Apple peel and flesh contain pro-neurogenic compounds
As mammals evolved exposed to particular diets, naturally abundant compounds may have become part of the set of environmental co-determinants that shaped brain structure and function. Here we investigated whether bioactive factors found in apples directly affect hippocampal neural stem cells and promote neurogenesis in the adult mouse. Whereas the consumption of apple juice per se neither altered adult hippocampal neurogenesis nor improved learning and memory, we did find specific direct effects of apple-derived factors on neural stem cell survival and differentiation. Our results revealed that quercetin, the most abundant flavanol in apple peel, was anti-proliferative at high concentrations but acted pro-neurogenically at low concentrations. This was confirmed in vivo, with intraperitoneally-delivered quercetin promoting survival and neuronal differentiation, without affecting proliferation, likely via the PI3 kinase-Akt and Nrf2-Keap1 pathways, respectively. Using a bio-assay-guided fractionation approach with high-resolution collision induced dissociation mass spectroscopy, we also identified additional pro-neurogenic compounds in apple flesh that were not related to flavonoids. In particular, we found that 3,5-dihydroxybenzoic acid, a weak agonist to the lactate receptor, significantly increased both in vitro and in vivo neural precursor cell proliferation and neurogenesis. Altogether, this work shows that both flavonoids and 3,5-dihydroxybenzoic acid are pro-neurogenic, not only by activating precursor cell proliferation but also through promoting cell cycle exit, cellular survival, and neuronal differentiation.