Electrical impedance myography for monitoring motor neuron loss in the SOD1 G93A amyotrophic lateral sclerosis rat

OBJECTIVE

Human studies have shown that electrical impedance myography (EIM), a technique based on the surface application of high-frequency, low-intensity electrical current to localized areas of muscle, is sensitive to muscle denervation. In this study, we examined the role of EIM as a potential biomarker for assessing ALS disease progression in the SOD1 transgenic rat by comparing it to motor unit number estimation (MUNE).

METHODS

Multi-frequency EIM and MUNE were performed twice weekly in 16 rats from approximately 10 weeks of age onward. Four different EIM measures were evaluated, including the previously studied 50 kHz phase and three condensed multi-frequency parameters.

RESULTS

The rate of deterioration in the multi-frequency phase data from 100-500 kHz had the strongest correlation to survival (ρ=0.79, p<0.001), surpassing that of MUNE (ρ=0.57, p=0.020). These two measures were also strongly correlated (ρ=-0.94, p<0.001) to one another.

CONCLUSIONS

These findings support that EIM is an effective tool for assessing disease progression in the ALS rat.

SIGNIFICANCE

Given its ease of application and ability to assess virtually any superficial muscle, EIM deserves further study as a biomarker in human ALS clinical therapeutic trials.

ALS-causing SOD1 mutations promote production of copper-deficient misfolded species

Point mutations scattered throughout the sequence of Cu,Zn superoxide dismutase (SOD1) cause a subset of amyotrophic lateral sclerosis (ALS) cases. SOD1 is a homodimer in which each subunit binds one copper atom and one zinc atom. Inclusions containing misfolded SOD1 are seen in motor neurons of SOD1-associated ALS cases. The mechanism by which these diverse mutations cause misfolding and converge on the same disease is still not well understood. Previously, we developed several time-resolved techniques to monitor structural changes in SOD1 as it unfolds in guanidine hydrochloride. By measuring the rates of Cu and Zn release using an absorbance-based assay, dimer dissociation through chemical cross-linking, and β-barrel conformation changes by tryptophan fluorescence, we established that wild-type SOD1 unfolds by a branched pathway involving a Zn-deficient monomer as the dominant intermediate of the major pathway, and with various metal-loaded and Cu-deficient dimers populated along the minor pathway. We have now compared the unfolding pathway of wild-type SOD1 with those of A4V, G37R, G85R, G93A, and I113T ALS-associated mutant SOD1. The kinetics of unfolding of the mutants were generally much faster than those of wild type. However, all of the mutants utilize the minority pathway to a greater extent than the wild-type protein, leading to greater populations of Cu-deficient intermediates and decreases in Zn-deficient intermediates relative to the wild-type protein. The greater propensity of the mutants to populate Cu-deficient states potentially implicates these species as a pathogenic form of SOD1 in SOD1-associated ALS and provides a novel target for therapeutic intervention.

Disease-related changes in the cerebrospinal fluid metabolome in amyotrophic lateral sclerosis detected by GC/TOFMS

Background/Aim

The changes in the cerebrospinal fluid (CSF) metabolome associated with the fatal neurodegenerative disease amyotrophic lateral sclerosis (ALS) are poorly understood and earlier smaller studies have shown conflicting results. The metabolomic methodology is suitable for screening large cohorts of samples. Global metabolomics can be used for detecting changes of metabolite concentrations in samples of fluids such as CSF.

Methodology

Using gas chromatography coupled to mass spectrometry (GC/TOFMS) and multivariate statistical modeling, we simultaneously studied the metabolome signature of ∼120 small metabolites in the CSF of patients with ALS, stratified according to hereditary disposition and clinical subtypes of ALS in relation to controls.

