In situ hybridization reveals co-expression of embryonic and adult alpha globin genes in the earliest murine erythrocyte progenitors

Discovering a sparse set of pairwise discriminating features in high-dimensional data

MOTIVATION

Recent technological advances produce a wealth of high-dimensional descriptions of biological processes, yet extracting meaningful insight and mechanistic understanding from these data remains challenging. For example, in developmental biology, the dynamics of differentiation can now be mapped quantitatively using single-cell RNA sequencing, yet it is difficult to infer molecular regulators of developmental transitions. Here, we show that discovering informative features in the data is crucial for statistical analysis as well as making experimental predictions.

RESULTS

We identify features based on their ability to discriminate between clusters of the data points. We define a class of problems in which linear separability of clusters is hidden in a low-dimensional space. We propose an unsupervised method to identify the subset of features that define a low-dimensional subspace in which clustering can be conducted. This is achieved by averaging over discriminators trained on an ensemble of proposed cluster configurations. We then apply our method to single-cell RNA-seq data from mouse gastrulation, and identify 27 key transcription factors (out of 409 total), 18 of which are known to define cell states through their expression levels. In this inferred subspace, we find clear signatures of known cell types that eluded classification prior to discovery of the correct low-dimensional subspace.

AVAILABILITY AND IMPLEMENTATION

https://github.com/smelton/SMD.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Analysis of extraembryonic mesodermal structure formation in the absence of morphological primitive streak

During mouse gastrulation, the primitive streak is formed on the posterior side of the embryo. Cells migrate out of the primitive streak to form the future mesoderm and endoderm. Fate mapping studies revealed a group of cell migrate through the proximal end of the primitive streak and give rise to the extraembryonic mesoderm tissues such as the yolk sac blood islands and allantois. However, it is not clear whether the formation of a morphological primitive streak is required for the development of these extraembryonic mesodermal tissues. Loss of the Cripto gene in mice dramatically reduces, but does not completely abolish, Nodal activity leading to the absence of a morphological primitive streak. However, embryonic erythrocytes are still formed and assembled into the blood islands. In addition, Cripto mutant embryos form allantoic buds. However, Drap1 mutant embryos have excessive Nodal activity in the epiblast cells before gastrulation and form an expanded primitive streak, but no yolk sac blood islands or allantoic bud formation. Lefty2 embryos also have elevated levels of Nodal activity in the primitive streak during gastrulation, and undergo normal blood island and allantois formation. We therefore speculate that low level of Nodal activity disrupts the formation of morphological primitive streak on the posterior side, but still allows the formation of primitive streak cells on the proximal side, which give rise to the extraembryonic mesodermal tissues formation. Excessive Nodal activity in the epiblast at pre-gastrulation stage, but not in the primitive streak cells during gastrulation, disrupts extraembryonic mesoderm development.

The modern primitives: applying new technological approaches to explore the biology of the earliest red blood cells

One of the most critical stages in mammalian embryogenesis is the independent production of the embryo's own circulating, functional red blood cells. Correspondingly, erythrocytes are the first cell type to become functionally mature during embryogenesis. Failure to achieve this invariably leads to in utero lethality. The recent application of technologies such as transcriptome analysis, flow cytometry, mutant embryo analysis, and transgenic fluorescent gene expression reporter systems has shed new light on the distinct erythroid lineages that arise early in development. Here, I will describe the similarities and differences between the distinct erythroid populations that must form for the embryo to survive. While much of the focus of this review will be the poorly understood primitive erythroid lineage, a discussion of other erythroid and hematopoietic lineages, as well as the cell types making up the different niches that give rise to these lineages, is essential for presenting an appropriate developmental context of these cells.

FGF signalling regulates chromatin organisation during neural differentiation via mechanisms that can be uncoupled from transcription

Changes in higher order chromatin organisation have been linked to transcriptional regulation; however, little is known about how such organisation alters during embryonic development or how it is regulated by extrinsic signals. Here we analyse changes in chromatin organisation as neural differentiation progresses, exploiting the clear spatial separation of the temporal events of differentiation along the elongating body axis of the mouse embryo. Combining fluorescence in situ hybridisation with super-resolution structured illumination microscopy, we show that chromatin around key differentiation gene loci Pax6 and Irx3 undergoes both decompaction and displacement towards the nuclear centre coincident with transcriptional onset. Conversely, down-regulation of Fgf8 as neural differentiation commences correlates with a more peripheral nuclear position of this locus. During normal neural differentiation, fibroblast growth factor (FGF) signalling is repressed by retinoic acid, and this vitamin A derivative is further required for transcription of neural genes. We show here that exposure to retinoic acid or inhibition of FGF signalling promotes precocious decompaction and central nuclear positioning of differentiation gene loci. Using the Raldh2 mutant as a model for retinoid deficiency, we further find that such changes in higher order chromatin organisation are dependent on retinoid signalling. In this retinoid deficient condition, FGF signalling persists ectopically in the elongating body, and importantly, we find that inhibiting FGF receptor (FGFR) signalling in Raldh2−/− embryos does not rescue differentiation gene transcription, but does elicit both chromatin decompaction and nuclear position change. These findings demonstrate that regulation of higher order chromatin organisation during differentiation in the embryo can be uncoupled from the machinery that promotes transcription and, for the first time, identify FGF as an extrinsic signal that can direct chromatin compaction and nuclear organisation of gene loci.

