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Background and aims During liver development, bipotent progenitor cells differentiate into hepatocytes and biliary epithelial cells to ensure a functional liver required to maintain organismal homeostasis. The developmental cues controlling the differentiation of committed progenitors into these cell types, however, are incompletely understood. Here, we discover an essential role for estrogenic regulation in vertebrate liver development to affect hepatobiliary fate decisions. Approach and results Exposure of zebrafish embryos to 17β-estradiol (E2) during liver development significantly decreased hepatocyte-specific gene expression, liver size, and hepatocyte number. In contrast, pharmacological blockade of estrogen synthesis or nuclear estrogen receptor (ESR) signaling enhanced liver size and hepatocyte marker expression. Transgenic reporter fish demonstrated nuclear ESR activity in the developing liver. Chemical inhibition and morpholino knockdown of nuclear estrogen receptor 2b (esr2b) increased hepatocyte gene expression and blocked the effects of E2 exposure. esr2b-/- mutant zebrafish exhibited significantly increased expression of hepatocyte markers with no impact on liver progenitors, other endodermal lineages, or vasculature. Significantly, E2-stimulated Esr2b activity promoted biliary epithelial differentiation at the expense of hepatocyte fate, whereas loss of esr2b impaired biliary lineage commitment. Chemical and genetic epistasis studies identified bone morphogenetic protein (BMP) signaling as a mediator of the estrogen effects. The divergent impact of estrogen on hepatobiliary fate was confirmed in a human hepatoblast cell line, indicating the relevance of this pathway for human liver development. Conclusions Our studies identify E2, esr2b, and downstream BMP activity as important regulators of hepatobiliary fate decisions during vertebrate liver development. These results have significant clinical implications for liver development in infants exposed to abnormal estrogen levels or estrogenic compounds during pregnancy.

Rationale: Genome editing by CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 is evolving rapidly. Recently, second-generation CRISPR/Cas9 activation systems based on nuclease inactive dead (d)Cas9 fused to transcriptional transactivation domains were developed for directing specific guide (g)RNAs to regulatory regions of any gene of interest, to enhance transcription. The application of dCas9 to activate cardiomyocyte transcription in targeted genomic loci in vivo has not been demonstrated so far. Objective: We aimed to develop a mouse model for cardiomyocyte-specific, CRISPR-mediated transcriptional modulation, and to demonstrate its versatility by targeting Mef2d and Klf15 loci (2 well-characterized genes implicated in cardiac hypertrophy and homeostasis) for enhanced transcription. Methods and Results: A mouse model expressing dCas9 with the VPR transcriptional transactivation domains under the control of the Myh (myosin heavy chain) 6 promoter was generated. These mice innocuously expressed dCas9 exclusively in cardiomyocytes. For initial proof-of-concept, we selected Mef2d , which when overexpressed, led to hypertrophy and heart failure, and Klf15 , which is lowly expressed in the neonatal heart. The most effective gRNAs were first identified in fibroblast (C3H/10T1/2) and myoblast (C2C12) cell lines. Using an improved triple gRNA expression system (TRISPR [triple gRNA expression construct]), up to 3 different gRNAs were transduced simultaneously to identify optimal conditions for transcriptional activation. For in vivo delivery of the validated gRNA combinations, we employed systemic administration via adeno-associated virus serotype 9. On gRNA delivery targeting Mef2d expression, we recapitulated the anticipated cardiac hypertrophy phenotype. Using gRNA targeting Klf15 , we could enhance its transcription significantly, although Klf15 is physiologically silenced at that time point. We further confirmed specific and robust dCas9VPR on-target effects. Conclusions: The developed mouse model permits enhancement of gene expression by using endogenous regulatory genomic elements. Proof-of-concept in 2 independent genomic loci suggests versatile applications in controlling transcription in cardiomyocytes of the postnatal heart.

