Ataxin-1 oligomers induce local spread of pathology and decreasing them by passive immunization slows Spinocerebellar ataxia type 1 phenotypes

Cross-species genetic screens identify transglutaminase 5 as a regulator of polyglutamine-expanded ataxin-1

Many neurodegenerative disorders are caused by abnormal accumulation of misfolded proteins. In spinocerebellar ataxia type 1 (SCA1), accumulation of polyglutamine-expanded (polyQ-expanded) ataxin-1 (ATXN1) causes neuronal toxicity. Lowering total ATXN1, especially the polyQ-expanded form, alleviates disease phenotypes in mice, but the molecular mechanism by which the mutant ATXN1 is specifically modulated is not understood. Here, we identified 22 mutant ATXN1 regulators by performing a cross-species screen of 7787 and 2144 genes in human cells and Drosophila eyes, respectively. Among them, transglutaminase 5 (TG5) preferentially regulated mutant ATXN1 over the WT protein. TG enzymes catalyzed cross-linking of ATXN1 in a polyQ-length-dependent manner, thereby preferentially modulating mutant ATXN1 stability and oligomerization. Perturbing Tg in Drosophila SCA1 models modulated mutant ATXN1 toxicity. Moreover, TG5 was enriched in the nuclei of SCA1-affected neurons and colocalized with nuclear ATXN1 inclusions in brain tissue from patients with SCA1. Our work provides a molecular insight into SCA1 pathogenesis and an opportunity for allele-specific targeting for neurodegenerative disorders.

Intercellular Propagation and Aggregate Seeding of Mutant Ataxin-1

Intercellular propagation of aggregated protein inclusions along actin-based tunneling nanotubes (TNTs) has been reported as a means of pathogenic spread in Alzheimer’s, Parkinson’s, and Huntington’s diseases. Propagation of oligomeric-structured polyglutamine-expanded ataxin-1 (Atxn1[154Q]) has been reported in the cerebellum of a Spinocerebellar ataxia type 1 (SCA1) knock-in mouse to correlate with disease propagation. In this study, we investigated whether a physiologically relevant polyglutamine-expanded ATXN1 protein (ATXN1[82Q]) could propagate intercellularly. Using a cerebellar-derived live cell model, we observed ATXN1 aggregates form in the nucleus, subsequently form in the cytoplasm, and finally, propagate to neighboring cells along actin-based intercellular connections. Additionally, we observed the facilitation of aggregate-resistant proteins into aggregates given the presence of aggregation-prone proteins within cells. Taken together, our results support a pathogenic role of intercellular propagation of polyglutamine-expanded ATXN1 inclusions.

Extracellular Vesicles Physiological Role and the Particular Case of Disease-Spreading Mechanisms in Polyglutamine Diseases

Recent research demonstrated pathological spreading of the disease-causing proteins from one focal point across other brain regions for some neurodegenerative diseases, such as Parkinson's and Alzheimer's disease. Spreading mediated by extracellular vesicles is one of the proposed disease-spreading mechanisms. Extracellular vesicles are cell membrane-derived vesicles, used by cells for cell-to-cell communication and excretion of toxic components. Importantly, extracellular vesicles carrying pathological molecules, when internalized by "healthy" cells, may trigger pathological pathways and, consequently, promote disease spreading to neighboring cells. Polyglutamine diseases are a group of genetic neurodegenerative disorders characterized by the accumulation of mutant misfolded proteins carrying an expanded tract of glutamines, including Huntington's and Machado-Joseph disease. The pathological spread of the misfolded proteins or the corresponding mutant mRNA has been explored. The understanding of the disease-spreading mechanism that plays a key role in the pathology progression of these diseases can result in the development of effective therapeutic approaches to stop disease progression, arresting the spread of the toxic components and disease aggravation. Therefore, the present review's main focus is the disease-spreading mechanisms with emphasis on polyglutamine diseases and the putative role played by extracellular vesicles in this process.

