Molecular pathogenesis of spinocerebellar ataxia type 6

From the disruption of RNA metabolism to the targeting of RNA-binding proteins: The case of polyglutamine spinocerebellar ataxias

Polyglutamine spinocerebellar ataxias (PolyQ SCAs) represent a group of monogenetic diseases in which the expanded polyglutamine repeats give rise to a mutated protein. The abnormally expanded polyglutamine protein produces aggregates and toxic species, causing neuronal dysfunction and neuronal death. The main symptoms of these disorders include progressive ataxia, motor dysfunction, oculomotor impairment, and swallowing problems. Nowadays, the current treatments are restricted to symptomatic alleviation, and no existing therapeutic strategies can reduce or stop the disease progression. Even though the origin of these disorders has been associated with polyglutamine-induced toxicity, RNA toxicity has recently gained relevance in polyQ SCAs molecular pathogenesis. Therefore, the research's focus on RNA metabolism has been increasing, especially on RNA-binding proteins (RBPs). The present review summarizes RNA metabolism, exposing the different processes and the main RBPs involved. We also explore the mechanisms by which RBPs are dysregulated in PolyQ SCAs. Finally, possible therapies targeting the RNA metabolism are presented as strategies to reverse neuropathological anomalies and mitigate physical symptoms.

The Role of Protein Quantity Control in Polyglutamine Spinocerebellar Ataxias

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

Phenotypic analysis of ataxia in spinocerebellar ataxia type 6 mice using DeepLabCut

This study emphasizes the benefits of open-source software such as DeepLabCut (DLC) and R to automate, customize and enhance data analysis of motor behavior. We recorded 2 different spinocerebellar ataxia type 6 mouse models while performing the classic beamwalk test, tracked multiple body parts using the markerless pose-estimation software DLC and analyzed the tracked data using self-written scripts in the programming language R. The beamwalk analysis script (BAS) counts and classifies minor and major hindpaw slips with an 83% accuracy compared to manual scoring. Nose, belly and tail positions relative to the beam, as well as the angle at the tail base relative to the nose and tail tip were determined to characterize motor deficits in greater detail. Our results found distinct ataxic abnormalities such as an increase in major left hindpaw slips and a lower belly and tail position in both SCA6 ataxic mouse models compared to control mice at 18 months of age. Furthermore, a more detailed analysis of various body parts relative to the beam revealed an overall lower body position in the SCA684Q compared to the CT-longQ27PC mouse line at 18 months of age, indicating a more severe ataxic deficit in the SCA684Q group.

Loss of Purkinje cells in the developing cerebellum strengthens the cerebellothalamic synapses

Cerebellar damage early in life often causes long-lasting motor, social, and cognitive impairments, suggesting the roles of the cerebellum in developing a broad spectrum of behaviors. This recent finding has promoted research on how cerebellar damage affects the development of the cerebral cortex, the brain region responsible for higher-order control of all behaviors. However, the cerebral cortex is not directly connected to the cerebellum. The thalamus is the direct postsynaptic target of the cerebellum, sending cerebellar outputs to the cerebral cortex. Despite its crucial position in cerebello-cerebral interaction, thalamic susceptibility to cerebellar damage remains largely unclear. Here, we studied the consequences of early cerebellar perturbation on thalamic development. Whole-cell patch-clamp recordings showed that the synaptic organization of the cerebellothlamic circuit is similar to that of the primary sensory thalamus, in which aberrant sensory activity alters synaptic circuit formation. The hemizygous deletion of the tuberous sclerosis complex-1 ( Tsc1 ) gene in the Purkinje cell-known to cause Purkinje cell hypoactivity and autistic behaviors-did not alter cerebellothalamic synapses or intrinsic membrane properties of thalamic neurons. However, the ablation of Purkinje cells in the developing cerebellum strengthened the cerebellothalamic synapses and enhanced thalamic suprathreshold activities. These results suggest that the cerebellothalamic circuit is resistant to moderate perturbation in the developing cerebellum, such as the reduced firing rate of Purkinje cells, and that autistic behaviors are not necessarily linked to thalamic abnormality. Still, Purkinje cell loss alters the thalamic circuit, suggesting the vulnerability of the thalamus to substantial disturbance in the developing cerebellum.

Therapeutic Potential of Targeting Regulated Intramembrane Proteolysis Mechanisms of Voltage-Gated Ion Channel Subunits and Cell Adhesion Molecules

