The culture of mouse embryonic stem cells and formation of embryoid bodies

Super-enhancers include classical enhancers and facilitators to fully activate gene expression

Super-enhancers are compound regulatory elements that control expression of key cell identity genes. They recruit high levels of tissue-specific transcription factors and co-activators such as the Mediator complex and contact target gene promoters with high frequency. Most super-enhancers contain multiple constituent regulatory elements, but it is unclear whether these elements have distinct roles in activating target gene expression. Here, by rebuilding the endogenous multipartite α-globin super-enhancer, we show that it contains bioinformatically equivalent but functionally distinct element types: classical enhancers and facilitator elements. Facilitators have no intrinsic enhancer activity, yet in their absence, classical enhancers are unable to fully upregulate their target genes. Without facilitators, classical enhancers exhibit reduced Mediator recruitment, enhancer RNA transcription, and enhancer-promoter interactions. Facilitators are interchangeable but display functional hierarchy based on their position within a multipartite enhancer. Facilitators thus play an important role in potentiating the activity of classical enhancers and ensuring robust activation of target genes.

Controlled Plasma Membrane Delivery of FGFR1 and Modulation of Signaling by a Novel Regulated Anterograde RTK Transport Pathway

How human FGFR1 localizes to the PM is unknown. Currently, it is assumed that newly synthesized FGFR1 is continuously delivered to the PM. However, evidence indicates that FGFR1 is mostly sequestered in intracellular post-Golgi vesicles (PGVs) under normal conditions. In this report, live-cell imaging and total internal reflection fluorescence microscopy (TIRFM) were employed to study the dynamics of these FGFR1-positive vesicles. We designed recombinant proteins to target different transport components to and from the FGFR1 vesicles. Mouse embryoid bodies (mEBs) were used as a 3D model system to confirm major findings. Briefly, we found that Rab2a, Rab6a, Rab8a, RalA and caveolins are integral components of FGFR1-positive vesicles, representing a novel compartment. While intracellular sequestration prevented FGFR1 activation, serum starvation and hypoxia stimulated PM localization of FGFR1. Under these conditions, FGFR1 C-terminus acts as a scaffold to assemble proteins to (i) inactivate Rab2a and release sequestration, and (ii) assemble Rab6a for localized activation of Rab8a and RalA-exocyst to deliver the receptor to the PM. This novel pathway is named Regulated Anterograde RTK Transport (RART). This is the first instance of RTK regulated through control of PM delivery.

HMCES modulates the transcriptional regulation of nodal/activin and BMP signaling in mESCs

Despite the fundamental roles of TGF-β family signaling in cell fate determination in all metazoans, the mechanism by which these signals are spatially and temporally interpreted remains elusive. The cell-context-dependent function of TGF-β signaling largely relies on transcriptional regulation by SMAD proteins. Here, we discover that the DNA repair-related protein, HMCES, contributes to early development by maintaining nodal/activin- or BMP-signaling-regulated transcriptional network. HMCES binds with R-SMAD proteins, co-localizing at active histone marks. However, HMCES chromatin occupancy is independent on nodal/activin or BMP signaling. Mechanistically, HMCES competitively binds chromatin to limit binding by R-SMAD proteins, thereby forcing their dissociation and resulting in repression of their regulatory effects. In Xenopus laevis embryo, hmces KD causes dramatic development defects with abnormal left-right axis asymmetry along with increasing expression of lefty1. These findings reveal HMCES transcriptional regulatory function in the context of TGF-β family signaling.

hnRNPLL controls pluripotency exit of embryonic stem cells by modulating alternative splicing of Tbx3 and Bptf

