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1.  SOD1 oxidation and formation of soluble aggregates in yeast: Relevance to sporadic ALS development 
Redox Biology  2014;2:632-639.
Misfolding and aggregation of copper–zinc superoxide dismutase (Sod1) are observed in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Mutations in Sod1 lead to familial ALS (FALS), which is a late-onset disease. Since oxidative damage to proteins increases with age, it had been proposed that oxidation of Sod1 mutants may trigger their misfolding and aggregation in FALS. However, over 90% of ALS cases are sporadic (SALS) with no obvious genetic component. We hypothesized that oxidation could also trigger the misfolding and aggregation of wild-type Sod1 and sought to confirm this in a cellular environment. Using quiescent, stationary-phase yeast cells as a model for non-dividing motor neurons, we probed for post-translational modification (PTM) and aggregation of wild-type Sod1 extracted from these cells. By size-exclusion chromatography (SEC), we isolated two populations of Sod1 from yeast: a low-molecular weight (LMW) fraction that is catalytically active and a catalytically inactive, high-molecular weight (HMW) fraction. High-resolution mass spectrometric analysis revealed that LMW Sod1 displays no PTMs but HMW Sod1 is oxidized at Cys146 and His71, two critical residues for the stability and folding of the enzyme. HMW Sod1 is also oxidized at His120, a copper ligand, which will promote loss of this catalytic metal cofactor essential for SOD activity. Monitoring the fluorescence of a Sod1-green-fluorescent-protein fusion (Sod1-GFP) extracted from yeast chromosomally expressing this fusion, we find that HMW Sod1-GFP levels increase up to 40-fold in old cells. Thus, we speculate that increased misfolding and inclusion into soluble aggregates is a consequence of elevated oxidative modifications of wild-type Sod1 as cells age. Our observations argue that oxidative damage to wild-type Sod1 initiates the protein misfolding mechanisms that give rise to SALS.
Highlights
•Key Sod1 catalytic and structure-stabilizing residues (Cys146, His120, His71) are oxidized in stationary-phase yeast.•Oxidized Sod1 is isolated in an inactive, high-molecular-weight, soluble aggregate.•Sod1 with native mass isolated from the same samples is not oxidized and is catalytically active.•Our results argue that oxidation triggers the formation of soluble Sod1-containing aggregates that may contribute to sporadic ALS development.
Graphical abstract
doi:10.1016/j.redox.2014.03.005
PMCID: PMC4052529  PMID: 24936435
Wild-type Sod1; Oxidative PTMs; Soluble aggregates; Sporadic ALS; Yeast
2.  Structural switching of Cu,Zn-superoxide dismutases at loop VI: insights from the crystal structure of 2-mercaptoethanol-modified enzyme 
Bioscience Reports  2012;32(Pt 6):539-548.
Cu,Zn SOD1 (superoxide dismutase 1) is implicated in FALS (familial amyotrophic lateral sclerosis) through the accumulation of misfolded proteins that are toxic to neuronal cells. Loop VI (residues 102–115) of the protein is at the dimer interface and could play a critical role in stability. The free cysteine residue, Cys111 in the loop, is readily oxidized and alkylated. We have found that modification of this Cys111 with 2-ME (2-mercaptoethanol; 2-ME-SOD1) stabilizes the protein and the mechanism may provide insights into destabilization and the formation of aggregated proteins. Here, we determined the crystal structure of 2-ME-SOD1 and find that the 2-ME moieties in both subunits interact asymmetrically at the dimer interface and that there is an asymmetric configuration of segment Gly108 to Cys111 in loop VI. One loop VI of the dimer forms a 310-helix (Gly108 to His110) within a unique β-bridge stabilized by a hydrogen bond between Ser105-NH and His110-CO, while the other forms a β-turn without the H-bond. The H-bond (H-type) and H-bond free (F-type) configurations are also seen in some wild-type and mutant human SOD1s in the Protein Data Bank suggesting that they are interconvertible and an intrinsic property of SOD1s. The two structures serve as a basis for classification of these proteins and hopefully a guide to their stability and role in pathophysiology.
doi:10.1042/BSR20120029
PMCID: PMC3497728  PMID: 22804629
superoxide dismutase 1 (SOD1); crystal structure; amyotrophic lateral sclerosis (ALS); asymmetric configuration; ALS, amyotrophic lateral sclerosis; FALS, familial amyotrophic lateral sclerosis; 2-ME, 2-mercaptoethanol; PDB, protein data bank; SOD1, superoxide dismutase 1; WT, wild-type
3.  Destabilizing Protein Polymorphisms in the Genetic Background Direct Phenotypic Expression of Mutant SOD1 Toxicity 
PLoS Genetics  2009;5(3):e1000399.
Genetic background exerts a strong modulatory effect on the toxicity of aggregation-prone proteins in conformational diseases. In addition to influencing the misfolding and aggregation behavior of the mutant proteins, polymorphisms in putative modifier genes may affect the molecular processes leading to the disease phenotype. Mutations in SOD1 in a subset of familial amyotrophic lateral sclerosis (ALS) cases confer dominant but clinically variable toxicity, thought to be mediated by misfolding and aggregation of mutant SOD1 protein. While the mechanism of toxicity remains unknown, both the nature of the SOD1 mutation and the genetic background in which it is expressed appear important. To address this, we established a Caenorhabditis elegans model to systematically examine the aggregation behavior and genetic interactions of mutant forms of SOD1. Expression of three structurally distinct SOD1 mutants in C. elegans muscle cells resulted in the appearance of heterogeneous populations of aggregates and was associated with only mild cellular dysfunction. However, introduction of destabilizing temperature-sensitive mutations into the genetic background strongly enhanced the toxicity of SOD1 mutants, resulting in exposure of several deleterious phenotypes at permissive conditions in a manner dependent on the specific SOD1 mutation. The nature of the observed phenotype was dependent on the temperature-sensitive mutation present, while its penetrance reflected the specific combination of temperature-sensitive and SOD1 mutations. Thus, the specific toxic phenotypes of conformational disease may not be simply due to misfolding/aggregation toxicity of the causative mutant proteins, but may be defined by their genetic interactions with cellular pathways harboring mildly destabilizing missense alleles.
