Research Projects

Lysosomes are the most catabolic organelles of eukaryotic cells. They play a central role in the degradation of pathogens that invade and replicate inside host cells. Lysosomes are a hub for various homeostatic and immune processes such as nutrient sensing, exocytosis, autophagy, innate immunity, and our understanding of these processes during infection is limited. The Q-fever causing bacterial pathogen, Coxiella burnetii, is a great model system to understand lysosomal biology. C. burnetii secretes effectors through a specialized type 4 secretion system (T4SS) that redefine lysosomal identity, subvert vesicle traffic and facilitate bacterial replication in the acidic lysosomal environment. In contrast, most pathogens evade their delivery to the lysosomes. C. burnetii effectors, in conjunction with host vesicle fusion-related proteins, SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) remodel conventional lysosomes into spacious, unified Coxiella-containing vacuole (CCV) that supports bacterial replication. However, mechanisms that underlie the maintenance of this vacuole are not well-understood. Being a lysosome-derived organelle, the substrates or catabolic products in the CCV may include toxic or antimicrobial elements that are detrimental for bacterial growth. Thus, it is possible that, during co-evolution with mammals, C. burnetii has co-opted host mechanisms to remove or exclude lysosomal content to maintain the permissiveness of the CCV. Lysosomal exocytosis, the process of lysosomal fusion with the plasma membrane (PM) to (a) secrete luminal content including enzymes and toxic granules and (b) repair and reseal plasma membrane is one plausible mechanism. Observations from my postdoctoral work and others’ strongly supports a role for lysosomal exocytosis in C. burnetii infection and bacterial and host regulation of this process. Data included in the proposal shows that (1) Inhibition of lysosomal exocytosis decreases intracellular C. burnetii replication. (2) Decrease in C. burnetii replication in HeLa cells expressing the SNARE STX11 correlates with reduced secretion of lysosomal proteins. STX11 is known to function in endo-lysosomal fusion, granule secretion and lysosomal exocytosis. Further, STX11 polymorphisms in humans leads to defective secretion of cytolytic granules, accompanied by macrophage dysregulation, but the precise mechanisms are not well-defined. (3) C. burnetii activates the transcriptional factor, TFEB through effector-dependent mechanisms and TFEB-deficiency adversely affects CCV biogenesis. Interestingly, TFEB is a key transcriptional activator of the lysosomal exocytosis pathway. I hypothesize that STX11 negatively regulates C. burnetii replication by suppressing lysosomal exocytosis. Owing to the importance of lysosomal exocytosis in promoting C. burnetii replication, I propose that C. burnetii upregulates lysosomal exocytosis through the activation of TFEB.