Receptor activity-modifying protein dependent and independent activation mechanisms in the coupling of calcitonin gene-related peptide and adrenomedullin receptors to Gs

Pharmacological characterisation of erenumab, Aimovig, at two calcitonin gene-related peptide responsive receptors

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) is involved in migraine pathophysiology. CGRP can signal through two receptors. The canonical CGRP receptor comprises the calcitonin receptor-like receptor and receptor activity-modifying protein 1 (RAMP1); the AMY1 receptor comprises the calcitonin receptor with RAMP1. Drugs that reduce CGRP activity, such as receptor antagonists, are approved for the treatment and prevention of migraine. Despite being designed to target the canonical CGRP receptor, emerging evidence suggests that these antagonists, including erenumab (a monoclonal antibody antagonist) can also antagonise the AMY1 receptor. However, it is difficult to estimate its selectivity because direct comparisons between receptors under matched conditions have not been made. We therefore characterised erenumab at both CGRP-responsive receptors with multiple ligands, including αCGRP and βCGRP.

EXPERIMENTAL APPROACH

Erenumab antagonism was quantified through IC50 and pKB experiments, measuring cAMP production. We used SK-N-MC cells which endogenously express the human CGRP receptor, and HEK293S and Cos7 cells transiently transfected to express either human CGRP or AMY1 receptors.

KEY RESULTS

Erenumab antagonised both the CGRP and AMY1 receptors with an ~20-120-fold preference for the CGRP receptor, depending on the cells, agonist, analytical approach and/or assay format. Erenumab antagonised both forms of CGRP equally, and appeared to act as a competitive reversible antagonist at both receptors.

CONCLUSION AND IMPLICATIONS

Despite being designed to target the CGRP receptor, erenumab can antagonise the AMY1 receptor. Its ability to antagonise CGRP activity at both receptors may be useful in better understanding the clinical profile of erenumab.

Shared and independent roles of CGRP and PACAP in migraine pathophysiology

The neuropeptides calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have emerged as mediators of migraine pathogenesis. Both are vasodilatory peptides that can cause migraine-like attacks when infused into people and migraine-like symptoms when injected into rodents. In this narrative review, we compare the similarities and differences between the peptides in both their clinical and preclinical migraine actions. A notable clinical difference is that PACAP, but not CGRP, causes premonitory-like symptoms in patients. Both peptides are found in distinct, but overlapping areas relevant to migraine, most notably with the prevalence of CGRP in trigeminal ganglia and PACAP in sphenopalatine ganglia. In rodents, the two peptides share activities, including vasodilation, neurogenic inflammation, and nociception. Most strikingly, CGRP and PACAP cause similar migraine-like symptoms in rodents that are manifested as light aversion and tactile allodynia. Yet, the peptides appear to act by independent mechanisms possibly by distinct intracellular signaling pathways. The complexity of these signaling pathways is magnified by the existence of multiple CGRP and PACAP receptors that may contribute to migraine pathogenesis. Based on these differences, we suggest PACAP and its receptors provide a rich set of targets to complement and augment the current CGRP-based migraine therapeutics.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Elucidating the Interactome of G Protein-Coupled Receptors and Receptor Activity-Modifying Proteins

G protein-coupled receptors (GPCRs) are known to interact with several other classes of integral membrane proteins that modulate their biology and pharmacology. However, the extent of these interactions and the mechanisms of their effects are not well understood. For example, one class of GPCR-interacting proteins, receptor activity-modifying proteins (RAMPs), comprise three related and ubiquitously expressed single-transmembrane span proteins. The RAMP family was discovered more than two decades ago, and since then GPCR-RAMP interactions and their functional consequences on receptor trafficking and ligand selectivity have been documented for several secretin (class B) GPCRs, most notably the calcitonin receptor-like receptor. Recent bioinformatics and multiplexed experimental studies suggest that GPCR-RAMP interactions might be much more widespread than previously anticipated. Recently, cryo-electron microscopy has provided high-resolution structures of GPCR-RAMP-ligand complexes, and drugs have been developed that target GPCR-RAMP complexes. In this review, we provide a summary of recent advances in techniques that allow the discovery of GPCR-RAMP interactions and their functional consequences and highlight prospects for future advances. We also provide an up-to-date list of reported GPCR-RAMP interactions based on a review of the current literature. SIGNIFICANCE STATEMENT: Receptor activity-modifying proteins (RAMPs) have emerged as modulators of many aspects of G protein-coupled receptor (GPCR)biology and pharmacology. The application of new methodologies to study membrane protein-protein interactions suggests that RAMPs interact with many more GPCRs than had been previously known. These findings, especially when combined with structural studies of membrane protein complexes, have significant implications for advancing GPCR-targeted drug discovery and the understanding of GPCR pharmacology, biology, and regulation.

