Involvement of calcitonin gene-related peptide in control of human fetoplacental vascular tone

Calcitonin gene-related peptide (CGRP), one of the most potent endogenous vasodilators known, has been implicated in vascular adaptations and placental functions during pregnancy. The present study was designed to examine the existence of CGRP-A receptor components, the calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1), in the human placenta and the vasoactivity of CGRP in the fetoplacental circulation. Immunofluorescent staining of the human placenta in term labor using polyclonal anti-CRLR and RAMP1 antibodies revealed that labeling specifically concentrated in the vascular endothelium and the underlying smooth muscle cells in the umbilical artery/vein, chorionic artery/vein, and stem villous vessels as well as in the trophoblast layer of the placental villi. In vitro isometric force measurement showed that CGRP dose dependently relaxes the umbilical artery/vein, chorionic artery/vein, and stem villous vessels. Furthermore, CGRP-induced relaxation of placental vessels are inhibited by a CGRP receptor antagonist (CGRP8–37), ATP-sensitive potassium (KATP) channel blocker (glybenclamide), and cAMP-dependent protein kinase A inhibitor (Rp-cAMPS) and partially inhibited by a nitric oxide inhibitor ( Nω-nitro-l-arginine methyl ester). We propose that CGRP may play a role in the control of human fetoplacental vascular tone, and the vascular dilations in response to CGRP may involve activation of KATP channels, cAMP, and a nitric oxide pathway.

Expression and distribution of the calcitonin receptor-like receptor in the developing rat heart

During ontogenesis the 52 amino acid peptide adrenomedullin is first expressed in the heart and it is essential for normal cardiovascular development. Recent work suggests that most adrenomedullin effects are conveyed via the calcitonin receptor-like receptor (CRLR) in combination with appropriate receptor activity-modifying proteins (RAMPs). Here, we investigated the expression of these components during the development of the rat heart, focusing on the period of coronary vascular development. Using RT-PCR, transcripts for CRLR, RAMP1 and RAMP2 were detected at all stages from E 14 to adulthood. The distribution of CRLR was investigated by immunohistochemistry, and endothelial cells and their precursors identified with monoclonal antibodies against RECA-1 and flk-1. On E 14, intense CRLR immunoreactivity was observed in endothelial cells of the large vessels and the endocardial cushions at the AV-junction. Small CRLR immunoreactive cell clusters were located in the wall of the outflow tract and subepicardially in the ventricular wall. On E 16, tubes of CRLR immunoreactive cells formed a subepicardial plexus, from which they penetrated radially towards the trabecular network and entered at E 18. Smooth muscle cells of coronary arteries gained a moderate CRLR immunoreactivity at E 20 which persisted at this intensity up to P 8 and then decreased. At the same time, CRLR immunoreactivity of endothelial cells in coronary arteries vanished while those of coronary veins still exhibited intense CRLR immunoreactivity. These data suggest multiple functions of the adrenomedullin/CRLR signaling pathway in cardiac development, among which the most prominent appears to be the early outgrowth and proliferation of the immature endothelial cells of the coronary vasculature.

Three receptor-activity-modifying proteins define calcitonin gene-related peptide or adrenomedullin selectivity of the mouse calcitonin-like receptor in COS-7 cells

Receptors for calcitonin gene-related peptide (CGRP) and adrenomedullin (AM) are heterodimeric complexes of the calcitonin-like receptor (CLR) together with associated receptor-activity-modifying proteins (RAMP)1, -2 or -3. The RAMP define the specificity of the CLR for CGRP or AM. Here, mouse (m)CLR/mRAMP1, -2 and -3 were expressed in COS-7 cells that lack detectable CGRP and AM receptors. myc epitope-tagged non-glycosylated mRAMP1 required V5-tagged mCLR for its translocation to the cell surface. The glycosylated myc-mRAMP2 and -3, on the other hand, were expressed at the cell surface in the absence of co-transfected mCLR. Selective binding of [125I]h alpha CGRP to mCLR/mRAMP1 expressing cells was inhibited by rat (r)alpha CGRP(1-37) and the CGRP antagonist r alpha CGRP(8-37) with IC(50) of 7.0+/-1.6 nM and 1.0+/-0.1 nM (mean+/-SEM). rAM(1-50) and the AM antagonist rAM(20-50) inhibited [125I]h alpha CGRP binding at over 36-fold higher concentrations than r alpha CGRP. In mCLR/mRAMP2 expressing cells, selective [125I]rAM binding was inhibited by rAM(1-50) and -(20-50) with IC(50) of 8.9+/-2.6 nM and 34+/-9 nM. r alpha CGRP(1-37) and -(8-37) displaced the binding at over 25-fold higher concentrations. mCLR/mRAMP3 expressing cells recognized both [125I]h alpha CGRP and -rAM. The IC(50) of rAM and r alpha CGRP(8-37) ranged between 5.8 and 7.0 nM, and those of r alpha CGRP and rAM(20-50) were only 4- to 8-fold higher. r alpha CGRP and rAM stimulated and r alpha CGRP(8-37) and rAM(20-50) antagonized mCLR/mRAMP1, -2 and -3 mediated cAMP formation with relative potencies that reflected the observed CGRP and AM selectivity of the three receptor types. In conclusion, mCLR/mRAMP1 and -2 are CGRP- and AM-selective receptors, respectively, whereas mCLR/mRAMP3 is an AM/CGRP receptor.

