The many faces of hematopoietic stem cell heterogeneity

Runx1+ vascular smooth muscle cells are essential for hematopoietic stem and progenitor cell development in vivo

Hematopoietic stem cells (HSCs) produce all essential cellular components of the blood. Stromal cell lines supporting HSCs follow a vascular smooth muscle cell (vSMC) differentiation pathway, suggesting that some hematopoiesis-supporting cells originate from vSMC precursors. These pericyte-like precursors were recently identified in the aorta-gonad-mesonephros (AGM) region; however, their role in the hematopoietic development in vivo remains unknown. Here, we identify a subpopulation of NG2+Runx1+ perivascular cells that display a sclerotome-derived vSMC transcriptomic profile. We show that deleting Runx1 in NG2+ cells impairs the hematopoietic development in vivo and causes transcriptional changes in pericytes/vSMCs, endothelial cells and hematopoietic cells in the murine AGM. Importantly, this deletion leads also to a significant reduction of HSC reconstitution potential in the bone marrow in vivo. This defect is developmental, as NG2+Runx1+ cells were not detected in the adult bone marrow, demonstrating the existence of a specialised pericyte population in the HSC-generating niche, unique to the embryo.

Stress-protecting harbors for hematopoietic stem cells

Hematopoietic stem cells (HSCs) rely on specialized microenvironments known as niches to maintain their self-renewal and multilineage potential to generate diverse types of blood cells continuously. Over the last two decades, substantial advancements have been made in unraveling the niche cell components and HSC localizations under homeostatic and stressed circumstances. Advances in imaging, combined with the discovery of phenotypic surface markers combinations and single cell sequencing, have greatly facilitated the systematic examination of HSC localizations. This review aims to present a summary of HSC localizations, highlighting potential distinctions between phenotypically and functionally defined HSCs, and explore the functionality of niches in ensuring the integrity and long-term maintenance of HSCs.

Hematopoietic stem cell heterogeneity in non-human primates revealed by five-lineage output bias analysis

Understanding hematopoietic stem cell (HSC) heterogeneity is crucial for treating malignant blood disorders. Compared with mice, we have limited knowledge of the heterogeneity of human HSCs. Fortunately, non-human primates (NHPs) have become the best animal models for studying human HSCs. Here, we employed a public dataset derived from NHP autologous bone marrow transplantation, and focused on a total of 820 HSC clones with reconstitution capacity of all available five lineages (granulocyte, monocyte, B cell, T cell, and natural killer cell) at two time points (11/12 and/or 42/43 months). Intriguingly, unsupervised clustering on these clones revealed six HSC subtypes, including a lymphoid/myeloid balanced (LM-balanced) subtype and five single-lineage-biased subtypes. We also observed that the subtypes of these HSC clones might change over time, and a given subtype could transition into any one of the other five subtypes, albeit with a certain degree of selectivity. Particularly, each of the six subtypes was more likely to turn into lymphoid-biased rather than myeloid-biased ones. Additionally, our five-lineage classification method exhibited strong correlation with traditional lymphoid/myeloid bias classification method. Specifically, our granulocyte- and monocyte-biased subtypes were predominantly attributed to α-HSCs, while LM-balanced, B cell-biased, and T cell-biased subtypes were primarily associated with β-HSCs. The γ-HSCs were composed of a small subset of B cell-biased and T cell-biased subtypes. In summary, our five-lineage classification identifies more finely tuned HSC subtypes based on lineage output bias. These findings enrich our understanding of HSC heterogeneity in NHPs and provide important insights for human research.

Zeb1 Regulates the Function of Lympho-Myeloid Primed Progenitors after Transplantation

Zeb1, a zinc finger E-box binding homeobox epithelial-mesenchymal (EMT) transcription factor, acts as a critical regulator of hematopoietic stem cell (HSC) self-renewal and multi-lineage differentiation. Whether Zeb1 directly regulates the function of multi-potent progenitors primed for hematopoietic lineage commitment remains ill defined. By using an inducible Mx-1 Cre conditional mouse model where Zeb1 was genetically engineered to be deficient in the adult hematopoietic system (hereafter Zeb1-/-), we found that the absolute cell number of immunophenotypically defined lympho-myeloid primed progenitors (LMPPs) from Zeb1-/- mice was reduced. Myeloid- and lymphoid-biased HSCs in Zeb1-/- mice were unchanged, implying that defective LMPP generation from Zeb1-/- mice was not directly caused by an imbalance of lineage-biased HSCs. Functional analysis of LMPP from Zeb1-/- mice, as judged by competitive transplantation, revealed an overall reduction in engraftment to hematopoietic organs over 4 weeks, which correlated with minimal T-cell engraftment, reduced B-cell and monocyte/macrophage engraftment, and unperturbed granulocyte engraftment. Thus, Zeb1 regulates LMPP differentiation potential to select lympho-myeloid lineages in the context of transplantation.

Haematopoietic stem and progenitor cell heterogeneity is inherited from the embryonic endothelium

Definitive haematopoietic stem and progenitor cells (HSPCs) generate erythroid, lymphoid and myeloid lineages. HSPCs are produced in the embryo via transdifferentiation of haemogenic endothelial cells in the aorta-gonad-mesonephros (AGM). HSPCs in the AGM are heterogeneous in differentiation and proliferative output, but how these intrinsic differences are acquired remains unanswered. Here we discovered that loss of microRNA (miR)-128 in zebrafish leads to an expansion of HSPCs in the AGM with different cell cycle states and a skew towards erythroid and lymphoid progenitors. Manipulating miR-128 in differentiating haemogenic endothelial cells, before their transition to HSPCs, recapitulated the lineage skewing in both zebrafish and human pluripotent stem cells. miR-128 promotes Wnt and Notch signalling in the AGM via post-transcriptional repression of the Wnt inhibitor csnk1a1 and the Notch ligand jag1b. De-repression of cskn1a1 resulted in replicative and erythroid-biased HSPCs, whereas de-repression of jag1b resulted in G2/M and lymphoid-biased HSPCs with long-term consequence on the respective blood lineages. We propose that HSPC heterogeneity arises in the AGM endothelium and is programmed in part by Wnt and Notch signalling.

