Adult insulin- and glucagon-producing cells differentiate from two independent cell lineages

Genes, Signals, and Lineages in Pancreas Development

Type I diabetes results from the autoimmune-mediated destruction of pancreatic beta cells, which regulate blood sugar levels by secretion of insulin. Recent clinical data suggest that the disease could be cured if an adequate supply of new beta-cells were available, and one goal of pancreatic developmental biology is to understand how endogenous beta-cells are made, with the hope of making them exogenously. Much is now known about the transcriptional regulation of pancreatic organ specification, growth, and lineage allocation; less is known about intercellular signals that regulate this process, but candidates continue to emerge. Additional insights, often contradicting older models, have come from the application of new lineage-tracing techniques. Altogether, these studies also shed light on the still-elusive pancreatic stem cell, which may participate in normal organ maintenance as well as recovery from injury. A rigorous proof of the existence of such a cell, whether in vivo or in vitro, would offer real hope for the prospect of controlled beta-cell generation in a clinical setting.

Hnf6 and Tcf2 (MODY5) are linked in a gene network operating in a precursor cell domain of the embryonic pancreas

During pancreatic organogenesis endocrine cells arise from non self-renewing progenitors that express Ngn3. The precursors that give rise to Ngn3+ cells are presumably located within duct-like structures. However, the nature of such precursors is poorly understood. We show that, at E13-E18, the embryonic stage during which the major burst of beta-cell neogenesis takes place, pancreatic duct cells express Hnf1beta, the product of the maturity-onset diabetes of the young type 5 (MODY5) gene. Ngn3+ cells at this stage invariably cluster with mitotically competent Hnf1beta+ cells, and are often intercalated with these cells in the epithelium that lines the lumen of primitive ducts. We present several observations that collectively indicate that Hnf1beta+ cells are the immediate precursors of Ngn3+ cells. We furthermore show that Hnf1beta expression is markedly reduced in early pancreatic epithelial cells of Hnf6-deficient mice, in which formation of Ngn3+ cells is defective. These findings define a precursor cellular stage of the embryonic pancreas and place Hnf1beta in a genetic hierarchy that regulates the generation of pancreatic endocrine cells.

Advances in islet cell biology: from stem cell differentiation to clinical transplantation: conference report

The 3rd Annual Rachmiel Levine Symposium entitled "Advances in Islet Cell Biology-From Stem Cell Differentiation to Clinical Transplantation" was organized by the Department of Diabetes, Endocrinology and Metabolism at the City of Hope National Medical Center, with the support of the Southern California Islet Cell Resources Center, American Diabetes Association-David Shapiro Research Fund, Ross Foundation, the National Center for Research Resources (NCRR), and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health. The symposium was held at the Hilton Anaheim Hotel in Anaheim, CA, in October 2002, and was attended by nearly 400 participants from 23 countries and 30 U.S. states. The symposium consisted of 11 sessions focusing on 3 areas: (1) pancreas and islet cell differentiation and islet generation, (2) beta cell biology and insulin synthesis and/or secretion, and (3) pancreatic islet transplantation in patients with type I diabetes. Thirty-nine world experts lectured on the most current information in each field. Fifty-three abstracts were selected for presentation and discussed at the poster session. The first author of each of the top 10 posters received a Young Investigator Travel Award provided by the National Center for Research Resources and the Southern California Islet Cell Resources Center. The symposium also offered special Meet the Professor sessions, which gave the attendees an opportunity to closely interact with the participating speakers of the day.

Betacellulin improves glucose metabolism by promoting conversion of intraislet precursor cells to beta-cells in streptozotocin-treated mice

Betacellulin (BTC) induces differentiation of pancreatic beta-cells and promotes regeneration of beta-cells in experimental diabetes. The present study was conducted to determine if BTC improved glucose metabolism in severe diabetes induced by a high dose of streptozotocin (STZ) in mice. Male ICR mice were injected with 200 microg/g ip STZ, and various doses of BTC were administered daily for 14 days. The plasma glucose concentration increased to a level of >500 mg/dl in STZ-injected mice. BTC (0.2 microg/g) significantly reduced the plasma glucose concentration, but a higher concentration was ineffective. The effect of BTC was marked by day 4 but became smaller on day 6 or later. The plasma insulin concentration and the insulin content were significantly higher in mice treated with 0.1 and 0.2 microg/g BTC. BTC treatment significantly increased the number of beta-cells in each islet as well as the number of insulin-positive islets. Within islets, the numbers of 5-bromo-2-deoxyuridine/somatostatin-positive cells and pancreatic duodenal homeobox-1/somatostatin-positive cells were significantly increased by BTC. These results indicate that BTC improved hyperglycemia induced by a high dose of STZ by promoting neoformation of beta-cells, mainly from somatostatin-positive islet cells.

