Project description:Small RNA-guided chromatin silencing, also referred to as nuclear RNAi, plays an essential role in genome surveillance in eukaryotes and provides a unique paradigm to explore the complexity in RNA-mediated chromatin regulation and transgenerational epigenetics. A well-recognized paradox in this research area is that transcription of the target loci is necessary for the initiation and maintenance of the silencing at the same loci. How the two opposing activities (transcriptional activation and repression) are coordinated during animal development is poorly understood. To resolve this gap, we took single-molecule RNA imaging, deep-sequencing, and genetic approaches towards delineating the developmental regulation and subcellular localization of RNA transcripts of two exemplary endogenous germline nuclear RNAi targets in C. elegans, Cer3 and Cer8 LTR retrotransposons. By examining the wild type and a collection of mutant strains, we found that transcription and silencing cycle of Cer3 and Cer8 is tightly coupled with the early embryogenesis and germline mitotic and meiotic cell cycles. Strikingly, Cer3 and Cer8 transcripts are exclusively localized in the nuclei of germ cells in both wild type and germline nuclear RNAi-defective mutant animals. RNA-sequencing analysis found that this nuclear enrichment feature is a general feature for the endogenous targets of the germline nuclear RNAi pathway. In addition, the germline and somatic repressions of Cer3 have different genetic requirement for the three H3K9 histone methyltransferases, MET-2, SET-25, and SET-32, in conjunction with the nuclear Argonaute protein WAGO-9/HRDE-1. These results provide a first comprehensive cellular and developmental characterization of the nuclear RNAi-targeted endogenous targets throughout animal reproductive cycle. Altogether, these results support a model in which (1) both the transcriptional activation and repression steps of the germline nuclear RNAi pathway are tightly coupled with animal development, (2) the endogenous targets exhibit a hallmark of nuclear enrichment of their transcripts, and (3) different heterochromatin enzymes play distinct roles in somatic and germline silencing of the endogenous targets.
Project description:Small RNA-guided chromatin silencing, also referred to as nuclear RNAi, plays an essential role in genome surveillance in eukaryotes and provides a unique paradigm to explore the complexity in RNA-mediated chromatin regulation and transgenerational epigenetics. A well-recognized paradox in this research area is that transcription of the target loci is necessary for the initiation and maintenance of the silencing at the same loci. How the two opposing activities (transcriptional activation and repression) are coordinated during animal development is poorly understood. To resolve this gap, we took single-molecule RNA imaging, deep-sequencing, and genetic approaches towards delineating the developmental regulation and subcellular localization of RNA transcripts of two exemplary endogenous germline nuclear RNAi targets in C. elegans, Cer3 and Cer8 LTR retrotransposons. By examining the wild type and a collection of mutant strains, we found that transcription and silencing cycle of Cer3 and Cer8 is tightly coupled with the early embryogenesis and germline mitotic and meiotic cell cycles. Strikingly, Cer3 and Cer8 transcripts are exclusively localized in the nuclei of germ cells in both wild type and germline nuclear RNAi-defective mutant animals. RNA-sequencing analysis found that this nuclear enrichment feature is a general feature for the endogenous targets of the germline nuclear RNAi pathway. In addition, the germline and somatic repressions of Cer3 have different genetic requirement for the three H3K9 histone methyltransferases, MET-2, SET-25, and SET-32, in conjunction with the nuclear Argonaute protein WAGO-9/HRDE-1. These results provide a first comprehensive cellular and developmental characterization of the nuclear RNAi-targeted endogenous targets throughout animal reproductive cycle. Altogether, these results support a model in which (1) both the transcriptional activation and repression steps of the germline nuclear RNAi pathway are tightly coupled with animal development, (2) the endogenous targets exhibit a hallmark of nuclear enrichment of their transcripts, and (3) different heterochromatin enzymes play distinct roles in somatic and germline silencing of the endogenous targets.
