Opposing actions of cis- and trans-ligands at a single GPCR link osmotic state to habitat choice

How internal physiological states are translated into altered behavioral responses to the same sensory cue remains poorly understood. Here we show that the glycosylphosphatidylinositol (GPI)-anchored Ly6/uPAR protein Belly roll (Bero) acts as a cis-ligand for the neuropeptide receptor Allatostatin C receptor 2 (AstC-R2) in Drosophila larvae. Bero--AstC-R2 signaling establishes a phospholipase C{beta}-dependent high-Ca2+ state in abdominal leucokinin-producing (ABLK) neurons that suppresses nociceptive sensory transmission. Desiccation raises hemolymph osmolality, which activates AstC-producing neurons; the secreted AstC peptide then opposes Bero--AstC-R2 signaling, lowers ABLK Ca2+ activity, and unlocks avoidance of dry substrates. As a consequence, dehydrated larvae bias their pupation sites toward humid environments, mitigating desiccation risk during the immobile pupal stage. Thus, a membrane-tethered cis- and a secreted trans-ligand can act in opposition at a single GPCR, providing a non-canonical mechanism by which organismal physiological state is encoded in receptor signaling and converted into an adaptive behavioral choice.