Proteasome Activity in the Plasma as a Novel Biomarker in Mild Cognitive Impairment with Chronic Tinnitus

Extracellular Vesicle-Mediated Delivery of 20S Proteasomes Enhances Tau Degradation in Recipient Cells

The 26S proteasome holoenzyme comprises 20S catalytic and 19S regulatory complexes. Accumulating evidence suggests that the majority of proteasomes in the extracellular space exist as free 20S proteasomes; however, their origin and pathophysiological function remain to be determined. Here, we report that cellular proteasomes are effectively packaged into the lumen of extracellular vesicles (EVs) and secreted in a structurally intact and enzymatically active 20S form. We further demonstrate that EV-encapsulated 20S proteasomes are delivered to recipient cells and facilitate the degradation of overexpressed tau proteins without disrupting global proteolytic pathways. These findings highlight a novel cell-to-cell communication system that transports the proteasomes to target cells for the clearance of proteotoxic substrates. Further characterisation of this homeostatic mechanism will improve our understanding of organismal stress response mechanisms and may provide a therapeutic approach to treat various proteinopathies, including Alzheimer's disease.

In silico identification of 20S proteasome-β5 subunit inhibitors using structure-based virtual screening

Proteasome inhibitors have effective anti-tumor activity in cell culture and can induce apoptosis by interfering with the degradation of cell cycle proteins. 20S Proteasome is acknowledged to be a satisfactory target that has persistent properties against the human immune defense and is obligatory for the degradation of some vital proteins. This study aimed to identify potential inhibitors against 20S proteasome, specifically the β5 subunit, using structure-based virtual screening and molecular docking to reduce the number of ligands that should be eligible for experimental assays. A total of 4961 molecules with anticancer activity were screened from the ASINEX database. The filtered compounds that showed higher docking affinity were then used in more sophisticated molecular docking simulations with AutoDock Vina for validation. Finally, six drug molecules (BDE 28974746, BDE 25657353, BDE 29746159, BDD 27844484, BDE 29746109, and BDE 29746162) exhibited highly significant interactions compared to the positive controls were retained. Among these six molecules, three molecules (BDE 28974746, BDE 25657353, and BDD 27844484) showed high binding affinity and binding energy compared with Carfilzomib and Bortezomib. Molecular simulation and dynamics studies of the top three drug molecules in each case allowed us to draw further conclusions about their stability with the β5 subunit. Computed absorption, distribution, metabolism, excretion and toxicity studies on these derivatives showed encouraging results with very low toxicity, distribution, and absorption. These compounds may serve as potential hits for further biological evaluation in the development of new proteasome inhibitors.Communicated by Ramaswamy H. Sarma.

Ramifications of POU4F3 variants associated with autosomal dominant hearing loss in various molecular aspects

POU4F3, a member of the POU family of transcription factors, commonly causes autosomal dominant deafness. Exome sequencing was used to identify four novel variants in POU4F3 (NM_002700.2), including c.564dupA: p.Ala189SerfsTer26, c.743T > C:p.Leu248Pro, c.879C > A:p.Phe293Leu, and c.952G > A:p.Val318Met, and diverse aspects of the molecular consequences of their protein expression, stability, subcellular localization, and transcriptional activity were investigated. The expression of three mutant proteins, encoded by missense variants, was reduced compared to the wild-type protein, demonstrating that the mutants were unstable and vulnerable to degradation. Additionally, all the mutant proteins had distinct subcellular localization patterns. A mutant protein carrying p.Ala189SerfsTer26, in which both mono- and bi-partite nuclear localization signals were disrupted, showed abnormal subcellular localization. Resultantly, all the mutant proteins significantly reduced the transcriptional activity required to regulate the downstream target gene expression. Furthermore, we identified the altered expression of 14 downstream target genes associated with inner ear development using patient-derived lymphoblastoid cell lines. There was a significant correlation of the expression profile between patient-derived cells and the cochlear hair cells, which provided a breakthrough for cases where the collection of human cochlear samples for transcriptome studies was unfeasible. This study expanded the genotypic spectrum of POU4F3 in DFNA15, and further refined the molecular mechanisms underlying POU4F3-associated DFNA15.

