Expanded Polyglutamine Stretch Research Articles

Activation of the IkappaB kinase complex and nuclear factor-kappaB contributes to mutant huntingtin neurotoxicity.

Transcriptional dysregulation by mutant huntingtin (Htt) protein has been implicated in the pathogenesis of Huntington's disease (HD). We find that cultured cells expressing mutant Htt and striatal cells from HD transgenic mice have elevated nuclear factor-kappaB (NF-kappaB) activity. Furthermore, NF-kappaB is concentrated in the nucleus of neurons in the brains of HD transgenic mice. In inducible PC12 cells and in HD transgenic mice, mutant Htt activates the IkappaB kinase complex (IKK), a key regulator of NF-kappaB. Activation of IKK is likely mediated by direct interaction with mutant Htt, because the expanded polyglutamine stretch and adjacent proline-rich motifs in mutant Htt interact with IKKgamma, a regulatory subunit of IKK. Activation of IKK may also influence the toxicity of mutant Htt, because expression of IKKgamma promotes aggregation and nuclear localization of mutant Htt exon-1. Moreover, in acute striatal slice cultures, inhibition of IKK activity with an N-terminally truncated form of IKKgamma blocks mutant Htt-induced toxicity in medium-sized spiny neurons (MSNs). In addition, blocking degradation of NF-kappaB inhibitors with a dominant-negative ubiquitin ligase beta-transducin repeat-containing protein also reduces the toxicity of mutant Htt in MSNs. Therefore, aberrant NF-kappaB activation may contribute to the neurodegeneration induced by mutant Htt.

Mutant huntingtin directly increases susceptibility of mitochondria to the calcium-induced permeability transition and cytochrome c release.

Huntington's disease (HD) is initiated by an abnormally expanded polyglutamine stretch in the huntingtin protein, conferring a novel property on the protein that leads to the loss of striatal neurons. Defects in mitochondrial function have been implicated in the pathogenesis of HD. Here, we have examined the hypothesis that the mutant huntingtin protein may directly interact with the mitochondrion and affect its function. In human neuroblastoma cells and clonal striatal cells established from HdhQ7 (wild-type) and HdhQ111 (mutant) homozygote mouse knock-in embryos, huntingtin was present in a purified mitochondrial fraction. Subfractionation of the mitochondria and limited trypsin digestion of the organelle demonstrated that huntingtin was associated with the outer mitochondrial membrane. We further demonstrated that a recombinant truncated mutant huntingtin protein, but not a wild-type, directly induced mitochondrial permeability transition (MPT) pore opening in isolated mouse liver mitochondria, an effect that was prevented completely by cyclosporin A (CSA) and ATP. Importantly, the mutant huntingtin protein significantly decreased the Ca2+ threshold necessary to trigger MPT pore opening. We found a similar increased susceptibility to the calcium-induced MPT in liver mitochondria isolated from a knock-in HD mouse model. The mutant huntingtin protein-induced MPT pore opening was accompanied by a significant release of cytochrome c, an effect completely inhibited by CSA. These findings suggest that the development of specific MPT inhibitors may be an interesting therapeutic avenue to delay the onset of HD.

Disruption of the toxic conformation of the expanded polyglutamine stretch leads to suppression of aggregate formation and cytotoxicity

The polyglutamine (polyQ) diseases are a class of inherited neurodegenerative diseases including Huntington’s disease, caused by the expansion of a polyQ stretch within each disease protein. This expansion is thought to cause a conformational change in the protein leading to aggregation of the protein, resulting in cytotoxicity. To analyze whether disrupting the toxic conformation of the polyQ protein can alter its aggregation propensity and cytotoxicity, we examined the effect of interruption of the expanded polyQ stretch by proline insertion, since prolines cause great alterations in protein conformation. Here, we show that insertion of prolines into the expanded polyQ stretch indeed disrupts its ordered secondary structure, leading to suppression of polyQ protein aggregation both in vitro and in cell culture, and reduction of cytotoxicity in correlation with the number of proline interruptions. Furthermore, we found that a short polyQ stretch with a proline interruption is able to inhibit aggregation of the expanded polyQ protein in trans. These results show that a gain in toxic conformation of the expanded polyQ protein is essential for aggregation and cytotoxicity, providing insight into establishing therapies against the polyQ diseases.

