Ataxin-2 regulates RGS8 translation in a new BAC-SCA2 transgenic mouse model

The polyglutamine protein ATXN2: from its molecular functions to its involvement in disease

A CAG repeat sequence in the ATXN2 gene encodes a polyglutamine (polyQ) tract within the ataxin-2 (ATXN2) protein, showcasing a complex landscape of functions that have been progressively unveiled over recent decades. Despite significant progresses in the field, a comprehensive overview of the mechanisms governed by ATXN2 remains elusive. This multifaceted protein emerges as a key player in RNA metabolism, stress granules dynamics, endocytosis, calcium signaling, and the regulation of the circadian rhythm. The CAG overexpansion within the ATXN2 gene produces a protein with an extended poly(Q) tract, inducing consequential alterations in conformational dynamics which confer a toxic gain and/or partial loss of function. Although overexpanded ATXN2 is predominantly linked to spinocerebellar ataxia type 2 (SCA2), intermediate expansions are also implicated in amyotrophic lateral sclerosis (ALS) and parkinsonism. While the molecular intricacies await full elucidation, SCA2 presents ATXN2-associated pathological features, encompassing autophagy impairment, RNA-mediated toxicity, heightened oxidative stress, and disruption of calcium homeostasis. Presently, SCA2 remains incurable, with patients reliant on symptomatic and supportive treatments. In the pursuit of therapeutic solutions, various studies have explored avenues ranging from pharmacological drugs to advanced therapies, including cell or gene-based approaches. These endeavours aim to address the root causes or counteract distinct pathological features of SCA2. This review is intended to provide an updated compendium of ATXN2 functions, delineate the associated pathological mechanisms, and present current perspectives on the development of innovative therapeutic strategies.

Development of an isogenic human cell trio that models polyglutamine disease

Polyglutamine (polyQ) diseases are rare autosomal-dominant neurodegenerative diseases associated with the expansion of glutamine-encoding triplet repeats in certain genes. To investigate the functional influence of repeat expansion on disease mechanisms, we applied a biallelic genome-engineering platform that we recently established, called Universal Knock-in System or UKiS, to develop a human cell trio, a set of three isogenic cell lines that are homozygous for two different numbers of repeats (first and second lines) or heterozygous for the two repeat numbers (third line). As an example of a polyQ disease, we chose spinocerebellar ataxia type 2 (SCA2). In a pseudodiploid human cell line, both alleles of the glutamine-encoding triplet repeat in the SCA2-causing gene, ataxin 2 or ATXN2, were first knocked in with a donor sequence encoding both thymidine kinase and either puromycin or blasticidin resistance proteins under dual drug selection. The knocked-in donor alleles were then substituted with a payload having either 22 or 76 triplet repeats in ATXN2 by ganciclovir negative selection. The two-step substitution and subsequent SNP typing and genomic sequencing confirmed that the SCA2-modeling isogenic cell trio was obtained: three clones of 22-repeat homozygotes, two clones of 22/76-repeat heterozygotes and two clones of 76-repeat homozygotes. Finally, RT-PCR and immunoblotting using the obtained clones showed that, consistent with previous observations, glutamine tract expansion reduced transcriptional and translational expression of ATXN2. The cell clones with homozygous long-repeat alleles, which are rarely obtained from patients with SCA2, showed more drastic reduction of ATXN2 expression than the heterozygous clones. This study thus demonstrates the potential of UKiS, which is a beneficial platform for the efficient development of cell models not only for polyQ diseases but also for any other genetic diseases, which may accelerate our deeper understanding of disease mechanisms and cell-based screening for therapeutic drugs.

Targeting Ion Channels and Purkinje Neuron Intrinsic Membrane Excitability as a Therapeutic Strategy for Cerebellar Ataxia

In degenerative neurological disorders such as Parkinson's disease, a convergence of widely varying insults results in a loss of dopaminergic neurons and, thus, the motor symptoms of the disease. Dopamine replacement therapy with agents such as levodopa is a mainstay of therapy. Cerebellar ataxias, a heterogeneous group of currently untreatable conditions, have not been identified to have a shared physiology that is a target of therapy. In this review, we propose that perturbations in cerebellar Purkinje neuron intrinsic membrane excitability, a result of ion channel dysregulation, is a common pathophysiologic mechanism that drives motor impairment and vulnerability to degeneration in cerebellar ataxias of widely differing genetic etiologies. We further propose that treatments aimed at restoring Purkinje neuron intrinsic membrane excitability have the potential to be a shared therapy in cerebellar ataxia akin to levodopa for Parkinson's disease.

Slc9a6 mutation causes Purkinje cell loss and ataxia in the shaker rat

The shaker rat carries a naturally occurring mutation leading to progressive ataxia characterized by Purkinje cell (PC) loss. We previously reported on fine-mapping the shaker locus to the long arm of the rat X chromosome. In this work, we sought to identify the mutated gene underlying the shaker phenotype and confirm its identity by functional complementation. We fine-mapped the candidate region and analyzed cerebellar transcriptomes, identifying a XM_217630.9 (Slc9a6):c.[191_195delinsA] variant in the Slc9a6 gene that segregated with disease. We generated an adeno-associated virus (AAV) targeting Slc9a6 expression to PCs using the mouse L7-6 (L7) promoter. We administered the AAV prior to the onset of PC degeneration through intracerebroventricular injection and found that it reduced the shaker motor, molecular and cellular phenotypes. Therefore, Slc9a6 is mutated in shaker and AAV-based gene therapy may be a viable therapeutic strategy for Christianson syndrome, also caused by Slc9a6 mutation.

Targeted long-read sequencing captures CRISPR editing and AAV integration outcomes in brain

Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing is an emerging therapeutic modality that shows promise in Huntington's disease and spinocerebellar ataxia (SCA) mouse models. However, advancing CRISPR-based therapies requires methods to fully define in vivo editing outcomes. Here, we use polymerase-free, targeted long-read nanopore sequencing and evaluate single- and dual-gRNA AAV-CRISPR editing of human ATXN2 in transgenic mouse models of SCA type 2 (SCA2). Unbiased high sequencing coverage showed 10%-25% editing. Along with intended edits there was AAV integration, 1%-2% of which contained the entire AAV genome and were largely unmethylated. More than 150 kb deletions at target loci and rearrangements of the transgenic allele (1%) were also found. In contrast, PCR-based nanopore sequencing showed bias for partial AAV fragments and inverted terminal repeats (ITRs) and failed to detect full-length AAV. Cumulatively this work defines the spectrum of outcomes of CRISPR editing in mouse brain after AAV gene transfer using an unbiased long-read sequencing approach.

Neuro-respiratory pathology in spinocerebellar ataxia

The spinocerebellar ataxias (SCA) are a heterogeneous group of neurodegenerative disorders with an autosomal dominant inheritance. Symptoms include poor coordination and balance, peripheral neuropathy, impaired vision, incontinence, respiratory insufficiency, dysphagia, and dysarthria. Although many patients with SCA have respiratory-related complications, the exact mechanism and extent of this pathology remain unclear. This review aims to provide an update on the recent clinical and preclinical scientific findings on neuropathology causing respiratory insufficiency in SCA.

