Characterization of a covalent polysulfane bridge in copper-zinc superoxide dismutase .
ABSTRACT: In the course of studies on human copper-zinc superoxide dismutase (SOD1), we observed a modified form of the protein whose mass was increased by 158 mass units. The covalent modification was characterized, and we established that it is a novel heptasulfane bridge connecting the two Cys111 residues in the SOD1 homodimer. The heptasulfane bridge was visualized directly in the crystal structure of a recombinant human mutant SOD1, H46R/H48Q, produced in yeast. The modification is reversible, with the bridge being cleaved by thiols, by cyanide, and by unfolding of the protein to expose the polysulfane. The polysulfane bridge can be introduced in vitro by incubation of purified SOD1 with elemental sulfur, even under anaerobic conditions and in the presence of a metal chelator. Because polysulfanes and polysulfides can catalyze the generation of reactive oxygen and sulfur species, the modification may endow SOD1 with a toxic gain of function.
Project description:Mutations in the Cu/Zn superoxide dismutase gene (SOD1) are responsible for 20% of familial forms of amyotrophic lateral sclerosis (ALS), and mutant SOD1 has been shown to have increased surface hydrophobicity in vitro. Mutant SOD1 may adopt a complex array of conformations with varying toxicity in vivo. We have used a novel fluorescence-based proteomic assay using 4,4'-bis-1-anilinonaphthalene-8-sulfonate (bisANS) to assess the surface hydrophobicity, and thereby distinguish between different conformations, of SOD1 and other proteins in situ.Covalent bisANS labeling of spinal cord extracts revealed that alterations in surface hydrophobicity of H46R/H48Q mutations in SOD1 provoke formation of high molecular weight SOD1 species with lowered solubility, likely due to increased exposure of hydrophobic surfaces. BisANS was docked on the H46R/H48Q SOD1 structure at the disordered copper binding and electrostatic loops of mutant SOD1, but not non-mutant WT SOD1. 16 non-SOD1 proteins were also identified that exhibited altered surface hydrophobicity in the H46R/H48Q mutant mouse model of ALS, including proteins involved in energy metabolism, cytoskeleton, signaling, and protein quality control. Heat shock proteins (HSPs) were also enriched in the detergent-insoluble fractions with SOD1. Given that chaperones recognize proteins with exposed hydrophobic surfaces as substrates and the importance of protein homeostasis in ALS, we crossed SOD1 H46R/H48Q mutant mice with mice over-expressing the heat shock factor 1 (HSF1) transcription factor. Here we showed that HSF1 over-expression in H46R/H48Q ALS mice enhanced proteostasis as evidenced by increased expression of HSPs in motor neurons and astrocytes and increased solubility of mutant SOD1. HSF1 over-expression significantly reduced body weight loss, delayed ALS disease onset, decreases cases of early disease, and increased survival for the 25th percentile in an H46R/H48Q SOD1 background. HSF1 overexpression did not affect macroautophagy in the ALS background, but was associated with maintenance of carboxyl terminus of Hsp70 interacting protein (CHIP) expression which declined in H46R/H48Q mice.Our results uncover the potential importance of changes in protein surface hydrophobicity of SOD1 and other non-SOD1 proteins in ALS, and how strategies that activate HSF1 are valid therapies for ALS and other age-associated proteinopathies.
Project description: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 the following: (1) an ablated copper-binding site, (2) a substantially weakened affinity for zinc, (3) a binding site for a 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 familial ALS SOD1 mutants, leading to a population of destabilized, off-pathway folding intermediates that are toxic to motor neurons.
Project description:A subset of superoxide dismutase 1 (Cu/Zn-SOD1) mutants that cause familial amyotrophic lateral sclerosis (FALS) have heightened reactivity with (-)ONOO and H(2)O(2) in vitro. This reactivity requires a copper ion bound in the active site and is a suggested mechanism of motor neuron injury. However, we have found that transgenic mice that express SOD1-H46R/H48Q, which combines natural FALS mutations at ligands for copper and which is inactive, develop motor neuron disease. Using a direct radioactive copper incorporation assay in transfected cells and the established tools of single crystal x-ray diffraction, we now demonstrate that this variant does not stably bind copper. We find that single mutations at copper ligands, including H46R, H48Q, and a quadruple mutant H46R/H48Q/H63G/H120G, also diminish the binding of radioactive copper. Further, using native polyacrylamide gel electrophoresis and a yeast two-hybrid assay, the binding of copper was found to be related to the formation of the stable dimeric enzyme. Collectively, our data demonstrate a relationship between copper and assembly of SOD1 into stable dimers and also define disease-causing SOD1 mutants that are unlikely to robustly produce toxic radicals via copper-mediated chemistry.
