Formation of granular cytoplasmic aggregates in COS7 cells expressing mutant Cu/Zn superoxide dismutase associated with familial amyotrophic lateral sclerosis

The biophysics of superoxide dismutase-1 and amyotrophic lateral sclerosis

Few proteins have come under such intense scrutiny as superoxide dismutase-1 (SOD1). For almost a century, scientists have dissected its form, function and then later its malfunction in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). We now know SOD1 is a zinc and copper metalloenzyme that clears superoxide as part of our antioxidant defence and respiratory regulation systems. The possibility of reduced structural integrity was suggested by the first crystal structures of human SOD1 even before deleterious mutations in the sod1 gene were linked to the ALS. This concept evolved in the intervening years as an impressive array of biophysical studies examined the characteristics of mutant SOD1 in great detail. We now recognise how ALS-related mutations perturb the SOD1 maturation processes, reduce its ability to fold and reduce its thermal stability and half-life. Mutant SOD1 is therefore predisposed to monomerisation, non-canonical self-interactions, the formation of small misfolded oligomers and ultimately accumulation in the tell-tale insoluble inclusions found within the neurons of ALS patients. We have also seen that several post-translational modifications could push wild-type SOD1 down this toxic pathway. Recently we have come to view ALS as a prion-like disease where both the symptoms, and indeed SOD1 misfolding itself, are transmitted to neighbouring cells. This raises the possibility of intervention after the initial disease presentation. Several small-molecule and biologic-based strategies have been devised which directly target the SOD1 molecule to change the behaviour thought to be responsible for ALS. Here we provide a comprehensive review of the many biophysical advances that sculpted our view of SOD1 biology and the recent work that aims to apply this knowledge for therapeutic outcomes in ALS.

Lipid molecules induce the cytotoxic aggregation of Cu/Zn superoxide dismutase with structurally disordered regions

Cu/Zn-superoxide dismutase (SOD1) is present in the cytosol, nucleus, peroxisomes and mitochondrial intermembrane space of human cells. More than 114 variants of human SOD1 have been linked to familial amyotrophic lateral sclerosis (ALS), which is also known as Lou Gehrig's disease. Although the ultimate mechanisms underlying SOD1-mediated cytotoxicity are largely unknown, SOD1 aggregates have been strongly implicated as a common feature in ALS. This study examined the mechanism for the formation of SOD1 aggregates in vitro as well as the nature of its cytotoxicity. The aggregation propensity of SOD1 species was investigated using techniques ranging from circular dichroism spectroscopy to fluorescence dye binding methods, as well as electron microscopic imaging. The aggregation of SOD1 appears to be related to its structural instability. The demetallated (apo)-SOD1 and aggregated SOD1 species, with structurally disordered regions, readily undergo aggregation in the presence of lipid molecules, whereas metallated (holo)-SOD1 does not. The majority of aggregated SOD1s that are induced by lipid molecules have an amorphous morphology and exhibit significant cytotoxicity. The lipid binding propensity of SOD1 was found to be closely related to the changes in surface hydrophobicity of the proteins, even at very low levels, which induced further binding and assembly with lipid molecules. These findings suggest that lipid molecules induce SOD1 aggregation under physiological conditions and exert cytotoxicity, and might provide a possible mechanism for the pathogenesis of ALS.

Loss of metal ions, disulfide reduction and mutations related to familial ALS promote formation of amyloid-like aggregates from superoxide dismutase

Mutations in the gene encoding Cu-Zn superoxide dismutase (SOD1) are one of the causes of familial amyotrophic lateral sclerosis (FALS). Fibrillar inclusions containing SOD1 and SOD1 inclusions that bind the amyloid-specific dye thioflavin S have been found in neurons of transgenic mice expressing mutant SOD1. Therefore, the formation of amyloid fibrils from human SOD1 was investigated. When agitated at acidic pH in the presence of low concentrations of guanidine or acetonitrile, metalated SOD1 formed fibrillar material which bound both thioflavin T and Congo red and had circular dichroism and infrared spectra characteristic of amyloid. While metalated SOD1 did not form amyloid-like aggregates at neutral pH, either removing metals from SOD1 with its intramolecular disulfide bond intact or reducing the intramolecular disulfide bond of metalated SOD1 was sufficient to promote formation of these aggregates. SOD1 formed amyloid-like aggregates both with and without intermolecular disulfide bonds, depending on the incubation conditions, and a mutant SOD1 lacking free sulfhydryl groups (AS-SOD1) formed amyloid-like aggregates at neutral pH under reducing conditions. ALS mutations enhanced the ability of disulfide-reduced SOD1 to form amyloid-like aggregates, and apo-AS-SOD1 formed amyloid-like aggregates at pH 7 only when an ALS mutation was also present. These results indicate that some mutations related to ALS promote formation of amyloid-like aggregates by facilitating the loss of metals and/or by making the intramolecular disulfide bond more susceptible to reduction, thus allowing the conversion of SOD1 to a form that aggregates to form resembling amyloid. Furthermore, the occurrence of amyloid-like aggregates per se does not depend on forming intermolecular disulfide bonds, and multiple forms of such aggregates can be produced from SOD1.

