Mouse Embryonic Head as a Site for Hematopoietic Stem Cell Development

Decoding the human prenatal immune system with single-cell multi-omics

The human immune system is made up of a huge variety of cell types each with unique functions. Local networks of resident immune cells are poised to sense and protect against pathogen entry, whereas more widespread innate and adaptive immune networks provide first rapid, then long-lasting and targeted responses. However, how we develop such a diverse and complex system remains unknown. Studying human development directly has been challenging in the past, but recent advances in single-cell and spatial genomics, together with the co-ordinated efforts of the Human Cell Atlas and other initiatives, have led to new studies that map the development of the human immune system in unprecedented detail. In this Review, we consider the timings, transitions, cell types and tissue microenvironments that are crucial for building the human immune system. We also compare and contrast the human system with model species and in vitro systems, and discuss how an understanding of prenatal immune system development will improve our knowledge of human disease.

Lineage tracing studies suggest that the placenta is not a de novo source of hematopoietic stem cells

Definitive hematopoietic stem and progenitor cells (HSPCs) arise from a small number of hemogenic endothelial cells (HECs) within the developing embryo. Understanding the origin and ontogeny of HSPCs is of considerable interest and potential therapeutic value. It has been proposed that the murine placenta contains HECs that differentiate into HSPCs. However, during human gestation HSPCs arise in the aorta considerably earlier than when they can first be detected in the placenta, suggesting that the placenta may primarily serve as a niche. We found that the Runx1 transcription factor, which is required to generate HSPCs from HECs, is not expressed by mouse placental ECs. To definitively determine whether the mouse placenta is a site of HSPC emergence, we performed lineage tracing experiments with a Hoxa13Cre allele that specifically labels ECs in the placenta and umbilical cord (UC), but not in the yolk sac or embryo. Immunostaining revealed Hoxa13Cre lineage-traced HECs and HSPCs in the UC, a known site of HECs, but not the placenta. Consistent with these findings, ECs harvested from the E10.5 aorta and UC, but not the placenta, gave rise to hematopoietic cells ex vivo, while colony forming assays using E14.5 fetal liver revealed only 2% of HSPCs arose from Hoxa13-expressing precursors. In contrast, the pan-EC Cdh5-CreERT2 allele labeled most HSPCs in the mouse placenta. Lastly, we found that RUNX1 and other HEC genes were not expressed in first-trimester human placenta villous ECs, suggesting that human placenta is not hemogenic. Our findings demonstrate that the placenta functions as a site for expansion of HSPCs that arise within the embryo proper and is not a primary site of HSPC emergence.

Endothelial Myosin IIA Is Required for the Maintenance of Blood-Brain Barrier Integrity

Brain endothelial cells (ECs) are essential elements of the blood-brain barrier (BBB), maintaining its integrity through both paracellular junctions and transcellular transport systems. Myosin IIA, a multifunctional protein, plays a significant role in various cellular processes, including cytoskeletal maintenance, cell division, and signal transduction. While Myosin IIA has been implicated in bleeding and ischemic stroke, its role in regulating BBB integrity under physiological conditions remains unclear. In this study, we investigated the impact of Myosin IIA deficiency on BBB integrity using intravenous tracer injections and models of epilepsy. Flow cytometry, Western blot, and real-time PCR were employed to isolate brain cells and assess changes in protein and mRNA levels. Additionally, immunofluorescence staining and electron microscopy were used to explore alterations in protein expression and the structure of BBB. Our results demonstrate that endothelial Myosin IIA deficiency increased BBB permeability and exacerbated symptoms in BBB-related diseases. Mechanistically, we found that Myosin IIA modulates β-catenin transcription and protein interactions. The overexpression of β-catenin in brain endothelial Myosin IIA deficiency mice improved BBB integrity and reduced disease severity. This study establishes Myosin IIA as a critical regulator of BBB integrity and suggests new therapeutic targets for vascular diseases.

The heart is a resident tissue for hematopoietic stem and progenitor cells in zebrafish

The contribution of endocardial cells (EdCs) to the hematopoietic lineages has been strongly debated. Here, we provide evidence that in zebrafish, the endocardium gives rise to and maintains a stable population of hematopoietic cells. Using single-cell sequencing, we identify an endocardial subpopulation expressing enriched levels of hematopoietic-promoting genes. High-resolution microscopy and photoconversion tracing experiments uncover hematopoietic cells, mainly hematopoietic stem and progenitor cells (HSPCs)/megakaryocyte-erythroid precursors (MEPs), derived from EdCs as well as the dorsal aorta stably attached to the endocardium. Emergence of HSPCs/MEPs in hearts cultured ex vivo without external hematopoietic sources, as well as longitudinal imaging of the beating heart using light sheet microscopy, support endocardial contribution to hematopoiesis. Maintenance of these hematopoietic cells depends on the adhesion factors Integrin α4 and Vcam1 but is at least partly independent of cardiac trabeculation or shear stress. Finally, blocking primitive erythropoiesis increases cardiac-residing hematopoietic cells, suggesting that the endocardium is a hematopoietic reservoir. Altogether, these studies uncover the endocardium as a resident tissue for HSPCs/MEPs and a de novo source of hematopoietic cells.

Hematopoietic cluster formation: an essential prelude to blood cell genesis

Adult blood cells are produced in the bone marrow by hematopoietic stem cells (HSCs), the origin of which can be traced back to fetal developmental stages. Indeed, during mouse development, at days 10-11 of gestation, the aorta-gonad-mesonephros (AGM) region is a primary site of HSC production, with characteristic cell clusters related to stem cell genesis observed in the dorsal aorta. Similar clusters linked with hematopoiesis are also observed in the other sites such as the yolk sac and placenta. In this review, I outline the formation and function of these clusters, focusing on the well-characterized intra-aortic hematopoietic clusters (IAHCs).

Capturing embryonic hematopoiesis in temporal and spatial dimensions

Hematopoietic stem cells (HSCs) possess the ability to sustain the continuous production of all blood cell types throughout an organism's lifespan. Although primarily located in the bone marrow of adults, HSCs originate during embryonic development. Visualization of the birth of HSCs, their developmental trajectory, and the specific interactions with their successive niches have significantly contributed to our understanding of the biology and mechanics governing HSC formation and expansion. Intravital techniques applied to live embryos or non-fixed samples have remarkably provided invaluable insights into the cellular and anatomical origins of HSCs. These imaging technologies have also shed light on the dynamic interactions between HSCs and neighboring cell types within the surrounding microenvironment or niche, such as endothelial cells or macrophages. This review delves into the advancements made in understanding the origin, production, and cellular interactions of HSCs, particularly during the embryonic development of mice and zebrafish, focusing on studies employing (live) imaging analysis.

