Résumé
Abstract only Several axonal growth inhibitors, such as Nogo-A, block functional recovery after neuronal injuries such as stroke. Nogo-A binds to the receptor NgR1, which also serves as a common receptor for a variety of axonal growth inhibitors such as myelin-associated glycoprotein, oligodendrocyte myelin glycoprotein, and chondroitin sulfate proteoglycans. Currently various pharmacological agents are evaluated to block NgR1 as well as its downstream signaling. Some safe and inexpensive natural products that cross the blood-brain barrier such as daidzein and green tea polyphenols are well suited for axonal regeneration and functional recovery after stroke. Furthermore, understanding the mechanisms by which these natural products act will help development of drugs with greater efficacy. In our current study, we have found that epigallocatechin-3-gallate, a green tea polyphenol induced endocytosis of NgR1 in neuronal-like cells and primary cortical neurons. It also prevented both the antineuritogenic and growth-cone collapsing activities of Nogo-A. The binding of EGCG to its cell-surface high affinity receptor resulted in an elevation of intracellular cAMP. This second messenger is apparently responsible for the endocytosis of NgR1. In line with this postulation, the addition of dibutyryl-cAMP and agents that elevate intracellular cAMP such as forskolin (adenylyl cyclase activator) and rolipram (cyclic nucleotide phosphodiesterase inhibitor) caused the endocytosis of NgR1. We also observed an internalization of NgR1 by 8-pCPT-2’- O -methyl-cAMP, a selective activator of Epac (exchange protein directly activated by cAMP). Moreover, ESI-09, an Epac-specific inhibitor blocked both EGCG and cAMP-induced endocytosis of NgR1, suggesting a crucial role of Epac in this process. On the contrary, NgR1 internalization was not observed with N 6 -benzoyl cAMP, a selective activator of protein kinase A (PKA), which is another effector of cAMP. Furthermore, EGCG and cAMP both induced endocytosis of NgR1 in neuronal cells deficient in PKA indicating a lack of role of PKA in this process. Conceivably, endocytosis of NgR1 desensitizes neurons to various axonal growth inhibitors, which may be exploited for drug development to enhance post-stroke recovery.