Different human copper-zinc superoxide dismutase mutants, SOD1G93A and SOD1H46R, exert distinct harmful effects on gross phenotype in 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.

Human Motor Neurons With SOD1-G93A Mutation Generated From CRISPR/Cas9 Gene-Edited iPSCs Develop Pathological Features of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by gradual degeneration and elimination of motor neurons (MNs) in the motor cortex, brainstem, and spinal cord. Some familial forms of ALS are caused by genetic mutations in superoxide dismutase 1 (SOD1) but the mechanisms driving MN disease are unclear. Identifying the naturally occurring pathology and understanding how this mutant SOD1 can affect MNs in translationally meaningful ways in a valid and reliable human cell model remains to be established. Here, using CRISPR/Cas9 genome editing system and human induced pluripotent stem cells (iPSCs), we generated highly pure, iPSC-derived MNs with a SOD1-G93A missense mutation. With the wild-type cell line serving as an isogenic control and MNs from a patient-derived iPSC line with an SOD1-A4V mutation as a comparator, we identified pathological phenotypes relevant to ALS. The mutant MNs accumulated misfolded and aggregated forms of SOD1 in cell bodies and processes, including axons. They also developed distinctive axonal pathologies. Mutants had axonal swellings with shorter axon length and less numbers of branch points. Moreover, structural and molecular abnormalities in presynaptic and postsynaptic size and density were found in the mutants. Finally, functional studies with microelectrode array demonstrated that the individual mutant MNs exhibited decreased number of spikes and diminished network bursting, but increased burst duration. Taken together, we identified spontaneous disease phenotypes relevant to ALS in mutant SOD1 MNs from genome-edited and patient-derived iPSCs. Our findings demonstrate that SOD1 mutations in human MNs cause cell-autonomous proteinopathy, axonopathy, synaptic pathology, and aberrant neurotransmission.

Antioxidant Alternatives in the Treatment of Amyotrophic Lateral Sclerosis: A Comprehensive Review

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that produces a selective loss of the motor neurons of the spinal cord, brain stem and motor cortex. Oxidative stress (OS) associated with mitochondrial dysfunction and the deterioration of the electron transport chain has been shown to be a factor that contributes to neurodegeneration and plays a potential role in the pathogenesis of ALS. The regions of the central nervous system affected have high levels of reactive oxygen species (ROS) and reduced antioxidant defenses. Scientific studies propose treatment with antioxidants to combat the characteristic OS and the regeneration of nicotinamide adenine dinucleotide (NAD+) levels by the use of precursors. This review examines the possible roles of nicotinamide riboside and pterostilbene as therapeutic strategies in ALS.

Biomechanistic insights into the roles of oxidative stress in generating complex neurological disorders

Neurological diseases like Alzheimer's disease, epilepsy, parkinsonism, depression, Huntington's disease and amyotrophic lateral sclerosis prevailing globally are considered to be deeply influenced by oxidative stress-based changes in the biochemical settings of the organs. The excess oxygen concentration triggers the production of reactive oxygen species, and even the intrinsic antioxidant enzyme system, i.e. SOD, CAT and GSHPx, fails to manage their levels and keep them under desirable limits. This consequently leads to oxidation of protein, lipids and nucleic acids in the brain resulting in apoptosis, proteopathy, proteasomes and mitochondrion dysfunction, glial cell activation as well as neuroinflammation. The present exploration deals with the evidence-based mechanism of oxidative stress towards development of key neurological diseases along with the involved biomechanistics and biomaterials.

Systemic overexpression of SQSTM1/p62 accelerates disease onset in a SOD1H46R-expressing ALS mouse model

