Fluspirilene Analogs Activate the 20S Proteasome and Overcome Proteasome Impairment by Intrinsically Disordered Protein Oligomers

20S proteasome enhancers prevent cytotoxic tubulin polymerization-promoting protein induced α-synuclein aggregation

Synucleinopathies are a class of neurodegenerative diseases defined by the presence of α-synuclein inclusions. The location and composition of these α-synuclein inclusions directly correlate to the disease pattern. The inclusions in Multiple System Atrophy are located predominantly in oligodendrocytes and are rich in a second protein, p25α. P25α plays a key role in neuronal myelination by oligodendrocytes. In healthy oligodendrocytes, there is little to no α-synuclein present. If aberrant α-synuclein is present, p25α leaves the myelin sheaths and quickly co-aggregates with α-synuclein, resulting in the disruption of the cellular process and ultimately cell death. Herein, we report that p25α is susceptible for 20S proteasome-mediated degradation and that p25α induces α-synuclein aggregation, resulting in proteasome impairment and cell death. In addition, we identified small molecules 20S proteasome enhancers that prevent p25α induced α-synuclein fibrilization, restore proteasome impairment, and enhance cell viability.

Proteasome activation: A novel strategy for targeting undruggable intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) are characterized by their inability to adopt well-defined tertiary structures under physiological conditions. Nonetheless, they often play pivotal roles in the progression of various diseases, including cancer, neurodegenerative disorders, and cardiovascular ailments. Owing to their inherent dynamism, conventional drug design approaches based on structural considerations encounter substantial challenges when applied to IDPs. Consequently, the pursuit of therapeutic interventions directed towards IDPs presents a complex endeavor. While there are indeed existing methodologies for targeting IDPs, they are encumbered by noteworthy constrains. Hence, there exists an imminent imperative to investigate more efficacious and universally applicable strategies for modulating IDPs. Here, we present an overview of the latest advancements in the research pertaining to IDPs, along with the indirect regulation approach involving the modulation of IDP degradation through proteasome. By comprehending these advancements in research, novel insights can be generated to facilitate the development of new drugs targeted at addressing the accumulation of IDPs in diverse pathological conditions.

Syrosingopine Enhances 20S Proteasome Activity and Degradation of α-Synuclein

Cellular proteins are degraded by the 26S proteasome in the ubiquitin-proteasome system in an ATP-dependent manner, whereas intrinsically disordered proteins (IDPs) are degraded by the 20S proteasome independent of ATP and ubiquitin. The accumulation and aggregation of IDPs are considered to be the etiology of neurodegenerative diseases. Notably, the 20S proteasome has a cylindrical structure, and its gate on the α-ring is closed in the inactive form. The compounds that open the gate promote the degradation of IDPs and prevent their accumulation, and therefore, such compounds may be promising therapeutic agents for neurodegenerative diseases. After screening the Prestwick Phytochemical Library, several yohimbine-type and ergot alkaloids were identified that enhance the 20S proteasome activity. Among them, syrosingopine was the most potent activator of the 20S proteasome and enhanced the degradation of fluorogenic substrates and α-synuclein, an IDP. Furthermore, in HeLa cells, syrosingopine enabled the binding of a membrane-permeable fluorescent probe to the catalytic site of the 20S proteasome by opening the gate.

Development of a Cell-Based AlphaLISA Assay for High-Throughput Screening for Small Molecule Proteasome Modulators

The balance between protein degradation and protein synthesis is a highly choreographed process generally called proteostasis. Most intracellular protein degradation occurs through the ubiquitin-proteasome system (UPS). This degradation takes place through either a ubiquitin-dependent or a ubiquitin-independent proteasomal pathway. The ubiquitin-independent pathway selectively targets unfolded proteins, including intrinsically disordered proteins (IDPs). Dysregulation of proteolysis can lead to the accumulation of IDPs, seen in the pathogenesis of various diseases, including cancer and neurodegeneration. Therefore, the enhancement of the proteolytic activity of the 20S proteasome using small molecules has been identified as a promising pathway to combat IDP accumulation. Currently, there are a limited number of known small molecules that enhance the activity of the 20S proteasome, and few are observed to exhibit enhanced proteasome activity in cell culture. Herein, we describe the development of a high-throughput screening assay to identify cell-permeable proteasome enhancers by utilizing an AlphaLISA platform that measures the degradation of a GFP conjugated intrinsically disordered protein, ornithine decarboxylase (ODC). Through the screening of the Prestwick and NIH Clinical Libraries, a kinase inhibitor, erlotinib, was identified as a new 20S proteasome enhancer, which enhances the degradation of ODC in cells and α-synuclein in vitro.

