Functional Studies Of Tachykinin,A Novel Glucose Sensor Identified In Drosophila,and Its Downstream Circuitry

Food consumption is essential for all animal species,this basic behavior make sure animals get enough energy and nutrients to support their life.Meanwhile,they are capable of maintain energy homeostasis,this amazing ability plays a crucial role to prevent them from metabolic disorders like diabetes and obesity.Food intake of the adult fruit fly Drosophila melanogaster,an intermittent feeder,is attributed to several behavioral elements including foraging,feeding initiation and termination,and food ingestion.Despite the development of various feeding assays in fruit flies,how each of these behavioral elements,particularly food ingestion,is regulated remains largely uncharacterized.To this end,we have developed a manual feeding(MAFE)assay that specifically measures food ingestion of an individual fly completely independent of the other behavioral elements.This assay reliably recapitulates the effects of known feeding modulators,and offers temporal resolution in the scale of seconds.Using this assay,we find that fruit flies can rapidly assess the nutritional value of sugars within 20–30 s,and increase the ingestion of nutritive sugars after prolonged periods of starvation.Two candidate nutrient sensors,SLC5A11 and Gr43a,are required for discriminating the nutritive sugars,D-glucose and D-fructose,from their non-nutritive enantiomers,respectively.This suggests that different sensing mechanisms play a key role in determining food nutritional value.To maintain the energy hemostasis,animals need to sense both energy intake from food and energy storage.Sensing satiety is a crucial survival skill for all animal species including human.Despite the discovery of numerous neuromodulators that regulate food intake in Drosophila,the mechanism of satiety sensing remains largely elusive.To study how the internal states regulated feeding behavior without the interference of food properties,we screened for regulators of food consumption using non-nutritive L-glucose.We investigated how neuropeptidergic circuitry conveyed satiety state to influence flies'food consumption.Drosophila tackykinin(DTK)and its receptor TAKR99D were identified in an RNAi screen as feeding suppressors.Two pairs of DTK~+neurons in the fly brain could be activated by elevated D-glucose in the hemolymph and imposed a suppressive effect on feeding.These DTK~+neurons formed a two-synapse circuitry targeting insulin-producing cells,a well-known feeding suppressor,via TAKR99D~+neurons,and this circuitry could be rapidly activated during food ingestion and cease feeding.Taken together,we identified a novel satiety sensor in the fly brain that could detect specific circulating nutrients and modulate feeding in response.Taken together,we describe a novel and quantitative feeding assay to examine the regulation of food ingestion with excellent temporal resolution,and identifies a fast-acting neural mechanism that assesses food nutritional value and modulates food intake.Meanwhile,we found a DTK-TAKR99D-IPCs circuitry functions as a satiety sensor,being activated by elevated circulating nutrients during feeding,and imposing robust suppressive effect on food consumption.