Molecular regulation of stem cell quiescence

Abstract

Subsets of mammalian adult stem cells reside in the quiescent state for prolonged periods of time. This state, which is reversible, has long been viewed as dormant and with minimal basal activity. Recent advances in adult stem cell isolation have provided insights into the epigenetic, transcriptional and post-transcriptional control of quiescence and suggest that quiescence is an actively maintained state in which signalling pathways are involved in maintaining a poised state that allows rapid activation. Deciphering the molecular mechanisms regulating adult stem cell quiescence will increase our understanding of tissue regeneration mechanisms and how they are dysregulated in pathological conditions and in ageing.

Figure 1. Molecular regulation of stem cell quiescence

Recent data suggest that the state of quiescence is actively regulated by different molecular mechanisms. a | Quiescence regulators such as RB, cyclin-dependent kinase inhibitors (CKIs) and p53 negatively regulate the activation of quiescent stem cells. In quiescent muscle stem cells, the loss of the downstream effector of the Notch signalling pathway, RBP-J, promotes spontaneous activation and terminal differentiation, in some cases without cell division. b | Quiescent stem cells are actively regulated by post-transcriptional mechanisms. The loss of the microRNA (miRNA)-processing enzyme Dicer or specific miRNAs promotes quiescent stem cell activation. c | Differential mRNA processing alters the susceptibility of mRNAs to miRNA regulation. In quiescent cells, distal polyadenylation signals (PASs) are used to generate mRNA transcripts with long 3′ untranslated regions (3′ UTRs). In proliferating cells, proximal PASs are used, which decrease the number of miRNA target sites on the transcripts, allowing some transcripts to escape miRNA-mediated inhibition and leading to increased protein expression.

Figure 2. Survival mechanisms of quiescent and activated stem cells

a| Metabolic sensors an d effectors such as forkhead box O (FOXO), hypoxia-inducible factor 1α (HIF1α) and liver kinase B1 (LKB1) are expressed in quiescent stem cells. Expression of these molecules is essential for metabolic functions and survival of quiescent stem cells in adverse environments. These factors protect quiescent stem cells from oxidative stress caused by the accumulation of reactive oxygen species (ROS). Quiescent stem cells devoid of these pathways have an increased propensity to become activated and fail to maintain the stem cell pool. b | Non-homologous end-joining (NHEJ) and homologous recombination are pathways that repair DNA double-strand breaks (DSBs). Homologous recombination is a high-fidelity mechanism that uses homologous templates as guides for DSB repair, whereas NHEJ directly ligates the ends of the DSBs. DSBs that have been repaired by NHEJ can be imprecise when the overhangs at the DSBs are not compatible. In quiescent stem cells, the error-prone NHEJ mechanism is used for DSB repair, which suggests that DNA mutations accumulate in these cells throughout their life. MYF5, myogenic factor 5.

Figure 3. Quiescent stem cells are poised for activation

A proposed model of how the quiescent state of a stem cell constitutes a poised state for activation. Quiescent stem cells are actively regulated at the epigenetic, transcriptional and post-transcriptional level. The epigenetic landscape keeps the chromatin in a permissive state, which allows rapid transcriptional activation. Additional layers of transcriptional and post-transcriptional control safeguard quiescent stem cells to enable precise stem cell activation when necessary.

Figure 4. Transcriptional control of stem cell quiescence

Current data suggest that RNA polymerase II (Pol II) is paused at transcription start sites where histone H3 is trimethylated at Lys 4 (H3K4me3; which is a permissive histone mark). Quiescent stem cells have low mRNA content. The carboxy-terminal domain (CTD) of PolII is phosphorylated at Ser5 but not the Ser2, which indicates transcriptional initiation but not transcriptional elongation. Many types of quiescent stem cells lack Ser2 phosphorylation at the Pol II CTD, which suggests that transcriptional elongation does not occur in these cells, and hence the mRNA levels are low. In quiescent stem cells, Pol II is associated with the negative elongation factors DSIF (DRB sensitivity inducing factor) and NELF. Upon stem cell activation, phosphorylation of DSIF, of NELF (negative elongation factor) and of Ser2 of the Pol II CTD, in combination with the expression of MYC, which is recruited to promoters together with transcription elongation factors (such as pTEFb (positive transcription elongation factor b)), transform the state of promoter proximal pausing into productive elongation and lead to mRNA synthesis.