A 26–30 kDa developmentally-regulated tauisoform localized within nuclei of mitotic humanneuroblastoma cells

Abstract

Tau isoforms migrating at 46–68 and 97–115 kDa were prominent within heat-stableTriton-soluble material, and were present in lesser concentration within Triton-insolublecytoskeletons, derived from undifferentiated SH-SY-5Y human neuroblastoma cells. Conversely,a 26–30 kDa tau isoform was enriched in the cytoskeleton and detected at relatively minor levelswithin cytosolic fractions. Pulse labeling with 35S-methionine indicated that this 26–30kDa small tau did not represent a breakdown product of larger isoforms. Since the nucleus isretained within the Triton-insoluble cytoskeleton, additional cultures were fractionated ontosucrose to obtain purified nuclei. The vast majority of small tau was recovered within purifiednuclei. Small tau was reactive with tau antibodies directed towards N-terminal, C-terminal andcentral epitopes, further confirming that this small isoform was not derived from proteolyticcleavage of larger tau isoforms. Small tau demonstrated alkaline phosphatase-sensitive reactivitywith multiple phospho-dependent tau antibodies. Small tau was depleted within 3 days of retinoicacid-induced differentiation, suggesting that the putative function of this isoform may be obsoletefollowing terminal differentiation of neurons.