Novel calcitonin-(8-32)-sensitive adrenomedullin receptors derived from co-expression of calcitonin receptor with receptor activity-modifying proteins

Inhibitory effects of two G protein-coupled receptor kinases on the cell surface expression and signaling of the human adrenomedullin receptor

Receptor activity-modifying protein 2 (RAMP2) enables the calcitonin receptor-like receptor (CLR, a family B GPCR) to form the type 1 adrenomedullin receptor (AM1 receptor). Here, we investigated the effects of the five non-visual GPCR kinases (GRKs 2 through 6) on the cell surface expression of the human (h)AM1 receptor by cotransfecting each of these GRKs into HEK-293 cells that stably expressed hRAMP2. Flow cytometric analysis revealed that when coexpressed with GRK4 or GRK5, the cell surface expression of the AM1 receptor was markedly decreased prior to stimulation with AM, thereby attenuating both the specific [(125)I]AM binding and AM-induced cAMP production. These inhibitory effects of both GRKs were abolished by the replacement of the cytoplasmic C-terminal tail (C-tail) of CLR with that of the calcitonin receptor (a family B GPCR) or β2-adrenergic receptor (a family A GPCR). Among the sequentially truncated CLR C-tail mutants, those lacking the five residues 449-453 (Ser-Phe-Ser-Asn-Ser) abolished the inhibition of the cell surface expression of CLR via the overexpression of GRK4 or GRK5. Thus, we provided new insight into the function of GRKs in agonist-unstimulated GPCR trafficking using a recombinant AM1 receptor and further determined the region of the CLR C-tail responsible for this GRK function.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Pharmacological characterization of rat amylin receptors: implications for the identification of amylin receptor subtypes

BACKGROUND AND PURPOSE

Amylin (Amy) is an important glucoregulatory peptide and AMY receptors are clinical targets for diabetes and obesity. Human (h) AMY receptor subtypes are complexes of the calcitonin (CT) receptor with receptor activity-modifying proteins (RAMPs); their rodent counterparts have not been characterized. To allow identification of the most clinically relevant receptor subtype, the elucidation of rat (r) AMY receptor pharmacology is necessary.

EXPERIMENTAL APPROACH

Receptors were transiently transfected into COS-7 cells and cAMP responses measured in response to different agonists, with or without antagonists. Competition binding experiments were performed to determine rAmy affinity.

KEY RESULTS

rCT was the most potent agonist of rCT((a)) receptors, whereas rAmy was most potent at rAMY(1(a)) and rAMY(3(a)) receptors. rAmy bound to these receptors with high affinity. Rat α-calcitonin gene-related peptide (CGRP) was equipotent to rAmy at both AMY receptors. Rat adrenomedullin (AM) and rAM2/intermedin activated all three receptors but were most effective at rAMY(3(a)) . AC187, AC413 and sCT(8-32) were potent antagonists at all three receptors. rαCGRP(8-37) displayed selectivity for rAMY receptors over rCT((a)) receptors. rAMY(8-37) was a weak antagonist but was more effective at rAMY(1(a)) than rAMY(3(a)) .

CONCLUSIONS AND IMPLICATIONS

AMY receptors were generated by co-expression of rCT((a)) with rRAMP1 or 3, forming rAMY(1(a)) and rAMY(3(a)) receptors, respectively. CGRP was more potent at rAMY than at hAMY receptors. No antagonist tested was able to differentiate the rAMY receptor subtypes. The data emphasize the need for and provide a useful resource for developing new CT or AMY receptor ligands as pharmacological tools or potential clinical candidates.

Flow cytometric analysis of the calcitonin receptor-like receptor domains responsible for cell-surface translocation of receptor activity-modifying proteins

The three receptor activity-modifying proteins (RAMPs1, -2, and -3) associate with a wide variety of G protein-coupled receptors (GPCRs), including calcitonin receptor-like receptor (CRLR). In this study, we used flow cytometry to measure RAMP translocation to the cell surface as a marker of RAMP-receptor interaction. Because VPAC2 does not interact with RAMPs, although, like CRLR, it is a Family B peptide hormone receptor, we constructed a set of chimeric CRLR/VPAC2 receptors to evaluate the trafficking interactions between CRLR domains and each RAMP. We found that CRLR regions extending from transmembrane domain 1 (TM1) through TM5 are necessary and sufficient for the transport of RAMPs to the plasma membrane. In addition, the extracellular N-terminal domain of CRLR, its 3rd intracellular loop and/or TM6 were also important for the cell-surface translocation of RAMP2, but not RAMP1 or RAMP3. Other regions within CRLR were not involved in trafficking interactions with RAMPs. These findings provide new insight into the trafficking interactions between accessory proteins such as RAMPs and their receptor partners.

