Targeting of vasoactive intestinal peptide receptor 2, VPAC2, a secretin family G-protein coupled receptor, to primary cilia

Primary cilia protrude from the cell surface of many cell types in the human body and function as cellular antennae via ciliary membrane localized receptors. Neurons and glial cells in the brain possess primary cilia, and the malfunction of primary cilia may contribute to neurological deficits present in many cilia-associated disorders. Several rhodopsin family G-protein coupled receptors (GPCRs) are specifically localized to a subset of neuronal primary cilia. However, whether other family GPCRs target to neuronal cilia and whether glial primary cilia harbor any GPCRs are not known. We conducted a screening of GPCRs to determine their ability to target to primary cilia, and identified a secretin family member, Vasoactive Intestinal Receptor 2 (VPAC2), as a novel ciliary GPCR. Here, we show that endogenous VPAC2 targets to primary cilia in various brain regions, including the suprachiasmatic nuclei and the thalamus. Surprisingly, VPAC2 not only localizes to neuronal cilia but also to glial cilia. In addition, we show that VPAC2's C-terminus is both necessary and sufficient for its ciliary targeting and we define a novel ciliary targeting signal: the tetrapeptide RDYR motif in the C-terminus of VPAC2. Furthermore, we demonstrate that VPAC2 ciliary targeting is dependent on Tubby, the BBSome (a complex of Bardet-Biedl syndrome proteins) and the BBSome targeting factor, Arl6.