Localization of a mutant SOD1 protein in E40K-heterozygous dogs: Implications for non-cell-autonomous pathogenesis of degenerative myelopathy

Intrinsic structural vulnerability in the hydrophobic core induces species-specific aggregation of canine SOD1 with degenerative myelopathy-linked E40K mutation

Canine degenerative myelopathy (DM), a fatal neurodegenerative disease in dogs, shares clinical and genetic features with amyotrophic lateral sclerosis (ALS), a human motor neuron disease. Mutations in the SOD1 gene encoding Cu/Zn superoxide dismutase (SOD1) cause canine DM and a subset of inherited human ALS. The most frequent DM causative mutation is homozygous E40K mutation which induces the aggregation of canine SOD1 but not of human SOD1. However, the mechanism through which canine E40K mutation induces species-specific aggregation of SOD1 remains unknown. By screening human/canine chimeric SOD1s, we identified that the humanized mutation of the 117th residue (M117L), encoded by exon 4, significantly reduced aggregation propensity of canine SOD1^E40K. Conversely, introducing a mutation of leucine 117 to methionine, a residue homologous to canine, promoted E40K-dependent aggregation in human SOD1. M117L mutation improved protein stability and reduced cytotoxicity of canine SOD1^E40K. Furthermore, crystal structural analysis of canine SOD1 proteins revealed that M117L increased the packing within the hydrophobic core of the β-barrel structure, contributing to the increased protein stability. Our findings indicate that the structural vulnerability derived intrinsically from Met 117 in the hydrophobic core of the β-barrel structure induces E40K-dependent species-specific aggregation in canine SOD1.

Molecular Epidemiological Survey for Degenerative Myelopathy in German Shepherd Dogs in Japan: Allele Frequency and Clinical Progression Rate

Canine degenerative myelopathy (DM) is an adult-onset, chronic, progressive neurodegenerative disease reported in multiple canine breeds, including the German Shepherd Dog (GSD). Clinical signs include progressive motor neuron paralysis, which begins in the pelvic limbs and eventually leads to respiratory distress, which may necessitate euthanasia. A common DM-associated mutation is a single nucleotide substitution that causes an amino acid substitution (c.118G>A, p.E40K) in the canine SOD1 gene. This SOD1 mutation and the clinical progression rate of A/A risk genotype in the Japanese GSD population have not been analyzed before. Therefore, the aim of this study was to determine the frequency of the mutated allele and analyze the clinical progression rate in the Japanese GSD population. We studied 541 GSDs registered with the Japanese German Shepherd Dog Registration Society between 2000 and 2019. Genotyping was performed using real-time PCR with DNA extracted from the hair roots of each dog. The study revealed 330 G/G dogs (61%), 184 G/A dogs (34%), and 27 A/A dogs (5%), indicating a frequency of the mutant allele of 0.220, which are in Hardy−Weinberg equilibrium. We analyzed the clinical signs in A/A dogs with an age limit of 10 years based on information obtained from the dogs’ owners. Of the seven A/A dogs older than 10 years, owners reported DM-related clinical signs, indicating a clinical progression rate of 100%. These results, further genotyping, and thorough clinical examinations of SOD1 A/A risk genotype will help control and prevent DM in the Japanese GSD population.

In vitro evidence of propagation of superoxide dismutase-1 protein aggregation in canine degenerative myelopathy

Canine degenerative myelopathy (DM) is a progressive and fatal neurodegenerative disorder that has been linked to mutations in the superoxide dismutase 1 (SOD1) gene. The accumulation of misfolded protein aggregates in spinal neurons and astrocytes is implicated as an important pathological process in DM; however, the mechanism of protein aggregate formation is largely unknown. In human neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), cell-to-cell propagation of disease-relevant proteins has been demonstrated. Therefore, in this study, propagation of aggregation-forming property of mutant SOD1 protein in DM in vitro was investigated. This study demonstrated that aggregates composed of canine wild type SOD1 protein were increased by co-transfection with canine mutant SOD1 (E40K SOD1), indicating intracellular propagation of SOD1 aggregates. Further, aggregated recombinant SOD1 proteins were released from the cells, taken up by other cells, and induced further aggregate formation of normally folded SOD1 proteins. These results suggest intercellular propagation of SOD1 aggregates. The hypothesis of cell-to-cell propagation of SOD1 aggregates proposed in this study may underly the progressive nature of DM pathology.

