Apolipoprotein E (apoE) has been recently implicated in the pathogenesis of Alzheimer's disease (A.D.) One of the apoE alleles, epsilon4, behaves as an autosomal co-dominant trait in the majority of late-onset familial and sporadic A.D. The apoE4 gene dose is a major risk factor susceptibility gene for A.D. with homozygosity for this allele virtually sufficient to cause disease by age 80. ApoE is found in the defining pathologic structures in the A.D. brain, neurofibrillary tangle-bearing neurons, the neuritic plaque, and congophilic angiopathy. The amount of Abeta peptide amyloid deposited in the senile plaque is increased in A.D. patients homozygous for the apoE4 allele. In vitro, apoE avidly binds Abeta peptide and binding properties are isoform-specific. The presence of apoE extracellularly in the senile plaque and angiopathy and intracellularly in neurons bearing neurofibrillary tangles implicates apoE in the biology of these lesions. The number of neurofibrillary tangles in A.D. correlates better with the degree of dementia than does the number of senile plaques. Therefore the mechanism of formation of these structures appears important in the biochemical mechanisms of this disease. Neurofibrillary tangles are pathologic filamentous structures formed from tau protein, which normally binds and stabilizes microtubules. The assembly of tau into paired helical filaments is preceded by extensive phosphorylation, and avid binding of the tandem-repeat binding domains of tau. We propose to examine the hypothesis that the isoform-specific interaction of apoE with tau alters the normal interaction of tau with the microfilament, alters the kinetics of tau phosphorylation, and alters the ability of tau to assemble the pathologic paired helical filament. We have demonstrated in vitro that apoE3, but not apoE4, binds tau to form a complex which is not dissociated by boiling in SDS. The demonstrated inability of apoE4 to bind tau may therefore result in increased tau phosphorylation, and the subsequent enhanced assembly of the pathologic paired helical filament. Pilot experiments on cultured neurons demonstrate that the addition of exogenous purified and delipidated apoE3 increases neurite extension, which is dependent on microtubules. We will determine whether exogenous apoE3 binds tau in the cultured neuron, as previously demonstrated in vitro, and will determine if apoE4 affects neurite outgrowth. In vitro experiments will determine whether the isoform-specific apoE interaction with tau alters tau binding to the microtubule, alters tau phosphorylation, or alters the formation of the paired helical filament. Alterations of tau metabolism or function by isoform-specific interaction with apoE may explain the mechanism of pathogenesis of apoE4 associated with late-onset Alzheimer's disease.
