Impairment of protein degradation and proteasome function in hereditary neuropathies

USP14 inhibition enhances Parkin-independent mitophagy in iNeurons

Loss of proteostasis is well documented during physiological aging and depends on the progressive decline in the activity of two major degradative mechanisms: the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway. This decline in proteostasis is exacerbated in age-associated neurodegenerative diseases, such as Parkinson's Disease (PD). In PD, patients develop an accumulation of aggregated proteins and dysfunctional mitochondria, which leads to ROS production, neuroinflammation and neurodegeneration. We recently reported that inhibition of the deubiquitinating enzyme USP14, which is known to enhance both the UPS and autophagy, increases lifespan and rescues the pathological phenotype of two Drosophila models of PD. Studies on the effects of USP14 inhibition in mammalian neurons have not yet been conducted. To close this gap, we exploited iNeurons differentiated from human embryonic stem cells (hESCs), and investigated the effect of inhibiting USP14 in these cultured neurons. Quantitative global proteomics analysis performed following genetic ablation or pharmacological inhibition of USP14 demonstrated that USP14 loss of function specifically promotes mitochondrial autophagy in iNeurons. Biochemical and imaging data also showed that USP14 inhibition enhances mitophagy. The mitophagic effect of USP14 inhibition proved to be PINK1/Parkin- independent, instead relying on expression of the mitochondrial E3 Ubiquitin Ligase MITOL/MARCH5. Notably, USP14 inhibition normalized the mitochondrial defects of Parkin KO human neurons.

Pharmacologically increasing cGMP improves proteostasis and reduces neuropathy in mouse models of CMT1

Increasing cyclic GMP activates 26S proteasomes via phosphorylation by Protein Kinase G and stimulates the intracellular degradation of misfolded proteins. Therefore, agents that raise cGMP may be useful therapeutics against neurodegenerative diseases and other diseases in which protein degradation is reduced and misfolded proteins accumulate, including Charcot Marie Tooth 1A and 1B peripheral neuropathies, for which there are no treatments. Here we increased cGMP in the S63del mouse model of CMT1B by treating for three weeks with either the phosphodiesterase 5 inhibitor tadalafil, or the brain-penetrant soluble guanylyl cyclase stimulator CYR119. Both molecules activated proteasomes in the affected peripheral nerves, reduced polyubiquitinated proteins, and improved myelin thickness and nerve conduction. CYR119 increased cGMP more than tadalafil in the peripheral nerves of S63del mice and elicited greater biochemical and functional improvements. To determine whether raising cGMP could be beneficial in other neuropathies, we first showed that polyubiquitinated proteins and the disease-causing protein accumulate in the sciatic nerves of the C3 mouse model of CMT1A. Treatment of these mice with CYR119 reduced the levels of polyubiquitinated proteins and the disease-causing protein, presumably by increasing their degradation, and improved myelination, nerve conduction, and motor coordination. Thus, pharmacological agents that increase cGMP are promising treatments for CMT1 neuropathies and may be useful against other proteotoxic and neurodegenerative diseases.

Pharmacological inhibition of USP14 delays proteostasis-associated aging in a proteasome-dependent but foxo-independent manner

Aging is often accompanied by a decline in proteostasis, manifested as an increased propensity for misfolded protein aggregates, which are prevented by protein quality control systems, such as the ubiquitin-proteasome system (UPS) and macroautophagy/autophagy. Although the role of the UPS and autophagy in slowing age-induced proteostasis decline has been elucidated, limited information is available on how these pathways can be activated in a collaborative manner to delay proteostasis-associated aging. Here, we show that activation of the UPS via the pharmacological inhibition of USP14 (ubiquitin specific peptidase 14) using IU1 improves proteostasis and autophagy decline caused by aging or proteostatic stress in Drosophila and human cells. Treatment with IU1 not only alleviated the aggregation of polyubiquitinated proteins in aging Drosophila flight muscles but also extended the fly lifespan with enhanced locomotive activity via simultaneous activation of the UPS and autophagy. Interestingly, the effect of this drug disappeared when proteasomal activity was inhibited, but was evident upon proteostasis disruption by foxo mutation. Overall, our findings shed light on potential strategies to efficiently ameliorate age-associated pathologies associated with perturbed proteostasis.Abbreviations: AAAs: amino acid analogs; foxo: forkhead box, sub-group O; IFMs: indirect flight muscles; UPS: ubiquitin-proteasome system; USP14: ubiquitin specific peptidase 14.

The nociceptive activity of peripheral sensory neurons is modulated by the neuronal membrane proteasome

Proteasomes are critical for peripheral nervous system (PNS) function. Here, we investigate mammalian PNS proteasomes and reveal the presence of the neuronal membrane proteasome (NMP). We show that specific inhibition of the NMP on distal nerve fibers innervating the mouse hind paw leads to reduction in mechanical and pain sensitivity. Through investigating PNS NMPs, we demonstrate their presence on the somata and proximal and distal axons of a subset of dorsal root ganglion (DRG) neurons. Single-cell RNA sequencing experiments reveal that the NMP-expressing DRGs are primarily MrgprA3+ and Cysltr2+. NMP inhibition in DRG cultures leads to cell-autonomous and non-cell-autonomous changes in Ca2+ signaling induced by KCl depolarization, αβ-meATP, or the pruritogen histamine. Taken together, these data support a model whereby NMPs are expressed on a subset of somatosensory DRGs to modulate signaling between neurons of distinct sensory modalities and indicate the NMP as a potential target for controlling pain.