The regulatory circuitry underlying embryonic stem (ES) cell self-renewal is well defined, but how this circuitry is disintegrated to enable lineage specification is unclear. RNA-binding proteins (RBPs) have essential roles in RNA-mediated gene regulation, and preliminary data suggest that they might regulate ES cell fate. By combining bioinformatic analyses with functional screening, we identified seven RBPs played important roles for the exit from pluripotency of ES cells. We characterized hnRNPLL, which mainly functions as a global regulator of alternative splicing in ES cells. Specifically, hnRNPLL promotes multiple ES cell-preferred exon skipping events during the onset of ES cell differentiation. hnRNPLL depletion thus leads to sustained expression of ES cell-preferred isoforms, resulting in a differentiation deficiency that causes developmental defects and growth impairment in hnRNPLL-KO mice. In particular, hnRNPLL-mediated alternative splicing of two transcription factors, Bptf and Tbx3, is important for pluripotency exit. These data uncover the critical role of RBPs in pluripotency exit and suggest the application of targeting RBPs in controlling ES cell fate.

Methods to label, image, and analyze the complex structural architectures of microvascular networks

Microvascular networks play key roles in oxygen transport and nutrient delivery to meet the varied and dynamic metabolic needs of different tissues throughout the body, and their spatial architectures of interconnected blood vessel segments are highly complex. Moreover, functional adaptations of the microcirculation enabled by structural adaptations in microvascular network architecture are required for development, wound healing, and often invoked in disease conditions, including the top eight causes of death in the Unites States. Effective characterization of microvascular network architectures is not only limited by the available techniques to visualize microvessels but also reliant on the available quantitative metrics that accurately delineate between spatial patterns in altered networks. In this review, we survey models used for studying the microvasculature, methods to label and image microvessels, and the metrics and software packages used to quantify microvascular networks. These programs have provided researchers with invaluable tools, yet we estimate that they have collectively attained low adoption rates, possibly due to limitations with basic validation, segmentation performance, and nonstandard sets of quantification metrics. To address these existing constraints, we discuss opportunities to improve effectiveness, rigor, and reproducibility of microvascular network quantification to better serve the current and future needs of microvascular research.

Stress Forces First Lineage Differentiation of Mouse Embryonic Stem Cells; Validation of a High-Throughput Screen for Toxicant Stress

Mouse Embryonic Stem Cells (mESCs) are unique in their self-renewal and pluripotency. Hypothetically, mESCs model gestational stress effects or stresses of in vitro fertilization/assisted reproductive technologies or drug/environmental exposures that endanger embryos. Testing mESCs stress responses should diminish and expedite in vivo embryo screening. Transgenic mESCs for green fluorescent protein (GFP) reporters of differentiation use the promoter for platelet-derived growth factor receptor (Pdgfr)a driving GFP expression to monitor hyperosmotic stress-forced mESC proliferation decrease (stunting), and differentiation increase that further stunts mESC population growth. In differentiating mESCs Pdgfra marks the first-lineage extraembryonic primitive endoderm (ExEndo). Hyperosmotic stress forces mESC differentiation gain (Pdgfra-GFP) in monolayer or three-dimensional embryoid bodies. Despite culture with potency-maintaining leukemia inhibitory factor (LIF), stress forces ExEndo as assayed using microplate readers and validated by coexpression of Pdgfra-GFP, Disabled 2 (Dab2), and laminin by immunofluorescence and GFP protein and Dab2 by immunoblot. In agreement with previous reports, Rex1 and Oct4 loss was inversely proportional to increased Pdgfra-GFP mESC after treatment with high hyperosmotic sorbitol despite LIF. The increase in subpopulations of Pdgfra-GFP⁺ cells>background at ∼23% was similar to the previously reported ∼25% increase in Rex1-red fluorescent protein (RFP)-negative subpopulation at matched high sorbitol doses. By microplate reader, there is a ∼7-11-fold increase in GFP at a high nonmorbid and a morbid dose despite LIF, compared with LIF alone. By flow cytometry (FACS), the subpopulation of Pdgfra-GFP⁺ cells>background increases ∼8-16-fold at these doses. Taken together, the microplate, FACS, immunoblot, and immunofluorescence data suggest that retinoic acid or hyperosmotic stress forces dose-dependent differentiation whether LIF is present or not and this is negatively correlated with and possibly compensates for stress-forced diminished ESC population expansion and potency loss.

