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

Impaired 26S proteasome causes learning and memory deficiency and induces neuroinflammation mediated by NF-κB in mice

A reduction in proteasome activity, loss of synapses and increased neuroinflammation in the brain are hallmarks of aging and many neurodegenerative disorders, including Alzheimer disease (AD); however, whether proteasome dysfunction is causative to neuroinflammation remains less understood. In this study, we investigated the impact of 26S proteasome deficiency on neuroinflammation in the Psmc1 knockout (KO) mice deficient in a 19S proteasome subunit limited to the forebrain region. Our results revealed that impaired 26S proteasome led to reduced learning and memory capability and overt neuroinflammation in the synapses of the Psmc1 KO brain at eight weeks of age. Moreover, pronounced neuroinflammation was also found in the whole brain cortex, which was confirmed by increased levels of several key immune response-related proteins, including Stat1, Trem2 and NF-κB, and by activation of astrocytes and microglia in the KO brain. To validate NF-κB mediating neuroinflammation, we administered a selective NF-κB inhibitor to the KO animals at 5 weeks of age for three weeks, and then, animal behaviors and neuroinflammation were assessed when they reached eight weeks of age. Following the treatment, the KO mice exhibited improved behaviors and reduced neuroinflammation compared to the control animals. These data indicate that impaired 26S proteasome causes AD-like cognitive deficiency and induces neuroinflammation mediated largely by NF-κB. These results may aid development of effective therapeutics and better understanding of the pathogenesis of AD and many other neurodegenerative disorders where impaired proteasome is consistently coupled with neuroinflammation.

Raw Inonotus obliquus polysaccharide counteracts Alzheimer's disease in a transgenic mouse model by activating the ubiquitin-proteosome system

BACKGROUND/OBJECTIVES

Inonotus obliquus has been used as antidiabetic herb around the world, especially in the Russian and Scandinavian countries. Diabetes is widely believed to be a key factor in Alzheimer's disease (AD), which is widely considered to be type III diabetes. To investigate whether I. obliquus can also ameliorate AD, it would be interesting to identify new clues for AD treatment. We tested the anti-AD effects of raw Inonotus obliquus polysaccharide (IOP) in a mouse model of AD (3×Tg-AD transgenic mice).

MATERIALS/METHODS

SPF-grade 3×Tg-AD mice were randomly divided into three groups (Control, Metformin, and raw IOP groups, n = 5 per group). β-Amyloid deposition in the brain was analyzed using immunohistochemistry for AD characterization. Gene and protein expression of pertinent factors of the ubiquitin-proteasome system (UPS) was determined using real-time quantitative polymerase chain reaction and Western blotting.

RESULTS

Raw IOP significantly reduced the accumulation of amyloid aggregates and facilitated UPS activity, resulting in a significant reduction in AD-related symptoms in an AD mouse model. The presence of raw IOP significantly enhanced the expression of ubiquitin, E1, and Parkin (E3) at both the mRNA and protein levels in the mouse hippocampus. The mRNA level of ubiquitin carboxyl-terminal hydrolase isozyme L1, a key factor involved in UPS activation, also increased by approximately 50%.

CONCLUSIONS

Raw IOP could contribute to AD amelioration via the UPS pathway, which could be considered as a new potential strategy for AD treatment, although we could not exclude other mechanisms involved in counteracting AD processing.

Effects of Phenylethanoid Glycosides Extracted from Herba Cistanches on the Learning and Memory of the APP/PSI Transgenic Mice with Alzheimer's Disease

Background

To investigate the effects of phenylethanoid glycosides (PhGs) extracted from Herba Cistanches on the behavioral and cognition capacity of the APP/PSI transgenic mice with Alzheimer's disease (AD).

Methods

AD mice were randomly divided into the control group, model group, donepezil group, PhG groups, and verbascose group, respectively. Three weeks later, the animals were subject to behavioral and cognition evaluation by the nesting test, Morris water maze test, and step-down test.

Results

The cognition capacity in these groups showed a significant increase compared with that in the model group. The step-down test indicated that the errors induced by the memory decrease in the PhG groups and verbascose group showed a significant decrease compared with those in the model group (P < 0.05).

Conclusions

PhGs attenuated the cognitive dysfunction features of the APP/PSI transgenic gene. Besides, PhGs were the active components for the anti-AD activity of H. Cistanches.

