Both ubiquitin ligases FBXW8 and PARK2 are sequestrated into insolubility by ATXN2 PolyQ expansions, but only FBXW8 expression is dysregulated

ATXN2L primarily interacts with NUFIP2, the absence of ATXN2L results in NUFIP2 depletion, and the ATXN2-polyQ expansion triggers NUFIP2 accumulation

The cytoplasmic Ataxin-2 (ATXN2) protein associates with TDP-43 in stress granules (SG) where RNA quality control occurs. Mutations in this pathway underlie Spinocerebellar Ataxia type 2 (SCA2) and Amyotrophic Lateral Sclerosis. In contrast, Ataxin-2-like (ATXN2L) is predominantly perinuclear, more abundant, and essential for embryonic life. Its sequestration into ATXN2 aggregates may contribute to disease. In this study, we utilized two approaches to clarify the roles of ATXN2L. First, we identified interactors through co-immunoprecipitation in both wild-type and ATXN2L-null murine embryonic fibroblasts. Second, we assessed the proteome profile effects using mass spectrometry in these cells. Additionally, we examined the accumulation of ATXN2L interactors in the SCA2 mouse model, Atxn2-CAG100-KnockIn (KIN). We observed that RNA-binding proteins, including PABPN1, NUFIP2, MCRIP2, RBMS1, LARP1, PTBP1, FMR1, RPS20, FUBP3, MBNL2, ZMAT3, SFPQ, CSDE1, HNRNPK, and HNRNPDL, exhibit a stronger association with ATXN2L compared to established interactors like ATXN2, PABPC1, LSM12, and G3BP2. Additionally, ATXN2L interacted with components of the actin complex, such as SYNE2, LMOD1, ACTA2, FYB, and GOLGA3. We noted that oxidative stress increased HNRNPK but decreased SYNE2 association, which likely reflects the relocalization of SG. Proteome profiling revealed that NUFIP2 and SYNE2 are depleted in ATXN2L-null fibroblasts. Furthermore, NUFIP2 homodimers and SYNE1 accumulate during the ATXN2 aggregation process in KIN 14-month-old spinal cord tissues. The functions of ATXN2L and its interactors are therefore critical in RNA granule trafficking and surveillance, particularly for the maintenance of differentiated neurons.

Key roles of ubiquitination in regulating critical regulators of cancer stem cell functionality

The ubiquitin (Ub) system, a ubiquitous presence across eukaryotes, plays a crucial role in the precise orchestration of diverse cellular protein processes. From steering cellular signaling pathways and orchestrating cell cycle progression to guiding receptor trafficking and modulating immune responses, this process plays a crucial role in regulating various biological functions. The dysregulation of Ub-mediated signaling pathways in prevalent cancers ushers in a spectrum of clinical outcomes ranging from tumorigenesis and metastasis to recurrence and drug resistance. Ubiquitination, a linchpin process mediated by Ub, assumes a central mantle in molding cellular signaling dynamics. It navigates transitions in biological cues and ultimately shapes the destiny of proteins. Recent years have witnessed an upsurge in the momentum surrounding the development of protein-based therapeutics aimed at targeting the Ub system under the sway of cancer stem cells. The article provides a comprehensive overview of the ongoing in-depth discussions regarding the regulation of the Ub system and its impact on the development of cancer stem cells. Amidst the tapestry of insights, the article delves into the expansive roles of E3 Ub ligases, deubiquitinases, and transcription factors entwined with cancer stem cells. Furthermore, the spotlight turns to the interplay with pivotal signaling pathways the Notch, Hedgehog, Wnt/β-catenin, and Hippo-YAP signaling pathways all play crucial roles in the regulation of cancer stem cells followed by the specific modulation of Ub-proteasome.

