For all animals, maintenance of life and speciation depend on the process of fertilization. Fusion of eggs and sperm to form a zygote is the culmination of a complex series of interactions between two highly specialized gametes. In mammals, interactions between gametes begin when free-swimming sperm reach the site of ovulated eggs in the oviduct and then bind to eggs.
Our research focuses on mammalian egg and sperm surface components that account for species-specific binding of sperm to eggs during fertilization. For example, one of these components is a glycoprotein, called ZP3, that is present in the ovulated egg extracellular coat, or zona pellucida (ZP). ZP3 serves as a receptor for sperm during fertilization. We discovered and characterized ZP3 biochemically and, subsequently, cloned and characterized the gene encoding ZP3. While much of our research has been carried out with mice, ZP3 is present in the ZP of all mammalian eggs, including human eggs, and a ZP3-like glycoprotein is present in the extracellular coat (vitelline envelope) of eggs from amphibians, birds, fish, and many invertebrates.
Our studies have revealed that ZP3 is synthesized, processed, and secreted by oocytes, the precursors of eggs, during the growth phase of oogenesis. During growth of the oocyte, ZP3 and two other glycoproteins, called ZP1 and ZP2, assemble into an extensive network of interconnected filaments that constitute the ZP. The filaments consist of ZP2 and ZP3, present every 150 Å, and are interconnected by ZP1. Free-swimming sperm recognize and bind to specific oligosaccharides present on ZP3 at its sperm combining-site. Binding to ZP3 activates the signal transduction pathway of sperm that culminates in exocytosis, the acrosome reaction, and enables bound sperm to penetrate the ZP. Therefore, ZP3 is a structural glycoprotein, a species-specific sperm receptor, and an activator of signal transduction when sperm bind to unfertilized eggs.
All ZP glycoproteins share a large region of polypeptide (approximately 260 amino acids), called the "ZP domain". A ZP domain is also present in a wide variety of other proteins of diverse origins and functions. For example, ZP domain containing proteins are components of the mammalian egg, inner ear, nose, and kidney, as well as the Drosophila cuticle, wing epithelium, and mechanosensory organ. Our research strongly suggests that the ZP domain is a conserved module used for polymerization of extracellular proteins. Since the domain is found in a number of proteins involved in human pathologies, we are also investigating relationships between mutations in the ZP domain and disease.
In our research, we use a wide variety of contemporary methodology, including transfection of mammalian cells, exon swapping, site-directed mutagenesis, transgenesis, and targeted gene disruption. Overall, the object of our research is to understand the molecular basis of the multiple functions of ZP glycoproteins during mammalian oogenesis, fertilization, and preimplantation development and to understand the role of the ZP domain in normal and diseased states.
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