Pharmacological intervention in a transgenic mouse model improves Alzheimer's-associated pathological phenotype: Involvement of proteasome activation

Alzheimer's disease (AD) is the most common form of dementia worldwide, characterized by a progressive decline in a variety of cognitive and non-cognitive functions. The amyloid beta protein cascade hypothesis places the formation of amyloid beta protein aggregates on the first position in the complex pathological cascade leading to neurodegeneration, and therefore AD might be considered to be a protein-misfolding disease. The Ubiquitin Proteasome System (UPS), being the primary protein degradation mechanism with a fundamental role in the maintenance of proteostasis, has been identified as a putative therapeutic target to delay and/or to decelerate the progression of neurodegenerative disorders that are characterized by accumulated/aggregated proteins. The purpose of this study was to test if the activation of proteasome in vivo can alleviate AD pathology. Specifically by using two compounds with complementary modes of proteasome activation and documented antioxidant and redox regulating properties in the 5xFAD transgenic mice model of AD, we ameliorated a number of AD related deficits. Shortly after proteasome activation we detected significantly reduced amyloid-beta load correlated with improved motor functions, reduced anxiety and frailty level. Essentially, to our knowledge this is the first report to demonstrate a dual activation of the proteasome and its downstream effects. In conclusion, these findings open up new directions for future therapeutic potential of proteasome-mediated proteolysis enhancement.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

Priming the Proteasome to Protect against Proteotoxicity

Increased proteotoxic stress (IPTS) resulting from the increased production or decreased removal of abnormally folded proteins is recognized as an important pathogenic factor for a large group of highly disabling and life-threatening human diseases, such as neurodegenerative disorders and many heart diseases. The proteasome is pivotal to the timely removal of abnormal proteins but its functional capacity often becomes inadequate in the disease conditions; consequently, proteasome functional insufficiency in return exacerbates IPTS. Recent research in proteasome biology reveals that the proteasome can be activated by endogenous protein kinases, making it possible to pharmacologically prime the proteasome for treating diseases with IPTS.

Targeting Ubiquitin-Proteasome Pathway by Natural Products: Novel Therapeutic Strategy for Treatment of Neurodegenerative Diseases

Misfolded proteins are the main common feature of neurodegenerative diseases, thereby, normal proteostasis is an important mechanism to regulate the neural survival and the central nervous system functionality. The ubiquitin-proteasome system (UPS) is a non-lysosomal proteolytic pathway involved in numerous normal functions of the nervous system, modulation of neurotransmitter release, synaptic plasticity, and recycling of membrane receptors or degradation of damaged and regulatory intracellular proteins. Aberrant accumulation of intracellular ubiquitin-positive inclusions has been implicated to a variety of neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease (HD), Amyotrophic Lateral Sclerosis (ALS), and Multiple Myeloma (MM). Genetic mutation in deubiquitinating enzyme could disrupt UPS and results in destructive effects on neuron survival. To date, various agents were characterized with proteasome-inhibitory potential. Proteins of the ubiquitin-proteasome system, and in particular, E3 ubiquitin ligases, may be promising molecular targets for neurodegenerative drug discovery. Phytochemicals, specifically polyphenols (PPs), were reported to act as proteasome-inhibitors or may modulate the proteasome activity. PPs modify the UPS by means of accumulation of ubiquitinated proteins, suppression of neuronal apoptosis, reduction of neurotoxicity, and improvement of synaptic plasticity and transmission. This is the first comprehensive review on the effect of PPs on UPS. Here, we review the recent findings describing various aspects of UPS dysregulation in neurodegenerative disorders. This review attempts to summarize the latest reports on the neuroprotective properties involved in the proper functioning of natural polyphenolic compounds with implication for targeting ubiquitin-proteasome pathway in the neurodegenerative diseases. We highlight the evidence suggesting that polyphenolic compounds have a dose and disorder dependent effects in improving neurological dysfunctions, and so their mechanism of action could stimulate the UPS, induce the protein degradation or inhibit UPS and reduce protein degradation. Future studies should focus on molecular mechanisms by which PPs can interfere this complex regulatory system at specific stages of the disease development and progression.

