Abstract

Tight regulation of RNA metabolism is essential for normal brain function. This includes co and post-transcriptional regulation, which are extremely prevalent in neurons. Recently, circular RNAs (circRNAs), a highly abundant new type of regulatory non-coding RNA have been found across the animal kingdom. Two of these RNAs have been shown to act as miRNA sponges but no function is known for the thousands of other circRNAs, indicating the existence of a widespread layer of previously unknown gene regulation.

Here we show that circRNAs are generated co-transcriptionally. We demonstrate that circularization and linear splicing compete against each other and that these mechanisms are tissue-specific and conserved from flies to humans. The second exon of the splicing factor muscleblind (MBL/MBNL1) is circularized in flies and humans. This circRNA (circMbl) and its flanking introns contain conserved muscleblind binding sites, which are strongly and specifically bound by MBL . Moreover, modulation of MBL levels strongly affects the biosynthesis of circMbl and this effect is dependent on the MBL binding sites. In addition, we show that circRNAs are highly expressed in the fly brain and their host genes are strongly enriched in genes with synaptic functions. By manipulating the levels of specific circRNAs in the brain we are determining functions of these new types of molecules at the physiological and behavioral levels. Last but not least we show that temperature dynamically alters the levels of circRNAs in the fly neural tissue. Our preliminary results indicate that this effect is mediated by editing by the enzyme dADAR. In sum, here we provide evidence for the mechanisms generating circRNAs and we perform the first systematic screening for circRNA function.