Tissue-specific mRNA expression of a calcitonin receptor-like receptor during fetal and postnatal development

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.

What makes a CGRP2 receptor?

1. Heterogeneity in the receptors for the neuropeptide calcitonin gene-related peptide (CGRP) has been apparent for nearly 20 years. This is most clearly manifested in the observation of CGRP(8-37)-sensitive and -insensitive populations of CGRP-activated receptors. The pA(2) values for CGRP(8-37) in excess of 7 are widely considered to be the result of antagonism of CGRP(1) receptors, whereas those below 7 are believed to be the consequence of antagonism of a second population of receptors, namely CGRP(2) receptors. 2. However, a multitude of pA(2) values exist for CGRP(8-37), spanning several log units, and as such no obvious clusters of values are apparent. Understanding the molecular nature of the receptors that underlie this phenomenon is likely to aid the development of selective pharmacological tools to progress our understanding of the physiology of CGRP and related peptides. Because there is active development of CGRP agonists and antagonists as therapeutics, such information would also further this pursuit. 3. The CGRP(1) receptor is pharmacologically and molecularly well defined as a heterodimer of the calcitonin receptor-like receptor (CL) and receptor activity modifying protein (RAMP) 1. The CL/RAMP1 complex is highly sensitive to CGRP(8-37). Conversely, the constituents of the CGRP(2) receptor have not been identified. In fact, there is little evidence for a distinct molecular entity that represents the CGRP(2) receptor. 4. Recent pharmacological characterization of receptors related to CGRP(1) has revealed that some of these receptors may explain CGRP(2) receptor pharmacology. Specifically, AMY(1(a)) (calcitonin receptor/RAMP1) and AM(2) (CL/RAMP3) receptors can be activated by CGRP but are relatively insensitive to CGRP(8-37). 5. This, along with other supporting data, suggests that the 'CGRP(2) receptor' that has been extensively reported in the literature may, in fact, be an amalgamation of contributions from a variety of CGRP-activated receptors. The use of appropriate combinations of agonists and antagonists, along with receptor expression studies, could allow such receptors to be separated.

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.

Female Sex Steroid Hormones and Pregnancy Regulate Receptors for Calcitonin Gene-Related Peptide in Rat Mesenteric Arteries, but Not in Aorta1

Calcitonin gene-related peptide (CGRP) is a potent vasodilator neuropeptide known to be involved in the regulation of vascular tone. Results of previous studies from our laboratory and others suggest that vascular sensitivity to CGRP is enhanced during pregnancy and that the female sex steroid hormones estradiol-17beta (E2) and progesterone (P4) may be involved in this process. We hypothesized that CGRP receptors in the mesenteric artery are increased during pregnancy and with sex steroid hormone treatments. In the present study, we investigated whether pregnancy and female sex steroid hormones modulate the CGRP-receptors CGRP-A and CGRP-B in the mesenteric artery in the rat. The CGRP-A receptor consists of calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1); however, the CGRP-B receptor needs to be further characterized. Messenger RNA levels for CRLR and RAMP1 were assessed by reverse transcription-polymerase chain reaction, and CGRP-B receptor proteins levels were determined by Western blot analysis. In addition, [125I]CGRP binding was measured by Scatchard analysis. Both mRNA for CGRP-A (CRLR and RAMP1) and the protein for CGRP-B receptors in mesenteric arteries were increased with pregnancy compared to nonpregnant, diestrous animals. A P4 antagonist, RU-486, downregulated and P4 upregulated these receptors in mesenteric arteries (P < 0.05) in pregnant rats. In adult ovariectomized rats, P4 upregulated CRLR and RAMP1 mRNA levels as well as [125I]CGRP-binding sites. The CGRP-B-receptor protein levels were significantly (P < 0.05) elevated by P4 and by combined E2 and P4 treatment. Together with earlier findings, these data suggest that increases in the expression of CGRP-A (CRLR and RAMP1) and CGRP-B receptors in mesenteric arteries may be important in reducing vascular resistance and in vascular adaptations that occur during pregnancy; in addition, P4 may be involved in this process.

Immunohistochemical detection of the calcitonin receptor-like receptor protein in the microvasculature of rat endothelium

Calcitonin-gene-related peptide and adrenomedullin have similar and potent vascular effects, which appear to be mediated by the G protein-coupled calcitonin receptor-like (CRL) receptor. Using immunohistochemical and Western blot analyses, we have obtained novel evidence that CRL receptor is expressed in the rat vascular endothelium using an antibody to rat CRL receptor that we have raised and fully characterised. These results are an important basis for further studies aimed at determining the so far ill-defined functional significance of the extensive distribution of CRL receptor in the vascular endothelium.

