Dynamic regulation of molecular chaperone gene expression in polyglutamine disease

Involvement of molecular chaperone in protein-misfolding brain diseases

Protein misfolding causes aggregation and build-up in a variety of brain diseases. There are numeral molecules that are linked with the protein homeostasis mechanism. Molecular chaperones are one of such molecules that are responsible for protection against protein misfolded and aggregation-induced neurotoxicity. Many studies have explored the participation of molecular chaperones in Parkinson's disease, Alzheimer's disease, Amyotrophic lateral sclerosis, and Huntington's diseases. In this review, we highlighted the constructive role of molecular chaperones in neurological diseases characterized by protein misfolding and aggregation and their capability to control aberrant protein interactions at an early stage thus successfully suppressing pathogenic cascades. A comprehensive understanding of the protein misfolding associated with brain diseases and the molecular basis of involvement of chaperone against aggregation-induced cellular stress might lead to the progress of new therapeutic intrusion-related to protein misfolding and aggregation.

The Transcriptional Response to Oxidative Stress is Independent of Stress-Granule Formation

Cells respond to stress with translational arrest, robust transcriptional changes, and transcription-independent formation of mRNP assemblies termed stress granules (SGs). Despite considerable interest in the role of SGs in oxidative, unfolded-protein and viral stress responses, whether and how SGs contribute to stress-induced transcription has not been rigorously examined. To address this, we characterized transcriptional changes in Drosophila S2 cells induced by acute oxidative-stress and assessed how these were altered under conditions that disrupted SG assembly. Oxidative stress for 3-hours predominantly resulted in induction or upregulation of stress-responsive mRNAs whose levels peaked during recovery after stress cessation. The stress-transcriptome is enriched in mRNAs coding for chaperones, including HSP70s, small heat shock proteins, glutathione transferases, and several non-coding RNAs. Oxidative stress also induced cytoplasmic SGs that disassembled 3-hours after stress cessation. As expected, RNAi-mediated knockdown of the conserved G3BP1/Rasputin protein inhibited SG assembly. However, this disruption had no significant effect on the stress-induced transcriptional response or stress-induced translational arrest. Thus, SG assembly and stress-induced gene expression alterations appear to be driven by distinctive signaling processes. We suggest that while SG assembly represents a fast, transient mechanism, the transcriptional response enables a slower, longer-lasting mechanism for adaptation to and recovery from cell stress.

FipoQ/FBXO33, a Cullin-1-based ubiquitin ligase complex component modulates ubiquitination and solubility of polyglutamine disease protein

Polyglutamine (polyQ) diseases describe a group of progressive neurodegenerative disorders caused by the CAG triplet repeat expansion in the coding region of the disease genes. To date, nine such diseases, including spinocerebellar ataxia type 3 (SCA3), have been reported. The formation of SDS-insoluble protein aggregates in neurons causes cellular dysfunctions, such as impairment of the ubiquitin-proteasome system, and contributes to polyQ pathologies. Recently, the E3 ubiquitin ligases, which govern substrate specificity of the ubiquitin-proteasome system, have been implicated in polyQ pathogenesis. The Cullin (Cul) proteins are major components of Cullin-RING ubiquitin ligases (CRLs) complexes that are evolutionarily conserved in the Drosophila genome. In this study, we examined the effect of individual Culs on SCA3 pathogenesis and found that the knockdown of Cul1 expression enhances SCA3-induced neurodegeneration and reduces the solubility of expanded SCA3-polyQ proteins. The F-box proteins are substrate receptors of Cul1-based CRL. We further performed a genetic modifier screen of the 19 Drosophila F-box genes and identified F-box involved in polyQ pathogenesis (FipoQ) as a genetic modifier of SCA3 degeneration that modulates the ubiquitination and solubility of expanded SCA3-polyQ proteins. In the human SK-N-MC cell model, we identified that F-box only protein 33 (FBXO33) exerts similar functions as FipoQ in modulating the ubiquitination and solubility of expanded SCA3-polyQ proteins. Taken together, our study demonstrates that Cul1-based CRL and its associated F-box protein, FipoQ/FBXO33, modify SCA3 protein toxicity. These findings will lead to a better understanding of the disease mechanism of SCA3 and provide insights for developing treatments against SCA3. Cover Image for this issue: doi: 10.1111/jnc.14510.

