LYVE1 Marks the Divergence of Yolk Sac Definitive Hemogenic Endothelium from the Primitive Erythroid Lineage

Single-cell dynamic RNA and glycosylation sequencing reveals the mechanism underlying the differentiation of pluripotent stem cells into hematopoietic stem cells

Studying the mechanism of hematopoietic stem cells' generation from induced pluripotent stem cells in vitro can be useful for understanding embryonic hematopoiesis, as well as for the application of related cell therapy. This study aimed to delineate the process of the differentiation of induced pluripotent stem cells into hematopoietic stem cells' models and provide a theoretical basis and clinical value for the production of hematopoietic stem cells in vitro. We analyzed the differentiation model by single-cell dynamic transcriptome and glycosylation sequencing, which was divided into three differentiation stages based on the new-to-total RNA ratio and glycosylation level. Two differentiation fates were found in the pseudo-time, including hematopoietic development and other tissue development. Precursor hematopoietic cells with a high glycosylation level greatly expressed hematopoietic regulation and vascular endothelial genes, suggesting that glycosylation is associated with angiogenesis and hematopoietic regulation. The multiple differentiation events in the in vitro model are similar to those in hematopoietic development in vivo, including yolk sac hematopoiesis, cellular communication between non-potential hematopoietic subsets and potential hematopoietic subsets, gene expression, and temporal deviations in hematopoietic fate. Our study has revealed the similar hematopoiesis process in the differentiation model via single-cell dynamic RNA and glycosylation sequencing, which provides an important theoretical basis for the study of hematopoietic stem cell development.

Lyve1-Driven NrasQ61R Causes Edema, Enlarged Lymphatic Vessels, and Hepatic Vascular Defects in Embryonic Mice

BACKGROUND

Kaposiform lymphangiomatosis (KLA) is a complex lymphatic anomaly associated with a somatic activating NRAS p.Q61R (NRASQ61R) mutation. KLA is characterized by malformed lymphatic vessels that can lead to effusions and coagulopathy. The goal of this study was to generate an in vivo mouse model to determine if prenatal expression of the NrasQ61R mutation in lymphatic endothelial cells induces disease characteristics found in KLA patients.

PROCEDURE

A Cre-loxP system was used to conditionally express NrasQ61R in cells expressing lymphatic vessel endothelial hyaluronan receptor 1 (Lyve1), a marker of lymphatic and other types of endothelial cells that starts being expressed at embryonic day (E) 7.5. Because pups did not survive birth, embryos were collected at E14.5, E15.5, and E18.5 for gross analysis, histology and immunostaining, and organ whole-mounts.

RESULTS

Staining for NRASQ61R demonstrated robust recombination in the NrasQ61R mutant embryos and localization of NrasQ61R at sites of vascular abnormalities. NrasQ61R mutant embryos had significant edema and dysmorphic jugular lymph sacs with abnormal Lyve1-positive cellular masses. The lymphatic vessel network in the back skin of the NrasQ61R mutant embryos had fewer branch points and increased vessel diameter. NrasQ61R mutant embryos had severe hepatic defects characterized by disordered and enlarged vessels. By E18.5, NrasQ61R mutant embryos were dead.

CONCLUSIONS

Conditional expression of NrasQ61R in Lyve1-positive cells caused edema, abnormal lymphatic development, and hepatic vascular defects in mouse embryos. These findings further support the role of NRASQ61R as a driver of the lymphatic overgrowth, vessel enlargement, and dysfunction in the pathophysiology of KLA.

Comprehensive analysis of myeloid reporter mice

Macrophages are a pivotal cell type within the synovial lining and sub-lining of the joint, playing a crucial role in maintaining homeostasis of synovium. Although fate-mapping techniques have been employed to differentiate synovial macrophages from other synovial myeloid cells, no comprehensive study has yet been conducted within the mouse synovial macrophage compartment. In this study, we present, for the first time, lineage tracing results from 18 myeloid-specific fate-mapping models in mouse peripheral blood (PB) and synovial tissue. The identification of synovial macrophages and monocyte-lineage cells through flow cytometry was further validated using cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) datasets. These findings provide a valuable methodological tool for researchers to select appropriate models for studying the function of synovial myeloid cells and serve as a reference for investigations in other tissue types.

Conversion of placental hemogenic endothelial cells to hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cells (HSPCs) are critical for the treatment of blood diseases in clinic. However, the limited source of HSPCs severely hinders their clinical application. In the embryo, hematopoietic stem cells (HSCs) arise from hemogenic endothelial (HE) cells lining the major arteries in vivo. In this work, by engineering vascular niche endothelial cells (VN-ECs), we generated functional HSPCs in vitro from ECs at various sites, including the aorta-gonad-mesonephros (AGM) region and the placenta. Firstly, we converted mouse embryonic HE cells from the AGM region (aHE) into induced HSPCs (iHSPCs), which have the abilities for multilineage differentiation and self-renewal. Mechanistically, we found that VN-ECs can promote the generation of iHSPCs via secretion of CX3CL1 and IL1A. Next, through VN-EC co-culture, we showed that placental HE (pHE) cells, a type of extra-embryonic HE cells, were successfully converted into iHSPCs (pHE-iHSPCs), which have multilineage differentiation capacity, but exhibit limited self-renewal ability. Furthermore, comparative transcriptome analysis of aHE-iHSPCs and pHE-iHSPCs showed that aHE-iHSPCs highly expressed HSC-specific and self-renewal-related genes. Moreover, experimental validation showed that retinoic acid (RA) treatment promoted the transformation of pHE cells into iHSPCs that have self-renewal ability. Collectively, our results suggested that pHE cells possess the potential to transform into self-renewing iHSPCs through RA treatment, which will facilitate the clinical application of placental endothelial cells in hematopoietic cell generation.

Mouse and human macrophages and their roles in cardiovascular health and disease

The past 15 years have witnessed a leap in understanding the life cycle, gene expression profiles, origins and functions of mouse macrophages in many tissues, including macrophages of the artery wall and heart that have critical roles in cardiovascular health. Here, we review the phenotypical and functional diversity of macrophage populations in multiple organs and discuss the roles that proliferation, survival, and recruitment and replenishment from monocytes have in maintaining macrophages in homeostasis and inflammatory states such as atherosclerosis and myocardial infarction. We also introduce emerging data that better characterize the life cycle and phenotypic profiles of human macrophages. We discuss the similarities and differences between murine and human macrophages, raising the possibility that tissue-resident macrophages in humans may rely more on bone marrow-derived monocytes than in mouse.

