Histone deacetylase inhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila

Huntington's disease: a decade beyond gene discovery

Huntington's disease is a dominantly inherited neurodegenerative disease that causes a progressive movement disorder, cognitive decline, and varying degrees of psychiatric dysfunction. The identification of the mutant gene in 1993 paved the way for a decade of basic research. The resultant advances in our understanding of the pathogenesis of the disorder are moving us toward rational therapies to slow the progression and delay the onset of the illness.

Aberrant histone acetylation, altered transcription, and retinal degeneration in a Drosophila model of polyglutamine disease are rescued by CREB-binding protein

Sequestration of the transcriptional coactivator CREB-binding protein (CBP), a histone acetyltransferase, has been implicated in the pathogenesis of polyglutamine expansion neurodegenerative disease. We used a Drosophila model to demonstrate that polyglutamine-induced neurodegeneration is accompanied by a defect in histone acetylation and a substantial alteration in the transcription profile. Furthermore, we demonstrate complete functional and morphological rescue by up-regulation of endogenous Drosophila CBP (dCBP). Rescue of the degenerative phenotype is associated with eradication of polyglutamine aggregates, recovery of histone acetylation, and normalization of the transcription profile. These findings suggest that histone acetylation is an early target of polyglutamine toxicity and indicate that transcriptional dysregulation is an important part of the pathogenesis of polyglutamine-induced neurodegeneration.

Leuprorelin rescues polyglutamine-dependent phenotypes in a transgenic mouse model of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset motor neuron disease that affects males. It is caused by the expansion of a polyglutamine (polyQ) tract in androgen receptors. Female carriers are usually asymptomatic. No specific treatment has been established. Our transgenic mouse model carrying a full-length human androgen receptor with expanded polyQ has considerable gender-related motor impairment. This phenotype was abrogated by castration, which prevented nuclear translocation of mutant androgen receptors. We examined the effect of androgen-blockade drugs on our mouse model. Leuprorelin, a lutenizing hormone-releasing hormone (LHRH) agonist that reduces testosterone release from the testis, rescued motor dysfunction and nuclear accumulation of mutant androgen receptors in male transgenic mice. Moreover, leuprorelin treatment reversed the behavioral and histopathological phenotypes that were once caused by transient increases in serum testosterone. Flutamide, an androgen antagonist promoting nuclear translocation of androgen receptors, yielded no therapeutic effect. Leuprorelin thus seems to be a promising candidate for the treatment of SBMA.

A cell-based assay for aggregation inhibitors as therapeutics of polyglutamine-repeat disease and validation in Drosophila

The formation of polyglutamine-containing aggregates and inclusions are hallmarks of pathogenesis in Huntington's disease that can be recapitulated in model systems. Although the contribution of inclusions to pathogenesis is unclear, cell-based assays can be used to screen for chemical compounds that affect aggregation and may provide therapeutic benefit. We have developed inducible PC12 cell-culture models to screen for loss of visible aggregates. To test the validity of this approach, compounds that inhibit aggregation in the PC12 cell-based screen were tested in a Drosophila model of polyglutamine-repeat disease. The disruption of aggregation in PC12 cells strongly correlates with suppression of neuronal degeneration in Drosophila. Thus, the engineered PC12 cells coupled with the Drosophila model provide a rapid and effective method to screen and validate compounds.

Polyglutamine expansion induces a protein-damaging stress connecting heat shock protein 70 to the JNK pathway

Polyglutamine diseases, including Huntington's disease, designate a group of nine neurodegenerative disorders characterized by the presence of a toxic polyglutamine expansion in specific target proteins. Using cell and mouse models, we have shown that expanded polyglutamine led to activation of the stress kinase JNK and the transcription factor AP-1, which are implicated in neuronal death. Polyglutamine expansion-induced stress shared common features with protein-damaging stress such as heat shock, because activation of JNK involved inhibition of JNK phosphatase activities. Indeed, expanded polyglutamine impaired the solubility of the dual-specificity JNK phosphatase M3/6. Aggregation of M3/6 by polyglutamine expansion appeared to be indirect, because M3/6 was not recruited into polyglutamine inclusions. The heat shock protein HSP70, which is known to inhibit JNK during the heat shock response, suppressed polyglutamine-mediated aggregation of M3/6 and activation of JNK. Interestingly, levels of HSP70 were down-regulated by polyglutamine expansion. We suggest that reduction of HSP70 by expanded polyglutamine is implicated in aggregation and inhibition of M3/6 and in activation of JNK and AP-1.

Transcriptional abnormalities in Huntington disease

Huntington disease (HD) is caused by a CAG repeat expansion that is translated into an abnormally long polyglutamine (polyQ) tract in the huntingtin protein. The precise mechanisms leading to neurodegeneration in HD have not been fully elucidated, but alterations in gene transcription could well be involved because the activities of several nuclear proteins are compromised by the polyQ mutation. Recent microarray studies also show relevant changes in gene expression profiles in HD models, providing useful information on the potential consequences of disrupted transcriptional pathways in HD.

[Huntington chorea in Drosophila and mice: toward new therapeutic steps]

Huntington's disease is an hereditary dominant neurodegenerative disorder clinically characterised by progressive dyskinesia, cognitive decline and psychiatric disturbances. One decade after the identification of the gene whose mutation is responsible for the disease, this pathology remains incurable. However, major insights into early cellular and molecular basis of Huntington's disease have arisen from transgenic models. Transcriptional dysregulation, abnormal degradation of misfolded proteins as well as excitotoxic processes and mitochondrial dysfunction are involved in Huntington's disease. The present review discusses the recent insights gained from mouse and Drosophila models towards the understanding of pathogenesis and the development of new therapeutic tools.

