Long-term reconstitution of the mouse hematopoietic system by embryonic stem cell-derived fetal liver

Genome-wide survey reveals dynamic widespread tissue-specific changes in DNA methylation during development

BACKGROUND

Changes in DNA methylation in the mammalian genome during development are frequent events and play major roles regulating gene expression and other developmental processes. It is necessary to identify these events so that we may understand how these changes affect normal development and how aberrant changes may impact disease.

RESULTS

In this study Methylated DNA ImmunoPrecipitation (MeDIP) was used in conjunction with a NimbleGen promoter plus CpG island (CpGi) array to identify Tissue and Developmental Stage specific Differentially Methylated DNA Regions (T-DMRs and DS-DMRs) on a genome-wide basis. Four tissues (brain, heart, liver, and testis) from C57BL/6J mice were analyzed at three developmental stages (15 day embryo, E15; new born, NB; 12 week adult, AD). Almost 5,000 adult T-DMRs and 10,000 DS-DMRs were identified. Surprisingly, almost all DS-DMRs were tissue specific (i.e. methylated in at least one tissue and unmethylated in one or more tissues). In addition our results indicate that many DS-DMRs are methylated at early development stages (E15 and NB) but are unmethylated in adult. There is a very strong bias for testis specific methylation in non-CpGi promoter regions (94%). Although the majority of T-DMRs and DS-DMRs tended to be in non-CpGi promoter regions, a relatively large number were also located in CpGi in promoter, intragenic and intergenic regions (>15% of the 15,979 CpGi on the array).

CONCLUSIONS

Our data suggests the vast majority of unique sequence DNA methylation has tissue specificity, that demethylation has a prominent role in tissue differentiation, and that DNA methylation has regulatory roles in alternative promoter selection and in non-promoter regions. Overall, our studies indicate changes in DNA methylation during development are a dynamic, widespread, and tissue-specific process involving both DNA methylation and demethylation.

W41/W41 blastocyst complementation: a system for genetic modeling of hematopoiesis

We report a rapid and highly efficient approach to generate mice in which the hematopoietic system is derived from embryonic stem (ES) cells. We show that ES cells injected into blastocysts from the c-kit-deficient W(41)/W(41) mouse strain have a clear advantage over the W(41)/W(41) blastocyst-derived inner cell mass cells in founding the hematopoietic system. Fetal liver hematopoietic stem cells from W(41)/W(41) blastocyst complementation embryos can be transplanted to establish large cohorts of bone marrow chimeras with hematopoiesis of practically pure ES-cell origin. Using ES cells with site-directed modifications, we show how this system can be used to drive inducible transgene expression in hematopoietic cells in a robust and reliable manner both in vitro and in vivo. The approach avoids the cost and time constraints associated with the creation of standard transgenic mouse strains while taking advantage of the sophisticated site-directed manipulations that are possible in ES cells.

Expression of AA4.1 marks lymphohematopoietic progenitors in early mouse development

The hematopoietic system of mice is established during the early to midgestational stage of development. However, the earliest lymphohematopoietic progenitors that appear during mouse development have been less well characterized compared with the hematopoietic stem cell compartment of fetal liver and bone marrow. We isolated the earliest lymphohematopoietic progenitors by using embryonic stem (ES) cell culture in vitro. Cells with the c-Kit(+)Lin(-) cell surface phenotype were present abundantly in ES cells cocultured with stromal cell lines. We further separated the cells into two distinct cell subsets based on AA4.1 expression. Although AA4.1(+) and AA4.1(-) cells had equivalent potency to generate myeloid cell lineages, the lymphoid potential in ES-cell-derived cells was largely restricted to the cells expressing AA4.1. The same cell type was present abundantly in the early yolk sac and in fewer numbers (approximately 5% of that in the yolk sac) in the caudal half of the developing embryos. These data suggest that AA4.1 is a cell surface marker that can identify the earliest lymphohematopoietic progenitors in mouse development.

