Intra-islet glucagon signalling regulates beta-cell connectivity, first-phase insulin secretion and glucose homoeostasis

Abstract

Objective: Type 2 diabetes (T2D) is characterised by the loss of first-phase insulin secretion. We studied mice with β-cell selective loss of the glucagon receptor (Gcgr^fl/fl X Ins-1^Cre), to investigate the role of intra-islet glucagon receptor (GCGR) signalling on pan-islet [Ca²⁺]_I activity and insulin secretion.

Methods: Metabolic profiling was conducted on Gcgr^β-cell-/- and littermate controls. Crossing with GCaMP6f (STOP flox) animals further allowed for β-cell specific expression of a fluorescent calcium indicator. These islets were functionally imaged in vitro and in vivo. Wild-type mice were transplanted with islets expressing GCaMP6f in β-cells into the anterior eye chamber and placed on a high fat diet. Part of the cohort received a glucagon analogue (GCG-analogue) for 40 days and the control group were fed to achieve weight matching. Calcium imaging was performed regularly during the development of hyperglycaemia and in response to GCG-analogue treatment.

Results: Gcgr^β-cell-/- mice exhibited higher glucose levels following intraperitoneal glucose challenge (control 12.7 mmol/L ± 0.6 vs. Gcgr^β-cell-/- 15.4 mmol/L ± 0.0 at 15 min, p = 0.002); fasting glycaemia was not different to controls. In vitro, Gcgr^β-cell-/- islets showed profound loss of pan-islet [Ca²⁺]_I waves in response to glucose which was only partially rescued in vivo. Diet induced obesity and hyperglycaemia also resulted in a loss of co-ordinated [Ca²⁺]_I waves in transplanted islets. This was reversed with GCG-analogue treatment, independently of weight-loss (n = 8).

Conclusion: These data provide novel evidence for the role of intra-islet GCGR signalling in sustaining synchronised [Ca²⁺]_I waves and support a possible therapeutic role for glucagonergic agents to restore the insulin secretory capacity lost in T2D.

Keywords: Calcium waves; Glucagon; Islet calcium imaging.