Cardiac remodelling is accompanied by silencing of genes necessary for cardiac homeostasis including Krüppel-like factor 15 ( Klf15 ), an anti-hypertrophic factor. Due to a lack of tools for gene activation, we aim to adapt a CRISPR-based approach for gene expression control for mammalian cardiomyocytes. We generated a transgenic mouse model (TG) expressing an enzymatically inactive, gRNA-programmable Cas9 protein under Myh6 promoter control fused to transcriptional activators (VPR) to drive gene expression. As a proof of concept, we systemically injected mice at postnatal day P4 with AAV9 carrying a triple gRNA expression cassette containing gRNAs targeted to the Klf15 promoter region. Significant Klf15 activation was observed at P10 up to 10-fold compared to controls in three independent TG lines. This Klf15 activation was sufficient to drive target gene expression of Aldh2 and Adhfe1 (controls = WT + saline, WT + AAV9, TG + saline, n ≥ 3 per group, ANOVA and Bonferroni correction). Importantly, at the neonatal stage, the Klf15 promoter is characterized by low H3K27ac presence indicating that endogenous gene activation can enhance transcription from an epigenetically inactive locus. We demonstrated that CRISPR/Cas9 mediated loss of KLF15 expression results in impaired contractile function in a 3D model of engineering heart muscle, similar to the Klf15 -/- mouse heart phenotype. Encouraged by the evolutionary conserved KLF15 -mediated mechanisms, we aimed to generate a human induced pluripotent stem cells, which constitutively express dCas9VPR (hIPSC-dCas9VPR) for gene activation in hIPSC-derived cardiomyocytes to complement the in vivo findings upon disease. We targeted the transgene integration to the AAVS1 site by CRISPR gene editing, confirmed correct integration by PCR followed by sequencing and hIPSC-dCas9VPR were successfully differentiated into beating cardiomyocytes. To this end, we have tested gRNAs targeted to the KLF15 promoter region in HEK293 cells and observed significant KLF15 activation of up to 5.5-fold (n = 3, ANOVA and Bonferroni correction). In summary, we report transgenic mammalian tools for endogenous gene (re-) activation purposes to identify potential therapeutic targets for preventing heart failure progression.

During development, hematopoietic stem cells (HSCs) emerge from aortic endothelial cells (ECs) through an intermediate stage called hemogenic endothelium by a process known as endothelial‐to‐hematopoietic transition (EHT). While Notch signaling, including its upstream regulator Vegf, is known to regulate this process, the precise molecular control and temporal specificity of Notch activity remain unclear. Here, we identify the zebrafish transcriptional regulator evi1 as critically required for Notch‐mediated EHT. In vivo live imaging studies indicate that evi1 suppression impairs EC progression to hematopoietic fate and therefore HSC emergence. evi1 is expressed in ECs and induces these effects cell autonomously by activating Notch via pAKT. Global or endothelial‐specific induction of notch, vegf, or pAKT can restore endothelial Notch and HSC formations in evi1 morphants. Significantly, evi1 overexpression induces Notch independently of Vegf and rescues HSC numbers in embryos treated with a Vegf inhibitor. In sum, our results unravel evi1 –pAKT as a novel molecular pathway that, in conjunction with the shh–vegf axis, is essential for activation of Notch signaling in VDA endothelial cells and their subsequent conversion to HSCs.

Myocardin-Related Transcription Factors A and B (MRTF-A and MRTF-B) are highly homologous proteins that function as powerful coactivators of serum response factor (SRF), a ubiquitously expressed transcription factor essential for cardiac development. The SRF/MRTF complex binds to CArG boxes found in the control regions of genes that regulate cytoskeletal dynamics and muscle contraction, among other processes. While SRF is required for heart development and function, the role of MRTFs in the developing or adult heart has not been explored. Through cardiac-specific deletion of MRTF alleles in mice, we show that either MRTF-A or MRTF-B is dispensable for cardiac development and function, whereas deletion of both MRTF-A and MRTF-B causes a spectrum of structural and functional cardiac abnormalities. Defects observed in MRTF-A/B null mice ranged from reduced cardiac contractility and adult onset heart failure to neonatal lethality accompanied by sarcomere disarray. RNA-seq analysis on neonatal hearts identified the most altered pathways in MRTF double knockout hearts as being involved in cytoskeletal organization. Together, these findings demonstrate redundant but essential roles of the MRTFs in maintenance of cardiac structure and function and as indispensible links in cardiac cytoskeletal gene regulatory networks.

