Effects of CGRP receptor antagonism on glucose and bone metabolism in mice with diet-induced obesity

The neuropeptide calcitonin gene-related peptide (CGRP) and its receptor, calcitonin receptor-like receptor (CLR) complexing with receptor activity-modifiying protein 1 (RAMP1), have been shown to be crucially involved in the pathogenesis of migraine. However, CGRP also plays a pivotal role in regulating bone turnover and was suggested to contribute to the development of the metabolic syndrome. Therefore, our study was designed to characterize the effects of CGRP antagonism on bone and glucose metabolism in a murine model of diet-induced obesity (DIO). A subcutaneous pellet releasing the CGRP receptor antagonist BIBN 4096 (BIBN; olcegepant) was implanted in WT mice with DIO. Metabolic effects were assessed through body- and organ-weights, oral glucose tolerance (oGT), serum lipids, and gene-expression studies. Bone turnover was assessed through histomorphometry of non-decalcified bone sections and analyses of bone turnover markers in serum samples. BIBN treatment did not alter body weight gain or the levels of serum lipids including triacylglycerol and cholesterol during DIO. BIBN led to a moderate improvement of oGT which was accompanied by an increased expression of stearoyl-CoA desaturase in the liver. In skeletal tissue, BIBN treatment resulted in reduced bone volume. This was explained by decreased parameters of bone formation whereas bone resorption was not affected. Our results indicate that inhibition of CGRP signaling only moderately affects glucose metabolism during DIO but significantly impairs bone formation. As novel agents blocking CGRP or its receptor are currently introduced clinically for the treatment of migraine disorders, their potential negative impact on bone metabolism requires further clinical studies.

Accelerated Development With Increased Bone Mass and Skeletal Response to Loading Suggest Receptor Activity Modifying Protein-3 as a Bone Anabolic Target

Knockout technologies provide insights into physiological roles of genes. Studies initiated into endocrinology of heteromeric G protein-coupled receptors included deletion of receptor activity modifying protein-3, an accessory protein that alters ligand selectivity of calcitonin and calcitonin-like receptors. Initially, deletion of Ramp3^-/- appeared phenotypically silent, but it has emerged that mice have a high bone mass phenotype, and more subtle alterations to angiogenesis, amylin homeostasis, and a small proportion of the effects of adrenomedullin on cardiovascular and lymphatic systems. Here we explore in detail, effects of Ramp3^-/- deletion on skeletal growth/development, bone mass and response of bone to mechanical loading mimicking exercise. Mouse pups lacking RAMP3 are healthy and viable, having accelerated development of the skeleton as assessed by degree of mineralisation of specific bones, and by microCT measurements. Specifically, we observed that neonates and young mice have increased bone volume and mineralisation in hindlimbs and vertebrae and increased thickness of bone trabeculae. These changes are associated with increased osteoblast numbers and bone apposition rate in Ramp3^-/- mice, and increased cell proliferation in epiphyseal growth plates. Effects persist for some weeks after birth, but differences in gross bone mass between RAMP3 and WT mice lose significance in older animals although architectural differences persist. Responses of bones of 17-week old mice to mechanical loading that mimics effects of vigorous exercise is increased significantly in Ramp3^-/- mice by 30% compared with WT control mice. Studies on cultured osteoblasts from Ramp3^-/- mice indicate interactions between mRNA expression of RAMPs1 and 3, but not RAMP2 and 3. Our preliminary data shows that Ramp3^-/- osteoblasts had increased expression β-catenin, a component of the canonical Wnt signalling pathway known to regulate skeletal homeostasis and mechanosensitivity. Given interactions of RAMPs with both calcitonin and calcitonin-like receptors to alter ligand selectivity, and with other GPCRs to change trafficking or ligand bias, it is not clear whether the bone phenotype of Ramp3^-/- mice is due to alterations in signalling mediated by one or more GPCRS. However, as antagonists of RAMP-interacting receptors are growing in availability, there appears the likelihood that manipulation of the RAMP3 signalling system could provide anabolic effects therapeutically.

[Manual Acupuncture Stimulation Regulates Expression of Receptor Activity-modifying Protein 1 and 5-HT 1 D Receptor Proteins and Genes in Migraine Rats]

OBJECTIVE

To observe the effect of liver-soothing and mental-activity-regulating (LSMAR) needling on the expression of receptor activity-modifying protein 1 (RAMP 1, receptor of calcitonin gene-related peptide), 5-hydroxytryptamine 1 D receptor(5-HT 1 DR) in the spinal trigeminal nucleus (STN) and midbrain in migraine rats, so as to explore its underlying mechanism in relieving migraine.

METHODS

A total of 40 male Wistar rats were randomly divided into control, model, LSMAR and conventional needling groups (n=10 rats in each). The migraine model was established by subcutaneous injection of nitroglycerin at the posterior neck. LSMAR was applied to "Baihui" (GV 20), bilateral "Fengchi" (GB 20), "Neiguan" (PC 6) and "Taichong" (LR 3) in the LSMAR group and conventional needling was applied to "Baihui" (GV 20) and bilateral "Fengchi" (GB 20) in the conventional acupuncture group for 30 min, once a day for 8 days before modeling. The expression levels of RAMP 1 and 5-HT 1 DR proteins and mRNAs in the STN and mesencephalon were detected by real-time fluorescent quantitative PCR and Western blot, separately.

RESULTS

Compared with the control group, the expression levels of RAMP 1 protein and mRNA in STN and mesencephalon were significantly increased (P<0.05) and those of 5-HT 1 DR protein and mRNA considerably decreased (P<0.05) in the model group. After the acupuncture treatment, the increased levels of RAMP 1 protein and mRNA and the decreased levels of 5-HT 1 DR protein and mRNA in the STN and midbrain were obviously reversed in the LSMAR and conventional needling groups relevant to the model group (P<0.05). The effect of LSMAR needling was significantly superior to that of conventional needling in down-regulating the expression levels of RAMP 1 mRNA and protein in the STN and mesencephalon (P<0.05) and in up-regulating the expression levels of 5-HT 1 DR mRNA and protein in the two brain regions (P<0.05).

CONCLUSION

Manual acupuncture stimulation of GV 20, GB 20, etc. can inhibit the expression of RAMP 1 protein and mRNA in the STN and midbrain, and up-regulate the expression of 5-HT 1 DR in the two brain regions of migraine rats, which may be related to its effect in relieving migraine.

[Impact and related mechanism of exogenous receptor activity modifying protein 1 on calcitonin gene-related peptide modified bone marrow mesenchymal stem cells on the migration of vascular smooth muscle cells in vitro]

OBJECTIVE

To investigate the impact of calcitonin gene-related peptide (CGRP) modified bone marrow mesenchymal stem cell (MSC) on the migration of vascular smooth muscle cell (VSMC) and related mechanisms.

METHODS

The MSC and VSMC were isolated from rats and cultured, CGRP was transfected to MSC with the high expression lentivirus vector, VSMC was transfected with high expression lentivirus vector of receptor activity modifying protein 1 (RAMP1) and the silence expression lentivirus vector of RAMP1. Then MSC was co-cultured with VSMC. Experimental groups were as follows: (1) Ang II group (MSC + VSMC + Ang II); (2) MSC(CGRP+) group (MSC(CGRP+) + VSMC + Ang II); (3) MSC(CGRP+) RAMP1(-) group (MSC(CGRP+) + VSMC(RAMP1-) + Ang II); (4) MSC(CGRP+) RAMP1(+) group (MSC(CGRP+) + VSMC(RAMP1+) + Ang II); (5) RAMP1(+) group (MSC + VSMC(RAMP1+) + Ang II). Transwell assay was applied to detect the migration of smooth muscle cells, Western blot was applied to detect the protein expression of cells in various groups.

RESULTS

VSMC migration number was significantly lower in MSC(CGRP+) group compared with Ang II group (50.8 ± 2.6 vs. 71.4 ± 2.3, P < 0.05), but higher than in MSC(CGRP+) RAMP1(+) group (50.8 ± 2.6 vs. 30.4 ± 3.0, P < 0.05). When RAMP1 expression reduced in VSMC, compared with MSC(CGRP+) RAMP1(+) group, VSMC migration increased in the MSC(CGRP+) RAMP1(-) group compared to MSC(CGRP+)RAMP1(+) (69.0 ± 5.6 vs. 30.4 ± 3.0, P < 0.05) and was similar to Ang II group (69.0 ± 5.6 vs. 71.4 ± 2.3, P > 0.05) and RAMP1(+) group (71.6 ± 3.4). According to the result of Western blot, P-P65 protein expression in MSC(CGRP+) group was lower than that in Ang II group (0.475 ± 0.022 vs.0.642 ± 0.035, P < 0.05). P-P65 protein expression in MSC(CGRP+)RAMP1(-) group was higher than that in MSC(CGRP+) RAMP1(+) group (0.670 ± 0.030 vs. 0.373 ± 0.041, P < 0.05), and there was no difference between MSC(CGRP+)RAMP1(-) group and Ang II group (P > 0.05). P-P65 protein expression was similar between RAMP1(+) group (0.643 ± 0.039) and Ang II group (P > 0.05).

