Sharing of polyglutamine localization by the neuronal nucleus and cytoplasm in CAG-repeat diseases

Altered Metabolic Signaling and Potential Therapies in Polyglutamine Diseases

Polyglutamine diseases comprise a cluster of genetic disorders involving neurodegeneration and movement disabilities. In polyglutamine diseases, the target proteins become aberrated due to polyglutamine repeat formation. These aberrant proteins form the root cause of associated complications. The metabolic regulation during polyglutamine diseases is not well studied and needs more attention. We have brought to light the significance of regulating glutamine metabolism during polyglutamine diseases, which could help in decreasing the neuronal damage associated with excess glutamate and nucleotide generation. Most polyglutamine diseases are accompanied by symptoms that occur due to excess glutamate and nucleotide accumulation. Along with a dysregulated glutamine metabolism, the Nicotinamide adenine dinucleotide (NAD+) levels drop down, and, under these conditions, NAD+ supplementation is the only achievable strategy. NAD+ is a major co-factor in the glutamine metabolic pathway, and it helps in maintaining neuronal homeostasis. Thus, strategies to decrease excess glutamate and nucleotide generation, as well as channelizing glutamine toward the generation of ATP and the maintenance of NAD+ homeostasis, could aid in neuronal health. Along with understanding the metabolic dysregulation that occurs during polyglutamine diseases, we have also focused on potential therapeutic strategies that could provide direct benefits or could restore metabolic homeostasis. Our review will shed light into unique metabolic causes and into ideal therapeutic strategies for treating complications associated with polyglutamine diseases.

Glutamatergic Synapse Dysfunction in Drosophila Neuromuscular Junctions Can Be Rescued by Proteostasis Modulation

Glutamate is the major excitatory neurotransmitter in the nervous system, and the Drosophila glutamatergic neuromuscular junctions (NMJs) offer a tractable platform to understand excitatory synapse biology both in health and disease. Synaptopathies are neurodegenerative diseases that are associated with synaptic dysfunction and often display compromised proteostasis. One such rare, progressive neurodegenerative condition, Spinocerebellar Ataxia Type 3 (SCA3) or Machado-Joseph Disease (MJD), is characterized by cerebellar ataxia, Parkinsonism, and degeneration of motor neuron synapses. While the polyQ repeat mutant protein ataxin-3 is implicated in MJD, it is unclear how it leads to impaired synaptic function. In this study, we indicated that a Drosophila model of MJD recapitulates characteristics of neurodegenerative disorders marked by motor neuron dysfunction. Expression of 78 polyQ repeats of mutant ataxin-3 protein in Drosophila motor neurons resulted in behavioral defects, such as impaired locomotion in both larval and adult stages. Furthermore, defects in eclosion and lifespan were observed in adult flies. Detailed characterization of larval glutamatergic neuromuscular junctions (NMJs) revealed defects in morphological features along with compromised NMJ functioning. Autophagy, one of the key proteostasis pathways, is known to be impaired in the case of several synaptopathies. Our study reveals that overexpression of the autophagy-related protein Atg8a rescued behavioral defects. Thus, we present a model for glutamatergic synapse dysfunction that recapitulates synaptic and behavioral deficits and show that it is an amenable system for carrying out genetic and chemical biology screens to identify potential therapeutic targets for synaptopathies.

The neostriatum in polyglutamine diseases: preferential decreases in large neurons in dentatorubral-pallidoluysian atrophy and Machado-Joseph disease and in small neurons in Huntington disease

The presence of polyglutamine-immunoreactive deposits in neurons of the neostriatum has been reported in dentatorubral-pallidoluysian atrophy (DRPLA), Machado-Joseph disease (MJD), and Huntington disease (HD). However, among these diseases, precise quantitative investigations on neurons have been performed only for HD. Changes in the number of neurons and the immunohistological features of polyglutamine deposits in the caudate head and putamen were examined in six patients with DRPLA, three with MJD, and four with HD. In the neostriatum in DRPLA, the numbers of large and small neurons were reduced to 33-38% and 48-68% relative to controls, respectively, whereas the corresponding figures in MJD were 19-26% and 65-76%, respectively, and those in HD were 34-35% and 12-16%, respectively. In DRPLA, 2-55% of neurons remaining in the neostriatum showed diffuse nuclear accumulation of polyglutamine, in contrast to 3-20% in MJD and a few percent in HD. These findings indicate that, in the neostriatum, a decrease in the number of small neurons is predominant in HD, whereas a decrease in the number of large neurons is predominant in DRPLA and MJD. Thus, it is suggested that disease processs differ among polyglutamine diseases.

A Novel SCA3 Knock-in Mouse Model Mimics the Human SCA3 Disease Phenotype Including Neuropathological, Behavioral, and Transcriptional Abnormalities Especially in Oligodendrocytes

Spinocerebellar ataxia type 3 is the most common autosomal dominant inherited ataxia worldwide, caused by a CAG repeat expansion in the Ataxin-3 gene resulting in a polyglutamine (polyQ)-expansion in the corresponding protein. The disease is characterized by neuropathological, phenotypical, and specific transcriptional changes in affected brain regions. So far, there is no mouse model available representing all the different aspects of the disease, yet highly needed for a better understanding of the disease pathomechanisms. Here, we characterized a novel Ataxin-3 knock-in mouse model, expressing a heterozygous or homozygous expansion of 304 CAACAGs in the murine Ataxin-3 locus using biochemical, behavioral, and transcriptomic approaches. We compared neuropathological, and behavioral features of the new knock-in model with the in SCA3 research mostly used YAC84Q mouse model. Further, we compared transcriptional changes found in cerebellar samples of the SCA3 knock-in mice and post-mortem human SCA3 patients. The novel knock-in mouse is characterized by the expression of a polyQ-expansion in the murine Ataxin-3 protein, leading to aggregate formation, especially in brain regions known to be vulnerable in SCA3 patients, and impairment of Purkinje cells. Along these neuropathological changes, the mice showed a reduction in body weight accompanied by gait and balance instability. Transcriptomic analysis of cerebellar tissue revealed age-dependent differential expression, enriched for genes attributed to myelinating oligodendrocytes. Comparing these changes with those found in cerebellar tissue of SCA3 patients, we discovered an overlap of differentially expressed genes pointing towards similar gene expression perturbances in several genes linked to myelin sheaths and myelinating oligodendrocytes.

