Joseph J. Gingell

Structural insights into RAMP modification of secretin family G protein-coupled receptors: implications for drug development

Secretin family G protein-coupled receptors (GPCRs) are important therapeutic targets for migraine, diabetes, bone disorders, inflammatory disorders and cardiovascular disease. They possess a large N-terminal extracellular domain (ECD) known to be the primary ligand-binding determinant. Structural determination of several secretin family GPCR ECDs in complex with peptide ligands has been achieved recently, providing insight into the molecular determinants of hormone binding. Some secretin family GPCRs associate with receptor activity-modifying proteins (RAMPs), resulting in changes to receptor pharmacology. Recently, the first crystal structure of a RAMP ECD in complex with a secretin family GPCR was solved, revealing the elegant mechanism governing receptor selectivity of small molecule antagonists of the calcitonin gene-related peptide (CGRP) receptor. Here we review the structural basis of ligand binding to secretin family GPCRs, concentrating on recent progress made on the structural basis of RAMP-modified GPCR pharmacology and its implications for rational drug design.

Calcitonin and calcitonin receptor-like receptors: common themes with family B GPCRs?

The calcitonin receptor (CTR) and calcitonin receptor‐like receptor (CLR) are two of the 15 human family B (or Secretin‐like) GPCRs. CTR and CLR are of considerable biological interest as their pharmacology is moulded by interactions with receptor activity‐modifying proteins. They also have therapeutic relevance for many conditions, such as osteoporosis, diabetes, obesity, lymphatic insufficiency, migraine and cardiovascular disease. In light of recent advances in understanding ligand docking and receptor activation in both the family as a whole and in CLR and CTR specifically, this review reflects how applicable general family B GPCR themes are to these two idiosyncratic receptors. We review the main functional domains of the receptors; the N‐terminal extracellular domain, the juxtamembrane domain and ligand interface, the transmembrane domain and the intracellular C‐terminal domain. Structural and functional findings from the CLR and CTR along with other family B GPCRs are critically appraised to gain insight into how these domains may function. The ability for CTR and CLR to interact with receptor activity‐modifying proteins adds another level of sophistication to these receptor systems but means careful consideration is needed when trying to apply generic GPCR principles. This review encapsulates current thinking in the realm of family B GPCR research by highlighting both conflicting and recurring themes and how such findings relate to two unusual but important receptors, CTR and CLR.

A key role for tryptophan 84 in receptor activity-modifying protein 1 in the amylin 1 receptor

Amylin (Amy) receptors are complexes of the calcitonin receptor with receptor activity-modifying proteins. RAMP1 with the calcitonin receptor forms the AMY(1) receptor; the insert negative isoform of the calcitonin receptor in this complex makes the AMY(1(a)) receptor. This receptor has high affinity for Amy and the related peptide calcitonin gene-related peptide (CGRP). Amy is a peptide that has a role in lowering blood glucose levels and therefore its receptors represent potential drug targets for the treatment of diabetes. It has been suggested that the peptides bind in a pocket formed between the long N-termini of the calcitonin receptor and RAMP1, although very few residues in either component have been assigned specific roles. Based on the crystal structure of the RAMP1 N-terminus, the RAMP1 residues Arg67, Asp71, Glu78, Trp74 and Trp84 were identified as potentially having a role in peptide binding. Here, Arg67, Asp71, Glu78 and Trp84 were individually mutated to alanine and the function of mutant AMY(1(a)) receptors was determined using a cAMP assay. ELISA was used to measure cell surface expression and western blotting for total expression. Mutation of Arg67, Asp71 and Glu78 had no significant effect on Amy or CGRP potency, cell surface or total expression. Trp84Ala, however, resulted in a significant reduction in agonist potency and cell surface expression. Interestingly, a Trp84Phe substitution was able to restore some of this function, without restoring cell surface expression, suggesting that the residue may be important for peptide interactions. The data reveal the importance of Trp84 in the AMY(1(a)) receptor.

