Spatial proximity between a photolabile residue in position 19 of salmon calcitonin and the amino terminus of the human calcitonin receptor

The processing intermediate of human amylin, pro-amylin(1-48), has in vivo and in vitro bioactivity

Amylin is released by pancreatic beta-cells in response to a meal and its major soluble mature form (37 amino acid-peptide) produces its biological effects by activating amylin receptors. Amylin is derived from larger propeptides that are processed within the synthesizing beta-cell. There are suggestions that a partially processed form, pro-amylin(1-48) is also secreted. We tested the hypothesis that pro-amylin(1-48) has biological activity and that human pro-amylin(1-48) may also form toxic pre-amyloid species. Amyloid formation, the ability to cross-seed and in vitro toxicity were similar between human pro-amylin(1-48) and amylin. Human pro-amylin(1-48) was active at amylin-responsive receptors, though its potency was reduced at rat, but not human amylin receptors. Pro-amylin(1-48) was able to promote anorexia by activating neurons of the area postrema, amylin's primary site of action, indicating that amylin can tolerate significant additions at the N-terminus without losing bioactivity. Our studies help to shed light on the possible roles of pro-amylin(1-48) which may be relevant for the development of future amylin-based drugs.

Calcitonin-typical suppression of osteoclastic activity by amphioxus calcitonin superfamily peptides and insights into the evolutionary conservation and diversity of their structures

Calcitonin (CT) is a hormone that decreases serum calcium level by suppressing osteoclastic activity in the vertebrate bone. In vertebrates, the structure-function relationship of CTs has been studied extensively. We recently identified three CT superfamily peptides, Bf-CTFP1 to 3, and clarified the molecular and functional characteristics of their receptor and receptor activity-modifying protein in amphioxus, Branchiostoma floridae. However, the CT activity of Bf-CTFPs has yet to be investigated. In the present study, a functional analysis of Bf-CTFPs was performed using goldfish scales having both osteoclasts and osteoblasts. All Bf-CTFPs suppressed osteoclastic activity via a goldfish CT receptor. Although the primary amino acid sequences of the Bf-CTFPs showed low sequence similarity to vertebrate CTs, Bf-CTFP1 to 3 share three amino acids, Thr²⁵, Thr²⁷, and Pro³²-NH₂, that are required for receptor binding, with salmon CT. Moreover, homology model analysis revealed that the Bf-CTFPs form alpha-helical structures. The alpha-helical position and length of Bf-CTFP1 and 2 were conserved with those of a highly potent ligand, teleost CT. Interestingly, the composition of the alpha-helix of Bf-CTFP3 differed from those of teleost CT, despite that the action of Bf-CTFP3 on goldfish scales was the same as that of Bf-CTFP1 and 2. Collectively, the present study provides new insights into the structure-function relationship of CT and its functional evolution in chordates.

An allosteric role for receptor activity-modifying proteins in defining GPCR pharmacology

G protein-coupled receptors are allosteric proteins that control transmission of external signals to regulate cellular response. Although agonist binding promotes canonical G protein signalling transmitted through conformational changes, G protein-coupled receptors also interact with other proteins. These include other G protein-coupled receptors, other receptors and channels, regulatory proteins and receptor-modifying proteins, notably receptor activity-modifying proteins (RAMPs). RAMPs have at least 11 G protein-coupled receptor partners, including many class B G protein-coupled receptors. Prototypic is the calcitonin receptor, with altered ligand specificity when co-expressed with RAMPs. To gain molecular insight into the consequences of this protein–protein interaction, we combined molecular modelling with mutagenesis of the calcitonin receptor extracellular domain, assessed in ligand binding and functional assays. Although some calcitonin receptor residues are universally important for peptide interactions (calcitonin, amylin and calcitonin gene-related peptide) in calcitonin receptor alone or with receptor activity-modifying protein, others have RAMP-dependent effects, whereby mutations decreased amylin/calcitonin gene-related peptide potency substantially only when RAMP was present. Remarkably, the key residues were completely conserved between calcitonin receptor and AMY receptors, and between subtypes of AMY receptor that have different ligand preferences. Mutations at the interface between calcitonin receptor and RAMP affected ligand pharmacology in a RAMP-dependent manner, suggesting that RAMP may allosterically influence the calcitonin receptor conformation. Supporting this, molecular dynamics simulations suggested that the calcitonin receptor extracellular N-terminal domain is more flexible in the presence of receptor activity-modifying protein 1. Thus, RAMPs may act in an allosteric manner to generate a spectrum of unique calcitonin receptor conformational states, explaining the pharmacological preferences of calcitonin receptor-RAMP complexes. This provides novel insight into our understanding of G protein-coupled receptor-protein interaction that is likely broadly applicable for this receptor class.

