Biochemical abnormalities and excitotoxicity in Huntington's disease brain.
The physiological role of huntingtin and the mechanisms by which the expanded CAG repeat in ITI5 and its polyglutamine stretch in mutant huntingtin induce Huntington's disease (HD) are unknown. Several techniques have now demonstrated abnormal metabolism in HD brain; direct measurement of respiratory chain enzyme activities has shown severe deficiency of complex II/III and a milder defect of complex IV. We confirm that these abnormalities appear to be confined to the striatum within the HD brain. Analysis of complex II/III activity in HD fibroblasts was normal, despite expression of mutant huntingtin. Although glyceraldehyde 3-phosphate dehydrogenase (a huntingtin binding protein) activity was normal in all areas studied, aconitase activity was decreased to 8% in HD caudate, 27% in putamen, and 52% in cerebral cortex, but normal in HD cerebellum and fibroblasts. We have demonstrated that although complexes II and III are those parts of the respiratory chain most vulnerable to inhibition in the presence of a nitric oxide (NO*) generator, aconitase activity was even more sensitive to inhibition. The pattern of these enzyme deficiencies and their parallel to the anatomical distribution of HD pathology support an important role for NO* and excitotoxicity in HD pathogenesis. Furthermore, based on the biochemical defects we have described, we suggest that NO* generation produces a graded response, with aconitase inhibition followed by complex II/III inhibition and the initiation of a self-amplifying cycle of free radical generation and aconitase inhibition, which results in severe ATP depletion. We propose that these events are important in determining neuronal cell death and are critical steps in the pathogenesis of HD.
Tabrizi
S J
SJ
University Department of Clinical Neurosciences, Royal Free and UCH School of Medicine, London, UK.
Cleeter
M W
MW
Xuereb
J
J
Taanman
J W
JW
Cooper
J M
JM
Schapira
A H
AH
D000154:Aconitate Hydratase / Q000378:metabolism; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001923:Brain Chemistry; D001927:Brain Diseases / Q000378:metabolism*; D004579:Electron Transport / Q000502:physiology; D042963:Electron Transport Complex II; D014450:Electron Transport Complex III / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D005347:Fibroblasts; D005978:Glutathione / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D000071058:Huntingtin Protein; D006816:Huntington Disease / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D008928:Mitochondria / Q000187:drug effects / Q000201:enzymology; D009097:Multienzyme Complexes / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000235:genetics*; D009447:Neuroblastoma; D018696:Neuroprotective Agents / Q000494:pharmacology; D009498:Neurotoxins / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism; D009603:Nitroso Compounds / Q000494:pharmacology; D009687:Nuclear Proteins / Q000235:genetics*; D010088:Oxidoreductases / Q000378:metabolism; D026422:S-Nitrosoglutathione; D013384:Succinate Cytochrome c Oxidoreductase / Q000378:metabolism; D013385:Succinate Dehydrogenase / Q000378:metabolism; D018911:Trinucleotide Repeats; D014407:Tumor Cells, Cultured / Q000201:enzymology
C086055:HTT protein, human; D000071058:Huntingtin Protein; D009097:Multienzyme Complexes; D009419:Nerve Tissue Proteins; D018696:Neuroprotective Agents; D009498:Neurotoxins; D009603:Nitroso Compounds; D009687:Nuclear Proteins; D009569:Nitric Oxide; D026422:S-Nitrosoglutathione; D010088:Oxidoreductases; D013384:Succinate Cytochrome c Oxidoreductase; D042963:Electron Transport Complex II; D013385:Succinate Dehydrogenase; D000154:Aconitate Hydratase; D014450:Electron Transport Complex III; D005978:Glutathione