Neuromuscular junction maturation defects precede impaired lower motor neuron connectivity in Charcot–Marie–Tooth type 2D mice

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Abstract

Dominant mutations inGARS, encoding the essential enzyme glycyl-tRNA synthetase (GlyRS), result in a form of Charcot–Marie–Tooth disease, type 2D (CMT2D), predominantly characterized by lower motor nerve degeneration. GlyRS charges the amino acid glycine with its cognate tRNA and is therefore essential for protein translation. However, the underlying mechanisms linking toxic gain-of-functionGARSmutations to lower motor neuron degeneration remain unidentified. The neuromuscular junction (NMJ) appears to be an early target for pathology in a number of peripheral nerve diseases and becomes denervated at later stages in two mouse models of CMT2D. We therefore performed a detailed longitudinal examination of NMJs in the distal lumbrical muscles and the proximal transversus abdominis (TVA) muscles of wild-type andGarsmutant mice. We determined that mutant lumbrical NMJs display a persistent defect in maturation that precedes a progressive, age-dependent degeneration. Conversely, the TVA remains relatively unaffected, with only a subtle, short-lived impairment in pre- and post-synaptic development and no reduction in lower motor neuron connectivity to muscle. Together, these observations suggest that mutantGarsis associated with compromised development of the NMJ prior to synaptic degeneration and highlight the neuromuscular synapse as an important site of early, selective pathology in CMT2D mice.

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