Telomere, DNA damage, and oxidative stress in stem cell aging

"Stem cell aging" is a novel concept that developed together with the advances of stem cell biology, especially the sophisticated prospectively isolation and characterization of multipotent somatic tissue stem cells. Although being immortal in principle, stem cells can also undergo aging processes and potentially contribute to organismal aging. The impact of an age-dependent decline of stem cell function weighs differently in organs with high or low rates of cell turnover. Nonetheless, most of the organ systems undergo age-dependent loss of homeostasis and functionality, and emerging evidence showed that this has to do with the aging of resident stem cells in the organ systems. The mechanisms of stem cell aging and its real contribution to human aging remain to be defined. Many antitumor mechanisms protect potential malignant transformation of stem cell by inducing apoptosis or senescence but simultaneously provoke stem cell aging. In this review, we try to discuss several concept of stem cell aging and summarize recent progression on the molecular mechanisms of stem cell aging.     

The mitochondrial unfolded protein response is activated upon hematopoietic stem cell exit from quiescence

The mitochondrial unfolded protein response (UPR^mt), a cellular protective program that ensures proteostasis in the mitochondria, has recently emerged as a regulatory mechanism for adult stem cell maintenance that is conserved across tissues. Despite the emerging genetic evidence implicating the UPR^mt in stem cell maintenance, the underlying molecular mechanism is unknown. While it has been speculated that the UPR^mt is activated upon stem cell transition from quiescence to proliferation, the direct evidence is lacking. In this study, we devised three experimental approaches that enable us to monitor quiescent and proliferating hematopoietic stem cells (HSCs) and provided the direct evidence that the UPR^mt is activated upon HSC transition from quiescence to proliferation, and more broadly, mitochondrial integrity is actively monitored at the restriction point to ensure metabolic fitness before stem cells are committed to proliferation.     

Genetic control of quiescence in hematopoietic stem cells

Cellular quiescence is a reversible cell cycle arrest that is poised to re-enter the cell cycle in response to a combination of cell-intrinsic factors and environmental cues. In hematopoietic stem cells, a coordinated balance between quiescence and differentiating proliferation ensures longevity and prevents both genetic damage and stem cell exhaustion. However, little is known about how all these processes are integrated at the molecular level. We will briefly review the environmental and intrinsic control of stem cell quiescence and discuss a new model that involves a protein-to-protein interaction between G0S2 and the phospho-nucleoprotein nucleolin in the cytosol.     

p57 controls adult neural stem cell quiescence and modulates the pace of lifelong neurogenesis

Throughout life, neural stem cells (NSCs) in the adult hippocampus persistently generate new neurons that modify the neural circuitry. Adult NSCs constitute a relatively quiescent cell population but can be activated by extrinsic neurogenic stimuli. However, the molecular mechanism that controls such reversible quiescence and its physiological significance have remained unknown. Here, we show that the cyclin‐dependent kinase inhibitor p57^kip2 (p57) is required for NSC quiescence. In addition, our results suggest that reduction of p57 protein in NSCs contributes to the abrogation of NSC quiescence triggered by extrinsic neurogenic stimuli such as running. Moreover, deletion of p57 in NSCs initially resulted in increased neurogenesis in young adult and aged mice. Long‐term p57 deletion, on the contrary, led to NSC exhaustion and impaired neurogenesis in aged mice. The regulation of NSC quiescence by p57 might thus have important implications for the short‐term (extrinsic stimuli‐dependent) and long‐term (age‐related) modulation of neurogenesis.     

Telomere dysfunction induces environmental alterations limiting hematopoietic stem cell function and engraftment

Cell-intrinsic checkpoints limit the proliferative capacity of primary cells in response to telomere dysfunction. It is not known, however, whether telomere dysfunction contributes to cell-extrinsic alterations that impair stem cell function and organ homeostasis. Here we show that telomere dysfunction provokes defects of the hematopoietic environment that impair B lymphopoiesis but increase myeloid proliferation in aging telomerase knockout (Terc(-/-)) mice. Moreover, the dysfunctional environment limited the engraftment of transplanted wild-type hematopoietic stem cells (HSCs). Dysfunction of the hematopoietic environment was age dependent and correlated with progressive telomere shortening in bone marrow stromal cells. Telomere dysfunction impaired mesenchymal progenitor cell function, reduced the capacity of bone marrow stromal cells to maintain functional HSCs, and increased the expression of various cytokines, including granulocyte colony-stimulating factor (G-CSF), in the plasma of aging mice. Administration of G-CSF to wild-type mice mimicked some of the defects seen in aging Terc(-/-) mice, including impairment of B lymphopoiesis and HSC engraftment. Conversely, inhibition of G-CSF improved HSC engraftment in aged Terc(-/-) mice. Taken together, these results show that telomere dysfunction induces alterations of the environment that can have implications for organismal aging and cell transplantation therapies.     

