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Egg CD9 protein tides correlated with sperm oscillations tune the gamete fusion ability in mammal Free
Benjamin Ravaux, Sophie Favier, Eric Perez, and Christine Gourier *
Laboratoire de Physique Statistique, Ecole Normale Superieure/PSL Research University, UPMC Univ Paris 06, Universite´ Paris Diderot, CNRS, 75005 Paris, France
*Correspondence to:* Correspondence to: Christine Gourier, E-mail: gourier@lps.ens.fr
J Mol Cell Biol, Volume 10, Issue 6, December 2018, Pages 494-502  https://doi.org/10.1093/jmcb/mjy005
Keyword: mammalian fertilization, membrane dynamics, real-time imaging, CD9, sperm oscillations, gamete fusion

Mammalian fertilization involves membrane events—adhesion, fusion, sperm engulfment, membrane block to polyspermy—whose causes remain largely unknown. Recently, specific oscillations of the sperm in contact with the egg were shown to be necessary for fusion. Using a microfluidic chip to impose the venue for the encounter of two gametes allowed real-time observation of the membrane remodelling occurring at the sperm/egg interface. The spatiotemporal mapping of egg CD9 revealed that this protein concentrates at the egg/sperm interface as a result of sperm oscillations, until a CD9-rich platform is nucleated on which fusion immediately takes place. Within 2–5 min after fusion, most of the CD9 leaves the egg for the external aqueous medium. Then an egg membrane wave engulfs the sperm head in ~25 min. These results show that sperm oscillations initiate the CD9 recruitment that causes gamete fusion after which CD9 and associated proteins leave the membrane in a process likely to contribute to block polyspermy. They highlight that the gamete fusion story in mammals is an unexpected interplay between mechanical constraints and proteins.