The ubiquitin-proteasome system and neurodegenerative disorders

Ubiquitination in rheumatoid arthritis

Rheumatoid arthritis is a chronic, inflammatory joint disease leading to inflammation of synovial membrane that lines the joints. This inflammation further progresses and results in destruction of joints and surrounding cartilages. The underlying factors can be oxidative stress, pro-inflammatory mediators, imbalance and attenuation between various enzymes and proteins (like nuclear factor erythroid 2 related factor 2/Nrf2 and ubiquitin). Protein degradation pathways comprises of lysosomal, proteasomal pathway, and autophagosome (that are carried out in mammalian cells) are regulated through ubiquitin. Ubiquitin proteasomal system is dominating pathway for carrying out non-lysosomal proteolysis of intracellularly proteins. Fundamental processes including cell cycle progression, process of division, apoptosis, modulation of immune responses and cell trafficking are regulated by process of ubiquitination. Ubiquitin proteasomal pathway (UPP) includes ubiquitin moieties which are covalently attached to proteins and guides them proteasome for degradation. Misfolded, oxidized and damaged proteins which are responsible for critical processes, are major targets of degradation process. Any alteration in this system leads to dysregulated cellular homeostasis; progressively leading to numerous diseases including rheumatoid arthritis. Factors including TAK1, TRAF6 undergo are required for the progression of disease and thus contributes towards pathology of inflammatory disorders such as rheumatoid arthritis. This review will include all linked aspects which contribute its major role in rheumatoid arthritis.

Molecular insights into an ancient form of Paget's disease of bone

We identify an ancient and atypical form of Paget’s disease of bone (PDB) in a collection of medieval skeletons exhibiting unusually extensive pathological changes, high disease prevalence, and low age-at-death estimations. Proteomic analysis of ancient bone-preserved proteins combined with analysis of small RNAs supports a retrospective diagnosis of PDB. Remains affected by other skeletal disorders may therefore hold a chemical memory amenable to similar molecular interrogation. Abnormalities in a contemporary PDB-linked protein detected in ancient tooth samples indicate that dentition may represent an unexplored storehouse for the study of skeletal disorders. Our work provides insights into the natural history of PDB and prompts a similar revaluation of other archaeological collections.

Paget’s disease of bone (PDB) is a chronic skeletal disorder that can affect one or several bones in individuals older than 55 y of age. PDB-like changes have been reported in archaeological remains as old as Roman, although accurate diagnosis and natural history of the disease is lacking. Six skeletons from a collection of 130 excavated at Norton Priory in the North West of England, which dates to medieval times, show atypical and extensive pathological changes resembling contemporary PDB affecting as many as 75% of individual skeletons. Disease prevalence in the remaining collection is high, at least 16% of adults, with age at death estimations as low as 35 y. Despite these atypical features, paleoproteomic analysis identified sequestosome 1 (SQSTM1) or p62, a protein central to the pathological milieu of PDB, as one of the few noncollagenous human sequences preserved in skeletal samples. Targeted proteomic analysis detected >60% of the ancient p62 primary sequence, with Western blotting indicating p62 abnormalities, including in dentition. Direct sequencing of ancient DNA excluded contemporary PDB-associated SQSTM1 mutations. Our observations indicate that the ancient p62 protein is likely modified within its C-terminal ubiquitin-associated domain. Ancient miRNAs were remarkably preserved in an osteosarcoma from a skeleton with extensive disease, with miR-16 expression consistent with that reported in contemporary PDB-associated bone tumors. Our work displays the use of proteomics to inform diagnosis of ancient diseases such as atypical PDB, which has unusual features presumably potentiated by yet-unidentified environmental or genetic factors.

