Cultivation of aorta-gonad-mesonephros-derived hematopoietic stem cells in the fetal liver microenvironment amplifies long-term repopulating activity and enhances engraftment to the bone marrow

Antenatal diagnosis of chorioamnionitis: A review of the potential role of fetal and placental imaging

Chorioamnionitis is present in up to 70% of spontaneous preterm births. It is defined as an acute inflammation of the chorion, with or without involvement of the amnion, and is evidence of a maternal immunological response to infection. A fetal inflammatory response can coexist and is diagnosed on placental histopathology postnatally. Fetal inflammatory response syndrome (FIRS) is associated with poorer fetal and neonatal outcomes. The only antenatal diagnostic test is amniocentesis which carries risks of miscarriage or preterm birth. Imaging of the fetal immune system, in particular the thymus and the spleen, and the placenta may give valuable information antenatally regarding the diagnosis of fetal inflammatory response. While ultrasound is largely limited to structural information, MRI can complement this with functional information that may provide insight into the metabolic activities of the fetal immune system and placenta. This review discusses fetal and placental imaging in pregnancies complicated by chorioamnionitis and their potential future use in achieving non-invasive antenatal diagnosis. This article is protected by copyright. All rights reserved.

Erythroid Lineage Cells in the Liver: Novel Immune Regulators and Beyond

The lineage of the erythroid cell has been revisited in recent years. Instead of being classified as simply inert oxygen carriers, emerging evidence has shown that they are a tightly regulated in immune potent population with potential developmental plasticity for lineage crossing. Erythroid cells have been reported to exert immune regulatory function through secreted cytokines, or cell-cell contact, depending on the conditions of the microenvironment and disease models. In this review, we explain the natural history of erythroid cells in the liver through a developmental lens, as it offers perspectives into newly recognized roles of this lineage in liver biology. Here, we review the known immune roles of erythroid cells and discuss the mechanisms in the context of disease models and stages. Then, we explore the capability of erythroid lineage as a cell source for regenerative medicine. We propose that the versatile lineage of erythroid cells provides an underappreciated and potentially promising area for basic and translational research in the field of liver disease.

Hematopoietic Stem and Progenitor Cells (HSPCs)

Hematopoietic stem/progenitor cells (HSPCs) isolated from bone marrow have been successfully employed for 50 years in hematological transplantations. Currently, these cells are more frequently isolated from mobilized peripheral blood or umbilical cord blood. In this chapter, we overview several topics related to these cells including their phenotype, methods for isolation, and in vitro and in vivo assays to evaluate their proliferative potential. The successful clinical application of HSPCs is widely understood to have helped establish the rationale for the development of stem cell therapies and regenerative medicine.

LMO2 is required for TAL1 DNA binding activity and initiation of definitive haematopoiesis at the haemangioblast stage

LMO2 is a bridging factor within a DNA binding complex and is required for definitive haematopoiesis to occur. The developmental stage of the block in haematopoietic specification is not known. We show that Lmo2^−/− mouse embryonic stem cells differentiated to Flk-1⁺ haemangioblasts, but less efficiently to haemogenic endothelium, which only produced primitive haematopoietic progenitors. Genome-wide approaches indicated that LMO2 is required at the haemangioblast stage to position the TAL1/LMO2/LDB1 complex to regulatory elements that are important for the establishment of the haematopoietic developmental program. In the absence of LMO2, the target site recognition of TAL1 is impaired. The lack of LMO2 resulted in altered gene expression levels already at the haemangioblast stage, with transcription factor genes accounting for ∼15% of affected genes. Comparison of Lmo2^−/− with Tal1^−/− Flk-1⁺ cells further showed that TAL1 was required to initiate or sustain Lmo2 expression.

Interferon-α signaling promotes embryonic HSC maturation

In the developing mouse embryo, the first hematopoietic stem cells (HSCs) arise in the aorta-gonad-mesonephros (AGM) and mature as they transit through the fetal liver (FL). Compared with FL and adult HSCs, AGM HSCs have reduced repopulation potential in irradiated adult transplant recipients but mechanisms underlying this deficiency in AGM HSCs are poorly understood. By co-expression gene network analysis, we deduced that AGM HSCs show lower levels of interferon-α (IFN-α)/Jak-Stat1-associated gene expression than FL HSCs. Treatment of AGM HSCs with IFN-α enhanced long-term hematopoietic engraftment and donor chimerism. Conversely, IFN-α receptor-deficient AGMs (Ifnαr1(-/-)), had significantly reduced donor chimerism. We identify adenine-thymine-rich interactive domain-3a (Arid3a), a factor essential for FL and B lymphopoiesis, as a key transcriptional co-regulator of IFN-α/Stat1 signaling. Arid3a occupies the genomic loci of Stat1 as well as several IFN-α effector genes, acting to regulate their expression. Accordingly, Arid3a(-/-) AGM HSCs had significantly reduced transplant potential, which was rescued by IFN-α treatment. Our results implicate the inflammatory IFN-α/Jak-Stat pathway in the developmental maturation of embryonic HSCs, whose manipulation may lead to increased potency of reprogrammed HSCs for transplantation.