Transcriptome Sequencing Reveal That Rno-Rsf1_0012 Participates in Levodopa-Induced Dyskinesia in Parkinson's Disease Rats via Binding to Rno-mir-298-5p

Levodopa-induced dyskinesia (LID) is a common complication of chronic dopamine replacement therapy in the treatment of Parkinson’s disease (PD), and a noble cause of disability in advanced PD patients. Circular RNA (circRNA) is a novel type of non-coding RNA with a covalently closed-loop structure, which can regulate gene expression and participate in many biological processes. However, the biological roles of circRNAs in LID are not completely known. In the present study, we established typical LID rat models by unilateral lesions of the medial forebrain bundle and repeated levodopa therapy. High-throughput next-generation sequencing was used to screen circRNAs differentially expressed in the brain of LID and non-LID (NLID) rats, and key circRNAs were selected according to bioinformatics analyses. Regarding fold change ≥2 and p < 0.05 as the cutoff value, there were a total of 99 differential circRNAs, including 39 up-regulated and 60 down-regulated circRNAs between the NLID and LID groups. The expression of rno-Rsf1_0012 was significantly increased in the striatum of LID rats and competitively bound rno-mir-298-5p. The high expression of target genes PCP and TBP in LID rats also supports the conclusion that rno-Rsf1_0012 may be related to the occurrence of LID.

The Emerging Key Role of the mGluR1-PKCγ Signaling Pathway in the Pathogenesis of Spinocerebellar Ataxias: A Neurodevelopmental Viewpoint

Spinocerebellar ataxias (SCAs) are a heterogeneous group of autosomal dominantly inherited progressive disorders with degeneration and dysfunction of the cerebellum. Although different subtypes of SCAs are classified according to the disease-associated causative genes, the clinical syndrome of the ataxia is shared, pointing towards a possible convergent pathogenic pathway among SCAs. In this review, we summarize the role of SCA-associated gene function during cerebellar Purkinje cell development and discuss the relationship between SCA pathogenesis and neurodevelopment. We will summarize recent studies on molecules involved in SCA pathogenesis and will focus on the mGluR1-PKCγ signaling pathway evaluating the possibility that this might be a common pathway which contributes to these diseases.

A quantitative high-throughput screen identifies compounds that lower expression of the SCA2-and ALS-associated gene ATXN2

CAG repeat expansions in the ATXN2 (ataxin-2) gene can cause the autosomal dominant disorder spinocerebellar ataxia type 2 (SCA2) as well as increase the risk of ALS. Abnormal molecular, motor, and neurophysiological phenotypes in SCA2 mouse models are normalized by lowering ATXN2 transcription, and reduction of nonmutant Atxn2 expression has been shown to increase the life span of mice overexpressing the TDP-43 (transactive response DNA-binding protein 43 kDa) ALS protein, demonstrating the potential benefits of targeting ATXN2 transcription in humans. Here, we describe a quantitative high-throughput screen to identify compounds that lower ATXN2 transcription. We screened 428,759 compounds in a multiplexed assay using an ATXN2-luciferase reporter in human embryonic kidney 293 (HEK-293) cells and identified a diverse set of compounds capable of lowering ATXN2 transcription. We observed dose-dependent reductions of endogenous ATXN2 in HEK-293 cells treated with procillaridin A, 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), and heat shock protein 990 (HSP990), known inhibitors of HSP90 and Na+/K+-ATPases. Furthermore, HEK-293 cells expressing polyglutamine-expanded ATXN2-Q58 treated with 17-DMAG had minimally detectable ATXN2, as well as normalized markers of autophagy and endoplasmic reticulum stress, including STAU1 (Staufen 1), molecular target of rapamycin, p62, LC3-II (microtubule-associated protein 1A/1B-light chain 3II), CHOP (C/EBP homologous protein), and phospho-eIF2α (eukaryotic initiation factor 2α). Finally, bacterial artificial chromosome ATXN2-Q22 mice treated with 17-DMAG or HSP990 exhibited highly reduced ATXN2 protein abundance in the cerebellum. Taken together, our study demonstrates inhibition of HSP90 or Na+/K+-ATPases as potentially effective therapeutic strategies for treating SCA2 and ALS.

CRISPR-Cas13-Mediated Knockdown of Regulator of G-Protein Signaling 8 (RGS8) Does Not Affect Purkinje Cell Dendritic Development

CRISPR-Cas13 technology is rapidly evolving as it is a very specific tool for RNA editing and interference. Since there are no significant off-target effects via the Cas13-mediated method, it is a promising tool for studying gene function in differentiating neurons. In this study, we designed two crRNA targeting regulator of G-protein signaling 8 (RGS8), which is a signaling molecule associated with spinocerebellar ataxias. Using CRISPR-Cas13 technology, we found that both of crRNAs could specifically achieve RGS8 knockdown. By observing and comparing the dendritic growth of Purkinje cells, we found that CRISPR-Cas13-mediated RGS8 knockdown did not significantly affect Purkinje cell dendritic development. We further tested the role of RGS8 by classical RNAi. Again, the results of the RNAi-mediated RGS8 knockdown showed that reduced RGS8 expression did not significantly affect the dendritic growth of Purkinje cells. This is the first example of CRISPR-Cas13-mediated gene function study in Purkinje cells and establishes CRISPR-Cas13-mediated knockdown as a reliable method for studying gene function in primary neurons.

ATXN2 intermediate expansions in amyotrophic lateral sclerosis

Intermediate CAG (polyQ) expansions in the gene ataxin-2 (ATXN2) are now recognized as a risk factor for amyotrophic lateral sclerosis. The threshold for increased risk is not yet firmly established, with reports ranging from 27 to 31 repeats. We investigated the presence of ATXN2 polyQ expansions in 9268 DNA samples collected from people with amyotrophic lateral sclerosis, amyotrophic lateral sclerosis with frontotemporal dementia, frontotemporal dementia alone, Lewy body dementia and age matched controls. This analysis confirmed ATXN2 intermediate polyQ expansions of ≥31 as a risk factor for amyotrophic lateral sclerosis with an odds ratio of 6.31. Expansions were an even greater risk for amyotrophic lateral sclerosis with frontotemporal dementia (odds ratio 27.59) and a somewhat lesser risk for frontotemporal dementia alone (odds ratio 3.14). There was no increased risk for Lewy body dementia. In a subset of 1362 patients with amyotrophic lateral sclerosis with complete clinical data, we could not confirm previous reports of earlier onset of amyotrophic lateral sclerosis or shorter survival in 25 patients with expansions. These new data confirm ≥31 polyQ repeats in ATXN2 increase the risk for amyotrophic lateral sclerosis, and also for the first time show an even greater risk for amyotrophic lateral sclerosis with frontotemporal dementia. The lack of a more aggressive phenotype in amyotrophic lateral sclerosis patients with expansions has implications for ongoing gene-silencing trials for amyotrophic lateral sclerosis.

