Vhnf1 acts downstream of Bmp, Fgf, and RA signals to regulate endocrine beta cell development in zebrafish

Identification of downstream effectors of retinoic acid specifying the zebrafish pancreas by integrative genomics

Retinoic acid (RA) is a key signal for the specification of the pancreas. Still, the gene regulatory cascade triggered by RA in the endoderm remains poorly characterized. In this study, we investigated this regulatory network in zebrafish by combining RNA-seq, RAR ChIP-seq and ATAC-seq assays. By analysing the effect of RA and of the RA receptor (RAR) inverse-agonist BMS493 on the transcriptome and on the chromatin accessibility of endodermal cells, we identified a large set of genes and regulatory regions regulated by RA signalling. RAR ChIP-seq further defined the direct RAR target genes in zebrafish, including hox genes as well as several pancreatic regulators like mnx1, insm1b, hnf1ba and gata6. Comparison of zebrafish and murine RAR ChIP-seq data highlighted the conserved direct target genes and revealed that some RAR sites are under strong evolutionary constraints. Among them, a novel highly conserved RAR-induced enhancer was identified downstream of the HoxB locus and driving expression in the nervous system and in the gut in a RA-dependent manner. Finally, ATAC-seq data unveiled the role of the RAR-direct targets Hnf1ba and Gata6 in opening chromatin at many regulatory loci upon RA treatment.

Retinoic acid-induced expression of Hnf1b and Fzd4 is required for pancreas development in Xenopus laevis

Retinoic acid (RA) is required for pancreas specification in Xenopus and other vertebrates. However, the gene network that is directly induced by RA signalling in this context remains to be defined. By RNA sequencing of in vitro-generated pancreatic explants, we identified the genes encoding the transcription factor Hnf1β and the Wnt-receptor Fzd4/Fzd4s as direct RA target genes. Functional analyses of Hnf1b and Fzd4/Fzd4s in programmed pancreatic explants and whole embryos revealed their requirement for pancreatic progenitor formation and differentiation. Thus, Hnf1β and Fzd4/Fzd4s appear to be involved in pre-patterning events of the embryonic endoderm that allow pancreas formation in Xenopus.

FGF1 Mediates Overnutrition-Induced Compensatory β-Cell Differentiation

Increased insulin demand resulting from insulin resistance and/or overnutrition induces a compensatory increase in β-cell mass. The physiological factors responsible for the compensation have not been fully characterized. In zebrafish, overnutrition rapidly induces compensatory β-cell differentiation through triggering the release of a paracrine signal from persistently activated β-cells. We identified Fgf1 signaling as a key component of the overnutrition-induced β-cell differentiation signal in a small molecule screen. Fgf1 was confirmed as the overnutrition-induced β-cell differentiation signal, as inactivation of fgf1 abolished the compensatory β-cell differentiation. Furthermore, expression of human FGF1 solely in β-cells in fgf1(-/-) animals rescued the compensatory response, indicating that β-cells can be the source of FGF1. Additionally, constitutive secretion of FGF1 with an exogenous signal peptide increased β-cell number in the absence of overnutrition. These results demonstrate that fgf1 is necessary and FGF1 expression in β-cells is sufficient for the compensatory β-cell differentiation. We further show that FGF1 is secreted during prolonged activation of cultured mammalian β-cells and that endoplasmic reticulum stress acts upstream of FGF1 release. Thus, the recently discovered antidiabetes function of FGF1 may act partially through increasing β-cell differentiation.

Full transcriptome analysis of early dorsoventral patterning in zebrafish

Understanding the molecular interactions that lead to the establishment of the major body axes during embryogenesis is one of the main goals of developmental biology. Although the past two decades have revolutionized our knowledge about the genetic basis of these patterning processes, the list of genes involved in axis formation is unlikely to be complete. In order to identify new genes involved in the establishment of the dorsoventral (DV) axis during early stages of zebrafish embryonic development, we employed next generation sequencing for full transcriptome analysis of normal embryos and embryos lacking overt DV pattern. A combination of different statistical approaches yielded 41 differentially expressed candidate genes and we confirmed by in situ hybridization the early dorsal expression of 32 genes that are transcribed shortly after the onset of zygotic transcription. Although promoter analysis of the validated genes suggests no general enrichment for the binding sites of early acting transcription factors, most of these genes carry “bivalent” epigenetic histone modifications at the time when zygotic transcription is initiated, suggesting a “poised” transcriptional status. Our results reveal some new candidates of the dorsal gene regulatory network and suggest that a plurality of the earliest upregulated genes on the dorsal side have a role in the modulation of the canonical Wnt pathway.