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by a selective loss of upper and lower motor neurons. Recent studies have shown that mutations in SQSTM1 are linked to ALS. SQSTM1 encodes SQSTM1/p62 that regulates not only autophagy via the association with MAP1LC3/LC3 and ubiquitinated proteins but also the KEAP1-NFE2L2/Nrf2 anti-oxidative stress pathway by interacting with KEAP1. Previously, we have demonstrated that loss of SQSTM1 exacerbates disease phenotypes in a SOD1^H46R-expressing ALS mouse model. To clarify the effects of SQSTM1 overexpression in this model, we generated SQSTM1 and SOD1 ^H46R double-transgenic (SQSTM1;SOD1 ^H46R ) mice. SQSTM1;SOD1 ^H46R mice exhibited earlier disease onset and shorter lifespan than did SOD1 ^H46R mice. Conversely, disease progression after the onset rather slightly but significantly slowed in SQSTM1;SOD1 ^H46R mice. However, there were observable differences neither in the number of Nissl positive neurons nor in the distribution of ubiquitin-positive and/or SQSTM1-positive aggregates between SOD1 ^H46R and SQSTM1;SOD1 ^H46R mice. It was noted that these protein aggregates were mainly observed in neuropil, and partly localized to astrocytes and/or microglia, but not to MAP2-positive neuronal cell bodies and dendrites at the end-stage of disease. Nonetheless, the biochemically-detectable insoluble SQSTM1 and poly-ubiquitinated proteins were significantly and progressively increased in the spinal cord of SQSTM1;SOD1 ^H46R mice compared to SOD1 ^H46R mice. These results suggest that overexpression of SQSTM1 in SOD1 ^H46R mice accelerates disease onset by compromising the protein degradation pathways.

Functional links between SQSTM1 and ALS2 in the pathogenesis of ALS: cumulative impact on the protection against mutant SOD1-mediated motor dysfunction in mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by a selective loss of motor neurons in the brain and spinal cord. Multiple toxicity pathways, such as oxidative stress, misfolded protein accumulation, and dysfunctional autophagy, are implicated in the pathogenesis of ALS. However, the molecular basis of the interplay between such multiple factors in vivo remains unclear. Here, we report that two independent ALS-linked autophagy-associated gene products; SQSTM1/p62 and ALS2/alsin, but not antioxidant-related factor; NFE2L2/Nrf2, are implicated in the pathogenesis in mutant SOD1 transgenic ALS models. We generated SOD1^H46R mice either on a Nfe2l2-null, Sqstm1-null, or Sqstm1/Als2-double null background. Loss of SQSTM1 but not NFE2L2 exacerbated disease symptoms. A simultaneous inactivation of SQSTM1 and ALS2 further accelerated the onset of disease. Biochemical analyses revealed that loss of SQSTM1 increased the level of insoluble SOD1 at the intermediate stage of the disease, whereas no further elevation occurred at the end-stage. Notably, absence of SQSTM1 rather suppressed the mutant SOD1-dependent accumulation of insoluble polyubiquitinated proteins, while ALS2 loss enhanced it. Histopathological examinations demonstrated that loss of SQSTM1 accelerated motor neuron degeneration with accompanying the preferential accumulation of ubiquitin-positive aggregates in spinal neurons. Since SQSTM1 loss is more detrimental to SOD1^H46R mice than lack of ALS2, the selective accumulation of such aggregates in neurons might be more insulting than the biochemically-detectable insoluble proteins. Collectively, two ALS-linked factors, SQSTM1 and ALS2, have distinct but additive protective roles against mutant SOD1-mediated toxicity by modulating neuronal proteostasis possibly through the autophagy-endolysosomal system.

Differences in protein quality control correlate with phenotype variability in 2 mouse models of familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disease of variable severity in terms of speed of progression of the disease course. We found a similar variability in disease onset and progression of 2 familial ALS mouse strains, despite the fact that they carry the same transgene copy number and express the same amount of mutant SOD1G93A messenger RNA and protein in the central nervous system. Comparative analysis of 2 SOD1G93A mouse strains highlights differences associated with the disease severity that are unrelated to the degree of motor neuron loss but that appear to promote early dysfunction of these cells linked to protein aggregation. Features of fast progressing phenotype are (1) abundant protein aggregates containing mutant SOD1 and multiple chaperones; (2) low basal expression of the chaperone alpha-B-crystallin (CRYAB) and β5 subunits of proteasome; and (3) downregulation of proteasome subunit expression at disease onset. In contrast, high levels of functional chaperones such as cyclophillin-A and CRYAB, combined with delayed alteration of expression of proteasome subunits and the sequestration of TDP43 into aggregates, are features associated with a more slowly progressing pathology. These data support the hypothesis that impairment of protein homeostasis caused by low-soluble chaperone levels, together with malfunction of the proteasome degradation machinery, contributes to accelerate motor neuron dysfunction and progression of disease symptoms. Therefore, modulating the activity of these systems could represent a rational therapeutic strategy for slowing down disease progression in SOD1-related ALS.