Mitochondrial impairment in patients and asymptomatic gene carriers of Huntington's disease

Background: Huntington's disease (HD) is clinically characterized by a progressive movement disorder, cognitive decline and mood disturbance. HD is caused by a CAG repeat expansion in the IT-15 gene resulting in an expanded polyglutamine stretch in the mutant huntingtin protein. The mechanism how polyglutamine expansions cause neurodegeneration is still unknown. Several lines of evidence point to impaired energy metabolism as a relevant factor in the pathogenesis of HD. Respiratory chain defects have been demonstrated in post mortem brains as well as in muscle homogenates of HD patients.

Using antibodies to analyze polyglutamine stretches.

Expansion of a homomeric stretch of glutamine residues beyond a critical threshold can produce neurodegenerative disease. This observation led to the idea that abnormal polyglutamine stretches can alter protein structure in ways that contribute to disease. Because they are prone to aggregation, proteins with abnormal polyglutamine expansions have been difficult to study with conventional biophysical approaches. Some of these proteins are also very large, complicating efforts to generate them in vitro or to purify them for biochemical studies. An alternative approach has been to use antibodies with known binding specificity as probes of protein folding and protein structure. Antibodies can often bind to specific protein epitopes in situ and are, therefore, one of the few tools that can be used to probe protein structure in a physiological context and in the presence of that protein's normal binding partners. However, antibodies are complex reagents, and an understanding of their binding properties, methods of use, and limitations is needed to interpret results properly. We have developed monoclonal antibodies that specifically recognize expanded polyglutamine stretches in mutant huntingtin. Here, we describe several methods for using one of these antibodies to explore the structure of abnormal polyglutamine expansions and the proteins that contain them.

Recent advances in understanding the pathogenesis of polyglutamine diseases: involvement of molecular chaperones and ubiquitin-proteasome pathway

Polyglutamine diseases consist of a group of familial neurodegenerative disorders caused by expression of proteins containing expanded polyglutamine stretch. Over the past several years, tremendous progress has been made in identifying the molecular mechanisms by which the expanded polyglutamine tract leads to neuronal dysfunction and neurodegeneration. A common feature of most polyglutamine disorders is the occurrence of ubiquitin-positive neuronal intranuclear inclusions. The appearance of ubiquitinated aggregates implies an underline incapability of the cellular chaperones and proteasome machinery that normally functions to prevent the accumulation of misfolded proteins. Here we review the recent studies that have revealed a critical role for molecular chaperones and ubiquitin-proteasome pathway in the pathogenesis of polyglutamine diseases.

SUMO-1 co-localized with mutant atrophin-1 with expanded polyglutamines accelerates intranuclear aggregation and cell death.

To investigate the implication of small ubiquitin-related modifier-1 (SUMO-1) in the formation of neuronal intranuclear inclusions in polyglutamine diseases, we examined the localization of SUMO-1 in dentatorubral-pallidoluysian atrophy (DRPLA) brain tissues and PC12 cells expressing truncated atrophin-1 with expanded poly-glutamine stretches. SUMO-1 was co-localized with neuronal intranuclear inclusions in DRPLA brain and the DRPLA model cells, which showed that the aggregates formed by expanded polyglutamine stretches were highly SUMOlylated. In addition, to examine the role of SUMO-1 in nuclear aggregate formation and cell death, either SUMO-1 or DeltaSUMO-1, which is a SUMOlylation defective mutant lacking the C-terminal motif, was co-transfected with atrophin-1 with expanded polyglutamine stretches. Co-transfection of DeltaSUMO-1 decreased number of the cells with nuclear aggregates and consequent apoptosis of PC12 cells, both of which were markedly enhanced by co-transfection of SUMO-1 with atrophin-1 with expanded polyglutamine stretches. These results suggest that SUMO-1 is implicated in the pathogenesis of DRPLA and accelerates aggregate formation and cell death.

Genotype-phenotype correlation in CAG-repeat diseases.