Principal Findings

The study is the first to report data validated over two sub-sets of ALS vs. control patients for a large set of metabolites analyzed by GC/TOFMS. We find that patients with sporadic amyotrophic lateral sclerosis (SALS) have a heterogeneous metabolite signature in the cerebrospinal fluid, in some patients being almost identical to controls. However, familial amyotrophic lateral sclerosis (FALS) without superoxide dismutase-1 gene (SOD1) mutation is less heterogeneous than SALS. The metabolome of the cerebrospinal fluid of 17 ALS patients with a SOD1 gene mutation was found to form a separate homogeneous group. Analysis of metabolites revealed that glutamate and glutamine were reduced, in particular in patients with a familial predisposition. There are significant differences in the metabolite profile and composition among patients with FALS, SALS and patients carrying a mutation in the SOD1 gene suggesting that the neurodegenerative process in different subtypes of ALS may be partially dissimilar.

Conclusions/Significance

Patients with a genetic predisposition to amyotrophic lateral sclerosis have a more distinct and homogeneous signature than patients with a sporadic disease.

A novel SOD1 splice site mutation associated with familial ALS revealed by SOD activity analysis

More than 145 mutations have been found in the gene CuZn-Superoxide dismutase (SOD1) in patients with amyotrophic lateral sclerosis (ALS). The vast majority are easily detected nucleotide mutations in the coding region. In a patient from a Swiss ALS family with half-normal erythrocyte SOD1 activity, exon flanking sequence analysis revealed a novel thymine to guanine mutation 7 bp upstream of exon 4 (c.240-7T>G). The results of splicing algorithm analyses were ambiguous, but five out of seven analysis tools suggested a potential novel splice site that would add six new base pairs to the mRNA. If translated, this mRNA would insert Ser and Ile between Glu78 and Arg79 in the SOD1 protein. In fibroblasts from the patient, the predicted mutant transcript and the mutant protein were both highly expressed, and despite the location of the insertion into the metal ion-binding loop IV, the SOD1 activity appeared high. In erythrocytes, which lack protein synthesis and are old compared with cultured fibroblasts, both SOD1 protein and enzymic activity was 50% of controls. Thus, the usage of the novel splice site is near 100%, and the mutant SOD1 shows the reduced stability typical of ALS-associated mutant SOD1s. The findings suggests that this novel intronic mutation is causing the disease and highlights the importance of wide exon-flanking sequencing and transcript analysis combined with erythrocyte SOD1 activity analysis in comprehensive search for SOD1 mutations in ALS. We find that there are potentially more SOD1 mutations than previously reported.

An examination of alpha B-crystallin as a modifier of SOD1 aggregate pathology and toxicity in models of familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a progressively paralytic neurodegenerative disease that can be caused by mutations in Cu,Zn-superoxide dismutase 1 (SOD1). Transgenic mice that over-express mutant SOD1 develop paralysis and accumulate aggregates of mutant protein in the brainstem and spinal cord. The present study uses a cell culture model to demonstrate alpha B-crystallin is capable of reducing aggregation of mutant SOD1. To test the role of alpha B-crystallin in modulating SOD1 aggregation in vivo, alpha B-crystallin deficient mice were bred to mice expressing two different SOD1 mutants (G37R and L126Z). Although completely eliminating alpha B-crystallin reduced the interval to disease endstage by 20-30 days in mice expressing either mutant, there were no detectable changes in the levels of sedimentable, SOD1 aggregates in the spinal cord of symptomatic mice. Because alpha B-crystallin is most abundantly expressed in muscle, we expected that the loss of this chaperone would leave this tissue vulnerable to mutant SOD1 aggregation. However, there was no evidence of mutant SOD1 aggregation in the muscle of mice lacking alpha B-crystallin. Our findings indicate that a significant perturbation to the protein homeostasis network of muscle is not sufficient to induce the aggregation of misfolded mutant SOD1. These outcomes have implications regarding the role of chaperones in modulating the tissue specific accumulations of misfolded SOD1.

