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Multiple signaling pathways have been reported to be altered in Myotonic Dystrophy type 1 (DM1) skeletal muscle, contributing to pathogenicity. In particular, previous work established that AMPK signaling, a key sensor of energy metabolism, is repressed in DM1 mouse muscle and that activating AMPK through exercise and/or with pharmacological activators is beneficial for the DM1 muscle phenotype. Here, we explored the effects of a newer, more specific allosteric AMPK activator acting directly on AMPK. We treated male and female HSALR mice for 1 and 4 weeks with a daily injection of the allosteric activator MK‐8722, the AMP mimetic AICAR, or vehicle. Our results show that 1 and 4 weeks of treatment with MK‐8722 improves alternative splicing toward wild‐type levels in male and female HSALR muscle. However, the effects of MK‐8722 were more modest compared to AICAR. In contrast, 4 weeks of treatment with MK‐8722 improved muscle histology to a greater extent than AICAR. As expected with AMPK activation, 4 weeks of treatment with MK‐8722 and AICAR promoted the expression of slower, more oxidative fibers. Finally, acute injections of MK‐8722 and AICAR triggered the rapid and transient increase in phospho‐AMPK in muscle. However, the peak of AMPK phosphorylation was lower with MK‐8722 compared to AICAR, thereby explaining the more modest effects of AMPK allosteric activation. Altogether, our data demonstrate that chronic activation of AMPK with specific pharmacological activators is beneficial for the DM1 muscle. They further indicate that at least a portion of the beneficial effects seen following the administration of these drugs occurs through AMPK. [ABSTRACT FROM AUTHOR]

The localized transcription of several muscle genes at the motor endplate is controlled by the Ets transcription factor GABP. To evaluate directly its contribution to the formation of the neuromuscular junction, we generated transgenic mice expressing a general Ets dominant-negative mutant specifically in skeletal muscle. Quantitative RT-PCR analysis demonstrated that the expression of genes containing an Ets-binding site was severely affected in the mutant mice. Conversely, the expression of other synaptic genes, including MuSK and Rapsyn, was unchanged. In these animals, muscles expressing the mutant transcription factor developed normally, but examination of the post-synaptic morphology revealed marked alterations of both the primary gutters and secondary folds of the neuro-muscular junction. Our results demonstrate that Ets transcription factors are crucial for the normal formation of the neuro-muscular junction. They further show that Ets-independent mechanisms control the synaptic expression of a distinct set of synaptic genes. [ABSTRACT FROM AUTHOR]

Spinal muscular atrophy (SMA) is characterized by the loss of alpha motor neurons in the spinal cord and a progressive muscle weakness and atrophy. SMA is caused by loss-of-function mutations and/or deletions in the survival of motor neuron (SMN) gene. The role of SMN in motor neurons has been extensively studied, but its function and the consequences of its loss in muscle have also emerged as a key aspect of SMA pathology. In this study, we explore the molecular mechanisms involved in muscle defects in SMA. First, we show in C2C12 myoblasts, that arginine methylation by CARM1 controls myogenic differentiation. More specifically, the methylation of HuR on K217 regulates HuR levels and subcellular localization during myogenic differentiation, and the formation of myotubes. Furthermore, we demonstrate that SMN and HuR interact in C2C12 myoblasts. Interestingly, the SMA-causing E134K point mutation within the SMN Tudor domain, and CARM1 depletion, modulate the SMN-HuR interaction. In addition, using the Smn2B/- mouse model, we report that CARM1 levels are markedly increased in SMA muscles and that HuR fails to properly respond to muscle denervation, thereby affecting the regulation of its mRNA targets. Altogether, our results show a novel CARM1-HuR axis in the regulation of muscle differentiation and plasticity as well as in the aberrant regulation of this axis caused by the absence of SMN in SMA muscle. With the recent developments of therapeutics targeting motor neurons, this study further indicates the need for more global therapeutic approaches for SMA., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Congenital myasthenic syndromes (CMS) are rare genetic diseases affecting the neuromuscular junction (NMJ) and are characterized by a dysfunction of the neurotransmission. They are heterogeneous at their pathophysiological level and can be classified in three categories according to their presynaptic, synaptic and postsynaptic origins. We report here the first case of a human neuromuscular transmission dysfunction due to mutations in the gene encoding a postsynaptic molecule, the muscle-specific receptor tyrosine kinase (MuSK). Gene analysis identified two heteroallelic mutations, a frameshift mutation (c.220insC) and a missense mutation (V790M). The muscle biopsy showed dramatic pre- and postsynaptic structural abnormalities of the neuromuscular junction and severe decrease in acetylcholine receptor (AChR) epsilon-subunit and MuSK expression. In vitro and in vivo expression experiments were performed using mutant MuSK reproducing the human mutations. The frameshift mutation led to the absence of MuSK expression. The missense mutation did not affect MuSK catalytic kinase activity but diminished expression and stability of MuSK leading to decreased agrin-dependent AChR aggregation, a critical step in the formation of the neuromuscular junction. In electroporated mouse muscle, overexpression of the missense mutation induced, within a week, a phenotype similar to the patient muscle biopsy: a severe decrease in synaptic AChR and an aberrant axonal outgrowth. These results strongly suggest that the missense mutation, in the presence of a null mutation on the other allele, is responsible for the dramatic synaptic changes observed in the patient.