Cerebrovascular clearance of amyloid-beta: Kinetics analysis, pathogenesis, and therapeutic approaches

Amyloid-beta (Abeta) is a by-product of neuronal metabolism that plays central pathological role in Alzheimer's disease (AD). Cerebral level of Abeta is regulated by its brain production and clearance. Mounting evidence suggests that Abeta accumulation in the brain of late-onset AD patients is related to its impaired clearance from the brain. Clearance of Abeta from the brain takes place by three pathways, transport across the blood-brain barrier (BBB), brain degradation, and bulk flow of cerebrospinal fluid. However, the contribution of each pathway remains unclear. In this work, we estimated that 62% of intracerebrally injected Abeta is cleared across the BBB while 38% is cleared by brain degradation in the brains of wild type mice. Given the major contribution of BBB to the total clearance of Abeta, evaluation of kinetic parameters of Abeta uptake, efflux and degradation by endothelial cells of the BBB is essential to understand key steps involved in Abeta clearance and its brain disposition. Using bEnd3 and hCMEC/D3 cells, as in vitro mouse and human BBB models, respectively, a mechanistic two-compartment model that describes Abeta disposition by endothelial cells was established. The established model demonstrated a significant role of the endothelial cells in Abeta clearance from the brain, and provided insights on the differences between mouse and human endothelial cells in Abeta clearance. Taking into consideration this important contribution of the BBB in maintaining Abeta brain homeostasis, it is suggestive that accumulation of Abeta and consequent formation of different Abeta aggregates in the brain of AD patients could disrupt the BBB integrity, which in turn would further reduce Abeta clearance and contribute to accumulation of more cerebral Abeta. This hypothesis was evaluated and our findings showed that a mixture of Abeta consisting of nanomolar levels of both Abeta40 monomer and Abeta42 oligomer exerts toxic effect against the endothelial cells of the BBB and reduced Abeta clearance. Then, and given the importance of functional BBB in maintaining Abeta homeostasis, we investigated the BBB as a therapeutic target to prevent and/or slow the progression of AD via enhancing Abeta clearance across the BBB. The dietary intake of extra-virgin olive oil (EVOO) has been linked to reduced risk of AD among Mediterranean population; however, the mechanism underlying this protective effect is incompletely understood. Therefore and according to preliminary in vitro data observed with oleocanthal, a phenolic compound present in EVOO, we hypothesized that EVOO exerts its protective effect against AD and CAA by maintaining Abeta brain homeostasis. A series of biochemical analyses of amyloid accumulation in the brains of TgSwDI mice were performed, and the results showed a significant reduction in the brains' levels of Abeta after feeding the mice with EVOO-enriched diet. This reduction was associated with improved behavioral functions and reduced Abeta related pathological alterations. To explain the observed reduction in A(3 levels with EVOO consumption, we studied possible alterations in its production and clearance pathways. Our results suggested that EVOO could provide its protection effect against Abeta pathogenesis mainly by enhancing Abeta clearance across the BBB and through ApoE-dependent clearance pathway. These findings support beneficial effect of EVOO, as a major part of the Mediterranean diet, against AD and CAA. Collectively, above findings support our premise that enhanced cerebral Abeta clearance as a promising therapeutic strategy to decrease Abeta load in the brains of AD patients and thus delay or slow the progression of AD. In addition, the above findings signify the importance of BBB in Abeta clearance, and the involvement of BBB breakdown in many neurological diseases. Thus, it would be of a great benefit to develop therapeutics that target the BBB. To do so, we utilized a cell-based BBB in vitro model to high-throughput screen (HTS) hundreds of molecules (Sigma LOPACRTM1280 library) for their effect on the BBB function. The screening results could identify 62 compounds as BBB integrity disruptors, and 50 hits as BBB integrity enhancers. The identification of BBB integrity enhancers is considered a compelling approach to develop therapeutic compounds that could be used for the treatment and/or prevention of BBB breakdown associated with neurological diseases such as AD and CAA.