Early appearance of pancreatic hormone-expressing cells in the zebrafish embryo

Dynamic glucoregulation and mammalian-like responses to metabolic and developmental disruption in zebrafish

Zebrafish embryos are emerging as models of glucose metabolism. However, patterns of endogenous glucose levels, and the role of the islet in glucoregulation, are unknown. We measured absolute glucose levels in zebrafish and mouse embryos, and demonstrate similar, dynamic glucose fluctuations in both species. Further, we show that chemical and genetic perturbations elicit mammalian-like glycemic responses in zebrafish embryos. We show that glucose is undetectable in early zebrafish and mouse embryos, but increases in parallel with pancreatic islet formation in both species. In zebrafish, increasing glucose is associated with activation of gluconeogenic phosphoenolpyruvate carboxykinase1 (pck1) transcription. Non-hepatic Pck1 protein is expressed in mouse embryos. We show using RNA in situ hybridization, that zebrafish pck1 mRNA is similarly expressed in multiple cell types prior to hepatogenesis. Further, we demonstrate that the Pck1 inhibitor 3-mercaptopicolinic acid suppresses normal glucose accumulation in early zebrafish embryos. This shows that pre- and extra-hepatic pck1 is functional, and provides glucose locally to rapidly developing tissues. To determine if the primary islet is glucoregulatory in early fish embryos, we injected pdx1-specific morpholinos into transgenic embryos expressing GFP in beta cells. Most morphant islets were hypomorphic, not a genetic, but embryos still exhibited persistent hyperglycemia. We conclude from these data that the early zebrafish islet is functional, and regulates endogenous glucose. In summary, we identify mechanisms of glucoregulation in zebrafish embryos that are conserved with embryonic and adult mammals. These observations justify use of this model in mechanistic studies of human metabolic disease.

ff1b, the SF1 ortholog, is important for pancreatic islet cell development in zebrafish

The adrenal cortex and pancreatic islets have endocrine functions, producing steroid-based hormones and insulin, respectively. Cells of the adrenal cortex originate in the mesoderm while the cells of pancreatic islets originate in the endoderm. The zebrafish is a powerful model for understanding organ development due to its ease of genetic and molecular manipulation, transparent embryos, and large number of progeny for statistically powerful experiments. Like humans, the zebrafish pancreas has both exocrine and endocrine functions; unlike humans, there is only one endocrine islet cell group, instead of multiple islets. Using an eGFP-transgenic line of zebrafish, we have observed that the steroidogenic factor 1 (SF1) ortholog, ff1b, which is critical for adrenal cortex development and function in the zebrafish, is also implicated in zebrafish pancreatic islet development. We show that interruption of ff1b expression using an ff1b-morpholino (MO) disrupts development of insulin expressing cells. We conclude that ff1b-MO alters pancreatic islet development in zebrafish, demonstrating the utility of the zebrafish as a model for studying pancreatic development. This work is consistent with previous studies in mouse and human that have suggested SF1 participates in the vascular and ductal development of the pancreas, and disruption of SF1 function leads to abnormal development of the pancreatic islets due to poor vascularization.

Dachshund homologues play a conserved role in islet cell development

All metazoans use insulin to control energy metabolism, but they secrete it from different cells: neurons in the central nervous system in invertebrates and endocrine cells in the gut or pancreas in vertebrates. Despite their origins in different germ layers, all of these insulin-producing cells share common functional features and gene expression patterns. In this study, we tested the role in insulin-producing cells of the vertebrate homologues of Dachshund, a transcriptional regulator that marks the earliest committed progenitors of the neural insulin-producing cells in Drosophila. Both zebrafish and mice expressed a single dominant Dachshund homologue in the pancreatic endocrine lineage, and in both species loss of this homologue reduced the numbers of all islet cell types including the insulin-producing β-cells. In mice, Dach1 gene deletion left the pancreatic progenitor cells unaltered, but blocked the perinatal burst of proliferation of differentiated β-cells that normally generates most of the β-cell mass. In β-cells, Dach1 bound to the promoter of the cell cycle inhibitor p27Kip1, which constrains β-cell proliferation. Taken together, these data demonstrate a conserved role for Dachshund homologues in the production of insulin-producing cells.

The serendipitous origin of chordate secretin peptide family members

Background

The secretin family is a pleotropic group of brain-gut peptides with affinity for class 2 G-protein coupled receptors (secretin family GPCRs) proposed to have emerged early in the metazoan radiation via gene or genome duplications. In human, 10 members exist and sequence and functional homologues and ligand-receptor pairs have been characterised in representatives of most vertebrate classes. Secretin-like family GPCR homologues have also been isolated in non-vertebrate genomes however their corresponding ligands have not been convincingly identified and their evolution remains enigmatic.

Results

In silico sequence comparisons failed to retrieve a non-vertebrate (porifera, cnidaria, protostome and early deuterostome) secretin family homologue. In contrast, secretin family members were identified in lamprey, several teleosts and tetrapods and comparative studies revealed that sequence and structure is in general maintained. Sequence comparisons and phylogenetic analysis revealed that PACAP, VIP and GCG are the most highly conserved members and two major peptide subfamilies exist; i) PACAP-like which includes PACAP, PRP, VIP, PH, GHRH, SCT and ii) GCG-like which includes GCG, GLP1, GLP2 and GIP. Conserved regions flanking secretin family members were established by comparative analysis of the Takifugu, Xenopus, chicken and human genomes and gene homologues were identified in nematode, Drosophila and Ciona genomes but no gene linkage occurred. However, in Drosophila and nematode genes which flank vertebrate secretin family members were identified in the same chromosome.

Conclusions

Receptors of the secretin-like family GPCRs are present in protostomes but no sequence homologues of the vertebrate cognate ligands have been identified. It has not been possible to determine when the ligands evolved but it seems likely that it was after the protostome-deuterostome divergence from an exon that was part of an existing gene or gene fragment by rounds of gene/genome duplication. The duplicate exon under different evolutionary pressures originated the chordate PACAP-like and GCG-like subfamily groups. This event occurred after the emergence of the metazoan secretin GPCRs and led to the establishment of novel peptide-receptor interactions that contributed to the generation of novel physiological functions in the chordate lineage.