The Role of Protein Quantity Control in Polyglutamine Spinocerebellar Ataxias

Polyglutamine spinocerebellar ataxias (polyQ SCAs) represent the most prevalent subtype of SCAs. The primary pathogenic mechanism is believed to be the gain-of-function neurotoxicity of polyQ proteins. Strategies such as enhancing the degradation or inhibiting the accumulation of these mutant proteins are pivotal for reducing their toxicity and slowing disease progression. The protein quality control (PQC) system, comprising primarily molecular chaperones and the ubiquitin‒proteasome system (UPS), is essential for maintaining protein homeostasis by regulating protein folding, trafficking, and degradation. Notably, polyQ proteins can disrupt the PQC system by sequestering its critical components and impairing its proteasomal functions. Therefore, restoring the PQC system through genetic or pharmacological interventions could potentially offer beneficial effects and alleviate the symptoms of the disease. Here, we will provide a review on the distribution, expression, and genetic or pharmacological intervention of protein quality control system in cellular or animal models of PolyQ SCAs.

Ataxin-2: a powerful RNA-binding protein

Ataxin-2 (ATXN2) was originally discovered in the context of spinocerebellar ataxia type 2 (SCA2), but it has become a key player in various neurodegenerative diseases. This review delves into the multifaceted roles of ATXN2 in human diseases, revealing its diverse molecular and cellular pathways. The impact of ATXN2 on diseases extends beyond functional outcomes; it mainly interacts with various RNA-binding proteins (RBPs) to regulate different stages of post-transcriptional gene expression in diseases. With the progress of research, ATXN2 has also been found to play an important role in the development of various cancers, including breast cancer, gastric cancer, pancreatic cancer, colon cancer, and esophageal cancer. This comprehensive exploration underscores the crucial role of ATXN2 in the pathogenesis of diseases and warrants further investigation by the scientific community. By reviewing the latest discoveries on the regulatory functions of ATXN2 in diseases, this article helps us understand the complex molecular mechanisms of a series of human diseases related to this intriguing protein.

Ataxin-2 sequesters Raptor into aggregates and impairs cellular mTORC1 signaling

Ataxin-2 (Atx2) is a polyglutamine (polyQ) protein, in which abnormal expansion of the polyQ tract can trigger protein aggregation and consequently cause spinocerebellar ataxia type 2 (SCA2), but the mechanism underlying how Atx2 aggregation leads to proteinopathy remains elusive. Here, we investigate the molecular mechanism and cellular consequences of Atx2 aggregation by molecular cell biology approaches. We have revealed that either normal or polyQ-expanded Atx2 can sequester Raptor, a component of mammalian target of rapamycin complex 1 (mTORC1), into aggregates based on their specific interaction. Further research indicates that the polyQ tract and the N-terminal region (residues 1-784) of Atx2 are responsible for the specific sequestration. Moreover, this sequestration leads to suppression of the mTORC1 activity as represented by down-regulation of phosphorylated P70S6K, which can be reversed by overexpression of Raptor. As mTORC1 is a key regulator of autophagy, Atx2 aggregation and sequestration also induces autophagy by upregulating LC3-II and reducing phosphorylated ULK1 levels. This study proposes that Atx2 sequesters Raptor into aggregates, thereby impairing cellular mTORC1 signaling and inducing autophagy, and will be beneficial for a better understanding of the pathogenesis of SCA2 and other polyQ diseases.

Sequestration of cellular native factors by biomolecular assemblies: Physiological or pathological?

In addition to native-state structures, biomolecules often form condensed supramolecular assemblies or cellular membraneless organelles that are critical for cell life. These biomolecular assemblies, generally including liquid-like droplets (condensates) and amyloid-like aggregates, can sequester or recruit their interacting partners, so as to either modulate various cellular behaviors or even cause disorders. This review article summarizes recent advances in the sequestration of native factors by biomolecular assemblies and discusses their potential consequences on cellular function, homeostasis, and disease pathology.

