Persistent expression of HNF6 in islet endocrine cells causes disrupted islet architecture and loss of beta cell function

We used transgenesis to explore the requirement for downregulation of hepatocyte nuclear factor 6 (HNF6) expression in the assembly, differentiation, and function of pancreatic islets. In vivo, HNF6 expression becomes downregulated in pancreatic endocrine cells at 18. 5 days post coitum (d.p.c.), when definitive islets first begin to organize. We used an islet-specific regulatory element (pdx1(PB)) from pancreatic/duodenal homeobox (pdx1) gene to maintain HNF6 expression in endocrine cells beyond 18.5 d.p.c. Transgenic animals were diabetic. HNF6-overexpressing islets were hyperplastic and remained very close to the pancreatic ducts. Strikingly, alpha, delta, and PP cells were increased in number and abnormally intermingled with islet beta cells. Although several mature beta cell markers were expressed in beta cells of transgenic islets, the glucose transporter GLUT2 was absent or severely reduced. As glucose uptake/metabolism is essential for insulin secretion, decreased GLUT2 may contribute to the etiology of diabetes in pdx1(PB)-HNF6 transgenics. Concordantly, blood insulin was not raised by glucose challenge, suggesting profound beta cell dysfunction. Thus, we have shown that HNF6 downregulation during islet ontogeny is critical to normal pancreas formation and function: continued expression impairs the clustering of endocrine cells and their separation from the ductal epithelium, disrupts the spatial organization of endocrine cell types within the islet, and severely compromises beta cell physiology, leading to overt diabetes.     

Notch-responsive cells initiate the secondary transition in larval zebrafish pancreas

Zebrafish provide a highly versatile model in which to study vertebrate development. Many recent studies have elucidated early events in the organogenesis of the zebrafish pancreas; however, several aspects of early endocrine pancreas formation in the zebrafish are not homologous to the mammalian system. To better identify mechanisms of islet formation in the zebrafish, with true homology to those observed in mammals, we have temporally and spatially characterized zebrafish secondary islet formation. As is the case in the mouse, we show that Notch inhibition leads to precocious differentiation of endocrine tissues. Furthermore, we have used transgenic fish expressing fluorescent markers under the control of a Notch-responsive element to observe the precursors of these induced endocrine cells. These pancreatic Notch-responsive cells represent a novel population of putative progenitors that are associated with larval pancreatic ductal epithelium, suggesting functional homology between secondary islet formation in zebrafish and the secondary transition in mammals. We also show that Notch-responsive cells persist in the adult pancreas and possess the classical characteristics of centroacinar cells, a cell type believed to be a multipotent progenitor cell in adult mammalian pancreas.     

Expression and function of alpha(v)beta(3) and alpha(v)beta(5) integrins in the developing pancreas: roles in the adhesion and migration of putative endocrine progenitor cells

Cell-cell and cell-matrix interactions play a critical role in tissue morphogenesis and in homeostasis of adult tissues. The integrin family of adhesion receptors regulates cellular interactions with the extracellular matrix, which provides three-dimensional information for tissue organization. It is currently thought that pancreatic islet cells develop from undifferentiated progenitors residing within the ductal epithelium of the fetal pancreas. This process involves cell budding from the duct, migration into the surrounding mesenchyme, differentiation, and clustering into the highly organized islet of Langerhans. Here we report that alpha(v)beta(3) and alpha(v)beta(5), two integrins known to coordinate epithelial cell adhesion and movement, are expressed in pancreatic ductal cells and clusters of undifferentiated cells emerging from the ductal epithelium. We show that expression and function of alpha(v)beta(3) and alpha(v)beta(5) integrins are developmentally regulated during pancreatic islet ontogeny, and mediate adhesion and migration of putative endocrine progenitor cells both in vitro and in vivo in a model of pancreatic islet development. Moreover, we demonstrate the expression of fibronectin and collagen IV in the basal membrane of pancreatic ducts and of cell clusters budding from the ductal epithelium. Conversely, expression of vitronectin marks a population of epithelial cells adjacent to, or emerging from, pancreatic ducts. Thus, these data provide the first evidence for the contribution of integrins alpha(v)beta(3) and alpha(v)beta(5) and their ligands to morphogenetic events in the human endocrine pancreas.     

