Spatial Organizing Principles of Cytokinesis Signaling

Abstract

Cytokinesis is the final step of cell division, in which the cytoplasm of one cell is physically divided into two. Animal cells integrate multiple positional cues from the mitotic spindle to decide where to position their cleavage plane. The unusually large egg cells from frogs face additional spatial organization challenges due to their large size compared to their mitotic spindle. How the spindle communicates to the cell cortex to precisely position the cleavage furrow at the cell midplane in these large cells is the focus of this dissertation. Chapter One introduces how cells across all domains of life position their division site, and the spatial organizing principles and molecular mechanisms that have been established for animal cells. The unique challenges faced by very large cells are discussed using eggs from the frog Xenopus laevis as a model system, and the large microtubule asters that organize the cytoplasm in these cells during cytokinesis are described. Chapter Two describes the development of a cell-free system using cytoplasmic extracts from Xenopus eggs to reconstitute signaling events from microtubules to the cell cortex that lead to cleavage furrow induction. This is the first time a biochemical extract system is reported for studying cytokinesis. The Chromosome Passenger Complex (CPC) is identified as the main furrow-inducing signal, which is delivered to the interaction zone between neighboring asters through a combined action of two kinesins, Kif4A and an embryonic paralog of Kif20A. Chapter Three describes experiments using the cell-free system to elucidate what happens when two neighboring asters meet and how they maintain an interaction zone that serves as a clear mutual boundary where cytokinesis proteins can be accumulated. The Prc1/Kif4A midzone module is identified as being essential for forming antiparallel microtubule overlaps that block further plus end growth, which both prevents microtubules from one aster from crossing into another and ensures that the majority of microtubules within a single aster grow radially outward. Chapter Four describes experiments in both extracts and whole embryos that identify initial conditions for CPC recruitment to aster interactions, such as proximity of chromatin and optimal starting distance between aster centers.