Nucleotides regulate the common molecular mechanisms that underlie neurodegenerative diseases; Therapeutic implications

Neurodegenerative diseases (ND) are a heterogeneous group of neurological disorders characterized by a progressive loss of neuronal function which results in neuronal death. Although a specific toxic factor has been identified for each ND, all of them share common pathological molecular mechanisms favouring the disease development. In the final stages of ND, patients become unable to take care of themselves and decline to a total functional incapacitation that leads to their death. Some of the main factors which contribute to the disease progression include proteasomal dysfunction, neuroinflammation, synaptic alterations, protein aggregation, and oxidative stress. Over recent years, evidence has been accumulated to suggest that purinergic signaling plays a key role in the aforementioned molecular pathways. In this review, we revise the implications of the purinergic signaling in the common molecular mechanism underlying the ND. In particular, we focus on the role of the purinergic receptors P2X7, P2Y₂ and the ectoenzyme tissue-nonspecific alkaline phosphatase (TNAP).

The CAG-polyglutamine repeat diseases: a clinical, molecular, genetic, and pathophysiologic nosology

Throughout the genome, unstable tandem nucleotide repeats can expand to cause a variety of neurologic disorders. Expansion of a CAG triplet repeat within a coding exon gives rise to an elongated polyglutamine (polyQ) tract in the resultant protein product, and accounts for a unique category of neurodegenerative disorders, known as the CAG-polyglutamine repeat diseases. The nine members of the CAG-polyglutamine disease family include spinal and bulbar muscular atrophy (SBMA), Huntington disease, dentatorubral pallidoluysian atrophy, and six spinocerebellar ataxias (SCA 1, 2, 3, 6, 7, and 17). All CAG-polyglutamine diseases are dominantly inherited, with the exception of SBMA, which is X-linked, and many CAG-polyglutamine diseases display anticipation, which is defined as increasing disease severity in successive generations of an affected kindred. Despite widespread expression of the different polyQ-expanded disease proteins throughout the body, each CAG-polyglutamine disease strikes a particular subset of neurons, although the mechanism for this cell-type selectivity remains poorly understood. While the different genes implicated in these disorders display amino acid homology only in the repeat tract domain, certain pathologic molecular processes have been implicated in almost all of the CAG-polyglutamine repeat diseases, including protein aggregation, proteolytic cleavage, transcription dysregulation, autophagy impairment, and mitochondrial dysfunction. Here we highlight the clinical and molecular genetic features of each distinct disorder, and then discuss common themes in CAG-polyglutamine disease pathogenesis, closing with emerging advances in therapy development.

Pharmacological Therapies for Machado-Joseph Disease

Machado-Joseph disease (MJD), also known as Spinocerebellar Ataxia type 3 (SCA3), is the most common autosomal dominant ataxia worldwide. MJD integrates a large group of disorders known as polyglutamine diseases (polyQ). To date, no effective treatment exists for MJD and other polyQ diseases. Nevertheless, researchers are making efforts to find treatment possibilities that modify the disease course or alleviate disease symptoms. Since neuroimaging studies in mutation carrying individuals suggest that in nervous system dysfunction begins many years before the onset of any detectable symptoms, the development of therapeutic interventions becomes of great importance, not only to slow progression of manifest disease but also to delay, or ideally prevent, its onset. Potential therapeutic targets for MJD and polyQ diseases can be divided into (i) those that are aimed at the polyQ proteins themselves, namely gene silencing, attempts to enhance mutant protein degradation or inhibition/prevention of aggregation; and (ii) those that intercept the toxic downstream effects of the polyQ proteins, such as mitochondrial dysfunction and oxidative stress, transcriptional abnormalities, UPS impairment, excitotoxicity, or activation of cell death. The existence of relevant animal models and the recent contributions towards the identification of putative molecular mechanisms underlying MJD are impacting on the development of new drugs. To date only a few preclinical trials were conducted, nevertheless some had very promising results and some candidate drugs are close to being tested in humans. Clinical trials for MJD are also very few to date and their results not very promising, mostly due to trial design constraints. Here, we provide an overview of the pharmacological therapeutic strategies for MJD studied in animal models and patients, and of their possible translation into the clinical practice.

Deregulation of autophagy in postmortem brains of Machado-Joseph disease patients

Autophagy, the major pathway for protein turnover, is critical to maintain cellular homeostasis and has been implicated in neurodegenerative diseases. The aim of this research was to analyze the expression of autophagy markers in postmortem brains from Machado-Joseph disease (MJD) patients. The expression of autophagy markers in the cerebellum and the oculomotor nucleus from MJD patients and age-matched controls with no signs of neuropathology was inspected postmortem by immunohistochemistry (IHC) and Western blot. Furthermore, autophagy was examined by means of transmission electron microscopy (TEM). Western blot and IHC revealed nuclear accumulation of misfolded ataxin-3 (ATXN3) and the presence of ubiquitin- and p62-positive aggregates in MJD patients as compared to controls. Moreover, the autophagic proteins, autophagy-related gene (Atg) protein (ATG)-7, ATG-12, ATG16L2 and autophagosomal microtubule-associated protein light chain 3 (LC3) were significantly increased in MJD brains relative to controls, while beclin-1 levels were reduced in MJD patients. Increase in the levels of lysosomal-associated membrane protein 2 (LAMP-2) and of the endosomal markers (Rab7 and Rab1A) were observed in MJD patients relatively to controls. In addition, these findings were further confirmed by TEM in brain tissue where large vesicles accumulating electron-dense materials were highly enriched in MJD patients. Postmortem brains with MJD exhibit increased markers of autophagy relative to age-matched control brains, therefore suggesting strong dysregulation of autophagy that may have an important role in the course of MJD pathogenesis.

