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The role of mRNA-processing bodies in ageing

Final Report Summary - PAGE (The role of mRNA-processing bodies in ageing)

Ageing is the process of time-dependent deterioration of biological systems. Continuous changes that take place with ageing at the molecular and cellular levels drive the loss of tissue physiology and anatomy, with a devastating impact on an organism’s healthy lifespan. In support to this, direct alterations in primary regulators of gene expression and cellular metabolism can greatly affect (increase or decrease) the lifespan of model organisms. Important regulators of gene expression and cellular metabolism, in all eukaryotes, are specific RNA-protein complexes (RNPs) that control the abundance and translation of mRNAs in the cytoplasm, under both normal and stress conditions. However, the ways in which ageing process affects the profile and function of such RNPs in somatic cells as well as how alterations in RNP-related components affect the ageing process remained a mystery for many years. We addressed these questions using as a model system the nematode C. elegans, the best tool for the genetic analysis of longevity. The main achievements of the project are outlined below:
-Generation of transgenic worms carrying various fluorescent reporters of proteins that localize to mRNA Processing bodies (PBs) or to stress granules (SGs), the two major cytoplasmic RNPs regulating the subcellular localization, degradation, silence or storage of bulk mRNA under certain conditions. We described the expression pattern of the related genes and observed the dynamic formation of each RNP in response to cellular stress. We further monitored the accumulation of PBs with age, in all somatic tissues of worms. Surprisingly, there was no age-dependent accumulation of SGs, in spite of their formation under oxidative stress, suggesting that the mild stress accompanying ageing is not sufficient to induce SG formation. Similarly, alterations in mRNA degradation could induce the aggregation of PBs, but not SGs, in young animals; thus providing a possible explanation for the accumulation of PBs in aged animals.
-Disruption of PB-related genes by mutations resulted in slow development, reduced reproduction, short lifespan and impaired stress response, highlighting their conserved role for animal biology. In addition post-developmental RNA interference (RNAi) of gene expression confirmed that genes directly related to PBs modulate adult lifespan, separately from their developmental role. Loss of such genes could also shorten the long life of several mutants affecting well-known longevity pathways. In contrast, disruption of the examined SG-related genes had varied consequences on animal development and survival, suggesting diverse functions, not previously explored. Our work revealed the tissue- and stress-specific properties of SG components and expands our knowledge on the cellular roles of these RNPs in the regulation of mRNA metabolism. We further extended our investigation to the role of the above PB- and SG-associated factors in the progression of neurodegenerative diseases.
-Study of the GCN2 kinase signaling that controls translation of mRNAs during stress and affects, the formation of SGs in response to specific insults. We established the conserved function of worm GCN-2 kinase in eIF2α phosphorylation under amino acid limitation and revealed a novel function of GCN-2 in the ageing process, affecting the lifespan of nutrient-deprived worms. Moreover, we provided for the first time genetic evidence that link GCN-2 with TOR kinase, the other major nutrient-sensing pathway, in the dietary restriction (DR)-induced longevity and stress response.
Overall, the project explored a new field in the ageing biology, related to the function of cytoplasmic mRNA metabolism factors in lifespan determination and survival of the organism under adverse conditions. The outcomes lead to significant enhancement of our understanding of cellular responses to ageing and contribute to the central goal of current ageing research, namely to identify strategies that maximize the healthy lifespan of humans.