Macrophage EP4 deficiency increases apoptosis and suppresses early atherosclerosis

Prostaglandin (PG) E(2), a major product of activated macrophages, has been implicated in atherosclerosis and plaque rupture. The PGE(2) receptors, EP2 and EP4, are expressed in atherosclerotic lesions and are known to inhibit apoptosis in cancer cells. To examine the roles of macrophage EP4 and EP2 in apoptosis and early atherosclerosis, fetal liver cell transplantation was used to generate LDLR(-/-) mice chimeric for EP2(-/-) or EP4(-/-) hematopoietic cells. After 8 weeks on a Western diet, EP4(-/-) --> LDLR(-/-) mice, but not EP2(-/-) --> LDLR(-/-) mice, had significantly reduced aortic atherosclerosis with increased apoptotic cells in the lesions. EP4(-/-) peritoneal macrophages had increased sensitivity to proapoptotic stimuli, including palmitic acid and free cholesterol loading, which was accompanied by suppression of activity of p-Akt, p-Bad, and NF-kappaB-regulated genes. Thus, EP4 deficiency inhibits the PI3K/Akt and NF-kappaB pathways compromising macrophage survival and suppressing early atherosclerosis, identifying macrophage EP4-signaling pathways as molecular targets for modulating the development of atherosclerosis.

Hematopoietic reconstitution with androgenetic and gynogenetic stem cells

Parthenogenetic embryonic stem (ES) cells with two oocyte-derived genomes (uniparental) have been proposed as a source of autologous tissue for transplantation. The therapeutic applicability of any uniparental cell type is uncertain due to the consequences of genomic imprinting that in mammalian uniparental tissues causes unbalanced expression of imprinted genes. We transplanted uniparental fetal liver cells into lethally irradiated adult mice to test their capacity to replace adult hematopoietic tissue. Both maternal (gynogenetic) and paternal (androgenetic) derived cells conveyed long-term, multilineage reconstitution of hematopoiesis in recipients, with no associated pathologies. We also establish that uniparental ES cells can differentiate into transplantable hematopoietic progenitors in vitro that contribute to long-term hematopoiesis in recipients. Hematopoietic tissue in recipients maintained fidelity of parent-of-origin methylation marks at the Igf2/H19 locus; however, variability occurred in the maintenance of parental-specific methylation marks at other loci. In summary, despite genomic imprinting and its consequences on development that are particularly evident in the androgenetic phenotype, uniparental cells of both parental origins can form adult-transplantable stem cells and can repopulate an adult organ.

Improved embryonic stem cell technologies

Murine embryonic stem (ES) cells have become an indispensable tool for investigating genetic function both in vitro and, importantly, in vivo. Recent advances, including tetraploid aggregation, new site-specific recombinases and RNAi, have enabled more sophisticated manipulation of the ES cell genome. For instance, it is now possible to control gene expression in both a temporally and spatially restricted manner. Such new technologies are answering complex questions surrounding the function and interaction of an increasing number of genes. This chapter will review both the history and recent technological progress that has been made in mouse ES cell derivation, genetic manipulation and the generation of ES cell-derived chimaeric animals.

Tyrosine phosphatase MEG2 modulates murine development and platelet and lymphocyte activation through secretory vesicle function

MEG2, a protein tyrosine phosphatase with a unique NH₂-terminal lipid-binding domain, binds to and is modulated by the polyphosphoinositides PI_(4,5)P₂ and PI_(3,4,5)P₃. Recent data implicate MEG2 in vesicle fusion events in leukocytes. Through the genesis of Meg2-deficient mice, we demonstrate that Meg2^−/−embryos manifest hemorrhages, neural tube defects including exencephaly and meningomyeloceles, cerebral infarctions, abnormal bone development, and >90% late embryonic lethality. T lymphocytes and platelets isolated from recombination activating gene 2^−/− mice transplanted with Meg2^−/− embryonic liver–derived hematopoietic progenitor cells showed profound defects in activation that, in T lymphocytes, was attributable to impaired interleukin 2 secretion. Ultrastructural analysis of these lymphocytes revealed near complete absence of mature secretory vesicles. Taken together, these observations suggest that MEG2-mediated modulation of secretory vesicle genesis and function plays an essential role in neural tube, vascular, and bone development as well as activation of mature platelets and lymphocytes.

