Axonal transport of mutant superoxide dismutase 1 and focal axonal abnormalities in the proximal axons of transgenic mice

Axonopathy Underlying Amyotrophic Lateral Sclerosis: Unraveling Complex Pathways and Therapeutic Insights

Amyotrophic Lateral Sclerosis (ALS) is a complex neurodegenerative disorder characterized by progressive axonopathy, jointly leading to the dying back of the motor neuron, disrupting both nerve signaling and motor control. In this review, we highlight the roles of axonopathy in ALS progression, driven by the interplay of multiple factors including defective trafficking machinery, protein aggregation, and mitochondrial dysfunction. Dysfunctional intracellular transport, caused by disruptions in microtubules, molecular motors, and adaptors, has been identified as a key contributor to disease progression. Aberrant protein aggregation involving TDP-43, FUS, SOD1, and dipeptide repeat proteins further amplifies neuronal toxicity. Mitochondrial defects lead to ATP depletion, oxidative stress, and Ca2+ imbalance, which are regarded as key factors underlying the loss of neuromuscular junctions and axonopathy. Mitigating these defects through interventions including neurotrophic treatments offers therapeutic potential. Collaborative research efforts aim to unravel ALS complexities, opening avenues for holistic interventions that target diverse pathological mechanisms.

Copper ion / H2O2 oxidation of Cu/Zn-Superoxide dismutase: Implications for enzymatic activity and antioxidant action

Copper ion-catalyzed oxidation of yeast SOD1 (ySOD1) was examined to determine early oxidative modifications, including oxidation of a crucial disulfide bond, and the structural and functional repercussions of these events. The study used distinct oxidative conditions: Cu²⁺/H₂O₂, Cu²⁺/H₂O₂/AscH⁻ and Cu²⁺/H₂O₂/glucose. Capillary electrophoresis experiments and quantification of protein carbonyls indicate that ySOD1 is highly susceptible to oxidative modification and that changes can be detected within 0.1 min of the initiation of the reaction. Oxidation-induced structural perturbations, characterized by circular dichroism, revealed the formation of partially-unfolded ySOD1 species in a dose-dependent manner. Consistent with these structural changes, pyrogallol assay indicates a partial loss of enzymatic activity. ESI-MS analyses showed seven distinct oxidized ySOD1 species under mild oxidation within 0.1 min. LC/MS analysis after proteolytic digestion demonstrated that the copper-coordinating active site histidine residues, His47 and His49, were converted into 2-oxo-histidine. Furthermore, the Cu and Zn bridging residue, His64 is converted into aspartate/asparagine. Importantly, the disulfide-bond Cys58-Cys147 which is critical for the structural and functional integrity of ySOD1 was detected as being oxidized at Cys147. We propose, based on LC/MS analyses, that disulfide-bond oxidation occurs without disulfide bond cleavage. Modifications were also detected at Met85 and five surface-exposed Lys residues. Based on these data we propose that the Cys58-Cys147 bond may act as a sacrificial target for oxidants and protect ySOD1 from oxidative inactivation arising from exposure to Cu²⁺/H₂O₂ and auto-inactivation during extended enzymatic turnover.

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Oxidation of yeast superoxide dismutase (ySOD1) by Cu²⁺/H₂O₂ is examined.

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Rapid modification of His, Met, Cys and Lys residues detected by LC-MS methods.

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Oxidation of active site His residues and partial protein unfolding are early events.

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The Cys58-Cys147 disulfide bond is oxidized and may act as a sacrificial target.

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Excess exogenous Cu²⁺ decreases protein damage and can reverse loss of activity.

Prion-like properties of disease-relevant proteins in amyotrophic lateral sclerosis

The hallmark of age-related neurodegenerative diseases is the appearance of cellular protein deposits and spreading of this pathology throughout the central nervous system. Growing evidence has shown the involvement and critical role of proteins with prion-like properties in the formation of these characteristic cellular aggregates. Prion-like domains of such proteins with their proposed function in the organization of membraneless organelles are prone for misfolding and promoting further aggregation. Spreading of these toxic aggregates between cells and across tissues can explain the progression of clinical phenotypes and pathology in a stereotypical manner, characteristic for almost every neurodegenerative disease. Here, we want to review the current evidence for the role of prion-like mechanisms in classical neurodegenerative diseases and ALS in particular. We will also discuss an intriguingly central role of the protein TDP-43 in the majority of cases of this devastating disease.

Kaplan-Meier Meets Chemical Kinetics: Intrinsic Rate of SOD1 Amyloidogenesis Decreased by Subset of ALS Mutations and Cannot Fully Explain Age of Disease Onset

Over 150 mutations in SOD1 (superoxide dismutase-1) cause amyotrophic lateral sclerosis (ALS), presumably by accelerating SOD1 amyloidogenesis. Like many nucleation processes, SOD1 fibrillization is stochastic (in vitro), which inhibits the determination of aggregation rates (and obscures whether rates correlate with patient phenotypes). Here, we diverged from classical chemical kinetics and used Kaplan-Meier estimators to quantify the probability of apo-SOD1 fibrillization (in vitro) from ∼10³ replicate amyloid assays of wild-type (WT) SOD1 and nine ALS variants. The probability of apo-SOD1 fibrillization (expressed as a Hazard ratio) is increased by certain ALS-linked SOD1 mutations but is decreased or remains unchanged by other mutations. Despite this diversity, Hazard ratios of fibrillization correlated linearly with (and for three mutants, approximately equaled) Hazard ratios of patient survival (R² = 0.67; Pearson's r = 0.82). No correlation exists between Hazard ratios of fibrillization and age of initial onset of ALS (R² = 0.09). Thus, Hazard ratios of fibrillization might explain rates of disease progression but not onset. Classical kinetic metrics of fibrillization, i.e., mean lag time and propagation rate, did not correlate as strongly with phenotype (and ALS mutations did not uniformly accelerate mean rate of nucleation or propagation). A strong correlation was found, however, between mean ThT fluorescence at lag time and patient survival (R² = 0.93); oligomers of SOD1 with weaker fluorescence correlated with shorter survival. This study suggests that SOD1 mutations trigger ALS by altering a property of SOD1 or its oligomers other than the intrinsic rate of amyloid nucleation (e.g., oligomer stability; rates of intercellular propagation; affinity for membrane surfaces; and maturation rate).

Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

Neurons affected in a wide variety of unrelated adult-onset neurodegenerative diseases (AONDs) typically exhibit a "dying back" pattern of degeneration, which is characterized by early deficits in synaptic function and neuritic pathology long before neuronal cell death. Consistent with this observation, multiple unrelated AONDs including Alzheimer's disease, Parkinson's disease, Huntington's disease, and several motor neuron diseases feature early alterations in kinase-based signaling pathways associated with deficits in axonal transport (AT), a complex cellular process involving multiple intracellular trafficking events powered by microtubule-based motor proteins. These pathogenic events have important therapeutic implications, suggesting that a focus on preservation of neuronal connections may be more effective to treat AONDs than addressing neuronal cell death. While the molecular mechanisms underlying AT abnormalities in AONDs are still being analyzed, evidence has accumulated linking those to a well-established pathological hallmark of multiple AONDs: altered patterns of neuronal protein phosphorylation. Here, we present a short overview on the biochemical heterogeneity of major motor proteins for AT, their regulation by protein kinases, and evidence revealing cell type-specific AT specializations. When considered together, these findings may help explain how independent pathogenic pathways can affect AT differentially in the context of each AOND.

