A Robust Platform for the Molecular Design of Potent, Protease-Stable, Long-Acting GIP Analogues

Glucose-dependent insulinotropic peptide (GIP) is a 42-amino acid peptide hormone that regulates postprandial glucose levels. GIP binds to its cognate receptor, GIPR, and mediates metabolic physiology by improved insulin sensitivity, β-cell proliferation, increased energy consumption, and stimulated glucagon secretion. Dipeptidyl peptidase-4 (DPP4) catalyzes the rapid inactivation of GIP within 6 min in vivo. Here, we report a molecular platform for the design of GIP analogues that are refractory to DPP4 action and exhibit differential activation of the receptor, thus offering potentially hundreds of GIP-based compounds to fine-tune pharmacology. The lead compound from our studies, which harbored a combination of N-terminal alkylation and side-chain lipidation, was equipotent and retained full efficacy at GIPR as the native peptide, while being completely refractory toward DPP4, and was resistant to trypsin. The GIP analogue identified from these studies was further evaluated in vivo and is one of the longest-acting GIPR agonists to date.

A Non-Perturbative Molecular Grafting Strategy for Stable and Potent Therapeutic Peptide Ligands

The gut-derived incretin hormone, glucagon-like peptide-1 (GLP1), plays an important physiological role in attenuating post-prandial blood glucose excursions in part by amplifying pancreatic insulin secretion. Native GLP1 is rapidly degraded by the serine protease, dipeptidyl peptidase-4 (DPP4); however, enzyme-resistant analogues of this 30-amino-acid peptide provide an effective therapy for type 2 diabetes (T2D) and can curb obesity via complementary functions in the brain. In addition to its medical relevance, the incretin system provides a fertile arena for exploring how to better separate agonist function at cognate receptors versus susceptibility of peptides to DPP4-induced degradation. We have discovered that novel chemical decorations can make GLP1 and its analogues completely DPP4 resistant while fully preserving GLP1 receptor activity. This strategy is also applicable to other therapeutic ligands, namely, glucose-dependent insulinotropic polypeptide (GIP), glucagon, and glucagon-like peptide-2 (GLP2), targeting the secretin family of receptors. The versatility of the approach offers hundreds of active compounds based on any template that target these receptors. These observations should allow for rapid optimization of pharmacological properties and because the appendages are in a position crucial to receptor stimulation, they proffer the possibility of conferring "biased" signaling and in turn minimizing side effects.

Mutation-Induced Functional Alterations of CCR6

The Cys-Cys chemokine receptor 6 (CCR6) is a well-established modulator of inflammation. Although several genetic associations have been identified between CCR6 polymorphisms and immune system disorders (e.g., rheumatoid arthritis and Crohn's disease), the pharmacological effects of naturally occurring missense mutations in this receptor have yet to be characterized. In this study, we initially assessed G protein-mediated signaling and observed that wild-type (WT) CCR6 exhibited ligand-independent activity. In addition, we found that the five most frequent CCR6 missense variants (A89T, A150V, R155W, G345S, and A369V) exhibited decreased basal and/or ligand induced Gαi protein signaling. To complement the study of these loss-of-function variants, we engineered a set of constitutively active CCR6 receptors. Selected mutations enhanced basal G protein-mediated signaling up to 3-fold relative to the WT value. Using a bioluminescence resonance energy transfer assay we investigated the ability of each naturally occurring and engineered CCR6 receptor mutant to recruit β-arrestin. In contrast to G protein-mediated signaling, β-arrestin mobilization was largely unperturbed by the naturally occurring loss-of-function CCR6 variants. Elevated recruitment of β-arrestin was observed in one of the engineered constitutively active mutants (T98P). Our results demonstrate that point mutations in CCR6 can result in either a gain or loss of receptor function. These observations underscore the need to explore how CCR6 natural variants may influence immune cell physiology and human disease.

Membrane tethered bursicon constructs as heterodimeric modulators of the Drosophila G protein-coupled receptor rickets

The study of complex heterodimeric peptide ligands has been hampered by a paucity of pharmacological tools. To facilitate such investigations, we have explored the utility of membrane tethered ligands (MTLs). Feasibility of this recombinant approach was explored with a focus on Drosophila bursicon, a heterodimeric cystine-knot protein that activates the G protein-coupled receptor rickets (rk). Rk/bursicon signaling is an evolutionarily conserved pathway in insects required for wing expansion, cuticle hardening, and melanization during development. We initially engineered two distinct MTL constructs, each composed of a type II transmembrane domain, a peptide linker, and a C terminal extracellular ligand that corresponded to either the α or β bursicon subunit. Coexpression of the two complementary bursicon MTLs triggered rk-mediated signaling in vitro. We were then able to generate functionally active bursicon MTLs in which the two subunits were fused into a single heterodimeric peptide, oriented as either α-β or β-α. Carboxy-terminal deletion of 32 amino acids in the β-α MTL construct resulted in loss of agonist activity. Coexpression of this construct with rk inhibited receptor-mediated signaling by soluble bursicon. We have thus generated membrane-anchored bursicon constructs that can activate or inhibit rk signaling. These probes can be used in future studies to explore the tissue and/or developmental stage-dependent effects of bursicon in the genetically tractable Drosophila model organism. In addition, our success in generating functionally diverse bursicon MTLs offers promise that such technology can be broadly applied to other complex ligands, including the family of mammalian cystine-knot proteins.

