The Blood-Brain Barrier as a Therapeutic Target for Alzheimer’s Disease and Related Disorders

Abstract

The blood-brain barrier (BBB) is a highly selective and protective membrane that preserves brain homeostasis and prevents the free entry of chemicals, toxins and drugs to the brain. The BBB barrier function is primarily provided by the endothelial cells lining the brain capillaries, which create a physical barrier separating the brain from blood. In aging and in numerous neurological diseases, including Alzheimer’s disease (AD), the BBB integrity is affected, as supported by several studies reported leaky BBB in the brains of aged and AD mice models, and brains of AD patients. These findings point toward the BBB as a potential therapeutic target to prevent or halt AD progression by restoring its integrity and function. In the first project, we utilized our recently developed high-throughput screening (HTS) assay to find and evaluate agents that improve BBB integrity from toxic amyloid. In this assay, an in-vitro BBB model composed of mouse brain endothelial cells (bEnd3) grown on 96-well plate inserts was used. Lucifer Yellow as a paracellular permeability marker was used to assess the effect of screened compounds on the intactness of the in-vitro BBB model with and without amyloid-β (Aβ). Using this model, we screened more than 3000 compounds from 3 different libraries namely, Sigma LOPAC1280 library (1280 compounds), NIH Clinical Collection (716 compounds) and NINDS Custom Collection (1040 compounds) to identify enhancers of the in-vitro BBB model intactness. Identified hits were FDA approved drugs, investigational compounds, and natural product compounds. Among the FDA approved drugs, granisetron emerged as a promising candidate selected for further evaluation. In my second project, I evaluated the ability of granisetron to enhance the integrity of the BBB in-vivo using aged and AD mouse models. Granisetron was tested in C57Bl/6J young and aged wild-type mice and in a transgenic mouse model of AD namely TgSwDI. Findings demonstrated granisetron enhanced the BBB integrity in both aged and AD mice. This effect was accompanied by an overall reduction in Aβ load and neuroinflammation in TgSwDI mice brains. Supported by proteomics analysis of mice brains, the effect of granisetron to reduce Aβ-induced calcium influx significantly in-vitro and modulate calcium dyshomeostasis could be mediated by inducing calmodulin-dependent protein kinase II (CaMKII)/cAMP-response element binding protein (CREB) pathway in the TgSwDI mice brains. These results suggest granisetron repurposing as a potential drug to withhold, slow and/or treat AD. Besides BBB dysfunction, AD is characterized by the accumulation of extracellular Aβ plaques and neurofibrillary tangles, neuroinflammation and impaired autophagy. Thus, novel strategies that target multiple disease pathways would be indispensable to prevent, halt and/or treat the disease. Growing body of evidence including our studies supported a protective effect of oleocanthal (OC) and extra-virgin olive oil (EVOO), a major component of the Mediterranean diet, at early AD stages before the pathology onset starts, however, the mechanism(s) by which OC and EVOO exert such effect, and whether this effect extends to a later stage of AD remain unknown. Thus, in my third project, I sought to evaluate the effect of OC-rich EVOO consumption on enhancing the BBB integrity at an advanced stage of AD in TgSwDI mice and on Aβ-related pathology, starting at 6 months of age for 3 months treatment ending at 9 months of age, and then to elucidate the potential mechanism(s) by which OC-rich EVOO exerts the observed beneficial effect. Overall our findings suggest OC-rich EVOO reduced Aβ-related pathology by reducing neuroinflammation through inhibition of NLRP3 inflammasome and inducing autophagy markers through activation of the AMPK/ULK1 pathway. Thus, diet supplementation with OC-rich EVOO could have the potential to slow and/or hold the progression of AD.