A switch from MafB to MafA expression accompanies differentiation to pancreatic beta-cells

Direct lineage conversions: unnatural but useful?

Classic experiments such as somatic cell nuclear transfer into oocytes and cell fusion demonstrated that differentiated cells are not irreversibly committed to their fate. More recent work has built on these conclusions and discovered defined factors that directly induce one specific cell type from another, which may be as distantly related as cells from different germ layers. This suggests the possibility that any specific cell type may be directly converted into any other if the appropriate reprogramming factors are known. Direct lineage conversion could provide important new sources of human cells for modeling disease processes or for cellular-replacement therapies. For future applications, it will be critical to carefully determine the fidelity of reprogramming and to develop methods for robustly and efficiently generating human cell types of interest.

Insulin biosynthesis in neuronal progenitors derived from adult hippocampus and the olfactory bulb

In the present study, we demonstrated that insulin is produced not only in the mammalian pancreas but also in adult neuronal cells derived from the hippocampus and olfactory bulb (OB). Paracrine Wnt3 plays an essential role in promoting the active expression of insulin in both hippocampal and OB-derived neural stem cells. Our analysis indicated that the balance between Wnt3, which triggers the expression of insulin via NeuroD1, and IGFBP-4, which inhibits the original Wnt3 action, is regulated depending on diabetic (DB) status. We also show that adult neural progenitors derived from DB animals retain the ability to give rise to insulin-producing cells and that grafting neuronal progenitors into the pancreas of DB animals reduces glucose levels. This study provides an example of a simple and direct use of adult stem cells from one organ to another, without introducing additional inductive genes.

MafB interacts with Gcm2 and regulates parathyroid hormone expression and parathyroid development

Serum calcium and phosphate homeostasis is critically regulated by parathyroid hormone (PTH) secreted by the parathyroid glands. Parathyroid glands develop from the bilateral parathyroid-thymus common primordia. In mice, the expression of transcription factor Glial cell missing 2 (Gcm2) begins in the dorsal/anterior part of the primordium on embryonic day 9.5 (E9.5), specifying the parathyroid domain. The parathyroid primordium then separates from the thymus primordium and migrates to its adult location beside the thyroid gland by E15.5. Genetic ablation of gcm2 results in parathyroid agenesis in mice, indicating that Gcm2 is essential for early parathyroid organogenesis. However, the regulation of parathyroid development at later stages is not well understood. Here we show that transcriptional activator v-maf musculoaponeurotic fibrosarcoma oncogene homologue B (MafB) is developmentally expressed in parathyroid cells after E11.5. MafB expression was lost in the parathyroid primordium of gcm2 null mice. The parathyroid glands of mafB(+/-) mice were mislocalized between the thymus and thyroid. In mafB(-/-) mice, the parathyroid did not separate from the thymus. Furthermore, in mafB(-/-) mice, PTH expression and secretion were impaired; expression levels of renal cyp27b1, one of the target genes of PTH, was decreased; and bone mineralization was reduced. We also demonstrate that although Gcm2 alone does not stimulate the PTH gene promoter, it associates with MafB to synergistically activate PTH expression. Taken together, our results suggest that MafB regulates later steps of parathyroid development, that is, separation from the thymus and migration toward the thyroid. MafB also regulates the expression of PTH in cooperation with Gcm2.

Stem cell approaches for diabetes: towards beta cell replacement

Stem cells hold great promise for pancreatic beta cell replacement therapy for diabetes. In type 1 diabetes, beta cells are mostly destroyed, and in type 2 diabetes beta cell numbers are reduced by 40% to 60%. The proof-of-principle that cellular transplants of pancreatic islets, which contain insulin-secreting beta cells, can reverse the hyperglycemia of type 1 diabetes has been established, and there is now a need to find an adequate source of islet cells. Human embryonic stem cells can be directed to become fully developed beta cells and there is expectation that induced pluripotent stem (iPS) cells can be similarly directed. iPS cells can also be generated from patients with diabetes to allow studies of the genomics and pathogenesis of the disease. Some alternative approaches for replacing beta cells include finding ways to enhance the replication of existing beta cells, stimulating neogenesis (the formation of new islets in postnatal life), and reprogramming of pancreatic exocrine cells to insulin-producing cells. Stem-cell-based approaches could also be used for modulation of the immune system in type 1 diabetes, or to address the problems of obesity and insulin resistance in type 2 diabetes. Herein, we review recent advances in our understanding of diabetes and beta cell biology at the genomic level, and we discuss how stem-cell-based approaches might be used for replacing beta cells and for treating diabetes.

MafA and MafB activity in pancreatic β cells

Analyses in mouse models have revealed crucial roles for MafA (musculoaponeurotic fibrosarcoma oncogene family A) and MafB in islet β cells, with MafB being required during development and MafA in adults. These two closely related transcription factors regulate many genes essential for glucose sensing and insulin secretion in a cooperative and sequential manner. Significantly, the switch from MafB to MafA expression also appears to be vital for functional maturation of β cells produced by human embryonic stem (hES) cell differentiation. This review summarizes the discovery, distribution, and function of MafA and MafB in rodent pancreatic β cells, and describes some key questions regarding their importance to β cells.

Activin, BMP and FGF pathways cooperate to promote endoderm and pancreatic lineage cell differentiation from human embryonic stem cells

The study of how human embryonic stem cells (hESCs) differentiate into insulin-producing beta cells has twofold significance: first, it provides an in vitro model system for the study of human pancreatic development, and second, it serves as a platform for the ultimate production of beta cells for transplantation into patients with diabetes. The delineation of growth factor interactions regulating pancreas specification from hESCs in vitro is critical to achieving these goals. In this study, we describe the roles of growth factors bFGF, BMP4 and Activin A in early hESC fate determination. The entire differentiation process is carried out in serum-free chemically-defined media (CDM) and results in reliable and robust induction of pancreatic endoderm cells, marked by PDX1, and cell clusters co-expressing markers characteristic of beta cells, including PDX1 and insulin/C-peptide. Varying the combinations of growth factors, we found that treatment of hESCs with bFGF, Activin A and BMP4 (FAB) together for 3-4days resulted in strong induction of primitive-streak and definitive endoderm-associated genes, including MIXL1, GSC, SOX17 and FOXA2. Early proliferative foregut endoderm and pancreatic lineage cells marked by PDX1, FOXA2 and SOX9 expression are specified in EBs made from FAB-treated hESCs, but not from Activin A alone treated cells. Our results suggest that important tissue interactions occur in EB-based suspension culture that contribute to the complete induction of definitive endoderm and pancreas progenitors. Further differentiation occurs after EBs are embedded in Matrigel and cultured in serum-free media containing insulin, transferrin, selenium, FGF7, nicotinamide, islet neogenesis associated peptide (INGAP) and exendin-4, a long acting GLP-1 agonist. 21-28days after embedding, PDX1 gene expression levels are comparable to those of human islets used for transplantation, and many PDX1(+) clusters are formed. Almost all cells in PDX1(+) clusters co-express FOXA2, HNF1ß, HNF6 and SOX9 proteins, and many cells also express CPA1, NKX6.1 and PTF1a. If cells are then switched to medium containing B27 and nicotinamide for 7-14days, then the number of insulin(+) cells increases markedly. Our study identifies a new chemically defined culture protocol for inducing endoderm- and pancreas-committed cells from hESCs and reveals an interplay between FGF, Activin A and BMP signaling in early hESC fate determination.