Abrogation of protein convertase 2 activity results in delayed islet cell differentiation and maturation, increased alpha-cell proliferation, and islet neogenesis

To date, the role of pancreatic hormones in pancreatic islet growth and differentiation is poorly understood. To address this issue, we examined mice with a disruption in the gene encoding prohormone convertase 2 (PC2). These mice are unable to process proglucagon, prosomatostatin, and other neuroendocrine precursors into mature hormones. Initiation of insulin (IN) expression during development was delayed in PC2 mutant mice. Cells containing IN were first detected in knockout embryos on d 15 of development, 5 d later than in wild-type littermates. However, the IN(+) cells of d 15 PC2 mutant mice coexpressed glucagon, as did the first appearing beta-cells of controls. In addition, lack of PC2 perturbed the pattern of expression of transcription factors presumed to be involved in the determination of the mature alpha-cell phenotype. Thus, in contrast to controls, alpha-cells of mutant mice had protracted expression of Nkx 6.1 and Pdx-1, but did not express Brn-4. Islets of adult mutant mice also contained cells coexpressing insulin and somatostatin, an immature cell type found only in islets of the wild-type strain during development. In addition to the effects on islet cell differentiation, the absence of PC2 activity resulted in a 3-fold increase in the rate of proliferation of proglucagon cells during the perinatal period. This increase contributed to the development of alpha-cell hyperplasia during postnatal life. Furthermore, the total beta-cell volume was increased 2-fold in adult mutants compared with controls. This increase was due to islet neogenesis, as the number of islets per section was significantly higher in knockout mice compared with wild-type mice, whereas both strains had similar rates of IN cell proliferation. These results indicate that hormones processed by PC2 affected processes that regulate islet cell differentiation and maturation in embryos and adults.

Activated Notch1 prevents differentiation of pancreatic acinar cells and attenuate endocrine development

Mice carrying loss-of-function mutations in certain Notch pathway genes display increased and accelerated pancreatic endocrine development, leading to depletion of precursor cells followed by pancreatic hypoplasia. Here, we have investigated the effect of expressing a constitutively active form of the Notch1 receptor (Notch1(ICD)) in the developing pancreas using the pdx1 promoter. At e10.5 to e12.5, we observe a disorganized pancreatic epithelium with reduced numbers of endocrine cells, confirming a repressive activity of Notch1 upon the early differentiation program. Subsequent branching morphogenesis is impaired and the pancreatic epithelium forms cyst-like structures with ductal phenotype containing a few endocrine cells but completely devoid of acinar cells. The endocrine cells that do form show abnormal expression of cell type-specific markers. Our observations show that sustained Notch1 signaling not only significantly represses endocrine development, but also fully prevents pancreatic exocrine development, suggesting that a possible role of Notch1 is to maintain the undifferentiated state of common pancreatic precursor cells.

Label-retaining cells in the rat pancreas: location and differentiation potential in vitro

Islets of Langerhans are micro-organs scattered throughout the pancreas that contain insulin-producing cells, called beta-cells. Although new light has been recently shed on beta-cell development, information on the phenotype and location of beta-stem cells remains scarce. Here, we provide evidence that beta-stem cells are slow-cycling cells located within and around the islets of Langerhans. First, using a bromodeoxyuridine (BrdU) pulse/chase approach, we detected BrdU-retaining cells in vivo in the islet area of rat pancreata. These cells were negative for endocrine markers but expressed Pdx1, a marker for pancreatic stem cells. Next, using an in vitro model that mimicked endocrine cell development, we found that BrdU-retaining cells were capable of differentiating into beta-cells. Taken together, these observations demonstrate that BrdU retention is a property of beta-stem cells.

Alternative sources of beta cells for cell therapy of diabetes

Type 1 and type 2 diabetes affect 150 million people worldwide. This is a result of the incapacity of pancreatic beta cells to produce and secrete enough insulin. Transplantation of pancreatic beta cells represents a potential therapeutic approach for type 1 diabetes. However, one limitation is the insufficient amount of beta cells available for grafts. Alternative sources of beta cells have yet to be defined. During the past years, progress has been made in the definition of new strategies to produce mature pancreatic beta cells. Different cell sources are currently tested for their capacity to differentiate into mature beta cells. In this review, I will summarize the status of our knowledge in the field of cell therapy for diabetes.

