Elucidation of the active conformation of the amino terminus of receptor-bound secretin using intramolecular disulfide bond constraints

Family B G protein-coupled receptors include several potentially important drug targets, yet our understanding of the molecular basis of ligand binding to and activation of these receptors is incomplete. While NMR and crystal structures exist for peptide ligand-associated amino-terminal domains of several family members, these only provide insights into the conformation of the carboxyl-terminal region of the peptides. The amino-terminal region of these peptides, critical for biological activity, is believed to interact with the helical bundle domain, and is, therefore, unconstrained in these structures. The aim of the current study was to provide insights into the conformation of the amino terminus of secretin as bound to its receptor. We prepared a series of conformationally constrained secretin peptides containing intramolecular disulfide bonds that were predicted by molecular modeling to approximate the conformation of the analogous region of PACAP bound to its receptor that had been determined using transfer-NOE NMR techniques. Secretin peptides with pairs of cysteine residues in positions 2-7, 3-5, 3-6, 4-7, 7-9, and 4-10 were studied as linear and disulfide-bonded forms. The analog with a disulfide bond connecting positions 7-9 had binding affinity and biological activity similar to natural secretin, supporting the relevance of this constraint to its active conformation. While this feature is shared between secretin and PACAP, absence of activity in other constrained peptides in this series also suggest that there are differences between these receptor-bound conformations. It will be critical to extend similar studies to other family members to learn what structural elements might be most conserved in this family.

Functional and structural diversification of the Anguimorpha lizard venom system

Venom has only been recently discovered to be a basal trait of the Anguimorpha lizards. Consequently, very little is known about the timings of toxin recruitment events, venom protein molecular evolution, or even the relative physical diversifications of the venom system itself. A multidisciplinary approach was used to examine the evolution across the full taxonomical range of this ∼130 million-year-old clade. Analysis of cDNA libraries revealed complex venom transcriptomes. Most notably, three new cardioactive peptide toxin types were discovered (celestoxin, cholecystokinin, and YY peptides). The latter two represent additional examples of convergent use of genes in toxic arsenals, both having previously been documented as components of frog skin defensive chemical secretions. Two other novel venom gland-overexpressed modified versions of other protein frameworks were also recovered from the libraries (epididymal secretory protein and ribonuclease). Lectin, hyaluronidase, and veficolin toxin types were sequenced for the first time from lizard venoms and shown to be homologous to the snake venom forms. In contrast, phylogenetic analyses demonstrated that the lizard natriuretic peptide toxins were recruited independently of the form in snake venoms. The de novo evolution of helokinestatin peptide toxin encoding domains within the lizard venom natriuretic gene was revealed to be exclusive to the helodermatid/anguid subclade. New isoforms were sequenced for cysteine-rich secretory protein, kallikrein, and phospholipase A(2) toxins. Venom gland morphological analysis revealed extensive evolutionary tinkering. Anguid glands are characterized by thin capsules and mixed glands, serous at the bottom of the lobule and mucous toward the apex. Twice, independently this arrangement was segregated into specialized serous protein-secreting glands with thick capsules with the mucous lobules now distinct (Heloderma and the Lanthanotus/Varanus clade). The results obtained highlight the importance of utilizing evolution-based search strategies for biodiscovery and emphasize the largely untapped drug design and development potential of lizard venoms.

Role of N-linked glycosylation in biosynthesis, trafficking, and function of the human glucagon-like peptide 1 receptor

The family B G protein-coupled glucagon-like peptide 1 (GLP-1) receptor is an important drug target for treatment of type 2 diabetes. Like other family members, the GLP-1 receptor is a glycosylated membrane protein that contains three potential sites for N-linked glycosylation within the functionally important extracellular amino-terminal domain. However, the roles for each potential site of glycosylation in receptor biosynthesis, trafficking, and function are not known. In this work, we demonstrated that tunicamycin inhibition of glycosylation of the GLP-1 receptor expressed in CHO cells interfered with biosynthesis and intracellular trafficking, thereby eliminating natural ligand binding. To further investigate the roles of each of the glycosylation sites, site-directed mutagenesis was performed to eliminate these sites individually and in aggregate. Our results showed that mutation of each of the glycosylation sites individually did not interfere with receptor expression on the cell surface, ligand binding, and biological activity. However, simultaneous mutation of two or three glycosylation sites resulted in almost complete loss of GLP-1 binding and severely impaired biological activity. Immunostaining studies demonstrated receptor biosynthesis but aberrant trafficking, with most of the receptor trapped in the endoplasmic reticulum and golgi compartments and little of the receptor expressed on the cell surface. Interestingly, surface expression, ligand binding, and biological activity of these mutants improved significantly when biosynthesis was slowed using low temperature (30 degrees C). These data suggest that N-linked glycosylation of the GLP-1 receptor is important for its normal folding and trafficking to the cell surface.

Allosteric ligands of the glucagon-like peptide 1 receptor (GLP-1R) differentially modulate endogenous and exogenous peptide responses in a pathway-selective manner: implications for drug screening

The glucagon-like peptide-1 (GLP-1) receptor is a key regulator of insulin secretion and a major therapeutic target for treatment of diabetes. However, GLP-1 receptor function is complex, with multiple endogenous peptides that can interact with the receptor, including full-length (1-37) and truncated (7-37) forms of GLP-1 that can each exist in an amidated form and the related peptide oxyntomodulin. We have investigated two GLP-1 receptor allosteric modulators, Novo Nordisk compound 2 (6,7-dichloro2-methylsulfonyl-3-tert-butylaminoquinoxaline) and quercetin, and their ability to modify binding and signaling (cAMP formation, intracellular Ca(2+) mobilization, and extracellular signal-regulated kinase 1/2 phosphorylation) of each of the naturally occurring endogenous peptide agonists, as well as the clinically used peptide mimetic exendin-4. We identified and quantified stimulus bias across multiple endogenous peptides, with response profiles for truncated GLP-1 peptides distinct from those of either the full-length GLP-1 peptides or oxyntomodulin, the first demonstration of such behavior at the GLP-1 receptor. Compound 2 selectively augmented cAMP signaling but did so in a peptide-agonist dependent manner having greatest effect on oxyntomodulin, weaker effect on truncated GLP-1 peptides, and negligible effect on other peptide responses; these effects were principally driven by parallel changes in peptide agonist affinity. In contrast, quercetin selectively modulated calcium signaling but with effects only on truncated GLP-1 peptides or exendin and not oxyntomodulin or full-length peptides. These data have significant implications for how GLP-1 receptor targeted drugs are screened and developed, whereas the allosterically driven, agonist-selective, stimulus bias highlights the potential for distinct clinical efficacy depending on the properties of individual drugs.

