Aging hematopoietic stem cells decline in function and exhibit epigenetic dysregulation

Hematopoiesis is not clonal in healthy elderly women

Clonality assays, based on X-chromosome inactivation, discriminate active from inactive alleles. Skewing of X-chromosome allelic usage, based on preferential methylation of one of the HUMARA alleles, was reported as evidence of clonal hematopoiesis in approximately 30% of elderly women. Using a quantitative, transcriptionally based clonality assay, we reported X-chromosome-transcribed allelic ratio in blood cells of healthy women consistent with random X-inactivation of 8 embryonic hematopoietic stem cells. Furthermore, we did not detect clonal hematopoiesis in more than 200 healthy nonelderly women. In view of the susceptibility of aging hematopoietic stem cells to epigenetic dysregulation, we reinvestigated the issue of clonality in elderly women. Forty healthy women (ages 65-92 years; mean, 81.3 years) were tested by a novel, quantitative polymerase chain reaction (qPCR) transcriptional clonality assay. We did not detect clonal hematopoiesis in any of the tested subjects. We also tested DNA from the same granulocyte samples using the methylation-based HUMARA assay, and confirmed previous reports of approximately 30% extensively skewed or monoallelic methylation, in agreement with likely age-related deregulated methylation of the HUMARA gene locus. We conclude that the transcriptionally based X-chromosome clonality assays are suitable for evaluation of clonal hematopoiesis in elderly women.

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.

Replicative senescence of mesenchymal stem cells: a continuous and organized process

Mesenchymal stem cells (MSC) comprise a promising tool for cellular therapy. These cells are usually culture expanded prior to their application. However, a precise molecular definition of MSC and the sequel of long-term in vitro culture are yet unknown. In this study, we have addressed the impact of replicative senescence on human MSC preparations. Within 43 to 77 days of cultivation (7 to 12 passages), MSC demonstrated morphological abnormalities, enlargement, attenuated expression of specific surface markers, and ultimately proliferation arrest. Adipogenic differentiation potential decreased whereas the propensity for osteogenic differentiation increased. mRNA expression profiling revealed a consistent pattern of alterations in the global gene expression signature of MSC at different passages. These changes are not restricted to later passages, but are continuously acquired with increasing passages. Genes involved in cell cycle, DNA replication and DNA repair are significantly down-regulated in late passages. Genes from chromosome 4q21 were over-represented among differentially regulated transcripts. Differential expression of 10 genes has been verified in independent donor samples as well as in MSC that were isolated under different culture conditions. Furthermore, miRNA expression profiling revealed an up-regulation of hsa-mir-371, hsa-mir-369-5P, hsa-mir-29c, hsa-mir-499 and hsa-let-7f upon in vitro propagation. Our studies indicate that replicative senescence of MSC preparations is a continuous process starting from the first passage onwards. This process includes far reaching alterations in phenotype, differentiation potential, global gene expression patterns, and miRNA profiles that need to be considered for therapeutic application of MSC preparations.

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.

Gone with the Wnt/Notch: stem cells in laminopathies, progeria, and aging

Specific mutations in the human gene encoding lamin A or in the lamin A–processing enzyme, Zmpste24, cause premature aging. New data on mice and humans suggest that these mutations affect adult stem cells by interfering with the Notch and Wnt signaling pathways.

Nuclear envelope defects cause stem cell dysfunction in premature-aging mice

Nuclear lamina alterations occur in physiological aging and in premature aging syndromes. Because aging is also associated with abnormal stem cell homeostasis, we hypothesize that nuclear envelope alterations could have an important impact on stem cell compartments. To evaluate this hypothesis, we examined the number and functional competence of stem cells in Zmpste24-null progeroid mice, which exhibit nuclear lamina defects. We show that Zmpste24 deficiency causes an alteration in the number and proliferative capacity of epidermal stem cells. These changes are associated with an aberrant nuclear architecture of bulge cells and an increase in apoptosis of their supporting cells in the hair bulb region. These alterations are rescued in Zmpste24^−/−Lmna^+/− mutant mice, which do not manifest progeroid symptoms. We also report that molecular signaling pathways implicated in the regulation of stem cell behavior, such as Wnt and microphthalmia transcription factor, are altered in Zmpste24^−/− mice. These findings establish a link between age-related nuclear envelope defects and stem cell dysfunction.

