Chlamydia trachomatis serovar L2 induces protein tyrosine phosphorylation during uptake by HeLa cells

Vitamin D effects on Chlamydia trachomatis infection: a case-control and experimental study

Introduction

Vitamin D deficiency is the most common nutritional deficiency worldwide. Chronic vitamin D deficiency causes immune system dysfunction, which increases susceptibility to pathogens such as bacteria, especially intracellular parasites, and viruses. Chlamydia trachomatis (C. t) is an obligate intracellular parasitic bacterium that causes a variety of sequelae. We speculated that vitamin D might be associated with C. t infection. This study aimed to address this gap in knowledge by investigating the relationship between vitamin D and C. t infection using both in vitro and in vivo models.

Methods and results

The addition of calcitriol to McCoy cell culture in vitro delayed and reduced the quantity and volume of inclusions compared to the control group. Macrophages of peritoneally lavaged mice co-cultured with McCoy decreased the infection rate and delayed the appearance of inclusions. In mice models of vitamin D deficiency, mice in the VD-group exhibited more severe genital tract inflammation and a longer duration of infection after inoculation with C. t in the genital tract. Supplementing these mice with vitamin D3 during treatment enhanced the therapeutic effect of antibiotics. We also conducted a case-control study involving 174 C. t-positive patients (95 males and 79 females) and 380 healthy volunteers (211 males and 169 females) aged 20-49 from January 2016 to March 15, 2017. Serum 25-(OH)D concentration was measured by assessing morning fasting blood samples of healthy volunteers and C. t-positive patients 1 day before antibiotic treatment and the next day after one course of treatment. The patients were followed up for 1 month and evaluated for recovery. The results showed that vitamin D deficiency was a risk factor for C. t infection and treatment failure.

Conclusion

In summary, findings from experimental and clinical studies indicate a close association between vitamin D levels and C. t infection and treatment outcomes. Given the affordability and safety of vitamin D, both healthy individuals and patients should focus on vitamin D intake. Vitamin D supplementation could enhance treatment success and should be used as an adjunctive therapy alongside antibiotic therapy for C. t infections, pending confirmation in larger, prospective, randomized controlled trials.

Lack of neutralization of Chlamydia trachomatis infection by high avidity monoclonal antibodies to surface-exposed major outer membrane protein variable domain IV

Chlamydia trachomatis is the leading cause of sexually transmitted diseases causing frequent, long-lasting, and often asymptomatic recurrent infections resulting in severe reproductive complications. C. trachomatis is an intracellular Gram-negative bacterium with a biphasic developmental cycle in which extracellular, infectious elementary bodies (EB) alternate with the intracellular replicating reticulate bodies (RB). The outer membrane of EB consists of a tight disulfide cross-linking protein network. The most essential protein is the 42 kDa major outer membrane protein (MOMP) that contributes to the rigid structural integrity of the outer membrane. MOMP is a transmembrane protein with a β-barrel structure consisting of four variable domains (VD) separated by five constant domains. VDIV possesses surface-exposed species-specific epitopes recognized by the immune system and, therefore, functions as a candidate for vaccine development. To analyze the protective contribution of antibodies for a MOMP vaccine, we investigated the specificity and binding characteristics of two monoclonal antibodies (MAb)224.2 and MAb244.4 directed against C. trachomatis serovar D MOMP. By immunoelectron microscopy, we found that both MAb bind to the surface of C. trachomatis EB. By epitope mapping, we characterized the MOMP epitope as linear consisting of 6 amino acids: 322TIAGAGD328. By ELISA it was shown that both antibodies bind with a higher avidity to the chlamydial surface compared to binding to monomeric MOMP, indicating that the antibodies bind divalently to the surface of C. trachomatis EB. Despite strong binding to the chlamydial surface, the antibodies only partially reduced the infectivity. This may be explained by the observation that even though both MAb covered the EB surface, antibodies could not be regularly detected on EB after the uptake into the host cell.

Chlamydia repurposes the actin-binding protein EPS8 to disassemble epithelial tight junctions and promote infection

Invasive microbial pathogens often disrupt epithelial barriers, yet the mechanisms used to dismantle tight junctions are poorly understood. Here, we show that the obligate pathogen Chlamydia trachomatis uses the effector protein TepP to transiently disassemble tight junctions early during infection. TepP alters the tyrosine phosphorylation status of host proteins involved in cytoskeletal regulation, including the filamentous actin-binding protein EPS8. We determined that TepP and EPS8 are necessary and sufficient to remodel tight junctions and that the ensuing disruption of epithelial barrier function promotes secondary invasion events. The genetic deletion of EPS8 renders epithelial cells and endometrial organoids resistant to TepP-mediated tight junction remodeling. Finally, TepP and EPS8 promote infection in murine models of infections, with TepP mutants displaying defects in ascension to the upper genital tract. These findings reveal a non-canonical function of EPS8 in the disassembly of epithelial junctions and an important role for Chlamydia pathogenesis.

Bacteria-Mediated Oncogenesis and the Underlying Molecular Intricacies: What We Know So Far

Cancers are known to have multifactorial etiology. Certain bacteria and viruses are proven carcinogens. Lately, there has been in-depth research investigating carcinogenic capabilities of some bacteria. Reports indicate that chronic inflammation and harmful bacterial metabolites to be strong promoters of neoplasticity. Helicobacter pylori-induced gastric adenocarcinoma is the best illustration of the chronic inflammation paradigm of oncogenesis. Chronic inflammation, which produces excessive reactive oxygen species (ROS) is hypothesized to cause cancerous cell proliferation. Other possible bacteria-dependent mechanisms and virulence factors have also been suspected of playing a vital role in the bacteria-induced-cancer(s). Numerous attempts have been made to explore and establish the possible relationship between the two. With the growing concerns on anti-microbial resistance and over-dependence of mankind on antibiotics to treat bacterial infections, it must be deemed critical to understand and identify carcinogenic bacteria, to establish their role in causing cancer.

