Expression of CD41 on hematopoietic progenitors derived from embryonic hematopoietic cells

Altered 3D genome reorganization mediates precocious myeloid differentiation of aged hematopoietic stem cells in inflammation

Inflammation is a driving force of hematopoietic stem cells (HSCs) aging, causing irreversible exhaustion of functional HSCs. However, the underlying mechanism of HSCs erosion by inflammatory insult remains poorly understood. Here, we find that transient LPS exposure primes aged HSCs to undergo accelerated differentiation at the expense of self-renewal, leading to depletion of HSCs. Meanwhile, the central regulator nuclear factor kappa B (NF-κB) mediating functional impairment by inflammation insult induces differential transcriptional response in aged HSCs compared with young HSCs, with precocious activation of myeloid lineage genes. Altered compartmentalization and chromatin loop formation are associated with aging-related differential transcriptional response in HSCs upon lipopolysaccharide (LPS) stimulation. Mechanistically, enhancer and promoter regions of myeloid lineage genes in aged HSCs are more accessible and display more rapid and prominent CTCF occupancy upon LPS stimulation. Our study provides comprehensive resources for the three-dimensional (3D) genome structure of HSCs and sheds light into the ordered genome organization and the associated transcriptome signature underlying HSCs aging.

Effect of fibrin on the expression of adhesion molecules (ICAM-1, ITGAV, and ITGB3) in unrestricted somatic stem cells

BACKGROUND

Hematopoietic stem cell expansion relies on direct cell-cell interactions mediated by adhesion molecules, integrins, and cytokines. Unrestricted somatic stem cells have emerged as novel stromal cells supporting hematopoietic stem cell expansion in co-culture conditions via secretion of hematopoiesis-related cytokines and the expression of adhesion molecules. Previous research showed fibrin increased hematopoiesis-related gene expression in these cells. This study focused on the adhesive characteristics of unrestricted somatic stem cells on 3D fibrin scaffolds.

METHODS

Unrestricted somatic stem cells were isolated from umbilical cord blood and characterized using flow cytometry and multilineage differentiation assays. Scanning electron microscopy and DAPI staining were employed to analyze cell attachment to fibrin. Viability on fibrin was assessed through MTT assays. Quantitative polymerase chain reaction was conducted to evaluate the expression of intercellular adhesion molecule 1 (ICAM-1), integrin subunit αv (ITGAV), and integrin subunit β3 (ITGB3) in cells cultured on 3D fibrin scaffolds.

RESULTS

Cells were positive for CD73, CD105, and CD166 but negative for CD45. Alizarin red and Oil red O stains confirmed calcium deposition and lipid vacuoles. MTT assays revealed that fibrin positively impacts viability. ITGAV expression was significantly increased in cells cultured on fibrin compared to those cultured on plastic tissue culture plates (Control Group). Furthermore, ITGB3 expression showed no significant change in both groups, while ICAM-1 expression was downregulated in cells cultured on fibrin.

CONCLUSIONS

Our study revealed that fibrin has a positive impact on the expression of ITGAV, which plays a crucial role in direct cell-cell interactions affecting hematopoietic stem cell expansion.

Flow cytometry immunophenotyping of healthy platelets and hospitalized patients with suspected platelet dysfunction: Challenges for establishing a cutoff value

INTRODUCTION AND OBJECTIVE

Flow Cytometry (FC) is one of the techniques, which allows the identification and characterization of platelets. The detection of absent or reduced expression of the glycoproteins is the main objective of this technique. Abnormalities of glycoproteins lead to hemorrhagic syndromes. Among the main diseases, the Bernard-Soulier syndrome (BSS) and Glanzmann thrombasthenia (GT) stand out. We aimed to show a FC-based platelet assessment test for diagnostic use, which measures the expression of markers in normal patients, and evaluate these markers in patients with platelet disorders.

METHODS

We examined a control group of 41 healthy adults to establish reference values and assess the variability of the relative expression of platelet markers and subsequently compared these findings to those of 30 patients with suspected platelet dysfunctions. We determined the mean fluorescent intensity (MFI) of the expressed parameters by FC using CD41, CD42a, CD42b and CD61 and SSC/FSC platelet-gated cells.

RESULTS

We determined our baseline panel of markers and compared them to suspected platelet dysfunctions. Patients with suspected BSS presented increased levels of the MFI for the GPIIIa (CD61) and GPIIb (CD41). They showed significantly reduced levels of the GPIb (CD42b) and GPIX (CD42a). Patients with suspected GT showed normal expression of the GPIX (CD42a), increased expression of the GPIb (CD42b) and reduced levels of the GPIIIa (CD61). In this case, with reduced levels of only one marker, the GPIIb (CD41), values showed normal expression.

CONCLUSIONS

We describe the FC assay to support the diagnosis of different platelet disorders. Our study made it possible to implement a technique that brought benefits to care.

Hematopoietic stem cells and extramedullary hematopoiesis in the lungs

Hematopoietic stem cells are key players in hematopoiesis as the body maintains a physiologic steady state, and the signaling pathways and control mechanisms of these dynamic cells are implicated in processes from inflammation to cancer. Although the bone marrow is commonly regarded as the site of hematopoiesis and hematopoietic stem cell residence, these cells also circulate in the blood and reside in extramedullary tissues, including the lungs. Flow cytometry is an invaluable tool in evaluating hematopoietic stem cells, revealing their phenotypes and relative abundances in both healthy and diseased states. This review outlines current protocols and cell markers used in flow cytometric analysis of hematopoietic stem and progenitor cell populations. Specific niches within the bone marrow are discussed, as are metabolic processes that contribute to stem cell self-renewal and differentiation, as well as the role of hematopoietic stem cells outside of the bone marrow at physiologic steady state. Finally, pulmonary extramedullary hematopoiesis and its associated disease states are outlined. Hematopoiesis in the lungs is a new and emerging concept, and discovering ways in which the study of lung-resident hematopoietic stem cells can be translated from murine models to patients will impact clinical treatment.

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

BACKGROUND

Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication initiation determinant protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING E3 ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation.

METHODS

The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis.

RESULTS

RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter.

CONCLUSION

This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production. Video Abstract.

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

Background Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication origin binding protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation. Methods The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis. Results RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter. Conclusion This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production.

Self-organized yolk sac-like organoids allow for scalable generation of multipotent hematopoietic progenitor cells from induced pluripotent stem cells

Although the differentiation of human induced pluripotent stem cells (hiPSCs) into various types of blood cells has been well established, approaches for clinical-scale production of multipotent hematopoietic progenitor cells (HPCs) remain challenging. We found that hiPSCs cocultured with stromal cells as spheroids (hematopoietic spheroids [Hp-spheroids]) can grow in a stirred bioreactor and develop into yolk sac-like organoids without the addition of exogenous factors. Hp-spheroid-induced organoids recapitulated a yolk sac-characteristic cellular complement and structures as well as the functional ability to generate HPCs with lympho-myeloid potential. Moreover, sequential hemato-vascular ontogenesis could also be observed during organoid formation. We demonstrated that organoid-induced HPCs can be differentiated into erythroid cells, macrophages, and T lymphocytes with current maturation protocols. Notably, the Hp-spheroid system can be performed in an autologous and xeno-free manner, thereby improving the feasibility of bulk production of hiPSC-derived HPCs in clinical, therapeutic contexts.

