The aging of lympho-hematopoietic stem cells

Animal models in gerontology research

Animal models have paved the way for the vast majority of advances in biomedical research. Studies on aged animals are essential for understanding the processes inherent in normal aging and the progression of age-related diseases. Animal models are used to identify physiological changes with age, to identify the genetic basis of normal aging and age-associated disease and degeneration, and to test potential therapeutic interventions. This chapter will focus on rodent models and will summarize important considerations for the use of animals in aging research in general and in modeling geriatric epilepsy.

P2X7-induced apoptosis decreases by aging in mice myeloblasts

In the current study, the ability of ATP to promote apoptosis in myeloblasts at different ages was investigated. We have observed that high concentration of extracellular ATP (>1mM), which activates P2X(7) receptor, produced cell shrinkage an increase in the number of events in the sub-G(0)/G(1) region of the cellular cycle and annexin-V/propidium iodide label, which characterizes the apoptotic cell death. In addition, BzATP produced apoptosis, but not ADP and UTP. Gr-1(+) cells express the P2X(7) receptor and oxidized ATP, a specific P2X(7) inhibitor, blocked the ATP-dependent apoptosis. ATP-dependent apoptosis is decreased by aging in myeloblasts of 12 and 22-month-old mice. Furthermore, P2X(7) expression decrease was observed in older mice, explaining apoptosis decrease. This decrease in apoptosis by aging may be related to some diseases in the myelocyte lineage.

Robustness and fragility in immunosenescence

We construct a model to study tradeoffs associated with aging in the adaptive immune system, focusing on cumulative effects of replacing naive cells with memory cells. Binding affinities are characterized by a stochastic shape space model. System loss arising from an individual infection is associated with disease severity, as measured by the total antigen population over the course of an infection. We monitor evolution of cell populations on the shape space over a string of infections, and find that the distribution of losses becomes increasingly heavy-tailed with time. Initially this lowers the average loss: the memory cell population becomes tuned to the history of past exposures, reducing the loss of the system when subjected to a second, similar infection. This is accompanied by a corresponding increase in vulnerability to novel infections, which ultimately causes the expected loss to increase due to overspecialization, leading to increasing fragility with age (i.e., immunosenescence). In our model, immunosenescence is not the result of a performance degradation of some specific lymphocyte, but rather a natural consequence of the built-in mechanisms for system adaptation. This "robust, yet fragile" behavior is a key signature of Highly Optimized Tolerance.

The telomere-telomerase axis and the heart

The preservation of myocyte number and cardiac mass throughout life is dependent on the balance between cell death and cell division. Rapidly emerging evidence indicates that new myocytes can be formed through the activation and differentiation of resident cardiac progenitor cells. The critical issue is the identification of mechanisms that define the aging of cardiac progenitor cells and, ultimately, their inability to replace dying myocytes. The most reliable marker of cellular senescence is the modification of the telomere-telomerase axis, together with the expression of the cell cycle inhibitors p16INK4a and p53. Cellular senescence is characterized by biochemical events that occur within the cell. In this regard, one of the most relevant processes is represented by repeated oxidative stress that may evolve into the activation of the cell death program or result in the development of a senescent phenotype. Thus, the modulation of telomerase activity and the control of telomeric length, together with the attenuation of the formation of reactive oxygen species, may represent important therapeutic tools in regenerative medicine and in prevention of aging and diabetic cardiomyopathies.

The impact of altered p53 dosage on hematopoietic stem cell dynamics during aging

A temporal decline in tissue stem cell functionality may be a key component of mammalian aging. The tumor suppressor p53 has recently been implicated as a potential regulator of aging. We examined age-associated hematopoietic stem cell (HSC) dynamics in mice with varying p53 activities. Reduced p53 activity in p53+/- mice was associated with higher numbers of proliferating hematopoietic stem and progenitor cells in old age compared with aged wild-type (p53+/+) mice. We also assessed HSC dynamics in a p53 mutant mouse model (p53+/m) with higher apparent p53 activity than wild-type mice. The p53 hypermorphic (p53+/m) mice display phenotypes of premature aging. Many aged p53+/m organs exhibit reduced cellularity and atrophy, suggesting defects in stem-cell regenerative capacity. HSC numbers from old p53+/m mice fail to increase with age, unlike those of their p53+/+ and p53+/- counterparts. Moreover, transplantation of 500 HSCs from old p53+/m mice into lethally irradiated recipients resulted in reduced engraftment compared with old wild-type p53+/+ and p53+/- HSCs. Thus, alteration of p53 activity affects stem-cell numbers, proliferation potential, and hematopoiesis in older organisms, supporting a model in which aging is caused in part by a decline in tissue stem cell regenerative function.

Clonogenic endothelial progenitor cells are sensitive to oxidative stress

Endothelial progenitor cells (EPCs) circulate in the peripheral blood and reside in blood vessel walls. A hierarchy of EPCs exists where progenitors can be discriminated based on their clonogenic potential. EPCs are exposed to oxidative stress during vascular injury as residents of blood vessel walls or as circulating cells homing to sites of neovascularization. Given the links between oxidative injury, endothelial cell dysfunction, and vascular disease, we tested whether EPCs were sensitive to oxidative stress using newly developed clonogenic assays. Strikingly, in contrast to previous reports, we demonstrate that the most proliferative EPCs (high proliferative potential-endothelial colony-forming cells and low proliferative potential-endothelial colony-forming cells) had decreased clonogenic capacity after oxidant treatment. In addition, EPCs exhibited increased apoptosis and diminished tube-forming ability in vitro and in vivo in response to oxidative stress, which was directly linked to activation of a redox-dependent stress-induced kinase pathway. Thus, this study provides novel insights into the effect of oxidative stress on EPCs. Furthermore, this report outlines a framework for understanding how oxidative injury leads to vascular disease and potentially limits the efficacy of transplantation of EPCs into ischemic tissues enriched for reactive oxygen species and oxidized metabolites.

