Caenorhabditis elegans RSD-2 and RSD-6 promote germ cell immortality by maintaining small interfering RNA populations

Abstract

Germ cells are maintained in a pristine non-aging state as they proliferate over generations. Here, we show that a novel function of the Caenorhabditis elegans RNA interference proteins RNAi spreading defective (RSD)-2 and RSD-6 is to promote germ cell immortality at high temperature. rsd mutants cultured at high temperatures became progressively sterile and displayed loss of small interfering RNAs (siRNAs) that target spermatogenesis genes, simple repeats, and transposons. Desilencing of spermatogenesis genes occurred in late-generation rsd mutants, although defective spermatogenesis was insufficient to explain the majority of sterility. Increased expression of repetitive loci occurred in both germ and somatic cells of lategeneration rsd mutant adults, suggesting that desilencing of many heterochromatic segments of the genome contributes to sterility. Nuclear RNAi defective (NRDE)-2 promotes nuclear silencing in response to exogenous double-stranded RNA, and our data imply that RSD-2, RSD-6, and NRDE-2 function in a common transgenerational nuclear silencing pathway that responds to endogenous siRNAs. We propose that RSD-2 and RSD-6 promote germ cell immortality at stressful temperatures by maintaining transgenerational epigenetic inheritance of endogenous siRNA populations that promote genome silencing.germ line | fertility | tudor domain protein

Movement of regulatory RNA between animal cells

“…For example, an early genetic screen looked for R NAi s preading d efective ( rsd ) genes under the premise that mutants defective in genes required for the spread of RNA to the germline would be capable of silencing in response to dsRNA injected into the germline but not in response to dsRNA ingested from the environment (Tijsterman et al, 2004). While this screen identified alleles of sid-1 (called rsd-8 in the study), it identified additional genes such as rsd-2 and rsd-6 that satisfy the screen criteria but were nevertheless later found to be required for the efficient execution of RNA silencing within cells (Han et al, 2008, Sakaguchi et al, 2014). One possible explanation for the earlier misclassification is that the dosage of dsRNA delivered by injection is far greater than that delivered by ingestion, which makes a mutant with a partial defect in the execution of RNA silencing appear to have a defect in the transport of RNA between cells.…”

Piwi mutant germ cells transmit a form of heritable stress that promotes longevity

“…Because RDE-4 promotes the biogenesis of endogenous small RNAs and rde-4 mutants have relatively minor overt phenotypes (11,40,41), we asked whether deficiency for rde-4 affects the lifespan of daf-2 mutants. We found that daf-2; rde-4 double mutant lifespan was not significantly different than that of daf-2 single mutant controls ( Fig 1A, S1 Table, p=1), indicating that the small RNA pathway that can be activated by DAF-16 in prg-1 mutants is dispensable for its function in adult longevity.…”

“…A second pathway that promotes germ cell immortality is a genomic silencing pathway where small RNAs associate with Argonaute proteins to promote heterochromatin formation (10)(11)(12). This pathway initiates with the perinuclear Argonaute protein PRG-1, the C. elegans ortholog of Piwi, which localizes to P granules of germ cells and interacts with thousands of 21 nucleotide piRNAs that scan the genome for viruses, transposons or foreign nucleic acids such as man-made transgenes (13)(14)(15)(16)(17)(18)(19).…”

Distinct nuclear and cytoplasmic machineries cooperatively promote the inheritance of RNAi in Caenorhabditis elegans

“…Inherited dsRNAs also spread between cells in the progeny to elicit a systemic effect. Consistent with this idea, RNAi spreading defective factors RSD‐2 and RSD‐6 have been found to promote genome silencing and maintain siRNA populations in progeny [Sakaguchi et al., ]. However, whether and how parentally acquired dsRNAs could engage in multigenerational RNAi inheritance requires further investigation because parentally acquired dsRNAs will be quickly diluted over generations.…”