Polyglutamine repeats of spinocerebellar ataxia 6 impair the cell-death-preventing effect of CaV2.1 Ca2+ channel—loss-of-function cellular model of SCA6

Polyglutamine disorders: Pathogenesis and potential drug interventions

Polyglutamine/poly(Q) diseases are a group nine hereditary neurodegenerative disorders caused due to abnormally expanded stretches of CAG trinucleotide in functionally distinct genes. All human poly(Q) diseases are characterized by the formation of microscopically discernable poly(Q) positive aggregates, the inclusion bodies. These toxic inclusion bodies are responsible for the impairment of several cellular pathways such as autophagy, transcription, cell death, etc., that culminate in disease manifestation. Although, these diseases remain largely without treatment, extensive research has generated mounting evidences that various events of poly(Q) pathogenesis can be developed as potential drug targets. The present review article briefly discusses the key events of disease pathogenesis, model system-based investigations that support the development of effective therapeutic interventions against pathogenesis of human poly(Q) disorders, and a comprehensive list of pharmacological and bioactive compounds that have been experimentally shown to alleviate poly(Q)-mediated neurotoxicity. Interestingly, due to the common cause of pathogenesis, all poly(Q) diseases share etiology, thus, findings from one disease can be potentially extrapolated to other poly(Q) diseases as well.

Invited review: decoding the pathophysiological mechanisms that underlie RNA dysregulation in neurodegenerative disorders: a review of the current state of the art

Altered RNA metabolism is a key pathophysiological component causing several neurodegenerative diseases. Genetic mutations causing neurodegeneration occur in coding and noncoding regions of seemingly unrelated genes whose products do not always contribute to the gene expression process. Several pathogenic mechanisms may coexist within a single neuronal cell, including RNA/protein toxic gain-of-function and/or protein loss-of-function. Genetic mutations that cause neurodegenerative disorders disrupt healthy gene expression at diverse levels, from chromatin remodelling, transcription, splicing, through to axonal transport and repeat-associated non-ATG (RAN) translation. We address neurodegeneration in repeat expansion disorders [Huntington's disease, spinocerebellar ataxias, C9ORF72-related amyotrophic lateral sclerosis (ALS)] and in diseases caused by deletions or point mutations (spinal muscular atrophy, most subtypes of familial ALS). Some neurodegenerative disorders exhibit broad dysregulation of gene expression with the synthesis of hundreds to thousands of abnormal messenger RNA (mRNA) molecules. However, the number and identity of aberrant mRNAs that are translated into proteins - and how these lead to neurodegeneration - remain unknown. The field of RNA biology research faces the challenge of identifying pathophysiological events of dysregulated gene expression. In conclusion, we discuss current research limitations and future directions to improve our characterization of pathological mechanisms that trigger disease onset and progression.

Canadian Association of Neurosciences Review: Polyglutamine Expansion Neurodegenerative Diseases

Since the early 1990s, DNA triplet repeat expansions have been found to be the cause in an ever increasing number of genetic neurologic diseases. A subset of this large family of genetic diseases has the expansion of a CAG DNA triplet in the open reading frame of a coding exon. The result of this DNA expansion is the expression of expanded glutamine amino acid repeat tracts in the affected proteins, leading to the term, Polyglutamine Diseases, which is applied to this sub-family of diseases. To date, nine distinct genes are known to be linked to polyglutamine diseases, including Huntington's disease, Machado-Joseph Disease and spinobulbar muscular atrophy or Kennedy's disease. Most of the polyglutamine diseases are characterized clinically as spinocerebellar ataxias. Here we discuss recent successes and advancements in polyglutamine disease research, comparing these different diseases with a common genetic flaw at the level of molecular biology and early drug design for a family of diseases where many new research tools for these genetic disorders have been developed. Polyglutamine disease research has successfully used interdisciplinary collaborative efforts, informative multiple mouse genetic models and advanced tools of pharmaceutical industry research to potentially serve as the prototype model of therapeutic research and development for rare neurodegenerative diseases.