Novel Fluorescently Labeled PACAP and VIP Highlight Differences between Peptide Internalization and Receptor Pharmacology

The related peptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) have diverse biological functions in peripheral tissues and the central nervous system. Therefore, these peptides and their three receptors represent potential drug targets for several conditions, including neurological and pain-related disorders. However, very little is known about how these peptides regulate their receptors through processes such as internalization. Therefore, we developed tools to study receptor regulation through the synthesis of fluorescently labeled analogues of PACAP-38, PACAP-27, and VIP using copper-mediated 1,3-dipolar cycloaddition of the Cy5 fluorophore. The functionality of Cy5-labeled peptides at their receptors was confirmed in cAMP accumulation assays. Internalization of the Cy5-labeled peptides was then examined and quantified at two distinct PAC₁ receptor splice variants, VPAC₁ and VPAC₂ receptors in transfected cells. All labeled peptides were functional, exhibiting comparable cAMP pharmacology to their unlabeled counterparts and underwent internalization in a time-dependent manner. Temporal differences in the internalization profiles were observed between Cy5-labeled peptides at the PAC_1n, PAC_1s, VPAC₁, and VPAC₂ receptors. Interestingly, the pattern of Cy5-labeled peptide activity differed for cAMP accumulation and internalization, indicating that these peptides differentially stimulate cAMP accumulation and internalization and therefore display biased agonism. This novel insight into PACAP-responsive receptor signaling and internalization may provide a unique avenue for future therapeutic development. The fluorescently labeled PACAP and VIP peptides described herein, which we validated as tools to study receptor internalization, will have utility across a broad range of applications and provide greater insight into this receptor family.

Lipidated Calcitonin Gene-Related Peptide (CGRP) Peptide Antagonists Retain CGRP Receptor Activity and Attenuate CGRP Action In Vivo

Signaling through calcitonin gene-related peptide (CGRP) receptors is associated with pain, migraine, and energy expenditure. Small molecule and monoclonal antibody CGRP receptor antagonists that block endogenous CGRP action are in clinical use as anti-migraine therapies. By comparison, the potential utility of peptide antagonists has received less attention due to suboptimal pharmacokinetic properties. Lipidation is an established strategy to increase peptide half-life in vivo. This study aimed to explore the feasibility of developing lipidated CGRP peptide antagonists that retain receptor antagonist activity in vitro and attenuate endogenous CGRP action in vivo. CGRP peptide analogues based on the archetypal CGRP receptor antagonist, CGRP_8-37, were palmitoylated at the N-terminus, position 24, and near the C-terminus at position 35. The antagonist activities of the lipidated peptide analogues were tested in vitro using transfected Cos-7 cells expressing either the human or mouse CGRP receptor, amylin subtype 1 (AMY₁) receptor, adrenomedullin (AM) receptors, or calcitonin receptor. Antagonist activities were also evaluated in SK-N-MC cells that endogenously express the human CGRP receptor. Lipidated peptides were then tested for their ability to antagonize endogenous CGRP action in vivo using a capsaicin-induced dermal vasodilation (CIDV) model in C57/BL6J mice. All lipidated peptides except for the C-terminally modified analogue retained potent antagonist activity compared to CGRP_8-37 towards the CGRP receptor. The lipidated peptides also retained, and sometimes gained, antagonist activities at AMY₁, AM₁ and AM₂ receptors. Several lipidated peptides produced robust inhibition of CIDV in mice. This study demonstrates that selected lipidated peptide antagonists based on αCGRP_8-37 retain potent antagonist activity at the CGRP receptor and are capable of inhibition of endogenous CGRP action in vivo. These findings suggest that lipidation can be applied to peptide antagonists, such as αCGRP_8-37 and are a potential strategy for antagonizing CGRP action.

Vasopressin-oxytocin-type signaling is ancient and has a conserved water homeostasis role in euryhaline marine planarians

Vasopressin/oxytocin (VP/OT)-related peptides are essential for mammalian antidiuresis, sociosexual behavior, and reproduction. However, the evolutionary origin of this peptide system is still uncertain. Here, we identify orthologous genes to those for VP/OT in Platyhelminthes, intertidal planarians that have a simple bilaterian body structure but lack a coelom and body-fluid circulatory system. We report a comprehensive characterization of the neuropeptide derived from this VP/OT-type gene, identifying its functional receptor, and name it the "platytocin" system. Our experiments with these euryhaline planarians, living where environmental salinities fluctuate due to evaporation and rainfall, suggest that platytocin functions as an "antidiuretic hormone" and also organizes diverse actions including reproduction and chemosensory-associated behavior. We propose that bilaterians acquired physiological adaptations to amphibious lives by such regulation of the body fluids. This neuropeptide-secreting system clearly became indispensable for life even without the development of a vascular circulatory system or relevant synapses.