Several integral membrane proteins undergo regulated intramembrane proteolysis (RIP), a tightly controlled process through which cells transmit information across and between intracellular compartments. RIP generates biologically active peptides by a series of proteolytic cleavage events carried out by two primary groups of enzymes: sheddases and intramembrane-cleaving proteases (iCLiPs). Following RIP, fragments of both pore-forming and non-pore-forming ion channel subunits, as well as immunoglobulin super family (IgSF) members, have been shown to translocate to the nucleus to function in transcriptional regulation. As an example, the voltage-gated sodium channel β1 subunit, which is also an IgSF-cell adhesion molecule (CAM), is a substrate for RIP. β1 RIP results in generation of a soluble intracellular domain, which can regulate gene expression in the nucleus. In this review, we discuss the proposed RIP mechanisms of voltage-gated sodium, potassium, and calcium channel subunits as well as the roles of their generated proteolytic products in the nucleus. We also discuss other RIP substrates that are cleaved by similar sheddases and iCLiPs, such as IgSF macromolecules, including CAMs, whose proteolytically generated fragments function in the nucleus. Importantly, dysfunctional RIP mechanisms are linked to human disease. Thus, we will also review how understanding RIP events and subsequent signaling processes involving ion channel subunits and IgSF proteins may lead to the discovery of novel therapeutic targets. SIGNIFICANCE STATEMENT: Several ion channel subunits and immunoglobulin superfamily molecules have been identified as substrates of regulated intramembrane proteolysis (RIP). This signal transduction mechanism, which generates polypeptide fragments that translocate to the nucleus, is an important regulator of gene transcription. RIP may impact diseases of excitability, including epilepsy, cardiac arrhythmia, and sudden death syndromes. A thorough understanding of the role of RIP in gene regulation is critical as it may reveal novel therapeutic strategies for the treatment of previously intractable diseases.

Activation of TrkB-Akt signaling rescues deficits in a mouse model of SCA6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease resulting in motor coordination deficits and cerebellar pathology. Expression of brain-derived neurotrophic factor (BDNF) is reduced in postmortem tissue from SCA6 patients. Here, we show that levels of cerebellar BDNF and its receptor, tropomyosin receptor kinase B (TrkB), are reduced at an early disease stage in a mouse model of SCA6 (SCA684Q/84Q). One month of exercise elevated cerebellar BDNF expression and improved ataxia and cerebellar Purkinje cell firing rate deficits. A TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), likewise improved motor coordination and Purkinje cell firing rate and elevated downstream Akt signaling. Prolonged 7,8-DHF administration persistently improved ataxia when treatment commenced near disease onset but was ineffective when treatment was started late. These data suggest that 7,8-DHF, which is orally bioavailable and crosses the blood-brain barrier, is a promising therapeutic for SCA6 and argue for the importance of early intervention for SCA6.

Cerebellar contribution to threat probability in a SCA6 mouse model

Fear and anxiety have proven to be essential during the evolutionary process. However, the mechanisms involved in recognizing and categorizing threat probability (i.e. low to high) to elicit the appropriate defensive behavior are yet to be determined. In this study, we investigated the cerebellar contribution in evoking appropriate defensive escape behavior using a purely cerebellar, neurodegenerative mouse model for spinocerebellar ataxia type 6 which is caused by an expanded CAG repeat in exon 47 of the P/Q type calcium channel α1A subunit. These mice overexpress the carboxy terminus (CT) of the P/Q type calcium channel containing an expanded 27 CAG repeat specifically in cerebellar Purkinje cells (CT-longQ27PC). We found that our CT-longQ27PC mice exhibit anxiolytic behavior in the open field, elevated plus maze and light/dark place preference tests, which could be recovered with more threatening conditions such as brighter lighting, meowing sounds and an ultrasound repellent. Their innate fear to find safety in the Barnes maze and visual cliff tests was also diminished with subsequent trials, which could be partially recovered with an ultrasound repellent in the Barnes maze. However, under higher threat conditions such as in the light/dark place preference with ultrasound repellent and in the looming tests, CT-longQ27PC mice responded with higher defensive escape behaviors as controls. Moreover, CT-longQ27PC mice displayed increased levels of CT-labeled aggregates compared with controls. Together these data suggest that cerebellar degeneration by overexpression of CT-longQ27PC is sufficient to impair defensive escape responses in those mice.

High-Resolution Structures of K+ Channels

Potassium channels are present in every living cell and essential to setting up a stable, non-zero transmembrane electrostatic potential which manifests the off-equilibrium livelihood of the cell. They are involved in other cellular activities and regulation, such as the controlled release of hormones, the activation of T-cells for immune response, the firing of action potential in muscle cells and neurons, etc. Pharmacological reagents targeting potassium channels are important for treating various human diseases linked to dysfunction of the channels. High-resolution structures of these channels are very useful tools for delineating the detailed chemical basis underlying channel functions and for structure-based design and optimization of their pharmacological and pharmaceutical agents. Structural studies of potassium channels have revolutionized biophysical understandings of key concepts in the field - ion selectivity, conduction, channel gating, and modulation, making them multi-modality targets of pharmacological regulation. In this chapter, I will select a few high-resolution structures to illustrate key structural insights, proposed allostery behind channel functions, disagreements still open to debate, and channel-lipid interactions and co-evolution. The known structural consensus allows the inference of conserved molecular mechanisms shared among subfamilies of K⁺ channels and makes it possible to develop channel-specific pharmaceutical agents.