Specification and function of hemogenic endothelium during embryogenesis

Hemogenic endothelium is a specialized subset of developing vascular endothelium that acquires hematopoietic potential and can give rise to multilineage hematopoietic stem and progenitor cells during a narrow developmental window in tissues such as the extraembryonic yolk sac and embryonic aorta-gonad-mesonephros. Herein, we review current knowledge about the historical and developmental origins of hemogenic endothelium, the molecular events that govern hemogenic specification of vascular endothelial cells, the generation of multilineage hematopoietic stem and progenitor cells from hemogenic endothelium, and the potential for translational applications of knowledge gained from further study of these processes.

Hematopoietic activity in putative mouse primordial germ cell populations

In the present paper, starting from the observation of heterogeneous expression of the GOF-18ΔPE-GFP Pou5f1 (Oct3/4) transgene in putative mouse PGC populations settled in the aorta-gonad-mesonephros (AGM) region, we identified various OCT3/4 positive populations showing distinct expression of PGC markers (BLIMP-1, AP, TG-1, STELLA) and co-expressing several proteins (CD-34, CD-41, FLK-1) and genes (Brachyury, Hox-B4, Scl/Tal-1 and Gata-2) of hematopoietic precursors. Moreover, we found that Oct3/4-GFP(weak) CD-34(weak/high) cells possess robust hematopoietic colony forming activity (CFU) in vitro. These data indicate that the cell population usually considered PGCs moving toward the gonadal ridges encompasses a subset of cells co-expressing several germ cell and hematopoietic markers and possessing hematopoietic activity. These results are discussed within of the current model of germline segregation.

Oncostatin M maintains the hematopoietic microenvironment in the bone marrow by modulating adipogenesis and osteogenesis

The bone marrow (BM) is an essential organ for hematopoiesis in adult, in which proliferation and differentiation of hematopoietic stem/progenitor cells (HSPC) is orchestrated by various stromal cells. Alterations of BM hematopoietic environment lead to various hematopoietic disorders as exemplified by the linking of fatty marrow with increased adipogenesis to anemia or pancytopenia. Therefore, the composition of mesenchymal stromal cell (MSC)-derived cells in the BM could be crucial for proper hematopoiesis, but the mechanisms underlying the MSC differentiation for hematopoiesis remain poorly understood. In this study, we show that Oncostatin M (OSM) knock out mice exhibited pancytopenia advancing fatty marrow with age. OSM strongly inhibited adipogenesis from BM MSC in vitro, whereas it enhanced their osteogenesis but suppressed the terminal differentiation. Intriguingly, OSM allowed the MSC-derived cells to support the ex vivo expansion of HSPC effectively as feeder cells. Furthermore, the administration of OSM in lethally irradiated wild-type mice blocked fatty marrow and enhanced the recovery of HSPC number in the BM and peripheral blood cells after engraftment of HSPC. Collectively, OSM plays multiple critical roles in the maintenance and development of the hematopoietic microenvironment in the BM at a steady state as well as after injury.

Human fetal liver stromal cells expressing erythropoietin promote hematopoietic development from human embryonic stem cells

Blood cells transfusion and hematopoietic stem cells (HSCs) transplantation are important methods for cell therapy. They are widely used in the treatment of incurable hematological disorder, infectious diseases, genetic diseases, and immunologic deficiency. However, their availability is limited by quantity, capacity of proliferation and the risk of blood transfusion complications. Recently, human embryonic stem cells (hESCs) have been shown to be an alternative resource for the generation of hematopoietic cells. In the current study, we describe a novel method for the efficient production of hematopoietic cells from hESCs. The stable human fetal liver stromal cell lines (hFLSCs) expressing erythropoietin (EPO) were established using the lentiviral system. We observed that the supernatant from the EPO transfected hFLSCs could induce the hESCs differentiation into hematopoietic cells, especially erythroid cells. They not only expressed fetal and embryonic globins but also expressed the adult-globin chain on further maturation. In addition, these hESCs-derived erythroid cells possess oxygen-transporting capacity, which indicated hESCs could generate terminally mature progenies. This should be useful for ultimately developing an animal-free culture system to generate large numbers of erythroid cells from hESCs and provide an experimental model to study early human erythropoiesis.