Impaired post-translational folding of familial ALS-linked Cu, Zn superoxide dismutase mutants

Over 110 structurally diverse missense mutations in the superoxide dismutase (SOD1) gene have been linked to the pathogenesis of familial amyotrophic lateral sclerosis (FALS), yet the mechanism by which these lead to cytotoxicity still remains unknown. We have synthesized wild-type and mutant SOD1 in synchronized cell-free reticulocyte extracts replete with the full complement of molecular chaperones and folding facilitators that are normally required to fold this metalloenzyme. Here, we report that, despite being a small, single-domain protein, human SOD1 folds post-translationally to a hyperstable native-like conformation without a requirement for ATP-dependent molecular chaperones. SOD1 folding requires tight Zn but not Cu binding and proceeds through at least three kinetically and biochemically distinct states. We find that all 11 FALS-associated SOD1 mutants examined using this system delay the kinetics of folding, but do not necessarily preclude the formation of native-like states. These data suggest a model whereby impaired post-translational folding increases the population of on- and off-pathway folding intermediates that could provide an important source of proto-toxic protein, and suggest a unifying mechanism for SOD1-linked FALS pathogenesis.

Mutant SOD1 instability: implications for toxicity in amyotrophic lateral sclerosis

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.

Alteration of familial ALS-linked mutant SOD1 solubility with disease progression: Its modulation by the proteasome and Hsp70

Accumulation of misfolded Cu/Zn superoxide dismutase (SOD1) occurs in patients with a subgroup of familial amyotrophic lateral sclerosis (fALS). To identify the conversion of SOD1 from a normally soluble form to insoluble aggregates, we investigated the change of SOD1 solubility with aging in fALS-linked H46R SOD1 transgenic mice. Mutant SOD1 specifically altered to insoluble forms, which were sequentially separated into Triton X-100-insoluble/sodium dodecyl sulfate (SDS)-soluble and SDS-insoluble/formic acid-soluble species. In spinal cords, the levels of SDS-dissociable soluble SOD1 monomers and SDS-stable soluble dimers were significantly elevated before motor dysfunction onset. In COS-7 cells expressing H46R SOD1, treatment with proteasome inhibitors recapitulated the alteration of SOD1 solubility in transgenic mice. In contrast, overexpression of Hsp70 reduced accumulation of mutant-specific insoluble SOD1. SDS-soluble low molecular weight species of H46R SOD1 may appear as early misfolded intermediates when their concentration exceeds the capacity of the proteasome and molecular chaperones.

Mutant superoxide dismutase 1 forms aggregates in the brain mitochondrial matrix of amyotrophic lateral sclerosis mice

An increasing body of evidence suggests that mitochondrial dysfunction plays an important role in the pathogenesis of familial amyotrophic lateral sclerosis associated with "gain of function" mutations in Cu/Zn superoxide dismutase 1 (SOD1). SOD1 is mostly a cytosolic protein, but a portion of SOD1 is localized in mitochondria of patients with familial amyotrophic lateral sclerosis and transgenic mouse models of the disease. Despite the finding that mutant SOD1 localizes in mitochondria, the pathogenic significance of the mitochondrial mutant SOD1 remains to be elucidated. Here, we demonstrate that both wild-type and mutant human SOD1 accumulate in brain mitochondria of transgenic mice and that SOD1 displays a very complex intramitochondrial compartmentalization. For the first time, we show that, in addition to being in the mitochondrial outer membrane and intermembrane space, SOD1 is also localized in the mitochondrial matrix. Importantly, we show that aberrant SOD1 macromolecular aggregates are formed in the matrix of brain mitochondria. This suggests that mutant SOD1 in the brain mitochondrial matrix is misfolded and prone to aggregation, which may contribute to selective neuronal degeneration.