Losing the Beat: Contribution of Purkinje Cell Firing Dysfunction to Disease, and Its Reversal

The cerebellum is a brain structure that is highly interconnected with other brain regions. There are many contributing factors to cerebellar-related brain disease, such as altered afferent input, local connectivity, and/or cerebellar output. Purkinje cells (PC) are the principle cells of the cerebellar cortex, and fire intrinsically; that is, they fire spontaneous action potentials at high frequencies. This review paper focuses on PC intrinsic firing activity, which is altered in multiple neurological diseases, including ataxia, Huntington Disease (HD) and autism spectrum disorder (ASD). Notably, there are several cases where interventions that restore or rescue PC intrinsic activity also improve impaired behavior in these mouse models of disease. These findings suggest that rescuing PC firing deficits themselves may be sufficient to improve impairment in cerebellar-related behavior in disease. We propose that restoring PC intrinsic firing represents a good target for drug development that might be of therapeutic use for several disorders.

Oxidative Stress in DNA Repeat Expansion Disorders: A Focus on NRF2 Signaling Involvement

DNA repeat expansion disorders are a group of neuromuscular and neurodegenerative diseases that arise from the inheritance of long tracts of nucleotide repetitions, located in the regulatory region, introns, or inside the coding sequence of a gene. Although loss of protein expression and/or the gain of function of its transcribed mRNA or translated product represent the major pathogenic effect of these pathologies, mitochondrial dysfunction and imbalance in redox homeostasis are reported as common features in these disorders, deeply affecting their severity and progression. In this review, we examine the role that the redox imbalance plays in the pathological mechanisms of DNA expansion disorders and the recent advances on antioxidant treatments, particularly focusing on the expression and the activity of the transcription factor NRF2, the main cellular regulator of the antioxidant response.

Cholesterol-Dependent Gating Effects on Ion Channels

Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.

Autosomal-dominant cerebellar ataxias

Spinocerebellar ataxias (SCAs) are a genetically diverse group of dominantly inherited disorders that share clinical features that result from dysfunction and degeneration of the cerebellum and its associated pathways. Although nearly 40 genes are currently recognized to result in SCA, shared mechanisms for disease pathogenesis exist among subsets of the SCAs. The most common SCAs result from a glutamine-encoding CAG repeat in the respective disease genes. This chapter discusses the varied genetic etiology of SCA and attempts to categorize these disorders based on shared mechanisms of disease. We also summarize evaluation and management for the SCAs.

Evaluation of parkinsonism and striatal dopamine transporter loss in patients with spinocerebellar ataxia type 6

It is unclear whether patients with spinocerebellar ataxia type 6 (SCA6) have parkinsonism and striatal dopamine transporter (DAT) loss, based on previously small size studies without well-matched controls. A study with a larger number of patients and both age- and gender-matched healthy controls (HCs) is needed for a better answer to this question. Twelve genetically confirmed ataxic SCA6 patients (six male six female, age 65.3 ± 11.2 years), and eight age- and gender-matched HCs (five male three female, age 71.3 ± 8.6 years) were enrolled during 2013-2015 from tertiary movement disorders and ataxia clinics. Clinical assessment for parkinsonism, and qualitative and quantitative assessment of DAT level on DaTscan™ imaging were conducted in SCA6 patients compared to HCs. We found no convincing parkinsonism in SCA6 patients, given generalized bradykinesia in the context of significant ataxia in all, with mild symmetric rigidity in five without resting tremor. Furthermore, we found no striatal DAT loss in anterior, posterior, and total putamen and caudate on imaging, assessed independently by qualitative visual inspection in a blinded manner by the nuclear medicine specialist and movement disorder specialist (kappa = 1). Additional quantitative analysis on these areas did not reveal significant DAT loss either in SCA6 patients compared to HCs. We conclude that there is no convincing parkinsonism or DAT loss in SCA6 patients in this unique study with a larger than previously reported number of patients compared to both age- and gender-matched HCs, suggesting that dopaminergic dysfunction is not usually involved in SCA6.

'Medusa-head ataxia': the expanding spectrum of Purkinje cell antibodies in autoimmune cerebellar ataxia. Part 1: Anti-mGluR1, anti-Homer-3, anti-Sj/ITPR1 and anti-CARP VIII

Serological testing for anti-neural autoantibodies is important in patients presenting with idiopathic cerebellar ataxia, since these autoantibodies may indicate cancer, determine treatment and predict prognosis. While some of them target nuclear antigens present in all or most CNS neurons (e.g. anti-Hu, anti-Ri), others more specifically target antigens present in the cytoplasm or plasma membrane of Purkinje cells (PC). In this series of articles, we provide a detailed review of the clinical and paraclinical features, oncological, therapeutic and prognostic implications, pathogenetic relevance, and differential laboratory diagnosis of the 12 most common PC autoantibodies (often referred to as 'Medusa-head antibodies' due to their characteristic somatodendritic binding pattern when tested by immunohistochemistry). To assist immunologists and neurologists in diagnosing these disorders, typical high-resolution immunohistochemical images of all 12 reactivities are presented, diagnostic pitfalls discussed and all currently available assays reviewed. Of note, most of these antibodies target antigens involved in the mGluR1/calcium pathway essential for PC function and survival. Many of the antigens also play a role in spinocerebellar ataxia. Part 1 focuses on anti-metabotropic glutamate receptor 1-, anti-Homer protein homolog 3-, anti-Sj/inositol 1,4,5-trisphosphate receptor- and anti-carbonic anhydrase-related protein VIII-associated autoimmune cerebellar ataxia (ACA); part 2 covers anti-protein kinase C gamma-, anti-glutamate receptor delta-2-, anti-Ca/RhoGTPase-activating protein 26- and anti-voltage-gated calcium channel-associated ACA; and part 3 reviews the current knowledge on anti-Tr/delta notch-like epidermal growth factor-related receptor-, anti-Nb/AP3B2-, anti-Yo/cerebellar degeneration-related protein 2- and Purkinje cell antibody 2-associated ACA, discusses differential diagnostic aspects and provides a summary and outlook.

