Application of protein knockdown strategy targeting β-sheet structure to multiple disease-associated polyglutamine proteins

Polyglutamine diseases are a class of neurodegenerative diseases associated with the accumulation of aggregated mutant proteins. We previously developed a class of degradation-inducing agents targeting the β-sheet-rich structure typical of such aggregates, and we showed that these agents dose-, time-, and proteasome-dependently decrease the intracellular level of mutant huntingtin with an extended polyglutamine tract, which correlates well with the severity of Huntington’s disease. Here, we demonstrate that the same agents also deplete other polyglutamine disease-related proteins: mutant ataxin-3 and ataxin-7 in cells from spino-cerebellar ataxia patients, and mutant atrophin-1 in cells from dentatorubral-pallidoluysian atrophy patients. Targeting cross-β-sheet structure could be an effective design strategy to develop therapeutic agents for multiple neurodegenerative diseases.     

Mutant huntingtin binds the mitochondrial fission GTPase dynamin-related protein-1 and increases its enzymatic activity

Huntington's disease is an inherited and incurable neurodegenerative disorder caused by an abnormal polyglutamine (polyQ) expansion in huntingtin (encoded by HTT). PolyQ length determines disease onset and severity, with a longer expansion causing earlier onset. The mechanisms of mutant huntingtin-mediated neurotoxicity remain unclear; however, mitochondrial dysfunction is a key event in Huntington's disease pathogenesis. Here we tested whether mutant huntingtin impairs the mitochondrial fission-fusion balance and thereby causes neuronal injury. We show that mutant huntingtin triggers mitochondrial fragmentation in rat neurons and fibroblasts of individuals with Huntington's disease in vitro and in a mouse model of Huntington's disease in vivo before the presence of neurological deficits and huntingtin aggregates. Mutant huntingtin abnormally interacts with the mitochondrial fission GTPase dynamin-related protein-1 (DRP1) in mice and humans with Huntington's disease, which, in turn, stimulates its enzymatic activity. Mutant huntingtin-mediated mitochondrial fragmentation, defects in anterograde and retrograde mitochondrial transport and neuronal cell death are all rescued by reducing DRP1 GTPase activity with the dominant-negative DRP1 K38A mutant. Thus, DRP1 might represent a new therapeutic target to combat neurodegeneration in Huntington's disease.     

Caspase-mediated proteolysis of the polyglutamine disease protein ataxin-3

Spinocerebellar ataxia type-3, also known as Machado-Joseph Disease, is one of many inherited neurodegenerative disorders caused by polyglutamine-encoding CAG repeat expansions in otherwise unrelated disease genes. Polyglutamine disorders are characterized by disease protein misfolding and aggregation; often within the nuclei of affected neurons. Although the precise mechanism of polyglutamine-mediated cell death remains elusive, evidence suggests that proteolysis of polyglutamine disease proteins by caspases contributes to pathogenesis. Using cellular models we now show that the endogenous spinocerebellar ataxia type-3 disease protein, ataxin-3, is proteolyzed in apoptotic paradigms, resulting in the loss of full-length ataxin-3 and the corresponding appearance of an approximately 28-kDa fragment containing the glutamine repeat. Broad-spectrum caspase inhibitors block ataxin-3 proteolysis and studies suggest that caspase-1 is a primary mediator of cleavage. Site-directed mutagenesis experiments eliminating three, six or nine potential caspase cleavage sites in the protein suggest redundancy in the site(s) at which cleavage can occur, as previously described for other disease proteins; but also map a major cleavage event to a cluster of aspartate residues within the ubiquitin-binding domain of ataxin-3 near the polyglutamine tract. Finally, caspase-mediated cleavage of expanded ataxin-3 resulted in increased ataxin-3 aggregation, suggesting a potential role for caspase-mediated proteolysis in spinocerebellar ataxia type-3 pathogenesis.     