Characterization of CD41+ cells in the lymph node

Lymph nodes (LNs) are the critical sites of immunity, and the stromal cells of LNs are crucial to their function. Our understanding of the stromal compartment of the LN has deepened recently with the characterization of nontraditional stromal cells. CD41 (integrin αIIb) is known to be expressed by platelets and hematolymphoid cells. We identified two distinct populations of CD41⁺Lyve1⁺ and CD41⁺Lyve1^- cells in the LNs. CD41⁺Lyve1^- cells appear in the LN mostly at the later stages of the lives of mice. We identified CD41⁺ cells in human LNs as well. We demonstrated that murine CD41⁺ cells express mesodermal markers, such as Sca-1, CD105 and CD29, but lack platelet markers. We did not observe the presence of platelets around the HEVs or within proximity to fibroblastic reticular cells of the LN. Examination of thoracic duct lymph fluid showed the presence of CD41⁺Lyve1^- cells, suggesting that these cells recirculate throughout the body. FTY720 reduced their trafficking to lymph fluid, suggesting that their egress is controlled by the S1P1 pathway. CD41⁺Lyve1^- cells of the LNs were sensitive to radiation, suggestive of their replicative nature. Single cell RNA sequencing data showed that the CD41⁺ cell population in naïve mouse LNs expressed largely stromal cell markers. Further studies are required to examine more deeply the role of CD41⁺ cells in the function of LNs.

Flow Cytometry Analysis of Blood Large Extracellular Vesicles in Patients with Multiple Sclerosis Experiencing Relapse of the Disease

The number of people living with multiple sclerosis (MS) in developed countries is increasing. The management of patients is hindered by the absence of reliable laboratory tests accurately reflecting the disease activity. Extracellular vesicles (EVs) of different cell origin were reportedly elevated in MS patients. We assessed the diagnostic potential, with flow cytometry analysis, of fresh large EVs (lEVs), which scattered more light than the 590 nm silica beads and were isolated from the blood plasma of relapsing remitting MS patients. Venous blood was collected from 15 patients and 16 healthy controls (HC). The lEVs were isolated from fresh platelet-free plasma by centrifugation, labelled with antibodies and the presence of platelet (CD41+, CD36+), endothelial (CD105+), erythrocyte (CD235a+), leukocyte (CD45+, CD19+, CD3+) and phosphatidylserine (Annexin V+) positive lEVs was analyzed using standard flow cytometry. Cryo-electron microscopy was used to verify the presence of EVs in the analyzed plasma fractions. MS patients experiencing acute relapse had slightly reduced relative levels (% of positive lEVs) of CD105+, CD45+, CD3+, CD45+CD3+ or CD19+ labelled lEVs in comparison to healthy controls. An analysis of other markers or a comparison of absolute lEV counts (count of lEVs/µL) did not yield any significant differences. Our data do not support the hypothesis that the exacerbation of the disease in RRMS patients leads to an increased numbers of circulating plasma lEVs which can be monitored by standard flow cytometry.

Dntt expression reveals developmental hierarchy and lineage specification of hematopoietic progenitors

Intrinsic and extrinsic cues determine developmental trajectories of hematopoietic stem cells (HSCs) towards erythroid, myeloid and lymphoid lineages. Using two newly generated transgenic mice that report and trace the expression of terminal deoxynucleotidyl transferase (TdT), transient induction of TdT was detected on a newly identified multipotent progenitor (MPP) subset that lacked self-renewal capacity but maintained multilineage differentiation potential. TdT induction on MPPs reflected a transcriptionally dynamic but uncommitted stage, characterized by low expression of lineage-associated genes. Single-cell CITE-seq indicated that multipotency in the TdT⁺ MPPs is associated with expression of the endothelial cell adhesion molecule ESAM. Stable and progressive upregulation of TdT defined the lymphoid developmental trajectory. Collectively, we here identify a new multipotent progenitor within the MPP4 compartment. Specification and commitment are defined by downregulation of ESAM which marks the progressive loss of alternative fates along all lineages.

Identification of Novel Low-Density Neutrophil Markers Through Unbiased High-Dimensional Flow Cytometry Screening in Non-Small Cell Lung Cancer Patients

Neutrophils have been described as a phenotypically heterogeneous cell type that possess both pro- and anti-tumor properties. Recently, a subset of neutrophils isolated from the peripheral blood mononuclear cell (PBMC) fraction has been described in cancer patients. These low-density neutrophils (LDNs) show a heterogeneous maturation state and have been associated with pro-tumor properties in comparison to mature, high-density neutrophils (HDNs). However, additional studies are necessary to characterize this cell population. Here we show new surface markers that allow us to discriminate between LDNs and HDNs in non-small cell lung cancer (NSCLC) patients and assess their potential as diagnostic/prognostic tool. LDNs were highly enriched in NSCLC patients (median=20.4%, range 0.3-76.1%; n=26) but not in healthy individuals (median=0.3%, range 0.1-3.9%; n=14). Using a high-dimensional human cell surface marker screen, we identified 12 surface markers that were downregulated in LDNs when compared to HDNs, while 41 surface markers were upregulated in the LDN subset. Using flow cytometry, we confirmed overexpression of CD36, CD41, CD61 and CD226 in the LDN fraction. In summary, our data support the notion that LDNs are a unique neutrophil population and provide novel targets to clarify their role in tumor progression and their potential as diagnostic and therapeutic tool.

Study of inherited thrombocytopenia resulting from mutations in ETV6 or RUNX1 using a human pluripotent stem cell model

Inherited thrombocytopenia results in low platelet counts and increased bleeding. Subsets of these patients have monoallelic germline mutations in ETV6 or RUNX1 and a heightened risk of developing hematologic malignancies. Utilizing CRISPR-Cas9, we compared the in vitro phenotype of hematopoietic progenitor cells and megakaryocytes derived from induced pluripotent stem cell (iPSC) lines harboring mutations in either ETV6 or RUNX1. Both mutant lines display phenotypes consistent with a platelet-bleeding disorder. Surprisingly, these cellular phenotypes were largely distinct. The ETV6-mutant iPSCs yield more hematopoietic progenitor cells and megakaryocytes, but the megakaryocytes are immature and less responsive to agonist stimulation. On the contrary, RUNX1-mutant iPSCs yield fewer hematopoietic progenitor cells and megakaryocytes, but the megakaryocytes are more responsive to agonist stimulation. However, both mutant iPSC lines display defects in proplatelet formation. Our work highlights that, while patients harboring germline ETV6 or RUNX1 mutations have similar clinical phenotypes, the molecular mechanisms may be distinct.

Zebrafish for thrombocytopoiesis- and hemostasis-related researches and disorders

Platelets play vital roles in hemostasis, inflammation, and vascular biology. Platelets are also active participants in the immune responses. As vertebrates, zebrafish have a highly conserved hematopoietic system in the developmental, cellular, functional, biochemical, and genetic levels with mammals. Thrombocytes in zebrafish are functional homologs of mammalian platelets. Here, we summarized thrombocyte development, function, and related research techniques in zebrafish, and reviewed available zebrafish models of platelet-associated disorders, including congenital amegakaryocytic thrombocytopenia, inherited thrombocytopenia, essential thrombocythemia, and blood coagulation disorders such as gray platelet syndrome. These elegant zebrafish models and methods are crucial for understanding the molecular and genetic mechanisms of thrombocyte development and function, and provide deep insights into related human disease pathophysiology and drug development.

