Porcine sperm capacitation involves tyrosine phosphorylation and activation of aldose reductase.

Mammalian sperm must be activated in the tubal isthmus through capacitation to induce the acrosome reaction and subsequent fertilization. Although the molecular mechanisms involved in capacitation have yet to be fully elucidated, increased concentrations of reactive oxygen species (ROS) and the extent of tyrosine phosphorylation of proteins have been suggested to play central roles in the completion of capacitation. In this study, aldose reductase was for the first time identified as one of the tyrosine-phosphorylated proteins involved in the capacitation of porcine cauda epididymal sperm. Both tyrosine phosphorylation and activity of aldose reductase associated with the particulate fraction of sperm cells were significantly enhanced during capacitation. Alrestatin, a membrane-permeable and specific inhibitor of aldose reductase, plays a role in the inhibition of aldose reductase activity, elevation of intracellular levels of ROS, and induction of hyperactivated motility, all at similar dose dependencies. Alrestatin canceled both the increase in the tyrosine phosphorylation of aldose reductase and the decrease in the glutathione levels in sperm-induced during capacitation. The hyperactivated motility was induced to a higher extent in the presence of glucose than in the presence of fructose. These results indicate that aldose reductase plays an important role in induction of hyperactivation and capacitation of sperm through the elevation of ROS in sperm cells. Furthermore, aldose reductase was shown to be added to sperm during transit through the epididymis, suggesting that aldose reductase is one of the key proteins that support the functional maturation of sperm.

Localization of a novel RNA-binding protein, SKIV2L2, to the nucleus in the round spermatids of mice.

In our previous study (Kawashima et al., Biol Reprod 2009; 80: 1293-1304), we suggested that the first cycle of spermatogenesis recovered from busulfan-induced temporary arrest was a good model system to analyze the proteins expressed at the specific stages of spermatogenesis in the mouse, and this has been confirmed in the present paper. Namely, six-week-old mice were injected with busulfan at 20 mg/kg body weight. The germ cells except for the undifferentiated spermatogonia disappeared by 32 days after injection. The surviving spermatogonia started to proliferate, and spermatogenesis was entirely recovered about 77 days after injection. By proteome analysis of the busulfan-treated testis during the process of recovery of spermatogenesis, we identified a protein that was expected to be expressed in the spermatogenic cells as Superkiller viralicidic activity-2-like-2 (SKIV2L2). Skiv2l2 mRNA was found in the kidney, epididymis and heart as well as the testis. In the testis, Skiv2l2 mRNA was shown to be highly expressed in the spermatocytes at stages I to VI. On the other hand, SKIV2L2 protein was found to be predominantly localized in the nuclei of round spermatids. In accordance with the fact that SKIV2L2 belongs to the Ski2 family within the superfamily 2 of RNA helicases, it has been shown that SKIV2L2 has both the RNA-binding and ATPase activities.