Serotonin signals through ciliary 5-HT6 to control insulin secretion

In the pancreatic islet, serotonin (5-HT) functions basally to modulate glucagon secretion, but its contribution to beta-cell signaling and function remains unclear. Here, we demonstrate that the Gs-coupled serotonin receptor 6 (5-HT6), previously thought to be expressed almost exclusively in the brain, localizes to the primary cilium and plasma membrane of islet cells and augments insulin secretion. Utilizing a transgenic model of beta-cell cilia loss, along with approaches to perturb cilia-specific 5-HT6 function without disrupting the native cilium architecture, we show that 5-HT6 action in beta cells is dependent on the primary cilium. Critically, the plasma membrane pool of 5-HT6 cannot stimulate insulin release when the cilium is absent, representing an instance of whole-cell regulation by a cilia-localized GPCR. These findings establish subcellular localization as a determinant of GPCR function in beta cells and identify ciliary 5-HT6 signaling as a distinct pathway for modulating insulin secretion.