Type II Turn of Receptor-bound Salmon Calcitonin Revealed by X-ray Crystallography

Calcitonin is a peptide hormone consisting of 32 amino acid residues and the calcitonin receptor is a Class B G protein-coupled receptor (GPCR). The crystal structure of the human calcitonin receptor ectodomain (CTR ECD) in complex with a truncated analogue of salmon calcitonin ([BrPhe(22)]sCT(8-32)) has been determined to 2.1-Å resolution. Parallel analysis of a series of peptide ligands showed that the rank order of binding of the CTR ECD is identical to the rank order of binding of the full-length CTR, confirming the structural integrity and relevance of the isolated CTR ECD. The structure of the CTR ECD is similar to other Class B GPCRs and the ligand binding site is similar to the binding site of the homologous receptors for the calcitonin gene-related peptide (CGRP) and adrenomedulin (AM) recently published (Booe, J. M., Walker, C. S., Barwell, J., Kuteyi, G., Simms, J., Jamaluddin, M. A., Warner, M. L., Bill, R. M., Harris, P. W., Brimble, M. A., Poyner, D. R., Hay, D. L., and Pioszak, A. A. (2015) Mol. Cell 58, 1040-1052). Interestingly the receptor-bound structure of the ligand [BrPhe(22)]sCT(8-32) differs from the receptor-bound structure of the homologous ligands CGRP and AM. They all adopt an extended conformation followed by a C-terminal β turn, however, [BrPhe(22)]sCT(8-32) adopts a type II turn (Gly(28)-Thr(31)), whereas CGRP and AM adopt type I turns. Our results suggest that a type II turn is the preferred conformation of calcitonin, whereas a type I turn is the preferred conformation of peptides that require RAMPs; CGRP, AM, and amylin. In addition the structure provides a detailed molecular explanation and hypothesis regarding ligand binding properties of CTR and the amylin receptors.

Transmembrane signal transduction by peptide hormones via family B G protein-coupled receptors

Although family B G protein-coupled receptors (GPCRs) contain only 15 members, they play key roles in transmembrane signal transduction of hormones. Family B GPCRs are drug targets for developing therapeutics for diseases ranging from metabolic to neurological disorders. Despite their importance, the molecular mechanism of activation of family B GPCRs remains largely unexplored due to the challenges in expression and purification of functional receptors to the quantity for biophysical characterization. Currently, there is no crystal structure available of a full-length family B GPCR. However, structures of key domains, including the extracellular ligand binding regions and seven-helical transmembrane regions, have been solved by X-ray crystallography and NMR, providing insights into the mechanisms of ligand recognition and selectivity, and helical arrangements within the cell membrane. Moreover, biophysical and biochemical methods have been used to explore functions, key residues for signaling, and the kinetics and dynamics of signaling processes. This review summarizes the current knowledge of the signal transduction mechanism of family B GPCRs at the molecular level and comments on the challenges and outlook for mechanistic studies of family B GPCRs.

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

Receptor activity-modifying protein-dependent impairment of calcitonin receptor splice variant Δ(1-47)hCT((a)) function

BACKGROUND AND PURPOSE

Alternative splicing expands proteome diversity to GPCRs. Distinct receptor variants have been identified for a secretin family GPCR, the calcitonin receptor (CTR). The possible functional contributions of these receptor variants are further altered by their potential interactions with receptor activity-modifying proteins (RAMPs). One variant of the human CTR lacks the first 47 residues at its N terminus [Δ(1-47)hCT((a)) ]. However, very little is known about the pharmacology of this variant or its ability to interact with RAMPs to form amylin receptors.

EXPERIMENTAL APPROACH

Δ(1-47)hCT((a)) was characterized both with and without RAMPs in Cos7 and/or HEK293S cells. The receptor expression (ELISA assays) and function (cAMP and pERK1/2 assays) for up to six agonists and two antagonists were determined.

