A multicomponent screen for feeding behaviour and nutritional status in Drosophila to interrogate mammalian appetite-related genes.

OBJECTIVE: More than 300 genetic variants have been robustly associated with measures of human adiposity. Highly penetrant mutations causing human obesity do so largely by disrupting satiety pathways in the brain and increasing food intake. Most of the common obesity-predisposing variants are in, or near, genes expressed highly in the brain, but little is known of their function. Exploring the biology of these genes at scale in mammalian systems is challenging. We sought to establish and validate the use of a multicomponent screen for feeding behaviour phenotypes, taking advantage of the tractable model organism Drosophila melanogaster. METHODS: We validated a screen for feeding behaviour in Drosophila by comparing results after disrupting the expression of centrally expressed genes that influence energy balance in flies to those of 10 control genes. We then used this screen to explore the effects of disrupted expression of genes either a) implicated in energy homeostasis through human genome-wide association studies (GWAS) or b) expressed and nutritionally responsive in specific populations of hypothalamic neurons with a known role in feeding/fasting. RESULTS: Using data from the validation study to classify responses, we studied 53 Drosophila orthologues of genes implicated by human GWAS in body mass index and found that 15 significantly influenced feeding behaviour or energy homeostasis in the Drosophila screen. We then studied 50 Drosophila homologues of 47 murine genes reciprocally nutritionally regulated in POMC and agouti-related peptide neurons. Seven of these 50 genes were found by our screen to influence feeding behaviour in flies. CONCLUSION: We demonstrated the utility of Drosophila as a tractable model organism in a high-throughput genetic screen for food intake phenotypes. This simple, cost-efficient strategy is ideal for high-throughput interrogation of genes implicated in feeding behaviour and obesity in mammals and will facilitate the process of reaching a functional understanding of obesity pathogenesis.

Hypothalamic transcriptome plasticity in two rodent species reveals divergent differential gene expression but conserved pathways.

We have addressed the question of how different rodent species cope with the life-threatening homeostatic challenge of dehydration at the level of transcriptome modulation in the supraoptic nucleus (SON), a specialised hypothalamic neurosecretory apparatus responsible for the production of the antidiuretic peptide hormone arginine vasopressin (AVP). AVP maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of AVP from SON axon terminals located in the posterior pituitary, and this is accompanied by a plethora of changes in the morphology, electrophysiological properties, biosynthetic and secretory activity of this structure. Microarray analysis was used to generate a definitive catalogue of the genes expressed in the mouse SON, and to describe how the gene expression profile changes in response to dehydration. Comparison of the genes differentially expressed in the mouse SON as a consequence of dehydration with those of the rat has revealed many similarities, pointing to common processes underlying the function-related plasticity in this nucleus. In addition, we have identified many genes that are differentially expressed in a species-specific manner. However, in many cases, we have found that the hyperosmotic cue can induce species-specific alterations in the expression of different genes in the same pathway. The same functional end can be served by different means, via differential modulation, in different species, of different molecules in the same pathway. We suggest that pathways, rather than specific genes, should be the focus of integrative physiological studies based on transcriptome data.

Identification of the global transcriptomic response of the hypothalamic arcuate nucleus to fasting and leptin.

Leptin plays a major role in coordinating the integrated response of the brain to changes in nutritional state. Leptin receptor expressing neurones within the arcuate nucleus (ARC) of the hypothalamus sense circulating leptin and densely innervate other regions of the hypothalamus, including the paraventricular nucleus (PVN). In the ARC, leptin is known to alter the expression of genes with important roles in the control of energy balance, and the aim of the present study was to obtain a more comprehensive picture of the action of leptin in these nuclei. Mice were ad libitum fed, or fasted for 48 h when receiving either sham or i.p. leptin treatment. We used laser capture microdissection and microarrays to identify leptin-regulated transcripts within the ARC. Expression of 639 genes are increased and 452 decreased within the fasted ARC. Leptin regulates 15% and 20% of these genes, respectively. In addition to expected changes in Pomc, Agrp, Npy and Cart, pathway analysis indicated that leptin regulated other genes concerned with energy homeostasis and endocrine function. As previously reported for the PVN, leptin also altered the expression of genes involved in nervous system development and synaptic function. However, aside from a small number of such genes (e.g. Gap43), leptin influenced the expression of different sets of neuronal developmental genes in the ARC and PVN. In conclusion, the present study identifies a set of genes that are regulated, at least in part, by leptin in the ARC, highlighting these as candidates for possible roles in leptin action and resistance.

Diet-induced gene expression of isolated pancreatic islets from a polygenic mouse model of the metabolic syndrome.

AIMS/HYPOTHESIS: Numerous new genes have recently been identified in genome-wide association studies for type 2 diabetes. Most are highly expressed in beta cells and presumably play important roles in their function. However, these genes account for only a small proportion of total risk and there are likely to be additional candidate genes not detected by current methodology. We therefore investigated islets from the polygenic New Zealand mouse (NZL) model of diet-induced beta cell dysfunction to identify novel genes and pathways that may play a role in the pathogenesis of diabetes. METHODS: NZL mice were fed a diabetogenic high-fat diet (HF) or a diabetes-protective carbohydrate-free HF diet (CHF). Pancreatic islets were isolated by laser capture microdissection (LCM) and subjected to genome-wide transcriptome analyses. RESULTS: In the prediabetic state, 2,109 islet transcripts were differentially regulated (>1.5-fold) between HF and CHF diets. Of the genes identified, 39 (e.g. Cacna1d, Chd2, Clip2, Igf2bp2, Dach1, Tspan8) correlated with data from the Diabetes Genetics Initiative and Wellcome Trust Case Control Consortium genome-wide scans for type 2 diabetes, thus validating our approach. HF diet induced early changes in gene expression associated with increased cell-cycle progression, proliferation and differentiation of islet cells, and oxidative stress (e.g. Cdkn1b, Tmem27, Pax6, Cat, Prdx4 and Txnip). In addition, pathway analysis identified oxidative phosphorylation as the predominant gene-set that was significantly upregulated in response to the diabetogenic HF diet. CONCLUSIONS/INTERPRETATION: We demonstrated that LCM of pancreatic islet cells in combination with transcriptional profiling can be successfully used to identify novel candidate genes for diabetes. Our data strongly implicate glucose-induced oxidative stress in disease progression.

