RESUMO
In eukaryotes, Suv39h H3K9 trimethyltransferases are required for pericentric heterochromatin formation and function. In early mouse preimplantation embryos, however, paternal pericentric heterochromatin lacks Suv39h-mediated H3K9me3 and downstream marks. Here we demonstrate Ezh2-independent targeting of maternally provided polycomb repressive complex 1 (PRC1) components to paternal heterochromatin. In Suv39h2 maternally deficient zygotes, PRC1 also associates with maternal heterochromatin lacking H3K9me3, thereby revealing hierarchy between repressive pathways. In Rnf2 maternally deficient zygotes, the PRC1 complex is disrupted, and levels of pericentric major satellite transcripts are increased at the paternal but not the maternal genome. We conclude that in early embryos, Suv39h-mediated H3K9me3 constitutes the dominant maternal transgenerational signal for pericentric heterochromatin formation. In absence of this signal, PRC1 functions as the default repressive back-up mechanism. Parental epigenetic asymmetry, also observed along cleavage chromosomes, is resolved by the end of the 8-cell stage--concurrent with blastomere polarization--marking the end of the maternal-to-embryonic transition.
Assuntos
Embrião de Mamíferos/metabolismo , Desenvolvimento Embrionário/fisiologia , Impressão Genômica , Heterocromatina/fisiologia , Metiltransferases/fisiologia , Camundongos/embriologia , Proteínas Repressoras/fisiologia , Animais , Apoptose/fisiologia , Blastômeros/metabolismo , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/fisiologia , Embrião de Mamíferos/ultraestrutura , Proteína Potenciadora do Homólogo 2 de Zeste , Feminino , Imunofluorescência , Heterocromatina/ultraestrutura , Histona-Lisina N-Metiltransferase/genética , Histona-Lisina N-Metiltransferase/fisiologia , Integrases/metabolismo , Masculino , Camundongos Endogâmicos C57BL , Camundongos Knockout , Oócitos/metabolismo , Complexo Repressor Polycomb 1 , Complexo Repressor Polycomb 2 , Proteínas do Grupo Polycomb , Proteínas/genética , Proteínas/fisiologia , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transcrição Gênica , Transgenes/fisiologia , Ubiquitina-Proteína LigasesRESUMO
One physiological function proposed for RNA interference (RNAi) is to constrain expression of repetitive elements and thereby reduce the incidence of retrotransposition. Consistent with this model is that inhibiting the RNAi pathway results in an increase in expression of repetitive elements in preimplantation mouse embryos. Mouse oocytes are essentially transcriptionally quiescent providing a unique opportunity to assess the stability of repetitive element-derived transcripts in these cells. We compared the transcriptome of freshly isolated fully grown germinal vesicle (GV)-intact oocytes to that of oocytes in which meiotic maturation in vitro was inhibited for 48 h by milrinone. Consistent with the aforementioned function for RNAi is that the abundance of only a relatively small number of transcripts decreased in the cultured oocytes, when compared to changes that occur during maturation or following fertilization, and of those, several belonged to mobile elements.