Understanding the cellular time machine that enables rejuvenation

PROJECT SUMMARY Nearly every human malaise is enhanced by age. Given the rise of the aging population, there is the urgent need for treatments that improve organ function and block or reverse aging. Such rejuvenation of organisms, tissues, and cells has recently become possible through approaches such as parabiosis and in vivo cellular reprogramming. For instance, the transcription factors OCT4, SOX2, KLF4, and cMYC (OSKM) can reprogram somatic cells to induced pluripotent stem cells (iPSCs) and reset the epigenetic profile, one of the most dependable hallmarks of aging, of the starting somatic cell to a new embryonic state. In an exciting modification of the original iPSC reprogramming method, partial OSKM-mediated reprogramming approaches were recently developed that rejuvenate tissues in vivo. In the initial proof-of-concept investigation, the cyclic induction of OSKM in aged mice yielded notable outcomes including enhanced expansion of pancreatic cells, improved glucose tolerance, and accelerated muscle regeneration after injury. Subsequently, OSKM-induced partial reprogramming was successfully implemented in a cell type-specific manner in vivo. Furthermore, recent advancements in vitro have demonstrated that partial reprogramming of ‘old’ fibroblasts by OSKM induced a rejuvenated reprogramming intermediate, wherein somatic identity is preserved. This rejuvenation was quantified to be the equivalent of approximately 30 years, as determined by epigenetic clock measurements. Thus, faithfully reprogramming cells undergo progressive and continuous rejuvenation of the epigenome, long before the pluripotent state is achieved. Intriguingly, the rejuvenated reprogramming intermediates can revert to the fibroblast state upon withdrawal of reprogramming factor expression and maintain its youthful state. Additionally, crucial functional attributes such as collagen production and migration reverted to a more youthful state, indicating comprehensive cellular rejuvenation. Partial reprogramming by OSKM may therefore have broad implications in regenerative medicine. However, the feasibility of partial reprogramming by OSKM in clinical settings is limited due to concerns regarding its potential to induce cells with a tumorigenic potential. We hypothesize that the mechanistic understanding of how partial reprogramming by OSKM induces epigenetic rejuvenation will lead to the discovery of strategies that can be more safely applied in vivo to induce rejuvenation. To unravel the molecular mechanisms underlying rejuvenation by partial OSKM reprogramming, we will 1) map the changes in the transcriptome, chromatin landscape, transcription factor binding, and epigenetic age across the OSKM-induced partial reprogramming, and based on these data, 2) identify the gene-regulatory and chromatin networks of rejuvenation and regulators of age. We expect that these approaches will begin to define rejuvenation factors that can be applied to other models and provide critical insights into the determinants of epigenetic aging.