Pancreatic islet plasticity: interspecies comparison of islet architecture and composition

The pancreatic islet displays diverse patterns of endocrine cell arrangement. The prototypic islet, with insulin-secreting beta-cells forming the core surrounded by other endocrine cells in the periphery, is largely based on studies of normal rodent islets. Recent reports on large animals, including humans, show a difference in islet architecture, in which the endocrine cells are randomly distributed throughout the islet. This particular species difference has raised concerns regarding the interpretation of data based on rodent studies to humans. On the other hand, further variations have been reported in marsupials and some nonhuman primates, which possess an inverted ratio of beta-cells to other endocrine cells. This review discusses the striking plasticity of islet architecture and cellular composition among various species including changes in response to metabolic states within a single species. We propose that this plasticity reflects evolutionary acquired adaptation induced by altered physiological conditions, rather than inherent disparities between species.

prep1.2 and aldh1a2 participate to a positive loop required for branchial arches development in zebrafish

Segmentation is a key step in embryonic development. Acting in all germ layers, it is responsible for the generation of antero-posterior asymmetries. Hox genes, with their diverse expression in individual segments, are fundamental players in the determination of different segmental fates. In vertebrates, Hox gene products gain specificity for DNA sequences by interacting with Pbx, Prep and Meis homeodomain transcription factors. In this work we cloned and analysed prep1.2 in zebrafish. In-situ hybridization experiments show that prep1.2 is maternally and ubiquitously expressed up to early somitogenesis when its expression pattern becomes more restricted to the head and trunk mesenchyme. Experiments of loss of function with prep1.2 morpholinos change the shape of the hyoid and third pharyngeal cartilages while arches 4-7 and pectoral fins are absent, a phenotype strikingly similar to that caused by loss of retinoic acid (RA). In fact, we show that prep1.2 is positively regulated by RA and required for the normal expression of aldh1a2 at later stages, particularly in tissues involved in the development of the branchial arches and pectoral fins. Thus, prep1.2 and aldh1a2 are members of an indirect positive feedback loop required for pharyngeal endoderm and posterior branchial arches development. As the paralogue gene prep1.1 is more important in hindbrain patterning and neural crest chondrogenesis, we provide evidence of a functional specialization of prep genes in zebrafish head segmentation and morphogenesis.

The Pax6b homeodomain is dispensable for pancreatic endocrine cell differentiation in zebrafish

Pax6 is a well conserved transcription factor that contains two DNA-binding domains, a paired domain and a homeodomain, and plays a key role in the development of eye, brain, and pancreas in vertebrates. The recent identification of the zebrafish sunrise mutant, harboring a mutation in the pax6b homeobox and presenting eye abnormalities but no obvious pancreatic defects, raised a question about the role of pax6b in zebrafish pancreas. We show here that pax6b does play an essential role in pancreatic endocrine cell differentiation, as revealed by the phenotype of a novel zebrafish pax6b null mutant and of embryos injected with pax6b morpholinos. Pax6b-depleted embryos have almost no beta cells, a strongly reduced number of delta cells, and a significant increase of epsilon cells. Through the use of various morpholinos targeting intron-exon junctions, pax6b RNA splicing was perturbed at several sites, leading either to retention of intronic sequences or to deletion of exonic sequences in the pax6b transcript. By this strategy, we show that deletion of the Pax6b homeodomain in zebrafish embryos does not disturb pancreas development, whereas lens formation is strongly affected. These data thus provide the explanation for the lack of pancreatic defects in the sunrise pax6b mutants. In addition, partial reduction of Pax6b function in zebrafish embryos performed by injection of small amounts of pax6b morpholinos caused a clear rise in alpha cell number and in glucagon expression, emphasizing the importance of the fine tuning of the Pax6b level to its biological activity.

Nkx6.1 and nkx6.2 regulate alpha- and beta-cell formation in zebrafish by acting on pancreatic endocrine progenitor cells

In mice, the Nkx6 genes are crucial to alpha- and beta-cell differentiation, but the molecular mechanisms by which they regulate pancreatic subtype specification remain elusive. Here it is shown that in zebrafish, nkx6.1 and nkx6.2 are co-expressed at early stages in the first pancreatic endocrine progenitors, but that their expression domains gradually segregate into different layers, nkx6.1 being expressed ventrally with respect to the forming islet while nkx6.2 is expressed mainly in beta-cells. Knockdown of nkx6.2 or nkx6.1 expression leads to nearly complete loss of alpha-cells but has no effect on beta-, delta-, or epsilon-cells. In contrast, nkx6.1/nkx6.2 double knockdown leads additionally to a drastic reduction of beta-cells. Synergy between the effects of nkx6.1 and nkx6.2 knockdown on both beta- and alpha-cell differentiation suggests that nkx6.1 and nkx6.2 have the same biological activity, the required total nkx6 threshold being higher for alpha-cell than for beta-cell differentiation. Finally, we demonstrate that the nkx6 act on the establishment of the pancreatic endocrine progenitor pool whose size is correlated with the total nkx6 expression level. On the basis of our data, we propose a model in which nkx6.1 and nkx6.2, by allowing the establishment of the endocrine progenitor pool, control alpha- and beta-cell 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.

Molecular regulation of pancreas development in zebrafish

The pancreas is a vertebrate-specific organ of endodermal origin which is responsible for production of digestive enzymes and hormones involved in regulating glucose homeostasis, in particular insulin, deficiency of which results in diabetes. Basic research on the genetic and molecular pathways regulating pancreas formation and function has gained major importance for the development of regenerative medical approaches aimed at improving diabetes treatment. Among the different model organisms that are currently used to elucidate the basic pathways of pancreas development and regeneration, the zebrafish is distinguished by its unique opportunities to combine genetic and pharmacological approaches with sophisticated live-imaging methodology, and by its ability to regenerate the pancreas within a short time. Here we review current perspectives and present methods for studying two important processes contributing to pancreas development and regeneration, namely cell migration via time-lapse micropscopy and cell proliferation via incorporation of nucleotide analog EdU, with a focus on the insulin-producing beta cells of the islet.

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.

T-cell factor 4 (tcf7l2) is the main effector of Wnt signaling during zebrafish intestine organogenesis

The Wnt pathway orchestrates cell fate decisions during embryonic development, organogenesis, and adult tissues homeostasis. T-cell factor (Tcf )/lymphoid enhancer-binding factor (Lef) transcription factors are the downstream effectors of canonical Wnt signaling. Upon Wnt signal activation, beta-catenin stabilizes and translocates to the nucleus, where it interacts with Tcfs activating the transcription of Wnt target genes. In the absence of Wnt, levels of stable beta-catenin are reduced by the action of adenomatous polyposis coli (Apc) and other cytoplasmic proteins. Mutations in Apc cause constitutive accumulation of beta-catenin and inappropriate activation of the Wnt pathway. apc(mcr/mcr) fish embryos show absence of expression of tissue-specific differentiation markers in the intestine, suggesting that inappropriate activation of Wnt signaling abrogates gut organogenesis. Which Tcf transcription factor mediates Wnt signaling during zebrafish gut organogenesis remains unclear. We studied the combined effect of loss of Tcf family members and Apc in the developing embryo. Tcf4 (tcf7l2) loss rescues the apc(mcr/mcr) phenotype in the intestine. Single depletion of Tcf1 (tcf7) and Tcf3 (tcf7l1a) function in an Apc mutant background had no effect on endoderm development. This study reveals that Tcf4 (tcf7l2) is the major effector of Wnt signaling in the intestine during zebrafish organogenesis.