Loss of alpha CGRP reduces sound-evoked activity in the cochlear nerve

alpha-Calcitonin gene-related peptide (alphaCGRP) is one of several neurotransmitters immunolocalized in the unmyelinated component of the cochlear efferent innervation, the lateral olivocochlear (OC) system, which makes axo-dendritic synapses with cochlear sensory neurons. In rodents, CGRP is also immunocolocalized in the myelinated medial OC system, which contacts cochlear outer hair cells (OHCs). To understand the role(s) of this neuropeptide in the OC system, we characterized the auditory phenotype of alphaCGRP-null mice. Cochlear threshold sensitivity was normal in mutant mice, both via a neural metric, the auditory brain stem response (ABR), and an OHC-based metric, distortion product otoacoustic emissions (DPOAEs). Medial OC function and resistance to acoustic injury were also unaffected by alphaCGRP deletion: the former was assessed by measuring cochlear response suppression with electrical stimulation of the OC bundle, the latter by measuring temporary threshold shifts after exposure to high level sound. However, significant abnormality in alphaCGRP-null mice was seen in the growth of cochlear neural responses with increasing stimulus level. This observation, contrasted with normal amplitude-versus-level functions for DPOAEs, is consistent with a selective, postsynaptic effect on cochlear neurons via alphaCGRP release from lateral OC terminals. This constitutes the most direct evidence to date for a functional role of the lateral OC system in the auditory periphery.

Changes in the expression of calcitonin receptor-like receptor, receptor activity-modifying protein (RAMP) 1, RAMP2, and RAMP3 in rat uterus during pregnancy, labor, and by steroid hormone treatments

Calcitonin gene-related peptide (CGRP) and its related peptide, adrenomedullin (AM), are potent smooth muscle relaxants in a variety of tissues. The CGRP has been reported to play an important role in maintaining uterine relaxation during pregnancy. We have previously reported that CGRP-induced uterine relaxation was gestationally regulated. Calcitonin receptor-like receptor (CRLR), a seven-domain transmembrane protein functions as CGRP-A receptor, in association with receptor activity-modifying protein (RAMP) 1, a single-domain transmembrane protein, whereas CRLR and RAMP2 or RAMP3 constitute a receptor for AM. In the present investigation, we examined the mRNA expression of CRLR, RAMP1, RAMP2, and RAMP3 in rat uterus (n = 8) by reverse transcriptional analysis and polymerase chain reaction to assess the changes in the expression of CGRP-A- and AM-receptor components during pregnancy and labor and by steroid hormone treatments in adult ovariectomized rats. The changes in mRNA are expressed relative to the 18S mRNA in the uterus of rats at various stages: nonpregnant, pregnant on Day 18, spontaneous labor at term, Day 2 postpartum, and in pregnant rats on treatment with RU486. Ovariectomized rats treated for 3 days twice daily s.c. with estradiol-17beta (2.5 microg/injection), progesterone (2 mg/injection), and the combination of estradiol-17beta and progesterone (same doses as above) were also examined for the expression of various receptor components. Results showed that mRNA expression of the receptor components was significantly higher (P < 0.001 for CRLR, P < 0.01 for RAMP1, P < 0.05 for RAMP2, and P < 0.01 for RAMP3) in pregnant compared to nonpregnant rats. Except for RAMP3, expression of all the other three genes decreased significantly (P < 0.05) during labor. A progesterone antagonist, RU486 significantly decreased (P < 0.01 for CRLR, P < 0.05 for RAMP1, RAMP2, and RAMP3) all the receptor components during pregnancy. In adult ovariectomized rats, progesterone caused significant increases in CRLR (P < 0.001), RAMP1 (P < 0.05), and RAMP2 (P < 0.01). Levels of RAMP3 were unaffected by the progesterone treatment. Estradiol-17beta treatment decreased all of the four receptor components significantly (P < 0.01 for CRLR, P < 0.05 for RAMP1, RAMP2, and RAMP3). Our results demonstrate that both CGRP and AM may play a role in uterine quiescence during pregnancy and that their receptor components are regulated by the steroid hormones.

