Abstract
Insulin-like peptides (ILPs) regulate growth, reproduction, metabolic homeostasis, life span and stress resistance in worms, flies and mammals. A set of insulin producing cells (IPCs) in the Drosophila brain that express three ILPs (DILP2, 3 and 5) have been the main focus of interest in hormonal DILP signaling. Little is, however, known about factors that regulate DILP production and release by these IPCs. Here we show that the IPCs express the metabotropic GABAB receptor (GBR), but not the ionotropic GABAA receptor subunit RDL. Diminishing the GBR expression on these cells by targeted RNA interference abbreviates life span, decreases metabolic stress resistance and alters carbohydrate and lipid metabolism at stress, but not growth in Drosophila. A direct effect of diminishing GBR on IPCs is an increase in DILP immunofluorescence in these cells, an effect that is accentuated at starvation. Knockdown of irk3, possibly part of a G protein-activated inwardly rectifying K+ channel that may link to GBRs, phenocopies GBR knockdown in starvation experiments. Our experiments suggest that the GBR is involved in inhibitory control of DILP production and release in adult flies at metabolic stress and that this receptor mediates a GABA signal from brain interneurons that may convey nutritional signals. This is the first demonstration of a neurotransmitter that inhibits insulin signaling in its regulation of metabolism, stress and life span in an invertebrate brain.
Highlights
Insulin and insulin-like peptides regulate development, growth, reproduction, metabolism, stress resistance and lifespan in animals from Caenorhabditis elegans to mammals [1,2,3,4,5,6]
There is a cluster of median neurosecretory cells in the Drosophila brain the produce DILP2, 3 and 5 [2,16,17]
In Drosophila, like in mammals, the metabotropic GABAB receptors (GBRs) are G-protein-coupled seven-transmembrane proteins composed of two subunits GABABR1 and GABABR2 [26,27]
Summary
Insulin and insulin-like peptides regulate development, growth, reproduction, metabolism, stress resistance and lifespan in animals from Caenorhabditis elegans to mammals [1,2,3,4,5,6]. In adult Drosophila DILP signaling plays an important role in metabolic homeostasis, resistance to various stresses and regulation of life span [3,12,13,14,15]. Ablation of the insulin producing cells (IPCs) in the brain results in retarded growth, increased glucose levels in the circulation, increased storage of lipid and carbohydrate of adults, reduced fecundity, and increased stress resistance [12,17]. This suggests that one or several of the DILPs of the IPCs mediate these responses. Recent experiments where individual DILPs were targeted showed that the peptides expressed in the IPCs display redundant functions [10]
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