Involvement of a Na⁺-coupled Oligopeptide Transport System for β-amyloid Peptide (Aβ_1–42) in Brain Cells

Purpose: A Na⁺-coupled transport system in mammalian cells is responsible for the uptake of oligopeptides consisting of 5 or more amino acids. Here we investigated if this transport system is expressed in brain cells and transports the 42-amino-acid β-amyloid peptide (Aβ_1–42). Methods: The human and mouse neuronal cell lines SK-N-SH and HT22, human microglial cell line HMC-3, and human blood-brain barrier endothelial cell line hCMEC/D3 were used to monitor the uptake of [³H]-deltorphin II (a heptapeptide) and fluorescence-labeled Aβ_1–42. Results: All four cell lines exhibited Na⁺-coupled uptake of deltorphin II. Aβ_1–42 competed with deltorphin II for the uptake. Uptake of fluorescence-labeled Aβ_1–42 was detectable in these cell lines, and the uptake was Na⁺-dependent and inhibitable by deltorphin II. The Na⁺-coupled uptake disappeared at high concentrations of Aβ_1–42 due to oligomerization of the peptide. Exposure of the cells to excess iron abolished the uptake. In hCMEC/D3 cells cultured on Transwell filters, the uptake was localized preferentially to the abluminal membrane. Conclusion: A Na⁺-coupled transport system mediates the uptake of Aβ_1–42 monomers in neuronal and microglial cells. The same system is also responsible for the uptake of Aβ_1–42 from brain into blood-brain barrier endothelial cells. These findings have relevance to Alzheimer’s disease.