Disrupted blood-brain barrier in 5×FAD mouse model of Alzheimer's disease can be mimicked and repaired in vitro with neural stem cell-derived exosomes

Alzheimer's disease (AD) is a devastating neurodegenerative disease and is associated with blood-brain barrier (BBB) disruption. AD mice and cell culture models play an essential role in understanding AD pathogenesis and validation of therapeutic reagents. One of the commonly used AD mice is the 5 × FAD mouse and previous studies have shown that BBB leakage occurs at 9 months of age in the mice. However, it remains unknown whether disrupted BBB also occurs in young animals and whether AD-caused BBB impairment can be replicated and further corrected in a cell culture model. Here, we examine BBB breakdown in the 5 × FAD mouse model at different ages including both pre-symptomatic and post-symptomatic ages and test an in vitro BBB model established with the 5 × FAD primary cerebral endothelial cells. Moreover, with the BBB in vitro model, we also examined the therapeutic effect of human neural stem cells (NSCs)-derived exosomes on AD-caused BBB leakage. Our result indicated that BBB breakdown in the 5 × FAD mice occurred at 4 months of age, which could be mimicked with an in vitro BBB model. Importantly, we further demonstrated that treatment of the in vitro BBB model with NSCs-derived exosomes reversed AD-caused BBB deficiency. The information should be useful for researchers to determine which ages of the AD mice should be employed in specific in vivo and in vitro studies and the data also suggest that AD-caused BBB disruption can be corrected at least by NSC-derived exosomes.

Modeling Sporadic Alzheimer's Disease by Efficient Direct Reprogramming of the Elderly Derived Disease Dermal Fibroblasts into Neural Stem Cells

Alzheimer's disease (AD) is a devastating neurodegenerative disorder, neuropathologically characterized by hyperphosphorylation of tau and formation of amyloid plaques. Most AD cases are sporadic with no clear cause. Cell models play an important role in understanding the pathogenesis of sporadic AD, and the cell reprogramming and epigenetic techniques have provided new avenue to model the disorder. However, since most sporadic AD patients are late-onset, it poses a challenge to reprogram elderly somatic cells into stem cells. Here, we report that combination of overexpressing a single transcription factor, hSOX, with nine small molecules, was able to directly reprogram elderly (55-75 years of age) sporadic AD and the age-matched healthy individual dermal fibroblasts into the induced neural stem cells (iNSCs). These cells possessed the typical neural stem cell properties and were able to be further differentiated into neurons and glia in vitro and in vivo. More importantly, AD iNSC-derived neurons showed hyperphosphorylation at several sites of tau and increased release of Aβ into culture medium, indicating the replication of the major neuropathological hallmarks. Thus, we described a new technique to directly convert elderly AD dermal fibroblasts into iNSCs that may serve as a useful tool for studying the pathogenesis of sporadic AD and for drug discovery to treat the disorder.

Genetic Pathways of Aging and Their Relevance in the Dog as a Natural Model of Human Aging

Aging research has experienced a burst of scientific efforts in the last decades as the growing ratio of elderly people has begun to pose an increased burden on the healthcare and pension systems of developed countries. Although many breakthroughs have been reported in understanding the cellular mechanisms of aging, the intrinsic and extrinsic factors that contribute to senescence on higher biological levels are still barely understood. The dog, Canis familiaris, has already served as a valuable model of human physiology and disease. The possible role the dog could play in aging research is still an open question, although utilization of dogs may hold great promises as they naturally develop age-related cognitive decline, with behavioral and histological characteristics very similar to those of humans. In this regard, family dogs may possess unmatched potentials as models for investigations on the complex interactions between environmental, behavioral, and genetic factors that determine the course of aging. In this review, we summarize the known genetic pathways in aging and their relevance in dogs, putting emphasis on the yet barely described nature of certain aging pathways in canines. Reasons for highlighting the dog as a future aging and gerontology model are also discussed, ranging from its unique evolutionary path shared with humans, its social skills, and the fact that family dogs live together with their owners, and are being exposed to the same environmental effects.

Proteasome Activation as a New Therapeutic Approach To Target Proteotoxic Disorders

Proteasomes are multienzyme complexes that maintain protein homeostasis (proteostasis) and important cellular functions through the degradation of misfolded, redundant, and damaged proteins. It is well established that aging is associated with the accumulation of damaged and misfolded proteins. This phenomenon is paralleled by declined proteasome activity. When the accumulation of redundant proteins exceed degradation, undesirable signaling and/or aggregation occurs and are the hallmarks of neurodegenerative diseases and many cancers. Thus, increasing proteasome activity has been recognized as a new approach to delay the onset or ameliorate the symptoms of neurodegenerative and other proteotoxic disorders. Enhancement of proteasome activity has many therapeutic potentials but is still a relatively unexplored field. In this perspective, we review current approaches, genetic manipulation, posttranslational modification, and small molecule proteasome agonists used to increase proteasome activity, challenges facing the field, and applications beyond aging and neurodegenerative diseases.