New insights into the endothelial origin of hematopoietic system inspired by "TIF" approaches

Hematopoietic stem progenitor cells (HSPCs) are derived from a specialized subset of endothelial cells named hemogenic endothelial cells (HECs) via a process of endothelial-to-hematopoietic transition during embryogenesis. Recently, with the usage of multiple single-cell technologies and advanced genetic lineage tracing techniques, namely, "TIF" approaches that combining transcriptome, immunophenotype and function/fate analyses, massive new insights have been achieved regarding the cellular and molecular evolution underlying the emergence of HSPCs from embryonic vascular beds. In this review, we focus on the most recent advances in the enrichment markers, functional characteristics, developmental paths, molecular controls, and the embryonic site-relevance of the key intermediate cell populations bridging embryonic vascular and hematopoietic systems, namely HECs and pre-hematopoietic stem cells, the immediate progenies of some HECs, in mouse and human embryos. Specifically, using expression analyses at both transcriptional and protein levels and especially efficient functional assays, we propose that the onset of Kit expression is at the HEC stage, which has previously been controversial.

Erythropoiesis in the mammalian embryo

Red blood cells (RBCs) comprise a critical component of the cardiovascular network, which constitutes the first functional organ system of the developing mammalian embryo. Examination of circulating blood cells in mammalian embryos revealed two distinct types of erythroid cells: large, nucleated "primitive" erythroblasts followed by smaller, enucleated "definitive" erythrocytes. This review describes the current understanding of primitive and definitive erythropoiesis gleaned from studies of mouse and human embryos and induced pluripotent stem cells (iPSCs). Primitive erythropoiesis in the mouse embryo comprises a transient wave of committed primitive erythroid progenitors (primitive erythroid colony-forming cells, EryP-CFC) in the early yolk sac that generates a robust cohort of precursors that mature in the bloodstream and enucleate. In contrast, definitive erythropoiesis has two distinct developmental origins. The first comprises a transient wave of definitive erythroid progenitors (burst-forming units erythroid, BFU-E) that emerge in the yolk sac and seed the fetal liver where they terminally mature to provide the first definitive RBCs. The second comprises hematopoietic stem cell (HSC)-derived BFU-E that terminally mature at sites colonized by HSCs particularly the fetal liver and subsequently the bone marrow. Primitive and definitive erythropoiesis are derived from endothelial identity precursors with distinct developmental origins. Although they share prototypical transcriptional regulation, primitive and definitive erythropoiesis are also characterized by distinct lineage-specific factors. The exquisitely timed, sequential production of primitive and definitive erythroid cells is necessary for the survival and growth of the mammalian embryo.

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).

New advances of NG2-expressing cell subset in marrow mesenchymal stem cells as novel therapeutic tools for liver fibrosis/cirrhosis

BACKGROUND

Bone marrow-derived mesenchymal stem cell (BMMSC)-based therapy has become a major focus for treating liver fibrosis/cirrhosis. However, although these cell therapies promote the treatment of this disease, the heterogeneity of BMMSCs, which causes insufficient efficacy during clinical trials, has not been addressed. In this study, we describe a novel Percoll-Plate-Wait procedure (PPWP) for the isolation of an active cell subset from BMMSC cultures that was characterized by the expression of neuroglial antigen 2 (NG2/BMMSCs).

METHODS

By using the key method of PPWP and other classical biological techniques we compared NG2/BMMSCs with parental BMMSCs in biological and functional characteristics within a well-defined diethylnitrosamine (DEN)-induced liver fibrosis/cirrhosis injury male C57BL/6 mouse model also in a culture system. Of note, the pathological alterations in the model is quite similar to humans'.

RESULTS

The NG2/BMMSCs revealed more advantages compared to parentalBMMSCs. They exhibited greater proliferation potential than parental BMMSCs, as indicated by Ki-67 immunofluorescence (IF) staining. Moreover, higher expression of SSEA-3 (a marker specific for embryonic stem cells) was detected in NG2/BMMSCs than in parental BMMSCs, which suggested that the "stemness" of NG2/BMMSCs was greater than that of parental BMMSCs. In vivo studies revealed that an injection of NG2/BMMSCs into mice with ongoing DEN-induced liver fibrotic/cirrhotic injury enhanced repair and functional recovery to a greater extent than in mice treated with parental BMMSCs. These effects were associated with the ability of NG2/BMMSCs to differentiate into bile duct cells (BDCs). In particular, we discovered for the first time that NG2/BMMSCs exhibit unique characteristics that differ from those of parental BMMSCs in terms of producing liver sinusoidal endothelial cells (LSECs) to reconstruct injured blood vessels and sinusoidal structures in the diseased livers, which are important for initiating hepatocyte regeneration. This unique potential may also suggest that NG2/BMMSCs could be an novel off-liver progenitor of LSECs. Ex vivo studies revealed that the NG2/BMMSCs exhibited a similar trend to that of their in vivo in terms of functional differentiation responding to the DEN-diseased injured liver cues. Additionally, the obvious core role of NG2/BMMSCs in supporting the functions of BMMSCs in bile duct repair and BDC-mediated hepatocyte regeneration might also be a novel finding.

CONCLUSIONS

Overall, the PPWP-isolated NG2/BMMSCs could be a novel effective cell subset with increased purity to serve as a new therapeutic tool for enhancing treatment efficacy of BMMSCs and special seed cell source (BDCs, LSECs) also for bioliver engineering.

Self-Organization of Sinusoidal Vessels in Pluripotent Stem Cell-derived Human Liver Bud Organoids

The induction of tissue-specific vessels in in vitro living tissue systems remains challenging. Here, we directly differentiated human pluripotent stem cells into CD32b+ putative liver sinusoidal progenitors (iLSEP) by dictating developmental pathways. By devising an inverted multilayered air-liquid interface (IMALI) culture, hepatic endoderm, septum mesenchyme, arterial and sinusoidal quadruple progenitors self-organized to generate and sustain hepatocyte-like cells neighbored by divergent endothelial subsets composed of CD32blowCD31high, LYVE1+STAB1+CD32bhighCD31lowTHBD-vWF-, and LYVE1-THBD+vWF+ cells. Wnt2 mediated sinusoidal-to-hepatic intercellular crosstalk potentiates hepatocyte differentiation and branched endothelial network formation. Intravital imaging revealed iLSEP developed fully patent human vessels with functional sinusoid-like features. Organoid-derived hepatocyte- and sinusoid-derived coagulation factors enabled correction of in vitro clotting time with Factor V, VIII, IX, and XI deficient patients' plasma and rescued the severe bleeding phenotype in hemophilia A mice upon transplantation. Advanced organoid vascularization technology allows for interrogating key insights governing organ-specific vessel development, paving the way for coagulation disorder therapeutics.

