Summary
Circular RNAs (CircRNAs) are a newly appreciated class of RNAs that lack free 5´ and 3´ ends, are expressed by the thousands in diverse forms of life, and are mostly of enigmatic function. Ostensibly due to their resistance to exonucleases, circRNAs are known to be exceptionally stable. Here, we examined the global profile of circRNAs in C. elegans during aging by performing ribo-depleted total RNA-seq from the fourth larval stage (L4) through 10-day old adults. Using stringent bioinformatic criteria and experimental validation, we annotated 1,166 circRNAs, including 575 newly discovered circRNAs. These circRNAs were derived from 797 genes with diverse functions, including genes involved in the determination of lifespan. A massive accumulation of circRNAs during aging was uncovered. Many hundreds of circRNAs were significantly increased among the aging time-points and increases of select circRNAs by over 40-fold during aging were quantified by qRT-PCR. The age-accumulation of circRNAs was not accompanied by increased expression of linear RNAs from the same host genes. We attribute the global scale of circRNA age-accumulation to the high composition of postmitotic cells in adult C. elegans, coupled with the high resistance of circRNAs to decay. These findings suggest that the exceptional stability of circRNAs might explain age-accumulation trends observed from neural tissues of other organisms, which also have a high composition of post-mitotic cells. Given the suitability of C. elegans for aging research, it is now poised as an excellent model system to determine if there are functional consequences of circRNA accumulation during aging.