Neurofilaments and Neurofilament Proteins in Health and Disease

SUMMARYNeurofilaments (NFs) are unique among tissue-specific classes of intermediate filaments (IFs) in being heteropolymers composed of four subunits (NF-L [neurofilament light]; NF-M [neurofilament middle]; NF-H [neurofilament heavy]; and α-internexin or peripherin), each having different domain structures and functions. Here, we review how NFs provide structural support for the highly asymmetric geometries of neurons and, especially, for the marked radial expansion of myelinated axons crucial for effective nerve conduction velocity. NFs in axons extensively cross-bridge and interconnect with other non-IF components of the cytoskeleton, including microtubules, actin filaments, and other fibrous cytoskeletal elements, to establish a regionally specialized network that undergoes exceptionally slow local turnover and serves as a docking platform to organize other organelles and proteins. We also discuss how a small pool of oligomeric and short filamentous precursors in the slow phase of axonal transport maintains this network. A complex pattern of phosphorylation and dephosphorylation events on each subunit modulates filament assembly, turnover, and organization within the axonal cytoskeleton. Multiple factors, and especially turnover rate, determine the size of the network, which can vary substantially along the axon. NF gene mutations cause several neuroaxonal disorders characterized by disrupted subunit assembly and NF aggregation. Additional NF alterations are associated with varied neuropsychiatric disorders. New evidence that subunits of NFs exist within postsynaptic terminal boutons and influence neurotransmission suggests how NF proteins might contribute to normal synaptic function and neuropsychiatric disease states.

Relationship between mutant Cu/Zn superoxide dismutase 1 maturation and inclusion formation in cell models

A common property of Cu/Zn superoxide dismutase 1 (SOD1), harboring mutations associated with amyotrophic lateral sclerosis, is a high propensity to misfold and form abnormal aggregates. The aggregation of mutant SOD1 has been demonstrated in vitro, with purified proteins, in mouse models, in human tissues, and in cultured cell models. In vitro translation studies have determined that SOD1 with amyotrophic lateral sclerosis mutations is slower to mature, and thus perhaps vulnerable to off-pathway folding that could generate aggregates. The aggregation of mutant SOD1 in living cells can be monitored by tagging the protein with fluorescent fluorophores. In this study, we have taken advantage of the Dendra2 fluorophore technology in which excitation can be used to switch the output color from green to red, thereby clearly creating a time stamp that distinguishes pre-existing and newly made proteins. In cells that transiently over-express the Ala 4 to Val variant of SOD1-Dendra2, we observed that newly made mutant SOD1 was rapidly captured by pathologic intracellular inclusions. In cell models of mutant SOD1 aggregation over-expressing untagged A4V-SOD1, we observed that immature forms of the protein, lacking a Cu co-factor and a normal intramolecular disulfide, persist for extended periods. Our findings fit with a model in which immature forms of mutant A4V-SOD1, including newly made protein, are prone to misfolding and aggregation.

Conformational Disorder of the Most Immature Cu, Zn-Superoxide Dismutase Leading to Amyotrophic Lateral Sclerosis

Misfolding of Cu,Zn-superoxide dismutase (SOD1) is a pathological change in the familial form of amyotrophic lateral sclerosis caused by mutations in the SOD1 gene. SOD1 is an enzyme that matures through the binding of copper and zinc ions and the formation of an intramolecular disulfide bond. Pathogenic mutations are proposed to retard the post-translational maturation, decrease the structural stability, and hence trigger the misfolding of SOD1 proteins. Despite this, a misfolded and potentially pathogenic conformation of immature SOD1 remains obscure. Here, we show significant and distinct conformational changes of apoSOD1 that occur only upon reduction of the intramolecular disulfide bond in solution. In particular, loop regions in SOD1 lose their restraint and become significantly disordered upon dissociation of metal ions and reduction of the disulfide bond. Such drastic changes in the solution structure of SOD1 may trigger misfolding and fibrillar aggregation observed as pathological changes in the familial form of amyotrophic lateral sclerosis.

Distinctive features of the D101N and D101G variants of superoxide dismutase 1; two mutations that produce rapidly progressing motor neuron disease

Mutations in superoxide dismutase 1 (SOD1) associated with familial amyotrophic lateral sclerosis induce misfolding and aggregation of the protein with the inherent propensity of mutant SOD1 to aggregate generally correlating, with a few exceptions, to the duration of illness in patients with the same mutation. One notable exception was the D101N variant, which has been described as wild-type-like. The D101N mutation is associated with rapidly progressing motor neuron degeneration but shows a low propensity to aggregate. By assaying the kinetics of aggregation in a well-characterized cultured cell model, we show that the D101N mutant is slower to initiate aggregation than the D101G mutant. In this cell system of protein over-expression, both mutants were equally less able to acquire Zn than WT SOD1. In addition, both of these mutants were equivalently less able to fold into the trypsin-resistant conformation that characterizes WT SOD1. A second major difference between the two mutants was that the D101N variant more efficiently formed a normal intramolecular disulfide bond. Overall, our findings demonstrate that the D101N and D101G variants exhibit clearly distinctive features, including a different rate of aggregation, and yet both are associated with rapidly progressing disease. We sought to better characterize the biochemical features of two SOD1 mutants associated with rapidly progressing disease, the D101G and wild-type like D101N mutants. We observed using our cell model that that although similarities were observed when comparing the ability to bind metals and resist trypsin digestion, these mutants differed in their ability to initiate aggregation and to form the normal intramolecular disulfide bond. We conclude that these mutants exhibit distinct properties despite producing similar disease phenotypes in patients.

Deamidation of asparagine to aspartate destabilizes Cu, Zn superoxide dismutase, accelerates fibrillization, and mirrors ALS-linked mutations

The reactivity of asparagine residues in Cu, Zn superoxide dismutase (SOD1) to deamidate to aspartate remains uncharacterized; its occurrence in SOD1 has not been investigated, and the biophysical effects of deamidation on SOD1 are unknown. Deamidation is, nonetheless, chemically equivalent to Asn-to-Asp missense mutations in SOD1 that cause amyotrophic lateral sclerosis (ALS). This study utilized computational methods to identify three asparagine residues in wild-type (WT) SOD1 (i.e., N26, N131, and N139) that are predicted to undergo significant deamidation (i.e., to >20%) on time scales comparable to the long lifetime (>1 year) of SOD1 in large motor neurons. Site-directed mutagenesis was used to successively substitute these asparagines with aspartate (to mimic deamidation) according to their predicted deamidation rate, yielding: N26D, N26D/N131D, and N26D/N131D/N139D SOD1. Differential scanning calorimetry demonstrated that the thermostability of N26D/N131D/N139D SOD1 is lower than WT SOD1 by ~2-8 °C (depending upon the state of metalation) and <3 °C lower than the ALS mutant N139D SOD1. The triply deamidated analog also aggregated into amyloid fibrils faster than WT SOD1 by ~2-fold (p < 0.008**) and at a rate identical to ALS mutant N139D SOD1 (p > 0.2). A total of 534 separate amyloid assays were performed to generate statistically significant comparisons of aggregation rates among WT and N/D SOD1 proteins. Capillary electrophoresis and mass spectrometry demonstrated that ~23% of N26 is deamidated to aspartate (iso-aspartate was undetectable) in a preparation of WT human SOD1 (isolated from erythrocytes) that has been used for decades by researchers as an analytical standard. The deamidation of asparagine--an analytically elusive, sub-Dalton modification--represents a plausible and overlooked mechanism by which WT SOD1 is converted to a neurotoxic isoform that has a similar structure, instability, and aggregation propensity as ALS mutant N139D SOD1.

