The Proteolysis of Apolipoprotein E in Alzheimer\u27s Disease

Harboring the apolipoprotein E4 (APOE4) allele is the greatest genetic risk factor associated with late-onset (sporadic) Alzheimer’s disease (AD), however the mechanism by which apoE4 contributes to the pathology of AD is unknown. The proteolysis of apoE4 has been suggested to contribute to AD pathology due to a possible toxic gain, or loss of function. In order to determine if apoE4 is being cleaved, we designed and characterized a site-directed cleavage antibody directed at position D151 of full length human apoE4. The antibody (nApoECFp17) detected a predicted ~17 kDa fragment following incubation with the proteases Type-1 collagenase, and matrix metalloproteinase (MMP)-1and 9. Once the nApoECFp17 amino-terminal antibody was applied to frontal cortex AD brain sections, it revealed nuclear labeling of glial cells and neurofibrillary tangles (NFTs). In addition, nApoECFp17 regionally localized with the protease MMP-9 in plaque-rich regions in vivo, suggesting apoE4 may be cleaved extracellularly. Together these data suggest a novel cleavage event of apoE4 by Type-I collagenase and MMP-1 and 9, generating an amino-terminal fragment that is then taken up by glial cells and localizes to the nucleus. This fragment localizing to the nucleus purposes a new role of apoE4 in AD pathology where apoE may alter gene expression