Importance of lipid-exposed residues in transmembrane segment four for family B calcitonin receptor homo-dimerization

Dimerization of the prototypic family B G protein-coupled secretin receptor is determined by the lipid-exposed face of transmembrane segment four (TM4), and has substantial functional importance, facilitating G protein coupling. Recently, we demonstrated that the human secretin receptor elicits an inter-receptor bioluminescence resonance energy transfer (BRET) signal with most other human family B peptide receptors, except for the calcitonin receptor. In this study we have explored the occurrence and importance of calcitonin receptor oligomerization. Static and saturation receptor BRET were utilized to demonstrate that, unlike the human calcitonin receptor that does not yield a significant homomeric BRET signal, the rabbit calcitonin receptor exhibits strong resonance energy transfer. Within the lipid-exposed face of TM4, rabbit and human calcitonin receptors differ by a single amino acid (Arg236 in human; His in rabbit), while Thr253 that occurs in human and rabbit calcitonin receptors is unique across family B receptors. Mutating Arg236 or Thr253 of the human calcitonin receptor to residues found in the rabbit calcitonin receptor or the human secretin receptor (R236H, R236Y and T253A) resulted in generation of significant BRET signals. Similarly, mutation of Val250 of the human calcitonin receptor to another key lipid-facing residue found in the secretin receptor (V250I) also increased the receptor BRET signal. These data support the consistent theme of lipid-exposed residues of TM4 being important for the dimerization of the calcitonin receptor. However, rabbit and human calcitonin receptor constructs bound calcitonin and stimulated cAMP similarly, suggesting that differences in BRET could reflect differences in orientation or in the stability of homo-dimeric receptor complexes, which were nevertheless similarly effective in eliciting the functions attributed to that complex. The likelihood of human calcitonin receptor dimerization, even in the absence of a significant BRET signal, was further supported by data demonstrating that the peptide representing TM4 of this receptor that disrupts the rabbit receptor BRET signal, produced a right shift in the cAMP concentration-response curves for both rabbit and human receptors.

Spatial approximations between residues 6 and 12 in the amino-terminal region of glucagon-like peptide 1 and its receptor: a region critical for biological activity

Understanding the molecular basis of natural ligand binding and activation of the glucagon-like peptide 1 (GLP1) receptor may facilitate the development of agonist drugs useful for the management of type 2 diabetes mellitus. We previously reported molecular approximations between carboxyl-terminal residues 24 and 35 within GLP1 and its receptor. In this work, we have focused on the amino-terminal region of GLP1, known to be critical for receptor activation. We developed two high-affinity, full agonist photolabile GLP1 probes having sites of covalent attachment in positions 6 and 12 of the 30-residue peptide (GLP1(7-36)). Both probes bound to the receptor specifically and covalently labeled single distinct sites. Chemical and protease cleavage of the labeled receptor identified the juxtamembrane region of its amino-terminal domain as the region of covalent attachment of the position 12 probe, whereas the region of labeling by the position 6 probe was localized to the first extracellular loop. Radiochemical sequencing identified receptor residue Tyr(145), adjacent to the first transmembrane segment, as the site of labeling by the position 12 probe, and receptor residue Tyr(205), within the first extracellular loop, as the site of labeling by the position 6 probe. These data provide support for a common mechanism for natural ligand binding and activation of family B G protein-coupled receptors. This region of interaction of peptide amino-terminal domains with the receptor may provide a pocket that can be targeted by small molecule agonists.

Refinement of the pharmacophore of an agonist ligand of the secretin receptor using conformationally constrained cyclic hexapeptides

There is a compelling need for the development of small molecule agonists acting at family B G protein-coupled receptors. A possible lead for the development of such drugs was reported when it was recognized that sequences endogenous to the amino terminus of the secretin receptor and certain other receptors in this family possess weak full agonist activity (Dong et al. Mol Pharmacol 2006;70:206-213). In the current report, we extended those observations by building the active dipeptide motif found in the secretin receptor (WD) into each position around a conformationally constrained d-amino acid-containing cyclic hexapeptide, and determining the biological activity of each peptide at the secretin receptor. Indeed, only two positions for WD around this constrained ring resulted in biological activity at the receptor, providing further insights into the structural specificity of this phenomenon. Molecular modeling supported the presence of a unique WD backbone conformation shared only by these active peptides, and provided a more constrained template for future receptor-active agonist drug development.

Modulation of cell surface expression of nonactivated cholecystokinin receptors using bivalent ligand-induced internalization

CCK(2) receptor antagonists potentiate pain relief by MOP receptor agonists. In an attempt to enhance this effect, we prepared bivalent ligands incorporating CCK(2) receptor antagonist and MOP receptor agonist pharmacophores. (9) Ligands with 16- to 22-atom spacers could simultaneously bind both receptors but provided no advantage in activity over individual ligands. We now examine the effect of these ligands on receptor internalization as a mechanism of receptor regulation. We prepared CHO cell lines expressing nonfluorescent halves (YN and YC) of yellow fluorescent protein attached to each receptor. Spatial approximation of constructs was needed to yield fluorescence. Monovalent MOP agonist 1 signaled normally and internalized the MOP receptor. Monovalent CCK(2) antagonist 2 did not stimulate receptor internalization. In the dual receptor-bearing cells, bivalent ligands 3a-c capable of simultaneously binding both receptors resulted in cell surface fluorescence and internalization of the fluorescent complex in a time- and temperature-dependent manner. Bivalent ligand 4 with spacer too short to occupy both receptors simultaneously yielded no signal. Receptor tethering with appropriate bivalent ligands can down-regulate signaling by moving a nonactivated receptor into the endocytic pathway.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.

Risk factors for pancreatic adenocarcinoma and prospects for screening

Pancreatic cancer has one of the worst survival rates of any cancer and is the fourth leading cause of cancer mortality. Early detection and surgery are the patient's best chance for cure. However, symptoms are typically vague and occur when the cancer is unresectable. Population-based mass screening is not practical for this rare disease, though screening and early detection in asymptomatic high-risk patient populations may be indicated.

Dimeric arrangement of the parathyroid hormone receptor and a structural mechanism for ligand-induced dissociation

The parathyroid hormone receptor (PTH1R) is a class B G protein-coupled receptor that is activated by parathyroid hormone (PTH) and PTH-related protein (PTHrP). Little is known about the oligomeric state of the receptor and its regulation by hormone. The crystal structure of the ligand-free PTH1R extracellular domain (ECD) reveals an unexpected dimer in which the C-terminal segment of both ECD protomers forms an alpha-helix that mimics PTH/PTHrP by occupying the peptide binding groove of the opposing protomer. ECD-mediated oligomerization of intact PTH1R was confirmed in living cells by bioluminescence and fluorescence resonance energy transfer experiments. As predicted by the structure, PTH binding disrupted receptor oligomerization. A receptor rendered monomeric by mutations in the ECD retained wild-type PTH binding and cAMP signaling ability. Our results are consistent with the hypothesis that PTH1R forms constitutive dimers that are dissociated by ligand binding and that monomeric PTH1R is capable of activating G protein.

Secretin occupies a single protomer of the homodimeric secretin receptor complex: insights from photoaffinity labeling studies using dual sites of covalent attachment

The secretin receptor, a prototypic family B G protein-coupled receptor, forms a constitutive homodimeric complex that is stable even in the presence of hormone. Recently, a model of this agonist-bound receptor was built based on high resolution structures reported for amino-terminal domains of other family members. Although this model provided the best solution for all extant data, including 10 photoaffinity labeling constraints, a new such constraint now obtained with a position 16 photolabile probe was inconsistent with this model. As the secretin receptor forms constitutive homodimers, we explored whether secretin might dock across both protomers of the complex, an observation that could also contribute to the negative cooperativity observed. To directly explore this, we prepared six secretin analogue probes that simultaneously incorporated two photolabile benzoylphenylalanines as sites of covalent attachment, in positions known to label distinct receptor subdomains. Each bifunctional probe was a full agonist that labeled the receptor specifically and saturably, with electrophoretic migration consistent with labeling a single protomer of the homodimeric secretin receptor. No band representing radiolabeled receptor dimer was observed with any bifunctional probe. The labeled monomeric receptor bands were cleaved with cyanogen bromide to demonstrate that both of the photolabile benzoylphenylalanines within a single probe had established covalent adducts with a single receptor in the complex. These data are consistent with a model of secretin occupying a single secretin receptor protomer within the homodimeric receptor complex. A new molecular model accommodating all constraints is now proposed.

