Formin-like 3 (FMNL3) is a member of the formin-likes (FMNLs), which belong to the formin family. As an F-actin nucleator, FMNL3 is essential for several cellular functions, such as polarity control, invasion, and migration. However, the roles of FMNL3 during oocytes meiosis remain unclear. In this study, we investigated the functions of FMNL3 during mouse oocyte maturation. Our results showed that FMNL3 mainly concentrated in the oocyte cortex and spindle periphery. Depleting FMNL3 led to the failure of polar body extrusion, and we also found large polar bodies in the FMNL3-deleted oocytes, indicating the occurrence of symmetric meiotic division. There was no effect of FMNL3 on spindle organization; however, we observed spindle migration defects at late metaphase I, which might be due to the decreased cytoplasmic actin. Microinjecting Fmnl3-EGFP mRNA into Fmnl3-depleted oocytes significantly rescued these defects. In addition, the results of co-immunoprecipitation and the perturbation of protein expression experiments suggested that FMNL3 interacted with the actin-binding protein FASCIN for the regulation of actin filaments in oocytes. Thus, our results provide the evidence that FMNL3 regulates FASCIN for actin-mediated spindle migration and cytokinesis during mouse oocyte meiosis.

Summary Sentence

The formin protein FMNL3 associates with FASCIN to promote actin assembly, which further involves into the meiotic spindle movement and polar body extrusion in mouse oocyte meiosis.