In vivo detection of microstructural spinal cord lesions in dogs with degenerative myelopathy using diffusion tensor imaging

Background

Degenerative myelopathy (DM) in dogs is a progressive neurodegenerative condition that causes white matter spinal cord lesions. These lesions are undetectable on standard magnetic resonance imaging (MRI), limiting diagnosis and monitoring of the disease. Spinal cord lesions cause disruption to the structural integrity of the axons causing water diffusion to become more random and less anisotropic. These changes are detectable by the technique of diffusion tensor imaging (DTI) which is highly sensitive to diffusion alterations secondary to white matter lesion development.

Objective

Perform spinal DTI on cohorts of dogs with and without DM to identify if lesions caused by DM will cause a detectable alteration in spinal cord diffusivity that correlates with neurological status.

Animals

Thirteen dogs with DM and 13 aged‐matched controls.

Methods

All animals underwent MRI with DTI of the entire spine. Diffusivity parameters fractional anisotropy (FA) and mean diffusivity (MD) were measured at each vertebral level and statistically compared between groups.

Results

Dogs with DM had significant decreases in FA within the regions of the spinal cord that had high expected lesion load. Decreases in FA were most significant in dogs with severe forms of the disease and correlated with neurological grade.

Conclusions and Clinical Importance

Findings suggest that FA has the potential to be a biomarker for spinal cord lesion development in DM and could play an important role in improving diagnosis and monitoring of this condition.

Degenerative Myelopathy in Hovawart Dogs: Molecular Characterization, Pathological Features and Accumulation of Mutant Superoxide Dismutase 1 Protein

Degenerative myelopathy (DM) is an adult-onset, progressive neurological disease affecting several breeds of dog. Homozygosity or compound heterozygosity for the canine superoxide dismutase 1 (SOD1) gene mutations, possibly modulated by the modifier SP110 locus, are associated with a high risk for DM. Although the pathophysiological mechanisms are largely unknown, a role for mutant SOD1 in causing neuronal degeneration has been postulated. Three Hovawart dogs, 9-12 years of age, developed slowly progressive incoordination and weakness of the pelvic limbs leading to non-ambulatory flaccid paraparesis and muscle atrophy. Neuropathological lesions comprised axonal degeneration and loss of ascending and descending spinal pathways, which were most severe in the mid- to caudal thoracic segments. Accumulation of mutant SOD1 protein in neurons and reactive astrocytes was demonstrated by immunolabelling with the 16G9 antibody against the mutant SOD1 protein (p.E40K amino acid substitution). All three dogs were homozygous for the c.118A allele, but none had the SP110 'risk' haplotype, suggesting a weak association of SP110 with the onset of DM in this breed. Our data suggest that the Hovawart breed is predisposed to the SOD1:c.118G>A mutation, which is associated with the development of DM. Prevention of DM could be achieved with the help of strategies based on epidemiological and genetic testing.

Up-regulated spinal microRNAs induce aggregation of superoxide dismutase 1 protein in canine degenerative myelopathy

Canine degenerative myelopathy (DM) is a fatal progressive neurodegenerative disease. Mutations in the superoxide dismutase 1 (SOD1) gene have been shown to be the major risk factor for DM, and it is hypothesized that neural degeneration is caused by a "gain of toxic function" of mutant SOD1. In this study, the spinal cord microRNA (miRNA) profiles of DM-affected dogs were investigated to elucidate the pathomechanisms of DM. Quantification of 277 miRNAs identified three up-regulated miRNAs and 18 down-regulated miRNAs in the spinal cords of DM-affected dogs. Based on gene ontology analysis, the target cluster of up-regulated miRNAs was associated with protein expression or modification and cellular response, and that of down-regulated miRNAs was associated with tissue development. In these clusters, we focused on the mechanism of protein ubiquitination. Polyubiquitination assay demonstrated that canine SOD1 proteins were polyubiquitinated and degraded by proteasomes. Immunohistochemistry of the spinal cords of DM-affected dogs showed that mutant SOD1 aggregations were not ubiquitin immunopositive. Using cultured cells, co-transfection of canine SOD1 and up-regulated miRNA in DM-affected dogs demonstrated that miR-23a, miR-142 and miR-221 significantly increased the proportion of cells with mutant SOD1 aggregation. These results suggested that up-regulated miRNAs in the spinal cords of DM-affected dogs may inhibit ubiquitination of misfolded SOD1 protein and induce mutant SOD1 aggregations, leading to further progression of degenerative processes in the DM pathology.