Autophagy in Spinocerebellar ataxia type 2, a dysregulated pathway, and a target for therapy

Spinocerebellar ataxia type 2 (SCA2) is an incurable and genetic neurodegenerative disorder. The disease is characterized by progressive degeneration of several brain regions, resulting in severe motor and non-motor clinical manifestations. The mutation causing SCA2 disease is an abnormal expansion of CAG trinucleotide repeats in the ATXN2 gene, leading to a toxic expanded polyglutamine segment in the translated ataxin-2 protein. While the genetic cause is well established, the exact mechanisms behind neuronal death induced by mutant ataxin-2 are not yet completely understood. Thus, the goal of this study is to investigate the role of autophagy in SCA2 pathogenesis and investigate its suitability as a target for therapeutic intervention. For that, we developed and characterized a new striatal lentiviral mouse model that resembled several neuropathological hallmarks observed in SCA2 disease, including formation of aggregates, neuronal marker loss, cell death and neuroinflammation. In this new model, we analyzed autophagic markers, which were also analyzed in a SCA2 cellular model and in human post-mortem brain samples. Our results showed altered levels of SQSTM1 and LC3B in cells and tissues expressing mutant ataxin-2. Moreover, an abnormal accumulation of these markers was detected in SCA2 patients' striatum and cerebellum. Importantly, the molecular activation of autophagy, using the compound cordycepin, mitigated the phenotypic alterations observed in disease models. Overall, our study suggests an important role for autophagy in the context of SCA2 pathology, proposing that targeting this pathway could be a potential target to treat SCA2 patients.

RNA Toxicity and Perturbation of rRNA Processing in Spinocerebellar Ataxia Type 2

BACKGROUND

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disease caused by expansion of a CAG repeat in Ataxin-2 (ATXN2) gene. The mutant ATXN2 protein with a polyglutamine tract is known to be toxic and contributes to the SCA2 pathogenesis.

OBJECTIVE

Here, we tested the hypothesis that the mutant ATXN2 transcript with an expanded CAG repeat (expATXN2) is also toxic and contributes to SCA2 pathogenesis.

METHODS

The toxic effect of expATXN2 transcripts on SK-N-MC neuroblastoma cells and primary mouse cortical neurons was evaluated by caspase 3/7 activity and nuclear condensation assay, respectively. RNA immunoprecipitation assay was performed to identify RNA binding proteins (RBPs) that bind to expATXN2 RNA. Quantitative PCR was used to examine if ribosomal RNA (rRNA) processing is disrupted in SCA2 and Huntington's disease (HD) human brain tissue.

RESULTS

expATXN2 RNA induces neuronal cell death, and aberrantly interacts with RBPs involved in RNA metabolism. One of the RBPs, transducin β-like protein 3 (TBL3), involved in rRNA processing, binds to both expATXN2 and expanded huntingtin (expHTT) RNA in vitro. rRNA processing is disrupted in both SCA2 and HD human brain tissue.

CONCLUSION

These findings provide the first evidence of a contributory role of expATXN2 transcripts in SCA2 pathogenesis, and further support the role of expHTT transcripts in HD pathogenesis. The disruption of rRNA processing, mediated by aberrant interaction of RBPs with expATXN2 and expHTT transcripts, suggest a point of convergence in the pathogeneses of repeat expansion diseases with potential therapeutic implications. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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.

ALS-associated genes in SCA2 mouse spinal cord transcriptomes

The spinocerebellar ataxia type 2 (SCA2) gene ATXN2 has a prominent role in the pathogenesis and treatment of amyotrophic lateral sclerosis (ALS). In addition to cerebellar ataxia, motor neuron disease is often seen in SCA2, and ATXN2 CAG repeat expansions in the long normal range increase ALS risk. Also, lowering ATXN2 expression in TDP-43 ALS mice prolongs their survival. Here we investigated the ATXN2 relationship with motor neuron dysfunction in vivo by comparing spinal cord (SC) transcriptomes reported from TDP-43 and SOD1 ALS mice and ALS patients with those from SCA2 mice. SC transcriptomes were determined using an SCA2 bacterial artificial chromosome mouse model expressing polyglutamine expanded ATXN2. SCA2 cerebellar transcriptomes were also determined, and we also investigated the modification of gene expression following treatment of SCA2 mice with an antisense oligonucleotide (ASO) lowering ATXN2 expression. Differentially expressed genes (DEGs) defined three interconnected pathways (innate immunity, fatty acid biosynthesis and cholesterol biosynthesis) in separate modules identified by weighted gene co-expression network analysis. Other key pathways included the complement system and lysosome/phagosome pathways. Of all DEGs in SC, 12.6% were also dysregulated in the cerebellum. Treatment of mice with an ATXN2 ASO also modified innate immunity, the complement system and lysosome/phagosome pathways. This study provides new insights into the underlying molecular basis of SCA2 SC phenotypes and demonstrates annotated pathways shared with TDP-43 and SOD1 ALS mice and ALS patients. It also emphasizes the importance of ATXN2 in motor neuron degeneration and confirms ATXN2 as a therapeutic target.

SCA7 Mouse Cerebellar Pathology Reveals Preferential Downregulation of Key Purkinje Cell-Identity Genes and Shared Disease Signature with SCA1 and SCA2

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disease mainly characterized by motor incoordination because of progressive cerebellar degeneration. SCA7 is caused by polyglutamine expansion in ATXN7, a subunit of the transcriptional coactivator SAGA, which harbors histone modification activities. Polyglutamine expansions in specific proteins are also responsible for SCA1-SCA3, SCA6, and SCA17; however, the converging and diverging pathomechanisms remain poorly understood. Using a new SCA7 knock-in mouse, SCA7140Q/5Q, we analyzed gene expression in the cerebellum and assigned gene deregulation to specific cell types using published datasets. Gene deregulation affects all cerebellar cell types, although at variable degree, and correlates with alterations of SAGA-dependent epigenetic marks. Purkinje cells (PCs) are by far the most affected neurons and show reduced expression of 83 cell-type identity genes, including these critical for their spontaneous firing activity and synaptic functions. PC gene downregulation precedes morphologic alterations, pacemaker dysfunction, and motor incoordination. Strikingly, most PC genes downregulated in SCA7 have also decreased expression in SCA1 and SCA2 mice, revealing converging pathomechanisms and a common disease signature involving cGMP-PKG and phosphatidylinositol signaling pathways and LTD. Our study thus points out molecular targets for therapeutic development, which may prove beneficial for several SCAs. Furthermore, we show that SCA7140Q/5Q males and females exhibit the major disease features observed in patients, including cerebellar damage, cerebral atrophy, peripheral nerves pathology, and photoreceptor dystrophy, which account for progressive impairment of behavior, motor, and visual functions. SCA7140Q/5Q mice represent an accurate model for the investigation of different aspects of SCA7 pathogenesis.SIGNIFICANCE STATEMENT Spinocerebellar ataxia 7 (SCA7) is one of the several forms of inherited SCAs characterized by cerebellar degeneration because of polyglutamine expansion in specific proteins. The ATXN7 involved in SCA7 is a subunit of SAGA transcriptional coactivator complex. To understand the pathomechanisms of SCA7, we determined the cell type-specific gene deregulation in SCA7 mouse cerebellum. We found that the Purkinje cells are the most affected cerebellar cell type and show downregulation of a large subset of neuronal identity genes, critical for their spontaneous firing and synaptic functions. Strikingly, the same Purkinje cell genes are downregulated in mouse models of two other SCAs. Thus, our work reveals a disease signature shared among several SCAs and uncovers potential molecular targets for their treatment.

