MOUSE OOCYTES PROMOTE STEROL BIOSYNTHESIS IN CUMULUS CELLS: MORE EVIDENCE FOR OOCYTEREGULATION OF METABOLIC COOPERATIVITY BETWEEN GRANULOSA CELLS AND OOCYTES

Abstract

To evaluate the function(s) of oocyte-derived BMP15 and GDF9 on the development and function of cumulus cells before LH-stimulation of cumulus expansion, we previously compared the transcriptomes of cumulus cells from eCG-primed wildtype (WT), Bmp15−/−, and Bmp15−/−Gdf9+/− double mutant (DM) mice by micro-array analysis, and found that transcripts encoding enzymes of metabolic pathways in cumulus cells, particularly glycolysis, sterol biosynthesis, inositol metabolism, and pentose phosphate, were significantly affected. Here we have focused on the sterol biosynthesis pathway and studied its regulation in cumulus cells by oocytes. Oocytectomy (OOX) of WT cumulus cell-oocyte complexes reduced the expression in cumulus cells of 15 of 17 mRNAs encoding enzymes for sterol biosynthesis. This reduction was prevented when OOX cumulus cells were co-cultured with fully-grown oocytes (FGOs). Associated with the down-regulation of these transcripts' expression, the rate of de novo cholesterol biosynthesis was reduced by more than 90% in OOX cumulus cells. The reduction in cholesterol biosynthesis was recovered to only 50% of the control level by co-culture with FGOs. This suggests that contact between oocytes and cumulus cells is necessary for cumulus cell cholesterol biosynthesis. The expression of 6 mRNAs encoding enzymes in this pathway, Mvd, Fdps, Sqle, Lss, and Tm7sf2, was also significantly lower in cumulus cells of Bmp15−/− and DM, compared to that in WT mice, and these low mRNA levels were reflected in reduced levels of cholesterol synthesis (55% and 25% of WT level in Bmp15−/− and DM cumulus cells, respectively). These results suggest that the oocyte-derived paracrine factors, including GDF9 and BMP15, enhance sterol biosynthesis in cumulus cells by promoting the expression of mRNAs encoding the key enzymes in this pathway. Comparison of the expression of all the transcripts encoding enzymes in this pathway in cumulus cells and oocytes revealed that 7 mRNAs, Mvk, Pmvk, Fdps, Sqle, Cyp51, Sc4mol, and Ebp, were robustly expressed in cumulus cells, but barely detectable in oocytes. Moreover, the level of de novo synthesized cholesterol in cumulus cell-enclosed oocytes was about 5-fold higher than that in denuded oocytes. We conclude that oocytes are deficient in synthesizing sterols and require cumulus cells to provide them with cholesterol and the other intermediates of the sterol biosynthetic pathway. Moreover, mouse oocyte-derived BMP15 and GDF9 profoundly affect the metabolic functions of cumulus cells before the LH surge. Supported by HD23839 (Y-QS, KS, JJE), HD21970 (KS, KW, JJE), HD33438 (SAP, MMM) and 5F32HD46335 (SAP). (platform)