BDNF/TrkB Signaling Increases Autophagy Flux in Cervical Spinal Cord

Abstract

Autophagy is a homeostatic process for the elimination of damaged, unnecessary proteins or organelles within cells. Our overarching hypothesis is that autophagy contributes to neuromuscular dysfunction, specifically increased NMTF seen with aging. Understanding the mechanisms regulating autophagy flux is essential to discovering therapeutic agents which may mitigate the effects of neuromuscular dysfunction across conditions including old age. In recent studies, we found that in 18 month old mice autophagy in the cervical spinal cord is accelerated and neuromuscular transmission failure (NMTF) in diaphragm muscle‐phrenic nerve preparations is increased. Neurotrophin activity, specifically brain‐derived neurotrophic factor (BDNF) signaling through its high affinity receptor tropomyosin‐related kinase receptor subtype B (TrkB) decreases NMTF. Thus, we predict BDNF/TrkB signaling suppresses autophagy in phrenic motor neurons located in the cervical spinal cord. We hypothesize that inhibiting TrkB kinase activity accelerates autophagy in phrenic motor neurons. TrkBF616A mice at 18 months of age were randomized to receive 1 week treatment with vehicle (n=4) or 1NMPP1 (TrkB kinase inhibitor; n=6). Additionally, 18 month old wild type C57BL/6 mice (n=2) were studied. Whole cervical spinal cord (CSC) was harvested for measurements of autophagy marker protein expression analyzed by Western blotting. Data for the vehicle‐treated TrkBF616A and wild type mice was combined given lack of differences for any measurement. p62, a marker associated with the initiation of autophagy, was significantly decreased in the 1NMPP1‐treated group compared to vehicle‐treated or wild type mice. LC3, a protein marking the autophagosome membrane elongation phase, was not significantly changed across groups indicating that LC3‐tagged autophagosomes proceed to lysosomal degradation and do not accumulate. These results support our hypothesis that inhibiting BDNF/TrkB signaling accelerates autophagy flux in the CSC and reveal trophic effects of BDNF/TrkB signaling in the neuromuscular system. Accelerated autophagy may underlie age‐related neuromuscular dysfunction in a BDNF/TrkB signaling dependent manner.Support or Funding InformationSupport: NIH R01 AG057052