Identification of genes that modify ataxin-1-induced neurodegeneration

Altered transcription in yeast expressing expanded polyglutamine

Expanded polyglutamine tracts are responsible for at least eight fatal neurodegenerative diseases. In mouse models, proteins with expanded polyglutamine cause transcriptional dysregulation before onset of symptoms, suggesting that this dysregulation may be an early event in polyglutamine pathogenesis. Transcriptional dysregulation and cellular toxicity may be due to interaction between expanded polyglutamine and the histone acetyltransferase CREB-binding protein. To determine whether polyglutamine-mediated transcriptional dysregulation occurs in yeast, we expressed polyglutamine tracts in Saccharomyces cerevisiae. Gene expression profiles were determined for strains expressing either a cytoplasmic or nuclear protein with 23 or 75 glutamines, and these profiles were compared to existing profiles of mutant yeast strains. Transcriptional induction of genes encoding chaperones and heat-shock factors was caused by expression of expanded polyglutamine in either the nucleus or cytoplasm. Transcriptional repression was most prominent in yeast expressing nuclear expanded polyglutamine and was similar to profiles of yeast strains deleted for components of the histone acetyltransferase complex Spt/Ada/Gcn5 acetyltransferase (SAGA). The promoter from one affected gene (PHO84) was repressed by expanded polyglutamine in a reporter gene assay, and this effect was mitigated by the histone deacetylase inhibitor, Trichostatin A. Consistent with an effect on SAGA, nuclear expanded polyglutamine enhanced the toxicity of a deletion in the SAGA component SPT3. Thus, an early component of polyglutamine toxicity, transcriptional dysregulation, is conserved in yeast and is pharmacologically antagonized by a histone deacetylase inhibitor. These results suggest a therapeutic approach for treatment of polyglutamine diseases and provide the potential for yeast-based screens for agents that reverse polyglutamine toxicity.

Of flies and men--studying human disease in Drosophila

During the past year, the Drosophila genome has been sequenced. More than 60% of genes implicated in human disease have Drosophila orthologues. Developments in RNA-mediated interference and homologous recombination have made 'reverse genetics' feasible in Drosophila. Conventional Drosophila genetics is being used increasingly to place human disease genes of unknown function in the context of functional pathways.

Histone deacetylase inhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila

Proteins with expanded polyglutamine repeats cause Huntington's disease and other neurodegenerative diseases. Transcriptional dysregulation and loss of function of transcriptional co-activator proteins have been implicated in the pathogenesis of these diseases. Huntington's disease is caused by expansion of a repeated sequence of the amino acid glutamine in the abnormal protein huntingtin (Htt). Here we show that the polyglutamine-containing domain of Htt, Htt exon 1 protein (Httex1p), directly binds the acetyltransferase domains of two distinct proteins: CREB-binding protein (CBP) and p300/CBP-associated factor (P/CAF). In cell-free assays, Httex1p also inhibits the acetyltransferase activity of at least three enzymes: p300, P/CAF and CBP. Expression of Httex1p in cultured cells reduces the level of the acetylated histones H3 and H4, and this reduction can be reversed by administering inhibitors of histone deacetylase (HDAC). In vivo, HDAC inhibitors arrest ongoing progressive neuronal degeneration induced by polyglutamine repeat expansion, and they reduce lethality in two Drosophila models of polyglutamine disease. These findings raise the possibility that therapy with HDAC inhibitors may slow or prevent the progressive neurodegeneration seen in Huntington's disease and other polyglutamine-repeat diseases, even after the onset of symptoms.

Triplet repeats, RNA secondary structure and toxic gain-of-function models for pathogenesis

Ten years after the discovery of human diseases caused by trinucleotide repeat expansions, searching for mechanistic links between gene mutation and pathological phenotype remains a fundamental and unsolved issue. Evidence accumulated so far indicates that the pathogenesis of repeat disorders is complex and multi-factorial. Diseases caused by CAG expansions coding for polyglutamine tracts have been extensively studied, and in most cases a toxic gain-of-function of the mutant protein was demonstrated. Most recently, tracking the effects of repeats along the pathway of gene expression is providing additional clues to understand how a triplet repeat expansion can cause disease. Expanded repeats form DNA secondary structures that confer genetic instability, and most likely contribute to alter the local chromatin configuration leading to transcriptional silencing. At the level of RNA, the expanded repeat may either interfere with processing of the primary transcript, resulting in deficit of the corresponding protein, or interact with RNA-binding proteins altering their normal activity. The latter mechanism, termed RNA gain-of-function, has no precedents in human genetics. Recent evidence suggests that expanded RNAs and associated RNA-binding proteins are potential contributors to the pathogenesis of several triplet repeat diseases.