TAT-mediated transduction of MafA protein in utero results in enhanced pancreatic insulin expression and changes in islet morphology

Alongside Pdx1 and Beta2/NeuroD, the transcription factor MafA has been shown to be instrumental in the maintenance of the beta cell phenotype. Indeed, a combination of MafA, Pdx1 and Ngn3 (an upstream regulator of Beta2/NeuroD) was recently reported to lead to the effective reprogramming of acinar cells into insulin-producing beta cells. These experiments set the stage for the development of new strategies to address the impairment of glycemic control in diabetic patients. However, the clinical applicability of reprogramming in this context is deemed to be poor due to the need to use viral vehicles for the delivery of the above factors. Here we describe a recombinant transducible version of the MafA protein (TAT-MafA) that penetrates across cell membranes with an efficiency of 100% and binds to the insulin promoter in vitro. When injected in utero into living mouse embryos, TAT-MafA significantly up-regulates target genes and induces enhanced insulin production as well as cytoarchitectural changes consistent with faster islet maturation. As the latest addition to our armamentarium of transducible proteins (which already includes Pdx1 and Ngn3), the purification and characterization of a functional TAT-MafA protein opens the door to prospective therapeutic uses that circumvent the use of viral delivery. To our knowledge, this is also the first report on the use of protein transduction in utero.

Pancreas organogenesis: from bud to plexus to gland

Pancreas oganogenesis comprises a coordinated and highly complex interplay of signaling events and transcriptional networks that guide a step-wise process of organ development from early bud specification all the way to the final mature organ state. Extensive research on pancreas development over the last few years, largely driven by a translational potential for pancreatic diseases (diabetes, pancreatic cancer, and so on), is markedly advancing our knowledge of these processes. It is a tenable goal that we will one day have a clear, complete picture of the transcriptional and signaling codes that control the entire organogenetic process, allowing us to apply this knowledge in a therapeutic context, by generating replacement cells in vitro, or perhaps one day to the whole organ in vivo. This review summarizes findings in the past 5 years that we feel are amongst the most significant in contributing to the deeper understanding of pancreas development. Rather than try to cover all aspects comprehensively, we have chosen to highlight interesting new concepts, and to discuss provocatively some of the more controversial findings or proposals. At the end of the review, we include a perspective section on how the whole pancreas differentiation process might be able to be unwound in a regulated fashion, or redirected, and suggest linkages to the possible reprogramming of other pancreatic cell-types in vivo, and to the optimization of the forward-directed-differentiation of human embryonic stem cells (hESC), or induced pluripotential cells (iPSC), towards mature β-cells.

MafA promotes the reprogramming of placenta-derived multipotent stem cells into pancreatic islets-like and insulin+ cells

MafA is a pancreatic transcriptional factor that controls β-cell-specific transcription of the insulin gene. However, the role of MafA in the regulation of pancreatic transdifferentiation and reprogramming in human stem cells is still unclear. In this study, we investigate the role of MafA in placenta-derived multipotent stem cells (PDMSCs) that constitutively expressed Oct-4 and Nanog. PDMSCs were isolated and transfected with MafA using a lentivector. Our results showed that overexpression of MafA in PDMSCs significantly up-regulated the expression of pancreatic development-related genes (Sox17, Foxa2, Pdx1 and Ngn3). Microarray analysis suggested that the gene expression profile of MafA-overexpressing PDMSCs was similar to that of pancreas and islet tissues. MafA increased the expression levels of the mRNAs of NKx2.2, Glut2, insulin, glucagons and somatostatin, and further facilitated the differentiation of PDMSCs into insulin⁺ cells. The glucose-stimulated responses to insulin and c-peptide production in MafA-overexpressing PDMSCs were significantly higher than in PDMSCs with vector control. Our results indicated that MafA-overexpressing PDMSCs were more resistant to oxidative damage and oxidative damage-induced apoptosis than PDMSCs carrying the vector control were. Importantly, the expression of MafA in PDMSCs xenotransplanted into immunocompromised mice improved the restoration of blood insulin levels to control values and greatly prolonged the survival of graft cells in immunocompromised mice with STZ-induced diabetes. In summary, these data suggest that MafA plays a novel role in the reprogramming of stem cells into pancreatic β-progenitors, promotes the islet-like characteristics of PDMSCs, as well as functionally enhances insulin production to restore the regulation of blood glucose levels in transplanted grafts.

Two distinct origins for Leydig cell progenitors in the fetal testis

During the differentiation of the mammalian embryonic testis, two compartments are defined: the testis cords and the interstitium. The testis cords give rise to the adult seminiferous tubules, whereas steroidogenic Leydig cells and other less well characterized cell types differentiate in the interstitium (the space between testis cords). Although the process of testis cord formation is essential for male development, it is not entirely understood. It has been viewed as a Sertoli-cell driven process, but growing evidence suggests that interstitial cells play an essential role during testis formation. However, little is known about the origin of the interstitium or the molecular and cellular diversity within this early stromal compartment. To better understand the process of mammalian gonad differentiation, we have undertaken an analysis of developing interstitial/stromal cells in the early mouse testis and ovary. We have discovered molecular heterogeneity in the interstitium and have characterized new markers of distinct cell types in the gonad: MAFB, C-MAF, and VCAM1. Our results show that at least two distinct progenitor lineages give rise to the interstitial/stromal compartment of the gonad: the coelomic epithelium and specialized cells along the gonad-mesonephros border. We demonstrate that both these populations give rise to interstitial precursors that can differentiate into fetal Leydig cells. Our analysis also reveals that perivascular cells migrate into the gonad from the mesonephric border along with endothelial cells and that these vessel-associated cells likely represent an interstitial precursor lineage. This study highlights the cellular diversity of the interstitial cell population and suggests that complex cell-cell interactions among cells in the interstitium are involved in testis morphogenesis.

Mafa expression enhances glucose-responsive insulin secretion in neonatal rat beta cells

AIM/HYPOTHESIS

Neonatal beta cells lack glucose-stimulated insulin secretion and are thus functionally immature. We hypothesised that this lack of glucose responsiveness results from a generalised low expression of genes characteristic of mature functional beta cells. Important glucose-responsive transcription factors, Mafa and Pdx1, regulate genes involved in insulin synthesis and secretion, and have been implicated in late beta cell development. The aim of this study was to assess whether Mafa and/or Pdx1 regulates the postnatal functional maturation of beta cells.

METHODS

By quantitative PCR we evaluated expression of these and other beta cell genes over the first month compared with adult. After infection with adenovirus expressing MAFA, Pdx1 or green fluorescent protein (Gfp), P2 rat islets were evaluated by RT-PCR and insulin secretion with static incubation and reverse haemolytic plaque assay (RHPA).