The autosomal dominant spinocerebellar ataxias: emerging mechanistic themes suggest pervasive Purkinje cell vulnerability

The spinocerebellar ataxias are a genetically heterogeneous group of disorders with clinically overlapping phenotypes arising from Purkinje cell degeneration, cerebellar atrophy and varying degrees of degeneration of other grey matter regions. For 22 of the 32 subtypes, a genetic cause has been identified. While recurring themes are emerging, there is no clear correlation between the clinical phenotype or penetrance, the type of genetic defect or the category of the disease mechanism, or the neuronal types involved beyond Purkinje cells. These phenomena suggest that cerebellar Purkinje cells may be a uniquely vulnerable neuronal cell type, more susceptible to a wider variety of genetic/cellular insults than most other neuron types.

Ataxias with Autosomal, X-Chromosomal or Maternal Inheritance

Heredoataxias are a group of genetic disorders with a cerebellar syndrome as the leading clinical manifestation. The current classification distinguishes heredoataxias according to the trait of inheritance into autosomal dominant, autosomal recessive, X-linked, and maternally inherited heredoataxias. The autosomal dominant heredoataxias are separated into spinocerebellar ataxias (SCA1-8, 10-15, 17-23, 25-30, and dentato-rubro-pallido-luysian atrophy), episodic ataxias (EA1-7), and autosomal dominant mitochondrial heredoataxias (Leigh syndrome, MIRAS, ADOAD, and AD-CPEO). The autosomal recessive ataxias are separated into Friedreich ataxia, ataxia due to vitamin E deficiency, ataxia due to Abeta-lipoproteinemia, Refsum disease, late-onset Tay-Sachs disease, cerebrotendineous xanthomatosis, spinocerebellar ataxia with axonal neuropathy, ataxia telangiectasia, ataxia telangiectasia-like disorder, ataxia with oculomotor apraxia 1 and 2, spastic ataxia of Charlevoix-Saguenay, Cayman ataxia, Marinesco-Sjögren syndrome, and autosomal recessive mitochondrial ataxias (AR-CPEO, SANDO, SCAE, AHS, IOSCA, MEMSA, LBSL CoQ-deficiency, PDC-deficiency). Only two of the heredoataxias, fragile X/tremor/ataxia syndrome, and XLSA/A are transmitted via an X-linked trait. Maternally inherited heredoataxias are due to point mutations in genes encoding for tRNAs, rRNAs, respiratory chain subunits or single large scale deletions/duplications of the mitochondrial DNA and include MELAS, MERRF, KSS, PS, MILS, NARP, and non-syndromic mitochondrial disorders. Treatment of heredoataxias is symptomatic and supportive and may have a beneficial effect in single patients.

**Please see page 424 for abbreviation list.

Paraneoplastic CDR2 and CDR2L antibodies affect Purkinje cell calcium homeostasis

Paraneoplastic cerebellar degeneration (PCD) is characterized by loss of Purkinje cells (PCs) associated with progressive pancerebellar dysfunction in the presence of onconeural Yo antibodies. These antibodies recognize the cerebellar degeneration-related antigens CDR2 and CDR2L. Response to PCD therapy is disappointing due to limited understanding of the neuropathological mechanisms. Here, we report the pathological role of CDR antibodies on the calcium homeostasis in PCs. We developed an antibody-mediated PCD model based on co-incubation of cerebellar organotypic slice culture with human patient serum or rabbit CDR2 and CDR2L antibodies. The CDR antibody-induced pathology was investigated by high-resolution multiphoton imaging and biochemical analysis. Both human and rabbit CDR antibodies were rapidly internalized by PCs and led to reduced immunoreactivity of calbindin D_28K (CB) and L7/Pcp-2 as well as reduced dendritic arborizations in the remaining PCs. Washout of the CDR antibodies partially recovered CB immunoreactivity, suggesting a transient structural change in CB calcium-binding site. We discovered that CDR2 and CB co-immunoprecipitate. Furthermore, the expression levels of voltage-gated calcium channel Cav2.1, protein kinase C gamma and calcium-dependent protease, calpain-2, were increased after CDR antibody internalization. Inhibition of these signaling pathways prevented or attenuated CDR antibody-induced CB and L7/Pcp-2 immunoreactivity loss, morphological changes and increased protein expression. These results signify that CDR antibody internalization causes dysregulation of cell calcium homeostasis. Hence, drugs that modulate these events may represent novel neuroprotective therapies that limit the damaging effects of CDR antibodies and prevent PC neurodegeneration.