Increasing p16INK4a expression decreases forebrain progenitors and neurogenesis during ageing

Mammalian ageing is associated with reduced regenerative capacity in tissues that contain stem cells. It has been proposed that this is at least partially caused by the senescence of progenitors with age; however, it has not yet been tested whether genes associated with senescence functionally contribute to physiological declines in progenitor activity. Here we show that progenitor proliferation in the subventricular zone and neurogenesis in the olfactory bulb, as well as multipotent progenitor frequency and self-renewal potential, all decline with age in the mouse forebrain. These declines in progenitor frequency and function correlate with increased expression of p16INK4a, which encodes a cyclin-dependent kinase inhibitor linked to senescence. Ageing p16INK4a-deficient mice showed a significantly smaller decline in subventricular zone proliferation, olfactory bulb neurogenesis, and the frequency and self-renewal potential of multipotent progenitors. p16INK4a deficiency did not detectably affect progenitor function in the dentate gyrus or enteric nervous system, indicating regional differences in the response of neural progenitors to increased p16INK4a expression during ageing. Declining subventricular zone progenitor function and olfactory bulb neurogenesis during ageing are thus caused partly by increasing p16INK4a expression.

Age-related accumulation of a novel CD44 + CD25lowgammadelta T-cell population in hematopoietic organs of the mouse

We discovered a novel population of gammadelta T cells in the mouse that accumulates with age in hematopoietic organs, but not in epithelia. These cells are CD25low (an unusual phenotype for gammadelta T cells in the mouse); express higher levels of TCRgammadelta and CD44 than do CD25- gammadelta T cells; mainly express Vgamma2, Vgamma3, and Vgamma4 chains; and are largely quiescent. A very similar cell population appears in the late stages of fetal thymus organ cultures, suggesting that the accumulation of CD44 + CD25lowTCRgammadelta + cells is a response to stress induced by aging in vivo or by culture in vitro. The precursors of CD44 + CD25lowTCRgammadelta + cells are generated during fetal or very young adult life, as this population was undetectable in aged recipients of bone marrow from old or young donors. CD44 + CD25lowTCRgammadelta + cells may be a biomarker of aging, but could also play a role in the inflammatory changes that accompany aging.

Telomere length changes after umbilical cord blood transplant

BACKGROUND

The establishment of donor-derived hematopoiesis in the recipients of hematopoietic stem cell (HSC) transplants involves extensive proliferation and differentiation of HSCs. Data from long-term survivors of HSC transplants suggest that these transplanted HSCs may experience a debilitating replicative senescence. A significant posttransplant shortening of peripheral blood mononuclear cell (PBMNC) telomeres has been observed in both marrow transplant and peripheral blood progenitor cell transplant recipients. Similar studies have not been performed for umbilical cord blood (UCB) HSC transplants, which might be expected to exhibit increased posttransplant replicative potential due to their inherently greater telomere length.

STUDY DESIGN AND METHODS

Blood was obtained from donor-recipient pairs of allogeneic PBHSC transplant and UCB HSC transplant, both before transplant and at follow-up treatments (minimum 1 year after transplant) after engraftment. Telomere restriction fragment length (TRFL) analysis was performed on the blood samples. The mean TRFL and posttransplant changes in the mean TRFL were analyzed.

RESULTS

Measurements of telomere lengths in the PBMNCs of transplant patients revealed a significant net decrease in telomere length in all transplant recipients compared with their respective donors. Our results also revealed that the PBMNCs of umbilical cord stem cell transplant patients retain a significantly longer posttransplant telomere length.

CONCLUSION

The significantly longer telomeres observed in the allogeneic UCB HSC transplant recipients compared to the allogeneic PBHSC transplant recipients in our study may be indicative of a replicative advantage inherent in the use of UCB HSC for transplant.

Diabetes promotes cardiac stem cell aging and heart failure, which are prevented by deletion of the p66shc gene

Diabetes leads to a decompensated myopathy, but the etiology of the cardiac disease is poorly understood. Oxidative stress is enhanced with diabetes and oxygen toxicity may alter cardiac progenitor cell (CPC) function resulting in defects in CPC growth and myocyte formation, which may favor premature myocardial aging and heart failure. We report that in a model of insulin-dependent diabetes mellitus, the generation of reactive oxygen species (ROS) leads to telomeric shortening, expression of the senescent associated proteins p53 and p16INK4a, and apoptosis of CPCs, impairing the growth reserve of the heart. However, ablation of the p66shc gene prevents these negative adaptations of the CPC compartment, interfering with the acquisition of the heart senescent phenotype and the development of heart failure with diabetes. ROS elicit 3 cellular reactions: low levels activate cell growth, intermediate quantities trigger cell apoptosis, and high amounts initiate cell necrosis. CPC replication predominates in diabetic p66shc-/-, whereas CPC apoptosis and myocyte apoptosis and necrosis prevail in diabetic wild type. Expansion of CPCs and developing myocytes preserves cardiac function in diabetic p66shc-/-, suggesting that intact CPCs can effectively counteract the impact of uncontrolled diabetes on the heart. The recognition that p66shc conditions the destiny of CPCs raises the possibility that diabetic cardiomyopathy is a stem cell disease in which abnormalities in CPCs define the life and death of the heart. Together, these data point to a genetic link between diabetes and ROS, on the one hand, and CPC survival and growth, on the other.

Increased hematopoietic stem cell mobilization in aged mice

Hematopoietic stem and progenitor cells (HSPCs) are located in the bone marrow in close association with a highly organized 3-dimensional structure formed by stroma cells, referred to as the niche. Mobilization of HSPCs from bone marrow to peripheral blood in response to granulocyte colony-stimulating factor (G-CSF) requires de-adhesion of HSPCs from the niche. The influence of aging of HSPCs on cell-stroma interactions has not been determined in detail. Using a mouse model of G-CSF-induced mobilization, we demonstrated that the ability to mobilize hematopoietic stem cells is approximately 5-fold greater in aged mice. Competitive mobilization experiments confirmed that enhanced mobilization ability was intrinsic to the stem cell. Enhanced mobilization efficiency of primitive hematopoietic cells from aged mice correlated with reduced adhesion of hematopoietic progenitor cells to stroma and with elevated levels of GTP-bound Cdc42. These results might indicate that stroma-stem cell interactions are dynamic over a lifetime and result in physiologically relevant changes in the biology of primitive hematopoietic cells with age.

