CAMP activates Rap1 in differentiating mouse male germ cells: a new signaling pathway mediated by the cAMP-activated exchange factor Epac?

Rap1, a Ras-like G-protein, is implicated in the signaling of various cellular processes as morphogenesis, differentiation, cell adhesion and spreading, and maintenance of T cell anergy and B cell activation. The effectors that mediate Rap1 signaling have not yet been definitely identified, with the exception of B-Raf which, however, is restricted to neuronal tissues and a small subset of other cell types, including in particular male germ cells. We previously showed that in mouse spermatids Rap1 could interact with B-Raf giving rise to a signaling complex. Here we investigated about the possible molecules which "switch on" Rap1 finding that cAMP could in vivo activate endogenous Rap1. Spermatid-enriched cell cultures stimulated with 8-(4-chlorophenylthio)-cyclic AMP yielded higher levels of GTP-bound Rap1 than unstimulated cells. Since cAMP-induced Rap1 activation is actually retained to occur through Epac, we checked whether this recently discovered Rap1 exchange factor is expressed in male germ cells. Our findings indicate that Epac is present in spermatogenic cells and exhibits a preferential subplasmalemmal localization, although it shows also an intracellular location, more or less pronounced depending on the type of spermatogenic cell examined. Taken together, our data show that cAMP activates Rap1 in differentiating male germ cells which express the cAMP sensor Epac, thus suggesting that this activation might occur directly through Epac.