Mutant SOD1 Toxicity Research Articles

Targeting low levels of MIF expression as a potential therapeutic strategy for ALS

Mutations in SOD1 cause amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by motor neuron (MN) loss. We previously discovered that macrophage migration inhibitory factor (MIF), whose levels are extremely low in spinal MNs, inhibits mutant SOD1 misfolding and toxicity. In this study, we show that a single peripheral injection of adeno-associated virus (AAV) delivering MIF into adult SOD1G37R mice significantly improves their motor function, delays disease progression, and extends survival. Moreover, MIF treatment reduces neuroinflammation and misfolded SOD1 accumulation, rescues MNs, and corrects dysregulated pathways as observed by proteomics and transcriptomics. Furthermore, we reveal low MIF levels in human induced pluripotent stem cell-derived MNs from familial ALS patients with different genetic mutations, as well as in post mortem tissues of sporadic ALS patients. Our findings indicate that peripheral MIF administration may provide a potential therapeutic mechanism for modulating misfolded SOD1 invivo and disease outcome in ALS patients.

MIF homolog d-dopachrome tautomerase (D-DT/MIF-2) does not inhibit accumulation and toxicity of misfolded SOD1

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of upper and lower motor neurons. About 20% of familial ALS cases are caused by dominant mutations in SOD1. It has been suggested that toxicity of mutant SOD1 results from its misfolding, however, it is unclear why misfolded SOD1 accumulates within specific tissues. We have demonstrated that macrophage migration inhibitory factor (MIF), a multifunctional protein with cytokine/chemokine and chaperone-like activity, inhibits the accumulation and aggregation of misfolded SOD1. Although MIF homolog, D-dopachrome tautomerase (D-DT/MIF-2), shares structural and genetic similarities with MIF, its biological function is not well understood. In the current study, we investigated, for the first time, the mechanism of action of D-DT in a model of ALS. We show that D-DT inhibits mutant SOD1 amyloid aggregation in vitro, promoting the formation of amorphous aggregates. Moreover, we report that D-DT interacts with mutant SOD1, but does not inhibit misfolded mutant SOD1 accumulation and toxicity in neuronal cells. Finally, we show that D-DT is expressed mainly in liver and kidney, with extremely low expression in brain and spinal cord of adult mice. Our findings contribute to better understanding of D-DT versus MIF function in the context of ALS.

A VDAC1-Derived N-Terminal Peptide Inhibits Mutant SOD1-VDAC1 Interactions and Toxicity in the SOD1 Model of ALS.

Mutations in superoxide dismutase (SOD1) are the second most common cause of familial amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the death of motor neurons in the brain and spinal cord. SOD1 neurotoxicity has been attributed to aberrant accumulation of misfolded SOD1, which in its soluble form binds to intracellular organelles, such as mitochondria and ER, disrupting their functions. Here, we demonstrate that mutant SOD1 binds specifically to the N-terminal domain of the voltage-dependent anion channel (VDAC1), an outer mitochondrial membrane protein controlling cell energy, metabolic and survival pathways. Mutant SOD1G93A and SOD1G85R, but not wild type SOD1, directly interact with VDAC1 and reduce its channel conductance. No such interaction with N-terminal-truncated VDAC1 occurs. Moreover, a VDAC1-derived N-terminal peptide inhibited mutant SOD1-induced toxicity. Incubation of motor neuron-like NSC-34 cells expressing mutant SOD1 or mouse embryonic stem cell-derived motor neurons with different VDAC1 N-terminal peptides resulted in enhanced cell survival. Taken together, our results establish a direct link between mutant SOD1 toxicity and the VDAC1 N-terminal domain and suggest that VDAC1 N-terminal peptides targeting mutant SOD1 provide potential new therapeutic strategies for ALS.

Metabolic Changes Associated With Muscle Expression of SOD1G93A.