Alleviating neurodegeneration in Drosophila models of PolyQ diseases

Polyglutamine (polyQ) diseases are a group of neurodegenerative conditions, induced from CAG trinucleotide repeat expansion within causative gene respectively. Generation of toxic proteins, containing polyQ-expanded tract, is the key process to cause neurodegeneration. Till now, although polyQ diseases remain uncurable, numerous therapeutic strategies with great potential have been examined and have been proven to be effective against polyQ diseases, including diverse small biological molecules and many pharmacological compounds mainly through prevention on formation of aggregates and inclusions, acceleration on degradation of toxic proteins and regulation of cellular function. We review promising therapeutic strategies by using Drosophila models of polyQ diseases including HD, SCA1, SCA3 and SBMA.

Autosomal dominant cerebellar ataxia type III: a review of the phenotypic and genotypic characteristics

Autosomal Dominant Cerebellar Ataxia (ADCA) Type III is a type of spinocerebellar ataxia (SCA) classically characterized by pure cerebellar ataxia and occasionally by non-cerebellar signs such as pyramidal signs, ophthalmoplegia, and tremor. The onset of symptoms typically occurs in adulthood; however, a minority of patients develop clinical features in adolescence. The incidence of ADCA Type III is unknown. ADCA Type III consists of six subtypes, SCA5, SCA6, SCA11, SCA26, SCA30, and SCA31. The subtype SCA6 is the most common. These subtypes are associated with four causative genes and two loci. The severity of symptoms and age of onset can vary between each SCA subtype and even between families with the same subtype. SCA5 and SCA11 are caused by specific gene mutations such as missense, inframe deletions, and frameshift insertions or deletions. SCA6 is caused by trinucleotide CAG repeat expansions encoding large uninterrupted glutamine tracts. SCA31 is caused by repeat expansions that fall outside of the protein-coding region of the disease gene. Currently, there are no specific gene mutations associated with SCA26 or SCA30, though there is a confirmed locus for each subtype. This disease is mainly diagnosed via genetic testing; however, differential diagnoses include pure cerebellar ataxia and non-cerebellar features in addition to ataxia. Although not fatal, ADCA Type III may cause dysphagia and falls, which reduce the quality of life of the patients and may in turn shorten the lifespan. The therapy for ADCA Type III is supportive and includes occupational and speech modalities. There is no cure for ADCA Type III, but a number of recent studies have highlighted novel therapies, which bring hope for future curative treatments.

Spinocerebellar degenerations

The spinocerebellar ataxias (SCA) are a large group of inherited disorders affecting the cerebellum and its afferent and efferent pathways. Their hallmark symptom is slowly progressive, symmetrical, midline, and appendicular ataxia. Some may also have associated hyperkinetic movements (chorea, dystonia, myoclonus, postural/action tremor, restless legs, rubral tremor, tics), which may aid in differential diagnosis and provide treatable targets to improve performance and quality of life in these progressive, incurable conditions. The typical dominant ataxias with associated hyperkinetic movements are SCA1-3, 6-8, 12, 14, 15, 17, 19-21, and 27. The common recessive ataxias with associated hyperkinetic movements are ataxia telangiectasia and Friedreich's ataxia. Fragile X tremor-ataxia syndrome (FXTAS) and multiple-system atrophy (a sporadic ataxia which is felt to have a genetic substrate) also have hyperkinetic features. A careful work-up should be done in all apparently sporadic cases, to rule out acquired causes of ataxia, some of which can cause hyperkinetic movements in addition to ataxia. Some testing should be done even in individuals with a confirmed genetic cause, as the presence of a secondary factor (nutritional deficiency, thyroid dysfunction) can contribute to the phenotype.