Clinical neurogenetics: autosomal dominant spinocerebellar ataxia

The autosomal dominant spinocerebellar ataxias are a diverse and clinically heterogeneous group of disorders characterized by degeneration and dysfunction of the cerebellum and its associated pathways. Clinical and diagnostic evaluation can be challenging because of phenotypic overlap among causes, and a stratified and systematic approach is essential. Recent advances include the identification of additional genes causing dominant genetic ataxia, a better understanding of cellular pathogenesis in several disorders, the generation of new disease models that may stimulate development of new therapies, and the use of new DNA sequencing technologies, including whole-exome sequencing, to improve diagnosis.

Recent advances in RNA interference therapeutics for CNS diseases

Over the last decade, RNA interference technology has shown therapeutic promise in rodent models of dominantly inherited brain diseases, including those caused by polyglutamine repeat expansions in the coding region of the affected gene. For some of these diseases, proof-of concept studies in model organisms have transitioned to safety testing in larger animal models, such as the nonhuman primate. Here, we review recent progress on RNA interference-based therapies in various model systems. We also highlight outstanding questions or concerns that have emerged as a result of an improved (and ever advancing) understanding of the technologies employed.

[Have centers of rare neurological diseases changed their practices and management of the hereditary cerebellar ataxias?]

The classification and management of hereditary cerebellar ataxias have been considerably changed by advances made in the field of genetics. Given the numerous genes implicated in the disorders, genetic analysis, which alone can confirm the diagnosis, needs to be based on phenotypically precise studies. Diagnostic algorithms including both recessive and dominant forms of ataxia have been proposed. The range of disease effects has been further expanded in the light of evidence of ataxias associated with permutations of the Fragile X gene, and ataxias linked to mutations of the nuclear genes coding for structural proteins of mitochondrial DNA. In the field of therapeutics, several studies are currently ongoing for Friedreich's ataxia.

A conserved eEF2 coding variant in SCA26 leads to loss of translational fidelity and increased susceptibility to proteostatic insult

The autosomal dominant spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of disorders exhibiting cerebellar atrophy and Purkinje cell degeneration whose subtypes arise from 31 distinct genetic loci. Our group previously published the locus for SCA26 on chromosome 19p13.3. In this study, we performed targeted deep sequencing of the critical interval in order to identify candidate causative variants in individuals from the SCA26 family. We identified a single variant that co-segregates with the disease phenotype that produces a single amino acid substitution in eukaryotic elongation factor 2. This substitution, P596H, sits in a domain critical for maintaining reading frame during translation. The yeast equivalent, P580H EF2, demonstrated impaired translocation, detected as an increased rate of -1 programmed ribosomal frameshift read-through in a dual-luciferase assay for observing translational recoding. This substitution also results in a greater susceptibility to proteostatic disruption, as evidenced by a more robust activation of a reporter gene driven by unfolded protein response activation upon challenge with dithiothreitol or heat shock in our yeast model system. Our results present a compelling candidate mutation and mechanism for the pathogenesis of SCA26 and further support the role of proteostatic disruption in neurodegenerative diseases.

Delayed postnatal loss of P/Q-type calcium channels recapitulates the absence epilepsy, dyskinesia, and ataxia phenotypes of genomic Cacna1a mutations

Inherited loss of P/Q-type calcium channel function causes human absence epilepsy, episodic dyskinesia, and ataxia, but the molecular "birthdate" of the neurological syndrome and its dependence on prenatal pathophysiology is unknown. Since these channels mediate transmitter release at synapses throughout the brain and are expressed early in embryonic development, delineating the critical circuitry and onset underlying each of the emergent phenotypes requires targeted control of gene expression. To visualize P/Q-type Ca(2+) channels and dissect their role in neuronal networks at distinct developmental stages, we created a novel conditional Cacna1a knock-in mouse by inserting the floxed green fluorescent protein derivative Citrine into the first exon of Cacna1a and then crossed it with a postnatally expressing PCP2-Cre line for delayed Purkinje cell (PC) gene deletion within the cerebellum and sparsely in forebrain (purky). PCs in purky mice lacked P/Q-type calcium channel protein and currents within the first month after birth, displayed altered spontaneous firing, and showed impaired neurotransmission. Unexpectedly, adult purky mice exhibited the full spectrum of neurological deficits seen in mice with genomic Cacna1a ablation. Our results show that the ataxia, dyskinesia, and absence epilepsy caused by inherited disorders of the P/Q-type channel arise from signaling defects beginning in late infancy, revealing an early window of opportunity for therapeutic intervention.

