Cpeb1b-mediated cytoplasmic polyadenylation of shha mRNA modulates zebrafish definitive hematopoiesis.

During vertebrate embryogenesis, hematopoietic stem and progenitor cell (HSPC) production through endothelial-to-hematopoietic transition requires suitable developmental signals, but how these signals are accurately regulated remains incompletely understood. Cytoplasmic polyadenylation, which is one of the posttranscriptional regulations, plays a crucial role in RNA metabolism. Here, we report that Cpeb1b-mediated cytoplasmic polyadenylation is important for HSPC specification by translational control of Hedgehog (Hh) signaling during zebrafish early development. Cpeb1b is highly expressed in notochord and its deficiency results in defective HSPC production. Mechanistically, Cpeb1b regulates hemogenic endothelium specification by the Hedgehog-Vegf-Notch axis. We demonstrate that the cytoplasmic polyadenylation element motif-dependent interaction between Cpeb1b and shha messenger RNA (mRNA) in the liquid-like condensates, which are induced by Pabpc1b phase separation, is required for cytoplasmic polyadenylation of shha mRNA. Intriguingly, the cytoplasmic polyadenylation regulates translation but not stability of shha mRNA, which further enhances the Shha protein level and Hh signal transduction. Taken together, our findings uncover the role of Cpeb1b-mediated cytoplasmic polyadenylation in HSPC development and provide insights into how posttranscriptional regulation can direct developmental signals with high fidelity to translate them into cell fate transition.

A single-cell resolution developmental atlas of hematopoietic stem and progenitor cell expansion in zebrafish.

During vertebrate embryogenesis, fetal hematopoietic stem and progenitor cells (HSPCs) exhibit expansion and differentiation properties in a supportive hematopoietic niche. To profile the developmental landscape of fetal HSPCs and their local niche, here, using single-cell RNA-sequencing, we deciphered a dynamic atlas covering 28,777 cells and 9 major cell types (23 clusters) of zebrafish caudal hematopoietic tissue (CHT). We characterized four heterogeneous HSPCs with distinct lineage priming and metabolic gene signatures. Furthermore, we investigated the regulatory mechanism of CHT niche components for HSPC development, with a focus on the transcription factors and ligand-receptor networks involved in HSPC expansion. Importantly, we identified an endothelial cell-specific G protein-coupled receptor 182, followed by in vivo and in vitro functional validation of its evolutionally conserved role in supporting HSPC expansion in zebrafish and mice. Finally, comparison between zebrafish CHT and human fetal liver highlighted the conservation and divergence across evolution. These findings enhance our understanding of the regulatory mechanism underlying hematopoietic niche for HSPC expansion in vivo and provide insights into improving protocols for HSPC expansion in vitro.

Smarca5-mediated epigenetic programming facilitates fetal HSPC development in vertebrates.

Nascent hematopoietic stem and progenitor cells (HSPCs) acquire definitive hematopoietic characteristics only when they develop into fetal HSPCs; however, the mechanisms underlying fetal HSPC development are poorly understood. Here, we profiled the chromatin accessibility and transcriptional features of zebrafish nascent and fetal HSPCs using ATAC-seq and RNA-seq and revealed dynamic changes during HSPC transition. Functional assays demonstrated that chromatin remodeler-mediated epigenetic programming facilitates fetal HSPC development in vertebrates. Systematical screening of chromatin remodeler-related genes identified that smarca5 is responsible for the maintenance of chromatin accessibility at promoters of hematopoiesis-related genes in fetal HSPCs. Mechanistically, Smarca5 interacts with nucleolin to promote chromatin remodeling, thereby facilitating genomic binding of transcription factors to regulate expression of hematopoietic regulators such as bcl11ab. Our results unravel a new role of epigenetic regulation and reveal that Smarca5-mediated epigenetic programming is responsible for fetal HSPC development, which will provide new insights into the generation of functional HSPCs both in vivo and in vitro.

Rab5c-mediated endocytic trafficking regulates hematopoietic stem and progenitor cell development via Notch and AKT signaling.

