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    • Family and twin studies have estimated the heritability of o

    2019-12-26

    Family and twin studies have estimated the heritability of obesity to be between 40 and 70% [5], [6], [7]. However, all known risk variants can only account for ∼3% of the variance in BMI [8], [9]. Therefore, alternate methods are needed to identify additional risk factors. In recent decades, Drosophila have been increasingly used as a model for metabolic disorders and the ability to perform large-scale genetic screens in flies has accelerated the identification of potential susceptibility genes. The ability to perform these screens in vivo is a requirement to study physiological processes at the whole-organism level, such as energy homeostasis, which involve cross-talk between multiple tissues. Drosophila also possess many of the same basic metabolic functions as mammals including the ability to maintain carbohydrate homeostasis, storing and mobilizing energy stores, and modulating food intake in response to nutritional cues [10], [11], [12], [13], [14]. In addition, many of the molecular mechanisms that regulate these metabolic processes are largely conserved including the insulin, target of rapamycin, and leptin pathways [14], [15], [16]. We performed a large-scale in vivo genetic screen in neurons and assayed for alterations to the levels of stored lipids. After three rounds of screening to verify the reproducibility and specificity of the adiposity phenotypes, we identified 24 genes. These include genes that have been previously associated with energy homeostasis in flies including regulation of carbohydrate and lipid metabolism and starvation response as well as genes not previously linked with energy balance. One of the genes identified was Diacylglycerol kinase whose homologues have been associated with obesity in human populations and have been shown to regulate insulin secretion in vitro. We performed the first functional study of Dgk in flies and showed that it is involved in lipid and carbohydrate metabolism. Further, we show that Dgk is likely functioning within the insulin-producing 3-Chlorotyrosine to regulate dILP secretion and impacting systemic insulin pathway activity.
    Materials and methods
    Results
    Discussion While studies in human populations have led to significant progress in the generation of an obesity gene map and broadened our understanding of the genetic basis of common obesity, there is still a large portion of heritability and etiology that remains unknown. An alternative approach to hasten the rate of gene discovery is to use the genetically tractable fruit fly. While there have been previous genetic screens in flies to identify metabolic regulators [57], [58], [59], [60], this study is the first to examine the role of specific genes in the central nervous system and to uncover a role for Dgk in energy homeostasis and insulin secretion. To identify neuronal genes involved in energy homeostasis in Drosophila, we systematically screened 1748 genes for effects on stored TAG levels when knocked down using a neuron-specific driver. After three rounds of screening, we identified 24 hits including several previously known to play a role in lipid and carbohydrate metabolism, response to starvation, dILP secretion and lifespan. Some of the hits also have homologues that are involved in mammalian energy homeostasis thereby validating the approach and the ability to identify factors relevant to energy balance in flies and mammals. Interestingly, all 24 genes identified resulted in increased TAG levels when knocked down in neurons using the fru-Gal4 driver. The fact that we did not identify any genes with decreased TAG levels could be attributed to the fact that silencing of Fru-Gal4-expressing neurons results in increased TAG levels whereas hyperactivation reduces levels [27] and knockdown of genes in the nervous system is more likely to result in a decrease rather than an increase in neuronal function. Thus, the screen seems to be unintentionally biased towards genes that negatively regulate TAG (and therefore would result in increased TAG when knocked out) and may have missed genes that positively regulate lipid storage.