Splice isoform-specific suppression of the Cav2.1 variant underlying spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is an inherited neurodegenerative disease caused by a polyglutamine (polyQ) expansion in the Ca(V)2.1 voltage-gated calcium channel subunit (CACNA1A). There is currently no treatment for this debilitating disorder and thus a pressing need to develop preventative therapies. RNA interference (RNAi) has proven effective at halting disease progression in several models of spinocerebellar ataxia (SCA), including SCA types 1 and 3. However, in SCA6 and other dominantly inherited neurodegenerative disorders, RNAi-based strategies that selectively suppress expression of mutant alleles may be required. Using a Ca(V)2.1 mini-gene reporter system, we found that pathogenic CAG expansions in Ca(V)2.1 enhance splicing activity at the 3'end of the transcript, leading to a CAG repeat length-dependent increase in the levels of a polyQ-encoding Ca(V)2.1 mRNA splice isoform and the resultant disease protein. Taking advantage of this molecular phenomenon, we developed a novel splice isoform-specific (SIS)-RNAi strategy that selectively targets the polyQ-encoding Ca(V)2.1 splice variant. Selective suppression of transiently expressed and endogenous polyQ-encoding Ca(V)2.1 splice variants was achieved in a variety of cell-based models including a human neuronal cell line, using a new artificial miRNA-like delivery system. Moreover, the efficacy of gene silencing correlated with effective intracellular recognition and processing of SIS-RNAi miRNA mimics. These results lend support to the preclinical development of SIS-RNAi as a potential therapy for SCA6 and other dominantly inherited diseases.

Autosomal dominant cerebellar ataxias: polyglutamine expansions and beyond

Cerebellar ataxias with autosomal dominant transmission are rare, but identification of the associated genes has provided insight into the mechanisms that could underlie other forms of genetic or non-genetic ataxias. In many instances, the phenotype is not restricted to cerebellar dysfunction but includes complex multisystemic neurological deficits. The designation of the loci, SCA for spinocerebellar ataxia, indicates the involvement of at least two systems: the spinal cord and the cerebellum. 11 of 18 known genes are caused by repeat expansions in the corresponding proteins, sharing the same mutational mechanism. All other SCAs are caused by either conventional mutations or large rearrangements in genes with different functions, including glutamate signalling (SCA5/SPTBN2) and calcium signalling (SCA15/16/ITPR1), channel function (SCA13/KCNC3, SCA14/PRKCG, SCA27/FGF14), tau regulation (SCA11/TTBK2), and mitochondrial activity (SCA28/AFG3L2) or RNA alteration (SCA31/BEAN-TK2). The diversity of underlying mechanisms that give rise to the dominant cerebellar ataxias need to be taken into account to identify therapeutic targets.

The carboxy-terminal fragment of alpha(1A) calcium channel preferentially aggregates in the cytoplasm of human spinocerebellar ataxia type 6 Purkinje cells

Spinocerebellar ataxia type 6 (SCA6) is an autosomal dominant neurodegenerative disease caused by a small polyglutamine (polyQ) expansion (control: 4–20Q; SCA6: 20–33Q) in the carboxyl(C)-terminal cytoplasmic domain of the α_1A voltage-dependent calcium channel (Ca_v2.1). Although a 75–85-kDa Ca_v2.1 C-terminal fragment (CTF) is toxic in cultured cells, its existence in human brains and its role in SCA6 pathogenesis remains unknown. Here, we investigated whether the small polyQ expansion alters the expression pattern and intracellular distribution of Ca_v2.1 in human SCA6 brains. New antibodies against the Ca_v2.1 C-terminus were used in immunoblotting and immunohistochemistry. In the cerebella of six control individuals, the CTF was detected in sucrose- and SDS-soluble cytosolic fractions; in the cerebella of two SCA6 patients, it was additionally detected in SDS-insoluble cytosolic and sucrose-soluble nuclear fractions. In contrast, however, the CTF was not detected either in the nuclear fraction or in the SDS-insoluble cytosolic fraction of SCA6 extracerebellar tissues, indicating that the CTF being insoluble in the cytoplasm or mislocalized to the nucleus only in the SCA6 cerebellum. Immunohistochemistry revealed abundant aggregates in cell bodies and dendrites of SCA6 Purkinje cells (seven patients) but not in controls (n = 6). Recombinant CTF with a small polyQ expansion (rCTF-Q28) aggregated in cultured PC12 cells, but neither rCTF-Q13 (normal-length polyQ) nor full-length Ca_v2.1 with Q28 did. We conclude that SCA6 pathogenesis may be associated with the CTF, normally found in the cytoplasm, being aggregated in the cytoplasm and additionally distributed in the nucleus.

The effect of 3,4-diaminopyridine on the patients with hereditary pure cerebellar ataxia

BACKGROUND

Downbeat nystagmus (DBN) is often seen in patients with pure cerebellar type of spinocerebellar ataxia (SCA) like spinocerebellar ataxia type 6 (SCA6). DBN frequently presents with other cerebellar symptoms such as postural imbalance or ataxia. A potassium channel blocker 3,4-diaminopyridine (3,4-DAP) has been reported to reduce DBN by increasing the excitability of Purkinje cells.

OBJECTIVE

The objective of this study is to determine whether 3,4-DAP has a beneficial effect on DBN along with postural imbalance and ataxic symptoms in 10 patients with SCA6 and five patients with chromosome 16q22.1-linked autosomal dominant cerebellar ataxia (16q-ADCA).