Changes in the position of genes within the nucleus and in their local organisation frequently correlate with whether or not genes are turned on. However, little is known about how such nuclear organisation is controlled and whether this can be separated from the mechanisms that promote transcription. We show here that central nuclear position and chromatin de-compaction correlate with onset of expression at key neural differentiation gene loci in the mouse embryo. Conversely, the locus of a gene that is down-regulated as neural differentiation commences exhibits a shift towards the nuclear periphery as this takes place. Importantly, we show that signalling through the fibroblast growth factor (FGF) pathway regulates changes at this level of nuclear organisation. FGF represses differentiation gene transcription and keeps differentiation gene loci compact and at the nuclear periphery. By blocking FGF signalling in a retinoid deficient embryo in which differentiation genes are not expressed, we further show that control of nuclear organisation by FGF is not just a consequence of gene transcription. These findings are the first to demonstrate that such higher order nuclear organisation is regulated in the developing embryo, that this takes place downstream of FGF signaling, and can be uncoupled from the machinery of gene transcription.

The first trimester human placenta is a site for terminal maturation of primitive erythroid cells

Embryonic hematopoiesis starts via the generation of primitive red blood cells (RBCs) that satisfy the embryo's immediate oxygen needs. Although primitive RBCs were thought to retain their nuclei, recent studies have shown that primitive RBCs in mice enucleate in the fetal liver. It has been unknown whether human primitive RBCs enucleate, and what hematopoietic site might support this process. Our data indicate that the terminal maturation and enucleation of human primitive RBCs occurs in first trimester placental villi. Extravascular ζ-globin(+) primitive erythroid cells were found in placental villi between 5-7 weeks of development, at which time the frequency of enucleated RBCs was higher in the villous stroma than in circulation. RBC enucleation was further evidenced by the presence of primitive reticulocytes and pyrenocytes (ejected RBC nuclei) in the placenta. Extravascular RBCs were found to associate with placental macrophages, which contained ingested nuclei. Clonogenic macrophage progenitors of fetal origin were present in the chorionic plate of the placenta before the onset of fetoplacental circulation, after which macrophages had migrated to the villi. These findings indicate that placental macrophages may assist the enucleation process of primitive RBCs in placental villi, implying an unexpectedly broad role for the placenta in embryonic hematopoiesis.

Developmental silencing of human zeta-globin gene expression is mediated by the transcriptional repressor RREB1

The mammalian embryonic zeta-globin genes, including that of humans, are expressed at the early embryonic stage and then switched off during erythroid development. This autonomous silencing of the zeta-globin gene transcription is probably regulated by the cooperative work of various protein-DNA and protein-protein complexes formed at the zeta-globin promoter and its upstream enhancer (HS-40). We present data here indicating that a protein-binding motif, ZF2, contributes to the repression of the HS-40-regulated human zeta-promoter activity in erythroid cell lines and in transgenic mice. Combined site-directed mutagenesis and EMSA suggest that repression of the human zeta-globin promoter is mediated through binding of the zinc finger factor RREB1 to ZF2. This model is further supported by the observation that human zeta-globin gene transcription is elevated in the human erythroid K562 cell line or the primary erythroid culture upon RNA interference (RNAi)(2) knockdown of RREB1 expression. These data together suggest that RREB1 is a putative repressor for the silencing of the mammalian zeta-globin genes during erythroid development. Because zeta-globin is a powerful inhibitor of HbS polymerization, our experiments have provided a foundation for therapeutic up-regulation of zeta-globin gene expression in patients with severe hemoglobinopathies.

Energetic differences at the subunit interfaces of normal human hemoglobins correlate with their developmental profile

A previously unrecognized function of normal human hemoglobins occurring during protein assembly is described, i.e. self-regulation of subunit pairings and their durations arising from the variable strengths of their subunit interactions. Although many mutant human hemoglobins are known to have altered subunit interface strengths, those of the normal embryonic, fetal, and adult human hemoglobins have not been considered to differ significantly. However, in a comprehensive study of both types of subunit interfaces of seven of the eight normal oxy human hemoglobins, we found that the strengths, i.e., the free energies of the tetramer-dimer interfaces, contrary to previous reports, differ by 3 orders of magnitude and display an undulating profile similar to the transitions ("switches") of various globin subunit types over time. The dimer interface strengths are also variable and correlate linearly with their developmental profile. Embryonic hemoglobins are the weakest; fetal hemoglobin is of intermediate strength, and adult hemoglobins are the strongest. The pattern also correlates generally with their different O(2) affinities and responses to allosteric regulatory molecules. Acetylation of fetal hemoglobin weakens its unusually strong subunit interactions and occurs progressively as its level of expression diminishes and adult hemoglobin A formation begins; a causal relationship is suggested. The relative contributions of globin gene order and competition among subunits due to differences in their interface strengths were found to be complementary and establish a connection among genetics, thermodynamics, and development.