Specification of hepatopancreas progenitors in zebrafish by hnf1ba and wnt2bb

Although the liver and ventral pancreas are thought to arise from a common multipotent progenitor pool, it is unclear whether these progenitors of the hepatopancreas system are specified by a common genetic mechanism. Efforts to determine the role of Hnf1b and Wnt signaling in this crucial process have been confounded by a combination of factors, including a narrow time frame for hepatopancreas specification, functional redundancy among Wnt ligands, and pleiotropic defects caused by either severe loss of Wnt signaling or Hnf1b function. Using a novel hypomorphic hnf1ba zebrafish mutant that exhibits pancreas hypoplasia, as observed in HNF1B monogenic diabetes, we show that hnf1ba plays essential roles in regulating β-cell number and pancreas specification, distinct from its function in regulating pancreas size and liver specification, respectively. By combining Hnf1ba partial loss of function with conditional loss of Wnt signaling, we uncover a crucial developmental window when these pathways synergize to specify the entire ventrally derived hepatopancreas progenitor population. Furthermore, our in vivo genetic studies demonstrate that hnf1ba generates a permissive domain for Wnt signaling activity in the foregut endoderm. Collectively, our findings provide a new model for HNF1B function, yield insight into pancreas and β-cell development, and suggest a new mechanism for hepatopancreatic specification.

Rargb regulates organ laterality in a zebrafish model of right atrial isomerism

Developmental signals determine organ morphology and position during embryogenesis. To discover novel modifiers of liver development, we performed a chemical genetic screen in zebrafish and identified retinoic acid as a positive regulator of hepatogenesis. Knockdown of the four RA receptors revealed that all receptors affect liver formation, however specific receptors exert differential effects. Rargb knockdown results in bilateral livers but does not impact organ size, revealing a unique role for Rargb in conferring left-right positional information. Bilateral populations of hepatoblasts are detectable in rargb morphants, indicating Rargb acts during hepatic specification to position the liver, and primitive endoderm is competent to form liver on both sides. Hearts remain at the midline and gut looping is perturbed in rargb morphants, suggesting Rargb affects lateral plate mesoderm migration. Overexpression of Bmp during somitogenesis similarly results in bilateral livers and midline hearts, and inhibition of Bmp signaling rescues the rargb morphant phenotype, indicating Rargb functions upstream of Bmp to regulate organ sidedness. Loss of rargb causes biliary and organ laterality defects as well as asplenia, paralleling symptoms of the human condition right atrial isomerism. Our findings uncover a novel role for RA in regulating organ laterality and provide an animal model of one form of human heterotaxia.

Depletion of Bhmt elevates sonic hedgehog transcript level and increases β-cell number in zebrafish

Betaine homocysteine S-methyltransferase (BHMT, EC 2.1.1.5) is a key enzyme in the methionine cycle and is highly expressed in the liver. Despite its important biochemical function, it is not known whether BHMT plays a role during organ development. In this report, we showed that early in development of zebrafish before endoderm organogenesis, bhmt is first expressed in the yolk syncytial layer and then after liver formation becomes a liver-enriched gene. By using the anti-bhmt morpholinos that deplete the Bhmt, we found that in morphant embryos, several endoderm-derived organs, including liver, exocrine pancreas, and intestine are hypoplastic. Strikingly, the number of β-cells in the pancreatic islet was increased rather than reduced in the morphant. Additional studies showed that Bhmt depletion elevates the sonic hedgehog (shh) transcript level in the morphant, whereas Bhmt-depletion in the Shh-deficient mutant syu failed to rescue the isletless phenotype. These molecular and genetic data strongly suggest that Shh functions downstream of Bhmt to promote β-cell development. Therefore, although there are still many intriguing questions to be answered, our finding may identify a novel function for Bhmt involving modulation of Shh signaling to control β-cell development.