The expansion of a CAG repeat is a common causative gene mutation in several hereditary neurodegenerative disorders, including dentatorubral-pallidoluysian atrophy (DRPLA). Although, in DRPLA, it is revealed that the variety of clinical manifestations is related to the variable expansion of the CAG repeat, there are still many problems in the correlation between the symptoms and neuropathologic findings. Recent immunohistochemical studies have revealed that diffuse intranuclear accumulation of mutant proteins with expanded polyglutamine stretches is a significant pathology in DRPLA, and involves a wide range of the CNS regions far beyond the lesion distribution previously established by neuronal loss. This novel pathology may become a clue for elucidating molecular mechanisms of neuronal dysfunction and establishing clinicopathological correlations in CAG-repeat diseases.

Involvement of lysosomes in the pathogenesis of CAG repeat diseases

In CAG repeat diseases, affected neurons possess many cytoplasmic granules immunopositive for expanded polyglutamine stretches. Electron microscopic immunohistochemistry showed that the granules corresponded to lysosomes of primitive type. The results suggest that, in addition to the ubiquitin/proteasome pathway, mutant proteins with expanded polyglutamine stretches are involved in the lysosomal pathway for protein degradation and that this processing mechanism may serve as a target for a new therapeutic approach to CAG repeat diseases.

Chemical Chaperones Reduce Aggregate Formation and Cell Death Caused by the Truncated Machado–Joseph Disease Gene Product with an Expanded Polyglutamine Stretch

Machado–Joseph disease/spinocerebellar ataxia-3 (MJD/SCA-3) is an inherited neurodegenerative disorder caused by expansion of the polyglutamine stretch in the MJD gene-encoded protein ataxin-3. The truncated form of mutated ataxin-3 causes aggregation and cell death in vitro and in vivo. Abnormal conformation and misfolding of the pathological protein are assumed critical to pathogenesis. To test this hypothesis, we transfected BHK-21 and Neuro2a cells transiently with N-terminal truncated ataxin-3 with an expanded polyglutamine stretch. We then studied the effects of organic solvent dimethyl sulfoxide (DMSO), cellular osmolytes glycerol, and trimethylamine N-oxide (TMAO) on aggregate formation and cell death. These reagents stabilize proteins in their native conformation and called chemical chaperones based on their influence on protein folding. Aggregate formation and cytotoxicity induced by truncated expanded ataxin-3 were reduced by exposing cells to these chemical chaperones. Our results indicate the potentially useful therapeutic strategy of the chemical chaperones in preventing cell death in MJD.

Intranuclear Aggregation of Nonexpanded Ataxin-3 in Marinesco Bodies of the Nonhuman Primate Substantia Nigra

Marinesco bodies (MB) are intranuclear inclusion bodies predominantly found in melanin-pigmented neurons of the substantia nigra. MB are demonstrable not only in humans but also in nonhuman primates. In the present study MB of aged rhesus monkeys (Macaca mulatta; n = 15; mean age 16 years) and aged baboons (Papio anubis; n = 13; mean age 25 years) were examined immunohistochemically. MB were found to be immunoreactive for ubiquitin, a protein involved in initiation of proteasome-mediated proteolysis. We also demonstrate that MB in monkeys are intensely immunoreactive for the protein ataxin-3 as detected by using two monoclonal anti-ataxin-3 antibodies (1H9 and 2B6). The abnormally expanded form of this polyglutamine protein is known to be causally involved in spinocerebellar ataxia type 3 or Machado–Joseph disease. The monoclonal antibody 1C2 was employed to examine whether ataxin-3 in MB in monkeys contains such an abnormally expanded polyglutamine stretch. MB were consistently 1C2-immunonegative, indicating that they are composed of normal wild-type ataxin-3. In conclusion MB in nonhuman primates permit experimental examination of mechanisms involved in transnuclear localization, intranuclear aggregation, and ubiquitination of nonexpanded polyglutamine proteins.

The IGF-1/Akt Pathway Is Neuroprotective in Huntington's Disease and Involves Huntingtin Phosphorylation by Akt

In the search for neuroprotective factors in Huntington's disease, we found that insulin growth factor 1 via activation of the serine/threonine kinase Akt/PKB is able to inhibit neuronal death specifically induced by mutant huntingtin containing an expanded polyglutamine stretch. The IGF-1/Akt pathway has a dual effect on huntingtin-induced toxicity, since activation of this pathway also results in a decrease in the formation of intranuclear inclusions of mutant huntingtin. We demonstrate that huntingtin is a substrate of Akt and that phosphorylation of huntingtin by Akt is crucial to mediate the neuroprotective effects of IGF-1. Finally, we show that Akt is altered in Huntington's disease patients. Taken together, these results support a potential role of the Akt pathway in Huntington's disease.

