Molecular Biology of Mammalian Gametogenesis, Fertilization and Early Development

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We use the mouse as a paradigm for investigating the developmental biology of mammalian gametogenesis and early embryogenesis. Our studies focus on molecular mechanisms of sex-specific factors that promote formation of ovulated eggs and mature spermatozoa, ensure fertilization, and support early development. Gametogenesis: A hallmark of female germ cell development is asymmetric cell divisions during meiosis that result in large, ovulated eggs containing maternal factors necessary for early development. A hallmark of male germ cells is post-meiotic spermiogenesis in which round spermatids are transmogrified into mature, motile spermatozoa capable of fertilization. We have identified downstream targets of a novel, oocyte-specific, basic helix-loop-helix transcription factor, FIGLA (factor in the germline, alpha) which has provided a wealth of information on the molecular basis of maternal effects that disrupt fertilization and pre-implantation development. We also have identified male-specific nuclear proteins that are required for maintenance of sperm stem cells and expression of pachytene piRNAs that are needed for successful spermiogenesis and male fertility. Fertilization: Monospermic fertilization is essential for the onset of mammalian development. One sperm is required, but two are lethal. A major arbiter of this constricted window of opportunity is the zona pellucida that surrounds ovulated eggs and pre-implantation embryos. The extracellular human and mouse zonae pellucidae are composed of four (huZP1-4) or three (moZP1-3) glycoproteins, respectively. We have developed models in which sperm-egg recognition is predicated on the N-terminus of ZP2 that is cleaved by ovastacin, a metalloendoprotease released from egg cortical granules following fertilization. Molecular biology and mouse transgenesis are being used to validate these models, learn more about cortical granule biology and investigate the preparation of gametes necessary for fertilization. Early Development: Although maternal effect genes encode proteins required for pre-implantation development, successful embryogenesis also is dependent on the partial or complete degradation of maternal proteins and RNA. We have identified a subcortical maternal complex (SCMC) and determined that maternal ablation of genes encoding individual components affects cleavage-stage embryogenesis, prevents activation of the embryonic genome and results in female infertility. We also have determined that the ubiquitin-proteasome system (UPS) partially degrades targeted maternal proteins required for normal development and are investigating the role of exosomes in mediating degradation of maternal RNAs associated with successful development.