Several studies have highlighted the close relationship between Alzheimer's disease (AD) and alterations in the bidirectional transport of amyloid-β (Aβ) peptides across the blood-brain barrier (BBB). The brain capillary endothelial cells (BCECs) that compose the BBB express the receptors and transporters that enable this transport process. There is significant in vivo evidence to suggest that P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) restrict Aβ peptides entry into the brain, whereas the receptor for advanced glycation end-products (RAGE) seems to mediate apical-to-basolateral passage across the BBB. However, deciphering the molecular mechanisms underlying these in vivo processes requires further in vitro characterization. Using an in vitro BBB model and specific competition experiments against RAGE, we have observed a significant decrease in apical-to-basolateral (but not basolateral-to-apical) transport of Aβ1-40 and Aβ1-42 peptides through BCECs. This transport is a caveolae-dependent process and fits with the apical location of RAGE observed in confocal microscopy experiments. Inhibition of P-gp and BCRP using different inhibitors increases transport of Aβ peptides suggesting that these efflux pumps are involved in Aβ peptide transport at the BCECs level. Taken as a whole, these results demonstrate the involvement of the caveolae-dependent transcytosis of Aβ peptides through the BBB in a RAGE-mediated transport process, reinforcing the hypothesis whereby this receptor is a potential drug target in AD.

In brain, excess cholesterol is metabolized into 24S-hydroxycholesterol (24S-OH-chol) and eliminated into the circulation across the blood-brain barrier. 24S-OH-chol is a natural agonist of the nuclear liver X receptors (LXRs) involved in peripheral cholesterol homeostasis. The effects of this oxysterol on the pericytes embedded in the basal lamina of this barrier (close to the brain compartment) have not been previously studied. We used primary cultures of brain pericytes to demonstrate that the latter express LXR nuclear receptors and their target gene ATP-binding cassette, sub-family A, member 1 (ABCA1), known to be one of the major transporters involved in peripheral lipid homeostasis. Treatment with 24S-OH-chol caused an increase in ABCA1 expression that correlated with a reverse cholesterol transfer to apolipoprotein E, apolipoprotein A-I, and high density lipoprotein particles. Inhibition of ABCA1 decreased this efflux. As pericytes are able to internalize the amyloid-β peptides which accumulate in brain of Alzheimer's disease patients, we then investigated the effects of 24S-OH-chol on this process. We found that the cellular accumulation process was not modified by 24S-OH-chol treatment. Overall, our results highlight the importance of the LXR/ABCA1 system in brain pericytes and suggest a new role for these cells in brain cholesterol homeostasis.