Naturally occurring HCA1 missense mutations result in loss of function: potential impact on lipid deposition

The hydroxy-carboxylic acid receptor (HCA1) is a G protein-coupled receptor that is highly expressed on adipocytes and considered a potential target for the treatment of dyslipidemia. In the current study, we investigated the pharmacological properties of naturally occurring variants in this receptor (H43Q, A110V, S172L, and D253H). After transient expression of these receptors into human embryonic kidney 293 cells, basal and ligand-induced signaling were assessed using luciferase reporter gene assays. The A110V, S172L, and D253 variants showed reduced basal activity; the S172L mutant displayed a decrease in potency to the endogenous ligand L-lactate. Both the S172L and D253H variants also showed impaired cell surface expression, which may in part explain the reduced activity of these receptors. The impact of a loss in HCA1 function on lipid accumulation was investigated in the adipocyte cell line, OP9. In these cells, endogenous HCA1 transcript levels rapidly increased and reached maximal levels 3 days after the addition of differentiation media. Knockdown of HCA1 using siRNA resulted in an increase in lipid accumulation as assessed by quantification of Nile Red staining and TLC analysis. Our data suggest that lipid homeostasis may be altered in carriers of selected HCA1 missense variants.

Selected melanocortin 1 receptor single-nucleotide polymorphisms differentially alter multiple signaling pathways

The melanocortin 1 receptor (MC1R) is a highly polymorphic G protein-coupled receptor, which is known to modulate pigmentation and inflammation. In the current study, we investigated the pharmacological effects of select single-nucleotide polymorphisms (SNPs) (V60L, R163Q, and F196L). After transient expression of MC1Rs in human embryonic kidney 293 cells, basal and ligand-induced cAMP signaling and mitogen-activated protein kinase (MAPK) activation were assessed by using luciferase reporter gene assays and Western blot analysis, respectively. All receptor variants showed decreased basal cAMP activity. With the V60L and F196L variants, the decrease in constitutive activity was attributable, at least in part, to a reduction in surface expression. The F196L variant also displayed a significant reduction in potency for both the peptide agonist α-melanocyte-stimulating hormone (α-MSH) and the small-molecule agonist 1-[1-(3-methyl-L-histidyl-O-methyl-D-tyrosyl)-4-phenyl-4-piperidinyl]-1-butanone (BMS-470539). In MAPK signaling assays, the F196L variant showed decreased phospho-extracellular signal-regulated kinase levels after stimulation with either α-MSH or BMS-470539. In contrast, the R163Q variant displayed a selective loss of α-MSH-induced MAPK activation; whereas responsiveness to the small-molecule agonist BMS-470539 was preserved. Further assessment of MC1R variants in A549 cells, an in vitro model of inflammation, revealed an enhanced inflammatory response resulting from expression of the F196L variant (versus the wild-type MC1R). This alteration in function was restored by treatment with BMS-470539. Overall, these studies illustrate novel signaling profiles linked to distinct MC1R SNPs. Furthermore, our investigations highlight the potential for small-molecule drugs to rescue the function of MC1R variants that show reduced basal and/or α-MSH stimulated activity.

Two naturally occurring mutations in the type 1 melanin-concentrating hormone receptor abolish agonist-induced signaling

The melanin-concentrating hormone (MCH) receptor type 1 (MCHR1) is a seven-transmembrane domain protein that modulates orexigenic activity of MCH, the corresponding endogenous peptide agonist. MCH antagonists are being explored as a potential treatment for obesity. In the current study, we examined the pharmacological impact of 11 naturally occurring mutations in the human MCHR1. Wild-type and mutant receptors were transiently expressed in human embryonic kidney 293 cells. MCHR1-mediated, Gα(i)-dependent signaling was monitored by using luciferase reporter gene assays. Two mutants, R210H and P377S, failed to respond to MCH. Five other variants showed significant alterations in MCH efficacy, ranging from 44 to 142% of the wild-type value. At each of the MCH-responsive mutants, agonist potency and inhibition by (S)-methyl 3-((3-(4-(3-acetamidophenyl)piperidin-1-yl)propyl)carbamoyl)-4-(3,4-difluorophenyl)-6-(methoxymethyl)-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (SNAP-7941), an established MCHR1 small-molecule antagonist, were similar to wild type. To explore the basis for inactivity of the R210H and P377S mutants, we examined expression levels of these receptors. Assessment by enzyme-linked immunosorbent assay revealed that cell surface expression of both nonfunctional receptors was comparable with wild type. Overnight treatment with SNAP-7941, followed by washout of antagonist, enhanced MCH induced signaling by the wild-type receptor and restored MCH responsiveness of the P377S but not the R210H variant. It is of note that the two loss-of-function mutants were identified in markedly underweight individuals, raising the possibility that a lean phenotype may be linked to deficient MCHR1 signaling. Formal association studies with larger cohorts are needed to explore the extent to which signaling-deficient MCHR1 variants influence the maintenance of body weight.

The μ-opioid receptor variant N190K is unresponsive to peptide agonists yet can be rescued by small-molecule drugs

The μ-opioid receptor (MOR) plays an important role in modulating analgesia, feeding behavior, and a range of autonomic functions. In the current study, we investigated the degree to which 13 naturally occurring missense mutations affect the pharmacological properties of the human MOR. After expression of each receptor in human embryonic kidney 293 cells, signaling (Gα(i/o)-mediated) induced by peptide agonists was assessed using luciferase reporter gene assays. Multiple mutants (S66F, S147C, R260H, R265C, R265H, and S268P) show a significant reduction in agonist potency. At the N190K variant, agonist-mediated signaling was essentially absent. Enzyme-linked immunosorbent assay, microscopic analysis, and radioligand binding assays revealed that this mutant shows markedly reduced cell-surface expression, whereas all other receptor variants were expressed at normal levels. Surface expression of the N190K variant could be increased by incubation with the alkaloid agonist buprenorphine or with either naltrexone or naloxone, structurally related MOR antagonists. We were surprised to find that both putative antagonists, despite being inactive at the wild-type MOR, triggered a concentration-dependent increase in N190K receptor-mediated signaling. In contrast, peptidic ligands failed to promote expression or rescue function of the N190K mutant. Subsequent analysis of the N190K variant in an ethnically diverse cohort identified this isoform in a subgroup of African Americans. Taken together, our studies reveal that the N190K mutation leads to severe functional alterations and, in parallel, changes the response to established MOR ligands. The extent to which this mutation results in physiological abnormalities or affects drug sensitivity in selected populations (e.g., those with chronic pain or addiction) remains to be investigated.

