Regulation of proteasome activity in health and disease

Base-CP proteasome can serve as a platform for stepwise lid formation

26S proteasome, a major regulatory protease in eukaryotes, consists of a 20S proteolytic core particle (CP) capped by a 19S regulatory particle (RP). The 19S RP is divisible into base and lid sub-complexes. Even within the lid, subunits have been demarcated into two modules: module 1 (Rpn5, Rpn6, Rpn8, Rpn9 and Rpn11), which interacts with both CP and base sub-complexes and module 2 (Rpn3, Rpn7, Rpn12 and Rpn15) that is attached mainly to module 1. We now show that suppression of RPN11 expression halted lid assembly yet enabled the base and 20S CP to pre-assemble and form a base-CP. A key role for Regulatory particle non-ATPase 11 (Rpn11) in bridging lid module 1 and module 2 subunits together is inferred from observing defective proteasomes in rpn11–m1, a mutant expressing a truncated form of Rpn11 and displaying mitochondrial phenotypes. An incomplete lid made up of five module 1 subunits attached to base-CP was identified in proteasomes isolated from this mutant. Re-introducing the C-terminal portion of Rpn11 enabled recruitment of missing module 2 subunits. In vitro, module 1 was reconstituted stepwise, initiated by Rpn11–Rpn8 heterodimerization. Upon recruitment of Rpn6, the module 1 intermediate was competent to lock into base-CP and reconstitute an incomplete 26S proteasome. Thus, base-CP can serve as a platform for gradual incorporation of lid, along a proteasome assembly pathway. Identification of proteasome intermediates and reconstitution of minimal functional units should clarify aspects of the inner workings of this machine and how multiple catalytic processes are synchronized within the 26S proteasome holoenzymes.

Defective proteasome 19S regulatory particles (RPs) were identified in rpn11f–m1, a proteasomal mutant with mitochondrial phenotypes. The Rpn11 subunit initiates assembly of a five-subunit lid module competent to integrate into pre-assembled base-20S core particle (CP), with subsequent recruitment of remaining lid subunits.

Mediation of organismal aging and somatic proteostasis by the germline

Experimental interventions that reduce reproduction cause an extension in lifespan. In invertebrates, such as Caenorhabditis elegans, the aging of the soma is regulated by signals from the germline. Indeed, ablation of germ cells significantly extends lifespan. Notably, germline-deficient animals exhibit heightened resistance to proteotoxic stress. This phenotype correlates with increased potential of intracellular clearance mechanisms such as the proteasome and autophagy in somatic tissues. Here we review the molecular mechanisms by which signals from the germline regulate lifespan in C. elegans with special emphasis on clearance mechanisms.

Autophagy and ubiquitination in cardiovascular diseases

A main function of the heart is to pump blood to the tissues and organs of the body. Although formed by different types of cells, the cardiomyocytes are the ones responsible for the coordinated and synchronized heart contraction. Given their low mitotic activity, cardiomyocytes largely depend on protein degradation mechanisms to maintain proteostasis and energetic balance. Autophagy, one of the main pathways whereby cells eliminate damaged, nonfunctional, or obsolete proteins, and organelles, is vital to ensure cell function, including in cardiomyocytes, both in rest and stress conditions. However, the impact of autophagy activation in the heart, being either protective or harmful, is not consensual and likely depends upon the severity of the stimuli and consequently the autophagy players involved. One of the signals that direct proteins for autophagy degradation, namely in the context of heart disorders, is ubiquitin. Indeed, the attachment of ubiquitin moieties to a target substrate and further recognition by autophagy adaptors constitute a main regulatory pathway that directs proteins to the lysosome. Therefore, a better understanding of the mechanisms and signals that regulate the autophagy process in the heart, including substrates targeting, is of utmost importance to design new approaches directed to this degradation pathway. We have previously shown that ubiquitination of the gap junction (GJ) protein Connexin43 (Cx43) triggers its degradation by autophagy through a process that requires the ubiquitin adaptors epidermal growth factor receptor substrate 15 (Eps15) and p62. This is particularly relevant in the heart because GJs, that form intercellular channels, are responsible for the rapid and efficient anisotropic propagation of the electrical impulse through the cardiomyocytes, essential for synchronized contraction of the cardiac muscle. In this review, we present recent studies devoted to the involvement of autophagy in heart homeostasis, with a particular focus on ubiquitin and GJs.

