are fundamental to the survival of a multi-cellular organism.
Control and diagnosis of normal and abnormal cell-cell
interactions is important in all areas of human health
ranging from embryonic development and immune function
to uncontrolled tumor growth in cancer. For example, understanding
the molecular mechanisms of sperm-egg recognition is required
for treating infertility and developing new, non-hormonal
methods of contraception.
We investigated the role of beta1 integrin
in mammalian fertilization and the mode of inhibition of
fertilinbeta-derived polymers. We determined that polymers
displaying the Glu-Cys-Asp peptide from the fertilinbeta
disintegrin domain mediate inhibition of mammalian fertilization
through a beta1 integrin receptor on the egg surface. Inhibition
of fertilization is a consequence of competition with sperm
binding to the cell surface, not activation of an egg-signaling
pathway. Most importantly, the presence of the beta1 integrin
on the egg surface increases the rate of sperm attachment,
but does not alter the total number of sperm that can attach
or fuse to the egg. We conclude that the presence of beta1
integrin enhances the initial adhesion of sperm to the
egg plasma membrane and that subsequent attachment and
fusion are mediated by additional egg and sperm proteins
present in the beta1 integrin complex. Therefore, the
mechanisms by which sperm fertilize wild-type and beta1
knockout eggs are different. Our work highlighted the
importance of using kinetics in the fertilization assays
to more fully assess the phenotype of knockout eggs.
Currently in the laboratory, we are using proteomic methods
with whole-animal isotope labeling to investigate additional
differences between wild-type and knockout eggs.
An important step in fertilization is binding of the sperm to the outer glycoprotein coat of the egg, the zona pellucida (ZP). The sperm must digest and traverse the ZP in order to reach the egg plasma membrane before fusion and fertilization can occur. The sperm acrosome reaction, proposed to initiate by binding to the ZP, results in release of digestive enzymes and matrix proteins from the sperm acrosomal vesicle. Oligosaccharides isolated from the ZP do not activate the acrosome reaction; they must be presented in a multivalent fashion. Many candidate ligands and receptors have been investigated; however, currently available biochemical and genetic methods have not definitively identified the ZP saccharides and sperm receptors that engage and lead to acrosome reaction. Defined molecular systems to unravel these complex cell-cell interactions are needed. Identification of the sugar moieties required for acrosome reaction and their receptors is necessary for controlling normal sperm function and diagnosing causes of abnormal sperm function.
We are developing
libraries of neoglycopolymers to screen for initiation
of the sperm acrosome reaction. We can control functional
group density, backbone flexibility, and functional group
variety in the polymer synthesis to provide a powerful,
molecular approach to further investigate the acrosome
reaction, and cell-cell interactions, in general. This
work builds on the polymer
developed in our lab.