Hematopoietic stem cells through the ages: A lifetime of adaptation to organismal demands

Direct megakaryopoiesis

PURPOSE OF REVIEW

Megakaryocytes are large, polyploid cells that produce platelets and originate from hematopoietic stem cells (HSCs) in the bone marrow. While in the classical paradigm, megakaryocytes are generated in a stepwise fashion through increasingly committed progenitor stages, studies using in-vivo barcoding, transplantation, and in-vitro culture have suggested that, in addition, a more direct pathway existed. The relevance of this direct pathway and its functional and phenotypic characteristics were unclear, however.

RECENT FINDINGS

Recent publications using fate-mapping and single-cell transplantation now unequivocally demonstrate the existence of a direct megakaryocyte differentiation pathway, provide molecular characterization, and indicate distinct roles and regulation of both pathways. The direct pathway originates from a separate subset of 'top' HSCs, is enhanced by hematopoietic stress, inflammation and aging, bypasses multipotential progenitors, may be more active in myeloproliferative neoplasms, and generates phenotypically distinct megakaryocyte progenitors and more reactive platelets.

SUMMARY

Novel insights into the direct megakaryocyte differentiation pathway provide a deeper understanding of HSC biology, hematological recovery after myeloablation, and aging of the hematopoietic system, and suggest that this pathway may contribute to the increase in thrombotic incidents with age and in myeloproliferative neoplasms.

Leukemogenic Kras mutation reprograms multipotent progenitors to facilitate its spread through the hematopoietic system

Leukemia-driving mutations are thought to arise in hematopoietic stem cells (HSC), yet the natural history of their spread is poorly understood. We genetically induced mutations within endogenous murine HSC and traced them in unmanipulated animals. In contrast to mutations associated with clonal hematopoiesis (such as Tet2 deletion), the leukemogenic KrasG12D mutation dramatically accelerated HSC contribution to all hematopoietic lineages. The acceleration was mediated by KrasG12D-expressing multipotent progenitors (MPP) that lacked self-renewal but showed increased proliferation and aberrant transcriptome. The deletion of osteopontin, a secreted negative regulator of stem/progenitor cells, delayed the early expansion of mutant progenitors. KrasG12D-carrying cells showed increased CXCR4-driven motility in the bone marrow, and the blockade of CXCR4 reduced the expansion of MPP in vivo. Finally, therapeutic blockade of KRASG12D spared mutant HSC but reduced the expansion of mutant MPP and their mature progeny. Thus, transforming mutations facilitate their own spread from stem cells by reprogramming MPP, creating a preleukemic state via a two-component stem/progenitor circuit.

Acetylation profiling by Iseq-Kac reveals insights into HSC aging and lineage decision

Profiling post-translational modifications face challenges with low-input samples. We developed Iseq-Kac (internal standard-assisted enrichment-free approach for high-throughput quantitative analysis of lysine acetylation) to profile the acetylome in as few as 103-104 cells. By using a hyperacetylated internal standard, Iseq-Kac can be used in mass spectrometry (MS) to enhance MS1 signals and facilitate MS2 fragmentation of acetylated peptides. Using Iseq-Kac, we quantified 675-1,471 acetylated peptides per analysis from 104 hematopoietic stem cells (HSCs) or multipotent progenitors. Validation by targeted MS, site-specific antibodies and functional assays linked aging-related proteome and acetylome changes to HSC lineage decision. A pronounced decrease in acetylation at H4 lysine 77 (H4K77ac) was observed in aged HSCs, linked to histone deacetylase 3 (HDAC3) activity. HDAC3 inhibition or knockdown in HSCs significantly promoted lymphocyte differentiation. Mimicking H4K77ac through H4K77Q expression enhanced B cell differentiation while repressing myeloid differentiation. Overall, Iseq-Kac enables robust low-input acetylome profiling and reveals epigenetic mechanisms underlying lineage skewing in aged HSCs.

Clonal tracing with somatic epimutations reveals dynamics of blood ageing

Current approaches used to track stem cell clones through differentiation require genetic engineering1,2 or rely on sparse somatic DNA variants3,4, which limits their wide application. Here we discover that DNA methylation of a subset of CpG sites reflects cellular differentiation, whereas another subset undergoes stochastic epimutations and can serve as digital barcodes of clonal identity. We demonstrate that targeted single-cell profiling of DNA methylation5 at single-CpG resolution can accurately extract both layers of information. To that end, we develop EPI-Clone, a method for transgene-free lineage tracing at scale. Applied to mouse and human haematopoiesis, we capture hundreds of clonal differentiation trajectories across tens of individuals and 230,358 single cells. In mouse ageing, we demonstrate that myeloid bias and low output of old haematopoietic stem cells6 are restricted to a small number of expanded clones, whereas many functionally young-like clones persist in old age. In human ageing, clones with and without known driver mutations of clonal haematopoieis7 are part of a spectrum of age-related clonal expansions that display similar lineage biases. EPI-Clone enables accurate and transgene-free single-cell lineage tracing on hematopoietic cell state landscapes at scale.

Embryonic macrophages orchestrate niche cell homeostasis for the establishment of the definitive hematopoietic stem cell pool

Embryonic macrophages emerge before the onset of definitive hematopoiesis, seed into discrete tissues and contribute to specialized resident macrophages throughout life. Presence of embryonic macrophages in the bone marrow and functional impact on hematopoietic stem cells (HSC) or the niche remains unclear. Here we show that bone marrow macrophages consist of two ontogenetically distinct cell populations from embryonic and adult origin. Newborn mice lacking embryonic macrophages have decreased HSC numbers in the bone marrow suggesting an important function for embryo-derived macrophages in orchestrating HSC trafficking around birth. The establishment of a normal cellular niche space in the bone marrow critically depends on embryonic macrophages that are important for the development of mesenchymal stromal cells, but not other non-hematopoietic niche cells, providing evidence for a specific role for embryo-derived macrophages in the establishment of the niche environment pivotal for the establishment of a normally sized HSC pool.

