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

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.

Cortactin: A universal host cytoskeletal target of Gram-negative and Gram-positive bacterial pathogens

Pathogenic bacteria possess a great potential of causing infectious diseases and represent a serious threat to human and animal health. Understanding the molecular basis of infection development can provide new valuable strategies for disease prevention and better control. In host-pathogen interactions, actin-cytoskeletal dynamics play a crucial role in the successful adherence, invasion, and intracellular motility of many intruding microbial pathogens. Cortactin, a major cellular factor that promotes actin polymerization and other functions, appears as a central regulator of host-pathogen interactions and different human diseases including cancer development. Various important microbes have been reported to hijack cortactin signaling during infection. The primary regulation of cortactin appears to proceed via serine and/or tyrosine phosphorylation events by upstream kinases, acetylation, and interaction with various other host proteins, including the Arp2/3 complex, filamentous actin, the actin nucleation promoting factor N-WASP, focal adhesion kinase FAK, the large GTPase dynamin-2, the guanine nucleotide exchange factor Vav2, and the actin-stabilizing protein CD2AP. Given that many signaling factors can affect cortactin activities, several microbes target certain unique pathways, while also sharing some common features. Here we review our current knowledge of the hallmarks of cortactin as a major target for eminent Gram-negative and Gram-positive bacterial pathogens in humans.

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.

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.

The Chlamydia trachomatis secreted effector TmeA hijacks the N-WASP-ARP2/3 actin remodeling axis to facilitate cellular invasion

As an obligate intracellular pathogen, host cell invasion is paramount to Chlamydia trachomatis proliferation. While the mechanistic underpinnings of this essential process remain ill-defined, it is predicted to involve delivery of prepackaged effector proteins into the host cell that trigger plasma membrane remodeling and cytoskeletal reorganization. The secreted effector proteins TmeA and TarP, have risen to prominence as putative key regulators of cellular invasion and bacterial pathogenesis. Although several studies have begun to unravel molecular details underlying the putative function of TarP, the physiological function of TmeA during host cell invasion is unknown. Here, we show that TmeA employs molecular mimicry to bind to the GTPase binding domain of N-WASP, which results in recruitment of the actin branching ARP2/3 complex to the site of chlamydial entry. Electron microscopy revealed that TmeA mutants are deficient in filopodia capture, suggesting that TmeA/N-WASP interactions ultimately modulate host cell plasma membrane remodeling events necessary for chlamydial entry. Importantly, while both TmeA and TarP are necessary for effective host cell invasion, we show that these effectors target distinct pathways that ultimately converge on activation of the ARP2/3 complex. In line with this observation, we show that a double mutant suffers from a severe entry defect nearly identical to that observed when ARP3 is chemically inhibited or knocked down. Collectively, our study highlights both TmeA and TarP as essential regulators of chlamydial invasion that modulate the ARP2/3 complex through distinct signaling platforms, resulting in plasma membrane remodeling events that are essential for pathogen uptake.

Epidermal Growth Factor Receptor and Transforming Growth Factor β Signaling Pathways Cooperate To Mediate Chlamydia Pathogenesis

Human genital Chlamydia infection is a major public health concern due to the serious reproductive system complications. Chlamydia binds several receptor tyrosine kinases (RTKs) on host cells, including the epidermal growth factor receptor (EGFR), and activates cellular signaling cascades for host invasion, cytoskeletal remodeling, optimal inclusion development, and induction of pathogenic epithelial-mesenchyme transition (EMT). Chlamydia also upregulates transforming growth factor beta (TGF-β) expression, whose signaling pathway synergizes with the EGFR cascade, but its role in infectivity, inclusions, and EMT induction is unknown. We hypothesized that the EGFR and TGF-β signaling pathways cooperate during chlamydial infection for optimal inclusion development and stable EMT induction. The results revealed that Chlamydia upregulated TGF-β expression as early as 6 h postinfection of epithelial cells and stimulated both the EGFR and TGF-β signaling pathways. Inhibition of either the EGFR or TGF-βR1 signaling substantially reduced inclusion development; however, the combined inhibition of both EGFR and TGF-βR1 signaling reduced inclusions by over 90% and prevented EMT induction. Importantly, EGFR inhibition suppressed TGF-β expression, and an inhibitory thrombospondin-1 (Tsp1)-based peptide inhibited chlamydia-induced EMT, revealing a major source of active TGF-β during infection. Finally, TGF-βR signaling inhibition suppressed the expression of transforming acidic coiled-coil protein-3 (TACC3), which stabilizes EGFR signaling, suggesting reciprocal regulation between TGF-β and EGFR signaling during chlamydial infection. Thus, RTK-mediated host invasion by chlamydia upregulated TGF-β expression and signaling, which cooperated with other cellular signaling cascades and cytoskeletal remodeling to support optimal inclusion development and EMT induction. This finding may provide new targets for chlamydial disease biomarkers and prevention.

