The physiological effects of glucagon-like peptide-1 (GLP-1) are of immense interest because of the potential clinical relevance of this peptide. Produced in intestinal L-cells through posttranslational processing of the proglucagon gene, GLP-1 is released from the gut in response to nutrient ingestion. Peripherally, GLP-1 is known to affect gut motility, inhibit gastric acid secretion, and inhibit glucagon secretion. In the central nervous system, GLP-1 induces satiety, leading to reduced weight gain. In the pancreas, GLP-1 is now known to induce expansion of insulin-secreting β-cell mass, in addition to its most well-characterized effect: the augmentation of glucose-stimulated insulin secretion. GLP-1 is believed to enhance insulin secretion through mechanisms involving the regulation of ion channels (including ATP-sensitive K+ channels, voltage-dependent Ca2+ channels, voltage-dependent K+ channels, and nonselective cation channels) and by the regulation of intracellular energy homeostasis and exocytosis. The present article will focus principally on the mechanisms proposed to underlie the glucose dependence of GLP-1’s insulinotropic effect.

The generation of insulin-producing β-cells from human pluripotent stem cells is dependent on efficient endoderm induction and appropriate patterning and specification of this germ layer to a pancreatic fate. In this study, we elucidated the temporal requirements for TGFβ family members and canonical WNT signaling at these developmental stages and show that the duration of nodal/activin A signaling plays a pivotal role in establishing an appropriate definitive endoderm population for specification to the pancreatic lineage. WNT signaling was found to induce a posterior endoderm fate and at optimal concentrations enhanced the development of pancreatic lineage cells. Inhibition of the BMP signaling pathway at specific stages was essential for the generation of insulin-expressing cells and the extent of BMP inhibition required varied widely among the cell lines tested. Optimal stage-specific manipulation of these pathways resulted in a striking 250-fold increase in the levels of insulin expression and yielded populations containing up to 25% C-peptide+ cells.

OBJECTIVE Zinc ions are essential for the formation of hexameric insulin and hormone crystallization. A nonsynonymous single nucleotide polymorphism rs13266634 in the SLC30A8 gene, encoding the secretory granule zinc transporter ZnT8, is associated with type 2 diabetes. We describe the effects of deleting the ZnT8 gene in mice and explore the action of the at-risk allele. RESEARCH DESIGN AND METHODS Slc30a8 null mice were generated and backcrossed at least twice onto a C57BL/6J background. Glucose and insulin tolerance were measured by intraperitoneal injection or euglycemic clamp, respectively. Insulin secretion, electrophysiology, imaging, and the generation of adenoviruses encoding the low- (W325) or elevated- (R325) risk ZnT8 alleles were undertaken using standard protocols. RESULTS ZnT8^−/− mice displayed age-, sex-, and diet-dependent abnormalities in glucose tolerance, insulin secretion, and body weight. Islets isolated from null mice had reduced granule zinc content and showed age-dependent changes in granule morphology, with markedly fewer dense cores but more rod-like crystals. Glucose-stimulated insulin secretion, granule fusion, and insulin crystal dissolution, assessed by total internal reflection fluorescence microscopy, were unchanged or enhanced in ZnT8^−/− islets. Insulin processing was normal. Molecular modeling revealed that residue-325 was located at the interface between ZnT8 monomers. Correspondingly, the R325 variant displayed lower apparent Zn²⁺ transport activity than W325 ZnT8 by fluorescence-based assay. CONCLUSIONS ZnT8 is required for normal insulin crystallization and insulin release in vivo but not, remarkably, in vitro. Defects in the former processes in carriers of the R allele may increase type 2 diabetes risks.

Aims/hypothesis Zinc is highly concentrated in pancreatic beta cells, is critical for normal insulin storage and may regulate glucagon secretion from alpha cells. Zinc transport family member 8 (ZnT8) is a zinc efflux transporter that is highly abundant in beta cells. Polymorphisms of ZnT8 (also known as SLC30A8) gene in man are associated with increased risk of type 2 diabetes. While global Znt8 knockout (Znt8KO) mice have been characterised, ZnT8 is also present in other islet cell types and extra-pancreatic tissues. Therefore, it is important to find ways of understanding the role of ZnT8 in beta and alpha cells without the difficulties caused by the confounding effects of ZnT8 in these other tissues. Methods We generated mice with beta cell-specific (Znt8BKO) and alpha cell-specific (Znt8AKO) knockout of Znt8, and performed in vivo and in vitro characterisation of the phenotypes to determine the functional and anatomical impact of ZnT8 in these cells. Thus we assessed zinc accumulation, insulin granule morphology, insulin biosynthesis and secretion, and glucose homeostasis. Results Znt8BKO mice are glucose-intolerant, have reduced beta cell zinc accumulation and atypical insulin granules. They also display reduced first-phase glucose-stimulated insulin secretion, reduced insulin processing enzyme transcripts and increased proinsulin levels. In contrast, Znt8AKO mice show no evident abnormalities in plasma glucagon and glucose homeostasis. Conclusions/interpretation This is the first report of specific beta and alpha cell deletion of Znt8. Our data indicate that while, under the conditions studied, ZnT8 is absolutely essential for proper beta cell function, it is largely dispensable for alpha cell function.