Different ataxin-2 antibodies display different immunoreactive profiles

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.

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.

The Multiple Faces of Spinocerebellar Ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is among the most common forms of autosomal dominant ataxias, accounting for 15% of the total families. Occurrence is higher in specific populations such as the Cuban and Southern Italian. The disease is caused by a CAG expansion in ATXN2 gene, leading to abnormal accumulation of the mutant protein, ataxin‐2, in intracellular inclusions. The clinical picture is mainly dominated by cerebellar ataxia, although a number of other neurological signs have been described, ranging from parkinsonism to motor neuron involvement, making the diagnosis frequently challenging for neurologists, particularly when information about the family history is not available. Although the functions of ataxin‐2 have not been completely elucidated, the protein is involved in mRNA processing and control of translation. Recently, it has also been shown that the size of the CAG repeat in normal alleles represents a risk factor for ALS, suggesting that ataxin‐2 plays a fundamental role in maintenance of neuronal homeostasis.

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.

Drosophila ataxin-2 gene encodes two differentially expressed isoforms and its function in larval fat body is crucial for development of peripheral tissues

Different isoforms of ataxin‐2 are predicted in Drosophila and may underlie different cellular processes. Here, we validated the isoforms B and C of Drosophila ataxin‐2 locus (dAtx2), which we found to be expressed in various tissues and at different levels during development. dAtx2‐B mRNA was detected at low amounts during all developmental stages, whereas dAtx2‐C mRNA levels increase by eightfold from L3 to pupal–adult stages. Higher amounts of dAtx2‐B protein were detected in embryos, while dAtx2‐C protein was also expressed in higher levels in pupal–adult stages, indicating post‐transcriptional control for isoform B and transcription induction for isoform C, respectively. Moreover, in the fat body of L3 larvae dAtx2‐C, but not dAtx2‐B, accumulates in cytoplasmic foci that colocalize with sec23, a marker of endoplasmic reticulum exit sites (ERES). Interestingly, animals subjected to selective knockdown of dAtx2 in the larval fat body do not complete metamorphosis and die at the third larval stage or early puparium. Additionally, larvae knocked down for dAtx2, grown at 29 °C, are significantly smaller than control animals due to reduction in DNA replication and cell growth, which are consistent with the decreased levels of phosphorylated‐AKT observed in the fat body. Based on the localization of ataxin‐2 (dAtx2‐C) in ERESs, and on the phenotypes observed by dAtx2 knockdown in the larval fat body, we speculate a possible role for this protein in processes that regulate ERES formation. These data provide new insights into the biological function of ataxin‐2 with potential relevance to neurodegenerative diseases.

Spinocerebellar ataxia type 2: clinical presentation, molecular mechanisms, and therapeutic perspectives

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant genetic disease characterized by cerebellar dysfunction associated with slow saccades, early hyporeflexia, severe tremor of postural or action type, peripheral neuropathy, cognitive disorders, and other multisystemic features. SCA2, one of the most common ataxias worldwide, is caused by the expansion of a CAG triplet repeat located in the N-terminal coding region of the ATXN2 gene, which results in the incorporation of a segment of polyglutamines in the mutant protein, being longer expansions associated with earlier onset and more sever disease in subsequent generations. In this review, we offer a detailed description of the clinical manifestations of SCA2 and compile the experimental evidence showing the participation of ataxin-2 in crucial cellular processes, including messenger RNA maturation and translation, and endocytosis. In addition, we discuss in the light of present data the potential molecular mechanisms underlying SCA2 pathogenesis. The mutant protein exhibits a toxic gain of function that is mainly attributed to the generation of neuronal inclusions of phosphorylated and/or proteolytic cleaved mutant ataxin-2, which might alter normal ataxin-2 function, leading to cell dysfunction and death of target cells. In the final part of this review, we discuss the perspectives of development of therapeutic strategies for SCA2. Based on previous experience with other polyglutamine disorders and considering the molecular basis of SCA2 pathogenesis, a nuclei-acid-based strategy focused on the specific silencing of the dominant disease allele that preserves the expression of the wild-type allele is highly desirable and might prevent toxic neurodegenerative sequelae.

Experimental and computational analysis of polyglutamine-mediated cytotoxicity

Expanded polyglutamine (polyQ) proteins are known to be the causative agents of a number of human neurodegenerative diseases but the molecular basis of their cytoxicity is still poorly understood. PolyQ tracts may impede the activity of the proteasome, and evidence from single cell imaging suggests that the sequestration of polyQ into inclusion bodies can reduce the proteasomal burden and promote cell survival, at least in the short term. The presence of misfolded protein also leads to activation of stress kinases such as p38MAPK, which can be cytotoxic. The relationships of these systems are not well understood. We have used fluorescent reporter systems imaged in living cells, and stochastic computer modeling to explore the relationships of polyQ, p38MAPK activation, generation of reactive oxygen species (ROS), proteasome inhibition, and inclusion body formation. In cells expressing a polyQ protein inclusion, body formation was preceded by proteasome inhibition but cytotoxicity was greatly reduced by administration of a p38MAPK inhibitor. Computer simulations suggested that without the generation of ROS, the proteasome inhibition and activation of p38MAPK would have significantly reduced toxicity. Our data suggest a vicious cycle of stress kinase activation and proteasome inhibition that is ultimately lethal to cells. There was close agreement between experimental data and the predictions of a stochastic computer model, supporting a central role for proteasome inhibition and p38MAPK activation in inclusion body formation and ROS-mediated cell death.

