BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor

A neural network-based model framework for cell-fate decisions and development

Gene regulatory networks (GRNs) fulfill the essential function of maintaining the stability of cellular differentiation states by sustaining lineage-specific gene expression, while driving the progression of development. However, accounting for the relative stability of intermediate differentiation stages and their divergent trajectories remains a major challenge for models of developmental biology. Here, we develop an empirical data-based associative GRN model (AGRN) in which regulatory networks store multilineage stage-specific gene expression profiles as associative memory patterns. These networks are capable of responding to multiple instructive signals and, depending on signal timing and identity, can dynamically drive the differentiation of multipotent cells toward different cell state attractors. The AGRN dynamics can thus generate diverse lineage-committed cell populations in a robust yet flexible manner, providing an attractor-based explanation for signal-driven cell fate decisions during differentiation and offering a readily generalizable modelling tool that can be applied to a wide variety of cell specification systems.

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

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

The role of BMP4 signaling in trophoblast emergence from pluripotency

The Bone Morphogenetic Protein (BMP) signaling pathway has established roles in early embryonic morphogenesis, particularly in the epiblast. More recently, however, it has also been implicated in development of extraembryonic lineages, including trophectoderm (TE), in both mouse and human. In this review, we will provide an overview of this signaling pathway, with a focus on BMP4, and its role in emergence and development of TE in both early mouse and human embryogenesis. Subsequently, we will build on these in vivo data and discuss the utility of BMP4-based protocols for in vitro conversion of primed vs. naïve pluripotent stem cells (PSC) into trophoblast, and specifically into trophoblast stem cells (TSC). PSC-derived TSC could provide an abundant, reproducible, and ethically acceptable source of cells for modeling placental development.

Effects of high-molecular-weight polyvinyl chloride on Xenopus laevis adults and embryos: the mRNA expression profiles of Myf5, Esr1, Bmp4, Pax6, and Hsp70 genes during early embryonic development

Microplastics and associated adverse effects have been on the global agenda in recent years. Because of its importance as a model organism for studies on developmental biology, Xenopus laevis has been chosen as the study animal in in vitro teratogenesis studies. FETAX test uses early-stage embryos of X. laevis to measure the potential of substances to cause mortality, malformation, and growth inhibition in developing embryos. The aim of this study was to examine the effects of high molecular weight polyvinyl chloride (HMW-PVC) on parental X. laevis frogs and their embryos using the FETAX test. To this purpose, a HMW-PVC dose of 1% of body weight/twice each week was provided to frogs by oral gavage throughout 6 weeks. After the procedure, oocytes and sperms of HMW-PVC-exposed frogs were fertilized and FETAX was applied to selected embryos. After the completion of a 96-h incubation period, tadpoles were examined, their live/dead status were determined, their lengths were measured, and their anomalies were photographed. Besides, excised organs of the parental frogs were referred to histopathology examination. On the other hand, the mRNA expression levels of Hsp70, Myf5, Bmp4, Pax6, and Esr1 genes were determined by applying real-time quantitative PCR method to cDNA which was synthesized from the total RNA of embryos. The results showed that treatment with HMW-PVC dose of 1% of body weight/twice each week caused malformations and decreased viability. Hsp70 and Pax6 gene expression levels significantly decreased in all assay groups, as compared with controls. Lung and intestine tissues showed normal appearance in histopatological examination. Further research is required to explain the whole effects of HMW-PVC exposure on X. laevis embryos.

Assessment of the Hematopoietic Differentiation Potential of Human Pluripotent Stem Cells in 2D and 3D Culture Systems

BACKGROUND

In vitro methods for hematopoietic differentiation of human pluripotent stem cells (hPSC) are a matter of priority for the in-depth research into the mechanisms of early embryogenesis. So-far, published results regarding the generation of hematopoietic cells come from studies using either 2D or 3D culture formats, hence, it is difficult to discern their particular contribution to the development of the concept of a unique in vitro model in close resemblance to in vivo hematopoiesis.

AIM OF THE STUDY

To assess using the same culture conditions and the same time course, the potential of each of these two formats to support differentiation of human pluripotent stem cells to primitive hematopoiesis without exogenous activation of Wnt signaling.

METHODS

We used in parallel 2D and 3D formats, the same culture environment and assay methods (flow cytometry, IF, qPCR) to investigate stages of commitment and specification of mesodermal, and hemogenic endothelial cells to CD34 hematopoietic cells and evaluated their clonogenic capacity in a CFU system.

RESULTS

We show an adequate formation of mesoderm, an efficient commitment to hemogenic endothelium, a higher number of CD34 hematopoietic cells, and colony-forming capacity potential only in the 3D format-supported differentiation.

CONCLUSIONS

This study shows that the 3D but not the 2D format ensures the induction and realization by endogenous mechanisms of human pluripotent stem cells' intrinsic differentiation program to primitive hematopoietic cells. We propose that the 3D format provides an adequate level of upregulation of the endogenous Wnt/β-catenin signaling.

BMP signaling is required for postnatal murine hematopoietic stem cell self-renewal

Life-long production of blood from hematopoietic stem cells (HSC) is a process of strict modulation. Intrinsic and extrinsic signals govern fate options like self-renewal – a cardinal feature of HSC. Bone morphogenetic proteins (BMP) have an established role in embryonic hematopoiesis, but less is known about its functions in adulthood. Previously, SMAD-mediated BMP signaling has been proven dispensable for HSC. However, the BMP type-II receptor (BMPR-II) is highly expressed in HSC, leaving the possibility that BMP function via alternative pathways. Here, we establish that BMP signaling is required for selfrenewal of adult HSC. Through conditional knockout we show that BMPR-II deficient HSC have impaired self-renewal and regenerative capacity. BMPR-II deficient cells have reduced p38 activation, implying that non-SMAD pathways operate downstream of BMP in HSC. Indeed, a majority of primitive hematopoietic cells do not engage in SMADmediated responses downstream of BMP in vivo. Furthermore, deficiency of BMPR-II results in increased expression of TJP1, a known regulator of self-renewal in other stem cells, and knockdown of TJP1 in primitive hematopoietic cells partly rescues the BMPR-II null phenotype. This suggests TJP1 may be a universal stem cell regulator. In conclusion, BMP signaling, in part mediated through TJP1, is required endogenously by adult HSC to maintain self-renewal capacity and proper resilience of the hematopoietic system during regeneration.