Identification of N-terminal receptor activity-modifying protein residues important for calcitonin gene-related peptide, adrenomedullin, and amylin receptor function

Calcitonin-family receptors comprise calcitonin receptor-like receptor (CL) or calcitonin receptor and receptor activity-modifying protein (RAMP) pairings. Calcitonin gene-related peptide (CGRP) receptors are CL/RAMP1, whereas adrenomedullin (AM) receptors are CL/RAMP2 (AM1 receptor) or CL/RAMP3 (AM2 receptor). Amylin (Amy) receptors are RAMP hetero-oligomers with the calcitonin receptor (AMY1, AMY2, and AMY3, respectively). How RAMPs change G protein-coupled receptor pharmacology is not fully understood. We exploited sequence differences between RAMP1 and RAMP3 to identify individual residues capable of altering receptor pharmacology. Alignment of human RAMPs revealed eight residues that are conserved in RAMP2 and RAMP3 but are different in RAMP1. We hypothesized that residues in RAMP2 and RAMP3, but not RAMP1, are responsible for making CL/RAMP2 and CL/RAMP3 AM receptors. Using site-directed mutagenesis, we introduced individual RAMP3 residues into RAMP1 and vice versa in these eight positions. Mutant or wild-type RAMPs were transfected into Cos7 cells with CL or the insert-negative form of the calcitonin receptor [CT(a)]. Agonist-stimulated cAMP production and cell-surface expression of constructs were measured. Position 74 in RAMP1 and RAMP3 was critical for determining AM potency and affinity, and Phe93 in RAMP1 was an important contributor to alphaCGRP potency at CGRP receptors. Mutant RAMP/CT(a) receptor complexes displayed different phenotypes. It is noteworthy that RAMP1 S103N and W74E mutations led to enhanced rAmy potency, probably related to increased cell-surface expression of these complexes. This differs from the effect on CL-based receptors where expression was unchanged. Targeted substitution has emphasized the importance of position 74 in RAMP1/RAMP3 as a key determinant of AM pharmacology.

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.

Receptor pharmacology

Despite clear evidence for a distinct amylin pharmacology and localization of such pharmacology to sites such as the nucleus accumbens,efforts to clone an amylin receptor were fruitless for over a decade. This enigma led many to doubt the status of amylin as a bona fide hormone. Yet it became apparent during those cloning efforts that, whatever the amylin receptor was, it was somehow similar to a calcitonin receptor. The enigma of the amylin receptor was solved following the identification of receptor activity modifying proteins (RAMPs). These single transmembrane spanning molecules, when associated with a calcitonin receptor, altered its pharmacology from calcitonin-preferring to amylin-preferring. With at least two forms of the calcitonin receptor and three forms of RAMP, there is the potential for six subtypes of amylin receptors. Of these, two appear to predominate. The CTa (shorter form) calcitonin receptor, dimerized with RAMP1 [amylin 1 (a) receptor], appears to represent binding sites at the nucleus accumbens and the subfornical organ. Binding sites at area postrema appear to be composed of CTa + RAMP3 [amylin3 (a) receptors]. Thus far, RAMP proteins have been associated in vivo only with the CT/CLR receptor system. It is presently unknown whether RAMPs are more general modulators of receptor function, dynamically modifying responsivity with time or across other receptor classes. The largest and first identified amylin-binding field was in the nucleus accumbens. The function of these receptors is yet undetermined, but because the nucleus accumbens is within the blood-brain barrier, the cognate ligand is unlikely to be circulating amylin. Dense amylin binding is present at the circumventricular organs, including the subfornical organ, the organum vasculosum lateralis terminalis (OVLT), and the area postrema. There is no diffusional (blood-brain) barrier at these structures, so they most likely respond to circulating (beta-cell-derived) amylin. Despite pharmacological evidence of amylin sensitivity in several peripheral tissues, selective amylin binding outside of the brain is observed only in the renal cortex. The newly designated amylinomimetic drug class was defined on the basis of its unique pharmacology prior to the molecular characterization of amylin receptors. Currently, the class includes any agent that acts as antagonist at characterized amylin receptors. Several peptides, typically analogs of truncated salmon calcitonin, have been developed as potent and selective amylin antagonists and have been useful in identifying amylinergic responses. Of these, AC187 (30Asn32Tyr[8-32]sCT; Amylin Pharmaceuticals Inc.) is particularly selective and potent, and has been most often cited in studies using amylin antagonists. Antagonism of a response with an order of potency of AC187> AC66 > CGRP[8-37] is suggestive that it is mediated via amylin receptors. Activation of a response with salmon calcitonin (sCT) > amylin >calcitonin gene-related peptide (CGRP) > mammalian CT suggests activation via the amylinl (a) receptor, while sCT = amylin >> CGRP >mammalian CT suggests activation via amylin3 (a) receptors. Absence of response to other ligands (e.g., adrenomedullin) is useful for excluding certain pharmacologies.

