Ghrelin is a recently identified growth hormone (GH) secretogogue whose administration not only induces GH release but also stimulates food intake, increases adiposity, and reduces fat utilization in mice. The effect on food intake appears to be independent of GH release and instead due to direct activation of orexigenic neurons in the arcuate nucleus of the hypothalamus. The effects of ghrelin administration on food intake have led to the suggestion that inhibitors of endogenous ghrelin could be useful in curbing appetite and combating obesity. To further study the role of endogenous ghrelin in appetite and body weight regulation, we generated ghrelin-deficient (ghrl ؊/؊ ) mice, in which the ghrelin gene was precisely replaced with a lacZ reporter gene. ghrl ؊/؊ mice were viable and exhibited normal growth rates as well as normal spontaneous food intake patterns, normal basal levels of hypothalamic orexigenic and anorexigenic neuropeptides, and no impairment of reflexive hyperphagia after fasting. These results indicate that endogenous ghrelin is not an essential regulator of food intake and has, at most, a redundant role in the regulation of appetite. However, analyses of ghrl ؊/؊ mice demonstrate that endogenous ghrelin plays a prominent role in determining the type of metabolic substrate (i.e., fat vs. carbohydrate) that is used for maintenance of energy balance, particularly under conditions of high fat intake.G hrelin is a 28-aa peptide produced predominantly in the stomach (1, 2) that has recently been identified as a ligand of the growth hormone (GH) secretogogue (GHS) receptor (GHS-R). Like other GHSs, activation of the receptor stimulates GH secretion from the pituitary gland (1). In addition to inducing GH release, administration of exogenous ghrelin also stimulates food intake and body weight gain (3-7), increases gastric motility and acid secretion (8, 9), and decreases lipid metabolism in mice and rats (3, 4). The effects of centrally administered ghrelin on food intake are independent of its ability to induce GH release and thought to result from its direct actions on the arcuate nucleus of the hypothalamus. Furthermore, recent studies have demonstrated that plasma ghrelin levels increase preceding meals and during fasting (10, 11). Thus, it has been suggested that ghrelin stimulates appetite and that inhibitors of endogenous ghrelin, therefore, could prove useful in reducing food intake and combating obesity (11).Supporting the possibility that ghrelin acts as a key regulator of appetite and food intake by actions on the hypothalamus, GHS-R is colocalized with neuropeptide Y (NPY)͞agouti-related protein (AgRP) neurons (12) in the arcuate nucleus, a region that is responsive to circulating peripheral nutrients and hormones and critically involved in the regulation of food intake (13). Indeed, ghrelin stimulates the spontaneous activity of these neurons (14), and central ghrelin administration increases NPY and AgRP gene expression (15). Moreover, ghrelin-immunoreactivity has been reported in the h...

The gut peptide ghrelin, the endogenous ligand for the growth hormone secretagogue receptor, has been implicated not only in the regulation of pituitary growth hormone (GH) secretion but in a number of endocrine and nonendocrine functions, including appetitive behavior and carbohydrate substrate utilization. Nevertheless, recent genetic studies have failed to show any significant defects in GH levels, food intake, or body weight in adult ghrelin-deficient (Ghrl −/− ) mice. Here we demonstrate that male Ghrl −/− mice are protected from the rapid weight gain induced by early exposure to a high-fat diet 3 weeks after weaning (6 weeks of age). This reduced weight gain was associated with decreased adiposity and increased energy expenditure and locomotor activity as the animals aged. Despite the absence of ghrelin, these Ghrl −/− mice showed a paradoxical preservation of the GH/IGF-1 axis, similar to that reported in lean compared with obese humans. These findings suggest an important role for endogenous ghrelin in the metabolic adaptation to nutrient availability.

Genetic ablation of Inppl1, which encodes SHIP2 (SH2-domain containing inositol 5-phosphatase 2), was previously reported to induce severe insulin sensitivity, leading to early postnatal death. In the previous study, the targeting construct left the first eighteen exons encoding Inppl1 intact, generating a Inppl1(EX19-28-/-) mouse, and apparently also deleted a second gene, Phox2a. We report a new SHIP2 knockout (Inppl1(-/-)) targeted to the translation-initiating ATG, which is null for Inppl1 mRNA and protein. Inppl1(-/-) mice are viable, have normal glucose and insulin levels, and normal insulin and glucose tolerances. The Inppl1(-/-) mice are, however, highly resistant to weight gain when placed on a high-fat diet. These results suggest that inhibition of SHIP2 would be useful in the effort to ameliorate diet-induced obesity, but call into question a dominant role of SHIP2 in modulating glucose homeostasis.

