The function of conserved cysteine residues in the extracellular domain of human receptor-activity-modifying protein

Development and validation of a reporter gene assay for bioactivity determination of Anti-CGRP monoclonal antibodies

Calcitonin gene-related peptide (CGRP) is critical for the pathophysiology of migraine, and four therapeutic antibodies targeting CGRP and its corresponding receptors have been approved by the Food and Drug Administration (FDA), while many others are in the different stages of clinical trials. Bioactivity determination is essential for the quality control and clinical application of therapeutic monoclonal antibodies (mAbs). However, no bioassay has been reported to date. In this study, we developed a reporter gene assay (RGA) based on SK-N-MC cells stably expressing firefly luciferase driven by cAMP response element (CRE). The key assay parameters were optimized according to signal-to-noise (SNR), the response value, and the fitted dose-response curve. Validation of the RGA in accordance with ICH-Q2 guidelines showed that the method had good specificity, accuracy, linearity, and precision. The established RGA can be utilized as a reference method for release testing and stability studies of relevant antibodies.

Introduction to RAMPs

Receptor activity modifying proteins (RAMPs) are single transmembrane proteins discovered for their role in the regulation of translocation of certain G-protein coupled receptors (GPCRs) to the plasma membrane. Since its discovery in 1998, several pivotal advances have been made in understanding the function of this family of proteins. This chapter provides a basic introduction to RAMPs as well as details on the various chapters in this book.

Shared and separate functions of the RAMP-based adrenomedullin receptors

Adrenomedullin (AM) is a novel hypotensive peptide that exerts a variety of strongly protective effects against multiorgan damage. AM-specific receptors were first identified as heterodimers composed of calcitonin-receptor-like receptor (CLR), a G protein coupled receptor, and one of two receptor activity-modifying proteins (RAMP2 or RAMP3), which are accessory proteins containing a single transmembrane domain. RAMPs are required for the surface delivery of CLR and the determination of its phenotype. CLR/RAMP2 (AM₁ receptor) is more highly AM-specific than CLR/RAMP3 (AM₂ receptor). Although there have been no reports showing differences in intracellular signaling via the two AM receptors, in vitro studies have shed light on their distinct trafficking and functionality. In addition, the tissue distributions of RAMP2 and RAMP3 differ, and their gene expression is differentially altered under pathophysiological conditions, which is suggestive of the separate roles played by AM₁ and AM₂ receptors in vivo. Both AM and the AM₁ receptor, but not the AM₂ receptor, are crucial for the development of the fetal cardiovascular system and are able to effectively protect against various vascular diseases. However, AM₂ receptors reportedly play an important role in maintaining a normal body weight in old age and may be involved in immune function. In this review article, we focus on the shared and separate functions of the AM receptor subtypes and also discuss the potential for related drug discovery. In addition, we mention their possible function as receptors for AM2 (or intermedin), an AM-related peptide whose biological functions are similar to those of AM.

Calcitonin gene-related peptide (CGRP) and its role in hypertension

Hypertension is still presently the number one "silent killer" in the Western World, and a major risk factor for the development of secondary diseases contributing to cardiovascular disease (CVD). However, despite a broad range of therapies, the mechanisms involved in the onset of hypertension remains unclear, therefore there is a real need to investigate the mechanisms involved. Calcitonin gene-related peptide (CGRP) is the most potent microvascular vasodilator known to date. Widely expressed in the nervous system, this peptide is considered to play a positive role in wound healing and protects against ischaemic and other traumas. However, whilst the protective mechanisms are not well understood, evidence indicates that these mechanisms become important in vascular-related stress. This review provides evidence that CGRP is both a potent vasodilator and hypotensive agent. However studies to date suggest that CGRP does not contribute to the physiological regulation of blood pressure. By comparing results from a range of human and animal studies, findings broadly suggest an association between CGRP and the pathophysiology of hypertension in terms of protective mechanisms, with possibly the RAMP1 component of the CGRP receptor playing a key role in the brain stem, in addition to peripheral receptors. The studies of agents that release CGRP agonists are at an early stage, with analogues for human use currently under development. However, at this stage, further research is required to establish the mechanisms by which CGRP is protective in the onset of hypertension, if novel and therapeutic modes of treatment are to be developed.