Unbiased screen identifies aripiprazole as a modulator of abundance of the polyglutamine disease protein, ataxin-3

No disease-modifying treatment exists for the fatal neurodegenerative polyglutamine disease known both as Machado-Joseph disease and spinocerebellar ataxia type 3. As a potential route to therapy, we identified small molecules that reduce levels of the mutant disease protein, ATXN3. Screens of a small molecule collection, including 1250 Food and Drug Administration-approved drugs, in a novel cell-based assay, followed by secondary screens in brain slice cultures from transgenic mice expressing the human disease gene, identified the atypical antipsychotic aripiprazole as one of the hits. Aripiprazole increased longevity in a Drosophila model of Machado-Joseph disease and effectively reduced aggregated ATXN3 species in flies and in brains of transgenic mice treated for 10 days. The aripiprazole-mediated decrease in ATXN3 abundance may reflect a complex response culminating in the modulation of specific components of cellular protein homeostasis. Aripiprazole represents a potentially promising therapeutic drug for Machado-Joseph disease and possibly other neurological proteinopathies.

CAG repeat length does not associate with the rate of cerebellar degeneration in spinocerebellar ataxia type 3

This cross-sectional study investigated the correlation between the CAG repeat length and the degeneration of cerebellum in spinocerebellar ataxia type 3 (SCA3) patients based on neuroimaging approaches. Forty SCA3 patients were recruited and classified into two subgroups according to their CAG repeat lengths (≥ 74 and < 74). We measured each patient's Scale for the Assessment and Rating of Ataxia (SARA) score, N-acetylaspartate (NAA)/creatine (Cr) ratios based on magnetic resonance spectroscopy (MRS), and 3-dimensional fractal dimension (3D-FD) values derived from magnetic resonance imaging (MRI) results. Furthermore, the 3D-FD values were used to construct structural covariance networks based on graph theoretical analysis. The results revealed that SCA3 patients with a longer CAG repeat length demonstrated earlier disease onset. However, the CAG repeat length did not significantly correlate with their SARA scores, cerebellar NAA/Cr ratios or cerebellar 3D-FD values. Network dissociation between cerebellar regions and parietal-occipital regions was found in SCA3 patients with CAG ≥ 74, but not in those with CAG < 74. In conclusion, the CAG repeat length is uncorrelated with the change of SARA score, cerebellar function and cerebellar structure in SCA3. Nevertheless, a longer CAG repeat length may indicate early structural covariance network dissociation.

Induced pluripotent stem cell - derived neurons for the study of spinocerebellar ataxia type 3

The neurodegenerative disease spinocerebellar ataxia type 3 (SCA3) is caused by a CAG-repeat expansion in the ATXN3 gene. In this study, induced pluripotent stem cell (iPSC) lines were established from two SCA3 patients. Dermal fibroblasts were reprogrammed using an integration-free method and the resulting SCA3 iPSCs were differentiated into neurons. These neuronal lines harbored the disease causing mutation, expressed comparable levels of several neuronal markers and responded to the neurotransmitters, glutamate/glycine, GABA and acetylcholine. Additionally, all neuronal cultures formed networks displaying synchronized spontaneous calcium oscillations within 28days of maturation, and expressed the mature neuronal markers NeuN and Synapsin 1 implying a relatively advanced state of maturity, although not comparable to that of the adult human brain. Interestingly, we were not able to recapitulate the glutamate-induced ataxin-3 aggregation shown in a previously published iPSC-derived SCA3 model. In conclusion, we have generated a panel of SCA3 patient iPSCs and a robust protocol to derive neurons of relatively advanced maturity, which could potentially be valuable for the study of SCA3 disease mechanisms.

Peripheral neuropathy and parkinsonism: a large clinical and pathogenic spectrum

Peripheral neuropathy (PN) has been reported in idiopathic and hereditary forms of parkinsonism, but the pathogenic mechanisms are unclear and likely heterogeneous. Levodopa-induced vitamin B12 deficiency has been discussed as a causal factor of PN in idiopathic Parkinson's disease, but peripheral nervous system involvement might also be a consequence of the underlying neurodegenerative process. Occurrence of PN with parkinsonism has been associated with a panel of mitochondrial cytopathies, more frequently related to a nuclear gene defect and mainly polymerase gamma (POLG1) gene. Parkin (PARK2) gene mutations are responsible for juvenile parkinsonism, and possible peripheral nervous system involvement has been reported. Rarely, an association of parkinsonism with PN may be encountered in other neurodegenerative diseases such as fragile X-associated tremor and ataxia syndrome related to premutation CGG repeat expansion in the fragile X mental retardation (FMR1) gene, Machado-Joseph disease related to an abnormal CAG repeat expansion in ataxin-3 (ATXN3) gene, Kufor-Rakeb syndrome caused by mutations in ATP13A2 gene, or in hereditary systemic disorders such as Gaucher disease due to mutations in the β-glucocerebrosidase (GBA) gene and Chediak-Higashi syndrome due to LYST gene mutations. This article reviews conditions in which PN may coexist with parkinsonism.

