Morphological Purkinje cell changes in spinocerebellar ataxia type 6

Viral-based animal models in polyglutamine disorders

Polyglutamine disorders are a complex group of incurable neurodegenerative disorders caused by an abnormal expansion in the trinucleotide cytosine-adenine-guanine tract of the affected gene. To better understand these disorders, our dependence on animal models persists, primarily relying on transgenic models. In an effort to complement and deepen our knowledge, researchers have also developed animal models of polyglutamine disorders employing viral vectors. Viral vectors have been extensively used to deliver genes to the brain, not only for therapeutic purposes but also for the development of animal models, given their remarkable flexibility. In a time- and cost-effective manner, it is possible to use different transgenes, at varying doses, in diverse targeted tissues, at different ages, and in different species, to recreate polyglutamine pathology. This paper aims to showcase the utility of viral vectors in disease modelling, share essential considerations for developing animal models with viral vectors, and provide a comprehensive review of existing viral-based animal models for polyglutamine disorders.

Mitochondrial damage and impaired mitophagy contribute to disease progression in SCA6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease that manifests in midlife and progressively worsens with age. SCA6 is rare, and many patients are not diagnosed until long after disease onset. Whether disease-causing cellular alterations differ at different disease stages is currently unknown, but it is important to answer this question in order to identify appropriate therapeutic targets across disease duration. We used transcriptomics to identify changes in gene expression at disease onset in a well-established mouse model of SCA6 that recapitulates key disease features. We observed both up- and down-regulated genes with the major down-regulated gene ontology terms suggesting mitochondrial dysfunction. We explored mitochondrial function and structure and observed that changes in mitochondrial structure preceded changes in function, and that mitochondrial function was not significantly altered at disease onset but was impaired later during disease progression. We also detected elevated oxidative stress in cells at the same disease stage. In addition, we observed impairment in mitophagy that exacerbates mitochondrial dysfunction at late disease stages. In post-mortem SCA6 patient cerebellar tissue, we observed metabolic changes that are consistent with mitochondrial impairments, supporting our results from animal models being translatable to human disease. Our study reveals that mitochondrial dysfunction and impaired mitochondrial degradation likely contribute to disease progression in SCA6 and suggests that these could be promising targets for therapeutic interventions in particular for patients diagnosed after disease onset.

Morphological and Molecular Changes in the Cortex and Cerebellum of Immunocompetent Mice Infected with Zika Virus

Zika virus (ZIKV) disease continues to be a threat to public health, and it is estimated that millions of people have been infected and that there have been more cases of serious complications than those already reported. Despite many studies on the pathogenesis of ZIKV, several of the genes involved in the malformations associated with viral infection are still unknown. In this work, the morphological and molecular changes in the cortex and cerebellum of mice infected with ZIKV were evaluated. Neonatal BALB/c mice were inoculated with ZIKV intraperitoneally, and the respective controls were inoculated with a solution devoid of the virus. At day 10 postinoculation, the mice were euthanized to measure the expression of the markers involved in cortical and cerebellar neurodevelopment. The infected mice presented morphological changes accompanied by calcifications, as well as a decrease in most of the markers evaluated in the cortex and cerebellum. The modifications found could be predictive of astrocytosis, dendritic pathology, alterations in the regulation systems of neuronal excitation and inhibition, and premature maturation, conditions previously described in other models of ZIKV infection and microcephaly.

Age-related differences of cerebellar cortex and nuclei: MRI findings in healthy controls and its application to spinocerebellar ataxia (SCA6) patients

Understanding cerebellar alterations due to healthy aging provides a reference point against which pathological findings in late-onset disease, for example spinocerebellar ataxia type 6 (SCA6), can be contrasted. In the present study, we investigated the impact of aging on the cerebellar nuclei and cerebellar cortex in 109 healthy controls (age range: 16 - 78 years) using 3 Tesla magnetic resonance imaging (MRI). Findings were compared with 25 SCA6 patients (age range: 38 - 78 years). A subset of 16 SCA6 (included: 14) patients and 50 controls (included: 45) received an additional MRI scan at 7 Tesla and were re-scanned after one year. MRI included T1-weighted, T2-weighted FLAIR, and multi-echo T2*-weighted imaging. The T2*-weighted phase images were converted to quantitative susceptibility maps (QSM). Since the cerebellar nuclei are characterized by elevated iron content with respect to their surroundings, two independent raters manually outlined them on the susceptibility maps. T1-weighted images acquired at 3T were utilized to automatically identify the cerebellar gray matter (GM) volume. Linear correlations revealed significant atrophy of the cerebellum due to tissue loss of cerebellar cortical GM in healthy controls with increasing age. Reduction of the cerebellar GM was substantially stronger in SCA6 patients. The volume of the dentate nuclei did not exhibit a significant relationship with age, at least in the age range between 18 and 78 years, whereas mean susceptibilities of the dentate nuclei increased with age. As previously shown, the dentate nuclei volumes were smaller and magnetic susceptibilities were lower in SCA6 patients compared to age- and sex-matched controls. The significant dentate volume loss in SCA6 patients could also be confirmed with 7T MRI. Linear mixed effects models and individual paired t-tests accounting for multiple comparisons revealed no statistical significant change in volume and susceptibility of the dentate nuclei after one year in neither patients nor controls. Importantly, dentate volumes were more sensitive to differentiate between SCA6 (Cohen's d = 3.02) and matched controls than the cerebellar cortex volume (d = 2.04). In addition to age-related decline of the cerebellar cortex and atrophy in SCA6 patients, age-related increase of susceptibility of the dentate nuclei was found in controls, whereas dentate volume and susceptibility was significantly decreased in SCA6 patients. Because no significant changes of any of these parameters was found at follow-up, these measures do not allow to monitor disease progression at short intervals.

Histopathology of the cerebellar cortex in essential tremor and other neurodegenerative motor disorders: comparative analysis of 320 brains

In recent years, numerous morphologic changes have been identified in the essential tremor (ET) cerebellar cortex, distinguishing ET from control brains. These findings have not been fully contextualized within a broader degenerative disease spectrum, thus limiting their interpretability. Building off our prior study and now doubling the sample size, we conducted comparative analyses in a postmortem series of 320 brains on the severity and patterning of cerebellar cortex degenerative changes in ET (n = 100), other neurodegenerative disorders of the cerebellum [spinocerebellar ataxias (SCAs, n = 47, including 13 SCA3 and 34 SCA1, 2, 6, 7, 8, 14); Friedreich's ataxia (FA, n = 13); multiple system atrophy (MSA), n = 29], and other disorders that may involve the cerebellum [Parkinson's disease (PD), n = 62; dystonia, n = 19] versus controls (n = 50). We generated data on 37 quantitative morphologic metrics, grouped into 8 broad categories: Purkinje cell (PC) loss, heterotopic PCs, PC dendritic changes, PC axonal changes (torpedoes), PC axonal changes (other than torpedoes), PC axonal changes (torpedo-associated), basket cell axonal hypertrophy, and climbing fiber-PC synaptic changes. Principal component analysis of z scored raw data across all diagnoses (11,651 data items) revealed that diagnostic groups were not uniform with respect to pathology. Dystonia and PD each differed from controls in only 4/37 and 5/37 metrics, respectively, whereas ET differed in 21, FA in 10, SCA3 in 10, MSA in 21, and SCA1/2/6/7/8/14 in 27. Pathological changes were generally on the milder end of the degenerative spectrum in ET, FA and SCA3, and on the more severe end of that spectrum in SCA1/2/6/7/8/14. Comparative analyses across morphologic categories demonstrated differences in relative expression, defining distinctive patterns of changes in these groups. In summary, we present a robust and reproducible method that identifies somewhat distinctive signatures of degenerative changes in the cerebellar cortex that mark each of these disorders.

Myelinating Glia: Potential Therapeutic Targets in Polyglutamine Spinocerebellar Ataxias

Human studies, in combination with animal and cellular models, support glial cells as both major contributors to neurodegenerative diseases and promising therapeutic targets. Among glial cells, oligodendrocytes and Schwann cells are the myelinating glial cells of the central and peripheral nervous system, respectively. In this review, we discuss the contributions of these central and peripheral myelinating glia to the pathomechanisms of polyglutamine (polyQ) spinocerebellar ataxia (SCA) types 1, 2, 3, 6, 7, and 17. First, we highlight the function of oligodendrocytes in healthy conditions and how they are disrupted in polyQ SCA patients and diseased model systems. We then cover the role of Schwann cells in peripheral nerve function and repair as well as their possible role in peripheral neuropathy in polyQ SCAs. Finally, we discuss potential polyQ SCA therapeutic interventions in myelinating glial.

