Ex vivo time-lapse confocal imaging of the mouse embryo aorta

Spatial analysis of transcript and protein levels in skeletal muscle

Skeletal muscle spatial analyses have revealed unexpected regionalized gene expression patterns challenging the understanding of muscle as a homogeneous tissue. Here, we present a protocol for the spatial analysis of transcript and protein levels in murine skeletal muscle. We describe steps for tibialis anterior dissection, formaldehyde fixation, tissue chopper cutting, and hybridization chain reaction (HCR) detection and amplification. We then detail procedures for immunostaining, tissue clearing, and imaging. This protocol is easily adaptable to other tissues.

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.

The Role of SCL Isoforms in Embryonic Hematopoiesis

Three waves of hematopoiesis occur in the mouse embryo. The primitive hematopoiesis appears as blood islands in the extra embryonic yolk sac at E7.5. The extra embryonic pro-definitive hematopoiesis launches in late E8 and the embryonic definitive one turns on at E10.5 indicated by the emergence of hemogenic endothelial cells on the inner wall of the extra embryonic arteries and the embryonic aorta. To study the roles of SCL protein isoforms in murine hematopoiesis, the SCL-large (SCL-L) isoform was selectively destroyed with the remaining SCL-small (SCL-S) isoform intact. It was demonstrated that SCL-S was specifically expressed in the hemogenic endothelial cells (HECs) and SCL-L was only detected in the dispersed cells after budding from HECs. The SCLΔ/Δ homozygous mutant embryos only survived to E10.5 with normal extra embryonic vessels and red blood cells. In wild-type mouse embryos, a layer of neatly aligned CD34+ and CD43+ cells appeared on the endothelial wall of the aorta of the E10.5 fetus. However, the cells at the same site expressed CD31 rather than CD34 and/or CD43 in the E10.5 SCLΔ/Δ embryo, indicating that only the endothelial lineage was developed. These results reveal that the SCL-S is sufficient to sustain the primitive hematopoiesis and SCL-L is necessary to launch the definitive hematopoiesis.

Clot Imaging Using Photostable Nanodiamond

While thrombosis is the leading cause of morbidity and mortality in the United States, an understanding of its triggers, progression, and response to anticoagulant therapy is lacking. Intravital fluorescence microscopy has advanced the study of thrombus formation by providing targeted, multi-color contrast. However, photodegradation of fluorophores limits the application in longitudinal studies (e.g., clot progression and/or dissolution). Fluorescent nanodiamond (FND) is a fluorophore which utilizes intrinsic fluorescence of chromogenic centers within and protected by the diamond crystalline lattice. Recent developments in diamond processing have allowed for the controlled production of nanodiamonds emitting in green or red. Here, the use of FND to label blood clots and/or clot lysis is demonstrated and compared to commonly used organic fluorophores. Model ex vivo clots were formed with incorporated labeled fibrinogen to allow imaging. FND was shown to match the morphology of organic fluorophore labels absent of photobleaching over time. The addition of tissue plasminogen activator (tPa) allowed visualization of the clot lysis stage, which is vital to studies of both DVT and pulmonary embolism resolution.

CLASP2 safeguards hematopoietic stem cell properties during mouse and fish development

Hematopoietic stem cells (HSCs) express a large variety of cell surface receptors that are associated with acquisition of self-renewal and multipotent properties. Correct expression of these receptors depends on a delicate balance between cell surface trafficking, recycling, and degradation and is controlled by the microtubule network and Golgi apparatus, whose roles have hardly been explored during embryonic/fetal hematopoiesis. Here we show that, in the absence of CLASP2, a microtubule-associated protein, the overall production of HSCs is reduced, and the produced HSCs fail to self-renew and maintain their stemness throughout mouse and zebrafish development. This phenotype can be attributed to decreased cell surface expression of the hematopoietic receptor c-Kit, which originates from increased lysosomal degradation in combination with a reduction in trafficking to the plasma membrane. A dysfunctional Golgi apparatus in CLASP2-deficient HSCs seems to be the underlying cause of the c-Kit expression and signaling imbalance.

