Chlamydia trachomatis Causes Centrosomal Defects Resulting in Chromosomal Segregation Abnormalities

Micronucleus formation detected by live-cell imaging

Although micronuclei (MNi) have been extensively used to evaluate genotoxic effects and chromosome instability, the most basic issue regarding their formation was not completely addressed until recently, due to limitations of traditional experimental methods. The development of live-cell imaging, combined with genetically engineered chromosome labelling techniques makes it possible to investigate the origin of a micronucleus in a single cell in a real-time and high-throughput manner. Here, we review all the available studies on the origins of MNi in live cells and discuss novel findings based on this recently emerged methodology. Some unsolved questions on MNi formation and limitations of live-cell imaging in the investigation of MNi have also been discussed.

Recent advances in Chlamydia subversion of host cytoskeletal and membrane trafficking pathways

Chlamydia species are obligate intracellular bacteria that cause sexually transmitted disease, ocular infections and atypical pneumonia. This review highlights recent advances describing the mechanisms by which Chlamydia subvert host cytoskeleton and membrane trafficking pathways to create a replication competent niche.

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.

Killing me softly: chlamydial use of proteolysis for evading host defenses

Chlamydial infections in humans cause severe health problems, including blinding trachoma and sexually transmitted diseases. Although the involved pathogenic mechanisms remain unclear, the ability to replicate and maintain long-term residence in the infected cells seems to significantly contribute to chlamydial pathogenicity. These obligate intracellular parasites maintain a delicate balance between exploiting and protecting their host: they occupy intracellular space and acquire nutrients from the infected cells, but at the same time they have to maintain the integrity of the host cells for the completion of their intracellular growth. For this purpose, chlamydiae hijack certain signaling pathways that prevent the host cells from undergoing apoptosis induced by intracellular stress and protect the infected cells from recognition and attack by host defenses. Interestingly, one of the strategies that chlamydiae use for these purposes is the induction of limited proteolysis of host proteins, which is the main focus of this article.

Centrosome abnormalities during a Chlamydia trachomatis infection are caused by dysregulation of the normal duplication pathway

The presence of supernumerary centrosomes in cells infected with Chlamydia trachomatis may provide a mechanism to explain the association of C. trachomatis genital infection with cervical cancer. We show that the amplified centrosomal foci induced during a chlamydial infection contain both centriolar and pericentriolar matrix markers, demonstrating that they are bona fide centrosomes. As there were multiple immature centrioles but approximately one mature centriole per cell, aborted cytokinesis alone cannot account for centrosome amplification during a chlamydial infection. Production of supernumerary centrosomes required the kinase activities of Cdk2 and Plk4, which are known regulators of centrosome duplication, and progression through S-phase, which is the stage in the cell cycle when duplication of the centrosome occurs. These requirements indicate that centrosome amplification during a chlamydial infection depends on the host centrosome duplication pathway, which normally produces a single procentriole from each template centriole. However, C. trachomatis induces a loss of numerical control so that multiple procentrioles are formed per template.

Cytokinesis is blocked in mammalian cells transfected with Chlamydia trachomatis gene CT223

Background

The chlamydiae alter many aspects of host cell biology, including the division process, but the molecular biology of these alterations remains poorly characterized. Chlamydial inclusion membrane proteins (Incs) are likely candidates for direct interactions with host cell cytosolic proteins, as they are secreted to the inclusion membrane and exposed to the cytosol. The inc gene CT223 is one of a sequential set of orfs that encode or are predicted to encode Inc proteins. CT223p is localized to the inclusion membrane in all tested C. trachomatis serovars.

Results

A plasmid transfection approach was used to examine the function of the product of CT223 and other Inc proteins within uninfected mammalian cells. Fluorescence microscopy was used to demonstrate that CT223, and, to a lesser extent, adjacent inc genes, are capable of blocking host cell cytokinesis and facilitating centromere supranumeracy defects seen by others in chlamydiae-infected cells. Both phenotypes were associated with transfection of plasmids encoding the carboxy-terminal tail of CT223p, a region of the protein that is likely exposed to the cytosol in infected cells.

Conclusion

These studies suggest that certain Inc proteins block cytokinesis in C. trachomatis-infected cells. These results are consistent with the work of others showing chlamydial inhibition of host cell cytokinesis.