A survey of protein interactions and posttranslational modifications that influence the polyglutamine diseases

The presence and aggregation of misfolded proteins has deleterious effects in the nervous system. Among the various diseases caused by misfolded proteins is the family of the polyglutamine (polyQ) disorders. This family comprises nine members, all stemming from the same mutation—the abnormal elongation of a polyQ repeat in nine different proteins—which causes protein misfolding and aggregation, cellular dysfunction and disease. While it is the same type of mutation that causes them, each disease is distinct: it is influenced by regions and domains that surround the polyQ repeat; by proteins with which they interact; and by posttranslational modifications they receive. Here, we overview the role of non-polyQ regions that control the pathogenicity of the expanded polyQ repeat. We begin by introducing each polyQ disease, the genes affected, and the symptoms experienced by patients. Subsequently, we provide a survey of protein-protein interactions and posttranslational modifications that regulate polyQ toxicity. We conclude by discussing shared processes and pathways that bring some of the polyQ diseases together and may serve as common therapeutic entry points for this family of incurable disorders.

Testosterone Levels Are Decreased and Associated with Disease Duration in Male Spinocerebellar Ataxia Type 2 Patients

Spinocerebellar ataxia type 2 (SCA2) is a progressive neurodegenerative disorder due to an unstable expansion of a CAG repeat in the ATXN2 gene. Despite clinical and experimental evidence indicating the relevance of the gonadotropic axis to the prognosis and therapeutics for several late-onset neurodegenerative disorders, its functioning and association with disease severity have not been previously explored in SCA2. To assess serum levels of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH), and their clinical relevance in SCA2 patients. A case-control study involving 94 Cuban SCA2 patients and 101 gender- and age-matched healthy controls was conducted. Testosterone, LH, and FSH serum levels were determined by radioimmunoassay or immunoradiometric assay systems. Clinical outcomes included age at onset, disease duration, Scale for the Assessment and Rating of Ataxia (SARA) score, and progression rate. Univariate general linear models were generated. Testosterone, LH, and FSH serum levels were significantly reduced in male SCA2 patients relative to control individuals. On average, there was a 35% reduction in testosterone levels in male patients versus male control individuals. Testosterone levels were associated with disease duration (r = 0.383; p = 0.025) and age at onset (r = 0.414; p = 0.011) in male SCA2 patients, but no association was observed between testosterone and CAG expansion size, SARA score, or progression rate. Testosterone levels might be a biomarker of disease progression in male SCA2 patients. Further studies are needed to explore the effects of low testosterone levels on non-motor symptoms, and to assess the potential of testosterone replacement therapy in male SCA2 patients.

In Human and Mouse Spino-Cerebellar Tissue, Ataxin-2 Expansion Affects Ceramide-Sphingomyelin Metabolism

Ataxin-2 (human gene symbol ATXN2) acts during stress responses, modulating mRNA translation and nutrient metabolism. Ataxin-2 knockout mice exhibit progressive obesity, dyslipidemia, and insulin resistance. Conversely, the progressive ATXN2 gain of function due to the fact of polyglutamine (polyQ) expansions leads to a dominantly inherited neurodegenerative process named spinocerebellar ataxia type 2 (SCA2) with early adipose tissue loss and late muscle atrophy. We tried to understand lipid dysregulation in a SCA2 patient brain and in an authentic mouse model. Thin layer chromatography of a patient cerebellum was compared to the lipid metabolome of Atxn2-CAG100-Knockin (KIN) mouse spinocerebellar tissue. The human pathology caused deficits of sulfatide, galactosylceramide, cholesterol, C22/24-sphingomyelin, and gangliosides GM1a/GD1b despite quite normal levels of C18-sphingomyelin. Cerebellum and spinal cord from the KIN mouse showed a consistent decrease of various ceramides with a significant elevation of sphingosine in the more severely affected spinal cord. Deficiency of C24/26-sphingomyelins contrasted with excess C18/20-sphingomyelin. Spinocerebellar expression profiling revealed consistent reductions of CERS protein isoforms, Sptlc2 and Smpd3, but upregulation of Cers2 mRNA, as prominent anomalies in the ceramide-sphingosine metabolism. Reduction of Asah2 mRNA correlated to deficient S1P levels. In addition, downregulations for the elongase Elovl1, Elovl4, Elovl5 mRNAs and ELOVL4 protein explain the deficit of very long-chain sphingomyelin. Reduced ASMase protein levels correlated to the accumulation of long-chain sphingomyelin. Overall, a deficit of myelin lipids was prominent in SCA2 nervous tissue at prefinal stage and not compensated by transcriptional adaptation of several metabolic enzymes. Myelination is controlled by mTORC1 signals; thus, our human and murine observations are in agreement with the known role of ATXN2 yeast, nematode, and mouse orthologs as mTORC1 inhibitors and autophagy promoters.

