PDGFR alpha expression during mouse embryogenesis: immunolocalization analyzed by whole-mount immunohistostaining using the monoclonal anti-mouse PDGFR alpha antibody APA5

A review of human pluripotent stem cell-derived cardiomyocytes for high-throughput drug discovery, cardiotoxicity screening, and publication standards

Drug attrition rates have increased in past years, resulting in growing costs for the pharmaceutical industry and consumers. The reasons for this include the lack of in vitro models that correlate with clinical results and poor preclinical toxicity screening assays. The in vitro production of human cardiac progenitor cells and cardiomyocytes from human pluripotent stem cells provides an amenable source of cells for applications in drug discovery, disease modeling, regenerative medicine, and cardiotoxicity screening. In addition, the ability to derive human-induced pluripotent stem cells from somatic tissues, combined with current high-throughput screening and pharmacogenomics, may help realize the use of these cells to fulfill the potential of personalized medicine. In this review, we discuss the use of pluripotent stem cell-derived cardiomyocytes for drug discovery and cardiotoxicity screening, as well as current hurdles that must be overcome for wider clinical applications of this promising approach.

Myocardin regulates BMP10 expression and is required for heart development

Myocardin is a muscle lineage-restricted transcriptional coactivator that has been shown to transduce extracellular signals to the nucleus required for SMC differentiation. We now report the discovery of a myocardin/BMP10 (where BMP10 indicates bone morphogenetic protein 10) signaling pathway required for cardiac growth, chamber maturation, and embryonic survival. Myocardin-null (Myocd) embryos and embryos harboring a cardiomyocyte-restricted mutation in the Myocd gene exhibited myocardial hypoplasia, defective atrial and ventricular chamber maturation, heart failure, and embryonic lethality. Cardiac hypoplasia was caused by decreased cardiomyocyte proliferation accompanied by a dramatic increase in programmed cell death. Defective chamber maturation and the block in cardiomyocyte proliferation were caused in part by a block in BMP10 signaling. Myocardin transactivated the Bmp10 gene via binding of a serum response factor-myocardin protein complex to a nonconsensus CArG element in the Bmp10 promoter. Expression of p57kip2, a BMP10-regulated cyclin-dependent kinase inhibitor, was induced in Myocd-/- hearts, while BMP10-activated cardiogenic transcription factors, including NKX2.5 and MEF2c, were repressed. Remarkably, when embryonic Myocd-/- hearts were cultured ex vivo in BMP10-conditioned medium, the defects in cardiomyocyte proliferation and p57kip2 expression were rescued. Taken together, these data identify a heretofore undescribed myocardin/BMP10 signaling pathway that regulates cardiomyocyte proliferation and apoptosis in the embryonic heart.

NO-β-catenin crosstalk modulates primitive streak formation prior to embryonic stem cell osteogenic differentiation

Nitric oxide (NO) has been shown to play a crucial role in bone formation in vivo. We sought to determine the temporal effect of NO on murine embryonic stem cells (ESCs) under culture conditions that promote osteogenesis. Expression profiles of NO pathway members and osteoblast-specific markers were analyzed using appropriate assays. We found that NO was supportive of osteogenesis specifically during an early phase of in vitro development (days 3-5). Furthermore, ESCs stably overexpressing the inducible NO synthase showed accelerated and enhanced osteogenesis in vitro and in bone explant cultures. To determine the role of NO in early lineage commitment, a stage in ESC differentiation equivalent to primitive streak formation in vivo, ESCs were transfected with a T-brachyury-GFP reporter. Expression levels of T-brachyury and one of its upstream regulators, β-catenin, the major effector in the canonical Wnt pathway, were responsive to NO levels in differentiating primitive streak-like cells. Our results indicate that NO may be involved in early differentiation through regulation of β-catenin and T-brachyury, controlling the specification of primitive-streak-like cells, which may continue through differentiation to later become osteoblasts.

APELIN promotes hematopoiesis from human embryonic stem cells

Transcriptional profiling of differentiating human embryonic stem cells (hESCs) revealed that MIXL1-positive mesodermal precursors were enriched for transcripts encoding the G-protein–coupled APELIN receptor (APLNR). APLNR-positive cells, identified by binding of the fluoresceinated peptide ligand, APELIN (APLN), or an anti-APLNR mAb, were found in both posterior mesoderm and anterior mesendoderm populations and were enriched in hemangioblast colony-forming cells (Bl-CFC). The addition of APLN peptide to the media enhanced the growth of embryoid bodies (EBs), increased the expression of hematoendothelial genes in differentiating hESCs, and increased the frequency of Bl-CFCs by up to 10-fold. Furthermore, APLN peptide also synergized with VEGF to promote the growth of hESC-derived endothelial cells. These studies identified APLN as a novel growth factor for hESC-derived hematopoietic and endothelial cells.

Phf14, a novel regulator of mesenchyme growth via platelet-derived growth factor (PDGF) receptor-α

The regulation of mesenchymal cell growth by signaling molecules plays an important role in maintaining tissue functions. Aberrant mesenchymal cell proliferation caused by disruption of this regulatory process leads to pathogenetic events such as fibrosis. In the current study we have identified a novel nuclear factor, Phf14, which controls the proliferation of mesenchymal cells by regulating PDGFRα expression. Phf14-null mice died just after birth due to respiratory failure. Histological analyses of the lungs of these mice showed interstitial hyperplasia with an increased number of PDGFRα(+) mesenchymal cells. PDGFRα expression was elevated in Phf14-null mesenchymal fibroblasts, resulting in increased proliferation. We demonstrated that Phf14 acts as a transcription factor that directly represses PDGFRα expression. Based on these results, we used an antibody against PDGFRα to successfully treat mouse lung fibrosis. This study shows that Phf14 acts as a negative regulator of PDGFRα expression in mesenchymal cells undergoing normal and abnormal proliferation, and is a potential target for new treatments of lung fibrosis.

The epicardium as a candidate for heart regeneration

The mammalian heart loses its regenerative capacity during early postnatal stages; consequently, individuals surviving myocardial infarction are at risk of heart failure due to excessive fibrosis and maladaptive remodeling. There is an urgent need, therefore, to develop novel therapies for myocardial and coronary vascular regeneration. The epicardium-derived cells present a tractable resident progenitor source with the potential to stimulate neovasculogenesis and contribute de novo cardiomyocytes. The ability to revive ordinarily dormant epicardium-derived cells lies in the identification of key stimulatory factors, such as Tβ4, and elucidation of the molecular cues used in the embryo to orchestrate cardiovascular development. myocardial infarction injury signaling reactivates the adult epicardium; understanding the timing and magnitude of these signals will enlighten strategies for myocardial repair.