Expression of an endoplasmic reticulum-resident chaperone, glucose-regulated stress protein 78, in the spinal cord of a mouse model of amyotrophic lateral sclerosis

The immunohistochemical localization of glucose-regulated protein 78/BiP (GRP78), a chaperone protein that primarily resides within the lumen of the endoplasmic reticulum, was investigated in the lumbar spinal cord of mutant copper/zinc superoxide dismutase (SOD1) transgenic mice. Re-staining techniques were used to determine the immunoreactivity with anti-GRP78 antibody of abnormal structures observed by hematoxylin and eosin staining. Besides its physiological localization in the neuronal and glial cytoplasm, GRP78 was expressed in Lewy body-like hyaline inclusions, in irregularly-shaped eosinophilic structures without an apparent halo, and in cord-like swollen neurites. These different sites were invariably also immunopositive for ubiquitin, suggesting them to be pathological structures. The topographic distribution of GRP78 expression closely resembled that of SOD1. Moreover, our chronological quantitative analysis demonstrated that virtually all the Lewy body-like hyaline inclusions were immunolabeled by the anti-GRP78 antibody, irrespective to the age of mice examined, even at the presymptomatic stages. These findings imply that GRP78 may bind to, or at least be closely associated with, SOD1, and may participate in the pathological processes leading to inclusion formation. Thus, the results suggest that dysfunction of GRP78 and subsequent derangement of the system responding to unfolded proteins may be involved in the pathogenesis of familial amyotrophic lateral sclerosis caused by a mutation of the human SOD1 gene.

Aberrantly Increased Hydrophobicity Shared by Mutants of Cu,Zn-Superoxide Dismutase in Familial Amyotrophic Lateral Sclerosis

More than 100 different mutations in the gene encoding Cu,Zn-superoxide dismutase (SOD1) cause preferential motor neuron degeneration in familial amyotrophic lateral sclerosis (ALS). Although the cellular target(s) of mutant SOD1 toxicity have not been precisely specified, evidence to date supports the hypothesis that ALS-related mutations may increase the burden of partially unfolded SOD1 species. Influences that may destabilize SOD1 in vivo include impaired metal ion binding, reduction of the intrasubunit disulfide bond, or oxidative modification. In this study, we observed that metal-deficient as-isolated SOD1 mutants (H46R, G85R, D124V, D125H, and S134N) with disordered electrostatic and zinc-binding loops exhibited aberrant binding to hydrophobic beads in the absence of other destabilizing agents. Other purified ALS-related mutants that can biologically incorporate nearly normal amounts of stabilizing zinc ions (A4V, L38V, G41S, D90A, and G93A) exhibited maximal hydrophobic behavior after exposure to both a disulfide reducing agent and a metal chelator, while normal SOD1 was more resistant to these agents. Moreover, we detected hydrophobic SOD1 species in lysates from affected tissues in G85R and G93A mutant but not wildtype SOD1 transgenic mice. These findings suggest that a susceptibility to the cellular disulfide reducing environment and zinc loss may convert otherwise stable SOD1 mutants into metal-deficient forms with locally destabilized electrostatic and zinc-binding loops. These abnormally hydrophobic SOD1 species may promote aberrant interactions of the enzyme with itself or with other cellular constituents to produce toxicity in familial ALS.

Overexpression of mutated Cu,Zn-SOD in neuroblastoma cells results in cytoskeletal change

Amyotrophic lateral sclerosis (ALS) involves the progressive degeneration of motor neurons in the spinal cord and the motor cortex. It has been shown that 15–20% of patients with familial ALS (FALS) have defects in the Sod1 gene, which encodes Cu,Zn-superoxide dismutase (SOD). To elucidate the pathological role of mutated Cu,Zn-SOD, we examined the issue of whether mutated Cu,Zn-SOD affects the cell cycle. Mouse neuroblastoma Neuro-2a cells were transfected with human wild-type or mutated (G37R, G93A) Cu,Zn-SOD. Mutated, Cu,Zn-SOD-transfected cells exhibited marked retardation in cell growth and G2/M arrest. They also displayed lower reactivity to phalloidin, indicating that the cytoskeleton was disrupted. Immunoprecipitation, two-dimensional gel electrophoresis, and Western blot analysis indicated that mutated Cu,Zn-SOD associates with actin. Similar results were obtained by in vitro incubation experiments with purified actin and mutated Cu,Zn-SOD (G93A). These results suggest that mutated Cu,Zn-SOD in FALS causes cytoskeletal changes by associating with actin, which subsequently causes G2/M arrest and growth retardation.