Metal-free ALS variants of dimeric human Cu,Zn-superoxide dismutase have enhanced populations of monomeric species

Amino acid replacements at dozens of positions in the dimeric protein human, Cu,Zn superoxide dismutase (SOD1) can cause amyotrophic lateral sclerosis (ALS). Although it has long been hypothesized that these mutations might enhance the populations of marginally-stable aggregation-prone species responsible for cellular toxicity, there has been little quantitative evidence to support this notion. Perturbations of the folding free energy landscapes of metal-free versions of five ALS-inducing variants, A4V, L38V, G93A, L106V and S134N SOD1, were determined with a global analysis of kinetic and thermodynamic folding data for dimeric and stable monomeric versions of these variants. Utilizing this global analysis approach, the perturbations on the global stability in response to mutation can be partitioned between the monomer folding and association steps, and the effects of mutation on the populations of the folded and unfolded monomeric states can be determined. The 2- to 10-fold increase in the population of the folded monomeric state for A4V, L38V and L106V and the 80- to 480-fold increase in the population of the unfolded monomeric states for all but S134N would dramatically increase their propensity for aggregation through high-order nucleation reactions. The wild-type-like populations of these states for the metal-binding region S134N variant suggest that even wild-type SOD1 may also be prone to aggregation in the absence of metals.

The epidemiology of CuZn-SOD mutations in Germany: a study of 217 families

We screened 217 patients from Germany (n = 213), Austria (n = 2) and Switzerland (n = 2) with a positive family history for amyotrophic lateral sclerosis (ALS) for mutations in the copper/zinc superoxide dismutase (SOD1) gene. We found that 13% of the families tested carried mutations. By analyzing inheritance, we detected a clear-cut co-segregation in 5 of the 28 families; however, in two families with an established mutation, co-segregation was absent. In Germany, the R115G mutation is comparatively frequent and exhibits a specific aggressive phenotype. The L144F mutation, which is the most prevalent mutation in the Balkan countries, and the D90A mutation which is the most frequent SOD1 mutation globally, seem to be the second most common disease-causing mutations in Germany.

Inducible nitric oxide synthase is present in motor neuron mitochondria and Schwann cells and contributes to disease mechanisms in ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of motor neurons (MNs). The molecular pathogenesis of ALS is not understood, thus effective therapies for this disease are lacking. Some forms of ALS are inherited by mutations in the superoxide dismutase-1 (SOD1) gene. Transgenic mice expressing human Gly93 --> Ala (G93A) mutant SOD1 (mSOD1) develop severe MN disease, oxidative and nitrative damage, and mitochondrial pathology that appears to involve nitric oxide-mediated mechanisms. We used G93A-mSOD1 mice to test the hypothesis that the degeneration of MNs is associated with an aberrant up-regulation of the inducible form of nitric oxide synthase (iNOS or NOS2) activity within MNs. Western blotting and immunoprecipitation showed that iNOS protein levels in mitochondrial-enriched membrane fractions of spinal cord are increased significantly in mSOD1 mice at pre-symptomatic stages of disease. The catalytic activity of iNOS was also increased significantly in mitochondrial-enriched membrane fractions of mSOD1 mouse spinal cord at pre-symptomatic stages of disease. Reverse transcription-PCR showed that iNOS mRNA was present in the spinal cord and brainstem MN regions in mice and was increased in pre-symptomatic and early symptomatic mice. Immunohistochemistry showed that iNOS immunoreactivty was up-regulated first in spinal cord and brainstem MNs in pre-symptomatic and early symptomatic mice and then later in the course of disease in numerous microglia and few astrocytes. iNOS accumulated in the mitochondria in mSOD1 mouse MNs. iNOS immunoreactivity was also up-regulated in Schwann cells of peripheral nerves and was enriched particularly at the paranodal regions of the nodes of Ranvier. Drug inhibitors of iNOS delayed disease onset and significantly extended the lifespan of G93A-mSOD1 mice. This work identifies two new potential early mechanisms for MN degeneration in mouse ALS involving iNOS at MN mitochondria and Schwann cells and suggests that therapies targeting iNOS might be beneficial in treating human ALS.