CONCLUSIONS

CGRP inhibits VSMC migration through RAMP1. NF-κB and RAMP1 play crucial role in the inhibiting effects of CGRP on VSMC migration. Thus, RAMP1-CGRP signaling inhibits VSMC migration through NF-κB signal pathways.

A key role for tryptophan 84 in receptor activity-modifying protein 1 in the amylin 1 receptor

Amylin (Amy) receptors are complexes of the calcitonin receptor with receptor activity-modifying proteins. RAMP1 with the calcitonin receptor forms the AMY(1) receptor; the insert negative isoform of the calcitonin receptor in this complex makes the AMY(1(a)) receptor. This receptor has high affinity for Amy and the related peptide calcitonin gene-related peptide (CGRP). Amy is a peptide that has a role in lowering blood glucose levels and therefore its receptors represent potential drug targets for the treatment of diabetes. It has been suggested that the peptides bind in a pocket formed between the long N-termini of the calcitonin receptor and RAMP1, although very few residues in either component have been assigned specific roles. Based on the crystal structure of the RAMP1 N-terminus, the RAMP1 residues Arg67, Asp71, Glu78, Trp74 and Trp84 were identified as potentially having a role in peptide binding. Here, Arg67, Asp71, Glu78 and Trp84 were individually mutated to alanine and the function of mutant AMY(1(a)) receptors was determined using a cAMP assay. ELISA was used to measure cell surface expression and western blotting for total expression. Mutation of Arg67, Asp71 and Glu78 had no significant effect on Amy or CGRP potency, cell surface or total expression. Trp84Ala, however, resulted in a significant reduction in agonist potency and cell surface expression. Interestingly, a Trp84Phe substitution was able to restore some of this function, without restoring cell surface expression, suggesting that the residue may be important for peptide interactions. The data reveal the importance of Trp84 in the AMY(1(a)) receptor.

Amylin(1-8) is devoid of anabolic activity in bone

Amylin(1-8), a cyclic peptide consisting of the eight N-terminal amino acids of the 37-amino acid peptide amylin, has been shown to induce proliferation of primary osteoblasts and to induce bone formation in healthy male mice, whereas no data on efficacy in bone disease-related models have been reported. Therefore, we evaluated any effects of amylin(1-8) in ovariectomized rats with established osteopenia, a model for postmenopausal osteoporosis. At doses up to 100 nmol/kg/day, a dose highly effective in healthy mice, amylin(1-8) was unable to increase bone mineral density in ovariectomized rats during an 8-week treatment period. Histomorphometric analysis of the tibia indicated that amylin(1-8) did not change bone histomorphometric parameters. In an attempt to verify any potential biological effects of amylin(1-8), we investigated the efficacy of this peptide in various in vitro assays. Experiments designed to confirm previously published results on the proliferative effects of amylin(1-8) on primary osteoblasts failed to show any response. Amylin(1-8) was able to partially displace (125)I-rat amylin(1-37) from amylin receptors composed of the calcitonin receptor and RAMP1, indicating specific interaction of the peptide with the amylin binding site. However, in vitro efficacy assays with amylin(1-8) in calcitonin receptor-RAMP-positive HEK293T and MCF7 cells failed to reveal any agonist activity of amylin(1-8), whereas amylin(1-37) showed the expected agonist activity. In conclusion, our results indicate that amylin(1-8) does not show agonist activity on amylin receptors, does not affect osteoblast proliferation, and is devoid of anabolic activity in bone.

Mapping interaction sites within the N-terminus of the calcitonin gene-related peptide receptor; the role of residues 23-60 of the calcitonin receptor-like receptor

The calcitonin receptor-like receptor (CLR) acts as a receptor for the calcitonin gene-related peptide (CGRP) but in order to recognize CGRP, it must form a complex with an accessory protein, receptor activity modifying protein 1 (RAMP1). Identifying the protein/protein and protein/ligand interfaces in this unusual complex would aid drug design. The role of the extreme N-terminus of CLR (Glu23-Ala60) was examined by an alanine scan and the results were interpreted with the help of a molecular model. The potency of CGRP at stimulating cAMP production was reduced at Leu41Ala, Gln45Ala, Cys48Ala and Tyr49Ala; furthermore, CGRP-induced receptor internalization at all of these receptors was also impaired. Ile32Ala, Gly35Ala and Thr37Ala all increased CGRP potency. CGRP specific binding was abolished at Leu41Ala, Ala44Leu, Cys48Ala and Tyr49Ala. There was significant impairment of cell surface expression of Gln45Ala, Cys48Ala and Tyr49Ala. Cys48 takes part in a highly conserved disulfide bond and is probably needed for correct folding of CLR. The model suggests that Gln45 and Tyr49 mediate their effects by interacting with RAMP1 whereas Leu41 and Ala44 are likely to be involved in binding CGRP. Ile32, Gly35 and Thr37 form a separate cluster of residues which modulate CGRP binding. The results from this study may be applicable to other family B GPCRs which can associate with RAMPs.

Placental gene expression of calcitonin gene-related peptide and nitric oxide synthases in preeclampsia: effects of magnesium sulfate

OBJECTIVE

To determine placental gene expression of calcitonin gene-related peptide (CGRP), calcitonin receptor-like receptor (CRLR), receptor activity modifying protein 1 (RAMP1), and endothelial and inducible nitric oxide synthases (eNOS and iNOS) in mild preeclampsia, and to assess the effects of magnesium sulfate (MgSO4).

METHODS

Term placentas were obtained from 10 normotensive (NT group), 10 preeclamptic (PE) patients treated with 0.9% NaCl solution (PES group), and 8 PE women who received MgSO4 (PEMgSO4 group). The levels of mRNA were evaluated by real-time PCR.

RESULTS

Placental gene expression of CRLR, RAMP1 and iNOS were significantly higher (p < 0.001) in the PES group than in the NT group, without changes in CGRP. In addition, eNOS expression was 67% lower (p < 0.001) in the PES group. When compared with the PES group, the PEMgSO4 group showed significantly higher expression (p < 0.05) of CGRP, CRLR and eNOS, while iNOS was significantly lower (p < 0.05).

CONCLUSION

Placental gene expression of CRLR, RAMP1 and iNOS is higher in preeclampsia than in normal pregnancy, and MgSO4 treatment increased CGRP and CRLR and presented opposite effects upon eNOS and iNOS.

The role of peptides and receptors of the calcitonin family in the regulation of bone metabolism

The 'calcitonin family' is a group of peptide hormones that share structural similarities with calcitonin, and includes calcitonin gene-related peptide (CGRP), amylin, adrenomedullin and adrenomedullin 2 (intermedin). These hormones are produced by different tissues, with calcitonin being produced in thyroid C cells, alphaCGRP predominantly in neural tissue, amylin in beta-islet cells of the pancreas and adrenomedullin in many tissues and cell types. Bone appears to be a common target for all the peptides of the calcitonin family, although the specific bone effects of the peptides vary. Administration of calcitonin produces rapid lowering of serum calcium levels, mainly through inhibition of bone resorption by osteoclasts. In vitro and in a number of animal experimental models, amylin and CGRP are also effective in inhibiting osteoclast activity and bone resorption. Amylin, adrenomedullin and CGRP can also affect cells of the osteoblast lineage, inducing osteoblast proliferation and promoting bone formation. Receptors for the peptides of the calcitonin family are formed by heterodimerization of the calcitonin receptor (CTR) or calcitonin receptor-like receptor (CLR) with receptor activity modifying proteins (RAMPs). Although the different combinations of these proteins create receptors with distinct ligand specificities, there is a degree of cross-reactivity and the receptors are able to bind other ligands from the family, usually with lower affinity. Analysis of the expression of the receptors for the calcitonin family in 16 samples of human osteoblasts showed high levels of CLR and RAMP1, low levels of RAMP2 and no expression of RAMP3 or CTR. Recent studies of the bone phenotype of knockout animals lacking the calcitonin, alphaCGRP or amylin gene indicated that in this experimental system the main physiological role of amylin in bone is the inhibition of bone resorption, that of CGRP is the activation of bone formation, while calcitonin, unexpectedly appears to be inhibiting bone formation without affecting bone resorption. Further investigations will be required to determine the mechanisms of action of calcitonin peptides in bone and their significance to human bone physiology.

Receptor activity-modifying proteins differentially modulate the G protein-coupling efficiency of amylin receptors

Receptor activity-modifying proteins (RAMPs) 1, 2, and 3 are prototypic G protein-coupled receptor accessory proteins that can alter not only receptor trafficking but also receptor phenotype. Specific RAMP interaction with the calcitonin receptor (CTR) generates novel and distinct receptors for the peptide amylin; however, the role of RAMPs in receptor signaling is not understood. The current study demonstrates that RAMP interaction with the CTRa in COS-7 or HEK-293 cells leads to selective modulation of signaling pathways activated by the receptor complex. There was a 20- to 30-fold induction in amylin potency at CTR/RAMP1 (AMY1) and CTR/RAMP3 (AMY3) receptors, compared with CTR alone, for formation of the second-messenger cAMP that parallels an increase in amylin binding affinity. In contrast, only 2- to 5-fold induction of amylin potency was seen for mobilization of intracellular Ca++ or activation of ERK1/2. In addition, in COS-7 cells, the increase in amylin potency for Ca++ mobilization was 2-fold greater for AMY3 receptors, compared with AMY1 receptors and this paralleled the relative capacity of overexpression of Galphaq proteins to augment induction of high affinity 125I-amylin binding. These data demonstrate that RAMP-complexed receptors have a different signaling profile to CTRs expressed in the absence of RAMPs, and this is likely due to direct effects of the RAMP on G protein-coupling efficiency.

