cell cytoplasm and nucleus

Chlamydiae are obligate intracellular bacteria, developing inside host cells within chlamydial inclusions. From these inclusions, the chlamydiae secrete proteins into the host cell cytoplasm. A pathway through which secreted proteins can be delivered is the type III secretion system (T3SS). The T3SS is common to several gram-negative bacteria and the secreted proteins serve a variety of functions often related to the modulation of host signalling. To identify new potentially secreted proteins, the cytoplasm was extracted from Chlamydia trachomatis L2-infected HeLa cells, and two-dimensional polyacrylamide gel electrophoresis profiles of [³⁵S]-labelled chlamydial proteins from this extract were compared with profiles of chlamydial proteins from the lysate of infected cells. In this way, CT621 was identified. CT621 is a member of a family of proteins containing a domain of unknown function DUF582 that is only found within the genus Chlamydia . Immunofluorescence microscopy and immunoblotting demonstrated that CT621 is secreted late in the chlamydial developmental cycle and that it is the first chlamydial protein found to be localized within both the host cell cytoplasm and the nucleus. To determine whether CT621 is secreted through the T3SS, an inhibitor of this apparatus was added to the infection medium, resulting in retention of the protein inside the chlamydiae. Hence, the so far uncharacterized CT621 is a new type III secretion effector protein.     

Penicillin Induced Persistence in Chlamydia trachomatis: High Quality Time Lapse Video Analysis of the Developmental Cycle

Background

Chlamydia trachomatis is a major human pathogen with a unique obligate intracellular developmental cycle that takes place inside a modified cytoplasmic structure known as an inclusion. Following entry into a cell, the infectious elementary body (EB) differentiates into a non - infectious replicative form known as a reticulate body (RB). RBs divide by binary fission and at the end of the cycle they redifferentiate into EBs. Treatment of C.trachomatis with penicillin prevents maturation of RBs which survive and enlarge to become aberrant RBs within the inclusion in a non - infective persistent state. Persistently infected individuals may be a reservoir for chlamydial infection. The C.trachomatis genome encodes the enzymes for peptidoglycan (PG) biosynthesis but a PG sacculus has never been detected. This coupled to the action of penicillin is known as the chlamydial anomaly. We have applied video microscopy and quantitative DNA assays to the chlamydial developmental cycle to assess the effects of penicillin treatment and establish a framework for investigating penicillin induced chlamydial persistence.

Principal Findings

Addition of penicillin at the time of cell infection does not prevent uptake and the establishment of an inclusion. EB to RB transition occurs but bacterial cytokinesis is arrested by the second binary fission. RBs continue to enlarge but not divide in the presence of penicillin. The normal developmental cycle can be recovered by the removal of penicillin although the large, aberrant RBs do not revert to the normal smaller size but remain present to the completion of the developmental cycle. Chromosomal and plasmid DNA replication is unaffected by the addition of penicillin but the arrest of bacterial cytokinesis under these conditions results in RBs accumulating multiple copies of the genome.

Conclusions

We have applied video time lapse microscopy to the study of the chlamydial developmental cycle. Linked with accurate measures of genome replication this provides a defined framework to analyse the developmental cycle and to investigate and provide new insights into the effects of antibiotic treatments. Removal of penicillin allows recovery of the normal developmental cycle by 10–20 hrs and the process occurs by budding from aberrant RBs.     

Disulfide-mediated interactions of the chlamydial major outer membrane protein: role in the differentiation of chlamydiae?

The effects of exogenous reducing agents on a number of biological properties of purified Chlamydia trachomatis LGV-434 and Chlamydia psittaci meningopneumonitis elementary bodies (EBs) have been examined in an attempt to identify in vitro correlates of early events in the differentiation of the infectious EB to the replicative cell type, the reticulate body (RB). Treatment of EBs with dithiothreitol elicited a number of changes normally associated with differentiation to the RB. EBs in the presence of 10 mM dithiothreitol displayed enhanced rates of [14C]glutamate oxidation, reduced infectivity, and decreased osmotic stability, and their Machiavello staining properties changed to those characteristic of the RB. A true differentiation of EB to RB did not take place under these conditions, since EBs treated in this manner and examined by transmission electron microscopy did not demonstrate increased size or decreased electron density as do isolated RBs. Additional studies were initiated to identify the macromolecules involved in this process. With polyacrylamide gel electrophoresis and immunoblotting procedures with monoclonal and polyclonal monospecific antibodies, the chlamydial major outer membrane protein was found to be the predominant component that varied under reducing versus nonreducing conditions. Furthermore, the extent of disulfide-mediated cross-linking of the major outer membrane protein varied between the infective and replicative forms of the C. trachomatis LGV-434 life cycle. Implications of disulfide interactions in the life cycle of chlamydiae are discussed.     

