Superoxide-dependent peroxidase activity of H48Q: a superoxide dismutase variant associated with familial amyotrophic lateral sclerosis

Regulation of CuZnSOD and its redox signaling potential: implications for amyotrophic lateral sclerosis

SIGNIFICANCE

Molecular oxygen is a Janus-faced electron acceptor for biological systems, serving as a reductant for respiration, or as the genesis for oxygen-derived free radicals that damage macromolecules. Superoxide is well known to perturb nonheme iron proteins, including Fe/S proteins such as aconitase and succinate dehydrogenase, as well as other enzymes containing labile iron such as the prolyl hydroxylase domain-containing family of enzymes; whereas hydrogen peroxide is more specific for two-electron reactions with thiols on glutathione, glutaredoxin, thioredoxin, and the peroxiredoxins.

RECENT ADVANCES

Over the past two decades, familial cases of amyotrophic lateral sclerosis (ALS) have been shown to have an association with commonly altered superoxide dismutase 1 (SOD1) activity, expression, and protein structure. This has led to speculation that an altered redox balance may have a role in creating the ALS phenotype.

CRITICAL ISSUES

While SOD1 alterations in familial ALS are manifold, they generally create perturbations in the flux of electrons. The nexus of SOD1 between one- and two-electron signaling processes places it at a key signaling regulatory checkpoint for governing cellular responses to physiological and environmental cues.

FUTURE DIRECTIONS

The manner in which ALS-associated mutations adjust SOD1's role in controlling the flow of electrons between one- and two-electron signaling processes remains obscure. Here, we discuss the ways in which SOD1 mutations influence the form and function of copper zinc SOD, the consequences of these alterations on free radical biology, and how these alterations might influence cell signaling during the onset of ALS.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

A role for copper in the toxicity of zinc-deficient superoxide dismutase to motor neurons in amyotrophic lateral sclerosis

In the 16 years since mutations to copper, zinc superoxide dismutase (SOD1) were first linked to familial amyotrophic lateral sclerosis (ALS), a multitude of apparently contradictory results have prevented any general consensus to emerge about the mechanism of toxicity. A decade ago, we showed that the loss of zinc from SOD1 results in the remaining copper in SOD1 to become extremely toxic to motor neurons in culture by a mechanism requiring nitric oxide. The loss of zinc causes SOD1 to become more accessible, more redox reactive, and a better catalyst of tyrosine nitration. Although SOD1 mutant proteins have a modestly reduced affinity for zinc, wild-type SOD1 can be induced to lose zinc by dialysis at slightly acidic pH. Our zinc-deficient hypothesis offers a compelling explanation for how mutant SOD1s have an increased propensity to become selectively toxic to motor neurons and also explains how wild-type SOD1 can be toxic in nonfamilial ALS patients. One critical prediction is that a therapeutic agent directed at zinc-deficient mutant SOD1 could be even more effective in treating sporadic ALS patients. Although transgenic mice experiments have yielded contradictory evidence to the zinc-deficient hypothesis, we will review more recent studies that support a role for copper in ALS. A more careful examination of the role of copper and zinc binding to SOD1 may help counter the growing disillusion in the ALS field about understanding the pathological role of SOD1.

An in vitro screening cascade to identify neuroprotective antioxidants in ALS

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease, characterized by progressive dysfunction and death of motor neurons. Although evidence for oxidative stress in ALS pathogenesis is well described, antioxidants have generally shown poor efficacy in animal models and human clinical trials. We have developed an in vitro screening cascade to identify antioxidant molecules capable of rescuing NSC34 motor neuron cells expressing an ALS-associated mutation of superoxide dismutase 1. We have tested known antioxidants and screened a library of 2000 small molecules. The library screen identified 164 antioxidant molecules, which were refined to the 9 most promising molecules in subsequent experiments. Analysis of the in silico properties of hit compounds and a review of published literature on their in vivo effectiveness have enabled us to systematically identify molecules with antioxidant activity combined with chemical properties necessary to penetrate the central nervous system. The top-performing molecules identified include caffeic acid phenethyl ester, esculetin, and resveratrol. These compounds were tested for their ability to rescue primary motor neuron cultures after trophic factor withdrawal, and the mechanisms of action of their antioxidant effects were investigated. Subsequent in vivo studies can be targeted using molecules with the greatest probability of success.

