Hematopoeisis in ageing

Editor's Notes

• #3: reduced energetic reserve, loss of fractality a reduced ability to perform complex activities , resulting from a diminished functional capacity of the adult stem cells
• #4: HSCs sit at the apex of a developmental hierarchy. HSCs are self-renewing and multipotent which means they give rise to all mature myeloid and lymphoid cell lineages. According to the classical HSC model, the myeloid lineage has bipotent megakaryocyte- erythroid and granulocyte macrophage progenitors which oroduce unipotent progenitors and ultimately mature blood cells.Although they are multipotent and self-renewing, HSCs are relatively quiescent in order to maintain an indefinite pool of HSCs [1]. The pool of HSCs contains multipotent progenitors (MPP), short-term HSCs (ST-HSCs), and long-term HSCs (LT-HSCs). The balance of HSC self-renewal with multi-lineage differentiation is critical for hematopoietic tissue homeostasis.
• #5: HSCs reside in the bone marrow surrounded by many cell types, including macrophages, vascular-forming endothelial cells, and cellular matrix proteins that can stimulate or inhibit hematopoietic niche function. This is known as the hematopoietic niche or microenvironment and is essential for the production of blood forming elements, remodeling of the skeleton, and the maintenance of HSCs.. Signals between the niche cells and HSCs can be transmitted via direct cell–cell contact (through adhesion receptors46 and gap junctions such as connexin 43 junctions)109–111, cytokines and chemokines (such as CXC-chemokine ligand 12 (CXCL12; also known as SDF1))138, growth factors and via components of the extracellular matrix (such as fibronectin). HSCs often reside close to the endosteum (the endosteal niche) and the vascular system (the vascular niche) in the bone marrow. In these niches, HSCs are in close proximity to reticular cells expressing high levels of CXCL12 (CXCL12-abundant reticular cells; CAR cells), Schwann-like cells and nestin-positive mesenchymal stem cells (NES+ MSCs), which are all closely surrounded by nerve fibres139. Niches comprise non-haematopoietic cells, such as endothelial cells, osteoblasts and osteoclasts, adipocytes and MSCs, but haematopoietic cells (mainly macrophages) also contribute to niche function140.
• #6: age-associated micro-environmental (extrinsic) and intrinsic changes of the hematopoietic system: first, increase in the number of phenotypically defined HSCs; second, more distant localization of HSCs from the endosteum; third, less supportive stroma; fourth, accumulation of adipocytes in the bone marrow; fifth, different cytokine milieu; sixth, increased DNA-damage; seventh, increased exposure to reactive oxygen species (ROS); and eighth, changes in gene expression and accumulation of epigenetic alterations.
• #7: This is known as the hematopoietic niche or microenvironment and is This niche is composed of CD150+, CD48-, CD41-, Lin- HSCs [44] as well as endothelial cells [3] and nestin-GFPbright NG2+ arteriolar pericytes [55] that are distinct from sinusoidal-associated LepR+ cells [44]. NG2+ arteriolar pericytes express high levels of stem cell factor (SCF) and deletion of SCF from endothelial cells decreases HSC numbers [55]. Functionally, loss of NG2+ cells result in increased HSC cycling and decreased HSC-repopulating activity – hallmarks of HSC aging – and alters the distribution of HSCs from the NG2+ arteriolar compartment to the LepR+ sinusoidal compartment. Sympathetic nerve denervation induces a rapid loss of bone marrow glial cells and a drastic reduction in HSC numbers, with the remaining HSCs identified as actively proliferating, indicating that sympathetic nerves function to maintain HSC dormancy in the central bone marrow.
