Activity-dependent Degeneration of Axotomized Neuromuscular Synapses in Wld Mice

Activity and disuse of synapses are thought to influence progression of several neurodegenerative diseases in which synaptic degeneration is an early sign. Here we tested whether stimulation or disuse renders neuromuscular synapses more or less vulnerable to degeneration, using axotomy as a robust trigger. We took advantage of the slow synaptic degeneration phenotype of axotomized neuromuscular junctions in flexor digitorum brevis (FDB) and deep lumbrical (DL) muscles of Wallerian degeneration-Slow (Wld) mutant mice. First, we maintained ex vivo FDB and DL nerve-muscle explants at 32 C for up to 48 h. About 90% of fibers from Wld mice remained innervated, compared with about 36% in wild-type muscles at the 24-h checkpoint. Periodic high-frequency nerve stimulation (100 Hz: 1 s/100 s) reduced synaptic protection inWld preparations by about 50%. This effect was abolished in reduced Ca solutions. Next, we assayed FDB and DL innervation after 7 days of complete tetrodotoxin (TTX)-block of sciatic nerve conduction in vivo, followed by tibial nerve axotomy. Five days later, only about 9% of motor endplates remained innervated in the paralyzed muscles, compared with about 50% in 5 day-axotomized muscles from saline-control-treated Wld mice with no conditioning nerve block. Finally, we gave mice access to running wheels for up to 4 weeks prior to axotomy. Surprisingly, exercising Wld mice ad libitum for 4 weeks increased about twofold the amount of subsequent axotomy-induced synaptic degeneration. Together, the data suggest that vulnerability of mature neuromuscular synapses to axotomy, a potent neurodegenerative trigger, may be enhanced bimodally, either by disuse or by hyperactivity. 2015 The Authors. Published by Elsevier http://dx.doi.org/10.1016/j.neuroscience.2015.01.018 0306-4522/ 2015 The Authors. Published by Elsevier Ltd. on behalf of IBRO. This is an open access article under the CC BY license (http://creativecommons.org *Corresponding author. Address: Euan MacDonald Centre for MND Research, School of Biomedical Sciences, University of Edinburgh, Hugh Robson Building, Edinburgh EH8 9XD, UK. Tel: +44-0131650-3257. E-mail address: [email protected] (R. R. Ribchester). Abbreviations: ALS, amyotrophic lateral sclerosis; ANOVA, analysis of variance; CME, confocal microendoscopy; DL, deep lumbrical muscle; EPP, endplate potential; FDB, flexor digitorum brevis muscle; MEPP, miniature endplate potential; MPS, mammalian physiological saline; NAD, nicotinamide adenine dinucleotide; NMJ, neuromuscular junction; NMN, nicotinamide mononucleotide; Nmnat, nicotinamide mononucleotide adenyl transferase; QC, quantal content; Sarm1, sterile alpha and TIR motif-containing protein-1; SOD, superoxide dismutase; TTX, tetrodotoxin; WD, Wallerian degeneration; Wld, Wallerian degeneration-Slow mutant; WT, wild-type; YFP, yellow fluorescent protein. 300 Ltd. on behalf of IBRO. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).