Intracellular events and signaling pathways involved in sperm acquisition of fertilizing capacity and acrosome reaction.

Two processes, namely capacitation and acrosome reaction, are of fundamental importance in the fertilization of oocyte by spermatozoon. Physiologically occurring in the female genital tract, capacitation is a complex process, which renders the sperm cell capable for specific interaction with the oocyte. During capacitation, modification of membrane characteristics, enzyme activity and motility properties of spermatozoa render these cells able to penetrate oocyte investments and responsive to stimuli that induce acrosome reaction prior to fertilization. Physiological acrosome reaction occurs upon interaction of the spermatozoon with the zona pellucida protein ZP3. This is followed by liberation of several acrosomal enzymes and other constituents that facilitate penetration of the zona and expose molecules on the sperm equatorial segment that allows fusion of sperm membrane with the oolemma. The molecular mechanisms and the signal transduction pathways mediating the processes of capacitation and acrosome reaction have been partially defined, and appear to involve modifications of intracellular calcium and other ions, lipid transfer and phospholipid remodeling in sperm plasma membrane as well as changes in protein phosphorylation. Some of the kinases and phosphorylated proteins that are involved in the processes of capacitation and acrosome reaction have been now characterized. Characterization of sperm receptors to physiological inducers of acrosome reaction is in progress. This review summarizes the main signal transduction pathways involved in capacitation and acrosome reaction. Furthermore, the mechanisms underlying sperm DNA fragmentation are also briefly reviewed.