Autophagy regulates the maturation of hematopoietic precursors in the embryo

An understanding of the mechanisms regulating embryonic hematopoietic stem cell (HSC) development would facilitate their regeneration. The aorta-gonad-mesonephros region is the site for HSC production from hemogenic endothelial cells (HEC). While several distinct regulators are involved in this process, it is not yet known whether macroautophagy (autophagy) plays a role in hematopoiesis in the pre-liver stage. Here, we show that different states of autophagy exist in hematopoietic precursors and correlate with hematopoietic potential based on the LC3-RFP-EGFP mouse model. Deficiency of autophagy-related gene 5 (Atg5) specifically in endothelial cells disrupts endothelial to hematopoietic transition (EHT), by blocking the autophagic process. Using combined approaches, including single-cell RNA-sequencing (scRNA-seq), we have confirmed that Atg5 deletion interrupts developmental temporal order of EHT to further affect the pre-HSC I maturation, and that autophagy influences hemogenic potential of HEC and the formation of pre-HSC I likely via the nucleolin pathway. These findings demonstrate a role for autophagy in the formation/maturation of hematopoietic precursors.

Mesenchymal stem/stromal cells from human pluripotent stem cell-derived brain organoid enhance the ex vivo expansion and maintenance of hematopoietic stem/progenitor cells

BACKGROUND

Mesenchymal stem/stromal cells (MSCs) are of great therapeutic value due to their role in maintaining the function of hematopoietic stem/progenitor cells (HSPCs). MSCs derived from human pluripotent stem cells represent an ideal alternative because of their unlimited supply. However, the role of MSCs with neural crest origin derived from HPSCs on the maintenance of HSPCs has not been reported.

METHODS

Flow cytometric analysis, RNA sequencing and differentiation ability were applied to detect the characteristics of stromal cells from 3D human brain organoids. Human umbilical cord blood CD34+ (UCB-CD34+) cells were cultured in different coculture conditions composed of stromal cells and umbilical cord MSCs (UC-MSCs) with or without a cytokine cocktail. The hematopoietic stroma capacity of stromal cells was tested in vitro with the LTC-IC assay and in vivo by cotransplantation of cord blood nucleated cells and stroma cells into immunodeficient mice. RNA and proteomic sequencing were used to detect the role of MSCs on HSPCs.

RESULTS

The stromal cells, derived from both H1-hESCs and human induced pluripotent stem cells forebrain organoids, were capable of differentiating into the classical mesenchymal-derived cells (osteoblasts, chondrocytes, and adipocytes). These cells expressed MSC markers, thus named pluripotent stem cell-derived MSCs (pMSCs). The pMSCs showed neural crest origin with CD271 expression in the early stage. When human UCB-CD34+ HSPCs were cocultured on UC-MSCs or pMSCs, the latter resulted in robust expansion of UCB-CD34+ HSPCs in long-term culture and efficient maintenance of their transplantability. Comparison by RNA sequencing indicated that coculture of human UCB-CD34+ HSPCs with pMSCs provided an improved microenvironment for HSC maintenance. The pMSCs highly expressed the Wnt signaling inhibitors SFRP1 and SFRP2, indicating that they may help to modulate the cell cycle to promote the maintenance of UCB-CD34+ HSPCs by antagonizing Wnt activation.

CONCLUSIONS

A novel method for harvesting MSCs with neural crest origin from 3D human brain organoids under serum-free culture conditions was reported. We demonstrate that the pMSCs support human UCB-HSPC expansion in vitro in a long-term culture and the maintenance of their transplantable ability. RNA and proteomic sequencing indicated that pMSCs provided an improved microenvironment for HSC maintenance via mechanisms involving cell-cell contact and secreted factors and suppression of Wnt signaling. This represents a novel method for large-scale production of MSCs of neural crest origin and provides a potential approach for development of human hematopoietic stromal cell therapy for treatment of dyshematopoiesis.

Cellular iron depletion enhances behavioral rhythm by limiting brain Per1 expression in mice

AIMS

Disturbances in the circadian rhythm are positively correlated with the processes of aging and related neurodegenerative diseases, which are also associated with brain iron accumulation. However, the role of brain iron in regulating the biological rhythm is poorly understood. In this study, we investigated the impact of brain iron levels on the spontaneous locomotor activity of mice with altered brain iron levels and further explored the potential mechanisms governing these effects in vitro.

RESULTS

Our results revealed that conditional knockout of ferroportin 1 (Fpn1) in cerebral microvascular endothelial cells led to brain iron deficiency, subsequently resulting in enhanced locomotor activity and increased expression of clock genes, including the circadian locomotor output cycles kaput protein (Clock) and brain and muscle ARNT-like 1 (Bmal1). Concomitantly, the levels of period circadian regulator 1 (PER1), which functions as a transcriptional repressor in regulating biological rhythm, were decreased. Conversely, the elevated brain iron levels in APP/PS1 mice inhibited autonomous rhythmic activity. Additionally, our findings demonstrate a significant decrease in serum melatonin levels in Fpn1cdh5 -CKO mice compared with the Fpn1flox/flox group. In contrast, APP/PS1 mice with brain iron deposition exhibited higher serum melatonin levels than the WT group. Furthermore, in the human glioma cell line, U251, we observed reduced PER1 expression upon iron limitation by deferoxamine (DFO; iron chelator) or endogenous overexpression of FPN1. When U251 cells were made iron-replete by supplementation with ferric ammonium citrate (FAC) or increased iron import through transferrin receptor 1 (TfR1) overexpression, PER1 protein levels were increased. Additionally, we obtained similar results to U251 cells in mouse cerebellar astrocytes (MA-c), where we collected cells at different time points to investigate the rhythmic expression of core clock genes and the impact of DFO or FAC treatment on PER1 protein levels.

CONCLUSION

These findings collectively suggest that altered iron levels influence the circadian rhythm by regulating PER1 expression and thereby modulating the molecular circadian clock. In conclusion, our study identifies the regulation of brain iron levels as a potential new target for treating age-related disruptions in the circadian rhythm.

Development of the hematopoietic system: expanding the concept of hematopoietic stem cell-independent hematopoiesis

Hematopoietic stem cells (HSCs) give rise to nearly all blood cell types and play a central role in blood cell production in adulthood. For many years it was assumed that these roles were similarly responsible for driving the formation of the hematopoietic system during the embryonic period. However, detailed analysis of embryonic hematopoiesis has revealed the presence of hematopoietic cells that develop independently of HSCs both before and after HSC generation. Furthermore, it is becoming increasingly clear that HSCs are less involved in the production of functioning blood cells during the embryonic period when there is a much higher contribution from HSC-independent hematopoietic processes. We outline the current understanding and arguments for HSC-dependent and -independent hematopoiesis, mainly focusing on mouse ontogeny.

Nlrc3 signaling is indispensable for hematopoietic stem cell emergence via Notch signaling in vertebrates

Hematopoietic stem and progenitor cells generate all the lineages of blood cells throughout the lifespan of vertebrates. The emergence of hematopoietic stem and progenitor cells is finely tuned by a variety of signaling pathways. Previous studies have revealed the roles of pattern-recognition receptors such as Toll-like receptors and RIG-I-like receptors in hematopoiesis. In this study, we find that Nlrc3, a nucleotide-binding domain leucine-rich repeat containing family gene, is highly expressed in hematopoietic differentiation stages in vivo and vitro and is required in hematopoiesis in zebrafish. Mechanistically, nlrc3 activates the Notch pathway and the downstream gene of Notch hey1. Furthermore, NF-kB signaling acts upstream of nlrc3 to enhance its transcriptional activity. Finally, we find that Nlrc3 signaling is conserved in the regulation of murine embryonic hematopoiesis. Taken together, our findings uncover an indispensable role of Nlrc3 signaling in hematopoietic stem and progenitor cell emergence and provide insights into inflammation-related hematopoietic ontogeny and the in vitro expansion of hematopoietic stem and progenitor cells.