Cannabinoids (CB) modulate adult hematopoietic stem and progenitor cell (HSPCs) function, however, impact on the production, expansion, or migration of embryonic HSCs is currently uncharacterized. Here, using chemical and genetic approaches targeting CB-signaling in zebrafish, we show that CB receptor (CNR) 2, but not CNR1, regulates embryonic HSC development. During HSC specification in the aorta-gonad-mesonephros (AGM) region, CNR2 stimulation by AM1241 increased runx1;cmyb+ HSPCs, through heightened proliferation, whereas CNR2 antagonism decreased HSPC number; FACS analysis and absolute HSC counts confirmed and quantified these effects. Epistatic investigations showed AM1241 significantly upregulated PGE2 synthesis in a Ptgs2-dependent manner to increase AGM HSCs. During the phases of HSC production and colonization of secondary niches, AM1241 accelerated migration to the caudal hematopoietic tissue (CHT), the site of embryonic HSC expansion, and the thymus; however these effects occurred independently of PGE2. Using a candidate approach for HSC migration and retention factors, P-selectin was identified as the functional target of CNR2 regulation. Epistatic analyses confirmed migration of HSCs into the CHT and thymus was dependent on CNR2-regulated P-selectin activity. Together, these data suggest CNR2-signaling optimizes the production, expansion, and migration of embryonic HSCs by modulating multiple downstream signaling pathways. Stem Cells 2015;33:2596—2612

The genetic control of hematopoietic stem and progenitor cell (HSPC) function is increasingly understood, however less is known about the interactions specifying the embryonic hematopoietic niche. Here, we report 17β-estradiol (E2) influences the production of runx1+ HSPCs in the AGM region by antagonizing VEGF-signaling and subsequent assignment of hemogenic endothelial (HE) identity. Exposure to exogenous E2 during vascular niche development (12–24hpf) significantly disrupted flk1+ vessel maturation, ephrinB2+ arterial identity and specification of scl+ HE by decreasing the expression of VEGFAa and downstream arterial Notch-pathway components; heat-shock induction of VEGFAa/Notch rescued E2-mediated hemato-vascular defects. Conversely, repression of endogenous E2-activity increased somitic VEGF expression and vascular-target regulation, shifting the assignment of arterial/venous fate and HE localization; blocking E2-signaling allowed venous production of scl+/runx1+ cells, independent of arterial identity acquisition. Together, these data suggest yolk-derived estrogen sets the ventral boundary of hemogenic vascular niche specification by antagonizing the dorsal-ventral limits of VEGF regulation.

Many pathways regulating blood formation have been elucidated, yet how each coordinates with embryonic biophysiology to modulate the spatiotemporal production of hematopoietic stem cells (HSCs) is currently unresolved. Here, we report that glucose metabolism impacts the onset and magnitude of HSC induction in vivo. In zebrafish, transient elevations in physiological glucose levels elicited dose-dependent effects on HSC development, including enhanced runx1 expression and hematopoietic cluster formation in the aorta-gonad-mesonephros region; embryonic-to-adult transplantation studies confirmed glucose increased functional HSCs. Glucose uptake was required to mediate the enhancement in HSC development; likewise, metabolic inhibitors diminished nascent HSC production and reversed glucose-mediated effects on HSCs. Increased glucose metabolism preferentially impacted hematopoietic and vascular targets, as determined by gene expression analysis, through mitochondrial-derived reactive oxygen species (ROS)-mediated stimulation of hypoxia-inducible factor 1α (hif1α). Epistasis assays demonstrated that hif1α regulates HSC formation in vivo and mediates the dose-dependent effects of glucose metabolism on the timing and magnitude of HSC production. We propose that this fundamental metabolic-sensing mechanism enables the embryo to respond to changes in environmental energy input and adjust hematopoietic output to maintain embryonic growth and ensure viability.

