Development of the adult endocrine pancreas during metamorphosis in the sea lamprey, Petromyzon marinus L. II. Electron microscopy and immunocytochemistry

Cellular reprogramming during mouse development

States of terminal cell differentiation are often considered to be fixed. There are examples, however, in which cells of one type can be converted to a completely different cell type. The process whereby one cell type can be converted to another is referred to as cellular reprogramming. Cellular reprogramming is also referred to in the literature as transdifferentiation (or the direct conversion of one cell type to another without dedifferentiation to an intermediate cell type). Where the conversion between cell types occurs in the developing embryo, the process is referred to as transdetermination. Herein we examine some well-defined examples of transdetermination. Defining the molecular and cellular basis of transdetermination will help us to understand the normal developmental biology of the cells that interconvert, as well as identifying key regulatory transcription factors (master switch genes) that may be important for the reprogramming of stem cells. Harnessing the therapeutic potential of reprogramming and master genes is an important goal in regenerative medicine.

Evolutionary expression of glucose-dependent-insulinotropic polypeptide (GIP)

Glucose-dependent insulinotropic polypeptide (GIP) is a mammalian incretin hormone released into the circulation following nutrient ingestion. We examined the functional evolution of GIP and its relationship with insulin to delineate their respective roles in promoting nutrient efficiency. Expression patterns were examined in the sea lamprey (Petromyzon marinus), a basal vertebrate lacking a distinct pancreas, and in the zebrafish, Xenopus laevis, chicken, and mouse, organisms possessing extraintestinal pancreata. Although sea lamprey genomic analysis predicted a potential GIP-like gene, transcripts were not detected, and insulin expression was confined to the caudal pancreatic bud. GIP was detected in both the intestine and pancreas of the zebrafish and X. laevis. In contrast, GIP and insulin expression were limited to the intestine and pancreas, respectively, in chicken and mouse. Phylogenetic analysis of the glucagon-like ligands suggested proglucagon as the common ancestor, supporting the theory that GIP arose as a gene duplication of proglucagon. Insulin-secreting cells in the sea lamprey intestine may have obviated the need for an enteroinsular axis, and zebrafish may represent an evolutionary transition where GIP does not yet function as an incretin hormone. These observations are consistent with the hypothesis that GIP and insulin influence survival advantage by enhancing the efficiency of nutrient absorption and energy storage.

Phylogenetic and regulatory region analysis of Wnt5 genes reveals conservation of a regulatory module with putative implication in pancreas development

BACKGROUND

Wnt5 genes belong to the large Wnt family, encoding proteins implicated into several tumorigenic and developmental processes. Phylogenetic analyses showed that Wnt5 gene has been duplicated at the divergence time of gnathostomata from agnatha. Interestingly, experimental data for some species indicated that only one of the two Wnt5 paralogs participates in the development of the endocrine pancreas. The purpose of this paper is to reexamine the phylogenetic history of the Wnt5 developmental regulators and investigate the functional shift between paralogs through comparative genomics.

RESULTS

In this study, the phylogeny of Wnt5 genes was investigated in species belonging to protostomia and deuterostomia. Furthermore, an in silico regulatory region analysis of Wnt5 paralogs was conducted, limited to those species with insulin producing cells and pancreas, covering the evolutionary distance from agnatha to gnathostomata. Our results confirmed the Wnt5 gene duplication and additionally revealed that this duplication event included also the upstream region. Moreover, within this latter region, a conserved module was detected to which a complex of transcription factors, known to be implicated in embryonic pancreas formation, bind.

CONCLUSIONS

Results and observations presented in this study, allow us to conclude that during evolution, the Wnt5 gene has been duplicated in early vertebrates, and that some paralogs conserved a module within their regulatory region, functionally related to embryonic development of pancreas. Interestingly, our results allowed advancing a possible explanation on why the Wnt5 orthologs do not share the same function during pancreas development. As a final remark, we suggest that an in silico comparative analysis of regulatory regions, especially when associated to published experimental data, represents a powerful approach for explaining shift of roles among paralogs.

Generation of Hepatocyte-Like Cells from in Vitro Transdifferentiated Human Fetal Pancreas