Change in the cortical complexity of spinocerebellar ataxia type 3 appears earlier than clinical symptoms

Patients with spinocerebellar ataxia type 3 (SCA3) have exhibited cerebral cortical involvement and various mental deficits in previous studies. Clinically, conventional measurements, such as the Mini-Mental State Examination (MMSE) and electroencephalography (EEG), are insensitive to cerebral cortical involvement and mental deficits associated with SCA3, particularly at the early stage of the disease. We applied a three-dimensional fractal dimension (3D-FD) method, which can be used to quantify the shape complexity of cortical folding, in assessing cortical degeneration. We evaluated 48 genetically confirmed SCA3 patients by employing clinical scales and magnetic resonance imaging and using 50 healthy participants as a control group. According to the Scale for the Assessment and Rating of Ataxia (SARA), the SCA3 patients were diagnosed with cortical dysfunction in the cerebellar cortex; however, no significant difference in the cerebral cortex was observed according to the patients’ MMSE ratings. Using the 3D-FD method, we determined that cortical involvement was more extensive than involvement of traditional olivopontocerebellar regions and the corticocerebellar system. Moreover, the significant correlation between decreased 3D-FD values and disease duration may indicate atrophy of the cerebellar cortex and cerebral cortex in SCA3 patients. The change of the cerebral complexity in the SCA3 patients can be detected throughout the disease duration, especially it becomes substantial at the late stage of the disease. Furthermore, we determined that atrophy of the cerebral cortex may occur earlier than changes in MMSE scores and EEG signals.

Polyglutamine (PolyQ) diseases: genetics to treatments

The polyglutamine (polyQ) diseases are a group of neurodegenerative disorders caused by expanded cytosine-adenine-guanine (CAG) repeats encoding a long polyQ tract in the respective proteins. To date, a total of nine polyQ disorders have been described: six spinocerebellar ataxias (SCA) types 1, 2, 6, 7, 17; Machado-Joseph disease (MJD/SCA3); Huntington's disease (HD); dentatorubral pallidoluysian atrophy (DRPLA); and spinal and bulbar muscular atrophy, X-linked 1 (SMAX1/SBMA). PolyQ diseases are characterized by the pathological expansion of CAG trinucleotide repeat in the translated region of unrelated genes. The translated polyQ is aggregated in the degenerated neurons leading to the dysfunction and degeneration of specific neuronal subpopulations. Although animal models of polyQ disease for understanding human pathology and accessing disease-modifying therapies in neurodegenerative diseases are available, there is neither a cure nor prevention for these diseases, and only symptomatic treatments for polyQ diseases currently exist. Long-term pharmacological treatment is so far disappointing, probably due to unwanted complications and decreasing drug efficacy. Cellular transplantation of stem cells may provide promising therapeutic avenues for restoration of the functions of degenerative and/or damaged neurons in polyQ diseases.

Lithium chloride alleviates neurodegeneration partly by inhibiting activity of GSK3β in a SCA3 Drosophila model

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG trinucelotide repeat that encodes an abnormal polyglutamine (PolyQ) tract in the disease protein, ataxin-3. The formation of neuronal intranuclear inclusions in the specific brain regions is one of the pathological hallmarks of SCA3. Acceleration of the degradation of the mutant protein aggregates is proven to produce beneficial effects in SCA3 and other PolyQ diseases. Lithium is known to be neuroprotective in various models of neurodegenerative disease and can reduce the mutant protein aggregates by inducing autophagy. In this study, we explored the therapeutic potential of lithium in a SCA3 Drosophila model. We showed that chronic treatment with lithium chloride at specific doses notably prevented eye depigmentation, alleviated locomotor disability, and extended the median life spans of SCA3 transgenic Drosophila. By means of genetic approaches, we showed that co-expressing the mutant S9E, which mimicked the phosphorylated S9 state of Shaggy as done by lithium, also partly decreased toxicity of gmr-SCA3tr-Q78. Taken together, our findings suggest that lithium is a promising therapeutic agent for the treatment of SCA3 and other PolyQ diseases.

White matter damage is related to ataxia severity in SCA3

Spinocerebellar ataxia type 3 (SCA3) is the most frequent inherited cerebellar ataxia in Europe, the US and Japan, leading to disability and death through motor complications. Although the affected protein ataxin-3 is found ubiquitously in the brain, grey matter atrophy is predominant in the cerebellum and the brainstem. White matter pathology is generally less severe and thought to occur in the brainstem, spinal cord, and cerebellar white matter. Here, we investigated both grey and white matter pathology in a group of 12 SCA3 patients and matched controls. We used voxel-based morphometry for analysis of tissue loss, and tract-based spatial statistics (TBSS) on diffusion magnetic resonance imaging to investigate microstructural pathology. We analysed correlations between microstructural properties of the brain and ataxia severity, as measured by the Scale for the Assessment and Rating of Ataxia (SARA) score. SCA3 patients exhibited significant loss of both grey and white matter in the cerebellar hemispheres, brainstem including pons and in lateral thalamus. On between-group analysis, TBSS detected widespread microstructural white matter pathology in the cerebellum, brainstem, and bilaterally in thalamus and the cerebral hemispheres. Furthermore, fractional anisotropy in a white matter network comprising frontal, thalamic, brainstem and left cerebellar white matter strongly and negatively correlated with SARA ataxia scores. Tractography identified the thalamic white matter thus implicated as belonging to ventrolateral thalamus. Disruption of white matter integrity in patients suffering from SCA3 is more widespread than previously thought. Moreover, our data provide evidence that microstructural white matter changes in SCA3 are strongly related to the clinical severity of ataxia symptoms.

