Expression and localization of type III secretion-related proteins of Chlamydia pneumoniae

Robust Heat Shock Response in Chlamydia Lacking a Typical Heat Shock Sigma Factor

Cells reprogram their transcriptome in response to stress, such as heat shock. In free-living bacteria, the transcriptomic reprogramming is mediated by increased DNA-binding activity of heat shock sigma factors and activation of genes normally repressed by heat-induced transcription factors. In this study, we performed transcriptomic analyses to investigate heat shock response in the obligate intracellular bacterium Chlamydia trachomatis, whose genome encodes only three sigma factors and a single heat-induced transcription factor. Nearly one-third of C. trachomatis genes showed statistically significant (≥1.5-fold) expression changes 30 min after shifting from 37 to 45°C. Notably, chromosomal genes encoding chaperones, energy metabolism enzymes, type III secretion proteins, as well as most plasmid-encoded genes, were differentially upregulated. In contrast, genes with functions in protein synthesis were disproportionately downregulated. These findings suggest that facilitating protein folding, increasing energy production, manipulating host activities, upregulating plasmid-encoded gene expression, and decreasing general protein synthesis helps facilitate C. trachomatis survival under stress. In addition to relieving negative regulation by the heat-inducible transcriptional repressor HrcA, heat shock upregulated the chlamydial primary sigma factor σ⁶⁶ and an alternative sigma factor σ²⁸. Interestingly, we show for the first time that heat shock downregulates the other alternative sigma factor σ⁵⁴ in a bacterium. Downregulation of σ⁵⁴ was accompanied by increased expression of the σ⁵⁴ RNA polymerase activator AtoC, thus suggesting a unique regulatory mechanism for reestablishing normal expression of select σ⁵⁴ target genes. Taken together, our findings reveal that C. trachomatis utilizes multiple novel survival strategies to cope with environmental stress and even to replicate. Future strategies that can specifically target and disrupt Chlamydia’s heat shock response will likely be of therapeutic value.

Insight into microtubule nucleation from tubulin-capping proteins

Nucleation is one of the least understood steps of microtubule dynamics. It is a kinetically unfavorable process that is templated in the cell by the γ-tubulin ring complex or by preexisting microtubules; it also occurs in vitro from pure tubulin. Here we study the nucleation inhibition potency of natural or artificial proteins in connection with their binding mode to the longitudinal surface of α- or β-tubulin. The structure of tubulin-bound CopN, a Chlamydia protein that delays nucleation, suggests that this protein may interfere with two protofilaments at the (+) end of a nucleus. Designed ankyrin repeat proteins that share a binding mode similar to that of CopN also impede nucleation, whereas those that target only one protofilament do not. In addition, an αRep protein predicted to target two protofilaments at the (-) end does not delay nucleation, pointing to different behaviors at both ends of the nucleus. Our results link the interference with protofilaments at the (+) end and the inhibition of nucleation.

Molecular Genetic Analysis of Chlamydia Species

Species of Chlamydia are the etiologic agent of endemic blinding trachoma, the leading cause of bacterial sexually transmitted diseases, significant respiratory pathogens, and a zoonotic threat. Their dependence on an intracellular growth niche and their peculiar developmental cycle are major challenges to elucidating their biology and virulence traits. The last decade has seen tremendous advances in our ability to perform a molecular genetic analysis of Chlamydia species. Major achievements include the generation of large collections of mutant strains, now available for forward- and reverse-genetic applications, and the introduction of a system for plasmid-based transformation enabling complementation of mutations; expression of foreign, modified, or reporter genes; and even targeted gene disruptions. This review summarizes the current status of the molecular genetic toolbox for Chlamydia species and highlights new insights into their biology and new challenges in the nascent field of Chlamydia genetics.

The Type III Secretion System-Related CPn0809 from Chlamydia pneumoniae

Chlamydia pneumoniae is an intracellular Gram-negative bacterium that possesses a type III secretion system (T3SS), which enables the pathogen to deliver, in a single step, effector proteins for modulation of host-cell functions into the human host cell cytosol to establish a unique intracellular niche for replication. The translocon proteins located at the top of the T3SS needle filament are essential for its function, as they form pores in the host-cell membrane. Interestingly, unlike other Gram-negative bacteria, C. pneumoniae has two putative translocon operons, named LcrH_1 and LcrH_2. However, little is known about chlamydial translocon proteins. In this study, we analyzed CPn0809, one of the putative hydrophobic translocators encoded by the LcrH_1 operon, and identified an 'SseC-like family' domain characteristic of T3S translocators. Using bright-field and confocal microscopy, we found that CPn0809 is associated with EBs during early and very late phases of a C. pneumoniae infection. Furthermore, CPn0809 forms oligomers, and interacts with the T3SS chaperone LcrH_1, via its N-terminal segment. Moreover, expression of full-length CPn0809 in the heterologous host Escherichia coli causes a grave cytotoxic effect that leads to cell death. Taken together, our data indicate that CPn0809 likely represents one of the translocon proteins of the C. pneumoniae T3SS, and possibly plays a role in the translocation of effector proteins in the early stages of infection.

