Involvement of Bicarbonate-induced Radical Signaling in Oxysterol Formation and Sterol Depletion of Capacitating Mammalian Sperm During in Vitro Fertilization

This study demonstrates for the first time that porcine and mouse sperm incubated in capacitation media supplemented with bicarbonate produce oxysterols. The production is dependent on a reactive oxygen species (ROS) signaling pathway that is activated by bicarbonate and can be inhibited or blocked by addition of vitamin E or vitamin A or induced in absence of bicarbonate with pro-oxidants. The oxysterol formation was required to initiate albumin dependent depletion of 30% of the total free sterol and >50% of the formed oxysterols. Incubation of bicarbonate treated sperm with oxysterol-binding proteins (ORP-1 or ORP-2) caused a reduction of >70% of the formed oxysterols in the sperm pellet but no free sterol depletion. Interestingly, both ORP and albumin treatments led to similar signs of sperm capacitation: hyperactivated motility, tyrosin phosphorylation, and aggregation of flotillin in the apical ridge area of the sperm head. However, only albumin incubations led to high in vitro fertilization rates of the oocytes, whereas the ORP-1 and ORP-2 incubations did not. A pretreatment of sperm with vitamin E or A caused reduced in vitro fertilization rates with 47% and 100%, respectively. Artificial depletion of sterols mediated by methyl-beta cyclodextrin bypasses the bicarbonate ROS oxysterol signaling pathway but resulted only in low in vitro fertilization rates and oocyte degeneration. Thus, bicarbonate-induced ROS formation causes at the sperm surface oxysterol formation and a simultaneous activation of reverse sterol transport from the sperm surface, which appears to be required for efficient oocyte fertilization.