CHIP protects from the neurotoxicity of expanded and wild-type ataxin-1 and promotes their ubiquitination and degradation

CHIP (C terminus of Hsc-70 interacting protein) is an E3 ligase that links the protein folding machinery with the ubiquitin-proteasome system and has been implicated in disorders characterized by protein misfolding and aggregation. Here we investigate the role of CHIP in protecting from ataxin-1-induced neurodegeneration. Ataxin-1 is a polyglutamine protein whose expansion causes spinocerebellar ataxia type-1 (SCA1) and triggers the formation of nuclear inclusions (NIs). We find that CHIP and ataxin-1 proteins directly interact and co-localize in NIs both in cell culture and SCA1 postmortem neurons. CHIP promotes ubiquitination of expanded ataxin-1 both in vitro and in cell culture. The Hsp70 chaperone increases CHIP-mediated ubiquitination of ataxin-1 in vitro, and the tetratricopeptide repeat domain, which mediates CHIP interactions with chaperones, is required for ataxin-1 ubitiquination in cell culture. Interestingly, CHIP also interacts with and ubiquitinates unexpanded ataxin-1. Overexpression of CHIP in a Drosophila model of SCA1 decreases the protein steady-state levels of both expanded and unexpanded ataxin-1 and suppresses their toxicity. Finally we investigate the ability of CHIP to protect against toxicity caused by expanded polyglutamine tracts in different protein contexts. We find that CHIP is not effective in suppressing the toxicity caused by a bare 127Q tract with only a short hemagglutinin tag, but it is very efficient in suppressing toxicity caused by a 128Q tract in the context of an N-terminal huntingtin backbone. These data underscore the importance of the protein framework for modulating the effects of polyglutamine-induced neurodegeneration.     

FAT10 protein binds to polyglutamine proteins and modulates their solubility

Expansion of polyglutamine (pQ) chain by expanded CAG repeat causes dominantly inherited neurodegeneration such as Huntington disease, dentatorubral-pallidoluysian atrophy (DRPLA), and numbers of other spinocerebellar ataxias. Expanded pQ disrupts the stability of the pQ-harboring protein and increases its susceptibility to aggregation. Aggregated pQ protein is recognized by the ubiquitin proteasome system, and the enzyme ubiquitin ligase covalently attaches ubiquitin, which serves as a degradation signal by the proteasome. However, accumulation of the aggregated proteins in the diseased brain suggests insufficient degradation machinery. Ubiquitin has several functionally related proteins that are similarly attached to target proteins through its C terminus glycine residue. They are called ubiquitin-like molecules, and some of them are similarly related to the protein degradation pathway. One of the ubiquitin-like molecules, FAT10, is known to accelerate protein degradation through a ubiquitin-independent manner, but its role in pQ aggregate degradation is completely unknown. Thus we investigated its role in a Huntington disease cellular model and found that FAT10 molecules were covalently attached to huntingtin through their C terminus glycine. FAT10 binds preferably to huntingtin with a short pQ chain and completely aggregated huntingtin was FAT10-negative. In addition, ataxin-1,3 and DRPLA proteins were both positive for FAT10, and aggregation enhancement was observed upon FAT10 knockdown. These findings were similar to those for huntingtin. Our new finding will provide a new role for FAT10 in the pathogenesis of polyglutamine diseases.     

Broadening the therapeutic scope for rapamycin treatment

The role of autophagy in the degradation of aggregate-prone proteins has been well established. As a result, autophagy upregulation has become an attractive therapeutic strategy for the treatment of proteinopathies, a group of diseases caused by the accumulation of mutant misfolded proteins. We have previously shown that rapamycin attenuates the phenotype in a mouse model of Huntington disease when administered pre-symptomatically and have recently extended this to demonstrate the effectiveness of rapamycin in a transgenic mouse model of spinocerebellar ataxia type 3, a polyglutamine disorder caused by mutations in the ataxin-3 gene. Rapamycin, administered from the initial onset of disease signs, improves motor coordination and results in a decrease in the levels of soluble mutant ataxin-3 and protein aggregates in the brain.     

RNAi suppresses polyglutamine-induced neurodegeneration in a model of spinocerebellar ataxia

The dominant polyglutamine expansion diseases, which include spinocerebellar ataxia type 1 (SCA1) and Huntington disease, are progressive, untreatable, neurodegenerative disorders. In inducible mouse models of SCA1 and Huntington disease, repression of mutant allele expression improves disease phenotypes. Thus, therapies designed to inhibit expression of the mutant gene would be beneficial. Here we evaluate the ability of RNA interference (RNAi) to inhibit polyglutamine-induced neurodegeneration caused by mutant ataxin-1 in a mouse model of SCA1. Upon intracerebellar injection, recombinant adeno-associated virus (AAV) vectors expressing short hairpin RNAs profoundly improved motor coordination, restored cerebellar morphology and resolved characteristic ataxin-1 inclusions in Purkinje cells of SCA1 mice. Our data demonstrate in vivo the potential use of RNAi as therapy for dominant neurodegenerative disease.     

