Evidence for a diffusional model of Alzheimer amyloid A4 (beta-amyloid) deposition during neuritic plaque formation

A recent study reported that Alzheimer senile plaques immunostained with monoclonal antibodies against the A4 (beta-amyloid) region of the amyloid precursor protein show gradients of density (Majocha R. E., Benes F. M., Reifel R. L., Rodenrys A. M. and Marotta C. A., Proc. natn. Acad. Sci. U.S.A. 85, 6182-6186, 1988). Although more than one explanation was suggested for this observation, the possible involvement of a diffusional process during plaque maturation was considered. In order to examine this hypothesis, specimens from prefrontal cortex, entorhinal area and hippocampal formation were immunoprocessed in a similar fashion and subjected to quantitative microdensitometric analyses of A4 amyloid reaction product. All plaques in the three brain areas examined showed a curvilinear relationship between the area of amyloid reaction product (expressed in pixel counts) and optical density (expressed as each of six grey scale levels). There was an increase in the area of amyloid at progressively lower density levels. When the area of amyloid reaction product at each density level was correlated with the overall size of individual plaques, it was found that there was a striking increase in the correlation coefficients at progressively lower grey scale levels, with r = 0.853 at the lowest level examined. When a second order derivation of these correlations was performed by expressing individual r-values with respect to an optical density index, an asymptotic relationship resulted with the lowest density levels showing an increasingly sharp rise toward unity. These data are consistent overall with a model for plaque maturation that involves diffusion of amyloid protein through the extracellular space from focal regions of high density where synthesis and/or release may occur.