Amyloid-beta impairs insulin signaling by accelerating autophagy-lysosomal degradation of LRP-1 and IR-ß in blood-brain barrier endothelial cells in vitro and in 3XTg-AD mice.

Aberrant insulin signaling constitutes an early change in Alzheimer's disease (AD). Insulin receptors (IR) and low-density lipoprotein receptor-related protein-1 (LRP-1) are expressed in brain capillary endothelial cells (BCEC) forming the blood-brain barrier (BBB). There, insulin may regulate the function of LRP-1 in Aß clearance from the brain. Changes in IR-ß and LRP-1 and insulin signaling at the BBB in AD are not well understood. Herein, we identified a reduction in cerebral and cerebrovascular IR-ß levels in 9-month-old male and female 3XTg-AD (PS1M146V, APPSwe, and tauP301L) as compared to NTg mice, which is important in insulin mediated signaling responses. Reduced cerebral IR-ß levels corresponded to impaired insulin signaling and LRP-1 levels in brain. Reduced cerebral and cerebrovascular IR-ß and LRP-1 levels in 3XTg-AD mice correlated with elevated levels of autophagy marker LC3B. In both genotypes, high-fat diet (HFD) feeding decreased cerebral and hepatic LRP-1 expression and elevated cerebral Aß burden without affecting cerebrovascular LRP-1 and IR-ß levels. In vitro studies using primary porcine (p)BCEC revealed that Aß peptides 1-40 or 1-42 (240 nM) reduced cellular levels and interaction of LRP-1 and IR-ß thereby perturbing insulin-mediated signaling. Further mechanistic investigation revealed that Aß treatment accelerated the autophagy-lysosomal degradation of IR-ß and LRP-1 in pBCEC. LRP-1 silencing in pBCEC decreased IR-ß levels through post-translational pathways further deteriorating insulin-mediated responses at the BBB. Our findings indicate that LRP-1 proves important for insulin signaling at the BBB. Cerebral Aß burden in AD may accelerate LRP-1 and IR-ß degradation in BCEC thereby contributing to impaired cerebral and cerebromicrovascular insulin effects.

Astaxanthin exerts protective effects similar to bexarotene in Alzheimer's disease by modulating amyloid-beta and cholesterol homeostasis in blood-brain barrier endothelial cells.

The pathogenesis of Alzheimer's disease (AD) is characterized by overproduction, impaired clearance, and deposition of amyloid-ß peptides (Aß) and connected to cholesterol homeostasis. Since the blood-brain barrier (BBB) is involved in these processes, we investigated effects of the retinoid X receptor agonist, bexarotene (Bex), and the peroxisome proliferator-activated receptor α agonist and antioxidant, astaxanthin (Asx), on pathways of cellular cholesterol metabolism, amyloid precursor protein processing/Aß production and transfer at the BBB in vitro using primary porcine brain capillary endothelial cells (pBCEC), and in 3xTg AD mice. Asx/Bex downregulated transcription/activity of amyloidogenic BACE1 and reduced Aß oligomers and ~80 kDa intracellular 6E10-reactive APP/Aß species, while upregulating non-amyloidogenic ADAM10 and soluble (s)APPα production in pBCEC. Asx/Bex enhanced Aß clearance to the apical/plasma compartment of the in vitro BBB model. Asx/Bex increased expression levels of ABCA1, LRP1, and/or APOA-I. Asx/Bex promoted cholesterol efflux, partly via PPARα/RXR activation, while cholesterol biosynthesis/esterification was suppressed. Silencing of LRP-1 or inhibition of ABCA1 by probucol reversed Asx/Bex-mediated effects on levels of APP/Aß species in pBCEC. Murine (m)BCEC isolated from 3xTg AD mice treated with Bex revealed elevated expression of APOE and ABCA1. Asx/Bex reduced BACE1 and increased LRP-1 expression in mBCEC from 3xTg AD mice when compared to vehicle-treated or non-Tg treated mice. In parallel, Asx/Bex reduced levels of Aß oligomers in mBCEC and Aß species in brain soluble and insoluble fractions of 3xTg AD mice. Our results suggest that both agonists exert beneficial effects at the BBB by balancing cholesterol homeostasis and enhancing clearance of Aß from cerebrovascular endothelial cells.