Upregulation of miR-370 and miR-543 is associated with reduced expression of heat shock protein 40 in spinocerebellar ataxia type 3

Molecular chaperones are important regulators of protein folding and proteasomal removal of misfolded proteins. In spinocerebellar ataxia type 3 (SCA3), the co-chaperone DnaJ homology subfamily B member 1 (DNAJB1 or heat shock protein 40) is recruited to protein aggregates formed by the disease-causing mutant polyglutamine (polyQ) protein ataxin-3 (ATXN3). Over-expression of DNAJB1 reduces polyQ protein toxicity. Here, we identified two miRNAs, miR-370 and miR-543, that function in posttranscriptional regulation of DNAJB1 expression. MiRNAs are small endogenously produced RNAs controlling mRNA stability and play a role in polyQ disease pathogenesis. In human neuronal cultures derived from SCA3 patient-specific induced pluripotent stem cell (iPSC) lines, miR-370 and miR-543 levels are upregulated, while DNAJB1 expression is concurrently reduced. These findings suggest that downregulation of DNAJB1 by these two miRNAs is an early event that could contribute to SCA3 pathogenesis. Inhibition of these two miRNAs in turn could stabilize DNAJB1 and thereby be beneficial in SCA3 disease.

Planar cell polarity gene Fuz triggers apoptosis in neurodegenerative disease models

Planar cell polarity (PCP) describes a cell-cell communication process through which individual cells coordinate and align within the plane of a tissue. In this study, we show that overexpression of Fuz, a PCP gene, triggers neuronal apoptosis via the dishevelled/Rac1 GTPase/MEKK1/JNK/caspase signalling axis. Consistent with this finding, endogenous Fuz expression is upregulated in models of polyglutamine (polyQ) diseases and in fibroblasts from spinocerebellar ataxia type 3 (SCA3) patients. The disruption of this upregulation mitigates polyQ-induced neurodegeneration in Drosophila We show that the transcriptional regulator Yin Yang 1 (YY1) associates with the Fuz promoter. Overexpression of YY1 promotes the hypermethylation of Fuz promoter, causing transcriptional repression of Fuz Remarkably, YY1 protein is recruited to ATXN3-Q84 aggregates, which reduces the level of functional, soluble YY1, resulting in Fuz transcriptional derepression and induction of neuronal apoptosis. Furthermore, Fuz transcript level is elevated in amyloid beta-peptide, Tau and α-synuclein models, implicating its potential involvement in other neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. Taken together, this study unveils a generic Fuz-mediated apoptotic cell death pathway in neurodegenerative disorders.

Stressing Out Hsp90 in Neurotoxic Proteinopathies

A toxic accumulation of proteins is the hallmark pathology of several neurodegenerative disorders. Protein accumulation is regularly prevented by the network of molecular chaperone proteins, including and especially Hsp90. For reasons not yet elucidated, Hsp90 and the molecular chaperones interact with, but do not degrade, these toxic proteins resulting in the pathogenic accumulation of proteins such as tau, in Alzheimer's Disease, and α-synuclein, in Parkinson's Disease. In this review, we describe the associations between Hsp90 and the pathogenic and driver proteins of several neurodegenerative disorders. We additionally describe how the inhibition of Hsp90 promotes the degradation of both mutant and pathogenic protein species in models of neurodegenerative diseases. We also examine the current state of Hsp90 inhibitors capable of crossing the blood-brain barrier; compounds which may be capable of slowing, preventing, and possible reversing neurodegenerative diseases.