Mutant huntingtin increases nuclear corepressor function and enhances ligand-dependent nuclear hormone receptor activation

There is increasing evidence that transcriptional dysregulation is important in Huntington's disease pathogenesis. The transcriptional protein, nuclear corepressor (NCoR), acts to repress transcription of nuclear hormone receptors, such as the thyroid hormone receptor (TR) and retinoic acid receptor, in the absence of their appropriate ligand. NCoR has been shown to bind to the mutated huntingtin protein in a yeast two-hybrid screen. This aberrant interaction may have profound effects on both the function of the NCoR protein and on its control of nuclear hormone receptor-mediated transcription. To test this hypothesis, reporter gene assays were performed in inducible PC12 cell lines expressing exon 1 of the human huntingtin protein (Htt) with either a 25 or 103 polyglutamine (Q) repeat. Expression of mutant 103Q protein appears to enhance the ability of NCoR to repress TR-mediated transcription in the absence of ligand. Western analyses indicated that the expression of the mutant 103Q Htt protein did not change the endogenous NCoR levels in the HD103Q PC12 cells when compared to uninduced cells. Interestingly, using GST pull-down assays we found that a mutant Htt exon 1 construct with 53 polyglutamine (HD53Q) did not bind to NCoR in a polyglutamine-dependent fashion. These findings suggest that an aberrant NCoR-Htt interaction does not exist in vitro. Expression of the mutant 103Q protein was also found to enhance ligand-dependent activation of TR and retinoic acid receptor. In vitro binding data shows that TR binds to HD53Q in the presence of ligand. Taken together these data suggest that Htt may function as a transcriptional coactivator of nuclear hormone receptors.

Domain-selective small-molecule inhibitor of histone deacetylase 6 (HDAC6)-mediated tubulin deacetylation

Protein acetylation, especially histone acetylation, is the subject of both research and clinical investigation. At least four small-molecule histone deacetylase inhibitors are currently in clinical trials for the treatment of cancer. These and other inhibitors also affect microtubule acetylation. A multidimensional, chemical genetic screen of 7,392 small molecules was used to discover "tubacin," which inhibits alpha-tubulin deacetylation in mammalian cells. Tubacin does not affect the level of histone acetylation, gene-expression patterns, or cell-cycle progression. We provide evidence that class II histone deacetylase 6 (HDAC6) is the intracellular target of tubacin. Only one of the two catalytic domains of HDAC6 possesses tubulin deacetylase activity, and only this domain is bound by tubacin. Tubacin treatment did not affect the stability of microtubules but did decrease cell motility. HDAC6 overexpression disrupted the localization of p58, a protein that mediates binding of Golgi elements to microtubules. Our results highlight the role of alpha-tubulin acetylation in mediating the localization of microtubule-associated proteins. They also suggest that small molecules that selectively inhibit HDAC6-mediated alpha-tubulin deacetylation, a first example of which is tubacin, might have therapeutic applications as antimetastatic and antiangiogenic agents.

Nuclear localization of a non-caspase truncation product of atrophin-1, with an expanded polyglutamine repeat, increases cellular toxicity

Dentatorubral and pallidoluysian atrophy (DRPLA) is an autosomal dominant neurodegenerative disorder similar to Huntington's disease, with clinical manifestations including chorea, incoordination, ataxia, and dementia. It is caused by an expansion of a CAG trinucleotide repeat encoding polyglutamine in the atrophin-1 gene. Both patients and DRPLA transgenic mice have nuclear accumulation of atrophin-1, especially an approximately 120-kDa fragment, which appears to represent a cleavage product. We now show that this is an N-terminal fragment that does not correspond to the previously described caspase-3 fragment, or any other known caspase cleavage product. The atrophin-1 sequence contains a putative nuclear localization signal in the N terminus of the protein and a putative nuclear export signal in the C terminus. We have tested the hypothesis that endogenous localization signals are functional in atrophin-1, and that nuclear localization and proteolytic cleavage contribute to atrophin-1 cell toxicity. In transient cell transfection experiments using a neuroblastoma cell line, full-length atrophin-1 with 26 (normal) or 65 (expanded) glutamines localized to both nucleus and cytoplasm, with no significant difference in toxicity between the normal and mutant proteins. A construct with 65 glutamine repeats encoding an N-terminal fragment (which removes an NES) of atrophin-1 similar in size to the truncation product in DRPLA patient tissue, showed increased nuclear labeling, and an increase in cellular toxicity, compared with a similar fragment with 26 glutamines. Full-length atrophin-1 with 65 polyglutamine repeats and mutations inactivating the NES also yielded increased nuclear localization and increased toxicity. These data suggest that truncation enhances cellular toxicity of the mutant protein, and that the NES is a relevant region deleted during truncation. Furthermore, mutating the NLS in the truncated protein shifted atrophin-1 more to the cytoplasm and eliminated the increased toxicity, consistent with the idea that nuclear localization enhances toxicity. In none of the experiments were inclusions visible in the nucleus or cytoplasm suggesting that inclusion formation is unrelated to cell death. These data indicate that truncation of atrophin-1 may alter its ability to shuttle between the nucleus and cytoplasm, leading to abnormal nuclear interactions and cell toxicity.