It is well known that various developmental signals play diverse roles in hematopoietic stem and progenitor cell (HSPC) production; however, how these signaling pathways are orchestrated remains incompletely understood. Here, we report that Rab5c is essential for HSPC specification by endocytic trafficking of Notch and AKT signaling in zebrafish embryos. Rab5c deficiency leads to defects in HSPC production. Mechanistically, Rab5c regulates hemogenic endothelium (HE) specification by endocytic trafficking of Notch ligands and receptor. We further show that the interaction between Rab5c and Appl1 in the endosome is required for the survival of HE in the ventral wall of the dorsal aorta through AKT signaling. Interestingly, Rab5c overactivation can also lead to defects in HSPC production, which is attributed to excessive endolysosomal trafficking inducing Notch signaling defect. Taken together, our findings establish a previously unrecognized role of Rab5c-mediated endocytic trafficking in HSPC development and provide new insights into how spatiotemporal signals are orchestrated to accurately execute cell fate transition.

m6A modulates haematopoietic stem and progenitor cell specification.

N6-methyladenosine (m6A) has been identified as the most abundant modification on eukaryote messenger RNA (mRNA). Although the rapid development of high-throughput sequencing technologies has enabled insight into the biological functions of m6A modification, the function of m6A during vertebrate embryogenesis remains poorly understood. Here we show that m6A determines cell fate during the endothelial-to-haematopoietic transition (EHT) to specify the earliest haematopoietic stem/progenitor cells (HSPCs) during zebrafish embryogenesis. m6A-specific methylated RNA immunoprecipitation combined with high-throughput sequencing (MeRIP-seq) and m6A individual-nucleotide-resolution cross-linking and immunoprecipitation with sequencing (miCLIP-seq) analyses reveal conserved features on zebrafish m6A methylome and preferential distribution of m6A peaks near the stop codon with a consensus RRACH motif. In mettl3-deficient embryos, levels of m6A are significantly decreased and emergence of HSPCs is blocked. Mechanistically, we identify that the delayed YTHDF2-mediated mRNA decay of the arterial endothelial genes notch1a and rhoca contributes to this deleterious effect. The continuous activation of Notch signalling in arterial endothelial cells of mettl3-deficient embryos blocks EHT, thereby repressing the generation of the earliest HSPCs. Furthermore, knockdown of Mettl3 in mice confers a similar phenotype. Collectively, our findings demonstrate the critical function of m6A modification in the fate determination of HSPCs during vertebrate embryogenesis.

Epigenetic regulation of left-right asymmetry by DNA methylation.

DNA methylation is a major epigenetic modification; however, the precise role of DNA methylation in vertebrate development is still not fully understood. Here, we show that DNA methylation is essential for the establishment of the left-right (LR) asymmetric body plan during vertebrate embryogenesis. Perturbation of DNA methylation by depletion of DNA methyltransferase 1 (dnmt1) or dnmt3bb.1 in zebrafish embryos leads to defects in dorsal forerunner cell (DFC) specification or collective migration, laterality organ malformation, and disruption of LR patterning. Knockdown of dnmt1 in Xenopus embryos also causes similar defects. Mechanistically, loss of dnmt1 function induces hypomethylation of the lefty2 gene enhancer and promotes lefty2 expression, which consequently represses Nodal signaling in zebrafish embryos. We also show that Dnmt3bb.1 regulates collective DFC migration through cadherin 1 (Cdh1). Taken together, our data uncover dynamic DNA methylation as an epigenetic mechanism to control LR determination during early embryogenesis in vertebrates.

Inflammatory signaling regulates hematopoietic stem and progenitor cell emergence in vertebrates.

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.

Ncor2 is required for hematopoietic stem cell emergence by inhibiting Fos signaling in zebrafish.

Nuclear receptor corepressors (Ncors) are important for developmental and homeostatic processes in vertebrates, which exert transcriptional repression by coordinating with histone deacetylases. However, little is known about their roles in definitive hematopoiesis. In this study, we show that in zebrafish, ncor2 is required for hematopoietic stem cell (HSC) development by repressing fos-vegfd signaling. ncor2 is specifically expressed in the aorta-gonad-mesonephros (AGM) region in zebrafish embryos. ncor2 deficiency reduced the population of HSCs in both the AGM region and T cells in the thymus. Mechanistically, ncor2 knockdown upregulated fos transcription by modulating the acetylation level in the fos promoter region, which then enhanced Vegfd signaling. Consequently, the augmented Vegfd signaling induced Notch signaling to promote the arterial endothelial fate, therefore, possibly repressing the hemogenic endothelial specification, which is a prerequisite for HSC emergence. Thus, our findings identify a novel regulatory mechanism for Ncor2 through Fos-Vegfd-Notch signaling cascade during HSC development in zebrafish embryos.

