Mitochondrial and redox modifications in early stages of Huntington's disease

Deficits in mitochondrial function and redox deregulation have been attributed to Huntington's disease (HD), a genetic neurodegenerative disorder largely affecting the striatum. However, whether these changes occur in early stages of the disease and can be detected in vivo is still unclear. In the present study, we analysed changes in mitochondrial function and production of reactive oxygen species (ROS) at early stages and with disease progression. Studies were performed in vivo in human brain by PET using [⁶⁴Cu]-ATSM and ex vivo in human skin fibroblasts of premanifest and prodromal (Pre-M) and manifest HD carriers. In vivo brain [⁶⁴Cu]-ATSM PET in YAC128 transgenic mouse and striatal and cortical isolated mitochondria were assessed at presymptomatic (3 month-old, mo) and symptomatic (6–12 mo) stages. Pre-M HD carriers exhibited enhanced whole-brain (with exception of caudate) [⁶⁴Cu]-ATSM labelling, correlating with CAG repeat number. Fibroblasts from Pre-M showed enhanced basal and maximal respiration, proton leak and increased hydrogen peroxide (H₂O₂) levels, later progressing in manifest HD. Mitochondria from fibroblasts of Pre-M HD carriers also showed reduced circularity, while higher number of mitochondrial DNA copies correlated with maximal respiratory capacity. In vivo animal PET analysis showed increased accumulation of [⁶⁴Cu]-ATSM in YAC128 mouse striatum. YAC128 mouse (at 3 months) striatal isolated mitochondria exhibited a rise in basal and maximal mitochondrial respiration and in ATP production, and increased complex II and III activities. YAC128 mouse striatal mitochondria also showed enhanced mitochondrial H₂O₂ levels and circularity, revealed by brain ultrastructure analysis, and defects in Ca²⁺ handling, supporting increased striatal susceptibility. Data demonstrate both human and mouse mitochondrial overactivity and altered morphology at early HD stages, facilitating redox unbalance, the latter progressing with manifest disease.

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Pre-manifest HD carriers and presymptomatic YAC128 mice show increased brain [⁶⁴Cu]-ATSM labelling.

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Increased [⁶⁴Cu]-ATSM brain retention correlates with raised ROS levels in human and mouse samples.

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Increased [⁶⁴Cu]-ATSM correlates with enhanced mitochondrial activity and mtDNA copy number.

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Presymptomatic YAC128 mouse striatal mitochondria show altered morphology and Ca²⁺ handling.

A whole brain longitudinal study in the YAC128 mouse model of Huntington's disease shows distinct trajectories of neurochemical, structural connectivity and volumetric changes

Huntington's disease (HD) is a neurodegenerative disorder causing cognitive and motor impairments, evolving to death within 15-20 years after symptom onset. We previously established a mouse model with the entire human HD gene containing 128 CAG repeats (YAC128) which accurately recapitulates the natural history of the human disease. Defined time points in this natural history enable the understanding of longitudinal trajectories from the neurochemical and structural points of view using non-invasive high-resolution multi-modal imaging. Accordingly, we designed a longitudinal structural imaging (MRI and DTI) and spectroscopy (1H-MRS) study in YAC128, at 3, 6, 9 and 12 months of age, at 9.4 T. Structural analysis (MRI/DTI), confirmed that the striatum is the earliest affected brain region, but other regions were also identified through connectivity analysis (pre-frontal cortex, hippocampus, globus pallidus and thalamus), suggesting a striking homology with the human disease. Importantly, we found for the first time, a negative correlation between striatal and hippocampal changes only in YAC128. In fact, the striatum showed accelerated volumetric decay in HD, as opposed to the hippocampus. Neurochemical analysis of the HD striatum suggested early neurometabolic alterations in neurotransmission and metabolism, with a significant increase in striatal GABA levels, and specifically anticorrelated levels of N-acetyl aspartate and taurine, suggesting that the later is homeostatically adjusted for neuroprotection, as neural loss, indicated by the former, is progressing. These results provide novel insights into the natural history of HD and prove a valuable role for longitudinal multi-modal panels of structural and metabolite/neurotransmission in the YAC128 model.

Valosin-containing protein (VCP/p97) is an activator of wild-type ataxin-3

Alterations in the ubiquitin-proteasome system (UPS) have been reported in several neurodegenerative disorders characterized by protein misfolding and aggregation, including the polylgutamine diseases. Machado-Joseph disease (MJD) or Spinocerebellar Ataxia type 3 is caused by a polyglutamine-encoding CAG expansion in the ATXN3 gene, which encodes a 42 kDa deubiquitinating enzyme (DUB), ataxin-3. We investigated ataxin-3 deubiquitinating activity and the functional relevance of ataxin-3 interactions with two proteins previously described to interact with ataxin-3, hHR23A and valosin-containing protein (VCP/p97). We confirmed ataxin-3 affinity for both hHR23A and VCP/p97. hHR23A and ataxin-3 were shown to co-localize in discrete nuclear foci, while VCP/p97 was primarily cytoplasmic. hHR23A and VCP/p97 recombinant proteins were added, separately or together, to normal and expanded ataxin-3 in in vitro deubiquitination assays to evaluate their influence on ataxin-3 activity. VCP/p97 was shown to be an activator specifically of wild-type ataxin-3, exhibiting no effect on expanded ataxin-3, In contrast, we observed no significant alterations in ataxin-3 enzyme kinetics or substrate preference in the presence of hHR23A alone or in combination with VCP. Based on our results we propose a model where ataxin-3 normally functions with its interactors to specify the cellular fate of ubiquitinated proteins.

Compromised mitochondrial complex II in models of Machado-Joseph disease

Machado-Joseph disease (MJD), also known as Spinocerebellar Ataxia type 3, is an inherited dominant autosomal neurodegenerative disorder. An expansion of Cytosine-Adenine-Guanine (CAG) repeats in the ATXN3 gene is translated as an expanded polyglutamine domain in the disease protein, ataxin-3. Selective neurodegeneration in MJD is evident in several subcortical brain regions including the cerebellum. Mitochondrial dysfunction has been proposed as a mechanism of neurodegeneration in polyglutamine disorders. In this study, we used different cell models and transgenic mice to assess the importance of mitochondria on cytotoxicity observed in MJD. Transiently transfected HEK cell lines with expanded (Q84) ataxin-3 exhibited a higher susceptibility to 3-nitropropionic acid (3-NP), an irreversible inhibitor of mitochondrial complex II. Increased susceptibility to 3-NP was also detected in stably transfected PC6-3 cells that inducibly express expanded (Q108) ataxin-3 in a tetracycline-regulated manner. Moreover, cerebellar granule cells from MJD transgenic mice were more sensitive to 3-NP inhibition than wild-type cerebellar neurons. PC6-3 (Q108) cells differentiated into a neuronal-like phenotype with nerve growth factor (NGF) exhibited a significant decrease in mitochondrial complex II activity. Mitochondria from MJD transgenic mouse model and lymphoblast cell lines derived from MJD patients also showed a trend toward reduced complex II activity. Our results suggest that mitochondrial complex II activity is moderately compromised in MJD, which may designate a common feature in polyglutamine toxicity.