Linking Benzene, in Utero Carcinogenicity and Fetal Hematopoietic Stem Cell Niches: A Mechanistic Review

Previous research reported that prolonged benzene exposure during in utero fetal development causes greater fetal abnormalities than in adult-stage exposure. This phenomenon increases the risk for disease development at the fetal stage, particularly carcinogenesis, which is mainly associated with hematological malignancies. Benzene has been reported to potentially act via multiple modes of action that target the hematopoietic stem cell (HSCs) niche, a complex microenvironment in which HSCs and multilineage hematopoietic stem and progenitor cells (HSPCs) reside. Oxidative stress, chromosomal aberration and epigenetic modification are among the known mechanisms mediating benzene-induced genetic and epigenetic modification in fetal stem cells leading to in utero carcinogenesis. Hence, it is crucial to monitor exposure to carcinogenic benzene via environmental, occupational or lifestyle factors among pregnant women. Benzene is a well-known cause of adult leukemia. However, proof of benzene involvement with childhood leukemia remains scarce despite previously reported research linking incidences of hematological disorders and maternal benzene exposure. Furthermore, accumulating evidence has shown that maternal benzene exposure is able to alter the developmental and functional properties of HSPCs, leading to hematological disorders in fetus and children. Since HSPCs are parental blood cells that regulate hematopoiesis during the fetal and adult stages, benzene exposure that targets HSPCs may induce damage to the population and trigger the development of hematological diseases. Therefore, the mechanism of in utero carcinogenicity by benzene in targeting fetal HSPCs is the primary focus of this review.

The (intra-aortic) hematopoietic cluster cocktail: what is in the mix?

The adult-definitive hematopoietic hierarchy and hematopoietic stem cells (HSCs) residing in the bone marrow are established during embryonic development. In mouse, human, and many other mammals, it is the sudden formation of so-called intra-aortic/arterial hematopoietic clusters (IAHCs) that best signifies and visualizes this de novo generation of HSCs and hematopoietic progenitor cells (HPCs). Cluster cells arise through an endothelial-to-hematopoietic transition and, for some time, express markers/genes of both tissue types, whilst acquiring more hematopoietic features and losing endothelial ones. Among several hundreds of IAHC cells, the midgestation mouse embryo contains only very few bona fide adult-repopulating HSCs, suggestive of a challenging cell fate to achieve. Most others are HPCs of various types, some of which have the potential to mature into HSCs in vitro. Based on the number of cells that reveal hematopoietic function, a fraction of IAHC cells is uncharacterized. This review aims to explore the current state of knowledge on IAHC cells. We will describe markers useful for isolation and characterization of these fleetingly produced, yet vitally important, cells and for the refined enrichment of the HSCs they contain, and speculate on the role of some IAHC cells that are as-yet functionally uncharacterized.

Hematopoietic Stem Cell Development in Mammalian Embryos

Hematopoietic stem cells (HSCs) are situated at the top of the adult hematopoietic hierarchy in mammals and give rise to the majority of blood cells throughout life. Recently, with the advance of multiple single-cell technologies, researchers have unprecedentedly deciphered the cellular and molecular evolution, the lineage relationships, and the regulatory mechanisms underlying HSC emergence in mammals. In this review, we describe the precise vascular origin of HSCs in mouse and human embryos, emphasizing the conservation in the unambiguous arterial characteristics of the HSC-primed hemogenic endothelial cells (HECs). Serving as the immediate progeny of some HECs, functional pre-HSCs of mouse embryos can now be isolated at single-cell level using defined surface marker combinations. Heterogeneity regrading cell cycle status or lineage differentiation bias within HECs, pre-HSCs, or emerging HSCs in mouse embryos has been figured out. Several epigenetic regulatory mechanisms of HSC generation, including long noncoding RNA, DNA methylation modification, RNA splicing, and layered epigenetic modifications, have also been recently uncovered. In addition to that of HSCs, the cellular and molecular events underlying the development of multiple hematopoietic progenitors in human embryos/fetus have been unraveled with the use of series of single-cell technologies. Specifically, yolk sac-derived myeloid-biased progenitors have been identified as the earliest multipotent hematopoietic progenitors in human embryo, serving as an important origin of fetal liver monocyte-derived macrophages. Moreover, the development of multiple hematopoietic lineages in human embryos such as T and B lymphocytes, innate lymphoid cells, as well as myeloid cells like monocytes, macrophages, erythrocytes, and megakaryocytes has also been depicted and reviewed here.

Immune System Influence on Hematopoietic Stem Cells and Leukemia Development

Hematopoietic stem cells (HSCs) are the source for all blood cells, including immune cells, and they interact dynamically with the immune system. This chapter will explore the nature of stem cells, particularly HSCs, in the context of their immune microenvironment. The dynamic interactions between stem cells and the immune system can have profound implications for current and future therapies, particularly regarding a potential "immune-privileged" HSC microenvironment. Immune/stem cell interactions change during times of stress and injury. Recent advances in cancer immunotherapy have overturned the long-standing belief that, being derived from the self, cancer cells should be immunotolerant. Instead, an immunosurveillance system recognizes and eliminates emergent pre-cancerous cells. Only in the context of a failing immunosurveillance system does cancer fully develop. Combined with the knowledge that stem cells or their unique properties can be critically important for cancer initiation, persistence, and resistance to therapy, understanding the unique immune properties of stem cells will be critical for the development of future cancer therapies. Accordingly, the therapeutic implications for leukemic stem cells (LSCs) inheriting an immune-privileged state from HSCs will be discussed. Through their dynamic interactions with a diverse immune system, stem cells serve as the light and dark root of cancer prevention vs. development.

Stemness-related genes revealed by single-cell profiling of naïve and stimulated human CD34+ cells from CB and mPB

BACKGROUND

Hematopoietic stem cells (HSCs) from different sources show varied repopulating capacity, and HSCs lose their stemness after long-time ex vivo culture. A deep understanding of these phenomena may provide helpful insights for HSCs.

METHODS

Here, we applied single-cell RNA-seq (scRNA-seq) to analyse the naïve and stimulated human CD34+ cells from cord blood (CB) and mobilised peripheral blood (mPB).

RESULTS

We collected over 16 000 high-quality single-cell data to construct a comprehensive inference map and characterised the HSCs under a quiescent state on the hierarchy top. Then, we compared HSCs in CB with those in mPB and HSCs of naïve samples to those of cultured samples, and identified stemness-related genes (SRGs) associated with cell source (CS-SRGs) and culture time (CT-SRGs), respectively. Interestingly, CS-SRGs and CT-SRGs share genes enriched in the signalling pathways such as mRNA catabolic process, translational initiation, ribonucleoprotein complex biogenesis and cotranslational protein targeting to membrane, suggesting dynamic protein translation and processing may be a common requirement for stemness maintenance. Meanwhile, CT-SRGs are enriched in pathways involved in glucocorticoid and corticosteroid response that affect HSCs homing and engraftment. In contrast, CS-SRGs specifically contain genes related to purine and ATP metabolic process, which is crucial for HSC homeostasis in the stress settings. Particularly, when CT-SRGs are used as reference genes for the construction of the development trajectory of CD34+ cells, lymphoid and myeloid lineages are clearly separated after HSCs/MPPs. Finally, we presented an application through a small-scale drug screening using Connectivity Map (CMap) against CT-SRGs. A small molecule, cucurbitacin I, was found to efficiently expand HSCs ex vivo while maintaining its stemness.

CONCLUSIONS

Our findings provide new perspectives for understanding HSCs, and the strategy to identify candidate molecules through SRGs may be applicable to study other stem cells.

