Characterization of Inhibin/Activin Subunit, Activin Receptor, and Follistatin Messenger Ribonucleic Acid in Human and Mouse Oocytes: Evidence for Activin's Paracrine Signaling from Granulosa Cells to Oocytes1

Inhibin, activin, and follistatin (FS) are gonadal proteins that appear to have a role in regulating folliculogenesis through possible paracrine and/or autocrine interactions. To further examine the potential role of activin in oocyte-granulosa cell communication, we developed a sensitive reverse transcription-polymerase chain reaction protocol to analyze mRNA for the α, βA, and βB inhibin/activin subunits, FS, and the four activin receptor subtypes in individual human and mouse oocytes. The resulting expression pattern was further compared to that in human cumulus granulosa cells. Our results indicate that neither βA nor βB mRNA was detectable in any human or mouse oocyte, that α subunit was marginally present in some of the human oocytes, and that FS mRNA was detectable in human but not mouse oocytes. On the other hand, inhibin/activin subunit and FS mRNAs were abundantly expressed in cumulus cells. In addition, mRNAs for all four activin receptor subtypes (ActRIA, ActRIB, ActRIIA, and ActRIIB) were easily detectable in both oocytes and granulosa cells and appeared to be differentially expressed in oocytes during nuclear maturation. Finally, RNAs for both zona pellucida 3 and growth-differentiation factor-9, which were originally used as oocyte-specific markers, were detected in human but not mouse cumulus cells, although at lower levels than observed in oocytes. Taken together with previous studies, these results indicate that oocytes may be capable of responding to, but not synthesizing, activin.