Increased aggregation of polyleucine compared with that of polyglutamine in dentatorubral-pallidoluysian atrophy protein

Polyglutamine (polyQ) diseases result from expansion of CAG trinucleotide repeats in their responsible genes. Although gene products with polyQ expansions undergo conformational changes to aggregate in neurons, the relationship between inclusions and neurotoxicity remains unclear. Dentatorubral-pallidoluysian atrophy (DRPLA) is a polyQ disease, and DRPLA protein, also known as atrophin-1 (ATN1), carries an expanded polyQ tract. To investigate how an expanded polyQ tract influences ATN1 aggregation and localization, we compared the aggregation of ATN1 with a polyQ tract to that of ATN1 with a polyleucine (polyL) tract. In COS-7 cells, polyL-ATN1 triggered more aggregation than polyQ-ATN1 of similar repeat sizes. Immunocytochemical and biochemical studies revealed that replacement of the polyQ tract with polyL alters ATN1 localization, leading to retention of polyL-ATN1 in the cytoplasm. Despite this change in localization, polyL-ATN1 and polyQ-ATN1 demonstrate comparable repeat length dependent toxicity. These results suggest that expanded polyQ repeats in ATN1 may contribute to neurodegeneration via alterations in both protein aggregation and intracellular localization.

Clinical features, neurogenetics and neuropathology of the polyglutamine spinocerebellar ataxias type 1, 2, 3, 6 and 7

The spinocerebellar ataxias type 1 (SCA1), 2 (SCA2), 3 (SCA3), 6 (SCA6) and 7 (SCA7) are genetically defined autosomal dominantly inherited progressive cerebellar ataxias (ADCAs). They belong to the group of CAG-repeat or polyglutamine diseases and share pathologically expanded and meiotically unstable glutamine-encoding CAG-repeats at distinct gene loci encoding elongated polyglutamine stretches in the disease proteins. In recent years, progress has been made in the understanding of the pathogenesis of these currently incurable diseases: Identification of underlying genetic mechanisms made it possible to classify the different ADCAs and to define their clinical and pathological features. Furthermore, advances in molecular biology yielded new insights into the physiological and pathophysiological role of the gene products of SCA1, SCA2, SCA3, SCA6 and SCA7 (i.e. ataxin-1, ataxin-2, ataxin-3, α-1A subunit of the P/Q type voltage-dependent calcium channel, ataxin-7). In the present review we summarize our current knowledge about the polyglutamine ataxias SCA1, SCA2, SCA3, SCA6 and SCA7 and compare their clinical and electrophysiological features, genetic and molecular biological background, as well as their brain pathologies. Furthermore, we provide an overview of the structure, interactions and functions of the different disease proteins. On the basis of these comprehensive data, similarities, differences and possible disease mechanisms are discussed.

Distribution and pattern of pathology in subjects with familial or sporadic late-onset cerebellar ataxia as assessed by p62/sequestosome immunohistochemistry

We investigated whether ubiquitin-binding protein p62/sequestosome-1 could be utilized to evaluate the pathology seen in patients with a clinical diagnosis of progressive late-onset cerebellar ataxia (LOCA). p62-immunoreactive (IR) lesions were assessed by means of immunohistochemistry in the brains of six LOCA cases, one with the spinocerebellar ataxia type 1 mutation (SCA1), ages at death ranging from 46 to 56 years. All cases fulfilled the criteria of olivopontocerebellar atrophy (OPCA), i.e., displaying cell loss in the predilection brain areas. One case, genetics unknown, exhibited p62-IR neuronal intranuclear inclusions (NIs). Similar NIs were labeled with the 1C2 antibody that recognizes proteins containing large polyglutamine stretches. In this case, also fused in sarcoma-IR NIs were seen. In the remaining LOCA cases, including the case with the SCA1 mutation, different kinds of nuclear and cytoplasmic p62 and 1C2 labeling but no NIs were seen. The immunoreactivity and distribution of lesions while applying p62 and 1C2 immunohistochemistry varied in our six LOCA cases fulfilling the criteria of OPCA. In all cases except in the SCA1, diffuse nuclear p62 labeling was seen, not previously reported in SCA or other neurodegenerative disorders. Due to the variability noted here as well as the limited number of cases, no assessment of progression and distributional pattern of pathology could be conducted. Based on a literature search, it is apparent that there is a need for clinico-pathologic-genetical studies of LOCA, especially to obtain a deeper understanding of the regional distribution and progression of pathology.

Spinocerebellar ataxia type 7

Spinocerebellar ataxia type 7 (SCA7) is associated with progressive blindness, dominant transmission, and marked anticipation. SCA7 represents one of the polyglutamine expansion diseases with increase of CAG repeats. The gene maps to chromosome 3p12-p21.1. Normal values of CAG repeats range from 4 to 18. The SCA7 gene encodes a protein of largely unknown function, called ataxin-7. SCA7 is reported in many countries and ethnic groups. Its phenotypic expression depends on the number of expanded repeats. The infantile phenotype is very severe, with more than 100 repeats. The classic type has 50 to 55 repeats and is characterized by a combination of visual and ataxic disturbances lasting for 20-40 years.When the number of CAG repeats is between 36 and 43, the evolution is much slower, with few or no retinal abnormalities. A CAG repeat number from 18 to 35 is asymptomatic but predisposes to the development of the disorder when expanding to the pathological range through transmission. The diagnosis is made by molecular genetics. The neuropathology of the disorder includes atrophy of the spinocerebellar pathways, pyramidal tracts, and motor nuclei in the brainstem and spinal cord, a cone-rod sytrophy of the retina, and ataxin-7 immunoreactive neuronal intranuclear inclusions. The neuropathological features vary as a function of the number of CAG repeats. Present research deals mainly with the study of ataxin-7 in transfected neural cells and transgenic mouse models.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