No abstract

Administration of chemically synthesized ghrelin (Ghr) peptide has been shown to increase food intake and body adiposity in most species. However, the biological role of endogenous Ghr in the molecular control of energy metabolism is far less understood. Mice deficient for either Ghr or its receptor (the growth hormone secretagogue receptor, GHS-R1a) seem to exhibit enhanced protection against high-fat diet-induced obesity but do not show a substantial metabolic phenotype on a standard diet. Here we present the first mouse mutant lacking both Ghr and the Ghr receptor. We demonstrate that simultaneous genetic disruption of both genes of the Ghr system leads to an enhanced energy metabolism phenotype. Ghr/Ghr receptor double knockout (dKO) mice exhibit decreased body weight, increased energy expenditure, and increased motor activity on a standard diet without exposure to a high caloric environment. Mice on the same genetic background lacking either the Ghr or the Ghr receptor gene did not exhibit such a phenotype on standard chow, thereby confirming earlier reports. No differences in food intake, meal pattern, or lean mass were observed between dKO, Ghr-deficient, Ghr receptor-deficient, and wild-type (WT) control mice. Only dKO showed a slight decrease in body length. In summary, simultaneous deletion of Ghr and its receptor enhances the metabolic phenotype of single gene-deficient mice compared with WT mice, possibly suggesting the existence of additional, as of yet unknown, molecular components of the endogenous Ghr system.

Endogenous modulators of the central melanocortin system, such as the agouti-related protein (AgRP), should hold a pivotal position in the regulation of energy intake and expenditure. Despite this, AgRP-deficient mice were recently reported to exhibit normal food intake, body weight gain, and energy expenditure. Here we demonstrate that 2- to 3-month-old Agrp null mice do in fact exhibit subtle changes in response to feeding challenges (fasting and MCR agonists) but, of more significance and magnitude, exhibit reduced body weight and adiposity after 6 months of age. This age-dependent lean phenotype is correlated with increased metabolic rate, body temperature, and locomotor activity and increased circulating thyroid hormone (T4 and T3) and BAT UCP-1 expression. These results provide further proof of the importance of the AgRP neuronal system in the regulation of energy homeostasis.

.-Previous reports implicate the orexins in eating and body weight regulation. This study investigated possible functional relationships between hypothalamic orexins and circulating hormones or metabolites. In situ hybridization and quantitative PCR were used to examine orexin expression in the perifornical hypothalamus (PF) of rats and mice on diets varying in fat content and with differential propensity toward obesity. The results showed that orexin gene expression was stimulated by a high-fat diet in close association with elevated triglyceride levels, suggesting a functional relationship between these measures. Results obtained in obesity-prone rats and mice revealed a similar increase in orexin in close relation to triglycerides. A direct test of this orexin-triglyceride link was performed with Intralipid, which increased PF orexin expression along with circulating triglycerides. Whereas PF galanin is similarly stimulated by dietary fat, double-labeling immunofluorescence studies showed that orexin and galanin neurons are anatomically distinct. This evidence suggests that the orexins, like galanin, are "fat-responsive" peptides that respond to circulating lipids. perifornical lateral hypothalamus; triglycerides; high-fat diet; obesity THE OREXIN (hypocretin) peptides A and B derive from a common precursor protein, prepro-orexin, which is expressed specifically in the lateral hypothalamus (LH), most densely within the perifornical region (PF) (15,16). In addition to their role in the control of arousal and sleep-wake cycle (34), there is evidence that the orexins are involved in body weight regulation. Food deprivation stimulates orexin gene expression (11, 48), and injection of orexin A causes a small enhancement of food intake in rats (32,48,52). Conversely, administration of an orexin receptor 1 antagonist (30, 31) or anti-orexin antibody (61) inhibits feeding behavior. That these pharmacological results reflect a physiological function is supported by a recent study showing that genetic ablation of orexin neurons in mice causes hypophagia (29). However, these mice also develop late-onset obesity, indicating that the orexins may have additional effects beyond the stimulation of food intake. Indeed, other studies indicate that orexin peptides can increase metabolic rate and sympathetic nervous system (SNS) activity (3,19,49,51) and have variable effects on lipid utilization depending on time of day (38).Orexin neurons express leptin receptor immunoreactivity (27), and leptin administration downregulates orexin A levels in this area (6, 37). Whereas these findings suggest an inhibitory influence of leptin on the orexins, other studies show that orexin gene expression is downregulated in ob/ob mice and fa/fa rats, both of which have disturbed leptin signaling (7,20,51,62).

No abstract