Intracellular Receptor Modulation: Novel Approach to Target GPCRs

Recent crystal structures have suggested a high diversity of allosteric binding sites, including novel pockets in the intracellular domain of GPCRs. These intracellular sites can potentially be targeted with small molecules, pepducins, and nanobodies. The recent X-ray structures of CCR2, CCR9, and β2AR have revealed a highly-conserved intracellular pocket for small molecules, suggesting its presence in most chemokine receptors and other class A GPCRs. Although many allosteric ligands for GPCRs have been described, only few allosteric drugs have reached the market. Yet, the number of allosteric modulators in development stages keeps increasing, including the number of intracellular ligands in (pre)clinical studies. The discovery of intracellular binding sites, combined with the array of strategies for targeting such sites, opens up new approaches to better study and target GPCRs. Recent crystal structures of multiple G protein-coupled receptors (GPCRs) have revealed a highly conserved intracellular pocket that can be used to modulate these receptors from the inside. This novel intracellular site partially overlaps with the G protein and β-arrestin binding site, providing a new manner of pharmacological intervention. Here we provide an update of the architecture and function of the intracellular region of GPCRs, until now portrayed as the signaling domain. We review the available evidence on the presence of intracellular binding sites among chemokine receptors and other class A GPCRs, as well as different strategies to target it, including small molecules, pepducins, and nanobodies. Finally, the potential advantages of intracellular (allosteric) ligands over orthosteric ligands are also discussed. Recent crystal structures of multiple G protein-coupled receptors (GPCRs) have revealed a highly conserved intracellular pocket that can be used to modulate these receptors from the inside. This novel intracellular site partially overlaps with the G protein and β-arrestin binding site, providing a new manner of pharmacological intervention. Here we provide an update of the architecture and function of the intracellular region of GPCRs, until now portrayed as the signaling domain. We review the available evidence on the presence of intracellular binding sites among chemokine receptors and other class A GPCRs, as well as different strategies to target it, including small molecules, pepducins, and nanobodies. Finally, the potential advantages of intracellular (allosteric) ligands over orthosteric ligands are also discussed. parameter that describes how strong a ligand binds to its target. a binding site nonoverlapping and topographically distinct from the orthosteric binding site. any ligand that binds to an allosteric binding site, from which they can modulate the activity of orthosteric ligands. mutation that leads to a permanent and agonist-independent active state of the GPCR, compared with the wild-type receptor. parameter that describes the degree of effect or response achieved by a specific ligand upon binding to its target. family of transmembrane proteins that transduce a variety of extracellular signals into intracellular responses via G protein-dependent or -independent signaling pathways. In vertebrates, GPCRs are divided in four different classes or subfamilies: class A (rhodopsin-like), class B (secretin), class C (metabotropic glutamate), and class F (frizzled/smoothened). intracellularly-expressed antibody fragments aimed at intracellular targets. recombinant small size antibody (12–15 kDa), containing a variable-domain fragment derived from camelid heavy-chain antibodies. the binding site recognized and used by the endogenous ligand for a corresponding receptor. any ligand that binds to the orthosteric binding site of the receptor. Orthosteric ligands include the endogenous ligands and non-endogenous agonists, antagonists, or inverse agonists. lipidated peptides derived from the intracellular loops or the C terminus of GPCRs, which specifically target their cognate receptor by acting as allosteric agonists or antagonists. the ability of a ligand to (purposely and) effectively bind to several targets. parameter that describes the activity of a drug by defining how much of a ligand (concentration) is needed to produce a half-maximal effect. numbering system for amino acid residues, which takes into account structural information to correct for bulges and constrictions. In this numbering scheme the first number denotes the transmembrane domain and the second number denotes the residue position in relation to the most conserved amino acid, the latter always in position 50. E.g. Y7x 53 indicates that this tyrosine in located in TM7, in position 53. This numbering system is currently used by the GPCR database Appendix Aii [15Isberg V. et al.Generic GPCR residue numbers – aligning topology maps while minding the gaps.Trends Pharmacol. Sci. 2015; 36: 22-31Abstract Full Text Full Text PDF PubMed Scopus (268) Google Scholar].