Understanding the Control of Processing Bodies by RhoA GTPase

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Viruses are useful tools that often reveal previously unrecognized levels of control within a cell. By exploiting Kaposi’s sarcoma-associated herpesvirus (KSHV), I identified that the cytoskeletal GTPase, RhoA is a key regulator of messenger RNA (mRNA) stability and cytoplasmic processing bodies (PBs). PBs are small ribonucleoprotein (RNP)-containing cytoplasmic granules and major sites of mRNA decay that significantly contribute to the precise post-transcriptional regulation of cellular gene expression. PBs are dynamic, responding to changes in mRNA accumulation or protein translation status by altering their size and number. PBs also respond to certain cellular stressors; however, many questions remain regarding the upstream signals controlling PB assembly and disassembly. The rho family of small GTPases are molecular switches that cycle between inactive GDP- and active GTP-bound forms and thereby control several fundamental cellular processes. RhoA regulates actin cytoskeleton dynamics to facilitate normal cell attachment, the formation of actin stress fibers, cell migration and angiogenesis. RhoA activation also positively impacts the transcription of genes containing serum-response elements (SREs), coupling changes to the actin cytoskeleton with increased transcription. RhoA-mediated control of other aspects of gene expression remains unclear. By exploiting KSHV, I showed that the viral protein Kaposin B (KapB) activates RhoA. RhoA activation is necessary for KapB to negatively regulate PB formation and decrease the turnover of labile cellular mRNA. Interestingly, KapB-mediated changes to the actin cytoskeleton can be uncoupled from its effect on PB assembly using an inhibitor of the RhoA downstream effector protein, Rho-associated kinase (ROCK). When ROCK is inhibited, KapB fails to promote actin polymerization yet it is still able to disrupt PBs. I hypothesize that RhoA mediates the dispersal of cellular PBs using a novel mechanism that is completely independent from its control of the actin cytoskeleton and microtubule network. Specific objectives are proposed to test this hypothesis: (1) to use time-lapse live cell microscopy to precisely track the changes that occur to PBs, the actin cytoskeleton, and microtubules immediately after RhoA activation (2) to determine if active RhoA modifies, degrades, or recruits PB component proteins (3) to use genetic approaches to identify the effector proteins downstream of active RhoA that are required for RhoA-mediated PB dispersal. My NSERC-funded research program will elucidate the upstream signals that link active RhoA to the control of PB formation and identify novel downstream effectors used by RhoA to mediate PB dispersion, providing insight into the poorly understood process of RhoA-controlled post-transcriptional gene expression.