Putative denitrosylase activity of Cu,Zn-superoxide dismutase

The Cu,Zn-superoxide dismutase (SOD1) has been reported to exert an S-nitrosylated glutathione (GSNO) denitrosylase activity that was augmented by a familial amyotrophic lateral sclerosis (FALS)-associated mutation in this enzyme. This putative enzymatic activity as well as the spontaneous decomposition of GSNO has been reexamined. The spontaneous decomposition of GSNO exhibited several peculiarities, such as a lag phase followed by an accelerating rate plus a marked dependence on GSNO concentration, suggestive of autocatalysis, and a greater rate in polypropylene than in glass vessels. Dimedone caused a rapid increase in absorbance likely due to reaction with GSNO, followed by a slower increase possibly due to reaction with an intermediate such as glutathione sulfenic acid. SOD1 weakly increased the rate of decomposition of GSNO, but did so only when GSH was present; and FALS-associated mutant forms of SOD1 were not more active in this regard than was the wild type. Decomposed GSNO, when added to fresh GSNO, hastened its decomposition, in accord with autocatalysis, and when added to GSH, generated GSNO in accord with the presence of nitrite. A mechanism is proposed that is in accord with these observations.

Disease-associated mutations at copper ligand histidine residues of superoxide dismutase 1 diminish the binding of copper and compromise dimer stability

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.

Protective effects of polysaccharide from Euphorbia kansui (Euphorbiaceae) on the swimming exercise-induced oxidative stress in mice

The present study examined the effects of derivatives of galactosides and glucosides in a polysaccharide extract from Euphorbia kansui (Euphorbiaceae) on exercise-induced oxidative stress in mice. Exhaustive swimming exercise significantly increases the degree of lipid peroxidation in terms of malondialdehyde content and reduces the antioxidant activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx). Our findings revealed that chronic oral treatment with the extract elevates enzymatic activities of SOD and GPx accompanied by a corresponding decrease in malondialdehyde. The antioxidative activities of these compounds against exercise-induced oxidative stress are correlated with various activities such as reducing the production of superoxide and hydroxyl radicals, inhibiting lipid peroxidation, enhancing antioxidative defenses, and increasing the production of SOD and GPx activity and expression in different tissues. These compounds may be involved in glycogen metabolism to meet the requirement of working skeletal muscles and act as antioxidants by terminating the chain reaction of lipid peroxidation to maintain the morphological stability of mitochondria in spinal motor neurons. These observations suggest that E. kansui has antioxidative and antifatigue properties and can be given as prophylactic and (or) therapeutic supplements for increasing antioxidant enzyme activities and preventing lipid peroxidation during strenuous exercise.

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.

Oxidative stress and motor neurone disease

The effects of oxidative stress within post mitotic cells such as neurones may be cumulative, and injury by free radical species is a major potential cause of the age-related deterioration in neuronal function seen in several neurodegenerative diseases. There is strong evidence that oxidative stress plays an important role in the pathogenesis of motor neurone disease (MND). Point mutations in the antioxidant enzyme Cu,Zn superoxide dismutase (SOD1) are found in some pedigrees with the familial form of MND. How mutations in this ubiquitous enzyme cause the relatively selective cell death of specific groups of motor neurones is not clear, although a number of hypotheses have been forwarded. These include (1) the formation of hydroxyl radicals, (2) the catalysis of reactions of the nitrogen centred oxidant species peroxynitrite, (3) toxicity of copper or zinc and (4) protein aggregation. Some experimental support for these different hypotheses has been produced by manipulating cells in culture to express the mutant SOD1 proteins and by generating transgenic mice which over-express mutant SOD1. Observations in these model systems are, in some cases at least, supported by observations made on pathological material from patients with similar SOD1 mutations. Furthermore, there are reports of evidence of free radical mediated damage to neurones in the sporadic form of MND. Several lines of evidence suggest that alterations in the glutamatergic neurotransmitter system may also play a key role in the injury to motor neurones in sporadic MND. There are several important subcellular targets, which may be preferentially impaired within motor neurones, including neurofilament proteins and mitochondria. Future research will need to identify the aspects of the molecular and physiological phenotype of human motor neurones that makes them susceptible to degeneration in MND, and to identify those genetic and environmental factors which combine to cause this disease in individuals and in familial pedigrees.

Endogenous zinc in neurological diseases

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'. Here neurobiological roles of endogenous zinc is summarized.

Copper-zinc superoxide dismutase and amyotrophic lateral sclerosis

Copper-zinc superoxide dismutase (CuZnSOD, SOD1 protein) is an abundant copper- and zinc-containing protein that is present in the cytosol, nucleus, peroxisomes, and mitochondrial intermembrane space of human cells. Its primary function is to act as an antioxidant enzyme, lowering the steady-state concentration of superoxide, but when mutated, it can also cause disease. Over 100 different mutations have been identified in the sod1 genes of patients diagnosed with the familial form of amyotrophic lateral sclerosis (fALS). These mutations result in a highly diverse group of mutant proteins, some of them very similar to and others enormously different from wild-type SOD1. Despite their differences in properties, each member of this diverse set of mutant proteins causes the same clinical disease, presenting a challenge in formulating hypotheses as to what causes SOD1-associated fALS. In this review, we draw together and summarize information from many laboratories about the characteristics of the individual mutant SOD1 proteins in vivo and in vitro in the hope that it will aid investigators in their search for the cause(s) of SOD1-associated fALS.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

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.