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.

A meta-analysis of affinity purification-mass spectrometry experimental systems used to identify eukaryotic and chlamydial proteins at the Chlamydia trachomatis inclusion membrane

The obligate intracellular bacterial pathogen, Chlamydia trachomatis, develops within a membrane-bound vacuole termed the inclusion. Affinity purification-mass spectrometry (AP-MS) experiments to study the interactions that occur at the chlamydial inclusion membrane have been performed and, more recently, combined with advances in C. trachomatis genetics. However, each of the four AP-MS published reports used either different experimental approaches or statistical tools to identify proteins that localize at the inclusion. We critically analyzed each experimental approach and performed a meta-analysis of the reported statistically significant proteins for each study, finding that only a few eukaryotic proteins were commonly identified between all four experimental approaches. The two similarly conducted in vivo labeling studies were compared using the same statistical analysis tool, Significance Analysis of INTeractome (SAINT), which revealed a disparity in the number of significant proteins identified by the original analysis. We further examined methods to identify potential background contaminant proteins that remain after statistical analysis. Overall, this meta-analysis highlights the importance of carefully controlling and analyzing the AP-MS data so that pertinent information can be obtained from these various AP-MS experimental approaches. This study provides important guidelines and considerations for using this methodology to study intracellular pathogens residing within a membrane-bound compartment. SIGNIFICANCE: Chlamydia trachomatis, an obligate intracellular pathogen, grows within a membrane-bound vacuole termed the inclusion. The inclusion is studded with bacterial membrane proteins that likely orchestrate numerous interactions with the host cell. Although maintenance of the intracellular niche is vital, an understanding of the host-pathogen interactions that occur at the inclusion membrane is limited by the difficulty in purifying membrane protein fractions from infected host cells. The experimental procedures necessary to solubilize hydrophobic proteins fail to maintain transient protein-protein interactions. Advances in C. trachomatis genetics has allowed us and others to use various experimental approaches in combination with affinity purification mass spectrometry (AP-MS) to study the interactions that occur at the chlamydial vacuolar, or inclusion, membrane. For the first time, two groups have published AP-MS studies using the same tool, the ascorbate peroxidase proximity labeling system (APEX2), which overcomes past experimental limitations because membrane protein interactions are labeled in vivo in the context of infection. The utility of this system is highlighted by its ability to study chlamydial type III secreted inclusion membrane protein (Inc) interactions. Incs act as the mediators of host-pathogen interactions at the inclusion during C. trachomatis infection. When carefully controlled and analyzed, the data obtained can yield copious amounts of useful information. Here, we critically analyzed four previously published studies, including statistical analysis of AP-MS datasets related to Chlamydia-host interactions, to contextualize the data and to identify the best practices in interpreting these types of complex outputs.

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.

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.

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.

Nitropropenyl benzodioxole, an anti-infective agent with action as a protein tyrosine phosphatase inhibitor

