Functional redundancy and compensation: Deletion of multiple murine Crisp genes reveals their essential role for male fertility.

Mammalian Cysteine-RIch Secretory Protein (CRISP) family includes four members present in sperm and reported to regulate Ca2+ channels and fertilization. Based on our previous observations using single knockouts models and suggesting the existence of functional compensation among CRISP proteins, we investigated their relevance for male fertility by generating multiple Crisp gene mutants by CRISPR/Cas9 technology. Whereas targeting of Crisp1 and Crisp3 yielded subfertile males with early embryo developmental defects, the same deletion in zygotes from fertile Crisp2-/- .Crisp4-/- mice led to the generation of both triple and quadruple knockout mice exhibiting a complete or severe disruption of male fertility due to a combination of sperm transport, fertilization, and embryo developmental defects linked to intracellular Ca2+ dysregulation. These observations reveal that CRISP proteins are essential for male fertility and organize in functional modules that contribute distinctly to fertility success, bringing insights into the mechanisms underlying functional redundancy/compensation in protein families and emphasizing the importance of generating multiple and not just single knockout which might be masking the true functional relevance of family genes.

Relevance of CRISP proteins for epididymal physiology, fertilization, and fertility.

BACKGROUND: The molecular mechanisms involved in the acquisition of mammalian sperm fertilizing ability are still poorly understood, reflecting the complexity of this process. OBJECTIVES: In this review, we describe the role of Cysteine RIch Secretory Proteins (CRISP1-4) in different steps of the sperm journey to the egg as well as their relevance for fertilization and fertility. MATERIALS AND METHODS: We analyze bibliography reporting the phenotypes of CRISP KO mice models and combine this search with recent findings from our team. RESULTS: Generation of individual KO for CRISP proteins reveals they are key mediators in different stages of the fertilization process. However, in spite of their important functional roles, KO males for each of these proteins remain fertile, supporting the existence of compensatory mechanisms between homologous CRISP family members. The development of mice lacking epididymal CRISP1 and CRISP4 simultaneously (DKO) revealed that mutant males exhibit an impaired fertility due to deficiencies in the sperm ability to fertilize the eggs in vivo, consistent with the proposed roles of the two proteins in fertilization. Interestingly, DKO males show clear defects in both epididymal epithelium differentiation and luminal acidification known to be critical for sperm maturation and storage. Whereas in most of the cases, these epithelium defects seem to specifically affect the sperm fertilizing ability, some animals exhibit a disruption of the characteristic immune tolerance of the organ with clear signs of inflammation and sperm viability defects. DISCUSSION AND CONCLUSION: Altogether, these observations confirm the relevance of CRISP proteins for male fertility and contribute to a better understanding of the fine-tuning mechanisms underlying sperm maturation and immune tolerance within the epididymis. Moreover, considering the existence of a human epididymal protein functionally equivalent to rodent CRISP1 and CRISP4, DKO mice may represent an excellent model for studying human epididymal physiology and pathology.

Human fertilization: epididymal hCRISP1 mediates sperm-zona pellucida binding through its interaction with ZP3.

Human epididymal CRISP1 (hCRISP1) associates with sperm during maturation and participates in gamete fusion through egg complementary sites. Its homology with both rodent epididymal CRISP1 and CRISP4 reported to participate in the previous stage of sperm binding to the zona pellucida (ZP), led us to further investigate the functional role of hCRISP1 by studying its involvement in human sperm-ZP interaction. Human hemizona (HZ) were inseminated with human capacitated sperm in the presence of either anti-hCRISP1 polyclonal antibody to inhibit sperm hCRISP1, or bacterially-expressed hCRISP1 (rec-hCRISP1) to block putative hCRISP1 binding sites in the ZP. Results revealed that both anti-hCRISP1 and rec-hCRISP1 produced a significant inhibition in the number of sperm bound per HZ compared with the corresponding controls. The finding that neither anti-hCRISP1 nor rec-hCRISP1 affected capacitation-associated events (i.e. sperm motility, protein tyrosine phosphorylation or acrosome reaction) supports a specific inhibition at the sperm-egg interaction level. Moreover, immunofluorescence experiments using human ZP-intact eggs revealed the presence of complementary sites for hCRISP1 in the ZP. To identify the ligand of hCRISP1 in the ZP, human recombinant proteins ZP2, ZP3 and ZP4 expressed in insect cells were co-incubated with hCRISP1 and protein-protein interaction was analyzed by ELISA. Results revealed that rec-hCRISP1 mainly interacted with ZP3 in a dose-dependent and saturable manner, supporting the specificity of this interaction. Altogether, these results indicate that hCRISP1 is a multifunctional protein involved not only in sperm-egg fusion but also in the previous stage of sperm-ZP binding through its specific interaction with human ZP3.