Neural crest delamination and migration: from epithelium-to-mesenchyme transition to collective cell migration

After induction and specification in the ectoderm, at the border of the neural plate, the neural crest (NC) population leaves its original territory through a delamination process. Soon afterwards, the NC cells migrate throughout the embryo and colonize a myriad of tissues and organs where they settle and differentiate. The delamination involves a partial or complete epithelium-to-mesenchyme transition (EMT) regulated by a complex network of transcription factors including several proto-oncogenes. Studying the relationship between these genes at the time of emigration, and their individual or collective impact on cell behavior, provides valuable information about their role in EMT in other contexts such as cancer metastasis. During migration, NC cells are exposed to large number of positive and negative regulators that control where they go by generating permissive and restricted areas and by modulating their motility and directionality. In addition, as most NC cells migrate collectively, cell-cell interactions play a crucial role in polarizing the cells and interpreting external cues. Cell cooperation eventually generates an overall polarity to the population, leading to directional collective cell migration. This review will summarize our current knowledge on delamination, EMT and migration of NC cells using key examples from chicken, Xenopus, zebrafish and mouse embryos. Given the similarities between neural crest migration and cancer invasion, these cells may represent a useful model for understanding the mechanisms of metastasis.

ER71 directs mesodermal fate decisions during embryogenesis

Er71 mutant embryos are nonviable and lack hematopoietic and endothelial lineages. To further define the functional role for ER71 in cell lineage decisions, we generated genetically modified mouse models. We engineered an Er71-EYFP transgenic mouse model by fusing the 3.9 kb Er71 promoter to the EYFP reporter gene. Using FACS and transcriptional profiling, we examined the EYFP(+) population of cells in Er71 mutant and wild-type littermates. In the absence of ER71, we observed an increase in the number of EYFP-expressing cells, increased expression of the cardiac molecular program and decreased expression of the hemato-endothelial program, as compared with wild-type littermate controls. We also generated a novel Er71-Cre transgenic mouse model using the same 3.9 kb Er71 promoter. Genetic fate-mapping studies revealed that the ER71-expressing cells give rise to the hematopoietic and endothelial lineages in the wild-type background. In the absence of ER71, these cell populations contributed to alternative mesodermal lineages, including the cardiac lineage. To extend these analyses, we used an inducible embryonic stem/embryoid body system and observed that ER71 overexpression repressed cardiogenesis. Together, these studies identify ER71 as a critical regulator of mesodermal fate decisions that acts to specify the hematopoietic and endothelial lineages at the expense of cardiac lineages. This enhances our understanding of the mechanisms that govern mesodermal fate decisions early during embryogenesis.

Isolation and characterization of neural crest-derived stem cells from dental pulp of neonatal mice

Dental pulp stem cells (DPSCs) are shown to reside within the tooth and play an important role in dentin regeneration. DPSCs were first isolated and characterized from human teeth and most studies have focused on using this adult stem cell for clinical applications. However, mouse DPSCs have not been well characterized and their origin(s) have not yet been elucidated. Herein we examined if murine DPSCs are neural crest derived and determined their in vitro and in vivo capacity. DPSCs from neonatal murine tooth pulp expressed embryonic stem cell and neural crest related genes, but lacked expression of mesodermal genes. Cells isolated from the Wnt1-Cre/R26R-LacZ model, a reporter of neural crest-derived tissues, indicated that DPSCs were Wnt1-marked and therefore of neural crest origin. Clonal DPSCs showed multi-differentiation in neural crest lineage for odontoblasts, chondrocytes, adipocytes, neurons, and smooth muscles. Following in vivo subcutaneous transplantation with hydroxyapatite/tricalcium phosphate, based on tissue/cell morphology and specific antibody staining, the clones differentiated into odontoblast-like cells and produced dentin-like structure. Conversely, bone marrow stromal cells (BMSCs) gave rise to osteoblast-like cells and generated bone-like structure. Interestingly, the capillary distribution in the DPSC transplants showed close proximity to odontoblasts whereas in the BMSC transplants bone condensations were distant to capillaries resembling dentinogenesis in the former vs. osteogenesis in the latter. Thus we demonstrate the existence of neural crest-derived DPSCs with differentiation capacity into cranial mesenchymal tissues and other neural crest-derived tissues. In turn, DPSCs hold promise as a source for regenerating cranial mesenchyme and other neural crest derived tissues.

Pdgfrα and Flk1 are direct target genes of Mixl1 in differentiating embryonic stem cells

The Mixl1 homeodomain protein plays a key role in mesendoderm patterning during embryogenesis, but its target genes remain to be identified. We compared gene expression in differentiating heterozygous Mixl1(GFP/w) and homozygous null Mixl1(GFP/Hygro) mouse embryonic stem cells to identify potential downstream transcriptional targets of Mixl1. Candidate Mixl1 regulated genes whose expression was reduced in GFP+ cells isolated from differentiating Mixl1(GFP/Hygro) embryoid bodies included Pdgfrα and Flk1. Mixl1 bound to ATTA sequences located in the Pdgfrα and Flk1 promoters and chromatin immunoprecipitation assays confirmed Mixl1 occupancy of these promoters in vivo. Furthermore, Mixl1 transactivated the Pdgfrα and Flk1 promoters through ATTA sequences in a DNA binding dependent manner. These data support the hypothesis that Mixl1 directly regulates Pdgfrα and Flk1 gene expression and strengthens the position of Mixl1 as a key regulator of mesendoderm development during mammalian gastrulation.

Defining the earliest step of cardiovascular progenitor specification during embryonic stem cell differentiation

Mesp1, the earliest marker of cardiovascular development in vivo, is used to isolate and characterize multipotent cardiovascular progenitors during ESC differentiation.

During embryonic development and embryonic stem cell (ESC) differentiation, the different cell lineages of the mature heart arise from two types of multipotent cardiovascular progenitors (MCPs), the first and second heart fields. A key question is whether these two MCP populations arise from differentiation of a common progenitor. In this paper, we engineered Mesp1–green fluorescent protein (GFP) ESCs to isolate early MCPs during ESC differentiation. Mesp1-GFP cells are strongly enriched for MCPs, presenting the ability to differentiate into multiple cardiovascular lineages from both heart fields in vitro and in vivo. Transcriptional profiling of Mesp1-GFP cells uncovered cell surface markers expressed by MCPs allowing their prospective isolation. Mesp1 is required for MCP specification and the expression of key cardiovascular transcription factors. Isl1 is expressed in a subset of early Mesp1-expressing cells independently of Mesp1 and acts together with Mesp1 to promote cardiovascular differentiation. Our study identifies the early MCPs residing at the top of the cellular hierarchy of cardiovascular lineages during ESC differentiation.

PDGF function in diverse neural crest cell populations

Activation of platelet derived growth factor (PDGF) receptors causes context-dependent cellular responses, including proliferation and migration, and studies in model organisms have demonstrated that this receptor family (PDGFRα and PDGFRβ) is required in many mesenchymal and migratory cell populations during embryonic development. One of these migratory cell populations is the neural crest, which forms cranial bone and mesenchyme, sympathetic neurons and ganglia, melanocytes, and smooth muscle. Mice with disruption of PDGF signaling exhibit defects in some of these neural crest derivatives including the palate, aortic arch, salivary gland, and thymus. Although many of these neural crest defects were identified many years ago, the mechanism of action of PDGF in neural crest remains controversial. In this review, we examine the current knowledge of PDGF function during neural crest cell (NCC) development, focusing on its role in the formation of different neural crest-derived tissues and the implications for PDGF receptors in NCC-related human birth defects.