KEY RESULTS

Despite lacking 47 residues at the N terminus, Δ(1-47)hCT((a)) was still able to express at the cell surface, but displayed a generalized reduction in peptide potency. Δ(1-47)hCT((a)) retained its ability to interact with RAMP1 and formed a functional amylin receptor; this also appeared to be the case with RAMP3. On the other hand, its interaction with RAMP2 and resultant amylin receptor was reduced to a greater extent.

CONCLUSIONS AND IMPLICATIONS

Δ(1-47)hCT((a)) acts as a functional receptor at the cell surface. It exhibits altered receptor function, depending on whether it associates with a RAMP and which RAMP it interacts with. Therefore, the presence of this variant in tissues will potentially contribute to altered peptide binding and signalling, depending on the RAMP distribution in tissues.

Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling

The glucagon-like peptide 1 (GLP1) receptor is an important drug target within the B family of G protein-coupled receptors. Its natural agonist ligand, GLP1, has incretin-like actions and the receptor is a recognized target for management of type 2 diabetes mellitus. Despite recent solution of the structure of the amino terminus of the GLP1 receptor and several close family members, the molecular basis for GLP1 binding to and activation of the intact receptor remains unclear. We previously demonstrated molecular approximations between amino- and carboxyl-terminal residues of GLP1 and its receptor. In this work, we study spatial approximations with the mid-region of this peptide to gain insights into the orientation of the intact receptor and the ligand-receptor complex. We have prepared two new photolabile probes incorporating a p-benzoyl-l-phenylalanine into positions 16 and 20 of GLP1(7-36). Both probes bound to the GLP1 receptor specifically and with high affinity. These were each fully efficacious agonists, stimulating cAMP accumulation in receptor-bearing CHO cells in a concentration-dependent manner. Each probe specifically labeled a single receptor site. Protease cleavage and radiochemical sequencing identified receptor residue Leu(141) above transmembrane segment one as its site of labeling for the position 16 probe, whereas the position 20 probe labeled receptor residue Trp(297) within the second extracellular loop. Establishing ligand residue approximation with this loop region is unique among family members and may help to orient the receptor amino-terminal domain relative to its helical bundle region.

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.

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.

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

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

Procalcitonin has bioactivity at calcitonin receptor family complexes: potential mediator implications in sepsis

CONTEXT

Sepsis is a major cause of death in the United States and accounts for approximately 50% of the fatalities in intensive care units. Serum procalcitonin (ProCT) levels are markedly elevated in sepsis and correlate positively with severity of the illness and mortality, however, little is known about the biological activity of ProCT.

OBJECTIVE

To explore the biological activity of purified human ProCT at the calcitonin (CT) family of receptors.

DESIGN

Human ProCT was purified from the TT medullary thyroid carcinoma cell line. Human CTa receptor or human CT receptor-like receptor (CLR) was transiently expressed in COS-7 cells alone or together with individual receptor activity-modifying proteins (RAMPs) to generate the CTa (CT) receptor, the AMY1 (amylin) receptor, the CGRP1 (CT gene-related peptide) receptor, and the AM1 and AM2 (adrenomedullin) receptors. Biological activity of ProCT was assessed by measurement of cAMP accumulation.

RESULTS

ProCT was effectively inert at CTa, AM1, and AM2 receptors. In contrast, it was a potent partial agonist (50-60% of the CGRP efficacy) of the CGRP1 receptor with an EC50 as high as 0.56 nM, although the potency was batch dependent. ProCT also displayed weak partial agonist activity at the AMY1 receptor with an EC50 of approximately 100 nM. Moreover, ProCT also robustly inhibited CGRP-dependent cyclic adenosine monophosphate responses at the CGRP1 receptor.

CONCLUSIONS

Our data provide a potential molecular mechanism for the observation that ProCT appears to be toxic while CGRP treatment appears to be beneficial in animal models of sepsis.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

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.

Agonist-dependent consequences of proline to alanine substitution in the transmembrane helices of the calcitonin receptor

BACKGROUND AND PURPOSE

Transmembrane proline (P) residues in family A G protein-coupled receptors (GPCRs) form functionally important kinks in their helices. These residues are little studied in family B GPCRs but experiments with the VPAC1 receptor and calcitonin receptor-like receptor (CL) show parallels with family A receptors. We sought to determine the function of these residues in the insert negative form of the human calcitonin receptor, a close relative of CL.