Splice isoforms of the polyglutamine disease protein ataxin-3 exhibit similar enzymatic yet different aggregation properties

Protein context clearly influences neurotoxicity in polyglutamine diseases, but the contribution of alternative splicing to this phenomenon has rarely been investigated. Ataxin-3, a deubiquitinating enzyme and the disease protein in SCA3, is alternatively spliced to encode either a C-terminal hydrophobic stretch or a third ubiquitin interacting motif (termed 2UIM and 3UIM isoforms, respectively). In light of emerging insights into ataxin-3 function, we examined the significance of this splice variation. We confirmed neural expression of several minor 5′ variants and both of the known 3′ ataxin-3 splice variants. Regardless of polyglutamine expansion, 3UIM ataxin-3 is the predominant isoform in brain. Although 2UIM and 3UIM ataxin-3 display similar in vitro deubiquitinating activity, 2UIM ataxin-3 is more prone to aggregate and more rapidly degraded by the proteasome. Our data demonstrate how alternative splicing of sequences distinct from the trinucleotide repeat can alter properties of the encoded polyglutamine disease protein and thereby perhaps contribute to selective neurotoxicity.

Polyglutamine diseases: where does toxicity come from? what is toxicity? where are we going?

Although the genetic basis of polyglutamine diseases has been recognized for 20 years, their molecular basis is still unclear. We have no therapeutic strategies for these intractable neurodegenerative disorders. To adequately treat patients, we must clarify the molecular basis of polyglutamine diseases. Three main issues address their molecular pathogenesis: whether the specific structure of expanded polyglutamine diseases results in cellular toxicity; what type of dysfunction causes them; and how the toxic structure causes dysfunction, that is, the link between structure and dysfunction. For structures, expanded polyglutamine proteins undergo transformation from monomers to oligomers and inclusions. One can hypothesize that one of these structures might cause the polyglutamine disease. Although the expanded polyglutamine protein is toxic, it does not explain the selective vulnerability of specific neurons in each polyglutamine disease. The normal function of each protein, including protein-protein interaction and modification, might also be crucial for pathogenesis. For dysfunction, various molecular mechanisms have been proposed, including dysregulation of transcription, impairment of the ubiquitin-proteasome system, mitochondrial dysfunction, dysregulation of intracellular Ca(2+) homeostasis, impairment of axonal transport and genotoxic stress. These hypotheses might correlate with each other. In addition, the disease pathogenesis of might not be exclusive to one particular structure or dysfunction. To develop a therapeutic strategy for patients with polyglutamine disease, identifying the most toxic structure and the earliest event in the pathogenesis is important. We review the current understanding of the toxic structure and dysfunction by expanded polyglutamine proteins and suggest directions for future studies of polyglutamine diseases.

The pathogenic mechanisms of polyglutamine diseases and current therapeutic strategies

Expansion of CAG trinucleotide repeat within the coding region of several genes results in the production of proteins with expanded polyglutamine (PolyQ) stretch. The expression of these pathogenic proteins leads to PolyQ diseases, such as Huntington's disease or several types of spinocerebellar ataxias. This family of neurodegenerative disorders is characterized by constant progression of the symptoms and molecularly, by the accumulation of mutant proteins inside neurons causing their dysfunction and eventually death. So far, no effective therapy actually preventing the physical and/or mental decline has been developed. Experimental therapeutic strategies either target the levels or processing of mutant proteins in an attempt to prevent cellular deterioration, or they are aimed at the downstream pathologic effects to reverse or ameliorate the caused damages. Certain pathomechanistic aspects of PolyQ disorders are discussed here. Relevance of disease models and recent knowledge of therapeutic possibilities is reviewed and updated.

Calcium signaling in neurodegeneration

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion. The exact definition at the molecular level of the versatility of this ion has made overwhelming progress in the past several years and has been extensively reviewed. In the brain, calcium is fundamental in the control of synaptic activity and memory formation, a process that leads to the activation of specific calcium-dependent signal transduction pathways and implicates key protein effectors, such as CaMKs, MAPK/ERKs, and CREB. Properly controlled homeostasis of calcium signaling not only supports normal brain physiology but also maintains neuronal integrity and long-term cell survival. Emerging knowledge indicates that calcium homeostasis is not only critical for cell physiology and health, but also, when deregulated, can lead to neurodegeneration via complex and diverse mechanisms involved in selective neuronal impairments and death. The identification of several modulators of calcium homeostasis, such as presenilins and CALHM1, as potential factors involved in the pathogenesis of Alzheimer's disease, provides strong support for a role of calcium in neurodegeneration. These observations represent an important step towards understanding the molecular mechanisms of calcium signaling disturbances observed in different brain diseases such as Alzheimer's, Parkinson's, and Huntington's diseases.

