Induction of sperm acrosome reaction by perivitelline membrane glycoprotein ZP1 in Japanese quail (Coturnix japonica)

The extracellular matrix surrounding avian oocytes, called the perivitelline membrane (PL), consists of at least two major glycoproteins, ZP3 and ZP1. Our previous study using Japanese quail had demonstrated that the PL obtained from the preovulatory follicles was incubated in vitro with spermatozoa, and perforations were observed. This result indicated that the PL might contain a constituent that possesses activity to initiate the acrosome reaction (AR) in quail. In order to elaborate upon our previous findings, we evaluated the effects of ZP3 and ZP1 on the induction of sperm AR in Japanese quail. Ejaculated sperm were incubated with or without the purified PL glycoprotein, and their acrosome status was determined based on the presence or absence of the acrosome. Treatment of spermatozoa with increasing doses of the purified monomeric ZP1 led to a concentration-dependent stimulation of AR. The purified dimeric ZP1 had similar effect. Moreover, we found that the ZP1-induced AR was significantly blocked by the digestion of the PL protein with PNGaseF. In contrast, the addition of purified ZP3 failed to induce AR at any doses tested. These results indicate that N-linked glycans on ZP1 play an important role in triggering the AR in Japanese quail.