Neurogenin 3-specific dipeptidyl peptidase-2 deficiency causes impaired glucose tolerance, insulin resistance, and visceral obesity

The control of glucose metabolism is a complex process, and dysregulation at any level can cause impaired glucose tolerance and insulin resistance. These two defects are well-known characteristics associated with obesity and onset of type 2 diabetes. Here we introduce the N-terminal dipeptidase, DPP2, as a novel regulator of the glucose metabolism. We generated mice with a neurogenin 3 (NGN3)-specific DPP2 knockdown (kd) to explore a possible role of DPP2 in maintaining metabolic homeostasis. These mice spontaneously developed hyperinsulinemia, glucose intolerance, and insulin resistance by 4 months of age. In addition, we observed an increase in food intake in DPP2 kd mice, which was associated with a significant increase in adipose tissue mass and enhanced liver steatosis but no difference in body weight. In accordance with these findings, the mutant mice had a higher rate of respiratory exchange than the control littermates. This phenotype was exacerbated with age and when challenged with a high-fat diet. We report, for the first time, that DPP2 enzyme activity is essential for preventing hyperinsulinemia and maintaining glucose homeostasis. Interestingly, the phenotype of NGN3-DPP2 kd mice is opposite that of DPP4 knockout mice with regard to glucose metabolism, namely the former have normal glucagon-like peptide 1 levels but present with glucose intolerance, whereas the latter have increased glucagon-like peptide 1, which is accompanied by augmented glucose tolerance.

Pharmacological characterization of human incretin receptor missense variants

Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gut-derived incretin hormones that regulate blood glucose levels. In addition to their widely accepted insulinotropic role, there is evidence that GLP-1 modulates feeding behavior and GIP regulates lipid metabolism, thereby promoting postprandial fat deposition. In this study, we investigated whether naturally occurring polymorphisms in the GLP-1 receptor (GLP-1R) and the GIP receptor (GIP-R) affect the pharmacological properties of these proteins. After transient expression of the receptors in human embryonic kidney 293 cells, basal and ligand-induced cAMP production were assessed by use of luciferase reporter gene assays. Our data reveal that the wild-type GIP-R displays a considerable degree of ligand-independent activity. In comparison, the GIP-R variants C46S, G198C, R316L, and E354Q show a marked decrease in basal signaling that may, at least in part, be explained by reduced cell surface expression. When stimulated with GIP, the C46S and R316L mutants display significantly reduced potency (>1000 and 25- fold, respectively) compared with wild type. Complementary competition binding assays further demonstrate that the C46S variant fails to bind radio-iodinated GIP, whereas all other GIP-R mutants maintain normal ligand affinity. In contrast to the GIP-R, the wild-type GLP-1R lacks constitutive activity. Furthermore, none of the 10 GLP-1R missense mutations showed an alteration in pharmacological properties versus wild type. The extent to which abnormalities in GIP-R function may lead to physiological changes or affect drug sensitivity in selected populations (e.g., obese, diabetic individuals) remains to be further investigated.

Influence of selective fluorination on the biological activity and proteolytic stability of glucagon-like peptide-1

The relative simplicity and high specificity of peptide therapeutics has fueled recent interest. However, peptide and protein drugs generally require injection and suffer from low metabolic stability. We report here the design, synthesis, and characterization of fluorinated analogues of the gut hormone peptide, GLP-1. Overall, fluorinated GLP-1 analogues displayed higher proteolytic stability with simultaneous retention of biological activity (efficacy). Fluorinated amino acids are useful for engineering peptide drug candidates and probing ligand-receptor interactions.

Search for alpha-helical propensity in the receptor-bound conformation of glucagon-like peptide-1

To elucidate the receptor-bound conformation of glucagon-like peptide-1 (GLP-1), a series of conformationally constrained GLP-1 analogues were synthesized by introducing lactam bridges between Lys(i) and Glu(i)(+4) to form alpha-helices at various positions. The activity and affinity of these analogues to GLP-1 receptors suggested that the receptor-bound conformation comprises two alpha-helical segments between residues 11-21 and 23-34. It is notable that the N-terminal alpha-helix is extended to Thr(11), and that Gly(22) plays a pivotal role in arranging the two alpha-helices. Based on these findings, a highly potent bicyclic GLP-1 analogue was synthesized which is the most conformationally constrained GLP-1 analogue reported to date.

Four Missense Mutations in the Ghrelin Receptor Result in Distinct Pharmacological Abnormalities

The growth hormone secretagogue receptor (GHSR) plays an important role in regulating food intake and energy homeostasis. In this study, we compared the pharmacological properties of four reported variants of the human GHSR (I134T, V160M, A204E, and F279L) with those of the wild-type receptor. Corresponding recombinant receptors were transiently expressed in either human embryonic kidney 293 or COS-7 cells. Basal as well as ligand-induced signaling was assessed by luciferase reporter gene assays and measurement of inositol phosphate production. In addition, receptor expression levels were monitored by whole-cell enzyme-linked immunosorbent assay. Ligand-independent signaling of the wild-type GHSR is significantly reduced with introduction of either the V160M or F279L substitutions, whereas basal activity of the A204E mutant is not detectable. Ghrelin potency is markedly increased at the V160M mutant, whereas the I134T variant is unresponsive to this endogenous agonist. In contrast, the I134T mutant responds to a known GHSR inverse agonist, [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P (SP-analog), albeit with reduced efficacy. Activity of the SP-analog at the V160M and F279L mutants is comparable to the wild type (WT) value. The overall expression level of each of the four GHSR variants is reduced relative to WT; however, the ratio between the intracellular and plasma membrane receptor density remains comparable. Treatment with the SP-analog significantly increases cell surface expression of each receptor with the exception of the A204E variant. Taken together, our studies reveal that naturally occurring GHSR mutations affect a wide range of pharmacologic properties. The physiological impact of these alterations within selected populations (e.g., obese, lean individuals) as well as the pharmacogenomic consequences of corresponding mutations remain to be further investigated.