PDGFRβ+ cells play a dual role as hematopoietic precursors and niche cells during mouse ontogeny

Hematopoietic stem cell (HSC) generation in the aorta-gonad-mesonephros region requires HSC specification signals from the surrounding microenvironment. In zebrafish, PDGF-B/PDGFRβ signaling controls hematopoietic stem/progenitor cell (HSPC) generation and is required in the HSC specification niche. Little is known about murine HSPC specification in vivo and whether PDGF-B/PDGFRβ is involved. Here, we show that PDGFRβ is expressed in distinct perivascular stromal cell layers surrounding the mid-gestation dorsal aorta, and its deletion impairs hematopoiesis. We demonstrate that PDGFRβ⁺ cells play a dual role in murine hematopoiesis. They act in the aortic niche to support HSPCs, and in addition, PDGFRβ⁺ embryonic precursors give rise to a subset of HSPCs that persist into adulthood. These findings provide crucial information for the controlled production of HSPCs in vitro.

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PDGFRβ deletion affects hematopoietic development in the AGM in vivo

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The transcriptome and hematopoietic support of the PDGFRβ-KO niche are altered

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The osteogenic gene profile and differentiation of KO AGM MSCs are affected

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PDGFRβ⁺ early embryonic precursors contribute to EC and HSPC lineages in vivo

Embryonic Origins of the Hematopoietic System: Hierarchies and Heterogeneity

The hierarchical framework of the adult blood system as we know it from current medical and hematology textbooks, displays a linear branching network of dividing and differentiated cells essential for the growth and maintenance of the healthy organism. This view of the hierarchy has evolved over the last 75 years. An amazing increase in cellular complexity has been realized; however, innovative single-cell technologies continue to uncover essential cell types and functions in animal models and the human blood system. The most potent cell of the hematopoietic hierarchy is the hematopoietic stem cell. Stem cells for adult tissues are the long-lived self-renewing cellular component, which ensure that differentiated tissue-specific cells are maintained and replaced through the entire adult lifespan. Although much blood research is focused on hematopoietic tissue homeostasis, replacement and regeneration during adult life, embryological studies have widened and enriched our understanding of additional developmental hierarchies and interacting cells of this life-sustaining tissue. Here, we review the current state of knowledge of the hierarchical organization and the vast heterogeneity of the hematopoietic system from embryonic to adult stages.

An in vitro model for discovery of osteoclast specific biomarkers towards identification of racehorses at risk for catastrophic fractures

BACKGROUND

Focal bone microcracks with osteoclast recruitment and bone lysis, may reduce fracture resistance in racehorses. As current imaging does not detect all horses at risk for fracture, the discovery of novel serum biomarkers of bone resorption or osteoclast activity could potentially address this unmet clinical need. The biology of equine osteoclasts on their natural substrate, equine bone, has never been studied in vitro and may permit identification of specific biomarkers of their activity.

OBJECTIVES

1) Establish osteoclast cultures on equine bone, 2) Measure biomarkers (tartrate resistant acid phosphatase isoform 5b (TRACP-5b) and C-terminal telopeptide of type I collagen (CTX-I)) in vitro and 3) Study the effects of inflammation.

STUDY DESIGN

In vitro experiments.

METHODS

Haematopoietic stem cells, from 5 equine sternal bone marrow aspirates, were differentiated into osteoclasts and cultured either alone or on equine bone slices, with or without pro-inflammatory stimulus (IL-1β or LPS). CTX-I and TRACP-5b were immunoassayed in the media. Osteoclast numbers and bone resorption area were assessed.

RESULTS

TRACP-5b increased over time without bone (p < 0.0001) and correlated with osteoclast number (r = 0.63, p < 0.001). CTX-I and TRACP-5b increased with time for cultures with bone (p = 0.002; p = 0.02 respectively), correlated with each other (r = 0.64, p < 0.002) and correlated with bone resorption (r = 0.85, p < 0.001; r = 0.82, p < 0.001 respectively). Inflammation had no measurable effects.

MAIN LIMITATIONS

Specimen numbers limited.

CONCLUSIONS

Equine osteoclasts were successfully cultured on equine bone slices and their bone resorption quantified. TRACP-5b was shown to be a biomarker of equine osteoclast number and bone resorption for the first time; CTX-I was also confirmed to be a biomarker of equine bone resorption in vitro. This robust equine specific in vitro assay will help the study of osteoclast biology.

Engineered Tissue Models to Replicate Dynamic Interactions within the Hematopoietic Stem Cell Niche

Hematopoietic stem cells are the progenitors of the blood and immune system and represent the most widely used regenerative therapy. However, their rarity and limited donor base necessitate the design of ex vivo systems that support HSC expansion without the loss of long-term stem cell activity. This review describes recent advances in biomaterials systems to replicate features of the hematopoietic niche. Inspired by the native bone marrow, these instructive biomaterials provide stimuli and cues from cocultured niche-associated cells to support HSC encapsulation and expansion. Engineered systems increasingly enable study of the dynamic nature of the matrix and biomolecular environment as well as the role of cell-cell signaling (e.g., autocrine feedback vs paracrine signaling between dissimilar cells). The inherent coupling of material properties, biotransport of cell-secreted factors, and cell-mediated remodeling motivate dynamic biomaterial systems as well as characterization and modeling tools capable of evaluating a temporally evolving tissue microenvironment. Recent advances in HSC identification and tracking, model-based experimental design, and single-cell culture platforms facilitate the study of the effect of constellations of matrix, cell, and soluble factor signals on HSC fate. While inspired by the HSC niche, these tools are amenable to the broader stem cell engineering community.

One Size Does Not Fit All: Heterogeneity in Developmental Hematopoiesis

Our knowledge of the complexity of the developing hematopoietic system has dramatically expanded over the course of the last few decades. We now know that, while hematopoietic stem cells (HSCs) firmly reside at the top of the adult hematopoietic hierarchy, multiple HSC-independent progenitor populations play variegated and fundamental roles during fetal life, which reflect on adult physiology and can lead to disease if subject to perturbations. The importance of obtaining a high-resolution picture of the mechanisms by which the developing embryo establishes a functional hematopoietic system is demonstrated by many recent indications showing that ontogeny is a primary determinant of function of multiple critical cell types. This review will specifically focus on exploring the diversity of hematopoietic stem and progenitor cells unique to embryonic and fetal life. We will initially examine the evidence demonstrating heterogeneity within the hemogenic endothelium, precursor to all definitive hematopoietic cells. Next, we will summarize the dynamics and characteristics of the so-called "hematopoietic waves" taking place during vertebrate development. For each of these waves, we will define the cellular identities of their components, the extent and relevance of their respective contributions as well as potential drivers of heterogeneity.

