A novel tau transcript in cultured human neuroblastoma cells expressing nuclear tau.

We previously reported the presence of the microtubule-associated protein, tau in the nuclei of primate cells in culture. The present study confirms the existence of nuclear tau in two human neuroblastoma cells lines by indirect immunofluorescence and Western blot using mAbs to tau. Northern blot analysis of poly A+ mRNA detects a novel 2-kb tau transcript coexpressed with the 6-kb message in cultured human cells and human frontal cortex. PCR and cDNA sequencing demonstrate that the 2-kb message contains the entire tau coding region. Furthermore, actinomycin D transcription inhibition experiments indicate that the 2-kb message is not derived from the 6-kb message, but instead arises from the original tau transcript. One of the human neuroblastoma cell lines examined contains both nuclear and cytoplasmic tau as assayed by both Western blot and indirect immunofluorescence. Northern blot analysis of this cell line indicates that copious amounts of the 2-kb message are present while little of the 6-kb transcript is obvious. Immunofluorescence analysis of this cell line demonstrates that the cytoplasmic tau is not localized to microtubules. Together, these results indicate that the 2-kb tau message in humans may specify tau for non-microtubule functions in both the cytoplasm and the nucleus. We hypothesize that this is accomplished via a message targeting mechanism mediated by the untranslated regions of the tau messages.