Expression of FSHD-related DUX4-FL alters proteostasis and induces TDP-43 aggregation

Objective

Pathogenesis in facioscapulohumeral muscular dystrophy (FSHD) appears to be due to aberrant expression, particularly in skeletal muscle nuclei, of the full-length isoform of DUX4 (DUX4-FL). Expression of DUX4-FL is known to alter gene expression and to be cytotoxic, but cell responses to DUX4-FL are not fully understood. Our study was designed to identify cellular mechanisms of pathogenesis caused by DUX4-FL expression.

Methods

We used human myogenic cell cultures to analyze the effects of DUX4-FL when it was expressed either from its endogenous promoter in FSHD cells or by exogenous expression using BacMam vectors. We focused on determining the effects of DUX4-FL on protein ubiquitination and turnover and on aggregation of TDP-43.

Results

Human FSHD myotubes with endogenous DUX4-FL expression showed both altered nuclear and cytoplasmic distributions of ubiquitinated proteins and aggregation of TDP-43 in DUX4-FL-expressing nuclei. Similar changes were found upon exogenous expression of DUX4-FL, but were not seen upon expression of the non-toxic short isoform DUX4-S. DUX4-FL expression also inhibited protein turnover in a model system and increased the amounts of insoluble ubiquitinated proteins and insoluble TDP-43. Finally, inhibition of the ubiquitin–proteasome system with MG132 produced TDP-43 aggregation similar to DUX4-FL expression.

Interpretations

Our results identify DUX4-FL-induced inhibition of protein turnover and aggregation of TDP-43, which are pathological changes also found in diseases such as amyotrophic lateral sclerosis and inclusion body myopathy, as potential pathological mechanisms in FSHD.

N-terminal acetylation and replicative age affect proteasome localization and cell fitness during aging

Specific degradation of proteins is essential for virtually all cellular processes and is carried out predominantly by the proteasome. The proteasome is important for clearance of damaged cellular proteins. Damaged proteins accumulate over time and excess damaged proteins can aggregate and induce the death of old cells. In yeast, the localization of the proteasome changes dramatically during aging, possibly in response to altered proteasome activity requirements. We followed two key parameters of this process: the distribution of proteasomes in nuclear and cytosolic compartments, and the formation of cytoplasmic aggregate-like structures called proteasome storage granules (PSGs). Whereas replicative young cells efficiently relocalized proteasomes from the nucleus to the cytoplasm and formed PSGs, replicative old cells were less efficient in relocalizing the proteasome and had less PSGs. By using a microscopy-based genome-wide screen, we identified genetic factors involved in these processes. Both relocalization of the proteasome and PSG formation were affected by two of the three N-acetylation complexes. These N-acetylation complexes also had different effects on the longevity of cells, indicating that each N-acetylation complex has different roles in proteasome location and aging.

Upregulation of Unc-51-like kinase 1 by nitric oxide stabilizes SIRT1, independent of autophagy

