Apolipoprotein E (apoE), a plasma apolipoprotein that plays a central role in lipoprotein metabolism, is localized in the senile plaques, congophilic angiopathy, and neurofibrillary tangles of Alzheimer disease. Late-onset familial and sporadic Alzheimer disease patients have an increased frequency of one of the three common apoE alleles, e4, suggesting apoE4 is associated with increased susceptibility to disease. To follow up on this suggestion, we compared the binding of synthetic amyloid .8 (.3/A4) peptide to purified apoE4 and apoE3, the most common isoform. Both isoforms bound synthetic P/A4 peptide, the primary constituent of the plaque and angiopathy, forming a complex that resisted dissociation by boiling in SDS. Oxygen-mediated complex formation was implicated because binding was increased in oxygenated buffer, reduced in nitrogen-purged buffer, and prevented by reduction with dithiothreitol or 2-mercaptoethanol. Binding of fI/A4 peptide was saturable at 10-4 M peptide and required residues 12-28. Examination of apoE fragments revealed that residues 244-272 are critical for complex formation. Both oxidized apoE4 and apoE3 bound fi/A4 peptide; however, binding to apoE4 was observed in minutes, whereas binding to apoE3 required hours. In addition, apoE4 did not bind (8/A4 peptide at pH < 6.6, whereas apoE3 bound P/A4 peptide from pH 7.6 to 4.6. Together these results indicate differences in the two isoforms in complexing with the f/A4 peptide. Binding of P/A4 peptide by oxidized apoE may determine the sequestration or targeting of either apoE or ./A4 peptide, and isoformspecific differences in apoE binding or oxidation may be involved in the pathogenesis of the intra-and extracellular lesions of Alzheimer disease.

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The apoipoprotein E (apoE) The inheritance of apolipoprotein E4 (APOE, gene; apoE, protein) is associated with late-onset familial and sporadic Alzheimer disease (1-3). The allele frequency of APOE4 is greatly increased in patients in late-onset Alzheimer disease families and is also markedly elevated (0.40 ± 0.026 in patients; 0.16 ± 0.027 in aged-matched controls; P < 0.00001) in a series of cases of sporadic Alzheimer disease (2). The increased allele frequency of APOE4 in Alzheimer disease has been confirmed in many studies from several countries. Corder et aL (4) reported that the risk of Alzheimer disease was increased as a function of the inherited dose of APOE4 and that the mean age of onset (i) was lowered with each APOE4 allele (= 68.4 ± 1.2 yr, APOE4/4;Kx= 75.5 + 1.0 yr, APOE3/4; x = 84.3 ± 1.3 yr, APOE3/3) in late-onset families. Inheritance of APOE2 alleles decreased the risk of disease and increased the age of onset (5), so that APOE alleles appear to affect the rate of disease expression (4, 5). ApoE3 is the most common isoform in the general population and contains a single cysteine residue at position 112; apoE4 contains an arginine at this position; additionally apoE2 contains an Arg-158--+ Cys difference (6, 7). Isoform-specific properties ofapoE have been described and include different binding properties with the low-density lipoprotein receptor, with P/A4 peptide, and with lipoprotein particles (8-10).Also, the single cysteine in apoE3 permits disulfide bond formation with itself or other molecules (refs. 11 and 12; for review, see ref. 7). In brain tissue from Alzheimer disease patients, apoE is localized to neuritic plaques, vascular amyloid, and some neurofibrillary tangle-bearing neurons (1, 13, 14). In vitro, P/A4 peptide binds more avidly to apoE4 than to apoE3 (8).A comparison of brain sections from Alzheimer disease patients homozygous for APOE4 and APOE3 has demonstrated an increase in the number of amyloid plaques, the density of (3/A4 peptide immunoreactivity, and the area of brain sections occupied by plaques in APOE4/4 patients (15). Therefore, the well-recognized neuropathological heterogeneity of B/A4 peptide immunoreactivity can be explained by the isoform of apoE inherited by the affected patients. These data suggest that apoE4 decorates 8/A4 peptide-containing plaques better than apoE3 in vivo.Dementia in Alzheimer disease is generally accepted to better correlate with the neurofibrillary pathology than with the extent of P/A4 peptide deposition (16). Neurofibrillary lesions contain paired helical filaments, in which the principal constituent is hyperphosphorylated tau, a microtubuleassociated protein (17-21). Because of the genetic relevance of APOE4 and the presence of immunoreactive apoE in neurons containing neurofibrillary tangles, we examined isoform-specific interactions of apoE with recombinant tau before and after phosphorylation with a brain extract. MATERIALS AND METHODSRecombinant tau-40 (441-aa tau isoform from human brain) (22) was expressed and pur...

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