Diverse contribution of amniogenic somatopleural cells to cardiovascular development: With special reference to thyroid vasculature

BACKGROUND

The somatopleure serves as the primordium of the amnion, an extraembryonic membrane surrounding the embryo. Recently, we have reported that amniogenic somatopleural cells (ASCs) not only form the amnion but also migrate into the embryo and differentiate into cardiomyocytes and vascular endothelial cells. However, detailed differentiation processes and final distributions of these intra-embryonic ASCs (hereafter referred to as iASCs) remain largely unknown.

RESULTS

By quail-chick chimera analysis, we here show that iASCs differentiate into various cell types including cardiomyocytes, smooth muscle cells, cardiac interstitial cells, and vascular endothelial cells. In the pharyngeal region, they distribute selectively into the thyroid gland and differentiate into vascular endothelial cells to form intra-thyroid vasculature. Explant culture experiments indicated sequential requirement of fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) signaling for endothelial differentiation of iASCs. Single-cell transcriptome analysis further revealed heterogeneity and the presence of hemangioblast-like cell population within ASCs, with a switch from FGF to VEGF receptor gene expression.

CONCLUSION

The present study demonstrates novel roles of ASCss especially in heart and thyroid development. It will provide a novel clue for understanding the cardiovascular development of amniotes from embryological and evolutionary perspectives.

Fate-Mapping Macrophages: From Ontogeny to Functions

Macrophages are vital to the physiological function of most tissues, but also contribute to disease through a multitude of pathological roles. They are thus highly plastic and heterogeneous. It is now well recognized that macrophages develop from several distinct progenitors from embryogenesis onwards and extending throughout life. Tissue-resident macrophages largely originate from embryonic sources and in many cases self-maintain independently without monocyte input. However, in certain tissues, monocyte-derived macrophages replace these over time or as a result of tissue injury and inflammation. This additional layer of heterogeneity has introduced many questions regarding the influence of origin on fate and function of macrophages in health and disease. To comprehensively address these questions, appropriate methods of tracing macrophage ontogeny are required. This chapter explores why ontogeny is of vital importance in macrophage biology and how to delineate macrophage populations by origin through genetic fate mapping. First, we summarize the current view of macrophage ontogeny and briefly discuss how origin may influence macrophage function in homeostasis and pathology. We go on to make the case for genetic fate mapping as the gold standard and briefly review different fate-mapping models. We then put forward our recommendations for fate-mapping strategies best suited to answer specific research questions and finally discuss the strengths and limitations of currently available models.

Temporal recording of mammalian development and precancer

Key to understanding many biological phenomena is knowing the temporal ordering of cellular events, which often require continuous direct observations [1, 2]. An alternative solution involves the utilization of irreversible genetic changes, such as naturally occurring mutations, to create indelible markers that enables retrospective temporal ordering [3-8]. Using NSC-seq, a newly designed and validated multi-purpose single-cell CRISPR platform, we developed a molecular clock approach to record the timing of cellular events and clonality in vivo , while incorporating assigned cell state and lineage information. Using this approach, we uncovered precise timing of tissue-specific cell expansion during murine embryonic development and identified new intestinal epithelial progenitor states by their unique genetic histories. NSC-seq analysis of murine adenomas and single-cell multi-omic profiling of human precancers as part of the Human Tumor Atlas Network (HTAN), including 116 scRNA-seq datasets and clonal analysis of 418 human polyps, demonstrated the occurrence of polyancestral initiation in 15-30% of colonic precancers, revealing their origins from multiple normal founders. Thus, our multimodal framework augments existing single-cell analyses and lays the foundation for in vivo multimodal recording, enabling the tracking of lineage and temporal events during development and tumorigenesis.

De novo hematopoiesis from the fetal lung

Hemogenic endothelial cells (HECs) are specialized cells that undergo endothelial-to-hematopoietic transition (EHT) to give rise to the earliest precursors of hematopoietic progenitors that will eventually sustain hematopoiesis throughout the lifetime of an organism. Although HECs are thought to be primarily limited to the aorta-gonad-mesonephros (AGM) during early development, EHT has been described in various other hematopoietic organs and embryonic vessels. Though not defined as a hematopoietic organ, the lung houses many resident hematopoietic cells, aids in platelet biogenesis, and is a reservoir for hematopoietic stem and progenitor cells (HSPCs). However, lung HECs have never been described. Here, we demonstrate that the fetal lung is a potential source of HECs that have the functional capacity to undergo EHT to produce de novo HSPCs and their resultant progeny. Explant cultures of murine and human fetal lungs display adherent endothelial cells transitioning into floating hematopoietic cells, accompanied by the gradual loss of an endothelial signature. Flow cytometric and functional assessment of fetal-lung explants showed the production of multipotent HSPCs that expressed the EHT and pre-HSPC markers EPCR, CD41, CD43, and CD44. scRNA-seq and small molecule modulation demonstrated that fetal lung HECs rely on canonical signaling pathways to undergo EHT, including TGFβ/BMP, Notch, and YAP. Collectively, these data support the possibility that post-AGM development, functional HECs are present in the fetal lung, establishing this location as a potential extramedullary site of de novo hematopoiesis.

Divergent expression of Neurl3 from hemogenic endothelial cells to hematopoietic stem progenitor cells during development

Prior to the generation of hematopoietic stem cells (HSCs) from the hemogenic endothelial cells (HECs) mainly in the dorsal aorta in midgestational mouse embryos, multiple hematopoietic progenitors including erythro-myeloid progenitors and lymphoid progenitors are generated from yolk sac HECs. These HSC-independent hematopoietic progenitors have recently been identified as major contributors to functional blood cell production until birth. However, little is known about yolk sac HECs. Here, combining integrative analyses of multiple single-cell RNA-sequencing datasets and functional assays, we reveal that Neurl3-EGFP, in addition to marking the continuum throughout the ontogeny of HSCs from HECs, can also serve as a single enrichment marker for yolk sac HECs. Moreover, while yolk sac HECs have much weaker arterial characteristics than either arterial endothelial cells in the yolk sac or HECs within the embryo proper, the lymphoid potential of yolk sac HECs is largely confined to the arterial-biased subpopulation featured by the Unc5b expression. Interestingly, the B lymphoid potential of hematopoietic progenitors, but not for myeloid potentials, is exclusively detected in Neurl3-negative subpopulations in midgestational embryos. Taken together, these findings enhance our understanding of blood birth from yolk sac HECs and provide theoretical basis and candidate reporters for monitoring step-wise hematopoietic differentiation.

Yolk sac cell atlas reveals multiorgan functions during human early development

The extraembryonic yolk sac (YS) ensures delivery of nutritional support and oxygen to the developing embryo but remains ill-defined in humans. We therefore assembled a comprehensive multiomic reference of the human YS from 3 to 8 postconception weeks by integrating single-cell protein and gene expression data. Beyond its recognized role as a site of hematopoiesis, we highlight roles in metabolism, coagulation, vascular development, and hematopoietic regulation. We reconstructed the emergence and decline of YS hematopoietic stem and progenitor cells from hemogenic endothelium and revealed a YS-specific accelerated route to macrophage production that seeds developing organs. The multiorgan functions of the YS are superseded as intraembryonic organs develop, effecting a multifaceted relay of vital functions as pregnancy proceeds.

