On the Control of Microtubule Reorganization in Caenorhabditis elegans Oocytes prior to Fertilization

Abstract

The microtubule cytoskeleton of most animal oocytes differs from that of somatic cells in that the centrioles are lost during oogenesis. In most animals, female meiotic spindles assemble in the absence of centrosomes; instead, microtubule nucleation by chromatin, motor activity, and microtubule dynamics drive the self-organization of a bipolar meiotic spindle. Meiotic spindle assembly commences when microtubules gain access to chromatin after nuclear envelope breakdown (NEBD) during meiotic maturation. While many studies have addressed the chromatin-based mechanism of female meiotic spindle assembly, it is less clear how signaling influences microtubule localization and dynamics prior to NEBD. This thesis work analyzes microtubule behavior in response to hormonal signaling in Caenorhabditis elegans oocytes at the early stages of the meiotic maturation process. Oocyte meiotic maturation is defined by the transition between diakinesis and metaphase I and is accompanied by MAP kinase activation, NEBD, and meiotic spindle assembly. In C. elegans, sperm trigger oocyte meiotic maturation and ovulation using the major sperm protein (MSP) as an extracellular signaling molecule. To examine the role of MSP in regulating the oocyte microtubule cytoskeleton, we investigated microtubule organization in oocytes in the presence and absence of sperm. When sperm are present, microtubules are dispersed evenly throughout the cytoplasm. In contrast, when sperm are absent, microtubules are enriched at the cortical edges between oocytes. Females injected with purified MSP demonstrate that MSP is sufficient to reorganize oocyte microtubules. Using confocal microscopy and live-cell imaging, we show that MSP signaling reorganizes oocyte microtubules prior to NEBD and fertilization by affecting their localization and dynamics. By analyzing regulators of meiotic maturation, we have discovered several genes involved in organizing oocyte microtubules in response to MSP. We present evidence that MSP reorganizes oocyte microtubules through a signaling network involving antagonistic GĄo/i and GĄs pathways and gap-junctional communication with somatic cells of the gonad. We propose several biological functions for microtubule reorganization including a role in promoting meiotic spindle assembly through facilitating the search and capture of microtubules by meiotic chromatin following NEBD.