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Targeting the VCP-binding motif of ataxin-3 improves phenotypes in Drosophila models of Spinocerebellar Ataxia Type 3.

ABSTRACT: Of the family of polyglutamine (polyQ) neurodegenerative diseases, Spinocerebellar Ataxia Type 3 (SCA3) is the most common. Like other polyQ diseases, SCA3 stems from abnormal expansions in the CAG triplet repeat of its disease gene resulting in elongated polyQ repeats within its protein, ataxin-3. Various ataxin-3 protein domains contribute to its toxicity, including the valosin-containing protein (VCP)-binding motif (VBM). We previously reported that VCP, a homo-hexameric protein, enhances pathogenic ataxin-3 aggregation and exacerbates its toxicity. These findings led us to explore the impact of targeting the SCA3 protein by utilizing a decoy protein comprising the N-terminus of VCP (N-VCP) that binds ataxin-3's VBM. The notion was that N-VCP would reduce binding of ataxin-3 to VCP, decreasing its aggregation and toxicity. We found that expression of N-VCP in Drosophila melanogaster models of SCA3 ameliorated various phenotypes, coincident with reduced ataxin-3 aggregation. This protective effect was specific to pathogenic ataxin-3 and depended on its VBM. Increasing the amount of N-VCP resulted in further phenotype improvement. Our work highlights the protective potential of targeting the VCP-ataxin-3 interaction in SCA3, a key finding in the search for therapeutic opportunities for this incurable disorder.

SUBMITTER: Johnson SL

PROVIDER: S-EPMC8693084 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Differential toxicity of ataxin-3 isoforms in Drosophila models of Spinocerebellar Ataxia Type 3.

Project description:The most commonly inherited dominant ataxia, Spinocerebellar Ataxia Type 3 (SCA3), is caused by a CAG repeat expansion that encodes an abnormally long polyglutamine (polyQ) repeat in the disease protein ataxin-3, a deubiquitinase. Two major full-length isoforms of ataxin-3 exist, both of which contain the same N-terminal portion and polyQ repeat, but differ in their C-termini; one (denoted here as isoform 1) contains a motif that binds ataxin-3's substrate, ubiquitin, whereas the other (denoted here as isoform 2) has a hydrophobic tail. Most SCA3 studies have focused on isoform 1, the predominant version in mammalian brain, yet both isoforms are present in brain and a better understanding of their relative pathogenicity in vivo is needed. We took advantage of the fruit fly, Drosophila melanogaster to model SCA3 and to examine the toxicity of each ataxin-3 isoform. Our assays reveal isoform 1 to be markedly more toxic than isoform 2 in all fly tissues. Reduced toxicity from isoform 2 is due to much lower protein levels as a result of its expedited degradation. Additional studies indicate that isoform 1 is more aggregation-prone than isoform 2 and that the C-terminus of isoform 2 is critical for its enhanced proteasomal degradation. According to our results, although both full-length, pathogenic ataxin-3 isoforms are toxic, isoform 1 is likely the primary contributor to SCA3 due to its presence at higher levels. Isoform 2, as a result of rapid degradation that is dictated by its tail, is unlikely to be a key player in this disease. Our findings provide new insight into the biology of this ataxia and the cellular processing of the underlying disease protein.

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Indirect Negative Effect of Mutant Ataxin-1 on Short- and Long-Term Synaptic Plasticity in Mouse Models of Spinocerebellar Ataxia Type 1

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Heterogeneous nonataxic phenotypes of spinocerebellar ataxia in a Taiwanese population.