Orexin expression is regulated by alpha-melanocyte-stimulating hormone.

The hypothalamic melanocortin system plays a fundamental role in the regulation of energy homeostasis. Orexins (hypocretins) are also involved in a diverse range of physiological processes, including food intake. Previous evidence has suggested that hypothalamic orexin expression may be influenced by the central melanocortin system. Here, we studied orexin mRNA levels in pro-opiomelanocortin-deficient (Pomc(-/-)) mice, a mouse model lacking all endogenously produced melanocortin peptides. Orexin expression in the lateral hypothalamus was significantly increased in corticosterone deficient Pomc(-/-) mice. Furthermore, when circulating glucocorticoids were restored to levels within the physiological range, orexin expression remained elevated. However, i.c.v. administration of the melanocortin alpha-melanocyte-stimulating hormone (MSH) to Pomc(-/-) mice reduced orexin expression back down to wild-type levels. This was independent of the effects of alpha-MSH on food intake because elevated orexin expression persisted in Pomc(-/-) mice pairfed to alpha-MSH-treated animals. These data indicate that alpha-MSH may play a role in the regulation of orexin expression in Pomc(-/-), with an elevation in orexin levels contributing to the hyperphagia seen in these animals.

Central responsiveness to a ghrelin mimetic (GHRP-6) is rapidly altered by acute changes in nutritional status in rats.

The hypothalamus appears to be more responsive to ghrelin and growth hormone secretagogues (GHS) in fasting, as reflected by a two- to three-fold increase in the number of cells detected that express Fos protein in the arcuate nucleus, in 48-h fasted rats compared to fed controls. Moreover, this increased hypothalamic responsiveness to GHS in fasting is regulated by the central action of exogenous leptin and insulin, although it is unknown whether these hormones mediate the changes in hypothalamic responsiveness to GHS associated with the fasting/fed state. In the present study, we show that refeeding with normal rat chow for only 2 h at the end of a 48-h fast reversed the potentiation of the Fos response to GHRP-6 observed in fasted rats. Circulating leptin and insulin levels remained significantly lower in refed rats compared to ad lib-fed rats, suggesting that the change in the hypothalamic sensitivity brought about by refeeding was independent of these hormones. By contrast, 2 h of chow refeeding at the end of a fast restored plasma glucose levels to those of the fed state. Refeeding with sugar alone for 2 h at the end of a 48-h fast also reduced the potentiated Fos response in fasting, indicating that elevated blood glucose can influence the central responsiveness to ghrelin/GHS. By contrast, infusion of the ileal satiety factor, PYY(3-36) (known to increase postprandially) did not alter the central responsiveness to GHRP-6, although it suppressed feeding and body weight as expected. This study highlights the importance of nutritional status in regulating the action of exogenous GHS (and presumably endogenous ghrelin) on the hypothalamic circuits controlling food intake.

Ghrelin treatment reverses the reduction in weight gain and body fat in gastrectomised mice.

BACKGROUND AND AIMS: The gastric hormone ghrelin has been reported to stimulate food intake, increase weight gain, and cause obesity but its precise physiological role remains unclear. We investigated the long term effects of gastrectomy evoked ghrelin deficiency and of daily ghrelin injections on daily food intake, body weight, fat mass, lean body mass, and bone mass in mice. METHODS: Ghrelin was given by subcutaneous injections (12 nmol/mouse once daily) for eight weeks to young female mice subjected to gastrectomy or sham operation one week previously. RESULTS: Gastrectomy reduced plasma concentrations of total ghrelin (octanoylated and des-octanoylated) and active (octanoylated) ghrelin by approximately 80%. Immediately after injection of ghrelin, the plasma concentration was supraphysiological and was still elevated 16 hours later. Daily food intake was not affected by either gastrectomy or ghrelin treatment. The effect of ghrelin on meal initiation was not studied. At the end point of the study, mean body weight was 15% lower in gastrectomised mice than in sham operated mice (p<0.001); daily ghrelin injections for eight weeks partially prevented this weight loss. In sham operated mice, ghrelin had no effect on body weight. The weight of fat was reduced in gastrectomised mice (-30%; p<0.01). This effect was reversed by ghrelin, enhancing the weight of fat in sham operated mice also (+20%; p<0.05). Gastrectomy reduced lean body mass (-10%; p<0.01) and bone mass (-20%; p<0.001) compared with sham operated mice. Ghrelin replacement prevented the gastrectomy induced decrease in lean body mass but did not affect bone. In sham operated mice, ghrelin affected neither of these two parameters. CONCLUSIONS: Ghrelin replacement partially reversed the gastrectomy induced reduction in body weight, lean body mass, and body fat but not in bone mass. In sham operated mice, ghrelin only increased fat mass. Our results suggest that ghrelin is mainly concerned with the control of fat metabolism and that ghrelin replacement therapy may alleviate the weight loss associated with gastrectomy.