• #8: Phenotypical and functional changes in HSCs upon ageing. a | Young haematopoietic stem cells (HSCs) home to the bone marrow and localize in close proximity to the endosteum. They have high self-renewal and regenerative capacities, and a balanced differentiation potential towards lymphoid and myeloid progenitor cells. b | The number of phenotypic HSCs in the bone marrow increases with ageing, and this is probably a consequence of an enhanced self-renewal activity of aged HSCs, even though the regenerative capacity (serial transplantability) of individual aged stem cells is reduced compared to young HSCs. The localization of aged HSCs in the bone marrow is different to that of young HSCs; aged HSCs localize away from the endosteal stem cell niche following their transplantation. This implies that aged HSCs select for niches that are distinct from those that young HSCs occupy. Another hallmark of aged HSCs is that following their transfer into recipient mice they exhibit about a two-fold reduced ability to home to the bone marrow compared with young HSCs (not shown). Moreover, aged HSCs can be mobilized in higher numbers than young HSCs in response to cytokine stimulation. Finally, a central hallmark of aged HSCs is their skewing towards myeloid cell differentiation, as they provide more myeloid progenitor cells and fewer lymphoid progenitor cells compared with young HSCs. CAR, CXC-chemokine ligand 12-abundant reticular.
• #10: Telomeres are repetitive DNA sequences at the ends of chromosomes that prevent the activation of the DNA damage response and DNA repair activity.
• #11: Two of the hallmarks of young haematopoietic stem cells (HSCs) are the low production of reactive oxygen species (ROS) by mitochondria and the establishment of cell polarity by the cell division control protein 42 (CDC42). CDC42 establishes cell polarity through both its localization and its activity status (as determined by the ratio of GTP-bound CDC42 to GDP-bound CDC42). In addition, in young HSCs DNA integrity is achieved through the maintenance of telomere length and through the activation of effective DNA damage repair responses. Also, specific chromatin modifications, including acetylation of lysine 16 on the tail of histone H4 (H4K16ac) and maintenance of high expression of lymphoid genes, are involved. b | In aged HSCs, increased CDC42 activity causes depolarization of planar cell polarity markers in the cytoplasm, as well as loss of epigenetic polarity for H4K16ac in the nucleus. The mitochondrial production of ROS is increased, leading to mitochondrial DNA damage and increased activation of p38 mitogen-activated protein kinase, which might be involved in HSC ageing. Moreover, the altered gene expression profiles in aged HSCs indicate a mechanistic role for nuclear factor-κB (NF-κB) and stress adhesion signalling in ageing. Increased DNA damage and telomere shortening in aged HSCs induce the expression of cyclin-dependent kinase inhibitor 2A (CDKN2A; also known as p16INK4A) and probably of B cell lymphoma 2 (BCL-2), B cell-activating transcription factor (BATF) and p53. The resulting DNA damage signalling might cause senescence, apoptosis or differentiation. Changes in the chromatin epigenetic state, including DNA or histone methylation, or histone acetylation (such as H4K16ac) indicate improper replication of epigenetic marks upon cell division or reduced genome-wide levels of acetylation upon ageing. ICAM1, intercellular cell adhesion molecule 1; mtDNA, mitochondrial DNA; PECAM, platelet/endothelial cell adhesion molecule.
• #12: The number of osteoblasts decreases, the number of adipocytes increases as a result of the skewed differentiation of aged mesenchymal stem cells (MSCs) and and the composition of the extracellular matrix (ECM) is altered.. Moreover, osteoblasts generate higher levels of reactive oxygen species (ROS), Increased ROS levels induce p38 mitogen-activated protein kinase (MAPK) signalling in HSCs (not shown), which is associated with a reduced self-renewal capacity. Increased adiposity and reduced osteogenesis might be connected to decreased CXC-chemokine 12 (CXCL12; also known as SDF1) levels in the niche upon ageing. However, it is still unclear how these changes in the number of niche cells affect HSC ageing. Extrinsic factors that are secreted by cells that form the HSC niche promote the skewed differentiation of aged HSCs towards myeloid progenitors, and inhibit the development of lymphoid progenitors. This might partially be the consequence of increased levels of several cytokines and chemokines, such as CC-chemokine ligand 5 (CCL5; also known as RANTES), in the niche. Dysregulated cytokine and chemokine expression, as in the case of CXCL12, in the bone marrow niche might also contribute to the altered mobilization of aged HSCs. The sympathetic nervous system, which controls the HSC niche by regulating the interactions of HSCs with nestin-positive (NES+) MSCs, might also be important for the regulation of HSC mobilization and might show an altered activation status upon ageing, although this has yet to be determined. It is also a possibility that the number of NES+ MSCs changes with age. In addition, changes in adhesion receptor-mediated interactions in the aged niche, as well as altered cell–cell communication as a result of the altered function of gap junction channels, might cause an aged phenotype in HSCs.