Activity of pramlintide, rat and human amylin but not Aβ1-42 at human amylin receptors.

Amylin is a neuroendocrine hormone involved in glucose regulation. An amylin analog, pramlintide, is used to treat insulin-requiring diabetes. Its anorexigenic actions give it potential as an obesity treatment. There are 3 amylin receptors (AMY1, AMY2, AMY3), comprising the calcitonin receptor and receptor activity-modifying proteins 1, 2, and 3, respectively. The pharmacology of pramlintide at each subtype has not been determined whereas the unrelated peptide β-amyloid 1-42 (Aβ1-42) has recently been proposed to be a specific agonist of the AMY3 receptor. We investigated the actions of Aβ1-42 and pramlintide, compared with human and rat amylin at the calcitonin receptor, AMY1, AMY2, and AMY3 receptors, measuring the cAMP response in human embryonic kidney 293S and Cos 7 cells. Pramlintide activated all receptors with a slight preference for AMY1. No cAMP response was detected with Aβ1-42 at any receptor, suggesting that it may not be a genuine agonist of AMY receptors.

Receptor Activity-modifying Proteins 2 and 3 Generate Adrenomedullin Receptor Subtypes with Distinct Molecular Properties.

Adrenomedullin (AM) is a peptide hormone with numerous effects in the vascular systems. AM signals through the AM1 and AM2 receptors formed by the obligate heterodimerization of a G protein-coupled receptor, the calcitonin receptor-like receptor (CLR), and receptor activity-modifying proteins 2 and 3 (RAMP2 and RAMP3), respectively. These different CLR-RAMP interactions yield discrete receptor pharmacology and physiological effects. The effective design of therapeutics that target the individual AM receptors is dependent on understanding the molecular details of the effects of RAMPs on CLR. To understand the role of RAMP2 and -3 on the activation and conformation of the CLR subunit of AM receptors, we mutated 68 individual amino acids in the juxtamembrane region of CLR, a key region for activation of AM receptors, and determined the effects on cAMP signaling. Sixteen CLR mutations had differential effects between the AM1 and AM2 receptors. Accompanying this, independent molecular modeling of the full-length AM-bound AM1 and AM2 receptors predicted differences in the binding pocket and differences in the electrostatic potential of the two AM receptors. Druggability analysis indicated unique features that could be used to develop selective small molecule ligands for each receptor. The interaction of RAMP2 or RAMP3 with CLR induces conformational variation in the juxtamembrane region, yielding distinct binding pockets, probably via an allosteric mechanism. These subtype-specific differences have implications for the design of therapeutics aimed at specific AM receptors and for understanding the mechanisms by which accessory proteins affect G protein-coupled receptor function.

An allosteric role for receptor activity-modifying proteins in defining GPCR pharmacology.