Loss of Flocculus Purkinje Cell Firing Precision Leads to Impaired Gaze Stabilization in a Mouse Model of Spinocerebellar Ataxia Type 6 (SCA6)

Spinocerebellar Ataxia Type 6 (SCA6) is a mid-life onset neurodegenerative disease characterized by progressive ataxia, dysarthria, and eye movement impairment. This autosomal dominant disease is caused by the expansion of a CAG repeat tract in the CACNA1A gene that encodes the α1A subunit of the P/Q type voltage-gated Ca²⁺ channel. Mouse models of SCA6 demonstrate impaired locomotive function and reduced firing precision of cerebellar Purkinje in the anterior vermis. Here, to further assess deficits in other cerebellar-dependent behaviors, we characterized the oculomotor phenotype of a knock-in mouse model with hyper-expanded polyQ repeats (SCA6^84Q). We found a reduction in the efficacy of the vestibulo-ocular reflex (VOR) and optokinetic reflex (OKR) in SCA6 mutant mice, without a change in phase, compared to their litter-matched controls. Additionally, VOR motor learning was significantly impaired in SCA6^84Q mice. Given that the floccular lobe of the cerebellum plays a vital role in the generation of OKR and VOR calibration and motor learning, we investigated the firing behavior and morphology of floccular cerebellar Purkinje cells. Overall, we found a reduction in the firing precision of floccular lobe Purkinje cells but no morphological difference between SCA6^84Q and wild-type mice. Taken together, our findings establish that gaze stabilization and motor learning are impaired in SCA6^84Q mice and suggest that altered cerebellar output contributes to these deficits.

Deep Brain Stimulation of the Interposed Nucleus Reverses Motor Deficits and Stimulates Production of Anti-inflammatory Cytokines in Ataxia Mice

Cerebellum is one of the major targets of autoimmunity and cerebellar damage that leads to ataxia characterized by the loss of fine motor coordination and balance, with no treatment available. Deep brain stimulation (DBS) could be a promising treatment for ataxia but has not been extensively investigated. Here, our study aims to investigate the use of interposed nucleus of deep cerebellar nuclei (IN-DCN) for ataxia. We first characterized ataxia-related motor symptom of a Purkinje cell (PC)-specific LIM homeobox (Lhx)1 and Lhx5 conditional double knockout mice by motor coordination tests, and spontaneous electromyogram (EMG) recording. To validate IN-DCN as a target for DBS, in vivo local field potential (LFP) multielectrode array recording of IN-DCN revealed abnormal LFP amplitude surges in PCs. By synchronizing the EMG and IN-DCN recordings (neurospike and LFP) with high-speed video recordings, ataxia mice showed poorly coordinated movements associated with low EMG amplitude and aberrant IN-DCN neural firing. To optimize IN-DCN-DBS for ataxia, we tested DBS parameters from low (30 Hz) to high stimulation frequency (130 or 150 Hz), and systematically varied pulse width values (60 or 80 µs) to maximize motor symptom control in ataxia mice. The optimal IN-DCN-DBS parameter reversed motor deficits in ataxia mice as detected by animal behavioral tests and EMG recording. Mechanistically, cytokine array analysis revealed that anti-inflammatory cytokines such as interleukin (IL)-13 and IL-4 were upregulated after IN-DCN-DBS, which play key roles in neural excitability. As such, we show that IN-DCN-DBS is a promising treatment for ataxia and possibly other movement disorders alike.

Is essential tremor a degenerative disorder or an electric disorder? Degenerative disorder

Essential tremor (ET) is a highly prevalent neurologic disease and is the most common of the many tremor disorders. ET is a progressive condition with marked clinical heterogeneity, associated with a spectrum of both motor and non-motor features. However, its disease mechanisms remain poorly understood. Much debate has centered on whether ET should be considered a degenerative disorder, with underlying pathological changes in brain causing progressive disease manifestations, or an electric disorder, with overactivity of intrinsically oscillatory motor networks that occur without underlying structural brain abnormalities. Converging data from clinical, neuroimaging and pathological studies in ET now provide considerable evidence for the neurodegenerative hypothesis. A major turning point in this debate is that rigorous tissue-based studies have recently identified a series of structural changes in the ET cerebellum. Most of these pathological changes are centered on the Purkinje cell and connected neuronal populations, which can result in partial loss of Purkinje cells and circuitry reorganizations that would disturb cerebellar function. There is significant overlap in clinical and pathological features of ET with other disorders of cerebellar degeneration, and an increased risk of developing other degenerative diseases in ET. The combined implication of these studies is that ET could be degenerative. The evidence in support of the degenerative hypothesis is presented.

Myosin 18Aα targets the guanine nucleotide exchange factor β-Pix to the dendritic spines of cerebellar Purkinje neurons and promotes spine maturation

Myosin 18Aα is a myosin 2-like protein containing unique N- and C-terminal protein interaction domains that co-assembles with myosin 2. One protein known to bind to myosin 18Aα is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42 that has been shown to promote dendritic spine maturation by activating the assembly of actin and myosin filaments in spines. Here, we show that myosin 18A⍺ concentrates in the spines of cerebellar Purkinje neurons via co-assembly with myosin 2 and through an actin binding site in its N-terminal extension. miRNA-mediated knockdown of myosin 18A⍺ results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18A⍺. Importantly, β-Pix co-localizes with myosin 18A⍺ in spines, and its spine localization is lost upon myosin 18A⍺ knockdown or when its myosin 18A⍺ binding site is deleted. Finally, we show that the spines of myosin 18A⍺ knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data argue that mixed filaments of myosin 2 and myosin 18A⍺ form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix's GEF activity.

Spatiotemporal Alterations in Gait in Humanized Transgenic Sickle Mice

Sickle cell disease (SCD) is a hemoglobinopathy affecting multiple organs and featuring acute and chronic pain. Purkinje cell damage and hyperalgesia have been demonstrated in transgenic sickle mice. Purkinje cells are associated with movement and neural function which may influence pain. We hypothesized that Purkinje cell damage and/or chronic pain burden provoke compensatory gait changes in sickle mice. We found that Purkinje cells undergoe increased apoptosis as shown by caspase-3 activation. Using an automated gait measurement system, MouseWalker, we characterized spatiotemporal gait characteristics of humanized transgenic BERK sickle mice in comparison to control mice. Sickle mice showed alteration in stance instability and dynamic gait parameters (walking speed, stance duration, swing duration and specific swing indices). Differences in stance instability may reflect motor dysfunction due to damaged Purkinje cells. Alterations in diagonal and all stance indices indicative of hesitation during walking may originate from motor dysfunction and/or arise from fear and/or anticipation of movement-evoked pain. We also demonstrate that stance duration, diagonal swing indices and all stance indices correlate with both mechanical and deep tissue hyperalgesia, while stance instability correlates with only deep tissue hyperalgesia. Therefore, objective analysis of gait in SCD may provide insights into neurological impairment and pain states.

Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Polyglutamine spinocerebellar ataxias (polyQ SCAs) include SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 and constitute a group of adult onset neurodegenerative disorders caused by the expansion of a CAG repeat sequence located within the coding region of specific genes, which translates into polyglutamine tract in the corresponding proteins. PolyQ SCAs are characterized by degeneration of the cerebellum and its associated structures and lead to progressive ataxia and other diverse symptoms. In recent years, gene and epigenetic deregulations have been shown to play a critical role in the pathogenesis of polyQ SCAs. Here, we provide an overview of the functions of wild type and pathogenic polyQ SCA proteins in gene regulation, describe the extent and nature of gene expression changes and their pathological consequences in diseases, and discuss potential avenues to further investigate converging and distinct disease pathways and to develop therapeutic strategies.