A structural modeling approach for the understanding of initiation and elongation of ALS-linked superoxide dismutase fibrils

Familial amyotrophic lateral sclerosis caused by mutations in copper-zinc superoxide dismutase (SOD1) is characterized by the presence of SOD1-rich inclusions in spinal cords. It has been shown that a reduced intra-subunit disulfide bridge apo-SOD1 can rapidly initiate fibrillation forming an inter-subunits disulfide under mild, physiologically accessible conditions. Once initiated, elongation can proceed via recruitment of either apo or partially metallated disulfide-intact SOD1 and the presence of copper, but not zinc, ions inhibit fibrillation. We propose a structural model, refined through molecular dynamics simulations, that, taking into account these experimental findings, provides a molecular explanation for the initiation and the elongation of SOD1 fibrils in physiological conditions. The model indicates the occurrence of a new dimeric unit, prone to interact one with the other due to the presence of a wide hydrophobic surface and specific electrostatic interactions. The model has dimensions consistent with the SOD1 fibril size observed through electron microscopy and provides a structural basis for the understanding of SOD1 fibrillation.

Cupric ions induce the oxidation and trigger the aggregation of human superoxide dismutase 1

BACKGROUND

Amyotrophic lateral sclerosis (ALS), partly caused by the mutations and aggregation of human copper, zinc superoxide dismutase (SOD1), is a fatal degenerative disease of motor neurons. Because SOD1 is a major copper-binding protein present at relatively high concentration in motor neurons and copper can be a harmful pro-oxidant, we want to know whether aberrant copper biochemistry could underlie ALS pathogenesis. In this study, we have investigated and compared the effects of cupric ions on the aggregation of ALS-associated SOD1 mutant A4V and oxidized wild-type SOD1.

METHODOLOGY/PRINCIPAL FINDINGS

As revealed by 90° light scattering, dynamic light scattering, SDS-PAGE, and atomic force microscopy, free cupric ions in solution not only induce the oxidation of either apo A4V or Zn2-A4V and trigger the oligomerization and aggregation of oxidized A4V under copper-mediated oxidative conditions, but also trigger the aggregation of non-oxidized form of such a pathogenic mutant. As evidenced by mass spectrometry and SDS-PAGE, Cys-111 is a primary target for oxidative modification of pathological human SOD1 mutant A4V by either excess Cu(2+) or hydrogen peroxide. The results from isothermal titration calorimetry show that A4V possesses two sets of independent binding sites for Cu(2+): a moderate-affinity site (10(6) M(-1)) and a high-affinity site (10(8) M(-1)). Furthermore, Cu(2+) binds to wild-type SOD1 oxidized by hydrogen peroxide in a way similar to A4V, triggering the aggregation of such an oxidized form.

CONCLUSIONS/SIGNIFICANCE

We demonstrate that excess cupric ions induce the oxidation and trigger the aggregation of A4V SOD1, and suggest that Cu(2+) plays a key role in the mechanism of aggregation of both A4V and oxidized wild-type SOD1. A plausible model for how pathological SOD1 mutants aggregate in ALS-affected motor neurons with the disruption of copper homeostasis has been provided.

Molecular chaperone Hsp110 rescues a vesicle transport defect produced by an ALS-associated mutant SOD1 protein in squid axoplasm

Mutant human Cu/Zn superoxide dismutase 1 (SOD1) is associated with motor neuron toxicity and death in an inherited form of amyotrophic lateral sclerosis (ALS; Lou Gehrig disease). One aspect of toxicity in motor neurons involves diminished fast axonal transport, observed both in transgenic mice and, more recently, in axoplasm isolated from squid giant axons. The latter effect appears to be directly mediated by misfolded SOD1, whose addition activates phosphorylation of p38 MAPK and phosphorylation of kinesin. Here, we observe that several different oligomeric states of a fusion protein, comprising ALS-associated human G85R SOD1 joined with yellow fluorescent protein (G85R SOD1YFP), which produces ALS in transgenic mice, inhibited anterograde transport when added to squid axoplasm. Inhibition was blocked both by an apoptosis signal-regulating kinase 1 (ASK1; MAPKKK) inhibitor and by a p38 inhibitor, indicating the transport defect is mediated through the MAPK cascade. In further incubations, we observed that addition of the mammalian molecular chaperone Hsc70, abundantly associated with G85R SOD1YFP in spinal cord of transgenic mice, exerted partial correction of the transport defect, associated with diminished phosphorylation of p38. Most striking, the addition of the molecular chaperone Hsp110, in a concentration substoichiometric to the mutant SOD1 protein, completely rescued both the transport defect and the phosphorylation of p38. Hsp110 has been demonstrated to act as a nucleotide exchange factor for Hsc70 and, more recently, to be able to cooperate with it to mediate protein disaggregation. We speculate that it can cooperate with endogenous squid Hsp(c)70 to mediate binding and/or disaggregation of mutant SOD1 protein, abrogating toxicity.

Neurogenetics of slow axonal transport: from cells to animals

Slow axonal transport is a multivariate phenomenon implicated in several neurodegenerative disorders. Recent reports have unraveled the molecular basis of the transport of certain slow component proteins, such as the neurofilament subunits, tubulin, and certain soluble enzymes such as Ca(2+)/calmodulin-dependent protein kinase IIa (CaM kinase IIa), etc., in tissue cultured neurons. In addition, genetic analyses also implicate microtubule-dependent motors and other housekeeping proteins in this process. However, the biological relevance of this phenomenon is not so well understood. Here, the authors have discussed the possibility of adopting neurogenetic analyses in multiple model organisms to correlate molecular level measurements of the slow transport phenomenon to animal behavior, thus facilitating the investigation of its biological efficacy.

Selective knockdown of mutant SOD1 in Schwann cells ameliorates disease in G85R mutant SOD1 transgenic mice

Mutants of superoxide dismutase type 1 (mtSOD1) that have full dismutase activity (e.g., G37R) as well as none (e.g., G85R) cause familial amyotrophic lateral sclerosis (FALS), indicating that mtSOD1-induced FALS results from a toxicity rather than loss in SOD1 enzymatic activity. Still, it has remained unclear whether mtSOD1 dismutase activity can influence disease. A previous study demonstrated that Cre-mediated knockdown of G37R expression in Schwann cells (SCs) of G37R transgenic mice shortened the late phase of disease and survival. These results suggested that the neuroprotective effect of G37R expressed in SCs was greater than its toxicity, presumably because its dismutase activity counteracted reactive oxygen species (ROS). In order to further investigate this, we knocked down G85R in SCs by crossing G85R(flox) mice with myelin-protein-zero (P(0)):Cre mice, which express Cre recombinase in SCs. Knockdown of G85R in SCs of G85R mice delayed disease onset and extended survival indicating that G85R expression in SCs is neurotoxic. These results demonstrate differences in the effect on disease of dismutase active vs. inactive mtSOD1 suggesting that both a loss as well as gain in function of mtSOD1 influence FALS pathogenesis. The results suggest that mtSOD1-induced FALS treatment may have to be adjusted depending on the cell type targeted and particular mtSOD1 involved.