G protein-coupled receptors are allosteric proteins that control transmission of external signals to regulate cellular response. Although agonist binding promotes canonical G protein signalling transmitted through conformational changes, G protein-coupled receptors also interact with other proteins. These include other G protein-coupled receptors, other receptors and channels, regulatory proteins and receptor-modifying proteins, notably receptor activity-modifying proteins (RAMPs). RAMPs have at least 11 G protein-coupled receptor partners, including many class B G protein-coupled receptors. Prototypic is the calcitonin receptor, with altered ligand specificity when co-expressed with RAMPs. To gain molecular insight into the consequences of this protein–protein interaction, we combined molecular modelling with mutagenesis of the calcitonin receptor extracellular domain, assessed in ligand binding and functional assays. Although some calcitonin receptor residues are universally important for peptide interactions (calcitonin, amylin and calcitonin gene-related peptide) in calcitonin receptor alone or with receptor activity-modifying protein, others have RAMP-dependent effects, whereby mutations decreased amylin/calcitonin gene-related peptide potency substantially only when RAMP was present. Remarkably, the key residues were completely conserved between calcitonin receptor and AMY receptors, and between subtypes of AMY receptor that have different ligand preferences. Mutations at the interface between calcitonin receptor and RAMP affected ligand pharmacology in a RAMP-dependent manner, suggesting that RAMP may allosterically influence the calcitonin receptor conformation. Supporting this, molecular dynamics simulations suggested that the calcitonin receptor extracellular N-terminal domain is more flexible in the presence of receptor activity-modifying protein 1. Thus, RAMPs may act in an allosteric manner to generate a spectrum of unique calcitonin receptor conformational states, explaining the pharmacological preferences of calcitonin receptor-RAMP complexes. This provides novel insight into our understanding of G protein-coupled receptor-protein interaction that is likely broadly applicable for this receptor class.G protein-coupled receptors are allosteric proteins that control transmission of external signals to regulate cellular response. Although agonist binding promotes canonical G protein signalling transmitted through conformational changes, G protein-coupled receptors also interact with other proteins. These include other G protein-coupled receptors, other receptors and channels, regulatory proteins and receptor-modifying proteins, notably receptor activity-modifying proteins (RAMPs). RAMPs have at least 11 G protein-coupled receptor partners, including many class B G protein-coupled receptors. Prototypic is the calcitonin receptor, with altered ligand specificity when co-expressed with RAMPs. To gain molecular insight into the consequences of this protein–protein interaction, we combined molecular modelling with mutagenesis of the calcitonin receptor extracellular domain, assessed in ligand binding and functional assays. Although some calcitonin receptor residues are universally important for peptide interactions (calcitonin, amylin and calcitonin gene-related peptide) in calcitonin receptor alone or with receptor activity-modifying protein, others have RAMP-dependent effects, whereby mutations decreased amylin/calcitonin gene-related peptide potency substantially only when RAMP was present. Remarkably, the key residues were completely conserved between calcitonin receptor and AMY receptors, and between subtypes of AMY receptor that have different ligand preferences. Mutations at the interface between calcitonin receptor and RAMP affected ligand pharmacology in a RAMP-dependent manner, suggesting that RAMP may allosterically influence the calcitonin receptor conformation. Supporting this, molecular dynamics simulations suggested that the calcitonin receptor extracellular N-terminal domain is more flexible in the presence of receptor activity-modifying protein 1. Thus, RAMPs may act in an allosteric manner to generate a spectrum of unique calcitonin receptor conformational states, explaining the pharmacological preferences of calcitonin receptor-RAMP complexes. This provides novel insight into our understanding of G protein-coupled receptor-protein interaction that is likely broadly applicable for this receptor class.

Receptor activity-modifying proteins; multifunctional G protein-coupled receptor accessory proteins.

Receptor activity-modifying proteins (RAMPs) are single pass membrane proteins initially identified by their ability to determine the pharmacology of the calcitonin receptor-like receptor (CLR), a family B G protein-coupled receptor (GPCR). It is now known that RAMPs can interact with a much wider range of GPCRs. This review considers recent developments on the structure of the complexes formed between the extracellular domains (ECDs) of CLR and RAMP1 or RAMP2 as these provide insights as to how the RAMPs direct ligand binding. The range of RAMP interactions is also considered; RAMPs can interact with numerous family B GPCRs as well as examples of family A and family C GPCRs. They influence receptor expression at the cell surface, trafficking, ligand binding and G protein coupling. The GPCR-RAMP interface offers opportunities for drug targeting, illustrated by examples of drugs developed for migraine.