Essential tremor pathology: neurodegeneration and reorganization of neuronal connections

Essential tremor (ET) is the most common tremor disorder globally and is characterized by kinetic tremor of the upper limbs, although other clinical features can also occur. Postmortem studies are a particularly important avenue for advancing our understanding of the pathogenesis of ET; however, until recently, the number of such studies has been limited. Several recent postmortem studies have made important contributions to our understanding of the pathological changes that take place in ET. These studies identified abnormalities in the cerebellum, which primarily affected Purkinje cells (PCs), basket cells and climbing fibres, in individuals with ET. We suggest that some of these pathological changes (for example, focal PC axonal swellings, swellings in and regression of the PC dendritic arbor and PC death) are likely to be primary and degenerative. By contrast, other changes, such as an increase in PC recurrent axonal collateral formation and hypertrophy of GABAergic basket cell axonal processes, could be compensatory responses to restore cerebellar GABAergic tone and cerebellar cortical inhibitory efficacy. Such compensatory responses are likely to be insufficient, enabling the disease to progress. Here, we review the results of recent postmortem studies of ET and attempt to place these findings into an anatomical-physiological disease model.

An Improved Method for Differentiating Mouse Embryonic Stem Cells into Cerebellar Purkinje Neurons

While mixed primary cerebellar cultures prepared from embryonic tissue have proven valuable for dissecting structure-function relationships in cerebellar Purkinje neurons (PNs), this technique is technically challenging and often yields few cells. Recently, mouse embryonic stem cells (mESCs) have been successfully differentiated into PNs, although the published methods are very challenging as well. The focus of this study was to simplify the differentiation of mESCs into PNs. Using a recently described neural differentiation media, we generate monolayers of neural progenitor cells from mESCs and differentiate them into PN precursors using specific extrinsic factors. These PN precursors are then differentiated into mature PNs by co-culturing them with granule neuron (GN) precursors also derived from neural progenitors using different extrinsic factors. The morphology of mESC-derived PNs is indistinguishable from PNs grown in primary culture in terms of gross morphology, spine length, and spine density. Furthermore, mESC-derived PNs express Calbindin D28K, IP3R1, IRBIT, PLCβ4, PSD93, and myosin IIB-B2, all of which are either PN-specific or highly expressed in PNs. Moreover, we show that mESC-derived PNs form synapses with GN-like cells as in primary culture, express proteins driven by the PN-specific promoter Pcp2/L7, and exhibit the defect in spine ER inheritance seen in PNs isolated from dilute-lethal (myosin Va-null) mice when expressing a Pcp2/L7-driven miRNA directed against myosin Va. Finally, we define a novel extracellular matrix formulation that reproducibly yields monolayer cultures conducive for high-resolution imaging. Our improved method for differentiating mESCs into PNs should facilitate the dissection of molecular mechanisms and disease phenotypes in PNs.

The proteasome regulator PI31 is required for protein homeostasis, synapse maintenance, and neuronal survival in mice

The conserved proteasome-binding protein PI31 serves as an adapter to couple proteasomes with cellular motors to mediate their transport to distal tips of neurons where protein breakdown occurs. We generated global and conditional PI31 knockout mouse strains and show that this protein is required for protein homeostasis, and that its conditional inactivation in neurons disrupts synaptic structures and long-term survival. This work establishes a critical role for PI31 and local protein degradation in the maintenance of neuronal architecture, circuitry, and function. Because mutations in the PI31 pathway cause neurodegenerative diseases in humans, reduced PI31 activity may contribute to the etiology of these diseases.

Proteasome-mediated degradation of intracellular proteins is essential for cell function and survival. The proteasome-binding protein PI31 (Proteasomal Inhibitor of 31kD) promotes 26S assembly and functions as an adapter for proteasome transport in axons. As localized protein synthesis and degradation is especially critical in neurons, we generated a conditional loss of PI31 in spinal motor neurons (MNs) and cerebellar Purkinje cells (PCs). A cKO of PI31 in these neurons caused axon degeneration, neuronal loss, and progressive spinal and cerebellar neurological dysfunction. For both MNs and PCs, markers of proteotoxic stress preceded axonal degeneration and motor dysfunction, indicating a critical role for PI31 in neuronal homeostasis. The time course of the loss of MN and PC function in developing mouse central nervous system suggests a key role for PI31 in human neurodegenerative diseases.

Creation of a myosin Va-TAP-tagged mouse and identification of potential myosin Va-interacting proteins in the cerebellum

The actin-based motor myosin Va transports numerous cargos, including the smooth endoplasmic reticulum (SER) in cerebellar Purkinje neurons (PNs) and melanosomes in melanocytes. Identifying proteins that interact with this myosin is key to understanding its cellular functions. Toward that end, we used recombineering to insert via homologous recombination a tandem affinity purification (TAP) tag composed of the immunoglobulin G-binding domain of protein A, a tobacco etch virus cleavage site, and a FLAG tag into the mouse MYO5A locus immediately after the initiation codon. Importantly, we provide evidence that the TAP-tagged version of myosin Va (TAP-MyoVa) functions normally in terms of SER transport in PNs and melanosome positioning in melanocytes. Given this and other evidence that TAP-MyoVa is fully functional, we purified it together with associated proteins directly from juvenile mouse cerebella and subjected the samples to mass spectroscopic analyses. As expected, known myosin Va-binding partners like dynein light chain were identified. Importantly, numerous novel interacting proteins were also tentatively identified, including guanine nucleotide-binding protein G(o) subunit alpha (Gnao1), a biomarker for schizophrenia. Consistently, an antibody to Gnao1 immunoprecipitates myosin Va, and Gnao1's localization to PN dendritic spines depends on myosin Va. The mouse model created here should facilitate the identification of novel myosin Va-binding partners, which in turn should advance our understanding of the roles played by this important myosin in vivo.

Heterotopic Purkinje Cells: a Comparative Postmortem Study of Essential Tremor and Spinocerebellar Ataxias 1, 2, 3, and 6

Essential tremor (ET) is among the most common neurological diseases. Postmortem studies have noted a series of pathological changes in the ET cerebellum. Heterotopic Purkinje cells (PCs) are those whose cell body is mis-localized in the molecular layer. In neurodegenerative settings, these are viewed as a marker of the progression of neuronal degeneration. We (1) quantify heterotopias in ET cases vs. controls, (2) compare ET cases to other cerebellar degenerative conditions (spinocerebellar ataxias (SCAs) 1, 2, 3, and 6), (3) compare these SCAs to one another, and (4) assess heterotopia within the context of associated PC loss in each disease. Heterotopic PCs were quantified using a standard LH&E-stained section of the neocerebellum. Counts were normalized to PC layer length (n-heterotopia count). It is also valuable to consider PC counts when assessing heterotopia, as loss of PCs extends both to normally located as well as heterotopic PCs. Therefore, we divided n-heterotopias by PC counts. There were 96 brains (43 ET, 31 SCA [12 SCA1, 7 SCA2, 7 SCA3, 5 SCA6], and 22 controls). The median number of n-heterotopias in ET cases was two times higher than that of the controls (2.6 vs. 1.2, p < 0.05). The median number of n-heterotopias in the various SCAs formed a spectrum, with counts being highest in SCA3 and SCA1. In analyses that factored in PC counts, ET had a median n-heterotopia/Purkinje cell count that was three times higher than the controls (0.35 vs. 0.13, p < 0.01), and SCA1 and SCA2 had counts that were 5.5 and 11 times higher than the controls (respective p < 0.001). The median n-heterotopia/PC count in ET was between that of the controls and the SCAs. Similarly, the median PC count in ET was between that of the controls and the SCAs; the one exception was SCA3, in which the PC population is well known to be preserved. Heterotopia is a disease-associated feature of ET. In comparison, several of the SCAs evidenced even more marked heterotopia, although a spectrum existed across the SCAs. The median n-heterotopia/PC count and median PC in ET was between that of the controls and the SCAs; hence, in this regard, ET could represent an intermediate state or a less advanced state of spinocerebellar atrophy.

Somatic sprouts of the Purkinje cells in a patient with multiple system atrophy

We describe the post mortem case of a 71-year-old Japanese woman diagnosed as having multiple system atrophy (MSA), showing somatic sprouting formation of Purkinje cells. The patient had suffered from frequent falling episodes and clumsiness of the left hand since the age of 67 years. Orthostatic hypotension and parkinsonism subsequently emerged. Typical neuropathological features of MSA, including degeneration of the striatum, pontine base and cerebellum with abundance of phosphorylated α-synuclein-positive neuronal and glial cytoplasmic and nuclear inclusions in the brain, were observed. In addition to gliosis of the cerebellar white matter and notable loss of Purkinje cells, several Purkinje cells showed somatic sprouting. Somatic sprouting of Purkinje cells has been demonstrated in several specific conditions, such as developing brains and several neurodegenerative disorders, including Menkes kinky hair disease, familial spinocerebellar ataxia, acute encephalopathy linked to familial hemiplegic migraine, and Huntington's disease; however, no MSA cases have been reported with sprouting from the soma of Purkinje cells. Axonal damage caused by oligodendroglial dysfunction could be crucial in the development of Purkinje cell loss in MSA. Moreover, no apparent α-synuclein accumulation has been described in the Purkinje cells of MSA. We propose that MSA is another degenerative disorder associated with somatic sprouts of Purkinje cells.

