Dynamic Regulation of Vacuole Homeostasis during Infection by Intracellular Dynamic Regulation of Vacuole Homeostasis during Infection by Intracellular Bacterial Pathogens.

Abstract

Pathogen-containing vacuoles (PCV) are a rich interface for interaction of intracellular pathogens and their mammalian host cells. This thesis investigates mechanisms by which host and pathogen regulate the vacuolar environment to manipulate the outcome of infection. A novel role for the mammalian IKK-family kinase, TANK-binding-kinase-1 (TBK1), in maintaining integrity of PCV during an intracellular bacterial infection was identified. In the absence of TBK1, invading pathogens were released from the phagosome and entered into the host cytosol. Transcriptional analysis revealed elevated levels of Aquaporin-1 (AQP1) in cells deficient in TBK1. AQP1, a water channel that regulates swelling of secretory vesicles, was associated with PCV. Overexpression of AQP1 led to PCV destabilization and bacterial release into the cytosol in a manner dependent on bacterially-induced membrane damage and ion flux. Inhibition of AQP1 physiological function in multiple cell types also led to increased instability of PCV. These results highlight aquaporins as key mediators of vacuole integrity and homeostasis to control bacterial infection. From these data, it led to the hypothesis that while homeostatic regulation of vacuolar membrane channels may be utilized by host cells to control bacterial infection, they might also be exploited by cytosolic bacterial pathogens such as Listeria monocytogenes (L. monocytogenes), to escape the vacuole. Listeriolysin O (LLO) is a critical L. monocytogenes virulence factor that forms pores in the phagosomal membrane, altering the vacuolar environment. LLO is required for L. monocytogenes cytosolic entry, but host mechanisms that contribute to vacuole rupture are poorly defined. Upon inhibition of chloride flux, L. monocytogenes was unable to escape the phagosome. Functional inhibition of cystic fibrosis transmembrane conductance regulator (CFTR) also suppressed L. monocytogenes escape into the cytosol. These results suggest the potential that in response to L. monocytogenes infection, CFTR transports chloride into the vacuole, enhancing LLO stability and activity, resulting rupture of the vacuole. In vitro data support this hypothesis, as LLO hemolytic activity increased upon exposure to high levels of sodium chloride. These results demonstrate an unanticipated role for CFTR and chloride transport in L. monocytogenes pathogenesis. Overall, these findings emphasize the importance of host homeostatic mechanisms in host-pathogen interactions.