Expanded polyglutamine stretches form an ‘aggresome’

To understand the pathogenetic mechanisms underlying polyglutamine (polyQ) diseases, we investigated the mechanisms of the formation of aggregate bodies containing expanded polyQ stretches, focusing on dentatorubral-pallidoluysian atrophy (DRPLA). We demonstrated that the expression of a truncated DRPLA protein containing expanded polyQ stretches in COS-7 cells resulted in the formation of perinuclear aggregate bodies that are co-localized with γ-tubulin, a protein marker for the microtubules-organizing center (MTOC). A collapsed vimentin network surrounded these aggregate bodies. Furthermore, disruption of the microtubules (MTs) with nocodazole resulted in the formation of small aggregate bodies that were scattered throughout the cytoplasm. These findings suggest that the truncated DRPLA proteins containing expanded polyQ stretches unfold and form small aggregate bodies in the cell periphery. These aggregates move on MTs to the MTOC, where they remain as distinct ‘aggresomes’.

Differential susceptibility of cultured cell lines to aggregate formation and cell death produced by the truncated Machado-Joseph disease gene product with an expanded polyglutamine stretch

Ataxin-3, a protein coded by the Machado-Joseph disease gene, possesses a polyglutamine stretch whose expansion is known to produce neuronal intranuclear inclusion and neurodegeneration. Although previous studies describe the aggregate formation and toxic effect of the expanded polyglutamine tract in vitro and in vivo, differences in the susceptibility of different cultured cell lines has not been reported. Using the plasmid expressing N-terminal truncated ataxin-3 with an expanded polyglutamine stretch, we evaluated the aggregate formation and cytotoxicity in eight cultured cell lines—HeLa, Swiss/3T3, P19, C2C12, COS-1, BHK-21, PC12, and Neuro2a—that demonstrated a diverse range of aggregate formation and cell death. Although aggregate frequency did not appear to be correlated with cell death, Neuro2a demonstrated a high frequency of both. Our data indicates that susceptibility to cell death produced by mutant truncated ataxin-3 differs significantly among different cell lines and provides useful information when using a cultured cell line as an in vitro cellular model of polyglutamine disease.

Amino acid sequences flanking polyglutamine stretches influence their potential for aggregate formation.

Expanded polyglutamine stretches have been shown to form aggregates and to be toxic to cells. In this study, we hypothesized that amino acid sequences flanking the polyglutamine stretches influence the aggregate formation potential of these stretches. Green fluorescent protein (GFP) fusion proteins containing glutamine repeats of various lengths and a fixed number of flanking amino acids of ataxin-2, huntingtin, dentatorubral-pallidoluysian atrophy protein (DRPLAP) or ataxin-3 were transiently expressed in COS-7 cells. The aggregate formation potential of ataxin-2 and DRPLAP increased in a CAG-repeat-length-dependent manner, with a threshold between 34 and 36. Truncated ataxin-2-Q56-GFP and truncated huntingtin-Q56-GFP showed a significantly higher aggregate formation potential than truncated DRPLAP-Q56-GFP or truncated ataxin-3-Q56-GFP. These results are in agreement with the clinical observation that ages of disease onset in patients with spinocerebellar ataxia type 2 or Huntington's disease are lower than those in patients with DRPLA or Machado-Joseph disease having expanded CAG repeats of the same length. Furthermore, mutagenesis of the flanking sequence of ataxin-2 markedly reduced its aggregate formation potential. These results indicate that the amino acid sequences flanking the polyglutamine stretches significantly influence their aggregate formation potential.