SIRT1 is central to the lifespan and vascular health, but undergoes degradation that contributes to several medical conditions, including diabetes. How SIRT1 turnover is regulated remains unclear. However, emerging evidence suggests that endothelial nitric oxide synthase (eNOS) positively regulates SIRT1 protein expression. We recently identified NO as an endogenous inhibitor of 26S proteasome functionality with a cellular reporter system. Here we extended this finding to a novel pathway that regulates SIRT1 protein breakdown. In cycloheximide (CHX)-treated endothelial cells, NONOate, an NO donor, and A23187, an eNOS activator, significantly stabilized SIRT1 protein. Similarly, NO enhanced SIRT1 protein, but not mRNA expression, in CHX-free cells. NO also stabilized an autophagy-related protein unc-51 like kinase (ULK1), but did not restore SIRT1 protein levels in ULK1-siRNA-treated cells or in mouse embryonic fibroblasts (MEF) from Ulk1-/- mice. This suggests that ULK1 mediated the NO regulation of SIRT1. Furthermore, adenoviral overexpression of ULK1 increased SIRT1 protein expression, while ULK1 siRNA treatment decreased it. Rapamycin-induced autophagy did not mimic these effects, suggesting that the effects of ULK1 were autophagy-independent. Treatment with MG132, a proteasome inhibitor, or siRNA of β-TrCP1, an E3 ligase, prevented SIRT1 reduction induced by ULK1-siRNA. Mechanistically, ULK1 negatively regulated 26S proteasome functionality, which was at least partly mediated by O-linked-GlcNAc transferase (OGT), probably by increased O-GlcNAc modification of proteasomal subunit Rpt2. The NO-ULK1-SIRT1 axis was likely operative in the whole animal: both ULK1 and SIRT1 protein levels were significantly reduced in tissue homogenates in eNOS-knockout mice (lung) and in db/db mice where eNOS is downregulated (lung and heart). Taken together, the results show that NO stabilizes SIRT1 by regulating 26S proteasome functionality through ULK1 and OGT, but not autophagy, in endothelial cells.

Regulation of the proteasome: evaluating the lung proteasome as a new therapeutic target

SIGNIFICANCE

Lung diseases are on the second rank worldwide with respect to morbidity and mortality. For most respiratory diseases, no effective therapies exist. Whereas the proteasome has been successfully evaluated as a novel target for therapeutic interventions in cancer, neurodegenerative, and cardiac disorders, there is a profound lack of knowledge on the regulation of proteasome activity in chronic and acute lung diseases.

RECENT ADVANCES

There are various means of how the amount of active proteasome complexes in the cell can be regulated such as transcriptional regulation of proteasomal subunit expression, association with different regulators, assembly and half-life of proteasomes and regulatory complexes, as well as post-translational modifications. It also becomes increasingly evident that proteasome activity is fine-tuned and depends on the state of the cell. We propose here that 20S proteasomes and their regulators can be regarded as dynamic building blocks, which assemble or disassemble in response to cellular needs. The composition of proteasome complexes in a cell may vary depending on tissue, cell type and compartment, stage of development, or pathological context.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Dissecting the expression and regulation of the various catalytic forms of 20S proteasomes, such as constitutive, immuno-, and mixed proteasomes, together with their associated regulatory complexes will not only greatly enhance our understanding of proteasome function in lung pathogenesis but will also pave the way to develop new classes of drugs that inhibit or activate proteasome function in a defined setting for treatment of lung diseases.

SCF ubiquitin ligase-targeted therapies

The clinical successes of proteasome inhibitors for the treatment of cancer have highlighted the therapeutic potential of targeting this protein degradation system. However, proteasome inhibitors prevent the degradation of numerous proteins, which may cause adverse effects. Increased specificity could be achieved by inhibiting the components of the ubiquitin-proteasome system that target specific subsets of proteins for degradation. F-box proteins are the substrate-targeting subunits of SKP1-CUL1-F-box protein (SCF) ubiquitin ligase complexes. Through the degradation of a plethora of diverse substrates, SCF ubiquitin ligases control a multitude of processes at the cellular and organismal levels, and their dysregulation is implicated in many pathologies. SCF ubiquitin ligases are characterized by their high specificity for substrates, and these ligases therefore represent promising drug targets. However, the potential for therapeutic manipulation of SCF complexes remains an underdeveloped area. This Review explores and discusses potential strategies to target SCF-mediated biological processes to treat human diseases.