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.

The evolving views of hematopoiesis: from embryo to adulthood and from in vivo to in vitro

The hematopoietic system composed of hematopoietic stem and progenitor cells (HSPCs) and their differentiated lineages serves as an ideal model to uncover generic principles of cell fate transitions. From gastrulation onwards, there successively emerge primitive hematopoiesis (that produces specialized hematopoietic cells), pro-definitive hematopoiesis (that produces lineage-restricted progenitor cells), and definitive hematopoiesis (that produces multipotent HSPCs). These nascent lineages develop in several transient hematopoietic sites and finally colonize into lifelong hematopoietic sites. The development and maintenance of hematopoietic lineages are orchestrated by cell-intrinsic gene regulatory networks and cell-extrinsic microenvironmental cues. Owing to the progressive methodology (e.g., high-throughput lineage tracing and single-cell functional and omics analyses), our understanding of the developmental origin of hematopoietic lineages and functional properties of certain hematopoietic organs has been updated; meanwhile, new paradigms to characterize rare cell types, cell heterogeneity and its causes, and comprehensive regulatory landscapes have been provided. Here, we review the evolving views of HSPC biology during developmental and postnatal hematopoiesis. Moreover, we discuss recent advances in the in vitro induction and expansion of HSPCs, with a focus on the implications for clinical applications.

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.

Hyperactive KRAS/MAPK signaling disrupts normal lymphatic vessel architecture and function

Complex lymphatic anomalies (CLAs) are sporadically occurring diseases caused by the maldevelopment of lymphatic vessels. We and others recently reported that somatic activating mutations in KRAS can cause CLAs. However, the mechanisms by which activating KRAS mutations cause CLAs are poorly understood. Here, we show that KRASG12D expression in lymphatic endothelial cells (LECs) during embryonic development impairs the formation of lymphovenous valves and causes the enlargement of lymphatic vessels. We demonstrate that KRASG12D expression in primary human LECs induces cell spindling, proliferation, and migration. It also increases AKT and ERK1/2 phosphorylation and decreases the expression of genes that regulate the maturation of lymphatic vessels. We show that MEK1/2 inhibition with the FDA-approved drug trametinib suppresses KRASG12D-induced morphological changes, proliferation, and migration. Trametinib also decreases ERK1/2 phosphorylation and increases the expression of genes that regulate the maturation of lymphatic vessels. We also show that trametinib and Cre-mediated expression of a dominant-negative form of MEK1 (Map2k1 K97M) suppresses KRASG12D-induced lymphatic vessel hyperplasia in embryos. Last, we demonstrate that conditional knockout of wild-type Kras in LECs does not affect the formation or function of lymphatic vessels. Together, our data indicate that KRAS/MAPK signaling must be tightly regulated during embryonic development for the proper development of lymphatic vessels and further support the testing of MEK1/2 inhibitors for treating CLAs.

Fetal liver development and implications for liver disease pathogenesis

The metabolic, digestive and homeostatic roles of the liver are dependent on proper crosstalk and organization of hepatic cell lineages. These hepatic cell lineages are derived from their respective progenitors early in organogenesis in a spatiotemporally controlled manner, contributing to the liver's specialized and diverse microarchitecture. Advances in genomics, lineage tracing and microscopy have led to seminal discoveries in the past decade that have elucidated liver cell lineage hierarchies. In particular, single-cell genomics has enabled researchers to explore diversity within the liver, especially early in development when the application of bulk genomics was previously constrained due to the organ's small scale, resulting in low cell numbers. These discoveries have substantially advanced our understanding of cell differentiation trajectories, cell fate decisions, cell lineage plasticity and the signalling microenvironment underlying the formation of the liver. In addition, they have provided insights into the pathogenesis of liver disease and cancer, in which developmental processes participate in disease emergence and regeneration. Future work will focus on the translation of this knowledge to optimize in vitro models of liver development and fine-tune regenerative medicine strategies to treat liver disease. In this Review, we discuss the emergence of hepatic parenchymal and non-parenchymal cells, advances that have been made in in vitro modelling of liver development and draw parallels between developmental and pathological processes.

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.

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.

GATA2 regulates blood/lymph separation in a platelet-dependent and lymphovenous valve-independent manner

INTRODUCTION

Lymphatic vessels collect interstitial fluid, immune cells, and digested lipids and return these bodily fluids to blood through two pairs of lymphovenous valves (LVVs). Like other cardiovascular valves LVVs prevent the backflow of blood into the lymphatic vessels. In addition to LVVs, platelets are necessary to prevent the entry of blood into the lymphatic vessels. Platelet thrombi are observed at LVVs suggesting that LVVs and platelets function in synergy to regulate blood/lymphatic separation.

OBJECTIVES

The primary objective of this work is to determine whether platelets can regulate blood/lymph separation independently of LVVs.

METHODS

The transcription factor GATA2 is necessary for the development of both LVVs and hematopoietic stem cells. Using various endothelial- and hematopoietic cell expressed Cre-lines, we conditionally deleted Gata2. We hypothesized that this strategy would identify the tissue- and time-specific roles of GATA2 and reveal whether platelets and LVVs can independently regulate blood/lymph separation.

RESULTS

Lymphatic vasculature-specific deletion of Gata2 results in the absence of LVVs without compromising blood/lymph separation. In contrast, deletion of GATA2 from both lymphatic vasculature and hematopoietic cells results in the absence of LVVs, reduced number of platelets and blood-filled lymphatic vasculature.

CONCLUSION

GATA2 promotes blood/lymph separation through platelets. Furthermore, LVVs are the only known sites of interaction between blood and lymphatic vessels. The fact that blood is able to enter the lymphatic vessels of mice lacking LVVs and platelets indicates that under these circumstances the lymphatic and blood vessels are connected at yet to be identified sites.