Ngn3(+) endocrine progenitor cells control the fate and morphogenesis of pancreatic ductal epithelium

During pancreas development, endocrine and exocrine cells arise from a common multipotent progenitor pool. How these cell fate decisions are coordinated with tissue morphogenesis is poorly understood. Here we have examined ductal morphology, endocrine progenitor cell fate and Notch signaling in Ngn3^−/− mice, which do not produce islet cells. Ngn3 deficiency results in reduced branching and enlarged pancreatic duct-like structures, concomitant with Ngn3 promoter activation throughout the ductal epithelium and reduced Notch signaling. Conversely, forced generation of surplus endocrine progenitor cells causes reduced duct caliber and an excessive number of tip cells. Thus, endocrine progenitor cells normally provide a feedback signal to adjacent multipotent ductal progenitor cells that activates Notch signaling, inhibits further endocrine differentiation and promotes proper morphogenesis. These results uncover a novel layer of regulation coordinating pancreas morphogenesis and endocrine/exocrine differentiation, and suggest ways to enhance the yield of beta cells from stem cells.     

Cellular distribution of insulin, glucagon, pancreatic polypeptide hormone and somatostatin in the fetal and adult pancreas of the guinea pig: a comparative immunohistochemical study

Antibodies to insulin, glucagon, pancreatic polypeptide hormone and somatostatin were utilized to demonstrate the cellular localization of the hormones in pancreatic tissue of fetal guinea pig of advanced gestation by immunofluorescence histochemistry. The topographical distribution of the 4 endocrine cell types was compared with those of the adult pancreas and was found to be significantly different particularly for cells immunostaining for insulin, glucagon and somatostatin. These observations suggest changes in histogenesis of pancreatic endocrine cells during transition from fetal to postnatal and adult life. The presence of the 4 islet hormones in the fetal pancreas of this species implies that they may be important in fetal metabolism and growth.     

Formation of human islands of Langerhans cells from the epithelium of the acini and ducts]

The contents of endocrine cells in the epithelium of ducts and the number of acino-insular elements in exocrine parenchyma and in pancreatic islets were estimated by means of the dotted method in semithin sections prepared from the pancreas of the human embryos (4--7 months of embryogenesis) and of adult persons (40--50 years of age). Endocrine cell formation was noted in all stages studied in ontogenesis. Quantitative data demonstrated that the epithelium of the ducts is the main source for insulocyte formation. In the pancreas of both human embryos and adult persons, acino-insular transformation participated in the formation of endocrine elements. The data obtained gave a certain evidence on entodermal origin of the pancreatic islets in the human pancreas.     

Combined activities of hedgehog signaling inhibitors regulate pancreas development

Hedgehog signaling is known to regulate tissue morphogenesis and cell differentiation in a dose-dependent manner. Loss of Indian hedgehog (Ihh) results in reduction in pancreas size, indicating a requirement for hedgehog signaling during pancreas development. By contrast, ectopic expression of sonic hedgehog (Shh) inhibits pancreatic marker expression and results in transformation of pancreatic mesenchyme into duodenal mesoderm. These observations suggest that hedgehog signaling activity has to be regulated tightly to ensure proper pancreas development. We have analyzed the function of two hedgehog inhibitors, Hhip and patched 1 (Ptch), during pancreas formation. Our results indicated that loss of Hhip results in increased hedgehog signaling within the pancreas anlage. Pancreas morphogenesis, islet formation and endocrine cell proliferation is impaired in Hhip mutant embryos. Additional loss of one Ptch allele in Hhip-/-Ptch+/- embryos further impairs pancreatic growth and endodermal cell differentiation. These results demonstrate combined requirements for Hhip and Ptch during pancreas development and point to a dose-dependent response to hedgehog signaling within pancreatic tissue. Reduction of Fgf10 expression in Hhip homozygous mutants suggests that at least some of the observed phenotypes result from hedgehog-mediated inhibition of Fgf signaling at early stages.     

