Distinct Uptake Kinetics of Alzheimer Disease Amyloid- 40 and 42 at the Blood-Brain Barrier Endothelium.

Blood-brain barrier (BBB) endothelial cells lining the cerebral microvasculature maintain dynamic equilibrium between soluble amyloid- (A) levels in the brain and plasma. The BBB dysfunction prevalent in Alzheimer disease contributes to the dysregulation of plasma and brain A and leads to the perturbation of the ratio between A42 and A40, the two most prevalent A isoforms in patients with Alzheimer disease. We hypothesize that BBB endothelium distinguishes between A40 and A42, distinctly modulates their trafficking kinetics between plasma and brain, and thereby contributes to the maintenance of healthy A42/A40 ratios. To test this hypothesis, we investigated A40 and A42 trafficking kinetics in hCMEC/D3 monolayers (human BBB cell culture model) in vitro as well as in mice in vivo. Although the rates of uptake of fluorescein-labeled A40 and A42 (F-A40 and F-A42) were not significantly different on the abluminal side, the luminal uptake rate of F-A42 was substantially higher than F-A40. Since higher plasma A levels were shown to aggravate BBB dysfunction and trigger cerebrovascular disease, we systematically investigated the dynamic interactions of luminal [I]A peptides and their trafficking kinetics at BBB using single-photon emission computed tomography/computed tomography imaging in mice. Quantitative modeling of the dynamic imaging data thus obtained showed that the rate of uptake of toxic [I]A42 and its subsequent BBB transcytosis is significantly higher than [I]A40. It is likely that the molecular mechanisms underlying these kinetic differences are differentially affected in Alzheimer and cerebrovascular diseases, impact plasma and brain levels of A40 and A42, engender shifts in the A42/A40 ratio, and unleash downstream toxic effects. SIGNIFICANCE STATEMENT: Dissecting the binding and uptake kinetics of A40 and A42 at the BBB endothelium will facilitate the estimation of A40 versus A42 exposure to the BBB endothelium and allow assessment of the risk of BBB dysfunction by monitoring A42 and A40 levels in plasma. This knowledge, in turn, will aid in elucidating the role of these predominant A isoforms in aggravating BBB dysfunction and cerebrovascular disease.