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Lv et al. show that in mice, 5-HT can be synthesized in the aorta-gonad-mesonephros and acts as a novel endogenous regulator of hematopoietic stem and progenitor cell (HSPC) development. The promoting effect of 5-HT on the survival of HSPCs in the intraaortic hematopoietic cluster is mediated through Htr5a-AKT-Foxo1 signaling., The in vitro or ex vivo production of transplantable hematopoietic stem cells (HSCs) holds great promise for the treatment of hematological diseases in the clinic. However, HSCs have not been produced from either embryonic or induced pluripotent stem cells. In this study, we report that 5-hydroxytryptamine (5-HT; also called serotonin) can enhance the generation of hematopoietic stem and progenitor cells (HSPCs) in vitro and is essential for the survival of HSPCs in vivo during embryogenesis. In tryptophan hydroxylase 2–deficient embryos, a decrease in 5-HT synthesized in the aorta-gonad-mesonephros leads to apoptosis of nascent HSPCs. Mechanistically, 5-HT inhibits the AKT-Foxo1 signaling cascade to protect the earliest HSPCs in intraaortic hematopoietic clusters from excessive apoptosis. Collectively, our results reveal an unexpected role of 5-HT in HSPC development and suggest that 5-HT signaling may be a potential therapeutic target for promoting HSPC survival.

The limitation of using mouse embryos for hematopoiesis studies is the added inconvenience in operations, which is largely due to the intrauterine development of the embryo. Although genetic data from knockout (KO) mice are convincing, it is not realistic to generate KO mice for all genes as needed. In addition, performing in vivo rescue experiments to consolidate the data obtained from KO mice is not convenient. To overcome these limitations, the Aorta-Gonad-Mesonephros (AGM) explant culture was developed as an appropriate system to study hematopoietic stem cell (HSC) development. Especially for rescue experiments, it can be used to recover the impaired hematopoiesis in KO mice. By adding the appropriate chemicals into the medium, the impaired signaling can be reactivated or up-regulated pathways can be inhibited. With the use of this method, many experiments can be performed to identify the critical regulators of HSC development, including HSC related gene expression at mRNA and protein levels, colony formation ability, and reconstitution capacity. This series of experiments would be helpful in defining the underlying mechanisms essential for HSC development in mammals.

Inflammatory signaling has been shown to be essential for stress hematopoiesis in adult bone marrow, either through increasing proliferation or by directing differentiation of hematopoietic stem and progenitor cells (HSPCs) toward myeloid or lymphoid lineages. However, its role in embryonic normal hematopoiesis has been unknown. Here, we demonstrate that in both zebrafish and mouse embryos, inflammatory signaling is necessary and sufficient for HSPC emergence, in the absence of infection or pathological inflammation. Mechanistically, inflammatory signaling regulates hemogenic endothelium-derived HSPC development through a conserved Toll-like receptor 4 (TLR4)-nuclear factor κ-light-chain enhancer of activated B core (NF-κB) signaling, which then promotes Notch activity, a well-known signal required for HSPC specification in vertebrates. Our findings establish a previously unrecognized link between inflammatory signaling and HSPC emergence, and provide new insights into regenerative medicine and novel therapies to treat innate immune-related diseases.

Summary In mammals, hematopoietic stem and progenitor cells (HSPCs) rapidly expand in the fetal liver (FL), but the underlying mechanism remains unclear. Here, we characterize zebrafish caudal hematopoietic tissue (CHT) and identify an important cellular and molecular mechanism of HSPC expansion. Time-lapse imaging showed that HSPCs localize adjacent to vascular endothelial cells (ECs), and their migration and expansion display caudal vein-specific orientation in the CHT. RNA sequencing and functional analysis identified that an EC-expressed transcription factor, Kruppel-like factor 6a (Klf6a), is essential for the CHT niche. We further demonstrated that Klf6a directly regulates the expression of the chemokine (C-C motif) ligand 25b to modulate HSPC lodgment and proliferation. Ex vivo culture results support the conserved role of Ccl21/Ccr7 signaling in promoting HSPC expansion in mammals. Together, we identify the Klf6a-Ccl25b/Ccr7 axis in controlling the complex HSPC-CHT niche interaction, which may be applicable to in vitro expansion or engraftment of HSPCs after transplantation.

During development, hematopoietic stem cells (HSCs) undergo a rapid expansion in the fetal liver (FL) after their emergence in the aorta–gonad–mesonephros (AGM) region. We recently reported that the endolysosomal trafficking factor BLOS2, encoded by the Bloc1s2 gene, regulates HSC/hematopoietic progenitor cell emergence in the AGM region; however, whether it plays a role in the FL remains unknown. Here, we show that BLOS2 plays an essential role in the regulation of HSC proliferation and differentiation in the FL. Bloc1s2 depletion leads to elevated Notch signaling, with an increased frequency but weakened self-renewal ability of FL HSCs. Functional assays show that Bloc1s2 −/− FL HSCs harbor impaired lymphoid and myeloid differentiation abilities. These findings reveal that balanced control of Notch signaling by BLOS2 is required for HSC homeostasis during FL hematopoiesis.

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Avian hepatitis E virus (HEV) is related genetically and antigenically to human and swine HEVs and capsid protein of avian HEV shares approximately 48-49% amino acid sequence identities with those of human and swine HEVs. Six monoclonal antibodies (MAbs) were produced and used to locate different epitopes in the ORF2 region of aa 339-570 of avian HEV Chinese isolate. The results showed that five epitopes were located in the aa 339-414 region and one in the aa 510-515 region. Two epitopes located in aa 339-355 and aa 384-414 regions are the immunodominant epitopes on the surface of the avian HEV particles as demonstrated by immune capture of viral particles and immunohistochemical detection of the ORF2 antigens with two MAbs.