The concerted activities of Pax4 and Nkx2.2 are essential to initiate pancreatic beta-cell differentiation

Specifying pancreatic endocrine cell fates

Cell replacement therapy could represent an attractive alternative to insulin injections for the treatment of diabetes. However, this approach requires a thorough understanding of the molecular switches controlling the specification of the different pancreatic cell-types in vivo. These are derived from an apparently identical pool of cells originating from the early gut endoderm, which are successively specified towards the pancreatic, endocrine, and hormone-expressing cell lineages. Numerous studies have outlined the crucial roles exerted by transcription factors in promoting the cell destiny, defining the cell identity and maintaining a particular cell fate. This review focuses on the mechanisms regulating the morphogenesis of the pancreas with particular emphasis on recent findings concerning the transcription factor hierarchy orchestrating endocrine cell fate allocation.

The effect of overexpression of Pdx1 and Foxa2 on the differentiation of human embryonic stem cells into pancreatic cells

Human embryonic stem cells (HESCs) are pluripotent cells that may serve as a source of cells for transplantation medicine and as a tool to study human embryogenesis. Using genetic manipulation methodologies, we have investigated the potential of HESCs to differentiate into the various pancreatic cell types. We initially created various HESCs carrying the enhanced green fluorescent protein (eGFP) reporter gene under the control of either the insulin promoter or the pancreatic and duodenal homeobox factor-1 (Pdx1) promoter. Our analysis revealed that during the differentiation of HESCs into embryoid bodies (EBs), we could detect green fluorescent cells when eGFP is regulated by Pdx1 promoter but not by insulin promoter. To examine whether we can induce differentiation into pancreatic cells, we have established human embryonic stem cell lines that constitutively express either Pdx1 or the endodermal transcription factor Foxa2. Following differentiation into EBs, the constitutive expression of Pdx1 enhanced the differentiation of HESCs toward pancreatic endocrine and exocrine cell types. Thus, we have demonstrated expression of several transcription factors that are downstream of Pdx1 and various molecular markers for the different pancreatic cell types. However, the expression of the insulin gene could be demonstrated only when the cells differentiated in vivo into teratomas. We conclude that although overexpression of Pdx1 enhanced expression of pancreatic enriched genes, induction of insulin expression may require additional signals that are only present in vivo.

Genome-wide mapping of Polycomb target genes unravels their roles in cell fate transitions

The Polycomb group (PcG) proteins form chromatin-modifying complexes that are essential for embryonic development and stem cell renewal and are commonly deregulated in cancer. Here, we identify their target genes using genome-wide location analysis in human embryonic fibroblasts. We find that Polycomb-Repressive Complex 1 (PRC1), PRC2, and tri-methylated histone H3K27 co-occupy >1000 silenced genes with a strong functional bias for embryonic development and cell fate decisions. We functionally identify 40 genes derepressed in human embryonic fibroblasts depleted of the PRC2 components (EZH2, EED, SUZ12) and the PRC1 component, BMI-1. Interestingly, several markers of osteogenesis, adipogenesis, and chrondrogenesis are among these genes, consistent with the mesenchymal origin of fibroblasts. Using a neuronal model of differentiation, we delineate two different mechanisms for regulating PcG target genes. For genes activated during differentiation, PcGs are displaced. However, for genes repressed during differentiation, we paradoxically find that they are already bound by the PcGs in nondifferentiated cells despite being actively transcribed. Our results are consistent with the hypothesis that PcGs are part of a preprogrammed memory system established during embryogenesis marking certain key genes for repressive signals during subsequent developmental and differentiation processes.

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

Major insulin gene transcription factors, such as PDX-1 or NeuroD1, have equally important roles in pancreatic development and the differentiation of pancreatic endocrine cells. Previously, we identified and cloned another critical insulin gene transcription factor MafA (RIPE3b1) and reported that other Maf factors were expressed in pancreatic endocrine cells. Maf factors are important regulators of cellular differentiation; to understand their role in differentiation of pancreatic endocrine cells, we analyzed the expression pattern of large-Maf factors in the pancreas of embryonic and adult mice. Ectopically expressed large-Maf factors, MafA, MafB, or cMaf, induced expression from insulin and glucagon reporter constructs, demonstrating a redundancy in their function. Yet in adult pancreas, cMaf was expressed in both alpha- and beta-cells, and MafA and MafB showed selective expression in the beta- and alpha-cells, respectively. Interestingly, during embryonic development, a significant proportion of MafB-expressing cells also expressed insulin. In embryos, MafB is expressed before MafA, and our results suggest that the differentiation of beta-cells proceeds through a MafB+ MafA- Ins+ intermediate cell to MafB- MafA+ Ins+ cells. Furthermore, the MafB to MafA transition follows induction of PDX-1 expression (Pdx-1(high)) in MafB+ Ins+ cells. We suggest that MafB may have a dual role in regulating embryonic differentiation of both beta- and alpha-cells while MafA may regulate replication/survival and function of beta-cells after birth. Thus, this redundancy in the function and expression of the large-Maf factors may explain the normal islet morphology observed in the MafA knockout mice at birth.