Twist1-induced invadopodia formation promotes tumor metastasis

The Twist1 transcription factor is known to promote tumor metastasis and induce Epithelial-Mesenchymal Transition (EMT). Here, we report that Twist1 is capable of promoting the formation of invadopodia, specialized membrane protrusions for extracellular matrix degradation. Twist1 induces PDGFRα expression, which in turn activates Src, to promote invadopodia formation. We show that Twist1 and PDGFRα are central mediators of invadopodia formation in response to various EMT-inducing signals. Induction of PDGFRα and invadopodia is essential for Twist1 to promote tumor metastasis. Consistent with PDGFRα being a direct transcriptional target of Twist1, coexpression of Twist1 and PDGFRα predicts poor survival in breast tumor patients. Therefore, invadopodia-mediated matrix degradation is a key function of Twist1 in promoting tumor metastasis.

Challenges and strategies for generating therapeutic patient-specific hemangioblasts and hematopoietic stem cells from human pluripotent stem cells

Recent characterization of hemangioblasts differentiated from human embryonic stem cells (hESC) has further confirmed evidence from murine, zebrafish and avian experimental systems that hematopoietic and endothelial lineages arise from a common progenitor. Such progenitors may provide a valuable resource for delineating the initial developmental steps of human hemato-endotheliogenesis, which is a process normally difficult to study due to the very limited accessibility of early human embryonic/fetal tissues. Moreover, efficient hemangioblast and hematopoietic stem cell (HSC) generation from patient-specific pluripotent stem cells has enormous potential for regenerative medicine, since it could lead to strategies for treating a multitude of hematologic and vascular disorders. However, significant scientific challenges remain in achieving these goals, and the generation of transplantable hemangioblasts and HSC derived from hESC currently remains elusive. Our previous work has suggested that the failure to derive engraftable HSC from hESC is due to the fact that current methodologies for differentiating hESC produce hematopoietic progenitors developmentally similar to those found in the human yolk sac, and are therefore too immature to provide adult-type hematopoietic reconstitution. Herein, we outline the nature of this challenge and propose targeted strategies for generating engraftable human pluripotent stem cell-derived HSC from primitive hemangioblasts using a developmental approach. We also focus on methods by which reprogrammed somatic cells could be used to derive autologous pluripotent stem cells, which in turn could provide unlimited sources of patient-specific hemangioblasts and HSC.

Three-dimensional cartography of hematopoietic clusters in the vasculature of whole mouse embryos

Hematopoietic cell clusters in the aorta of vertebrate embryos play a pivotal role in the formation of the adult blood system. Despite their importance, hematopoietic clusters have not been systematically quantitated or mapped because of technical limitations posed by the opaqueness of whole mouse embryos. Here, we combine an approach to make whole mouse embryos transparent, with multicolor marking, to allow observation of hematopoietic clusters using high-resolution 3-dimensional confocal microscopy. Our method provides the first complete map and temporal quantitation of all hematopoietic clusters in the mouse embryonic vasculature. We show that clusters peak in number at embryonic day 10.5, localize to specific vascular subregions and are heterogeneous, indicating a basal endothelial to non-basal (outer cluster) hematopoietic cell transition. Clusters enriched with the c-Kit(+)CD31(+)SSEA1(-) cell population contain functional hematopoietic progenitors and stem cells. Thus, three-dimensional cartography of transparent mouse embryos provides novel insight into the vascular subregions instrumental in hematopoietic progenitor/stem cell development, and represents an important technological advancement for comprehensive in situ hematopoietic cluster analysis.

Epicardial-myocardial signaling directing coronary vasculogenesis

The establishment of the coronary circulation is critical for the development of the embryonic heart. Over the last several years, there has been tremendous progress in elucidating the pathways that control coronary development. Interestingly, many of the pathways that regulate the development of the coronary vasculature are distinct from those governing vasculogenesis in the rest of the embryo. It is becoming increasingly clear that coronary development depends on a complex communication between the epicardium, the subepicardial mesenchyme, and the myocardium mediated in part by secreted growth factors. This communication coordinates the growth of the myocardium with the formation of the coronary vasculature. This review summarizes our present understanding of the role of these growth factors in the regulation of coronary development. Continued progress in this field holds the potential to lead to novel therapeutics for the treatment of patients with coronary artery disease.

Differentiation of mesodermal cells from pluripotent stem cells

The pluripotency of embryonic stem cells has been well demonstrated by a vast variety of studies showing the induction of differentiation into desired cell types that have the potential to be used not only in basic studies but also in medical applications. The induction of mesodermal cells, especially blood cells, from embryonic stem cells is notable from the point of view of transplantation, and the methods for this induction have improved over the last few years, with more defined culture conditions in place. Concurrently, the generation of induced pluripotent stem cells from somatic cells opens the possibility of autologous transplantation. In fact, there are a growing number of reports demonstrating that several mesodermal cells can be differentiated from induced pluripotent stem cells using the same methods used for embryonic stem cells. This review summarizes recent advances in the differentiation of mesodermal cells from embryonic stem cells and induced pluripotent stem cells.

Prospective identification, isolation, and systemic transplantation of multipotent mesenchymal stem cells in murine bone marrow

Mesenchymal stem cells (MSCs) are defined as cells that undergo sustained in vitro growth and can give rise to multiple mesenchymal lineages. Because MSCs have only been isolated from tissue in culture, the equivalent cells have not been identified in vivo and little is known about their physiological roles or even their exact tissue location. In this study, we used phenotypic, morphological, and functional criteria to identify and prospectively isolate a subset of MSCs (PDGFRα⁺Sca-1⁺CD45⁻TER119⁻) from adult mouse bone marrow. Individual MSCs generated colonies at a high frequency and could differentiate into hematopoietic niche cells, osteoblasts, and adipocytes after in vivo transplantation. Naive MSCs resided in the perivascular region in a quiescent state. This study provides the useful method needed to identify MSCs as defined in vivo entities.

BMP inhibition stimulates WNT-dependent generation of chondrogenic mesoderm from embryonic stem cells

WNT and bone morphogenetic protein (BMP) signaling are known to stimulate hemogenesis from pluripotent embryonic stem (ES) cells. However, osteochondrogenic mesoderm was generated effectively when BMP signaling is kept to a low level, while WNT signaling was strongly activated. When mesoderm specification from ES cells was exogenous factor dependent, WNT3a addition supported the generation of cardiomyogenic cells expressing lateral plate/extraembryonic mesoderm genes, and this process involved endogenous BMP activities. Exogenous BMP4 showed a similar effect that depended on endogenous WNT activities. However, neither factor induced robust chondrogenic activity. In support, ES cell differentiation in the presence of either WNT3a or BMP4 was associated with elevated levels of both Bmp and Wnt mRNAs, which appeared to provide sufficient levels of active BMPs and WNTs to promote the nonchondrogenic mesoderm specification. The osteochondrogenic mesoderm expressed PDGFRalpha, which also expressed genes that mark somite and rostral presomitic mesoderm. A strong WNT signaling was required for generating the mesodermal progeny, while approximately 50- to 100-fold lower concentration of WNT3a was sufficient for specifying axial mes(end)oderm. Thus, depending on the dose and cofactor (BMP), WNT signaling stimulates the generation of different biological activities and specification of different types of mesodermal progeny from ES cells.