Recombinant Cpn 0810 stimulates proinflammatory cytokine expression and apoptosis in human monocytes

The aim of the present study was to express the recombinant Chlamydophila pneumoniae (C. pneumoniae) protein, Cpn 0810, in Escherichia coli (E. coli) BL21, and investigate the effects of Cpn 0810 on inflammatory and apoptotic processes in human monocytic (THP-1) cells. An ELISA was performed to detect the levels of the proinflammatory cytokines, tumor necrosis factor (TNF)-α and interleukin (IL)-6. In addition, Hoechst 33258 staining and annexin V binding analyses were performed to measure the rates of apoptosis. Purified glutathione S-transferase (GST)-Cpn 0810 recombinant proteins were obtained from the E. coli BL21 cells carrying the pGEX6p-2/Cpn 0810 plasmid, and were shown to stimulate the expression of TNF-α and IL-6 in the THP-1 cells in a dose- and time-dependent manner. TNF-α and IL-6 levels peaked at 24 h after GST-Cpn 0810 stimulation. Furthermore, GST-Cpn 0810 significantly promoted the apoptosis of THP-1 cells. In conclusion, recombinant GST-Cpn 0810 was shown to stimulate the expression of TNF-α and IL-6, inhibit proliferation and induce apoptosis in THP-1 cells. Therefore, Cpn 0810 may interact with host cells following C. pneumoniae infection, functioning as an important pathogenic factor.

Biochemical and structural insights into microtubule perturbation by CopN from Chlamydia pneumoniae

Although the actin network is commonly hijacked by pathogens, there are few reports of parasites targeting microtubules. The proposed member of the LcrE protein family from some Chlamydia species (e.g. pCopN from C. pneumoniae) binds tubulin and inhibits microtubule assembly in vitro. From the pCopN structure and its similarity with that of MxiC from Shigella, we definitively confirm CopN as the Chlamydia homolog of the LcrE family of bacterial proteins involved in the regulation of type III secretion. We have also investigated the molecular basis for the pCopN effect on microtubules. We show that pCopN delays microtubule nucleation and acts as a pure tubulin-sequestering protein at steady state. It targets the β subunit interface involved in the tubulin longitudinal self-association in a way that inhibits nucleotide exchange. pCopN contains three repetitions of a helical motif flanked by disordered N- and C-terminal extensions. We have identified the pCopN minimal tubulin-binding region within the second and third repeats. Together with the intriguing observation that C. trachomatis CopN does not bind tubulin, our data support the notion that, in addition to the shared function of type III secretion regulation, these proteins have evolved different functions in the host cytosol. Our results provide a mechanistic framework for understanding the C. pneumoniae CopN-specific inhibition of microtubule assembly.

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.

Penicillin kills Chlamydia following the fusion of bacteria with lysosomes and prevents genital inflammatory lesions in C. muridarum-infected mice

The obligate intracellular bacterium Chlamydia exists as two distinct forms. Elementary bodies (EBs) are infectious and extra-cellular, whereas reticulate bodies (RBs) replicate within a specialized intracellular compartment termed an ‘inclusion’. Alternative persistent intra-cellular forms can be induced in culture by diverse stimuli such as IFNγ or adenosine/EHNA. They do not grow or divide but revive upon withdrawal of the stimulus and are implicated in several widespread human diseases through ill-defined in vivo mechanisms. β-lactam antibiotics have also been claimed to induce persistence in vitro. The present report shows that upon penicillin G (pG) treatment, inclusions grow as fast as those in infected control cells. After removal of pG, Chlamydia do not revert to RBs. These effects are independent of host cell type, serovar, biovar and species of Chlamydia. Time-course experiments demonstrated that only RBs were susceptible to pG. pG-treated bacteria lost their control over host cell apoptotic pathways and no longer expressed pre-16S rRNA, in contrast to persistent bacteria induced with adenosine/EHNA. Confocal and live-video microscopy showed that bacteria within the inclusion fused with lysosomal compartments in pG-treated cells. That leads to recruitment of cathepsin D as early as 3 h post pG treatment, an event preceding bacterial death by several hours. These data demonstrate that pG treatment of cultured cells infected with Chlamydia results in the degradation of the bacteria. In addition we show that pG is significantly more efficient than doxycycline at preventing genital inflammatory lesions in C. muridarum-C57Bl/6 infected mice. These in vivo results support the physiological relevance of our findings and their potential therapeutic applications.