Drug-tolerant persister cells in acute myeloid leukemia: pressing challenge and promising new strategies for treatment

Acute myeloid leukemia (AML) exhibits a pronounced ability to develop drug resistance and undergo disease relapse. Recent research has noticed that resistance to treatments could substantially be attributed to drug-tolerant persister (DTP) cells, which are capable of surviving under therapeutic pressures. These are transient, reversibly dormant cells with the capability to act as a reservoir for disease relapse. DTP cells utilize diverse adaptive strategies to optimize the ecological niche, undergo metabolic reprogramming, and interact with microenvironment. The persister state of AML is established through transient cellular reprogramming, thus allowing cells to survive the initial phase of drug therapy and develop drug resistance. Our review explores the identification and phenotypic characteristics of AML DTP cells, as well as their clinical relevance. We summarize the mechanisms underlying the persistence of AML DTP cells and the molecular attributes that define the DTP state. We further address the current challenges and future prospects of DTP-targeting approaches. Understanding these features may provide critical insights into novel therapeutic strategies aimed at targeting AML DTP cells, especially in the new era of immunotherapy against AML.

A near death experience: The secret stem cell life of caspase-3

Caspase-3 is known to play a pivotal role in mediating apoptosis, a key programmed cell death pathway. While extensive research has focused on understanding how caspase-3 is activated and functions during apoptosis, emerging evidence has revealed its significant non-apoptotic roles across various cell types, including stem cells. This review explores the critical involvement of caspase-3 in regulating stem cell properties, maintaining stem cell populations, and facilitating tissue regeneration. We also explore the potential pathological consequences of caspase-3 dysfunction in stem cells and cancer cells alongside the therapeutic opportunities of targeting caspase-3.

Phase separation participates in the genetic regulation mechanism of hematopoietic stem cells: potential therapeutic methods

Hematopoietic stem cells (HSCs) are the primitive cells that give rise to common precursors for all blood cell lineages. Abnormalities in their number and/or function are important factors leading to the decline of immune function and the occurrence of various systemic diseases. Phase separation refers to a physicochemical mechanism in which intracellular liquid-liquid phase separation (LLPS) forms membrane-less organelles. It participates in various physiological activities and is related to the occurrence of diseases. Studies have shown that the functional activity of HSCs is regulated by complex mechanisms, and phase separation is closely related to these complex mechanisms such as genetic regulation, epigenetic regulation, microenvironment regulation, gene expression, autophagy degradation, and cell proliferation. With the deepening of research, the importance of phase separation in the pathogenesis and treatment of diseases such as leukemia and tumors has gradually emerged, but the deep mechanism of its regulation of HSCs genetic regulation still lacks exploration, and the direction of clinical targeted therapy is not yet clear. Here, we will summarize and elaborate the genetic regulation mechanism of HSCs, discuss the relationship between phase separation and the functional regulation of HSCs, and analyze the possibility of phase separation participating in the genetic regulation of HSCs to treat diseases, in order to provide help for the clinical implementation of targeted therapy for HSCs regulation.

Clinical hematopoietic stem cell-based gene therapy

Hematopoietic stem cell (HSC)-based gene therapies have seen extraordinary progress since their initial conception, now fundamentally transforming the treatment paradigms for various inherited hematologic, immunologic, and metabolic conditions-with additional use cases under exploration. Decades worth of work with advances in viral vector technologies and cell manufacturing have paved the way for HSC gene therapy with marked improvement in the safety and efficiency of gene delivery into HSCs. These have been augmented by the recent rise of innovative genome-editing techniques, particularly using clustered regularly interspaced short palindromic repeats CRISPR-associated proteins (CRISPR-Cas)-based technologies, which have enabled more precise and reproducible genome alterations in HSCs and fostered opportunities for targeted gene modification or gene correction. These breakthroughs have led to the development of many active clinical trials and culminated in the recent federal regulatory-agency approvals of multiple clinical HSC gene therapies for various indications that are now becoming available across different geographies. These treatments aim to offer significant, long-lasting benefits to patients worldwide without the toxicities of alternative treatment approaches. This review explores the history and advancements in HSC gene therapies and provides a comprehensive overview of the latest clinical innovations and cell-therapy products. Further, it concludes with a discussion of the persistent challenges that have limited adoption and potential future opportunities that aspire to enable curative treatment of many different patients through such personalized medicines.

Hematopoietic stem cell state and fate in trained immunity

Trained immunity serves as a de facto memory for innate immune responses, resulting in long-term functional reprogramming of innate immune cells. It enhances resistance to pathogens and augments immunosurveillance under physiological conditions. Given that innate immune cells typically have a short lifespan and do not divide, persistent innate immune memory may be mediated by epigenetic and metabolic changes in long-lived hematopoietic stem cells (HSCs) in the bone marrow. HSCs fine-tune their state and fate in various training conditions, thereby generating functionally adapted progeny cells that orchestrate innate immune plasticity. Notably, both beneficial and maladaptive trained immunity processes can comprehensively influence HSC state and fate, leading to divergent hematopoiesis and immune outcomes. However, the underlying mechanisms are still not fully understood. In this review, we summarize recent advances regarding HSC state and fate in the context of trained immunity. By elucidating the stem cell-intrinsic and extrinsic regulatory network, we aim to refine current models of innate immune memory and provide actionable insights for developing targeted therapies against infectious diseases and chronic inflammation. Furthermore, we propose a conceptual framework for engineering precision-trained immunity through HSC-targeted interventions.

KLF4 enhances transplantation-induced hematopoiesis by inhibiting TLRs and noncanonical NFκB signaling at a steady state

The transcription factor Krüppel-like factor 4 (KLF4) acts as a transcriptional activator and repressor. KLF4 plays a role in various cellular processes, including the dedifferentiation of somatic cells into induced pluripotent stem cells. Although it has been shown to enhance self-renewal in embryonic and leukemia stem cells, its role in adult hematopoietic stem cells (HSCs) remains underexplored. We demonstrate that conditional deletion of the Klf4 gene in hematopoietic cells led to an increased frequency of immunophenotypic HSCs in the bone marrow, along with a normal distribution of lymphoid and myeloid progenitor cells. Noncompetitive bone marrow transplants showed normal engraftment and multilineage reconstitution, except for monocytes and T cells. However, the loss of KLF4 hindered hematologic reconstitution in competitive serial bone marrow transplants, highlighting a critical role for KLF4 in stress-induced hematopoiesis. Transcriptome analysis revealed an upregulation of NFκB2 and toll-like receptors (e.g., TLR4) in Klf4-null HSCs during homeostasis. Flow cytometry and immunoblot analysis confirmed the increased cell surface expression of TLR4 and the activation of NFκB2 in HSCs under homeostatic conditions, whereas NFκB2 expression drops after radiation compared with steady-state levels. Our findings suggest that the constitutive activation of the TLR4-NFκB2 pathway inhibits the ability of HSCs to regenerate blood after transplantation in cytoablated bone marrow.

