Amyloid-beta peptides 40 and 42 employ distinct molecular pathways for cell entry and intracellular transit at the BBB endothelium

Blood-brain barrier (BBB) is a critical portal regulating the bidirectional transport of amyloid beta (Aβ) proteins between blood and brain. Disrupted trafficking at the BBB may not only promote the build-up of Aβ plaques in the brain parenchyma, but also facilitate Aβ accumulation within the BBB endothelium, which aggravates BBB dysfunction. Soluble Aβ42:Aβ40 ratios in plasma and cerebrospinal fluid have been reported to decrease during Alzheimer’s disease (AD) progression. Our previous publications demonstrated that trafficking of Aβ42 and Aβ40 at the BBB is distinct and is disrupted under various pathophysiological conditions. However, the intracellular mechanisms that allow BBB endothelium to differentially handle Aβ40 and Aβ42 have not been clearly elucidated. In this study, we identified mechanisms of fluorescently labeled Aβ (F-Aβ) endocytosis in polarized human cerebral microvascular endothelial (hCMEC/D3) cell monolayers using pharmacological inhibition and siRNA knock-down approaches. Further, intracellular transit of F-Aβ following endocytosis was tracked using live cell imaging. Our studies demonstrated that both F-Aβ peptides were internalized by BBB endothelial cells via energy, dynamin and actin dependent endocytosis. Interestingly, endocytosis of F-Aβ40 is found to be clathrin-mediated, whereas F-Aβ42 endocytosis is caveolae-mediated. Following endocytosis, both isoforms were sorted by the endo-lysosomal system. While Aβ42 was shown to accumulate more in the lysosome which could lead to its higher degradation and/or aggregation at lower lysosomal pH, Aβ40 demonstrated robust accumulation in recycling endosomes which may facilitate its transcytosis across the BBB. These results provide a mechanistic insight into the selective ability of BBB endothelium to transport Aβ40 versus Aβ42. This knowledge contributes to the understanding of molecular pathways underlying Aβ accumulation in the BBB endothelium and associated cerebrovascular dysfunction as well as amyloid deposition in the brain parenchyma which are implicated in AD pathogenesis. ### Competing Interest Statement The authors have declared no competing interest.