Collaborative Research: Cytokinetic Furrow Specification in Sea Urchin Embryos

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Intellectual merit. Animal cells divide themselves by assembling a contractile ring of actin and myosin around the cell equator, which constricts the cell surface between duplicated chromosome sets at the end of mitosis. Classical experiments suggest that cells "know" where to build the contractile ring because microtubules of the mitotic apparatus, the cellular machine that sorts chromosomes, convey spatial cues to the cell surface. The nature of the relevant spatial cues, and the mechanism that conveys them to the cell surface, have been elusive and controversial. Recent research shows that the signaling protein Rho is a key physiological link between the mitotic apparatus and recruitment of contractile proteins to the cell equator during division. Molecular genetic studies implicate microtubule-associated Rho regulators in the control of cell division, supporting a widely-favored hypothesis: that molecular motors traveling along microtubules bring information about events deep in the cytoplasm to the cell surface, thereby somehow creating a spatial pattern of Rho activity that favors actin and myosin recruitment at the right place and time to form the contractile ring. This project will test the causal relationship between distinct subsets of cellular microtubules and Rho activity, elucidate how microtubule geometry and behavior collaborate with Rho regulators to identify the division plane on the cell surface, and will seek to explain how cells rapidly respond to perturbations during division. The working hypothesis is that one population of microtubules -- the asters, which radiate toward the cell surface from each pole of the mitotic apparatus -- confine a diffusible signal that is released, after chromosome segregation, from another population of microtubules -- the midzone of the mitotic apparatus -- deep in the cell. The project will test the hypothesis that either population suffices to localize Rho activation, but that synergy between them makes cell division accurate and precise. Accuracy and precision in cell division are fundamentally important to the lives of cells and the organisms they compose. Even small errors in genome partitioning can be irrevocably disastrous. This project relies on sea urchin embryos as a model system, using fluorescent imaging of live cells at high spatial and temporal resolution to measure intracellular dynamics in normal and experimentally-perturbed cells. In so doing, the studies will resolve long-standing debates about the mechanism of cell division. Many classical results on cell division derive from studies of sea urchin eggs and similar embryonic cells, but several significant conclusions from classical work seem to disagree with recent work on the molecular genetics of cell division. By re-examining cell division in the sea urchin embryo using molecular probes, this research will show whether large embryonic cells follow different rules than small somatic cells, or whether animal cells of all types adapt a common mechanism to the diverse demands of their biology. Broader impacts. This project will train graduate and undergraduate students in state-of-the-art live-cell imaging and high-resolution confocal microscopy. At the same time the research will advance technique development in this area by developing widely-applicable fluorescent protein probes for visualizing subcellular organization and dynamics. The project also includes a major educational component. The research group, which includes an active public high school science teacher, will prepare high-resolution time-lapse films of normal cell behavior in embryos expressing fluorescent probes for key cellular constituents such as microtubules, actin filaments, and chromosomes. Such films will be annotated and disseminated along with didactic materials appropriate for classroom use in high-school and undergraduate cell biology curriculum. In addition, the principal investigators are actively involved in bringing scientific research into the public sphere through websites, public lectures and school outreach, and museum exhibitions.