Mechanisms of fertilization--a view from the study of gene-manipulated mice

Recent gene knockout approaches have revealed that many of the factors previously considered to be important were largely dispensable in gene-knockout animals, and that previously unknown factors are emerging. Unexpectedly, sperm from 5 different gene-disrupted mouse lines (calmegin (Clgn), Adam1a, Adam2, Adam3, and Ace) all have defective zona-binding ability and oviduct-migrating ability. We have disrupted a new testis-specific molecular chaperone, calsperin (Calr3), which became the sixth gene sharing the same infertile phenotype. The relationships among these 6 factors are discussed. After zona penetration, sperm needs to fuse with eggs. We reported that sperm require IZUMO1 and eggs require CD9 for sperm-egg fusion. However, the distribution of IZUMO1 is not limited to the equatorial segment where it is believed the fusion takes place. Therefore, we produced a mouse line that lacks an equatorial segment-specific antigen, Spesp1. The Spesp1 +/- and -/- sperm showed a decreased fusing ability compared with wild-type sperm, but the cause of the impaired fusion may not directly relate to the mechanism involving IZUMO1. In order to study the mechanism of fertilization, the visualization of sperm in vivo provides a powerful tool. We made a new transgenic mouse line that produces sperm with green fluorescent protein-tagged acrosome and DSRed2-tagged mitochondria. Studies of fertilization using gene-manipulated animals are described in the present review.