Molecular basis of association of receptor activity-modifying protein 3 with the family B G protein-coupled secretin receptor

The three receptor activity-modifying proteins (RAMPs) have been recognized as being important for the trafficking and function of a subset of family B G protein-coupled receptors, although the structural basis for this has not been well established. In the current work, we use morphological fluorescence techniques, bioluminescence resonance energy transfer, and bimolecular fluorescence complementation to demonstrate that the secretin receptor associates specifically with RAMP3, but not with RAMP1 or RAMP2. We use truncation constructs, peptide competition experiments, and chimeric secretin-GLP1 receptor constructs to establish that this association is structurally specific, dependent on the intramembranous region of the RAMP and TM6 and TM7 of this receptor. There were no observed changes in secretin-stimulated cAMP, intracellular calcium, ERK1/2 phosphorylation, or receptor internalization in receptor-bearing COS or CHO-K1 cells in the presence or absence of exogenous RAMP transfection, although the secretin receptor trafficks normally to the cell surface in these cells in a RAMP-independent manner, resulting in both free and RAMP-associated receptor on the cell surface. RAMP3 association with this receptor was shown to be capable of rescuing a receptor mutant (G241C) that is normally trapped intracellularly in the biosynthetic machinery. Similarly, secretin receptor expression had functional effects on adrenomedullin activity, with increasing secretin receptor expression competing for RAMP3 association with the calcitonin receptor-like receptor to yield a functional adrenomedullin receptor. These data provide important new insights into the structural basis for RAMP3 interaction with a family B G protein-coupled receptor, potentially providing a highly selective target for drug action. This may be representative of similar interactions between other members of this receptor family and RAMP proteins.

Therapeutic potential for novel drugs targeting the type 1 cholecystokinin receptor

Cholecystokinin (CCK) is a physiologically important gastrointestinal and neuronal peptide hormone, with roles in stimulating gallbladder contraction, pancreatic secretion, gastrointestinal motility and satiety. CCK exerts its effects via interactions with two structurally related class I guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs), the CCK(1) receptor and the CCK(2) receptor. Here, we focus on the CCK(1) receptor, with particular relevance to the broad spectrum of signalling initiated by activation with the natural full agonist peptide ligand, CCK. Distinct ligand-binding pockets have been defined for the natural peptide ligand and for some non-peptidyl small molecule ligands. While many CCK(1) receptor ligands have been developed and have had their pharmacology well described, their clinical potential has not yet been fully explored. The case is built for the potential importance of developing more selective partial agonists and allosteric modulators of this receptor that could have important roles in the treatment of common clinical syndromes.

Molecular basis of glucagon-like peptide 1 docking to its intact receptor studied with carboxyl-terminal photolabile probes

The glucagon-like peptide 1 (GLP1) receptor is a member of Family B G protein-coupled receptors and represents an important drug target for type 2 diabetes. Despite recent solution of the structure of the amino-terminal domain of this receptor and that of several close family members, understanding of the molecular basis of natural ligand GLP1 binding to its intact receptor remains limited. The goal of this study was to explore spatial approximations between specific receptor residues within the carboxyl terminus of GLP1 and its receptor as normally docked. Therefore, we developed and characterized two high affinity, full-agonist photolabile GLP1 probes having sites for covalent attachment in positions 24 and 35. Both probes labeled the receptor specifically and saturably. Subsequent peptide mapping using chemical and proteinase cleavages of purified wild-type and mutant GLP1 receptor identified that the Arg(131)-Lys(136) segment at the juxtamembrane region of the receptor amino terminus contained the site of labeling for the position 24 probe, and the specific receptor residue labeled by this probe was identified as Glu(133) by radiochemical sequencing. Similarly, nearby residue Glu(125) within the same region of the receptor amino-terminal domain was identified as the site of labeling by the position 35 probe. These data represent the first direct demonstration of spatial approximation between GLP1 and its intact receptor as docked, providing two important constraints for the modeling of this interaction. This should expand our understanding of the molecular basis of natural agonist ligand binding to the GLP1 receptor and may be relevant to other family members.

Elucidation of the molecular basis of cholecystokinin Peptide docking to its receptor using site-specific intrinsic photoaffinity labeling and molecular modeling

G protein-coupled receptors represent the largest family of receptors and the major target of current drug development efforts. Understanding of the mechanisms of ligand binding and activation of these receptors remains limited, despite recent advances in structural determination of family members. This work focuses on the use of photoaffinity labeling and molecular modeling to elucidate the structural basis of binding a natural peptide ligand to a family A G protein-coupled receptor, the type 1 cholecystokinin receptor. Two photolabile cholecystokinin analogues were developed and characterized as representing high-affinity, fully biologically active probes with sites of covalent attachment at positions 28 and 31. The sites of receptor labeling were identified by purification, proteolytic peptide mapping, and radiochemical sequencing of labeled wild-type and mutant cholecystokinin receptors. The position 28 probe labeled second extracellular loop residue Leu(199), while the position 31 probe labeled first extracellular loop residue Phe(107). Along with five additional spatial approximation constraints coming from previous photoaffinity labeling studies and 12 distance restraints from fluorescence resonance energy transfer studies, these were built into two homology models of the cholecystokinin receptor, based on the recent crystal structures of the beta2-adrenergic receptor and A2a-adenosine receptor. The resultant agonist ligand-occupied receptor models fully accommodate all existing experimental data and represent the best refined models of a peptide hormone receptor in this important family.

CCK(2) receptor splice variant with intron 4 retention in human gastrointestinal and lung tumours

The wild-type cholecystokinin type 2 (CCK₂) receptor is expressed in many gastrointestinal and lung tumours. A splice variant of the CCK₂ receptor with retention of intron 4 (CCK₂Ri4sv) showing constitutive activity associated with increased tumour growth was described in few colorectal, pancreatic and gastric cancers. Given the potential functional and clinical importance of this spliceoform, its occurrence was quantitatively characterized in a broad collection of 81 gastrointestinal and lung tumours, including insulinomas, ileal carcinoids, gastrointestinal stromal tumours (GIST), gastric, colorectal and pancreatic ductal adenocarcinomas, cholangiocellular and hepatocellular carcinomas, small cell lung cancers (SCLC), non-SCLC (nSCLC) and bronchopulmonary carcinoids, as well as 21 samples of corresponding normal tissues. These samples were assessed for transcript expression of total CCK₂ receptor, wild-type CCK₂ receptor and CCK₂Ri4sv with end-point and real-time RT-PCR, and for total CCK₂ receptor protein expression on the basis of receptor binding with in vitro receptor autoradiography. Wild-type CCK₂ receptor transcripts were found in the vast majority of tumours and normal tissues. CCK₂Ri4sv mRNA expression was present predominantly in insulinomas (incidence 100%), GIST (100%) and SCLC (67%), but rarely in pancreatic, colorectal and gastric carcinomas and nSCLC. It was not found in wild-type CCK₂ receptor negative tumours or any normal tissues tested. CCK₂Ri4sv transcript levels in individual tumours were low, ranging from 0.02% to 0.14% of total CCK₂ receptor transcripts. In conclusion, the CCK₂Ri4sv is a marker of specific gastrointestinal and lung tumours. With its high selectivity for and high incidence in SCLC and GIST, it may represent an attractive clinical target.