Importance of the amino terminus in secretin family G protein-coupled receptors. Intrinsic photoaffinity labeling establishes initial docking constraints for the calcitonin receptor

The calcitonin receptor is a member of the class B family of G protein-coupled receptors, closely related to secretin and parathyroid hormone receptors. Although mechanisms of ligand binding have been directly explored for those receptors, current knowledge of the molecular basis of calcitonin binding to its receptor is based only on receptor mutagenesis. In this work we have utilized the more direct approach of photoaffinity labeling to explore spatial approximations between distinct residues within calcitonin and its receptor. For this we have developed two human calcitonin analogues incorporating a photolabile p-benzoyl-l-phenylalanine residue in the mid-region and carboxyl-terminal half of the peptide in positions 16 and 26, respectively. Both probes specifically bound to the human calcitonin receptor with high affinity and were potent stimulants of cAMP accumulation in calcitonin receptor-bearing human embryonic kidney 293 cells. They covalently labeled the calcitonin receptor in a saturable and specific manner. Further purification, deglycosylation, specific chemical and enzymatic cleavage, and sequencing of labeled wild type and mutant calcitonin receptors identified the sites of labeling for the position 16 and 26 probes as receptor residues Phe137 and Thr30, respectively. Both were within the extracellular amino terminus of the calcitonin receptor, with the former adjacent to the first transmembrane segment and the latter within the distal amino-terminal tail of the receptor. These data are consistent with affinity labeling of other members of the class B G protein-coupled receptors using analogous probes and may suggest a common ligand binding mechanism for this family.

Tissue-specific activity of the proximal human calcitonin receptor promoter is mediated by Sp1 and an epigenetic phenomenon

To identify cis-acting sequences transcriptionally regulating the human calcitonin receptor (hCTR) gene, hCTR promoter/luciferase gene constructs were transiently or stably transfected into hCTR-positive and -negative cell lines. Luciferase assays demonstrated that the proximal hCTR promoter (hCTRP1) was transcriptionally active in all cell lines tested. High-level hCTRP1 activity depended on an 11 bp Sp1/Sp3 binding site. Electrophoretic mobility shift assay showed that this region bound the transcription factors Sp1 and Sp3. We further showed that hCTRP1 was strongly activated by the 11 bp Sp1/Sp3 binding site in hCTRP1/luciferase-, Sp1-transfected Drosophila S2 cells. Bisulphite-mediated sequencing of genomic DNA from hCTR-expressing and -non-expressing cell lines demonstrated that the endogenous hCTRP1 was hypomethylated in all cell lines tested. These results suggest that the hCTRP1 is activated by the tissue-ubiquitous transcription factor Sp1 and that an epigenetic process unrelated to CpG methylation represses its activity in hCTR-negative tissues.

CL/RAMP2 and CL/RAMP3 produce pharmacologically distinct adrenomedullin receptors: a comparison of effects of adrenomedullin22-52, CGRP8-37 and BIBN4096BS

