GATA2 heterozygosity causes an epigenetic feedback mechanism resulting in myeloid and erythroid dysplasia.

The transcription factor GATA2 has a pivotal role in haematopoiesis. Heterozygous germline GATA2 mutations result in a syndrome characterized by immunodeficiency, bone marrow failure and predispositions to myelodysplastic syndrome (MDS) and acute myeloid leukaemia. Clinical symptoms in these patients are diverse and mechanisms driving GATA2-related phenotypes are largely unknown. To explore the impact of GATA2 haploinsufficiency on haematopoiesis, we generated a zebrafish model carrying a heterozygous mutation of gata2b (gata2b+/-), an orthologue of GATA2. Morphological analysis revealed myeloid and erythroid dysplasia in gata2b+/- kidney marrow. Because Gata2b could affect both transcription and chromatin accessibility during lineage differentiation, this was assessed by single-cell (sc) RNA-seq and single-nucleus (sn) ATAC-seq. Sn-ATAC-seq showed that the co-accessibility between the transcription start site (TSS) and a -3.5-4.1 kb putative enhancer was more robust in gata2b+/- zebrafish HSPCs compared to wild type, increasing gata2b expression and resulting in higher genome-wide Gata2b motif use in HSPCs. As a result of increased accessibility of the gata2b locus, gata2b+/- chromatin was also more accessible during lineage differentiation. scRNA-seq data revealed myeloid differentiation defects, that is, impaired cell cycle progression, reduced expression of cebpa and cebpb and increased signatures of ribosome biogenesis. These data also revealed a differentiation delay in erythroid progenitors, aberrant proliferative signatures and down-regulation of Gata1a, a master regulator of erythropoiesis, which worsened with age. These findings suggest that cell-intrinsic compensatory mechanisms, needed to obtain normal levels of Gata2b in heterozygous HSPCs to maintain their integrity, result in aberrant lineage differentiation, thereby representing a critical step in the predisposition to MDS.

Gut Isolation from Zebrafish Larvae for Single-cell RNA Sequencing.

The gastrointestinal (GI) tract performs a range of functions essential for life. Congenital defects affecting its development can lead to enteric neuromuscular disorders, highlighting the importance to understand the molecular mechanisms underlying GI development and dysfunction. In this study, we present a method for gut isolation from zebrafish larvae at 5 days post fertilization to obtain live, viable cells which can be used for single-cell RNA sequencing (scRNA-seq) analysis. This protocol is based on the manual dissection of the zebrafish intestine, followed by enzymatic dissociation with papain. Subsequently, cells are submitted to fluorescence-activated cell sorting, and viable cells are collected for scRNA-seq. With this method, we were able to successfully identify different intestinal cell types, including epithelial, stromal, blood, muscle, and immune cells, as well as enteric neurons and glia. Therefore, we consider it to be a valuable resource for studying the composition of the GI tract in health and disease, using the zebrafish.

Unbiased characterization of the larval zebrafish enteric nervous system at a single cell transcriptomic level.

The enteric nervous system (ENS) regulates many gastrointestinal functions including peristalsis, immune regulation and uptake of nutrients. Defects in the ENS can lead to severe enteric neuropathies such as Hirschsprung disease (HSCR). Zebrafish have proven to be fruitful in the identification of genes involved in ENS development and HSCR pathogenesis. However, composition and specification of enteric neurons and glial subtypes at larval stages, remains mainly unexplored. Here, we performed single cell RNA sequencing of zebrafish ENS at 5 days post-fertilization. We identified vagal neural crest progenitors, Schwann cell precursors, and four clusters of differentiated neurons. In addition, a previously unrecognized elavl3+/phox2bb-population of neurons and cx43+/phox2bb-enteric glia was found. Pseudotime analysis supported binary neurogenic branching of ENS differentiation, driven by a notch-responsive state. Taken together, we provide new insights on ENS development and specification, proving that the zebrafish is a valuable model for the study of congenital enteric neuropathies.