Pharmacological characterisation of mouse calcitonin and calcitonin receptor-like receptors reveals differences compared with human receptors

BACKGROUND AND PURPOSE

The calcitonin (CT) receptor family is complex, comprising two receptors (the CT receptor [CTR] and the CTR-like receptor [CLR]), three accessory proteins (RAMPs) and multiple endogenous peptides. This family contains several important drug targets, including CGRP, which is targeted by migraine therapeutics. The pharmacology of this receptor family is poorly characterised in species other than rats and humans. To facilitate understanding of translational and preclinical data, we need to know the receptor pharmacology of this family in mice.

EXPERIMENTAL APPROACH

Plasmids encoding mouse CLR/CTR and RAMPs were transiently transfected into Cos-7 cells. cAMP production was measured in response to agonists in the absence or presence of antagonists.

KEY RESULTS

We report the first synthesis and characterisation of mouse adrenomedullin, adrenomedullin 2 and βCGRP and of mouse CTR without or with mouse RAMPs. Receptors containing m-CTR had subtly different pharmacology than human receptors; they were promiscuous in their pharmacology, both with and without RAMPs. Several peptides, including mouse αCGRP and mouse adrenomedullin 2, were potent agonists of the m-CTR:m-RAMP3 complex. Pharmacological profiles of receptors comprising m-CLR:m-RAMPs were generally similar to those of their human counterparts, albeit with reduced specificity.

CONCLUSION AND IMPLICATIONS

Mouse receptor pharmacology differed from that in humans, with mouse receptors displaying reduced discrimination between ligands. This creates challenges for interpreting which receptor may underlie an effect in preclinical models and thus translation of findings from mice to humans. It also highlights the need for new ligands to differentiate between these complexes.

LINKED ARTICLES

This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary).. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Molecular simulations reveal the impact of RAMP1 on ligand binding and dynamics of calcitonin gene-related peptide receptor (CGRPR) heterodimer

Calcitonin gene-related peptide receptor (CGRPR) is a heterodimer protein complex consisting of a class-B G protein-coupled receptor (GPCR) named calcitonin receptor-like receptor (CLR) and an accessory protein, receptor activity modifying protein type 1 (RAMP1). Here in this study, with several molecular modeling approaches and molecular dynamics (MD) simulations, the structural and dynamical effects of RAMP1 on the binding of small molecule CGRPR inhibitors (namely rimegepant and telcagepant) to the CGRPR extracellular ectodomain complex site (site 1) and transmembrane binding site (site 2) are investigated. Results showed that although these molecules stay stable at site 1, they can also bind to site 2, which may be interpreted as non-specificity of the ligands, however, most of these interactions at transmembrane binding site are not sustainable or are weak. Furthermore, to examine the site 2 for gepant binding, different in silico experiments (i.e., alanine scanning mutagenesis, SiteMap, ligand decomposition binding free energy analyses) are also conducted and the results confirmed the putative binding pocket (site 2) of the gepants at the CGRPRs.

Characterisation of agonist signalling profiles and agonist-dependent antagonism at PACAP-responsive receptors: Implications for drug discovery

BACKGROUND AND PURPOSE

The pituitary adenylate cyclase-activating peptide (PACAP) family is of clinical interest for the treatment of migraine. These peptides activate three different PACAP-responsive class B G protein-coupled receptors: the PAC₁ , VPAC₁ and VPAC₂ receptors. The PAC₁ receptor may be alternatively spliced, generating variants that can differ in their pharmacological or signalling profiles. To inform drug discovery efforts targeting migraine, we need to better understand how the different PACAP-responsive receptors signal and how effectively these responses can be blocked by antagonists.

EXPERIMENTAL APPROACH

The signalling profiles of the human PAC_1n , PAC_1s , VPAC₁ and VPAC₂ receptors were examined in transfected Cos7 cells for cAMP, IP₁ , pAkt, pERK and pCREB. Biased signalling was then quantified. The ability of antagonists to block PACAP-38, PACAP-27 or VIP stimulated cAMP accumulation at PACAP-responsive receptors was also determined.

KEY RESULTS

PACAP-responsive receptors exhibited varied pharmacological profiles but activated signalling in a similar manner. The PAC_1n and PAC_1s receptors displayed distinct pharmacology. At the PAC_1s receptor, VIP and PHM were more potent than at the PAC_1n receptor. PACAP-responsive receptors displayed agonist-dependent antagonism where PACAP-38 was less effectively antagonised compared to PACAP-27 and VIP.

