Exercise Modulates Remodeling of the Diaphragm of the Neuromuscular Junction with Aging in Mice

Background Remodeling of the neuromuscular junction (NMJ) occurs throughout the lifespan and is modulated by physical activity. Exercise leads to hypertrophy of the NMJ and improves recovery from peripheral nerve injury, whereas decreased physical activity leads to degenerative changes in the NMJ. Aging also affects the structure and function of the NMJ as evidenced by its recurrent degeneration and regeneration, likely through variations in physical activity (Fahim, 1997, J. Appl. Physiol. 83:59–66). However, in aging, it is not clear whether modification of the NMJ reflects general age‐related processes or an overall decrease in physical activity, nor how exercise interacts with age to affect the NMJ. The diaphragm, a ventilation muscle, is continuously active regardless of age, and serves as a useful tool in addressing changes of the NMJ related to activity or age. The purpose here is to assess the contribution of age and physical activity level to morphological changes of the NMJ, along with the effect of endurance exercise during aging. Methods In the present study we investigated structural age‐related changes in the diaphragm muscle in a total of 20 male C57BL/6NNia mice, divided into two groups: young ‐ aged 7 months (n = 10), and old ‐ aged 28 months (n = 10). Mice were exercised on a rodent treadmill, starting at 7 or 25 months of age, at a speed of 28 m/min for 60 min/day, 5 days/wk over 12 weeks. For nerve terminal assessment, a zinc‐ iodide osmium method for light microscopy was used, and for NMJ morphometry, conventional electron microscopy was used. Each variable of interest was analyzed with a 2‐way ANOVA on age x endurance training, and significance was set at an alpha of 0.05. Results [Aging] when comparing the NMJ of older versus younger mice structurally, the presynaptic portion of the NMJ displayed regions of abnormal distention and sprouting. Dispersion of terminal branches into a series of regions was observed, consistent with increased remodeling. Electron microscopy analysis shows regions of abnormal distension, complexity, a 30% decrease in nerve terminal area, a 74% decrease in synaptic vesicle density, a 225% increase in the number of coated vesicles, a 52% increase in the width of the synaptic cleft, and engorgement of cytoskeleton organelles (microtubules and neurofilaments). [Exercise] exercise led to an increase in nerve terminal area by 40% in young mice but a decrease of 29% in old mice, along with an increase in vesicle numbers by 133% in old mice, but a decrease of 5% in young mice. Conclusions Findings indicate that the mouse NMJ undergoes a process of morphological change and exhibits increased complexity and sprouting regardless of age; all of which suggest regenerative processes. These changes do not dissociate between pre‐or post‐synaptic sources. Results support the positive impact of exercise in both young and old groups. Moreover, exercise increases the remodeling pace to maintain the integrity and efficiency of NMJs with age.