Impaired extracellular secretion of mutant superoxide dismutase 1 associates with neurotoxicity in familial amyotrophic lateral sclerosis

Mutations in the intracellular metalloenzyme superoxide dismutase 1 (SOD1) are linked to neurotoxicity in familial amyotrophic lateral sclerosis (ALS) by an unclear mechanism. Golgi fragmentation and endoplasmic reticulum stress are early hallmarks of spinal motor neuron pathology in transgenic mice overexpressing mutant SOD1, suggesting that dysfunction of the neuronal secretory pathway may contribute to ALS pathogenesis. We therefore proposed that mutant SOD1 directly engages and modulates the secretory pathway based on recent evidence of SOD1 secretion in diverse human cell lines. Here, we demonstrate that a fraction of active endogenous SOD1 is secreted by NSC-34 motor neuron-like cells via a brefeldin-A (BFA)-sensitive pathway. Expression of enhanced green fluorescent protein-tagged mutant human SOD1 (hSOD1-EGFP) in NSC-34 cells induced frequent cytoplasmic inclusions and protein insolubility that correlated with toxicity. In contrast, transfection of non-neuronal COS-7 cells resulted in mutant hSOD1-EGFP cytoplasmic inclusions, oligomerization, and fragmentation without detectable toxicity. Importantly, impaired secretion of hSOD1-EGFP was common to all 10 SOD1 mutants tested relative to wild-type protein in NSC-34 cells. Treatment with BFA inhibited hSOD1-EGFP secretion with pronounced BFA-induced toxicity in mutant cells. Extracellular targeting of mutant hSOD1-EGFP via SOD3 signal peptide fusion attenuated cytoplasmic inclusion formation and toxicity. The effect of elevated extracellular SOD1 was then evaluated in a transgenic rat model of ALS. Chronic intraspinal infusion of exogenous wild-type hSOD1 significantly delayed disease progression and endpoint in transgenic SOD1(G93A) rats. Collectively, these results suggest novel extracellular roles for SOD1 in ALS and support a causal relationship between mutant SOD1 secretion and intraneuronal toxicity.

Disruption of the structure of the Golgi apparatus and the function of the secretory pathway by mutants G93A and G85R of Cu, Zn superoxide dismutase (SOD1) of familial amyotrophic lateral sclerosis

The Golgi apparatus of motor neurons (GA) is fragmented in sporadic amyotrophic lateral sclerosis (ALS), in familial ALS with SOD1 mutations, and in mice that express SOD1G93A of familial ALS, in which it was detected months before paralysis. In paralyzed transgenic mice expressing SOD1G93A or SOD1G85R, mutant proteins aggregated not only in the cytoplasm of motor neurons, but also in astrocytes and oligodendrocytes. Furthermore, aggregation of the G85R protein damaged astrocytes and was associated with rapidly progressing disease. In order to gain insight into the functional state of the fragmented GA, we examined the effects of S0D1 mutants G93A and G85R in Chinese Hamster Ovary Cells (CHO). In contrast to cells expressing the wt and G93A, the G85R expressers had no SOD1 activity. However, cells expressing both mutants, and to a lesser degree the wt, showed decreased survival, fragmentation of the GA, and dysfunction of the secretory pathway, which was assessed by measuring the amount of cell surface co-expressed CD4, a glycoprotein processed through the GA. The G93A and wt proteins were partially recovered in detergent insoluble fractions; while the recovery of G85R was minimal. Both mutants showed equal reductions of cell survival and function of the secretory pathway, in comparison to the wt and cells expressing mutant alsin, a protein found in rare cases of fALS. These results are consistent with the conclusion that the two SOD1 mutants, by an unknown mechanism, promote the dispersion of the GA and the dysfunction of the secretory pathway. This and other in vitro models of mutant SOD1 toxicity may prove useful in the elucidation of pathogenetic mechanisms.