Adrenomedullin (AM) has two known receptors formed by the calcitonin receptor-like receptor (CL) and receptor activity-modifying protein (RAMP) 2 or 3: we report the effects of the antagonist fragments of human AM and CGRP (AM22-52 and CGRP8-37) in inhibiting AM at human (h), rat (r) and mixed species CL/RAMP2 and CL/RAMP3 receptors transiently expressed in Cos 7 cells or endogenously expressed as rCL/rRAMP2 complexes by Rat 2 and L6 cells. AM22-52 (10 microM) antagonised AM at all CL/RAMP2 complexes (apparent pA2 values: 7.34+/-0.14 (hCL/hRAMP2), 7.28+/-0.06 (Rat 2), 7.00+/-0.05 (L6), 6.25+/-0.17 (rCL/hRAMP2)). CGRP8-37 (10 microM) resembled AM22-52 except on the rCL/hRAMP2 complex, where it did not antagonise AM (apparent pA2 values: 7.04+/-0.13 (hCL/hRAMP2), 6.72+/-0.06 (Rat2), 7.03+/-0.12 (L6)). On CL/RAMP3 receptors, 10 microM CGRP8-37 was an effective antagonist at all combinations (apparent pA2 values: 6.96+/-0.08 (hCL/hRAMP3), 6.18+/-0.18 (rCL/rRAMP3), 6.48+/-0.20 (rCL/hRAMP3)). However, 10 microM AM22-52 only antagonised AM at the hCL/hRAMP3 receptor (apparent pA2 6.73+/-0.14). BIBN4096BS (10 microM) did not antagonise AM at any of the receptors. Where investigated (all-rat and rat/human combinations), the agonist potency order on the CL/RAMP3 receptor was AM approximately betaCGRP>alphaCGRP. rRAMP3 showed three apparent polymorphisms, none of which altered its coding sequence. This study shows that on CL/RAMP complexes, AM22-52 has significant selectivity for the CL/RAMP2 combination over the CL/RAMP3 combination. On the mixed species receptor, CGRP8-37 showed the opposite selectivity. Thus, depending on the species, it is possible to discriminate pharmacologically between CL/RAMP2 and CL/RAMP3 AM receptors.

N-Glycosylation and conserved cysteine residues in RAMP3 play a critical role for the functional expression of CRLR/RAMP3 adrenomedullin receptor

The calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein-3 (RAMP3) can assemble into a CRLR/RAMP3 heterodimeric receptor that exhibits the characteristics of a high affinity adrenomedullin receptor. RAMP3 participates in adrenomedullin (AM) binding via its extracellular N-terminus characterized by the presence of six highly conserved cysteine residues and four N-glycosylation consensus sites. Here, we assessed the usage of these conserved residues in cotranslational modifications of RAMP3 and addressed their role in functional expression of the CRLR/RAMP3 receptor. Using a Xenopus oocyte expression system, we show that (i) RAMP3 is assembled with CRLR as a multiple N-glycosylated species in which two, three, or four consensus sites are used; (ii) elimination of all N-glycans in RAMP3 results in a significant inhibition of receptor [(125)I]AM binding and an increase in the EC(50) value for AM; (iii) several lines of indirect evidence indicate that each of the six cysteines is involved in disulfide bond formation; (iv) when all cysteines are mutated to serines, RAMP3 is N-glycosylated at all four consensus sites, suggesting that disulfide bond formation inhibits N-gylcosylation; and (v) elimination of all cysteines abolishes adrenomedullin binding and leads to a complete loss of receptor function. Our data demonstrate that cotranslational modifications of RAMP3 play a critical role in the function of the CRLR/RAMP3 adrenomedullin receptor.

Regulation of osteoclasts by calcitonin and amphiphilic calcitonin conjugates: role of cytosolic calcium

The peptide hormone calcitonin is a potent inhibitor of osteoclastic resorption, but it is unstable and poorly absorbed following oral administration. Conjugates of salmon calcitonin covalently linked to low-molecular-weight amphiphilic polymers show improved stability and absorption. The purpose of this study was to investigate the biological activity of these conjugates in vitro using rat osteoclasts and HEK-293 cells transfected with the C1a isoform of the calcitonin receptor. Salmon calcitonin or its conjugates (10 pM-10 nM) caused rapid arrest of osteoclast membrane ruffling and subsequent retraction. The same amphiphilic polymer attached to an unrelated protein had no effect on osteoclast morphology or motility. Since calcitonin-induced retraction of osteoclasts is thought to be mediated by Ca2+ signaling, we investigated the effects of calcitonin and its conjugates on cytosolic free Ca2+ concentration ([Ca24]i). In HEK-293 cells transfected with the calcitonin receptor, these agents induced transient elevations of [Ca2+]i. However, the rise of [Ca2+]i in HEK-293 cells occurred at concentrations 100-1000-fold higher than those required to elicit osteoclast retraction. To investigate the role of Ca2+ in osteoclast retraction, we preloaded cells with BAPTA to buffer changes in [Ca2+]i. BAPTA decreased the initial rate of calcitonin-induced osteoclast retraction, but it did not affect the degree of retraction 2-3 hours following calcitonin, indicating that retraction is mediated primarily by Ca(2+)-independent processes. We conclude that calcitonin conjugates cause osteoclast retraction and [Ca2+]i signaling in a manner similar to that elicited by calcitonin. Thus, orally bioavailable calcitonin conjugates show potential for use as antiresorptive agents.