Spinocerebellar ataxia 2 (SCA2)

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominantly inherited, neurodegenerative disease. It can manifest either with a cerebellar syndrome or as Parkinson's syndrome, while later stages involve mainly brainstem, spinal cord and thalamus. This particular atrophy pattern resembles sporadic multi-system-atrophy (MSA) and results in some clinical features indicative of SCA2, such as early saccade slowing, early hyporeflexia, severe tremor of postural or action type, and early myoclonus. For treatment, levodopa is temporarily useful for rigidity/bradykinesia and for tremor, magnesium for muscle cramps, but neuroprotective therapy will depend on the elucidation of pathogenesis. The disease cause lies in the polyglutamine domain of the protein ataxin-2, which can expand in families over successive generations resulting in earlier onset age and faster progression. Genetic testing in SCA2 and other polyglutamine disorders like the well-studied Huntington's disease is now readily available for family planning. Although these disorders differ clinically and in the affected neuron populations, it is not understood how the different polyglutamine proteins mediate such tissue specificity. The neuronal intranuclear inclusion bodies described in other polyglutamine disorders are not frequent in SCA2. For the quite ubiquitously expressed ataxin-2, a subcellular localization at the Golgi, the endoplasmic reticulum and the plasma membrane, in interaction with proteins of mRNA translation and of endocytosis have been observed. As a first victim of SCA2 degeneration, cerebellar Purkinje neurons may be preferentially susceptible to alterations of these subcellular pathways, and therefore our review aims to portray the particular profile of the SCA2 disease process and correlate it to the specific features of ataxin-2.

Ataxin-2 associates with rough endoplasmic reticulum

Ataxin-2 is a novel protein, normally with a domain of 22 consecutive glutamine (Q) residues, which may expand beyond a threshold of (Q)(32), causing a neurodegenerative disease named Spinocerebellar ataxia type 2 (SCA2). To obtain clues about the functions of ataxin-2, we used fluorescence microscopy and centrifugation fractionation analyses. Immunocytochemical detection in non-neuronal and neuronal cells showed endogenous and transfected ataxin-2 distributed throughout the cytoplasm, with perinuclear preference and a granular appearance. Triple-labelling and confocal microscopy demonstrated co-localisation with the endoplasmic reticulum (ER) markers calreticulin, calnexin and CFP-ER. The pathogenic form of ataxin-2 with an expanded polyQ domain showed the same distribution pattern. Subcellular fractionation of mouse brain homogenates showed endogenous ataxin-2 associated with rough ER (rER) membranes, in a manner dependent on RNA, salt and phosphorylation. Our data are in agreement with recent findings that ataxin-2 directly interacts with poly(A)-binding protein (PABP), thus associating with polyribosomes under normal conditions and being recruited to stress granules under environmental stress. These data, in conjunction with the presence of Lsm domains within ataxin-2, suggest that ataxin-2 is involved in the processing of mRNA and/or the regulation of translation.

Inclusion formation by ataxins -1, -2, -3, and -7

The authors studied inclusion formation in vitro using transiently transfected PC12 cells, with epitope-tagged and untagged full-length and truncated wild-type and expanded ataxins -1, -2, -3, and -7. At 72 hours, no inclusions were seen with wild-type full-length or truncated ataxins -2, -3, or -7, and only one with ataxin-1. Truncation abolished nuclear localization of ataxins -1 and -7, and allowed nuclear entry of ataxin-2. Of the expanded ataxins, only -1 and -2 formed inclusions, and those of ataxin-2 were rare and exclusively cytoplasmic. Truncation resulted in inclusion formation by ataxins -3 and -7, increased ataxin-1 inclusions, and enabled formation of nuclear ataxin-2 inclusions. There was no recruitment of wild-type ataxin-1 to expanded ataxin-1 inclusions.

Partial resistance of ataxin-2-containing olivary and pontine neurons to axotomy-induced degeneration

Spinocerebellar ataxia type 2 (SCA2) is caused by the expansion of a polyglutamine tract in ataxin-2, the SCA2 gene product. In spite of the identification of the genetic defect and the coded protein, the function of wild-type ataxin-2 has not been clarified. In order to identify the possible resistance of ataxin-2-containing neurons to degeneration, we investigated in this study the distribution and the characteristics of cell reaction to axotomy in ataxin-2-positive olivary and pontine neurons in a model of cerebellar damage represented by hemicerebellectomy. We also performed double immunofluorescence studies of ataxin-2 and purinergic receptors to characterize ataxin-2-positive surviving neurons. The present data demonstrated that after axotomy olivary and pontine ataxin-2-expressing neurons survived longer than the ataxin-2-negative cell population. Cell counting performed in the different olivary subdivisions failed to reveal any topographical prevalence in the distribution of ataxin-2-positive neurons. Therefore, the relative resistance to axotomy appears to be an intrinsic property of the ataxin-2 cell population. In addition, the capacity to modify the pattern of purinergic receptor expression in response to damage was present in only one subset of ataxin-2-positive surviving neurons. These data suggest that ataxin-2 is involved in resistance to degeneration phenomena which may be lost after mutation.