Reproduction Abstracts

Oocytes are arrested in prophase I until the LH surge, which stimulates an EGF-like growth factor signaling cascade critical for cumulus expansion, and releases meiotic arrest while inhibiting the expression of natriuretic peptide C (NPPC). In rodents, there is evidence that KIT ligand (KITLG) secreted from granulosa/cumulus cells may also enhance nuclear maturation of the oocyte. The objective of the present study was to measure the expression of KITLG in bovine cumulus–oocyte-complexes (COCs) during in vitro maturation (IVM), and to determine whether KITLG alters nuclear maturation. Bovine ovaries were collected from an abattoir, and COCs were aspirated from follicles 3 to 8 mm diameter and cultured for 22 h in a serum-free IVM medium with FSH and LH. Abundance of KITLG mRNA in cumulus cells increased with time during IVM, with a maximum at 12 h. The KITLG receptor, KIT, was expressed in oocytes, but mRNA levels did not change during IVM. Addition of KITLG to COCs for 22 h did not alter cumulus expansion, but significantly increased the proportion of oocytes progressing to metaphase II (54±3% vs 66±3% for 0 and 50 ng/ml KITLG). We then assessed potential mechanisms of action of KITLG. Addition of graded doses of KITLG to COCs significantly inhibited NPPC mRNA levels in cumulus cells. KITLG did not alter levels of mRNA encoding oocyte secreted factors (BMP15, GDF9) in the oocyte, but significantly increased abundance of mRNA encoding the maturation-associated protein, Y box binding protein 2 (YBX2). Finally, we assessed the effect of known oocyte-secreted factors on the expression of KITLG in cumulus cells; BMP15 and FGF8 increased KITLG mRNA levels, whereas FGF17 had no effect. The present study shows that KITLG enhances nuclear maturation of bovine oocytes, and may do so in part by decreasing levels of NPPC in cumulus cells and by increasing YBX2 levels in oocytes.