The beta-cell specific transcription factor Nkx6.1 inhibits glucagon gene transcription by interfering with Pax6

Glucagon-producing α-cell transcriptional identity and reprogramming towards insulin production

β-Cell replacement by in situ reprogramming of non-β-cells is a promising diabetes therapy. Following the observation that near-total β-cell ablation in adult mice triggers the reprogramming of pancreatic α-, δ-, and γ-cells into insulin (INS)-producing cells, recent studies are delving deep into the mechanisms controlling adult α-cell identity. Systematic analyses of the α-cell transcriptome and epigenome have started to pinpoint features that could be crucial for maintaining α-cell identity. Using different transgenic and chemical approaches, significant advances have been made in reprogramming α-cells in vivo into INS-secreting cells in mice. The recent reprogramming of human α-cells in vitro is an important step forward that must now be complemented with a comprehensive molecular dissection of the mechanisms controlling α-cell identity.

Deficiency of transcription factor Nkx6.1 does not prevent insulin secretion in INS-1E cells

Pancreatic-β-cell-specifying transcription factor Nkx6.1, indispensable for embryonic development of the pancreatic epithelium and commitment to β-cell lineage, directly controls the expression of a glucose transporter (Glut2), pyruvate carboxylase (Pcx), and genes for insulin processing (endoplasmic reticulum oxidoreductase-1β, Ero1lb; zinc transporter-8, Slc30a8). The Nkx6.1 decline in aging diabetic Goto-Kakizaki rats contributes to β-cell trans-differentiation into δ-cells. Elucidating further Nkx6.1 roles, we studied Nkx6.1 ablation in rat INS-1E cells, prepared by CRISPR/Cas9 gene editing from single colonies. INS-1E^Nkx6.1-/- cells exhibited unchanged glucose-stimulated insulin secretion (GSIS), moderately decreased phosphorylating/non-phosphorylating respiration ratios at high glucose; unchanged but delayed ATP-elevation responses to glucose; delayed uptake of fluorescent glucose analog, but slightly improved cytosolic Ca²⁺-oscillations, induced by glucose; despite approximately halved Glut2, Pcx, Ero1lb, and Slc30a8 expression, and reduced nuclear receptors Nr4a1 and Nr4a3. Thus, ATP synthesis was time-compensated, despite the delayed GLUT2-mediated glucose uptake and crippled pyruvate-malate redox shuttle (owing to the PCX-deficiency) in INS-1E^Nkx6.1-/- cells. Nkx6.1 thus controls the expression of genes that are not essential for acute insulin secretion, the function of which can be compensated for. Considerations that Nkx6.1 deficiency is an ultimate determinant of β-cell pathology beyond cell trans-(de-)differentiation or β-cell identity are not supported by our results.

Trans-differentiation of mouse mesenchymal stem cells into pancreatic β-like cells by a traditional anti-diabetic medicinal herb Medicago sativa L

Background and aim

Medicago sativa L. is a medicinal herb first cultivated in ancient Iran. Traditionally, it has been utilized for the treatment of several disorders. The plant has been in the human diet for at least 1500 years. Although the hypoglycaemic and anti-diabetic effects of the plant have been approved in traditional medicine, further investigations are needed to support the rational use of M. sativa by humans. This project aimed to evaluate the trans-differentiation potential of bone marrow mesenchymal stem cells (MSCs) to pancreatic β-like cells (insulin-producing cells; IPCs) under the influence of M. sativa extract.

Experimental procedure

Bone marrow MSCs isolated, characterized, and then treated by flower or leaf extract of M. sativa. Beta-cell characteristics of the differentiated cells were evaluated by several techniques, including specific staining, QPCR, immunofluorescence, and ELISA.

Results

The results showed that the differentiated cells were able to express some specific pancreatic genes (PDX-1, insulin1, and insulin2) and proteins (insulin receptor beta, insulin, proinsulin, and C peptide). Furthermore, ELISA analysis indicated the ability of these cells in the production and secretion of insulin, after exposure to glucose.