Receptor activity modifying proteins and their potential as drug targets

The receptor activity modifying proteins (RAMPs) are a family of membrane proteins that interact with some G protein-coupled receptors (GPCRs) to regulate their function. RAMPs can alter GPCR pharmacology and can regulate their signaling as well as trafficking to and from the cell surface. GPCRs have been successfully exploited as drug targets for many years. RAMPs therefore provide a new avenue for drug development, offering opportunities for regulating the function of therapeutically relevant RAMP-interacting GPCRs. RAMPs could be directly targeted themselves or advantage could be taken of the unique RAMP/GPCR interfaces for generating more selective drugs.

Structure-function analysis of RAMP1 by alanine mutagenesis

Receptor activity modifying protein 1 (RAMP1) is an integral component of several receptors including the calcitonin gene-related peptide (CGRP) receptor. It forms a complex with the calcitonin receptor-like receptor (CLR) and is required for receptor trafficking and ligand binding. The N-terminus of RAMP1 comprises three helices. The current study investigated regions of RAMP1 important for CGRP or CLR interactions by alanine mutagenesis. Modeling suggested the second and third helices were important in protein-protein interactions. Most of the conserved residues in the N-terminus (M48, W56, Y66, P85, N66, H97, F101, D113, P114, P115), together with a further 13 residues spread throughout three helices of RAMP1, were mutated to alanine and coexpressed with CLR in Cos 7 cells. None of the mutations significantly reduced RAMP expression. Of the nine mutants from helix 1, only M48A had any effect, producing a modest reduction in trafficking of CLR to the cell surface. In helix 2 Y66A almost completely abolished CLR trafficking; L69A and T73A reduced the potency of CGRP to produce cAMP. In helix 3, H97A abolished CLR trafficking; P85A, N86A, and F101A had caused modest reductions in CLR trafficking and also reduced the potency of CGRP on cAMP production. F93A caused a modest reduction in CLR trafficking alone and L94A increased cAMP production. The data are consistent with a CLR recognition site particularly involving Y66 and H97, with lesser roles for adjacent residues in helix 3. L69 and T73 may contribute to a CGRP recognition site in helix 2 also involving nearby residues.

Crystal structure of the human receptor activity-modifying protein 1 extracellular domain

Receptor activity-modifying protein (RAMP) 1 forms a heterodimer with calcitonin receptor-like receptor (CRLR) and regulates its transport to the cell surface. The CRLR.RAMP1 heterodimer functions as a specific receptor for calcitonin gene-related peptide (CGRP). Here, we report the crystal structure of the human RAMP1 extracellular domain. The RAMP1 structure is a three-helix bundle that is stabilized by three disulfide bonds. The RAMP1 residues important for cell-surface expression of the CRLR.RAMP1 heterodimer are clustered to form a hydrophobic patch on the molecular surface. The hydrophobic patch is located near the tryptophan residue essential for binding of the CGRP antagonist, BIBN4096BS. These results suggest that the hydrophobic patch participates in the interaction with CRLR and the formation of the ligand-binding pocket when it forms the CRLR.RAMP1 heterodimer.

Assembly and Signaling of CRLR and RAMP1 Complexes Assessed by BRET†

Biochemical and functional evidence suggest that the calcitonin receptor-like receptor (CRLR) interacts with receptor activity-modifying protein-1 (RAMP1) to generate a calcitonin gene-related peptide (CGRP) receptor. Using bioluminescence resonance energy transfer (BRET), we investigated the oligomeric assembly of the CRLR-RAMP1 signaling complex in living cells. As for their wild-type counterparts, fusion proteins linking CRLR and RAMP1 to the energy donor Renilla luciferase (Rluc) and energy acceptor green fluorescent protein (GFP) reach the cell surface only upon coexpression of CRLR and RAMP1. Radioligand binding and cAMP production assays also confirmed that the fusion proteins retained normal functional properties. BRET titration experiments revealed that CRLR and RAMP1 associate selectively to form heterodimers. This association was preserved for a mutated RAMP1 that cannot reach the cell surface, even in the presence of CRLR, indicating that the deficient targeting resulted from the altered conformation of the complex rather than a lack of heterodimerization. BRET analysis also showed that, in addition to associate with one another, both CRLR and RAMP1 can form homodimers. The homodimerization of the coreceptor was further confirmed by the ability of RAMP1 to prevent cell surface targeting of a truncated RAMP1 that normally exhibits receptor-independent plasma membrane delivery. Although the role of such dimerization remains unknown, BRET experiments clearly demonstrated that CRLR can engage signaling partners, such as G proteins and beta-arrestin, following CGRP stimulation, only in the presence of RAMP1. In addition to shed new light on the CRLR-RAMP1 signaling complex, the BRET assays developed herein offer new biosensors for probing CGRP receptor activity.

