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

CK2 activity is crucial for proper glucagon expression

AIMS/HYPOTHESIS

Protein kinase CK2 acts as a negative regulator of insulin expression in pancreatic beta cells. This action is mainly mediated by phosphorylation of the transcription factor pancreatic and duodenal homeobox protein 1 (PDX1). In pancreatic alpha cells, PDX1 acts in a reciprocal fashion on glucagon (GCG) expression. Therefore, we hypothesised that CK2 might positively regulate GCG expression in pancreatic alpha cells.

METHODS

We suppressed CK2 kinase activity in αTC1 cells by two pharmacological inhibitors and by the CRISPR/Cas9 technique. Subsequently, we analysed GCG expression and secretion by real-time quantitative RT-PCR, western blot, luciferase assay, ELISA and DNA pull-down assays. We additionally studied paracrine effects on GCG secretion in pseudoislets, isolated murine islets and human islets. In vivo, we examined the effect of CK2 inhibition on blood glucose levels by systemic and alpha cell-specific CK2 inhibition.

RESULTS

We found that CK2 downregulation reduces GCG secretion in the murine alpha cell line αTC1 (e.g. from 1094±124 ng/l to 459±110 ng/l) by the use of the CK2-inhibitor SGC-CK2-1. This was due to a marked decrease in Gcg gene expression through alteration of the binding of paired box protein 6 (PAX6) and transcription factor MafB to the Gcg promoter. The analysis of the underlying mechanisms revealed that both transcription factors are displaced by PDX1. Ex vivo experiments in isolated murine islets and pseudoislets further demonstrated that CK2-mediated reduction in GCG secretion was only slightly affected by the higher insulin secretion after CK2 inhibition. The kidney capsule transplantation model showed the significance of CK2 for GCG expression and secretion in vivo. Finally, CK2 downregulation also reduced the GCG secretion in islets isolated from humans.

CONCLUSIONS/INTERPRETATION

These novel findings not only indicate an important function of protein kinase CK2 for proper GCG expression but also demonstrate that CK2 may be a promising target for the development of novel glucose-lowering drugs.

LINC MIR503HG Controls SC-β Cell Differentiation and Insulin Production by Targeting CDH1 and HES1

Stem cell-derived pancreatic progenitors (SC-PPs), as an unlimited source of SC-derived β (SC-β) cells, offers a robust tool for diabetes treatment in stem cell-based transplantation, disease modeling, and drug screening. Whereas, PDX1+/NKX6.1+ PPs enhances the subsequent endocrine lineage specification and gives rise to glucose-responsive SC-β cells in vivo and in vitro. To identify the regulators that promote induction efficiency and cellular function maturation, single-cell RNA-sequencing is performed to decipher the transcriptional landscape during PPs differentiation. The comprehensive evaluation of functionality demonstrated that manipulating LINC MIR503HG using CRISPR in PP cell fate decision can improve insulin synthesis and secretion in mature SC-β cells, without effects on liver lineage specification. Importantly, transplantation of MIR503HG-/- SC-β cells in recipients significantly restored blood glucose homeostasis, accompanied by serum C-peptide release and an increase in body weight. Mechanistically, by releasing CtBP1 occupying the CDH1 and HES1 promoters, the decrease in MIR503HG expression levels provided an excellent extracellular niche and appropriate Notch signaling activation for PPs following differentiation. Furthermore, this exhibited higher crucial transcription factors and mature epithelial markers in CDH1High expressed clusters. Altogether, these findings highlighted MIR503HG as an essential and exclusive PP cell fate specification regulator with promising therapeutic potential for patients with diabetes.

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.

MAFB promotes the malignant phenotypes by IGFBP6 in esophageal squamous cell carcinomas

Esophageal squamous cell carcinoma (ESCC) is one of the most common malignant diseases in the world. Although the somatic alterations have been fully identified, there are still no targeted drugs at present. Our previous studies revealed that loss of grand H3K27me3 domains mediated transcriptional activation of a series of genes in ESCC. Among them, we focus on the investigation of MAFB, as its high expression is associated with a poor prognosis in ESCC. Functional assays show that knockdown of MAFB significantly suppresses cell growth, migration and invasion. Mechanistic investigation demonstrates that MAFB exerts its function by upregulating IGFBP6. Our findings suggest that MAFB may play a tumor-promoting role and may act as a potential therapeutic target for ESCC.

Growth differentiation factor-15 prevents glucotoxicity and connexin-36 downregulation in pancreatic beta-cells

Pancreatic beta cell dysfunction is a hallmark of type 2 diabetes. Growth differentiation factor 15 (GDF15), which is an energy homeostasis regulator, has been shown to improve several metabolic parameters in the context of diabetes. However, its effects on pancreatic beta-cell remain to be identified. We, therefore, performed experiments using cell models and histological sectioning of wild-type and knock-out GDF15 mice to determine the effect of GDF15 on insulin secretion and cell viability. A bioinformatics analysis was performed to identify GDF15-correlated genes. GDF15 prevents glucotoxicity-mediated altered glucose-stimulated insulin secretion (GSIS) and connexin-36 downregulation. Inhibition of endogenous GDF15 reduced GSIS in cultured mouse beta-cells under standard conditions while it had no impact on GSIS in cells exposed to glucolipotoxicity, which is a diabetogenic condition. Furthermore, this inhibition exacerbated glucolipotoxicity-reduced cell survival. This suggests that endogenous GDF15 in beta-cell is required for cell survival but not GSIS in the context of glucolipotoxicity.

Profiling of RNA-binding Proteins Interacting With Glucagon and Adipokinetic Hormone mRNAs

OBJECTIVE

Glucagon in mammals and its homolog (adipokinetic hormone [AKH] in Drosophila melanogaster) are peptide hormones which regulate lipid metabolism by breaking down triglycerides. Although regulatory mechanisms of glucagon and AKH expression have been widely studied, post-transcriptional gene expression of glucagon has not been investigated thoroughly. In this study, we aimed to profile proteins binding with Gcg messenger RNA (mRNA) in mouse and Akh mRNA in Drosophila.

METHODS

Drosophila Schneider 2 (S2) and mouse 3T3-L1 cell lysates were utilized for affinity pull down of Akh and Gcg mRNA respectively using biotinylated anti-sense DNA oligoes against target mRNAs. Mass spectrometry and computational network analysis revealed mRNA-interacting proteins residing in functional proximity.

