FNDC5 and Alzheimer disease: The tight interconnection between dopamine signaling, irisin, and AD is further suggested not only by the evidence that early neuron loss in the VTA is reported in AD-like mice [157] and the dopamine-evoked neprilysin activity [166] but also by the captivating finding that irisin was able to reduce Aβ pathology using a neprilysin-mediated mechanism [176].