Spinocerebellar [corrected] Ataxia Type 6: Molecular Mechanisms and Calcium Channel Genetics

Spinocerebellar ataxia (SCA) type 6 is an autosomal dominant disease affecting cerebellar degeneration. Clinically, it is characterized by pure cerebellar dysfunction, slowly progressive unsteadiness of gait and stance, slurred speech, and abnormal eye movements with late onset. Pathological findings of SCA6 include a diffuse loss of Purkinje cells, predominantly in the cerebellar vermis. Genetically, SCA6 is caused by expansion of a trinucleotide CAG repeat in the last exon of longest isoform CACNA1A gene on chromosome 19p13.1-p13.2. Normal alleles have 4-18 repeats, while alleles causing disease contain 19-33 repeats. Due to presence of a novel internal ribosomal entry site (IRES) with the mRNA, CACNA1A encodes two structurally unrelated proteins with distinct functions within an overlapping open reading frame (ORF) of the same mRNA: (1) α1A subunit of P/Q-type voltage gated calcium channel; (2) α1ACT, a newly recognized transcription factor, with polyglutamine repeat at C-terminal end. Understanding the function of α1ACT in physiological and pathological conditions may elucidate the pathogenesis of SCA6. More importantly, the IRES, as the translational control element of α1ACT, provides a potential therapeutic target for the treatment of SCA6.

Vulnerability of Purkinje Cells Generated from Spinocerebellar Ataxia Type 6 Patient-Derived iPSCs

Spinocerebellar ataxia type 6 (SCA6) is a dominantly inherited neurodegenerative disease characterized by loss of Purkinje cells in the cerebellum. SCA6 is caused by CAG trinucleotide repeat expansion in CACNA1A, which encodes Cav2.1, α1A subunit of P/Q-type calcium channel. However, the pathogenic mechanism and effective therapeutic treatments are still unknown. Here, we have succeeded in generating differentiated Purkinje cells that carry patient genes by combining disease-specific iPSCs and self-organizing culture technologies. Patient-derived Purkinje cells exhibit increased levels of full-length Cav2.1 protein but decreased levels of its C-terminal fragment and downregulation of the transcriptional targets TAF1 and BTG1. We further demonstrate that SCA6 Purkinje cells exhibit thyroid hormone depletion-dependent degeneration, which can be suppressed by two compounds, thyroid releasing hormone and Riluzole. Thus, we have constructed an in vitro disease model recapitulating both ontogenesis and pathogenesis. This model may be useful for pathogenic investigation and drug screening.

Distinct cerebellar regions related to motor and cognitive performance in SCA6 patients

OBJECTIVE

To demonstrate a correlation between anatomic regional changes in Spinocerebellar Ataxia type 6 (SCA6) patients and measures of cognitive performance on neuropsychological tests.

METHODS

Neurocognitive testing was conducted on 24 SCA6 and 28 control subjects. For each cognitive test, SCA6 patients were compared against the controls using Student's t-test. For the cerebellar patients, using voxel based morphometry, correlations between cerebellar gray matter volume at each voxel and performance on the neuropsychological exams were calculated using the Pearson correlation coefficient implemented in SPM8.

RESULTS

Compared to controls, SCA6 patients exhibited significantly impaired performance on the following cognitive tests: Rey-Auditory Verbal Learning Test Trial V, Controlled Oral Word Association phonemic test and semantic-verb test, Rey-Osterrieth Complex Figure copy test as well as immediate and delayed visuo-spatial memory recall test, Trail Making Test (TMT) Part A and Part B, Stroop Color Task completion time, Stroop Color-Word Task score, and Grooved Pegboard Test (GPT) Dominant and Non-Dominant Hand time. Correlations of gray matter density with cognitive test performance were determined for all SCA6 subjects. Using a p-value threshold of 0.001 and family-wise small volume error correction, significant correlations were found for GPT Non-Dominant, GPT Dominant, TMT Part A, and TMT Part B.

CONCLUSION

Different regional patterns of cerebellar involvement were found for the motoric GPT task and the executive version of the TMT. The results for the GPT strongly indicated that the integrity of medial superior hemispheric regions was associated with motor task performance, whereas executive cognitive function was localized in distinctly different inferior regions. This is the first VBM study to differentiate cognitive and motor contributions of the cerebellum.

Transient Developmental Purkinje Cell Axonal Torpedoes in Healthy and Ataxic Mouse Cerebellum

Information is carried out of the cerebellar cortical microcircuit via action potentials propagated along Purkinje cell axons. In several human neurodegenerative diseases, focal axonal swellings on Purkinje cells – known as torpedoes – have been associated with Purkinje cell loss. Interestingly, torpedoes are also reported to appear transiently during development in rat cerebellum. The function of Purkinje cell axonal torpedoes in health as well as in disease is poorly understood. We investigated the properties of developmental torpedoes in the postnatal mouse cerebellum of wild-type and transgenic mice. We found that Purkinje cell axonal torpedoes transiently appeared on axons of Purkinje neurons, with the largest number of torpedoes observed at postnatal day 11 (P11). This was after peak developmental apoptosis had occurred, when Purkinje cell counts in a lobule were static, suggesting that most developmental torpedoes appear on axons of neurons that persist into adulthood. We found that developmental torpedoes were not associated with a presynaptic GABAergic marker, indicating that they are not synapses. They were seldom found at axonal collateral branch points, and lacked microglia enrichment, suggesting that they are unlikely to be involved in axonal refinement. Interestingly, we found several differences between developmental torpedoes and disease-related torpedoes: developmental torpedoes occurred largely on myelinated axons, and were not associated with changes in basket cell innervation on their parent soma. Disease-related torpedoes are typically reported to contain neurofilament; while the majority of developmental torpedoes did as well, a fraction of smaller developmental torpedoes did not. These differences indicate that developmental torpedoes may not be functionally identical to disease-related torpedoes. To study this further, we used a mouse model of spinocerebellar ataxia type 6 (SCA6), and found elevated disease-related torpedo number at 2 years. However, we found normal levels of developmental torpedoes in these mice. Our findings suggest that the transient emergence of Purkinje cell axonal torpedoes during the second postnatal week in mice represents a normal morphological feature in the developing cerebellar microcircuit.

Transient cerebellar alterations during development prior to obvious motor phenotype in a mouse model of spinocerebellar ataxia type 6

KEY POINTS

Spinocerebellar ataxia type 6 (SCA6) is a midlife-onset neurodegenerative disease caused by a CACNA1A mutation; CACNA1A is also implicated in cerebellar development. We have previously shown that when disease symptoms are present in midlife in SCA6^84Q/84Q mice, cerebellar Purkinje cells spike with reduced rate and precision. In contrast, we find that during postnatal development (P10-13), SCA6^84Q/84Q Purkinje cells spike with elevated rate and precision. Although surplus climbing fibres are linked to ataxia in other mouse models, we found surplus climbing fibre inputs on developing (P10-13) SCA6^84Q/84Q Purkinje cells when motor deficits were not detected. Developmental alterations were transient and were no longer observed in weanling (P21-24) SCA6^84Q/84Q Purkinje cells. Our results suggest that changes in the developing cerebellar circuit can occur without detectable motor abnormalities, and that changes in cerebellar development may not necessarily persist into adulthood.

