Formic acid treatment exposes hidden neurofilament and tau epitopes in abnormal cytoskeletal filaments from patients with progressive supranuclear palsy and Alzheimer's disease.

In cases of progressive supranuclear palsy (PSP) and Alzheimer's disease (AD) the treatment of tissue sections with formic acid (FA) disclosed a neurofilament epitope in subcortical straight (SF) and paired helical (PHF) filaments. The same treatment produced a two-fold increase of tau-reactive neuropil threads and plaque-related neurites in AD cortical tissue. These findings indicate that epitopes of cytoskeletal proteins are hidden by the beta-pleated conformation of SF and PHF components. FA treatment should be used in immunohistochemical detection of intraneuronal abnormal cytoskeletal filaments.