Expression of bone morphogenetic proteins and receptors in porcine cumulus–oocyte complexes during in vitro maturation

In vitro oocyte growth is the essential technology which enables oocytes to achieve maturation and acquire the competence for subsequent manipulation. There is increasing evidence that members of the transforming growth factor-β (TGF-β) superfamily are expressed in a variety of cell types within the ovary in a developmental stage-related manner and function as crucial factors in oocyte growth and follicular development. However, the expression of TGF-β family members has been studied extensively in follicular compartment cells in the ovaries while poorly explored in the cumulus–oocytes complex (COC) within culture systems. Using semi-quantitative RT-PCR, we investigated the temporal and spatial expression patterns of several bone morphogenetic proteins (BMP-4, BMP-6, BMP-15 and GDF-9), as well as BMP receptors (BMPRIA, BMPRIB, BMPRII and ActRII), in porcine COCs throughout in vitro maturation (IVM). In oocytes, the transcription of BMP-6, BMP-15, GDF-9 and BMPRII were down-regulated, while BMP-4, BMPRIA and BMPRIB remained unchanged during IVM. In cumulus cells, BMP-4 mRNA expression increased significantly, while BMP-6 and ActRII was down-regulated during IVM. Nevertheless, mRNAs of BMPRIA, BMPRIB and BMPRII were constantly expressed in cumulus cells in the process. However, BMP-15 was absent in cumulus cells and ActRII was not detected in oocytes. In addition, hardly any transcription of BMP-2, BMP-5, BMP-7, ActRIA was found in porcine COCs throughout IVM. These data demonstrate a complex BMP-signaling system for member gene expression within porcine COCs during IVM and indicate the need for further functional characterization of these factors during oocyte maturation.