Antagonistic roles for Ataxin-2 structured and disordered domains in RNP condensation

Ataxin-2 (Atx2) is a translational control molecule mutated in spinocerebellar ataxia type II and amyotrophic lateral sclerosis. While intrinsically disordered domains (IDRs) of Atx2 facilitate mRNP condensation into granules, how IDRs work with structured domains to enable positive and negative regulation of target mRNAs remains unclear. Using the Targets of RNA-Binding Proteins Identified by Editing technology, we identified an extensive data set of Atx2-target mRNAs in the Drosophila brain and S2 cells. Atx2 interactions with AU-rich elements in 3'UTRs appear to modulate stability/turnover of a large fraction of these target mRNAs. Further genomic and cell biological analyses of Atx2 domain deletions demonstrate that Atx2 (1) interacts closely with target mRNAs within mRNP granules, (2) contains distinct protein domains that drive or oppose RNP-granule assembly, and (3) has additional essential roles outside of mRNP granules. These findings increase the understanding of neuronal translational control mechanisms and inform strategies for Atx2-based interventions under development for neurodegenerative disease.

Body Mass Index Is Significantly Associated With Disease Severity in Spinocerebellar Ataxia Type 2 Patients

BACKGROUND

Spinocerebellar ataxia type 2 is a progressive neurodegenerative disorder due to an unstable expansion of a CAG repeat in the ATXN2 gene. Although weight loss has been associated with disease progression in several neurodegenerative conditions, it has been barely assessed in patients with spinocerebellar ataxia type 2.

OBJECTIVE

The objective of this study was to test whether body mass index is altered in patients with spinocerebellar ataxia type 2 with varying expansion sizes from early to late disease stages.

METHODS

A cross-sectional case-control study was performed, which included 222 clinically and molecularly diagnosed patients and 214 sex- and age-matched healthy individuals. ATXN2 genotypes and sex were considered as risk factors. Clinical outcomes included the body mass index, age at onset, disease duration, Scale for the Assessment and Rating of Ataxia score, disease stage, dysphagia, and progression rate. Multiple linear regression models were generated.

RESULTS

Body mass index was significantly decreased in male patients, but not in female patients, relative to control subjects. In addition to sex, body mass index was significantly associated with age at onset and progression rate. Conversely, body mass index, along with repeat length in ATXN2 expanded alleles and disease duration, was associated with Scale for the Assessment and Rating of Ataxia score. In addition, body mass index, along with the age at onset and the repeat length in ATXN2 normal and expanded alleles, has a significant influence on progression rate.

CONCLUSIONS

Body mass index might be a useful biomarker of disease severity, particularly in male patients with spinocerebellar ataxia type 2 in the context of nutritional interventions or clinical trials assessing the efficacy of promising new drugs. © 2021 International Parkinson and Movement Disorder Society.

Modulation of Increased mGluR1 Signaling by RGS8 Protects Purkinje Cells From Dendritic Reduction and Could Be a Common Mechanism in Diverse Forms of Spinocerebellar Ataxia

Spinocerebellar ataxias (SCAs) are a group of hereditary neurodegenerative diseases which are caused by diverse genetic mutations in a variety of different genes. We have identified RGS8, a regulator of G-protein signaling, as one of the genes which are dysregulated in different mouse models of SCA (e.g., SCA1, SCA2, SCA7, and SCA14). In the moment, little is known about the role of RGS8 for pathogenesis of spinocerebellar ataxia. We have studied the expression of RGS8 in the cerebellum in more detail and show that it is specifically expressed in mouse cerebellar Purkinje cells. In a mouse model of SCA14 with increased PKCγ activity, RGS8 expression was also increased. RGS8 overexpression could partially counteract the negative effects of DHPG-induced mGluR1 signaling for the expansion of Purkinje cell dendrites. Our results suggest that the increased expression of RGS8 is an important mediator of mGluR1 pathway dysregulation in Purkinje cells. These findings provide new insights in the role of RGS8 and mGluR1 signaling in Purkinje cells and for the pathology of SCAs.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

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.

Altered Capicua expression drives regional Purkinje neuron vulnerability through ion channel gene dysregulation in spinocerebellar ataxia type 1

Selective neuronal vulnerability in neurodegenerative disease is poorly understood. Using the ATXN1[82Q] model of spinocerebellar ataxia type 1 (SCA1), we explored the hypothesis that regional differences in Purkinje neuron degeneration could provide novel insights into selective vulnerability. ATXN1[82Q] Purkinje neurons from the anterior cerebellum were found to degenerate earlier than those from the nodular zone, and this early degeneration was associated with selective dysregulation of ion channel transcripts and altered Purkinje neuron spiking. Efforts to understand the basis for selective dysregulation of channel transcripts revealed modestly increased expression of the ATXN1 co-repressor Capicua (Cic) in anterior cerebellar Purkinje neurons. Importantly, disrupting the association between ATXN1 and Cic rescued the levels of these ion channel transcripts, and lentiviral overexpression of Cic in the nodular zone accelerated both aberrant Purkinje neuron spiking and neurodegeneration. These findings reinforce the central role for Cic in SCA1 cerebellar pathophysiology and suggest that only modest reductions in Cic are needed to have profound therapeutic impact in SCA1.

Therapeutic antisense oligonucleotides for movement disorders

Movement disorders are a group of neurological conditions characterized by abnormalities of movement and posture. They are broadly divided into akinetic and hyperkinetic syndromes. Until now, no effective symptomatic or disease-modifying therapies have been available. However, since many of these disorders are monogenic or have some well-defined genetic component, they represent strong candidates for antisense oligonucleotide (ASO) therapies. ASO therapies are based on the use of short synthetic single-stranded ASOs that bind to disease-related target RNAs via Watson-Crick base-pairing and pleiotropically modulate their function. With information arising from the RNA sequence alone, it is possible to design ASOs that not only alter the expression levels but also the splicing defects of any protein, far exceeding the intervention repertoire of traditional small molecule approaches. Following the regulatory approval of ASO therapies for spinal muscular atrophy and Duchenne muscular dystrophy in 2016, there has been tremendous momentum in testing such therapies for other neurological disorders. This review article initially focuses on the chemical modifications aimed at improving ASO effectiveness, the mechanisms by which ASOs can interfere with RNA function, delivery systems and pharmacokinetics, and the common set of toxicities associated with their application. It, then, describes the pathophysiology and the latest information on preclinical and clinical trials utilizing ASOs for the treatment of Parkinson's disease, Huntington's disease, and ataxias 1, 2, 3, and 7. It concludes with issues that require special attention to realize the full potential of ASO-based therapies.