Increased expression and activation of cytosolic phospholipase A2 in the spinal cord of patients with sporadic amyotrophic lateral sclerosis

Compelling evidence identifies a link between cytotoxic effects of cytosolic phospholipase A2 (cPLA2) activity and neuron death in cell cultures. cPLA2 catalyzes the hydrolysis of membrane phospholipids to produce and release arachidonate, leading to plasma membrane injury, inflammatory response and subsequent cell death. To assess a role for cPLA2 in the pathomechanism of amyotrophic lateral sclerosis (ALS), we performed immunohistochemical, immunoblot, and densitometric analyses of cPLA2 and its active form phosphorylated at S505 (p-cPLA2) on spinal cords obtained at autopsy from ten sporadic ALS patients and ten age-matched controls. On sections, immunoreactivities for cPLA2 and p-cPLA2 were distinct and localized in almost all of the motor neurons, reactive astrocytes, and activated microglia in the ALS cases, while immunoreactivities were only weak or not at all observed in neurons and glia in the control cases. On immunoblots, both the cPLA2/β-actin density ratio and the p-cPLA2/cPLA2 density ratio were significantly increased in the ALS group compared to the control group. There was no significant link between the densitometric data and the clinical phenotypes, age at death or disease duration of the ALS patients. These results provide in vivo evidence for increased expression and activation of cPLA2 in motor neurons, reactive astrocytes, and activated microglia in ALS, suggesting occurrence of arachidonate cascade-induced motor neuron death via cell-autonomous and/or non-cell-autonomous mechanisms.

Metalloproteinase alterations in the bone marrow of ALS patients

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease, nowadays considered as suitable candidate for autologous stem therapy with bone marrow (BM). A careful characterization of BM stem cell (SC) compartment is mandatory before its extensive application to clinic. Indeed, widespread systemic involvement has been recently advocated given that non-neuronal neighboring cells actively influence the pathological neuronal loss. We therefore investigated BM samples from 21 ALS patients and reported normal hematopoietic biological properties while an atypical behavior and impaired SC capabilities affected only the mesenchymal compartment. Moreover, by quantitative real-time approach, we observed altered Collagen IV and Metalloproteinase-9 levels in patients' derived mesenchymal stem cells (MSCs). Widespread metalloproteinase (MMPs) and their tissue inhibitor (TIMPs) alterations were established by multiplex ELISA analysis, demonstrating diffuse enzymatic variations in MSC compartment. Since MMPs act as fundamental effectors of extra-cellular matrix remodeling and stem cell mobilization, their modifications in ALS may influence reparative mechanisms effective in counteracting the pathology. In conclusion, ALS is further confirmed to be a systemic disease, not restricted to the nervous system, but affecting also the BM stromal compartment, even in sporadic cases. Therefore, therapeutic implantation of autologous BM derived SC in ALS patients needs to be carefully reevaluated.

Familial amyotrophic lateral sclerosis with a novel G85S mutation of superoxide dismutase 1 gene: clinical features of lower motor neuron disease

Amyotrophic lateral sclerosis (ALS) is a devastating disease characterized by upper and lower motor neuron damage. Mutations of Cu/Zn superoxide dismutase gene (SOD1) account for 20% of familial ALS (FALS). We report a unique clinicogenotype of a Japanese family with a novel SOD 1 mutation. A 37-year-old woman (the proband) noticed muscle weakness in the left lower limb. Her mother had developed progressive lower motor neuron signs in four extremities at 38 years of age. Subsequently she was diagnosed as ALS and died of respiratory failure at 15 months after clinical onset. Neurological examination of the proband showed absent muscle stretch reflexes in the left knee and the left ankle without Babinski signs. Mild to moderate degree of muscle weakness existed in the left lower extremity. Muscle atrophy was presented in the left thigh. Initial pulmonary function revealed forced vital capacity of 91.1%. Electromyography disclosed ongoing denervation muscle potentials in the left lower extremity. SOD1 analysis demonstrated amino acid substitution of glycine by serine at codon 85 (G85S) in exon 4. Six months later, marked muscle weakness and atrophy expanded to four extremities. All muscle stretch reflexes were absent. Three months later, ventilator support with a tracheostomy was needed. The patient died at 18 months after clinical onset. Clinical hallmarks of this FALS family indicate that G85S mutation of SOD1 may cause rapidly progressive form of pure lower motor neuron signs.

Characterization of detergent-insoluble proteins in ALS indicates a causal link between nitrative stress and aggregation in pathogenesis

Background

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal motor neuron disease, and protein aggregation has been proposed as a possible pathogenetic mechanism. However, the aggregate protein constituents are poorly characterized so knowledge on the role of aggregation in pathogenesis is limited.