Tetrahydroxy stilbene glucoside rejuvenates aging hematopoietic stem cells with predilection for lymphoid differentiation via AMPK and Tet2

INTRODUCTION

Aging of hematopoietic stem cells (HSCs) has emerged as an important challenge to human health. Recent advances have raised the prospect of rejuvenating aging HSCs via specific medical interventions, including pharmacological treatments. Nonetheless, efforts to develop such drugs are still in infancy until now.

OBJECTIVES

We aimed to screen the prospective agents that can rejuvenate aging HSCs and explore the potential mechanisms.

METHODS

We screened a set of natural anti-aging compounds through oral administration to sub-lethally irradiated mice, and identified 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG) as a potent rejuvenating agent for aging HSCs. Then naturally aged mice were used for the follow-up assessment to determine the HSC rejuvenating potential of TSG. Finally, based on the transcriptome and DNA methylation analysis, we validated the role of the AMP-activated protein kinase (AMPK)-ten-eleven-translocation 2 (Tet2) axis (the AMPK-Tet2 axis) as the underlying mechanisms of TSG for ameliorating HSCs aging.

RESULTS

TSG treatment not only significantly increased the absolute number of common lymphoid progenitors (CLPs) along with B lymphocytes, but also boosted the HSCs/CLPs repopulation potential of aging mice. Further elaborated mechanism research demonstrated that TSG supplementation restored the stemness of aging HSCs, as well as promoted an epigenetic reprograming that was associated with an improved regenerative capacity and an increased rate of lymphopoiesis. Such effects were diminished when the mice were co-treated with an AMPK inhibitor, or when it was performed in Tet2 knockout mice as well as senescent cells assay.

CONCLUSION

TSG is effective in rejuvenating aging HSCs by modulating the AMPK- Tet2 axis and thus represents a potential candidate for developing effective HSC rejuvenating therapies.

Ontogeny Dictates Oncogenic Potential, Lineage Hierarchy, and Therapy Response in Pediatric Leukemia

Accumulating evidence links pediatric cancers to prenatal transformation events, yet the influence of the developmental stage on oncogenesis remains elusive. We investigated how hematopoietic stem cell developmental stages affect leukemic transformation, disease progression, and therapy response using a novel, humanized model of NUP98∷NSD1-driven pediatric acute myeloid leukemia, that is particularly aggressive with WT1 co-mutations. Fetal-derived hematopoietic stem cells readily transform into leukemia, and WT1 mutations further enhance stemness and alter lineage hierarchy. In contrast, stem cells from later developmental stages become progressively resistant to transformation. Single-cell analyses revealed that fetal-origin leukemia stem cells exhibit greater quiescence and reliance on oxidative phosphorylation than their postnatal counterparts. These differences drive distinct therapeutic responses, despite identical oncogenic mutations. In patients, onco-fetal transcriptional programs correlate with worse outcomes. By targeting key vulnerabilities of fetal-origin leukemia cells, we identified combination therapies that significantly reduce aggressiveness, highlighting the critical role of ontogeny in pediatric cancer treatment.

Reconsidering the usual suspects in age-related hematologic disorders: is stem cell dysfunction a root cause of aging?

Aging exerts a profound impact on the hematopoietic system, leading to increased susceptibility to infections, autoimmune diseases, chronic inflammation, anemia, thrombotic events, and hematologic malignancies. Within the field of experimental hematology, the functional decline of hematopoietic stem cells (HSCs) is often regarded as a primary driver of age-related hematologic conditions. However, aging is clearly a complex multifaceted process involving not only HSCs but also mature blood cells and their interactions with other tissues. This review reappraises an HSC-centric view of hematopoietic aging by exploring how the entire hematopoietic hierarchy, from stem cells to mature cells, contributes to age-related disorders. It highlights the decline of both innate and adaptive immunity, leading to increased susceptibility to infections and cancer, and the rise of autoimmunity as peripheral immune cells undergo aging-induced changes. It explores the concept of "inflammaging," where persistent, low-grade inflammation driven by old immune cells creates a cycle of tissue damage and disease. Additionally, this review delves into the roles of inflammation and homeostatic regulation in age-related conditions such as thrombotic events and anemia, arguing that these issues arise from broader dysfunctions rather than stemming from HSC functional attrition alone. In summary, this review highlights the importance of taking a holistic approach to studying hematopoietic aging and its related pathologies. By looking beyond just stem cells and considering the full spectrum of age-associated changes, one can better capture the complexity of aging and attempt to develop preventative or rejuvenation strategies that target multiple facets of this process.

Two distinct fetal-type signatures characterize juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is an aggressive clonal myeloproliferative neoplasm that affects infants and young children. The narrow window of onset suggests that age-related factors are involved in leukemogenesis. To investigate whether ontogeny-related features are involved in JMML oncogenesis, we compared the gene expression profile of hematopoietic progenitor cells isolated from JMML patients with that of healthy individuals at different stages of ontogeny. This analysis identified two main groups of JMML patients. In the first group, JMML progenitors exhibited a gene expression profile similar to that of embryo-fetal progenitors. Progenitors showed a strong monocytic identity as evidenced by the overexpression of monocytic/dendritic, inflammasome, and innate immune markers. This resembled the monocyte-predominant myelopoiesis characteristic of normal fetal hematopoiesis. However, in the second group, despite evidence of developmental dysregulation as indicated by the aberrant signature of the master oncofetal regulator LIN28B, JMML clustered separately from healthy prenatal and postnatal fractions. These findings highlight the intricate relationship between JMML and development, which will help inform future therapeutic approaches for this rare but severe form of leukemia.