CD63 acts as a functional marker in maintaining hematopoietic stem cell quiescence through supporting TGFβ signaling in mice

Hematopoietic stem cell (HSC) fate is tightly controlled by various regulators, whereas the underlying mechanism has not been fully uncovered due to the high heterogeneity of these populations. In this study, we identify tetraspanin CD63 as a novel functional marker of HSCs in mice. We show that CD63 is unevenly expressed on the cell surface in HSC populations. Importantly, HSCs with high CD63 expression (CD63^hi) are more quiescent and have more robust self-renewal and myeloid differentiation abilities than those with negative/low CD63 expression (CD63^-/lo). On the other hand, using CD63 knockout mice, we find that loss of CD63 leads to reduced HSC numbers in the bone marrow. In addition, CD63-deficient HSCs exhibit impaired quiescence and long-term repopulating capacity, accompanied by increased sensitivity to irradiation and 5-fluorouracil treatment. Further investigations demonstrate that CD63 is required to sustain TGFβ signaling activity through its interaction with TGFβ receptors I and II, thereby playing an important role in regulating the quiescence of HSCs. Collectively, our data not only reveal a previously unrecognized role of CD63 but also provide us with new insights into HSC heterogeneity.

Intact in situ Preparation of the Drosophila melanogaster Lymph Gland for a Comprehensive Analysis of Larval Hematopoiesis

Blood cells have a limited lifespan and are replenished by a small number of hematopoietic stem and progenitor cells (HSPCs). Adult vertebrate hematopoiesis occurs in the bone marrow, liver, and spleen, rendering a comprehensive analysis of the entire HSPC pool nearly impossible. The Drosophila blood system is well studied and has developmental, molecular, and functional parallels with that of vertebrates. Unlike vertebrates, post-embryonic hematopoiesis in Drosophila is essentially restricted to the larval lymph gland (LG), a multi-lobed organ that flanks the dorsal vessel. Because the anterior-most or primary lobes of the LG are easy to dissect out, their cellular and molecular characteristics have been studied in considerable detail. The 2-3 pairs of posterior lobes are more delicate and fragile and have largely been ignored. However, posterior lobes harbor a significant blood progenitor pool, and several hematopoietic mutants show differences in phenotype between the anterior and posterior lobes. Hence, a comprehensive analysis of the LG is important for a thorough understanding of Drosophila hematopoiesis. Most studies focus on isolating the primary lobes by methods that generally dislodge and damage other lobes. To obtain preparations of the whole LG, including intact posterior lobes, here we provide a detailed protocol for larval fillet dissection. This allows accessing and analyzing complete LG lobes, along with dorsal vessel and pericardial cells. We demonstrate that tissue architecture and integrity is maintained and provide methods for quantitative analysis. This protocol can be used to quickly and effectively isolate complete LGs from first instar larval to pupal stages and can be implemented with ease.

Probiotics and Trained Immunity

The characteristics of innate immunity have recently been investigated in depth in several research articles, and original findings suggest that innate immunity also has a memory capacity, which has been named “trained immunity”. This notion has revolutionized our knowledge of the innate immune response. Thus, stimulation of trained immunity represents a therapeutic alternative that is worth exploring. In this context, probiotics, live microorganisms which when administered in adequate amounts confer a health benefit on the host, represent attractive candidates for the stimulation of trained immunity; however, although numerous studies have documented the beneficial proprieties of these microorganisms, their mechanisms of action are not yet fully understood. In this review, we propose to explore the putative connection between probiotics and stimulation of trained immunity.

Differential H4K16ac levels ensure a balance between quiescence and activation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are able to reconstitute the bone marrow while retaining their self-renewal property. Individual HSCs demonstrate heterogeneity in their repopulating capacities. Here, we found that the levels of the histone acetyltransferase MOF (males absent on the first) and its target modification histone H4 lysine 16 acetylation are heterogeneous among HSCs and influence their proliferation capacities. The increased proliferative capacities of MOF-depleted cells are linked to their expression of CD93. The CD93+ HSC subpopulation simultaneously shows transcriptional features of quiescent HSCs and functional features of active HSCs. CD93+ HSCs were expanded and exhibited an enhanced proliferative advantage in Mof +/- animals reminiscent of a premalignant state. Accordingly, low MOF and high CD93 levels correlate with poor survival and increased proliferation capacity in leukemia. Collectively, our study indicates H4K16ac as an important determinant for HSC heterogeneity, which is linked to the onset of monocytic disorders.

The transformative potential of HSC gene therapy as a genetic medicine

Hematopoietic stem cells (HSCs) are precursor cells that give rise to blood, immune and tissue-resident progeny in humans. Their position at the starting point of hematopoiesis offers a unique therapeutic opportunity to treat certain hematologic diseases by implementing corrective changes that are subsequently directed through to multiple cell lineages. Attempts to exploit HSCs clinically have evolved over recent decades, from initial approaches that focused on transplantation of healthy donor allogeneic HSCs to treat rare inherited monogenic hematologic disorders, to more contemporary genetic modification of autologous HSCs offering the promise of benefits to a wider range of diseases. We are on the cusp of an exciting new era as the transformative potential of HSC gene therapy to offer durable delivery of gene-corrected cells to a range of tissues and organs, including the central nervous system, is beginning to be realized. This article reviews the rationale for targeting HSCs, the approaches that have been used to date for delivering therapeutic genes to these cells, and the latest technological breakthroughs in manufacturing and vector design. The challenges faced by the biotechnology cell and gene therapy sector in the commercialization of HSC gene therapy are also discussed.

Differential activation of JAK-STAT signaling reveals functional compartmentalization in Drosophila blood progenitors

Blood cells arise from diverse pools of stem and progenitor cells. Understanding progenitor heterogeneity is a major challenge. The Drosophila larval lymph gland is a well-studied model to understand blood progenitor maintenance and recapitulates several aspects of vertebrate hematopoiesis. However in-depth analysis has focused on the anterior lobe progenitors (AP), ignoring the posterior progenitors (PP) from the posterior lobes. Using in situ expression mapping and developmental and transcriptome analysis, we reveal PP heterogeneity and identify molecular-genetic tools to study this abundant progenitor population. Functional analysis shows that PP resist differentiation upon immune challenge, in a JAK-STAT-dependent manner. Upon wasp parasitism, AP downregulate JAK-STAT signaling and form lamellocytes. In contrast, we show that PP activate STAT92E and remain undifferentiated, promoting survival. Stat92E knockdown or genetically reducing JAK-STAT signaling permits PP lamellocyte differentiation. We discuss how heterogeneity and compartmentalization allow functional segregation in response to systemic cues and could be widely applicable.

Implications of hematopoietic stem cells heterogeneity for gene therapies

Hematopoietic stem cell transplantation (HSCT) is the therapeutic concept to cure the blood/immune system of patients suffering from malignancies, immunodeficiencies, red blood cell disorders, and inherited bone marrow failure syndromes. Yet, allogeneic HSCT bear considerable risks for the patient such as non-engraftment, or graft-versus host disease. Transplanting gene modified autologous HSCs is a promising approach not only for inherited blood/immune cell diseases, but also for the acquired immunodeficiency syndrome. However, there is emerging evidence for substantial heterogeneity of HSCs in situ as well as ex vivo that is also observed after HSCT. Thus, HSC gene modification concepts are suggested to consider that different blood disorders affect specific hematopoietic cell types. We will discuss the relevance of HSC heterogeneity for the development and manufacture of gene therapies and in exemplary diseases with a specific emphasis on the key target HSC types myeloid-biased, lymphoid-biased, and balanced HSCs.