Homology modeling of GPCRs

Over 1,000 sequences likely to encode G protein-coupled receptors (GPCRs) are currently available in publicly accessible and proprietary databases and this number may grow with the refinement of a number of different genomes. However, despite recent efforts in the crystallization of these proteins, homology modeling approaches are becoming widely used as a method for obtaining quantitative and qualitative information for structure-based drug design as well as the interpretation of experimental data.

Alternative splicing of pre-mRNA in cancer: focus on G protein-coupled peptide hormone receptors

Through alternative splicing, multiple different transcripts can be generated from a single gene. Alternative splicing represents an important molecular mechanism of gene regulation in physiological processes such as developmental programming as well as in disease. In cancer, splicing is significantly altered. Tumors express a different collection of alternative spliceoforms than normal tissues. Many tumor-associated splice variants arise from genes with an established role in carcinogenesis or tumor progression, and their functions can be oncogenic. This raises the possibility that products of alternative splicing play a pathogenic role in cancer. Moreover, cancer-associated spliceoforms represent potential diagnostic biomarkers and therapeutic targets. G protein-coupled peptide hormone receptors provide a good illustration of alternative splicing in cancer. The wild-type forms of these receptors have long been known to be expressed in cancer and to modulate tumor cell functions. They are also recognized as attractive clinical targets. Recently, splice variants of these receptors have been increasingly identified in various types of cancer. In particular, alternative cholecystokinin type 2, secretin, and growth hormone-releasing hormone receptor spliceoforms are expressed in tumors. Peptide hormone receptor splice variants can fundamentally differ from their wild-type receptor counterparts in pharmacological and functional characteristics, in their distribution in normal and malignant tissues, and in their potential use for clinical applications.

Effects of pH and temperature on photoaffinity labeling of Family B G protein-coupled receptors

The efficiency of covalent labeling of a receptor by a photolabile analogue of its natural ligand is dependent on the spatial approximation of the probe and its target. Systematic application of intrinsic photoaffinity labeling to the secretin receptor, a prototypic Family B G protein-coupled receptor, demonstrated reduced efficiency of labeling for amino-terminal and mid-region sites of labeling relative to carboxyl-terminal sites. Reduction of pH from 7.4 to 5.5 and reduction of temperature from 25 degrees C to 4 degrees C improved the efficiency of covalent labeling of the receptor with these probes. This correlated with sites of labeling at the interface between the receptor amino terminus and the receptor core, a region containing histidine residues that have their ionization affected in this pH range. Application to the calcitonin receptor, another Family B G protein-coupled receptor, yielded analogous results. These results support the consistent mode of docking peptide ligands to this group of receptors.

Juxtamembranous region of the amino terminus of the family B G protein-coupled calcitonin receptor plays a critical role in small-molecule agonist action

The Family B G protein-coupled calcitonin receptor is an important drug target. The aim of this work was to elucidate the molecular mechanism of action of small-molecule agonist ligands acting at this receptor, comparing it with the action mechanism of the receptor's natural peptide ligand. cAMP responses to four non-peptidyl ligands and calcitonin were studied in COS-1 cells expressing wild-type and chimeric calcitonin-secretin receptors. All compounds were full agonists at the calcitonin receptor with no activity at the secretin receptor. Only chimeric constructs including the calcitonin receptor amino terminus exhibited responses to any of these ligands. We progressively truncated this domain and tested constructs for cAMP responses. Although calcitonin was able to activate the calcitonin receptor fully with the first 58 residues absent, its potency was 3 orders of magnitude lower than that at the wild-type receptor. After truncation of 114 residues, there was no response to calcitonin. In contrast, small-molecule ligands were fully active at receptors having up to 149 amino-terminal residues absent. Those compounds finally became inactive after truncation of 153 residues. Deletion and/or alanine replacement of the region of the calcitonin receptor between residues 150 and 153 resulted in marked reduction in cAMP responses to these compounds, with some compound-specific differences observed, supporting a critical role for this region. Binding studies further supported distinct sites of action of small molecules relative to that of calcitonin. These findings focus attention on the potential importance of the juxtamembranous region of the amino terminus of the Family B calcitonin receptor for agonist drug action.

Functional importance of a structurally distinct homodimeric complex of the family B G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been described, but its structural basis and functional importance have been inconsistent. Here, we demonstrate that the agonist occupied wild-type secretin receptor is predominantly in a guanine nucleotide-sensitive high-affinity state and exhibits negative cooperativity, whereas the monomeric receptor is primarily in a guanine nucleotide-insensitive lower affinity state. We previously demonstrated constitutive homodimerization of this receptor through the lipid-exposed face of transmembrane (TM) IV. We now use cysteine-scanning mutagenesis of 14 TM IV residues, bioluminescence resonance energy transfer (BRET), and functional analysis to map spatial approximations and functional importance of specific residues in this complex. All, except for three helix-facing mutants, trafficked to the cell surface, where secretin was shown to bind and elicit cAMP production. Cells expressing complementary-tagged receptors were treated with cuprous phenanthroline to establish disulfide bonds between spatially approximated cysteines. BRET was measured as an indication of receptor oligomerization and was repeated after competitive disruption of oligomers with TM IV peptide to distinguish covalent from noncovalent associations. Although all constructs generated a significant BRET signal, this was disrupted by peptide in all except for single-site mutants replacing five residues with cysteine. Of these, covalent stabilization of receptor homodimers through positions of Gly(243), Ile(247), and Ala(250) resulted in a GTP-sensitive high-affinity state of the receptor, whereas the same procedure with Ala(246) and Phe(240) mutants resulted in a GTP-insensitive lower affinity state. We propose the existence of a functionally important, structurally specific high-affinity dimeric state of the secretin receptor, which may be typical of family B G protein-coupled receptors.

A system for monitoring and improving animal visibility and its implications for zoological parks

With the growing trend in zoos to build complex, naturalistic exhibits comes the potential for exhibits to be so densely vegetated or complex that animals are not easily seen by zoo visitors. This can negatively impact the visitor's visiting experience and the zoo's ability to communicate conservation and education messages. Over the past 9 years, Disney's Animal Kingdom has developed a process for monitoring and improving the visibility of animals on display to the public. This animal visibility process utilizes a data collection system whereby systematic observations are collected each week. The percentage of observations where at least one animal was visible is recorded for each species and compared to an 80% visibility criterion. Species that do not reach this criterion for 4 consecutive weeks are discussed at animal management meetings. If the problems associated with animal visibility cannot be easily solved, the animal-care teams partner with the research team to conduct a second process, called the Visibility Issues Process. This process provides additional information for the animal-care team to utilize in developing a plan to improve visibility. Although the processes described here are specific to the infrastructure at Disney's Animal Kingdom, the basic concepts of (1) a formalized visibility data collection process, (2) a visibility criterion to which managers of species are held accountable, and (3) a process for planning to improve animal visibility without negatively impacting animal welfare are fundamental concepts that can be developed at individual institutions and incorporated into that zoo's existing infrastructure.