Differential Hematopoietic Supportive Potential and Gene Expression of Stroma Cell Lines from Midgestation Mouse Placenta and Adult Bone Marrow

During mouse embryogenesis, hematopoietic development takes place in several distinct anatomic locations. The microenvironment of different hematopoietic organs plays an important role in the proliferation and maturation of the hematopoietic cells. We hypothesized that fetal stromal cells would be distinct to adult bone marrow (BM)-derived stromal cells because the BM contributes mainly to the homeostasis of hematopoietic stem cells (HSCs), while extensive expansion of HSCs occurs during fetal development. Here we report the establishment of stromal cell lines from fetal hematopoietic organs, namely aorta-gonad-mesonephros (AGM), midgestation placenta (PL), and fetal liver (FL) together with adult bone marrow (BM). The growth patterns and hematopoietic supportive potential were studied. Their phenotypic and molecular gene expression profiles were also determined. Stromal cell lines from each tissue were able to support cobblestone area formation of BM c-Kit+Sca-1+ hematopoietic cells: 22 (22/47) from AGM, three (3/4) from PL, three (3/4) from FL, and three (3/3) from BM. There were similar levels of expansion of total mononuclear cells (TMNs) when HSCs were cocultured with fetal stroma and adult BM stroma. However, PL-derived stromal cells supported higher levels of generation of colony-forming progenitor cell (CFU-C), indicated by more colonies and colonies with significantly larger size. Flow cytometric analysis of the PL1 cells demonstrated a phenotype of CD45-, CD105+, Sca-1+, CD34+, and CD49d+, compared to adult BM1 cells, which were CD45-, CD105+, Sca-1+, CD34-, and CD49d-. Using Affymetrix microarray analysis, we identified that genes specifically express in endothelial cells, such as Tie1, Tek, Kdr, Flt4, Emcn, Pecam1, Icam2, Cdh5, Esam1, Prom1, Cd34, and Sele were highly expressed in stroma PL1, consistent with an endothelial phenotype, while BM1 expressed a mesenchymal stromal phenotype. In summary, these data demonstrate distinct characteristics of stromal cells that provide insights into the microenvironmental control of HSCs.

Establishment and regulation of the HSC niche: Roles of osteoblastic and vascular compartments

Hematopoietic stem cells (HSC) are multi-potent cells that function to generate a lifelong supply of all blood cell types. During mammalian embryogenesis, sites of hematopoiesis change over the course of gestation: from extraembryonic yolk sac and placenta, to embryonic aorta-gonad-mesonephros region, fetal liver, and finally fetal bond marrow where HSC reside postnatally. These tissues provide microenviroments for de novo HSC formation, as well as HSC maturation and expansion. Within adult bone marrow, HSC self-renewal and differentiation are thought to be regulated by two major cellular components within their so-called niche: osteoblasts and vascular endothelial cells. This review focuses on HSC generation within, and migration to, different tissues during development, and also provides a summary of major regulatory factors provided by osteoblasts and vascular endothelial cells within the adult bone marrow niche.

Relationships between hematopoiesis and hepatogenesis in the midtrimester fetal liver characterized by dynamic transcriptomic and proteomic profiles

In fetal hematopoietic organs, the switch from hematopoiesis is hypothesized to be a critical time point for organogenesis, but it is not yet evidenced. The transient coexistence of hematopoiesis will be useful to understand the development of fetal liver (FL) around this time and its relationship to hematopoiesis. Here, the temporal and the comparative transcriptomic and proteomic profiles were observed during the critical time points corresponding to the initiation (E11.5), peak (E14.5), recession (E15.5), and disappearance (3 ddp) of mouse FL hematopoiesis. We found that E11.5-E14.5 corresponds to a FL hematopoietic expansion phase with distinct molecular features, including the expression of new transcription factors, many of which are novel KRAB (Kruppel-associated box)-containing zinc finger proteins. This time period is also characterized by extensive depression of some liver functions, especially catabolism/utilization, immune and defense, classical complement cascades, and intrinsic blood coagulation. Instead, the other liver functions increased, such as xenobiotic and sterol metabolism, synthesis of carbohydrate and glycan, the alternate and lectin complement cascades and extrinsic blood coagulation, and etc. Strikingly, all of the liver functions were significantly increased at E14.5-E15.5 and thereafter, and the depression of the key pathways attributes to build the hematopoietic microenvironment. These findings signal hematopoiesis emigration is the key to open the door of liver maturation.

Conserved expression of the PRELI domain containing 2 gene (Prelid2) during mid-later-gestation mouse embryogenesis

Prelid2, which belongs to the PRELI domain containing family, is identified as a conserved evolution gene. The expression and regulation during embryonic development of the prelid2 gene is unknown. In this study, we investigated the prelid2 gene expression and regulation using mouse embryos model, by in situ hybridization analysis, RT-PCR and bisulfite sequencing. In situ hybridization analysis showed that prelid2 gene expression were found in midbrain, spinal cord, optic eminence, otic vesicle and tail at E9.5 and E10.5 embryos, in forebrain, hindbrain, heart, lung, liver and kidney at E13.5 and E15.5 embryos. Real-time quantitative RT-PCR results verified the expression pattern in the four major mouse organs, brain, heart, lung, and liver during organs differentiation and formation. Bisulfite sequencing illustrated the consistent result of expression and its unmethylation status in the genomic promoter region at E12.5, E18.5, and new born. Thus, the prelid2 gene is a widely-spread, persistently expressed and unmethylated gene in mouse embryonic development. Our results suggest that the PRELI domain containing 2 gene is involved in mouse embryonic development.