Knockout of PA200 improves proteasomal degradation and myelination in a proteotoxic neuropathy

The cellular response to a decrease in protein degradation by 26S proteasomes in chronic diseases is poorly understood. Pharmacological inhibition of proteasomes increases the expression of proteasome subunits and Proteasome Activator 200 (PA200), an alternative proteasome activator. In the S63del mouse model of the peripheral neuropathy Charcot Marie Tooth 1B (CMT1B), proteasomal protein degradation is decreased and proteasome gene expression is increased. Here, we show an increase in PA200 and PA200-bound proteasomes in the peripheral nerves of S63del mice. To test genetically whether the upregulation of PA200 was compensatory, we generated S63del//PA200-/- mice. Unexpectedly, in the sciatic nerves of these mice, there was greater proteasomal protein degradation than in S63del, less polyubiquitinated proteins and markers of the unfolded protein response, and a greater amount of assembled, active 26S proteasomes. These changes were not seen in PA200-/- controls and were therefore specific to the neuropathy. Furthermore, in S63del//PA200-/- mice, myelin thickness and nerve conduction were restored to WT levels. Thus, the upregulation of PA200 is maladaptive in S63del mice and its genetic ablation prevented neuropathy.

Proteostasis plays an important role in demyelinating Charcot Marie Tooth disease

Type 1 Charcot-Marie-Tooth disease (CMT1) is the most common demyelinating peripheral neuropathy. Patients suffer from progressive muscle weakness and sensory problems. The underlying disease mechanisms of CMT1 are still unclear and no therapy is currently available, hence patients completely rely on supportive care. Balancing protein levels is a complex multistep process fundamental to maintain cells in their healthy state and a disrupted proteostasis is a hallmark of several neurodegenerative diseases. When protein misfolding occurs, protein quality control systems are activated such as chaperones, the lysosomal-autophagy system and proteasomal degradation to ensure proper degradation. However, in pathological circumstances, these mechanisms are overloaded and thereby become inefficient to clear the load of misfolded proteins. Recent evidence strongly indicates that a disbalance in proteostasis plays an important role in several forms of CMT1. In this review, we present an overview of the protein quality control systems, their role in CMT1, and potential treatment strategies to restore proteostasis.

Inhibition of the Proteasome Regulator PA28 Aggravates Oxidized Protein Overload in the Diabetic Rat Brain

Oxidized protein overloading caused by diabetes is one accelerating pathological pathway in diabetic encephalopathy development. To determine whether the PA28-regulated function of the proteasome plays a role in diabetes-induced oxidative damaged protein degradation, brain PA28α and PA28β interference experiments were performed in a high-fat diet (HFD) and streptozotocin (STZ)-induced rat model. The present results showed that proteasome activity was changed in the brains of diabetic rats, but the constitutive subunits were not. In vivo PA28α and PA28β inhibition via adeno-associated virus (AAV) shRNA infection successfully decreased PA28 protein levels and further exacerbated oxidized proteins load by regulating proteasome catalytic activity. These findings suggest that the proteasome plays a role in the elimination of oxidized proteins and that PA28 is functionally involved in the regulation of proteasome activity in vivo. This study suggests that abnormal protein turbulence occurring in the diabetic brain could be explained by the proteasome-mediated degradation pathway. Changes in proteasome activity regulator PA28 could be a reason to induce oxidative aggregation in diabetic brain. Proteasome regulator PA28 inhibition in vivo by AAV vector injection could aggravate oxidized proteins abundance in brain of HFD-STZ diabetic rat model.

26S proteasomes become stably activated upon heat shock when ubiquitination and protein degradation increase

Heat shock (HS) promotes protein unfolding, and cells respond by stimulating HS gene expression, ubiquitination of cell proteins, and proteolysis by the proteasome. Exposing HeLa and other cells to 43 °C for 2 h caused a twofold increase in the 26S proteasomes' peptidase activity assayed at 37 °C. This increase in activity occurred without any change in proteasome amount and did not require new protein synthesis. After affinity-purification from HS cells, 26S proteasomes still hydrolyzed peptides, adenosine 5'-triphosphate, and ubiquitinated substrates more rapidly without any evident change in subunit composition, postsynthetic modification, or association with reported proteasome-activating proteins. After returning HS cells to 37 °C, ubiquitin conjugates and proteolysis fell rapidly, but proteasome activity remained high for at least 16 h. Exposure to arsenite, which also causes proteotoxic stress in the cytosol, but not tunicamycin, which causes endoplasmic reticulum stress, also increased ubiquitin conjugate levels and 26S proteasome activity. Although the molecular basis for the enhanced proteasomal activity remains elusive, we studied possible signaling mechanisms. Proteasome activation upon proteotoxic stress required the accumulation of ubiquitinated proteins since blocking ubiquitination by E1 inhibition during HS or arsenite exposure prevented the stimulation of 26S activity. Furthermore, increasing cellular content of ubiquitin conjugates at 37 °C by inhibiting deubiquitinating enzymes with RA190 or b-AP15 also caused proteasome activation. Thus, cells respond to proteotoxic stresses, apparently in response to the accumulation of ubiquitinated proteins, by activating 26S proteasomes, which should help promote the clearance of damaged cell proteins.

