Characterization of host and bacterial factors critical for Shigella flexneri pathogenesis

Abstract

Shigella species are Gram-negative bacterial pathogens acquired by fecal-oral spread. A common cause of bacillary dysentery worldwide, particularly in developing countries, Shigella invade colonic mucosal cells and employ a repertoire of proteins secreted by a type III secretion system (T3SS) to manipulate host cytoskeleton and signaling pathways. The bacterium escapes from the vacuole, enters the host cell cytoplasm, evades host recognition, and spreads intercellularly. In this thesis, I describe two independent projects that aim to clarify the complex interplay between bacterial effectors and host processes critical to Shigella flexneri pathogenesis. Primarily, I examined the role of host small GTPase ADP-ribosylation factor 6 (ARF6) in S. flexneri entry. ARF6 functions in membrane trafficking at the plasma membrane and activates membrane ruffle formation. I show here that ARF6 is required for efficient entry, and together with the host guanine nucleotide exchange factor (GEF) ARF nucleotide binding site opener (ARNO) and the T3SS effector IpgD, constitutes a positive feedback loop that amplifies ARF6 activation at S. flexneri entry sites to promote efficient bacterial uptake. I also investigated the outer Shigella protein C (OspC) effectors OspC1, OspC2, and OspC3, highly homologous T3SS effectors with potentially disparate functions. Preliminary evidence suggested that OspC2 and/or OspC3 interact with host protein Toca-1, which is implicated in actin polymerization and pathogen recognition by host innate immune processes. My evidence suggests the OspC proteins are important for evasion of autophagy and/or other mechanisms of intracellular survival and replication.

This work identifies new cellular targets and activities of the Shigella effector IpgD and provides insight into mechanisms of pathogenesis dependent on secreted bacterial effectors. The lines of investigation presented here demonstrate the intricate ways in which S. flexneri both exploits and subverts host processes in order to survive in the intracellular environment.