Unaltered mRNA expression of calcitonin-like receptor and receptor activity modifying proteins in human arteries in stroke and myocardial infarction

Calcitonin-like receptor (CL-R) is a functional CGRP1-receptor when complexed with RAMP1 or an adrenomedullin-receptor or when complexed with RAMP2 or RAMP3. This study was carried out 1. to set up a method to examine the relative quantity of mRNA of CL-R, RAMP1, RAMP2 and RAMP3 in human coronary (CA), pulmonary (PA) and middle cerebral arteries (MCA), and 2. to examine the level of mRNA expression in cerebra- and cardiovascular diseases. The method was validated with respect to the use of postmortem tissue and we compared beta-actin and GAPDH as housekeeping genes. There was no time-dependent change in total RNA and level of mRNA for p-actin or GAPDH could be detected in vessels removed from 1 and 5 days post mortem. The expression of beta-actin appears lower in coronary artery than in pulmonary artery and middle cerebral artery with no significant difference for GAPDH; both worked well. There were some differences in mRNA expression for CL-R (higher) and RAMP3 (lower) in middle cerebral artery compared to coronary artery and pulmonary artery. There was no significant difference in mRNA for RAMP1 and RAMP2 in the three types of arteries. We did not observe any difference in mRNA for CL-R and RAMPs in arteries from patients with hemorrhagic stroke, arteriosclerosis and acute myocardial infarction when compared to patients without these diagnoses. Thus the mRNA expression seems to be unaltered in these disorders.

The effects of C-terminal truncation of receptor activity modifying proteins on the induction of amylin receptor phenotype from human CTb receptors

Receptor activity modifying proteins (RAMPs) interact with calcitonin receptors to produce novel amylin receptor phenotypes. We have recently demonstrated that the short intracellular C-terminus of RAMPs plays a key role in the function of amylin receptors derived from the CTa calcitonin receptor through the use of chimeric RAMPs and RAMPs that are truncated at the C-terminus [15, Udawela M, Christopoulos G, Morfis M, Christopoulos A, Ye S, Tilakaratne N, Sexton PM. A critical role for the short intracellular C terminus in receptor activity modifying protein function. Mol Pharmacol 2006;70:1750-60., 18, Udawela M, Christopoulos G, Tilakaratne N, Christopoulos A, Albiston A, Sexton PM. Distinct receptor activity-modifying protein domains differentially modulate interaction with calcitonin receptors. Mol Pharmacol 2006;69:1984-89.]. The calcitonin receptor in humans is expressed as two major alternatively spliced isoforms termed CTa and CTb. Relatively little is known about how alternate splicing of the receptor affects the interaction between calcitonin receptors and RAMPs. We have examined the effect of RAMP truncation, through use of mutant constructs that delete the last 8 amino acids of each of the 3 known human RAMPs, and characterised these for interaction with CTb receptors through co-expression in COS-7 cells. As seen with the CTa receptor isoform, RAMP truncation caused a marked loss in induction of AMYb receptor phenotypes as characterised by (125)I-rat amylin radioligand binding assays and cAMP accumulation assays; the latter as a marker of receptor signalling. The effect was most pronounced for RAMP1 and RAMP2 deletion mutants, but attenuated responses were also observed with co-expressed RAMP3 deletion mutants. These data support a direct role for the RAMP C-terminus in the interaction of RAMP/calcitonin receptor complexes with intracellular accessory proteins involved in signalling and/or receptor trafficking.

Ligand binding and activation of the CGRP receptor

The receptor for CGRP (calcitonin gene-related peptide) is a heterodimer between a GPCR (G-protein-coupled receptor), CLR (calcitonin receptor-like receptor) and an accessory protein, RAMP1 (receptor activity-modifying protein 1). Models have been produced of RAMP1 and CLR. It is likely that the C-terminus of CGRP interacts with the extracellular N-termini of CLR and RAMP1; the extreme N-terminus of CLR is particularly important and may interact directly with CGRP and also with RAMP1. The N-terminus of CGRP interacts with the TM (transmembrane) portion of the receptor; the second ECL (extracellular loop) is especially important. Receptor activation is likely to involve the relative movements of TMs 3 and 6 to create a G-protein-binding pocket, as in Family A GPCRs. Pro321 in TM6 appears to act as a pivot. At the base of TMs 2 and 3, Arg151, His155 and Glu211 may form a loose equivalent of the Family A DRY (Asp-Arg-Tyr) motif. Although the details of this proposed activation mechanism clearly do not apply to all Family B GPCRs, the broad outlines may be conserved.

Calcitonin receptor-like receptor and receptor activity modifying protein 1 in the rat dorsal horn: localization in glutamatergic presynaptic terminals containing opioids and adrenergic alpha2C receptors

Calcitonin gene-related peptide (CGRP) is abundant in the central terminals of primary afferents. However, the function of CGRP receptors in the spinal cord remains unclear. CGRP receptors are heterodimers of calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1). We studied the localization of CRLR and RAMP1 in the rat dorsal horn using well-characterized antibodies against them, which labeled numerous puncta in laminae I-II. In addition, RAMP1 was found in cell bodies, forming patches at the cell surface. The CRLR- and RAMP1-immunoreactive puncta were further characterized using double and triple labeling. Colocalization was quantified in confocal stacks using Imaris software. CRLR did not colocalize with primary afferent markers, indicating that these puncta were not primary afferent terminals. CRLR- and RAMP1-immunoreactive puncta contained synaptophysin and vesicular glutamate transporter-2 (VGLUT2), showing that they were glutamatergic presynaptic terminals. Electron microscopic immunohistochemistry confirmed that CRLR immunoreactivity was present in axonal boutons that were not in synaptic glomeruli. Using tyramide signal amplification for double labeling with the CRLR and RAMP1 antibodies, we found some clear instances of colocalization of CRLR with RAMP1 in puncta, but their overall colocalization was low. In particular, CRLR was absent from RAMP1-containing cells. Many of the puncta stained for CRLR and RAMP1 were labeled by anti-opioid and anti-enkephalin antibodies. CRLR and, to a lesser extent, RAMP1 also colocalized with adrenergic alpha(2C) receptors. Triple label studies demonstrated three-way colocalization of CRLR-VGLUT2-synaptophysin, CRLR-VGLUT2-opioids, and CRLR-opioids-alpha(2C) receptors. In conclusion, CRLR is located in glutamatergic presynaptic terminals in the dorsal horn that contain alpha(2C) adrenergic receptors and opioids. Some of these terminals contain RAMP1, which may form CGRP receptors with CRLR, but in others CRLR may form other receptors, possibly by dimerizing with RAMP2 or RAMP3. These findings suggest that CGRP or adrenomedullin receptors modulate opioid release in the dorsal horn.

Adrenomedullin is induced by hypoxia and enhances pancreatic cancer cell invasion

Adrenomedullin (ADM) is synthesized by different types of cells and acts by binding calcitonin receptor-like receptor (CRLR) and members of the receptor activity-modifying protein (RAMP) family. In this study, the expression and functional role of ADM and its signaling components were investigated in pancreatic adenocarcinoma (PDAC). By QRT-PCR, median mRNA levels of ADM and CRLR were 1.5- and 2.4-fold higher, respectively, in PDAC tissues compared to normal pancreatic tissues. By immunohistochemistry, ADM, CRLR, RAMP1 and RAMP2, but not RAMP3, were expressed in pancreatic cancer cells. ADM serum levels were significantly increased in PDAC patients compared to healthy controls and chronic pancreatitis (CP) patients, with an area under the ROC curve of 0.83 and 0.98, respectively. At a cut-off level of 30.6 ng/ml, the specificity of ADM to differentiate PDAC from controls and CP patients was 85.5 and 83.6%, with a sensitivity of 80 and 100%. All 5 evaluated pancreatic cancer cells lines expressed ADM, CRLR, RAMP1 and RAMP2, whereas RAMP3 was expressed in only 1/5 pancreatic cancer cell lines. ADM was strongly induced by hypoxia and significantly increased invasiveness in 3/5 human pancreatic cancer cells. Blocking of CRLR decreased invasiveness in 4/5 human pancreatic cancer cells. In addition, rADM slightly up-regulated vascular endothelial growth factor secretion in 3/5 cell lines. In conclusion, ADM is induced by hypoxia and over-expressed in PDAC and might therefore serve as a potential tumor marker. Furthermore, ADM increases invasiveness of some pancreatic cancer cells and might influence angiogenesis, suggesting that blocking this pathway might have a therapeutic potential.

