Modulating huntingtin half-life alters polyglutamine-dependent aggregate formation and cell toxicity

A common finding among the expanded polyglutamine disor-ders is intracellular protein aggregates. Although the precise role of these aggregates in the disease process is unclear, they are generally ubiquitinated, implicating the ubiquitin– proteasome pathway in neuronal degeneration. To investigate the mechanism of aggregate formation, we have developed a cell culture model to express huntingtin designed to have an altered degradation rate through the ubiquitin-dependent N-end rule pathway. We fused the first 171 amino acids of huntingtin, containing either a pathogenic or normal polyglu-tamine tract, to the enhanced green fluorescent protein (EGFP). The half-life of soluble huntingtin-EGFP was dependent on the degradation signal and the polyglutamine tract length. However, once huntingtin-EGFP with a patho-genic tract had aggregated, the protein was extremely stable. Huntingtin-EGFP with a pathogenic glutamine tract and a shorter half-life displayed a delayed onset of aggregate for-mation and was more toxic to transfected cells. These data suggest that rapid clearance through the ubiquitin–protea-some pathway slows aggregate formation, yet increases cel-lular toxicity. Polyglutamine-induced neurotoxicity may therefore be triggered by non-aggregated protein, and aggregate formation itself may be a cellular defense mech-anism