The markers to delineate different phenotypes of macrophages related to metabolic disorders

Macrophages have a wide variety of roles in physiological and pathological conditions, making them promising diagnostic and therapeutic targets in diseases, especially metabolic disorders, which have attracted considerable attention in recent years. Owing to their heterogeneity and polarization, the phenotypes and functions of macrophages related to metabolic disorders are diverse and complicated. In the past three decades, the rapid progress of macrophage research has benefited from the emergence of specific molecular markers to delineate different phenotypes of macrophages and elucidate their role in metabolic disorders. In this review, we analyze the functions and applications of commonly used and novel markers of macrophages related to metabolic disorders, facilitating the better use of these macrophage markers in metabolic disorder research.

Tracing the developmental origin of tissue stem cells

Tissue stem cells are vital for organ homeostasis and regeneration owing to their ability to self-renew and differentiate into the various cell types that constitute organ tissue. These stem cells are formed during complex and dynamic organ development, necessitating spatial-temporal coordination of morphogenetic events and cell fate specification during this process. In recent years, technological advances have enabled the tracing of the cellular dynamics, states, and lineages of individual cells over time in relation to tissue morphological changes. These dynamic data have not only revealed the origin of tissue stem cells in various organs but have also led to an understanding of the molecular, cellular, and biophysical bases of tissue stem cell formation. Herein, we summarize recent findings on the developmental origin of tissue stem cells in the hair follicles, intestines, brain, skeletal muscles, and hematopoietic system, and further discuss how stem cell fate specification is coordinated with tissue topology.

Single-cell transcriptome analysis of embryonic and adult endothelial cells allows to rank the hemogenic potential of post-natal endothelium

Hematopoietic stem cells (HSCs) are crucial for the continuous production of blood cells during life. The transplantation of these cells is one of the most common treatments to cure patient suffering of blood diseases. However, the lack of suitable donors is a major limitation. One option to get HSCs matching perfectly a patient is cellular reprogramming. HSCs emerge from endothelial cells in blood vessels during embryogenesis through the endothelial to hematopoietic transition. Here, we used single-cell transcriptomics analysis to compare embryonic and post-natal endothelial cells to investigate the potential of adult vasculature to be reprogrammed in hematopoietic stem cells. Although transcriptional similarities have been found between embryonic and adult endothelial cells, we found some key differences in term of transcription factors expression. There is a deficit of expression of Runx1, Tal1, Lyl1 and Cbfb in adult endothelial cells compared to their embryonic counterparts. Using a combination of gene expression profiling and gene regulatory network analysis, we found that endothelial cells from the pancreas, brain, kidney and liver appear to be the most suitable targets for cellular reprogramming into HSCs. Overall, our work provides an important resource for the rational design of a reprogramming strategy for the generation of HSCs.

Specification of the haematopoietic stem cell lineage: From blood-fated mesodermal angioblasts to haemogenic endothelium

Haematopoietic stem and progenitor cells emerge from specialized haemogenic endothelial cells in select vascular beds during embryonic development. Specification and commitment to the blood lineage, however, occur before endothelial cells are endowed with haemogenic competence, at the time of mesoderm patterning and production of endothelial cell progenitors (angioblasts). Whilst early blood cell fate specification has long been recognized, very little is known about the mechanisms that induce endothelial cell diversification and progressive acquisition of a blood identity by a subset of these cells. Here, we review the endothelial origin of the haematopoietic system and the complex developmental journey of blood-fated angioblasts. We discuss how recent technological advances will be instrumental to examine the diversity of the embryonic anatomical niches, signaling pathways and downstream epigenetic and transcriptional processes controlling endothelial cell heterogeneity and blood cell fate specification. Ultimately, this will give essential insights into the ontogeny of the cells giving rise to haematopoietic stem cells, that may aid in the development of novel strategies for their in vitro production for clinical purposes.

Differential Etv2 threshold requirement for endothelial and erythropoietic development

Endothelial and erythropoietic lineages arise from a common developmental progenitor. Etv2 is a master transcriptional regulator required for the development of both lineages. However, the mechanisms through which Etv2 initiates the gene-regulatory networks (GRNs) for endothelial and erythropoietic specification and how the two GRNs diverge downstream of Etv2 remain incompletely understood. Here, by analyzing a hypomorphic Etv2 mutant, we demonstrate different threshold requirements for initiation of the downstream GRNs for endothelial and erythropoietic development. We show that Etv2 functions directly in a coherent feedforward transcriptional network for vascular endothelial development, and a low level of Etv2 expression is sufficient to induce and sustain the endothelial GRN. In contrast, Etv2 induces the erythropoietic GRN indirectly via activation of Tal1, which requires a significantly higher threshold of Etv2 to initiate and sustain erythropoietic development. These results provide important mechanistic insight into the divergence of the endothelial and erythropoietic lineages.

The zinc finger transcription factor Sall1 is required for the early developmental transition of microglia in mouse embryos

Microglia play many critical roles in neural development. Recent single-cell RNA-sequencing studies have found diversity of microglia both across different stages and within the same stage in the developing brain. However, how such diversity is controlled during development is poorly understood. In this study, we first found the expression of the macrophage mannose receptor CD206 in early-stage embryonic microglia on mouse brain sections. This expression showed a sharp decline between E12.5 and E13.5 across the central nervous system. We next tested the roles of the microglia-expressed zinc finger transcription factor SALL1 in this early transition of gene expression. By deleting Sall1 specifically in microglia, we found that many microglia continued to express CD206 when it is normally downregulated. In addition, the mutant microglia continued to show less ramified morphology in comparison with controls even into postnatal stages. Thus, SALL1 is required for early microglia to transition into a more mature status during development.

Hyaluronic Acid: Known for Almost a Century, but Still in Vogue

Hyaluronic acid (HA) has a special position among glycosaminoglycans. As a major component of the extracellular matrix (ECM). This simple, unbranched polysaccharide is involved in the regulation of various biological cell processes, whether under physiological conditions or in cases of cell damage. This review summarizes the history of this molecule's study, its distinctive metabolic pathway in the body, its unique properties, and current information regarding its interaction partners. Our main goal, however, is to intensively investigate whether this relatively simple polymer may find applications in protecting against ionizing radiation (IR) or for therapy in cases of radiation-induced damage. After exposure to IR, acute and belated damage develops in each tissue depending upon the dose received and the cellular composition of a given organ. A common feature of all organ damage is a distinct change in composition and structure of the ECM. In particular, the important role of HA was shown in lung tissue and the variability of this flexible molecule in the complex mechanism of radiation-induced lung injuries. Moreover, HA is also involved in intermediating cell behavior during morphogenesis and in tissue repair during inflammation, injury, and would healing. The possibility of using the HA polymer to affect or treat radiation tissue damage may point to the missing gaps in the responsible mechanisms in the onset of this disease. Therefore, in this article, we will also focus on obtaining answers from current knowledge and the results of studies as to whether hyaluronic acid can also find application in radiation science.