A Practical Review of Proteasome Pharmacology

The ubiquitin proteasome system (UPS) degrades individual proteins in a highly regulated fashion and is responsible for the degradation of misfolded, damaged, or unneeded cellular proteins. During the past 20 years, investigators have established a critical role for the UPS in essentially every cellular process, including cell cycle progression, transcriptional regulation, genome integrity, apoptosis, immune responses, and neuronal plasticity. At the center of the UPS is the proteasome, a large and complex molecular machine containing a multicatalytic protease complex. When the efficiency of this proteostasis system is perturbed, misfolded and damaged protein aggregates can accumulate to toxic levels and cause neuronal dysfunction, which may underlie many neurodegenerative diseases. In addition, many cancers rely on robust proteasome activity for degrading tumor suppressors and cell cycle checkpoint inhibitors necessary for rapid cell division. Thus, proteasome inhibitors have proven clinically useful to treat some types of cancer, especially multiple myeloma. Numerous cellular processes rely on finely tuned proteasome function, making it a crucial target for future therapeutic intervention in many diseases, including neurodegenerative diseases, cystic fibrosis, atherosclerosis, autoimmune diseases, diabetes, and cancer. In this review, we discuss the structure and function of the proteasome, the mechanisms of action of different proteasome inhibitors, various techniques to evaluate proteasome function in vitro and in vivo, proteasome inhibitors in preclinical and clinical development, and the feasibility for pharmacological activation of the proteasome to potentially treat neurodegenerative disease.

Misframed ubiquitin and impaired protein quality control: an early event in Alzheimer's disease

Amyloid β (Aβ) plaque formation is a prominent cellular hallmark of Alzheimer's disease (AD). To date, immunization trials in AD patients have not been effective in terms of curing or ameliorating dementia. In addition, γ-secretase inhibitor strategies await clinical improvements in AD. These approaches were based upon the idea that autosomal dominant mutations in amyloid precursor protein (APP) and Presenilin 1 (PS1) genes are predictive for treatment of all AD patients. However most AD patients are of the sporadic form which partly explains the failures to treat this multifactorial disease. The major risk factor for developing sporadic AD (SAD) is aging whereas the Apolipoprotein E polymorphism (ε4 variant) is the most prominent genetic risk factor. Other medium-risk factors such as triggering receptor expressed on myeloid cells 2 (TREM2) and nine low risk factors from Genome Wide Association Studies (GWAS) were associated with AD. Recently, pooled GWAS studies identified protein ubiquitination as one of the key modulators of AD. In addition, a brain site specific strategy was used to compare the proteomes of AD patients by an Ingenuity Pathway Analysis. This strategy revealed numerous proteins that strongly interact with ubiquitin (UBB) signaling, and pointing to a dysfunctional ubiquitin proteasome system (UPS) as a causal factor in AD. We reported that DNA-RNA sequence differences in several genes including ubiquitin do occur in AD, the resulting misframed protein of which accumulates in the neurofibrillary tangles (NFTs). This suggests again a functional link between neurodegeneration of the AD type and loss of protein quality control by the UPS. Progress in this field is discussed and modulating the activity of the UPS opens an attractive avenue of research towards slowing down the development of AD and ameliorating its effects by discovering prime targets for AD therapeutics.

Screening for E3-ubiquitin ligase inhibitors: challenges and opportunities

The ubiquitin proteasome system (UPS) plays a role in the regulation of most cellular pathways, and its deregulation has been implicated in a wide range of human pathologies that include cancer, neurodegenerative and immunological disorders and viral infections. Targeting the UPS by small molecular regulators thus provides an opportunity for the development of therapeutics for the treatment of several diseases. The proteasome inhibitor Bortezomib was approved for treatment of hematologic malignancies by the FDA in 2003, becoming the first drug targeting the ubiquitin proteasome system in the clinic. Development of drugs targeting specific components of the ubiquitin proteasome system, however, has lagged behind, mainly due to the complexity of the ubiquitination reaction and its outcomes. However, significant advances have been made in recent years in understanding the molecular nature of the ubiquitination system and the vast variety of cellular signals that it produces. Additionally, improvement of screening methods, both in vitro and in silico, have led to the discovery of a number of compounds targeting components of the ubiquitin proteasome system, and some of these have now entered clinical trials. Here, we discuss the current state of drug discovery targeting E3 ligases and the opportunities and challenges that it provides.

Oxidative Stress and Protein Quality Control Systems in the Aged Canine Brain as a Model for Human Neurodegenerative Disorders

Aged dogs are considered the most suitable spontaneous animal model for studying normal aging and neurodegenerative diseases. Elderly canines naturally develop cognitive dysfunction and neuropathological hallmarks similar to those seen in humans, especially Alzheimer's disease-like pathology. Pet dogs also share similar living conditions and diets to humans. Oxidative damage accumulates in the canine brain during aging, making dogs a valid model for translational antioxidant treatment/prevention studies. Evidence suggests the presence of detective protein quality control systems, involving ubiquitin-proteasome system (UPS) and Heat Shock Proteins (HSPs), in the aged canine brain. Further studies on the canine model are needed to clarify the role of age-related changes in UPS activity and HSP expression in neurodegeneration in order to design novel treatment strategies, such as HSP-based therapies, aimed at improving chaperone defences against proteotoxic stress affecting brain during aging.

