ATP-dependent Chromatin Remodeling Factors Regulate Nuclear and Epigenetic Maturation of Bovine Oocytes.

Abstract

Chromatin remodeling involves in dynamic transition of chromatin structure and includes alterations for the DNA methylation, histone modification, histone composition and chromatin conformation through the action of ATP-dependent chromatin remodeling enzymes. During oocyte maturation, transition of chromatin to chromosome occurs via a series of processes including germinal vesicle (GV), germinal vesicle breakdown (GVBD), metaphase I and metaphase II. However, dynamics of chromatin remodeling factors in the progress is unclear. This study was performed to understand how ATP-dependent chromatin remodeling factors (ACRFs) act during the oocyte maturation. To inhibit the activity of ACRFs, oocytes were first treated with 20 U/ml apyrase, an enzyme for ATP hydrolysis. Oocytes that were obtained at 1 h, 9 h, 24 h and 36 h after maturation, respectively, were immunostained with various antibodies specific for ACRFs and epigenetic markers such as Brg-1, BAF-170, Mi-2, hSNF2H, 5-MeC, AcH3K9, AcH4K5, mono-, di- and tri-methyl H3K9. After apyrase treatment, the localization and the distribution of ACRFs such as Brg-1, BAF-170, Mi-2 and hSNF2H showed normal patterns in chromatin or metaphase chromosomes of maturing oocytes, but the timing of their chromatin remodeling was retarded. In contrast to normal oocytes, Mi-2 didn't disappear at 1 h after maturation in the nucleus of apyrase-treated oocytes. These results demonstrate that suppression of the activity of ACRFs give rise to delayed chromatin remodeling in the nuclear maturation. The distribution patterns of epigenetic markers, including DNA methylation of 5-MeC, histone acetylation of H3K9 and H4K5, and histone mono-, di-, tri-methylation of H3K9, were temporally consistent with the timing of chromatin remodeling in apyrase-treated oocytes. Thus, nuclear maturation of bovine oocytes was delayed by inhibition of ACRFs. Developmental competence of apyrase-treated oocytes fertilized in vitro was significantly decreased compared with that of normal oocytes. Our findings suggest that ACRFs control timely nuclear maturation by chromatin remodeling, thereby leading to oocyte maturation and early embryonic development.