Bicarbonate induces the membranes reorganization and the CB1R and TRPV1 endocannabionid receptors migration in lipid microdomains in capacitating boar spermatozoa.

Mammalian spermatozoa acquire the full fertilizing ability only after a morpho-functional maturation called the capacitation. During this process the high level of bicarbonate present within the upper female genital tract or in cultural media induces a marked reorganization of sperm membranes characterized by a biphasic behavior: in a few minutes, it promotes membrane phospholipid scrambling preliminary to the apical translocation of sterol that, 2-4h later, enables spermatozoa to recognize zona pellucida (ZP) after albumin-mediated cholesterol extraction. In the present research it was demonstrated that spermatozoa incubated with bicarbonate in protein-free media, underwent a marked reorganization of lipid microdomains present in detergent-resistant membrane fraction (DRM) isolated by ultracentrifugation on sucrose density gradient. In fact, bicarbonate exposed cells (ES), compared with ejaculated spermatozoa (non exposed cells, nES), displayed an increase in protein DRM content and, in particular, in Cav-1 and CD55, markers of caveolae and lipid rafts, as well in acrosin-2, marker of the outer acrosomal membrane (OAM). Moreover, the amount of certain proteins involved in capacitation, such as the endocannabinoid system receptors cannabinoid receptor type 1 (CBR1) and transient receptor potential cation channel 1 (TRPV1), increased in DRM obtained from ES. These data allow to hypothesize that the sperm membrane reorganization takes place even in absence of extracellular proteins; not only PM but also OAM participate to this process; important molecules playing a key role in inside-out signaling, such as endocannbinoid receptors TRPV1 and CB1R, are involved in this event, with potential important consequences on sperm function.