Dynamic interaction of formin proteins and cytoskeleton in mouse oocytes during meiotic maturation

Formin-2 (Fmn2) nucleates actin filaments required for spindle migration during the metaphase of meiosis I in mouse oocytes. While recent studies showed that Fmn2 is involved in the formation of a dynamic actin meshwork on meiotic spindle and the migration of chromosomes, the precise location and the mechanism of action of Fmn2 in the mouse oocyte is not known. In this work, we show that Fmn2 is colocalized with spindle during metaphase I (MI) and this pattern is lost in nocodazole-treated oocytes. Fmn2 directly interacts with polymerized microtubules (MTs) in vitro via a well-conserved domain called formin homology 2 (FH2). Microinjection of mRNA encoding formin homology 1 (FH1)FH2 domains of Fmn2 into Fmn2-/- oocytes partially rescued the defect of polar body extrusion, while mRNAs encoding FH2 domain alone could not rescue the defect. mDia1 and mDia2, Diaphanous (Dia) subfamily of formin proteins, exhibit unique patterns of expression in mouse oocytes. While mDia1 is localized on meiotic spindle, mDia2 localization is confined in spindle poles similar to γ-tubulin. Collectively, our results suggest that the ability of Fmn2 to directly interact with MTs and to polymerize actins via the conserved FH1FH2 domains is crucial for chromosomal migration in MI oocytes. We also show that mDia1 and mDia2 are dynamic components of meiotic spindle and pole complex during meiotic maturation of oocytes.