Cav-1 deletion impaired hematopoietic stem cell function

A tightly controlled balance between hematopoietic stem and progenitor cell compartments is required to maintain normal blood cell homeostasis throughout life, and this balance is regulated by intrinsic and extrinsic cellular factors. Cav-1 is a 22-kDa protein that is located in plasma membrane invaginations and is implicated in regulating neural stem cell and embryonic stem cell proliferation. However, the role of Cav-1 in hematopoietic stem cell (HSC) function is largely unknown. In this study, we used Cav-1^−/− mice to investigate the role of Cav-1 in HSCs function during aging. The results showed that Cav-1^−/− mice displayed a decreased percentage of B cells and an increased percentage of M cells in the bone marrow and peripheral blood, and these changes were due to an increased number of HSCs. FACS analysis showed that the numbers of Lin⁻Sca1⁺c-kit⁺ cells (LSKs), long-term HSCs (LT-HSCs), short-term HSCs and multipotent progenitors were increased in Cav-1^−/− mice compared with Cav-1^+/+ mice, and this increase became more pronounced with aging. An in vitro clonogenic assay showed that LT-HSCs from Cav-1^−/− mice had reduced ability to self-renew. Consistently, an in vivo competitive transplantation assay showed that Cav-1^−/− mice failed to reconstitute hematopoiesis. Moreover, a Cav-1 deletion disrupted the quiescence of LSKs and promoted cell cycle progression through G2/M phase. In addition, we found that Cav-1 deletion impaired the ability of HSCs to differentiate into mature blood cells. Taken together, these data suggest that Cav-1-deficient cells impaired HSCs quiescence and induced environmental alterations, which limited HSCs self-renewal and function.     

Telomeric epigenetic response mediated by Gadd45a regulates stem cell aging and lifespan

Progressive attrition of telomeres triggers DNA damage response (DDR) and limits the regenerative capacity of adult stem cells during mammalian aging. Intriguingly, telomere integrity is not only determined by telomere length but also by the epigenetic status of telomeric/sub‐telomeric regions. However, the functional interplay between DDR induced by telomere shortening and epigenetic modifications in aging remains unclear. Here, we show that deletion of Gadd45a improves the maintenance and function of intestinal stem cells (ISCs) and prolongs lifespan of telomerase‐deficient mice (G3Terc^?/?). Mechanistically, Gadd45a facilitates the generation of a permissive chromatin state for DDR signaling by inducing base excision repair‐dependent demethylation of CpG islands specifically at sub‐telomeric regions of short telomeres. Deletion of Gadd45a promotes chromatin compaction in sub‐telomeric regions and attenuates DDR initiation at short telomeres of G3Terc^?/? ISCs. Treatment with a small molecule inhibitor of base excision repair reduces DDR and improves the maintenance and function of G3Terc^?/? ISCs. Taken together, our study proposes a therapeutic approach to enhance stem cell function and prolong lifespan by targeting epigenetic modifiers.     

Heterochronic parabiosis induces stem cell revitalization and systemic rejuvenation across aged tissues

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Proliferative stem cells maintain quiescence of their niche by secreting the Activin inhibitor Follistatin

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Cytokines and hematopoietic stem cell mobilization

Hematopoietic stem cell transplantation (HSCT) has become the standard of care for the treatment of many hematologic malignancies, chemotherapy sensitive relapsed acute leukemias or lymphomas, multiple myeloma; and for some non-malignant diseases such as aplastic anemia and immunodeficient states. The hematopoietic stem cell (HSC) resides in the bone marrow (BM). A number of chemokines and cytokines have been shown in vivo and in clinical trials to enhance trafficking of HSC into the peripheral blood. This process, termed stem cell mobilization, results in the collection of HSC via apheresis for both autologous and allogeneic transplantation. Enhanced understanding of HSC biology, processes involved in HSC microenvironmental interactions and the critical ligands, receptors and cellular proteases involved in HSC homing and mobilization, with an emphasis on G-CSF induced HSC mobilization, form the basis of this review. We will describe the key features and dynamic processes involved in HSC mobilization and focus on the key ligand-receptor pairs including CXCR4/SDF1, VLA4/VCAM1, CD62L/PSGL, CD44/HA, and Kit/KL. In addition we will describe food and drug administration (FDA) approved and agents currently in clinical development for enhancing HSC mobilization and transplantation outcomes.     