The pharmacology of CGRP-responsive receptors in cultured and transfected cells

Historically, CGRP receptors have been classified as CGRP(1) or CGRP(2) subtypes, chiefly depending on their affinity for the antagonist CGRP(8-37). It has been shown that the complex between calcitonin receptor-like receptor (CRLR or CL) and receptor activity modifying protein (RAMP) 1 provides a molecular correlate for the CGRP(1) receptor; however, this does not explain the range of affinities seen for CGRP(8-37) in isolated tissues. It is suggested that these may largely be explained by a combination of methodological factors and CGRP-responsive receptors generated by CL and RAMP2 or RAMP3 and complexes of RAMPs with the calcitonin receptor.

Adrenomedullin receptors: pharmacological features and possible pathophysiological roles

Three receptor activity modifying proteins (RAMPs) chaperone calcitonin-like receptor (CLR) to the cell surface. RAMP2 enables CLR to form an adrenomedullin (AM)-specific receptor that is sensitive to AM-(22-52) (AM(1) receptor). RAMP3 enables CLR to form an AM receptor sensitive to both calcitonin gene-related peptide (CGRP)-(8-37) and AM-(22-52) (AM(2) receptor), though rat and mouse AM(2) receptors show a clear preference for CGRP alpha-(8-37) over AM-(22-52). RAMP1 enables CRL to form the CGRP-(8-37)-sensitive CGRP(1) receptor, which can also be activated by higher concentrations of AM. Here we review the available information on the pharmacological features and possible pathophysiological roles of the aforementioned AM receptors.

Amylin receptors: molecular composition and pharmacology

Several receptors which bind the hormone AMY (amylin) with high affinity have now been identified. The minimum binding unit is composed of the CT (calcitonin) receptor at its core, plus a RAMP (receptor activity modifying protein). The receptors have been named AMY1(a), AMY2(a) and AMY3(a) in accordance with the association of the CT receptor (CT(a)) with RAMP1, RAMP2 and RAMP3 respectively. The challenge is now to determine the localization and pharmacological nature of each of these receptors. Recent attempts to achieve these aims will be briefly discussed.

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Characterization of the Human Calcitonin Gene-Related Peptide Receptor Subtypes Associated with Receptor Activity-Modifying Proteins