Analysis of beta cell proliferation dynamics in zebrafish

Among the different mechanisms invoked to explain the beta cell mass expansion during postnatal stages and adulthood, self-replication is being considered the major cellular event occurring both under physiological conditions and in regenerating pancreas after partial pancreactomy. Neogenesis, i.e. differentiation from pancreatic progenitors, has been demonstrated to act concurrently with beta cell replication during pancreatic regeneration. Both phenomena have been largely elucidated in higher vertebrates (mouse, rat and guinea pig), but an extensive description of beta cell dynamics in other animal models is currently lacking. We, therefore, explored in zebrafish the cellular origins of new beta cells in both adult and larval stages. By integrating the results from in vivo time lapse analysis and immunostaining, we provide a detailed reconstruction of the major processes involved in fish beta cell genesis and maintenance. Moreover, by establishing the selective ablation of proliferating beta cells, through the ganciclovir-HSVTK system, we could show that in larval stages self-replication is the main mechanism of beta cells expansion. Since the same mechanism of proliferation has been observed to occur during early and late life stages, we suggest that zebrafish larvae can be used as an alternative tool for an in vivo exploration and screening of new potential mitogens specifically targeting beta cells.

Molecular cloning, ontogeny and tissue distribution of zebrafish (Danio rerio) prohormone convertases: pcsk1 and pcsk2

Prohormone convertase subtilisin/kexin (PCSK) enzymes are a family of nine related serine proteases, found in a multitude of tissues, and responsible for the maturation of a variety of protein and peptide precursors. Pcsk1 and Pcsk2 are found within dense core secretory granules in endocrine and neuroendocrine cells and are responsible for cleaving several hormones and neuropeptide precursors. In this work, we cloned and sequenced the cDNA of pcsk1 and pcsk2 from zebrafish (Danio rerio). pcsk1 is a 2268bp ORF, whose 755 amino acid protein product is identical to that predicted from the genome sequence. pcsk2 is a 1941bp ORF, encoding a 646 amino acid peptide. Both Pcsk1 and Pcsk2 display high degrees of similarity to their counterparts in other species, including the conservation of the catalytic triad and other essential residues. The brain contained the highest expression levels of both pcsk1 (1.49+/-0.21) (displayed as ratio to EF-1a), and pcsk2 (0.23+/-0.04). Both transcripts were also detectable in the fore, mid and distal gut. pcsk1 and 2 were detectable at 4.5h post-fertilization, and while pcsk1 expression increased throughout development (0.12+/-0.01 maximum at 3 days post-fertilization), pcsk2 expression was highest at day 5 post-fertilization (0.03+/-0.01), and decreased prior. For the first time, we have identified and characterized a pcsk1 transcript in fish. We have also identified and characterized the pcsk2 transcript in zebrafish, and have assessed the tissue distribution and ontogeny of both.

Cyp26 enzymes function in endoderm to regulate pancreatic field size

The control of organ size and position relies, at least in part, upon appropriate regulation of the signals that specify organ progenitor fields. Pancreatic cell fates are specified by retinoic acid (RA), and proper size and localization of the pancreatic field are dependent on tight control of RA signaling. Here we show that the RA-degrading Cyp26 enzymes play a critical role in defining the normal anterior limit of the pancreatic field. Disruption of Cyp26 function causes a dramatic expansion of pancreatic cell types toward the anterior of the embryo. The cyp26a1 gene is expressed in the anterior trunk endoderm at developmental stages when RA is signaling to specify pancreas, and analysis of cyp26a1/giraffe (gir) mutant zebrafish embryos confirms that cyp26a1 plays the primary role in setting the anterior limit of the pancreas. Analysis of the gir mutants further reveals that cyp26b1 and cyp26c1 function redundantly to partially compensate for loss of Cyp26a1 function. We used cell transplantation to determine that Cyp26a1 functions directly in endoderm to modulate RA signaling and limit the pancreatic field. Taken together with our finding that endodermal expression of cyp26 genes is subject to positive regulation by RA, our data reveal a feedback loop within the endoderm. Such feedback can maintain consistent levels of RA signaling, despite environmental fluctuations in RA concentration, thus ensuring a consistent size and location of the pancreatic field.

On the diabetic menu: zebrafish as a model for pancreas development and function

Development of the vertebrate pancreas is a complex stepwise process comprising regionalization, cell differentiation, and morphogenesis. Studies in zebrafish are contributing to an emerging picture of pancreas development in which extrinsic signaling molecules influence intrinsic transcriptional programs to allow ultimate differentiation of specific pancreatic cell types. Zebrafish experiments have revealed roles for several signaling molecules in aspects of this process; for example our own work has shown that retinoic acid signals specify the pre-pancreatic endoderm. Time-lapse imaging of live zebrafish embryos has started to provide detailed information about early pancreas morphogenesis. In addition to modeling embryonic development, the zebrafish has recently been used as a model for pancreas regeneration studies. Here, we review the significant progress in these areas and consider the future potential of zebrafish as a diabetes research model.

Exdpf is a key regulator of exocrine pancreas development controlled by retinoic acid and ptf1a in zebrafish

Both endocrine and exocrine pancreatic cells arise from pancreatic-duodenal homeobox 1 (pdx1)-positive progenitors. The molecular mechanisms controlling cell fate determination and subsequent proliferation, however, are poorly understood. Unlike endocrine cells, less is known about exocrine cell specification. We report here the identification and characterization of a novel exocrine cell determinant gene, exocrine differentiation and proliferation factor (exdpf), which is highly expressed in the exocrine cell progenitors and differentiated cells of the developing pancreas in zebrafish. Knockdown of exdpf by antisense morpholino caused loss or significant reduction of exocrine cells due to lineage-specific cell cycle arrest but not apoptosis, whereas the endocrine cell mass appeared normal. Real-time PCR results demonstrated that the cell cycle arrest is mediated by up-regulation of cell cycle inhibitor genes p21(Cip), p27(Kip), and cyclin G1 in the exdpf morphants. Conversely, overexpression of exdpf resulted in an overgrowth of the exocrine pancreas and a severe reduction of the endocrine cell mass, suggesting an inhibitory role for exdpf in endocrine cell progenitors. We show that exdpf is a direct target gene of pancreas-specific transcription factor 1a (Ptf1a), a transcription factor critical for exocrine formation. Three consensus Ptf1a binding sites have been identified in the exdpf promoter region. Luciferase assay demonstrated that Ptf1a promotes transcription of the exdpf promoter. Furthermore, exdpf expression in the exocrine pancreas was lost in ptf1a morphants, and overexpression of exdpf successfully rescued exocrine formation in ptf1a-deficient embryos. Genetic evidence places expdf downstream of retinoic acid (RA), an instructive signal for pancreas development. Knocking down exdpf by morpholino abolished ectopic carboxypeptidase A (cpa) expression induced by RA. On the other hand, exdpf mRNA injection rescued endogenous cpa expression in embryos treated with diethylaminobenzaldehyde, an inhibitor of RA signaling. Moreover, exogenous RA treatment induced anterior ectopic expression of exdpf and trypsin in a similar pattern. Our study provides a new understanding of the molecular mechanisms controlling exocrine cell specification and proliferation by a novel gene, exdpf. Highly conserved in mammals, the expression level of exdpf appears elevated in several human tumors, suggesting a possible role in tumor pathogenesis.