Synthesis and in vitro characterization of radioiodinatable benzodiazepines selective for type 1 and type 2 cholecystokinin receptors

Radiolabeled antagonists of specific peptide receptors identify a higher number of receptor binding sites than agonists and may thus be preferable for in vivo tumor targeting. In this study, two novel radioiodinated 1,4-benzodiazepines, (S)-1-(3-iodophenyl)-3-(1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)urea (9) and (R)-1-(3-iodophenyl)-3-(1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)urea (7), were developed. They were characterized in vitro as high affinity selective antagonists at cholecystokinin types 1 and 2 (CCK(1) and CCK(2)) receptors using receptor binding, calcium mobilization, and internalization studies. Their binding to human tumor tissues was assessed with in vitro receptor autoradiography and compared with an established peptidic CCK agonist radioligand. The (125)I-labeled CCK(1) receptor-selective compound 9 often revealed a substantially higher amount of CCK(1) receptor binding sites in tumors than the agonist (125)I-CCK. Conversely, the radioiodinated CCK(2) receptor-selective compound 7 showed generally weaker tumor binding than (125)I-CCK. In conclusion, compound 9 is an excellent radioiodinated nonpeptidic antagonist ligand for direct and selective labeling of CCK(1) receptors in vitro. Moreover, it represents a suitable candidate to test antagonist binding to CCK(1) receptor-expressing tumors in vivo.

Induced association of mu opioid (MOP) and type 2 cholecystokinin (CCK2) receptors by novel bivalent ligands

Both mu-opioid (MOP) and type 2 cholecystokinin (CCK2) receptors are present in areas of the central nervous system that are involved in modulation of pain processing. We conducted bioluminescence resonance energy transfer (BRET) studies on COS cells coexpressing MOP and CCK2 receptors to determine whether receptor heterodimerization is involved in such modulation. These studies revealed the absence of constitutive or monovalent ligand-induced heterodimerization. Heterodimerization of MOP and CCK2 receptors therefore is unlikely to be responsible for the opposing effects between morphine and CCK in the CNS. However, association was induced, as indicated by a positive BRET signal, on exposure of the cells to bivalent ligands containing mu-opioid agonist and CCK2 receptor antagonist pharmacophores linked through spacers containing 16-22 atoms but not with a shorter (9-atom) spacer. These studies demonstrate for the first time that an appropriately designed bivalent ligand is capable of inducing association of G-protein-coupled receptors. The finding that opioid tolerance studies with these ligands in mice showed no correlation with the BRET data is consistent with the absence of association of MOP and CCK2 receptors in vivo.

G protein-coupled receptor structures, molecular associations, and modes of regulation

G protein-coupled receptors are present on every excitable and regulatable cell in the body and represent important potential sites for drug action. This group of molecules already represents the most common target for currently approved drugs. With recent advances in our understanding of detailed molecular structure, mechanisms of activation, unique molecular associations, and cellular and biochemical mechanisms of regulation of these receptors, development and refinement of receptor-active drugs should benefit substantially. In this report, key past accomplishments and possible future directions for this field of research are reviewed.

Monitoring the state of cholecystokinin receptor oligomerization after ligand binding using decay of time-resolved fluorescence anisotropy

Oligomeric complexes of G protein-coupled receptors (GPCRs) are now commonly recognized and can provide a mechanism for regulation of signaling systems. Receptor oligomerization has been most extensively studied using coimmunoprecipitation and bioluminescence or fluorescence resonance energy-transfer techniques. Here, we have utilized decay of time-resolved fluorescence anisotropy of yellow fluorescent protein-labeled cholecystokinin receptor constructs to examine the state of oligomerization of this receptor in living cells. The rotational correlation times established that the cholecystokinin receptor is constitutively present in an oligomeric state that is dissociated in response to agonist occupation. In contrast, antagonist occupation failed to modify this signal, leaving the oligomeric structure intact. This dynamic technique complements the other biochemical and steady-state fluorescence techniques to establish the presence of oligomeric receptor complexes in living cells.

Application of fluorescence resonance energy transfer techniques to establish ligand-receptor orientation

Fluorescence resonance energy transfer (FRET) has been utilized to determine distances between a fluorescence donor and a fluorescence acceptor having appropriately overlapping spectra. In this chapter, we utilize this approach to establish distances between a fluorescence donor situated in a distinct position within a docked ligand and a fluorescence acceptor situated in a distinct position within its receptor. This technique is applicable to receptor expressed in the environment of an intact cell containing the full complement of signaling and regulatory proteins. A number of controls are necessary, including those establishing the normal function of the modified ligand and receptor, the absence of energy transfer to non-receptor proteins, and the specificity of transfer between the donor of interest and the acceptor of interest. We have utilized the example of FRET between a secretin peptide incorporating Alexa(488) and a secretin receptor construct derivatized with Alexa(568). The latter was prepared by the derivatization of a mono-cysteine-reactive receptor construct with a fluorescent methanethiosulfonate reagent. This approach can provide important spatial information that can be useful in the meaningful docking of a ligand at its receptor.

Effects of cholecystokinin-58 on type 1 cholecystokinin receptor function and regulation

Cholecystokinin, like many peptide hormones, is present as multiple molecular forms. CCK-58 has been identified as the dominant form in the circulation, whereas most of the studies of CCK-receptor interactions have been performed with CCK-8. Despite both sharing the pharmacophoric region of CCK, representing its carboxy terminal heptapeptide amide, studies in vivo have demonstrated biological diversity of action of the two peptides, with CCK-58, but not CCK-8, stimulating pancreatic fluid secretion and lengthening the interval between meals. Here, we have directly studied the ability of these two CCK peptides to bind to the type 1 CCK receptor and to stimulate it to elicit an intracellular calcium response. The calcium response relative to receptor occupation was identical for CCK-58 and CCK-8, with the longer peptide binding with approximately fivefold lower affinity. We also examined the ability of the two peptides to elicit receptor internalization using morphological techniques and to disrupt the constitutive oligomerization of the CCK receptor using receptor bioluminescence resonance energy transfer. Here, both full agonist peptides had similar effects on these regulatory processes. These data suggest that both molecular forms of CCK act at the CCK1 receptor quite similarly and elicit similar regulatory processes for that receptor, suggesting that the differences in biological activity observed in vivo most likely reflect differences in the clearance and/or metabolism of these long and short forms of CCK peptides.