Cannibalized erythroblasts accelerate developmental neurogenesis by regulating mitochondrial dynamics

Metabolic support was long considered to be the only developmental function of hematopoiesis, a view that is gradually changing. Here, we disclose a mechanism triggered during neurulation that programs brain development by donation of sacrificial yolk sac erythroblasts to neuroepithelial cells. At embryonic day (E) 8.5, neuroepithelial cells transiently integrate with the endothelium of yolk sac blood vessels and cannibalize intravascular erythroblasts as transient heme-rich endosymbionts. This cannibalistic behavior instructs precocious neuronal differentiation of neuroepithelial cells in the proximity of blood vessels. By experiments in vitro, we show that access to erythroblastic heme accelerates the pace of neurogenesis by induction of a truncated neurogenic differentiation program from a poised state. Mechanistically, the poised state is invoked by activation of the mitochondrial electron transport chain that leads to amplified production of reactive oxygen species in addition to omnipresent guanosine triphosphate (GTP) with consequential upregulation of pro-differentiation β-catenin.

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

Background

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

Results

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

Conclusions

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

Supplementary Information

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

BMP4 overexpression induces the upregulation of APP/Tau and memory deficits in Alzheimer's disease

Alzheimer's disease (AD) is a chronic progressive degenerative disease of the nervous system. Its pathogenesis is complex and is related to the abnormal expression of the amyloid β (Aβ), APP, and Tau proteins. Evidence has demonstrated that bone morphogenetic protein 4 (BMP4) is highly expressed in transgenic mouse models of AD and that endogenous levels of BMP4 mainly affect hippocampal function. To determine whether BMP4 participates in AD development, transgenic mice were constructed that overexpress BMP4 under the control of the neuron-specific enolase (NSE) promoter. We also performed MTT, FACS, transfection, TUNEL, and Western blotting assays to define the role of BMP4 in cells. We found that middle-aged BMP4 transgenic mice exhibited impaired memory via the Morris water maze experiment. Moreover, their hippocampal tissues exhibited high expression levels of AD-related proteins, including APP, Aβ, PSEN-1, Tau, P-Tau (Thr181), and P-Tau (Thr231). Furthermore, in multiple cell lines, the overexpression of BMP4 increased the expression of AD-related proteins, whereas the downregulation of BMP4 demonstrated opposing effects. Consistent with these results, BMP4 modulation affected cell apoptosis via the regulation of BAX and Bcl-2 expression in cells. Our findings indicate that BMP4 overexpression might be a potential factor to induce AD.

Yap1 promotes proliferation of transiently amplifying stress erythroid progenitors during erythroid regeneration

In contrast to steady-state erythropoiesis, which generates new erythrocytes at a constant rate, stress erythropoiesis rapidly produces a large bolus of new erythrocytes in response to anemic stress. In this study, we illustrate that Yes-associated protein (Yap1) promotes the rapid expansion of a transit-amplifying population of stress erythroid progenitors in vivo and in vitro. Yap1-mutated erythroid progenitors failed to proliferate in the spleen after transplantation into lethally irradiated recipient mice. Additionally, loss of Yap1 impaired the growth of actively proliferating erythroid progenitors in vitro. This role in proliferation is supported by gene expression profiles showing that transiently amplifying stress erythroid progenitors express high levels of genes associated with Yap1 activity and genes induced by Yap1. Furthermore, Yap1 promotes the proliferation of stress erythroid progenitors in part by regulating the expression of key glutamine-metabolizing enzymes. Thus, Yap1 acts as an erythroid regulator that coordinates the metabolic status with the proliferation of erythroid progenitors to promote stress erythropoiesis.

Inhibition of bone morphogenetic protein signaling reduces viability, growth and migratory potential of non-small cell lung carcinoma cells

PURPOSE

BMP signaling has an oncogenic and tumor-suppressing activity in lung cancer that makes the prospective therapeutic utility of BMP signaling in lung cancer treatment complex. A more in-depth analysis of lung cancer subtypes is needed to identify BMP-related therapeutic targets. We sought to examine the influence of BMP signaling on the viability, growth and migration properties of the cell line LCLC-103H, which originates from a large cell lung carcinoma with giant cells and an extended aneuploidy.

METHODS

We used BMP-4 and LDN-214117 as agonist/antagonist system for the BMP receptor type I signaling. Using flow cytometry, wound healing assay, trans-well assay and spheroid culture, we examined the influence of BMP signaling on cell viability, growth and migration. Molecular mechanisms underlying observed changes in cell migration were investigated via gene expression analysis of epithelial-mesenchymal transition (EMT) markers.

RESULTS

BMP signaling inhibition resulted in LCLC-103H cell apoptosis and necrosis 72 h after LDN-214117 treatment. Cell growth and proliferation are markedly affected by BMP signaling inhibition. Chemotactic motility and migratory ability of LCLC-103H cells were clearly hampered by LDN-214117 treatment. Cell migration changes after BMP signaling inhibition were shown to be coupled with considerable down-regulation of transcription factors involved in EMT, especially Snail.

CONCLUSIONS

BMP signaling inhibition in LCLC-103H cells leads to reduced growth and proliferation, hindered migration and accelerated cell death. The findings contribute to the pool of evidence on BMP signaling in lung cancer with a possibility of introducing BMP signaling inhibition as a novel therapeutic approach for the disease.

Zebrafish miR-462-731 regulates hematopoietic specification and pu.1-dependent primitive myelopoiesis

MicroRNAs (miRNAs) play significant roles in both embryonic hematopoiesis and hematological malignancy. Zebrafish miR-462-731 cluster is orthologous of miR-191-425 in human which regulates proliferation and tumorigenesis. In our previous work, miR-462-731 was found highly and ubiquitously expressed during early embryogenesis. In this study, by loss-of-function analysis (morpholino knockdown combined with CRISRP/Cas9 knockout) and mRNA profiling, we suggest that miR-462-731 is required for normal embryonic development by regulating cell survival. We found that loss of miR-462/miR-731 caused a remarkable decrease in the number of erythroid cells as well as an ectopic myeloid cell expansion at 48 hpf, suggesting a skewing of myeloid-erythroid lineage differentiation. Mechanistically, miR-462-731 provides an instructive input for pu.1-dependent primitive myelopoiesis through regulating etsrp/scl signaling combined with a novel pu.1/miR-462-731 feedback loop. On the other hand, morpholino (MO) knockdown of miR-462/miR-731 resulted in an expansion of posterior blood islands at 24 hpf, which is a mild ventralization phenotype resulted from elevation of BMP signaling. Rescue experiments with both BMP type I receptor inhibitor dorsomorphin and alk8 MO indicate that miR-462-731 acts upstream of alk8 within the BMP/Smad signaling pathway and functions as a novel endogenous BMP antagonist. Besides, an impairment of angiogenesis was observed in miR-462/miR-731 morphants. The specification of arteries and veins was also perturbed, as characterized by the irregular patterning of efnb2a and flt4 expression. Our study unveils a previously unrecognized role of miR-462-731 in BMP/Smad signaling mediated hematopoietic specification of mesodermal progenitors and demonstrates a miR-462-731 mediated regulatory mechanism driving primitive myelopoiesis in the ALPM. We also show a requirement for miR-462-731 in regulating arterial-venous specification and definitive hematopoietic stem cell (HSC) production. The current findings might provide further insights into the molecular mechanistic basis of miRNA regulation of embryonic hematopoiesis and hematological malignancy.

