Mechanisms of acentrosomal spindle assembly and stability during oocyte meiosis

Project Summary Organisms that reproduce sexually utilize a specialized cell division program called meiosis to reduce their chromosome number by half to generate haploid gametes. Proper execution of this process is crucial for a successful pregnancy, since errors in meiotic chromosome segregation result in aneuploidy (incorrect chromosome number in the embryos), the leading known cause of miscarriages and birth defects in humans. Meiosis in females is especially error prone and this vulnerability has a profound impact on human health: it is estimated that 10-25% of human embryos are chromosomally abnormal, and the vast majority of these defects arise from problems with the female meiotic cells (called oocytes). However, despite the importance of female meiosis for successful reproduction and human health, surprisingly little is known about the mechanisms that act to ensure accurate chromosome partitioning in oocytes. Oocytes have some special features that necessitate the use of novel cell division mechanisms. Perhaps most significantly, oocytes lack centrosomes, which define and organize the spindle poles in other cell types; therefore, spindles in these cells must assemble using different mechanisms. Using C. elegans as a model, we previously demonstrated that acentrosomal spindle assembly proceeds by 1) nucleation/stabilization of microtubules adjacent to the disassembling nuclear envelope, 2) sorting of microtubules such that their minus ends are positioned at the periphery of the array, 3) organization of these ends into nascent poles, and 4) coalescence of these sites until bipolarity is achieved. Moreover, we have identified proteins required for key events in this pathway, shedding light on the molecular mechanisms underlying this form of spindle assembly. Building on these discoveries, the goals of the proposed work are to: 1) deepen our understanding of acentrosomal spindle assembly and organization, and 2) investigate mechanisms that promote the formation and stability of acentrosomal spindle poles in both C. elegans and mammalian oocytes. These approaches will enable us to gain a mechanistic understanding of oocyte meiosis, an important yet poorly understood form of specialized cell division.