Past, present, and future of circRNAs

Abstract

Exonic circular RNAs (circRNAs) are covalently closed RNA molecules generated by a process named back-splicing. circRNAs are highly abundant in eukaryotes, and many of them are evolutionary conserved. In metazoans, circular RNAs are expressed in a tissue-specific manner, are highly stable, and accumulate with age in neural tissues. circRNA biogenesis can regulate the production of the linear RNA counterpart in cis as back-splicing competes with linear splicing. Recent reports also demonstrate functions for some circRNAs in trans: Certain circRNAs interact with microRNAs, some are translated, and circRNAs have been shown to regulate immune responses and behavior. Here, we review current knowledge about animal circRNAs and summarize new insights into potential circRNA functions, concepts of their origin, and possible future directions in the field.

Keywords: RNA processing; circRNAs; circular RNAs; non-coding RNAs; splicing.

Figure 1. circRNAs at a glance

circRNA can be generated either with the help of reverse complementary repeats or RNA‐binding proteins and exported from the nucleus (1). In the cytoplasm, the circRNA might be bound by multiple factors. These can be RNA‐binding proteins (2), Argonaute proteins loaded with miRNAs as sponge or scaffold (3) or for direct degradation (5), ribosomes (4) or endonucleases that would cause degradation of the circRNA (6). From the non‐degradative binding, the circRNA‐factor complex might diffuse in the cytoplasm or been actively transported in into particular regions of the cell [e.g., the synapse (7)] where it can release its bound cargo or starts to be translated. The enclosure of circRNAs or circRNA factor complexes in vesicle that would be released into the extracellular space would remove circRNAs from the cytoplasm (8). However, protected by the vesicle, the circRNAs or circRNA complexes could reach other cells or tissues and therefore act as messenger molecules or fulfill other unknown functions (9).

Figure 2. Multiple levels of circRNAs’ evolutionary conservation

To determine the level of conservation of a circRNA between two or more organisms, we offer to ask several general questions: 1. Are circRNAs present in both organisms? 2. Is a circRNA produced from the same (homolog or orthologue) gene in both organisms? 3. Is a circRNA produced from the same exon(s)? 4. Are the potential features conserved? This would imply the potential presence of an RBP and microRNA‐binding sites, presence of IRES(s) and translational stop(s). 5. Is a potential 3D structure conserved? 6. If the circRNA is translated, is also the peptide sequence homolog?