Receptor activity modifying protein 2 distribution in the rat central nervous system and regulation by changes in blood pressure

The neuropeptide, adrenomedullin, acts in the central nervous system (CNS) to regulate blood pressure, at least partly through an adrenomedullin receptor which is composed of receptor activity modifying protein 2 (RAMP-2) and calcitonin receptor-like receptor (CRLR). We used in situ hybridization to localize RAMP-2 mRNA throughout the brain, and we performed reverse transcription-polymerase chain reaction to detect CRLR mRNA in the brain. We found that RAMP-2 mRNA is expressed in numerous areas, including autonomic nuclei such as the paraventricular, supraoptic, arcuate and ventromedial nuclei, as well as the nucleus of the solitary tract (NTS), area postrema and dorsal motor nucleus of the vagus. Many regions expressing RAMP-2 mRNA also express low levels of CRLR mRNA. We examined changes in the mRNA expression of RAMP-2 and preproadrenomedullin in the brain in response to blood pressure manipulations. Rats received intravenous infusions of nitroprusside or phenylephrine to decrease or increase blood pressure, respectively. Decreased blood pressure elicited an increase in RAMP-2 mRNA expression in the NTS and a decrease in preproadrenomedullin mRNA expression in the paraventricular nucleus (PVN). Increased blood pressure elicited a decrease in RAMP-2 mRNA expression in the PVN and NTS. The CNS distribution and modulation of adrenomedullin signalling components by changes in blood pressure provide anatomical and physiological evidence for a homeostatic role for adrenomedullin in the brain.

Adrenomedullin receptor binding sites in rat brain and peripheral tissues

The existence of specific adrenomedullin receptor binding sites was investigated using the agonist peptide fragment [125I]human adrenomedullin-(13-52) in rat brain, lung and vas deferens homogenates. Saturation-binding experiments suggest that [125I]human adrenomedullin-(13-52) binds to an apparent single population of sites with similar affinities (K(D) of 0.3 to 0.6 nM) but with different maximal binding capacity in the rat brain, lung and vas deferens homogenates (B(max) of 73, 1760 and 144 fmol/mg protein, respectively). Competition-binding experiments using various analogues and fragments of calcitonin gene-related peptide (CGRP) and adrenomedullin were also performed using this radioligand. Competition-binding profiles suggest the possible existence of heterogeneous populations of adrenomedullin receptor binding sites. For example, in rat brain, human adrenomedullin-(1-52) and human adrenomedullin-(13-52) competed against specific [125I]human adrenomedullin-(13-52) sites with competition curves best fitted to a two-site model. Additionally, human calcitonin gene-related peptide alpha (hCGRPalpha), [Cys(Et)(2,7)]hCGRPalpha and [[R-(R,(R*,S*)]-N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-,1-Piperidinecarboxamide] (BIBN4096BS) competed against specific [125I]human adrenomedullin-(13-52) binding with profiles that were also best fitted to a two-site model. Furthermore, binding assays performed in the presence of GTPgammaS (100 microM) revealed that this compound inhibited 20% of specific [125I]human adrenomedullin-(13-52) sites in rat brain homogenates and competition curves of human adrenomedullin-(1-52) and [Cys(Et)(2,7)]hCGRPalpha against specific [125I]human adrenomedullin-(13-52) sites remained best fitted to a two-site model. Moreover, the existence of specific [125I]human adrenomedullin-(13-52) binding sites that are resistant to human adrenomedullin-(22-52) and human CGRP-(8-37) is suggested in the rat brain and vas deferens. Taken together, these data provide evidence for the possible existence of heterogeneous populations of adrenomedullin binding sites in rat brain and peripheral tissues.

Calcitonin receptor-like receptor is expressed on gastrointestinal immune cells

BACKGROUND/AIMS

Pharmacological and morphological studies suggest that the gut mucosal immune system and local neuropeptide-containing neurones interact. We aimed to determine whether gut immune cells are targets for calcitonin gene-related peptide (CGRP), which has potent immune regulatory properties.

METHODS

Using density gradient centrifugation, rat lamina propria mononuclear cells (LP-MNCs) and intra-epithelial lymphocytes (IELs) were isolated. RT-PCR was employed for the detection of mRNA of rat calcitonin receptor-like receptor (CRLR), which is considered to represent the pharmacologically defined CGRP receptor-1 subtype, as well as mRNA of the receptor activity-modifying proteins, which are essential for CRLR function and determine ligand specificity. A radioreceptor assay was employed for the detection of specific CGRP binding sites.

RESULTS

RT-PCR and DNA sequencing showed that LP-MNCs and IELs express CRLR. Incubation of isolated LP-MNCs with radiolabelled alphaCGRP revealed the existence of specific binding sites for CGRP.