The proteasome function reporter GFPu accumulates in young brains of the APPswe/PS1dE9 Alzheimer's disease mouse model

Alzheimer's disease (AD), the most common cause of dementia, is neuropathologically characterized by accumulation of insoluble fibrous inclusions in the brain in the form of intracellular neurofibrillary tangles and extracellular senile plaques. Perturbation of the ubiquitin-proteasome system (UPS) has long been considered an attractive hypothesis to explain the pathogenesis of AD. However, studies on UPS functionality with various methods and AD models have achieved non-conclusive results. To get further insight into UPS functionality in AD, we have crossed a well-documented APPswe/PS1dE9 AD mouse model with a UPS functionality reporter, GFPu, mouse expressing green fluorescence protein (GFP) fused to a constitutive degradation signal (CL-1) that facilitates its rapid turnover in conditions of a normal UPS. Our western blot results indicate that GFPu reporter protein was accumulated in the cortex and hippocampus, but not striatum in the APPswe/PS1dE9 AD mouse model at 4 weeks of age, which is confirmed by fluorescence microscopy and elevated levels of p53, an endogenous UPS substrate. In accordance with this, the levels of ubiquitinated proteins were elevated in the AD mouse model. These results suggest that UPS is either impaired or functionally insufficient in specific brain regions in the APPswe/PS1dE9 AD mouse model at a very young age, long before senile plaque formation and the onset of memory loss. These observations may shed new light on the pathogenesis of AD.

Brain site-specific proteome changes in aging-related dementia

This study is aimed at gaining insights into the brain site-specific proteomic senescence signature while comparing physiologically aged brains with aging-related dementia brains (for example, Alzheimer's disease (AD)). Our study of proteomic differences within the hippocampus (Hp), parietal cortex (pCx) and cerebellum (Cb) could provide conceptual insights into the molecular mechanisms involved in aging-related neurodegeneration. Using an isobaric tag for relative and absolute quantitation (iTRAQ)-based two-dimensional liquid chromatography coupled with tandem mass spectrometry (2D-LC-MS/MS) brain site-specific proteomic strategy, we identified 950 proteins in the Hp, pCx and Cb of AD brains. Of these proteins, 31 were significantly altered. Most of the differentially regulated proteins are involved in molecular transport, nervous system development, synaptic plasticity and apoptosis. Particularly, proteins such as Gelsolin (GSN), Tenascin-R (TNR) and AHNAK could potentially act as novel biomarkers of aging-related neurodegeneration. Importantly, our Ingenuity Pathway Analysis (IPA)-based network analysis further revealed ubiquitin C (UBC) as a pivotal protein to interact with diverse AD-associated pathophysiological molecular factors and suggests the reduced ubiquitin proteasome degradation system (UPS) as one of the causative factors of AD.

Protein quality control system in neurodegeneration: a healing company hard to beat but failure is fatal

A common feature in most neurodegenerative diseases and aging is the progressive accumulation of damaged proteins. Proteins are essential for all crucial biological functions. Under some notorious conditions, proteins loss their three dimensional native conformations and are converted into disordered aggregated structures. Such changes rise into pathological conditions and eventually cause serious protein conformation disorders. Protein aggregation and inclusion bodies formation mediated multifactorial proteotoxic stress has been reported in the progression of Parkinson's disease (PD), Huntington's disease (HD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) and Prion disease. Ongoing studies have been remarkably informative in providing a systematic outlook for better understanding the concept and fundamentals of protein misfolding and aggregations. However, the precise role of protein quality control system and precursors of this mechanism remains elusive. In this review, we highlight recent insights and discuss emerging cytoprotective strategies of cellular protein quality control system implicated in protein deposition diseases. Our current review provides a clear, understandable framework of protein quality control system that may offer the more suitable therapeutic strategies for protein-associated diseases.