Targeting the prodromal stage of spinocerebellar ataxia type 17 mice: G-CSF in the prevention of motor deficits via upregulating chaperone and autophagy levels

Spinocerebellar ataxia type 17 (SCA17), an autosomal dominant cerebellar ataxia, is a devastating, incurable disease caused by the polyglutamine (polyQ) expansion of transcription factor TATA binding protein (TBP). The polyQ expansion causes misfolding and aggregation of the mutant TBP, further leading to cytotoxicity and cell death. The well-recognized prodromal phase in many forms of neurodegeneration suggests a prolonged period of partial neuronal dysfunction prior to cell loss that may be amenable to therapeutic intervention. The objective of this study was to assess the effects and molecular mechanisms of granulocyte-colony stimulating factor (G-CSF) therapy during the pre-symptomatic stage in SCA17 mice. Treatment with G-CSF at the pre-symptomatic stage improved the motor coordination of SCA17 mice and reduced the cell loss, insoluble mutant TBP protein, and vacuole formation in the Purkinje neurons of these mice. The neuroprotective effects of G-CSF may be produced by increases in Hsp70, Beclin-1, LC3-II and the p-ERK survival pathway. Upregulation of chaperone and autophagy levels further enhances the clearance of mutant protein aggregation, slowing the progression of pathology in SCA17 mice. Therefore, we showed that the early intervention of G-CSF has a neuroprotective effect, delaying the progression of SCA17 in mutant mice via increases in the levels of chaperone expression and autophagy.

Assessing a peptidylic inhibitor-based therapeutic approach that simultaneously suppresses polyglutamine RNA- and protein-mediated toxicities in patient cells and Drosophila

Polyglutamine (polyQ) diseases represent a group of progressive neurodegenerative disorders that are caused by abnormal expansion of CAG triplet nucleotides in disease genes. Recent evidence indicates that not only mutant polyQ proteins, but also their corresponding mutant RNAs, contribute to the pathogenesis of polyQ diseases. Here, we describe the identification of a 13-amino-acid peptide, P3, which binds directly and preferentially to long-CAG RNA within the pathogenic range. When administered to cell and Drosophila disease models, as well as to patient-derived fibroblasts, P3 inhibited expanded-CAG-RNA-induced nucleolar stress and suppressed neurotoxicity. We further examined the combined therapeutic effect of P3 and polyQ-binding peptide 1 (QBP1), a well-characterized polyQ protein toxicity inhibitor, on neurodegeneration. When P3 and QBP1 were co-administered to disease models, both RNA and protein toxicities were effectively mitigated, resulting in a notable improvement of neurotoxicity suppression compared with the P3 and QBP1 single-treatment controls. Our findings indicate that targeting toxic RNAs and/or simultaneous targeting of toxic RNAs and their corresponding proteins could open up a new therapeutic strategy for treating polyQ degeneration.

Alternative splicing of Drosophila Nmnat functions as a switch to enhance neuroprotection under stress

Nicotinamide mononucleotide adenylyltransferase (NMNAT) is a conserved enzyme in the NAD synthetic pathway. It has also been identified as an effective and versatile neuroprotective factor. However, it remains unclear how healthy neurons regulate the dual functions of NMNAT and achieve self-protection under stress. Here we show that Drosophila Nmnat (DmNmnat) is alternatively spliced into two mRNA variants, RA and RB, which translate to protein isoforms with divergent neuroprotective capacities against spinocerebellar ataxia 1-induced neurodegeneration. Isoform PA/PC translated from RA is nuclear-localized with minimal neuroprotective ability, and isoform PB/PD translated from RB is cytoplasmic and has robust neuroprotective capacity. Under stress, RB is preferably spliced in neurons to produce the neuroprotective PB/PD isoforms. Our results indicate that alternative splicing functions as a switch that regulates the expression of functionally distinct DmNmnat variants. Neurons respond to stress by driving the splicing switch to produce the neuroprotective variant and therefore achieve self-protection.

