168 EFFECT OF WORTMANNIN IN IN VITRO MATURATION OF BOVINE OOCYTES

Abstract

It has been shown that phosphatidylinositol 3 kinase (PI3K) participates in oocyte maturation by regulating the activity of important reactions related to the resumption of meiosis and energy metabolism. Changes in the enzyme activity caused by in vitro conditions may impair important events of oocyte maturation and consequently the production of blastocysts. The study aimed to verify the effect of the addition of wortmannin (a specific inhibitor of PI3K) to the in vitro maturation medium on nuclear and cytoplasmic maturation of bovine oocyte, as well as on the production of blastocysts. Cumulus-oocyte complexes (COCs) were matured in vitro in medium supplemented with FCS and 0 (control) or 20 nM of wortmannin and then fertilized and cultured in vitro in the absence of inhibitor. Twenty-two hours after in vitro maturation the determination of PI3K activity of oocytes by Western blot was performed using anti-PI3K subunit P85 antibody/peroxidase. The activity quantification was performed by densitometry of the bands using the Gel Perfect software (Bozzo and Retamal 1991 Arch. Biol. Med. Exp. 63, 510). To check the effect of treatment on energy metabolism, glucose and glycogen concentration of oocytes was quantified by Glucox 500 test (Doles Reagentes e Equipamentos Para Laboratórios Ltd., Goiânia, Brazil). The oocyte viability determination was performed by double labelling with propidium iodide and calcein AM. To determine the nuclear maturation, the oocytes were stained with 2% acetic orcein, being considered matured those with chromosomes in metaphase plate. To assess the meiotic spindle organisation, the oocytes were labelled with anti-α tubulin-FITC and propidium iodide. The distribution of actin filaments and mitochondria was determined by rhodamine-phalloidin labelling. The distribution of cortical granules was observed by labelling the oocytes with Lens culinaris–fluorescein isothiocyanate (FITC). The cleavage and blastocyst rates were determined at 48 and 168 h post-fertilization, respectively, both calculated on the number of oocytes placed to mature. The treatment means were compared by t-test (SAS Institute Inc., Cary, NC, USA, 2003). Wortmannin produced a reduction around 40% of PI3K activity; however, the levels of glucose and glycogen were not altered. No changes were found in the viability of in vitro matured oocytes or in nuclear maturation rate (P ≥ 0.05). Treatment did not promote any change in the organisation of meiotic spindle, distribution of actin filaments, and positioning of mitochondria. However, oocytes treated with wortmannin showed a significant increase (P ≤ 0.05) in the migration of cortical granules compared with controls (87.4% ± 11.4 and 72.8 ± 11.8%, respectively). The cleavage rate was not influenced by treatment, but the blastocyst rate was higher when oocytes were matured in presence of wortmannin (34.2 ± 6.4% v. 20.0 ± 5.0%, respectively; P ≤ 0.05). The results indicate that partial inhibition of PI3K activity in bovine oocytes treated with wortmannin during in vitro maturation did not affect nuclear maturation, but improved the migration of cortical granules, which seems to have contributed to the increased the blastocyst rate.