Dicer is an essential component of RNA interference (RNAi) pathways, which have broad functions in gene regulation and genome organization. Probing the consequences of tissue-restricted Dicer loss in mice indicates a critical role for Dicer during meiosis in the female germline. Mouse oocytes lacking Dicer arrest in meiosis I with multiple disorganized spindles and severe chromosome congression defects. Oogenesis and early development are times of significant post-transcriptional regulation, with controlled mRNA storage, translation, and degradation. Our results suggest that Dicer is essential for turnover of a substantial subset of maternal transcripts that are normally lost during oocyte maturation. Furthermore, we find evidence that transposon-derived sequence elements may contribute to the metabolism of maternal transcripts through a Dicer-dependent pathway. Our studies identify Dicer as central to a regulatory network that controls oocyte gene expression programs and that promotes genomic integrity in a cell type notoriously susceptible to aneuploidy.

An increase in the incidence of aneuploidy is well documented with increasing maternal age, in particular in human females. Remarkably, little is known regarding the underlying molecular basis for the age-associated increase in aneuploidy, which is a major source of decreased fertility in humans. Using mouse as a model system we find that eggs obtained from old mice (60-70 weeks of age) display a 6-fold increase in the incidence of hyperploidy as assessed by chromosome spreads. Expression profiling of transcripts in oocytes and eggs obtained from young and old mice reveals that approximately 5% of the transcripts are differentially expressed in oocytes obtained from old females when compared to oocytes obtained from young females (6-12 weeks of age) and that this fraction increases to approximately 33% in eggs. The latter finding indicates that the normal pattern of degradation of maternal mRNAs that occurs during oocyte maturation is dramatically altered in eggs obtained from old mice and could therefore be a contributing source to the decline in fertility. Analysis of the differentially expressed transcripts also indicated that the strength of the spindle assembly checkpoint is weakened and that higher errors of microtubule-kinetochore interactions constitute part of molecular basis for the age-associated increase in aneuploidy in females. Last, BRCA1 expression is reduced in oocytes obtained from old females and RNAi-mediated reduction of BRCA1 in oocytes obtained from young females results in perturbing spindle formation and chromosome congression following maturation.

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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 ...