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.

FipoQ/FBXO33, a Cullin-1-based ubiquitin ligase complex component modulates ubiquitination and solubility of polyglutamine disease protein

Polyglutamine (polyQ) diseases describe a group of progressive neurodegenerative disorders caused by the CAG triplet repeat expansion in the coding region of the disease genes. To date, nine such diseases, including spinocerebellar ataxia type 3 (SCA3), have been reported. The formation of SDS-insoluble protein aggregates in neurons causes cellular dysfunctions, such as impairment of the ubiquitin-proteasome system, and contributes to polyQ pathologies. Recently, the E3 ubiquitin ligases, which govern substrate specificity of the ubiquitin-proteasome system, have been implicated in polyQ pathogenesis. The Cullin (Cul) proteins are major components of Cullin-RING ubiquitin ligases (CRLs) complexes that are evolutionarily conserved in the Drosophila genome. In this study, we examined the effect of individual Culs on SCA3 pathogenesis and found that the knockdown of Cul1 expression enhances SCA3-induced neurodegeneration and reduces the solubility of expanded SCA3-polyQ proteins. The F-box proteins are substrate receptors of Cul1-based CRL. We further performed a genetic modifier screen of the 19 Drosophila F-box genes and identified F-box involved in polyQ pathogenesis (FipoQ) as a genetic modifier of SCA3 degeneration that modulates the ubiquitination and solubility of expanded SCA3-polyQ proteins. In the human SK-N-MC cell model, we identified that F-box only protein 33 (FBXO33) exerts similar functions as FipoQ in modulating the ubiquitination and solubility of expanded SCA3-polyQ proteins. Taken together, our study demonstrates that Cul1-based CRL and its associated F-box protein, FipoQ/FBXO33, modify SCA3 protein toxicity. These findings will lead to a better understanding of the disease mechanism of SCA3 and provide insights for developing treatments against SCA3. Cover Image for this issue: doi: 10.1111/jnc.14510.

FBXW2 localizes with osteocalcin in bovine periosteum on culture dishes as visualized by double immunostaining

Osteocalcin (OC) is a well-known protein related to bone, however, the role of F-box and WD-40 domain-containing protein 2 (FBXW2) in bone remains unclear. In 2016, the presence of FBXW2 in bovine periosteum was reported. In this study, double immunostaining was used to investigate the relationship between OC and FBXW2. FBXW2 showed tubular structures, and OC showed a similar localization pattern as FBXW2. Double immunostaining findings suggested that FBXW2 tubes were coated with OC. To the author's knowledge, this is the first study to reveal the interaction between OC and FBXW2.