CONCLUSIONS AND IMPLICATIONS

The distinct pharmacological profile displayed by the PAC_1s receptor suggests that it can act as a dual receptor for VIP and PACAP. Furthermore, the effectiveness of blocking a signalling pathway can be influenced by which endogenous PACAP family agonist is present. These effects have potential implications for the development and effectiveness of drugs targeting the PACAP system.

To Probe the Binding Interactions between Two FDA Approved Migraine Drugs (Ubrogepant and Rimegepant) and Calcitonin-Gene Related Peptide Receptor (CGRPR) Using Molecular Dynamics Simulations

Recently, the FDA approved ubrogepant and rimegepant as oral drugs to treat migraines by targeting the calcitonin-gene related peptide receptor (CGRPR). Unfortunately, there is no high-resolution complex structure with these two drugs; thus the detailed interaction between drugs and the receptor remains elusive. This study uses molecular docking and molecular dynamics simulation to model the drug-receptor complex and analyze their binding interactions at a molecular level. The complex crystal structure (3N7R) of the gepant drugs' predecessor, olcegepant, was used for our molecular docking of the two drugs and served as a control system. The three systems, with ubrogepant, rimegepant, and crystal olcegepant, were subject to 3 × 1000 ns molecular dynamics simulations and followed by the simulation interaction diagram (SID), structural clustering, and MM-GBSA binding energy analyses. Our MD data revealed that olcegepant binds most strongly to the CGRPR, followed by ubrogepant and then rimegepant, largely due to changes in hydrophobic and electrostatic interactions. The order of our MM-GBSA binding energies of these three compounds is consistent with their experimental IC₅₀ values. SID analysis revealed the pharmacophore of the gepant class to be the dihydroquinazolinone group derivative. Subtle differences in interaction profile have been noted, including interactions with the W74 and W72 residues. The ubrogepant and rimegepant both contact A70 and M42 of the receptor, while olcegepant does not. The results of this study elucidate the interactions in the binding pocket of CGRP receptor and can assist in further development for orally available antagonists of the CGRP receptor.

CGRP, adrenomedullin and adrenomedullin 2 display endogenous GPCR agonist bias in primary human cardiovascular cells

Agonist bias occurs when different ligands produce distinct signalling outputs when acting at the same receptor. However, its physiological relevance is not always clear. Using primary human cells and gene editing techniques, we demonstrate endogenous agonist bias with physiological consequences for the calcitonin receptor-like receptor, CLR. By switching the receptor-activity modifying protein (RAMP) associated with CLR we can “re-route” the physiological pathways activated by endogenous agonists calcitonin gene-related peptide (CGRP), adrenomedullin (AM) and adrenomedullin 2 (AM2). AM2 promotes calcium-mediated nitric oxide signalling whereas CGRP and AM show pro-proliferative effects in cardiovascular cells, thus providing a rationale for the expression of the three peptides. CLR-based agonist bias occurs naturally in human cells and has a fundamental purpose for its existence. We anticipate this will be a starting point for more studies into RAMP function in native environments and their importance in endogenous GPCR signalling.

Clark et al. explore the ability of ligands to activate the calcitonin-like receptor (CLR) in primary endothelial cells, and the influence of co-expressed receptor-activity modifying proteins (RAMPs). Their study reveals that GPCR agonist bias occurs naturally in human cells and plays a fundamental role in providing unique functions to endogenous agonists.

PACAP Induces Light Aversion in Mice by an Inheritable Mechanism Independent of CGRP