4-hydroxynonenal and neurodegenerative diseases

The development of oxidative stress, in which production of highly reactive oxygen species (ROS) overwhelms antioxidant defenses, is a feature of many neurological diseases: ischemic, inflammatory, metabolic and degenerative. Oxidative stress is increasingly implicated in a number of neurodegenerative disorders characterized by abnormal filament accumulation or deposition of abnormal forms of specific proteins in affected neurons, like Alzheimer's disease (AD), Pick's disease, Lewy bodies related diseases, amyotrophic lateral sclerosis (ALS), and Huntington disease. Causes of neuronal death in neurodegenerative diseases are multifactorial. In some familiar cases of ALS mutation in the gene for Cu/Zn superoxide dismutase (SOD1) can be identified. In other neurodegenerative diseases ROS have some, usually not clear, role in early pathogenesis or implications on neuronal death in advanced stages of illness. The effects of oxidative stress on "post-mitotic cells", such as neurons may be cumulative, hence, it is often unclear whether oxidative damage is a cause or consequence of neurodegeneration. Peroxidation of cellular membrane lipids, or circulating lipoprotein molecules generates highly reactive aldehydes among which one of most important is 4-hydroxynonenal (HNE). The presence of HNE is increased in brain tissue and cerebrospinal fluid of AD patients, and in spinal cord of ALS patients. Immunohistochemical studies show presence of HNE in neurofibrilary tangles and in senile plaques in AD, in the cytoplasm of the residual motor neurons in sporadic ALS, in Lewy bodies in neocortical and brain stem neurons in Parkinson's disease (PD) and in diffuse Lewy bodies disease (DLBD). Thus, increased levels of HNE in neurodegenerative disorders and immunohistochemical distribution of HNE in brain tissue indicate pathophysiological role of oxidative stress in these diseases, and especially HNE in formation of abnormal filament deposites.

Mutant Cu,Zn superoxide dismutases and familial amyotrophic lateral sclerosis: evaluation of oxidative hypotheses

FALS-associated missense mutations of SOD1 exhibit a toxic gain of function that leads to the death of motor neurons. The explanations for this toxicity fall into two broad categories. One involves a gain of some oxidative activity, while the second involves a gain of protein: protein interactions. Among the postulated oxidative activities are the following: (i) peroxidase action; (ii) superoxide reductase action; and, (iii) the enhancement of production of O2- by partial reversal of the normal SOD activity, which then leads to increased formation of ONOO(-). We will herein concentrate on evaluating the relative merits of these oxidative hypotheses and consider whether the experiments with transgenic animals that purport to disprove these oxidative explanations really do so.

Misfolded CuZnSOD and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease of motor neurons. The inherited form of the disease, familial ALS, represents 5-10% of the total cases, and the best documented of these are due to lesions in SOD1, the gene encoding copper-zinc superoxide dismutase (CuZnSOD). The mechanism by which mutations in SOD1 cause familial ALS is currently unknown. Two hypotheses have dominated recent discussion of the toxicity of ALS mutant CuZnSOD proteins: the oligomerization hypothesis and the oxidative damage hypothesis. The oligomerization hypothesis maintains that mutant CuZnSOD proteins are, or become, misfolded and consequently oligomerize into increasingly high-molecular-weight species that ultimately lead to the death of motor neurons. The oxidative damage hypothesis maintains that ALS mutant CuZnSOD proteins catalyze oxidative reactions that damage substrates critical for viability of the affected cells. This perspective reviews some of the properties of both wild-type and mutant CuZnSOD proteins, suggests how these properties may be relevant to these two hypotheses, and proposes that these two hypotheses are not necessarily mutually exclusive.

Fibrillar inclusions and motor neuron degeneration in transgenic mice expressing superoxide dismutase 1 with a disrupted copper-binding site

Mutations in Cu/Zn superoxide dismutase 1 (SOD1) have been linked to dominantly inherited forms of amyotrophic lateral sclerosis (FALS). To test the hypothesis that the toxicity of mutant SOD1 originates in Cu(2+)-mediated formation of toxic radicals, we generated transgenic mice that express human SOD1 that encodes disease-linked mutations at two of the four histidine residues that are crucial for the coordinated binding of copper (H46R/H48Q). We demonstrate that mice expressing this mutant, which possesses little or no superoxide scavenging activity, develop motor neuron disease. Hence, mutations in SOD1 that disrupt the copper-binding site do not eliminate toxicity. We note that the pathology of the H46R/H48Q mice is dominated by fibrillar (Thioflavin-S-positive) inclusions and that similar inclusions were evident in mouse models that express the G37R, G85R, and G93A variants of human SOD1. Overall, our data are consistent with the hypothesis that the aberrant folding/aggregation of mutant SOD1 is a prominent feature in the pathogenesis of motor neuron disease.