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder, classified into sporadic or familial forms and characterized by motor neurons death, muscle atrophy, weakness, and paralysis. Among the familial cases of ALS, approximately 20% are caused by dominant mutations in the gene coding for superoxide dismutase (SOD1) protein. Of note, mutant SOD1 toxicity is not necessarily limited to the central nervous system. ALS is indeed a multi-systemic and multifactorial disease that affects whole body physiology and induces severe metabolic changes in several tissues, including skeletal muscle. Nevertheless, whether alterations in the plasticity, heterogeneity, and metabolism of muscle fibers are the result of motor neuron degeneration or alternatively occur independently of it remain to be elucidated. To address this issue, we made use of a mouse model (MLC/SOD1G93A) that overexpresses the SOD1 mutant gene selectively in skeletal muscle. We found an alteration in the metabolic properties of skeletal muscle characterized by alteration in fiber type composition and metabolism. Indeed, we observed an alteration of muscle glucose metabolism associated with the induction of Phosphofructokinases and Pyruvate dehydrogenase kinase 4 expression. The upregulation of Pyruvate dehydrogenase kinase 4 led to the inhibition of Pyruvate conversion into Acetyl-CoA. Moreover, we demonstrated that the MLC/SOD1G93A transgene was associated with an increase of lipid catabolism and with the inhibition of fat deposition inside muscle fibers. All together these data demonstrate that muscle expression of the SOD1G93A gene induces metabolic changes, along with a preferential use of lipid energy fuel by muscle fibers. We provided evidences that muscle metabolic alterations occurred before disease symptoms and independently of motor neuron degeneration, indicating that skeletal muscle is likely an important therapeutic target in ALS.

Poloxamer 188 decreases membrane toxicity of mutant SOD1 and ameliorates pathology observed in SOD1 mouse model for ALS

Here we report a gain in function for mutant (mt) superoxide dismutase I (SOD1), a cause of familial amyotrophic lateral sclerosis (FALS), wherein small soluble oligomers of mtSOD1 acquire a membrane toxicity. Phosphatidylglycerol (PG) lipid domains are selectively targeted, which could result in membrane damage or “toxic channels” becoming active in the bilayer. This PG-selective SOD1-mediated membrane toxicity is largely reversible in vitro by a widely-available FDA-approved surfactant and membrane-stabilizer P188. Treatment of G93ASOD1 transgenic mice with P188 significantly delayed symptoms onset, extended survival and decreased motoneuron death. The use of P188 or an analogue, which targets mtSOD1 misfolding-induced membrane toxicity, may provide a new direction for ALS treatment.

MIF inhibits the formation and toxicity of misfolded SOD1 amyloid aggregates: implications for familial ALS

Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the progressive loss of motor neurons in the brain and spinal cord. It has been suggested that toxicity of mutant SOD1 results from its misfolding, however, it is yet unclear why misfolded SOD1 accumulates specifically within motor neurons. We recently demonstrated that macrophage migration inhibitory factor (MIF)—a multifunctional protein with cytokine/chemokine activity and cytosolic chaperone-like properties—inhibits the accumulation of misfolded SOD1. Here, we show that MIF inhibits mutant SOD1 nuclear clearance when overexpressed in motor neuron-like NSC-34 cells. In addition, MIF alters the typical SOD1 amyloid aggregation pathway in vitro, and, instead, promotes the formation of disordered aggregates, as measured by Thioflavin T (ThT) assay and transmission electron microscopy (TEM) imaging. Moreover, we report that MIF reduces the toxicity of misfolded SOD1 by directly interacting with it, and that the chaperone function and protective effect of MIF in neuronal cultures do not require its intrinsic catalytic activities. Importantly, we report that the locked-trimeric MIFN110C mutant, which exhibits strongly impaired CD74-mediated cytokine functions, has strong chaperone activity, dissociating, for the first time, these two cellular functions. Altogether, our study implicates MIF as a potential therapeutic candidate in the treatment of ALS.