We report on the activities of a broad spectrum antimicrobial compound,nitropropenyl benzodioxole (NPBD) which are of relevance to its potential as an anti-infective drug. These investigations support the proposal that a major mechanism of NPBD is action as a tyrosine mimetic, competitively inhibiting bacterial and fungal protein tyrosine phosphatases (PTP). NPBD did not affect major anti-bacterial drug targets, namely, ATP production, cell wall or cell membrane integrity, or transcription and translation of RNA. NPBD inhibited bacterial YopH and human PTP1B and not human CD45 in enzyme assays. NPBD inhibited PTP-associated bacterial virulence factors, namely, endospore formation in Bacillus cereus, prodigiosin secretion in Serratia marcescens , motility in Proteus spp., and adherence and invasion of mammalian cells by Yersinia enterocolitica . NPBD acts intracellularly to inhibit the early development stages of the Chlamydia trachomatis infection cycle in mammalian cells known to involve sequestration of host cell PTPs. NPBD thus both kills pathogens and inhibits virulence factors relevant to early infection, making it a suitable candidate for development as an anti-infective agent, particularly for pathogens that enter through, or cause infections at, mucosal surfaces. Though much is yet to be understood about bacterial PTPs, they are proposed as suitable anti-infective targets and have been linked to agents similar to NPBD. The structural and functional diversity and heterogeneous distribution of PTPs across microbial species make them suitably selective targets for the development of both broadly active and pathogen-specific drugs.

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.

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.

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.

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.

Characterization and intracellular localization of putative Chlamydia pneumoniae effector proteins

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

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.

Protein kinase Cdelta and calmodulin regulate epidermal growth factor receptor recycling from early endosomes through Arp2/3 complex and cortactin

The intracellular trafficking of the epidermal growth factor receptor (EGFR) is regulated by a cross-talk between calmodulin (CaM) and protein kinase Cdelta (PKCdelta). On inhibition of CaM, PKCdelta promotes the formation of enlarged early endosomes and blocks EGFR recycling and degradation. Here, we show that PKCdelta impairs EGFR trafficking due to the formation of an F-actin coat surrounding early endosomes. The PKCdelta-induced polymerization of actin is orchestrated by the Arp2/3 complex and requires the interaction of cortactin with PKCdelta. Accordingly, inhibition of actin polymerization by using cytochalasin D or by overexpression of active cofilin, restored the normal morphology of the organelle and the recycling of EGFR. Similar results were obtained after down-regulation of cortactin and the sequestration of the Arp2/3 complex. Furthermore we demonstrate an interaction of cortactin with CaM and PKCdelta, the latter being dependent on CaM inhibition. In summary, this study provides the first evidence that CaM and PKCdelta organize actin dynamics in the early endosomal compartment, thereby regulating the intracellular trafficking of EGFR.

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.

Src, cortactin and Arp2/3 complex are required for E-cadherin-mediated internalization of Listeria into cells

Listeria monocytogenes is a food-borne pathogen able to invade non-phagocytic cells. InlA, a L. monocytogenes surface protein, interacts with human E-cadherin to promote bacterial entry. L. monocytogenes internalization is a dynamic process involving co-ordinated actin cytoskeleton rearrangements and host cell membrane remodelling at the site of bacterial attachment. Interaction between E-cadherin and catenins is required to promote Listeria entry, and for the establishment of adherens junctions in epithelial cells. Although several molecular factors promoting E-cadherin-mediated Listeria internalization have been identified, the proteins regulating the transient actin polymerization required at the bacterial entry site are unknown. Here we show that the Arp2/3 complex acts as an actin nucleator during the InlA/E-cadherin-dependent internalization. Using a variety of approaches including siRNA, expression of dominant negative derivatives and pharmacological inhibitors, we demonstrate the crucial role of cortactin in the activation of the Arp2/3 complex during InlA-mediated entry. We also show the requirement of the small GTPase Rac1 and that of Src-tyrosine kinase activity to promote Listeria internalization. Together, these data suggest a model in which Src tyrosine kinase and Rac1 promote recruitment of cortactin and activation of Arp2/3 at Listeria entry site, mimicking events that occur during adherens junction formation.

Mechanisms of Chlamydia trachomatis entry into nonphagocytic cells

The mechanisms of entry for the obligate intracellular bacterium C. trachomatis were examined by functional disruption of proteins essential for various modes of entry. RNA interference was used to disrupt proteins with established roles in clathrin-mediated endocytosis (clathrin heavy chain, dynamin-2, heat shock 70-kDa protein 8, Arp2, cortactin, and calmodulin), caveola-mediated endocytosis (caveolin-1, dynamin-2, Arp2, NSF, and annexin II), phagocytosis (RhoA, dynamin-2, Rac1, and Arp2), and macropinocytosis (Pak1, Rac1, and Arp2). Comparative quantitative PCR analysis was performed on small interfering RNA-transfected HeLa cells to accurately determine the extent of C. trachomatis entry after these treatments. Key structural and regulatory factors associated with clathrin-mediated endocytosis were found to be involved in Chlamydia entry, whereas those for caveola-mediated endocytosis, phagocytosis, and macropinocytosis were not. Thus, clathrin and its coordinate accessory factors were required for entry of C. trachomatis, although additional, uncharacterized mechanisms are also utilized.

