Age-dependent patterns of microRNA RISC loading

sequence-specific posttranscriptional regulators of gene expression, acting via RNA-induced silencing complexes (RISCs). They are termed small interfering RNAs (siRNAs) and microRNAs (miRNAs). Despite initial discovery from unrelated studies, these RNA classes are related in their biogenesis and assembly into RISC RNA–protein complexes, and are able to regulate gene transcripts negatively in diverse eukaryotes miRNAs are small RNA molecules of 20–24 nucleotides in length generated from endogenous transcripts and produced by Dicer, a double-stranded RNA-specific enzyme of the RNAse III family. They are typically loaded into the Ago1-RISC complex, where they trigger the silencing of complementary mRNA targets, acting through translational repression and mRNA cleavage. Following the discovery of the first miRNA, lin-4, in the nematode Canaerobditis elegans, hundreds of miRNAs have now been identified across multiple species, from plants to humans. miRNAs affect a multitude of biological processes at different stages, from development to aging. Their involvement in aging was initially measured by related changes in abundance. Coincidentally, lin-4 in C. elegans was later found to play a critical role, like miR-34 in Drosophila, in organismal and brain aging 482:519-523). siRNAs are also small RNAs of 21–25 nucleotides in length produced by Dicer. siRNAs are loaded into Ago2-RISC and trigger the silencing of their complementary RNA targets, classically functioning in the antiviral response. Small RNAs loaded into Ago1-RISC and Ago2-RISC differ in modification: small RNAs loaded into Ago1-RISC remain unmodified whereas those loaded into Ago2-RISC undergo 2'-O-methylation at the 3' terminal ribose (Czech et al. It also plays a protective role in animals, as loss of 2'-O-methylation leads to destabilization, tailing and trimming of siRNAs Previous studies on the role of Drosophila miR-34 in aging and age-associated brain degeneration highlighted an intriguing pattern: miR-34 isoforms of different lengths were seen, with only the short isoform accumulating with age in the brain (Liu et al., Nature, 2012, 482:519-523). These forms are generated by a novel 3'-to-5' exonuclease, Nibbler, which trims miR-34 as well as a number of other miRNAs, generating a diversity of isoforms differing by length at the 3'end The finding of a pattern of miR-34 isoforms with age raised the potential importance of regulation of miRNA length, in addition to miRNA abundance. We therefore explored the diversity of miRNA length and isoform pattern with age. By examining a number of miRNAs that present with a diversity of isoform lengths, we unexpectedly found a diversity of isoform patterns with age. …