Aβ40 Reduces P-Glycoprotein at the Blood-Brain Barrier through the Ubiquitin-Proteasome Pathway

Failure to clear amyloid-beta (A beta) from the brain is in part responsible for A beta brain accumulation in Alzheimer's disease (AD). A critical protein for clearing A beta across the blood-brain barrier is the efflux transporter P-glycoprotein (P-gp) in the luminal plasma membrane of the brain capillary endothelium. P-gp is reduced at the blood-brain barrier in AD, which has been shown to be associated with A beta brain accumulation. However, the mechanism responsible for P-gp reduction in AD is not well understood. Here we focused on identifying critical mechanistic steps involved in reducing P-gp in AD. We exposed isolated rat brain capillaries to 100 nM A beta 40, A beta 40, aggregated A beta 40, and A beta 42. We observed that only A beta 40 triggered reduction of P-gp protein expression and transport activity levels; this occurred in a dose-and time-dependent manner. To identify the steps involved in A beta-mediated P-gp reduction, we inhibited protein ubiquitination, protein trafficking, and the ubiquitin-proteasome system, and monitored P-gp protein expression, transport activity, and P-gp-ubiquitin levels. Thus, exposing brain capillaries to A beta 40 triggers ubiquitination, internalization, and proteasomal degradation of P-gp. These findings may provide potential therapeutic targets within the blood-brain barrier to limit P-gp degradation in AD and improve A beta brain clearance.