RESULTS

At P2 most beta cell genes were expressed at about 10% of adult, but by P7 Pdx1 and Neurod1 no longer differ from adult; by contrast, Mafa expression remained significantly lower than adult through P21. Overexpression of Pdx1 increased Mafa, Neurod1, glucokinase (Gck) mRNA and insulin content but failed to enhance glucose responsiveness. Similar overexpression of MAFA resulted in increased Neurod1, Nkx6-1, Gck and Glp1r mRNAs and no change in insulin content but, importantly, acquisition of glucose-responsive insulin secretion. Both the percentage of secreting beta cells and the amount of insulin secreted per beta cell increased, approaching that of adult beta cells.

CONCLUSIONS/INTERPRETATION

In the process of functional maturation acquiring glucose-responsive insulin secretion, neonatal beta cells undergo a coordinated gene expression programme in which Mafa plays a crucial role.

Proteasome activator PA28{gamma} stimulates degradation of GSK3-phosphorylated insulin transcription activator MAFA

MAFA is a member of the MAF family of basic leucine zipper transcription factors and is a critical regulator of insulin gene expression and islet β-cell function. To be degraded by the proteasome, MAFA must be phosphorylated by GSK3 and MAP kinases at multiple serine and threonine residues (Ser49, Thr53, Thr57, Ser61, and Ser65) within its amino-terminal domain. In this study, we report that MAFA degradation is stimulated by PA28γ (REGγ and PSME3), a member of a family of proteasome activators that bind and activate the 20S proteasome. To date, only a few PA28γ-proteasome pathway substrates have been identified, including steroid receptor coactivator 3 (SRC3) and the cell cycle inhibitor p21 (CIP1). PA28γ binds to MAFA, induces its proteasomal degradation, and thereby attenuates MAFA-driven transcriptional activation of the insulin promoter. Co-expression of GSK3 enhanced the PA28γ-mediated degradation of MAFA, but mutants that contained alanine substitutions at the MAFA phosphorylation sites did not bind PA28γ and were resistant to degradation. We also found that a PA28γ mutant (N151Y) that did not stimulate p21 degradation enhanced MAFA degradation, and another mutant (K188D) that promoted greater p21 degradation did not enhance MAFA degradation. These results suggest that PA28γ stimulates MAFA degradation through a novel molecular mechanism that is distinct from that for the degradation of p21.

The pancreatic islet β-cell-enriched transcription factor Pdx-1 regulates Slc30a8 gene transcription through an intronic enhancer

The SLC30A8 gene encodes the zinc transporter ZnT-8, which provides zinc for insulin-hexamer formation. Genome-wide association studies have shown that a polymorphic variant in SLC30A8 is associated with altered susceptibility to Type 2 diabetes and we recently reported that glucose-stimulated insulin secretion is decreased in islets isolated from Slc30a8-knockout mice. The present study examines the molecular basis for the islet-specific expression of Slc30a8. VISTA analyses identified two conserved regions in Slc30a8 introns 2 and 3, designated enhancers A and B respectively. Transfection experiments demonstrated that enhancer B confers elevated fusion gene expression in both βTC-3 cells and αTC-6 cells. In contrast, enhancer A confers elevated fusion gene expression selectively in βTC-3 and not αTC-6 cells. These data suggest that enhancer A is an islet β-cell-specific enhancer and that the mechanisms controlling Slc30a8 expression in α- and β-cells are overlapping, but distinct. Gel retardation and ChIP (chromatin immunoprecipitation) assays revealed that the islet-enriched transcription factor Pdx-1 binds enhancer A in vitro and in situ respectively. Mutation of two Pdx-1-binding sites in enhancer A markedly reduces fusion gene expression suggesting that this factor contributes to Slc30a8 expression in β-cells, a conclusion consistent with developmental studies showing that restriction of Pdx-1 to pancreatic islet β-cells correlates with the induction of Slc30a8 gene expression and ZnT-8 protein expression in vivo.

Hnf1α (MODY3) regulates β-cell-enriched MafA transcription factor expression

The expression pattern of genes important for pancreatic islet cell function requires the actions of cell-enriched transcription factors. Musculoaponeurotic fibrosarcoma homolog A (MafA) is a β-cell-specific transcriptional activator critical to adult islet β-cell function, with MafA mutant mice manifesting symptoms associated with human type 2 diabetes. Here, we describe that MafA expression is controlled by hepatocyte nuclear factor 1-α (Hnf1α), the transcription factor gene mutated in the most common monoallelic form of maturity onset diabetes of the young. There are six conserved sequence domains in the 5'-flanking MafA promoter, of which one, region 3 (R3) [base pair (bp) -8118/-7750] is principally involved in controlling the unique developmental and adult islet β-cell-specific expression pattern. Chromatin immunoprecipitation analysis demonstrated that Hnf1α bound specifically within R3. Furthermore, in vitro DNA-binding experiments localized an Hnf1α regulatory element between bp -7822 and -7793, an area previously associated with stimulation by the islet developmental regulator, Islet1. However, site-directed mutational studies showed that Hnf1α was essential to R3-driven reporter activation through bp -7816/-7811. Significantly, MafA levels were dramatically reduced in the insulin(+) cell population remaining in embryonic and adult Hnf1α(-/-) pancreata. Our results demonstrate that Hnf1α regulates MafA in β-cells and suggests that compromised MafA expression contributes to β-cell dysfunction in maturity onset diabetes of the young.

Pancreatic β-cell neogenesis by direct conversion from mature α-cells

Because type 1 and type 2 diabetes are characterized by loss of β-cells, β-cell regeneration has garnered great interest as an approach to diabetes therapy. Here, we developed a new model of β-cell regeneration, combining pancreatic duct ligation (PDL) with elimination of pre-existing β-cells with alloxan. In this model, in which virtually all β-cells observed are neogenic, large numbers of β-cells were generated within 2 weeks. Strikingly, the neogenic β-cells arose primarily from α-cells. α-cell proliferation was prominent following PDL plus alloxan, providing a large pool of precursors, but we found that β-cells could form from α-cells by direct conversion with or without intervening cell division. Thus, classical asymmetric division was not a required feature of the process of α- to β-cell conversion. Intermediate cells coexpressing α-cell- and β-cell-specific markers appeared within the first week following PDL plus alloxan, declining gradually in number by 2 weeks as β-cells with a mature phenotype, as defined by lack of glucagon and expression of MafA, became predominant. In summary, these data revealed a novel function of α-cells as β-cell progenitors. The high efficiency and rapidity of this process make it attractive for performing the studies required to gain the mechanistic understanding of the process of α- to β-cell conversion that will be required for eventual clinical translation as a therapy for diabetes.

MafA and MafB regulate genes critical to beta-cells in a unique temporal manner

OBJECTIVE

Several transcription factors are essential to pancreatic islet β-cell development, proliferation, and activity, including MafA and MafB. However, MafA and MafB are distinct from others in regard to temporal and islet cell expression pattern, with β-cells affected by MafB only during development and exclusively by MafA in the adult. Our aim was to define the functional relationship between these closely related activators to the β-cell.

