Selective vulnerability of motor neurons and dissociation of pre- and post-synaptic pathology at the neuromuscular junction in mouse models of spinal muscular atrophy

Abstract

Proximal spinal muscular atrophy (SMA) is a common autosomal recessive childhood form of motor neuron disease. Previous studies have highlighted nerve- and muscle-specific events in SMA, including atrophy of muscle fibres and post-synaptic motor endplates, loss of lower motor neuron cell bodies and denervation of neuromuscular junctions caused by loss of pre-synaptic inputs. Here we have undertaken a detailed morphological investigation of neuromuscular synaptic pathology in the Smn-/-;SMN2 and Smn-/-;SMN2;Delta7 mouse models of SMA. We show that neuromuscular junctions in the transversus abdominis (TVA), levator auris longus (LAL) and lumbrical muscles were disrupted in both mouse models. Pre-synaptic inputs were lost and abnormal accumulations of neurofilament were present, even in early/mid-symptomatic animals in the most severely affected muscle groups. Neuromuscular pathology was more extensive in the postural TVA muscle compared with the fast-twitch LAL and lumbrical muscles. Pre-synaptic pathology in Smn-/-;SMN2;Delta7 mice was reduced compared with Smn-/-;SMN2 mice at late-symptomatic time-points, although post-synaptic pathology was equally severe. We demonstrate that shrinkage of motor endplates does not correlate with loss of motor nerve terminals, signifying that one can occur in the absence of the other. We also demonstrate selective vulnerability of a subpopulation of motor neurons in the caudal muscle band of the LAL. Paralysis with botulinum toxin resulted in less terminal sprouting and ectopic synapse formation in the caudal band compared with the rostral band, suggesting that motor units conforming to a Fast Synapsing (FaSyn) phenotype are likely to be more vulnerable than those with a Delayed Synapsing (DeSyn) phenotype.

Dysregulation of synaptogenesis genes antecedes motor neuron pathology in spinal muscular atrophy

“…S3), indicating that the SMA mice are capable of expressing Z + agrin. However, the skipping of Z exons and the resulting Z + agrin protein deficiency in SMA MNs between P1 and P3 would impair postsynaptic AChR clustering, as observed at P5 in this SMA model (7)(8)(9).…”

“…Major morphological and biochemical deficits have been found at neuromuscular junctions (NMJs) and sensory-motor synapses. NMJ defects are first detectable at postnatal day 5 (P5), including presynaptic defects of terminal arborization and intermediate neurofilament aggregation in MNs, poor postsynaptic organization of AChRs in muscle, as well as reduced synaptic vesicle density and release at the NMJ (7)(8)(9). Importantly, similar NMJ defects have been reported in type I (the most severe type) SMA human fetuses (10).…”

Severe SMA mice show organ impairment that cannot be rescued by therapy with the HDACi JNJ-26481585

“…49 We examined TVA preparations of SMA and control littermates at P0, P5 and P8 to assess when NMJ defects become apparent. NMJ endplate size of the TVA began to lag behind at P5 in SMA as compared with control animals (Figures 5a, c and d).…”

Motor neuron cell-nonautonomous rescue of spinal muscular atrophy phenotypes in mild and severe transgenic mouse models

“…However, how suboptimal levels of SMN lead to SMA is largely unknown. Multiple studies in SMA mouse models revealed widespread synaptic defects in neuromuscular junctions (NMJs), including neurofilament accumulation, poor terminal arborization, immature endplates, reduced quantal content, disturbed calcium homeostasis, and decreased remodeling potential; these defects precede motor neuron death (Cifuentes-Diaz et al 2002;Le et al 2005;Jablonka et al 2007;Kariya et al 2008;Murray et al 2008Murray et al , 2012Kong et al 2009;Ling et al 2010;Ruiz et al 2010;Lee et al 2011), suggesting that the NMJ alterations are the initial consequence of SMN deficiency, Ó 2015 Hua et al This article is distributed exclusively by Cold Spring Harbor Laboratory Press for the first six months after the full-issue publication date (see http://genesdev.cshlp.org/site/misc/terms.xhtml). After six months, it is available under a Creative Commons License (Attribution-NonCommercial 4.0 International), as described at http:// creativecommons.org/licenses/by-nc/4.0/.…”