Treatment with IFB-088 Improves Neuropathy in CMT1A and CMT1B Mice

Charcot-Marie-Tooth disease type 1A (CMT1A), caused by duplication of the peripheral myelin protein 22 (PMP22) gene, and CMT1B, caused by mutations in myelin protein zero (MPZ) gene, are the two most common forms of demyelinating CMT (CMT1), and no treatments are available for either. Prior studies of the MpzSer63del mouse model of CMT1B have demonstrated that protein misfolding, endoplasmic reticulum (ER) retention and activation of the unfolded protein response (UPR) contributed to the neuropathy. Heterozygous patients with an arginine to cysteine mutation in MPZ (MPZR98C) develop a severe infantile form of CMT1B which is modelled by MpzR98C/ + mice that also show ER stress and an activated UPR. C3-PMP22 mice are considered to effectively model CMT1A. Altered proteostasis, ER stress and activation of the UPR have been demonstrated in mice carrying Pmp22 mutations. To determine whether enabling the ER stress/UPR and readjusting protein homeostasis would effectively treat these models of CMT1B and CMT1A, we administered Sephin1/IFB-088/icerguestat, a UPR modulator which showed efficacy in the MpzS63del model of CMT1B, to heterozygous MpzR98C and C3-PMP22 mice. Mice were analysed by behavioural, neurophysiological, morphological and biochemical measures. Both MpzR98C/ + and C3-PMP22 mice improved in motor function and neurophysiology. Myelination, as demonstrated by g-ratios and myelin thickness, improved in CMT1B and CMT1A mice and markers of UPR activation returned towards wild-type values. Taken together, our results demonstrate the capability of IFB-088 to treat a second mouse model of CMT1B and a mouse model of CMT1A, the most common form of CMT. Given the recent benefits of IFB-088 treatment in amyotrophic lateral sclerosis and multiple sclerosis animal models, these data demonstrate its potential in managing UPR and ER stress for multiple mutations in CMT1 as well as in other neurodegenerative diseases.

Raising cGMP restores proteasome function and myelination in mice with a proteotoxic neuropathy

Agents that raise cyclic guanosine monophosphate (cGMP) by activating protein kinase G increase 26S proteasome activities, protein ubiquitination and degradation of misfolded proteins. Therefore, they may be useful in treating neurodegenerative and other diseases caused by an accumulation of misfolded proteins. Mutations in myelin protein zero (MPZ) cause the peripheral neuropathy Charcot-Marie-Tooth type 1B (CMT1B). In peripheral nerves of a mouse model of CMT1B, where the mutant MPZS63del is expressed, proteasome activities are reduced, mutant MPZS63del and polyubiquitinated proteins accumulate and the unfolded protein response (p-eif2α) is induced. In HEK293 cells, raising cGMP stimulated ubiquitination and degradation of MPZS63del, but not of wild-type MPZ. Treating S63del mice with the phosphodiesterase 5 inhibitor, sildenafil-to raise cGMP-increased proteasome activity in sciatic nerves and reduced the levels of polyubiquitinated proteins, the proteasome reporter ubG76V-GFP and p-elF2α. Furthermore, sildenafil treatment reduced the number of amyelinated axons, and increased myelin thickness and nerve conduction velocity in sciatic nerves. Thus, agents that raise cGMP, including those widely used in medicine, may be useful therapies for CMT1B and other proteotoxic diseases.

Wrestling and Wrapping: A Perspective on SUMO Proteins in Schwann Cells

Schwann cell development and peripheral nerve myelination are finely orchestrated multistep processes; some of the underlying mechanisms are well described and others remain unknown. Many posttranslational modifications (PTMs) like phosphorylation and ubiquitination have been reported to play a role during the normal development of the peripheral nervous system (PNS) and in demyelinating neuropathies. However, a relatively novel PTM, SUMOylation, has not been studied in these contexts. SUMOylation involves the covalent attachment of one or more small ubiquitin-like modifier (SUMO) proteins to a substrate, which affects the function, cellular localization, and further PTMs of the conjugated protein. SUMOylation also regulates other proteins indirectly by facilitating non-covalent protein–protein interaction via SUMO interaction motifs (SIM). This pathway has important consequences on diverse cellular processes, and dysregulation of this pathway has been reported in several diseases including neurological and degenerative conditions. In this article, we revise the scarce literature on SUMOylation in Schwann cells and the PNS, we propose putative substrate proteins, and we speculate on potential mechanisms underlying the possible involvement of this PTM in peripheral myelination and neuropathies.