Hepatocyte-like cells generated by direct reprogramming from murine somatic cells can repopulate decellularized livers

Direct reprogramming represents an easy technique to generate induced hepatocytes (iHeps) from somatic cells. However, current protocols are accompanied by several drawbacks as iHeps are heterogenous and lack fully mature phenotypes of primary hepatocytes. Here, we established a polycistronic expression system to induce the direct reprogramming of mouse embryonic fibroblasts towards hepatocytes. The resulting iHeps are homogenous and display key properties of primary hepatocytes, such as expression of hepatocyte markers, albumin secretion, and presence of liver transaminases. iHeps also possess the capacity to repopulate decellularized liver tissue and exhibit enhanced hepatic maturation. As such, we present a novel strategy to generate homogenous and functional iHeps for applications in tissue engineering and cell therapy.

Arrayed mutant haploid embryonic stem cell libraries facilitate phenotype-driven genetic screens

Forward genetic screens using mammalian embryonic stem (ES) cells have identified genes required for numerous cellular processes. However, loss-of-function screens are more difficult to conduct in diploid cells because, in most cases, both alleles of a gene must be mutated to exhibit a phenotype. Recently, mammalian haploid ES cell lines were successfully established and applied to several recessive genetic screens. However, all these screens were performed in mixed pools of mutant cells and were mainly based on positive selection. In general, negative screening is not easy to apply to these mixed pools, although quantitative deep sequencing of mutagen insertions can help to identify some ‘missing’ mutants. Moreover, the interplay between different mutant cells in the mixed pools would interfere with the readout of the screens. Here, we developed a method for rapidly generating arrayed haploid mutant libraries in which the proportion of homozygous mutant clones can reach 85%. After screening thousands of individual mutant clones, we identified a number of novel factors required for the onset of differentiation in ES cells. A negative screen was also conducted to discover mutations conferring cells with increased sensitivity to DNA double-strand breaks induced by the drug doxorubicin. Both of these screens illustrate the value of this system.

Purging Exhausted Virus-Specific CD8 T Cell Phenotypes by Somatic Cell Reprogramming

Cytotoxic T cells are critical in controlling virus infections. However, continuous antigen stimulation and negative regulatory factors cause CD8 T cells to enter a dysfunctional state (T cell exhaustion), resulting in viral persistence. We hypothesized that the exhausted T cell state could be molecularly rejuvenated using a somatic cell reprogramming technology, which is technically able to convert any types of cells to induced pluripotent stem cells (iPSCs), to regenerate functional T cells capable of purging chronic infection. We generated a new mouse line (B6/129^OKSM) in which every somatic cell contains four doxycycline-inducible reprogramming genes (Oct4, Klf4, Sox2, and c-Myc: OKSM), and infected them with lymphocytic choriomeningitis virus (LCMV) clone 13 to establish chronic infection. Exhausted LCMV-specific T cells isolated by flow sorting were successfully reprogrammed ex vivo into iPSCs in the presence of doxycycline. Upon injection into blastocysts and subsequent transfer into foster females, the reprogrammed cells differentiated into functional naive T cells that maintained their original antigen specificity. These results provide proof of concept that somatic cell reprogramming of exhausted T cells into iPSCs can erase imprints of their previous exhausted state and in turn regenerate functional virus-specific T cells.