Proximal prostatic stem cells are programmed to regenerate a proximal-distal ductal axis

Prostate carcinoma and benign prostatic hypertrophy may both originate in stem cells, highlighting the importance of the characterization of these cells. The prostate gland contains a network of ducts each of which consists of a proximal (adjacent to the urethra), an intermediate, and a distal region. Here, we report that two populations of cells capable of regenerating prostatic tissue in an in vivo prostate reconstitution assay are present in different regions of prostatic ducts. The first population (with considerable growth potential) resides in the proximal region of ducts and in the urethra, and the survival of these cells does not require the presence of androgens. The second population (with more limited growth potential) is found in the remaining ductal regions and requires androgen for survival. In addition, we find that primitive proximal prostate cells that are able to regenerate functional prostatic tissue in vivo are also programmed to re-establish a proximal-distal ductal axis. Similar to their localization in the intact prostate, cells with the highest regenerative capacity are found in the proximal region of prostatic ducts formed in an in vivo prostate reconstitution assay. The primitive proximal cells can be passaged through four generations of subrenal capsule grafts. Together, these novel findings illustrate features of primitive prostate cells that may have implications for the development of therapies for treating proliferative prostatic diseases.

Decreased number of circulating CD34+KDR+ cells in asymptomatic subjects with preclinical atherosclerosis

OBJECTIVES

To assess whether circulating endothelial progenitor cells (CEPCs) can be considered as a cardiovascular risk marker before event has occurred, that is less firmly established than in clinically overt atherosclerosis.

METHODS

Number of CD34+KDR+ cell number per ml blood was measured by flow cytometry in 84 untreated subjects without cardiovascular disease. Atherosclerotic plaque was detected by ultrasound in carotid, abdominal aortic and femoral sites and the number of sites affected by plaque among these three sites was counted as 0, 1, 2 or 3. Additionally, intima-media thickness (IMT) was measured by computerized ultrasound imaging of both common carotid segments.

RESULTS

CD34+KDR+ cell number decreased by 48, 29 or 30% in the presence of carotid, aortic or femoral plaque (p<0.001, 0.05, 0.05, respectively) as compared to the absence of plaque and by 70% in the presence of three sites affected with plaque as compared with 0 site with plaque (p<0.01) but did not change with increasing IMT tertiles. Adjustment for Framingham risk score, that was also associated with decreased CD34+KDR+ cell number (p<0.001), made CD34+KDR+ cell number associations with plaque insignificant, except at the carotid site (p<0.01).

CONCLUSIONS

Reduced CEPC number may participate to preclinical stage of atherosclerosis and provide additional information to traditional risk factors as regards global risk assessment.

Physiologic aspects of aging: impact on cancer management and decision making, part II

In this second article of our two-part review, we focus on age-associated physiologic changes involving the nervous, endocrine, hematologic, immune, and musculoskeletal systems, with close attention to the interconnected nature of these systems. There is a well-known connection between the neuroendocrine and immune systems via the hypothalamic-pituitary-adrenal axis and via interaction by means of cytokines, hormones, and neurotransmitters. These changes may lead to a loss of integration and resiliency with age, thus decreasing the ability of the elderly patient with cancer to adapt to stressful circumstances. Prominent changes include decline in memory and cognition, and increased susceptibility to peripheral neuropathy. Hematologic and immune changes like reduced bone marrow reserve and increased susceptibility to infections have far reaching implications for cancer care in the elderly. Gradual decline in hormone levels, and changes in muscle and body composition, can lead to functional decline and frailty. Use of the clinical interventions suggested in this article, along with an appreciation of the interplay of these age-related physiologic changes and their consequences, allows oncology professionals to customize therapy and minimize side effects in the geriatric oncology patient.

Women and Ischemic Heart Disease

The current review highlights gender-specific issues in ischemic heart disease (IHD) presentation, evaluation, and outcomes with a special focus on the results derived from the National Institutes of Health-National Heart, Lung, and Blood Institute-sponsored Women's Ischemia Syndrome Evaluation (WISE) study. New evidence on gender-based differences in vascular wall, atherosclerotic plaque deposition, pathophysiology, and innovative cardiovascular imaging techniques are reviewed. Critical areas of further inquiry needed to advance new gender-specific IHD understanding are detailed.

Effects of radiation on the longitudinal trends of hemoglobin levels in the Japanese atomic bomb survivors

The late effects of radiation on the hematopoietic system have not been fully evaluated. We examined the long-term effects of radiation exposure on hemoglobin levels in the Japanese atomic bomb survivors over a 40-year period from 1958 to 1998. Compared to the unexposed survivors, the mean hemoglobin levels for those exposed to a bone marrow dose of 1 Gy were significantly reduced by 0.10 g/dl (95% CI: 0.04 to 0.16) or 0.67% at 40 years of age (P < 0.0001) and by 0.24 g/dl (95% CI: 0.08 to 0.40) or 1.8% at 80 years of age. Radiation effects are greater for smokers than for nonsmokers at age less than 35 years (P < 0.01), although cigarette smoking was associated with increased hemoglobin levels. Sex and birth cohort differences in radiation effects were not found after adjusting for smoking. The radiation-induced reduction in hemoglobin levels could not be explained by the presence of certain anemia-associated diseases.

The hematopoietic stem compartment consists of a limited number of discrete stem cell subsets

Hematopoietic stem cells (HSCs) display extensive heterogeneity in their behavior even when isolated as phenotypically homogeneous populations. It is not clear whether this heterogeneity reflects inherently diverse subsets of HSCs or a homogeneous population of HSCs diversified by their response to different external stimuli. To address this, we analyzed 97 individual HSCs in long-term transplantation assays. HSC clones were obtained from unseparated bone marrow (BM) through limiting dilution approaches. Following transplantation into individual hosts, donor-type cells in blood were measured bimonthly and the resulting repopulation kinetics were grouped according to overall shape. Only 16 types of repopulation kinetics were found among the HSC clones even though combinatorially 54 groups were possible. All HSC clones, regardless of their origin, could be assigned to this subset of groups, and the probability of finding new patterns is negligible. Thus, the full repertoire of repopulating HSCs was covered. These data indicate that the HSC compartment consists of a limited number of distinct HSC subsets, each with predictable behavior. Enrichment of HSCs (Lin- Rho- SP) changes the representation of HSC types by selecting for distinct subsets of HSCs. These data from the steady-state HSC repertoire could provide a basis for the diagnosis of perturbed patterns of HSCs potentially caused by disease or aging.

MacrophAging: a cellular and molecular review

Aging is associated with the deterioration of several physiological functions, which leads to aged-related pathologies and, ultimately, to death. The immune system is affected by aging, causing an increased susceptibility to infections and mortality, as well as a major incidence of immune diseases and cancer in the elderly. Because macrophages are an essential component of both innate and adaptive immunity, altered function of these phagocytic cells with aging may play a key role in immunosenescence. Here we summarize data about the effects of aging on macrophages and we discuss the molecular events that could be involved in this process.

