MYCT1 controls environmental sensing in human haematopoietic stem cells.

The processes that govern human haematopoietic stem cell (HSC) self-renewal and engraftment are poorly understood and challenging to recapitulate in culture to reliably expand functional HSCs1-3. Here we identify MYC target 1 (MYCT1; also known as MTLC) as a crucial human HSC regulator that moderates endocytosis and environmental sensing in HSCs. MYCT1 is selectively expressed in undifferentiated human haematopoietic stem and progenitor cells (HSPCs) and endothelial cells but becomes markedly downregulated during HSC culture. Lentivirus-mediated knockdown of MYCT1 prevented human fetal liver and cord blood (CB) HSPC expansion and engraftment. By contrast, restoring MYCT1 expression improved the expansion and engraftment of cultured CB HSPCs. Single-cell RNA sequencing of human CB HSPCs in which MYCT1 was knocked down or overexpressed revealed that MYCT1 governs important regulatory programmes and cellular properties essential for HSC stemness, such as ETS factor expression and low mitochondrial activity. MYCT1 is localized in the endosomal membrane in HSPCs and interacts with vesicle trafficking regulators and signalling machinery. MYCT1 loss in HSPCs led to excessive endocytosis and hyperactive signalling responses, whereas restoring MYCT1 expression balanced culture-induced endocytosis and dysregulated signalling. Moreover, sorting cultured CB HSPCs on the basis of lowest endocytosis rate identified HSPCs with preserved MYCT1 expression and MYCT1-regulated HSC stemness programmes. Our work identifies MYCT1-moderated endocytosis and environmental sensing as essential regulatory mechanisms required to preserve human HSC stemness. Our data also pinpoint silencing of MYCT1 as a cell-culture-induced vulnerability that compromises human HSC expansion.

Progesterone receptor in the vascular endothelium triggers physiological uterine permeability preimplantation.

Vascular permeability is frequently associated with inflammation and is triggered by a cohort of secreted permeability factors such as vascular endothelial growth factor (VEGF). Here, we show that the physiological vascular permeability that precedes implantation is directly controlled by progesterone receptor (PR) and is independent of VEGF. Global or endothelial-specific deletion of PR blocks physiological vascular permeability in the uterus, whereas misexpression of PR in the endothelium of other organs results in ectopic vascular leakage. Integration of an endothelial genome-wide transcriptional profile with chromatin immunoprecipitation sequencing revealed that PR induces an NR4A1 (Nur77/TR3)-dependent transcriptional program that broadly regulates vascular permeability in response to progesterone. Silencing of NR4A1 blocks PR-mediated permeability responses, indicating a direct link between PR and NR4A1. This program triggers concurrent suppression of several junctional proteins and leads to an effective, timely, and venous-specific regulation of vascular barrier function that is critical for embryo implantation.

Mapping human haematopoietic stem cells from haemogenic endothelium to birth.

The ontogeny of human haematopoietic stem cells (HSCs) is poorly defined owing to the inability to identify HSCs as they emerge and mature at different haematopoietic sites1. Here we created a single-cell transcriptome map of human haematopoietic tissues from the first trimester to birth and found that the HSC signature RUNX1+HOXA9+MLLT3+MECOM+HLF+SPINK2+ distinguishes HSCs from progenitors throughout gestation. In addition to the aorta-gonad-mesonephros region, nascent HSCs populated the placenta and yolk sac before colonizing the liver at 6 weeks. A comparison of HSCs at different maturation stages revealed the establishment of HSC transcription factor machinery after the emergence of HSCs, whereas their surface phenotype evolved throughout development. The HSC transition to the liver marked a molecular shift evidenced by suppression of surface antigens reflecting nascent HSC identity, and acquisition of the HSC maturity markers CD133 (encoded by PROM1) and HLA-DR. HSC origin was tracked to ALDH1A1+KCNK17+ haemogenic endothelial cells, which arose from an IL33+ALDH1A1+ arterial endothelial subset termed pre-haemogenic endothelial cells. Using spatial transcriptomics and immunofluorescence, we visualized this process in ventrally located intra-aortic haematopoietic clusters. The in vivo map of human HSC ontogeny validated the generation of aorta-gonad-mesonephros-like definitive haematopoietic stem and progenitor cells from human pluripotent stem cells, and serves as a guide to improve their maturation to functional HSCs.

