Morphological characteristics of motor neurons do not determine their relative susceptibility to degeneration in a mouse model of severe spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality, resulting primarily from the degeneration and loss of lower motor neurons. Studies using mouse models of SMA have revealed widespread heterogeneity in the susceptibility of individual motor neurons to neurodegeneration, but the underlying reasons remain unclear. Data from related motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), suggest that morphological properties of motor neurons may regulate susceptibility: in ALS larger motor units innervating fast-twitch muscles degenerate first. We therefore set out to determine whether intrinsic morphological characteristics of motor neurons influenced their relative vulnerability to SMA. Motor neuron vulnerability was mapped across 10 muscle groups in SMA mice. Neither the position of the muscle in the body, nor the fibre type of the muscle innervated, influenced susceptibility. Morphological properties of vulnerable and disease-resistant motor neurons were then determined from single motor units reconstructed in Thy.1-YFP-H mice. None of the parameters we investigated in healthy young adult mice - including motor unit size, motor unit arbor length, branching patterns, motor endplate size, developmental pruning and numbers of terminal Schwann cells at neuromuscular junctions - correlated with vulnerability. We conclude that morphological characteristics of motor neurons are not a major determinant of disease-susceptibility in SMA, in stark contrast to related forms of motor neuron disease such as ALS. This suggests that subtle molecular differences between motor neurons, or extrinsic factors arising from other cell types, are more likely to determine relative susceptibility in SMA

Review: neuromuscular synaptic vulnerability in motor neurone disease: amyotrophic lateral sclerosis and spinal muscular atrophy.

Amid the great diversity of neurodegenerative conditions, there is a growing body of evidence that non-somatic (that is, synaptic and distal axonal) compartments of neurones are early and important subcellular sites of pathological change. In this review we discuss experimental data from human patients, animal models and in vitro systems showing that neuromuscular synapses are targeted in different forms of motor neurone disease (MND), including amyotrophic lateral sclerosis and spinal muscular atrophy. We highlight important developments revealing the heterogeneous nature of vulnerability in populations of lower motor units in MND and examine how progress in our understanding of the molecular pathways underlying MND may provide insights into the regulation of synaptic vulnerability and pathology. We conclude that future experiments developing therapeutic approaches specifically targeting neuromuscular synaptic vulnerability are likely to be required to prevent or delay disease onset and progression in human MND patients