Conclusion

Overall, both the flower and leaf extract of M. sativa had the potential of differentiation induction of MSCs into IPCs with the characteristics of pancreatic β–like cells. Therefore, M. sativa, as an herbal drug, may be beneficial for the treatment of diseases including diabetes.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Spheroid Fabrication Using Concave Microwells Enhances the Differentiation Efficacy and Function of Insulin-Producing Cells via Cytoskeletal Changes

Pancreatic islet transplantation is the fundamental treatment for insulin-dependent diabetes; however, donor shortage is a major hurdle in its use as a standard treatment. Accordingly, differentiated insulin-producing cells (DIPCs) are being developed as a new islet source. Differentiation efficiency could be enhanced if the spheroid structure of the natural islets could be recapitulated. Here, we fabricated DIPC spheroids using concave microwells, which enabled large-scale production of spheroids of the desired size. We prepared DIPCs from human liver cells by trans-differentiation using transcription factor gene transduction. Islet-related gene expression and insulin secretion levels were higher in spheroids compared to those in single-cell DIPCs, whereas actin–myosin interactions significantly decreased. We verified actin–myosin-dependent insulin expression in single-cell DIPCs by using actin–myosin interaction inhibitors. Upon transplanting cells into the kidney capsule of diabetic mouse, blood glucose levels decreased to 200 mg/dL in spheroid-transplanted mice but not in single cell-transplanted mice. Spheroid-transplanted mice showed high engraftment efficiency in in vivo fluorescence imaging. These results demonstrated that spheroids fabricated using concave microwells enhanced the engraftment and functions of DIPCs via actin–myosin-mediated cytoskeletal changes. Our strategy potentially extends the clinical application of DIPCs for improved differentiation, glycemic control, and transplantation efficiency of islets.

NKX6.1 transcription factor: a crucial regulator of pancreatic β cell development, identity, and proliferation

Understanding the biology underlying the mechanisms and pathways regulating pancreatic β cell development is necessary to understand the pathology of diabetes mellitus (DM), which is characterized by the progressive reduction in insulin-producing β cell mass. Pluripotent stem cells (PSCs) can potentially offer an unlimited supply of functional β cells for cellular therapy and disease modeling of DM. Homeobox protein NKX6.1 is a transcription factor (TF) that plays a critical role in pancreatic β cell function and proliferation. In human pancreatic islet, NKX6.1 expression is exclusive to β cells and is undetectable in other islet cells. Several reports showed that activation of NKX6.1 in PSC-derived pancreatic progenitors (MPCs), expressing PDX1 (PDX1⁺/NKX6.1⁺), warrants their future commitment to monohormonal β cells. However, further differentiation of MPCs lacking NKX6.1 expression (PDX1⁺/NKX6.1⁻) results in an undesirable generation of non-functional polyhormonal β cells. The importance of NKX6.1 as a crucial regulator in MPC specification into functional β cells directs attentions to further investigating its mechanism and enhancing NKX6.1 expression as a means to increase β cell function and mass. Here, we shed light on the role of NKX6.1 during pancreatic β cell development and in directing the MPCs to functional monohormonal lineage. Furthermore, we address the transcriptional mechanisms and targets of NKX6.1 as well as its association with diabetes.

Impaired DNA-binding affinity of novel PAX6 mutations

Mutations in human PAX6 gene are associated with various congenital eye malformations including aniridia, foveal hypoplasia, and congenital nystagmus. These various phenotypes may depend on the mutation spectrums that can affect DNA-binding affinity, although this hypothesis is debatable. We screened PAX6 mutations in two unrelated patients with congenital nystagmus, and measured DNA-binding affinity through isothermal titration calorimetry (ITC). To elucidate phenotypic differences according to DNA-binding affinity, we also compared DNA-binding affinity among the previously reported PAX6 missense mutations within the linker region between two subdomains of the paired domain (PD). We identified two novel mutations of PAX6 gene: c.214 G > T (p.Gly72Cys) and c.249_250delinsCGC (p.Val84Alafs*8). Both were located within the linker region between the two subdomains of the PD. ITC measurement revealed that the mutation p.Val84Alafs*8 had no DNA-binding affinity, while the p.Gly72Cys mutation showed a decreased binding affinity (Kd = 0.58 μM) by approximately 1.4 times compared to the wild type-PAX6 (Kd = 0.41 μM). We also found that there was no close relationship between DNA-binding affinity and phenotypic differences. Our results suggest that the DNA-binding affinity alone might be insufficient to determine PAX6-related phenotypes, and that other modifier genes or environmental factors might affect phenotypes of the PAX6 gene.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Pancreatic islet-autonomous insulin and smoothened-mediated signalling modulate identity changes of glucagon+ α-cells

