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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - StemProteostasis (Mediation of stem cell identity and aging by proteostasis)

Teaser

Summary

By 2050, the global population over the age of 80 will triple. Thus, research for improving the quality of life at older age can be of enormous benefit for our ever-aging society. To address this challenge, we propose an innovative approach based on a combination of stem cell research with genetic experiments in the organismal model C. elegans. Mechanisms that promote protein homeostasis (proteostasis) slow down aging and decrease the incidence of age-related diseases. Since human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) replicate continuously in the absence of aging, we hypothesize that they can provide a novel paradigm to study proteostasis and its demise in aging. We previously found that hESCs/iPSCs exhibit increased proteasome activity, a machinery that terminates damaged/toxic proteins. Moreover, we uncovered that the proteasome subunit RPN-6 is required for this activity and sufficient to extend healtshpan in C. elegans. However, the mechanisms by which the proteasome regulates hESC/iPSC function remain unknown. Our first aim is to define how the proteasome regulates not only hESC/iPSC identity but also aging and the onset of age-related diseases. Moreover, one of the next challenges is to define how other proteostasis pathways impinge upon hESC/iPSC function. We hypothesize that, in addition to the proteasome, hESCs differentially regulate other subcellular stress response pathways designed to protect them from disequilibrium in the folding and degradation of their proteome. For this reason, we perform a comprehensive study of proteostasis of hESCs/iPSCs and mimic this network in somatic cells to alleviate age-related diseases. Finally, we seek to determine whether loss of proteostasis promotes somatic stem cell (SC) exhaustion, which is one of the most obvious characteristics of the aging process and contributes to tissue degeneration. By using mouse models, we will examine whether sustained proteostasis delays neural SC exhaustion. Our research can have an impact in several fields such as stem cell research, neurogenesis, proteostasis, aging and age-related diseases.

Work performed

We have now defined E3 ubiquitin ligases (i.e., the enzymes that target specific proteins for proteasomal degradation) up-regulated in hESCs. Systematic characterization of E3 interactome and loss-of-function experiments suggest a link with hESC identity. Moreover, global proteasome inhibition impairs diverse processes required for hESC identity, including telomere maintenance (Saez et al, Scientific Reports, 2018). Moreover, we found that the ubiquitin ligase UBR5 suppresses proteostasis collapse in stem cells and Huntingtonâ€™s disease (HD) models (Koyuncu et al, Nature Communications, 2018). In addition, we found that hESCs exhibit enhanced assembly of the TRiC/CCT complex, a chaperonin that facilitates protein folding. We observed that somatic increase of CCT8 mimics proteostasis of hESCs and extends C. elegans lifespan (Noormohammadi, Nature Communications, 2016). With the strong link between TRiC/CTT complex and HD, we examined the impact of CCT8 in HD models and found that ectopic expression of CCT8 ameliorates HD-related changes. We also obtained unexpected results that show an increase of distinct components of the protein synthesis network in hESCs. In particular, we uncovered CSDE1 as a post-transcriptional regulator of hESC identity and neurogenesis (Lee et al, Nature Communications, 2017).

Final results

During this period, we have made significant advances to define the regulation of proteostasis in human pluripotent stem cells and how this network impinges upon pluripotency and differentiation. Particularly, we defined several proteostasis components that suppress disease-related protein aggregation in human pluripotent stem cells and their role in differentiation. Among them, we have found the UBR5 enzyme, the chaperonin subunit CCT8 and the RNA-binding protein CSDE1. We also defined the molecular mechanisms by which these factors regulate proteostasis. UBR5 plays a key role in tagging aggregation-prone proteins for proteasomal degradation. We also found that CCT8 is a central activator of the assembly of TRiC/CCT complex, a chaperonin that facilitates the folding of 15% of the human proteome. CSDE1 is a post-transcriptional regulator that regulates the synthesis rates of numerous proteins. Then, we applied our interdisciplinary approach to uncover new mechanisms of longevity that might, in turn, protect from the symptoms associated to human age-related diseases. Indeed, modulation of either UBR5 or CCT8 can suppress the accumulation of disease-related proteins in the neurons of HD invertebrate models and extend organismal healthspan. Thus, by using our interdisciplinary innovative approach that combines hESC/iPSC research with genetics in C. elegans, we have obtained important results to address two important challenges: 1) defining stem cell identity and 2) improving the quality of life at older age. Moreover, these results are a proof-of-concept of our approach, supporting that other proteostasis components could also protect from misfolded protein-aggregation and slow down the aging process. Thus, we will now define other enhanced proteostasis components in pluripotent stem cells and mimic them in somatic cells of model organisms to extend healthspan. Our research proposal has many potential outcomes and our findings could have a significant impact in several fields such as proteostasis, stem cell research, cell reprogramming, cell therapy, neurogenesis, aging and age-related diseases.