RESULTS

We observed that 1) 91 proteins interact with Akh mRNA from S2 cell lysates, 2) 34 proteins interact with Gcg mRNA from 3T3-L1 cell lysates. 3) Akh mRNA interactome revealed clusters of ribosomes and known RNA-binding proteins (RBPs). 4) Gcg mRNA interactome revealed mRNA-binding proteins including Plekha7, zinc finger protein, carboxylase, lipase, histone proteins and a cytochrome, Cyp2c44. 5) Levels of Gcg mRNA and its interacting proteins are elevated in skeletal muscles isolated from old mice compared to ones from young mice.

CONCLUSION

Akh mRNA in S2 cells are under active translation in a complex of RBPs and ribosomes. Gcg mRNA in mouse precursor adipocyte is in a condition distinct from Akh mRNA due to biochemical interactions with a subset of RBPs and histones. We anticipate that our study contributes to investigating regulatory mechanisms of Gcg and Akh mRNA decay, translation, and localization.

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.

Aberrant development of pancreatic beta cells derived from human iPSCs with FOXA2 deficiency

FOXA2 has been identified as an essential factor for pancreas development and emerging evidence supports an association between FOXA2 and diabetes. Although the role of FOXA2 during pancreatic development is well-studied in animal models, its role during human islet cell development remains unclear. Here, we generated induced pluripotent stem cells (iPSCs) from a patient with FOXA2 haploinsufficiency (FOXA2^+/- iPSCs) followed by beta-cell differentiation to understand the role of FOXA2 during pancreatic beta-cell development. Our results showed that FOXA2 haploinsufficiency resulted in aberrant expression of genes essential for the differentiation and proper functioning of beta cells. At pancreatic progenitor (PP2) and endocrine progenitor (EPs) stages, transcriptome analysis showed downregulation in genes associated with pancreatic development and diabetes and upregulation in genes associated with nervous system development and WNT signaling pathway. Knockout of FOXA2 in control iPSCs (FOXA2^-/- iPSCs) led to severe phenotypes in EPs and beta-cell stages. The expression of NGN3 and its downstream targets at EPs as well as INSUILIN and GLUCAGON at the beta-cell stage, were almost absent in the cells derived from FOXA2^-/- iPSCs. These findings indicate that FOXA2 is crucial for human pancreatic endocrine development and its defect may lead to diabetes based on FOXA2 dosage.

The Long Noncoding RNA Paupar Modulates PAX6 Regulatory Activities to Promote Alpha Cell Development and Function

Many studies have highlighted the role of dysregulated glucagon secretion in the etiology of hyperglycemia and diabetes. Accordingly, understanding the mechanisms underlying pancreatic islet α cell development and function has important implications for the discovery of new therapies for diabetes. In this study, comparative transcriptome analyses between embryonic mouse pancreas and adult mouse islets identified several pancreatic lncRNAs that lie in close proximity to essential pancreatic transcription factors, including the Pax6-associated lncRNA Paupar. We demonstrate that Paupar is enriched in glucagon-producing α cells where it promotes the alternative splicing of Pax6 to an isoform required for activation of essential α cell genes. Consistently, deletion of Paupar in mice resulted in dysregulation of PAX6 α cell target genes and corresponding α cell dysfunction, including blunted glucagon secretion. These findings illustrate a distinct mechanism by which a pancreatic lncRNA can coordinate glucose homeostasis by cell-specific regulation of a broadly expressed transcription factor.

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.

Functional Urate-Associated Genetic Variants Influence Expression of lincRNAs LINC01229 and MAFTRR

Genetic variation in the genomic regulatory landscape likely plays a crucial role in the pathology of disease. Non-coding variants associated with disease can influence the expression of long intergenic non-coding RNAs (lincRNAs), which in turn function in the control of protein-coding gene expression. Here, we investigate the function of two independent serum urate-associated signals (SUA1 and SUA2) in close proximity to lincRNAs and an enhancer that reside ∼60 kb and ∼300 kb upstream of MAF, respectively. Variants within SUA1 are expression quantitative trait loci (eQTL) for LINC01229 and MAFTRR, both co-expressed with MAF. We have also identified that variants within SUA1 are trans-eQTL for genes that are active in kidney- and serum urate-relevant pathways. Serum urate-associated variants rs4077450 and rs4077451 within SUA2 lie within an enhancer that recruits the transcription factor HNF4α and forms long range interactions with LINC01229 and MAFTRR. The urate-raising alleles of rs4077450 and rs4077451 increase enhancer activity and associate with increased expression of LINC01229. We show that the SUA2 enhancer region drives expression in the zebrafish pronephros, recapitulating endogenous MAF expression. Depletion of MAFTRR and LINC01229 in HEK293 cells in turn lead to increased MAF expression. Collectively, our results are consistent with serum urate variants mediating long-range transcriptional regulation of the lincRNAs LINC01229 and MAFTRR and urate relevant genes (e.g., SLC5A8 and EHHADH) in trans.

miR-132-3p is a positive regulator of alpha-cell mass and is downregulated in obese hyperglycemic mice

Objective

Diabetes is a complex disease implicating several organs and cell types. Within the islets, dysregulation occurs in both alpha- and beta-cells, leading to defects of insulin secretion and increased glucagon secretion. Dysregulation of alpha-cells is associated with transcriptome changes. We hypothesized that microRNAs (miRNAs) which are negative regulators of mRNA stability and translation could be involved in alpha-cell alterations or adaptations during type 2 diabetes.

Methods

miRNA microarray analyses were performed on pure alpha- and beta-cells from high-fat diet fed obese hyperglycemic mice and low-fat diet fed controls. Then, the most regulated miRNA was overexpressed or inhibited in primary culture of mouse and human alpha-cells to determine its molecular and functional impact.

Results

16 miRNAs were significantly regulated in alpha-cells of obese hyperglycemic mice and 28 in beta-cells. miR-132-3p had the strongest regulation level in alpha-cells, where it was downregulated, while we observed an opposite upregulation in beta-cells. In vitro experiments showed that miR-132-3p, which is inversely regulated by somatostatin and cAMP, is a positive modulator of alpha-cell proliferation and implicated in their resistance to apoptosis. These effects are associated with the regulation of a series of genes, including proliferation and stress markers Mki67 and Bbc3 in mouse and human alpha-cells, potentially involved in miR-132-3p functions.

Conclusions

Downregulation of miR-132-3p in alpha-cells of obese diabetic mice may constitute a compensatory mechanism contributing to keep glucagon-producing cell number constant in diabetes.

•

Alpha- and beta-cells present specific microRNA signatures.

•

16 microRNAs are significantly regulated in alpha-cells of obese hyperglycemic mice.

•

miR-132-3p is downregulated in alpha-cells of obese hyperglycemic mice.

•

miR-132-3p stimulates alpha-cells proliferation and resistance to apoptosis.