WNT and BMP signaling are both required for hematopoietic cell development from human ES cells

Pluripotent human embryonic stem (hES) cells are capable of generating a variety of mature cell types, including hematopoietic cells in vitro. However, the precise signaling mechanisms that regulate hematopoietic cell development from hES cells are still poorly documented. Here we demonstrate that hemoangiogenic cells derived from hES cells are defined by their high-level expression of KDR and low-level expression of PDGFRalpha (KDR(+)PDGFRalpha(lo)), and that the generation of such cells from hES cells is significantly elevated by the addition of WNT3a or BMP4 during differentiation. The addition of WNT3a caused the induction of both hemogenic and angiogenic activities, and the addition of BMP4 preferentially increased angiogenic activity, all enriched in the KDR(+)PDGFRalpha(lo) cell fraction. Interestingly, WNT3a stimulation of hemoangiogenic cell genesis was virtually abolished in the presence of a BMP inhibitor. On the other hand, the BMP4-induced angiogenic cell genesis was suppressed by coaddition of a WNT inhibitor. Thus, WNT and BMP signaling coordinately direct the differentiation of hES cells into KDR(+)PDGFRalpha(lo) hemoangiogenic cells.

Both alleles of PSF1 are required for maintenance of pool size of immature hematopoietic cells and acute bone marrow regeneration

Hematopoietic stem cells (HSCs) have a very low rate of cell division in the steady state; however, under conditions of hematopoietic stress, these cells can begin to proliferate at high rates, differentiate into mature hematopoietic cells, and rapidly reconstitute ablated bone marrow (BM). Previously, we isolated a novel evolutionarily conserved DNA replication factor, PSF1 (partner of SLD5-1), from an HSC-specific cDNA library. In the steady state, PSF1 is expressed predominantly in CD34+KSL (c-kit+/Sca-1+/Lineage−) cells and progenitors, whereas high levels of PSF1 expression are induced in KSL cells after BM ablation. In 1-year-old PSF1+/− mice, the pool size of stem cells and progenitors is decreased. Whereas young PSF1+/− mutant mice develop normally, are fertile, and have no obvious differences in hematopoiesis in the steady state compared with wild-type mice, intravenous injection of 5-fluorouracil (5-FU) is lethal in PSF1+/− mice, resulting from a delay in induction of HSC proliferation during ablated BM reconstitution. Overexpression studies revealed that PSF1 regulates molecular stability of other GINS components, including SLD5, PSF2, and PSF3. Our data indicate that PSF1 is required for acute proliferation of HSCs in the BM of mice.

Localization of mesenchymal cells in adult mouse thymus: their abnormal distribution in mice with disorganization of thymic medullary epithelium

Thymic mesenchymal cells are known to be important for the development of the early fetal thymus into a functionally mature organ supporting T cell differentiation. We examined the expression of mesenchymal markers: pan-mesenchymal marker ER-TR7, desmin, alpha-smooth muscle actin (alpha-SMA), and alpha- and beta-chain of platelet-derived growth factor receptor (PDGFRalpha, PDGFRbeta) in thymi of normal adult mice. Desmin and ER-TR7 revealed specific staining in the capsule, septa, and perivascular cells. Most perivascular cells highly expressed PDGFRbeta at the same levels as desmin. Low expression of PDGFRalpha was detected in the capsule, intralobular septa, and some perivascular cells of normal adult thymi. alpha-SMA, used to identify vascular smooth muscle cells, was detectable on arterioles and some large venules but not on capillaries. Thus, desmin, PDGFRalpha, and PDGFRbeta were localized in the capsule, septa, and perivascular cells in thymus of adult mouse, although there were differences in the expression level among these markers. On the other hand, the expression of mesenchymal markers was detectable in the region of the thymic medullary epithelium of lymphotoxin beta receptor-deficient mice and plt/plt mice, indicating that mesenchymal cells were abnormally localized in the region. These results suggest that disorganization of the medullary epithelium may be accompanied by aberrant distribution of mesenchyme in adult mouse thymus.

Mesodermal patterning activity of SCL

The transcription factor SCL is critically required for establishing hematopoiesis and for proper endothelial development, but not for maintenance of hematopoietic stem cells or endothelial cells in the adult. Conflicting data exists regarding the developmental function of SCL, namely, whether it acts as a master regulator, actively patterning mesoderm toward hematopoietic development at the expense of other lineages, or is merely necessary to maintain the earliest committed hematopoietic precursors. To answer this question, we have engineered murine embryonic stem cells with a conditional doxycycline-inducible SCL transgene, and evaluated the effects of SCL expression at defined time points during in vitro development. Continual SCL expression during differentiation results in markedly increased hematopoiesis. By using pulses of gene expression over a 6-day differentiation time course, we map and characterize windows of SCL responsiveness. Notably, a pulse of SCL expression during early mesodermal patterning (48 to 72 hours of differentiation) promoted Flk1+ PDGFRalphaneg presumptive extraembryonic/lateral plate mesoderm at the expense of PDGFRalpha+ Flk1neg presumptive paraxial mesoderm. Consistent with this, the early pulse of SCL expression also expanded hematopoietic colony-forming cell numbers, while concomitantly repressing expression of paraxial and cardiac markers, and inhibited development of beating cardiomyocytes. By mixing the inducible embryonic stem cells with fluorescently labeled wild-type cells in chimeric embryoid bodies, we show that these effects of SCL are cell autonomous. These data support a master-regulatory role for SCL in mesodermal patterning, in addition to its established later roles in hematopoietic differentiation.

Efficient propagation of single cells Accutase-dissociated human embryonic stem cells

Human embryonic stem cells (hESCs) hold great promise for cell-based therapies and drug screening applications. However, growing and processing large quantities of undifferentiated hESCs is a challenging task. Conventionally, hESCs are passaged as clusters, which can limit their growth efficiency and use in downstream applications. This study demonstrates that hESCs can be passaged as single cells using Accutase, a formulated mixture of digestive enzymes. In contrast to trypsin treatment, Accutase treatment does not significantly affect the viability and proliferation rate of hESC dissociation into single cells. Accutase-dissociated single cells can be separated by FACS and proliferate as fully pluripotent hESCs. An Oct4-eGFP reporter construct engineered into hESCs was used to monitor the pluripotency of hESCs passaged with Accutase. Compared to collagenase-passaged hESCs, Accutase-treated cultures contained a larger proportion of undifferentiated (Oct4-positive) cells. Additionally, Accutase-passaged undifferentiated hESCs could be grown as monolayers without the need for monitoring and/or selection for quality hESC colonies.