RESEARCH DESIGN AND METHODS

The distribution of MafA and MafB in the β-cell population was determined immunohistochemically at various developmental and perinatal stages in mice. To identify genes regulated by MafB, microarray profiling was performed on wild-type and MafB(-/-) pancreata at embryonic day 18.5, with candidates evaluated by quantitative RT-PCR and in situ hybridization. The potential role of MafA in the expression of verified targets was next analyzed in adult islets of a pancreas-wide MafA mutant (termed MafA(ΔPanc)).

RESULTS

MafB was produced in a larger fraction of β-cells than MafA during development and found to regulate potential effectors of glucose sensing, hormone processing, vesicle formation, and insulin secretion. Notably, expression from many of these genes was compromised in MafA(ΔPanc) islets, suggesting that MafA is required to sustain expression in adults.

CONCLUSIONS

Our results provide insight into the sequential manner by which MafA and MafB regulate islet β-cell formation and maturation.

Impact of co-culture on pancreatic differentiation of embryonic stem cells

Promise of cellular therapy for type 1 diabetes has inspired the search for transplantable cell sources, and embryonic stem cells (ESCs) have emerged as strong candidates. We have developed a directed differentiation protocol to obtain insulin-producing cells from ESCs. The ESCs are first induced towards a homogeneous monolayer of definitive endoderm-like cells by co-culture with primary hepatocytes. Pancreatic commitment is induced by plating the ESC-derived endoderms on Matrigel, along with Sonic hedgehog inhibition and retinoid induction. More than 70% of differentiated cells positively upregulated Pdx-1, along with pro-endocrine transcription factors Ngn3, β2/neroD1, Nkx2.2 and Nkx6.1. Final maturation to islet-specific cells is achieved by co-culturing the ESC-derived pancreatic endocrine cells with endothelial cells, which resulted in Insulin 1 upregulation in 60% of the cell population, along with high levels of IAPP and Glut2. The differentiated cell population also secreted high levels of insulin. Our findings illustrate the significant effect of co-culture in different stages of differentiation and maturation of ESCs in vitro. Such a high yield of pancreatic islet cells has not yet been reported. Our findings establish a robust protocol for islet differentiation.

The new generation of beta-cells: replication, stem cell differentiation, and the role of small molecules

Diabetic patients suffer from the loss of insulin-secreting β-cells, or from an improper working β-cell mass. Due to the increasing prevalence of diabetes across the world, there is a compelling need for a renewable source of cells that could replace pancreatic β-cells. In recent years, several promising approaches to the generation of new β-cells have been developed. These include directed differentiation of pluripotent cells such as embryonic stem (ES) cells or induced pluripotent stem (iPS) cells, or reprogramming of mature tissue cells. High yield methods to differentiate cell populations into β-cells, definitive endoderm, and pancreatic progenitors, have been established using growth factors and small molecules. However, the final step of directed differentiation to generate functional, mature β-cells in sufficient quantities has yet to be achieved in vitro. Beside the needs of transplantation medicine, a renewable source of β-cells would also be important in terms of a platform to study the pathogenesis of diabetes, and to seek alternative treatments. Finally, by generating new β-cells, we could learn more details about pancreatic development and β-cell specification. This review gives an overview of pancreas ontogenesis in the perspective of stem cell differentiation, and highlights the critical aspects of small molecules in the generation of a renewable β-cell source. Also, it discusses longer term challenges and opportunities in moving towards a therapeutic goal for diabetes.

Generation of insulin-producing cells from pluripotent stem cells: from the selection of cell sources to the optimization of protocols

The pancreas arises from Pdx1-expressing progenitors in developing foregut endoderm in early embryo. Expression of Ngn3 and NeuroD1 commits the cells to form endocrine pancreas, and to differentiate into subsets of cells that constitute islets of Langerhans. β-cells in the islets transcribe gene-encoding insulin, and subsequently process and secrete insulin, in response to circulating glucose. Dysfunction of β-cells has profound metabolic consequences leading to hyperglycemia and diabetes mellitus. β-cells are destroyed via autoimmune reaction in type 1 diabetes (T1D). Type 2 diabetes (T2D), characterized by impaired β-cell functions and reduced insulin sensitivity, accounts for 90% of all diabetic patients. Islet transplantation is a promising treatment for T1D. Pluripotent stem cells provide an unlimited cell source to generate new β-cells for patients with T1D. Furthermore, derivation of induced pluripotent stem cells (iPSCs) from patients captures "disease-in-a-dish" for autologous cell replacement therapy, disease modeling, and drug screening for both types of diabetes. This review highlights essential steps in pancreas development, and potential stem cell applications in cell regeneration therapy for diabetes mellitus.

The transcription factor MafB antagonizes antiviral responses by blocking recruitment of coactivators to the transcription factor IRF3

Viral infection induces type I interferons (IFN-alpha and IFN-beta) that recruit unexposed cells in a self-amplifying response. We report that the transcription factor MafB thwarts auto-amplification by a metastable switch activity. MafB acted as a weak positive basal regulator of transcription at the IFNB1 promoter through activity at transcription factor AP-1-like sites. Interferon elicitors recruited the transcription factor IRF3 to the promoter, whereupon MafB acted as a transcriptional antagonist, impairing the interaction of coactivators with IRF3. Mathematical modeling supported the view that prepositioning of MafB on the promoter allows the system to respond rapidly to fluctuations in IRF3 activity. Higher expression of MafB in human pancreatic islet beta cells might increase cellular vulnerability to viral infections associated with the etiology of type 1 diabetes.

Reciprocal modulation of adult beta cell maturity by activin A and follistatin

AIMS/HYPOTHESIS

The functional maturity of pancreatic beta cells is impaired in diabetes mellitus. We sought to define factors that can influence adult beta cell maturation status and function.

METHODS

MIN6 cells labelled with a Pdx1 monomeric red fluorescent protein-Ins1 enhanced green fluorescent protein dual reporter lentivirus were used to screen candidate growth and/or differentiation factors using image-based approaches with confirmation by real-time RT-PCR and assays of beta cell function using primary mouse islets.

RESULTS

Activin A strikingly decreased the number of mature beta cells and increased the number of immature beta cells. While activins are critical for pancreatic morphogenesis, their role in adult beta cells remains controversial. In primary islets and MIN6 cells, activin A significantly decreased the expression of insulin and several genes associated with beta cell maturity (e.g. Pdx1, Mafa, Glut2 [also known as Slc2a2]). Genes found in immature beta cells (e.g. Mafb) tended to be upregulated by activin A. Insulin secretion was also reduced by activin A. In addition, activin A-treated MIN6 cells proliferated faster than non-treated cells. The effects of endogenous activin A on beta cells were completely reversed by exogenous follistatin.

CONCLUSIONS/INTERPRETATION

These results suggest that autocrine and/or paracrine activin A signalling exerts a suppressive effect on adult beta cell maturation and function. Thus, the maturation state of adult beta cells can be modulated by external factors in culture. Interventions inhibiting activin or its signalling pathways may improve beta cell function. Understanding of maturation and plasticity of adult pancreatic tissue has significant implications for islet regeneration and for in vitro generation of functional beta cells.

