Dietary compounds activate an insect gustatory receptor on enteroendocrine cells to elicit myosuppressin secretion

Sensing of midgut internal contents is important for ensuring appropriate hormonal response and digestion following the ingestion of dietary components. Studies in mammals have demonstrated that taste receptors (TRs), a subgroup of G protein-coupled receptors (GPCRs), are expressed in gut enteroendocrine cells (EECs) to sense dietary compounds and regulate the production and/or secretion of peptide hormones. Although progress has been made in identifying expression patterns of gustatory receptors (GRs) in gut EECs, it is currently unknown whether these receptors, which act as ligand-gated ion channels, serve similar functions as mammalian GPCR TRs to elicit hormone production and/or secretion. A Bombyx mori Gr, BmGr6, has been demonstrated to express in cells by oral sensory organs, midgut and nervous system; and to sense isoquercitrin and chlorogenic acid, which are non-nutritional secondary metabolites of host mulberry. Here, we show that BmGr6 co-expresses with Bommo-myosuppressin (BMS) in midgut EECs, responds to dietary compounds and is involved in regulation of BMS secretion. The presence of dietary compounds in midgut lumen after food intake resulted in an increase of BMS secretions in hemolymph of both wild-type and BmGr9 knockout larvae, but BMS secretions in BmGr6 knockout larvae decreased relative to wild-type. In addition, loss of BmGr6 led to a significant decrease in weight gain, excrement, hemolymph carbohydrates levels and hemolymph lipid levels. Interestingly, although BMS is produced in both midgut EECs and brain neurosecretory cells (NSCs), BMS levels in tissue extracts suggested that the increase in hemolymph BMS during feeding conditions is primarily due to secretion from midgut EECs. Our studies indicate that BmGr6 expressed in midgut EECs responds to the presence of dietary compounds in the lumen by eliciting BMS secretion in B. mori larvae.