Abnormal phosphorylation of tau precedes ubiquitination in neurofibrillary pathology of Alzheimer disease.

On tissue sections of Alzheimer brain, 4 antibodies to tau immunolabel not only neurofibrillary tangles, neuritic plaques and neuropil threads but also the tangle-free cytoplasm of a subset of hippocampal and cortical neurons we believe to be at a stage of alteration preceding the formation of paired helical filaments (PHF). Pretreatment of tissue sections with alkaline phosphatase leads to an increase in staining intensity and in number of immunoreactive lesions with antibodies directed to an amino terminal and to a mid-region of the tau molecule. The diffuse neuronal staining could not be observed with any of 7 monoclonal antibodies recognizing ubiquitin. We conclude (1) that abnormal phosphorylation of tau occurs prior to its incorporation into PHF and leads to its accumulation in the nerve cell body and (2) that ubiquitin is seen associated only when a neurofibrillary tangle is already formed.