Scl represses cardiomyogenesis in prospective hemogenic endothelium and endocardium.

Endothelium in embryonic hematopoietic tissues generates hematopoietic stem/progenitor cells; however, it is unknown how its unique potential is specified. We show that transcription factor Scl/Tal1 is essential for both establishing the hematopoietic transcriptional program in hemogenic endothelium and preventing its misspecification to a cardiomyogenic fate. Scl(-/-) embryos activated a cardiac transcriptional program in yolk sac endothelium, leading to the emergence of CD31+Pdgfrα+ cardiogenic precursors that generated spontaneously beating cardiomyocytes. Ectopic cardiogenesis was also observed in Scl(-/-) hearts, where the disorganized endocardium precociously differentiated into cardiomyocytes. Induction of mosaic deletion of Scl in Scl(fl/fl)Rosa26Cre-ER(T2) embryos revealed a cell-intrinsic, temporal requirement for Scl to prevent cardiomyogenesis from endothelium. Scl(-/-) endothelium also upregulated the expression of Wnt antagonists, which promoted rapid cardiomyocyte differentiation of ectopic cardiogenic cells. These results reveal unexpected plasticity in embryonic endothelium such that loss of a single master regulator can induce ectopic cardiomyogenesis from endothelial cells.

The genesis of human hematopoietic stem cells.

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to HSCs.

MLLT3 governs human haematopoietic stem-cell self-renewal and engraftment.

Limited knowledge of the mechanisms that govern the self-renewal of human haematopoietic stem cells (HSCs), and why this fails in culture, have impeded the expansion of HSCs for transplantation1. Here we identify MLLT3 (also known as AF9) as a crucial regulator of HSCs that is highly enriched in human fetal, neonatal and adult HSCs, but downregulated in culture. Depletion of MLLT3 prevented the maintenance of transplantable human haematopoietic stem or progenitor cells (HSPCs) in culture, whereas stabilizing MLLT3 expression in culture enabled more than 12-fold expansion of transplantable HSCs that provided balanced multilineage reconstitution in primary and secondary mouse recipients. Similar to endogenous MLLT3, overexpressed MLLT3 localized to active promoters in HSPCs, sustained levels of H3K79me2 and protected the HSC transcriptional program in culture. MLLT3 thus acts as HSC maintenance factor that links histone reader and modifying activities to modulate HSC gene expression, and may provide a promising approach to expand HSCs for transplantation.

Lymphoid priming in human bone marrow begins before expression of CD10 with upregulation of L-selectin.

Expression of the cell-surface antigen CD10 has long been used to define the lymphoid commitment of human cells. Here we report a unique lymphoid-primed population in human bone marrow that was generated from hematopoietic stem cells (HSCs) before onset of the expression of CD10 and commitment to the B cell lineage. We identified this subset by high expression of the homing molecule L-selectin (CD62L). CD10(-)CD62L(hi) progenitors had full lymphoid and monocytic potential but lacked erythroid potential. Gene-expression profiling placed the CD10(-)CD62L(hi) population at an intermediate stage of differentiation between HSCs and lineage-negative (Lin(-)) CD34(+)CD10(+) progenitors. CD62L was expressed on immature thymocytes, and its ligands were expressed at the cortico-medullary junction of the thymus, which suggested a possible role for this molecule in homing to the thymus. Our studies identify the earliest stage of lymphoid priming in human bone marrow.

VEGF-C protects the integrity of the bone marrow perivascular niche in mice.