Obese and sleepy without orexin

Using novel genetic techniques to cause degeneration of a subset of hypothalamic neurons in mice, researchers from the University of Tsukuba in Japan and the University of Texas at Dallas have shown that orexin-containing cells regulate sleep/wake states and energy metabolism. Orexins, or hypocretins, are neuropeptides expressed in the lateral hypothalamic area implicated in both feeding behavior and sleep dysfunction. A toxic Machado–Joseph disease gene product (ataxin-3) with an expanded polyglutamine stretch was linked to an orexin-specific promotor to cause postnatal degeneration of orexinergic neurons in transgenic mice. These mice exhibited a phenotype similar to human narcolepsy, including premature onset of REM sleep, cataplexic arrests, and fragmented wake and sleep episodes. Furthermore, these mice also developed late-onset obesity, despite eating less. This study suggests these neurons regulate wakefulness, energy and metabolism in complex ways. The full paper is in the May issue of Neuron. LO

Hints about Huntington's disease

Scientists from Johns Hopkins University have defined a mechanism linking abnormal huntingtin protein to neuronal cell death. Mutant huntingtin, with an expanded polyglutamine stretch, interacts with CREB binding protein (CBP), recruits it into cellular aggregates, and prevents it from participating as a co-activator in CREB-mediated transcription. This nuclear depletion and aggregate enrichment of CBP was observed in Huntington's disease (HD) culture and transgenic mice models as well as in human postmortem HD brain tissue. Furthermore, the cell death seen with mutant huntingtin expression in cells grown in culture is decreased following overexpression of CBP, supporting the current hypothesis that the toxic gain-of-function seen with mutant huntingtin involves transcriptional disruption of cell-survival programs. The full paper is published in the March 23 issue of Science. LO

Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity.

Orexins (hypocretins) are a pair of neuropeptides implicated in energy homeostasis and arousal. Recent reports suggest that loss of orexin-containing neurons occurs in human patients with narcolepsy. We generated transgenic mice in which orexin-containing neurons are ablated by orexinergic-specific expression of a truncated Machado-Joseph disease gene product (ataxin-3) with an expanded polyglutamine stretch. These mice showed a phenotype strikingly similar to human narcolepsy, including behavioral arrests, premature entry into rapid eye movement (REM) sleep, poorly consolidated sleep patterns, and a late-onset obesity, despite eating less than nontransgenic littermates. These results provide evidence that orexin-containing neurons play important roles in regulating vigilance states and energy homeostasis. Orexin/ataxin-3 mice provide a valuable model for studying the pathophysiology and treatment of narcolepsy.

Involvement of the cerebral cortex and autonomic ganglia in Machado-Joseph disease.

Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative disorder caused by expansion of a polyglutamine tract in the disease protein. In this study of brains from four autopsied MJD patients, we have demonstrated immunohistochemically that expanded polyglutamine stretches largely accumulate as inclusions in neuronal nuclei, and less frequently are distributed in the nucleoplasm in a diffuse pattern. These nuclear abnormalities involved many neurons covering a wide range of central and peripheral nervous system regions, including the cerebral cortex, thalamus and autonomic ganglia that have been categorized previously as spared regions by conventional pathological studies. These lesions, newly recognized by polyglutamine immunohistochemistry, may be responsible for the cerebral cortical dysfunctions or autonomic abnormalities pointed out in MJD patients by the recent clinical and neuroradiological studies.

Pathology of CAG repeat diseases.

Neuronal intranuclear inclusions have become the neuropathological signature of the CAG repeat diseases, although their cytotoxicity is a matter of controversy. It has been demonstrated that the inclusions in dentatorubral-pallidoluysian atrophy (DRPLA) and Machado-Joseph disease (MJD) were immunopositive for several transcription factors such as TATA-binding protein (TBP), TBP-associated factor (TAF(II)130), Sp1, cAMP-responsive element-binding protein (CREB) and CREB-binding protein, suggesting that neuronal degeneration in polyglutamine diseases may result from nuclear depletion of transcription factors containing the glutamine-rich domain. It was also revealed that, in the DRPLA brain, expanded polyglutamine stretches were diffusely accumulated in neuronal nucleoplasm. This nuclear pathology involved many neurons in various nervous system regions, such as the cerebral cortex, thalamus, substantia nigra, pontine nuclei, reticular formation and inferior olive, in addition to the previously recognized affected regions. The diffuse nuclear labeling was also detected in MJD, Huntington's disease, and spinal and bulbar muscular atrophy, suggesting that this nuclear pathology may be a characteristic feature and may exert certain influence on certain nuclear functions of many neurons in the CAG repeat diseases.