Differential expression of preprosomatostatin- and somatostatin receptor-encoding mRNAs in association with the growth hormone-insulin-like growth factor system during embryonic development of rainbow trout (Oncorhynchus mykiss)

Rainbow trout were used to evaluate the relationship between the somatostatin (SS) signaling and the growth hormone (GH)-insulin-like growth factor (IGF) systems during pre-hatch and post-hatch embryonic development. The expression of preprosomatostatins (PPSS), SS receptors (SSTR), GH receptors (GHR), IGF-1, IGF-2, and IGF type 1 receptors (IGFR1) was examined in various regions at the eyed-egg (29 days post-fertilization, dpf;), post-hatch (53dpf), swim-up (68dpf), and complete yolk-absorbed (90dpf) stages. In head, PPSSI mRNA abundance increased during development while that of PPSSII' decreased and that of PPSSII'' remained unchanged. In body and tail, mRNA abundance of all PPSSs remained unchanged except that of PPSSII'' which declined in the tail. SSTR expression increased as development progressed in all regions with the exception of SSTR1A mRNA which remained unchanged. mRNA levels of GHR1 declined in all regions of post-hatch embryos, whereas those of GHR2 remained unchanged. Expression of IGF-1 and IGF-2 in head and tail regions increased immediately after hatching, and then declined, whereas the expression of neither IGF changed during development in the body. The expression of IGFR1 mRNAs declined in all regions, reaching their lowest levels at 90dpf, with the exception of IGFR1A mRNA in the body which remained unchanged. The general decline in the expression of GH-IGF system components during development appears inversely related to a general increase in the expression of SS system elements, and suggests that these two systems interact to regulate the tissue expansion and tissue regression of embryogenesis.

Organ-specific requirements for Hdac1 in liver and pancreas formation

Liver, pancreas and lung originate from the presumptive foregut in temporal and spatial proximity. This requires precisely orchestrated transcriptional activation and repression of organ-specific gene expression within the same cell. Here, we show distinct roles for the chromatin remodelling factor and transcriptional repressor Histone deacetylase 1 (Hdac1) in endodermal organogenesis in zebrafish. Loss of Hdac1 causes defects in timely liver specification and in subsequent differentiation. Mosaic analyses reveal a cell-autonomous requirement for hdac1 within the hepatic endoderm. Our studies further reveal specific functions for Hdac1 in pancreas development. Loss of hdac1 causes the formation of ectopic endocrine clusters anteriorly to the main islet, as well as defects in exocrine pancreas specification and differentiation. In addition, we observe defects in extrahepatopancreatic duct formation and morphogenesis. Finally, loss of hdac1 results in an expansion of the foregut endoderm in the domain from which the liver and pancreas originate. Our genetic studies demonstrate that Hdac1 is crucial for regulating distinct steps in endodermal organogenesis. This suggests a model in which Hdac1 may directly or indirectly restrict foregut fates while promoting hepatic and exocrine pancreatic specification and differentiation, as well as pancreatic endocrine islet morphogenesis. These findings establish zebrafish as a tractable system to investigate chromatin remodelling factor functions in controlling gene expression programmes in vertebrate endodermal organogenesis.

Expression of zebrafish pax6b in pancreas is regulated by two enhancers containing highly conserved cis-elements bound by PDX1, PBX and PREP factors

Background

PAX6 is a transcription factor playing a crucial role in the development of the eye and in the differentiation of the pancreatic endocrine cells as well as of enteroendocrine cells. Studies on the mouse Pax6 gene have shown that sequences upstream from the P0 promoter are required for expression in the lens and the pancreas; but there remain discrepancies regarding the precise location of the pancreatic regulatory elements.

Results

Due to genome duplication in the evolution of ray-finned fishes, zebrafish has two pax6 genes, pax6a and pax6b. While both zebrafish pax6 genes are expressed in the developing eye and nervous system, only pax6b is expressed in the endocrine cells of the pancreas. To investigate the cause of this differential expression, we used a combination of in silico, in vivo and in vitro approaches. We show that the pax6b P0 promoter targets expression to endocrine pancreatic cells and also to enteroendocrine cells, retinal neurons and the telencephalon of transgenic zebrafish. Deletion analyses indicate that strong pancreatic expression of the pax6b gene relies on the combined action of two conserved regulatory enhancers, called regions A and C. By means of gel shift assays, we detected binding of the homeoproteins PDX1, PBX and PREP to several cis-elements of these regions. In constrast, regions A and C of the zebrafish pax6a gene are not active in the pancreas, this difference being attributable to sequence divergences within two cis-elements binding the pancreatic homeoprotein PDX1.

Conclusion

Our data indicate a conserved role of enhancers A and C in the pancreatic expression of pax6b and emphasize the importance of the homeoproteins PBX and PREP cooperating with PDX1, in activating pax6b expression in endocrine pancreatic cells. This study also provides a striking example of how adaptative evolution of gene regulatory sequences upon gene duplication progressively leads to subfunctionalization of the paralogous gene pair.

New insight into the molecular evolution of the somatostatin family

The present review describes the molecular evolution of the somatostatin family and its relationships with that of the urotensin II family. Most of the somatostatin sequences collected from different vertebrate species can be grouped as the products of at least four loci. The somatostatin 1 (SS1) gene is present in all vertebrate classes from agnathans to mammals. The SS1 gene has given rise to the somatostatin 2 (SS2) gene by a segment/chromosome duplication that is probably the result of a tetraploidization event according to the 2R hypothesis. The somatostatin-related peptide cortistatin, first identified in rodents and human, is the counterpart of SS2 in placental mammals. In fish, the existence of two additional somatostatin genes has been reported. The first gene, which encodes a peptide usually named somatostatin II (SSII), exists in almost all teleost species investigated so far and is thought to have arisen through local duplication of the SS1 gene. The second gene, which has been characterized in only a few teleost species, encodes a peptide also named SSII that exhibits a totally atypical structure. The origin of this gene is currently unknown. Nevertheless, because the two latter genes are clearly paralogous genes, we propose to rename them SS3 and SS4, respectively, in order to clarify the current confusing nomenclature. The urotensin II family consists of two genes, namely the urotensin II (UII) gene and the UII-related peptide (URP) gene. Both UII and URP exhibit limited structural identity to somatostatin so that UII was originally described as a "somatostatin-like peptide". Recent comparative genomics studies have revealed that the SS1 and URP genes, on the one hand, and the SS2 and UII genes, on the other hand, are closely linked on the same chromosomes, thus confirming that the SS1/SS2 and the UII/URP genes belong to the same superfamily. According to these data, it appears that an ancestral somatostatin/urotensin II gene gave rise by local duplication to a somatostatin ancestor and a urotensin II ancestor, whereupon this pair was duplicated (presumably by a segment/chromosome duplication) to give rise to the SS1-UII pair and the SS2-URP pair.

