Expression levels of endoglin distinctively identify hematopoietic and endothelial progeny at different stages of yolk sac hematopoiesis

Single-nuclei transcriptome analysis of IgM+ cells isolated from channel catfish (Ictalurus punctatus) spleen

Catfish production is the primary aquaculture sector in the United States, and the key cultured species is channel catfish (Ictalurus punctatus). The major causes of production losses are pathogenic diseases, and the spleen, an important site of adaptive immunity, is implicated in these diseases. To examine the channel catfish immune system, single-nuclei transcriptomes of sorted and captured IgM+ cells were produced from adult channel catfish. Three channel catfish (~1 kg) were euthanized, the spleen dissected, and the tissue dissociated. The lymphocytes were isolated using a Ficoll gradient and IgM+ cells were then sorted with flow cytometry. The IgM+ cells were lysed and single-nuclei libraries generated using a Chromium Next GEM Single Cell 3' GEM Kit and the Chromium X Instrument (10x Genomics) and sequenced with the Illumina NovaSeq X Plus sequencer. The reads were aligned to the I. punctatus reference assembly (Coco_2.0) using Cell Ranger, and normalization, cluster analysis, and differential gene expression analysis were carried out with Seurat. Across the three samples, approximately 753.5 million reads were generated for 18,686 cells. After filtering, 10,637 cells remained for the cluster analysis. The cluster analysis identified 16 clusters which were classified as B cells (10,276), natural killer-like (NK-like) cells (178), T cells or natural killer cells (45), hematopoietic stem and progenitor cells (HSPC)/megakaryocytes (MK) (66), myeloid/epithelial cells (40), and plasma cells (32). The B cell clusters were further defined as different populations of mature B cells, cycling B cells, and plasma cells. The plasma cells highly expressed ighm and we demonstrated that the secreted form of the transcript was largely being expressed by these cells. This atlas provides insight into the gene expression of IgM+ immune cells in channel catfish. The atlas is publicly available and could be used garner more important information regarding the gene expression of splenic immune cells.

Altered BMP2/4 Signaling in Stem Cells and Their Niche: Different Cancers but Similar Mechanisms, the Example of Myeloid Leukemia and Breast Cancer

Understanding mechanisms of cancer development is mandatory for disease prevention and management. In healthy tissue, the microenvironment or niche governs stem cell fate by regulating the availability of soluble molecules, cell-cell contacts, cell-matrix interactions, and physical constraints. Gaining insight into the biology of the stem cell microenvironment is of utmost importance, since it plays a role at all stages of tumorigenesis, from (stem) cell transformation to tumor escape. In this context, BMPs (Bone Morphogenetic Proteins), are key mediators of stem cell regulation in both embryonic and adult organs such as hematopoietic, neural and epithelial tissues. BMPs directly regulate the niche and stem cells residing within. Among them, BMP2 and BMP4 emerged as master regulators of normal and tumorigenic processes. Recently, a number of studies unraveled important mechanisms that sustain cell transformation related to dysregulations of the BMP pathway in stem cells and their niche (including exposure to pollutants such as bisphenols). Furthermore, a direct link between BMP2/BMP4 binding to BMP type 1 receptors and the emergence and expansion of cancer stem cells was unveiled. In addition, a chronic exposure of normal stem cells to abnormal BMP signals contributes to the emergence of cancer stem cells, or to disease progression independently of the initial transforming event. In this review, we will illustrate how the regulation of stem cells and their microenvironment becomes dysfunctional in cancer via the hijacking of BMP signaling with main examples in myeloid leukemia and breast cancers.

Endoglin in the Spotlight to Treat Cancer

A spotlight has been shone on endoglin in recent years due to that fact of its potential to serve as both a reliable disease biomarker and a therapeutic target. Indeed, endoglin has now been assigned many roles in both physiological and pathological processes. From a molecular point of view, endoglin mainly acts as a co-receptor in the canonical TGFβ pathway, but also it may be shed and released from the membrane, giving rise to the soluble form, which also plays important roles in cell signaling. In cancer, in particular, endoglin may contribute to either an oncogenic or a non-oncogenic phenotype depending on the cell context. The fact that endoglin is expressed by neoplastic and non-neoplastic cells within the tumor microenvironment suggests new possibilities for targeted therapies. Here, we aimed to review and discuss the many roles played by endoglin in different tumor types, as well as the strong evidence provided by pre-clinical and clinical studies that supports the therapeutic targeting of endoglin as a novel clinical strategy.