Interaction of receptor-activity-modifying protein1 with tubulin

Receptor-activity-modifying protein (RAMP) 1 is an accessory protein of the G protein-coupled calcitonin receptor-like receptor (CLR). The CLR/RAMP1 heterodimer defines a receptor for the potent vasodilatory calcitonin gene-related peptide. A wider tissue distribution of RAMP1, as compared to that of the CLR, is consistent with additional biological functions. Here, glutathione S-transferase (GST) pull-down, coimmunoprecipitation and yeast two-hybrid experiments identified beta-tubulin as a novel RAMP1-interacting protein. GST pull-down experiments indicated interactions between the N- and C-terminal domains of RAMP1 and beta-tubulin. Yeast two-hybrid experiments confirmed the interaction between the N-terminal region of RAMP1 and beta-tubulin. Interestingly, alpha-tubulin was co-extracted with beta-tubulin in pull-down experiments and immunoprecipitation of RAMP1 coprecipitated alpha- and beta-tubulin. Confocal microscopy indicated colocalization of RAMP1 and tubulin predominantly in axon-like processes of neuronal differentiated human SH-SY5Y neuroblastoma cells. In conclusion, the findings point to biological roles of RAMP1 beyond its established interaction with G protein-coupled receptors.

Sensitization of calcitonin gene-related peptide receptors by receptor activity-modifying protein-1 in the trigeminal ganglion

The neuropeptide calcitonin gene-related peptide (CGRP) from the trigeminal ganglion has been established as a key player in the pathogenesis of migraine. In this study, we provide evidence that the responsiveness of neuronal CGRP receptors is strongly enhanced in vitro and in vivo by expression of human receptor activity-modifying protein-1 (hRAMP1), an obligatory subunit of the CGRP receptor. We first demonstrated that activation of CGRP receptors on cultured trigeminal ganglion neurons increased endogenous CGRP mRNA levels and promoter activity. The promoter activation was cAMP dependent and blocked by the antagonist BIBN4096BS [1-piperidinecarboxamide, N-[2-[[5-amino-l-[[4-(4-pyridinyl)-l-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)], a new antimigraine drug. Gene transfer using an adenoviral hRAMP1 expression vector increased the maximal production of cAMP by 1.8 +/- 0.2-fold and decreased the EC50 to 2.3 +/- 0.8 nM from 9.0 +/- 5.9 nM and 15.6 +/- 5.2 nM in uninfected and control-infected cultures, respectively. To establish whether RAMP1 is limiting in vivo as indicated from the culture studies, a transgenic mouse expressing hRAMP1 in the nervous system was generated. After CGRP injection into the whiskerpad, the hRAMP1 transgenic mice displayed 2.2 +/- 0.2-fold greater plasma extravasation, which is a measure of neurogenic inflammation. These results demonstrate that RAMP1 is functionally rate limiting for CGRP receptor activity in the trigeminal ganglion, which raises the possibility that elevated RAMP1 might sensitize some individuals to CGRP actions in migraine.

Post-endocytic sorting of calcitonin receptor-like receptor and receptor activity-modifying protein 1

Calcitonin receptor-like receptor (CLR) and the receptor activity-modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene-related peptide (CGRP). Although CGRP induces endocytosis of CLR/RAMP1, little is known about post-endocytic sorting of these proteins. We observed that the duration of stimulation with CGRP markedly affected post-endocytic sorting of CLR/RAMP1. In HEK and SK-N-MC cells, transient stimulation (10(-7) M CGRP, 1 h), induced CLR/RAMP1 recycling with similar kinetics (2-6 h), demonstrated by labeling receptors in living cells with antibodies to extracellular epitopes. Recycling of CLR/RAMP1 correlated with resensitization of CGRP-induced increases in [Ca(2+)](i). Cycloheximide did not affect resensitization, but bafilomycin A(1), an inhibitor of vacuolar H(+)-ATPases, abolished resensitization. Recycling CLR and RAMP1 were detected in endosomes containing Rab4a and Rab11a, and expression of GTPase-defective Rab4aS22N and Rab11aS25N inhibited resensitization. After sustained stimulation (10(-7) M CGRP, >2 h), CLR/RAMP1 trafficked to lysosomes. RAMP1 was degraded approximately 4-fold more rapidly than CLR (RAMP1, 45% degradation, 5 h; CLR, 54% degradation, 16 h), determined by Western blotting. Inhibitors of lysosomal, but not proteasomal, proteases prevented degradation. Sustained stimulation did not induce detectable mono- or polyubiquitination of CLR or RAMP1, determined by immunoprecipitation and Western blotting. Moreover, a RAMP1 mutant lacking the only intracellular lysine (RAMP1K142R) internalized and was degraded normally. Thus, after transient stimulation with CGRP, CLR and RAMP1 traffic from endosomes to the plasma membrane, which mediates resensitization. After sustained stimulation, CLR and RAMP1 traffic from endosomes to lysosomes by ubiquitin-independent mechanisms, where they are degraded at different rates.

[Alterations of intermedin and its receptor system in oleic acid-induced acute lung injury of rats]

OBJECTIVE

To investigate the changes of pulmonary IMD and its receptor system-calcitonin receptor-like receptor (CL) and receptor activity modifying proteins (RAMPs) mRNA in acute lung injury(ALI) induced by oleic acid of rats.

METHODS

Contents of IMD in plasma and lung homogenates were measured by radioimmunoassay(RIA). The lung mRNA of IMD, CL and RAMPs was determined by semi-quantitative RT-PCR.

RESULTS

Compared with control group, in ALI group, the contents of IMD1-53 in plasma and lung homogenates were decreased by 20.8% and 74.5% (all P < 0.05) , respectively. Furthermore , it was found that the levels of IMD, CL, RAMP1 and RAMP2 mRNA in lung were decreased by 30%, 38%, 26% and 37.9% (all P < 0. 05) , respectively. The levels of CL , RAMP1 or RAMP2 mRNA were positively correlated with down-regulations of IMD mRNA in ALI. However, compared with control group, the maximum binding capacity of IMD1-53 to plasma membranes was significantly increased in ALI group, and the affinity of IMD1-53 for its receptor almost had no change.

CONCLUSION

The amount of IMD 1-53 is down-regulated and IMD receptor system also down-regulated in Oliec acid induced ALI of rats. These changes suggest that IMD and its receptor system probably are involved in the development of ALI.

A critical role for the short intracellular C terminus in receptor activity-modifying protein function

Receptor activity-modifying proteins (RAMPs) interact with and modify the behavior of the calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR). We have examined the contribution of the short intracellular C terminus, using constructs that delete the last eight amino acids of each RAMP. C-Terminal deletion of individual RAMPs had little effect on the signaling profile induced when complexed with CLR in COS-7 or human embryonic kidney (HEK)293 cells. Likewise, confocal microscopy revealed each of the mutant RAMPs translocated hemagglutinin-tagged CLR to the cell surface. In contrast, a pronounced effect of RAMP C-terminal truncation was seen for RAMP/CTRa complexes, studied in COS-7 cells, with significant attenuation of amylin receptor phenotype induction that was stronger for RAMP1 and -2 than RAMP3. The loss of amylin binding upon C-terminal deletion could be partially recovered with overexpression of Galpha(s), suggesting an impact of the RAMP C terminus on coupling of G proteins to the receptor complex. In HEK293 cells the c-Myc-RAMP1 C-terminal deletion mutant showed high receptor-independent cell surface expression; however, this construct showed low cell surface expression when expressed alone in COS-7 cells, indicating interaction of RAMPs with other cellular components via the C terminus. This mutant also had reduced cell surface expression when coexpressed with CTR. Thus, this study reveals important functionality of the RAMP C-terminal domain and identifies key differences in the role of the RAMP C terminus for CTR versus CLR-based receptors.

Adrenomedullin Peptide: Gene Expression of Adrenomedullin, its Receptors and Receptor Activity Modifying Proteins, and Receptor Binding in Rat Testis—Actions on Testosterone Secretion1

Adrenomedullin (ADM) has been shown to be present in the human and rat male reproductive systems. This study demonstrates the expression of ADM in the rat testis and its effect on the secretion of testosterone. Whole testicular extracts had 5.43 +/- 0.42 fmol of immunoreactive ADM per milligram of protein and 84 +/- 8 fg of ADM mRNA per picogram of Actb (beta-actin) mRNA. Immunocytochemical studies showed positive ADM immunostaining in the Leydig cells and in the Sertoli cells. Gel filtration chromatography of testicular extracts showed two peaks, with the predominant one eluting at the position of the ADM precursor. Furthermore, the testis was shown to coexpress mRNAs encoding the calcitonin receptor-like receptor and receptor activity modifying protein 1 (Ramp1), Ramp2, and Ramp3. These account for the specific binding of ADM to the testis, which was partially inhibited by human ADM (22-52) and by human calcitonin gene-related peptide (8-37), the ADM and calcitonin gene-related peptide receptor antagonists, respectively. Administration of ADM to testicular blocks in vitro resulted in a dose-dependent inhibition of hCG-stimulated release of testosterone, which was abolished by the administration of ADM (22-52). Our results suggest a paracrine effect of ADM on testicular steroidogenesis.

