Modulating huntingtin half-life alters polyglutamine-dependent aggregate formation and cell toxicity

Antibody-drug conjugates in urothelial carcinoma: scientometric analysis and clinical trials analysis

In 2020, bladder cancer, which commonly presents as urothelial carcinoma, became the 10th most common malignancy. For patients with metastatic urothelial carcinoma, the standard first-line treatment remains platinum-based chemotherapy, with immunotherapy serving as an alternative in cases of programmed death ligand 1 expression. However, treatment options become limited upon resistance to platinum and programmed death 1 or programmed death ligand 1 agents. Since the FDA's approval of Enfortumab Vedotin and Sacituzumab Govitecan, the therapeutic landscape has expanded, heralding a shift towards antibody-drug conjugates as potential first-line therapies. Our review employed a robust scientometric approach to assess 475 publications on antibody-drug conjugates in urothelial carcinoma, revealing a surge in related studies since 2018, predominantly led by U.S. institutions. Moreover, 89 clinical trials were examined, with 36 in Phase II and 13 in Phase III, exploring antibody-drug conjugates as both monotherapies and in combination with other agents. Promisingly, novel targets like HER-2 and EpCAM exhibit substantial therapeutic potential. These findings affirm the increasing significance of antibody-drug conjugates in urothelial carcinoma treatment, transitioning them from posterior-line to frontline therapies. Future research is poised to focus on new therapeutic targets, combination therapy optimization, treatment personalization, exploration of double antibody-coupled drugs, and strategies to overcome drug resistance.

Drug conjugates for the treatment of lung cancer: from drug discovery to clinical practice

A drug conjugate consists of a cytotoxic drug bound via a linker to a targeted ligand, allowing the targeted delivery of the drug to one or more tumor sites. This approach simultaneously reduces drug toxicity and increases efficacy, with a powerful combination of efficient killing and precise targeting. Antibody‒drug conjugates (ADCs) are the best-known type of drug conjugate, combining the specificity of antibodies with the cytotoxicity of chemotherapeutic drugs to reduce adverse reactions by preferentially targeting the payload to the tumor. The structure of ADCs has also provided inspiration for the development of additional drug conjugates. In recent years, drug conjugates such as ADCs, peptide‒drug conjugates (PDCs) and radionuclide drug conjugates (RDCs) have been approved by the Food and Drug Administration (FDA). The scope and application of drug conjugates have been expanding, including combination therapy and precise drug delivery, and a variety of new conjugation technology concepts have emerged. Additionally, new conjugation technology-based drugs have been developed in industry. In addition to chemotherapy, targeted therapy and immunotherapy, drug conjugate therapy has undergone continuous development and made significant progress in treating lung cancer in recent years, offering a promising strategy for the treatment of this disease. In this review, we discuss recent advances in the use of drug conjugates for lung cancer treatment, including structure-based drug design, mechanisms of action, clinical trials, and side effects. Furthermore, challenges, potential approaches and future prospects are presented.

Insights into the recognition mechanism in the UBR box of UBR4 for its specific substrates

The N-end rule pathway is a proteolytic system involving the destabilization of N-terminal amino acids, known as N-degrons, which are recognized by N-recognins. Dysregulation of the N-end rule pathway results in the accumulation of undesired proteins, causing various diseases. The E3 ligases of the UBR subfamily recognize and degrade N-degrons through the ubiquitin-proteasome system. Herein, we investigated UBR4, which has a distinct mechanism for recognizing type-2 N-degrons. Structural analysis revealed that the UBR box of UBR4 differs from other UBR boxes in the N-degron binding sites. It recognizes type-2 N-terminal amino acids containing an aromatic ring and type-1 N-terminal arginine through two phenylalanines on its hydrophobic surface. We also characterized the binding mechanism for the second ligand residue. This is the report on the structural basis underlying the recognition of type-2 N-degrons by the UBR box with implications for understanding the N-end rule pathway.