Nicotinamide mononucleotide adenylyltransferase (NMNAT) acts in the NAD biosynthesis pathway and has neuroprotective activity. Ruan et al. show that the neuroprotective activity of NMNAT is restricted to a splice variant of the enzyme, and that this variant is preferentially spliced in response to stress.

Focused cerebellar laser light induced hyperthermia improves symptoms and pathology of polyglutamine disease SCA1 in a mouse model

Spinocerebellar ataxia 1 (SCA1) results from pathologic glutamine expansion in the ataxin-1 protein (ATXN1). This misfolded ATXN1 causes severe Purkinje cell (PC) loss and cerebellar ataxia in both humans and mice with the SCA1 disease. The molecular chaperone heat-shock proteins (HSPs) are known to modulate polyglutamine protein aggregation and are neuroprotective. Since HSPs are induced under stress, we explored the effects of focused laser light induced hyperthermia (HT) on HSP-mediated protection against ATXN1 toxicity. We first tested the effects of HT in a cell culture model and found that HT induced Hsp70 and increased its localization to nuclear inclusions in HeLa cells expressing GFP-ATXN1[82Q]. HT treatment decreased ATXN1 aggregation by making GFP-ATXN1[82Q] inclusions smaller and more numerous compared to non-treated cells. Further, we tested our HT approach in vivo using a transgenic (Tg) mouse model of SCA1. We found that our laser method increased cerebellar temperature from 38 to 40 °C without causing any neuronal damage or inflammatory response. Interestingly, mild cerebellar HT stimulated the production of Hsp70 to a significant level. Furthermore, multiple exposure of focused cerebellar laser light induced HT to heterozygous SCA1 transgenic (Tg) mice significantly suppressed the SCA1 phenotype as compared to sham-treated control animals. Moreover, in treated SCA1 Tg mice, the levels of PC calcium signaling/buffering protein calbindin-D28k markedly increased followed by a reduction in PC neurodegenerative morphology. Taken together, our data suggest that laser light induced HT is a novel non-invasive approach to treat SCA1 and maybe other polyglutamine disorders.

Therapeutic Approaches for Inhibition of Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a late-onset and progressive neurodegenerative disorder that is caused by aggregation of mutant huntingtin protein which contains expanded-polyglutamine. The molecular chaperones modulate the aggregation in early stage and known for the most potent protector of neurodegeneration in animal models of HD. Over the past decades, a number of studies have demonstrated molecular chaperones alleviate the pathogenic symptoms by polyQ-mediated toxicity. Moreover, chaperone-inducible drugs and anti-aggregation drugs have beneficial effects on symptoms of disease. Here, we focus on the function of molecular chaperone in animal models of HD, and review the recent therapeutic approaches to modulate expression and turn-over of molecular chaperone and to develop anti-aggregation drugs.

Chaperone-dependent Neurodegeneration: A Molecular Perspective on Therapeutic Intervention

Maintenance of cellular homeostasis is regulated by the molecular chaperones. Under pathogenic conditions, aberrant proteins are triaged by the chaperone network. These aberrant proteins, known as "clients," have major roles in the pathogenesis of numerous neurological disorders, including tau in Alzheimer's disease, α-synuclein and LRRK2 in Parkinson's disease, SOD-1, TDP-43 and FUS in amyotrophic lateral sclerosis, and polyQ-expanded proteins such as huntingtin in Huntington's disease. Recent work has demonstrated that the use of chemical compounds which inhibit the activity of molecular chaperones subsequently alter the fate of aberrant clients. Inhibition of Hsp90 and Hsc70, two major molecular chaperones, has led to a greater understanding of how chaperone triage decisions are made and how perturbing the chaperone system can promote clearance of these pathogenic clients. Described here are major pathways and components of several prominent neurological disorders. Also discussed is how treatment with chaperone inhibitors, predominately Hsp90 inhibitors which are selective for a diseased state, can relieve the burden of aberrant client signaling in these neurological disorders.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