Myocardial aging

This review questions the old paradigm that describes the heart as a post-mitotic organ and introduces the notion of the heart as a self-renewing organ regulated by a compartment of multipotent cardiac stem cells (CSCs) capable of regenerating myocytes and coronary vessels throughout life. Because of this dramatic change in cardiac biology, the objective is to provide an alternative perspective of the aging process of the heart and stimulate research in an area that pertains to all of us without exception. The recent explosion of the field of stem cell biology, with the recognition that the possibility exists for extrinsic and intrinsic regeneration of myocytes and coronary vessels, necessitates reevaluation of cardiac homeostasis and myocardial aging. From birth to senescence, the mammalian heart is composed of non-dividing and dividing cells. Loss of telomeric DNA is minimal in fetal and neonatal myocardium but rather significant in the senescent heart. Aging affects the growth and differentiation potential of CSCs interfering not only with their ability to sustain physiological cell turnover but also with their capacity to adapt to increases in pressure and volume loads. The recognition of factors enhancing the activation of the CSC pool, their mobilization, and translocation, however, suggests that the detrimental effects of aging on the heart might be prevented or reversed by local stimulation of CSCs or the intramyocardial delivery of CSCs following their expansion and rejuvenation in vitro. CSC therapy may become, perhaps, a novel strategy for the devastating problem of heart failure in the old population.

Reduced number of circulating endothelial progenitor cells predicts future cardiovascular events: proof of concept for the clinical importance of endogenous vascular repair

BACKGROUND

The maintenance of endothelial integrity plays a critical role in preventing atherosclerotic disease progression. Endothelial progenitor cells (EPCs) were experimentally shown to incorporate into sites of neovascularization and home to sites of endothelial denudation. Circulating EPCs may thus provide an endogenous repair mechanism to counteract ongoing risk factor-induced endothelial injury and to replace dysfunctional endothelium.

METHODS AND RESULTS

In 120 individuals (43 control subjects, 44 patients with stable coronary artery disease, and 33 patients with acute coronary syndromes), circulating EPCs were defined by the surface markers CD34+KDR+ and analyzed by flow cytometry. Cardiovascular events (cardiovascular death, unstable angina, myocardial infarction, PTCA, CABG, or ischemic stroke) served as outcome variables over a median follow-up period of 10 months. Patients suffering from cardiovascular events had significantly lower numbers of EPCs (P<0.05). Reduced numbers of EPCs were associated with a significantly higher incidence of cardiovascular events by Kaplan-Meier analysis (P=0.0009). By multivariate analysis, reduced EPC levels were a significant, independent predictor of poor prognosis, even after adjustment for traditional cardiovascular risk factors and disease activity (hazard ratio, 3.9; P<0.05).

CONCLUSIONS

Reduced levels of circulating EPCs independently predict atherosclerotic disease progression, thus supporting an important role for endogenous vascular repair to modulate the clinical course of coronary artery disease.

Impaired progenitor cell activity in age-related endothelial dysfunction

OBJECTIVES

We investigated whether human age-related endothelial dysfunction is accompanied by quantitative and qualitative alterations of the endothelial progenitor cell (EPC) pool.

BACKGROUND

Circulating progenitor cells with an endothelial phenotype contribute to the regeneration and repair of the vessel wall. An association between the loss of endothelial integrity and EPC modification may provide a background to study the mechanistic nature of such age-related vascular changes.

METHODS

In 20 old and young healthy individuals (61 +/- 2 years and 25 +/- 1 year, respectively) without major cardiovascular risk factors, endothelial function, defined by flow-mediated dilation of the brachial artery via ultrasound, as well as the number and function of EPCs isolated from peripheral blood, were determined.

RESULTS

Older subjects had significantly impaired endothelium-dependent dilation of brachial artery (flow-mediated dilation [FMD] 5.2 +/- 0.5% vs. 7.1 +/- 0.6%; p < 0.05). Endothelium-independent dilation after glycerol trinitrate (GTN) was not different, but the FMD/GTN ratio was significantly lower in old subjects (49 +/- 4% vs. 37 +/- 3%; p < 0.05), suggesting endothelial dysfunction. There were no differences in the numbers of circulating EPCs, defined as CD34/KDR or CD133/KDR double-positive cells in peripheral blood. In contrast, lower survival (39 +/- 6 cells/mm(2) vs. 65 +/- 11 cells/mm(2); p < 0.05), migration (80 +/- 12 vs. 157 +/- 16 cells/mm(2); p < 0.01), and proliferation (0.20 +/- 0.04 cpm vs. 0.44 +/- 0.07 cpm; p < 0.05) implicate functional impairment of EPCs from old subjects. The FMD correlated univariately with EPC migration (r = 0.52, p < 0.05) and EPC proliferation (r = 0.49, p < 0.05). Multivariate analysis showed that both functional features represent independent predictors of endothelial function.

CONCLUSIONS

Maintenance of vascular homeostasis by EPCs may be attenuated with age based on functional deficits rather than depletion of CD34/KDR or CD133/KDR cells.

Transplantation of a multipotent cell population from human adipose tissue induces dystrophin expression in the immunocompetent mdx mouse

Here, we report the isolation of a human multipotent adipose-derived stem (hMADS) cell population from adipose tissue of young donors. hMADS cells display normal karyotype; have active telomerase; proliferate >200 population doublings; and differentiate into adipocytes, osteoblasts, and myoblasts. Flow cytometry analysis indicates that hMADS cells are CD44+, CD49b+, CD105+, CD90+, CD13+, Stro-1(-), CD34-, CD15-, CD117-, Flk-1(-), gly-A(-), CD133-, HLA-DR(-), and HLA-I(low). Transplantation of hMADS cells into the mdx mouse, an animal model of Duchenne muscular dystrophy, results in substantial expression of human dystrophin in the injected tibialis anterior and the adjacent gastrocnemius muscle. Long-term engraftment of hMADS cells takes place in nonimmunocompromised animals. Based on the small amounts of an easily available tissue source, their strong capacity for expansion ex vivo, their multipotent differentiation, and their immune-privileged behavior, our results suggest that hMADS cells will be an important tool for muscle cell-mediated therapy.