Small Molecule Antagonist of Cell Surface Glycosaminoglycans Restricts Mouse Embryonic Stem Cells in a Pluripotent State

Recently, the field of stem cell-based regeneration has turned its attention toward chemical approaches for controlling the pluripotency and differentiation of embryonic stem cells (ESCs) using drug-like small molecule modulators. Growth factor receptors or their associated downstream kinases that regulate intracellular signaling pathways during differentiation are typically the targets for these molecules. The glycocalyx, which plays an essential role in actuating responses to growth factors at the cellular boundary, offers an underexplored opportunity for intervention using small molecules to influence differentiation. Here, we show that surfen, an antagonist of cell-surface glycosaminoglycans required for growth factor association with cognate receptors, acts as a potent and general inhibitor of differentiation and promoter of pluripotency in mouse ESCs. This finding shows that drugging the stem cell Glycome with small molecules to silence differentiation cues can provide a powerful new alternative to existing techniques for controlling stem cell fate. Stem Cells 2018;36:45-54.

Injection of embryonic stem cell derived macrophages ameliorates fibrosis in a murine model of liver injury

Chronic liver injury can be caused by viral hepatitis, alcohol, obesity, and metabolic disorders resulting in fibrosis, hepatic scarring, and cirrhosis. Novel therapies are urgently required and previous work has demonstrated that treatment with bone marrow derived macrophages can improve liver regeneration and reduce fibrosis in a murine model of hepatic injury and fibrosis. Here, we describe a protocol whereby pure populations of therapeutic macrophages can be produced in vitro from murine embryonic stem cells on a large scale. Embryonic stem cell derived macrophages display comparable morphology and cell surface markers to bone marrow derived macrophages but our novel imaging technique revealed that their phagocytic index was significantly lower. Differences were also observed in their response to classical induction protocols with embryonic stem cell derived macrophages having a reduced response to lipopolysaccharide and interferon gamma and an enhanced response to IL4 compared to bone marrow derived macrophages. When their therapeutic potential was assessed in a murine, carbon tetrachloride-induced injury and fibrosis model, embryonic stem cell derived macrophages significantly reduced the amount of hepatic fibrosis to 50% of controls, down-regulated the number of fibrogenic myofibroblasts and activated liver progenitor cells. To our knowledge, this is the first study that demonstrates a therapeutic effect of macrophages derived in vitro from pluripotent stem cells in a model of liver injury. We also found that embryonic stem cell derived macrophages repopulated the Kupffer cell compartment of clodronate-treated mice more efficiently than bone marrow derived macrophages, and expressed comparatively lower levels of Myb and Ccr2, indicating that their phenotype is more comparable to tissue-resident rather than monocyte-derived macrophages.

Aneuploid embryonic stem cells exhibit impaired differentiation and increased neoplastic potential

Aneuploidy leads to severe developmental defects in mammals and is also a hallmark of cancer. However, whether aneuploidy is a driving cause or a consequence of tumor formation remains controversial. Paradoxically, existing studies based on aneuploid yeast and mouse fibroblasts have shown that aneuploidy is usually detrimental to cellular fitness. Here, we examined the effects of aneuploidy on mouse embryonic stem (ES) cells by generating a series of cell lines that each carries an extra copy of single chromosomes, including trisomy 6, 8, 11, 12, or 15. Most of these aneuploid cell lines had rapid proliferation rates and enhanced colony formation efficiencies. They were less dependent on growth factors for self-renewal and showed a reduced capacity to differentiate in vitro Moreover, trisomic stem cells formed teratomas more efficiently, from which undifferentiated cells can be recovered. Further investigations demonstrated that co-culture of wild-type and aneuploid ES cells or supplementation with extracellular BMP4 rescues the differentiation defects of aneuploid ES cells.