Human and murine amniotic fluid c-Kit+Lin- cells display hematopoietic activity

We have isolated c-Kit(+)Lin(-) cells from both human and murine amniotic fluid (AF) and investigated their hematopoietic potential. In vitro, the c-Kit(+)Lin(-) population in both species displayed a multilineage hematopoietic potential, as demonstrated by the generation of erythroid, myeloid, and lymphoid cells. In vivo, cells belonging to all 3 hematopoietic lineages were found after primary and secondary transplantation of murine c-Kit(+)Lin(-) cells into immunocompromised hosts, thus demonstrating the ability of these cells to self-renew. Gene expression analysis of c-Kit(+) cells isolated from murine AF confirmed these results. The presence of cells with similar characteristics in the surrounding amnion indicates the possible origin of AF c-Kit(+)Lin(-) cells. This is the first report showing that cells isolated from the AF do have hematopoietic potential; our results support the idea that AF may be a new source of stem cells for therapeutic applications.

Efficient hematopoietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches

Hematopoietic stem cells derived from human embryonic stem cells (hESCs) could provide a therapeutic alternative to bone marrow transplants, but the efficiency of currently available derivation protocols is low. In this study, we investigated whether coculture with monolayers of cells derived from mouse AGM and fetal liver, or with stromal cell lines derived from these tissues, can enhance hESC hematopoietic differentiation. We found that under such conditions hESC-derived differentiating cells formed early hematopoietic progenitors, with a peak at day 18-21 of differentiation that corresponded to the highest CD34 expression. These hESC-derived hematopoietic cells were capable of primary and secondary hematopoietic engraftment into immunocompromised mice at substantially higher levels than described previously. Transcriptional and functional analysis identified TGF-beta1 and TGF-beta3 as positive enhancers of hESC hematopoietic differentiation that can further stimulate this process when added to the culture. Overall, our findings represent significant progress toward the goal of deriving functional hematopoietic stem cells from hESCs.

Of lineage and legacy: the development of mammalian hematopoietic stem cells

The hematopoietic system is one of the first complex tissues to develop in the mammalian conceptus. Of particular interest in the field of developmental hematopoiesis is the origin of adult bone marrow hematopoietic stem cells. Tracing their origin is complicated because blood is a mobile tissue and because hematopoietic cells emerge from many embryonic sites. The origin of the adult mammalian blood system remains a topic of lively discussion and intense research. Interest is also focused on developmental signals that induce the adult hematopoietic stem cell program, as these may prove useful for generating and expanding these clinically important cell populations ex vivo. This review presents a historical overview of and the most recent data on the developmental origins of hematopoiesis.

Junctional adhesion molecule-A, JAM-A, is a novel cell-surface marker for long-term repopulating hematopoietic stem cells

Junctional adhesion molecule-A (JAM-A/JAM-1/F11R) is a cell adhesion molecule expressed in epithelial and endothelial cells, and also hematopoietic cells, such as leukocytes, platelets, and erythrocytes. Here, we show that JAM-A is expressed at a high level in the enriched hematopoietic stem cell (HSC) fraction; that is, CD34(+)c-Kit(+) cells in embryonic day 11.5 (E11.5) aorta-gonod-mesonephros (AGM) and E11.5 fetal liver (FL), as well as c-Kit(+)Sca-1(+)Lineage(-) (KSL) cells in E14.5 FL, E18.5FL, and adult bone marrow (BM). Although the percentage of JAM-A(+) cells in those tissues decreases during development, the expression in the HSC fraction is maintained throughout life. Colony-forming assays reveal that multilineage colony-forming activity in JAM-A(+) cells is higher than that in JAM-A(-) cells in the enriched HSC fraction in all of those tissues. Transplantation assays show that long-term reconstituting HSC (LTR-HSC) activity is exclusively in the JAM-A(+) population and is highly enriched in the JAM-A(+) cells sorted directly from whole BM cells by anti-JAM-A antibody alone. Together, these results indicate that JAM-A is expressed on hematopoietic precursors in various hematopoietic tissues and is an excellent marker to isolate LTR-HSCs.

The regulatory role of stromal microenvironments in fetal hematopoietic ontogeny

Fetal hematopoietic development occurs through the successive expansion and differentiation of hematopoietic stem cells in distinct anatomic sites. The temporal pattern of fetal hematopoietic ontogeny suggests a coordinated developmental sequence whereby the preceding organ sustains the basic, immediate hematopoietic needs of the embryo allowing time for the development of niches within the subsequent organ with more complex supportive functions. We examine the hypothesis that there is a period of stromal genesis and circulating mesenchymal precursor cells, which gives rise to specialized niches within each of the definitive fetal hematopoietic organs, and these niches regulate hematopoietic stem cells fate determination. This article reviews fetal hematopoietic and stromal development and the current understanding of the development, composition, and regulation of the fetal stem cell niche.