Infection-provoked psoriasis: Induced or aggravated (Review)

Psoriasis is a common chronic, immune-mediated, inflammatory skin disorder, with a reported prevalence of 0.0-2.1% among children and 0.91-8.50% among adults, worldwide. Psoriasis is induced by several environmental factors, including infection, alcohol consumption, drugs, trauma, acute withdrawal of systemic or potent topical corticosteroids, body mass index and endocrine disorders. Increasing evidence suggest that a variety of microorganisms play key roles in the induction and exacerbation of psoriasis. Pathogens, such as streptococci and staphylococci are considered causal factors, presumably via superantigen activation of skin-seeking T cells. In addition, fungal pathogens, such as Candida and Malassezia, and viral agents, such as human immunodeficiency virus, hepatitis C virus infection and human papillomavirus, are also closely associated with psoriasis. Recently, several types of pathogens, such as Helicobacter pylori infection, Zika virus and scabies, have been reported to potentially trigger psoriasis. The present review discusses the underlying molecular mechanisms by which these infections influence psoriasis to provide a better understanding of the pathogenesis of psoriasis.

Got mutants? How advances in chlamydial genetics have furthered the study of effector proteins

Chlamydia trachomatis is the leading cause of infectious blindness and a sexually transmitted infection. All chlamydiae are obligate intracellular bacteria that replicate within a membrane-bound vacuole termed the inclusion. From the confines of the inclusion, the bacteria must interact with many host organelles to acquire key nutrients necessary for replication, all while promoting host cell viability and subverting host defense mechanisms. To achieve these feats, C. trachomatis delivers an arsenal of virulence factors into the eukaryotic cell via a type 3 secretion system (T3SS) that facilitates invasion, manipulation of host vesicular trafficking, subversion of host defense mechanisms and promotes bacteria egress at the conclusion of the developmental cycle. A subset of these proteins intercalate into the inclusion and are thus referred to as inclusion membrane proteins. Whereas others, referred to as conventional T3SS effectors, are released into the host cell where they localize to various eukaryotic organelles or remain in the cytosol. Here, we discuss the functions of T3SS effector proteins with a focus on how advances in chlamydial genetics have facilitated the identification and molecular characterization of these important factors.

Analysis of complement deposition and processing on Chlamydia trachomatis

Chlamydia trachomatis (C. trachomatis) is the leading cause of sexually transmitted bacterial infections worldwide, with over 120 million annual cases. C. trachomatis infections are associated with severe reproductive complications in women such as extrauterine pregnancy and tubal infertility. The infections are often long lasting, associated with immunopathology, and fail to elicit protective immunity which makes recurrent infections common. The immunological mechanisms involved in C. trachomatis infections are only partially understood. Murine infection models suggest that the complement system plays a significant role in both protective immunity and immunopathology during primary Chlamydia infections. However, only limited structural and mechanistic evidence exists on complement-mediated immunity against C. trachomatis. To expand our current knowledge on this topic, we analyzed global complement deposition on C. trachomatis using comprehensive in-depth mass spectrometry-based proteomics. We show that factor B, properdin, and C4b bind to C. trachomatis demonstrating that C. trachomatis-induced complement activation proceeds through at least two activation pathways. Complement activation leads to cleavage and deposition of C3 and C5 activation products, causing initiation of the terminal complement pathway and deposition of C5b, C6, C7, C8, C9 on C. trachomatis. Interestingly, using immunoelectron microscopy, we show that C5b-9 deposition occurred sporadically and only in rare cases formed complete lytic terminal complexes, possibly caused by the presence of the negative regulators vitronectin and clusterin. Finally, cleavage analysis of C3 demonstrated that deposited C3b is degraded to the opsonins iC3b and C3dg and that this complement opsonization facilitates C. trachomatis binding to human B-cells.

Pathogenic Puppetry: Manipulation of the Host Actin Cytoskeleton by Chlamydia trachomatis

The actin cytoskeleton is crucially important to maintenance of the cellular structure, cell motility, and endocytosis. Accordingly, bacterial pathogens often co-opt the actin-restructuring machinery of host cells to access or create a favorable environment for their own replication. The obligate intracellular organism Chlamydia trachomatis and related species exemplify this dynamic: by inducing actin polymerization at the site of pathogen-host attachment, Chlamydiae induce their own uptake by the typically non-phagocytic epithelium they infect. The interaction of chlamydial adhesins with host surface receptors has been implicated in this effect, as has the activity of the chlamydial effector TarP (translocated actin recruitment protein). Following invasion, C. trachomatis dynamically assembles and maintains an actin-rich cage around the pathogen’s membrane-bound replicative niche, known as the chlamydial inclusion. Through further induction of actin polymerization and modulation of the actin-crosslinking protein myosin II, C. trachomatis promotes egress from the host via extrusion of the inclusion. In this review, we present the experimental findings that can inform our understanding of actin-dependent chlamydial pathogenesis, discuss lingering questions, and identify potential avenues of future study.

The multiple functions of the numerous Chlamydia trachomatis secreted proteins: the tip of the iceberg

Chlamydia trachomatis serovars are obligate intracellular bacterial pathogens mainly causing ocular and urogenital infections that affect millions of people worldwide and which can lead to blindness or sterility. They reside and multiply intracellularly within a membrane-bound vacuolar compartment, known as inclusion, and are characterized by a developmental cycle involving two morphologically and physiologically distinct chlamydial forms. Completion of the developmental cycle involves the secretion of > 70 C. trachomatis proteins that function in the host cell cytoplasm and nucleus, in the inclusion membrane and lumen, and in the extracellular milieu. These proteins can, for example, interfere with the host cell cytoskeleton, vesicular and non-vesicular transport, metabolism, and immune signalling. Generally, this promotes C. trachomatis invasion into, and escape from, host cells, the acquisition of nutrients by the chlamydiae, and evasion of cell-autonomous, humoral and cellular innate immunity. Here, we present an in-depth review on the current knowledge and outstanding questions about these C. trachomatis secreted proteins.