Alzheimer's disease: redox dysregulation as a common denominator for diverse pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

Natural product-like macrocyclic N-methyl-peptide inhibitors against a ubiquitin ligase uncovered from a ribosome-expressed de novo library

Naturally occurring peptides often possess macrocyclic and N-methylated backbone. These features grant them structural rigidity, high affinity to targets, proteolytic resistance, and occasionally membrane permeability. Because such peptides are produced by either nonribosomal peptide synthetases or enzymatic posttranslational modifications, it is yet a formidable challenge in degenerating sequence or length and preparing libraries for screening bioactive molecules. Here, we report a new means of synthesizing a de novo library of "natural product-like" macrocyclic N-methyl-peptides using translation machinery under the reprogrammed genetic code, which is coupled with an in vitro display technique, referred to as RaPID (random nonstandard peptides integrated discovery) system. This system allows for rapid selection of strong binders against an arbitrarily chosen therapeutic target. Here, we have demonstrated the selection of anti-E6AP macrocyclic N-methyl-peptides, one of which strongly inhibits polyubiqutination of proteins such as p53.

HSP70 mediates dissociation and reassociation of the 26S proteasome during adaptation to oxidative stress

We report an entirely new role for the HSP70 chaperone in dissociating 26S proteasome complexes (into free 20S proteasomes and bound 19S regulators), preserving 19S regulators, and reconstituting 26S proteasomes in the first 1-3h after mild oxidative stress. These responses, coupled with direct 20S proteasome activation by poly(ADP ribose) polymerase in the nucleus and by PA28αβ in the cytoplasm, instantly provide cells with increased capacity to degrade oxidatively damaged proteins and to survive the initial effects of stress exposure. Subsequent adaptive (hormetic) processes (3-24h after stress exposure), mediated by several signal transduction pathways and involving increased transcription/translation of 20S proteasomes, immunoproteasomes, and PA28αβ, abrogate the need for 26S proteasome dissociation. During this adaptive period, HSP70 releases its bound 19S regulators, 26S proteasomes are reconstituted, and ATP-stimulated proteolysis is restored. The 26S proteasome-dependent, and ATP-stimulated, turnover of ubiquitinylated proteins is essential for normal cell metabolism, and its restoration is required for successful stress adaptation.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Intracellular aggregation of human stefin B: confocal and electron microscopy study

BACKGROUND

Protein aggregation is a major contributor to the pathogenic mechanisms of human neurodegenerative diseases. Mutations in the CSTB (cystatin B) gene [StB (stefin B)] cause EPM1 (progressive myoclonus epilepsy of type 1), an epilepsy syndrome with features of neurodegeneration and increased oxidative stress. Oligomerization and aggregation of StB in mammalian cells have recently been reported. It has also been observed that StB is overexpressed after seizures and in certain neurodegenerative conditions, which could potentially lead to its aggregation. Human StB proved to be a good model system to study amyloid fibril formation in vitro and, as we show here, to study protein aggregation in cells.

RESULTS

Endogenous human StB formed smaller, occasional cytoplasmic aggregates and chemical inhibition of the UPS (ubiquitin-proteasome system) led to an increase in the amount of the endogenous protein and also increased its aggregation. Further, we characterized both the untagged and T-Sapphire-tagged StB on overexpression in mammalian cells. Compared with wild-type StB, the EPM1 missense mutant (G4R), the aggregate-prone EPM1 mutant (R68X) and the Y31 StB variant (both tagged and untagged) formed larger cytosolic and often perinuclear aggregates accompanied by cytoskeletal reorganization. Non-homogeneous morphology of these large aggregates was revealed using TEM (transmission electron microscopy) with StB detected by immunogold labelling. StB-positive cytoplasmic aggregates were partially co-localized with ubiquitin, proteasome subunits S20 and S26 and components of microfilament and microtubular cytoskeleton using confocal microscopy. StB aggregates also co-localized with LC3 and the protein adaptor p62, markers of autophagy. Flow cytometry showed that protein aggregation was associated with reduced cell viability.

CONCLUSIONS

We have shown that endogenous StB aggregates within cells, and that aggregation is increased upon protein overexpression or proteasome inhibition. From confocal and TEM analyses, we conclude that aggregates of StB show some of the molecular characteristics of aggresomes and may be eliminated from the cell by autophagy. Intracellular StB aggregation shows a negative correlation with cell survival.