Amyotrophic lateral sclerosis: a proposed mechanism

Missense mutations in Cu,Zn-superoxide dismutase (SOD1) account for approximately 20% of familial amyotrophic lateral sclerosis (FALS) through some, as yet undefined, toxic gain of function that leads to gradual death of motor neurons. Mitochondrial swelling and vacuolization are early signs of incipient motor neuron death in FALS. We previously reported that SOD1 exists in the intermembrane space of mitochondria. Herein, we demonstrate that the entry of SOD1 into mitochondria depends on demetallation and that heat shock proteins (Hsp70, Hsp27, or Hsp25) block the uptake of the FALS-associated mutant SOD1 (G37R, G41D, or G93A), while having no effect on wild-type SOD1. The binding of mutant SOD1 to Hsps in the extract of neuroblastoma cells leads to formation of sedimentable aggregates. Many antiapoptotic effects of Hsps have been reported. We now propose that this binding of Hsps to mutant forms of a protein abundant in motor neurons, such as SOD1, makes Hsps unavailable for their antiapoptotic functions and leads ultimately to motor neuron death. It also appears that the Hsp-SOD1 complex recruits other proteins present in the neuroblastoma cell and presumably in motor neurons to form sedimentable aggregates.

Decreased metallation and activity in subsets of mutant superoxide dismutases associated with familial amyotrophic lateral sclerosis

Over 90 different mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) cause approximately 2% of amyotrophic lateral sclerosis (ALS) cases by an unknown mechanism. We engineered 14 different human ALS-related SOD1 mutants and obtained high yields of biologically metallated proteins from an Sf21 insect cell expression system. Both the wild type and mutant "as isolated" SOD1 variants were deficient in copper and were heterogeneous by native gel electrophoresis. By contrast, although three mutant SOD1s with substitutions near the metal binding sites (H46R, G85R, and D124V) were severely deficient in both copper and zinc ions, zinc deficiency was not a consistent feature shared by the as isolated mutants. Eight mutants (A4V, L38V, G41S, G72S, D76Y, D90A, G93A, and E133 Delta) exhibited normal SOD activity over pH 5.5-10.5, per equivalent of copper, consistent with the presumption that bound copper was in the proper metal-binding site and was fully active. The H48Q variant contained a high copper content yet was 100-fold less active than the wild type enzyme and exhibited a blue shift in the visible absorbance peak of bound Cu(II), indicating rearrangement of the Cu(II) coordination geometry. Further characterization of these as-isolated SOD1 proteins may provide new insights regarding mutant SOD1 enzyme toxicity in ALS.

Copper,zinc superoxide dismutase as a univalent NO(-) oxidoreductase and as a dichlorofluorescin peroxidase

Nitroxyl (NO(-)) may be produced by nitric-oxide synthase and by the reduction of NO by reduced Cu,Zn-SOD. The ability of NO(-) to cause oxidations and of SOD to inhibit such oxidations was therefore explored. The decomposition of Angeli's salt (AS) produces NO(-) and that in turn caused the aerobic oxidation of NADPH, directly or indirectly. O(2) was produced concomitant with the aerobic oxidation of NADPH by AS, as evidenced by the SOD-inhibitable reduction of cytochrome c. Both Cu,Zn-SOD and Mn-SOD inhibited the aerobic oxidation of NADPH by AS, but the amounts required were approximately 100-fold greater than those needed to inhibit the reduction of cytochrome c. This inhibition was not due to a nonspecific protein effect or to an effect of those large amounts of the SODs on the rate of decomposition of AS. NO(-) caused the reduction of the Cu(II) of Cu,Zn-SOD, and in the presence of O(2), SOD could catalyze the oxidation of NO(-) to NO. The reverse reaction, i.e. the reduction of NO to NO(-) by Cu(I),Zn-SOD, followed by the reaction of NO(-) with O(2) would yield ONOO(-) and that could explain the oxidation of dichlorofluorescin (DCF) by Cu(I),Zn-SOD plus NO. Cu,Zn-SOD plus H(2)O(2) caused the HCO(3)(-)-dependent oxidation of DCF, casting doubt on the validity of using DCF oxidation as a reliable measure of intracellular H(2)O(2) production.

