Decreased cofilin1 expression is important for compaction during early mouse embryo development

Compaction, occurring at the eight-cell stage of mouse development, is the process of cell flattening and polarization by which cellular asymmetry is first established. During this process many molecules and organelles undergo polarized distribution, but the cytoskeletal basis for these distribution specifications remains to be explored. The present study focused on cofilin1, an actin-binding protein that depolymerizes actin filaments. We showed that cofilin1 expression decreased at the compaction stage, and that down-regulation of cofilin1 expression by siRNA microinjection accelerated compaction. Continuous observation using time-lapse video miscroscopy confirmed these findings. That is, the embryonic cells microinjected with anti-cofilin1 antibody exhibit earlier adherence properties compared to uninjected cells. Pronuclear microinjection of a site-directed mutated cofilin1 plasmid, in which cofilin1 is sustained in its active form produced embryos with blastomeres that did not adhere, suggesting that inactivation of cofilin1 is critical for cell flattening and adherence. Fluorescein-phalloidin staining indicated that decreased cofilin1 expression promoted the formation of the apical pole, which is a marker for polarity. Scanning electron microscopy results demonstrated the appearance of microvilli on the outer face of blastomeres in cofilin1 knockdown embryos. Our results suggest that cofilin1 plays an important role in cortical cytoplasmic organization during embryo compaction.