An internal nutrient sensor detects specific dietary amino acids and promotes food consumption in Drosophila

Adequate protein intake is crucial for animals. Despite the recent progress in understanding protein hunger and satiety in the fruit fly Drosophila melanogaster, how fruit flies assess prospective dietary protein sources and ensure protein consumption remains elusive. We show here that three specific amino acids, L-glutamate (L-Glu), L-alanine (L-Ala), and L-aspartate (L-Asp), but not the D-enantiomers, rapidly promote food consumption in fruit flies when present in food. The effect of dietary amino acids to promote food consumption is independent of mating experience and internal nutritional status. Calcium imaging experiments show that six brain neurons expressing diuretic hormone 44 (DH44) can be rapidly and directly activated by these three amino acids during feeding. Genetic analysis shows that DH44+ neurons are both necessary and sufficient for dietary amino acids to promote food consumption. By conducting single cell RNAseq analysis, we also identify a amino acid transporter, CG13248, which is highly expressed in DH44+ neurons and is required for dietary amino acids to promote food consumption. Therefore, these data suggest that dietary amino acids may enter DH44+ neurons via CG13248 and modulate their activity and hence food consumption. Taken together, these data identify an internal amino acid sensor in the fly brain that evaluate food sources post-ingestively and facilitates adequate protein intake. These results shed critical light on the regulation of protein homeostasis at organismal levels by the nervous system. As described previously (Yu et al., 2016), individual DH44+ cells (visualized by GFP experession) were harvested from dissected fly brain under a fluorescence microscope with a glass micropipette pulled from thick-walled borosilicate capillaries (BF120-69-10, Sutter Instruments). Individual cells were immediately transferred to lysate buffer and underwent reverse transcription and cDNA amplification (SMARTer Ultra Low RNA Kit for Sequencing, Clonetech). The amplified cDNA were sonicated to ~250 bp fragments by the Covaris S2 system and then subjected to end-repair, dA-tail, adaptor ligation, and 12 cycles of PCR amplification using the library preparation kit (NEBNext Ultra II DNA Library Prep Kit, NEB). The cDNA libraries were sequenced by Illumina Hiseq 2500 platform. The sequenced raw data were first pre-processed to remove low-quality reads, adaptor sequences and amplification primer. Reads were mapped to Drosophila genome and mapped reads were selected for further analysis. FPKM (Fragments Per Kilobase Of Exon Per Million Fragments Mapped) was used to quantify gene expression.