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Epigenetic determinants of muscle stem cell quiescence and chronological aging

ABSTRACT: Skeletal muscle is a post-mitotic tissue that exhibits an extremely low turnover in the absence of disease or injury. At the same time, muscle possesses remarkable regenerative capacity mediated by satellite cells (SCs) that reside in close association with individual myofibers, underneath the fiber’s basal lamina. Consistent with the low turnover of the muscle, SCs in adult animals are mitotically quiescent and therefore provide an excellent model to study stem cell quiescence. As an organism grows older, the resident stem cells are exposed to a deteriorating environment and experience chronological aging. In stem cells with high turnover, the effects of chronological aging are superimposed upon the effects of the replicative aging that results from DNA replication and cell division. On the contrary, SCs experience minimal replicative aging due to their low turnover. They are thus a good model to study the consequence of chronological aging of quiescent stem cells. We performed microarray analysis of quiescent and activated SCs from both young and aged mice to understand the global gene expression profile underlying stem cell properties such as quiecence and self-renewal, and to understand how the transcriptome of a quiescent stem cell pouplation changes with age.

ORGANISM(S): Mus musculus

PROVIDER: GSE47177 | GEO | 2013/05/23

SECONDARY ACCESSION(S): PRJNA205033

REPOSITORIES: GEO

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Project description:Aging is a complex process characterized by a progressive decline in physiological integrity that leads to impaired cellular and tissue function. Adult stem cells play a critical role in organismal health and aging. Their age-related deterioration contributes to a reduced homeostatic and regenerative capacity. Notably, most studies of stem cell aging focus on the mechanisms of replicative aging in stem cells with high cellular turnover. Yet, the therapeutic potential of stem cells with low cellular turnover, such as adipose-derived stem cells (ASC), is increasingly recognized as potentially superior. The mechanism of aging in low turnover stem cells is thought to differ from those with high turnover and to more closely reflect chronological aging. The latter, however, is exceedingly difficult to study in slowly replicating primary human stem cells and thus remains poorly understood. Here, we employ our unique model of chronological aging in primary human ASCs to examine genome-wide transcriptional networks in early chronological aging using RNA-seq analyses. Our findings demonstrate that the transcriptome of aging ASCs is more stable than that of age-matched fibroblasts. Limited transcriptional modifications in aging ASCs reveal more active transcriptional profiles of cell cycle genes and translation initiation genes when compared with aging differentiated cells. Accordingly, nascent protein synthesis, measured by incorporation of op-puromycin, is increased in ASCs from older individuals, concurrent with a decreased phosphorylation at ser-51 of eIF2, a mechanism of inhibiting translation initiation. A shortened G1 phase observed in the old ASCs could be linked to the increased protein synthesis activity, potentially resulting in more active cell proliferation. This effect, however, is not detected in aging fibroblasts. The altered regulation of cell cycle in aging ASCs could allow a more active cell proliferation to meet an increase demand to preserve tissue and organ functions. These observations are consistent with data supporting the maintenance of ASC integrity in aging human adipose tissue and reveal early chronological aging mechanisms in ASCs that are inherently different from other cell types.

Project description:A unique property of many adult stem cells is their ability to exist in a non-cycling, quiescent state. Although quiescence serves an essential role in preserving stem cell function until the stem cell is needed in tissue homeostasis or repair, defects in quiescence can lead to an impairment in tissue function, the extent to which stem cells can regulate quiescence is unknown. Here, we show that the stem cell quiescent state is composed of two distinct functional phases: G0 and an “alert” phase we term GAlert, and that stem cells actively and reversibly transition between these phases in response to injury-induced, systemic signals. Using genetic models specific to muscle stem cells (or satellite cells (SCs)), we show that mTORC1 activity is necessary and sufficient for the transition of SCs from G0 into GAlert and that signaling through the HGF receptor, cMet is also necessary. We also identify G0-to-GAlert transitions in several populations of quiescent stem cells. Quiescent stem cells that transition into GAlert possess enhanced tissue regenerative function. We propose that the transition of quiescent stem cells into GAlert functions as an 'alerting' mechanism, a novel adaptive response that positions stem cells to respond rapidly under conditions of injury and stress without requiring cell cycle entry or a cell fate commitment. We performed microarray analysis on muscle stem cells, harvested immediately after isolation, to investigate the transcriptional response these cells have to injury and the role of mTORC1 and cMet have in this response At least 2 weeks after tamoxifen treatment, to induce recombination and expression of the Rosa26rYFP lineage tracer in Pax7 expressing muscle stem cells using a Pax7-CreER driver, YFP+ cells were FACS purified from hindlimb muscles of animals that were noninjured (quiescent satellite cells (QSCs)) or from hindlimb muscle contralateral to muscles where we induced a BaCl2 injury (contralateral satellite cells (CSCs)). We compared the response of wild-type (WT) muscle stem cells to the response elicited by muscle stem cells with conditional ablation of TSC1, Rptor, or cMet genes. Immediately after FACS purification of cells, total RNA was extracted, processed, labeled, and hybridized to Affymetrix Mouse Gene 1.0 ST arrays. Each biological replicate is a pooled sample of muscle stem cells isolated from 3-4 mice.

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Epigenetic determinants of muscle stem cell quiescence and chronological aging

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