Targeting immunoproteasome in neurodegeneration: A glance to the future

Mechanisms of ubiquitin-independent proteasomal degradation and their roles in age-related neurodegenerative disease

Neurodegenerative diseases are characterized by the progressive breakdown of neuronal structure and function and the pathological accumulation of misfolded protein aggregates and toxic protein oligomers. A major contributor to the deterioration of neuronal physiology is the disruption of protein catabolic pathways mediated by the proteasome, a large protease complex responsible for most cellular protein degradation. Previously, it was believed that proteolysis by the proteasome required tagging of protein targets with polyubiquitin chains, a pathway called the ubiquitin-proteasome system (UPS). Because of this, most research on proteasomal roles in neurodegeneration has historically focused on the UPS. However, additional ubiquitin-independent pathways and their importance in neurodegeneration are increasingly recognized. In this review, we discuss the range of ubiquitin-independent proteasome pathways, focusing on substrate identification and targeting, regulatory molecules and adaptors, proteasome activators and alternative caps, and diverse proteasome complexes including the 20S proteasome, the neuronal membrane proteasome, the immunoproteasome, extracellular proteasomes, and hybrid proteasomes. These pathways are further discussed in the context of aging, oxidative stress, protein aggregation, and age-associated neurodegenerative diseases, with a special focus on Alzheimer's Disease, Huntington's Disease, and Parkinson's Disease. A mechanistic understanding of ubiquitin-independent proteasome function and regulation in neurodegeneration is critical for the development of therapies to treat these devastating conditions. This review summarizes the current state of ubiquitin-independent proteasome research in neurodegeneration.

PA28γ, the ring that makes tumors invisible to the immune system?

PA28γ is a proteasomal interactor whose main and most known function is to stimulate the hydrolytic activity of the 20 S proteasome independently of ubiquitin and ATP. Unlike its two paralogues, PA28α and PA28β, PA28γ is largely present in the nuclear compartment and plays pivotal functions in important pathways such as cellular division, apoptosis, neoplastic transformation, chromatin structure and organization, fertility, lipid metabolism, and DNA repair mechanisms. Although it is known that a substantial fraction of PA28γ is found in the cell in a free form (i.e. not associated with 20 S), almost all of the studies so far have focused on its ability to modulate proteasomal enzymatic activities. In this respect, the ability of PA28γ to strongly stimulate degradation of proteins, especially if intrinsically disordered and therefore devoid of three-dimensional tightly folded structure, appears to be the main molecular mechanism underlying its multiple biological effects. Initial studies, conducted more than 20 years ago, came to the conclusion that among the many biological functions of PA28γ, the immunological ones were rather limited and circumscribed. In this review, we focus on recent evidence showing that PA28γ fulfills significant functions in cell-mediated acquired immunity, with a particular role in attenuating MHC class I antigen presentation, especially in relation to neoplastic transformation and autoimmune diseases.

The Delayed Turnover of Proteasome Processing of Myocilin upon Dexamethasone Stimulation Introduces the Profiling of Trabecular Meshwork Cells' Ubiquitylome

Glaucoma is chronic optic neuropathy whose pathogenesis has been associated with the altered metabolism of Trabecular Meshwork Cells, which is a cell type involved in the synthesis and remodeling of the trabecular meshwork, the main drainage pathway of the aqueous humor. Starting from previous findings supporting altered ubiquitin signaling, in this study, we investigated the ubiquitin-mediated turnover of myocilin (MYOC/TIGR gene), which is a glycoprotein with a recognized role in glaucoma pathogenesis, in a human Trabecular Meshwork strain cultivated in vitro in the presence of dexamethasone. This is a validated experimental model of steroid-induced glaucoma, and myocilin upregulation by glucocorticoids is a phenotypic marker of Trabecular Meshwork strains. Western blotting and native-gel electrophoresis first uncovered that, in the presence of dexamethasone, myocilin turnover by proteasome particles was slower than in the absence of the drug. Thereafter, co-immunoprecipitation, RT-PCR and gene-silencing studies identified STUB1/CHIP as a candidate E3-ligase of myocilin. In this regard, dexamethasone treatment was found to downregulate STUB1/CHIP levels by likely promoting its proteasome-mediated turnover. Hence, to strengthen the working hypothesis about global alterations of ubiquitin-signaling, the first profiling of TMCs ubiquitylome, in the presence and absence of dexamethasone, was here undertaken by diGLY proteomics. Application of this workflow effectively highlighted a robust dysregulation of key pathways (e.g., phospholipid signaling, β-catenin, cell cycle regulation) in dexamethasone-treated Trabecular Meshwork Cells, providing an ubiquitin-centered perspective around the effect of glucocorticoids on metabolism and glaucoma pathogenesis.