A role for cortactin in Listeria monocytogenes invasion of NIH 3T3 cells, but not in its intracellular motility

Cortactin is an F-actin binding protein that binds to the Arp2/3 complex, stimulates its actin nucleation activity, and inhibits actin filament debranching. Using RNA interference directed against cortactin, we explored the importance of cortactin for several processes involving dynamic actin assembly. Silencing cortactin expression was efficiently achieved in HeLa and NIH 3T3 cells, with less than 5% of cortactin expression in siRNA-treated cells. Surprisingly, endocytosis in HeLa and NIH 3T3 cells, and cell migration rates, were not altered by RNAi-mediated cortactin silencing. Listeria utilizes actin-based motility to move within and spread among mammalian host cells; its actin-clouds and tails recruit cortactin. We explored the role of cortactin during the Listeria life cycle in cortactin "knockdown" NIH 3T3 cells. Interestingly, cortactin siRNA-treated cells showed a significant reduction in the efficiency of the bacteria invasion in NIH 3T3 cells. However, cortactin depletion did not interfere with assembly of Listeria actin clouds or actin tails, or Listeria intracellular motility or speed. Therefore, our findings suggest that cortactin plays a role in Listeria internalization, but not in the formation of actin clouds and tails, or in bacteria intracellular motility.

Identification and characterization of secreted effector proteins of Chlamydophila pneumoniae TW183

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

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-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.

Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens

Cortactin is an actin-binding protein and a central regulator of the actin cytoskeleton. Importantly, cortactin is also a common target exploited by microbes during infection. Its involvement in disease development is exemplified by a variety of pathogenic processes, such as pedestal formation [enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC)], invasion (Shigella, Neisseria, Rickettsia, Chlamydia, Staphylococcus and Cryptosporidium), actin-based motility (Listeria, Shigella and vaccinia virus) and cell scattering (Helicobacter). Recent progress turns our attention to how cortactin function can be regulated by serine and tyrosine phosphorylation. This has an important impact on how pathogens abuse cortactin to modulate the architecture of the host actin cytoskeleton.

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.

Cortactin signalling and dynamic actin networks

Cortactin was first identified over a decade ago, and its initial characterization as both an F-actin binding protein and v-Src substrate suggested that it was likely to be a key regulator of actin rearrangements in response to tyrosine kinase signalling. The recent discovery that cortactin binds and activates the actin related protein (Arp)2/3 complex, and thus regulates the formation of branched actin networks, together with the identification of multiple protein targets of the cortactin SH3 domain, have revealed diverse cellular roles for this protein. This article reviews current knowledge regarding the role of cortactin in signalling to the actin cytoskeleton in the context of these developments.

Lipid raft-mediated entry is not required for Chlamydia trachomatis infection of cultured epithelial cells

Using pharmacologic and biochemical criteria, we evaluated whether uptake of four different Chlamydia trachomatis serovars, D, E, K, and L2, was dependent upon lipid rafts. Our data suggest that lipid raft-mediated entry is not required for C. trachomatis infection of cultured epithelial cells.

Diagnosis and assessment of trachoma

Trachoma is caused by Chlamydia trachomatis. Clinical grading with the WHO simplified system can be highly repeatable provided graders are adequately trained and standardized. At the community level, rapid assessments are useful for confirming the absence of trachoma but do not determine the magnitude of the problem in communities where trachoma is present. New rapid assessment protocols incorporating techniques for obtaining representative population samples (without census preparation) may give better estimates of the prevalence of clinical trachoma. Clinical findings do not necessarily indicate the presence or absence of C. trachomatis infection, particularly as disease prevalence falls. The prevalence of ocular C. trachomatis infection (at the community level) is important because it is infection that is targeted when antibiotics are distributed in trachoma control campaigns. Methods to estimate infection prevalence are required. While culture is a sensitive test for the presence of viable organisms and nucleic acid amplification tests are sensitive and specific tools for the presence of chlamydial nucleic acids, the commercial assays presently available are all too expensive, too complex, or too unreliable for use in national programs. There is an urgent need for a rapid, reliable test for C. trachomatis to assist in measuring progress towards the elimination of trachoma.

