CD9 tides correlated with sperm oscillations tune the gamete fusion ability

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 to 5 minutes after fusion, most of the CD9 leaves the egg for the external aqueous medium. Then an egg membrane wave engulfs the sperm head in approximately 25 minutes. 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. JMCB, 2018

Dynamics of CD9-EGFP during successful IIVF. A- recruitment of CD9-EGFP (yellow arrow) in the sperm egg interface (dashed red line contour) before the gamete fusion reaction. B- depletion of CD9-EGFP (white arrow) from the contact area after gamete fusion. C- Release of CD9-EGFP in the egg surrounding medium (red arrow) a few minutes after gamete fusion. The black mask with red dashed contours hides the fluorescence of the egg membrane to better reveal CD9-EGFP outside the egg. The timer gives the time from the onset of the gamete contact.