The neuropeptides CGRP (calcitonin gene-related peptide) and PACAP (pituitary adenylate cyclase-activating polypeptide) have emerged as mediators of migraine, yet the potential overlap of their mechanisms remains unknown. Infusion of PACAP, like CGRP, can cause migraine in people, and both peptides share similar vasodilatory and nociceptive functions. In this study, we have used light aversion in mice as a surrogate for migraine-like photophobia to compare CGRP and PACAP and ask whether CGRP or PACAP actions were dependent on each other. Similar to CGRP, PACAP induced light aversion in outbred CD-1 mice. The light aversion was accompanied by increased resting in the dark, but not anxiety in a light-independent open field assay. Unexpectedly, about one-third of the CD-1 mice did not respond to PACAP, which was not seen with CGRP. The responder and nonresponder phenotypes were stable, inheritable, and not sex linked, although there was a trend for greater responses among male mice. RNA-sequencing analysis of trigeminal ganglia yielded hierarchical clustering of responder and nonresponder mice and revealed a number of candidate genes, including greater expression of the Trpc5 and Kcnk12 ion channels and glycoprotein hormones and receptors in a subset of male responder mice. Importantly, an anti-PACAP monoclonal antibody could block PACAP-induced light aversion but not CGRP-induced light aversion. Conversely, an anti-CGRP antibody could not block PACAP-induced light aversion. Thus, we propose that CGRP and PACAP act by independent convergent pathways that cause a migraine-like symptom in mice.SIGNIFICANCE STATEMENT The relationship between the neuropeptides CGRP (calcitonin gene-related peptide) and PACAP (pituitary adenylate cyclase-activating polypeptide) in migraine is relevant given that both peptides can induce migraine in people, yet to date only drugs that target CGRP are available. Using an outbred strain of mice, we were able to show that most, but not all, mice respond to PACAP in a preclinical photophobia assay. Our finding that CGRP and PACAP monoclonal antibodies do not cross-inhibit the other peptide indicates that CGRP and PACAP actions are independent and suggests that PACAP-targeted drugs may be effective in patients who do not respond to CGRP-based therapeutics.

Agonist bias and agonist-dependent antagonism at corticotrophin releasing factor receptors

The corticotropin-releasing factor (CRF) receptors represent potential drug targets for the treatment of anxiety, stress, and other disorders. However, it is not known if endogenous CRF receptor agonists display biased signaling, how effective CRF receptor antagonists are at blocking different agonists and signaling pathways or how receptor activity-modifying proteins (RAMPs) effect these processes. This study aimed to address this by investigating agonist and antagonist action at CRF₁ and CRF₂ receptors. We used CRF₁ and CRF₂ receptor transfected Cos7 cells to assess the ability of CRF and urocortin (UCN) peptides to activate cAMP, inositol monophosphate (IP₁ ), and extracellular signal-regulated kinase 1/2 signaling and determined the ability of antagonists to block agonist-stimulated cAMP and IP₁ accumulation. The ability of RAMPs to interact with CRF receptors was also examined. At the CRF₁ receptor, CRF and UCN1 activated signaling in the same manner. However, at the CRF₂ receptor, UCN1 and UCN2 displayed similar signaling profiles, whereas CRF and UCN3 displayed bias away from IP₁ accumulation over cAMP. The antagonist potency was dependent on the receptor, agonist, and signaling pathway. CRF₁ and CRF₂ receptors had no effect on RAMP1 or RAMP2 surface expression. The presence of biased agonism and agonist-dependent antagonism at the CRF receptors offers new avenues for developing drugs tailored to activate a specific signaling pathway or block a specific agonist. Our findings suggest that the already complex CRF receptor pharmacology may be underappreciated and requires further investigation.

Structure and Dynamics of Adrenomedullin Receptors AM1 and AM2 Reveal Key Mechanisms in the Control of Receptor Phenotype by Receptor Activity-Modifying Proteins

Adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) receptors are critically important for metabolism, vascular tone, and inflammatory response. AM receptors are also required for normal lymphatic and blood vascular development and angiogenesis. They play a pivotal role in embryo implantation and fertility and can provide protection against hypoxic and oxidative stress. CGRP and AM receptors are heterodimers of the calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) (CGRPR), as well as RAMP2 or RAMP3 (AM1R and AM2R, respectively). However, the mechanistic basis for RAMP modulation of CLR phenotype is unclear. In this study, we report the cryo-EM structure of the AM1R in complex with AM and Gs at a global resolution of 3.0 Å, and structures of the AM2R in complex with either AM or intermedin/adrenomedullin 2 (AM2) and Gs at 2.4 and 2.3 Å, respectively. The structures reveal distinctions in the primary orientation of the extracellular domains (ECDs) relative to the receptor core and distinct positioning of extracellular loop 3 (ECL3) that are receptor-dependent. Analysis of dynamic data present in the cryo-EM micrographs revealed additional distinctions in the extent of mobility of the ECDs. Chimeric exchange of the linker region of the RAMPs connecting the TM helix and the ECD supports a role for this segment in controlling receptor phenotype. Moreover, a subset of the motions of the ECD appeared coordinated with motions of the G protein relative to the receptor core, suggesting that receptor ECD dynamics could influence G protein interactions. This work provides fundamental advances in our understanding of GPCR function and how this can be allosterically modulated by accessory proteins.

