UCSF

Huntington's disease is a devastating inherited neurodegenerative disorder without a cure. Mutations expanding a stretch of glutamines in the first exon of the widely-expressed huntingtin protein are responsible for Huntington's disease. The disease is characterized by neuronal death within the striatum and cortex. At the cellular level, abnormal inclusions of ubiquitinated huntingtin are present in affected brain areas. The role of inclusions in cellular pathogenesis has been controversial, with inclusions potentially playing protective, detrimental, or incidental roles. We developed a high-throughput robotic imaging and analysis platform that enables us to follow the fates of individual cells and intracellular proteins over time. This platform represents a significant improvement over standard approaches of cell-population based biochemistry and conventional time-lapse microscopy in being able to monitor molecular changes as they unfold in large numbers of live cells and in quantifying the extent to which these changes predict cellular outcomes. Using this platform, we followed individual neurons expressing mutant huntingtin and determined that cells that form inclusions survive better than those that do not. Levels of mutant protein were higher in those cells that formed inclusions and in those cells that died earlier. Through the use of fluorescent reporters of ubiquitin-proteasome system function, we determined that impairment of the ubiquitin-proteasome system is toxic to neurons and is higher in cells that go on to form inclusions compared to those that do not. Following inclusion formation, ubiquitin-proteasome system function is improved. Our data unambiguously shows that inclusions are beneficial to neuronal survival and that part of this beneficial effect may be through improving ubiquitin-proteasome system function. These findings shift the focus to steps preceding inclusion formation in the search for the molecular species responsible for Huntington's disease.