Antisense oligonucleotide therapeutics in neurodegenerative diseases: the case of polyglutamine disorders

Polyglutamine (polyQ) disorders are a group of nine neurodegenerative diseases that share a common genetic cause, which is an expansion of CAG repeats in the coding region of the causative genes that are otherwise unrelated. The trinucleotide expansion encodes for an expanded polyQ tract in the respective proteins, resulting in toxic gain-of-function and eventually in neurodegeneration. Currently, no disease-modifying therapies are available for this group of disorders. Nevertheless, given their monogenic nature, polyQ disorders are ideal candidates for therapies that target specifically the gene transcripts. Antisense oligonucleotides (ASOs) have been under intense investigation over recent years as gene silencing tools. ASOs are small synthetic single-stranded chains of nucleic acids that target specific RNA transcripts through several mechanisms. ASOs can reduce the levels of mutant proteins by breaking down the targeted transcript, inhibit mRNA translation or alter the maturation of the pre-mRNA via splicing correction. Over the years, chemical optimization of ASO molecules has allowed significant improvement of their pharmacological properties, which has in turn made this class of therapeutics a very promising strategy to treat a variety of neurodegenerative diseases. Indeed, preclinical and clinical strategies have been developed in recent years for some polyQ disorders using ASO therapeutics. The success of ASOs in several animal models, as well as encouraging results in the clinic for Huntington's disease, points towards a promising future regarding the application of ASO-based therapies for polyQ disorders in humans, offering new opportunities to address unmet medical needs for this class of disorders. This review aims to present a brief overview of key chemical modifications, mechanisms of action and routes of administration that have been described for ASO-based therapies. Moreover, it presents a review of the most recent and relevant preclinical and clinical trials that have tested ASO therapeutics in polyQ disorders.

Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Polyglutamine spinocerebellar ataxias (polyQ SCAs) include SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 and constitute a group of adult onset neurodegenerative disorders caused by the expansion of a CAG repeat sequence located within the coding region of specific genes, which translates into polyglutamine tract in the corresponding proteins. PolyQ SCAs are characterized by degeneration of the cerebellum and its associated structures and lead to progressive ataxia and other diverse symptoms. In recent years, gene and epigenetic deregulations have been shown to play a critical role in the pathogenesis of polyQ SCAs. Here, we provide an overview of the functions of wild type and pathogenic polyQ SCA proteins in gene regulation, describe the extent and nature of gene expression changes and their pathological consequences in diseases, and discuss potential avenues to further investigate converging and distinct disease pathways and to develop therapeutic strategies.

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.

Current Opinions and Consensus for Studying Tremor in Animal Models

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Molecular Mechanisms and Therapeutics for Spinocerebellar Ataxia Type 2

The effective therapeutic treatment and the disease-modifying therapy for spinocerebellar ataxia type 2 (SCA2) (a progressive hereditary disease caused by an expansion of polyglutamine in the ataxin-2 protein) is not available yet. At present, only symptomatic treatment and methods of palliative care are prescribed to the patients. Many attempts were made to study the physiological, molecular, and biochemical changes in SCA2 patients and in a variety of the model systems to find new therapeutic targets for SCA2 treatment. A better understanding of the uncovered molecular mechanisms of the disease allowed the scientific community to develop strategies of potential therapy and helped to create some promising therapeutic approaches for SCA2 treatment. Recent progress in this field will be discussed in this review article.

[Antisense therapies for neurological diseases]

Despite identification of many genes causing neurodegenerative diseases in the last decades, development of disease-modifying treatments has been slow. Antisense oligonucleotide (ASO) therapeutics for spinal muscular atrophy, Duchenne muscular dystrophy and transthyretin amyloidosis predict a robust future for ASOs in medicine. Perhaps the most significant advantage of ASO therapeutics over other small molecule approaches is that acquisition of the target sequence provides immediate knowledge of possible complementary oligonucleotide therapeutics. This review article describes the various types of ASOs, their therapeutic use and the current preclinical efforts to develop new ASO treatments.

Generation of an Atxn2-CAG100 knock-in mouse reveals N-acetylaspartate production deficit due to early Nat8l dysregulation

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disorder caused by CAG-expansion mutations in the ATXN2 gene, mainly affecting motor neurons in the spinal cord and Purkinje neurons in the cerebellum. While the large expansions were shown to cause SCA2, the intermediate length expansions lead to increased risk for several atrophic processes including amyotrophic lateral sclerosis and Parkinson variants, e.g. progressive supranuclear palsy. Intense efforts to pioneer a neuroprotective therapy for SCA2 require longitudinal monitoring of patients and identification of crucial molecular pathways. The ataxin-2 (ATXN2) protein is mainly involved in RNA translation control and regulation of nutrient metabolism during stress periods. The preferential mRNA targets of ATXN2 are yet to be determined. In order to understand the molecular disease mechanism throughout different prognostic stages, we generated an Atxn2-CAG100-knock-in (KIN) mouse model of SCA2 with intact murine ATXN2 expression regulation. Its characterization revealed somatic mosaicism of the expansion, with shortened lifespan, a progressive spatio-temporal pattern of pathology with subsequent phenotypes, and anomalies of brain metabolites such as N-acetylaspartate (NAA), all of which mirror faithfully the findings in SCA2 patients. Novel molecular analyses from stages before the onset of motor deficits revealed a strong selective effect of ATXN2 on Nat8l mRNA which encodes the enzyme responsible for NAA synthesis. This metabolite is a prominent energy store of the brain and a well-established marker for neuronal health. Overall, we present a novel authentic rodent model of SCA2, where in vivo magnetic resonance imaging was feasible to monitor progression and where the definition of earliest transcriptional abnormalities was possible. We believe that this model will not only reveal crucial insights regarding the pathomechanism of SCA2 and other ATXN2-associated disorders, but will also aid in developing gene-targeted therapies and disease prevention.

Antisense therapies for movement disorders

Currently, few disease-modifying therapies exist for degenerative movement disorders. Antisense oligonucleotides are small DNA oligonucleotides, usually encompassing ∼20 base pairs, that can potentially target any messenger RNA of interest. Antisense oligonucleotides often contain modifications to the phosphate backbone, the sugar moiety, and the nucleotide base. The development of antisense oligonucleotide therapies spinal muscular atrophy and Duchenne muscular dystrophy suggest potentially wide-ranging therapeutic applications for antisense oligonucleotides in neurology. Successes with these two diseases have heightened interest in academia and the pharmaceutical industry to develop antisense oligonucleotides for several movement disorders, including, spinocerebellar ataxias, Huntington's disease, and Parkinson's disease. Compared to small molecules, antisense oligonucleotide-based therapies have an advantage because the target disease gene sequence is the immediate path to identifying the therapeutically effective complementary antisense oligonucleotide. In this review we describe the different types of antisense oligonucleotide chemistries and their potential use for the treatment of human movement disorders. © 2019 International Parkinson and Movement Disorder Society.