Orderly hematopoietic development of induced pluripotent stem cells via Flk-1(+) hemoangiogenic progenitors

Induced pluripotent stem (iPS) cells, reprogrammed somatic cells with embryonic stem (ES) cell-like characteristics, are generated by the introduction of combinations of specific transcription factors. Little is known about the differentiation of iPS cells in vitro. Here we demonstrate that murine iPS cells produce various hematopoietic cell lineages when incubated on a layer of OP9 stromal cells. During this differentiation, iPS cells went through an intermediate stage consisting of progenitor cells that were positive for the early mesodermal marker Flk-1 and for the sequential expression of other genes that are associated with hematopoietic and endothelial development. Flk-1(+) cells differentiated into primitive and definitive hematopoietic cells, as well as into endothelial cells. Furthermore, Flk-1(+) populations contained common bilineage progenitors that could generate both hematopoietic and endothelial lineages from single cells. Our results demonstrate that iPS cell-derived cells, like ES cells, can follow a similar hematopoietic route to that seen in normal embryogenesis. This finding highlights the potential use of iPS cells in clinical areas such as regenerative medicine, disease investigation, and drug screening.

The embryonic aorta-gonad-mesonephros region as a generator of haematopoietic stem cells

During mammalian embryonic development the definitive haematopoietic stem cells (HSCs) may arise either in the extra-embryonic mesoderm or in the aorta-gonad-mesonephros (AGM) region that forms in close proximity to the assembling urogenital system, generating the gonad, cortex of the adrenal gland and metanephros. Researchers have been attempting for a long time to define the region of importance for generating the definitive HSCs that colonize the fetal liver and bone marrow, the two major sites where haematopoiesis takes place in the adult. The fetal liver might gain HSCs from both of the primary haematopoietic sources, but the extra-embryonic HSCs seem not to be able to colonize adult bone marrow directly. It is known that the microenvironment around the HSCs is important for directing cell fates, but we do not yet have much idea about the cell-cell interactions, tissue interactions and molecules that regulate cell behaviour in the AGM. We will here discuss the contribution of the AGM to definitive haematopoiesis in mammals and review some of the cell-cell interactions and associated signalling systems involved in the development of AGM stem cells.

Defects in yolk sac vasculogenesis, chorioallantoic fusion, and embryonic axis elongation in mice with targeted disruption of Yap65

YAP is a multifunctional adapter protein and transcriptional coactivator with several binding partners well described in vitro and in cell culture. To explore in vivo requirements for YAP, we generated mice carrying a targeted disruption of the Yap gene. Homozygosity for the Yap(tm1Smil) allele (Yap-/-) caused developmental arrest around E8.5. Phenotypic characterization revealed a requirement for YAP in yolk sac vasculogenesis. Yolk sac endothelial and erythrocyte precursors were specified as shown by histology, PECAM1 immunostaining, and alpha globin expression. Nonetheless, development of an organized yolk sac vascular plexus failed in Yap-/- embryos. In striking contrast, vasculogenesis proceeded in both the allantois and the embryo proper. Mutant embryos showed patterned gene expression domains along the anteroposterior neuraxis, midline, and streak/tailbud. Despite this evidence of proper patterning and tissue specification, Yap-/- embryos showed developmental perturbations that included a notably shortened body axis, convoluted anterior neuroepithelium, caudal dysgenesis, and failure of chorioallantoic fusion. These results reveal a vital requirement for YAP in the developmental processes of yolk sac vasculogenesis, chorioallantoic attachment, and embryonic axis elongation.

Stomatin is mis-trafficked in the erythrocytes of overhydrated hereditary stomatocytosis, and is absent from normal primitive yolk sac-derived erythrocytes

The 32 kD lipid-raft-associated membrane protein 'stomatin' is deficient from the erythrocyte membrane in the Na+-K+ leaky haemolytic anaemia, overhydrated hereditary stomatocytosis (OHSt). To date, no mutation in the gene coding for this protein has so far been found in OHSt. In this study, we have analysed the distribution of stomatin in both cultured erythroid cells from OHSt patients and in normal embryological and fetal erythroid development. In erythroid cell cultures from OHSt patients, stomatin-immunoreactivity (stomatin-IR) was present in progenitor cells but remained restricted to the area of the multivesicular complexes and the nucleus in the developing cells and was not seen in the plasma membrane. This could be consistent with the idea that stomatin is an innocent passenger in a more fundamental trafficking abnormality. In normal embryonic development, we found that, in extraembryonic (yolk sac) erythropoiesis, neither the nucleated red cells nor their enucleated mature derivatives displayed any stomatin-IR. In contrast, all haemangiopoietic progenitor cells of intraembryonic haematopoiesis, starting with the mesodermal precursors in the aorta-gonad-mesonephros region, exhibited strong stomatin-IR. The significance of this observation on these poorly understood cells is currently unclear.