Histone acetyltransferases and deacetylases in the control of cell proliferation and differentiation

Histone acetylation and deacetylation are chromatin-modifying processes that have fundamental importance for transcriptional regulation. Transcriptionally active chromatin regions show a high degree of histone acetylation, whereas deacetylation events are generally linked to transcriptional silencing. Many of the acetylating and deacetylating enzymes were originally identified as transcriptional coactivators or repressors. Their histone-modifying enzymatic activity was discovered more recently, opening up a whole new area of research. Histone acetyltransferases such as CREB-binding protein (CBP) and PCAF are involved in processes as diverse as promoting cell cycle progression and regulating differentiation. A controlled balance between histone acetylation and deacetylation seems to be essential for normal cell growth. Both histone acetyltransferases and deacetylases are involved in the development of diseases, including neurodegenerative disorders and cancer. Treatments that target these enzymes are already under clinical investigation.

Histone deacetylase inhibitors prevent oxidative neuronal death independent of expanded polyglutamine repeats via an Sp1-dependent pathway

Oxidative stress is believed to be an important mediator of neurodegeneration. However, the transcriptional pathways induced in neurons by oxidative stress that activate protective gene responses have yet to be fully delineated. We report that the transcription factor Sp1 is acetylated in response to oxidative stress in neurons. Histone deacetylase (HDAC) inhibitors augment Sp1 acetylation, Sp1 DNA binding, and Sp1-dependent gene expression and confer resistance to oxidative stress-induced death in vitro and in vivo. Sp1 activation is necessary for the protective effects of HDAC inhibitors. Together, these results demonstrate that HDAC inhibitors inhibit oxidative death independent of polyglutamine expansions by activating an Sp1-dependent adaptive response.

Collaborative spirit of histone deacetylases in regulating chromatin structure and gene expression

The flexible N-terminal tails of core histones are subject to dynamic, reversible lysine acetylation. At least 10 histone deacetylases have been identified in Saccharomyces cerevisiae and 19 in humans. Emerging themes regarding the function and regulation of these enzymes include the following: targeted and non-targeted chromatin deacetylation; their collaboration with each other and with other chromatin regulators to promote transcriptional repression and silencing; deacetylation of transcription factors and other non-histone proteins; and regulation by subcellular compartmentalization and subunit association. Histone deacetylases are important targets for drugs with potential therapeutic value in the treatment of cancer, neurodegenerative disorders, cardiac hypertrophy and other human diseases.

blue cheese mutations define a novel, conserved gene involved in progressive neural degeneration

A common feature of many human neurodegenerative diseases is the accumulation of insoluble ubiquitin-containing protein aggregates in the CNS. Although Drosophila has been helpful in understanding several human neurodegenerative disorders, a loss-of-function mutation has not been identified that leads to insoluble CNS protein aggregates. The study of Drosophila mutations may identify unique components that are associated with human degenerative diseases. The Drosophila blue cheese (bchs) gene defines such a novel degenerative pathway. bchs mutants have a reduced adult life span with the age-dependent formation of protein aggregates throughout the neuropil of the CNS. These inclusions contain insoluble ubiquitinated proteins and amyloid precursor-like protein. Progressive loss of CNS size and morphology along with extensive neuronal apoptosis occurs in aged bchs mutants. BCHS protein is widely expressed in the cytoplasm of CNS neurons and is present over the entire length of axonal projections. BCHS is nearly 3500 amino acids in size, with the last 1000 amino acids consisting of three functional protein motifs implicated in vesicle transport and protein processing. This region along with previously unidentified proteins encoded in the human, mouse, and nematode genomes shows striking homology along the full length of the BCHS protein. The high degree of conservation between Drosophila and human bchs suggests that study of the functional pathway of BCHS and associated mutant phenotype may provide useful insights into human neurodegenerative disorders.

Cdc42-interacting protein 4 binds to huntingtin: neuropathologic and biological evidence for a role in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease caused by polyglutamine (polyQ) expansion in the protein huntingtin (htt). Pathogenesis in HD seems to involve the formation of neuronal intranuclear inclusions and the abnormal regulation of transcription and signal transduction. To identify previously uncharacterized htt-interacting proteins in a simple model system, we used a yeast two-hybrid screen with a Caenorhabditis elegans activation domain library. We found a predicted SH3 domain protein (K08E3.3b) that interacts with N-terminal htt in two-hybrid tests. A human homolog of K08E3.3b is the Cdc42-interacting protein 4 (CIP4), a protein involved in Cdc42 and Wiskott-Aldrich syndrome protein-dependent signal transduction. CIP4 interacted in vitro with full-length htt from lymphoblastoid cells. Neuronal CIP4 immunoreactivity increased with neuropathological severity in the neostriatum of HD patients and partially colocalized to ubiquitin-positive aggregates. Marked CIP4 overexpression also was observed in Western blot from human HD brain striatum. The overexpression of CIP4 induced the death of striatal neurons. Our data suggest that CIP4 accumulation and cellular toxicity may have a role in HD pathogenesis.

