Permeability of Proteins at the Blood–Brain Barrier in the Normal Adult Mouse and Double Transgenic Mouse Model of Alzheimer's Disease

Abstract

The permeability of albumin, insulin, and human Aβ 1–40 at the blood–brain barrier (BBB) was determined in the normal adult mouse (B6/SJL) and in the double transgenic Alzheimer mouse (APP, PS1) by using an I.V. bolus injection technique to quantify the permeability coefficient-surface area (PS) product for each protein after correction for the residual plasma volume (Vp) occupied by the protein in the blood vessels of different brain regions using a second aliquot of the same protein radiolabeled with a different isotope of iodine (125I vs 131I) as a vascular space marker. This technology for quantifying BBB permeability of proteins was adapted from the rat to the mouse and involved catheterizing the femoral artery and vein of the mouse instead of the brachial artery and vein as for the rat. Because of the smaller blood volume in the mouse, serial sampling (20 μl) of blood from the femoral artery of the mouse was performed and directly TCA precipitated to generate a whole blood washout curve for the intact protein. When similar blood sampling techniques were used in the rat, the PS values for albumin and insulin at the BBB were similar in these two species. In the double transgenic mouse, the Vp values for albumin were significantly increased 1.4- to 1.6-fold in five of six brain regions compared to the normal adult mouse, which indicated increased adherence of albumin to vessel walls. As a result, the PS values were significantly decreased, from 1.4- to 3.2-fold, which likely reflected decreased transport of albumin by passive diffusion. In contrast, insulin, which is taken up into the brain by a receptor-mediated transport mechanism at the BBB, showed no significant difference in the Vp values but a significant increase in the PS values in four of six brain regions. This suggests a compensatory mechanism in the Alzheimer's transgenic brain whereby there is an increased permeability to insulin at the BBB. Surprisingly, there was no significant difference in the Vp or PS values for human Aβ 1–40 at the BBB in the double transgenic Alzheimer mouse at 24, 32, or 52 weeks of age, when there is both significant Aβ levels in the plasma and amyloid burden in the brains of these animals. These data suggest that there is not an alteration in permeability to human Aβ 1–40 at the BBB with increasing amyloid burden in the double transgenic Alzheimer mouse. Although these observations suggest structural alterations at the BBB, they do not support the concept of extensive BBB damage with substantial increases in BBB permeability in Alzheimer's disease.