Biology of the Extracellular Proteasome

Proteasomes are traditionally considered intracellular complexes that play a critical role in maintaining proteostasis by degrading short-lived regulatory proteins and removing damaged proteins. Remarkably, in addition to these well-studied intracellular roles, accumulating data indicate that proteasomes are also present in extracellular body fluids. Not much is known about the origin, biological role, mode(s) of regulation or mechanisms of extracellular transport of these complexes. Nevertheless, emerging evidence indicates that the presence of proteasomes in the extracellular milieu is not a random phenomenon, but rather a regulated, coordinated physiological process. In this review, we provide an overview of the current understanding of extracellular proteasomes. To this end, we examine 143 proteomic datasets, leading us to the realization that 20S proteasome subunits are present in at least 25 different body fluids. Our analysis also indicates that while 19S subunits exist in some of those fluids, the dominant proteasome activator in these compartments is the PA28α/β complex. We also elaborate on the positive correlations that have been identified in plasma and extracellular vesicles, between 20S proteasome and activity levels to disease severity and treatment efficacy, suggesting the involvement of this understudied complex in pathophysiology. In addition, we address the considerations and practical experimental methods that should be taken when investigating extracellular proteasomes. Overall, we hope this review will stimulate new opportunities for investigation and thoughtful discussions on this exciting topic that will contribute to the maturation of the field.

Improper Proteostasis: Can It Serve as Biomarkers for Neurodegenerative Diseases?

Cells synthesize new proteins after multiple molecular decisions. Damage of existing proteins, accumulation of abnormal proteins, and basic requirement of new proteins trigger protein quality control (PQC)-based alternative strategies to cope against proteostasis imbalance. Accumulation of misfolded proteins is linked with various neurodegenerative disorders. However, how deregulated components of this quality control system and their lack of general mechanism-based long-term changes can serve as biomarkers for neurodegeneration remains largely unexplored. Here, our article summarizes the chief findings, which may facilitate the search of novel and relevant proteostasis mechanism-based biomarkers associated with neuronal disorders. Understanding the abnormalities of PQC coupled molecules as possible biomarkers can help to determine neuronal fate and their contribution to the aetiology of several nervous system disorders.

Evaluation of Immunoproteasome-Specific Proteolytic Activity Using Fluorogenic Peptide Substrates

The 26S proteasome irreversibly hydrolyzes polyubiquitylated substrates to maintain protein homeostasis; it also regulates immune responses by generating antigenic peptides. An alternative form of the 26S proteasome is the immunoproteasome, which contains substituted catalytic subunits (β1i/PSMB9, β2i/PSMB10, and β5i/PSMB8) instead of constitutively expressed counterparts (β1/PSMB6, β2/PSMB7, and β5/PSMB5). The immunoproteasome expands the peptide repertoire presented on MHC class I molecules. However, how its activity changes in this context is largely elusive, possibly due to the lack of a standardized methodology to evaluate its specific activity. Here, we describe an assay protocol that measures the immunoproteasome activity of whole-cell lysates using commercially available fluorogenic peptide substrates. Our results showed that the most accurate assessment of immunoproteasome activity could be achieved by combining β5i-targeting substrate Ac-ANW-AMC and immunoproteasome inhibitor ONX-0914. This simple and reliable protocol may contribute to future studies of immunoproteasomes and their pathophysiological roles during viral infection, inflammation, and tumorigenesis.