Zinc and copper in the pathogenesis of amyotrophic lateral sclerosis

1. Missense mutations in the gene encoding Cu,Zn superoxide dismutase (SOD1) are responsible for causing one form of familial amyotrophic lateral sclerosis (FALS) linked to chromosome 21q. 2. Mutant SOD1-induced disease is clearly related to a toxic gain of function for the abnormal enzyme, and recent work has begun to investigate the mechanisms underlying this toxicity. In addition to its well known and likely beneficial dismutase activity, wild type SOD1 also possesses the ability to participate in other enzymatic reactions that may be injurious to cells including peroxidation or nitration. 3. Many of the SOD1 mutations associated with FALS appear to increase the likelihood that the enzyme will perform either one of these potentially harmful functions resulting in increased hydroxyl radical formation or the addition of nitro groups to tyrosine residues within cellular proteins.

Redox-modulating gene therapies for human diseases

Baseline levels of reactive oxygen species (ROS) are generated as an integral component of cellular function. Under certain conditions, e.g., the presence of an elevated concentration of transition metal (Fe/Cu) ions, drug metabolism, or ischemia-reperfusion, ROS generation is exaggerated to an extent that overwhelms cellular antioxidant defenses and results in oxidative stress. Oxidative stress has been characterized by the assessment of oxidative damage to cellular components, e.g., protein, lipid, and nucleic acid. More recent studies have determined that at a concentration much below that required for inflicting oxidative damage, ROS may serve as cellular second messengers through the regulation of numerous signal transduction pathways. For this reason, much of the current medical focus in this area has been directed toward the understanding of redox-driven physiological and pathophysiological processes in the cell. The goal of such research is to formulate effective strategies for manipulating the cellular redox environment in a manner that is beneficial for restoring normal cell functions in the setting of disease.

Formation of advanced glycation end-product-modified superoxide dismutase-1 (SOD1) is one of the mechanisms responsible for inclusions common to familial amyotrophic lateral sclerosis patients with SOD1 gene mutation, and transgenic mice expressing human SOD1 gene mutation

Neuronal Lewy body-like hyaline inclusions (LBHI) and astrocytic hyaline inclusions (Ast-HI) are morphological hallmarks of certain familial amyotrophic lateral sclerosis (FALS) patients with superoxide dismutase-1 (SOD1) gene mutations, and transgenic mice expressing the human SOD1 gene mutation. The ultrastructure of inclusions in both diseases is identical: the essential common constituents are granule-coated fibrils approximately 15-25nm in diameter and granular materials. Detailed immunohistochemical analyses have shown that the essential common protein of the inclusions in both diseases is an SOD1 protein. This finding, together with the immunoelectron microscopy finding that the abnormal granule-coated fibrils comprising the inclusions are positive for SOD1, indicates that these granule-coated fibrils containing SOD1 are important evidence for mutant SOD1-linked disease in human and mouse. For immunoelectron microscopy, the granule-coated fibrils are modified by advanced glycation endproducts (AGE) such as N(epsilon)-carboxymethyl lysine, pyrraline and pentosidine (Maillard reaction). Based on the fact that AGE themselves are insoluble molecules with direct cytotoxic effects, the granule-coated fibrils and granular materials are not digested by the lysosomal and ubiquitin systems. The neurons and astrocytes of the normal individuals and non-transgenic mice show no significant immunoreactivity for AGE. Considered with the mutant-SOD1 aggregation toxicity, a portion of the SOD1 comprising both types of the inclusion is modified by the AGE, and the formation of the AGE-modified SOD1 (probably AGE-modified mutant SOD1) is one of the mechanisms responsible for the aggregation (i.e. granule-coated fibril formation).

The pro-oxidative activity of SOD and nitroxide SOD mimics

Native Cu,Zn-SOD and synthetic SOD mimics sometimes demonstrate an apparently anomalous bell-shaped dose-response relationship when protecting various biological systems from oxidative stress. Several mechanisms have been proposed to account for such an effect, including: overproduction of H(2)O(2), peroxidative activity of SOD, and opposing roles played by O(2)(*-) in both initiation and termination of radical chain reactions. In the present study, ferrocyanide and thiols, which are susceptible to one-electron and two-electron oxidation, respectively, were subjected to a flux of superoxide in the presence and absence of SOD or SOD mimics. The results show that 1) either O(2)(*-)/HO(2)(*) or H(2)O(2) alone partially inactivates papain, whereas when combined they act synergistically; 2) nitroxide SOD mimics, but not SOD, exhibit a bell-shaped dose-response relationship in protecting papain from inactivation; 3) SOD, which at low dose inhibits superoxide-induced oxidation of ferrocyanide, loses its antioxidative effect as its concentration increases. These findings offer an additional explanation for the pro-oxidative activity of SOD and SOD mimics without invoking any dual activity of O(2)(*-) or a combined effect of SOD and H(2)O(2). The most significant outcome of an increase in SOD level is a decrease of [O(2)(*-)](steady state), rather than any notable elevation of [H(2)O(2)](steady state). As a result, the reaction kinetics of the high oxidation state of each catalyst is altered. In the presence of ultra-low [O(2)(*-)](steady state), the oxidized form of SOD [Cu(II),Zn-SOD] or SOD mimic (oxo-ammonium cation) does not react with O(2)(*-) but rather oxidizes the target molecule that it was supposed to have protected. Consequently, these catalysts exert an anti- or pro-oxidative effect depending on their concentration.