Aspartate availability limits hematopoietic stem cell function during hematopoietic regeneration

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Inflammatory exposure drives long-lived impairment of hematopoietic stem cell self-renewal activity and accelerated aging

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Optimization of physical microenvironment to maintain the quiescence of human induced pluripotent stem cell-derived hepatic stellate cells

Hepatic stellate cells (HSCs) play a crucial role in liver fibrosis by producing excessive extracellular matrix (ECM) following chronic inflammation. However, studying HSC function has been challenging due to the limited availability of primary human quiescent HSCs (qHSCs) in vitro, and the fact that primary qHSCs quickly activate when cultured on plastic plates. Advances in stem cell technology have allowed for the generation of qHSCs from human induced pluripotent stem cells (hiPSCs) with the potential to provide an unlimited source of cells. However, differentiated quiescent-like HSCs (iqHSCs) also activate spontaneously on conventional plastic plates. In this study, we generated iqHSCs from hiPSCs and developed a culture method to maintain such iqHSCs in a lowly activated state for up to 5 days by optimizing their physical culture microenvironment. We observed that three-dimensional (3D) culture of iqHSCs in soft type 1 collagen hydrogels significantly inhibited their spontaneous activation in vitro while maintaining their ability to convert to activated state. Activation of iqHSC was successfully modeled by stimulating them with the fibrotic cytokine TGFβ1. Hence, our culture method can be used to generate HSCs with functions comparable to those in a healthy liver, facilitating the development of accurate in vitro liver models for identifying novel therapeutic agents.     

CTC1 deletion results in defective telomere replication, leading to catastrophic telomere loss and stem cell exhaustion

The proper maintenance of telomeres is essential for genome stability. Mammalian telomere maintenance is governed by a number of telomere binding proteins, including the newly identified CTC1-STN1-TEN1 (CST) complex. However, the in vivo functions of mammalian CST remain unclear. To address this question, we conditionally deleted CTC1 from mice. We report here that CTC1 null mice experience rapid onset of global cellular proliferative defects and die prematurely from complete bone marrow failure due to the activation of an ATR-dependent G2/M checkpoint. Acute deletion of CTC1 does not result in telomere deprotection, suggesting that mammalian CST is not involved in capping telomeres. Rather, CTC1 facilitates telomere replication by promoting efficient restart of stalled replication forks. CTC1 deletion results in increased loss of leading C-strand telomeres, catastrophic telomere loss and accumulation of excessive ss telomere DNA. Our data demonstrate an essential role for CTC1 in promoting efficient replication and length maintenance of telomeres.     

Osteopontin attenuates aging-associated phenotypes of hematopoietic stem cells

Upon aging, hematopoietic stem cells (HSCs) undergo changes in function and structure, including skewing to myeloid lineages, lower reconstitution potential and loss of protein polarity. While stem cell intrinsic mechanisms are known to contribute to HSC aging, little is known on whether age-related changes in the bone marrow niche regulate HSC aging. Upon aging, the expression of osteopontin (OPN) in the murine bone marrow stroma is reduced. Exposure of young HSCs to an OPN knockout niche results in a decrease in engraftment, an increase in long-term HSC frequency and loss of stem cell polarity. Exposure of aged HSCs to thrombin-cleaved OPN attenuates aging of old HSCs, resulting in increased engraftment, decreased HSC frequency, increased stem cell polarity and a restored balance of lymphoid and myeloid cells in peripheral blood. Thus, our data suggest a critical role for reduced stroma-derived OPN for HSC aging and identify thrombin-cleaved OPN as a novel niche informed therapeutic approach for ameliorating HSC phenotypes associated with aging.     

Harnessing matrix stiffness to engineer a bone marrow niche for hematopoietic stem cell rejuvenation

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