Background: Aging is a pleiotropic process affecting many aspects of organismal and cellular 1 physiology. Mammalian organisms are composed of a constellation of distinct cell type and state 2 identities residing within different tissue environments. Due to technological limitations, the study 3 of aging has traditionally focused on changes within individual cell types, or the aggregate changes 4 across cell types within a tissue. The influence of cell identity and tissue environment on the trajectory 5 of aging therefore remains unclear.6Results: Here, we perform single cell RNA-seq on >50,000 individual cells across three tissues in 7 young and aged mice. These molecular profiles allow for comparison of aging phenotypes across cell 8 types and tissue environments. We find transcriptional features of aging common across many cell 9 types, as well as features of aging unique to each type. Leveraging matrix factorization and optimal 10 transport methods, we compute a trajectory and magnitude of aging for each cell type. We find that 11 cell type exerts a larger influence on these measures than tissue environment.
12Conclusion: In this study, we use single cell RNA-seq to dissect the influence of cell identity and 13 tissue environment on the aging process. Single cell analysis reveals that cell identities age in unique 14 ways, with some common features of aging shared across identities. We find that both cell identities 15 and tissue environments exert influence on the trajectory and magnitude of aging, with cell identity 16 influence predominating. These results suggest that aging manifests with unique directionality and 17 magnitude across the diverse cell identities in mammals.
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Introduction
19Aging is a gradual process of functional and homeostatic decline in living systems. This decline results in increased 20 mortality risk and disease prevalence, eventually resulting in death. Aging appears to be a conserved feature of 21 eukaryotic biology, affecting organisms as phylogenetically diverse as the single celled S. cerevisiea, the eutelic 22 nematode C. elegans, mice, and humans [45, 63, 91]. Despite the near universal nature of the aging process, the 23 underlying causes of aging are poorly understood. Aging phenotypes have been observed and hypotheses have been 24 proposed for more than a hundred years [97, 50, 37, 64], but we do not yet know the cellular and molecular players 25 that cause aging or how they differ between biological contexts. Both the fundamental nature of aging and its negative 26 effects provide motivation to enumerate these players and establish causal relationships among aging phenotypes.
27Mammalian aging phenotypes manifest at the organismal, tissue, cellular, and molecular levels [100]. Extensive 28 research has produced catalogs of aging phenotypes at the physiological level, providing functional and behavioral 29 * Equal Contribution † Current Address: DuPont Nutrition and Biosciences, 200 Powder Mill Rd, Wilmington, DE, 19803 PREPRINT hallmarks of age related decline. Likewi...