Proteasomal Stimulation by MK886 and Its Derivatives Can Rescue Tau-Induced Neurite Pathology

Anthricin-induced hyperactive proteasome and its molecular mechanism

Recently, targeted protein degradation has attracted increasing interest as a new drug discovery approach. This method aims to control the function of drug targets by inducing their degradation through protein degradation systems such as the proteasome. Concurrently, compounds that enhance proteasome activity have also garnered attention. In 2023, we reported that anthricin (also known as 4-deoxypodophyllotoxin), a natural product that belongs to the lignan family, enhances proteasome activity. However, whether this enhancement was because of increased proteasome expression or improved proteasome function remains unclear. In this study, we investigated the structure-activity relationship of anthricin and its analogs in enhancing proteasome activity, the effects of anthricin on proteasome-related gene expression, and the direct binding between anthricin and the proteasome using pull-down assay. Moreover, we assessed the interaction between anthricin and the proteasome using molecular dynamics (MD) simulations. The results showed that anthricin does not induce proteasome-related gene expression, but instead binds to the β-subunit of the proteasome, bringing the side chains of three amino acid residues (Thr1, Asp17, and Lys33) at the catalytic site closer together, thereby inducing a hyperactive state. To the best of our knowledge, this study is the first to suggest the mechanism of proteasome activity enhancement by anthricin at the molecular level. The findings could contribute to the development of new chemotypes to enhance the effects of targeted protein degraders by regulating proteasome activity.

Rpt5-Derived Analogs Stimulate Human Proteasome Activity in Cells and Degrade Proteins Forming Toxic Aggregates in Age-Related Diseases

Aging and age-related diseases are associated with a decline in the capacity of protein turnover. Intrinsically disordered proteins, as well as proteins misfolded and oxidatively damaged, prone to aggregation, are preferentially digested by the ubiquitin-independent proteasome system (UIPS), a major component of which is the 20S proteasome. Therefore, boosting 20S activity constitutes a promising strategy to counteract a decrease in total proteasome activity during aging. One way to enhance the proteolytic removal of unwanted proteins appears to be the use of peptide-based activators of the 20S. In this study, we synthesized a series of peptides and peptidomimetics based on the C-terminus of the Rpt5 subunit of the 19S regulatory particle. Some of them efficiently stimulated human 20S proteasome activity. The attachment of the cell-penetrating peptide TAT allowed them to penetrate the cell membrane and stimulate proteasome activity in HEK293T cells, which was demonstrated using a cell-permeable substrate of the proteasome, TAS3. Furthermore, the best activator enhanced the degradation of aggregation-prone α-synuclein and Tau-441. The obtained compounds may therefore have the potential to compensate for the unbalanced proteostasis found in aging and age-related diseases.

Neurodegenerative Disease Tauopathies

Tauopathies are a diverse group of progressive and fatal neurodegenerative diseases characterized by aberrant tau inclusions in the central nervous system. Tau protein forms pathologic fibrillar aggregates that are typically closely associated with neuronal cell death, leading to varied clinical phenotypes including dementia, movement disorders, and motor neuron disease. In this review, we describe the clinicopathologic features of tauopathies and highlight recent advances in understanding the mechanisms that lead to spread of pathologic aggregates through interconnected neuronal pathways. The cell-to-cell propagation of tauopathy is then linked to posttranslational modifications, tau fibril structural variants, and the breakdown of cellular protein quality control.