Gata2-regulated Gfi1b expression controls endothelial programming during endothelial-to-hematopoietic transition.

The first hematopoietic stem cells (HSCs) are formed through endothelial-to-hematopoietic transition (EHT) during embryonic development. The transcription factor GATA2 is a crucial regulator of EHT and HSC function throughout life. Because patients with GATA2 haploinsufficiency have inborn mutations, prenatal defects are likely to influence disease development. In mice, Gata2 haploinsufficiency (Gata2+/-) reduces the number and functionality of embryonic hematopoietic stem and progenitor cells (HSPCs) generated through EHT. However, the embryonic HSPC pool is heterogeneous and the mechanisms underlying this defect in Gata2+/- embryos remain unclear. Here, we investigated whether Gata2 haploinsufficiency selectively affects a cellular subset undergoing EHT. We showed that Gata2+/- HSPCs initiate, but cannot fully activate, hematopoietic programming during EHT. In addition, due to the reduced activity of the endothelial repressor Gfi1b, Gata2+/- HSPCs cannot repress endothelial identity to complete maturation. Finally, we showed that hematopoietic-specific induction of gfi1b could restore HSC production in gata2b-null (gata2b-/-) zebrafish embryos. This study illustrates the pivotal role of Gata2 in the regulation of the transcriptional network governing HSPC identity throughout the EHT.

Essential role for Gata2 in modulating lineage output from hematopoietic stem cells in zebrafish.

The differentiation of hematopoietic stem cells (HSCs) is tightly controlled to ensure a proper balance between myeloid and lymphoid cell output. GATA2 is a pivotal hematopoietic transcription factor required for generation and maintenance of HSCs. GATA2 is expressed throughout development, but because of early embryonic lethality in mice, its role during adult hematopoiesis is incompletely understood. Zebrafish contains 2 orthologs of GATA2: Gata2a and Gata2b, which are expressed in different cell types. We show that the mammalian functions of GATA2 are split between these orthologs. Gata2b-deficient zebrafish have a reduction in embryonic definitive hematopoietic stem and progenitor cell (HSPC) numbers, but are viable. This allows us to uniquely study the role of GATA2 in adult hematopoiesis. gata2b mutants have impaired myeloid lineage differentiation. Interestingly, this defect arises not in granulocyte-monocyte progenitors, but in HSPCs. Gata2b-deficient HSPCs showed impaired progression of the myeloid transcriptional program, concomitant with increased coexpression of lymphoid genes. This resulted in a decrease in myeloid-programmed progenitors and a relative increase in lymphoid-programmed progenitors. This shift in the lineage output could function as an escape mechanism to avoid a block in lineage differentiation. Our study helps to deconstruct the functions of GATA2 during hematopoiesis and shows that lineage differentiation flows toward a lymphoid lineage in the absence of Gata2b.

Deletion of a conserved Gata2 enhancer impairs haemogenic endothelium programming and adult Zebrafish haematopoiesis.

Gata2 is a key transcription factor required to generate Haematopoietic Stem and Progenitor Cells (HSPCs) from haemogenic endothelium (HE); misexpression of Gata2 leads to haematopoietic disorders. Here we deleted a conserved enhancer (i4 enhancer) driving pan-endothelial expression of the zebrafish gata2a and showed that Gata2a is required for HE programming by regulating expression of runx1 and of the second Gata2 orthologue, gata2b. By 5 days, homozygous gata2aΔi4/Δi4 larvae showed normal numbers of HSPCs, a recovery mediated by Notch signalling driving gata2b and runx1 expression in HE. However, gata2aΔi4/Δi4 adults showed oedema, susceptibility to infections and marrow hypo-cellularity, consistent with bone marrow failure found in GATA2 deficiency syndromes. Thus, gata2a expression driven by the i4 enhancer is required for correct HE programming in embryos and maintenance of steady-state haematopoietic stem cell output in the adult. These enhancer mutants will be useful in exploring further the pathophysiology of GATA2-related deficiencies in vivo.