Replicative stress, stem cells and aging

DNA synthesis is a remarkably vulnerable phase in the cell cycle. In addition to introduction of errors during semi-conservative replication, the inherently labile structure of the replication fork, as well as numerous pitfalls encountered in the course of fork progression, make the normally stable double stranded molecule susceptible to collapse and recombination. As described in this issue, maintenance of genome integrity in the face of such events is essential to prevent the premature onset of age-related diseases. At the organismal level, the roles for such maintenance are numerous; however, the preservation of stem and progenitor cell pools may be particularly important as indicated by several genetically engineered mouse models. Stresses on stem and progenitor cell pools, in the form of telomere shortening (Terc(-/-)) or other genome maintenance failures (ATR(mKO), Ku86(-/-), LIG4(Y288C), XPD(R722W/R722W), etc.), have been shown to degrade tissue renewal capacity and accelerate the appearance of age-related phenotypes. In the case of telomere shortening, exhaustion of replicative potential appears to be at least partially dependent on the cell cycle regulatory component of the DNA damage response. Therefore, both the genome maintenance mechanisms that counter DNA damage and the cell cycle checkpoint responses to damage strongly influence the onset of age-related diseases and do so, at least in part, by affecting long-term stem and progenitor cell potential.

How does suppression of IGF-1 signaling by DNA damage affect aging and longevity?

Long-lived animals have evolved a robust set of defenses to maintain genomic integrity over their entire lifespan. The DNA damage response and DNA repair pathways are critical pillars of organismal defenses, minimizing somatic mutations in both post-mitotic and mitotic cells. These genomic maintenance systems not only prevent the premature emergence of cancers but may also maintain normal tissue function and organismal longevity. Genetic defects in a number of DNA repair and DNA damage response genes often leads to a dramatic increase in cancer incidence; in other cases, premature aging or progeroid syndromes may be induced. In this review, we discuss two recent reports of two nucleotide excision repair-deficient models that exhibit dramatic premature aging and shortened longevity. The DNA repair defects were also associated with a significant inhibition of the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis, an endocrine signaling pathway shown to influence aging and longevity in both vertebrates and invertebrates. Potential mechanisms of how DNA damage might affect IGF-1 signaling and aging are discussed, with a particular emphasis on the role of such signaling alterations in the adult tissue stem cell compartments.

Cytotoxicity and mutagenicity of endogenous DNA base lesions as potential cause of human aging

Endogenous factors constitute a substantial source of damage to the genomic DNA. The type of damage includes a number of different base lesions and single- and double-strand breaks. Unrepaired DNA damage can give rise to mutations and may cause cell death. A number of studies have demonstrated an association between aging and the accumulation of DNA damage. This may be attributed to reduced DNA repair with age, although this is apparently not a general feature for all types of damage and repair mechanisms. Therefore, detailed studies that improve our knowledge of DNA repair systems as well as mutagenic and toxic effects of DNA lesions will help us to gain a better insight into the mechanisms of aging. The aim of this review is to provide a brief description of cytotoxic and mutagenic endogenous DNA lesions that are mainly repaired by base excision repair and single-strand break repair pathways and to discuss the potential role of DNA lesions and DNA repair dysfunction in the onset of human aging.

Intrinsic changes and extrinsic influences of myogenic stem cell function during aging

The myogenic stem cell (satellite cell) is almost solely responsible for the remarkable regeneration of adult skeletal muscle fibers after injury. The availability and the functionality of satellite cells are the determinants of efficient muscle regeneration. During aging, the efficiency of muscle regeneration declines, suggesting that the functionality of satellite cells and their progeny may be altered. Satellite cells do not sit in isolation but rather are surrounded by, and influenced by, many extrinsic factors within the muscle tissue that can alter their functionality. These factors likely change during aging and impart both reversible and irreversible changes to the satellite cells and on their proliferating progeny. In this review, we discuss the possible mechanisms of impaired muscle regeneration with respect to the biology of satellite cells. Future studies that enhance our understanding of the interactions between stem cells and the environment in which they reside will offer promise for therapeutic applications in age-related diseases.

Hematopoietic stem cell aging: wrinkles in stem cell potential

Hematopoietic stem cells (HSC) continuously replenish the blood and immune systems. Their activity must be sustained throughout life to support optimal immune responses. It has been thought that stem cells may be somewhat protected from age because of their perpetual requirement to replenish the blood, however studies over the past 10 years have revealed dramatic changes in HSC function and phenotype with respect to age. When the number of HSC within murine bone marrow is measured, an increase in concentration and absolute number of HSC within the bone marrow is observed as the animal ages, paralleled with increased homogeneity of stem cell marker expression. Results from transplantation studies demonstrate that although there is a decline in hematopoietic output on a per-cell basis, the increase in number provides sufficient, yet abnormal, blood production throughout the lifespan of the animal. HSC may play a role in immunosenescence through cell-fate decisions leading to an overproduction of myeloid cells and an underproduction of lymphocytes. When examining gene expression of aged HSC, recent studies have highlighted several key factors contributing to increased inflammation, stress response and genomic instability. Here, we will review the general phenotype observed with aging of the hematopoietic system, focusing on the HSC, and compile recent expression profiling efforts that have examined HSC aging.

