APP and Alzheimer disease: Modern pharmacological studies indicate that there are many common pathological mechanisms between AD and OP, such as estrogen deficiency, Wnt signaling pathway inhibition, OPG/RANK/RANKL axis disorder, NF-κB signaling pathway activation, amyloid precursor protein cleavage, vitamin D deficiency, inflammation, parathyroid hormone (PTH) deficiency, calcitonin gene-related peptide (CGRP) expression, and autonomic nervous system (ANS) dysfunction (Figure 2).