Identification of second and third calcitonin receptor-stimulating peptides in porcine brain

We identified two cDNAs encoding new calcitonin receptor-stimulating peptides (CRSPs) in porcine hypothalamus cDNA library by cross-hybridization with the CRSP cDNA, and designated the second and third peptides as CRSP-2 and CRSP-3. The putative amino acid sequences of prepro-CRSP-2 and prepro-CRSP-3 showed higher identity with that of prepro-CRSP-1 than that of prepro-calcitonin gene-related peptide (CGRP), respectively, and these three CRSPs are considered to form a new family in the CGRP superfamily. RT-PCR analysis demonstrated that both CRSP-2 and CRSP-3 gene transcripts were expressed mainly in the central nervous system and thyroid gland. Synthetic CRSP-2 and CRSP-3 stimulated cAMP production very weakly in LLC-PK(1) cells compared with CRSP and calcitonin (CT). Furthermore, CRSP-2 and CRSP-3 did not elicit a cAMP elevation at all in the COS-7 cells expressing CT receptor or CT-like receptor with or without one of receptor activity-modifying proteins. These results suggest the presence of still unidentified action mechanisms and functions of the peptides in the CGRP superfamily.

Impaired differentiation of osteoclasts in TREM-2-deficient individuals

TREM-2 is an immunoglobulin-like cell surface receptor associated with DAP12/KARAP that activates monocyte-derived dendritic cells (DCs) in vitro. Recently, it has been shown that genetic defects of human DAP12/KARAP and TREM-2 result in a rare syndrome characterized by bone cysts and presenile dementia called Nasu-Hakola disease. This observation suggests that TREM-2 may function in myeloid cells other than DCs, most probably osteoclasts (OCs) and microglial cells, which are involved in bone modeling and brain function. Consistent with this prediction, here we show that OC differentiation is dramatically arrested in TREM-2-deficient patients, resulting in large aggregates of immature OCs that exhibit impaired bone resorptive activity. These results demonstrate a critical role for TREM-2 in the differentiation of mononuclear myeloid precursors into functional multinucleated OCs.

Angiotensin II modulates gene expression of adrenomedullin receptor components in rat cardiomyocytes

Both adrenomedullin (AM) and angiotensin II (Ang II) are locally-acting hormones in the cardiac ventricles. Previously we reported that AM inhibits Ang II-induced hypertrophy of cultured rat neonatal cardiomyocytes. In this study, we examined whether Ang II affects the gene expression of the AM receptor components of calcitonin-receptor-like receptor (CRLR) and receptor-activity-modifying protein (RAMP) in rat cardiomyocytes. The mRNA levels of RAMP1 and RAMP3 were significantly elevated following 24-h treatment with Ang II without a change of those of RAMP2 and CRLR. AM increased the intracellular cAMP level and the cAMP accumulation by AM was significantly amplified by the 24-h preincubation with Ang II. The effects of Ang II on RAMP1 and RAMP3 expression were abolished by an Ang II type 1 (AT1) receptor antagonist, but not by an AT2 receptor antagonist. Thus, Ang II modulates gene expression of the AM receptor components via AT1 receptor, suggesting alteration of AM actions by Ang II in cultured rat cardiomyocytes.

