The neuromuscular junctions (NMJs) undergo maturation process after the initiation of nerve-muscle interaction. The morphological changes of NMJs during the process were meticulously studied. At the presynaptic side, SV clusters formed before the establishment of NMJ was enhanced at the nerve-muscle contact site. At the postsynaptic side, small puncta of nerve-induced AChR clusters consolidated into large patches. In addition, the apposition of pre- and postsynaptic specializations was also strengthened during the maturation process. Interestingly, chronic application of brain-derived neurotrophic factors (BDNF) or elevation of intracellular cyclic AMP (cAMP) level caused inhibitory effects on the NMJ maturation process. The density of nerve-induced AChR clusters as well as the...[ Read more ]

The neuromuscular junctions (NMJs) undergo maturation process after the initiation of nerve-muscle interaction. The morphological changes of NMJs during the process were meticulously studied. At the presynaptic side, SV clusters formed before the establishment of NMJ was enhanced at the nerve-muscle contact site. At the postsynaptic side, small puncta of nerve-induced AChR clusters consolidated into large patches. In addition, the apposition of pre- and postsynaptic specializations was also strengthened during the maturation process. Interestingly, chronic application of brain-derived neurotrophic factors (BDNF) or elevation of intracellular cyclic AMP (cAMP) level caused inhibitory effects on the NMJ maturation process. The density of nerve-induced AChR clusters as well as the apposition of SV and AChR clusters were significantly suppressed. However, the inhibitory effects of BDNF and cAMP were strongly reversed by TrkB and PKA inhibitors. It turns out that the TrkB signaling activated by BDNF plays an important role in the process through the mediation of PKA. FRET measurement of cAMP level in spinal cord neurons revealed that TrkB activation by BDNF raises intracellular cAMP level through the mediation of the MAPK pathway, which in turn leads to PKA activation. In addition, the PI3k-Akt pathway downstream of TrkB was also found to inhibit NMJ development. When the Akt activity was increased by PTEN inhibition, NMJ formation was compromised. This process was partially rescued by rapamycin, which means that the target of rapamycin (TOR) mediates the inhibitory effects of the PI3k-Akt pathway on NMJ formation. As a result, the downstream effector molecules of PKA and TOR may together regulate the inhibitory effects of TrkB signaling on NMJ development. These results suggest that the TrkB signaling, though essential for neuronal survival and growth, needs to be turned down for NMJ formation and maturation.