UM171 expands distinct types of myeloid and NK progenitors from human pluripotent stem cells

Scaling up blood cell production from hPSCs is critical to advancing hPSC technologies for blood transfusion, immunotherapy, and transplantation. Here we explored the potential of the HSC agonist pyrimido-indole derivative UM171, to expand hematopoietic progenitors (HPs) derived from hPSCs in chemically defined conditions. We revealed that culture of hPSC-HPs in HSC expansion conditions (SFEM with added TPO, SCF, FLT3L, IL3 and IL6) in the presence of UM171 predominantly expanded HPs with a unique CD34⁺CD41a^loCD45⁺ phenotype that were enriched in granulocytic progenitors (G-CFCs). In contrast, in lymphoid cultures on OP9-DLL4, in the presence of SCF, FLT3L, and IL7, UM171 selectively expanded CD34⁺CD45⁺CD7⁺ lymphoid progenitors with NK cell potential, and increased NK cell output up to 10-fold. These studies should improve our understanding of the effect of UM171 on de novo generated HPs, and facilitate development of protocols for robust granulocyte and lymphoid cell production from hPSCs, for adoptive immunotherapies.

The pluripotency factor NANOG controls primitive hematopoiesis and directly regulates Tal1

Progenitors of the first hematopoietic cells in the mouse arise in the early embryo from Brachyury-positive multipotent cells in the posterior-proximal region of the epiblast, but the mechanisms that specify primitive blood cells are still largely unknown. Pluripotency factors maintain uncommitted cells of the blastocyst and embryonic stem cells in the pluripotent state. However, little is known about the role played by these factors during later development, despite being expressed in the postimplantation epiblast. Using a dual transgene system for controlled expression at postimplantation stages, we found that Nanog blocks primitive hematopoiesis in the gastrulating embryo, resulting in a loss of red blood cells and downregulation of erythropoietic genes. Accordingly, Nanog-deficient embryonic stem cells are prone to erythropoietic differentiation. Moreover, Nanog expression in adults prevents the maturation of erythroid cells. By analysis of previous data for NANOG binding during stem cell differentiation and CRISPR/Cas9 genome editing, we found that Tal1 is a direct NANOG target. Our results show that Nanog regulates primitive hematopoiesis by directly repressing critical erythroid lineage specifiers.

Insights into Endothelial Progenitor Cells: Origin, Classification, Potentials, and Prospects

With the discovery of endothelial progenitor cells (EPCs) in the late 1990s, a paradigm shift in the concept of neoangiogenesis occurred. The identification of circulating EPCs in peripheral blood marked the beginning of a new era with enormous potential in the rapidly transforming regenerative field. Overwhelmed with the revelation, researchers across the globe focused on isolating, defining, and interpreting the role of EPCs in various physiological and pathological conditions. Consequently, controversies emerged regarding the isolation techniques and classification of EPCs. Nevertheless, the potential of using EPCs in tissue engineering as an angiogenic source has been extensively explored. Concomitantly, the impact of EPCs on various diseases, such as diabetes, cancer, and cardiovascular diseases, has been studied. Within the limitations of the current knowledge, this review attempts to delineate the concept of EPCs in a sequential manner from the speculative history to a definitive presence (origin, sources of EPCs, isolation, and identification) and significance of these EPCs. Additionally, this review is aimed at serving as a guide for investigators, identifying potential research gaps, and summarizing our current and future prospects regarding EPCs.

The Effect of β-Aminopropionitrile on Skeletal Micromorphology and Osteogenesis

Collagen cross-linking, as a form of collagen post-translational modification, plays a crucial role in maintaining bone mechanical properties as well as in regulating cell biological functions. Shifts in cross-links profile are found apparently correlated to kinds of skeletal pathology and diseases, whereas little is known about the relationship between collagen cross-links and osteogenesis. Here, we hypothesized that the inhibition of collagen cross-links could impair skeletal microstructure and inhibit osteogenesis. A mouse model of collagen cross-linking defects has been established using subcutaneous injection of 350 mg/kg β-aminopropionitrile (BAPN) daily for 4 weeks, and same dose of phosphate buffered saline (PBS) served as control group. The analysis of bone microstructural parameters revealed a significant decrease of bone volume fraction (BV/TV) and trabecular thickness (Tb.Th), and increase of bone surface ratio (BS/BV), structure model index (SMI) as well as trabecular separation (Tb.Sp) in the experimental group (p < 0.05), whereas there was no difference observed in bone mineral density (BMD). Histological staining displayed that the BAPN treatment caused thinner trabeculae and decrease of collagen content in proximal tibiae. The analysis of osteogenesis PCR (Polymerase Chain Reaction) array reflected that BAPN remarkably influenced the expression of Alpl, Bglap, Bgn, Bmp5, Col10a1, Col1a1, Col1a2, Col5a1, Itga2b, and Serpinh1. The results of immunohistochemistry displayed a significant reduction in the mean optical densities of OCN and COL1 at the presence of BAPN. The overall results of this study suggested that BAPN alters bone microstructure and hinders the expression of osteogenic genes without affecting mineralization processes, indicating the influences of collagen cross-links on osteogenesis may be a potential pathological mechanism in skeletal diseases.

Fully Dedifferentiated Chondrocytes Expanded in Specific Mesenchymal Stem Cell Growth Medium with FGF2 Obtains Mesenchymal Stem Cell Phenotype In Vitro but Retains Chondrocyte Phenotype In Vivo

Given recent progress in regenerative medicine, we need a means to expand chondrocytes in quantity without losing their regenerative capability. Although many reports have shown that growth factor supplementation can have beneficial effects, the use of growth factor-supplemented basal media has widespread effect on the characteristics of chondrocytes. Chondrocytes were in vitro cultured in the 2 most widely used chondrocyte growth media, conventional chondrocyte culture medium and mesenchymal stem cell (MSC) culture medium, both with and without fibroblast growth factor-2 (FGF2) supplementation. Their expansion rates, expressions of extracellular matrix-related factors, senescence, and differentiation potentials were examined in vitro and in vivo. Our results revealed that chondrocytes quickly dedifferentiated during expansion in all tested media, as assessed by the loss of type II collagen expression. The 2 basal media (chondrocyte culture medium vs. MSC culture medium) were associated with distinct differences in cell senescence. Consistent with the literature, FGF2 was associated with accelerated dedifferentiation during expansion culture and superior redifferentiation upon induction. However, chondrocytes expanded in FGF2-containing conventional chondrocyte culture medium showed MSC-like features, as indicated by their ability to direct ectopic bone formation and cartilage formation. In contrast, chondrocytes cultured in FGF2-supplemented MSC culture medium showed potent chondrogenesis and almost no bone formation. The present findings show that the chosen basal medium can exert profound effects on the characteristics and activity of in vitro-expanded chondrocytes and indicate that right growth factor/medium combination can help chondrocytes retain a high-level chondrogenic potential without undergoing hypertrophic transition.