Hematopoietic Stem Cell Metabolism during Development and Aging

Cellular metabolism in hematopoietic stem cells (HSCs) is an area of intense research interest, but the metabolic requirements of HSCs and their adaptations to their niches during development have remained largely unaddressed. Distinctive from other tissue stem cells, HSCs transition through multiple hematopoietic sites during development. This transition requires drastic metabolic shifts, insinuating the capacity of HSCs to meet the physiological demand of hematopoiesis. In this review, we highlight how mitochondrial metabolism determines HSC fate, and especially focus on the links between mitochondria, endoplasmic reticulum (ER), and lysosomes in HSC metabolism.

Neglected No More: Emerging Cellular Therapies in Traumatic Injury

Traumatic injuries are a leading cause of death and disability in both military and civilian populations. Given the complexity and diversity of traumatic injuries, novel and individualized treatment strategies are required to optimize outcomes. Cellular therapies have potential benefit for the treatment of acute or chronic injuries, and various cell-based pharmaceuticals are currently being tested in preclinical studies or in clinical trials. Cellular therapeutics may have the ability to complement existing therapies, especially in restoring organ function lost due to tissue disruption, prolonged hypoxia or inflammatory damage. In this article we highlight the current status and discuss future directions of cellular therapies for the treatment of traumatic injury. Both published research and ongoing clinical trials are discussed here.

Mitophagy: A New Player in Stem Cell Biology

The fundamental importance of functional mitochondria in the survival of most eukaryotic cells, through regulation of bioenergetics, cell death, calcium dynamics and reactive oxygen species (ROS) generation, is undisputed. However, with new avenues of research in stem cell biology these organelles have now emerged as signaling entities, actively involved in many aspects of stem cell functions, including self-renewal, commitment and differentiation. With this recent knowledge, it becomes evident that regulatory pathways that would ensure the maintenance of mitochondria with state-specific characteristics and the selective removal of organelles with sub-optimal functions must play a pivotal role in stem cells. As such, mitophagy, as an essential mitochondrial quality control mechanism, is beginning to gain appreciation within the stem cell field. Here we review and discuss recent advances in our knowledge pertaining to the roles of mitophagy in stem cell functions and the potential contributions of this specific quality control process on to the progression of aging and diseases.

An Overview of Novel Unconventional Mechanisms of Hematopoietic Development and Regulators of Hematopoiesis - a Roadmap for Future Investigations

Hematopoietic stem cells (HSCs) are the best-characterized stem cells in adult tissues. Nevertheless, as of today, many open questions remain. First, what is the phenotype of the most primitive "pre-HSC" able to undergo asymmetric divisions during ex vivo expansion that gives rise to HSC for all hemato-lymphopoietic lineages. Next, most routine in vitro assays designed to study HSC specification into hematopoietic progenitor cells (HPCs) for major hematopoietic lineages are based on a limited number of peptide-based growth factors and cytokines, neglecting the involvement of several other regulators that are endowed with hematopoietic activity. Examples include many hormones, such as pituitary gonadotropins, gonadal sex hormones, IGF-1, and thyroid hormones, as well as bioactive phosphosphingolipids and extracellular nucleotides (EXNs). Moreover, in addition to regulation by stromal-derived factor 1 (SDF-1), trafficking of these cells during mobilization or homing after transplantation is also regulated by bioactive phosphosphingolipids, EXNs, and three ancient proteolytic cascades, the complement cascade (ComC), the coagulation cascade (CoA), and the fibrinolytic cascade (FibC). Finally, it has emerged that bone marrow responds by "sterile inflammation" to signals sent from damaged organs and tissues, systemic stress, strenuous exercise, gut microbiota, and the administration of certain drugs. This review will address the involvement of these unconventional regulators and present a broader picture of hematopoiesis.

Current Trends on Hemopoietic Stem Cells

Advances in single-cell technology and genetic mouse models have resulted in the identification of new types of hemopoietic stem cells (HSC), resulting in baffling observations, suggesting a reconsideration of the long-held notion that all hematopoietic cells in the adult are derived from HSCs. The existence of long-lived HSC-independent hematopoiesis has led to the conclusion that despite the single hierarchical differentiation route that generates functional blood types, other differentiation routes exist in-vivo. Heterogeneity in the HSC population and the evolving knowledge around HSC has translated to it's improved application as a therapeutic tool for various blood disorders. The reprogramming of non-hematopoietic somatic and mature blood cells to pluripotency with their subsequent differentiation into hematopoietic stem cells/progenitor cells and the introduction of new generation sequencing holds the potential for the resolution of ambiguities involved in HSC bone marrow transplantation. There is a change in the paradigm for HSC transplantation donor selection. Donor choice favors haploidentical HCT than cord blood. This review provides a general overview of the current events around hemopoietic stem cells, with emphasis on the rising trend of HSC transplantation, especially haploidentical stem cell transplantation.

Hematopoietic stem cells from pluripotent stem cells: Clinical potential, challenges, and future perspectives

The generation of hematopoietic stem cells (HSCs) from induced pluripotent stem cells (iPSCs) is an active and promising area of research; however, generating engraftable HSCs remains a major obstacle. Ex vivo HSC derivation from renewable sources such as iPSCs offers an experimental tool for studying developmental hematopoiesis, disease modeling, and drug discovery, and yields tremendous therapeutic potential for malignant and nonmalignant hematological disorders. Although initial attempts mostly recapitulated yolk sac primitive/definitive hematopoiesis with inability to engraft, recent advances suggest the feasibility of engraftable HSC derivation from iPSCs utilizing ectopic transcription factor expression. Strategic development for de novo HSC generation includes further investigations of HSC ontogeny, and elucidation of critical signaling pathways, epigenetic modulations, HSC and iPSC microenvironment, and cell‐cell interactions that contribute to stem cell biology and function.

In vivo single-cell lineage tracing in zebrafish using high-resolution infrared laser-mediated gene induction microscopy

Heterogeneity broadly exists in various cell types both during development and at homeostasis. Investigating heterogeneity is crucial for comprehensively understanding the complexity of ontogeny, dynamics, and function of specific cell types. Traditional bulk-labeling techniques are incompetent to dissect heterogeneity within cell population, while the new single-cell lineage tracing methodologies invented in the last decade can hardly achieve high-fidelity single-cell labeling and long-term in-vivo observation simultaneously. In this work, we developed a high-precision infrared laser-evoked gene operator heat-shock system, which uses laser-induced CreERT2 combined with loxP-DsRedx-loxP-GFP reporter to achieve precise single-cell labeling and tracing. In vivo study indicated that this system can precisely label single cell in brain, muscle and hematopoietic system in zebrafish embryo. Using this system, we traced the hematopoietic potential of hemogenic endothelium (HE) in the posterior blood island (PBI) of zebrafish embryo and found that HEs in the PBI are heterogeneous, which contains at least myeloid unipotent and myeloid-lymphoid bipotent subtypes.