A Role for MOSPD1 in Mesenchymal Stem Cell Proliferation and Differentiation

Mesenchymal stem cells (MSCs) isolated from many tissues including bone marrow and fat can be expanded in vitro and can differentiate into a range of different cell types such as bone, cartilage, and adipocytes. MSCs can also exhibit immunoregulatory properties when transplanted but, although a number of clinical trials using MSCs are in progress, the molecular mechanisms that control their production, proliferation, and differentiation are poorly understood. We identify MOSPD1 as a new player in this process. We generated MOSPD1-null embryonic stem cells (ESCs) and demonstrate that they are deficient in their ability to differentiate into a number of cell lineages including osteoblasts, adipocytes, and hematopoietic progenitors. The self-renewal capacity of MOSPD1-null ESCs was normal and they exhibited no obvious defects in early germ layer specification nor in epithelial to mesenchymal transition (EMT), indicating that MOSPD1 functions after these key steps in the differentiation process. Mesenchymal stem cell (MSC)-like cells expressing CD73, CD90, and CD105 were generated from MOSPD1-null ESCs but their growth rate was significantly impaired implying that MOSPD1 plays a role in MSC proliferation. Phenotypic deficiencies exhibited by MOSPD1-null ESCs were rescued by exogenous expression of MOSPD1, but not MOSPD3 indicating distinct functional properties of these closely related genes. Our in vitro studies were supported by RNA-sequencing data that confirmed expression of Mospd1 mRNA in cultured, proliferating perivascular pre-MSCs isolated from human tissue. This study adds to the growing body of knowledge about the function of this largely uncharacterized protein family and introduces a new player in the control of MSC proliferation and differentiation.

Embryonic Stem Cells License a High Level of Dormant Origins to Protect the Genome against Replication Stress

Maintaining genomic integrity during DNA replication is essential for stem cells. DNA replication origins are licensed by the MCM2–7 complexes, with most of them remaining dormant. Dormant origins (DOs) rescue replication fork stalling in S phase and ensure genome integrity. However, it is not known whether DOs exist and play important roles in any stem cell type. Here, we show that embryonic stem cells (ESCs) contain more DOs than tissue stem/progenitor cells such as neural stem/progenitor cells (NSPCs). Partial depletion of DOs does not affect ESC self-renewal but impairs their differentiation, including toward the neural lineage. However, reduction of DOs in NSPCs impairs their self-renewal due to accumulation of DNA damage and apoptosis. Furthermore, mice with reduced DOs show abnormal neurogenesis and semi-embryonic lethality. Our results reveal that ESCs are equipped with more DOs to better protect against replicative stress than tissue-specific stem/progenitor cells.

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ESCs possess more dormant origins than tissue stem/progenitor cells

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The greater number of dormant origins in ESCs effectively protects genome integrity

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Reduction of dormant origins impairs ESC differentiation, but not self-renewal

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Reduction of dormant origins severely affects neurogenesis and embryonic viability

Derivation of Neural Precursor Cells from Human Embryonic Stem Cells for DNA Methylomic Analysis

Embryonic stem cells are self-renewing pluripotent cells with competency to differentiate into all three-germ lineages. Many studies have demonstrated the importance of genetic and epigenetic molecular mechanisms in the maintenance of self-renewal and pluripotency. Stem cells are under unique molecular and cellular regulations different from somatic cells. Proper regulation should be ensured to maintain their unique self-renewal and undifferentiated characteristics. Understanding key mechanisms in stem cell biology will be important for the successful application of stem cells for regenerative therapeutic medicine. More importantly practical use of stem cells will require our knowledge on how to properly direct and differentiate stem cells into the necessary type of cells. Embryonic stem cells and adult stem cells have been used as study models to unveil molecular and cellular mechanisms in various signaling pathways. They are especially beneficial to developmental studies where in vivo molecular/cellular study models are not available. We have derived neural stem cells from human embryonic stem cells as a model to study the effect of teratogen in neural development. We have tested commercial neural differentiation system and successfully derived neural precursor cells exhibiting key molecular features of neural stem cells, which will be useful for experimental application.

