Role of TrkB kinase activity in aging diaphragm neuromuscular junctions

Brain derived neurotrophic factor (BDNF) acting through the tropomyosin-related kinase receptor B (TrkB) enhances neuromuscular transmission in the diaphragm muscle of adult mice, reflecting presynaptic effects. With aging, BDNF enhancement of neuromuscular transmission is lost. We hypothesize that disrupting BDNF/TrkB signaling in early old age will reveal a period of susceptibility evident by morphological changes at neuromuscular junctions (NMJ). Adult, male TrkBF616A mice (n = 25) at 6 and 18 months of age, were used to examine the structural properties of diaphragm muscle NMJs (n = 1097). Confocal microscopy was used to compare pre- and post-synaptic morphology and denervation following a 7 day treatment with the phosphoprotein phosphatase-1 derivative 1NMPP1, which inhibits TrkB kinase activity in TrkBF616A mice vs. vehicle treatment. In early old age (18 months), presynaptic terminal volume decreased compared to 6 month old diaphragm NMJs (~ 20%). Inhibition of TrkB kinase activity significantly decreased the presynaptic terminal volume (~ 20%) and motor end-plate 2D planar area (~ 10%), independent of age group. Inhibition of TrkB kinase activity in early old age significantly reduced overlap of pre- and post-synaptic structures and increased the proportion of denervated NMJs (to ~ 20%). Collectively these results support a period of susceptibility in early old age when BDNF/TrkB signaling at diaphragm NMJs supports the maintenance of NMJs structure and muscle innervation.