Early expression of the type III secretion system of Parachlamydia acanthamoebae during a replicative cycle within its natural host cell Acanthamoeba castellanii

The type three secretion system (T3SS) operons of Chlamydiales bacteria are distributed in different clusters along their chromosomes and are conserved at both the level of sequence and genetic organization. A complete characterization of the temporal expression of multiple T3SS components at the transcriptional and protein levels has been performed in Parachlamydia acanthamoebae, replicating in its natural host cell Acanthamoeba castellanii. The T3SS components were classified in four different temporal clusters depending on their pattern of expression during the early, mid- and late phases of the infectious cycle. The putative T3SS transcription units predicted in Parachlamydia are similar to those described in Chlamydia trachomatis, suggesting that T3SS units of transcriptional expression are highly conserved among Chlamydiales bacteria. The maximal expression and activation of the T3SS of Parachlamydia occurred during the early to mid-phase of the infectious cycle corresponding to a critical phase during which the intracellular bacterium has (1) to evade and/or block the lytic pathway of the amoeba, (2) to differentiate from elementary bodies (EBs) to reticulate bodies (RBs), and (3) to modulate the maturation of its vacuole to create a replicative niche able to sustain efficient bacterial growth.

Genus-optimized strategy for the identification of chlamydial type III secretion substrates

Among chlamydial virulence factors are the type III secretion (T3S) system and its effectors. T3S effectors target host proteins to benefit the infecting chlamydiae. The assortment of effectors, each with a unique function, varies between species. This variation likely contributes to differences in host specificity and disease severity. A dozen effectors of Chlamydia trachomatis have been identified; however, estimates suggest that more exist. A T3S prediction algorithm, SVM-based Identification and Evaluation of Virulence Effectors (SIEVE), along with a Yersinia surrogate secretion system helped to identify a new T3S substrate, CT082, which rather than functioning as an effector associates with the chlamydial envelope after secretion. SIEVE was modified to improve/expand effector predictions to include all sequenced genomes. Additional adjustments were made to the existing surrogate system whereby the N terminus of putative effectors was fused to a known effector lacking its own N terminus and was tested for secretion. Expansion of effector predictions by cSIEVE and modification of the surrogate system have also assisted in identifying a new T3S substrate from C. psittaci. The expanded predictions along with modifications to improve the surrogate secretion system have enhanced our ability to identify novel species-specific effectors, which upon characterization should provide insight into the unique pathogenic properties of each species.

Immunogenicity of Cpn0425 and its localization in cells infected with Chlamydophila pneumoniae

The present study aimed to determine the intracellular localization of Cpn0425 in Chlamydophila pneumoniae-infected cells. The recombinant plasmid pGEX-6p/Cpn0425 was transformed into E.coli Bl21 cells to express the fusion protein. Following purification with glutathione S-transferase (GST) resin chromatography, the Cpn0425 fusion protein was used to induce immunity in mice to develop monoclonal and polyclonal antibodies, which were subsequently used to localize the endogenous Cpn0425 protein by indirect immunofluorescence assay (IFA). ELISA was used to determine the immunogenicity of the Cpn0425 plasmid protein by recognizing the pool sera of patients infected with Chlamydia trachomatis and the pool sera of mice immunized with the Cpn0425 fusion protein. The Cpn0425 gene was expressed as the GST-Cpn0425 fusion protein in E. coli and its antibody was prepared by immunizing mice with the fusion protein. An anti-GST-Cpn0425 antibody was used to localize the protein in cells infected with Chlamydophila pneumoniae AR-39 using an IFA. The anti-GST-CT058 antibody detected an inclusion signal in the IFA. Cpn0425 protein strongly reacted with antiserum. Although Cpn0425 protein is not a secreted protein, it has good immunogenicity. Therefore, this protein may be useful for developing vaccines against Chlamydophila pneumoniae infection.

Biochemical and localization analyses of putative type III secretion translocator proteins CopB and CopB2 of Chlamydia trachomatis reveal significant distinctions

Chlamydia spp. are among the many pathogenic Gram-negative bacteria that employ a type III secretion system (T3SS) to overcome host defenses and exploit available resources. Significant progress has been made in elucidating contributions of T3S to the pathogenesis of these medically important, obligate intracellular parasites, yet important questions remain. Chief among these is how secreted effector proteins traverse eukaryotic membranes to gain access to the host cytosol. Due to a complex developmental cycle, it is possible that chlamydiae utilize a different complement of proteins to accomplish translocation at different stages of development. We investigated this possibility by extending the characterization of C. trachomatis CopB and CopB2. CopB is detected early during infection but is depleted and not detected again until about 20 h postinfection. In contrast, CopB2 was detectible throughout development. CopB is associated with the inclusion membrane. Biochemical and ectopic expression analyses were consistent with peripheral association of CopB2 with inclusion membranes. This interaction correlated with development and required both chlamydial de novo protein synthesis and T3SS activity. Collectively, our data indicate that it is unlikely that CopB serves as the sole chlamydial translocation pore and that CopB2 is capable of association with the inclusion membrane.

