Mutant SOD1 Instability: Implications for Toxicity in Amyotrophic Lateral Sclerosis

Department of Neurology, University of Massachusetts Medical School, Worcester, 01655, USA.
Neurodegenerative Diseases (Impact Factor: 3.51). 02/2005; 2(3-4):115-27. DOI: 10.1159/000089616
Source: PubMed


The biological basis of preferential motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains incompletely understood, and effective therapies to prevent the lethal consequences of this disorder are not yet available. Since 1993, more than 100 mutant variants of the antioxidant enzyme Cu/Zn superoxide dismutase (SOD1) have been identified in familial ALS. Many studies have sought to distinguish abnormal properties shared by these proteins that may contribute to their toxic effects and cause age-dependent motor neuron loss. Complex networks of cellular interactions and changes associated with aging may link mutant SOD1s and other stresses to motor neuron death in ALS. Our laboratory and collaborators have compared physicochemical properties of biologically metallated wild-type and mutant SOD1 proteins to discern specific vulnerabilities that may be relevant to the mutant toxicity in vivo. X-ray crystal structures obtained from metallated 'wild-type-like' (WTL) SOD1 mutants, which retain the ability to bind copper and zinc and exhibit normal specific activity, indicate a native-like structure with only subtle changes to the backbone fold. In contrast, a group of 'metal-binding region' (MBR) SOD1 mutants that are deficient in copper and zinc exhibit severe thermal destabilization and structural disorder of conserved loops near the metal-binding sites. A growing body of evidence highlights specific stresses in vivo that may perturb well-folded, metallated SOD1 variants and thereby favor an increased burden of partially unfolded, metal-deficient species. For example, WTL SOD1 mutants are more susceptible than wild-type SOD1 to reduction of the intrasubunit disulfide bond between Cys-57 and Cys-146 at physiological pH and temperature. This bond anchors the disulfide loop to the SOD1 beta-barrel and helps to maintain the dimeric configuration of the protein. Cleavage of the disulfide linkage renders the well-folded WTL mutants vulnerable to metal loss and monomerization such that they may resemble the destabilized and locally misfolded MBR mutant species. SOD1 proteins with disordered loops or monomeric structure are expected to be more susceptible to aberrant self-association or detrimental interactions with other cellular constituents. The challenge for future investigations is to relate these abnormal properties of partially unfolded SOD1 to specific mechanisms of toxicity in motor neurons, supporting cells, or target tissues.