Thrombopoietin induces hematopoiesis from mouse ES cells via HIF-1α-dependent activation of a BMP4 autoregulatory loop

Understanding the molecular mechanisms underlying hematopoietic differentiation of embryonic stem (ES) cells may help to ascertain the conditions for the in vitro generation of hematopoietic cells. Previously, we found that patients with congenital amegakaryocytic thrombocytopenia (CAMT), who develop pancytopenia early after birth, harbor mutations within the thrombopoietin (TPO) receptor, c-MPL. This knowledge, together with observations in vitro and in vivo, suggests that TPO/c-MPL signaling promotes early hematopoiesis. However, the mechanisms underlying TPO signaling are not fully elucidated. Here, we describe a direct connection between TPO and bone morphogenetic protein 4 (BMP4) signaling pathways in determining the hematopoietic fate of ES cells. Morphogen BMP4 is known to induce early hematopoietic differentiation of ES cells. Treatment of ES cells with TPO induced the autocrine production of BMP4 with concomitant upregulation of the BMP receptor BMPR1A, phosphorylation of SMAD1, 5, 8, and activation of specific BMP4 target genes; this was mediated by TPO-dependent binding of transcription factor HIF-1α to the BMP4 gene promoter. Treatment of ES cells with the BMP antagonist noggin substantially reduced TPO-dependent hematopoietic differentiation of ES cells. Thus, our findings contribute to the establishment of techniques for generating hematopoietic cells from ES cells.

Cancer Is to Embryology as Mutation Is to Genetics: Hypothesis of the Cancer as Embryological Phenomenon

Despite numerous advances in cell biology, genetics, and developmental biology, cancer origin has been attributed to genetic mechanisms primarily involving mutations. Embryologists have expressed timidly cancer embryological origin with little success in leveraging the discussion that cancer could involve a set of conventional cellular processes used to build the embryo during morphogenesis. Thus, this "cancer process" allows the harmonious and coherent construction of the embryo structural base, and its implementation as the embryonic process involves joint regulation of differentiation, proliferation, cell invasion, and migration, enabling the human being recreation of every generation. On the other hand, "cancer disease" is the representation of an abnormal state of the cell that might happen in the stem cells of an adult person, in which the mechanism for joint gene regulating of differentiation, proliferation, cell invasion, and migration could be reactivated in an entirely inappropriate context.

Fetal hematopoietic stem cells express MFG-E8 during mouse embryogenesis

The milk fat globule-EGF-factor 8 protein (MFG-E8) has been identified in various tissues, where it has an important role in intercellular interactions, cellular migration, and neovascularization. Previous studies showed that MFG-E8 is expressed in different cell types under normal and pathophysiological conditions, but its expression in hematopoietic stem cells (HSCs) during hematopoiesis has not been reported. In the present study, we investigated MFG-E8 expression in multiple hematopoietic tissues at different stages of mouse embryogenesis. Using immunohistochemistry, we showed that MFG-E8 was specifically expressed in CD34⁺ HSCs at all hematopoietic sites, including the yolk sac, aorta-gonad-mesonephros region, placenta and fetal liver, during embryogenesis. Fluorescence-activated cell sorting and polymerase chain reaction analyses demonstrated that CD34⁺ cells, purified from the fetal liver, expressed additional HSC markers, c-Kit and Sca-1, and that these CD34⁺ cells, but not CD34⁻ cells, highly expressed MFG-E8. We also found that MFG-E8 was not expressed in HSCs in adult mouse bone marrow, and that its expression was confined to F4/80⁺ macrophages. Together, this study demonstrates, for the first time, that MFG-8 is expressed in fetal HSC populations, and that MFG-E8 may have a role in embryonic hematopoiesis.

Are neural crest stem cells the missing link between hematopoietic and neurogenic niches?

Hematopoietic niches are defined as cellular and molecular microenvironments that regulate hematopoietic stem cell (HSC) function together with stem cell autonomous mechanisms. Many different cell types have been characterized as contributors to the formation of HSC niches, such as osteoblasts, endothelial cells, Schwann cells, and mesenchymal progenitors. These mesenchymal progenitors have themselves been classified as CXC chemokine ligand (CXCL) 12-abundant reticular (CAR) cells, stem cell factor expressing cells, or nestin-positive mesenchymal stem cells (MSCs), which have been recently identified as neural crest-derived cells (NCSCs). Together, these cells are spatially associated with HSCs and believed to provide appropriate microenvironments for HSC self-renewal, differentiation, mobilization and hibernation both by cell-cell contact and soluble factors. Interestingly, it appears that regulatory pathways governing the hematopoietic niche homeostasis are operating in the neurogenic niche as well. Therefore, this review paper aims to compare both the regulation of hematopoietic and neurogenic niches, in order to highlight the role of NCSCs and nervous system components in the development and the regulation of the hematopoietic system.

ZNF423: Transcriptional modulation in development and cancer

Krüppel-like zinc finger proteins are versatile players in biology that have been implicated in mammalian development and disease. Among these proteins, ZNF423 and its mouse ortholog Zfp423 were initially implicated in midline patterning of the central nervous system but have emerged as critical transcriptional modulators in cancer. Epigenetically uncurbed ZNF423 interferes with lymphopoiesis by sequestration of the essential early B-cell factor 1 (EBF1) causing B-cell maturation arrest, a hallmark of acute lymphoblastic leukemia. Conversely, its presence in neuroblastoma, a primitive neuroectodermal tumor of childhood, allows retinoic acid-induced differentiation and is associated with a favorable outcome of neuroblastoma patients. Such opposing effects may be explained by the cellular context, but also by the multifunctionality of ZNF423 that is mediated by 30 zinc fingers forming various functional domains. This review summarizes current knowledge of ZNF423, focusing on its role in development and cancer.