A novel variant of human superoxide dismutase 1 harboring amyotrophic lateral sclerosis-associated and experimental mutations in metal-binding residues and free cysteines lacks toxicity in vivo

Mutations in superoxide dismutase 1 (SOD1) cause familial amyotrophic lateral sclerosis. The Cu-binding capacity of SOD1 has spawned hypotheses that implicate metal-mediated production of reactive species as a potential mechanism of toxicity. In past experiments, we have tested such hypotheses by mutating residues in SOD1 that normally coordinate the binding of Cu, finding that such mutants retain the capacity to induce motor neuron disease. We now describe the lack of disease in mice that express a variant of human SOD1 in which residues that coordinate the binding of Cu and Zn have been mutated (SODMD). SODMD encodes three disease-causing and four experimental mutations that ultimately eliminate all histidines involved in the binding of metals; and includes one disease-causing and one experimental mutation that eliminate secondary metal binding at C6 and C111. We show that the combined effect of these mutations produces a protein that is unstable but does not aggregate on its own, is not toxic, and does not induce disease when co-expressed with high levels of wild-type SOD1. In cell culture models, we determine that the combined mutation of C6 and C111 to G and S, respectively, dramatically reduces the aggregation propensity of SODMD and may account for the lack of toxicity for this mutant.

PATHOLOGIES OF AXONAL TRANSPORT IN NEURODEGENERATIVE DISEASES

Gene products such as organelles, proteins and RNAs are actively transported to synaptic terminals for the remodeling of pre-existing neuronal connections and formation of new ones. Proteins described as molecular motors mediate this transport and utilize specialized cytoskeletal proteins that function as molecular tracks for the motor based transport of cargos. Molecular motors such as kinesins and dynein's move along microtubule tracks formed by tubulins whereas myosin motors utilize tracks formed by actin. Deficits in active transport of gene products have been implicated in a number of neurological disorders. We describe such disorders collectively as "transportopathies". Here we review current knowledge of critical components of active transport and their relevance to neurodegenerative diseases.

Superoxide dismutase 1 encoding mutations linked to ALS adopts a spectrum of misfolded states

Background

Mutations in superoxide dismutase 1 (SOD1), which are one cause of familial amyotrophic lateral sclerosis (fALS), induce misfolding and aggregation of the protein. Misfolding can be detected by the binding of antibodies raised against peptide epitopes that are normally buried in the native conformation, shifts in solubility in non-ionic detergents, and the formation of macromolecular inclusions. In the present study, we investigate the relationship between detergent-insoluble and sedimentable forms of mutant SOD1, forms of mutant SOD1 with aberrantly accessible epitopes, and mutant protein in inclusions with the goal of defining the spectrum of misfolded states that mutant SOD1 can adopt.

Results

Using combined approaches in cultured cell models, we demonstrate that a substantial fraction of mutant SOD1 adopts a non-native conformation that remains soluble and freely mobile. We also show that mutant SOD1 can produce multimeric assemblies of which some are insoluble in detergent and large enough to sediment by ultracentrifugation and some are large enough to detect visually. Three conformationally restricted antibodies were found to be useful in discriminating mal-folded forms of mutant SOD1. An antibody termed C4F6 displays properties consistent with recognition of soluble, freely mobile, mal-folded mutant SOD1. An antibody termed SEDI, which recognizes C-terminal residues, detects larger inclusion structures as well as soluble misfolded entities. An antibody termed hSOD1, which recognizes aa 24-36, detects an epitope shared by soluble non-natively folded WT and mutant SOD1. This epitope becomes inaccessible in aggregates of mutant SOD1.

Conclusions

Our studies demonstrate how different methods of detecting misfolding and aggregation of mutant SOD1 reveal different forms of aberrantly folded protein. Immunological and biochemical methods can be used in combination to detect soluble and insoluble misfolded forms of mutant SOD1. Our findings support the view that mutant SOD1 can adopt multiple misfolded conformations with the potential that different structural variants mediate different aspects of fALS.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Neuronal intermediate filaments and neurodegenerative disorders

Intermediate filaments represent the most abundant cytoskeletal element in mature neurons. Mutations and/or accumulations of neuronal intermediate filament proteins are frequently observed in several human neurodegenerative disorders. Although it is now admitted that disorganization of the neurofilament network may be directly involved in neurodegeneration, certain type of perikaryal intermediate filament aggregates confer protection in motor neuron disease. The use of various mouse models provided a better knowledge of the role played by the disorganization of intermediate filaments in the pathogenesis of neurodegenerative disorders, but the mechanisms leading to the formation of these aggregates remain elusive. Here, we will review some neurodegenerative diseases involving intermediate filaments abnormalities and possible mechanisms susceptible to provoke them.

Variation in aggregation propensities among ALS-associated variants of SOD1: correlation to human disease

To date, 146 different mutations in superoxide dismutase 1 (SOD1) have been identified in patients with familial amyotrophic lateral sclerosis (ALS). The mean age of disease onset in patients inheriting mutations in SOD1 is 45-47 years of age. However, although the length of disease duration is highly variable, there are examples of consistent disease durations associated with specific mutations (e. g. A4V, less than 2 years). In the present study, we have used a large set of data from SOD1-associated ALS pedigrees to identify correlations between disease features and biochemical/biophysical properties of more than 30 different variants of mutant SOD1. Using a reliable cell culture assay, we show that all ALS-associated mutations in SOD1 increase the inherent aggregation propensity of the protein. However, the relative propensity to do so varied considerably among mutants. We were not able to explain the variation in aggregation rates by differences in known protein properties such as enzyme activity, protein thermostability, mutation position or degree of change in protein charge. Similarly, we were not able to explain variability in the duration of disease in SOD1-associated ALS pedigrees by these properties. However, we find that the majority of pedigrees in which patients exhibit reproducibly short disease durations are associated with mutations that show a high inherent propensity to induce aggregation of SOD1.

Role of mutant SOD1 disulfide oxidation and aggregation in the pathogenesis of familial ALS

Transgenic mice that model familial (f)ALS, caused by mutations in superoxide dismutase (SOD)1, develop paralysis with pathology that includes the accumulation of aggregated forms of the mutant protein. Using a highly sensitive detergent extraction assay, we traced the appearance and abundance of detergent-insoluble and disulfide cross-linked aggregates of SOD1 throughout the disease course of SOD1-fALS mice (G93A, G37R, and H46R/H48Q). We demonstrate that the accumulation of disulfide cross-linked, detergent-insoluble, aggregates of mutant SOD1 occurs primarily in the later stages of the disease, concurrent with the appearance of rapidly progressing symptoms. We find no evidence for a model in which aberrant intermolecular disulfide bonding has an important role in promoting the aggregation of mutant SOD1, instead, such cross-linking appears to be a secondary event. Also, using both cell culture and mouse models, we find that mutant protein lacking the normal intramolecular disulfide bond is a major component of the insoluble SOD1 aggregates. Overall, our findings suggest a model in which soluble forms of mutant SOD1 initiate disease with larger aggregates implicated only in rapidly progressing events in the final stages of disease. Within the final stages of disease, abnormalities in the oxidation of a normal intramolecular disulfide bond in mutant SOD1 facilitate the aggregation of mutant protein.

