Genetic regulation of primitive hematopoietic stem cell senescence

Factor 3 regulates airway engraftment by human bronchial basal cells

Cystic fibrosis transmembrane conductance regulator (CFTR) gene editing and transplantation of CFTR-gene corrected airway basal cells has the potential to cure CF lung disease. Although mouse studies established that cell transplantation was feasible, the engraftment rate was typically low and frequently less than the estimated therapeutic threshold. The purpose of this study was to identify genes and culture conditions that regulate the therapeutic potential of human bronchial basal cells. Factor 3 (F3, Tissue Factor 1) is a component of the extrinsic coagulation pathway and activates a cascade of proteases that convert fibrinogen to fibrin. Based on reports that F3 was necessary for human basal cell survival and adhesion in vitro, the present study evaluated F3 as a potential determinant of therapeutic fitness. The gene expression profile of F3 mRNA-positive human bronchial basal cells was evaluated by scRNAseq and the impact of the lung environment on F3 expression was modeled by varying in vitro culture conditions. F3 necessity for adhesion, proliferation, and differentiation was determined by CRISPR/Cas9 knockout (KO) of the F3 gene. Finally, the impact of F3 manipulation on engraftment was determined by orthotropic co-transplantation of wild-type and F3-KO cells into the airways of immunocompromised mice. In contrast with the hypothesis that F3 increases the therapeutic fitness of basal cells, F3 expression decreased engraftment. These studies guide the ongoing development of cellular therapies by showing that in vitro assessments may not predict therapeutic potential and that the lung milieu influences the functional properties of transplanted bronchial basal cells.

Clonal dynamics and somatic evolution of haematopoiesis in mouse

Haematopoietic stem cells maintain blood production throughout life. While extensively characterised using the laboratory mouse, little is known about how the population is sustained and evolves with age. We isolated stem cells and progenitors from young and old mice, identifying 221,890 somatic mutations genome-wide in 1845 single cell-derived colonies, and used phylogenetic analysis to infer the ontogeny and population dynamics of the stem cell pool. Mouse stem cells and progenitors accrue ~45 somatic mutations per year, a rate only about 2-fold greater than human progenitors despite the vastly different organismal sizes and lifespans. Phylogenetic patterns reveal that stem and multipotent progenitor cell pools are both established during embryogenesis, after which they independently self-renew in parallel over life. The stem cell pool grows steadily over the mouse lifespan to approximately 70,000 cells, self-renewing about every six weeks. Aged mice did not display the profound loss of stem cell clonal diversity characteristic of human haematopoietic ageing. However, targeted sequencing revealed small, expanded clones in the context of murine ageing, which were larger and more numerous following haematological perturbations and exhibited a selection landscape similar to humans. Our data illustrate both conserved features of population dynamics of blood and distinct patterns of age-associated somatic evolution in the short-lived mouse.

A Review of the Impact of Calorie Restriction on Stem Cell Potency

Calorie restriction (CR) prolongs lifespan in various species and also minimises pathologies caused by aging. One of the characteristics seen in age-related pathologies is stem cell exhaustion. Here, we review the various impacts of CR on mammalian health mediated through stem cell potency in various tissues. This study comprised of a literature search through NCBI, Science Direct, Google Scholar and PubMed, focusing on the impact of CR on pluripotency. In the skeletal muscle, CR acts as an anti-inflammatory agent and increases the presence of satellite cells endogenously to improve regeneration, thus causing a metabolic shift to oxidation to meet oxygen demand. In the intestinal epithelium, CR suppresses the mechanistic target of rapamycin complex 1 (mTORC1) signalling in Paneth cells to shift the stem cell equilibrium towards self-renewal at the cost of differentiation. In haematopoiesis, CR prevents deterioration or maintains the function of haematopoietic stem cells (HSCs) depending on the genetic variation of the mice. In skin and hair follicles, CR increases the thickness of the epidermis and hair growth and improves hair retention through stem cells. CR mediates the proliferation and self-renewal of stem cells in various tissues, thus increasing its regenerative ability.

Stem Cell DNA Damage and Genome Mutation in the Context of Aging and Cancer Initiation

Adult stem cells fuel tissue homeostasis and regeneration through their unique ability to self-renew and differentiate into specialized cells. Thus, their DNA provides instructions that impact the tissue as a whole. Since DNA is not an inert molecule, but rather dynamic, interacting with a myriad of chemical and physical factors, it encounters damage from both endogenous and exogenous sources. Damage to DNA introduces deviations from its normal intact structure and, if left unrepaired, may result in a genetic mutation. In turn, mutant genomes of stem and progenitor cells are inherited in cells of the lineage, thus eroding the genetic information that maintains homeostasis of the somatic cell population. Errors arising in stem and progenitor cells will have a substantially larger impact on the tissue in which they reside than errors occurring in postmitotic differentiated cells. Therefore, maintaining the integrity of genomic DNA within our stem cells is essential to protect the instructions necessary for rebuilding healthy tissues during homeostatic renewal. In this review, we will first discuss DNA damage arising in stem cells and cell- and tissue-intrinsic mechanisms that protect against harmful effects of this damage. Secondly, we will examine how erroneous DNA repair and persistent DNA damage in stem and progenitor cells impact stem cells and tissues in the context of cancer initiation and aging. Finally, we will discuss the use of invertebrate and vertebrate model systems to address unanswered questions on the role that DNA damage and mutation may play in aging and precancerous conditions.