Characterization of the structure of RAMP1 by mutagenesis and molecular modeling

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

The gene expression of adrenomedullin, calcitonin-receptor-like receptor and receptor activity modifying proteins (RAMPs) in CCl4-induced rat liver cirrhosis

This study was undertaken to determine AM expression in carbon tetrachloride (CCl4)-induced liver cirrhosis developed with peritoneal ascites. Sprague-Dawley rats received subcutaneous injections of CCl4 twice weekly in olive oil (1:1, 0.3 ml per kg body weight) for 6 or 12 weeks until ascites developed, or saline in olive oil as control. At 6 weeks, fibrosis developed and at 12 weeks cirrhosis developed with ascites formation. At both 6 and 12 weeks, increases in plasma renin and AM were evident, as was the gene expression of AM. At 12 weeks after CCl4 injection, the gene expression of calcitonin-like-receptor (CRLR) and receptor activity modifying proteins (RAMP1, RAMP2 and RAMP3) were all elevated when compared to the control. The results suggest that liver cirrhosis increases mRNA expressions of AM, CRLR and RAMP1, RAMP2 and RAMP3 and that the increase in AM gene expression precedes the development of cirrhosis. The increase in AM synthesis as reflected by an increase in AM gene expression, together with a lack of increase in AM peptide at both 6 and 12 weeks may suggest an elevation of AM release. Given the potent vasodilatory action of AM, the increase in the synthesis and release of AM in the cirrhotic liver may also contribute to peripheral vasodilatation in liver cirrhosis.

Expression of calcitonin receptor in rat mammary gland during lactation

Calcitonin (CT) and calcitonin receptor (CTR) have been reported to play an important role in mammary tissue during pregnancy, lactation, and involution. In the present study, the expression and distribution of CTR mRNA in rat mammary tissue during pregnancy and lactation were investigated. As measured by real-time RT-PCR, CTR mRNA levels were increased only slightly during pregnancy, but increased markedly immediately postpartum and remained elevated through lactation, with the highest levels observed 14 days postpartum. In situ hybridization analysis showed that intense CTR mRNA signals were detected in the whole mammary gland. We performed immunohistochemistry to determine distribution of CTR in the mammary epithelium. CTR has been reported to act as an amylin receptor when heterodimerized with receptor activity modifying protein-1 (RAMP1) or RAMP3. mRNA expression of RAMP1 and RAMP3 in mammary tissue decreased during pregnancy and lactation, and amylin mRNA was undetectable, suggesting that up-regulated CTR in lactating mammary tissues binds CT rather than amylin. In primary cultures of mammary cells isolated from rat dams 14 days postpartum, CT produced a statistically significant decrease in thymidine incorporation. These results suggest that up-regulation of CTR during lactation may contribute to inhibition of mammary epithelial cell proliferation.

Identification and pharmacological characterization of domains involved in binding of CGRP receptor antagonists to the calcitonin-like receptor

The calcitonin-like receptor (CLR) and the calcitonin receptor (CTR) interact with receptor activity-modifying protein 1 (RAMP1) at the cell surface to form heterodimeric receptor complexes. CLR and CTR are members of the class II (family B) G-protein-coupled receptors (GPCR) and bind calcitonin gene-related peptide (CGRP) with similar affinities when coexpressed with RAMP1. The observation that various nonpeptide CGRP receptor antagonists display a higher affinity for the CLR/RAMP1 complex than for CTR/RAMP1 provided an opportunity to investigate the molecular determinants of the differential receptor affinities of these antagonists. A chimeric receptor approach was utilized to identify key domains within CLR responsible for conferring high-affinity antagonist binding. Initial chimera experiments implicated distinct regions within CLR as responsible for the affinities of structurally diverse CGRP receptor antagonists. Dissection of these key regions implicated amino acids 37-63 located in the amino terminus of CLR as responsible for the high-affinity interaction of one structural class, while transmembrane domain (TM) 7 was responsible for the interaction of a second class of antagonist. A unique binding interaction in the amino terminus of CLR is consistent with the observation that these compounds also interact with the extracellular region of RAMP1 and could suggest the formation of a binding pocket between the two proteins. Conversely, a compound which interacted with TM7 did not display a similar RAMP1 dependence, suggesting an allosteric mechanism of antagonism. Collectively, these data provide insight into two alternative mechanisms of antagonism for this unique heterodimeric receptor complex.

Calcitonin gene-related peptide receptor activation by receptor activity-modifying protein-1 gene transfer to vascular smooth muscle cells

The neuropeptide calcitonin gene-related peptide (CGRP) is a potent vasodilator that plays a protective role in the cardiovascular system. The receptor for CGRP is an unusual complex of the G protein-coupled calcitonin-like receptor and an obligate receptor activity modifying protein-1 (RAMP1). In this report we provide the first evidence that RAMP1 is rate limiting in vascular smooth muscle cells. Although cultured rat aorta smooth muscle cells express calcitonin like-receptor and RAMP1, we found that CGRP is not a potent activator of the receptor. After overexpression of RAMP1 by adenoviral gene transfer, there was a striking increase in CGRP-induced production of cAMP, with a 75-fold decrease in the EC(50) and a 1.5-fold increase in the maximal response. The biological consequence of this increased receptor activity was observed in three different paradigms. First, RAMP1 gene transfer caused a CGRP-dependent decrease in cell proliferation. Second, RAMP1 and CGRP treatment led to a 3-fold greater free radical-induced reduction in cell number. Finally, RAMP1 gene transfer resulted in a 5-fold CGRP-dependent increase in terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate nick end labeling-positive apoptotic cells upon serum withdrawal. The mechanisms underlying these effects involved cAMP-dependent pathways. We propose that RAMP1 gene transfer may be an effective strategy for increasing the effectiveness of CGRP-induced decrease in restenosis after aortic angioplasty.

Distinct receptor activity-modifying protein domains differentially modulate interaction with calcitonin receptors

Calcitonin receptors (CTRs) dimerize with receptor activity-modifying proteins (RAMPs) to generate high-affinity amylin (AMY) receptors; however, the relative contribution of individual RAMP domains to the formation of AMY receptors is poorly understood. We have used chimeras between RAMP1 and RAMP2 that specifically exchanged the N-terminal, transmembrane, or C-terminal domain and examined these in assays of [(125)I]amylin binding or peptide-induced cAMP signaling in COS-7 cells transiently transfected with wild-type or chimeric RAMPs and human CTRa. The specificity of peptides in competition for [(125)I]amylin binding was principally dictated by the N-terminal domain present in the chimeras; however, the maximal level of binding induced was dictated by the transmembrane domain present. This extended previous data (Zumpe et al., 2000) to provide a distinction between the transmembrane domain and the C terminus in this function. In contrast to the effects on binding, each of the RAMP domains played a role in the signaling phenotype of the receptors. In particular, the potency of calcitonin gene-related peptide (CGRP) was most influenced by the C-terminal domain present, in which the presence of the RAMP1 C-terminal domain led to increased potency over CTRa alone, whereas chimeras with the RAMP2 C-terminal domain did not induce increased CGRP potency. The data provide additional support for the importance of the N terminus in determining binding affinity but reveal a prominent role of the transmembrane domain in the strength of amylin binding and a unique role for the C terminus in signaling by peptides to stimulate cAMP production.

Potentiated response to adrenomedullin in myocardia and aortas in spontaneously hypertensive rat

Adrenomedullin (AM) is a multifunctional regulatory peptide, and endogenous AM is an important factor in regulating cardiovascular and renal homeostasis as a potent cardio-reno-protective factor. To illustrate the protective mechanism of adrenomedullin (AM) on the cardiovascular system by observing (1) the changes in mRNA and protein levels of AM and its receptor-calcitonin receptor-like receptor (CL) and receptor activity-modifying proteins (RAMPs)-in myocardia and aortas of spontaneously hypertensive rats (SHRs) and (2) the response of cardiovascular tissue to AM. The AM content and cyclic adenosine monophosphate (cAMP) production in myocardia and aortas were measured in SHRs and Wistar Kyoto (WKY) rats (11-week-old) by radioimmunoassay (RIA). The mRNA levels of brain natriuretic peptide (BNP), AM, CL, RAMP1, -2, -3 were determined by semi-quantitative RTPCR. Protein levels of CL, RAMP1, -2, -3 were assayed by Western blotting. SHRs had severe hypertension, and the tail-blood pressure was 76.7% higher, the ratio of heart weight to body weight (heart coefficient) 45.5% higher, and the BNP gene expression 4.5-fold higher than that of WKY rats (all p < 0.01). The AM-ir content in plasma, myocardia and aortas of SHRs increased by 42.5%, 68.3% and 80.4%, respectively (all p < 0.01) compared with WKY rats. Furthermore, the mRNA levels of AM, CL, RAMP1, RAMP2 and RAMP3 were elevated by 46% (p < 0.01), 62% (p < 0.05), 51.2% (p < 0.01), 41% (p < 0.01) and 54% (p < 0.01), respectively, in myocardia and by 72%, 87%, 155%, 53% and 74% (all p < 0.01), respectively, in aortas. The elevated mRNA level of CL, RAMP1 RAMP2 and RAMP3 correlated positively with that of AM mRNA in hypertrophic myocardia (r= 0.943, 0.621, 0.688 and 0.633, respectively, all p < 0.01) and aortas (r = 0.762, 0.892, 0.828 and 0.736, respectively, all p < 0.01). The protein levels of CL, RAMP1, RAMP2 and RAMP3 in myocardia and aortas of SHRs were increased compared with that of WKY rats. The response to AM was potentiated in myocardia and aortas in SHRs, and the production of cAMP was increased by 47% and 65% (both p < 0.01), respectively. AM-stimulated cAMP generation in myocardia and aortas was blocked by both AM(22-52), the specific antagonist of AM, and calcitonin gene-related peptide (CGRP)(8-37), the antagonist of the CGRP1 receptor. In myocardia and aortas of SHRs, the gene expressions and protein levels of AM, CL, RAMP1, RAMP2 and RAMP3 were increased, and the response to AM was potentiated. AM-stimulated cAMP generation in myocardia and aortas was blocked by both AM(22-52) and CGRP(8-37). The results suggest that the changes of AM and its receptors in cardiovascular tissue, and the increased response of cardiovascular tissue to AM might importantly impact the pathogenesis of hypertension.