Genes expressed in the developing endocrine pancreas and their importance for stem cell and diabetes research

The genes that regulate endocrine pancreas development, maintain adult endocrine cells, and stimulate progenitor/stem cells during regeneration remain largely unstudied. There is ample evidence that many of the genes involved in endocrine pancreas development also function in the homeostasis of the adult islet. In light of the potential benefits to diabetic research, it is surprising that there is little information about the genes expressed throughout the ontogeny of the endocrine pancreas. In the past few years, efforts have been made to establish the Endocrine Pancreas Consortium database (EPConDB), in which many of the genes expressed in the developing endocrine pancreas are in a database with a corresponding publicly available clone bank. In addition, advances in microarray technology now allow for a quantitative expression analysis of thousands of genes simultaneously, which makes it possible to generate a quantitative catalog of the genes expressed at each step of endocrine differentiation, from embryonic endoderm to mature beta cells. In this review, I will discuss how genes discovered by virtue of their role in endocrine pancreas development may function in the maintenance of pancreatic stem cells and the regeneration of islets. I will further summarize the recent advances in genomics-based studies of the developing endocrine pancreas and will discuss how they might impact on the discovery of diagnostics and research into stem cell-based approaches for the treatment of diabetes.

General and conditional replacement of connexin43-coding DNA by a lacZ reporter gene for cell-autonomous analysis of expression

Using the Cre/loxP system, we have circumvented early postnatal lethality and possible pleiotropic effects of general Cx43 gene deletion, in order to determine the expression and function of connexin43 (Cx43) in defined cell types. General or cell type-specific, Cre-mediated deletion of the floxed (i.e. flanked by loxP sites) Cx43-coding region led to activation of the inserted lacZ reporter gene in cells with transcriptional activity of the Cx43 gene. As deduced from lacZ expression in mice with general deletion, transcriptional activity of the Cx43 gene was not only found in a broad range of cell types known to a express Cx43, but also inpancreatic duct cells and vascular cells of the gut and skeletal muscle. Cre-mediated deletion restricted to defined cell types led to lacZ activation highlighting corresponding subsets of cells expressing Cx43, such as vascular endothelial cells, hepatic duct cells and putative neural crest cells, which were otherwise masked by strong Cx43 expression in neighbouring cells. In Cx43 expressing cell types, the floxed Cx43 allele was useful as a Cre-excision reporter for the characterization of Cre transgenes.

Single-cell transcript analysis of pancreas development

DNA microarray analysis was combined with a modified single-cell PCR procedure to study gene expression profiles of single cells at different stages of pancreatic development. This method identifies distinct cell types at embryonic day 10.5, a stage when the pancreatic epithelium is morphologically uniform. Some cells express unexpected combinations of genes, and these expression patterns provide new insights into pancreas development. Following on these findings, we use PCR products from different cell types to identify novel pancreatic genes, some of which mark subtypes of developing pancreatic cells. By integrating these data with previous genetic and biochemical studies, we propose a pathway for pancreatic cell development. This form of single-cell transcriptional analysis can be applied to any developmental process or tissue to characterize distinct cell types.

Transcription factors direct the development and function of pancreatic beta cells

Transcription factors orchestrate intricate pathways of cellular growth and differentiation by regulating the rate of transcription of an array of genes. Genetic and biochemical studies have begun to unravel the complex cascade of factors that controls the proliferation and differentiation of cells in the developing pancreas. The specific pathway leading to the development of the insulin-secreting beta cell has been a focus of many of these studies because an understanding of the transcription factors governing this pathway will be crucial to the engineering of new beta cells to cure diabetes. In recent years, the number of transcription factors that has been implicated in beta-cell differentiation and function has grown considerably. Here, we outline the known role of transcription factors in beta-cell development, and describe how these factors form a network of gene activation signals that mediates insulin transcription.

Cre-loxp fate-mapping of Pax6 enhancer active retinal and pancreatic progenitors

Pax6 plays important roles in the control of ocular and pancreatic development. We identified a 450 bp Pax6 enhancer that contains two interacting sequences: a 274 bp fragment sufficient for expression in retinal progenitors and an adjacent 156 bp fragment required for expression in pancreatic progenitors. Since this enhancer is only transiently expressed during embryogenesis, a Cre-loxP fate-mapping strategy was used to investigate the developmental potential of these progenitors. Surprisingly, the labeled retinal precursors predominantly gave rise to horizontal cells, indicating a cell lineage role in horizontal cell differentiation. In the pancreas, all enhancer-specific cells were restricted to endocrine and ductal cell lineages. This result lends support to a model whereby Pax6-expressing progenitors contribute to the adult pancreatic islets and ducts. The progenitor cell-specificity of this enhancer will be useful in studies that require either cell-specific expression or conditional gene inactivation in these cell populations.