Analysis of Chlamydia pneumoniae infection in mononuclear cells by reverse transcription-PCR targeted to chlamydial gene transcripts

Chlamydia pneumoniae (C. pneumoniae) is an important etiological agent of respiratory infections including pneumonia. C. pneumoniae DNA can be detected in peripheral blood mononuclear cells indicating that monocytes can assist the spread of infection to other anatomical sites. Persistent infection established at these sites could promote inflammation and enhance pathology. Thus, the mononuclear cells are in a strategic position in the development of persistent infection. To investigate the intracellular replication and fate of C. pneumoniae in mononuclear cells, we have established an in vitro model in the human Mono Mac 6 cell line. In the present study, we analyzed the transcription of 11 C. pneumoniae genes in Mono Mac 6 cells during infection by real-time RT-PCR. Our results suggest that the transcriptional profile of the studied genes in monocytes is different from that seen in epithelial cells. Furthermore, our study shows that genes related to secretion are transcribed, and secreted bacterial proteins are also translated during infection of monocytes, creating novel opportunities for the management of chlamydial infection of monocytes.

The Chlamydial Type III Secretion Mechanism: Revealing Cracks in a Tough Nut

Present-day members of the Chlamydiaceae contain parasitic bacteria that have been co-evolving with their eukaryotic hosts over hundreds of millions of years. Likewise, a type III secretion system encoded within all genomes has been refined to complement the unique obligate intracellular niche colonized so successfully by Chlamydia spp. All this adaptation has occurred in the apparent absence of the horizontal gene transfer responsible for creating the wide range of diversity in other Gram-negative, type III-expressing bacteria. The result is a system that is, in many ways, uniquely chlamydial. A critical mass of information has been amassed that sheds significant light on how the chlamydial secretion system functions and contributes to an obligate intracellular lifestyle. Although the overall mechanism is certainly similar to homologous systems, an image has emerged where the chlamydial secretion system is essential for both survival and virulence. Numerous apparent differences, some subtle and some profound, differentiate chlamydial type III secretion from others. Herein, we provide a comprehensive review of the current state of knowledge regarding the Chlamydia type III secretion mechanism. We focus on the aspects that are distinctly chlamydial and comment on how this important system influences chlamydial pathogenesis. Gaining a grasp on this fascinating system has been challenging in the absence of a tractable genetic system. However, the surface of this tough nut has been scored and the future promises to be fruitful and revealing.

A comparative study of RNA and DNA as internal gene expression controls early in the developmental cycle of Chlamydia pneumoniae

Many microbial pathogens invade and proliferate within host cells and the molecular mechanism underlying this behavior is currently being revealed for several bacterial species. Testing clinically relevant antibacterial compounds and elucidating their effects on gene expression requires adequate controls, especially when studying genetically intractable organisms such as Chlamydia spp., for which various gene fusions cannot be constructed. Until now, relative mRNA levels in Chlamydia have been measured using different internal gene expression controls, including 16S rRNA, mRNAs, and DNA. Here, we compared the advantages and disadvantages of various internal expression controls during the early phase of Chlamydia pneumoniae development. The relative abundance of target mRNAs varied using the different internal control RNAs. This was partly due to variation in the transcript stability of the RNA species. Also, seven out of nine of the analyzed RNAs increased fivefold or more between 2 and 14 h postinfection, while the amount of DNA and number of cells remained essentially unaltered. Our results suggest that RNA should not be used as a gene expression control during the early phase of Chlamydia development, and that intrinsic bacterial DNA is preferable for that purpose because it is stable, abundant, and its relative amount is generally correlated with bacterial numbers.

Chemical tools for dissecting bacterial physiology and virulence

Increasingly, chemical biology is being used in the context of bacterial virulence and the host-pathogen interaction, as small molecule inhibitors provide a number of unique advantages for the study of bacterial pathogens that complement powerful, existing classical genetic approaches. Small molecules have the potential to inhibit targets rapidly and reversibly, with a high degree of specificity. They are therefore well suited for studying the role of essential genes in bacterial physiology and virulence in both genetically tractable and intractable organisms, with the capacity to reveal novel phenotypes and insights into the function of essential factors during infection. The use of small molecule inhibitors during infection is also deepening our understanding of the role that host factors play in bacterial pathogenesis. In the future, the utility of chemical biology will grow as technologies for rapid identification of targets of interesting bioactive small molecules are developed. In this review, we highlight recent work in which small molecule inhibitors are used to study essential genes and genetically intractable organisms, to reveal novel phenotypes related to bacterial physiology, and to probe the role of bacterial and host factors during infection. In addition, we review recent advances in biochemical, genetic, and genomic techniques for target identification.

