Galactosyltransferase Function during Mammalian Fertilization

Abstract

Gamete recognition has been studied extensively in the mouse. In this system, it is generally believed that sperm bind to a class of O-linked oligosaccharides on the zona pellucida glycoprotein, ZP3. The best characterized sperm receptor for ZP3 is β1,4-galactosyltransferase (GalT), which functions in a lectin-like capacity by binding to N-terminal N-acetylglucosamine residues on ZP3 oligosaccharides. Multivalent oligosaccharides on ZP3, as well as synthetic polymers terminating in N-acetylglucosamine aggregate GalT, leading to activation of a heterotrimeric G protein cascade and culminating in the acrosome reaction. Following fertilization, cortical granules release N-acetylglucosaminidase, which removes the binding site for sperm GalT and facilitates the zona block to polyspermic binding. Genetic manipulation of GalT expression has confirmed its function as a ZP3 receptor. Overexpressing GalT on sperm leads to increased binding of ZP3, increased G protein activation, and precocious acrosome reactions. In contrast, sperm from mice made null for GalT by homologous recombination are refractory to ZP3, in that they are unable to bind soluble ZP3 and fail to undergo the acrosome reaction in response to zona glycoproteins. Surprisingly, GalT null sperm still bind to the zona and achieve low rates of fertilization in vitro. This then suggests that sperm-egg binding involves receptor-ligand interactions independent of GalT and ZP3. The current model suggests that GalT functions as the ZP3 receptor that is responsible for inducing the acrosome reaction, whereas initial sperm-zona binding is dictated by other sperm surface receptors. Consistent with this, at least three other zona pellucida monosaccharides have been implicated in sperm binding, and novel sperm surface glycoproteins have been suggested to function in gamete binding. A large scaffolding protein has been identified that associates with the GalT cytoplasmic domain and may be responsible for orchestrating its signal transduction capacities that lead to the acrosome reaction.

Molecular Basis of Mammalian Gamete Binding

“…Using polyacrylamide as a matrix, N-acetylglucosamine on polyacrylamide triggers acrosomal exocytosis. Free N-acetylglucosamine, polyacrylamide, or polyacrylamide derivatized with galactose have no effect (Nixon et al, 2001).…”

“…Sperm bind to this formidable barrier and must exocytose the acrosome in order to pass through the matrix (Yanagimachi, 1994). Considerable evidence produced during the last 20 years suggests that sperm binding to the zona pellucida is a carbohydrate-mediated process (Nixon et al, 2001;Wassarman and Litscher, 2001;Wassarman et al, 2001). Lectin-like proteins on sperm bind to oligosaccharides on the zona pellucida proteins.…”

“…This review will present our work and that of others aimed at understanding this process. Excellent reviews emphasizing other aspects of fertilization have been published recently (Rankin and Dean, 2000;Nixon et al, 2001;Wassarman et al, 2001).…”

More Than a Simple Lock and Key Mechanism: Unraveling the Intricacies of Sperm-Zona Pellucida Binding

“…Consistent with the notion that primary sperm-zona pellucida interaction involves engagement with specific carbohydrate structures on ZP3, a number of the identified sperm receptors possess lectin-like affinity for specific sugar residues (McLeskey, et al, 1998, Topfer-Petersen, 1999, Wassarman, 1992. In the mouse, the most widely studied model, these receptors include, but are not limited to: -1,4-galatosyltransferase (GalT1) (Lopez, et al, 1985, Nixon, et al, 2001, Shur and Bennett, 1979, Shur and Hall, 1982a, ZP3R (or sp56) (Bookbinder, et al, 1995, Cheng, et al, 1994, Cohen and Wassarman, 2001, -D-mannosidase (Cornwall, et al, 1991) and zonadhesin (Gao and Garbers, 1998, Tardif and Cormier, 2011, Topfer-Petersen, et al, 1998) (see Table 1). However, despite the wealth of knowledge accumulated about each of these putative zona pellucida receptors it is now apparent that none are uniquely capable of directing sperm-zona pellucida adhesion.…”

Efecto de la Vitrificación de Ovocitos Humanos sobre la Capacidad de Unión y el Estado Acrosomal de Espermatozoides Humanos

“…El espermatozoide tras unirse a la ZP sufre la RA para atravesarla (Yanagimachi, 1994). Se dice que la unión del espermatozoide a la ZP es un proceso mediado por glicoproteínas (Nixon et al, 2001), pero los oligosacáridos precisos a los que se une el esper- …”