Genetic evidence of PEBP2beta-independent activation of Runx1 in the murine embryo

The Runx1/AML1 transcription factor is required for the generation of hematopoietic stem cells and is one of the most frequently targeted genes in human leukemia. Runx1-deficient mice die around embryonic day (E)12.5 due to severe hemorrhage in the central nervous system and the complete absence of definitive hematopoietic cells. Since mice lacking the heterodimeric partner of Runx1, PEBP2beta/CBFbeta, are almost identical in phenotype to Runx1 (-/-) mice, PEBP2beta was believed to be essential for the in vivo function of Runx1. Here we show that transgenic overexpression of Runx1 partially rescues the lethal phenotype of PEBP2beta-deficient mice at E12.5. Some of the rescued mice escaped from the severe hemorrhage at E11.5-12.5, although definitive hematopoiesis was not restored. Thus, PEBP2beta-independent Runx1 activation can occur in vivo. This observation sheds new light on the mechanism(s) that regulate the activity of Runx transcription factors.

PDGF-A as an epicardial mitogen during heart development

In the developing heart, reciprocal interactions between the epicardium and myocardium drive further sublineage specification and ventricular chamber morphogenesis. Several observations suggest that the epicardium is a source of secreted factors that influence cardiomyocyte proliferation, and these factors may have other roles as well. However, the identity of these epicardial factors remains mostly unknown. We have identified platelet-derived growth factor-A (PDGF-A) as one of several mitogens expressed by the rat EMC epicardial cell line (epicardial mesothelial cells), by embryonic epicardium and myocardium during mouse heart development, and by adult epicardium. Expression of the cognate receptor gene Pdgfra was detected in the epicardium, although a low level of expression in myocardium could not be ruled out. To address the potential role of PDGF signaling in heart development, we mutated both PDGF receptor genes in the myocardial and mesodermal compartments of the heart; however, this did not result in an observable cardiac phenotype. This finding suggests that mesodermal PDGF signaling is not essential in heart development, although its role may be redundant with other signaling pathways. Indeed, our results demonstrate the presence of additional mitogens that may have such an overlapping role.

A nonneural epithelial domain of embryonic cranial neural folds gives rise to ectomesenchyme

The neural crest is generally believed to be the embryonic source of skeletogenic mesenchyme (ectomesenchyme) in the vertebrate head and other derivatives, including pigment cells and neurons and glia of the peripheral nervous system. Although classical transplantation experiments leading to this conclusion assumed that embryonic neural folds were homogeneous epithelia, we reported that embryonic cranial neural folds contain spatially and phenotypically distinct domains, including a lateral nonneural domain with cells that coexpress E-cadherin and PDGFRalpha and a thickened mediodorsal neuroepithelial domain where these proteins are reduced or absent. We now show that Wnt1-Cre is expressed in the lateral nonneural epithelium of rostral neural folds and that cells coexpressing Cre-recombinase and PDGFRalpha delaminate precociously from some of this nonneural epithelium. We also show that ectomesenchymal cells exhibit beta-galactosidase activity in embryos heterozygous for an Ecad-lacZ reporter knock- in allele. We conclude that a lateral nonneural domain of the neural fold epithelium, which we call "metablast," is a source of ectomesenchyme distinct from the neural crest. We suggest that closer analysis of the origin of ectomesenchyme might help to understand (i) the molecular-genetic regulation of development of both neural crest and ectomesenchyme lineages; (ii) the early developmental origin of skeletogenic and connective tissue mesenchyme in the vertebrate head; and (iii) the presumed origin of head and branchial arch skeletal and connective tissue structures during vertebrate evolution.

Disruption of PDGFRalpha-initiated PI3K activation and migration of somite derivatives leads to spina bifida

Spina bifida, or failure of the vertebrae to close at the midline, is a common congenital malformation in humans that is often synonymous with neural tube defects (NTDs). However, it is likely that other etiologies exist. Genetic disruption of platelet-derived growth factor receptor (PDGFR) alpha results in spina bifida, but the underlying mechanism has not been identified. To elucidate the cause of this birth defect in PDGFRalpha mutant embryos, we examined the developmental processes involved in vertebrae formation. Exposure of chick embryos to the PDGFR inhibitor imatinib mesylate resulted in spina bifida in the absence of NTDs. We next examined embryos with a tissue-specific deletion of the receptor. We found that loss of the receptor from chondrocytes did not recapitulate the spina bifida phenotype. By contrast, loss of the receptor from all sclerotome and dermatome derivatives or disruption of PDGFRalpha-driven phosphatidyl-inositol 3' kinase (PI3K) activity resulted in spina bifida. Furthermore, we identified a migration defect in the sclerotome as the cause of the abnormal vertebral development. We found that primary cells from these mice exhibited defects in PAK1 activation and paxillin localization. Taken together, these results indicate that PDGFRalpha downstream effectors, especially PI3K, are essential for cell migration of a somite-derived dorsal mesenchyme and disruption of receptor signaling in these cells leads to spina bifida.

Comparative transcriptome analysis of embryonic and adult stem cells with extended and limited differentiation capacity

Comparison of the transcriptomes of pluripotent embryonic stem cells, multipotent adult progenitor cells and lineage restricted mesenchymal stem cells identified a unique gene expression profile of multipotent adult progenitor cells.

Background

Recently, several populations of postnatal stem cells, such as multipotent adult progenitor cells (MAPCs), have been described that have broader differentiation ability than classical adult stem cells. Here we compare the transcriptome of pluripotent embryonic stem cells (ESCs), MAPCs, and lineage-restricted mesenchymal stem cells (MSCs) to determine their relationship.

Results

Applying principal component analysis, non-negative matrix factorization and k-means clustering algorithms to the gene-expression data, we identified a unique gene-expression profile for MAPCs. Apart from the ESC-specific transcription factor Oct4 and other ESC transcripts, some of them associated with maintaining ESC pluripotency, MAPCs also express transcripts characteristic of early endoderm and mesoderm. MAPCs do not, however, express Nanog or Sox2, two other key transcription factors involved in maintaining ESC properties. This unique molecular signature was seen irrespective of the microarray platform used and was very similar for both mouse and rat MAPCs. As MSC-like cells isolated under MAPC conditions are virtually identical to MSCs, and MSCs cultured in MAPC conditions do not upregulate MAPC-expressed transcripts, the MAPC signature is cell-type specific and not merely the result of differing culture conditions.

Conclusion

Multivariate analysis techniques clustered stem cells on the basis of their expressed gene profile, and the genes determining this clustering reflected the stem cells' differentiation potential in vitro. This comparative transcriptome analysis should significantly aid the isolation and culture of MAPCs and MAPC-like cells, and form the basis for studies to gain insights into genes that confer on these cells their greater developmental potency.

Functional skeletal muscle regeneration from differentiating embryonic stem cells

Little progress has been made toward the use of embryonic stem (ES) cells to study and isolate skeletal muscle progenitors. This is due to the paucity of paraxial mesoderm formation during embryoid body (EB) in vitro differentiation and to the lack of reliable identification and isolation criteria for skeletal muscle precursors. Here we show that expression of the transcription factor Pax3 during embryoid body differentiation enhances both paraxial mesoderm formation and the myogenic potential of the cells within this population. Transplantation of Pax3-induced cells results in teratomas, however, indicating the presence of residual undifferentiated cells. By sorting for the PDGF-alpha receptor, a marker of paraxial mesoderm, and for the absence of Flk-1, a marker of lateral plate mesoderm, we derive a cell population from differentiating ES cell cultures that has substantial muscle regeneration potential. Intramuscular and systemic transplantation of these cells into dystrophic mice results in extensive engraftment of adult myofibers with enhanced contractile function without the formation of teratomas. These data demonstrate the therapeutic potential of ES cells in muscular dystrophy.

