G. da Silva Xavier

Ablation of AMP-activated protein kinase α1 and α2 from mouse pancreatic beta cells and RIP2.Cre neurons suppresses insulin release in vivo

Aims/hypothesisAMP-activated protein kinase (AMPK) is an evolutionarily conserved enzyme and a target of glucose-lowering agents, including metformin. However, the precise role or roles of the enzyme in controlling insulin secretion remain uncertain.MethodsThe catalytic α1 and α2 subunits of AMPK were ablated selectively in mouse pancreatic beta cells and hypothalamic neurons by breeding Ampkα1 [also known as Prkaa1]-knockout mice, bearing floxed Ampkα2 [also known as Prkaa2] alleles (Ampkα1−/−,α2fl/fl,), with mice expressing Cre recombinase under the rat insulin promoter (RIP2). RIP2 was used to express constitutively activated AMPK selectively in beta cells in transgenic mice. Food intake, body weight and urinary catecholamines were measured using metabolic cages. Glucose and insulin tolerance were determined after intraperitoneal injection. Beta cell mass and morphology were analysed by optical projection tomography and confocal immunofluorescence microscopy, respectively. Granule docking, insulin secretion, membrane potential and intracellular free Ca2+ were measured with standard techniques.ResultsTrigenic Ampkα1−/−,α2fl/fl expressing Cre recombinase and lacking both AMPKα subunits in the beta cell, displayed normal body weight and increased insulin sensitivity, but were profoundly insulin-deficient. Secreted catecholamine levels were unchanged. Total beta cell mass was unaltered, while mean islet and beta cell volume were reduced. AMPK-deficient beta cells displayed normal glucose-induced changes in membrane potential and intracellular free Ca2+, while granule docking and insulin secretion were enhanced. Conversely, βAMPK transgenic mice were glucose-intolerant and displayed defective insulin secretion.Conclusions/interpretationInhibition of AMPK activity within the beta cell is necessary, but not sufficient for stimulation of insulin secretion by glucose to occur. AMPK activation in extrapancreatic RIP2.Cre-expressing cells might also influence insulin secretion in vivo.

Increased expression of miR-187 in human islets from individuals with type 2 diabetes is associated with reduced glucose-stimulated insulin secretion

Aims/hypothesisType 2 diabetes is characterised by progressive beta cell dysfunction, with changes in gene expression playing a crucial role in its development. MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression and therefore alterations in miRNA levels may be involved in the deterioration of beta cell function.MethodsGlobal TaqMan arrays and individual TaqMan assays were used to measure islet miRNA expression in discovery (n = 20) and replication (n = 20) cohorts from individuals with and without type 2 diabetes. The role of specific dysregulated miRNAs in regulating insulin secretion, content and apoptosis was subsequently investigated in primary rat islets and INS-1 cells. Identification of miRNA targets was assessed using luciferase assays and by measuring mRNA levels.ResultsIn the discovery and replication cohorts miR-187 expression was found to be significantly increased in islets from individuals with type 2 diabetes compared with matched controls. An inverse correlation between miR-187 levels and glucose-stimulated insulin secretion (GSIS) was observed in islets from normoglycaemic donors. This correlation paralleled findings in primary rat islets and INS-1 cells where overexpression of miR-187 markedly decreased GSIS without affecting insulin content or apoptotic index. Finally, the gene encoding homeodomain-interacting protein kinase-3 (HIPK3), a known regulator of insulin secretion, was identified as a direct target of miR-187 and displayed reduced expression in islets from individuals with type 2 diabetes.Conclusions/interpretationOur findings suggest a role for miR-187 in the blunting of insulin secretion, potentially involving regulation of HIPK3, which occurs during the pathogenesis of type 2 diabetes.

An alternative polyadenylation signal in TCF7L2 generates isoforms that inhibit T cell factor/lymphoid-enhancer factor (TCF/LEF)-dependent target genes

Aims/hypothesisIntronic single nucleotide polymorphisms within the transcription factor 7-like 2 (TCF7L2) gene are associated with risk of type 2 diabetes. It is widely hypothesised that the predisposing variation is involved in cis-regulation of TCF7L2 activity. The aim of this study was to seek evidence for the existence of novel TCF7L2 isoforms encoded within the type 2 diabetes-associated genomic region.MethodsWe searched expressed sequence tag (EST) databases for novel TCF7L2 transcripts and sought to validate the function and integrity of any isoforms found using a combination of RT-PCR, western blotting and reporter gene techniques.ResultsAnalysis of EST databases suggested the presence of an alternative polyadenylation site located in intron 4 of TCF7L2. We used 3′ rapid amplification of cDNA ends and real-time PCR to validate the integrity of this polyadenylation signal and show its wide use across human tissues. Western blotting results are consistent with the use of this polyadenylation signal to generate novel protein isoforms. The alternative polyadenylation signal results in the production of isoforms that retain the β-catenin binding domain but do not possess the high-mobility group box DNA-binding domain. Promoter–reporter gene assays suggest that these isoforms inhibit TCF7L2-dependent target genes by sequestering β-catenin.Conclusions/interpretationWe have identified a novel polyadenylation signal within TCF7L2 that can result in the production of isoforms that act to repress TCF/LEF-dependent target genes. These findings may provide new insights into the association of TCF7L2 with susceptibility to type 2 diabetes.

