Antioxidants in Huntington's disease

Abstract

Huntington's disease (HD) is a prototypical neurodegenerative disease in which there is selective neuronal degeneration, which leads to progressive disability, manifesting itself as a movement disorder, with both psychiatric and cognitive impairment. The disease is caused by a cytosine-adenine-guanine (CAG) repeat expansion in the huntingtin gene, which causes an expanded polyglutamine repeat in the huntingtin protein, resulting in a protein with a novel gain of function. The mutant huntingtin protein causes neuronal dysfunction and eventual cell death in which transcriptional impairment, excitotoxicity, oxidative damage, inflammation, apoptosis and mitochondrial dysfunction are all implicated. A critical transcriptional impairment may be impaired expression and function of peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), a master co-regulator of mitochondrial biogenesis and expression of antioxidant enzymes. A deficiency of PGC-1α leads to increased vulnerability to oxidative stress and to striatal degeneration. The extent and severity of the oxidative damage in HD are features well recognized but perhaps under-appreciated. Oxidative damage occurs to lipids, proteins and deoxyribonucleic acid (DNA), and it has been suggested that the latter may contribute to CAG repeat expansion during DNA repair [1]. A marked elevation of oxidized DNA bases occurs in patients' plasma, which may provide a biomarker of disease progression. Antioxidants are effective in slowing disease progression in transgenic mouse models of HD, and show promise in human clinical trials. Strategies to transcriptionally increase expression of antioxidant enzymes by modulating the Nrf-2/ARE pathway, or by increasing expression of PGC-1α hold great promise for developing new treatments to slow or halt the progression of HD. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Figure 1

(Top) Vicious cycle induced by mutant htt (mhtt): mhtt causes damage to mitochondria, which then produce harmful reactive oxygen species (ROS), which in turn damage more mitochondria. Mitochondria in mutant cells are thus particularly vulnerable to stresses such as respiratory chain complex inhibitors, Ca⁺² and ROS. (Centre) mhtt binds more tightly to the mitochondrial fission protein Drp-1, causing mitochondrial fragmentation, mhtt also impairs mitochondrial movement thus reducing ATP at nerve terminals. (Bottom) mhtt interferes with the function and/or transcription of PGC-1α, thus resulting in downregulation of PGC-1α target genes such as those involved in mitochondrial biogenesis and antioxidant defense.