Molecular Mechanisms of Class B GPCR Activation: Insights from Adrenomedullin Receptors

Adrenomedullin (AM) is a 52 amino acid peptide that plays a regulatory role in the vasculature. Receptors for AM comprise the class B G protein-coupled receptor, the calcitonin-like receptor (CLR), in complex with one of three receptor activity-modifying proteins (RAMPs). The C-terminus of AM is involved in binding to the extracellular domain of the receptor, while the N-terminus is proposed to interact with the juxtamembranous portion of the receptor to activate signaling. There is currently limited information on the molecular determinants involved in AM signaling, thus we set out to define the importance of the AM N-terminus through five signaling pathways (cAMP production, ERK phosphorylation, CREB phosphorylation, Akt phosphorylation, and IP1 production). We characterized the three CLR:RAMP complexes through the five pathways, finding that each had a distinct repertoire of intracellular signaling pathways that it is able to regulate. We then performed an alanine scan of AM from residues 15-31 and found that most residues could be substituted with only small effects on signaling, and that most substitutions affected signaling through all receptors and pathways in a similar manner. We identify F18, T20, L26, and I30 as being critical for AM function, while also identifying an analogue (AM15-52 G19A) which has unique signaling properties relative to the unmodified AM. We interpret our findings in the context of new structural information, highlighting the complementary nature of structural biology and functional assays.

RAMPs as allosteric modulators of the calcitonin and calcitonin-like class B G protein-coupled receptors

Receptor activity-modifying proteins (RAMPs) are a family of three single span transmembrane proteins in humans that interact with many GPCRs and can modulate their function. RAMPs were discovered as key components of the calcitonin gene-related peptide and adrenomedullin receptors. They are required for transport of this class B GPCR, calcitonin receptor-like receptor (CLR), to the cell surface and determine its peptide ligand binding preferences. Soon thereafter RAMPs were shown to modulate the binding of calcitonin and amylin peptides to the related calcitonin receptor (CTR) and in the years since an ever-growing number of RAMP-interacting receptors have been identified including most if not all of the 15 class B GPCRs and several GPCRs from other families. Studies of CLR, CTR, and a handful of other GPCRs revealed that RAMPs are able to modulate various aspects of receptor function including trafficking, ligand binding, and signaling. Here, we review RAMP interactions and functions with an emphasis on class B receptors for which our understanding is most advanced. A key focus is to discuss recent evidence that RAMPs serve as endogenous allosteric modulators of CLR and CTR. We discuss structural studies of RAMP-CLR complexes and CTR and biochemical and pharmacological studies that collectively have significantly expanded our understanding of the mechanistic basis for RAMP modulation of these class B GPCRs. Last, we consider the implications of these findings for drug development targeting RAMP-CLR/CTR complexes.

The CGRP Pathway in Migraine as a Viable Target for Therapies

The neuropeptide calcitonin gene-related peptide is well established as a key player in the pathogenesis of migraine. Clinical studies show calcitonin gene-related peptide levels correlate with migraine attacks, and decreases in this neuropeptide can indicate antimigraine therapy effectiveness. Research has revealed a wide distribution of expression sites for calcitonin gene-related peptide in the central and peripheral nervous system. Of these, the calcitonin gene-related peptide receptor, which binds calcitonin gene-related peptide with high affinity, has attracted growing interest as a viable target for antimigraine therapies. An incentive to pursue such research is the continuing unmet medical need of patients. Triptans have offered some clinical benefit, but many patients do not respond and these drugs have important safety considerations. Initial calcitonin gene-related peptide-focused research led to development of the "gepant" small-molecule calcitonin gene-related peptide receptor blockers. Positive efficacy reports concerning the gepants have been tempered by safety findings which led to the discontinuation of some of these agents. Currently, there is considerable excitement regarding monoclonal antibodies against calcitonin gene-related peptide (eptinezumab, galcanezumab, fremanezumab) and the calcitonin gene-related peptide receptor (erenumab). To date, these monoclonal antibodies have shown promising efficacy in clinical trials, with no major safety concerns. If ongoing long-term studies show that their efficacy can be maintained, this may herald a new era for effective antimigraine therapies.

Deconvoluting the Molecular Control of Binding and Signaling at the Amylin 3 Receptor: RAMP3 Alters Signal Propagation through Extracellular Loops of the Calcitonin Receptor