Expression of calcitonin gene-related peptide receptor components, calcitonin receptor-like receptor and receptor activity modifying protein 1, in the rat placenta during pregnancy and their cellular localization

Calcitonin gene-related peptide (CGRP), one of the most potent endogenous vasodilators known, has been implicated in vascular adaptations and placental function during pregnancy. The present study was aimed to investigate mRNA expression of CGRP-A receptor components, calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP(1)) in the rat placenta. Immunohistochemical staining of rat placentas obtained on day 18 of pregnancy using polyclonal anti-CRLR and RAMP(1) antibodies revealed that labelling was specifically concentrated in the cytotrophoblast and syncytium in labyrinth, trophoblastic giant cells and basophilic cells in trophospongial cell layer, and endothelium and smooth muscle cells in fetal vessels. The intensity of staining was reduced in all these cells except in the syncytium in placentas obtained during labour. RT-PCR analysis showed that mRNA expression of CRLR and RAMP(1) was significantly higher in the rat placenta from gestation day 17 to day 22, than during labour. During pregnancy, 17beta-estradiol inhibits, while progesterone stimulates, placental mRNA and proteins for CRLR and RAMP(1). Antiestrogen, ICI 182780, increased, whereas antiprogesterone, RU 486, inhibited the expression of both CRLR and RAMP(1). In summary, we demonstrate the presence and cellular localization of CRLR and RAMP(1) in the rat placenta. Elevated placental CRLR and RAMP(1) may be involved in CGRP-related increases in blood flow and therefore fetal growth and decreases at term labour may help minimize the blood loss.

Presence of calcitonin receptors in shell gland of the guineafowl and changes in binding property during an oviposition cycle

Radioligand binding assays of membrane fraction of the shell gland endometrium of guineafowl revealed the presence of receptor for calcitonin. The equilibrium dissociation constant (Kd) obtained by Scatchard analyses was 0.50 to 1.25 nM in laying birds and 1.12 to 1.19 nM in nonlaying birds. The maximum binding capacity (Bmax) per milligram of protein was 33.1 to 107.5 femtomoles in laying birds and 101.4 to 114.9 femtomoles in nonlaying birds. Both Kd and Bmax values changed during eggshell formation in laying birds. The results suggest that calcitonin receptor binding may be related to eggshell formation in guineafowl.

Upregulation of ligand, receptor system, and amidating activity of adrenomedullin in left ventricular hypertrophy of severely hypertensive rats: effects of angiotensin-converting enzyme inhibitors and diuretic

OBJECT

We investigated the pathophysiological role of the cardiac adrenomedullin (AM) system, including the ligand, receptor and amidating activity in the hypertrophied heart in severe hypertension.

METHOD

We studied the following four groups: control Wistar-Kyoto rats (WKY), spontaneously hypertensive stroke-prone rats (SHR-SP), 8 weeks captopril-treated SHR-SP, and 8 weeks trichlormethiazide-treated SHR-SP. AM precursor is converted to inactive glycine-extended AM (AM-Gly) and subsequently AM-Gly is converted to active mature AM (AM-m) by enzymatic amidation. We measured AM-m, AM-total (AM-T; AM-T = AM-m + AM-Gly), and atrial natriuretic peptide (ANP) in the plasma and left ventricle (LV) by immunoradiometric assay. We also measured gene expression of AM and ANP was and gene expression and protein levels of AM receptor system components such as calcitonin receptor-like receptor (CRLR), receptor-activity modifying protein (RAMP) 2 and RAMP3.

RESULTS

At 7 weeks old, SHR-SP had higher blood pressure and ANP mRNA levels and lower plasma AM-T compared with WKY, however, there were no differences in other indices between the two groups. At 17 weeks old, SHR-SP had increased blood pressure, LV weight, plasma and LV ANP levels and mRNA levels of ANP compared with WKY. AM-m and AM-T levels in plasma (AM-m: + 31%; AM-T: + 56%) and the LV (AM-m: + 84%; AM-T: + 31%) were significantly higher in SHR-SP than in WKY. The LV tissue AM-m/AM-T ratio was significantly higher in SHR-SP (93.2%) than in WKY. The mRNA levels of AM, CRLR, and RAMP2 in the LV were significantly higher in SHR-SP than in WKY. Captopril and trichlormethiazide similarly decreased blood pressure and LV hypertrophy with the reduction of the LV AM-m and AM-T levels and mRNA abundance of AM and its receptor component.

CONCLUSION

These results suggest that cardiac AM system is upregulated in the hypertrophied heart in this hypertension model. Considering that AM acts as an anti-remodeling autocrine and/or paracrine factor, upregulation of the AM system may modulate the pathophysiology in LV hypertrophy.

