APP and hyperparathyroidism: Reduced VDR activity enhances the expression of the APP, BACE1, and PSEN1 genes and increases the production of Aβ42, which directly or indirectly activates GABAB1R/CaSR heterodimers to increase cAMP production and enhance the expression and phosphorylation of Tau, leading to PTH hypersecretion and the manifestations of hyperparathyroidism.