Phenotypic effects of expanded ataxin-1 polyglutamines with interruptions in vitro

Spinocerebellar ataxia type 1 is a neurodegenerative disease caused by expansion of an uninterrupted glutamine repeat in ataxin-1 protein. Protein aggregation and immunoreactivity to 1C2 monoclonal antibody are two distinct pathognomonic features of expanded ataxin-1, as well as of other polyglutamine disorders. Rare cases of non-affected elderly subjects carrying expanded ataxin-1 alleles were found in random population. However, in these alleles the glutamine stretch was interrupted by histidines. Due to lack of phenotype, these alleles should be considered "normal". Most importantly, occurrence of these unusual alleles provides a unique opportunity to investigate which molecular properties of expanded ataxin-1 are not coupled to polyglutamine pathogenesis. Towards this goal, we compared in vitro the immunoreactivity to 1C2 antibody and the ability to form aggregates of interrupted and uninterrupted alleles. Immunoblotting showed that expanded-interrupted ataxin-1 had an affinity to 1C2 resembling that of normal ataxin-1. On the contrary, filter assay showed that aggregation rate of expanded-interrupted ataxin-1 resembles that of expanded-uninterrupted ataxin-1. These observations indicate that affinity for 1C2 does not directly correlate with self-aggregation of ataxin-1. Moreover, self-aggregation is not directly affected by histidine interruptions. In conclusion, these results support the hypothesis that mechanisms underlying neuronal degeneration are triggered by protein misfolding rather than by protein aggregation.

Polyglutamine expansion neurodegenerative disease

Kennedy's disease was the first of eight neurodegenerative disorders found to be caused by expanded polyglutamine repeats. Each of these disorders is likely caused by a toxic gain of function in the disease gene product, often associated with inclusions of mutant protein in susceptible neurons. The mechanism of toxicity may involve sequestration and depletion of a polyglutamine-containing protein that is important to neuronal survival, such as CREB-binding protein. Recent insights into the biochemistry and cellular pathology of the polyglutamine expansion neurodegenerative diseases provide the opportunity for systematic drug screens and a rational approach to effective therapy.

PQBP-1 (Np/PQ): a polyglutamine tract-binding and nuclear inclusion-forming protein

Polyglutamine(Q) tract binding protein-1 (PQBP-1) was isolated on the basis of its interaction with polyglutamine tracts and localizes predominantly to the nucleus where it suppresses transcriptional activation by a neuron-specific transcription factor, Brn-2. Its C-terminal domain is highly conserved and binds to a component of the spliceosome. PQBP-1 possesses unique repetitive sequences that may fold as polar zippers. Interestingly, PQBP-1 also forms nuclear inclusion bodies, which are similar to those nucleated by the protein products of polyglutamine disease genes. Furthermore, because PQBP-1 is highly conserved in simple animal metazoans and plants (Caenorhabditis elegans and Arabidopsis), it may perform a basic function in cells. By the same token, disruption of the basic function could be critical to the disease process. Collectively, PQBP-1 might be a candidate molecule involved in the pathology of polyglutamine diseases. In this review, we discuss the structure and function of the PQBP-1 protein, the relevance of its aggregation and possible roles in normal and disease processes.

Analysis of heat shock transcription factor for suppression of polyglutamine toxicity

Individually over-expressed chaperones can interfere with cytotoxicity and aggregation of polyglutamine proteins in disease models. As chaperones cooperate, the analysis of suppression or reversal of polyglutamine pathology may require ways to up-regulate multiple chaperone coding genes. This condition might be achieved by exogenous expression of de-repressed forms of heat shock transcription factor 1 (HSF1), which mediates induction of several genes coding cytosolic and nuclear chaperones. Here we present the rationale behind this possible approach and the caveats, and employ a non-neuronal cell system to test whether Ataxin-1 aggregation can be modulated by de-repressed HSF1 mutants through augmented expression of chaperone coding genes. In our experiments, HSF1 mutants have induced heat shock protein 70 and Human DnaJ (HDJ)-1 to intermediate levels. Cells expressing such mutants also showed partial reduction of Ataxin-1 [31Q] aggregation. A consolidated positive outcome of these tests in cellular models would encourage experiments in transgenic mice and prospects for pharmacological modulation of HSF1 activity or delivery.

Transgenic invertebrate models of age-associated neurodegenerative diseases

Transgenic Drosophila melanogaster and Caenorhabditis elegans strains have been engineered to express human proteins associated with neurodegenerative diseases. These model systems include transgenic animals expressing beta-amyloid peptide (Alzheimer's disease), polyglutamine repeat proteins (Huntington's disease, Spinocerebellar ataxia), and alpha-synuclein (Parkinson's disease). In most of these invertebrate models, some aspects of the human diseases are reproduced. Although expression of all these proteins in transgenic mice has been instructive, the invertebrate models offer experimental advantages (e.g. forward genetic screens) that can potentially address some of the outstanding questions regarding the cellular processes underlying these diseases. This review considers what has been learned from these invertebrate models, and speculates what further insight may be gained from them.