Antibody drug conjugate: the "biological missile" for targeted cancer therapy

Antibody–drug conjugate (ADC) is typically composed of a monoclonal antibody (mAbs) covalently attached to a cytotoxic drug via a chemical linker. It combines both the advantages of highly specific targeting ability and highly potent killing effect to achieve accurate and efficient elimination of cancer cells, which has become one of the hotspots for the research and development of anticancer drugs. Since the first ADC, Mylotarg^® (gemtuzumab ozogamicin), was approved in 2000 by the US Food and Drug Administration (FDA), there have been 14 ADCs received market approval so far worldwide. Moreover, over 100 ADC candidates have been investigated in clinical stages at present. This kind of new anti-cancer drugs, known as “biological missiles”, is leading a new era of targeted cancer therapy. Herein, we conducted a review of the history and general mechanism of action of ADCs, and then briefly discussed the molecular aspects of key components of ADCs and the mechanisms by which these key factors influence the activities of ADCs. Moreover, we also reviewed the approved ADCs and other promising candidates in phase-3 clinical trials and discuss the current challenges and future perspectives for the development of next generations, which provide insights for the research and development of novel cancer therapeutics using ADCs.

Role of Polyphenols on Gut Microbiota and the Ubiquitin-Proteasome System in Neurodegenerative Diseases

Today, neurodegenerative diseases have become a remarkable public health challenge due to their direct relation with aging. Accordingly, understanding the molecular and cellular mechanisms occurring in the pathogenesis of them is essential. Both protein aggregations as a result of the ubiquitin-proteasome system (UPS) inefficiency and gut microbiota alternation are the main pathogenic hallmarks. Polyphenols upregulating this system may decrease the developing rate of neurodegenerative diseases. Most of the dietary intake of polyphenols is converted into other microbial metabolites, which have completely different biological properties from the original polyphenols and should be thoroughly investigated. Herein, several prevalent neurodegenerative diseases are pinpointed to explain the role of gut microbiota alternations and the role of molecular changes, especially UPS down-regulation in their pathogenesis. Some of the most important polyphenols found in our diet are explained along with their microbial metabolites in the body.

A slipped-CAG DNA-binding small molecule induces trinucleotide-repeat contractions in vivo

In many repeat diseases, such as Huntington's disease (HD), ongoing repeat expansions in affected tissues contribute to disease onset, progression and severity. Inducing contractions of expanded repeats by exogenous agents is not yet possible. Traditional approaches would target proteins driving repeat mutations. Here we report a compound, naphthyridine-azaquinolone (NA), that specifically binds slipped-CAG DNA intermediates of expansion mutations, a previously unsuspected target. NA efficiently induces repeat contractions in HD patient cells as well as en masse contractions in medium spiny neurons of HD mouse striatum. Contractions are specific for the expanded allele, independently of DNA replication, require transcription across the coding CTG strand and arise by blocking repair of CAG slip-outs. NA-induced contractions depend on active expansions driven by MutSβ. NA injections in HD mouse striatum reduce mutant HTT protein aggregates, a biomarker of HD pathogenesis and severity. Repeat-structure-specific DNA ligands are a novel avenue to contract expanded repeats.

PIN1 Modulates Huntingtin Levels and Aggregate Accumulation: An In vitro Model

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder characterized by a polyglutamine expansion within the N-terminal region of huntingtin protein (HTT). Cellular mechanisms promoting mutant huntingtin (mHTT) clearance are of great interest in HD pathology as they can lower the level of the mutant protein and its toxic aggregated species, thus affecting disease onset and progression. We have previously shown that the prolyl-isomerase PIN1 represents a promising negative regulator of mHTT aggregate accumulation using a genetically precise HD mouse model, namely Hdh^Q111 mice. Therefore, the current study aims at underpinning the mechanism by which PIN1 affects huntingtin's aggregates. We found that PIN1 overexpression led to a reduction of mHTT aggregates in HEK293 cells, and that this could be linked to a negative regulation of mHTT half-life by PIN1. Furthermore, we show that PIN1 has the ability to stimulate the proteasome presenting evidence of a mechanism regulating this phenomenon. Our findings provide a rationale for future investigation into PIN1 with the potential for the development of novel therapeutic strategies.

