| Abstract |
SUMOylation (the attachment of a small ubiquitin related modifier (SUMO) to a target protein) is employed by the cell to govern diverse and vital processes. The consequences of SUMOylation vary from changing the localization of proteins within the cell, through influencing protein-protein interactions to determining protein stability. The function of SUMO has already been implicated in DNA damage response, cell division, development and carcinogenesis, among others. However, our understanding of SUMOylation in relation to ageing remains elusive. Here we show that the amount of SUMOylated proteins is increasing in C. elegans during ageing. Reducing the expression of the sole SUMO protein (SMO-1) by RNAi, leads to a shortened lifespan, while a modest overexpression of SUMO extends the life expectancy of C. elegans. Furthermore, we find that the shortened lifespan is conveyed via the intestine and the nervous system. The regulation of lifespan is achieved via conserved signaling pathways, and there are several main transcription factors, like DAF-16/FOXO, SKN-1/NRF and HSF-1 which influence the process of ageing. The SUMOylated status of HSF-1 and its inhibitory function has been already demonstrated. In silico analysis predicted putative SUMOylation sites in the other two transcription factors, DAF-16 and SKN-1, as well. We find that the lifespan influencing effect of SUMOylation is achieved via DAF-16 and SKN-1. In order to show that SUMOylation affects the transcriptional activity of DAF-16 and SKN-1, we analyzed the expression of their target genes, using reporter constructs, in animals subjected to smo-1 RNAi and we found an increased expression under normal conditions. Moreover, in a SMO-1 overexpressing background, the mRNA levels of DAF-16 target genes are significantly reduced. Intriguingly, this change in the mRNA levels of SKN-1 target genes did not happen in the same genetic background. Using transgenic lines, we were able to observe co-localization of DAF-16 and SMO-1 in the nucleus. Furthermore, SMO-1 overexpression confers stress resistance; these animals survived longer under oxidative and heat stress conditions due to their ability to mount a more robust stress response in early adulthood compared to wild type. Besides their role as master regulators of lifespan, DAF-16 and SKN-1 regulate mitochondrial homeostasis as well. Interestingly, reduced smo-1 expression causes the preservation of interconnected mitochondrial network in the intestine of aged animals and this phenotype is DAF-16 dependent. Moreover, we find that SUMOylation is required for the fission of mitochondria. As a consequence, the process of mitophagy is blocked upon smo-1(RNAi). Additionally, smo-1(RNAi) treated animals have more active mitochondria and they produce more ROS as well in a DAF-16 dependent manner. At the same time, animals with reduced expression of smo-1 display no change in their mitochondrial DNA copy number compared to control levels. We conclude that SUMOylation is required for healthy ageing and maintenance of mitochondrial homeostasis.
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