Although the appearance of hepatic foci in the pancreas has been described in animal experiments and in human pathology, evidence for the conversion of human pancreatic cells to liver cells is still lacking. We therefore investigated the developmental plasticity between human embryonic pancreatic cells and liver cells. Cells were isolated and expanded from 7–8-week-old human fetal pancreata (HFP) and were characterized for the absence and presence of pancreatic and hepatic markers. In vitro expanded HFP were treated with fibroblast growth factor 2 (FGF2) and dexamethasone (DX) to induce a liver phenotye in the cells. These treated cells in various passages were further studied for their capacity to be functional in hepatic parenchyma following retrorsine-induced injury in nude C57 black mice. Amylase- and EPCAM-positive-enriched cells isolated from HFP and treated with FGF2 and DX lost expression of pancreatic markers and gained a liver phenotype. Hepatic differentiation was based on the expression (both at the mRNA and protein level) of liver markers albumin and cytokeratin 19. When transplanted in vivo into nude mice treated with retrorsine, both cell types successfully engrafted and functionally differentiated into hepatic cells expressing human albumin, glycogen, dipeptidyl peptidase, and γ-glutamyltranspeptidase. These data indicate that human fetal pancreatic cells have a capacity to alter their gene expression profile in response to exogenous treatment with FGF2 and DX. It may be possible to generate an unlimited supply of hepatocytes in vitro for cell therapy.

Characterization of cells in the developing human liver

Human hepatic progenitor cells (HPCs) have been shown to co-express the hematopoietic stem cell (HSC) markers, CD117 and CD34. These cells differentiate not only into hepatocytes and cholangiocytes but also into pancreatic ductal and acinar cells under certain conditions. The fetal liver (FL) is rich in precursor/stem cells; however, little is known about (i) the markers expressed by liver cells during fetal development and (ii) whether an equivalent to the adult liver stem-like progenitors exists in the FL. Here, (i) FL tissue obtained from human 5-18-week-old fetuses were evaluated by means of flow cytometry, immunocyto-, and histochemistry for the emergence of cells expressing and co-expressing known hematopoietic, hepatic, and pancreatic cell markers, and (ii) isolated putative HPCs were phenotypically and molecularly characterized. We report that (i) red blood and endothelial cell precursors were most abundant in early gestation. Cells expressing HSC and pancreatic markers were found in the first trimester, while cells expressing hepatic markers appeared in the second trimester. Very few committed cells were present in FLs obtained early in the first trimester. In addition, cells expressing pancreatic markers co-expressed the HSC marker CD117. (ii) Isolated CD117+/CD34+/CD90- cells in vitro expressed both the genes and proteins for the hepatic markers such as albumin, alpha feto protein (AFP), alpha1-antitrypsin, and cytokeratin 19 (CK19). Our study suggests that hepatoblast and ductal plate/bile duct development mainly occurs during the second trimester. FLs in gestation weeks 5-9 had the highest numbers of precursor cells and the least committed cells. Cells that differentiate into Alb+ or CK19+ can be isolated from early FLs and may be appropriate progenitors for establishing novel systems to investigate basic mechanisms for cell therapy.

Bile ducts as a source of pancreatic ? cells

In recent years, there have been a number of well-documented examples demonstrating that one cell type can be converted to another. Two such examples are the appearance of ectopic pancreas in the liver and formation of hepatic tissue in the pancreas. The conversion of liver to pancreas raises the intriguing possibility of generating insulin-producing beta cells for therapeutic transplantation into diabetics. There is now a striking addition to the growing list of pancreatic conversions: the formation of pancreatic tissue in the developing biliary system.

Conversion of biliary system to pancreatic tissue in Hes1-deficient mice

The biliary system, pancreas and liver all develop from the nearby foregut at almost the same time in mammals. The molecular mechanisms that determine the identity of each organ in this complex area are unknown. Hes1 encodes the basic helix-loop-helix protein Hes1 (ref. 1), which represses positive basic helix-loop-helix genes such as Neurog3 (ref. 3). Expression of Hes1 is controlled by the evolutionarily conserved Notch pathway. Hes1 operates as a general negative regulator of endodermal endocrine differentiation, and defects in Notch signaling lead to accelerated pancreatic endocrine differentiation. Mutations in JAG1, encoding a Notch ligand, cause the Alagille syndrome in humans, characterized by poor development of the biliary system, suggesting that the Notch pathway is also involved in normal biliary development. Here we show that Hes1 is expressed in the extrahepatic biliary epithelium throughout development and that Hes1-deficient mice have gallbladder agenesis and severe hypoplasia of extrahepatic bile ducts. Biliary epithelium in Hes1-/- mice ectopically expresses the proendocrine gene Neurog3 (refs. 12,13), differentiates into endocrine and exocrine cells and forms acini and islet-like structures in the mutant bile ducts. Thus, biliary epithelium has the potential for pancreatic differentiation and Hes1 determines biliary organogenesis by preventing the pancreatic differentiation program, probably by directly repressing transcription of Neurog3.

Regulatory phases of early liver development: paradigms of organogenesis

Genetic analysis, embryonic tissue explantation and in vivo chromatin studies have together identified the distinct regulatory steps that are necessary for the development of endoderm into a bud of liver tissue and, subsequently, into an organ. In this review, I discuss the acquisition of competence to express liver-specific genes by the endoderm, the control of early hepatic growth, the coordination of hepatic and vascular development and the cell differentiation that is necessary to generate a functioning liver. The regulatory mechanisms that underlie these phases are common to the development of many organ systems and might be recapitulated or disrupted during stem-cell differentiation and adult tissue pathogenesis.