Monomeric Cu,Zn-superoxide dismutase is a common misfolding intermediate in the oxidation models of sporadic and familial amyotrophic lateral sclerosis

Proteinacious intracellular aggregates in motor neurons are a key feature of both sporadic and familial amyotrophic lateral sclerosis (ALS). These inclusion bodies are often immunoreactive for Cu,Zn-superoxide dismutase (SOD1) and are implicated in the pathology of ALS. On the basis of this and a similar clinical presentation of symptoms in the familial (fALS) and sporadic forms of ALS, we sought to investigate the possibility that there exists a common disease-related aggregation pathway for fALS-associated mutant SODs and wild type SOD1. We have previously shown that oxidation of fALS-associated mutant SODs produces aggregates that have the same morphological, structural, and tinctorial features as those found in SOD1 inclusion bodies in ALS. Here, we show that oxidative damage of wild type SOD at physiological concentrations ( approximately 40 microm) results in destabilization and aggregation in vitro. Oxidation of either mutant or wild type SOD1 causes the enzyme to dissociate to monomers prior to aggregation. Only small changes in secondary and tertiary structure are associated with monomer formation. These results indicate a common aggregation prone monomeric intermediate for wild type and fALS-associated mutant SODs and provides a link between sporadic and familial ALS.

Inducible superoxide dismutase 1 aggregation in transgenic amyotrophic lateral sclerosis mouse fibroblasts

High molecular weight detergent-insoluble complexes of superoxide dismutase 1 (SOD1) enzyme are a biochemical abnormality associated with mutant SOD1-linked familial amyotrophic lateral sclerosis (FALS). In the present study, SOD1 protein from spinal cords of transgenic FALS mice was fractionated according to solubility in saline, zwitterionic, non-ionic or anionic detergents. Both endogenous mouse SOD1 and mutant human SOD1 were least soluble in SDS, followed by NP-40 and CHAPS, with an eight-fold greater detergent resistance of mutant protein overall. Importantly, high molecular weight mutant SOD1 complexes were isolated with SDS-extraction only. To reproduce SOD1 aggregate pathology in vitro, primary fibroblasts were isolated and cultured from neonatal transgenic FALS mice. Fibroblasts expressed abundant mutant SOD1 without spontaneous aggregation over time with passage. Proteasomal inhibition of cultures using lactacystin induced dose-dependent aggregation and increased the SDS-insoluble fraction of mutant SOD1, but not endogenous SOD1. In contrast, paraquat-mediated superoxide stress in fibroblasts promoted aggregation of endogenous SOD1, but not mutant SOD1. Treatment of cultures with peroxynitrite or the copper chelator diethyldithiocarbamate (DDC) alone did not modulate aggregation. However, DDC inhibited lactacystin-induced mutant SOD1 aggregation in transgenic fibroblasts, while exogenous copper slightly augmented aggregation. These data suggest that SOD1 aggregates may derive from proteasomal or oxidation-mediated oligomerisation pathways from mutant and endogenous subunits respectively. Furthermore, these pathways may be affected by copper availability. We propose that non-neural cultures such as these transgenic fibroblasts with inducible SOD1 aggregation may be useful for rapid screening of compounds with anti-aggregation potential in FALS.