Gene expression cascades in pancreatic development

The specialized endocrine and exocrine cells of the pancreas originally derive from a pool of apparently identical cells in the early gut endoderm. Serial changes in their gene expression program, controlled by a hierarchy of pancreatic transcription factors, direct this progression from multipotent progenitor cell to mature pancreatic cell. When the cells differentiate, this hierarchy of factors coalesces into a network of factors that maintain the differentiated phenotype of the cells. As we develop an understanding of the pancreatic transcription factors, we are also acquiring the tools with which we can ultimately control pancreatic cell differentiation.

Direct lineage tracing reveals the ontogeny of pancreatic cell fates during mouse embryogenesis

Lineage tracing follows the progeny of labeled cells through development. This technique identifies precursors of mature cell types in vivo and describes the cell fate restriction steps they undergo in temporal order. In the mouse pancreas, direct cell lineage tracing reveals that Pdx1- expressing progenitors in the early embryo give rise to all pancreatic cells. The progenitors for the mature pancreatic ducts separate from the endocrine/exocrine tissues before E12.5. Expression of Ngn3 and pancreatic polypeptide marks endocrine cell lineages during early embryogenesis, and these cells behave as transient progenitors rather than stem cells. In adults, Ngn3 is expressed within the endocrine islets, and the NGN3+ cells seem to contribute to pancreatic islet renewal. These results indicate the stage at which each progenitor population is restricted to a particular fate and provide markers for isolating progenitors to study their growth, differentiation, and the genes necessary for their development.

Growth inhibitors promote differentiation of insulin-producing tissue from embryonic stem cells

The use of embryonic stem cells for cell-replacement therapy in diseases like diabetes mellitus requires methods to control the development of multipotent cells. We report that treatment of mouse embryonic stem cells with inhibitors of phosphoinositide 3-kinase, an essential intracellular signaling regulator, produced cells that resembled pancreatic beta cells in several ways. These cells aggregated in structures similar, but not identical, to pancreatic islets of Langerhans, produced insulin at levels far greater than previously reported, and displayed glucose-dependent insulin release in vitro. Transplantation of these cell aggregates increased circulating insulin levels, reduced weight loss, improved glycemic control, and completely rescued survival in mice with diabetes mellitus. Graft removal resulted in rapid relapse and death. Graft analysis revealed that transplanted insulin-producing cells remained differentiated, enlarged, and did not form detectable tumors. These results provide evidence that embryonic stem cells can serve as the source of insulin-producing replacement tissue in an experimental model of diabetes mellitus. Strategies for producing cells that can replace islet functions described here can be adapted for similar uses with human cells.

Osteoblast-specific knockout of the insulin-like growth factor (IGF) receptor gene reveals an essential role of IGF signaling in bone matrix mineralization

To examine the local actions of IGF signaling in skeletal tissue in a physiological context, we have used Cre-mediated recombination to disrupt selectively in mouse osteoblasts the gene encoding the type 1 IGF receptor (Igf1r). Mice carrying this bone-specific mutation were of normal size and weight but, in comparison with normal siblings, demonstrated a striking decrease in cancellous bone volume, connectivity, and trabecular number, and an increase in trabecular spacing. These abnormalities correlated with a striking decrease in the rate of mineralization of osteoid that occurred despite an unexpected osteoblast and osteoclast hyperactivity, detected from the significant increments in both osteoblast and erosion surfaces. Our findings indicate that IGF1 is essential for coupling matrix biosynthesis to sustained mineralization. This action is likely to be particularly important during the pubertal growth spurt when rapid bone formation and consolidation are required.

Glucagon is required for early insulin-positive differentiation in the developing mouse pancreas

The embryonic pancreas is thought to develop from pluripotent endodermal cells that give rise to endocrine and exocrine cells. A key guidance mechanism for pancreatic development has previously been found to be epithelial-mesenchymal interaction. Interactions within the epithelium, however, have not been well studied. Glucagon is the earliest peptide hormone present at appreciable levels in the developing pancreatic epithelium (embryonic day [E]-9.5 in mouse). Insulin accumulation begins slightly later (E11 in mouse), followed by a rapid accumulation during the "second wave" of insulin differentiation ( approximately E15). Here we found that blocking early expression and function of glucagon, but not GLP-1, an alternate gene product of preproglucagon mRNA, prevented insulin-positive differentiation in early embryonic (E11) pancreas. These results suggest a novel concept and a key role for glucagon in the paracrine induction of differentiation of other pancreatic components in the early embryonic pancreas.