Therapeutic correction of hemophilia A by transplantation of hPSC-derived liver sinusoidal endothelial cell progenitors

Liver sinusoidal endothelial cells (LSECs) form the predominant microvasculature in the liver where they carry out many functions including the secretion of coagulation factor VIII (FVIII). To investigate the early origins of this lineage, we develop an efficient and scalable protocol to produce human pluripotent stem cell (hPSC)-derived LSEC progenitors characterized as venous endothelial cells (VECs) from different mesoderm subpopulations. Using a sensitive and quantitative vascular competitive transplantation assay, we demonstrate that VECs generated from BMP4 and activin A-induced KDR⁺CD235a/b⁺ mesoderm are 50-fold more efficient at LSEC engraftment than venous cells from BMP4 and WNT-induced KDR⁺CD235a/b^- mesoderm. When transplanted into immunocompromised hemophilia A mice (NSG-HA), these VECs engraft the liver, proliferate, and mature to functional LSECs that secrete bioactive FVIII capable of correcting the bleeding phenotype. Together, these findings highlight the importance of appropriate mesoderm induction for generating hPSC-derived LSECs capable of functioning in a preclinical model of hemophilia A.

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.

Endothelial MEKK3-KLF2/4 signaling integrates inflammatory and hemodynamic signals during definitive hematopoiesis

The hematopoietic stem cells (HSCs) that produce blood for the lifetime of an animal arise from RUNX1+ hemogenic endothelial cells (HECs) in the embryonic vasculature through a process of endothelial-to-hematopoietic transition (EHT). Studies have identified inflammatory mediators and fluid shear forces as critical environmental stimuli for EHT, raising the question of how such diverse inputs are integrated to drive HEC specification. Endothelial cell MEKK3-KLF2/4 signaling can be activated by both fluid shear forces and inflammatory mediators, and plays roles in cardiovascular development and disease that have been linked to both stimuli. Here we demonstrate that MEKK3 and KLF2/4 are required in endothelial cells for the specification of RUNX1+ HECs in both the yolk sac and dorsal aorta of the mouse embryo and for their transition to intra-aortic hematopoietic cluster cells (IAHCs). The inflammatory mediators lipopolysaccharide and interferon gamma increase RUNX1+ HECs in an MEKK3-dependent manner. Maternal administration of catecholamines that stimulate embryo cardiac function and accelerate yolk sac vascular remodeling increases EHT by wild-type but not MEKK3-deficient endothelium. These findings identify MEKK-KLF2/4 signaling as an essential pathway for EHT and provide a molecular basis for the integration of diverse environmental inputs, such as inflammatory mediators and hemodynamic forces, during definitive hematopoiesis.

Stem Cell-Based Disease Models for Inborn Errors of Immunity

The intrinsic capacity of human hematopoietic stem cells (hHSCs) to reconstitute myeloid and lymphoid lineages combined with their self-renewal capacity hold enormous promises for gene therapy as a viable treatment option for a number of immune-mediated diseases, most prominently for inborn errors of immunity (IEI). The current development of such therapies relies on disease models, both in vitro and in vivo, which allow the study of human pathophysiology in great detail. Here, we discuss the current challenges with regards to developmental origin, heterogeneity and the subsequent implications for disease modeling. We review models based on induced pluripotent stem cell technology and those relaying on use of adult hHSCs. We critically review the advantages and limitations of current models for IEI both in vitro and in vivo. We conclude that existing and future stem cell-based models are necessary tools for developing next generation therapies for IEI.

LYVE1+ macrophages of murine peritoneal mesothelium promote omentum-independent ovarian tumor growth

Two resident macrophage subsets reside in peritoneal fluid. Macrophages also reside within mesothelial membranes lining the peritoneal cavity, but they remain poorly characterized. Here, we identified two macrophage populations (LYVE1hi MHC IIlo-hi CX3CR1gfplo/- and LYVE1lo/- MHC IIhi CX3CR1gfphi subsets) in the mesenteric and parietal mesothelial linings of the peritoneum. These macrophages resembled LYVE1+ macrophages within surface membranes of numerous organs. Fate-mapping approaches and analysis of newborn mice showed that LYVE1hi macrophages predominantly originated from embryonic-derived progenitors and were controlled by CSF1 made by Wt1+ stromal cells. Their gene expression profile closely overlapped with ovarian tumor-associated macrophages previously described in the omentum. Indeed, syngeneic epithelial ovarian tumor growth was strongly reduced following in vivo ablation of LYVE1hi macrophages, including in mice that received omentectomy to dissociate the role from omental macrophages. These data reveal that the peritoneal compartment contains at least four resident macrophage populations and that LYVE1hi mesothelial macrophages drive tumor growth independently of the omentum.

Endothelial-specific depletion of TGF-β signaling affects lymphatic function

BACKGROUND

Transforming growth factor (TGF)-β is a multifunctional cytokine involved in cell differentiation, cell proliferation, and tissue homeostasis. Although TGF-β signaling is essential for maintaining blood vessel functions, little is known about the role of TGF-β in lymphatic homeostasis.

METHODS

To delineate the role of TGF-β signaling in lymphatic vessels, TβRII^fl/fl mice were crossed with Prox1-Cre^ERT2 mice to generate TβRII^fl/fl; Prox1-Cre^ERT2 mice. The TβRII gene in the lymphatic endothelial cells (LECs) of the conditional knockout TβRII^iΔLEC mice was selectively deleted using tamoxifen. The effects of TβRII gene deletion on embryonic lymphangiogenesis, postnatal lymphatic structure and drainage function, tumor lymphangiogenesis, and lymphatic tumor metastasis were investigated.