EXPERIMENTAL APPROACH

Proline residues within the transmembrane domains of the calcitonin receptor (P246, P249, P280, P326, P336) were individually mutated to alanine (A) using site-directed mutagenesis. Receptors were transiently transfected into Cos-7 cells using polyethylenimine and salmon and human calcitonin-induced cAMP responses measured. Salmon and human calcitonin competition binding experiments were also performed and receptor cell-surface expression assessed by whole cell ELISA.

KEY RESULTS

P246A, P249A and P280A were wild-type in terms of human calcitonin-induced cAMP activation. P326A and P336A had reduced function (165 and 12-fold, respectively). In membranes, human calcitonin binding was not detectable for any mutant receptor but in whole cells, binding was detected for all mutants apart from P326A. Salmon calcitonin activated mutant and wild-type receptors equally, although B(max) values were reduced for all mutants apart from P326A.

CONCLUSIONS AND IMPLICATIONS

P326 and P336 are important for the function of human calcitonin receptors and are likely to be involved in generating receptor conformations appropriate for agonist binding and receptor activation. However, agonist-specific effects were observed , implying distinct conformations of the human calcitonin receptor.

Molecular Approximations between Residues 21 and 23 of Secretin and Its Receptor: Development of a Model for Peptide Docking with the Amino Terminus of the Secretin Receptor

The structurally unique amino-terminal domain of class II G protein-coupled receptors is critically important for ligand binding and receptor activation. Understanding the precise role it plays requires detailed insights into the molecular basis of its ligand interactions and the conformation of the ligand-receptor complex. In this work, we used two high-affinity, full-agonist, secretin-like photolabile probes having sites for covalent attachment in positions 21 and 23 and used sequential proteolysis and sequencing of the labeled region of the receptor to identify two new spatial approximation constraints. The position 21 probe labeled receptor residue Arg(15), whereas the position 23 probe labeled receptor residue Arg(21). A homology model of the amino-terminal domain of the secretin receptor was developed using the NMR structure of the analogous domain of the corticotropin-releasing factor receptor. This was attached to a homology model of the secretin receptor transmembrane bundle, with the two domains oriented relative to each other based on continuity of the peptide backbone and by imposing a distance restraint recently identified between the amino-terminal WDN sequence and the region of the helical bundle above transmembrane segment six. Secretin was docked to this model using seven sets of spatial approximation constraints identified in previous photoaffinity labeling studies. This model was found to fully accommodate all existing constraints, as well as the two new approximations identified in this work.

Determinants of 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) (BIBN4096BS) Affinity for Calcitonin Gene-Related Peptide and Amylin Receptors—The Role of Receptor Activity Modifying Protein 1

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) (BIBN4096BS), a calcitonin gene-related peptide (CGRP) receptor antagonist, can alleviate the symptoms of migraine and is highly selective for CGRP over adrenomedullin (AM) receptors. These receptors are heterodimers of the calcitonin receptor-like receptor (CL) and receptor activity modifying proteins (RAMPs), with the pharmacological properties determined by the RAMP subunit. BIBN4096BS-sensitive CGRP(1) receptors are CL/RAMP1, whereas BIBN4096BS-insensitive AM receptors are CL/RAMP2 or CL/RAMP3 (AM(1) and AM(2), respectively), implicating RAMP1 in conferring BIB-N4096BS sensitivity. Because calcitonin receptors [CT((a))] also interact with RAMP1 [AMY(1(a)) receptors], BIBN4096BS could also have affinity for these receptors. To test this, receptors were transfected into COS-7 cells and agonist-stimulated cAMP levels measured in the presence and absence of antagonists. We found that AMY(1(a)) receptors were approximately 150-fold less sensitive to BIBN4096BS antagonism than CGRP(1) receptors. In contrast, AMY(3(a)) [CT((a))/RAMP3] or AM(2) receptors were not sensitive to BIBN4096BS antagonism. We investigated Trp74 in RAMP1, a residue implicated in the species selectivity of BIBN4096BS. BIBN4096BS affinity was reduced at AMY(1(a)) and CGRP(1) receptors when this residue was mutated to lysine or alanine. The equivalent residue in RAMP3, Glu74, when mutated to tryptophan (E74W), induced BIBN4096BS sensitivity at AM(2) and AMY(3(a)) receptors. It is interesting that a selective reduction in AM potency was observed at E74W AM(2) receptors, implicating this residue in AM interactions with this receptor. These data support the importance of Trp74 in RAMP1 in the interaction of BIBN4096BS with CGRP(1) and AMY(1(a)) receptors and identified Glu74 in RAMP3 as the first amino acid in RAMP important for agonist interactions with calcitonin-family receptors.