Calcitonin acts as a first messenger to regulate adenylyl cyclase/cAMP and mammalian sperm function

Calcitonin stimulates capacitation in uncapacitated mouse spermatozoa and then inhibits spontaneous acrosome loss in capacitated cells, responses similar to those elicited by fertilization promoting peptide (FPP), a peptide known to regulate the adenylyl cyclase/cAMP pathway. This study investigated the hypothesis that calcitonin also modulates this pathway. Calcitonin significantly stimulated cAMP production in uncapacitated spermatozoa and then inhibited it in capacitated cells; the magnitude of both stimulatory and inhibitory changes was similar to that obtained with FPP but the inhibitory responses to FPP preceded those of calcitonin. This possibly reflects the involvement of two different adenosine receptors in response to FPP compared with one calcitonin receptor. Calcitonin receptors were located on the acrosomal cap and the flagellum, the midpiece having a greater abundance than the principal piece. Although both calcitonin and adenosine receptors are found in the head and flagellum, there was no evidence for cross-talk between them. Chlortetracycline investigations to determine the minimum extracellular Ca(2+) requirement for responses to calcitonin revealed that calcitonin significantly stimulated capacitation in Ca(2+)-deficient medium but FPP did not. Calcitonin also significantly stimulated cAMP production under these conditions, and similarly preincubated suspensions, when diluted into +Ca(2+) medium, were significantly more fertile in vitro than untreated controls. These results indicate that calcitonin, like FPP, acts as a first messenger to regulate the production of cAMP and mammalian sperm function, but the differences in Ca(2+) requirements suggest that calcitonin and FPP may regulate different isoforms of adenylyl cyclase.

Molecular pharmacology of the calcitonin receptor

The peptide hormone calcitonin is widely used therapeutically in the treatment of bone disorders such as Paget's disease, osteoporosis, and the hypercalcemia of some malignancies. However, emerging evidence suggests the actions of calcitonin via its G protein-coupled receptor, the calcitonin receptor, may not be limited to bone. Calcitonin receptors have also been identified in the central nervous system, testes, skeletal muscle, lymphocytes, and the placenta. We are now becoming aware that the complexity of the calcitonin response mediated by the calcitonin receptor can be influenced by accessory proteins, receptor isoforms, genetic polymorphisms, developmental and/or transcriptional regulation, feedback inhibition, and the specific cellular or tissue background. This article discusses what is known about the molecular and pharmacological actions of the calcitonin receptor and highlights areas of current research.

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.

Solid-phase synthesis of peptide radiopharmaceuticals using Fmoc-N-epsilon-(hynic-Boc)-lysine, a technetium-binding amino acid: application to Tc-99m-labeled salmon calcitonin

Labeling of proteins with metallic radionuclides for use in radiopharmaceuticals involves covalently attaching a bifunctional chelator. In principle, use of smaller peptides allows this chelator to be incorporated during solid-phase peptide synthesis (SPPS) with total site specificity. To realize the advantages of this approach, a lysine-hynic conjugate Fmoc-N-epsilon-(Hynic-Boc)-Lys was synthesized for incorporating the well-known technetium-99m-binding hydrazinonicotinamide ligand into peptides during SPPS. It was used to synthesize a technetium-99m-labeled salmon calcitonin with the hynic-linked amino acid in place of lysine-18. A trifluoroacetate group protected the hynic during alkaline oxidation to the cyclic disulfide and was readily removed by mild acid treatment. The peptide was efficiently labeled (91-98% radiochemical yield) with Tc-99m in the presence of tricine and SnCl(2) with high specific activity (>100 MBq/microg). The product showed good serum stability and specific affinity for human calcitonin receptors. Fmoc-N-epsilon-(Hynic-Boc)-Lys is a highly versatile technetium-binding amino acid for incorporation into peptides during SPPS. This allows total flexibility and control in the site of attachment and is suitable for a combinatorial approach to peptide radiopharmaceuticals.

The extracellular domain of receptor activity-modifying protein 1 is sufficient for calcitonin receptor-like receptor function

A functional calcitonin gene-related peptide (CGRP) receptor requires dimerization of calcitonin receptor-like receptor (CRLR) with receptor activity-modifying protein 1 (RAMP 1). To determine the function of the three domains (extracellular, ECD; transmembrane, TM; and tail domains) of human RAMP 1, three mutants were constructed: RAMP 1 without the cytoplasmic tail, a chimera consisting of the ECD of RAMP 1 and the TM and tail of the platelet-derived growth factor receptor, and the ECD of RAMP 1 alone. These RAMP 1 mutants were examined for their ability to associate with CRLR to effect CGRP-stimulated cAMP accumulation, CGRP binding, CRLR trafficking, and cell surface expression. All RAMP 1 mutants were able to associate with CRLR with full efficacy for CGRP-stimulated cAMP accumulation. However, the RAMP 1/platelet-derived growth factor receptor chimera demonstrated a 10-fold decrease in potency for CGRP signaling and binding, and the RAMP 1-ECD mutant had a 4000-fold decrease in potency. In conclusion, the ECD of RAMP 1 is sufficient for normal CRLR association and efficacy. The presence of a TM domain and the specific sequence of the RAMP 1 TM domain contribute to CGRP affinity and potency. The C-terminal tail of RAMP 1 is unnecessary for CRLR function.