Hematopoiesis in the yolk sac: more than meets the eye

The first blood cells observed in the embryo are large nucleated erythroblasts generated in blood islands of the extraembryonic yolk sac. These unique red cells have been termed primitive because of their resemblance to nucleated erythroblasts of nonmammalian species. It is now widely assumed that hematopoiesis in the yolk sac is "primitive" and that "definitive" hematopoiesis has its origins in the aorta/gonad/mesonephros (AGM) region. Recent studies of yolk sac hematopoiesis have challenged several aspects of this paradigm. First, primitive erythropoiesis in mammals shares many features with definitive erythropoiesis, including progressive erythroblast maturation leading to the circulation of enucleated erythrocytes. Second, the emergence of primitive erythroid progenitors in the yolk sac prior to somitogenesis may be associated with the macrophage and megakaryocyte lineages, raising the possibility that "primitive" hematopoiesis may be multilineage in nature. Third, a second wave of hematopoietic progenitors emerge from the yolk sac during early somitogenesis that consists of multiple myeloid lineages that are temporally and spatially associated with definitive erythroid progenitors. These "definitive" hematopoietic progenitors expand in numbers in the yolk sac and are thought to seed the fetal liver and generate the first definitive blood cells that rapidly emerge from the liver. Recent findings support a model of hematopoietic ontogeny in which the conceptus' first maturing blood cells and committed progenitors are provided by the yolk sac, allowing survival until AGM-derived hematopoietic stem cells can emerge, seed the liver and differentiate into mature blood cells.

"Maturational" globin switching in primary primitive erythroid cells

Mammals have 2 distinct erythroid lineages. The primitive erythroid lineage originates in the yolk sac and generates a cohort of large erythroblasts that terminally differentiate in the bloodstream. The definitive erythroid lineage generates smaller enucleated erythrocytes that become the predominant cell in fetal and postnatal circulation. These lineages also have distinct globin expression patterns. Our studies in primary murine primitive erythroid cells indicate that betaH1 is the predominant beta-globin transcript in the early yolk sac. Thus, unlike the human, murine beta-globin genes are not up-regulated in the order of their chromosomal arrangement. As primitive erythroblasts mature from proerythroblasts to reticulocytes, they undergo a betaH1- to epsilony-globin switch, up-regulate adult beta1- and beta2-globins, and down-regulate zeta-globin. These changes in transcript levels correlate with changes in RNA polymerase II density at their promoters and transcribed regions. Furthermore, the epsilony- and betaH1-globin genes in primitive erythroblasts reside within a single large hyperacetylated domain. These data suggest that this "maturational" betaH1- to epsilony-globin switch is dynamically regulated at the transcriptional level. Globin switching during ontogeny is due not only to the sequential appearance of primitive and definitive lineages but also to changes in globin expression as primitive erythroblasts mature in the bloodstream.

zeta-/- Thalassemic mice are affected by two modifying loci and display unanticipated somatic recombination leading to inherited variation

Thalassemia is a disease caused by a variety of mutations affecting both the adult and embryonic alpha- and beta-globin loci. A mouse strain carrying an embryonic zeta-globin gene disrupted by the insertion of a PGK-Neo cassette displays an alpha-thalassemia-like syndrome. Embryonic survival of this zeta-null mouse is variable and strongly influenced by genetic background, the 129/SvEv mouse strain displaying a more severe phenotype than C57BL/6. We have identified two modifying loci on C57BL/6 chromosomes 2 and 5, which affect the penetrance of embryonic lethality in the 129/SvEv mouse. Through this work, we were able to observe an interesting effect on somatic recombination events in thalassemic embryos. We show that these events can occur on multiple chromosomes in very early embryonic cells, prior to their allocation to the germline. Our results demonstrate that somatic recombination events can be transmitted to subsequent generations.

Generation of a floxed allele of Smad5 for cre-mediated conditional knockout in the mouse

Smad5 is a member of the Smad family of intracellular mediators of BMP signals and in endothelial cells of TGF-beta signals. We and others previously showed that loss of Smad5 in the mouse results in embryonic lethality (between E9.5-E11.5) due to multiple embryonic and extraembryonic defects. To circumvent the early embryonic lethality and to allow tissue- and time-specific Smad5 inactivation, we created a conditional Smad5 allele in the mouse. Floxed Smad5 (Smad5(flE2,Neo/flE2,Neo)) mice were generated in which both exon2 and the Neo-cassette were flanked by loxP sites. Here we demonstrate that embryos with ubiquitous Cre-mediated deletion of Smad5 (Smad5(flDeltaE2/flDeltaE2)) phenocopy the conventional Smad5 knockout mice. Smad5(flE2/flE2) mice are now available and will be a valuable tool to analyze the role of Smad5 beyond its crucial early embryonic function throughout development and postnatal life.

