Ectopic expression of glucagon receptor in skeletal muscles improves glucose homeostasis in a mouse model of diabetes

Abstract

Aims/hypothesis Excessive secretion of glucagon partially contributes to the development of diabetic hyperglycaemia. However, complete blocking of glucagon action will lead to adverse effects, since glucagon exerts certain beneficial effects via its receptor in many organs. We aimed to study the effects of a 'decoy receptor' for circulating glucagon on modulating beta cell function and glucose homeostasis in mice by over-producing the glucagon receptor (GCGR) in skeletal muscles. Methods We generated transgenic mice in which the expression of Gcgr is driven by the muscle specific creatine kinase (Mck) promoter, and assessed the effects of glucagon on the modulation of glucose homeostasis under conditions of extremes of glucose influx or efflux.Results Mck/Gcgr mice showed increased circulating levels of glucagon and insulin, resulting in an unchanged ratio of glucagon-to-insulin. The levels of hepatic glucose-6-phosphatase (G6PC) and fructose-1,6-bisphosphatase (F1,6P2ase) were significantly decreased, whereas the phosphorylation level of pancreatic cAMP-response-element-binding-protein (CREB) was significantly increased in these transgenic mice. Under basal conditions, the mice displayed normal blood glucose levels and unchanged glucose tolerance and insulin sensitivity when compared with their age-matched wild-type (WT) littermates. However, following multiple lowdose streptozotocin injections, Mck/Gcgr mice exhibited a delay in the onset of hyperglycaemia compared with the WT controls. This was associated with preserved beta cell mass and beta cell secretory capacity in response to glucose challenge.A. Maharaj and L. Zhu contributed equally to this study. Electronic supplementary material The online version of this article

Intra-islet glucagon secretion and action in the regulation of glucose homeostasis

“…Furthermore, pancreas perfusion studies using specific GCGR antagonists suggested that the intra-islet glucagon-induced trophic effects on the β-cells are mainly through GCGR rather than GLP-1 receptors on islet β-cells (Kawai et al, 1995), although the gut-derived insulinotropic hormone is recently found to be also produced by the α-cells and exerts local actions (Marchetti et al, 2012). The intra-islet trophic effects of glucagon have been further studied using transgenic mice expressing GCGR under control of the muscle specific creatine kinase (Mck) promoter (Maharaj et al, 2012). The Mck/Gcgr mice, which displayed mild hyperglucagonemia (but unchanged circulating GLP-1 levels), are euglycemic under basal conditions but are resistant to streptozotocin-induced β-cell injury partially due to enhanced intra-islet action of both glucagon and insulin on β-cell competence (Maharaj et al, 2012).…”

“…The intra-islet trophic effects of glucagon have been further studied using transgenic mice expressing GCGR under control of the muscle specific creatine kinase (Mck) promoter (Maharaj et al, 2012). The Mck/Gcgr mice, which displayed mild hyperglucagonemia (but unchanged circulating GLP-1 levels), are euglycemic under basal conditions but are resistant to streptozotocin-induced β-cell injury partially due to enhanced intra-islet action of both glucagon and insulin on β-cell competence (Maharaj et al, 2012). …”

“…Although ablation of hepatic glucagon actions can prevent the occurrence of diabetic hyperglycemia and metabolic manifestations following β-cell destruction (Lee et al, 2011, 2012), complete elimination of glucagon action can produce adverse effects, which are exemplified by the phenotypes of Gcgr −/− mice who have increased fetal lethality, defective islet development and impaired glucose competence in the β-cells (Sorensen et al, 2006; Vuguin et al, 2006). To investigate the effects of attenuating hepatic glucagon actions while enhancing intra-islet glucagon actions, our laboratory has generated Mck/Gcgr mice, a line with muscle-specifically expressing GCGR (Maharaj et al, 2012). The rationale behind this approach is that the ectopic overexpression of GCGR in the skeletal muscle, where little GCGRs are produced endogenously, creates a decoy receptor to neutralize excess circulating glucagon and elevates intra-islet glucagon action as a consequence of α-cell compensation to the trapping of glucagon in the muscle in Mck/Gcgr mice (Maharaj et al, 2012).…”

“…To investigate the effects of attenuating hepatic glucagon actions while enhancing intra-islet glucagon actions, our laboratory has generated Mck/Gcgr mice, a line with muscle-specifically expressing GCGR (Maharaj et al, 2012). The rationale behind this approach is that the ectopic overexpression of GCGR in the skeletal muscle, where little GCGRs are produced endogenously, creates a decoy receptor to neutralize excess circulating glucagon and elevates intra-islet glucagon action as a consequence of α-cell compensation to the trapping of glucagon in the muscle in Mck/Gcgr mice (Maharaj et al, 2012). These transgenic mice displayed a significant decrease in hepatic glucose-6-phosphatase and fructose-1,6-bisphosphatase mRNA levels, suggesting a reduction in gluconeogenesis and glycogenolysis.…”

“…These transgenic mice displayed a significant decrease in hepatic glucose-6-phosphatase and fructose-1,6-bisphosphatase mRNA levels, suggesting a reduction in gluconeogenesis and glycogenolysis. Remarkably, the Mck/Gcgr mice are euglycemic and possess higher capability in maintaining glycemic stability, particularly under extreme conditions of glucose influx or efflux exemplified by β-cell injury (Maharaj et al, 2012) and excessive nutrients feeding (Maharaj and Wang unpublished data). The reduced liver gluconeogenesis, glycogenolysis and elevated intra-islet glucagon action as a consequence of glucagon trapping and α-cell compensation, respectively, suggest that neutralizing peripheral glucagon actions, while enhancing intra-islet glucagon actions, may present a useful therapeutic approach for diabetes.…”