Protocols for Investigating the Epithelial Properties of Cardiac Progenitor Cells in the Mouse Embryo

Epithelial cardiac progenitor cells of the second heart field (SHF) contribute to growth of the vertebrate heart tube by progressive addition of cells from the dorsal pericardial wall to the cardiac poles. Perturbation of SHF development, including defects in apicobasal or planar polarity, results in shortening of the heart tube and a spectrum of congenital heart defects. Here, we provide detailed protocols for fixed section and wholemount immunofluorescence and live imaging approaches to studying the epithelial properties of cardiac progenitors in the dorsal pericardial wall during mouse heart development. Whole-embryo culture and electroporation methods are also presented, allowing for pharmacological and genetic perturbation of SHF development, as well as image analysis approaches to quantify cell features across the progenitor cell epithelium. These protocols are broadly applicable to the study of epithelia in the early embryo.

Multispecies RNA tomography reveals regulators of hematopoietic stem cell birth in the embryonic aorta

The defined location of a stem cell within a niche regulates its fate, behavior, and molecular identity via a complex extrinsic regulation that is far from being fully elucidated. To explore the molecular characteristics and key components of the aortic microenvironment, where the first hematopoietic stem cells are generated during development, we performed genome-wide RNA tomography sequencing on zebrafish, chicken, mouse, and human embryos. The resulting anterior-posterior and dorsal-ventral transcriptional maps provided a powerful resource for exploring genes and regulatory pathways active in the aortic microenvironment. By performing interspecies comparative RNA sequencing analyses and functional assays, we explored the complexity of the aortic microenvironment landscape and the fine-tuning of various factors interacting to control hematopoietic stem cell generation, both in time and space in vivo, including the ligand-receptor couple ADM-RAMP2 and SVEP1. Understanding the regulatory function of the local environment will pave the way for improved stem cell production in vitro and clinical cell therapy.

Leukocyte Trafficking and Hemostasis in the Mouse Fetus in vivo: A Practical Guide

In vivo observations of blood cells and organ compartments within the fetal mammalian organism are difficult to obtain. This practical guide describes a mouse model for in vivo observation of the fetal yolk-sac and corporal microvasculature throughout murine gestation, including imaging of various organ compartments, microvascular injection procedures, different methods for staining of blood plasma, vessel wall and circulating cell subsets. Following anesthesia of pregnant mice, the maternal abdominal cavity is opened, the uterus horn exteriorized, and the fetus prepared for imaging while still connected to the placenta. Microinjection methods allow delivery of substances directly into the fetal circulation, while substances crossing the placenta can be easily administered via the maternal circulation. Small volume blood sample collection allows for further in vitro workup of obtained results. The model permits observation of leukocyte-endothelial interactions, hematopoietic niche localization, platelet function, endothelial permeability studies, and hemodynamic changes in the mouse fetus, using appropriate strains of fluorescent protein expressing reporter mice and various sophisticated intravital microscopy techniques. Our practical guide is of interest to basic physiologists, developmental biologists, cardiologists, and translational neonatologists and reaches out to scientists focusing on the origin and regulation of hematopoietic niches, thrombopoiesis and macrophage heterogeneity.

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.

In vivo generation of haematopoietic stem/progenitor cells from bone marrow-derived haemogenic endothelium

It is well established that haematopoietic stem and progenitor cells (HSPCs) are generated from a transient subset of specialized endothelial cells termed haemogenic, present in the yolk sac, placenta and aorta, through an endothelial-to-haematopoietic transition (EHT). HSPC generation via EHT is thought to be restricted to the early stages of development. By using experimental embryology and genetic approaches in birds and mice, respectively, we document here the discovery of a bone marrow haemogenic endothelium in the late fetus/young adult. These cells are capable of de novo producing a cohort of HSPCs in situ that harbour a very specific molecular signature close to that of aortic endothelial cells undergoing EHT or their immediate progenies, i.e., recently emerged HSPCs. Taken together, our results reveal that HSPCs can be generated de novo past embryonic stages. Understanding the molecular events controlling this production will be critical for devising innovative therapies.