Protein aggregation in motor neurone disorders

Toxicity associated with abnormal protein folding and protein aggregation are major hypotheses for neurodegeneration. This article comparatively reviews the experimental and human tissue-based evidence for the involvement of such mechanisms in neuronal death associated with the motor system disorders of X-linked spinobulbar muscular atrophy (SBMA; Kennedy's disease) and amyotrophic lateral sclerosis (ALS), especially disease related to mutations in the superoxide dismutase (SOD1) gene. Evidence from transgenic mouse, Drosophila and cell culture models of SBMA, in common with other trinucleotide repeat expansion disorders, show protein aggregation of the mutated androgen receptor, and intraneuronal accumulation of aggregated protein, to be obligate mechanisms. Strong experimental data link these phenomena with downstream biochemical events involving gene transcription pathways (CREB-binding protein) and interactions with protein chaperone systems. Manipulations of these pathways are already established in experimental systems of trinucleotide repeat disorders as potential beneficial targets for therapeutic activity. In contrast, the evidence for the role of protein aggregation in models of SOD1-linked familial ALS is less clear-cut. Several classes of intraneuronal inclusion body have been described, some of which are invariably present. However, the lack of understanding of the biochemical basis of the most frequent inclusion in sporadic ALS, the ubiquitinated inclusion, has hampered research. The toxicity associated with expression of mutant SOD1 has been intensively studied however. Abnormal protein aggregation and folding is the only one of the four major hypotheses for the mechanism of neuronal degeneration in this disorder currently under investigation (the others comprise oxidative stress, axonal transport and cytoskeletal dysfunctions, and glutamatergic excitotoxicity). Whilst hyaline inclusions, which are strongly immunoreactive to SOD1, are linked to degeneration in SOD1 mutant mouse models, the evidence from human tissue is less consistent and convincing. A role for mutant SOD1 aggregation in the mitochondrial dysfunction associated with ALS, and in potentially toxic interactions with heat shock proteins, both leading to apoptosis, are supported by some experimental data. Direct in vitro data on mutant SOD1 show evidence for spontaneous oligomerization, but the role of such oligomers remains to be elucidated, and therapeutic strategies are less well developed for this familial variant of ALS.

Copper-binding-site-null SOD1 causes ALS in transgenic mice: aggregates of non-native SOD1 delineate a common feature

Cu/Zn superoxide dismutase (SOD1), a crucial cellular antioxidant, can in certain settings mediate toxic chemistry through its Cu cofactor. Whether this latter property explains why mutations in SOD1 cause FALS has been debated. Here, we demonstrate motor neuron disease in transgenic mice expressing a SOD1 variant that mutates the four histidine residues that coordinately bind Cu. In-depth analyses of this new mouse model, previously characterized models and FALS human tissues revealed that the accumulation of detergent-insoluble forms of SOD1 is a common feature of the disease. These insoluble species include full-length SOD1 proteins, peptide fragments, stable oligomers and ubiquitinated entities. Moreover, chaperones Hsp25 and alphaB-crystallin specifically co-fractionated with insoluble SOD1. In cultured cells, all 11 of the FALS variants tested produced insoluble forms of mutant SOD1. Importantly, expression of recombinant peptide fragments of wild-type SOD1 in cultured cells also produced insoluble species, suggesting that SOD1 possesses elements with an intrinsic propensity to aggregate. Thus, modifications to the protein, such as FALS mutations, fragmentation and possibly covalent modification, may simply act to augment a natural, but potentially toxic, propensity to aggregate.

Mutant SOD1 linked to familial amyotrophic lateral sclerosis, but not wild-type SOD1, induces ER stress in COS7 cells and transgenic mice

Mutations in a Cu, Zn-superoxide dismutase (SOD1) cause motor neuron death in human familial amyotrophic lateral sclerosis (FALS) and its mouse model, suggesting that mutant SOD1 has a toxic effect on motor neurons. However, the question of how the toxic function is gained has not been answered. Here, we report that the mutant SOD1s linked to FALS, but not wild-type SOD1, aggregated in association with the endoplasmic reticulum (ER) and induced ER stress in the cDNA-transfected COS7 cells. These cells showed an aberrant intracellular localization of mitochondria and microtubules, which might lead to a functional disturbance of the cells. Motor neurons of the spinal cord in transgenic mice with a FALS-linked mutant SOD1 also showed a marked increase of GRP78/BiP, an ER-resident chaperone, just before the onset of motor symptoms. These data suggest that ER stress is involved in the pathogenesis of FALS with an SOD1 mutation.

Oxidation-induced misfolding and aggregation of superoxide dismutase and its implications for amyotrophic lateral sclerosis

The presence of intracellular aggregates that contain Cu/Zn superoxide dismutase (SOD1) in spinal cord motor neurons is a pathological hallmark of amyotrophic lateral sclerosis (ALS). Although SOD1 is abundant in all cells, its half-life in motor neurons far exceeds that in any other cell type. On the basis of the premise that the long half-life of the protein increases the potential for oxidative damage, we investigated the effects of oxidation on misfolding/aggregation of SOD1 and ALS-associated SOD1 mutants. Zinc-deficient wild-type SOD1 and SOD1 mutants were extremely prone to form visible aggregates upon oxidation as compared with wild-type holo-protein. Oxidation of select histidine residues that bind metals in the active site mediates SOD1 aggregation. Our results provide a plausible model to explain the accumulation of SOD1 aggregates in motor neurons affected in ALS.