Coexpression of receptor activity-modifying proteins (RAMPs) with calcitonin receptor 2 (CTR2) or calcitonin receptor-like receptor (CRLR) leads to the formation of four functional heterodimeric receptors for human calcitonin gene-related peptide (hCGRP). In this study, we transfected hCGRP receptors into human embryonic kidney 293 cells and examined their pharmacological profiles using three dominant-negative (DN) RAMP mutants and various hCGRPalpha analogs. Fluorescence-activated cell-sorting analysis revealed that their cotransfection with CTR2 induced cell surface expression of all three RAMPs, and the three CTR2/RAMP heterodimers mediated equivalent levels of cAMP production in response to hCGRPalpha that were approximately 50-fold greater than were seen with CTR2 alone. By contrast, [Tyr0]hCGRPalpha binding and signaling were markedly weaker with CTR2/RAMP2 or -3 than with CTR2/RAMP1 or CRLR/RAMP1; likewise, 125I-[His10]hCGRPalpha bound most potently to CTR2/RAMP1. When CTR2 was coexpressed with DN RAMP1 or -2, hCGRPalpha-evoked responses were similar to those seen with CTR2 alone, despite the expression of both CTR2 and DN RAMP at the cell surface. But coexpression of DN RAMP3 with CTR2 significantly diminished hCGRPalpha signaling compared with that seen with CTR2 alone, indicating that DN RAMP3 is able to function as a negative regulator of CTR2 function. Competition experiments showed the relative agonist sensitivity of the four receptors to be hCGRPalpha > [Tyr0]hCGRPalpha > [Cys(Et)2,7]hCGRPalpha > [Cys(ACM)2,7]hCGRPalpha. Of the linear analogs, [Cys(ACM)2,7]hCGRPalpha (ACM, acetylmethoxy) enhanced cAMP formation only via CTR2/RAMP1, whereas [Cys(Et2,7)]hCGRPalpha acted via CRLR/RAMP1 and somewhat less potently via CTR2/RAMP1. Thus, among the three CGRP8-37-insensitive receptors, CTR2/RAMP1 is most sensitive to the two linear analogs, suggesting that it could be classified as a CGRP2 receptor. Moreover, the combined use of iodinated CGRPalpha analogs may be useful for defining the CGRP1 receptor.

Calcitonin gene-related peptide as inflammatory mediator

Sensory neuropeptides have been proposed to play a key role in the pathogenesis of a number of respiratory diseases such as asthma, chronic obstructive pulmonary disease or chronic cough. Next to prominent neuropeptides such as tachykinins or vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP) has long been suggested to participate in airway physiology and pathophysiology. CGRP is a 37 amino-acid peptide which is expressed by nerve fibers projecting to the airways and by pulmonary neuroendocrine cells. The most prominent effects of CGRP in the airways are vasodilatation and in a few instances bronchoconstriction. A further pulmonary effect of CGRP is the induction of eosinophil migration and the stimulation of beta-integrin-mediated T cell adhesion to fibronectin at the site of inflammation. By contrast, CGRP inhibits macrophage secretion and the capacity of macrophages to activate T-cells, indicating a potential anti-inflammatory effect. Due to the complex pulmonary effects of CGRP with bronchoconstriction and vasodilatation and diverse immunomodulatory actions, potential anti-asthma drugs based on this peptide have not been established so far. However, targeting the effects of CGRP may be of value for future strategies in nerve modulation.

Identification of the human receptor activity-modifying protein 1 domains responsible for agonist binding specificity

When co-expressed with receptor activity-modifying protein (RAMP) 1, calcitonin receptor-like receptor (CRLR) can function as a receptor for both calcitonin gene-related peptide (CGRP) and adrenomedullin (AM). To investigate the structural determinants of ligand binding specificity, we examined the extracellular domain of human (h) RAMP1 using various deletion mutants. Co-expression of the hRAMP1 mutants with hCRLR in HEK-293 cells revealed that deletion of residues 91-94, 96-100, or 101-103 blocked [125I]CGRP binding and completely abolished intracellular cAMP accumulation normally elicited by CGRP or AM. On the other hand, the deletion of residues 78-80 or 88-90 significantly attenuated only AM-evoked responses. In all of these cases, the receptor heterodimers were fully expressed at the cell surface. Substituting alanine for residues 91-103 one at a time had little effect on CGRP-induced responses, indicating that although this segment is essential for high affinity agonist binding to the receptors, none of the residues directly interacts with either CGRP or AM. This finding suggests that RAMPs probably determine ligand specificity by contributing to the structure of the ligand-binding pocket or by allosteric modulation of the conformation of the receptor. Interestingly, the L94A mutant up-regulated surface expression of the receptor heterodimer to a greater degree than wild-type hRAMP1, thereby increasing CGRP binding and signaling. L94A also significantly increased cell surface expression of the hRAMP1 deletion mutant D101-103 when co-transfected with hCRLR, and expression of a L94A/D101-103 double mutant markedly attenuated the activity of endogenous RAMP1 in HEK-293T cells.