[Acrosome reaction in mammalian spermatozoa. Biochemical aspects]

Abstract

Mammalian sperm capacitation and the acrosome reaction (AR) are essential prerequisites for fertilization. This report examines part of the molecular events developed during capacitation and the AR of mammalian spermatozoa; especially those events related to sperm head membrane bound enzymes and phospholipids. For this purpose, it has been analysed results obtained from an in vitro capacitation/acrosome reaction inducing system for golden hamster spermatozoa. First of all, the analysis is focused in the phospholipid transmethylation reactions possibly occurring at plasma membrane level during capacitation and the AR; it is suggested too, that this pathway could provide the substrate for a sperm head membrane bound phospholipase A2 which is able to produce a lysophospholipid (a fusogen) and fatty acids; both of them, very likely involved in the late steps of the AR. These assumptions are confirmed by experiments demonstrating that exogenous lysophospholipids and/or cis-unsaturated fatty acids are able to accelerate AR in previously capacitated spermatozoa. It is also suggested future research in this field, which could involve a sperm phospholipase C specific for phosphatydil-inositol, 4.5 bisphosphate; its products, Inositol trisphosphate and diacylglycerol could act as second messengers with a probable physiological function during capacitation. Finally, an integrative mechanism for the AR-involving phospholipid methylation, acrosin activation, phospholipase A2 activation and endogenous lysophospholipids and fatty acids production is proposed as a model for discussion.