Enhanced Vascular Responses to Adrenomedullin in Mice Overexpressing Receptor-Activity–Modifying Protein 2

Adrenomedullin (AM) levels are elevated in cardiovascular disease, but little is known of the role of specific receptor components. AM acts via the calcitonin receptor-like receptor (CLR) interacting with a receptor-activity–modifying protein (RAMP). The AM 1 receptor is composed of CLR and RAMP2, and the calcitonin gene–related peptide (CGRP) receptor of CLR and RAMP1, as determined by molecular and cell-based analysis. This study examines the relevance of RAMP2 in vivo. Transgenic (TG) mice that overexpress RAMP2 in smooth muscle were generated. The role of RAMP2 in the regulation of blood pressure and in vascular function was investigated. Basal blood pressure, acute angiotensin II–raised blood pressure, and cardiovascular properties were similar in wild-type (WT) and TG mice. However, the hypotensive effect of IV AM, unlike CGRP, was enhanced in TG mice ( P <0.05), whereas a negative inotropic action was excluded by left-ventricular pressure–volume analysis. In aorta relaxation studies, TG vessels responded in a more sensitive manner to AM (EC 50 , 8.0±1.5 nmol/L) than WT (EC 50 , 17.9±3.6 nmol/L). These responses were attenuated by the AM receptor antagonist, AM 22-52 , such that residual responses were identical in all mice. Remaining relaxations were further inhibited by CGRP receptor antagonists, although neither affected AM responses when given alone. Mesenteric and cutaneous resistance vessels were also more sensitive to AM in TG than WT mice. Thus RAMP2 plays a key role in the sensitivity and potency of AM-induced hypotensive responses via the AM 1 receptor, providing evidence that this receptor is a selective target for novel therapeutic approaches.

Functions of the cytoplasmic tails of the human receptor activity-modifying protein components of calcitonin gene-related peptide and adrenomedullin receptors

Receptor activity-modifying proteins (RAMPs) enable calcitonin receptor-like receptor (CRLR) to function as a calcitonin gene-related peptide receptor (CRLR/RAMP1) or an adrenomedullin (AM) receptor (CRLR/RAMP2 or -3). Here we investigated the functions of the cytoplasmic C-terminal tails (C-tails) of human RAMP1, -2, and -3 (hRAMP1, -2, and -3) by cotransfecting their C-terminal deletion or progressive truncation mutants into HEK-293 cells stably expressing hCRLR. Deletion of the C-tail from hRAMP1 had little effect on the surface expression, function, or intracellular trafficking of the mutant heterodimers. By contrast, deletion of the C-tail from hRAMP2 disrupted transport of hCRLR to the cell surface, resulting in significant reductions in (125)I-hAM binding and evoked cAMP accumulation. The transfection efficiency for the hRAMP2 mutant was comparable with that for wild-type hRAMP2; moreover, immunocytochemical analysis showed that the mutant hRAMP2 remained within the endoplasmic reticulum. FACS analysis revealed that deleting the C-tail from hRAMP3 markedly enhances AM-evoked internalization of the mutant heterodimers, although there was no change in agonist affinity. Truncating the C-tails by removing the six C-terminal amino acids of hRAMP2 and -3 or exchanging their C-tails with one another had no effect on surface expression, agonist affinity, or internalization of hCRLR, which suggests that the highly conserved Ser-Lys sequence within hRAMP C-tails is involved in cellular trafficking of the two AM receptors. Notably, deleting the respective C-tails from hRAMPs had no effect on lysosomal sorting of hCRLR. Thus, the respective C-tails of hRAMP2 and -3 differentially affect hCRLR surface delivery and internalization.

Differential expression of components of the cardiomyocyte adrenomedullin/intermedin receptor system following blood pressure reduction in nitric oxide-deficient hypertension

Adrenomedullin (AM) and intermedin (IMD; adrenomedulln-2) are vasodilator peptides related to calcitonin gene-related peptide (CGRP). The actions of these peptides are mediated by the calcitonin receptor-like receptor (CLR) in association with one of three receptor activity-modifying proteins. CGRP is selective for CLR/receptor activity modifying protein (RAMP)1, AM for CLR/RAMP2 and -3, and IMD acts at both CGRP and AM receptors. In a model of pressure overload induced by inhibition of nitric-oxide synthase, up-regulation of AM was observed previously in cardiomyocytes demonstrating a hypertrophic phenotype. The current objective was to examine the effects of blood pressure reduction on cardiomyocyte expression of AM and IMD and their receptor components. Nomega-nitro-L-arginine methyl ester (L-NAME) (35 mg/kg/day) was administered to rats for 8 weeks, with or without concurrent administration of hydralazine (50 mg/kg/day) and hydrochlorothiazide (7.5 mg/kg/day). In left ventricular cardiomyocytes from L-NAME-treated rats, increases (-fold) in mRNA expression were 1.6 (preproAM), 8.4 (preproIMD), 3.4 (CLR), 4.1 (RAMP1), 2.8 (RAMP2), and 4.4 (RAMP3). Hydralazine/hydrochlorothiazide normalized systolic blood pressure (BP) and abolished mRNA up-regulation of hypertrophic markers sk-alpha-actin and BNP and of preproAM, CLR, RAMP2, and RAMP3 but did not normalize cardiomyocyte width nor preproIMD or RAMP1 mRNA expression. The robust increase in IMD expression indicates an important role for this peptide in the cardiac pathology of this model but, unlike AM, IMD is not associated with pressure overload upon the myocardium. The concordance of IMD and RAMP1 up-regulation indicates a CGRP-type receptor action; considering also a lack of response to BP reduction, IMD may, like CGRP, have an anti-ischemic function.

Localization of calcitonin receptor-like receptor and receptor activity modifying protein 1 in enteric neurons, dorsal root ganglia, and the spinal cord of the rat

Calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene related peptide (CGRP) and intermedin. Although CGRP is widely expressed in the nervous system, less is known about the localization of CLR and RAMP1. To localize these proteins, we raised antibodies to CLR and RAMP1. Antibodies specifically interacted with CLR and RAMP1 in HEK cells coexpressing rat CLR and RAMP1, determined by Western blotting and immunofluorescence. Fluorescent CGRP specifically bound to the surface of these cells and CGRP, CLR, and RAMP1 internalized into the same endosomes. CLR was prominently localized in nerve fibers of the myenteric and submucosal plexuses, muscularis externa and lamina propria of the gastrointestinal tract, and in the dorsal horn of the spinal cord of rats. CLR was detected at low levels in the soma of enteric, dorsal root ganglia (DRG), and spinal neurons. RAMP1 was also localized to enteric and DRG neurons and the dorsal horn. CLR and RAMP1 were detected in perivascular nerves and arterial smooth muscle. Nerve fibers containing CGRP and intermedin were closely associated with CLR fibers in the gastrointestinal tract and dorsal horn, and CGRP and CLR colocalized in DRG neurons. Thus, CLR and RAMP1 may mediate the effects of CGRP and intermedin in the nervous system. However, mRNA encoding RAMP2 and RAMP3 was also detected in the gastrointestinal tract, DRG, and dorsal horn, suggesting that CLR may associate with other RAMPs in these tissues to form a receptor for additional peptides such as adrenomedullin.

Antiapoptotic effect of calcitonin gene-related peptide on oxidative stress-induced injury in H9c2 cardiomyocytes via the RAMP1/CRLR complex

Calcitonin gene-related peptide (CGRP) plays an important role in the mediation of protective effects observed in situations such as ischemic preconditioning in rat hearts. In this study, we investigated in H9c2 rat cardiomyoblasts if the protective effect of CGRP could be linked to an inhibitory effect on the apoptotic pathway. We also determined the specificity of observed effects by treatment with adrenomedullin (ADM) in stress conditions generated by 100 microM hydrogen peroxide. Using MTT assays, we demonstrate that a pretreatment with CGRP decreases by half the loss of cell viability induced by H(2)O(2). CGRP inhibits phosphatidylserine externalization, caspase 3 activation and DNA fragmentation due to oxidative stress. Using RT-PCR, we observed an increase in Bcl-2 mRNA expression induced by CGRP treatment. Dot blotting experiments showed that, in stress conditions, Bcl-2 protein level decreases while Bax is increased. CGRP administration prior to stress prevents these effects. The three-receptor activity modifying protein (RAMP) isotypes were detected by RT-PCR in H9c2 cells and in left ventricle rat tissue, RAMP1 and RAMP3 being the most abundant in both cases. RAMP1 expression was upregulated by CGRP while RAMP3 mRNA level was decreased. Cell viability assessed by MTT indicates that, contrary to CGRP, pretreatment of stressed cells with ADM, a RAMP2 agonist, fails to protect them while treatment with CGRP(8-37) (a RAMP1 and 2 inhibitor) abolished CGRP protective effect. Taken together, these data suggest that CGRP has antiapoptotic properties through the RAMP1/CRLR complex. CGRP could be used to prevent apoptosis in an ischemia-reperfusion context.

