Research in my laboratory is focused on the fundamental question of how cells generate cellular asymmetry to carry out their specific function. Elements of the cytoskeleton, known as microtubules, have been found to play a central role in this process. We utilize motile fibroblasts as a model system to study how microtubules contribute to cell polarity. In these cells there are two sources of microtubule polarity: the selective stabilization of microtubules oriented towards the leading edge, and the reorientation of microtubule organizing center (MTOC) towards the leading edge. We are currently investigating how the polarization of the microtubule array is signaled in the cytoplasm, and how the polarization of the microtubule array contributes to other cell polarity processes. We employ biochemical, molecular and cell biological approaches to address these questions, including real time microscopic observation of the behavior of fluorescent molecules introduced into cells by microinjection or by transfection. The stabilization of microtubules in specific locations in cells is central to the idea that microtubules drive cellular polarization. We have developed a serum starved fibroblast system to identify extracellular factors and intracellular signal transduction pathways involved in triggering microtubule stabilization in crawling fibroblasts. We have used this system to show that the small GTP-binding protein, Rho, a member of the ras superfamily, is critically involved in the selective stabilization of microtubules in the lamella of crawling cells. Recently we identified the mDia, a member of the formin family of proteins, as the downstream target of Rho that mediates MT stabilization. In budding yeast formins have been genetically implicated in the capture and shrinkage of the plus end of microtubules in the cortex of the bud. We are currently testing whether other proteins from the "capture shrinkage pathway" (i.e. EB1 and APC) are involved in microtubule stabilization in fibroblasts. The subunit protein of microtubules, tubulin, undergoes a unique post translation modification, known as detyrosination, when microtubules are stabilized. We have recently found that detyrosination acts as a signal for the interaction of stable microtubules with other organelles in the cell. Thus, detyrosinated microtubules act as preferential sites for the establishment of an extended array of vimentin intermediate filaments in fibroblasts. In addition, detyrosinated microtubules are preferentially used for export from the endocytic recycling compartment while tyrosinated microtubules are are preferentially used for import from the plasma membrane to this compartment. Other cellular organelles, e.g., mitochondria and endoplasmic reticulum, may also be dependent on detyrosinated microtubules. With these results, we are now able to suggest a general mechanism for how cells establish internal organization: 1) dynamic microtubules are locally stabilized 2) the stable microtubules are post-translationally modified, and 3) the modified microtubules interact with other cellular organelles. Recently we used the serum starved fibroblast system to show that MTOC reorientation is regulated by a specific signal transduction pathway. Similar to the microtubule stabilization pathway, MTOC reorientation is triggered by a small G-protein, CDC42. By activating MTOC orientation with serum factors or active CDC42 we have been able to determine that dynein and the dynactin complex act downstream of CDC42 to provide the force required to reposition the MTOC in response to extracellular cues. Importantly we have shown that MTOC reorientation and selective stabilization of microtubules towards the cell edge are each controlled by distinct and independent signaling pathways despite the fact that these two polarization events act to rearrange a single microtubule array.