Small-Molecule Inhibitors Targeting Proteasome-Associated Deubiquitinases

The 26S proteasome is the principal protease for regulated intracellular proteolysis. This multi-subunit complex is also pivotal for clearance of harmful proteins that are produced throughout the lifetime of eukaryotes. Recent structural and kinetic studies have revealed a multitude of conformational states of the proteasome in substrate-free and substrate-engaged forms. These conformational transitions demonstrate that proteasome is a highly dynamic machinery during substrate processing that can be also controlled by a number of proteasome-associated factors. Essentially, three distinct family of deubiquitinases–USP14, RPN11, and UCH37–are associated with the 19S regulatory particle of human proteasome. USP14 and UCH37 are capable of editing ubiquitin conjugates during the process of their dynamic engagement into the proteasome prior to the catalytic commitment. In contrast, RPN11-mediated deubiquitination is directly coupled to substrate degradation by sensing the proteasome’s conformational switch into the commitment steps. Therefore, proteasome-bound deubiquitinases are likely to tailor the degradation events in accordance with substrate processing steps and for dynamic proteolysis outcomes. Recent chemical screening efforts have yielded highly selective small-molecule inhibitors for targeting proteasomal deubiquitinases, such as USP14 and RPN11. USP14 inhibitors, IU1 and its progeny, were found to promote the degradation of a subset of substrates probably by overriding USP14-imposed checkpoint on the proteasome. On the other hand, capzimin, a RPN11 inhibitor, stabilized the proteasome substrates and showed the anti-proliferative effects on cancer cells. It is highly conceivable that these specific inhibitors will aid to dissect the role of each deubiquitinase on the proteasome. Moreover, customized targeting of proteasome-associated deubiquitinases may also provide versatile therapeutic strategies for induced or repressed protein degradation depending on proteolytic demand and cellular context.

Mechanisms That Activate 26S Proteasomes and Enhance Protein Degradation

Although ubiquitination is widely assumed to be the only regulated step in the ubiquitin–proteasome pathway, recent studies have demonstrated several important mechanisms that regulate the activities of the 26S proteasome. Most proteasomes in cells are inactive but, upon binding a ubiquitinated substrate, become activated by a two-step mechanism requiring an association of the ubiquitin chain with Usp14 and then a loosely folded protein domain with the ATPases. The initial activation step is signaled by Usp14’s UBL domain, and many UBL-domain-containing proteins (e.g., Rad23, Parkin) also activate the proteasome. ZFAND5 is a distinct type of activator that binds ubiquitin conjugates and the proteasome and stimulates proteolysis during muscle atrophy. The proteasome’s activities are also regulated through subunit phosphorylation. Agents that raise cAMP and activate PKA stimulate within minutes Rpn6 phosphorylation and enhance the selective degradation of short-lived proteins. Likewise, hormones, fasting, and exercise, which raise cAMP, activate proteasomes and proteolysis in target tissues. Agents that raise cGMP and activate PKG also stimulate 26S activities but modify different subunit(s) and stimulate also the degradation of long-lived cell proteins. Both kinases enhance the selective degradation of aggregation-prone proteins that cause neurodegenerative diseases. These new mechanisms regulating proteolysis thus have clear physiological importance and therapeutic potential.

Role of UPP pathway in amelioration of diabetes-associated complications

Type 2 diabetes (T2D) is one of the most widely spread metabolic disorder also called as "life style" disease. Due to the alarming number of patients, there is great need to therapies targeting functions which can help in maintaining the homeostasis of glucose levels and improving insulin sensitivity. Detailed analysis was done through various research and review papers which was searched using MEDLINE, BIOSIS, and EMBASE using various keywords. This search retrieved the most appropriate content on these molecules targeting UPP pathway. From this extensive review involving UPP pathway, it was concluded that the role of ubiquitin's is not only limited to neurodegenerative disorders but also plays a critical role in progression of diabetes including obesity, insulin resistance, and various neurogenerative disorders but it also targets proteasomal degradation including mediation of cellular signaling pathways. Thus, drugs targeting UPP not only may show effect against diabetes but also are therapeutically beneficial in the treatment of diabetes-associated complications which may be obtained. Thus, based on the available information and data on UPP functions, it can be concluded that regulation of UPP pathway via downstream regulators mainly E1, E2, and E3 may bring promising results. Drugs targeting these transcriptional factors may emerge as a novel therapy in the treatment of diabetes and diabetes-associated complications.

cGMP via PKG activates 26S proteasomes and enhances degradation of proteins, including ones that cause neurodegenerative diseases

Most studies of the regulation of proteolysis by the ubiquitin proteasome system have focused on the control of ubiquitination. However, it is now clear that the activity of the 26S proteasome and rates of protein degradation in cells are also tightly regulated through proteasome phosphorylation. Here we demonstrate that agents that raise cGMP and activate cGMP-dependent protein kinase (e.g., widely used phosphodiesterase 5 inhibitors) stimulate proteasome activities and intracellular proteolysis without affecting autophagy. Furthermore, we showed that raising cGMP reduced the levels of the disease-causing mutant tau in a zebrafish model by increasing its degradation, and also decreased the associated morphological abnormalities. Thus, activating the proteasome via cGMP is a promising strategy to prevent the progression of neurodegenerative diseases.