Ageing and atherosclerosis: vascular intrinsic and extrinsic factors and potential role of IL-6

The number of old people is rising worldwide, and advancing age is a major risk factor for atherosclerotic cardiovascular disease. However, the mechanisms underlying this phenomenon remain unclear. In this Review, we discuss vascular intrinsic and extrinsic mechanisms of how ageing influences the pathology of atherosclerosis. First, we focus on factors that are extrinsic to the vasculature. We discuss how ageing affects the development of myeloid cells leading to the expansion of certain myeloid cell clones and induces changes in myeloid cell functions that promote atherosclerosis via inflammation, including a potential role for IL-6. Next, we describe vascular intrinsic factors by which ageing promotes atherogenesis - in particular, the effects on mitochondrial function. Studies in mice and humans have shown that ageing leads to a decline in vascular mitochondrial function and impaired mitophagy. In mice, ageing is associated with an elevation in the levels of the inflammatory cytokine IL-6 in the aorta, which participates in a positive feedback loop with the impaired vascular mitochondrial function to accelerate atherogenesis. We speculate that vascular and myeloid cell ageing synergize, via IL-6 signalling, to accelerate atherosclerosis. Finally, we propose future avenues of clinical investigation and potential therapeutic approaches to reduce the burden of atherosclerosis in old people.

CBP/Catenin antagonists: Targeting LSCs' Achilles heel

Cancer stem cells (CSCs), including leukemia stem cells (LSCs), exhibit self-renewal capacity and differentiation potential and have the capacity to maintain or renew and propagate a tumor/leukemia. The initial isolation of CSCs/LSCs was in adult myelogenous leukemia, although more recently, the existence of CSCs in a wide variety of other cancers has been reported. CSCs, in general, and LSCs, specifically with respect to this review, are responsible for initiation of disease, therapeutic resistance and ultimately disease relapse. One key focus in cancer research over the past decade has been the development of therapies that safely eliminate the LSC/CSC population. One major obstacle to this goal is the identification of key mechanisms that distinguish LSCs from normal endogenous hematopoietic stem cells. An additional daunting feature that has recently come to light with advances in next-generation sequencing and single-cell sequencing is the heterogeneity within leukemias/tumors, with multiple combinations of mutations, gain and loss of function of genes, and so on being capable of driving disease, even within the CSC/LSC population. The focus of this review/perspective is on our work in identifying and validating, in both chronic myelogenous leukemia and acute lymphoblastic leukemia, a safe and efficacious mechanism to target an evolutionarily conserved signaling nexus, which constitutes a common "Achilles heel" for LSCs/CSCs, using small molecule-specific CBP/catenin antagonists.

DNA Damage Response in Hematopoietic Stem Cell Ageing

Maintenance of tissue-specific stem cells is vital for organ homeostasis and organismal longevity. Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic system. They divide asymmetrically and give rise to daughter cells with HSC identity (self-renewal) and progenitor progenies (differentiation), which further proliferate and differentiate into full hematopoietic lineages. Mammalian ageing process is accompanied with abnormalities in the HSC self-renewal and differentiation. Transcriptional changes and epigenetic modulations have been implicated as the key regulators in HSC ageing process. The DNA damage response (DDR) in the cells involves an orchestrated signaling pathway, consisting of cell cycle regulation, cell death and senescence, transcriptional regulation, as well as chromatin remodeling. Recent studies employing DNA repair-deficient mouse models indicate that DDR could intrinsically and extrinsically regulate HSC maintenance and play important roles in tissue homeostasis of the hematopoietic system. In this review, we summarize the current understanding of how the DDR determines the HSC fates and finally contributes to organismal ageing.

Kat3 coactivators in somatic stem cells and cancer stem cells: biological roles, evolution, and pharmacologic manipulation

Long-lived somatic stem cells regenerate adult tissues throughout our lifetime. However, with aging, there is a significant deterioration in the function of stem and progenitor cells, which contribute to diseases of aging. The decision for a long-lived somatic stem cell to become activated and subsequently to undergo either a symmetric or an asymmetric division is a critical cellular decision process. The decision to preferentially divide symmetrically or asymmetrically may be the major fundamental intrinsic difference between normal somatic stem cells and cancer stem cells. Based upon work done primarily in our laboratory over the past 15 years, this article provides a perspective on the critical role of somatic stem cells in aging. In particular, we discuss the importance of symmetric versus asymmetric divisions in somatic stem cells and the role of the differential usage of the highly similar Kat3 coactivators, CREB-binding protein (CBP) and p300, in stem cells. We describe and propose a more complete model for the biological mechanism and roles of these two coactivators, their evolution, and unique roles and importance in stem cell biology. Finally, we discuss the potential to pharmacologically manipulate Kat3 coactivator interactions in endogenous stem cells (both normal and cancer stem cells) to potentially ameliorate the aging process and common diseases of aging.

Critical Roles of Reactive Oxygen Species in Age-Related Impairment in Ischemia-Induced Neovascularization by Regulating Stem and Progenitor Cell Function

Reactive oxygen species (ROS) regulate bone marrow microenvironment for stem and progenitor cells functions including self-renewal, differentiation, and cell senescence. In response to ischemia, ROS also play a critical role in mediating the mobilization of endothelial progenitor cells (EPCs) from the bone marrow to the sites of ischemic injury, which contributes to postnatal neovascularization. Aging is an unavoidable biological deteriorative process with a progressive decline in physiological functions. It is associated with increased oxidative stress and impaired ischemia-induced neovascularization. This review discusses the roles of ROS in regulating stem and progenitor cell function, highlighting the impact of unbalanced ROS levels on EPC dysfunction and the association with age-related impairment in ischemia-induced neovascularization. Furthermore, it discusses strategies that modulate the oxidative levels of stem and progenitor cells to enhance the therapeutic potential for elderly patients with cardiovascular disease.

Potential relationship between inadequate response to DNA damage and development of myelodysplastic syndrome

Hematopoietic stem cells (HSCs) are responsible for the continuous regeneration of all types of blood cells, including themselves. To ensure the functional and genomic integrity of blood tissue, a network of regulatory pathways tightly controls the proliferative status of HSCs. Nevertheless, normal HSC aging is associated with a noticeable decline in regenerative potential and possible changes in other functions. Myelodysplastic syndrome (MDS) is an age-associated hematopoietic malignancy, characterized by abnormal blood cell maturation and a high propensity for leukemic transformation. It is furthermore thought to originate in a HSC and to be associated with the accrual of multiple genetic and epigenetic aberrations. This raises the question whether MDS is, in part, related to an inability to adequately cope with DNA damage. Here we discuss the various components of the cellular response to DNA damage. For each component, we evaluate related studies that may shed light on a potential relationship between MDS development and aberrant DNA damage response/repair.