Genetic isolation of stem cell-derived pacemaker-nodal cardiac myocytes

Dysfunction of the cardiac pacemaker tissues due to genetic defects, acquired diseases, or aging results in arrhythmias. When arrhythmias occur, artificial pacemaker implants are used for treatment. However, the numerous limitations of electronic implants have prompted studies of biological pacemakers that can integrate into the myocardium providing a permanent cure. Embryonic stem (ES) cells cultured as three-dimensional (3D) spheroid aggregates termed embryoid bodies possess the ability to generate all cardiac myocyte subtypes. Here, we report the use of a SHOX2 promoter and a Cx30.2 enhancer to genetically identify and isolate ES cell-derived sinoatrial node (SAN) and atrioventricular node (AVN) cells, respectively. The ES cell-derived Shox2 and Cx30.2 cardiac myocytes exhibit a spider cell morphology and high intracellular calcium loading characteristic of pacemaker-nodal myocytes. These cells express abundant levels of pacemaker genes such as endogenous HCN4, Cx45, Cx30.2, Tbx2, and Tbx3. These cells were passaged, frozen, and thawed multiple times while maintaining their pacemaker-nodal phenotype. When cultured as 3D aggregates in an attempt to create a critical mass that simulates in vivo architecture, these cell lines exhibited an increase in the expression level of key regulators of cardiovascular development, such as GATA4 and GATA6 transcription factors. In addition, the aggregate culture system resulted in an increase in the expression level of several ion channels that play a major role in the spontaneous diastolic depolarization characteristic of pacemaker cells. We have isolated pure populations of SAN and AVN cells that will be useful tools for generating biological pacemakers.

SCF/KIT inhibition has a cumulative but reversible effect on the self-renewal of embryonic stem cells and on the survival of differentiating cells

The receptor tyrosine kinase c-KIT is expressed in embryonic stem cells (ESCs) and adult stem cells, and many functional studies have demonstrated the importance of the SCF/KIT signaling pathway in adult stem cell maintenance. In this study, we show that a high level of KIT expression in wild-type ESCs correlates with an enhanced self-renewal and that inhibition of KIT signaling in ESCs for extended periods of time has a cumulative but reversible effect on self-renewal. Together these data suggest that continued KIT signaling in some cells within a self-renewing ESC population is required for optimal ESC function. Using a KIT blocking antibody, we recapitulated the phenotype we previously reported for genetically deficient KIT-null cells, demonstrating that SCF/KIT signaling is essential for the survival of differentiating ESCs. Here we show that this phenotype is also reversible. Pharmacological inhibition of JNK also had a cumulative but reversible detrimental effect on the survival of differentiating cells, thus recapitulating the Kit null phenotype and implicating JNK as a downstream mediator of KIT signaling. In contrast, the self-renewal of ESCs was unaffected by prolonged exposure to the JNK inhibitor, suggesting that JNK-independent downstream pathways are involved in KIT-mediated ESC self-renewal whereas KIT-mediated survival of differentiating ESC is likely to be JNK dependent.

Formation of embryoid bodies using dielectrophoresis

Embryoid body (EB) formation forms an important step in embryonic stem cell differentiation invivo. In murine embryonic stem cell (mESC) cultures EB formation is inhibited by the inclusion of leukaemic inhibitory factor (LIF) in the medium. Assembly of mESCs into aggregates by positive dielectrophoresis (DEP) in high field regions between interdigitated oppositely castellated electrodes was found to initiate EB formation. Embryoid body formation in aggregates formed with DEP occurred at a more rapid rate-in fact faster compared to conventional methods-in medium without LIF. However, EB formation also occurred in medium in which LIF was present when the cells were aggregated with DEP. The optimum characteristic size for the electrodes for EB formation with DEP was found to be 75-100 microns; aggregates smaller than this tended to merge, whilst aggregates larger than this tended to split to form multiple EBs. Experiments with ESCs in which green fluorescent protein (GFP) production was targeted to the mesodermal gene brachyury indicated that differentiation within embryoid bodies of this size may preferentially occur along the mesoderm lineage. As hematopoietic lineages during normal development derive from mesoderm, the finding points to a possible application of DEP formed EBs in the production of blood-based products from ESCs.