Maternal Diabetes Induces Immune Dysfunction in Autistic Offspring Through Oxidative Stress in Hematopoietic Stem Cells

Autism spectrum disorders (ASD) have been found to be associated with immune dysfunction and elevated cytokines, although the detailed mechanism remains unknown. In this study, we aim to investigate the potential mechanisms through a maternal diabetes-induced autistic mouse model. We found that maternal diabetes-induced autistic offspring have epigenetic changes on the superoxide dismutase 2 (SOD2) promoter with subsequent SOD2 suppression in both hematopoietic stem cells (HSC) and peripheral blood mononuclear cells (PBMC). Bone marrow transplantation of normal HSC to maternal diabetes-induced autistic offspring transferred epigenetic modifications to PBMC and significantly reversed SOD2 suppression and oxidative stress and elevated inflammatory cytokine levels. Further, in vivo human study showed that SOD2 mRNA expression from PBMC in the ASD group was reduced to ~12% compared to typically developing group, and the SOD2 mRNA level-based ROC (Receiver Operating Characteristic) curve shows a very high sensitivity and specificity for ASD patients. We conclude that maternal diabetes induces immune dysfunction in autistic offspring through SOD2 suppression and oxidative stress in HSC. SOD2 mRNA expression in PBMC may be a good biomarker for ASD diagnosis.

Traditional and Advanced Cell Cultures in Hematopoietic Stem Cell Studies

Hematopoiesis is the main function of bone marrow. Human hematopoietic stem and progenitor cells reside in the bone marrow microenvironment, making it a hotspot for the development of hematopoietic diseases. Numerous alterations that correspond to disease progression have been identified in the bone marrow stem cell niche. Complex interactions between the bone marrow microenvironment and hematopoietic stem cells determine the balance between the proliferation, differentiation and homeostasis of the stem cell compartment. Changes in this tightly regulated network can provoke malignant transformation. However, our understanding of human hematopoiesis and the associated niche biology remains limited due to accessibility to human material and the limits of in vitro culture models. Traditional culture systems for human hematopoietic studies lack microenvironment niches, spatial marrow gradients, and dense cellularity, rendering them incapable of effectively translating marrow physiology ex vivo. This review will discuss the importance of 2D and 3D culture as a physiologically relevant system for understanding normal and abnormal hematopoiesis.

Thrombopoietin Metabolically Primes Hematopoietic Stem Cells to Megakaryocyte-Lineage Differentiation

During acute myelosuppression or thrombocytopenia, bone marrow (BM) hematopoietic cells respond rapidly to replenish peripheral blood platelets. While the cytokine thrombopoietin (Thpo) both regulates platelet production and maintains HSC potential, whether Thpo controls megakaryocyte (Mk)-lineage differentiation of HSCs is unclear. Here, we show that Thpo rapidly upregulates mitochondrial activity in HSCs, an activity accompanied by differentiation to an Mk lineage. Moreover, in unperturbed hematopoiesis, HSCs with high mitochondrial activity exhibit Mk-lineage differentiation in vitro and myeloid lineage-biased reconstitution in vivo. Furthermore, Thpo skewed HSCs to express the tetraspanin CD9, a pattern correlated with mitochondrial activity. Mitochondria-active HSCs are resistant to apoptosis and oxidative stress upon Thpo stimulation. Thpo-regulated mitochondrial activity associated with mitochondrial translocation of STAT3 phosphorylated at serine 727. Overall, we report an important role for Thpo in regulating rapid Mk-lineage commitment. Thpo-dependent changes in mitochondrial metabolism prime HSCs to undergo direct differentiation to an Mk lineage.

Mesenchymal stem cell perspective: cell biology to clinical progress

The terms MSC and MSCs have become the preferred acronym to describe a cell and a cell population of multipotential stem/progenitor cells commonly referred to as mesenchymal stem cells, multipotential stromal cells, mesenchymal stromal cells, and mesenchymal progenitor cells. The MSCs can differentiate to important lineages under defined conditions in vitro and in limited situations after implantation in vivo. MSCs were isolated and described about 30 years ago and now there are over 55,000 publications on MSCs readily available. Here, we have focused on human MSCs whenever possible. The MSCs have broad anti-inflammatory and immune-modulatory properties. At present, these provide the greatest focus of human MSCs in clinical testing; however, the properties of cultured MSCs in vitro suggest they can have broader applications. The medical utility of MSCs continues to be investigated in over 950 clinical trials. There has been much progress in understanding MSCs over the years, and there is a strong foundation for future scientific research and clinical applications, but also some important questions remain to be answered. Developing further methods to understand and unlock MSC potential through intracellular and intercellular signaling, biomedical engineering, delivery methods and patient selection should all provide substantial advancements in the coming years and greater clinical opportunities. The expansive and growing field of MSC research is teaching us basic human cell biology as well as how to use this type of cell for cellular therapy in a variety of clinical settings, and while much promise is evident, careful new work is still needed.

An integrated cell printing system for the construction of heterogeneous tissue models

A new three-dimensional (3D) cell printing system was developed and investigated to organize multiple cells/biomaterials with a control precision within 100 μm. This system can be used for the in vitro construction of heterogeneous tissue models. The proposed printing system was achieved by the integration of extrusion printing and alternating viscous and inertial force jetting (AVIFJ) techniques using dual-nozzle switching. In this technique, hydrogels containing high cell densities were extruded using extrusion printing, while droplets containing single cells were precisely manipulated using AVIFJ. The droplets that contained single cells were at the scale of pico-liters and could be accurately positioned at the micron scale. Stable hydrogel structures with adjustable diameters were also printed, with cell viabilities exceeding 90% after printing. A heterogeneous tumor model that contained spheroids and human umbilical vein endothelial cells (HUVECs) was then constructed using the established integrated cell printing system in a stepwise or simultaneous fashion. HUVEC-loaded droplets were observed to locate around the preformed tumor spheroids as designed. Cells and spheroids in the model maintained high cell viability and sustained growth throughout the culture period. The ELISA results of albumin production also proved that the spheroids maintained increased cellular function during the culture. These results demonstrated the feasibility of this integrated 3D printing system for the engineering of in vitro heterogeneous tissue models for future biological and pathological studies. STATEMENT OF SIGNIFICANCE: Addressing the challenge of multi-scale printing in the construction of heterogeneous tissue models, a new 3D cell printing system was developed to organize cells/biomaterials of a control precision within 100 μm. AVIFJ was integrated with extrusion printing, thereby achieving the construction of cell interactions between single cells and spheroids, the manipulation of single cells in a 3D microenvironment with high accuracy, and the real-time on-demand printing. The printed heterogeneous tumor model maintained cell viability, sustained cell growth, and increased cell function during 7 days of culture. We believed that this work would benefit the production of functional artificial tissues, enabling the construction of more biomimetic cell arrangements and microenvironment to support cell functions.