Immunobiology of monocytes and macrophages during Chlamydia trachomatis infection

Infections caused by the intracellular bacterium Chlamydia trachomatis are a global health burden affecting more than 100 million people annually causing damaging long-lasting infections. In this review, we will present and discuss important aspects of the interaction between C. trachomatis and monocytes/macrophages.

The Loss of Expression of a Single Type 3 Effector (CT622) Strongly Reduces Chlamydia trachomatis Infectivity and Growth

Invasion of epithelial cells by the obligate intracellular bacterium Chlamydia trachomatis results in its enclosure inside a membrane-bound compartment termed an inclusion. The bacterium quickly begins manipulating interactions between host intracellular trafficking and the inclusion interface, diverging from the endocytic pathway and escaping lysosomal fusion. We have identified a previously uncharacterized protein, CT622, unique to the Chlamydiaceae, in the absence of which most bacteria failed to establish a successful infection. CT622 is abundant in the infectious form of the bacteria, in which it associates with CT635, a putative novel chaperone protein. We show that CT622 is translocated into the host cytoplasm via type three secretion throughout the developmental cycle of the bacteria. Two separate domains of roughly equal size have been identified within CT622 and a 1.9 Å crystal structure of the C-terminal domain has been determined. Genetic disruption of ct622 expression resulted in a strong bacterial growth defect, which was due to deficiencies in proliferation and in the generation of infectious bacteria. Our results converge to identify CT622 as a secreted protein that plays multiple and crucial roles in the initiation and support of the C. trachomatis growth cycle. They reveal that genetic disruption of a single effector can deeply affect bacterial fitness.

Complement C3 opsonization of Chlamydia trachomatis facilitates uptake in human monocytes

Chlamydia trachomatis is an obligate intracellular bacterium that causes severe infections, which can lead to infertility and ectopic pregnancy. Although both innate and adaptive immune responses are elicited during chlamydial infection the bacterium succeeds to evade host defense mechanisms establishing chronic infections. Thus, studying the host-pathogen interaction during chlamydial infection is of importance to understand how C. trachomatis can cause chronic infections. Both the complement system and monocytes play essential roles in anti-bacterial defense, and, therefore, we investigated the interaction between the complement system and the human pathogens C. trachomatis D and L2. Complement competent serum facilitated rapid uptake of both chlamydial serovars into monocytes. Using immunoelectron microscopy, we showed that products of complement C3 were loosely deposited on the bacterial surface in complement competent serum and further characterization demonstrated that the deposited C3 product was the opsonin iC3b. Using C3-depleted serum we confirmed that complement C3 facilitates rapid uptake of chlamydiae into monocytes in complement competent serum. Complement facilitated uptake did not influence intracellular survival of C. trachomatis or C. trachomatis-induced cytokine secretion. Hence, C. trachomatis D and L2 activate the complement system leading to chlamydial opsonization by iC3b and subsequent phagocytosis, activation and bacterial elimination by human monocytes.

Manipulation of the Host Cell Cytoskeleton by Chlamydia

Chlamydiae are obligate intracellular pathogens. They undergo a biphasic developmental cycle differentiating between the infectious but metabolically quiescent elementary body and the vegetative, but non-infectious reticulate body. Chlamydia spends a significant portion of its development in the non-infectious stage, demanding an effective strategy of manipulating the host cells to ensure its intracellular survival and replication. A common target of all Chlamydia species studied so far is the host cell cytoskeleton, with past and recent findings revealing crucial roles in invasion, inclusion maintenance, nutrient acquisition, and egress. The molecular details of how Chlamydia co-opts the cytoskeleton is becoming clearer, with bacterial factors and their corresponding host cell targets identified.

Activation of epidermal growth factor receptor is required for Chlamydia trachomatis development

Background

Chlamydia trachomatis (C. trachomatis) is a clinically significant human pathogen and one of the leading causative agents of sexually transmitted diseases. As obligate intracellular bacteria, C. trachomatis has evolved strategies to redirect the host’s signaling and resources for its own survival and propagation. Despite the clinical notoriety of Chlamydia infections, the molecular interactions between C. trachomatis and its host cell proteins remain elusive.

Results

In this study, we focused on the involvement of the host cell epidermal growth factor receptor (EGFR) in C. trachomatis attachment and development. A combination of molecular approaches, pharmacological agents and cell lines were used to demonstrate distinct functional requirements of EGFR in C. trachomatis infection. We show that C. trachomatis increases the phosphorylation of EGFR and of its downstream effectors PLCγ1, Akt and STAT5. While both EGFR and platelet-derived growth factor receptor-β (PDGFRβ) are partially involved in bacterial attachment to the host cell surface, it is only the knockdown of EGFR and not PDGFRβ that affects the formation of C. trachomatis inclusions in the host cells. Inhibition of EGFR results in small immature inclusions, and prevents C. trachomatis-induced intracellular calcium mobilization and the assembly of the characteristic F-actin ring at the inclusion periphery. By using complementary approaches, we demonstrate that the coordinated regulation of both calcium mobilization and F-actin assembly by EGFR are necessary for maturation of chlamydial inclusion within the host cells. A particularly important finding of this study is the co-localization of EGFR with the F-actin at the periphery of C. trachomatis inclusion where it may function to nucleate the assembly of signaling protein complexes for cytoskeletal remodeling required for C. trachomatis development.

Conclusion

Cumulatively, the data reported here connect the function of EGFR to C. trachomatis attachment and development in the host cells, and this could lead to new venues for targeting C. trachomatis infections and associated diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0277-4) contains supplementary material, which is available to authorized users.

