Validation of circRNAs and their expression during mouse brain development and neuronal maturation

Circular RNAs (circRNAs) are a new type of mostly non-coding RNA species that unlike linear RNAs form covalently closed, continuous loops and have recently been shown to be highly abundant, evolutionarily conserved and relatively stable in the cytoplasm1. circRNAs that are formed by backsplicing events can consist of one or more exonic and/or intronic sequences and are mainly involved in gene regulating processes. The key-function of the circRNAs that has been proven until now is their activity as miRNA-sponges2, 3, 7. Genome-wide screenings of circRNA expression have shown that they are enriched in the mammalian brain, and dynamically expressed during in vitro neuronal differentiation and in vivo brain development1. Strikingly, we noticed similar features also in the radiation responsive genes that had been identified in a previous study done in embryonic mouse brains. These radiation responsive genes are also generally highly induced during in vivo brain development and in vitro neuronal differentiation4, and most intriguingly, we learned that many of these genes are predicted to express circRNAs. In this study, we aimed at validating the expression of the circular isoforms of two of those genes, Pvt1 and Ano3, and at the characterization of their expression profiles in comparison to their cognate linear mRNAs. We successfully validated the expression of stable circRNAs for both Pvt1 and Ano3, and found them to be enriched in the mouse brain. Moreover, we observed an induction of the circular transcripts (of both genes) during neuronal maturation and brain development, and found very first indications of radiation-responsive circRNAs. Based on our results, we suspect that the circular transcripts in particular may be deeply involved in brain development related matters. The deviating and dynamically regulated expression profiles of the circRNAs from their cognate mRNAs in any case suggest that they play a unique and significant role in cellular functioning.