Serial transplantation reveals a critical role for endoglin in hematopoietic stem cell quiescence

Transforming growth factor β (TGF-β) is well known for its important function in hematopoietic stem cell (HSC) quiescence. However, the molecular mechanism underlining this function remains obscure. Endoglin (Eng), a type III receptor for the TGF-β superfamily, has been shown to selectively mark long-term HSCs; however, its necessity in adult HSCs is unknown due to embryonic lethality. Using conditional deletion of Eng combined with serial transplantation, we show that this TGF-β receptor is critical to maintain the HSC pool. Transplantation of Eng-deleted whole bone marrow or purified HSCs into lethally irradiated mice results in a profound engraftment defect in tertiary and quaternary recipients. Cell cycle analysis of primary grafts revealed decreased frequency of HSCs in G₀, suggesting that lack of Eng impairs reentry of HSCs to quiescence. Using cytometry by time of flight (CyTOF) to evaluate the activity of signaling pathways in individual HSCs, we find that Eng is required within the Lin^-Sca⁺Kit⁺-CD48^- CD150⁺ fraction for canonical and noncanonical TGF-β signaling, as indicated by decreased phosphorylation of SMAD2/3 and the p38 MAPK-activated protein kinase 2, respectively. These findings support an essential role for Eng in positively modulating TGF-β signaling to ensure maintenance of HSC quiescence.

In vivo identification of Bmp2-correlation networks during fracture healing by means of a limb-specific conditional inactivation of Bmp2

Bmp2 is known to play an essential role in the initiation of fracture healing via periosteal activation. Specifically, activation and subsequent differentiation of periosteal progenitor cells requires Bmp2 signaling for activation of the osteo-chondrogenic pathway. Here, we explored the interactive transcriptional gene-gene interplays between Bmp2 and 150 known candidate genes during fracture repair. We constructed the interactive Bmp2 signaling pathways in vivo, by comparing gene expression levels prior and 24 h post femur fracture, in presence (wild type) and in absence of Bmp2 (Bmp2c/c;Prx1::cre limb-specific conditional knockout). Twenty-six differentially expressed genes (pre- vs. post-fracture), which demonstrated high correlations within each experimental condition, were used to construct the co-expression networks. Topological dynamic shifts across different co-expression networks characterized the 26 differentially expressed genes as non-redundant focal linking hubs, redundant connecting hubs, periphery genes, or non-existent. Top-ranked up- or down-regulated genes were identified and discussed. Protein-protein interactions in public databases support our findings. Thus, the co-expression networks from this study can be used for future experimental hypotheses.

The BMP pathway: A unique tool to decode the origin and progression of leukemia

The microenvironment (niche) governs the fate of stem cells (SCs) by balancing self-renewal and differentiation. Increasing evidence indicates that the tumor niche plays an active role in cancer, but its important properties for tumor initiation progression and resistance remain to be identified. Clinical data show that leukemic stem cell (LSC) survival is responsible for disease persistence and drug resistance, probably due to their sustained interactions with the tumor niche. Bone morphogenetic protein (BMP) signaling is a key pathway controlling stem cells and their niche. BMP2 and BMP4 are important in both the normal and the cancer context. Several studies have revealed profound alterations of the BMP signaling in cancer SCs, with major deregulations of the BMP receptors and their downstream signaling elements. This was illustrated in the hematopoietic system by pioneer studies in chronic myelogenous leukemia that may now be expanded to acute myeloid leukemia and lymphoid leukemia, as reviewed here. At diagnosis, cells from the leukemic microenvironment are the major providers of soluble BMPs. Conversely, LSCs display altered receptors and downstream BMP signaling elements accompanied by altered functional responses to BMPs. These studies reveal the role of BMPs in tumor initiation, in addition to their known effects in later stages of transformation and progression. They also reveal the importance of BMPs in fueling cell transformation and expansion by overamplifying a natural SC response. This mechanism may explain the survival of LSCs independently of the initial oncogenic event and therefore may be involved in resistance processes.

Arrested Development: Infantile Hemangioma and the Stem Cell Teratogenic Hypothesis

BACKGROUND

Early-life programming is defined by the adaptive changes made by the fetus in response to an adverse in utero environment. Infantile hemangioma (IH), a vascular anomaly, is the most common tumor of infancy. Here we take IH as the tumor model to propose the stem cell teratogenic hypothesis of tumorigenesis and the potential involvement of the immune system.

OBJECTIVES

Teratogenic agents include chemicals, heavy metals, pathogens, and ionizing radiation. To investigate the etiology and pathogenesis of IH, we hypothesized that they result from a teratogenic mechanism. Immature, incompletely differentiated, dysregulated progenitor cells (multipotential stem cells) are arrested in development with vasculogenic, angiogenic, and tumorigenic potential due to exposure to teratogenic agents such as extrinsic factors that disrupt intrinsic factors via molecular mimicry. During the critical period of immunological tolerance, environmental exposure to immunotoxic agents may harness the teratogenic potential in the developing embryo or fetus and modify the early-life programming algorithm by altering normal fetal development, causing malformations, and inducing tumorigenesis. Specifically, exposure to environmental agents may interfere with physiological signaling pathways and contribute to the generation of IH, by several mechanisms.