Machado-Joseph Disease: from first descriptions to new perspectives

Machado-Joseph Disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), represents the most common form of SCA worldwide. MJD is an autosomal dominant neurodegenerative disorder of late onset, involving predominantly the cerebellar, pyramidal, extrapyramidal, motor neuron and oculomotor systems; although sharing features with other SCAs, the identification of minor, but more specific signs, facilitates its differential diagnosis. MJD presents strong phenotypic heterogeneity, which has justified the classification of patients into three main clinical types. Main pathological lesions are observed in the spinocerebellar system, as well as in the cerebellar dentate nucleus. MJD's causative mutation consists in an expansion of an unstable CAG tract in exon 10 of the ATXN3 gene, located at 14q32.1. Haplotype-based studies have suggested that two main founder mutations may explain the present global distribution of the disease; the ancestral haplotype is of Asian origin, and has an estimated age of around 5,800 years, while the second mutational event has occurred about 1,400 years ago. The ATXN3 gene encodes for ataxin-3, which is ubiquitously expressed in neuronal and non-neuronal tissues, and, among other functions, is thought to participate in cellular protein quality control pathways. Mutated ATXN3 alleles consensually present about 61 to 87 CAG repeats, resulting in an expanded polyglutamine tract in ataxin-3. This altered protein gains a neurotoxic function, through yet unclear mechanisms. Clinical variability of MJD is only partially explained by the size of the CAG tract, which leaves a residual variance that should be explained by still unknown additional factors. Several genetic tests are available for MJD, and Genetic Counseling Programs have been created to better assist the affected families, namely on what concerns the possibility of pre-symptomatic testing. The main goal of this review was to bring together updated knowledge on MJD, covering several aspects from its initial descriptions and clinical presentation, through the discovery of the causative mutation, its origin and dispersion, as well as molecular genetics aspects considered essential for a better understanding of its neuropathology. Issues related with molecular testing and Genetic Counseling, as well as recent progresses and perspectives on genetic therapy, are also addressed.

An autopsy case of an aged patient with spinocerebellar ataxia type 2

We report the case of a woman who developed limb clumsiness in her fifties and gait disturbance in her sixties. She was bedridden after bone fractures at age 75 and showed disorientation, slow eye movement, gaze palsy, ataxic speech, muscle atrophy and weakness, and areflexia with pathological reflex. She died of respiratory failure at age 85. This patient was diagnosed genetically as having spinocerebellar ataxia type 2 (SCA2), and the number of expanded CAG repeats was 41. At autopsy, the brain weighed 965 g, and the brainstem, cerebellum, frontal convexity and spinal cord were atrophic. Neuronal loss and gliosis were severe in the pontine nucleus, inferior olivary nucleus, cerebellar cortex, gracile and cuneate nuclei and moderate in the substantia nigra, cerebellar dentate nucleus, anterior horns of the spinal cord and dorsal root ganglia. Axonal loss was observed in the middle and inferior cerebellar peduncles, pyramidal tract and posterior column of the spinal cord. Senile plaques and neurofibrillary tangles (NFTs) were diffusely found in the cerebrum (plaque stage C; NFT stage IV). Expanded polyglutamine-immunoreactive inclusions in the neuronal cytoplasm were widely distributed in the CNS, and neuronal intranuclear inclusions were observed in the pontine nucleus and cerebral cortex. This patient in this autopsy case is a late-onset and aged patient with SCA2, and this is the first report of SCA2 combined with Alzheimer's disease (AD) pathology. Neuropathological findings in this patient, except for AD pathology, were consistent with those of reported SCA2 cases. However, the olivo-ponto-cerebellar system of this patient was relatively preserved and the cerebellar dentate nucleus was more involved as compared with previously reported cases. These results suggest that age at onset or the number of CAG repeat expansions could correlate with the distribution pattern of SCA2 neurodegeneration.

Proteolytic processing regulates pathological accumulation in dentatorubral-pallidoluysian atrophy

Dentatorubral-pallidoluysian atrophy is caused by polyglutamine (polyQ) expansion in atrophin-1 (ATN1). Recent studies have shown that nuclear accumulation of ATN1 and cleaved fragments with expanded polyQ is the pathological process underlying neurodegeneration in dentatorubral-pallidoluysian atrophy. However, the mechanism underlying the proteolytic processing of ATN1 remains unclear. In the present study, we examined the proteolytic processing of ATN1 aiming to understand the mechanisms of ATN1 accumulation with polyQ expansion. Using COS-7 and Neuro2a cells that express the ATN1 gene, in which ATN1 was accumulated by increasing the number of polyQs, we identified a novel C-terminal fragment containing a polyQ tract. The mutant C-terminal fragment with expanded polyQ selectively accumulated in the cells, and this was also demonstrated in the brain tissues of patients with dentatorubral-pallidoluysian atrophy. Immunocytochemical and biochemical studies revealed that full-length ATN1 and C-terminal fragments displayed individual localization. The mutant C-terminal fragment was preferentially found in the cytoplasmic membrane/organelle and insoluble fractions. Accordingly, it is assumed that the proteolytic processing of ATN1 regulates the localization of C-terminal fragments. Accumulation of the C-terminal fragment was enhanced by inhibition of caspases in the cytoplasm of COS-7 cells. Collectively, these results suggest that the C-terminal fragment plays a principal role in the pathological accumulation of ATN1 in dentatorubral-pallidoluysian atrophy.