Disassembly of Lys11 and mixed linkage polyubiquitin conjugates provides insights into function of proteasomal deubiquitinases Rpn11 and Ubp6

Protein homeostasis is largely dependent on proteolysis by the ubiquitin-proteasome system. Diverse polyubiquitin modifications are reported to target cellular proteins to the proteasome. At the proteasome, deubiquitination is an essential preprocessing event that contributes to degradation efficiency. We characterized the specificities of two proteasome-associated deubiquitinases (DUBs), Rpn11 and Ubp6, and explored their impact on overall proteasome DUB activity. This was accomplished by constructing a panel of well defined ubiquitin (Ub) conjugates, including homogeneous linkages of varying lengths as well as a heterogeneously modified target. Rpn11 and Ubp6 processed Lys(11) and Lys(63) linkages with comparable efficiencies that increased with chain length. In contrast, processing of Lys(48) linkages by proteasome was inversely correlated to chain length. Fluorescently labeled tetra-Ub chains revealed endo-chain preference for Ubp6 acting on Lys(48) and random action for Rpn11. Proteasomes were more efficient at deconjugating identical substrates than their constituent DUBs by roughly 2 orders of magnitude. Incorporation into proteasomes significantly enhanced enzymatic efficiency of Rpn11, due in part to alleviation of the autoinhibitory role of its C terminus. The broad specificity of Rpn11 could explain how proteasomes were more effective at disassembling a heterogeneously modified conjugate compared with homogeneous Lys(48)-linked chains. The reduced ability to disassemble homogeneous Lys(48)-linked chains longer than 4 Ub units may prolong residency time on the proteasome.

A chaperome subnetwork safeguards proteostasis in aging and neurodegenerative disease

Chaperones are central to the proteostasis network (PN) and safeguard the proteome from misfolding, aggregation, and proteotoxicity. We categorized the human chaperome of 332 genes into network communities using function, localization, interactome, and expression data sets. During human brain aging, expression of 32% of the chaperome, corresponding to ATP-dependent chaperone machines, is repressed, whereas 19.5%, corresponding to ATP-independent chaperones and co-chaperones, are induced. These repression and induction clusters are enhanced in the brains of those with Alzheimer's, Huntington's, or Parkinson's disease. Functional properties of the chaperome were assessed by perturbation in C. elegans and human cell models expressing Aβ, polyglutamine, and Huntingtin. Of 219 C. elegans orthologs, knockdown of 16 enhanced both Aβ and polyQ-associated toxicity. These correspond to 28 human orthologs, of which 52% and 41% are repressed, respectively, in brain aging and disease and 37.5% affected Huntingtin aggregation in human cells. These results identify a critical chaperome subnetwork that functions in aging and disease.

Regulating the 20S proteasome ubiquitin-independent degradation pathway

For many years, the ubiquitin-26S proteasome degradation pathway was considered the primary route for proteasomal degradation. However, it is now becoming clear that proteins can also be targeted for degradation by the core 20S proteasome itself. Degradation by the 20S proteasome does not require ubiquitin tagging or the presence of the 19S regulatory particle; rather, it relies on the inherent structural disorder of the protein being degraded. Thus, proteins that contain unstructured regions due to oxidation, mutation, or aging, as well as naturally, intrinsically unfolded proteins, are susceptible to 20S degradation. Unlike the extensive knowledge acquired over the years concerning degradation by the 26S proteasome, relatively little is known about the means by which 20S-mediated proteolysis is controlled. Here, we describe our current understanding of the regulatory mechanisms that coordinate 20S proteasome-mediated degradation, and highlight the gaps in knowledge that remain to be bridged.

