Mechanisms of Rickettsia parkeri invasion of host cells and early actin-based motility

Rickettsiae are obligate intracellular pathogens that are transmitted to humans by arthropod vectors and cause diseases such as spotted fever and typhus. Spotted fever group (SFG) Rickettsia hijack the host actin cytoskeleton to invade, move within, and spread between eukaryotic host cells during their obligate intracellular life cycle. Rickettsia express two bacterial proteins that can activate actin polymerization: RickA activates the host actin-nucleating Arp2/3 complex while Sca2 directly nucleates actin filaments. In this thesis, I aimed to resolve which host proteins were required for invasion and intracellular motility, and to determine how the bacterial proteins RickA and Sca2 contribute to these processes. Although rickettsiae require the host cell actin cytoskeleton for invasion, the cytoskeletal proteins that mediate this process have not been completely described. To identify the host factors important during cell invasion by Rickettsia parkeri, a member of the SFG, I performed an RNAi screen targeting 105 proteins in Drosophila melanogaster S2R+ cells. The screen identified 34 proteins important for invasion, including a signal transduction pathway involving Abl tyrosine kinase, the RhoGEF Vav, the GTPases Rac1, Rac2, and Cdc42, the WAVE nucleation promoting factor (NPF) complex and the Arp2/3 complex. In mammalian cells, including HMEC-1 endothelial cells, the natural targets of R. parkeri, the Arp2/3 complex was also crucial for invasion, while requirements for WAVE2 as well as Rho GTPases depended on the particular cell type. I propose that R. parkeri invades S2R+ arthropod cells through a primary pathway leading to actin nucleation, whereas invasion of mammalian endothelial cells occurs via redundant pathways that may involve the activity of host and bacterial proteins. Our results reveal a key role for the WAVE and Arp2/3 complexes, as well as a higher degree of variation than previously appreciated in actin nucleation pathways activated during Rickettsia invasion. Most pathogens undergo actin-based motility during a single phase of their life cycle, using the force of actin tails to push into neighboring cells without accessing the extracellular milieu. In contrast, I found that R. parkeri undergo two phases of motility, early and late, during the infectious cycle. Early actin tails are formed between 15 and 60 minutes after infection, have a distinctive short and curved appearance, are decorated with Arp2/3 complex proteins Arp3 and ARPC5, and are associated with polar localization of the Rickettsia Arp2/3 NPF, RickA. Late actin tails, as previously described (Haglund et al. 2010, Serio et al. 2010) are long, composed of helical bundles of actin, and associated with polar localization of the Rickettsia formin-like actin nucleator, Sca2. Early motility is Arp2/3-dependent and is significantly slower, and less efficient when compared to late motility. Finally, isolation of R. parkeri strains with transposon insertions in either the rickA or sca2 genes revealed that RickA is required for robust early actin tail formation, while Sca2 is required for late actin tail formation and efficient cell-to-cell spread. Thus, Rickettsia appear to be unique in their ability to promote two temporally and mechanistically distinct phases of actin-based motility during infection. Continued investigation of invasion and actin-based motility may shed light on the pathogenesis of Rickettsia, the function of actin in the host cell, and the purpose of actin tail formation during intracellular infection.