Enhanced Degradation of Mutant C9ORF72-Derived Toxic Dipeptide Repeat Proteins by 20S Proteasome Activation Results in Restoration of Proteostasis and Neuroprotection

A hexanucleotide repeat expansion (HRE) in an intron of gene C9ORF72 is the most common cause of familial amyotrophic lateral sclerosis and frontotemporal dementia. The HRE undergoes noncanonical translation (repeat-associated non-ATG translation) resulting in the production of five distinct dipeptide repeat (DPR) proteins. Arginine-rich DPR proteins have shown to be toxic to motor neurons, and recent evidence suggests this toxicity is associated with disruption of the ubiquitin-proteasome system. Here we report the ability of known 20S proteasome activator, TCH-165, to enhance the degradation of DPR proteins and overcome proteasome impairment evoked by DPR proteins. Furthermore, the 20S activator protects rodent motor neurons from DPR protein toxicity and restores proteostasis in cortical neuron cultures. This study suggests that 20S proteasome enhancers may have therapeutic efficacy in neurodegenerative diseases that display proteostasis defects.

Silybins are stereospecific regulators of the 20S proteasome

A reduced proteasome activity tiles excessive amyloid growth during the progress of protein conformational diseases (PCDs). Hence, the development of safe and effective proteasome enhancers represents an attractive target for the therapeutic treatment of these chronic disorders. Here we analyze two natural diastereoisomers belonging to the family of flavonolignans, Sil A and Sil B, by evaluating their capacity to increase proteasome activity. Enzyme assays carried out on yeast 20S (y20S) proteasome and in parallel on a permanently "open gate" mutant (α3ΔN) evidenced that Sil B is a more efficient 20S activator than Sil A. Conversely, in the case of human 20S proteasome (h20S) a higher affinity and more efficient activation is observed for Sil A. Driven by experimental data, computational studies further demonstrated that the taxifolin group of both diastereoisomers plays a crucial role in their anchoring to the α5/α6 groove of the outer α-ring. However, due to the different stereochemistry at C-7" and C-8" of ring D, only Sil A was able to reproduce the interactions responsible for h20S proteasome activation induced by their cognate regulatory particles. The provided silybins/h20S interaction models allowed us to rationalize their different ability to activate the peptidase activities of h20S and y20S. Our results provide structural details concerning the important role played by stereospecific interactions in driving Sil A and Sil B binding to the 20S proteasome and may support future rational design of proteasome enhancers.

Genetic and pharmacologic proteasome augmentation ameliorates Alzheimer's-like pathology in mouse and fly APP overexpression models

The proteasome has key roles in neuronal proteostasis, including the removal of misfolded and oxidized proteins, presynaptic protein turnover, and synaptic efficacy and plasticity. Proteasome dysfunction is a prominent feature of Alzheimer's disease (AD). We show that prevention of proteasome dysfunction by genetic manipulation delays mortality, cell death, and cognitive deficits in fly and cell culture AD models. We developed a transgenic mouse with neuronal-specific proteasome overexpression that, when crossed with an AD mouse model, showed reduced mortality and cognitive deficits. To establish translational relevance, we developed a set of TAT-based proteasome-activating peptidomimetics that stably penetrated the blood-brain barrier and enhanced 20S/26S proteasome activity. These agonists protected against cell death, cognitive decline, and mortality in cell culture, fly, and mouse AD models. The protective effects of proteasome overexpression appear to be driven, at least in part, by the proteasome's increased turnover of the amyloid precursor protein along with the prevention of overall proteostatic dysfunction.