Hierarchical and Ontogenic Positions Serve to Define the Molecular Basis of Human Hematopoietic Stem Cell Behavior

The molecular basis governing functional behavior of human hematopoietic stem cells (HSCs) is largely unknown. Here, using in vitro and in vivo assays, we isolate and define progenitors versus repopulating HSCs from multiple stages of human development for global gene expression profiling. Accounting for both the hierarchical relationship between repopulating cells and their progenitors, and the enhanced HSC function unique to early stages of ontogeny, the human homologs of Hairy Enhancer of Split-1 (HES-1) and Hepatocyte Leukemia Factor (HLF) were identified as candidate regulators of HSCs. Transgenic human hematopoietic cells expressing HES-1 or HLF demonstrated enhanced in vivo reconstitution ability that correlated to increased cycling frequency and inhibition of apoptosis, respectively. Our report identifies regulatory factors involved in HSC function that elicit their effect through independent systems, suggesting that a unique orchestration of pathways fundamental to all human cells is capable of controlling stem cell behavior.

Senescent cells, tumor suppression, and organismal aging: good citizens, bad neighbors

Cells from organisms with renewable tissues can permanently withdraw from the cell cycle in response to diverse stress, including dysfunctional telomeres, DNA damage, strong mitogenic signals, and disrupted chromatin. This response, termed cellular senescence, is controlled by the p53 and RB tumor suppressor proteins and constitutes a potent anticancer mechanism. Nonetheless, senescent cells acquire phenotypic changes that may contribute to aging and certain age-related diseases, including late-life cancer. Thus, the senescence response may be antagonistically pleiotropic, promoting early-life survival by curtailing the development of cancer but eventually limiting longevity as dysfunctional senescent cells accumulate.

Effects of aging on the homing and engraftment of murine hematopoietic stem and progenitor cells

To test the hypothesis that aging has negative effects on stem-cell homing and engraftment, young or old C57BL/6 bone marrow (BM) cells were injected, using a limiting-dilution, competitive transplantation method, into old or young Ly5 congenic mice. Numbers of hematopoietic stem cells (HSCs) and progenitor cells (HPCs) recovered from BM or spleen were measured and compared with the numbers initially transplanted. Although the frequency of marrow competitive repopulation units (CRUs) increased approximately 2-fold from 2 months to 2 years of age, the BM homing efficiency of old CRUs was approximately 3-fold lower than that of young CRUs. Surprisingly, the overall size of individual stem-cell clones generated in recipients receiving a single CRU was not affected by donor age. However, the increased ages of HSC donors and HSC transplant recipients caused marked skewing of the pattern of engraftment toward the myeloid lineage, indicating that HSC-intrinsic and HSC-extrinsic (microenvironmental) age-related changes favor myelopoiesis. This correlated with changes after transplantation in the rate of recovery of circulating leukocytes, erythrocytes, and platelets. Recovery of the latter was especially blunted in aged recipients. Collectively, these findings may have implications for clinical HSC transplantation in which older persons increasingly serve as donors for elderly patients.

Regulation of hematopoietic stem cell aging in vivo by a distinct genetic element

Until recently, stem cells were thought to be endowed with unlimited self-renewal capacity and, thus, assumed exempt from aging. But accumulating evidence over the past decade compellingly argues that a measurable and progressive replicative impairment in the hematopoietic, intestinal, and muscle stem cell activity exists from adulthood to old age, resulting in a decline in stem cell function and rendering stem cell aging as the possible link between cellular aging and organismal aging. By using a previously uncharacterized congenic animal model to study genetic regulation of hematopoietic stem cell aging, we have demonstrated definitively that a locus on murine chromosome 2 regulates hematopoietic stem cell aging. In addition to demonstrating that hematopoietic stem cell aging is regulated by a distinct genetic element, experimental evidence links the response of hematopoietic stem cells to DNA double-strand breaks to cellular aging, suggesting DNA integrity influences stem cell aging.

In vitro functional defects of bone marrow-derived CD34+ progenitors from newly diagnosed mature B-cell malignancies with bone marrow tumor involvement

OBJECTIVE

We hypothesized that the presence of tumor cells in bone marrow (BM) could alter hematopoietic progenitor cell functions. Therefore, we evaluated phenotypic and in vitro functional properties of BM-derived CD34+ progenitors issued from untreated and newly diagnosed patients presenting a mature B-lymphoproliferative disorder (LPD) involving the BM (Inv+).

PATIENTS AND METHODS

In vitro proliferation and differentiation capacities of primitive and committed progenitors were evaluated by cobblestone area-forming cell (CAFC) and colony-forming cell (CFC) assays, and ex vivo cell expansion. Migratory capacities of CD34+ cells were explored by chemotaxis assays using a CXCL12alpha gradient.

RESULTS

Our results showed that CD34+ cells from Inv+ patients overexpressed CD117 and had a significant decrease of week-3 and -6 CAFC, and CFC frequencies, compared to cells obtained from healthy volunteers and LPD patients without BM involvement (Inv-). In addition, progenitors from Inv+ patients maintained a significantly decreased CFC capacity after ex vivo cell expansion, compared to healthy volunteers. However, the former cells held their migratory capacity in response to CXCL12alpha.

CONCLUSION

Functional defects of primitive and committed CD34+ progenitors detected among LPD patients with BM tumor involvement suggest either that tumor cells may induced bystander effects on progenitors or that "unusual" CD34+ cells may exist in the BM that could belong to the proliferating tumor tissue.

Disruption of PLZP in mice leads to increased T-lymphocyte proliferation, cytokine production, and altered hematopoietic stem cell homeostasis

Deregulated function of members of the POK (POZ and Kruppel) family of transcriptional repressors, such as promyelocytic leukemia zinc finger (PLZF) and B-cell lymphoma 6 (BCL-6), plays a critical role in the pathogenesis of acute promyelocytic leukemia (APL) and non-Hodgkin's lymphoma, respectively. PLZP, also known as TZFP, FAZF, or ROG, is a novel POK protein that displays strong homology with PLZF and has been implicated in the pathogenesis of the cancer-predisposing syndrome, Fanconi's anemia, and of APL, in view of its ability to heterodimerize with the FANC-C and PLZF proteins, respectively. Here we report the generation and characterization of mice in which we have specifically inactivated the PLZP gene through in-frame insertion of a lacZ reporter and without perturbing the expression of the neighboring MLL2 gene. We show that PLZP-deficient mice display defects in cell cycle control and cytokine production in the T-cell compartment. Importantly, PLZP inactivation perturbs the homeostasis of the hematopoietic stem and/or progenitor cell. On the basis of our data, a deregulation of PLZP function in Fanconi's anemia and APL may affect the biology of the hematopoietic stem cell, in turn contributing to the pathogenesis of these disorders.

