Comparative analysis of molecular events in mammalian oocytes

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Project summary Egg generation entails a unique cell division, Meiosis I (a process also known as oocyte maturation), during which the oocyte acquires its developmental competence. For poorly understood reasons, mammalian oocytes (including human, bovine and porcine oocytes) are notoriously prone to reduced quality. Because oocyte quality is a major determinant of embryo developmental competence and pregnancy success, the long-term research goal of this application is to dissect basic molecular mechanisms that regulate meiosis I in human, bovine and porcine oocytes to understand why female gametes are notoriously prone to reduced quality. Work from our group has demonstrated that preferential initial nucleus and spindle positioning at the center of mouse oocytes is an insurance mechanism to avoid the premature exposure of the spindle to the cortical signaling that hinders proper chromosome-microtubule attachments, thereby protecting against aneuploidy, the major genetic cause of infertility and congenital defects. These findings implicate initial peripheral nucleus and spindle positioning as a major risk factor for aneuploidy in mice. Strikingly, in contrast to mouse oocytes, the majority of human, bovine and porcine oocytes have a peripheral nucleus and, subsequently, peripheral spindle formation. Interestingly, in human oocytes, peripheral nucleus positioning correlates with poor maturation rates. How the nucleus/spindle behaves when it is initially positioned peripherally in those species (humans, pigs and cattle) and whether this behavior correlates with aneuploidy are unknown. This proposal builds on these findings via two complementary albeit independent aims: (1) determine the underlying molecular mechanisms regulating nucleus behavior in human, bovine and porcine oocytes, and (2) determine whether nucleus/spindle positioning relates to aneuploidy. To carry out these aims, we will employ super-resolution microscopy, multiphoton laser ablation, genetic, optochemical and integrative multiomic approaches. These conceptually and technically innovative aims are expected to have a broad impact on the field by filling a substantial gap in our knowledge of how the nucleus/spindle behaves in human, bovine and porcine oocytes to ensure the development of good-quality gametes with clear clinical implications for assisted reproductive technologies. Such understanding is relevant to human and animal reproductive health because mammalian oocytes are notoriously associated with poor developmental competence and aneuploidy, major causes of early pregnancy loss (a high priority area of the Fertility and Infertility Branch of the NICHD).