Parkinson Disease from Mendelian Forms to Genetic Susceptibility: New Molecular Insights into the Neurodegeneration Process

Parkinson disease (PD) is known as a common progressive neurodegenerative disease which is clinically diagnosed by the manifestation of numerous motor and nonmotor symptoms. PD is a genetically heterogeneous disorder with both familial and sporadic forms. To date, researches in the field of Parkinsonism have identified 23 genes or loci linked to rare monogenic familial forms of PD with Mendelian inheritance. Biochemical studies revealed that the products of these genes usually play key roles in the proper protein and mitochondrial quality control processes, as well as synaptic transmission and vesicular recycling pathways within neurons. Despite this, large number of patients affected with PD typically tends to show sporadic forms of disease with lack of a clear family history. Recent genome-wide association studies (GWAS) meta-analyses on the large sporadic PD case-control samples from European populations have identified over 12 genetic risk factors. However, the genetic etiology that underlies pathogenesis of PD is also discussed, since it remains unidentified in 40% of all PD-affected cases. Nowadays, with the emergence of new genetic techniques, international PD genomics consortiums and public online resources such as PDGene, there are many hopes that future large-scale genetics projects provide further insights into the genetic etiology of PD and improve diagnostic accuracy and therapeutic clinical trial designs.

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.

Quantitative Global Proteomics of Yeast PBP1 Deletion Mutants and Their Stress Responses Identifies Glucose Metabolism, Mitochondrial, and Stress Granule Changes

The yeast protein PBP1 is implicated in very diverse pathways. Intriguingly, its deletion mitigates the toxicity of human neurodegeneration factors. Here, we performed label-free quantitative global proteomics to identify crucial downstream factors, either without stress or under cell stress conditions (heat and NaN₃). Compared to the wildtype BY4741 strain, PBP1 deletion always triggered downregulation of the key bioenergetics enzyme KGD2 and the prion protein RNQ1 as well as upregulation of the leucine biosynthesis enzyme LEU1. Without stress, enrichment of stress response factors was consistently detected for both deletion mutants; upon stress, these factors were more pronounced. The selective analysis of components of stress granules and P-bodies revealed a prominent downregulation of GIS2. Our yeast data are in good agreement with a global proteomics and metabolomics publication that the PBP1 ortholog ATAXIN-2 (ATXN2) knockout (KO) in mouse results in mitochondrial deficits in leucine/fatty acid catabolism and bioenergetics, with an obesity phenotype. Furthermore, our data provide the completely novel insight that PBP1 mutations in stress periods involve GIS2, a plausible scenario in view of previous data that both PBP1 and GIS2 relocalize from ribosomes to stress granules, interact with poly(A)-binding protein in translation regulation and prevent mitochondrial precursor overaccumulation stress (mPOS). This may be relevant for human diseases like spinocerebellar ataxias, amyotrophic lateral sclerosis, and the metabolic syndrome.

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.

Search for SCA2 blood RNA biomarkers highlights Ataxin-2 as strong modifier of the mitochondrial factor PINK1 levels

Ataxin-2 (ATXN2) polyglutamine domain expansions of large size result in an autosomal dominantly inherited multi-system-atrophy of the nervous system named spinocerebellar ataxia type 2 (SCA2), while expansions of intermediate size act as polygenic risk factors for motor neuron disease (ALS and FTLD) and perhaps also for Levodopa-responsive Parkinson's disease (PD). In view of the established role of ATXN2 for RNA processing in periods of cell stress and the expression of ATXN2 in blood cells such as platelets, we investigated whether global deep RNA sequencing of whole blood from SCA2 patients identifies a molecular profile which might serve as diagnostic biomarker. The bioinformatic analysis of SCA2 blood global transcriptomics revealed various significant effects on RNA processing pathways, as well as the pathways of Huntington's disease and PD where mitochondrial dysfunction is crucial. Notably, an induction of PINK1 and PARK7 expression was observed. Conversely, expression of Pink1 was severely decreased upon global transcriptome profiling of Atxn2-knockout mouse cerebellum and liver, in parallel to strong effects on Opa1 and Ghitm, which encode known mitochondrial dynamics regulators. These results were validated by quantitative PCR and immunoblots. Starvation stress of human SH-SY5Y neuroblastoma cells led to a transcriptional phasic induction of ATXN2 in parallel to PINK1, and the knockdown of one enhanced the expression of the other during stress response. These findings suggest that ATXN2 may modify the known PINK1 roles for mitochondrial quality control and autophagy during cell stress. Given that PINK1 is responsible for autosomal recessive juvenile PD, this genetic interaction provides a concept how the degeneration of nigrostriatal dopaminergic neurons and the Parkinson phenotype may be triggered by ATXN2 mutations.