Antisense oligonucleotides: A primer

There are few disease-modifying therapeutics for neurodegenerative diseases, but successes on the development of antisense oligonucleotide (ASO) therapeutics for spinal muscular atrophy and Duchenne muscular dystrophy predict a robust future for ASOs in medicine. Indeed, existing pipelines for the development of ASO therapies for spinocerebellar ataxias, Huntington disease, Alzheimer disease, amyotrophic lateral sclerosis, Parkinson disease, and others, and increased focus by the pharmaceutical industry on ASO development, strengthen the outlook for using ASOs for neurodegenerative diseases. Perhaps the most significant advantage to ASO therapeutics over other small molecule approaches is that acquisition of the target sequence provides immediate knowledge of putative complementary oligonucleotide therapeutics. In this review, we describe the various types of ASOs, how they are used therapeutically, and the present efforts to develop new ASO therapies that will contribute to a forthcoming toolkit for treating multiple neurodegenerative diseases.

MTSS1/Src family kinase dysregulation underlies multiple inherited ataxias

The genetically heterogeneous spinocerebellar ataxias (SCAs) are caused by Purkinje neuron dysfunction and degeneration, but their underlying pathological mechanisms remain elusive. The Src family of nonreceptor tyrosine kinases (SFK) are essential for nervous system homeostasis and are increasingly implicated in degenerative disease. Here we reveal that the SFK suppressor Missing-in-metastasis (MTSS1) is an ataxia locus that links multiple SCAs. MTSS1 loss results in increased SFK activity, reduced Purkinje neuron arborization, and low basal firing rates, followed by cell death. Surprisingly, mouse models for SCA1, SCA2, and SCA5 show elevated SFK activity, with SCA1 and SCA2 displaying dramatically reduced MTSS1 protein levels through reduced gene expression and protein translation, respectively. Treatment of each SCA model with a clinically approved Src inhibitor corrects Purkinje neuron basal firing and delays ataxia progression in MTSS1 mutants. Our results identify a common SCA therapeutic target and demonstrate a key role for MTSS1/SFK in Purkinje neuron survival and ataxia progression.

ALS/FTD-Linked Mutation in FUS Suppresses Intra-axonal Protein Synthesis and Drives Disease Without Nuclear Loss-of-Function of FUS

Through the generation of humanized FUS mice expressing full-length human FUS, we identify that when expressed at near endogenous murine FUS levels, both wild-type and ALS-causing and frontotemporal dementia (FTD)-causing mutations complement the essential function(s) of murine FUS. Replacement of murine FUS with mutant, but not wild-type, human FUS causes stress-mediated induction of chaperones, decreased expression of ion channels and transporters essential for synaptic function, and reduced synaptic activity without loss of nuclear FUS or its cytoplasmic aggregation. Most strikingly, accumulation of mutant human FUS is shown to activate an integrated stress response and to inhibit local, intra-axonal protein synthesis in hippocampal neurons and sciatic nerves. Collectively, our evidence demonstrates that human ALS/FTD-linked mutations in FUS induce a gain of toxicity that includes stress-mediated suppression in intra-axonal translation, synaptic dysfunction, and progressive age-dependent motor and cognitive disease without cytoplasmic aggregation, altered nuclear localization, or aberrant splicing of FUS-bound pre-mRNAs. VIDEO ABSTRACT.

Conserved Pbp1/Ataxin-2 regulates retrotransposon activity and connects polyglutamine expansion-driven protein aggregation to lifespan-controlling rDNA repeats

Ribosomal DNA (rDNA) repeat instability and protein aggregation are thought to be two major and independent drivers of cellular aging. Pbp1, the yeast ortholog of human ATXN2, maintains rDNA repeat stability and lifespan via suppression of RNA-DNA hybrids. ATXN2 polyglutamine expansion drives neurodegeneration causing spinocerebellar ataxia type 2 and promoting amyotrophic lateral sclerosis. Here, molecular characterization of Pbp1 revealed that its knockout or subjection to disease-modeling polyQ expansion represses Ty1 (Transposons of Yeast) retrotransposons by respectively promoting Trf4-depedendent RNA turnover and Ty1 Gag protein aggregation. This aggregation, but not its impact on retrotransposition, compromises rDNA repeat stability and shortens lifespan by hyper-activating Trf4-dependent turnover of intergenic ncRNA within the repeats. We uncover a function for the conserved Pbp1/ATXN2 proteins in the promotion of retrotransposition, create and describe powerful yeast genetic models of ATXN2-linked neurodegenerative diseases, and connect the major aging mechanisms of rDNA instability and protein aggregation.

Staufen1 links RNA stress granules and autophagy in a model of neurodegeneration

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disease caused by expansion of polyglutamine tract in the ATXN2 protein. We identified Staufen1 (STAU1) as an interactor of ATXN2, and showed elevation in cells from SCA2 patients, amyotrophic lateral sclerosis (ALS) patients, and in SCA2 mouse models. We demonstrated recruitment of STAU1 to mutant ATXN2 aggregates in brain tissue from patients with SCA2 human brain and in an SCA2 mouse model, and association of STAU1 elevation with dysregulation of SCA2-related transcript abundances. Targeting STAU1 in vitro by RNAi restored PCP2 transcript levels and lowering mutant ATXN2 also normalized STAU1 levels. Reduction of Stau1 in vivo improved motor behavior in an SCA2 mouse model, normalized the levels of several SCA2-related proteins, and reduced aggregation of polyglutamine-expanded ATXN2. These findings suggest a function for STAU1 in aberrant RNA metabolism associated with ATXN2 mutation, suggesting STAU1 is a possible novel therapeutic target for SCA2.

Oligonucleotide therapeutics in neurodegenerative diseases

Therapeutics that directly target RNAs are promising for a broad spectrum of disorders, including the neurodegenerative diseases. This is exemplified by the FDA approval of Nusinersen, an antisense oligonucleotide (ASO) therapeutic for spinal muscular atrophy (SMA). RNA targeting therapeutics are currently under development for amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and spinocerebellar ataxias. We have used an ASO approach toward developing a treatment for spinocerebellar ataxia type 2 (SCA2), for targeting the causative gene ATXN2. We demonstrated that reduction of ATXN2 expression in SCA2 mice treated by intracerebroventicular injection (ICV) of ATXN2 ASO delayed motor phenotype onset, improved the expression of several genes demonstrated abnormally reduced by transcriptomic profiling of SCA2 mice, and restored abnormal Purkinje cell firing frequency in acute cerebellar sections. Here we discuss RNA abnormalities in disease and the prospects of targeting neurodegenerative diseases at the level of RNA control using ASOs and other RNA-targeted therapeutics.