ABSTRACT

Although some neurodegenerative diseases are caused by mutations in genes that are known to regulate neuronal development, surprisingly, patients may not present disease symptoms until adulthood. Spinocerebellar ataxia type 6 (SCA6) is one such midlife-onset disorder in which the mutated gene, CACNA1A, is implicated in cerebellar development. We wondered whether changes were observed in the developing cerebellum in SCA6 prior to the detection of motor deficits. To address this question, we used a transgenic mouse with a hyper-expanded triplet repeat (SCA6^84Q/84Q ) that displays late-onset motor deficits at 7 months, and measured cerebellar Purkinje cell synaptic and intrinsic properties during postnatal development. We found that firing rate and precision were enhanced during postnatal development in P10-13 SCA6^84Q/84Q Purkinje cells, and observed surplus multiple climbing fibre innervation without changes in inhibitory input or dendritic structure during development. Although excess multiple climbing fibre innervation has been associated with ataxic symptoms in several adult transgenic mice, we observed no detectable changes in cerebellar-related motor behaviour in developing SCA6^84Q/84Q mice. Interestingly, we found that developmental alterations were transient, as both Purkinje cell firing properties and climbing fibre innervation from weanling-aged (P21-24) SCA6^84Q/84Q mice were indistinguishable from litter-matched control mice. Our results demonstrate that significant alterations in neuronal circuit development may be observed without any detectable behavioural read-out, and that early changes in brain development may not necessarily persist into adulthood in midlife-onset diseases.

Case Study: Somatic Sprouts and Halo-Like Amorphous Materials of the Purkinje Cells in Huntington's Disease

We described a 63-year-old Japanese female with genetically confirmed Huntington's disease who showed unusual pathological findings in the cerebellum. This case exhibited typical neuropathological features as Huntington's disease, including severe degeneration of the neostriatum and widespread occurrence of ubiquitin and expanded polyglutamine-positive neuronal intranuclear and intracytoplasmic inclusions. The cerebellum was macroscopically unremarkable; however, somatic sprouts and halo-like amorphous materials of Purkinje cell with a large amount of torpedoes were noteworthy. Furthermore, the Purkinje cells were found to have granular cytoplasmic inclusions. Somatic sprouting is a form of degenerated Purkinje cell exhibited in several specific conditions. Although this finding usually appeared in developmental brains, several neurodegenerative disorders, including Menkes kinky hair disease, familial spinocerebellar ataxia, acute encephalopathy linked to familial hemiplegic migraine, and several other conditions, have been reported showing sprouting from the soma of Purkinje cell. We propose that Huntington's disease is another degenerative condition associated with these distinct neuropathological findings of Purkinje cell. Abnormally accumulated huntingtin protein in the cytoplasm could be related to the development of these structures.

4-aminopyridine reverses ataxia and cerebellar firing deficiency in a mouse model of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a devastating midlife-onset autosomal dominant motor control disease with no known treatment. Using a hyper-expanded polyglutamine (84Q) knock-in mouse, we found that cerebellar Purkinje cell firing precision was degraded in heterozygous (SCA6^84Q/+) mice at 19 months when motor deficits are observed. Similar alterations in firing precision and motor control were observed at disease onset at 7 months in homozygous (SCA6^84Q/84Q) mice, as well as a reduction in firing rate. We further found that chronic administration of the FDA-approved drug 4-aminopyridine (4-AP), which targets potassium channels, alleviated motor coordination deficits and restored cerebellar Purkinje cell firing precision to wildtype (WT) levels in SCA6^84Q/84Q mice both in acute slices and in vivo. These results provide a novel therapeutic approach for treating ataxic symptoms associated with SCA6.

Early Cerebellar Network Shifting in Spinocerebellar Ataxia Type 6

Spinocerebellar ataxia 6 (SCA6), an autosomal dominant degenerative disease, is characterized by diplopia, gait ataxia, and incoordination due to severe progressive degeneration of Purkinje cells in the vestibulo- and spinocerebellum. Ocular motor deficits are common, including difficulty fixating on moving objects, nystagmus and disruption of smooth pursuit movements. In presymptomatic SCA6, there are alterations in saccades and smooth-pursuit movements. We sought to assess functional and structural changes in cerebellar connectivity associated with a visual task, hypothesizing that gradual changes would parallel disease progression. We acquired functional magnetic resonance imaging and diffusion tensor imaging data during a passive smooth-pursuit task in 14 SCA6 patients, representing a range of disease duration and severity, and performed a cross-sectional comparison of cerebellar networks compared with healthy controls. We identified a shift in activation from vermis in presymptomatic individuals to lateral cerebellum in moderate-to-severe cases. Concomitantly, effective connectivity between regions of cerebral cortex and cerebellum was at its highest in moderate cases, and disappeared in severe cases. Finally, we noted structural differences in the cerebral and cerebellar peduncles. These unique results, spanning both functional and structural domains, highlight widespread changes in SCA6 and compensatory mechanisms associated with cerebellar physiology that could be utilized in developing new therapies.

Rapid Onset of Motor Deficits in a Mouse Model of Spinocerebellar Ataxia Type 6 Precedes Late Cerebellar Degeneration

Spinocerebellar ataxia type 6 (SCA6) is an autosomal-dominant cerebellar ataxia that has been associated with loss of cerebellar Purkinje cells. Disease onset is typically at midlife, although it can vary widely from late teens to old age in SCA6 patients. Our study focused on an SCA6 knock-in mouse model with a hyper-expanded (84X) CAG repeat expansion that displays midlife-onset motor deficits at ∼7 months old, reminiscent of midlife-onset symptoms in SCA6 patients, although a detailed phenotypic analysis of these mice has not yet been reported. Here, we characterize the onset of motor deficits in SCA6^84Q mice using a battery of behavioral assays to test for impairments in motor coordination, balance, and gait. We found that these mice performed normally on these assays up to and including at 6 months, but motor impairment was detected at 7 months with all motor coordination assays used, suggesting that motor deficits emerge rapidly during a narrow age window in SCA6^84Q mice. In contrast to what is seen in SCA6 patients, the decrease in motor coordination was observed without alterations in gait. No loss of cerebellar Purkinje cells or striatal neurons were observed at 7 months, the age at which motor deficits were first detected, but significant Purkinje cell loss was observed in 2-year-old SCA6^84Q mice, arguing that Purkinje cell death does not significantly contribute to the early stages of SCA6.

DnaJ-1 and karyopherin α3 suppress degeneration in a new Drosophila model of Spinocerebellar Ataxia Type 6

Spinocerebellar ataxia type 6 (SCA6) belongs to the family of CAG/polyglutamine (polyQ)-dependent neurodegenerative disorders. SCA6 is caused by abnormal expansion in a CAG trinucleotide repeat within exon 47 of CACNA1A, a bicistronic gene that encodes α1A, a P/Q-type calcium channel subunit and a C-terminal protein, termed α1ACT. Expansion of the CAG/polyQ region of CACNA1A occurs within α1ACT and leads to ataxia. There are few animal models of SCA6. Here, we describe the generation and characterization of the first Drosophila melanogaster models of SCA6, which express the entire human α1ACT protein with a normal or expanded polyQ. The polyQ-expanded version of α1ACT recapitulates the progressively degenerative nature of SCA6 when expressed in various fly tissues and the presence of densely staining aggregates. Additional studies identify the co-chaperone DnaJ-1 as a potential therapeutic target for SCA6. Expression of DnaJ-1 potently suppresses α1ACT-dependent degeneration and lethality, concomitant with decreased aggregation and reduced nuclear localization of the pathogenic protein. Mutating the nuclear importer karyopherin α3 also leads to reduced toxicity from pathogenic α1ACT. Little is known about the steps leading to degeneration in SCA6 and the means to protect neurons in this disease are lacking. Invertebrate animal models of SCA6 can expand our understanding of molecular sequelae related to degeneration in this disorder and lead to the rapid identification of cellular components that can be targeted to treat it.