The mechanisms that restrict regeneration and maintain cell identity following injury are poorly characterized in higher vertebrates. Following β-cell loss, 1-2% of the glucagon-producing α-cells spontaneously engage in insulin production in mice. Here we explore the mechanisms inhibiting α-cell plasticity. We show that adaptive α-cell identity changes are constrained by intra-islet insulin- and Smoothened-mediated signalling, among others. The combination of β-cell loss or insulin-signalling inhibition, with Smoothened inactivation in α- or δ-cells, stimulates insulin production in more α-cells. These findings suggest that the removal of constitutive 'brake signals' is crucial to neutralize the refractoriness to adaptive cell-fate changes. It appears that the maintenance of cell identity is an active process mediated by repressive signals, which are released by neighbouring cells and curb an intrinsic trend of differentiated cells to change.

Endocrine lineage biases arise in temporally distinct endocrine progenitors during pancreatic morphogenesis

Decoding the molecular composition of individual Ngn3 + endocrine progenitors (EPs) during pancreatic morphogenesis could provide insight into the mechanisms regulating hormonal cell fate. Here, we identify population markers and extensive cellular diversity including four EP subtypes reflecting EP maturation using high-resolution single-cell RNA-sequencing of the e14.5 and e16.5 mouse pancreas. While e14.5 and e16.5 EPs are constantly born and share select genes, these EPs are overall transcriptionally distinct concomitant with changes in the underlying epithelium. As a consequence, e16.5 EPs are not the same as e14.5 EPs: e16.5 EPs have a higher propensity to form beta cells. Analysis of e14.5 and e16.5 EP chromatin states reveals temporal shifts, with enrichment of beta cell motifs in accessible regions at later stages. Finally, we provide transcriptional maps outlining the route progenitors take as they make cell fate decisions, which can be applied to advance the in vitro generation of beta cells.

Endocrine progenitors form early in pancreatic development but the diversity of this cell population is unclear. Here, the authors use single cell RNA sequencing of the mouse pancreas at e14.5 and e16.5 to show that endocrine progenitors are temporally distinct and those formed later are more likely to become beta cells

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Glucose-dependent downregulation of glucagon gene expression mediated by selective interactions between ALX3 and PAX6 in mouse alpha cells

AIMS/HYPOTHESIS

The stimulation of glucagon secretion in response to decreased glucose levels has been studied extensively. In contrast, little is known about the regulation of glucagon gene expression in response to fluctuations in glucose concentration. Paired box 6 (PAX6) is a key transcription factor that regulates the glucagon promoter by binding to the G1 and G3 elements. Here, we investigated the role of the transcription factor aristaless-like homeobox 3 (ALX3) as a glucose-dependent modulator of PAX6 activity in alpha cells.

METHODS

Experiments were performed in wild-type or Alx3-deficient islets and alphaTC1 cells. We used chromatin immunoprecipitations and electrophoretic mobility shift assays for DNA binding, immunoprecipitations and pull-down assays for protein interactions, transfected cells for promoter activity, and small interfering RNA and quantitative RT-PCR for gene expression.

RESULTS

Elevated glucose concentration resulted in stimulated expression of Alx3 and decreased glucagon gene expression in wild-type islets. In ALX3-deficient islets, basal glucagon levels were non-responsive to changes in glucose concentration. In basal conditions ALX3 bound to the glucagon promoter at G3, but not at G1. ALX3 could form heterodimers with PAX6 that were permissive for binding to G3 but not to G1. Thus, increasing the levels of ALX3 in response to glucose resulted in the sequestration of PAX6 by ALX3 for binding to G1, thus reducing glucagon promoter activation and glucagon gene expression.

CONCLUSIONS/INTERPRETATION

Glucose-stimulated expression of ALX3 in alpha cells provides a regulatory mechanism for the downregulation of glucagon gene expression by interfering with PAX6-mediated transactivation on the glucagon G1 promoter element.

