The role of polyglutamine aggregate cytotoxicity in Huntington’s Disease

Polyglutamine diseases refer to a group of neuro-degenerative diseases including Huntington's disease , that share the same mutational basis: expansion of a CAG repeat coding sequence that encodes for a polyglutamine repeat in the respective proteins. Neuronal intranuclear inclusions (NII) formed by the aggregation of the expanded polyglutamine proteins is a hallmark of polyglutamine diseases. However, it is still under debate whether the polyglutamine aggregates are toxic to the cells, and if so, which aggregated forms are the toxic species. While important data has been provided by the existing transfected cell and transgenic animal models, technical aspects have limited our ability to rigorously interpret this data in terms of molecular and cellular mechanism. A number of comprehensive studies on in vitro polyglutamine aggregation kinetics, and features of the aggregates produced in vitro, provided the foundation to directly address the question of the cytotoxicity of polyglutamine aggregates. In the present study, we designed a new cell model by directly introducing in vitro synthesized polyglutamine aggregates into mammalian cells, both PC-12 and Cos-7, to study the cytotoxicity of different forms of aggregates. We found that cell viability was largely unaffected when polyglutamine aggregates are localized to the cytoplasm; aggregates of polyglutamine peptides containing a nuclear localization signal, however, are localized to nuclei and lead to dramatic cell death. This cytotoxicity is polyglutamine-sequence-specific. We also found that a short polyglutamine peptide (Q20), once aggregated and delivered to the cell nucleus, seems to be equally cytotoxic. Further studies suggest that the mechanism of cell death in this novel model system seems to be apoptotic. Our results support a direct role for polyglutamine aggregates in cytotoxicity and are consistent with a recruitmentsequestration disease mechanism. This is further supported by studies, also described here, in which a polyglutamine aggregation elongation inhibitor protects against cell death induced by nuclear polyglutamine aggregates.