A gain of superoxide dismutase (SOD) activity obtained with CCS, the copper metallochaperone for SOD1

The incorporation of copper ions into the cytosolic superoxide dismutase (SOD1) is accomplished in vivo by the action of the copper metallochaperone CCS (copper chaperone for SOD1). Mammalian CCS is comprised of three distinct protein domains, with a central region exhibiting remarkable homology (approximately 50% identity) to SOD1 itself. Conserved in CCS are all the SOD1 zinc binding ligands and three of four histidine copper binding ligands. In CCS the fourth histidine is replaced by an aspartate (Asp(200)). Despite this conservation of sequence between SOD1 and CCS, CCS exhibited no detectable SOD activity. Surprisingly, however, a single D200H mutation, targeting the fourth potential copper ligand in CCS, granted significant superoxide scavenging activity to this metallochaperone that was readily detected with CCS expressed in yeast. This mutation did not inhibit the metallochaperone capacity of CCS, and in fact, D200H CCS appears to represent a bifunctional SOD that can self-activate itself with copper. The aspartate at CCS position 200 is well conserved among mammalian CCS molecules, and we propose that this residue has evolved to preclude deleterious reactions involving copper bound to CCS.

Light-induced retinal damage in mice carrying a mutated SOD I gene

Transgenic mice expressing mutated mouse Cu/Zn superoxide dismutase (SOD I), corresponding to a mutation associated with familial amyotrophic lateral sclerosis, develop a fatal motorneuron degeneration that resembles the human disease. The biochemical properties of some mutant SOD I enzymes indicate that a gain of catalytic functions, (such as increased peroxidase activity) may be the pathologic factor(s). However, at the present time there is little in vivo evidence that a mutation-induced change in the catalytic activity of SOD I is directly involved in neuronal cell death or that vulnerability to cell death is related to the level of functional/metabolic activity of cells carrying mutated SOD I. In pigmented mice carrying the G86R mutation of mouse SOD I, exposure to constant bright light for 20 days caused a diminution of electroretinographic activity and specific degeneration of photoreceptor cells, while no pathological effects were seen in transgenic littermates not exposed to bright light or in light exposed non-transgenic littermates. These findings are the first to indicate that one mechanism for neuronal cell death by mutated SOD I is use-dependent and/or related to metabolic activity, and therefore may be due to a gain in function of catalytic activities involving superoxide/hydrogen peroxide. The light-exposure pathology in this transgenic mouse model indicates an essential role for SOD I in the protection of photoreceptors from light-damage.

The Alzheimer's disease amyloid precursor protein modulates copper-induced toxicity and oxidative stress in primary neuronal cultures

The amyloid precursor protein (APP) of Alzheimer's disease can reduce copper (II) to copper (I) in a cell-free system potentially leading to increased oxidative stress in neurons. We used neuronal cultures derived from APP knock-out (APP(-/-)) and wild-type (WT) mice to examine the role of APP in copper neurotoxicity. WT cortical, cerebellar, and hippocampal neurons were significantly more susceptible than their respective APP(-/-) neurons to toxicity induced by physiological concentrations of copper but not by zinc or iron. There was no difference in copper toxicity between APLP2(-/-) and WT neurons, demonstrating specificity for APP-associated copper toxicity. Copper uptake was the same in WT and APP(-/-) neurons, suggesting APP may interact with copper to induce a localized increase in oxidative stress through copper (I) production. This was supported by significantly higher levels of copper-induced lipid peroxidation in WT neurons. Treatment of neuronal cultures with a peptide corresponding to the human APP copper-binding domain (APP142-166) potentiated copper but not iron or zinc toxicity. Incubation of APP142-166 with low-density lipoprotein (LDL) and copper resulted in significantly increased lipid peroxidation compared to copper and LDL alone. Substitution of the copper coordinating histidine residues with asparagines (APP142-166(H147N, H149N, H151N)) abrogated the toxic effects. A peptide corresponding to the zinc-binding domain (APP181-208) failed to induce copper or zinc toxicity in neuronal cultures. These data support a role for the APP copper-binding domain in APP-mediated copper (I) generation and toxicity in primary neurons, a process that has important implications for Alzheimer's disease and other neurodegenerative disorders.