Measuring the aging process in stem cells

Stem cells persist in replenishing functional mature cells throughout life by self-renewal and multilineage differentiation. Hematopoietic stem cells (HSCs) are among the best-characterized and understood stem cells, and they are responsible for the life-long production of all lineages of blood cells. HSCs are a heterogeneous population containing lymphoid-biased, myeloid-biased, and balanced subsets. HSCs undergo age-associated phenotypic and functional changes, and the composition of the HSC pool alters with aging. HSCs and their lineage-biased subfractions can be identified and analyzed by flow cytometry based on cell surface makers. Fluorescence-activated cell sorting (FACS) enables the isolation and purification of HSCs that greatly facilitates the mechanistic study of HSCs and their aging process at both cellular and molecular levels. The mouse model has been extensively used in HSC aging study. Bone marrow cells are isolated from young and old mice and stained with fluorescence-conjugated antibodies specific for differentiated and stem cells. HSCs are selected based on the negative expression of lineage markers and positive selection for several sets of stem cell markers. Lineage-biased HSCs can be further distinguished by the level of SLAM/CD150 expression and the extent of Hoechst efflux.

Age-related Deterioration of Hematopoietic Stem Cells

Aging is the process of system deterioration over time in the whole body. Stem cells are self-renewing and therefore have been considered exempt from the aging process. Earlier studies by Hayflick showed that there is an intrinsic limit to the number of divisions that mammalian somatic cells can undergo, and cycling kinetics and ontogeny-related studies strongly suggest that even the most primitive stem cell functions exhibit a certain degree of aging. Despite these findings, studies on the effects of aging on stem cell functions are inconclusive. Here we review the age-related properties of hematopoietic stem cells in terms of intrinsic and extrinsic alterations, proliferative potential, signaling molecules, telomere and telomerase, senescence and cancer issues, regenerative potential and other indications of stem cell aging are discussed in detail.

Hematopoietic stem cell aging

Aging is organized in a hierarchy, in which aging of cells results in aged tissues, ultimately limiting lifespan. For organ systems that also in the adult depend on stem cells for tissue homeostasis like the hematopoietic system that forms immune cells, it is believed that aging of the stem cells strongly contributes to aging-associated dysfunction. In this review, we summarize current aspects on cellular and molecular mechanisms that are associated with aging of hematopoietic stem cells, the role of the stem cell niche for stem cell aging as well as novel and encouraging experimental approaches to attenuate aging of hematopoietic stem cells to target immunosenescence.

Iron regulatory protein-1 protects against mitoferrin-1-deficient porphyria

Mitochondrial iron is essential for the biosynthesis of heme and iron-sulfur ([Fe-S]) clusters in mammalian cells. In developing erythrocytes, iron is imported into the mitochondria by MFRN1 (mitoferrin-1, SLC25A37). Although loss of MFRN1 in zebrafish and mice leads to profound anemia, mutant animals showed no overt signs of porphyria, suggesting that mitochondrial iron deficiency does not result in an accumulation of protoporphyrins. Here, we developed a gene trap model to provide in vitro and in vivo evidence that iron regulatory protein-1 (IRP1) inhibits protoporphyrin accumulation. Mfrn1(+/gt);Irp1(-/-) erythroid cells exhibit a significant increase in protoporphyrin levels. IRP1 attenuates protoporphyrin biosynthesis by binding to the 5'-iron response element (IRE) of alas2 mRNA, inhibiting its translation. Ectopic expression of alas2 harboring a mutant IRE, preventing IRP1 binding, in Mfrn1(gt/gt) cells mimics Irp1 deficiency. Together, our data support a model whereby impaired mitochondrial [Fe-S] cluster biogenesis in Mfrn1(gt/gt) cells results in elevated IRP1 RNA-binding that attenuates ALAS2 mRNA translation and protoporphyrin accumulation.

Cdc42 and aging of hematopoietic stem cells

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) continuously provide mature blood cells during the lifespan of a mammal. The functional decline in hematopoiesis in the elderly, which involves a progressive reduction in the immune response and an increased incidence of myeloid malignancy, is partly linked to HSC aging. Molecular mechanisms of HSC aging remain unclear, hindering rational approaches to slow or reverse the decline of HSC function with age. Identifying conditions under which aged HSCs become equivalent to young stem cells might result in treatments for age-associated imbalances in lymphopoiesis and myelopoiesis and in blood regeneration.

RECENT FINDINGS

Aging of HSCs has been for a long time thought to be an irreversible process imprinted in stem cells due to the intrinsic nature of HSC aging. Mouse model studies have found that aging is associated with elevated activity of the Rho GTPase Cdc42 in HSCs that is causative for loss of polarity, altered epigenetic modifications and functional deficits of aged HSCs. The work suggests that inhibition of Cdc42 activity in aged HSCs may reverse a number of phenotypes associated with HSC aging.

SUMMARY

Maintaining the regenerative capacity of organs or organ systems may be a useful way to ensure healthy aging. A defined set of features phenotypically separate young from aged HSCs. Aging of HSCs has been thought to be irreversible. Recent findings support the hypothesis that functional decline of aged HSCs may be reversible by pharmacological intervention of age altered signaling pathways and epigenetic modifications.

Mechanisms that regulate stem cell aging and life span

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.

Immunosenescence and organ transplantation

Increasing numbers of elderly transplant recipients and a growing demand for organs from older donors impose pressing challenges on transplantation medicine. Continuous and complex modifications of the immune system in parallel to aging have a major impact on transplant outcome and organ quality. Both, altered alloimmune responses and increased immunogenicity of organs present risk factors for inferior patient and graft survival. Moreover, a growing body of knowledge on age-dependent modifications of allorecognition and alloimmune responses may require age-adapted immunosuppression and organ allocation. Here, we summarize relevant aspects of immunosenescence and their possible clinical impact on organ transplantation.