N-Glycosylation of Asparagine 130 in the Extracellular Domain of the Human Calcitonin Receptor Significantly Increases Peptide Hormone Affinity

The calcitonin receptor (CTR) is a class B G protein-coupled receptor that is activated by the peptide hormones calcitonin and amylin. Calcitonin regulates bone remodeling through CTR, whereas amylin regulates blood glucose and food intake by activating CTR in complex with receptor activity-modifying proteins (RAMPs). These receptors are targeted clinically for the treatment of osteoporosis and diabetes. Here, we define the role of CTR N-glycosylation in hormone binding using purified calcitonin and amylin receptor extracellular domain (ECD) glycoforms and fluorescence polarization/anisotropy and isothermal titration calorimetry peptide-binding assays. N-Glycan-free CTR ECD produced in Escherichia coli exhibited ∼10-fold lower peptide affinity than CTR ECD produced in HEK293T cells, which yield complex N-glycans, or in HEK293S GnTI- cells, which yield core N-glycans (Man5GlcNAc2). PNGase F-catalyzed removal of N-glycans at N73, N125, and N130 in the CTR ECD decreased peptide affinity ∼10-fold, whereas Endo H-catalyzed trimming of the N-glycans to single GlcNAc residues had no effect on peptide binding. Similar results were observed for an amylin receptor RAMP2-CTR ECD complex. Characterization of peptide-binding affinities of purified N → Q CTR ECD glycan site mutants combined with PNGase F and Endo H treatment strategies and mass spectrometry to define the glycan species indicated that a single GlcNAc residue at CTR N130 was responsible for the peptide affinity enhancement. Molecular modeling suggested that this GlcNAc functions through an allosteric mechanism rather than by directly contacting the peptide. These results reveal an important role for N-linked glycosylation in the peptide hormone binding of a clinically relevant class B GPCR.

Photoaffinity cross-linking and unnatural amino acid mutagenesis reveal insights into calcitonin gene-related peptide binding to the calcitonin receptor-like receptor/receptor activity-modifying protein 1 (CLR/RAMP1) complex

Calcitonin gene-related peptide (CGRP) binds to the complex of the calcitonin receptor-like receptor (CLR) with receptor activity-modifying protein 1 (RAMP1). How CGRP interacts with the transmembrane domain (including the extracellular loops) of this family B receptor remains unclear. In this study, a photoaffinity cross-linker, p-azido l-phenylalanine (azF), was incorporated into CLR, chiefly in the second extracellular loop (ECL2) using genetic code expansion and unnatural amino acid mutagenesis. The method was optimized to ensure efficient photolysis of azF residues near the transmembrane bundle of the receptor. A CGRP analogue modified with fluorescein at position 15 was used for detection of ultraviolet-induced cross-linking. The methodology was verified by confirming the known contacts of CGRP to the extracellular domain of CLR. Within ECL2, the chief contacts were I284 on the loop itself and L291, at the top of the fifth transmembrane helix (TM5). Minor contacts were noted along the lip of ECL2 between S286 and L290 and also with M223 in TM3 and F349 in TM6. Full length molecular models of the bound receptor complex suggest that CGRP sits at the top of the TM bundle, with Thr6 of the peptide making contacts with L291 and H295. I284 is likely to contact Leu12 and Ala13 of CGRP, and Leu16 of CGRP is at the ECL/extracellular domain boundary of CLR. The reduced potency, Emax, and affinity of [Leu16Ala]-human α CGRP are consistent with this model. Contacts between Thr6 of CGRP and H295 may be particularly important for receptor activation.

Class B GPCR: Receptors and RAMPs

Receptor activity-modifying proteins (RAMPs) are a family of three single transmembrane spanning proteins. They were first identified based on their ability to facilitate the cell surface expression of a G protein-coupled receptor (GPCR), named the calcitonin receptor-like receptor (CLR). They also determine whether CLR can be activated by calcitonin-gene related peptide (CGRP) or adrenomedullin (AM). We now know that RAMPs can interact with a number of GPCRs, including the calcitonin receptor. The interaction between a GPCR and RAMP can influence cell surface expression and trafficking, determine hormone specificity and regulate intracellular signaling and/or G protein-coupling in a receptor-specific manner. This chapter will review recent advances in the RAMP field, paying particular attention to receptors formed by the co-expression of CLR and RAMPs. We will detail the known GPCR partners for RAMPs and describe how these interactions with RAMP can influence GPCR function.