Tripeptidyl Peptidase II Regulates Sperm Function by Modulating Intracellular Ca2+ Stores via the Ryanodine Receptor

Recent studies have identified Ca(2+) stores in sperm cells; however, it is not clear whether these Ca(2+) stores are functional and how they are mobilized. Here, in vitro and in vivo, we determined that tripeptidyl peptidase II antagonists strongly activated the cAMP/PKA signaling pathway that drives sperm capacitation-associated protein tyrosine phosphorylation. We demonstrated that in the absence of Ca(2+), TPIII antagonists elevated the intracellular Ca(2+) levels in sperm, resulting in a marked improvement in sperm movement, capacitation, acrosome reaction, and the in vitro fertilizing ability. This antagonist-induced release of intracellular Ca(2+) could be blocked by the inhibitors of ryanodine receptors (RyRs) which are the main intracellular Ca(2+) channels responsible for releasing stored Ca(2+). Consistent with these results, indirect immunofluorescence assay using anti-RyR antibodies further validated the presence of RyR3 in the acrosomal region of mature sperm. Thus, TPPII can regulate sperm maturation by modulating intracellular Ca(2+) stores via the type 3 RyR.