Investigating the role of Huntingtin in development and disease using the zebrafish model organism.

Abstract

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disorder of typically mid-life onset, for which there is currently no cure. HD is one of nine neurological disorders caused by the expansion of a CAG trinucleotide repeat that encodes an extended polyglutamine tract within the respective disease proteins (which, in the case of HD, is Huntingtin). Curiously, despite these proteins having mostly widespread patterns of expression in the brain, a specific subset of neurons is preferentially affected in each disease, whilst other neurons also expressing the mutant protein are relatively unaffected. Furthermore, although the expression patterns of these disease proteins often overlap in distribution within the brain, the population of neurons that is most vulnerable differs from one disease to the next. Knowledge of what determines the specificity of neuronal vulnerability is likely to provide insight into the molecular mechanism(s) underlying the pathology in these diseases. The aim of this work was to use the zebrafish model organism to investigate two factors hypothesised to contribute to the specificity of neuronal vulnerability in HD: 1) region-specific somatic expansion of the disease allele, and 2) disruption of normal Huntingtin (Htt) protein function. The most significant findings of this study resulted from the investigation into the normal function of Htt. Antisense morpholino oligonucleotides were used to specifically knock down Htt expression in early zebrafish development, resulting in a wide variety of developmental defects. Most notably, Htt-deficient zebrafish had pale blood due to a decrease in haemoglobin production, despite the presence of (apparently unavailable) iron within these cells. Provision of additional iron in a bio-available form to the cytoplasm restored haemoglobin production in Htt-deficient embryos. Since blood cells acquire iron via receptor-mediated endocytosis of transferrin, these results suggest a role for Htt in the release of iron from endocytic compartments into the cytosol. Iron is required for the function of many cellular proteins and enzymes that play key roles in oxidative energy production. Disrupted iron homeostasis and decreased energy metabolism are features of HD pathogenesis that correlate to the major sites of degeneration in the HD brain. The findings of this study raise the possibility that perturbation of normal Htt function (by polyglutamine expansion) may contribute to these defects, thereby providing a novel link between Htt function and specificity of neuronal vulnerability in HD.