The neuromuscular junction (NMJ) undergoes a maturation process physiologically after initial nerve-muscle contact, which is characterized by spontaneous synaptic current (SSC) amplitude increase and rise time decrease, as well as the development of well-defined quantal ACh release. Chronic application of BDNF and elevation of intracellular cAMP resulted in inhibition in the maturation process, while NT-3, NT-4, GDNF, HB-GAM and HGF appeared to promote it. The response to BDNF was mediated by TrkB receptor. PKA inhibitors could reverse the inhibitory effects of cAMP and BDNF, which suggests that cAMP and BDNF suppress NMJ maturation through cAMP-PKA-dependent signaling pathway. Further investigations revealed that CaMKs mediate the inhibitory effects of cAMP and BDNF. That led...[ Read more ]

The neuromuscular junction (NMJ) undergoes a maturation process physiologically after initial nerve-muscle contact, which is characterized by spontaneous synaptic current (SSC) amplitude increase and rise time decrease, as well as the development of well-defined quantal ACh release. Chronic application of BDNF and elevation of intracellular cAMP resulted in inhibition in the maturation process, while NT-3, NT-4, GDNF, HB-GAM and HGF appeared to promote it. The response to BDNF was mediated by TrkB receptor. PKA inhibitors could reverse the inhibitory effects of cAMP and BDNF, which suggests that cAMP and BDNF suppress NMJ maturation through cAMP-PKA-dependent signaling pathway. Further investigations revealed that CaMKs mediate the inhibitory effects of cAMP and BDNF. That led us to study the function of Ca²⁺ in this regulation. Ca²⁺-free culture medium had no effect on cAMP and BDNF-mediated inhibition of NMJ maturation. However, depletion of cytosolic Ca²⁺ potently reversed the effects, suggesting that Ca²⁺ released from intracellular Ca²⁺ stores regulates neurotrophic actions on NMJ maturation. Since Ca²⁺ released from intracellular Ca²⁺ stores through IP3 receptors and ryanodine receptors, their roles in regulating NMJ maturation were studied. In the presence of 2-APB (IP3 receptor inhibitor) or TMB-8 (ryanodine receptor inhibitor) alone, the inhibitory effects of cAMP and BDNF were still present. However, co-treatment with both 2-APB and TMB-8 reversed their effects, suggesting that mobilization of intracellular Ca²⁺ mediated by either IP3Rs or RYRs contributes to the effect of cAMP/BDNF in inhibiting NMJ maturation.