Immunology

Rat Membrane Cofactor Protein (MCP; CD46) Is Expressed Only in the Acrosome of Developing and Mature Spermatozoa and Mediates Binding to Immobilized Activated C3

Abstract
The rat analogue of the complement regulator membrane cofactor protein (MCP; CD46) was recently cloned and analysis at the mRNA level suggested that expression was restricted to testis. In light of the proposed roles of human MCP in sperm-egg interaction, we undertook to analyse rat MCP expression at the protein level in order better to address its putative role in fertilisation. Recombinant fusion proteins comprising antibody Fc and specific domains of rat MCP were generated and used to develop a mAb (MM.1) specific for rat MCP. Immunohistochemistry using these reagents confirmed the reported testis-specific expression of MCP in sexually mature rats and demonstrated that MCP was expressed only by spermatozoa and their immediate precursors in spermiogenesis, spermatids. Pre-pubertal male rats did not express MCP and there was no evidence of MCP expression at any site in the embryo. Spermatozoal MCP expression was restricted to the inner acrosomal membrane, exposed only after fixation or induction of the acrosome reaction. Acrosome-reacted, but not un-reacted spermatozoa, bound methylamine-activated C3 immobilised on plastic. The retention of MCP at this subcellular site which is crucial to sperm-egg interaction, and the functional demonstration of binding to activated C3, strengthen suggestions from human studies that MCP may play an important role in fertilisation. The reagents and results described here will enable studies of the role of spermatozoal MCP in sperm-egg interaction using a relevant animal model system.

Key words: Immunology • Testis • Acrosome reaction • Sperm • Spermatogenesis

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