Dissecting the role of glutamine in seeding peptide aggregation

Poly glutamine and glutamine-rich peptides play a central role in a plethora of pathological aggregation events. However, biophysical characterization of soluble oligomers -the most toxic species involved in these processes- remains elusive due to their structural heterogeneity and dynamical nature. Here, we exploit the high spatio-temporal resolution of coarse-grained simulations as a computational microscope to characterize the aggregation propensity and morphology of a series of polyglutamine and glutamine-rich peptides. Comparative analysis of ab-initio aggregation pinpointed a double role for glutamines. In the first phase, glutamines mediate seeding by pairing monomeric peptides, which serve as primers for higher-order nucleation. According to the glutamine content, these low molecular-weight oligomers may then proceed to create larger aggregates. Once within the aggregates, buried glutamines continue to play a role in their maturation by optimizing solvent-protected hydrogen bonds networks.

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.

Formation of Toxic Oligomeric Assemblies of RNA-binding Protein: Musashi in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated with structural and functional alterations of brain cells causing progressive deterioration of memory and other cognitive functions. Recent studies demonstrate that several neurodegenerative diseases, including AD exhibit RNA-binding proteins (RBPs) pathologies, including TAR DNA -binding protein (TDP-43), fused in sarcoma (FUS), superoxide dismutase (SOD1) and T-interacting antigen-1 (TIA-1), highlighting the role of RBPs in neurodegeneration. One such group of RBPs, Musashi proteins comprised of MSI1 and MSI2, has been long studied in neurogenesis and cancer biology. Herein, we have investigated the aggregation properties of MSI1 and MSI2 by in vitro assays, their expression and accumulation as well as their possible interactions with other cellular proteins, such as tau in AD pathology. We have performed atomic force microscopy, Western blot, and immunoprecipitation to demonstrate the aggregation properties of recombinant Musashi proteins. Furthermore, we have studied cortical brain sections from AD (N = 4) and age-matched non-demented subjects (N = 4) by Western blot and immunofluorescence microscopy to investigate MSI1 and MSI2 levels and their localization in human brain tissues. Musashi proteins showed in vitro aggregation properties by forming oligomers. We have observed an increase in Musashi proteins levels in AD brain tissues as compared with age-matched non-demented subjects. Moreover, Musashi proteins are observed to form oligomers in the diseased brain tissues. Interestingly, the co-immunofluorescence study has revealed a change in fluorescence pattern of oligomeric Musashi proteins and tau with a high association in the perinuclear area of the cells suggesting changes in function of Musashi proteins. Our data have demonstrated for the first time that MSI1 and MSI2 are present in an oligomeric state in AD brains compared to the age-matched non-demented subjects and that these large assemblies co-localize with tau contributing to the neurodegenerative pathogenesis.

A Druggable Genome Screen Identifies Modifiers of α-Synuclein Levels via a Tiered Cross-Species Validation Approach

Accumulation of α-Synuclein (α-Syn) causes Parkinson's disease (PD) as well as other synucleopathies. α-Syn is the major component of Lewy bodies and Lewy neurites, the proteinaceous aggregates that are a hallmark of sporadic PD. In familial forms of PD, mutations or copy number variations in SNCA (the α-Syn gene) result in a net increase of its protein levels. Furthermore, common risk variants tied to PD are associated with small increases of wild-type α-Syn levels. These findings are further bolstered by animal studies which show that overexpression of α-Syn is sufficient to cause PD-like features. Thus, increased α-Syn levels are intrinsically tied to PD pathogenesis and underscore the importance of identifying the factors that regulate its levels. In this study, we establish a pooled RNAi screening approach and validation pipeline to probe the druggable genome for modifiers of α-Syn levels and identify 60 promising targets. Using a cross-species, tiered validation approach, we validate six strong candidates that modulate α-Syn levels and toxicity in cell lines, Drosophila, human neurons, and mouse brain of both sexes. More broadly, this genetic strategy and validation pipeline can be applied for the identification of therapeutic targets for disorders driven by dosage-sensitive proteins.SIGNIFICANCE STATEMENT We present a research strategy for the systematic identification and validation of genes modulating the levels of α-Synuclein, a protein involved in Parkinson's disease. A cell-based screen of the druggable genome (>7,500 genes that are potential therapeutic targets) yielded many modulators of α-Synuclein that were subsequently confirmed and validated in Drosophila, human neurons, and mouse brain. This approach has broad applicability to the multitude of neurological diseases that are caused by mutations in genes whose dosage is critical for brain function.

