Progesterone Receptor Availability in Mouse Spermatozoa During Epididymal Transit and Capacitation: Ligand Blot Detection of Progesterone-Binding Protein

ABSTRACT: The goals of the present study were to determine theavailability of progesterone (P4) receptor (P4r) in mouse sperm dur-ing maturation and capacitation and to make the first steps towarda characterization of P4r. It has been proposed that P4 is able toinduce an acrosomal reaction (AR) by using a membrane P4r. Thisinduction was verified in sperm isolated from the cauda epididymis(fully mature) when incubated in specific conditions that capacitatesperm. First, we set up the conditions in our laboratory to induce anAR in mature and capacitated sperm triggered by P4 that was de-tected by a chlortetracycline (CTC) assay. Then, we examinedsperm isolated from the caput epididymis (immature) incubated un-der conditions that support cauda sperm capacitation and found thatthe AR could not be detected. Moreover, P4 was unable to inducethe AR when it was applied to sperm isolated from either region andincubated under conditions that did not support capacitation. Theseresults can be explained by changes in P4r availability. A suitablemarker for P4r is the gold (Au)-P4 complex. This marker shows abinding capacity that can be visualized directly by electron micros-copy (EM) and indirectly by silver-enhanced methods with light mi-croscopy. The Au-P4 complex was localized in capacitated caudasperm at the dorsal edge of the head. Using these techniques, weobserved a significant decrease in both noncapacitatedcaudaspermand caput sperm (whether incubated in capacitating media or not).Genomic P4r could be responsible for the signal detected, but an-tibodies against the P4 nuclear receptor did not recognize any sitesin the sperm by immunostaining methodology. Instead, a 44-kd pro-tein band was detected in the sperm by a ligand blot assay. In con-clusion, P4 promotes the AR in capacitated cauda sperm but is un-able to do so in noncapacitated or immature sperm because theavailability of P4r increases during epididymal transit and after ca-pacitation. The P4r responsible for this behavior is different from aclassical nuclear receptor—on the basis of the immunostaining re-sults—and is probably a protein close to 44 kd—on the basis of theligand assay results.Key words: Acrosome reaction, sperm, maturation.J Androl 2003;24:612–620