Endogenous macrophage migration inhibitory factor reduces the accumulation and toxicity of misfolded SOD1 in a mouse model of ALS

Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons in the brain and spinal cord. It has been suggested that the toxicity of mutant SOD1 results from its misfolding and accumulation on the cytoplasmic faces of intracellular organelles, including the mitochondria and endoplasmic reticulum (ER) of ALS-affected tissues. Recently, macrophage migration inhibitory factor (MIF) was shown to directly inhibit the accumulation of misfolded SOD1 and its binding to intracellular membranes, but the role of endogenous MIF in modulating SOD1 misfolding in vivo remains unknown. To elucidate this role, we bred MIF-deficient mice with SOD1(G85R) mice, which express a dismutase-inactive mutant of SOD1 and are considered a model of familial ALS. We found that the accumulation of misfolded SOD1, its association with mitochondrial and ER membranes, and the levels of sedimentable insoluble SOD1 aggregates were significantly higher in the spinal cords of SOD1(G85R)-MIF(-/-) mice than in their SOD1(G85R)-MIF(+/+) littermates. Moreover, increasing MIF expression in neuronal cultures inhibited the accumulation of misfolded SOD1 and rescued from mutant SOD1-induced cell death. In contrast, the complete elimination of endogenous MIF accelerated disease onset and late disease progression and shortened the lifespan of the SOD1(G85R) mutant mice. These findings indicate that MIF plays a significant role in the folding and misfolding of SOD1 in vivo, and they have implications for the potential therapeutic role of up-regulating MIF within the nervous system to modulate the selective accumulation of misfolded SOD1.

Mutant SOD1 accumulation in sensory neurons does not associate with endoplasmic reticulum stress features: Implications for differential vulnerability of sensory and motor neurons to SOD1 toxicity

Mutations in Cu/Zn-superoxide dismutase (SOD1) cause familial amyotrophic lateral sclerosis (ALS). Previous papers showed that mutant SOD1 accumulates and undergoes misfolding in motor neurons and that the specific interaction of mutant SOD1 with derlin-1 leads to endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). Because evidence shows that mutant SOD1 expression also damages sensory neurons, we hypothesized that, similarly to motor neurons, the sensory neurons of ALS mouse model SOD1G93A accumulate mutant/misfolded SOD1 and suffer from ER stress and UPR activation.Our results reveal that SOD1G93A sensory neurons accumulate mutant/misfolded SOD1 but, surprisingly, do not suffer from ER stress and UPR activation. Moreover, the sensory neurons do not express detectable levels of the SOD1 interactor derlin-1. These results suggest a potential molecular mechanism underlying the differential vulnerability of motor and sensory neurons to mutant SOD1 toxicity.

Copper Homeostasis as a Therapeutic Target in Amyotrophic Lateral Sclerosis with SOD1 Mutations.

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease affecting both upper and lower motor neurons, and currently, there is no cure or effective treatment. Mutations in a gene encoding a ubiquitous antioxidant enzyme, Cu,Zn-superoxide dismutase (SOD1), have been first identified as a cause of familial forms of ALS. It is widely accepted that mutant SOD1 proteins cause the disease through a gain in toxicity but not through a loss of its physiological function. SOD1 is a major copper-binding protein and regulates copper homeostasis in the cell; therefore, a toxicity of mutant SOD1 could arise from the disruption of copper homeostasis. In this review, we will briefly review recent studies implying roles of copper homeostasis in the pathogenesis of SOD1-ALS and highlight the therapeutic interventions focusing on pharmacological as well as genetic regulations of copper homeostasis to modify the pathological process in SOD1-ALS.

A cysteine residue affects the conformational state and neuronal toxicity of mutant SOD1 in mice: relevance to the pathogenesis of ALS.

We previously showed by in vitro experiments that the cysteine residue (Cys111) near the dimer interface is critical for monomerization and resultant aggregate formation of mutant Cu, Zn-superoxide dismutase (SOD1) protein, which is toxic to motor neurons in familial amyotrophic lateral sclerosis (ALS). To verify the importance of Cys111 in the mutant SOD1-associated ALS pathogenesis in vivo, we analyzed the disease phenotype of SOD1 transgenic mice harboring H46R mutation alone (H46R mice) or H46R/C111S double mutations (H46R/C111S mice). Behavioral, histological and biochemical analyses of the spinal cord showed that the onset and progression of the disease phenotype were delayed in H46R/C111S mice compared with H46R mice. We found that peroxidized Cys111 of H46R SOD1 plays a role in promoting formation of high molecular weight insoluble SOD1 species that is correlated with the progression of the motor neuron disease phenotype. These results support that Cys111 is a critical residue for the neuronal toxicity of mutant SOD1 in vivo, and the blockage of peroxidation of this residue in mutant SOD1 may constitute a future target for developing ALS treatment.