Author Summary
Correct folding and stability are essential for protein function. In cells, a network of molecular chaperones and degradative enzymes facilitate folding, prevent aggregation and ensure degradation of the misfolded proteins, thus maintaining protein homeostasis. In many diseases, including Amyotrophic Lateral Sclerosis (ALS), expression of a single mutant protein that misfolds and aggregates causes cellular toxicity that is strongly dependent on the genetic background. To address the influence of genetic background on the toxicity of aggregation-prone proteins, we established a C. elegans model of misfolding and aggregation of several distinct ALS-related mutants of superoxide dismutase 1 (SOD1). In one wild type genetic background (N2), these proteins exhibited only mild cellular toxicity despite strong, mutant-specific aggregation phenotypes. However, when SOD1 mutants were expressed in the background of mildly destabilized protein polymorphisms, their toxicity was enhanced and a number of distinct phenotypes were exposed. These synthetic phenotypes reflected the loss-of-function of the destabilized polymorphic proteins. Furthermore, the degree to which each of these phenotypes was exposed depended on the nature of the SOD1 mutation. These data suggest that the presence of mildly destabilizing polymorphisms in the genetic background may modulate and direct the specific toxic phenotypes in protein aggregation diseases.
doi:10.1371/journal.pgen.1000399
PMCID: PMC2642731  PMID: 19266020
4.  Structural and thermodynamic effects of post-translational modifications in mutant and wild type Cu, Zn superoxide dismutase 
Journal of molecular biology  2011;408(3):555-567.
Aggregation of Cu, Zn superoxide dismutase (SOD1) is implicated in Amyotrophic Lateral Sclerosis (ALS). Glutathionylation and phosphorylation of SOD1 is omnipresent in the human body, even in healthy individuals, and has been shown to increase SOD1 dimer dissociation, which is the first step on the pathway toward SOD1 aggregation. We find that post-translational modification of SOD1, especially glutathionylation, promotes dimer dissociation. We discover an intermediate state in the pathway to dissociation, a conformational change that involves a “loosening” of the β-barrels and a loss or shift of dimer interface interactions. In modified SOD1, this intermediate state is stabilized as compared to unmodified SOD1. The presence of post-translational modifications could explain the environmental factors involved in the speed of disease progression. Because post-translational modifications such as glutathionylation are often induced by oxidative stress, post-translational modification of SOD1 could be a factor in the occurrence of sporadic cases of ALS, which make up 90% of all cases of the disease.
doi:10.1016/j.jmb.2011.03.004
PMCID: PMC3082150  PMID: 21396374
5.  An Analysis of Interactions between Fluorescently-Tagged Mutant and Wild-Type SOD1 in Intracellular Inclusions 
PLoS ONE  2013;8(12):e83981.
Background
By mechanisms yet to be discerned, the co-expression of high levels of wild-type human superoxide dismutase 1 (hSOD1) with variants of hSOD1 encoding mutations linked familial amyotrophic lateral sclerosis (fALS) hastens the onset of motor neuron degeneration in transgenic mice. Although it is known that spinal cords of paralyzed mice accumulate detergent insoluble forms of WT hSOD1 along with mutant hSOD1, it has been difficult to determine whether there is co-deposition of the proteins in inclusion structures.
Methodology/Principal Findings
In the present study, we use cell culture models of mutant SOD1 aggregation, focusing on the A4V, G37R, and G85R variants, to examine interactions between WT-hSOD1 and misfolded mutant SOD1. In these studies, we fuse WT and mutant proteins to either yellow or red fluorescent protein so that the two proteins can be distinguished within inclusions structures.
Conclusions/Significance
Although the interpretation of the data is not entirely straightforward because we have strong evidence that the nature of the fused fluorophores affects the organization of the inclusions that form, our data are most consistent with the idea that normal dimeric WT-hSOD1 does not readily interact with misfolded forms of mutant hSOD1. We also demonstrate the monomerization of WT-hSOD1 by experimental mutation does induce the protein to aggregate, although such monomerization may enable interactions with misfolded mutant SOD1. Our data suggest that WT-hSOD1 is not prone to become intimately associated with misfolded mutant hSOD1 within intracellular inclusions that can be generated in cultured cells.
doi:10.1371/journal.pone.0083981
PMCID: PMC3877123  PMID: 24391857
6.  Computational methods for identifying a layered allosteric regulatory mechanism for ALS-causing mutations of Cu-Zn superoxide dismutase 1 
Proteins  2011;79(2):417-427.
The most prominent form of familial amyotrophic lateral sclerosis (fALS, Lou Gehrig’s Disease) is caused by mutations of Cu-Zn superoxide dismutase 1 (SOD1). SOD1 maintains antioxidant activity under fALS causing mutations, suggesting that the mutations introduce a new, toxic, function. There are 100+ such known mutations that are chemically diverse and spatially distributed across the structure. The common phenotype leads us to propose an allosteric regulatory mechanism hypothesis: SOD1 mutants alter the correlated dynamics of the structure and differentially signal across an inherent allosteric network, thereby driving the disease mechanism at varying rates of efficiency.