Membrane Mucin Muc4 promotes blood cell association with tumor cells and mediates efficient metastasis in a mouse model of breast cancer

Mucin-4 (Muc4) is a large cell surface glycoprotein implicated in the protection and lubrication of epithelial structures. Previous studies suggest that aberrantly expressed Muc4 can influence the adhesiveness, proliferation, viability and invasiveness of cultured tumor cells, as well as the growth rate and metastatic efficiency of xenografted tumors. Although it has been suggested that one of the major mechanisms by which Muc4 potentiates tumor progression is via its engagement of the ErbB2/HER2 receptor tyrosine kinase, other mechanisms exist and remain to be delineated. Moreover, the requirement for endogenous Muc4 for tumor growth progression has not been previously explored in the context of gene ablation. To assess the contribution of endogenous Muc4 to mammary tumor growth properties, we first created a genetically engineered mouse line lacking functional Muc4 (Muc4ko), and then crossed these animals with the NDL (Neu DeLetion mutant) model of ErbB2-induced mammary tumorigenesis. We observed that Muc4ko animals are fertile and develop normally, and adult mice exhibit no overt tissue abnormalities. In tumor studies, we observed that although some markers of tumor growth such as vascularity and cyclin D1 expression are suppressed, primary mammary tumors from Muc4ko/NDL female mice exhibit similar latencies and growth rates as Muc4wt/NDL animals. However, the presence of lung metastases is markedly suppressed in Muc4ko/NDL mice. Interestingly, histological analysis of lung lesions from Muc4ko/NDL mice revealed a reduced association of disseminated cells with platelets and white blood cells. Moreover, isolated cells derived from Muc4ko/NDL tumors interact with fewer blood cells when injected directly into the vasculature or diluted into blood from wild type mice. We further observed that blood cells more efficiently promote the viability of non-adherent Muc4wt/NDL cells than Muc4ko/NDL cells. Together, our observations suggest that Muc4 may facilitate metastasis by promoting the association of circulating tumor cells with blood cells to augment tumor cell survival in circulation.

FLI1 level during megakaryopoiesis affects thrombopoiesis and platelet biology

Friend leukemia virus integration 1 (FLI1), a critical transcription factor (TF) during megakaryocyte differentiation, is among genes hemizygously deleted in Jacobsen syndrome, resulting in a macrothrombocytopenia termed Paris-Trousseau syndrome (PTSx). Recently, heterozygote human FLI1 mutations have been ascribed to cause thrombocytopenia. We studied induced-pluripotent stem cell (iPSC)-derived megakaryocytes (iMegs) to better understand these clinical disorders, beginning with iPSCs generated from a patient with PTSx and iPSCs from a control line with a targeted heterozygous FLI1 knockout (FLI1^+/-). PTSx and FLI1^+/- iMegs replicate many of the described megakaryocyte/platelet features, including a decrease in iMeg yield and fewer platelets released per iMeg. Platelets released in vivo from infusion of these iMegs had poor half-lives and functionality. We noted that the closely linked E26 transformation-specific proto-oncogene 1 (ETS1) is overexpressed in these FLI1-deficient iMegs, suggesting FLI1 negatively regulates ETS1 in megakaryopoiesis. Finally, we examined whether FLI1 overexpression would affect megakaryopoiesis and thrombopoiesis. We found increased yield of noninjured, in vitro iMeg yield and increased in vivo yield, half-life, and functionality of released platelets. These studies confirm FLI1 heterozygosity results in pleiotropic defects similar to those noted with other critical megakaryocyte-specific TFs; however, unlike those TFs, FLI1 overexpression improved yield and functionality.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Role of stem cell factor in the placental niche

Stem cell factor (SCF) is a cytokine found in hematopoietic stem cells (HSCs) and causes proliferation and differentiation of cells by binding to its receptor (c-kit). It is produced in the yolk sac, fetal liver and bone marrow during the development of the fetus and, together with its signaling pathway, plays an important role in the development of these cells. The placenta, an important hematopoiesis site before the entry of cells into the liver, is rich in HSCs, with definitive hematopoiesis in a variety of HSC types and embryonic stem cells. Chorionic-plate-derived mesenchymal stem cells (CP-MSCs) isolated from the placenta show stem cell markers such as CD41 and cause the self-renewal of cells under hypoxic conditions. In contrast, hypoxia can result in apoptosis and autophagy via oxidative stress in stem cells. As a hypoxia-induced factor, SCF causes a balance between cell survival and death by autophagy in CP-MSCs. Stromal cells and MSCs have a crucial function in the development of HSCs in the placenta via SCF expression in the placental vascular niche. Defects in hematopoietic growth factors (such as SCF and its signaling pathways) lead to impaired hematopoiesis, resulting in fetal death and abortion. Therefore, an awareness of the role of the SCF/c-kit pathway in the survival, apoptosis and development of stem cells can significantly contribute to the exploration of stem cell production pathways during the embryonic period and in malignancies and in the further generation of these cells to facilitate therapeutic approaches. In this review, we discuss the role of SCF in the placental niche.

A Quantitative Proteomic Analysis of Hemogenic Endothelium Reveals Differential Regulation of Hematopoiesis by SOX17

The in vitro derivation of hematopoietic stem cells (HSCs) from pluripotent stem cells (PSCs) is complicated by the existence of multiple overlapping embryonic blood cell programs called primitive, erythromyeloid progenitor (EMP), and definitive. As HSCs are only generated during the definitive stage of hematopoiesis, deciphering the regulatory pathways that control the emergence of this program and identifying markers that distinguish it from the other programs are essential. To identify definitive specific pathways and marker sets, we used label-free proteomics to determine the proteome of embryo-derived and mouse embryonic stem cell-derived VE-CADHERIN(+)CD45(-) definitive hematopoietic progenitors. With this approach, we identified Stat1 as a marker that distinguishes the definitive erythroid lineage from the primitive- and EMP-derived lineages. Additionally, we provide evidence that the generation of the Stat1(+) definitive lineage is dependent on Sox17. These findings establish an approach for monitoring the emergence of definitive hematopoiesis in the PSC differentiation cultures.

Integrin-Alpha IIb Identifies Murine Lymph Node Lymphatic Endothelial Cells Responsive to RANKL

Microenvironment and activation signals likely imprint heterogeneity in the lymphatic endothelial cell (LEC) population. Particularly LECs of secondary lymphoid organs are exposed to different cell types and immune stimuli. However, our understanding of the nature of LEC activation signals and their cell source within the secondary lymphoid organ in the steady state remains incomplete. Here we show that integrin alpha 2b (ITGA2b), known to be carried by platelets, megakaryocytes and hematopoietic progenitors, is expressed by a lymph node subset of LECs, residing in medullary, cortical and subcapsular sinuses. In the subcapsular sinus, the floor but not the ceiling layer expresses the integrin, being excluded from ACKR4⁺ LECs but overlapping with MAdCAM-1 expression. ITGA2b expression increases in response to immunization, raising the possibility that heterogeneous ITGA2b levels reflect variation in exposure to activation signals. We show that alterations of the level of receptor activator of NF-κB ligand (RANKL), by overexpression, neutralization or deletion from stromal marginal reticular cells, affected the proportion of ITGA2b⁺ LECs. Lymph node LECs but not peripheral LECs express RANK. In addition, we found that lymphotoxin-β receptor signaling likewise regulated the proportion of ITGA2b⁺ LECs. These findings demonstrate that stromal reticular cells activate LECs via RANKL and support the action of hematopoietic cell-derived lymphotoxin.