ZnII(atsm) is protective in amyotrophic lateral sclerosis model mice via a copper delivery mechanism

Mutations in the metalloprotein Cu,Zn-superoxide dismutase (SOD1) cause approximately 20% of familial cases of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease for which effective therapeutics do not yet exist. Transgenic rodent models based on over-expression of mutant SOD1 have been developed and these have provided opportunity to test new therapeutic strategies and to study the mechanisms of mutant SOD1 toxicity. Although the mechanisms of mutant SOD1 toxicity are yet to be fully elucidated, incorrect or incomplete metallation of SOD1 confers abnormal folding, aggregation and biochemical properties, and improving the metallation state of SOD1 provides a viable therapeutic option. The therapeutic effects of delivering copper (Cu) to mutant SOD1 have been demonstrated recently. The aim of the current study was to determine if delivery of zinc (Zn) to SOD1 was also therapeutic. To investigate this, SOD1G37R mice were treated with the metal complex diacetyl-bis(4-methylthiosemicarbazonato)zincII [ZnII(atsm)]. Treatment resulted in an improvement in locomotor function and survival of the mice. However, biochemical analysis of spinal cord tissue collected from the mice revealed that the treatment did not increase overall Zn levels in the spinal cord nor the Zn content of SOD1. In contrast, overall levels of Cu in the spinal cord were elevated in the ZnII(atsm)-treated SOD1G37R mice and the Cu content of SOD1 was also elevated. Further experiments demonstrated transmetallation of ZnII(atsm) in the presence of Cu to form the Cu-analogue CuII(atsm), indicating that the observed therapeutic effects for ZnII(atsm) in SOD1G37R mice may in fact be due to in vivo transmetallation and subsequent delivery of Cu.

Macrophage migration inhibitory factor as a component of selective vulnerability of motor neurons in ALS.

Amyotrophic lateral sclerosis (ALS) is a progressive adult-onset neurodegenerative disorder characterized by the selective loss of upper and lower motor neurons. Mutations in superoxide dismutase (SOD1) cause about 20 percent of familial ALS which is accompanied by accumulation of misfolded SOD1 onto intracellular organelles. Recently, we identified the 12 kDa macrophage migration inhibitory factor (MIF) as a chaperone for mutant SOD1 which is abundant in non-neuronal tissues. Purified recombinant MIF was shown to directly inhibit mutant SOD1 misfolding and association with mitochondria and ER. Elevating MIF in neuronal cells inhibited the accumulation of misfolded SOD1 and its association with mitochondria and ER, and extended survival of mutant SOD1-expressing motor neurons. Our results revealed that the levels of MIF protein are very low in motor neurons, implicating low chaperone activity as a component of selective vulnerability of motor neurons to mutant SOD1 misfolding and toxicity.

Direct and indirect mechanisms for wild-type SOD1 to enhance the toxicity of mutant SOD1 in bigenic transgenic mice

Co-expression of wild-type human superoxide dismutase 1 (WT-hSOD1) with ALS mutant hSOD1 accelerates disease onset relative to mice expressing only mutant protein. Here, we analyzed the effect of co-expressed WT-hSOD1 in two established mutant mouse models (L126Z and G37R), and a new model that expresses the first 102 amino acids of SOD1 with mutations at histidines 46, 48 and 63 to eliminate Cu binding (Cu-V103Z). A subset of Cu-V103Z mice developed paralysis between 500 and 730 days. Similar to mice expressing L126Z-SOD1, the spinal cords of this new model showed SOD1 immunoreactive fibrillar inclusions. Co-expression of WT-hSOD1 with Cu-V103Z SOD1 moderately accelerated the age to paralysis, similar in magnitude to WT/L126Z mice. In either combination of these bigenic mice, the severity of fibrillar inclusion pathology was diminished and unreactive to antibodies specific for the C terminus of WT protein. Co-expression of WT-hSOD1 fused to yellow fluorescent protein (WT-hSOD1:YFP) with G37R-hSOD1 produced earlier disease, and spinal cords of paralyzed bigenic mice showed YFP fluorescent inclusion-like structures. In bigenic L126Z/WT-hSOD1:YFP mice, disease was not accelerated and WT-hSOD1:YFP remained diffusely distributed. A combination of split luciferase complementation assays and affinity capture-binding assays demonstrated that soluble G37R-hSOD1 efficiently and tightly bound soluble WT-hSOD1, whereas soluble forms of the Cu-V103Z and L126Z variants demonstrated low affinity. These data indicate that WT-hSOD1 may indirectly augment the toxicity of mutant protein by competing for protective factors, but disease onset seems to be most accelerated when WT-hSOD1 interacts with mutant SOD1 and becomes misfolded.