Therapeutic benefits of putrescine-modified catalase in a transgenic mouse model of familial amyotrophic lateral sclerosis

Dominant mutations in the copper/zinc superoxide dismutase (SOD1) gene have been observed in 15-20% of familial amyotrophic lateral sclerosis (FALS) cases. The mechanism by which SOD1 mutations result in motor neuron degeneration in FALS mice partly involves oxidative damage and an increased peroxidase activity of the mutant SOD1. A new therapeutic approach designed to eliminate the substrate of this peroxidase activity was examined in two lines of transgenic mice expressing the FALS-linked mutation glycine to alanine (G93A). We investigated the ability of putrescine-modified catalase (PUT-CAT), an antioxidant enzyme that removes hydrogen peroxide and has increased permeability at the blood-brain barrier, to modify the time course of the SOD1 mutation-induced motor neuron disease in these FALS mice. Continuous, subcutaneous administration of PUT-CAT significantly delayed the age at which onset of clinical disease occurred (indicated by loss of splay and/or tremors of hindlimbs) in a high-expressor line of FALS transgenic mice. Intraperitoneal injection of PUT-CAT given two times per week also significantly delayed the onset of clinical disease in a low-expressor line of FALS mice. PUT-CAT also significantly delayed the age at which clinical weakness developed (quantified by measuring the shortening of stride length) in both lines of FALS animals. No significant changes were observed in the survival times of the high-expressor FALS mice in any of the treatment groups. However, a trend toward a prolongation of survival was observed in the PUT-CAT-treated low-expressor FALS mice. These results support the role of free radical-mediated damage in the cascade of events leading to motor neurodegeneration in FALS and indicate that PUT-CAT interacts with a critical step in this cascade to delay the onset of clinical disease as well as the development of clinical weakness in FALS transgenic mice.

Variation in the biochemical/biophysical properties of mutant superoxide dismutase 1 enzymes and the rate of disease progression in familial amyotrophic lateral sclerosis kindreds

Mutations in superoxide dismutase 1 (SOD1) polypeptides cause a form of familial amyotrophic lateral sclerosis (FALS). In different kindreds, harboring different mutations, the duration of illness tends to be similar for a given mutation. For example, patients inheriting a substitution of valine for alanine at position four (A4V) average a 1.5 year life expectancy after the onset of symptoms, whereas patients harboring a substitution of arginine for histidine at position 46 (H46R) average an 18 year life expectancy after disease onset. Here, we examine a number of biochemical and biophysical properties of nine different FALS variants of SOD1 polypeptides, including enzymatic activity (which relates indirectly to the affinity of the enzyme for copper), polypeptide half-life, resistance to proteolytic degradation and solubility, in an effort to determine whether a specific property of these enzymes correlates with clinical progression. We find that although all the mutants tested appear to be soluble, the different mutants show a remarkable degree of variation with respect to activity, polypeptide half-life and resistance to proteolysis. However, these variables do not stratify in a manner that correlates with clinical progression. We conclude that the basis for the different life expectancies of patients in different kindreds of sod1-linked FALS may result from an as yet unidentified property of these mutant enzymes.

Reactions of hydrogen peroxide with familial amyotrophic lateral sclerosis mutant human copper-zinc superoxide dismutases studied by pulse radiolysis

Mutations in copper-zinc superoxide dismutase (CuZn-SOD) have been implicated in the familial form of the motor neuron disease amyotrophic lateral sclerosis (Lou Gehrig's disease). We have expressed and purified recombinant human wild type (hWT) and G93A (hG93A) CuZn-SOD, and we have used pulse radiolysis to measure their superoxide dismutase activities and their rates of deactivation upon exposure to hydrogen peroxide or heat. Both hG93A and hWT CuZn-SOD were found to have high SOD activities in their copper and zinc containing as-isolated forms as well as when remetallated entirely with copper (CuCu). Rates of deactivation by hydrogen peroxide of the as-isolated hWT and hG93A enzymes were determined and were found to be similar, suggesting that the FALS mutant enzyme is not inactivated at a higher rate than wild type by generation of and subsequent reaction with hydroxyl radical, .OH, when it is in the CuZn form. However, rates of deactivation by hydrogen peroxide of the CuCu derivatives of both hWT and hG93A were significantly greater than those of the copper and zinc containing as-isolated enzymes. Rates of thermal deactivation were also similar for the mutant and hWT as-isolated CuZn forms but were greater for the CuCu derivatives of both enzymes. Reactions of hydrogen peroxide with the Cu(II)Cu(II) derivative of the WT enzyme demonstrate that the copper ion in the copper site is reduced much more rapidly than the copper in the zinc site, leading to the conclusion that reaction of hydrogen peroxide with Cu(I) in the copper site is the source of deactivation in the CuCu as well as the CuZn enzymes.