Altered Co-Translational Processing Plays a Role in Huntington's Pathogenesis-A Hypothesis

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG codon repeat region in the HTT gene's first exon that results in huntingtin protein aggregation and neuronal cell death. The development of therapeutic treatments for HD is hindered by the fact that while the etiology and symptoms of HD are understood, the molecular processes connecting this genotype to its phenotype remain unclear. Here, we propose the novel hypothesis that the perturbation of a co-translational process affects mutant huntingtin due to altered translation-elongation kinetics. These altered kinetics arise from the shift of a proline-induced translational pause site away from Htt's localization sequence due to the expansion of the CAG-repeat segment between the poly-proline and localization sequences. Motivation for this hypothesis comes from recent experiments in the field of protein biogenesis that illustrate the critical role that temporal coordination of co-translational processes plays in determining the function, localization, and fate of proteins in cells. We show that our hypothesis is consistent with various experimental observations concerning HD pathology, including the dependence of the age of symptom onset on CAG repeat number. Finally, we suggest three experiments to test our hypothesis.

The ubiquitin-proteasome system in neurodegenerative diseases: precipitating factor, yet part of the solution

The ubiquitin-proteasome system (UPS) has been implicated in neurodegenerative diseases based on the presence of deposits consisting of ubiquitylated proteins in affected neurons. It has been postulated that aggregation-prone proteins associated with these disorders, such as α-synuclein, β-amyloid peptide, and polyglutamine proteins, compromise UPS function, and delay the degradation of other proteasome substrates. Many of these substrates play important regulatory roles in signaling, cell cycle progression, or apoptosis, and their inadvertent stabilization due to an overloaded and improperly functioning UPS may thus be responsible for cellular demise in neurodegeneration. Over the past decade, numerous studies have addressed the UPS dysfunction hypothesis using various model systems and techniques that differ in their readout and sensitivity. While an inhibitory effect of some disease proteins on the UPS has been demonstrated, increasing evidence attests that the UPS remains operative in many disease models, which opens new possibilities for treatment. In this review, we will discuss the paradigm shift that repositioned the UPS from being a prime suspect in the pathophysiology of neurodegeneration to an attractive therapeutic target that can be harnessed to accelerate the clearance of disease-linked proteins.

Expanded polyglutamine-containing N-terminal huntingtin fragments are entirely degraded by mammalian proteasomes

Huntington disease is a neurodegenerative disorder caused by an expanded polyglutamine (polyQ) repeat within the protein huntingtin (Htt). N-terminal fragments of the mutant Htt (mHtt) proteins containing the polyQ repeat are aggregation-prone and form intracellular inclusion bodies. Improving the clearance of mHtt fragments by intracellular degradation pathways is relevant to obviate toxic mHtt species and subsequent neurodegeneration. Because the proteasomal degradation pathway has been the subject of controversy regarding the processing of expanded polyQ repeats, we examined whether the proteasome can efficiently degrade Htt-exon1 with an expanded polyQ stretch both in neuronal cells and in vitro. Upon targeting mHtt-exon1 to the proteasome, rapid and complete clearance of mHtt-exon1 was observed. Proteasomal degradation of mHtt-exon1 was devoid of polyQ peptides as partial cleavage products by incomplete proteolysis, indicating that mammalian proteasomes are capable of efficiently degrading expanded polyQ sequences without an inhibitory effect on the proteasomal activity.

Proteostasis of polyglutamine varies among neurons and predicts neurodegeneration

In polyglutamine (polyQ) diseases, only certain neurons die, despite widespread expression of the offending protein. PolyQ expansion may induce neurodegeneration by impairing proteostasis, but protein aggregation and toxicity tend to confound conventional measurements of protein stability. Here, we used optical pulse labeling to measure effects of polyQ expansions on the mean lifetime of a fragment of huntingtin, the protein that causes Huntington's disease, in living neurons. We show that polyQ expansion reduced the mean lifetime of mutant huntingtin within a given neuron and that the mean lifetime varied among neurons, indicating differences in their capacity to clear the polypeptide. We found that neuronal longevity is predicted by the mean lifetime of huntingtin, as cortical neurons cleared mutant huntingtin faster and lived longer than striatal neurons. Thus, cell type-specific differences in turnover capacity may contribute to cellular susceptibility to toxic proteins, and efforts to bolster proteostasis in Huntington's disease, such as protein clearance, could be neuroprotective.