Early signaling events involved in the entry of Rickettsia conorii into mammalian cells

Rickettsia conorii, the causative agent of Mediterranean spotted fever, is able to attach to and invade a variety of cell types both in vitro and in vivo. Although previous studies show that entry of R. conorii into non-phagocytic cells relies on actin polymerization, little else is known about the molecular details governing Rickettsia-host cell interactions and actin rearrangements. We determined that R. conorii recruits the Arp2/3 complex to the site of entry foci and that expression of an Arp 2/3 binding derivative of the WASP-family member, Scar, inhibited bacterial entry into Vero cells, establishing that Arp2/3 is an active component of this process. Using transient transfection with plasmids expressing dominant negative versions of small GTPases, we showed that Cdc42, but not Rac1 is involved in R. conorii invasion into Vero cells. Using pharmacological approaches, we show that this invasion is dependent on phosphoinositide (PI) 3-kinase and on protein tyrosine kinase (PTK) activities, in particular Src-family kinases. C-Src and its downstream target, p80/85 cortactin, colocalize at entry sites early in the infection process. R. conorii internalization correlated with the tyrosine phosphorylation of several other host proteins, including focal adhesion kinase (FAK), within minutes of R. conorii infection. Our results reveal that R. conorii entry into nonphagocytic cells is dependent on the Arp2/3 complex and that the interplay of pathways involving Cdc42, PI 3-kinase, c-Src, cortactin and tyrosine-phosphorylated proteins regulates Arp2/3 activation leading to the localized actin rearrangements observed during bacterial entry. This is the first report that documents the mechanism of entry of a rickettsial species into mammalian cells.

Inhibiting the Arp2/3 complex limits infection of both intracellular mature vaccinia virus and primate lentiviruses

Characterizing cellular factors involved in the life cycle of human immunodeficiency virus type 1 (HIV-1) is an initial step toward controlling replication of HIV-1. Actin polymerization mediated by the Arp2/3 complex has been found to play a critical role in some pathogens' intracellular motility. We have asked whether this complex also contributes to the viral life cycles including that of HIV-1. We have used both the acidic domains from actin-related protein (Arp) 2/3 complex-binding proteins such as the Wiscott-Aldrich syndrome protein (N-WASP) or cortactin, and siRNA directing toward Arp2 to inhibit viral infection. HIV-1, simian immunodeficiency virus (SIV), and intracellular mature vaccinia virus (IMV) were sensitive to inhibition of the Arp2/3 complex, whereas MLV, HSV-1, and adenovirus were not. Interestingly, pseudotyping HIV-1 with vesicular stomatitis virus G protein (VSV-G) overcame this inhibition. Constitutive inhibition of the Arp2/3 complex in the T-cell line H9 also blocked replication of HIV-1. These data suggested the existence of an Arp2/3 complex-dependent event during the early phase of the life cycles of both primate lentiviruses and IMV. Inhibiting the HIV-1's ability to activate Arp2/3 complex could be a potential chemotherapeutic intervention for acquired immunodeficiency syndrome (AIDS).

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.

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.

Filamentous actin binding ability of cortactin isoforms is responsible for their cell-cell junctional localization in epithelial cells

Cortactin is an F-actin binding protein that contributes to cytoskeleton remodelling. We identified five isoforms of mouse cortactin that differ in the number of tandem 37-amino acid repeats, named cortactin repeats. The transcription of minor isoforms with 4.5, 3.5 or 2.5 cortactin repeats was low in most adult tissues whereas an isoform with 4.5 cortactin repeats was highly transcribed in the adult brain. In accordance with the brain-specific upregulation of a minor isoform, a brain-specific novel 72-kDa cortactin protein was identified. Major isoforms with 6.5 or 5.5 cortactin repeats bound F-actin more robustly than minor isoforms in vitro. All isoforms were concentrated at cell-cell junction sites in epithelial cells. Deletion mutants lacking whole cortactin repeats did not bind F-actin and were not concentrated at cell-cell junction sites. Thus, the F-actin binding ability is mostly correlated with the number of cortactin repeats and is required for the cell-cell junctional localization.