Use of multidimensional fluorescence resonance energy transfer to establish the orientation of cholecystokinin docked at the type A cholecystokinin receptor

Fluorescence resonance energy transfer (FRET) represents a powerful tool to establish relative distances between donor and acceptor fluorophores. By utilizing several donors situated in distinct positions within a docked full agonist ligand and several acceptors distributed at distinct sites within its receptor, multiple interdependent dimensions can be determined. These can provide a unique method to establish or confirm three-dimensional structure of the molecular complex. In this work, we have utilized full agonist analogues of cholecystokinin (CCK) with Aladan distributed throughout the pharmacophore in positions 24, 29, and 33, along with receptor constructs derivatized with Alexa (546) at positions 94, 102, 204, and 341 in the helical bundle and first, second, and third extracellular loops, respectively. These provided 12 FRET distances to overlay on working models of the CCK-occupied receptor. These established that the carboxyl terminus of CCK resides at the external surface of the lipid bilayer, adjacent to the receptor amino-terminal tail, rather than being inserted into the helical bundle. They also provide important experimentally derived constraints for understanding spatial relationships between the docked ligand and the flexible extracellular loop regions. Multidimensional FRET provides a new independent method to establish and refine structural insights into ligand-receptor complexes.

Dimerization in the absence of higher-order oligomerization of the G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been proposed to affect receptor function and regulation; however, little is known about the molecular nature of such complexes. We previously utilized bioluminescence resonance energy transfer (BRET) to demonstrate that the prototypic Family B secretin receptor can form oligomers. We now explore the order of oligomerization present utilizing unique bimolecular fluorescence complementation and energy transfer techniques. The non-fluorescent carboxyl-terminal and amino-terminal halves of yellow fluorescent protein (YFP) were fused to the carboxyl terminus of the secretin receptor. These constructs bound secretin normally and signaled in response to secretin like wild type receptor. When co-expressed on COS cells, these constructs physically interacted to yield typical YFP fluorescence in biosynthetic compartments and at the plasma membrane, reflecting receptor homo-dimerization. However, the addition of another potential partner in form of Rlu- or CFP-tagged secretin receptor yielded no significant BRET or FRET signal, respectively, under conditions in which intact YFP-tagged secretin receptor yielded such a signal. Absence of higher-order receptor oligomers was further confirmed using saturation BRET techniques. Absence of significant resonance transfer to the secretin receptor homo-dimer was true for carboxyl-terminally-tagged secretin receptor, as well as for receptor incorporating the transfer partner into each of the three distinct intracellular loop domains. These results suggest that the secretin receptor can exist only as a structurally-specific homo-dimer, without being present as higher-order oligomers.

The micelle-associated 3D structures of Boc-Y(SO3)-Nle-G-W-Nle-D-2-phenylethylester (JMV-180) and CCK-8(s) share conformational elements of a calculated CCK1 receptor-bound model

JMV-180 ( 1) and CCK-8(s) are high affinity ligands at the CCK 1 receptor that have similar and different actions via this receptor. Here we calculate the tertiary structure of 1 or CCK-8(s) in the presence of dodecylphosphocholine micelles at pH 5.0 and 35 degrees C from 2D (1)H NMR data recorded at 600 MHz. The NMR derived 3D structures of 1 and CCK-8(s) share a common type I beta-turn around residues Nle3/M3 and G4 and diverge from each other structurally at the N- and C-termini. The fluorescence and circular dichroism spectral properties of these peptides are consistent with their NMR derived structures. The structures determined in the presence of DPC micelles are compared to available models of 1 or CCK-8(s) bound to the CCK 1 receptor. For CCK and 1, these comparisons show that DPC micelle associated structures duplicate some important aspects of the models calculated from cross-linking derived constraints at the CCK 1 receptor.

Extent and diversity of human alternative splicing established by complementary database annotation and microarray analysis

Alternative splicing generates functional diversity in higher organisms through alternative first and last exons, skipped and included exons, intron retentions and alternative donor, and acceptor sites. In large-scale microarray studies in humans and the mouse, emphasis so far has been placed on exon-skip events, leaving the prevalence and importance of other splice types largely unexplored. Using a new human splice variant database and a genome-wide microarray to probes thousands of splice events of each type, we measured differential expression of splice types across six pair of diverse cell lines and validated the database annotation process. Results suggest that splicing in humans is more complex than simple exon-skip events, which account for a minority of splicing differences. The relative frequency of differential expression of the splice types correlates with what is found by our annotation efforts. In conclusion, alternative splicing in human cells is considerably more complex than the canonical example of the exon skip. The complementary approaches of genome-wide annotation of alternative splicing in human and design of genome-wide splicing microarrays to measure differential splicing in biological samples provide a powerful high-throughput tool to study the role of alternative splicing in human biology.

Structural basis of cholecystokinin receptor binding and regulation

Two structurally-related guanine nucleotide-binding protein-coupled receptors for two related peptides, cholecystokinin (CCK) and gastrin, have evolved to exhibit substantial diversity in specificity of ligand recognition, in their molecular basis of binding these ligands, and in their mechanisms of biochemical and cellular regulation. Consistent with this, the CCK1 and CCK2 receptors also play unique and distinct roles in physiology and pathophysiology. The paradigms for ligand recognition and receptor regulation and function are reviewed in this article, and should be broadly applicable to many members of this remarkable receptor superfamily. This degree of specialization is instructive and provides an encouraging basis for the diversity of potential drugs targeting these receptors and their actions that can be developed.

Spatial approximation between secretin residue five and the third extracellular loop of its receptor provides new insight into the molecular basis of natural agonist binding

The amino terminus of class II G protein-coupled receptors plays an important role in ligand binding and receptor activation. Understanding of the conformation of the amino-terminal domain of these receptors has been substantially advanced with the solution of nuclear magnetic resonance and crystal structures of this region of receptors for corticotrophin-releasing factor, pituitary adenylate cyclase-activating polypeptide, and gastric inhibitory polypeptide. However, the orientation of the amino terminus relative to the receptor core and how the receptor gets activated upon ligand binding remain unclear. In this work, we have used photoaffinity labeling to identify a critical spatial approximation between residue five of secretin and a residue within the proposed third extracellular loop of the secretin receptor. This was achieved by purification, deglycosylation, cyanogen bromide cleavage, and sequencing of labeled wild-type and mutant secretin receptors. This constraint has been used to refine our evolving molecular model of secretin docked at the intact receptor, which for the first time includes refined helical bundle and loop regions and reflects a peptide-binding groove within the receptor amino terminus that directs the amino terminus of the peptide toward the receptor body. This model is fully consistent with the endogenous agonist mechanism for class II G protein-coupled receptor activation, where ligand binding promotes the interaction of a portion of the receptor amino terminus with the receptor body to activate it.

Pattern of intra-family hetero-oligomerization involving the G-protein-coupled secretin receptor

Oligomerization of G-protein-coupled receptors (GPCRs) is emerging as a mechanism for regulation and functional modification, although it has been studied most extensively for Family A receptors. Family B receptors have clear structural differences from Family A. In this paper, we have systematically evaluated GPCRs that are capable of association with the prototypic Family B secretin receptor. All of the receptor constructs were shown to traffic normally to the plasma membrane. We utilized receptor bioluminescence resonance energy transfer (BRET) to determine the presence of constitutive and ligand-dependent receptor association. Extensive intra-family and no cross-family association was observed. Of the nine Family B receptors studied, all constitutively yielded a significant BRET signal with the secretin receptor, except for the calcitonin receptor. Each of the associating hetero-oligomeric receptor pairs generated a BRET signal of similar intensity, less than that of homo-oligomeric secretin receptors. BRET signals from some receptor pairs were reduced by ligand occupation, but none were increased by this treatment. Thus, Family B GPCR oligomerization occurs, with many structurally related members associating with each other. The specific functional implications of this need to be further evaluated.