Two recently developed computational methods for identifying allosteric control sites are applied to the wild type crystal structure, 4 fALS mutant crystal structures, 20 computationally generated fALS mutants and 1 computationally generated non-fALS mutant. The ensemble of mutant structures is used to generate an ensemble of dynamics, from which two allosteric control networks are identified. One network is connected to the catalytic site and thus may be involved in the natural antioxidant function. The second allosteric control network has a locus bordering the dimer interface and thus may serve as a mechanism to modulate dimer stability. Though the toxic function of mutated SOD1 is unknown and likely due to several contributing factors, this study explains how diverse mutations give rise to a common function. This new paradigm for allostery controlled function has broad implications across allosteric systems and may lead to the identification of the key chemical activity of SOD1-linked ALS.
doi:10.1002/prot.22892
PMCID: PMC3058251  PMID: 21104697
mutation ensemble; elastic network; normal mode analysis (NMA); correlated dynamics; network flow; amyotrophic lateral sclerosis (ALS)
7.  SOD1 and Amyotrophic Lateral Sclerosis: Mutations and Oligomerization 
PLoS ONE  2008;3(2):e1677.
There are about 100 single point mutations of copper, zinc superoxide dismutase 1 (SOD1) which are reported (http://alsod.iop.kcl.ac.uk/Als/index.aspx) to be related to the familial form (fALS) of amyotrophic lateral sclerosis (ALS). These mutations are spread all over the protein. It is well documented that fALS produces protein aggregates in the motor neurons of fALS patients, which have been found to be associated to mitochondria. We selected eleven SOD1 mutants, most of them reported as pathological, and characterized them investigating their propensity to aggregation using different techniques, from circular dichroism spectra to ThT-binding fluorescence, size-exclusion chromatography and light scattering spectroscopy. We show here that these eleven SOD1 mutants, only when they are in the metal-free form, undergo the same general mechanism of oligomerization as found for the WT metal-free protein. The rates of oligomerization are different but eventually they give rise to the same type of soluble oligomeric species. These oligomers are formed through oxidation of the two free cysteines of SOD1 (6 and 111) and stabilized by hydrogen bonds, between beta strands, thus forming amyloid-like structures. SOD1 enters the mitochondria as demetallated and mitochondria are loci where oxidative stress may easily occur. The soluble oligomeric species, formed by the apo form of both WT SOD1 and its mutants through an oxidative process, might represent the precursor toxic species, whose existence would also suggest a common mechanism for ALS and fALS. The mechanism here proposed for SOD1 mutant oligomerization is absolutely general and it provides a common unique picture for the behaviors of the many SOD1 mutants, of different nature and distributed all over the protein.
doi:10.1371/journal.pone.0001677
PMCID: PMC2250751  PMID: 18301754
8.  Colocalization of 14-3-3 Proteins with SOD1 in Lewy Body-Like Hyaline Inclusions in Familial Amyotrophic Lateral Sclerosis Cases and the Animal Model 
PLoS ONE  2011;6(5):e20427.
Background and Purpose
Cu/Zn superoxide dismutase (SOD1) is a major component of Lewy body-like hyaline inclusion (LBHI) found in the postmortem tissue of SOD1-linked familial amyotrophic lateral sclerosis (FALS) patients. In our recent studies, 14-3-3 proteins have been found in the ubiquitinated inclusions inside the anterior horn cells of spinal cords with sporadic amyotrophic lateral sclerosis (ALS). To further investigate the role of 14-3-3 proteins in ALS, we performed immunohistochemical analysis of 14-3-3 proteins and compared their distributions with those of SOD1 in FALS patients and SOD1-overexpressing mice.
Methods
We examined the postmortem brains and the spinal cords of three FALS cases (A4V SOD1 mutant). Transgenic mice expressing the G93A mutant human SOD1 (mutant SOD1-Tg mice), transgenic mice expressing the wild-type human SOD1 (wild-type SOD1-Tg mice), and non-Tg wild-type mice were also subjected to the immunohistochemical analysis.
Results
In all the FALS patients, LBHIs were observed in the cytoplasm of the anterior horn cells, and these inclusions were immunopositive intensely for pan 14-3-3, 14-3-3β, and 14-3-3γ. In the mutant SOD1-Tg mice, a high degree of immunoreactivity for misfolded SOD1 (C4F6) was observed in the cytoplasm, with an even greater degree of immunoreactivity present in the cytoplasmic aggregates of the anterior horn cells in the lumbar spinal cord. Furthermore, we have found increased 14-3-3β and 14-3-3γ immunoreactivities in the mutant SOD1-Tg mice. Double immunofluorescent staining showed that C4F6 and 14-3-3 proteins were partially co-localized in the spinal cord with FALS and the mutant SOD1-Tg mice. In comparison, the wild-type SOD1-Tg and non-Tg wild-type mice showed no or faint immunoreactivity for C4F6 and 14-3-3 proteins (pan 14-3-3, 14-3-3β, and 14-3-3γ) in any neuronal compartments.
Discussion
These results suggest that 14-3-3 proteins may be associated with the formation of SOD1-containing inclusions, in FALS patients and the mutant SOD1-Tg mice.
doi:10.1371/journal.pone.0020427
PMCID: PMC3105059  PMID: 21655264
9.  Features of wild-type human SOD1 limit interactions with misfolded aggregates of mouse G86R Sod1 
Mutations in the gene encoding superoxide dismutase 1 (SOD1) account for about 20% of the cases of familial amyotrophic lateral sclerosis (fALS). It is well established that mutations in SOD1, associated with fALS, heighten the propensity of the protein to misfold and aggregate. Although aggregation appears to be a factor in the toxicity of mutant SOD1s, the precise nature of this toxicity has not been elucidated. A number of other studies have now firmly established that raising the levels of wild-type (WT) human SOD1 (hSOD1) proteins can in some manner augment the toxicity of mutant hSOD1 proteins. However, a recent study demonstrated that raising the levels of WT-hSOD1 did not affect disease in mice that harbor a mouse Sod1 gene (mSod1) encoding a well characterized fALS mutation (G86R). In the present study, we sought a potential explanation for the differing effects with WT-hSOD1 on the toxicity of mutant hSOD1 versus mutant mSod1. In the cell culture models used here, we observe poor interactions between WT-hSOD1 and misfolded G86R-mSod1, possibly explaining why over-expression of WT-hSOD1 does not synergize with mutant mSod1 to accelerate the course of the disease in mice.
doi:10.1186/1750-1326-8-46
PMCID: PMC3881023  PMID: 24341866
10.  The Effects of Glutaredoxin and Copper Activation Pathways on the Disulfide and Stability of Cu,Zn Superoxide Dismutase*… 
The Journal of biological chemistry  2006;281(39):28648-28656.
