Zinc depletion activates porcine metaphase II oocytes independently of the protein kinase C pathway

Zinc is an important trace element that regulates several biological functions. This study investigated the role of zinc in the metaphase (M) II arrest of porcine oocytes. N, N, N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), a Zn2+ chelator, was used to deplete free zinc from porcine MII stage oocytes. TPEN treatment significantly (P < 0.01) reduced the zinc content in the cytoplasm. The percentages of oocytes in which second polar body emission and pronuclear formation occurred increased as the concentration of TPEN increased (P < 0.01), and reached 93.64 +/- 5.53% and 90.61 +/- 9.10%, respectively, following treatment with 10 mu M TPEN. Zinc depletion also resulted in cortical granule release and spindle depolarization. Maturation-promoting factor activity, as assessed by examining p34(cdc2) activity, decreased (P < 0.05) following zinc depletion. Following TPEN treatment, embryos developed to the 4-cell stage but failed to reach the blastocyst stage. Zinc release is a common event in protein kinase C (PKC) activation. Therefore, we examined the impact of zinc depletion on phosphorylation of PKC substrates. Phosphorylation of PKC substrates was reduced (P < 0.05) in zinc-depleted oocytes, and this was rescued by phorbol 12-myristate 13-acetate (PMA) treatment. However, treatment of oocytes with both PMA and TPEN did not affect pronuclear formation or second polar body emission. These data are inconsistent with the hypothesis that oocyte activation caused by zinc depletion is mediated by the PKC pathway. This study shows that zinc has a novel role in maintaining MII arrest in porcine oocytes, but this is not mediated by the PKC pathway.