Antagonistic interactions of hedgehog, Bmp and retinoic acid signals control zebrafish endocrine pancreas development

Pancreatic organogenesis is promoted or restricted by different signaling pathways. In amniotes, inhibition of hedgehog (Hh) activity in the early embryonic endoderm is a prerequisite for pancreatic specification. However, in zebrafish, loss of Hh signaling leads to a severe reduction of β-cells, leading to some ambiguity as to the role of Hh during pancreas development and whether its function has completely diverged between species. Here, we have employed genetic and pharmacological manipulations to temporally delineate the role of Hh in zebrafish endocrine pancreas development and investigate its relationship with the Bmp and retinoic acid (RA) signaling pathways. We found that Hh is required at the start of gastrulation for the medial migration and differentiation of pdx1-expressing pancreatic progenitors at later stages. This early positive role of Hh promotes β-cell lineage differentiation by restricting the repressive effects of Bmp. Inhibition of Bmp signaling in the early gastrula leads to increased β-cell numbers and partially rescued β-cell formation in Hh-deficient embryos. By the end of gastrulation, Hh switches to a negative role by antagonizing RA-mediated specification of the endocrine pancreas, but continues to promote differentiation of exocrine progenitors. We show that RA downregulates the Hh signaling components ptc1 and smo in endodermal explants, indicating a possible molecular mechanism for blocking axial mesoderm-derived Hh ligands from the prepancreatic endoderm during the specification stage. These results identify multiple sequential roles for Hh in pancreas development and highlight an unexpected antagonistic relationship between Hh and other signaling pathways to control pancreatic specification and differentiation.

Rfx6 is an Ngn3-dependent winged helix transcription factor required for pancreatic islet cell development

The transcription factor neurogenin 3 (Neurog3 or Ngn3) controls islet cell fate specification in multipotent pancreatic progenitor cells in the mouse embryo. However, our knowledge of the genetic programs implemented by Ngn3, which control generic and islet subtype-specific properties, is still fragmentary. Gene expression profiling in isolated Ngn3-positive progenitor cells resulted in the identification of the uncharacterized winged helix transcription factor Rfx6. Rfx6 is initially expressed broadly in the gut endoderm, notably in Pdx1-positive cells in the developing pancreatic buds, and then becomes progressively restricted to the endocrine lineage, suggesting a dual function in both endoderm development and islet cell differentiation. Rfx6 is found in postmitotic islet progenitor cells in the embryo and is maintained in all developing and adult islet cell types. Rfx6 is dependent on Ngn3 and acts upstream of or in parallel with NeuroD, Pax4 and Arx transcription factors during islet cell differentiation. In zebrafish, the Rfx6 ortholog is similarly found in progenitors and hormone expressing cells of the islet lineage. Loss-of-function studies in zebrafish revealed that rfx6 is required for the differentiation of glucagon-, ghrelin- and somatostatin-expressing cells, which, in the absence of rfx6, are blocked at the progenitor stage. By contrast, beta cells, whose number is only slightly reduced, were no longer clustered in a compact islet. These data unveil Rfx6 as a novel regulator of islet cell development.

Roles of hepatocyte nuclear factors (HNF) in the regulation of reproduction in teleosts

Hepatocyte nuclear factor (HNF) families are composed of liver-enriched transcription factors and upstream regulators of many liver-specific genes. HNF are involved in liver-specific gene expression, metabolism, development, cell growth and many cellular functions in the body. HNF genes can be activated or influenced by several hormones and insulin-like growth factors (IGF), and different combinations of the four HNF factors form a network in controlling the expression of liver-specific or liver-enriched genes. The functions of these factors and their interactions within the gonads of bony fishes, however, are not well understood, and the related literature is scant. Recently, several members of the HNF families have been detected in teleost gonads together with their downstream genes (IGF-I and IGF-II), suggesting that these HNF could be upregulated in vitro by steroid hormones. Thus, the hormone-HNF-IGF-gonad interaction may be an alternative axis in the reproductive mechanism that acts in concert with the conventional hypothalamus-pituitary-gonad pathway. This may help the early development and maturation of the gonad or gamete, sexual maturity or reversion and spawning-regulating mechanisms among fishes to be understood.