•

miR-132-3p is regulated by somatostatin in alpha-cells.

MafB Is Critical for Glucagon Production and Secretion in Mouse Pancreatic α Cells In Vivo

The MafB transcription factor is expressed in pancreatic α and β cells during development but becomes exclusive to α cells in adult rodents. Mafb-null (Mafb^-/- ) mice were reported to have reduced α- and β-cell numbers throughout embryonic development. To further analyze the postnatal function of MafB in the pancreas, we generated endocrine cell-specific (Mafb^ΔEndo ) and tamoxifen-dependent (Mafb^ΔTAM ) Mafb knockout mice. Mafb^ΔEndo mice exhibited reduced populations of insulin-positive (insulin⁺) and glucagon⁺ cells at postnatal day 0, but the insulin⁺ cell population recovered by 8 weeks of age. In contrast, the Arx⁺ glucagon⁺ cell fraction and glucagon expression remained decreased even in adulthood. Mafb^ΔTAM mice, with Mafb deleted after pancreas maturation, also demonstrated diminished glucagon⁺ cells and glucagon content without affecting β cells. A decreased Arx⁺ glucagon⁺ cell population in Mafb^ΔEndo mice was compensated for by an increased Arx⁺ pancreatic polypeptide⁺ cell population. Furthermore, gene expression analyses from both Mafb^ΔEndo and Mafb^ΔTAM islets revealed that MafB is a key regulator of glucagon expression in α cells. Finally, both mutants failed to respond to arginine, likely due to impaired arginine transporter gene expression and glucagon production ability. Taken together, our findings reveal that MafB is critical for the functional maintenance of mouse α cells in vivo, including glucagon production and secretion, as well as in development.

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 Homeostasis: Regulation by Peripheral Circadian Clocks in Rodents and Humans

Most organisms, including humans, have developed an intrinsic system of circadian oscillators, allowing the anticipation of events related to the rotation of Earth around its own axis. The mammalian circadian timing system orchestrates nearly all aspects of physiology and behavior. Together with systemic signals, emanating from the central clock that resides in the hypothalamus, peripheral oscillators orchestrate tissue-specific fluctuations in gene expression, protein synthesis, and posttranslational modifications, driving overt rhythms in physiology and behavior. There is increasing evidence on the essential roles of the peripheral oscillators, operative in metabolically active organs in the regulation of body glucose homeostasis. Here, we review some recent findings on the molecular and cellular makeup of the circadian timing system and its implications in the temporal coordination of metabolism in health and disease.

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.

α-Cell Dysfunctions and Molecular Alterations in Male Insulinopenic Diabetic Mice Are Not Completely Corrected by Insulin

Glucagon and α-cell dysfunction are critical in the development of hyperglycemia during diabetes both in humans and rodents. We hypothesized that α-cell dysfunction leading to dysregulated glucagon secretion in diabetes is due to both a lack of insulin and intrinsic defects. To characterize α-cell dysfunction in diabetes, we used glucagon-Venus transgenic male mice and induced insulinopenic hyperglycemia by streptozotocin administration leading to alterations of glucagon secretion. We investigated the in vivo impact of insulinopenic hyperglycemia on glucagon-producing cells using FACS-sorted α-cells from control and diabetic mice. We demonstrate that increased glucagonemia in diabetic mice is mainly due to increases of glucagon release and biosynthesis per cell compared with controls without changes in α-cell mass. We identified genes coding for proteins involved in glucagon biosynthesis and secretion, α-cell differentiation, and potential stress markers such as the glucagon, Arx, MafB, cMaf, Brain4, Foxa1, Foxa3, HNF4α, TCF7L2, Glut1, Sglt2, Cav2.1, Cav2.2, Nav1.7, Kir6.2/Sur1, Pten, IR, NeuroD1, GPR40, and Sumo1 genes, which were abnormally regulated in diabetic mice. Importantly, insulin treatment partially corrected α-cell function and expression of genes coding for proglucagon, or involved in glucagon secretion, glucose transport and insulin signaling but not those coding for cMAF, FOXA1, and α-cell differentiation markers as well as GPR40, NEUROD1, CAV2.1, and SUMO1. Our results indicate that insulinopenic diabetes induce marked α-cell dysfunction and molecular alteration, which are only partially corrected by in vivo insulin treatment.

Multiple functions of Maf in the regulation of cellular development and differentiation

Cellular muscular aponeurotic fibrosarcoma (c-Maf) is a member of the large macrophage-activating factor family. C-Maf plays important roles in the morphogenetic processes and cellular differentiation of the lens, kidneys, liver, T cells and nervous system, and it is particularly important in pancreatic islet and erythroblastic island formation. However, the exact role of c-Maf remains to be elucidated. In this review, we summarize the research to clarify the functions of c-Maf in the cellular development and differentiation. The expression of c-Maf is higher in pancreatic duct cells than in pancreatic islet cells. Therefore, we suggest that pancreatic duct cells may be converted to the functional insulin-secreting cells by regulating c-Maf.

DICER Inactivation Identifies Pancreatic β-Cell "Disallowed" Genes Targeted by MicroRNAs

Pancreatic β-cells are the body's sole source of circulating insulin and essential for the maintenance of blood glucose homeostasis. Levels of up to 66 “disallowed” genes, which are strongly expressed and play housekeeping roles in most other mammalian tissues, are unusually low in β-cells. The molecular mechanisms involved in repressing these genes are largely unknown. Here, we explore the role in gene disallowance of microRNAs (miRNAs), a type of small noncoding RNAs that silence gene expression at the posttranscriptional level and are essential for β-cell development and function. To selectively deplete miRNAs from adult β-cells, the miRNA-processing enzyme DICER was inactivated by deletion of the RNase III domain with a tamoxifen-inducible Pdx1CreER transgene. In this model, β-cell dysfunction was apparent 2 weeks after recombination and preceded a decrease in insulin content and loss of β-cell mass. Of the 14 disallowed genes studied, quantitative RT-quantitative real-time PCR revealed that 6 genes (Fcgrt, Igfbp4, Maf, Oat, Pdgfra, and Slc16a1) were up-regulated (1.4- to 2.1-fold, P < .05) at this early stage. Expression of luciferase constructs bearing the 3′-untranslated regions of the corresponding mRNAs in wild-type or DICER-null β-cells demonstrated that Fcgrt, Oat, and Pdgfra are miRNA direct targets. We thus reveal a role for miRNAs in the regulation of disallowed genes in β-cells and provide evidence for a novel means through which noncoding RNAs control the functional identity of these cells independently of actions on β-cell mass.

