Evidence for the autocrine induction of capacitation of mammalian spermatozoa

Mammalian spermatozoa require a maturational event after ejaculation that a llows them to acquire the capacity for fertilization. This process, known a s capacitation, occurs spontaneously in simple defined medium implicating a potential role of autocrine induction. This study shows that the ether pho spholipid 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphocholine (PAF) meets the c riteria for an autocrine mediator of capacitation. Sperm released PAF after their dilution into capacitation medium and expressed a receptor for PAF o n their membranes. PAF stimulated changes in the motility of sperm and enha nced fertilization in vitro. These actions were inhibited by a PAF receptor antagonist (UR-12519) and by extracellular recombinant PAF:acetylhydrolase (an enzyme that degrades PAF to a biologically inert form). Seminal plasma contained an acid-labile PAF: acetylhydrolase, whereas capacitation was in hibited by an acid-labile factor within seminal plasma, implicating this fa ctor as a potential decapacitation factor within seminal plasma. Sperm from a PAF receptor knock-out mouse strain failed to express the receptor and d isplayed a significantly (p < 0.01) reduced rate of capacitation, as assess ed by the spontaneous onset of the acrosome reaction in vitro. When used fo r in vitro fertilization, sperm from PAF receptor knock-out mice gave a sig nificantly lower rate of fertilization (21.5%) than did wild-type sperm (66 .7%). The study shows for the first time the operation of an autocrine Poop that induces capacitation in sperm in vitro and shows that this loop acts in concert with other mediators of capacitation to promote efficient fertil ization.