Activation of pancreatic-duct-derived progenitor cells during pancreas regeneration in adult rats

The adult pancreas has considerable capacity to regenerate in response to injury. We hypothesized that after partial pancreatectomy (Px) in adult rats, pancreatic-duct cells serve as a source of regeneration by undergoing a reproducible dedifferentiation and redifferentiation. We support this hypothesis by the detection of an early loss of the ductal differentiation marker Hnf6 in the mature ducts, followed by the transient appearance of areas composed of proliferating ductules, called foci of regeneration, which subsequently form new pancreatic lobes. In young foci, ductules express markers of the embryonic pancreatic epithelium - Pdx1, Tcf2 and Sox9 - suggesting that these cells act as progenitors of the regenerating pancreas. The endocrine-lineage-specific transcription factor Neurogenin3, which is found in the developing embryonic pancreas, was transiently detected in the foci. Islets in foci initially resemble embryonic islets in their lack of MafA expression and lower percentage of beta-cells, but with increasing maturation have increasing numbers of MafA(+) insulin(+) cells. Taken together, we provide a mechanism by which adult pancreatic duct cells recapitulate aspects of embryonic pancreas differentiation in response to injury, and contribute to regeneration of the pancreas. This mechanism of regeneration relies mainly on the plasticity of the differentiated cells within the pancreas.

New insights into endocrine pancreatic development: the role of environmental factors

The pancreas is a mixed gland that contains endocrine and exocrine components. Within the pancreatic islets, beta cells produce insulin and control the glycemia. Their deficiency leads to diabetes and several potential complications. In the last decade, numerous studies have focused on pancreas development. The objective was to characterize the cellular and molecular factors that control the differentiation of endocrine and exocrine cell types. Investigation of the role of transcription factors by using genetic approaches led to the discovery of key molecules that are expressed both in rodents and humans. Some of them are ubiquitous, and some others are specifically involved in endocrine or exocrine specification. In addition to these intrinsic factors, recent studies have focused on the role of environmental factors. In the present review, we describe the roles of nutrients and oxygen in the embryonic pancreas. Interestingly, these extrinsic parameters can interfere with beta-cell differentiation and function. Altogether, these data should help to generate beta cells in vitro and define strategies for a cell-based therapy of type 1 diabetes.

Islet beta-cell-specific MafA transcription requires the 5'-flanking conserved region 3 control domain

MafA is a key transcriptional activator of islet beta cells, and its exclusive expression within beta cells of the developing and adult pancreas is distinct among pancreatic regulators. Region 3 (base pairs -8118 to -7750 relative to the transcription start site), one of six conserved 5' cis domains of the MafA promoter, is capable of directing beta-cell-line-selective expression. Transgenic reporters of region 3 alone (R3), sequences spanning regions 1 to 6 (R1-6; base pairs -10428 to +230), and R1-6 lacking R3 (R1-6(DeltaR3)) were generated. Only the R1-6 transgene was active in MafA(+) insulin(+) cells during development and in adult cells. R1-6 also mediated glucose-induced MafA expression. Conversely, pancreatic expression was not observed with the R3 or R1-6(DeltaR3) line, although much of the nonpancreatic expression pattern was shared between the R1-6 and R1-6(DeltaR3) lines. Further support for the importance of R3 was also shown, as the islet regulators Nkx6.1 and Pax6, but not NeuroD1, activated MafA in gel shift, chromatin immunoprecipitation (ChIP), and transfection assays and in vivo mouse knockout models. Lastly, ChIP demonstrated that Pax6 and Pdx-1 also bound to R1 and R6, potentially functioning in pancreatic and nonpancreatic expression. These data highlight the nature of the cis- and trans-acting factors controlling the beta-cell-specific expression of MafA.

Transcriptional regulation of glucose sensors in pancreatic β-cells and liver: an update

Pancreatic β-cells and the liver play a key role in glucose homeostasis. After a meal or in a state of hyperglycemia, glucose is transported into the β-cells or hepatocytes where it is metabolized. In the β-cells, glucose is metabolized to increase the ATP:ADP ratio, resulting in the secretion of insulin stored in the vesicle. In the hepatocytes, glucose is metabolized to CO(2), fatty acids or stored as glycogen. In these cells, solute carrier family 2 (SLC2A2) and glucokinase play a key role in sensing and uptaking glucose. Dysfunction of these proteins results in the hyperglycemia which is one of the characteristics of type 2 diabetes mellitus (T2DM). Thus, studies on the molecular mechanisms of their transcriptional regulations are important in understanding pathogenesis and combating T2DM. In this paper, we will review a recent update on the progress of gene regulation of glucose sensors in the liver and β-cells.

Cellular plasticity within the pancreas--lessons learned from development

The pancreas has been the subject of intense research due to the debilitating diseases that result from its dysfunction. In this review, we summarize current understanding of the critical tissue interactions and intracellular regulatory events that take place during formation of the pancreas from a small cluster of cells in the foregut domain of the mouse embryo. Importantly, an understanding of principles that govern the development of this organ has equipped us with the means to manipulate both embryonic and differentiated adult cells in the context of regenerative medicine. The emerging area of lineage modulation within the adult pancreas is of particular interest, and this review summarizes recent findings that exemplify how lessons learned from development are being applied to reveal the potential of fully differentiated cells to change fate.

Phosphorylation within the MafA N terminus regulates C-terminal dimerization and DNA binding

Phosphorylation regulates transcription factor activity by influencing dimerization, cellular localization, activation potential, and/or DNA binding. Nevertheless, precisely how this post-translation modification mediates these processes is poorly understood. Here, we examined the role of phosphorylation on the DNA-binding properties of MafA and MafB, closely related transcriptional activators of the basic-leucine zipper (b-Zip) family associated with cell differentiation and oncogenesis. Many common phosphorylation sites were identified by mass spectrometry. However, dephosphorylation only precluded the detection of MafA dimers and consequently dramatically reduced DNA-binding ability. Analysis of MafA/B chimeras revealed that sensitivity to the phosphorylation status of MafA was imparted by sequences spanning the C-terminal dimerization region (amino acids (aa) 279-359), whereas the homologous MafB region (aa 257-323) conveyed phosphorylation-independent DNA binding. Mutational analysis showed that formation of MafA dimers capable of DNA binding required phosphorylation within the distinct N-terminal transactivation domain (aa 1-72) and not the C-terminal b-Zip region. These results demonstrate a novel relationship between the phosphoamino acid-rich transactivation and b-Zip domains in controlling MafA DNA-binding activity.

Stem cell therapy for type 1 diabetes mellitus

The use of stem cells in regenerative medicine holds great promise for the cure of many diseases, including type 1 diabetes mellitus (T1DM). Any potential stem-cell-based cure for T1DM should address the need for beta-cell replacement, as well as control of the autoimmune response to cells which express insulin. The ex vivo generation of beta cells suitable for transplantation to reconstitute a functional beta-cell mass has used pluripotent cells from diverse sources, as well as organ-specific facultative progenitor cells from the liver and the pancreas. The most effective protocols to date have produced cells that express insulin and have molecular characteristics that closely resemble bona fide insulin-secreting cells; however, these cells are often unresponsive to glucose, a characteristic that should be addressed in future protocols. The use of mesenchymal stromal cells or umbilical cord blood to modulate the immune response is already in clinical trials; however, definitive results are still pending. This Review focuses on current strategies to obtain cells which express insulin from different progenitor sources and highlights the main pathways and genes involved, as well as the different approaches for the modulation of the immune response in patients with T1DM.