Potentiation of amyotrophic lateral sclerosis (ALS)-associated TDP-43 aggregation by the proteasome-targeting factor, ubiquilin 1

TDP-43 (43-kDa TAR DNA-binding domain protein) is a major constituent of ubiquitin-positive cytoplasmic aggregates present in neurons of patients with fronto-temporal lobular dementia and amyotrophic lateral sclerosis (ALS). The pathologic significance of TDP-43 aggregation is not known; however, dominant mutations in TDP-43 cause a subset of ALS cases, suggesting that misfolding and/or altered trafficking of TDP-43 is relevant to the disease process. Here, we show that the presenilin-binding protein ubiquilin 1 (UBQLN) plays a role in TDP-43 aggregation. TDP-43 interacted with UBQLN both in yeast and in vitro, and the carboxyl-terminal ubiquitin-associated domain of UBQLN was both necessary and sufficient for binding to polyubiquitylated forms of TDP-43. Overexpression of UBQLN recruited TDP-43 to detergent-resistant cytoplasmic aggregates that colocalized with the autophagosomal marker, LC3. UBQLN-dependent aggregation required the UBQLN UBA domain, was mediated by non-overlapping regions of TDP-43, and was abrogated by a mutation in UBQLN previously linked to Alzheimer disease. Four ALS-associated alleles of TDP-43 also coaggregated with UBQLN, and the extent of aggregation correlated with in vitro UBQLN binding affinity. Our findings suggest that UBQLN is a polyubiquitin-TDP-43 cochaperone that mediates the autophagosomal delivery and/or proteasome targeting of TDP-43 aggregates.

Wild-type SOD1 overexpression accelerates disease onset of a G85R SOD1 mouse

Approximately 10% of amyotrophic lateral sclerosis (ALS) cases are familial (FALS), and approximately 25% of FALS cases are caused by mutations in Cu/Zn superoxide dismutase type 1 (SOD1). Mutant (MT) SOD1 is thought to be pathogenic because it misfolds and aggregates. A number of transgenic mice have been generated that express different MTSOD1s as transgenes and exhibit an ALS-like disease. Although one study found that overexpression of human wild-type (WT) SOD1 did not affect disease in G85R transgenic mice, more recent reports claim that overexpression of WTSOD1 in other MTSOD1 transgenic mice hastened disease, raising a possibility that the effect of WTSOD1 overexpression in this FALS mouse model is mutant-specific. In order to clarify this issue, we studied the effect of WTSOD1 overexpression in a G85R transgenic mouse that we recently generated. We found that G85R/WTSOD1 double transgenic mice had an acceleration of disease onset and shortened survival compared with G85R single transgenic mice; in addition, there was an earlier appearance of pathological and immunohistochemical abnormalities. The spinal cord insoluble fraction from G85R/WTSOD1 mice had evidence of G85R-WTSOD1 heterodimers and WTSOD1 homodimers (in addition to G85R homodimers) with intermolecular disulfide bond cross-linking. These studies suggest that WTSOD1 can be recruited into disease-associated aggregates by redox processes, providing an explanation for the accelerated disease seen in G85R mice following WTSOD1 overexpression, and suggesting the importance of incorrect disulfide-linked protein as key to MTSOD1 toxicity.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterised by progressive muscular paralysis reflecting degeneration of motor neurones in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Incidence (average 1.89 per 100,000/year) and prevalence (average 5.2 per 100,000) are relatively uniform in Western countries, although foci of higher frequency occur in the Western Pacific. The mean age of onset for sporadic ALS is about 60 years. Overall, there is a slight male prevalence (M:F ratio approximately 1.5:1). Approximately two thirds of patients with typical ALS have a spinal form of the disease (limb onset) and present with symptoms related to focal muscle weakness and wasting, where the symptoms may start either distally or proximally in the upper and lower limbs. Gradually, spasticity may develop in the weakened atrophic limbs, affecting manual dexterity and gait. Patients with bulbar onset ALS usually present with dysarthria and dysphagia for solid or liquids, and limbs symptoms can develop almost simultaneously with bulbar symptoms, and in the vast majority of cases will occur within 1-2 years. Paralysis is progressive and leads to death due to respiratory failure within 2-3 years for bulbar onset cases and 3-5 years for limb onset ALS cases. Most ALS cases are sporadic but 5-10% of cases are familial, and of these 20% have a mutation of the SOD1 gene and about 2-5% have mutations of the TARDBP (TDP-43) gene. Two percent of apparently sporadic patients have SOD1 mutations, and TARDBP mutations also occur in sporadic cases. The diagnosis is based on clinical history, examination, electromyography, and exclusion of 'ALS-mimics' (e.g. cervical spondylotic myelopathies, multifocal motor neuropathy, Kennedy's disease) by appropriate investigations. The pathological hallmarks comprise loss of motor neurones with intraneuronal ubiquitin-immunoreactive inclusions in upper motor neurones and TDP-43 immunoreactive inclusions in degenerating lower motor neurones. Signs of upper motor neurone and lower motor neurone damage not explained by any other disease process are suggestive of ALS. The management of ALS is supportive, palliative, and multidisciplinary. Non-invasive ventilation prolongs survival and improves quality of life. Riluzole is the only drug that has been shown to extend survival.

An ALS-linked mutant SOD1 produces a locomotor defect associated with aggregation and synaptic dysfunction when expressed in neurons of Caenorhabditis elegans

The nature of toxic effects exerted on neurons by misfolded proteins, occurring in a number of neurodegenerative diseases, is poorly understood. One approach to this problem is to measure effects when such proteins are expressed in heterologous neurons. We report on effects of an ALS-associated, misfolding-prone mutant human SOD1, G85R, when expressed in the neurons of Caenorhabditis elegans. Stable mutant transgenic animals, but not wild-type human SOD1 transgenics, exhibited a strong locomotor defect associated with the presence, specifically in mutant animals, of both soluble oligomers and insoluble aggregates of G85R protein. A whole-genome RNAi screen identified chaperones and other components whose deficiency increased aggregation and further diminished locomotion. The nature of the locomotor defect was investigated. Mutant animals were resistant to paralysis by the cholinesterase inhibitor aldicarb, while exhibiting normal sensitivity to the cholinergic agonist levamisole and normal muscle morphology. When fluorescently labeled presynaptic components were examined in the dorsal nerve cord, decreased numbers of puncta corresponding to neuromuscular junctions were observed in mutant animals and brightness was also diminished. At the EM level, mutant animals exhibited a reduced number of synaptic vesicles. Neurotoxicity in this system thus appears to be mediated by misfolded SOD1 and is exerted on synaptic vesicle biogenesis and/or trafficking.

A new animal model of the human neurodegenerative disease amyotrophic lateral sclerosis (ALS; Lou Gehrig's Disease) is presented. Two percent of ALS cases result from heritable mutations affecting the abundant enzyme superoxide dismutase (SOD1). Such mutations have been indicated to impair the folding and stability of the enzyme, leading it to misfold and aggregate in motor neurons, associated with the paralyzing disease. Here, when a mutant form of human SOD1 was produced in neurons of C. elegans worms, it led to a severe locomotor defect—the worms were essentially paralyzed. The protein formed aggregates in the neurons, including an intermediate form of aggregate, soluble oligomers, that has been linked to toxicity to cells. By contrast, worms expressing the normal version of human SOD1 protein exhibited normal movement and no aggregation. The movement defect was further analyzed using chemical inhibitors and found to result from defective function of synapses, the connections made between neurons, and between neurons and muscle. Finally, in a screen using RNA interference, we observed that the worms' aggregation and locomotor condition was worsened when a class of molecules called molecular chaperones, which assist protein folding in the cell, were impaired in function. This is consistent with the idea that misfolded SOD1 is directly involved with causing the neuronal dysfunction.

