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Submitted on June 23, 2004
Accepted on July 23, 2004
Department of Animal and Poultry Science (J.C., H.Z., P.W.D., J.L.), Department of Biomedical Science (J.P.), University of Guelph, Guelph, Ontario, Canada N1G 2W1
* To whom correspondence should be addressed. E-mail: jli{at}uoguelph.ca.
Recent studies have suggested leptin has a central role in female reproduction, including ovarian function. The leptin receptor (Ob-R) has six isoforms and can signal through either the MAPK, or the JAK/STAT signal transduction pathway, depending on the isoform. Expression of Ob-R has been reported in human and mouse oocytes; however the physiological role of leptin during follicular development and oocyte maturation is largely unknown. In the current study, expression of Ob-R during oocyte growth and maturation was investigated in porcine oocytes from small, medium and large follicles, and in germinal vesicle (GV), germinal vesicle break down (GVBD), and metaphase II (MII) stage oocytes at both the mRNA and protein level. The proportion of oocytes expressing Ob-R was maximal in oocytes from medium follicles and at the GVBD stage (P < 0.05), while proportion of Ob-Rb expressing oocytes was found to be consistently low throughout growth and maturation. When included in oocyte maturation medium, leptin significantly increased the proportion of oocytes reaching MII (P < 0.01), elevated cyclin B1 protein content in MII stage oocytes (P < 0.05) and enhanced embryo developmental potential (P < 0.05), suggesting that leptin plays a role in both nuclear and cytoplasmic maturation. During oocyte maturation, leptin increased phosphorylated MAPK content by 2.8 fold (P < 0.05), and leptin-stimulated oocyte maturation was blocked when leptin-induced MAPK phosphorylation was suppressed by a specific MAPK activation inhibitor, U0126 (P < 0.01), demonstrating that leptin enhances nuclear maturation via activation of the MAPK pathway.
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