Precursor cells in mouse islets generate new beta-cells in vivo during aging and after islet injury

Whereas it is believed that the pancreatic duct contains endocrine precursors, the presence of insulin progenitor cells residing in islets remain controversial. We tested whether pancreatic islets of adult mice contain precursor beta-cells that initiate insulin synthesis during aging and after islet injury. We used bigenic mice in which the activation of an inducible form of Cre recombinase by a one-time pulse of tamoxifen results in the permanent expression of a floxed human placental alkaline phosphatase (PLAP) gene in 30% of pancreatic beta-cells. If islets contain PLAP(-) precursor cells that differentiate into beta-cells (PLAP(-)IN(+)), a decrease in the percentage of PLAP(+)IN(+) cells per total number of IN(+) cells would occur. Conversely, if islets contain PLAP(+)IN(-) precursors that initiate synthesis of insulin, the percentage of PLAP(+)IN(+) cells would increase. Confocal microscope analysis revealed that the percentage of PLAP(+)IN(+) cells in islets increased from 30 to 45% at 6 months and to 60% at 12 months. The augmentation in the level of PLAP in islets with time was confirmed by real-time PCR. Our studies also demonstrate that the percentage of PLAP(+)IN(+) cells in islets increased after islet injury and identified putative precursors in islets. We postulate that PLAP(+)IN(-) precursors differentiate into insulin-positive cells that participate in a slow renewal of the beta-cell mass during aging and replenish beta-cells eliminated by injury.

Regenerating pancreatic beta-cells: plasticity of adult pancreatic cells and the feasibility of in-vivo neogenesis

PURPOSE OF REVIEW

Diabetes results from inadequate functional mass of pancreatic beta-cells and therefore replenishing with new glucose-responsive beta-cells is an important therapeutic option. In addition to replication of pre-existing beta-cells, new beta-cells can be produced from differentiated adult cells using in-vitro or in-vivo approaches. This review will summarize recent advances in in-vivo generation of beta-cells from cells that are not beta-cells (neogenesis) and discuss ways to overcome the limitations of this process.

RECENT FINDINGS

Multiple groups have shown that adult pancreatic ducts, acinar and even endocrine cells exhibit cellular plasticity and can differentiate into beta-cells in vivo. Several different approaches, including misexpression of transcription factors and tissue injury, have induced neogenesis of insulin-expressing cells in vivo and ameliorated diabetes.

SUMMARY

Recent breakthroughs demonstrating cellular plasticity of adult pancreatic cells to form new beta-cells are a positive first step towards developing in-vivo regeneration-based therapy for diabetes. Currently, neogenesis processes are inefficient and do not generate sufficient amounts of beta-cells required to normalize hyperglycemia. However, an improved understanding of mechanisms regulating neogenesis of beta-cells from adult pancreatic cells and of their maturation into functional glucose-responsive beta-cells can make therapies based on in-vivo regeneration a reality.

c-Maf and MafB transcription factors are differentially expressed in Huxley's and Henle's layers of the inner root sheath of the hair follicle and regulate cuticle formation

BACKGROUND

The hair follicle of mammalian skin consists of a group of concentric epithelial cell layers. The inner root sheath (IRS), which surrounds the hardening hair shaft beneath the skin surface, is subdivided into three layers, termed the cuticle of the IRS, Huxley's layer, and Henle's layer. The IRS forms a follicular wall in the hair canal and helps guide the developing hair shaft. c-Maf and MafB, members of the Maf family of transcription factors, play important roles in the developmental processes of various tissues and in cell type-specific gene expression.

OBJECTIVE

The aim of this study is to reveal the pattern of expression and functional roles of c-Maf and MafB in the hair follicle.

METHODS

We determined the precise location of c-Maf and MafB expression using immunofluorescent staining of mouse skin sections with layer-specific markers. We also analyzed whiskers of c-maf- and mafB-null mice (c-maf(-/-) and mafB(-/-), respectively) using scanning electron microscopy.

RESULTS

c-Maf and MafB were differentially expressed in the Huxley's and Henle's layers of the IRS. Scanning electron microscopic analysis showed irregular cuticle patterning of whiskers of c-maf(-/-) and mafB(-/-) mice. The cuticles of mafB(-/-) mice were also thinner than those of wild-type mice.

CONCLUSION

c-Maf and MafB are expressed in the IRS layers in a lineage-restricted manner and are involved in hair morphogenesis.

Identification of markers for newly formed beta-cells in the perinatal period: a time of recognized beta-cell immaturity

Markers of beta-cell maturity would be useful in staging the differentiation of stem/progenitor cells to beta-cells whether in vivo or in vitro. We previously identified markers for newly formed beta-cells in regenerating rat pancreases after 90% partial pancreatectomy. To test the generality of these markers of newly formed beta-cells, we examined their expression during the perinatal period, a time of recognized beta-cell immaturity. We show by semiquantitative RT-PCR and immunostaining over the time course from embryonic day 18/20 to birth, 1 day, 2 days, 3 days, 7 days, and adult that MMP-2, CK-19, and SPD are truly markers of new and immature beta-cells and that their expression transiently peaks in the perinatal period and is not entirely synchronous. The shared expression of these markers among fetal, newborn, and newly regenerated beta-cells, but not adult, strongly supports their use as potential markers for new beta-cells in the assessment of both the maturity of stem cell-derived insulin-producing cells and the presence of newly formed islets (neogenesis) in the adult pancreas.

Identification of known and novel pancreas genes expressed downstream of Nkx2.2 during development

BACKGROUND

The homeodomain containing transcription factor Nkx2.2 is essential for the differentiation of pancreatic endocrine cells. Deletion of Nkx2.2 in mice leads to misspecification of islet cell types; insulin-expressing beta cells and glucagon-expressing alpha cells are replaced by ghrelin-expressing cells. Additional studies have suggested that Nkx2.2 functions both as a transcriptional repressor and activator to regulate islet cell formation and function. To identify genes that are potentially regulated by Nkx2.2 during the major wave of endocrine and exocrine cell differentiation, we assessed gene expression changes that occur in the absence of Nkx2.2 at the onset of the secondary transition in the developing pancreas.

RESULTS

Microarray analysis identified 80 genes that were differentially expressed in e12.5 and/or e13.5 Nkx2.2-/- embryos. Some of these genes encode transcription factors that have been previously identified in the pancreas, clarifying the position of Nkx2.2 within the islet transcriptional regulatory pathway. We also identified signaling factors and transmembrane proteins that function downstream of Nkx2.2, including several that have not previously been described in the pancreas. Interestingly, a number of known exocrine genes are also misexpressed in the Nkx2.2-/- pancreas.