RESULTS

Deficiency of LEC-specific TGF-β signaling in embryos, where lymphangiogenesis is active, caused dorsal edema with dilated lymphatic vessels at E13.5. Postnatal mice in which lymphatic vessels had already been formed displayed dilation and increased bifurcator of lymphatic vessels after tamoxifen administration. Similar dilation was also observed in tumor lymphatic vessels. The drainage of FITC-dextran, which was subcutaneously injected into the soles of the feet of the mice, was reduced in TβRII^iΔLEC mice. Furthermore, Lewis lung carcinoma cells constitutively expressing GFP (LLC-GFP) transplanted into the footpads of the mice showed reduced patellar lymph node metastasis.

CONCLUSION

These data suggest that TGF-β signaling in LECs maintains the structure of lymphatic vessels and lymphatic homeostasis, in addition to promoting tumor lymphatic metastasis. Therefore, suppression of TGF-β signaling in LECs might be effective in inhibiting cancer metastasis.

Spatiotemporal and Functional Heterogeneity of Hematopoietic Stem Cell-Competent Hemogenic Endothelial Cells in Mouse Embryos

Hematopoietic stem cells (HSCs) are derived from hemogenic endothelial cells (HECs) during embryogenesis. The HSC-primed HECs increased to the peak at embryonic day (E) 10 and have been efficiently captured by the marker combination CD41^-CD43^-CD45^-CD31⁺CD201⁺Kit⁺CD44⁺ (PK44) in the aorta-gonad-mesonephros (AGM) region of mouse embryos most recently. In the present study, we investigated the spatiotemporal and functional heterogeneity of PK44 cells around the time of emergence of HSCs. First, PK44 cells in the E10.0 AGM region could be further divided into three molecularly different populations showing endothelial- or hematopoietic-biased characteristics. Specifically, with the combination of Kit, the expression of CD93 or CD146 could divide PK44 cells into endothelial- and hematopoietic-feature biased populations, which was further functionally validated at the single-cell level. Next, the PK44 population could also be detected in the yolk sac, showing similar developmental dynamics and functional diversification with those in the AGM region. Importantly, PK44 cells in the yolk sac demonstrated an unambiguous multilineage reconstitution capacity after in vitro incubation. Regardless of the functional similarity, PK44 cells in the yolk sac displayed transcriptional features different from those in the AGM region. Taken together, our work delineates the spatiotemporal characteristics of HECs represented by PK44 and reveals a previously unknown HSC competence of HECs in the yolk sac. These findings provide a fundamental basis for in-depth study of the different origins and molecular programs of HSC generation in the future.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

Single-cell RNA-seq revealed diverse cell types in the mouse placenta at mid-gestation

The mammalian placenta consists of a set of cells to ensure normal placental functions throughout gestation. Dysfunctional placentae are considered as the origin of a series of pregnancy complications. Therefore, it is urgent for detailed information about the molecular recipes of the cell types within the normal placenta. In the past years, gene expression analysis via single-cell RNA-seq (scRNA-seq) offers opportunities to identify new cell types in a variety of organs and tissues. In this study, scRNA-seq was used to explore the cell heterogeneity within the E10.5 mouse placenta and unravel their discrepancies in cell composition and communications. We identified sixteen cell clusters, including some cell clusters that originated from the maternal tissue. Moreover, we traced the developmental trajectories of trophoblasts and Hofbauer-like cells. Further analysis revealed cell connections between the endothelial cells and pericytes, syncytiotrophoblasts, as well as decidual cells. Besides, we highlighted several signaling pathways, such as the EGF, FGF, canonical, and non-canonical WNT signaling pathways, which mediated the potential crosstalk between different cell types within placenta. Our research provides an in-depth understanding of placental development, cellular composition, and communications at the maternal-fetal interface.

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.

Lymphatic Type 1 Interferon Responses Are Critical for Control of Systemic Reovirus Dissemination

Mammalian orthoreovirus (reovirus) spreads from the site of infection to every organ system in the body via the blood. However, mechanisms that underlie reovirus hematogenous spread remain undefined. Nonstructural protein σ1s is a critical determinant of reovirus bloodstream dissemination that is required for efficient viral replication in many types of cultured cells. Here, we used the specificity of the σ1s protein for promoting hematogenous spread as a platform to uncover a role for lymphatic type 1 interferon (IFN-1) responses in limiting reovirus systemic dissemination. We found that replication of a σ1s-deficient reovirus was restored to wild-type levels in cells with defective interferon-α receptor (IFNAR1) signaling. Reovirus spreads systemically following oral inoculation of neonatal mice, whereas the σ1s-null virus remains localized to the intestine. We found that σ1s enables reovirus spread in the presence of a functional IFN-1 response, as the σ1s-deficient reovirus disseminated comparably to wild-type virus in IFNAR1^-/- mice. Lymphatics are hypothesized to mediate reovirus spread from the intestine to the bloodstream. IFNAR1 deletion from cells expressing lymphatic vessel endothelium receptor 1 (LYVE-1), a marker for lymphatic endothelial cells, enabled the σ1s-deficient reovirus to disseminate systemically. Together, our findings indicate that IFN-1 responses in lymphatics limit reovirus dissemination. Our data further suggest that the lymphatics are an important conduit for reovirus hematogenous spread.IMPORTANCE Type 1 interferons (IFN-1) are critical host responses to viral infection. However, the contribution of IFN-1 responses to control of viruses in specific cell and tissue types is not fully defined. Here, we identify IFN-1 responses in lymphatics as important for limiting reovirus dissemination. We found that nonstructural protein σ1s enhances reovirus resistance to IFN-1 responses, as a reovirus mutant lacking σ1s was more sensitive to IFN-1 than wild-type virus. In neonatal mice, σ1s is required for reovirus systemic spread. We used tissue-specific IFNAR1 deletion in combination with the IFN-1-sensitive σ1s-null reovirus as a tool to test how IFN-1 responses in lymphatics affect reovirus systemic spread. Deletion of IFNAR1 in lymphatic cells using Cre-lox technology enabled dissemination of the IFN-1-sensitive σ1s-deficient reovirus. Together, our results indicate that IFN-1 responses in lymphatics are critical for controlling reovirus systemic spread.