Origins of mammalian hematopoiesis: in vivo paradigms and in vitro models

Though a topic of medical interest for centuries, our understanding of vertebrate hematopoietic or "blood-forming" tissue development has improved greatly only in recent years and given a series of scientific and technical milestones. Key among these observations was the description of procedures that allowed the transplantation of blood-forming activity. Beyond this, other advances include the creation of a variety of knock-out animals (mice and more recently zebrafish), microdissection of embryonic and fetal blood-forming tissues, hematopoietic stem (HSC) and progenitor cell (HPC) colony-forming assays, the discovery of cytokines with defined hematopoietic activities, gene transfer technologies, and the description of lineage-specific surface antigens for the identification and purification of pluripotent and differentiated blood cells. The availability of both murine and human embryonic stem cells (ESC) and the delineation of in vitro systems to direct their differentiation have now been added to this analytical arsenal. Such tools have allowed researchers to interrogate the complex developmental processes behind both primitive (yolk sac or extraembryonic) and definitive (intraembryonic) hematopoietic tissue formation. Using ES cells, we hope to not only gain additional basic insights into hematopoietic development but also to develop platforms for therapeutic use in patients suffering from hematological disease. In this review, we will focus on points of convergence and divergence between murine and human hematopoiesis in vivo and in vitro, and use these observations to evaluate the literature regarding attempts to create hematopoietic tissue from embryonic stem cells, the pitfalls encountered therein, and what challenges remain.

Appearance of erythrocyte-like globules in the mouse visceral yolk sac endodermal cells on embryonic day 12, with special reference to blood islands

The mouse visceral yolk sac (VYS) is widely known to play an important role as erythropoietic tissue during embryonic periods. Mouse VYS from embryonic days 9 to 12 was examined by light microscopy, electron microscopy and histochemical analysis with benzidine to detect the presence of hemoglobin with special reference to the development of VYS, the disappearance of the blood islands in VYS, and the appearance of a novel structure in the form of erythrocyte-like globules in VYS endodermal cells. The villous appearance of VYS became complicated by the development of VYS endodermal cells. The blood islands positive for the benzidine reaction were light microscopically detected on embryonic days 9, 10, and 11. They disappeared on embryonic day 12, however. Erythrocyte-like globules positive for the benzidine reaction were not observed in VYS endodermal cells on embryonic days 9, 10, and 11, but then appeared on embryonic day 12, by light and electron microscopy. Erythrocyte-like globules in VYS endodermal cells, which appear after the disappearance of blood islands in VYS, may participate in erythropoiesis during embryonic development.

Generation of definitive hematopoietic stem cells from murine early yolk sac and paraaortic splanchnopleures by aorta-gonad-mesonephros region-derived stromal cells

There is controversy as to whether murine definitive hematopoiesis originates from yolk sac (YS) or the intraembryonic region. This study reports the generation of definitive hematopoietic stem cells (HSCs) from both early YS and intraembryonic paraaortic splanchnopleures (P-Sp) on AGM-S3 stromal cells derived from the aorta-gonad-mesonephros (AGM) region at 10.5 days post coitum (dpc). YS and P-Sp cells at 8.5 dpc generated no definitive hematopoiesis-derived colony-forming cells in cocultures with AGM-S3 cells, but spleen colony-forming cells and HSCs capable of reconstituting definitive hematopoiesis in adult mice simultaneously appeared on day 4 of coculture. Precursors for definitive HSCs were present in YS and P-Sp at 8.0 dpc, a time when YS and embryo were not connected by blood vessels. It is proposed that precursors with the potential to generate definitive HSCs appear independently in YS and intraembryonic P-Sp and that the P-Sp or AGM region affords the microenvironment that facilitates generation of definitive hematopoiesis from precursors.

Indian hedgehog signaling in extraembryonic endoderm and ectoderm differentiation in ES embryoid bodies

We previously demonstrated that a member of the Hedgehog gene family, Indian hedgehog (Ihh), is expressed in the visceral endoderm of EC and ES cell embryoid bodies and mouse embryos. Overexpression studies suggested that Ihh was involved in visceral endoderm differentiation. We now provide evidence for a Hh response in the embryoid body core and in the mesothelial layer of the visceral yolk sac. We also demonstrate that treatment of ES embryoid bodies with the Hh antagonists cAMP and forskolin results in downregulation of the Hh response and altered embryoid body differentiation. The outer endoderm layer undergoes a transition to parietal endoderm while formation of an embryonic ectoderm layer surrounding a cavity is inhibited. These treatments also result in a decrease in the expression of markers for the mesoderm derivatives, blood and endothelial cells. We present a model to explain how Ihh and BMP signaling may regulate extraembryonic endoderm and embryonic ectoderm differentiation.