Wild-type and splice-variant secretin receptors in lung cancer: overexpression in carcinoid tumors and peritumoral lung tissue

Gastrointestinal peptide hormone receptors, like somatostatin receptors, are often overexpressed in human cancer, allowing receptor-targeted tumor imaging and therapy. A novel candidate for these applications is the secretin receptor recently identified in pancreatic and cholangiocellular carcinomas. In the present study, secretin receptors were assessed in a non-gastrointestinal tissue, the human lung. Non-small-cell lung cancers (n=26), small-cell lung cancers (n=10), bronchopulmonary carcinoid tumors (n=29), and non-neoplastic lung (n=46) were investigated for secretin receptor protein expression with in vitro receptor autoradiography, using (125)I-[Tyr(10)] rat secretin and for secretin receptor transcripts with RT-PCR. Secretin receptor protein expression was found in 62% of bronchopulmonary carcinoids in moderate to high density, in 12% of non-small cell lung cancers in low density, but not in small cell lung cancers. In tumors found to be secretin receptor positive by autoradiography, RT-PCR revealed transcripts for the wild-type secretin receptor and for novel secretin receptor splice variants. In the non-neoplastic lung, secretin receptor protein expression was observed in low density along the alveolar septa in direct tumor vicinity in cases of acute inflammation, but not in histologically normal lung. In the autoradiographically positive peritumoral lung, RT-PCR showed transcripts for the wild-type secretin receptor and for a secretin receptor spliceoform different from those occurring in lung and gut tumors. In conclusion, secretin receptors are new markers for bronchopulmonary carcinoid tumors, and represent the molecular basis for an in vivo targeting of carcinoid tumors for diagnosis and therapy. Furthermore, secretin receptors may play a role in peritumoral lung pathophysiology. Secretin receptor mis-splicing specifically occurs in tumor and non-tumor lung pathology.

Insights into the structural basis of endogenous agonist activation of family B G protein-coupled receptors

Agonist drugs targeting the glucagon-like peptide-1 (GLP1) receptor represent important additions to the clinical management of patients with diabetes mellitus. In the current report, we have explored whether the recently described concept of a receptor-active endogenous agonist sequence within the amino terminus of the secretin receptor may also be applicable to the GLP1 receptor. If so, this could provide a lead for the development of additional small molecule agonists targeting this and other important family members. Indeed, the region of the GLP1 receptor analogous to that containing the active WDN within the secretin receptor was found to possess full agonist activity at the GLP1 receptor. The minimal fragment within this region that had full agonist activity was NRTFD. Despite having no primary sequence identity with the WDN, it was also active at the secretin receptor, where it had similar potency and efficacy to WDN, suggesting common structural features. Molecular modeling demonstrated that an intradomain salt bridge between the side chains of arginine and aspartate could yield similarities in structure with cyclic WDN. This directly supports the relevance of the endogenous agonist concept to the GLP1 receptor and provides new insights into the rational development and refinement of new types of drugs activating this important receptor.

Distinctive heavy metal composition of pancreatic juice in patients with pancreatic carcinoma

Epidemiologic studies have shown the health risks of exposure to cigarette smoke and air pollution, with heavy metal composition implicated as contributing to both. Environmental exposure to cigarette smoke has been epidemiologically associated with pancreatic cancer, but the pathophysiologic basis for this is not yet clear. In the current work, we have used inductively coupled plasma mass spectrometry to quantify the metal composition of pancreatic juice collected in response to secretin stimulation in successive patients evaluated for abdominal pain (35 with pancreatic cancer, 30 with chronic pancreatitis, and 35 with normal pancreas). Indeed, metal composition of pancreatic juice was distinctive in patients with pancreatic cancer relative to those without such a cancer. The metal concentrations that were found to have the strongest association with pancreatic cancer were chromium, selenium, and molybdenum, with 1 SD increases in the concentrations of each associated with substantial increases in the odds of having pancreatic cancer relative to those in patients with normal pancreas (210%, 160%, and 76%, respectively). Of note, elevations in concentrations of chromium and selenium did not correlate in individuals, whereas those having a 1 SD increase in the sum of the concentrations of these two metals in their pancreatic juice had a 480% increase in the odds of having pancreatic cancer. Elevations of nickel and zinc correlated with elevated chromium in individuals, with each of these metals known to be present in cigarette smoke, whereas other recognized metal components of cigarette smoke were not elevated. An understanding of why these metals are elevated in pancreatic juice and what effects they might have on pancreatic cells may have important implications for the diagnosis, treatment, and even prevention of pancreatic cancer.

Mutational analysis of predicted intracellular loop domains of human motilin receptor

Motilin is an important endogenous regulator of gastrointestinal motor function, mediated by the class I G protein-coupled motilin receptor. Motilin and erythromycin, two chemically distinct full agonists of the motilin receptor, are known to bind to distinct regions of this receptor, based on previous systematic mutagenesis of extracellular regions that dissociated the effects on these two agents. In the present work, we examined the predicted intracellular loop regions of this receptor for effects on motilin- and erythromycin-stimulated activity. We prepared motilin receptor constructs that included sequential deletions throughout the predicted first, second, and third intracellular loops, as well as replacing the residues in key regions with alanine, phenylalanine, or histidine. Each construct was transiently expressed in COS cells and characterized for motilin- and erythromycin-stimulated intracellular calcium responses and for motilin binding. Deletions of receptor residues 63-66, 135-137, and 296-301 each resulted in substantial loss of intracellular calcium responses to stimulation by both motilin and erythromycin. Constructs with mutations of residues Tyr66, Arg136, and Val299 were responsible for the negative impact on biological activity stimulated by both agonists. These data suggest that action by different chemical classes of agonists that are known to interact with distinct regions of the motilin receptor likely yield a common activation state of the cytosolic face of this receptor that is responsible for interaction with its G protein. The identification of functionally important residues in the predicted cytosolic face provides strong candidates for playing roles in receptor-G protein interaction.

Fluorescence Resonance Energy Transfer Analysis of Secretin Docking to Its Receptor

Full structural characterization of G protein-coupled receptors has been limited to rhodopsin, with its uniquely stable structure and ability to be crystallized. For other members of this important superfamily, direct structural insights have been limited to NMR structures of soluble domains. Two members of the Class II family have recently had the structures of their isolated amino-terminal regions solved by NMR, yet it remains unclear how that domain is aligned with the heptahelical transmembrane bundle domain of those receptors. Indeed, three distinct orientations have been suggested for different members of this family. In the current work, we have utilized fluorescence resonance energy transfer to establish the distances between four residues distributed throughout fully biologically active, high affinity analogues of secretin and distinct residues in each of four extracellular regions of the intact secretin receptor. These 16 distance constraints were utilized along with nine photoaffinity labeling spatial approximation constraints to study the three proposed orientations of the peptide-binding amino terminus and helical bundle domains of this receptor. In the best model, the carboxyl terminus of secretin was found to bind in a groove above the beta-hairpin region of the receptor amino terminus, with its amino-terminal end adjacent to the third extracellular loop and top of transmembrane segment VI. This refined model of the intact receptor was also fully consistent with the spatial approximation of the Trp(48)-Asp(49)-Asn(50) endogenous agonist segment with the third extracellular loop region that it has been shown to photolabel. This provides strong evidence for the orientation of peptide-binding and signaling domains of a prototypic Class II G protein-coupled receptor.