Oncostatin m maintains the hematopoietic microenvironment and retains hematopoietic progenitors in the bone marrow

Bone marrow (BM) functions as the primary hematopoietic tissue throughout adult life by providing a microenvironment for the proliferation, differentiation, and retention of hematopoietic stem cells and progenitors. We describe novel roles for oncostatin M (OSM) in the BM hematopoietic microenvironment. Hematopoietic progenitor activity in OSM-deficient mice was reduced in BM but elevated in the spleen and peripheral blood. The level of circulating granulocyte colony-stimulating factor (G-CSF) was increased, whereas that of stromal cell-derived factor 1 (SDF-1) was decreased in OSM-deficient mice. Moreover, the ability of OSM-deficient BM stromal cells to support hematopoiesis in vitro was significantly reduced. These results indicate that OSM plays a unique role in hematopoiesis by maintaining the proper microenvironment for BM hematopoiesis; it also retains hematopoietic progenitors in BM by regulating G-CSF and SDF-1 levels.

Transcription elongation factor S-II is required for definitive hematopoiesis

Transcription elongation factor S-II/TFIIS promotes readthrough of transcriptional blocks by stimulating nascent RNA cleavage activity of RNA polymerase II in vitro. The biologic significance of S-II function in higher eukaryotes, however, remains unclear. To determine its role in mammalian development, we generated S-II-deficient mice through targeted gene disruption. Homozygous null mutants died at midgestation with marked pallor, suggesting severe anemia. S-II(-/-) embryos had a decreased number of definitive erythrocytes in the peripheral blood and disturbed erythroblast differentiation in fetal liver. There was a dramatic increase in apoptotic cells in S-II(-/-) fetal liver, which was consistent with a reduction in Bcl-x(L) gene expression. The presence of phenotypically defined hematopoietic stem cells and in vitro colony-forming hematopoietic progenitors in S-II(-/-) fetal liver indicates that S-II is dispensable for the generation and differentiation of hematopoietic stem cells. S-II-deficient fetal liver cells, however, exhibited a loss of long-term repopulating potential when transplanted into lethally irradiated adult mice, indicating that S-II deficiency causes an intrinsic defect in the self-renewal of hematopoietic stem cells. Thus, S-II has critical and nonredundant roles in definitive hematopoiesis.

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.

Improved Safety of Hematopoietic Transplantation with Monkey Embryonic Stem Cells in the Allogeneic Setting

Cynomolgus monkey embryonic stem cell (cyESC)-derived in vivo hematopoiesis was examined in an allogeneic transplantation model. cyESCs were induced to differentiate into the putative hematopoietic precursors in vitro, and the cells were transplanted into the fetal cynomolgus liver at approximately the end of the first trimester (n = 3). Although cyESC-derived hematopoietic colony-forming cells were detected in the newborns (4.1%-4.7%), a teratoma developed in all newborns. The risk of tumor formation was high in this allogeneic transplantation model, given that tumors were hardly observed in immunodeficient mice or fetal sheep that had been xeno-transplanted with the same cyESC derivatives. It turned out that the cyESC-derived donor cells included a residual undifferentiated fraction positive for stage-specific embryonic antigen (SSEA)-4 (38.2% +/- 10.3%) despite the rigorous differentiation culture. When an SSEA-4-negative fraction was transplanted (n = 6), the teratoma was no longer observed, whereas the cyESC-derived hematopoietic engraftment was unperturbed (2.3%-5.0%). SSEA-4 is therefore a clinically relevant pluripotency marker of primate embryonic stem cells (ESCs). Purging pluripotent cells with this surface marker would be a promising method of producing clinical progenitor cell preparations using human ESCs.

Designer blood: creating hematopoietic lineages from embryonic stem cells

Embryonic stem (ES) cells exhibit the remarkable capacity to become virtually any differentiated tissue upon appropriate manipulation in culture, a property that has been beneficial for studies of hematopoiesis. Until recently, the majority of this work used murine ES cells for basic research to elucidate fundamental properties of blood-cell development and establish methods to derive specific mature lineages. Now, the advent of human ES cells sets the stage for more applied pursuits to generate transplantable cells for treating blood disorders. Current efforts are directed toward adapting in vitro hematopoietic differentiation methods developed for murine ES cells to human lines, identifying the key interspecies differences in biologic properties of ES cells, and generating ES cell-derived hematopoietic stem cells that are competent to repopulate adult hosts. The ultimate medical goal is to create patient-specific and generic ES cell lines that can be expanded in vitro, genetically altered, and differentiated into cell types that can be used to treat hematopoietic diseases.