Project description:Nuclear RNA interference provides a unique approach to the study of RNA-mediated transgenerational epigenetic inheritance. A paradox in the field is that expression of target loci is necessary for the initiation and maintenance of their silencing. How expression and repression are coordinated during animal development is poorly understood. To resolve this gap, we took imaging, deep-sequencing and genetic approaches towards delineating the developmental regulation and subcellular localization of RNA transcripts of two representative endogenous targets, the LTR retrotransposons Cer3 and Cer8. By examining wild-type worms and a collection of mutant strains, we found that the expression and silencing cycle of Cer3 and Cer8 is coupled with embryonic and germline development. Strikingly, endogenous targets exhibit a hallmark of nuclear enrichment of their RNA transcripts. In addition, germline and somatic repressions of Cer3 have different genetic requirements for three heterochromatin enzymes, MET-2, SET-25 and SET-32, in conjunction with the nuclear Argonaute protein HRDE-1. These results provide the first comprehensive cellular and developmental characterization of nuclear RNAi activities throughout the animal reproductive cycle.
Project description:Mammals have one Dicer gene required for biogenesis of small RNAs in microRNA (miRNA) and RNA interference (RNAi) pathways. Yet, endogenous RNAi is highly active in oocytes but not in somatic cells. Here, we provide a mechanistical explanation for high RNAi activity in mouse oocytes. The main Dicer isoform in oocytes is transcribed from an intronic MT-C retrotransposon, which functions as a promoter of an oocyte-specific Dicer isoform (denoted DicerO). DicerO lacks an N-terminal helicase domain and has a higher cleavage activity than the full-length Dicer from somatic cells. DicerO can rescue the miRNA pathway and, in addition, it efficiently produces small RNAs from long dsRNA substrates. Thus, control of endogenous RNAi activity in mice occurs via alternative Dicer isoform and the phylogenetic origin of DicerO demonstrates evolutionary plasticity of RNA silencing pathways.
Project description:Mammals have one Dicer gene required for biogenesis of small RNAs in microRNA (miRNA) and RNA interference (RNAi) pathways. Yet, endogenous RNAi is highly active in oocytes but not in somatic cells. Here, we provide a mechanistical explanation for high RNAi activity in mouse oocytes. The main Dicer isoform in oocytes is transcribed from an intronic MT-C retrotransposon, which functions as a promoter of an oocyte-specific Dicer isoform (denoted DicerO). DicerO lacks an N-terminal helicase domain and has a higher cleavage activity than the full-length Dicer from somatic cells. DicerO can rescue the miRNA pathway and, in addition, it efficiently produces small RNAs from long dsRNA substrates. Thus, control of endogenous RNAi activity in mice occurs via alternative Dicer isoform and the phylogenetic origin of DicerO demonstrates evolutionary plasticity of RNA silencing pathways. NIH3T3 cells or mouse embryonic stem cells expressing oocyte-specific or somatic form of Dicer were transiently transfected with a plasmid expressing long double-stranded RNA (within the 3'-UTR of EGFP reporter) or left without transfection for controls.
Project description:Some long noncoding RNAs (ncRNAs) transcribed by RNA polymerase II (RNAPII) are retained on chromatin, where they regulate RNAi and chromatin structure. The molecular basis of this retention remains unknown. We show that in fission yeast serine 7 (Ser7) of the C-terminal domain (CTD) of RNAPII is required for efficient siRNA generation for RNAi-dependent heterochromatin formation. Surprisingly, Ser7 facilitates chromatin retention of nascent heterochromatic RNAs (hRNAs). Chromatin retention of hRNAs and siRNA generation requires both Ser7 and an RNA-binding activity of the chromodomain of Chp1, a subunit of the RNA-induced transcriptional silencing (RITS) complex. Furthermore, RITS associates with RNAPII in a Ser7- dependent manner. We propose that Ser7 promotes cotranscriptional chromatin retention of hRNA by recruiting the RNA-chromatin connector protein Chp1, which facilitates RNAi-dependent heterochromatin formation. Our findings reveal a function of the CTD code: linking ncRNA transcription to RNAi for heterochromatin formation.