Mammalian ataxin-2 modulates translation control at the pre-initiation complex via PI3K/mTOR and is induced by starvation

Ataxin-2 is a cytoplasmic protein, product of the ATXN2 gene, whose deficiency leads to obesity, while its gain-of-function leads to neural atrophy. Ataxin-2 affects RNA homeostasis, but its effects are unclear. Here, immunofluorescence analysis suggested that ataxin-2 associates with 48S pre-initiation components at stress granules in neurons and mouse embryonic fibroblasts, but is not essential for stress granule formation. Coimmunoprecipitation analysis showed associations of ataxin-2 with initiation factors, which were concentrated at monosome fractions of polysome gradients like ataxin-2, unlike its known interactor PABP. Mouse embryonic fibroblasts lacking ataxin-2 showed increased phosphorylation of translation modulators 4E-BP1 and ribosomal protein S6 through the PI3K-mTOR pathways. Indeed, human neuroblastoma cells after trophic deprivation showed a strong induction of ATXN2 transcript via mTOR inhibition. Our results support the notion that ataxin-2 is a nutritional stress-inducible modulator of mRNA translation at the pre-initiation complex.

Sequestration of cellular interacting partners by protein aggregates: implication in a loss-of-function pathology

Protein misfolding and aggregation are a hallmark of several neurodegenerative diseases (NDs). However, how protein aggregation leads to cytotoxicity and neurodegeneration is still controversial. Emerging evidence demonstrates that sequestration of cellular-interacting partners by protein aggregates contributes to the pathogenesis of these diseases. Here, we review current research on sequestration of cellular proteins by protein aggregates and its relation to proteinopathies. Based on different interaction modes, we classify these protein sequestrations into four types: protein coaggregation, domain/motif-mediated sequestration, RNA-assisted sequestration, and sequestration of molecular chaperones. Thus, the cellular essential proteins and/or RNA hijacked by protein aggregates may lose their biological functions, consequently resulting in cytotoxicity and neurodegeneration. We have proposed a hijacking model recapitulating the sequestration process and the loss-of-function pathology of ND.

Ataxin-2 (Atxn2)-Knock-Out Mice Show Branched Chain Amino Acids and Fatty Acids Pathway Alterations

Human Ataxin-2 (ATXN2) gene locus variants have been associated with obesity, diabetes mellitus type 1,and hypertension in genome-wide association studies, whereas mouse studies showed the knock-out of Atxn2 to lead to obesity, insulin resistance, and dyslipidemia. Intriguingly, the deficiency of ATXN2 protein orthologs in yeast and flies rescues the neurodegeneration process triggered by TDP-43 and Ataxin-1 toxicity. To understand the molecular effects of ATXN2 deficiency by unbiased approaches, we quantified the global proteome and metabolome of Atxn2-knock-out mice with label-free mass spectrometry. In liver tissue, significant downregulations of the proteins ACADS, ALDH6A1, ALDH7A1, IVD, MCCC2, PCCA, OTC, together with bioinformatic enrichment of downregulated pathways for branched chain and other amino acid metabolism, fatty acids, and citric acid cycle were observed. Statistical trends in the cerebellar proteome and in the metabolomic profiles supported these findings. They are in good agreement with recent claims that PBP1, the yeast ortholog of ATXN2, sequestrates the nutrient sensor TORC1 in periods of cell stress. Overall, ATXN2 appears to modulate nutrition and metabolism, and its activity changes are determinants of growth excess or cell atrophy.