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) stem cell niche, which provides a vital source of HSC regulatory signals. Radiation and chemotherapy disrupt the HSC niche, including its sinusoidal vessels and perivascular cells, contributing to delayed hematopoietic recovery. Thus, identification of factors that can protect the HSC niche during an injury could offer a significant therapeutic opportunity to improve hematopoietic regeneration. In this study, we identified a critical function for vascular endothelial growth factor-C (VEGF-C), that of maintaining the integrity of the BM perivascular niche and improving BM niche recovery after irradiation-induced injury. Both global and conditional deletion of Vegfc in endothelial or leptin receptor-positive (LepR+) cells led to a disruption of the BM perivascular niche. Furthermore, deletion of Vegfc from the microenvironment delayed hematopoietic recovery after transplantation by decreasing endothelial proliferation and LepR+ cell regeneration. Exogenous administration of VEGF-C via an adenoassociated viral vector improved hematopoietic recovery after irradiation by accelerating endothelial and LepR+ cell regeneration and by increasing the expression of hematopoietic regenerative factors. Our results suggest that preservation of the integrity of the perivascular niche via VEGF-C signaling could be exploited therapeutically to enhance hematopoietic regeneration.

Genetic Regulation of Fibroblast Activation and Proliferation in Cardiac Fibrosis.

BACKGROUND: Genetic diversity and the heterogeneous nature of cardiac fibroblasts (CFbs) have hindered characterization of the molecular mechanisms that regulate cardiac fibrosis. The Hybrid Mouse Diversity Panel offers a valuable tool to examine genetically diverse cardiac fibroblasts and their role in fibrosis. METHODS: Three strains of mice (C57BL/6J, C3H/HeJ, and KK/HlJ) were selected from the Hybrid Mouse Diversity Panel and treated with either isoproterenol (ISO) or saline by an intraperitoneally implanted osmotic pump. After 21 days, cardiac function and levels of fibrosis were measured by echocardiography and trichrome staining, respectively. Activation and proliferation of CFbs were measured by in vitro and in vivo assays under normal and injury conditions. RNA sequencing was done on isolated CFbs from each strain. Results were analyzed by Ingenuity Pathway Analysis and validated by reverse transcription-qPCR, immunohistochemistry, and ELISA. RESULTS: ISO treatment in C57BL/6J, C3H/HeJ, and KK/HlJ mice resulted in minimal, moderate, and extensive levels of fibrosis, respectively (n=7-8 hearts per condition). Isolated CFbs treated with ISO exhibited strain-specific increases in the levels of activation but showed comparable levels of proliferation. Similar results were found in vivo, with fibroblast activation, and not proliferation, correlating with the differential levels of cardiac fibrosis after ISO treatment. RNA sequencing revealed that CFbs from each strain exhibit unique gene expression changes in response to ISO. We identified Ltbp2 as a commonly upregulated gene after ISO treatment. Expression of LTBP2 was elevated and specifically localized in the fibrotic regions of the myocardium after injury in mice and in human heart failure patients. CONCLUSIONS: This study highlights the importance of genetic variation in cardiac fibrosis by using multiple inbred mouse strains to characterize CFbs and their response to ISO treatment. Our data suggest that, although fibroblast activation is a response that parallels the extent of scar formation, proliferation may not necessarily correlate with levels of fibrosis. In addition, by comparing CFbs from multiple strains, we identified pathways as potential therapeutic targets and LTBP2 as a marker for fibrosis, with relevance to patients with underlying myocardial fibrosis.

Scl binds to primed enhancers in mesoderm to regulate hematopoietic and cardiac fate divergence.

Scl/Tal1 confers hemogenic competence and prevents ectopic cardiomyogenesis in embryonic endothelium by unknown mechanisms. We discovered that Scl binds to hematopoietic and cardiac enhancers that become epigenetically primed in multipotent cardiovascular mesoderm, to regulate the divergence of hematopoietic and cardiac lineages. Scl does not act as a pioneer factor but rather exploits a pre-established epigenetic landscape. As the blood lineage emerges, Scl binding and active epigenetic modifications are sustained in hematopoietic enhancers, whereas cardiac enhancers are decommissioned by removal of active epigenetic marks. Our data suggest that, rather than recruiting corepressors to enhancers, Scl prevents ectopic cardiogenesis by occupying enhancers that cardiac factors, such as Gata4 and Hand1, use for gene activation. Although hematopoietic Gata factors bind with Scl to both activated and repressed genes, they are dispensable for cardiac repression, but necessary for activating genes that enable hematopoietic stem/progenitor cell development. These results suggest that a unique subset of enhancers in lineage-specific genes that are accessible for regulators of opposing fates during the time of the fate decision provide a platform where the divergence of mutually exclusive fates is orchestrated.

