title: Synaptic Deficits at Neuromuscular Junctions in Two Mouse Models of Charcot-Marie-Tooth Type 2d.
abstract: Patients with Charcot-Marie-Tooth Type 2D (CMT2D), caused by dominant mutations in Glycl tRNA synthetase (GARS), present with progressive weakness, consistently in the hands, but often in the feet also. Electromyography shows denervation, and patients often report that early symptoms include cramps brought on by cold or exertion. Based on reported clinical observations, and studies of mouse models of CMT2D, we sought to determine whether weakened synaptic transmission at the neuromuscular junction (NMJ) is an aspect of CMT2D. Quantal analysis of NMJs in two different mouse models of CMT2D (Gars(P278KY), Gars(C201R)), found synaptic deficits that correlated with disease severity and progressed with age. Results of voltage-clamp studies revealed presynaptic defects characterized by: (1) decreased frequency of spontaneous release without any change in quantal amplitude (miniature endplate current), (2) reduced amplitude of evoked release (endplate current) and quantal content, (3) age-dependent changes in the extent of depression in response to repetitive stimulation, and (4) release failures at some NMJs with high-frequency, long-duration stimulation. Drugs that modify synaptic efficacy were tested to see whether neuromuscular performance improved. The presynaptic action of 3,4 diaminopyridine was not beneficial, whereas postsynaptic-acting physostigmine did improve performance. Smaller mutant NMJs with correspondingly fewer vesicles and partial denervation that eliminates some release sites also contribute to the reduction of release at a proportion of mutant NMJs. Together, these voltage-clamp data suggest that a number of release processes, while essentially intact, likely operate suboptimally at most NMJs of CMT2D mice.
  
  
  CONCLUSIONS
  We have uncovered a previously unrecognized aspect of axonal Charcot-Marie-Tooth disease in mouse models of CMT2D. Synaptic dysfunction contributes to impaired neuromuscular performance and disease progression. This suggests that drugs which improve synaptic efficacy at the NMJ could be considered in treating the pathophysiology of CMT2D patients.
authors.affiliation: The Jackson Laboratory, Bar Harbor, Maine 04609, Graduate School of Biomedical Science and Engineering, University of Maine, Orono, Maine 04469.
authors.forename: Emily L
authors.initials: EL
authors.lastname: Spaulding
authors.affiliation: Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, United Kingdom, and.
authors.forename: James N
authors.identifier: http://orcid.org/0000-0002-3782-9045
authors.initials: JN
authors.lastname: Sleigh
authors.affiliation: The Jackson Laboratory, Bar Harbor, Maine 04609, Graduate School of Biomedical Science and Engineering, University of Maine, Orono, Maine 04469.
authors.forename: Kathryn H
authors.initials: KH
authors.lastname: Morelli
authors.affiliation: Emory University, Atlanta, Georgia 30322.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Pinter
authors.affiliation: The Jackson Laboratory, Bar Harbor, Maine 04609.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Burgess
authors.affiliation: The Jackson Laboratory, Bar Harbor, Maine 04609, kevin.seburn@jax.org.
authors.forename: Kevin L
authors.identifier: http://orcid.org/0000-0001-6132-5982
authors.initials: KL
authors.lastname: Seburn
mesh_terms: D000367:Age Factors; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D002607:Charcot-Marie-Tooth Disease / Q000235:genetics / Q000473:pathology* / Q000503:physiopathology; D004195:Disease Models, Animal; D004558:Electric Stimulation; D006032:Glycine-tRNA Ligase / Q000235:genetics*; D021621:Imaging, Three-Dimensional; D051379:Mice; D008822:Mice, Transgenic; D009045:Motor Endplate / Q000235:genetics / Q000503:physiopathology; D053580:Muscle Strength / Q000235:genetics; D018482:Muscle, Skeletal / Q000473:pathology / Q000648:ultrastructure; D009154:Mutation / Q000235:genetics*; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009469:Neuromuscular Junction / Q000187:drug effects / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D018408:Patch-Clamp Techniques; D011950:Receptors, Cholinergic / Q000378:metabolism; D055366:Synaptic Potentials / Q000187:drug effects / Q000235:genetics; D009435:Synaptic Transmission / Q000235:genetics*; D013572:Synaptic Vesicles / Q000473:pathology / Q000648:ultrastructure
chemical_list: D000631:Aminopyridines; C498441:Bsn protein, mouse; D009419:Nerve Tissue Proteins; D011950:Receptors, Cholinergic; D006032:Glycine-tRNA Ligase
keywords: 3,4 diaminopyridine; axonal neuropathy; neuromuscular transmission; physostigmine; quantal content; voltage clamp