HTR6 and SSTR3 targeting to primary cilia

Abstract

Primary cilia are hair-like projections of the cell membrane supported by an inner microtubule scaffold, the axoneme, which polymerizes out of a membrane-docked centriole at the ciliary base. By working as specialized signaling compartments, primary cilia provide an optimal environment for many G protein-coupled receptors (GPCRs) and their effectors to efficiently transmit their signals to the rest of the cell. For this to occur, however, all necessary receptors and signal transducers must first accumulate at the ciliary membrane. Serotonin receptor 6 (HTR6) and Somatostatin receptor 3 (SSTR3) are two GPCRs whose signaling in brain neuronal cilia affects cognition and is implicated in psychiatric, neurodegenerative, and oncologic diseases. Over a decade ago, the third intracellular loops (IC3s) of HTR6 and SSTR3 were shown to contain ciliary localization sequences (CLSs) that, when grafted onto non-ciliary GPCRs, could drive their ciliary accumulation. Nevertheless, these CLSs were dispensable for ciliary targeting of HTR6 and SSTR3, suggesting the presence of additional CLSs, which we have recently identified in their C-terminal tails. Herein, we review the discovery and mapping of these CLSs, as well as the state of the art regarding how these CLSs may orchestrate ciliary accumulation of these GPCRs by controlling when and where they interact with the ciliary entry and exit machinery via adaptors such as TULP3, RABL2 and the BBSome.

IFT-A Structure Reveals Carriages for Membrane Protein Transport into Cilia

“…In COPI, membrane-associated β-propeller domains interact with the dilysine motifs on cargoes destined for transport from the Golgi to the ER (Jackson et al, 2012; Ma and Goldberg, 2013). The sequence determinants for ciliary membrane protein targeting are less well defined, but can involve distributed, partially redundant motifs [for example, in the third intracellular loop and C-terminal region of ciliary GPCRs; (Barbeito and Garcia-Gonzalo, 2021)]. It is possible that the β-propellers of IFT-A and TUBBY domain of TULP work in conjunction to recognize different targeting elements on ciliary membrane proteins.…”

Multiple Ciliary Localization Signals Control INPP5E Ciliary Targeting

“…On the other hand, CLS4 is needed for association to PDE6D, RPGR and, in cooperation with CLS1, to ATG16L1. Altogether, our data represent a major step forward in the field and reveal an unprecedented degree of complexity in the ciliary targeting mechanisms of INPP5E, as compared to other known ciliary cargoes, for which a single or at most two CLSs suffice to explain ciliary accumulation 1, [70][71][72][73] .…”

“…Altogether, our data show that INPP5E ciliary targeting is a surprisingly complex process involving four different cis-acting sequences (CLS1-4), and multiple trans-acting factors (like PDE6D, RPGR, ARL13B, TULP3, CEP164 and ATG16L1). This level of complexity is unusual, especially when compared to other ciliary cargoes, whose targeting is more straightforward, typically involving a single CLS 1, [70][71][72][73] . The complexity and redundancy in INPP5E ciliary targeting suggest this is a very important process, subject to fine regulation.…”

Ciliary membrane, localised lipid modification and cilia function

“…Several GPCRs, particularly the classes A, B and F types, are selectively localised to the cilia (McIntyre et al, 2016; Schou et al, 2015). Amongst them, the Rhodopsin, GPR161, HTR6 and Somatostatin receptor 3 (SSTR3) are targeted due to specific ciliary localisation sequences at the intracellular loops and the C‐terminal ends of these receptors (for review, see Barbeito & Garcia‐Gonzalo, 2021). Cholesterol binding modulates many GPCR functions (reviewed in Sarkar & Chattopadhyay, 2020).…”

“…The cilia‐dependent signalling repertoire includes some of the primary regulators of cell proliferation, such as the hedgehog (Hh), wingless (Wnt), G‐protein coupled receptors (GPCRs) and a variety of growth factor receptors active during development and in differentiated tissues (Wingfield et al, 2018). The ciliary localisation of GPCRs like the serotonin (HTR6) and somatostatin (SSTR3) receptors, the transient receptor potential (TRP) superfamily members like the polycystin (PC1/2) family, and cyclic nucleotide‐gated (CNG) channels also contribute to the physiological conditioning of the cell under different environmental situations (Barbeito & Garcia‐Gonzalo, 2021; Yanardag & Pugacheva, 2021). The growth, stability and resorption of cilia are strongly correlated with cell proliferation, homeostasis and function.…”