Fellow 1995-96

David J. Miller

Animal Sciences

Signaling through sperm receptors for eggs during mammalian fertilization

The overall goal of these studies is to gain a clearer understanding of the process of fertilization at a molecular level which will result in a greater ability to increase or decrease fertilization rates. In order to manipulate this process, it is imperative that we identify receptors on sperm and eggs that interact, allowing sperm to bind to eggs. These studies build upon previous work that has identified a receptor on mammalian sperm that binds an egg coat glycoprotein. ZP3 is a glycoprotein found in the egg coat that binds to β1,4-galactosyltransferase (GalTase) found on the surface of sperm. Following binding, ZP3 causes sperm to release the acrosome, an enzyme-filled vesicle whose contents digest a pathway for sperm through the egg coat. The acrosome reaction is required for fertilization. However, the mechanism by which ZP3 induces the acrosome reaction is not understood. With the recent discovery that sperm GalTase serves as a specific receptor for ZP3, signaling events following binding may now be studied. Importantly, recent evidence suggests that aggregation of sperm GalTase by ZP3 elicits the release of the sperm acrosome. Thus, GalTase may act not only to bind ZP3 but also to serve as a receptor to initiate signaling mechanisms leading to the acrosome reaction.

Because it is difficult to study signaling during the acrosome reaction in sperm, an analogous model system will be used that can be made to express GalTase, Xenopus laevis oocytes. Both sperm and oocytes release vesicles (acrosome or cortical granules) in response to specific activators. For a variety of technical reasons, vesicle release in Xenopus oocytes is easier to study.

In these experiments, Professor Miller's team will: 1) confirm that GalTase binding in sperm results in signal transduction and release of the sperm acrosome, 2) determin3 if Xenopus oocytes expressing murine GalTase on their surface specifically bind ZP3, and 3) ascertain if GalTase initiates signaling events in Xenopus oocytes causing release of vesicles.

Collectively, these experiments will determine if GalTase is simply a non-signaling adhesion molecule or if, like some other adhesion molecules, it also transmits a signal intracellularly. These studies will identify important signals during vesicle release. These aims will also determine the feasibility of studying acrosome release using a new and innovative system. This system should enable Professor Miller and his team to understand more clearly how a sperm receptor for the egg receives a cue from the zona pellucida and initiates the process of the acrosome release.