Protein tyrosine phosphatase epsilon increases the risk of mammary hyperplasia and mammary tumors in transgenic mice

Accurate phosphorylation of tyrosine residues in proteins plays a central role in regulation of cellular function. Although connections between aberrant tyrosine kinase activity and malignancy are well-established, significantly less is known about the roles of protein tyrosine phosphatases (PTPases) in tumorigenesis. We have previously shown that the transmembranal form of PTPase Epsilon (PTPepsilon) is upregulated in mouse mammary tumors initiated specifically by ras or neu, suggesting that PTPepsilon may play a role in transformation by these two oncogenes. In order to test this notion in vivo, we created transgenic mice that express elevated levels of PTPepsilon in their mammary epithelium by use of the MMTV promoter/enhancer. Following several cycles of pregnancy female MMTV-PTPepsilon mice uniformly developed pronounced and persistent mammary hyperplasia which was accompanied by residual milk production. Solitary mammary tumors were often detected secondary to mammary hyperplasia. The sporadic nature of the tumors, the long latency period prior to their development, and low levels of transgene expression in the tumors indicate that PTPepsilon provides a necessary, but insufficient, signal for oncogenesis. The results provide genetic evidence that PTPepsilon plays an accessory role in production of mammary tumors in a manner consistent with its upregulation in mammary tumors induced by ras or neu.

Transforming growth factor-beta signalling in extraembryonic mesoderm is required for yolk sac vasculogenesis in mice

We have analysed the function of transforming growth factor beta (TGF-beta) in yolk sac development in mice by generating somatic chimaeras in which the extraembryonic mesoderm, which gives rise to the endothelial and haematopoietic cells of the yolk sac vasculature, is derived from embryonic stem (ES) cells. The ES cells were stably transfected and express either the full-length type II binding receptor or a kinase-deficient mutant of this receptor. Examination of yolk sacs from chimaeras between E8.5 and 9.5, and analysis of marker expression in embryoid bodies from these mutant ES cell lines in prolonged suspension culture demonstrated that (1) a major function of TGF-beta in yolk sac mesoderm is to regulate production and deposition of fibronectin in the extracellular matrix that maintains yolk sac integrity, (2) TGF-beta signalling is not required for differentiation of extraembryonic mesoderm into endothelial cells but is necessary for their subsequent organisation into robust vessels, and (3) TGF-beta signalling must be tightly regulated for the differentiation of primitive haematopoietic cells to take place normally. Together, these results show that defective TGF-beta signalling in the extraembryonic mesoderm alone is sufficient to account for the extraembryonic phenotype reported previously in TGF-beta1(−/−) mice (Dickson, M. C., Martin, J. S., Cousins, F. M., Kulkarni, A. B., Karlsson, S. and Akhurst, R. J. (1995) Development 121, 1845–1854).

Smad5 knockout mice die at mid-gestation due to multiple embryonic and extraembryonic defects

Smad5 has been implicated as a downstream signal mediator for several bone morphogenetic proteins (BMPs). To understand the in vivo function of Smad5, we generated mice deficient in Smad5 using embryonic stem (ES) cell technology. Homozygous mutant embryos die between E9.5 and E11.5, and display variable phenotypes. Morphological defects are first detected at E8.0 in the developing amnion, gut and heart (the latter defect being similar to BMP-2 knockout mice). At later stages, mutant embryos fail to undergo proper turning, have craniofacial and neural tube abnormalities, and are edematous. In addition, several extraembryonic lesions are observed. After E9.0, the yolk sacs of the mutants contain red blood cells but lack a well-organized vasculature, which is reminiscent of BMP-4, TGF-beta1 and TGF-beta type II receptor knockout mice. In addition, the allantois of many Smad5 mutants is fused to the chorion, but is not well-elongated. A unique feature of the Smad5 mutant embryos is that ectopic vasculogenesis and hematopoiesis is observed in the amnion, likely due to mislocation of allantois tissue. Despite the expression of Smad5 from gastrulation onwards, and in contrast to knockouts of Smad2 and Smad4, Smad5 only becomes essential later in extraembryonic and embryonic development.