Receptor-activity-modifying proteins are required for forward trafficking of the calcium-sensing receptor to the plasma membrane

The calcium-sensing receptor (CaSR) is a class III G-protein-coupled receptor (GPCR) that responds to changes in extracellular calcium concentration and plays a crucial role in calcium homeostasis. The mechanisms controlling CaSR trafficking and surface expression are largely unknown. Using a CaSR tagged with the pH-sensitive GFP super-ecliptic pHluorin (SEP-CaSR), we show that delivery of the GPCR to the cell surface is dependent on receptor-activity-modifying proteins (RAMPs). We demonstrate that SEP-CaSRs are retained in the endoplasmic reticulum (ER) in COS7 cells that do not contain endogenous RAMPs whereas they are delivered to the plasma membrane in HEK 293 cells that do express RAMP1. Coexpression of RAMP1 or RAMP3, but not RAMP2, in COS7 cells was sufficient to target the CaSR to the cell surface. RAMP1 and RAMP3 colocalised and coimmunoprecipitated with the CaSR suggesting that these proteins associate within the cell. Our results indicate that RAMP expression promotes the forward trafficking of the GPCR from the ER to the Golgi apparatus and results in mature CaSR glycosylation, which is not observed in RAMP-deficient cells. Finally, silencing of RAMP1 in the endogenously expressing HEK293 cells using siRNA resulted in altered CaSR traffic. Taken together, our results show that the association with RAMPs is necessary and sufficient to transfer the immature CaSR retained in the ER towards the Golgi where it becomes fully glycosylated prior to delivery to the plasma membrane and demonstrate a role for RAMPs in the trafficking of a class III GPCR.

The N-terminal extracellular domain 23-60 of the calcitonin receptor-like receptor in chimeras with the parathyroid hormone receptor mediates association with receptor activity-modifying protein 1

The calcitonin receptor-like receptor (CLR) requires the associated receptor activity-modifying protein (RAMP)1 to reveal a calcitonin gene-related peptide (CGRP) receptor. Here, the subdomain of the CLR that associates with RAMP1 has been identified in chimeras between the CLR and the parathyroid hormone (PTH) receptor 1 (PTHR). The PTHR alone does not interact with RAMP1. RAMP1 requires the CLR for its transport to the cell surface. Thus, receptor-dependent RAMP1 delivery to the plasma membrane and coimmunoprecipitation from the cell surface were used as measures for receptor/RAMP1 interaction. Several chimeric CLR-PTHR included the N-terminal amino acids 23-60 of the CLR transported RAMP1 to the surface of COS-7 cells much like the intact CLR. Moreover, RAMP1 coimmunoprecipitated with these receptors from the cell surface. A CLR deletion mutant, consisting of the N-terminal extracellular domain, the first transmembrane domain, and the C-terminal intracellular region, revealed the same results. Cyclic AMP was stimulated by CGRP in CLR/RAMP1 expressing cells (58 +/- 19-fold, EC(50) = 0.12 +/- 0.03 nM) and by PTH-related protein in cells expressing the PTHR (50 +/- 10-fold, EC(50) = 0.25 +/- 0.03 nM) or a PTHR with the N-terminal amino acids 23-60 of the CLR (23 +/- 5-fold, EC(50) > 1000 nM). Other chimeric CLR-PTHR were inactive. In conclusion, structural elements in the extreme N-terminus of the CLR between amino acids 23-60 are required and sufficient for CLR/RAMP1 cotransport to the plasma membrane and heterodimerization.

Receptor Activity-modifying Protein (RAMP) Isoform-specific Regulation of Adrenomedullin Receptor Trafficking by NHERF-1

Receptor activity-modifying proteins (RAMPs 1-3) are single transmembrane accessory proteins critical to various G-protein coupled receptors for plasma membrane expression and receptor phenotype. A functional receptor for the vasodilatory ligand, adrenomedullin (AM), is comprised of RAMP2 or RAMP3 and calcitonin receptor-like receptor (CRLR). It is now known that RAMP3 protein-protein interactions regulate the recycling of the AM2 receptor. The major aim of this study was to identify other interaction partners of RAMP3 and determine their role in CRLR-RAMP3 trafficking. Trafficking of G-protein-coupled receptors has been shown to be regulated by the Na+/H+ exchanger regulatory factor-1 (NHERF-1), an adaptor protein containing two tandem PSD-95/Discs-large/ZO-1 homology (PDZ) domains. In HEK 293T cells expressing the AM2 receptor, the complex undergoes agonist-induced desensitization and internalization. However, in the presence of NHERF-1, although the AM receptor (CRLR/RAMP3) undergoes desensitization, the internalization of the receptor complex is blocked. Overlay assays and mutational analysis indicated that RAMP3 and NHERF-1 interact via a PDZ type I domain on NHERF-1. The internalization of the CRLR-RAMP complex was not affected by NHERF-1 when CRLR was co-expressed with RAMP1 or RAMP2. Mutation of the ezrin/radixin/moesin (ERM) domain on NHERF-1 indicated that NHERF-1 inhibits CRLR/RAMP3 complex internalization by tethering the complex to the actin cytoskeleton. When examined in a primary culture of human proximal tubule cells endogenously expressing the CRLR-RAMP3 complex and NHERF-1, the CRLR-RAMP complex desensitizes but is unable to internalize upon agonist stimulation. Knock-down of either RAMP3 or NHERF-1 by RNA interference technology enabled agonist-induced internalization of the CRLR-RAMP complex. These results, using both endogenous and overexpressed cellular models, indicate a novel function for NHERF-1 and RAMP3 in the internalization of the AM receptor and suggest additional regulatory mechanisms for receptor trafficking.

Evidence for decreased calcitonin gene-related peptide (CGRP) receptors and compromised responsiveness to CGRP of fetoplacental vessels in preeclamptic pregnancies

Calcitonin gene-related peptide (CGRP) is a potent vasodilatory peptide, and its concentration is increased in both maternal and fetal circulation during late pregnancy. The present study was designed to investigate the expression of CGRP receptor components, calcitonin receptor-like receptor (CRLR), and receptor activity modifying protein 1 (RAMP1), and the relaxation response to CGRP in fetoplacental vessels from normotensive pregnant women and women with preeclampsia. Results showed that: 1) mRNA for both CRLR and RAMP1 was expressed in fetoplacental vessels from normal pregnancies; however, these mRNA expressions were substantially reduced in the vessels from preeclamptic women; 2) CRLR and RAMP1 proteins were abundantly expressed in the endothelium and smooth muscle layer of the fetoplacental vessels, as well as the trophoblast cells in normal placentas. In contrast, both vascular tissues and trophoblasts showed decreased expressions for CRLR and RAMP1 proteins and declined CGRP binding sites in preeclamptic placentas; and 3) CGRP produced a dose-dependent relaxation of serotonin-induced contraction of umbilical and chorionic arteries from normal pregnancies, but the response to CGRP was significantly attenuated in the vessels from preeclampsia. We concluded that CGRP may contribute to the low fetoplacental vascular resistance in normal pregnancies; however, CGRP-dependent vascular relaxation appears to be compromised in preeclamptic pregnancies.

Novel Function for Receptor Activity-modifying Proteins (RAMPs) in Post-endocytic Receptor Trafficking

RAMPs (1-3) are single transmembrane accessory proteins crucial for plasma membrane expression, which also determine receptor phenotype of various G-protein-coupled receptors. For example, adrenomedullin receptors are comprised of RAMP2 or RAMP3 (AM1R and AM2R, respectively) and calcitonin receptor-like receptor (CRLR), while a CRLR heterodimer with RAMP1 yields a calcitonin gene-related peptide receptor. The major aim of this study was to determine the role of RAMPs in receptor trafficking. We hypothesized that a PDZ type I domain present in the C terminus of RAMP3, but not in RAMP1 or RAMP2, leads to protein-protein interactions that determine receptor trafficking. Employing adenylate cyclase assays, radioligand binding, and immunofluorescence microscopy, we observed that in HEK293 cells the CRLR-RAMP complex undergoes agonist-stimulated desensitization and internalization and fails to resensitize (i.e. degradation of the receptor complex). Co-expression of N-ethylmaleimide-sensitive factor (NSF) with the CRLR-RAMP3 complex, but not CRLR-RAMP1 or CRLR-RAMP2 complex, altered receptor trafficking to a recycling pathway. Mutational analysis of RAMP3, by deletion and point mutations, indicated that the PDZ motif of RAMP3 interacts with NSF to cause the change in trafficking. The role of RAMP3 and NSF in AM2R recycling was confirmed in rat mesangial cells, where RNA interference with RAMP3 and pharmacological inhibition of NSF both resulted in a lack of receptor resensitization/recycling after agonist-stimulated desensitization. These findings provide the first functional difference between the AM1R and AM2R at the level of post-endocytic receptor trafficking. These results indicate a novel function for RAMP3 in the post-endocytic sorting of the AM-R and suggest a broader regulatory role for RAMPs in receptor trafficking.