The voltage‐gated calcium channel blocker lomerizine is neuroprotective in motor neurons expressing mutant SOD1, but not TDP‐43

Excitotoxicity and disruption of Ca(2+) homeostasis have been implicated in amyotrophic lateral sclerosis (ALS) and limiting Ca(2+) entry is protective in models of ALS caused by mutation of SOD1. Lomerizine, an antagonist of L- and T-type voltage-gated calcium channels and transient receptor potential channel 5 transient receptor potential channels, is well tolerated clinically, making it a potential therapeutic candidate. Lomerizine reduced glutamate excitotoxicity in cultured motor neurons by reducing the accumulation of cytoplasmic Ca(2+) and protected motor neurons against multiple measures of mutant SOD1 toxicity: Ca(2+) overload, impaired mitochondrial trafficking, mitochondrial fragmentation, formation of mutant SOD1 inclusions, and loss of viability. To assess the utility of lomerizine in other forms of ALS, calcium homeostasis was evaluated in culture models of disease because of mutations in the RNA-binding proteins transactive response DNA-binding protein 43 (TDP-43) and Fused in Sarcoma (FUS). Calcium did not play the same role in the toxicity of these mutant proteins as with mutant SOD1 and lomerizine failed to prevent cytoplasmic accumulation of mutant TDP-43, a hallmark of its pathology. These experiments point to differences in the pathogenic pathways between types of ALS and show the utility of primary culture models in comparing those mechanisms and effectiveness of therapeutic strategies.

MicroRNA-206: A Potential Circulating Biomarker Candidate for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a lethal motor neuron disease that progressively debilitates neuronal cells that control voluntary muscle activity. Biomarkers are urgently needed to facilitate ALS diagnosis and prognosis, and as indicators of therapeutic response in clinical trials. microRNAs (miRNAs), small posttranscriptional modifiers of gene expression, are frequently altered in disease conditions. Besides their important regulatory role in variety of biological processes, miRNAs can also be released into the circulation by pathologically affected tissues and display remarkable stability in body fluids. In a mouse model of ALS that expresses mutated human superoxide dismutase 1 (SOD1-G93A) skeletal muscle is one of the tissues affected early by mutant SOD1 toxicity. To find biomarkers for ALS, we studied miRNA alterations from skeletal muscle and plasma of SOD1-G93A mice, and subsequently tested the levels of the affected miRNAs in the serum from human ALS patients. Fast-twitch and slow-twitch muscles from symptomatic SOD1-G93A mice (age 90 days) and their control littermates were first studied using miRNA microarrays and then evaluated with quantitative PCR from five age groups from neonatal to the terminal disease stage (10–120 days). Among those miRNA changed in various age/gender/muscle groups (miR-206, -1, -133a, -133b, -145, -21, -24), miR-206 was the only one consistently altered during the course of the disease pathology. In both sexes, mature miR-206 was increased in fast-twitch muscles preferably affected in the SOD1-G93A model, with highest expression towards the most severely affected animals. Importantly, miR-206 was also increased in the circulation of symptomatic animals and in a group of 12 definite ALS patients tested. We conclude that miR-206 is elevated in the circulation of symptomatic SOD1-G93A mice and possibly in human ALS patients. Although larger scale studies on ALS patients are warranted, miR-206 is a promising candidate biomarker for this motor neuron disease.

