Inclusion formation in Huntington's disease models

Huntington's disease (HD) is a progressive genetic neurodegenerative disease caused by the expansion of the CAG repeat in the HD gene and by the expression of pathological polyglutamine tracts in the huntingtin protein. Abnormal aggregates of protein (known as neuronal intranuclear inclusions) are found in the brains of HD patients and mouse models of HD. However, the role of inclusions in HD is controversial. To understand better the role of inclusions in HD, aggregate formation was studied using two well-established transgenic models of HD: an in vitro PCI 2 cell model carrying inducible transgenes with normal or pathological length CAG repeats and a transgenic mouse model, the R6/2 line. For the first time, time-lapse microscopy of inducible transgenic HD cells made it possible to visualise a disease-associated process in living cells. The experiments described in Chapter 3 showed that HD aggregate formation was dynamic, complicated and inversely related to cell death. Further investigations with these cells in Chapters 4-7 showed that some mechanisms underlying HD aggregate formation include proteasome dysfunction, calcineurin inhibition and cell differentiation. HD aggregate formation was also studied in R6/2 mouse brain. The experiments described in Chapter 8 showed that in vivo aggregate formation was dynamic and strikingly similar to that seen in vitro. Huntingtin aggregation was an early and ordered event that could occur as early as two weeks after birth, considerably earlier than that reported previously and well before any observable R6/2 phenotype. Huntingtin aggregation preceded ubiquitin labelling in vitro and in vivo, suggesting that the timing of ubiquitin labelling may be a determinant in the onset of HD pathology. Together, the work presented here suggests that aggregating huntingtin may not be immediately recognised by HD cells as 'toxic'. Thus, these studies support the idea that aggregate formation may have bimodal effects in HD, by being protective and toxic at different stages of aggregate development.