Above image is a super resolution 3D image of a fluorescently labeled chlamydial inclusion containing Chlamydia trachomatis transformed with a plasmid encoding IncA-APEX2, infected onto a HeLa cell monolayer. The red fluorescence is labeling IncA-APEX2, pink indicating endogenous IncA, green is labeling biotinylated proteins, and blue are chlamydial organisms within the inclusion
Chlamydia trachomatis is a highly successful pathogen of significant medical importance. The CDC estimates that 10% of women between the ages of 15 to 19 test positive for C. trachomatis and 50-70% of these infections are asymptomatic. This increases the risk of widespread transmission and untreated infections, which can lead to pelvic inflammatory disease or infertility in a significant portion of women of childbearing age. In developing countries with limited hygiene resources, C. trachomatis is also associated with the leading cause of preventable blindness. In these scenarios, Chlamydia is not sexually transmitted, rather it is transmitted between infected individuals by direct contact with infected ocular secretions or from flies coming in contact with infected then uninfected individuals. Other Chlamydia species associated primarily with human respiratory tract infections include C. pneumoniae, a major cause of community acquired pneumonia, and C. psittaci, often acquired as a zoonotic infection from birds and manifesting as a severe pneumonia. Because of the importance of chlamydial diseases, there is a great need to identify strategies to reduce/prevent transmission and limit infections to the primary site of inoculation.
Chlamydia is a Gram-negative, obligate intracellular bacterium, which means it must grow in a host cell unlike many other bacteria that can grow on an agar-based substrate. Interestingly, Chlamydia is a developmentally regulated bacterium that alternates between functional and morphological forms called the EB (elementary body) and RB (reticulate body). The EB is the infectious form that mediates attachment and entry into a susceptible host cell. The infecting EB remains within a vacuole that is rapidly diverted from the phagolysosomal pathway and begins differentiating into the RB. The RB is the non-infectious form that grows and divides within a pathogen-specified organelle termed the chlamydial inclusion. After multiple rounds of division, the RBs differentiate to EBs, which are released from the host cell to initiate a new round of infection. In evolving to this obligate intracellular niche, Chlamydia has significantly reduced its genome (~1Mbp with ~1000 genes vs E. coli with ~4.5Mbp and ~4500 genes) yet manages to perform a variety of functions that allows it to usurp the host cell in such a way as to preserve its viability.
Drs. Ouellette and Rucks are both well trained within the field of Chlamydia biology. Dr. Ouellette has been investigating how Chlamydia functions with its reduced genome by specifically examining mechanisms governing chlamydial cell division and how Chlamydia responds to stress without the requisite regulatory elements or genes common to other bacteria. Dr. Rucks has been investigating the role of eukaryotic SNAREs at the chlamydial inclusion by examining inclusion-host trafficking dynamics, chlamydial acquisition of host-derived lipids, and chlamydial growth and development in the absence of these proteins. Additionally, Drs. Ouellette and Rucks have funding to study their individual projects and are currently looking for interested trainees to join their group.
Team Chlamydia’s Collaborative Project: Inclusion Membrane Dynamics
Dr. Ouellette’s expertise in cellular microbiology is from the perspective of microorganisms, while Dr. Rucks’s expertise in cellular microbiology is from the perspective of the host cell. A natural collaboration is to combine both of their expertise to better characterize chlamydial inclusion membrane structure and function.
The integrity of the inclusion membrane is inextricably linked with chlamydial fitness, but the fundamental cellular processes (both eukaryotic and bacterial) that support inclusion membrane biogenesis are not understood. The inclusion membrane is studded with proteins termed ‘Inc’ proteins. Incs are defined as proteins containing at least two large hydrophobic transmembrane domains flanked by termini that are exposed on the host cytosolic face of the chlamydial inclusion. We hypothesize that Inc proteins have two functional domains, the transmembrane and cytosolic domains, which serve to organize the inclusion membrane and recruit necessary host cell proteins, respectively. The inclusion membrane is the platform that supports chlamydial-host interactions, as such this project is the perfect intersection of the combined expertise of Team Chlamydia, examining this problem simultaneously from the host (Rucks) and pathogen (Ouellette). We are using novel approaches to capture protein-protein interactions, as well as advanced fluorescent microscopy techniques. Further, this project exploits newly developed genetic tools (such as Dr. Ouellette’s CRISPRi system) to help test our overall hypothesis of how specific Incs contribute to the structure and function of the inclusion membrane. This project has the potential to make significant contributions to the field of Chlamydia biology in that not much is known about the inclusion membrane