Involvement of Onuf's nucleus in Machado-Joseph disease: a morphometric and immunohistochemical study

Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative disease caused by an expansion of CAG repeats in the MJD1 gene, in which lower urinary tract dysfunction is known to be the most commonly encountered autonomic failure. However, it remains unclear whether Onuf's nucleus (ON), which plays major roles in the micturition reflex and voluntary continence, degenerates during the disease process. In the present study, we conducted a morphometric and immunohistochemical study of ON, together with the lateral nuclear group (LNG) of the sacral anterior horns, in seven patients with MJD. When compared with controls, the number of lower motor neurons in both ON and LNG was significantly smaller in the MJD patients, the former being inversely correlated with the size of the expanded CAG repeats. Notably, MJD patients with a large CAG-repeat expansion showed an ON-predominant pattern of neuronal loss, while in the remaining patients, ON and LNG were affected to a similar degree, or rather an LNG-predominant pattern of neuronal loss was evident. Moreover, when adjusted for age, the degree of neuronal loss in both ON and LNG was significantly correlated with the extent of expansion of the CAG repeats. In MJD, the remaining lower motor neurons in ON often exhibited ataxin-3- or 1C2-immunoreactive (ir) neuronal intranuclear inclusions, while no pTDP-43-ir neuronal cytoplasmic inclusions were present in these neurons. In conclusion, the present findings strongly suggest that neuronal loss in ON, the degree of which is highly influenced by the extent of expansion of CAG repeats, is a consistent feature in MJD.

Selective occurrence of TDP-43-immunoreactive inclusions in the lower motor neurons in Machado-Joseph disease

Pathological transactivation-responsive DNA-binding protein 43 (TDP-43) has been identified as a component of ubiquitinated inclusions in frontotemporal lobar degeneration with motor neuron disease, as well as in sporadic and some forms of familial amyotrophic lateral sclerosis. To clarify whether pathological TDP-43 is present in other neurodegenerative diseases involving the motor neuron system, we immunohistochemically examined the brain and spinal cord affected by two CAG repeat (polyglutamine) diseases, Machado-Joseph disease (MJD) and spinal and bulbar muscular atrophy (SBMA), using polyclonal antibody against TDP-43. In all the MJD cases, TDP-43-immunoreactive (ir) neuronal cytoplasmic inclusions (NCIs), although few in number, were found only in the lower motor neurons in the brainstem and spinal cord. TDP-43-ir NCIs appeared as linear wisp-like, skein-like, or thick, somewhat rod-like bodies. These inclusions were also visualized with antibodies against phosphoserines 409 and 410 of TDP-43, and ubiquitin, but were not recognized by antibody against expanded polyglutamine stretches or ataxin-3. The ultrastructure of the TDP-43-ir NCIs was similar to that of the inclusions seen in sporadic ALS, consisting of bundles of parallel filaments. None of the SBMA cases showed abnormal TDP-43 immunoreactivity in any of the regions examined. Immunoblot analysis failed to recognize hyperphosphorylated TDP-43 at ~23 kDa in two MJD cases examined. However, the immunohistochemical findings strongly suggested that in MJD, in addition to the polyglutamine-dependent disease process, TDP-43-related pathogenesis is associated with degeneration and death of the lower motor neurons.

New insights into the pathoanatomy of spinocerebellar ataxia type 3 (Machado-Joseph disease)

PURPOSE OF REVIEW

This review summarizes recent neuropathological findings in spinocerebellar ataxia type 3 and discusses their relevance for clinical neurology.

RECENT FINDINGS

The extent of the spinocerebellar ataxia type 3 related central nervous neurodegenerative changes has been recently systematically investigated in a series of pathoanatomical studies. These studies showed that the extent of the central nervous degenerative changes of spinocerebellar ataxia type 3 has been underestimated so far. The newly described pattern of central nervous neurodegeneration includes the visual, auditory, vestibular, somatosensory, ingestion-related, dopaminergic and cholinergic systems. These pathological findings were correlated with clinical findings and explain a variety of the spinocerebellar ataxia type 3 symptoms observed in clinical practice.

SUMMARY

Systematic pathoanatomical analysis of spinocerebellar ataxia type 3 brains helps to understand the structural basis of this neurodegenerative disease and offers explanations for a variety of disease symptoms. This better understanding of the neuropathology of the condition has implications for the treatment of spinocerebellar ataxia type 3 patients and represents a basis for further biochemical and molecular biological studies aimed at deciphering the pathomechanisms of this progressive ataxic disorder.

CAG repeat disorder models and human neuropathology: similarities and differences

CAG repeat diseases are hereditary neurodegenerative disorders caused by expansion of a polyglutamine tract in each respective disease protein. They include at least nine disorders, including Huntington's disease (HD), dentatorubral pallidoluysian atrophy (DRPLA), spinal and bulbar muscular atrophy (SBMA), and the spinocerebellar ataxias SCA1, SCA2, SCA3 (also known as Machado-Joseph disease), SCA6, SCA7, and SCA17. It is thought that a gain of toxic function resulting from the protein mutation plays important and common roles in the pathogenesis of these diseases. Recent studies have disclosed that, in addition to the presence of clinical phenotypes and conventional neuropathology in each disease, human brains affected by CAG repeat diseases share several polyglutamine-related changes in their neuronal nuclei and cytoplasm including the formation of intranuclear inclusions. Although these novel pathologic changes also show a distribution pattern characteristic to each disease, they are generally present beyond the lesion distribution of neuronal loss, suggesting that neurons are affected much more widely than has been recognized previously. Various mouse models of CAG repeat diseases have revealed that CAG repeat lengths, which are responsible for polyglutamine diseases in humans, are not sufficient for creating the conditions characteristic of each disease in mice. Although high expression of mutant proteins in mice results in the successful generation of polyglutamine-related changes in the brain, there are still some differences from human pathology in the lesion distribution or cell types that are affected. In addition, no model has yet successfully reproduced the specific neuronal loss observed in humans. Although there are no models that fully represent the neuropathologic changes present in humans, the data obtained have provided evidence that clinical onset is not clearly associated with neuronal cell death, but depends on intranuclear accumulation of mutant proteins in neurons.