Expanded polyglutamine domain possesses nuclear export activity which modulates subcellular localization and toxicity of polyQ disease protein via exportin-1

Polyglutamine (polyQ) diseases are a group of late-onset, progressive neurodegenerative disorders caused by CAG trinucleotide repeat expansion in the coding region of disease genes. The cell nucleus is an important site of pathology in polyQ diseases, and transcriptional dysregulation is one of the pathologic hallmarks observed. In this study, we showed that exportin-1 (Xpo1) regulates the nucleocytoplasmic distribution of expanded polyQ protein. We found that expanded polyQ protein, but not its unexpanded form, possesses nuclear export activity and interacts with Xpo1. Genetic manipulation of Xpo1 expression levels in transgenic Drosophila models of polyQ disease confirmed the specific nuclear export role of Xpo1 on expanded polyQ protein. Upon Xpo1 knockdown, the expanded polyQ protein was retained in the nucleus. The nuclear disease protein enhanced polyQ toxicity by binding to heat shock protein (hsp) gene promoter and abolished hsp gene induction. Further, we uncovered a developmental decline of Xpo1 protein levels in vivo that contributes to the accumulation of expanded polyQ protein in the nucleus of symptomatic polyQ transgenic mice. Taken together, we first showed that Xpo1 is a nuclear export receptor for expanded polyQ domain, and our findings establish a direct link between protein nuclear export and the progressive nature of polyQ neurodegeneration.

Hsp70 and its molecular role in nervous system diseases

Heat shock proteins (HSPs) are induced in response to many injuries including stroke, neurodegenerative disease, epilepsy, and trauma. The overexpression of one HSP in particular, Hsp70, serves a protective role in several different models of nervous system injury, but has also been linked to a deleterious role in some diseases. Hsp70 functions as a chaperone and protects neurons from protein aggregation and toxicity (Parkinson disease, Alzheimer disease, polyglutamine diseases, and amyotrophic lateral sclerosis), protects cells from apoptosis (Parkinson disease), is a stress marker (temporal lobe epilepsy), protects cells from inflammation (cerebral ischemic injury), has an adjuvant role in antigen presentation and is involved in the immune response in autoimmune disease (multiple sclerosis). The worldwide incidence of neurodegenerative diseases is high. As neurodegenerative diseases disproportionately affect older individuals, disease-related morbidity has increased along with the general increase in longevity. An understanding of the underlying mechanisms that lead to neurodegeneration is key to identifying methods of prevention and treatment. Investigators have observed protective effects of HSPs induced by preconditioning, overexpression, or drugs in a variety of models of brain disease. Experimental data suggest that manipulation of the cellular stress response may offer strategies to protect the brain during progression of neurodegenerative disease.

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.

Sodium dodecyl sulfate-insoluble oligomers are involved in polyglutamine degeneration

In polyglutamine (polyQ) degeneration, disease protein that carries an expanded polyQ tract is neurotoxic. Expanded polyQ protein exists in different conformations that display distinct solubility properties. In this study, an inducible transgenic Drosophila model is established to define the pathogenic form of polyQ protein at an early stage of degeneration in vivo. We show that microscopic polyQ aggregates are neither pathogenic nor protective. Further, no toxic effect of sodium dodecyl sulfate (SDS) -soluble polyQ protein is observed in our model. By means of filtration, 2 forms of SDS-insoluble protein species are identified according to their size. Coexpression of an ATPase-defective form of the molecular chaperone Hsc70 (Hsc70-K71S) selectively reduces the abundance of the large SDS-insoluble polyQ species, but such modulation has no modifying effects on degeneration. Notably, we detect a distinct Hsc70-K71S-resistant, small, SDS-insoluble polyQ oligomeric species that is closely correlated with degeneration. Our data highlight the toxic role of SDS-insoluble oligomers in polyQ degeneration in vivo.