Proteasome inhibitors and cardiac cell growth

Activation of the ubiquitin-proteasome system has been described in different models of cardiac hypertrophy. Cardiac cell growth in response to pressure or volume overload, as well as physiological adaptive hypertrophy, is accompanied by an increase in protein ubiquitination, proteasome subunit expression, and proteasome activity. Importantly, an inhibition of proteasome activity prevents and reverses cardiac hypertrophy and remodelling in vivo. The focus of this review is to provide an update about the mechanisms by which proteasome inhibitors affect cardiac cell growth in adaptive and maladaptive models of cardiac hypertrophy. In the first part, we summarize how the proteasome affects both proteolysis and protein synthesis in a context of cardiac cell growth. In the second part, we show how proteasome inhibition can prevent and reverse cardiac hypertrophy and remodelling in response to different conditions of overload.

Intermediate filament transcription in astrocytes is repressed by proteasome inhibition

Increased expression of the astrocytic intermediate filament protein glial fibrillary acidic protein (GFAP) is a characteristic of astrogliosis. This process occurs in the brain during aging and neurodegeneration and coincides with impairment of the ubiquitin proteasome system. Inhibition of the proteasome impairs protein degradation; therefore, we hypothesized that the increase in GFAP may be the result of impaired proteasomal activity in astrocytes. We investigated the effect of proteasome inhibitors on GFAP expression and other intermediate filament proteins in human astrocytoma cells and in a rat brain model for astrogliosis. Extensive quantitative RT-PCR, immunocytochemistry, and Western blot analysis resulted unexpectedly in a strong decrease of GFAP mRNA to <4% of control levels [Control (DMSO) 100+/-19.2%; proteasome inhibitor (epoxomicin) 3.5+/-1.3%, n=8; P < or = 0.001] and a loss of GFAP protein in astrocytes in vitro. We show that the proteasome alters GFAP promoter activity, possibly mediated by transcription factors as demonstrated by a GFAP promoter-luciferase assay and RT(2) Profiler PCR array for human transcription factors. Most important, we demonstrate that proteasome inhibitors also reduce GFAP and vimentin expression in a rat model for induced astrogliosis in vivo. Therefore, proteasome inhibitors could serve as a potential therapy to modulate astrogliosis associated with CNS injuries and disease.

The E163K DJ-1 mutant shows specific antioxidant deficiency

Recent discoveries of genetic mutations linked to familial forms of Parkinson's disease (PD), including mutations in DJ-1, have provided insights into the pathogenesis of sporadic PD. Recently, a novel homozygous missense mutation in the gene encoding human DJ-1 protein resulting in the E163K amino acid substitution has been reported. This mutation is associated with early-onset and clinical presentations that include parkinsonism, cognitive decline, and amyotrophic lateral sclerosis. The specific effect of this mutation on the function of DJ-1 protein as it relates to disease pathogenesis is currently unknown. Herein we show that the E163K pathogenic mutant retains similar properties to wild-type DJ-1 protein as it relates to protein stability, solubility, and dimerization. However, we show that the E163K mutant loses the ability to protect against oxidative stress while demonstrating a reduced redistribution towards mitochondria, but retains the ability to mitigate toxicity due to mitochondrial stress and proteasomal impairment. These findings suggest that DJ-1 influences several neuroprotective pathways and that the E163K mutation impairs the mechanism that is more specific to oxidative stress.