The Chlamydia trachomatis type III secretion chaperone Slc1 engages multiple early effectors, including TepP, a tyrosine-phosphorylated protein required for the recruitment of CrkI-II to nascent inclusions and innate immune signaling

Chlamydia trachomatis, the causative agent of trachoma and sexually transmitted infections, employs a type III secretion (T3S) system to deliver effector proteins into host epithelial cells to establish a replicative vacuole. Aside from the phosphoprotein TARP, a Chlamydia effector that promotes actin re-arrangements, very few factors mediating bacterial entry and early inclusion establishment have been characterized. Like many T3S effectors, TARP requires a chaperone (Slc1) for efficient translocation into host cells. In this study, we defined proteins that associate with Slc1 in invasive C. trachomatis elementary bodies (EB) by immunoprecipitation coupled with mass spectrometry. We identified Ct875, a new Slc1 client protein and T3S effector, which we renamed TepP (Translocated early phosphoprotein). We provide evidence that T3S effectors form large molecular weight complexes with Scl1 in vitro and that Slc1 enhances their T3S-dependent secretion in a heterologous Yersinia T3S system. We demonstrate that TepP is translocated early during bacterial entry into epithelial cells and is phosphorylated at tyrosine residues by host kinases. However, TepP phosphorylation occurs later than TARP, which together with the finding that Slc1 preferentially engages TARP in EBs leads us to postulate that these effectors are translocated into the host cell at different stages during C. trachomatis invasion. TepP co-immunoprecipitated with the scaffolding proteins CrkI-II during infection and Crk was recruited to EBs at entry sites where it remained associated with nascent inclusions. Importantly, C. trachomatis mutants lacking TepP failed to recruit CrkI-II to inclusions, providing genetic confirmation of a direct role for this effector in the recruitment of a host factor. Finally, endocervical epithelial cells infected with a tepP mutant showed altered expression of a subset of genes associated with innate immune responses. We propose a model wherein TepP acts downstream of TARP to recruit scaffolding proteins at entry sites to initiate and amplify signaling cascades important for the regulation of innate immune responses to Chlamydia.

Diverse requirements for SRC-family tyrosine kinases distinguish chlamydial species

Chlamydiae are well known for their species specificity and tissue tropism, and yet the individual species and strains show remarkable genomic synteny and share an intracellular developmental cycle unique in the microbial world. Only a relatively few chlamydial genes have been linked to specific disease or tissue tropism. Here we show that chlamydial species associated with human infections, Chlamydia trachomatis and C. pneumoniae, exhibit unique requirements for Src-family kinases throughout their developmental cycle. Utilization of Src-family kinases by C. trachomatis includes tyrosine phosphorylation of the secreted effector Tarp during the entry process, a functional role in microtubule-dependent trafficking to the microtubule organizing center, and a requirement for Src-family kinases for successful initiation of development. Nonhuman chlamydial species C. caviae and C. muridarum show none of these requirements and, instead, appear to be growth restricted by the activities of Src-family kinases. Depletion of Src-family kinases triggers a more rapid development of C. caviae with up to an 800% increase in infectious progeny production. Collectively, the results suggest that human chlamydial species have evolved requirements for tyrosine phosphorylation by Src-family kinases that are not seen in other chlamydial species. The requirement for Src-family kinases thus represents a fundamental distinction between chlamydial species that would not be readily apparent in genomic comparisons and may provide insights into chlamydial disease association and species specificity.

IMPORTANCE

Chlamydiae are well known for their species specificity and tissue tropism as well as their association with unique diseases. A paradox in the field relates to the remarkable genomic synteny shown among chlamydiae and the very few chlamydial genes linked to specific diseases. We have found that different chlamydial species exhibit unique requirements for Src-family kinases. These differing requirements for Src-family kinases would not be apparent in genomic comparisons and appear to be a previously unrecognized distinction that may provide insights to guide research in chlamydial pathogenesis.

Abl family of tyrosine kinases and microbial pathogenesis

Abl nonreceptor tyrosine kinases are activated by multiple stimuli and regulate cytoskeletal reorganization, cell proliferation, survival, and stress responses. Several downstream pathways have direct impact on physiological processes, including development and maintenance of the nervous and immune systems and epithelial morphogenesis. Recent studies also indicated that numerous viral and bacterial pathogens highjack Abl signaling for different purposes. Abl kinases are activated to reorganize the host actin cytoskeleton and promote the direct tyrosine phosphorylation of viral surface proteins and injected bacterial type-III and type-IV effector molecules. However, Abl kinases also play other roles in infectious processes of bacteria, viruses, and prions. These activities have crucial impact on microbial invasion and release from host cells, actin-based motility, pedestal formation, as well as cell-cell dissociation involved in epithelial barrier disruption and other responses. Thus, Abl kinases exhibit important functions in pathological signaling during microbial infections. Here, we discuss the different signaling pathways activated by pathogens and highlight possible therapeutic intervention strategies.

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.

Inflammasome-dependent caspase-1 activation in cervical epithelial cells stimulates growth of the intracellular pathogen Chlamydia trachomatis

Inflammasomes have been extensively characterized in monocytes and macrophages, but not in epithelial cells, which are the preferred host cells for many pathogens. Here we show that cervical epithelial cells express a functional inflammasome. Infection of the cells by Chlamydia trachomatis leads to activation of caspase-1, through a process requiring the NOD-like receptor family member NLRP3 and the inflammasome adaptor protein ASC. Secretion of newly synthesized virulence proteins from the chlamydial vacuole through a type III secretion apparatus results in efflux of K(+) through glibenclamide-sensitive K(+) channels, which in turn stimulates production of reactive oxygen species. Elevated levels of reactive oxygen species are responsible for NLRP3-dependent caspase-1 activation in the infected cells. In monocytes and macrophages, caspase-1 is involved in processing and secretion of pro-inflammatory cytokines such as interleukin-1beta. However, in epithelial cells, which are not known to secrete large quantities of interleukin-1beta, caspase-1 has been shown previously to enhance lipid metabolism. Here we show that, in cervical epithelial cells, caspase-1 activation is required for optimal growth of the intracellular chlamydiae.

Chlamydia trachomatisand male fertility

There is increasing evidence that the function of human spermatozoa can be significantly affected by direct exposure to the bacterium Chlamydia trachomatis. This may contribute to sub-fertility in infected individuals by a route that is independent of any damage to the reproductive epithelium. In addition, if a C. trachomatis infection is undiagnosed it could contribute to poor outcomes in assisted conception techniques such as in vitro fertilization. The antibiotics routinely used in IVF culture systems are largely ineffective against chlamydia, emphasizing the importance of screening patients prior to treatment. Moreover, given the many thousands of semen samples provided for analysis by men in primary care (many of which will never undergo assisted conception treatment), it is suggested that this may represent a wasted opportunity to provide screening (and treatment) for the infection using an appropriate test specimen and without the need for additional hospital visits.