An in vitro perspective on the molecular mechanisms underlying mutant huntingtin protein toxicity

Huntington's disease (HD) is a devastating neurodegenerative disorder whose main hallmark is brain atrophy. However, several peripheral organs are considerably affected and their symptoms may, in fact, manifest before those resulting from brain pathology. HD is of genetic origin and caused by a mutation in the huntingtin gene. The mutated protein has detrimental effects on cell survival, but whether the mutation leads to a gain of toxic function or a loss of function of the altered protein is still highly controversial. Most currently used in vitro models have been designed, to a large extent, to investigate the effects of the aggregation process in neuronal-like cells. However, as the pathology involves several other organs, new in vitro models are critically needed to take into account the deleterious effects of mutant huntingtin in peripheral tissues, and thus to identify new targets that could lead to more effective clinical interventions in the early course of the disease. This review aims to present current in vitro models of HD pathology and to discuss the knowledge that has been gained from these studies as well as the new in vitro tools that have been developed, which should reflect the more global view that we now have of the disease.

Changes in BiP availability reveal hypersensitivity to acute endoplasmic reticulum stress in cells expressing mutant huntingtin

Huntington's disease (HD) is caused by expanded glutamine repeats within the huntingtin (Htt) protein. Mutant Htt (mHtt) in the cytoplasm has been linked to induction of the luminal endoplasmic reticulum (ER) stress pathway, the unfolded protein response (UPR). How mHtt impacts the susceptibility of the ER lumen to stress remains poorly understood. To investigate molecular differences in the ER in cells expressing mHtt, we used live-cell imaging of a sensitive reporter of the misfolded secretory protein burden, GFP fused to the ER chaperone BiP (also known as GRP78), which decreases in mobility as it binds increasing amounts of misfolded proteins. Striatal neurons expressing full-length mHtt showed no differences in BiP-GFP mobility and no evidence of UPR activation compared with wild-type cells at steady state. However, mHtt-expressing cells were acutely sensitive to misfolded secretory proteins. Treatment with ER stressors, tunicamycin or DTT, rapidly decreased BiP-GFP mobility in mHtt striatal cells and accelerated UPR activation compared with wild-type cells. mHtt-expressing cells exhibited decreased misfolded protein flux as a result of ER associated degradation (ERAD) dysfunction. Furthermore, UPR-adapted mHtt cells succumbed to misfolded protein stresses that could be tolerated by adapted wild-type cells. Thus, mHtt expression impairs misfolded secretory protein turnover, decreases the ER stress threshold, and increases cell vulnerability to insults.

Pathogenic mechanisms in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder presenting in midlife. Multiple pathogenic mechanisms which hypothesise how the expanded CAG repeat causes manifest disease have been suggested since the mutation was first detected. These mechanisms include events that operate at both the gene and protein levels. It has been proposed that somatic instability of the CAG repeat could underlie the striatal-specific pathology observed in HD, although how this occurs and what consequences this has in the disease state remain unknown. The form in which the Htt protein exists within the cell has been extensively studied in terms of both its role in aggregate formation and its cellular processing. Protein-protein interactions, post-translational modifications and protein cleavage have all been suggested to contribute to HD pathogenesis. The potential downstream effects of the mutant Htt protein are also noted here. In particular, the adverse effect of the mutant Htt protein on cellular protein degradation, subcellular transport and transcription are explored, and its role in energy metabolism and excitotoxicity investigated. Elucidating the mechanisms at work in HD pathogenesis and determining when they occur in relation to disease is an important step in the pathway to therapeutic interventions.