Accurate prediction of secreted substrates and identification of a conserved putative secretion signal for type III secretion systems

The type III secretion system is an essential component for virulence in many Gram-negative bacteria. Though components of the secretion system apparatus are conserved, its substrates--effector proteins--are not. We have used a novel computational approach to confidently identify new secreted effectors by integrating protein sequence-based features, including evolutionary measures such as the pattern of homologs in a range of other organisms, G+C content, amino acid composition, and the N-terminal 30 residues of the protein sequence. The method was trained on known effectors from the plant pathogen Pseudomonas syringae and validated on a set of effectors from the animal pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) after eliminating effectors with detectable sequence similarity. We show that this approach can predict known secreted effectors with high specificity and sensitivity. Furthermore, by considering a large set of effectors from multiple organisms, we computationally identify a common putative secretion signal in the N-terminal 20 residues of secreted effectors. This signal can be used to discriminate 46 out of 68 total known effectors from both organisms, suggesting that it is a real, shared signal applicable to many type III secreted effectors. We use the method to make novel predictions of secreted effectors in S. Typhimurium, some of which have been experimentally validated. We also apply the method to predict secreted effectors in the genetically intractable human pathogen Chlamydia trachomatis, identifying the majority of known secreted proteins in addition to providing a number of novel predictions. This approach provides a new way to identify secreted effectors in a broad range of pathogenic bacteria for further experimental characterization and provides insight into the nature of the type III secretion signal.

Induction of type III secretion by cell-free Chlamydia trachomatis elementary bodies

Chlamydiae secrete type III effector proteins at two distinct stages of their developmental cycle. Elementary bodies (EBs) secrete at least one pre-formed effector protein, Tarp, across the host plasma membrane from an extracellular location. Once internalized, a set of newly transcribed proteins are secreted to modify the inclusion membrane. In an effort to better understand the triggers for chlamydial type III secretion and develop means to identify new effectors, we investigated various inducers of T3SS in other Gram-negative bacterial systems to determine if they were able to activate chlamydial type III secretion from EBs using Tarp secretion as an indicator of activation. Chlamydial EBs are induced to secrete Tarp by exposure to FBS, BSA, or sphingolipid and cholesterol-rich liposomes (SCRLs). The induction by FBS and BSA, but not SCRL, is enhanced in the presence of the calcium-chelator, EGTA. This secretion was temperature dependent and inhibited by paraformaldehyde fixation of the EBs.

The essential role of the CopN protein in Chlamydia pneumoniae intracellular growth

Bacterial virulence determinants can be identified, according to the molecular Koch's postulates, if inactivation of a gene associated with a suspected virulence trait results in a loss in pathogenicity. This approach is commonly used with genetically tractable organisms. However, the current lack of tools for targeted gene disruptions in obligate intracellular microbial pathogens seriously hampers the identification of their virulence factors. Here we demonstrate an approach to studying potential virulence factors of genetically intractable organisms, such as Chlamydia. Heterologous expression of Chlamydia pneumoniae CopN in yeast and mammalian cells resulted in a cell cycle arrest, presumably owing to alterations in the microtubule cytoskeleton. A screen of a small molecule library identified two compounds that alleviated CopN-induced growth inhibition in yeast. These compounds interfered with C. pneumoniae replication in mammalian cells, presumably by 'knocking out' CopN function, revealing an essential role of CopN in the support of C. pneumoniae growth during infection. This work demonstrates the role of a specific chlamydial protein in virulence. The chemical biology approach described here can be used to identify virulence factors, and the reverse chemical genetic strategy can result in the identification of lead compounds for the development of novel therapeutics.

Chlamydial effector proteins localized to the host cell cytoplasmic compartment

Disease-causing microbes utilize various strategies to modify their environment in order to create a favorable location for growth and survival. Gram-negative bacterial pathogens often use specialized secretion systems to translocate effector proteins directly into the cytosol of the eukaryotic cells they infect. These bacterial proteins are responsible for modulating eukaryotic cell functions. Identification of the bacterial effectors has been a critical step toward understanding the molecular basis for the pathogenesis of the bacteria that use them. Chlamydiae are obligate intracellular bacterial pathogens that have a type III secretion system believed to translocate virulence effector proteins into the cytosol of their host cells. Selective permeabilization of the eukaryotic cell membrane was used in conjunction with metabolic labeling of bacterial proteins to identify chlamydial proteins that localize within the cytosol of infected cells. More than 20 Chlamydia trachomatis and C. pneumoniae proteins were detected within the cytoplasmic compartment of infected cells. While a number of cytosolic proteins were shared, others were unique to each species, suggesting that variation among cytosolic chlamydial proteins contributes to the differences in the pathogenesis of the chlamydial species. The spectrum of chlamydial proteins exported differed concomitant with the progress of the developmental cycle. These data confirm that a dynamic relationship exists between Chlamydia and its host and that translocation of bacterial proteins into the cytosol is developmentally dependent.

