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

Abstract

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 Aβ clearance from the brain. Changes in IR-β 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-β levels in 9-month-old male and female 3XTg-AD (PS1_M146V, APP_Swe, and tau_P301L) as compared to NTg mice, which is important in insulin mediated signaling responses. Reduced cerebral IR-β levels corresponded to impaired insulin signaling and LRP-1 levels in brain. Reduced cerebral and cerebrovascular IR-β 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 Aβ burden without affecting cerebrovascular LRP-1 and IR-β levels. In vitro studies using primary porcine (p)BCEC revealed that Aβ peptides 1-40 or 1-42 (240 nM) reduced cellular levels and interaction of LRP-1 and IR-β thereby perturbing insulin-mediated signaling. Further mechanistic investigation revealed that Aβ treatment accelerated the autophagy-lysosomal degradation of IR-β and LRP-1 in pBCEC. LRP-1 silencing in pBCEC decreased IR-β 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 Aβ burden in AD may accelerate LRP-1 and IR-β degradation in BCEC thereby contributing to impaired cerebral and cerebromicrovascular insulin effects.

Keywords: Alzheimer's disease; Amyloid-β peptides; Autophagy-lysosomal pathway; Blood-brain barrier; Endothelial cells; Insulin receptor-beta; Insulin signaling; Low-density lipoprotein receptor-related protein-1.