DISCUSSION

An adverse in utero environment no longer serves as a sustainable environment for proper embryogenesis and normal development. Targeted disruption of stem cells by extrinsic factors can alter the genetic program.

CONCLUSIONS

This article offers new perspectives to stimulate discussion, explore novel experimental approaches (such as immunotoxicity/vasculotoxicity assays and novel isogenic models), and to address the questions raised to convert the hypotheses into nontoxic, noninvasive treatments.

Endoglin: a novel target for therapeutic intervention in acute leukemias revealed in xenograft mouse models

Endoglin (CD105), a receptor of the transforming growth factor-β superfamily, has been reported to identify functional long-term repopulating hematopoietic stem cells, and has been detected in certain subtypes of acute leukemias. Whether this receptor plays a functional role in leukemogenesis remains unknown. We identified endoglin expression on the majority of blasts from patients with acute myeloid leukemia (AML) and acute B-lymphoblastic leukemia (B-ALL). Using a xenograft model, we find that CD105⁺ blasts are endowed with superior leukemogenic activity compared with the CD105^- population. We test the effect of targeting this receptor using the monoclonal antibody TRC105, and find that in AML, TRC105 prevented the engraftment of primary AML blasts and inhibited leukemia progression following disease establishment, but in B-ALL, TRC105 alone was ineffective due to the shedding of soluble CD105. However, in both B-ALL and AML, TRC105 synergized with reduced intensity myeloablation to inhibit leukemogenesis, indicating that TRC105 may represent a novel therapeutic option for B-ALL and AML.

Infantile Hemangioma Originates From A Dysregulated But Not Fully Transformed Multipotent Stem Cell

Infantile hemangioma (IH) is the most common tumor of infancy. Its cellular origin and biological signals for uncontrolled growth are poorly understood, and specific pharmacological treatment is unavailable. To understand the process of hemangioma-genesis we characterized the progenitor hemangioma-derived stem cell (HemSC) and its lineage and non-lineage derivatives. For this purpose we performed a high-throughput (HT) phenotypic and gene expression analysis of HemSCs, and analyzed HemSC-derived tumorspheres. We found that IH is characterized by high expression of genes involved in vasculogenesis, angiogenesis, tumorigenesis and associated signaling pathways. These results show that IH derives from a dysregulated stem cell that remains in an immature, arrested stage of development. The potential biomarkers we identified can afford the development of diagnostic tools and precision-medicine therapies to "rewire" or redirect cellular transitions at an early stage, such as signaling pathways or immune response modifiers.

Endoglin integrates BMP and Wnt signalling to induce haematopoiesis through JDP2

Mechanisms of haematopoietic and cardiac patterning remain poorly understood. Here we show that the BMP and Wnt signalling pathways are integrated in an endoglin (Eng)-dependent manner in cardiac and haematopoietic lineage specification. Eng is expressed in early mesoderm and marks both haematopoietic and cardiac progenitors. In the absence of Eng, yolk sacs inappropriately express the cardiac marker, Nkx2.5. Conversely, high levels of Eng in vitro and in vivo increase haematopoiesis and inhibit cardiogenesis. Levels of Eng determine the activation of both BMP and Wnt pathways, which are integrated downstream of Eng by phosphorylation of Smad1 by Gsk3. By interrogating Eng-dependent Wnt-mediated transcriptional changes, we identify Jdp2 as a key Eng-dependent Wnt target, sufficient to establish haematopoietic fate in early mesoderm when BMP and Wnt crosstalk is disturbed. These studies provide mechanistic insight into the integration of BMP and Wnt signalling in the establishment of haematopoietic and cardiac progenitors during embryogenesis.

How both BMP and Wnt signalling pathways regulate lineage specification early in development is unclear. Here, the authors show that endoglin via Jdp2, an AP-1 family member, modulates BMP and Wnt signalling to commit mesodermal progenitors to a haematopoietic fate at the expense of the cardiac lineage.

Definitive Hematopoiesis in the Yolk Sac Emerges from Wnt-Responsive Hemogenic Endothelium Independently of Circulation and Arterial Identity