Growth Factor Independence (Gfi) transcription factors play essential roles in hematopoiesis, differentially activating and repressing transcriptional programs required for hematopoietic stem/progenitor cell (HSPC) development and lineage specification. In mammals, Gfi1a regulates hematopoietic stem cells (HSC), myeloid and lymphoid populations, while its paralog, Gfi1b, regulates HSC, megakaryocyte and erythroid development. In zebrafish, gfi1aa is essential for primitive hematopoiesis; however, little is known about the role of gfi1aa in definitive hematopoiesis or about additional gfi factors in zebrafish. Here, we report the isolation and characterization of an additional hematopoietic gfi factor, gfi1b. We show that gfi1aa and gfi1b are expressed in the primitive and definitive sites of hematopoiesis in zebrafish. Our functional analyses demonstrate that gfi1aa and gfi1b have distinct roles in regulating primitive and definitive hematopoietic progenitors, respectively. Loss of gfi1aa silences markers of early primitive progenitors, scl and gata1. Conversely, loss of gfi1b silences runx-1, c-myb, ikaros and cd41, indicating that gfi1b is required for definitive hematopoiesis. We determine the epistatic relationships between the gfi factors and key hematopoietic transcription factors, demonstrating that gfi1aa and gfi1b join lmo2, scl, runx-1 and c-myb as critical regulators of teleost HSPC. Our studies establish a comparative paradigm for the regulation of hematopoietic lineages by gfi transcription factors.

The recently identified 2-component clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system has revolutionized the ease with which genome editing can be performed, greatly simplifying the process of generating knockout animal models.1,2 However, in spite of the great advances made, fewer studies have focused on the efficacy of the system for editing postnatal or adult tissues as a means to model adult onset diseases or to provide potential therapeutic intervention. In particular, the use of the system to perform genome editing in a tissue-specific manner remains to be thoroughly evaluated, especially for tissues that are less amenable to viral delivery of the CRISPR/Cas9 components, a common strategy for delivery in adult tissues.3 Because the heart is responsible for a large disease burden, it represents a particularly attractive tissue to target for therapeutic purposes.4 Although several studies have shown that cardiomyocytes can be edited in the postnatal murine heart using viral delivery of CRISPR/Cas9 system components,5–7 the efficiency of this phenomenon has been less well characterized. Article, see p 1168 A study in this issue by Johansen et al8 from Eva van Rooij’s group performed an in-depth analysis of CRISPR/Cas9-mediated gene editing in postnatal mouse myocytes. Cardiac-restricted expression of Cas9 was inducing using mice containing a floxed Cas9 cassette with a GFP (green fluorescent protein) reporter knocked into the Rosa26 locus together with an αMHC-Cre (alpha-myosin heavy chain-Cre) transgene. Consistent with other models that have expressed Cas9 in either a broad or tissue restricted manner,5,6,9 no changes in baseline cardiac function were observed after Cas9 overexpression, suggesting that Cas9 …