Neural stem cells as therapeutic agents for age-related brain repair

Neurogenesis occurs in two germinal centres of the adult brain and persists with increasing age, although at a reduced level. This observation, that the mature brain can support neurogenesis, has given rise to the hope that neural stem cells could be used to repair the brain by repopulating regions suffering from neuronal loss as a result of injury or disease. The aging brain is vulnerable to mild cognitive impairment, increasing incidence of stroke, and a variety of neurodegenerative diseases. However, most studies to date have focused on the young adult brain, and relatively little information is available about the regulation of neurogenesis in the aged brain or the potential of using neural stem cells to repair the aged brain. This review summarizes the current state of knowledge on neurogenesis in the young adult brain and discusses the information available on age-related changes in neurogenesis. Possible therapeutic strategies using neural stem cells for repair of the aging brain are considered.

Genetic programs regulating HSC specification, maintenance and expansion

All mature blood cells originate from a small population of self-renewing pluripotent hematopoietic stem cells (HSCs). The capacity to self-renew characterizes all stem cells, whether normal or neoplastic. Interestingly, recent studies suggest that self-renewal is essential for tumor cell maintenance, implicating that this process has therapeutic relevance. Unfortunately, the molecular bases for self-renewal of vertebrate cells remain poorly defined. This article will focus on the developmental mechanisms underlying fetal and adult HSC homeostasis. Specifically, distinctions between genetic programs regulating HSC specification (identity), self-renewal (in both fetal and adult) and differentiation/commitment will be discussed with a special emphasis on transcriptional and chromatin regulators.

Stem cells, aging, and cancer: inevitabilities and outcomes

Given the unique abilities of a stem cell to self-renew, differentiate, and proliferate, it is no wonder that they are critically important to an organism during development and to maintain homeostasis. Likewise, when something goes awry within a stem cell, it is likely to have far-reaching effects, since stem cells persist throughout the lifetime of the individual. Two significant biological phenomena that involve stem cells are the inevitable process of aging and a major health issue whose incidence increases with aging: cancer. In this review, we summarize evidence and theories concerning these two stem cell processes. The inability of stem cells to be passaged indefinitely in mice and the data supporting regular replication of the quiescent stem cell pool are discussed. Further, the current evidence indicating a stem cell origin of acute myeloid leukemia, including examples from both experimental mouse models and human clinical samples, is evaluated. Finally, we propose a model in which aging of the stem cell population of the hematopoietic system in particular can create conditions that are permissive to leukemia development; in fact, we suggest that aging is a secondary event in leukemogenesis.

Flow cytometric detection of accelerated telomere shortening in myelodysplastic syndromes: correlations with aetiological and clinical-biological findings*

Using quantitative fluorescence in situ hybridisation and flow cytometry (flow-FISH), we investigated the biological and clinical relevance of telomere length in 55 patients affected by myelodysplastic syndromes (MDS) compared with 55 sex- and age-matched controls. We found that telomere fluorescence in MDS granulocytes, and CD34+ cells did not decline with age as in normal controls and that MDS granulocytes and CD34+ cells had significantly shorter telomeres than healthy controls. A significant higher incidence of cases with intermediate-unfavourable cytogenetics and International Prognostic Scoring System (IPSS) int-2/high-risk group was observed among patients with lower telomere fluorescence. We also found that apoptosis in CD34+ cells was significantly higher in IPSS int-1 low-risk patients when compared with IPSS int-2 high-risk cases and healthy controls and that CD34+ cell telomere fluorescence directly correlated with CD34+ cell apoptosis. Reduced telomere fluorescence was associated with a history of occupational exposure to toxic agents and with worse survival in univariate and multivariate analyses. Our results suggest that flow-cytometry assessment of telomere dynamics may represent a valuable tool in the biological and clinical-prognostic characterisation of MDS disorders.

Selective pressure for a decreased rate of asymmetrical divisions within stem cell niches may contribute to age-related alterations in stem cell function

Most mammalian continuously renewing tissues are maintained by stem cells located within stem cell niches. Each niche contains a number of stem cells that replicate asymmetrically to give differentiated cells and also undergo periodic deletion and compensatory replacement by symmetrical "self-renewal" divisions of stem cells remaining within the niche. It has been recognized that there is selective pressure for an increased rate of self-renewal divisions and that the increasingly likely consequence is neoplasia. However, to date it has not been appreciated that there is also an independent selective pressure for a decreased rate of asymmetrical divisions. In this article, the origin of this second type of selective pressure is explained and its consequences explored through the use of computer modeling. It is shown that age-related changes in a range of mammalian stem cell compartments can be understood in the context of a decreased rate of asymmetrical stem cell divisions with an increased propensity for self-renewal divisions. It is proposed that a decreased rate of asymmetrical divisions impairs the ability of old stem cell compartments to respond effectively to stress.

A new human somatic stem cell from placental cord blood with intrinsic pluripotent differentiation potential

Here a new, intrinsically pluripotent, CD45-negative population from human cord blood, termed unrestricted somatic stem cells (USSCs) is described. This rare population grows adherently and can be expanded to 10¹⁵ cells without losing pluripotency. In vitro USSCs showed homogeneous differentiation into osteoblasts, chondroblasts, adipocytes, and hematopoietic and neural cells including astrocytes and neurons that express neurofilament, sodium channel protein, and various neurotransmitter phenotypes. Stereotactic implantation of USSCs into intact adult rat brain revealed that human Tau-positive cells persisted for up to 3 mo and showed migratory activity and a typical neuron-like morphology. In vivo differentiation of USSCs along mesodermal and endodermal pathways was demonstrated in animal models. Bony reconstitution was observed after transplantation of USSC-loaded calcium phosphate cylinders in nude rat femurs. Chondrogenesis occurred after transplanting cell-loaded gelfoam sponges into nude mice. Transplantation of USSCs in a noninjury model, the preimmune fetal sheep, resulted in up to 5% human hematopoietic engraftment. More than 20% albumin-producing human parenchymal hepatic cells with absence of cell fusion and substantial numbers of human cardiomyocytes in both atria and ventricles of the sheep heart were detected many months after USSC transplantation. No tumor formation was observed in any of these animals.