Osteoclast-like cells express receptor activity modifying protein 2: application of laser capture microdissection

Receptor activity modifying proteins (RAMPs) act as receptor modulators that determine the ligand specificity of receptors for the calcitonin (CT) family. The purpose of this study was to analyze the expression of RAMPs in osteoclast-like cells using the laser capture microdissection (LCM) technique. Mouse bone marrow and spleen cells were co-cultured on a film designed for LCM. After 10 days, 250 osteoclast-like cells were captured using the LCM system. Total RNA from these cells was used to synthesize cDNA and RT-PCR analysis was performed. Osteoclast-like cells expressed CT receptor (CTR), CT receptor-like receptor (CRLR) and RAMP2, but did not express RAMP1 or RAMP3. These results indicated (1) that a pure population of osteoclast-like cells can be prepared by LCM and gene expression of this population can be analyzed by RT-PCR and (2) that RT-PCR shows that osteoclast-like cells express RAMP2, CTR and CRLR, suggesting the potential for adrenomedullin binding to osteoclast-like cells. This is the first report that osteoclast-like cells express RAMP2.

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.

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

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

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.

Progesterone upregulates calcitonin gene-related peptide and adrenomedullin receptor components and cyclic adenosine 3'5'-monophosphate generation in Eker rat uterine smooth muscle cell line

Calcitonin gene-related peptide (CGRP) and adrenomedullin (AM), two potent smooth-muscle relaxants, have been shown to cause uterine relaxation. Both CGRP- and AM-binding sites in the uterus increase during pregnancy and decrease at labor and postpartum. These changes in binding sites appear to be related to the changes in calcitonin receptor-like receptor (CRLR), receptor activity-modified protein 1 (RAMP1), RAMP2, and RAMP3 mRNA levels. It is not clear, however, whether the changes in the receptor components occur in the myometrial cells and whether the steroid hormones can directly alter these receptor components in the muscle cells. In addition, the mechanism of CGRP and AM signaling in the rat myometrium is not well understood. Therefore, we examined the mRNA expression of CGRP- and AM-receptor components, G protein Galphas, CGRP, and AM stimulation of cAMP and cGMP, and the effects of progesterone on these parameters in the Eker rat uterine myometrial smooth-muscle cell line (ELT3). ELT3 cells expressed CGRP- and AM-receptor components CRLR, RAMP1, RAMP2, and RAMP3. Expression of CRLR and RAMP1 mRNA increased with progesterone treatment and decreased with estradiol-17beta treatment. However, RAMP2 and RAMP3 mRNA expressions were unaltered by both progesterone and estradiol. Progesterone increased (P<0.05) Galphas expression and augmented CGRP- and AM-induced increases in cAMP levels. In uterine smooth-muscle cells, the antagonist to Galphas protein NF449 decreased basal as well as CGRP- and AM-stimulated cAMP levels. None of the cell treatments affected cyclic GMP production. Our results suggest that the progesterone-stimulated increases in CGRP and AM receptors, Galphas protein levels, and cAMP generation in the myometrial cells may be responsible for increased uterine relaxation sensitivity to CGRP and AM during pregnancy.

RAMPs and CRLR expressions in osteoblastic cells after dexamethasone treatment

Adrenomedullin (ADM) is a potent stimulator of osteoblastic activity and promotes bone growth in vivo. ADM receptors are formed by heterodimerization of the CRLR and a RAMP2 or RAMP3 molecule. Since glucocorticoid responsive elements were recently identified in the human CRLR promoter and that glucocorticoids exert a major action in bones, we investigated the acute effect of dexamethasone (Dex) treatment on ADM receptor components in osteoblastic cell types: the MC3T3-E1 cells and calvaria-derived osteoblastic cells. Changes in expression of CRLR and RAMPs molecules were evaluated at mRNA levels using RT-PCR and at protein levels by Western blot analysis. We found that Dex increased expression of RAMP1 and RAMP2 mRNA in a time-dependent but dose-independent manner, while RAMP3 was unchanged. In contrast, Dex decreased the CRLR mRNA expression and these changes were reflected at protein levels. We suggest that Dex, in osteoblastic cells, altered ADM receptor by inhibition of CRLR expression and consequently could impair the ADM anabolic effect on bone. Our findings could explain in part, the detrimental side effects observed at bone level during glucocorticoid therapy.

Aspartate(69) of the calcitonin-like receptor is required for its functional expression together with receptor-activity-modifying proteins 1 and -2

The calcitonin-like receptor (CLR) associated with receptor-activity-modifying proteins (RAMP) 1 or -2 recognizes calcitonin gene-related peptide (CGRP) and adrenomedullin (AM), respectively. The amino acid sequence CNRTWDGWLCW corresponding to residues 64-74 in the extracellular N-terminus of the CLR is conserved. The Asp(69) (D(69)) is present in all family B1 G-protein-coupled receptors. Here the D(69) of a V5-tagged mouse CLR has been mutated to Ala (A), Glu (E), and Asn (N). The function of the intact and the mutant CLR was investigated in COS-7 cells coexpressing myc-tagged mouse RAMP1 or -2. In CLR/RAMP1 and -2 expressing cells CGRP and AM stimulated cAMP formation with an EC(50) of 0.17 and 0.50 nM, respectively. The expression of the D69A, D69E, and D69N mutants at the cell surface was comparable to that of the intact CLR. cAMP stimulation by CGRP and AM was abolished in the D69A mutant. With the D69E mutant the EC(50) of CGRP and AM were 1000-fold higher than those with the intact CLR. With the D69N mutant the EC(50) of CGRP was 0.48 nM and that of AM 0.44 nM, but the maximal cAMP formation was reduced to 24% and to 12% of cells with the intact CLR. Co-immunoprecipitation of RAMP1 with the CLR, indicating complex formation, was reduced with the D69A, D69N, and D69E mutants. RAMP2 co-precipitated with the mutant receptors indistinguishable from the intact CLR. In conclusion, mutation of D69 to N, E or A in the CLR did not affect its expression at the cell surface, but impaired or abolished the CGRP and AM receptor function in the presence of RAMP1 and -2, respectively.

[Increased atria expression of receptor activity-modifying proteins in heart failure patients]

OBJECTIVE

Receptor activity-modifying proteins (RAMPs) determine the ligand specificity of the calcitonin receptor-like receptor (CRLR); co-expression of RAMP1 and CRLR results in a calcitonin gene related peptide (CGRP) receptor, whereas the association of RAMP2 or RAMP3 with CRLR gives an adrenomedullin(ADM) receptor. As CGRP and ADM may play a beneficial role in heart failure, this study aimed at the question whether RAMPs mRNAs are changed in heart failure.

METHODS

Semi-quantitative reverse transcription-PCR (RT-PCR) was used to detect and quantify the mRNAs of RAMP1 and RAMP3 in the atria of heart failing patients.

RESULTS

It was found that the expressions of RAMP1, RAMP2 and RAMP3 mRNAs increased with the worsening of heart function, but the expressions of RAMP1 and RAMP2 mRNA decreased at level IV of heart failure.

CONCLUSION

The above results demonstrated in the atria of heart failure patients an up-regulation of CGRP receptor by an increase of RAMP1 in association with CRLR and an up-regulation of ADM receptor by an increase of RAMP2 expression in association with CRLR, thus suggesting that CGRP and ADM receptors be playing a functional role in compensating the chronic heart failure in human.

Expression of calcitonin gene-related peptide type 1 receptor mRNA and their activity-modifying proteins in the rat nucleus accumbens

The calcitonin receptor-like receptor (CRLR) and the orphan receptor RDC-1 have been proposed to be calcitonin gene-related peptide type 1 (CGRP1) receptors, and receptor activity-modifying proteins (RAMPs) determine the ligand specificity of CRLR. Coexpression of RAMP1 and CRLR resulted in functional CGRP1 receptors; the complex of RAMP2 or RAMP3 and CRLR created functional adrenomedullin receptor. Although high levels of CGRP binding sites in the nucleus accumbens have been reported, little is known about the expression of these novel CGRP receptors. In the present study, we used real-time quantitative RT-PCR to detect and quantitate the relative expression of CGRP, CRLR, RAMP1-3 and RDC-1 in the nucleus accumbens of intact rats and rats with inflammation. Our results demonstrate that CGRP, CRLR, RAMP1 and RAMP2 exist in the nucleus accumbens of intact rats, and that they were significantly upregulated in rats with inflammation. In contrast, no expression was detected for RDC-1 and RAMP3. These findings indicated a functional role for CGRP and its receptors in inflammation and pain modulation.

Tumor necrosis factor-alpha downregulates adrenomedullin receptors in human coronary artery smooth muscle cells

We examined the effects of tumor necrosis factor (TNF)-alpha on the expression and functionality of adrenomedullin (AM) receptors in cultured human coronary artery smooth muscle cells. Analysis of real-time quantitative polymerase chain reactions showed that these cells abundantly express two AM receptors comprised of calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1) or RAMP2. TNF-alpha induced time- and dose-dependent decreases in the expression of CRLR and RAMP1/2 mRNAs, thereby diminishing AM-evoked cAMP production. The suppression of these three mRNAs was unaffected by inhibiting NOS, protein kinase G, protein kinase A, superoxide formation or NF-kappaB activation.

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.