Jonasz Jeremiasz Weber

Calpain-mediated ataxin-3 cleavage in the molecular pathogenesis of spinocerebellar ataxia type 3 (SCA3)

Spinocerebellar ataxia type 3 (SCA3) is pathologically characterized by the formation of intranuclear aggregates which contain ataxin-3, the mutated protein in SCA3, in a specific subtype of neurons. It has been proposed that ataxin-3 is cleaved by proteolytic enzymes, in particular by calpains and caspases, eventually leading to the formation of aggregates. In our study, we examined the ability of calpains to cleave ataxin-3 in vitro and in vivo. We demonstrated in cell culture and mouse brain homogenates that cleavage of overexpressed ataxin-3 by calpains and in particular by calpain-2 occur and that polyglutamine expanded ataxin-3 is more sensitive to calpain degradation. Based on these results, we investigated the influence of calpains on the pathogenesis of SCA3 in vivo. For this purpose, we enhanced calpain activity in a SCA3 transgenic mouse model by knocking out the endogenous calpain inhibitor calpastatin. Double-mutant mice demonstrated an aggravated neurological phenotype with an increased number of nuclear aggregates and accelerated neurodegeneration in the cerebellum. This study confirms the critical importance of calcium-dependent calpain-type proteases in the pathogenesis of SCA3 and suggests that the manipulation of the ataxin-3 cleavage pathway and the regulation of intracellular calcium homeostasis may represent novel targets for therapeutic intervention in SCA3.

From Pathways to Targets: Understanding the Mechanisms behind Polyglutamine Disease

The history of polyglutamine diseases dates back approximately 20 years to the discovery of a polyglutamine repeat in the androgen receptor of SBMA followed by the identification of similar expansion mutations in Huntingtons disease, SCA1, DRPLA, and the other spinocerebellar ataxias. This common molecular feature of polyglutamine diseases suggests shared mechanisms in disease pathology and neurodegeneration of disease specific brain regions. In this review, we discuss the main pathogenic pathways including proteolytic processing, nuclear shuttling and aggregation, mitochondrial dysfunction, and clearance of misfolded polyglutamine proteins and point out possible targets for treatment.

Olesoxime suppresses calpain activation and mutant huntingtin fragmentation in the BACHD rat.

Huntingtons disease is a fatal human neurodegenerative disorder caused by a CAG repeat expansion in the HTT gene, which translates into a mutant huntingtin protein. A key event in the molecular pathogenesis of Huntingtons disease is the proteolytic cleavage of mutant huntingtin, leading to the accumulation of toxic protein fragments. Mutant huntingtin cleavage has been linked to the overactivation of proteases due to mitochondrial dysfunction and calcium derangements. Here, we investigated the therapeutic potential of olesoxime, a mitochondria-targeting, neuroprotective compound, in the BACHD rat model of Huntingtons disease. BACHD rats were treated with olesoxime via the food for 12 months. In vivo analysis covered motor impairments, cognitive deficits, mood disturbances and brain atrophy. Ex vivo analyses addressed olesoximes effect on mutant huntingtin aggregation and cleavage, as well as brain mitochondria function. Olesoxime improved cognitive and psychiatric phenotypes, and ameliorated cortical thinning in the BACHD rat. The treatment reduced cerebral mutant huntingtin aggregates and nuclear accumulation. Further analysis revealed a cortex-specific overactivation of calpain in untreated BACHD rats. Treated BACHD rats instead showed significantly reduced levels of mutant huntingtin fragments due to the suppression of calpain-mediated cleavage. In addition, olesoxime reduced the amount of mutant huntingtin fragments associated with mitochondria, restored a respiration deficit, and enhanced the expression of fusion and outer-membrane transport proteins. In conclusion, we discovered the calpain proteolytic system, a key player in Huntingtons disease and other neurodegenerative disorders, as a target of olesoxime. Our findings suggest that olesoxime exerts its beneficial effects by improving mitochondrial function, which results in reduced calpain activation. The observed alleviation of behavioural and neuropathological phenotypes encourages further investigations on the use of olesoxime as a therapeutic for Huntingtons disease.