Genotype-phenotype correlation of PAX6 gene mutations in aniridia

The objective of this study was to investigate the genotype-phenotype correlation of the PAX6 gene in aniridia. We clinically examined 5 families and 16 sporadic patients with aniridia. We performed chromosomal analysis and PCR analysis of the PAX6 gene using patient genomic DNA. Chromosomal analysis demonstrated deletions at 11p13 in one allele in four sporadic patients. Seven nonsense mutations, two frameshifts (two insertions), four splice junction errors and two missense mutations were found, and all were heterozygous. The iris phenotype ranged from total to normal in each patient, and the characteristic phenotypes, including cataract, glaucoma or optic nerve hypoplasia, varied widely even among members of the same family. Foveal hypoplasia was detected in all patients except for one. No obvious genotype-phenotype correlation was identified; however, the aniridia phenotype between the two eyes in each patient was quite similar in all patients. Because PAX6 regulates numerous downstream genes and its expression is regulated by several factors during eye development, the aniridia phenotype may be complex even in family members. However, because PAX6 regulation, resulting from both paternal and maternal alleles associated with PAX6, is considered to be roughly similar in both eyes of each patient, the aniridia phenotype may be similar in both eyes of each patient.

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

Lineage tracing studies have revealed that transcription factors play a cardinal role in pancreatic development, differentiation and function. Three transitions define pancreatic organogenesis, differentiation and maturation. In the primary transition, when pancreatic organogenesis is initiated, there is active proliferation of pancreatic progenitor cells. During the secondary transition, defined by differentiation, there is growth, branching, differentiation and pancreatic cell lineage allocation. The tertiary transition is characterized by differentiated pancreatic cells that undergo further remodeling, including apoptosis, replication and neogenesis thereby establishing a mature organ. Transcription factors function at multiple levels and may regulate one another and auto-regulate. The interaction between extrinsic signals from non-pancreatic tissues and intrinsic transcription factors form a complex gene regulatory network ultimately culminating in the different cell lineages and tissue types in the developing pancreas. Mutations in these transcription factors clinically manifest as subtypes of diabetes mellitus. Current treatment for diabetes is not curative and thus, developmental biologists and stem cell researchers are utilizing knowledge of normal pancreatic development to explore novel therapeutic alternatives. This review summarizes current knowledge of transcription factors involved in pancreatic development and β-cell differentiation in rodents.

Grg3/TLE3 and Grg1/TLE1 induce monohormonal pancreatic β-cells while repressing α-cell functions

In the pancreas, α- and β-cells possess a degree of plasticity. In vitro differentiation of pluripotent cells yields mostly α- and polyhormonal β-like cells, indicating a gap in understanding of how functional monohormonal β-cells are formed and of the endogenous repressive mechanisms used to maintain β-cell identity. We show that the corepressor Grg3 is expressed in almost all β-cells throughout embryogenesis to adulthood. However, Grg3 is expressed in fewer nascent α-cells and is progressively lost from α-cells as endocrine cells mature into adulthood. We show that mouse Grg3(+/-) β-cells have increased α-specific gene expression, and Grg3(+/-) pancreata have more α-cells and more polyhormonal cells, indicating that Grg3 is required for the physiologic maintenance of monohormonal β-cell identity. Ectopic expression of Grg3 in α-cells represses glucagon and Arx, and the addition of Pdx1 induces Glut2 expression and glucose-responsive insulin secretion. Furthermore, we found that Grg1 is the predominant Groucho expressed in human β-cells but acts functionally similarly to Grg3. Overall, we find that Grg3 and Grg1 establish a monohormonal β-cell identity, and Groucho family members may be useful tools or markers for making functional β-cells.

The functional and molecular characterisation of human embryonic stem cell-derived insulin-positive cells compared with adult pancreatic beta cells

AIMS/HYPOTHESIS

Using a novel directed differentiation protocol, we recently generated up to 25% insulin-producing cells from human embryonic stem cells (hESCs) (insulin(+) cells). At this juncture, it was important to functionally and molecularly characterise these hESC-derived insulin(+) cells and identify key differences and similarities between them and primary beta cells.

