EXPRESSION OF MANNOSE-BINDING SITES ON HUMAN SPERMATOZOA AND THEIR ROLE IN SPERM-ZONA PELLUCIDA BINDING

A D-mannosylated albumin (DMA) neoglycoprotein was assessed to validat e experimentally a probe capable of detecting mannose-binding sperm re ceptors involved in human sperm-egg interaction. DMA specifically bloc ked zona binding of swim-up human spermatozoa in a concentration-depen dent manner. While no considerable effect was observed on sperm-zona i nitial contact, almost 50% of spermatozoa bound to the zona during a 2 -hour period detached from it when DMA was introduced in the incubatio n medium. DMA inhibition was evident when 10% fetal bovine serum, but not 3.5% human serum albumin (HSA), was used as Ham's F10 medium suppl ementation. This may be due to the amount of free calcium in the mediu m since addition of 40 mM CaCl2 to F10-HSA restored DMA inhibition. Fu rthermore, the higher the calcium concentration in the incubation buff er, the greater the DMA blockage of sperm-zona binding. Unfixed sperm presented fluorescent DMA label over the entire acrosomal area (cap pa ttern), or concentrated at the equatorial segment (bar pattern). These patterns increased during capacitation, appearing on an average of 20 % of the sperm after overnight incubation. They also increased, especi ally the bar pattern, following calcium ionophore treatment. Nearly al l of methanol-fixed spermatozoa displayed the fluorescent label at the head level. Concomitant assessment of sperm membrane integrity and DM A fluorescent patterns revealed that DMA fluorescence coincided mostly with permeabilized or altered sperm plasma membrane. In conclusion, D MA is a suitable probe to identify human sperm mannose-binding sites c rucially involved in sperm-zona interaction. These sites appear to req uire free calcium concentrations to operate, and their expression chan ges with capacitation and acrosome reaction. Precise location on human spermatozoa, however, warrants further investigation.