Cell-cell contact and anatomical compatibility in stromal cell-mediated HSC support during development

Hematopoietic stem cells (HSCs) are able to generate the wide variety of blood cells found in the adult and are maintained in the bone marrow (BM) stromal microenvironment. In the aorta-gonads-mesonephros (AGM), which autonomously generates the first HSCs, the stromal microenvironment is largely uncharacterized. We have previously made an extensive panel of stromal clones from AGM subregions and have found that clones from the urogenital ridges (UG) provide the most potent support for adult BM HSCs. However, it is unknown to what extent the stroma from this developmentally and anatomically distinct microenvironment can support HSCs from other regions of the embryo, such as yolk sac. Moreover, it is unknown whether cell-cell contact is necessary in this microenvironment. Here, we show that the HSCs from the embryonic aorta are the most potently supported HSCs in UG stromal clone co-cultures and that contact is required for the maintenance and expansion of embryo-derived HSCs.

The emergence of definitive hematopoietic stem cells in the mammal

PURPOSE OF REVIEW

Hematopoietic stem cells (HSC) are the basis for blood formation during adult life. The amazing potency of HSCs has been exploited for over 30 years in regenerative therapies for patients with blood-related genetic disease and leukemia. As clinically important cells and also as the most widely studied cell differentiation system, they have been the focus of intense fundamental research. Indeed, HSC research has established many paradigms in the more general field of stem cells. Recently, the study of the embryonic origins of HSCs and their genetic program is beginning to provide unique insights into how these stem cells are formed, maintained, and expanded, and how they contribute to the complex adult hematopoietic system. Although many short-lived hematopoietic progenitors are present in early stage mammalian embryos, this review will focus on the events leading to emergence of the most potent cells of the hematopoietic system, HSCs and on their developmental lineage relationships.

RECENT FINDINGS

Developmental and genetic studies further our understanding of the fate determination events occurring in several embryonic tissues leading to the generation of potent HSCs--those cells with the ability to long-term, high-level repopulate all hematopoietic lineages of the adult.

SUMMARY

Several mammalian embryonic tissues contribute to the growth and/or generation of potent HSCs that are the source of blood cells throughout the lifespan of the individual. Insight into how mammalian HSC fate is determined has been provided through functional, phenotypic, and genetic studies at early developmental stages.

The unfolded protein response sensor IRE1alpha is required at 2 distinct steps in B cell lymphopoiesis

B lymphocyte differentiation is coordinated with the induction of high-level Ig secretion and expansion of the secretory pathway. Upon accumulation of unfolded proteins in the lumen of the ER, cells activate an intracellular signaling pathway termed the unfolded protein response (UPR). Two major proximal sensors of the UPR are inositol-requiring enzyme 1alpha (IRE1alpha), an ER transmembrane protein kinase/endoribonuclease, and ER-resident eukaryotic translation initiation factor 2alpha (eIF2alpha) kinase (PERK). To elucidate whether the UPR plays an important role in lymphopoiesis, we carried out reconstitution of recombinase-activating gene 2-deficient (rag2-/-) mice with hematopoietic cells defective in either IRE1alpha- or PERK-mediated signaling. IRE1alpha-deficient (ire1alpha-/-) HSCs can proliferate and give rise to pro-B cells that home to bone marrow. However, IRE1alpha, but not its catalytic activities, is required for Ig gene rearrangement and production of B cell receptors (BCRs). Analysis of rag2-/- mice transplanted with IRE1alpha trans-dominant-negative bone marrow cells demonstrated an additional requirement for IRE1alpha in B lymphopoiesis: both the IRE1alpha kinase and RNase catalytic activities are required to splice the mRNA encoding X-box-binding protein 1 (XBP1) for terminal differentiation of mature B cells into antibody-secreting plasma cells. Furthermore, UPR-mediated translational control through eIF2alpha phosphorylation is not required for B lymphocyte maturation and/or plasma cell differentiation. These results suggest specific requirements of the IRE1alpha-mediated UPR subpathway in the early and late stages of B lymphopoiesis.

The unfolded protein response sensor IRE1alpha is required at 2 distinct steps in B cell lymphopoiesis

B lymphocyte differentiation is coordinated with the induction of high-level Ig secretion and expansion of the secretory pathway. Upon accumulation of unfolded proteins in the lumen of the ER, cells activate an intracellular signaling pathway termed the unfolded protein response (UPR). Two major proximal sensors of the UPR are inositol-requiring enzyme 1alpha (IRE1alpha), an ER transmembrane protein kinase/endoribonuclease, and ER-resident eukaryotic translation initiation factor 2alpha (eIF2alpha) kinase (PERK). To elucidate whether the UPR plays an important role in lymphopoiesis, we carried out reconstitution of recombinase-activating gene 2-deficient (rag2-/-) mice with hematopoietic cells defective in either IRE1alpha- or PERK-mediated signaling. IRE1alpha-deficient (ire1alpha-/-) HSCs can proliferate and give rise to pro-B cells that home to bone marrow. However, IRE1alpha, but not its catalytic activities, is required for Ig gene rearrangement and production of B cell receptors (BCRs). Analysis of rag2-/- mice transplanted with IRE1alpha trans-dominant-negative bone marrow cells demonstrated an additional requirement for IRE1alpha in B lymphopoiesis: both the IRE1alpha kinase and RNase catalytic activities are required to splice the mRNA encoding X-box-binding protein 1 (XBP1) for terminal differentiation of mature B cells into antibody-secreting plasma cells. Furthermore, UPR-mediated translational control through eIF2alpha phosphorylation is not required for B lymphocyte maturation and/or plasma cell differentiation. These results suggest specific requirements of the IRE1alpha-mediated UPR subpathway in the early and late stages of B lymphopoiesis.