Mechanisms of developmental control of transcription in the murine alpha- and beta-globin loci

We have characterized mRNA expression and transcription of the mouse alpha- and beta-globin loci during development. S1 nuclease and primary transcript in situ hybridization analyses demonstrate that all seven murine globin genes (zeta, alpha1, alpha2, epsilony, betaH1, betamaj, and betamin) are transcribed during primitive erythropoiesis, however transcription of the zeta, epsilony, and betaH1 genes is restricted to the primitive erythroid lineage. Transcription of the betamaj and betamin genes in primitive cells is EKLF-dependent demonstrating EKLF activity in embryonic red cells. Novel kinetic analyses suggest that multigene expression in the beta locus occurs via alternating single-gene transcription whereas coinitiation cannot be ruled out in the alpha locus. Transcriptional activation of the individual murine beta genes in primitive cells correlates inversely with their distance from the locus control region, in contrast with the human beta locus in which the adult genes are only activated in definitive erythroid cells. The results suggest that the multigene expression mechanism of alternating transcription is evolutionarily conserved between mouse and human beta globin loci but that the timing of activation of the adult genes is altered, indicating important fundamental differences in globin gene switching.

In vitro expansion of murine multipotential hematopoietic progenitors from the embryonic aorta-gonad-mesonephros region

The origin of hematopoietic stem cells (HSCs) and their growth factor requirement are poorly understood. Here we describe a new in vitro culture system of the aorta-gonad-mesonephros (AGM) region, where long-term repopulating HSCs first arise. We demonstrate that oncostatin M (OSM) is expressed in the AGM and is absolutely required for the expansion of multipotential hematopoietic progenitors in vitro. In addition, OSM enhances the formation of endothelial cell clusters. Thus, OSM appears to be a key cytokine for the development of multipotential hematopoietic progenitors in the AGM, possibly acting on common precursor cells between HSCs and endothelial cells. By using the AGM culture derived from macrophage colony-stimulating factor (M-CSF)-deficient op/op mutant embryos, we also show a pivotal role for M-CSF in fetal myelopoiesis.

Mouse ζ- and α-Globin Genes: Embryonic Survival, α-Thalassemia, and Genetic Background Effects

A classical notion regarding the expression of murine embryonic ζ- and adult α-globin genes holds that there is a switch in globin production from the embryonic to the adult form during fetal development. Our previous in situ hybridization studies challenged this view, since both ζ- and α-globin mRNAs can be detected simultaneously in the earliest erythrocyte populations. This finding raises the possibility that ζ-globin production might be wholly or partially redundant in embryos in which the adult α-globin is also expressed. To test this possibility, we created a null mutation of the ζ-globin gene using homologous recombination in embryonic stem cells. Many outbred mice homozygous for the ζ-null mutation were able to develop normally, undermining the notion that there is an absolute need for ζ-globin and indicating that α-globin alone can serve the survival needs of the fetus. Interestingly, insertion of the PGK-Neo cassette (used to create the null mutation) into the ζ-globin gene appears to influence the expression of the nearby α-globin genes, giving rise to reduced α-globin production and to an α-thalassemia–like syndrome. There is also evidence indicating the strong influence of genetic background on the ζ-null and α1-null phenotypes, both of which are much more severe in the 129/SvEv inbred genetic background. These quantitative differences can potentially be exploited to identify genes important for erythropoiesis.

Initiation of Murine Embryonic Erythropoiesis: A Spatial Analysis

Hematopoiesis in the mouse conceptus begins in the visceral yolk (VYS), with primitive erythroblasts first evident in blood islands at the headfold stage (E8.0). VYS erythropoiesis is decreased or abrogated by targeted disruption of the hematopoietic transcription factors tal-1, rbtn2, GATA-1, and GATA-2. To better understand the potential roles of these genes, and to trace the initial temporal and spatial development of mammalian embryonic hematopoiesis, we examined their expression patterns, and that of βH1-globin, in normal mouse conceptuses by means of in situ hybridization. Attention was focused on the 36-hour period from mid-primitive streak to early somite stages (E7.25 to E8.5), when the conceptus undergoes rapid morphologic changes with formation of the yolk sac and blood islands. Each of these genes was expressed in extraembryonic mesoderm, from which blood islands are derived. This VYS expression occurred in a defined temporal sequence: tal-1 and rbtn2 transcripts were detected earlier than the others, followed by GATA-2 and GATA-1, and then by βH1-globin. Transcripts for all of these genes were present in VYS mesoderm cell masses at the neural plate stage (E7.5), indicating commitment of these cells to the erythroid lineage before the appearance of morphologically recognizable erythroblasts. By early somite stages (E8.5), GATA-2 mRNA expression is downregulated in VYS blood islands as terminal primitive erythroid differentiation proceeds. We conclude that primitive mammalian erythropoiesis arises during gastrulation through the ordered temporal expression of tal-1, rbtn2, GATA2, and GATA-1 in a subset of extraembryonic mesoderm cells. During the stages analyzed, tal-1 and rbtn2 expression was also present in posterior embryonic mesoderm, while GATA-1 and GATA-2 expression was evident in extraembryonic tissues of ectodermal origin.

