Decoy tricks sperm, prevents fertilization in mice

A fragment of a protein found outside of egg cells can serve as a decoy for sperm, getting the wiggly sex cells to bind to it instead of an egg, according to a new study (Sci. Transl. Med. 2016, DOI: 10.1126/scitranslmed.aad9946).

This peptide could lead to new strategies for reversible contraceptives or methods to select sperm for in vitro fertilization, the study’s authors write.

In mammals, a spongy matrix consisting of glycoproteins surrounds egg cells. During fertilization, a sperm cell binds to this so-called zona pellucida and then penetrates through it on its way to fuse with the egg. Studies in mice have suggested that the sperm’s specific target in the matrix is a protein called ZP2.

Jurrien Dean of the National Institutes of Health and colleagues tested whether agarose microbeads decorated with the N-terminal fragment of ZP2 could snare sperm in the uterus of mice.

Mice receiving the ZP2 beads had a significantly lower fertilization rate compared with animals receiving naked beads—less than 10% versus almost 85%, respectively.

Dean sees the study as a proof of principle and doesn’t think the peptide beads themselves would be used as contraceptives. One option, he says, could be to add the decoy peptide to spermicidal sponges or vaginal rings that release steroids as a way to increase the efficacy of existing contraceptive methods.

Still, the beads could find other uses. The team determined that sperm captured by the beads were better at binding to and penetrating the zona pellucida than uncaptured sperm, suggesting that the beads could be used to select competent sperm for in vitro fertilization.