Human sperm plasma membrane progesterone receptor(s) and the acrosome reaction

Progesterone initiation of the human sperm acrosome reaction (AR) includes stimulation of a transient Ca2+ influx and a transient Cl- efflux. A role for one or more plasma membrane receptors has been suggested, but, except for evidence supporting the involvement of a sperm GABA(A)-like receptor/Cl- channel, there is little information about possible receptor identity. Here, we attempt to identify plasma membrane progesterone receptors in human sperm using a mouse monoclonal antibody (mAb C-262) raised against the C-terminal steroid-binding domain of the human intracellular progesterone receptor, C-262 inhibited the progesterone-initiated AR in a dose-dependent manner. Maximum inhibition was 77% as detected by fluorescein isothiocyanate (FITC)-concanavalin A (ConA). Motility was unaffected. A control mouse mAb (h-151) raised against the human estrogen receptor did not inhibit the progesterone-initiated AR. Western blotting with C-262, but not with h-151, detected a major sperm protein band of 50-52 kDa. In indirect immunofluorescence localization studies, live and ethanol-fixed uncapacitated sperm and fixed capacitated sperm incubated with C-262, but not with h-151, displayed fluorescence at the equatorial segment region of the sperm head plasma membrane. In spectrofluorometric studies using capacitated sperm loaded with the Ca2+ probe Fura-2 or the Cl- probe MEQ, 0-262 but not h-151 inhibited both Ca2+ influx and Cl- efflux. These ion fluxes could be due to the binding of progesterone to two different receptor/channels or to its binding to one and cross talk with the other. Our results strongly support the involvement of sperm plasma membrane receptors in the progesterone-initiated AR and provide a candidate for one such receptor.