Defective insulin secretion in pancreatic beta cells lacking type 1 IGF receptor

Defective insulin secretion is a feature of type 2 diabetes that results from inadequate compensatory increase of beta cell mass and impaired glucose-dependent insulin release. beta cell proliferation and secretion are thought to be regulated by signaling through receptor tyrosine kinases. In this regard, we sought to examine the potential proliferative and/or antiapoptotic role of IGFs in beta cells by tissue-specific conditional mutagenesis ablating type 1 IGF receptor (IGF1R) signaling. Unexpectedly, lack of functional IGF1R did not affect beta cell mass, but resulted in age-dependent impairment of glucose tolerance, associated with a decrease of glucose- and arginine-dependent insulin release. These observations reveal a requirement of IGF1R-mediated signaling for insulin secretion.

The role of the transcriptional regulator Ptf1a in converting intestinal to pancreatic progenitors

Pancreas development begins with the formation of buds at specific sites in the embryonic foregut endoderm. We used recombination-based lineage tracing in vivo to show that Ptf1a (also known as PTF1-p48) is expressed at these early stages in the progenitors of pancreatic ducts, exocrine and endocrine cells, rather than being an exocrine-specific gene as previously described. Moreover, inactivation of Ptf1a switches the character of pancreatic progenitors such that their progeny proliferate in and adopt the normal fates of duodenal epithelium, including its stem-cell compartment. Consistent with the proposal that Ptf1a supports the specification of precursors of all three pancreatic cell types, transgene-based expression of Pdx1, a gene essential to pancreas formation, from Ptf1a cis-regulatory sequences restores pancreas tissue to Pdx1-null mice that otherwise lack mature exocrine and endocrine cells because of an early arrest in organogenesis. These experiments provide evidence that Ptf1a expression is specifically connected to the acquisition of pancreatic fate by undifferentiated foregut endoderm.

Development and diseases of the pancreas

The pancreas is a vital gland of exocrine and endocrine function. It is the target of two main affections: diabetes and pancreatic cancer. We describe the tissue interactions, signaling pathways and intracellular targets that are involved in the emergence of the pancreas primordium and its proliferation, morphogenesis and differentiation. It appears that several genes of developmental relevance have an adult function and are involved in pancreas affections. Embryological experimentation in animals contributed to provide candidate genes for human disease and holds promise for future treatments.

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.

Overexpression of c-Myc in beta-cells of transgenic mice causes proliferation and apoptosis, downregulation of insulin gene expression, and diabetes

To test the hypothesis that c-Myc plays an important role in beta-cell growth and differentiation, we generated transgenic mice overexpressing c-Myc in beta-cells under control of the rat insulin II promoter. F(1) transgenic mice from two founders developed neonatal diabetes (associated with reduced plasma insulin levels) and died of hyperglycemia 3 days after birth. In pancreata of transgenic mice, marked hyperplasia of cells with an altered phenotype and amorphous islet organization was displayed: islet volume was increased threefold versus wild-type littermates. Apoptotic nuclei were increased fourfold in transgenic versus wild-type mice, suggesting an increased turnover of beta-cells. Very few cells immunostained for insulin; pancreatic insulin mRNA and content were markedly reduced. GLUT2 mRNA was decreased, but other beta-cell-associated genes (IAPP [islet amyloid pancreatic polypeptide], PDX-1 [pancreatic and duodenal homeobox-1], and BETA2/NeuroD) were expressed at near-normal levels. Immunostaining for both GLUT2 and Nkx6.1 was mainly cytoplasmic. The defect in beta-cell phenotype in transgenic embryos (embryonic days 17-18) and neonates (days 1-2) was similar and, therefore, was not secondary to overt hyperglycemia. When pancreata were transplanted under the kidney capsules of athymic mice to analyze the long-term effects of c-Myc activation, beta-cell depletion was found, suggesting that, ultimately, apoptosis predominates over proliferation. In conclusion, these studies demonstrate that activation of c-Myc in beta-cells leads to 1) increased proliferation and apoptosis, 2) initial hyperplasia with amorphous islet organization, and 3) selective downregulation of insulin gene expression and the development of overt diabetes.

Regulation of pancreatic cell differentiation and morphogenesis

Organogenesis requires tissue interactions to initiate the cascade of inductive and repressive signals necessary for normal organ development. Tissue interactions initiate the pancreatic lineage within the primitive foregut endodermal epithelium and continue to direct the morphogenesis and differentiation of the endocrine, exocrine and ductal portions of the pancreas. An understanding of the mechanisms controlling pancreatic growth would enable the development of alternative therapies for diseases such as type 1 diabetes.

