Expression of TrkB at the neuromuscular junction during aging

There is increasing evidence that age associated alterations in innervation may underlie many of the changes that occur in skeletal muscle during senescence. Aging of the neuromuscular system is associated with loss of motor neurons, altered neurotransmitter release, disruption of post-synaptic acetylcholine receptor (AChR) organization, denervation and reinnervation of muscle fibers, and motor unit remodeling. Currently the cellular mechanisms involved in the reorganization of neural innervation remain elusive. Experimental evidence supports the role of the neurotrophins, brain derived neurotrophic factor (BDNF) and neurotrophin 4/5 (NT-4/5), and their receptor TrkB, as modulators which influence the morphology and physiological function of the neuromuscular system. Recent work has shown that BDNF and NT-4/5, signaling via TrkB influences maintenance of the post-synaptic apparatus, motor neuron survival, and neuromuscular transmission. Therefore we chose to investigate whether TrkB expression at the neuromuscular junction is altered with aging and if changes in TrkB expression are correlated with disorganization of the neuromuscular junction. Immunohistochemical methods were used to label nerve terminals, AChR's, and TrkB in mouse limb muscle at age 2, 12, or 24 months. AChR area and number of discrete receptor regions were quantified to assess age-associated changes in the post-synaptic neuromuscular junction. The extent of TrkB expression and structural denervation were assessed by determining TrkB/AChR colocalization and nerve terminal/AChR colocalization respectively. As expected the AChR area expanded and fragmented with age. Changes in the soleus were greater than the changes which occurred in the EDL. Nerve terminal/AChR colocalization was decreased in the soleus but not the EDL suggesting that structural denervation was more significant in the soleus. Both EDL and soleus showed decreased TrkB. A significant linear relationship was seen between TrkB/AChR colocalization and nerve terminal/AChR colocalization in both muscle groups, however r2 values were relatively low (r2 = 0.35-0.66). Surprisingly no relationship was seen between TrkB/AChR colocalization and the number of discrete AChR regions. Together this data indicates that a loss of TrkB expression may be one of the mechanisms which contribute to structural alteration of the neuromuscular junction with aging.