Return to youth with Sox17.

Maturation of hematopoietic stem cells (HSCs) from fetal to adult state and differentiation to progenitors are thought to follow a one-way street. In this issue of Genes & Development, He and colleagues (pp. 1613-1627) show that overexpression of Sox17 can convert adult multipotential progenitors to self-renewing HSCs that possess fetal properties. These findings challenge the irreversibility of hematopoietic development, and open up new perspectives to understand the different forms of HSC self-renewal at distinct stages of ontogeny and during transformation.

Sex hormone drives blood stem cell reproduction.

Stem cells ensure the maintenance of tissue homeostasis throughout life by tightly regulating their self-renewal and differentiation. In a recent study published in Nature, Nakada et al, 2014 unveil an unexpected endocrine mechanism that regulates hematopoietic stem cell (HSC) self-renewal.

Critical requirement of VEGF-C in transition to fetal erythropoiesis.

Vascular endothelial growth factor C (VEGF-C) is a major driver of lymphangiogenesis in embryos and adults. Vegfc gene deletion in mouse embryos results in failure of lymphangiogenesis, fluid accumulation in tissues, and lethality. The VEGF-C receptors VEGFR3 and VEGFR2 are required for embryonic blood vessel formation. The related VEGF is essential for both blood vessel formation and embryonic hematopoiesis, whereas the possible involvement of VEGF-C in hematopoiesis is unknown. Here we unveil a novel hematopoietic function of VEGF-C in fetal erythropoiesis. Deletion of Vegfc in embryonic day 7.5 (E7.5) embryos in the C57BL6 mouse genetic background led to defective fetal erythropoiesis, characterized by anemia and lack of enucleated red blood cells in blood circulation. Macrophages and erythroid cells in the fetal liver (FL) were also decreased after midgestation because of decreased cell proliferation and increased apoptosis. However, the Lin(-)Sca-1(+)c-Kit(+) stem cell compartment in E14.5 FL was not affected by Vegfc deletion. VEGF-C loss did not disrupt the generation of primitive erythroid cells or erythro-myeloid progenitors (EMPs) in the yolk sac, but it decreased the expression of α4-integrin on EMPs and compromised EMP colonization of the FL. The distribution, maturation, and enucleation of primitive erythroblasts were also impaired by Vegfc deletion. In contrast, Vegfc deletion from E10.5 onward did not compromise definitive hematopoiesis in the liver, and Vegfc deletion in adult mice did not cause anemia. These results reveal an unexpected role for VEGF-C, a major lymphangiogenic growth factor, in the transition to FL erythropoiesis.

GLI2 inhibition abrogates human leukemia stem cell dormancy.

BACKGROUND: Dormant leukemia stem cells (LSC) promote therapeutic resistance and leukemic progression as a result of unbridled activation of stem cell gene expression programs. Thus, we hypothesized that 1) deregulation of the hedgehog (Hh) stem cell self-renewal and cell cycle regulatory pathway would promote dormant human LSC generation and 2) that PF-04449913, a clinical antagonist of the GLI2 transcriptional activator, smoothened (SMO), would enhance dormant human LSC eradication. METHODS: To test these postulates, whole transcriptome RNA sequencing (RNA-seq), microarray, qRT-PCR, stromal co-culture, confocal fluorescence microscopic, nanoproteomic, serial transplantation and cell cycle analyses were performed on FACS purified normal, chronic phase (CP) chronic myeloid leukemia (CML), blast crisis (BC) phase CML progenitors with or without PF-04449913 treatment. RESULTS: Notably, RNA-seq analyses revealed that Hh pathway and cell cycle regulatory gene overexpression correlated with leukemic progression. While lentivirally enforced GLI2 expression enhanced leukemic progenitor dormancy in stromal co-cultures, this was not observed with a mutant GLI2 lacking a transactivation domain, suggesting that GLI2 expression prevented cell cycle transit. Selective SMO inhibition with PF-04449913 in humanized stromal co-cultures and LSC xenografts reduced downstream GLI2 protein and cell cycle regulatory gene expression. Moreover, SMO inhibition enhanced cell cycle transit and sensitized BC LSC to tyrosine kinase inhibition in vivo at doses that spare normal HSC. CONCLUSION: In summary, while GLI2, forms part of a core HH pathway transcriptional regulatory network that promotes human myeloid leukemic progression and dormant LSC generation, selective inhibition with PF-04449913 reduces the dormant LSC burden thereby providing a strong rationale for clinical trials predicated on SMO inhibition in combination with TKIs or chemotherapeutic agents with the ultimate aim of obviating leukemic therapeutic resistance, persistence and progression.