Murine adipose tissue-derived stromal cell apoptosis and susceptibility to oxidative stress in vitro are regulated by genetic background

Adipose tissue-derived stromal cells (ADSCs) are of interest for regenerative medicine as they are isolated easily and can differentiate into multiple cell lineages. Studies of their in vitro proliferation, survival, and differentiation are common; however, genetic effects on these phenotypes remain unknown. To test if these phenotypes are genetically regulated, ADSCs were isolated from three genetically diverse inbred mouse strains- C57BL/6J (B6), BALB/cByJ (BALB), and DBA/2J (D2)- in which genetic regulation of hematopoietic stem function is well known. ADSCs from all three strains differentiated into osteogenic and chondrogenic lineages in vitro. ADSCs from BALB grew least well in vitro, probably due to apoptotic cell death after several days in culture. BALB ADSCs were also the most susceptible to the free radical inducers menadione and H₂O₂. ADSCs from the three possible F1 hybrids were employed to further define genetic regulation of ADSC phenotypes. D2, but not B6, alleles stimulated ADSC expansion in BALB cells. In contrast, B6, but not D2, alleles rescued BALB H₂O₂ resistance. We conclude that low oxidative stress resistance does not limit BALB ADSC growth in vitro, as these phenotypes are genetically regulated independently. In addition, ADSCs from these strains are an appropriate model system to investigate genetic regulation of ADSC apoptosis and stress resistance in future studies. Such investigations are essential to optimize cell expansion and differentiation and thus, potential for regenerative medicine.

Kidney transplantation and the ageing immune system

The world population is rapidly growing and ageing at a pace that is projected to continue for at least three decades. This shift towards an older populace has invariably increased the number of individuals with diseases related to ageing, such as chronic kidney disease. The increase in chronic kidney disease is associated with a growing number of elderly patients receiving kidney transplants. Understanding how the immune system changes with increasing age will help to define the risks of rejection and infection in the elderly population and will focus attention on the need for individualized immunosuppression for patients in this age group. This Review addresses what is currently known about ageing and the immune system, highlighting age-related changes that affect the outcome of transplantation in elderly individuals. The need for new strategies to improve outcomes in this growing population of elderly renal transplant recipients is also emphasized.

Changes in primary lymphoid organs with aging

Aging is associated with decreased immune function that leads to increased morbidity and mortality in the elderly. Immune senescence is accompanied by age-related changes in two primary lymphoid organs, bone marrow and thymus, that result in decreased production and function of B and T lymphocytes. In bone marrow, hematopoietic stem cells exhibit reduced self-renewal potential, increased skewing toward myelopoiesis, and decreased production of lymphocytes with aging. These functional sequelae of aging are caused in part by increased oxidative stress, inflammation, adipocyte differentiation, and disruption of hypoxic osteoblastic niches. In thymus, aging is associated with tissue involution, exhibited by a disorganization of the thymic epithelial cell architecture and increased adiposity. This dysregulation correlates with a loss of stroma-thymocyte 'cross-talk', resulting in decreased export of naïve T cells. Mounting evidence argues that with aging, thymic inflammation, systemic stress, local Foxn1 and keratinocyte growth factor expression, and sex steroid levels play critical roles in actively driving thymic involution and overall adaptive immune senescence across the lifespan. With a better understanding of the complex mechanisms and pathways that mediate bone marrow and thymus involution with aging, potential increases for the development of safe and effective interventions to prevent or restore loss of immune function with aging.

Fetal myocardium in the kidney capsule: an in vivo model of repopulation of myocytes by bone marrow cells

Debate surrounds the question of whether the heart is a post-mitotic organ in part due to the lack of an in vivo model in which myocytes are able to actively regenerate. The current study describes the first such mouse model — a fetal myocardial environment grafted into the adult kidney capsule. Here it is used to test whether cells descended from bone marrow can regenerate cardiac myocytes. One week after receiving the fetal heart grafts, recipients were lethally irradiated and transplanted with marrow from green fluorescent protein (GFP)-expressing C57Bl/6J (B6) donors using normal B6 recipients and fetal donors. Levels of myocyte regeneration from GFP marrow within both fetal myocardium and adult hearts of recipients were evaluated histologically. Fetal myocardium transplants had rich neovascularization and beat regularly after 2 weeks, continuing at checkpoints of 1, 2, 4, 6, 8 and12 months after transplantation. At each time point, GFP-expressing rod-shaped myocytes were found in the fetal myocardium, but only a few were found in the adult hearts. The average count of repopulated myocardium with green rod-shaped myocytes was 996.8 cells per gram of fetal myocardial tissue, and 28.7 cells per adult heart tissue, representing a thirty-five fold increase in fetal myocardium compared to the adult heart at 12 months (when numbers of green rod-shaped myocytes were normalized to per gram of myocardial tissue). Thus, bone marrow cells can differentiate to myocytes in the fetal myocardial environment. The novel in vivo model of fetal myocardium in the kidney capsule appears to be valuable for testing repopulating abilities of potential cardiac progenitors.

Age-dependent response of murine female bone marrow cells to hyperbaric oxygen

Consequences of age on the effects of hyperbaric oxygen (HBO) on bone marrow (BM) derived stem cells and progenitors (SCPs) are largely unknown. We treated 2- and 18-month old C57BL/6 female mice by HBO. Hematopoietic stem cells and progenitors, enumerated as colony-forming units in culture, were doubled only in peripheral leukocytes and BM cells of young mice receiving HBO. In old mice colony-forming unit fibroblast numbers, a measure of mesenchymal stromal cells (MSCs) from BM, were high but unaffected by HBO. To further explore this finding, in BM-MSCs we quantified the transcripts of adipocyte early-differentiation genes peroxisome proliferator-activated receptor-γ, CCAAT/enhancer binding protein-β and fatty-acid binding protein 4; these transcripts were not affected by age or HBO. However, osteoblast gene transcripts runt-related transcription factor 2, osterix (OSX) and alkaline phosphatase (AP) were twofold to 20-fold more abundant in MSCs from old control mice relative to those of young control mice. HBO affected expression of osteoblast markers only in old MSCs (OSX gene expression was reduced by twofold and AP expression was increased threefold). Our data demonstrate the impact of aging on the response of BM SCPs to HBO and indicate the potentially different age-related benefit of HBO in wound healing and tissue remodeling.