Complex kinase requirements for Chlamydia trachomatis Tarp phosphorylation

Chlamydia trachomatis translocates the effector protein Tarp (translocated actin-recruiting phosphoprotein) into the host cell cytoplasm where it is quickly tyrosine phosphorylated. Abl and Src kinases have been implicated in Tarp phosphorylation; however, we observed that the situation is more complex. Chemical inhibition of Src family kinases confirmed a role for these kinases in Tarp phosphorylation. Infection of Src, Yes, Fyn (SYF)-deficient cells showed a dampened, but incompletely blocked, Tarp phosphorylation. Inhibition of Abl in an SYF background still did not completely block Tarp phosphorylation. Consequently, we tested additional kinases and found that Syk, but not Btk or Jak2, is a potent kinase of Tarp in vitro. Inhibition of Syk in an SYF background further blocked Tarp phosphorylation. Under these conditions, inclusion formation still proceeded normally. These data reveal a highly promiscuous substrate property of Tarp and set the stage for further functional characterization of Tarp phosphorylation during host cell infection.

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.

Chlamydophila pneumoniae PknD exhibits dual amino acid specificity and phosphorylates Cpn0712, a putative type III secretion YscD homolog

Chlamydophila pneumoniae is an obligate intracellular bacterium that causes bronchitis, pharyngitis, and pneumonia and may be involved in atherogenesis and Alzheimer's disease. Genome sequencing has identified three eukaryote-type serine/threonine protein kinases, Pkn1, Pkn5, and PknD, that may be important signaling molecules in Chlamydia. Full-length PknD was cloned and expressed as a histidine-tagged protein in Escherichia coli. Differential centrifugation followed by sodium carbonate treatment of E. coli membranes demonstrated that His-PknD is an integral membrane protein. Fusions of overlapping PknD fragments to alkaline phosphatase revealed that PknD contains a single transmembrane domain and that the kinase domain is in the cytoplasm. To facilitate solubility, the kinase domain was cloned and expressed as a glutathione S-transferase (GST) fusion protein in E. coli. Purified GST-PknD kinase domain autophosphorylated, and catalytic mutants (K33G, D156G, and K33G-D156G mutants) and activation loop mutants (T185A and T193A) were inactive. PknD phosphorylated recombinant Cpn0712, a type III secretion YscD homolog that has two forkhead-associated domains. Thin-layer chromatography revealed that the PknD kinase domain autophosphorylated on threonine and tyrosine and phosphorylated the FHA-2 domain of Cpn0712 on serine and tyrosine. To our knowledge, this is the first demonstration of a bacterial protein kinase with amino acid specificity for both serine/threonine and tyrosine residues and this is the first study to show phosphorylation of a predicted type III secretion structural protein.

Tyrosine-phosphorylated bacterial effector proteins: the enemies within

The tyrosine phosphorylation of proteins has a central role during signal transduction in eukaryotes. Recent progress shows that tyrosine phosphorylation is also a common feature of several effector proteins translocated by bacterial type III and type IV secretion systems. The involvement of these secretion systems in disease development is exemplified by a variety of pathogenic processes: pedestal formation (Tir of EPEC and Citrobacter), cell scattering (CagA of Helicobacter), invasion (Tarp of Chlamydia) and possibly proinflammatory responses and cell proliferation (BepD-F of Bartonella). The discovery that different bacterial pathogens use this common strategy to subvert host-cell function suggests that more examples will soon emerge.

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.

Increased cystic fibrosis transmembrane conductance regulator (CFTR) expression in the human hydrosalpinx

BACKGROUND

Hydrosalpinx (HSP), characterized by abnormal fluid accumulation in the Fallopian tube, is one of the main causes of infertility in women; however, the mechanism underlying the formation of hydrosalpinx fluid (HF) remains elusive. The present study investigated the possible involvement of cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-dependent chloride channel, in the pathogenesis of hydrosalpinx.

METHODS

Masson's trichrome staining was used to characterize epithelial transformation in human HSP; RT-PCR, immunohistochemistry and immunofluorescence staining were used for CFTR expression and localization.

RESULTS

Masson's trichrome staining showed areas of epithelial transformation, focally attenuated and pseudostratified. Immunostaining showed enhanced CFTR immunoreactivity in the focally attenuated and pseudostratified areas of HSP epithelium. RT-PCR revealed that CFTR expression in HSP was significantly greater than that in normal Fallopian tubes.

CONCLUSIONS

These results indicate that HSP epithelium undergoes epithelial transformation with elevated CFTR expression, which may lead to increased transepithelial electrolyte and fluid secretion resulting in HF formation. The present findings may lead to the development of new treatment strategies for infertile patients with HSP.

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

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

Analysis of Chlamydia caviae entry sites and involvement of Cdc42 and Rac activity

In epithelial cells, endocytic activity is mostly dedicated to nutrient and macromolecule uptake. To invade these cells, Chlamydiaceae, like other pathogens, have evolved strategies that utilise the existing endocytic machineries and signalling pathways, but little is known about the host cell molecules involved. In this report, we show that within five minutes of infection of HeLa cells by Chlamydia caviae GPIC strain several events take place in the immediate vicinity of invasive bacteria: GM1-containing microdomains cluster, tyrosine-phosphorylated proteins accumulate, and intense actin polymerization occurs. We show that actin polymerization is controlled by the small GTPases Cdc42 and Rac, which become activated upon infection. Expression of dominant negative forms of these GTPases inhibits C. caviae entry and leads to abnormal actin polymerization. In contrast, the small GTPase Rho does not seem essential for bacterial entry. Finally, phosphatidylinositol 3-kinase activity is also required for internalization of C. caviae, probably downstream of the other molecular events reported here. We present the first scheme of the events occurring at the sites of invasion of epithelial cells by a member of the Chlamydiaceae family.