Assessment of Prenatal Kynurenine Metabolism Using Tissue Slices: Focus on the Neosynthesis of Kynurenic Acid in Mice

Several lines of evidence support the hypothesis that abnormally elevated brain levels of kynurenic acid (KYNA), a metabolite of the kynurenine pathway (KP) of tryptophan degradation, play a pathophysiologically significant role in schizophrenia and other major neurodevelopmental disorders. Studies in experimental animal models suggest that KP impairments in these diseases may originate already in utero since prenatal administration of KYNA's bioprecursor, kynurenine, leads to biochemical and structural abnormalities as well as distinct cognitive impairments in adulthood. As KP metabolism during pregnancy is still insufficiently understood, we designed this study to examine the de novo synthesis of KYNA and 3-hydroxykynurenine (3-HK), an alternative biologically active product of kynurenine degradation, in tissue slices obtained from pregnant mice on gestational day (GD) 18. Fetal brain and liver, placenta, and maternal brain and liver were collected, and the tissues were incubated in vitroin the absence or presence of micromolar concentrations of kynurenine. KYNA and 3-HK were measured in the extracellular milieu. Basal and newly produced KYNA was detected in all cases. As KYNA formation exceeded 3-HK production by 2-3 orders of magnitude in the placenta and maternal brain, and as very little 3-HK neosynthesis was detectable in fetal brain tissue, detailed follow-up experiments focused on KYNA only. The fetal brain produced 3-4 times more KYNA than the maternal brain and placenta, though less than the maternal and fetal liver. No significant differences were observed when using tissues obtained on GD 14 and GD 18. Pharmacological inhibition of KYNA's main biosynthetic enzymes, kynurenine aminotransferase (KAT) I and KAT II, revealed qualitative and quantitative differences between the tissues, with a preferential role of KAT I in the fetal and maternal brain and of KAT II in the fetal and maternal liver. Findings using tissue slices from KAT II knockout mice confirmed these conclusions. Together, these results clarify the dynamics of KP metabolism during pregnancy and provide the basis for the conceptualization of interventions aimed at manipulating cerebral KP function in the prenatal period.

Single-cell transcriptomics reveal the dynamic of haematopoietic stem cell production in the aorta

Haematopoietic stem cells (HSCs) are generated from haemogenic endothelial (HE) cells via the formation of intra-aortic haematopoietic clusters (IAHCs) in vertebrate embryos. The molecular events controlling endothelial specification, endothelial-to-haematopoietic transition (EHT) and IAHC formation, as it occurs in vivo inside the aorta, are still poorly understood. To gain insight in these processes, we performed single-cell RNA-sequencing of non-HE cells, HE cells, cells undergoing EHT, IAHC cells, and whole IAHCs isolated from mouse embryo aortas. Our analysis identified the genes and transcription factor networks activated during the endothelial-to-haematopoietic switch and IAHC cell maturation toward an HSC fate. Our study provides an unprecedented complete resource to study in depth HSC generation in vivo. It will pave the way for improving HSC production in vitro to address the growing need for tailor-made HSCs to treat patients with blood-related disorders.

In vivo single cell analysis reveals Gata2 dynamics in cells transitioning to hematopoietic fate

Eich et al. reveal the dynamic expression of the Gata2 transcription factor in single aortic cells transitioning to hematopoietic fate by vital imaging of Gata2Venus mouse embryos. Pulsatile expression level changes highlight an unstable genetic state during hematopoietic cell generation.

