The varied roles of nuclear argonaute-small RNA complexes and avenues for therapy

Abstract

Argonautes are highly conserved proteins found in almost all eukaryotes and some bacteria and archaea. In humans, there are eight argonaute proteins evenly distributed across two clades, the Ago clade (AGO1-4) and the Piwi clade (PIWIL1-4). The function of Ago proteins is best characterized by their role in RNA interference (RNAi) and cytoplasmic post-transcriptional gene silencing (PTGS) - which involves the loading of siRNA or miRNA into argonaute to direct silencing of genes at the posttranscriptional or translational level. However, nuclear-localized, as opposed to cytoplasmic, argonaute-small RNA complexes may also orchestrate the mechanistically very different process of transcriptional gene silencing, which results in prevention of transcription from a gene locus by the formation of silent chromatin domains. More recently, the role of argonaute in other aspects of epigenetic regulation of chromatin, alternative splicing and DNA repair is emerging. This review focuses on the activity of nuclear-localized short RNA-argonaute complexes in a mammalian setting and discusses recent in vivo studies employing nuclear-directed sRNA for therapeutic interventions. These studies heed the potential development of RNA-based drugs which induce epigenetic changes in the cell.

Figure 1

The role of argonaute in transcriptional gene silencing (TGS) and activation (RNAa). (a) The dicer-generated small RNA (sRNA) species which are loaded into argonaute (gray box) can be derived from shRNA expressed from plasmids or viral vectors transfected into the cell, or from endogenous processes, such as miRNA biogenesis or dsRNA resulting from bivalent transcription. Alternatively, siRNA directly transfected into the cell may be loaded. The sRNA species have a single “guide” strand loaded into argonaute while the other “passenger” strand is degraded. (b) Loaded Ago1 (Ago1+sRNA) may enter the nucleus and together with a nuclear RISC (nucRISC) and in association with active RNAPII and sense transcripts, initiate chromatin remodelling and transcriptional gene silencing (TGS) by such enzymes as histone 3 lysine 9 methyltransferases (H3K9 HMTase). The loaded Ago2 (Ago2+sRNA) is catalytically active and may direct the silencing of mature mRNA in the cytoplasm via the post-transcriptional gene silencing (PTGS) pathway, or it may be migrate to the periphery of the nucleus and (perhaps in association with a nucRISC), degrade primarily antisense transcripts. This process is associated with remodeling to active chromatin by histone 3 lysine 4 histone methyltransferases (H3K4 HMTase) and subsequent transcriptional gene activation (RNAa). There is also evidence nuclear Ago2 is associated with TGS.

Figure 2

Required and enriched factors for transcriptional gene silencing and activation.