How cells change their phenotype

Recent attention has focused on the remarkable ability of adult stem cells to produce differentiated cells from embryologically unrelated tissues. This phenomenon is an example of metaplasia and shows that embryological commitments can be reversed or erased under certain circumstances. In some cases, even fully differentiated cells can change their phenotype (transdifferentiation). This review examines recently discovered cases of metaplasia, and speculates on the potential molecular and cellular mechanisms that underlie the switches, and their significance to developmental biology and medicine.

Programming of the pancreas

The pancreas, as most of the digestive tract, derives from the endoderm. Differentiation of these early gut endoderm cells into the endocrine cells forming the pancreatic islets of Langerhans depends on a cascade of gene activation events. These are controlled by different classes of transcription factors including the homeodomain, the basic helix-loop-helix (bHLH) and the winged helix proteins. Recently, considerable progress has been made delineating this cascade. The present review focuses on the role of the different transcription factors during pancreas development, with a particular emphasis on the newly identified bHLH transcription factor neurogenin3.

Conditional control of gene expression in the mouse

One of the most powerful tools that the molecular biology revolution has given us is the ability to turn genes on and off at our discretion. In the mouse, this has been accomplished by using binary systems in which gene expression is dependent on the interaction of two components, resulting in either transcriptional transactivation or DNA recombination. During recent years, these systems have been used to analyse complex and multi-staged biological processes, such as embryogenesis and cancer, with unprecedented precision. Here, I review these systems and discuss certain studies that exemplify the advantages and limitations of each system.

Transdifferentiation and metaplasia--switching cell types

States of developmental commitment are encoded as combinations of transcription factors and changes in their expression can bring about transdifferentiation or metaplasia. For example, ectopic expressions of Vestigial can convert Drosophila leg to wing; of C/EBPbeta can convert pancreatic exocrine cells to hepatocytes; and expression of C/EBPalpha and PPARgamma can convert myoblasts to adipocytes.

In-vitro differentiation of pancreatic beta-cells

Stem cell biology is a new field that holds promise for in-vitro mass production of pancreatic beta-cells, which are responsible for insulin synthesis, storage, and release. Lack or defect of insulin produces diabetes mellitus, a devastating disease suffered by 150 million people in the world. Transplantation of insulin-producing cells could be a cure for type 1 and some cases of type 2 diabetes, however this procedure is limited by the scarcity of material. Obtaining pancreatic beta-cells from embryonic stem cells would overcome this problem. We have derived insulin-producing cells from mouse embryonic stem cells by a 3-step in-vitro differentiation method consisting of directed differentiation, cell-lineage selection, and maturation. These insulin-producing cells normalize blood glucose when transplanted into streptozotocin-diabetic mice. Strategies to increase islet precursor cells from embryonic stem cells include the expression of relevant transcription factors (Pdx1, Ngn3, Isl-1, etc), together with the use of extracellular factors. Once a high enough proportion of islet precursors has been obtained there is a need for cell-lineage selection in order to purify the desired cell population. For this purpose, we designed a cell-trapping method based on a chimeric gene that fuses the human insulin gene regulatory region with the structural gene that confers resistance to neomycin. When incorporated into embryonic stem cells, this fusion gene will generate neomycin resistance in those cells that initiate the synthesis of insulin. Not only embryonic, but also adult stem cells are potential sources for insulin-containing cells. Duct cells from the adult pancreas are committed to differentiate into the four islet cell types; other possibilities may include nestin-positive cells from islets and adult pluripotent stem cells from other origins. Whilst the former are committed to be islet cells but have a reduced capacity to expand, the latter are more pluripotent and more expandable, but a longer pathway separates them from the insulin-producing stage. The aim of this review is to discuss the different strategies that may be followed to in-vitro differentiate pancreatic beta-cells from stem cells.

Insulin production by human embryonic stem cells

Type 1 diabetes generally results from autoimmune destruction of pancreatic islet beta-cells, with consequent absolute insulin deficiency and complete dependence on exogenous insulin treatment. The relative paucity of donations for pancreas or islet allograft transplantation has prompted the search for alternative sources for beta-cell replacement therapy. In the current study, we used pluripotent undifferentiated human embryonic stem (hES) cells as a model system for lineage-specific differentiation. Using hES cells in both adherent and suspension culture conditions, we observed spontaneous in vitro differentiation that included the generation of cells with characteristics of insulin-producing beta-cells. Immunohistochemical staining for insulin was observed in a surprisingly high percentage of cells. Secretion of insulin into the medium was observed in a differentiation-dependent manner and was associated with the appearance of other beta-cell markers. These findings validate the hES cell model system as a potential basis for enrichment of human beta-cells or their precursors, as a possible future source for cell replacement therapy in diabetes.