Project description:Mutations in the metalloenzyme copper-zinc superoxide dismutase (SOD1) cause one form of familial amyotrophic lateral sclerosis (ALS), and metals are suspected to play a pivotal role in ALS pathology. To learn more about metals in ALS, we determined the metallation states of human wild-type or mutant (G37R, G93A, and H46R/H48Q) SOD1 proteins from SOD1-ALS transgenic mice spinal cords. SOD1 was gently extracted from spinal cord and separated into insoluble (aggregated) and soluble (supernatant) fractions, and then metallation states were determined by HPLC inductively coupled plasma MS. Insoluble SOD1-rich fractions were not enriched in copper and zinc. However, the soluble mutant and WT SOD1s were highly metallated except for the metal-binding-region mutant H46R/H48Q, which did not bind any copper. Due to the stability conferred by high metallation of G37R and G93A, it is unlikely that these soluble SOD1s are prone to aggregation in vivo, supporting the hypothesis that immature nascent SOD1 is the substrate for aggregation. We also investigated the effect of SOD1 overexpression and disease on metal homeostasis in spinal cord cross-sections of SOD1-ALS mice using synchrotron-based x-ray fluorescence microscopy. In each mouse genotype, except for the H46R/H48Q mouse, we found a redistribution of copper between gray and white matters correlated to areas of high SOD1. Interestingly, a disease-specific increase of zinc was observed in the white matter for all mutant SOD1 mice. Together these data provide a picture of copper and zinc in the cell as well as highlight the importance of these metals in understanding SOD1-ALS pathology.
Project description:The His46Arg (H46R) mutant of human copper-zinc superoxide dismutase (SOD1) is associated with an unusual, slowly progressing form of familial amyotrophic lateral sclerosis (FALS). Here we describe in detail the crystal structures of pathogenic H46R SOD1 in the Zn-loaded (Zn-H46R) and metal-free (apo-H46R) forms. The Zn-H46R structure demonstrates a novel zinc coordination that involves only three of the usual four liganding residues, His 63, His 80, and Asp 83 together with a water molecule. In addition, the Asp 124 "secondary bridge" between the copper- and zinc-binding sites is disrupted, and the "electrostatic loop" and "zinc loop" elements are largely disordered. The apo-H46R structure exhibits partial disorder in the electrostatic and zinc loop elements in three of the four dimers in the asymmetric unit, while the fourth has ordered loops due to crystal packing interactions. In both structures, nonnative SOD1-SOD1 interactions lead to the formation of higher-order filamentous arrays. The disordered loop elements may increase the likelihood of protein aggregation in vivo, either with other H46R molecules or with other critical cellular components. Importantly, the binding of zinc is not sufficient to prevent the formation of nonnative interactions between pathogenic H46R molecules. The increased tendency to aggregate, even in the presence of Zn, arising from the loss of the secondary bridge is consistent with the observation of an increased abundance of hyaline inclusions in spinal motor neurons and supporting cells in H46R SOD1 transgenic rats.
Project description:Determining the composition of aggregated superoxide dismutase 1 (SOD1) species associated with amyotrophic lateral sclerosis (ALS), especially with respect to co-aggregated proteins and post-translational modifications, could identify cellular or biochemical factors involved in the formation of these aggregates and explain their apparent neurotoxicity. The results of mass spectrometric and shotgun-proteomic analyses of SOD1-containing aggregates isolated from spinal cords of symptomatic transgenic ALS mice using two different isolation strategies are presented, including 1) resistance to detergent extraction and 2) size exclusion-coupled anti-SOD1 immunoaffinity chromatography. Forty-eight spinal cords from three different ALS-SOD1 mutant mice were analyzed, namely G93A, G37R, and the unnatural double mutant H46R/H48Q. The analysis consistently revealed that the most abundant proteins recovered from aggregate species were full-length unmodified SOD1 polypeptides. Although aggregates from some spinal cord samples contained trace levels of highly abundant proteins, such as vimentin and neurofilament-3, no proteins were consistently found to co-purify with mutant SOD1 in stoichiometric quantities. The results demonstrate that the principal protein in the high molecular mass aggregates whose appearance correlates with symptoms of the disease is the unmodified, full-length SOD1 polypeptide.
