G-Protein-Coupled Receptor Signaling in Cilia

Abstract

G-protein-coupled receptors (GPCRs) are the largest and most versatile family of signaling receptors in humans. They respond to diverse external signals, such as photons, proteins, peptides, chemicals, hormones, lipids, and sugars, and mediate a myriad of functions in the human body. Signaling through GPCRs can be optimized by enriching receptors and downstream effectors in discrete cellular domains. Many GPCRs have been found to be selectively targeted to cilia on numerous mammalian cell types. Moreover, investigations into the pathophysiology of human ciliopathies have implicated GPCR ciliary signaling in a number of developmental and cellular pathways. Thus, cilia are now appreciated as an increasingly important nexus for GPCR signaling. Yet, we are just beginning to understand the precise signaling pathways mediated by most ciliary GPCRs and how they impact cellular function and mammalian physiology.

Figure 1.

Overview of G-protein-coupled receptor (GPCR) signaling at the plasma membrane. (A) Ligand binding to a GPCR facilitates GDP release from the G-protein α-subunit and stimulates GTP binding to the α-subunit, which leads to dissociation of the α-subunit from the βγ-complex. Both the α-subunit and βγ-complex can then regulate various intracellular effectors. (B) Activated GPCRs are phosphorylated at specific sites on their intracellular domains predominantly by G-protein-coupled receptor kinases (GRKs). Phosphorylated receptors are targets for the recruitment of β-arrestins, which prevent further G-protein activation and mediate internalization of receptors by promoting clathrin-mediated endocytosis. β-Arrestins bind to numerous intracellular signaling proteins and can act as signal transducers independently of G-protein coupling. In some cases, GPCR signaling can be sustained or enhanced on endocytosis.

Figure 2.

Examples of G-protein-coupled receptors (GPCRs) and effectors that are enriched in primary cilia. (A) Image of a day 7 mouse hippocampal neuron immunolabeled with an antibody to somatostatin receptor subtype 3 (SSTR3) showing an SSTR3-positive cilium projecting from the cell body. (B) Adult mouse brain section corresponding to the medial hypothalamus immunolabeled with an antibody to kisspeptin receptor 1 (KISS1R). Note the presence of multiple KISS1R-positive cilia. (C) Image of a day 7 mouse hippocampal neuron treated with somatostatin and immunolabeled with an antibody to β-arrestin. Arrow indicates β-arrestin ciliary localization. (D) Image of a day 7 mouse hippocampal neuron immunolabeled with an antibody to type 3 adenylyl cyclase (AC3) showing an AC3-positive cilium projecting from the cell body. Scale bars, 5 µm.

Figure 3.

Overview of odorant receptor signaling in olfactory sensory neurons. (A) Scanning electron microscopy image of the surface of the mouse olfactory epithelium. Scale bar, 1 µm (courtesy of Jeff Martens). (B) Schematic of a single olfactory sensory neuron with cilia projecting into the olfactory epithelium. (C) Odorant activation of olfactory G-protein-coupled receptors (GPCRs) triggers the activation of the stimulatory G protein Gα_olf, which then activates type 3 adenylyl cyclase (AC3) and increases cAMP levels within the cilium. The cAMP binds to and activates cyclic-nucleotide-gated (CNG) channels on the ciliary membrane, leading to an increase in Ca²⁺ levels, subsequent activation of Ca²⁺-gated chloride channels, and depolarization of the neuron. (D) β-Arrestin binding to activated odorant receptors mediates desensitization. The type 3 muscarinic (M3) acetylcholine receptor can inhibit the recruitment of β-arrestin to odorant receptors, thereby potentiating odor-induced signaling.

Figure 4.

Overview of ciliary signaling in renal cilia. (A) Cross section of a renal tubule showing primary cilia projecting into the lumen of the tubule (top). Schematic of flow-induced Ca²⁺ signaling (bottom). (B) Vasopressin binding to vasopressin receptor 2 (V2R) on the ciliary membrane activates adenylyl cyclase. The increase in local cAMP concentrations activates a cation-selective channel, possibly through protein kinase A, thereby regulating intraciliary Ca²⁺ signals. (C) Agonist binding to dopamine receptor 5 on the ciliary membrane results in Ca_V1.2 channel activation, possibly through the action of dissociated Gβγ, which increases intraciliary Ca²⁺ levels.

Figure 5.

Overview of G-protein-coupled receptor (GPCR) signaling on neuronal cilia. (A) Ligand binding to neuropeptide Y receptor 2 (NPY2R) on the ciliary membrane may activate Gα_i and inhibit adenylyl cyclase, thereby leading to a reduction in cAMP levels. Ligand treatment also leads to a reduction in NPY2R ciliary localization, suggesting that activated receptor exits the cilium. (B) Somatostatin treatment stimulates endogenous β-arrestin recruitment into somatostatin receptor subtype 3 (SSTR3)-positive cilia. Somatostatin treatment also causes a β-arrestin-dependent decrease in SSTR3 ciliary localization, suggesting that β-arrestin mediates SSTR3 ciliary export.