Helper component-proteinase enhances the activity of 1-deoxy-D-xylulose-5-phosphate synthase and promotes the biosynthesis of plastidic isoprenoids in Potato virus Y-infected tobacco.

Virus-infected plants show strong morphological and physiological alterations. Many physiological processes in chloroplast are affected, including the plastidic isoprenoid biosynthetic pathway [the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway]; indeed, isoprenoid contents have been demonstrated to be altered in virus-infected plants. In this study, we found that the levels of photosynthetic pigments and abscisic acid (ABA) were altered in Potato virus Y (PVY)-infected tobacco. Using yeast two-hybrid assays, we demonstrated an interaction between virus protein PVY helper component-proteinase (HC-Pro) and tobacco chloroplast protein 1-deoxy-D-xylulose-5-phosphate synthase (NtDXS). This interaction was confirmed using bimolecular fluorescence complementation (BiFC) assays and pull-down assays. The Transket_pyr domain (residues 394-561) of NtDXS was required for interaction with HC-Pro, while the N-terminal region of HC-Pro (residues 1-97) was necessary for interaction with NtDXS. Using in vitro enzyme activity assays, PVY HC-Pro was found to promote the synthase activity of NtDXS. We observed increases in photosynthetic pigment contents and ABA levels in transgenic plants with HC-Pro accumulating in the chloroplasts. During virus infection, the enhancement of plastidic isoprenoid biosynthesis was attributed to the enhancement of DXS activity by HC-Pro. Our study reveals a new role of HC-Pro in the host plant metabolic system and will contribute to the study of host-virus relationships.

Foxn1 maintains thymic epithelial cells to support T-cell development via mcm2 in zebrafish.

The thymus is mainly comprised of thymic epithelial cells (TECs), which form the unique thymic epithelial microenvironment essential for intrathymic T-cell development. Foxn1, a member of the forkhead transcription factor family, is required for establishing a functional thymic rudiment. However, the molecular mechanisms underlying the function of Foxn1 are still largely unclear. Here, we show that Foxn1 functions in thymus development through Mcm2 in the zebrafish. We demonstrate that, in foxn1 knockdown embryos, the thymic rudiment is reduced and T-cell development is impaired. Genome-wide expression profiling shows that a number of genes, including some known thymopoiesis genes, are dysregulated during the initiation of the thymus primordium and immigration of T-cell progenitors to the thymus. Functional and epistatic studies show that mcm2 and cdca7 are downstream of Foxn1, and mcm2 is a direct target gene of Foxn1 in TECs. Finally, we find that the thymus defects in foxn1 and mcm2 morphants might be attributed to reduced cell proliferation rather than apoptosis. Our results reveal that the foxn1-mcm2 axis plays a central role in the genetic regulatory network controlling thymus development in zebrafish.

Activation of lineage competence in hemogenic endothelium precedes the formation of hematopoietic stem cell heterogeneity.

Hematopoietic stem and progenitor cells (HSPCs) are considered as a heterogeneous population, but precisely when, where and how HSPC heterogeneity arises remain largely unclear. Here, using a combination of single-cell multi-omics, lineage tracing and functional assays, we show that embryonic HSPCs originate from heterogeneous hemogenic endothelial cells (HECs) during zebrafish embryogenesis. Integrated single-cell transcriptome and chromatin accessibility analysis demonstrates transcriptional heterogeneity and regulatory programs that prime lymphoid/myeloid fates at the HEC level. Importantly, spi2+ HECs give rise to lymphoid/myeloid-primed HSPCs (L/M-HSPCs) and display a stress-responsive function under acute inflammation. Moreover, we uncover that Spi2 is required for the formation of L/M-HSPCs through tightly controlling the endothelial-to-hematopoietic transition program. Finally, single-cell transcriptional comparison of zebrafish and human HECs and human induced pluripotent stem cell-based hematopoietic differentiation results support the evolutionary conservation of L/M-HECs and a conserved role of SPI1 (spi2 homolog in mammals) in humans. These results unveil the lineage origin, biological function and molecular determinant of HSPC heterogeneity and lay the foundation for new strategies for induction of transplantable lineage-primed HSPCs in vitro.