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

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

Lipid rafts, caveolae, caveolin-1, and entry by Chlamydiae into host cells

Obligate intracellular bacterial pathogens of the genus Chlamydia are reported to enter host cells by both clathrin-dependent and clathrin-independent processes. C. trachomatis serovar K recently was shown to enter cells via caveolae-like lipid raft domains. We asked here how widespread raft-mediated entry might be among the Chlamydia. We show that C. pneumoniae, an important cause of respiratory infections in humans that additionally is associated with cardiovascular disease, and C. psittaci, an important pathogen in domestic mammals and birds that also infects humans, each enter host cells via cholesterol-rich lipid raft microdomains. Further, we show that C. trachomatis serovars E and F also use these domains to enter host cells. The involvement of these membrane domains in the entry of these organisms was indicated by the sensitivity of their entry to the raft-disrupting agents Nystatin and filipin, and by their intracellular association with caveolin-1, a 22-kDa protein associated with the formation of caveolae in rafts. In contrast, caveolin-marked lipid raft domains do not mediate entry of C. trachomatis serovars A, 36B, and C, nor of LGV serovar L2 and MoPn. Finally, we show that entry of each of these chlamydial strains is independent of cellular expression of caveolin-1. Thus, entry via the Nystatin and filipin-sensitive pathway is dependent on lipid rafts containing cholesterol, rather than invaginated caveolae per se.

The chlamydial inclusion: escape from the endocytic pathway

Chlamydiae, bacterial obligate intracellular pathogens, are the etiologic agents of several human diseases. A large part of the chlamydial intracellular survival strategy involves the formation of a unique organelle called the inclusion that provides a protected site within which they replicate. The chlamydial inclusion is effectively isolated from endocytic pathways but is fusogenic with a subset of exocytic vesicles that deliver sphingomyelin from the Golgi apparatus to the plasma membrane. A combination of host and parasite functions contribute to the biogenesis of this compartment. Establishment of the mature inclusion is accompanied by the insertion of multiple chlamydial proteins, suggesting that chlamydiae actively modify the inclusion to define its interactions with the eukaryotic host cell. Despite being sequestered within a membrane-bound vacuole, chlamydiae clearly communicate with and manipulate the host cell from within this privileged intracellular niche.

Chlamydia trachomatis - the agent

Chlamydiae are obligate intracellular bacteria, parasitizing eukaryotic cells. Chlamydia trachomatis, C. psittaci and C. pneumoniae are the three species of chlamydiae pathogenic to humans. C. trachomatis shows a tropism for the genital and conjunctival epithelia and consists of 19 different serovars which are pathogenic predominantly for the urogenital tract.A distinguishing feature of chlamydiae is their transition between the infectious elementary body that enters the host cell and the non-infectious reticulate body that replicates intracellularly within an inclusion that does not fuse with lysosomes. Chlamydiae depend for some functions upon the host cell; in particular, chlamydiae have little capacity for generating energy. The complete sequence of the 1000-kb chromosome of C. trachomatis is known, as are most of the genes located on the 7.5-kb cryptic plasmid. Recently, several concepts about the biology and the metabolic pathways of C. trachomatis have been revised in relation to the genome sequence, and different novel proteins have been described.

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

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

Association of caveolin with Chlamydia trachomatis inclusions at early and late stages of infection

The mechanism by which the intracellular bacterial pathogen Chlamydia trachomatis enters eukaryotic cells is poorly understood. There are conflicting reports of entry occurring by clathrin-dependent and clathrin-independent processes. We report here that C. trachomatis serovar K enters HEp-2 and HeLa 229 epithelial cells and J-774A.1 mouse macrophage/monocyte cells via caveolin-containing sphingolipid and cholesterol-enriched raft microdomains in the host cell plasma membranes. First, filipin and nystatin, drugs that specifically disrupt raft function by cholesterol chelation, each impaired entry of C. trachomatis serovar K. In control experiments, filipin did not impair entry of the same organism by an antibody-mediated opsonic process, nor did it impair entry of BSA-coated microspheres. Second, the chlamydia-containing endocytic vesicles specifically reacted with antisera against the caveolae marker protein caveolin. These vesicles are known to become the inclusions in which parasite replication occurs. They avoid fusion with lysosomes and instead traffic to the Golgi region, where they intercept Golgi-derived vesicles that recycle sphingolipids and cholesterol to the plasma membrane. We also report that late-stage C. trachomatis inclusions continue to display high levels of caveolin, which they likely acquire from the exocytic Golgi vesicles. We suggest that the atypical raft-mediated entry process may have important consequences for the host-pathogen interaction well after entry has occurred. These consequences include enabling the chlamydial vesicle to avoid acidification and fusion with lysosomes, to traffic to the Golgi region, and to intercept sphingolipid-containing vesicles from the Golgi.

Mammalian 14-3-3beta associates with the Chlamydia trachomatis inclusion membrane via its interaction with IncG

Chlamydiae replicate intracellularly within a vacuole that is modified early in infection to become fusogenic with a subset of exocytic vesicles. We have recently identified four chlamydial inclusion membrane proteins, IncD-G, whose expression is detected within the first 2 h after internalization. To gain a better understanding of how these Inc proteins function, a yeast two-hybrid screen was employed to identify interacting host proteins. One protein, 14-3-3beta, was identified that interacted specifically with IncG. The interaction between 14-3-3beta and IncG was confirmed in infected HeLa cells by indirect immunofluorescence microscopy and interaction with a GFP-14-3-3beta fusion protein. 14-3-3 proteins are phosphoserine-binding proteins. Immunoprecipitation studies with [32P]-orthophosphate-labelled cells demonstrated that IncG is phosphorylated in both chlamydia-infected HeLa cells and in yeast cells expressing IncG. Site-directed mutagenesis of predicted 14-3-3 phosphorylation sites demonstrated that IncG binds to 14-3-3beta via a conserved 14-3-3-binding motif (RS164RS166F). Finally, indirect immunofluorescence demonstrated that 14-3-3beta interacts with Chlamydia trachomatis inclusions but not C. psittaci or C. pneumoniae inclusions. 14-3-3beta is the first eukaryotic protein found to interact with the chlamydial inclusion; however, its unique role in C. trachomatis pathogenesis remains to be determined.