Amylin is coexpressed with insulin in pancreatic islet β-cells and has potent effects on gastric emptying and food intake. The effect of amylin on satiation has been postulated to involve AMY3 receptors (AMY3R) that are heteromers of the calcitonin receptor (CTR) and receptor activity-modifying protein 3 (RAMP3). Understanding the molecular control of signaling through the AMY3R is thus important for peptide drug targeting of this receptor. We have previously used alanine scanning mutagenesis to study the contribution of the extracellular surface of the CTR to binding and signaling initiated by calcitonin (CT) and related peptides (Dal Maso, E., et al. (2019) The molecular control of calcitonin receptor signaling. ACS Pharmacol. Transl. Sci. 2, 31-51). That work revealed ligand- and pathway-specific effects of mutation, with extracellular loops (ECLs) 2 and 3 particularly important in the distinct propagation of signaling mediated by individual peptides. In the current study, we have used equivalent alanine scanning of ECL2 and ECL3 of the CTR in the context of coexpression with RAMP3 to form AMY3Rs, to examine functional affinity and efficacy of peptides in cAMP accumulation and extracellular signal-regulated kinase (ERK) phosphorylation (pERK). The effect of mutation was determined on representatives of the three major distinct classes of CT peptide, salmon CT (sCT), human CT (hCT), and porcine CT (pCT), as well as rat amylin (rAmy) or human α-CGRP (calcitonin gene-related peptide, hCGRP) whose potency is enhanced by RAMP interaction. We demonstrate that the dynamic nature of CTR ECL2 and ECL3 in propagation of signaling is fundamentally altered when complexed with RAMP3 to form the AMY3R, despite only having predicted direct interactions with ECL2. Moreover, the work shows that the role of these loops in receptor signaling is highly peptide dependent, illustrating that even subtle changes to peptide sequence may change signaling output downstream of the receptor.

Peeking at G-protein-coupled receptors through the molecular dynamics keyhole

Molecular dynamics is a state of the art computational tool for the investigation of biophysics phenomenon at a molecular scale, as it enables the modeling of dynamic processes, such as conformational motions, molecular solvation and ligand binding. The recent advances in structural biology have led to a bloom in published G-protein-coupled receptor structures, representing a solid and valuable resource for molecular dynamics studies. During the last decade, indeed, a plethora of physiological and pharmacological facets of this membrane protein superfamily have been addressed by means of molecular dynamics simulations, including the activation mechanism, allosterism and, very recently, biased signaling. Here, we try to recapitulate some of the main contributions that molecular dynamics has recently produced in the field.

The Structure of the CGRP and Related Receptors

The canonical CGRP receptor is a complex between calcitonin receptor-like receptor (CLR), a family B G-protein-coupled receptor (GPCR) and receptor activity-modifying protein 1 (RAMP1). A third protein, receptor component protein (RCP) is needed for coupling to Gs. CGRP can interact with other RAMP-receptor complexes, particularly the AMY1 receptor formed between the calcitonin receptor (CTR) and RAMP1. Crystal structures are available for the binding of CGRP_27-37 [D³¹,P³⁴,F³⁵] to the extracellular domain (ECD) of CLR and RAMP1; these show that extreme C-terminal amide of CGRP interacts with W84 of RAMP1 but the rest of the analogue interacts with CLR. Comparison with the crystal structure of a fragment of the allied peptide adrenomedullin bound to the ECD of CLR/RAMP2 confirms the importance of the interaction of the ligand C-terminus and the RAMP in determining pharmacology specificity, although the RAMPs probably also have allosteric actions. A cryo-electron microscope structure of calcitonin bound to the full-length CTR associated with Gs gives important clues as to the structure of the complete receptor and suggests that the N-terminus of CGRP makes contact with His^5.40b, high on TM5 of CLR. However, it is currently not known how the RAMPs interact with the TM bundle of any GPCR. Major challenges remain in understanding how the ECD and TM domains work together to determine ligand specificity, and how G-proteins influence this and the role of RCP. It seems likely that allosteric mechanisms are particularly important as are the dynamics of the receptors.

Assessment of Conformational State Transitions of Class B GPCRs Using Molecular Dynamics

Class B G protein-coupled receptors (GPCRs) comprise a family of 15 peptide-binding members, which are crucial targets for endocrine, metabolic, and stress-related disorders. While their protein structures and dynamics remain largely unclear, computer modeling and simulations represent a promising means to help solve such puzzles. Herein, we present a basic introduction to the methodology of molecular dynamics (MD) simulations and two analytical methods to assess the conformational ensembles and transitions of Class B GPCRs, using our recent studies of the human pituitary adenylate cyclase activating polypeptide (PAC₁) receptor as an example. From long MD simulations, conformational ensembles with different roles in ligand binding and receptor activation are sampled to establish four states identified as either "open" or "closed" for the PAC₁ receptor. Next, the dynamical network can be applied to analyze the simulations and identify key features within each conformational ensemble, which help distinguish the ligand-bound states of the PAC₁ receptor from the ligand-free one. Further, the Markov State Model has emerged as a key approach to construct the transition network and connect the GPCR ensembles, providing detailed information for the transition pathways and kinetics. For the ligand-free PAC₁ receptor, the transitions within the closed states are near 10-30 times faster than the open-closed transitions, which is likely related to the activation mechanism of the receptor. Overall, long MD simulations and analyses are useful to assess conformational transitions for the Class B GPCRs and to gain mechanistic insight, which is difficult to obtain using other methods.

