Passage of amyloid β protein antibody across the blood–brain barrier in a mouse model of Alzheimer’s disease
Introduction
Alzheimer’s disease is the leading cause of dementia in Western countries. Histological hallmarks include neurofibrillary tangles and amyloid plaques. The latter is composed of fibrillated amyloid β protein (AβP) and has been proposed as playing a causal role in Alzheimer’s disease [14], [29]. Transgenic mice which overexpress amyloid precursor peptide (APP) have elevated brain levels of AβP and an abundance of amyloid plaques [15]. These mice also have been reported to have impairments in learning and memory [10].
Passive and active immunization against AβP has been shown to decrease the number of amyloid plaques and to reverse to some degree the learning and memory deficits in transgenic mice [10], [20], [27]. However, it is usually assumed that antibodies cannot cross the blood–brain barrier (BBB). Serum proteins in general are excluded from the brain unless there is a specific transport system for them at the BBB. In general, the CSF/serum ratio for most antibodies is similar to that for albumin, a substance with very limited entry into the brain. Indeed, it may be important to exclude antibodies from the CNS as their Fc region is capable of microglial and endothelial activation with resulting immune cell invasion [16].
However, some antibodies appear capable of crossing the BBB when they are directed against transporter systems located there [7], [13]. This has raised the question of how antibodies might be working and whether antibodies directed at AβP might indeed be able to cross the BBB, especially in mice which overexpress APP [2], [5], [30].
Here, we determined whether an antibody directed against AβP could cross the BBB. We compared the permeation of the antibody across the BBB to that of albumin, a serum protein known to cross the BBB using the extracellular pathways [3], [8], [9]. We also determined whether overexpression of APP altered the uptake by the brain of this antibody.
Section snippets
Influx rates
Antibody directed against the 1–17 region of AβP (Ant; catalog no. sc-7497, Santa Cruz Biotechnology, Santa Cruz, CA), a region of AβP that is identical in human and mouse, and human serum albumin were labeled with either or by the chloramine T method and purified on a column of G-10 Sephadex. The antibody is an affinity-purified goat polyclonal antibody and was custom purified into a vehicle which only contained sterile phosphate buffer. -Ant and -Albumin or -Ant and
Results
Brain uptake of both albumin and antibody increased with time (Fig. 1). Both substances entered the brain at about the same rate for the first hour after injection (Fig. 1, upper panel), with no statistically significant difference between the lines. This was followed by a much slower uptake phase for both compounds (Fig. 1, lower panel), with antibody entering much slower than albumin and at less than 1% its initial uptake rate.
Antibody had a biphasic clearance from blood with the half-time
Discussion
Besides treatment of Alzheimer’s disease, peripherally administered antibodies have also been proposed in the treatment of prion disease, cocaine addiction, and imipramine neurotoxicity [23], [25], [26]. The studies here show that initially after intravenous injection, antibody slowly enters the brain. This rate of uptake is not statistically different from that of albumin. Similar early uptake rates show that antibody most likely enters the brain by the extracellular pathways used by albumin
Acknowledgements
This work was supported by VA Merit Review, R01 NS41863, and RO1 AA12743.
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