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) 9 genomic editing has revolutionized the generation of mutant animals by simplifying the creation of null alleles in virtually any organism. However, most current approaches with this method require zygote injection, making it difficult to assess the adult, tissue-specific functions of genes that are widely expressed or which cause embryonic lethality when mutated. Here, we describe the generation of cardiac-specific Cas9 transgenic mice, which express high levels of Cas9 in the heart, but display no overt defects. In proof-of-concept experiments, we used Adeno-Associated Virus 9 (AAV9) to deliver single-guide RNA (sgRNA) that targets the Myh6 locus exclusively in cardiomyocytes. Intraperitoneal injection of postnatal cardiac-Cas9 transgenic mice with AAV9 encoding sgRNA against Myh6 resulted in robust editing of the Myh6 locus. These mice displayed severe cardiomyopathy and loss of cardiac function, with elevation of several markers of heart failure, confirming the effectiveness of this method of adult cardiac gene deletion. Mice with cardiac-specific expression of Cas9 provide a tool that will allow rapid and accurate deletion of genes following a single injection of AAV9-sgRNAs, thereby circumventing embryonic lethality. This method will be useful for disease modeling and provides a means of rapidly editing genes of interest in the heart.

The Evi1 locus was originally identified as a common site of retroviral integration in murine myeloid tumors. Several reports associate Evi1 expression with aggressiveness in myeloid leukemia. Since developmental pathways often reactivate in cancer, we hypothesized that Evi1 also plays critical roles during developmental hematopoiesis. Here, we employ the zebrafish model to study how evi1 modulates early blood development. We find that indeed zebrafish evi1 co-localizes with the hematopoietic markers scl, gata1, pu.1 and gata2 in the posterior lateral mesoderm and the rostral blood islands, indicating involvement in primitive hematopoiesis. Knockdown of evi1 via three independent Morpholino Oligonucleotides impairs embryonic myelopoiesis as shown by reduced pu.1, mpo and l-plastin staining, while not affecting hemangioblast formation and primitive erythropoiesis. Additionally, we observe reduced levels of cd41 expression upon evi1 knockdown, indicating that megakaryopoiesis is also impaired. Notably, at later time-points, evi1 is also expressed in the AGM region and evi1 morphants show strong reduction of runx1/c-myb expression in this region, demonstrating an additional role in hematopoietic stem cell (HSC) formation. Consistently, evi1 morphants lack ikaros+ lymphocyte precursor cells and rag1+ T-lymphocytes, and less circulating globin+, lyz+ and cd41+ cells are detected in transgenic fish analyzed by flow cytometry at 5 days post fertilization. To dissect the mechanisms by which evi1 regulates HSC formation, we furthermore analyzed genes specifically expressed in the dorsal aorta, where HSCs emerge from the hemogenic endothelium. We hypothesize that evi1 is indeed regulating HSC specification from hemogenic endothelial cells, since detailed analyses show defective aortic expression of efnb2a and dlc. How this effect is mediated is currently being investigated. Furthermore, TUNEL, anti-activated Caspase-3, anti-phospho Histone H3 and BrdU assays show both increased apoptosis and decreased proliferation in the AGM region of evi1 morphants as compared to control injected fish, suggesting important additional effects of evi1 on HSC biology beyond the specification step. Similar effects are seen in the rostral blood islands and are possibly responsible for the observed reduction of primitive myeloid progenitors. We also examined potential downstream targets of evi1. Previous reports in adult murine hematopoietic cells suggest that Evi1 affects HSC proliferation through regulation of Gata2. Indeed, gata2 co-injection enhances hematopoietic cell viability also in zebrafish embryos, and co-injection of gata2 mRNA is able to fully rescue both primitive myelopoiesis and HSC formation. We are currently investigating whether gata2 expression can compensate evi1 requirement during HSC specification or, alternatively, might rescue HSC numbers by amplifying residual HSCs that escape evi1 inhibition. Taken together, our data demonstrate that evi1 plays multiple roles during hematopoietic development, critically regulating the biology of primitive myeloid progenitors and pre-formed HSCs, as well as HSC specification from the hemogenic endothelium. Currently, we analyze the molecular mechanisms that mediate evi1 effects during these different regulatory steps and use the CRISPR/Cas9 system to generate evi1 mutant fish for further validation of our findings. Disclosures No relevant conflicts of interest to declare.

