The molecular basis of sperm-oocyte interaction in the mouse

The initial interaction between the mammalian spermatozoon and oocyte represents a fundamental cellular communication event in the process of fertilisation; however, the molecular basis of this interaction remains poorly understood. Employing a mouse model, studies presented in this thesis were targeted towards elucidation of the relationship between the sperm signalling pathways activated during capacitation and the acquisition of fertilising ability. Further, the nature of the sperm receptor for the zona pellucida was examined. Data presented in this thesis demonstrated a direct causal relationship between sperm protein tyrosine phosphorylation during capacitation and zona pellucida (ZP) binding. In addition, the concept of phosphorylation of superficially expressed sperm head proteins during capacitation as a prerequisite for gamete interaction was introduced. Molecular chaperones heat shock protein 60 (HSP60) and endoplasmin (ERP99) were identified as tyrosine phosphoproteins expressed on the surface of the capacitated sperm head. Further investigation suggested that the role of tyrosine phosphoproteins in zona adhesion is indirect, leading to the proposition of a revised model for the zona pellucida receptor on the surface of spermatozoa. This hypothesis states that sperm possess a multimeric ZP receptor complex within their plasma membrane comprising chaperones HSP60 and ERP99 in addition to distinct zona recognition molecule(s). Tyrosine phosphorylation of HSP60 and ERP99 during capacitation may provide a signal for conformational changes in this complex such that both phosphotyrosine residues and ZP recognition moieties are available on the cell surface for interaction with the oocyte. The metalloprotease, a disintegrin and a metalloprotease domain with thrombospondin repeats 10 (ADAMTS l 0), was identified as a putative member of this complex. Future work is required to confirm this finding and to identify other proteins that cooperate to facilitate ZP recognition. Examination of HSP60 and ERP99 expression in the mouse testis and epididymis further supported a hypothesised role for these proteins in initial gamete interaction and suggested an additional function in the maturation of spermatozoa within the epididymal milieu. Forthcoming investigation of chaperone function in the epididymis should inform our understanding of how this organ confers functionality upon the spermatozoon. As a result of the studies presented in this thesis, we can now suggest a molecular basis for the acquisition of sperm fertilising ability during capacitation and offer a revised model for the murine zona pellucida receptor.