Toward a control theory analysis of aging

Aging is due to the accumulation of damage over time that affects the function and survival of the organism; however, it has proven difficult to infer the relative importance of the many processes that contribute to aging. To address this, here we outline an approach that may prove useful in analyzing aging. In this approach, the function of the organism is described as a set of interacting physiological systems. Degradation of their outputs leads to functional decline and death as a result of aging. In turn, degradation of the system outputs is attributable to changes at the next hierarchical level down, the cell, through changes in cell number or function, which are in turn a consequence of the metabolic history of the cell. Within this framework, we then adapt the methods of metabolic control analysis (MCA) to determine which modifications are important for aging. This combination of a hierarchical framework and the methodologies of MCA may prove useful both for thinking about aging and for analyzing it experimentally.

The role of DNA damage repair in aging of adult stem cells

DNA repair maintains genomic stability and the loss of DNA repair capacity results in genetic instability that may lead to a decline of cellular function. Adult stem cells are extremely important in the long-term maintenance of tissues throughout life. They regenerate and renew tissues in response to damage and replace senescent terminally differentiated cells that no longer function. Oxidative stress, toxic byproducts, reduced mitochondrial function and external exposures all damage DNA through base modification or mis-incorporation and result in DNA damage. As in most cells, this damage may limit the survival of the stem cell population affecting tissue regeneration and even longevity. This review examines the hypothesis that an age-related loss of DNA damage repair pathways poses a significant threat to stem cell survival and longevity. Normal stem cells appear to have strict control of gene expression and DNA replication whereas stem cells with loss of DNA repair may have altered patterns of proliferation, quiescence and differentiation. Furthermore, stem cells with loss of DNA repair may be susceptible to malignant transformation either directly or through the emergence of cancer-prone stem cells. Human diseases and animal models of loss of DNA repair provide longitudinal analysis of DNA repair processes in stem cell populations and may provide links to the physiology of aging.

CD150- side population cells represent a functionally distinct population of long-term hematopoietic stem cells

Hematopoietic stem cells (HSCs) are a self-renewing population of bone marrow cells that replenish the cellular elements of blood throughout life. HSCs represent a paradigm for the study of stem-cell biology, because robust methods for prospective isolation of HSCs have facilitated rigorous characterization of these cells. Recently, a new isolation method was reported, using the SLAM family of cell-surface markers, including CD150 (SlamF1), to offer potential advantages over established protocols. We examined the overlap between SLAM family member expression with an established isolation scheme based on Hoechst dye efflux (side population; SP) in conjunction with canonical HSC cell-surface markers (Sca-1, c-Kit, and lineage markers). Importantly, we find that stringent gating of SLAM markers is essential to achieving purity in HSC isolation and that the inclusion of canonical HSC markers in the SLAM scheme can greatly augment HSC purity. Furthermore, we observe that both CD150(+) and CD150(-) cells can be found within the SP population and that both populations can contribute to long-term multilineage reconstitution. Thus, using SLAM family markers to isolate HSCs excludes a substantial fraction of the marrow HSC compartment. Interestingly, these 2 subpopulations are functionally distinct, with respect to lineage output as well as proliferative status.

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.

Relationships between stem cell exhaustion, tumour suppression and ageing

Ageing is a complex process characterised by a variety of disorders associated with general organismal decline and an inability to maintain tissue homoeostasis. As described in this review, recent studies indicate that ageing may be caused, in part, by the depletion of stem and progenitor cells that govern tissue renewal. The potential causes of stem and progenitor cell attrition are numerous; however, a commonly accepted theory is that these cells are lost as a result of naturally occurring DNA damage and the obligate checkpoint responses that follow. Failure to launch appropriate responses to DNA damage is strongly associated with cancer initiation and progression. Therefore, it is at this nexus, the response to DNA damage, that an important organismal fate may be determined: to degrade regenerative potential for the purpose of preventing cancer. According to this viewpoint, ageing may be the unfortunate mark of successful cancer suppression in stem cells and other cell types. In this review, we will describe how degeneration of tissue renewal capacity links ageing and cancer suppression.