Maternal and zygotic aldh1a2 activity is required for pancreas development in zebrafish

We have isolated and characterized a novel zebrafish pancreas mutant. Mutant embryos lack expression of isl1 and sst in the endocrine pancreas, but retain isl1 expression in the CNS. Non-endocrine endodermal gene expression is less affected in the mutant, with varying degrees of residual expression observed for pdx1, carbA, hhex, prox1, sid4, transferrin and ifabp. In addition, mutant embryos display a swollen pericardium and lack fin buds. Genetic mapping revealed a mutation resulting in a glycine to arginine change in the catalytic domain of the aldh1a2 gene, which is required for the production of retinoic acid from vitamin A. Comparison of our mutant (aldh1a2^um22) to neckless (aldh1a2ⁱ²⁶), a previously identified aldh1a2 mutant, revealed similarities in residual endodermal gene expression. In contrast, treatment with DEAB (diethylaminobenzaldehyde), a competitive reversible inhibitor of Aldh enzymes, produces a more severe phenotype with complete loss of endodermal gene expression, indicating that a source of Aldh activity persists in both mutants. We find that mRNA from the aldh1a2^um22 mutant allele is inactive, indicating that it represents a null allele. Instead, the residual Aldh activity is likely due to maternal aldh1a2, since we find that translation-blocking, but not splice-blocking, aldh1a2 morpholinos produce a phenotype similar to DEAB treatment. We conclude that Aldh1a2 is the primary Aldh acting during pancreas development and that maternal Aldh1a2 activity persists in aldh1a2^um22 and aldh1a2ⁱ²⁶ mutant embryos.

Zebrafish pancreas development

An accurate understanding of the molecular events governing pancreas development can have an impact on clinical medicine related to diabetes, obesity and pancreatic cancer, diseases with a high impact in public health. Until 1996, the main animal models in which pancreas formation and differentiation could be studied were mouse and, for some instances related to early development, chicken and Xenopus. Zebrafish has penetrated this field very rapidly offering a new model of investigation; by joining functional genomics, genetics and in vivo whole mount visualization, Danio rerio has allowed large scale and fine multidimensional analysis of gene functions during pancreas formation and differentiation.

Heat-shock inducible Cre strains to study organogenesis in transgenic Xenopus laevis

The frog Xenopus is a well established vertebrate model to study the processes involved in embryogenesis and organogenesis, as it can be manipulated easily with a whole series of methods. We have expanded these approaches by establishing two transgenic Xenopus strains that allow specific interference with the activity of defined genes using a heat-shock inducible Cre recombinase that can induce upon heat-shock expression of a reporter gene in crossings to a corresponding reporter strain. We have applied this binary technique of gene interference in Xenopus development to overexpress the mutated HNF1 beta transcription factor at distinct developmental stages. Induction of HNF1 beta P328L329del by heat-shock at the gastrula stage resulted in a dramatic phenotype including malformation of the pronephros, gut, stomach, abnormal tail development and massive edemas indicative for kidney dysfunction. Thus, we have established the first binary inducible gene expression system in Xenopus laevis that can be used to study organogenesis.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

Crucial role of vHNF1 in vertebrate hepatic specification

Mouse liver induction occurs via the acquisition of ventral endoderm competence to respond to inductive signals from adjacent mesoderm, followed by hepatic specification. Little is known about the regulatory circuit involved in these processes. Through the analysis of vHnf1 (Hnf1b)-deficient embryos, generated by tetraploid embryo complementation, we demonstrate that lack of vHNF1 leads to defective hepatic bud formation and abnormal gut regionalization. Thickening of the ventral hepatic endoderm and expression of known hepatic genes do not occur. At earlier stages, hepatic specification of vHnf1-/- ventral endoderm is disrupted. More importantly, mutant ventral endoderm cultured in vitro loses its responsiveness to inductive FGF signals and fails to induce the hepatic-specification genes albumin and transthyretin. Analysis of liver induction in zebrafish indicates a conserved role of vHNF1 in vertebrates. Our results reveal the crucial role of vHNF1 at the earliest steps of liver induction: the acquisition of endoderm competence and the hepatic specification.