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.

Structural Determinants of Salmon Calcitonin Bioactivity

Salmon calcitonin (sCT) forms an amphipathic helix in the region 9-19, with the C-terminal decapeptide interacting with the helix (Amodeo, P., Motta, A., Strazzullo, G., Castiglione Morelli, M. A. (1999) J. Biomol. NMR 13, 161-174). To uncover the structural requirements for the hormone bioactivity, we investigated several sCT analogs. They were designed so as to alter the length of the central helix by removal and/or replacement of flanking residues and by selectively mutating or deleting residues inside the helix. The helix content was assessed by circular dichroism and NMR spectroscopies; the receptor binding affinity in human breast cancer cell line T 47D and the in vivo hypocalcemic activity were also evaluated. In particular, by NMR spectroscopy and molecular dynamics calculations we studied Leu(23),Ala(24)-sCT in which Pro(23) and Arg(24) were replaced by helix inducing residues. Compared with sCT, it assumes a longer amphipathic alpha-helix, with decreased binding affinity and one-fifth of the hypocalcemic activity, therefore supporting the idea of a relationship between a definite helix length and bioactivity. From the analysis of other sCT mutants, we inferred that the correct helix length is located in the 9-19 region and requires long range interactions and the presence of specific regions of residues within the sequence for high binding affinity and hypocalcemic activity. Taken together, the structural and biological data identify well defined structural parameters of the helix for sCT bioactivity.

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.

Possible endogenous agonist mechanism for the activation of secretin family G protein-coupled receptors

The class B family of G protein-coupled receptors contains several potentially important drug targets, yet our understanding of the molecular basis of ligand binding and receptor activation remains incomplete. Although a key role is recognized for the cysteine-rich, disulfide-bonded amino-terminal domain of these receptors, detailed insights into ligand docking and resultant conformational changes are not clear. We postulate that binding natural ligands to this domain results in a conformational change that exposes an endogenous ligand which interacts with the body of the receptor to activate it. In this work, we examined whether a synthetic peptide corresponding to a candidate region between the first and third conserved cysteines could act as an agonist. Indeed, this peptide was a weakly potent but fully efficacious agonist, stimulating a concentration-dependent cAMP response in secretin receptor-bearing cells. This effect was maintained as the peptide length was reduced from 30 to 5, and ultimately, three residues focused on the conserved residue Asp49. The agonist potency was enhanced by cyclization through a diaminopropionic acid linker and by amino-terminal fatty acid acylation. Both ends of the cyclic peptide were shown to interact with the top of transmembrane segment 6 of the receptor, using probes with a photolabile benzoyl-phenylalanine on each end. Analogous observations were also made for two other members of this family, the vasoactive intestinal polypeptide type 1 and calcitonin receptors. These data may provide a unique molecular mechanism and novel leads for the development of small-molecule agonists acting at potential drug targets within this physiologically important receptor family.

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.

Mechanisms of ligand binding to the parathyroid hormone (PTH)/PTH-related protein receptor: selectivity of a modified PTH(1-15) radioligand for GalphaS-coupled receptor conformations