Project description: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 ?-bridge stabilized by a hydrogen bond between Ser105-NH and His110-CO, while the other forms a ?-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.
Project description:It is proposed that conformational changes induced in proteins by oxidation can lead to loss of activity or protein aggregation through exposure of hydrophobic residues and alteration in surface hydrophobicity. Because increased oxidative stress and protein aggregation are consistently observed in amyotrophic lateral sclerosis (ALS), we used a 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (BisANS) photolabeling approach to monitor changes in protein unfolding in vivo in skeletal muscle proteins in ALS mice. We find two major proteins, creatine kinase (CK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), conformationally affected in the ALS G93A mouse model concordant with a 43% and 41% reduction in enzyme activity, respectively. This correlated with changes in conformation and activity that were detected in CK and GAPDH with in vitro oxidation. Interestingly, we found that GAPDH, but not CK, is conformationally and functionally affected in a longer-lived ALS model (H46R/H48Q), exhibiting a 22% reduction in enzyme activity. We proposed a reaction mechanism for BisANS with nucleophilic amino acids such as lysine, serine, threonine, and tyrosine, and BisANS was found to be primarily incorporated to lysine residues in GAPDH. We identified the specific BisANS incorporation sites on GAPDH in nontransgenic (NTg), G93A, and H46R/H48Q mice using liquid chromatography-tandem mass spectrometry analysis. Four BisANS-containing sites (K52, K104, K212, and K248) were found in NTg GAPDH, while three out of four of these sites were lost in either G93A or H46R/H48Q GAPDH. Conversely, eight new sites (K2, K63, K69, K114, K183, K251, S330, and K331) were found on GAPDH for G93A, including one common site (K114) for H46R/H48Q, which is not found on GAPDH from NTg mice. These data show that GAPDH is differentially affected structurally and functionally in vivo in accordance with the degree of oxidative stress associated with these two models of ALS.
Project description:Transgenic mice that model familial (f)ALS, caused by mutations in superoxide dismutase (SOD)1, develop paralysis with pathology that includes the accumulation of aggregated forms of the mutant protein. Using a highly sensitive detergent extraction assay, we traced the appearance and abundance of detergent-insoluble and disulfide cross-linked aggregates of SOD1 throughout the disease course of SOD1-fALS mice (G93A, G37R, and H46R/H48Q). We demonstrate that the accumulation of disulfide cross-linked, detergent-insoluble, aggregates of mutant SOD1 occurs primarily in the later stages of the disease, concurrent with the appearance of rapidly progressing symptoms. We find no evidence for a model in which aberrant intermolecular disulfide bonding has an important role in promoting the aggregation of mutant SOD1, instead, such cross-linking appears to be a secondary event. Also, using both cell culture and mouse models, we find that mutant protein lacking the normal intramolecular disulfide bond is a major component of the insoluble SOD1 aggregates. Overall, our findings suggest a model in which soluble forms of mutant SOD1 initiate disease with larger aggregates implicated only in rapidly progressing events in the final stages of disease. Within the final stages of disease, abnormalities in the oxidation of a normal intramolecular disulfide bond in mutant SOD1 facilitate the aggregation of mutant protein.
Project description:An important consequence of protein misfolding related to neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), is the formation of proteinaceous inclusions or aggregates within the central nervous system. We have previously shown that several familial ALS-linked copper-zinc superoxide dismutase (SOD1) mutants (A4V, G85R, and G93A) interact and co-localize with the dynein-dynactin complex in cultured cells and affected tissues of ALS mice. In this study, we report that the interaction between mutant SOD1 and the dynein motor plays a critical role in the formation of large inclusions containing mutant SOD1. Disruption of the motor by overexpression of the p50 subunit of dynactin in neuronal and non-neuronal cell cultures abolished the association between aggregation-prone SOD1 mutants and the dynein-dynactin complex. The p50 overexpression also prevented mutant SOD1 inclusion formation and improved the survival of cells expressing A4V SOD1. Furthermore, we observed that two ALS-linked SOD1 mutants, H46R and H48Q, which showed a lower propensity to interact with the dynein motor, also produced less aggregation and fewer large inclusions. Overall, these data suggest that formation of large inclusions depends upon association of the abnormal SOD1s with the dynein motor. Whether the misfolded SOD1s directly perturb axonal transport or impair other functional properties of the dynein motor, this interaction could propagate a toxic effect that ultimately causes motor neuron death in ALS.