Tetraploid development in the mouse

Spontaneous duplication of the mammalian genome occurs in approximately 1% of fertilizations. Although one or more whole genome duplications are believed to have influenced vertebrate evolution, polyploidy of contemporary mammals is generally incompatible with normal development and function of all but a few tissues. The production of tetraploid (4n) embryos has become a common experimental manipulation in the mouse. Although development of tetraploid mice has generally not been observed beyond midgestation, tetraploid:diploid (4n:2n) chimeras are widely used as a method for rescuing extraembryonic defects. The tolerance of tissues to polyploidy appears to be dependent on genetic background. Indeed, the recent discovery of a naturally tetraploid rodent species suggests that, in rare genetic backgrounds, mammalian genome duplications may be compatible with the development of viable and fertile adults. Thus, the range of developmental potentials of tetraploid embryos remains in large part unexplored. Here, we review the biological consequences and experimental utility of tetraploid mammals, in particular the mouse.

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.

The ultimate source of human hematopoietic stem cells: thinking outside the marrow

The ability to reconstitute cellular components of the hematopoietic system has immense utility in several areas of clinical medicine. These include replacement of cells responsible for innate and acquired immunity, providing red cells for oxygen transport, and ultimately the ability to recover hematopoietic function by repopulating all lineages comprising the entire blood system. This latter property functionally defines the mammalian hematopoietic stem cell (HSC). Recently, human embryonic stem cells (ESCs) have been suggested to be a viable source of transplantable hematopoietic cells. Although the number of human ESCs is virtually unlimited, the ability to efficiently differentiate adequate numbers of cells that possess hematopoietic repopulating ability remains to be determined. Achieving this goal is confounded by the difficulty of experimentally generating murine hematopoietic cell types capable of in vivo reconstitution from mouse ESC, suggesting that similar limitations may arise using human counterparts. Although the use of human ESCs and adult somatic HSCs have their independent merits, a direct comparison between HSCs derived from each source using similar assays will ultimately be required to determine the best source for clinical use. Here we will summarize the results from efforts to differentiate and assay primitive hematopoietic cells derived from ESCs, and compare these findings to similar parameters using putative mammalian HSCs harvested from the adult.

Cytokines and BMP-4 promote hematopoietic differentiation of human embryonic stem cells

Human embryonic stem cells (hESCs) randomly differentiate into multiple cell types during embryoid body (EB) development. To date, characterization of specific factors capable of influencing hematopoietic cell fate from hESCs remains elusive. Here, we report that the treatment of hESCs during EB development with a combination of cytokines and bone morphogenetic protein-4 (BMP-4), a ventral mesoderm inducer, strongly promotes hematopoietic differentiation. Hematopoietic progenitors of multiple lineages were generated from EBs and were found to be restricted to the population of progeny expressing cell surface CD45. Addition of BMP-4 had no statistically significant effect on hematopoietic differentiation but enabled significant enhancement in progenitor self-renewal, independent of cytokine treatment. Hematopoietic commitment was characterized as the temporal emergence of single CD45+ cells first detectable after day 10 of culture and was accompanied by expression of hematopoietic transcription factors. Despite the removal of cytokines at day 10, hematopoietic differentiation of hESCs continued, suggesting that cytokines act on hematopoietic precursors as opposed to differentiated hematopoietic cells. Our study establishes the first evidence for the role of cytokines and BMP-4 in promoting hematopoietic differentiation of hESC lines and provides an unprecedented system to study early developmental events that govern the initiation of hematopoiesis in the human.