Targeted inhibition of miRNA maturation with morpholinos reveals a role for miR-375 in pancreatic islet development

Several vertebrate microRNAs (miRNAs) have been implicated in cellular processes such as muscle differentiation, synapse function, and insulin secretion. In addition, analysis of Dicer null mutants has shown that miRNAs play a role in tissue morphogenesis. Nonetheless, only a few loss-of-function phenotypes for individual miRNAs have been described to date. Here, we introduce a quick and versatile method to interfere with miRNA function during zebrafish embryonic development. Morpholino oligonucleotides targeting the mature miRNA or the miRNA precursor specifically and temporally knock down miRNAs. Morpholinos can block processing of the primary miRNA (pri-miRNA) or the pre-miRNA, and they can inhibit the activity of the mature miRNA. We used this strategy to knock down 13 miRNAs conserved between zebrafish and mammals. For most miRNAs, this does not result in visible defects, but knockdown of miR-375 causes defects in the morphology of the pancreatic islet. Although the islet is still intact at 24 hours postfertilization, in later stages the islet cells become scattered. This phenotype can be recapitulated by independent control morpholinos targeting other sequences in the miR-375 precursor, excluding off-target effects as cause of the phenotype. The aberrant formation of the endocrine pancreas, caused by miR-375 knockdown, is one of the first loss-of-function phenotypes for an individual miRNA in vertebrate development. The miRNA knockdown strategy presented here will be widely used to unravel miRNA function in zebrafish.

siRNA specific to Pdx-1 disturbed the formation of the islet in early zebrafish embryos

Pdx-1, an important transcription factor highlighting in the early pancreatic development, islet functions and pancreatic disorders, needs to be more investigated in zebrafish, and siRNA is still seldom applied in zebrafish embryo-related research. Our aim was to explore the role of pdx-1 in pancreatic development of zebrafish embryos by using siRNA approach. Microinjection, reverse transcriptase-PCR (RT-PCR), in situ hybridization and immunofluorescent staining were used in this research, and the morphology of the islet in normal zebrafish embryos, and in those treated with the siRNA specific to pdx-1 (siPDX-1) or siGFP was observed and compared. The expression of pdx-1 was detected in the stages of 1-cell, 2-cell, 4-cell, 8-cell, 16-cell, 16-hour by RT-PCT. The in situ hybridization and immunofluorescent staining results showed that siPDX-1 disturbed the formation of the islet in zebrafish embryos. Pdx-1 played multiple roles in maintaining the phenotype of the islet during embryogenesis in zebrafish.

Cdx4 is required in the endoderm to localize the pancreas and limit beta-cell number

Cdx transcription factors have crucial roles in anteroposterior patterning of the nervous system and mesoderm. Here we focus on the role of cdx4 in patterning the endoderm in zebrafish. We show that cdx4 has roles in determining pancreatic beta-cell number, directing midline convergence of beta-cells during early pancreatic islet formation, and specifying the anteroposterior location of foregut organs. Embryos deficient in cdx4 have a posteriorly shifted pancreas, liver and small intestine. The phenotype is more severe with knockdown of an additional Cdx factor, cdx1a. We show that cdx4 functions within the endoderm to localize the pancreas. Morpholino knockdown of cdx4 specifically in the endoderm recapitulates the posteriorly shifted pancreas observed in cdx4 mutants. Conversely, overexpression of cdx4 specifically in the endoderm is sufficient to shift the pancreas anteriorly. Together, these results suggest a model in which cdx4 confers posterior identity to the endoderm. Cdx4 might function to block pancreatic identity by preventing retinoic acid (RA) signal transduction in posterior endoderm. In support of this, we demonstrate that in cdx4-deficient embryos treated with RA, ectopic beta-cells are located well posterior to the normal pancreatic domain.

Expression of unconventional myosin genes during neuronal development in zebrafish

Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (1, 2, 3, 5, 6, 7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

PACAP, VIP and their receptors in the metazoa: insights about the origin and evolution of the ligand-receptor pair

The evolution, function and interaction of ligand-receptor pairs are of major pharmaceutical interest. Comparative sequence analysis approaches using data from phylogenetically distant organisms can provide insights into their origin and possible physiological roles. The present review focuses on the pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and their receptors in the metazoa. A PACAP-like peptide is present in tunicates and chordates while VIP- and PACAP/VIP-specific receptors have only been isolated in the latter phyla. The apparently disparate evolution of the ligands and their specific receptors raises questions about their evolution during the metazoan radiation and also about how the ligands may have acquired new functions.

Function and regulation of zebrafish nkx2.2a during development of pancreatic islet and ducts

In the mouse Nkx2.2 is expressed in the entire pancreatic anlage. Nevertheless, absence of Nkx2.2 only perturbs the development of endocrine cell types, notably beta-cells which are completely absent. In order to test the possibility that Nkx2.2 might fulfil additional functions during pancreas development we analysed its zebrafish homologue nkx2.2a using gene targeting and GFP-transgenic fish lines. Our results suggest similar roles for nkx2.2a and Nkx2.2 during the development of the endocrine pancreas. Morpholino-based knock-down of nkx2.2a leads to a reduction of alpha- and beta-cell number and an increase of ghrelin-producing cells but, as in mice, does not affect delta-cells. Moreover, like in the mouse, two spatially distinct promoters regulate expression of nkx2.2a in precursors and differentiated islet cells. In addition we found that in zebrafish nkx2.2a is also expressed in the anterior pancreatic bud and, later, in the differentiated pancreatic ducts. A nkx2.2a-transgenic line in which pancreatic GFP expression is restricted to the pancreatic ducts revealed that single GFP-positive cells leave the anterior pancreatic bud and move towards the islet where they form intercellular connections between each other. Subsequently, these cells generate the branched network of the larval pancreatic ducts. Morpholinos that block nkx2.2a function also lead to the absence of the pancreatic ducts. We observed the same phenotype in ptf1a-morphants that are additionally characterized by a reduced number of nkx2.2a-positive duct precursors. Whereas important details of the molecular program leading to the differentiation of endocrine cell types are conserved between mammals and zebrafish, our results reveal a new function for nkx2.2a in the development of the pancreatic ducts.

Calsenilin is required for endocrine pancreas development in zebrafish

Calsenilin/DREAM/Kchip3 is a neuronal calcium-binding protein. It is a multifunctional protein, mainly expressed in neural tissues and implicated in regulation of presenilin processing, repression of transcription, and modulation of A-type potassium channels. Here, we performed a search for new genes expressed during pancreatic development and have studied the spatiotemporal expression pattern and possible role of calsenilin in pancreatic development in zebrafish. We detected calsenilin transcripts in the pancreas from 21 somites to 39 hours postfertilization stages. Using double in situ hybridization, we found that the calsenilin gene was expressed in pancreatic endocrine cells. Loss-of-function experiments with anti-calsenilin morpholinos demonstrated that injected morphants have a significant decrease in the number of pancreatic endocrine cells. Furthermore, the knockdown of calsenilin leads to perturbation in islet morphogenesis, suggesting that calsenilin is required for early islet cell migration. Taken together, our results show that zebrafish calsenilin is involved in endocrine cell differentiation and morphogenesis within the pancreas.

Anatomical and histological observation on the pancreas in adult zebrafish

OBJECTIVES

To investigate the anatomic structure of the pancreas and the distribution of the islets in adult zebrafish.

METHODS

In situ immunofluorescent staining, electron microscopy, and serial paraffin-embedded sectioning with hematoxylin/eosin staining were applied.