Role of large MAF transcription factors in the mouse endocrine pancreas

The members of the MAF family of transcription factors are homologs of v-Maf –the oncogenic component of the avian retrovirus AS42. The MAF family is subdivided into 2 groups, small and large MAFs. To elucidate the role of the large MAF transcription factors in the endocrine pancreas, we analyzed large MAF gene knockout mice. It has been shown that Mafa^−/− mice develop phenotypes including abnormal islet structure soon after birth. This study revealed that Ins1 and Ins2 transcripts and the protein contents were significantly reduced in Mafa^−/− mice at embryonic day 18.5. In addition, Mafa^−/−;Mafb^−/− mice contained less than 10% of the insulin transcript and protein of those of wild-type mice, suggesting that Mafa and Mafb cooperate to maintain insulin levels at the embryonic stage. On the other hand, the number of insulin-positive cells in Mafa^−/− mice was comparable to that of wild-type mice, and even under a Mafb-deficient background the number of insulin-positive cells was not decreased, suggesting that Mafb plays a dominant role in embryonic β-cell development. We also found that at 20 weeks of age Mafa^−/−;Mafb^+/− mice showed a higher fasting blood glucose level than single Mafa^−/− mice. In summary, our results indicate that Mafa is necessary for the maintenance of normal insulin levels even in embryos and that Mafb is important for the maintenance of fasting blood glucose levels in the Mafa-deficient background in adults.

Role of transcription factors in the transdifferentiation of pancreatic islet cells

The α and β cells act in concert to maintain blood glucose. The α cells release glucagon in response to low levels of glucose to stimulate glycogenolysis in the liver. In contrast, β cells release insulin in response to elevated levels of glucose to stimulate peripheral glucose disposal. Despite these opposing roles in glucose homeostasis, α and β cells are derived from a common progenitor and share many proteins important for glucose sensing and hormone secretion. Results from recent work have underlined these similarities between the two cell types by revealing that β-to-α as well as α-to-β transdifferentiation can take place under certain experimental circumstances. These exciting findings highlight unexpected plasticity of adult islets and offer hope of novel therapeutic paths to replenish β cells in diabetes. In this review, we focus on the transcription factor networks that establish and maintain pancreatic endocrine cell identity and how they may be perturbed to facilitate transdifferentiation.

Circulating microRNAs in Diabetes Progression: Discovery, Validation, and Research Translation

Diabetes, in all of its forms, is a disease state that demonstrates wide ranging pathological effects throughout the body. Until now, the only method of diagnosing and monitoring the progression of diabetes was through the measurement of blood glucose. Unfortunately, beta cell dysfunction initiates well before the clinical onset of diabetes, and so the development of an effective biomarker signature is of paramount importance to predict and monitor the progression of this disease. MicroRNAs (miRNAs/miRs) are small (18-22 nucleotide) noncoding (nc)RNAs that post-transcriptionally regulate endogenous gene expression by targeted inhibition or degradation of messenger (m)RNA. Recently, miRNAs have shown great promise as biomarkers as some exhibit differential expression in multiple disease states, including type 1 and type 2 diabetes (T1D/T2D). Furthermore, miRNAs are quite stable in circulation, resistant to freeze-thaw and pH-mediated degradation, and are relatively easy to detect using quantitative (q)PCR. Here, we discuss microRNAs that may form a diabetes biomarker signature. To identify these transcripts we outline miRNAs that play a central role in pancreas development and diabetes, as well as previously identified miRNAs with differential expression in individuals with T1D and T2D. Validation and refinement of a miRNA biomarker signature for diabetes would allow identification and intervention of individuals at risk of this disease, as well as stratification and monitoring of patients with established diabetes.

Foxa1 and Foxa2 regulate α-cell differentiation, glucagon biosynthesis, and secretion

The Forkhead box A transcription factors are major regulators of glucose homeostasis. They show both distinct and redundant roles during pancreas development and in adult mouse β-cells. In vivo ablation studies have revealed critical implications of Foxa1 on glucagon biosynthesis and requirement of Foxa2 in α-cell terminal differentiation. In order to examine the respective role of these factors in mature α-cells, we used small interfering RNA (siRNA) directed against Foxa1 and Foxa2 in rat primary pancreatic α-cells and rodent α-cell lines leading to marked decreases in Foxa1 and Foxa2 mRNA levels and proteins. Both Foxa1 and Foxa2 control glucagon gene expression specifically through the G2 element. Although we found that Foxa2 controls the expression of the glucagon, MafB, Pou3f4, Pcsk2, Nkx2.2, Kir6.2, and Sur1 genes, Foxa1 only regulates glucagon gene expression. Interestingly, the Isl1 and Gipr genes were not controlled by either Foxa1 or Foxa2 alone but by their combination. Foxa1 and Foxa2 directly activate and bind the promoter region the Nkx2.2, Kir6.2 and Sur1, Gipr, Isl1, and Pou3f4 genes. We also demonstrated that glucagon secretion is affected by the combined effects of Foxa1 and Foxa2 but not by either one alone. Our results indicate that Foxa1 and Foxa2 control glucagon biosynthesis and secretion as well as α-cell differentiation with both common and unique target genes.

The L-cell isolation from heterogonous population of intestinal cell line using antibiotic selection method

Enteroendocrine cells are the largest population of hormone-producing cells in the body and play important roles in many aspects of body functions. The enteroendocrine cell population is divided into different subpopulations that secrete different hormones and peptides. Characterization of each subpopulation is particularly useful for analyzing the cellular mechanisms responsible for specific cell types. Therefore, the necessity of a pure cell line for a specific study purpose was the important motivation for the separation of cell lines for each subpopulation of enteroendocrine cells. The present research introduces a method for the isolation of L-cells, one of the important subpopulations of enteroendocrine cells. The antibiotic selection method was conducted in order to isolate the L-cells from a heterogonous population of intestinal cell line. In this method, a neomycin resistance gene (as selected marker) was expressed under the control of a specific promoter of L-cells. After transfection of manipulated plasmid, only the cells which determine the specific promoter and express neomycin resistance protein would be able to survive under Geneticin antibiotic treatment condition. In order to confirm that the isolated cells were L-cells, reverse transcriptase polymerase chain reaction (PCR) and quantitative PCR assays were performed. Based on the results, the isolated cells were pure L-cells that could be able to express specific mRNA of L-cells efficiently. This technique provides a unique method for the isolation and purification of any cell line. The purified isolated L-cells by this method can be used for future studies and for analyzing cellular mechanisms that involve L-cells' functions.

