Journal Article

Development of abnormalities at the neuromuscular junction in the SOD1-G93A mouse model of ALS: dysfunction then disruption of postsynaptic structure precede overt motor symptoms

Giuraniuc, Claudiu V., Miles, Gareth B., Banks, Robert W., Mekrouda, Imane, Dashti, Maryam, Bewick, Guy S., McIntosh, Jayne

2023

Overview

The ultimate deficit in amyotrophic lateral sclerosis (ALS) is neuromuscular junction (NMJ) loss, producing permanent paralysis, ultimately in respiratory muscles. However, understanding the functional and structural deficits at NMJs prior to this loss is crucial for therapeutic strategy design. Should early interventions focus on reversing denervation, or supporting largely intact NMJs that are functionally impaired? We therefore determined when functional and structural deficits appeared in diaphragmatic NMJs relative to the onset of hindlimb tremor (the first overt motor symptoms) in the SOD1-G93A mouse model of ALS. We employed electrophysiological recording of NMJ postsynaptic potentials for spontaneous and nerve stimulation-evoked responses. This was correlated with fluorescent imaging microscopy of the postsynaptic acetylcholine receptor (AChR) distribution throughout the postnatal developmental timecourse from 2 weeks to early symptomatic ages. Significant reduction in the amplitudes of spontaneous miniature endplate potentials (mEPPs) and evoked EPPs emerged only at early symptomatic ages (in our colony, 18-22 weeks). Reductions in mEPP frequency, number of vesicles per EPP, and EPP rise time were seen earlier, at 16weeks, but this reversed by early symptomatic ages. However, the earliest and most striking impairment was an inability to maintain EPP amplitude during a 20 Hz stimulus train, which appeared 6 weeks before overt motor symptoms. Despite this, fluorescent -bungarotoxin labelling revealed no systematic, progressive changes in 11 comprehensive NMJ morphological parameters (area, shape, compactness, number of acetylcholine receptor, AChR, regions, etc.) with disease progression. Rather, while NMJs were largely normally-shaped, from 16 weeks there was a progressive and substantial disruption in AChR concentration and distribution within the NMJ footprint. Thus, NMJ functional deficits appear at least 6 weeks before motor symptoms , while structural deficits occur 4 weeks later, and predominantly within NMJs. These data suggest initial therapies focused on rectifying suboptimal NMJ function could produce effective relief of symptoms of weakness.

• Publisher: Frontiers Media SA,Frontiers Media S.A
• Subject: acetylcholine receptor / Acetylcholine receptors / Age / ALS / Amyotrophic lateral sclerosis / Animals / Bungarotoxin / Denervation / Diaphragm (Anatomy) / Electrodes / Labeling / Molecular Neuroscience / Morphology / motor neuron disease / Muscles / neuromuscular junction / Neuromuscular junctions / neuromuscular transmission / Paralysis / SOD1-G93A / Structure-function relationships / Superoxide dismutase / Tremor