Mechanisms of Copper Ion Mediated Huntington's Disease Progression

Abstract

Huntington's disease (HD) is caused by a dominant polyglutamine expansion within the N-terminus of huntingtin protein and results in oxidative stress, energetic insufficiency and striatal degeneration. Copper and iron are increased in the striata of HD patients, but the role of these metals in HD pathogenesis is unknown. We found, using inductively-coupled-plasma mass spectroscopy, that elevations of copper and iron found in human HD brain are reiterated in the brains of affected HD transgenic mice. Increased brain copper correlated with decreased levels of the copper export protein, amyloid precursor protein. We hypothesized that increased amounts of copper bound to low affinity sites could contribute to pro-oxidant activities and neurodegeneration. We focused on two proteins: huntingtin, because of its centrality to HD, and lactate dehydrogenase (LDH), because of its documented sensitivity to copper, necessity for normoxic brain energy metabolism and evidence for altered lactate metabolism in HD brain. The first 171 amino acids of wild-type huntingtin, and its glutamine expanded mutant form, interacted with copper, but not iron. N171 reduced Cu2+ in vitro in a 1∶1 copper∶protein stoichiometry indicating that this fragment is very redox active. Further, copper promoted and metal chelation inhibited aggregation of cell-free huntingtin. We found decreased LDH activity, but not protein, and increased lactate levels in HD transgenic mouse brain. The LDH inhibitor oxamate resulted in neurodegeneration when delivered intra-striatially to healthy mice, indicating that LDH inhibition is relevant to neurodegeneration in HD. Our findings support a role of pro-oxidant copper-protein interactions in HD progression and offer a novel target for pharmacotherapeutics.

Huntington disease arises from a combinatory toxicity of polyglutamine and copper binding

“…However, excess copper also possesses toxicity (32); e.g., it has been established that brain copper accumulation is associated with neurodegeneration (33). Previous studies suggested that there might be a close relationship between copper and the progress of Huntington disease (13)(14)(15). In this study, we provided in vivo genetic evidence that manipulating copper transporters such as Ctr1B or DmATP7 in brains can greatly modulate HD progress.…”

“…This prompted us to investigate the interaction between Htt exon1 and copper. In line with this thinking, it has been reported that copper directly binds the first 171 amino acids of Htt (N171) in vitro (13). Both His82 and His98 residues of rodent Htt appear essential for the interaction.…”

“…Both His82 and His98 residues of rodent Htt appear essential for the interaction. Nevertheless, human exon-1 (17Q) Htt (N84), which lacks His98, interacts with copper (II) as well, suggesting it has a conformation and mode of interaction with copper different from N171 (13).…”

“…Many studies offered evidence that iron may be an important contributor to HD progression (11). Besides iron, the copper level is also increased in the CNS of human HD brains, HD mouse models, and a rat HD model (12)(13)(14). It was reported that in vitro copper could significantly accelerate the fibrillation and aggregation of purified recombinant Htt exon1 with polyQ tract (15), and the fragment containing the first 171 amino acids of human wild-type Htt and its glutamine-expanded mutant form directly interact with copper (13).…”

“…Besides iron, the copper level is also increased in the CNS of human HD brains, HD mouse models, and a rat HD model (12)(13)(14). It was reported that in vitro copper could significantly accelerate the fibrillation and aggregation of purified recombinant Htt exon1 with polyQ tract (15), and the fragment containing the first 171 amino acids of human wild-type Htt and its glutamine-expanded mutant form directly interact with copper (13). Clioquinol (CQ), an antibiotic with divalent metal ion-binding ability and whose action mechanism is still controversial (16,17), improved cell survival and behavioral and pathologic phenotypes in human Htt transgenic mice (18).…”