Human ES cells--haematopoiesis and transplantation strategies

Human embryonic stem (ES) cells provide a novel opportunity to study early developmental events in a human system. We have used human ES cell lines, including clonally derived lines, to evaluate haematopoiesis. Co-culture of the human ES cells with irradiated bone marrow stromal cell lines in the presence of fetal bovine serum (FBS), but without other exogenous cytokines, leads to differentiation of the human ES cells within a matter of days. A portion of these differentiated cells express CD34, the best-defined marker for early haematopoietic cells. Haematopoietic colony-forming cells (CFCs) are demonstrated by methylcellulose assay. Myeloid, erythroid, megakaryocyte and multipotential CFCs can all be derived under these conditions. Enrichment of CD34+ cells derived from the human ES cells markedly increases the yield of CFCs, as would be expected for cells derived from adult bone marrow or umbilical cord blood. Transcription factors are also expressed in a manner consistent with haematopoietic differentiation. This system now presents the potential to evaluate specific conditions needed to induce or support events in early human blood development. Human ES cells are also a novel source of cells for transplantation therapies. The immunogenicity of ES cell-derived cells is unknown. The unique properties of ES cells afford the opportunity to explore novel mechanisms to prevent immune-mediated rejection. Potential strategies to overcome rejection will be presented, including creation of haematopoietic chimerism as a means to successfully transplant cells and tissues derived from human ES cells.

The Hematopoietic Microenvironment: Phylogeny and Ontogeny of the Hematopoietic Microenvironment

Although there is no formalized area of study called phylogeny or ontogeny of the hematopoietic microenvironment, new models and molecular tools are now available for such studies. The concept of a hematopoietic microenvironment has developed from the need to answer basic questions about migration, control of proliferation and differentiation of lymphohematopoietic cells; e.g. how are cells with the same genes induced to express different sets of these genes which lead to differentiation. These questions were first approached when cells could only be identified morphologically. The ontogeny of hematopoiesis was traced from the blood islands of the embryonic yolk sac, to the fetal liver, spleen, and bone marrow. Cells with reticular morphology were associated with areas of hematopoiesis and, in the embryo, they were thought to give rise to both hematopoietic and supportive cells. In the 1960's the classic work of McCulloch, Till and Siminovitch led the study of hematopoietic precursors which have no distinctive morphological identity and are too infrequent to study microscopically. These cells were identified by their functions; e.g. colony formation in culture in the presence of certain factors, production of spleen colonies or rescue of lethally irradiated mice. Cells with these functions were also found sequentially in the yolk sac blood islands, in the aorta/mesonephros, fetal liver, spleen, and bone marrow during development. The question remained, what regulates the proliferation and differentiation of these cells and why do they home to different sites in different stages of development? Among the laboratories studying spleen colonies, a controversy arose as to whether differentiation decisions were stochastic or induced by extra cellular factors. Dexter and Greenberger developed the long-term bone marrow culture system which has aided in studying the roles of factors such as cell-cell contact and extracellular matrix in hematopoietic differentiation. The molecular identification of ligand/receptor pairs such as ckit and KL as well as transactivating factors that control whole sets of lineage related genes such as the GATAs and Ikaros, may lead to clarification of the stochastic versus induced differentiation issue. Chimeric bird and frog embryos and analysis of mutations effecting hematopoiesis in frogs and zebrafish have helped to trace the earliest hematopoietic development in the embryo and to determine what influences it. The identification of genes that alter development of hematopoiesis opens the possibility of comparing microenvironmental control mechanisms in various present day organisms and relating these to evolutionary events. Many basic questions relevant to the interaction between hematopoietic cells and their microenvironment can be addressed by studying "simple" organisms in which the answers may be more easily determined than in mice or humans. Examples of possibly useful organisms, range from the teliosts such as zebrafish to algae such as Volvox, a two cell organism, to Dictyostelium which change from 1 to many cell types and in the process, migrate, adhere and differentiate.