Development of endocrine pancreatic islets in embryos of the grass snake Natrix natrix (Lepidosauria,Serpentes)

Differentiation of the pancreatic islets in grass snake Natrix natrix embryos, was analyzed using light, transmission electron microscopy, and immuno-gold labeling. The study focuses on the origin of islets, mode of islet formation, and cell arrangement within islets. Two waves of pancreatic islet formation in grass snake embryos were described. The first wave begins just after egg laying when precursors of endocrine cells located within large cell agglomerates in the dorsal pancreatic bud differentiate. The large cell agglomerates were divided by mesenchymal cells thus forming the first islets. This mode of islet formation is described as fission. During the second wave of pancreatic islet formation which is related to the formation of the duct mantle, we observed four phases of islet formation: (a) differentiation of individual endocrine cells from the progenitor layer of duct walls (budding) and their incomplete delamination; (b) formation of two types of small groups of endocrine cells (A/D and B) in the wall of pancreatic ducts; (c) joining groups of cells emerging from neighboring ducts (fusion) and rearrangement of cells within islets; (d) differentiated pancreatic islets with characteristic arrangement of endocrine cells. Mature pancreatic islets of the grass snake contained mainly A endocrine cells. Single B and D or PP–cells were present at the periphery of the islets. This arrangement of endocrine cells within pancreatic islets of the grass snake differs from that reported from most others vertebrate species. Endocrine cells in the pancreas of grass snake embryos were also present in the walls of intralobular and intercalated ducts. At hatching, some endocrine cells were in contact with the lumen of the pancreatic ducts.     

Genetic analysis of early endocrine pancreas formation in zebrafish

Endocrine pancreas of zebrafish consist of at least four different cell types that function similarly to mammalian pancreatic islet. No mutants specifically affecting formation of the endocrine pancreas have been identified during the previous large-scale mutagenesis screens in zebrafish due to invisibility of a pancreatic islet. We combined in situ hybridization method to visualize pancreatic islet with an ethyl-nitroso-urea mutagenesis screen to identify novel genes involved in pancreatic islet formation in zebrafish. We screened 900 genomes and identified 11 mutations belonging to nine different complementation groups. These mutants fall into three major phenotypic classes displaying severely reduced insulin expression, reduced insulin expression with abnormal islet morphology, or abnormal islet morphology with relatively normal number of insulin expressing cells. Seven of these mutants do not have any other visible phenotypes associated. These mutations affect different processes in pancreatic islet development. Additional analysis on glucagon and somatostatin cell specification revealed that somatostatin cells are specified at a separate domain from insulin cells whereas glucagon cells are specified adjacent to insulin cells. Furthermore, glucagon cells and somatostatin cells are always associated with insulin cells in mutants that have scattered insulin expression. These data indicate that there are separate mechanisms regulating endocrine cell migration, proliferation, and differentiation. Further study on these mutants will reveal important information on novel genes involved in pancreatic islet cell specification and morphogenesis.     

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.     

Islet Formation during the Neonatal Development in Mice

The islet of Langerhans is a unique micro-organ within the exocrine pancreas, which is composed of insulin-secreting beta-cells, glucagon-secreting alpha-cells, somatostatin-secreting delta-cells, pancreatic polypeptide-secreting PP cells and ghrelin-secreting epsilon-cells. Islets also contain non-endocrine cell types such as endothelial cells. However, the mechanism(s) of islet formation is poorly understood due to technical difficulties in capturing this dynamic event in situ. We have developed a method to monitor beta-cell proliferation and islet formation in the intact pancreas using transgenic mice in which the beta-cells are specifically tagged with a fluorescent protein. Endocrine cells proliferate contiguously, forming branched cord-like structures in both embryos and neonates. Our study has revealed long stretches of interconnected islets located along large blood vessels in the neonatal pancreas. Alpha-cells span the elongated islet-like structures, which we hypothesize represent sites of fission and facilitate the eventual formation of discrete islets. We propose that islet formation occurs by a process of fission following contiguous endocrine cell proliferation, rather than by local aggregation or fusion of isolated beta-cells and islets. Mathematical modeling of the fission process in the neonatal islet formation is also presented.     