The genesis of human hematopoietic stem cells

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to HSCs.

Mast cell ontogeny: From fetal development to life-long health and disease

Mast cells (MCs) are evolutionarily ancient innate immune cells with important roles in protective immunity against bacteria, parasites, and venomous animals. They can be found in most organs of the body, where they also contribute to normal tissue functioning, for example by engaging in crosstalk with nerves. Despite this, they are most widely known for their detrimental roles in allergy, anaphylaxis, and atopic disease. Just like macrophages, mast cells were conventionally thought to originate from the bone marrow. However, they are already present in fetal tissues before the onset of bone marrow hematopoiesis, questioning this dogma. In recent years, our view of myeloid cell ontogeny has been revised. We now know that the first mast cells originate from progenitors made in the extra-embryonic yolk sac, and later get supplemented with mast cells produced from subsequent waves of hematopoiesis. In most connective tissues, sizeable populations of fetal-derived mast cells persist into adulthood, where they self-maintain largely independently from the bone marrow. These developmental origins are highly reminiscent of macrophages, which are known to have critical functions in development. Mast cells too may thus support healthy development. Their fetal origins and longevity also make mast cells susceptible to genetic and environmental perturbations, which may render them pathological. Here, we review our current understanding of mast cell biology from a developmental perspective. We first summarize how mast cell populations are established from distinct hematopoietic progenitor waves, and how they are subsequently maintained throughout life. We then discuss what functions mast cells may normally have at early life stages, and how they may be co-opted to cause, worsen, or increase susceptibility to disease.

Multiple waves of fetal-derived immune cells constitute adult immune system

It has been over three decades since Drs. Herzenberg and Herzenberg proposed the layered immune system hypothesis, suggesting that different types of stem cells with distinct hematopoietic potential produce specific immune cells. This layering of immune system development is now supported by recent studies showing the presence of fetal-derived immune cells that function in adults. It has been shown that various immune cells arise at different embryonic ages via multiple waves of hematopoiesis from special endothelial cells (ECs), referred to as hemogenic ECs. However, it remains unknown whether these fetal-derived immune cells are produced by hematopoietic stem cells (HSCs) during the fetal to neonatal period. To address this question, many advanced tools have been used, including lineage-tracing mouse models, cellular barcoding techniques, clonal assays, and transplantation assays at the single-cell level. In this review, we will review the history of the search for the origins of HSCs, B-1a progenitors, and mast cells in the mouse embryo. HSCs can produce both B-1a and mast cells within a very limited time window, and this ability declines after embryonic day (E) 14.5. Furthermore, the latest data have revealed that HSC-independent adaptive immune cells exist in adult mice, which implies more complicated developmental pathways of immune cells. We propose revised road maps of immune cell development.

Hematopoietic Stem Cells and the Immune System in Development and Aging

Hematopoietic stem cells (HSCs) support haematopoiesis throughout life and give rise to the whole variety of cells of the immune system. Developing in the early embryo, passing through the precursor stage, and maturing into the first HSCs, they undergo a fairly large number of divisions while maintaining a high regenerative potential due to high repair activity. This potential is greatly reduced in adult HSCs. They go into a state of dormancy and anaerobic metabolism to maintain their stemness throughout life. However, with age, changes occur in the pool of HSCs that negatively affect haematopoiesis and the effectiveness of immunity. Niche aging and accumulation of mutations with age reduces the ability of HSCs to self-renew and changes their differentiation potential. This is accompanied by a decrease in clonal diversity and a disturbance of lymphopoiesis (decrease in the formation of naive T- and B-cells) and the predominance of myeloid haematopoiesis. Aging also affects mature cells, regardless of HSC, therefore, phagocytic activity and the intensity of the oxidative burst decrease, and the efficiency of processing and presentation of antigens by myeloid cells is impaired. Aging cells of innate and adaptive immunity produce factors that form a chronic inflammatory background. All these processes have a serious negative impact on the protective properties of the immune system, increasing inflammation, the risk of developing autoimmune, oncological, and cardiovascular diseases with age. Understanding the mechanisms of reducing the regenerative potential in a comparative analysis of embryonic and aging HSCs, the features of inflammatory aging will allow us to get closer to deciphering the programs for the development, aging, regeneration and rejuvenation of HSCs and the immune system.

Dynamic crosstalk between hematopoietic stem cells and their niche from emergence to aging

The behavior of somatic stem cells is regulated by their niche. Interaction between hematopoietic stem cells (HSCs) and their niches are a representative model to understand stem cell-niche interplay. Here, we provide an overview of crosstalk between HSCs and their niches in bone marrow and extramedullary organs following the life journey of HSCs from emergence, development, maturation until aging. We highlight the unique differences of HSC niches in different life stages within various organs focusing on recent literature to propose new speculations and hypotheses.

Hepatic leukemia factor-expressing paraxial mesoderm cells contribute to the developing brain vasculature

Recent genetic lineage tracing studies reveal heterogeneous origins of vascular endothelial cells and pericytes in the developing brain vasculature, despite classical experimental evidence for a mesodermal origin. Here we provide evidence through a genetic lineage tracing experiment that cephalic paraxial mesodermal cells give rise to endothelial cells and pericytes in the developing mouse brain. We show that Hepatic leukemia factor (Hlf) is transiently expressed by cephalic paraxial mesenchyme at embryonic day (E) 8.0-9.0 and the genetically-marked E8.0 Hlf-expressing cells mainly contribute to the developing brain vasculature. Interestingly, the genetically-marked E10.5 Hlf-expressing cells, which have been previously reported to contain embryonic hematopoietic stem cells, fail to contribute to the vascular cells. Combined, our genetic lineage tracing data demonstrate that a transient expression of Hlf marks a cephalic paraxial mesenchyme contributing to the developing brain vasculature.

p57Kip2 regulates embryonic blood stem cells by controlling sympathoadrenal progenitor expansion

Hematopoietic stem cells (HSCs) are of major clinical importance, and finding methods for their in vitro generation is a prime research focus. We show here that the cell cycle inhibitor p57Kip2/Cdkn1c limits the number of emerging HSCs by restricting the size of the sympathetic nervous system (SNS) and the amount of HSC-supportive catecholamines secreted by these cells. This regulation occurs at the SNS progenitor level and is in contrast to the cell-intrinsic function of p57Kip2 in maintaining adult HSCs, highlighting profound differences in cell cycle requirements of adult HSCs compared with their embryonic counterparts. Furthermore, this effect is specific to the aorta-gonad-mesonephros (AGM) region and shows that the AGM is the main contributor to early fetal liver colonization, as early fetal liver HSC numbers are equally affected. Using a range of antagonists in vivo, we show a requirement for intact β2-adrenergic signaling for SNS-dependent HSC expansion. To gain further molecular insights, we have generated a single-cell RNA-sequencing data set of all Ngfr+ sympathoadrenal cells around the dorsal aorta to dissect their differentiation pathway. Importantly, this not only defined the relevant p57Kip2-expressing SNS progenitor stage but also revealed that some neural crest cells, upon arrival at the aorta, are able to take an alternative differentiation pathway, giving rise to a subset of ventrally restricted mesenchymal cells that express important HSC-supportive factors. Neural crest cells thus appear to contribute to the AGM HSC niche via 2 different mechanisms: SNS-mediated catecholamine secretion and HSC-supportive mesenchymal cell production.