METHODS

We used a new reporter hESC line with green fluorescent protein (GFP) cDNA targeted to the INS locus by homologous recombination (INS (GFP/w)) and an untargeted hESC line (HES2). INS (GFP/w) allowed efficient identification and purification of GFP-producing (INS:GFP(+)) cells. Insulin(+) cells were examined for key features of adult beta cells using microarray, quantitative PCR, secretion assays, imaging and electrophysiology.

RESULTS

Immunofluorescent staining showed complete co-localisation of insulin with GFP; however, cells were often multihormonal, many with granules containing insulin and glucagon. Electrophysiological recordings revealed variable K(ATP) and voltage-gated Ca(2+) channel activity, and reduced glucose-induced cytosolic Ca(2+) uptake. This translated into defective glucose-stimulated insulin secretion but, intriguingly, appropriate glucagon responses. Gene profiling revealed differences in global gene expression between INS:GFP(+) cells and adult human islets; however, INS:GFP(+) cells had remarkably similar expression of endocrine-lineage transcription factors and genes involved in glucose sensing and exocytosis.

CONCLUSIONS/INTERPRETATION

INS:GFP(+) cells can be purified from differentiated hESCs, providing a superior source of insulin-producing cells. Genomic analyses revealed that INS:GFP(+) cells collectively resemble immature endocrine cells. However, insulin(+) cells were heterogeneous, a fact that translated into important functional differences within this population. The information gained from this study may now be used to generate new iterations of functioning beta cells that can be purified for transplant.

Catalase rs769214 SNP in elderly malnutrition and during renutrition: is glucagon to blame?

Impaired glucose tolerance is common during aging. The transcription factor PAX6 is involved in glucose homeostasis. Computational promoter sequence analysis of the catalase gene highlighted a putative PAX6 binding site on the rs769214 polymorphism A allele. Creation of this binding site has been suggested to explain renutrition inefficiency in malnourished elderly patients. Our aim was to evaluate the link between the rs769214 polymorphism of the catalase gene and glucose homeostasis in malnourished elderly patients at inclusion and during renutrition. Thirty-three malnourished elderly Caucasian inpatients were recruited. Nutritional and inflammatory statuses were assessed and a multiplex adipokine analysis was conducted at inclusion and discharge from the Geriatric Nutritional Care Unit at Charles-Foix Hospital (Ivry-sur-Seine, France). Serum glucagon, PAI-1, and TNF-α levels were significantly lower in the A-allele carriers at inclusion. During renutrition, A-allele carriers exhibited increased serum glucagon, PAI-1, and TNF-α variation. After renutrition, levels of these parameters were similar for A-allele carriers and G-allele carriers. A logistic ordinal multivariate regression analysis linked only variation of glucagon to rs769214 SNP. These results support a role for catalase SNP in the efficiency of renutrition in malnourished elderly patients via the modulation of glucagon secretion, probably involving PAX6.

Glucagon gene expression in the endocrine pancreas: the role of the transcription factor Pax6 in α-cell differentiation, glucagon biosynthesis and secretion

The glucagon gene is expressed in α-cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α-cell-specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α-cell differentiation and glucagon gene expression; in addition, the absence of the β-cell-specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G(1) which confers α-cell specificity to the promoter and to G(3) and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α-cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion.

Nkx6.2 synergizes with Cdx-2 in stimulating proglucagon gene expression

AIM: To investigate whether the transactivator of the proglucagon gene (Gcg), Cdx-2, synergizes with other transcription factors in stimulating Gcg expression and the trans-differentiation of Gcg-expressing cells.

METHODS: We conducted affinity chromatography to identify proteins that interact with Cdx-2, using GST-tagged Cdx-2 against cell lysates from pancreatic InR1-G9 and intestinal GLUTag cell lines. This was followed by a mass-spectrometry analysis. From a potential Cdx-2 interaction protein identified, we examined its expression in pancreatic and gut endocrine cells, confirmed its interaction with Cdx-2 by GST-pull down and determined its effect in provoking Gcg expression in cell lines that do not express endogenous Gcg.