Megakaryocytopoiesis in vitro: from the stem cells' perspective

Megakaryocytopoiesis is a complex network regulated by different megakaryocyte (MK)-stimulating factors (i.e., thrombopoietin [TPO], stem cell factor [SCF], interleukin 3 [IL-3], IL-6, IL-11 and GM-CSF). Although all of these factors can affect human and murine megakaryocytopoiesis at different levels of MK development, the effect on very primitive hematopoietic stem cells (HSC) is not well understood. We have further characterized the in vitro biological activity of recombinant murine TPO, SCF and IL-3 on the maturation and proliferation of MK progenitors from different murine primitive hematopoietic cells in a fibrin clot system under serum-free conditions. Neither TPO nor SCF alone induced MK colony formation (CFU-MK) from Lin- Sca+ cells. However, isolated large and mature MKs were observed in the presence of TPO. In contrast, IL-3 exerted a potent effect on CFU-MK formation from Lin- Sca+ cells. On this population of HSC, a significant increase of large MK colonies with mature MK were obtained under those conditions in which TPO was combined with IL-3 or SCF plus IL-3. Similar results were obtained with murine bone marrow cells enriched by primitive progenitors from day 3 post-5-fluorouracil treated mice (5-FUBMC). In contrast, TPO-sensitive precursors were detected in fetal liver cells (FLC). These cells differentiate and proliferate to MK progenitors in the presence of TPO. A significant increase in the number of CFU-MK was induced when TPO was combined with either IL-3 or SCF. On these populations of primitive hematopoietic progenitors, IL-3 induced both the proliferation and differentiation of MK progenitors. Because erythropoietin and TPO share similarities between their molecules and their receptors, we studied whether these growth factors may modulate megakaryocytopoiesis from FLC. Flow cytometry analysis of FLC expressing erythroid markers demonstrated that these cells expressed c-Mpl receptor. In our in vitro studies, although EPO by itself did not induce MK colonies from FLC, it enhanced the proliferative activity of TPO. High ploidy and proplatelet-shedding MK were observed in Lin- Sca+ cells, 5-FUBMC and FLC stimulated with TPO alone or in combination with other MK-stimulating factors. Based on these observations, we propose that TPO, IL-3 and SCF constitute early MK-acting factors with differential proliferative and differentiative activities on murine stem cells. TPO by itself does not appear to be involved in the proliferation of MK progenitors from bone marrow HSC. TPO appears to induce in these cells the commitment toward MK differentiation. However, this growth factor may enhance the proliferative activity of IL-3. IL-3 is an early MK-stimulating factor able to induce in vitro the proliferation and differentiation of MK progenitors from HSC.

Characterization of the stage in natural killer cell development in 14.5-day mouse fetal liver using adult bone marrow stroma

Nonstimulated fetal liver (FL) from 14.5-day gestation mice had no natural killer (NK) cell activity and <3% expressed NK1.1. Even after short-term (3-4 day) culture of FL with the late-acting cytokines, interleukin (IL)-15 or IL-2, little or no NK activity was detected. However, longer-term (13 day) culture with IL-2 plus stroma derived from bone marrow (BM) of adult mice, resulted in extensive proliferation and differentiation to mature NK cells. Cell numbers began to increase after 4 days, and by day 13, they had increased 40-fold and 69% of the cells were NK1.1+ with high NK activity and 5%-10% were NK1.1- B220+. With stroma, but no IL-2, equivalent proliferation occurred, but differentiated cells were predominantly NK1.1- B220+, not NK cells. Culture for 13 days without stroma, but with either IL-2, IL-15, FLTK3-ligand (L) or stroma-conditioned medium, resulted in less than fivefold expansion, and minimal NK activity. Culture with combinations of FLTK3-L or ckit-L plus IL-15 or IL-2 increased both cell number and NK activity, but the increase in cell number was less than that seen with stroma plus IL-2. By limiting dilution assay on stroma plus IL-2, the precursor frequency was 1/(2660+/-292) whole FL cells and the absolute number, but not the frequency, increased during culture on stroma without IL-2. The NK cell progenitors were found in sorted NK1.1- and Sca-1+ c-kit+ lineage- subpopulations at a frequency of 1/(156+/-52.5). Together, these data suggest that the NK lineage cells in FL are primarily in early stages of development. They are highly proliferative, respond to early acting cytokines and express stem cell markers.