RESULTS

The pancreas along the intestine included 4 relatively independent and concentrated lobes, in which 4 kinds of islets-principal islets, Brockmann bodies, diffusely existing islets, and single beta-cell-were observed. Some islets contained both alpha and beta cells, whereas some contained only beta cells. The islet number in each adult zebrafish averaged 84.53 +/- 43.77; and the lower quartile, median, and upper quartile were 55.25, 70.50, and 112.00, respectively (n = 40). The different islets were differently distributed in the 4 pancreatic lobes with statistical significance (P < 0.05). Meanwhile, 3 kinds of secretory granules were found in the cytoplasm of different islet cells.

CONCLUSIONS

According to the distinct distribution, concentration of the pancreas, and different contents of the islets within the pancreas, 4 lobes of the pancreas along the intestine-the gallbladder-spleen lobe, the middle lobe, the left lobe, and the ventral lobe-were identified in adult zebrafish.

Dose-dependent effects of chemical immobilization on the heart rate of embryonic zebrafish

The small size and optical transparency of zebrafish embryos and larvae greatly facilitate modern intravital microscopic phenotyping of these experimentally tractable laboratory animals. Neither the experimentally derived dose-response relationships for chemicals commonly used in the mounting of live fish larvae, nor their effect on the stress of the animal, are currently available in the research literature. This is particularly problematic for IACUCs attempting to maintain the highest ethical standards of animal care in the face of a recent spate in investigator-initiated requests to use embryonic zebrafish as experimental models. The authors address this issue by describing the dose-dependent efficacy of several commonly used chemical mounting treatments and their effect on one stress parameter, embryo heart rate. The results of this study empirically define, for the first time, effective, minimally stressful treatments for immobilization and in vivo visualization during early zebrafish development.

Targeted ablation of beta cells in the embryonic zebrafish pancreas using E. coli nitroreductase

In order to generate a zebrafish model of beta cell regeneration, we have expressed an Escherichia coli gene called nfsB in the beta cells of embryonic zebrafish. This bacterial gene encodes a nitroreductase (NTR) enzyme, which can convert prodrugs such as metronidazole (Met) to cytotoxins. By fusing nfsB to mCherry, we can simultaneously render beta cells susceptible to prodrug and visualize Met dependent cell ablation. We show that the neighboring alpha and delta cells are unaffected by prodrug treatment and that ablation is beta cell specific. Following drug removal and 36h of recovery, beta cells regenerate. Using ptf1a morphants, it is clear that this beta cell recovery occurs independently of the presence of the exocrine pancreas. Also, by using photoconvertible Kaede to cell lineage trace and BrdU incorporation to label proliferation, we investigate mechanisms for beta regeneration. Therefore, we have developed a unique resource for the study of beta cell regeneration in a living vertebrate organism, which will provide the opportunity to conduct large-scale screens for pharmacological and genetic modifiers of beta cell regeneration.

Distinct delta and jagged genes control sequential segregation of pancreatic cell types from precursor pools in zebrafish

The different cell types of the vertebrate pancreas arise asynchronously during organogenesis. Beta-cells producing insulin, alpha-cells producing glucagon, and exocrine cells secreting digestive enzymes differentiate sequentially from a common primordium. Notch signaling has been shown to be a major mechanism controlling these cell-fate choices. So far, the pleiotropy of Delta and Jagged/Serrate genes has hindered the evaluation of the roles of specific Notch ligands, as the phenotypes of knock-out mice are lethal before complete pancreas differentiation. Analyses of gene expression and experimental manipulations of zebrafish embryos allowed us to determine individual contributions of Notch ligands to pancreas development. We have found that temporally distinct phases of both endocrine and exocrine cell type specification are controlled by different delta and jagged genes. Specifically, deltaA knock-down embryos lack alpha cells, similarly to mib (Delta ubiquitin ligase) mutants and embryos treated with DAPT, a gamma secretase inhibitor able to block Notch signaling. Conversely, jagged1b morphants develop an excess of alpha-cells. Moreover, the pancreas of jagged2 knock-down embryos has a decreased ratio of exocrine-to-endocrine compartments. Finally, overexpression of Notch1a-intracellular-domain in the whole pancreas primordium or specifically in beta-cells helped us to refine a model of pancreas differentiation in which cells exit the precursor state at defined stages to form the pancreatic cell lineages, and, by a feedback mediated by different Notch ligands, limit the number of other cells that can leave the precursor state.

Ontogeny of the endocrine pancreatic cells of the gilthead sea bream, Sparus aurata (Teleost)

The development of the gilthead sea bream, Sparus aurata, endocrine pancreas was studied from hatching to 114 days, using immunocytochemical techniques. Bonito insulin (INS)-, synthetic somatostatin-14 (SS-14)-, salmon somatostatin-25 (SS-25)-, mammalian somatostatin-28 (1-12) (SS-28)-, porcine glucagon (GLU)-, glucagon-like peptide-1 (1-19) (GLP-1)-, synthetic porcine peptide tyrosine tyrosine (PYY)-, and neuropeptide Y (NPY)-like immunoreactivities were demonstrated. The different types of endocrine cells appear at distinct stages of development and differ in their arrangement. The coexistence of INS and SS-25 immunoreactivities was demonstrated in the cells of one strand or primordial cord and a primordial islet that appeared close to the dorsal epithelium of the anterior region of the undifferentiated gut or next to the gut at hatching and one day after hatching, respectively. INS- and SS-25- immunoreactive (ir) cells were located in the core and at the periphery of the single islet found in 2-day-old larvae, while SS-28-ir cells were found in the single islet in 4- to 11-day-old larvae. GLU/GLP-1-ir cells were located next to the outer SS-25-ir cells in the single islet of 12- and 16-day-old larvae. SS-14/SS-25- and SS-14/SS-28-ir cells were detected in the outer region and in the inner area of the single islet, respectively, in 17- to 23-day-old larvae. One big islet and several small islets and isolated or clustered cells next to the pancreatic duct were present in 24- and 25-day-old larvae. The islets were similar in cell composition to the single islet seen in the previous stage, while the isolated and grouped cells showed the coexistence of INS and diverse SSs immunoreactivities. Nerve fibers showing PYY immunoreactivity were identified in the islets from 17 days onwards. In 30- to 44-day-old larvae, GLU and NPY immunoreactivities coexisted in a few cells at the periphery of some small islets. PYY-ir cells were first detected at day 51. One big islet, several intermediate islets and numerous small islets were present from 51-day-old-larvae to juveniles. GLU was colocalized with PYY and NPY in a few cells in a small peripheral area in the big islet and a few intermediate islets. The outer region of small islets and other intermediate islets showed the complete coexistence of GLU, PYY, and NPY.