RESULTS: We identified 18 potential Cdx-2 interacting proteins. One of them is Nkx6.2. This homeodomain (HD) protein is expressed in pancreatic α and intestinal endocrine L cells but not in insulin producing cell lines, including In111. Nkx6.2, but not Nkx6.1, was shown to interact with Cdx-2, detected by GST-pull down. Furthermore, Nkx6.2 was found to synergize with Cdx-2 in provoking Gcg expression when they were ectopically expressed in the In111 cell line. Finally, when Cdx-2 and Nkx6.2 were co-transfected into the undifferentiated rat intestinal IEC-6 cell line, it produced detectable amount of Gcg mRNA.

CONCLUSION: Cdx-2 recruits Nkx6.2 in exerting its effect in stimulating Gcg expression. Our observations further support the notion that multiple HD proteins, including Cdx-2 and Nkx6.2, are involved in the regulation of Gcg expression and the genesis of Gcg-producing cells.

NKX6.1 promotes PDX-1-induced liver to pancreatic β-cells reprogramming

Reprogramming adult mammalian cells is an attractive approach for generating cell-based therapies for degenerative diseases, such as diabetes. Adult human liver cells exhibit a high level of developmental plasticity and have been suggested as a potential source of pancreatic progenitor tissue. An instructive role for dominant pancreatic transcription factors in altering the hepatic developmental fate along the pancreatic lineage and function has been demonstrated. Here we analyze whether transcription factors expressed in mature pancreatic β-cells preferentially activate β-cell lineage differentiation in liver. NKX6.1 is a transcription factor uniquely expressed in β-cells of the adult pancreas, its potential role in reprogramming liver cells to pancreatic lineages has never been analyzed. Our results suggest that NKX6.1 activates immature pancreatic markers such as NGN-3 and ISL-1 but not pancreatic hormones gene expression in human liver cells. We hypothesized that its restricted capacity to activate a wide pancreatic repertoire in liver could be related to its incapacity to activate endogenous PDX-1 expression in liver cells. Indeed, the complementation of NKX6.1 by ectopic PDX-1 expression substantially and specifically promoted insulin expression and glucose regulated processed hormone secretion to a higher extent than that of PDX-1 alone, without increasing the reprogrammed cells. This may suggest a potential role for NKX6.1 in promoting PDX-1 reprogrammed cells maturation along the β-cell-like lineage. By contrast, NKX6.1 repressed PDX-1 induced proglucagon gene expression. The individual and concerted effects of pancreatic transcription factors in adult extra-pancreatic cells, is expected to facilitate developing regenerative medicine approaches for cell replacement therapy in diabetics.

Distinct regulation of hepatic nuclear factor 1alpha by NKX6.1 in pancreatic beta cells

Hepatic nuclear factor 1alpha (HNF1alpha) is a key regulator of development and function in pancreatic beta cells and is specifically involved in regulation of glycolysis and glucose-stimulated insulin secretion. Abnormal expression of HNF1alpha leads to development of MODY3 (maturity-onset diabetes of the young 3). We report that NK6 homeodomain 1 (NKX6.1) binds to a cis-regulatory element in the HNF1alpha promoter and is a major regulator of this gene in beta cells. We identified an NKX6.1 recognition sequence in the distal region of the HNF1alpha promoter and demonstrated specific binding of NKX6.1 in beta cells by electrophoretic mobility shift and chromatin immunoprecipitation assays. Site-directed mutagenesis of the NKX6.1 core-binding sequence eliminated NKX6.1-mediated activation and substantially decreased activity of the HNF1alpha promoter in beta cells. Overexpression or small interfering RNA-mediated knockdown of the Nkx6.1 gene resulted in increased or diminished HNF1alpha gene expression, respectively, in beta cells. We conclude that NKX6.1 is a novel regulator of HNF1alpha in pancreatic beta cells. This novel regulatory mechanism for HNF1alpha in beta cells may provide new molecular targets for the diagnosis of MODY3.

On the origin of the beta cell

The major forms of diabetes are characterized by pancreatic islet beta-cell dysfunction and decreased beta-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of beta cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and beta-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine beta cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing beta-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships.

