Mice lacking ghrelin receptors resist the development of diet-induced obesity

Abstract

Ghrelin is the endogenous ligand for the growth hormone secretagogue receptor (GHSR; ghrelin receptor). Since its discovery, accumulating evidence has suggested that ghrelin may play a role in signaling and reversing states of energy insufficiency. For example, ghrelin levels rise following food deprivation, and ghrelin administration stimulates feeding and increases body weight and adiposity. However, recent loss-of-function studies have raised questions regarding the physiological significance of ghrelin in regulating these processes. Here, we present results of a study using a novel GHSR-null mouse model, in which ghrelin administration fails to acutely stimulate food intake or activate arcuate nucleus neurons. We show that when fed a high-fat diet, both female and male GHSR-null mice eat less food, store less of their consumed calories, preferentially utilize fat as an energy substrate, and accumulate less body weight and adiposity than control mice. Similar effects on body weight and adiposity were also observed in female, but not male, GHSR-null mice fed standard chow. GHSR deletion also affected locomotor activity and levels of glycemia. These findings support the hypothesis that ghrelin-responsive pathways are an important component of coordinated body weight control. Moreover, our data suggest that ghrelin signaling is required for development of the full phenotype of diet-induced obesity.

Figure 1

Gene targeting of the Ghsr locus. Upper left panel: Schematic diagram of the derivation of GHSR-null mice by homologous recombination. Lower left panel: Southern blot analysis of genomic DNA extracted from representative progeny of mating animals heterozygous for the recombinant Ghsr allele. Right panels: Representative dark-field photomicrographs of in situ hybridization histochemistry experiments performed on mouse brains using a mouse GHSR–specific riboprobe. Ghsr mRNA expression is evidenced by the white-appearing silver granules. 1–3, Ghsr exons 1–3; 3V, third ventricle; AP, area postrema; Arc, arcuate nucleus; DMV, dorsal motor nucleus of the vagus; NTS, nucleus of the solitary tract; P, Southern probe. Scale bar: 200 μm (applies to all 4 panels).

Figure 2

Effects of icv administration of ghrelin to GHSR-null mice. Two-month-old female GHSR-null mice and wild-type littermates were injected icv with 2 μg (2 μg/μl) ghrelin or 1 μl saline in a cross-over fashion. Upper panel: The intake of standard chow during the 2 hours immediately following the administration of ghrelin or saline. Data points and error bars correspond to the mean 2-hour food intake per body weight ± SEM. The only significant difference among the 4 groups is noted by an asterisk; *P < 0.01. Lower panels: Immunohistochemical analysis for Fos performed on tuberal hypothalamic brain sections from representative study animals following the 2-hour food intake observation period on the second drug administration day. Scale bar: 200 μm (applies to all 4 panels).

Figure 3

Body weights and body compositions of mice fed a Western-type HFD. Weekly body weights (upper panels) of GHSR-null mice and wild-type littermates fed HFD for 19 weeks, beginning at 4 weeks of age, and their body composition at the end of the 19-week study, as determined both by carcass analysis (middle panels) and by DEXA (lower panels). Data for both female (left panels) and male (right panels) mice are presented. Data points and error bars correspond to the mean ± SEM. Body weight and DEXA mean values were calculated from 19–22 animals per group. Carcass analysis mean values were calculated from 8–13 animals per group. Significant differences between groups noted in the body composition analyses are denoted by asterisks; *P < 0.05.

Figure 4

Weekly food intake and cumulative feed efficiency of mice fed a Western-type HFD. Weekly amounts of food intake (upper panels) and cumulative feed efficiencies (lower panels) of female (left panels) and male (right panels) GHSR-null mice and wild-type littermates fed HFD for 19 weeks, beginning at 4 weeks of age. Data points and error bars correspond to the mean ± SEM. Mean values were calculated from 19–22 animals per group.

Figure 5

Respiratory quotients and locomotor activity of mice fed a Western-type HFD. Respiratory quotient (left panel) and locomotor activity (right panel) were determined for a cohort of 8 male GHSR-null mice and 8 of their male wild-type littermates, after they had been fed HFD for 19 weeks, by use of a Columbus Instruments CLAMS. Data points and error bars correspond to the mean values ± SEM gathered over 48 hours. Statistically significant differences between GHSR-null mice and wild-type mice are indicated by asterisks; *P < 0.05; **P < 0.005. Statistical trends are denoted by a cross; ^†P = 0.054.

Figure 6

Body weights and body composition of mice maintained on a standard chow diet. Weekly body weights (upper panels) and body compositions (lower panels; as determined by DEXA) of female (left panels) and male (right panels) GHSR-null mice and wild-type littermates included in a 19-week-long standard chow diet study, beginning at 4 weeks of age. Data points and error bars correspond to the mean ± SEM. Mean values were calculated from 17–21 animals (for body weight) or 13–21 animals (for DEXA) per each group. Statistical trends are denoted by a cross; ^†P = 0.087.