Concept and application of circulating proteasomes

Proteostasis is primarily a function of protein synthesis and degradation. Although the components and processes involved in intracellular proteostasis have been studied extensively, it is apparent that extracellular proteostasis is equitably crucial for the viability of organisms. The 26S proteasome, a unique ATP-dependent proteolytic complex in eukaryotic cells, contributes to the majority of intracellular proteolysis. Accumulating evidence suggests the presence of intact 20S proteasomes in the circulatory system (c-proteasomes), and similar to other plasma proteins, the levels of these c-proteasomes may vary, potentially reflecting specific pathophysiological conditions. Under normal conditions, the concentration of c-proteasomes has been reported to be in the range of ~0.2-2 μg/mL, which is ~2-4-fold lower than that of functional plasma proteins but markedly higher than that of signaling proteins. The characterization of c-proteasomes, such as their origin, structure, role, and clearance, has been delayed mainly due to technical limitations. In this review, we summarize the current perspectives pertaining to c-proteasomes, focusing on the methodology, including our experimental understanding. We believe that once the pathological relevance of c-proteasomes is revealed, these unique components may be utilized in the diagnosis and prognosis of diverse human diseases.

Formation of Non-Nucleoplasmic Proteasome Foci during the Late Stage of Hyperosmotic Stress

Cellular stress induces the formation of membraneless protein condensates in both the nucleus and cytoplasm. The nucleocytoplasmic transport of proteins mainly occurs through nuclear pore complexes (NPCs), whose efficiency is affected by various stress conditions. Here, we report that hyperosmotic stress compartmentalizes nuclear 26S proteasomes into dense nuclear foci, independent of signaling cascades. Most of the proteasome foci were detected between the condensed chromatin mass and inner nuclear membrane. The proteasome-positive puncta were not colocalized with other types of nuclear bodies and were reversibly dispersed when cells were returned to the isotonic medium. The structural integrity of 26S proteasomes in the nucleus was slightly affected under the hyperosmotic condition. We also found that these insulator-body-like proteasome foci were possibly formed through disrupted nucleus-to-cytosol transport, which was mediated by the sequestration of NPC components into osmostress-responding stress granules. These data suggest that phase separation in both the nucleus and cytosol may be a major cell survival mechanism during hyperosmotic stress conditions.

Tinnitus and Neuropsychological Dysfunction in the Elderly: A Systematic Review on Possible Links

INTRODUCTION

Tinnitus is a common and disabling symptom often associated with hearing loss. While clinical practice frequently shows that a certain degree of psychological discomfort often characterizes tinnitus suffers, it has been recently suggested in adults as a determining factor for cognitive decline affecting attention and memory domains. The aim of our systematic review was to provide evidence for a link between tinnitus, psychological distress, and cognitive dysfunction in older patients and to focus on putative mechanisms of this relationship.

METHODS

We performed a systematic review, finally including 192 articles that were screened. This resulted in 12 manuscripts of which the full texts were included in a qualitative analysis.

RESULTS

The association between tinnitus and psychological distress, mainly depression, has been demonstrated in older patients, although only few studies addressed the aged population. Limited studies on cognitive dysfunction in aged patients affected by chronic tinnitus are hardly comparable, as they use different methods to validate cognitive impairment. Actual evidence does not allow us with certainty to establish if tinnitus matters as an independent risk factor for cognitive impairment or evolution to dementia.

CONCLUSION

Tinnitus, which is usually associated with age-related hearing loss, might negatively affect emotional wellbeing and cognitive capacities in older people, but further studies are required to improve the evidence.

Emerging Topics in the Behavioral Neuroscience of Tinnitus

This volume has highlighted the many recent advances in tinnitus theory, models, diagnostics, therapies, and therapeutics. But tinnitus knowledge is far from complete. In this chapter, contributors to the Behavioral Neuroscience of Tinnitus consider emerging topics and areas of research needed in light of recent findings. New research avenues and methods to explore are discussed. Issues pertaining to current assessment, treatment, and research methods are outlined, along with recommendations on new avenues to explore with research.