CONCLUSION

These novel data indicate that mucosal immune cells of the rat gut are a target for CGRP and provide significant evidence that CGRP functions as an immune regulator in the gut mucosa.

Evidence for the existence of a new receptor for CGRP, which is not CRLR

Receptors for calcitonin gene-related peptide (CGRP), a neuropeptide known to be the most potent vasodilator, are abundantly expressed in cerebellum. A monoclonal antibody to cerebellar CGRP receptors specifically detects a 66 kDa protein from rat cerebellum and other rat and human tissues, but not from SK-N-MC cells which express calcitonin receptor-like receptor (CRLR), a recently described component of CGRP receptors. In contrast, mRNA expression for CRLR was abundant in SK-N-MC cells, but it was undetectable in rat cerebellum. Furthermore, the antibody could not detect any immunoreactive protein in HEK 293 cells transiently transfected with CRLR and receptor activity-modifying protein 1 (RAMP(1)) indicating the possible existence of another CGRP receptor, which does not involve CRLR. Due to the absence of biochemical or structural data on the existence of a CGRP(2) receptor and the new data provided in this paper, we suggest to identify the two CGRP receptors as CGRP-A and CGRP-B.

Calcitonin gene-related peptide in pregnancy and its emerging receptor heterogeneity

Calcitonin gene-related peptide (CGRP) is the most potent vasodilator, and there is a growing body of evidence that this peptide might have multiple other functions. During pregnancy, circulating CGRP levels in rats increase up to the time of delivery, followed by a sharp decline at term and postpartum. In addition, the sensitivity of various vascular beds to CGRP in rats appears to increase with advancing pregnancy. This increased sensitivity might be involved in regulating uteroplacental blood flow, in addition to other vascular adaptations that occur during normal pregnancy. Furthermore, the uterine relaxation response to CGRP is elevated during pregnancy and decreased at term. Sex steroid hormones, estrogens and progesterone, regulate CGRP synthesis and its effects on both myometrial and uterine vascular tissues. These changes in smooth muscle relaxation sensitivity to CGRP appear to be a consequence of changes in CGRP-receptor levels in these tissues. There appear to be two receptors for CGRP: the CGRP-A receptor, a well-characterized receptor consisting of calcitonin receptor-like receptor and receptor activity modifying protein 1, and the CGRP-B receptor. The CGRP system might play a role in the maintenance of normal pregnancy, and a defect in this system might lead to complications.

Adrenomedullin and related peptides: receptors and accessory proteins

Adrenomedullin (AM), alpha- and beta-calcitonin gene-related peptide (CGRP), amylin and calcitonin (CT) are structurally and functionally related peptides. The structure of a receptor for CT (CTR) was elucidated in 1991 through molecular cloning, but the structures of the receptors for the other three peptides had yet to be elucidated. The discovery of receptor-activity-modifying proteins (RAMP) 1 and -2 and their co-expression with an orphan receptor, calcitonin receptor-like receptor (CRLR) has led to the elucidation of functional CGRP and AM receptors, respectively. RAMP1 and -3 which are co-expressed with CTR revealed two amylin receptor isotypes. Molecular interactions between CRLR and RAMPs are involved in their transport to the cell surface. Heterodimeric complexes between CRLR or CTR and RAMPs are required for ligand recognition.

A review of the biological properties and clinical implications of adrenomedullin and proadrenomedullin N-terminal 20 peptide (PAMP), hypotensive and vasodilating peptides

Adrenomedullin (AM), identified from pheochromocytoma and having 52 amino acids, elicits a long-lasting vasodilatation and diuresis. AM is mainly mediated by the intracellular adenylate cyclase coupled with cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) -cyclic guanosine monophosphate (cGMP) pathway through its specific receptor. The calcitonin receptor-like receptor (CLCR) and receptor-activity modifying protein (RAMP) 2 or RAMP3 models have been proposed as the candidate receptor. AM is produced mainly in cardiovascular tissues in response to stimuli such as shear stress and stretch, hormonal factors and cytokines. Recently established AM knockout mice lines revealed that AM is essential for development of vitelline vessels of embryo. Plasma AM levels elevate in cardiovascular diseases such as heart failure, hypertension and septic shock, where AM may play protective roles through its characteristic biological activities. Human AM gene delivery improves hypertension, renal function, cardiac hypertrophy and nephrosclerosis in the hypertensive rats. AM decreases cardiac preload and afterload and improves cardiac contractility and diuresis in patients with heart failure and hypertension. Advances in gene engineering and receptor studies may contribute to further understandings of biological implication and therapeutic availability of AM.

