Glucagon-like peptide 1 (GLP1) is postulated to regulate blood glucose and satiety, but the biological importance of GLP1 as an incretin and neuropeptide remains controversal. The regulation of nutrient-induced insulin secretion is dependent on the secretion of incretins, gut-derived peptides that potentiate insulin secretion from the pancreatic islets. To ascertain the relative physiological importance of GLP1 as a regulator of feeding behavior and insulin secretion, we have generated mice with a targeted disruption of the GLP1 receptor gene (GLP1R). These GLP1R-/- mice are viable, develop normally but exhibit increased levels of blood glucose following oral glucose challenge in association with diminished levels of circulating insulin. It is surprising that they also exhibit abnormal levels of blood glucose following intraperitoneal glucose challenge. Intracerebroventricular administration of GLP1 inhibited feeding in wild-type mice but not in GLP1R-/- mice; however, no evidence for abnormal body weight or feeding behavior was observed in GLP1R-/- mice. These observations demonstrate that GLP1 plays a central role in the regulation of glycemia; however, disruption of GLP1/GLP1R signaling in the central nervous system is not associated with perturbation of feeding behavior or obesity in vivo.

No abstract

Incretins are gastrointestinal hormones that act on the pancreas to potentiate glucose-stimulated insulin secretion. Despite the physiological importance of the enteroinsular axis, disruption of glucagon-like peptide (GLP)-1 action is associated with only modest glucose intolerance in GLP-1 receptor -/- (GLP-1R -/-) mice. We show here that GLP-1R -/- mice exhibit compensatory changes in the enteroinsular axis via increased glucose-dependent insulinotropic polypeptide (GIP) secretion and enhanced GIP action. Serum GIP levels in GLP-1R -/- mice were significantly elevated versus those in +/+ control mice after an oral glucose tolerance test (369 +/- 40 vs. 236 +/- 28 pmol/l; P < or = 0.02). Furthermore, GIP perfusion of mice pancreas and isolated islets in the presence of elevated glucose concentrations elicited a significantly greater insulin response in GLP-1R -/- than in +/+ mice (P < or = 0.02-0.05). In contrast, no significant perturbation in the insulin response to perfused glucagon was detected under conditions of low (4.4 mmol/l) or high (16.6 mmol/l) glucose in GLP-1R -/- mice. Total pancreatic insulin but not glucagon content was significantly reduced in GLP-1R -/- compared with in +/+ mice (77 +/- 9 vs. 121 +/- 10 pmol/mg protein; P < or = 0.005). These observations suggest that upregulation of the GIP component of the enteroinsular axis, at the levels of GIP secretion and action, modifies the phenotype resulting from interruption of the insulinotropic activity of GLP-1 in vivo.

Late-stage neuropathological hallmarks of Alzheimer's disease (AD) are β-amyloid (βA) and hyperphosphorylated tau peptides, aggregated into plaques and tangles, respectively. Corresponding phenotypes have been mimicked in existing transgenic mice, however, the translational value of aggressive over-expression has recently been questioned. As controlled gene expression may offer animal models with better predictive validity, we set out to design a transgenic mouse model that circumvents complications arising from pronuclear injection and massive over-expression, by targeted insertion of human mutated amyloid and tau transgenes, under the forebrain- and neurone-specific CaMKIIα promoter, termed PLB1Double. Crossing with an existing presenilin 1 line resulted in PLB1Triple mice. PLB1Triple mice presented with stable gene expression and age-related pathology of intra-neuronal amyloid and hyperphosphorylated tau in hippocampus and cortex from 6 months onwards. At this early stage, pre-clinical 18FDG PET/CT imaging revealed cortical hypometabolism with increased metabolic activity in basal forebrain and ventral midbrain. Quantitative EEG analyses yielded heightened delta power during wakefulness and REM sleep, and time in wakefulness was already reliably enhanced at 6 months of age. These anomalies were paralleled by impairments in long-term and short-term hippocampal plasticity and preceded cognitive deficits in recognition memory, spatial learning, and sleep fragmentation all emerging at ∼12 months. These data suggest that prodromal AD phenotypes can be successfully modelled in transgenic mice devoid of fibrillary plaque or tangle development. PLB1Triple mice progress from a mild (MCI-like) state to a more comprehensive AD-relevant phenotype, which are accessible using translational tools such as wireless EEG and microPET/CT.

