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

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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.

Section snippets

Secretin family GPCRs

GPCRs are proven drug targets [1]. Mammalian GPCRs are subdivided into five classes based on sequence and structural homologies: rhodopsin (family A, class 1), secretin (family B, class 2), glutamate (family C, class 3), frizzled, and adhesion [2]. This review will focus on the secretin receptor family, which contains 15 GPCRs. These form the basis for a larger number of pharmacologically distinct receptors due to interactions with receptor activity-modifying proteins (RAMPs) (Table 1). They

Structural insight into peptide binding to secretin family GPCRs

The secretin family GPCR ECDs have a conserved overall fold consisting of two antiparallel β-sheets and an N-terminal α-helix that is stabilized by three disulfide bonds (Figure 1a). The structures solved include the human glucose-dependent insulinotropic polypeptide (GIP) receptor [13], human glucagon-like peptide-1 (GLP-1) receptor 14, 15, human PTH1 receptor 16, 17, human type 1 corticotropin-releasing factor (CRF1) receptor 18, 19 and the human CRF receptor [20].

These show that the

Oligomerization of secretin family GPCRs

Homodimerization and heterodimerization of GPCRs has been shown across rhodopsin and glutamate family GPCRs, although the functional and pharmacological consequences of such dimerization are frequently elusive [32]. There are examples of homodimerization and heterodimerization within secretin family GPCRs, but this might not be universal across the family 16, 33, 34, 35, 36, 37, 38, 39, 40, 41. Where dimerization does occur, it appears to involve the lipid-exposed helix of TM 4 33, 34, 37, 38.

Secretin family GPCR oligomerization with RAMPs

Some secretin family GPCRs can undergo oligomerization with RAMPs 42, 43, 44, 45. The three RAMPs (1–3) each comprise a single TM spanning helix with an ECD of ∼120–160 amino acids in length and a short intracellular C-terminus. The RAMP2 ECD is extended by approximately 20 amino acids. Although there are other membrane-spanning GPCR accessory proteins, RAMPs appear to be unique in their breadth of modifications to GPCR function [45]. Within secretin family GPCRs, RAMPs interact with the CT

Structural insight into RAMP interactions with GPCRs

Recent crystal structures provide important insights into RAMP interactions with GPCRs and their modification of pharmacology. These structures are all isolated ECDs. They are: human RAMP1 [protein data bank (PDB) code: 2YX8] [47], human CLR/RAMP1 (CGRP receptor; PDB codes: 3N7P, 3N7R, 3N7S) [12] and, most recently, human RAMP2 (PDB code: 2XVT; showing residues 58–135) [Quigley, A. et al., unpublished data].

The CLR/RAMP1 complex in conjunction with the new RAMP2 structure provides the most

Structural insight into modification of CLR pharmacology by RAMPs

These structures give us the opportunity to try to decipher how RAMPs modify secretin family GPCR pharmacology. Our focus is CLR because there are currently insufficient data to explain how RAMPs modify CT receptor pharmacology. We will initially consider the pharmacology of the cognate peptides for CLR/RAMP complexes, followed by small molecule ligands.

Implications for future drug development

The CGRP receptor structure potentially provides a gateway to the development of other molecules. For example, selective antagonists for other RAMP/GPCR complexes could be developed, initially through exploitation of homology models of these receptors, followed by further crystal structures. Olcegepant and telcagepant are essentially bivalent ligands. That is, the molecules can be considered to have CLR-binding components and RAMP1-binding components, a feature which could be effectively

Concluding remarks

Significant recent progress has been made in determining many ECD structures of secretin family GPCRs, including the structure of a CLR/RAMP1 complex. This structure demonstrates the importance of RAMPs for ligand binding, and provides the first structural insight into how RAMPs can alter receptor pharmacology. Further insight into how RAMPs modulate the pharmacology of secretin family GPCRs should arise from determination of the crystal structures of additional receptors.

Nevertheless, the

Acknowledgments

The authors acknowledge the Maurice Wilkins Centre and the University of Auckland Biopharma Thematic Research Initiative for their support. J.K.A. is supported by the National Health and Medical Research Council of Australia (C.J. Martin Fellowship).

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