Cell fate is established through coordinated gene expression programs in individual cells. Regulatory networks that include the Gata2 transcription factor play central roles in hematopoietic fate establishment. Although Gata2 is essential to the embryonic development and function of hematopoietic stem cells that form the adult hierarchy, little is known about the in vivo expression dynamics of Gata2 in single cells. Here, we examine Gata2 expression in single aortic cells as they establish hematopoietic fate in Gata2Venus mouse embryos. Time-lapse imaging reveals rapid pulsatile level changes in Gata2 reporter expression in cells undergoing endothelial-to-hematopoietic transition. Moreover, Gata2 reporter pulsatile expression is dramatically altered in Gata2^+/− aortic cells, which undergo fewer transitions and are reduced in hematopoietic potential. Our novel finding of dynamic pulsatile expression of Gata2 suggests a highly unstable genetic state in single cells concomitant with their transition to hematopoietic fate. This reinforces the notion that threshold levels of Gata2 influence fate establishment and has implications for transcription factor–related hematologic dysfunctions.

Cell cycle dynamics and complement expression distinguishes mature haematopoietic subsets arising from hemogenic endothelium

The emergence of haematopoietic stem and progenitor cells (HSPCs) from hemogenic endothelium results in the formation of sizeable HSPC clusters attached to the vascular wall. We evaluate the cell cycle and proliferation of HSPCs involved in cluster formation, as well as the molecular signatures from their initial appearance to the point when cluster cells are capable of adult engraftment (definitive HSCs). We uncover a non-clonal origin of HSPC clusters with differing cell cycle, migration, and cell signaling attributes. In addition, we find that the complement cascade is highly enriched in mature HSPC clusters, possibly delineating a new role for this pathway in engraftment.

Restricted intra-embryonic origin of bona fide hematopoietic stem cells in the chicken

Hematopoietic stem cells (HSCs), which are responsible for blood cell production, are generated during embryonic development. Human and chicken embryos share features that position the chicken as a reliable and accessible alternative model to study developmental hematopoiesis. However, the existence of HSCs has never been formally proven in chicken embryos. Here, we have established a complete cartography and quantification of hematopoietic cells in the aorta during development. We demonstrate the existence of bona fide HSCs, originating from the chicken embryo aorta (and not the yolk sac, allantois or head), through an in vivo transplantation assay. Embryos transplanted in ovo with GFP embryonic tissues on the chorio-allantoic membrane provided multilineage reconstitution in adulthood. Historically, most breakthrough discoveries in the field of developmental hematopoiesis were first made in birds and later extended to mammals. Our study sheds new light on the avian model as a valuable system to study HSC production and regulation in vivo.

Understanding Hematopoietic Stem Cell Development through Functional Correlation of Their Proliferative Status with the Intra-aortic Cluster Architecture

During development, hematopoietic stem cells (HSCs) emerge in the aorta-gonad-mesonephros (AGM) region through a process of multi-step maturation and expansion. While proliferation of adult HSCs is implicated in the balance between self-renewal and differentiation, very little is known about the proliferation status of nascent HSCs in the AGM region. Using Fucci reporter mice that enable in vivo visualization of cell-cycle status, we detect increased proliferation during pre-HSC expansion followed by a slowing down of cycling once cells start to acquire a definitive HSC state, similar to fetal liver HSCs. We observe time-specific changes in intra-aortic hematopoietic clusters corresponding to HSC maturation stages. The proliferative architecture of the clusters is maintained in an orderly anatomical manner with slowly cycling cells at the base and more actively proliferating cells at the more apical part of the cluster, which correlates with c-KIT expression levels, thus providing an anatomical basis for the role of SCF in HSC maturation.