Mutations in Cu,Zn superoxide dismutase (SOD1) can cause amyotrophic lateral sclerosis (ALS) through mechanisms proposed to involve SOD1 misfolding, but the intracellular factors that modulate folding and stability of SOD1 are largely unknown. By using yeast and mammalian expression systems, we demonstrate here that SOD1 stability is governed by post-translational modification factors that target the SOD1 disulfide. Oxidation of the human SOD1 disulfide in vivo was found to involve both the copper chaperone for SOD1 (CCS) and the CCS-independent pathway for copper activation. When both copper pathways were blocked, wild type SOD1 stably accumulated in yeast cells with a reduced disulfide, whereas ALS SOD1 mutants A4V, G93A, and G37R were degraded. We describe here an unprecedented role for the thiol oxidoreductase glutaredoxin in reducing the SOD1 disulfide and destabilizing ALS mutants. Specifically, the major cytosolic glutaredoxin of yeast was seen to reduce the intramolecular disulfide of ALS SOD1 mutant A4V SOD1 in vivo and in vitro. By comparison, glutaredoxin was less reactive toward the disulfide of wild type SOD1. The apo-form of A4V SOD1 was highly reactive with glutaredoxin but not SOD1 containing both copper and zinc. Glutaredoxin therefore preferentially targets the immature form of ALS mutant SOD1 lacking metal co-factors. Overall, these studies implicate a critical balance between cellular reductants such as glutaredoxin and copper activation pathways in controlling the disulfide and stability of SOD1 in vivo.
doi:10.1074/jbc.M600138200
PMCID: PMC2757158  PMID: 16880213
11.  Superoxide dismutase 1 encoding mutations linked to ALS adopts a spectrum of misfolded states 
Background
Mutations in superoxide dismutase 1 (SOD1), which are one cause of familial amyotrophic lateral sclerosis (fALS), induce misfolding and aggregation of the protein. Misfolding can be detected by the binding of antibodies raised against peptide epitopes that are normally buried in the native conformation, shifts in solubility in non-ionic detergents, and the formation of macromolecular inclusions. In the present study, we investigate the relationship between detergent-insoluble and sedimentable forms of mutant SOD1, forms of mutant SOD1 with aberrantly accessible epitopes, and mutant protein in inclusions with the goal of defining the spectrum of misfolded states that mutant SOD1 can adopt.
Results
Using combined approaches in cultured cell models, we demonstrate that a substantial fraction of mutant SOD1 adopts a non-native conformation that remains soluble and freely mobile. We also show that mutant SOD1 can produce multimeric assemblies of which some are insoluble in detergent and large enough to sediment by ultracentrifugation and some are large enough to detect visually. Three conformationally restricted antibodies were found to be useful in discriminating mal-folded forms of mutant SOD1. An antibody termed C4F6 displays properties consistent with recognition of soluble, freely mobile, mal-folded mutant SOD1. An antibody termed SEDI, which recognizes C-terminal residues, detects larger inclusion structures as well as soluble misfolded entities. An antibody termed hSOD1, which recognizes aa 24-36, detects an epitope shared by soluble non-natively folded WT and mutant SOD1. This epitope becomes inaccessible in aggregates of mutant SOD1.
Conclusions
Our studies demonstrate how different methods of detecting misfolding and aggregation of mutant SOD1 reveal different forms of aberrantly folded protein. Immunological and biochemical methods can be used in combination to detect soluble and insoluble misfolded forms of mutant SOD1. Our findings support the view that mutant SOD1 can adopt multiple misfolded conformations with the potential that different structural variants mediate different aspects of fALS.
doi:10.1186/1750-1326-6-77
PMCID: PMC3248846  PMID: 22094223
12.  An examination of wild-type SOD1 in modulating the toxicity and aggregation of ALS-associated mutant SOD1 
Human Molecular Genetics  2010;19(24):4774-4789.
Mutations in superoxide dismutase 1 (SOD1) are associated with familial cases of amyotrophic lateral sclerosis (fALS). Studies in transgenic mice have suggested that wild-type (WT) SOD1 can modulate the toxicity of mutant SOD1. In the present study, we demonstrate that the effects of WT SOD1 on the age at which transgenic mice expressing mutant human SOD1 (hSOD1) develop paralysis are influenced by the nature of the ALS mutation and the expression levels of WT hSOD1. We show that regardless of whether WT SOD1 changes the course of disease, both WT and mutant hSOD1 accumulate as detergent-insoluble aggregates in symptomatic mice expressing both proteins. However, using a panel of fluorescently tagged variants of SOD1 in a cell model of mutant SOD1 aggregation, we demonstrate that the interactions between mutant and WT SOD1 in aggregate formation are not simply a co-assembly of mutant and WT proteins. Overall, these data demonstrate that the product of the normal SOD1 allele in fALS has potential to influence the toxicity of mutant SOD1 and that complex interactions with the mutant protein may influence the formation of aggregates and inclusion bodies generated by mutant SOD1.