Methodology/Principal Findings

We carried out a proteomic analysis of the protein composition of the insoluble fraction, as a model of protein aggregates, from familial ALS (fALS) mouse model at different disease stages. We identified several proteins enriched in the detergent-insoluble fraction already at a preclinical stage, including intermediate filaments, chaperones and mitochondrial proteins. Aconitase, HSC70 and cyclophilin A were also significantly enriched in the insoluble fraction of spinal cords of ALS patients. Moreover, we found that the majority of proteins in mice and HSP90 in patients were tyrosine-nitrated. We therefore investigated the role of nitrative stress in aggregate formation in fALS-like murine motor neuron-neuroblastoma (NSC-34) cell lines. By inhibiting nitric oxide synthesis the amount of insoluble proteins, particularly aconitase, HSC70, cyclophilin A and SOD1 can be substantially reduced.

Conclusion/Significance

Analysis of the insoluble fractions from cellular/mouse models and human tissues revealed novel aggregation-prone proteins and suggests that nitrative stress contribute to protein aggregate formation in ALS.

Structural and biophysical properties of metal-free pathogenic SOD1 mutants A4V and G93A

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disease characterized by the destruction of motor neurons in the spinal cord and brain. A subset of ALS cases are linked to dominant mutations in copper-zinc superoxide dismutase (SOD1). The pathogenic SOD1 variants A4V and G93A have been the foci of multiple studies aimed at understanding the molecular basis for SOD1-linked ALS. The A4V variant is responsible for the majority of familial ALS cases in North America, causing rapidly progressing paralysis once symptoms begin and the G93A SOD1 variant is overexpressed in often studied murine models of the disease. Here we report the three-dimensional structures of metal-free A4V and of metal-bound and metal-free G93A SOD1. In the metal-free structures, the metal-binding loop elements are observed to be severely disordered, suggesting that these variants may share mechanisms of aggregation proposed previously for other pathogenic SOD1 proteins.

Multiple roles of HDAC inhibition in neurodegenerative conditions

Histone deacetylases (HDACs) play a key role in homeostasis of protein acetylation in histones and other proteins and in regulating fundamental cellular activities such as transcription. A wide range of brain disorders are associated with imbalances in protein acetylation levels and transcriptional dysfunctions. Treatment with various HDAC inhibitors can correct these deficiencies and has emerged as a promising new strategy for therapeutic intervention in neurodegenerative disease. Here, we review and discuss intriguing recent developments in the use of HDAC inhibitors to combat neurodegenerative conditions in cellular and disease models. HDAC inhibitors have neuroprotective, neurotrophic and anti-inflammatory properties; improvements in neurological performance, learning/memory and other disease phenotypes are frequently seen in these models. We discuss the targets and mechanisms underlying these effects of HDAC inhibition and comment on the potential for some HDAC inhibitors to prove clinically effective in the treatment of neurodegenerative disorders.

Evaluation of matrix metalloproteinases in serum of patients with amyotrophic lateral sclerosis with pattern recognition methods

Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases, which are present in central and peripheral nervous system. They are considered to be involved in the pathogenesis of several neurological diseases, as multiple sclerosis, Alzheimer disease, and amyotrophic lateral sclerosis (ALS). The aim of the present study was to evaluate the application of the pattern recognition methods for the assessment of MMPs in serum of patients with ALS. Thirty patients with amyotrophic lateral sclerosis (ALS), in two subgroups: (i) with mild and (ii) severe progressing ALS, and 15 control healthy subjects were studied. The metalloproteinases MT-MMP-1, MMP-2, MMP-9 were examined. Additional variables (age of subjects and disease duration) were also analyzed by using a standard, parallel and hierarchical classifiers. Our results indicate that: (i) MMP-2 in serum may be an important marker for the evaluation of ALS progress; (ii) the set of two features {MT-MMP-1, MMP-9} may be helpful in differentiation between ALS and healthy subjects; (iii) the error rates obtained for the pair-wise linear classifier were similar to those received for the classifiers (standard, parallel, and hierarchical) based on k-NN rule. We conclude that the pattern recognition methods may be useful for the evaluation of significance MMPs as markers in neurodegenerative diseases, such as ALS.

