Morphological localisation of sulfonylurea receptor 1 in endocrine cells of human, mouse and rat pancreas

AIMS/HYPOTHESIS

Sulfonylurea receptor 1 (SUR1) is the regulatory subunit of ATP-sensitive K channels in beta cells. Morphological methods (immunohistochemistry and sulfonylurea binding) were used to establish the cellular and subcellular location of SUR1 in human and rodent islets.

RESULTS

In the human, mouse and rat pancreas, all endocrine cells of the islets were immunolabelled with an anti-SUR1 antibody, whereas tissues containing SUR2 were consistently negative, as were those from Sur1 (also known as Abcc8)(-/-) mice. In beta cells of the three species, the plasma membrane was distinctly stained, but SUR1 was mainly present over the cytoplasm, with an intensity that varied between cells. Electron microscopy showed that SUR1 was immunolocalised in insulin, glucagon and somatostatin granules. In rat beta cells degranulated by in vivo treatment with glibenclamide (known as glyburide in the USA and Canada), the insulin and SUR1 staining intensity was similarly decreased by approximately 45%, whereas SUR1 staining was not changed in non-beta cells. In all islet cells, binding of glibenclamide labelled with fluorescent dipyrromethane boron difluoride (BODIPY-FL) was punctate over the cytoplasm, compatible with the labelling of endocrine granules. A faint labelling persisted in Sur1 (-/-) mice, but it was not different from that obtained with BODIPY-FL alone used as negative control.

CONCLUSIONS/INTERPRETATION

Our study immunolocalised SUR1 in alpha, beta and delta cells of human, mouse and rat islets, and for the first time visualised it in the plasma membrane. We also show that SUR1 is abundant in endocrine granules, where its function remains to be established. No specific sulfonylurea-binding sites other than SUR1 are identified in islet cells by the glibenclamide-BODIPY-FL technique.

Effect of creatine on the pancreatic beta-cell

We report on the stimulatory effect of creatine on insulin secretion and ATP concentration in MIN-6 beta-cells. The addition of creatine (5 mM) to MIN-6 cells in the presence of glucose (1-10 mM) elicited a significant (p<0.001) increase in insulin secretion, but no effect was demonstrated in the absence of glucose. The lack of effect of creatine in the absence of glucose suggests that creatine may act as a potentiator of insulin secretion rather than as an initiator. The potentiatory effect of creatine is specific for glucose since no effect was found in the presence of other known initiators of insulin secretion (K(+), 2-ketoisocaproic acid and tolbutamide). Cellular ATP content was markedly increased by glucose (1-15 mM). Creatine (5 and 10 mM) further increased the ATP level at all glucose concentrations, and the effect was observed even in the absence of glucose. The results from this study demonstrate the ability of creatine to increase insulin secretion only in the presence of glucose, while its effect on increased cellular ATP was independent of the presence of glucose. The mechanism whereby creatine potentiates insulin release is yet to be investigated. However, our data suggest possible unique interactions between creatine and the glucose-dependent insulin secretory pathway.

Human calcium/calmodulin-dependent protein kinase II gamma gene (CAMK2G): cloning, genomic structure and detection of variants in subjects with type II diabetes

AIMS/HYPOTHESIS

Ca(2+)/calmodulin-dependent protein kinase II, is expressed in the pancreatic beta cells and is activated by glucose and other secretagogues in a manner correlating with insulin secretion. The activation of Ca(2+)/calmodulin-dependent protein kinase II mediates some of the actions of Ca(2+) on the exocytosis of insulin. We therefore investigated the gene encoding the gamma isoform ( CAMK2G) which has been shown to be expressed in human beta cells as a candidate gene for Type II (non-insulin-dependent) diabetes mellitus.

METHODS

Human CAMK2G was cloned from a total human P1 artificial chromosome library using a partial Ca(2+)/calmodulin-dependent protein kinase gamma(E) cDNA probe. Positive PAC clones were localised to chromosome 10q22 by fluorescence in situ hybridisation. To obtain structural information and the sequences of the exon-intron boundaries, the published genomic structures of the rat and mouse genes allowed the putative exon-intron boundaries of human CAMK2G to be amplified by vectorette polymerase chain reaction and sequenced. Sequence variants in each exon were identified using single stranded conformational polymorphism analysis.

RESULTS

The human CAMK2G gene comprises 22 exons which range in size between 43 to 230 bp. Screening of the exons and exon-intron boundaries identified two single nucleotide polymorphisms. These did not show association with diabetes in 122 patients and 144 control subjects.

CONCLUSIONS/INTERPRETATION

We have identified the genomic structure of CAMK2G to enable further study of this potential candidate gene. Variation in this gene is not strongly associated with diabetes in Caucasians in the United Kingdom. We have identified two single nucleotide polymorphisms which, with appropriately large case control studies, can be used to assess the role of CAMK2G in the susceptibility to Type II diabetes.

Metabotropic glutamate and GABA(B) receptors contribute to the modulation of glucose-stimulated insulin secretion in pancreatic beta cells

AIMS/HYPOTHESIS

The neurotransmitters glutamate and gamma-aminobutyric acid (GABA) could participate in the regulation of the endocrine functions of islets of Langerhans. We investigated the role of the metabotropic glutamate (mGluRs) and GABA(B) (GABA(B)Rs) receptors in this process.

METHODS

We studied the expression of mGluRs and GABA(B)Rs in rat and human islets of Langerhans and in pancreatic alpha-cell and beta-cell lines using RT-PCR and immunoblot analysis. Effects of mGluR and GABA(B) R agonists on insulin secretion were determined by radioimmunoassays and enzyme-linked immunoadsorbent assays (ELISAs).

RESULTS

We detected mGluR3 and mGluR5 (but not mGluR1, 6 and 7) mRNAs in all of the samples examined. Trace amount of mGluR2 was found in MIN6 beta cells; mGluR4 was identified in rat islets; and mGluR8 expression was detected in rat islets, RINm5F and MIN6 cells. GABA(B)R1 a/b and 2 mRNAs were identified in islets of Langerhans and MIN6 cells. The expression of mGluR3, mGluR5, GABA(B)R1 a/b and GABA(B)R2 proteins was confirmed using specific antibodies. Group I (mGluR1/5) and group II (mGluR2/3) specific mGluR agonists increased the release of insulin in the presence of 3 to 10 mmol/l or 3 to 25 mmol/l glucose, respectively, whereas a group III (mGluR4/6-8) specific agonist inhibited insulin release at high (10-25 mmol/l) glucose concentrations. Baclofen, a GABA(B)R agonist, also inhibited the release of insulin but only in the presence of 25 mmol/l glucose.

CONCLUSION/INTERPRETATION

These data suggest that mGluRs and GABA(B)Rs play a role in the regulation of the endocrine pancreas with mechanisms probably involving direct activation or inhibition of voltage dependent Ca(2+)-channels, cAMP generation and G-protein-mediated modulation of K(ATP) channels.