Stuck on You

Science Signaling  13 Jul 2010:
Vol. 3, Issue 130, pp. ec214
DOI: 10.1126/scisignal.3130ec214

Many sperm bind to an unfertilized egg, and the entry of one sperm into the egg triggers exocytosis of the egg's cortical granules, which prevents sperm binding to the fertilized egg. One model of sperm-egg recognition proposes that cleavage of the egg zona pellucida (ZP) component ZP2 by a protease released from the cortical granules prevents sperm binding after fertilization. Another model proposes that sperm bind to glycans of the ZP component ZP3, which are removed by a cortical granule glycosidase. To distinguish between these models, Gahlay et al. tested whether sperm binding to egg in mouse required cleavage of ZP2 or the presence of O-glycosylated ZP3. Wild-type sperm bound to eggs derived from Zp3-null females that carried a transgene encoding a form of ZP3 that lacked glycosylation sites. Likewise, sperm bound to eggs from Zp2-null females that carried a transgene encoding a form of ZP2 that could not be cleaved. However, sperm also bound to eggs that contained the uncleavable ZP2 even after fertilization and cortical granule release. Therefore, cleavage of ZP2, rather than deglycosylation of ZP3, was important for preventing sperm-egg recognition after fertilization. The authors propose that ZP2 is the primary determinant of sperm-egg recognition and that cortical granule exocytosis triggers cleavage of ZP2, thus releasing bound sperm and preventing subsequent binding.

G. Gahlay, L. Gauthier, B. Baibakov, O. Epifano, J. Dean, Gamete recognition in mice depends on the cleavage status of an egg's zona pellucida protein. Science 329, 216–219 (2010). [Abstract] [Full Text]