MBNL splicing factors regulate the microtranscriptome of skeletal muscles (human, skeleta muscle, DM1 and non-DM)
Muscleblind-like proteins (MBNLs) regulate 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 showed the significance of MBNLs in the regulation of microtranscriptome dynamics during postnatal development of skeletal muscles and in microRNA (miRNA) misregulation observed in mouse models and patients with DM1. We identified multiple miRNAs sensitive to insufficiency of MBNL proteins and revealed that many of them were postnatally regulated, which was correlated 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 identified two mechanisms through which MBNLs influence miRNA levels. First, MBNL loss induces transcriptional changes in miRNA precursors. 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. Overall design: MicroRNA (miRNA) sequencing from skeletal muscles of DM1 (n=3) and non-DM (n=2) individuals