doi:10.1093/hmg/ddq408
PMCID: PMC2989888  PMID: 20871097
13.  Redox environment is an intracellular factor to operate distinct pathways for aggregation of Cu,Zn-superoxide dismutase in amyotrophic lateral sclerosis 
Dominant mutations in Cu,Zn-superoxide dismutase (SOD1) cause a familial form of amyotrophic lateral sclerosis (fALS). Misfolding and aggregation of mutant SOD1 proteins are a pathological hallmark of SOD1-related fALS cases; however, the molecular mechanism of SOD1 aggregation remains controversial. Here, I have used E. coli as a model organism and shown multiple distinct pathways of SOD1 aggregation that are dependent upon its thiol-disulfide status. Overexpression of fALS-mutant SOD1s in the cytoplasm of E. coli BL21 and SHuffleTM, where redox environment is reducing and oxidizing, respectively, resulted in the formation of insoluble aggregates with notable differences; a disulfide bond of SOD1 was completely reduced in BL21 or abnormally formed between SOD1 molecules in SHuffleTM. Depending upon intracellular redox environment, therefore, mutant SOD1 is considered to misfold/aggregate through distinct pathways, which would be relevant in description of the pathological heterogeneity of SOD1-related fALS cases.
doi:10.3389/fncel.2013.00240
PMCID: PMC3841916  PMID: 24348334
SOD1; ALS; aggregation; disulfide bond
14.  MODULATION OF MUTANT SUPEROXIDE DISMUTASE 1 AGGREGATION BY CO-EXPRESSION OF WILD-TYPE ENZYME 
Journal of neurochemistry  2008;108(4):1009-1018.
Mutations in superoxide dismutase 1 (SOD1, EC 1.15.1.1) cause familial amyotrophic lateral sclerosis (fALS); with aggregated forms of mutant protein accumulating in spinal cord tissues of transgenic mouse models and human patients. Mice over-expressing wild-type human SOD1 (WT hSOD1) do not develop ALS-like disease, but co-expression of WT enzyme at high levels with mutant SOD1 accelerates the onset of motor neuron disease compared to mice expressing mutant hSOD1 alone. Spinal cords of mice expressing both proteins contain aggregated forms of mutant protein and, in some cases, evidence of co-aggregation of WT hSOD1 enzyme. In the present study, we used a cell culture model of mutant SOD1 aggregation to examine how the presence of WT SOD1 affects mutant protein aggregation, finding that co-expression of WT SOD1, human (hSOD1) or mouse (mSOD1), delayed the formation of mutant hSOD1 aggregates; in essence appearing to slow the aggregation rate. In some combinations of WT and mutant hSOD1 co-expression, the aggregates that did eventually form appeared to contain WT hSOD1 protein. However, WT mSOD1 did not co-aggregate with mutant hSOD1 despite displaying a similar ability to slow mutant hSOD1 aggregation. Together, these studies indicate that WT SOD1 (human or mouse), when expressed at levels equivalent to the mutant protein, modulates aggregation of FALS-mutant hSOD1.
doi:10.1111/j.1471-4159.2008.05839.x
PMCID: PMC2801375  PMID: 19077113
superoxide; dismutase; aggregation; amyotrophic lateral sclerosis
15.  Structural and Biophysical Properties of the Pathogenic SOD1 Variant H46R/H48Q 
Biochemistry  2009;48(15):3436-3447.
Over 100 mutations in the gene encoding human copper-zinc superoxide dismutase (SOD1) cause an inherited form of the fatal neurodegenerative disease amyotrophic lateral sclerosis (ALS). Two pathogenic SOD1 mutations, His46Arg (H46R) and His48Gln (H48Q), affect residues that act as copper ligands in the wild type enzyme. Transgenic mice expressing a human SOD1 variant containing both mutations develop paralytic disease akin to ALS. Here we show that H46R/H48Q SOD1 possesses multiple characteristics that distinguish it from the wild type. These properties include: 1) an ablated copper-binding site; 2) a substantially weakened affinity for zinc; 3) a binding site for calcium ion; 4) the ability to form stable heterocomplexes with the Copper Chaperone for SOD1 (CCS); and 5) compromised CCS-mediated oxidation of the intrasubunit disulfide bond in vivo. The results presented here, together with data on pathogenic SOD1 proteins coming from cell culture and transgenic mice, suggest that incomplete posttranslational modification of nascent SOD1 polypeptides via CCS may be a characteristic shared by fALS SOD1 mutants, leading to a population of destabilized, off-pathway folding intermediates that are toxic to motor neurons.
doi:10.1021/bi8021735
PMCID: PMC2757159  PMID: 19227972
16.  Glutathionylation potentiates benign superoxide dismutase 1 variants to the toxic forms associated with amyotrophic lateral sclerosis 
Scientific Reports  2013;3:3275.
Dissociation of superoxide dismutase 1 dimers is enhanced by glutathionylation, although the dissociation constants reported to date are imprecise. We have quantified the discreet dissociation constants for wild-type superoxide dismutase 1 and six naturally occurring sequence variants, in their unmodified and glutathionylated forms, at the ratios expressed. Unmodified superoxide dismutase 1 variants that shared similar dissociation constants with SOD1WT had disproportionately increased dissociation constants when glutathionylated. This defines a key role for glutathionylation in superoxide dismutase 1 associated familial amyotrophic lateral sclerosis.