Because raising cAMP enhances 26S proteasome activity and the degradation of cell proteins, including the selective breakdown of misfolded proteins, we investigated whether agents that raise cGMP may also regulate protein degradation. Treating various cell lines with inhibitors of phosphodiesterase 5 or stimulators of soluble guanylyl cyclase rapidly enhanced multiple proteasome activities and cellular levels of ubiquitinated proteins by activating protein kinase G (PKG). PKG stimulated purified 26S proteasomes by phosphorylating a different 26S component than is modified by protein kinase A. In cells and cell extracts, raising cGMP also enhanced within minutes ubiquitin conjugation to cell proteins. Raising cGMP, like raising cAMP, stimulated the degradation of short-lived cell proteins, but unlike cAMP, also markedly increased proteasomal degradation of long-lived proteins (the bulk of cell proteins) without affecting lysosomal proteolysis. We also tested if raising cGMP, like cAMP, can promote the degradation of mutant proteins that cause neurodegenerative diseases. Treating zebrafish models of tauopathies or Huntington’s disease with a PDE5 inhibitor reduced the levels of the mutant huntingtin and tau proteins, cell death, and the resulting morphological abnormalities. Thus, PKG rapidly activates cytosolic proteasomes, protein ubiquitination, and overall protein degradation, and agents that raise cGMP may help combat the progression of neurodegenerative diseases.

The Proteasome and Its Network: Engineering for Adaptability

The proteasome, the most complex protease known, degrades proteins that have been conjugated to ubiquitin. It faces the unique challenge of acting enzymatically on hundreds and perhaps thousands of structurally diverse substrates, mechanically unfolding them from their native state and translocating them vectorially from one specialized compartment of the enzyme to another. Moreover, substrates are modified by ubiquitin in myriad configurations of chains. The many unusual design features of the proteasome may have evolved in part to endow this enzyme with a robust ability to process substrates regardless of their identity. The proteasome plays a major role in preserving protein homeostasis in the cell, which requires adaptation to a wide variety of stress conditions. Modulation of proteasome function is achieved through a large network of proteins that interact with it dynamically, modify it enzymatically, or fine-tune its levels. The resulting adaptability of the proteasome, which is unique among proteases, enables cells to control the output of the ubiquitin-proteasome pathway on a global scale.

Physiological State Dictates the Proteasomal-Mediated Purging of Misfolded Protein Fragments

A pivotal feature that underlies the development of neurodegeneration is the accumulation of protein aggregates. In response, eukaryotic cells have evolved sophisticated quality control mechanisms to identify, repair and/or eliminate the misfolded abnormal proteins. Chaperones identify any otherwise abnormal conformations in proteins and often help them to regain their correct conformation. However, if repair is not an option, the abnormal protein is selectively degraded to prevent its oligomerization into toxic multimeric complexes. Autophagiclysosomal system and the ubiquitin-proteasome system mediate the targeted degradation of the aberrant protein fragments. Despite the increasing understanding of the molecular counteracting responses toward the accumulation of dysfunctional misfolded proteins, the molecular links between the upstream physiological inputs and the clearance of abnormal misfolded proteins is relatively poorly understood. Recent work has demonstrated that certain physiological states such as vigorous exercise and fasting may enhance the ability of mammalian cells to clear misfolded, toxic and aberrant protein fragments. These findings unveil a novel mechanism that activates the cells' protein-disposal machinery, facilitating the adaptation process of cellular proteome to fluctuations in cellular demands and alterations of environmental cues. Herein, we briefly discuss the molecular interconnection between certain physiological cues and proteasomal degradation pathway in the context of these interesting findings and highlight some of the future prospects.

Natural alkaloid harmine promotes degradation of alpha-synuclein via PKA-mediated ubiquitin-proteasome system activation

BACKGROUND

Parkinson's disease (PD) is an age-dependent progressive movement disorder characterized by a profound and selective loss of nigrostriatal dopaminergic neurons. Accumulation of <alpha>-synuclein (<alpha>-syn) positive protein aggregates in the substantia nigra is a pathological hallmark of PD, indicating that protein turnover defect is implicated in PD pathogenesis.

PURPOSE

This study aims to identify neuroprotective compounds which can alleviate the accumulation of <alpha>-syn in neuronal cells and dissect the underlying mechanisms.

METHODS

High throughput screening was performed by dot blot assay. The degradation of different forms of <alpha>-syn by candidate compounds were assessed by western blot. The autophagy lysosome pathway and ubiquitin-proteasome system were examined to dissect the degradation pathway. The UPS activity was assessed by cellular UPS substrates degradation assay and biochemical proteasome activity assay. Q-PCR was performed to test the mRNA level of different proteasome subunits. Furthermore, Neuroprotective effect of candidate compound was tested by LDH assay and PI staining.

