Elucidating the roles for discrete actin filaments in maintenance of organelle and cellular homeostasis - Project Summary: The cytoskeleton comprised of the actin filaments, microtubules and the intermediate filaments plays important roles in organelle dynamics including positioning, transport, and crosstalk. While the microtubules have been significantly defined for their roles in regulating the dynamics of several organelles including Endoplasmic Reticulum (ER) and mitochondria, the role of actin filaments in dynamics and interactions of the ER and mitochondria is largely under-studied. The dominant actin structures visible in a cultured mammalian cell include the cortical actin, the stress fibers, and the filaments at the leading edge (lamellipodia) whose prime function is to generate force. However, underneath these overwhelming structures are a myriad of distinct and dynamic pools of actin filaments which are specifically associated with different organelles and whose role in organelle and cellular physiology have not been explored. My research has explored two such actin structures. One of them assembles around the ER, polymerized by an ER-bound actin assembly factor, INF2. The other pool polymerizes strictly around dysfunctional mitochondria mediated by the Arp2/3 complex and formin FMNL. My research showed that both these actin structures are polymerized by distinct set of actin assembly factors and affected ER and mitochondrial dynamics and interactions in contrasting ways. However, the underlying mechanisms and the downstream consequences of these distinct pools of actin filaments on the organelles and overall cellular physiology remain to be defined. My proposed work will unify concepts from both the fields of actin dynamics and organelle biology to understand, both mechanistically and functionally, the specific roles these actin filaments play in controlling ER and mitochondrial biology, dynamics, and crosstalk. My proposed research plan will tackle the following key questions: 1) How do dynamic actin pools act as key regulators of ER-mitochondrial crosstalk; 2) How does the ER associated actin filaments regulate ER function, and 3) What are the distinct cellular effects of these dynamic actin pools on cellular homeostasis. The overall goal of my research program is to identify, characterize and understand the roles that these dynamic pools of actin play in regulating various cellular processes specifically related to organelle dynamics and crosstalk, critically differentiating, and uncoupling them from being the functional consequences of the cortical actin cytoskeleton. In the future we aim to apply these concepts in understanding alteration in organelle dynamics in various pathological system with a hope for specific therapeutic interventions through perturbation of these specific actin filaments along with its associated assembly factors and other actin binding proteins.