Download full-text


Available from: Lawrence J Hayward, Oct 13, 2015
21 Reads
  • Source
    • "Different mutations in the SOD1 gene can be associated with different properties of the protein with regard to hydrophobicity, di- and oligomerization, aggregation propensity, and toxicity [33–35]. Localization and tracking of mutated SOD1 offers the opportunity to examine several aspects of ALS, as selective vulnerability of specific cells, cell to cell transmission, or co-occurrence of inclusions and neurodegeneration. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Objectives The paraffin-embedded tissue (PET) blot technique followed by limited protease digestion has been established to detect protein aggregates in prion diseases, alpha-synucleopathies, and tauopathies. We analyzed whether the scope of the method can be extended to analyze aggregates in mouse and human tissue with amyotrophic lateral sclerosis (ALS) associated with superoxide dismutase 1 (SOD1) mutation.Methods Formalin-fixed and paraffin-embedded brain and spinal cord tissue from SOD1G93A mice was first analyzed for the expression of SOD1, aggregated SOD1, ubiquitin, and p62 by convential immunohistochemistry and then used to establish the PET blot technique, limited protease digest, and immunodetection of SOD1 aggregates. The method was then transferred to spinal cord from an ALS patient with SOD1E100G mutation.ResultsMouse and human paraffin-embedded brain and spinal cord tissue can be blotted to membranes and stained with anti-SOD1 antibodies. The SOD1 labelling is abolished after limited proteolytic digest in controls, whereas under identical conditions SOD1 aggregates are detected the SOD1G93A mouse model of ALS and in human familial ALS. The most prominent areas where aggregates could be detected are the brainstem and the anterior horn of the spinal cord.DiscussionApplicability of the PET blot technique to demonstrate SOD1 aggregates in ALS tissue associated with mutations in the SOD1 gene offers a new approach to examine potential spreading of aggregates in the course of ALS.
    08/2014; 2(1):130. DOI:10.1186/PREACCEPT-8556289621364030
  • Source
    • "We also reproduced an in vitro characteristic of SOD1 dynamics in C. elegans by showing that mutant SOD1 is susceptible to changes in temperature and that it exacerbates its toxic effects at higher temperatures. This occurrence was also depicted in the temperature-dependent response in locomotion (Rodriguez et al., 2002; Tiwari and Hayward, 2005). Furthermore, because aging represents the single greatest risk factor for ALS and other neurodegenerative diseases, it is therefore significant that SOD1 G85R locomotion defects are improved by mutants or RNAi versus select targets in the daf-2/insulin-like signaling and aging regulatory pathway of C. elegans (Boccitto et al., 2012). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Molecular mechanisms underlying neurodegenerative diseases converge at the interface of pathways impacting cellular stress, protein homeostasis, and aging. Targeting the intrinsic capacities of neuroprotective proteins to restore neuronal function and/or attenuate degeneration represents a potential means toward therapeutic intervention. The product of the human DYT1 gene, torsinA, is a member of the functionally diverse AAA+ family of proteins and exhibits robust molecular chaperone-like activity, both in vitro and in vivo. While mutations in DYT1 are associated with a rare form of heritable generalized dystonia, the native function of torsinA appears cytoprotective in maintaining the cellular threshold to endoplasmic reticulum (ER) stress. Here we explore the potential for torsinA to serve as a buffer to attenuate the cellular consequences of misfolded protein stress as it pertains to the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). The selective vulnerability of motor neurons to degeneration in ALS mice models harboring mutations in the superoxide dismutase, SOD1, has been found to correlate with regional-specific ER stress in brains. Using Caenorhabditis elegans as a system to model ER stress, we generated transgenic nematodes overexpressing either wildtype or mutant human SOD1 to evaluate their relative impact on ER stress induction in vivo. These studies revealed a mutant SOD1-specific increase in ER stress that was further exacerbated by changes in temperature, all of which was robustly attenuated by co-expression of torsinA. Moreover, through complementary behavioral analysis, torsinA was able to restore normal neuronal function in mutant G85R SOD1 animals. Furthermore, torsinA targeted mutant SOD1 for degradation via the proteasome, representing a mechanistic insight into the activity torsinA has on aggregate prone proteins. These results expand our understanding of proteostatic mechanisms influencing neuronal dysfunction in ALS, while simultaneously highlighting the potential for torsinA as a novel target for therapeutic development.
    Disease Models and Mechanisms 12/2013; 7(2). DOI:10.1242/dmm.013615 · 4.97 Impact Factor
  • Source
    • "However, in seeded reactions, there was no appreciable correlation of protein stability to lag time although there was a trend that higher stability may promote seeded fibrillization. This is in line with the aggregation theory proposed by others where destabilization of SOD1 promotes formation of aggregates, which are detrimental to motor neurons [33], [34], [35], [36], [37], [38], [39]. Here, it is demonstrated that formation of fibrils is an alternative misfolding property of destabilized SOD1 resulting in higher order structures with the intrinsic ability to autocatalyze the formation of more fibrils as opposed to the formation of poorly-defined or unstructured aggregates. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that specifically affects motor neurons and leads to a progressive and ultimately fatal loss of function, resulting in death typically within 3 to 5 years of diagnosis. The disease starts with a focal centre of weakness, such as one limb, and appears to spread to other parts of the body. Mutations in superoxide dismutase 1 (SOD1) are known to cause disease and it is generally accepted they lead to pathology not by loss of enzymatic activity but by gain of some unknown toxic function(s). Although different mutations lead to varying tendencies of SOD1 to aggregate, we suggest abnormal proteins share a common misfolding pathway that leads to the formation of amyloid fibrils. Here we demonstrate that misfolding of superoxide dismutase 1 leads to the formation of amyloid fibrils associated with seeding activity, which can accelerate the formation of new fibrils in an autocatalytic cascade. The time limiting event is nucleation to form a stable protein "seed" before a rapid linear polymerisation results in amyloid fibrils analogous to other protein misfolding disorders. This phenomenon was not confined to fibrils of recombinant protein as here we show, for the first time, that spinal cord homogenates obtained from a transgenic mouse model that overexpresses mutant human superoxide dismutase 1 (the TgSOD1(G93A) mouse) also contain amyloid seeds that accelerate the formation of new fibrils in both wildtype and mutant SOD1 protein in vitro. These findings provide new insights into ALS disease mechanism and in particular a mechanism that could account for the spread of pathology throughout the nervous system. This model of disease spread, which has analogies to other protein misfolding disorders such as prion disease, also suggests it may be possible to design assays for therapeutics that can inhibit fibril propagation and hence, possibly, disease progression.
    PLoS ONE 05/2010; 5(5):e10627. DOI:10.1371/journal.pone.0010627 · 3.23 Impact Factor
Show more