Amyloid Beta Accumulation in Mouse Brain in Response to Vitamin D Receptor Ligands, Cholesterol and Age

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by the accumulation of amyloid beta (Aβ) plaques. Aging, vitamin D deficiency and high cholesterol are AD risk factors. Activation of the vitamin D receptor (VDR) increases P-gp expression for Aβ efflux and lowers peripheral cholesterol. However, the effect on neprilysin for Aβ degradation and β-secretase protein (BACE-1) for Aβ synthesis is not entirely defined. We described in vitro strategies for identifying selective VDR analogs for activation of brain P-gp but not intestinal calcium channel, TRPV6. Upon improving the GAL4-hVDR luciferase reporter assay by adding ketoconazole to inhibit the bioactivation and degradation enzymes, we showed that the Teijin analogs induce brain P-gp but remain hypercalcemic. We then demonstrated, with various hypercholesterolemic mouse models: C57BL/6 mice fed high fat/high cholesterol (WD) or vitamin D deficient diets or Ldlr-/- mice; that plasma and liver cholesterol but not brain cholesterol levels were elevated with WD and decreased with 1,25(OH)2D3 or dietary vitamin D3. 1,25(OH)2D3 and atorvastatin co-administered to C57BL/6 mice lowered brain Aβ40 significantly, without affecting brain cholesterol. 1,25(OH)2D3 treatment increased P-gp expression in the young (8-weeks) and middle-aged (4-8 months) but not old (10-months) C57BL/6 mice and decreased the Aβ42/Aβ40 ratio for the middle-aged and old mice. Significant, negative correlations were observed for P-gp protein and mRNA expressions of neprilysin and Cyp46a1, the brain cholesterol metabolic enzyme whereas a significant, positive correlation was observed for Bace-1 protein vs. brain Aβ levels. Overall, brain cholesterol did not change nor correlate with brain Aβ although plasma cholesterol and brain Aβ levels were positively correlated. It is concluded that high peripheral cholesterol and age-related changes in P-gp, neprilysin, and Cyp46a1 are risk factors for brain Aβ accumulation. Effects in brain cholesterol may be revealed in future experiments with longer duration of WD and 1,25(OH)2D3 and atorvastatin treatment.Ph.D.2020-11-16 00:00:0