Up-regulated inflammatory signatures of the spinal cord in canine degenerative myelopathy

Canine degenerative myelopathy (DM) is an adult-onset fatal disease characterized by progressive degeneration of the spinal cord. Affected dogs have homozygous mutations in superoxide dismutase 1, and thus DM is a potential spontaneous animal model of human familial amyotrophic lateral sclerosis (ALS). Neuroinflammation is the pathological hallmark of ALS, whereby proinflammatory cytokines and chemokines are overproduced by activated glial cells such as astrocytes and microglia. However, the detailed pathogenesis of spinal cord degeneration in DM remains unknown. To further characterize the pathological mechanism of DM, we analyzed the caudal cervical cords of ten Pembroke Welsh Corgis pathologically diagnosed with DM by quantitative real-time reverse transcription polymerase chain reaction, immunohistochemistry (IHC), and double immunofluorescence. Compared to control spinal cord tissues, we found significantly enhanced transcriptions of interleukin-1β, tumor necrosis factor-α, CC motif chemokine ligand (CCL) 2 and vascular cell adhesion molecule -1 mRNA in the spinal cords of DM dogs. Moreover, IHC for the class II major histocompatibility complex molecules HLA-DR and CCL2 indicated that the immunopositive areas of activated macrophages/microglia and CCL2 protein were significantly increased in DM, and CCL2 protein was mainly overproduced by astrocytes. Our results suggest a proinflammatory state of the microenvironment in the DM spinal cord in which activated microglia and astrocytes play important roles by secreting a set of cytokines, chemokines, and expressing adhesion molecules.

Canine SOD1 harboring E40K or T18S mutations promotes protein aggregation without reducing the global structural stability

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease associated with aggregation of superoxide dismutase 1 (SOD1) protein. More than 160 mutations in human SOD1 have been identified in familial ALS and extensively characterized in previous studies. Here, we investigated the effects of T18S and E40K mutations on protein aggregation of canine SOD1. These two mutations are exclusively found in canine degenerative myelopathy (an ALS-like neurodegenerative disease in dogs), whose phenotype is unknown at the level of protein folding. Interestingly, the T18S and E40K mutations did not alter far-UV CD spectrum, enzymatic activity, or global structural stability of canine SOD1. However, thioflavin-T assay and transmission electron microscopy analysis revealed that these mutations promote formation of fibrous aggregates, in particular in the Cu²⁺/Zn²⁺-unbound state. These evidence suggested that the T18S and E40K mutations promote protein aggregation through a unique mechanism, possibly involving destabilization of the local structure, reduction of net negative charge, or production of disulfide-linked oligomers.

Species-specific consequences of an E40K missense mutation in superoxide dismutase 1 (SOD1)

A glutamic acid to lysine (E40K) residue substitution in superoxide dismutase 1 (SOD1) is associated with canine degenerative myelopathy: the only naturally occurring large animal model of amyotrophic lateral sclerosis (ALS). The E40 residue is highly conserved across mammals, except the horse, which naturally carries the (dog mutant) K40 residue. Here we hypothesized that in vitro expression of mutant dog SOD1 would recapitulate features of human ALS (ie, SOD1 protein aggregation, reduced cell viability, perturbations in mitochondrial morphology and membrane potential, reduced ATP production, and increased superoxide ion levels); further, we hypothesized that an equivalent equine SOD1 variant would share similar perturbations in vitro, thereby explain horses' susceptibility to certain neurodegenerative diseases. As in human ALS, expression of mutant dog SOD1 was associated with statistically significant increased aggregate formation, raised superoxide levels (ROS), and altered mitochondrial morphology (increased branching (form factor)), when compared to wild-type dog SOD1-expressing cells. Similar deficits were not detected in cells expressing the equivalent horse SOD1 variant. Our data helps explain the ALS-associated cellular phenotype of dogs expressing the mutant SOD1 protein and reveals that species-specific sequence conservation does not necessarily predict pathogenicity. The work improves understanding of the etiopathogenesis of canine degenerative myelopathy.

In vitro evidence consistent with an interaction between wild-type and mutant SOD1 protein associated with canine degenerative myelopathy

Canine degenerative myelopathy (DM) is a progressive neurological disorder that may be considered to be a large animal model for specific forms of the fatal human disease, familial amyotrophic lateral sclerosis (fALS). DM is associated with a c118G>A mutation of the superoxide dismutase 1 (Sod1) gene, and a significant proportion of cases are inherited in an autosomal recessive manner in contrast to the largely, but not exclusively, dominant mode of inheritance in fALS. The consensus view is that these Sod1/SOD1 mutations result in a toxic gain of function but the mechanisms remain unclear. Here we used an in vitro neuroblastoma cell line transfection system to monitor wild-type and mutant forms of SOD1 fusion proteins containing either a Cherry or an enhanced green fluorescent protein (EGFP) tag. These fusion proteins retained SOD1 enzymatic activity on a native gel assay system. We demonstrate that SOD1 aggregate density is significantly higher in DM transfectants compared to wild-type. In addition, we show by co-immunoprecipitation and confocal microscopy, evidence for a potential interaction between wild-type and mutant forms of SOD1 in co-transfected cells. While in vitro studies have shown SOD1 heterodimer formation in fALS models, this is the first report for DM SOD1. Therefore, despite for the majority of cases there is a difference in the mode of inheritance between fALS and DM, a similar interaction between wild-type and mutant SOD1 forms can occur. Clarifying the role of SOD1 in DM may also be of benefit to understanding the role of SOD1 in fALS.

