Sperm IZUMO1 Is Required for Binding Preceding Fusion With Oolemma in Mice and Rats

Abstract

Fertilization occurs as the culmination of multi-step complex processes. First, mammalian spermatozoa undergo the acrosome reaction to become fusion-competent. Then, the acrosome-reacted spermatozoa penetrate the zona pellucida and adhere to and finally fuse with the egg plasma membrane. IZUMO1 is the first sperm protein proven to be essential for sperm-egg fusion in mammals, as <i>Izumo1</i> knockout mouse spermatozoa adhere to but fail to fuse with the oolemma. However, the IZUMO1 function in other species remains largely unknown. Here, we generated <i>Izumo1</i> knockout rats by CRISPR/Cas9 and found the male rats were infertile. Unlike in mice, <i>Izumo1</i> knockout rat spermatozoa failed to bind to the oolemma. Further investigation revealed that the acrosome-intact sperm binding conceals a decreased number of the acrosome-reacted sperm bound to the oolemma in <i>Izumo1</i> knockout mice. Of note, we could not see any apparent defects in the binding of the acrosome-reacted sperm to the oolemma in the mice lacking recently found fusion-indispensable genes, <i>Fimp</i>, <i>Sof1</i>, <i>Spaca6</i>, or <i>Tmem95</i>. Collectively, our data suggest that IZUMO1 is required for the sperm-oolemma binding prior to fusion at least in rat.