Could a Common Mechanism of Protein Degradation Impairment Underlie Many Neurodegenerative Diseases?

At the cellular level, many neurodegenerative diseases (NDs), often considered proteinopathies, are characterized by the accumulation of misfolded and damaged proteins into large insoluble aggregates. Prominent species that accumulate early and play fundamental roles in disease pathogenesis are amyloid β (Aβ) and tau in Alzheimer disease, α-synuclein (α-syn) in Parkinson disease, and polyQ-expanded huntingtin (Htt) in Huntington disease. Although significant efforts have focused on how the cell deals with these protein aggregates, why is it that these misfolded proteins are not degraded normally in the first place? A vast body of literature supports the notion that the cell's protein degradation system for individual proteins-the ubiquitin proteasome system (UPS)-does not function sufficiently in many NDs. The proteasome itself has received significant focus for years due to its obvious failure to degrade misfolded proteins in ND, but no general mechanism has been uncovered. We have recently found that specific pathologically relevant oligomers can potently and directly inhibit the proteasome. What is most interesting is that the misfolded protein's primary amino acid sequence was irrelevant to its ability to inhibit. Instead, the culprit is the 3-dimensional shape of the misfolded oligomers. It turns out that many misfolded proteins in ND can take on this proteasome-impairing shape suggesting that there could be a common mechanism for UPS impairment in many NDs. The proteasome is already an important target for treating cancer, could it also be targeted to broadly treat ND?

Overexpression of Ubiquilin-1 Alleviates Alzheimer's Disease-Caused Cognitive and Motor Deficits and Reduces Amyloid-β Accumulation in Mice

Ubiquilin-1 (Ubqln1) is a ubiquitin-like protein that has been implicated in Alzheimer's disease (AD). However, whether Ubqln1 modulates learning and memory and alters AD-like behavior and/or pathology has not been determined in animal models. To understand the function of Ubqln1 in vivo, we previously generated Ubqln1 transgenic (TG) mice that overexpress mouse Ubqln1. With the model, we here characterized the TG mouse cognitive behaviors and found that Ubqln1 TG mice showed better spatial learning and memory capabilities than their wild-type littermates in both radial arm water maze and Y-maze tests. Additionally, we crossed the Ubqln1 TG mice with the AβPPswe/PSEN1dE9 double transgenic AD mouse to generate the AD/Ubqln1 triple TG (AD/TG) mice. Our results suggest that at 12 months of age following the onset of AD, AD/TG mice showed better spatial learning and memory than AD mice. AD/TG mice also exhibited better motor function than AD mice at the same age. Furthermore, compared to AD mice, AD/TG mice showed significant reduction in amyloid-β 40 (Aβ40) and Aβ42 levels in the cerebral cortex and in the hippocampus at the post-onset stage. The number of Aβ plaques was significantly decreased in the cerebral cortex of AD/TG mice at this post-onset stage. Moreover, mature AβPP level in AD/TG hippocampus was lower than that in AD hippocampus. These data not only provide a direct link between overexpression of Ubqln1 and altered learning and memory, but also raise the possibility that Ubqln1 is a potential therapeutic target for treating AD and possibly other neurodegenerative disorders.

Proteasome stress leads to APP axonal transport defects by promoting its amyloidogenic processing in lysosomes

Alzheimer Disease (AD) pathology includes the accumulation of poly-ubiquitinated proteins and failures in proteasome-dependent degradation. Whereas the distribution of proteasomes and its role in synaptic function have been studied, whether proteasome activity regulates the axonal transport and metabolism of the amyloid precursor protein (APP), remains elusive. Using live imaging in primary hippocampal neurons, we showed that proteasome inhibition rapidly and severely impairs the axonal transport of APP. Fluorescent cross-correlation analyses and membrane internalization blockage showed that plasma membrane APP do not contribute to transport defects. Moreover, by western blots and double-color APP imaging we demonstrated that proteasome inhibition precludes APP axonal transport by enhancing its endo-lysosomal delivery where β-cleavage is induced. Together, we found that proteasomes controls the distal transport of APP and can re-distribute Golgi-derived vesicles to the endo-lysosomal pathway. This crosstalk between proteasomes and lysosomes regulates APP intracellular dynamics, and defects in proteasome activity can be considered a contributing factor that lead to abnormal APP metabolism in AD.

