Mechanisms of mitosis and size control in Xenopus

University Of California Berkeley, Berkeley CA

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Abstract

PROJECT SUMMARY Mechanisms of Mitosis and Size Control in Xenopus Research in my laboratory is focused on two major areas: Cell division is arguably the most dramatic event in the life of a cell. Chromosomes condense, organelles vesiculate, and the microtubule cytoskeleton rearranges into a bipolar spindle that attaches to chromosomes at their kinetochores and segregates a complete genome to each daughter cell. Although the morphological changes that occur during mitosis were first observed over a century ago, we still do not understand how these dynamic events are orchestrated. Many factors have been identified that contribute to spindle assembly and function, but the molecular and biophysical mechanisms and interactions that ensure mitotic fidelity remain unclear. Our current projects address outstanding questions including 1) What are the molecular underpinnings and functional consequences of different spindle architectures? Spindle size and organization vary dramatically across cell types and organisms, and factors known to affect these parameters are altered in many cancers, but how specific spindle features are established and their effects on chromosome segregation and cell division are poorly understood. We will leverage morphometric and phylogenetic comparisons together with biochemical and functional assays to investigate the dramatic changes in spindle architecture that occur between oocyte meiosis and the mitotic divisions of early development in Xenopus and the sea squirt Ciona intestinalis. We will elucidate the role of specific factors in this transition, and examine the consequences of altering spindle architecture on embryo cell division. 2) What defects in cell division mechanisms underlie speciation? We have observed chromosome mis-segregation in inviable hybrids generated by fertilizing Xenopus tropicalis eggs with X. laevis sperm, and identified incompatibility between a subset of paternal centromeres and maternal cytoplasm as one underlying cause. We will elucidate the molecular basis of inter-species conflicts that impact cell division and contribute to reproductive isolation. 3) What is the molecular basis of mitotic chromosome condensation? We have developed a novel approach using optical tweezers to measure the dynamics of single DNA molecules in real-time in Xenopus egg extracts with high spatial and temporal precision and will use this system to dissect the roles of key factors in driving mitotic chromosome assembly. Absolute and relative size of biological entities varies widely, both within and among species at all levels of organization above the atomic/molecular: the organism, the cells that make up the organism, and the cellular components. How does scaling occur so that everything fits and functions properly? Correct scaling inside cells is crucial for cell function, architecture, and division, but until recently the control systems that a cell uses to regulate the size of its internal structures were virtually unknown. We have established assays to elucidate mechanisms of intracellular scaling between different-sized frog species and during the rapid, reductive cell divisions of early embryogenesis. We are further developing these systems to ask: 1) What scales mitotic chromosome size to cell size? We are testing the hypothesis that a surface area to volume sensor acting on the interphase nucleus and the mitotic spindle also coordinately adjusts mitotic chromosomes to cell size during Xenopus development. 2) What are the connections between genome size, cell size, physiology, and development? Cell size correlates strongly with genome size across evolution, but underlying mechanisms are unknown. We will utilize different ploidy frog embryos to address how altering genome size affects gene expression, and a variety of species including the dodecaploid frog Xenopus longipes to investigate relationships between genome size, cell division mechanisms, development, and physiology. The means to address these fundamental cell biological questions is enabled by powerful experimental systems based on cytoplasmic extracts and functional in vivo assays in vertebrate (Xenopus) embryos. We have established productive collaborations and apply diverse techniques including high-resolution microscopy, single molecule assays, genomics, proteomics, microfluidics and computational modeling to fill important conceptual gaps in an innovative, rigorous, and interdisciplinary manner. Our research will continue to provide novel insight into cell division and size control, processes essential for viability and development, and defective in human diseases including cancer. Although introduced as distinct topics, cell division and size control are intimately linked. We are increasingly focused on how cross-species comparisons can elucidate molecular mechanisms underlying cell division and size control, as well as how biological constraints related to these processes have shaped evolution. Together, these projects uniquely advance our understanding of long-standing questions in biology.