Transgenic bovine chimeric offspring produced from somatic cell-derived stem-like cells

We have developed a method, using nuclear transplantation, to produce transgenic embryonic stem (ES)-like cells from fetal bovine fibroblasts. These cells, when reintroduced into preimplantation embryos, differentiated into derivatives from the three embryonic germ layers, ectoderm, mesoderm, and endoderm, in 5-month-old animals. Six out of seven (86%) calves born were found to be chimeric for at least one tissue. These experiments demonstrate that somatic cells can be genetically modified and then de-differentiated by nuclear transfer into ES-like cells, opening the possibility of using them in differentiation studies and human cell therapy.

Conditional genome alteration in mice

The recent ability to inactivate specific genes in mice has significantly accelerated our understanding of molecular, cellular, and even behavioral aspects of normal and disease processes. However, this ability has also demonstrated the extreme complexity of genetic determination in mammals, in particular, that genes in the same family or pathway can be functionally redundant and that a given gene often has multiple roles. Thus, inactivation of a gene often does not indicate its complete spectrum of functions. To circumvent this problem, many new tools and novel applications of classic techniques have been developed to place spatial and temporal restrictions on the genomic alterations. These approaches include chimera and mosaic studies, organ transplantation, complementation assays, dominant negative mutants, conditional gene knockouts, and lineage-specific gene rescue. Not only has this opened up more sophisticated ways to make genomic alterations, but it has provided the opportunity to create animal models for sporadic human genetic diseases.

Generation of completely embryonic stem cell-derived mutant mice using tetraploid blastocyst injection

Embryonic stem (ES) cells provide a unique tool for producing specifically designed mutations in mice. Here, we describe an alternative approach toward the generation of mice which are derived completely from ES cells (ES mice), as judged by glucose phosphate isomerase (GPI) analysis, without prior passage through the germline. By injecting wild-type and mutant ES cells into tetraploid blastocysts, viable and fertile ES mice were generated, suggesting that totipotency of ES cells was not affected by long-term culture and experimental manipulation in vitro. When ES cell clones harboring a lacZ reporter gene introduced by either targeted insertion or a gene-trap approach were used, the expression pattern of the lacZ gene in ES fetuses was identical to that of fetuses that were derived from breeding of chimeric mice. Thus, this technique can be considered as a useful and rapid approach to produce fetuses and mice directly from ES cells carrying predetermined genetic changes and offers many applications for studies in molecular genetics and developmental biology.

Developmental-stage-specific expression and regulation of an amphotropic retroviral receptor in hematopoietic cells

Expression of the transmembrane receptor protein Ram-1 may be critical to optimizing retroviral gene transfer. Ram-1 acts as both a sodium-dependent phosphate transporter and a receptor for amphotropic retroviruses. We previously reported detectable Ram-1 in murine hematopoietic fetal liver cells (FLC) despite resistance of these cells to amphotropic retroviral transduction (infection). We document here that Ram-1 expression is completely absent in murine yolk sac cells from days 9.5 through 13.5 of ontogeny and first appears at low levels in midgestational FLC between days 13.5 and 14.5. In addition, Ram-1 expression is detected only in more differentiated populations within FLC, day 14.5, and not in those highly enriched for stem cells, indicating developmental regulation of Ram-1 during murine hematopoiesis. Others have reported the in vitro use of phosphate-free medium as a stimulus to increase levels of Ram-1 mRNA in nonhematopoietic cells. We now demonstrate that Ram-1 poly(A)+ mRNA increases significantly following culture of FLC in phosphate-free medium. Further, transduction of FLC in phosphate-free medium with an amphotropic retrovirus containing the multiple drug resistance gene leads to gene transfer not observed previously. These data demonstrate that (i) the normal resistance of FLC to amphotropic transduction is most likely due to an insufficient number of Ram-1 molecules for efficient retroviral recognition and binding, and (ii) Ram-1 can be upregulated by increasing the need for phosphate transport across the cell membrane.