In vivo assessment of riluzole as a potential therapeutic drug for spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominantly inherited neurodegenerative disorder for which no curative therapy is available. The cause of this disease is the expansion of a CAG repeat in the so‐called ATXN3 gene leading to an expanded polyglutamine stretch in the ataxin‐3 protein. Although the function of ataxin‐3 has been defined as a deubiquitinating enzyme, the pathogenic pathway underlying SCA3 remains to be deciphered. Besides others, also the glutamatergic system seems to be altered in SCA3. The antiglutamatergic substance riluzole has thus been suggested as a potential therapeutic agent for SCA3. To assess whether riluzole is effective in the treatment of SCA3 in vivo, we used a phenotypically well‐characterized conditional mouse model previously generated by us. Treatment with 10 mg/kg riluzole in the drinking water was started when mice showed impairment in rotarod performance. Post‐symptomatic treatment with riluzole carried out for a period of 10 months led to reduction of the soluble ataxin‐3 level and an increase in ataxin‐3 positive accumulations, but did not improve motor deficits measured by rotarod. There was also no positive effect on home cage behavior or body weight. We even observed a pronounced reduction of calbindin expression in Purkinje cells in riluzole‐treated mice. Thus, long‐term treatment with riluzole was not able to alleviate disease symptoms observed in transgenic SCA3 mice and should be considered with caution in the treatment of human patients.

A combinatorial approach to identify calpain cleavage sites in the Machado-Joseph disease protein ataxin-3

Ataxin-3, the disease protein in Machado-Joseph disease, is known to be proteolytically modified by various enzymes including two major families of proteases, caspases and calpains. This processing results in the generation of toxic fragments of the polyglutamine-expanded protein. Although various approaches were undertaken to identify cleavage sites within ataxin-3 and to evaluate the impact of fragments on the molecular pathogenesis of Machado-Joseph disease, calpain-mediated cleavage of the disease protein and the localization of cleavage sites remained unclear. Here, we report on the first precise localization of calpain cleavage sites in ataxin-3 and on the characterization of the resulting breakdown products. After confirming the occurrence of calpain-derived fragmentation of ataxin-3 in patient-derived cell lines and post-mortem brain tissue, we combined in silico prediction tools, western blot analysis, mass spectrometry, and peptide overlay assays to identify calpain cleavage sites. We found that ataxin-3 is primarily cleaved at two sites, namely at amino acid positions D208 and S256 and mutating amino acids at both cleavage sites to tryptophan nearly abolished ataxin-3 fragmentation. Furthermore, analysis of calpain cleavage-derived fragments showed distinct aggregation propensities and toxicities of C-terminal polyglutamine-containing breakdown products. Our data elucidate the important role of ataxin-3 proteolysis in the pathogenesis of Machado-Joseph disease and further emphasize the relevance of targeting this disease pathway as a treatment strategy in neurodegenerative disorders.

Cerebellar Soluble Mutant Ataxin-3 Level Decreases during Disease Progression in Spinocerebellar Ataxia Type 3 Mice

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.

The calpain-suppressing effects of olesoxime in Huntington's disease

ABSTRACT Olesoxime, a small molecule drug candidate, has recently attracted attention due to its significant beneficial effects in models of several neurodegenerative disorders including Huntingtons disease. Olesoximes neuroprotective effects have been assumed to be conveyed through a direct, positive influence on mitochondrial function. In a long-term treatment study in BACHD rats, the latest rat model of Huntingtons disease, olesoxime revealed a positive influence on mitochondrial function and improved specific behavioral and neuropathological phenotypes. Moreover, a novel target of the compound was discovered, as olesoxime was found to suppress the activation of the calpain proteolytic system, a major contributor to the cleavage of the disease-causing mutant huntingtin protein into toxic fragments, and key player in degenerative processes in general. Results from a second model of Huntingtons disease, the HdhQ111 knock-in mouse, confirm olesoximes calpain-suppressing effects and support the therapeutic value of olesoxime for Huntingtons disease and other disorders involving calpain overactivation.

