BDNF and major depressive disorder: For example, exercise-induced miRNAs, such as miR-125b-5p, miR-126, and miR-146a, target the BDNF gene, including the Val66Met polymorphism, which is linked to major depressive disorder (MDD) with Alzheimer’s disease (AD) [43], by improving synaptic remodeling and differentiation via BDNF/TrkB binding in MDD and AD by activating its downstream signaling, such as MAPK/ERK [44].