Characterization of Chlamydia trachomatis l2-induced tyrosine-phosphorylated HeLa cell proteins by two-dimensional gel electrophoresis

Proteomics paves the way for Q fever diagnostics

Q fever is a worldwide zoonosis caused by Coxiella burnetii. The disease most frequently manifests clinically as a self-limited febrile illness, as pneumonia (acute Q fever) or as a chronic illness that presents mainly as infective endocarditis. The extreme infectivity of the bacterium results in large outbreaks, and the recent outbreak in the Netherlands underlines its impact on public health. Recent studies on the bacterium have included genome sequencing, the investigation of host-bacterium interactions, the development of cellular and animal models of infection, and the comprehensive analysis of different clinical isolates by whole genome and proteomic approaches. Current approaches for diagnosing Q fever are based on serological methods and PCR techniques, but the diagnosis of early stage disease lacks specificity and sensitivity. Consequently, different platforms have been created to explore Q fever biomarkers. Several studies using a combination of proteomics and recombinant protein screening approaches have been undertaken for the development of diagnostics and vaccines. In this review, we highlight advances in the field of C. burnetii proteomics, focusing mainly on the contribution of these technologies to the development and improvement of Q fever diagnostics.

Specific chlamydial inclusion membrane proteins associate with active Src family kinases in microdomains that interact with the host microtubule network

Chlamydiae are Gram-negative obligate intracellular bacteria that cause diseases with significant medical and economic impact. Chlamydia trachomatis replicates within a vacuole termed an inclusion, which is extensively modified by the insertion of a number of bacterial effector proteins known as inclusion membrane proteins (Incs). Once modified, the inclusion is trafficked in a dynein-dependent manner to the microtubule-organizing centre (MTOC), where it associates with host centrosomes. Here we describe a novel structure on the inclusion membrane comprised of both host and bacterial proteins. Members of the Src family of kinases are recruited to the chlamydial inclusion in an active form. These kinases display a distinct, localized punctate microdomain-like staining pattern on the inclusion membrane that colocalizes with four chlamydial inclusion membrane proteins (Incs) and is enriched in cholesterol. Biochemical studies show that at least two of these Incs stably interact with one another. Furthermore, host centrosomes associate with these microdomain proteins in C. trachomatis-infected cells and in uninfected cells exogenously expressing one of the chlamydial effectors. Together, the data suggest that a specific structure on the C. trachomatis inclusion membrane may be responsible for the known interactions of chlamydiae with the microtubule network and resultant effects on centrosome stability.

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.

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

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

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

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

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

Recent insights into the mechanisms of Chlamydia entry

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

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

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

A live and inactivated Chlamydia trachomatis mouse pneumonitis strain induces the maturation of dendritic cells that are phenotypically and immunologically distinct

The intracellular bacterial pathogen Chlamydia trachomatis is a major cause of sexually transmitted disease worldwide. While protective immunity does appear to develop following natural chlamydial infection in humans, early vaccine trials using heat-killed C. trachomatis resulted in limited and transient protection with possible enhanced disease during follow-up. Thus, immunity following natural infection with live chlamydia may differ from immune responses induced by immunization with inactivated chlamydia. To study this differing immunology, we used murine bone marrow-derived dendritic cells (DC) to examine DC maturation and immune effector function induced by live and UV-irradiated C. trachomatis elementary bodies (live EBs and UV-EB, respectively). DC exposed to live EBs acquired a mature DC morphology; expressed high levels of major histocompatibility complex (MHC) class II, CD80, CD86, CD40, and ICAM-1; produced elevated amounts of interleukin-12 and tumor necrosis factor alpha; and were efficiently recognized by Chlamydia-specific CD4+ T cells. In contrast, UV-EB-pulsed DC expressed low levels of CD40 and CD86 but displayed high levels of MHC class II, ICAM-1, and CD80; secreted low levels of proinflammatory cytokines; and exhibited reduced recognition by Chlamydia-specific CD4+ T cells. Adoptive transfer of live EB-pulsed DC was more effective than that of UV-EB-pulsed DC at protecting mice against challenge with live C. trachomatis. The expression of DC maturation markers and immune protection induced by UV-EB could be significantly enhanced by costimulation of DC ex vivo with UV-EB and oligodeoxynucleotides containing cytosine phosphate guanosine; however, the level of protection was significantly less than that achieved by using DC pulsed ex vivo with viable EBs. Thus, exposure of DC to live EBs results in a mature DC phenotype which is able to promote protective immunity, while exposure to UV-EB generates a semimature DC phenotype with less protective potential. This result may explain in part the differences in protective immunity induced by natural infection and immunization with whole inactivated organisms and is relevant to rational chlamydia vaccine design strategies.

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

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

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.

The reprogrammed host: Chlamydia trachomatis-induced up-regulation of glycoprotein 130 cytokines, transcription factors, and antiapoptotic genes

OBJECTIVE

Infection with Chlamydia trachomatis is a known cause of sexually transmitted diseases, eye infections (including trachoma), and reactive arthritis (ReA). Because the mechanisms of Chlamydia-induced changes leading to ReA are poorly defined, this study sought to identify the target genes involved at the molecular level.