Identification of amino acid determinants of dopamine 2 receptor synthetic agonist function

The human dopamine 2 receptor (hD2R) modulates locomotor activity, hormone secretion, and neuropsychiatric function. Current knowledge of the hD2R structure is in large part derived from mutagenesis studies and molecular pharmacologic analysis together with homology modeling using bovine rhodopsin as a template. In this study, we utilized comparison of the Drosophila D2-like receptor (DD2R) with the hD2R as a novel approach for identifying candidate amino acids that are determinants of ligand potency and/or efficacy. We focused our studies on four dopaminergic ligands that are used in the treatment of Parkinson's disease: bromocriptine, pergolide, piribedil, and ropinirole. All four ligands are potent agonists at the wild-type hD2R, whereas only bromocriptine shows comparable function at the DD2R. We performed site-directed mutagenesis to replace hD2R amino acids (modeled to project into the ligand binding pocket) with corresponding fly residues, and vice versa. Substitution of three amino acids in the hD2R with the homologous DD2R residues (V91A, C118S, and L170I) led to a pronounced loss of pergolide potency and efficacy. A converse triple amino acid substitution of human residues into the fly receptor (DD2R-A133V/S160C/I211L) markedly enhanced pergolide efficacy and potency at the mutant DD2R. The same substitutions also converted piribedil and ropinirole, which lacked appreciable activity on the DD2R, to partial agonists. These findings show the important role of these three residues in drug-receptor interactions. Our study illustrates that comparison of a mammalian receptor with an invertebrate homolog complements previously described strategies for defining G protein-coupled receptor structure-function relationships.

Point mutations in either subunit of the GABAB receptor confer constitutive activity to the heterodimer

The GABA receptor (GABABR) is a class C G protein-coupled receptor (GPCR) that functions as an obligate heterodimer, composed of two heptahelical subunits, GABABR subunit 1 (R1) and GABABR subunit 2 (R2). In this study, we generated and pharmacologically characterized constitutively active GABABR mutants as novel tools to explore the molecular mechanisms underlying receptor function. A single amino acid substitution, T290K, in the R1 agonist binding domain results in ligand-independent signaling when this mutant subunit is coexpressed with wild-type R2. Introduction of a Y690V mutation in the putative G protein-coupling domain of R2 is sufficient to confer moderate constitutive activity when this subunit is expressed alone. Activity of the Y690V mutant can be markedly enhanced with coexpression of wild-type R1. Coexpression of both mutant subunits (R1-T290K and R2-Y690K) leads to a further increase in basal signaling. Potencies of the full agonists R-(+)-beta-(aminomethyl)-4-chlorobenzenepropanoic acid hydrochloride (baclofen) and GABA are increased at the constitutively active versus the corresponding wild-type receptors. The mutant GABABR variants provided a sensitive probe enabling detection of inverse or partial agonist activity of molecules previously considered neutral antagonists. Our studies using constitutively active isoforms provide independent support for a model of GABABR function that takes into account 1) ligand binding by R1, 2) signal transduction by R2, and 3) modulation of R2-induced function by R1. Furthermore, we demonstrate that certain hallmark features of constitutive activity as originally established with class A GPCRs (e.g., enhanced agonist potency and affinity), are more generally applicable, as suggested by our finding with a class C heterodimeric receptor.

Class B GPCRs: a hidden agonist within?

Class B G protein-coupled receptors (GPCRs) regulate a wide range of endocrine and neuroendocrine functions and are endogenously stimulated by moderately large peptide hormones. Current evidence suggests that the carboxyl termini of cognate peptides bind to the amino terminus of their G protein-coupled receptors (GPCRs) and that the peptides' amino terminal segments then dock to the heptahelical receptor portion to induce signaling. In this issue of Molecular Pharmacology, Dong et al. (p. 206) propose an alternative model of ligand-induced class B GPCR activation. Based primarily on studies with the secretin receptor, a prototype class B family member, they provide evidence that the endogenous peptide hormone does not function as an activator per se. Instead, this hormone (secretin) exposes a hidden, built-in agonist epitope that is present within the amino terminus of its target GPCR. Isolated oligopeptide fragments containing this epitope act as full agonists on the secretin receptor despite their lack of amino acid homology with the secretin hormone. These nonconventional agonists can be minimized to tripeptide molecules and still maintain biological activity. The study to be discussed introduces a novel paradigm of class B GPCR function, and may facilitate the elusive goal of finding small molecule agonist drugs for this therapeutically attractive group of receptors.

A human glucagon-like peptide-1 receptor polymorphism results in reduced agonist responsiveness

Glucagon-like peptide-1 (GLP-1) and its cognate receptor play an important physiological role in maintaining blood glucose homeostasis. A GLP-1 receptor (GLP-1R) polymorphism in which threonine 149 is substituted with a methionine residue has been recently identified in a patient with type 2 diabetes but was not found in non-diabetic control subjects. We have functionally assessed the recombinant GLP-1R variant after transient expression in COS-7 and HEK 293 cells. Compared to the wild type receptor, the variant GLP-1R showed (i) similar expression levels, (ii) 60-and 5-fold reduced binding affinities, respectively, for two GLP-1R full agonists, GLP-1 and exendin-4, and (iii) markedly decreased potencies of these peptides in triggering cAMP-mediated signaling (despite conserved efficacies). In contrast to full agonists, the efficacy of the primary GLP-1 metabolite/GLP-1R partial agonist, GLP-1 (9-36) amide, was essentially abolished by the T149M substitution. By hydropathy analysis, the polymorphism localizes to transmembrane domain 1, suggesting this receptor segment as a novel determinant of agonist affinity/efficacy. These findings reveal that naturally occurring sequence variability of the GLP-1R within the human population can result in substantial loss-of-function. A genetic link between the T149M variant and increased susceptibility to type 2 diabetes remains to be established.