While the genetic control of hematopoietic stem cell (HSC) function is increasingly understood, less is known about factors that pattern the embryonic hematopoietic niche and specify the location of HSC emergence. 17beta-estradiol (E2) and related estrogenic compounds were identified in a zebrafish chemical genetic screen as modifiers of the number of runx1+ HSCs in the Aorta-Gonad-Mesonephros region. Exposure to exogenous E2 during hematopoietic specification significantly decreased production of runx1+ HSCs by in situ hybridization (ISH) and qPCR (p Disclosures: No relevant conflicts of interest to declare.

Abstract 3379 The Growth Factor Independence (Gfi) zinc finger transcription factors play essential roles in hematopoiesis, differentially activating and repressing transcriptional programs required for hematopoietic lineage specification. In mammals, Gfi1 regulates hematopoietic stem cell (HSC) and lymphoid populations, while Gfi1b is required for megakaryocyte and erythroid development (van der Meer, et al. 2010 Leukemia 11:1834–43). In zebrafish, gfi1.1 plays an essential role in primitive hematopoiesis, preserving primitive HSC populations and regulating the erythroid-myeloid balance (Wei, et al. 2008 Cell Res. 6:677–85). However, little is known about the role of gfi1.1 in definitive hematopoiesis or about the role of additional hematopoietic gfi family members in zebrafish. Here, we report the isolation and characterization of an additional zebrafish gfi family transcription factor, gfi1.2b. We compare and contrast gfi1.1 and gfi1.2b, showing that they are highly expressed in the intermediate cell mass (ICM) and aorta-gonad-mesonephros (AGM), the respective sites of primitive and definitive hematopoiesis in zebrafish. Using antisense morpholino oligos (MO), whole mount in situ hybridization (WISH) and fluorescent activated cell sorting (FACS) of transgenic reporter fish, we demonstrate that gfi1.1 and gfi1.2b have distinct, essential roles in preserving primitive and definitive HSC populations, respectively. Loss of gfi1.1 specifically silences expression of scl and gata-1, markers of primitive HSC and erythroid progenitors. Conversely, loss of gfi1.2b silences expression of Tg(cd41:eGFPlo) cells, indicating an essential role for gfi1.2b in preserving definitive hematopoietic progenitors (Ma, et al. 2011 Blood 118:289–297). Consistent with the discrete roles of gfi1.1 and gfi1.2b in primitive and definitive lineages, knockdown of gfi1.2b silences lymphocyte rag-1 expression in the developing thymus, while knockdown of gfi1.1 has no effect on the thymic lymphocyte population. gfi1.1 and gfi1.2b have overlapping roles in erythropoiesis, as loss of either gfi factor reduces erythrocyte populations, while loss of both gfi paralogs results in a more profound silencing of erythrocytes. We further demonstrate that loss of gata-1 reduces gfi1.1 expression and silences gfi1.2b, suggesting that gata-1 plays an essential role in regulating the transcription of both genes. Together, these studies demonstrate that gfi1.1 and gfi1.2b have distinct and overlapping roles in zebrafish hematopoiesis and establish a new paradigm for the regulation of primitive and definitive hematopoietic lineages by gfi transcription factors. Disclosures: No relevant conflicts of interest to declare.

