Professional phagocytes undergo shifts in cell shape in response to a variety of cues, and this plasticity requires the ability to remodel the cytoskeleton in extreme ways. Historically, many new insights into cell biology have emerged from studying intracellular pathogens that have evolved to survive by modulating the subcellular organization and function of host pathways. The goal of this proposal is to better understand how mycobacteria manipulate the host cytoskeleton and the role of the WASH complex, an Arp2/3 nucleation promoting factor, in this process. We have discovered an ancient mycobacterial effector, EsxM, that promotes changes to the macrophage cytoskeleton through a putative interaction with WASHC4, a member of the WASH complex. These changes enhance the dissemination of mycobacterial disease via migrating macrophages. The striking shift in macrophage morphology and behavior induced by EsxM have led us to probe the extent of actin rearrangements during infection (Aim 1) and whether the WASH complex may have unknown roles in regulating cell shape and motility (Aim 2). Although the importance of cytoskeletal rearrangement in migrating cells is established, little is known about how mycobacterial effectors may manipulate this axis during infection. Additionally, the WASH complex has been studied primarily for its role in endocytic trafficking, but less is known about its potential role in macrophage migration, or as a target of intracellular bacteria. Thus, new investigations into the host cytoskeleton and the WASH complex are needed to understand their roles in cell migration and immunity. The overall hypothesis of this proposal is that mycobacterial effector EsxM induces changes in the actin cytoskeleton that lead to enhanced migratory capacity of macrophages via the WASH complex. In order to test this hypothesis, I will first take advantage of our established zebrafish model which offers genetic tractability and optical transparency. In Aim 1, I will utilize an established macrophage F-actin reporter zebrafish line to observe changes in the actin cytoskeleton induced by EsxM in the context of cell migration and innate immunity. I will combine high resolution imaging, pharmacological treatment, and genetic tools to functionally test modes of macrophage migration during these processes. In Aim 2 I will test how genetic disruption of the WASH complex changes macrophage morphology and migration in vivo using zebrafish knockouts. I will also take advantage of a mouse line in which the WASH complex is disrupted to study its role in mycobacterial infection in a mammalian model. Overall, these studies will both take advantage of the zebrafish system to provide insights into the dynamic actin cytoskeleton in real time, and interrogate a role for the WASH complex in macrophage migration. The completion of the proposed work will represent a significant advancement in our understanding of how the actin cytoskeleton is altered in the context of...