Mechanisms of ligand binding to the PTH/PTHrP receptor (PTHR) were explored using PTH fragment analogs as radioligands in binding assays. In particular, the modified amino-terminal fragment analog, (125)I-[Aib(1,3),Nle8,Gln10,homoarginine11,Ala12,Trp14,Tyr15]rPTH(1-15)NH2, (125)I-[Aib(1,3),M]PTH(1-15), was used as a radioligand that we hypothesized to bind solely to the juxtamembrane (J) portion of the PTHR containing the extracellular loops and transmembrane helices. We also employed (125)I-PTH(1-34) as a radioligand that binds to both the amino-terminal extracellular (N) and J domains of the PTHR. Binding was examined in membranes derived from cells expressing either wild-type or mutant PTHRs. We found that the binding of (125)I-[Aib(1,3),M]PTH(1-15) to the wild-type PTHR was strongly (approximately 90%) inhibited by guanosine 5'-O-(3-thio)triphosphate (GTPgammaS), whereas the binding of (125)I-PTH(1-34) was only mildly (approximately 25%) inhibited by GTPgammaS. Of these two radioligands, only (125)I-[Aib(1,3),M]PTH(1-15) bound to PTHR-delNt, which lacks most of the receptor's N domain, and again this binding was strongly inhibited by GTPgammaS. Binding of (125)I-[Aib(1,3),M]PTH(1-15) to the constitutively active receptor, PTHR-H223R, was only mildly (approximately 20%) inhibited by GTPgammaS, as was the binding of (125)I-PTH(1-34). In membranes prepared from cells lacking Galpha(S) via knockout mutation of Gnas, no binding of (125)I-[Aib(1,3),M]PTH(1-15) was observed, but binding of (125)I-[Aib(1,3),M]PTH(1-15) was recovered by virally transducing the cells to heterologously express Galpha(S). (125)I-PTH(1-34) bound to the membranes with or without Galpha(S). The overall findings confirm the hypothesis that (125)I-[Aib(1,3),M]PTH(1-15) binds solely to the J domain of the PTHR. They further show that this binding is strongly dependent on coupling of the receptor to Galpha(S)-containing heterotrimeric G proteins, whereas the binding of (125)I-PTH(1-34) can occur in the absence of such coupling. Thus, (125)I-[Aib(1,3),M]PTH(1-15) appears to function as a selective probe of Galpha(S)-coupled, active-state PTHR conformations.

Photoaffinity Cross-Linking of the Corticotropin-Releasing Factor Receptor Type 1 with Photoreactive Urocortin Analogues

Interaction of natural peptide ligands with class 2 GPCRs, which are targets of biologically important hormones such as glucagon, secretin, and corticotropin-releasing factor (CRF), occurs with a common orientation, in that the ligand C-terminus binds to the extracellular receptor N-terminus, whereas the ligand N-terminus binds to the receptor juxtamembrane domain. N-Terminal truncation, by eight amino acids in the case of CRF, leads to antagonists, suggesting those residues constitute the receptor activating sequence. Here, we identified by photoaffinity cross-linking using p-benzoyl-l-phenylalanine (Bpa) analogues of urocortin (Ucn) the most affine CRF receptor agonist, interaction domains of CRF(1) receptor with Bpa residues at exclusive positions. Specific cleavage patterns of the corresponding ligand-receptor complexes, obtained using several cleavage methods in combination with SDS-PAGE for fragment size determination, showed that a Bpa group located N-terminally or in position 12 binds at the second and such in position 17 or 22 at the first extracellular receptor loop. Our results indicate that the very N-terminal ligand residues (1-11), which are responsible for receptor activation, are oriented to the juxtamembrane domain by interaction of amino acid residues 12, 17, and 22. Our findings contradict a recently proposed interaction model derived from ligand interaction with a soluble receptor N-terminus, indicating that conclusions drawn from such a reduced system may be of limited value to understand the interaction with the full-length receptor.

Insights into interactions between the alpha-helical region of the salmon calcitonin antagonists and the human calcitonin receptor using photoaffinity labeling

Fish-like calcitonins (CTs), such as salmon CT (sCT), are widely used clinically in the treatment of bone-related disorders; however, the molecular basis for CT binding to its receptor, a class II G protein-coupled receptor, is not well defined. In this study we have used photoaffinity labeling to identify proximity sites between CT and its receptor. Two analogues of the antagonist sCT(8-32) containing a single photolabile p-benzoyl-l-phenylalanine (Bpa) residue in position 8 or 19 were used. Both analogues retained high affinity for the CT receptor and potently inhibited agonist-induced cAMP production. The [Bpa(19)]sCT(8-32) analogue cross-linked to the receptor at or near the equivalent cross-linking site of the full-length peptide, within the fragment Cys(134)-Lys(141) (within the amino terminus of the receptor, adjacent to transmembrane 1) (Pham, V., Wade, J. D., Purdue, B. W., and Sexton, P. M. (2004) J. Biol. Chem. 279, 6720-6729). In contrast, proteolytic mapping and mutational analysis identified Met(49) as the cross-linking site for [Bpa(8)]sCT(8-32). This site differed from the previously identified cross-linking site of the agonist [Bpa(8)]human CT (Dong, M., Pinon, D. I., Cox, R. F., and Miller, L. J. (2004) J. Biol. Chem. 279, 31177-31182) and may provide evidence for conformational differences between interaction with active and inactive state receptors. Molecular modeling suggests that the difference in cross-linking between the two Bpa(8) analogues can be accounted for by a relatively small change in peptide orientation. The model was also consistent with cooperative interaction between the receptor amino terminus and the receptor core.

