Chd9 mediates highly loosened chromatin structure in growing mouse oocytes

During oogenesis, oocytes prepare for embryonic development following fertilization. The mechanisms underlying this process are still unknown. Recently, it has been suggested that a loosened chromatin structure is involved in pluripotency and totipotency in embryonic stem (ES) cells and early preimplantation embryos, respectively. Here, we explored chromatin looseness in oocytes by fluorescence recovery after photobleaching (FRAP) using enhanced green fluorescent protein-tagged histone H2B. The results indicated that the chromatin in growing oocytes was already highly loosened to a level comparable to that in early preimplantation embryos. To elucidate the mechanism underlying the loosened chromatin structure in oocytes, we focused on chromodomain helicase DNA binding protein 9 (Chd9), which is highly expressed in growing oocytes. The oocytes from Chd9 knockout mice (Chd9^-/-) generated using the CRISPR/Cas9 system exhibited a less loosened chromatin structure than that of wild-type mice, suggesting that Chd9 is involved in the loosened chromatin structure in growing oocytes. These results suggest that a loosened chromatin structure, which is mediated by Chd9, is a prerequisite for the acquisition of totipotency after fertilization.