Microglia centered pathogenesis in ALS: insights in cell interconnectivity

Amyotrophic lateral sclerosis (ALS) is the most common and most aggressive form of adult motor neuron (MN) degeneration. The cause of the disease is still unknown, but some protein mutations have been linked to the pathological process. Loss of upper and lower MNs results in progressive muscle paralysis and ultimately death due to respiratory failure. Although initially thought to derive from the selective loss of MNs, the pathogenic concept of non-cell-autonomous disease has come to the forefront for the contribution of glial cells in ALS, in particular microglia. Recent studies suggest that microglia may have a protective effect on MN in an early stage. Conversely, activated microglia contribute and enhance MN death by secreting neurotoxic factors, and impaired microglial function at the end-stage may instead accelerate disease progression. However, the nature of microglial–neuronal interactions that lead to MN degeneration remains elusive. We review the contribution of the neurodegenerative network in ALS pathology, with a special focus on each glial cell type from data obtained in the transgenic SOD1G93A rodents, the most widely used model. We further discuss the diverse roles of neuroinflammation and microglia phenotypes in the modulation of ALS pathology. We provide information on the processes associated with dysfunctional cell–cell communication and summarize findings on pathological cross-talk between neurons and astroglia, and neurons and microglia, as well as on the spread of pathogenic factors. We also highlight the relevance of neurovascular disruption and exosome trafficking to ALS pathology. The harmful and beneficial influences of NG2 cells, oligodendrocytes and Schwann cells will be discussed as well. Insights into the complex intercellular perturbations underlying ALS, including target identification, will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression of this devastating disease.

Genetically altering organismal metabolism by leptin-deficiency benefits a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, neurodegenerative disease that causes death of motor neurons. ALS patients and mouse models of familial ALS display organismal level metabolic dysfunction, which includes increased energy expenditure despite decreased lean mass. The pathophysiological relevance of abnormal energy homeostasis to motor neuron disease remains unclear. Leptin is an adipocyte-derived hormone that regulates whole-animal energy expenditure. Here, we report that placing mutant superoxide dismutase 1 (SOD1) mice in a leptin-deficient background improves energy homeostasis and slows disease progression. Leptin-deficient mutant SOD1 mice possess increased bodyweight and fat mass, as well as decreased energy expenditure. These observations coincide with enhanced survival, improved strength and decreased motor neuron loss. These results suggest that altering whole-body energy metabolism in mutant SOD1 mice can mitigate disease progression. We propose that manipulations that increase fat mass and reduce energy expenditure will be beneficial in the setting of motor neuron disease.

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

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

Microglia and motor neurons during disease progression in the SOD1G93A mouse model of amyotrophic lateral sclerosis: changes in arginase1 and inducible nitric oxide synthase

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting the motor system. Although the etiology of the disease is not fully understood, microglial activation and neuroinflammation are thought to play a role in disease progression.

METHODS

We examined the immunohistochemical expression of two markers of microglial phenotype, the arginine-metabolizing enzymes inducible nitric oxide synthase (iNOS) and arginase1 (Arg1), in the spinal cord of a mouse model carrying an ALS-linked mutant human superoxide dismutase transgene (SOD1(G93A)) and in non-transgenic wild-type (WT) mice. Immunolabeling for iNOS and Arg1 was evaluated throughout disease progression (6 to 25 weeks), and correlated with body weight, stride pattern, wire hang duration and ubiquitin pathology. For microglia and motor neuron counts at each time point, SOD1(G93A) and WT animals were compared using an independent samples t-test. A Welch t-test correction was applied if Levene's test showed that the variance in WT and SOD1G93A measurements was substantially different.

RESULTS

Disease onset, measured as the earliest change in functional parameters compared to non-transgenic WT mice, occurred at 14 weeks of age in SOD1(G93A) mice. The ventral horn of the SOD1(G93A) spinal cord contained more microglia than WT from 14 weeks onwards. In SOD1(G93A) mice, Arg1-positive and iNOS-positive microglia increased 18-fold and 7-fold, respectively, between 10 and 25 weeks of age (endpoint) in the lumbar spinal cord, while no increase was observed in WT mice. An increasing trend of Arg1- and iNOS-expressing microglia was observed in the cervical spinal cords of SOD1(G93A) mice. Additionally, Arg1-negative motor neurons appeared to selectively decline in the spinal cord of SOD1(G93A) mice, suggesting that Arg1 may have a neuroprotective function.

