Gamete Biology

Insulin Signaling in Mammalian Oocytes

Abstract
Continual exposure of follicles/oocytes to elevated insulin compromises embryonic developmental competence, yet cellular mechanisms are unknown. Objectives of present studies were to determine if mouse oocytes have insulin receptors, a functional insulin signaling cascade, and whether insulin exposure during oocyte growth or maturation influences meiotic progression and chromatin remodeling. Immunoblot and immunocytochemical analyses of germinal vesicle-intact (GVI) oocytes demonstrated presence of insulin receptor-beta. Insulin receptor expression was increased in oocytes following gonadotropin stimulation, and remained elevated throughout meiotic maturation. Fully-grown GVI oocytes contained 3-phosphoinositide dependent protein kinase-1 (PDPK1), thymoma viral proto-oncogene 1 (AKT1) and glycogen synthase kinase 3 (GSK3). In vitro maturation of GVI oocytes in 5 µg/ml insulin had no influence on meiotic progression or incidence of normal MII chromosome condensation. Treatment of oocytes during maturation had no effect on GSK3A/B protein expression or phosphorylation on S21/9. However, culture of pre-antral follicles for 10 days with 5 µg/ml insulin increased phosphorylation of oocyte GSK3B, indicating GSK3 inactivation. Rate of development to MI was similar between oocytes obtained from insulin treated follicles and controls, yet incidence of abnormal MI chromatin condensation was significantly higher in oocytes obtained from follicles cultured with insulin compared to no insulin. These results demonstrate that oocytes contain a functional insulin signaling pathway and that insulin exposure during oocyte growth results in chromatin remodeling aberrations. These findings begin to elucidate mechanisms by which chronic elevated insulin influences oocyte meiosis, chromatin remodeling, and embryonic developmental competence.

Key words: Gamete Biology • Insulin • Kinases • Meiosis • Oocyte development