Gene- and cell-based approaches for neurodegenerative disease

Neurodegenerative diseases comprise an important group ofchronic diseases that increase in incidence with rising age. In particular, the two most common neurodegenerative diseases are Alzheimer's disease and Parkinson's disease, both of which will be discussed below. A third, Huntington's disease, occurs infrequently, but has been studied intensely. Each of these diseases shares characteristics which are also generalizeable to other neurodegenerative diseases: accumulation ofproteinaceous substances that leads inexorably to selective neuronal death and decline in neural function. Treatments for these diseases have historically focused on symptomatic relief, but recent advances in molecular research have identified more specific targets. Additionally, stem cell therapy, immunotherapy and trophic-factor delivery provide avenues for neuronal protection that may alter the natural progression of these devastating illnesses. Upcoming clinical trials will evaluate treatment strategies and provide hope that translational research will decrease the onset of debilitating disability associated with neurodegenerative disease.

Mimicking proteasomal release of polyglutamine peptides initiates aggregation and toxicity

Several neurodegenerative disorders, including Huntington's disease, are caused by expansion of the polyglutamine (polyQ) tract over 40 glutamines in the disease-related protein. Fragments of these proteins containing the expanded polyQ tract are thought to initiate aggregation and represent the toxic species. Although it is not clear how these toxic fragments are generated, in vitro data suggest that proteasomes are unable to digest polyQ tracts. To examine whether the resulting polyQ peptides could initiate aggregation in living cells, we mimicked proteasomal release of monomeric polyQ peptides. These peptides lack the commonly used starting methionine residue or any additional tag. Only expanded polyQ peptides seem to be peptidase resistant, and their accumulation initiated the aggregation process. As observed in polyQ disorders, these aggregates subsequently sequestered proteasomes, ubiquitin and polyQ proteins, and recruited Hsp70. The generated expanded polyQ peptides were toxic to neuronal cells. Our approach mimics proteasomal release of pure polyQ peptides in living cells, and represents a valuable tool to screen for proteins and compounds that affect aggregation and toxicity.

The ubiquitin-proteasome reporter GFPu does not accumulate in neurons of the R6/2 transgenic mouse model of Huntington's disease

Impairment of the ubiquitin-proteasome system (UPS) has long been considered an attractive hypothesis to explain the selective dysfunction and death of neurons in polyglutamine disorders such as Huntington's disease (HD). The fact that inclusion bodies in HD mouse models and patient brains are rich in ubiquitin and proteasome components suggests that the UPS may be hindered directly or indirectly by inclusion bodies or their misfolded monomeric or oligomeric precursors. However, studies into UPS function in various polyglutamine disease models have yielded conflicting results, suggesting mutant polyglutamine tracts may exert different effects on the UPS depending on protein context, expression level, subcellular localisation and cell-type. To investigate UPS function in a well-characterised mouse model of HD, we have crossed R6/2 HD mice with transgenic UPS reporter mice expressing the GFPu construct. The GFPu construct comprises GFP fused to a constitutive degradation signal (CL-1) that promotes its rapid degradation under conditions of a healthy UPS. Using a combination of immunoblot analysis, fluorescence and immunofluorescence microscopy studies, we found that steady-state GFPu levels were not detectably different between R6/2 and non-R6/2 brain. We observed no correlation between inclusion body formation and GFPu accumulation, suggesting no direct relationship between protein aggregation and global UPS inhibition in R6/2 mice. These findings suggest that while certain branches of the UPS can be impaired by mutant polyglutamine proteins, such proteins do not necessarily cause total blockade of UPS-dependent degradation. It is therefore likely that the relationship between mutant polyglutamine proteins and the UPS is more complex than originally anticipated.

Misfolding of proteins with a polyglutamine expansion is facilitated by proteasomal chaperones

Deposition of misfolded proteins with a polyglutamine expansion is a hallmark of Huntington disease and other neurodegenerative disorders. Impairment of the proteolytic function of the proteasome has been reported to be both a cause and a consequence of polyglutamine accumulation. Here we found that the proteasomal chaperones that unfold proteins to be degraded by the proteasome but also have non-proteolytic functions co-localized with huntingtin inclusions both in primary neurons and in Huntington disease patients and formed a complex independently of the proteolytic particle. Overexpression of Rpt4 or Rpt6 facilitated aggregation of mutant huntingtin and ataxin-3 without affecting proteasomal degradation. Conversely, reducing Rpt6 or Rpt4 levels decreased the number of inclusions in primary neurons, indicating that endogenous Rpt4 and Rpt6 facilitate inclusion formation. In vitro reconstitution experiments revealed that purified 19S particles promote mutant huntingtin aggregation. When fused to the ornithine decarboxylase destabilizing sequence, proteins with expanded polyglutamine were efficiently degraded and did not aggregate. We propose that aggregation of proteins with expanded polyglutamine is not a consequence of a proteolytic failure of the 20S proteasome. Rather, aggregation is elicited by chaperone subunits of the 19S particle independently of proteolysis.