Disruption of ubiquitin-mediated processes in diseases of the brain and bone

A role for ubiquitin in the pathogenesis of human diseases was first suggested some two decades ago, from studies that localized the protein to intracellular protein aggregates, which are a feature of the major human neurodegenerative disorders. Although several different mechanisms have been proposed to connect impairment of the UPS (ubiquitin-proteasome system) to the presence of these 'ubiquitin inclusions' within diseased neurones, their significance in the disease process remains to be fully clarified. Ubiquitin inclusions also contain ubiquitin-binding proteins, such as the p62 protein [also known as SQSTM1 (sequestosome 1)], which non-covalently interacts with the ubiquitinated protein aggregates and may serve to mediate their autophagic clearance. p62 is a multifunctional protein and, in the context of bone-resorbing osteoclasts, is an important scaffold in the RANK [receptor activator of NF-kappaB (nuclear factor kappaB)]-NF-kappaB signalling pathway. Further, mutations affecting the UBA domain (ubiquitin-associated domain) of p62 are commonly found in patients with the skeletal disorder PDB (Paget's disease of bone). These mutations impair the ability of p62 to bind to ubiquitin and result in disordered osteoclast NF-kappaB signalling that may underlie the disease aetiology. Recent structural insights into the unusual mechanism of ubiquitin recognition by the p62 UBA domain have helped rationalize the mechanisms by which different PDB mutations exert their negative effects on ubiquitin binding by p62, as well as providing an indication of the ubiquitin-binding selectivity of p62 and, by extension, its normal biological functions.

Abeta inhibits the proteasome and enhances amyloid and tau accumulation

The accumulation of misfolded protein aggregates is a common feature of numerous neurodegenerative disorders including Alzheimer disease (AD). Here, we examined the effects of different assembly states of amyloid beta (Abeta) on proteasome function. We find that Abeta oligomers, but not monomers, inhibit the proteasome in vitro. In young 3xTg-AD mice, we observed impaired proteasome activity that correlates with the detection of intraneuronal Abeta oligomers. Blocking proteasome function in pre-pathological 3xTg-AD mice with specific inhibitors causes a marked increase in Abeta and tau accumulation, highlighting the adverse consequences of impaired proteasome activity for AD. Lastly, we show that Abeta immunotherapy in the 3xTg-AD mice reduces Abeta oligomers and reverses the deficits in proteasome activity. Taken together, our results indicate that Abeta oligomers impair proteasome activity, contributing to the age-related pathological accumulation of Abeta and tau. These findings provide further evidence that the proteasome represents a viable target for therapeutic intervention in AD.

Compensatory changes in the ubiquitin-proteasome system, brain-derived neurotrophic factor and mitochondrial complex II/III in YAC72 and R6/2 transgenic mice partially model Huntington's disease patients

Intraneuronal protein aggregates of the mutated huntingtin in Huntington's disease (HD) brains suggest an overload and/or dysfunction of the ubiquitin-proteasome system (UPS). There is a general inhibition of the UPS in many brain regions (cerebellum, cortex, substantia nigra and caudate-putamen) and skin fibroblasts from HD patients. In the current experiment, the widely used mutant huntingtin-exon 1 CAG repeat HD transgenic mice model (R6/2) (with 144 CAG repeat and exon 1) during late-stage pathology, had increases in proteasome activity in the striatum. However, this discrepancy with HD patient tissue was not apparent in the mutant CAG repeat huntingtin full-length HD (YAC72) transgenic mouse model during post-symptomatic and late-stage pathology, which then also showed UPS inhibition similar to HD patients' brains. In both types of HD model mice, we determined biochemical changes, including expression of brain-derived neurotrophic factor (BDNF) and mitochondrial complex II/III (MCII/III) activities related to HD pathology. We found increases of both BDNF expression, and MCII/III activities in YAC72 transgenic mice, and no change of BDNF expression in R6/2 mice. Our data show that extreme CAG repeat lengths in R6/2 mice is paradoxically associated with increased proteasome activity, probably as a cellular compensatory biochemical change in response to the underlying mutation. Changes in HD patients for UPS function, BDNF expression and MCII/III activity are only partially modeled in R6/2 and YAC72 mice, with the latter at 16 months of age being most congruent with the human disease.

Translational gene mapping of cognitive decline

The ability to maintain cognitive function during aging is a complex process subject to genetic and environmental influences. Alzheimer's disease (AD) is the most common disorder causing cognitive decline among the elderly. Among those with AD, there is broad variation in the relationship between AD neuropathology and clinical manifestations of dementia. Differences in expression of genes involved in neural processing pathways may contribute to individual differences in maintenance of cognitive function. We performed whole genome expression profiling of RNA obtained from frontal cortex of clinically non-demented and AD subjects to identify genes associated with brain aging and cognitive decline. Genetic mapping information and biological function annotation were incorporated to highlight genes of particular interest. The candidate genes identified in this study were compared with those from two other studies in different tissues to identify common underlying transcriptional profiles. In addition to confirming sweeping transcriptomal differences documented in previous studies of cognitive decline, we present new evidence for up-regulation of actin-related processes and down-regulation of translation, RNA processing and localization, and vesicle-mediated transport in individuals with cognitive decline.