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

Endoglin (Eng), an ancillary receptor of the transforming growth factor beta (TGFβ) signaling pathway superfamily, has been well recognized for its important function in vascular development and angiogenesis since its discovery more than a decade ago. Recent studies show that this receptor is also critical for the emergence of blood during embryonic development, and that at E7.5, endoglin together with Flk-1 identifies early mesoderm progenitors that are endowed with hematopoietic and endothelial potential. These two lineages emerge in very close association during embryogenesis, and because they share the expression of the same surface markers, it has been difficult to distinguish the earliest hematopoietic from endothelial cells. Here, we evaluated the function of endoglin in hematopoiesis as development progresses past E7.5, and found that the hematopoietic and endothelial progenitors can be distinguished by the levels of endoglin in E9.5 yolk sacs. Whereas endothelial cells are Eng(bright), hematopoietic activity is primarily restricted to a subset of cells that display dim expression of endoglin (Eng(dim)). Molecular characterization of these subfractions showed that endoglin-mediated induction of hematopoiesis occurs in concert with BMP2/BMP4 signaling. This pathway is highly active in Eng(dim) cells but significantly downregulated in the Eng knockout. Taken together, our findings show an important function for endoglin in mediating BMP2/BMP4 signaling during yolk sac hematopoietic development and suggest that the levels of this receptor modulate TGFβ versus bone morphogenetic protein (BMP) signaling.

Primitive CML cell expansion relies on abnormal levels of BMPs provided by the niche and on BMPRIb overexpression

Leukemic stem cells in chronic phase chronic myelogenous leukemia (CP-CML) are responsible for disease persistence and eventual drug resistance, most likely because they survive, expand, and are sustained through interactions with their microenvironment. Bone morphogenetic proteins 2 (BMP2) and 4 (BMP4) regulate the fate and proliferation of normal hematopoietic stem cells, as well as interactions with their niche. We show here that the intrinsic expression of members of the BMP response pathway are deregulated in CML cells with differences exhibited in mature (CD34(-)) and immature (CD34(+)) compartments. These changes are accompanied by altered functional responses of primitive leukemic cells to BMP2 and BMP4 and strong increases in soluble BMP2 and BMP4 in the CML bone marrow. Using primary cells and a cell line mimicking CP-CML, we found that myeloid progenitor expansion is driven by the exposure of immature cells overexpressing BMP receptor Ib to BMP2 and BMP4. In summary, we demonstrate that deregulation of intracellular BMP signaling in primary CP-CML samples corrupts and amplifies their response to exogenous BMP2 and BMP4, which are abnormally abundant within the tumor microenvironment. These results provide new insights with regard to leukemic stem cell biology and suggest possibilities for the development of novel therapeutic tools specifically targeting the CML niche.

BMP-mediated specification of the erythroid lineage suppresses endothelial development in blood island precursors

The developmental relationship between the blood and endothelial cell (EC) lineages remains unclear. In the extra-embryonic blood islands of birds and mammals, ECs and blood cells are closely intermixed, and blood island precursor cells in the primitive streak express many of the same molecular markers, leading to the suggestion that both lineages arise from a common precursor, called the hemangioblast. Cells within the blood island of Xenopus also coexpress predifferentiation markers of the blood and EC lineages. However, using multiple assays, we find that precursor cells in the Xenopus blood island do not normally differentiate into ECs, suggesting that classic hemangioblasts are rare or nonexistent in Xenopus. What prevents these precursor cells from developing into mature ECs? We have found that bone morphogenetic protein (BMP) signaling is essential for erythroid differentiation, and in the absence of BMP signaling, precursor cells adopt an EC fate. Furthermore, inhibition of the erythroid transcription pathway leads to endothelial differentiation. Our results indicate that bipotential endothelial/erythroid precursor cells do indeed exist in the Xenopus blood island, but BMP signaling normally acts to constrain EC fate. More generally, these results provide evidence that commitment to the erythroid lineage limits development of bipotential precursors toward an endothelial fate.

BMP4 inhibits the proliferation of breast cancer cells and induces an MMP-dependent migratory phenotype in MDA-MB-231 cells in 3D environment

Background

Bone morphogenetic protein 4 (BMP4) belongs to the transforming growth factor β (TGF-β) family of proteins. BMPs regulate cell proliferation, differentiation and motility, and have also been reported to be involved in cancer pathogenesis. We have previously shown that BMP4 reduces breast cancer cell proliferation through G1 cell cycle arrest and simultaneously induces migration in a subset of these cell lines. Here we examined the effects of BMP4 in a more physiological environment, in a 3D culture system.

Methods

We used two different 3D culture systems; Matrigel, a basement membrane extract from mouse sarcoma cells, and a synthetic polyethylene glycol (PEG) gel. AlamarBlue reagent was used for cell proliferation measurements and immunofluorescence was used to determine cell polarity. Expression of cell cycle regulators was examined by Western blot and matrix metalloproteinase (MMP) expression by qRT-PCR.

Results

The MCF-10A normal breast epithelial cells formed round acini with correct apicobasal localization of α6 integrin in Matrigel whereas irregular structures were seen in PEG gel. The two 3D matrices also supported dissimilar morphology for the breast cancer cells. In PEG gel, BMP4 inhibited the growth of MCF-10A and the three breast cancer cell lines examined, thus closely resembling the 2D culture conditions, but in Matrigel, no growth inhibition was observed in MDA-MB-231 and MDA-MB-361 cells. Furthermore, BMP4 induced the expression of the cell cycle inhibitor p21 both in 2D and 3D culture, thereby partly explaining the growth arrest. Interestingly, MDA-MB-231 cells formed large branching, stellate structures in response to BMP4 treatment in Matrigel, suggestive of increased cell migration or invasion. This effect was reversed by Batimastat, a broad-spectrum MMP inhibitor, and subsequent analyses showed BMP4 to induce the expression of MMP3 and MMP14, that are thus likely to be responsible for the stellate phenotype.

Conclusions

Taken together, our results show that Matrigel provides a more physiological environment for breast epithelial cells than PEG gel. Moreover, BMP4 partly recapitulates in 3D culture the growth suppressive abilities previously seen in 2D culture and induces an MMP-dependent migratory phenotype in MDA-MB-231 cells.

Regulation of primitive hematopoiesis by class I histone deacetylases

BACKGROUND

Histone deacetylases (HDACs) regulate multiple developmental processes and cellular functions. However, their roles in blood development have not been determined, and in Xenopus laevis a specific function for HDACs has yet to be identified. Here, we employed the class I selective HDAC inhibitor, valproic acid (VPA), to show that HDAC activity is required for primitive hematopoiesis.

RESULTS

VPA treatment during gastrulation resulted in a complete absence of red blood cells (RBCs) in Xenopus tadpoles, but did not affect development of other mesodermal tissues, including myeloid and endothelial lineages. These effects of VPA were mimicked by Trichostatin A (TSA), a well-established pan-HDAC inhibitor, but not by valpromide, which is structurally similar to VPA but does not inhibit HDACs. VPA also caused a marked, dose-dependent loss of primitive erythroid progenitors in mouse yolk sac explants at clinically relevant concentrations. In addition, VPA treatment inhibited erythropoietic development downstream of bmp4 and gata1 in Xenopus ectodermal explants.

