TSSK6, a Member of the Testis-Specific Serine Kinase (TSSK) Family, Is Present in Mature Sperm and Required for Sperm-Egg Fusion in the Mouse.

Abstract

The Testis Specific Serine Kinases (TSSKs) are a family of proteins with a testis specific pattern of expression. Male mice null for Tssk6, one member of this protein family, have been reported to be sterile. In the present work we analyze the factors that make these mice infertile. Sperm from the TSSK6 null mice were unable to fertilize eggs in vitro but were able to activate development by Intra Citoplasmatic Sperm Injection. To further analyze this mechanism, Zonae Pellucidae-free eggs from Wild Type mice were used in vitro. In this facilitated condition, the Tssk6-/- sperm were able to attach to the egg oolema; however, contrary with other knock out sperm phenotypes such as the CatSper-/- or the sperm-specific protein kinase A, the Tssk6-/- sperm were unable to fuse. IZUMO, a protein involved in sperm-egg fusion was found to be present at normal levels in the Tssk6-/- sperm and to immunolocalize to the sperm acrosome similarly to wild type sperm. However, different to wild type sperm, the acrosome reaction-associated change in the apparent localization pattern of this protein was not observed in the null sperm. This discovery strongly suggests that changes in the IZUMO immunofluorescent pattern after the acrosome reaction are needed for sperm-egg fusion. It also demonstrates that the acrosome reaction alone is not sufficient to cause IZUMO changes. In this work, we have also investigated potential mechanisms for IZUMO movements and the possibility that IZUMO is phosphorylated in sperm. Altogether, these results provide insights into the molecular mechanisms of sperm-egg fusion and the signal transduction cascades associated with it.