Proteasome inhibition protects blood-brain barrier P-glycoprotein and lowers Aβ brain levels in an Alzheimer's disease model

BackgroundLoss of P-glycoprotein (P-gp) at the blood-brain barrier contributes to amyloid-beta (A beta) brain accumulation in Alzheimer's disease (AD). Using transgenic human amyloid precursor protein (hAPP)-overexpressing mice (Tg2576), we previously showed that A beta triggers P-gp loss by activating the ubiquitin-proteasome pathway, which leads to P-gp degradation. Furthermore, we showed that inhibiting the ubiquitin-activating enzyme (E1) prevents P-gp loss and lowers A beta accumulation in the brain of hAPP mice. Based on these data, we hypothesized that repurposing the FDA-approved proteasome inhibitor, bortezomib (Velcade (R); BTZ), protects blood-brain barrier P-gp from degradation in hAPP mice in vivo.MethodsWe treated hAPP mice with the proteasome inhibitor BTZ or a combination of BTZ with the P-gp inhibitor cyclosporin A (CSA) for 2 weeks. Vehicle-treated wild-type (WT) mice were used as a reference for normal P-gp protein expression and transport activity. In addition, we used the opioid receptor agonist loperamide as a P-gp substrate in tail flick assays to indirectly assess P-gp transport activity at the blood-brain barrier in vivo. We also determined P-gp protein expression by Western blotting, measured P-gp transport activity levels in isolated brain capillaries with live cell confocal imaging and assessed A beta plasma and brain levels with ELISA.ResultsWe found that 2-week BTZ treatment of hAPP mice restored P-gp protein expression and transport activity in brain capillaries to levels found in WT mice. We also observed that hAPP mice displayed significant loperamide-induced central antinociception compared to WT mice indicating impaired P-gp transport activity at the blood-brain barrier of hAPP mice in vivo. Furthermore, BTZ treatment prevented loperamide-induced antinociception suggesting BTZ protected P-gp loss in hAPP mice. Further, BTZ-treated hAPP mice had lower A beta 40 and A beta 42 brain levels compared to vehicle-treated hAPP mice.ConclusionsOur data indicate that BTZ protects P-gp from proteasomal degradation in hAPP mice, which helps to reduce A beta brain levels. Our data suggest that the proteasome system could be exploited for a novel therapeutic strategy in AD, particularly since increasing A beta transport across the blood-brain barrier may prove an effective treatment for patients.

Automated summary

This study suggests that the proteasome inhibitor BTZ can protect blood-brain barrier P-gp from degradation in hAPP mice, leading to a reduction in Aβ brain levels.