Cellular senescence and tumor suppressor gene p16

Cellular senescence is an irreversible arrest of cell growth. Biochemical and morphological changes occur during cellular senescence, including the formation of a unique cellular morphology such as flattened cytoplasm. Function of mitochondria, endoplasmic reticulum and lysosomes are affected resulting in the inhibition of lysosomal and proteosomal pathways. Cellular senescence can be triggered by a number of factors including, aging, DNA damage, oncogene activation and oxidative stress. While the molecular mechanism of senescence involves p16 and p53 tumor suppressor genes and telomere shortening, this review is focused on the mechanism of p16 control. The p16-mediated senescence acts through the retinoblastoma (Rb) pathway inhibiting the action of the cyclin dependant kinases leading to G1 cell cycle arrest. Rb is maintained in a hypophosphorylated state resulting in the inhibition of transcription factor E2F1. Regulation of p16 expression is complex and involves epigenetic control and multiple transcription factors. PRC1 (Pombe repressor complex (1) and PRC2 (Pombe repressor complex (2) proteins and histone deacetylases play an important role in the promoter hypermethylation for suppressing p16 expression. While transcription factors YY1 and Id1 suppress p16 expression, transcription factors CTCF, Sp1 and Ets family members activate p16 transcription. Senescence occurs with the inactivation of suppressor elements leading to the enhanced expression of p16.

Aging-associated changes in hematopoiesis and leukemogenesis: what's the connection?

Aging is associated with a marked increase in a number of diseases, including many types of cancer. Due to the complex and multi-factorial nature of both aging and cancer, accurate deciphering of causative links between aging and cancer remains a major challenge. It is generally accepted that initiation and progression of cancers are driven by a process of clonal evolution. In principle, this somatic evolution should follow the same Darwinian logic as evolutionary processes in populations in nature: diverse heritable types arising as a result of mutations are subjected to selection, resulting in expansion of the fittest clones. However, prevalent paradigms focus primarily on mutational aspects in linking aging and cancer. In this review, we will argue that age-related changes in selective pressures are likely to be equally important. We will focus on aging-related changes in the hematopoietic system, where age-associated alterations are relatively well studied, and discuss the impact of these changes on the development of leukemias and other malignancies.

Oxidative stress, mitochondrial dysfunction, and aging

Aging is an intricate phenomenon characterized by progressive decline in physiological functions and increase in mortality that is often accompanied by many pathological diseases. Although aging is almost universally conserved among all organisms, the underlying molecular mechanisms of aging remain largely elusive. Many theories of aging have been proposed, including the free-radical and mitochondrial theories of aging. Both theories speculate that cumulative damage to mitochondria and mitochondrial DNA (mtDNA) caused by reactive oxygen species (ROS) is one of the causes of aging. Oxidative damage affects replication and transcription of mtDNA and results in a decline in mitochondrial function which in turn leads to enhanced ROS production and further damage to mtDNA. In this paper, we will present the current understanding of the interplay between ROS and mitochondria and will discuss their potential impact on aging and age-related diseases.

The critical role of agrin in the hematopoietic stem cell niche

Hematopoiesis is the process leading to the sustained production of blood cells by hematopoietic stem cells (HSCs). Growth, survival, and differentiation of HSCs occur in specialized microenvironments called "hematopoietic niches," through molecular cues that are only partially understood. Here we show that agrin, a proteoglycan involved in the neuromuscular junction, is a critical niche-derived signal that controls survival and proliferation of HSCs. Agrin is expressed by multipotent nonhematopoietic mesenchymal stem cells (MSCs) and by differentiated osteoblasts lining the endosteal bone surface, whereas Lin(-)Sca1(+)c-Kit(+) (LSK) cells express the α-dystroglycan receptor for agrin. In vitro, agrin-deficient MSCs were less efficient in supporting proliferation of mouse Lin(-)c-Kit(+) cells, suggesting that agrin plays a role in the hematopoietic cell development. These results were indeed confirmed in vivo through the analysis of agrin knockout mice (Musk-L;Agrn(-/-)). Agrin-deficient mice displayed in vivo apoptosis of CD34(+)CD135(-) LSK cells and impaired hematopoiesis, both of which were reverted by an agrin-sufficient stroma. These data unveil a crucial role of agrin in the hematopoietic niches and in the cross-talk between stromal and hematopoietic stem cells.

Age-related changes in human hematopoietic stem/progenitor cells

Adult stem cells are critical for maintaining cellular homeostasis throughout life, yet the effects of age on their regenerative capacity are poorly understood. All lymphoid and myeloid blood cell lineages are continuously generated from hematopoietic stem cells present in human bone marrow. With age, significant changes in the function and composition of mature blood cells are observed. In this study, we report that age-related changes also occur in the human hematopoietic stem cell compartment. We find that the proportion of multipotent CD34(+) CD38(-) cells increases in the bone marrow of elderly (>70 years) individuals. CD34(+) CD38(+) CD90(-) CD45RA(+/-) CD10(-) and CD34(+) CD33(+) myeloid progenitors persist at the same level in the bone marrow, while the frequency of early CD34(+) CD38(+) CD90(-) CD45RA(+) CD10(+) and committed CD34(+) CD19(+) B-lymphoid progenitors decreases with age. In contrast to mice models of aging, transplantation experiments with immunodeficient NOD/SCID/IL-2Rγ null (NSG) mice showed that the frequency of NSG repopulating cells does not change significantly with age, and there is a decrease in myeloid lineage reconstitution. An age-related decrease in the capacity of CD34(+) cells to generate myeloid cells was also seen in colony-forming assays in vitro. Thus, with increasing age, human hematopoietic stem/progenitor cells undergo quantitative changes as well as functional modifications.