Adrenomedullin is an autocrine/paracrine growth factor for rat vascular smooth muscle cells

Adrenomedullin is a potent vasodilator peptide secreted by vascular endothelial and smooth muscle cells. Adrenomedullin stimulates the proliferation of quiescent rat vascular smooth muscle cells (VSMCs) via p42/p44 ERK/MAP kinase activation. Recently, receptor-activity-modifying proteins (RAMPs) have been shown to transport calcitonin-receptor-like-receptor (CRLR) to the cell surface to present either as CGRP receptor or adrenomedullin receptor. We investigated whether adrenomedullin acts as an autocrine/paracrine growth factor for cultured rat VSMCs and whether coexpressions of RAMP isoform and CRLR may mediate p42/p44 ERK/MAP kinase activation by adrenomedullin. Adrenomedullin dose-dependently stimulated the proliferation of quiescent rat VSMCs, and this effect was inhibited by an adrenomedullin receptor antagonist, a MAP kinase kinase inhibitor and phosphatidylinositol 3-kinase inhibitors. Addition of either CGRP(8-37) or anti-adrenomedullin antibody to exponentially growing rat VSMCs inhibited the serum-induced cell proliferation, suggesting its role as an autocrine/paracrine growth factor. Cotransfection of RAMP2 or RAMP3 with CRLR into rat VSMCs potentiated activation of cAMP activity, but not of p42/p44 ERK/MAP kinase activity in response to adrenomedullin. Our results suggest that adrenomedullin is an autocrine/paracrine growth factor for rat VSMCs via p42/p44 ERK/MAP kinase and phosphatidylinositol 3-kinase pathways and that it is not mediated by human RAMP-CRLR receptors.

Calcitonin receptor-stimulating peptide, a new member of the calcitonin gene-related peptide family. Its isolation from porcine brain, structure, tissue distribution, and biological activity

We isolated a novel biologically active peptide, designated calcitonin receptor-stimulating peptide (CRSP), from the acid extract of the porcine brain by monitoring cAMP production in the porcine kidney cell line LLC-PK(1). Determination of the amino acid sequence and cDNA analysis encoding a CRSP precursor showed that this peptide has approximately 60% identity in the amino acid sequence with human calcitonin gene-related peptide type-alpha (alphaCGRP), type-beta (betaCGRP), and porcine CGRP. Northern blot analysis and radioimmunoassay demonstrated that CRSP is expressed mainly in the thyroid gland and the central nervous system, in which the calcitonin receptor was abundantly expressed. Synthetic CRSP elicited a potent stimulatory effect on the cAMP production in LLC-PK(1) cells. Although it shows significant sequence similarity with CGRPs, this peptide did not elicit cAMP elevation in cells that endogenously expressed a CGRP receptor or an adrenomedullin receptor or were transfected with either of these recombinant receptors. Administration of CRSP into anesthetized rats did not alter the blood pressure but induced a transient decrease in the plasma calcium concentration. In fact, this peptide potently increased the intracellular cAMP concentration in COS-7 cells that expressed the recombinant calcitonin receptor. These unique properties indicate that CRSP is not a porcine counterpart of betaCGRP and probably elicits its biological effects via the calcitonin receptor.

Binding of filamin to the C-terminal tail of the calcitonin receptor controls recycling

Many G protein-coupled receptors undergo endocytosis, but the mechanisms involved in endocytic sorting and recycling remain to be fully elucidated. We found that the G protein-coupled calcitonin receptor (CTR) undergoes tonic internalization and accumulates within the cell. Using a fluorescence loss in photobleaching assay, we classified these vesicles functionally as recycling vesicles. In a two-hybrid screening, we found that the actin-binding protein filamin interacted with the C-terminal tail of the CTR. The degradation of the receptor was profoundly increased in the absence of filamin or the CTR-filamin interaction. The absence of filamin was also associated with a marked decrease in recycling of the receptor from the endosomes to the cell surface. In contrast, calcitonin-induced inhibition of spontaneous filamin proteolysis was associated with increased recycling of the receptor to the cell surface and decreased degradation of the CTR, suggesting an important role for filamin in the endocytic sorting and recycling of the internalized CTR.