Learning, memory, and transcription factors

Cognitive disorders in children have traditionally been described in terms of clinical phenotypes or syndromes, chromosomal lesions, metabolic disorders, or neuropathology. Relatively little is known about how these disorders affect the chemical reactions involved in learning and memory. Experiments in fruit flies, snails, and mice have revealed some highly conserved pathways that are involved in learning, memory, and synaptic plasticity, which is the primary substrate for memory storage. These can be divided into short-term memory storage through local changes in synapses, and long-term storage mediated by activation of transcription to translate new proteins that modify synaptic function. This review summarizes evidence that disruptions in these pathways are involved in human cognitive disorders, including neurofibromatosis type I, Coffin-Lowry syndrome, Rubinstein-Taybi syndrome, Rett syndrome, tuberous sclerosis-2, Down syndrome, X-linked alpha-thalassemia/mental retardation, cretinism, Huntington disease, and lead poisoning.

Dying for a cause: invertebrate genetics takes on human neurodegeneration

If invertebrate neurons are injured by hostile environments or aberrant proteins they die much like human neurons, indicating that the powerful advantages of invertebrate molecular genetics might be successfully used for testing specific hypotheses about human neurological diseases, for drug discovery and for non-biased screens for suppressors and enhancers of neurodegeneration. Recent molecular dissection of the genetic requirements for hypoxia, excitotoxicity and death in models of Alzheimer disease, polyglutamine-expansion disorders, Parkinson disease and more, is providing mechanistic insights into neurotoxicity and suggesting new therapeutic interventions. An emerging theme is that neuronal crises of distinct origins might converge to disrupt common cellular functions, such as protein folding and turnover.

Suberoylanilide hydroxamic acid, a histone deacetylase inhibitor, ameliorates motor deficits in a mouse model of Huntington's disease

Huntington's disease (HD) is an inherited, progressive neurological disorder that is caused by a CAG/polyglutamine repeat expansion and for which there is no effective therapy. Recent evidence indicates that transcriptional dysregulation may contribute to the molecular pathogenesis of this disease. Supporting this view, administration of histone deacetylase (HDAC) inhibitors has been shown to rescue lethality and photoreceptor neurodegeneration in a Drosophila model of polyglutamine disease. To further explore the therapeutic potential of HDAC inhibitors, we have conducted preclinical trials with suberoylanilide hydroxamic acid (SAHA), a potent HDAC inhibitor, in the R6/2 HD mouse model. We show that SAHA crosses the blood-brain barrier and increases histone acetylation in the brain. We found that SAHA could be administered orally in drinking water when complexed with cyclodextrins. SAHA dramatically improved the motor impairment in R6/2 mice, clearly validating the pursuit of this class of compounds as HD therapeutics.

Dysfunction of wild-type huntingtin in Huntington disease

Huntingtin is the protein involved in Huntington disease (HD), an inherited neurodegenerative disease. Research activities have focused on the abnormal functions of mutant huntingtin. However, recent results indicate that wild-type huntingtin has important activities in brain neurons, suggesting that loss of these activities may play a role in HD.

Trinucleotide repeat disease. The androgen receptor in spinal and bulbar muscular atrophy

It has been more than 10 years since the discovery that the expansion of a simple CAG trinucleotide repeat within the coding region of the androgen receptor gene leads to the motor neuronopathy spinal and bulbar muscular atrophy (SBMA). A flurry of investigation into this and the other, more recently discovered, polyglutamine diseases has led to an understanding of many aspects of the molecular pathogenesis of this family of diseases. A characteristics pathological feature of the polyglutamine diseases is the occurrence in affected neurons of ubiquitinated aggregates; such aggregates also contain, among others, proteins involved in the folding and degradation of the mutant proteins. Aggregates themselves are likely not directly cytotoxic, but rather mark the accumulation of all or part of the mutant protein. Furthermore, aggregation occurs because of the inefficient clearance of the mutant protein by the ubiquitin-proteasome pathway for protein degradation. These findings are common to the polyglutamine diseases and reflect the general problem of folding/degrading expanded polyglutamines. In SBMA, the altered metabolism of the androgen receptor is ligand dependent. How the accumulation of the mutant protein causes neuronal dysfunction and disease is not well understood, but several cellular processes have been implicated. Although these findings provide insight into the toxic function of the expanded polyglutamine protein, additional investigations have led to the finding that intrinsic AR transactivational function is somewhat diminished in the presence of the expanded polyglutamine; this likely leads to the partial androgen insensitivity that characterizes patients with SBMA. The recent development of useful animal and cell models of SBMA will lead to increased understanding of disease pathogenesis, as well as to the development of new and better therapeutic strategies.

Evidence for an epigenetic mechanism by which Hsp90 acts as a capacitor for morphological evolution

Morphological alterations have been shown to occur in Drosophila melanogaster when function of Hsp90 (heat shock 90-kDa protein 1alpha, encoded by Hsp83) is compromised during development. Genetic selection maintains the altered phenotypes in subsequent generations. Recent experiments have shown, however, that phenotypic variation still occurs in nearly isogenic recombinant inbred strains of Arabidopsis thaliana. Using a sensitized isogenic D. melanogaster strain, iso-Kr(If-1), we confirm this finding and present evidence supporting an epigenetic mechanism for Hsp90's capacitor function, whereby reduced activity of Hsp90 induces a heritably altered chromatin state. The altered chromatin state is evidenced by ectopic expression of the morphogen wingless in eye imaginal discs and a corresponding abnormal eye phenotype, both of which are epigenetically heritable in subsequent generations, even when function of Hsp90 is restored. Mutations in nine different genes of the trithorax group that encode chromatin-remodeling proteins also induce the abnormal phenotype. These findings suggest that Hsp90 acts as a capacitor for morphological evolution through epigenetic and genetic mechanisms.

