HEAD-SPECIFIC MANNOSE LIGAND-RECEPTOR EXPRESSION IN HUMAN SPERMATOZOAIS DEPENDENT ON CAPACITATION-ASSOCIATED MEMBRANE CHOLESTEROL LOSS

D-Mannose binding lectins appear on the human sperm head following in- vitro incubation under capacitating conditions. Surface expression of lectin is dependent on reduction of the sterol content of sperm membra nes. Mannose-specific lectin distribution over the head differs in acr osome-intact and acrosome-reacted spermatozoa. Sugar competition exper iments show that mannose is the only saccharide competitive with D-man nosylated albumin binding and that free mannose induces an acrosome re action in capacitated spermatozoa. A total of 183 men with normozoospe rmic semen parameters were screened for the ability of their spermatoz oa to bind fluorescein isothiocyanate (FITC)-labelled mannosylated alb umins, before and after incubation in vitro. The spermatozoa from 176 men 'responded' to incubation by exhibiting time-dependent increases i n head-directed mannosylated albumin binding, accompanied by increases in the percentage of spermatozoa showing spontaneous acrosome loss. M otile spermatozoa from the remaining seven men failed to express manno se-lectin binding activity after 18 h of incubation and only low perce ntages of their spermatozoa showed spontaneous acrosome loss. These se ven men were classified as 'non-responders'. The relative amounts of n on-esterified cholesterol in the sperm membranes of the responder and non-responder males were analysed by gas-liquid chromatography. Respon der spermatozoa showed decreases in free cholesterol content whereas n on-responder spermatozoa exhibited either no decrease or increases in relative free cholesterol per cell. Fresh swim-up spermatozoa contain sub-plasma membrane stores of mannose lectins which are revealed by FT TC-mannosylated albumin labelling before and after removal of the plas ma membrane by vortexing. In contrast, the mannose-lectin binding acti vity of capacitated spermatozoa is entirely limited to the sperm surfa ce. Western blots of proteins isolated from sperm plasma membranes aft er capacitation revealed two molecular species reactive with polyclona l antibodies against human macrophage mannose receptors. A model is pr oposed for the molecular mechanism whereby mannose lectins are transpo sed from sub-plasma membrane sites to an integral membrane position as a consequence of the loss of cholesterol from the sperm membrane.