Schematic illustration of neurotrophin signaling. (NT = neurotrophins, BDNF = brain-derived neurotrophic factor, TRK = tyrosine receptor kinase, GDNF = glial-derived neurotrophic factor, PLC = phospholipase C, RAS = Ras proteins, GRB = growth factor receptor-bound protein 2, PIK3 = phosphatidylinositol-4,5-bisphosphate 3-kinase, AKT = AKT serine/threonine–protein kinases, FOXO = The forkhead box O transcription factor, PKC = protein kinase C, CREB = the cAMP-responsive element-binding protein)