Maternal BRG1 regulates zygotic genome activation in the mouse

Abstract

Zygotic genome activation (ZGA) is a nuclear reprogramming event that transforms the genome from transcriptional quiescence at fertilization to robust transcriptional activity shortly thereafter. The ensuing gene expression profile in the cleavage-stage embryo establishes totipotency and is required for further development. Although little is known about the molecular basis of ZGA, oocyte-derived mRNAs and proteins that alter chromatin structure are likely crucial. To test this hypothesis, we generated a maternal-effect mutation of Brg1, which encodes a catalytic subunit of SWI/SNF-related complexes, utilizing Cre-loxP gene targeting. In conditional-mutant females, BRG1-depleted oocytes completed meiosis and were fertilized. However, embryos conceived from BRG1-depleted eggs exhibited a ZGA phenotype including two-cell arrest and reduced transcription for ∼30% of expressed genes. Genes involved in transcription, RNA processing, and cell cycle regulation were particularly affected. The early embryonic arrest is not a consequence of a defective oocyte because depleting maternal BRG1 after oocyte development is complete by RNA interference (RNAi) also resulted in two-cell arrest. To our knowledge, Brg1 is the first gene required for ZGA in mammals. Depletion of maternal BRG1 did not affect global levels of histone acetylation, whereas dimethyl-H3K4 levels were reduced. These data provide a framework for understanding the mechanism of ZGA.[Keywords: Two-cell embryo; BRG1; SWI/SNF; maternal-effect mutation; transcriptional regulation; zygotic genome activation] Supplemental material is available at http://www.genesdev.org. In Drosophila and Xenopus, mRNAs stockpiled in the oocyte are stable after fertilization and regulate many aspects of embryonic development (Wolpert et al. 2002). Conversely, in mammals, oocyte-derived mRNAs are degraded shortly after fertilization and cannot direct more than the first few cell divisions . Therefore, zygotic genome activation (ZGA) must occur very early during mammalian development. In the mouse, a minor burst of ZGA toward the end of the one-cell stage is followed by a major burst during the two-cell stage (Latham et al. 1992;Vernet et al. 1992;Aoki et al. 1997;Thompson et al. 1998;Schultz 2002). In a variety of other mammals, including humans, the embryonic genome is activated in a similar stepwise manner at the four-to eight-cell stage or the eight-to 16-cell stage (Telford et al. 1990;Kanka 2003). ZGA is required for continued development because mRNAs common to the oocyte and embryo (e.g., genes involved in the maternal-to-zygotic transition and housekeeping genes) are replenished and many genes not transcribed in the oocyte are expressed for the first time. The outcome of ZGA is a novel gene expression profile that establishes the totipotent state of each blastomere in the cleavagestage embryo. This step is a prerequisite for future cell lineage commitments and differentiation events that underlie pattern formation and organogenesis.Changes in chromatin structure are thought ...

Paternal H3K4 methylation is required for minor zygotic gene activation and early mouse embryonic development

“…Nuclear accumulation of RNA polymerase II (RNAP II) CTD, including the non-phosphorylated form, the phosphorylated form at serine 5 representing transcriptional initiation, and the phosphorylated form at serine 2 representing transcriptional elongation, was also maintained in the knockdown embryos ( Supplementary Fig S5A-C). Similarly, BRG-1, a catalytic subunit of the SWI-SNF chromatin remodeling complex, and previously reported to be involved in H3K4me2 and major ZGA at the 2-cell stage [35], was unaffected by siMll3/4 ( Supplementary Fig S5C). These results suggested at least three possibilities.…”

“…Collectively, we conclude that paternal H3K4me1 catalyzed by the Mll3/4 complex is required for minor ZGA in the paternal genome, and its alteration causes developmental arrest in 4-to 8-cell stage embryos, which is distinct from major ZGA phenotype (i.e., arrest in 2-to 4-cell stages) induced by a-amanitin treatment, or depletion of Mll2 or Brg-1 [23,35]. Interestingly, this delayed growth retardation may be explained by a "stepwise" gene activation model; that is, one transcriptional wave triggers the following transcriptional waves during preimplantation development, and thus, ZGA transcripts and their protein products would be required for the progression of embryos beyond the 4-cell stage [36].…”

Brg1 is required for stem cell maintenance in the murine intestinal epithelium in a tissue-specific manner

“…In line with the vast network of protein interactions involving Brg1, the latter has been reported to be involved in regulation of various physiological processes. Consistent with early developmental failure of Brg1 deficient mice [4], Brg1 has been implicated in a number of early developmental events such as zygotic genome activation [5], the segregation of the inner cell mass and trophectoderm [6], and transcriptional control of the key regulators of embryonic stem cell pluripotency and self-renewal [7,8]. In addition to its role in early development, Brg1 has also has been implicated in various processes at later stages of embryonic development, mainly in aiding differentiation along various cell lineages (reviewed in [9]) as well as in maintenance and differentiation of certain somatic stem cells including neural [10] and mesenchymal stem cells [11,12].…”

CFTR mediates bicarbonate-dependent activation of miR-125b in preimplantation embryo development

“…The observed inhibitory effect of HCO 3 − deprivation, CFTR inhibitor and sAC inhibitor on two-cell to fourcell transition is interesting, since two-cell stage is the time for mouse zygotic genome activation (ZGA), failure of which results in two-cell arrest. ZGA is the global reprogramming of gene expression in early embryos, which transforms the genome from transcription quiescence at fertilization to robust transcriptional activity [45]. The initiation of transcription requires the activation of a variety of transcription factors.…”