Mammalian sperm, including human, must undergo several physiological and biochemical changes, collectively called capacitation, to be fertilization-competent. Capacitated sperm actively generate reactive oxygen species (ROS). A low level of ROS facilitates capacitation whereas an excessive ROS impairs capacitation. Hypotaurine (HT) is a precursor of taurine, and is abundant in the oviduct. HT is known to mitigate oxidative stress in hamster sperm, and is transported by taurine transporter (TauT) in a Na⁺- and Cl^-- dependent manner. However, how HT protect sperm from oxidative stress remains unknown. This study aimed to elucidate the antioxidant mechanisms by HT in hamster sperm, focusing on the involvement of TauT.
We first examined the effects of HT on sperm motility, intra- and extra- cellular ROS levels. HT was shown to be necessary to maintain motility. HT lowered intracellular ROS levels, but had no effect on extracellular ROS levels at the concentration tested, although HT itself has an antioxidative capacity at higher concentrations. Incorporation and enrichment of HT in sperm were confirmed by LC-MS/MS analysis. Next, the involvement of TauT was investigated. TauT was present in hamster sperm. β-alanine, a blocker of TauT, inhibited HT transport, increased intracellular ROS levels and impaired sperm motility. Moreover, the elimination of Na⁺ and Cl^- inhibited HT transport, and increased intracellular ROS levels.
In conclusions, the results indicate that hamster sperm incorporate and concentrate HT via TauT to protect themselves from ROS.