Amylin and adrenomedullin: novel regulators of bone growth

Bone growth is regulated by circulating hormones and locally generated factors. Understanding their mechanisms of action enables us to obtain a better appreciation of the cellular and molecular basis of bone remodelling, and could therefore be valuable in approaches to new therapies. In this review, we consider the actions on bone tissue of the peptide hormones amylin and adrenomedullin, known to circulate at picomolar concentrations. Adrenomedullin is also produced locally in bone. Amylin and adrenomedullin are related peptides with some homology to both calcitonin and calcitonin gene-related peptide. These peptides have recently been found to stimulate the proliferation of osteoblasts in vitro, and to increase indices of bone formation when administered either locally or systemically in vivo. In addition, amylin inhibits bone resorption. Both amylin and adrenomedullin have also been found to act on chondrocytes, stimulating their proliferation in culture and increasing tibial growth plate thickness when administered systemically to adult mice. Like the peptides themselves, the receptors for the calcitonin family are also related to each other. Each peptide seems to act through its own distinct high affinity receptor, as well as through other receptors for the family, usually with lower affinity. Characterisation of the putative receptors expressed in osteoblasts, has provided some understanding of the physiological effects of amylin and adrenomedullin in these cells. Studies of structure-activity relationships have demonstrated that osteotropic effects of amylin and adrenomedullin can be retained in peptide fragments of the molecule whilst losing the parent molecule's effects on carbohydrate metabolism or vasodilatory properties respectively. Thus, these small peptides, or their analogues, are attractive candidates as anabolic therapies for osteoporosis.

Human osteoblast-like cell proliferation induced by calcitonin-related peptides involves PKC activity

The calcitonin peptides [calcitonin (CT), calcitonin gene-related peptide (CGRP), amylin] share many biological actions, including activity on bone cells. In the present study, CT (10(-11) to 10(-9) M) stimulated [(3)H]thymidine incorporation in primary cultures of human osteoblasts (hOB), as already demonstrated for CGRP and amylin. RT-PCR analysis showed that the calcitonin receptor and the calcitonin receptor-like receptor are both expressed in hOB. In these cells, CT (10(-10) M) and amylin (10(-9) M), in contrast to CGRP (10(-8) M), did not increase cAMP production. All three peptides stimulated protein kinase C (PKC) activity. To evaluate PKC involvement in hOB proliferation, cells were incubated with phorbol 12,13-dibutyrate, a stimulator of PKC activity; cell proliferation was increased in a dose-dependent manner (EC(50) = 3.4 x 10(-8) M). Staurosporine (10(-9) M), a PKC inhibitor, blocked phorbol 12,13-dibutyrate-induced PKC activity and cell proliferation. Inhibition of PKC by staurosporine also counteracted the stimulatory effect of CT, CGRP, and amylin on hOB proliferation. From these data, it is deduced that the activation of PKC is important for hOB proliferation and that it is involved in the anabolic effect of CT peptides on bone.

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

We tested whether heterodimers comprised of calcitonin (CT) receptor lacking the 16-amino acid insert in intracellular domain 1 (CTR(I1-)) and receptor activity-modifying protein (RAMP) can function not only as calcitonin gene-related peptide (CGRP) receptors but also as adrenomedullin (AM) receptors. Whether transfected alone or together with RAMP, human (h)CTR(I1-) appeared mainly at the surface of HEK-293 cells. Expression of CTR(I1-) alone led to significant increases in cAMP in response to hCGRP or hAM, though both peptides remained about 100-fold less potent than hCT. However, the apparent potency of AM, like that of CGRP, approached that of CT when CTR(I1-) was co-expressed with RAMP. CGRP- or AM-evoked cAMP production was strongly inhibited by salmon CT-(8-32), a selective amylin receptor antagonist, but not by hCGRP-(8-37) or hAM-(22-52), antagonists of CGRP and AM receptors, respectively. Moreover, the inhibitory effects of CT-(8-32) were much stronger in cells co-expressing CTR(I1-) and RAMP than in cells expressing CTR(I1-) alone. Co-expression of CTR(I1-) with RAMP thus appears to produce functional CT-(8-32)-sensitive AM receptors.