Isolation of cardiac cells from E8.5 yolk sac by ALCAM (CD166) expression

It is known that the adhesion molecule ALCAM (CD166) mediates metastasis of malignant cells and organogenesis in embryos. We show here that embryonic day 8.5 (E8.5) murine yolk sac cells express ALCAM protein and that ALCAM expression can be used to define endothelial and cardiac precursors from hematopoietic precursors in E8.5 yolk sacs. ALCAM high+ cells exclusively give rise to endothelial and cardiac cells in matrigel assays but generate no hematopoietic colonies in methylcellulose assays. ALCAM low+ and ALCAM- populations predominantly give rise to hematopoietic cells in methylcellulose, but do not generate any cell clusters in matrigel. The ALCAM high+ population contains both Flk-1+ and Flk-1- cells. The former population exclusively contains endothelial cells whereas the latter give rise to cardiac cells when cultured on OP9 stromal cells. We also show that cardiac lineage marker genes such as Nkx-2.5, and the endothelial marker VE-cadherin are expressed in the ALCAM high+ fraction, whereas the hematopoietic marker GATA1 and Runx1 are expressed in the ALCAM low+/- fraction. However, we did not detect expression of the cardiac structural protein cTn-T in cells from yolk sac cells until these had had been differentiated on OP9 for 5 days. Altogether, these results indicate that cells retaining a potential to differentiate to the cardiac lineage are present in E8.5 yolk sacs and can be isolated as ALCAM high+, Flk-1- cells. Our report provides novel insights into the origin and differentiation process of cardiac cells in the formation of the circulatory system.

Neuroepithelial cells supply an initial transient wave of MSC differentiation

Mesenchymal stem cells (MSCs) are defined as cells that undergo sustained in vitro growth and are able to give rise to multiple mesenchymal lineages. Although MSCs are already used in regenerative medicine little is known about their in vivo behavior and developmental derivation. Here, we show that the earliest wave of MSC in the embryonic trunk is generated from Sox1+ neuroepithelium but not from mesoderm. Using lineage marking by direct gfp knock-in and Cre-recombinase mediated lineage tracing, we provide evidence that Sox1+ neuroepithelium gives rise to MSCs in part through a neural crest intermediate stage. This pathway can be distinguished from the pathway through which Sox1+ cells give rise to oligodendrocytes by expression of PDGFRbeta and A2B5. MSC recruitment from this pathway, however, is transient and is replaced by MSCs from unknown sources. We conclude that MSC can be defined as a definite in vivo entity recruited from multiple developmental origins.

Coexpression of platelet-derived growth factor receptor alpha and fetal liver kinase 1 enhances cardiogenic potential in embryonic stem cell differentiation in vitro

Nascent mesodermal cells derived from EB5 embryonic stem (ES) cells were sorted in terms of cardiogenic potential on the basis of their expression levels of platelet-derived growth factor receptor alpha (PDGFRalpha) and fetal liver kinase 1 (Flk-1). The sorted cells were cocultured with OP9 stromal cells to induce terminal differentiation into contractile cardiac colonies. A significant number of cardiac colonies were found in the Flk-1+/PDGFRalpha+ fraction. The enrichment double-positive fraction produced approximately fivefold more cardiac colonies than the Flk-1+/PDGFRalpha- fraction and 10-fold more than the Flk-1-/PDGFRalpha+ fraction. To investigate the involvement of these markers in embryonic cardiogenesis, the cells that disseminated from the E7.5-7.75 embryos were fractionated and seeded on OP9 cells. The cardiogenic potential was markedly enhanced in the Flk-1+/PDGFRalpha+ fraction. These results suggest that some of the precursor cells coexpressing these markers are selectively involved in cardiogenic events, and that the identification of ES-cell-derived precursors with these markers will contribute to the effective production of cardiomyocytes for cell therapies.

Mesenchymal cells are required for functional development of thymic epithelial cells

Epithelial-mesenchymal interactions have essential roles in thymus organogenesis. Mesenchymal cells are known to be required for epithelial cell proliferation. However, the contribution of mesenchymal cells to thymic epithelial cell differentiation is still unclear. In the present study, we have investigated the roles of mesenchymal cells in functional development of epithelial cells in the thymus anlage in patch (ph) mutant mice, which have a primarily defect in mesenchymal cells caused by the absence of platelet-derived growth factor receptor alpha expression. In the ph/ph thymus anlage, T cell progenitors migrate normally among the epithelial cells, however, they are severely impaired to proliferate and differentiate to CD25-positive cells. Epithelial cells of the ph/ph thymus anlage show severely impaired proliferation and expression of functional molecules, such as SCF, Delta-like 4 and MHC class II, which have crucial roles in T cell development. Moreover, the cultured ph/ph thymus anlage fails to develop into a mature organ supporting full T cell development. Addition of intact thymic mesenchymal cells to organ culture induces development of the ph/ph thymus anlage. In the cultured lobes, added mesenchymal cells contribute to form not only the capsule but also the meshwork structure mingled with epithelial cells. Our present results strongly suggest the roles of mesenchymal cells in functional development of epithelial cells in thymus organogenesis. In addition, our data suggest that mesenchymal cells are required to create the thymic microenvironment and to maintain epithelial architecture and function.

Evidence that a late-emerging population of trunk neural crest cells forms the plastron bones in the turtle Trachemys scripta

The origin of the turtle plastron is not known, but these nine bones have been homologized to the exoskeletal components of the clavicles, the interclavicular bone, and gastralia. Earlier evidence from our laboratory showed that the bone-forming cells of the plastron were positive for HNK-1 and PDGFRalpha, two markers of the skeletogenic neural crest. This study looks at the embryonic origin of these plastron-forming cells. We show that the HNK-1+ cells are also positive for p75 and FoxD3, confirming their neural crest identity, and that they originate from the dorsal neural tube of stage 17 turtle embryos, several days after the original wave of neural crest cells have migrated and differentiated. DiI studies show that these are migratory cells, and they can be observed in the lateral regions of the embryo and can be seen forming intramembranous bone in the ventral (plastron) regions. Before migrating ventrally, these late-emerging neural crest cells reside for over a week in a carapacial staging area above the neural tube and vertebrae. It is speculated that this staging area is where they lose the inability to form skeletal cells.

Potential of dental mesenchymal cells in developing teeth

The tooth, composed of dentin and enamel, develops through epithelium-mesenchyme interactions. Neural crest (NC) cells contribute to the dental mesenchyme in the developing tooth and differentiate into dentin-secreting odontoblasts. NC cells are known to differentiate into chondrocytes and osteoblasts in the craniofacial region. However, it is not clear whether the dental mesenchymal cells in the developing tooth possess the potential to differentiate into a lineage(s) other than the odontoblast lineage. In this study, we prepared mesenchymal cells from E13.5 tooth germ cells and assessed their potential for differentiation in culture. They differentiated into odontoblasts, chondrocyte-like cells, and osteoblast-like cells. Their derivation was confirmed by tracing NC-derived cells as LacZ(+) cells using P0-Cre/Rosa26R mice. Using the flow cytometry-fluorescent di-beta-D-galactosidase system, which makes it possible to detect LacZ(+) cells as living cells, cell surface molecules of dental mesenchymal cells were characterized. Large number of LacZ(+) NC-derived cells expressed platelet-derived growth factor receptor alpha and integrins. Taken together, these results suggest that NC-derived cells with the potential to differentiate into chondrocyte-like and osteoblast-like cells are present in the developing tooth, and these cells may contribute to tooth organogenesis.

