Double Incretin Receptor Knockout (DIRKO) Mice Reveal an
Double Incretin Receptor Knockout (DIRKO) Mice Reveal an
Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are gut-derived incretins that potentiate glucose clearance following nutrient ingestion. Elimination of incretin receptor action in GIPR or GLP-1R mice produces only modest impairment in glucose homeostasis, perhaps due to compensatory upregulation of the remaining incretin. We have now studied glucose homeostasis in double incretin receptor knockout (DIRKO) mice. DIRKO mice exhibit normal body weight and fail to exhibit an improved glycemic response after exogenous administration of GIP or the GLP-1R agonist exendin-4. Plasma glucagon and the hypoglycemic response to exogenous insulin were normal in DIRKO mice. Glycemic excursion was abnormally increased and levels of glucose-stimulated insulin secretion were decreased following oral but not intraperitoneal glucose challenge in DIRKO compared with GIPR or GLP-1R mice. Similarly, glucose-stimulated insulin secretion and the response to forskolin were well preserved in perifused DIRKO islets. Although the dipeptidyl peptidase-IV (DPP-IV) inhibitors valine pyrrolidide (Val-Pyr) and SYR106124 lowered glucose and increased plasma insulin in wild-type and single incretin receptor knockout mice, the glucose-lowering actions of DPP-IV inhibitors were eliminated in DIRKO mice. These findings demonstrate that glucose-stimulated insulin secretion is maintained despite complete absence of both incretin receptors, and they delineate a critical role for incretin receptors as essential downstream targets for the acute glucoregulatory actions of DPP-IV inhibitors.
The observation that a glycemic stimulus derived from enteral nutrients exerts a greater insulinotropic response than a comparable isoglycemic challenge achieved through parenteral glucose administration has been termed the incretin effect. The first enteroendocrine-derived incretin to be identified, glucose-dependent insulinotropic peptide (GIP), is secreted from duodenal K-cells and rapidly potentiates glucose-dependent insulin secretion. However, the finding that immunoneutralization of GIP could not completely eliminate the incretin response strongly suggested the existence of additional gut-derived incretins. A second peptide with incretin activity, glucagon-like peptide 1 (GLP-1), was subsequently identified following elucidation of the nucleotide sequence for preproglucagon in the 1980s.
Human subjects with type 2 diabetes exhibit significant defects in meal-stimulated insulin secretion, leading to the suggestion that diminished incretin action or subnormal incretin secretion may contribute to the pathogenesis of β-cell dysfunction in specific patients. This hypothesis is supported in part by observations demonstrating resistance to GIP action and reductions in meal-stimulated GLP-1 secretion in diabetic subjects. The physiological importance of incretin action for glucose control is further illustrated by results of experiments directed at eliminating incretin action in vivo. A combination of peptide antagonists and immunoneutralizing antisera directed against either GIP, GLP-1, or their respective receptors have demonstrated that both GIP and GLP-1 are independently essential for regulation of glucose-dependent insulin secretion.
A complementary approach for analysis of incretin biology involves the development of mouse models of disrupted incretin receptor action. GIP receptor null mice develop normally and exhibit only modest glucose intolerance following an oral glucose challenge. Similarly, mice with a null mutation in the GLP-1 receptor gene do not develop severe diabetes but exhibit defective glucose-stimulated insulin secretion and glucose intolerance. The unexpectedly modest phenotypes of both GIPR and GLP-1R mice have prompted suggestions that one or more compensatory mechanisms have evolved to supplant the role normally subserved by individual incretin receptors in control of glucose homeostasis.
Evidence supporting the upregulation of compensatory mechanisms derives from findings that GLP-1R mice exhibit significantly enhanced β-cell sensitivity to the actions of GIP, whereas GIPR mice exhibit an enhanced insulin secretory response to GLP-1. Accordingly, we reasoned that the phenotype arising from disruption of single incretin receptor genes in mice may be partially modified as a result of complementary upregulation of the remaining intact incretin receptor axis. To identify the essential roles of GIP and GLP-1 for glucose homeostasis and to determine whether incretin receptors are key downstream targets essential for the action of dipeptidyl peptidase-IV (DPP-IV) inhibitors, we have now generated and characterized double incretin receptor knockout (DIRKO) mice with complete loss of both GIP and GLP-1 receptor action.
Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are gut-derived incretins that potentiate glucose clearance following nutrient ingestion. Elimination of incretin receptor action in GIPR or GLP-1R mice produces only modest impairment in glucose homeostasis, perhaps due to compensatory upregulation of the remaining incretin. We have now studied glucose homeostasis in double incretin receptor knockout (DIRKO) mice. DIRKO mice exhibit normal body weight and fail to exhibit an improved glycemic response after exogenous administration of GIP or the GLP-1R agonist exendin-4. Plasma glucagon and the hypoglycemic response to exogenous insulin were normal in DIRKO mice. Glycemic excursion was abnormally increased and levels of glucose-stimulated insulin secretion were decreased following oral but not intraperitoneal glucose challenge in DIRKO compared with GIPR or GLP-1R mice. Similarly, glucose-stimulated insulin secretion and the response to forskolin were well preserved in perifused DIRKO islets. Although the dipeptidyl peptidase-IV (DPP-IV) inhibitors valine pyrrolidide (Val-Pyr) and SYR106124 lowered glucose and increased plasma insulin in wild-type and single incretin receptor knockout mice, the glucose-lowering actions of DPP-IV inhibitors were eliminated in DIRKO mice. These findings demonstrate that glucose-stimulated insulin secretion is maintained despite complete absence of both incretin receptors, and they delineate a critical role for incretin receptors as essential downstream targets for the acute glucoregulatory actions of DPP-IV inhibitors.
The observation that a glycemic stimulus derived from enteral nutrients exerts a greater insulinotropic response than a comparable isoglycemic challenge achieved through parenteral glucose administration has been termed the incretin effect. The first enteroendocrine-derived incretin to be identified, glucose-dependent insulinotropic peptide (GIP), is secreted from duodenal K-cells and rapidly potentiates glucose-dependent insulin secretion. However, the finding that immunoneutralization of GIP could not completely eliminate the incretin response strongly suggested the existence of additional gut-derived incretins. A second peptide with incretin activity, glucagon-like peptide 1 (GLP-1), was subsequently identified following elucidation of the nucleotide sequence for preproglucagon in the 1980s.
Human subjects with type 2 diabetes exhibit significant defects in meal-stimulated insulin secretion, leading to the suggestion that diminished incretin action or subnormal incretin secretion may contribute to the pathogenesis of β-cell dysfunction in specific patients. This hypothesis is supported in part by observations demonstrating resistance to GIP action and reductions in meal-stimulated GLP-1 secretion in diabetic subjects. The physiological importance of incretin action for glucose control is further illustrated by results of experiments directed at eliminating incretin action in vivo. A combination of peptide antagonists and immunoneutralizing antisera directed against either GIP, GLP-1, or their respective receptors have demonstrated that both GIP and GLP-1 are independently essential for regulation of glucose-dependent insulin secretion.
A complementary approach for analysis of incretin biology involves the development of mouse models of disrupted incretin receptor action. GIP receptor null mice develop normally and exhibit only modest glucose intolerance following an oral glucose challenge. Similarly, mice with a null mutation in the GLP-1 receptor gene do not develop severe diabetes but exhibit defective glucose-stimulated insulin secretion and glucose intolerance. The unexpectedly modest phenotypes of both GIPR and GLP-1R mice have prompted suggestions that one or more compensatory mechanisms have evolved to supplant the role normally subserved by individual incretin receptors in control of glucose homeostasis.
Evidence supporting the upregulation of compensatory mechanisms derives from findings that GLP-1R mice exhibit significantly enhanced β-cell sensitivity to the actions of GIP, whereas GIPR mice exhibit an enhanced insulin secretory response to GLP-1. Accordingly, we reasoned that the phenotype arising from disruption of single incretin receptor genes in mice may be partially modified as a result of complementary upregulation of the remaining intact incretin receptor axis. To identify the essential roles of GIP and GLP-1 for glucose homeostasis and to determine whether incretin receptors are key downstream targets essential for the action of dipeptidyl peptidase-IV (DPP-IV) inhibitors, we have now generated and characterized double incretin receptor knockout (DIRKO) mice with complete loss of both GIP and GLP-1 receptor action.