Cellular Basis of Embryonic Hematopoiesis and Its Implications in Prenatal Erythropoiesis

Primitive erythrocytes are the first hematopoietic cells observed during ontogeny and are produced specifically in the yolk sac. Primitive erythrocytes express distinct hemoglobins compared with adult erythrocytes and circulate in the blood in the nucleated form. Hematopoietic stem cells produce adult-type (so-called definitive) erythrocytes. However, hematopoietic stem cells do not appear until the late embryonic/early fetal stage. Recent studies have shown that diverse types of hematopoietic progenitors are present in the yolk sac as well as primitive erythroblasts. Multipotent hematopoietic progenitors that arose in the yolk sac before hematopoietic stem cells emerged likely fill the gap between primitive erythropoiesis and hematopoietic stem-cell-originated definitive erythropoiesis and hematopoiesis. In this review, we discuss the cellular origin of primitive erythropoiesis in the yolk sac and definitive hematopoiesis in the fetal liver. We also describe mechanisms for developmental switches that occur during embryonic and fetal erythropoiesis and hematopoiesis, particularly focusing on recent studies performed in mice.

Location, Location, Location: How Vascular Specialization Influences Hematopoietic Fates During Development

During embryonic development, sequential waves of hematopoiesis give rise to blood-forming cells with diverse lineage potentials and self-renewal properties. This process must accomplish two important yet divergent goals: the rapid generation of differentiated blood cells to meet the needs of the developing embryo and the production of a reservoir of hematopoietic stem cells to provide for life-long hematopoiesis in the adult. Vascular beds in distinct anatomical sites of extraembryonic tissues and the embryo proper provide the necessary conditions to support these divergent objectives, suggesting a critical role for specialized vascular niche cells in regulating disparate blood cell fates during development. In this review, we will examine the current understanding of how organ- and stage-specific vascular niche specialization contributes to the development of the hematopoietic system.

Identifying 8-mRNAsi Based Signature for Predicting Survival in Patients With Head and Neck Squamous Cell Carcinoma via Machine Learning

Cancer stem cells (CSCs) have been characterized by several exclusive features that include differentiation, self-renew, and homeostatic control, which allows tumor maintenance and spread. Recurrence and therapeutic resistance of head and neck squamous cell carcinomas (HNSCC) have been identified to be attributed to CSCs. However, the biomarkers led to the development of HNSCC stem cells remain less defined. In this study, we quantified cancer stemness by mRNA expression-based stemness index (mRNAsi), and found that mRNAsi indices were higher in HNSCC tissues than that in normal tissue. A significantly higher mRNAsi was observed in HPV positive patients than HPV negative patients, as well as in male patients than in female patients. The 8-mRNAsi signature was identified from the genes in two modules which were mostly related to mRNAsi screened by weighted gene co-expression network analysis. In this prognostic signatures, high expression of RGS16, LYVE1, hnRNPC, ANP32A, and AIMP1 focus in promoting cell proliferation and tumor progression. While ZNF66, PIK3R3, and MAP2K7 are associated with a low risk of death. The riskscore of eight signatures have a powerful capacity for 1-, 3-, 5-year of overall survival prediction (5-year AUC 0.77, 95% CI 0.69-0.85). These findings based on stemness indices may provide a novel understanding of target therapy for suppressing HNSCC stem cells.

Lymphatics in bone arise from pre-existing lymphatics

Bones do not normally have lymphatics. However, individuals with generalized lymphatic anomaly (GLA) or Gorham-Stout disease (GSD) develop ectopic lymphatics in bone. Despite growing interest in the development of tissue-specific lymphatics, the cellular origin of bone lymphatic endothelial cells (bLECs) is not known and the development of bone lymphatics has not been fully characterized. Here, we describe the development of bone lymphatics in mouse models of GLA and GSD. Through lineage-tracing experiments, we show that bLECs arise from pre-existing Prox1-positive LECs. We show that bone lymphatics develop in a stepwise manner where regional lymphatics grow, breach the periosteum and then invade bone. We also show that the development of bone lymphatics is impaired in mice that lack osteoclasts. Last, we show that rapamycin can suppress the growth of bone lymphatics in our models of GLA and GSD. In summary, we show that bLECs can arise from pre-existing LECs and that rapamycin can prevent the growth of bone lymphatics.

Epigenetic priming by Dot1l in lymphatic endothelial progenitors ensures normal lymphatic development and function

Proper functioning of the lymphatic system is required for normal immune responses, fluid balance, and lipid reabsorption. Multiple regulatory mechanisms are employed to ensure the correct formation and function of lymphatic vessels; however, the epigenetic modulators and mechanisms involved in this process are poorly understood. Here, we assess the regulatory role of mouse Dot1l, a histone H3 lysine (K) 79 (H3K79) methyltransferase, in lymphatic formation. Genetic ablation of Dot1l in Tie2(+) endothelial cells (ECs), but not in Lyve1(+) or Prox1(+) lymphatic endothelial cells (LECs) or Vav1(+) definitive hematopoietic stem cells, leads to catastrophic lymphatic anomalies, including skin edema, blood–lymphatic mixing, and underdeveloped lymphatic valves and vessels in multiple organs. Remarkably, targeted Dot1l loss in Tie2(+) ECs leads to fully penetrant lymphatic aplasia, whereas Dot1l overexpression in the same cells results in partially hyperplastic lymphatics in the mesentery. Genetic studies reveal that Dot1l functions in c-Kit(+) hemogenic ECs during mesenteric lymphatic formation. Mechanistically, inactivation of Dot1l causes a reduction of both H3K79me2 levels and the expression of genes important for LEC development and function. Thus, our study establishes that Dot1l-mediated epigenetic priming and transcriptional regulation in LEC progenitors safeguard the proper lymphatic development and functioning of lymphatic vessels.

Two distinct interstitial macrophage populations coexist across tissues in specific subtissular niches

Macrophages are a heterogeneous cell population involved in tissue homeostasis, inflammation, and various pathologies. Although the major tissue-resident macrophage populations have been extensively studied, interstitial macrophages (IMs) residing within the tissue parenchyma remain poorly defined. Here we studied IMs from murine lung, fat, heart, and dermis. We identified two independent IM subpopulations that are conserved across tissues: Lyve1^loMHCII^hiCX3CR1^hi (Lyve1^loMHCII^hi) and Lyve1^hiMHCII^loCX3CR1^lo (Lyve1^hiMHCII^lo) monocyte-derived IMs, with distinct gene expression profiles, phenotypes, functions, and localizations. Using a new mouse model of inducible macrophage depletion (Slco2b1 ^flox/DTR), we found that the absence of Lyve1^hiMHCII^lo IMs exacerbated experimental lung fibrosis. Thus, we demonstrate that two independent populations of IMs coexist across tissues and exhibit conserved niche-dependent functional programming.

