Neuronal toxicity of expanded polyglutamine depends on intracellular distribution in addition to the expression level

The proteins that accumulate in pathologic lesions of neurodegenerative disorders are thought to be closely associated with neuronal cell damage. However, whether or not the formation of cytoplasmic or nuclear inclusions by expanded polyglutamine (polyQ) is directly toxic to neurons has been controversial to date. We prepared a culture model system in which polyQ tracts were transfected into Neuro2a, cells of neuronal origin, to study novel factors involved in cell toxicity of polyQ tracts to neuronal cells. Pathogenic polyQ tracts of 79 repeats (Q79C) when expressed in cytoplasm of Neuro2a cells changed in their intracellular distribution patterns from homogeneous, via punctate aggregates, to massive aggregates with incubation time. Some polyQ tracts formed nuclear inclusions. Cytoplasmic massive aggregates of Q79C tended to be associated with apoptotic fate of Neuro2a cells. Cells exhibiting cytoplasmic massive inclusions had the highest expression level of polyQ tracts among cells with four patterns of intracellular distribution. The elevation in the expression levels of polyQ tracts was not due to the difference in the initial transfection efficiency. When compared among cells expressing polyQ tracts at similar levels, damages were most remarkable in cells with cytoplasmic massive aggregate in terms of shrunken cellular and nuclear sizes. Cells with the other patterns of polyQ tract distribution such as cytoplasmic homogeneous, cytoplasmic punctate and nuclear inclusions were relatively spared. These data suggest that the severity of cell damages depends on the type of intracellular distribution of polyQ tracts, in addition to the expression level of polyQ tracts.