CONCLUSIONS

These findings suggest an important role for class I HDACs in primitive hematopoiesis. Our work also demonstrates that specific developmental defects associated with exposure to VPA, a significant teratogen in humans, arise through inhibition of class I HDACs.

Activin and BMP4 synergistically promote formation of definitive endoderm in human embryonic stem cells

Human embryonic stem cells (hESCs) herald tremendous promise for the production of clinically useful cell types for the treatment of injury and disease. Numerous reports demonstrate their differentiation into definitive endoderm (DE) cells, the germ layer from which pancreatic β cells and hepatocytes arise, solely from exposure to a high dose of recombinant Activin/Nodal. We show that combining a second related ligand, BMP4, in combination with Activin A yields 15%-20% more DE as compared with Activin A alone. The addition of recombinant BMP4 accelerates the downregulation of pluripotency genes, particularly SOX2, and results in upregulation of endogenous BMP2 and BMP4, which in turn leads to elevated levels of phospho-SMAD1/5/8. Combined Activin A and BMP4 treatment also leads to an increase in the expression of DE genes CXCR4, SOX17, and FOXA2 when compared with Activin A addition alone. Comparative microarray studies between DE cells harvested on day 3 of differentiation further reveal a novel set of genes upregulated in response to initial BMP4 exposure. Several of these, including APLNR, LRIG3, MCC, LEPREL1, ROR2, and LZTS1, are expressed in the mouse primitive streak, the site of DE formation. Thus, this synergism between Activin A and BMP4 during the in vitro differentiation of hESC into DE suggests a complex interplay between BMP and Activin/Nodal signaling during the in vivo allocation and expansion of the endoderm lineage.

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

Bone morphogenetic protein 4-a fascinating regulator of cancer cell behavior

Bone morphogenetic proteins (BMPs) are extracellular signaling molecules that belong to the transforming growth factor β (TGFβ) superfamily and are well-known for their indispensable roles in vertebrate development. In recent years, important new information has been generated on the contribution of BMP family members, such as BMP4, in cancer pathogenesis. First of all, BMP4 gene variants have been shown to predispose to colorectal cancer. In sporadic cancer, BMP4 expression levels are commonly altered in many tumor types and have been linked to patient prognosis in hepatocellular and ovarian cancer. In terms of BMP4 function in cancer cells, the majority of studies demonstrate that BMP4 suppresses cell growth both in vitro and in vivo, and at the same time is able to induce migration, invasion, and epithelial-mesenchymal transition. These latter phenotypes are typically associated with cancer metastasis and progression, and thus BMP4 seems to elicit effects that are both detrimental and beneficial for the cancer cells. The functional effects of BMP4 are not restricted to the control of cell proliferation and mobility, since it also contributes to the regulation of differentiation, apoptosis, and angiogenesis. The latter is especially intriguing since the formation of new blood vessels is a prerequisite for sustained tumor growth and cancer progression. Mainly due to its growth suppressive abilities, BMP4 has been suggested as a possible therapeutic target in cancer cells. However, the other functional characteristics of BMP4, especially the promotion of cell mobility, make such strategies less appealing. Improved knowledge of the downstream mediators of BMP4 effects in cancer cells may allow dissection of the different BMP4-induced phenotypes and thereby generation of specific targeted therapies.

Effect of endoglin overexpression during embryoid body development

Increasing evidence points to endoglin (Eng), an accessory receptor for the transforming growth factor-β superfamily commonly associated with the endothelial lineage, as an important regulator of the hematopoietic lineage. We have shown that lack of Eng results in reduced numbers of primitive erythroid colonies as well as downregulation of key hematopoietic genes. To determine the effect of Eng overexpression in hematopoietic development, we generated a doxycycline-inducible embryonic stem cell line. Our results demonstrate that induction of Eng during embryoid body differentiation leads to a significant increase in the frequency of hematopoietic progenitors, in particular, the erythroid lineage, which correlated with upregulation of Scl, Gata1, Runx1, and embryonic globin. Interestingly, activation of the hematopoietic program happened at the expense of endothelial and cardiac cells, as differentiation into these mesoderm lineages was compromised. Eng-induced enhanced erythroid activity was accompanied by high levels of Smad1 phosphorylation. This effect was attenuated by addition of a bone morphogenetic protein (BMP) signaling inhibitor to these cultures. Among the BMPs, BMP4 is well known for its role in hematopoietic specification from mesoderm by promoting expression of several hematopoietic genes, including Scl. Because Scl is considered the master regulator of the hematopoietic program, we investigated whether Scl would be capable of rescuing the defective hematopoietic phenotype observed in Eng(-/-) embryonic stem cells. Scl expression in Eng-deficient embryonic stem cells resulted in increased erythroid colony-forming activity and upregulation of Gata1 and Gata2, positioning Eng upstream of Scl. Taken together, these findings support the premise that Eng modulates the hematopoietic transcriptional network, most likely through regulation of BMP4 signaling.

A critical role for endoglin in the emergence of blood during embryonic development

Much remains unknown about the signals that induce early mesoderm to initiate hematopoietic differentiation. Here, we show that endoglin (Eng), a receptor for the TGFβ superfamily, identifies all cells with hematopoietic fate in the early embryo. These arise in an Eng(+)Flk1(+) mesodermal precursor population at embryonic day 7.5 (E7.5), a cell fraction also endowed with endothelial potential. In Eng-knockout embryos, hematopoietic colony activity and numbers of CD71(+)Ter119(+) erythroid progenitors were severely reduced. This coincided with severely reduced expression of embryonic globin and key bone morphogenic protein (BMP) target genes, including the hematopoietic regulators Scl, Gata1, Gata2, and Msx-1. To interrogate molecular pathways active in the earliest hematopoietic progenitors, we applied transcriptional profiling to sorted cells from E7.5 embryos. Eng(+)Flk-1(+) progenitors coexpressed TGFβ and BMP receptors and target genes. Furthermore, Eng(+)Flk-1(+) cells presented high levels of phospho-SMAD1/5, indicating active TGFβ and/or BMP signaling. Remarkably, under hematopoietic serum-free culture conditions, hematopoietic outgrowth of Eng-expressing cells was dependent on the TGFβ superfamily ligands BMP4, BMP2, or TGF-β1. These data demonstrate that the E(+)F(+) fraction at E7.5 represents mesodermal cells competent to respond to TGFβ1, BMP4, or BMP2, shaping their hematopoietic development, and that Eng acts as a critical regulator in this process by modulating TGF/BMP signaling.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

Life is a pattern: vascular assembly within the embryo

The formation of the vascular system is one of the earliest and most important events during organogenesis in the developing embryo because the growing organism needs a transportation system to supply oxygen and nutrients and to remove waste products. Two distinct processes termed vasculogenesis and angiogenesis lead to a complex vasculature covering the entire body. Several cellular mechanisms including migration, proliferation, differentiation and maturation are involved in generating this hierarchical vascular tree. To achieve this aim, a multitude of signaling pathways need to be activated and coordinated in spatio-temporal patterns. Understanding embryonic molecular mechanism in angiogenesis further provides insight for therapeutic approaches in pathological conditions like cancer or ischemic diseases in the adult. In this review, we describe the current understanding of major signaling pathways that are necessary and active during vascular development.