RESULTS

The patients took 20mg of 3,4-DAP twice a day for a week. DBN was observed in seven patients with SCA6 and two with 16q-ADCA. Although 3,4-DAP significantly reduced DBN (P<0.05), other ataxic symptoms did not improved. However, 3,4-DAP showed benefit in two patients with oscillopsia.

CONCLUSION

3,4-DAP may be effective on DBN and oscillopsia, although it was not proved to be effective on other symptoms of ataxia in SCA patients.

The spinocerebellar ataxias

Slowly progressive ataxia accompanied by cerebellar degeneration is often genetic in origin. The past 15 years have witnessed a revolution in our understanding of the causes of dominantly inherited ataxias, now known as the spinocerebellar ataxias (SCAs). Nearly 30 distinct genetic causes of SCA are known, numbered chronologically in order of discovery. All SCAs display classic cerebellar signs, and many display disabling noncerebellar features, most commonly brainstem dysfunction. Eye movement abnormalities are common, reflecting cerebellar and brainstem degeneration. Visual loss from retinal degeneration is rare in SCA, occurring most commonly and profoundly in SCA7. Although the SCAs are relentlessly progressive and currently untreatable, recent scientific advances have begun to shed light on various disease mechanisms that may lead to preventive therapies.

Physiologic alterations in ataxia: channeling changes into novel therapies

The ataxias constitute a heterogeneous group of diseases in which cerebellar dysfunction typically underlies the major neurologic manifestations. It is increasingly clear that ataxia can result directly from mutations in ion channels or from perturbations in ion channel physiology in the absence of a primary channel defect. Neuronal dysfunction stemming from perturbed channel activity likely explains some motor deficits in episodic and degenerative ataxias. Understanding these pathophysiologic changes may reveal novel therapeutic targets for symptomatic treatment of ataxia.

Deficit of heat shock transcription factor 1-heat shock 70 kDa protein 1A axis determines the cell death vulnerability in a model of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is caused by a small expansion of polyglutamine (polyQ)-encoding CAG repeat in Ca(v)2.1 calcium channel gene. To gain insights into pathogenic mechanism of SCA6, we used HEK293 cells expressing fusion protein of enhanced green fluorescent protein and Ca(v)2.1 carboxyl terminal fragment (EGFP-Ca(v)2.1CT) [L24 and S13 cells containing 24 polyQ (disease range) and 13 polyQ (normal range), respectively] and examined their responses to some stressors. When exposed to CdCl(2), L24 cells showed lower viability than the control S13 cells and caspase-dependent apoptosis was enhanced more in L24 cells. Localization of EGFP-Ca(v)2.1CT was almost confined to the nucleus, where it existed as speckle-like structures. Interestingly, CdCl(2) treatment resulted in disruption of more promyelocytic leukemia nuclear bodies (PML-NBs) in L24 cells than in S13 cells and in cells where PML-NBs were disrupted, aggregates of EGFP-Ca(v)2.1CT became larger. Furthermore, a large number of aggregates were formed in L24 cells than in S13 cells. Results of RNAi experiments indicated that HSPA1A determined the difference against CdCl(2) toxicity. Furthermore, protein expression of heat shock transcription factor 1 (HSF1), which activates HSPA1A expression, was down-regulated in L24 cells. Therefore, HSF1-HSPA1A axis is critical for the vulnerability in L24 cells.

Spinocerebellar ataxia type 6 in Brazil

Spinocerebellar ataxia type 6 (SCA 6) is an autosomal dominant cerebellar ataxia caused by CAG repeat expansion in the SCA6 gene, a alpha 1A voltage-dependent calcium channel subunit gene on chromosome 19p13. SCA-6 is characterized predominantly by slowly progressive pure cerebellar ataxia with late onset. We report three index patients, with pure, late onset, cerebellar ataxia, belonging to three different Brazilian families, all of them with Japanese ancestry, from Hokkaido island of Japan.

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.

An update on inherited ataxias

This article provides an overview of recent advances in the field of inherited ataxias. In the past few years, new causative mutations that broaden the diagnostic spectrum of ataxias have been described. In addition, important advances have unveiled the molecular pathology of these disorders, resulting in a classification based on the pathogenetic pathways rather than clinical or genetic features. As concepts of treatment principles emerge, debate continues as to whether such concepts might be applicable to more than one genetically defined disorder or whether each ataxia disorder requires its own unique therapeutic approach. New clinical assessment instruments have been developed that will facilitate future interventional trials. A recent phase 2 clinical trial suggested a positive effect of high-dose idebenone in Friedreich's ataxia, raising hopes that a treatment option will soon be available for this disorder.

