LYPD4, mouse homolog of a human acrosome protein, is essential for sperm fertilizing ability and male fertility†

Fertilization is one of the fundamental biological processes, but so far, we still do not have a full understanding of the underlying molecular mechanism. We have identified a human acrosome protein, LY6/PLAUR domain containing 4 (LYPD4), expressed specifically in human testes and sperm, and conserved within mammals. Mouse Lypd4, also specific to the testis and sperm, is essential for male fertility. LYPD4 protein first appeared in round spermatids during acrosome biogenesis and became part of acrosomes during spermatogenesis and in mature sperm. Lypd4 knockout mice are infertile with normal sperm number and motility. Mutant sperm, however, failed to reach oviduct during sperm migration inside the female reproductive tract, leading to fertilization failure and infertility. In addition, Lypd4 mutant sperms were unable to fertilize denuded egg via IVF (in vitro fertilization) but could fertilize eggs within intact Cumulus-Oocyte Complex, supporting an additional role in sperm-zona interaction. Out of more than five thousand spermatozoa proteins identified by mass spectrometry analysis, only a small subset of proteins (26 proteins) was changed in the absence of LYPD4, revealing a whole proteome picture of mutant sperm defective in sperm migration and sperm-zona binding. ADAM3, a key component of fertilization complex, as well as other sperm ADAM proteins are significantly reduced. We hence propose that LYPD4 plays an essential role in mammalian fertilization, and further investigation of its function and its interaction with other sperm membrane complexes may yield insights into human fertilization and novel strategy to improve IVF success.