Targeting telomerase in hematologic malignancy

Over the past two decades, it has become increasingly apparent that telomerase-mediated telomere maintenance plays a crucial role in hematopoiesis. Supporting evidence is underscored by recent findings of mutations in genes involved in telomerase-mediated telomere maintenance that contribute to the pathogenesis of bone marrow failure syndromes. More recently described telomere-independent functions of telomerase are also likely to contribute to both normal hematopoiesis and hematologic diseases. The high levels of telomerase detected in aggressive leukemias have fueled fervent investigation into diverse approaches to targeting telomerase in hematologic malignancies. Successful preclinical investigations that employed genetic strategies, oligonucleotides, small-molecule inhibitors and immunotherapy have resulted in a rapid translation to clinical trials. Further investigation of telomere-independent functions of telomerase and detailed preclinical studies of telomerase inhibition in both normal and malignant hematopoiesis will be invaluable for refining treatments to effectively and safely exploit telomerase as a therapeutic target in hematologic malignancies.

Aging is not associated with bone marrow-resident progenitor cell depletion

Changes in progenitor cell biology remain at the forefront of many theories of biologic aging, but there are limited studies evaluating this in humans. Aging has been associated with a progressive depletion of circulating progenitor cells, but age-related bone marrow-resident progenitor cell depletion has not been systematically determined in humans. Patients undergoing total hip replacement were consented, and bone marrow and peripheral progenitor cells were enumerated based on aldehyde dehydrogenase activity and CD34 and CD133 expression. Circulating progenitors demonstrated an age-dependent decline. In contrast, marrow-resident progenitor cell content demonstrated no age association with any progenitor cell subtype. In humans, aging is associated with depletion of circulating, but not marrow-resident, progenitors. This finding has impact on the mechanism(s) responsible for age-related changes in circulating stem cells and important implications for the use of autologous marrow for the treatment of age-related diseases.

A quantitative trait locus on chr.4 regulates thymic involution

The mechanisms underlying age-associated thymic involution are unknown. In mice, thymic involution shows mouse strain-dependent genetic variation. Identification of the underlying genes would provide mechanistic insight into this elusive process. We previously showed that responsiveness of hematopoietic stem and progenitor cells (HSPCs) to transforming growth factor-beta 2, a positive regulator of HSPC proliferation, is regulated by a quantitative trait locus (QTL) on chr. 4, Tb2r1. Interestingly, Tgfb2(+/-) mice have delayed thymic involution. Therefore, we tested the hypothesis that a QTL on chr. 4 might regulate thymic involution. Aged, but not young, B6.D2-chr.4 congenic mice, where the telomeric region of chr. 4 was introgressed from DBA/2 to C57BL/6 mice, had larger thymi, and better maintenance of early thymic precursors than C57BL/6 control mice. These observations unequivocally demonstrate that the telomeric region of chr. 4 contains a QTL, Ti1 (thymic involution 1) that regulates thymic involution, and suggest the possibility that Ti1 may be identical to Tb2r1.

Genetic background affects properties of satellite cells and mdx phenotypes

Duchenne muscular dystrophy (DMD) is the most common lethal genetic disorder of children. The mdx (C57BL/10 background, C57BL/10-mdx) mouse is a widely used model of DMD, but the histopathological hallmarks of DMD, such as the smaller number of myofibers, accumulation of fat and fibrosis, and insufficient regeneration of myofibers, are not observed in adult C57BL/10-mdx except for in the diaphragm. In this study, we showed that DBA/2 mice exhibited decreased muscle weight, as well as lower myofiber numbers after repeated degeneration-regeneration cycles. Furthermore, the self-renewal efficiency of satellite cells of DBA/2 is lower than that of C57BL/6. Therefore, we produced a DBA/2-mdx strain by crossing DBA/2 and C57BL/10-mdx. The hind limb muscles of DBA/2-mdx mice exhibited lower muscle weight, fewer myofibers, and increased fat and fibrosis, in comparison with C57BL/10-mdx. Moreover, remarkable muscle weakness was observed in DBA/2-mdx. These results indicate that the DBA/2-mdx mouse is a more suitable model for DMD studies, and the efficient satellite cell self-renewal ability of C57BL/10-mdx might explain the difference in pathologies between humans and mice.

Aging in the lympho-hematopoietic stem cell compartment

Cells of the immune system are progeny of a single primitive cell type, the hematopoietic stem cell (HSC). Aging in most strains of mice is associated with a reduction in HSC frequency and a reduction in HSC function. Aged HSCs demonstrate reduced differentiation toward the lymphoid lineage, and this might be a relevant factor influencing immunosenescence. The molecular mechanisms of HSC aging need to be determined in more detail, but current studies have identified, among others, a role for telomere dysfunction in inducing cell intrinsic checkpoints and environmental alterations, which both skews and reduces stem cell differentiation and function. Reverting or ameliorating aging of HSCs might be a crucial step to restoring immuno-competence in the elderly.

Effects of aging on hematopoietic stem and progenitor cells

Although relationships between cellular and organismal aging are not well understood, several studies describe age-related changes in hematopoietic stem cells (HSCs) with functional consequences for the hematopoietic system. Importantly, aged hematopoietic stem and progenitor cells (HSPCs) differ from their younger counterparts in functional capacity, the complement of proteins on the cell surface, transcriptional activity, and genome integrity. These changes, however, are likely the result of a combination of cell-intrinsic and microenvironment-derived influences. Evolving views of the composition of the HSC compartment suggest that changes in HSCs may reflect the effects of the aging process on individual HSCs or a shift in the clonal composition of the HSC pool with age.