Ca2+ signaling and spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a neural disorder, which is caused by degenerative changes in the cerebellum. SCA is primarily characterized by gait ataxia, and additional clinical features include nystagmus, dysarthria, tremors and cerebellar atrophy. Forty-four hereditary SCAs have been identified to date, along with >35 SCA-associated genes. Despite the great diversity and distinct functionalities of the SCA-related genes, accumulating evidence supports the occurrence of a common pathophysiological event among several hereditary SCAs. Altered calcium (Ca²⁺) homeostasis in the Purkinje cells (PCs) of the cerebellum has been proposed as a possible pathological SCA trigger. In support of this, signaling events that are initiated from or lead to aberrant Ca²⁺ release from the type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1), which is highly expressed in cerebellar PCs, seem to be closely associated with the pathogenesis of several SCA types. In this review, we summarize the current research on pathological hereditary SCA events, which involve altered Ca²⁺ homeostasis in PCs, through IP₃R1 signaling.

Potassium channel dysfunction underlies Purkinje neuron spiking abnormalities in spinocerebellar ataxia type 2

Alterations in Purkinje neuron firing often accompany ataxia, but the molecular basis for these changes is poorly understood. In a mouse model of spinocerebellar ataxia type 2 (SCA2), a progressive reduction in Purkinje neuron firing frequency accompanies cell atrophy. We investigated the basis for altered Purkinje neuron firing in SCA2. A reduction in the expression of large-conductance calcium-activated potassium (BK) channels and Kv3.3 voltage-gated potassium channels accompanies the inability of Purkinje neurons early in disease to maintain repetitive spiking. In association with prominent Purkinje neuron atrophy, repetitive spiking is restored, although at a greatly reduced firing frequency. In spite of a continued impairment in spike repolarization and a persistently reduced BK channel mediated afterhyperpolarization (AHP), repetitive spiking is maintained, through the increased activity of barium-sensitive potassium channels, most consistent with inwardly rectifying potassium (Kir) channels. Increased activity of Kir channels results in the generation of a novel AHP not seen in wild-type Purkinje neurons that also accounts for the reduced firing frequency late in disease. Homeostatic changes in Purkinje neuron morphology that help to preserve repetitive spiking can also therefore have deleterious consequences for spike frequency. These results suggest that the basis for spiking abnormalities in SCA2 differ depending on disease stage, and interventions targeted towards correcting potassium channel dysfunction in ataxia need to be tailored to the specific stage in the degenerative process.

Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2) is autosomal dominantly inherited and caused by CAG repeat expansion in the ATXN2 gene. Because the CAG repeat expansion is localized to an encoded region of ATXN2, the result is an expanded polyglutamine (polyQ) tract in the ATXN2 protein. SCA2 is characterized by progressive ataxia, and slow saccades. No treatment for SCA2 exists. ATXN2 mutation causes gains of new or toxic functions for the ATXN2 protein, resulting in abnormally slow Purkinje cell (PC) firing frequency and ultimately PC loss. This chapter describes the characteristics of SCA2 patients briefly, and reviews ATXN2 molecular features and progress toward the identification of a treatment for SCA2.

Regulating the regulators: Epigenetic, transcriptional, and post-translational regulation of RGS proteins

Regulators of G protein signaling (RGS) are a family of proteins classically known to accelerate the intrinsic GTPase activity of G proteins, which results in accelerated inactivation of heterotrimeric G proteins and inhibition of G protein coupled receptor signaling. RGS proteins play major roles in essential cellular processes, and dysregulation of RGS protein expression is implicated in multiple diseases, including cancer, cardiovascular and neurodegenerative diseases. The expression of RGS proteins is highly dynamic and is regulated by epigenetic, transcriptional and post-translational mechanisms. This review summarizes studies that report dysregulation of RGS protein expression in disease states, and presents examples of drugs that regulate RGS protein expression. Additionally, this review discusses, in detail, the transcriptional and post-transcriptional mechanisms regulating RGS protein expression, and further assesses the therapeutic potential of targeting these mechanisms. Understanding the molecular mechanisms controlling the expression of RGS proteins is essential for the development of therapeutics that indirectly modulate G protein signaling by regulating expression of RGS proteins.

Polyglutamine spinocerebellar ataxias - from genes to potential treatments

The dominantly inherited spinocerebellar ataxias (SCAs) are a large and diverse group of neurodegenerative diseases. The most prevalent SCAs (SCA1, SCA2, SCA3, SCA6 and SCA7) are caused by expansion of a glutamine-encoding CAG repeat in the affected gene. These SCAs represent a substantial portion of the polyglutamine neurodegenerative disorders and provide insight into this class of diseases as a whole. Recent years have seen considerable progress in deciphering the clinical, pathological, physiological and molecular aspects of the polyglutamine SCAs, with these advances establishing a solid base from which to pursue potential therapeutic approaches.

Gene co-expression network analysis for identifying modules and functionally enriched pathways in SCA2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disease caused by CAG repeat expansion in the ATXN2 gene. The repeat resides in an encoded region of the gene resulting in polyglutamine (polyQ) expansion which has been assumed to result in gain of function, predominantly, for the ATXN2 protein. We evaluated temporal cerebellar expression profiles by RNA sequencing of ATXN2Q127 mice versus wild-type (WT) littermates. ATXN2Q127 mice are characterized by a progressive motor phenotype onset, and have progressive cerebellar molecular and neurophysiological (Purkinje cell firing frequency) phenotypes. Our analysis revealed previously uncharacterized early and progressive abnormal patterning of cerebellar gene expression. Weighted Gene Coexpression Network Analysis revealed four gene modules that were significantly correlated with disease status, composed primarily of genes associated with GTPase signaling, calcium signaling and cell death. Of these genes, few overlapped with differentially expressed cerebellar genes that we identified in Atxn2-/- knockout mice versus WT littermates, suggesting that loss-of-function is not a significant component of disease pathology. We conclude that SCA2 is a disease characterized by gain of function for ATXN2.

ATXN2-AS, a gene antisense to ATXN2, is associated with spinocerebellar ataxia type 2 and amyotrophic lateral sclerosis

OBJECTIVE

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disease caused by a CAG repeat expansion in the gene ataxin-2 (ATXN2). ATXN2 intermediate-length CAG expansions were identified as a risk factor for amyotrophic lateral sclerosis (ALS). The ATXN2 CAG repeat is translated into polyglutamine, and SCA2 pathogenesis has been thought to derive from ATXN2 protein containing an expanded polyglutamine tract. However, recent evidence of bidirectional transcription at multiple CAG/CTG disease loci has led us to test whether additional mechanisms of pathogenesis may contribute to SCA2.

METHODS

In this work, using human postmortem tissue, various cell models, and animal models, we provide the first evidence that an antisense transcript at the SCA2 locus contributes to SCA2 pathogenesis.

RESULTS

We demonstrate the expression of a transcript, containing the repeat as a CUG tract, derived from a gene (ATXN2-AS) directly antisense to ATXN2. ATXN2-AS transcripts with normal and expanded CUG repeats are expressed in human postmortem SCA2 brains, human SCA2 fibroblasts, induced SCA2 pluripotent stem cells, SCA2 neural stem cells, and lymphoblastoid lines containing an expanded ATXN2 allele associated with ALS. ATXN2-AS transcripts with a CUG repeat expansion are toxic in an SCA2 cell model and form RNA foci in SCA2 cerebellar Purkinje cells. Finally, we detected missplicing of amyloid beta precursor protein and N-methyl-D-aspartate receptor 1 in SCA2 brains, consistent with findings in other diseases characterized by RNA-mediated pathogenesis.

