Neuronal GLP1R mediates liraglutide's anorectic but not glucose-lowering effect

Abstract

Glucose control and weight loss are cornerstones of type 2 diabetes treatment. Currently, only glucagon-like peptide-1 (GLP1) analogs are able to achieve both weight loss and glucose tolerance. Both glucose and body weight are regulated by the brain, which contains GLP1 receptors (GLP1R). Even though the brain is poised to mediate the effects of GLP1 analogs, it remains unclear whether the glucose- and body weight-lowering effects of long-acting GLP1R agonists are via direct action on CNS GLP1R or the result of downstream activation of afferent neuronal GLP1R. We generated mice with either neuronal or visceral nerve-specific deletion of Glp1r and then administered liraglutide, a long-acting GLP1R agonist. We found that neither reduction of GLP1R in the CNS nor in the visceral nerves resulted in alterations in body weight or food intake in animals fed normal chow or a high-fat diet. Liraglutide treatment provided beneficial glucose-lowering effects in both chow- and high-fat-fed mice lacking GLP1R in the CNS or visceral nerves; however, liraglutide was ineffective at altering food intake, body weight, or causing a conditioned taste aversion in mice lacking neuronal GLP1R. These data indicate that neuronal GLP1Rs mediate body weight and anorectic effects of liraglutide, but are not required for glucose-lowering effects.

Figure 1. Characterization of GLP1R KD^ΔNestin and GLP1R KD^ΔPhox2b mice.

(A) Glp1r floxed construct. (B) Body weight analysis of chow-fed Glp1r mutant mice. (C) Body composition analysis on chow diet. (D) Body weight analysis of high-fat–fed Glp1r mutant mice. (E) Body composition analysis after 7 weeks of high-fat diet. (F) Seven-day high-fat diet intake. Statistical analysis was with 2-way ANOVA (B–E) or 1-way ANOVA (F) with Tukey’s post-hoc. Black, controls; red, GLP1R KD^ΔNestin; gray, GLP1R KD^ΔPhox2b.

Figure 2. Acute effects of liraglutide in GLP1R KD^ΔNestin and GLP1R KD^ΔPhox2b mice.

(A and B) Anorectic effects of liraglutide at 4 (A) and 24 (B) hours in chow-fed mice. (C and D) Anorectic effects of liraglutide in GLP1R KD^ΔNestin high-fat–fed mice at 4 (C) and 24 (D) hours. (E) CTA test after acute liraglutide administration in chow-fed mice. Statistical analysis was with 2-way ANOVA with repeated measures (A and B) or without repeated measures (C–E) with Tukey’s post-hoc. *P < 0.05 vs. same genotype; ^§P < 0.05 vs. same drug, different genotype

Figure 3. Liraglutide requires CNS GLP1R for anorectic and weight-lowering effects in high-fat–fed mice.

(A) Longitudinal body weights of Glp1r mutant mice treated with a long-acting GLP1R agonist or saline. (B) Cumulative weight loss of 14 days. (C) Longitudinal 14-day cumulative food intake of mice. (D) Total food intake during last 7 days of treatment. (E) Fourteen-day fat-mass change of mice. (F) Fourteen-day lean-mass change in mice. Black, controls; red, GLP1R KD^ΔNestin; gray, GLP1R KD^ΔPhox2b; solid, saline treatment; patterned, liraglutide treatment. Statistical analysis was with 2-way ANOVA with Tukey’s post-hoc with repeated measures (A and C) or without repeated measures (B and D–F). *P < 0.05 compared with saline treatment, same genotype unless otherwise indicated.

Figure 4. Glucose tolerance phenotypes of Glp1r mutant mice.

(A) i.p. GTT on chow diet. (B) AUC for A. (C) Oral GTT on chow diet. Main effect of genotype. (D) AUC for C. (E) i.p. GTT on high-fat diet. (F) AUC for E. (G) Insulin response to i.p. glucose. (H) Oral GTT on high-fat diet. (I) AUC for H. Statistical analysis was with 2-way ANOVA with repeated measures (A, C, E, G, and H) or 1-way ANOVA (B, D, F, and I) with Tukey’s post-hoc.

Figure 5. Liraglutide lowers glucose tolerance despite KD of central GLP1 receptors.

(A) i.p. GTT on chow diet. (B) i.p. GTT on high-fat diet after 14 days of chronic liraglutide (from Figure 3A). (C) Insulin level during the GTT in B. Statistical analysis was with 2-way ANOVA with repeated measures with Tukey post-hoc.