Ménétrier's disease in a patient with Helicobacter pylori infection is linked to elevated glucagon-like peptide-2 activity

This report focuses on a 59-year-old male Japanese patient with Ménétrier's disease who suffered from severe hypoproteinemia and tested positive for Helicobacter pylori when initially admitted to hospital. Blood levels of intact glucagon-like peptide-2 (GLP-2) were determined by specific bioassay, using serum-induced cAMP production in COS-7 cells expressing recombinant human GLP-2 receptors as a functional readout. Eradication of H. pylori led to remission of Ménétrier's disease as well as a partial yet significant decrease in GLP-2 levels, and also improved hypoproteinemia. These observations suggest a possible link between excess systemic endogenous production of GLP-2, a gut hormone that induces mucosal growth, and the hypertrophic gastropathy in a Ménétrier's disease patient with H. pylori infection.

Basal receptor activation by locally produced glucagon-like peptide-1 contributes to maintaining beta-cell function

Glucagon-like peptide 1 (GLP-1) is a physiological stimulus of pancreatic beta-cell function. This enteroendocrine hormone is produced by intestinal L cells, and is delivered via the bloodstream to GLP-1 receptors (GLP-1Rs) on pancreatic beta-cells. In addition, there is evidence that beta-cell GLP-1Rs maintain sustained basal activity even in the absence of intestinal peptide, an observation that has raised the question whether these receptors have some degree of ligand-independent function. Here, we provide an alternative explanation for basal receptor activity based on our finding that biologically relevant amounts of fully processed GLP-1 are locally generated by insulinoma cell lines, as well as by alpha-cells of isolated rat islets in primary culture. Presence of GLP-1 was established by immunocytochemistry, as well as by selective ELISAs and bioassays of cell supernatants. A GLP-1R antagonist significantly reduced insulin secretion/production in beta-TC-6 insulinoma cells and isolated rat islets, suggesting a functionally important loop between locally produced GLP-1 and its cognate receptor. Treatment with this antagonist also inhibited the growth of beta-TC-6 cells. These observations provide novel insight into the function of insulin-producing cell lines and native beta-cells during in vitro culture, and they support the idea that locally produced GLP-1 may play a role in intra-islet regulation.

Interleukin-1 upregulates anaphylatoxin receptors on mononuclear cells

BACKGROUND

The anaphylatoxins, C3a and C5a, that are generated during trauma, major surgery, or infection are potent proinflammatory mediators that increase interleukin (IL-1) cytokine synthesis. We investigated the effects of IL-1 on anaphylatoxin receptor expression in monocytes.

METHODS

A human monocytic cell line, MONO-MAC-6, was used. C3a and C5a binding sites were assayed by competitive binding. Levels of messenger RNA for the C3a and C5a receptors were analyzed by reverse transcriptase-polymerase chain reaction. Changes of free cytosolic Ca(2+) concentration ([Ca(2+)]i) in response to C3a and C5a were measured.

RESULTS

Basal MONO-MAC-6 cell sites for C3a and C5a binding were 10900 C3aR/cell (K(d)=2.0 nmol/L), 8700 C5aR/cell (K(d)=0.9 nmol/L). IL-1alpha increased sites for both C3a (61% increase; P <.01) and C5a (71% increase; P <.001). Levels of C3aR and C5aR messenger RNA also increased in IL-1alpha-stimulated cells. Receptors were coupled to functional responses, which were demonstrated by C3a- or C5a-induced [Ca(2+)]i increases. IL-1 receptor antagonist blocked the effects of IL-1alpha upregulation of anaphylatoxin receptors.

CONCLUSION

These results suggest that there is an additional link between IL-1 and anaphylatoxins to amplify proinflammatory effects through monocytes and macrophages. Although C3a and C5a can increase the monocyte production of IL-1, IL-1 increases monocyte expression of receptors for these anaphylatoxins, which further amplifies inflammation.

Ligand function at constitutively active receptor mutants is affected by two distinct yet interacting mechanisms

It has been proposed that mutations that induce constitutive activity in G-protein-coupled receptors (GPCRs) concomitantly enhance the ability of partial agonists to trigger second-messenger signaling. Using the cholecystokinin type 2 receptor (CCK-2R) as a model system, we have explored whether this association applies to a diverse set of activating mutations. Consistent with established principles, constitutively active CCK-2Rs resulting from amino acid substitutions within the third intracellular loop each systematically increased partial agonist activities versus corresponding wild-type values. In contrast, activating mutations within transmembrane domain segments near the extracellular loops led to an increase in efficacy of only a subset of compounds but decreased or did not change the function of others. When transmembrane domain amino acid substitutions were introduced in combination with intracellular amplifying mutations, observed changes in ligand activity were defined by the product of two discernible factors 1) systematic amplification caused by an equilibrium shift from the inactive to the active receptor conformation and 2) ligand-specific alterations in signaling, which probably result from mutation-induced changes in the putative binding pocket. These findings illustrate functional heterogeneity among GPCR mutants with ligand-independent signaling. A subgroup of activating mutations facilitates receptor isomerization to the active state and in parallel perturbs ligand receptor interactions. These mutants do not adhere to the previously proposed "hallmark criteria" of constitutive activity.