RESULTS

Through the high throughput screening, harmine was identified as a potent <alpha>-syn lowering compound. The time-dependent and dose-dependent effects of harmine on the degradation of different forms of <alpha>-syn were further confirmed. Harmine could dramatically promote the degradation of UPS substrates GFP-CL1, Ub-R-GFP and Ub-G76V-GFP, and activate cellular proteasome activity. Mechanistically, harmine dramatically enhanced PKA phosphorylation to enhance proteasome subunit PSMD1 expression. PKA inhibitor blocked the effects of harmine in activating UPS, up regulating PSMD1 and promoting <alpha>-syn degradation, indicating that harmine enhances UPS function via PKA activation. Moreover, harmine efficiently rescued cell death induced by over-expression of <alpha>-syn, via UPS-dependent manner.

CONCLUSION

Harmine, as a new proteasome enhancer, may have potential to be developed into therapeutic agent against neurodegenerative diseases associated with UPS dysfunction and aberrant proteins accumulation.

Measurement of the Multiple Activities of 26S Proteasomes

Because proteasomes catalyze most of the protein degradation in mammalian cells, and their functioning is essential for cellular homeostasis, proteasome structure, biochemical mechanisms, and regulation in normal and disease states are now widely studied and are of major importance. In addition, inhibitors of the proteasome's peptidase activity have proven to be very valuable as research tools and in the treatment of hematologic malignancies, and a number of newer pharmacological agents that alter proteasome function are being developed. The rapid degradation of ubiquitinated proteins by the 26S proteasome involves multiple enzymatic and non-enzymatic steps, including the binding of ubiquitinated substrates to the 19S particle (Subheading 3.2), opening the gated substrate entry channel into the 20S particle (Subheading 3.3), disassembly of the Ub chain (Subheading 3.4), ATP hydrolysis (Subheading 3.5), substrate unfolding and translocation, and proteolysis within the 20S particle (Subheadings 3.3 and 3.7). Assaying each of these processes is important if we are to fully understand the physiological regulation of proteasome function and the effects of disease or drugs. Here, we describe several methods that we have found useful to measure many of these individual activities using purified proteasomes. Studies using these approaches have already provided valuable new insights into the effects of post-synthetic modifications to 26S subunits, the physiological regulation of the ubiquitin-proteasome system, and the impairment of proteasome activity in neurodegenerative disease. These advances would not have been possible if only the standard assays of peptidase activity were used.

Schwann cells ER-associated degradation contributes to myelin maintenance in adult nerves and limits demyelination in CMT1B mice

In the peripheral nervous system (PNS) myelinating Schwann cells synthesize large amounts of myelin protein zero (P0) glycoprotein, an abundant component of peripheral nerve myelin. In humans, mutations in P0 cause the demyelinating Charcot-Marie-Tooth 1B (CMT1B) neuropathy, one of the most diffused genetic disorders of the PNS. We previously showed that several mutations, such as the deletion of serine 63 (P0-S63del), result in misfolding and accumulation of P0 in the endoplasmic reticulum (ER), with activation of the unfolded protein response (UPR). In addition, we observed that S63del mouse nerves display the upregulation of many ER-associated degradation (ERAD) genes, suggesting a possible involvement of this pathway in the clearance of the mutant P0. In ERAD in fact, misfolded proteins are dislocated from the ER and targeted for proteasomal degradation. Taking advantage of inducible cells that express the ER retained P0, here we show that the P0-S63del glycoprotein is degraded via ERAD. Moreover, we provide strong evidence that the Schwann cell-specific ablation of the ERAD factor Derlin-2 in S63del nerves exacerbates both the myelin defects and the UPR in vivo, unveiling a protective role for ERAD in CMT1B neuropathy. We also found that lack of Derlin-2 affects adult myelin maintenance in normal nerves, without compromising their development, pinpointing ERAD as a previously unrecognized player in preserving Schwann cells homeostasis in adulthood. Finally, we provide evidence that treatment of S63del peripheral nerve cultures with N-Acetyl-D-Glucosamine (GlcNAc), known to enhance protein quality control pathways in C.elegans, ameliorates S63del nerve myelination ex vivo. Overall, our study suggests that potentiating adaptive ER quality control pathways might represent an appealing strategy to treat both conformational and age-related PNS disorders.

Charcot-Marie-Tooth neuropathies are a large family of peripheral nerve disorders, showing extensive clinical and genetic heterogeneity. Although strong advances have been made in the identification of genes and mutations involved, effective therapies are still lacking. Intracellular retention of abnormal proteins has been recently suggested as one of the pathogenetic events that might underlie several conformational neuropathies. To limit the toxic effects of accumulated mutant proteins, cells have developed efficient protein quality control systems aimed at optimizing both protein folding and degradation. Here we show that ER-associated degradation limits Schwann cells stress and myelin defects caused by the accumulation of a mutant myelin protein into the ER. In addition, we also describe for the first time the importance of Schwann cells ERAD in preserving myelin integrity in adult nerves, showing that genetic ERAD impairment leads to a late onset, motor-predominant, peripheral neuropathy in vivo. Effort in the design of strategies that potentiate ERAD and ER quality controls is therefore highly desirable.