Adult-repopulating hematopoietic stem cells (HSCs) emerge in low numbers in the midgestation mouse embryo from a subset of arterial endothelium, through an endothelial-to-hematopoietic transition. HSC-producing arterial hemogenic endothelium relies on the establishment of embryonic blood flow and arterial identity, and requires β-catenin signaling. Specified prior to and during the formation of these initial HSCs are thousands of yolk sac-derived erythro-myeloid progenitors (EMPs). EMPs ensure embryonic survival prior to the establishment of a permanent hematopoietic system, and provide subsets of long-lived tissue macrophages. While an endothelial origin for these HSC-independent definitive progenitors is also accepted, the spatial location and temporal output of yolk sac hemogenic endothelium over developmental time remain undefined. We performed a spatiotemporal analysis of EMP emergence, and document the morphological steps of the endothelial-to-hematopoietic transition. Emergence of rounded EMPs from polygonal clusters of Kit(+) cells initiates prior to the establishment of arborized arterial and venous vasculature in the yolk sac. Interestingly, Kit(+) polygonal clusters are detected in both arterial and venous vessels after remodeling. To determine whether there are similar mechanisms regulating the specification of EMPs with other angiogenic signals regulating adult-repopulating HSCs, we investigated the role of embryonic blood flow and Wnt/β-catenin signaling during EMP emergence. In embryos lacking a functional circulation, rounded Kit(+) EMPs still fully emerge from unremodeled yolk sac vasculature. In contrast, canonical Wnt signaling appears to be a common mechanism regulating hematopoietic emergence from hemogenic endothelium. These data illustrate the heterogeneity in hematopoietic output and spatiotemporal regulation of primary embryonic hemogenic endothelium.

Endoglin potentiates nitric oxide synthesis to enhance definitive hematopoiesis

During embryonic development, hematopoietic cells develop by a process of endothelial-to hematopoietic transition of a specialized population of endothelial cells. These hemogenic endothelium (HE) cells in turn develop from a primitive population of FLK1(+) mesodermal cells. Endoglin (ENG) is an accessory TGF-β receptor that is enriched on the surface of endothelial and hematopoietic stem cells and is also required for the normal development of hemogenic precursors. However, the functional role of ENG during the transition of FLK1(+) mesoderm to hematopoietic cells is ill defined. To address this we used a murine embryonic stem cell model that has been shown to mirror the temporal emergence of these cells in the embryo. We noted that FLK1(+) mesodermal cells expressing ENG generated fewer blast colony-forming cells but had increased hemogenic potential when compared with ENG non-expressing cells. TIE2(+)/CD117(+) HE cells expressing ENG also showed increased hemogenic potential compared with non-expressing cells. To evaluate whether high ENG expression accelerates hematopoiesis, we generated an inducible ENG expressing ES cell line and forced expression in FLK1(+) mesodermal or TIE2(+)/CD117(+) HE cells. High ENG expression at both stages accelerated the emergence of CD45(+) definitive hematopoietic cells. High ENG expression was associated with increased pSMAD2/eNOS expression and NO synthesis in hemogenic precursors. Inhibition of eNOS blunted the ENG induced increase in definitive hematopoiesis. Taken together, these data show that ENG potentiates the emergence of definitive hematopoietic cells by modulating TGF-β/pSMAD2 signalling and increasing eNOS/NO synthesis.

Expression of podocalyxin separates the hematopoietic and vascular potentials of mouse embryonic stem cell-derived mesoderm

In the mouse embryo and differentiating embryonic stem cells, the hematopoietic, endothelial, and cardiomyocyte lineages are derived from Flk1+ mesodermal progenitors. Here, we report that surface expression of Podocalyxin (Podxl), a member of the CD34 family of sialomucins, can be used to subdivide the Flk1+ cells in differentiating embryoid bodies at day 4.75 into populations that develop into distinct mesodermal lineages. Definitive hematopoietic potential was restricted to the Flk1+Podxl+ population, while the Flk1-negative Podxl+ population displayed only primitive erythroid potential. The Flk1+Podxl-negative population contained endothelial cells and cardiomyocyte potential. Podxl expression distinguishes Flk1+ mesoderm populations in mouse embryos at days 7.5, 8.5, and 9.5 and is a marker of progenitor stage primitive erythroblasts. These findings identify Podxl as a useful tool for separating distinct mesodermal lineages.

Hemogenic endothelium generates mesoangioblasts that contribute to several mesodermal lineages in vivo

The embryonic endothelium is a known source of hematopoietic stem cells. Moreover, vessel-associated progenitors/stem cells with multilineage mesodermal differentiation potential, such as the ‘embryonic mesoangioblasts’, originate in vitro from the endothelium. Using a genetic lineage tracing approach, we show that early extra-embryonic endothelium generates, in a narrow time-window and prior to the hemogenic endothelium in the major embryonic arteries, hematopoietic cells that migrate to the embryo proper, and are subsequently found within the mesenchyme. A subpopulation of these cells, distinct from embryonic macrophages, co-expresses mesenchymal and hematopoietic markers. In addition, hemogenic endothelium-derived cells contribute to skeletal and smooth muscle, and to other mesodermal cells in vivo, and display features of embryonic mesoangioblasts in vitro. Therefore, we provide new insights on the distinctive characteristics of the extra-embryonic and embryonic hemogenic endothelium, and we identify the putative in vivo counterpart of embryonic mesoangioblasts, suggesting their identity and developmental ontogeny.