Glucagon: the renewal of an old hormone in the pathophysiology of diabetes

Type 2 diabetes (T2D) is one of the most common diseases, affecting 5-10% of the population in most countries; the progression of its prevalence has been constant over the past 50 years in all countries worldwide, creating a major public health problem in terms of disease management and financial burden. Although the pathophysiology of T2D has been attributed for decades to insulin resistance and decreased insulin secretion, particularly in response to glucose, the contributing role of glucagon in hyperglycemia has been highlighted since the early 1970s by demonstrating its glycogenolytic, gluconeogenic and ketogenic properties. More recently, the importance of glucagon in diabetes has been highlighted in a model of streptozotocin-induced diabetic mice becoming euglycemic in the absence of glucagon receptors and without insulin treatment. Understanding the dysregulation of α-cells in diabetes will be critical to better define the pathophysiology of diabetes and develop new antidiabetic treatment.

Microarray analysis of novel candidate genes responsible for glucose-stimulated insulin secretion in mouse pancreatic β cell line MIN6

Elucidating the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic islet β cells is important for understanding and treating diabetes. MIN6 cells, a transformed β-cell line derived from a mouse insulinoma, retain GSIS and are a popular in vitro model for insulin secretion. However, in long-term culture, MIN6 cells' GSIS capacity is lost. We previously isolated a subclone, MIN6 clone 4, from the parental MIN6 cells, that shows well-regulated insulin secretion in response to glucose, glybenclamide, and KCl, even after prolonged culture. To investigate the molecular mechanisms responsible for preserving GSIS in this subclone, we compared four groups of MIN6 cells: Pr-LP (parental MIN6, low passage number), Pr-HP (parental MIN6, high passage number), C4-LP (MIN6 clone 4, low passage number), and C4-HP (MIN6 clone 4, high passage number). Based on their capacity for GSIS, we designated the Pr-LP, C4-LP, and C4-HP cells as “responder cells.” In a DNA microarray analysis, we identified a group of genes with high expression in responder cells (“responder genes”), but extremely low expression in the Pr-HP cells. Another group of genes (“non-responder genes”) was expressed at high levels in the Pr-HP cells, but at extremely low levels in the responder cells. Some of the responder genes were involved in secretory machinery or glucose metabolism, including Chrebp, Scgn, and Syt7. Among the non-responder genes were Car2, Maf, and Gcg, which are not normally expressed in islet β cells. Interestingly, we found a disproportionate number of known imprinted genes among the responder genes. Our findings suggest that the global expression profiling of GSIS-competent and GSIS-incompetent MIN6 cells will help delineate the gene regulatory networks for insulin secretion.

MicroRNA expression in alpha and beta cells of human pancreatic islets

microRNAs (miRNAs) play an important role in pancreatic development and adult β-cell physiology. Our hypothesis is based on the assumption that each islet cell type has a specific pattern of miRNA expression. We sought to determine the profile of miRNA expression in α-and β-cells, the main components of pancreatic islets, because this analysis may lead to a better understanding of islet gene regulatory pathways. Highly enriched (>98%) subsets of human α-and β-cells were obtained by flow cytometric sorting after intracellular staining with c-peptide and glucagon antibody. The method of sorting based on intracellular staining is possible because miRNAs are stable after fixation. MiRNA expression levels were determined by quantitative high throughput PCR-based miRNA array platform screening. Most of the miRNAs were preferentially expressed in β-cells. From the total of 667 miRNAs screened, the Significant Analysis of Microarray identified 141 miRNAs, of which only 7 were expressed more in α-cells (α-miRNAs) and 134 were expressed more in β-cells (β-miRNAs). Bioinformatic analysis identified potential targets of β-miRNAs analyzing the Beta Cell Gene Atlas, described in the T1Dbase, the web platform, supporting the type 1 diabetes (T1D) community. cMaf, a transcription factor regulating glucagon expression expressed selectively in α-cells (TFα) is targeted by β-miRNAs; miR-200c, miR-125b and miR-182. Min6 cells treated with inhibitors of these miRNAs show an increased expression of cMaf RNA. Conversely, over expression of miR-200c, miR-125b or miR-182 in the mouse alpha cell line αTC6 decreases the level of cMAF mRNA and protein. MiR-200c also inhibits the expression of Zfpm2, a TFα that inhibits the PI3K signaling pathway, at both RNA and protein levels.In conclusion, we identified miRNAs differentially expressed in pancreatic α- and β-cells and their potential transcription factor targets that could add new insights into different aspects of islet biology and pathophysiology.

A novel DNA binding mechanism for maf basic region-leucine zipper factors inferred from a MafA-DNA complex structure and binding specificities

MafA is a proto-oncoprotein and is critical for insulin gene expression in pancreatic β-cells. Maf proteins belong to the AP1 superfamily of basic region-leucine zipper (bZIP) transcription factors. Residues in the basic helix and an ancillary N-terminal domain, the Extended Homology Region (EHR), endow maf proteins with unique DNA binding properties: binding a 13 bp consensus site consisting of a core AP1 site (TGACTCA) flanked by TGC sequences and binding DNA stably as monomers. To further characterize maf DNA binding, we determined the structure of a MafA-DNA complex. MafA forms base-specific hydrogen bonds with the flanking G(-5)C(-4) and central C(0)/G(0) bases, but not with the core-TGA bases. However, in vitro binding studies utilizing a pulse-chase electrophoretic mobility shift assay protocol revealed that mutating either the core-TGA or flanking-TGC bases dramatically increases the binding off rate. Comparing the known maf structures, we propose that DNA binding specificity results from positioning the basic helix through unique phosphate contacts. The EHR does not contact DNA directly but stabilizes DNA binding by contacting the basic helix. Collectively, these results suggest a novel multistep DNA binding process involving a conformational change from contacting the core-TGA to contacting the flanking-TGC bases.

Evaluation of insulin expression and secretion in genetically engineered gut K and L-cells

Background

Gene therapy could provide an effective treatment of diabetes. Previous studies have investigated the potential for several cell and tissue types to produce mature and active insulin. Gut K and L-cells could be potential candidate hosts for gene therapy because of their special features.

Results

In this study, we isolated gut K and L-cells to compare the potential of both cell types to produce insulin when exposed to similar conditions. The isolated pure K and L-cells were transfected with recombinant plasmids encoding insulin and with specific promoters for K or L-cells. Insulin expression was studied in response to glucose or meat hydrolysate. We found that glucose and meat hydrolysate efficiently induced insulin secretion from K and L-cells. However, the effects of meat hydrolysate on insulin secretion were more potent in both cells compared with glucose. Results of enzyme-linked immunosorbent assays showed that L-cells secreted more insulin compared with K-cells regardless of the stimulator, although this difference was not statistically significant.