Donor Dependent Variations in Hematopoietic Differentiation among Embryonic and Induced Pluripotent Stem Cell Lines

Hematopoiesis generated from human embryonic stem cells (ES) and induced pluripotent stem cells (iPS) are unprecedented resources for cell therapy. We compared hematopoietic differentiation potentials from ES and iPS cell lines originated from various donors and derived them using integrative and non-integrative vectors. Significant differences in differentiation toward hematopoietic lineage were observed among ES and iPS. The ability of engraftment of iPS or ES-derived cells in NOG mice varied among the lines with low levels of chimerism. iPS generated from ES cell-derived mesenchymal stem cells (MSC) reproduce a similar hematopoietic outcome compared to their parental ES cell line. We were not able to identify any specific hematopoietic transcription factors that allow to distinguish between good versus poor hematopoiesis in undifferentiated ES or iPS cell lines. There is a relatively unpredictable variation in hematopoietic differentiation between ES and iPS cell lines that could not be predicted based on phenotype or gene expression of the undifferentiated cells. These results demonstrate the influence of genetic background in variation of hematopoietic potential rather than the reprogramming process.

Distinct Sources of Hematopoietic Progenitors Emerge before HSCs and Provide Functional Blood Cells in the Mammalian Embryo

Hematopoietic potential arises in mammalian embryos before adult-repopulating hematopoietic stem cells (HSCs). At embryonic day 9.5 (E9.5), we show the first murine definitive erythro-myeloid progenitors (EMPs) have an immunophenotype distinct from primitive hematopoietic progenitors, maturing megakaryocytes and macrophages, and rare B cell potential. EMPs emerge in the yolk sac with erythroid and broad myeloid, but not lymphoid, potential. EMPs migrate to the fetal liver and rapidly differentiate, including production of circulating neutrophils by E11.5. Although the surface markers, transcription factors, and lineage potential associated with EMPs overlap with those found in adult definitive hematopoiesis, they are present in unique combinations or proportions that result in a specialized definitive embryonic progenitor. Furthermore, we find that embryonic stem cell (ESC)-derived hematopoiesis recapitulates early yolk sac hematopoiesis, including primitive, EMP, and rare B cell potential. EMPs do not have long-term potential when transplanted in immunocompromised adults, but they can provide transient adult-like RBC reconstitution.

Inducible Gata1 suppression expands megakaryocyte-erythroid progenitors from embryonic stem cells

Transfusion of donor-derived platelets is commonly used for thrombocytopenia, which results from a variety of clinical conditions and relies on a constant donor supply due to the limited shelf life of these cells. Embryonic stem (ES) and induced pluripotent stem (iPS) cells represent a potential source of megakaryocytes and platelets for transfusion therapies; however, the majority of current ES/iPS cell differentiation protocols are limited by low yields of hematopoietic progeny. In both mice and humans, mutations in the gene-encoding transcription factor GATA1 cause an accumulation of proliferating, developmentally arrested megakaryocytes, suggesting that GATA1 suppression in ES and iPS cell-derived hematopoietic progenitors may enhance megakaryocyte production. Here, we engineered ES cells from WT mice to express a doxycycline-regulated (dox-regulated) shRNA that targets Gata1 transcripts for degradation. Differentiation of these cells in the presence of dox and thrombopoietin (TPO) resulted in an exponential (at least 10¹³-fold) expansion of immature hematopoietic progenitors. Dox withdrawal in combination with multilineage cytokines restored GATA1 expression, resulting in differentiation into erythroblasts and megakaryocytes. Following transfusion into recipient animals, these dox-deprived mature megakaryocytes generated functional platelets. Our findings provide a readily reproducible strategy to exponentially expand ES cell-derived megakaryocyte-erythroid progenitors that have the capacity to differentiate into functional platelet-producing megakaryocytes.

Decoding the regulatory network of early blood development from single-cell gene expression measurements

Reconstruction of the molecular pathways controlling organ development has been hampered by a lack of methods to resolve embryonic progenitor cells. Here we describe a strategy to address this problem that combines gene expression profiling of large numbers of single cells with data analysis based on diffusion maps for dimensionality reduction and network synthesis from state transition graphs. Applying the approach to hematopoietic development in the mouse embryo, we map the progression of mesoderm toward blood using single-cell gene expression analysis of 3,934 cells with blood-forming potential captured at four time points between E7.0 and E8.5. Transitions between individual cellular states are then used as input to develop a single-cell network synthesis toolkit to generate a computationally executable transcriptional regulatory network model of blood development. Several model predictions concerning the roles of Sox and Hox factors are validated experimentally. Our results demonstrate that single-cell analysis of a developing organ coupled with computational approaches can reveal the transcriptional programs that underpin organogenesis.

Direct reprogramming of murine fibroblasts to hematopoietic progenitor cells

Recent reports have shown that somatic cells, under appropriate culture conditions, could be directly reprogrammed to cardiac, hepatic, or neuronal phenotype by lineage-specific transcription factors. In this study, we demonstrate that both embryonic and adult somatic fibroblasts can be efficiently reprogrammed to clonal multilineage hematopoietic progenitors by the ectopic expression of the transcription factors ERG, GATA2, LMO2, RUNX1c, and SCL. These reprogrammed cells were stably expanded on stromal cells and possessed short-term reconstitution ability in vivo. Loss of p53 function facilitated reprogramming to blood, and p53(-/-) reprogrammed cells efficiently generated erythroid, megakaryocytic, myeloid, and lymphoid lineages. Genome-wide analyses revealed that generation of hematopoietic progenitors was preceded by the appearance of hemogenic endothelial cells expressing endothelial and hematopoietic genes. Altogether, our findings suggest that direct reprogramming could represent a valid alternative approach to the differentiation of embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) for disease modeling and autologous blood cell therapies.

RUNX1 positively regulates a cell adhesion and migration program in murine hemogenic endothelium prior to blood emergence

During ontogeny, the transcription factor RUNX1 governs the emergence of definitive hematopoietic cells from specialized endothelial cells called hemogenic endothelium (HE). The ultimate consequence of this endothelial-to-hematopoietic transition is the concomitant activation of the hematopoietic program and downregulation of the endothelial program. However, due to the rare and transient nature of the HE, little is known about the initial role of RUNX1 within this population. We, therefore, developed and implemented a highly sensitive DNA adenine methyltransferase identification-based methodology, including a novel data analysis pipeline, to map early RUNX1 transcriptional targets in HE cells. This novel transcription factor binding site identification protocol should be widely applicable to other low abundance cell types and factors. Integration of the RUNX1 binding profile with gene expression data revealed an unexpected early role for RUNX1 as a positive regulator of cell adhesion- and migration-associated genes within the HE. This suggests that RUNX1 orchestrates HE cell positioning and integration prior to the release of hematopoietic cells. Overall, our genome-wide analysis of the RUNX1 binding and transcriptional profile in the HE provides a novel comprehensive resource of target genes that will facilitate the precise dissection of the role of RUNX1 in early blood development.

