The β-cell primary cilium is an autonomous Ca2+ compartment for paracrine GABA signalling
The primary cilium is an organelle present in most adult mammalian cells and is thought of as an antenna for detection of a variety of signals. Here we use intact mouse pancreatic islets of Langerhans to investigate signalling properties of the primary cilium in {beta}-cells. Using cilia-targeted Ca2+ indicators we find that the resting Ca2+ concentration in the cilium is lower than that of the cytosol, and we uncover a Ca2+ extrusion mechanism in the cilium that effectively insulates the cilium from changes in cytosolic Ca2+. Stimuli that give rise to pronounced cytosolic Ca2+ concentration increases, such as glucose- and depolarization-induced Ca2+ influx, and mobilization of Ca2+ from the ER, was accompanied by minor increases in cilia Ca2+ concentrations that were spatially restricted to a small compartment at the base. Conversely, we observe pronounced Ca2+ concentration changes in the primary cilia of islet {beta}-cells that do not propagate into the cytosol and show that paracrine GABA signalling via cilia-localized GABA-B1-receptors is responsible for this Ca2+ signalling. Finally, we demonstrate that the cilia response to GABA involves ligand-dependent transport of GABA-B1 receptors into the cilium.