A drosophila model for amyotrophic lateral sclerosis reveals motor neuron damage by human SOD1

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease that leads to loss of motor function and early death. About 5% of cases are inherited, with the majority of identified linkages in the gene encoding copper, zinc-superoxide dismutase (SOD1). Strong evidence indicates that the SOD1 mutations confer dominant toxicity on the protein. To provide new insight into mechanisms of ALS, we have generated and characterized a model for familial ALS in Drosophila with transgenic expression of human SOD1. Expression of wild type or disease-linked (A4V, G85R) mutants of human SOD1 selectively in motor neurons induced progressive climbing deficits. These effects were accompanied by defective neural circuit electrophysiology, focal accumulation of human SOD1 protein in motor neurons, and a stress response in surrounding glia. However, toxicity was not associated with oligomerization of SOD1 and did not lead to neuronal loss. These studies uncover cell-autonomous injury by SOD1 to motor neurons in vivo, as well as non-autonomous effects on glia, and provide the foundation for new insight into injury and protection of motor neurons in ALS.

Activation of brain calcineurin (Cn) by Cu-Zn superoxide dismutase (SOD1) depends on direct SOD1-Cn protein interactions occurring in vitro and in vivo

Cn (calcineurin) activity is stabilized by SOD1 (Cu-Zn superoxide dismutase), a phenomenon attributed to protection from superoxide (O2*-). The effects of O2*- on Cn are still controversial. We found that O2*-, generated either in vitro or in vivo did not affect Cn activity. Yet native bovine, recombinant human or rat, and two chimaeras of human SOD1-rat SOD1, all activated Cn, but SOD2 (Mn-superoxide dismutase) did not affect Cn activity. There was also a poor correlation between SOD1 dismutase activity and Cn activation. A chimaera of human N-terminal SOD1 and rat C-terminal SOD1 had little detectable dismutase activity, yet stimulated Cn activity the same as full-length human or rat SOD1. Nevertheless, there was evidence that the active site of SOD1 was involved in Cn activation based on the loss of activation following chelation of Cu from the active site of SOD1. Also, SOD1 engaged in the catalysis of O2*- dismutation was ineffective in activating Cn. SOD1 activation of Cn resulted from a 90-fold decrease in phosphatase K(m) without a change in V(max). A possible mechanism for the activation of Cn was identified in our studies as the prevention of Fe and Zn losses from the active site of Cn, suggesting a conformation-dependent SOD1-Cn interaction. In neurons, SOD1 and Cn were co-localized in cytoplasm and membranes, and SOD1 co-immunoprecipitated with Cn from homogenates of brain hippocampus and was present in immunoprecipitates as large multimers. Pre-incubation of pure SOD1 with Cn caused SOD1 multimer formation, an indication of an altered conformational state in SOD1 upon interaction with Cn.

Rapid and intermittent cotransport of slow component-b proteins

After synthesis in neuronal perikarya, proteins destined for synapses and other distant axonal sites are transported in three major groups that differ in average velocity and protein composition: fast component (FC), slow component-a (SCa), and slow component-b (SCb). The FC transports mainly vesicular cargoes at average rates of approximately 200-400 mm/d. SCa transports microtubules and neurofilaments at average rates of approximately 0.2-1 mm/d, whereas SCb translocates approximately 200 diverse proteins critical for axonal growth, regeneration, and synaptic function at average rates of approximately 2-8 mm/d. Several neurodegenerative diseases are characterized by abnormalities in one or more SCb proteins, but little is known about mechanisms underlying SCb compared with FC and SCa. Here, we use live-cell imaging to visualize and quantify the axonal transport of three SCb proteins, alpha-synuclein, synapsin-I, and glyceraldehyde-3-phosphate dehydrogenase in cultured hippocampal neurons, and directly compare their transport to synaptophysin, a prototypical FC protein. All three SCb proteins move rapidly but infrequently with pauses during transit, unlike synaptophysin, which moves much more frequently and persistently. By simultaneously visualizing the transport of proteins at high temporal and spatial resolution, we show that the dynamics of alpha-synuclein transport are distinct from those of synaptophysin but similar to other SCb proteins. Our observations of the cotransport of multiple SCb proteins in single axons suggest that they move as multiprotein complexes. These studies offer novel mechanistic insights into SCb and provide tools for further investigating its role in disease processes.

The genetics of axonal transport and axonal transport disorders

Neurons are specialized cells with a complex architecture that includes elaborate dendritic branches and a long, narrow axon that extends from the cell body to the synaptic terminal. The organized transport of essential biological materials throughout the neuron is required to support its growth, function, and viability. In this review, we focus on insights that have emerged from the genetic analysis of long-distance axonal transport between the cell body and the synaptic terminal. We also discuss recent genetic evidence that supports the hypothesis that disruptions in axonal transport may cause or dramatically contribute to neurodegenerative diseases.

Oxidative stress in ALS: A mechanism of neurodegeneration and a therapeutic target

The cause(s) of amyotrophic lateral sclerosis (ALS) is not fully understood in the vast majority of cases and the mechanisms involved in motor neuron degeneration are multi-factorial and complex. There is substantial evidence to support the hypothesis that oxidative stress is one mechanism by which motor neuron death occurs. This theory becomes more persuasive with the discovery that mutation of the anti-oxidant enzyme, superoxide dismutase 1 (SOD1), causes disease in a significant minority of cases. However, the precise mechanism(s) by which mutant SOD1 leads to motor neuron degeneration have not been defined with certainty, and trials of anti-oxidant therapies have been disappointing. Here, we review the evidence implicating oxidative stress in ALS pathogenesis, discuss how oxidative stress may affect and be affected by other proposed mechanisms of neurodegeneration, and review the trials of various anti-oxidants as potential therapies for ALS.

Molecular biology of amyotrophic lateral sclerosis: insights from genetics

Amyotrophic lateral sclerosis (ALS) is a paralytic disorder caused by motor neuron degeneration. Mutations in more than 50 human genes cause diverse types of motor neuron pathology. Moreover, defects in five Mendelian genes lead to motor neuron disease, with two mutations reproducing the ALS phenotype. Analyses of these genetic effects have generated new insights into the diverse molecular pathways involved in ALS pathogenesis. Here, we present an overview of the mechanisms for motor neuron death and of the role of non-neuronal cells in ALS.

Disease progression of human SOD1 (G93A) transgenic ALS model rats

The recent development of a rat model of amyotrophic lateral sclerosis (ALS) in which the rats harbor a mutated human SOD1 (G93A) gene has greatly expanded the range of potential experiments, because the rats' large size permits biochemical analyses and therapeutic trials, such as the intrathecal injection of new drugs and stem cell transplantation. The precise nature of this disease model remains unclear. We described three disease phenotypes: the forelimb-, hindlimb-, and general-types. We also established a simple, non-invasive, and objective evaluation system using the body weight, inclined plane test, cage activity, automated motion analysis system (SCANET), and righting reflex. Moreover, we created a novel scale, the Motor score, which can be used with any phenotype and does not require special apparatuses. With these methods, we uniformly and quantitatively assessed the onset, progression, and disease duration, and clearly presented the variable clinical course of this model; disease progression after the onset was more aggressive in the forelimb-type than in the hindlimb-type. More importantly, the disease stages defined by our evaluation system correlated well with the loss of spinal motor neurons. In particular, the onset of muscle weakness coincided with the loss of approximately 50% of spinal motor neurons. This study should provide a valuable tool for future experiments to test potential ALS therapies.