Do oxidatively modified proteins cause ALS?

Over 90 individual mutations in SOD1 are known to cause familial amyotrophic lateral sclerosis (FALS). It is widely accepted that these mutations exert their toxic effects by a gain of function mechanism, but the nature of these toxic effects is as yet unknown. It has been proposed by several laboratories that reactions of FALS-mutant CuZnSOD are the source of elevated oxidative stress in CuZnSOD-linked FALS. It has also been proposed that aggregates of CuZnSOD are somehow involved in the disease. The hypothesis that aggregates of CuZnSOD cause ALS is particularly attractive because protein aggregates are frequently associated with other neurodegenerative diseases. Recent evidence increasingly suggests that protein aggregates containing CuZnSOD protein play a role in CuZnSOD-linked ALS, but it is not yet know why the aggregates form nor if the CuZnSOD proteins in the aggregates are cleaved, oxidized, demetallated, or otherwise covalently modified.

Dorfin ubiquitylates mutant SOD1 and prevents mutant SOD1-mediated neurotoxicity

Amyotrophic lateral sclerosis (ALS) is a progressive paralytic disorder resulting from the degeneration of motor neurons in the cerebral cortex, brainstem, and spinal cord. The cytopathological hallmark in the remaining motor neurons of ALS is the presence of ubiquitylated inclusions consisting of insoluble protein aggregates. In this paper we report that Dorfin, a RING finger-type E3 ubiquitin ligase, is predominantly localized in the inclusion bodies of familial ALS with a copper/zinc superoxide dismutase (SOD1) mutation as well as sporadic ALS. Dorfin physically bound and ubiquitylated various SOD1 mutants derived from familial ALS patients and enhanced their degradation, but it had no effect on the stability of the wild-type SOD1. The overexpression of Dorfin protected against the toxic effects of mutant SOD1 on neural cells and reduced SOD1 inclusions. Our results indicate that Dorfin protects neurons by recognizing and then ubiquitylating mutant SOD1 proteins followed by targeting them for proteasomal degradation.

Mutant Cu/Zn-superoxide dismutase proteins have altered solubility and interact with heat shock/stress proteins in models of amyotrophic lateral sclerosis

Mutations in the Cu/Zn-superoxide dismutase (SOD-1) gene are responsible for a familial form of amyotrophic lateral sclerosis. In humans and experimental models, death of motor neurons is preceded by formation of cytoplasmic aggregates containing mutant SOD-1 protein. In our previous studies, heat shock protein 70 (HSP70) prolonged viability of cultured motor neurons expressing mutant human SOD-1 and reduced formation of aggregates. In this paper, we report that mutant SOD-1 proteins have altered solubility in cells relative to wild-type SOD-1 and can form a direct association with HSP70 and other stress proteins. Whereas wild-type human and endogenous mouse SOD-1 were detergent-soluble, a portion of mutant SOD-1 was detergent-insoluble in protein extracts of NIH3T3 transfected with SOD-1 gene constructs, spinal cord cultures established from G93A SOD-1 transgenic mouse embryos, and lumbar spinal cord from adult G93A transgenic mice. A direct association of HSP70, HSP40, and alphaB-crystallin with mutant SOD-1 (G93A or G41S), but not wild-type or endogenous mouse SOD-1, was demonstrated by coimmunoprecipitation. Mutant SOD-1.HSP70 complexes were predominantly in the detergent-insoluble fraction. However, only a small percentage of total cellular mutant SOD-1 was detergent-insoluble, suggesting that mutation-induced alteration of protein conformation may not in itself be sufficient for direct interaction with heat shock proteins.

Transgenic mouse model for familial amyotrophic lateral sclerosis with superoxide dismutase-1 mutation