Structurally-defined deubiquitinase inhibitors provide opportunities to investigate disease mechanisms

The Ubiquitin/Proteasome System comprises an essential cellular mechanism for regulated protein degradation. Ubiquitination may also promote the assembly of protein complexes that initiate intracellular signaling cascades. Thus, proper regulation of substrate protein ubiquitination is essential for maintaining normal cellular physiology. Deubiquitinases are the class of enzymes responsible for removing ubiquitin modifications from target proteins and have been implicated in regulating human disease. As such, deubiquitinases are now recognized as emerging drug targets. Small molecule deubiquitinase inhibitors have been developed; among those, inhibitors for the deubiquitinases USP7 and USP14 are the best-characterized given that they are structurally validated. In this review we discuss the normal physiological roles of the USP7 and USP14 deubiquitinases as well as the pathological conditions associated with their dysfunction, with a focus on oncology and neurodegenerative diseases. We also review structural biology of USP7 and USP14 enzymes and the characterization of their respective inhibitors, highlighting the various molecular mechanisms by which these deubiquitinases may be functionally inhibited. Finally, we summarize the cellular and in vivo studies performed using the structurally-validated USP7 and USP14 inhibitors.

A Pathogen and a Non-pathogen Spotted Fever Group Rickettsia Trigger Differential Proteome Signatures in Macrophages

We have previously reported that Rickettsia conorii and Rickettsia montanensis have distinct intracellular fates within THP-1 macrophages, suggesting that the ability to proliferate within macrophages may be a distinguishable factor between pathogenic and non-pathogenic Spotted fever group (SFG) members. To start unraveling the molecular mechanisms underlying the capacity (or not) of SFG Rickettsia to establish their replicative niche in macrophages, we have herein used quantitative proteomics by SWATH-MS to profile the alterations resulted by the challenge of THP-1 macrophages with R. conorii and R. montanensis. We show that the pathogenic, R. conorii, and the non-pathogenic, R. montanensis, member of SFG Rickettsia trigger differential proteomic signatures in macrophage-like cells upon infection. R. conorii specifically induced the accumulation of several enzymes of the tricarboxylic acid cycle, oxidative phosphorylation, fatty acid β-oxidation, and glutaminolysis, as well as of several inner and outer membrane mitochondrial transporters. These results suggest a profound metabolic rewriting of macrophages by R. conorii toward a metabolic signature of an M2-like, anti-inflammatory activation program. Moreover, several subunits forming the proteasome and immunoproteasome are found in lower abundance upon infection with both rickettsial species, which may help bacteria to escape immune surveillance. R. conorii-infection specifically induced the accumulation of several host proteins implicated in protein processing and quality control in ER, suggesting that this pathogenic Rickettsia may be able to increase the ER protein folding capacity. This work reveals novel aspects of macrophage-Rickettsia interactions, expanding our knowledge of how pathogenic rickettsiae explore host cells to their advantage.

26S Proteasomes are rapidly activated by diverse hormones and physiological states that raise cAMP and cause Rpn6 phosphorylation

Pharmacological agents that raise cAMP and activate protein kinase A (PKA) stimulate 26S proteasome activity, phosphorylation of subunit Rpn6, and intracellular degradation of misfolded proteins. We investigated whether a similar proteasome activation occurs in response to hormones and under various physiological conditions that raise cAMP. Treatment of mouse hepatocytes with glucagon, epinephrine, or forskolin stimulated Rpn6 phosphorylation and the 26S proteasomes' capacity to degrade ubiquitinated proteins and peptides. These agents promoted the selective degradation of short-lived proteins, which are misfolded and regulatory proteins, but not the bulk of cell proteins or lysosomal proteolysis. Proteasome activities and Rpn6 phosphorylation increased similarly in working hearts upon epinephrine treatment, in skeletal muscles of exercising humans, and in electrically stimulated rat muscles. In WT mouse kidney cells, but not in cells lacking PKA, treatment with antidiuretic hormone (vasopressin) stimulated within 5-minutes proteasomal activity, Rpn6 phosphorylation, and the selective degradation of short-lived cell proteins. In livers and muscles of mice fasted for 12-48 hours cAMP levels, Rpn6 phosphorylation, and proteasomal activities increased without any change in proteasomal content. Thus, in vivo cAMP-PKA-mediated proteasome activation is a common cellular response to diverse endocrine stimuli and rapidly enhances the capacity of target tissues to degrade regulatory and misfolded proteins (e.g., proteins damaged upon exercise). The increased destruction of preexistent regulatory proteins may help cells adapt their protein composition to new physiological conditions.