Mitochondria and autophagy in adult stem cells: proliferate or differentiate

Adult stem cells are undifferentiated cells that are found in many different tissues after development. They are responsible for regenerating and repairing tissues after injury, as well as replacing cells when needed. Adult stem cells maintain a delicate balance between self-renewal to prevent depletion of the stem cell pool and differentiation to continually replenish downstream lineages. The important role of mitochondria in generating energy, calcium storage and regulating cell death is well established. However, new research has linked mitochondria to stem cell maintenance and fate. In addition, efficient mitochondrial quality control is critical for stem cell homeostasis to ensure their long-term survival in tissues. In this review, we discuss the latest evidence linking mitochondrial function, remodeling and turnover via autophagy to regulation of adult stem cell self-renewal and differentiation.

Hematopoietic Stem Cell Dynamics Are Regulated by Progenitor Demand: Lessons from a Quantitative Modeling Approach

The prevailing view on murine hematopoiesis and on hematopoietic stem cells (HSCs) in particular derives from experiments that are related to regeneration after irradiation and HSC transplantation. However, over the past years, different experimental techniques have been developed to investigate hematopoiesis under homeostatic conditions, thereby providing access to proliferation and differentiation rates of hematopoietic stem and progenitor cells in the unperturbed situation. Moreover, it has become clear that hematopoiesis undergoes distinct changes during aging with large effects on HSC abundance, lineage contribution, asymmetry of division, and self-renewal potential. However, it is currently not fully resolved how stem and progenitor cells interact to respond to varying demands and how this balance is altered by an aging-induced shift in HSC polarity. Aiming toward a conceptual understanding, we introduce a novel in silico model to investigate the dynamics of HSC response to varying demand. By introducing an internal feedback within a heterogeneous HSC population, the model is suited to consistently describe both hematopoietic homeostasis and regeneration, including the limited regulation of HSCs in the homeostatic situation. The model further explains the age-dependent increase in phenotypic HSCs as a consequence of the cells' inability to preserve divisional asymmetry. Our model suggests a dynamically regulated population of intrinsically asymmetrically dividing HSCs as suitable control mechanism that adheres with many qualitative and quantitative findings on hematopoietic recovery after stress and aging. The modeling approach thereby illustrates how a mathematical formalism can support both the conceptual and the quantitative understanding of regulatory principles in HSC biology.

Effect of cell sample size in atomic force microscopy nanoindentation

Single-cell technologies are powerful tools to evaluate cell characteristics. In particular, Atomic Force Microscopy (AFM) nanoindentation experiments have been widely used to study single cell mechanical properties. One important aspect related to single cell techniques is the need for sufficient statistical power to obtain reliable results. This aspect is often overlooked in AFM experiments were sample sizes are arbitrarily set. The aim of the present work was to propose a tool for sample size estimation in the context of AFM nanoindentation experiments of single cell. To this aim, a retrospective approach was used by acquiring a large dataset of experimental measurements on four bone cell types and by building saturation curves for increasing sample sizes with a bootstrap resampling method. It was observed that the coefficient of variation (CV_%) decayed with a function of the form y = ax^b with similar parameters for all samples tested and that sample sizes of 21 and 83 cells were needed for the specific cells and protocol employed if setting a maximum threshold on CV_% of 10% or 5%, respectively. The developed tool is made available as an open-source repository and guidelines are provided for its use for AFM nanoindentation experimental design.

Implications and Current Limitations of Oogenesis from Female Germline or Oogonial Stem Cells in Adult Mammalian Ovaries

A now large body of evidence supports the existence of mitotically active germ cells in postnatal ovaries of diverse mammalian species, including humans. This opens the possibility that adult stem cells naturally committed to a germline fate could be leveraged for the production of female gametes outside of the body. The functional properties of these cells, referred to as female germline or oogonial stem cells (OSCs), in ovaries of women have recently been tested in various ways, including a very recent investigation of the differentiation capacity of human OSCs at a single cell level. The exciting insights gained from these experiments, coupled with other data derived from intraovarian transplantation and genetic tracing analyses in animal models that have established the capacity of OSCs to generate healthy eggs, embryos and offspring, should drive constructive discussions in this relatively new field to further exploring the value of these cells to the study, and potential management, of human female fertility. Here, we provide a brief history of the discovery and characterization of OSCs in mammals, as well as of the in-vivo significance of postnatal oogenesis to adult ovarian function. We then highlight several key observations made recently on the biology of OSCs, and integrate this information into a broader discussion of the potential value and limitations of these adult stem cells to achieving a greater understanding of human female gametogenesis in vivo and in vitro.

Hematopoietic Stem Cell Heterogeneity

Hematopoietic stem cells (HSCs) maintain lifelong production of mature blood cells and regenerate the hematopoietic system after cytotoxic injury. Use of expanding cell surface marker panels and advanced functional analyses have revealed the presence of several immunophenotypically different HSC subsets with distinct self-renewal and repopulating capacity and bias toward selective lineage differentiation. This chapter summarizes current understanding of the phenotypic and functional heterogeneity within the HSC pool, with emphasis on the immunophenotypes and functional features of several known HSC subsets, and their roles in steady-state and emergency hematopoiesis, and in aging. The chapter also highlights some of the future research directions to elucidate further the biology and function of different HSC subsets in health and disease states.

Cell Cycle Regulation of Stem Cells by MicroRNAs

MicroRNAs (miRNAs) are a class of small non-coding RNA molecules involved in the regulation of gene expression. They are involved in the fine-tuning of fundamental biological processes such as proliferation, differentiation, survival and apoptosis in many cell types. Emerging evidence suggests that miRNAs regulate critical pathways involved in stem cell function. Several miRNAs have been suggested to target transcripts that directly or indirectly coordinate the cell cycle progression of stem cells. Moreover, previous studies have shown that altered expression levels of miRNAs can contribute to pathological conditions, such as cancer, due to the loss of cell cycle regulation. However, the precise mechanism underlying miRNA-mediated regulation of cell cycle in stem cells is still incompletely understood. In this review, we discuss current knowledge of miRNAs regulatory role in cell cycle progression of stem cells. We describe how specific miRNAs may control cell cycle associated molecules and checkpoints in embryonic, somatic and cancer stem cells. We further outline how these miRNAs could be regulated to influence cell cycle progression in stem cells as a potential clinical application.

