Ins and outs of GPCR signaling in primary cilia

Abstract

Primary cilia are specialized microtubule-based signaling organelles that convey extracellular signals into a cellular response in most vertebrate cell types. The physiological significance of primary cilia is underscored by the fact that defects in assembly or function of these organelles lead to a range of severe diseases and developmental disorders. In most cell types of the human body, signaling by primary cilia involves different G protein-coupled receptors (GPCRs), which transmit specific signals to the cell through G proteins to regulate diverse cellular and physiological events. Here, we provide an overview of GPCR signaling in primary cilia, with main focus on the rhodopsin-like (class A) and the smoothened/frizzled (class F) GPCRs. We describe how such receptors dynamically traffic into and out of the ciliary compartment and how they interact with other classes of ciliary GPCRs, such as class B receptors, to control ciliary function and various physiological and behavioral processes. Finally, we discuss future avenues for developing GPCR-targeted drug strategies for the treatment of ciliopathies.

Keywords: G protein‐coupled receptors; ciliopathies; intraflagellar transport; neuronal signaling; primary cilia.

Figure 1. Overview on GPCR signaling and function of ciliary GPCRs in vision and olfaction

(A) Examples of sensory inputs for GPCR-mediated signaling pathways through activation of G proteins. (B) List of known ciliary GPCRs in photoreceptor and olfactory receptor neurons. (C) A cartoon of a rod photoreceptor neuron and localization of light-sensitive GPCRs (rhodopsin) in the disk array of the outer segment, which comprises a modified primary cilium. (D) Cartoon of olfactory receptor cells and localization of ciliary GPCRs that detect odorants. Please see text for details and references.

Figure 2. Primary cilia are sensory organelles that coordinate GPCR signaling during development and in tissue homeostasis

(A) Electron micrographs (EMs) of the primary cilium: (i) transmission EM of a longitudinal section of a neuronal primary cilium emerging from the centrosomal mother centriole, which functions as a basal body (BB). The region between the ciliary membrane and plasma membrane, referred to as the periciliary membrane, is often infolded to produce a ciliary pocket (CiPo) that comprises an active site for exocytosis and clathrin-dependent endocytosis of ciliary receptors (courtesy of Joseph Gleeson). (ii) Scanning EM of a fibroblast primary cilium with a ciliary pocket; asterisks mark microvilli (courtesy of Peter Satir). (iii) Transmission EM of a cross section of a fibroblast primary cilium at the ciliary transition zone showing the 9 + 0 microtubules arrangement of the axoneme (with permission 144). (B) Cartoon illustrating IFT and trafficking processes at the primary cilium to control ciliary assembly and targeting of GPCRs to the ciliary membrane. Receptor transport from the TGN to the ciliary pocket is mediated by ARF4, IFT20, and TCTEX-1 together with a complex consisting of FIP3, ASAP1, and RAB11/RAB8/Rabin8 subcomplex. The docking of vesicles near the periciliary membrane switches to IFT in a process involving the IFT-B components IFT57 and IFT20 as well as ARL6 and the RAB8-binding protein, Rabaptin5. Please see text for further details. (C) List of GPCRs known to localize to the primary cilium. (D) Immunofluorescence micrographs with examples of GPCRs localizing along the axis of the primary cilium in various cell types: (i) localization of melanin-concentrating hormone receptor 1 (MCHR1) in cultured mouse hypothalamic neurons (courtesy of Nicholas Berbari and Kirk Mykytyn); (ii) localization of somatostatin receptor 3 (SSTR3) in neuronal primary cilia of the mouse hindbrain (with permission 145); (iii) localization of dopamine 5 receptor (DR5) to primary cilia in porcine kidney proximal tubule (LLC-PK1) cells (with permission 146); (iv) localization of kisspeptin receptor 1 (KISSR1) to primary cilia the medial hypothalamus in adult Cilia^GFP mice (with permission 77); and (v) localization of smoothened (SMO) to primary cilia in SMO agonist (SAG)-stimulated human embryonic stem cells (hESC) (with permission 147). Keys: β-tubulin III (neuronal marker), ACIII (adenylate cyclase III, neuronal primary cilium marker), Ac-tub (acetylated α-tubulin, primary cilium marker), Glu-tub (glutamylated α-tubulin, primary cilium marker), and DAPI/DRAQ5 (stains DNA, nuclear marker). Please see text for references and further details

Figure 3. Examples of ciliary GPCR signaling

(A) GPCR signaling in olfactory cilia relies on the cAMP-dependent opening of ion channels, leading to an influx of Na⁺ and Ca²⁺ ions into the ciliary compartment, which in turn activates chloride channels, causing efflux of Cl⁻, which results in a further depolarization of the cell. Abbreviations: Olf: ligands for olfactory receptors. (B) Outline of trafficking and signaling processes associated with MCHR1 and SSTR3 signaling in neuronal primary cilia. The BBSome and TULP3, IFT-A, and KIF3A control the localization of the receptors to the ciliary base and further into the ciliary membrane. Abbreviations: M: melanin-concentrating hormone; S: somatostatin. (C) Outline of signaling processes associated with D1R and GPR88 signaling in primary cilia. D1R is activated by catecholamines both in the cilium and at the plasma membrane, but receptor activation is specifically inhibited within the cilium by GPR88. (D) Outline of trafficking and signaling processes associated with HH signaling in primary cilia. Please see text for references and further details.