How age affects human hematopoietic stem and progenitor cells and the strategies to mitigate aging

Hematopoietic stem cells (HSCs) are central to blood formation and play a pivotal role in hematopoietic and systemic aging. With aging, HSCs undergo significant functional changes, such as an increased stem cell pool, declined homing and reconstitution capacity, and skewed differentiation toward myeloid and megakaryocyte/platelet progenitors. These phenotypic alterations are likely due to the expansion of certain clones, known as clonal hematopoiesis (CH), which leads to disrupted hematopoietic homeostasis, including anemia, impaired immunity, higher risks of hematological malignancies, and even associations with cardiovascular disease, highlighting the broader impact of HSC aging on overall health. HSC aging is driven by a range of mechanisms involving both intrinsic and extrinsic factors, such as DNA damage accumulation, epigenetic remodeling, inflammaging and metabolic regulation. In this review, we summarize the updated understanding of age-related changes in hematopoietic stem and progenitor cells (HSPCs) and the mechanisms underlying the aging process in mammalian models, especially in human study. Additionally, we provide insights into potential therapeutic strategies to counteract aging process and enhance HSC regenerative capacity, which will support therapeutic interventions and promote healthy aging.

IFN alpha signaling drives hematopoietic stem cells malfunction under acute inflammation

Inflammation stimulation regulates the activity of hematopoietic stem cells (HSCs) through direct-sensing and cytokine-mediation. It is known that HSCs directly sense lipopolysaccharide (LPS), a classical infection-related inflammatory signal, via toll like receptor 4 (TLR4) and subsequently become active. However, the mechanism underlying the activity change of HSCs induced by LPS remains incompletely disclosed. Here we explored that under LPS stimulation, the activation of interferon alpha (IFNα) signal pathway resulted in the activation and exhaustion of HSCs in vitro, indicating HSCs directly responded to LPS through the downstream IFNα signal pathway. We also discovered the increased production of IFNα in mice bone marrow and expression of interferon-α/β receptor (IFNAR) on mice HSCs after LPS stimulation. Creatine, an IFNα inhibitor, could reverse the activation and prevent the exhaustion of HSCs caused by LPS by suppressing the expressions of genes associated with the IFNα signal pathway both in vitro and in vivo. Furthermore, we found that the IFNAR deficiency in mice effectively protected HSCs from activation, elevated apoptosis and impaired reconstitution ability under LPS stimulation in vivo. This finding further supports the notion that LPS activates and injures HSCs indirectly via promoting IFNα secretion in the bone marrow environment. Overall, our findings reveal that LPS causes the injury to HSCs either through direct or cytokine-mediated indirect activation of the IFNα signal pathway.

Unraveling the pathogenesis of bone marrow hematopoietic injury and the therapeutic potential of natural products

Bone marrow hematopoietic injury encompasses a range of pathological conditions that disrupt the normal function of the hematopoietic system, primarily through the impaired production and differentiation of bone marrow hematopoietic cells. Key pathogenic mechanisms include aging, radiation damage, chemical induction, infection and inflammation, and cross-talk with non-hematopoietic diseases. These pathological factors often lead to myelosuppression and myeloid skewing. Furthermore, we explored the potential and application prospects of natural products in the treatment of bone marrow hematopoietic injury. Natural products, particularly those derived from Chinese herbal medicines and other natural sources, have emerged as promising therapeutic options due to their distinctive mechanisms and minimal side effects. A deeper understanding of the underlying mechanisms of bone marrow hematopoietic injury could illuminate how natural products exert their effects, thereby optimizing treatment strategies and offering safer, more effective options for patients. Future research should leverage emerging technologies to further elucidate the composition and interactions within the bone marrow microenvironment, as well as the specific pathways through which natural products modulate hematopoietic dysfunction.

Bone Marrow Niche in Cardiometabolic Disease: Mechanisms and Therapeutic Potential

Cardiovascular and cardiometabolic diseases are leading causes of morbidity and mortality worldwide, driven in part by chronic inflammation. Emerging research suggests that the bone marrow microenvironment, or marrow niche, plays a critical role in both immune system regulation and disease progression. The bone marrow niche is essential for maintaining hematopoietic stem cells (HSCs) and orchestrating hematopoiesis. Under normal conditions, this niche ensures a return to immune homeostasis after acute stress. However, in the setting of inflammatory conditions such as those seen in cardiometabolic diseases, it becomes dysregulated, leading to enhanced myelopoiesis and immune activation. This review explores the reciprocal relationship between the bone marrow niche and cardiometabolic diseases, highlighting how alterations in the niche contribute to disease development and progression. The niche regulates HSCs through complex interactions with stromal cells, endothelial cells, and signaling molecules. However, in the setting of chronic diseases such as hypertension, atherosclerosis, and diabetes, inflammatory signals disrupt the balance between HSC self-renewal and differentiation, promoting the excessive production of proinflammatory myeloid cells that exacerbate the disease. Key mechanisms discussed include the effects of hyperlipidemia, hyperglycemia, and sympathetic nervous system activation on HSC proliferation and differentiation. Furthermore, the review emphasizes the role of epigenetic modifications and metabolic reprogramming in creating trained immunity, a phenomenon whereby HSCs acquire long-term proinflammatory characteristics that sustain disease states. Finally, we explore therapeutic strategies aimed at targeting the bone marrow niche to mitigate chronic inflammation and its sequelae. Novel interventions that modulate hematopoiesis and restore niche homeostasis hold promise for the treatment of cardiometabolic diseases. By interrupting the vicious cycle of inflammation and marrow dysregulation, such therapies may offer new avenues for reducing cardiovascular risk and improving patient outcomes.

Deciphering the Complexities of Adult Human Steady State and Stress-Induced Hematopoiesis: Progress and Challenges

Human hematopoietic stem cells (HSCs) have traditionally been viewed as self-renewing, multipotent cells with enormous potential in sustaining essential steady state blood and immune cell production throughout life. Indeed, around 86% (1011-1012) of new cells generated daily in a healthy young human adult are of hematopoietic origin. Therapeutically, human HSCs have contributed to over 1.5 million hematopoietic cell transplants (HCTs) globally, making this the most successful regenerative therapy to date. We will commence this review by briefly highlighting selected key achievements (from 1868 to the end of the 20th century) that have contributed to this accomplishment. Much of our knowledge of hematopoiesis is based on small animal models that, despite their enormous importance, do not always recapitulate human hematopoiesis. Given this, we will critically review the progress and challenges faced in identifying adult human HSCs and tracing their lineage differentiation trajectories, referring to murine studies as needed. Moving forward and given that human hematopoiesis is dynamic and can readily adjust to a variety of stressors, we will then discuss recent research advances contributing to understanding (i) which HSPCs maintain daily steady state human hematopoiesis, (ii) where these are located, and (iii) which mechanisms come into play when homeostatic hematopoiesis switches to stress-induced or emergency hematopoiesis.

