Ribonuclear foci at the neuromuscular junction in myotonic dystrophy type 1

In myotonic dystrophy type 1 (DM1) the muscle fibers express RNA containing an expanded CUG repeat (CUGexp). The CUGexp RNA is retained in the nucleus, forming ribonuclear foci. Splicing factors in the muscleblind (MBNL) family are sequestered in ribonuclear foci, resulting in abnormal regulation of alternative splicing. In extrajunctional nuclei, these effects on splicing regulation lead to reduced chloride conductance and altered insulin receptor signaling. Here we show that CUGexp RNA is also expressed in subsynaptic nuclei of muscle fibers and in motor neurons in DM1, causing sequestration of MBNL1 protein in both locations. In a transgenic mouse model, expression of CUGexp RNA at high levels in extrajunctional nuclei replicates many features of DM1, but the toxic RNA is poorly expressed in subsynaptic nuclei and the mice fail to develop denervation-like features of DM1 myopathology. Our findings indicate that subsynaptic nuclei and motor neurons are at risk for DM1-induced spliceopathy, which may affect function or stability of the neuromuscular junction.