Runx1 is required for progression of CD41+ embryonic precursors into HSCs but not prior to this

Haematopoiesis in adult animals is maintained by haematopoietic stem cells (HSCs), which self-renew and can give rise to all blood cell lineages. The AGM region is an important intra-embryonic site of HSC development and a wealth of evidence indicates that HSCs emerge from the endothelium of the embryonic dorsal aorta and extra-embryonic large arteries. This, however, is a stepwise process that occurs through sequential upregulation of CD41 and CD45 followed by emergence of fully functional definitive HSCs. Although largely dispensable at later stages, the Runx1 transcription factor is crucially important during developmental maturation of HSCs; however, exact points of crucial involvement of Runx1 in this multi-step developmental maturation process remain unclear. Here, we have investigated requirements for Runx1 using a conditional reversible knockout strategy. We report that Runx1 deficiency does not preclude formation of VE-cad+CD45-CD41+ cells, which are phenotypically equivalent to precursors of definitive HSCs (pre-HSC Type I) but blocks transition to the subsequent CD45+ stage (pre-HSC Type II). These data emphasise that developmental progression of HSCs during a very short period of time is regulated by precise stage-specific molecular mechanisms.

Expression levels of endoglin distinctively identify hematopoietic and endothelial progeny at different stages of yolk sac hematopoiesis

Endoglin (Eng), an ancillary receptor of the transforming growth factor beta (TGFβ) signaling pathway superfamily, has been well recognized for its important function in vascular development and angiogenesis since its discovery more than a decade ago. Recent studies show that this receptor is also critical for the emergence of blood during embryonic development, and that at E7.5, endoglin together with Flk-1 identifies early mesoderm progenitors that are endowed with hematopoietic and endothelial potential. These two lineages emerge in very close association during embryogenesis, and because they share the expression of the same surface markers, it has been difficult to distinguish the earliest hematopoietic from endothelial cells. Here, we evaluated the function of endoglin in hematopoiesis as development progresses past E7.5, and found that the hematopoietic and endothelial progenitors can be distinguished by the levels of endoglin in E9.5 yolk sacs. Whereas endothelial cells are Eng(bright), hematopoietic activity is primarily restricted to a subset of cells that display dim expression of endoglin (Eng(dim)). Molecular characterization of these subfractions showed that endoglin-mediated induction of hematopoiesis occurs in concert with BMP2/BMP4 signaling. This pathway is highly active in Eng(dim) cells but significantly downregulated in the Eng knockout. Taken together, our findings show an important function for endoglin in mediating BMP2/BMP4 signaling during yolk sac hematopoietic development and suggest that the levels of this receptor modulate TGFβ versus bone morphogenetic protein (BMP) signaling.

Common developmental pathway for primitive erythrocytes and multipotent hematopoietic progenitors in early mouse development

Development of the hematopoietic system proceeds in a multistep manner. Primitive erythrocytes are the first hematopoietic cells to be observed that were produced transiently in developing embryos. Multilineage lymphohematopoiesis occurs after the primitive erythropoiesis. However, the lineage relationship of cells that comprise embryonic hematopoietic system is not well characterized. To clarify this process, careful analyses of the embryonic cells that differentiate into these cell lineages are necessary. We identified the common precursors of primitive erythrocytes and multipotent hematopoietic cells in mouse embryonic stem cell cultures and mouse embryos. A subset defined as CD45⁻CD41⁺AA4.1⁻ cells showed bipotential capability to produce primitive erythrocytes and lymphomyeloid cells at the single-cell level. The cell population was present in vivo before hematopoietic stem cells (HSCs) appeared. Our results show that primitive erythrocytes and lymphomyeloid cells are not completely separate cell lineages, and these precursors comprise the embryonic hematopoietic system before HSC emergence.

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Primitive erythrocytes and lymphomyeloid progenitors have bipotent precursors

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The precursors form primitive erythroid and lymphomyeloid cells in distinct waves

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The common precursors are noted in the yolk sac and embryo proper

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Primitive erythrocytes and lymphocytes can be derived from a single precursor cell

Targeting connective tissue growth factor (CTGF) in acute lymphoblastic leukemia preclinical models: anti-CTGF monoclonal antibody attenuates leukemia growth

Connective tissue growth factor (CTGF/CCN2) is involved in extracellular matrix production, tumor cell proliferation, adhesion, migration, and metastasis. Recent studies have shown that CTGF expression is elevated in precursor B-acute lymphoblastic leukemia (ALL) and that increased expression of CTGF is associated with inferior outcome in B-ALL. In this study, we characterized the functional role and downstream signaling pathways of CTGF in ALL cells. First, we utilized lentiviral shRNA to knockdown CTGF in RS4;11 and REH ALL cells expressing high levels of CTGF mRNA. Silencing of CTGF resulted in significant suppression of leukemia cell growth compared to control vector, which was associated with AKT/mTOR inactivation and increased levels of cyclin-dependent kinase inhibitor p27. CTGF knockdown sensitized ALL cells to vincristine and methotrexate. Treatment with an anti-CTGF monoclonal antibody, FG-3019, significantly prolonged survival of mice injected with primary xenograft B-ALL cells when co-treated with conventional chemotherapy (vincristine, L-asparaginase and dexamethasone). Data suggest that CTGF represents a targetable molecular aberration in B-ALL, and blocking CTGF signaling in conjunction with administration of chemotherapy may represent a novel therapeutic approach for ALL patients.

Single site-specific integration targeting coupled with embryonic stem cell differentiation provides a high-throughput alternative to in vivo enhancer analyses

Comprehensive analysis of cis-regulatory elements is key to understanding the dynamic gene regulatory networks that control embryonic development. While transgenic animals represent the gold standard assay, their generation is costly, entails significant animal usage, and in utero development complicates time-course studies. As an alternative, embryonic stem (ES) cells can readily be differentiated in a process that correlates well with developing embryos. Here, we describe a highly effective platform for enhancer assays using an Hsp68/Venus reporter cassette that targets to the Hprt locus in mouse ES cells. This platform combines the flexibility of Gateway® cloning, live cell trackability of a fluorescent reporter, low background and the advantages of single copy insertion into a defined genomic locus. We demonstrate the successful recapitulation of tissue-specific enhancer activity for two cardiac and two haematopoietic enhancers. In addition, we used this assay to dissect the functionality of the highly conserved Ets/Ets/Gata motif in the Scl+19 enhancer, which revealed that the Gata motif is not required for initiation of enhancer activity. We further confirmed that Gata2 is not required for endothelial activity of the Scl+19 enhancer using Gata2^−/− Scl+19 transgenic embryos. We have therefore established a valuable toolbox to study gene regulatory networks with broad applicability.

Integrin αIIb (CD41) plays a role in the maintenance of hematopoietic stem cell activity in the mouse embryonic aorta

Integrins are transmembrane receptors that play important roles as modulators of cell behaviour through their adhesion properties and the initiation of signaling cascades. The α_IIb integrin subunit (CD41) is one of the first cell surface markers indicative of hematopoietic commitment. α_IIb pairs exclusively with β₃ to form the α_IIbβ₃ integrin. β₃ (CD61) also pairs with α_v (CD51) to form the α_vβ₃ integrin. The expression and putative role of these integrins during mouse hematopoietic development is as yet unknown. We show here that hematopoietic stem cells (HSCs) differentially express α_IIbβ₃ and α_vβ₃ integrins throughout development. Whereas the first HSCs generated in the aorta at mid-gestation express both integrins, HSCs from the placenta only express α_vβ₃, and most fetal liver HSCs do not express either integrin. By using α_IIb deficient embryos, we show that α_IIb is not only a reliable HSC marker but it also plays an important and specific function in maintaining the HSC activity in the mouse embryonic aorta.