Generation of an induced pluripotent stem cell line from a patient with spinocerebellar ataxia type 3 (SCA3): HIHCNi002-A

A skin biopsy of a patient with spinocerebellar ataxia type 3 (SCA3, also known as Machado-Joseph disease (MJD)) caused by a CAG trinucleotide repeat expansion in the ATXN3 gene, was used to generate an induced pluripotent stem cell line, HIHCNi002-A (iPSC-SCA3). Skin fibroblasts were reprogrammed using episomal plasmids carrying hOCT4, hSOX2, hKLF4, hL-MYC, and hLIN28. The iPSC-SCA3 line exhibits chromosomal stability with conservation of the ATXN3 repeat expansion, expresses pluripotency markers and differentiates into endo-, meso-, and ectodermal cells in vitro.

Karyopherin α-3 is a key protein in the pathogenesis of spinocerebellar ataxia type 3 controlling the nuclear localization of ataxin-3

Significance Ataxin-3 is the affected protein in the neurodegenerative disorder spinocerebellar ataxia type 3 (SCA3). Nuclear ataxin-3 has been linked to disease progression and formation of aggregates. Our present findings implicate karyopherin alpha 3 (KPNA3) in the in vitro transport of ataxin-3 and in the SCA3-related phenotypes in Drosophila and mouse models. We have demonstrated that altering transport proteins has an effect on both pathogenic mechanisms (e.g., the intracellular localization and the formation of aggregates) and key features of ataxin-3 toxicity such as anxiety, total activity, and gait abnormalities. A better appreciation of this cellular mechanism can enhance our understanding of polyglutamine diseases and the role of nuclear/cytoplasmic compartments in toxicity and clearance of ataxin-3. Spinocerebellar ataxia type 3 (SCA3) is a neurodegenerative disorder caused by a CAG expansion in the ATXN3 gene leading to a polyglutamine expansion in the ataxin-3 protein. The nuclear presence and aggregation of expanded ataxin-3 are critical steps in disease pathogenesis. To identify novel therapeutic targets, we investigated the nucleocytoplasmic transport system by screening a collection of importins and exportins that potentially modulate this nuclear localization. Using cell, Drosophila, and mouse models, we focused on three transport proteins, namely, CRM1, IPO13, KPNA3, and their respective Drosophila orthologs Emb, Cdm, and Kap-α3. While overexpression of CRM1/Emb demonstrated positive effects in Drosophila, KPNA3/Kap-α3 emerged as the most promising target, as knockdown via multiple RNAi lines demonstrated its ability to shuttle both truncated and full-length expanded ataxin-3, rescue neurodegeneration, restore photoreceptor formation, and reduce aggregation. Furthermore, KPNA3 knockout in SCA3 mice resulted in an amelioration of molecular and behavioral disturbances such as total activity, anxiety, and gait. Since KPNA3 is known to function as an import protein and recognize nuclear localization signals (NLSs), this work unites ataxin-3 structure to the nuclear pore machinery and provides a link between karyopherins, NLS signals, and polyglutamine disease, as well as demonstrates that KPNA3 is a key player in the pathogenesis of SCA3.

Environment-dependent striatal gene expression in the BACHD rat model for Huntington disease

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene which results in progressive neurodegeneration in the striatum, cortex, and eventually most brain areas. Despite being a monogenic disorder, environmental factors influence HD characteristics. Both human and mouse studies suggest that mutant HTT (mHTT) leads to gene expression changes that harbor potential to be modulated by the environment. Yet, the underlying mechanisms integrating environmental cues into the gene regulatory program have remained largely unclear. To better understand gene-environment interactions in the context of mHTT, we employed RNA-seq to examine effects of maternal separation (MS) and environmental enrichment (EE) on striatal gene expression during development of BACHD rats. We integrated our results with striatal consensus modules defined on HTT-CAG length and age-dependent co-expression gene networks to relate the environmental factors with disease progression. While mHTT was the main determinant of expression changes, both MS and EE were capable of modulating these disturbances, resulting in distinctive and in several cases opposing effects of MS and EE on consensus modules. This bivalent response to maternal separation and environmental enrichment may aid in explaining their distinct effects observed on disease phenotypes in animal models of HD and related neurodegenerative disorders.