Structural and functional MRI abnormalities of cerebellar cortex and nuclei in SCA3, SCA6 and Friedreich's ataxia

Spinocerebellar ataxia type 3, spinocerebellar ataxia type 6 and Friedreich's ataxia are common hereditary ataxias. Different patterns of atrophy of the cerebellar cortex are well known. Data on cerebellar nuclei are sparse. Whereas cerebellar nuclei have long been thought to be preserved in spinocerebellar ataxia type 6, histology shows marked atrophy of the nuclei in Friedreich's ataxia and spinocerebellar ataxia type 3. In the present study susceptibility weighted imaging was used to assess atrophy of the cerebellar nuclei in patients with spinocerebellar ataxia type 6 (n = 12, age range 41-76 years, five female), Friedreich's ataxia (n = 12, age range 21-55 years, seven female), spinocerebellar ataxia type 3 (n = 10, age range 34-67 years, three female), and age- and gender-matched controls (total n = 23, age range 22-75 years, 10 female). T1-weighted magnetic resonance images were used to calculate the volume of the cerebellum. In addition, ultra-high field functional magnetic resonance imaging was performed with optimized normalization methods to assess function of the cerebellar cortex and nuclei during simple hand movements. As expected, the volume of the cerebellum was markedly reduced in spinocerebellar ataxia type 6, preserved in Friedreich's ataxia, and mildy reduced in spinocerebellar ataxia type 3. The volume of the cerebellar nuclei was reduced in the three patient groups compared to matched controls (P-values < 0.05; two-sample t-tests). Atrophy of the cerebellar nuclei was most pronounced in spinocerebellar ataxia type 6. On a functional level, hand-movement-related cerebellar activation was altered in all three disorders. Within the cerebellar cortex, functional magnetic resonance imaging signal was significantly reduced in spinocerebellar ataxia type 6 and Friedreich's ataxia compared to matched controls (P-values < 0.001, bootstrap-corrected cluster-size threshold; two-sample t-tests). The difference missed significance in spinocerebellar ataxia type 3. Within the cerebellar nuclei, reductions were significant when comparing spinocerebellar ataxia type 6 and Friedreich's ataxia to matched controls (P < 0.01, bootstrap-corrected cluster-size threshold; two-sample t-tests). Susceptibility weighted imaging allowed depiction of atrophy of the cerebellar nuclei in patients with Friedreich's ataxia and spinocerebellar ataxia type 3. In spinocerebellar ataxia type 6, pathology was not restricted to the cerebellar cortex but also involved the cerebellar nuclei. Functional magnetic resonance imaging data, on the other hand, revealed that pathology in Friedreich's ataxia and spinocerebellar ataxia type 3 is not restricted to the cerebellar nuclei. There was functional involvement of the cerebellar cortex despite no or little structural changes.

A case-control proton magnetic resonance spectroscopy study confirms cerebellar dysfunction in benign adult familial myoclonic epilepsy

BACKGROUND

Benign adult familial myoclonic epilepsy (BAFME) is a rare form of epilepsy syndrome. The pathogenesis of BAFME remains unclear, though it seems to involve dysfunction of the cerebellum.

OBJECTIVES

The purpose of this study was to use proton magnetic resonance spectroscopy ((1)H-MRS) to investigate whether neurochemical changes underlie abnormal brain function in BAFME.

METHODS

Twelve BAFME patients from one family and 12 age- and sex-matched healthy controls were enrolled in this study. The ratios of NAA/Cr, NAA/Cho, Cho/Cr, and NAA/(Cr+Cho) were analyzed.

RESULTS

The BAFME patients exhibited a decreased N-acetylaspartate (NAA)/choline (Cho) ratio in the cerebellar cortex, whereas there were no significant differences in the NAA/creatine (Cr), Cho/Cr, and NAA/(Cr+Cho) ratios compared with healthy controls. There were no significant differences in (1)H-MRS values in the frontal cortex or thalamus between the BAFME patients and controls. No correlation was detected between the NAA/Cho ratio in the cerebellar cortex and disease duration, myoclonus severity, or tremor severity.

CONCLUSION

Our results indicate clear cerebellar dysfunction in BAFME. (1)H-MRS is a useful tool for the diagnosis of BAFME in combination with family history and electrophysiological examination.

Macro- and microstructural changes in patients with spinocerebellar ataxia type 6: assessment of phylogenetic subdivisions of the cerebellum and the brain stem

BACKGROUND AND PURPOSE

Site-specific degeneration patterns of the infratentorial brain in relation to phylogenetic origins may relate to symptoms in patients with spinocerebellar degeneration, but the patterns are still unclear. We investigated macro- and microstructural changes of the infratentorial brain based on phylogenetic origins and their correlation with symptoms in patients with spinocerebellar ataxia type 6.

MATERIALS AND METHODS

MR images of 9 patients with spinocerebellar ataxia type 6 and 9 age- and sex-matched controls were obtained. We divided the infratentorial brain on the basis of phylogenetic origins and performed an atlas-based analysis. Comparisons of the 2 groups and a correlation analysis assessed with the International Cooperative Ataxia Rating Scale excluding age effects were performed.

RESULTS

A significant decrease of fractional volume and an increase of mean diffusivity were seen in all subdivisions of the cerebellum and in all the cerebellar peduncles except mean diffusivity in the inferior cerebellar peduncle in patients compared with controls (P < .0001 to <.05). The bilateral anterior lobes showed the strongest atrophy. Fractional volume decreased mainly in old regions, whereas mean diffusivity increased mainly in new regions of the cerebellum. Reflecting this tendency, the International Cooperative Ataxia Rating Scale total score showed strong correlations in fractional volume in the right flocculonodular lobe and the bilateral deep structures and in mean diffusivity in the bilateral posterior lobes (r = 0.73 to ±0.87).

CONCLUSIONS

We found characteristic macro- and microstructural changes, depending on phylogenetic regions of the infratentorial brain, that strongly correlated with clinical symptoms in patients with spinocerebellar ataxia type 6.

Ataxia and hypogonadism caused by the loss of ubiquitin ligase activity of the U box protein CHIP

Gordon Holmes syndrome (GHS) is a rare Mendelian neurodegenerative disorder characterized by ataxia and hypogonadism. Recently, it was suggested that disordered ubiquitination underlies GHS though the discovery of exome mutations in the E3 ligase RNF216 and deubiquitinase OTUD4. We performed exome sequencing in a family with two of three siblings afflicted with ataxia and hypogonadism and identified a homozygous mutation in STUB1 (NM_005861) c.737C→T, p.Thr246Met, a gene that encodes the protein CHIP (C-terminus of HSC70-interacting protein). CHIP plays a central role in regulating protein quality control, in part through its ability to function as an E3 ligase. Loss of CHIP function has long been associated with protein misfolding and aggregation in several genetic mouse models of neurodegenerative disorders; however, a role for CHIP in human neurological disease has yet to be identified. Introduction of the Thr246Met mutation into CHIP results in a loss of ubiquitin ligase activity measured directly using recombinant proteins as well as in cell culture models. Loss of CHIP function in mice resulted in behavioral and reproductive impairments that mimic human ataxia and hypogonadism. We conclude that GHS can be caused by a loss-of-function mutation in CHIP. Our findings further highlight the role of disordered ubiquitination and protein quality control in the pathogenesis of neurodegenerative disease and demonstrate the utility of combining whole-exome sequencing with molecular analyses and animal models to define causal disease polymorphisms.

Neuropathology of cervical dystonia

The aim of this study was to search for neuropathological changes in postmortem brain tissue of individuals with cervical dystonia (CD). Multiple regions of formalin-preserved brains were collected from patients with CD and controls and examined with an extensive battery of histopathological stains in a two-stage study design. In stage one, 4 CD brains underwent a broad screening neuropathological examination. In stage two, these 4 CD brains were combined with 2 additional CD brains, and the subjective findings were quantified and compared to 16 age-matched controls. The initial subjective neuropathological assessment revealed only two regions with relatively consistent changes. The substantia nigra had frequent ubiquitin-positive intranuclear inclusions known as Marinesco bodies. Additionally, the cerebellum showed patchy loss of Purkinje cells, areas of focal gliosis and torpedo bodies. Other brain regions showed minor or inconsistent changes. In the second stage of the analysis, quantitative studies failed to reveal significant differences in the numbers of Marinesco bodies in CD versus controls, but confirmed a significantly lower Purkinje cell density in CD. Molecular investigations revealed 4 of the CD cases and 2 controls to harbor sequence variants in non-coding regions of THAP1, and these cases had lower Purkinje cell densities regardless of whether they had CD. The findings suggest that subtle neuropathological changes such as lower Purkinje cell density may be found in primary CD when relevant brain regions are investigated with appropriate methods.

Development of Purkinje cell degeneration in a knockin mouse model reveals lysosomal involvement in the pathogenesis of SCA6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease caused by the expansion of a polyglutamine tract in the Ca(v)2.1 voltage-gated calcium channel. To elucidate how the expanded polyglutamine tract in this plasma membrane protein causes the disease, we created a unique knockin mouse model that modestly overexpressed the mutant transcripts under the control of an endogenous promoter (MPI-118Q). MPI-118Q mice faithfully recapitulated many features of SCA6, including selective Purkinje cell degeneration. Surprisingly, analysis of inclusion formation in the mutant Purkinje cells indicated the lysosomal localization of accumulated mutant Ca(v)2.1 channels in the absence of autophagic response. The lack of cathepsin B, a major lysosomal cysteine proteinase, exacerbated the loss of Purkinje cells and was accompanied by an acceleration of inclusion formation in this model. Thus, the pathogenic mechanism of SCA6 involves the endolysosomal degradation pathway, and unique pathological features of this model further illustrate the pivotal role of protein context in the pathogenesis of polyglutamine diseases.