ATF3 expression precedes death of spinal motoneurons in amyotrophic lateral sclerosis-SOD1 transgenic mice and correlates with c-Jun phosphorylation, CHOP expression, somato-dendritic ubiquitination and Golgi fragmentation

To obtain insight into the morphological and molecular correlates of motoneuron degeneration in amyotrophic lateral sclerosis (ALS) mice that express G93A mutant superoxide dismutase (SOD)1 (G93A mice), we have mapped and characterized 'sick' motoneurons labelled by the 'stress transcription factors' ATF3 and phospho-c-Jun. Immunocytochemistry and in situ hybridization showed that a subset of motoneurons express ATF3 from a relatively early phase of disease before the onset of active caspase 3 expression and motoneuron loss. The highest number of ATF3-expressing motoneurons occurred at symptom onset. The onset of ATF3 expression correlated with the appearance of ubiquitinated neurites. Confocal double-labelling immunofluorescence showed that all ATF3-positive motoneurons were immunoreactive for phosphorylated c-Jun. Furthermore, the majority of ATF3 and phospho-c-Jun-positive motoneurons were also immunoreactive for CHOP (GADD153) and showed Golgi fragmentation. A subset of ATF3 and phosphorylated c-Jun-immunoreactive motoneurons showed an abnormal appearance characterized by a number of distinctive features, including an eccentric flattened nucleus, perikaryal accumulation of ubiquitin immunoreactivity, juxta-nuclear accumulation of the Golgi apparatus and the endoplasmic reticulum, and intense Hsp70 immunoreactivity. These abnormal cells were not immunoreactive for active caspase 3. We conclude that motoneurons in ALS-SOD1 mice prior to their death and disappearance experience a prolonged sick phase, characterized by the gradual accumulation of ubiquitinated material first in the neurites and subsequently the cell body.

The WldS gene modestly prolongs survival in the SOD1G93A fALS mouse

The "slow Wallerian degeneration" (Wld(S)) gene is neuroprotective in numerous models of axonal degeneration. Axonal degeneration is an early feature of disease progression in the SOD1G93A mouse, a widely used model of familial amyotrophic lateral sclerosis (fALS). We crossed the Wld(S) mouse with the SOD1G93A mouse to investigate whether the Wld(S) gene could prolong survival and modify neuropathology in these mice. SOD/Wld(S) mice showed levels of motor axon loss similar to that seen in SOD1G93A mice. The presence of the Wld(S) gene, however, modestly prolonged survival and delayed denervation at the neuromuscular junction. Prolonged survival was more prominent in female mice and did not depend on whether animals were heterozygous or homozygous for the Wld(S) gene. We also report that SOD1G93A mice show significant degeneration of sensory axons during the course of disease, supporting previous data from humans demonstrating that ALS is not purely a motor disorder.

Somatodendritic accumulation of misfolded SOD1-L126Z in motor neurons mediates degeneration: alphaB-crystallin modulates aggregation

Mice expressing variants of superoxide dismutase-1 (SOD1) encoding C-terminal truncation mutations linked to familial amyotrophic lateral sclerosis (FALS) have begun to define the role of misfolding and aggregation in the pathogenesis of disease. Here, we examine transgenic mice expressing SOD1-L126Z (Z = stop-truncation of last 28 amino acids), finding that detergent-insoluble mutant protein specifically accumulates in somatodendritic compartments. Soluble forms of the SOD1-L126Z were virtually undetectable in spinal cord at any age and the levels of accumulated protein directly correlated with disease symptoms. Neither soluble nor insoluble forms of SOD1-L126Z were transported to distal axons. In vitro, small heat shock protein (Hsp) alphaB-crystallin suppressed the in vitro aggregation of SOD1-L126Z. In vivo, alphaB-crystallin immunoreactivity was most abundant in oligodendrocytes and up-regulated in astrocytes of symptomatic mice; neither of these cell-types accumulated mutant SOD1 immunoreactivity. These results suggest that damage to motor neuron cell bodies and dendrites within the spinal cord can be sufficient to induce motor neuron disease and that the activities of chaperones may modulate the cellular specificity of mutant SOD1 accumulation.

Amyotrophic Lateral Sclerosis Mutations Have the Greatest Destabilizing Effect on the Apo- and Reduced Form of SOD1, Leading to Unfolding and Oxidative Aggregation

Mutant forms of Cu,Zn-superoxide dismutase (SOD1) that cause familial amyotrophic lateral sclerosis (ALS) exhibit toxicity that promotes the death of motor neurons. Proposals for the toxic properties typically involve aberrant catalytic activities or protein aggregation. The striking thermodynamic stability of mature forms of the ALS mutant SOD1 (Tm>70 degrees C) is not typical of protein aggregation models that involve unfolding. Over 44 states of the polypeptide are possible, depending upon metal occupancy, disulfide status, and oligomeric state; however, it is not clear which forms might be responsible for toxicity. Recently the intramolecular disulfide has been shown to be required for SOD1 activity, leading us to examine these states of several disease-causing SOD1 mutants. We find that ALS mutations have the greatest effect on the most immature form of SOD1, destabilizing the metal-free and disulfide-reduced polypeptide to the point that it is unfolded at physiological temperatures (Tm<37 degrees C). We also find that immature states of ALS mutant (but not wild type) proteins readily form oligomers at physiological concentrations. Furthermore, these oligomers are more susceptible to mild oxidative stress, which promotes incorrect disulfide cross-links between conserved cysteines and drives aggregation. Thus it is the earliest disulfide-reduced polypeptides in the SOD1 assembly pathway that are most destabilized with respect to unfolding and oxidative aggregation by ALS-causing mutations.

BACE1 cytoplasmic domain interacts with the copper chaperone for superoxide dismutase-1 and binds copper

The amyloidogenic pathway leading to the production and deposition of Abeta peptides, major constituents of Alzheimer disease senile plaques, is linked to neuronal metal homeostasis. The amyloid precursor protein binds copper and zinc in its extracellular domain, and the Abeta peptides also bind copper, zinc, and iron. The first step in the generation of Abeta is cleavage of amyloid precursor protein by the aspartic protease BACE1. Here we show that BACE1 interacts with CCS (the copper chaperone for superoxide dismutase-1 (SOD1)) through domain I and the proteins co-immunoprecipitate from rat brain extracts. We have also been able to visualize the co-transport of membranous BACE1 and soluble CCS through axons. BACE1 expression reduces the activity of SOD1 in cells consistent with direct competition for available CCS as overexpression of CCS restores SOD1 activity. Finally, we demonstrate that the twenty-four residue C-terminal domain of BACE1 binds a single Cu(I) atom with high affinity through cysteine residues.

