Mutant huntingtin directly increases susceptibility of mitochondria to the calcium-induced permeability transition and cytochrome c release

Abstract

Huntington's disease (HD) is initiated by an abnormally expanded polyglutamine stretch in the huntingtin protein, conferring a novel property on the protein that leads to the loss of striatal neurons. Defects in mitochondrial function have been implicated in the pathogenesis of HD. Here, we have examined the hypothesis that the mutant huntingtin protein may directly interact with the mitochondrion and affect its function. In human neuroblastoma cells and clonal striatal cells established from HdhQ7 (wild-type) and HdhQ111 (mutant) homozygote mouse knock-in embryos, huntingtin was present in a purified mitochondrial fraction. Subfractionation of the mitochondria and limited trypsin digestion of the organelle demonstrated that huntingtin was associated with the outer mitochondrial membrane. We further demonstrated that a recombinant truncated mutant huntingtin protein, but not a wild-type, directly induced mitochondrial permeability transition (MPT) pore opening in isolated mouse liver mitochondria, an effect that was prevented completely by cyclosporin A (CSA) and ATP. Importantly, the mutant huntingtin protein significantly decreased the Ca2+ threshold necessary to trigger MPT pore opening. We found a similar increased susceptibility to the calcium-induced MPT in liver mitochondria isolated from a knock-in HD mouse model. The mutant huntingtin protein-induced MPT pore opening was accompanied by a significant release of cytochrome c, an effect completely inhibited by CSA. These findings suggest that the development of specific MPT inhibitors may be an interesting therapeutic avenue to delay the onset of HD.

Dithiol-based Compounds Maintain Expression of Antioxidant Protein Peroxiredoxin 1 That Counteracts Toxicity of Mutant Huntingtin

“…Although mitochondria produce most of the cellular ATP, they are also a major source of ROS production via electron leakage from the respiratory chain (especially complexes I and III). Several studies have shown that mHtt is found in association with the outer mitochondrial membrane in brain tissue from HD transgenic mice and in isolated mitochondria from both lymphoblasts and postmortem brain tissue from HD patients (5)(6)(7). In addition, isolated mitochondria from HD mice exhibit decreased membrane potential, increased propensity to depolarize at lower calcium loads, and elevated sensitivity to calcium-induced cytochrome c release compared with controls (5,6).…”

“…Several studies have shown that mHtt is found in association with the outer mitochondrial membrane in brain tissue from HD transgenic mice and in isolated mitochondria from both lymphoblasts and postmortem brain tissue from HD patients (5)(6)(7). In addition, isolated mitochondria from HD mice exhibit decreased membrane potential, increased propensity to depolarize at lower calcium loads, and elevated sensitivity to calcium-induced cytochrome c release compared with controls (5,6). Transcription of peroxisome proliferator-activated receptor, a coactivator 1␣ (PGC1␣), a key transcriptional co-activator that induces expression of genes that regulate mitochondrial respiration and oxidative stress, is repressed in mHtt-expressing neurons (8).…”

Bioenergetic dysfunction in Huntington's disease human cybrids

“…Mutant huntingtin may cause mitochondrial dysfunction by either perturbing transcription of nuclear-encoded mitochondrial proteins or by directly interacting with the organelle, thus evoking defects in mitochondrial dynamics, organelle trafficking and fission and fusion, which, in turn, may result in bioenergetic failure in HD (Bossy-Wetzel et al, 2008). In fact, mutant huntingtin was previously shown to interact with neuronal mitochondria of YAC72 transgenic mice (Panov et al, 2002), suggesting that mitochondrial calcium abnormalities associated with HD pathogenesis may be due to a direct effect of mutant huntingtin on the organelle (Choo et al, 2004;Panov et al, 2002).…”

“…Moreover, mutant huntingtin fragments can directly induce the opening of the mitochondrial permeability transition pore in isolated mouse liver mitochondria, with the consequent release of cytochrome c, as well as a decrease in the calcium threshold necessary to induce it (Choo et al, 2004), favoring the hypothesis that mutant huntingtin interacting with mitochondria may well lead to mitochondrial modifications independently of damage on mtDNA.…”

Age‐related changes in regional brain mitochondria from Fischer 344 rats

“…In the nervous system, the mPT is believed to occur in response to acute insults such as ischemia, stroke, head injury or excitotoxicity (Dubinsky & Levi, 1998;Matsumoto et al ., 1999;Sullivan et al ., 1999;Budd et al ., 2000;Lifshitz et al ., 2003). In neurodegenerative diseases, the mPT has been implicated in Huntington's disease as a consequence of abnormal gene expression (Panov et al ., 2002;Choo et al ., 2004). Depolarized mitochondrial membrane potential by itself would reduce the driving force for Ca 2+ entry and could possibly lessen the mitochondrial contribution to clearance of cytosolic Ca 2+ loads.…”