Established theory of radiation-induced decay is not generalizable to Bolton-Hunter labeled peptides

Peptide hormones radiolabeled with 125I are widely used for the pharmacological characterization of cognate receptors. As a prerequisite for calculating ligand affinities from competition binding assays, and for estimating receptor densities from such studies, it is necessary to know the concentration of bioactive radioligand that is used in respective experiments. It has been demonstrated previously that radioiodinated peptides undergo decay catastrophe, i.e. disintegration of the radioactive label leads to the concomitant destruction of the carrier peptide. Here, we demonstrate that decay catastrophe does not apply to two peptide hormones that are iodinated by Bolton-Hunter conjugation: cholecystokinin octapeptide and glucagon-like peptide 2. The function of aged samples of these radioligands at corresponding recombinantly expressed receptors was assessed by measuring ligand-induced inositol phosphate production or generation of cyclic AMP, respectively. Both of the tested compounds, although predicted by decay catastrophe to contain little or subthreshold remaining bioactivity, stimulated an unexpectedly high level of receptor-mediated second messenger signaling. Quantitative comparison of observed functions with those of corresponding unlabeled peptides suggested that the bioactivity of each radioligand had been largely conserved despite the radioactive decay of the iodine label. Consistent with an apparent absence of decay catastrophe, we noted that the specific radioactivity, when determined immediately following peptide iodination, was close to the theoretical maximum but exponentially decreased over time. These findings raise the possibility that attachment of a Bolton-Hunter conjugate may shield labeled peptides from radiation-induced damage, a scenario that should be considered when performing radioligand binding experiments.

Identification of a series of CCK-2 receptor nonpeptide agonists: sensitivity to stereochemistry and a receptor point mutation

The search for small-molecule drugs that act at peptide hormone receptors has resulted in the identification of a wide variety of antagonists. In contrast, the discovery of nonpeptide agonists has been far more elusive. We have used a constitutively active mutant of the cholecystokinin 2 receptor (CCK-2R) as a sensitive screen to detect ligand activity. Functional assessment of structural analogs of the prototype CCK-2R antagonist, L-365,260 [3R-N- (2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl)-N'-(3-methylphenyl)urea], resulted in the identification of a series of agonists. Each of the active molecules is an S enantiomer, whereas the corresponding R stereoisomers have little or no activity. Further in vitro and in vivo assessment at the wild-type receptor indicated that efficacy of the two most active ligands approached that of the endogenous hormone. The function of selected R and S enantiomers was differentially sensitive to a point mutation, N353L, within the putative CCK-2R ligand pocket. The results of this study highlight the potential of constitutively active receptors as drug screening tools and the interdependence of ligand stereochemistry and receptor conformation in defining drug efficacy.

Conserved cholecystokinin receptor transmembrane domain IV amino acids confer peptide affinity

The Cholecystokinin type 1 and type 2 receptors (CCK-1R and CCK-2R) share >50% amino acid identity, as well as subnanomolar affinity for the endogenous peptide cholecystokinin octapeptide (CCK-8). Although it is likely that these two receptor subtypes share amino acids that confer CCK-8 affinity, it has been difficult to identify such residues. We have examined the role of several transmembrane domain (TMD) IV residues that are common to both CCK receptor subtypes. In both the CCK-1R and CCK-2R, we demonstrate that alanine substitution of two TMD IV residues, which are highly conserved among all known CCK receptor subtypes and species homologs, significantly decrease CCK-8 affinity. Despite the observed decrease in peptide binding, the mutant receptors maintain close to wild-type affinity for the respective subtype selective nonpeptide ligands, 3H-labeled L-364,714 (CCK-1R) and 3H-labeled L-365,260 (CCK-2R), suggesting conserved tertiary structure of these mutants. Assessment of CCK-8-induced inositol phosphate production at each of the mutant CCK receptors revealed normal peptide efficacy. In contrast, peptide potencies are reduced in parallel with the observed decreases in affinity. Taken together, these findings suggest that important peptide affinity determinants are localized on TMD IV, a region that has not previously been considered a major contributor to ligand affinity in either CCK receptors or other G protein-coupled peptide receptors.

A Drosophila dopamine 2-like receptor: Molecular characterization and identification of multiple alternatively spliced variants

Dopamine is an important neurotransmitter in the central nervous system of both Drosophila and mammals. Despite the evolutionary distance, functional parallels exist between the fly and mammalian dopaminergic systems, with both playing roles in modulating locomotor activity, sexual function, and the response to drugs of abuse. In mammals, dopamine exerts its effects through either dopamine 1-like (D1-like) or D2-like G protein-coupled receptors. Although pharmacologic data suggest the presence of both receptor subtypes in insects, only cDNAs encoding D1-like proteins have been isolated previously. Here we report the cloning and characterization of a newly discovered Drosophila dopamine receptor. Sequence analysis reveals that this putative protein shares highest homology with known mammalian dopamine 2-like receptors. Eight isoforms of the Drosophila D2-like receptor (DD2R) transcript have been identified, each the result of alternative splicing. The encoded heptahelical receptors range in size from 461 to 606 aa, with variability in the length and sequence of the third intracellular loop. Pharmacologic assessment of three DD2R isoforms, DD2R-606, DD2R-506, and DD2R-461, revealed that among the endogenous biogenic amines, dopamine is most potent at each receptor. As established for mammalian D2-like receptors, stimulation of the Drosophila homologs with dopamine triggers pertussis toxin-sensitive Gi/o-mediated signaling. The D2-like receptor agonist, bromocriptine, has nanomolar potency at DD2R-606, -506, and -461, whereas multiple D2-like receptor antagonists (as established with mammalian receptors) have markedly reduced if any affinity when assessed at the fly receptor isoforms. The isolation of cDNAs encoding Drosophila D2-like receptors extends the range of apparent parallels between the dopaminergic system in flies and mammals. Pharmacologic and genetic manipulation of the DD2Rs will provide the opportunity to better define the physiologic role of these proteins in vivo and further explore the utility of invertebrates as a model system for understanding dopaminergic function in higher organisms.