Characterization of the putative type III secretion ATPase CdsN (Cpn0707) of Chlamydophila pneumoniae

Type III secretion (T3S) is utilized by a wide range of gram-negative bacterial pathogens to allow the efficient delivery of effector proteins into the host cell cytoplasm through the use of a syringe-like injectisome. Chlamydophila pneumoniae is a gram-negative, obligate intracellular pathogen that has the structural genes coding for a T3S system, but the functionality of the system has not yet been demonstrated. T3S is dependent on ATPase activity, which catalyzes the unfolding of proteins and the secretion of effector proteins through the injectisome. CdsN (Cpn0707) is predicted to be the T3S ATPase of C. pneumoniae based on sequence similarity to other T3S ATPases. Full-length CdsN and a C-terminal truncation of CdsN were cloned as glutathione S-transferase (GST)-tagged constructs and expressed in Escherichia coli. The GST-tagged C-terminal truncation of CdsN possessed ATPase activity, catalyzing the release of ADP and P(i) from ATP at a rate of 0.55 +/- 0.07 micromol min(-1) mg(-1) in a time- and dose-dependent manner. CdsN formed oligomers and high-molecular-weight multimers, as assessed by formaldehyde fixation and nondenaturing polyacrylamide gel electrophoresis. Using bacterial two-hybrid and GST pull-down assays, CdsN was shown to interact with CdsD, CdsL, CdsQ, and CopN, four putative structural components of the C. pneumoniae T3S system. CdsN also interacted with an unannotated protein, Cpn0706, a putative CdsN chaperone. Interactions between CdsN, CdsD, and CopN represent novel interactions not previously reported for other bacterial T3S systems and may be important in the localization and/or function of the ATPase at the inner membrane of C. pneumoniae.

Immunoproteomic identification and serological responses to novel Chlamydia pneumoniae antigens that are associated with persistent C. pneumoniae infections

The controversial discussion about the role of Chlamydia pneumoniae in atherosclerosis cannot be solved without a reliable diagnosis that allows discrimination between past and persistent infections. Using a proteomic approach and immunoblotting with human sera, we identified 31 major C. pneumoniae Ags originating from 27 different C. pneumoniae proteins. More than half of the proteins represent Chlamydia Ags not described previously. Using a comparative analysis of spot reactivity Pmp6, OMP2, GroEL, DnaK, RpoA, EF-Tu, as well as CpB0704 and CpB0837, were found to be immunodominant. The comparison of Ab-response patterns of sera from subjects with and without evidence for persisting C. pneumoniae, determined by multiple PCR analysis of PBMC and vasculatory samples, resulted in differential reactivity for 12 proteins, which is not reflected by reactivity of the sera in the microimmunofluorescence test, the current gold standard for serodiagnosis. Although reactivity of sera from PCR-positive donors was increased toward RpoA, MOMP, YscC, Pmp10, PorB, Pmp21, GroEL, and Cpaf, the reactivity toward YscL, Rho, LCrE, and CpB0837 was decreased, reflecting the altered protein expression of persisting C. pneumoniae in vitro. Our data provide the first evidence of a unique Ab-response pattern associated with persistent C. pneumoniae infections, which is a prerequisite for the serological determination of persistently infected patients.

Characterization and intracellular localization of putative Chlamydia pneumoniae effector proteins

We here describe four proteins of Chlamydia pneumoniae, which might play a role in host-pathogen interaction. The hypothetical bacterial proteins CPn0708 and CPn0712 were detected in Chlamydia pneumoniae-infected host cells by indirect immunofluorescence tests with polyclonal antisera raised against the respective proteins. While CPn0708 was localized within the inclusion body, CPn0712 was identified in the inclusion membrane and in the surrounding host cell cytosol. CPn0712 colocalizes with actin, indicating its possible interaction with components of the cytoskeleton. Investigations on CPn0809 and CPn1020, two Chlamydia pneumoniae proteins previously described to be secreted into the host cell cytosol, revealed colocalization with calnexin, a marker for the ER. Neither CPn0712, CPn0809 nor CPn1020 were able to inhibit host cell apoptosis. Furthermore, transient expression of CPn0712, CPn0809 and CPn1020 by the host cell itself had no effect on subsequent infection with Chlamydia pneumoniae. However, microarray analysis of CPn0712-expressing host cells revealed six host cell genes which were regulated as in host cells infected with Chlamydia pneumoniae, indicating the principal usefulness of heterologous expression to study the effect of Chlamydia pneumoniae proteins on host cell modulation.