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Expansion of HSC precursors is accompanied by increased proliferation

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Final steps of HSC maturation are accompanied by decelerating proliferation

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Proliferative architecture of intra-aortic clusters is maintained during HSC development

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c-Kit expression levels correlate with the proliferative status of HSC precursors

Cooperative binding of AP-1 and TEAD4 modulates the balance between vascular smooth muscle and hemogenic cell fate

The transmission of extracellular signals into the nucleus involves inducible transcription factors, but how different signalling pathways act in a cell type-specific fashion is poorly understood. Here, we studied the regulatory role of the AP-1 transcription factor family in blood development using embryonic stem cell differentiation coupled with genome-wide transcription factor binding and gene expression analyses. AP-1 factors respond to MAP kinase signalling and comprise dimers of FOS, ATF and JUN proteins. To examine genes regulated by AP-1 and to examine how it interacts with other inducible transcription factors, we abrogated its global DNA-binding activity using a dominant-negative FOS peptide. We show that FOS and JUN bind to and activate a specific set of vascular genes and that AP-1 inhibition shifts the balance between smooth muscle and hematopoietic differentiation towards blood. Furthermore, AP-1 is required for de novo binding of TEAD4, a transcription factor connected to Hippo signalling. Our bottom-up approach demonstrates that AP-1- and TEAD4-associated cis-regulatory elements form hubs for multiple signalling-responsive transcription factors and define the cistrome that regulates vascular and hematopoietic development by extrinsic signals.

Chromatin programming by developmentally regulated transcription factors: lessons from the study of haematopoietic stem cell specification and differentiation

Although the body plan of individuals is encoded in their genomes, each cell type expresses a different gene expression programme and therefore has access to only a subset of this information. Alterations to gene expression programmes are the underlying basis for the differentiation of multiple cell types and are driven by tissue-specific transcription factors (TFs) that interact with the epigenetic regulatory machinery to programme the chromatin landscape into transcriptionally active and inactive states. The haematopoietic system has long served as a paradigm for studying the molecular principles that regulate gene expression in development. In this review article, we summarize the current knowledge on the mechanism of action of TFs regulating haematopoietic stem cell specification and differentiation, and place this information into the context of general principles governing development.

EphrinB2 regulates the emergence of a hemogenic endothelium from the aorta

Adult-type intraembryonic hematopoiesis arises from specialized endothelial cells of the dorsal aorta (DA). Despite the critical importance of this specialized endothelium for establishment of hematopoietic stem cells and adult hematopoietic lineages, the mechanisms regulating its emergence are incompletely understood. We show that EphrinB2, a principal regulator of endothelial cell function, controls the development of endothelium producing adult-type hematopoiesis. The absence of EphrinB2 impairs DA-derived hematopoiesis. Transmembrane EphrinB2 and its EphB4 receptor interact in the emerging DA, which transiently harbors EphrinB2⁺ and EphB4⁺ endothelial cells, thereby providing an opportunity for bi-directional cell-to-cell signaling to control the emergence of the hemogenic endothelium. Embryonic Stem (ES) cell-derived EphrinB2⁺ cells are enriched with hemogenic endothelial precursors. EphrinB2 silencing impairs ES generation of hematopoietic cells but not generation of endothelial cells. The identification of EphrinB2 as an essential regulator of adult hematopoiesis provides important insight in the regulation of early hematopoietic commitment.

TBX1 regulates epithelial polarity and dynamic basal filopodia in the second heart field

Elongation of the vertebrate heart occurs by progressive addition of second heart field (SHF) cardiac progenitor cells from pharyngeal mesoderm to the poles of the heart tube. The importance of these cells in the etiology of congenital heart defects has led to extensive research into the regulation of SHF deployment by signaling pathways and transcription factors. However, the basic cellular features of these progenitor cells, including epithelial polarity, cell shape and cell dynamics, remain poorly characterized. Here, using immunofluorescence, live imaging and embryo culture, we demonstrate that SHF cells constitute an atypical, apicobasally polarized epithelium in the dorsal pericardial wall, characterized by apical monocilia and dynamic actin-rich basal filopodia. We identify the 22q11.2 deletion syndrome gene Tbx1, required in the SHF for outflow tract development, as a regulator of the epithelial properties of SHF cells. Cell shape changes in mutant embryos include increased circularity, a reduced basolateral membrane domain and impaired filopodial activity, and are associated with elevated aPKCζ levels. Activation of aPKCζ in embryo culture similarly impairs filopodia activity and phenocopies proliferative defects and ectopic differentiation observed in the SHF of Tbx1 null embryos. Our results reveal that epithelial and progenitor cell status are coupled in the SHF, identifying control of cell shape as a regulatory step in heart tube elongation and outflow tract morphogenesis.