Spinocerebellar ataxia type 6

The autosomal dominant spinocerebellar ataxias (SCA) are a genetically heterogeneous group of neurodegenerative disorders characterized by progressive motor incoordination, in some cases with ataxia alone and in others in association with additional progressive neurological deficits. Spinocerebellar ataxia type 6 (SCA6) is the prototype of a pure cerebellar ataxia, associated with a severe form of progressive ataxia and cerebellar dysfunction. SCA6, originally classified as such by Zhuchenko et al. (1997), is caused by a CAG repeat expansion in the CACNA1A gene which encodes the α1A subunit of the P/Q-type voltage-gated calcium channel. SCA6 is one of ten polyglutamine-encoding CAG nucleotide repeat expansion disorders comprising other neurodegenerative disorders such as Huntington's disease. The present review describes clinical, genetic, and pathological manifestations associated with this illness. Currently, there is no treatment for this neurodegenerative disease. Successful therapeutic strategies must target a valid pathological mechanism; thus, understanding the underlying mechanisms of disease is crucial to finding a proper treatment. Hence, this chapter will discuss as well the molecular mechanisms possibly associated with SCA6 pathology and their implication for the development of future treatment.

Increased number of Purkinje cell dendritic swellings in essential tremor

BACKGROUND AND PURPOSE

Essential Tremor (ET) is among the most prevalent neurologic disorders. Growing clinical and neuro-imaging evidence implicates cerebellar dysfunction in the pathogenesis of ET and emerging postmortem studies have identified structural changes in the cerebellum, particularly in Purkinje cells. In this study we systematically quantified focal Purkinje cell dendritic swellings (DS) in 20 ET vs. 19 control brains.

METHODS

In each brain, a standard parasagittal neocerebellar tissue block was harvested. DS were quantified in one 7-μm thick section stained with Luxol Fast Blue/Hematoxylin and Eosin (LH&E) and one section stained with Bielschowsky method.

RESULTS

The number of DS were higher in cases than controls by LH&E (1.50 ± 1.79 vs. 0.05 ± 0.23, P = 0.002) and Bielschowsky methods (2.70 ± 3.10 vs. 0.37 ± 0.50, P = 0.002). The number of DS was correlated with the number of torpedoes and marginally inversely correlated with the number of Purkinje cells.

CONCLUSION

The current study documents and quantifies an additional structural abnormality in the ET cerebellum, adding to the growing list of such changes in this disease. The mechanisms that underlie this and other structural changes observed in ET are currently unknown, and they deserve additional exploration.

Apoptosis inducing factor deficiency causes reduced mitofusion 1 expression and patterned Purkinje cell degeneration

Alteration in mitochondrial dynamics has been implicated in many neurodegenerative diseases. Mitochondrial apoptosis inducing factor (AIF) plays a key role in multiple cellular and disease processes. Using immunoblotting and flow cytometry analysis with Harlequin mutant mice that have a proviral insertion in the AIF gene, we first revealed that mitofusion 1 (Mfn1), a key mitochondrial fusion protein, is significantly diminished in Purkinje cells of the Harlequin cerebellum. Next, we investigated the cerebellar pathology of Harlequin mice in an age-dependent fashion, and identified a striking process of progressive and patterned Purkinje cell degeneration. Using immunohistochemistry with zebrin II, the most studied compartmentalization marker in the cerebellum, we found that zebrin II-negative Purkinje cells first started to degenerate at 7 months of age. By 11 months of age, almost half of the Purkinje cells were degenerated. Subsequently, most of the Purkinje cells disappeared in the Harlequin cerebellum. The surviving Purkinje cells were concentrated in cerebellar lobules IX and X, where these cells were positive for heat shock protein 25 and resistant to degeneration. We further showed that the patterned Purkinje cell degeneration was dependent on caspase but not poly(ADP-ribose) polymerase-1 (PARP-1) activation, and confirmed the marked decrease of Mfn1 in the Harlequin cerebellum. Our results identified a previously unrecognized role of AIF in Purkinje cell degeneration, and revealed that AIF deficiency leads to altered mitochondrial fusion and caspase-dependent cerebellar Purkinje cell loss in Harlequin mice. This study is the first to link AIF and mitochondrial fusion, both of which might play important roles in neurodegeneration.

Periodic alternating nystagmus and periodic alternating skew deviation in spinocerebellar ataxia type 6

The combination of periodic alternating nystagmus (PAN) and periodic alternating skew deviation (PASD) is rare. We report a case of PAN and PASD in a patient with spinocerebellar ataxia type 6 (SCA-6) and discuss the role of the cerebellum as a plausible mechanism for this combined pathologic condition.

In vitro complementation of Tdp1 deficiency indicates a stabilized enzyme-DNA adduct from tyrosyl but not glycolate lesions as a consequence of the SCAN1 mutation

A homozygous H493R mutation in the active site of tyrosyl-DNA phosphodiesterase (TDP1) has been implicated in hereditary spinocerebellar ataxia with axonal neuropathy (SCAN1), an autosomal recessive neurodegenerative disease. However, it is uncertain how the H493R mutation elicits the specific pathologies of SCAN1. To address this question, and to further elucidate the role of TDP1 in repair of DNA end modifications and general physiology, we generated a Tdp1 knockout mouse and carried out detailed behavioral analyses as well as characterization of repair deficiencies in extracts of embryo fibroblasts from these animals. While Tdp1(-/-) mice appear phenotypically normal, extracts from Tdp1(-/-) fibroblasts exhibited deficiencies in processing 3'-phosphotyrosyl single-strand breaks and 3'-phosphoglycolate double-strand breaks (DSBs), but not 3'-phosphoglycolate single-strand breaks. Supplementing Tdp1(-/-) extracts with H493R TDP1 partially restored processing of 3'-phosphotyrosyl single-strand breaks, but with evidence of persistent covalent adducts between TDP1 and DNA, consistent with a proposed intermediate-stabilization effect of the SCAN1 mutation. However, H493R TDP1 supplementation had no effect on phosphoglycolate (PG) termini on 3' overhangs of double-strand breaks; these remained completely unprocessed. Altogether, these results suggest that for 3'-phosphoglycolate overhang lesions, the SCAN1 mutation confers loss of function, while for 3'-phosphotyrosyl lesions, the mutation uniquely stabilizes a reaction intermediate.

Current advances in lesion-symptom mapping of the human cerebellum

While high-resolution structural magnetic resonance imaging (MRI) combined with newer analysis methods has become a powerful tool in human cerebral lesion studies, comparatively few studies have used these advanced imaging techniques to study lesions of the human cerebellum and their associated symptoms. This review will summarize the methodology of MRI-based lesion-symptom mapping of the human cerebellum and discuss its potential for gaining insights into cerebellar function. The investigation of patients with defined focal lesions yields the greatest potential for obtaining meaningful correlations between lesion site and behavioral deficits. In smaller groups of patients overlay plots and subtraction analysis are good options. If larger groups of patients are available, different statistical techniques have been introduced to compare behavior and lesion site on a voxel-by-voxel basis. Although localization in degenerative cerebellar disorders is less accurate because of the diffuse nature of the disease, certain information about the supposed function of larger subdivisions of the cerebellum can be gained. Examples are given which show that lesion-symptom mapping allows to investigate the function of the intermediate zone and cerebellar nuclei. We conclude that meaningful correlations between lesion site and behavioral data can be obtained in patients with degenerative as well as focal cerebellar disorders.

The emerging neuropathology of essential tremor

Essential tremor (ET) is one of the most prevalent neurological disorders. At the same time, it is among the most poorly-understood of these disorders. The underlying anatomical pathology of ET has been elusive until recently. Postmortem studies have begun to display some of the underlying brain changes in patients with this disease. These types of investigations are likely to lead the way to additional insights into the pathophysiology of ET and guide the development of therapies for this common movement disorder.