Identification and characterization of a type III secretion system in Chlamydophila psittaci

Chlamydiaceae are obligate intracellular Gram-negative bacteria replicating in vacuoles inside eukaryotic cells. It has been proven that most of them possess a type III secretion system (T3SS) allowing them to transfer effector molecules in the host cell. We examined the existence of a T3SS in Chlamydophila psittaci by studying the expression of three essential structural proteins SctW, SctC, and SctN, and one putative effector protein IncA. Immunofluorescence assays showed SctW and IncA to be associated with the bacteria and the inclusion membrane, while SctC and SctN were only localized to the bacteria itself. Immuno electron microscopy could confirm these results for SctW, IncA, and SctC. Unfortunately, SctN was not investigated with this technique. Additionally, we sequenced 14 full-length T3S genes (scc1, sctW, sctJ, sctL, sctR, sctS, scc2, copD1, sctN, sctQ, sctC, incA, ca037, and cadd) and examined the transcription of 26 Cp. psittaci T3S genes namely cluster 1 (scc1, sctW, sctV, sctU), cluster 2 (sctJ, sctL, sctR, sctS, sctT, scc2, copB1, copD1), cluster 3 (sctD, sctN, ca037, sctQ, pkn5, sctC) and non-clustered genes (incA, incC, scc3, copD2, cap1, tarp, ca530, cadd). The gene expression study indicated the T3S structural protein encoding genes to be transcribed from mid-cycle (12-18 h post infection (p.i.)) on. Genes encoding effector proteins and putative T3S related proteins were expressed early (1.5 h-8 h p.i.) or late (>24 h p.i.) during the developmental cycle. We hereby provided evidence for the existence of a T3SS and possible effectors in avian Cp. psittaci.

Interactions between CdsD, CdsQ, and CdsL, three putative Chlamydophila pneumoniae type III secretion proteins

Chlamydophila pneumoniae is a gram-negative obligate intracellular bacterial pathogen that causes pneumonia and bronchitis and may contribute to atherosclerosis. The developmental cycle of C. pneumoniae includes a morphological transition from an infectious extracellular elementary body (EB) to a noninfectious intracellular reticulate body (RB) that divides by binary fission. The C. pneumoniae genome encodes a type III secretion (T3S) apparatus that may be used to infect eukaryotic cells and to evade the host immune response. In the present study, Cpn0712 (CdsD), Cpn0704 (CdsQ), and Cpn0826 (CdsL), three C. pneumoniae genes encoding yersiniae T3S YscD, YscQ, and YscL homologs, respectively, were cloned and expressed as histidine- and glutathione S-transferase (GST)-tagged proteins in Escherichia coli. Purified recombinant proteins were used to raise hyper-immune polyclonal antiserum and were used in GST pull-down and copurification assays to identify protein-protein interactions. CdsD was detected in both EB and RB lysates by Western blot analyses, and immunofluorescent staining demonstrated the presence of CdsD within inclusions. Triton X-114 solubilization and phase separation of chlamydial EB proteins indicated that CdsD partitions with cytoplasmic proteins, suggesting it is not an integral membrane protein. GST pull-down assays indicated that recombinant CdsD interacts with CdsQ and CdsL, and copurification assays with chlamydial lysates confirmed that native CdsD interacts with CdsQ and CdsL. To the best of our knowledge, this is the first report demonstrating interactions between YscD, YscQ, and YscL homologs of bacterial T3S systems. These novel protein interactions may play important roles in the assembly or function of the chlamydial T3S apparatus.

Type III secretion systems and disease

Type III secretion systems (T3SSs) are complex bacterial structures that provide gram-negative pathogens with a unique virulence mechanism enabling them to inject bacterial effector proteins directly into the host cell cytoplasm, bypassing the extracellular milieu. Although the effector proteins vary among different T3SS pathogens, common pathogenic mechanisms emerge, including interference with the host cell cytoskeleton to promote attachment and invasion, interference with cellular trafficking processes, cytotoxicity and barrier dysfunction, and immune system subversion. The activity of the T3SSs correlates closely with infection progression and outcome, both in animal models and in human infection. Therefore, to facilitate patient care and improve outcomes, it is important to understand the T3SS-mediated virulence processes and to target T3SSs in therapeutic and prophylactic development efforts.

Small molecule inhibitors of type III secretion inYersiniablock theChlamydia pneumoniaeinfection cycle

Intracellular parasitism by Chlamydiales is a complex process involving transmission of metabolically inactive particles that differentiate, replicate, and re-differentiate within the host cell. A type three secretion system (T3SS) has been implicated in this process. We have here identified small molecules of a chemical class of acylated hydrazones of salicylaldehydes that specifically blocks the T3SS of Chlamydia. These compounds also affect the developmental cycle showing that the T3SS has a pivotal role in the pathogenesis of Chlamydia. Our results suggest a previously unexplored avenue for development of novel anti-chlamydial drugs.