Oxidative stress promotes mutant huntingtin aggregation and mutant huntingtin-dependent cell death by mimicking proteasomal malfunction

Huntington's disease (HD) is a familial neurodegenerative disorder caused by an abnormal expansion of CAG repeats in the coding region of huntingtin gene. A major hallmark of HD is the proteolytic production of N-terminal fragments of huntingtin containing polyglutamine repeats that form ubiquitinated aggregates in the nucleus and cytoplasm of the affected neurons. However, the mechanism by which the mutant huntingtin causes neurodegeneration is not well understood. Here, we found that oxidative stimuli enhance the polyglutamine-expanded truncated N-terminal huntingtin (mutant huntingtin) aggregation and mutant huntingtin-induced cell death. Oxidative stimuli also lead to rapid proteasomal dysfunction in the mutant huntingtin expressing cells as compared to normal glutamine repeat expressing cells. Overexpression of Cu/Zn superoxide dismutase (SOD1), Hsp40 or Hsp70 reverses the oxidative stress-induced proteasomal malfunction, mutant huntingtin aggregation, and death of the mutant huntingtin expressing cells. Finally, we show the higher levels of expression of SOD1 and DJ-1 in the mutant huntingtin expressing cells. Our result suggests that oxidative stress-induced proteasomal malfunction might be linked with mutant huntingtin-induced cell death.     

Molecular genetics of spinocerebellar ataxia type 8 (SCA8)

We previously reported that a transcribed but untranslated CTG expansion causes a novel form of ataxia, spinocerebellar ataxia type 8 (SCA8) (Koob et al., 1999). SCA8 was the first example of a dominant spinocerebellar ataxia that is not caused by the expansion of a CAG repeat translated into a polyglutamine tract. This slowly progressive form of ataxia is characterized by dramatic repeat instability and a high degree of reduced penetrance. The clinical and genetic features of the disease are discussed below.     

Recruitment and the Role of Nuclear Localization in Polyglutamine-mediated Aggregation

The inherited neurodegenerative diseases caused by an expanded glutamine repeat share the pathologic feature of intranuclear aggregates or inclusions (NI). Here in cell-based studies of the spinocerebellar ataxia type-3 disease protein, ataxin-3, we address two issues central to aggregation: the role of polyglutamine in recruiting proteins into NI and the role of nuclear localization in promoting aggregation. We demonstrate that full-length ataxin-3 is readily recruited from the cytoplasm into NI seeded either by a pathologic ataxin-3 fragment or by a second unrelated glutamine-repeat disease protein, ataxin-1. Experiments with green fluorescence protein/polyglutamine fusion proteins show that a glutamine repeat is sufficient to recruit an otherwise irrelevant protein into NI, and studies of human disease tissue and a Drosophila transgenic model provide evidence that specific glutamine-repeat–containing proteins, including TATA-binding protein and Eyes Absent protein, are recruited into NI in vivo. Finally, we show that nuclear localization promotes aggregation: an ataxin-3 fragment containing a nonpathologic repeat of 27 glutamines forms inclusions only when targeted to the nucleus. Our findings establish the importance of the polyglutamine domain in mediating recruitment and suggest that pathogenesis may be linked in part to the sequestering of glutamine-containing cellular proteins. In addition, we demonstrate that the nuclear environment may be critical for seeding polyglutamine aggregates.     

Protective roles for induction of autophagy in multiple proteinopathies

Many late-onset neurodegenerative diseases, including Parkinson's disease, tauopathies, Huntington's disease and forms of spinocerebellar ataxia, are caused by aggregate-prone proteins. Previously we showed that mutant huntingtin is an autophagy substrate and that autophagy induction reduced soluble and aggregated huntingtin levels and attenuated its toxicity in cell, fly and mouse models of disease. We have recently shown in cell and fly models that autophagy induction may have general protective effects across a range of diseases caused by aggregate-prone intracellular proteins. First, we showed that this strategy reduces the levels of the primary toxin, the aggregate-prone mutant protein. Second, our recent work suggests that autophagy induction may have additional cytoprotective effects by protecting cells against a range of subsequent pro-apoptotic insults.     