Synthesis of disulfide-bonded outer membrane proteins during the developmental cycle of Chlamydia psittaci and Chlamydia trachomatis.

The disulfide bond cross-linked major outer membrane protein (MOMP) of the extracellular elementary bodies (EBs) of Chlamydia psittaci was reduced to its monomeric form within 1 h of entry of EBs into host cells by a process which was inhibited by chloramphenicol, while monomeric forms of three cross-linked cysteine-rich proteins could not be detected in Sarkosyl outer membrane complexes at any time in either extracellular or intracellular forms of C. psittaci. Synthesis and incorporation of the MOMP into outer membrane complexes were detected early in the infection cycle (12 h postinfection), while synthesis and incorporation of the cysteine-rich proteins were not observed until reticulate bodies had begun to reorganize into EBs at 20 to 22 h postinfection. By 46 h postinfection, the intracellular population of C. psittaci consisted mainly of EBs, the outer membrane complexes of which were replete with monomeric MOMP and cross-linked cysteine-rich proteins. Upon lysis of infected cells at 46 h, the MOMP was rapidly cross-linked, and infectious EBs were released. The status of the MOMP of intracellular Chlamydia trachomatis was similar to the status of the MOMP of C. psittaci in that the MOMP was largely uncross-linked at 24 and 48 h postinfection, but formed interpeptide disulfide bonds when it was exposed to an extracellular environment late in the developmental cycle. In contrast to C. psittaci, only a fraction of the cross-linked MOMP of infecting EBs of C. trachomatis was reduced by 4 h postinfection, and reduction of the MOMP was not inhibited by chloramphenicol. Exposure of extracellular EBs of C. trachomatis and C. psittaci to dithiothreitol reduced the MOMP but failed to stimulate metabolic activities normally associated with reticulate bodies.     

Productive Chlamydia trachomatis lymphogranuloma venereum 434 infection in cells with augmented or inactivated autophagic activities

Autophagy, a eukaryotic cellular activity leading to the degradation of cellular components, serves as a defense mechanism against facultative intracellular bacteria as well as a growth niche for the obligate intracellular bacterium Coxiella burnetii . We here demonstrate that the obligate intracellular bacterial pathogen Chlamydia trachomatis lymphogranuloma venereum strongly induced autophagy in the middle of the chlamydial developmental cycle (24 h after infection), a time point with maximal level of chlamydial replication, but not during the early stages with low overall chlamydial metabolism (before 8 h). No autophagy induction was evident in cells exposed to heat- and UV-inactivated elementary bodies (EBs, the infectious form of Chlamydia ) or to inocula from which EBs had been removed before inoculation. Blocking chlamydial development with chloramphenicol also prevented autophagy induction in cells infected with infectious EBs. It appears that autophagy is activated primarily in response to the metabolic stress consequent to chlamydial replication. However, autophagy-defective ATG5^−/− cells supported chlamydial development as efficiently as autophagy-proficient ATG5^+/+ cells.     

Immunoelectron microscopic localization of chlamydial lipopolysaccharide (LPS) in McCoy cells inoculated with Chlamydia trachomatis.

Interactions between Chlamydia trachomatis, host cells, and the immune system are believed to involve lipopolysaccharide (LPS). We used immunogold techniques to study the distribution of chlamydial LPS in cultured cells infected with C. trachomatis LGV-L1. McCoy cells inoculated with C. trachomatis were cultured and then fixed and embedded in situ with acrylic resins. Sections were immunolabeled with a protein A-gold method using antisera to the genus-specific, periodate-sensitive epitope on chlamydial LPS. Pre-embedding immunogold labeling on permeabilized cells was also done. By post-embedding methods, labeling for LPS was equally abundant over the outer membranes of elementary (EB) and reticulate bodies (RB). By post-embedding labeling, the sub-surface side of the EB outer membrane was more heavily labeled than the surface side. By pre-embedding labeling, LPS was found to be less abundant on the surface of EBs than RBs. Labeling for LPS was found over apparent lysosomes in McCoy cells and over electron-dense blebs on or near the surface of the plasma membranes of McCoy cells. These results indicate that the concentration of LPS in chlamydial membranes is constant during development but that with development its location changes from being mostly cell-surface to sub-surface. These results show that the post-embedding immunogold technique can be a useful approach for the cell culture-based study of chlamydial LPS.     

