Deletion of Eqtn in mice reduces male fertility and sperm-egg adhesion

A number of sperm proteins are involved in the processes from gamete adhesion to fusion, but the underlying mechanism is still unclear. Here, we established a mouse mutant, the EQUATORIN-knockout (EQTN-KO, Eqtn(-/-)) mouse model and found that the EQTN-KO males have reduced fertility and sperm-egg adhesion, while the EQTN-KO females are fertile. Eqtn sperm were normal in morphology and motility. Eqtn(-/-)-Tg (Acr-Egfp) sperm, which were produced as the acrosome reporter by crossing Eqtn(-/-) with Eqtn(+/+)-Tg(Acr-Egfp) mice, traveled to the oviduct ampulla and penetrated the egg zona pellucida of WT females. However, Eqtn(-/-) males mated with WT females showed significant reduction in both fertility and the number of sperm attached to the zona-free oocyte. Sperm IZUMO1 and egg CD9 behaved normally in Eqtn(-/-) sperm when they were fertilized with WT egg. Another acrosomal protein, SPESP1, behaved aberrantly in Eqtn(-/-) sperm during the acrosome reaction. The fertility impairment of EQTN/SPESP1-double KO males lacking Eqtn and Spesp1 (Eqtn/Spesp1(-/-)) was more severe compared with that of Eqtn(-/-) males. Eqtn(-/-)-Tg (Eqtn) males, which were generated to rescue Eqtn(-/-) males, restored the reduced fertility.