Biology subjects

Belew, M. D.

Publications and source records attributed to Belew, M. D..

4 recordsLinked to original sources

bioRxiv2025

A role for the nuclear RNAi pathway in global transcriptional silencing in C. elegans primordial germ cells

In C. elegans, if embryos hatch into an environment lacking nutrients the primordial germ cells (PGCs) will arrest mRNA transcription and enter quiescence until food becomes available. Our previous work had shown that the global transcriptional silencing (GTS) that occurs in PGCs during L1 starvation requires a "hyperdeposition" of the repressive histone mark H3K9me3 in germline chromatin. Hyperdeposition is defined as an increase in the amount of H3K9me3 embedded in germline chromatin, relative to neighboring somatic nuclei. Hyperdeposition initiates in embryonic PGCs and is maintained in starved L1s. Here, we show that the nuclear RNAi pathway contributes to both the initiation and maintenance of H3K9me3 hyperdeposition. Interestingly, both known nuclear Argonautes, HRDE-1 and NRDE-3, play a role in hyperdeposition and they do so in a sequential manner. We show that HRDE-1 acts in embryonic PGCs to initiate hyperdeposition and then, at the embryo-to-L1 transition, HRDE-1 becomes dispensable and NRDE-3 is required to maintain H3K9me3 levels, and for GTS. We also examine the timing of H3K9me3 hyperdeposition and find that it initiates soon after germline zygotic genome activation (ZGA) occurs. Our data suggest a model where ZGA promotes both gene expression and H3K9me3 deposition at active loci, and under starvation conditions these H3K9me3 marks are then employed to silence the germline genome.

developmental biology↗

bioRxiv2024

Transcriptome analysis in C. elegans early embryos upon depletion of the Topoisomerase 2/condensin II axis.

In C. elegans, the chromosome compaction factors topoisomerase 2 (TOP-2) and condensin II have been shown to globally repress transcription in multiple contexts. Our group has previously reported that TOP-2 and condensin II repress transcription in the C. elegans germline during larval starvation, oocyte maturation, and in germline progenitor cells of the early embryo. Here, we assess the transcriptome of early embryos treated with RNAi against TOP-2 and the condensin II subunit CAPG-2. We found 144 upregulated and 172 downregulated genes. Further analysis showed that the upregulated genes are mostly somatic, with a host of neuronal cells present in our tissue enrichment analysis.

developmental biology↗

bioRxiv2022

The TOP-2-condensin II axis silences transcription during germline specification in C. elegans

In C. elegans, the germline is specified via a preformation mechanism that relies on the PIE-1 proteins ability to globally silence mRNA transcription in germline precursor cells, also known as the P-lineage. Recent work from our group has identified additional genome silencing events in C. elegans during oogenesis and in starved L1 larvae, and these require the condensin II complex, topoisomerase II (TOP-2), and components of the H3K9me/heterochromatin pathway. Interestingly, silencing in oocytes also requires PIE-1, but this is not the case in starved L1s. Here, we ask if additional genome silencing components besides PIE-1 are required to repress gene expression in the P-lineage of early embryos, and we find that condensin II and TOP-2 are required and the H3K9me/heterochromatin pathway is not. We show that depletion of condensin II/TOP-2 activates the normally suppressed RNA polymerase II to inappropriately transcribe somatic genes in the P-lineage. We also present evidence that while both PIE-1 and condensin II/TOP-2 are required for genome silencing in the P-lineage, PIE-1 can silence transcription independently of condensin II/TOP-2 when misexpressed in somatic cells. Thus, in oocytes, all three genome silencing systems (TOP-2/condensin II, H3K9me, and PIE-1) are operational while in both early embryos and starved L1s two of the three are active (TOP- 2/condensin II and PIE-1 for early embryos, TOP-2/condensin II and H3K9me for starved L1s). Our data show that multiple, redundantly acting genome silencing mechanisms act in a mix and match manner to repress transcription at different developmental stages in the C. elegans germline.

developmental biology↗

bioRxiv2022

The global chromatin compaction pathway silences transcription during meiotic prophase in C. elegans

Abstract/SummaryWhile 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.

developmental biology↗