Geraniol (GER) attenuated chronic sleep restriction (CSR)-induced neuroinflammation in adolescent mice

Sleep insufficiency is a significant health problem worldwide, and adolescent sleep restriction (SR) could induce multiple neurodevelopmental disorders in the central nervous system (CNS). Microglial-mediated neuroinflammation plays a vital role in multiple neurological diseases, and recent research showed the regulation effect of immunoproteasome on microglia functions. Geraniol (GER), an important ingredient in many essential oils, possesses diverse pharmacological properties like anti-inflammatory and antioxidant. The present study was designed to evaluate the neuroprotective effect of GER on SR in adolescent mice and further investigate the underlying mechanisms. Our results displayed that 14 days of chronic sleep restriction (CSR) induced cognitive decline, and anxiety-like and attention-deficit behaviors, which were mitigated by GER pretreatment. GER administration also reversed microglial pro-inflammatory response under CSR stimulation in the anterior cingulate cortex (ACC) regions by reducing the expression and secretion of cytokines like IL-1β and TNF-α. Mechanism research showed that LMP7 mRNA was selectively up-regulated under CSR treatment but down-regulated by GER administration. Proteasome activity and protein expression of LMP7 were consistent with mRNA data. ONX-0914 was applied to inhibit LMP7 selectively, and data validated that GER might alleviate CSR-induced neuroinflammation by regulating LMP7. Our study provides evidence that LMP7 is a critical regulator of CSR-induced proinflammation, and geraniol might be a promising therapy against CSR-induced neurodevelopmental disorders.

The role of the immunoproteasome in cardiovascular disease

The ubiquitinproteasome system (UPS) is the main mechanism responsible for the intracellular degradation of misfolded or damaged proteins. Under inflammatory conditions, the immunoproteasome, an isoform of the proteasome, can be induced, enhancing the antigen-presenting function of the UPS. Furthermore, the immunoproteasome also serves nonimmune functions, such as maintaining protein homeostasis and regulating signalling pathways, and is involved in the pathophysiological processes of various cardiovascular diseases (CVDs). This review aims to provide a comprehensive summary of the current research on the involvement of the immunoproteasome in cardiovascular diseases, with the ultimate goal of identifying novel strategies for the treatment of these conditions.

Novel frontiers in neuroprotective therapies in glaucoma: Molecular and clinical aspects

In the last years, neuroprotective therapies have attracted the researcher interests as modern and challenging approach for the treatment of neurodegenerative diseases, aimed at protecting the nervous system from injuries. Glaucoma is a neurodegenerative disease characterized by progressive excavation of the optic nerve head, retinal axonal injury and corresponding vision loss that affects millions of people on a global scale. The molecular basis of the pathology is largely uncharacterized yet, and the therapeutic approaches available do not change the natural course of the disease. Therefore, in accordance with the therapeutic regimens proposed for other neurodegenerative diseases, a modern strategy to treat glaucoma includes prescription of drugs with neuroprotective activities. With respect to this, several preclinical and clinical investigations on a plethora of different drugs are currently ongoing. In this review, first, the conceptualization of the rationale for the adoption of neuroprotective strategies for retina is summarized. Second, the molecular aspects highlighting glaucoma as a neurodegenerative disease are reported. In conclusion, the molecular and pharmacological properties of most promising direct neuroprotective drugs used to delay glaucoma progression are examined, including: neurotrophic factors, NMDA receptor antagonists, the α2-adrenergic agonist, brimonidine, calcium channel blockers, antioxidant agents, nicotinamide and statins.

The Insulin-Degrading Enzyme from Structure to Allosteric Modulation: New Perspectives for Drug Design

The insulin-degrading enzyme (IDE) is a Zn2+ peptidase originally discovered as the main enzyme involved in the degradation of insulin and other amyloidogenic peptides, such as the β-amyloid (Aβ) peptide. Therefore, a role for the IDE in the cure of diabetes and Alzheimer's disease (AD) has been long envisaged. Anyway, its role in degrading amyloidogenic proteins remains not clearly defined and, more recently, novel non-proteolytic functions of the IDE have been proposed. From a structural point of view, the IDE presents an atypical clamshell structure, underscoring unique enigmatic enzymological properties. A better understanding of the structure-function relationship may contribute to solving some existing paradoxes of IDE biology and, in light of its multifunctional activity, might lead to novel therapeutic approaches.

Design and synthesis of amino-substituted N-arylpiperidinyl-based inhibitors of the (immuno)proteasome

The constitutive proteasome and the immunoproteasome represent validated targets for pharmacological intervention in the context of various diseases, such as cancer, inflammation, and autoimmune diseases. The development of novel chemical scaffolds of non-peptidic nature, capable of inhibiting different catalytically active subunits of both isoforms, is a viable approach against these diseases. Such compounds are also useful as leads for the development of biochemical probes that enable the studies of the roles of both isoforms in various biological contexts. Here, we present a ligand-based computational design of (immuno)proteasome inhibitors, which resulted in the amino-substituted N-arylpiperidine-based compounds that can inhibit different subunits of the (immuno)proteasome in the low micromolar range. The compounds represent a useful starting point for further structure-activity relationship studies that will, hopefully, lead to non-peptidic compounds that could be used in pharmacological and biochemical studies of both proteasomes.