Hemogenic and aortic endothelium arise from a common hemogenic angioblast precursor and are specified by the Etv2 dosage

The embryonic zebrafish brain is exclusively colonized by pu.1-dependent and lymphatic-independent population of microglia

Microglia, the crucial immune cells inhabiting the central nervous system (CNS), perform a range of vital functions, encompassing immune defense and neuronal regulation. Microglia subsets with diverse functions and distinct developmental regulations have been identified recently. It is generally accepted that all microglia originate from hematopoiesis and depend on the myeloid transcription factor PU.1. However, a recent study reported the existence of mrc1+ microglia in zebrafish embryos, which are seemingly independent of Pu.1 and reliant on lymphatic vessels, sparking great interest in the possibility of lymphatic-originated microglia. To address this, we took advantage of a pu.1 knock-in zebrafish allele for a detailed investigation. Our results conclusively showed that almost all zebrafish embryonic microglia (~95% on average) express pu.1. Further, lineage tracing and mutant analysis revealed that these microglia neither emerged from nor depended on lymphatic vessels. In essence, our study refutes the presence of pu.1-independent but lymphatic-dependent microglia.

Histone H2A deubiquitinase BAP1 is essential for endothelial cell differentiation from human pluripotent stem cells

Polycomb group proteins (PcGs) add repressive post translational histone modifications such as H2AK119ub1, and histone H2A deubiquitinases remove it. Mice lacking histone H2A deubiquitinases such as Usp16 and Bap1 die in embryonic stage, while mice lacking Usp3, Mysm1, Usp12, and Usp21 have been shown to be deficient in hematopoietic lineage differentiation, cell cycle regulation, and DNA repair. Thus, it is likely that histone deubiquitinases may also be required for human endothelial cell differentiation; however, there are no reports about the role of histone H2A deubiquitinase BAP1 in human endothelial cell development. We differentiated human pluripotent stem cells into the endothelial lineage which expressed stable inducible shRNA against BAP1. Our results show that BAP1 is required for human endothelial cell differentiation.

Systematic single-cell analysis reveals dynamic control of transposable element activity orchestrating the endothelial-to-hematopoietic transition

BACKGROUND

The endothelial-to-hematopoietic transition (EHT) process during definitive hematopoiesis is highly conserved in vertebrates. Stage-specific expression of transposable elements (TEs) has been detected during zebrafish EHT and may promote hematopoietic stem cell (HSC) formation by activating inflammatory signaling. However, little is known about how TEs contribute to the EHT process in human and mouse.

RESULTS

We reconstructed the single-cell EHT trajectories of human and mouse and resolved the dynamic expression patterns of TEs during EHT. Most TEs presented a transient co-upregulation pattern along the conserved EHT trajectories, coinciding with the temporal relaxation of epigenetic silencing systems. TE products can be sensed by multiple pattern recognition receptors, triggering inflammatory signaling to facilitate HSC emergence. Interestingly, we observed that hypoxia-related signals were enriched in cells with higher TE expression. Furthermore, we constructed the hematopoietic cis-regulatory network of accessible TEs and identified potential TE-derived enhancers that may boost the expression of specific EHT marker genes.

CONCLUSIONS

Our study provides a systematic vision of how TEs are dynamically controlled to promote the hematopoietic fate decisions through transcriptional and cis-regulatory networks, and pre-train the immunity of nascent HSCs.

Tuning apicobasal polarity and junctional recycling in the hemogenic endothelium orchestrates the morphodynamic complexity of emerging pre-hematopoietic stem cells

Hematopoietic stem cells emerge in the embryo from an aortic-derived tissue called the hemogenic endothelium (HE). The HE appears to give birth to cells of different nature and fate but the molecular principles underlying this complexity are largely unknown. Here we show, in the zebrafish embryo, that two cell types emerge from the aortic floor with radically different morphodynamics. With the support of live imaging, we bring evidence suggesting that the mechanics underlying the two emergence types rely, or not, on apicobasal polarity establishment. While the first type is characterized by reinforcement of apicobasal polarity and maintenance of the apical/luminal membrane until release, the second type emerges via a dynamic process reminiscent of trans-endothelial migration. Interfering with Runx1 function suggests that the balance between the two emergence types depends on tuning apicobasal polarity at the level of the HE. In support of this and unexpectedly, we show that Pard3ba - one of the four Pard3 proteins expressed in the zebrafish - is sensitive to interference with Runx1 activity, in aortic endothelial cells. This supports the idea of a signaling cross talk controlling cell polarity and its associated features, between aortic and hemogenic cells. In addition, using new transgenic fish lines that express Junctional Adhesion Molecules and functional interference, we bring evidence for the essential role of ArhGEF11/PDZ-RhoGEF in controlling the HE-endothelial cell dynamic interface, including cell-cell intercalation, which is ultimately required for emergence completion. Overall, we highlight critical cellular and dynamic events of the endothelial-to-hematopoietic transition that support emergence complexity, with a potential impact on cell fate.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Cross-species transcriptomics reveals bifurcation point during the arterial-to-hemogenic transition

