Investigating the role of RNA Polymerase III and the Unfolded Protein Response

Over time, the transcriptome of eukaryotes ranging from yeast to humans has been shown to become dysregulated, and ensuing changes in gene expression have been linked to several hallmarks of ageing, such as mitochondrial dysfunction, altered intracellular communication, and deregulated nutrient sensing. Three RNA polymerases generate the transcripts that make up the transcriptome, with each polymerase synthesising specific RNA products. RNA Polymerase III (Pol III) synthesises various non-coding RNAs: tRNAs and 5S rRNAs, which are fundamental components of the translation machinery, and other small RNAs such as snoRNA, SINEs, 7SL RNA, Y RNA, and U6 spliceosomal RNA that are involved in RNA processing and regulatory functions. Pol III transcription is regulated by cell cycle stage, nutrient availability, and cell stress. Dysregulation of Pol III is implicated in several pathological states, including Alzheimer's, Fragile X syndrome and many forms of cancer. Pol III inhibition has recently been shown to extend lifespan in yeast, worms and flies. However, the mechanism underpinning longevity is not known. The unfolded protein response (UPR) is a signalling system that detects misfolded proteins within the endoplasmic reticulum (ER) and coordinates a cellular response to restore protein homeostasis. Three key UPR signal activators-IRE1, PERK and ATF6-give rise to separate branches of the response. All the branches are conserved in the soil-dwelling nematode Caenorhabditis elegans (C. elegans) and have been shown to respond to elevated levels of misfolded proteins. This report explores the relationship between Pol III and the UPR in C. elegans. Using mutant strains, RNAi and microscopy, it was found that Pol III inhibition induced levels of the protein chaperone HSP-4 (mammalian BiP). This activation of the UPR was found to be specific to the ER and dependent on downstream transcription factors XBP-1 and ATF-4. Also explored in this report is how Pol III targets-tRNA synthetases-control the UPR-ER.

The ability to activate the UPR-ER declines with age, and the inability to ensure proper folding of proteins has been implicated as a contributing factor to the ageing process. Linking RNA Pol III and the UPR-ER uncovers a novel regulatory mechanism that may have implications for age-related diseases, particularly those involving protein misfolding.