Role of the ubiquitin proteasome system in Alzheimer's disease

Though Alzheimer's disease (AD) is a syndrome with well-defined clinical and neuropathological manifestations, an array of molecular defects underlies its pathology. A role for the ubiquitin proteasome system (UPS) was suspected in the pathogenesis of AD since the presence of ubiquitin immunoreactivity in AD-related neuronal inclusions, such as neurofibrillary tangles, is seen in all AD cases. Recent studies have indicated that components of the UPS could be linked to the early phase of AD, which is marked by synaptic dysfunction, as well as to the late stages of the disease, characterized by neurodegeneration. Insoluble protein aggregates in the brain of AD patients could result from malfunction or overload of the UPS, or from structural changes in the protein substrates, which prevent their recognition and degradation by the UPS. Defective proteolysis could cause the synaptic dysfunction observed early in AD since the UPS is known to play a role in the normal functioning of synapses. In this review, we discuss recent observations on possible links between the UPS and AD, and the potential for utilizing UPS components as targets for treatment of this disease.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Immediate early gene-X1 interferes with 26 S proteasome activity by attenuating expression of the 19 S proteasomal components S5a/Rpn10 and S1/Rpn2

The stress response gene IEX-1 (immediate early gene-X-1) is involved in the regulation of cell growth and cellular viability. To some extent, these effects include an interference with the proteasomal turnover of certain regulatory proteins. Here, we show that IEX-1 directly attenuates the activity and formation of the 26 S proteasome in HEK-293 cells (human embryonic kidney cells). We further demonstrate that IEX-1 reduces the overall expression levels of certain protein components of the 19 S proteasomal subunit such as S5a/Rpn10 and S1/Rpn2, whereas the expression of other proteasomal proteins was less or not affected. In contrast with direct apoptotic stimuli, such as the anti-cancer drug etoposide, leading to caspase-dependent degradation of S1 and S5a, the effect of IEX-1 is independent of proteolytic cleavage of these proteins. Furthermore, the decreasing effect of IEX-1 on S5a and S1 expression is still seen in the presence of cycloheximide, but not in the presence of actinomycin D, and quantitative real-time PCR revealed lower mRNA levels of S5a and S1 in IEX-1-overexpressing cells, suggesting an interference of IEX-1 with the gene transcription of S5a and S1. Additionally, luciferase assays confirmed an interference of IEX-1 with the activity of the S5a promoter. These findings indicate a role of IEX-1 in the maintenance and assembly of the 26 S proteasome, obviously involving an altered gene expression of certain proteasomal proteins. Thereby, IEX-1 may essentially modulate signalling pathways related to 26 S proteasome activity and involved in cellular growth control and apoptosis.

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 PA28gamma, but not subunit S5a, recovered proteasome function in the HD cells. PA28gamma 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.

Conserved signal peptide of Notch3 inhibits interaction with proteasome

The Notch3 N-terminal sequence is conserved across several mammalian species but diverges from the three other Notch proteins. We determined the significance of the N-terminal sequence using deletion mutants. The first 39 amino acids are required for Notch3 receptor expression, processing, and functional activity. In contrast, the first 14 amino acids do not appear to enhance function, yet are required to reduce ectopic cytoplasmic expression of Notch3. We screened binding partners for cytoplasmic expressed Notch3 using a yeast two-hybrid assay. Notch3 binds specifically to the proteasome subunit PSMA1, and increased cytoplasmic expression of Notch3 results in inhibition of proteasome activity. Our findings support a multifunctional role for the conserved N-terminal sequence of Notch3: targeting of the protein to the secretory pathway and reduction of cytoplasmic Notch3 expression which may inhibit cytoplasmic functions.

Prostaglandin E2 receptor subtype 2 (EP2) regulates microglial activation and associated neurotoxicity induced by aggregated alpha-synuclein

BACKGROUND

The pathogenesis of idiopathic Parkinson's disease (PD) remains elusive, although evidence has suggested that neuroinflammation characterized by activation of resident microglia in the brain may contribute significantly to neurodegeneration in PD. It has been demonstrated that aggregated alpha-synuclein potently activates microglia and causes neurotoxicity. However, the mechanisms by which aggregated alpha-synuclein activates microglia are not understood fully.

