Circular RNAs (circRNAs) are for the most part generated by back-splicing of exons in protein-coding genes. They have been discovered in many species, but only few have been functionally characterized. Weigelt et al. now find that the circRNA generated by the sulfateless gene (circSfl) controls the lifespan of flies.

The nutrient-sensing insulin/insulin-like growth factor signalling pathway is a key regulator of metabolism and ageing. As inhibition of this pathway has been shown to extend lifespan, the authors went on to analyse the links between insulin-mediated lifespan extension and circRNAs.

Credit: V. Summersby/Springer Nature Limited

Whereas global circRNA expression increased with age in brain tissue of wild-type Drosophila melanogaster, the increase was significantly lower in the brain of long-lived dilp 2–3,5 mutant flies (which are defective in the expression of insulin-like peptides). This observation suggests that ageing-dependent accumulation of circRNAs is slowed down by reduced insulin signalling. The authors focused on characterizing circSfl, as it was strongly upregulated in dilp 2–3,5 mutant flies and other long-lived insulin mutants and is regulated independently of its cognate gene.

When experimentally overexpressed in wild-type flies, the ubiquitous expression of circSfl led to up to 15% increase in median lifespan. The generation of circSfl was linked with the expression of one of two alternative splicing isoforms of the cognate, linear mRNA (Sfl RA), and when expression of circSfl and Sfl RA were reduced in insulin-mutant flies, the effect on lifespan was abolished. Thus, the insulin-mediated lifespan extension requires circSfl and Sfl RA.

circSfl was found to be translated in vivo, and circSfl protein levels increased significantly in dilp 2–3,5 mutants. Moreover, overexpression of the circSfl ORF from a linear transcript extended lifespan of female flies, together indicating that the protein encoded by circSfl is sufficient to increase lifespan. The sfl linear transcripts encode a Golgi enzyme that catalyses the synthesis of heparan sulfate, whereas the circSfl protein is a truncated version that lacks catalytic activity. Interestingly, the full length protein induced lifespan extension only if it was overexpressed specifically in neurons, suggesting that the two proteins might have both common and unique roles in ageing.

This study shows that increased expression of circSfl and linear sfl, and of the proteins they encode, can extend lifespan in flies. It will be interesting to elucidate the mechanisms leading to longer lifespan and how the different transcripts and proteins interact.