Clair DK. Regulation of superoxide dismutase genes: implications in disease

Numerous short-lived and highly reactive oxygen species (ROS) such as superoxide (O2(.-)), hydroxyl radical, and hydrogen peroxide are continuously generated in vivo. Depending upon concentration, location, and intracellular conditions, ROS can cause toxicity or act as signaling molecules. The cellular levels of ROS are controlled by antioxidant enzymes and small-molecule antioxidants. As major antioxidant enzymes, superoxide dismutases (SODs), including copper-zinc superoxide dismutase (Cu/ZnSOD), manganese superoxide dismutase, and extracellular superoxide dismutase, play a crucial role in scavenging O2(.-). This review focuses on the regulation of the sod genes coding for these enzymes, with an emphasis on the human genes. Current knowledge about sod structure and regulation is summarized and depicted as diagrams. Studies to date on genes coding for Cu/ZnSOD (sod1) are mostly focused on alterations in the coding region and their associations with amyotrophic lateral sclerosis. Evaluation of nucleotide sequences reveals that regulatory elements of the sod2 gene reside in both the noncoding and the coding region. Changes associated with sod2 lead to alterations in expression levels as well as protein function. We also discuss the structural basis for the changes in SOD expression associated with pathological conditions and where more work is needed to establish the relationship between SODs and diseases.

Sixteen novel mutations in the Cu/Zn superoxide dismutase gene in amyotrophic lateral sclerosis: a decade of discoveries, defects and disputes

OBJECTIVE

Since the discovery of mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1) ten years ago, testing for SOD1 gene mutations has become a part of the investigation of patients with suspected motor neuron disease. We searched for novel SOD1 mutations and for clinical characteristics of patients with these mutations.

METHODS

Analysis was made of patient files at the Neurogenetic DNA Diagnostic Laboratory at Massachusetts General Hospital. We also scrutinized available medical records and examined patients with the different SOD1 mutations.

RESULTS

One hundred and forty eight (148) of 2045 amyotrophic lateral sclerosis (ALS) patients carried a disease-associated mutation in the SOD1 gene. The most prevalent was the A4V missense mutation, found in 41% of those patients. Sixteen novel exonic mutations (L8V, F20C, Q22L, H48R, T54R, S591, V87A, T88deltaTAD, A89T, V97M, S105deltaSL, V118L, D124G, G141X, G147R, 11515) were found, bringing the total number of SOD1 gene mutations in ALS to 105.

CONCLUSIONS

Mutations in the SOD1 gene are found both in sporadic and familial ALS cases without any definite predilection for any part of the gene. A common structural denominator for the 16 novel mutations or previously reported mutations is not obvious. Similarly, the nature of the putative acquired toxic function of mutant SOD1 remains unresolved. We conclude that patients with SOD1 mutations may infrequently show symptoms and signs unrelated to the motor systems, sometimes obscuring the diagnosis of ALS.

CGP 3466B has no effect on disease course of (G93A) mSOD1 transgenic mice

BACKGROUND

There is an accumulating body of evidence that apoptosis is involved in the motor neuron death that occurs in ALS, and in the (G93A) mSOD1 transgenic mouse model (mSOD1 mice). CGP 3466B, a tricyclic propargylamine structurally related to (-)-deprenyl, was found to inhibit apoptosis in a wide variety of in vitro and in vivo models. We therefore studied the effect of CGP 3466B in mSOD1 mice.

METHODS

As the effect of CGP 3466B was previously reported to have a bell-shaped curve, we performed a dose-ranging study. High-copy G93A mSOD1 mice were treated subcutaneously from the age of 50 days until death with four concentrations of CGP 3466B (0.39 microg kg(-1), 3.9 microg kg(-1), 39 microg kg(-1), and 390 microg kg(-1)). Behavioural tests were performed daily to determine disease onset, disease progression and survival. At the age of 110 days, two mice per group were sacrificed for histopathological analysis of the lumbar ventral horn and for semiquantitative analysis of motor neuron number.

