steroids 73 ( 2 0 0 8 ) 1416–1423 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/steroids Membrane progestin receptor beta (mPR-): A protein related to cumulus expansion that is involved in in vitro maturation of pig cumulus–oocyte complexes H.B. Qiu, S.S. Lu, K.L. Ji, X.M. Song, Y.Q. Lu, M. Zhang, K.H. Lu ∗ Animal Reproduction Institute, Guangxi Key Laboratory of Subtropical Bioresource Conservation and Utilization, Guangxi University, No. 100, Daxue Road, Nanning 530004, China article info Article history: Received 17 February 2008 Received in revised form 11 July 2008 Accepted 11 July 2008 Published on line 5 August 2008 Keywords: Membrane progestin receptor beta In vitro maturation Subcellular localization Endoplasmic reticulum Cumulus expansion Exocytosis abstract 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-,- and -). In the present study we have investigated the role of mPR- 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-. The levels of both transcripts and proteins increased between 0 and 20 h IVM, but then decreased between 20 and 44 h. The luteiniz- ing hormone (LH) and follicle-stimulating hormone (FSH) did not affect mPR- expression during IVM. Immunofluorescence analysis indicated that the mPR- was localized in the plasma membrane of cumulus cell. However, in mouse embryonic fibroblasts (MEFs), mPR- was detected at the endoplasmic reticulum (ER) rather than the plasma membrane. Cumu- lus expansion was impaired significantly (P < 0.05) when COCs were incubated in maturation medium containing 10% (v/v) anti-mPR- serum during IVM. Bioinformatics analysis pre- dicted that mPR- had an ER retention motif and an endocytosis internalization motif. These results suggest that the mPR- is a molecule related to cumulus expansion and it might func- tion by regulation of exocytosis. In conclusion, this is the first description of the expression patterns and subcellular localization of mPR- in COCs and might shed light on the function of the protein. © 2008 Published by Elsevier Inc. Although the non-genomic or rapid action of progesterone was found over 20 years ago [1], efforts to determine the structures of non-genomic progesterone receptors were unsuccessful until recently [2–6]. In 2003, Zhu et al. [5,6] iso- lated a group of proteins from fish and other vertebrates including humans that they called membrane progestin recep- tors (mPRs), because they were localized at the plasma membrane and bound progestin with high affinity [5]. These proteins were classified into three groups called membrane ∗ Corresponding author. Tel.: +86 771 3235724; fax: +86 771 3238064. E-mail address: khlu@gxu.edu.cn (K.H. Lu). progestin receptor alpha, beta and gamma (mPR-,- and -) [5]. All of these mPRs have seven deduced trans-membrane domains, which is the characteristic of G-protein coupled receptors (GPCRs) [5]. Furthermore, the N-terminus regions of mPRs are extracellular and C-terminus regions are intracellu- lar [5], which are also characteristics of GPCRs. Some reports suggest that the mPRs are physiological progesterone recep- tors involved in oocyte maturation of fish [5,6] and Xenopus [7]. 0039-128X/$ – see front matter © 2008 Published by Elsevier Inc. doi:10.1016/j.steroids.2008.07.007