RNAi screen in Drosophila cells reveals the involvement of the Tom complex in Chlamydia infection

Chlamydia spp. are intracellular obligate bacterial pathogens that infect a wide range of host cells. Here, we show that C. caviae enters, replicates, and performs a complete developmental cycle in Drosophila SL2 cells. Using this model system, we have performed a genome-wide RNA interference screen and identified 54 factors that, when depleted, inhibit C. caviae infection. By testing the effect of each candidate's knock down on L. monocytogenes infection, we have identified 31 candidates presumably specific of C. caviae infection. We found factors expected to have an effect on Chlamydia infection, such as heparansulfate glycosaminoglycans and actin and microtubule remodeling factors. We also identified factors that were not previously described as involved in Chlamydia infection. For instance, we identified members of the Tim-Tom complex, a multiprotein complex involved in the recognition and import of nuclear-encoded proteins to the mitochondria, as required for C. caviae infection of Drosophila cells. Finally, we confirmed that depletion of either Tom40 or Tom22 also reduced C. caviae infection in mammalian cells. However, C. trachomatis infection was not affected, suggesting that the mechanism involved is C. caviae specific.

Chlamydia spp. are intracellular bacterial pathogens that infect a wide range of hosts and cause various diseases, including preventable blindness in developing countries, sexually transmitted disease, and pneumonia. Chlamydia spp. are able to establish their replication niche inside the host cell, residing in a membrane-bound compartment that serves as a protector shield against immune surveillance and antimicrobial agents but also acts as a “filter” to exchange factors with the host cell. Despite the primary importance of Chlamydia for human health, little is known about the mechanisms underlying the infection process. The study of Chlamydia pathogenesis is challenging because Chlamydia spp. are not amenable to genetic manipulation and it is difficult to conduct extensive genetic approaches in the mammalian host. To circumvent these difficulties, we have used Drosophila cells to model Chlamydia infection. We conducted a genome-wide RNA interference screen and identified host factors that, when depleted, reduce Chlamydia infection. Validating our approach, we further showed that the identified factors were also required for infection in mammalian cells. This work will help us better understand the complex interaction between Chlamydia and its host and potentially identify novel targets for therapeutic treatment.

Human GCIP interacts with CT847, a novel Chlamydia trachomatis type III secretion substrate, and is degraded in a tissue-culture infection model

The obligate intracellular bacterium Chlamydia trachomatis occupies a parasitophorous vacuole and employs a type III secretion mechanism to translocate host-interactive proteins. These proteins most likely contribute to pathogenesis through modulation of host cell mechanisms crucial for the establishment and maintenance of a permissive intracellular environment. Using a surrogate Yersinia type III secretion system (T3SS), we have identified the conserved gene product CT847 as a chlamydial T3SS substrate. Yeast two-hybrid studies using CT847 as bait to screen a HeLa cell cDNA library identified an interaction with mammalian Grap2 cyclin D-interacting protein (GCIP). Immunoblot analyses of C. trachomatis-infected HeLa cells showed that GCIP levels begin to decrease (as compared with mock-infected HeLa cells) between 8 h and 12 h post infection. GCIP was virtually undetectable in 24 h time point material. This decrease was inhibited by proteasome inhibitors lactacystin and MG-132, and the T3SS inhibitor Compound 1. CT847 was detectible in purified reticulate body but not elementary body lysates, and reverse transcription polymerase chain reaction (RT-PCR) expression analyses indicate a mid-cycle expression pattern. Both of these findings are consistent with CT847 contributing to the observed effect on GCIP. Given the established roles of GCIP, we believe that we have discovered a novel C. trachomatis antihost protein whose activity is relevant to chlamydial pathogenesis.

Loss of centrosome integrity induces p38-p53-p21-dependent G1-S arrest

Centrosomes organize the microtubule cytoskeleton for both interphase and mitotic functions. They are implicated in cell-cycle progression but the mechanism is unknown. Here, we show that depletion of 14 out of 15 centrosome proteins arrests human diploid cells in G1 with reduced Cdk2-cyclin A activity and that expression of a centrosome-disrupting dominant-negative construct gives similar results. Cell-cycle arrest is always accompanied by defects in centrosome structure and function (for example, duplication and primary cilia assembly). The arrest occurs from within G1, excluding contributions from mitosis and cytokinesis. The arrest requires p38, p53 and p21, and is preceded by p38-dependent activation and centrosomal recruitment of p53. p53-deficient cells fail to arrest, leading to centrosome and spindle dysfunction and aneuploidy. We propose that loss of centrosome integrity activates a checkpoint that inhibits G1-S progression. This model satisfies the definition of a checkpoint in having three elements: a perturbation that is sensed, a transducer (p53) and a receiver (p21).

New genotoxin shows diversity of bacterial attack mechanisms

Bacteria make a wide range of toxic products that interact with eukaryotic cellular machinery in a precise way. These toxins interfere with key eukaryotic processes, such as cellular signalling components, and some directly attack the genome. Nougayrède and colleagues have recently identified a novel hybrid peptide-polyketide compound from Escherichia coli that leads to DNA damage. This novel compound is produced by pathogenic and, most interestingly, commensal isolates. Although it is not yet clear how the peptide-polyketide compound functions at the molecular level, it is possible that it contributes to bacterial pathogenesis and bacterially induced carcinogenesis.