Familial amyotrophic lateral sclerosis (ALS) with mutations in the gene for superoxide dismutase-1 (SOD1) is clinicopathologically reproduced by transgenic mice expressing mutant forms of SOD1 detectable in familial ALS patients. Motor neuron degeneration associated with SOD1 mutation has been thought to result from a novel neurotoxicity of mutant SOD1, but not from a reduction in activity of this enzyme, based on autosomal dominant transmission of SOD1 mutant familial ALS and its transgenic mouse model, clinical severity of the ALS patients independent to enzyme activity, no ALS-like disease in SOD1 knockout or wild-type SOD1-overexpressing mice, and clinicopathological severity of mutant SOD1 transgenic mice dependent on transgene copy numbers. Proposed mechanisms of motor neuron degeneration such as oxidative injury, peroxynitrite toxicity, cytoskeletal disorganization, glutamate excitotoxicity, disrupted calcium homeostasis, SOD1 aggregation, carbonyl stress and apoptosis have been discussed. Intracytoplasmic vacuoles, indicative of increased oxidative damage to the mitochondria and endoplasmic reticulum, in the neuropil and motor neurons appear in high expressors of mutant SOD1 transgenic mice but not in low expressors of the mice or familial ALS patients, suggesting that overexpression of mutant SOD1 in mice may enhance oxidative stress generation from this enzyme. Thus, transgenic mice carrying small transgene copy numbers of mutant SOD1 would provide a beneficial animal model for SOD1 mutant familial ALS. Such a model would contribute to elucidating the pathomechanism of this disease and establishing new therapeutic agents.

Japanese familial amyotrophic lateral sclerosis family with a two-base deletion in the superoxide dismutase-1 gene

The clinical characteristics of members of a familial amyotrophic lateral sclerosis (FALS) family from Oki Island, whose members have a 2-bp deletion at codon 126 of Cu/Zn superoxide dismutase (SOD1) gene, are presented here. Mean age of the onset in the members was 42 years. Mean disease duration among the members who had not been placed on a respirator was approximately 2 years. Long-term survivors with respiratory support presented disturbances in eye movement and urination toward the end stages of the disease. They predominantly exhibited lower motor neuron symptoms. In addition, the authors focused on frameshift, nonsense and non-amino-acid-altering mutations. Frameshift and nonsense mutations were all found within exon 4, exon 5 and intron 4. These amyotrophic lateral sclerosis cases were likely to have shorter disease duration than the FALS patients with single substitution. Several hypotheses were presented on the pathogenesis of FALS with SOD1 mutation.

Formation of high molecular weight complexes of mutant Cu, Zn-superoxide dismutase in a mouse model for familial amyotrophic lateral sclerosis

Deposition of aggregated protein into neurofilament-rich cytoplasmic inclusion bodies is a common cytopathological feature of neurodegenerative disease. How-or indeed whether-protein aggregation and inclusion body formation cause neurotoxicity are presently unknown. Here, we show that the capacity of superoxide dismutase (SOD) to aggregate into biochemically distinct, high molecular weight, insoluble protein complexes (IPCs) is a gain of function associated with mutations linked to autosomal dominant familial amyotrophic lateral sclerosis. SOD IPCs are detectable in spinal cord extracts from transgenic mice expressing mutant SOD several months before inclusion bodies and motor neuron pathology are apparent. Sequestration of mutant SOD into cytoplasmic inclusion bodies resembling aggresomes requires retrograde transport on microtubules. These data indicate that aggregation and inclusion body formation are mechanistically and temporally distinct processes.

Gain in functions of mutant Cu,Zn-superoxide dismutases as a causative factor in familial amyotrophic lateral sclerosis: less reactive oxidant formation but high spontaneous aggregation and precipitation

Eight mutant Cu,Zn-superoxide dismutases (SODs) related to familial amyotrophic lateral sclerosis (FALS) were produced in a baculovirus/insect cell expression system and their molecular properties in terms of hydroxyl radical formation and aggregation were compared with the wild-type enzyme. Treatment of the enzymes with Chelex 100 resin decreased Cu contents as well as SOD activities in all mutant Cu,Zn-SODs, indicating that the affinities of the enzymes for copper ion were decreased. Contrary to previous reports, all the mutant Cu,Zn-SODs exhibited less reactive oxidant producing ability in the presence of hydrogen peroxide than the wild-type enzyme. Both SOD activities and their reactive oxidant forming correlated well with the copper ion content of the molecules. In addition, the proteins spontaneously aggregated and were precipitated by simple centrifugation at 12,000g for 20 min in keeping their enzyme activities. Since hyaline inclusions found in FALS patients with SOD1 mutations contained components which were reactive to anti-Cu,Zn-SOD antibody, a primary reaction caused by mutant SOD1 may be attributed to their propensity to form aggregates. Aggregated but still active mutant SOD1 would be expected to mediate the formation of reactive oxygen species and nitrosylation in a more condensed state.