A small molecule ligand of the glucagon-like peptide 1 receptor targets its amino-terminal hormone binding domain

The glucagon-like peptide 1 receptor (GLP-1R) belongs to a distinct subgroup of G protein-coupled peptide hormone receptors (class B) that has been difficult to target by small molecule drugs. Here, we report that a non-peptide compound, T-0632, binds with micromolar affinity to the human GLP-1R and blocks GLP-1-induced cAMP production. Furthermore, the observation that T-0632 has almost 100-fold selectivity for the human versus the highly homologous rat GLP-1R provided an opportunity to map determinants of non-peptide binding. Radioligand competition experiments utilizing a series of chimeric human/rat GLP-1R constructs revealed that partial substitution of the amino terminus of the rat GLP-1R with the corresponding sequence from the human homolog was sufficient to confer high T-0632 affinity. Follow-up analysis of receptors where individual candidate amino acids had been exchanged between the human and rat GLP-1Rs identified a single residue that explained species selectivity of non-peptide binding. Replacement of tryptophan 33 in the human GLP-1R by serine (the homologous amino acid in the rat GLP-1R) resulted in a 100-fold loss of T-0632 affinity, whereas the converse mutation in the rat GLP-1R led to a reciprocal gain-of-function phenotype. These observations suggest that in a class B receptor, important determinants of non-peptide affinity reside within the extracellular amino-terminal domain. Compound T-0632 may mimic, and thereby interfere with, the putative "pseudo-tethering" mechanism by which the amino terminus of class B receptors initiates the binding of cognate hormones.

Investigation of gastroprotective compounds at subcellular level in isolated gastric mucosal cells

We tested the hypothesis that recognized gastroprotective agents exert direct protection against ethanol-induced injury in isolated rat gastric mucosal cells in vitro. If protection exists, we also wanted to identify subcellular targets in the reversible and/or irreversible stages of cell injury. Ethanol-induced cell injury was quantified by measuring plasma membrane leakage (trypan blue exclusion and lactate dehydrogenase release), mitochondrial integrity (succinic dehydrogenase), and nuclear damage (ethidium bromide-DNA fluorescence). Initial cell viability and responsiveness were estimated by the effects of carbachol, carbachol + atropine, or 16,16-dimethyl-PGE(2) on chief cell pepsinogen secretion. Enriched parietal cells were stimulated by histamine, carbachol, or histamine + IBMX. Preincubation of cells with PG, sucrose octasulfate, or the sulfhydryl compounds N-acetylcysteine, taurine, or cysteamine increased cell resistance </=21% against ethanol. Similar protection was found with low histamine concentrations, but a higher concentration aggravated ethanol toxicity. Other naturally occurring or synthetic gastroprotective agents offered partial protection or aggravated ethanol-induced cell injury. Only a few in vivo gastroprotective agents demonstrated in vitro direct cytoprotection, which involved mainly the reversible stage of cell injury (e.g., plasma membrane changes) and, less often, irreversible (e.g., mitochondrial and nuclear) damage. Our findings also indicate that a major part of the beneficial effect of gastroprotective agents is expressed at the tissue level.

Inhibition of morphine tolerance development by a substance P-opioid peptide chimera

The neuropeptide substance P (SP), apart from its traditional role in spinal nociceptive processing, is an important regulatory effector of opioid-dependent analgesic processes. The present study stems from our original findings indicating that 1) pharmacologically administered SP mediates a strong inhibitory activity on the development of morphine tolerance in rats, and that 2) a novel SP-opioid peptide chimera YPFFGLM-NH(2), designated ESP7, produces opioid-dependent analgesia without tolerance development. To further examine the effects of simultaneous activation of two distinct opposing spinal systems on opioid tolerance and the mechanisms underlying chimeric peptide function, a second SP-opioid chimera was synthesized. This chimera, designated ESP6 (YPFFPLM-NH(2)), contains overlapping domains of endomorphin-2 and SP, respectively. ESP6 is distinguished from ESP7 by a glycine to proline substitution at position 5. Intrathecal administration of morphine sulfate (MS) with ESP6 leads to a prolongation of MS analgesia over a 5-day period. The analgesia produced by ESP6 and MS is opioid receptor-dependent, due to the ability of naltrexone to block the analgesic response. Furthermore, when ESP6 and MS are administered with concurrent NK-1 receptor blockade, a decay in analgesic potency similar to that seen with MS alone results. The presence of a proline in ESP6 appears to reduce its conformational flexibility, limit its potency at the micro-opioid receptor, and hinder its analgesic effectiveness alone. However, ESP6 represents a novel adjuvant for the maintenance of opioid analgesia over time and provides a means to predict the pharmacological properties of a chimera from its structure.

CCK receptor polymorphisms: an illustration of emerging themes in pharmacogenomics

Polymorphisms in G-protein-coupled receptors can alter drug affinity and/or activity. In addition, genetic differences in amino acid sequences can induce ligand-independent signaling, which in turn can lead to disease. With growing efforts in the field of pharmacogenomics, it is anticipated that polymorphism-induced alterations in drug and/or receptor function will be a focus of increasing concern during the course of future drug-development efforts. In this review, the spectrum of pharmacological consequences that result from polymorphisms in the cholecystokinin CCK2 receptor will be discussed, thereby illustrating emerging themes in pharmacogenomics.