Hemogenic endothelium (HE) with hematopoietic stem cell (HSC)-forming potential emerge from specialized arterial endothelial cells (AECs) undergoing the endothelial-to-hematopoietic transition (EHT) in the aorta-gonad-mesonephros (AGM) region. Characterization of this AECs subpopulation and whether this phenomenon is conserved across species remains unclear. Here we introduce HomologySeeker, a cross-species method that leverages refined mouse information to explore under-studied human EHT. Utilizing single-cell transcriptomic ensembles of EHT, HomologySeeker reveals a parallel developmental relationship between these two species, with minimal pre-HSC signals observed in human cells. The pre-HE stage contains a conserved bifurcation point between the two species, where cells progress towards HE or late AECs. By harnessing human spatial transcriptomics, we identify ligand modules that contribute to the bifurcation choice and validate CXCL12 in promoting hemogenic choice using a human in vitro differentiation system. Our findings advance human arterial-to-hemogenic transition understanding and offer valuable insights for manipulating HSC generation using in vitro models.

Vascular Progenitor Cells: From Cancer to Tissue Repair

Vascular progenitor cells are activated to repair and form a neointima following vascular damage such as hypertension, atherosclerosis, diabetes, trauma, hypoxia, primary cancerous lesions and metastases as well as catheter interventions. They play a key role not only in the resolution of the vascular lesion but also in the adult neovascularization and angiogenesis sprouting (i.e., the growth of new capillaries from pre-existing ones), often associated with carcinogenesis, favoring the formation of metastases, survival and progression of tumors. In this review, we discuss the biology, cellular plasticity and pathophysiology of different vascular progenitor cells, including their origins (sources), stimuli and activated pathways that induce differentiation, isolation and characterization. We focus on their role in tumor-induced vascular injury and discuss their implications in promoting tumor angiogenesis during cancer proliferation and migration.

Atf7ip and Setdb1 interaction orchestrates the hematopoietic stem and progenitor cell state with diverse lineage differentiation

Hematopoietic stem and progenitor cells (HSPCs) are a heterogeneous group of cells with expansion, differentiation, and repopulation capacities. How HSPCs orchestrate the stemness state with diverse lineage differentiation at steady condition or acute stress remains largely unknown. Here, we show that zebrafish mutants that are deficient in an epigenetic regulator Atf7ip or Setdb1 methyltransferase undergo excessive myeloid differentiation with impaired HSPC expansion, manifesting a decline in T cells and erythroid lineage. We find that Atf7ip regulates hematopoiesis through Setdb1-mediated H3K9me3 modification and chromatin remodeling. During hematopoiesis, the interaction of Atf7ip and Setdb1 triggers H3K9me3 depositions in hematopoietic regulatory genes including cebpβ and cdkn1a, preventing HSPCs from loss of expansion and premature differentiation into myeloid lineage. Concomitantly, loss of Atf7ip or Setdb1 derepresses retrotransposons that instigate the viral sensor Mda5/Rig-I like receptor (RLR) signaling, leading to stress-driven myelopoiesis and inflammation. We find that ATF7IP or SETDB1 depletion represses human leukemic cell growth and induces myeloid differentiation with retrotransposon-triggered inflammation. These findings establish that Atf7ip/Setdb1-mediated H3K9me3 deposition constitutes a genome-wide checkpoint that impedes the myeloid potential and maintains HSPC stemness for diverse blood cell production, providing unique insights into potential intervention in hematological malignancy.

The ETS transcription factor Spi2 regulates hematopoietic cell development in zebrafish

The E26 transformation-specific or E-twenty-six (ETS) genes encode a superfamily of transcription factors involved in diverse biological processes. Here, we report the identification and characterization of a previously unidentified member of the ETS transcription factors, Spi2, that is found exclusively in the ray-finned fish kingdom. We show that the expression of spi2 is restricted to hemogenic endothelial cells (HECs) and to hematopoietic stem and progenitor cells (HSPCs) in zebrafish. Using bacteria artificial chromosome transgenesis, we generate a spi2 reporter line, TgBAC(spi2:P2a-GFP), which manifests the GFP pattern recapitulating the endogenous spi2 expression. Genetic ablation of spi2 has little effect on HEC formation and the endothelial-to-hematopoietic transition, but results in compromised proliferation of HSPCs in the caudal hematopoietic tissue (CHT) during early development and in severe myeloid lineage defect in adulthood. Epistatic analysis shows that spi2 acts downstream of runx1 in regulating HSPC development in the CHT. Our study identifies Spi2 as an essential regulator for definitive hematopoietic cell development and creates a TgBAC(spi2:P2a-GFP) reporter line for tracking HECs, HSPCs, myeloid cells and thrombocytes from early development to adulthood.

Clonal Hematopoiesis: Role in Hematologic and Non-Hematologic Malignancies

Hematopoietic stem cells (HSCs) ensure the coordinated and balanced production of all hematopoietic cell types throughout life. Aging is associated with a gradual decline of the self-renewal and regenerative potential of HSCs and with the development of clonal hematopoiesis. Clonal hematopoiesis of indeterminate potential (CHIP) defines the clonal expansion of genetically variant hematopoietic cells bearing one or more gene mutations and/or structural variants (such as copy number alterations). CHIP increases exponentially with age and is associated with cancers, including hematologic neoplasia, cardiovascular and other diseases. The presence of CHIP consistently increases the risk of hematologic malignancy, particularly in individuals who have CHIP in association with peripheral blood cytopenia.