ALS-Causing SOD1 Mutations Promote Production of Copper-Deficient Misfolded Species

ArticleinJournal of Molecular Biology 409(5):839-52 · June 2011with33 Reads
DOI: 10.1016/j.jmb.2011.04.027 · Source: PubMed
Abstract
Point mutations scattered throughout the sequence of Cu,Zn superoxide dismutase (SOD1) cause a subset of amyotrophic lateral sclerosis (ALS) cases. SOD1 is a homodimer in which each subunit binds one copper atom and one zinc atom. Inclusions containing misfolded SOD1 are seen in motor neurons of SOD1-associated ALS cases. The mechanism by which these diverse mutations cause misfolding and converge on the same disease is still not well understood. Previously, we developed several time-resolved techniques to monitor structural changes in SOD1 as it unfolds in guanidine hydrochloride. By measuring the rates of Cu and Zn release using an absorbance-based assay, dimer dissociation through chemical cross-linking, and β-barrel conformation changes by tryptophan fluorescence, we established that wild-type SOD1 unfolds by a branched pathway involving a Zn-deficient monomer as the dominant intermediate of the major pathway, and with various metal-loaded and Cu-deficient dimers populated along the minor pathway. We have now compared the unfolding pathway of wild-type SOD1 with those of A4V, G37R, G85R, G93A, and I113T ALS-associated mutant SOD1. The kinetics of unfolding of the mutants were generally much faster than those of wild type. However, all of the mutants utilize the minority pathway to a greater extent than the wild-type protein, leading to greater populations of Cu-deficient intermediates and decreases in Zn-deficient intermediates relative to the wild-type protein. The greater propensity of the mutants to populate Cu-deficient states potentially implicates these species as a pathogenic form of SOD1 in SOD1-associated ALS and provides a novel target for therapeutic intervention.
    • "At higher concentrations (>20 M) Zn 2+ produces a voltagedependent inhibition of NMDAR currents, probably by binding at the Mg 2+ -blocking site inside the pore. Similar to Cu 2+ , other than inducing conformational changes in mutant and wild-type SOD1 (Ip et al., 2011; Homma et al., 2013), Zn 2+ dyshomeostasis can greatly affects synaptic function particularly at the central synapse where Zn 2+ modulation has been fully demonstrated (Sensi et al., 2011). We have investigated the modulatory effects that a brief exposure to extracellular Zn 2+ has on NMDAR-mediated neurotoxicity in cortical cultures carrying the SOD1G93A mutation (Nutini et al., 2011b ). "
    [Show abstract] [Hide abstract] ABSTRACT: Amyotrophic lateral sclerosis (ALS) is now recognized as a multisystem disorder, in which the primary pathology is the degeneration of motor neurons, with cognitive and/or behavioral dysfunctions that constitutes the non-motor manifestations of ALS. The combination of clinical, neuroimaging, and neuropathological data, and detailed genetic studies suggest that ALS and frontotemporal dementia (FTD) might form part of a disease continuum, with pure ALS and pure FTD at the two extremes. Mutations in the superoxide dismutase 1 (SOD1) gene were the first genetic mutations linked to the insurgence of ALS. Since that discovery numerous animal models carrying SOD1 mutations have been created. Despite their limitations these animal models, particularly the mice, have broaden our knowledge on the system alterations occurring in the ALS spectrum of disorders. The present review aims at providing an overview of the data obtained with the SOD1 animal models first and foremost on the cortical and subcortical regions, the cortico-striatal and hippocampal synaptic plasticity, dendritic branching and glutamate receptors function.
    Full-text · Article · Nov 2015
    • "Some SOD1-mutations reduce metal content to the effect of destabilizing the holoprotein, but in some cases not the apoprotein[40][41]. Since protein misfolding is generally related to metal release[25][39], these mutations, even if they do not destabilize the apoprotein, may be pathogenic for the same reason as the apoprotein-destabilizing mutations: They would increase the pool of non-native apoprotein targeted for degradation, and thus, increase the burden of protein turnover. This may explain why metal-imbalance in the form defined recently (i.e. "
    [Show abstract] [Hide abstract] ABSTRACT: Late-onset neurodegenerative diseases remain poorly understood as search continues for the perceived pathogenic protein species. Previously, variants in Superoxide Dismutase 1 (SOD1) causing Amyotrophic Lateral Sclerosis (ALS) were found to destabilize and reduce net charge, suggesting a pathogenic aggregation mechanism. This paper reports analysis of compiled patient data and experimental and computed protein properties for variants of human SOD1, a major risk factor of ALS. Both stability and reduced net charge correlate significantly with disease, with larger significance than previously observed. Using two independent methods and two data sets, a probability < 3% (t-statistical test) is found that ALScausing mutations share average stability with all possible 2907 SOD1 mutations. Most importantly, un-weighted patient survival times correlate strongly with the misfolded/unfolded protein copy number, expressed as an exponential function of the experimental stabilities (R2 = 0.31, p = 0.002), and this phenotype is further aggravated by charge (R2 = 0.51, p = 1.8 x 10-5). This finding suggests that disease relates to the copy number of misfolded proteins. Exhaustion of motor neurons due to expensive protein turnover of misfolded protein copies is consistent with the data but can further explain e.g. the expression-dependence of SOD1 pathogenicity, the lack of identification of a molecular toxic mode, elevated SOD1 mRNA levels in sporadic ALS, bioenergetic effects and increased resting energy expenditure in ALS patients, genetic risk factors affecting RNA metabolism, and recent findings that a SOD1 mutant becomes toxic when proteasome activity is recovered after washout of a proteasome inhibitor. Proteome exhaustion is also consistent with energy-producing mitochondria accumulating at the neuromuscular junctions where ALS often initiates. If true, this exhaustion mechanism implies a complete change of focus in treatment of ALS towards actively nursing the energy state and protein turnover of the motor neurons.
    Full-text · Article · Mar 2015
    • "When used to treat a SOD1G93A mouse model of ALS, the compound significantly delayed locomotor deficit onset and improved survival, decreased levels of peroxynitrite-induced protein nitration and increased SOD1 activity in spinal cord tissue [57] . Collectively, these studies indicated that a therapeutic agent capable of increasing copper bioavailability could protect against mutant SOD1 toxicity, a possibility consistent with previous in vitro studies which had described the potential for copper deficiency to promote SOD1 pathology via increased misfolding and altered hydrophobicity [59, 60]. It was however a more recent study [61] involving Cu II (atsm) treatment in a SOD1G37R mouse model of ALS that illustrated the potential link between copper-deficient SOD1 and ALS pathogenesis in vivo. "
    [Show abstract] [Hide abstract] ABSTRACT: Mutations to the ubiquitous antioxidant enzyme Cu/Zn superoxide dismutase (SOD1) were the first established genetic cause of the fatal, adult-onset neurodegenerative disease amyotrophic lateral sclerosis (ALS). It is widely accepted that these mutations do not cause ALS via a loss of antioxidant function, but elucidating the alternate toxic gain of function has proven to be elusive. Under physiological conditions, SOD1 binds one copper ion and one zinc ion per monomer to form a highly stable and functional homodimer, but there is now ample evidence to indicate aberrant persistence of SOD1 in an intermediate metal-deficient state may contribute to the protein's involvement in ALS. This review briefly discusses some of the data to support a role for metal-deficient SOD1 in the development of ALS and some of the outcomes from drug development studies that have aimed to modify the symptoms of ALS by targeting the metal state of SOD1. The implications for the metal state of SOD1 in cases of sporadic ALS that do not involve mutant SOD1 are also discussed.
    Full-text · Article · Mar 2015
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