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.

Chlamydia-infected cells continue to undergo mitosis and resist induction of apoptosis

Both anti- and proapoptotic activities have been reported to occur during chlamydial infection. To reconcile the apparent controversy, we compared host cell apoptotic responses to infection with 17 different chlamydial serovars and strains. None of the serovars caused any biologically significant apoptosis in the infected host cells. Host cells in chlamydia-infected cultures can continue to undergo DNA synthesis and mitosis. Chlamydia-infected cells are resistant to apoptosis induction, although the extent of the antiapoptotic ability varied between serovars. These observations have demonstrated that an anti- but not proapoptotic activity is the prevailing event in chlamydia-infected cultures.

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

The entire developmental cycle of the obligate intracellular bacteria Chlamydia pneumoniae takes place within the inclusion body. As many gram negative bacteria, Chlamydia possess a type III-secretion system (TTSS), which allows them to target effector molecules into the host cell. The expression and localization of several proteins constituting the TTSS apparatus and of proteins supposed to be secreted by the TTSS have been investigated. For the TTSS-constituting proteins, we selected representatives such as YscN (ATPase), LcrE (putative "lid" of the TTSS) and LcrH1 (postulated to be a chaperone). Furthermore, we focused on the putative effector proteins IncA, IncB, IncC, Cpn0809 and Cpn1020. Expression of these proteins was detected by reverse transcriptase-PCR followed by immunoblot analysis using antisera that were generated against the corresponding recombinant proteins. Thereby, expression could be detected on the RNA and/or protein level. Intracellular localization of proteins under investigation was determined by immunofluorescence assays using the respective antisera. YscN was shown to be distributed equally throughout the inclusion body, whereas LcrE gave a more prominent staining of the inclusion membrane. IncA was detected mainly on the membrane of the inclusion body, whereas IncB and IncC were shown to be located within the inclusion. Immunofluorescence assays with antisera raised against Cpn0809 and Cpn1020 showed completely different labeling. Signals corresponding to Cpn0809 and Cpn1020 were distributed within the host cell rather than inside the inclusions. Taken together, the different localization patterns of the effector proteins indicate differences in function and interplay with the host cell.

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.

Cryptosporidium parvum invasion of biliary epithelia requires host cell tyrosine phosphorylation of cortactin via c-Src

BACKGROUND & AIMS

Cryptosporidium parvum invasion of epithelia requires polymerization of host cell actin at the attachment site. We analyzed the role of host cell c-Src, a cytoskeleton-associated protein tyrosine kinase, in C. parvum invasion of biliary epithelia.

METHODS

In vitro models of biliary cryptosporidiosis using a human biliary epithelial cell line were used to assay the role of c-Src signaling pathway in C. parvum invasion.

RESULTS

c-Src and cortactin, an actin-binding protein and a substrate for c-Src, were recruited to the parasite-host cell interface during C. parvum invasion. Tyrosine phosphorylation of cortactin in infected cells was also detected. Inhibition of host cell c-Src significantly blocked C. parvum -induced accumulation and tyrosine phosphorylation of cortactin and actin polymerization at the attachment sites, thereby inhibiting C. parvum invasion of biliary epithelial cells. A triple mutation of tyrosine of cortactin in the epithelia also diminished C. parvum invasion. In addition, proteins originating from the parasite were detected within infected cells at the parasite-host cell interface. Antiserum against C. parvum membrane proteins blocked accumulation of c-Src and cortactin and significantly decreased C. parvum invasion. No accumulation of the endocytosis-related proteins, dynamin 2 and clathrin, was found at the parasite-host cell interface; also, inhibition of dynamin 2 did not block C. parvum invasion.

CONCLUSIONS

C. parvum invasion of biliary epithelial cells requires host cell tyrosine phosphorylation of cortactin by a c-Src-mediated signaling pathway to induce actin polymerization at the attachment site, a process associated with microbial secretion but independent of host cell endocytosis.

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.