The role of Xenopus membrane progesterone receptor β in mediating the effect of progesterone on oocyte maturation

Rapid, nongenomic membranal effects of progesterone were demonstrated in amphibian oocytes more than 30 y ago. Recently, a distinct family of membrane progestin receptors (mPRs) has been cloned in fish and other vertebrate species. In this study we explore the role of mPR in promoting oocyte maturation in Xenopus laevis. RT-PCR analysis indicates that Xenopus oocytes contain transcripts for the mPR beta ortholog, similar to what has been reported in zebrafish oocytes, and Western blotting shows that the protein is expressed on the oocyte plasma membrane. Microinjection of mPR beta-specific antibodies into oocytes resulted in a dramatic inhibition of progesterone-dependent oocyte maturation, whereas microinjection of mRNA encoding Myc-Xenopus mPR (XmPR)beta resulted in an accelerated rate of progesterone-induced oocyte maturation, concomitant with membranal localization of the protein. Binding studies in mammalian cells expressing XmPR beta confirmed specific binding of progesterone by the expressed protein. These results suggest that XmPR beta is a physiological progesterone receptor involved in initiating the resumption of meiosis during maturation of Xenopus oocytes.