Insights into the structure and molecular basis of ligand docking to the G protein-coupled secretin receptor using charge-modified amino-terminal agonist probes

The amino terminus and third loop regions of class B G protein-coupled receptors play critical roles in ligand docking and action. For the prototypic secretin receptor, the hormone amino terminus is spatially approximated with receptor region high in transmembrane segment 6 (TM6), whereas residues ranging from position 6 through 26 label the amino terminus. Here, we focus on the role of charge of the secretin amino terminus, using a series of full-agonist, acetylated probes. Sites of covalent labeling were examined using sequential purification, chemical and enzymatic cleavage, and Edman degradation. High-affinity amino-terminally-blocked probes labeled the distal amino-terminal tail, rather than TM6, while adding a basic residue, again labeled TM6. These data suggest that the secretin amino terminus docks between the amino terminus and TM6 of the receptor, with this region of secretin likely interacting with an acidic residue within the receptor TM6 and the third extracellular loop. To explore this, candidate acidic residues were mutated to Ala (E341A, D342A, E345A, E351A). The E351A mutant markedly interfered with binding, biological activity, and internalization, whereas all others bound secretin and signaled and internalized normally. This supports the possibility that there is a charge-charge interaction between this residue and the amino terminus of secretin that is critical to its normal docking.

Pharmacological discrimination of calcitonin receptor: receptor activity-modifying protein complexes

Calcitonin (CT) receptors dimerize with receptor activity-modifying proteins (RAMPs) to create high-affinity amylin (AMY) receptors, but there is no reliable means of pharmacologically distinguishing these receptors. We used agonists and antagonists to define their pharmacology, expressing the CT(a) receptor alone or with RAMPs in COS-7 cells and measuring cAMP accumulation. Intermedin short, otherwise known as adrenomedullin 2, mirrored the action of alpha CGRP, being a weak agonist at CT(a), AMY(2a), and AMY(3a) receptors but considerably more potent at AMY(1a) receptors. Likewise, the linear calcitonin gene-related peptide (CGRP) analogs (Cys(ACM)(2,7))h alpha CGRP and (Cys(Et)(2,7))h alpha CGRP were only effective at AMY(1a) receptors, but they were partial agonists. As previously observed in COS-7 cells, there was little induction of the AMY(2a) receptor phenotype; thus, AMY(2a) was not examined further in this study. The antagonist peptide salmon calcitonin(8-32) (sCT(8-32)) did not discriminate strongly between CT and AMY receptors; however, AC187 was a more effective antagonist of AMY responses at AMY receptors, and AC413 additionally showed modest selectivity for AMY(1a) over AMY(3a) receptors. CGRP(8-37) also demonstrated receptor-dependent effects. CGRP(8-37) more effectively antagonized AMY at AMY(1a) than AMY(3a) receptors, although it was only a weak antagonist of both, but it did not inhibit responses at the CT(a) receptor. Low CGRP(8-37) affinity and agonism by linear CGRP analogs at AMY(1a) are the classic signature of a CGRP2 receptor. Our data indicate that careful use of combinations of agonists and antagonists may allow pharmacological discrimination of CT(a), AMY(1a), and AMY(3a) receptors, providing a means to delineate the physiological significance of these receptors.

Covalent capture: a natural complement to self-assembly

The utility of peptide self-assembly can be extended by covalent capture of these supramolecular materials. Disulfide bond formation, native chemical ligation, olefin metathesis, radical capture and oxidative lysine cross-linking have been used recently to help stabilize and characterize a variety of self-assembled peptides. These include natural peptides, proteins and protein mimics such as alpha-helical coiled coils, amyloid-like beta-sheet fibres, portions of p53, glutathione S-transferase and elastin as well as unnatural peptide constructs such as cyclic peptide nanotubes and cylindrical micelles of peptide amphiphiles.