MafB: an activator of the glucagon gene expressed in developing islet alpha- and beta-cells

The large Maf family of basic leucine-zipper-containing transcription factors are known regulators of key developmental and functional processes in various cell types, including pancreatic islets. Here, we demonstrate that within the adult pancreas, MafB is only expressed in islet alpha-cells and contributes to cell type-specific expression of the glucagon gene through activation of a conserved control element found between nucleotides -77 to -51. MafB was also shown to be expressed in developing alpha- and beta-cells as well as in proliferating hormone-negative cells during pancreatogenesis. In addition, MafB expression is maintained in the insulin(+) and glucagon(+) cells remaining in mice lacking either the Pax4 or Pax6 developmental regulators, implicating a potentially early role for MafB in gene regulation during islet cell development. These results indicate that MafB is not only important to islet alpha-cell function but may also be involved in regulating genes required in both endocrine alpha- and beta-cell differentiation.

Activin A-induced expression of PAX4 in AR42J-B13 cells involves the increase in transactivation of E47/E12

Pax4 is a paired-homeodomain containing transcriptional factor that controls the differentiation of pancreatic beta cells. The aim of this study was to investigate the mechanism of PAX4 expression by activin A. By reporter gene analysis using AR42J-B13 cells, in which treatment with activin A induced PAX4 mRNA expression, we identified that a short sequence located approximately 1930 bp upstream of the transcriptional start site is essential for activin A induced PAX4 promoter activation. This region contains an E box and binding sites for hepatocyte nuclear factor (HNF)-1alpha. Mutation introduced in each binding site markedly reduced activin A responsiveness. It has been reported that HNF-1alpha synergizes with basic helix-loop-helix (bHLH) proteins in activating the PAX4 promoter, and we demonstrated that activin A strongly enhanced the functional activity of E47/E12 without the increase in its binding ability. In addition, suppression of E47/E12 expression in AR42J-B13 cells using siRNA oligonucleotides results in the significant decrease in the intrinsic activin A-induced PAX4 expression. Our results suggest that activin A enhances PAX4 expression by enhanced transactivation of E47/E12 proteins and might result in a cumulative transactivation of the promoter.

Prox1 activity controls pancreas morphogenesis and participates in the production of "secondary transition" pancreatic endocrine cells

The development of the mammalian pancreas is governed by various signaling processes and by a cascade of gene activation events controlled by different transcription factors. Here we show that the divergent homeodomain transcription factor Prox1 is a novel, crucial regulator of mouse pancreas organogenesis. Loss of Prox1 function severely disrupted epithelial pancreas morphology and hindered pancreatic growth without affecting significantly the genesis of endocrine cells before E11.5. Conversely, the lack of Prox1 activity substantially decreased the formation of islet cell precursors after E13.5, during a period known as the "secondary transition". Notably, this defect occurred concurrently with an abnormal increment of exocrine cells. Hence, it is possible that Prox1 contributes to the allocation of an adequate supply of islet cells throughout pancreas ontogeny by preventing exocrine cell differentiation of multipotent pancreatic progenitors. Prox1 thus appears to be an essential component of a genetic program destined to produce the cellular complexity of the mammalian pancreas.

Genetic determinants of pancreatic epsilon-cell development

Recently, the expression of the peptide hormone ghrelin was detected in alpha-cells of the islets of Langerhans as well as in epsilon-cells, a newly discovered endocrine cell type, but it remains unclear how the latter is related in lineage to the four classical islet cell types, alpha-, beta-, delta-, and PP-cells. Here, we provide further evidence that ghrelin is predominantly produced in the alpha-cells of mouse islets but also in single hormone ghrelin-secreting epsilon-cells. We additionally demonstrate that pancreatic epsilon-cells derive from Neurogenin3-expressing precursor cells and their genesis depends on Neurogenin3 activity. Furthermore, our data indicate that the number of ghrelin-producing cells is differentially regulated during pancreas morphogenesis by the homeodomain-containing transcription factors Arx, Pax4, and Pax6. Arx mutants lack ghrelin+ glucagon+ alpha-cells whereas Pax4 mutants develop an excess of these cells. Importantly, the ghrelin+ glucagon- epsilon-cell population is not affected following Arx or Pax4 disruption. In contrast, the loss of Pax6 provokes an unexpected increase of the ghrelin+ glucagon- epsilon-cell number which is not due to increased proliferation. Thus, we demonstrate that the development of ghrelin-producing cells is differentially dependent on Neurogenin3 in different domains of the gastrointestinal tract and that, in the endocrine pancreas, epsilon-cell genesis does not require Arx or Pax4 activities but is antagonized by Pax6.

Can diabetes be cured by therapeutic cloning?