Dynamic distribution of linker histone H1.5 in cellular differentiation.

Linker histones are essential components of chromatin, but the distributions and functions of many during cellular differentiation are not well understood. Here, we show that H1.5 binds to genic and intergenic regions, forming blocks of enrichment, in differentiated human cells from all three embryonic germ layers but not in embryonic stem cells. In differentiated cells, H1.5, but not H1.3, binds preferentially to genes that encode membrane and membrane-related proteins. Strikingly, 37% of H1.5 target genes belong to gene family clusters, groups of homologous genes that are located in proximity to each other on chromosomes. H1.5 binding is associated with gene repression and is required for SIRT1 binding, H3K9me2 enrichment, and chromatin compaction. Depletion of H1.5 results in loss of SIRT1 and H3K9me2, increased chromatin accessibility, deregulation of gene expression, and decreased cell growth. Our data reveal for the first time a specific and novel function for linker histone subtype H1.5 in maintenance of condensed chromatin at defined gene families in differentiated human cells.

Nipah virus envelope-pseudotyped lentiviruses efficiently target ephrinB2-positive stem cell populations in vitro and bypass the liver sink when administered in vivo.

Sophisticated retargeting systems for lentiviral vectors have been developed in recent years. Most seek to suppress the viral envelope's natural tropism while modifying the receptor-binding domain such that its tropism is determined by the specificity of the engineered ligand-binding motif. Here we took advantage of the natural tropism of Nipah virus (NiV), whose attachment envelope glycoprotein has picomolar affinity for ephrinB2, a molecule proposed as a molecular marker of "stemness" (present on embryonic, hematopoietic, and neural stem cells) as well as being implicated in tumorigenesis of specific cancers. NiV entry requires both the fusion (F) and attachment (G) glycoproteins. Truncation of the NiV-F cytoplasmic tail (T5F) alone, combined with full-length NiV-G, resulted in optimal titers of NiV-pseudotyped particles (NiVpp) (∼10(6) IU/ml), even without ultracentrifugation. To further enhance the infectivity of NiVpp, we engineered a hyperfusogenic NiV-F protein lacking an N-linked glycosylation site (T5FΔN3). T5FΔN3/wt G particles exhibited enhanced infectivity on less permissive cell lines and efficiently targeted ephrinB2(+) cells even in a 1,000-fold excess of ephrinB2-negative cells, all without any loss of specificity, as entry was abrogated by soluble ephrinB2. NiVpp also transduced human embryonic, hematopoietic, and neural stem cell populations in an ephrinB2-dependent manner. Finally, intravenous administration of the luciferase reporter NiVpp-T5FΔN3/G to mice resulted in signals being detected in the spleen and lung but not in the liver. Bypassing the liver sink is a critical barrier for targeted gene therapy. The extraordinary specificity of NiV-G for ephrinB2 holds promise for targeting specific ephrinB2(+) populations in vivo or in vitro.

Regulated expression of microRNAs-126/126* inhibits erythropoiesis from human embryonic stem cells.