METHODS

We investigated the role of prostaglandin E2 receptor subtype 2 (EP2) in alpha-synuclein aggregation-induced microglial activation using ex vivo, in vivo and in vitro experimental systems.

RESULTS

Results demonstrated that ablation of EP2(EP2-/-) significantly enhanced microglia-mediated ex vivo clearance of alpha-synuclein aggregates (from mesocortex of Lewy body disease patients) while significantly attenuating neurotoxicity and extent of alpha-synuclein aggregation in mice treated with a parkinsonian toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Furthermore, we report that reduced neurotoxicity by EP2-/- microglia could be attributed to suppressed translocation of a critical cytoplasmic subunit (p47-phox) of NADPH oxidase (PHOX) to the membranous compartment after exposure to aggregated alpha-synuclein.

CONCLUSION

Thus, it appears that microglial EP2 plays a critical role in alpha-synuclein-mediated neurotoxicity.

The UBL domain of PLIC-1 regulates aggresome formation

Defects in protein folding and the proteasomal pathway have been linked with many neurodegenerative diseases. PLIC-1 (protein linking IAP to the cytoskeleton) is a ubiquitin-like protein that binds to the ubiquitin-interacting motif (UIM) of the proteasomal subunit S5a. Here, we show that PLIC-1 also binds to the UIM proteins ataxin 3--a deubiquitinating enzyme--HSJ1a--a co-chaperone--and EPS15 (epidermal growth factor substrate 15)--an endocytic protein. Using a polyglutamine (polyQ) disease model, we found that both endogenous PLIC-1 and EPS15 localize to perinuclear aggresomes, and that polyQ enhances their in vivo interaction. We show that knockdown of PLIC-1 and EPS15 by RNA interference reduces aggresome formation. In addition, PLIC-1(DeltaUBL) functions as a dominant-negative mutant, blocking both polyQ transport to aggresomes and the association of EPS15 with dispersed aggregates. We also show that PLIC-1 is upregulated by arsenite-induced protein misfolding. These results indicate a role for PLIC-1 in the protein aggregation-stress pathway, and we propose a novel function for the ubiquitin-like (UBL) domain--by means of UBL-UIM interactions--in transport to aggresomes.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

Proteasome inhibitor MG-132 induces dopaminergic degeneration in cell culture and animal models

Impairment in ubiquitin-proteasome system (UPS) has recently been implicated in Parkinson's disease, as demonstrated by reduced proteasomal activities, protein aggregation and mutation of several genes associated with UPS. However, experimental studies with proteasome inhibitors failed to yield consensus regarding the effect of proteasome inhibition on dopaminergic degeneration. In this study, we systematically examined the effect of the proteasome inhibitor MG-132 on dopaminergic degeneration in cell culture and animal models of Parkinson's disease. Exposure of immortalized dopaminergic neuronal cells (N27) to low doses of MG-132 (2-10 microM) resulted in dose- and time-dependent cytotoxicity. Further, exposure to MG-132 (5 microM) for 10 min led to dramatic reduction of proteasomal activity (>70%) accompanied by a rapid accumulation of ubiquitinated proteins in these cells. MG-132 treatment also induced increases in caspase-3 activity in a time-dependent manner, with significant activation occurring between 90 and 150 min. We also noted a 12-fold increase in DNA fragmentation in MG-132 treated N27 cells. Similarly, primary mesencephalic neurons exposed to 5 microM MG-132 also induced >60% loss of TH positive neurons but only a minimal loss of non-dopaminergic cells. Stereotaxic injection of MG-132 (0.4 microg in 4 microl) into the substantia nigra compacta (SNc) in C57 black mice resulted in significant depletion of ipisilateral striatal dopamine and DOPAC content as compared to the vehicle-injected contralateral control sides. Also, we observed a significant decrease in the number of TH positive neurons in the substantia nigra of MG-132-injected compared to the vehicle-injected sites. Collectively, these results demonstrate that the proteasomal inhibitor MG-132 induces dopamine depletion and nigral dopaminergic degeneration in both cell culture and animal models, and suggest that proteasomal dysfunction may promote nigral dopaminergic degeneration in Parkinson's disease.