Platelet derived growth factor receptor alpha is essential for establishing a microenvironment that supports definitive erythropoiesis

The hematopoietic system undergoes a qualitative change during the embryogenesis of most vertebrates. It is designated as the shift of primitive to definitive hematopoiesis and suitable microenvironment must be established to support this shift. While studying the role of platelet derived growth factor receptor alpha (PDGFR alpha) in embryonic hematopoiesis, we found that it was expressed in a stromal cell component of liver, a major site of this shift, but not in the yolk sac, the site of primitive hematopoiesis. Thus, we considered that development of PDGFRalpha positive stromal cells is an essential requirement for this shift. Without PDFGRalpha positive cell component, erythropoiesis was suppressed in the culture of fetal liver. Moreover, injection of an antagonistic anti-PDGFRalpha monoclonal antibody during embryogenesis suppressed the production of definitive erythrocytes. These indicated that PDGF exerts its effect on a subset of stromal components to prepare a microenvironment that can support the definitive erythropoiesis.

Physiological pathway of differentiation of hematopoietic stem cell population into mural cells

Endothelial cells (ECs), which are a major component of blood vessels, have been reported to develop in adulthood from hematopoietic cell populations, especially those of the monocyte lineage. Here we show that mural cells (MCs), another component of blood vessels, develop physiologically during embryogenesis from a hematopoietic stem cell (HSC) population, based on the in vitro culture of HSCs and histological examination of acute myeloid leukemia 1 mutant embryos, which lack HSCs. As in the embryo, HSCs in adult bone marrow differentiate into CD45⁺CD11b⁺ cells before differentiating into MCs. Moreover, CD45⁺CD11b⁺ cells are composed of two populations, CD11b^high and CD11b^low cells, both of which can differentiate into MCs as well as ECs. Interestingly, in a murine ischemia model, MCs and ECs derived from the CD11b^low population had a long-term potential to contribute to the formation of newly developed blood vessels in vivo compared with the CD11^high population, which could not. Moreover, injection of the CD11b^high population induced leaky blood vessels, but the CD11b^low population did not. With respect to the permeability of vessels, we found that angiopoietin 1, which is a ligand for Tie2 receptor tyrosine kinase expressed on ECs and is suggested to induce cell adhesion between ECs and MCs, is produced by the CD11b^low population and plays a critical role in the formation of nonleaky vessels. These observations suggested that the CD11^low cell population serves as a good source of cells for in vivo blood vessel regeneration.

Microarray analysis of PDGFR alpha+ populations in ES cell differentiation culture identifies genes involved in differentiation of mesoderm and mesenchyme including ARID3b that is essential for development of embryonic mesenchymal cells

An inherent difficulty in using DNA microarray technology on the early mouse embryo is its relatively small size. In this study, we investigated whether use of ES cell differentiation culture, which has no theoretical limit in the number of cells that can be generated, can improve this situation. Seven distinct ES-cell-derived populations were analyzed by DNA microarray and examined for genes whose distribution patterns are similar to those of PDGFRalpha, a gene implicated in differentiation of mesoderm/mesenchymal lineages. Using software developed in our laboratory, we formed a group of 30 genes which showed the highest similarity to PDGFRalpha, 18 of these genes were shown to be involved in development of either mesodermal, mesenchymal or neural crest cells. This list also contains several genes whose role in embryogenesis has not yet been fully identified. One such molecule is mARID3b. The mARID3b expression is found in the paraxial mesoderm and cranial mesenchyme. mARID3b-null mouse showed early embryonic lethality, and most phenotypes of this mutant appear to develop from a failure to generate a sufficient number of cranial mesenchymal cells. These results demonstrate the potential use of ES cell differentiation culture in identifying novel genes playing an indispensable role in embryogenesis.

In Vitro Modeling of Paraxial and Lateral Mesoderm Differentiation Reveals Early Reversibility

Endothelial cells (ECs) are thought to be derived mainly from the vascular endothelial growth factor receptor 2 (VEGFR-2)+ lateral mesoderm during early embryogenesis. In this study, we specified several pathways for EC differentiation using a murine embryonic stem (ES) cell differentiation culture system that is a model for cellular processes during early embryogenesis. Based on the results of in vitro fate analysis, we show that, in the main pathway, committed ECs are differentiated through the VEGFR-2+ platelet-derived growth factor receptor alpha (PDGFR-alpha)- single-positive (VSP) population that is derived from the VEGFR-2+ PDGFR-alpha+ double-positive (DP) population. This major differentiation course was also confirmed using DNA microarray analysis. In addition to this main pathway, however, ECs also can be generated from the VEGFR-2- PDGFR-alpha+ single-positive (PSP) population, which represents the paraxial mesodermal lineage and is also derived from the DP population. Our results strongly suggest that, even after differentiation from the common progenitor DP population into the VSP and PSP populations, these two populations continue spontaneous switching of their surface phenotype, which results in switching of their eventual fates. The rate of this interlineage conversion between VSP and PSP is unexpectedly high. Because of this potential to undergo fate switch, we conclude that ECs can be generated via multiple pathways in in vitro ES cell differentiation.

Growth factors in lung development

Organized and coordinated lung development follows transcriptional regulation of a complex set of cell-cell and cell-matrix interactions resulting in a blood-gas interface ready for physiologic gas exchange at birth. Transcription factors, growth factors, and various other signaling molecules regulate epithelial-mesenchymal interactions by paracrine and autocrine mechanisms. Transcriptional control at the earliest stages of lung development results in cell differentiation and cell commitment in the primitive lung bud, in essence setting up a framework for pattern formation and branching morphogenesis. Branching morphogenesis results in the formation of the conductive airway system, which is critical for alveolization. Lung development is influenced at all stages by spatial and temporal distribution of various signaling molecules and their receptors and also by the positive and negative control of signaling by paracrine, autocrine, and endocrine mechanisms. Lung bud formation, cell differentiation, and its interaction with the splanchnic mesoderm are regulated by HNF-3beta, Shh, Nkx2.1, HNF-3/Forkhead homolog-8 (HFH-8), Gli, and GATA transcription factors. HNF-3beta regulates Nkx2.1, a transcription factor critical to the formation of distal pulmonary structures. Nkx2.1 regulates surfactant protein genes that are important for the development of alveolar stability at birth. Shh, produced by the foregut endoderm, regulates lung morphogenesis signaling through Gli genes expressed in the mesenchyme. FGF10, produced by the mesoderm, regulates branching morphogenesis via its receptors on the lung epithelium. Alveolization and formation of the capillary network are influenced by various factors that include PDGF, vascular endothelial growth factor (VEGF), and retinoic acid. Epithelial-endothelial interactions during lung development are important in establishing a functional blood-gas interface. The effects of various growth factors on lung development have been demonstrated by gain- or loss-of-function studies in null mutant and transgenic mice models. Understanding the role of growth factors and various other signaling molecules and their cellular interactions in lung development will provide us with new insights into the pathogenesis of bronchopulmonary dysplasia and disorders of lung morphogenesis.