Techniques: fruit flies as models for neuropharmacological research

An unlikely animal model is gaining popularity in neuropharmacological research: the 2-mm fruit fly (Drosophila melanogaster). Drugs have been administered to adult flies in their food and, more recently, via gasses and injections. Pharmacological tools have introduced behavioral alterations in Drosophila reminiscent of human behavior, rescued flies from gene-alteration-triggered neuropathologies, and triggered gene silencing. Combined, these methods hold promise for significant neuropharmacological advancement.

Molecular chaperones as modulators of polyglutamine protein aggregation and toxicity

The formation of insoluble protein aggregates in neurons is a hallmark of neurodegenerative diseases caused by proteins with expanded polyglutamine (polyQ) repeats. However, the mechanistic relationship between polyQ aggregation and its toxic effects on neurons remains unclear. Two main hypotheses have been put forward for how polyQ expansions may cause cellular dysfunction. In one model neurotoxicity results from the ability of polyQ-expanded proteins to recruit other important cellular proteins with polyQ stretches into the aggregates. In the other model, aggregating polyQ proteins partially inhibit the ubiquitin-proteasome system for protein degradation. These two mechanisms are not exclusive but may act in combination. In general, protein misfolding and aggregation are prevented by the machinery of molecular chaperones. Some chaperones such as the members of the Hsp70 family also modulate polyQ aggregation and suppress its toxicity. These recent findings suggest that an imbalance between the neuronal chaperone capacity and the production of potentially dangerous polyQ proteins may trigger the onset of polyQ disease.

A Drosophila homolog of the polyglutamine disease gene SCA2 is a dosage-sensitive regulator of actin filament formation

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by the expansion of a CAG repeat encoding a polyglutamine tract in ataxin-2, the SCA2 gene product. The normal cellular function of ataxin-2 and the mechanism by which polyglutamine expansion of ataxin-2 causes neurodegeneration remain unknown. In this study we have used genetic and molecular approaches to investigate the function of a Drosophila homolog of the SCA2 gene (Datx2). Like human ataxin-2, Datx2 is found throughout development in a variety of tissue types and localizes to the cytoplasm. Mutations that reduce Datx2 activity or transgenic overexpression of Datx2 result in female sterility, aberrant sensory bristle morphology, loss or degeneration of tissues, and lethality. These phenotypes appear to result from actin filament formation defects occurring downstream of actin synthesis. Further studies demonstrate that Datx2 does not assemble with actin filaments, suggesting that the role of Datx2 in actin filament formation is indirect. These results indicate that Datx2 is a dosage-sensitive regulator of actin filament formation. Given that loss of cytoskeleton-dependent dendritic structure defines an early event in SCA2 pathogenesis, our findings suggest the possibility that dysregulation of actin cytoskeletal structure resulting from altered ataxin-2 activity is responsible for neurodegeneration in SCA2.

Ataxin-3 Is a Histone-binding Protein with Two Independent Transcriptional Corepressor Activities

The mechanisms of pathology for the family of polyglutamine disease proteins are unknown; however, recently it was shown that several of these proteins inhibit transcription suggesting that transcriptional repression may be a potential mechanism for pathology. In the present study we use cell transfections, in vitro binding, co-immunoprecipitations, and reporter assays to show that the polyglutamine disease protein, ataxin-3, interacts with the major histone acetyltransferases cAMP-response-element binding protein (CREB)-binding protein, p300, and p300/CREB-binding protein-associated factor and inhibits transcription by these coactivators. Importantly, endogenous ataxin-3 is co-immunoprecipitated with each of these coactivators in non-transfected cells. The C-terminal polyglutamine-containing domain of ataxin-3 inhibits coactivator-dependent transcription and is required for binding coactivators. The N-terminal domain of ataxin-3 inhibits histone acetylation by p300 in vitro and inhibits transcription in vivo. Histone binding and blocking access of coactivators to acetylation sites on histones appears to be the mechanism of inhibition. Together, our data provide a novel mechanism of transcriptional regulation by ataxin-3 that involves targeting histones, coactivators, and an independent mode of direct repression of transcription, and suggests that its physiological function and possibly pathological effects are linked to its interactions with these proteins.

Altered histone acetylation at glutamate receptor 2 and brain-derived neurotrophic factor genes is an early event triggered by status epilepticus

The mechanisms underlying seizure-induced changes in gene expression are unclear. Using a chromatin immunoprecipitation assay, we found that acetylation of histone H4 in rat hippocampal CA3 neurons was reduced at the glutamate receptor 2 (GluR2; GRIA2) glutamate receptor promoter but increased at brain-derived neurotrophic factor promoter P2 as soon as 3 hr after induction of status epilepticus by pilocarpine. This result indicates that status epilepticus rapidly activates different signal pathways to modulate histone acetylation in a promoter-specific manner. H4 deacetylation preceded seizure-induced GluR2 mRNA downregulation. The histone deacetylase inhibitor trichostatin A prevented and quickly reversed deacetylation of GluR2-associated histones. Trichostatin A also blunted seizure-induced downregulation of GluR2 mRNA in CA3. Thus, rapid gene-specific changes in histone acetylation patterns may be a key early step in the pathological processes triggered by status epilepticus.

Mouse and fly models of neurodegeneration

One of the most surprising discoveries of the past decade (at least in the field of neurodegeneration) was that protein misfolding underlies several seemingly disparate neurological diseases. Animal models were crucial to this discovery. In this article, we will discuss the CAG repeat diseases, the tauopathies and Parkinson disease, highlighting how mouse and fly models have contributed to our understanding of pathogenesis. In each case, we will stress what has been learned about the role of protein clearance and the questions that remain about how misfolded proteins acquire their toxicity.

