Receptor-mediated transport of human amyloid β-protein 1-40 and 1-42 at the blood-brain barrier

Since amyloid β-protein (Aβ) is the primary component of both vascular and parenchymal amyloid deposits in Alzheimer's disease, information regarding its permeability at the blood-brain barrier (BBB) will help elucidate the contribution of circulating Aβ to vascular and parenchymal Aβ deposition in this disease and in brain aging. The permeability of the D- and L-enantiomers of Aβ 1-40 and L-Aβ 1-42 at the BBB was determined in the normal adult rat by quantifying the permeability coefficient-surface area product (PS) for each protein after correction for the residual plasma volume (V(p)) occupied by the protein [labeled with a different isotope of iodine (¹²⁵I vs ¹³¹I)] in blood vessels of different brain regions. After a single iv bolus injection, the plasma pharmacokinetics determined by TCA precipitation, paper chromatography, and SDS-PAGE were similar for both ¹²⁵I-L-Aβ 1-40 and ¹²⁵I-L-Aβ 1-42. The PS at the BBB for L-Aβ 1-42 was significantly (1.4- to 1.8-fold) higher than for L-Aβ 1-40 and ranged from 17.7 to 26.4 x 10^-6 ml/g/s for different brain regions. A comparison of the PS values at the BBB for L-Aβ 1-40 showed no significant difference when determined at 15 or 30 min after iv bolus injection, times that reflect different levels of degradation in plasma (37.9% at 15 min and 65.5% at 30 min). The PS values obtained, therefore, were representative of the intact protein rather than degradation products. The PS values obtained for the all- D-enantiomer of Aβ 1-40 were very low and comparable to that of albumin and IgG, whose mechanism of transport is by passive diffusion. Taken together, these data imply a stereoisomer-specific, ligand-receptor interaction at the BBB for the L-Aβ proteins. The high PS values observed for L-Aβ 1-40 and 1- 42 compare to insulin, whose uptake is decidedly by a receptor-mediated transport process, and suggest a similar mechanism for L-Aβ entry into the brain.