Evidence that Brain-Reactive Autoantibodies Contribute to Chronic Neuronal Internalization of Exogenous Amyloid-β1-42 and Key Cell Surface Proteins During Alzheimer's Disease Pathogenesis

Blood-brain barrier (BBB) permeability is a recognized early feature of Alzheimer's disease (AD). In the present study, we examined consequences of increased BBB permeability on the development of AD-related pathology by tracking selected leaked plasma components and their interactions with neurons in vivo and in vitro. Histological sections of cortical regions of postmortem AD brains were immunostained to determine the distribution of amyloid-beta(1-42) (A beta(42)), cathepsin D, IgG, GluR2/3, and alpha7 nicotinic acetylcholine receptor (alpha 7nAChR). Results revealed that chronic IgG binding to pyramidal neurons coincided with internalization of A beta(42), IgG, GluR2/3, and alpha 7nAChR as well as lysosomal compartment expansion in these cells in regions of AD pathology. To test possible mechanistic interrelationships of these phenomena, we exposed differentiated SH-SY5Y neuroblastoma cells to exogenous, soluble A beta(42) peptide and serum from AD and control subjects. The rate and extent of A beta(42) internalization in these cells was enhanced by serum containing neuron-binding IgG auto antibodies. This was confirmed by treating cells with individual antibodies specific for alpha 7nAChR, purified IgG from AD or non-AD sera, and sera devoid of IgG, in the presence of 100 nM A beta(42). Initial co-localization of IgG, alpha 7nAChR, and A beta(42) was temporally and spatially linked to early endosomes (Rab11) and later to lysosomes (LAMP-1). A beta(42) internalization was attenuated by treatment with monovalent F(ab) antibody fragments generated from purified IgG from AD serum and then rescued by coupling F(ab) fragments with divalent human anti-Fab. Overall, results suggest that cross-linking of neuronbinding autoantibodies targeting cell surface proteins can accelerate intraneuronal A beta(42) deposition in AD.