RESULTS

We observed no effect on disease onset, disease progression, or survival of the mice. We also did not observe a significant effect on the number of motor neurons due to CGP 3466B.

CONCLUSIONS

We conclude that in high-copy G93A mSOD1 mice, chronic subcutaneous treatment with CGP 3466B offers no clinical benefit.

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.

Aggregation of copper-zinc superoxide dismutase in familial and sporadic ALS

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease characterized by the selective death of motor neurons. While the most common form of ALS is sporadic and has no known cause, a small subset of cases is familial because of underlying genetic mutations. The best-studies example of familial ALS is that caused by mutations in the protein copper-zinc superoxide dismutase. The formation of SOD1-rich inclusions in the spinal cord is an early and prominent feature of SOD1-linked familial ALS in human patients and animal models of this disease. These inclusions have been shown to consist of SOD1-rich fibrils, suggesting that the conversion of soluble SOD1 into amyloid fibrils may play an important role in the etiology of familial ALS. SOD1 is also present in inclusions found in spinal cords of sporadic ALS patients, allowing speculations to arise regarding a possible involvement of SOD1 in the sporadic form of this disease. We here review the recent research on the significance, causes, and mechanisms of SOD1 fibril formation from a biophysical perspective.

Reduced expression of the Kinesin-Associated Protein 3 (KIFAP3) gene increases survival in sporadic amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a degenerative disorder of motor neurons that typically develops in the 6th decade and is uniformly fatal, usually within 5 years. To identify genetic variants associated with susceptibility and phenotypes in sporadic ALS, we performed a genome-wide SNP analysis in sporadic ALS cases and controls. A total of 288,357 SNPs were screened in a set of 1,821 sporadic ALS cases and 2,258 controls from the U.S. and Europe. Survival analysis was performed using 1,014 deceased sporadic cases. Top results for susceptibility were further screened in an independent sample set of 538 ALS cases and 556 controls. SNP rs1541160 within the KIFAP3 gene (encoding a kinesin-associated protein) yielded a genome-wide significant result (P = 1.84 x 10(-8)) that withstood Bonferroni correction for association with survival. Homozygosity for the favorable allele (CC) conferred a 14.0 months survival advantage. Sequence, genotypic and functional analyses revealed that there is linkage disequilibrium between rs1541160 and SNP rs522444 within the KIFAP3 promoter and that the favorable alleles of rs1541160 and rs522444 correlate with reduced KIFAP3 expression. No SNPs were associated with risk of sporadic ALS, site of onset, or age of onset. We have identified a variant within the KIFAP3 gene that is associated with decreased KIFAP3 expression and increased survival in sporadic ALS. These findings support the view that genetic factors modify phenotypes in this disease and that cellular motor proteins are determinants of motor neuron viability.

The effect of mutant SOD1 dismutase activity on non-cell autonomous degeneration in familial amyotrophic lateral sclerosis

Mutant superoxide dismutase type 1 (MTSOD1), the most common known cause of familial amyotrophic lateral sclerosis (FALS), is believed to cause FALS as a result of a toxicity of the protein. MTSOD1s with full dismutase enzymatic activity (e.g., G37R) and without any enzymatic activity (e.g., G85R) cause FALS, demonstrating that the ability of MTSOD1 to cause FALS is not dependent on the dismutase activity; however, it remains unclear whether MTSOD1 dismutase activity can influence disease phenotype. In the present study, we selectively knocked down G85R expression in particular cell types of G85R mice. Results following knockdown of G85R in motor neurons (MNs)/interneurons of G85R mice were similar to results from a published study involving knockdown of G37R in G37R mice; however, G85R knockdown in microglia/macrophages induced a prolonged early and late disease phase while G37R knockdown in the same cells only affected late phase. These results show that: (i) MN as well as non-MN expression of G85R, like G37R, has a significant effect on disease in transgenic mice - indicating the role of non-cell autonomous degeneration in both dismutase-active and inactive MTSOD1s. (ii) The effect of MTSOD1 expression in microglia/macrophages varies with different mutants, and may be influenced by the MTSOD1's dismutase activity.

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.