Enhanced Proteostasis in Post-ischemic Stroke Mouse Brains by Ubiquilin-1 Promotes Functional Recovery

Stroke is pathologically associated with oxidative stress, protein damage, and neuronal loss. We previously reported that overexpression of a ubiquitin-like protein, ubiquilin-1 (Ubqln), protects neurons against ischemia-caused brain injury, while knockout of the gene exacerbates cerebral ischemia-caused neuronal damage and delays functional recovery. Although these observations indicate that Ubqln is a potential therapeutic target, transgenic manipulation-caused overexpression of Ubqln occurs before the event of ischemic stroke, and it remains unknown whether delayed Ubqln overexpression in post-ischemic brains within a clinically relevant time frame is still beneficial. To address this question, we generated lentiviruses (LVs) either overexpressing or knocking down mouse Ubqln, and treated post-ischemic stroke mice 6 h following the middle cerebral artery occlusion with the LVs before animal behaviors were evaluated at day 1, 3, 5, and 7. Our data indicate that post-ischemic overexpression of Ubqln significantly promoted functional recovery, whereas post-ischemic downregulation of Ubqln expression delays functional recovery. To further understand the mechanisms underlying how Ubqln functions, we also isolated protein aggregates from the brains of wild-type mice or the mice overexpressing Ubqln following ischemia/reperfusion. Western blot analysis indicates that overexpression of Ubqln significantly reduced the accumulation of protein aggregates. These observations not only suggest that Ubqln is a useful candidate for therapeutic intervention for ischemic stroke but also highlight the significance of proteostasis in functional recovery following stroke.

Immunoproteasome deficiency alters microglial cytokine response and improves cognitive deficits in Alzheimer's disease-like APPPS1 mice

The immunoproteasome (iP) represents a specialized type of proteasomes, which plays an important role in the clearance of oxidant-damaged proteins under inflammatory and pathological conditions determining the outcome of various diseases. In Alzheimer’s disease (AD)-like APPPS1 mice Aβ-deposition is paralleled by iP upregulation, most likely mediated through type I interferon induction. To define the impact of increased iP expression we crossed APPPS1 mice with mice deficient in the iP subunit LMP7 resulting in impaired iP function. While LMP7 deficient APPPS1 mice showed no major change in cerebral Aβ-pathology, we observed an altered cytokine response in microglia isolated from LMP7 deficient APPPS1 mice compared to LMP7 expressing APPPS1 control mice. The altered microglial cytokine profile upon iP deficiency in the presence of extracellular Aβ-pathology was associated with an improvement of Aβ-associated cognitive deficits typically present in APPPS1 mice. Our findings suggest a role for iP in the regulation of the innate immune response towards extracellular Aβ-pathology and indicate that inhibition of iP function can modulate the cognitive phenotype upon overexpression of Aβ.

Reduced body weight gain in ubiquilin-1 transgenic mice is associated with increased expression of energy-sensing proteins

Ubiquilin-1 (Ubqln1), a ubiquitin-like protein, is implicated in a variety of pathophysiological processes, but its role in mediating body weight gain or metabolism has not been determined. Here, we demonstrate that global overexpression of Ubqln1 in a transgenic (Tg) mouse reduces the animal's body weight gain. The decreased body weight gain in Tg mice is associated with lower visceral fat content and higher metabolic rate. The Ubqln1 Tg mice exhibited reduced leptin and insulin levels as well as increased insulin sensitivity manifested by homeostatic model assessment of insulin resistance. Additionally, the reduced body weight in Tg mice was associated with the upregulation of two energy-sensing proteins, sirtuin1 (SIRT1) in the hypothalamus and AMP-activated protein kinase (AMPK) in the skeletal muscle. Consistent with the in vivo results, overexpression of Ubqln1 significantly increased SIRT1 and AMPK levels in the mouse embryonic fibroblast cell culture. Thus, our results not only establish the link between Ubqln1 and body weight regulation but also indicate that the metabolic function of Ubqln1 on body weight may be through regulating energy-sensing proteins.