KIT is dispensable for physiological organ vascularisation in the embryo

Blood vessels form vast networks in all vertebrate organs to sustain tissue growth, repair and homeostatic metabolism, but they also contribute to a range of diseases with neovascularisation. It is, therefore, important to define the molecular mechanisms that underpin blood vessel growth. The receptor tyrosine kinase KIT is required for the normal expansion of hematopoietic progenitors that arise during embryogenesis from hemogenic endothelium in the yolk sac and dorsal aorta. Additionally, KIT has been reported to be expressed in endothelial cells during embryonic brain vascularisation and has been implicated in pathological angiogenesis. However, it is neither known whether KIT expression is widespread in normal organ endothelium nor whether it promotes blood vessel growth in developing organs. Here, we have used single-cell analyses to show that KIT is expressed in endothelial cell subsets of several organs, both in the adult and in the developing embryo. Knockout mouse analyses revealed that KIT is dispensable for vascularisation of growing organs in the midgestation embryo, including the lung, liver and brain. By contrast, vascular changes emerged during late-stage embryogenesis in these organs from KIT-deficient embryos, concurrent with severe erythrocyte deficiency and growth retardation. These findings suggest that KIT is not required for developmental tissue vascularisation in physiological conditions, but that KIT deficiency causes foetal anaemia at late gestation and thereby pathological vascular remodelling.

A common epigenetic mechanism across different cellular origins underlies systemic immune dysregulation in an idiopathic autism mouse model

Immune dysregulation plays a key role in the pathogenesis of autism. Changes occurring at the systemic level, from brain inflammation to disturbed innate/adaptive immune in the periphery, are frequently observed in patients with autism; however, the intrinsic mechanisms behind them remain elusive. We hypothesize a common etiology may lie in progenitors of different types underlying widespread immune dysregulation. By single-cell RNA sequencing (sc-RNA seq), we trace the developmental origins of immune dysregulation in a mouse model of idiopathic autism. It is found that both in aorta-gonad-mesonephros (AGM) and yolk sac (YS) progenitors, the dysregulation of HDAC1-mediated epigenetic machinery alters definitive hematopoiesis during embryogenesis and downregulates the expression of the AP-1 complex for microglia development. Subsequently, these changes result in the dysregulation of the immune system, leading to gut dysbiosis and hyperactive microglia in the brain. We further confirm that dysregulated immune profiles are associated with specific microbiota composition, which may serve as a biomarker to identify autism of immune-dysregulated subtypes. Our findings elucidate a shared mechanism for the origin of immune dysregulation from the brain to the gut in autism and provide new insight to dissecting the heterogeneity of autism, as well as the therapeutic potential of targeting immune-dysregulated autism subtypes.

Astrocyte-derived hepcidin controls iron traffic at the blood-brain-barrier via regulating ferroportin 1 of microvascular endothelial cells

Brain iron dysregulation associated with aging is closely related to motor and cognitive impairments in neurodegenerative diseases. The regulation of iron traffic at the blood-brain barrier (BBB) is crucial to maintain brain iron homeostasis. However, the specific mechanism has not been clarified in detail. Using various conditional gene knockout and overexpression mice, as well as cell co-culture of astrocyte and bEND.3 in the transwell, we found that astrocyte hepcidin knockdown increased the expression of ferroportin 1 (FPN1) of brain microvascular endothelial cells (BMVECs), and that it also induced brain iron overload and cognitive decline in mice. Moreover, BMVECs FPN1 knockout decreased iron contents in the cortex and hippocampus. Furthermore, hepcidin regulates the level of FPN1 of BMVECs with conditional gene overexpression in vivo and in vitro. Our results revealed that astrocytes responded to the intracellular high iron level and increased the secretion of hepcidin, which in turn diminished iron uptake at BBB from circulation through directly regulating FPN1 of BMVECs. Our results demonstrate that FPN1 of BMVECs is a gateway for iron transport into the brain from circulation, and the controller of this gateway is hepcidin secreted by astrocyte at its endfeet through physical contact with BMVECs. This regulation is indeed the major checkpoint for iron transport from the blood circulation to the brain. This study delineates the pathway and regulation of iron entry into the brain, providing potential therapeutic targets for iron dysregulation-related neurological diseases.

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

Temporal-spatial low shear stress induces heterogenous distribution of hematopoietic stem cell budding in zebrafish

Hematopoietic stem/progenitor cells (HSPCs) originate from endothelial cells (ECs) localized on the ventral side of the dorsal aorta (DA), and hemodynamic parameters may suffer sharp changes in DA at HSPCs development stage for intersegmental vessel formation. However, the temporal-spatial shear stress parameters and biomechanics mechanisms of HSPC budding remain unknown. Here, we found that the hematopoietic endothelium (HE) in the aorta-gonad-mesonephros was heterogeneous; that is, HEs were mainly distributed at the ventral side of the vascular bifurcation in zebrafish embryos, which was found to show low shear stress (LSS) through numerical simulation analysis. Furthermore, HSPCs localized in the posterior somite of aorta-gonad-mesonephros with slow velocity. On the temporal scale, there was a slow velocity and LSS during HE budding from 36 h post-fertilization and decreased shear stress with drug expanded HSPC numbers. Mechanistically, matrix metalloproteinase (MMP) expression and macrophage chemotaxis were significantly increased in HEs by RNA-seq. After treatment with an MMP13 inhibitor, HSPCs were significantly reduced in both the aorta-gonad-mesonephros and caudal hematopoietic tissue in embryos. Our results show that HSPC budding is heterogeneous, and the mechanism is that physiological LSS controls the emergence of HSPCs by promoting the accumulation of macrophages and subsequent MMP expression.

The embryonic zebrafish brain is seeded by a lymphatic-dependent population of mrc1+ microglia precursors

Microglia are the resident macrophages of the CNS that serve critical roles in brain construction. Although human brains contain microglia by 4 weeks gestation, an understanding of the earliest microglia that seed the brain during its development remains unresolved. Using time-lapse imaging in zebrafish, we discovered a mrc1a+ microglia precursor population that seeds the brain before traditionally described microglia. These early microglia precursors are dependent on lymphatic vasculature that surrounds the brain and are independent of pu1+ yolk sac-derived microglia. Single-cell RNA-sequencing datasets reveal Mrc1+ microglia in the embryonic brains of mice and humans. We then show in zebrafish that these early mrc1a+ microglia precursors preferentially expand during pathophysiological states in development. Taken together, our results identify a critical role of lymphatics in the microglia precursors that seed the early embryonic brain.