The spinocerebellar ataxias: order emerges from chaos

In the past decade, the genetic etiologies accounting for most cases of adult-onset dominant cerebellar ataxia have been discovered. This group of disorders, generally referred to as the spinocerebellar ataxias (SCAs), can now be classified by a simple genetic nosology, essentially a sequential list in which each new SCA is given a number. However, recent advances in the elucidation of SCA pathogenesis provide the opportunity to subclassify the disorders into three discrete groups based on pathogenesis: 1) the polyglutamine disorders, SCAs 1, 2, 3, 7, and 17, which result from proteins with toxic stretches of polyglutamine; 2) the channelopathies, SCA6 and episodic ataxia types 1 and 2 (EA1 and EA2), which result from disruption of calcium or potassium channel function; and 3) the gene expression disorders, SCAs 8, 10, and 12, which result from repeat expansions outside of coding regions that may quantitatively alter gene expression. SCAs 4, 5, 9, 11, 13-16, 19, 21, and 22 are of unknown etiology, and may or may not fit into one of these three groups. At present, most diagnostic and therapeutic strategies apply equally to all of the SCAs. Therapy specific for individual diseases or types of diseases is a realistic goal in the foreseeable future.

Androgen-dependent neurodegeneration by polyglutamine-expanded human androgen receptor in Drosophila

Spinal and bulbar muscular atrophy (SBMA) is an X-linked, adult-onset, neurodegenerative disorder affecting only males and is caused by expanded polyglutamine (polyQ) stretches in the N-terminal A/B domain of human androgen receptor (hAR). Although no overt phenotype was detected in adult fly eye photoreceptor neurons expressing mutant hAR (polyQ 52), ingestion of androgen or its known antagonists caused marked neurodegeneration with nuclear localization and structural alteration of the hAR mutant. Ligand-independent toxicity was detected with a truncated polyQ-expanded A/B domain alone, which was attenuated with cytosolic trapping by coexpression of the unliganded hAR E/F ligand binding domain. Thus, our findings suggest that the full binding of androgen to the polyQ-expanded hAR mutants leads to structural alteration with nuclear translocation that eventually results in the onset of SBMA in male patients.

Testosterone reduction prevents phenotypic expression in a transgenic mouse model of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a polyglutamine disease caused by the expansion of a CAG repeat in the androgen receptor (AR) gene. We generated a transgenic mouse model carrying a full-length AR containing 97 CAGs. Three of the five lines showed progressive muscular atrophy and weakness as well as diffuse nuclear staining and nuclear inclusions consisting of the mutant AR. These phenotypes were markedly pronounced in male transgenic mice, and dramatically rescued by castration. Female transgenic mice showed only a few manifestations that markedly deteriorated with testosterone administration. Nuclear translocation of the mutant AR by testosterone contributed to the phenotypic difference with gender and the effects of hormonal interventions. These results suggest the therapeutic potential of hormonal intervention for SBMA.

Intranuclear inclusions, neuronal loss and CAG mosaicism in two patients with Machado-Joseph disease

The presence of neuronal intranuclear inclusions (NIIs) and neuronal mosaicism has been described in some autosomal dominant spinocerebellar ataxias (SCA), but their implication in neurodegenerative mechanisms still remains unclear.

OBJECTIVE

To investigate the correlation between neuronal loss and NIIs, and the size of CAG triplet expansion in selected areas of the CNS in two SCA3 patients.

MATERIAL AND METHODS

Postmortem neuropathological study was carried out, and the regional distribution of neuronal loss was compared with NIIs. CAG expansion was analysed by PCR amplification in the same regions.

RESULTS

Marked neuronal loss was seen in the anterior horn of the spinal cord, pontine nuclei and motor nuclei of the brain stem. Moderate neurone loss was found in the locus ceruleus, colliculus and substantia nigra. Loss of granule and Purkinje cells was found in the cerebellum, mainly in the vermis. NIIs were present in neurones of the involved nuclei of the anterior horn of the spinal cord, medulla oblongata and pons, but not in the locus ceruleus, substantia nigra and cerebellum. A few NIIs were found in the striatum. The number of CAG repeats was 27/70 in the first patient and 21/74 in the second patient. The variation of the expanded allele size among different cerebral areas was +/-1-3 CAG repeats.

CONCLUSION

The partial correlation between neuronal loss and NIIs suggests that other factors distinct from NII formation may be involved in the neuronal death. Moreover, the low degree of mosaicism between regions without neuronal loss and regions with marked neuronal loss points to the existence of selective cellular vulnerability to the genetic defect.

Function and regulation of CREB family transcription factors in the nervous system

CREB and its close relatives are now widely accepted as prototypical stimulus-inducible transcription factors. In many cell types, these factors function as effector molecules that bring about cellular changes in response to discrete sets of instructions. In neurons, a wide range of extracellular stimuli are capable of activating CREB family members, and CREB-dependent gene expression has been implicated in complex and diverse processes ranging from development to plasticity to disease. In this review, we focus on the current level of understanding of where, when, and how CREB family members function in the nervous system.

Recent advances in Huntington's disease: implications for experimental therapeutics

PURPOSE OF REVIEW

In this article we have set out to critically review recent advances in the basic and clinical understanding of Huntington's disease, with specific emphasis on those findings that are most relevant to the planning, design, and conduct of future clinical trials for this devastating disorder.