Exogenous Mtv-7 superantigen transgene expression in major histocompatibility complex class II I-E- mice reconstituted with embryonic stem cell-derived hematopoietic stem cells

Direct genetic manipulation of hematopoietic cells is limited by the lack of an established hematopoietic stem cell line. It has been demonstrated that embryonic stem (ES) cell<-->tetraploid embryos are completely ES cell-derived and that fetal liver (FL) cells from these embryos support hematopoiesis in lethally irradiated recipients. In this report, we demonstrate that FL cells from ES cell<-->tetraploid embryos support normal lymphopoiesis and T-cell repertoire development. Moreover, the introduction of the Mtv-7 superantigen transgene coding for minor lymphocyte stimulatory antigen 1 into murine hematopoietic cells via reconstitution with ES cell<-->tetraploid FL cells demonstrates that this method can effectively confer stable genetic changes into the hematopoietic tissues without going through the germ line. Long-term and secondary reconstitution with ES cell<-->tetraploid FL cells expressing the Mtv-7 superantigen transgene clonally deleted minor lymphocyte stimulatory antigen 1-reactive T-cell receptor V beta 6+, -8.1+, and -9+ T cells, but not V beta 7+ T cells, in H-2b (I-E-) mice. This model system will be extremely important for analyzing structure-function relationships of molecules involved in proliferation, differentiation, and selection of hematopoietic cells in vivo and for examining hematopoiesis-specific effects of mutations that are lethal during embryogenesis.

Derivation of completely cell culture-derived mice from early-passage embryonic stem cells

Several newly generated mouse embryonic stem (ES) cell lines were tested for their ability to produce completely ES cell-derived mice at early passage numbers by ES cell <==> tetraploid embryo aggregation. One line, designated R1, produced live offspring which were completely ES cell-derived as judged by isoenzyme analysis and coat color. These cell culture-derived animals were normal, viable, and fertile. However, prolonged in vitro culture negatively affected this initial totipotency of R1, and after passage 14, ES cell-derived newborns died at birth. However, one of the five subclones (R1-S3) derived from single cells at passage 12 retained the original totipotency and gave rise to viable, completely ES cell-derived animals. The total in vitro culture time of the sublines at the time of testing was equivalent to passage 24 of the original line. Fully potent early passage R1 cells and the R1-S3 subclone should be very useful not only for ES cell-based genetic manipulations but also in defining optimal in vitro culture conditions for retaining the initial totipotency of ES cells.

ES cells have only a limited lymphopoietic potential after adoptive transfer into mouse recipients

While hematopoietic stem cells from adult and fetal stages of murine development are capable of long term reconstitution of all mature blood lineages in vivo, embryonic hematopoietic stem cell repopulation in vivo has proved difficult. It is thought that there are many fewer hematopoietic stem cells in the embryo than in the fetal/adult stages of mouse development and that these cells possess a different developmental potential. One source of such cells are embryonic stem (ES) cells which can differentiate into most mature blood lineages in vitro. We have therefore used transplantation of differentiated ES cells to assess the hematopoietic potential of embryonic hematopoietic cells in vivo. We demonstrate here that precursors obtained from in vitro cultures of normal ES cells can contribute only to restricted and limited hematopoiesis in a mouse without leading to tumour formation. Repopulation occurs for greater than 6.5 months at levels ranging from 0.1% to 6% in B and T cell lineages in peripheral blood. In contrast to in vitro colony data demonstrating the myeloid lineage developmental potential of ES cells, no donor-derived myeloid repopulation was observed in CFU-S assays and no macrophage and mast cells were found in long term repopulated recipients. Thus, the hematopoietic potential of ES cells in vivo is limited to low levels of repopulation and is restricted to the lymphoid lineage.

Molecular genetic approaches to the elucidation of hematopoietic stem cell function

The past few years have seen considerable advances in the development of the methodologies for discovering novel genes critical to hematopoietic stem cell function and for analyzing their biological role in hematopoiesis. This review briefly discusses some common themes that are emerging from the molecular genetic approaches to hematopoietic stem cell function.

flk-1, an flt-related receptor tyrosine kinase is an early marker for endothelial cell precursors