Discovery of a vaccine antigen that protects mice from Chlamydia pneumoniae infection

Chlamydiae are atypical intracellular bacteria that infect via mucosal surfaces causing, for example, trachoma, pneumonia, cervicitis, urethritis and infertility. Existing antibiotics are only partially effective and no vaccines are available. Using surface expressed or secreted proteins previously identified by genomics and proteomics we tested five as vaccines against intranasal challenge with Chlamydia pneumoniae. One antigen, LcrE, induced CD4+ and CD8+ T cell activation, type 1 cytokine secretion and neutralising antibodies and was completely effective in eliminating infection. Such antigens are highly conserved and essential to all Chlamydial species. The discovery of an effective vaccine for Chlamydiae pneumoniae has potential wide benefits for human health.

Intranasal administration of chlamydial outer protein N (CopN) induces protection against pulmonary Chlamydia pneumoniae infection in a mouse model

Chlamydia pneumoniae is an intracellular pathogen that grows inside a vacuole, referred to as an inclusion. C. pneumoniae possess a type III secretion system (TTSS), which allows them to secrete effector molecules into the inclusion membrane and to the host cell cytosol. Proteins such as chlamydial outer protein N (CopN) that associate with the inclusion membrane are potential targets for the host's MHC-dependent antigen presentation, thereby representing ideal antigen candidates for T cell-based vaccination. The results of this study showed that intranasal immunization of BALB/c mice with heat-aggregated CopN protein and an Escherichia coli heat-labile toxin (LT) induced a strong immune response, detected as antigen-specific antibody production, lymphocyte proliferation and IFN-gamma production. Furthermore, the immunization induced statistically significant protection against intranasal C. pneumoniae challenge, the level of which correlated with the magnitude of CopN-specific lymphocyte proliferation. Both heat-aggregation of the antigen and the presence of LT adjuvant were required for maximal protective effect.

Identification and characterization of secreted effector proteins of Chlamydophila pneumoniae TW183

We report the expression of several chlamydial effector proteins in Chlamydophila pneumoniae, as well as their time-dependent secretion into the inclusion membrane. Localization of the respective genes within type III secretion gene clusters as well as bioinformatic analysis suggest that the identified proteins are type III-secreted effector proteins. Immunocytochemistry with antisera raised against CpMip (C. pneumoniae macrophage infectivity potentiator, Cpn0661), Pkn5 (Cpn0703), Cpn0709, Cpn0712 and Cpn0827 showed secretion of the respective proteins into the inclusion membrane at 20 h postinfection (hpi). CpMip was detected within the inclusion membrane from 20 to 72 hpi, whereas Cpn0324 (CopN) was located in this compartment at 72 hpi only. This was confirmed by co-localization of the respective proteins with IncA, an inclusion membrane marker protein. These data illustrate the fact that different effectors are being expressed and secreted during different time intervals of the infection cycle. Proteins Cpn0706 and Cpn0808 were not secreted by C. pneumoniae. The immunophilin FK506, known to inhibit the activity of Legionella, C. trachomatis and C. psittaci Mip proteins, was shown to interfere with chlamydial infection. Here we report the putatively type III-dependent secretion of CpMip into the inclusion membrane as well as the effect of its inhibition on C. pneumoniae infection of HEp-2 cells.

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.

Secretion of Cpn0796 from Chlamydia pneumoniae into the host cell cytoplasm by an autotransporter mechanism

By comparison of proteome profiles of purified Chlamydia pneumoniae and whole lysates of C. pneumoniae infected HEp-2 cells, an N-terminal fragment of the previously uncharacterized chlamydial protein Cpn0796 was identified as a secreted protein. A 38 kDa cleavage product of Cpn0796 was present in infected cells, whereas only the 65 kDa full-length Cpn0796 could be detected in purified Chlamydia. Confocal immunofluorescence microscopy demonstrated that Cpn0796 was localized in the Chlamydia membrane in young inclusions. However, at 36 h post infection and later Cpn0796 was detected in the cytoplasm of C. pneumoniae infected HEp-2 and BHK cells. Furthermore, Cpn0796 was detected in the cytoplasm of infected cells in the lungs of C. pneumoniae infected C57Bl mice. When cleavage was inhibited, Cpn0796 was retained in the chlamydiae. We propose that Cpn0796 is an autotransporter the N-terminal of which is translocated to the host cell cytoplasm. This is the first example of secretion of a Chlamydia autotransporter passenger domain into the host cell cytoplasm. Cpn0796 is specific for C. pneumoniae, where five homologous proteins are encoded by clustered genes. None of these five proteins were found to be secreted.

Bioinformatics, genomics and evolution of non-flagellar type-III secretion systems: a Darwinian perpective

We review the biology of non-flagellar type-III secretion systems from a Darwinian perspective, highlighting the themes of evolution, conservation, variation and decay. The presence of these systems in environmental organisms such as Myxococcus, Desulfovibrio and Verrucomicrobium hints at roles beyond virulence. We review newly discovered sequence homologies (e.g., YopN/TyeA and SepL). We discuss synapomorphies that might be useful in formulating a taxonomy of type-III secretion. The problem of information overload is likely to be ameliorated by launch of a web site devoted to the comparative biology of type-III secretion ().