Project description:BACKGROUND:Spinocerebellar ataxia (SCA) presents with variable clinical presentations in addition to ataxia. The aim of this study was to reappraise the diverse nonataxic clinical characteristics of the five most common SCA subtypes in the Asian population. METHODS:The clinical presentations of 90 patients with genetically confirmed SCA1, SCA2, SCA3, SCA6, or SCA17 were assessed retrospectively between November 2008 and September 2018 at a tertiary referral center in Taiwan. RESULTS:Parkinsonism was the most common nonataxic phenotype (21.1%), with a greater prevalence than Caucasian and other Asian SCA carriers. Patients with parkinsonism feature had fewer CAG repeats in SCA2 (31.0 ± 4.5 vs. 36.9 ± 6.0, p = .03) and SCA3 (65.6 ± 7.9 vs. 70.0 ± 4.2, p = .02) compared to those with pure ataxia presentation. The average age of symptom onset was significantly higher in the parkinsonism group of SCA2 (51.5 ± 8.9 vs. 35.3 ± 12.6 years, p = .007) than those with pure ataxia. Focal or segmental dystonia was identified in 4.4% of SCA patients (n = 2 each SCA2 and SCA3). Nonmotor symptoms, including impaired cognition (6.1% of SCA2 and 8.3% of SCA3 patients) and depression (9.1% of SCA2 and 8.3% of SCA3 patients), were also common nonataxic features in our SCA patients. CONCLUSIONS:Parkinsonism, dystonia, and cognitive-psychiatric symptoms are common features in patients with SCA mutations in our population. Our study identifies a different clinical spectrum of SCA1, SCA2, SCA3, SCA6, and SCA17 compared to Caucasians.

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Project description:Spinocerebellar Ataxia Type 3 (SCA3), also known as Machado-Joseph disease, is an autosomal dominantly inherited neurodegenerative disease caused by an expanded polyglutamine stretch in the ataxin-3 protein. A pathological hallmark of the disease is cerebellar and brainstem atrophy, which correlates with the formation of intranuclear aggregates in a specific subset of neurons. Several studies have demonstrated that the formation of aggregates depends on the generation of aggregation-prone and toxic intracellular ataxin-3 fragments after proteolytic cleavage of the full-length protein. Despite this observed increase in aggregated mutant ataxin-3, information on soluble mutant ataxin-3 levels in brain tissue is lacking. A quantitative method to analyze soluble levels will be a useful tool to characterize disease progression or to screen and identify therapeutic compounds modulating the level of toxic soluble ataxin-3. In the present study we describe the development and application of a quantitative and easily applicable immunoassay for quantification of soluble mutant ataxin-3 in human cell lines and brain samples of transgenic SCA3 mice. Consistent with observations in Huntington disease, transgenic SCA3 mice reveal a tendency for decrease of soluble mutant ataxin-3 during disease progression in fractions of the cerebellum, which is inversely correlated with aggregate formation and phenotypic aggravation. Our analyses demonstrate that the time-resolved Förster resonance energy transfer immunoassay is a highly sensitive and easy method to measure the level of soluble mutant ataxin-3 in biological samples. Of interest, we observed a tendency for decrease of soluble mutant ataxin-3 only in the cerebellum of transgenic SCA3 mice, one of the most affected brain regions in Spinocerebellar Ataxia Type 3 but not in whole brain tissue, indicative of a brain region selective change in mutant ataxin-3 protein homeostasis.

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Project description:Spinocerebellar ataxia type 7 (SCA7) is an autosomal-dominant neurodegenerative disorder that results from polyglutamine expansion of the ataxin-7 (ATXN7) protein. Remarkably, although mutant ATXN7 is expressed throughout the body, pathology is restricted primarily to the cerebellum and retina. One major goal has been to identify factors that contribute to the tissue specificity of SCA7. Here we describe the development and use of a human astrocyte cell culture model to identify reelin, a factor intimately involved in the development and maintenance of Purkinje cells and the cerebellum as a whole, as an ATXN7 target gene. We found that polyglutamine expansion decreased ATXN7 occupancy, which correlated with increased levels of histone H2B monoubiquitination, at the reelin promoter. Treatment with trichostatin A, but not other histone deacetylase inhibitors, partially restored reelin transcription and promoted the accumulation of mutant ATXN7 into nuclear inclusions. Our findings suggest that reelin could be a previously unknown factor involved in the tissue specificity of SCA7 and that trichostatin A may ameliorate deleterious effects of the mutant ATXN7 protein by promoting its sequestration away from promoters into nuclear inclusions.

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