FPP MODULATES MAMMALIAN SPERM FUNCTION VIA TCP-11 AND THE ADENYLYL-CYCLASE CAMP PATHWAY

Fertilization promoting peptide (FPP; pGlu-Glu-ProNH(2)), which is fou nd in seminal plasma, promotes capacitation but inhibits spontaneous a crosome loss in mammalian spermatozoa in vitro. Adenosine, known to mo dulate the adenylyl cyclase (AC)/cAMP pathway, elicits these same resp onses whereas FPP + adenosine produces an enhanced response, leading t o the hypothesis that FPP and adenosine modulate the same signal trans duction pathway but act via different receptors. TCP-11, the product o f a t-complex gene, is the putative receptor for FPP: Fab fragments of anti-TCP-11 antibodies have the same effect as FPP on mouse spermatoz oa and Gln-FPP, a competitive inhibitor of FPP, also competitively inh ibits responses to the Fab fragments. In the present study, specific b inding of H-3-FPP to sperm membranes was significantly inhibited by 20 0 nM Gln-FPP and anti-TCP-11 Fab fragments (1/25 dilution), thus confi rming that FPP, Gln-FPP, and Fab fragments compete for the same bindin g site. In addition, spermatozoa treated with A23187 to induce the acr osome reaction bound significantly less H-3-FPP than untreated cells, suggesting that a large proportion of the FPP binding sites are associ ated with the acrosomal cap region; TCP-11 is located in this region. In other experiments, 100 nM FPP significantly stimulated cAMP product ion in mouse sperm membranes, permeabilized cells and intact cells. Fu rthermore, Gln-FPP inhibited production of cAMP in response to FPP but not to adenosine (10 mu M) or its analogue NECA (100 nM), supporting the involvement of two different receptors. Finally, anti-TCP-Il Fab f ragments (1/25 dilution) significantly stimulated cAMP production, whe reas low Fab (1/200; nonstimulatory when used alone) plus adenosine (1 0 mu M) significantly enhanced the stimulation of capacitation by aden osine. These results support the hypotheses that TCP-11 is the recepto r for FPP and that FPP<-->TCP-11 interactions modulate AC/cAMP. Mol. R eprod. Dev. 51:468-476, 1998. (C) 1998 Wiley-Liss, Inc.