The fish endocrine pancreas: review, new data, and future research directions in ontogeny and phylogeny

The literature on the ontogeny and phylogeny of the endocrine pancreas of ray-finned fishes is summarized since the latest review in fish [Youson, J.H., Al-Mahrouki, A.A., 1999. Review. Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fishes. Gen. Comp. Endocrinol. 116, 303-335]. A basic description and a demonstration of the diversity of the fish islet organ is provided through new immunohistochemical data on islet tissue from a basal teleost, an osteoglossomorph, and a more derived teleost, a perciforme. Unlike the previous review, the present report provides a review and discussion of the utility of sequence data of insulin, somatostatin, and NPY- and glucagon-family peptides in phylogenetic analyses of jawed and jawless fishes. The present study also provides the first comparative analysis of sequences of preprohormones of endocrine peptides from closely related basal teleost species. Some nucleotide and deduced amino acid sequence data for preprosomatostatins (PPSS-I and/or -II) are compared for four species of bonytongues, Osteoglossomorpha, and with PPSSs of the white sucker, Catostomus commersoni, representing Cypriniformes, a more generalized teleost order. Phylogenetic analysis of deduced amino acid sequences of the PPSSs of these species and others from databases indicates good support for the monophyly of Osteoglossomorpha and some support for the present taxonomic grouping of the osteoglossomorphs examined, and also the white sucker. However, PPSS may have limited phylogenetic utility due to the relative short sequence, particularly in resolving relationships among lineages that diverged over a short period of time. Since in the few fish species examined we have just touched the surface in describing the diversity of structure of the islet organ, and likely the nature of the products of its cells, this report promotes the continued study of this organ.

A mutation in the zebrafish Na,K-ATPase subunitatp1a1a.1provides genetic evidence that the sodium potassium pump contributes to left-right asymmetry downstream or in parallel to nodal flow

While there is a good conceptual framework of dorsoventral and anterioposterior axes formation in most vertebrate groups, understanding of left-right axis initiation is fragmentary. Diverse mechanisms have been implied to contribute to the earliest steps of left-right asymmetry, including small molecule signals, gap junctional communication, membrane potential, and directional flow of extracellular liquid generated by monocilia in the node region. Here we demonstrate that a mutation in the zebrafish Na,K-ATPase subunit atp1a1a causes left-right defects including isomerism of internal organs at the anatomical level. The normally left-sided Nodal signal spaw as well as its inhibitor lefty are expressed bilaterally, while pitx2 may appear random or bilateral. Monocilia movement and fluid circulation in Kupffer's vesicle are normal in atp1a1a(m883) mutant embryos. Therefore, the Na,K-ATPase is required downstream or in parallel to monocilia function during initiation of left-right asymmetry in zebrafish.

Analyses of pancreas development by generation of gfp transgenic zebrafish using an exocrine pancreas-specific elastaseA gene promoter

In contrast to what we know on development of endocrine pancreas, the formation of exocrine pancreas remains poorly understood. To create an animal model that allows observation of exocrine cell differentiation, proliferation, and morphogenesis in living animals, we used the zebrafish elastaseA (elaA) regulatory sequence to develop transgenic zebrafish that display highly specific exocrine pancreas expression of GFP in both larvae and adult. By following GFP expression, we found that the pancreas in early development was a relatively compact organ and later extended posterior along the intestine. By transferring the elaA:gfp transgene into slow muscle omitted mutant that is deficient in receiving Hedgehog signals, we further showed that Hedgehog signaling is required for exocrine morphogenesis but not for cell differentiation. We also applied the morpholino knockdown and toxin-mediated cell ablation approaches to this transgenic line. We showed that the development of exocrine pancreas is Islet-1 dependent. Injection of the diphtheria toxin A (DTA) construct under the elastaseA promoter resulted in selective ablation of exocrine cells while the endocrine cells and other endodermal derivatives (liver and intestine) were not affected. Thus, our works demonstrated the new transgenic line provided a useful experimental tool in analyzing exocrine pancreas development.

sox4b is a key player of pancreatic alpha cell differentiation in zebrafish

Pancreas development relies on a network of transcription factors belonging mainly to the Homeodomain and basic Helix-Loop-Helix families. We show in this study that, in zebrafish, sox4, a member of the SRY-like HMG-box (SOX) family, is required for proper endocrine cell differentiation. We found that two genes orthologous to mammalian Sox4 are present in zebrafish and that only one of them, sox4b, is strongly expressed in the pancreatic anlage. Transcripts of sox4b were detected in mid-trunk endoderm from the 5-somite stage, well before the onset of expression of the early pancreatic gene pdx-1. Furthermore, by fluorescent double in situ hybridization, we found that expression of sox4b is mostly restricted to precursors of the endocrine compartment. This expression is not maintained in differentiated cells although transient expression can be detected in alpha cells and some beta cells. That sox4b-expressing cells belong to the endocrine lineage is further illustrated by their absence from the pancreata of slow-muscle-omitted mutant embryos, which specifically lack all early endocrine markers while retaining expression of exocrine markers. The involvement of sox4b in cell differentiation is suggested firstly by its up-regulation in mind bomb mutant embryos displaying accelerated pancreatic cell differentiation. In addition, sox4b knock-down leads to a drastic reduction in glucagon expression, while other pancreatic markers including insulin, somatostatin, and trypsin are not significantly affected. This disruption of alpha cell differentiation is due to down-regulation of the homeobox arx gene specifically in the pancreas. Taken together, these data demonstrate that, in zebrafish, sox4b is expressed transiently during endocrine cell differentiation and plays a crucial role in the generation of alpha endocrine cells.

Wnt5 signaling in vertebrate pancreas development

Background

Signaling by the Wnt family of secreted glycoproteins through their receptors, the frizzled (Fz) family of seven-pass transmembrane proteins, is critical for numerous cell fate and tissue polarity decisions during development.

Results

We report a novel role of Wnt signaling in organogenesis using the formation of the islet during pancreatic development as a model tissue. We used the advantages of the zebrafish to visualize and document this process in living embryos and demonstrated that insulin-positive cells actively migrate to form an islet. We used morpholinos (MOs), sequence-specific translational inhibitors, and time-lapse imaging analysis to show that the Wnt-5 ligand and the Fz-2 receptor are required for proper insulin-cell migration in zebrafish. Histological analyses of islets in Wnt5a^-/- mouse embryos showed that Wnt5a signaling is also critical for murine pancreatic insulin-cell migration.

Conclusion

Our results implicate a conserved role of a Wnt5/Fz2 signaling pathway in islet formation during pancreatic development. This study opens the door for further investigation into a role of Wnt signaling in vertebrate organ development and disease.

Expression analysis of jagged genes in zebrafish embryos

The interaction of transmembrane Delta and Jagged/Serrate ligands with Notch receptors on neighboring cells is critically involved in cell specification during development. In zebrafish, the early expression of delta but not of jagged genes has been investigated in some detail. We have analyzed the sequence and embryonic expression pattern of the three zebrafish genes jagged1a, jagged1b, and jagged2. These genes, whose transcripts are detectable by in situ hybridization from early somitogenesis, are widely and dynamically expressed in embryos. Coexpression is limited, however, to the notochord and lens (jagged1a and jagged1b) and to the otic vesicle and pronephros (jagged1b and jagged2). Conversely, jagged1a and jagged2, both widely expressed in the central nervous system, are not coexpressed. jagged2 is also detected in the epidermis, newly formed somites, pharyngeal pouches, and pancreatic exocrine anlage and jagged1b in otic placodes and cell clusters close to the pancreatic islet. The similarities of the expression patterns of jagged and delta genes in zebrafish suggest that the Jagged and Delta ligands are functionally redundant or required in specific combinations in many differentiation processes.