Differential and cell-specific expression of calcitonin receptor-like receptor and receptor activity modifying proteins in the human uterus

The calcitonin receptor-like receptor (CRLR) can function as a receptor for either calcitonin gene-related peptide (CGRP) or adrenomedullin (AM), depending upon co-expression with members of a novel family of receptor activity-modifying proteins (RAMP). RAMP1 presents the CRLR at the cell surface as a CGRP/AM receptor. RAMP2- and RAMP3-transported CRLR receptors act as AM-specific receptors. However, it is still unknown if this signalling system operates in vivo. Of particular interest is the uterus, where both peptides and their binding sites are known to be present and where both mitogenic and vasodilatory responses to AM and CGRP have been demonstrated. In this study, we examined whether CRLR and RAMP are co-expressed in the same populations of cells in human uterine tissue. Analysis by in-situ hybridization and immunocytochemistry revealed a heterogeneous and cell type-specific distribution of components of this AM/CGRP signalling system. Adrenomedullin mRNA was expressed and evenly distributed across all cell types. CRLR mRNA was predominantly found in blood vessels. RAMP1 expression was specific to myometrial myocytes and vascular smooth muscle cells in uterine arteries. RAMP2 and RAMP3 mRNA were not detectable by in-situ hybridization. The pattern of differential and cell-specific expression of CRLR and RAMP suggests the involvement of CRLR/RAMP1 in the processes of vasodilation, smooth muscle relaxation and angiogenesis in response to AM and CGRP in the human uterus, but also indicates that other receptors may be implicated.

Characterization of CGRP(1) receptors in the guinea pig basilar artery

The purpose of the present study was to characterise receptors mediating calcitonin gene-related peptide (CGRP)-induced relaxation of guinea pig basilar artery. This was done by investigating vasomotor responses in vitro and performing autoradiographic binding studies. We also intended to study the importance of an intact endothelium. Agonist studies showed that peptides of the CGRP family induced relaxation of the guinea pig basilar artery with the following order of potency: human beta-CGRP=human alpha-CGRP>>adrenomedullin=[acetamidomethyl-Cys(2,7)]alpha-human CGRP ([Cys(ACM)(2,7)]CGRP)=amylin. These data are in concord with those of the autoradiographic binding studies that showed displacement of [125I] human alpha-CGRP binding with the following order of potency: human alpha-CGRP=human beta-CGRP>>adrenomedullin=human alpha-CGRP-(8-37)>>Cys(ACM)(2,7)]CGRP. In blockade experiments, the relaxant responses to human alpha- and human beta-CGRP were competitively blocked by the CGRP(1) receptor antagonist human alpha-CGRP-(8-37), while those of adrenomedullin and amylin were blocked non-competitively. In order to examine whether amylin induced relaxation via amylin or CGRP receptors, we studied the antagonistic effect of amylin-(8-37) on the weak relaxant response to amylin and found that it was not blocked by amylin-(8-37). These findings, together with the finding that the CGRP(2) receptor agonist [Cys(ACM)(2,7)]CGRP only induced a weak relaxation in the highest concentrations examined, suggest that the CGRP family of peptides mediate relaxation by CGRP(1)-type receptors. Removal of the endothelium, the addition of N(G)-nitro-L-arginine methyl ester (L-NAME), methylene blue or indomethacin did not affect the concentration-response curves of the CGRP analogues, neither in the presence nor in the absence of human CGRP-(8-37). The study shows the presence of a relaxant CGRP(1) receptor on the smooth muscle cells of guinea pig basilar artery. Various endothelial factors did not influence relaxant responses.

Mouse receptor-activity-modifying proteins 1, -2 and -3: amino acid sequence, expression and function