Early Fetal Liver Readily Repopulates B Lymphopoiesis in Adult Bone Marrow

Fetal liver (FL) becomes a major organ of hematopoiesis at mouse embryonic day (E) 11 and E12, when definitive hematopoietic stem cells, originating from the aorta-gonads-mesonephros region, colonize the hepatic tissue. Unipotent B-cell progenitors are very rare in FL by day 12, whereas erythropoiesis prevails. We have studied hematopoiesis in FL from different gestational ages, with special emphasis on B lymphopoiesis. The mRNA levels of selected liver-specific genes, hematopoietic lineage-specific genes, and genes for selected cytokines/hormones as well as for their receptors were evaluated by real-time polymerase chain reaction in FL from E12.5, E14.5, and E17.5, adult liver and adult bone marrow (BM). The level of B lineage-related gene expression in FL was very low at E12.5. There was also a significantly lower fraction of B220+ and CD19+ B cells in E12.5 FL compared with E17.5 FL. To analyze whether these differences reflect different stem cell potentials occurring during FL development, 10(6) or 5 x 10(6) of FL cells collected from embryos at E12.5 or E17.5 and those from adult BM were transplanted into sublethally irradiated (3- or 6-Gy) congenic mice. Short-term and long-term repopulation of B and T cells and granulocyte/macrophage lineages from donor FL or adult BM cells were evaluated in competition to adult hematopoiesis of sublethally irradiated recipients. In short-term repopulation, the transplantation of E12.5 FL cells resulted in a lower blood chimerism compared with that of E17.5 FL cells. However, the proportion of B lymphopoiesis exerted by E12.5 FL cells was not different from that of E17.5 FL or adult BM. This study demonstrates that E12.5 FL contains hematopoietic stem cells with fully developed B-cell repopulating capacity and that the developmental period of fetal hematopoiesis between E12.5 and E17.5 is not an obligatory phase for the adult B lymphopoiesis.

Oncostatin M, a multifunctional cytokine

Oncostatin M (OSM) is a multifunctional cytokine that belongs to the Interleukin (IL)-6 subfamily. Among the family members, OSM is most closely related to leukemia inhibitory factor (LIF) and it in fact utilizes the LIF receptor in addition to its specific receptor in the human. While OSM was originally recognized by its unique activity to inhibit the proliferation of tumor cells, accumulating evidence now indicates that OSM exhibits many unique biological activities in inflammation, hematopoiesis, and development. Here, we review the profile of OSM activities, receptors, and signal transduction.

A new bone to pick: osteoblasts and the haematopoietic stem-cell niche

Two recent publications highlight the role of bone-forming cells, the osteoblasts, in controlling the development of neighboring haematopoietic stem cells (HSCs).1,2 Using two distinct transgenic mouse models, one using the conditional deletion of the Bone Morphogenetic Protein Receptor 1A (BMPR1A) gene, the other using over-expression of an active PTH/PTHrP receptor (PPR) mutant within osteoblasts, the authors show parallel, concordant increases in the generation of trabecular osteoblasts and the number of HSCs. In situ staining showed that rarely cycling HSCs sporadically attach to endosteal osteoblasts, while in vitro assays indicated that ligation of Jag1 on osteoblasts by Notch1 on HSCs promotes HSC proliferation. These two independent works have revived and revitalized the notion that osteoblasts are a major, defining component of the HSC niche within the bone marrow (BM). This minireview discusses these results in the context of other recent studies of mesenchymal cells within the BM microenvironment, presents one potential unified model of the functional anatomy of the BM HSC niche, and highlights new questions raised by these and other studies of osteoblasts and HSCs.

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.