Conclusion

The responses of K and L-cells to stimulation with glucose or meat hydrolysate were generally comparable. Therefore, both K and L-cells show similar potential to be used as surrogate cells for insulin gene expression in vitro. The potential use of these cells for diabetic gene therapy warrants further investigation.

Pax6 is a key component of regulated glucagon secretion

The Pax6 transcription factor is crucial for pancreatic α-cells. Indeed, Pax6-deficient mouse models are characterized by markedly altered α-cell differentiation. Our objective was to investigate the role of Pax6 in glucagon secretion process. We used a Pax6-deficient model in rat primary enriched-α cells with specific small interfering RNA leading to a 70% knockdown of Pax6 expression. We first showed that Pax6 knockdown decreases glucagon biosynthesis as well as glucagon release. Through physiological assays, we demonstrated that the decrease of Pax6 affects specifically acute glucagon secretion in primary α-cell in response to glucose, palmitate, and glucose-dependent insulinotropic peptide (GIP) but not the response to arginine and epinephrine. We identified in Pax6 knockdown model that genes involved in glucagon secretion such as the glucokinase (GCK), G protein-coupled receptor (GPR40), and GIP receptor (GIPR) as well as the corresponding proteins were significantly decreased whereas the insulin receptor (IR) Kir6.2/Sur1, and glucose transporter 1 genes were not affected. We demonstrated that Pax6 directly binds and activates specific elements on the promoter region of the GPR40, GCK, and GIPR genes. Finally, through site-directed mutagenesis experiments, we showed that disruption of Pax6 binding on the GCK, GPR40, and GIPR gene promoters led to specific decreases of their activities in the αTC1.9 glucagon-producing cell line. Hence our results indicate that Pax6 acts on the regulation of glucagon secretion at least through the transcriptional control of GCK, GPR40, and GIPR. We propose that Pax6 is not only critical for glucagon biosynthesis but also for glucagon secretion particularly in response to nutrients.

Catalase polymorphisms and metabolic diseases

PURPOSE OF REVIEW

This review summarizes our current understanding of the implication of catalase polymorphisms in the occurrence, control and comorbidities of metabolic diseases.

RECENT FINDINGS

Whatever impaired glucose tolerance, insulin resistance on diabetes and whatever their occurrence or implications, the studies taken together converge toward the hypothesis that catalase polymorphisms play a role in glucose disorders. -262C/T and -844A>G single nucleotide polymorphisms are associated to hypertension susceptibility and/or onset. Concerning dyslipidemia, very recent studies requiring confirmation report a -262C/T implication. Finally, a role of catalase polymorphisms in bone metabolism is described.

SUMMARY

Plethora of studies on catalase SNPs and their link with diseases exist. It is now clear that genetic variations in the catalase gene and its promoter are putative risk factors for metabolic disease. The question of how these polymorphisms actively play a role in various metabolisms remains unanswered. Further functional studies are required in order to gain a deeper insight into the direct role of catalase.

Pax6 is crucial for β-cell function, insulin biosynthesis, and glucose-induced insulin secretion

The Pax6 transcription factor is crucial for endocrine cell differentiation and function. Indeed, mutations of Pax6 are associated with a diabetic phenotype and a drastic decrease of insulin-positive cell number. Our aim was to better define the β-cell Pax6 transcriptional network and thus provide further information concerning the role of Pax6 in β-cell function. We developed a Pax6-deficient model in rat primary β-cells with specific small interfering RNA leading to a 75% knockdown of Pax6 expression. Through candidate gene approach, we confirmed that Pax6 controls the mRNA levels of the insulin 1 and 2, Pdx1, MafA, GLUT2, and PC1/3 genes in β-cells. Importantly, we identified new Pax6 target genes coding for GK, Nkx6.1, cMaf, PC2, GLP-1R and GIPR which are all involved in β-cell function. Furthermore, we demonstrated that Pax6 directly binds and activates specific elements on the promoter region of these genes. We also demonstrated that Pax6 knockdown led to decreases in insulin cell content, in insulin processing, and a specific defect of glucose-induced insulin secretion as well as a significant reduction of GLP-1 action in primary β-cells. Our results strongly suggest that Pax6 is crucial for β-cells through transcriptional control of key genes coding for proteins that are involved in insulin biosynthesis and secretion as well as glucose and incretin actions on β-cells. We provide further evidence that Pax6 represents a key element of mature β-cell function.

Functional redundancy of two Pax-like proteins in transcriptional activation of cyst wall protein genes in Giardia lamblia

The protozoan Giardia lamblia differentiates from a pathogenic trophozoite into an infectious cyst to survive outside of the host. During encystation, genes encoding cyst wall proteins (CWPs) are coordinately induced. Pax family transcription factors are involved in a variety of developmental processes in animals. Nine Pax proteins have been found to play an important role in tissue and organ development in humans. To understand the progression from primitive to more complex eukaryotic cells, we tried to identify putative pax genes in the G. lamblia genome and found two genes, pax1 and pax2, with limited similarity. We found that Pax1 may transactivate the encystation-induced cwp genes and interact with AT-rich initiatior elements that are essential for promoter activity and transcription start site selection. In this study, we further characterized Pax2 and found that, like Pax1, Pax2 was present in Giardia nuclei and it may specifically bind to the AT-rich initiator elements of the encystation-induced cwp1-3 and myb2 genes. Interestingly, overexpression of Pax2 increased the cwp1-3 and myb2 gene expression and cyst formation. Deletion of the C-terminal paired domain or mutation of the basic amino acids of the paired domain resulted in a decrease of nuclear localization, DNA-binding activity, and transactivation activity of Pax2. These results are similar to those found in the previous Pax1 study. In addition, the profiles of gene expression in the Pax2 and Pax1 overexpressing cells significantly overlap in the same direction and ERK1 associated complexes may phosphorylate Pax2 and Pax1, suggesting that Pax2 and Pax1 may be downstream components of a MAPK/ERK1 signaling pathway. Our results reveal functional redundancy between Pax2 and Pax1 in up-regulation of the key encystation-induced genes. These results illustrate functional redundancy of a gene family can occur in order to increase maintenance of important gene function in the protozoan organism G. lamblia.

Engineering an L-cell line that expresses insulin under the control of the glucagon-like peptide-1 promoter for diabetes treatment

Background

Diabetes mellitus is a complicated disease with a pathophysiology that includes hyperinsulinemia, hyperglycemia and other metabolic impairments leading to many clinical complications. It is necessary to develop appropriate treatments to manage the disease and reduce possible acute and chronic side effects. The advent of gene therapy has generated excitement in the medical world for the possible application of gene therapy in the treatment of diabetes. The glucagon-like peptide-1 (GLP-1) promoter, which is recognised by gut L-cells, is an appealing candidate for gene therapy purposes. The specific properties of L-cells suggest that L-cells and the GLP-1 promoter would be useful for diabetes therapy approaches.