The role of the haematopoietic stem cell niche in development and ageing

Blood production depends on rare haematopoietic stem cells (HSCs) and haematopoietic stem and progenitor cells (HSPCs) that ultimately take up residence in the bone marrow during development. HSPCs and HSCs are subject to extrinsic regulation by the bone marrow microenvironment, or niche. Studying the interactions between HSCs and their niche is critical for improving ex vivo culturing conditions and genetic manipulation of HSCs, which is pivotal for improving autologous HSC therapies and transplantations. Additionally, understanding how the complex molecular network in the bone marrow is altered during ageing is paramount for developing novel therapeutics for ageing-related haematopoietic disorders. HSCs are unique amongst stem and progenitor cell pools in that they engage with multiple physically distinct niches during their ontogeny. HSCs are specified from haemogenic endothelium in the aorta, migrate to the fetal liver and, ultimately, colonize their final niche in the bone marrow. Recent studies employing single-cell transcriptomics and microscopy have identified novel cellular interactions that govern HSC specification and engagement with their niches throughout ontogeny. New lineage-tracing models and microscopy tools have raised questions about the numbers of HSCs specified, as well as the functional consequences of HSCs interacting with each developmental niche. Advances have also been made in understanding how these niches are modified and perturbed during ageing, and the role of these altered interactions in haematopoietic diseases. In this Review, we discuss these new findings and highlight the questions that remain to be explored.

Molecular regulators of chemotaxis in human hematopoietic stem cells

Hematopoietic stem cells (HSCs), essential for lifelong blood cell regeneration, are clinically utilized to treat various hematological disorders. These cells originate in the aorta-gonad-mesonephros region, expand in the fetal liver, and mature in the bone marrow. Chemotaxis, involving gradient sensing, polarization, and migration, directs HSCs and is crucial for their homing and mobilization. The molecular regulation of HSC chemotaxis involves chemokines, chemokine receptors, signaling pathways, and cytoskeletal proteins. Recent advances in understanding these regulatory mechanisms have deepened insights into HSC development and hematopoiesis, offering new avenues for therapeutic innovations. Strategies including glucocorticoid receptor activation, modulation of histone acetylation, stimulation of nitric oxide signaling, and interference with m6A RNA modification have shown potential in enhancing CXCR4 expression, thereby improving the chemotactic response and homing capabilities of human HSCs. This review synthesizes current knowledge on the molecular regulation of human HSC chemotaxis and its implications for health and disease.

Deciphering the evolving niche interactome of human hematopoietic stem cells from ontogeny to aging

Hematopoietic stem cells (HSC) reside within specialized microenvironments that undergo dynamic changes throughout development and aging to support HSC function. However, the evolving cell-cell communication networks within these niches remain largely unexplored. This study integrates single-cell RNA sequencing datasets to systematically characterize the HSC niche interactome from ontogeny to aging. We reconstructed single-cell atlases of HSC niches at different developmental stages, revealing stage-specific cellular compositions and interactions targeting HSC. During HSC maturation, our analysis identified distinct patterns of ligand-receptor interactions and signaling pathways that govern HSC emergence, expansion, and maintenance. HSC aging was accompanied by a decrease in supportive niche interactions, followed by an adaptive increase in interaction strength in old adult bone marrow. This complex aging process involved the emergence of interactions associated with inflammation, altered stem cell function, and a decline in the efficacy of key signaling pathways. Our findings provide a comprehensive understanding of the dynamic remodeling of the HSC niche interactome throughout life, paving the way for targeted interventions to maintain HSC function and promote healthy aging. This study offers valuable insights into the intricate cell-cell communication networks that govern HSC behavior and fate, with implications for hematological disorders and regenerative medicine.

Platelet Factor 4 (PF4) Regulates Hematopoietic Stem Cell Aging

Hematopoietic stem cells (HSCs) responsible for blood cell production and their bone marrow regulatory niches undergo age-related changes, impacting immune responses and predisposing individuals to hematologic malignancies. Here, we show that the age-related alterations of the megakaryocytic niche and associated downregulation of Platelet Factor 4 (PF4) are pivotal mechanisms driving HSC aging. PF4-deficient mice display several phenotypes reminiscent of accelerated HSC aging, including lymphopenia, increased myeloid output, and DNA damage, mimicking physiologically aged HSCs. Remarkably, recombinant PF4 administration restored old HSCs to youthful functional phenotypes characterized by improved cell polarity, reduced DNA damage, enhanced in vivo reconstitution capacity, and balanced lineage output. Mechanistically, we identified LDLR and CXCR3 as the HSC receptors transmitting the PF4 signal, with double knockout mice showing exacerbated HSC aging phenotypes similar to PF4-deficient mice. Furthermore, human HSCs across various age groups also respond to the youthful PF4 signaling, highlighting its potential for rejuvenating aged hematopoietic systems. These findings pave the way for targeted therapies aimed at reversing age-related HSC decline with potential implications in the prevention or improvement of the course of age-related hematopoietic diseases.

Key Points

Age-related attrition of the megakaryocytic niche and associated PF4 downregulation is a central mechanism in HSC aging.PF4 supplementation, acting on LDLR and CXCR3 receptors, rejuvenates the function of aged HSCs.

Delivery of Stem Cell-Rejuvenating Compounds via Subcutaneous Osmotic Pumps

Aging is widely regarded as an irreversible physiological process throughout the mammalian lifespan, characterized by functional tissue deterioration and increased disease incidence. One hallmark of mammalian aging is reduced tissue regeneration, primarily attributed to the declining function of tissue-specific stem cells. In recent years, various strategies aimed at rejuvenating stem cells through drug-delivery systems have been extensively explored. Here we describe a method for the long-term, controlled, systemic delivery of drugs using subcutaneous implantations of osmotic pumps.