Development of the islets, exocrine pancreas, and related ducts in the Nile tilapia, Oreochromis niloticus (Pisces: Cichlidae)

Pancreatic development and the relationship of the islets with the pancreatic, hepatic, and bile ducts were studied in the Nile tilapia, Oreochromis niloticus, from hatching to the onset of maturity at 7 months. The number of islets formed during development was counted, using either serial sections or dithizone staining of isolated islets. There was a general increase in islet number with both age and size. Tilapia housed in individual tanks grew more quickly and had more islets than siblings of the same age left in crowded conditions. The pancreas is a compact organ in early development, and at 1 day posthatch (dph) a single principal islet, positive for all hormones tested (insulin, SST-14, SST-28, glucagon, and PYY), is partially surrounded by exocrine pancreas. However, the exocrine pancreas becomes more disseminated in older fish, following blood vessels along the mesenteries and entering the liver to form a hepatopancreas. The epithelium of the pancreatic duct system from the intercalated ducts to the main duct entering the duodenum was positive for glucagon and SST-14 in 8 and 16 dph tilapia. Individual insulin-immunopositive cells were found in one specimen. At this early stage in development, therefore, the pancreatic duct epithelial cells appear to be pluripotent and may give rise to the small islets found near the pancreatic ducts in 16-37 dph tilapia. Glucagon, SST-14, and some PPY-positive enteroendocrine cells were present in the intestine of the 8 dph larva and in the first part of the intestine of the 16 dph juvenile. Glucagon and SST-14-positive inclusions were found in the apical cytoplasm of the mid-gut epithelium of the 16 dph tilapia. These hormones may have been absorbed from the gut lumen, since they are produced in both the pancreatic ducts and the enteroendocrine cells. At least three hepatic ducts join the cystic duct to form the bile duct, which runs alongside the pancreatic duct to the duodenum.

Differential requirement for ptf1a in endocrine and exocrine lineages of developing zebrafish pancreas

Mammalian studies have implicated important roles for the basic helix-loop-helix transcription factor Ptf1a-p48 in the development of both exocrine and endocrine pancreas. We have cloned the Ptf1a-p48 ortholog in Danio rerio. Early zebrafish ptf1a expression is observed in developing hindbrain and in endodermal pancreatic precursors. Analysis of ptf1a and insulin expression reveals a population of exocrine precursors that, throughout early development, are temporally and spatially segregated from endocrine elements. Morpholino-mediated knockdown of ptf1a confirms early divergence of these endocrine and exocrine lineages. Ptf1a morphants lack differentiated exocrine pancreas, but maintain normal differentiation and organization of the principal islet. In addition to the exocrine phenotype, ptf1a knockdown also reduces the prevalence of a small population of anterior endocrine cells normally found outside the principal islet. Together, these findings suggest the presence of distinct ptf1a-dependent and ptf1a-independent precursor populations in developing zebrafish pancreas.

A conserved role for retinoid signaling in vertebrate pancreas development

Retinoic acid (RA) signaling plays critical roles in the regionalization of the central nervous system and mesoderm of all vertebrates that have been examined. However, to date, a role for RA in pancreas and liver development has only been demonstrated for the teleost zebrafish. Here, we demonstrate that RA signaling is required for development of the pancreas but not the liver in the amphibian Xenopus laevis and the avian quail. We disrupted RA signaling in Xenopus tadpoles, using both a pharmacological and a dominant-negative strategy. RA-deficient quail embryos were obtained from hens with a dietary deficiency in vitamin A. In both species we found that pancreas development was dependent on RA signaling. Furthermore, treatment of Xenopus tadpoles with exogenous RA led to an expansion of the pancreatic field. By contrast, liver development was not perturbed by manipulation of RA signaling. Taken together with our previous finding that RA signaling is necessary and sufficient for zebrafish pancreas development, these data support the hypothesis that a critical role for RA signaling in pancreas development is a conserved feature of the vertebrates.

Notch inhibits Ptf1 function and acinar cell differentiation in developing mouse and zebrafish pancreas

Notch signaling regulates cell fate decisions in a variety of adult and embryonic tissues, and represents a characteristic feature of exocrine pancreatic cancer. In developing mouse pancreas, targeted inactivation of Notch pathway components has defined a role for Notch in regulating early endocrine differentiation, but has been less informative with respect to a possible role for Notch in regulating subsequent exocrine differentiation events. Here, we show that activated Notch and Notch target genes actively repress completion of an acinar cell differentiation program in developing mouse and zebrafish pancreas. In developing mouse pancreas, the Notch target gene Hes1 is co-expressed with Ptf1-P48 in exocrine precursor cells, but not in differentiated amylase-positive acinar cells. Using lentiviral delivery systems to induce ectopic Notch pathway activation in explant cultures of E10.5 mouse dorsal pancreatic buds, we found that both Hes1 and Notch1-IC repress acinar cell differentiation, but not Ptf1-P48 expression, in a cell-autonomous manner. Ectopic Notch activation also delays acinar cell differentiation in developing zebrafish pancreas. Further evidence of a role for endogenous Notch in regulating exocrine pancreatic differentiation was provided by examination of zebrafish embryos with homozygous mindbomb mutations, in which Notch signaling is disrupted. mindbomb-deficient embryos display accelerated differentiation of exocrine pancreas relative to wild-type clutchmate controls. A similar phenotype was induced by expression of a dominant-negative Suppressor of Hairless [Su(H)] construct, confirming that Notch actively represses acinar cell differentiation during zebrafish pancreatic development. Using transient transfection assays involving a Ptf1-responsive reporter gene, we further demonstrate that Notch and Notch/Su(H) target genes directly inhibit Ptf1 activity, independent of changes in expression of Ptf1 component proteins. These results define a normal inhibitory role for Notch in the regulation of exocrine pancreatic differentiation.

Differential requirement for ptf1a in endocrine and exocrine lineages of developing zebrafish pancreas

Mammalian studies have implicated important roles for the basic helix-loop-helix transcription factor Ptf1a-p48 in the development of both exocrine and endocrine pancreas. We have cloned the Ptf1a-p48 ortholog in Danio rerio. Early zebrafish ptf1a expression is observed in developing hindbrain and in endodermal pancreatic precursors. Analysis of ptf1a and insulin expression reveals a population of exocrine precursors that, throughout early development, are temporally and spatially segregated from endocrine elements. Morpholino-mediated knockdown of ptf1a confirms early divergence of these endocrine and exocrine lineages. Ptf1a morphants lack differentiated exocrine pancreas, but maintain normal differentiation and organization of the principal islet. In addition to the exocrine phenotype, ptf1a knockdown also reduces the prevalence of a small population of anterior endocrine cells normally found outside the principal islet. Together, these findings suggest the presence of distinct ptf1a-dependent and ptf1a-independent precursor populations in developing zebrafish pancreas.