Wild-type Cu/Zn superoxide dismutase stabilizes mutant variants by heterodimerization

Mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1) are responsible for a subset of amyotrophic lateral sclerosis cases presumably by the acquisition of as yet unknown toxic properties. Additional overexpression of wild-type SOD1 in mutant SOD1 transgenic mice did not improve but rather accelerated the disease course. Recently, it was documented that the presence of wild-type SOD1 (SODWT) reduced the aggregation propensity of mutant SOD1 by the formation of heterodimers between mutant and SOD1WT and that these heterodimers displayed at least a similar toxicity in cellular and animal models. In this study we investigated the biochemical and biophysical properties of obligate SOD1 dimers that were connected by a peptide linker. Circular dichroism spectra indicate an increased number of unstructured residues in SOD1 mutants. However, SOD1WT stabilized the folding of heterodimers compared to mutant homodimers as evidenced by an increase in resistance against proteolytic degradation. Heterodimerization also reduced the affinity of mutant SOD1 to antibodies detecting misfolded SOD1. In addition, the formation of obligate dimers resulted in a detection of substantial dismutase activity even of the relatively labile SOD1G85R mutant. These data indicate that soluble, dismutase-active SOD1 dimers might contribute at least partially to mutant SOD1 toxicity.

Mutant Copper-Zinc Superoxide Dismutase (SOD1) Induces Protein Secretion Pathway Alterations and Exosome Release in Astrocytes: IMPLICATIONS FOR DISEASE SPREADING AND MOTOR NEURON PATHOLOGY IN AMYOTROPHIC LATERAL SCLEROSIS

Amyotrophic lateral sclerosis is the most common motor neuron disease and is still incurable. The mechanisms leading to the selective motor neuron vulnerability are still not known. The interplay between motor neurons and astrocytes is crucial in the outcome of the disease. We show that mutant copper-zinc superoxide dismutase (SOD1) overexpression in primary astrocyte cultures is associated with decreased levels of proteins involved in secretory pathways. This is linked to a general reduction of total secreted proteins, except for specific enrichment in a number of proteins in the media, such as mutant SOD1 and valosin-containing protein (VCP)/p97. Because there was also an increase in exosome release, we can deduce that astrocytes expressing mutant SOD1 activate unconventional secretory pathways, possibly as a protective mechanism. This may help limit the formation of intracellular aggregates and overcome mutant SOD1 toxicity. We also found that astrocyte-derived exosomes efficiently transfer mutant SOD1 to spinal neurons and induce selective motor neuron death. We conclude that the expression of mutant SOD1 has a substantial impact on astrocyte protein secretion pathways, contributing to motor neuron pathology and disease spread.

Establishing a novel C. elegans model to investigate the role of autophagy in amyotrophic lateral sclerosis

Aim:To develop a C. elegans model of amyotrophic lateral sclerosis (ALS) and to evaluate the role of autophagy in the disease.Methods:Stable transgenic worms expressing the G93A mutant form of Cu,Zn-superoxide dismutase (SOD1) in GABAergic motor neurons were generated. Axon guidance and protein aggregation in the motor neurons were observed with fluorescence microscopy. A paralysis assay was performed to evaluate the motor function of the transgenic worms. The expression of autophagic genes in daf-2(e1370) mutants was analyzed using real-time PCR. The reporter GFP::LGG-1 was used to verify whether autophagy was induced in motor neurons.Results:Expression of G93A SOD1 in motor neurons caused age-dependent motor defects accompanied by significant SOD1 aggregation and axon guidance failure. After 12 d, over 80% of the G93A worms became paralyzed, whereas less than 10% of the controls showed a paralytic phenotype. In the daf-2(e1370) mutants of C. elegans, the levels of autophagic genes bec-1, atg-7, lgg-1, and atg-18 were upregulated by approximately 1.5-fold, the level of unc-51 increased by approximately fourfold, and autophagosomes in motor neurons was markedly increased. Crossing the daf-2(e1370) mutation into the G93A SOD1 mutant worms significantly ameliorated the motor defects, SOD1 aggregation, and axon guidance failure.Conclusion:G93A SOD1 expression in motor neurons of C. elegans results in characteristic alterations of ALS. Increased autophagy protects C. elegans motor neurons against the toxicity of mutant SOD1.