• #17: reduced NOTCH1 and GATA3 and regulator pathways IL7- STAT5 pathway, IL7 receptor, . Reduced expression of B cell master regulator genes such as Pax5 and Ebf1. Activation of STAT5 restores B cell maturation
• #18: EPCR, endothelial protein C receptor; FLK2, fetal liver kinase 2; HSC, haematopoietic stem cell; LIN, lineage; LT-HSC, long-term HSC; SCA1, stem cell antigen 1. *Also described as side population, as determined by Hoechst blue versus Hoechst red staining. ‡Also described as tip of side population, as determined by Hoechst blue versus Hoechst red staining. .as aged mice have increased numbers of CMP cells and decreased numbers of CLP cells compared with young mice. Elderly individuals also have reduced numbers of both CLP cells and early B lymphoid progenitor cells but, contrary to what has been observed in mice, this reduction in the number of CLP cells is not accompanied by an increase in the CMP compartment12,34.
• #19: Left column shows surface marker HSC-identification strategy from whole bone marrow based on lineage-negative, c-kit, Scal-1 expression, as well as Flk-2 and CD34. Top middle panel demonstrates delineation of SP. Top right shows regions designated as lower-SP and upper-SP and below that, c-kit and Sca-1 cells expression of cells gated on these and through a lineage-negative gate. Surface markers on these cells, designated as SPKLS are shown below. CD150 expression is heterogeneous, with CD150+ cells more prevalent in the lower-SP
• #20: (A) The traditional clonal succession model (left) in which all mature blood cells are the progeny of a single uniform pool of LT-HSCs, and the clonal diversity model (right), supported by our data, in which distinct HSC subtypes are capable of contributing to all lineages, but are stably programmed to do so in a highly biased fashion (B) The SP allows visual representation of the continuum of HSC subtypes encompassing the spectrum from the most myeloid-biased CD150+ lower-SPKLS to the most lymphoid-biased CD150− upper-SPKLS. The HSC subtypes exhibit additional cellular, molecular, and functional distinctions. A parental unbiased HSC likely exists during development, and conceivably in the adult.
• #21: The defective generation of naive T cells, and the accumulation of effector and memory T cells, results in decreased diversity in the T cell repertoire after the age of 70
• #22: T cell generation and maintenance during aging. early in life, providing a diverse T cell receptor (TCR) repertoire. The maintenance of the naïve T cell pool during adult life is entirely dependent on homeostatic proliferation. In old age, aberrant homeostatic proliferation results in contraction of the T cell pool (especially CD8+ T cells), decrease in TCR repertoire diversity and generation of virtual memory cells (VM).
• #23: hematopoietic stem cells (HSC) are skewed towards the myeloid lineage, reducing the number of lymphoid progenitors and subsequently, B cell precursors in the bone marrow. B cell precursors undergo a series of selection events, where initial pre-B cells are positively (+, green bars) selected for functional BCR heavy chain arrangements followed by negative selection (-, red bars) of self-reactive immature B cells, prior to exit from the bone marrow Reduced naïve B cell output from the bone marrow in the elderly leads to memory cell expansion by homeostatic proliferation and contraction of repertoire diversity. In addition, there is accumulation of age-associated B cells (ABCs) and autoreactive antibodies.