Novel parathyroid hormone (PTH) antagonists that bind to the juxtamembrane portion of the PTH/PTH-related protein receptor

Current antagonists for the parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (PTHR) are N-terminally truncated or N-terminally modified analogs of PTH(1-34) or PTHrP(1-34) and are thought to bind predominantly to the N-terminal extracellular (N) domain of the receptor. We hypothesized that ligands that bind only to PTHR region comprised of the extracellular loops and seven transmembrane helices (the juxtamembrane or J domain) could also antagonize the PTHR. To test this, we started with the J domain-selective agonists [Gln(10),Ala(12),Har(11),Trp(14),Arg(19) (M)]PTH(1-21), [M]PTH(1-15), and [M]PTH(1-14), and introduced substitutions at positions 1-3 that were predicted to dissociate PTHR binding and cAMP signaling activities. Strong dissociation was observed with the tri-residue sequence diethylglycine (Deg)(1)-para-benzoyl-l-phenylalanine (Bpa)(2)-Deg(3). In HKRK-B7 cells, which express the cloned human PTHR, [Deg(1,3),Bpa(2),M]PTH(1-21), [Deg(1,3),Bpa(2),M]PTH(1-15), and [Deg(1,3),Bpa(2),M]PTH(1-14) fully inhibited (IC(50)s = 100-700 nm) the binding of (125)I-[alpha-aminoisobutyric acid(1,3),M]PTH(1-15) and were severely defective for stimulating cAMP accumulation. In ROS 17/2.8 cells, which express the native rat PTHR, [Deg(1,3),Bpa(2),M]PTH(1-21) and [Deg(1,3),Bpa(2),M]PTH(1-15) antagonized the cAMP-agonist action of PTH(1-34), as did PTHrP(5-36) (IC(50)s = 0.7 microm, 2.6 microm, and 36 nm, respectively). In COS-7 cells expressing PTHR-delNt, which lacks the N domain of the receptor, [Deg(1,3),Bpa(2), M]PTH(1-21) and [Deg(1,3),Bpa(2),M]PTH(1-15) inhibited the agonist actions of [alpha-aminoisobutyric acid(1,3)]PTH(1-34) and [M]PTH(1-14) (IC(50)s approximately 1 microm), whereas PTHrP(5-36) failed to inhibit. [Deg(1,3),Bpa(2),M]PTH(1-14) inhibited the constitutive cAMP-signaling activity of PTHR-tether-PTH(1-9), in which the PTH(1-9) sequence is covalently linked to the PTHR J domain, as well as that of PTHR(cam)H223R. Thus, the J-domain-selective N-terminal PTH fragment analogs can function as antagonists as well as inverse agonists for the PTHR. The new ligands described should be useful for further studies of the ligand binding and activation mechanisms that operate in the critical PTHR J domain.

Molecular approximation between a residue in the amino-terminal region of calcitonin and the third extracellular loop of the class B G protein-coupled calcitonin receptor

The calcitonin receptor is a member of the class B family of G protein-coupled receptors, which contains numerous potentially important drug targets. Delineation of themes for agonist binding and activation of these receptors will facilitate the rational design of receptor-active drugs. We reported previously that a photolabile residue within the carboxyl-terminal half (residue 26) and mid-region (residue 16) of calcitonin covalently label the extracellular amino-terminal domain of this receptor (Dong, M., Pinon, D. I., Cox, R. F., and Miller, L. J. (2004) J. Biol. Chem. 279, 1167-1175). Chimeric receptor studies support the importance of this region and suggest important contributions of extracellular loop domains. To examine whether other parts of the ligand may contact those loops, we developed another probe that has its photolabile site of labeling within the amino-terminal half in position 8 of the ligand. This probe was a full agonist (EC(50) = 563 +/- 67 pm), stimulating cAMP accumulation in receptor-bearing human embryonic kidney 293 cells in a concentration-dependent manner. It bound specifically and saturably (K(i) = 14.3 +/- 1.9 nm) and was able to efficiently label the calcitonin receptor. By purification, specific cleavage, and sequencing of labeled wild-type and mutant calcitonin receptors, the site of attachment was identified as residue Leu(368) within the third extracellular loop of the receptor, a domain distinct from that labeled by previous probes. These data are consistent with a common ligand binding mechanism for receptors in this important family.