Toxicity and endocytosis of spinocerebellar ataxia type 6 polyglutamine domains: role of myosin IIb

Spinocerebellar ataxia type 6 (SCA6) is a dominantly inherited neurodegenerative disease caused by a small expansion of CAG repeats in the sequence coding for the cytoplasmic C-terminal region of the Ca(v)2.1 subunit of P/Q-type calcium channels. We have tested the toxicity of mutated Ca(v)2.1 C-terminal domains expressed in the plasma membrane. In COS-7 cells, CD4-green fluorescent protein fused to Ca(v)2.1 C-terminal domains containing expanded 24 polyglutamine (Q) tracts displayed increased toxicity and stronger expression at the cell surface relative to 'normal' 12 Q tracts, partially because of reduced endocytosis. Glutathione S-transferase pull-down and proteomic analysis indicated that Ca(v)2.1 C-termini interact with the heavy and light chains of cerebellar myosin IIB, a molecular motor protein. This interaction was confirmed by coimmunoprecipitation from rat cerebellum and COS-7 cells and shown to be direct by binding of in vitro-translated (35)S-myosin IIB heavy chain. In COS-7 cells, incremented polyglutamine tract length increased the interaction with myosin IIB. Furthermore, the myosin II inhibitor blebbistatin reversed the effects of polyglutamine expansion on plasma membrane expression. Our findings suggest a key role of myosin IIB in promoting accumulation of mutant Ca(v)2.1Ct at the plasma membrane and suggest that this gain of function might contribute to the pathogenesis of SCA6.

Prediction of drug combination chemosensitivity in human bladder cancer

The choice of therapy for metastatic cancer is largely empirical because of a lack of chemosensitivity prediction for available combination chemotherapeutic regimens. Here, we identify molecular models of bladder carcinoma chemosensitivity based on gene expression for three widely used chemotherapeutic agents: cisplatin, paclitaxel, and gemcitabine. We measured the growth inhibition elicited by these three agents in a series of 40 human urothelial cancer cell lines and correlated the GI(50) (50% of growth inhibition) values with quantitative measures of global gene expression to derive models of chemosensitivity using a misclassification-penalized posterior approach. The misclassification-penalized posterior-derived models predicted the growth response of human bladder cancer cell lines to each of the three agents with sensitivities of between 0.93 and 0.96. We then developed an in silico approach to predict the cellular growth responses for each of these agents in the clinically relevant two-agent combinations. These predictions were prospectively evaluated on a series of 15 randomly chosen bladder carcinoma cell lines. Overall, 80% of the predicted combinations were correct (P = 0.0002). Together, our results suggest that chemosensitivity to drug combinations can be predicted based on molecular models and provide the framework for evaluation of such models in patients undergoing combination chemotherapy for cancer. If validated in vivo, such predictive models have the potential to guide therapeutic choice at the level of an individual's tumor.

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.

PATHOMECHANISMS IN CHANNELOPATHIES OF SKELETAL MUSCLE AND BRAIN

Ion channelopathies are a diverse array of human disorders caused by mutations in ion channel genes. This review focuses on the pathogenic mechanisms of channelopathies affecting skeletal muscle and brain arising from mutations of voltage-gated ion channels and fast ligand-gated ion channels expressed at the surface membrane. Derangements in channel function alter the electrical excitability of the cell and thereby increase susceptibility to transient symptomatic attacks including myasthenia, periodic paralysis, myotonic stiffness, seizures, headache, dyskinesia, or episodic ataxia. Although these disorders are rare, they stand out as exemplary cases for which disease pathogenesis can be traced from a point mutation to altered protein function, to altered cellular activity, and to clinical phenotype. The study of these disorders has provided insights on channel structure-function relations, the physiological roles of ion channels, and rational approaches toward therapeutic intervention for many disorders of cellular excitability.