Embryonic vascular establishment requires protein C receptor-expressing endothelial progenitors

Vascular establishment is one of the early events in embryogenesis. It is believed that vessel-initiating endothelial progenitors cluster to form the first primitive vessel. Understanding the molecular identity of these progenitors is crucial in order to elucidate lineage hierarchy. In this study, we identify protein C receptor (Procr) as an endothelial progenitor marker and investigate the role of Procr+ progenitors during embryonic vascular development. Using a ProcrmGFP-2A-lacZ reporter, we reveal a much earlier Procr expression (embryonic day 7.5) than previously acknowledged (embryonic day 13.5). Genetic fate-mapping experiments using ProcrCre and ProcrCreER demonstrate that Procr+ cells give rise to blood vessels throughout the entire embryo proper. Single-cell RNA-sequencing analyses place Procr+ cells at the start of endothelial commitment and maturation. Furthermore, targeted ablation of Procr+ cells results in failure of vessel formation and early embryonic lethality. Notably, genetic fate mapping and scRNA-seq pseudotime analysis support the view that Procr+ progenitors can give rise to hemogenic endothelium. In this study, we establish a Procr expression timeline and identify Procr+ vessel-initiating progenitors, and demonstrate their indispensable role in establishment of the vasculature during embryo development.

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.

Mapping human haematopoietic stem cells from haemogenic endothelium to birth

The ontogeny of human haematopoietic stem cells (HSCs) is poorly defined owing to the inability to identify HSCs as they emerge and mature at different haematopoietic sites¹. Here we created a single-cell transcriptome map of human haematopoietic tissues from the first trimester to birth and found that the HSC signature RUNX1⁺HOXA9⁺MLLT3⁺MECOM⁺HLF⁺SPINK2⁺ distinguishes HSCs from progenitors throughout gestation. In addition to the aorta-gonad-mesonephros region, nascent HSCs populated the placenta and yolk sac before colonizing the liver at 6 weeks. A comparison of HSCs at different maturation stages revealed the establishment of HSC transcription factor machinery after the emergence of HSCs, whereas their surface phenotype evolved throughout development. The HSC transition to the liver marked a molecular shift evidenced by suppression of surface antigens reflecting nascent HSC identity, and acquisition of the HSC maturity markers CD133 (encoded by PROM1) and HLA-DR. HSC origin was tracked to ALDH1A1⁺KCNK17⁺ haemogenic endothelial cells, which arose from an IL33⁺ALDH1A1⁺ arterial endothelial subset termed pre-haemogenic endothelial cells. Using spatial transcriptomics and immunofluorescence, we visualized this process in ventrally located intra-aortic haematopoietic clusters. The in vivo map of human HSC ontogeny validated the generation of aorta-gonad-mesonephros-like definitive haematopoietic stem and progenitor cells from human pluripotent stem cells, and serves as a guide to improve their maturation to functional HSCs.

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

The past five decades have seen significant progress in our understanding of human hematopoiesis. This has in part been due to the unprecedented development of advanced technologies, which have allowed the identification and characterization of rare subsets of human hematopoietic stem and progenitor cells and their lineage trajectories from embryonic through to adult life. Additionally, surrogate in vitro and in vivo models, although not fully recapitulating human hematopoiesis, have spurred on these scientific advances. These approaches have heightened our knowledge of hematological disorders and diseases and have led to their improved diagnosis and therapies. Here, we review human hematopoiesis at each end of the age spectrum, during embryonic and fetal development and on aging, providing exemplars of recent progress in deciphering the increasingly complex cellular and molecular hematopoietic landscapes in health and disease. This review concludes by highlighting links between chronic inflammation and metabolic and epigenetic changes associated with aging and in the development of clonal hematopoiesis.

Developmental programming of macrophages by early life adversity

Macrophages are central elements of all organs, where they have a multitude of physiological and pathological functions. The first macrophages are produced during fetal development, and most adult organs retain populations of fetal-derived macrophages that self-maintain without major input of hematopoietic stem cell-derived monocytes. Their developmental origins make macrophages highly susceptible to environmental perturbations experienced in early life, in particular the fetal period. It is now well recognized that such adverse developmental conditions contribute to a wide range of diseases later in life. This chapter explores the notion that macrophages are key targets of environmental adversities during development, and mediators of their long-term impact on health and disease. We first briefly summarize our current understanding of macrophage ontogeny and their biology in tissues and consider potential mechanisms by which environmental stressors may mediate fetal programming. We then review evidence for programming of macrophages by adversities ranging from maternal immune activation and diet to environmental pollutants and toxins, which have disease relevance for different organ systems. Throughout this chapter, we contemplate appropriate experimental strategies to study macrophage programming. We conclude by discussing how our current knowledge of macrophage programming could be conceptualized, and finally highlight open questions in the field and approaches to address them.

New insights into Human Hematopoietic Stem and Progenitor Cells via Single-Cell Omics

Residing at the apex of the hematopoietic hierarchy, hematopoietic stem and progenitor cells (HSPCs) give rise to all mature blood cells. In the last decade, significant progress has been made in single-cell RNA sequencing as well as multi-omics technologies that have facilitated elucidation of the heterogeneity of previously defined human HSPCs. From the embryonic stage through the adult stage to aging, single-cell studies have enabled us to trace the origins of hematopoietic stem cells (HSCs), demonstrating different hematopoietic differentiation during development, as well as identifying novel cell populations. In both hematological benign diseases and malignancies, single-cell omics technologies have begun to reveal tissue heterogeneity and have permitted mapping of microenvironmental ecosystems and tracking of cell subclones, thereby greatly broadening our understanding of disease development. Furthermore, advances have also been made in elucidating the molecular mechanisms for relapse and identifying therapeutic targets of hematological disorders and other non-hematological diseases. Extensive exploration of hematopoiesis at the single-cell level may thus have great potential for broad clinical applications of HSPCs, as well as disease prognosis.

Macrophage compartmentalization in the brain and cerebrospinal fluid system

Macrophages reside within the diverse anatomical compartments of the central nervous system (CNS). Within each compartment, these phagocytes are exposed to unique combinations of niche signals and mechanical stimuli that instruct their tissue-specific identities. Whereas most CNS macrophages are tissue-embedded, the macrophages of the cerebrospinal fluid (CSF) system are bathed in an oscillating liquid. Studies using multiomics technologies have recently uncovered the transcriptomic and proteomic profiles of CSF macrophages, enhancing our understanding of their cellular characteristics in both rodents and humans. Here, we review the relationships between CNS macrophage populations, with a focus on the origins, phenotypes, and functions of CSF macrophages in health and disease.

Notch Signaling in HSC Emergence: When, Why and How

The hematopoietic stem cell (HSC) sustains blood homeostasis throughout life in vertebrates. During embryonic development, HSCs emerge from the aorta-gonads and mesonephros (AGM) region along with hematopoietic progenitors within hematopoietic clusters which are found in the dorsal aorta, the main arterial vessel. Notch signaling, which is essential for arterial specification of the aorta, is also crucial in hematopoietic development and HSC activity. In this review, we will present and discuss the evidence that we have for Notch activity in hematopoietic cell fate specification and the crosstalk with the endothelial and arterial lineage. The core hematopoietic program is conserved across vertebrates and here we review studies conducted using different models of vertebrate hematopoiesis, including zebrafish, mouse and in vitro differentiated Embryonic stem cells. To fulfill the goal of engineering HSCs in vitro, we need to understand the molecular processes that modulate Notch signaling during HSC emergence in a temporal and spatial context. Here, we review relevant contributions from different model systems that are required to specify precursors of HSC and HSC activity through Notch interactions at different stages of development.