A life-time of hematopoietic cell function: ascent, stability, and decline

Aging is a set of complex processes that occur temporally and continuously. It is generally a unidirectional progression of cellular and molecular changes occurring during the life stages of cells, tissues and ultimately the whole organism. In vertebrate organisms, this begins at conception from the first steps in blastocyst formation, gastrulation, germ layer differentiation, and organogenesis to a continuum of embryonic, fetal, adolescent, adult, and geriatric stages. Tales of the "fountain of youth" and songs of being "forever young" are dominant ideas informing us that growing old is something science should strive to counteract. Here, we discuss the normal life stages of the blood system, particularly the historical recognition of its importance in the early growth stages of vertebrates, and what this means with respect to progressive gain and loss of hematopoietic function in the adult.

Mitochondrial serine catabolism safeguards maintenance of the hematopoietic stem cell pool in homeostasis and injury

Hematopoietic stem cells (HSCs) employ a very unique metabolic pattern to maintain themselves, while the spectrum of their metabolic adaptations remains incompletely understood. Here, we uncover a distinct and heterogeneous serine metabolism within HSCs and identify mouse HSCs as a serine auxotroph whose maintenance relies on exogenous serine and the ensuing mitochondrial serine catabolism driven by the hydroxymethyltransferase 2 (SHMT2)-methylene-tetrahydrofolate dehydrogenase 2 (MTHFD2) axis. Mitochondrial serine catabolism primarily feeds NAD(P)H generation to maintain redox balance and thereby diminishes ferroptosis susceptibility of HSCs. Dietary serine deficiency, or genetic or pharmacological inhibition of the SHMT2-MTHFD2 axis, increases ferroptosis susceptibility of HSCs, leading to impaired maintenance of the HSC pool. Moreover, exogenous serine protects HSCs from irradiation-induced myelosuppressive injury by fueling mitochondrial serine catabolism to mitigate ferroptosis. These findings reframe the canonical view of serine from a nonessential amino acid to an essential niche metabolite for HSC pool maintenance.

Childhood B cell leukemia: Intercepting the paths to progression

B-cell Acute Lymphoblastic Leukemia (B-ALL) is the most common pediatric cancer, arising most often in children aged 2-5 years. This distinctive age distribution hints at an association between B-ALL development and disrupted immune system function during a susceptible period during childhood, possibly triggered by early exposure to infection. While cure rates for childhood B-ALL surpass 90% in high-income nations, survivors suffer from diminished quality of life due to the side effects of treatment. Consequently, understanding the origins and evolution of B-ALL, and how to prevent this prevalent childhood cancer, is paramount to alleviate this substantial health burden. This article provides an overview of our current understanding of the etiology of childhood B-ALL and explores how this knowledge can inform preventive strategies.

Sex-dependent niche responses modulate steady-state and regenerative hematopoiesis

Hematopoietic stem cells (HSCs) adapt to organismal blood production needs by balancing self-renewal and differentiation, adjusting to physiological demands and external stimuli. Although sex differences have been implicated in differential hematopoietic function in males versus females, the mediators responsible for these effects require further study. Here, we characterized hematopoiesis at a steady state and during regeneration following hematopoietic stem cell transplantation (HST). RNA sequencing of lineage(-) bone marrow cells from C57/Bl6 mice revealed a broad transcriptional similarity between the sexes. However, we identified distinct sex differences in key biological pathways, with female cells showing reduced expression of signatures involved in inflammation and enrichment of genes related to glycolysis, hypoxia, and cell cycle regulation, suggesting a more quiescent and less inflammatory profile compared with male cells. To determine the functional impacts of the observed transcriptomic differences, we performed sex-matched and mismatched transplantation studies of lineage(-) donor cells. During short-term 56-day HST recovery, we found a male donor cell proliferative advantage, coinciding with elevated serum TNF-α, and a male recipient engraftment advantage, coinciding with increased serum CXCL12. Together, we show that sex-specific cell responses, marked by differing expression of pathways regulating metabolism, hypoxia, and inflammation, shape normal and regenerative hematopoiesis, with implications for the clinical understanding of hematopoietic function.

Ageing-related bone and immunity changes: insights into the complex interplay between the skeleton and the immune system

Ageing as a natural irreversible process inherently results in the functional deterioration of numerous organ systems and tissues, including the skeletal and immune systems. Recent studies have elucidated the intricate bidirectional interactions between these two systems. In this review, we provide a comprehensive synthesis of molecular mechanisms of cell ageing. We further discuss how age-related skeletal changes influence the immune system and the consequent impact of immune system alterations on the skeletal system. Finally, we highlight the clinical implications of these findings and propose potential strategies to promote healthy ageing and reduce pathologic deterioration of both the skeletal and immune systems.

SDHAF1 confers metabolic resilience to aging hematopoietic stem cells by promoting mitochondrial ATP production

Aging generally predisposes stem cells to functional decline, impairing tissue homeostasis. Here, we report that hematopoietic stem cells (HSCs) acquire metabolic resilience that promotes cell survival. High-resolution real-time ATP analysis with glucose tracing and metabolic flux analysis revealed that old HSCs reprogram their metabolism to activate the pentose phosphate pathway (PPP), becoming more resistant to oxidative stress and less dependent on glycolytic ATP production at steady state. As a result, old HSCs can survive without glycolysis, adapting to the physiological cytokine environment in bone marrow. Mechanistically, old HSCs enhance mitochondrial complex II metabolism during stress to promote ATP production. Furthermore, increased succinate dehydrogenase assembly factor 1 (SDHAF1) in old HSCs, induced by physiological low-concentration thrombopoietin (TPO) exposure, enables rapid mitochondrial ATP production upon metabolic stress, thereby improving survival. This study provides insight into the acquisition of resilience through metabolic reprogramming in old HSCs and its molecular basis to ameliorate age-related hematopoietic abnormalities.

Autophagy counters inflammation-driven glycolytic impairment in aging hematopoietic stem cells

Autophagy is central to the benefits of longevity signaling programs and to hematopoietic stem cell (HSC) response to nutrient stress. With age, a subset of HSCs increases autophagy flux and preserves regenerative capacity, but the signals triggering autophagy and maintaining the functionality of autophagy-activated old HSCs (oHSCs) remain unknown. Here, we demonstrate that autophagy is an adaptive cytoprotective response to chronic inflammation in the aging murine bone marrow (BM) niche. We find that inflammation impairs glucose uptake and suppresses glycolysis in oHSCs through Socs3-mediated inhibition of AKT/FoxO-dependent signaling, with inflammation-mediated autophagy engagement preserving functional quiescence by enabling metabolic adaptation to glycolytic impairment. Moreover, we show that transient autophagy induction via a short-term fasting/refeeding paradigm normalizes glycolytic flux and significantly boosts oHSC regenerative potential. Our results identify inflammation-driven glucose hypometabolism as a key driver of HSC dysfunction with age and establish autophagy as a targetable node to reset oHSC regenerative capacity.