Canine Cognitive Dysfunction and Alzheimer's Disease - Two Facets of the Same Disease?

Neurodegenerative diseases present a major and increasing burden in the societies worldwide. With aging populations, the prevalence of neurodegenerative diseases is increasing, yet there are no effective cures and very few treatment options are available. Alzheimer’s disease is one of the most prevalent neurodegenerative conditions and although the pathology is well studied, the pathogenesis of this debilitating illness is still poorly understood. This is, among other reasons, also due to the lack of good animal models as laboratory rodents do not develop spontaneous neurodegenerative diseases and human Alzheimer’s disease is only partially mimicked by transgenic rodent models. On the other hand, older dogs commonly develop canine cognitive dysfunction, a disease that is similar to Alzheimer’s disease in many aspects. Dogs show cognitive deficits that could be paralleled to human symptoms such as disorientation, memory loss, changes in behavior, and in their brains, beta amyloid plaques are commonly detected both in extracellular space as senile plaques and around the blood vessels. Dogs could be therefore potentially a very good model for studying pathological process and novel treatment options for Alzheimer’s disease. In the present article, we will review the current knowledge about the pathogenesis of canine cognitive dysfunction, its similarities and dissimilarities with Alzheimer’s disease, and developments of novel treatments for these two diseases with a focus on canine cognitive dysfunction.

Plasma microRNA miR-26b as a potential diagnostic biomarker of degenerative myelopathy in Pembroke welsh corgis

BACKGROUND

Degenerative myelopathy (DM) is a progressive neurodegenerative disease frequently found in Pembroke Welsh Corgis (PWCs). Most DM-affected PWCs are homozygous for the mutant superoxide dismutase 1 (SOD1) allele; however, the genetic examination for the SOD1 mutation does not exclusively detect symptomatic dogs. In order to identify novel biomarkers, the plasma microRNA (miRNA) profiles of PWCs with DM were investigated.

RESULTS

Quantification of the plasma levels of 277 miRNAs by an RT-qPCR array identified 11 up-regulated miRNAs and 7 down-regulated miRNAs in DM-affected PWCs from those in wild-type SOD1 PWCs. A pathway analysis identified 3 miRNAs: miR-26b, miR-181a, and miR-196a, which potentially regulate several genes associated with SOD1. In order to validate the diagnostic accuracy of the candidate miRNAs in the aged PWC population, candidate miRNAs in plasma were measured by RT-qPCR and a receiver operating characteristic (ROC) curve analysis was performed. miR-26b had the largest area under the ROC curve for distinguishing DM PWCs from healthy PWCs (sensitivity, 66.7%; specificity, 87.0%). The plasma level of miR-26b was significantly higher in the DM group than in the healthy control group. A positive correlation was observed between increases in the plasma level of miR-26b and disease progression.

CONCLUSIONS

These results suggest that plasma miR-26b is a potential novel diagnostic biomarker of DM.

The Role of Glia in Canine Degenerative Myelopathy: Relevance to Human Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive degeneration of motor neurons and grim prognosis. Over the last decade, studies on neurodegenerative diseases pointed on the role of glia in supporting the proper function of neurons. Particularly, oligodendrocytes were shown to be essential through myelin production and supplying axons with energy metabolites via monocarboxylate transporters (MCT). We have used dogs with naturally occurring degenerative myelopathy (DM) which closely resembles features observed in human ALS. We have performed two types of analysis of spinal cord tissue samples: histology and molecular analysis. Histology included samples collected from dogs that succumbed to the DM at different disease stages, which were compared to age-matched controls as well as put in the context of young spinal cords. Molecular analysis was performed on spinal cords with advanced DM and age-matched samples and included real-time PCR analysis of selected gene products related to the function of neurons, oligodendrocytes, myelin, and MCT. Demyelination has been detected in dogs with DM through loss of eriochrome staining and decreased expression of genes related to myelin including MBP, Olig1, and Olig2. The prominent reduction of MCT1 and MCT2 and increased MCT4 expression is indicative of disturbed energy supply to neurons. While Rbfox3 expression was not altered, the ChAT production was negatively affected. DM in dogs reproduces main features of human ALS including loss of motor neurons, dysregulation of energy supply to neurons, and loss of myelin, and as such is an ideal model system for highly translational studies on therapeutic approaches for ALS.