Cryo-EM structure of the active, Gs-protein complexed, human CGRP receptor

Calcitonin gene-related peptide (CGRP) is a widely expressed neuropeptide that has a major role in sensory neurotransmission. The CGRP receptor is a heterodimer of the calcitonin receptor-like receptor (CLR) class B G-protein-coupled receptor and a type 1 transmembrane domain protein, receptor activity-modifying protein 1 (RAMP1). Here we report the structure of the human CGRP receptor in complex with CGRP and the Gs-protein heterotrimer at 3.3 Å global resolution, determined by Volta phase-plate cryo-electron microscopy. The receptor activity-modifying protein transmembrane domain sits at the interface between transmembrane domains 3, 4 and 5 of CLR, and stabilizes CLR extracellular loop 2. RAMP1 makes only limited direct contact with CGRP, consistent with its function in allosteric modulation of CLR. Molecular dynamics simulations indicate that RAMP1 provides stability to the receptor complex, particularly in the positioning of the extracellular domain of CLR. This work provides insights into the control of G-protein-coupled receptor function.

Structure-function analyses reveal a triple β-turn receptor-bound conformation of adrenomedullin 2/intermedin and enable peptide antagonist design

The cardioprotective vasodilator peptide adrenomedullin 2/intermedin (AM2/IMD) and the related adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) signal through three heterodimeric receptors comprising the calcitonin receptor-like class B G protein-coupled receptor (CLR) and a variable receptor activity-modifying protein (RAMP1, -2, or -3) that determines ligand selectivity. The CGRP receptor (RAMP1:CLR) favors CGRP binding, whereas the AM₁ (RAMP2:CLR) and AM₂ (RAMP3:CLR) receptors favor AM binding. How AM2/IMD binds the receptors and how RAMPs modulate its binding is unknown. Here, we show that AM2/IMD binds the three purified RAMP-CLR extracellular domain (ECD) complexes with a selectivity profile that is distinct from those of CGRP and AM. AM2/IMD bound all three ECD complexes but preferred the CGRP and AM₂ receptor complexes. A 2.05 Å resolution crystal structure of an AM2/IMD antagonist fragment-bound RAMP1-CLR ECD complex revealed that AM2/IMD binds the complex through a unique triple β-turn conformation that was confirmed by peptide and receptor mutagenesis. Comparisons of the receptor-bound conformations of AM2/IMD, AM, and a high-affinity CGRP analog revealed differences that may have implications for biased signaling. Guided by the structure, enhanced-affinity AM2/IMD antagonist variants were developed, including one that discriminates the AM₁ and AM₂ receptors with ∼40-fold difference in affinities and one stabilized by an intramolecular disulfide bond. These results reveal differences in how the three peptides engage the receptors, inform development of AM2/IMD-based pharmacological tools and therapeutics, and provide insights into RAMP modulation of receptor pharmacology.

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.

Receptor Activity Modifying Proteins Have Limited Effects on the Class B G Protein-Coupled Receptor Calcitonin Receptor-Like Receptor Stalk

The calcitonin receptor-like receptor (CLR) is a class B G protein-coupled receptor (GPCR) that forms the basis of three pharmacologically distinct receptors, the calcitonin gene-related peptide (CGRP) receptor, and two adrenomedullin (AM) receptors. These three receptors are created by CLR interacting with three receptor activity-modifying proteins (RAMPs). Class B GPCRs have an N-terminal extracellular domain (ECD) and transmembrane bundle that are both important for binding endogenous ligands. These two domains are joined together by a stretch of amino acids that is referred to as the "stalk". Studies of other class B GPCRs suggest that the stalk may act as hinge, allowing the ECD to adopt multiple conformations. It is unclear what the role of the stalk is within CLR and whether RAMPs can influence its function. Therefore, this study investigated the role of this region using an alanine scan. Effects of mutations were measured with all three RAMPs through cell surface expression, cAMP production and, in select cases, radioligand binding and total cell expression assays. Most mutants did not affect expression or cAMP signaling. CLR C127A, N140A, F142A, and L144A impaired cell surface expression with all three RAMPs. T125A decreased the potency of all peptides at all receptors. N128A, V135A, and L139A showed ligand-dependent effects. While the stalk appears to play a role in CLR function, the effect of RAMPs on this region seems limited, in contrast to their effects on the structure of CLR in other receptor regions.