Lentiviral vector-mediated overexpression of mutant ataxin-7 recapitulates SCA7 pathology and promotes accumulation of the FUS/TLS and MBNL1 RNA-binding proteins

Viral-based animal models in polyglutamine disorders

Polyglutamine disorders are a complex group of incurable neurodegenerative disorders caused by an abnormal expansion in the trinucleotide cytosine-adenine-guanine tract of the affected gene. To better understand these disorders, our dependence on animal models persists, primarily relying on transgenic models. In an effort to complement and deepen our knowledge, researchers have also developed animal models of polyglutamine disorders employing viral vectors. Viral vectors have been extensively used to deliver genes to the brain, not only for therapeutic purposes but also for the development of animal models, given their remarkable flexibility. In a time- and cost-effective manner, it is possible to use different transgenes, at varying doses, in diverse targeted tissues, at different ages, and in different species, to recreate polyglutamine pathology. This paper aims to showcase the utility of viral vectors in disease modelling, share essential considerations for developing animal models with viral vectors, and provide a comprehensive review of existing viral-based animal models for polyglutamine disorders.

Key Modulators of the Stress Granule Response TIA1, TDP-43, and G3BP1 Are Altered by Polyglutamine-Expanded ATXN7

Spinocerebellar ataxia type 7 (SCA7) and other polyglutamine (polyQ) diseases are caused by expansions of polyQ repeats in disease-specific proteins. Aggregation of the polyQ proteins resulting in various forms of cellular stress, that could induce the stress granule (SG) response, is believed to be a common pathological mechanism in these disorders. SGs can contribute to cell survival but have also been suggested to exacerbate disease pathology by seeding protein aggregation. In this study, we show that two SG-related proteins, TDP-43 and TIA1, are sequestered into the aggregates formed by polyQ-expanded ATXN7 in SCA7 cells. Interestingly, mutant ATXN7 also localises to induced SGs, and this association altered the shape of the SGs. In spite of this, neither the ability to induce nor to disassemble SGs, in response to arsenite stress induction or relief, was affected in SCA7 cells. Moreover, we could not observe any change in the number of ATXN7 aggregates per cell following SG induction, although a small, non-significant, increase in total aggregated ATXN7 material could be detected using filter trap. However, mutant ATXN7 expression in itself increased the speckling of the SG-nucleating protein G3BP1 and the SG response. Taken together, our results indicate that the SG response is induced, and although some key modulators of SGs show altered behaviour, the dynamics of SGs appear normal in the presence of mutant ATXN7.

Consensus Paper: Strengths and Weaknesses of Animal Models of Spinocerebellar Ataxias and Their Clinical Implications

Spinocerebellar ataxias (SCAs) represent a large group of hereditary degenerative diseases of the nervous system, in particular the cerebellum, and other systems that manifest with a variety of progressive motor, cognitive, and behavioral deficits with the leading symptom of cerebellar ataxia. SCAs often lead to severe impairments of the patient's functioning, quality of life, and life expectancy. For SCAs, there are no proven effective pharmacotherapies that improve the symptoms or substantially delay disease progress, i.e., disease-modifying therapies. To study SCA pathogenesis and potential therapies, animal models have been widely used and are an essential part of pre-clinical research. They mainly include mice, but also other vertebrates and invertebrates. Each animal model has its strengths and weaknesses arising from model animal species, type of genetic manipulation, and similarity to human diseases. The types of murine and non-murine models of SCAs, their contribution to the investigation of SCA pathogenesis, pathological phenotype, and therapeutic approaches including their advantages and disadvantages are reviewed in this paper. There is a consensus among the panel of experts that (1) animal models represent valuable tools to improve our understanding of SCAs and discover and assess novel therapies for this group of neurological disorders characterized by diverse mechanisms and differential degenerative progressions, (2) thorough phenotypic assessment of individual animal models is required for studies addressing therapeutic approaches, (3) comparative studies are needed to bring pre-clinical research closer to clinical trials, and (4) mouse models complement cellular and invertebrate models which remain limited in terms of clinical translation for complex neurological disorders such as SCAs.

Molecular, functional, and pathological aspects of TDP-43 fragmentation

Transactive response DNA binding protein 43 (TDP-43) is a DNA/RNA binding protein involved in transcriptional regulation and RNA processing. It is linked to sporadic and familial amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 is predominantly nuclear, but it translocates to the cytoplasm under pathological conditions. Cytoplasmic accumulation, phosphorylation, ubiquitination and truncation of TDP-43 are the main hallmarks of TDP-43 proteinopathies. Among these processes, the pathways leading to TDP-43 fragmentation remain poorly understood. We review here the molecular and biochemical properties of several TDP-43 fragments, the mechanisms and factors mediating their production, and their potential role in disease progression. We also address the presence of TDP-43 C-terminal fragments in several neurological disorders, including Alzheimer's disease, and highlight their respective implications. Finally, we discuss features of animal models expressing TDP-43 fragments as well as recent therapeutic strategies to approach TDP-43 truncation.