The calcitonin receptor-like receptor (CRLR) requires novel receptor-activity-modifying proteins (RAMPs) for its function as an adrenomedullin (ADM) or a calcitonin (CT) gene-related peptide (CGRP) receptor. Here, mouse cDNA clones representing expressed sequence tags (ESTs) in the GenEMBL database have been identified. They encode for proteins with 70, 68 and 84% amino acid sequence identity with respect to human RAMP1, -2 and -3. On Northern blot analysis of polyA(+) RNA mouse RAMP1 (mRAMP1) encoding mRNA with an apparent size of 0.8 kb was predominantly observed in embryonic and adult brain and lung and in adult skeletal muscle. Mouse RAMP2 encoding 0.8 and 1.2 kb mRNA were recognized in all tissues analyzed with the highest levels in embryonic brain, lung and gut and in adult heart, lung, skeletal muscle and brain. A single 1.2 kb mRAMP3 encoding transcript was mainly expressed in embryonic and adult brain. In COS-7 cells co-expressing rat CRLR (rCRLR) and mRAMP1, [125I]halphaCGRP binding was inhibited by ralphaCGRP(8-37), ralphaCGRP and rbetaCGRP with IC(50) of 1.4+/-0.5, 4.5+/-0.6 and 7+/-0.3 nM, respectively. CyclicAMP accumulation was maximally stimulated tenfold by rbetaCGRP and ralphaCGRP with EC(50) of 0. 65+/-0.67 and 0.86+/-0.6 nM. In the same cells co-expressing rCRLR and mRAMP2, binding of [125I]rADM was displaced by rADM and rADM(20-50) with IC(50) of 1.9+/-0.5 and 3.4+/-1.4 nM, respectively, and a maximal sevenfold stimulation of cAMP accumulation was observed with rADM with an EC(50) of 0.82+/-0.85 nM. In the cells co-expressing rCRLR and mRAMP3, [125I]halphaCGRP binding was inhibited by ralphaCGRP(8-37), rbetaCGRP, ralphaCGRP, rADM and rADM(20-50) with IC(50) between 4 and 22 nM. cAMP accumulation was stimulated by rADM with an EC(50) of 5.1+/-2.7 nM that was 12-fold and 11-fold lower than that of ralphaCGRP and rbetaCGRP. In conclusion, mouse RAMP1, -2 and -3 exhibit high amino acid sequence homology to the corresponding human RAMPs. Co-expression of rCRLR with mRAMP1, -2 or -3 in COS-7 cells revealed distinct CGRP-, ADM- or ADM/CGRP receptors.

A receptor activity modifying protein (RAMP)2-dependent adrenomedullin receptor is a calcitonin gene-related peptide receptor when coexpressed with human RAMP1

Adrenomedullin (ADM) and alpha- and beta-calcitonin (CT) gene-related peptide (alpha-, betaCGRP) are structurally related vasodilatory peptides with homology to CT and amylin. An originally orphan human CT receptor-like receptor (hCRLR) is a Gs protein-coupled CGRP or ADM receptor when coexpressed with recently identified human single transmembrane domain receptor activity modifying proteins 1 (hRAMP1) or -2 (hRAMP2), respectively. Here, the function of the rat CRLR homologue (rCRLR) has been investigated in rat osteoblast-like UMR-106 cells and in COS-7 cells, in the absence and presence of hRAMP1 and -2 and combinations thereof. Transient expression of rCRLR in UMR-106 cells revealed an ADM receptor, and [125I]rat (r) ADM binding was enhanced with hRAMP2 and inhibited by 50% when hRAMP1 was coexpressed. Detectable [125I]h alphaCGRP binding required the presence of hRAMP1, and the expression of CGRP binding sites was unaffected by coexpressed hRAMP2. Specificity of ADM binding sites in [125I]rADM binding inhibition experiments was reflected by an over 1000-fold higher potency of rADM [half-maximal effective concentration = 0.19 +/- 0.05 nM (mean +/- SEM, n = 4)], compared with r alphaCGRP and r betaGRP, to induce a cAMP-responsive luciferase reporting gene (CRE-luc). In rCRLR and hRAMP1 cotransfected cells, expressing predominantly CGRP binding sites, r betaCGRP, r alphaCGRP, and rADM induced CRE-luc with half-maximal effective concentration of 0.27 +/- 0.17 nM, 0.37 +/- 0.27 nM, and 1.4 +/- 0.9 nM, respectively. In COS-7 cells, the results were comparable, but rCRLR required coexpressed hRAMP2 for ADM receptor function. This is consistent with higher levels of endogenous RAMP2 encoding messenger RNA in UMR-106, compared with COS-7 cells. In conclusion, the recognition of RAMP1 and -2 as mediators of CRLR expression as a CGRP or ADM receptor has been extended, with evidence that endogenous RAMP2 is sufficient to reveal an ADM receptor in UMR-106 cells. Inhibition of RAMP2-evoked ADM receptor expression by RAMP1 and generation of a CGRP receptor is consistent with competitive interactions of the different RAMPs with rCRLR.