Pathogen–host reorganization during Chlamydia invasion revealed by cryo‐electron tomography

Invasion of host cells is a key early event during bacterial infection, but the underlying pathogen–host interactions are yet to be fully visualized in three‐dimensional detail. We have captured snapshots of the early stages of bacterial‐mediated endocytosis in situ by exploiting the small size of chlamydial elementary bodies (EBs) for whole‐cell cryo‐electron tomography. Chlamydiae are obligate intracellular bacteria that infect eukaryotic cells and cause sexually transmitted infections and trachoma, the leading cause of preventable blindness. We demonstrate that Chlamydia trachomatis LGV2 EBs are intrinsically polarized. One pole is characterized by a tubular inner membrane invagination, while the other exhibits asymmetric periplasmic expansion to accommodate an array of type III secretion systems (T3SSs). Strikingly, EBs orient with their T3SS‐containing pole facing target cells, enabling the T3SSs to directly contact the cellular plasma membrane. This contact induces enveloping macropinosomes, actin‐rich filopodia and phagocytic cups to zipper tightly around the internalizing bacteria. Once encapsulated into tight early vacuoles, EB polarity and the T3SSs are lost. Our findings reveal previously undescribed structural transitions in both pathogen and host during the initial steps of chlamydial invasion.     

Genetic differences in the Chlamydia trachomatis tryptophan synthase alpha-subunit can explain variations in serovar pathogenesis

The human pathogen Chlamydia trachomatis is an obligate intracellular bacterium, characterized by a developmental cycle that alternates between the infectious, extracellular elementary bodies and intracellular, metabolically active reticulate bodies. The cellular immune effector interferon gamma (IFN-gamma) inhibits chlamydial multiplication in human epithelial cells by induction of the tryptophan degrading enzyme indoleamine 2,3 dioxygenase. IFN-gamma causes persistent C. trachomatis serovar A infections with atypical reticulate bodies that are unable to redifferentiate into elementary bodies and show diminished expression of important immunogens, but not of GroEL. However, the sensitivity to IFN-gamma varies among serovars of C. trachomatis. In our previous study significant IFN-gamma-specific, but tryptophan reversible, induction of proteins in C. trachomatis A and L2 with molecular masses of approximately 30 and 40 kDa was observed on 2D-gels. The 30-kDa protein from C. trachomatis L2 migrated with a significantly lower molecular weight in C. trachomatis A. In this paper we include C. trachomatis B, C and D in our investigations and identify the proteins as alpha- and beta-subunits of the chlamydial tryptophan synthase using matrix-assisted laser desorption/ionization mass spectrometry. DNA sequencing of the trpA genes from C. trachomatis A and C shows that the TrpA in these serovars is a 7.7-kDa truncated version of C. trachomatis D and L2 TrpA. The truncation probably impairs the TrpA activity, thus elucidating a possible molecular mechanism behind variations in the pathogenesis of C. trachomatis serovars.     

Localization and characterization of a putative cysteine desulfurase in Chlamydia psittaci

Chlamydia psittaci is an obligate intracellular pathogen with a biphasic developmental life cycle. It is auxotrophic for a variety of essential metabolites and obtains amino acids from eukaryotic host cells. Chlamydia can develop inside host cells within chlamydial inclusions. A pathway secreting proteins from inclusions into the host cellular cytoplasm is the type III secretion system (T3SS). The T3SS is universal among several Gram-negative bacteria. Here, we show that CPSIT_0959 of C. psittaci is expressed midcycle and secreted into the infected cellular cytoplasm via the T3SS. Recombinant CPSIT_0959 possesses cysteine desulfurase and PLP-binding activity, which removes sulfur from cysteine to produce alanine, and helps chlamydial replication. Our study shows that CPSIT_0959 improve the infectivity of offspring elementary bodies and seems to promote the replication by its product. This phenomenon has inhibited by the PLP-dependent enzymes inhibitor. Moreover, CPSIT_0959 increased expression of Bim and tBid, and decreased the mitochondrial membrane potential of host mitochondria to induce apoptosis in the latecycle for release of offspring. These results demonstrate that CPSIT_0959 has cysteine desulfurase and PLP-binding activity and is likely to contribute to apoptosis of the infected cells via a mitochondria-mediated pathway to improve the infectivity of progeny.     