Ubiquitin-proteasome system alterations in a striatal cell model of Huntington's disease

Huntington's disease (HD) is a progressive, autosomal dominant neurodegenerative disease caused by an abnormally expanded CAG repeat in the HD gene. Ubiquitylated aggregates containing mutant huntingtin protein in neurons are hallmarks of HD. Misfolded mutant huntingtin monomers, oligomers, or aggregates may be a result of, and cause, ubiquitin- proteasome dysfunction. To investigate the ubiquitin-proteasome system we designed a series of firefly luciferase reporters targeted selectively to different points along this pathway. These reporters were used to monitor ubiquitin-proteasome system function in a striatal cell culture model of HD. Ubiquitylation processes were not reduced in mutant huntingtin cells but recognition or degradation of ubiquitylated substrates was decreased. We also found mutant huntingtin expressing cells had slight but significant decreases in chymotrypsin-like and caspase-like activities, and an unexpected increase in trypsin-like activity of the proteasome core. General proteasome core inhibitors, as well as selective caspase-like activity inhibitors, were less effective in mutant cells. Finally, treatment with 3-nitropropionic acid, a succinate dehydrogenase inhibitor, had opposite effects on the ubiquitin-proteasome system with activation in wild-type and decreased activity in mutant huntingtin expressing cells. The results of these experiments show clearly selective disruption of the ubiquitin-proteasome system in this cell culture model of HD. The high throughput tools that we have designed and optimized will also be useful in identifying compounds that alter ubiquitin-proteasome system function and to investigate other neurodegenerative diseases such Alzheimer's disease and Parkinson's disease.

Role of Ubiquitin Protein Ligases in the Pathogenesis of Polyglutamine Diseases

The accumulation of intracellular protein deposits as inclusion bodies is the common pathological hallmark of most age related neurodegenerative disorders including polyglutamine diseases. Appearances of aggregates of the misfolded mutant disease proteins suggest that the cells are unable to efficiently degrade them, and failure of clearance leads to the severe disturbances of the cellular quality control system. The quality control ubiquitin ligases are now increasingly implicated in the biology of polyglutamine diseases, Parkinson's diseases, Amyotrophic lateral sclerosis and Alzheimer's disease. Here we review the recent studies that have revealed a critical role of E3 ubiquitin ligases in understanding the pathogenesis of polyglutamine diseases.

Proteasome activator enhances survival of Huntington's disease neuronal model cells

In patients with Huntington's disease (HD), the proteolytic activity of the ubiquitin proteasome system (UPS) is reduced in the brain and other tissues. The pathological hallmark of HD is the intraneuronal nuclear protein aggregates of mutant huntingtin. We determined how to enhance UPS function and influence catalytic protein degradation and cell survival in HD. Proteasome activators involved in either the ubiquitinated or the non-ubiquitinated proteolysis were overexpressed in HD patients' skin fibroblasts or mutant huntingtin-expressing striatal neurons. Following compromise of the UPS, overexpression of the proteasome activator subunit PA28γ, but not subunit S5a, recovered proteasome function in the HD cells. PA28γ also improved cell viability in mutant huntingtin-expressing striatal neurons exposed to pathological stressors, such as the excitotoxin quinolinic acid and the reversible proteasome inhibitor MG132. These results demonstrate the specific functional enhancements of the UPS that can provide neuroprotection in HD cells.