RECENT FINDINGS

The exact mechanisms underlying neuronal death in Huntington's disease remain unknown. Over the past 10 years, the leading models of neurodegeneration in the disease have involved mitochondrial dysfunction and subsequent excitotoxic injury, oxidative stress, and apoptosis. Recent studies have lent support to these models, but additional theories involving abnormalities of protein metabolism and transcriptional dysregulation have emerged as well. As progress is made toward clarifying the pathophysiological mechanisms leading to Huntington's disease, and new therapies are proposed, investigators have begun to develop improved outcome measures for potential use in future clinical trials aimed at slowing the progression of the disorder.

SUMMARY

Recent advances in the understanding of the molecular biology and pathophysiology of Huntington's disease have suggested new therapeutic strategies aimed at slowing progression or forestalling onset of this neurodegenerative disease. In preparation for future clinical trials, clinical studies have begun to provide more quantitative measures of disease onset and progression. This progress in both the basic science and clinical realms raises real hope for effective therapies in the near future.

Early mitochondrial calcium defects in Huntington's disease are a direct effect of polyglutamines

Huntington's disease (HD) is caused by an expansion of exonic CAG triplet repeats in the gene encoding huntingtin protein (Htt), but the mechanisms by which this mutant protein causes neurodegeneration remain unknown. Here we show that lymphoblast mitochondria from patients with HD have a lower membrane potential and depolarize at lower calcium loads than do mitochondria from controls. We found a similar defect in brain mitochondria from transgenic mice expressing full-length mutant huntingtin, and this defect preceded the onset of pathological or behavioral abnormalities by months. By electron microscopy, we identified N-terminal mutant huntingtin on neuronal mitochondrial membranes, and by incubating normal mitochondria with a fusion protein containing an abnormally long polyglutamine repeat, we reproduced the mitochondrial calcium defect seen in human patients and transgenic animals. Thus, mitochondrial calcium abnormalities occur early in HD pathogenesis and may be a direct effect of mutant huntingtin on the organelle.

Animal models of PD: pieces of the same puzzle?

Parkinson's disease (PD) is a common neurodegenerative disorder with no known cure. The etiology of PD is likely due, in part, to combinations of genetic susceptibilities and environmental factors. In rare familial cases, PD is due to genetic mutations. A number of new genetic and toxin models of PD and advances in older models are yielding important new information about the pathogenesis of PD. This has prompted us to critically review the current animal models for PD and discuss how these models may yield fresh insights into the pathogenesis of PD, as well as new therapeutic opportunities.

Live-cell imaging reveals divergent intracellular dynamics of polyglutamine disease proteins and supports a sequestration model of pathogenesis

Protein misfolding and aggregation are central features of the polyglutamine neurodegenerative disorders, but the dynamic properties of expanded polyglutamine proteins are poorly understood. Here, we use fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) with green fluorescent protein fusion proteins to study polyglutamine protein kinetics in living cells. Our results reveal markedly divergent mobility states for an expanded polyglutamine protein, ataxin-3, and establish that nuclear inclusions formed by this protein are aggregates. Additional studies of green fluorescent protein-tagged cAMP response element binding protein coexpressed with either of two mutant polyglutamine proteins, ataxin-3 and huntingtin, support a model of disease in which coaggregation of transcriptional components contributes to pathogenesis. Finally, studies of a third polyglutamine disease protein, ataxin-1, reveal unexpected heterogeneity in the dynamics of inclusions formed by different disease proteins, a finding which may help explain disease-specific elements of pathogenesis in these neurodegenerative disorders.

Spinocerebellar degenerations: an update

Over the past decade, the spinocerebellar degenerations have gone from a diverse group of loosely defined phenotypes to a family of diseases with many identifiable genotypes and the promise of gene-specific treatments. The evaluation of the spinocerebellar ataxias has been simplified, and the counseling of patients and families has been enhanced by the growing number of molecular diagnostic tests now available. Management strategies remain symptomatic and focused on rehabilitation, with empirical use of antioxidants based on research in other neurogenetic diseases.

Nuclear localization of N-terminal mutant huntingtin is cell cycle dependent

Unlike normal huntingtin (htt) which is located predominantly in the cytoplasm, mutant htt is also found in the nucleus of affected neurons. Nuclear localization of toxic polyglutamine-containing proteins has been postulated to be necessary for the pathogenesis of triplet repeat disorders. However, little is known about the mechanism by which mutant htt enters the nucleus. We have recently reported exclusive nuclear localization of exon 1 mutant htt in striatal primary neuronal cultures from the HD94 conditional mouse model of HD. This seemed to contradict the predominant cytoplasmic localization of N-terminal htt reported from transfection experiments and prompted us to hypothesize that subcellular localization of the toxic htt fragment might be favoured in nondividing cells. To test this, we analyzed subcellular localization of mutant htt in HD94 mixed neuron-glia cultures. Subconfluent glial cells showed cytoplasmic localization. However, nuclear localization was prompted by confluence, by serum withdrawal, and by treatment with cell cycle progression inhibitors such as Ara C or lactacystin. BrdU labelling experiments further confirmed that nuclear localization does not occur in dividing cells. Our findings offer an explanation for the neuronal specific toxicity of mutant htt despite its ubiquitous expression. Unraveling the mechanism of this cell cycle arrest-dependent entrance into the nucleus may offer new opportunities for therapeutic intervention.