Vascular repair by circulating endothelial progenitor cells: the missing link in atherosclerosis?

The integrity and functional activity of the endothelial monolayer play a crucial role in the prevention of atherosclerosis. Increasing evidence suggests that risk factors for coronary artery disease increase endothelial cell apoptosis and lead to a disturbance in the endothelial monolayer. Recent insights suggest that the injured endothelial monolayer is regenerated by circulating bone marrow derived endothelial progenitor cells, which accelerates reendothelialization and limits atherosclerotic lesion formation. However, risk factors for coronary artery disease such as age and diabetes reduce the number and functional activity of these circulating endothelial progenitor cells, thus limiting the regenerative capacity. The impairment of stem/progenitor cells by risk factors may contribute to atherogenesis and atherosclerotic disease progression. We discuss this novel concept of endothelial regeneration and highlight possible novel strategies to interfere with the balance of injury and repair mechanisms.

Inhibition of angiotensin I–converting enzyme induces radioprotection by preserving murine hematopoietic short-term reconstituting cells

Angiotensin I–converting enzyme (ACE) inhibitors can affect hematopoiesis by several mechanisms including inhibition of angiotensin II formation and increasing plasma concentrations of AcSDKP (acetyl-N-Ser-Asp-Lys-Pro), an ACE substrate and a negative regulator of hematopoiesis. We tested whether ACE inhibition could decrease the hematopoietic toxicity of lethal or sublethal irradiation protocols. In all cases, short treatment with the ACE inhibitor perindopril protected against irradiation-induced death. ACE inhibition accelerated hematopoietic recovery and led to a significant increase in platelet and red cell counts. Pretreatment with perindopril increased bone marrow cellularity and the number of hematopoietic progenitors (granulocyte macrophage colony-forming unit [CFU-GM], erythroid burst-forming unit [BFU-E], and megakaryocyte colony-forming unit [CFU-MK]) from day 7 to 28 after irradiation. Perindopril also increased the number of hematopoietic stem cells with at least a short-term reconstitutive activity in animals that recovered from irradiation. To determine the mechanism of action involved, we evaluated the effects of increasing AcSDKP plasma concentrations and of an angiotensin II type 1 (AT1) receptor antagonist (telmisartan) on radioprotection. We found that the AT1-receptor antagonism mediated similar radioprotection as the ACE inhibitor. These results suggest that ACE inhibitors and AT1-receptor antagonists could be used to decrease the hematopoietic toxicity of irradiation.

The Positive Regulatory Effect of TGF-β2 on Primitive Murine Hemopoietic Stem and Progenitor Cells Is Dependent on Age, Genetic Background, and Serum Factors

TGF-beta is considered a negative regulator of hemopoietic stem and progenitor cells. We have previously shown that one TGF-beta isoform, TGF-beta2, is, in fact, a positive regulator of murine hemopoietic stem cell function in vivo. In vitro, TGF-beta2, but not TGF-beta1 and TGF-beta3, had a biphasic dose response on the proliferation of purified lin-Sca1(++)kit(+) (LSK) cells, with a stimulatory effect at low concentrations, which was subject to mouse strain-dependent variation. In this study we report that the stimulatory effect of TGF-beta2 on the proliferation of LSK cells increases with age and after replicative stress in C57BL/6, but not in DBA/2, mice. The age-related changes in the TGF-beta2 effect correlated with life span in BXD recombinant strains. The stimulatory effect of TGF-beta2 on the proliferation of LSK cells requires one or more nonprotein, low m.w. factors present in fetal calf and mouse sera. The activity of this factor(s) in mouse serum increases with age. Taken together, our data suggest a role for TGF-beta2 and as yet unknown serum factors in the aging of the hemopoietic stem cell compartment and possibly in organismal aging.

In patients with myelodysplastic syndromes response to rHuEPO and G-CSF treatment is related to an increase of cytogenetically normal CD34+ cells

The in vivo effect of recombinant human erythropoietin (rHuEpo) and granulocyte colony-stimulating factor (G-CSF) combined treatment on CD34(+) cells was evaluated by fluorescence in situ hybridization (FISH) in 13 myelodysplastic syndrome (MDS) patients with known cytogenetic abnormalities. After treatment, responsive patients presented a significantly lower proportion of FISH abnormal CD34(+) cells than before treatment (P = 0.003), and in comparison with unresponsive cases (P = 0.007). Response to treatment was associated with a reduced degree of apoptosis in CD34(+) cells (P = 0.021): however, no difference in telomere length was observed in responsive patients after growth factor administration. Although the number of patients analysed was relatively small, the present data suggest that, in MDS patients, response to rHuEpo and G-CSF may be related to the proliferation of karyotypically normal but potentially defective CD34(+) progenitor cells.

Myocardial aging and senescence: where have the stem cells gone?

Heart failure remains a leading cause of hospital admissions and mortality in the elderly, and current interventional approaches often fail to treat the underlying cause of pathogenesis. Preservation of structure and function in the aging myocardium is most likely to be successful via ongoing cellular repair and replacement, as well as survival of existing cardiomyocytes that generate contractile force. Research has led to a paradigm shift driven by application of stem cells to generate cardiovascular cell lineages. Early controversial findings of pluripotent precursors adopting cardiac phenotypes are now widely accepted, and current debate centers upon the efficiency of progenitor cell incorporation into the myocardium. Much work remains to be done in determining the relevant progenitor cell population and optimizing conditions for efficient differentiation and integration. Significant implications exist for treatment of pathologically damaged or aging myocardium since future interventional approaches will capitalize upon the use of cardiac stem cells as therapeutic reagents.