With the increasing incidence of diabetes mellitus (DM), it is imperative to develop novel treatments. Stem cells offer the potential for use as renewable sources of glucose-responsive, insulin-secreting cells. However, developing a consistent protocol to enrich beta-cells is not a trivial issue. The question whether embryonic, fetal, or adult stem cells offer particular advantages as the starting material remains to be resolved experimentally. While somatic cell nuclear transfer avoids many of the problems associated with heterologous transplantation, the problem of autoimmune destruction of the beta-cells in type 1 DM might still remain. This review summarizes the innovative treatment strategies for DM and considers the possible advantages and problems.

Stage-specific expression of myelin basic protein in oligodendrocytes involves Nkx2.2-mediated repression that is relieved by the Sp1 transcription factor

The homeodomain-containing protein Nkx2.2 is critical for the development of oligodendrocyte lineage cells, but the target genes of Nkx2.2 regulation have not been identified. In the present study, we found that the myelin basic protein gene is one of the genes that is regulated by Nkx2.2. Expression of Nkx2.2 represses the expression of myelin basic protein in oligodendrocyte progenitors. Two regulatory elements in the myelin basic protein promoter were identified and found to interact with Nkx2.2 in vitro. Despite their sequence divergence, both sites were involved in the Nkx2.2-mediated repression of the myelin basic protein promoter. Binding of Nkx2.2 also blocked and disrupted the binding of the transcriptional activator Puralpha to the myelin basic protein promoter. Additionally Nkx2.2 recruited a histone deacetylase 1-mSin3A complex to the myelin basic protein promoter. We also found that the transcription factor Sp1 was able to compete off the binding of Nkx2.2 to its consensus binding site in vitro and reversed the repressive effect of Nkx2.2 in vivo. Our data revealed a novel role for Nkx2.2 in preventing the precocious expression of myelin basic protein in immature oligodendrocytes. Based on this study and our previous reports, a model for myelin basic protein gene control is proposed.

PAX4 gene variations predispose to ketosis-prone diabetes

Ketosis-prone diabetes (KPD) is a rare form of type 2 diabetes, mostly observed in subjects of west African origin (west Africans and African-Americans), characterized by fulminant and phasic insulin dependence, but lacking markers of autoimmunity observed in type 1 diabetes. PAX4 is a transcription factor essential for the development of insulin-producing pancreatic beta-cells. Recently, a missense mutation (Arg121Trp) of PAX4 has been implicated in early and insulin deficient type 2 diabetes in Japanese subjects. The phenotype similarities between KPD and Japanese carriers of Arg121Trp have prompted us to investigate the role of PAX4 in KPD. We have screened 101 KPD subjects and we have found a new variant in the PAX4 gene (Arg133Trp), specific to the population of west African ancestry, and which predisposes to KPD under a recessive model. Homozygous Arg133Trp PAX4 carriers were found in 4% of subjects with KPD but not in 355 controls or 147 subjects with common type 2 or type 1 diabetes. In vitro, the Arg133Trp variant showed a decreased transcriptional repression of target gene promoters in an alpha-TC1.6 cell line. In addition, one KPD patient was heterozygous for a rare PAX4 variant (Arg37Trp) that was not found in controls and that showed a more severe biochemical phenotype than Arg133Trp. Clinical investigation of the homozygous Arg133Trp carriers and of the Arg37Trp carrier demonstrated a more severe alteration in insulin secretory reserve, during a glucagon-stimulation test, compared to other KPD subjects. Together these data provide the first evidence that ethnic-specific gene variants may contribute to the predisposition to this particular form of diabetes and suggest that KPD, like maturity onset diabetes of the young, is a rare, phenotypically defined but genetically heterogeneous form of type 2 diabetes.

Ghrelin cells replace insulin-producing beta cells in two mouse models of pancreas development

The pancreatic islet is necessary for maintaining glucose homeostasis. Within the pancreatic islet, the homeodomain protein Nkx2.2 is essential for the differentiation of all insulin-producing beta cells and a subset of glucagon-producing alpha cells (1). Mice lacking Nkx2.2 have relatively normal sized islets, but a large number of cells within the mutant islet fail to produce any of the four major islet hormones. In this study we demonstrate that Nkx2.2 mutant endocrine cells have been replaced by cells that produce ghrelin, an appetite-promoting peptide predominantly found in the stomach. Intriguingly, normal mouse pancreas also contains a small population of ghrelin-producing cells, defining a new islet "epsilon" cell population. The expansion of ghrelin-producing cells at the expense of beta cells may be a general phenomenon, because we demonstrate that Pax4 mutant mice display a similar phenotype. We propose that insulin and ghrelin cells share a common progenitor and that Nkx2.2 and Pax4 are required to specify or maintain differentiation of the beta cell fate. This finding also suggests that there is a genetic component underlying the balance between insulin and ghrelin in regulating glucose metabolism.