Sp1 and TAFII130 transcriptional activity disrupted in early Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disease caused by expansion of a polyglutamine tract in the huntingtin protein. Transcriptional dysregulation has been implicated in HD pathogenesis. Here, we report that huntingtin interacts with the transcriptional activator Sp1 and coactivator TAFII130. Coexpression of Sp1 and TAFII130 in cultured striatal cells from wild-type and HD transgenic mice reverses the transcriptional inhibition of the dopamine D2 receptor gene caused by mutant huntingtin, as well as protects neurons from huntingtin-induced cellular toxicity. Furthermore, soluble mutant huntingtin inhibits Sp1 binding to DNA in postmortem brain tissues of both presymptomatic and affected HD patients. Understanding these early molecular events in HD may provide an opportunity to interfere with the effects of mutant huntingtin before the development of disease symptoms.

Differential D1 and D2 receptor-mediated effects on immediate early gene induction in a transgenic mouse model of Huntington's disease

The diminished expression of D1 and D2 dopamine receptors is a well-documented hallmark of Huntington's disease (HD), but relatively little is known about how these changes in receptor populations affect the dopaminergic responses of striatal neurons. Using transgenic mice expressing an N-terminal portion of mutant huntingtin (R6/2 mice), we have examined immediate early gene (IEG) expression as an index of dopaminergic signal transduction. c-fos, jun B, zif268, and N10 mRNA levels and expression patterns were analyzed using quantitative in situ hybridization histochemistry following intraperitoneal administration of selective D1 and D2 family pharmacological agents (SKF-82958 and eticlopride). Basal IEG levels were generally lower in the dorsal subregion of R6/2 striata relative to wild-type control striata at 10-11 weeks of age, a finding in accord with previously reported decreases in D1 and adenosine A2A receptors. D2-antagonist-stimulated IEG expression was significantly reduced in the striata of transgenic animals. In contrast, D1-agonist-induced striatal R6/2 IEG mRNA levels were either equivalent or significantly enhanced relative to control levels, an unexpected result given the reduced level of D1 receptors in R6/2 animals. Understanding the functional bases for these effects may further elucidate the complex pathophysiology of Huntington's disease.

Toxic proteins in neurodegenerative disease

A broad range of neurodegenerative disorders is characterized by neuronal damage that may be caused by toxic, aggregation-prone proteins. As genes are identified for these disorders and cell culture and animal models are developed, it has become clear that a major effect of mutations in these genes is the abnormal processing and accumulation of misfolded protein in neuronal inclusions and plaques. Increased understanding of the cellular mechanisms for disposal of abnormal proteins and of the effects of toxic protein accumulation on neuronal survival may allow the development of rational, effective treatment for these disorders.

A long CAG repeat in the mouse Sca1 locus replicates SCA1 features and reveals the impact of protein solubility on selective neurodegeneration

To faithfully recreate the features of the human neurodegenerative disease spinocerebellar ataxia type 1 (SCA1) in the mouse, we targeted 154 CAG repeats into the endogenous mouse locus. Sca1(154Q/2Q) mice developed a progressive neurological disorder that resembles human SCA1, featuring motor incoordination, cognitive deficits, wasting, and premature death, accompanied by Purkinje cell loss and age-related hippocampal synaptic dysfunction. Mutant ataxin-1 solubility varied with brain region, being most soluble in the neurons most vulnerable to degeneration. Solubility decreased overall as the mice aged; Purkinje cells, the most affected in SCA1, did not form aggregates of mutant protein until an advanced stage of disease. It appears that those neurons that cannot sequester the mutant protein efficiently and thereby curb its toxicity suffer the worst damage from polyglutamine-induced toxicity.

PQBP-1 increases vulnerability to low potassium stress and represses transcription in primary cerebellar neurons

PQBP-1 is a polyglutamine tract binding protein implicated in transcription. We previously reported that PQBP-1 and mutant ataxin-1, product of the spinocerebellar atrophy type 1 (SCA1) causative gene, cooperatively induce cell death in culture cells. Simultaneously, we showed that mutant ataxin-1 promoted interaction between PQBP-1 and RNA polymerase II and enhanced repression of the basal transcription by PQBP-1. In this study, we have examined the effects of overexpression of PQBP-1 to the primary-cultured cerebellar neurons. Our results indicate that overexpression of PQBP-1 inhibits the basal transcription in cerebellar neurons and increases their vulnerability to low potassium conditions.

Ultrastructure of nuclear aggregates formed by expressing an expanded polyglutamine

Intranuclear inclusions have been observed in the brains of patients affected with Huntington's disease (HD). Neuro 2A cells that transiently expressed HD exon 1 bearing 74 glutamine repeats linked to the green fluorescent protein (GFP) and the nuclear localization sequence (NLS) contained aggregates in nuclei. The aggregates were purified by fractionation with centrifugation followed by fluorescence-activated cell sorting (FACS). Heat treatment of the aggregate in an SDS sample buffer caused the dense aggregate cores to disappear and generated a basket-like structure composed of fibrils. Biochemical analysis of the aggregates revealed that the HD exon 1-GFP fusion protein was the major component. The heterogeneous nuclear ribonucleoproteins F and H, histones and ubiquitin were found to be associated with the aggregates. Our observations suggest that the N-terminal fragment of huntingtin may organize the skeletal structure of the aggregates and may disturb normal cellular functions by trapping other proteins within the aggregates.