RAMPs: the past, present and future

The discovery of receptor-activity-modifying proteins (RAMPs) as accessory proteins required for the appropriate localization and function of certain G-protein coupled receptors (GPCRs) produced a paradigm shift in our understanding of GPCR regulation. Three RAMPs have now been demonstrated to be crucial for various aspects of the life cycle of calcitonin-like receptor (CLR) including endoplasmic reticulum-to-Golgi translocation, internalization and recycling. Although the RAMP-CLR interaction was the first to be identified, other GPCRs belonging to both the class B and C families of GPCRs also seem to be regulated by RAMPs. The recent advances in our knowledge of the cellular and biochemical regulation of RAMPs and how they in turn regulate the life cycle of GPCRs could lead to therapeutic advances in several diseases.

Characterization of the structure of RAMP1 by mutagenesis and molecular modeling

Receptor activity modifying proteins (RAMPs) are a family of single-pass transmembrane proteins that dimerize with G-protein-coupled receptors. They may alter the ligand recognition properties of the receptors (particularly for the calcitonin receptor-like receptor, CLR). Very little structural information is available about RAMPs. Here, an ab initio model has been generated for the extracellular domain of RAMP1. The disulfide bond arrangement (Cys27-Cys82, Cys40-Cys72, and Cys57-Cys104) was determined by site-directed mutagenesis. The secondary structure (alpha-helices from residues 29-51, 60-80, and 87-100) was established from a consensus of predictive routines. Using these constraints, an assemblage of 25,000 structures was constructed and these were ranked using an all-atom statistical potential. The best 1000 conformations were energy minimized. The lowest scoring model was refined by molecular dynamics simulation. To validate our strategy, the same methods were applied to three proteins of known structure; PDB:1HP8, PDB:1V54 chain H (residues 21-85), and PDB:1T0P. When compared to the crystal structures, the models had root mean-square deviations of 3.8 A, 4.1 A, and 4.0 A, respectively. The model of RAMP1 suggested that Phe93, Tyr100, and Phe101 form a binding interface for CLR, whereas Trp74 and Phe92 may interact with ligands that bind to the CLR/RAMP1 heterodimer.

GPCR modulation by RAMPs

Our conceptual understanding of the molecular architecture of G-protein coupled receptors (GPCRs) has transformed over the last decade. Once considered as largely independent functional units (aside from their interaction with the G-protein itself), it is now clear that a single GPCR is but part of a multifaceted signaling complex, each component providing an additional layer of sophistication. Receptor activity-modifying proteins (RAMPs) provide a notable example of proteins that interact with GPCRs to modify their function. They act as pharmacological switches, modifying GPCR pharmacology for a particular subset of receptors. However, there is accumulating evidence that these ubiquitous proteins have a broader role, regulating signaling and receptor trafficking. This article aims to provide the reader with a comprehensive appraisal of RAMP literature and perhaps some insight into the impact that their discovery has had on those who study GPCRs.

Heterodimers and family-B GPCRs: RAMPs, CGRP and adrenomedullin

RAMPs (receptor activity-modifying proteins) are single-pass transmembrane proteins that associate with certain family-B GPCRs (G-protein-coupled receptors). Specifically for the CT (calcitonin) receptor-like receptor and the CT receptor, this results in profound changes in ligand binding and receptor pharmacology, allowing the generation of six distinct receptors with preferences for CGRP (CT gene-related peptide), adrenomedullin, amylin and CT. There are three RAMPs: RAMP1–RAMP3. The N-terminus appears to be the main determinant of receptor pharmacology, whereas the transmembrane domain contributes to association of the RAMP with the GPCR. The N-terminus of all members of the RAMP family probably contains two disulphide bonds; a potential third disulphide is found in RAMP1 and RAMP3. The N-terminus appears to be in close proximity to the ligand and plays a key role in its binding, either directly or indirectly. BIBN4096BS, a CGRP antagonist, targets RAMP1 and this gives the compound very high selectivity for the human CGRP1 receptor.