Membrane progestin receptor beta (mPR-beta): a protein related to cumulus expansion that is involved in in vitro maturation of pig cumulus-oocyte complexes.

A new group of putative membrane receptors have now been isolated from fish and other vertebrates, including human. These proteins are classified into three groups known as membrane progestin receptor alpha, beta and gamma (mPR-alpha, -beta and -gamma). In the present study we have investigated the role of mPR-beta in regulating in vitro maturation (IVM) of pig cumulus-oocyte complexes (COCs). RT-PCR and Western blot analysis indicated that COCs contain transcripts and proteins for mPR-beta. The levels of both transcripts and proteins increased between 0 and 20h IVM, but then decreased between 20 and 44h. The luteinizing hormone (LH) and follicle-stimulating hormone (FSH) did not affect mPR-beta expression during IVM. Immunofluorescence analysis indicated that the mPR-beta was localized in the plasma membrane of cumulus cell. However, in mouse embryonic fibroblasts (MEFs), mPR-beta was detected at the endoplasmic reticulum (ER) rather than the plasma membrane. Cumulus expansion was impaired significantly (P<0.05) when COCs were incubated in maturation medium containing 10% (v/v) anti-mPR-beta serum during IVM. Bioinformatics analysis predicted that mPR-beta had an ER retention motif and an endocytosis internalization motif. These results suggest that the mPR-beta is a molecule related to cumulus expansion and it might function by regulation of exocytosis. In conclusion, this is the first description of the expression patterns and subcellular localization of mPR-beta in COCs and might shed light on the function of the protein.