Objective Maintenance of glucose homeostasis requires the precise regulation of hormone secretion from the endocrine pancreas. Free fatty acid receptor 4 (FFAR4/GPR120) is a G protein-coupled receptor whose activation in islets of Langerhans promotes insulin and glucagon secretion and inhibits somatostatin secretion. However, the contribution of individual islet cell types (α, β, and δ cells) to the insulinotropic and glucagonotropic effects of GPR120 remains unclear. As gpr120 mRNA is enriched in somatostatin-secreting δ cells, we hypothesized that GPR120 activation stimulates insulin and glucagon secretion via inhibition of somatostatin release. Methods Glucose tolerance tests were performed in mice after administration of selective GPR120 agonist Compound A. Insulin, glucagon, and somatostatin secretion were measured in static incubations of isolated mouse islets in response to endogenous (ω-3 polyunsaturated fatty acids) and/or pharmacological (Compound A and AZ-13581837) GPR120 agonists. The effect of Compound A on hormone secretion was tested further in islets isolated from mice with global or somatostatin cell-specific knock-out of gpr120 . Gpr120 expression was assessed in pancreatic sections by RNA in situ hybridization. Cyclic AMP (cAMP) and calcium dynamics in response to pharmacological GPR120 agonists were measured specifically in α, β, and δ cells in intact islets using cAMPER and GCaMP6 reporter mice, respectively. Results Acute exposure to Compound A increased glucose tolerance, circulating insulin, and glucagon levels in vivo. Endogenous and/or pharmacological GPR120 agonists reduced somatostatin secretion in isolated islets and concomitantly demonstrated dose-dependent potentiation of glucose-stimulated insulin secretion and arginine-stimulated glucagon secretion. Gpr120 was enriched in δ cells. Pharmacological GPR120 agonists reduced cAMP and calcium levels in δ cells but increased these signals in α and β cells. Compound A-mediated inhibition of somatostatin secretion was insensitive to pertussis toxin. The effect of Compound A on hormone secretion was completely absent in islets from mice with either global or somatostatin cell-specific deletion of gpr120 and partially reduced upon blockade of somatostatin receptor signaling by cyclosomatostatin. Conclusions Inhibitory GPR120 signaling in δ cells contributes to both insulin and glucagon secretion in part by mitigating somatostatin release.

Proliferation of pancreatic b-cells has long been known to reach its peak in the neonatal stages and decline during adulthood. However, b-cell proliferation has been studied under the assumption that all b-cells constitute a single, homogenous population. It is unknown whether a subpopulation of b-cells retains the capacity to proliferate at a higher rate and thus contributes disproportionately to the maintenance of mature b-cell mass in adults. We therefore assessed the proliferative capacity and turnover potential of virgin b-cells, a novel population of immature b-cells found at the islet periphery. We demonstrate that virgin b-cells can proliferate but do so at rates similar to those of mature b-cells from the same islet under normal and challenged conditions. Virgin b-cell proliferation rates also conform to the agedependent decline previously reported for b-cells at large. We further show that virgin b-cells represent a long-lived, stable subpopulation of b-cells with low turnover into mature b-cells under healthy conditions. Our observations indicate that virgin b-cells at the islet periphery can divide but do not contribute disproportionately to the maintenance of adult b-cell mass. More than 30 million Americans currently have diabetes, which results from progressive b-cell loss caused by either autoimmune attack or lifestyle and genetic factors (1). Because b-cells are the only cells in the body capable of secreting insulin, a major therapeutic goal for diabetes has been to replace b-cells lost during disease. Islet or wholepancreas transplantation has had some success in achievement of insulin independence, but limited donor availability, immunological complications, and transplant survival make this process not a viable option for the majority of patients with diabetes (2). Thus, a great interest continues in strategies that promote the regeneration of b-cells from various cell sources (3,4). Despite these efforts, clinically meaningful restoration of b-cell mass has not yet been achieved, illustrating the ongoing need for strategies to regenerate b-cells. During late pancreas development, b-cell mass expands rapidly by self-replication of young b-cells. Seminal experiments by Dor et al. (5) established, by pulse-chase labeling of b-cells, that self-replication is the main mechanism to maintain b-cell mass-a conclusion that is noncontroversial (6,7). However, b-cell proliferation declines rapidly with age in mice (8-10), with even lower proliferation rates observed in human islets (11), which complicates the challenge of restoring b-cell mass secondary to diabetesassociated b-cell loss. Against this backdrop, numerous reports of insulin-positive multipotent precursors (12,13), transdifferentiation of aand d-cells to b-cells (14-16), transdifferentiation of exocrine cells to b-cells (17,18), and, most recently, the existence of a population of protein C receptor (ProCr)-positive islet-resident stem cells ( 19) continue to raise the prospect that alternative paths to generation of b-cells exist and coul...