Hepatocyte differentiation: from the endoderm and beyond

Hepatocytes differentiate from the endoderm during embryonic development. Recent studies show, however, that hepatocytes can also be derived from rare cells that reside in the pancreas, bone marrow, and brain. Indeed, the latest discoveries indicate that embryonic hepatocytes normally arise by diversion of an endodermal cell population that would otherwise default to a pancreatic fate. Convergent FGF and BMP signals from distinct mesodermal cell types control this transition. Molecular signals that govern the differentiation of hepatocytes from non-endodermal cells and the role of such cells in normal liver physiology remain to be discovered.

A bipotential precursor population for pancreas and liver within the embryonic endoderm

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Comparative morphology of the gallbladder and biliary tract in vertebrates: variation in structure, homology in function and gallstones

A review of investigations on the morphology of the gallbladder and biliary tract in fish, reptiles, amphibians, birds, and mammals was performed. Scanning electron microscopy, transmission electron microscopy, and light microscopy observations by the authors were also included. Variations in the presence or absence of a gallbladder, surface epithelium of the gallbladder, and differences in the morphology of the biliary tract in vertebrates were reported. Many differences were diet-related. Despite some dissimilarities observed, analogous functioning of the biliary system was accomplished by its various components, with the biliary ducts performing the function of the gallbladder when this organ was absent. In addition, the occurrence of peculiar parasitism and gallstones among some cases of vertebrates, including humans, was presented.

Development of pancreas

Pancreatic development is reviewed in man, mammals, and birds. Anatomical differences and differing topography of pancreatic excretory ducts are described in a series of mammalian species. Species differences are discussed with respect to their embryological significance. The developmental potency of the hepatopancreatic ring is stressed. Cytodifferentiation of exocrine and endocrine cells is considered.

A different transformation

The pattern of mRNA expression for liver-specific proteins and liver-enriched transcription factors were studied in two models of facultative gut epithelial progenitor cells activation: D-galactosa mine (GalN)-induced liver injury and dietary copper depletion leading to pancreatic acinar atrophy. After 5 weeks of copper deficiency (CuD, pancreatic acini of Fischer 344 rats underwent atrophy, associated with intense proliferation of small ductlike cells with oval-shaped nuclei. These cells resemble morphologically epithelial progenitor cells of the liver that proliferate after GalN administration. Activated pancreatic epithelial cells express mRNAs for five liver specific genes normally expressed in fetal liver, including α 1 antitrypsin, glucose-6-phosphatase, and others, but not genes that are turned on after birth such as serine dehydratase, tyrosine aminotransferase, and multidrug resistance gene-1b. They express mRNAs for liver-enriched transcription factors including HNF-1α, HNF-3β and γ, HNF-4, and members of the CCAAT-enhancer binding protein (C/EBP) family. The only mRNA for a liver-enriched transcription factor not detected in the pancreas of CuD animals was HNF-3α. Expression of HNF-3α, β and γ, and C/EBP-β mRNA was highly activated in proliferating liver epithelial cells on days 2 and 3 after GalN injury. Increased expression of C/EBP-δ was observed first in the liver on day 1 after GalN administration and in the pancreas at 4 weeks after initiating CuD. We suggest that C/EBP-δ could not be involved in the initial activation of epithelial progenitor cells and that HNF-3α, β and γ, and C/EBP-β might participate in their maturation. We conclude further that pancreatic epithelial progeni tor cells undertake differentiation through the hepatocyte lineage but cannot complete the differ entiation program within the pancreatic milieu.

Development of the adult endocrine pancreas during metamorphosis in the sea lamprey, Petromyzon marinus L. I. Light microscopy and autoradiography

The results of this study were found to support the hypothesis put forth by Boenig (Z. Mikrosk-Anat. Forsch., 17:125-184, 1929) that the caudal pancreas of adult lamprey develops from the epithelium of the extrahepatic common bile duct in that the bile duct cells were found to undergo a great proliferation during the early stages of metamorphosis, with a large number of the cells incorporating 3H-thymidine. If the bile duct degenerated as suggested by Barrington (The Biology of Lampreys. Academic Press, London, pp. 135-169, 1972), this uptake would not be expected. The cranial pancreas was determined to develop in a similar manner to the larval islets, with formation of the islets taking place within the intestinal/diverticular epithelium. The newly formed islets would migrate into the surrounding connective tissue. During the later stages of metamorphosis a small number of cells was found to incorporate the tritiated thymidine within mature islets.