CD41 expression marks myeloid-biased adult hematopoietic stem cells and increases with age

The hematopoietic stem cell (HSC) compartment is heterogeneous, yet our understanding of the identities of different HSC subtypes is limited. Here we show that platelet integrin CD41 (αIIb), currently thought to only transiently mark fetal HSCs, is expressed on an adult HSC subtype that accumulates with age. CD41+ HSCs were largely quiescent and exhibited myeloerythroid and megakaryocyte gene priming, governed by Gata1, whereas CD41- HSCs were more proliferative and exhibited lymphoid gene priming. When isolated without the use of blocking antibodies, CD41+ HSCs possessed long-term repopulation capacity on serial transplantations and showed a marked myeloid bias compared with CD41- HSCs, which yielded a more lymphoid-biased progeny. CD41-knockout (KO) mice displayed multilineage hematopoietic defects coupled with decreased quiescence and survival of HSCs, suggesting that CD41 is functionally relevant for HSC maintenance and hematopoietic homeostasis. Transplantation experiments indicated that CD41-KO-associated defects are long-term transplantable, HSC-derived and, in part, mediated through the loss of platelet mass leading to decreases in HSC exposure to important platelet released cytokines, such as transforming growth factor β1. In summary, our data provide a novel marker to identify a myeloid-biased HSC subtype that becomes prevalent with age and highlights the dogma of HSC regulation by their progeny.

Generation of megakaryocytic progenitors from human embryonic stem cells in a feeder- and serum-free medium

BACKGROUND

The production of human platelets from embryonic stem cells in a defined culture system is a prerequisite for the generation of platelets for therapeutic use. As an important step towards this goal, we report the differentiation of human embryonic stem cells (hESCs) towards the megakaryocyte (Mk) lineage using a 'spin embryoid body' method in serum-free differentiation medium.

METHODOLOGY AND PRINCIPAL FINDINGS

Immunophenotypic analyses of differentiating hESC identified a subpopulation of cells expressing high levels of CD41a that expressed other markers associated with the Mk lineage, including CD110, CD42b and CD61. Differentiated cells were sorted on the basis of their expression of CD41a, CD34 and CD45 and assessed for Mk colony formation, expression of myeloid and Mk genes and ability to endoreplicate DNA. In a collagen-based colony assay, the CD41a⁺ cells sorted from these differentiation cultures produced 100-800 Mk progenitors at day 13 and 25-160 Mk progenitors at day 20 of differentiation per 100,000 cells assayed. Differentiated Mk cells produced platelet-like particles which expressed CD42b and were activated by ADP, similar to platelets generated from precursors in cord blood. These studies were complemented by real time PCR analyses showing that subsets of cells enriched for CD41a⁺ Mk precursors expressed high levels of Mk associated genes such as PF4 and MPL. Conversely, high levels of myeloid and erythroid related transcripts, such as GATA1, TAL1/SCL and PU.1, were detected in sorted fractions containing CD34⁺ and CD45⁺ cells.

CONCLUSIONS

We describe a serum- and feeder-free culture system that enabled the generation of Mk progenitors from human embryonic stem cells. These cells formed colonies that included differentiated Mks that fragmented to form platelet-like particles. This protocol represents an important step towards the generation of human platelets for therapeutic use.

Biological effects of T315I-mutated BCR-ABL in an embryonic stem cell-derived hematopoiesis model

The occurrence of T315I mutation during the course of targeted therapies of chronic myeloid leukemia is a major concern because it confers resistance to all currently approved tyrosine kinase inhibitors. The exact phenotype of the hematopoietic stem cell and the hierarchical level of the occurrence of this mutation in leukemic hematopoiesis has not been determined. To study the effects of T315I-mutated breakpoint cluster region-abelson (BCR-ABL) in a primitive hematopoietic stem cell, we have used the murine embryonic stem cell (mESC)-derived hematopoiesis model. Native and T315I-mutated BCR-ABL were introduced retrovirally in mESC-derived embryonic bodies followed by induction of hematopoiesis. In several experiments, T315I-mutated and nonmutated BCR-ABL-transduced embryonic bodies rapidly generated hematopoietic cells on OP-9 feeders, with evidence of hematopoietic stem cell markers. After injection into NOD/SCID mice, these cells induced myeloid and lymphoid leukemias, whereas transplantation of control (nontransduced) hematopoietic cells failed to produce any hematopoietic reconstitution in vivo. Moreover, the expression of native and T315I-mutated BCR-ABL conferred to mESC-derived hematopoietic cells a self-renewal capacity demonstrated by the generation of leukemias after secondary transplantations. Secondary leukemias were more aggressive with evidence of extramedullary tumors. The expression of stem cell regulator Musashi-2 was found to be increased in bone marrow of leukemic mice. These data show that T315I-mutated BCR-ABL is functional at the stem cell level, conferring to mESC-derived leukemic cells a long-term hematopoietic repopulation ability. This model could be of interest to test the efficiency of drugs at the stem cell level in leukemias with T315I mutation.

Megakaryopoiesis and thrombopoiesis: an update on cytokines and lineage surface markers

Megakaryopoiesis is the process by which mature megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs). The biological function of MKs is to produce platelets, which play critical roles in hemostasis and contribute to angiogenesis and wound healing. The generation of platelets from MKs is termed thrombopoiesis. The cytokine thrombopoietin (TPO) is the major regulator of megakaryopoiesis and thrombopoiesis. It binds to its surface receptor, c-Mpl, and acts through multiple downstream signaling pathways, including the PI-3 kinase-Akt, MAPK, and ERK1/ERK2 pathways. However, non-TPO pathways, such as the SDF1/CXCR4 axis, Notch signaling, src family kinases, integrin signaling, and Platelet Factor 4/low-density lipoprotein receptor-related protein 1, have more recently been recognized to influence megakaryopoiesis and thrombopoiesis in vitro and in vivo. In this chapter, we review megakaryopoiesis and thrombopoiesis with emphasis on cell surface marker changes during their differentiation from HSCs, and the classical cytokines that affect these developmental stages. We also discuss non-TPO regulators and their effects on in vitro culture systems.

Promotion of hematopoietic differentiation from mouse induced pluripotent stem cells by transient HoxB4 transduction

Ectopic expression of HoxB4 in embryonic stem (ES) cells leads to an efficient production of hematopoietic cells, including hematopoietic stem/progenitor cells. Previous studies have utilized a constitutive HoxB4 expression system or tetracycline-regulated HoxB4 expression system to induce hematopoietic cells from ES cells. However, these methods cannot be applied therapeutically due to the risk of transgenes being integrated into the host genome. Here, we report the promotion of hematopoietic differentiation from mouse ES cells and induced pluripotent stem (iPS) cells by transient HoxB4 expression using an adenovirus (Ad) vector. Ad vector could mediate efficient HoxB4 expression in ES cell-derived embryoid bodies (ES-EBs) and iPS-EBs, and its expression was decreased during cultivation, showing that Ad vector transduction was transient. A colony-forming assay revealed that the number of hematopoietic progenitor cells with colony-forming potential in HoxB4-transduced cells was significantly increased in comparison with that in non-transduced cells or LacZ-transduced cells. HoxB4-transduced cells also showed more efficient generation of CD41-, CD45-, or Sca-1-positive cells than control cells. These results indicate that transient, but not constitutive, HoxB4 expression is sufficient to augment the hematopoietic differentiation of ES and iPS cells, and that our method would be useful for clinical applications, such as cell transplantation therapy.