Mutations of HNF-1beta inhibit epithelial morphogenesis through dysregulation of SOCS-3

Hepatocyte nuclear factor-1beta (HNF-1beta) is a Pit-1, Oct-1/2, Unc-86 (POU) homeodomain-containing transcription factor expressed in the kidney, liver, pancreas, and other epithelial organs. Mutations of HNF-1beta cause maturity-onset diabetes of the young, type 5 (MODY5), which is characterized by early-onset diabetes mellitus and congenital malformations of the kidney, pancreas, and genital tract. Knockout of HNF-1beta in the mouse kidney results in cyst formation. However, the signaling pathways and transcriptional programs controlled by HNF-1beta are poorly understood. Using genome-wide chromatin immunoprecipitation and DNA microarray (ChIP-chip) and microarray analysis of mRNA expression, we identified SOCS3 (suppressor of cytokine signaling-3) as a previously unrecognized target gene of HNF-1beta in the kidney. HNF-1beta binds to the SOCS3 promoter and represses SOCS3 transcription. The expression of SOCS3 is increased in HNF-1beta knockout mice and in renal epithelial cells expressing dominant-negative mutant HNF-1beta. Increased levels of SOCS-3 inhibit HGF-induced tubulogenesis by decreasing phosphorylation of Erk and STAT-3. Conversely, knockdown of SOCS-3 in renal epithelial cells expressing dominant-negative mutant HNF-1beta rescues the defect in HGF-induced tubulogenesis by restoring phosphorylation of Erk and STAT-3. Thus, HNF-1beta regulates tubulogenesis by controlling the levels of SOCS-3 expression. Manipulating the levels of SOCS-3 may be a useful therapeutic approach for human diseases induced by HNF-1beta mutations.

Directed Differentiation of Human Embryonic Stem Cells into the Pancreatic Endocrine Lineage

Human embryonic stem (hES) cells represent a potentially unlimited source of transplantable beta-cells for the treatment of diabetes. Here we describe a differentiation strategy that reproducibly directs HES3, an National Institutes of Health (NIH)-registered hES cell line, into cells of the pancreatic endocrine lineage. HES3 cells are removed from their feeder layer and cultured as embryoid bodies in a three-dimensional matrix in the presence of Activin A and Bmp4 to induce definitive endoderm. Next, growth factors known to promote the proliferation and differentiation of pancreatic ductal epithelial cells to glucose-sensing, insulin-secreting beta-cells are added. Pdx1 expression, which identifies pancreatic progenitors, is detected as early as day 12 of differentiation. By day 34, Pdx1+ cells comprise between 5% and 20% of the total cell population and Insulin gene expression is up-regulated, with release of C-peptide into the culture medium. Unlike another recent report of the induction of insulin+ cells in differentiated hES cell populations, we are unable to detect the expression of other pancreatic hormones in insulin+ cells. When transplanted into severe combined immunodeficiency (SCID) mice, differentiated cell populations retain their endocrine identity and synthesize insulin.

Origin of the zebrafish endocrine and exocrine pancreas

Here, we report a detailed fate map of the zebrafish pancreas at the early gastrula stage of development (6 hours postfertilization; hpf). We show that, at this stage, both pancreas and liver progenitors are symmetrically localized in two broad domains relative to the dorsal organizer. We demonstrate that the dorsal and ventral pancreatic buds can derive from common progenitor pools at 6 hpf, but often derive from independent populations. Endocrine vs. exocrine pancreas show a similar pattern of progenitors, consistent with descriptions of the dorsal bud being strictly endocrine and the ventral bud primarily exocrine. In general, we find that endocrine/dorsal bud progenitors are located more dorsally than the exocrine pancreas/ventral bud progenitors. Later in gastrulation (10 hpf), pancreas progenitors have migrated to bilateral domains at the equator of the embryo. Our fate map will assist with design and interpretation of future experiments to understand early pancreas development.