The biological effects of simple tandem repeats: lessons from the repeat expansion diseases

Tandem repeats are common features of both prokaryote and eukaryote genomes, where they can be found not only in intergenic regions but also in both the noncoding and coding regions of a variety of different genes. The repeat expansion diseases are a group of human genetic disorders caused by long and highly polymorphic tandem repeats. These disorders provide many examples of the effects that such repeats can have on many biological processes. While repeats in the coding sequence can result in the generation of toxic or malfunctioning proteins, noncoding repeats can also have significant effects including the generation of chromosome fragility, the silencing of the genes in which they are located, the modulation of transcription and translation, and the sequestering of proteins involved in processes such as splicing and cell architecture.

Familial hemiplegic migraine

Familial hemiplegic migraine (FHM) is a rare and genetically heterogeneous autosomal dominant subtype of migraine with aura. Mutations in the genes CACNA1A and SCNA1A, encoding the pore-forming alpha(1) subunits of the neuronal voltage-gated Ca2+ channels Ca(V)2.1 and Na+ channels Na(V)1.1, are responsible for FHM1 and FHM3, respectively, whereas mutations in ATP1A2, encoding the alpha2 subunit of the Na+, K+ adenosinetriphosphatase (ATPase), are responsible for FHM2. This review discusses the functional studies of two FHM1 knockin mice and of several FHM mutants in heterologous expression systems (12 FHM1, 8 FHM2, and 1 FHM3). These studies show the following: (1) FHM1 mutations produce gain-of-function of the Ca(V)2.1 channel and, as a consequence, increased Ca(V)2.1-dependent neurotransmitter release from cortical neurons and facilitation of in vivo induction and propagation of cortical spreading depression (CSD: the phenomenon underlying migraine aura); (2) FHM2 mutations produce loss-of-function of the alpha2 Na+,K+-ATPase; and (3) the FHM3 mutation accelerates recovery from fast inactivation of Na(V)1.5 (and presumably Na(V)1.1) channels. These findings are consistent with the hypothesis that FHM mutations share the ability of rendering the brain more susceptible to CSD by causing either excessive synaptic glutamate release (FHM1) or decreased removal of K+ and glutamate from the synaptic cleft (FHM2) or excessive extracellular K+ (FHM3). The FHM data support a key role of CSD in migraine pathogenesis and point to cortical hyperexcitability as the basis for vulnerability to CSD and to migraine attacks. Hence, they support novel therapeutic strategies that consider CSD and cortical hyperexcitability as key targets for preventive migraine treatment.

Episodic ataxia type 2

Episodic ataxia type 2 (EA 2) is a rare neurological disorder of autosomal dominant inheritance resulting from dysfunction of a voltage-gated calcium channel. It manifests with recurrent disabling attacks of imbalance, vertigo, and ataxia, and can be provoked by physical exertion or emotional stress. In the spell-free interval, patients present with central ocular motor dysfunction, mainly downbeat nystagmus. A slow progression of cerebellar signs accompanied by a slight atrophy of midline cerebellar structures is commonly observed during the course of the disease. EA 2 is caused most often by the loss of function mutations of the calcium channel gene CACNA1A, which encodes the Ca(v)2.1 subunit of the P/Q-type calcium channel and is primarily expressed in Purkinje cells. To date, more than 30 mutations have been described. Two effective treatment options have been established for EA 2: acetazolamide (ACTZ), which probably changes the intracellular pH and thereby the transmembraneous potential, and 4-aminopyridine (4-AP), a potassium channel blocker. Approximately 70% of all patients respond to treatment with ACTZ, but the effect is often only transient. In an open trial, 4-AP prevented attacks in five of six patients with EA 2, most likely by increasing the resting activity and excitability of the Purkinje cells. These findings were confirmed by experiments in animal models of EA 2. Many aspects of the pathophysiology (e.g., induction of the attacks) and treatment of EA 2 (e.g., mode of action of ACTZ and 4-AP) still remain unclear and need to be addressed in further animal and clinical studies.

Autoimmune and paraneoplastic channelopathies

Thirty years ago, antibodies against the muscle acetylcholine receptor (AChR) were recognized as the cause of myasthenia gravis. Since then, there has been great interest in identifying other neurological disorders associated with autoantibodies. Several other antibody-mediated neuromuscular disorders have been identified, each associated with an antibody against a ligand- or voltage-gated ion channel. The Lambert-Eaton syndrome is caused by antibodies against voltage-gated calcium channels and often occurs in patients with small cell lung cancer. Acquired neuromyotonia is caused by voltage-gated potassium channel antibodies, and autoimmune autonomic ganglionopathy is caused by antibodies against the neuronal AChR in autonomic ganglia. There is good evidence that antibodies in these disorders cause changes in synaptic function or neuronal excitability by directly inhibiting ion channel function. More recently, studies have identified ion channel antibodies in patients with certain CNS disorders, such as steroid-responsive encephalitis and paraneoplastic cerebellar ataxia. It remains unclear if antibodies can gain access to the CNS and directly cause ion channel dysfunction. Treatment of autoimmune channelopathies includes drugs that help restore normal neuronal function and treatments to remove pathogenic antibodies (plasma exchange) or modulate the immune response (steroids or immunosuppressants). These disabling neurological disorders may be dramatically responsive to immunomodulatory therapy. Future studies will likely lead to identification of other ion channel antibodies and other autoimmune channelopathies.