We have used RT-PCR to screen pluripotent murine embryonic stem cells to identify receptor tyrosine kinases (RTKs) potentially involved in the determination or differentiation of cell lineages during early mouse development. Fourteen different tyrosine kinase sequences were identified. The expression patterns of four RTKs have been examined and all are expressed in the mouse embryo during, or shortly after, gastrulation. We report here the detailed expression pattern of one such RTK, the flt-related gene flk-1. In situ hybridization analysis of the late primitive streak stage embryo revealed that flk-1 was expressed in the proximal-lateral embryonic mesoderm; tissue fated to become heart. By headfold stages, staining was confined to the endocardial cells of the heart primordia as well as to the blood islands of the visceral yolk sac and the developing allantois. Patchy, speckled staining was detected in the endothelium of all the major embryonic and extraembryonic blood vessels as they formed. During early organogenesis, expression was detected in the blood vessels of highly vascularized tissues such as the brain, liver, lungs and placenta. Since flk-1 was expressed in early mesodermal cells prior to any morphological evidence for endothelial cell differentiation (vasculogenesis), as well as in cells that form blood vessels from preexisting ones (angiogenesis), it appears to be a very early marker of endothelial cell precursors. We have previously reported that another novel RTK, designated tek, was expressed in differentiating endothelial cells. We show here that flk-1 transcripts are expressed one full embryonic day earlier than the first tek transcripts. The expression of these two RTKs appear to correlate with the specification and early differentiation of the endothelial cell lineage respectively, and therefore may play important roles in the establishment of this lineage.

The infusion of human fetal liver cells

Only transient engraftment of infused fetal liver cells has been demonstrated in a small proportion of patients with hypoplastic bone marrow or patients undergoing treatment for acute leukemia. This presumably reflects the ability of the recipient to reject the infused cells, the infusion of too few viable stem cells or the availability of too few accessory cells; it is clear from the successful engraftment of infused fetal liver cells in a high proportion of infants and fetuses with severe immunodeficiency diseases that, under favorable circumstances, cells derived from human fetal liver are capable of establishing effective grafts and making a substantial contribution to hematopoiesis comparable to that of transplanted cells derived from the liver of the fetal mouse, rat, rabbit or dog. Significant clinical and hematological improvements have been described following infusions of fetal liver cells without evidence of engraftment. These improvements have been attributed to the ability of the infused cells to promote regeneration of autologous hematopoiesis and to inhibit the growth of tumor cells. These possibilities are worthy of evaluation in relation to the production of putative regulators of cellular proliferation in the liver. Meanwhile a suppressor of tumor growth is being used to purge bone marrow prior to autologous transplantation. The generation in vitro of cells which possess the properties of hematopoietic stem cells generated in the liver--from cells which can be maintained as permanent cell lines--would transform hematopoietic cell replacement therapy, and the possibility may not be too unrealistic to contemplate.

Embryonic stem cells and in vitro hematopoiesis

To study hematopoietic differentiation a variety of in vitro systems have been established using hematopoietic precursors derived from various explanted adult and fetal tissues. In this prospective we describe and discuss the potential of a novel system for studying the earliest stages of hematopoietic development. In addition, some of the applications of this system as a unique in vitro model for studying other developmental systems are discussed. Murine embryonic stem cells (ESC), which are totipotent and can be maintained undifferentiated indefinitely in vitro, have the capacity to differentiate in vitro into hematopoietic precursors of most, if not all, of the colony forming cells found in normal bone marrow. This potential can be exploited to study the control of the early stages of hematopoietic induction and differentiation. Recent results have indicated that there is a strong transcriptional activation, in a well defined temporal order, of many of the hematopoietically relevant genes. Examples of the genes expressed early during the induction of hematopoiesis include erythropoietin (Epo) and its receptor as well as the Steel (SI) factor (SLF) and its receptor (c-kit). Several other genes, including CSF-1, IL-1, and G-CSF were expressed during the later stages of hematopoietic differentiation. Contrasting with these observations, IL-3 and GM-CSF were not expressed during the first 24 days of ES cell differentiation suggesting that neither factor is necessary for the induction of hematopoietic precursors. Although these studies are just beginning, this system is easily manipulated and gives us an approach to understanding the control of the induction and differentiation of the hematopoietic system in ways not previously possible.