Results

In this study, L-cells were isolated from a primary intestinal cell line to create suitable target cells for insulin expression studies. The isolated cells displayed L-cell properties and were therefore used as an L-cell surrogate. Next, the isolated L-cells were transfected with the recombinant plasmid consisting of an insulin gene located downstream of the GLP-1 promoter. The secretion tests revealed that an increase in glucose concentration from 5 mM to 25 mM induced insulin gene expression in the L-cells by 2.7-fold. Furthermore, L-cells quickly responded to the glucose stimulation; the amount of insulin protein increased 2-fold in the first 30 minutes and then reached a plateau after 90 minutes.

Conclusion

Our data showed that L-cells efficiently produced the mature insulin protein. In addition, the insulin protein secretion was positively regulated with glucose induction. In conclusion, GLP-1 promoter and L-cell could be potential candidates for diabetes gene therapy agents.

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.

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.

A novel pax-like protein involved in transcriptional activation of cyst wall protein genes in Giardia lamblia

Giardia lamblia differentiates into infectious cysts to survive outside of the host. It is of interest to identify factors involved in up-regulation of cyst wall proteins (CWPs) during this differentiation. Pax proteins are important regulators of development and cell differentiation in Drosophila and vertebrates. No member of this gene family has been reported to date in yeast, plants, or protozoan parasites. We have identified a pax-like gene (pax1) encoding a putative paired domain in the G. lamblia genome. Epitope-tagged Pax1 localized to nuclei during both vegetative growth and encystation. Recombinant Pax1 specifically bound to the AT-rich initiator elements of the encystation-induced cwp1 to -3 and myb2 genes. Interestingly, overexpression of Pax1 increased cwp1 to -3 and myb2 gene expression and cyst formation. Deletion of the C-terminal paired domain or mutation of the basic amino acids of the paired domain resulted in a decrease of the transactivation function of Pax1. Our results indicate that the Pax family has been conserved during evolution, and Pax1 could up-regulate the key encystation-induced genes to regulate differentiation of the protozoan eukaryote, G. lamblia.

Pax6 controls the expression of critical genes involved in pancreatic {alpha} cell differentiation and function

The paired box homeodomain Pax6 is crucial for endocrine cell development and function and plays an essential role in glucose homeostasis. Indeed, mutations of Pax6 are associated with diabetic phenotype. Importantly, homozygous mutant mice for Pax6 are characterized by markedly decreased β and δ cells and absent α cells. To better understand the critical role that Pax6 exerts in glucagon-producing cells, we developed a model of primary rat α cells. To study the transcriptional network of Pax6 in adult and differentiated α cells, we generated Pax6-deficient primary rat α cells and glucagon-producing cells, using either specific siRNA or cells expressing constitutively a dominant-negative form of Pax6. In primary rat α cells, we confirm that Pax6 controls the transcription of the Proglucagon and processing enzyme PC2 genes and identify three new target genes coding for MafB, cMaf, and NeuroD1/Beta2, which are all critical for Glucagon gene transcription and α cell differentiation. Furthermore, we demonstrate that Pax6 directly binds and activates the promoter region of the three genes through specific binding sites and that constitutive expression of a dominant-negative form of Pax6 in glucagon-producing cells (InR1G9) inhibits the activities of the promoters. Finally our results suggest that the critical role of Pax6 action on α cell differentiation is independent of those of Arx and Foxa2, two transcription factors that are necessary for α cell development. We conclude that Pax6 is critical for α cell function and differentiation through the transcriptional control of key genes involved in glucagon gene transcription, proglucagon processing, and α cell differentiation.

Relative roles of the different Pax6 domains for pancreatic alpha cell development

Background

The transcription factor Pax6 functions in the specification and maintenance of the differentiated cell lineages in the endocrine pancreas. It has two DNA binding domains, the paired domain and the homeodomain, in addition to a C-terminal transactivation domain. The phenotype of Pax6^-/- knockout mice suggests non-redundant functions of the transcription factor in the development of glucagon-expressing α-cells as this cell type is absent in the mutants. We ask the question of how the differentiation of pancreatic endocrine cells, in particular that of α-cells, is affected by selective inactivation of either one of the three major domains of Pax6.

Results

The Pax6^Aey18 mutant mouse line, in which the paired domain is inactivated, showed a phenotype similar to that of Pax6^-/- knockout mice with a near complete absence of glucagon-positive α-cells (0-4 cells/section; ≤1% of wt), reduced β-cell area (74% of wt) and disorganized islets. The proportion of ghrelin-positive ε-cells was expanded. In Pax6^Sey-Neu mutants, which lack the transactivation domain, α-and β-cells where reduced to 25 and 40% of wt, respectively. We also studied two mouse lines with mutations in the homeodomain, Pax6^4Neu and Pax6^132-14Neu. Neighboring amino acids are affected in the two lines and both point mutations abolish DNA binding of the classical P3 homeodomain target sequence. The pancreatic phenotype of the two mutants however was divergent. While Pax6^4Neu homozygotes showed a reduction of α- and β-cells to 59 and 61%, respectively, pancreatic endocrine development was unaltered in the Pax6^132-14Neu mutant strain.

Conclusions

We show that inactivation of the Pax6 paired domain leads to a more severe phenotype with regards to the differentiation of pancreatic α-cells than the loss of the transactivation domain. The analysis of two different homeodomain mutants suggests that the binding of Pax6 to P3 homeodomain consensus sequences is not required for α-cell development. It rather seems that the homeodomain has a modulating role in Pax6 function, possibly by facilitating a PH0-like binding confirmation on paired domain target genes like proglucagon. This function is differentially affected by the two homeodomain mutations analyzed in this study.

