Hydrophilic antioxidant ergothioneine promotes neuronal differentiation through activation of mTORC1 and NT5/TrkB signaling in neural stem cells

Abstract

Ergothioneine (ERGO) is a food-derived hydrophilic antioxidant and classified into α-amino acid. After oral ingestion, ERGO is distributed to the brain across the blood-brain barrier, and taken up into neural stem cells (NSCs) by carnitine/organic cation transporter OCTN1/SLC22A4. We have recently reported that OCTN1-mediated ERGO uptake in mouse NSCs promotes neuronal differentiation through unidentified mechanisms minimally associated with antioxidant action of ERGO. Elucidation of detailed mechanisms underlying promotion of neuronal differentiation by ERGO may lead to find a novel target for promotion of neurogenesis with an aim to treat neuropsychiatric diseases. We first focused on activation of mammalian target of rapamycin complex 1 (mTORC1) signaling known as an amino acids sensor. Exposure of cultured NSCs to ERGO increased the number of neuronal marker βIII-tubulin positive cells whereas the addition of the mTORC1 inhibitor rapamycin inhibited the increase by ERGO. ERGO promoted phosphorylation of mTOR and S6K1, which is the downstream positive effector of the mTORC1 signaling. On the other hand, rapamycin significantly suppressed the phosphorylation of mTOR and S6K1 by ERGO. These results suggest that ERGO may promote neuronal differentiation through activation of mTORC1 signaling in NSCs. Next, we examined whether neurotrophic factors were induced after activation of mTORC1 signaling. Exposure of cultured NSCs to ERGO markedly increased expression of mRNA and gene product of neurotrophin5 (NT5). ERGO promoted phosphorylation of tropomyosin receptor kinase B (TrkB), which is a receptor for NT5, whereas the addition of the S6K1 inhibitor PF4708671 inhibited the promotion by ERGO. PF4708671 or the TrkB inhibitor GNF5837 significantly suppressed the increase in βIII-tubulin positive cells by the ERGO treatment in cultured NSCs. Furthermore, oral administration of ERGO promoted phosphorylation of S6K1 and TrkB, and increased expression of NT5, in the hippocampal dentate gyrus of mice. These results suggest that ERGO may promote neuronal differentiation thorough activation of mTORC1 and NT5/TrkB signaling in NSCs. ERGO or its derivatives could be a possible candidate for treatment and/or prevention of certain neuropsychiatric diseases including depression because defects in mTORC1 and NT5/TrkB signaling are related with their deterioration and/or onset.