Oocytes prevent bovine cumulus cell apoptosis by maintaining a morphogenic paracrine gradient of bone morphogenetic proteins

Abstract

Paracrine factors secreted by the oocyte regulate a broad range of cumulus cell (CC) functions. Previously we have shown that the low incidence of apoptosis in CCs is due to unidentified oocyte-secreted factors (OSF) acting in an anti-apoptotic manner. Here we examine the nature of the paracrine network of oocyte BMP growth factors and their binding proteins regulating CC apoptosis. Bovine cumulus–oocyte complexes (COC) were aspirated from abattoir-derived ovaries and oocytes microsurgically removed to create oocytectomized (OOX) complexes. OOX were treated with denuded oocytes (DO) or various growth factors for 24 h, then CC apoptosis was assessed using TUNEL together with confocal microscopy plus image analysis and by Western blotting for Bcl-2 and Bax. CC apoptosis was significantly (P < 0.001) reduced by DO, bone morphogenetic protein 15 (BMP15), BMP6 or BMP7 as assessed by TUNEL. Accordingly, expression of anti-apoptotic Bcl-2 was high in OOX+DO and OOX+BMP15, and low with OOX+GDF9 and OOX alone, whereas the reverse was observed for pro-apoptotic Bax. Combined treatment of OOXs with BMP6 and BMP15 did not further decrease apoptosis levels beyond that of BMP15 alone (P > 0.05), suggesting no additive effect of these two BMPs. Follistatin (FS) effectively antagonized BMP15 anti-apoptotic effects, and likewise, a BMP6 neutralizing antibody (NAb) antagonized the inhibitory effect of BMP6. Gremlin blocked BMP7 anti-apoptotic effects on CCs, but had no significant effect on BMP15. FS or BMP6. NAb antagonized ~50% of the anti-apoptotic activity of oocytes; however, these effects were not additive suggesting the additional involvement of other OSF. These results indicate for the first time that OSF (BMP15 and BMP6 in particular) maintains the low incidence of CC apoptosis by establishing a localized morphogenic gradient of bone morphogenetic proteins.