Recent Advances in Developmental Hematopoiesis: Diving Deeper With New Technologies

The journey of a hematopoietic stem cell (HSC) involves the passage through successive anatomical sites where HSCs are in direct contact with their surrounding microenvironment, also known as niche. These spatial and temporal cellular interactions throughout development are required for the acquisition of stem cell properties, and for maintaining the HSC pool through balancing self-renewal, quiescence and lineage commitment. Understanding the context and consequences of these interactions will be imperative for our understanding of HSC biology and will lead to the improvement of in vitro production of HSCs for clinical purposes. The aorta-gonad-mesonephros (AGM) region is in this light of particular interest since this is the cradle of HSC emergence during the embryonic development of all vertebrate species. In this review, we will focus on the developmental origin of HSCs and will discuss the novel technological approaches and recent progress made to identify the cellular composition of the HSC supportive niche and the underlying molecular events occurring in the AGM region.

Wild-Type p53-Induced Phosphatase 1 Plays a Positive Role in Hematopoiesis in the Mouse Embryonic Head

The first adult repopulating hematopoietic stem cells (HSCs) are found in the aorta-gonad-mesonephros (AGM) region, which are produced from hemogenic endothelial cells. Embryonic head is the other site for HSC development. Wild-type p53-induced phosphatase 1 (Wip1) is a type-2Cδ family serine/threonine phosphatase involved in various cellular processes such as lymphoid development and differentiation of adult HSCs. Most recently, we have shown that Wip1 modulates the pre-HSC maturation in the AGM region. However, it is not clear whether Wip1 regulates hematopoiesis in the embryonic head. Here we reported that disruption of Wip1 resulted in a decrease of hematopoietic progenitor cell number in the embryonic head. In vivo transplantation assays showed a reduction of HSC function after Wip1 ablation. We established that Wip1 deletion reduced the frequency and cell number of microglia in the embryonic head. Further observations revealed that Wip1 absence enhanced the gene expression of microglia-derived pro-inflammatory factors. Thus, it is likely that Wip1 functions as a positive regulator in HSC development by regulating the function of microglia in the embryonic head.

Infant leukaemia - faithful models, cell of origin and the niche

For patients and their families, the diagnosis of infant leukaemia is devastating. This disease has not seen the improvements in outcomes experienced with other paediatric leukaemias and it is becoming ever more apparent that infant leukaemia is a distinct biological entity. Insights into some of the distinguishing features of infant leukaemia, such as a single mutation - the MLL-gene rearrangement, the biology of disease aggressiveness and lineage plasticity, and the high incidence of central nervous system involvement, are likely to be gained from understanding the interactions between leukaemic cells and their environment or niche. The origins of infant leukaemia lie in the embryonic haematopoietic system, which is characterised by shifting locations and dynamic changes in the microenvironment. Understanding this foetal or embryonic context is integral to understanding infant leukaemia development. Owing to its rarity and prenatal origins, developing accurate modelling systems for further investigation of infant leukaemia is essential. In this Review, we discuss how available in vitro, ex vivo and in vivo infant leukaemia models contribute to our current understanding of the leukaemia niche in embryonic development, established disease and specialised non-haematopoietic niches. The mechanistic insights provided by accurate models will help identify viable novel therapeutic options.

Hlf Expression Marks Early Emergence of Hematopoietic Stem Cell Precursors With Adult Repopulating Potential and Fate

In the aorta-gonad-mesonephros (AGM) region of mouse embryos, pre-hematopoietic stem cells (pre-HSCs) are generated from rare and specialized hemogenic endothelial cells (HECs) via endothelial-to-hematopoietic transition, followed by maturation into bona fide hematopoietic stem cells (HSCs). As HECs also generate a lot of hematopoietic progenitors not fated to HSCs, powerful tools that are pre-HSC/HSC-specific become urgently critical. Here, using the gene knockin strategy, we firstly developed an Hlf-tdTomato reporter mouse model and detected Hlf-tdTomato expression exclusively in the hematopoietic cells including part of the immunophenotypic CD45^– and CD45⁺ pre-HSCs in the embryonic day (E) 10.5 AGM region. By in vitro co-culture together with long-term transplantation assay stringent for HSC precursor identification, we further revealed that unlike the CD45^– counterpart in which both Hlf-tdTomato-positive and negative sub-populations harbored HSC competence, the CD45⁺ E10.5 pre-HSCs existed exclusively in Hlf-tdTomato-positive cells. The result indicates that the cells should gain the expression of Hlf prior to or together with CD45 to give rise to functional HSCs. Furthermore, we constructed a novel Hlf-CreER mouse model and performed time-restricted genetic lineage tracing by a single dose induction at E9.5. We observed the labeling in E11.5 AGM precursors and their contribution to the immunophenotypic HSCs in fetal liver (FL). Importantly, these Hlf-labeled early cells contributed to and retained the size of the HSC pool in the bone marrow (BM), which continuously differentiated to maintain a balanced and long-term multi-lineage hematopoiesis in the adult. Therefore, we provided another valuable mouse model to specifically trace the fate of emerging HSCs during development.

Discovering Myeloid Cell Heterogeneity in Mandibular Bone - Cell by Cell Analysis

The myeloid-derived bone marrow progenitor populations from different anatomical locations are known to have diverse osteoclastogenesis potential. Specifically, myeloid progenitors from the tibia and femur have increased osteoclast differentiation potential compared to myeloid progenitors from the alveolar process. In this study, we explored the differences in the myeloid lineage progenitor cell populations in alveolar (mandibular) bone versus long (femur) bone using flow cytometry and high-throughput single cell RNA sequencing (scRNA-seq) to provide a comprehensive transcriptional landscape. Results indicate that mandibular bone marrow-derived cells exhibit consistent deficits in myeloid differentiation, including significantly fewer myeloid-derived suppressor cell (MDSC)-like populations (CD11b+Ly6C+, CD11b+Ly6G+), as well as macrophages (CD11b+F4/80+). Although significantly fewer in number, MDSCs from mandibular bone exhibited increased immunosuppressive activity compared to MDSCs isolated from long bone. Using flow cytometry panels specific for bone marrow progenitors, analysis of hematopoietic stem cells showed no defects in mandibular bone marrow in LSK (Lin-Sca1+cKit+) cell and LK (Lin-Sca1-cKit+) cell populations. While there was no significant difference in granulocyte progenitors, the granulocyte-monocyte progenitors and monocyte progenitor population were significantly decreased in the mandibular bone marrow. T-lymphocyte subsets were not significantly different between mandibular and femoral bone, except for CD4+CD25+Foxp3+ regulatory T lymphocytes, which were significantly increased in the mandible. In addition, B lymphocytes were significantly increased in mandible. Single cell RNA sequencing from mandible and femur BM revealed distinct differences in transcriptomic profiles in myeloid populations establishing previously unappreciated aspects of mandibular bone marrow populations. These analyses reveal site-specific differences in the myeloid progenitor cellular composition and transcriptional programs providing a deeper appreciation of the complex differences in myeloid cell heterogeneity from different anatomical bone marrow sites.