Widespread aggregation of mutant VAPB associated with ALS does not cause motor neuron degeneration or modulate mutant SOD1 aggregation and toxicity in mice

BackgroundA proline-to-serine substitution at position-56 (P56S) of vesicle-associated membrane protein-associated protein B (VAPB) causes a form of dominantly inherited motor neuron disease (MND), including typical and atypical amyotrophic lateral sclerosis (ALS) and a mild late-onset spinal muscular atrophy (SMA). VAPB is an integral endoplasmic reticulum (ER) protein and has been implicated in various cellular processes, including ER stress, the unfolded protein response (UPR) and Ca2+ homeostasis. However, it is unclear how the P56S mutation leads to neurodegeneration and muscle atrophy in patients. The formation of abnormal VAPB-positive inclusions by mutant VAPB suggests a possible toxic gain of function as an underlying mechanism. Furthermore, the amount of VAPB protein is reported to be reduced in sporadic ALS patients and mutant SOD1G93A mice, leading to the hypothesis that wild type VAPB plays a role in the pathogenesis of ALS without VAPB mutations.ResultsTo investigate the pathogenic mechanism in vivo, we generated human wild type (wtVAPB) and mutant VAPB (muVAPB) transgenic mice that expressed the transgenes broadly in the CNS. We observed robust VAPB-positive aggregates in the spinal cord of muVAPB transgenic mice. However, we failed to find an impairment of motor function and motor neuron degeneration. We also did not detect any change in the endogenous VAPB level or evidence for induction of the unfolded protein response (UPR) and coaggregation of VAPA with muVAPB. Furthermore, we crossed these VAPB transgenic mice with mice that express mutant SOD1G93A and develop motor neuron degeneration. Overexpression of neither wtVAPB nor muVAPB modulated the protein aggregation and disease progression in the SOD1G93A mice.ConclusionOverexpression of VAPBP56S mutant to approximately two-fold of the endogenous VAPB in mouse spinal cord produced abundant VAPB aggregates but was not sufficient to cause motor dysfunction or motor neuron degeneration. Furthermore, overexpression of either muVAPB or wtVAPB does not modulate the course of ALS in SOD1G93A mice. These results suggest that changes in wild type VAPB do not play a significant role in ALS cases that are not caused by VAPB mutations. Furthermore, these results suggest that muVAPB aggregates are innocuous and do not cause motor neuron degeneration by a gain-of-toxicity, and therefore, a loss of function may be the underlying mechanism.

An emerging role for misfolded wild-type SOD1 in sporadic ALS pathogenesis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that targets motor neurons, leading to paralysis and death within a few years of disease onset. While several genes have been linked to the inheritable, or familial, form of ALS, much less is known about the cause(s) of sporadic ALS, which accounts for ~90% of ALS cases. Due to the clinical similarities between familial and sporadic ALS, it is plausible that both forms of the disease converge on a common pathway and, therefore, involve common factors. Recent evidence suggests the Cu,Zn-superoxide dismutase (SOD1) protein to be one such factor that is common to both sporadic and familial ALS. In 1993, mutations were uncovered in SOD1 that represent the first known genetic cause of familial ALS. While the exact mechanism of mutant-SOD1 toxicity is still not known today, most evidence points to a gain of toxic function that stems, at least in part, from the propensity of this protein to misfold. In the wild-type SOD1 protein, non-genetic perturbations such as metal depletion, disruption of the quaternary structure, and oxidation, can also induce SOD1 to misfold. In fact, these aforementioned post-translational modifications cause wild-type SOD1 to adopt a “toxic conformation” that is similar to familial ALS-linked SOD1 variants. These observations, together with the detection of misfolded wild-type SOD1 within human post-mortem sporadic ALS samples, have been used to support the controversial hypothesis that misfolded forms of wild-type SOD1 contribute to sporadic ALS pathogenesis. In this review, we present data from the literature that both support and contradict this hypothesis. We also discuss SOD1 as a potential therapeutic target for both familial and sporadic ALS.