Effects of in vivo injury on the neuromuscular junction in healthy and dystrophic muscles

Authors


R. Lovering: University of Maryland School of Medicine, Department of Orthopaedics, 100 Penn St, AHB, Room 540, Baltimore, MD 21201, USA. Email: rlovering@som.umaryland.edu

Key points

  • • Strength loss induced by lengthening contractions is typically attributed to damaged force-bearing structures within skeletal muscle. Muscle lacking the structural protein dystrophin, as in Duchenne muscular dystrophy, is particularly susceptible to contraction-induced injury.
  • • We tested the hypothesis that changes in neuromuscular junctions (NMJs) contribute to strength loss following lengthening contractions in wild-type and in dystrophic skeletal muscle.
  • • NMJs in dystrophic (mdx) mice, the murine model of Duchenne muscular dystrophy, show discontinuous and dispersed motor end-plate morphology. Following lengthening contractions, mdx quadriceps muscles show a greater loss in force, increased neuromuscular transmission failure and decreased electromyographic measures compared to wild-type.
  • • Consistent with NMJ disruption as a mechanism contributing to this force loss, only mdx showed increased motor end-plate discontinuity and dispersion of acetylcholine receptor aggregates.
  • • Our results indicate that the NMJ in mdx muscle is particularly susceptible to damage, and might play a role in the exacerbated response to injury in dystrophic muscles.

Abstract  The most common and severe form of muscular dystrophy is Duchenne muscular dystrophy (DMD), a disorder caused by the absence of dystrophin, a structural protein found on the cytoplasmic surface of the sarcolemma of striated muscle fibres. Considerable attention has been dedicated to studying myofibre damage and muscle plasticity, but there is little information to determine if damage from contraction-induced injury occurs at or near the nerve terminal axon. We used α-bungarotoxin to compare neuromuscular junction (NMJ) morphology in healthy (wild-type, WT) and dystrophic (mdx) mouse quadriceps muscles and evaluated transcript levels of the post-synaptic muscle-specific kinase signalling complex. Our focus was to study changes in NMJs after injury induced with an established in vivo animal injury model. Neuromuscular transmission, electromyography (EMG), and NMJ morphology were assessed 24 h after injury. In non-injured muscle, muscle-specific kinase expression was significantly decreased in mdx compared to WT. Injury resulted in a significant loss of maximal torque in WT (39 ± 6%) and mdx (76 ± 8%) quadriceps, but significant changes in NMJ morphology, neuromuscular transmission and EMG data were found only in mdx following injury. Compared with WT mice, motor end-plates of mdx mice demonstrated less continuous morphology, more disperse acetylcholine receptor aggregates and increased number of individual acetylcholine receptor clusters, an effect that was exacerbated following injury. Neuromuscular transmission failure increased and the EMG measures decreased after injury in mdx mice only. The data show that eccentric contraction-induced injury causes morphological and functional changes to the NMJs in mdx skeletal muscle, which may play a role in excitation–contraction coupling failure and progression of the dystrophic process.

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