Sharing of polyglutamine localization by the neuronal nucleus and cytoplasm in CAG-repeat diseases

The expansion of a trinucleotide cytosine adenine and guanine (CAG) repeat that codes for polyglutamine is a common gene mutation in the family of hereditary neurodegenerative diseases that includes Machado-Joseph disease (MJD) and dentatorubral-pallidoluysian atrophy (DRPLA). The presence of ubiquitinated neuronal intranuclear inclusions (NIIs) has been recognized as a neuropathological hallmark of these diseases, although the significance of NIIs in the pathogenesis remains a matter of controversy. In a previous study of DRPLA, we proposed that intranuclear diffuse accumulation of mutant proteins is another pathological characteristic of neurones, and that the variable prevalence of this characteristic may be relevant to the variation of clinical symptoms in patients with different repeat sizes. Recently, we also disclosed that polyglutamine tracts are localized in a subset of lysosomes in affected neurones. The present immunohistochemical study of autopsied MJD and DRPLA brains shows that the nucleus and cytoplasm of affected neurones share the subcellular distribution of expanded polyglutamine tracts, the pattern of distribution being specific to each diseased brain. The results suggest that in CAG-repeat diseases, mutant proteins are involved in both the ubiquitin/proteasome and endosomal/lysosomal pathways for protein degradation in different intraneuronal compartments, where their accumulation may exert distinct influences on neuronal physiology.