Ubiquitin is detected in neurofibrillary tangles and senile plaque neurites of Alzheimer disease brains

Neurofibrillary tangles (NFT) and neurites associated with senile plaques (SP) in Alzheimer disease-affected brain tissues were specifically immunostained with affinity-purified antibody preparations directed against ubiquitin. In addition, a class of neurites seen in brain regions containing NFT and SP were also specifically stained. Cross-reactivity of the ubiquitin antisera for tau protein, neurofilament proteins, and high molecular weight microtubule-associated proteins (MAPs) were ruled out by (i) the inability of the ubiquitin antisera to stain these proteins in immunoblotting experiments and (ii) the inability of tau, neurofilament, and MAP preparations, when preincubated with the ubiquitin antisera, to inhibit the selective neurofibrillar staining observed. Our results are consistent with the suggestion that ubiquitin is covalently associated with the insoluble neurofibrillary material of NFT and SP. We propose that the ubiquitin-mediated degradative pathway may be ineffective in removing these fibrillar structures in Alzheimer disease brain.