A genome-wide association study identifies susceptibility variants for type 2 diabetes in Han Chinese

To investigate the underlying mechanisms of T2D pathogenesis, we looked for diabetes susceptibility genes that increase the risk of type 2 diabetes (T2D) in a Han Chinese population. A two-stage genome-wide association (GWA) study was conducted, in which 995 patients and 894 controls were genotyped using the Illumina HumanHap550-Duo BeadChip for the first genome scan stage. This was further replicated in 1,803 patients and 1,473 controls in stage 2. We found two loci not previously associated with diabetes susceptibility in and around the genes protein tyrosine phosphatase receptor type D (PTPRD) (P = 8.54×10⁻¹⁰; odds ratio [OR] = 1.57; 95% confidence interval [CI] = 1.36–1.82), and serine racemase (SRR) (P = 3.06×10⁻⁹; OR = 1.28; 95% CI = 1.18–1.39). We also confirmed that variants in KCNQ1 were associated with T2D risk, with the strongest signal at rs2237895 (P = 9.65×10⁻¹⁰; OR = 1.29, 95% CI = 1.19–1.40). By identifying two novel genetic susceptibility loci in a Han Chinese population and confirming the involvement of KCNQ1, which was previously reported to be associated with T2D in Japanese and European descent populations, our results may lead to a better understanding of differences in the molecular pathogenesis of T2D among various populations.

Type 2 diabetes (T2D) is a complex disease that involves many genes and environmental factors. Genome-wide and candidate-gene association studies have thus far identified at least 19 regions containing genes that may confer a risk for T2D. However, most of these studies were conducted with patients of European descent. We studied Chinese patients with T2D and identified two genes, PTPRD and SRR, that were not previously known to be involved in diabetes and are involved in biological pathways different from those implicated in T2D by previous association reports. PTPRD is a protein tyrosine phosphatase and may affect insulin signaling on its target cells. SRR encodes a serine racemase that synthesizes D-serine from L-serine. Both D-serine (coagonist) and the neurotransmitter glutamate bind to NMDA receptors and trigger excitatory neurotransmission in the brain. Glutamate signaling also regulates insulin and glucagon secretion in pancreatic islets. Thus, SRR and D-serine, in addition to regulating insulin and glucagon secretion, may play a role in the etiology of T2D. Our study suggests that, in different patient populations, different genes may confer risks for diabetes. Our findings may lead to a better understanding of the molecular pathogenesis of T2D.

Foxa1 and Foxa2 control the differentiation of goblet and enteroendocrine L- and D-cells in mice

BACKGROUND & AIMS

The winged helix transcription factors Foxa1 and Foxa2 are expressed in all epithelia of the gastrointestinal tract from its embryonic origin into adulthood. In vitro studies have shown that Foxa1/a2 can transactivate the promoters of Mucin 2 (Muc2), which is expressed in goblet cells, and of preproglucagon, which is expressed in enteroendocrine cells. These findings suggest Foxa1/a2 as critical factors in the differentiation of gut epithelial cells.

METHODS

Mice with intestine-specific simultaneous deletion of Foxa1 and Foxa2 were derived using the Cre-loxP system and analyzed using histologic and molecular means.

RESULTS

Both Foxa1 and Foxa2 were deleted successfully in the epithelia of the small intestine and colon using Villin-Cre mice. Immunohistochemical staining showed that Foxa1/a2 mutants lack glucagon-like peptide-1- and peptide-2-expressing cells (L-cells), and have reduced numbers of somatostatin (D-cells) and peptide YY-expressing cells (L-cells). Preproglucagon, somatostatin, and peptide YY messenger RNA (mRNA) levels also were reduced significantly in Foxa1/a2 mutants. Thus, Foxa1 and Foxa2 are essential regulators of these enteroendocrine lineages in vivo. The mRNA levels of transcription factors Islet-1 and Pax6 were reduced significantly in the small intestine, showing that Foxa1 and Foxa2 impact on a transcription factor network in the enteroendocrine lineage. In addition, deletion of Foxa1/a2 caused a reduction in goblet cell number with altered expression of the secretory mucins Muc2, Mucin5b, Mucin5ac, and Mucin 6.

CONCLUSIONS

The winged helix factors Foxa1 and Foxa2 are essential members of the transcription factor network that govern secretory cell differentiation in the mammalian gastrointestinal tract.

c-Maf increases apoptosis in peripheral CD8 cells by transactivating Caspase 6

In addition to transactivation of interleukin-4 (IL-4), cellular muscular aponeurotic fibrosarcoma (c-Maf) enhances CD4 cell apoptosis by limiting Bcl-2 expression. The CD8 cells also express c-Maf and peripheral CD8 cell numbers are reduced in c-Maf transgenic mice, suggesting that c-Maf may influence CD8 cell survival in a manner similar to CD4 cells. Here we confirm that, similar to CD4 cells, c-Maf enhances CD8 cell susceptibility to apoptosis induced by multiple stimuli, independent of IL-4. However, unlike CD4 cells, c-Maf enhancement of apoptosis is independent of Bcl-2, suggesting that c-Maf uses other mechanisms to regulate CD8 cell apoptosis. Real-time reverse transcription-polymerase chain reaction reveals that the pro-apoptotic gene Caspase 6 is upregulated in c-Maf transgenic CD8 cells, suggesting that Caspase 6 is a novel c-Maf target gene. Luciferase reporter assays and site-directed mutagenesis reveal a functional c-Maf recognition element (MARE) within the first intron of Caspase 6. Binding of c-Maf to the MARE site is detectable by chromatin immunoprecipitation using non-transgenic T-cell lysates, so c-Maf can interact with the Caspase 6 MARE site in normal T cells. Furthermore, caspase 6 activity is increased among CD8 cells from c-Maf transgenic mice following T-cell receptor engagement. As expected, activity of the downstream caspases 3 and 7 is also increased. Consistent with the ability of caspase 6 to participate in positive feedback loops, cytochrome c release and caspase 8 activation are also increased. Together these results indicated that c-Maf increases CD8 cell sensitivity to apoptotic stimuli, at least in part, by direct transactivation of Caspase 6, providing increased substrate for Caspase 6-dependent apoptosis pathways.

Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2

Pax6 is important in the development of the pancreas and was previously shown to regulate pancreatic endocrine differentiation, as well as the insulin, glucagon, and somatostatin genes. Prohormone convertase 2 (PC2) is the main processing enzyme in pancreatic alpha cells, where it processes proglucagon to produce glucagon under the spatial and temporal control of 7B2, which functions as a molecular chaperone. To investigate the role of Pax6 in glucagon biosynthesis, we studied potential target genes in InR1G9 alpha cells transfected with Pax6 small interfering RNA and in InR1G9 clones expressing a dominant-negative form of Pax6. We now report that Pax6 controls the expression of the PC2 and 7B2 genes. By binding and transactivation studies, we found that Pax6 indirectly regulates PC2 gene transcription through cMaf and Beta2/NeuroD1 while it activates the 7B2 gene both directly and indirectly through the same transcription factors, cMaf and Beta2/NeuroD1. We conclude that Pax6 is critical for glucagon biosynthesis and processing by directly and indirectly activating the glucagon gene through cMaf and Beta2/NeuroD1, as well as the PC2 and 7B2 genes.