Rapid, transient phosphatidylserine externalization induced in host cells by infection with Chlamydia spp

Chlamydia organisms are obligate intracellular bacterial pathogens responsible for a range of human diseases. Persistent infection or reinfection with Chlamydia trachomatis leads to scarring of ocular or genital tissues, and Chlamydia pneumoniae infection is associated with the development of atherosclerosis. We demonstrate that C. trachomatis and C. pneumoniae infection in vitro elicits the externalization of the lipid phosphatidylserine on the surface of human epithelial, endothelial, granulocytic, and monocytic cells. Phosphatidylserine externalization is associated with cellular development, differentiation, and death. Infection-induced phosphatidylserine externalization was immediate, transient, calcium dependent, and infectious dose dependent and was unaffected by a broad-spectrum caspase inhibitor. Chlamydia-infected cells accelerated plasma clotting and increased the macrophage phagocytosis of infected cells that was phosphatidylserine dependent. The rapid externalization of phosphatidylserine by infected cells may be an important factor in the pathogenesis of chlamydial infections.

Coincubation of human spermatozoa with Chlamydia trachomatis in vitro causes increased tyrosine phosphorylation of sperm proteins

Elementary bodies (EBs) of the obligate intracellular bacterium Chlamydia trachomatis are responsible for the first step of attachment to host cells. We have studied the effects of EBs on human sperm protein tyrosine phosphorylation, which is important to sperm function. Indirect immunofluorescence using antiphosphotyrosine antibodies showed that serovar E, but not LGV, caused increased tyrosine phosphorylation which was localized to the sperm tail region. Immunoblotting revealed that serovar E caused a marked increase in tyrosine phosphorylation of 80- and 95-kDa sperm proteins, whereas serovar LGV caused increased phosphorylation of only the 80-kDa moiety. Considering the importance of tyrosine phosphorylation for sperm capacitation and other aspects of sperm function, we conclude that EBs may affect these events.

Intracellular survival by Chlamydia

Chlamydiae are obligate intracellular bacterial pathogens whose entry into mucosal epithelial cells is required for intracellular survival and subsequent growth. After a seemingly stealthy entry, chlamydiae quickly modify their vacuole (i) for exit from the endosomal pathway to the exocytic pathway and (ii) to permit fusion with intercepted endoplasmic reticulum- and Golgi-derived vesicles carrying glycerophospholipids and sphingolipids for chlamydiae-containing vacuole membrane expansion. Chlamydiae possess novel hollow proteinaceous structures, termed projections, which they use to pierce the inclusion membrane, possibly to acquire from the epithelial cytoplasm nutrients they cannot synthesize; whether or not these truncated flagellar-like structures serve a dual exchange function for secretion of molecules to programme host cell signalling is unknown. Despite the accumulation of some 500-1000 progeny in the enormously enlarged inclusion, host cell function is surprisingly little disrupted, and progeny escape can be unobtrusive. This elegant adaptive pathogen strategy, which leads to silent, chronic human infection, is fascinating from a cellular microbiology perspective.

Eukaryotic cell uptake of heparin-coated microspheres: a model of host cell invasion by Chlamydia trachomatis

Using polystyrene microspheres coated with heparin or heparan sulfate, it was shown that coated microspheres specifically bound eukaryotic cells and were endocytosed by nonprofessional phagocytic cells. Coated microspheres displayed properties of binding to eukaryotic cells that were similar to those of chlamydiae, and the microspheres were competitively inhibited by chlamydial organisms. Endocytosis of heparin-coated beads resulted in the tyrosine phosphorylation of a similar set of host proteins as did endocytosis of chlamydiae; however, unlike viable chlamydial organisms, which prevent phagolysosomal fusion, endocytosed beads were trafficked to a lysosomal compartment. These findings suggest that heparin-coated beads and Chlamydia trachomatis enter eukaryotic cells by similar pathways.

Significance of host cell kinesin in the development of Chlamydia psittaci

The influence of the microtubule-associated motor protein kinesin on Chlamydia psittaci inclusion development in epithelial and fibroblast cell lines was addressed. Kinesin was blocked early after chlamydial internalization (4 h postinfection [p.i.]) and before the initiation of active chlamydial multiplication (8 h p.i.). Chlamydia development was monitored by fluorescence and transmission electron microscopy at different times during the cycle. In both host cell lines, kinesin blockage restricted mitochondria from the chlamydial vacuole. The effects of kinesin blockage on the C. psittaci replication cycle included the presence of multiple inclusions up to late in the cycle, the presence of enlarged pleomorphic reticulate bodies, and a delayed reappearance of elementary bodies. The last effect seems to be greater when kinesin is blocked early after infection. Our results show that kinesin activity is required for optimal development of these microorganisms, most probably acting through the apposition of mitochondria to the C. psittaci inclusions.

Legionella pneumophila invasion of MRC-5 cells induces tyrosine protein phosphorylation

After uptake and intracellular multiplication of Legionella pneumophila in MRC-5 lung fibroblasts, important cytoskeletal filament structures, like actin, tubulin, or vimentin, and a cell membrane-associated fibronectin were rearranged during early infection, resulting in a loss of cell adhesion and collapse of the cytoskeleton. Dysregulation of the cellular phosphorylation and dephosphorylation cascade may contribute to the observed changes and may support intracellular survival and multiplication of L. pneumophila. We therefore studied expression of phosphoproteins during intracellular growth of L. pneumophila. By using an anti-tyrosine phosphoprotein antibody we showed that proteins phosphorylated on tyrosine residues accumulated progressively during late infection exclusively around or in phagosomes filled with bacteria. In contrast, expression of serine/threonine phosphoproteins did not change. To discern the origin of phosphorylated proteins, the host cells were treated with cycloheximide, an inhibitor of eukaryotic protein synthesis. The newly synthesized proteins were labeled metabolically with [(35)S]methionine-cysteine and immunoprecipitated with a phosphotyrosine-specific antibody. Sodium dodecyl sulfate gel electrophoresis gave evidence for synthesis of at least three protein clusters (160 to 200, 35 to 60, and 19 to 28 kDa) of Legionella origin that were phosphorylated on tyrosine residues 24 h after infection. Treatment of infected host cells with genistein, a tyrosine kinase inhibitor, revealed that tyrosine protein phosphorylation was not important for bacterial uptake but contributed to intracellular growth of L. pneumophila. Bacterial tyrosine phosphoproteins and the observed intracellular structural changes may be important to understanding the process involved in intracellular growth of L. pneumophila.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Signal transduction during Legionella pneumophila entry into human monocytes