Depleting Trim28 in adult mice is well tolerated and reduces levels of α-synuclein and tau

Alzheimer's and Parkinson's disease are late onset neurodegenerative diseases that will require therapy over decades to mitigate the effects of disease-driving proteins such tau and α-synuclein (α-Syn). Previously we found that TRIM28 regulates the levels and toxicity of α-Syn and tau (Rousseaux et al., 2016). However, it was not clear how TRIM28 regulates α-Syn and it was not known if its chronic inhibition later in life was safe. Here, we show that TRIM28 may regulate α-Syn and tau levels via SUMOylation, and that genetic suppression of Trim28 in adult mice is compatible with life. We were surprised to see that mice lacking Trim28 in adulthood do not exhibit behavioral or pathological phenotypes, and importantly, adult reduction of TRIM28 results in a decrease of α-Syn and tau levels. These results suggest that deleterious effects from TRIM28 depletion are limited to development and that its inhibition adulthood provides a potential path for modulating α-Syn and tau levels.

Motor neuron degeneration correlates with respiratory dysfunction in SCA1

Spinocerebellar ataxia type 1 (SCA1) is characterized by adult-onset cerebellar degeneration with attendant loss of motor coordination. Bulbar function is eventually impaired and patients typically die from an inability to clear the airway. We investigated whether motor neuron degeneration is at the root of bulbar dysfunction by studying SCA1 knock-in (Atxn1^154Q/+) mice. Spinal cord and brainstem motor neurons were assessed in Atxn1^154Q/+ mice at 1, 3 and 6 months of age. Specifically, we assessed breathing physiology, diaphragm histology and electromyography, and motor neuron histology and immunohistochemistry. Atxn1^154Q/+ mice show progressive neuromuscular respiratory abnormalities, neurogenic changes in the diaphragm, and motor neuron degeneration in the spinal cord and brainstem. Motor neuron degeneration is accompanied by reactive astrocytosis and accumulation of Atxn1 aggregates in the motor neuron nuclei. This observation correlates with previous findings in SCA1 patient tissue. Atxn1^154Q/+ mice develop bulbar dysfunction because of motor neuron degeneration. These findings confirm the Atxn1^154Q/+ line as a SCA1 model with face and construct validity for this understudied disease feature. Furthermore, this model is suitable for studying the pathogenic mechanism driving motor neuron degeneration in SCA1 and possibly other degenerative motor neuron diseases. From a clinical standpoint, the data indicate that pulmonary function testing and employment of non-invasive ventilator support could be beneficial in SCA1 patients. The physiological tests used in this study might serve as valuable biomarkers for future therapeutic interventions and clinical trials.

This article has an associated First Person interview with the first author of the paper.

Summary: In this manuscript, we discovered motor neuron degeneration which correlates with respiratory failure in a knock-in mouse model of spinocerebellar ataxia type 1 (SCA1).

Polyglutamine expansion diseases: More than simple repeats

Polyglutamine (polyQ) repeat-containing proteins are widespread in the human proteome but only nine of them are associated with highly incapacitating neurodegenerative disorders. The genetic expansion of the polyQ tract in disease-related proteins triggers a series of events resulting in neurodegeneration. The polyQ tract plays the leading role in the aggregation mechanism, but other elements modulate the aggregation propensity in the context of the full-length proteins, as implied by variations in the length of the polyQ tract required to trigger the onset of a given polyQ disease. Intrinsic features such as the presence of aggregation-prone regions (APRs) outside the polyQ segments and polyQ-flanking sequences, which synergistically participate in the aggregation process, are emerging for several disease-related proteins. The inherent polymorphic structure of polyQ stretches places the polyQ proteins in a central position in protein-protein interaction networks, where interacting partners may additionally shield APRs or reshape the aggregation course. Expansion of the polyQ tract perturbs the cellular homeostasis and contributes to neuronal failure by modulating protein-protein interactions and enhancing toxic oligomerization. Post-translational modifications further regulate self-assembly either by directly altering the intrinsic aggregation propensity of polyQ proteins, by modulating their interaction with different macromolecules or by modifying their withdrawal by the cell quality control machinery. Here we review the recent data on the multifaceted aggregation pathways of disease-related polyQ proteins, focusing on ataxin-3, the protein mutated in Machado-Joseph disease. Further mechanistic understanding of this network of events is crucial for the development of effective therapies for polyQ diseases.