Regulation by Per-Arnt-Sim (PAS) kinase of pancreatic duodenal homeobox-1 nuclear import in pancreatic β-cells

The transcription factor PDX-1 (pancreatic duodenal homeobox-1) is required for normal pancreatic development and for the function of insulin-producing islet beta-cells in mammals. We have shown previously that glucose regulates insulin gene expression in part through the activation and translocation of PDX-1 from the nuclear periphery to the nucleoplasm. We have also found that PASK [PAS (Per-Arnt-Sim) kinase], a member of the nutrient-regulated family of protein kinases, is activated in response to glucose challenge in beta-cells and is involved in the regulation of expression of PDX-1. Purified PASK efficiently phosphorylated recombinant PDX-1 in vitro on a single site (Thr-152). To determine the impact of phosphorylation at this site, we generated wild-type and mutant (T152A, T152D and T152E) forms of PDX-1 and examined the distribution of each of these in clonal MIN6 beta-cells by immunocytochemical analysis. Unexpectedly, only the T152D mutation significantly affected subcellular distribution, increasing the ratio of nuclear/cytosolic labelling at low and high glucose concentrations, suggesting that phosphorylation at Thr-152 inhibits nuclear uptake in response to glucose. Based on these results, experiments to examine the contribution of Thr-152 to the overall phosphorylation of PDX-1 in intact cells will be undertaken.

Ablation of AMPKα1 and α2 from pancreatic beta cells and RIP.Cre neurons suppresses insulin release in vivo

Aims/hypothesisAMP-activated protein kinase (AMPK) is an evolutionarily conserved enzyme and a target of glucose-lowering agents, including metformin. However, the precise role or roles of the enzyme in controlling insulin secretion remain uncertain.MethodsThe catalytic α1 and α2 subunits of AMPK were ablated selectively in mouse pancreatic beta cells and hypothalamic neurons by breeding Ampkα1 [also known as Prkaa1]-knockout mice, bearing floxed Ampkα2 [also known as Prkaa2] alleles (Ampkα1−/−,α2fl/fl,), with mice expressing Cre recombinase under the rat insulin promoter (RIP2). RIP2 was used to express constitutively activated AMPK selectively in beta cells in transgenic mice. Food intake, body weight and urinary catecholamines were measured using metabolic cages. Glucose and insulin tolerance were determined after intraperitoneal injection. Beta cell mass and morphology were analysed by optical projection tomography and confocal immunofluorescence microscopy, respectively. Granule docking, insulin secretion, membrane potential and intracellular free Ca2+ were measured with standard techniques.ResultsTrigenic Ampkα1−/−,α2fl/fl expressing Cre recombinase and lacking both AMPKα subunits in the beta cell, displayed normal body weight and increased insulin sensitivity, but were profoundly insulin-deficient. Secreted catecholamine levels were unchanged. Total beta cell mass was unaltered, while mean islet and beta cell volume were reduced. AMPK-deficient beta cells displayed normal glucose-induced changes in membrane potential and intracellular free Ca2+, while granule docking and insulin secretion were enhanced. Conversely, βAMPK transgenic mice were glucose-intolerant and displayed defective insulin secretion.Conclusions/interpretationInhibition of AMPK activity within the beta cell is necessary, but not sufficient for stimulation of insulin secretion by glucose to occur. AMPK activation in extrapancreatic RIP2.Cre-expressing cells might also influence insulin secretion in vivo.

Ablation of AMP-activated protein kinase α1 and α2 from mouse pancreatic beta cells and RIP2.Cre

Aims/hypothesisAMP-activated protein kinase (AMPK) is an evolutionarily conserved enzyme and a target of glucose-lowering agents, including metformin. However, the precise role or roles of the enzyme in controlling insulin secretion remain uncertain.MethodsThe catalytic α1 and α2 subunits of AMPK were ablated selectively in mouse pancreatic beta cells and hypothalamic neurons by breeding Ampkα1 [also known as Prkaa1]-knockout mice, bearing floxed Ampkα2 [also known as Prkaa2] alleles (Ampkα1−/−,α2fl/fl,), with mice expressing Cre recombinase under the rat insulin promoter (RIP2). RIP2 was used to express constitutively activated AMPK selectively in beta cells in transgenic mice. Food intake, body weight and urinary catecholamines were measured using metabolic cages. Glucose and insulin tolerance were determined after intraperitoneal injection. Beta cell mass and morphology were analysed by optical projection tomography and confocal immunofluorescence microscopy, respectively. Granule docking, insulin secretion, membrane potential and intracellular free Ca2+ were measured with standard techniques.ResultsTrigenic Ampkα1−/−,α2fl/fl expressing Cre recombinase and lacking both AMPKα subunits in the beta cell, displayed normal body weight and increased insulin sensitivity, but were profoundly insulin-deficient. Secreted catecholamine levels were unchanged. Total beta cell mass was unaltered, while mean islet and beta cell volume were reduced. AMPK-deficient beta cells displayed normal glucose-induced changes in membrane potential and intracellular free Ca2+, while granule docking and insulin secretion were enhanced. Conversely, βAMPK transgenic mice were glucose-intolerant and displayed defective insulin secretion.Conclusions/interpretationInhibition of AMPK activity within the beta cell is necessary, but not sufficient for stimulation of insulin secretion by glucose to occur. AMPK activation in extrapancreatic RIP2.Cre-expressing cells might also influence insulin secretion in vivo.