Final Report Summary - PROTEOSTASIS OF HESC (Proteostasis of aging and stem cells)
With age, post-mitotic cells lose extensive control of the protein homeostasis (proteostasis) equilibrium: wide spread, aberrant changes in translation, a loss of function in protein degradation machineries, and a generalized down regulation of chaperones often appear in differentiated cells across time. This demise in proteostasis is considered one of the hallmarks of aging. Human pluripotent stem cells (hPSCs) demonstrate a striking capacity to avoid senescence – a capacity that necessarily demands avoidance of any imbalance in proteostasis that would otherwise compromise their function during replication. Thus, we hypothesize that hPSCs can provide a novel paradigm to study proteostasis and its demise during the aging process. For this purpose, our laboratory wants to define the regulation of proteostasis in hPSCs and how this network impinges upon pluripotency and differentiation. In addition, we seek to determine how these proteostasis mechanisms can be adapted to alleviate age-related pathologies, with particular interest in Huntington’s disease (HD). For this purpose, we use an innovative approach based on a combination of stem cell research with genetic analysis in C. elegans. In our previous work, we discovered that hPSCs exhibit high proteasome activity compared with their differentiated counterparts. Furthermore, we uncovered that PSMD11/RPN-6, a key proteasomal subunit, is required for increased proteasome activity. Interestingly, ectopic expression of RPN-6 in somatic tissues is sufficient to induce proteotoxic resistance and extend healtshpan in C. elegans. However, the mechanisms by which the ubiquitin proteasome system regulates hPSC function and aging remain largely unknown. Besides the ubiquitin proteasome system, we hypothesize that other proteostasis mechanisms are also increased in hPSCs. In this project, we performed quantitative proteomics to define increased components of the ubiquitin proteasome system and other proteostasis nodes in hPSCs compared with their differentiated counterparts. Notably, we found that hPSCs exhibit profound differences in the levels of a significant number of proteostasis regulators. This systematic analysis of the proteostasis network was critical to establish our laboratory and future research lines. In this grant, in particular, we focused on E2 enzymes, the main determinants for selection of the lysine to construct ubiquitin chains, which thereby directly control the cellular fate of the substrate. Interestingly, we found an increase in three E2 enzymes in hPSCs. Thus, we examined whether these increased E2 enzymes impinge upon hPSC function. We observed that one of these E2 enzymes is required for neuronal differentiation of hPSCs. Moreover, this E2 enzyme bound to specific H3 histones and its loss induced an increase in H3K9me3, a repressive transcriptional mark. Interestingly, we find that the E2 enzyme directly modulates H3 ubiquitination, a process that determines the trimethylation state of H3 and, in turn, transcriptional changes in pluripotency and differentiation genes. Notably, alterations in histone methylation induced by modulation of our target E2 hasten the aging process in C. elegans. Thus, our results indicate a novel connection between E2 enzymes, histone modification, hPSC identity and organismal aging. Since our target E2 enzyme interacts with mutant huntingtin (HTT), the protein underlying HD, we assessed a potential role of HTT in H3K9me3 regulation. Remarkably, loss of HTT also triggers H3K9me3 levels in hPSCs, a process modulated by its interaction with the chromatin factor ATF7IP. Our research can have important implications in several fields such as stem cell research, neurogenesis, epigenetics and proteostasis. Moreover, our findings could lead to defining novel regulators of the aging process that might, in turn, protect from the symptoms associated with human age-related diseases.