Neural stem cell detection, characterization, and age-related changes in the subventricular zone of mice

The mammalian brain contains neural stem cells (NSCs) that allow continued neurogenesis throughout the life of the animal. However, neurogenesis is known to decline during aging and, to the extent that neurogenesis is required for normal CNS function, this may contribute to neurodegenerative disease. Decreased neurogenesis could result from loss of NSCs or dysfunction at some later step, and distinguishing these possibilities is important for understanding the cause of the decline. However, because of the inability to distinguish NSCs from their rapidly dividing progeny in situ, it has not been possible to quantitatively assess the NSC populations in young and old animals. In this report we show that the G1 phase-specific expression of the replication factor Mcm2 is a useful marker for detecting slowly cycling putative NSCs in situ and confirm the identity of these cells using both cytosine beta-D-arabinofuranoside (Ara-C) treatment and a double nucleoside analog-labeling technique. The ability to distinguish NSCs from proliferative progenitors has allowed characterization of the expression of several markers including Nestin, Musashi, and GFAP in these different cell types. Furthermore, comparison of the NSC populations in the subventricular zones of young (2-4 months) and old (24-26 months) mice demonstrates an approximately twofold reduction in the older mice. A similar twofold reduction is also observed in the number of neurospheres recovered in culture from old relative to young animals. The reduction in the neural stem cell population documented here is sufficient to account for the reduced level of neurogenesis in old animals.

Lung Cells Transplanted to Irradiated Recipients Generate Lymphohematopoietic Progeny

Bone marrow (stem) cells can differentiate into cells in multiple tissues, including lung. Conversely, there are reports that cells of nonhematopoietic tissues (brain, muscle) can give rise to lymphohematopoietic cells. Here we show that the lung contains cells capable of giving rise to lymphohematopoietic cells when transplanted to irradiated recipients. Whole lung cell suspensions, lung side population (SP) cells, and CD45(+/-) lung cells obtained from male transgenic enhanced green fluorescent protein-expressing mice were transplanted intravenously to total body irradiated female mice. Green fluorescent cells were recovered from the circulation and phenotyped for their expression of lymphohematopoietic markers (CD3, CD4, CD8, B220, Gr-1, and Mac-1). Lung SP cells were composed of heterogeneous populations and had less ability to give rise to lymphohematopoietic cells than did bone marrow SP cells. Furthermore, the ability of cells from the lung of aged mice to generate lymphohematopoietic progeny was equivalent to that of cells from young mice. Cells from lung with radioprotective and lymphohematopoietic reconstituting abilities were CD45(+). CD45(+) cells in the lung cells have lymphohematopoietic stem/progenitor cell characteristics, and this has implications for cell or gene therapy applications.

Profoundly reduced neovascularization capacity of bone marrow mononuclear cells derived from patients with chronic ischemic heart disease

BACKGROUND

Cell therapy with bone marrow-derived stem/progenitor cells is a novel option for improving neovascularization and cardiac function in ischemic heart disease. Circulating endothelial progenitor cells in patients with coronary heart disease are impaired with respect to number and functional activity. However, whether this impairment also extends to bone marrow-derived mononuclear cells (BM-MNCs) in patients with chronic ischemic cardiomyopathy (ICMP) is unclear.

METHODS AND RESULTS

BM-MNCs were isolated from bone marrow aspirates in 18 patients with ICMP (ejection fraction, 38+/-11%) and 8 healthy control subjects (controls). The number of hematopoietic stem/progenitor cells (CD34+/CD133+), CD49d(+) (VLA-4) cells, and CXCR4+ cells did not differ between the 2 groups. However, the colony-forming capacity of BM-MNCs from patients with ICMP was significantly lower compared with BM-MNCs from healthy controls (37.3+/-25.0 versus 113.8+/-70.4 granulocyte-macrophage colony-forming units; P=0.009). Likewise, the migratory response to stromal cell-derived factor 1 (SDF-1) and vascular endothelial growth factor (VEGF) was significantly reduced in BM-MNCs derived from patients with ICMP compared with BM-MNCs from healthy controls (SDF-1, 46.3+/-26.2 versus 108.6+/-40.4 cells/microscopic field, P<0.001; VEGF, 34+/-24.2 versus 54.8+/-29.3 cells/microscopic field, P=0.027). To assess the in vivo relevance of these findings, we tested the functional activity of BM-MNCs to improve neovascularization in a hindlimb animal model using nude mice. Two weeks after ligation of the femoral artery and intravenous injection of 5x10(5) BM-MNCs, laser Doppler-derived relative limb blood flow in mice treated with BM-MNCs from patients with ICMP was significantly lower compared with mice treated with BM-MNCs from healthy controls (0.45+/-0.14 versus 0.68+/-0.15; P<0.001). The in vivo neovascularization capacity of BM-MNCs closely correlated with the in vitro assessment of SDF-1-induced migration (r=0.78; P<0.001) and colony-forming capacity (r=0.74; P<0.001).

CONCLUSIONS

BM-MNCs isolated from patients with ICMP have a significantly reduced migratory and colony-forming activity in vitro and a reduced neovascularization capacity in vivo despite similar content of hematopoietic stem cells. This functional impairment of BM-MNCs from patients with ICMP may limit their therapeutic potential for clinical cell therapy.

Age-related changes in lymphocyte development and function

The effects of aging on the immune system are widespread and extend from hematopoietic stem cells and lymphoid progenitors in the bone marrow and thymus to mature lymphocytes in secondary lymphoid organs. These changes combine to result in a diminution of immune responsiveness in the elderly. This review aims to provide an overview of age-related changes in lymphocyte development and function and discusses current controversies in the field of aging research.

Quantitative genetic variation in the hematopoietic stem cell and progenitor cell compartment and in lifespan are closely linked at multiple loci in BXD recombinant inbred mice

The number of bone marrow hematopoietic stem and progenitor cells as defined by the lineage(-), Sca1(++), c-kit(+) (LSK) phenotype and their proliferative capacity in vitro are subject to quantitative genetic variation, and several quantitative trait loci (QTL) have been identified in young mice. Because some traits affecting hematopoiesis also change with age in a mouse strain-dependent fashion, we performed quantitative trait analysis in aged BXD recombinant inbred (RI) mice for the number and frequency of LSK cells, and for their proliferative capacity in vitro. Several novel QTL were identified. The number and frequency of LSK cells in old mice correlated inversely with lifespan. Furthermore, 4 of 7 lifespan QTL overlap with QTL contributing to the number, frequency, or proliferative capacity of LSK cells in young or old mice. Taken together, these data establish a close genetic, and perhaps functional, link between genetic variation in lifespan and characteristics of stem and progenitor cells.

Impaired Angiogenesis in the Aged

The process of angiogenesis, during which new blood vessels are formed, is impaired during aging. This Perspective describes many of the myriad components of the angiogenic response that are altered with age. In addition, the impact of impaired angiogenesis on wound healing, vascular disease, and cancer in the aged is discussed.