Synergistic effects on dopamine cell death in a Drosophila model of chronic toxin exposure

The neurodegenerative effects of Parkinson's disease (PD) are marked by a selective loss of dopaminergic (DA) neurons. Epidemiological studies suggest that chronic exposure to the pesticide paraquat may increase the risk for PD and DA cell loss. However, combined exposure with additional fungicide(s) including maneb and/or ziram may be required for pathogenesis. To explore potential pathogenic mechanisms, we have developed a Drosophila model of chronic paraquat exposure. We find that while chronic paraquat exposure alone decreased organismal survival and motor function, combined chronic exposure to both paraquat and maneb was required for DA cell death in the fly. To initiate mechanistic studies of this interaction, we used additional genetic reagents to target the ubiquitin proteasome system, which has been implicated in some rare familial forms of PD and the toxic effects of ziram. Genetic inhibition of E1 ubiquitin ligase, but not the proteasome itself, increased DA cell death in combination with maneb but not paraquat. These studies establish a model for long-term exposure to multiple pesticides, and support the idea that pesticide interactions relevant to PD may involve inhibition of protein ubiquitination.

Ubiquitin proteasome system-mediated degradation of synaptic proteins: An update from the postsynaptic side

The ubiquitin proteasome system is one of the principle mechanisms for the regulation of protein homeostasis in mammalian cells. In dynamic cellular structures such as neuronal synapses, ubiquitin proteasome system and protein translation provide an efficient way for cells to respond promptly to local stimulation and regulate neuroplasticity. The majority of research related to long-term plasticity has been focused on the postsynapses and has shown that ubiquitination and subsequent degradation of specific proteins are involved in various activity-dependent plasticity events. This review summarizes recent achievements in understanding ubiquitination of postsynaptic proteins and its impact on synapse plasticity and discusses the direction for advancing future research in the field.

Transferrin surface-modified PLGA nanoparticles-mediated delivery of a proteasome inhibitor to human pancreatic cancer cells

The aim of this study was to develop a drug delivery system based on poly(lactic-co-glycolic acid) (PLGA) nanoparticles for an efficient and targeted action of the proteasome inhibitor bortezomib against pancreatic cancer cells. The PLGA nanoparticles were formulated with a poloxamer, and further surface-modified with transferrin for tumor targeting. The nanoparticles were characterized as polymer carriers of bortezomib, and the cellular uptake and growth inhibitory effects were evaluated in pancreatic cells. Cellular internalization of nanoparticles was observed in normal and cancer cells, but with higher uptake by cancer cells. The sustained release of the loaded bortezomib from PLGA nanoparticles showed cytotoxic effects against pancreatic normal and cancer cells. Noteworthy differential cytotoxicity was attained by transferrin surface-modified PLGA nanoparticles since significant cell growth inhibition by delivered bortezomib was only observed in cancer cells. These findings demonstrate that the ligand transferrin enhanced the targeted delivery of bortezomib-loaded PLGA nanoparticles to pancreatic cancer cells. These in vitro results highlight the transferrin surface-modified PLGA nanoparticles as a promising system for targeted delivery of anticancer drugs.

Untangling the web of systemic autoinflammatory diseases

The innate immune system is involved in the pathophysiology of systemic autoinflammatory diseases (SAIDs), an enlarging group of disorders caused by dysregulated production of proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α, in which autoreactive T-lymphocytes and autoantibodies are indeed absent. A widely deranged innate immunity leads to overactivity of proinflammatory cytokines and subsequent multisite inflammatory symptoms depicting various conditions, such as hereditary periodic fevers, granulomatous disorders, and pyogenic diseases, collectively described in this review. Further research should enhance our understanding of the genetics behind SAIDs, unearth triggers of inflammatory attacks, and result in improvement for their diagnosis and treatment.