doi:10.1038/srep03275
PMCID: PMC3834562  PMID: 24253732
17.  Early steps in thermal unfolding of superoxide dismutase 1 are similar to the conformational changes associated with the ALS-associated A4V mutation 
There are over 100 mutations in Cu/Zn superoxide dismutase (SOD1) that result in a subset of familial amyotrophic lateral sclerosis (fALS) cases. The hypothesis that dissociation of the dimer, misfolding of the monomer and subsequent aggregation of mutant SOD1 leads to fALS has been gaining support as an explanation for how these disparate missense mutations cause the same disease. These forms are only responsible for a fraction of the ALS cases; however, the rest are sporadic. Starting with a folded apo monomer, the species considered most likely to be involved in misfolding, we used high-temperature all-atom molecular dynamics simulations to explore the events of the wild-type protein unfolding through the denatured state. All simulations showed early loss of structure along the β5–β6 edge of the β-sandwich, supporting earlier findings of instability in this region. Transition state structures identified from the simulations are in good agreement with experiment, providing detailed, validated molecular models for this elusive state. Furthermore, we compare the process of thermal unfolding investigated here to that of the lethal A4V mutant-induced unfolding at physiological temperature and find that the pathways are very similar.
doi:10.1093/protein/gzt030
PMCID: PMC3711394  PMID: 23784844
ALS; misfolding; molecular dynamics; SOD1; unfolding
18.  S-Glutathionylation: From Molecular Mechanisms to Health Outcomes 
Antioxidants & Redox Signaling  2011;15(1):233-270.
Abstract
Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms. Antioxid. Redox Signal. 15, 233–270.
I. Introduction
A. Glutathione homeostasis
B. Proximal donors for S-glutathionylation reactions
II. Detection of S-Glutathionylation
A. Antibody detection of S-glutathionylation
B. Analytical detection and quantification of P-SSG
III. Enzymes That Catalyze the S-Glutathionylation Cycle
A. Proteins with S-glutathionylase activity
1. Glutathione S-transferases
2. Gamma-glutamyl transpeptidase
3. Grx1 and Grx2
B. Proteins with deglutathionylase activity
IV. Redox Regulation of Kinase Signaling Pathways
A. S-glutathionylation and modulation of mitogenic signaling
1. Ras-MEK-ERK pathway
2. Protein tyrosine phosphatases
3. Protein kinase A
B. Phosphatidylinositol 3-kinase-Akt-p53 pathway
C. I kappa B kinase-nuclear factor kappa B pathway
D. JNK-c-Jun pathway
V. S-Glutathionylation and Modulation of Survival/Apoptosis
A. S-glutathionylation of death receptors
B. S-glutathionylation of caspases
VI. Redox Regulation of Calcium-Dependent Proteins
A. Protein kinase C
B. Sarco/ER calcium ATPase
C. Nitric oxide synthase
VII. S-Glutathionylation and Ubiquitin-Proteasome Pathway
VIII. S-Glutathionylation and Unfolded Protein Response
A. Signaling pathways in the unfolded protein response
B. Protein disulfide isomerase
IX. Redox Regulation of Cell Migration and Mobilization
A. S-glutathionylation of cytoskeletal proteins
B. Redox regulation of bone marrow mobilization
X. Cancer and Redox Homeostasis
A. Energy metabolism
B. S-glutathionylation and tumor metastasis
C. S100 proteins in cancer and leukocyte migration
1. S100B
2. S100A8 and S100A9
XI. Redox Dysregulation in Pathophysiology
A. Liver injury
B. Diabetes mellitus
C. Cardiovascular disease
D. Traumatic brain injury
XII. Neurological Diseases and Redox Pathways
A. Parkinson's disease
B. Alzheimer's disease
C. Huntington's disease
D. Friedreich's ataxia
E. Amylotrophic lateral sclerosis
XIII. Conclusions
doi:10.1089/ars.2010.3540
PMCID: PMC3110090  PMID: 21235352
19.  FUS and TARDBP but Not SOD1 Interact in Genetic Models of Amyotrophic Lateral Sclerosis 
PLoS Genetics  2011;7(8):e1002214.
Mutations in the SOD1 and TARDBP genes have been commonly identified in Amyotrophic Lateral Sclerosis (ALS). Recently, mutations in the Fused in sarcoma gene (FUS) were identified in familial (FALS) ALS cases and sporadic (SALS) patients. Similarly to TDP-43 (coded by TARDBP gene), FUS is an RNA binding protein. Using the zebrafish (Danio rerio), we examined the consequences of expressing human wild-type (WT) FUS and three ALS–related mutations, as well as their interactions with TARDBP and SOD1. Knockdown of zebrafish Fus yielded a motor phenotype that could be rescued upon co-expression of wild-type human FUS. In contrast, the two most frequent ALS–related FUS mutations, R521H and R521C, unlike S57Δ, failed to rescue the knockdown phenotype, indicating loss of function. The R521H mutation caused a toxic gain of function when expressed alone, similar to the phenotype observed upon knockdown of zebrafish Fus. This phenotype was not aggravated by co-expression of both mutant human TARDBP (G348C) and FUS (R521H) or by knockdown of both zebrafish Tardbp and Fus, consistent with a common pathogenic mechanism. We also observed that WT FUS rescued the Tardbp knockdown phenotype, but not vice versa, suggesting that TARDBP acts upstream of FUS in this pathway. In addition we observed that WT SOD1 failed to rescue the phenotype observed upon overexpression of mutant TARDBP or FUS or upon knockdown of Tardbp or Fus; similarly, WT TARDBP or FUS also failed to rescue the phenotype induced by mutant SOD1 (G93A). Finally, overexpression of mutant SOD1 exacerbated the motor phenotype caused by overexpression of mutant FUS. Together our results indicate that TARDBP and FUS act in a pathogenic pathway that is independent of SOD1.