Pdx1 level defines pancreatic gene expression pattern and cell lineage differentiation

The absence of Pdx1 and the expression of brain-4 distinguish alpha-cells from other pancreatic endocrine cell lineages. To define the transcription factor responsible for pancreatic cell differentiation, we employed the reverse tetracycline-dependent transactivator system in INS-I cell-derived subclones INSralphabeta and INSrbeta to achieve tightly controlled and conditional expression of wild type Pdx1 or its dominant-negative mutant, as well as brain-4. INSralphabeta cells express not only insulin but also glucagon and brain-4, while INSrbeta cells express only insulin. Overexpression of Pdx1 eliminated glucagon mRNA and protein in INSralphabeta cells and promoted the expression of beta-cell-specific genes in INSrbeta cells. Induction of dominant-negative Pdx1 in INSralphabeta cells resulted in differentiation of insulin-producing beta-cells into glucagon-containing alpha-cells without altering brain4 expression. Loss of Pdx1 function alone in INSrbeta cells, which do not express endogenous brain-4 and glucagon, was also sufficient to abolish the expression of genes restricted to beta-cells and to cause alpha-cell differentiation. In contrast, induction of brain-4 in INSrbeta cells initiated detectable expression of glucagon but did not affect beta-cell-specific gene expression. In conclusion, Pdx1 confers the expression of pancreatic beta-cell-specific genes, such as genes encoding insulin, islet amyloid polypeptide, Glut2, and Nkx6.1. Pdx1 defines pancreatic cell lineage differentiation. Loss of Pdx1 function rather than expression of brain4 is a prerequisite for alpha-cell differentiation.

Stem cells and diabetes

Diabetes mellitus is a metabolic disorder affecting 2-5% of the population. Transplantation of isolated islets of Langerhans from donor pancreata could be a cure for diabetes; however, such an approach is limited by the scarcity of the transplantation material and the long-term side effects of immunosuppressive therapy. These problems may be overcome by using a renewable source of cells, such as islet cells derived from stem cells. Stem cells are defined as clonogenic cells capable of both self-renewal and multilineage differentiation. This mean that these cells can be expanded in vivo or in vitro and differentiated to produce the desired cell type. There exist several sources of stem cells that have been demonstrated to give rise to pluripotent cell lines: 1) embryonic stem cells; 2) embryonic germ cells; 3) embryonic carcinoma cells; and 4) adult stem cells. By using in vitro differentiation and selection protocols, embryonic stem cells can be guided into specific cell lineages and selected by applying genetic selection when a marker gene is expressed. Recently, differentiation and cell selection protocols have been used to generate embryonic stem cell-derived insulin-secreting cells that normalise blood glucose when transplanted into diabetic animals. Some recent reports suggest that functional plasticity of adult stem cells may be greater than expected. The use of adult stem cells will circumvent the ethical dilemma surrounding embryonic stem cells and will allow autotransplantation. These investigations have increased the expectations that cell therapy could be one of the solutions to diabetes.

Pancreas development in zebrafish: early dispersed appearance of endocrine hormone expressing cells and their convergence to form the definitive islet

To begin to understand pancreas development and the control of endocrine lineage formation in zebrafish, we have examined the expression pattern of several genes shown to act in vertebrate pancreatic development: pdx-1, insulin (W. M. Milewski et al., 1998, Endocrinology 139, 1440-1449), glucagon, somatostatin (F. Argenton et al., 1999, Mech. Dev. 87, 217-221), islet-1 (Korzh et al., 1993, Development 118, 417-425), nkx2.2 (Barth and Wilson, 1995, Development 121, 1755-1768), and pax6.2 (Nornes et al., 1998, Mech. Dev. 77, 185-196). To determine the spatial relationship between the exocrine and the endocrine compartments, we have cloned the zebrafish trypsin gene, a digestive enzyme expressed in differentiated pancreatic exocrine cells. We found expression of all these genes in the developing pancreas throughout organogenesis. Endocrine cells first appear in a scattered fashion in two bilateral rows close to the midline during mid-somitogenesis and converge during late-somitogenesis to form a single islet dorsal to the nascent duodenum. We have examined development of the endocrine lineage in a number of previously described zebrafish mutations. Deletion of chordamesoderm in floating head (Xnot homolog) mutants reduces islet formation to small remnants, but does not delete the pancreas, indicating that notochord is involved in proper pancreas development, but not required for differentiation of pancreatic cell fates. In the absence of knypek gene function, which is involved in convergence movements, the bilateral endocrine primordia do not merge. Presence of trunk paraxial mesoderm also appears to be instrumental for convergence since the bilateral endocrine primordia do not merge in spadetail mutants. We discuss our findings on zebrafish pancreatogenesis in the light of evolution of the pancreas in chordates.

