Inhibition of 26S proteasome activity by α-synuclein is mediated by the proteasomal chaperone Rpn14/PAAF1

Parkinson's disease (PD) is characterized by aggregation of α-synuclein (α-syn) into protein inclusions in degenerating brains. Increasing amounts of aggregated α-syn species indicate significant perturbation of cellular proteostasis. Altered proteostasis depends on α-syn protein levels and the impact of α-syn on other components of the proteostasis network. Budding yeast Saccharomyces cerevisiae was used as eukaryotic reference organism to study the consequences of α-syn expression on protein dynamics. To address this, we investigated the impact of overexpression of α-syn and S129A variant on the abundance and stability of most yeast proteins using a genome-wide yeast library and a tandem fluorescent protein timer (tFT) reporter as a measure for protein stability. This revealed that the stability of in total 377 cellular proteins was altered by α-syn expression, and that the impact on protein stability was significantly enhanced by phosphorylation at Ser129 (pS129). The proteasome assembly chaperone Rpn14 was identified as one of the top candidates for increased protein stability by expression of pS129 α-syn. Elevated levels of Rpn14 enhanced the growth inhibition by α-syn and the accumulation of ubiquitin conjugates in the cell. We found that Rpn14 interacts physically with α-syn and stabilizes pS129 α-syn. The expression of α-syn along with elevated levels of Rpn14 or its human counterpart PAAF1 reduced the proteasome activity in yeast and in human cells, supporting that pS129 α-syn negatively affects the 26S proteasome through Rpn14. This comprehensive study into the alternations of protein homeostasis highlights the critical role of the Rpn14/PAAF1 in α-syn-mediated proteasome dysfunction.

Trans-omics Impact of Thymoproteasome in Cortical Thymic Epithelial Cells

The thymic function to produce self-protective and self-tolerant T cells is chiefly mediated by cortical thymic epithelial cells (cTECs) and medullary TECs (mTECs). Recent studies including single-cell transcriptomic analyses have highlighted a rich diversity in functional mTEC subpopulations. Because of their limited cellularity, however, the biochemical characterization of TECs, including the proteomic profiling of cTECs and mTECs, has remained unestablished. Utilizing genetically modified mice that carry enlarged but functional thymuses, here we show a combination of proteomic and transcriptomic profiles for cTECs and mTECs, which identified signature molecules that characterize a developmental and functional contrast between cTECs and mTECs. Our results reveal a highly specific impact of the thymoproteasome on proteasome subunit composition in cTECs and provide an integrated trans-omics platform for further exploration of thymus biology.

Ohigashi et al. show that the use of cyclin D1-transgenic mice allows quantitative proteomic analysis of cortical and medullary thymic epithelial cells (TECs). Results provide a trans-omics platform for further exploration of TEC biology and reveal the specific impact of the thymoproteasome on proteasome subunit composition in cortical TECs.

Defective induction of the proteasome associated with T-cell receptor signaling underlies T-cell senescence

The proteasome degradation machinery is essential for a variety of cellular processes including senescence and T-cell immunity. Decreased proteasome activity is associated with the aging process; however, the regulation of the proteasome in CD4⁺ T cells in relation to aging is unclear. Here, we show that defects in the induction of the proteasome in CD4⁺ T cells upon T-cell receptor (TCR) stimulation underlie T-cell senescence. Proteasome dysfunction promotes senescence-associated phenotypes, including defective proliferation, cytokine production and increased levels of PD-1⁺ CD44^High CD4⁺ T cells. Proteasome induction by TCR signaling via MEK-, IKK- and calcineurin-dependent pathways is attenuated with age and decreased in PD-1⁺ CD44^High CD4⁺ T cells, the proportion of which increases with age. Our results indicate that defective induction of the proteasome is a hallmark of CD4⁺ T-cell senescence.

Dynamic Regulation of Proteasome Expression

The 26S proteasome is a multisubunit complex that catalyzes the degradation of ubiquitinated proteins. The proteasome comprises 33 distinct subunits, all of which are essential for its function and structure. Proteasomes are necessary for various biological processes in cells; therefore, precise regulation of proteasome expression and activity is essential for maintaining cellular health and function. Two decades of research revealed that transcription factors such as Rpn4 and Nrf1 control expression of proteasomes. In this review, we focus on the current understanding and recent findings on the mechanisms underlying the regulation of proteasome expression, as well as the translational regulation of proteasomes.

Foxn1-β5t transcriptional axis controls CD8+ T-cell production in the thymus

The thymus is an organ that produces functionally competent T cells that protect us from pathogens and malignancies. Foxn1 is a transcription factor that is essential for thymus organogenesis; however, the direct target for Foxn1 to actuate thymic T-cell production is unknown. Here we show that a Foxn1-binding cis-regulatory element promotes the transcription of β5t, which has an essential role in cortical thymic epithelial cells to induce positive selection of functionally competent CD8⁺ T cells. A point mutation in this genome element results in a defect in β5t expression and CD8⁺ T-cell production in mice. The results reveal a Foxn1-β5t transcriptional axis that governs CD8⁺ T-cell production in the thymus.

Foxn1 is involved in thymic epithelial cell (TEC) and CD8⁺ T cell development. Here the authors show this development requires Foxn1 binding proximal to, and inducing transcription of, the gene encoding β5t in cortical TECs.