Mitochondrial Mechanisms of Neuromuscular Junction Degeneration with Aging

Abstract

Introduction Mitochondrial 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). Findings Our 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. Conclusion Our 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 altera...

Apitherapy for Age-Related Skeletal Muscle Dysfunction (Sarcopenia): A Review on the Effects of Royal Jelly, Propolis, and Bee Pollen

“…Neuronal denervation is a key factor that contributes to skeletal muscle loss, and it is related to a plethora of pathological conditions [ 3 , 149 ]. Experimental induction of oxidative stress and inflammation results in skeletal muscle atrophy through induction of denervation e.g., of sciatic nerve [ 150 ].…”

“…The release of acetylcholine (a neurotransmitter that regulates cognition) at the synaptic cleft of the neuromuscular junction is essential for motor neurotransmission, which controls excitation-contraction coupling and cell size. However, free radicals, cytokines, and AGEs impair neurotransmission by altering the production of acetylcholine [ 6 , 21 , 149 , 153 ]. On the other hand, upregulation of acetylcholine receptors improves neurotransmission [ 154 ].…”

mtDNA Mutation Accumulation in Muscle Is Not a Major Cause of Fiber Loss. Reply to “Comment on: Mitochondrial Mechanisms of Neuromuscular Junction Degeneration with Aging. Cells 2020, 9, 197”

“…However, I am not aware of any data directly addressing this possibility and look forward to studies addressing this and other related issues. I hope this clarifies the basis for the strongly worded statement in our review [1] dismissing a quantitatively important role for mtDNA deletion accumulation within muscle fibers in causing muscle atrophy.…”

“…I thank you for your comment on our review paper [1]. To be clear, I am not dismissing a role for mtDNA mutation accumulation in aging altogether, but rather specifically challenge the importance of mtDNA mutation accumulation within muscle fibers as a quantitatively important cause of muscle atrophy.…”

Comment on: “Mitochondrial Mechanisms of Neuromuscular Junction Degeneration with Aging. Cells 2020, 9, 197”

“…In their manuscript, Anagnostou and Hepple declare, “it may be time to retire the idea that mtDNA mutation accumulation in muscle is causally related to atrophy [ 2 ].” The authors conflate the global reductions in fiber size with the segmental intrafiber atrophy caused by mtDNA deletions that leads to fiber loss. The “ageing atrophy” described by Lexell et al encompasses two distinct processes (i.e., the loss of fibers and reductions in fiber size).…”