Wnt-inhibitory factor 1 dysregulation of the bone marrow niche exhausts hematopoietic stem cells

The role of Wnt signaling in hematopoietic stem cell fate decisions remains controversial. We elected to dysregulate Wnt signaling from the perspective of the stem cell niche by expressing the pan Wnt inhibitor, Wnt inhibitory factor 1 (Wif1), specifically in osteoblasts. Here we report that osteoblastic Wif1 overexpression disrupts stem cell quiescence, leading to a loss of self-renewal potential. Primitive stem and progenitor populations were more proliferative and elevated in bone marrow and spleen, manifesting an impaired ability to maintain a self-renewing stem cell pool. Exhaustion of the stem cell pool was apparent only in the context of systemic stress by chemotherapy or transplantation of wild-type stem cells into irradiated Wif1 hosts. Paradoxically this is mediated, at least in part, by an autocrine induction of canonical Wnt signaling in stem cells on sequestration of Wnts in the environment. Additional signaling pathways are dysregulated in this model, primarily activated Sonic Hedgehog signaling in stem cells as a result of Wif1-induced osteoblastic expression of Sonic Hedgehog. We find that dysregulation of the stem cell niche by overexpression of an individual component impacts other unanticipated regulatory pathways in a combinatorial manner, ultimately disrupting niche mediated stem cell fate decisions.

Modulation of TGF-β signaling by endoglin in murine hemangioblast development and primitive hematopoiesis

Endoglin (Eng), an accessory receptor for the transforming growth factor β (TGF-β) superfamily, is required for proper hemangioblast and primitive hematopoietic development. However the mechanism by which endoglin functions at this early developmental stage is currently unknown. Transcriptional analyses of differentiating eng(-/-) and eng(+/+) ES cells revealed that lack of endoglin leads to profound reductions in the levels of key hematopoietic regulators, including Scl, Lmo2, and Gata2. We also detected lower levels of phosphorylated Smad1 (pSmad1), a downstream target signaling molecule associated with the TGF-β pathway. Using doxycycline-inducible ES cell lines, we interrogated the TGF-β signaling pathway by expressing activated forms of ALK-1 and ALK-5, type I receptors for TGF-β. Our results indicate that ALK-1 signaling promotes hemangioblast development and hematopoiesis, as evidenced by colony assays, gene expression and FACS analyses, whereas signaling by ALK-5 leads to the opposite effect, inhibition of hemangioblast and hematopoietic development. In Eng(-/-) ES cells, ALK-1 rescued both the defective hemangioblast development, and primitive erythropoiesis, indicating that ALK-1 signaling can compensate for the absence of endoglin. We propose that endoglin regulates primitive hematopoiesis by modulating the activity of the Smad1/5 signaling pathway in early stages of development.

The role of Smad signaling in hematopoiesis and translational hematology

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) of adult individuals and function to produce and regenerate the entire blood and immune system over the course of an individual's lifetime. Historically, HSCs are among the most thoroughly characterized tissue-specific stem cells. Despite this, the regulation of fate options, such as self-renewal and differentiation, has remained elusive, partly because of the expansive plethora of factors and signaling cues that govern HSC behavior in vivo. In the BM, HSCs are housed in specialized niches that dovetail the behavior of HSCs with the need of the organism. The Smad-signaling pathway, which operates downstream of the transforming growth factor-β (TGF-β) superfamily of ligands, regulates a diverse set of biological processes, including proliferation, differentiation and apoptosis, in many different organ systems. Much of the function of Smad signaling in hematopoiesis has remained nebulous due to early embryonic lethality of most knockout mouse models. However, recently new data have been uncovered, suggesting that the Smad-signaling circuitry is intimately linked to HSC regulation. In this review, we bring the Smad-signaling pathway into focus, chronicling key concepts and recent advances with respect to TGF-β-superfamily signaling in normal and leukemic hematopoiesis.

ID1 and ID3 represent conserved negative regulators of human embryonic and induced pluripotent stem cell hematopoiesis

Mechanisms that govern hematopoietic lineage specification, as opposed to the expansion of committed hematopoietic progenitors, from human pluripotent stem cells (hPSCs) have yet to be fully defined. Here, we show that within the family of genes called inhibitors of differentiation (ID), ID1 and ID3 negatively regulate the transition from lineage-specified hemogenic cells to committed hematopoietic progenitors during hematopoiesis of both human embryonic stem cells (hESCs) and human induced pluripotent stem cell (hiPSCs). Upon hematopoietic induction of hPSCs, levels of ID1 and ID3 transcripts rapidly increase, peaking at the stage of hemogenic precursor emergence, and then exclusively decrease during subsequent hematopoietic commitment. Suppression of ID1 and ID3 expression in hemogenic precursors using specific small interfering RNAs augments differentiation into committed hematopoietic progenitors, with dual suppression of ID1 and ID3 further increasing hematopoietic induction compared with upon knockdown of each gene alone. This inhibitory role of ID1 and ID3 directly affects hemogenic precursors and is not dependent on non-hemogenic cells of other lineages within developing human embryoid bodies from hESCs or hiPSCs. Our study uniquely identifies ID1 and ID3 as negative regulators of the hPSC-hematopoietic transition from a hemogenic to a committed hematopoietic fate, and demonstrates that this is conserved between hESCs and hiPSCs.

MicroRNA programs in normal and aberrant stem and progenitor cells

Emerging evidence suggests that microRNAs (miRNAs), an abundant class of ∼22-nucleotide small regulatory RNAs, play key roles in controlling the post-transcriptional genetic programs in stem and progenitor cells. Here we systematically examined miRNA expression profiles in various adult tissue-specific stem cells and their differentiated counterparts. These analyses revealed miRNA programs that are common or unique to blood, muscle, and neural stem cell populations and miRNA signatures that mark the transitions from self-renewing and quiescent stem cells to proliferative and differentiating progenitor cells. Moreover, we identified a stem/progenitor transition miRNA (SPT-miRNA) signature that predicts the effects of genetic perturbations, such as loss of PTEN and the Rb family, AML1-ETO9a expression, and MLL-AF10 transformation, on self-renewal and proliferation potentials of mutant stem/progenitor cells. We showed that some of the SPT-miRNAs control the self-renewal of embryonic stem cells and the reconstitution potential of hematopoietic stem cells (HSCs). Finally, we demonstrated that SPT-miRNAs coordinately regulate genes that are known to play roles in controlling HSC self-renewal, such as Hoxb6 and Hoxa4. Together, these analyses reveal the miRNA programs that may control key processes in normal and aberrant stem and progenitor cells, setting the foundations for dissecting post-transcriptional regulatory networks in stem cells.