DNA repair in the trinucleotide repeat disorders

BACKGROUND

Inherited diseases caused by unstable repeated DNA sequences are rare, but together represent a substantial cause of morbidity. Trinucleotide repeat disorders are severe, usually life-shortening, neurological disorders caused by nucleotide expansions, and most have no disease-modifying treatments. Longer repeat expansions are associated with genetic anticipation (ie, earlier disease onset in successive generations), although the differences in age at onset are not entirely accounted for by repeat length. Such phenotypic variation within disorders implies the existence of additional modifying factors in pathways that can potentially be modulated to treat disease.

RECENT DEVELOPMENTS

A genome-wide association study detected genetic modifiers of age at onset in Huntington's disease. Similar findings were seen in the spinocerebellar ataxias, indicating an association between DNA damage-response and repair pathways and the age at onset of disease. These studies also suggest that a common genetic mechanism modulates age at onset across polyglutamine diseases and could extend to other repeat expansion disorders. Genetic defects in DNA repair underlie other neurodegenerative disorders (eg, ataxia-telangiectasia), and DNA double-strand breaks are crucial to the modulation of early gene expression, which provides a mechanistic link between DNA repair and neurodegeneration. Mismatch and base-excision repair are important in the somatic expansion of repeated sequences in mouse models of trinucleotide repeat disorders, and somatic expansion of the expanded CAG tract in HTT correlates with age at onset of Huntington's disease and other trinucleotide repeat disorders. WHERE NEXT?: To understand the common genetic architecture of trinucleotide repeat disorders and any further genetic susceptibilities in individual disorders, genetic analysis with increased numbers of variants and sample sizes is needed, followed by sequencing approaches to define the phenotype-modifying variants. The findings must then be translated into cell biology analyses to elucidate the mechanisms through which the genetic variants operate. Genes that have roles in the DNA damage response could underpin a common DNA repeat-based mechanism and provide new therapeutic targets (and hence therapeutics) in multiple trinucleotide repeat disorders.

TRIM28 regulates the nuclear accumulation and toxicity of both alpha-synuclein and tau

Several neurodegenerative diseases are driven by the toxic gain-of-function of specific proteins within the brain. Elevated levels of alpha-synuclein (α-Syn) appear to drive neurotoxicity in Parkinson's disease (PD); neuronal accumulation of tau is a hallmark of Alzheimer's disease (AD); and their increased levels cause neurodegeneration in humans and model organisms. Despite the clinical differences between AD and PD, several lines of evidence suggest that α-Syn and tau overlap pathologically. The connections between α-Syn and tau led us to ask whether these proteins might be regulated through a shared pathway. We therefore screened for genes that affect post-translational levels of α-Syn and tau. We found that TRIM28 regulates α-Syn and tau levels and that its reduction rescues toxicity in animal models of tau- and α-Syn-mediated degeneration. TRIM28 stabilizes and promotes the nuclear accumulation and toxicity of both proteins. Intersecting screens across comorbid proteinopathies thus reveal shared mechanisms and therapeutic entry points.

DOI:http://dx.doi.org/10.7554/eLife.19809.001

Behind many neurodegenerative diseases are specific proteins that abnormally accumulate inside neurons and damage the cells. In Parkinson’s disease, the protein alpha-synuclein accumulates; in Alzheimer’s disease, the protein tau is one of the toxic culprits; and in other neurodegenerative diseases, alpha-synuclein and tau both accumulate. Genetic studies suggest that accumulation of the two proteins may be linked, but little is known about the factors that regulate the levels of these proteins inside neurons.

