Neuromuscular Junction Structure and Redox Status of Skeletal Muscle Fiber: Effect on Schwann Cells

Abstract

Pre‐synaptic nerve terminal and post‐synaptic acetylcholine receptor clusters of neuromuscular junctions (NMJs) perfectly overlap in muscles of healthy adult animals. These junctions are supported by Schwann cells (SCs). NMJs undergo drastic alterations under certain circumstances in which redox status of the muscle is altered, including ageing, disease and peripheral nerve injury. SCs have been implicated in NMJ stability and re‐modelling. In non‐mammalian models hydrogen peroxide (H2O2) has been reported to control axonal guidance through activation of SCs.This study has examined changes in redox status of the muscle fibers in relation to changes in SC distribution and disruption of NMJ structure.Hindlimb skeletal muscles from adult and old WT mice, muscles from Sod1 KO (a model of accelerated muscle loss) and denervated muscles were used in this study. Changes in the redox state of the muscles were examined by use of the Amplex red assay to study mitochondrial peroxide production by permeabilised fiber bundles and intravital‐imaging of the ratio‐metric fluorescent probe cyto‐Hyper2 to specifically assess cytosolic H2O2 in muscle fibers. Fixed, whole EDL muscles were stained with s100‐beta antibody to visualise SCs alongside alpha‐bungarotoxin staining to visualise the NMJs within the intact muscle.Muscles from old mice had an increased rate of mitochondrial peroxide production compared with those from adult mice and this increased significantly when the muscle was denervated. In adult mice peroxide generation in muscle was increased significantly at 7 day time point post‐denervation and although it remained elevated, levels began to fall at 14 and 21 days post‐denervation.Immuno‐staining revealed s100‐beta expression along the axons and overlapped with the post‐synaptic bungarotoxin staining in muscle from adult and old WT mice, however s100‐beta expression appeared reduced in samples from old mice in comparison to adult. In denervated muscle, SC staining in the form of neuronal ‘tubes’ were evident for several days following surgery but staining was reduced at 14 and 21 days. When considered against the profile of peroxide generation in this model of denervation, these data suggest that at around 14 days post‐denervation, the potential initial protective adaptations have given way to degenerative processes.The pattern of SC staining in muscle of Sod1 KO mice was unlike the pattern observed in either the old mice or the denervated model despite evidence of an altered redox status and disrupted NMJs.These data suggest that the redox environment of the muscle influences SC location and potentially function which could contribute to the NMJ structural alterations seen in these models.Support or Funding InformationSupported by NIA AG051442This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.