A requirement for FGF signalling in the formation of primitive streak-like intermediates from primitive ectoderm in culture

BACKGROUND

Embryonic stem (ES) cells hold considerable promise as a source of cells with therapeutic potential, including cells that can be used for drug screening and in cell replacement therapies. Differentiation of ES cells into the somatic lineages is a regulated process; before the promise of these cells can be realised robust and rational methods for directing differentiation into normal, functional and safe cells need to be developed. Previous in vivo studies have implicated fibroblast growth factor (FGF) signalling in lineage specification from pluripotent cells. Although FGF signalling has been suggested as essential for specification of mesoderm and endoderm in vivo and in culture, the exact role of this pathway remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using a culture model based on early primitive ectoderm-like (EPL) cells we have investigated the role of FGF signalling in the specification of mesoderm. We were unable to demonstrate any mesoderm inductive capability associated with FGF1, 4 or 8 signalling, even when the factors were present at high concentrations, nor any enhancement in mesoderm formation induced by exogenous BMP4. Furthermore, there was no evidence of alteration of mesoderm sub-type formed with addition of FGF1, 4 or 8. Inhibition of endogenous FGF signalling, however, prevented mesoderm and favoured neural differentiation, suggesting FGF signalling was required but not sufficient for the differentiation of primitive ectoderm into primitive streak-like intermediates. The maintenance of ES cell/early epiblast pluripotent marker expression was also observed in cultures when FGF signalling was inhibited.

CONCLUSIONS/SIGNIFICANCE

FGF signalling has been shown to be required for the differentiation of primitive ectoderm to neurectoderm. This, coupled with our observations, suggest FGF signalling is required for differentiation of the primitive ectoderm into the germ lineages at gastrulation.

CpG-oligodeoxynucleotide inhibits Smad-dependent bone morphogenetic protein signaling: effects on myeloma cell apoptosis and in vitro osteoblastogenesis

The TLR9 agonist CpG-oligodeoxynucleotide (CpG-ODN) with a phosphorothioate backbone (PTO-CpG-ODN) is evaluated in clinical trials as a vaccine adjuvant or as treatment of cancers. Bone morphogenetic proteins (BMPs) regulate growth and differentiation of several cell types, and also induce apoptosis of cancer cells. Cross-talk between BMP- and TLR-signaling has been reported, and we aimed to investigate whether CpG-ODN influenced BMP-induced osteoblast differentiation or BMP-induced apoptosis of malignant plasma cells. We found that PTO-CpG-ODN inhibited BMP-2-induced osteoblast differentiation from human mesenchymal stem cells. Further, PTO-CpG-ODN counteracted BMP-2- and BMP-6-induced apoptosis of the human myeloma cell lines IH-1 and INA-6, respectively. In contrast, PTO-CpG-ODN did not antagonize the antiproliferative effect of BMP-2 on hMSCs or IH-1 cells. Inhibition of Smad-signaling and p38 MAPK-signaling indicated that apoptosis of IH-1 cells is dependent on Smad-signaling downstream of BMP, whereas the antiproliferative effect of BMP-2 on IH-1 cells also involves p38 MAPK-signaling. Together, the data suggested a specific inhibition by PTO-CpG-ODN on BMP-Smad-signaling. Supporting this we found that PTO-CpG-ODN inhibited BMP-induced phosphorylation of receptor-Smads in human mesenchymal stem cells and myeloma cell lines. This effect appeared to be independent of TLR9 because GpC-ODN and other ODNs with the ability to form multimeric structures inhibited Smad-signaling as efficiently as PTO-CpG-ODNs, and because knockdown of TLR9 by small interfering RNA in INA-6 cells did not blunt the effect of PTO-CpG-ODN. In conclusion, our results demonstrate that PTO-CpG-ODN inhibits BMP-signaling, and thus might provoke unwanted TLR9-independent side effects in patients.

Canonical BMP signaling is dispensable for hematopoietic stem cell function in both adult and fetal liver hematopoiesis, but essential to preserve colon architecture

Numerous publications have described the importance of bone morphogenetic protein (BMP) signaling in the specification of hematopoietic tissue in developing embryos. Here we investigate the full role of canonical BMP signaling in both adult and fetal liver hematopoiesis using conditional knockout strategies because conventional disruption of components of the BMP signaling pathway result in early death of the embryo. By targeting both Smad1 and Smad5, we have generated a double-knockout mouse with complete disruption of canonical BMP signaling. Interestingly, concurrent deletion of Smad1 and Smad5 results in death because of extrahematopoietic pathologic changes in the colon. However, Smad1/Smad5-deficient bone marrow cells can compete normally with wild-type cells and display unaffected self-renewal and differentiation capacity when transplanted into lethally irradiated recipients. Moreover, although BMP receptor expression is increased in fetal liver, fetal liver cells deficient in both Smad1 and Smad5 remain competent to long-term reconstitute lethally irradiated recipients in a multilineage manner. In conclusion, canonical BMP signaling is not required to maintain either adult or fetal liver hematopoiesis, despite its crucial role in the initial patterning of hematopoiesis in early embryonic development.

Signalling molecules involved in mouse bladder smooth muscle cellular differentiation

Mouse bladder mesenchyme differentiates into smooth muscle under the influence of urothelium at gestational day 13.5 (E13.5). Sonic hedgehog (Shh) is considered to be the upstream gene arising from the urothelium, which induces smooth muscle in the peripheral bladder mesenchyme. We hypothesize differential gene expression across the full thickness of bladder mesenchyme as a function of proximity to the inducing bladder urothelium and the peripheral location of the smooth muscle. Embryonic bladders from FVB mice were collected at E12.5, 13.5, 15 and 16 and cryosectioned followed by microdissection with a PixCell II laser capture microscope. RNA extraction was performed at the laser captured sites and mRNA expression profiles were measured using SYBR Green quantitative RT-PCR. Smooth muscle a-actin (SMAA) and smooth muscle myosin heavy chain (SM-MHC) were expressed in the E13.5, E15 and E16 bladders in the peripheral layer of mesenchyme, but not in the prospective submucosa. Patched 1 (Ptc1), Gli1 and bone morphogenetic protein (Bmp) 4 expression was consistently elevated in the mesenchymal layer immediately adjacent to the urothelium compared to the peripheral location at E12.5. After E12.5, Ptc1 expression decreased to an undetectable level throughout the bladder mesenchyme. The level of TGF-beta1 was highest in the mesenchymal layer adjacent to the serosa at E13.5. The level of expression of serum response factor (SRF) was also highest at E15 in the peripheral mesenchyme. Genes downstream of Shh are differentially expressed in the prospective submucosa vs. the peripheral bladder mesenchyme as a function gestation age and smooth muscle differentiation.