Oxidative stress in neurodegeneration in dentatorubral-pallidoluysian atrophy

Dentatorubral-pallidoluysian atrophy (DRPLA) is one of the CAG-repeat diseases, and is classified into juvenile and early adult types showing progressive myoclonus epilepsy (PME) in addition to late adult type. We immunohistochemically examined accumulation of oxidative products and expression of superoxide dismutase (SOD) in autopsy cases of DRPLA. Oxidative products to nucleosides, 8-hydroxy-2'-deoxyguanosine and 8-hydroxyguanosine, were accumulated in the lenticulate nucleus predominantly in DRPLA cases having PME. Neuronal accumulation of 4-hydroxy nonenal, a reactive lipid aldehyde, was found in the hippocampus, globus pallidus and cerebellar dentate nucleus in adult DRPLA cases and controls. Cytoplasmic immunoreactivity for Cu/ZnSOD was reduced in the external segment of globus pallidus, dentate nucleus and cerebellar cortex in DRPLA cases. Mitochondrial immunoreactivity for MnSOD was reduced in the lenticulate nucleus and cerebellum in DRPLA cases having PME. Some DRPLA cases showed reduced immunoreactivity for MnSOD in the cerebral cortex. Coexistence of reduced SOD expression and polyglutamine was observed in a few cases. It has been discussed in Huntington's disease that expanded polyglutamine can lead to oxidative neurodegeneration. It is likely that oxidative stress can be involved in DRPLA, although relationship with expanded polyglutamine remains to be elusive.

Degeneration of ingestion-related brainstem nuclei in spinocerebellar ataxia type 2, 3, 6 and 7

Dysphagia, which can lead to nutritional deficiencies, weight loss and dehydration, represents a risk factor for aspiration pneumonia. Although clinical studies have reported the occurrence of dysphagia in patients with spinocerebellar ataxia type 2 (SCA2), type 3 (SCA3), type 6 (SCA6) and type 7 (SCA7), there are neither detailed clinical records concerning the kind of ingestive malfunctions which contribute to dysphagia nor systematic pathoanatomical studies of brainstem regions involved in the ingestive process. In the present study we performed a systematic post mortem study on thick serial tissue sections through the ingestion-related brainstem nuclei of 12 dysphagic patients who suffered from clinically diagnosed and genetically confirmed spinocerebellar ataxias assigned to the CAG-repeat or polyglutamine diseases (two SCA2, seven SCA3, one SCA6 and two SCA7 patients) and evaluated their medical records. Upon pathoanatomical examination in all of the SCA2, SCA3, SCA6 and SCA7 patients, a widespread neurodegeneration of the brainstem nuclei involved in the ingestive process was found. The clinical records revealed that all of the SCA patients were diagnosed with progressive dysphagia and showed dysfunctions detrimental to the preparatory phase of the ingestive process, as well as the lingual, pharyngeal and oesophageal phases of swallowing. The vast majority of the SCA patients suffered from aspiration pneumonia, which was the most frequent cause of death in our sample. The findings of the present study suggest (i) that dysphagia in SCA2, SCA3, SCA6 and SCA7 patients may be associated with widespread neurodegeneration of ingestion-related brainstem nuclei; (ii) that dysphagic SCA2, SCA3, SCA6 and SCA7 patients may suffer from dysfunctions detrimental to all phases of the ingestive process; and (iii) that rehabilitative swallow therapy which takes specific functional consequences of the underlying brainstem lesions into account might be helpful in preventing aspiration pneumonia, weight loss and dehydration in SCA2, SCA3, SCA6 and SCA7 patients.

Spinocerebellar ataxia type 3 (SCA3): thalamic neurodegeneration occurs independently from thalamic ataxin-3 immunopositive neuronal intranuclear inclusions

In the last years progress has been made regarding the involvement of the thalamus during the course of the currently known polyglutamine diseases. Although recent studies have shown that the thalamus consistently undergoes neurodegeneration in Huntington's disease (HD) and spinocerebellar ataxia type 2 (SCA2) it is still unclear whether it is also a consistent target of the pathological process of spinocerebellar ataxia type 3 (SCA3). Accordingly we studied the thalamic pathoanatomy and distribution pattern of ataxin-3 immunopositive neuronal intranuclear inclusions (NI) in nine clinically diagnosed and genetically confirmed SCA3 patients and carried out a detailed statistical analysis of our findings. During our pathoanatomical study we disclosed (i) a consistent degeneration of the ventral anterior, ventral lateral and reticular thalamic nuclei; (ii) a degeneration of the ventral posterior lateral nucleus and inferior and lateral subnuclei of the pulvinar in the majority of these SCA3 patients; and (iii) a degeneration of the ventral posterior medial and lateral posterior thalamic nuclei, the lateral geniculate body and some of the limbic thalamic nuclei in some of them. Upon immunocytochemical analysis we detected NI in all of the thalamic nuclei of all of our SCA3 patients. According to our statistical analysis (i) thalamic neurodegeneration and the occurrence of ataxin-3 immunopositive thalamic NI was not associated with the individual length of the CAG-repeats in the mutated SCA3 allele, the patients age at disease onset and the duration of SCA3 and (ii) thalamic neurodegeneration was not correlated with the occurrence of ataxin-3 immunopositive thalamic NI. This lack of correlation may suggest that ataxin-3 immunopositive NI are not immediately decisive for the fate of affected nerve cells but rather represent unspecific and pathognomonic morphological markers of SCA3.