Recognising the potential of large animals for modelling neuromuscular junction physiology and disease

Abstract

The aetiology and pathophysiology of many diseases of the motor unit remain poorly understood and the role of the neuromuscular junction (NMJ) in this group of disorders is particularly overlooked, especially in humans, when these diseases are comparatively rare. However, elucidating the development, function and degeneration of the NMJ is essential to uncover its contribution to neuromuscular disorders, and to explore potential therapeutic avenues to treat these devastating diseases. Until now, an understanding of the role of the NMJ in disease pathogenesis has been hindered by inherent differences between rodent and human NMJs: stark contrasts in body size and corresponding differences in associated axon length underpin some of the translational issues in animal models of neuromuscular disease. Comparative studies in large mammalian models, including examination of naturally occurring, highly prevalent animal diseases and evaluation of their treatment, might provide more relevant insights into the pathogenesis and therapy of equivalent human diseases. This review argues that large animal models offer great potential to enhance our understanding of the neuromuscular system in health and disease, and in particular, when dealing with diseases for which nerve length dependency might underly the pathogenesis.

Keywords: NMDs; NMJ disorders; large animals; peripheral neuropathy.

FIGURE 1

A schematic diagram of the healthy motor axon. The brainstem is shown as this houses the motor neurons for the longest motor nerves of many large mammals, including horses.

FIGURE 2

A schematic diagram of the unhealthy motor axon. The brainstem is shown as this houses the motor neurons for the longest motor nerves of many large mammals, including horses.

FIGURE 3

Heterogeneity of the mammalian NMJ. Confocal micrographs representing average NMJ morphology in soleus, a predominantly slow‐twitch pelvic/hind/lower limb muscle, across mammalian species arranged in ascending body size: the mouse, sheep, pig, human and pony. The upper panel depicts composite images of pre‐ (cyan) and post‐synapse (magenta), pseudo‐coloured in Fiji. The bottom panels depict the pre‐ and post‐synapse individually in greyscale. SV2, synaptic vesicle protein 2 (cyan); 2H3 and 3A10, neurofilament (cyan); α‐BTX (α‐bungarotoxin), acetylcholine receptors (magenta); Scale bar = 10 μm across all images.

FIGURE 4

Evolutionary divergence of large mammalian models in comparison to mouse and human. (a) Phylogenetic tree depicting the timeline of divergence in million years ago (MYA) between mouse and human (both part of the superorder Euarchontoglires), sheep and pig (both part of the order Artiodactyla) and pony (all three part of the superorder Laurasiatheria). It is evident that despite mouse and human sharing the same clade, they diverged many million years sooner than the here listed domestic animals. Phylogenetic tree generated on http://www.timetree.org. (b) This so‐called tanglegram showcases the difference between two dendrograms. In this case, the individual dendrograms depict pre‐ and post‐synaptic components of the NMJ. Comparison via such a tanglegram allows to assess the differences or similarities between species, across pre‐synaptic, or post‐synaptic components of the NMJ. Both dendrograms showcase clustering of species according to the similarity in post‐synaptic (left dendrogram) or pre‐synaptic (right dendrogram) NMJ variables and their associated and derived variables resulting from analysis with NMJ‐morph/aNMJ‐morph (Jones et al., ; Minty et al., 2020). Red lines indicate similarities in subtree branches: the mouse is most different from the other species in post‐ and pre‐synaptic morphology. Thick black lines at the edges of the dendrogram indicate differences in branch distance from their node of origin: whilst sheep and human are most similar in their post‐synaptic morphology, the sheep and pony are most similar in their pre‐synaptic morphology. Mouse and human data were reproduced from (Jones et al., 2017). Sheep and pig data were reproduced from (Boehm, Alhindi, et al., ; Boehm, Miller, et al., 2020). Pony data yet unpublished (Cahalan et al., , under review). Tanglegram was generated in RStudio (version 1.4.0) using the packages tidverse, usedist, vegan, magrittr and dendextend (Galili, 2015).