Two parallel sRNA amplification cycles contribute to RNAi inheritance in C. elegans

Abstract

RNA-mediated interference (RNAi) is a conserved mechanism that uses small RNAs (sRNAs) to tune gene expression. In C. elegans, exposure to dsRNA induces the production of gene-specific sRNAs that are propagated to progeny not exposed to the dsRNA trigger. We present evidence that RNAi inheritance is mediated by two parallel sRNA amplification loops. The first loop, dependent on the nuclear Argonaute HRDE-1, targets nascent transcripts, and reduces but does not eliminate productive transcription at the locus. The second loop, dependent on the conserved helicase ZNFX-1, targets mature transcripts and concentrates them in perinuclear condensates (nuage). Each amplification loop generates a distinct class of sRNAs, with the ZNFX-1 loop responsible for the bulk of sRNA production on the region targeted by the trigger. By independently targeting nascent and mature transcripts, the HRDE-1 and ZNFX-1 loops ensure maximum silencing in progeny not exposed to the trigger.

Intergenerational transport of double-stranded RNA limits heritable epigenetic changes

“…1D), likely due to reduction of mCherry::h2b::gfp::h2b pre-mRNA ( 28 ). The pattern of mCherry::h2b silencing is similar to the spatial pattern observed for the RME-2-dependent entry of dsRNA delivered into the parental circulation ( 25 ) and is consistent with the pattern of mRNA degradation in the germline by extracellular dsRNA ( 29 ).…”

Inheritance and maintenance of small RNA‐mediated epigenetic effects

“…The Z granule contains two factors, ZNFX-1, and the Argonaute WAGO-4, which are essential for the inheritance of post-transcriptional silencing effects in the embryo, complementing the nuclear function Argonaute HRDE-1, which transmit heritable transcriptional silencing. [34,49] The mutator foci contain proteins involved in small RNA synthesis, including the RdRP and factors involved in RNA pUGylation, an RNA substrate that recruits RdRP to generate small RNAs essential for heritable RNAi. [44,45,50] Therefore, besides being a vehicle for maternal small RNA inheritance in the zygote, germ granules can also be necessary for reinitiating the de novo synthesis of small RNAs in embryos (Figure 1).…”