Chlamydia trachomatis has penicillin-binding proteins but not detectable muramic acid.

Chlamydia trachomatis LGV-434 was grown in HeLa 229 cells. Benzylpenicillin completely inhibited the formation of infectious elementary bodies (EBs) at a concentration of 19 pmol/ml or higher and produced abnormally large reticulate bodies (RBs) in the inclusions at 30 pmol/ml or higher. The possible targets for penicillin in C. trachomatis were three penicillin-binding proteins (PBPs) which were identified in the Sarkosyl-soluble fractions of both RBs and EBs. The apparent subunit molecular weights were 88,000 (PBP 1), 61,000 (BPB 2), and 36,000 (PBP 3). The 50% binding concentrations of [3H]penicillin for PBPs 1 to 3 in EBs and RBs were between 7 and 70 pmol/ml. Such high susceptibility to penicillin was shown by an organism that did not have detectable muramic acid (less than 0.02% by weight) in preparations of either whole cells or sodium dodecyl sulfate-insoluble residues.     

Electron tomography and cryo-SEM characterization reveals novel ultrastructural features of host-parasite interaction during Chlamydia abortus infection

Chlamydia (C.) abortus is a widely spread pathogen among ruminants that can be transmitted to women during pregnancy leading to severe systemic infection with consecutive abortion. As a member of the Chlamydiaceae, C. abortus shares the characteristic feature of an obligate intracellular biphasic developmental cycle with two morphological forms including elementary bodies (EBs) and reticulate bodies (RBs). In contrast to other chlamydial species, C. abortus ultrastructure has not been investigated yet. To do so, samples were fixed by high-pressure freezing and processed by different electron microscopic methods. Freeze-substituted samples were analysed by transmission electron microscopy, scanning transmission electron microscopical tomography and immuno-electron microscopy, and freeze-fractured samples were analysed by cryo-scanning electron microscopy. Here, we present three ultrastructural features of C. abortus that have not been reported up to now. Firstly, the morphological evidence that C. abortus is equipped with the type three secretion system. Secondly, the accumulation and even coating of whole inclusion bodies by membrane complexes consisting of multiple closely adjacent membranes which seems to be a C. abortus specific feature. Thirdly, the formation of small vesicles in the periplasmic space of RBs in the second half of the developmental cycle. Concerning the time point of their formation and the fact that they harbour chlamydial components, these vesicles might be morphological correlates of an intermediate step during the process of redifferentiation of RBs into EBs. As this feature has also been shown for C. trachomatis and C. pneumoniae, it might be a common characteristic of the family of Chlamydiaceae.     

Detection of surface-exposed epitopes on Chlamydia trachomatis by immune electron microscopy

The cell surfaces of two Chlamydia trachomatis serovars were explored by immune electron microscopy with monoclonal antibodies that recognize a number of chlamydial outer-membrane components. Species, subspecies and serovar-reactive epitopes on the major outer-membrane protein (MOMP) of a lymphogranuloma venereum biovar strain, L2/434/Bu, and a trachoma biovar strain, F/UW-6/Cx, were exposed on the surfaces of both elementary bodies (EBs) and reticulate bodies (RBs). Three epitopes on MOMP were inaccessible on EBs and RBs of both strains. These included a genus-reactive, species-reactive, and a subspecies-reactive epitope. In contrast, genus-specific epitopes on lipopolysaccharide (LPS) were not detected on the EB surface, but were clearly expressed on RBs of both L2/434/Bu and F/UW-6/Cx chlamydiae. Antibodies specific for the 60 kDa and 12 kDa 'cysteine-rich' outer-membrane proteins did not react with surface epitopes on either EBs or RBs. These data provide evidence that MOMP is a major surface antigen of both morphological forms, whereas some portions of the LPS molecule are exposed on the RB surface but become inaccessible to antibody after conversion to the infectious EB form.