Huntington's disease

Huntington's disease is an autosomal-dominant, progressive neurodegenerative disorder with a distinct phenotype, including chorea and dystonia, incoordination, cognitive decline, and behavioural difficulties. Typically, onset of symptoms is in middle-age after affected individuals have had children, but the disorder can manifest at any time between infancy and senescence. The mutant protein in Huntington's disease--huntingtin--results from an expanded CAG repeat leading to a polyglutamine strand of variable length at the N-terminus. Evidence suggests that this tail confers a toxic gain of function. The precise pathophysiological mechanisms of Huntington's disease are poorly understood, but research in transgenic animal models of the disorder is providing insight into causative factors and potential treatments.

Proteasome impairment does not contribute to pathogenesis in R6/2 Huntington's disease mice: exclusion of proteasome activator REGgamma as a therapeutic target

Huntington's disease (HD) is one of a group of neurodegenerative disorders caused by the pathological expansion of a glutamine tract. A hallmark of these so-called polyglutamine diseases is the presence of ubiquitylated inclusion bodies, which sequester various components of the 19S and 20S proteasomes. In addition, the ubiquitin-proteasome system (UPS) has been shown to be severely impaired in vitro in cells overexpressing mutant huntingtin. Thus, because of its fundamental housekeeping function, impairment of the UPS in neurons could contribute to neurotoxicity. We have recently proposed that the proteasome activator REGgamma could contribute to UPS impairment in polyglutamine diseases by suppressing the proteasomal catalytic sites responsible for cleaving Gln-Gln bonds. Capping of proteasomes with REGgamma could therefore contribute to a potential 'clogging' of the proteasome by pathogenic polyglutamines. We show here that genetic reduction of REGgamma has no effect on the well-defined neurological phenotype of R6/2 HD mice and does not affect inclusion body formation in the R6/2 brain. Surprisingly, we observe increased proteasomal 'chymotrypsin-like' activity in 13-week-old R6/2 brains relative to non-R6/2, irrespective of REGgamma levels. However, assays of 26S proteasome activity in mouse brain extracts reveal no difference in proteolytic activity regardless of R6/2 or REGgamma genotype. These findings suggest that REGgamma is not a viable therapeutic target in polyglutamine disease and that overall proteasome function is not impaired by trapped mutant polyglutamine in R6/2 mice.

Diseases of unstable repeat expansion: mechanisms and common principles

The list of developmental and degenerative diseases that are caused by expansion of unstable repeats continues to grow, and is now approaching 20 disorders. The pathogenic mechanisms that underlie these disorders involve either loss of protein function or gain of function at the protein or RNA level. Common themes have emerged within and between these different classes of disease; for example, among disorders that are caused by gain-of-function mechanisms, altered protein conformations are central to pathogenesis, leading to changes in protein activity or abundance. In all these diseases, the context of the expanded repeat and the abundance, subcellular localization and interactions of the proteins and RNAs that are affected have key roles in disease-specific phenotypes.

Progressive phenotype and nuclear accumulation of an amino-terminal cleavage fragment in a transgenic mouse model with inducible expression of full-length mutant huntingtin

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized behaviorally by chorea, incoordination, and shortened lifespan and neuropathologically by huntingtin inclusions and neuronal degeneration. In order to facilitate studies of pathogenesis and therapeutics, we have generated a new inducible mouse model of HD expressing full-length huntingtin (Htt) using a tetracycline-regulated promoter. In double transgenic mice Htt was expressed widely in the brain under the control of the tet-transactivator (tTA) driven by the prion promoter PrP (in the absence of doxycycline). Mice expressing full-length mutant Htt, but not full-length normal Htt, displayed a progressive behavioral phenotype, consisting of slowed and irregular voluntary movements, gait ataxia, tremor and jerky movements, incoordination, and weight loss, with a shortened lifespan. Neuropathology included prominent intranuclear inclusions in cortex and striatum as well as cytoplasmic aggregates. This phenotype is very similar to the phenotypes of previous transgenic mice expressing N-terminal fragments of mutant Htt. The current HD-transgenic mice had nuclear accumulation of Htt, particularly an approximately 60-kDa fragment, which appears to represent an N-terminal cleavage product. This fragment is smaller than calpain or caspase-derived cleavage products of Htt, but it is comparable to a product, termed cp-A, which accumulates in nuclei of cells in a previously described cell model. This new mouse model may be useful in the future for pathogenic and preclinical therapeutic studies related to HD. The data suggest that proteolytic processing could be a part of the pathogenesis of HD, potentially representing an attractive therapeutic target.

