Mechanisms that regulate stem cell aging and life span

Abstract

Mammalian aging is associated with reduced tissue regeneration, increased degenerative disease, and cancer. Because stem cells regenerate many adult tissues and contribute to the development of cancer by accumulating mutations, age-related changes in stem cells likely contribute to age-related morbidity. Consistent with this, stem cell function declines with age in numerous tissues as a result of gate-keeping tumor suppressor expression, DNA damage, changes in cellular physiology, and environmental changes in tissues. It remains unknown whether declines in stem cell function during aging influence organismal longevity. However, mechanisms that influence longevity also modulate age-related morbidity, partly through effects on stem cells.

Figure 1. Multiple Sources of Damage to Biological Macromolecules Reduce Stem Cell Function during Aging

Sources of damage (top row) including ROS, exogenous mutagens, proliferation, infidelity of DNA replication, and errors in protein translation can damage macromolecules or organelles within a cell (middle row). Damage accumulates in DNA, proteins, mitochondria, and lipids during aging and contributes to declines in stem cell function, tissue regeneration, and life span. The cellular consequences of this damage (bottom row) include cell death, cellular senescence, differentiation, altered cellular physiology, and cancer. All of these mechanisms are interrelated; damage to one component, such as telomeres, can influence the function of other components, such as mitochondria (Sahin and Depinho, 2010).

Figure 2. Heterochronic Genes Regulate Increases in the Expression of GateKeeping Tumor Suppressors and Declines in Aging Stem Cell Function

Stem cell self-renewal and stem cell aging are regulated by networks of proto-oncogenes (green) and tumor suppressors (red). (A) let-7 microRNA is an evolutionarily conserved heterochronic gene that regulates the timing of developmental events from C. elegans to mammals (Pasquinelli et al., 2000). let-7b expression increases with age in mammals, reducing the expression of the Hmga2 chromatin-associated factor, and increasing the expression of the JunB, p16^Ink4a, and p19^Arf tumor suppressors (Nishino et al., 2008). The increase in p16^Ink4a expression during aging reduces stem cell function in multiple tissues (Janzen et al., 2006; Krishnamurthy et al., 2006; Molofsky et al., 2006). let-7 microRNA also increases with age in Drosophila, acting in the niche to non-cell-autonomously reduce spermatogonial stem cell function by impairing the secretion of Unpaired (Toledano et al., 2012). (B) p16^Ink4a and p19^Arf expression are also repressed in mammalian stem cells by polycomb proteins, including Bmi-1 and Ezh2 (Jacobs et al., 1999; Lessard and Sauvageau, 2003; Molofsky et al., 2003; Park et al., 2003; Chen et al., 2009b). In the absence of Bmi-1, p16^Ink4a and p19^Arf expression are induced in postnatal stem cells from multiple tissues, inducing cell death, cellular senescence, or premature differentiation. These pathways illustrate how networks of proto-oncogenes and tumor suppressors regulate stem cell maintenance and homeostasis in adult tissues. The way in which proto-oncogenic and tumor suppressor signals are balanced within these networks changes with age in stem cells.

Figure 3. The Multifaceted and Context-Dependent Effects of p53 in Stem Cells

The consequences of tumor suppressor expression in stem cells can be context dependent. p53 can have both positive (blue) and negative (red) effects on stem cell function, depending on context and expression level. When expressed at low levels, p53 can promote stem cell maintenance by promoting the maintenance of genomic integrity and by regulating metabolism. When expressed at high levels, p53 can promote stem cell depletion through cell death or cellular senescence. The aggregate effect of these functions influences longevity, cancer incidence, and tissue regeneration during aging (Tyner et al., 2002; TeKippe et al., 2003; van Heemst et al., 2005; Dumble et al., 2007; Schoppy et al., 2010; Gannon et al., 2011).

Figure 4. Many Components of the Insulin/PI3K Signaling Pathway Regulate Stem Cell Function and Aging

A variety of tyrosine kinase receptors, including the insulin receptor, activate the PI3K pathway, which leads to the activation of both mTORC1 and mTOCR2 (Laplante and Sabatini, 2012). mTORC2 can phosphorylate and activate Akt, SGK, and protein kinase C (PKC). Activated Akt can phosphorylate FoxO transcription factors, restricting their localization to the cytosol. FoxOs that translocate to the nucleus can transcriptionally activate the expression of a variety of genes, including protein folding chaperones, antioxidant enzymes, and metabolic regulators (Salih and Brunet, 2008). Activated Akt can also activate mTORC1 by phosphorylating TSC2, which relieves the inhibitory effects of the TSC1/TSC2 complex on Rheb. mTORC1 activates mechanisms that promote protein translation and lipid and nucleic acid synthesis and inhibit autophagy. The components of these pathways that have not yet been studied in stem cells are likely to regulate stem cell function and perhaps even stem cell aging.

Figure 5. Proteostasis Is Required for Cellular Homeostasis during Aging

Proteostasis is regulated by protein translation rates, which are controlled by ribosome biogenesis, recruitment, and loading. Chaperones promote folding of nascent polypeptides or re-folding of misfolded proteins to prevent protein aggregation. Misfolded or damaged proteins can be ubiquitylated and targeted for proteosomal degradation or engulfed and degraded by auto-phagosomes. Interventions that promote proteostasis can slow aging, reduce the incidence of age-related diseases, and increase life span (Cohen et al., 2009; Durieux et al., 2011; Taylor and Dillin, 2011). These mechanisms are likely to influence tissue regeneration and stem cell function during aging, but this remains largely unstudied.