Effect of ubiquitin expression on neuropathogenesis in a mouse model of familial amyotrophic lateral sclerosis

The ubiquitin-proteasome system (UPS) is a central component in the cellular defence against potentially toxic protein aggregates. UPS dysfunction is linked to the pathogenesis of both sporadic and inherited neurodegenerative diseases, including dominantly inherited familial amyotrophic lateral sclerosis (fALS). To investigate the role of the UPS in fALS pathogenesis, transgenic mice expressing mutant G9 3A Cu,Zn superoxide dismutase (SOD1) were crossed with transgenic mice expressing epitope tagged, wild-type or dominant-negative mutant ubiquitin (Ub(K48R)). RNase protection assays were used to confirm expression of the Ub transgenes in spinal cord and ubiquitin transgene levels were estimated to account for 9-12% of total ubiquitin. Mice expressing the G9 3A transgene exhibited neurological symptoms and histopathological changes typical of this model irrespective of ubiquitin transgene status. Impaired rotarod performance was observed in all G9 3A transgenics by 7 weeks of age irrespective of ubiquitin genotype. The presence of wild-type or mutant ubiquitin transgenes resulted in a small but significant delay in the onset of clinical symptoms and mild acceleration of disease progression, without influencing overall survival. These data suggest that relatively small changes in ubiquitin expression can influence the development of neurodegenerative disease and are consistent with a neuroprotective role for the UPS.

Cyclin-dependent kinase inhibitors attenuate protein hyperphosphorylation, cytoskeletal lesion formation, and motor defects in Niemann-Pick Type C mice

Dysregulation of cyclin-dependent kinases (cdks) and cytoskeletal protein hyperphosphorylation characterizes a subset of human neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, and Niemann-Pick Type C (NPC). It is thought that these cytoskeletal changes lead eventually to development of hallmark cytoskeletal lesions such as neurofibrillary tangles and axonal spheroids. Although many studies support an involvement of cdks in these neurodegenerative cascades, it is not known whether cdk activity is essential. The naturally occurring npc-1 mutant mouse mimics human NPC, in displaying activation of cdk5, mitotic cdc2, and cdk4, with concomitant cytoskeletal pathology and neurodegeneration. We availed of this model and specific pharmacological inhibitors of cdk activity, to determine whether cdks are necessary for NPC neuropathology. The inhibitors were infused intracerebroventricularly for a 2-week period, initiated at a pathologically incipient stage. While an inactive stereoisomer, iso-olomoucine, was ineffective, two potent inhibitors, roscovitine and olomoucine, attenuated significantly the hyperphosphorylation of neurofilament, tau, and mitotic proteins, reduced the number of spheroids, modulated Purkinje neuron death, and ameliorated motor defects in npc mice. These results suggest that cdk activity is required for neuropathology and subsequent motor impairment in NPC. Studies aimed at knocking down individual cdks in these mice will help identify the specific cdk(s) that are essential, and delineate their precise roles in the neurodegenerative process.

The G336S variant in the human neurofilament-M gene does not affect its assembly or distribution: importance of the functional analysis of neurofilament variants

The human neurofilament medium (hNFM) subunit is one of the 3 neurofilament (NF) polypeptides, which are the most abundant intermediate filament (IF) proteins in post-mitotic neurons. The formation of neurofilamentous aggregates is a pathological hallmark of many neurodegenerative diseases, including the Lewy bodies found in Parkinson disease (PD). A Gly336Ser (G336S) variant in the rod domain of hNFM has recently been described in a patient with early-onset autosomal-dominant PD. In this study, we have generated a mammalian expression vector encoding the variant hNFM cDNA and characterized its effects on the formation of heteropolymeric IFs in heterologous cell lines. We have also investigated the distribution of the (G336S) hNFM variant protein in neuronal CAD cells, as well as the effects of the variant on the distribution of other cellular organelles and proteins. Our results demonstrate that the G336S variant does not affect the formation of IF networks nor the distribution of the variant hNFM protein. Our data suggest that if the G336S variant is involved in the development of PD, it does not appear to be due to defects in the assembly and distribution of NFs.

A novel CDK5-dependent pathway for regulating GSK3 activity and kinesin-driven motility in neurons

Neuronal transmission of information requires polarized distribution of membrane proteins within axonal compartments. Membrane proteins are synthesized and packaged in membrane-bounded organelles (MBOs) in neuronal cell bodies and later transported to axons by microtubule-dependent motor proteins. Molecular mechanisms underlying targeted delivery of MBOs to discrete axonal subdomains (i.e. nodes of Ranvier or presynaptic terminals) are poorly understood, but regulatory pathways for microtubule motors may be an essential step. In this work, pharmacological, biochemical and in vivo experiments define a novel regulatory pathway for kinesin-driven motility in axons. This pathway involves enzymatic activities of cyclin-dependent kinase 5 (CDK5), protein phosphatase 1 (PP1) and glycogen synthase kinase-3 (GSK3). Inhibition of CDK5 activity in axons leads to activation of GSK3 by PP1, phosphorylation of kinesin light chains by GSK3 and detachment of kinesin from transported cargoes. We propose that regulating the activity and localization of components in this pathway allows nerve cells to target organelle delivery to specific subcellular compartments. Implications of these findings for pathogenesis of neurodegenerative diseases such as Alzheimer's disease are discussed.

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.

Axonal transport of human α-synuclein slows with aging but is not affected by familial Parkinson's disease-linked mutations

Biochemical and genetic abnormalities of alpha-synuclein (alpha-Syn) are implicated in the pathogenesis of Parkinson's disease (PD) and other alpha-synucleinopathies. The abnormal intraneuronal accumulations of alpha-Syn in Lewy bodies (LBs) and Lewy neurites (LNs) have implicated defects in axonal transport of alpha-Syn in the alpha-synucleinopathies. Using human (Hu) alpha-Syn transgenic (Tg) mice, we have examined whether familial PD (FPD)-linked mutations (A30P and A53T) alter axonal transport of Hualpha-Syn. Our studies using peripheral nerves show that Hualpha-Syn and Moalpha-Syn are almost exclusively transported in the slow component (SC) of axonal transport and that the FPD-linked alpha-Syn mutations do not have obvious effects on the axonal transport of alpha-Syn. Moreover, older pre-symptomatic A53T Hualpha-Syn Tg mice do not show gross alterations in the axonal transport of alpha-Syn and other proteins in the SC, indicating that the early stages of alpha-synucleinopathy in A53T alpha-Syn Tg mice are not associated with gross alterations in the slow axonal transport. However, the axonal transport of alpha-Syn slows significantly with aging. Because the rate of axonal transport affects the stability and accumulation of proteins in axons, age-dependent-slowing alpha-Syn is a likely contributor to axonal aggregation of alpha-Syn in alpha-synucleinopathy.

Neuromuscular accumulation of mutant superoxide dismutase 1 aggregates in a transgenic mouse model of familial amyotrophic lateral sclerosis

Superoxide dismutase 1 (SOD1) aggregates are a histological and biochemical correlate of disease progression in neural tissues from mutant SOD1-linked forms of familial amyotrophic lateral sclerosis (FALS). In the present study, we assayed the monomeric and high molecular weight mutant SOD1 content of nervous, muscle and visceral tissues from transgenic SOD1(G93A) mice using immunoblotting and zymograms. A progressive age-dependent increase in mutant SOD1 level, aggregation and stabilisation by cross-species heterodimers was determined in lumbar spinal cord, sciatic nerve and gastrocnemius muscle. Such biochemical abnormalities were not present in cervical spinal cord, brainstem and diaphragm muscle, nor common to endogenous mouse SOD1. Mutant dismutase activity in general did not increase correspondingly with accumulating protein at later ages. These results suggest that peripheral targets such as hindlimb skeletal muscle and nerve accumulate mutant SOD1 aggregates and may therefore be susceptible to mutant SOD1-mediated toxicity, in addition to lower and upper motor neurons of the central nervous system in transgenic FALS mice.