Identification of positionally distinct astrocyte subtypes whose identities are specified by a homeodomain code

Astrocytes constitute the most abundant cell type in the central nervous system (CNS) and play diverse functional roles, but the ontogenetic origins of this phenotypic diversity are poorly understood. We have investigated whether positional identity, a fundamental organizing principle governing the generation of neuronal subtype diversity, is also relevant to astrocyte diversification. We identified three positionally distinct subtypes of white-matter astrocytes (WMA) in the spinal cord, which can be distinguished by the combinatorial expression of Reelin and Slit1. These astrocyte subtypes derive from progenitor domains expressing the homeodomain transcription factors Pax6 and Nkx6.1, respectively. Loss- and gain-of-function experiments indicate that the positional identity of these astrocyte subtypes is controlled by Pax6 and Nkx6.1 in a combinatorial manner. Thus, positional identity is an organizing principle underlying astrocyte, as well as neuronal, subtype diversification and is controlled by a homeodomain transcriptional code whose elements are reutilized following the specification of neuronal identity earlier in development.

Concise review: Pax6 transcription factor contributes to both embryonic and adult neurogenesis as a multifunctional regulator

Pax6 is a highly conserved transcription factor among vertebrates and is important in various developmental processes in the central nervous system (CNS), including patterning of the neural tube, migration of neurons, and formation of neural circuits. In this review, we focus on the role of Pax6 in embryonic and postnatal neurogenesis, namely, production of new neurons from neural stem/progenitor cells, because Pax6 is intensely expressed in these cells from the initial stage of CNS development and in neurogenic niches (the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricle) throughout life. Pax6 is a multifunctional player regulating proliferation and differentiation through the control of expression of different downstream molecules in a highly context-dependent manner.

An illustrated review of early pancreas development in the mouse

Pancreas morphogenesis and cell differentiation are highly conserved among vertebrates during fetal development. The pancreas develops through simple budlike structures on the primitive gut tube to a highly branched organ containing many specialized cell types. This review presents an overview of key molecular components and important signaling sources illustrated by an extensive three-dimensional (3D) imaging of the developing mouse pancreas at single cell resolution. The 3D documentation covers the time window between embryonic days 8.5 and 14.5 in which all the pancreatic cell types become specified and therefore includes gene expression patterns of pancreatic endocrine hormones, exocrine gene products, and essential transcription factors. The 3D perspective provides valuable insight into how a complex organ like the pancreas is formed and a perception of ventral and dorsal pancreatic growth that is otherwise difficult to uncover. We further discuss how this global analysis of the developing pancreas confirms and extends previous studies, and we envisage that this type of analysis can be instrumental for evaluating mutant phenotypes in the future.

Pax-6 and c-Maf functionally interact with the alpha-cell-specific DNA element G1 in vivo to promote glucagon gene expression

Specific expression of the glucagon gene in the rat pancreas requires the presence of the G1 element localized at -100/-49 base pairs on the promoter. Although it is known that multiple transcription factors such as Pax-6, Cdx-2/3, c-Maf, Maf-B, and Brain-4 can activate the glucagon gene promoter through G1, their relative importance in vivo is unknown. We first studied the expression of Maf-B, c-Maf, and Cdx-2/3 in the developing and adult mouse pancreas. Although Maf-B was detectable in a progressively increasing number of alpha-cells throughout development and in adulthood, c-Maf and Cdx-2/3 were expressed at low and very low levels, respectively. However, c-Maf but not Cdx-2/3 was detectable in adult islets by Western blot analyses. We then demonstrated the in vivo interactions of Pax-6, Cdx-2/3, Maf-B, and c-Maf but not Brain-4 with the glucagon gene promoter in glucagon-producing cells. Although Pax-6, Cdx-2/3, Maf-B, and c-Maf were all able to bind G1 by themselves, we showed that Pax-6 could interact with Maf-B, c-Maf, and Cdx-2/3 and activate transcription of the glucagon gene promoter. Overexpression of dominant negative forms of Cdx-2/3 and Mafs in alpha-cell lines indicated that Cdx-2/3 and the Maf proteins interact on an overlapping site within G1 and that this binding site is critical in the activation of the glucagon gene promoter. Finally, we show that specific inhibition of Pax-6 and c-Maf but not Cdx-2/3 or Maf-B led to decreases in endogenous glucagon gene expression and that c-Maf binds the glucagon gene promoter in mouse islets. We conclude that Pax-6 and c-Maf interact with G1 to activate basal expression of the glucagon gene.