Telomere length maintenance in stem cell populations

The maintenance of telomere length is essential for upholding the integrity of the genome. There is good evidence to suggest that telomere length maintenance in stem cell populations is important to facilitate the cell division required for tissue homeostasis. This is balanced against the requirement in long lived species for proliferative life span barriers for tumour suppression; the gradual erosion of telomeres provides one such barrier. The dynamics of telomeres in stem cell populations may thus be crucial in the balance between tumour suppression and tissue homeostasis. Here we briefly discuss our current understanding of telomere dynamics in stem cell populations, and provide some data to indicate that telomeres in human embryonic stem cells may be more stable and less prone to large-scale stochastic telomeric deletion.

A quantitative trait locus on chromosome 4 affects cycling of hematopoietic stem and progenitor cells through regulation of TGF-beta 2 responsiveness

The hematopoietic stem and progenitor cell (HSPC) compartment is subject to extensive quantitative genetic variation. We have previously shown that TGF-beta2 at low concentrations enhances flt3 ligand-induced growth of HSPCs, while it is potently antiproliferative at higher concentrations. This in vitro enhancing effect was subject to quantitative genetic variation, for which a quantitative trait locus (QTL) was tentatively mapped to chromosome 4 (chr.4). Tgfb2(+/-) mice have a smaller and more slowly cycling HSPC compartment, which has a decreased serial repopulation capacity, and are less susceptible to the lethal effect of high doses of 5-fluorouracil. To unequivocally demonstrate that these phenotypes can be attributed to the enhancing effect of TGF-beta2 on HSPC proliferation observed in vitro and are therefore subject to mouse strain-dependent variation as well, we generated congenic mice where the telomeric region of chr.4 was introgressed from DBA/2 into C57BL/6 mice. In these mice, the enhancing effect of TGF-beta2 on flt3 signaling, but not the generic antiproliferative effect of high concentrations of TGF-beta2, was abrogated, confirming the location of this QTL, which we named tb2r1, on chr.4. These mice shared a smaller and more slowly cycling HSPC compartment and increased 5-fluorouracil resistance but not a decreased serial repopulation capacity with Tgfb2(+/-) mice. The concordance of phenotypes between Tgfb2(+/-) and congenic mice indicates that HSPC frequency and cycling are regulated by tb2r1, while an additional QTL in the telomeric region of chr.4 may regulate the serial repopulation capacity of hematopoietic stem cells.

Oxidative stress in the regulation of normal and neoplastic hematopoiesis

Recent evidence suggests that oxidative stress contributes significantly to the regulation of hematopoietic cell homeostasis. In particular, red blood cells and hematopoietic stem cells are highly sensitive to deregulated accumulation of reactive oxygen species (ROS). Unchecked ROS accumulation often leads to hemolysis, that is, to destruction and shortened life span of red blood cells. In addition, the process of erythroid cell formation is sensitive to ROS accumulation. Similarly, ROS buildup in hematopoietic stem cells compromises their function as a result of potential damage to their DNA leading to loss of quiescence and alterations of hematopoietic stem cell cycling. These abnormalities may lead to accelerated aging of hematopoietic stem cells or to hematopoietic malignancies.

Characterization and quantification of clonal heterogeneity among hematopoietic stem cells: a model-based approach

Hematopoietic stem cells (HSCs) show pronounced heterogeneity in self-renewal and differentiation behavior, which is reflected in their repopulation kinetics. Here, a single-cell-based mathematical model of HSC organization is used to examine the basis of HSC heterogeneity. Our modeling results, which are based on the analysis of limiting dilution competitive repopulation experiments in mice, demonstrate that small quantitative but clonally fixed differences of cellular properties are necessary and sufficient to account for the observed functional heterogeneity. The model predicts, and experimental data validate, that competitive pressures will amplify small clonal differences into large changes in the number of differentiated progeny. We further predict that the repertoire of HSC clones will evolve over time. Last, our results suggest that larger differences in cellular properties have to be assumed to account for genetically determined differences in HSC behavior as observed in different inbred mice strains. The model provides comprehensive systemic and quantitative insights into the clonal heterogeneity among HSCs with potential applications in predicting the behavior of malignant and/or genetically modified cells.

Combining transcriptional profiling and genetic linkage analysis to uncover gene networks operating in hematopoietic stem cells and their progeny

Stem cells are unique in that they possess both the capacity to self-renew and thereby maintain their original pool as well as the capacity to differentiate into mature cells. In the past number of years, transcriptional profiling of enriched stem cell populations has been extensively performed in an attempt to identify a universal stem cell gene expression signature. While stem-cell-specific transcripts were identified in each case, this approach has thus far been insufficient to identify a universal group of core “stemness” genes ultimately responsible for self-renewal and multipotency. Similarly, in the hematopoietic system, comparisons of transcriptional profiles between different hematopoietic cell stages have had limited success in revealing core genes ultimately responsible for the initiation of differentiation and lineage specification. Here, we propose that the combined use of transcriptional profiling and genetic linkage analysis, an approach called “genetical genomics”, can be a valuable tool to assist in the identification of genes and gene networks that specify “stemness” and cell fate decisions. We review past studies of hematopoietic cells that utilized transcriptional profiling and/or genetic linkage analysis, and discuss several potential future applications of genetical genomics.

A new mechanism for the aging of hematopoietic stem cells: aging changes the clonal composition of the stem cell compartment but not individual stem cells

Whether hematopoietic stem cells (HSCs) change with aging has been controversial. Previously, we showed that the HSC compartment in young mice consists of distinct subsets, each with predetermined self-renewal and differentiation behavior. Three classes of HSCs can be distinguished based on their differentiation programs: lymphoid biased, balanced, and myeloid biased. We now show that aging causes a marked shift in the representation of these HSC subsets. A clonal analysis of repopulating HSCs demonstrates that lymphoid-biased HSCs are lost and long-lived myeloid-biased HSCs accumulate in the aged. Myeloid-biased HSCs from young and aged sources behave similarly in all aspects tested. This indicates that aging does not change individual HSCs. Rather, aging changes the clonal composition of the HSC compartment. We show further that genetic factors contribute to the age-related changes of the HSC subsets. In comparison with B6 mice, aged D2 mice show a more pronounced shift toward myeloid-biased HSCs with a corresponding reduction in the number of both T- and B-cell precursors. This suggests that low levels of lymphocytes in the blood can be a marker for HSC aging. The loss of lymphoid-biased HSCs may contribute to the impaired immune response to infectious diseases and cancers in the aged.

