Dispersion of Cyclin B mRNA Aggregation Is Coupled with Translational Activation of the mRNA during Zebrafish Oocyte Maturation

Abstract

AbstractCyclin B mRNA stored in immature zebrafish oocytes is translationally activated upon the stimulation of 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP), an event prerequisite for initiating oocyte maturation in this species. We investigated localization of cyclin B mRNA in zebrafish oocytes. Cyclin B mRNA was found to be exclusively localized as an aggregation along the cytoplasm at the animal pole of full-grown immature oocytes. When oocytes were treated with 17α,20β-DP, a meshwork of microfilaments in the oocyte cortex disappeared and the aggregation of cyclin B mRNA dispersed just prior to the initiation of cyclin B synthesis and germinal vesicle breakdown (GVBD). Cytochalasin B, but not nocodazole or taxol, deformed the aggregation of cyclin B mRNA, indicating the involvement of microfilaments in organizing this form. Like 17α,20β-DP, cytochalasin B (10 μg/ml) induced both complete dispersion of the aggregation and translational activation of cyclin B mRNA, forcing the oocytes to undergo GVBD without 17α,20β-DP. Conversely, disturbance of the aggregation of cyclin B mRNA with a low concentration (1 μg/ml) of cytochalasin B inhibited 17α,20β-DP-induced GVBD. These results suggest that the direct change in cyclin B mRNA from the aggregated form to the dispersed form is responsible for translational activation of the mRNA during zebrafish oocyte maturation