No abstract

No abstract

Glucagon-like peptide-1 (GLP-1) acts to control blood glucose via multiple mechanisms, including regulation of insulin and glucagon secretion, gastric emptying, satiety, and peripheral insulin sensitivity. However, the relative importance of these actions for regulation of blood glucose remains unclear. We demonstrate here a gene dosage effect for the incretin action of GLP-1, as heterozygous GLP-1R +/- mice exhibit an abnormal glycemic response to oral glucose challenge in association with reduced circulating levels of glucose-stimulated insulin. In contrast, GLP-1 signaling is not required for normal control of fasting and postabsorptive glucagon levels, and no significant changes were detected in the tissue content of pancreatic and intestinal proglucagon mRNA, glucagon-like immunoreactivity, or GLP-1 in GLP-1R -/- or +/- mice. Despite the demonstration that GLP-1 stimulates proinsulin gene transcription, pancreatic insulin mRNA transcripts were similar in wild-type and GLP-1R -/- mice. Furthermore, despite suggestions that GLP-1 regulates peripheral glucose disposal, whole-body glucose utilization was similar in wild-type and GLP-1R -/- mice under both basal and hyperinsulinemic conditions. These observations demonstrate that of the numerous physiological activities ascribed to GLP-1, only the incretin effect on pancreatic beta-cells appears essential for regulation of glucose homeostasis in vivo.

1-Cells from rodents and humans express different receptors recognizing hormones of the secretin-glucagon family, which--when activated--synergize with glucose in the control of insulin release. We have recently reported that isolated islets from mice homozygous for a GLP-1 receptor null mutation (GLP-1R(-/-)) exhibit a well-preserved insulin-secretory response to glucose. This observation can be interpreted in two different ways: 1) the presence of GLP-1R is not essential for the secretory response of isolated islets to glucose alone; 2) beta-cells in GLP-1R(-/-) pancreases underwent compensatory changes in response to the null mutation. To explore these possibilities, we studied islets from control GLP-IR(+/+) mice in the absence or presence of 1 pmol/l exendin (9-39)amide, a specific and potent GLP-1R antagonist. Exendin (9-39)amide (15-min exposure) reduced glucose-induced insulin secretion from both perifused and statically incubated GLP-1R(+/+) islets by 50% (P < 0.05), and reduced islet cAMP production in parallel (P < 0.001). Furthermore, GLP-1R(-/-) islets exhibited: 1) reduced cAMP accumulation in the presence of 20 mmol/l glucose (knockout islets versus control islets, 12 +/- 1 vs. 27 +/- 3 fmol x islet(-1) x 15 min(-1); P < 0.001) and exaggerated acceleration of cAMP production by 10 nmol/l glucose-dependent insulinotropic peptide (GIP) (increase over 20 mmol/l glucose by GIP in knockout islets versus control islets: 66 +/- 5 vs. 14 +/- 3 fmol x islet(-1) x 15 min(-1); P < 0.001); 2) increased mean cytosolic [Ca2+] ([Ca2+]c) at 7, 10, and 15 mmol/l glucose in knockout islets versus control islets; and 3) signs of asynchrony of [Ca2+]c oscillations between different islet subregions. In conclusion, disruption of GLP-1R signaling is associated with reduced basal but enhanced GIP-stimulated cAMP production and abnormalities in basal and glucose-stimulated [Ca2+]c. These abnormalities suggest that GLP-1R signaling is an essential upstream component of multiple beta-cell signaling pathways.