Identification of HSC/MPP expansion units in fetal liver by single-cell spatiotemporal transcriptomics.

Limited knowledge of cellular and molecular mechanisms underlying hematopoietic stem cell and multipotent progenitor (HSC/MPP) expansion within their native niche has impeded the application of stem cell-based therapies for hematological malignancies. Here, we constructed a spatiotemporal transcriptome map of mouse fetal liver (FL) as a platform for hypothesis generation and subsequent experimental validation of novel regulatory mechanisms. Single-cell transcriptomics revealed three transcriptionally heterogeneous HSC/MPP subsets, among which a CD93-enriched subset exhibited enhanced stem cell properties. Moreover, by employing integrative analysis of single-cell and spatial transcriptomics, we identified novel HSC/MPP 'pocket-like' units (HSC PLUS), composed of niche cells (hepatoblasts, stromal cells, endothelial cells, and macrophages) and enriched with growth factors. Unexpectedly, macrophages showed an 11-fold enrichment in the HSC PLUS. Functionally, macrophage-HSC/MPP co-culture assay and candidate molecule testing, respectively, validated the supportive role of macrophages and growth factors (MDK, PTN, and IGFBP5) in HSC/MPP expansion. Finally, cross-species analysis and functional validation showed conserved cell-cell interactions and expansion mechanisms but divergent transcriptome signatures between mouse and human FL HSCs/MPPs. Taken together, these results provide an essential resource for understanding HSC/MPP development in FL, and novel insight into functional HSC/MPP expansion ex vivo.

De novo generation of macrophage from placenta-derived hemogenic endothelium.

Macrophages play pivotal roles in immunity, hematopoiesis, and tissue homeostasis. In mammals, macrophages have been shown to originate from yolk-sac-derived erythro-myeloid progenitors and aorta-gonad-mesonephros (AGM)-derived hematopoietic stem cells. However, whether macrophages can arise from other embryonic sites remains unclear. Here, using single-cell RNA sequencing, we profile the transcriptional landscape of mouse fetal placental hematopoiesis. We uncover and experimentally validate that a CD44+ subpopulation of placental endothelial cells (ECs) exhibits hemogenic potential. Importantly, lineage tracing using the newly generated Hoxa13 reporter line shows that Hoxa13-labeled ECs can produce placental macrophages, named Hofbauer cell (HBC)-like cells. Furthermore, we identify two subtypes of HBC-like cells, and cell-cell interaction analysis identifies their potential roles in angiogenesis and antigen presentation, separately. Our study provides a comprehensive understanding of placental hematopoiesis and highlights the placenta as a source of macrophages, which has important implications for both basic and translational research.

Generation of foxn1/Casper Mutant Zebrafish for Allograft and Xenograft of Normal and Malignant Cells.

Cell transplantation into immunodeficient recipients is a widely used approach to study stem cell and cancer biology; however, studying cell states post transplantation in vivo is inconvenient in mammals. Here, we generated a foxn1/Casper mutant zebrafish that is transparent and exhibits T cell deficiency. By employing the line for hematopoietic stem cell (HSC) transplantation (HSCT), we could achieve nonconditioned transplantation. Meanwhile, we found that fetal HSCs from 3 days post fertilization zebrafish embryos produce a better transplant outcome in foxn1/Casper mutants, compared with adult HSCs. In addition to HSCT, the foxn1/Casper mutant is feasible for allografts of myelodysplastic syndrome-like and muscle cells, as well as xenografts of medaka muscle cells. In summary, foxn1/Casper mutants permit the nonconditioned engraftment of multiple cell types and visualized characterization of transplanted cells in vivo.

Primary cilia regulate hematopoietic stem and progenitor cell specification through Notch signaling in zebrafish.