Objective: Maintenance of glucose homeostasis requires the precise regulation of hormone secretion from the endocrine pancreas. Free-fatty acid receptor 4 (FFAR4/Gpr120) is a G protein-coupled receptor whose activation in islets of Langerhans promotes insulin and glucagon secretion and inhibits somatostatin secretion. However, the contribution of individual islet cell types (α, β, and δ cells) to the insulinotropic and glucagonotropic effects of Gpr120 remains unclear. As Gpr120 mRNA is enriched in somatostatin-secreting δ cells, we hypothesized that Gpr120 activation stimulates insulin and glucagon secretion via inhibition of somatostatin release. Methods: Glucose tolerance tests were performed in mice after administration of the selective Gpr120 agonist Cpd A. Gpr120 mRNA levels were assessed in pancreatic section by RNA in situ hybridization. Insulin, glucagon and somatostatin secretion were measured in static incubations of isolated mouse islets in response to endogenous (ω-3 polyunsaturated fatty acids) and pharmacological (Compound A and AZ-13581837) Gpr120 agonists. The effect of Compound A on hormone secretion was tested in islets isolated from mice with global or somatostatin cell-specific knockout (KO) of Gpr120. Calcium and cAMP dynamics in response to pharmacological Gpr120 agonists were measured in islets isolated from α, β, and δ cell-specific GCaMP6 and CAMPER reporter mice, respectively. Results: Acute exposure to Compound A increased glucose tolerance and circulating insulin and glucagon levels in vivo. Both pharmacological and endogenous Gpr120 agonists dose-dependently potentiated glucose-stimulated insulin secretion and arginine-stimulated glucagon secretion and concomitantly reduced somatostatin secretion in isolated islets. The effect of Compound A on hormone secretion was completely absent in islets from mice with either global or somatostatin cell-specific deletion of Gpr120, and was partially reduced upon blockade of somatostatin receptor signaling by cyclosomatostatin. Gpr120 agonists reduced forskolin-stimulated cAMP levels in δ cells, but did not evoke calcium signaling in islet cells. Conclusions: This study supports a key contribution of inhibitory Gpr120-Gαi/o signaling in δ cells in both insulin and glucagon secretion in part via mitigating somatostatin release.

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The long-chain fatty-acid receptor FFA4 exerts beneficial effects on glucose homeostasis and insulin secretion and is considered a potential therapeutic target for type 2 diabetes. FFA4 is expressed in islets, but its precise mechanism of action remains unknown. Previous studies from our group suggest that FFA4 is expressed in delta cells and regulates somatostatin secretion. The objective of this study was to test the hypothesis that FFA4 agonists indirectly stimulate insulin secretion via inhibition of somatostatin release. In 1-h static incubations of isolated, wild-type mouse islets, the FFA4 agonist CpdA dose-dependently potentiated glucose-induced insulin secretion from 2.2±0.2 to 3.4±0.2 % of insulin content at the concentration of 50 μM (n=8; p<0.001), and simultaneously decreased somatostatin secretion from 28.4±2.1 to 10.5±0.9 pM (n=8; p<0.0001). No effect of CpdA on insulin or somatostatin secretion was observed in islets from mice that do not express somatostatin (insulin: 3.2±0.1 vs 2.9±0.4, n=6, NS; somatostatin: 11.4±1.2 vs 11.9±1.2, n=6, NS). No sex differences were observed. Likewise, in delta-cell deficient islets isolated from diphtheria toxin-treated male mice expressing the diphtheria toxin receptor under the somatostatin promoter, no effect of CpdA was observed on insulin or somatostatin release (insulin: 4.9±0.9 vs 4.1±0.8, n=3, NS; somatostatin: 10.1±4.9 vs 9.2±5.3, n=6, NS). We conclude that FFA4 stimulation of insulin secretion is exclusively mediated by inhibition of somatostatin secretion from delta cells. Disclosure L.Reininger: None. M.Ethier: None. C.Tremblay: None. J.Ghislain: None. M.Huising: Consultant; AstraZeneca, Research Support; Crinetics Pharmaceuticals, Inc. V.Poitout: Consultant; AstraZeneca. M.Flisher: None. Funding National Institutes of Health (R01DK132597)