Résumé
Abstract only Recovery of stroke and neuronal injuries requires the promotion of axonal regeneration from the remaining neurons. However, axonal regeneration is inhibited by diverse axonal growth inhibitors, such as Nogo-A. C-terminal domain of Nogo-A, Nogo-66 binds to the Nogo-A receptor 1 (NgR1) and induces the collapse of growth cones and inhibition of neurite outgrowth. NgR1 is also a receptor for additional axonal growth inhibitors. In this study, by using indirect immunofluorescence and biotinylation method, we have found that a cell-permeable cAMP analog (dibutyryl-cAMP) and other intracellular cAMP-elevating agents, such as forskolin, which directly activates adenylyl cyclase, and rolipram, which inhibits cyclic nucleotide phosphodiesterase, all induced rapid internalization of the cell surface NgR1 in Neuroscreen-1 (NS-1) cells. This endocytosis of NgR1 is lipid raft mediated. These cAMP-elevating agents induced a reversible distribution of NgR1 between the cell surface and intracellular compartment; NgR1 distributed to the cell surface at low levels of cAMP and distributed to an intracellular compartment at high levels of cAMP. Using pharmacological activators and inhibitors of protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac), we found that NgR1 internalization is independent of PKA but dependent on Epac. There is a correlation between the decrease in cell surface expression of NgR1 decreased sensitivity of NS-1 cells to Nogo-66-induced growth cone collapse. Therefore, besides axonal growth inhibitors affecting neurons, neurons by themselves self-regulate their own sensitivity to extracellular cues such as axonal growth inhibitors. This normal cellular regulatory mechanism may be therapeutically applied to overcome axonal growth inhibitors and enhance functional recovery after stroke and neuronal injuries.