MicroRNAs (miRs) play an important role in cell differentiation and maintenance of cell identity, but relatively little is known of their functional role in modulating human hematopoietic lineage differentiation. Human embryonic stem cells (hESCs) provide a model system to study early human hematopoiesis. We differentiated hESCs by embryoid body (EB) formation and compared the miR expression profile of undifferentiated hESCs to CD34(+) EB cells. miRs-126/126* were the most enriched of the 7 miRs that were up-regulated in CD34(+) cells, and their expression paralleled the kinetics of hematopoietic transcription factors RUNX1, SCL, and PU.1. To define the role of miRs-126/126* in hematopoiesis, we created hESCs overexpressing doxycycline-regulated miRs-126/126* and analyzed their hematopoietic differentiation. Induction of miRs-126/126* during both EB differentiation and colony formation reduced the number of erythroid colonies, suggesting an inhibitory role of miRs-126/126* in erythropoiesis. Protein tyrosine phosphatase, nonreceptor type 9 (PTPN9), a protein tyrosine phosphatase that is required for growth and expansion of erythroid cells, is one target of miR-126. PTPN9 restoration partially relieved the suppressed erythropoiesis caused by miRs-126/126*. Our results define an important function of miRs-126/126* in negative regulation of erythropoiesis, providing the first evidence for a role of miR in hematopoietic differentiation of hESCs.

The first trimester human placenta is a site for terminal maturation of primitive erythroid cells.

Embryonic hematopoiesis starts via the generation of primitive red blood cells (RBCs) that satisfy the embryo's immediate oxygen needs. Although primitive RBCs were thought to retain their nuclei, recent studies have shown that primitive RBCs in mice enucleate in the fetal liver. It has been unknown whether human primitive RBCs enucleate, and what hematopoietic site might support this process. Our data indicate that the terminal maturation and enucleation of human primitive RBCs occurs in first trimester placental villi. Extravascular ζ-globin(+) primitive erythroid cells were found in placental villi between 5-7 weeks of development, at which time the frequency of enucleated RBCs was higher in the villous stroma than in circulation. RBC enucleation was further evidenced by the presence of primitive reticulocytes and pyrenocytes (ejected RBC nuclei) in the placenta. Extravascular RBCs were found to associate with placental macrophages, which contained ingested nuclei. Clonogenic macrophage progenitors of fetal origin were present in the chorionic plate of the placenta before the onset of fetoplacental circulation, after which macrophages had migrated to the villi. These findings indicate that placental macrophages may assist the enucleation process of primitive RBCs in placental villi, implying an unexpectedly broad role for the placenta in embryonic hematopoiesis.

Yolk sac steps up to the plate.

Atkins et al. (2022. J. Exp. Med.https://doi.org/10.1084/jem.20211924) create a PSC differentiation model for human yolk sac hematopoiesis and discover multipotent progenitors with erythro-myeloid and T lymphoid potential. The multipotent progenitors emerge via hemogenic endothelium and share origin with primitive erythroid wave in KDR+CD235a/b+ mesoderm.

Mef2C is a lineage-restricted target of Scl/Tal1 and regulates megakaryopoiesis and B-cell homeostasis.

The basic helix-loop-helix transcription factor stem cell leukemia gene (Scl) is a master regulator for hematopoiesis essential for hematopoietic specification and proper differentiation of the erythroid and megakaryocyte lineages. However, the critical downstream targets of Scl remain undefined. Here, we identified a novel Scl target gene, transcription factor myocyte enhancer factor 2 C (Mef2C) from Scl(fl/fl) fetal liver progenitor cell lines. Analysis of Mef2C(-/-) embryos showed that Mef2C, in contrast to Scl, is not essential for specification into primitive or definitive hematopoietic lineages. However, adult VavCre(+)Mef2C(fl/fl) mice exhibited platelet defects similar to those observed in Scl-deficient mice. The platelet counts were reduced, whereas platelet size was increased and the platelet shape and granularity were altered. Furthermore, megakaryopoiesis was severely impaired in vitro. Chromatin immunoprecipitation microarray hybridization analysis revealed that Mef2C is directly regulated by Scl in megakaryocytic cells, but not in erythroid cells. In addition, an Scl-independent requirement for Mef2C in B-lymphoid homeostasis was observed in Mef2C-deficient mice, characterized as severe age-dependent reduction of specific B-cell progenitor populations reminiscent of premature aging. In summary, this work identifies Mef2C as an integral member of hematopoietic transcription factors with distinct upstream regulatory mechanisms and functional requirements in megakaryocyte and B-lymphoid lineages.