Bone marrow niches for hematopoietic stem cells: life span dynamics and adaptation to acute stress

ABSTRACT

Hematopoietic stem cells (HSCs) are instrumental for organismal survival because they are responsible for lifelong production of mature blood lineages in homeostasis and response to external stress. To fulfill their function, HSCs rely on reciprocal interactions with specialized tissue microenvironments, termed HSC niches. From embryonic development to advanced aging, HSCs transition through several hematopoietic organs in which they are supported by distinct extrinsic cues. Here, we describe recent discoveries on how HSC niches collectively adapt to ensure robust hematopoietic function during biological aging and after exposure to acute stress. We also discuss the latest strategies leveraging niche-derived signals to revert aging-associated phenotypes and enhance hematopoietic recovery after myeloablation.

Ex Vivo Evaluation of the Function of Hematopoietic Stem Cells in Toxicology of Metals

A variety of metals, e.g., lead (Pb), cadmium (Cd), and lithium (Li), are in the environment and are toxic to humans. Hematopoietic stem cells (HSCs) reside at the apex of hematopoiesis and are capable of generating all kinds of blood cells and self-renew to maintain the HSC pool. HSCs are sensitive to environmental stimuli. Metals may influence the function of HSCs by directly acting on HSCs or indirectly by affecting the surrounding microenvironment for HSCs in the bone marrow (BM) or niche, including cellular and extracellular components. Investigating the impact of direct and/or indirect actions of metals on HSCs contributes to the understanding of immunological and hematopoietic toxicology of metals. Treatment of HSCs with metals ex vivo, and the ensuing HSC transplantation assays, are useful for evaluating the impacts of the direct actions of metals on the function of HSCs. Investigating the mechanisms involved, given the rarity of HSCs, methods that require large numbers of cells are not suitable for signal screening; however, flow cytometry is a useful tool for signal screening HSCs. After targeting signaling pathways, interventions ex vivo and HSCs transplantation are required to confirm the roles of the signaling pathways in regulating the function of HSCs exposed to metals. Here, we describe protocols to evaluate the mechanisms of direct and indirect action of metals on HSCs. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Identify the impact of a metal on the competence of HSCs Basic Protocol 2: Identify the impact of a metal on the lineage bias of HSC differentiation Basic Protocol 3: Screen the potential signaling molecules in HSCs during metal exposure Alternate Protocol 1: Ex vivo treatment with a metal on purified HSCs Alternate Protocol 2: Ex vivo intervention of the signaling pathway regulating the function of HSCs during metal exposure.

Epigenetics of hematopoietic stem cell aging

PURPOSE OF REVIEW

The development of new antiaging medicines is of great interest to the current elderly and aging population. Aging of the hematopoietic system is attributed to the aging of hematopoietic stem cells (HSCs), and epigenetic alterations are the key effectors driving HSC aging. Understanding the epigenetics of HSC aging holds promise of providing new insights for combating HSC aging and age-related hematological malignancies.

RECENT FINDINGS

Aging is characterized by the progressive loss of physiological integrity, leading to impaired function and increased vulnerability to death. During aging, the HSCs undergo both quantitative and qualitative changes. These functional changes in HSCs cause dysregulated hematopoiesis, resulting in anemia, immune dysfunction, and an increased risk of hematological malignancies. Various cell-intrinsic and cell-extrinsic effectors influencing HSC aging have also been identified. Epigenetic alterations are one such mechanism.

SUMMARY

Cumulative epigenetic alterations in aged HSCs affect their fate, leading to aberrant self-renewal, differentiation, and function of aged HSCs. In turn, these factors provide an opportunity for aged HSCs to expand by modulating their self-renewal and differentiation balance, thereby contributing to the development of hematological malignancies.

Lethal and sublethal effects of programmed cell death pathways on hematopoietic stem cells

Programmed cell death is an evolutionally conserved cellular process in multicellular organisms that eliminates unnecessary or rogue cells during development, infection, and carcinogenesis. Hematopoietic stem cells (HSCs) are a rare, self-renewing, and multipotent cell population necessary for the establishment and regeneration of the hematopoietic system. Counterintuitively, key components necessary for programmed cell death induction are abundantly expressed in long-lived HSCs, which often survive myeloablative stress by engaging a prosurvival response that counteracts cell death-inducing stimuli. Although HSCs are well known for their apoptosis resistance, recent studies have revealed their unique vulnerability to certain types of programmed necrosis, such as necroptosis and ferroptosis. Moreover, emerging evidence has shown that programmed cell death pathways can be sublethally activated to cause nonlethal consequences such as innate immune response, organelle dysfunction, and mutagenesis. In this review, we summarized recent findings on how divergent cell death programs are molecularly regulated in HSCs. We then discussed potential side effects caused by sublethal activation of programmed cell death pathways on the functionality of surviving HSCs.

From Marrow to Bone and Fat: Exploring the Multifaceted Roles of Leptin Receptor Positive Bone Marrow Mesenchymal Stromal Cells

The bone marrow (BM) stromal cell microenvironment contains non-hematopoietic stromal cells called mesenchymal stromal cells (MSCs). MSCs are plastic adherent, form CFU-Fs, and give rise to osteogenic, adipogenic, chondrogenic progenitors, and most importantly provide HSC niche factor chemokine C-X-C motif ligand 12 (CXCL12) and stem cell factor (SCF). Different authors have defined different markers for mouse MSC identification like PDGFR+Sca-1+ subsets, Nestin+, or LepR+ cells. Of these, the LepR+ cells are the major source of SCF and CXCL12 in the BM microenvironment and play a major role in HSC maintenance and hematopoiesis. LepR+ cells give rise to most of the bones and BM adipocytes, further regulating the microenvironment. In adult BM, LepR+ cells are quiescent but after fracture or irradiation, they proliferate and differentiate into mesenchymal lineage osteogenic, adipogenic and/or chondrogenic cells. They also play a crucial role in the steady-state hematopoiesis process, as well as hematopoietic regeneration and the homing of hematopoietic stem cells (HSCs) after myeloablative injury and/or HSC transplantation. They line the sinusoidal cavities, maintain the trabeculae formation, and provide the space for HSC homing and retention. However, the LepR+ cell subset is heterogeneous; some subsets have higher adipogenic potential, while others express osteollineage-biased genes. Different transcription factors like Early B cell factor 3 (EBF3) or RunX2 help maintain this balance between the self-renewing and committed states, whether osteogenic or adipogenic. The study of LepR+ MSCs holds immense promise for advancing our understanding of HSC biology, tissue regeneration, metabolic disorders, and immune responses. In this review, we will discuss the origin of the BM resident LepR+ cells, different subtypes, and the role of LepR+ cells in maintaining hematopoiesis, osteogenesis, and BM adipogenesis following their multifaceted impact.