Stem cells and aging in the hematopoietic system

The effector cells of the blood have limited lifetimes and must be replenished continuously throughout life from a small reserve of hematopoietic stem cells (HSCs) in the bone marrow. Although serial bone marrow transplantation experiments in mice suggest that the replicative potential of HSCs is finite, there is little evidence that replicative senescence causes depletion of the stem cell pool during the normal lifespan of either mouse or man. Studies conducted in murine genetic models defective in DNA repair, intracellular ROS management, and telomere maintenance indicate that all these pathways are critical to the longevity and stress response of the stem cell pool. With age, HSCs show an increased propensity to differentiate towards myeloid rather than lymphoid lineages, which may contribute to the decline in lymphopoiesis that attends aging. Challenges for the future include assessing the significance of 'lineage skewing' to immune dysfunction, and investigating the role of epigenetic dysregulation in HSC aging.

Intrinsic and microenvironmental defects are involved in the age-related changes of Lin - c-kit+ hematopoietic progenitor cells

The aim of this study was to evaluate through cross-transplantation models the effect of aging on the number of Lin(-)c-kit+ hematopoietic progenitor cells, on their ability to differentiate towards a lymphocyte phenotype, and on the role of the microenvironment in hematopoietic differentiation. The absolute number of purified Lin(-)c-kit+ cells from bone marrow was significantly lower in aged than in young mice. When transplanted in young recipients, Lin(-)c-kit+ hematopoietic progenitor cells from aged mice showed a reduced differentiation capacity in T cells and NK cells, compared to Lin(-)c-kit+ cells from young animals. The role of microenvironment in Lin(-)c-kit+ hematopoietic progenitor cells differentiation was evaluated by injecting young Lin(-)c-kit+ cells in young or aged recipients, the latter transplanted or not with a young thymus. In these conditions, the differentiation of Lin(-)c-kit+ cells from young mice in T and NK cells was less efficient in aged than in young recipients, independently of thymus grafting in aged recipients. In addition to quantitative defects qualitative alterations were also present in Lin(-)c-kit+ cells from aged mice, as evidenced by the fact that the injection of Lin(-)c-kit+ cells from aged donors in young recipients differentiated in CD4+ T cells that retained an interleukin-4 (IL-4) production in-between young and old control values. In conclusion, we have demonstrated that aging is associated with numerical and functional alterations of Lin(-)c-kit+ hematopoietic progenitor cells as well as with an altered microenvironment that is required for Lin(-)c-kit+ cells differentiation toward a lymphocyte phenotype.

Differential effects of age on circulating and splenic leukocyte populations in C57BL/6 and BALB/c male mice

BACKGROUND

Despite several reports on age-related phenotypic changes of the immune system's cells, studies that use a multipoint age comparison between the specific and innate immune cell populations of prototypical Th1- and Th2-type polarized mouse strains are still lacking.

RESULTS

Using a multipoint age comparison approach, cells from the two major immune system compartments, peripheral blood and spleen, and flow cytometry analysis, we found several principal differences in T cell and professional antigen presenting cell (APC) populations originating from a prototypical T helper (Th) 1 mouse strain, C57BL/6, and a prototypical Th2 strain, BALB/c. For example, regardless of age, there were strain differences in both peripheral blood mononuclear cells (PBMC) and spleens in the proportion of CD4+ (higher in the BALB/c strain), CD8+ T cells and CD11b+/CD11c+ APC (greater in C57BL/6 mice). Other differences were present only in PBMC (MHC class II + and CD19+ were greater in C57BL/6 mice) or differences were evident in the spleens but not in circulation (CD3+ T cells were greater in C57BL/6 mice). There were populations of cells that increased with age in PBMC and spleens of both strains (MHC class II+), decreased in the periphery and spleens of both strains (CD11b+) or did not change in the PBMC and spleens of both strains (CD8+). We also found strain and age differences in the distribution of naïve and memory/activated splenic T cells, e.g., BALB/c mice had more memory/activated and less naive CD8+ and CD4+ T cells and the C57BL/6 mice.

CONCLUSION

Our data provide important information on the principal differences, within the context of age, in T cell and professional APC populations between the prototypical Th1 mouse strain C57BL/6 and the prototypical Th2 strain BALB/c. Although the age-related changes that occur may be rather subtle, they may be very relevant in conditions of disease and stress. Importantly, our data indicate that age and strain should be considered in concert in the selection of appropriate mouse models for immunological research.

Hematopoietic stem cell aging and self-renewal

A functional decline of the immune system occurs during organismal aging that is attributable, in large part, to changes in the hematopoietic stem cell (HSC) compartment. In the mouse, several hallmark age-dependent changes in the HSC compartment have been identified, including an increase in HSC numbers, a decrease in homing efficiency, and a myeloid skewing of differentiation potential. Whether these changes are caused by gradual intrinsic changes within individual HSCs or by changes in the cellular composition of the HSC compartment remains unclear. However, of note, many of the aging properties of HSCs are highly dependent on their genetic background. In particular, the widely used C57Bl/6 strain appears to have unique HSC aging characteristics compared with those of other mouse strains. These differences can be exploited by using recombinant inbred strains to further our understanding of the genetic basis for HSC aging. The mechanism(s) responsible for HSC aging have only begun to be elucidated. Recent studies have reported co-ordinated variation in gene expression of HSCs with age, possibly as a result of epigenetic changes. In addition, an accumulation of DNA damage, in concert with an increase in intracellular reactive oxygen species, has been associated with aged HSCs. Nevertheless, whether age-related changes in HSCs are programmed to occur in a certain predictable fashion, or whether they are simply an accumulation of random changes over time remains unclear. Further, whether the genetic dysregulation observed in old HSCs is a cause or an effect of cellular aging is unknown.