Role of the hepatocyte nuclear factor 4alpha in control of the pregnane X receptor during fetal liver development

The fetal liver, the major site of hematopoiesis during embryonic development, acquires additional functions near birth. Among the important liver functions is the response to xenobiotic exposure due to expression of several cytochromes P450 (CYP) and drug efflux transporters. Expression of these genes is regulated by nuclear receptors such as the pregnane X receptor (PXR). In this study, regulation of xenobiotic responses during fetal liver development was analyzed using a fetal hepatocyte primary culture system derived from embryonic day 15 (E15) livers. Hepatocyte nuclear factor (HNF) 4alpha regulates the expression of many genes preferentially in the liver. Expression of several xenobiotic response genes as well as HNF4alpha was increased in fetal hepatocytes stimulated by the hepatic maturation factors oncostatin M (OSM) and Matrigel. To determine the contribution of HNF4alpha to xenobiotic responses in the fetal liver, fetal hepatocytes containing floxed HNF4alpha alleles were cultured and the HNF4alpha gene was inactivated by infection with an adenovirus containing the Cre gene. Expression of CYP3A11 and PXR was suppressed by inactivation of HNF4alpha. An HNF4alpha binding site was characterized in the PXR promoter and found to be required for activation of the PXR promoter in fetal hepatocytes. In conclusion, HNF4alpha is the key transcription factor regulating responses to xenobiotics through activation of the PXR gene during fetal liver development.

Ontogenic emergence of definitive hematopoietic stem cells

Research in the past 10 years has dramatically increased our knowledge of the development of the mammalian hematopoietic system and has provided insight into the embryonic sites of hematopoietic cell generation, the variety of hematopoietic cell types produced, and some of the microenvironmental influences on the rapidly growing blood system. Indeed, within mammalian embryos, it is now widely accepted that the embryo proper produces the first adult repopulating hematopoietic stem cells. This mesodermally derived intraembryonic region, known as the aorta-gonad-mesonephros region or, at a slightly earlier developmental stage, the paraaortic splanchnopleura, produces, respectively, potent hematopoietic stem cells and multipotent progenitor cells before their appearance in the yolk sac. This review focuses on the most recent findings concerning qualitative and quantitative aspects of hematopoietic stem-cell development, the endothelium as a possible direct precursor population of hematopoietic stem cells, and the microenvironment leading to the onset and maintenance of hematopoietic stem cells in the mammalian embryo.

Isolation and therapeutic potential of human haemopoietic stem cells

The haemopoietic stem cell (HSC) has long been regarded as an archetypal, tissue specific, stem cell, capable of completely regenerating haemopoiesis after myeloablation. It has proved relatively easy to harvest HSC, from bone marrow or peripheral blood. In turn, isolation of these cells has allowed therapeutic stem cell transplantation protocols to be developed, that capitalise on their prodigious self renewal and proliferative capabilities. Ex vivo approaches have been described to isolate, genetically manipulateand expand pluripotent stem cell subsets. These techniques have been crucial to the development of gene therapy, and may allow adults to enjoy the potential advantages of cord blood transplantation. Recently, huge conceptual changes have occurred in stem cell biology. In particular, the dogma that, in adults, stem cells are exclusively tissue restricted has been questioned and there is great excitement surrounding the potential plasticity of these cells, with the profound implications that this has, for developing novel cellular therapies. Mesenchymal stem cells, multipotent adult progenitor cells and embryonic stem cells are potential sources of cells for transplantation purposes. These cells may be directed toproduce HSC, in vitro and in the future may be used for therapeutic, or drug development, purposes.

Cytokine regulation of liver development

Liver development is a sequential array of distinct biological events. Each step of differentiation is regulated by intrinsically programmed mechanisms as well as by extracellular signals. The establishment of cell culture systems that recapitulate each stage of liver development has led to the identification of several extracellular signals that affect hepatocytic differentiation. Furthermore, studies on genetically engineered animals, especially knockout and transgenic mice, have highlighted a number of molecules essential for liver development. By applying primary culture techniques to analyses of mutant mice, it is now possible to link extracellular signals to intracellular pathways that provoke cellular responses of differentiation. Improvement in gene transfer technology utilizing viral vectors has further expanded the molecular analysis of liver development. In this review article, we summarize recent advances and attempt to describe the molecular basis of liver development from beginning to end as a sequential event.

The essential roles of the chemokine SDF-1 and its receptor CXCR4 in human stem cell homing and repopulation of transplanted immune-deficient NOD/SCID and NOD/SCID/B2m(null) mice

Hematopoietic stem cells are identified based on their functional ability to migrate via the blood circulation of transplanted recipients, to home to the host bone marrow and to durably repopulate this organ with high levels of maturing myeloid and lymphoid cells. While a small pool of undifferentiated stem cells with the potential to repeat the entire process in serially transplanted recipients is maintained within the bone marrow, maturing cells are continuously released into the circulation. In recent years pre-clinical, functional in vivo models for human stem cells have been developed, using immune-deficient mice or pre-immune, fetal sheep as recipients. The mechanism of human stem cell migration, homing and repopulation in transplanted immune-deficient NOD/SCID and NOD/SCID/B2m(null) mice as well as the accessory mediators that facilitate these processes, will be reviewed. In particular, the essential roles of the chemokine SDF-1 and its receptor CXCR4 which mediate and regulate stem cell homing and repopulation will be discussed.