Aggregation and motor neuron toxicity of an ALS-linked SOD1 mutant independent from wild-type SOD1

Analysis of transgenic mice expressing familial amyotrophic lateral sclerosis (ALS)-linked mutations in the enzyme superoxide dismutase (SOD1) have shown that motor neuron death arises from a mutant-mediated toxic property or properties. In testing the disease mechanism, both elimination and elevation of wild-type SOD1 were found to have no effect on mutant-mediated disease, which demonstrates that the use of SOD mimetics is unlikely to be an effective therapy and raises the question of whether toxicity arises from superoxide-mediated oxidative stress. Aggregates containing SOD1 were common to disease caused by different mutants, implying that coaggregation of an unidentified essential component or components or aberrant catalysis by misfolded mutants underlies a portion of mutant-mediated toxicity.

A glutamate bridge is essential for dimer stability and metal selectivity in manganese superoxide dismutase

In Escherichia coli manganese superoxide dismutase (MnSOD), the absolutely conserved Glu170 of one monomer is hydrogen-bonded to the Mn ligand His171 of the other monomer, forming a double bridge at the dimer interface. Point mutation of Glu170 --> Ala destabilizes the dimer structure, and the mutant protein occurs as a mixture of dimer and monomer species. The purified E170A MnSOD contains exclusively Fe and is devoid of superoxide dismutase activity. E170A Fe2-MnSOD closely resembles authentic FeSOD in terms of spectroscopic properties, anion interactions and pH titration behavior. Reconstitution of E170A Fe2-MnSOD with Mn(II) salts does not restore superoxide dismutase activity despite the spectroscopic similarity between E170A Mn2-MnSOD and wild type Mn2-MnSOD. Growth of sodA+ and sodA- E. coli containing the mutant plasmid pDT1-5(E170A) is impaired, suggesting that expression of mutant protein is toxic to the host cells.

Reexamination of the mechanism of hydroxyl radical adducts formed from the reaction between familial amyotrophic lateral sclerosis-associated Cu,Zn superoxide dismutase mutants and H2O2

Amyotrophic lateral sclerosis (ALS) involves the progressive degeneration of motor neurons in the spinal cord and motor cortex. Mutations to Cu,Zn superoxide dismutase (SOD) linked with familial ALS are reported to increase hydroxyl radical adduct formation from hydrogen peroxide as measured by spin trapping with 5, 5'-dimethyl-1-pyrrolline N-oxide (DMPO). In the present study, we have used oxygen-17-enriched water and H2O2 to reinvestigate the mechanism of DMPO/.OH formation from the SOD and SOD mutants. The relative ratios of DMPO/.17OH and DMPO/.16OH formed in the Fenton reaction were 90% and 10%, respectively, reflecting the ratios of H217O2 to H216O2. The reaction of the WT SOD with H217O2 in bicarbonate/CO2 buffer yielded 63% DMPO/.17OH and 37% DMPO/.16OH. Similar results were obtained from the reaction between familial ALS SOD mutants and H217O2: DMPO/.17OH (64%); DMPO/.16OH (36%) from A4V and DMPO/.17OH (62%); and DMPO/.16OH (38%) from G93A. These results were confirmed further by using 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide spin trap, a phosphorylated analog of DMPO. Contrary to earlier reports, the present results indicate that a significant fraction of DMPO/.OH formed during the reaction of SOD and familial ALS SOD mutants with H2O2 is derived from the incorporation of oxygen from water due to oxidation of DMPO to DMPO/.OH presumably via DMPO radical cation. No differences were detected between WT and mutant SODs, neither in the concentration of DMPO/.OH or DEPMPO/.OH formed nor in the relative incorporation of oxygen from H2O2 or water.

The familial amyotrophic lateral sclerosis-associated amino acid substitutions E100G, G93A, and G93R do not influence the rate of inactivation of copper- and zinc-containing superoxide dismutase by H2O2

Inactivation of copper- and zinc-containing superoxide dismutase (Cu,ZnSOD) by H2O2 is the consequence of several sequential reactions: reduction of the active site Cu(II) to Cu(I) by H2O2; oxidation of the Cu(I) by a second H2O2, thus generating a powerful oxidant, which may be Cu(I)O or Cu(II)OH or Cu(III); and finally oxidation of one of the histidines in the ligand field, causing loss of SOD activity. Three familial amyotrophic lateral sclerosis (FALS)-associated mutant Cu,ZnSODs, i.e., E100G, G93A, and G93R, did not differ from the control enzyme in susceptibility to inactivation by H2O2. It thus appears that an increased peroxidase activity of the FALS-associated Cu,ZnSOD variants might not be a factor in the development of this disease. This leaves the loss of Zn, and the consequent increase in peroxidase activity, or in nitration activity, as a viable explanation (J. P. Crow et al., 1997, J. Neurochem. 69, 1936-1944), among other possibilities.