Correlation of cerebellar volume with eyeblink conditioning in healthy subjects and in patients with cerebellar cortical degeneration

In the present study, acquisition and timing of conditioned eyeblink responses (CRs) were correlated with magnetic resonance imaging (MRI)-based cerebellar volume both in healthy human subjects and patients with cerebellar disease. Thirty-three healthy subjects and 25 patients with pure cortical cerebellar degeneration participated. Cerebellar volumes were measured for the cortex of the anterior lobe, the cortex of the posterior lobe, the white matter of the cerebellum and the cerebrum based on 3D MR images. CR parameters were assessed in a standard delay paradigm. In healthy subjects CR acquisition was significantly related to the volume of the grey matter of the posterior lobe, but neither to the volume of the grey matter of the anterior lobe, nor to the cerebellar white matter and nor to the cerebral volume. As expected, CR acquisition and volume of the cortex of the posterior lobe showed age-related decline in the controls. Furthermore, CR acquisition was significantly reduced in patients with cerebellar degeneration compared to controls. In the cerebellar group, however, no significant correlations between CR acquisition and any of the cerebellar volumes were observed. Floor effects are most likely responsible for this observation. Although CRs occurred significantly earlier in cerebellar patients compared to controls, no significant correlations between CR timing parameters and any of the cerebellar volumes were observed. Extending previous findings in healthy human subjects, age-related decline of the cerebellar cortex of the posterior lobe was related with a reduction of CR acquisition. Findings provide further evidence that the cerebellar cortex plays an important role in the acquisition of eyeblink conditioning in humans.

Decreased cortical inhibition and yet cerebellar pathology in ‘familial cortical myoclonic tremor with epilepsy’

Cortical hyperexcitability is a feature of "familial cortical myoclonic tremor with epilepsy" (FCMTE). However, neuropathological investigations in a single FCMTE patient showed isolated cerebellar pathology. Pathological investigations in a second FCMTE patient, reported here, confirmed cerebellar Purkinje cell degeneration and a normal sensorimotor cortex. Subsequently, we sought to explore the nature of cerebellar and motor system pathophysiology in FCMTE. Eye movement recordings and transcranial magnetic stimulation performed in six related FCMTE patients showed impaired saccades and smooth pursuit and downbeat nystagmus upon hyperventilation, as in patients with spinocerebellar ataxia type 6. In FCMTE patients short-interval intracortical inhibition (SICI) was significantly reduced. Resting motor threshold, recruitment curve, silent period, and intracortical facilitation were normal. The neuropathological and ocular motor abnormalities indicate cerebellar involvement in FCMTE patients. Decreased SICI is compatible with intracortical GABA(A)-ergic dysfunction. Cerebellar and intracortical functional changes could result from a common mechanism such as a channelopathy. Alternatively, decreased cortical inhibition may be caused by dysfunction of the cerebello-thalamo-cortical loop as a result of primary cerebellar pathology.

Molecular pathogenesis of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disorder caused by abnormal expansions of a trinucleotide CAG repeat in exon 47 of the CACNA1A gene, which encodes the alpha1A subunit of the P/Q-type voltage-gated calcium channel. The CAG repeat expansion is translated into an elongated polyglutamine tract in the carboxyl terminus of the alpha1A subunit. The alpha1A subunit is the main pore-forming subunit of the P/Q-type calcium channel. Patients with SCA6 suffer from a severe form of progressive ataxia and cerebellar dysfunction. Design of treatments for this disorder will depend on better definition of the mechanism of disease. As a disease arising from a mutation in an ion channel gene, SCA6 may behave as an ion channelopathy, and may respond to attempts to modulate or correct ion channel function. Alternatively, as a disease in which the mutant protein contains an expanded polyglutamine tract, SCA6 may respond to the targets of drug therapies developed for Huntington's disease and other polyglutamine disorders. In this review we will compare SCA6 to other polyglutamine diseases and channelopathies, and we will highlight recent advances in our understanding of alpha1A subunits and SCA6 pathology. We also propose a mechanism for how two seemingly divergent hypotheses can be combined into a cohesive model for disease progression.

Nonapoptotic cell death caused by the inhibition of RNA polymerase disrupts organelle distribution

It is controversial whether the mode of cell death induced by CAG repeat diseases is apoptotic. One technical problem that affects this issue is that the very methods used for DNA injection may induce artificial apoptosis. A recent study demonstrated that the functions of RNA polymerase II are disrupted in spinocerebellar ataxia type 1 (SCA 1) pathology, one of the CAG repeat diseases, and that alpha-amanitin can inhibit the activity of RNA polymerase. To examine the cell death mechanisms involved in CAG repeat diseases, we treated cultured rat neurons with alpha-amanitin to avoid the artifacts caused by DNA transfection. Mature and immature rat neurons were treated with alpha-amanitin for 4-6 days and the effects of the treatment on the elongation of neurites, the distribution or morphology of organelles, and the nature of cell death were assessed by immunocytochemistry and quantitative analysis. Neurons exhibited a disruption of neurite elongation and eventually died by day 15 of the treatment. However, apoptosis was not detected. When the neurons survived well, but showed altered neurites, Golgi complexes and lysosomes exhibited changes in their normal intracellular distribution or morphology, but the endoplasmic reticulum and mitochondria did not. The distribution of phosphorylated Trk receptors was also disrupted in the neurites of treated neurons. The signal intensity of the dynein intermediate chain was markedly decreased in the treated neurons. Thus, organelle transport systems, particularly a minus-end-directed microtubule-dependent pathway, would be disrupted by the inhibition of RNA polymerase, and this change is likely to be an early event involved in SCA 1 pathology.

Spatiotemporal analysis of purkinje cell degeneration relative to parasagittal expression domains in a model of neonatal viral infection

Infection of newborn Lewis rats with Borna disease virus (neonatal Borna disease [NBD]) results in cerebellar damage without the cellular inflammation associated with infections in later life. Purkinje cell (PC) damage has been reported for several models of early-life viral infection, including NBD; however, the time course and distribution of PC pathology have not been investigated rigorously. This study examined the spatiotemporal relationship between PC death and zonal organization in NBD cerebella. Real-time PCR at postnatal day 28 (PND28) revealed decreased cerebellar levels of mRNAs encoding the glycolytic enzymes aldolase C (AldoC, also known as zebrin II) and phosphofructokinase C and the excitatory amino acid transporter 4 (EAAT4). Zebrin II and EAAT4 immunofluorescence analysis in PND21, PND28, PND42, and PND84 NBD rat cerebella revealed a complex pattern of PC degeneration. Early cell loss (PND28) was characterized by preferential apoptotic loss of zebrin II/EAAT4-negative PC subsets in the anterior vermis. Consistent with early preferential loss of zebrin II/EAAT4-negative PCs in the vermis, the densities of microglia and the Bergmann glial expression of metallothionein I/II and the hyaluronan receptor CD44 were higher in zebrin II/EAAT4-negative zones. In contrast, early loss in lateral cerebellar lobules did not reflect a similar discrimination between PC phenotypes. Patterns of vermal PC loss became more heterogeneous at PND42, with the loss of both zebrin II/EAAT4-negative and zebrin II/EAAT4-positive neurons. At PND84, zebrin II/EAAT4 patterning was abolished in the anterior cerebellum, with preferential PC survival in lobule X. Our investigation reveals regional discrimination between patterns of PC subset loss, defined by zebrin II/EAAT4 expression domains, following neonatal viral infection. These findings suggest a differential vulnerability of PC subsets during the early stages of virus-induced neurodegeneration.

Mitochondrial trafficking and morphology in healthy and injured neurons

Mitochondria are the primary generators of ATP and are important regulators of intracellular calcium homeostasis. These organelles are dynamically transported along lengthy neuronal processes, presumably for appropriate distribution to cellular regions of high metabolic demand and elevated intracellular calcium, such as synapses. The removal of damaged mitochondria that produce harmful reactive oxygen species and promote apoptosis is also thought to be mediated by transport of mitochondria to autophagosomes. Mitochondrial trafficking is therefore important for maintaining neuronal and mitochondrial health while cessation of movement may lead to neuronal and mitochondrial dysfunction. Mitochondrial morphology is also dynamic and is remodeled during neuronal injury and disease. Recent studies reveal different manifestations and mechanisms of impaired mitochondrial movement and altered morphology in injured neurons. These are likely to cause varied courses toward neuronal degeneration and death. The goal of this review is to provide an appreciation of the full range of mitochondrial function, morphology and trafficking, and the critical role these parameters play in neuronal physiology and pathophysiology.