d-Mannose-binding sites are putative sperm determinants of human oocyte recognition and fertilization

The aim of the present study was to further evaluate the participation of d-mannose in the process of human sperm–egg interaction. Zona pellucida binding competitive assays in the presence of d-mannose were carried out using discarded oocytes from IVF. Spermatozoa were capacitated and d-mannose-binding site (MBS) expression, sperm viability and follicular fluid-induced acrosome reaction (AR) were evaluated. MBS were visualized using a fluorescein-neoglycoprotein probe. The capacity of free d-mannose and mannosylated albumin to induce the AR was also tested. MBS and the IVF outcome were also analysed. The involvement of d-mannose in sperm binding to the zona pellucida was verified by the inhibitory effect produced when the sugar was present during binding assays. MBS expression increased during capacitation, in parallel with the ability to undergo the induced AR. Mannosylated albumin, but not the free sugar, induced the AR. In acrosome-reacted spermatozoa, the MBS was located at the plasma membrane, as shown by confocal analysis. No significant difference in the increase in MBS expression was observed among the different IVF groups of patients. The data show that d-mannose is involved in the sperm–zona pellucida interaction, and that the expression of MBS on the sperm surface occurs during the acquisition of in-vitro sperm fertilizing ability.