Constitutive Alzheimer's-type tau epitopes in a neuritogenic rat CNS cell line

Paired helical filaments (PHFs) of Alzheimer's disease (AD) largely comprise hyperphosphorylated forms of the cytoskeletal protein tau. AD-type tau phosphoepitopes, detected by various monoclonal antibodies, are absent from normal adult neurons, but recent studies have shown that their expression may contribute to neuritogenesis and axon differentiation in the developing nervous system. Therefore, we have examined a brain nerve cell line that is spontaneously neuritogenic for possible expression of AD-type tau epitopes. The neuritogenic rat brain cell line B103 was found to constitutively produce two-AD related epitopes of tau, detected by cellular immunofluorescence studies with the PHF-1 and Alz-50 monoclonal antibodies. Biochemical studies showed that the antibodies bound to proteins within the molecular, weight range expected for phosphorylated tau isoforms. Further verification was established by use of tau antisense oligomers, which eliminated cellular immunofluorescence due to the AD-related monoclonals and polyclonal anti-tau but did not eliminate fluorescence due to anti-tubulin. Cells treated with tau antisense were not neurite-free. Neurites that remained, however, were abnormal, generally short and wavy in appearance. Cellular distribution of the tau epitopes was found to be particularly interesting. Alz-50 recognized only cytoplasmic tau whereas PHF-1 recognized nuclear tau as well as cytoplasmic. Thus, the two epitopes, are morphologically segregated within the cell. Because subcellular segregation of tau is compromised in Alzheimer's disease, mechanisms that segregate AD-type phosphotau epitopes in B103 cells may have relevance to this neurodegenerative disorder.