USP14 inhibitor attenuates cerebral ischemia/reperfusion-induced neuronal injury in mice

Stroke is associated with over-production of misfolded and aggregating proteins. However, it remains largely unclear whether enhanced removal of protein aggregates following ischemic stroke is neuroprotective. Deubiquitinating enzymes (DUBs) are a large group of proteases that regulate protein degradation. The ubiquitin-specific protease 14 (USP14) is a DUB that is associated with the proteasome and negatively regulates proteasome activity. In this study, we examined the effect of 1-[1-(4-fluorophenyl)-2,5-dimethylpyrrol-3-yl]-2-pyrrolidin-1-ylethanone (IU1), a specific small molecule inhibitor of USP14, on mouse focal cerebral ischemic stroke-induced neuronal injury in mice. We found that IU1 treatment attenuated ischemic stroke-caused neuronal injury, which was reflected by increased survival rate, reduced infarct volume, as well as decreased neuronal loss in the IU1-treated mice compared to the control-treated mice. Additionally, IU1 treatment is associated with reduced protein aggregates and enhanced proteasome functionality. These data not only highlight the significance of protein homeostasis in cerebral ischemia/reperfusion-induced neuronal injury but also extend the therapeutic role of DUB inhibitors.

Proteotoxicity and cardiac dysfunction

Baseline physiological function of the mammalian heart is under the constant threat of environmental or intrinsic pathological insults. Cardiomyocyte proteins are thus subject to unremitting pressure to function optimally, and this depends on them assuming and maintaining proper conformation. This review explores the multiple defenses a cell may use for its proteins to assume and maintain correct protein folding and conformation. There are multiple quality control mechanisms to ensure that nascent polypeptides are properly folded and mature proteins maintain their functional conformation. When proteins do misfold, either in the face of normal or pathological stimuli or because of intrinsic mutations or post-translational modifications, they must either be refolded correctly or recycled. In the absence of these corrective processes, they may become toxic to the cell. Herein, we explore some of the underlying mechanisms that lead to proteotoxicity. The continued presence and chronic accumulation of misfolded or unfolded proteins can be disastrous in cardiomyocytes because these misfolded proteins can lead to aggregation or the formation of soluble peptides that are proteotoxic. This in turn leads to compromised protein quality control and precipitating a downward spiral of the cell's ability to maintain protein homeostasis. Some underlying mechanisms are discussed and the therapeutic potential of interfering with proteotoxicity in the heart is explored.

Failure of ubiquitin proteasome system: risk for neurodegenerative diseases

The ubiquitin proteasome system (UPS) is the primary proteolytic quality control system in cells and has an essential function in the nervous system. UPS dysfunction has been linked to neurodegenerative conditions, including Alzheimer's, Parkinson's and Huntington's diseases. The pathology of neurodegenerative diseases is characterized by the abnormal accumulation of insoluble protein aggregates or inclusion bodies within neurons. The failure or dysregulation of the UPS prevents the degradation of misfolded/aberrant proteins, leading to deficient synaptic function that eventually affects the nervous system. In this review, we discuss the UPS and its physiological roles in the nervous system, its influence on neuronal function, and how UPS dysfunction contributes to the development of neurodegenerative diseases.

The proteasome activity reporter GFP-Cl1 is degraded by autophagy in the aging model Podospora anserina

The degradation of damaged proteins is an important vital function especially during aging and stress. The ubiquitin proteasome system is one of the major cellular machineries for protein degradation. Health and longevity are associated with high proteasome activity. To demonstrate such a role in aging of Podospora anserina, we first analyzed the transcript and protein abundance of selected proteasome components in wild-type cultures of different age. No significant differences were observed. Next, in order to increase the overall proteasome abundance we generated strains overexpressing the catalytic proteasome subunits PaPRE2 and PaPRE3. Although transcript levels were strongly increased, no substantial effect on the abundance of the corresponding proteins was observed. Finally, the analysis of the P. anserina strains expressing the sequence coding for the CL1 degron fused to the Gfp gene revealed no evidence for degradation of the GFP-CL1 fusion protein by the proteasome. Instead, our results demonstrate the degradation of the CL1-degron sequence via autophagy, indicating that basal autophagy appears to be a very effective protein quality control pathway in P. anserina.

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.