Structural basis of natural ligand binding and activation of the Class II G-protein-coupled secretin receptor

The secretin receptor is prototypic of Class II GPCRs (G-protein-coupled receptors), based on its structural and functional characteristics and those of its natural agonist ligand. Secretin represents a linear 27-residue peptide with diffuse pharmacophoric domain. The secretin receptor includes the typical signature sequences for this receptor family within its predicted transmembrane segments and the highly conserved six cysteine residues contributing to three intradomain disulfide bonds within its long N-terminus. This domain is critical for secretin binding based on receptor mutagenesis and photoaffinity labelling studies. Full agonist analogues of secretin incorporating a photolabile moiety at various positions throughout the pharmacophore covalently label residues within this region, while only N-terminal probes have labelled the core helical bundle domain. Combining insights coming from receptor structural studies, peptide structure-activity relationship considerations, photoaffinity labelling, and application of fluorescence techniques has resulted in the development of a working model of the secretin-receptor complex. This supports the initial docking of the peptide agonist within a cleft in the receptor N-terminus, providing the opportunity for an endogenous sequence within that domain to interact with the core of the receptor. This interaction is believed to be key in the molecular basis of conformational change associated with activation of this receptor. The site of action of this endogenous agonist could also provide a possible target for small molecule agonists to act.

A novel secretin receptor splice variant potentially useful for early diagnosis of pancreatic carcinoma

BACKGROUND & AIMS

Pancreatic and bile duct carcinomas represent highly aggressive malignancies that evolve from secretin receptor-rich ductular cells. With premessenger RNA splicing abnormalities common in cancer, we evaluated whether an abnormal secretin receptor spliceoform were present, characterized it, and developed a serum assay for it.

METHODS

Cancer cell lines and healthy and neoplastic tissue were studied by nested reverse-transcription polymerase chain reaction and sequencing. A promising spliceoform was isolated and characterized, and monoclonal antibodies were raised to 2 distinct regions. A dual antibody enzyme-linked immunosorbent assay was developed and applied to blinded serum samples from 26 patients with pancreatic carcinoma, 10 patients with chronic pancreatitis, and 14 controls.

RESULTS

Each of 9 pancreatic cancer specimens and no normal tissue expressed a secretin receptor variant with exons 3 and 4 deleted. This encoded a 111-residue peptide with its first 43 residues identical to wild-type receptor, but, subsequent to a shift in coding frame and early truncation, the next 68 residues were unique in the transcriptome/proteome. This nonfunctional soluble protein did not bind or signal in response to secretin and was secreted from transfected MiaPaCa-2 cells. Elevated serum levels of this variant were present in 69% of pancreatic cancer patients, 60% of chronic pancreatitis patients, and 1 of 14 controls.

CONCLUSIONS

We identified a novel abnormal spliceoform of the secretin receptor in pancreatic and bile duct cancers and developed a dual antibody sandwich enzyme-linked immunosorbent assay to measure it in the circulation. Initial application of this assay in patients with pancreatic cancer and chronic pancreatitis was promising, but additional validation will be required to evaluate its clinical utility.

Transmembrane segment IV contributes a functionally important interface for oligomerization of the Class II G protein-coupled secretin receptor

Oligomerization of the Class II G protein-coupled secretin receptor has been reported, but the molecular basis for this and its functional significance have not been determined. In the current work, we have examined the possible contribution of each of the transmembrane (TM) segments of this receptor to its homo-oligomerization, using the method of competitive disruption screening for inhibition of receptor bioluminescence resonance energy transfer signal. TM IV was the only segment that was found to disrupt receptor bioluminescence resonance energy transfer. Evaluation of predicted interhelical and lipid-exposed faces of this TM segment demonstrated that its lipid-exposed face represented the determinant for oligomerization. This was further confirmed by mutagenesis of the intact secretin receptor. Morphological FRET was utilized to demonstrate that secretin receptor oligomerization occurred at the cell surface and that this oligomerization was disrupted by mutating Gly(243) and Ile(247), key residues within the lipid-exposed face of TM IV. Although disruption of the receptor oligomerization interface had no effect on secretin binding parameters, it reduced the ability of secretin to stimulate intracellular cAMP. This supports a clear functional effect of oligomerization of this receptor. Such an effect might be particularly relevant to clinical situations in which this receptor is overexpressed, such as in certain neoplasms.

Biochemical and Cell Biological Mechanisms of Cholecystokinin Receptor Regulation

Regulation of the cholecystokinin receptor is accomplished by biochemical and cell biological mechanisms. The major mechanism for biochemical regulation involves phosphorylation of serine and threonine residues within the receptor's intracellular third loop and carboxyl-terminal tail. This form of rapid desensitization is achieved by protein kinase C, a kinase activated in the normal signaling cascade of this Gq-coupled receptor, and/or a member of the G protein-coupled receptor kinase family that recognizes the active conformation of the receptor. Conversely, a type 2A serine protein phosphatase has been shown to selectively act on this receptor in an agonist-regulated manner, resulting in receptor dephosphorylation and resensitization. Cell biological mechanisms for desensitization involve the net removal of receptors from exposure to circulating hormone via insulation within a specialized plasma membrane domain or internalization into the cell within endocytic compartments. Internalization has been shown to occur by both clathrin-dependent and clathrin-independent mechanisms, the latter believed to represent potocytosis in caveolae, that lead to recycling of receptors to the plasma membrane for resensitization or shuttling of receptors to lysosomes for degratory down-regulation. Interestingly, receptor phosphorylation has been shown to affect intracellular domain accessibility and receptor trafficking, although the specific proteins regulating this have not yet been identified. The cholecystokinin receptor can exist in the plasma membrane as oligomeric superstructures, namely as homomers with themselves or as heteromers with closely related class I G protein-coupled receptors. Agonist occupation results in oligomer dissociation, but the functional significance of this observation has yet to be defined.

Mapping the architecture of secretin receptors with intramolecular fluorescence resonance energy transfer using acousto-optic tunable filter-based spectral imaging

The molecular structure and agonist-induced conformational changes of class II G protein-coupled receptors are poorly understood. In this work, we developed and characterized a series of dual cyan fluorescent protein (CFP)-tagged and yellow fluorescent protein (YFP)-tagged secretin receptor constructs for use in various functional and fluorescence analyses of receptor structural variants. CFP insertions within the first or second intracellular loop domains of this receptor were tolerated poorly or partially, respectively, in receptors tagged with a carboxyl-terminal yellow fluorescent protein that itself had no effect on secretin binding or cAMP production. A similar CFP insertion into the third intracellular loop resulted in a plasma membrane-localized receptor that bound secretin and signaled normally. This fully active third-loop variant exhibited a significant decrease in fluorescence resonance energy transfer signals that were recorded with an acousto-optic tunable filter microscope after exposure to secretin agonist but not to a receptor antagonist. These data demonstrate changes in the relative positions of intracellular structures that support a model for secretin receptor activation.

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.