Lymphoid potential, probed before circulation in mouse, is restricted to caudal intraembryonic splanchnopleura

Emergence of hemopoietic stem cells in the mammalian embryo has yet to be definitively allocated. Previously, we detected multipotent hemopoietic precursors in the region surrounding the dorsal aorta (paraaortic splanchnopleura) beginning at 8.5 days postcoitum (dpc). However, as circulation is already established, it remained unclear whether hemopoietic precursors arise in situ or are blood-delivered. By adding an organotypic step to our former culture system, we now detect lymphocyte and multipotent myeloid precursors from the intraembryonic splanchnopleura as early as 7.5 dpc. Under identical conditions, yolk sacs from the same embryos are unable to generate lymphoid progeny and have a reduced potential for myeloid differentiation and maintenance. Thus, if isolated before circulation, the yolk sac does not produce multipotent precursors and therefore does not contribute to definitive hemopoiesis in the mouse.

In vitro development of primitive and definitive erythrocytes from different precursors

During mouse embryogenesis the production of "primitive" erythrocytes (EryP) precedes the production of "definitive" erythrocytes (EryD) in parallel with the transition of the hematopoietic site from the yolk sac to the fetal liver. On a macrophage colony-stimulating factor-deficient stromal cell line OP9, mouse embryonic stem cells were shown to give rise to EryP and EryD sequentially with a time course similar to that seen in murine ontogeny. Studies of the different growth factor requirements and limiting dilution analysis of precursor frequencies indicate that most EryP and EryD probably developed from different precursors by way of distinct differentiation pathways.

Differential gene expression during early murine yolk sac development

The visceral yolk sac (VYS), composed of extraembryonic mesoderm and visceral endoderm, is the initial site of blood cell development and serves important nutritive and absorptive functions. In the mouse, the visceral endoderm becomes a morphologically distinct tissue at the time of implantation (E4.5), while the extraembryonic mesoderm arises during gastrulation (E6.5-8.5). To isolate genes differentially expressed in the developing yolk sac, polymerase chain reaction (PCR) methods were used to construct cDNA from late primitive streak to neural plate stage (E7.5) murine VYS mesoderm and VYS endoderm tissues. Differential screening led to the identification of six VYS mesoderm-enriched clones: ribosomal protein L13a, the heat shock proteins hsc 70 and hsp 86, guanine-nucleotide binding protein-related gene, cellular nucleic acid binding protein, and alpha-enolase. One VYS endoderm-specific cDNA was identified as apolipoprotein C2. In situ hybridization studies confirmed the differential expression of these genes in E7.5 yolk sac tissues. These results indicate that representative cDNA populations can be obtained from small numbers of cells and that PCR methodologies permit the study of gene expression during early mammalian postimplantation development. While all of the mesoderm-enriched genes were ubiquitously expressed in the embryo proper, apolipoprotein C2 expression was confined to the visceral endoderm. These results are consistent with the hypothesis that at E7.5, the yolk sac endoderm provides differentiated liver-like functions, while the newly developing extraembryonic mesoderm is still a largely undifferentiated tissue.

Novel insights into erythroid development revealed through in vitro differentiation of GATA-1 embryonic stem cells

Mouse embryonic stem (ES) cells lacking the transcription factor GATA-1 do not produce mature red blood cells either in vivo or in vitro. To define the consequences of GATA-1 loss more precisely, we used an in vitro ES cell differentiation assay that permits enumeration of primitive (EryP) and definitive (EryD) erythroid precursors and recovery of pure erythroid colonies. In contrast to normal ES cells, GATA-1- ES cells fail to generate EryP precursors. EryD precursors, however, are normal in number but undergo developmental arrest and death at the proerythroblast stage. Contrary to initial expectations, arrested GATA-1(-)-definitive proerythroblasts express GATA target genes at normal levels. Transcripts of the related factor GATA-2 are remarkably elevated in GATA-1- proerythroblasts. These findings imply substantial interchangeability of GATA factors in vivo and suggest that GATA-1 normally serves to repress GATA-2 during erythropoiesis. The approach used here is a paradigm for the phenotypic analysis of targeted mutations affecting hematopoietic development.

Origin of extraembryonic mesoderm in experimental animals: relevance to chorionic mosaicism in humans

Confined chorionic mosaicism, a discordance in the karyotype between the fetus and placenta, occurs in 1% of chorionic villus sampling (CVS) cases. While the cytogenetic discrepancies occurring between different fetal tissues may pose clinical dilemmas, they can also be viewed as a natural experiment to determine early cell lineage relationships in the human. We reviewed extensive data in experimental animals to define the origin of the human extraembryonic mesoderm. The extraembryonic mesoderm in humans is an important component of the CVS culture preparation. Previously, the extraembryonic mesoderm was thought to originate in the cytotrophoblast or primitive streak. More recent evidence supports its origin from the yolk sac, which does not always correlate with the fetal karyotype. We formulated a model of early human cell lineage and employed it to clarify clinical cases of chorionic mosaicism in two large published studies.