CONCLUSIONS

Expression profiling of Nkx2.2-/- mice during embryogenesis has allowed us to identify known and novel pancreatic genes that function downstream of Nkx2.2 to regulate pancreas development. Several of the newly identified signaling factors and transmembrane proteins may function to influence islet cell fate decisions. These studies have also revealed a novel function for Nkx2.2 in maintaining appropriate exocrine gene expression. Most importantly, Nkx2.2 appears to function within a complex regulatory loop with Ngn3 at a key endocrine differentiation step.

Transcriptional analysis of intracytoplasmically stained, FACS-purified cells by high-throughput, quantitative nuclease protection

Exploring the pathophysiology underlying diabetes mellitus requires characterizing the cellular constituents of pancreatic islets, primarily insulin-producing β-cells. Such efforts have been limited by inadequate techniques for purifying islet cellular subsets for further biochemical and gene-expression studies. Using intracytoplasmic staining and fluorescence-activated cell-sorting (FACS) followed by quantitative nuclease protection assay (qNPA^™) technology, we examined 30 relevant genes expressed by islet subpopulations. Purified islet cell subsets expressed all four tested “housekeeping” genes with a surprising variability, dependent on both cell lineage and developmental stage, suggesting caution when interpreting housekeeping gene-normalized mRNA quantifications. Our new approach confirmed expected islet cell lineage-specific gene expression patterns at the transcriptional level, but also detected new phenotypes, including mRNA-profiles (supported by immunohistology) demonstrating that during pregnancy, some β-cells express Mafb, previously found only in immature β-cells during embryonic development. Overall, qNPA^™ gene expression analysis using intracellular-stained then FACS-sorted cells has broad applications beyond islet cell biology.

Expression of MafA in pancreatic progenitors is detrimental for pancreatic development

The transcription factor MafA regulates glucose-responsive expression of insulin. MafA-deficient mice have a normal proportion of insulin+ cells at birth but develop diabetes gradually with age, suggesting that MafA is required for maturation and not specification of pancreatic beta-cells. However, several studies show that ectopic expression of MafA may have a role in specification as it induces insulin+ cells in chicken gut epithelium, reprograms adult murine acinar cells into insulin+ cells in combination with Ngn3 and Pdx1, and triggers the lens differentiation. Hence, we examined whether MafA can induce specification of beta-cells during pancreatic development. When the MafA transgene is expressed in Pdx1+ pancreatic progenitors, both pancreatic mass and proliferation of progenitors are reduced, at least partially due to induction of cyclin kinase inhibitors p27 and p57. Expression of MafA in Pdx1+ cells until E12.5 was sufficient to cause these effects and to disproportionately inhibit the formation of endocrine cells in the remnant pancreas. Thus, in mice, MafA expression in Pdx1+ pancreatic progenitors is not sufficient to specify insulin+ cells but in fact deters pancreatic development and the differentiation of endocrine cells. These findings imply that MafA should be used to enhance maturation, rather than specification, of beta-cells from stem/progenitor cells.

Islet-1 is required for the maturation, proliferation, and survival of the endocrine pancreas

OBJECTIVE

The generation of mature cell types during pancreatic development depends on the expression of many regulatory and signaling proteins. In this study, we tested the hypothesis that the transcriptional regulator Islet-1 (Isl-1), whose expression is first detected in the mesenchyme and epithelium of the developing pancreas and is later restricted to mature islet cells, is involved in the terminal differentiation of islet cells and maintenance of islet mass.

RESEARCH DESIGN AND METHODS

To investigate the role of Isl-1 in the pancreatic epithelium during the secondary transition, Isl-1 was conditionally and specifically deleted from embryonic day 13.5 onward using Cre/LoxP technology.

RESULTS

Isl-1-deficient endocrine precursors failed to mature into functional islet cells. The postnatal expansion of endocrine cell mass was impaired, and consequently Isl-1 deficient mice were diabetic. In addition, MafA, a potent regulator of the Insulin gene and beta-cell function, was identified as a direct transcriptional target of Isl-1.

CONCLUSIONS

These results demonstrate the requirement for Isl-1 in the maturation, proliferation, and survival of the second wave of hormone-producing islet cells.

MafA-deficient and beta cell-specific MafK-overexpressing hybrid transgenic mice develop human-like severe diabetic nephropathy

Transcription factor MafA is a key molecule in insulin secretion and the development of pancreatic islets. Previously, we demonstrated that some of the MafA-deficient mice develop overt diabetes mellitus, and the phenotype of these mice seems to be mild probably because of redundant functions of other Maf proteins. In this study, we generated hybrid transgenic mice that were MafA-deficient and also over-expressed MafK specifically in beta cells (MafA(-/-)MafK(+)). MafA(-/-)MafK(+) mice developed severe overt diabetes mellitus within 5weeks old, and showed higher levels of proteinuria and serum creatinine. Histological analysis revealed that embryonic development of beta cells in the MafA(-/-)MafK(+) mice was significantly suppressed and the reduced number of beta cells was responsible for the early onset of diabetes. Furthermore, after uninephrectomy, these mice demonstrated three characteristics of human diabetic nephropathy: diffuse, nodular, and exudative lesions. MafA(-/-)MafK(+) mice might be a useful model for the analysis of human diabetic nephropathy.

Distinct populations of quiescent and proliferative pancreatic beta-cells identified by HOTcre mediated labeling

Pancreatic beta-cells are critical regulators of glucose homeostasis, and they vary dramatically in their glucose stimulated metabolic response and levels of insulin secretion. It is unclear whether these parameters are influenced by the developmental origin of individual beta-cells. Using HOTcre, a Cre-based genetic switch that uses heat-induction to precisely control the temporal expression of transgenes, we labeled two populations of beta-cells within the developing zebrafish pancreas. These populations originate in distinct pancreatic buds and exhibit gene expression profiles suggesting distinct functions during development. We find that the dorsal bud derived beta-cells are quiescent and exhibit a marked decrease in insulin expression postembryonically. In contrast, ventral bud derived beta-cells proliferate actively, and maintain high levels of insulin expression compared with dorsal bud derived beta-cells. Therapeutic strategies to regulate beta-cell proliferation and function are required to cure pathological states that result from excessive beta-cell proliferation (e.g., insulinoma) or insufficient beta-cell mass (e.g., diabetes mellitus). Our data reveal the existence of distinct populations of beta-cells in vivo and should help develop better strategies to regulate beta-cell differentiation and proliferation.

p38 MAPK is a major regulator of MafA protein stability under oxidative stress

Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of beta-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and beta-cell function. Because MafA is a highly phosphorylated protein, we examined whether regulating activity of protein kinases can increase MafA expression by enhancing its stability. We demonstrate that MafA protein stability in MIN6 cells and isolated mouse islets is regulated by both p38 MAPK and glycogen synthase kinase 3. Inhibiting p38 MAPK enhanced MafA stability in cells grown under both low and high concentrations of glucose. We also show that the N-terminal domain of MafA plays a major role in p38 MAPK-mediated degradation; simultaneous mutation of both threonines 57 and 134 into alanines in MafA was sufficient to prevent this degradation. Under oxidative stress, a condition detrimental to beta-cell function, a decrease in MafA stability was associated with a concomitant increase in active p38 MAPK. Interestingly, inhibiting p38 MAPK but not glycogen synthase kinase 3 prevented oxidative stress-dependent degradation of MafA. These results suggest that the p38 MAPK pathway may represent a common mechanism for regulating MafA levels under oxidative stress and basal and stimulatory glucose concentrations. Therefore, preventing p38 MAPK-mediated degradation of MafA represents a novel approach to improve beta-cell function.

