Characterizing New Molecules and Mechanisms of Semaphorin/Plexin/MICAL Signaling

The mechanisms that regulate the cellular behaviors including morphology, motility, navigation, and connectivity that are critical for normal human health are still incompletely understood. These types of behaviors are regulated through the ability of guidance cues that are present outside of cells to exert precise effects on the cell’s internal actin cytoskeleton. To help characterize an underlying logic for these cellular changes and proper cellular function I have been characterizing how one of the largest families of extracellular guidance cues, members of the Semaphorin family of proteins, induces actin cytoskeletal changes. Interestingly, Semaphorins have been found to inhibit the movement of cells and their membranous processes but the molecules linking them to these specific behaviors have remained poorly understood. Therefore, during my graduate work, I began further characterizing a new family of proteins, the MICALs that were identified as cytoplasmic binding partners of the Semaphorin receptor Plexin. Combining Drosophila genetics with in vitro biochemical assays, my work revealed that Mical regulates actin organization both in vivo and in vitro and is a novel actin disassembly factor. These results provide a new basis for understanding how extracellular guidance cues regulate the actin cytoskeleton. I then went on to further explore Mical-mediated actin filament (F-actin) disassembly. In one line of investigation, my work revealed that Mical plays an antagonistic role to F-actin stabilizing/bundling proteins including fascin and espin in regulating the F-actin cytoskeleton in vivo. This work also indicated that Semaphorin/Plexin/Mical activity not only directly disassembles the F-actin cytoskeleton but also triggers other actin regulatory proteins to reorganize a more complex F-actin network, resulting in increased cellular plasticity. In another line of investigation, I found that the Abl non-receptor tyrosine kinase is a new Mical-interacting protein. My functional assays revealed that Abl and Semaphorin/Plexin/Mical work together to regulate F-actin arrangements and these interactions are conserved in many different contexts including bristle cell morphology, axon guidance, and cancer cell survival and invasion. Thus, I have found that MICAL family proteins are novel controllers of the actin cytoskeleton, functioning directly on F-actin and with other actin regulatory proteins to modulate diverse cellular behaviors.