• #26: Increased liver retention of oral iron
• #27: . increased erythropoietin levels in non-anemic elderly subjects include decreased clearance due to reduced numbers of erythropoietin receptors in a relatively hypoplastic marrow, or an increase in the hypoxic stimulus for erythropoietin production. Potential underlying pathologic mechanisms of reduced erythropoietin levels in patients with diabetes and/or hypertension include the loss of hypoxic response as a stimulus for erythropoietin secretion, possibly due to functional and/or structural changes in the proximal tubule and cortical interstitium [73,74], effects of advanced glycation end products [75], or autonomic dysfunctionby a relative erythroid resistance to erythropoietin, leading to a need for increased erythropoietin secretion in order to maintain the red cell mass at physiologic levelsThe effects of growth hormone on erythropoiesis appear to be mediated through insulin-like growth factor-I [111] and include inhibition of apoptosis [112,113] and potentiation of the effects of erythropoietin [114] and other cytokines. Growth hormone may increase erythropoietin levels, possibly due to increased hepatic synthesis of erythropoietin. it is now known that inflammatory mediators, particularly interleukin-6 (IL-6) and interleukin-1 (IL-1), lead to increased hepatic synthesis of hepcidin. Hepcidin in turn acts to prevent absorption of iron from the gut and release of iron from macrophages in the bone marrow to erythroid progenitors and precursors
• #31: Ferritin is considered the best of the listed tests for iron status. The chief source of diagnostic confusion is with anemia of chronic disease/chronic inflammation, which raises serum ferritin but sometimes lowers % transferrin saturation. Therefore, in the elderly, who often have low-grade inflammation, a serum ferritin <45 g/L and sometimes even <100 g/l is accepted as indicating possible IDA. Serum TfR differentiates well between the two anemias, and a formula based on a ratio of serum TfR to log serum ferritin can enhance differentiation between the two anemias or identification of their coexistence. RBC folate (normal 140 g/L, but cutoff points have varied from 100 to 180) does not fluctuate as serum levels do and reflects long-term tissue status. Nevertheless, RBC folate is often falsely low in cobalamin deficiency, falsely high in reticulocytosis or in hemolyzed blood samples, and inaccurate in the transfused patient
• #33: The finding that higher Epo levels are required to sustain normal hemoglobin concentrations as a person ages may reflect an evolving resistance to Epo or a diminished responsiveness of the erythroid marrow to Epo.
• #35: There are several subfamilies of NOD-like receptors (NLR) but emerg-ing data indicates that the NLRP subfamily, in particular the NLRP3 member, is the major sensor for “intracellular danger-associated molecular patterns” (DAMPs). In the case of NLRP3, the activated receptor interacts with the adaptor protein ASC that recruits the inflammatory caspase-1 (CASP-1) to the complex, which subsequently oligomerizes into penta- or heptameric inflammasomes [26].
• #43: Summary of conserved concepts in aged BMHSCs between humans and mice.AgingoftheHSC-enriched compartmentofthebonemarrowisassociatedwithadecreasedlymphoidspecification,whereasmegakaryocytic/erythroid specificationisincreased
• #44: frequenciesof the HSC-enriched compartment(CD123low/−CD45RA−CD90+, hereafter referred toas human HSCs,h-HSCs)[41–43],themegakaryocytic/erythroid progenitor-enriched compartment (h-MEPs; CD123low/−CD45RA−CD110+)[44], thecommon lymphoid progenitor-enrichedcompartment(h-CLPs; CD45RA+CD10+)[45], andof the granulocyte/macrophage progenitor-enriched compartment(h-GMPs; CD123low/−CD45RA+) [46]within the CD34+Lin− populationsof CB,young BMand agedBM