Peptidomimetics Based on C-Terminus of Blm10 Stimulate Human 20S Proteasome Activity and Promote Degradation of Proteins

Degradation of misfolded, redundant and oxidatively damaged proteins constitutes one of the cellular processes which are influenced by the 20S proteasome. However, its activity is generally thought to decrease with age which leads to the gradual accumulation of abnormal proteins in cells and their subsequent aggregation. Therefore, increasing proteasomal degradation constitutes a promising strategy to delay the onset of various age-related diseases, including neurodegenerative disorders. In this study we designed and obtained a series of peptidomimetic stimulators of 20S comprising in their sequences the C-terminal fragment of Blm10 activator. Some of the compounds were capable of enhancing the degradation of natively unfolded and oxidatively damaged proteins, such as α-synuclein and enolase, whose applicability as proteasome substrates was evaluated by microscale thermophoresis (MST). Furthermore, they increased the ChT-L activity of the proteasome in HEK293T cell extracts. Our studies indicate that the 20S proteasome-mediated protein substrates hydrolysis may be selectively increased by peptide-based stimulators acting in an allosteric manner. These compounds, after further optimization, may have the potential to counteract proteasome impairment in patients suffering from age-related diseases.

Design, Synthesis, and Biological Evaluation of Potent 20S Proteasome Activators for the Potential Treatment of α-Synucleinopathies

While neurodegenerative diseases affect millions of patients worldwide, there are insufficient available therapeutics to halt or slow down the progression of these diseases. A key pathological feature of several neurodegenerative diseases is the oligomerization and aggregation of specific intrinsically disordered proteins (IDPs) creating neuronal deposits, such as Lewy bodies in Parkinson's disease. Clearance of these pathogenic, aggregation-prone IDPs is mediated by the 20S isoform of the human proteasome. Thus, enhancing the 20S proteasome-mediated proteolysis could be a very useful therapeutic pathway to prevent neurotoxicity. Here, we report the successful development of sub-microM 20S proteasome activators based on a phenothiazine scaffold. This class of compounds prevented the accumulation of pathologically relevant IDPs, such as the pathogenic A53T mutated α-synuclein, in vitro and in mammalian cell lines.

Small Molecule 20S Proteasome Enhancer Regulates MYC Protein Stability and Exhibits Antitumor Activity in Multiple Myeloma

Despite the addition of several new agents to the armamentarium for the treatment of multiple myeloma (MM) in the last decade and improvements in outcomes, the refractory and relapsing disease continues to take a great toll, limiting overall survival. Therefore, additional novel approaches are needed to improve outcomes for MM patients. The oncogenic transcription factor MYC drives cell growth, differentiation and tumor development in many cancers. MYC protein levels are tightly regulated by the proteasome and an increase in MYC protein expression is found in more than 70% of all human cancers, including MM. In addition to the ubiquitin-dependent degradation of MYC by the 26S proteasome, MYC levels are also regulated in a ubiquitin-independent manner through the REGγ activation of the 20S proteasome. Here, we demonstrate that a small molecule activator of the 20S proteasome, TCH-165, decreases MYC protein levels, in a manner that parallels REGγ protein-mediated MYC degradation. TCH-165 enhances MYC degradation and reduces cancer cell growth in vitro and in vivo models of multiple myeloma by enhancing apoptotic signaling, as assessed by targeted gene expression analysis of cancer pathways. Furthermore, 20S proteasome enhancement is well tolerated in mice and dogs. These data support the therapeutic potential of small molecule-driven 20S proteasome activation for the treatments of MYC-driven cancers, especially MM.

Advances in Proteasome Enhancement by Small Molecules

The proteasome system is a large and complex molecular machinery responsible for the degradation of misfolded, damaged, and redundant cellular proteins. When proteasome function is impaired, unwanted proteins accumulate, which can lead to several diseases including age-related and neurodegenerative diseases. Enhancing proteasome-mediated substrate degradation with small molecules may therefore be a valuable strategy for the treatment of various neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington's diseases. In this review, we discuss the structure of proteasome and how proteasome's proteolytic activity is associated with aging and various neurodegenerative diseases. We also summarize various classes of compounds that are capable of enhancing, directly or indirectly, proteasome-mediated protein degradation.