Polyglutamine-expanded ataxin-7 antagonizes CRX function and induces cone-rod dystrophy in a mouse model of SCA7

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant disorder caused by a CAG repeat expansion. To determine the mechanism of neurotoxicity, we produced transgenic mice and observed a cone-rod dystrophy. Nuclear inclusions were present, suggesting that the disease pathway involves the nucleus. When yeast two-hybrid assays indicated that cone-rod homeobox protein (CRX) interacts with ataxin-7, we performed further studies to assess this interaction. We found that ataxin-7 and CRX colocalize and coimmunoprecipitate. We observed that polyglutamine-expanded ataxin-7 can dramatically suppress CRX transactivation. In SCA7 transgenic mice, electrophoretic mobility shift assays indicated reduced CRX binding activity, while RT-PCR analysis detected reductions in CRX-regulated genes. Our results suggest that CRX transcription interference accounts for the retinal degeneration in SCA7 and thus may provide an explanation for how cell-type specificity is achieved in this polyglutamine repeat disease.

Tauopathy in Drosophila: neurodegeneration without neurofibrillary tangles

The microtubule-binding protein tau has been implicated in the pathogenesis of Alzheimer's disease and related disorders. However, the mechanisms underlying tau-mediated neurotoxicity remain unclear. We created a genetic model of tau-related neurodegenerative disease by expressing wild-type and mutant forms of human tau in the fruit fly Drosophila melanogaster. Transgenic flies showed key features of the human disorders: adult onset, progressive neurodegeneration, early death, enhanced toxicity of mutant tau, accumulation of abnormal tau, and relative anatomic selectivity. However, neurodegeneration occurred without the neurofibrillary tangle formation that is seen in human disease and some rodent tauopathy models. This fly model may allow a genetic analysis of the cellular mechanisms underlying tau neurotoxicity.

Impairment of the ubiquitin-proteasome system by protein aggregation

Intracellular deposition of aggregated and ubiquitylated proteins is a prominent cytopathological feature of most neurodegenerative disorders. Whether protein aggregates themselves are pathogenic or are the consequence of an underlying molecular lesion is unclear. Here, we report that protein aggregation directly impaired the function of the ubiquitin-proteasome system. Transient expression of two unrelated aggregation-prone proteins, a huntingtin fragment containing a pathogenic polyglutamine repeat and a folding mutant of cystic fibrosis transmembrane conductance regulator, caused nearly complete inhibition of the ubiquitin-proteasome system. Because of the central role of ubiquitin-dependent proteolysis in regulating fundamental cellular events such as cell division and apoptosis, our data suggest a potential mechanism linking protein aggregation to cellular disregulation and cell death.

A ubiquitin-interacting motif conserved in components of the proteasomal and lysosomal protein degradation systems

Ubiquitination generally serves as a signal for targeting cytoplasmic and nuclear proteins to the proteasome for subsequent degradation. Recently, evidence has accumulated indicating that ubiquitination also plays an important role in targeting integral membrane proteins for degradation by the lytic vacuole or the lysosome. This article describes a conserved protein motif, based on a sequence of the proteasomal component Rpn10/S5a, that is known to recognize ubiquitin. The presence of this motif in Eps15, Epsin and HRS, proteins involved in ligand-activated receptor endocytosis and degradation, suggest a more general role in ubiquitin recognition.

Therapeutic opportunities in polyglutamine disease

Polyglutamine diseases comprise a class of familial neurodegenerative disorders caused by expression of proteins containing expanded polyglutamine tracts. Great progress has been made in elucidating the molecular mechanisms contributing to polyglutamine pathology, and in identifying potential drug targets. Although much remains to be learned, these advances provide an opportunity for rational approaches to target-based drug discovery.

Damage control--a possible non-proteolytic role for ubiquitin in limiting neurodegeneration

Ubiquitin can be detected in the neuronal and glial inclusions that are the diagnostic hallmarks of a number of human neurodegenerative diseases. It has been assumed that the presence of ubiquitin signifies the failed attempt of the cell to remove abnormal protein structures, which have been allowed to aggregate. The burden of abnormal protein arising from genetic mutations or cumulative oxidative damage might in the course of time overwhelm the ubiquitin-proteasome pathway (whose responsibility it is to eliminate misfolded or damaged proteins). However, ubiquitin may still serve a protective purpose distinct from its role in proteolysis. The physical properties of ubiquitin are such that a surface coating of ubiquitin should preclude further growth of the aggregate, prevent non-productive interactions, and conceal the contents from detection mechanisms that might ultimately kill the cell. This 'nonstick coating' hypothesis makes predictions about the nature of the conjugated ubiquitin and the consequences of removing it.