Formin mDia1, a downstream molecule of FMNL1, regulates Profilin1 for actin assembly and spindle organization during mouse oocyte meiosis.

Mammalian diaphanous1 (mDia1) is a homologue of Drosophila diaphanous and belongs to the Formin-homology family of proteins that catalyze actin nucleation and polymerization. Although Formin family proteins, such as Drosophila diaphanous, have been shown to be essential for cytokinesis, whether and how mDia1 functions during meiosis remain uncertain. In this study, we explored possible roles and the signaling pathway involved for mDia1 using a mouse oocyte model. mDia1 depletion reduced polar body extrusion, which may have been due to reduced cortical actin assembly. mDia1 and Profilin1 had similar localization patterns in mouse oocytes and mDia1 knockdown resulted in reduced Profilin1 expression. Depleting FMNL1, another Formin family member, resulted in reduced mDia1 expression, while RhoA inhibition did not alter mDia1 expression, which indicated that there was a FMNL1-mDia1-Profilin1 signaling pathway in mouse oocytes. Additionally, mDia1 knockdown resulted in disrupting oocyte spindle morphology, which was confirmed by aberrant p-MAPK localization. Thus, these results demonstrated indispensable roles for mDia1 in regulating mouse oocyte meiotic maturation through its effects on actin assembly and spindle organization.