Progressive maturation toward hematopoietic stem cells in the mouse embryo aorta

Clusters of cells attached to the endothelium of the main embryonic arteries were first observed a century ago. Present in most vertebrate species, such clusters, or intraaortic hematopoietic clusters (IAHCs), derive from specialized hemogenic endothelial cells and contain the first few hematopoietic stem cells (HSCs) generated during embryonic development. However, some discrepancies remained concerning the spatio-temporal appearance and the numbers of IAHCs and HSCs. Therefore, the exact cell composition and function of IAHCs remain unclear to date. We show here that IAHCs contain pre-HSCs (or HSC precursors) that can mature into HSCs in vivo (as shown by the successful long-term multilineage reconstitution of primary neonates and secondary adult recipients). Such IAHC pre-HSCs could contribute to the HSC pool increase observed at midgestation. The novel insights in pre-HSC to HSC transition represent an important step toward generating transplantable HSCs in vitro that are needed for autologous HSC transplantation therapies.

Integrin αIIb (CD41) plays a role in the maintenance of hematopoietic stem cell activity in the mouse embryonic aorta

Integrins are transmembrane receptors that play important roles as modulators of cell behaviour through their adhesion properties and the initiation of signaling cascades. The α_IIb integrin subunit (CD41) is one of the first cell surface markers indicative of hematopoietic commitment. α_IIb pairs exclusively with β₃ to form the α_IIbβ₃ integrin. β₃ (CD61) also pairs with α_v (CD51) to form the α_vβ₃ integrin. The expression and putative role of these integrins during mouse hematopoietic development is as yet unknown. We show here that hematopoietic stem cells (HSCs) differentially express α_IIbβ₃ and α_vβ₃ integrins throughout development. Whereas the first HSCs generated in the aorta at mid-gestation express both integrins, HSCs from the placenta only express α_vβ₃, and most fetal liver HSCs do not express either integrin. By using α_IIb deficient embryos, we show that α_IIb is not only a reliable HSC marker but it also plays an important and specific function in maintaining the HSC activity in the mouse embryonic aorta.

Pharmacological manipulation of blood and lymphatic vascularization in ex vivo-cultured mouse embryos

Formation of new blood and lymphatic vessels is involved in many physiological and pathological processes, including organ and tumor growth, cancer cell metastasis, fluid drainage and lymphedema. Therefore, the ability to manipulate vascularization in a mammalian system is of particular interest to researchers. Here we describe a method for pharmacological manipulation of de novo and sprouting blood and lymphatic vascular development in ex vivo-cultured mouse embryos. The described protocol can also be used to evaluate the properties of pharmacological agents in growing mammalian tissues and to manipulate other developmental processes. The whole procedure, from embryo isolation to image quantification, takes 3-5 d, depending on the analysis and age of the embryos.

Studying cell behavior in whole zebrafish embryos by confocal live imaging: application to hematopoietic stem cells

Confocal live imaging is a key tool for studying cell behavior in the whole zebrafish embryo. Here we provide a detailed protocol that is adaptable for imaging any progenitor cell behavior in live zebrafish embryos. As an example, we imaged the emergence of the first hematopoietic stem cells from the aorta. We discuss the importance of selecting the appropriate zebrafish transgenic line as well as methods for immobilization of embryos to be imaged. In addition, we highlight the confocal microscopy acquisition parameters required for stem cell imaging and the software tools we used to analyze 4D movies. The whole protocol takes 2 h 15 min and allows confocal live imaging from a few hours to several days.