Polyglutamine gene function and dysfunction in the ageing brain

The coordinated regulation of gene expression and protein interactions determines how mammalian nervous systems develop and retain function and plasticity over extended periods of time such as a human life span. By studying mutations that occur in a group of genes associated with chronic neurodegeneration, the polyglutamine (polyQ) disorders, it has emerged that CAG/glutamine stretches play important roles in transcriptional regulation and protein-protein interactions. However, it is still unclear what the many structural and functional roles of CAG and other low-complexity sequences in eukaryotic genomes are, despite being the most commonly shared peptide fragments in such proteomes. In this review we examine the function of genes responsible for at least 10 polyglutamine disorders in relation to the nervous system and how expansion mutations lead to neuronal dysfunction, by particularly focusing on Huntington's disease (HD). We argue that the molecular and cellular pathways that turn out to be dysfunctional during such diseases, as a consequence of a CAG expansion, are also involved in the ageing of the central nervous system. These are pathways that control protein degradation systems (including molecular chaperones), axonal transport, redox-homeostasis and bioenergetics. CAG expansion mutations confer novel properties on proteins that lead to a slow-progressing neuronal pathology and cell death similar to that found in other age-related conditions such as Alzheimer's and Parkinson's diseases.

Polyglutamine-expanded ataxin-3 causes cerebellar dysfunction of SCA3 transgenic mice by inducing transcriptional dysregulation

In the present study, we prepared a SCA3 animal model by generating transgenic mice expressing polyglutamine-expanded ataxin-3-Q79. Ataxin-3-Q79 was expressed in brain areas implicated in SCA3 neurodegeneration, including cerebellum, pontine nucleus and substantia nigra. Ataxin-3-Q79 transgenic mice displayed motor dysfunction with an onset age of 5-6 months, and neurological symptoms deteriorated in the following months. A prominent neuronal loss was not found in the cerebellum of 10 to 11-month-old ataxin-3-Q79 mice displaying pronounced ataxic symptoms, suggesting that instead of neuronal demise, ataxin-3-Q79 causes neuronal dysfunction of the cerebellum and resulting ataxia. To test the involvement of transcriptional dysregulation in ataxin-3-Q79-induced cerebellar malfunction, microarray analysis and real-time RT-PCR assays were performed to identify altered cerebellar mRNA expressions of ataxin-3-Q79 mice. Compared to non-transgenic mice or mice expressing wild-type ataxin-3-Q22, 10 to 11-month-old ataxin-3-Q79 mice exhibited downregulated mRNA expressions of proteins involved in glutamatergic neurotransmission, intracellular calcium signaling/mobilization or MAP kinase pathways, GABA(A/B) receptor subunits, heat shock proteins and transcription factor regulating neuronal survival and differentiation. Upregulated expressions of Bax, cyclin D1 and CDK5-p39, which may mediate neuronal death, were also observed in ataxin-3-Q79 transgenic mice. The involvement of transcriptional abnormality in initiating the pathological process of SCA3 was indicated by the finding that 4 to 5-month-old ataxin-3-Q79 mice, which did not display neurological phenotype, exhibited downregulated mRNA levels of genes involved in glutamatergic signaling and signal transduction. Our study suggests that polyglutamine-expanded ataxin-3 causes cerebellar dysfunction and ataxia by disrupting the normal pattern of gene transcriptions.

Transcriptional malfunctioning of heat shock protein gene expression in spinocerebellar ataxias

Among the various dominantly-inherited spinocerebellar ataxias (SCAs), at least seven of them belong to the polyglutamine disease group and are caused by glutamine-coding CAG triplet repeat expansion. The expanded coding CAG repeat translates into a polyglutamine stretch in the disease protein, which leads to late-onset and progressive neurodegeneration. Expanded polyglutamine adopts a misfolded protein conformation, and is itself a cellular stressor which induces robust heat shock response (HSR). Under polyglutamine stress, heat shock proteins (Hsps) are produced in neurons to assist refolding and/or promote the degradation of misfolded proteins. Along with the progressive nature of polyglutamine degeneration, a gradual decline of HSR in degenerating neurons was observed. Such kind of reduction can be observed in a large family of hsp gene expression, including hsp22, 26, 27, and 70. This underscores an intimate relationship between the inducibility of hsp gene expression and the disease progression. In this review, we describe the current understandings of hsp gene dysregulation in polyglutamine disease.