Hematopoietic stem and progenitor cells (HSPCs) are capable of producing all mature blood lineages, as well as maintaining the self-renewal ability throughout life. The hairy-like organelle, cilium, is present in most types of vertebrate cells, and plays important roles in various biological processes. However, it is unclear whether and how cilia regulate HSPC development in vertebrates. Here, we show that cilia-specific genes, involved in primary cilia formation and function, are required for HSPC development, especially in hemogenic endothelium (HE) specification in zebrafish embryos. Blocking primary cilia formation or function by genetic or chemical manipulations impairs HSPC development. Mechanistically, we uncover that primary cilia in endothelial cells transduce Notch signal to the earliest HE for proper HSPC specification during embryogenesis. Altogether, our findings reveal a pivotal role of endothelial primary cilia in HSPC development, and may shed lights into in vitro directed differentiation of HSPCs.

HC-Pro protein of Potato virus Y can interact with three Arabidopsis 20S proteasome subunits in planta.

The multifunctional protein helper component proteinase (HC-Pro) is thought to interfere with the activity of the 20S proteasome; however, no sites of interaction have been identified for either protein. Here, we first show that the Potato virus Y (PVY) HC-Pro protein can interact with three Arabidopsis 20S proteasome subunits (PAA, PBB, and PBE), using a yeast two-hybrid system and the bimolecular fluorescence complement assay. In addition, yeast two-hybrid analysis of the interaction between several mutant subunits of the 20S proteasome and PVY HC-Pro confirmed that residues 81 to 140 of PAA, 1 to 80 of PBB, and 160 to 274 of PBE are necessary for binding PAA, PBB, and PBE to PVY HC-Pro, respectively. Deletion mutant analysis of PVY HC-Pro showed that the N terminus (residues 1 to 97) is necessary for its interaction with three Arabidopsis 20S proteasome subunits. The ability of HC-Pro to interact and interfere with the activity of the 20S proteasome may help explain the molecular basis of its multifunctional character.

The HC-pro protein of potato virus Y interacts with NtMinD of tobacco.

Potato virus Y (PVY) infections often lead to altered numbers of host plant chloroplasts, as well as changes in morphology and inhibited photosynthesis. The multifunctional protein helper component-proteinase, HC-Pro, has been identified in PVY-infected leaf chloroplasts. We used yeast two-hybrid and bimolecular fluorescence complementation assays to demonstrate that HC-Pro can interact with the chloroplast division-related factor NtMinD in yeast and tobacco cells, respectively. In addition, we confirmed that residues 271 to 314 in NtMinD are necessary for its interaction with PVY HC-Pro in a yeast two-hybrid analysis using four NtMinD deletion mutants. These residues are necessary for the dimerization of NtMinD, which plays a vital role in chloroplast division. Thus, PVY HC-Pro may affect NtMinD activity by inhibiting the formation of NtMinD homodimers, and this may interfere with chloroplast division and contribute to changes in the numbers of chloroplast per cell observed in PVY-infected plants.

The Vascular Niche Regulates Hematopoietic Stem and Progenitor Cell Lodgment and Expansion via klf6a-ccl25b.

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, Krüppel-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.

Essential role for SUN5 in anchoring sperm head to the tail.

SUN (Sad1 and UNC84 domain containing)-domain proteins are reported to reside on the nuclear membrane playing distinct roles in nuclear dynamics. SUN5 is a new member of the SUN family, with little knowledge regarding its function. Here, we generated Sun5-/- mice and found that male mice were infertile. Most Sun5-null spermatozoa displayed a globozoospermia-like phenotype but they were actually acephalic spermatozoa. Additional studies revealed that SUN5 was located in the neck of the spermatozoa, anchoring sperm head to the tail, and without functional SUN5 the sperm head to tail coupling apparatus was detached from nucleus during spermatid elongation. Finally, we found that healthy heterozygous offspring could be obtained via intracytoplasmic injection of Sun5-mutated sperm heads for both male mice and patients. Our studies reveal the essential role of SUN5 in anchoring sperm head to the tail and provide a promising way to treat this kind of acephalic spermatozoa-associated male infertility.