INTERPRETATION

These results suggest that ATXN2-AS has a role in SCA2 and possibly ALS pathogenesis, and may therefore provide a novel therapeutic target for these diseases. Ann Neurol 2016;80:600-615.

A positive feedback loop linking enhanced mGluR function and basal calcium in spinocerebellar ataxia type 2

Metabotropic glutamate receptor 1 (mGluR1) function in Purkinje neurons (PNs) is essential for cerebellar development and for motor learning and altered mGluR1 signaling causes ataxia. Downstream of mGluR1, dysregulation of calcium homeostasis has been hypothesized as a key pathological event in genetic forms of ataxia but the underlying mechanisms remain unclear. We find in a spinocerebellar ataxia type 2 (SCA2) mouse model that calcium homeostasis in PNs is disturbed across a broad range of physiological conditions. At parallel fiber synapses, mGluR1-mediated excitatory postsynaptic currents (EPSCs) and associated calcium transients are increased and prolonged in SCA2 PNs. In SCA2 PNs, enhanced mGluR1 function is prevented by buffering [Ca²⁺] at normal resting levels while in wildtype PNs mGluR1 EPSCs are enhanced by elevated [Ca²⁺]. These findings demonstrate a deleterious positive feedback loop involving elevated intracellular calcium and enhanced mGluR1 function, a mechanism likely to contribute to PN dysfunction and loss in SCA2.

DOI:http://dx.doi.org/10.7554/eLife.26377.001

Degenerative ataxias, from genes to therapies: The 2015 Cotzias Lecture

OBJECTIVE

To review progress in spinocerebellar ataxias (SCAs) and novel approaches to treatment.

RESULTS AND CONCLUSIONS

Autosomal dominant ataxias are now referred to as SCAs, with polyglutamine expansion mutations constituting the most common cause of SCAs. Phenotypic variation in patients with SCA is remarkable even in patients with identical mutations. In patients with SCA2, cerebellar ataxia is typically associated with slowed saccadic eye movements. In addition to classic cerebellar and brainstem signs, however, SCA2 can also present as a parkinsonian syndrome or as amyotrophic lateral sclerosis. After identifying the SCA2 gene (gene symbol ATXN2) in 1996, we generated several mouse models that recapitulated salient features of the human disease. In these models, behavioral and physiologic changes preceded cell death. Modified antisense oligonucleotides (ASOs) provide a unique tool to target mRNA transcripts in vivo with extended stability of ASOs and better activation of RNAse H. We generated methoxyethyl group-gapmer ASOs that reduced ATXN2 expression >80% in vitro and then progressed the lead ASO to in vivo testing in an SCA2 mouse model. Compared to intraventricular injection of saline, treatment with ASO resulted in significant knockdown of endogenous mouse and human transgenic ATXN2. In addition, progression of the motor phenotype was slowed and Purkinje cell firing in the acute cerebellar slice normalized. ASO-based therapies are underway in humans providing hope that this approach will also be applicable to patients with cerebellar degenerations.

Antisense oligonucleotide therapy for spinocerebellar ataxia type 2

Adult human neurodegenerative diseases have no disease-modifying treatments. We used spinocerebellar ataxia type 2 (SCA2), an autosomal dominant polyglutamine disease1, as a model to test RNA-targeted therapies² in two SCA2 mouse models. Both models recreate progressive adult-onset dysfunction and degeneration of a neuronal network including decreased firing frequency of cerebellar Purkinje cells (PCs) and decline in motor function^3,4. We developed a potential therapy directed at the ATXN2 gene by screening 152 antisense oligonucleotides (ASOs). Here we show that the most promising lead, ASO7, downregulated ATXN2 mRNA and protein resulting in delayed onset of SCA2 phenotypes. After delivery by intracerebroventricular injection (ICV) to ATXN2-Q127 mice, ASO7 localized to PCs, reduced cerebellar ATXN2 expression below 75% for >10 weeks without microglial activation, and reduced cerebellar ataxin-2 protein. ASO7 treatment of symptomatic mice improved motor functioning compared to saline-treated mice. ASO7 had a similar effect in the BAC-Q72 SCA2 mouse model, and in both mouse models it normalized protein levels of several SCA2-related PC proteins including Rgs8, Pcp2, Pcp4, Homer3, Cep76, and Fam107b. Most surprisingly, firing frequency of PCs returned to normal even when treatment was initiated >12 weeks after motor phenotype onset in BAC-Q72 mice. These findings support ASOs as a promising approach for treating some human neurodegenerative diseases.

Unraveling the Role of Ataxin-2 in Metabolism

Ataxin-2 is a polyglutamine protein implicated in several biological processes such as RNA metabolism and cytoskeleton reorganization. Ataxin-2 is highly expressed in various tissues including the hypothalamus, a brain region that controls food intake and energy balance. Ataxin-2 expression is influenced by nutritional status. Emerging studies discussed here now show that ataxin-2 deficiency correlates with insulin resistance and dyslipidemia, an action mediated via the mTOR pathway, suggesting that ataxin-2 might play key roles in metabolic homeostasis including body weight regulation, insulin sensitivity, and cellular stress responses. In this review we also discuss the relevance of ataxin-2 in the hypothalamic regulation of energy balance, and its potential as a therapeutic target in metabolic disorders such as obesity.

Motor Dysfunctions and Neuropathology in Mouse Models of Spinocerebellar Ataxia Type 2: A Comprehensive Review

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant ataxia caused by an expansion of CAG repeats in the exon 1 of the gene ATXN2, conferring a gain of toxic function that triggers the appearance of the disease phenotype. SCA2 is characterized by several symptoms including progressive gait ataxia and dysarthria, slow saccadic eye movements, sleep disturbances, cognitive impairments, and psychological dysfunctions such as insomnia and depression, among others. The available treatments rely on palliative care, which mitigate some of the major symptoms but ultimately fail to block the disease progression. This persistent lack of effective therapies led to the development of several models in yeast, C. elegans, D. melanogaster, and mice to serve as platforms for testing new therapeutic strategies and to accelerate the research on the complex disease mechanisms. In this work, we review 4 transgenic and 1 knock-in mouse that exhibit a SCA2-related phenotype and discuss their usefulness in addressing different scientific problems. The knock-in mice are extremely faithful to the human disease, with late onset of symptoms and physiological levels of mutant ataxin-2, while the other transgenic possess robust and well-characterized motor impairments and neuropathological features. Furthermore, a new BAC model of SCA2 shows promise to study the recently explored role of non-coding RNAs as a major pathogenic mechanism in this devastating disorder. Focusing on specific aspects of the behavior and neuropathology, as well as technical aspects, we provide a highly practical description and comparison of all the models with the purpose of creating a useful resource for SCA2 researchers worldwide.