Drosophila melanogaster as a Model to Dissect Obesity- Associated Mechanisms

Obesity affects a tremendous number of adults and children in the United States and worldwide, enough to be classified as a global epidemic by the World Health Organization (WHO). Given the serious complications associated with obesity, including heart disease, diabetes and cancer, it is critical to improve our understanding of the mechanisms underlying the response to obesity. In order to explore the pathogenesis of diet-induced obesity, we use Drosophila melanogaster as a model to evaluate the phenotypic, metabolic and transcriptional changes occurring in response to a high-fat diet. Similar to obesity in humans, Drosophila fed a high-fat diet (HFD) have increased triglyceride and glucose levels and decreased lifespan. Their tolerances to anoxic and cold stresses are diminished to only two-thirds and one-quarter, respectively, that of flies on regular diets (RD). In light of the multiple obesity-associated complications that involve hypoxia, we examined the effect of hypoxia on flies and found that intermittent hypoxia (IH), in particular, causes dramatic changes in phenotype. When flies spend a week in IH while on either RD or HFD, their triglyceride levels decrease and cold stress tolerance increases to nearly 100% survival. Meanwhile, IH seems to exacerbate the glucose levels in flies on both diets, and even more so in flies on HFD. We hypothesized that the phenotypic changes resulting from HFD or IH were likely accompanied by transcriptional changes in metabolic pathways. Indeed, microarray analysis reveals changes in amino acid metabolic pathways in flies on HFD and a candidate gene involved in arginine metabolism, CG9510, is able to simulate the HFD phenotype even when on regular diet (RD). Meanwhile, carbohydrate metabolic pathways are affected by IH in flies both on RD and HFD. The expression of several candidate genes, including CG9510, are differentially regulated by HFD and IH, suggesting that perhaps the two stresses are causing vastly different phenotypes by altering gene expression in opposite directions. Analysis of metabolite abundance using mass spectrometry of normoxic flies on HFD compared to RD reveals changes in similar pathways as those showing transcriptional changes. Flies on a HFD have altered abundances of metabolites associated with fatty acid, amino acid, and carbohydrate metabolism. We confirmed that the candidate gene identified from the HFD microarray has a similar function as its human homologue, argininosuccinate lyase (ASL), as it alters levels of urea and TCA cycle intermediates and thus lies at a key junction between carbon and nitrogen metabolism. The importance of this gene is further verified when up- regulation of CG9510 reverses aspects of the HFD phenotype. Reminiscent of the metabolic consequences that accompany diet-induced obesity in humans, Drosophila on a high-fat diet demonstrate a severely worsened phenotype and disrupted metabolic homeostasis. As a part of the response to HFD, flies on HFD alter CG9510 as a key enzyme at the junction of amino acid and carbohydrate metabolism. Importantly, manipulation of this single gene directly affects triglyceride storage and cold tolerance. As a result of these studies, we have identified a novel link between amino acid metabolism and obesit