Brain endothelial PTEN/AKT/NEDD4-2/MFSD2A axis regulates blood-brain barrier permeability

The low level of transcytosis is a unique feature of cerebrovascular endothelial cells (ECs), ensuring restrictive blood-brain barrier (BBB) permeability. Major facilitator superfamily domain-containing 2a (MFSD2A) is a key regulator of the BBB function by suppressing caveolae-mediated transcytosis. However, the mechanisms regulating MFSD2A at the BBB have been barely explored. Here, we show that cerebrovascular EC-specific deletion of Pten (phosphatase and tensin homolog) results in a dramatic increase in vesicular transcytosis by the reduction of MFSD2A, leading to increased transcellular permeability of the BBB. Mechanistically, AKT signaling inhibits E3 ubiquitin ligase NEDD4-2-mediated MFSD2A degradation. Consistently, cerebrovascular Nedd4-2 overexpression decreases MFSD2A levels, increases transcytosis, and impairs BBB permeability, recapitulating the phenotypes of Pten-deficient mice. Furthermore, Akt deletion decreases phosphorylated NEDD4-2 levels, restores MFSD2A levels, and normalizes BBB permeability in Pten-mutant mice. Altogether, our work reveals the essential physiological function of the PTEN/AKT/NEDD4-2/MFSD2A axis in the regulation of BBB permeability.

Adult-repopulating lymphoid potential of yolk sac blood vessels is not confined to arterial endothelial cells

During embryogenesis, hematopoietic stem progenitor cells (HSPCs) are believed to be derived from hemogenic endothelial cells (HECs). Moreover, arterial feature is proposed to be a prerequisite for HECs to generate HSPCs with lymphoid potential. Although the molecular basis of hematopoietic stem cell-competent HECs has been delicately elucidated within the embryo proper, the functional and molecular characteristics of HECs in the extraembryonic yolk sac (YS) remain largely unresolved. In this study, we initially identified six molecularly different endothelial populations in the midgestational YS through integrated analysis of several single-cell RNA sequencing (scRNA-seq) datasets and validated the arterial vasculature distribution of Gja5⁺ ECs using a Gja5-EGFP reporter mouse model. Further, we explored the hemogenic potential of different EC populations based on their Gja5-EGFP and CD44 expression levels. The hemogenic potential was ubiquitously detected in spatiotemporally different vascular beds on embryonic days (E)8.5-E9.5 and gradually concentrated in CD44-positive ECs from E10.0. Unexpectedly, B-lymphoid potential was detected in the YS ECs as early as E8.5 regardless of their arterial features. Furthermore, the capacity for generating hematopoietic progenitors with in vivo lymphoid potential was found in nonarterial as well as arterial YS ECs on E10.0-E10.5. Importantly, the distinct identities of E10.0-E10.5 HECs between YS and intraembryonic caudal region were revealed by further scRNA-seq analysis. Cumulatively, these findings extend our knowledge regarding the hemogenic potential of ECs from anatomically and molecularly different vascular beds, providing a theoretical basis for better understanding the sources of HSPCs during mammalian development.

First blood: the endothelial origins of hematopoietic progenitors

Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine.

The migratory pathways of the cells that form the endocardium, dorsal aortae, and head vasculature in the mouse embryo

Background

Vasculogenesis in amniotes is often viewed as two spatially and temporally distinct processes, occurring in the yolk sac and in the embryo. However, the spatial origins of the cells that form the primary intra-embryonic vasculature remain uncertain. In particular, do they obtain their haemato-endothelial cell fate in situ, or do they migrate from elsewhere? Recently developed imaging techniques, together with new Tal1 and existing Flk1 reporter mouse lines, have allowed us to investigate this question directly, by visualising cell trajectories live and in three dimensions.

Results

We describe the pathways that cells follow to form the primary embryonic circulatory system in the mouse embryo. In particular, we show that Tal1-positive cells migrate from within the yolk sac, at its distal border, to contribute to the endocardium, dorsal aortae and head vasculature. Other Tal1 positive cells, similarly activated within the yolk sac, contribute to the yolk sac vasculature. Using single-cell transcriptomics and our imaging, we identify VEGF and Apela as potential chemo-attractants that may regulate the migration into the embryo. The dorsal aortae and head vasculature are known sites of secondary haematopoiesis; given the common origins that we observe, we investigate whether this is also the case for the endocardium. We discover cells budding from the wall of the endocardium with high Tal1 expression and diminished Flk1 expression, indicative of an endothelial to haematopoietic transition.

Conclusions

In contrast to the view that the yolk sac and embryonic circulatory systems form by two separate processes, our results indicate that Tal1-positive cells from the yolk sac contribute to both vascular systems. It may be that initial Tal1 activation in these cells is through a common mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-021-00239-3.

Fetal-Derived Immune Cells at the Roots of Lifelong Pathophysiology

Tissue-resident innate immune cells exert a wide range of functions in both adult homeostasis and pathology. Our understanding of when and how these cellular networks are established has dramatically changed with the recognition that many lineages originate at least in part from fetal sources and self-maintain independently from hematopoietic stem cells. Indeed, fetal-derived immune cells are found in most organs and serous cavities of our body, where they reside throughout the entire lifespan. At the same time, there is a growing appreciation that pathologies manifesting in adulthood may be caused by adverse early life events, a concept known as “developmental origins of health and disease” (DOHaD). Yet, whether fetal-derived immune cells are mechanistically involved in DOHaD remains elusive. In this review, we summarize our knowledge of fetal hematopoiesis and its contribution to adult immune compartments, which results in a “layered immune system.” Based on their ontogeny, we argue that fetal-derived immune cells are prime transmitters of long-term consequences of prenatal adversities. In addition to increasing disease susceptibility, these may also directly cause inflammatory, degenerative, and metabolic disorders. We explore this notion for cells generated from erythro-myeloid progenitors (EMP) produced in the extra-embryonic yolk sac. Focusing on macrophages and mast cells, we present emerging evidence implicating them in lifelong disease by either somatic mutations or developmental programming events resulting from maternal and early environmental perturbations.

Contributions of Embryonic HSC-Independent Hematopoiesis to Organogenesis and the Adult Hematopoietic System

During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.

The hemogenic endothelium: a critical source for the generation of PSC-derived hematopoietic stem and progenitor cells

In vitro generation of hematopoietic cells and especially hematopoietic stem cells (HSCs) from human pluripotent stem cells (PSCs) are subject to intensive research in recent decades, as these cells hold great potential for regenerative medicine and autologous cell replacement therapies. Despite many attempts, in vitro, de novo generation of bona fide HSCs remains challenging, and we are still far away from their clinical use, due to insufficient functionality and quantity of the produced HSCs. The challenges of generating PSC-derived HSCs are already apparent in early stages of hemato-endothelial specification with the limitation of recapitulating complex, dynamic processes of embryonic hematopoietic ontogeny in vitro. Further, these current shortcomings imply the incompleteness of our understanding of human ontogenetic processes from embryonic mesoderm over an intermediate, specialized hemogenic endothelium (HE) to their immediate progeny, the HSCs. In this review, we examine the recent investigations of hemato-endothelial ontogeny and recently reported progress for the conversion of PSCs and other promising somatic cell types towards HSCs with the focus on the crucial and inevitable role of the HE to achieve the long-standing goal—to generate therapeutically applicable PSC-derived HSCs in vitro.