Circular RNAs-one of the enigmas of the brain.

Circular RNAs (circRNAs) provide a new and relatively unexplored class of noncoding RNAs that are predominantly found in mammalian cells. In this review, we present the latest data regarding the structural organization, possible mechanisms of synthesis, and functions of circRNAs. These transcripts were isolated as an RNA fraction that was resistant to RNase R treatment, which selectively destroys the linear forms of RNA molecules. circRNAs are encoded by orthologous genes in different organisms and show tissue- and organ-specific expression. Currently, the biogenesis and functional properties of circRNAs remain unclear; transcripts of this class, however, remain highly promising targets of research. Some of them have been ascribed the function of "molecular sponges" that can absorb microRNAs, RNA-binding proteins, and small nuclear RNAs. circRNAs are often formed from the RNA portions of protein-coding genes in the course of alternative splicing. Some features of the circRNAs of mammals were demonstrated using 11 circRNAs of the human sphingomyelin synthase 1 gene (SGMS1), which were discovered by us in the brain. These circRNAs consist mainly of portions of the multi-exon 5' untranslated region (5'UTR) of the SGMS1 gene and include one to five exons. The synthesis of circRNAs may be new, previously unknown, function of the multi-exon 5'UTR of genes. This feature is most clearly manifested in the brain, where the level of circRNAs is significantly higher.