In vitro analysis of proteasome-associated USP14 activity for substrate degradation and deubiquitylation

The ubiquitin-proteasome pathway plays an essential role in maintaining protein homeostasis and regulates almost every aspect of cellular processes in eukaryotes. Emerging evidence indicates that the proteasome does not work as a simple unidirectional molecular machinery for substrate proteolysis. In fact, proteasome activity should be tightly regulated, and the proteasome itself can be dynamically engaged in the degradation cycle. Proteasome-mediated degradation can occur through multistep mechanisms such as ubiquitin-dependent substrate recognition, deubiquitination, and ATP-driven unfolding and translocation of the substrate into 20S chamber for proteolytic cleavage. Deubiquitination is particularly interesting because this reaction may impose a critical checkpoint for substrate turnover on the proteasome. Notably, there are three major deubiquitinating enzymes (DUBs) on human proteasomes: USP14, UCH37, and RPN11. USP14 can spare the substrate from degradation prior to the proteasome's commitment step, suggesting that USP14 inhibition may stimulate proteasomal degradation of undesirable proteins under certain proteotoxic conditions. Furthermore, USP14 deubiquitinates multichain conjugates, the first among ~100 DUBs found to have this striking specificity. In this chapter, we describe in vitro methods to test proteasome-associated USP14 activity for substrate degradation and deubiquitylation.

Nrf1 is paved as a new strategic avenue to prevent and treat cancer, neurodegenerative and other diseases

Transcription factor Nrf1 acts as a unique vital player in maintaining cellular homeostasis and organ integrity during normal development and growth throughout the life process. Loss-of-function of Nrf1 results in severe oxidative stress, genomic instability, embryonic lethality, developmental disorders, and adult diseases such as non-alcoholic steatohepatitis, hepatocellular carcinoma, diabetes and neurogenerative diseases. Thereby, Nrf1 is critically implicated in a variety of important physio-pathological processes by governing robust target genes in order to reinforce antioxidant, detoxification and cytoprotective responses to cellular stress. Notably, there also exists a proteasomal 'bounce-back' response mediated by Nrf1, insofar as to enhance the drug resistance to proteasomal inhibitors in clinical treatment of neuroblastoma, multiple myeloma and triple-negative breast cancers. Recently, several drugs or chemicals are found or re-found in new ways to block the proteasomal compensatory process through inhibiting the multistep processing of Nrf1. Conversely, activation of Nrf1 induced by some drugs or chemicals leads to cytoprotection from cell apoptosis and promotes cell viability. This is the start of constructive and meaningful studies, approaching to explore the mechanism(s) by which Nrf1 is activated to protect neurons and other cells from malignant and degenerative diseases. Overall, Nrf1 has appealed attentions as a new attractive therapeutic strategy for human diseases including cancers.

Exploring the Regulation of Proteasome Function by Subunit Phosphorylation

Rates of degradation by the ubiquitin proteasome system depend not only on rates of ubiquitination, but also on the level of proteasome activity which can be regulated through phosphorylation of proteasome subunits. Many protein kinases have been proposed to influence proteasomal activity. However, for only two is there strong evidence that phosphorylation of a specific 26S subunit enhances the proteasome's capacity to degrade ubiquitinated proteins and promotes protein breakdown in cells: (1) protein kinase A (PKA), which after a rise in cAMP phosphorylates the 19S subunit Rpn6, and (2) dual tyrosine receptor kinase 2 (DYRK2), which during S through M phases of the cell cycle phosphorylates the 19S ATPase subunit Rpt3. In this chapter, we review and discuss the different methods used to assess the impact of phosphorylation by these two kinases on proteasomal activity and intracellular protein degradation. In addition, we present one method to determine if phosphorylation is responsible for an observed increase in proteasomal activity and another to evaluate by Phos-tag gel electrophoresis whether a specific proteasome subunit is modified by phosphorylation. The methods reviewed and presented here should be useful in clarifying the roles of other kinases and other posttranslational modifications of proteasome subunits.

Methods to Rapidly Prepare Mammalian 26S Proteasomes for Biochemical Analysis

Rapid, gentle isolation of 26S proteasomes from cells or tissues is an essential step for studies of the changes in proteasome activity and composition that can occur under different physiological or pathological conditions and in response to pharmacological agents. We present here three different approaches to affinity purify or to prepare proteasome-rich cell fractions. The first method uses affinity tags fused to proteasome subunits and has been useful in several cell lines for studies of proteasome structure by cryo-electron microscopy and composition by mass spectrometry. A second method uses the proteasome's affinity for a ubiquitin-like (UBL) domain and can be used to purify these particles from any cell or tissue. This method does not require expression of a tagged subunit and has proven to be very useful to investigate how proteasomal activity changes in different physiological states (e.g., fasting or aging), with neurodegenerative diseases, and with drugs or hormones that cause subunit phosphorylation. A third, simple method that is based on the 26S proteasome's high molecular weight (about 2.5 MDa) concentrates these particles greatly by differential centrifugation. This method maintains the association of proteasomes with ubiquitin (Ub) conjugates and many other loosely associated regulatory proteins and is useful to study changes in proteasome composition under different conditions.