Pancreatic mesenchyme regulates epithelial organogenesis throughout development

The developing pancreatic epithelium gives rise to all endocrine and exocrine cells of the mature organ. During organogenesis, the epithelial cells receive essential signals from the overlying mesenchyme. Previous studies, focusing on ex vivo tissue explants or complete knockout mice, have identified an important role for the mesenchyme in regulating the expansion of progenitor cells in the early pancreas epithelium. However, due to the lack of genetic tools directing expression specifically to the mesenchyme, the potential roles of this supporting tissue in vivo, especially in guiding later stages of pancreas organogenesis, have not been elucidated. We employed transgenic tools and fetal surgical techniques to ablate mesenchyme via Cre-mediated mesenchymal expression of Diphtheria Toxin (DT) at the onset of pancreas formation, and at later developmental stages via in utero injection of DT into transgenic mice expressing the Diphtheria Toxin receptor (DTR) in this tissue. Our results demonstrate that mesenchymal cells regulate pancreatic growth and branching at both early and late developmental stages by supporting proliferation of precursors and differentiated cells, respectively. Interestingly, while cell differentiation was not affected, the expansion of both the endocrine and exocrine compartments was equally impaired. To further elucidate signals required for mesenchymal cell function, we eliminated β-catenin signaling and determined that it is a critical pathway in regulating mesenchyme survival and growth. Our study presents the first in vivo evidence that the embryonic mesenchyme provides critical signals to the epithelium throughout pancreas organogenesis. The findings are novel and relevant as they indicate a critical role for the mesenchyme during late expansion of endocrine and exocrine compartments. In addition, our results provide a molecular mechanism for mesenchymal expansion and survival by identifying β-catenin signaling as an essential mediator of this process. These results have implications for developing strategies to expand pancreas progenitors and β-cells for clinical transplantation.     

NRP1 and NRP2 cooperate to regulate gangliogenesis, axon guidance and target innervation in the sympathetic nervous system

During mouse pancreas development, the transient expression of Neurogenin3 (Neurog3) in uncommitted pancreas progenitors is required to determine endocrine destiny. However it has been reported that Neurog3-expressing cells can eventually adopt acinar or ductal fates and that Neurog3 levels were important to secure the islet destiny. It is not known whether the competence of Neurog3-induced cells to give rise to non-endocrine lineages is an intrinsic property of these progenitors or depends on pancreas developmental stage. Using temporal genetic labeling approaches we examined the dynamic of endocrine progenitor differentiation and explored the plasticity of Neurog3-induced cells throughout development. We found that Neurog3(+) progenitors develop into hormone-expressing cells in a fast process taking less then 10h. Furthermore, fate-mapping studies in heterozygote (Neurog3(CreERT/+)) and Neurog3-deficient (Neurog3(CreERT/CreERT)) embryos revealed that Neurog3-induced cells have different potential over time. At the early bud stage, failed endocrine progenitors can adopt acinar or ductal fate, whereas later in the branching pancreas they do not contribute to the acinar lineage but Neurog3-deficient cells eventually differentiate into duct cells. Thus these results provide evidence that the plasticity of Neurog3-induced cells becomes restricted during development. Furthermore these data suggest that during the secondary transition, endocrine progenitor cells arise from bipotent precursors already committed to the duct/endocrine lineages and not from domain of cells having distinct potentialities.     

Generation of living color transgenic zebrafish to trace somatostatin-expressing cells and endocrine pancreas organization

In the present study, both gfp and rfp transgenic zebrafish lines using a 2.5-kb zebrafish somatostain2 (sst2) promoter were generated. During embryonic development, expression of GFP/RFP in the endocrine pancreas of transgenic embryos was initiated at ∼20 hpf and the number of GFP/RFP positive cells in the pancreas increased in subsequent stages; thus, our newly generated Tg(sst2:gfp) and Tg(sst2:rfp) lines faithfully recapitulated sst2 expression in endocrine pancreatic cells and provided a useful tool in analyzing the development of Sst2-producing δ-cells in the pancreas. By crossing these new transgenic lines with previously available transgenic lines targeted in insulin (Ins)-producing β-cells, Tg(ins:gfp) and Tg(ins:rfp), in combination with immunodetection of glucagon (Gcg)-producing α-cells and pancreatic polypeptide (PP)-producing PP-cells, the organization and composition of endocrine islets were investigated in both embryonic and adult pancreas. We found that there was always a big cluster of endocrine cells (principal islet) in the anterior-dorsal pancreas, followed by numerous smaller clusters (variable in size) of endocrine cells (secondary islets) along the anterior–posterior axis of the pancreas. All four types of endocrine cells were found in the principal islet, but secondary islets may or may not contain PP-cells. In addition, there were also discrete endocrine cells throughout the pancreas. In all co-localization experiments, we did not find any endocrine cells positive for more than one hormone markers, suggesting that these endocrine cells produce only a single hormone. In both principal and secondary islets, we found that β-cells were generally located in the center and non-β cells in the periphery; reminiscent of the “mantel–core” organization of islets of Langerhans in mammals where β-cells form the core and non-β-cells the mantel. In zebrafish primary islet, β-cells constitute most of the mass (∼50%), followed by δ-cells and α-cells (20–25% each), and PP-cells (1–2%); this is also similar to the composition of mammalian islets.