Mechanisms of Oocyte Meiotic Arrest – MOMA

Meiosis is fundamental for sexual reproduction. It generates haploid gametes, spermatozoa or oocytes, through two successive divisions, meiosis I followed by meiosis II, without an intermediate S-phase. At fertilization, fusion of the gametes restores diploidy, ensuring the proper development and survival of the embryo. In females, meiosis arrests at specific cell cycle stages to allow the oocyte to await fertilization. If these arrests fail, or occur prematurely, parthenogenic or aneuploid embryos will be generated. They are usually not viable and represent a significant cause of infertility. Importantly, this meiotic arrest occurs at different cell-cycle stages depending on the species. In vertebrates, it takes place in metaphase of meiosis II, while in other species, it occurs in prophase or metaphase of meiosis I, or even after meiosis completion. In the MOMA project, we will address the molecular mechanisms responsible for these different arrests, focusing on the role of the Cyclin B3.
It is well established that both meiosis and mitosis are driven by the kinase Cdk1 in association with Cyclin B1/2. Recent studies in mice, frogs, and nematodes have brought to light specific roles for the more divergent Cyclin B3 in regulating metaphase-anaphase transitions during meiosis and embryonic mitosis. In vertebrates, Cyclin B3 plays a unique role during female meiosis. Cdk1-Cyclin B3 activity triggers the exit from meiosis I by counteracting the signaling pathway responsible for metaphase II arrest. Cyclin B3 degradation following meiosis I allows the oocyte to arrest at metaphase II. In addition, maintaining Cyclin B3 expression in meiosis II experimentally leads to supernumerary cell divisions (our unpublished data). Thus, Cyclin B3 enables the oocyte to distinguish two functionally distinct metaphases to arrest in metaphase II, thereby limiting the number of oocyte divisions. In other animals, the role of Cyclin B3 remains poorly understood due to the limited number of species studied, but studies in Drosophila and C. elegans point to functions in meiosis and embryonic mitoses. We hypothesize that Cyclin B3 acted as a super-oscillator regulating female meiosis and embryonic mitoses in ancestral metazoans. Divergent features of molecular regulation have then emerged across species to promote cell-cycle arrest at distinct meiotic transitions for fertilization. To test this hypothesis, we will exploit the complementary expertise of our consortium. Cyclin B3 protein levels will be manipulated in the oocyte, combining biochemistry and imaging approaches, to address three main objectives: 1) Understanding the molecular mechanisms supporting Cyclin B3's super-oscillatory function in vertebrates, 2) Deciphering how Cyclin B3 is degraded to implement metaphase II arrest in vertebrates, and 3) Testing its function in two non-vertebrate species with distinct meiotic arrest points. We will compare and exchange results between four experimentally tractable species: two vertebrates (mouse and Xenopus; team 1), the ascidian Phallusia as an example of oocyte metaphase I arrest (team 2), and the cnidarian Clytia, which exhibits post-meiotic oocyte arrest in G1 (team 3). In vertebrates, our focus will be on how Cyclin B3 triggers meiosis I exit and deciphering its degradation mechanism, essential to implement the arrest in metaphase II and to suppress its super-oscillatory function. In parallel, we will test the function and regulation of Cyclin B3 during female meiosis and embryonic mitosis in Phallusia and Clytia. This novel comparative approach will provide new insights into the roles and the regulation of Cyclin B3, to control and implement distinct meiotic arrests for fertilization. Moreover, it will enhance our understanding of how finely regulated cell cycle processes contribute to the production of healthy oocytes for proper embryo development.