Morphological changes in pancreatic islets of KATP channel-deficient mice: the involvement of KATP channels in the survival of insulin cells and the maintenance of islet architecture

The ATP-sensitive potassium channel (KATP channel) is an essential ion channel involved in glucose-induced insulin secretion. The KATP channel is composed of an inwardly rectifying potassium channel, Kir6.2, and the sulfonylurea receptor (SUR 1); in the pancreas it is reported to be shared by all endocrine cell types. A previous study by our research group showed that Kir 6.2-knockout mice lacked KATP channel activities and failed to secrete insulin in response to glucose, but displayed normal blood glucose levels and only mild impairment in glucose tolerance at younger ages. In some aged knockout mice, however, obesity and hyperglycemia were recognizable. The present study aimed to reveal morphological changes in pancreatic islets of Kir 6.2-knockout mice throughout life. At birth, there were no significant differences in the islet cell arrangement between the knockout mice and controls. At 14 postnatal weeks glucagon cells appeared in the central parts of islets, and this image became more pronounced with aging. In animals older than 50 weeks insulin cells decreased in numbers and intensity of insulin immunoreactivity; most islets in 70- and 80-week-old mice were predominantly composed of glucagon cells and peptide YY (PYY)-containing cells. Staining of serial sections and double staining of single sections from these old mice demonstrated the frequent coexpression of glucagon and PYY, which is a phenotype for the earliest progenitor cells of pancreatic endocrine cells. These findings suggest that the KATP channel is important for insulin cell survival and also regulates the differentiation of islet cells.

Homeobox gene Nkx6.1 lies downstream of Nkx2.2 in the major pathway of beta-cell formation in the pancreas

Most insulin-producing beta-cells in the fetal mouse pancreas arise during the secondary transition, a wave of differentiation starting at embryonic day 13. Here, we show that disruption of homeobox gene Nkx6.1 in mice leads to loss of beta-cell precursors and blocks beta-cell neogenesis specifically during the secondary transition. In contrast, islet development in Nkx6. 1/Nkx2.2 double mutant embryos is identical to Nkx2.2 single mutant islet development: beta-cell precursors survive but fail to differentiate into beta-cells throughout development. Together, these experiments reveal two independently controlled pathways for beta-cell differentiation, and place Nkx6.1 downstream of Nkx2.2 in the major pathway of beta-cell differentiation.

Glucagon-like peptide-1 induces cell proliferation and pancreatic-duodenum homeobox-1 expression and increases endocrine cell mass in the pancreas of old, glucose-intolerant rats

Glucose homeostasis in mammals is maintained by insulin secretion from the beta-cells of the islets of Langerhans. Type 2 diabetes results either from primary beta-cell failure alone and/or a failure to secrete enough insulin to overcome insulin resistance. Here, we show that continuous infusion of glucagon-like peptide-1 (7-36) (GLP-1; an insulinotropic agent), to young and old animals, had effects on the beta-cell of the pancreas other than simply on the insulin secretory apparatus. Our previous studies on a rodent model of glucose intolerance, the aging Wistar rat, show that a plateau in islet size, insulin content, and beta-cell mass is reached at 13 months, despite a continuing increase in body weight. Continuous sc infusion of GLP-1 (1.5 pM/kg x min), over 5 days, resulted in normal glucose tolerance. Our current results in both young and old rats demonstrate that treatment caused an up-regulation of pancreatic-duodenum homeobox-1 (PDX-1) expression in islets and total pancreas, induced pancreatic cell proliferation, and beta-cell neogenesis. The effects on levels of PDX-1 messenger RNA were abrogated by simultaneous infusion of Exendin (9-39), a specific antagonist of GLP-1. PDX-1 protein levels increased 4-fold in whole pancreata and 6-fold in islets in response to treatment. Beta-cell mass increased to 7.2 +/- 0.58 from 4.88 +/- 0.38 mg, treated vs. control, respectively, P < 0.02. Total pancreatic insulin content also increased from 0.55 +/- 0.02 to 1.32 +/- 0.11 microg/mg total pancreatic protein. Therefore, GLP-1 would seem to be a unique therapy that can stimulate pancreatic cell proliferation and beta-cell differentiation in the pancreas of rodents.

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.