CONCLUSIONS

This study suggests that the increase in spinal cord microglia occurs around and after disease onset and is preceded by cellular pathology. The results show that Arg1 and iNOS, thought to have opposing inflammatory properties, are upregulated in microglia during disease progression and that Arg1 in motor neurons may confer protection from disease processes. Further understanding of the neuroinflammatory response, and the Arg1/iNOS balance in motor neurons, may provide suitable therapeutic targets for ALS.

Spinal but not cortical microglia acquire an atypical phenotype with high VEGF, galectin-3 and osteopontin, and blunted inflammatory responses in ALS rats

Activation of microglia, CNS resident immune cells, is a pathological hallmark of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting motor neurons. Despite evidence that microglia contribute to disease progression, the exact role of these cells in ALS pathology remains unknown. We immunomagnetically isolated microglia from different CNS regions of SOD1(G93A) rats at three different points in disease progression: presymptomatic, symptom onset and end-stage. We observed no differences in microglial number or phenotype in presymptomatic rats compared to wild-type controls. Although after disease onset there was no macrophage infiltration, there were significant increases in microglial numbers in the spinal cord, but not cortex. At disease end-stage, microglia were characterized by high expression of galectin-3, osteopontin and VEGF, and concomitant downregulated expression of TNFα, IL-6, BDNF and arginase-1. Flow cytometry revealed the presence of at least two phenotypically distinct microglial populations in the spinal cord. Immunohistochemistry showed that galectin-3/osteopontin positive microglia were restricted to the ventral horns of the spinal cord, regions with severe motor neuron degeneration. End-stage SOD1(G93A) microglia from the cortex, a less affected region, displayed similar gene expression profiles to microglia from wild-type rats, and displayed normal responses to systemic inflammation induced by LPS. On the other hand, end-stage SOD1(G93A) spinal microglia had blunted responses to systemic LPS suggesting that in addition to their phenotypic changes, they may also be functionally impaired. Thus, after disease onset, microglia acquired unique characteristics that do not conform to typical M1 (inflammatory) or M2 (anti-inflammatory) phenotypes. This transformation was observed only in the most affected CNS regions, suggesting that overexpression of mutated hSOD1 is not sufficient to trigger these changes in microglia. These novel observations suggest that microglial regional and phenotypic heterogeneity may be an important consideration when designing new therapeutic strategies targeting microglia and neuroinflammation in ALS.

The effect of SOD1 mutation on cellular bioenergetic profile and viability in response to oxidative stress and influence of mutation-type

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the progressive degeneration of motor neurons. Substantial evidence implicates oxidative stress and mitochondrial dysfunction as early events in disease progression. Our aim was to ascertain whether mutation of the SOD1 protein increases metabolic functional susceptibility to oxidative stress. Here we used a motor neuron-like cell line (NSC34) stably transfected with various human mutant SOD1 transgenes (G93A, G37R, H48Q) to investigate the impact of oxidative stress on cell viability and metabolic function within intact cells. NSC34 cells expressing mutant SOD1 showed a dose dependent reduction in cell viability when exposed to oxidative stress induced by hydrogen peroxide, with variation between mutations. The G93A transfectants showed greater cell death and LDH release compared to cells transfected with the other SOD1 mutations, and H48Q showed an accelerated decline at later time points. Differences in mitochondrial bioenergetics, including mitochondrial respiration, coupling efficiency and proton leak, were identified between the mutations, consistent with the differences observed in viability. NSC34 cells expressing G93A SOD1 displayed reduced coupled respiration and mitochondrial membrane potential compared to controls. Furthermore, the G93A mutation had significantly increased metabolic susceptibility to oxidative stress, with hydrogen peroxide increasing ROS production, reducing both cellular oxygen consumption and glycolytic flux in the cell. This study highlights bioenergetic defects within a cellular model of ALS and suggests that oxidative stress is not only detrimental to oxygen consumption but also glycolytic flux, which could lead to an energy deficit in the cell.