Upon adhesion to the zona pellucida, mammalian sperm undergo regulated exocytosis of the acrosome. Despite the difference in size, some parallels can be drawn concerning the signal transduction processes controlling the sperm acrosome reaction and synaptic vesicle exocytosis. Since components of signal transduction pathways are often organized in multiprotein signalling complexes, attempts were made to identify scaffolding proteins expressed in the acrosomal region of mammalian spermatozoa. Using RT-PCR approaches and immunohistochemical experiments, the Multi-PDZ domain Protein MUPP1, which comprises 13 potential protein interaction modules, was identified in mouse testis. Immunocytochemical experiments combined with immunogold electron microscopy revealed that MUPP1 is exclusively detectable within the acrosomal region of different mammalian spermatozoa and that the MUPP1 protein is most prominent at the outer acrosomal membrane. To assess the possible function of MUPP1, the acrosome reaction was monitored using the photosensitive calcium chelator NP-EGTA-AM and an inhibitory anti-MUPP1 antibody. This functional assay revealed that antibody treatment significantly reduces acrosome reaction compared to control conditions. These results together with the observation that MUPP1 co-migrates in detergent-insoluble lipid rafts along with proteins involved in acrosomal exocytosis, like syntaxin-2, indicates that MUPP1 in different mammalian species may assemble similar, if not identical signaling molecules controlling acrosomal exocytosis.
28 Apr - 02 May 2007
European Society of Endocrinology