Characterization of factors that underlie transcriptional silencing in C. elegans oocytes

Abstract

While it has been appreciated for decades that prophase-arrested oocytes are transcriptionally silenced on a global level, the molecular pathways that promote silencing have remained elusive. Previous work in C. elegans has shown that both topoisomerase II (TOP-2) and condensin II collaborate with the H3K9me heterochromatin pathway to silence gene expression in the germline during L1 starvation, and that the PIE-1 protein silences the genome in the P-lineage of early embryos. Here, we show that all three of these silencing systems, TOP-2/condensin II, H3K9me, and PIE-1, are required for transcriptional repression in oocytes. We find that H3K9me3 marks increase dramatically on chromatin during silencing, and that silencing is under cell cycle control. We also find that PIE-1 localizes to the nucleolus just prior to silencing, and that nucleolar dissolution during silencing is dependent on TOP-2/condensin II. Our data identify both the molecular components and the trigger for genome silencing in oocytes and establish a link between PIE-1 nucleolar residency and its ability to repress transcription.

The condensin II/TOP-2 axis silences transcription during germline specification inC. elegans

“…Next, we used RNAi to deplete TOP-2. Our previous work, with these same RNAi conditions, has shown that loss of TOP-2 promotes unscheduled transcription in oocytes, spermatocytes, and starved Z2/Z3 (2,3,8). However, in one-or two-cell embryos, TOP-2 depletion had no impact on transcriptional repression (Fig 2C).…”

“…One possibility is that TOP-2 requires the presence of PIE-1 to silence the genome. We have previously shown that this is true in oocytes, where loss of either PIE-1 or TOP-2 prevents transcriptional repression (2). By contrast, PIE-1 is not expressed in the Z2/Z3 PGCs and TOP-2 is nonetheless required for genome silencing during L1 starvation (8).…”

“…Starting with the gametes, we and others have shown that for both oocytes and spermatocytes transcription is globally silenced as these cells enter meiotic prophase (1)(2)(3). Silencing during gametogenesis requires three distinct pathways.…”

“…One is comprised of topoisomerase II (TOP-2 in worms) and condensin II, which work together to compact chromatin and to silence gene expression (2,3). The second is comprised of components of the H3K9me/heterochromatin pathway, for example the methyltransferases SET-25 and MET-2, and is characterized by a largescale buildup of H3K9me marks on chromatin as the genome is silenced (2). Lastly, the PIE-1 protein is also required for genome silencing during oogenesis (2).…”

“…The second is comprised of components of the H3K9me/heterochromatin pathway, for example the methyltransferases SET-25 and MET-2, and is characterized by a largescale buildup of H3K9me marks on chromatin as the genome is silenced (2). Lastly, the PIE-1 protein is also required for genome silencing during oogenesis (2). Upon fertilization, transcription continues to be repressed in the newly formed zygote, via the activities of the OMA-1/2 proteins, and this continues through first mitosis into the 2-cell embryo (4).…”