ALCAM (CD166) Is a Surface Marker for Early Murine Cardiomyocytes

ALCAM (activated leukocyte cell adhesion molecule, CD166) belongs to the immunoglobulin superfamily and is involved in axon guidance, hematopoiesis, immune response and tumor metastasis. During embryogenesis, mRNA encoding ALCAM was expressed in the cardiac crescent and the neural groove at embryonic day (E) 7.75 and predominately in the tubular heart at E8.5. A newly generated monoclonal antibody against the ALCAM molecule (ALC-48) exclusively stained cardiomyocytes at E8.25-10.5. However, ALCAM expression was lost by cardiomyocytes by E12.5 and its expression shifts to a variety of organs during later stages. ALCAM was found to be a prominent surface marker for cardiomyocytes in early embryonic hearts. The transient expression of ALCAM during early developmental stages marks specific developmental stages in cardiomyocyte differentiation.

Platelet-derived growth factors in the developing avian heart and maturating coronary vasculature

Platelet-derived growth factors (PDGFs) are important in embryonic development. To elucidate their role in avian heart and coronary development, we investigated protein expression patterns of PDGF-A, PDGF-B, and the receptors PDGFR-alpha and PDGFR-beta using immunohistochemistry on sections of pro-epicardial quail-chicken chimeras of Hamburger and Hamilton (HH) 28-HH35. PDGF-A and PDGFR-alpha were expressed in the atrial septum, sinus venosus, and throughout the myocardium, with PDGFR-alpha retreating to the trabeculae at later stages. Additionally, PDGF-A and PDGFR-alpha were present in outflow tract cushion mesenchyme and myocardium, respectively. Small cardiac nerves and (sub)epicardial cells expressed PDGF-B and PDGFR-beta. Furthermore, endothelial cells expressed PDGF-B, while vascular smooth muscle cells and interstitial epicardium-derived cells expressed PDGFR-beta, indicating a role in coronary maturation. PDGF-B is also present in ventricular septal development, in the absence of any PDGFR. Epicardium-derived cells in the atrioventricular cushions expressed PDGFR-beta. We conclude that all four proteins are involved in myocardial development, whereas PDGF-B and PDGFR-beta are specifically important in coronary maturation.

Loss of Tie2 receptor compromises embryonic stem cell-derived endothelial but not hematopoietic cell survival

Tie2 is a receptor-type tyrosine kinase expressed on hematopoietic stem cells and endothelial cells. We used cultured embryonic stem (ES) cells to determine the function of Tie2 during early vascular development and hematopoiesis. Upon differentiation, the ES cell-derived Tie2+ Flk1+ fraction was enriched for hematopoietic and endothelial progenitor cells. To investigate lymphatic differentiation, we used a monoclonal antibody against LYVE-1 and found that LYVE-1+ cells derived from Tie2+ Flk1+ cells possessed various characteristics of lymphatic endothelial cells. To determine whether Tie2 played a role in this process, we analyzed differentiation of Tie2-/- ES cells. Although the initial numbers of LYVE-1+ and PECAM-1+ cells derived from Tie2-/- cells did not vary significantly, the number of both decreased dramatically upon extended culturing. Such decreases were rescued by treatment with a caspase inhibitor, suggesting that reductions were due to apoptosis as a consequence of a lack of Tie2 signaling. Interestingly, Tie2-/- ES cells did not show measurable defects in development of the hematopoietic system, suggesting that Tie2 is not essential for hematopoietic cell development.

Ascorbic acid promotes osteoclastogenesis from embryonic stem cells

Ascorbic acid (AA) is known to regulate cell differentiation; however, the effects of AA on osteoclastogenesis, especially on its early stages, remain unclear. To examine the effects of AA throughout the process of osteoclast development, we established a culture system in which tartrate-resistant acid phosphate (TRAP)-positive osteoclasts were induced from embryonic stem cells without stromal cell lines. In this culture system, the number of TRAP-positive cells was strongly increased by the addition of AA during the development of osteoclast precursors, and reducing agents, 2-mercaptoethanol, monothioglycerol, and dithiothreitol, failed to substitute for AA. The effect of AA was stronger when it was added during the initial 4 days during the development of mesodermal cells than when it was added during the last 4 days. On day 4 of the culture period, AA increased the total cell recovery and frequency of osteoclast precursors. Magnetic cell sorting using anti-Flk-1 antibody enriched osteoclast precursors on day 4, and the proportion of Flk-1-positive cells but not that of platelet-derived growth factor receptor alpha-positive cells was increased by the addition of AA. These results suggest that AA might promote osteoclastogenesis of ES cells through increasing Flk-1-positive cells, which then give rise to osteoclast precursors.

Presence and distribution of neural crest-derived cells in the murine developing thymus and their potential for differentiation

Neural crest (NC) cells are multipotent cells that can differentiate into melanocytes, neurons, glias and myofibroblasts. They migrate into the fetal thymus on embryonic day (E) 12 in mice and may participate in thymic organogenesis. Although the abnormality of migration and distribution of NC cells in the thymus results in immunodeficiency, the spatial and temporal presence of their progeny cells has not been defined in detail. In this study, we traced NC-derived cells based on the myelin protein zero gene promoter-Cre-mediated excision. We demonstrated that large numbers of NC-derived cells in the thymus were detected on E11.5 to E16.5 but rarely on E17.5. A colony formation assay of single thymic cells demonstrated that multipotent cells with the potential to differentiate into melanocytes, neurons and/or glias were present in the E14.5 and E15.5 but not in the E17.5 fetal thymus. Furthermore, we confirmed that these multipotent cells were NC-derived cells. Taken together, these findings imply that multipotent NC-derived cells are present in the developing thymus, but rarely in this organ at a later stage, suggesting that NC-derived cells may play roles in thymic organogenesis at an early embryonic stage.

Characterization of Progenitor-Cell-Specific Genes Identified by Subtractive Suppression Hybridization

We have utilized subtractive suppression hybridization (SSH) to identify differentially expressed genes present in either neuroepithelial (NEP) cells or glial restricted precursor (GRP) cells. Eighteen clones enriched in GRP cells and 28 in NEP cells were identified. Five of the GRP-specific clones (tenascin C, cystatin C, GABA transporter 3, extracellular matrix molecule 2 and H2-4) were characterized further, and their glial specificity was confirmed by RT-PCR, in situ hybridization and immunocytochemistry. H2-4 (an expressed sequence tag) was shown to be part of chondroitin sulfate proteoglycan 3. Overall, our results show that SSH can be used to identify lineage- and stage-specific markers and that extracellular matrix molecules likely play important roles in the migration and differentiation of GRPs.