Repeat expansion and autosomal dominant neurodegenerative disorders: consensus and controversy

Repeat-expansion mutations cause 13 autosomal dominant neurodegenerative disorders falling into three groups. Huntington's disease (HD), dentatorubral pallidoluysian atrophy (DRPLA), spinal and bulbar muscular atrophy (SBMA), and spinocerebellar ataxias (SCAs) types 1, 2, 3, 7 and 17 are each caused by a CAG repeat expansion that encodes polyglutamine. Convergent lines of evidence demonstrate that neurodegeneration in these diseases is a consequence of the neurotoxic effects of abnormally long stretches of glutamines. How polyglutamine induces neurodegeneration, and why neurodegeneration occurs in only select neuronal populations, remains a matter of intense investigation. SCA6 is caused by a CAG repeat expansion in CACNA1A, a gene that encodes a subunit of the P/Q-type calcium channel. The threshold length at which the repeat causes disease is much shorter than in the other polyglutamine diseases, and neurodegeneration may arise from expansion-induced change of function in the calcium channel. Huntington's disease-like 2 (HDL2) and SCAs 8, 10 and 12 are rare disorders in which the repeats (CAG, CTG or ATTCT) are not in protein-coding regions. Investigation into these diseases is still at an early stage, but it is now reasonable to hypothesise that the net effect of each expansion is to alter gene expression. The different pathogenic mechanisms in these three groups of diseases have important implications for the development of rational therapeutics.     

Does tissue transglutaminase play a role in Huntington's disease?

Tissue transglutaminase (tTG) likely plays a role in numerous processes in the nervous system. tTG posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines (Glns). This reaction results in the incorporation of polyamines into substrate proteins or the formation of protein crosslinks, modifications that likely have significant effects on neural function. Huntington's disease is a genetic disorder caused by an expansion of the polyglutamine domain in the huntingtin protein. Because a polypeptide bound Gln is the determining factor for a tTG substrate, and mutant huntingtin aggregates have been found in Huntington's disease brain, it has been hypothesized that tTG may contribute to the pathogenesis of Huntington's disease. In vitro, polyglutamine constructs and huntingtin are substrates of tTG. Further, the levels of tTG and TG activity are elevated in Huntington's disease brain and immunohistochemical studies have demonstrated that there is an increase in tTG reactivity in affected neurons in Huntington's disease. These findings suggest that tTG may play a role in Huntington's disease. However in situ, neither wild type nor mutant huntingtin is modified by tTG. Further, immunocytochemical analysis revealed that tTG is totally excluded from the huntingtin aggregates, and modulation of the expression level of tTG had no effect on the frequency of the aggregates in the cells. Therefore, tTG is not required for the formation of huntingtin aggregates, and likely does not play a role in this process in Huntington's disease brain. However, tTG interacts with truncated huntingtin, and selectively polyaminates proteins that are associated with mutant truncated huntingtin. Given the fact that the levels of polyamines in cells is in the millimolar range and the crosslinking and polyaminating reactions catalyzed by tTG are competing reactions, intracellularly polyamination is likely to be the predominant reaction. Polyamination of proteins is likely to effect their function, and therefore it can be hypothesized that tTG may play a role in the pathogenesis of Huntington's disease by modifying specific proteins and altering their function and/or localization. Further research is required to define the specific role of tTG in Huntington's disease.     

Induction of autophagy with catalytic mTOR inhibitors reduces huntingtin aggregates in a neuronal cell model

Huntington's disease is a progressive neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the huntingtin gene. This expansion produces a mutant form of the huntingtin protein, which contains an elongated polyglutamine stretch at its amino-terminus. Mutant huntingtin may adopt an aberrant, aggregation-prone conformation predicted to start the pathogenic process leading to neuronal dysfunction and cell death. Thus, strategies reducing mutant huntingtin may lead to disease-modifying therapies. We investigated the mechanisms and molecular targets regulating huntingtin degradation in a neuronal cell model. We first found that mutant and wild-type huntingtin displayed strikingly diverse turn-over kinetics and sensitivity to proteasome inhibition. Then, we show that autophagy induction led to accelerate degradation of mutant huntingtin aggregates. In our neuronal cell model, allosteric inhibition of mTORC1 by everolimus, a rapamycin analogue, did not induce autophagy or affect aggregate degradation. In contrast, this occurred in the presence of catalytic inhibitors of both mTOR complexes mTORC1 and mTORC2. Our data demonstrate the existence of an mTOR-dependent but everolimus-independent mechanism regulating autophagy and huntingtin-aggregate degradation in cells of neuronal origin.