A metabolic interplay coordinated by HLX regulates myeloid differentiation and AML through partly overlapping pathways

The H2.0-like homeobox transcription factor (HLX) regulates hematopoietic differentiation and is overexpressed in Acute Myeloid Leukemia (AML), but the mechanisms underlying these functions remain unclear. We demonstrate here that HLX overexpression leads to a myeloid differentiation block both in zebrafish and human hematopoietic stem and progenitor cells (HSPCs). We show that HLX overexpression leads to downregulation of genes encoding electron transport chain (ETC) components and upregulation of PPARδ gene expression in zebrafish and human HSPCs. HLX overexpression also results in AMPK activation. Pharmacological modulation of PPARδ signaling relieves the HLX-induced myeloid differentiation block and rescues HSPC loss upon HLX knockdown but it has no effect on AML cell lines. In contrast, AMPK inhibition results in reduced viability of AML cell lines, but minimally affects myeloid progenitors. This newly described role of HLX in regulating the metabolic state of hematopoietic cells may have important therapeutic implications.

HLX transcription factor regulates haematopoietic stem and progenitor cell (HSPC) differentiation and is overexpressed in acute myeloid leukemia. Here the authors show that HLX overexpression leads to myeloid differentiation block in zebrafish and human HSPCs by direct regulation of metabolic pathways.

The Making of Hematopoiesis: Developmental Ancestry and Environmental Nurture

Evidence from studies of the behaviour of stem and progenitor cells and of the influence of cytokines on their fate determination, has recently led to a revised view of the process by which hematopoietic stem cells and their progeny give rise to the many different types of blood and immune cells. The new scenario abandons the classical view of a rigidly demarcated lineage tree and replaces it with a much more continuum-like view of the spectrum of fate options open to hematopoietic stem cells and their progeny. This is in contrast to previous lineage diagrams, which envisaged stem cells progressing stepwise through a series of fairly-precisely described intermediate progenitors in order to close down alternative developmental options. Instead, stem and progenitor cells retain some capacity to step sideways and adopt alternative, closely related, fates, even after they have “made a lineage choice.” The stem and progenitor cells are more inherently versatile than previously thought and perhaps sensitive to lineage guidance by environmental cues. Here we examine the evidence that supports these views and reconsider the meaning of cell lineages in the context of a continuum model of stem cell fate determination and environmental modulation.

Hematopoietic Hierarchy - An Updated Roadmap

The classical roadmap of hematopoietic hierarchy has been proposed for nearly 20 years and has become a dogma of stem cell research for most types of adult stem cells, including hematopoietic stem cells (HSCs). However, with the development of new technologies such as omics approaches at single-cell resolution, recent studies in vitro and in vivo have suggested that heterogeneity is a common feature of HSCs and their progenies. While these findings broaden our understanding of hematopoiesis, they also challenge the well-accepted hematopoietic hierarchy roadmap. Here, we review recent advances in the hematopoiesis field and provide an updated view to incorporate these new findings as well as to reflect on the complexity of HSCs and their derivatives in development and adulthood.

Secretome within the bone marrow microenvironment: A basis for mesenchymal stem cell treatment and role in cancer dormancy

The secretome produced by cells within the bone marrow is significant to homeostasis. The bone marrow, a well-studied organ, has multiple niches with distinct roles for supporting stem cell functions. Thus, an understanding of mediators involved in the regulation of stem cells could serve as a model for clinical problems and solutions such as tissue repair and regeneration. The exosome secretome of bone marrow stem cells is a developing area of research with respect to the regenerative potential by bone marrow cell, particularly the mesenchymal stem cells. The bone marrow niche regulates endogenous processes such as hematopoiesis but could also support the survival of tumors such as facilitating the cancer stem cells to exist in dormancy for decades. The bone marrow-derived secretome will be critical to future development of therapeutic strategies for oncologic diseases, in addition to regenerative medicine. This article discusses the importance for parallel studies to determine how the same secretome may compromise safety during the use of stem cells in regenerative medicine.

Mitochondria and the dynamic control of stem cell homeostasis

The maintenance of cellular identity requires continuous adaptation to environmental changes. This process is particularly critical for stem cells, which need to preserve their differentiation potential over time. Among the mechanisms responsible for regulating cellular homeostatic responses, mitochondria are emerging as key players. Given their dynamic and multifaceted role in energy metabolism, redox, and calcium balance, as well as cell death, mitochondria appear at the interface between environmental cues and the control of epigenetic identity. In this review, we describe how mitochondria have been implicated in the processes of acquisition and loss of stemness, with a specific focus on pluripotency. Dissecting the biological functions of mitochondria in stem cell homeostasis and differentiation will provide essential knowledge to understand the dynamics of cell fate modulation, and to establish improved stem cell-based medical applications.

Autophagy in stem cells: repair, remodelling and metabolic reprogramming

Autophagy is a catabolic pathway by which cellular components are delivered to the lysosome for degradation and recycling. Autophagy serves as a crucial intracellular quality control and repair mechanism but is also involved in cell remodelling during development and cell differentiation. In addition, mitophagy, the process by which damaged mitochondria undergo autophagy, has emerged as key regulator of cell metabolism. In recent years, a number of studies have revealed roles for autophagy and mitophagy in the regulation of stem cells, which represent the origin for all tissues during embryonic and postnatal development, and contribute to tissue homeostasis and repair throughout adult life. Here, we review these studies, focussing on the latest evidence that supports the quality control, remodelling and metabolic functions of autophagy during the activation, self-renewal and differentiation of embryonic, adult and cancer stem cells.

The hematopoietic stem cell niche: from embryo to adult

Hematopoietic stem cells (HSCs) develop in discrete anatomical niches, migrating during embryogenesis from the aorta-gonad-mesonephros (AGM) region to the fetal liver, and finally to the bone marrow, where most HSCs reside throughout adult life. These niches provide supportive microenvironments that specify, expand and maintain HSCs. Understanding the constituents and molecular regulation of HSC niches is of considerable importance as it could shed new light on the mechanistic principles of HSC emergence and maintenance, and provide novel strategies for regenerative medicine. However, controversy exists concerning the cellular complexity of the bone marrow niche, and our understanding of the different HSC niches during development remains limited. In this Review, we summarize and discuss what is known about the heterogeneity of the HSC niches at distinct stages of their ontogeny, from the embryo to the adult bone marrow, drawing predominantly on data from mouse studies.

Mechanisms of erythrocyte development and regeneration: implications for regenerative medicine and beyond

Hemoglobin-expressing erythrocytes (red blood cells) act as fundamental metabolic regulators by providing oxygen to cells and tissues throughout the body. Whereas the vital requirement for oxygen to support metabolically active cells and tissues is well established, almost nothing is known regarding how erythrocyte development and function impact regeneration. Furthermore, many questions remain unanswered relating to how insults to hematopoietic stem/progenitor cells and erythrocytes can trigger a massive regenerative process termed 'stress erythropoiesis' to produce billions of erythrocytes. Here, we review the cellular and molecular mechanisms governing erythrocyte development and regeneration, and discuss the potential links between these events and other regenerative processes.