The carbohydrate-binding domain of overexpressed STBD1 is important for its stability and protein-protein interactions

STBD1 (starch-binding domain-containing protein 1) belongs to the CBM20 (family 20 carbohydrate binding module) group of proteins, and is implicated in glycogen metabolism and autophagy. However, very little is known about its regulation or interacting partners. Here, we show that the CBM20 of STBD1 is crucial for its stability and ability to interact with glycogen-associated proteins. Mutation of a conserved tryptophan residue (W293) in this domain abolished the ability of STBD1 to bind to the carbohydrate amylose. Compared with the WT (wild-type) protein, this mutant exhibited rapid degradation that was rescued upon inhibition of the proteasome. Furthermore, STBD1 undergoes ubiquitination when expressed in COS cells, and requires the N-terminus for this process. In contrast, inhibition of autophagy did not significantly affect protein stability. In overexpression experiments, we discovered that STBD1 interacts with several glycogen-associated proteins, such as GS (glycogen synthase), GDE (glycogen debranching enzyme) and Laforin. Importantly, the W293 mutant of STBD1 was unable to do so, suggesting an additional role for the CBM20 domain in protein–protein interactions. In HepG2 hepatoma cells, overexpressed STBD1 could associate with endogenous GS. This binding increased during glycogenolysis, suggesting that glycogen is not required to bridge this interaction. Taken together, our results have uncovered new insights into the regulation and binding partners of STBD1.

STBD1 is a protein with a carbohydrate-binding domain that is implicated in autophagy and glycogen metabolism. Here we show the carbohydrate-binding domain is crucial for its stability and ability to bind to several glycogen-associated proteins.

Survival of mycobacteria depends on proteasome-mediated amino acid recycling under nutrient limitation

Intracellular protein degradation is an essential process in all life domains. While in all eukaryotes regulated protein degradation involves ubiquitin tagging and the 26S-proteasome, bacterial prokaryotic ubiquitin-like protein (Pup) tagging and proteasomes are conserved only in species belonging to the phyla Actinobacteria and Nitrospira. In Mycobacterium tuberculosis, the Pup-proteasome system (PPS) is important for virulence, yet its physiological role in non-pathogenic species has remained an enigma. We now report, using Mycobacterium smegmatis as a model organism, that the PPS is essential for survival under starvation. Upon nitrogen limitation, PPS activity is induced, leading to accelerated tagging and degradation of many cytoplasmic proteins. We suggest a model in which the PPS functions to recycle amino acids under nitrogen starvation, thereby enabling the cell to maintain basal metabolic activities. We also find that the PPS auto-regulates its own activity via pupylation and degradation of its components in a manner that promotes the oscillatory expression of PPS components. As such, the destructive activity of the PPS is carefully balanced to maintain cellular functions during starvation.

An iron-regulated and glycosylation-dependent proteasomal degradation pathway for the plasma membrane metal transporter ZIP14

Protein degradation is instrumental in regulating cellular function. Plasma membrane proteins targeted for degradation are internalized and sorted to multivesicular bodies, which fuse with lysosomes, where they are degraded. ZIP14 is a newly identified iron transporter with multitransmembrane domains. In an attempt to dissect the molecular mechanisms by which iron regulates ZIP14 levels, we found that ZIP14 is endocytosed, extracted from membranes, deglycosylated, and degraded by proteasomes. This pathway did not depend on the retrograde trafficking to the endoplasmic reticulum and thus did not involve the well-defined endoplasmic reticulum-associated protein degradation pathway. Iron inhibited membrane extraction of internalized ZIP14, resulting in higher steady-state levels of ZIP14. Asparagine-linked (N-linked) glycosylation of ZIP14, particularly the glycosylation at N102, was required for efficient membrane extraction of ZIP14 and therefore is necessary for its iron sensitivity. These findings highlight the importance of proteasomes in the degradation of endocytosed plasma membrane proteins.

Proteasomal degradation of O-GlcNAc transferase elevates hypoxia-induced vascular endothelial inflammatory response†

AIMS

Hypoxia induces vascular inflammation by a mechanism not fully understood. Emerging evidence implicates O-GlcNAc transferase (OGT) in inflammation. This study explored the role of OGT in hypoxia-induced vascular endothelial inflammatory response.