Sustained Neurog3 expression in hormone-expressing islet cells is required for endocrine maturation and function

Neurog3 (Neurogenin 3 or Ngn3) is both necessary and sufficient to induce endocrine islet cell differentiation from embryonic pancreatic progenitors. Since robust Neurog3 expression has not been detected in hormone-expressing cells, Neurog3 is used as an endocrine progenitor marker and regarded as dispensable for the function of differentiated islet cells. Here we used 3 independent lines of Neurog3 knock-in reporter mice and mRNA/protein-based assays to examine Neurog3 expression in hormone-expressing islet cells. Neurog3 mRNA and protein are detected in hormone-producing cells at both embryonic and adult stages. Significantly, inactivating Neurog3 in insulin-expressing beta cells at embryonic stages or in Pdx1-expressing islet cells in adults impairs endocrine function, a phenotype that is accompanied by reduced expression of several Neurog3 target genes that are essential for islet cell differentiation, maturation, and function. These findings demonstrate that Neurog3 is required not only for initiating endocrine cell differentiation, but also for promoting islet cell maturation and maintaining islet function.

Xenopus pancreas development

Understanding how the pancreas develops is vital to finding new treatments for a range of pancreatic diseases, including diabetes and pancreatic cancer. Xenopus is a relatively new model organism for the elucidation of pancreas development, and has already made contributions to the field. Recent studies have shown benefits of using Xenopus for understanding both early patterning and lineage specification aspects of pancreas organogenesis. This review focuses specifically on Xenopus pancreas development, and covers events from the end of gastrulation, when regional specification of the endoderm is occurring, right through metamorphosis, when the mature pancreas is fully formed. We have attempted to cover pancreas development in Xenopus comprehensively enough to assist newcomers to the field and also to enable those studying pancreas development in other model organisms to better place the results from Xenopus research into the context of the field in general and their studies specifically. Developmental Dynamics 238:1271-1286, 2009. (c) 2009 Wiley-Liss, Inc.

Reduced serum concentration is permissive for increased in vitro endocrine differentiation from murine embryonic stem cells

Embryonic stem cells (ESCs) have been shown to be capable of differentiating into pancreatic progenitors and insulin-producing cells in vitro. However, before ESC derivatives can be used in clinical settings, efficient selective differentiation needs to be achieved. Essential to improving ESC differentiation to islet endocrine cells is an understanding of the influences of extrinsic signals and transcription factors on cell specification. Herein, we investigate the influence of serum-supplemented growth conditions on the differentiation of murine ESCs to endocrine lineages in the context of over-expression of two pancreatic transcription factors, Pdx1 and Ngn3. To study the effect of different serum formulations and concentrations on the ability of murine ESCs to differentiate into endocrine cells in vitro, cells were grown into embryoid bodies and then differentiated in various serum replacement (SR), fetal calf serum (FCS) and serum-free conditions. Using immunohistochemistry and quantitative real-time RT-PCR (QPCR), we found that, of the conditions tested, 1% SR differentiation medium resulted in the highest levels of insulin-1 mRNA and significantly increased the total number of insulin-expressing cells. Applying this knowledge to cell lines in which Pdx1 or Ngn3 transgene expression could be induced by exposure to doxycycline we differentiated TetPDX1 and TetNgn3 ESCs under conditions of either 10% FCS or 1% SR medium. In the presence of 10% serum, induced expression of either Pdx1 or Ngn3 in differentiating ESCs resulted in modest increases in hormone transcripts and cell counts. However, changing the serum formulation from 10% FCS to 1% SR significantly enhanced the number of insulin+/C-peptide+ cells in parallel with increased insulin-1 transcript levels in both inducible cell lines. In summary, these data demonstrate that induced expression of key pancreatic transcription factors in combination with low serum/SR concentrations increases endocrine cell differentiation from murine ESCs.

Notch signaling as gatekeeper of rat acinar-to-beta-cell conversion in vitro

BACKGROUND & AIMS

Exocrine acinar cells in the pancreas are highly differentiated cells that retain a remarkable degree of plasticity. After isolation and an initial phase of dedifferentiation in vitro, rodent acinar cells can convert to endocrine beta-cells when cultured in the presence of appropriate factors. The mechanisms regulating this phenotypic conversion are largely unknown.

METHODS

Using rat acinar cell cultures, we studied the role of Notch signaling in a model of acinar-to-beta-cell conversion.

RESULTS

We report a novel lectin-based cell labeling method to demonstrate the acinar origin of newly formed insulin-expressing beta-cells. This method allows for specific tracing of the acinar cells. We demonstrate that growth factor-induced conversion of adult acinar cells to beta-cells is negatively regulated by Notch1 signaling. Activated Notch1 signaling prevents the reexpression of the proendocrine transcription factor Neurogenin-3, the key regulator of endocrine development in the embryonic pancreas. Interfering with Notch1 signaling allows modulating the acinar cell susceptibility to the differentiation-inducing factors. Its inhibition significantly improves beta-cell neoformation with approximately 30% of acinar cells that convert to beta-cells. The newly formed beta-cells mature when transplanted ectopically and are capable of restoring normal blood glycemia in diabetic recipients.

CONCLUSIONS

We report for the first time an efficient way to reprogram one third of the acinar cells to beta-cells by adult cell type conversion. This could find application in cell replacement therapy of type 1 diabetes, provided that it can be translated from rodent to human models.

Stem cells to pancreatic beta-cells: new sources for diabetes cell therapy

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

Endocrine cell clustering during human pancreas development

The development of efficient, reproducible protocols for directed in vitro differentiation of human embryonic stem (hES) cells into insulin-producing beta cells will benefit greatly from increased knowledge regarding the spatiotemporal expression profile of key instructive factors involved in human endocrine cell generation. Human fetal pancreases 7 to 21 weeks of gestational age, were collected following consent immediately after pregnancy termination and processed for immunostaining, in situ hybridization, and real-time RT-PCR expression analyses. Islet-like structures appear from approximately week 12 and, unlike the mixed architecture observed in adult islets, fetal islets are initially formed predominantly by aggregated insulin- or glucagon-expressing cells. The period studied (7-22 weeks) coincides with a decrease in the proliferation and an increase in the differentiation of the progenitor cells, the initiation of NGN3 expression, and the appearance of differentiated endocrine cells. The present study provides a detailed characterization of islet formation and expression profiles of key intrinsic and extrinsic factors during human pancreas development. This information is beneficial for the development of efficient protocols that will allow guided in vitro differentiation of hES cells into insulin-producing cells.