MBNL splicing factors regulate the microtranscriptome of skeletal muscles.

Muscleblind like splicing regulators (MBNLs) govern various RNA-processing steps, including alternative splicing, polyadenylation, RNA stability and mRNA intracellular localization. In myotonic dystrophy type 1 (DM1), the most common muscular dystrophy in adults, MBNLs are sequestered on toxic RNA containing expanded CUG repeats, which leads to disruption of MBNL-regulated processes and disease features of DM1. Herein, we show the significance of MBNLs in regulating microtranscriptome dynamics during the postnatal development of skeletal muscles and in microRNA (miRNA) misregulation observed in mouse models and patients with DM1. We identify multiple miRNAs sensitive to MBNL proteins insufficiency and reveal that many of them were postnatally regulated, which correlates with increases in the activity of these proteins during this process. In adult Mbnl1-knockout mice, miRNA expression exhibited an adult-to-newborn shift. We hypothesize that Mbnl1 deficiency influences miRNA levels through a combination of mechanisms. First, the absence of Mbnl1 protein results in alterations to the levels of pri-miRNAs. Second, MBNLs affect miRNA biogenesis by regulating the alternative splicing of miRNA primary transcripts. We propose that the expression of miR-23b, miR-27b and miR-24-1, produced from the same cluster, depends on the MBNL-sensitive inclusion of alternative exons containing miRNA sequences. Our findings suggest that MBNL sequestration in DM1 is partially responsible for altered miRNA activity. This study provides new insights into the biological roles and functions of MBNL proteins as regulators of miRNA expression in skeletal muscles.