METHODS

Chlamydia-induced changes in host cells were investigated by combining a screening technique, which utilized complementary DNA arrays on C trachomatis-infected and mock-infected epithelial HeLa cells, with real-time reverse transcription-polymerase chain reaction or enzyme-linked immunosorbent assay of gene products. Some responses were additionally demonstrated on human primary chondrocytes and a human synovial fibroblast cell line, both of which served as model cells for ReA.

RESULTS

Eighteen genes (of 1,176) were found to be up-regulated after 24 hours of infection with this obligate intracellular bacterium, among them the glycoprotein 130 family members IL-11 and LIF, the chemokine gene MIP2-alpha, the transcription factor genes EGR1, ETR101, FRA1, and c-jun, the apoptosis-related genes IEX-1L and MCL-1, adhesion molecule genes such as ICAM1, and various other functionally important genes. In the context of this rheumatic disease, the cytokines and transcription factors seem to be especially involved, since various connections to chondrocytes, synoviocytes, bone remodeling, joint pathology, and other rheumatic diseases have been demonstrated.

CONCLUSION

Infection with C trachomatis seems to reprogram the host cells (independent of activation by lipopolysaccharide or other ultraviolet-resistant bacterial components) at various key positions that act as intra- or intercellular switches, suggesting that these changes and similar Chlamydia-induced functional alterations constitute an important basis of the pathogenic inflammatory potential of these cells in ReA. Our results suggest that this approach is generally useful for the broad analysis of host-pathogen interactions involving obligate intracellular bacteria, and for the identification of target genes for therapeutic intervention in this rheumatic disease.

Proteome analysis of bacterial pathogens

Combining two-dimensional electrophoresis with mass spectrometry resulted in a powerful technology ideally suited to recognize and identify proteins of pathogenic microorganisms. This classical proteome analysis is now complemented by capillary chromatography/mass spectrometry combinations, miniaturization by chip technology and protein interaction investigations. Comparative proteomics is used to reveal vaccine candidates and pathogenicity factors. Immunoproteomics identifies specific and nonspecific antigens. For the management of the huge data amounts, bioinformatics is a valuable instrument for the construction of complex protein databases.

Evidence for the secretion of Chlamydia trachomatis CopN by a type III secretion mechanism

The medically significant, obligate intracellular pathogen Chlamydia trachomatis replicates within vacuoles termed inclusions. A developmental cycle is initiated after entry into a host cell and is manifested by the transformation of infectious elementary bodies (EBs) to larger, non-infectious reticulate bodies (RBs). Analysis of the C. trachomatis genome has revealed that chlamydiae possess genes that may encode a type III secretion apparatus. In other Gram-negative pathogens, the type III secretion mechanism is used to target virulence factors directly to the host cell cytoplasm and is essential for full virulence. To evaluate the possibility of a functional type III secretion mechanism in C. trachomatis, we initially focused on a locus containing genes encoding products with similarity to chaperones (Scc1), secretion pore components (Cds1 and Cds2) and secreted proteins (CopN) from other type III systems. Gene expression was tested by reverse transcriptase-polymerase chain reaction (RT-PCR) of total RNA extracted from infected HeLa cell monolayers at 2, 6, 12 and 20 h after infection and normalized for the number of C. trachomatis genomes present. Message was detected for Scc1 at all times, whereas message for all other tested genes was detected in significant amounts at 12 h and 20 h. Immunoblot analysis with Scc1- and CopN-specific antibodies revealed that CopN and Scc1 were present in EBs, RBs and whole-culture extracts harvested 20 h after infection. CopN is homologous to the secreted protein YopN of Yersinia sp., and analysis of monolayers 20 h after infection via indirect immunofluorescence showed specific labelling of inclusion membranes when probed with CopN-specific antibodies but not with Scc1-specific antibodies. His-tagged CopN and a chlamydial cytoplasmic control protein (NrdB) were expressed in Yersinia enterocolitica containing or lacking the virulence plasmid pYV. CopN, but not NrdB, was secreted by Y. enterocolitica in a Ca2+- and pYV-dependent fashion. These data indicate that components of the putative type III apparatus of C. trachomatis are expressed and that at least one of these products is secreted by chlamydiae to the inclusion membrane. The observation that CopN is also secreted by the Yersinia type III apparatus provides support for the notion that chlamydiae secrete proteins via a type III mechanism.

Mapping and identification of HeLa cell proteins separated by immobilized pH-gradient two-dimensional gel electrophoresis and construction of a two-dimensional polyacrylamide gel electrophoresis database

The HeLa cell line, a human adenocarcinoma, is used in many research fields, since it can be infected with a wide range of viruses and intracellular bacteria. Therefore, the mapping of HeLa cell proteins is useful for the investigation of parasite host cell interactions. Because of the recent improvements of two-dimensional gel electrophoresis with immobilized pH gradients (IPG) compared to isoelectric focusing with carrier ampholytes, a highly reproducible method for examining global changes in HeLa cell protein expression due to different stimuli is now available. Therefore, we have initiated the mapping of [35S]methionine/cysteine-labeled HeLa cell proteins with the 2-D PAGE (IPG)-system, using matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and N-terminal sequencing for protein identification. To date 21 proteins have been identified and mapped. In order to make these and future data accessible for interlaboratory comparison, we constructed a 2-D PAGE database on the World Wide Web.