Identification and characterization of a novel transcriptional target of RUNX1/AML1 at the onset of hematopoietic development

Although the critical requirement for the transcription factor RUNX1/AML1 at the onset of hematopoietic development is well established, little is known about its transcriptional targets at this pivotal stage of blood development. Using microarrays, we identified the uncharacterized gene AI467606 as a gene whose expression level is dramatically reduced in the absence of RUNX1. We further demonstrated by chromatin immunoprecipitation and promoter assay a direct regulation of its transcription by RUNX1. Using a bacterial artificial chromosome transgenic approach, we established that AI467606 is expressed during the development of the hematopoietic system in vivo and in vitro and that its expression is detected within the CD41(+) population and marks definitive hematopoietic potential. Similarly, in the adult mouse, all hematopoietic cell lineages, except mature erythrocytes, express AI467606. Taken together, these findings indicate that AI467606 is a novel transcriptional target of RUNX1/AML1 at the onset of hematopoietic development that is extensively expressed within the hematopoietic system.

The identification and characterization of zebrafish hematopoietic stem cells

HSCs are defined by their ability to self-renew and maintain hematopoiesis throughout the lifespan of an organism. The optical clarity of their embryos and the ease of genetic manipulation make the zebrafish (Danio rerio) an excellent model for studying hematopoiesis. Using flow cytometry, we identified 2 populations of CD41-GFP(+) cells (GFP(hi) and GFP(lo)) in the whole kidney marrow of Tg(CD41:GFP) zebrafish. Past studies in humans and mice have shown that CD41 is transiently expressed in the earliest hematopoietic progenitors and is then silenced, reappearing in the platelet/thrombocyte lineage. We have transplanted flow-sorted GFP(hi) and GFP(lo) cells into irradiated adult zebrafish and assessed long-term hematopoietic engraftment. Transplantation of GFP(hi) cells did not reconstitute hematopoiesis. In contrast, we observed multilineage hematopoiesis up to 68 weeks after primary and secondary transplantation of GFP(lo) cells. We detected the CD41-GFP transgene in all major hematopoietic lineages and CD41-GFP(+) cells in histologic sections of kidneys from transplant recipients. These studies show that CD41-GFP(lo) cells fulfill generally accepted criteria for HSCs. The identification of fluorescent zebrafish HSCs, coupled with our ability to transplant them into irradiated adult recipients, provide a valuable new tool to track HSC homing, proliferation, and differentiation into hematopoietic cells.

CD41 is developmentally regulated and differentially expressed on mouse hematopoietic stem cells

CD41 expression is associated with the earliest stages of mouse hematopoiesis. It is notably expressed on some cells of the intra-aortic hematopoietic clusters, an area where the first adult-repopulating hematopoietic stem cells (HSCs) are generated. Although it is generally accepted that CD41 expression marks the onset of primitive/definitive hematopoiesis, there are few published data concerning its expression on HSCs. It is as yet uncertain whether HSCs express CD41 throughout development, and if so, to what level. We performed a complete in vivo transplantation analysis with yolk sac, aorta, placenta, and fetal liver cells, sorted based on CD41 expression level. Our data show that the earliest emerging HSCs in the aorta express CD41 in a time-dependent manner. In contrast, placenta and liver HSCs are CD41⁻. Thus, differential and temporal expression of CD41 by HSCs in the distinct hematopoietic territories suggests a developmental/dynamic regulation of this marker throughout development.

RIP3 mediates the embryonic lethality of caspase-8-deficient mice

Apoptosis and necroptosis are complementary pathways controlled by common signalling adaptors, kinases and proteases; among these, caspase-8 (Casp8) is critical for death receptor-induced apoptosis. This caspase has also been implicated in non-apoptotic pathways that regulate Fas-associated via death domain (FADD)-dependent signalling and other less defined biological processes as diverse as innate immune signalling and myeloid or lymphoid differentiation patterns. Casp8 suppresses RIP3-RIP1 (also known as RIPK3-RIPK1) kinase complex-dependent necroptosis that follows death receptor activation as well as a RIP3-dependent, RIP1-independent necrotic pathway that has emerged as a host defence mechanism against murine cytomegalovirus. Disruption of Casp8 expression leads to embryonic lethality in mice between embryonic days 10.5 and 11.5 (ref. 7). Thus, Casp8 may naturally hold alternative RIP3-dependent death pathways in check in addition to promoting apoptosis. We find that RIP3 is responsible for the mid-gestational death of Casp8-deficient embryos. Remarkably, Casp8(-/-)Rip3(-/-) double mutant mice are viable and mature into fertile adults with a full immune complement of myeloid and lymphoid cell types. These mice seem immunocompetent but develop lymphadenopathy by four months of age marked by accumulation of abnormal T cells in the periphery, a phenotype reminiscent of mice with Fas-deficiency (lpr/lpr; also known as Fas). Thus, Casp8 contributes to homeostatic control in the adult immune system; however, RIP3 and Casp8 are together completely dispensable for mammalian development.

Influencing hematopoietic differentiation of mouse embryonic stem cells using soluble heparin and heparan sulfate saccharides

Heparan sulfate proteoglycans (HSPG) encompass some of the most abundant macromolecules on the surface of almost every cell type. Heparan sulfate (HS) chains provide a key interaction surface for the binding of numerous proteins such as growth factors and morphogens, helping to define the ability of a cell to respond selectively to environmental cues. The specificity of HS-protein interactions are governed predominantly by the order and positioning of sulfate groups, with distinct cell types expressing unique sets of HS epitopes. Embryos deficient in HS-synthesis (Ext1(-/-)) exhibit pre-gastrulation lethality and lack recognizable organized mesoderm and extraembryonic tissues. Here we demonstrate that embryonic stem cells (ESCs) derived from Ext1(-/-) embryos are unable to differentiate into hematopoietic lineages, instead retaining ESC marker expression throughout embryoid body (EB) culture. However hematopoietic differentiation can be restored by the addition of soluble heparin. Consistent with specific size and composition requirements for HS:growth factor signaling, chains measuring at least 12 saccharides were required for partial rescue of hematopoiesis with longer chains (18 saccharides or more) required for complete rescue. Critically N- and 6-O-sulfate groups were essential for rescue. Heparin addition restored the activity of multiple signaling pathways including bone morphogenic protein (BMP) with activation of phospho-SMADs re-established by the addition of heparin. Heparin addition to wild-type cultures also altered the outcome of differentiation, promoting hematopoiesis at low concentrations, yet inhibiting blood formation at high concentrations. Thus altering the levels of HS and HS sulfation within differentiating ESC cultures provides an attractive and accessible mechanism for influencing cell fate.