Collagen type IV and Perlecan exhibit dynamic localization in the Allantoic Core Domain, a putative stem cell niche in the murine allantois

A body of evidence suggests that the murine allantois contains a stem cell niche, the Allantoic Core Domain (ACD), that may contribute to a variety of allantoic and embryonic cell types. Given that extracellular matrix (ECM) regulates cell fate and function in niches, the allantois was systematically examined for Collagen type IV (ColIV) and Perlecan, both of which are associated with stem cell proliferation and differentiation. Not only was localization of ColIV and Perlecan more widespread during gastrulation than previously reported, but protein localization profiles were particularly robust and dynamic within the allantois and associated visceral endoderm as the ACD formed and matured. We propose that these data provide further evidence that the ACD is a stem cell niche whose activity is synchronized with associated visceral endoderm, possibly via ECM proteins.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.

Complex and context dependent regulation of hematopoiesis by TGF-beta superfamily signaling

The transforming growth factor (TGF)-beta superfamily of growth factors, including the TGF-betas, activins, and bone morphogenetic proteins (BMPs), provide cells with a broad spectrum of regulatory signals through the intracellular Smad pathway. Since loss-of-function studies of a majority of the TGF-beta superfamily members result in embryonic lethality, much of our current knowledge of the TGF-beta superfamily's role in hematopoiesis is generated from studies performed in vitro, or in very early stages of embryonic development. TGF-beta is well documented as a potent inhibitor of hematopoietic stem cell (HSC) proliferation in vitro, while its role in vivo is largely unknown. BMP signaling is crucial for the initiation of hematopoiesis in the developing embryo, although its role in adult hematopoiesis remains elusive. More recently we and others have used conditional knockout models to unravel the role of several components of TGF-beta family signaling in adult hematopoiesis. Here we review the currently known functions for the major factors of this signaling family in embryonic and adult hematopoietic regulation and discuss the context dependency and complexity that permeate this regulation.

BMP4 regulates the hematopoietic stem cell niche

Bone morphogenetic protein 4 (BMP4) is required for mesoderm commitment to the hematopoietic lineage during early embryogenesis. However, deletion of BMP4 is early embryonically lethal and its functional role in definitive hematopoiesis is unknown. Consequently, we used a BMP4 hypomorph to investigate the role of BMP4 in regulating hematopoietic stem cell (HSC) function and maintaining steady-state hematopoiesis in the adult. Reporter gene expression shows that Bmp4 is expressed in cells associated with the hematopoietic microenvironment including osteoblasts, endothelial cells, and megakaryocytes. Although resting hematopoiesis is normal in a BMP4-deficient background, the number of c-Kit+, Sca-1+, Lineage- cells is significantly reduced. Serial transplantation studies reveal that BMP4-deficient recipients have a microenvironmental defect that reduces the repopulating activity of wild-type HSCs. This defect is even more pronounced in a parabiosis model that demonstrates a profound reduction in wild-type hematopoietic cells within the bone marrow of BMP4-deficient recipients. Furthermore, wild-type HSCs that successfully engraft into the BMP4-deficient bone marrow show a marked decrease in functional stem cell activity when tested in a competitive repopulation assay. Taken together, these findings indicate BMP4 is a critical component of the hematopoietic microenvironment that regulates both HSC number and function.

Primordial germ cell specification from embryonic stem cells

Background

Primordial germ cell (PGC) specification is the first crucial step in germ line development. However, owing to significant challenges regarding the in vivo system, such as the complex cellular environment and potential problems with embryo manipulation, it is desirable to generate embryonic stem (ES) cells that are capable of overcoming these aforementioned limitations in order to provide a potential in vitro model to recapitulate the developmental processes in vivo.

Methodology and Principal Findings

Here, we studied the detailed process of PGC specification from stella-GFP ES cells. We first observed the heterogeneous expression of stella in ES cells. However, neither Stella-positive ES cells nor Stella-negative ES cells shared a similar gene expression pattern with either PGCs or PGC precursors. Second, we derived PGCs from ES cells using two differentiation methods, namely the attachment culture technique and the embryoid body (EB) method. Compared with PGCs derived via the attachment culture technique, PGCs derived via the EB method that had undergone the sequential erasure of Peg3 followed by Igf2r resulted in a cell line in which the expression dynamics of T, Fgf8 and Sox17, in addition to the expression of the epiblast markers, were more similar to the in vivo expression, thus demonstrating that the process of PGC derivation was more faithfully recapitulated using the EB method. Furthermore, we developed an in vitro model of PGC specification in a completely chemically defined medium (CDM) that indicated that BMP4 and Wnt3a promoted PGC derivation, whereas BMP8b and activinA had no observable effect on PGC derivation.

Conclusions and Significance

The in vitro model we have established can recapitulate the developmental processes in vivo and provides new insights into the mechanism of PGC specification.

Regulation of hematopoiesis by the BMP signaling pathway in adult zebrafish

OBJECTIVE

The zebrafish is an established model system for studying the embryonic emergence of tissues and organs, including the hematopoietic system. We hypothesized that key signaling pathways controlling embryonic hematopoiesis continue to be important in the adult, and we sought to develop approaches to test this in zebrafish, focused on the bone morphogenetic protein (BMP) signaling pathway. Functions for this pathway in adult hematopoiesis have been challenging to probe in other models.

MATERIALS AND METHODS

Several approaches tested the function of BMP signaling during adult zebrafish hematopoiesis. First, we evaluated steady-state hematopoiesis in adult fish that are heterozygous for mutant alleles of Smad5, or are homozygous for mutant alleles, and rescued to adulthood by injection of RNA encoding Smad5. Second, we tested the relative ability of smad5 mutant fish to recover from hemolytic anemia. Third, we generated a transgenic line that targets the expression of a dominant-negative BMP receptor to adult-stage Gata1+ progenitor cells.

RESULTS

Adult fish with a strong mutant smad5 allele are anemic at steady state and, in addition, respond to hemolytic anemia with kinetics that are altered compared to wild-type fish. Fish expressing a mutant BMP receptor in early Gata1+ definitive progenitors generate excessive eosinophils.

CONCLUSIONS

Our study provides proof of principle that regulation of adult hematopoiesis can be studied in zebrafish by altering specific pathways. We show that the BMP signaling pathway is relevant for adult hematopoiesis to maintain steady state erythropoiesis, control the erythropoietic response following stress anemia, and to generate normal numbers of eosinophils.