Expression of maturation genes and their receptors during in vitro maturation of sheep COCs in the presence and absence of somatic cells of cumulus origin

The purpose of this study was to investigate the effects of somatic cells of cumulus origin (sCC) on gene expression and maturation of cumulus oocyte complexes (COCs) in vitro. Good quality (i.e., healthy-looking) isolated sheep COCs were randomly divided into two treatment groups: control (COC with no sCC) and coculture (COC with sCC). Nuclear maturation statuses of oocytes were assessed after 27 hours of in vitro culture. Moreover, the expression levels of growth differentiation factor 9 (GDF9), bone morphogenetic protein (BMP)15, BMP6, bone morphogenetic protein receptor II (BMPRII), activin like kinase 5 (ALK5) (transforming growth factor β receptor 1: TGFβR1), ALK6 (BMPR1b), activin A receptor, type IIB (ActRIIB), and ALK3 (BMPR1a), as well as hyaluronan synthase 2 (HAS2) and prostaglandin endoperoxide synthase 2 (Ptgs2) in the COCs were assessed in both treatment groups after 3 h and 27 h of culture. The results showed that the proportion of metaphase II (MII) stage oocytes was significantly higher in the coculture group compared with the controls (77.21% ± 1.17 vs. 67.49% ± 1.80; P < 0.05). The relative expressions of BMPRII, ALK6, and ActRIIB in control group and GDF9 and ActRIIB in coculture group showed significant differences during culture as assessed by real time polymerase chain reaction (P < 0.05). The mean expression levels of BMPRII, ALK5, ALK6, and ActRIIB mRNA were decreased in the coculture group compared with those in the control group after 27 h of culture (P < 0.05). In conclusion, we propose that in vitro maturation of sheep COCs alone disrupted the normal gene expression levels of both TGFβ ligands and receptors, and also reduced the maturation rate. Coculture with sCC enhanced the maturation rate of oocytes concomitantly with reduced gene expression levels of a number of TGFβ ligands and receptors.