Polyglutamine expanded Ataxin-7 induces DNA damage and alters FUS localization and function

Polyglutamine (polyQ) diseases, such as Spinocerebellar ataxia type 7 (SCA7), are caused by expansions of polyQ repeats in disease specific proteins. The sequestration of vital proteins into aggregates formed by polyQ proteins is believed to be a common pathological mechanism in these disorders. The RNA-binding protein FUS has been observed in polyQ aggregates, though if disruption of this protein plays a role in the neuronal dysfunction in SCA7 or other polyQ diseases remains unclear. We therefore analysed FUS localisation and function in a stable inducible PC12 cell model expressing the SCA7 polyQ protein ATXN7. We found that there was a high degree of FUS sequestration, which was associated with a more cytoplasmic FUS localisation, as well as a decreased expression of FUS regulated mRNAs. In contrast, the role of FUS in the formation of γH2AX positive DNA damage foci was unaffected. In fact, a statistical increase in the number of γH2AX foci, as well as an increased trend of single and double strand DNA breaks, detected by comet assay, could be observed in mutant ATXN7 cells. These results were further corroborated by a clear trend towards increased DNA damage in SCA7 patient fibroblasts. Our findings suggest that both alterations in the RNA regulatory functions of FUS, and increased DNA damage, may contribute to the pathology of SCA7.

Antisense Transcription across Nucleotide Repeat Expansions in Neurodegenerative and Neuromuscular Diseases: Progress and Mysteries

Unstable repeat expansions and insertions cause more than 30 neurodegenerative and neuromuscular diseases. Remarkably, bidirectional transcription of repeat expansions has been identified in at least 14 of these diseases. More remarkably, a growing number of studies has been showing that both sense and antisense repeat RNAs are able to dysregulate important cellular pathways, contributing together to the observed clinical phenotype. Notably, antisense repeat RNAs from spinocerebellar ataxia type 7, myotonic dystrophy type 1, Huntington's disease and frontotemporal dementia/amyotrophic lateral sclerosis associated genes have been implicated in transcriptional regulation of sense gene expression, acting either at a transcriptional or posttranscriptional level. The recent evidence that antisense repeat RNAs could modulate gene expression broadens our understanding of the pathogenic pathways and adds more complexity to the development of therapeutic strategies for these disorders. In this review, we cover the amazing progress made in the understanding of the pathogenic mechanisms associated with repeat expansion neurodegenerative and neuromuscular diseases with a focus on the impact of antisense repeat transcription in the development of efficient therapies.

Motor Performances of Spontaneous and Genetically Modified Mutants with Cerebellar Atrophy

Chance discovery of spontaneous mutants with atrophy of the cerebellar cortex has unearthed genes involved in optimizing motor coordination. Rotorod, stationary beam, and suspended wire tests are useful in delineating behavioral phenotypes of spontaneous mutants with cerebellar atrophy such as Grid2^Lc, Grid2^ho, Rora^sg, Agtpbp1^pcd, Reln^rl, and Dab1^scm. Likewise, transgenic or null mutants serving as experimental models of spinocerebellar ataxia (SCA) are phenotyped with the same tests. Among experimental models of autosomal dominant SCA, rotorod deficits were reported in SCA1 to 3, SCA5 to 8, SCA14, SCA17, and SCA27 and stationary beam deficits in SCA1 to 3, SCA5, SCA6, SCA13, SCA17, and SCA27. Beam tests are sensitive to experimental therapies of various kinds including molecules affecting glutamate signaling, mesenchymal stem cells, anti-oligomer antibodies, lentiviral vectors carrying genes, interfering RNAs, or neurotrophic factors, and interbreeding with other mutants.

Molecular Targets and Therapeutic Strategies in Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a rare autosomal dominant neurodegenerative disorder characterized by progressive neuronal loss in the cerebellum, brainstem, and retina, leading to cerebellar ataxia and blindness as major symptoms. SCA7 is due to the expansion of a CAG triplet repeat that is translated into a polyglutamine tract in ATXN7. Larger SCA7 expansions are associated with earlier onset of symptoms and more severe and rapid disease progression. Here, we summarize the pathological and genetic aspects of SCA7, compile the current knowledge about ATXN7 functions, and then focus on recent advances in understanding the pathogenesis and in developing biomarkers and therapeutic strategies. ATXN7 is a bona fide subunit of the multiprotein SAGA complex, a transcriptional coactivator harboring chromatin remodeling activities, and plays a role in the differentiation of photoreceptors and Purkinje neurons, two highly vulnerable neuronal cell types in SCA7. Polyglutamine expansion in ATXN7 causes its misfolding and intranuclear accumulation, leading to changes in interactions with native partners and/or partners sequestration in insoluble nuclear inclusions. Studies of cellular and animal models of SCA7 have been crucial to unveil pathomechanistic aspects of the disease, including gene deregulation, mitochondrial and metabolic dysfunctions, cell and non-cell autonomous protein toxicity, loss of neuronal identity, and cell death mechanisms. However, a better understanding of the principal molecular mechanisms by which mutant ATXN7 elicits neurotoxicity, and how interconnected pathogenic cascades lead to neurodegeneration is needed for the development of effective therapies. At present, therapeutic strategies using nucleic acid-based molecules to silence mutant ATXN7 gene expression are under development for SCA7.

Mutant CAG Repeats Effectively Targeted by RNA Interference in SCA7 Cells

Spinocerebellar ataxia type 7 (SCA7) is a human neurodegenerative polyglutamine (polyQ) disease caused by a CAG repeat expansion in the open reading frame of the ATXN7 gene. The allele-selective silencing of mutant transcripts using a repeat-targeting strategy has previously been used for several polyQ diseases. Herein, we demonstrate that the selective targeting of a repeat tract in a mutant ATXN7 transcript by RNA interference is a feasible approach and results in an efficient decrease of mutant ataxin-7 protein in patient-derived cells. Oligonucleotides (ONs) containing specific base substitutions cause the downregulation of the ATXN7 mutant allele together with the upregulation of its normal allele. The A2 ON shows high allele selectivity at a broad range of concentrations and also restores UCHL1 expression, which is downregulated in SCA7.