Hematopoietic stem cell-independent hematopoiesis and the origins of innate-like B lymphocytes

The current paradigm that a single long-term hematopoietic stem cell can regenerate all components of the mammalian immune system has been challenged by recent findings in mice. These findings show that adult tissue-resident macrophages and innate-like lymphocytes develop early in fetal hematopoiesis from progenitors that emerge prior to, and apparently independently of, conventional long-term hematopoietic stem cells. Here, we discuss these recent findings, which show that an early and distinct wave of hematopoiesis occurs for all major hematopoietic lineages. These data provide evidence that fetal hematopoietic progenitors not derived from the bona fide long-term hematopoietic stem cells give rise to tissue-resident immune cells that persist throughout adulthood. We also discuss recent insights into B lymphocyte development and attempt to synthesize seemingly contradictory recent findings on the origins of innate-like B-1a lymphocytes during fetal hematopoiesis.

Hlf marks the developmental pathway for hematopoietic stem cells but not for erythro-myeloid progenitors

Hematopoietic stem cells (HSCs) and HSC-independent progenitors are generated from hemogenic endothelium. Yokomizo et al. show that Hlf expression distinguishes nascent HSCs from HSC-independent progenitors. HSC specification, regulated by the Evi-1/Hlf axis, is activated only within Hlf⁺ nascent hematopoietic clusters.

Before the emergence of hematopoietic stem cells (HSCs), lineage-restricted progenitors, such as erythro-myeloid progenitors (EMPs), are detected in the embryo or in pluripotent stem cell cultures in vitro. Although both HSCs and EMPs are derived from hemogenic endothelium, it remains unclear how and when these two developmental programs are segregated during ontogeny. Here, we show that hepatic leukemia factor (Hlf) expression specifically marks a developmental continuum between HSC precursors and HSCs. Using the Hlf-tdTomato reporter mouse, we found that Hlf is expressed in intra-aortic hematopoietic clusters and fetal liver HSCs. In contrast, EMPs and yolk sac hematopoietic clusters before embryonic day 9.5 do not express Hlf. HSC specification, regulated by the Evi-1/Hlf axis, is activated only within Hlf⁺ nascent hematopoietic clusters. These results strongly suggest that HSCs and EMPs are generated from distinct cohorts of hemogenic endothelium. Selective induction of the Hlf⁺ lineage pathway may lead to the in vitro generation of HSCs from pluripotent stem cells.

Chasing Mavericks: The quest for defining developmental waves of hematopoiesis

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

Murine hematopoietic stem cell activity is derived from pre-circulation embryos but not yolk sacs

The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is well supported, the yolk sac (YS) contribution to adult hematopoiesis remains controversial. The same developmental origin makes it difficult to identify specific markers that discern between an intraembryonic versus YS-origin using a lineage trace approach. Additionally, the highly migratory nature of blood cells and the inability of pre-circulatory embryonic cells (i.e., 5-7 somite pairs (sp)) to robustly engraft in transplantation, even after culture, has precluded scientists from properly answering these questions. Here we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2-7sp murine embryonic explants (Em-Ex). dHSC are undetectable in 2-7sp YS explants. Additionally, the engraftment from Em-Ex is confined to an emerging CD31⁺CD45⁺c-Kit⁺CD41^- population. In sum, our work supports a model in which the embryo, not the YS, is the major source of lifelong definitive hematopoiesis.

What do the lineage tracing studies tell us? Consideration for hematopoietic stem cell origin, dynamics, and leukemia-initiating cells

The recent advance of technologies enables us to trace the cell fate in vivo by marking the cells that express the gene of interest or by barcoding them at a single cell level. Various tamoxifen-inducible Cre-recombinase mice combined with Rosa-floxed lines are utilized. In this review, with the results revealed by lineage tracing assays, we re-visit the long-standing debate for the origin of hematopoietic stem cells in the mouse embryo, and introduce the view of native hematopoiesis, and possible leukemic-initiating cells emerged during fetal stages.

A novel podoplanin-GFPCre mouse strain for gene deletion in lymphatic endothelial cells

The lymphatic vascular system is a one-direction network of thin-walled capillaries and larger vessels covered by a continuous layer of endothelial cells responsible for maintaining fluid homeostasis. Some of the main functions of the lymphatic vasculature are to drain fluid from the extracellular spaces and return it back to the blood circulation, lipid absorption from the intestinal tract, and transport of immune cells to lymphoid organs. A number of genes controlling the development of the mammalian lymphatic vasculature have been identified in the last few years, and their functional roles started to be characterized using gene inactivation approaches in mice. Unfortunately, only few mouse Cre strains relatively specific for lymphatic endothelial cells (LECs) are currently available. In this article, we report the generation of a novel Podoplanin (Pdpn) GFPCre transgenic mouse strain using its 5' regulatory region. Pdpn encodes a transmembrane mucin-type O-glycoprotein that is expressed on the surface of embryonic and postnatal LECs, in addition to few other cell types. Our detailed characterization of this novel strain indicates that it will be a valuable additional genetic tool for the analysis of gene function in LECs.

Defining murine organogenesis at single-cell resolution reveals a role for the leukotriene pathway in regulating blood progenitor formation

During gastrulation, cell types from all three germ layers are specified and the basic body plan is established 1 . However, molecular analysis of this key developmental stage has been hampered by limited cell numbers and a paucity of markers. Single-cell RNA sequencing circumvents these problems, but has so far been limited to specific organ systems 2 . Here, we report single-cell transcriptomic characterization of >20,000 cells immediately following gastrulation at E8.25 of mouse development. We identify 20 major cell types, which frequently contain substructure, including three distinct signatures in early foregut cells. Pseudo-space ordering of somitic progenitor cells identifies dynamic waves of transcription and candidate regulators, which are validated by molecular characterization of spatially resolved regions of the embryo. Within the endothelial population, cells that transition from haemogenic endothelial to erythro-myeloid progenitors specifically express Alox5 and its co-factor Alox5ap, which control leukotriene production. Functional assays using mouse embryonic stem cells demonstrate that leukotrienes promote haematopoietic progenitor cell generation. Thus, this comprehensive single-cell map can be exploited to reveal previously unrecognized pathways that contribute to tissue development.

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.