Author Summary
Mutations in the SOD1, TARDBP, and FUS genes have been commonly identified in Amyotrophic Lateral Sclerosis (ALS). However, possible interactions between these ALS–causative genetic mutations have not been examined. Here we expressed each of three human FUS mutations (R521H, R521C, and S57Δ) in zebrafish embryos, with or without knocking down the zebrafish homolog Fus, and observed a motor phenotype consisting of significant behavioral (touch-evoked escape response) and cellular (shortened axonal projections from motor neurons) deficits due to loss of function for the R521H and R521C mutations and/or toxic gain of function solely for the R521H mutation. Wild-type FUS could rescue the Tardbp knockdown phenotype, but not vice versa, suggesting that TARDBP is upstream of FUS in this pathway responsible for motor neuron disorder. Furthermore, neither TARDBP nor FUS were able to modify and/or rescue the motor phenotype caused by mutant SOD1, and likewise SOD1 failed to rescue the phenotype of zebrafish expressing mutant TARDBP or FUS. Our results indicate that TARDBP acts upstream of FUS in a pathogenic pathway that is distinct from that of SOD1.
doi:10.1371/journal.pgen.1002214
PMCID: PMC3150442  PMID: 21829392
20.  Pathological Roles of Wild-Type Cu, Zn-Superoxide Dismutase in Amyotrophic Lateral Sclerosis 
Dominant mutations in a Cu, Zn-superoxide dismutase (SOD1) gene cause a familial form of amyotrophic lateral sclerosis (ALS). While it remains controversial how SOD1 mutations lead to onset and progression of the disease, many in vitro and in vivo studies have supported a gain-of-toxicity mechanism where pathogenic mutations contribute to destabilizing a native structure of SOD1 and thus facilitate misfolding and aggregation. Indeed, abnormal accumulation of SOD1-positive inclusions in spinal motor neurons is a pathological hallmark in SOD1-related familial ALS. Furthermore, similarities in clinical phenotypes and neuropathology of ALS cases with and without mutations in sod1 gene have implied a disease mechanism involving SOD1 common to all ALS cases. Although pathogenic roles of wild-type SOD1 in sporadic ALS remain controversial, recent developments of novel SOD1 antibodies have made it possible to characterize wild-type SOD1 under pathological conditions of ALS. Here, I have briefly reviewed recent progress on biochemical and immunohistochemical characterization of wild-type SOD1 in sporadic ALS cases and discussed possible involvement of wild-type SOD1 in a pathomechanism of ALS.
doi:10.1155/2012/323261
PMCID: PMC3395178  PMID: 22830015
21.  Distinctive features of the D101N and D101G variants of superoxide dismutase 1; two mutations that produce rapidly progressing motor neuron disease 
Journal of neurochemistry  2013;128(2):305-314.
Mutations in superoxide dismutase 1 (SOD1) associated with familial amyotrophic lateral sclerosis (fALS) induce misfolding and aggregation of the protein with the inherent propensity of mutant SOD1 to aggregate generally correlating, with a few exceptions, to the duration of illness in patients with the same mutation. One notable exception was the D101N variant, which has been described as wild-type-like. The D101N mutation is associated with rapidly progressing motor neuron degeneration but shows a low propensity to aggregate. By assaying the kinetics of aggregation in a well characterized cultured cell model, we show that the D101N mutant is slower to initiate aggregation than the D101G mutant. In this cell system of protein over-expression, both mutants were equally less able to acquire Zn than WT SOD1. Additionally, both of these mutants were equivalently less able to fold into the trypsin-resistant conformation that characterizes WT SOD1. A second major difference between the two mutants was that the D101N variant more efficiently formed a normal intramolecular disulfide bond. Overall, our findings demonstrate that the D101N and D101G variants exhibit clearly distinctive features, including a different rate of aggregation, and yet both are associated with rapidly progressing disease.
doi:10.1111/jnc.12451
PMCID: PMC3971632  PMID: 24032979
ALS; SOD1; aggregation; oxidation; stability
22.  Structural and Functional Analysis of Human SOD1 in Amyotrophic Lateral Sclerosis 
PLoS ONE  2013;8(12):e81979.
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with familial inheritance (fALS) in 5% to 10% of cases; 25% of those are caused by mutations in the superoxide dismutase 1 (SOD1) protein. More than 100 mutations in the SOD1 gene have been associated with fALS, altering the geometry of the active site, protein folding and the interaction between monomers. We performed a functional analysis of non-synonymous single nucleotide polymorphisms (nsSNPs) in 124 fALS SOD1 mutants. Eleven different algorithms were used to estimate the functional impact of the replacement of one amino acid on protein structure: SNPs&GO, PolyPhen-2, SNAP, PMUT, Sift, PhD-SNP, nsSNPAnalyzer, TANGO, WALTZ, LIMBO and FoldX. For the structural analysis, theoretical models of 124 SNPs of SOD1 were created by comparative modeling using the MHOLline workflow, which includes Modeller and Procheck. Models were aligned with the native protein by the TM-align algorithm. A human-curated database was developed using the server side include in Java, JMOL. The results of this functional analysis indicate that the majority of the 124 natural mutants are harmful to the protein structure and thus corroborate the correlation between the reported mutations and fALS. In the structural analysis, all models showed conformational changes when compared to wild-type SOD1, and the degree of structural alignment varied between them. The SOD1 database converge structural and functional analyses of SOD1; it is a vast resource for the molecular analysis of amyotrophic lateral sclerosis, which allows the user to expand his knowledge on the molecular basis of the disease. The SOD1 database is available at http://bioinfogroup.com/database.