Toxic stress prioritizes and imbalances stem cell differentiation: implications for new biomarkers and in vitro toxicology tests

This hypothesis and review introduces rules of stem cell stress responses that provide biomarkers and alternative testing that replaces or reduces gestational tests using whole animals. These rules for the stress responses of cultured stem cells validate the organismal strategy of the stress response and show that it emulates what must happen if the conceptus implants during a response to stress in vivo. Specifically there is a profound threshold during a stress dose response where stem cell accumulation is significantly reduced. Below this threshold stress enzymes manage the stress response by converting anabolic to catabolic processes and by suppressing apoptosis, without affecting differentiation. However above this threshold the stem cell survival response converts to an organismal survival response where stress enzymes switch to new substrates and mediate loss of potency factors, gain of early essential differentiated lineages, and suppression of later essential lineages. Stressed stem cells 'compensate' for lower accumulation rates by differentiating a higher fraction of cells, and the organismal survival response further enhances adaptation by prioritizing the differentiation of early essential lineages. Thus compensatory and prioritized differentiation and the sets of markers produced are part of a response of cultured embryos and stem cells that emulate what must happen during implantation of a stressed gestation. Knowledge of these markers and use of stressed stem cell assays in culture should replace or reduce the number of animals needed for developmental toxicity and should produce biomarkers for stressed development in vitro and in vivo.

HOXB4 can enhance the differentiation of embryonic stem cells by modulating the hematopoietic niche

Hematopoietic differentiation of embryonic stem cells (ESCs) in vitro has been used as a model to study early hematopoietic development, and it is well documented that hematopoietic differentiation can be enhanced by overexpression of HOXB4. HOXB4 is expressed in hematopoietic progenitor cells (HPCs) where it promotes self-renewal, but it is also expressed in the primitive streak of the gastrulating embryo. This led us to hypothesize that HOXB4 might modulate gene expression in prehematopoietic mesoderm and that this property might contribute to its prohematopoietic effect in differentiating ESCs. To test our hypothesis, we developed a conditionally activated HOXB4 expression system using the mutant estrogen receptor (ER(T2)) and showed that a pulse of HOXB4 prior to HPC emergence in differentiating ESCs led to an increase in hematopoietic differentiation. Expression profiling revealed an increase in the expression of genes associated with paraxial mesoderm that gives rise to the hematopoietic niche. Therefore, we considered that HOXB4 might modulate the formation of the hematopoietic niche as well as the production of hematopoietic cells per se. Cell mixing experiments supported this hypothesis demonstrating that HOXB4 activation can generate a paracrine as well as a cell autonomous effect on hematopoietic differentiation. We provide evidence to demonstrate that this activity is partly mediated by the secreted protein FRZB.

Embryonic stem cell-derived cardiomyocytes harbor a subpopulation of niche-forming Sca-1+ progenitor cells

The adult mammalian heart is known to contain a population of cardiac progenitor cells. It has not been unambiguously determined, however, whether these cells form as part of the developmental program of the heart or migrate there by way of the circulatory system. This study was done in order to determine the origin of this population of cells. A population of cardiomyocytes was established from mouse embryonic stem (ES) cells using a genetic selection technique. In order to determine whether cardiac progenitor cells exist within this ES cell-derived cardiomyocyte population, the cells were analyzed by fluorescence activated cell sorting (FACS) using an antibody directed against stem cell antigen-1 (Sca-1). We observed that approximately 4% of the cardiomyocyte population was composed of Sca-1(+) cells. When the Sca-1(+) cells were isolated by magnetic cell sorting and differentiated as cellular aggregates, contractions were observed in 100% of the aggregates. Gene expression studies using quantitative RT-PCR showed that these cells expressed terminally differentiated cardiac-specific genes. When three-dimensional cellular aggregates were formed from ES cell-derived cardiomyocytes co-cultured with adult HL-1 cardiomyocytes, the Sca-1(+) cells were found to "sort out" and form niches within the cell aggregates. Our data demonstrate that cardiac progenitor cells in the adult heart originate as part of the developmental program of the heart and that Sca-1(+) progenitor cells can provide an important in vitro model system to study the formation of cellular niches in the heart.