Clinico-pathological rescue of a model mouse of Huntington's disease by siRNA

Huntington's disease (HD) is an autosomal dominant inheritable neurodegenerative disorder currently without effective treatment. It is caused by an expanded polyglutamine (poly Q) tract in the corresponding protein, huntingtin (htt), and therefore suppressing the huntingtin expression in brain neurons is expected to delay the onset and mitigate the severity of the disease. Here, we have used small interfering RNAs (siRNAs) directed against the huntingtin gene to repress the transgenic mutant huntingtin expression in an HD mouse model, R6/2. Results showed that intraventricular injection of siRNAs at an early postnatal period inhibited transgenic huntingtin expression in brain neurons and induced a decrease in the numbers and sizes of intranuclear inclusions in striatal neurons. Treatments using this siRNA significantly prolonged model mice longevity, improved motor function and slowed down the loss of body weight. This work suggests that siRNA-based therapy is promising as a future treatment for HD.

Neuronal dysfunction in a polyglutamine disease model occurs in the absence of ubiquitin-proteasome system impairment and inversely correlates with the degree of nuclear inclusion formation

The accumulation of protein deposits in neurons, in vitro proteasome assays and over-expression studies suggest that impairment of the ubiquitin-proteasome system (UPS) may be a common mechanism of pathogenesis in polyglutamine diseases such as Huntington disease and spinocerebellar ataxias (SCAs). Using a knock-in mouse model that recapitulates the clinical features of human SCA7, including selective neuronal dysfunction, we assessed the UPS at cellular resolution using transgenic mice that express a green fluorescent protein (GFP)-based reporter substrate (Ub(G76V)-GFP) of the UPS. The levels of the reporter remained low during the initial phase of disease, suggesting that neuronal dysfunction occurs in the presence of a functional UPS. Late in disease, we observed a significant increase in reporter levels specific to the most vulnerable neurons. Surprisingly, the basis for the increase in Ub(G76V)-GFP protein can be explained by a corresponding increase in Ub(G76V)-GFP mRNA in the vulnerable neurons. An in vitro assay also showed normal proteasome proteolytic activity in the vulnerable neurons. Thus, no evidence for general UPS impairment or reduction of proteasome activity was seen. The differential increase of Ub(G76V)-GFP among individual neurons directly correlated with the down-regulation of a marker of selective pathology and neuronal dysfunction in SCA7. Furthermore, we observed a striking inverse correlation between the neuropathology revealed by this reporter and ataxin-7 nuclear inclusions in the vulnerable neurons. Altogether, these data show a protective role against neuronal dysfunction for polyglutamine nuclear inclusions and exclude significant impairment of the UPS as a necessary step for polyglutamine neuropathology.

Polyglutamine expansion of huntingtin impairs its nuclear export

Proteins with polyglutamine (polyQ) expansions accumulate in the nucleus and affect gene expression. The mechanism by which mutant huntingtin (htt) accumulates intranuclearly is not known; wild-type htt, a 350-kDa protein of unknown function, is normally found in the cytoplasm. N-terminal fragments of mutant htt, which contain a polyQ expansion (>37 glutamines), have no conserved nuclear localization sequences or nuclear export sequences but can accumulate in the nucleus and cause neurological problems in transgenic mice. Here we report that N-terminal htt shuttles between the cytoplasm and nucleus in a Ran GTPase-independent manner. Small N-terminal htt fragments interact with the nuclear pore protein translocated promoter region (Tpr), which is involved in nuclear export. PolyQ expansion and aggregation decrease this interaction and increase the nuclear accumulation of htt. Reducing the expression of Tpr by RNA interference or deletion of ten amino acids of N-terminal htt, which are essential for the interaction of htt with Tpr, increased the nuclear accumulation of htt. These results suggest that Tpr has a role in the nuclear export of N-terminal htt and that polyQ expansion reduces this nuclear export to cause the nuclear accumulation of htt.