DIETARY-CHOLESTEROL INDUCES ALZHEIMER-LIKE BETA-AMYLOID IMMUNOREACTIVITY IN RABBIT BRAIN

The premature occurrence of Alzheimer-like beta-amyloid senile plaques in non-demented subjects with coronary artery disease prompted neurop athologic investigations of rabbits fed high cholesterol diet compared to animals fed control diet. Initial studies in this accepted animal model of coronary artery disease suggest a relationship between the le ngth of rime on the cholesterol diet and increasing widespread accumul ation of neuronal beta-amyloid(1-16) immunoreactivity in the brain on a regional basis; extracellular deposition was rarely observed In the current studies, regional levels of cholesterol were established in fr esh brains from rabbits fed 2% cholesterol or control diet 8 weeks. In the same animals, activity of the free radical scavenger enzymes supe roxide dismutase (SOD) and glutathione peroxidase (GSH-Px) was determi ned in blood and brain. Adjacent sections of 4% paraformaldehyde perfu sed brain from rabbits fed 2% cholesterol 4, 6 and 8 weeks and control diet 8 weeks were immunohistochemically investigated using probes for SOD, beta-amyloid(1-12), beta-amyloid precursor protein (beta-APP; N- terminal specific), and the lysosomal enzyme cathepsin D. Cholesterol concentration increased on a regional basis in the brain after 8 weeks of experimental diet compared to controls. Dietary cholesterol induce d alterations in the activity of SOD and GSH-Px in brain paralleled ch anges in the blood The activity of SOD was increased and GSH-Px was de creased There was an identifiable relationship between regional choles terol levels and SOD activity in both experimental and control rabbits . This relationship did not occur SOY brain cholesterol levels and GSH -Px activity. Accumulation of beta-amyloid inzmunoreactivity induced b y dietary cholesterol was confirmed using another specific beta-amyloi d antibody (beta-amyloid(1-12)). In Contrast to studies utilizing beta -amyloid(1-16) antibody the incidence of extracellular beta-amyloid(1- 12) immunoreactive deposits increased with longer duration of choleste rol diet. There was no beta-APP immunoreactivity demonstrable in the b rain of any rabbit, and therefore, accumulation of this peptide did no t contribute to the enhanced beta-amyloid immunoreactivity produced by dietary cholesterol. Increased neuronal SOD immunoreactivity occurred early in the time-course of cholesterol administration and enhanced c athepsin D immunoreactivity occurred well after neuronal accumulation of beta-amyloid immrmoreactivity. As a result, increased cathepsin D e nzymatic activity probably does not initiate beta-amyloid production i n the cholesterol fed rabbit, but may potentiate the process. These st udies suggest that elevated circulating levels of cholesterol causes i ncreased levels in the brain which seems to be correlated to the level of neuronal beta-amyloid accumulation on a regional basis. By direct and/or indirect mechanisms increased brain cholesterol likely leads to enhanced free mrtical activity and the observed neuronal oxidative st ress. Via unknown mechanism(s), this oxidative stress may initiate the pathologic and neuronal accumulation of Alzheimer-like beta-amyloid p eptides, which thereafter may be deposited in the neuropil. Because ma ny of the neurophatologic alterations induced in the brains of these a nimals are similar to those found in AD brain, the data support the hy pothesis that the cholesterol Sed rabbit may be an animal model to inv estigate this dementing human disorder. (C) 1997, Medikal Press.