METHODS AND RESULTS

Hypoxia was either induced (1% O2 chamber) or mimicked by exposure to hypoxia-mimetic agents in cultured endothelial cells. Hypoxia increased hypoxia-inducible factor (HIF-1α) and inflammatory response (gene and protein expression of interleukin (IL)-6, IL-8, monocyte chemoattractant protein-1, and E-selectin) but, surprisingly, reduced OGT protein (not mRNA) levels. Hypoxia-mimetic CoCl2 failed to reduce OGT when proteasome inhibitors were present, suggesting proteasome involvement. Indeed, CoCl2 enhanced 26S proteasome functionality evidenced by diminished reporter (Ub(G76V)-GFP) proteins in proteasome reporter cells, likely due to increased chymotrypsin-like activities. Mechanistically, β-TrCP1 mediated OGT degradation, since siRNA ablation of this E3 ubiquitin ligase stabilized OGT. Administration of the oxidative stress inhibitors reversed both proteasome activation and OGT degradation. Furthermore, up-regulation of OGT by stabilization, overexpression, or activation mitigated CoCl2-elicited inflammatory response. These observations were recapitulated in a mouse (C57BL/6J) model mimicking hypoxia, in which lung tissues presented higher levels of HIF-1α, proteasome activity, and inflammatory response, but lower levels of OGT (n = 5/group, hypoxia vs. normoxia, P < 0.05). However, administration of an activator of OGT (glucosamine: 1 mg/g/day, vehicle: saline, ip, 5 days) abolished the up-regulation of proteasome activity and inflammatory response (n = 5/group, the treated vs. untreated hypoxia groups, P < 0.05).

CONCLUSIONS

26S proteasome-mediated OGT reduction contributed to hypoxia-induced vascular endothelial inflammatory response. Modulation of OGT may represent a new approach to treat diseases characterized by hypoxic inflammation.

The Mechanistic Links Between Proteasome Activity, Aging and Age-related Diseases

Damaged and misfolded proteins accumulate during the aging process, impairing cell function and tissue homeostasis. These perturbations to protein homeostasis (proteostasis) are hallmarks of age-related neurodegenerative disorders such as Alzheimer’s, Parkinson’s or Huntington’s disease. Damaged proteins are degraded by cellular clearance mechanisms such as the proteasome, a key component of the proteostasis network. Proteasome activity declines during aging, and proteasomal dysfunction is associated with late-onset disorders. Modulation of proteasome activity extends lifespan and protects organisms from symptoms associated with proteostasis disorders. Here we review the links between proteasome activity, aging and neurodegeneration. Additionally, strategies to modulate proteasome activity and delay the onset of diseases associated to proteasomal dysfunction are discussed herein.

Genetics of proteasome diseases

The proteasome is a large, multiple subunit complex that is capable of degrading most intracellular proteins. Polymorphisms in proteasome subunits are associated with cardiovascular diseases, diabetes, neurological diseases, and cancer. One polymorphism in the proteasome gene PSMA6 (-8C/G) is associated with three different diseases: type 2 diabetes, myocardial infarction, and coronary artery disease. One type of proteasome, the immunoproteasome, which contains inducible catalytic subunits, is adapted to generate peptides for antigen presentation. It has recently been shown that mutations and polymorphisms in the immunoproteasome catalytic subunit PSMB8 are associated with several inflammatory and autoinflammatory diseases including Nakajo-Nishimura syndrome, CANDLE syndrome, and intestinal M. tuberculosis infection. This comprehensive review describes the disease-related polymorphisms in proteasome genes associated with human diseases and the physiological modulation of proteasome function by these polymorphisms. Given the large number of subunits and the central importance of the proteasome in human physiology as well as the fast pace of detection of proteasome polymorphisms associated with human diseases, it is likely that other polymorphisms in proteasome genes associated with diseases will be detected in the near future. While disease-associated polymorphisms are now readily discovered, the challenge will be to use this genetic information for clinical benefit.