doi:10.1371/journal.pone.0081979
PMCID: PMC3846731  PMID: 24312616
23.  An emerging role for misfolded wild-type SOD1 in sporadic ALS pathogenesis 
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that targets motor neurons, leading to paralysis and death within a few years of disease onset. While several genes have been linked to the inheritable, or familial, form of ALS, much less is known about the cause(s) of sporadic ALS, which accounts for ~90% of ALS cases. Due to the clinical similarities between familial and sporadic ALS, it is plausible that both forms of the disease converge on a common pathway and, therefore, involve common factors. Recent evidence suggests the Cu,Zn-superoxide dismutase (SOD1) protein to be one such factor that is common to both sporadic and familial ALS. In 1993, mutations were uncovered in SOD1 that represent the first known genetic cause of familial ALS. While the exact mechanism of mutant-SOD1 toxicity is still not known today, most evidence points to a gain of toxic function that stems, at least in part, from the propensity of this protein to misfold. In the wild-type SOD1 protein, non-genetic perturbations such as metal depletion, disruption of the quaternary structure, and oxidation, can also induce SOD1 to misfold. In fact, these aforementioned post-translational modifications cause wild-type SOD1 to adopt a “toxic conformation” that is similar to familial ALS-linked SOD1 variants. These observations, together with the detection of misfolded wild-type SOD1 within human post-mortem sporadic ALS samples, have been used to support the controversial hypothesis that misfolded forms of wild-type SOD1 contribute to sporadic ALS pathogenesis. In this review, we present data from the literature that both support and contradict this hypothesis. We also discuss SOD1 as a potential therapeutic target for both familial and sporadic ALS.
doi:10.3389/fncel.2013.00253
PMCID: PMC3863749  PMID: 24379756
amyotrophic lateral sclerosis (ALS); sporadic amyotrophic lateral sclerosis; SOD1; protein misfolding; immunotherapy
24.  Local unfolding of Cu, Zn Superoxide Dismutase monomer determines the morphology of fibrillar aggregates 
Journal of Molecular Biology  2011;421(4-5):548-560.
Aggregation of Cu, Zn Superoxide Dismutase (SOD1) is often found in Amyotrophic Lateral Sclerosis (ALS) patients. The fibrillar aggregates formed by wildtype and various disease-associated mutants have recently been found to have distinct cores and morphologies. Previous computational and experimental studies of wildtype SOD1 suggest that the apo-monomer, highly aggregation-prone, displays substantial local unfolding dynamics. The residual folded structure of locally unfolded apoSOD1 corresponds to peptide segments forming the aggregation core as identified by a combination of proteolysis and mass spectroscopy. Therefore, we hypothesize that the destabilization of apoSOD1 caused by various mutations leads to distinct local unfolding dynamics. The partially unfolded structure, exposing the hydrophobic core and backbone hydrogen bond donors and acceptors, is prone to aggregate. The peptide segments in the residual folded structures form the “building block” for aggregation, which in turn determines the morphology of the aggregates. To test this hypothesis, we apply a multiscale simulation approach to study the aggregation of three typical SOD1 variants: wildtype, G37R, and I149T. Each of these SOD1 variants has distinct peptide segments forming the core structure and features different aggregate morphologies. We perform atomistic molecular dynamics simulations to study the conformational dynamics of apoSOD1 monomer, and coarse-grained molecular dynamics simulations to study the aggregation of partially unfolded SOD1 monomers. Our computational studies of monomer local unfolding and the aggregation of different SOD1 variants are consistent with experiments, supporting the hypothesis of the formation of aggregation “building blocks” via apo-monomer local unfolding as the mechanism of SOD1 fibrillar aggregation.
doi:10.1016/j.jmb.2011.12.029
PMCID: PMC3320695  PMID: 22210350
SOD1 misfolding and aggregation; fibrillar aggregate; aggregation building block; molecular dynamics; multiscale modeling
25.  Structural Characterization of Zinc-deficient Human Superoxide Dismutase and Implications for ALS 
Journal of molecular biology  2007;373(4):877-890.
Over 130 mutations to copper, zinc superoxide dismutase (SOD) are implicated in the selective death of motor neurons found in 25% of familial amyotrophic lateral sclerosis (ALS) patients. Despite their widespread distribution, ALS mutations appear positioned to cause structural and misfolding defects. Such defects decrease SOD’s affinity for zinc, and loss of zinc from SOD is sufficient to induce apoptosis in motor neurons in vitro. To examine the importance of the zinc site in the structure and pathogenesis of human SOD, we determined the 2.0 Å resolution crystal structure of a designed zinc-deficient human SOD, in which two zinc-binding ligands have been mutated to hydrogen-bonding serine residues. This structure revealed a 9° twist of the subunits, which opens the SOD dimer interface and represents the largest inter-subunit rotational shift observed for a human SOD variant. Furthermore, the electrostatic loop and zinc-binding sub-loop were partly disordered, the catalytically important Arg143 was rotated away from the active site, and the normally rigid intramolecular Cys57-Cys146 disulfide bridge assumed two conformations. Together, these changes allow small molecules greater access to the catalytic copper, consistent with the observed increased redox activity of zinc-deficient SOD. Moreover, the dimer interface is weakened and the Cys57-Cys146 disulfide is more labile, as demonstrated by the increased aggregation of zinc-deficient SOD in the presence of a thiol reductant. However, equimolar Cu,Zn SOD rapidly forms heterodimers with zinc-deficient SOD (t1/2 ≈ 15 min) and prevents aggregation. The stabilization of zinc-deficient SOD as a heterodimer with Cu,Zn SOD may thus be a contributing factor to the dominant inheritance of ALS mutations. These results have general implications for the importance of framework stability on normal metalloenzyme function and specific implications for the role of zinc ion in the fatal neuropathology associated with SOD mutations.
doi:10.1016/j.jmb.2007.07.043
PMCID: PMC2175016  PMID: 17888947
Amyotrophic lateral sclerosis; Cu; Zn superoxide dismutase; Lou Gehrig’s disease; zinc-deficient superoxide dismutase; crystal structure

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