• #45: Frequenciesofhumanandmurinehematopoieticstemandprogenitorcellsatdifferent ontogenicstages.(A-B)GatingstrategyandHSPCfrequenciesforhumandonors.(A)Flowcytometric profileandgatingstrategyforh-CLPs(CD10+CD45RA+),h-HSCs(CD123low/-CD45RA-CD90+),h-MEPs (CD123low/-CD45RA-CD110+)andh-GMPs(CD123low/-CD45RA+)inoneagedbonemarrowsample.PregatedonviableCD34+Lin- singlets.(B)Frequenciesofdepictedearlyhematopoieticcelltypeswithinthe primitiveCD34+Lin- fraction.Eachpointrepresentsonedonor.H-HSCs:agedn=44,youngn=52,CB n=58.H-GMPs:agedn=42,youngn=52,CBn=45.H-MEPs:agedn=29,youngn=46,CBn=13. H-CLPs:agedn=42,youngn=52,CBn=49.(C-D)GatingstrategyandHSPCfrequenciesformouse donors.(C)Flowcytometricprofileandgatingstrategyform-HSCs(Sca1+KithighCD150+CD48-),m-CLPs (Sca1+/-cKitlowFlt3+IL7Ra+),m-MkPs(Sca1-cKithighCD150+CD41+),m-pGM/GMPs (Sca1-cKithighCD41-CD150-CD105-),m-pMegEs(Sca1-cKithighCD41-CD150+CD105-),m-pCFU-Es (Sca1-cKithighCD41-CD150+CD105+)andCFU-Es(Sca1-cKithighCD41-CD150-CD105+)inoneoldmurine bonemarrowsample.Pre-gatedonviablecKit+Lin-singlets.(D)Frequenciesofdepictedearlyhematopoietic celltypeswithintheprimitiveLin-cKit+ fraction.Eachpointrepresentsonedonor.Youngn=31,agedn=9. Analyseswereperformedwithunpairedt-tests.*p0.05,**p0.01,***p0.001.Referencelines depictmeans±SEM.
• #49: in aged mouse HSCs.
• #50: mammalian target of rapamycin
• #54: Aging negatively affects several aspects of HSC function through both intrinsic and microenvironment-mediated mechanisms, leading to decreased self renewal, loss of cell polarity, impaired homing ability and a biased differentiation into the myeloid lineage. Intrinsic effects include increased ROS levels leading to DNA damage and replicative senescence. Additional consequences of intrinsic changes include stem cell exhaustion and decreased hematopoietic cell repopulation capacity, as well as reduced survival rates. Observed extrinsic changes include decreased adhesion to bone marrow stromal cells and increased adipocyte numbers with aging, leading to reduced hematopoietic activity. Additionally, HSCs localize farther from the bone surface and other key structures with age, potentially affecting their ability to remain quiescent. Intrinsic changes often yield HSC phenotypes similar to those from extrinsic effects, and their interplay awaits further resolution. CMP, common myeloid progenitor; CLP, common lymphoid progenitor; mtDNA, mitochondrial DNA.
• #55: Various cell types have been implicated for their roles in promoting HSC maintenance, including perivascular stromal cells, endothelial cells (ECs), macrophages, CAR cells, sympathetic neurons and nonmyelinating Schwann cells. Although established regulators of HSC maintenance include CXCL12 and SCF, other regulatory factors include pleiotrophin, angiopoietin 1 (ANGPT1) and TGF-β and signaling pathways including Notch and Wnt. Recent studies have suggested that osteoblasts are dispensable for HSC maintenance but may have a role in regulating lymphoid progenitor cells. Certain cell types have been shown to negatively affect HSC maintenance, including adipocytes, which are increasingly present after chemotherapy and radiation. Regional localization of HSCs during quiescence and after activation revealed that quiescent HSCs associate with arterioles ensheathed with NG2+ pericytes but after activation relocate near the Lepr-expressing perisinusoidal area. Although the identification of these various constituents is a major advancement in the field, the potential for discovering other unresolved cell populations or overlapping populations exists. Figures index
• #57: Model depicting Pak2 mechanism in HSPC function. Pak2 kinase activity mediates CDC42 activation to regulate filopodia formation, directional migration and its interaction with β-Pix regulates proper actin remodeling, velocity of migration to control homing, and engraftment of HSPCs in mice. Model depicting Pak2 mechanism in HSPC function. Pak2 kinase activity mediates CDC42 activation to regulate filopodia formation, directional migration and its interaction with β-Pix regulates proper actin remodeling, velocity of migration to control homing, and engraftment of HSPCs in mice.