Complementary and Inducible creERT2 Mouse Models for Functional Evaluation of Endothelial Cell Subtypes in the Bone Marrow

In the adult bone marrow (BM), endothelial cells (ECs) are an integral component of the hematopoietic stem cell (HSC)-supportive niche, which modulates HSC activity by producing secreted and membrane-bound paracrine signals. Within the BM, distinct vascular arteriole, transitional, and sinusoidal EC subtypes display unique paracrine expression profiles and create anatomically-discrete microenvironments. However, the relative contributions of vascular endothelial subtypes in supporting hematopoiesis is unclear. Moreover, constitutive expression and off-target activity of currently available endothelial-specific and endothelial-subtype-specific murine cre lines potentially confound data analysis and interpretation. To address this, we describe two tamoxifen-inducible cre-expressing lines, Vegfr3-creERT2 and Cx40-creERT2, that efficiently label sinusoidal/transitional and arteriole endothelium respectively in adult marrow, without off-target activity in hematopoietic or perivascular cells. Utilizing an established mouse model in which cre-dependent recombination constitutively-activates MAPK signaling within adult endothelium, we identify arteriole ECs as the driver of MAPK-mediated hematopoietic dysfunction. These results define complementary tamoxifen-inducible creERT2-expressing mouse lines that label functionally-discrete and non-overlapping sinusoidal/transitional and arteriole EC populations in the adult BM, providing a robust toolset to investigate the differential contributions of vascular subtypes in maintaining hematopoietic homeostasis.

Hematopoietic Stem Cells as an Integrative Hub Linking Lifestyle to Cardiovascular Health

Despite breakthroughs in modern medical care, the incidence of cardiovascular disease (CVD) is even more prevalent globally. Increasing epidemiologic evidence indicates that emerging cardiovascular risk factors arising from the modern lifestyle, including psychosocial stress, sleep problems, unhealthy diet patterns, physical inactivity/sedentary behavior, alcohol consumption, and tobacco smoking, contribute significantly to this worldwide epidemic, while its underpinning mechanisms are enigmatic. Hematological and immune systems were recently demonstrated to play integrative roles in linking lifestyle to cardiovascular health. In particular, alterations in hematopoietic stem cell (HSC) homeostasis, which is usually characterized by proliferation, expansion, mobilization, megakaryocyte/myeloid-biased differentiation, and/or the pro-inflammatory priming of HSCs, have been shown to be involved in the persistent overproduction of pro-inflammatory myeloid leukocytes and platelets, the cellular protagonists of cardiovascular inflammation and thrombosis, respectively. Furthermore, certain lifestyle factors, such as a healthy diet pattern and physical exercise, have been documented to exert cardiovascular protective effects through promoting quiescence, bone marrow retention, balanced differentiation, and/or the anti-inflammatory priming of HSCs. Here, we review the current understanding of and progression in research on the mechanistic interrelationships among lifestyle, HSC homeostasis, and cardiovascular health. Given that adhering to a healthy lifestyle has become a mainstream primary preventative approach to lowering the cardiovascular burden, unmasking the causal links between lifestyle and cardiovascular health from the perspective of hematopoiesis would open new opportunities to prevent and treat CVD in the present age.

The Chaperone Protein Cct5 is Essential for Hematopoietic Stem Cell Maintenance

Proper proteostasis is indispensable for the long-term maintenance of hematopoietic stem and progenitor cells (HSPCs). The TRiC/CCT (chaperonin-containing TCP-1) complex, a key constituent of cellular machinery facilitating accurate protein folding, has remained enigmatic in its specific function within HSPCs. Here we show that conditional knockout (KO) of Cct5 significantly impairs the maintenance of murine HSPCs. Primary and secondary transplantation experiments unequivocally demonstrate the incapacity of Cct5 KO HSPCs to reconstitute both myeloid and lymphoid lineage cells in recipient mice, highlighting the pivotal role of the TRiC/CCT complex in governing these cellular lineages. Furthermore, leveraging an integrated approach that merges a Protein-Protein Interaction (PPI) database with RNA sequencing (RNA-seq) data of HSPCs, our analysis reveals intricate interactions between Cct5 and vital transcription factors crucial for HSC homeostasis. Notably, Cct5 engages with MYC, PIAS1, TP53, ESR1, HOXA1, and JUN, intricately regulating the transcriptional landscape governing autophagy, myeloid differentiation, and cytoskeleton organization within HSPCs. Our study unveils the profound significance of TRiC/CCT complex-mediated proteostasis in orchestrating the maintenance and functionality of HSPCs.

KBTBD4-mediated reduction of MYC is critical for hematopoietic stem cell expansion upon UM171 treatment

ABSTRACT

Ex vivo expansion of hematopoietic stem cells (HSCs) is gaining importance for cell and gene therapy, and requires a shift from dormancy state to activation and cycling. However, abnormal or excessive HSC activation results in reduced self-renewal ability and increased propensity for myeloid-biased differentiation. We now report that activation of the E3 ligase complex CRL3KBTBD4 by UM171 not only induces epigenetic changes through CoREST1 degradation but also controls chromatin-bound master regulator of cell cycle entry and proliferative metabolism (MYC) levels to prevent excessive activation and maintain lympho-myeloid potential of expanded populations. Furthermore, reconstitution activity and multipotency of UM171-treated HSCs are specifically compromised when MYC levels are experimentally increased despite degradation of CoREST1.