Abstract 1617 The intrinsic signaling pathways regulating hematopoietic stem cells (HSC) are increasingly well recognized. However, less is known about how in utero exposure to common environmental xenobiotic compounds may alter HSC development and increase the risk of carcinogenesis. RUNX1 (AML1), required for definitive HSC induction in all vertebrates, is the target of frequent chromosomal alterations associated with leukemia. Through a chemical genetic screen for modifiers of runx1 expression in the zebrafish, estrogen-related compounds were identified. Here, we found that exposure to 17β-estradiol (E2) throughout the initial waves of hematopoietic development (5 somites (som) to 36 hours post fertilization (hpf)) significantly altered the number of runx1+ HSCs in the zebrafish Aorta-Gonad-Mesonephros Region (AGM) compared to controls (n≥25-50 embryos /condition). Other physiological estrogens, such as estrone and estriol, elicited a similar hematopoietic response. However, treatment with either the isomer 17α-estradiol, or the related steroid hormones testosterone or progesterone, could not mimic the effect of E2 on HSCs. Use of the aromatase inhibitor anastrozole and the pan-estrogen receptor inhibitor fulvestrant confirmed that estrogen was both required for nascent HSC regulation and functioned through classical estrogen receptor (esr) signaling. Microarray analysis of FACS-sorted cell populations during zebrafish development demonstrated differential spatio-temporal regulation of esr1 (esrα) and esr2a/b (esrβ) in vascular and hematopoietic cell types. During the primitive wave of hematopoiesis, exposure to E2 and the esr1-agonist PPT significantly enhanced red blood cell number as seen by in situ hybridization for embryonic globin (hbbe3) and quantified by fluorescent microscopy and FACS analysis of the Tg(globin:GFP) line. Conversely, the esr2-specific agonist DPN diminished definitive HSC formation after exposure from 5 som to 24 hpf; this phenotype was mediated by disruption of vessel formation, as indicated by flk1 (kdrl) expression, and alteration in the assignment of artery-vein identity. Interestingly, when exposure to E2 or DPN occurred from 24 – 36 hpf, after the establishment of ephb2+ arteries and the initiation of blood flow, estrogen treatment enhanced HSC formation; this was confirmed by FACS analysis and fluorescent microscopy using the Tg(cmyb:eGFP) and Tg(-6.0itga2b:eGFP)la2 (CD41:GFP) HSC-reporter lines. E2 treatment was found to elicit both pro-apoptotic (TUNEL+) and pro-proliferative (BrdU+) effects on HSCs and the vascular niche depending on the timing of exposure, but independent of the concentration of E2 over the physiological range and above (10nM to 10mM). Morpholino-mediated gene knockdown of esr1 and the two esr2 alleles alone and in combination with E2 confirmed that esr2 was responsible for the effects on definitive hematopoiesis. Using the Tg(TOP:GFP)w25 line, alterations in estrogen signaling were shown to mediate effects on wnt activity. To determine whether exposure to environmental estrogens could mediate similar alterations in HSC specification and proliferation, we exposed embryos to the phytoestrogen genistein, the synthetic estrogen ethinylestradiol, and the xenoestrogen bisphenol A (BPA) and found results reminiscent of E2; using fulvestrant, we confirmed that the phenotype elicited by each was dependent on estrogen receptor stimulation. In an adult zebrafish marrow injury model, E2 significantly enhanced stem and progenitor cell regeneration in males and females by day 10 post irradiation (n≥10 /condition). Intriguingly, we found that females, with higher circulating estrogen levels, recovered better after injury than male siblings, both in the presence and absence of exogenous estrogen. Finally, murine bone marrow treated with E2 or DPN produced significantly (n=10 /condition, p Disclosures: Goessling: Fate Therapeutics: Consultancy, Patents & Royalties. North:Fate Therapeutics: Consultancy, Patents & Royalties.

SummaryThe liver and pancreas arise from common endodermal progenitors. How these distinct cell fates are specified is poorly understood. Here we describe prostaglandin E2 (PGE2) as a regulator of endodermal fate specification during development. Modulating PGE2 activity has opposing effects on liver versus pancreas specification in zebrafish embryos as well as mouse endodermal progenitors. The PGE2 synthetic enzyme cox2a and receptor ep2a are patterned such that cells closest to PGE2 synthesis acquire a liver fate, whereas more distant cells acquire a pancreas fate. PGE2 interacts with the bmp2b pathway to regulate fate specification. At later stages of development, PGE2 acting via the ep4a receptor promotes outgrowth of both the liver and pancreas. PGE2 remains important for adult organ growth, as it modulates liver regeneration. This work provides in vivo evidence that PGE2 may act as a morphogen to regulate cell-fate decisions and outgrowth of the embryonic endodermal anlagen.