Mitochondrial DNA Mutations Induce Neuromuscular Junction Degeneration
Authors: Anagnostou Maria‐Eleni , Burke Sarah S. , Konokhova Yana , Ferguson Cole , Fenton Andrew , Kim Mi‐Jung , Someya Shinichi , Hepple Russell T. 
Published: June 2021

IntroductionMitochondrial dysfunction is frequently posited as a cause of aging muscle atrophy. One of the proposed mechanisms involves an age‐related accumulation of mitochondrial DNA (mtDNA) mutations and subsequent mitochondrial dysfunction. Consistent with this notion, previous studies have reported co‐localization of fiber segments with high mtDNA mutation load, severe oxidative impairment, up‐regulation of necrotic and apoptotic pathways, fiber atrophy and breakage in aged muscle. However, the significance of this mechanism is dubious as the majority of fibers harbouring high mtDNA mutation loads and oxidative damage do not exhibit atrophy. In contrast, we have reported previously that persistent denervation of muscle fibers correlates with fiber atrophy in advanced age, where over 90% of severely atrophied fibers show evidence of persistent denervation. These latter findings raise the question of what causes denervation in aging muscle. As virtually nothing is known about the potential for accumulation of mtDNA mutations to cause denervation in aging muscle, we studied neuromuscular junction (NMJ) morphology in a mouse model, exhibiting a rapid accumulation of mtDNA mutations and markedly accelerated muscle atrophy, the polymerase gamma mutator (PolGmut).FindingsOur analysis revealed significantly lower (p<0.05) muscle mass in 16‐mo old PolGmut (n=4) versus 5‐mo old PolGmut mice (n=8) in tibialis anterior (TA; mean ±SD: 25.5±7.4 mg versus 44.3 ±4.7 mg), soleus (Sol: 4.5 ±0.6 mg versus 6.5 mg ±1.1 mg) and gastrocnemius (90.9 ±8.5 mg versus 122.9 ±19.7 mg). In contrast, there was not a significant reduction in muscle weight in 15 to 17‐mo old (n=11) versus 5‐mo old (n=8) WT mice in the TA (42.5 ±7.9 mg versus 43.3 ±9.1 mg), soleus (7.3 ±1.8 mg versus 5.9 ±1.9 mg) and gastrocnemius (mean weight=123.1 ±29.6 mg versus 136.4 ±26.5 mg). Furthermore, we tested the impact of aging on NMJ morphology by immunofluorescent labelling and quantitative assessment of NMJ morphology from the TA of these mice. Significant alterations (p<0.05) were found in WT mice between 5‐mo and 16‐mo including reductions in pre‐synaptic variables: nerve terminal area, nerve terminal perimeter; and in post‐synaptic variables: AChR area, AChR perimeter, AChR compactness and nerve terminal area outside the endplate. Increased nerve terminal area outside AChRs and axonal area within the endplate were also detected. Some of these changes were potentiated in 16‐mo old PolGmut, characterized by increased pre‐synaptic axon diameter and post‐synaptic fragmentation, and reductions in post‐synaptic: unoccupied AChR area, endplate area and endplate perimeter. Most of the NMJ morphology changes were associated with post‐synaptic variables in both WT and PolGmut.ConclusionOur findings show that PolGmut mice exhibit an accelerated appearance of NMJ degeneration, consistent with the possibility that mtDNA mutations may be a cause of denervation in aging muscle. Further work is underway to determine whether NMJ degeneration is related to mtDNA alterations and mitochondrial dysfunction in motor neurons versus muscle fibers.Support or Funding InformationSupported by UF start‐up funds to RTH.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.