The SARS-Coronavirus Membrane protein induces apoptosis through modulating the Akt survival pathway

A number of viral gene products are capable of triggering apoptotic cell death through interfering with cellular signaling cascades, including the Akt kinase pathway. In this study, the pro-apoptotic role of the SARS-CoV Membrane (M) structural protein is described. We found that the SARS-CoV M protein induced apoptosis in both HEK293T cells and transgenic Drosophila. We further showed that M protein-induced apoptosis involved mitochondrial release of cytochrome c protein, and could be suppressed by caspase inhibitors. Over-expression of M caused a dominant rough-eye phenotype in adult Drosophila. By performing a forward genetic modifier screen, we identified phosphoinositide-dependent kinase-1 (PDK-1) as a dominant suppressor of M-induced apoptotic cell death. Both PDK-1 and Akt kinases play essential roles in the cell survival signaling pathway. Altogether, our data show that SARS-CoV M protein induces apoptosis through the modulation of the cellular Akt pro-survival pathway and mitochondrial cytochrome c release.

Neurotoxic protein expression reveals connections between the circadian clock and mating behavior in Drosophila

To investigate the functions of circadian neurons, we added two strategies to the standard Drosophila behavioral genetics repertoire. The first was to express a polyglutamine-expanded neurotoxic protein (MJDtr78Q; MJD, Machado-Joseph disease) in the major timeless (tim)-expressing cells of the adult brain. These Tim-MJD flies were viable, in contrast to the use of cell-death gene expression for tim neuron inactivation. Moreover, they were more arrhythmic than flies expressing other neurotoxins and had low but detectable tim mRNA levels. The second extended standard microarray technology from fly heads to dissected fly brains. By combining the two approaches, we identified a population of Tim-MJD-affected mRNAs. Some had been previously identified as sex-specific and relevant to courtship, including mRNAs localized to brain-proximal fat-body tissue and brain courtship centers. Finally, we found a decrease in the number of neurons that expressed male-specific forms of the fruitless protein in the laterodorsal region of the brain. The decrease was not a consequence of toxic protein expression within these specialized cells but a likely effect of communication with neighboring TIM-expressing neurons. The data suggest a functional interaction between adjacent circadian and mating circuits within the fly brain, as well as an interaction between circadian circuits and brain-proximal fat body.

Biochemical investigation of Tau protein phosphorylation status and its solubility properties in Drosophila

Tau hyperphosphorylation and insoluble aggregate formation are two cellular features of tauopathies. However, the contribution of Tau protein hyperphosphorylation and its aggregation to Tau pathology still remain controversial. Overexpression of human tau transgenes in the Drosophila eye is toxic and causes neuronal degeneration. We showed that human Tau protein was phosphorylated by endogenous protein kinases in flies, and overexpression of either GSK3beta or Cdk5 enhanced tau-induced toxicity. Using a dominant-negative approach, we showed that kinase activity is important for the enhancement of tau-induced toxicity. Interestingly, such enhancement was accompanied with hyperphosphorylation and alteration of protein solubility properties of Tau. This situation was reminiscent of that observed in pre-tangle neurons in tauopathies patients. We also observed age-dependent Tau aggregate formation in aged transgenic flies. In summary, tau-induced toxicity is enhanced when the human Tau protein undergoes hyperphosphorylation, and we further demonstrated that aging contributes to Tau aggregate formation. Our data also underscore the utilization of transgenic Drosophila Tau models for the studies of pre-tangle events in tauopathies.