Rousseaux et al. set out to identify how these proteins are regulated in the hope of finding new ways of targeting them and reducing their toxicity. Screening a subset of human genes led to one that encodes a protein called TRIM28, which regulates the levels of both alpha-synuclein and tau. When the TRIM28 protein was depleted in human and mouse cells, the levels of alpha-synuclein and tau also went down. This effect was specific because the levels of other proteins with the potential to cause neurodegeneration remained unaffected.

Models of neurodegenerative disease in fruit flies and mice were then used to explore how TRIM28 affects the levels of tau and alpha-synuclein in animals. In each case, the proteins’ levels dropped when TRIM28 was suppressed and this in turn protected the neurons from damage. Rousseaux et al. went on to show that TRIM28 affected how alpha-synuclein and tau were cleared in cells. Overexpressing TRIM28 revealed that it could encourage both alpha-synuclein and tau to accumulate in the nucleus of cells over time.

Finally, Rousseaux et al. compared post-mortem brain tissue from people who had neurodegenerative conditions that are driven by or associated with tau and alpha-synuclein with tissue from those who did not. The cell nuclei in the diseased tissue had much more TRIM28 associated with alpha-synuclein and tau than those in the healthy tissues.

Overall, the findings show that TRIM28 promotes the accumulation and damaging effects of both alpha-synuclein and tau. The next steps will be to understand how TRIM28 does this. It will also be important to determine if this effect can be targeted, whilst leaving others roles of TRIM28 intact, in order to explore it as a potential target to treat or prevent neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.19809.002

Stabilization and Degradation Mechanisms of Cytoplasmic Ataxin-1

Aggregation-prone proteins in neurodegenerative disease disrupt cellular protein stabilization and degradation pathways. The neurodegenerative disease spinocerebellar ataxia type 1 (SCA1) is caused by a coding polyglutamine expansion in the Ataxin-1 gene (ATXN1), which gives rise to the aggregation-prone mutant form of ATXN1 protein. Cerebellar Purkinje neurons, preferentially vulnerable in SCA1, produce ATXN1 protein in both cytoplasmic and nuclear compartments. Cytoplasmic stabilization of ATXN1 by phosphorylation and 14-3-3-mediated mechanisms ultimately drive translocation of the protein to the nucleus where aggregation may occur. However, experimental inhibition of phosphorylation and 14-3-3 binding results in rapid degradation of ATXN1, thus preventing nuclear translocation and cellular toxicity. The exact mechanism of cytoplasmic ATXN1 degradation is currently unknown; further investigation of degradation may provide future therapeutic targets. This review examines the present understanding of cytoplasmic ATXN1 stabilization and potential degradation mechanisms during normal and pathogenic states.

Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease

Key challenges faced by all cells include how to spatiotemporally organize complex biochemistry and how to respond to environmental fluctuations. The budding yeast Saccharomyces cerevisiae harnesses alternative protein folding mediated by yeast prion domains (PrDs) for rapid evolution of new traits in response to environmental stress. Increasingly, it is appreciated that low complexity domains similar in amino acid composition to yeast PrDs (prion-like domains; PrLDs) found in metazoa have a prominent role in subcellular cytoplasmic organization, especially in relation to RNA homeostasis. In this review, we highlight recent advances in our understanding of the role of prions in enabling rapid adaptation to environmental stress in yeast. We also present the complete list of human proteins with PrLDs and discuss the prevalence of the PrLD in nucleic-acid binding proteins that are often connected to neurodegenerative disease, including: ataxin 1, ataxin 2, FUS, TDP-43, TAF15, EWSR1, hnRNPA1, and hnRNPA2. Recent paradigm-shifting advances establish that PrLDs undergo phase transitions to liquid states, which contribute to the structure and biophysics of diverse membraneless organelles. This structural functionality of PrLDs, however, simultaneously increases their propensity for deleterious protein-misfolding events that drive neurodegenerative disease. We suggest that even these PrLD-misfolding events are not irreversible and can be mitigated by natural or engineered protein disaggregases, which could have important therapeutic applications.