Legionella pneumophila causes Legionnaires' disease by replication in alveolar macrophages and monocytes. The bacteria are internalized most efficiently by opsonin-dependent, CR3-mediated phagocytosis. This investigation focused on determining the role of actin polymerization and phosphorylation signals in this uptake mechanism. Uptake inhibition assays and confocal microscopic analysis indicated that entry of L. pneumophila activated tyrosine kinase (TK) and protein kinase C (PKC) and induced actin polymerization at the site of bacterial entry. Upon L. pneumophila entry, six major cellular proteins (75, 71, 59, 56, 53, and 52 kDa) were TK phosphorylated in soluble fractions of monocytes, and three of these proteins (52, 53, and 56 kDa) were consistently found in insoluble (i.e., cytoskeletal) fractions of monocytes as well. Tyrosine phosphorylation was suppressed when cells were pretreated with the kinase inhibitor genistein, tyrphostin, or staurosporine. A similar tyrosine-phosphorylated protein pattern was observed with CR3-mediated entry of avirulent L. pneumophila, Escherichia coli, or zymosan into monocytes. This study has shown that PKC and TK signals which activate actin polymerization during the process of phagocytosis are induced upon L. pneumophila entry. In addition, CR3 receptor-mediated phagocytosis into monocytes may involve tyrosine phosphorylation of similar proteins, regardless of the particle being phagocytosed. Therefore, the tyrosine-induced phosphorylation observed during opsonized L. pneumophila entry is not a virulence-associated event.

Inhibition of apoptosis in chlamydia-infected cells: blockade of mitochondrial cytochrome c release and caspase activation

We report that chlamydiae, which are obligate intracellular bacterial pathogens, possess a novel antiapoptotic mechanism. Chlamydia-infected host cells are profoundly resistant to apoptosis induced by a wide spectrum of proapoptotic stimuli including the kinase inhibitor staurosporine, the DNA-damaging agent etoposide, and several immunological apoptosis-inducing molecules such as tumor necrosis factor-alpha, Fas antibody, and granzyme B/perforin. The antiapoptotic activity was dependent on chlamydial but not host protein synthesis. These observations suggest that chlamydia may encode factors that interrupt many different host cell apoptotic pathways. We found that activation of the downstream caspase 3 and cleavage of poly (ADP-ribose) polymerase were inhibited in chlamydia-infected cells. Mitochondrial cytochrome c release into the cytosol induced by proapoptotic factors was also prevented by chlamydial infection. These observations suggest that chlamydial proteins may interrupt diverse apoptotic pathways by blocking mitochondrial cytochrome c release, a central step proposed to convert the upstream private pathways into an effector apoptotic pathway for amplification of downstream caspases. Thus, we have identified a chlamydial antiapoptosis mechanism(s) that will help define chlamydial pathogenesis and may also provide information about the central mechanisms regulating host cell apoptosis.

Infection with Chlamydia trachomatis alters the tyrosine phosphorylation and/or localization of several host cell proteins including cortactin

Infection of epithelial cells by two biovars of Chlamydia trachomatis results in the tyrosine phosphorylation of several host proteins. The most prominent change in host protein tyrosine phosphorylation involves a complex of proteins with molecular masses of 75 to 85 kDa (pp75/85) and 100 kDa (pp100). The C. trachomatis-induced tyrosine phosphorylation of pp75/85 and pp100 is observed in several cell lines, including epithelial cells, fibroblasts, and macrophages. Subcellular fractionation and detergent solubility properties of pp75/85 are consistent with its association with the cytoskeleton. Phosphoamino acid analysis demonstrates that the pp75/85 complex is phosphorylated on both tyrosine and serine residues. Immunofluorescence studies of chlamydia-infected cells by using fluorescein isothiocyanate-phalloidin and antibodies to phosphotyrosine and cortactin demonstrate that tyrosine-phosphorylated proteins, as well as cortactin, are localized to the chlamydial vacuole and that this process is facilitated by actin.

Internalization of Escherichia coli by human renal epithelial cells is associated with tyrosine phosphorylation of specific host cell proteins

Human renal epithelial cells are capable of internalizing Escherichia coli regardless of whether the bacteria are isolated from individuals with pyelonephritis or from healthy volunteers. In this study, we investigated the role of host cell tyrosine kinase activity in internalization. We found that internalization of both fecal and pyelonephritis isolates is blocked by tyrosine kinase inhibitors. We found increased intensity of two tyrosine-phosphorylated proteins, with relative mobilities of approximately 123,000 and 110,000, in Western blots of extracts from human renal epithelial cells infected with E. coli. The increased intensity of these tyrosine-phosphorylated proteins was observed only in the Triton X-100-insoluble fraction, suggesting that these proteins could be associated with the cytoskeleton. Increased tyrosine phosphorylation of these proteins upon E. coli infection was observed in both transformed and primary human renal epithelial cells and in cells infected with several different strains of E. coli isolated from the feces of healthy individuals or from the blood or urine of patients with pyelonephritis. The increased tyrosine phosphorylation of these proteins required live bacteria and was blocked by tyrosine kinase inhibition but not by protein synthesis inhibitors or cytochalasin D. These experiments establish a strong link between E. coli internalization and host cell signaling through tyrosine kinases in human kidney cells and provide evidence that specific proteins are involved in these processes.