CCK-B/Gastrin receptor transmembrane domain mutations selectively alter synthetic agonist efficacy without affecting the activity of endogenous peptides

Recent efforts have focused on identifying small nonpeptide molecules that can mimic the activity of endogenous peptide hormones. Understanding the molecular basis of ligand-induced receptor activation by these divergent classes of ligands should expedite the process of drug development. Using the cholecystokinin-B/gastrin receptor (CCK-BR) as a model system, we have recently shown that both affinity and efficacy of nonpeptide ligands are markedly affected by amino acid alterations within a putative transmembrane domain (TMD) ligand pocket. In this report, we examine whether residues projecting into the TMD pocket determine the pharmacologic properties of structurally diverse CCK-BR ligands, including peptides and synthetic peptide-derived partial agonists (peptoids). Nineteen mutant human CCK-BRs, each including a single TMD amino acid substitution, were transiently expressed in COS-7 cells and characterized. Binding affinities as well as ligand-induced inositol phosphate production at the mutant CCK-BRs were assessed for peptides (CCK-8 and CCK-4) and for peptoids (PD-135,158 and PD-136, 450). Distinct as well as overlapping determinants of peptide and peptoid binding affinity were identified, supporting that both classes of ligands, at least in part, interact with the CCK-BR TMD ligand pocket. Eight point mutations resulted in marked increases or decreases in the functional activity of the synthetic peptoid ligands. In contrast, the functional activity of both peptides, CCK-8 and CCK-4, was not affected by any of the CCK-BR mutations. These findings suggest that the mechanisms underlying activation of G-protein-coupled receptors by endogenous peptide hormones versus synthetic ligands may markedly differ.

A substance P-opioid chimeric peptide as a unique nontolerance-forming analgesic

To elucidate mechanisms of acute and chronic pain, it is important to understand how spinal excitatory systems influence opioid analgesia. The tachykinin substance P (SP) represents the prototypic spinal excitatory peptide neurotransmitter/neuromodulator, acting in concert with endogenous opioid systems to regulate analgesic responses to nociceptive stimuli. We have synthesized and pharmacologically characterized a chimeric peptide containing overlapping NH(2)- and COOH-terminal functional domains of the endogenous opioid endomorphin-2 (EM-2) and the tachykinin SP, respectively. Repeated administration of the chimeric molecule YPFFGLM-NH(2), designated ESP7, into the rat spinal cord produces opioid-dependent analgesia without loss of potency over 5 days. In contrast, repeated administration of ESP7 with concurrent SP receptor (SPR) blockade results in a progressive loss of analgesic potency, consistent with the development of tolerance. Furthermore, tolerant animals completely regain opioid sensitivity after post hoc administration of ESP7 alone, suggesting that coactivation of SPRs is essential to maintaining opioid responsiveness. Radioligand binding and signaling assays, using recombinant receptors, confirm that ESP7 can coactivate mu-opioid receptors (MOR) and SPRs in vitro. We hypothesize that coincidental activation of the MOR- and SPR-expressing systems in the spinal cord mimics an ongoing state of reciprocal excitation and inhibition, which is normally encountered in nociceptive processing. Due to the ability of ESP7 to interact with both MOR and SPRs, it represents a unique prototypic, anti-tolerance-forming analgesic with future therapeutic potential.

The cholecystokinin-A receptor mediates inhibition of food intake yet is not essential for the maintenance of body weight

Food intake and body weight are determined by a complex interaction of regulatory pathways. To elucidate the contribution of the endogenous peptide cholecystokinin, mice lacking functional cholecystokinin-A receptors were generated by targeted gene disruption. To explore the role of the cholecystokinin-A receptor in mediating satiety, food intake of cholecystokinin-A receptor-/- mice was compared with the corresponding intakes of wild-type animals and mice lacking the other known cholecystokinin receptor subtype, cholecystokinin-B/gastrin. Intraperitoneal administration of cholecystokinin failed to decrease food intake in mice lacking cholecystokinin-A receptors. In contrast, cholecystokinin diminished food intake by up to 90% in wild-type and cholecystokinin-B/gastrin receptor-/- mice. Together, these findings indicate that cholecystokinin-induced inhibition of food intake is mediated by the cholecystokinin-A receptor. To explore the long-term consequences of either cholecystokinin-A or cholecystokinin-B/gastrin receptor absence, body weight as a function of age was compared between freely fed wild-type and mutant animals. Both cholecystokinin-A and cholecystokinin-B/gastrin receptor-/- mice maintained normal body weight well into adult life. In addition, each of the two receptor-/- strains had normal pancreatic morphology and were normoglycemic. Our results suggest that although cholecystokinin plays a role in the short-term inhibition of food intake, this pathway is not essential for the long-term maintenance of body weight.

Mutations within the cholecystokinin-B/gastrin receptor ligand 'pocket' interconvert the functions of nonpeptide agonists and antagonists

We have reported previously that the transmembrane domains of the cholecystokinin-B/gastrin receptor (CCK-BR) comprise a putative ligand binding pocket. In the present study, we examined whether amino acid substitutions within the CCK-BR pocket altered the affinities and/or functional activities of L-365,260 (the prototypical nonpeptide CCK-BR antagonist) and two structural derivatives, YM022 (a higher affinity antagonist) and L-740,093S (a partial agonist). Eight amino acids that project into the CCK-BR pocket were individually replaced by alanine, using site-directed mutagenesis. Affinities for the nonpeptide molecules, as well as ligand-induced inositol phosphate production, were assessed with the wild-type and mutant receptors. For each of the nonpeptide ligands examined, a distinct series of mutations altered the affinity, suggesting that each ligand possessed a characteristic pattern of interactions within the CCK-BR pocket. Basal signaling levels and inositol phosphate formation induced by the full agonist CCK octapeptide were comparable for the wild-type receptor and all of the mutant CCK-BR forms. In contrast to the peptide agonist CCK octapeptide, the functional activities of the nonpeptide molecules were selectively altered by single point mutations within the CCK-BR pocket, resulting in interconversion of agonists and antagonists. These findings suggest that interactions between nonpeptide molecules and transmembrane domain amino acids of the CCK-BR can determine the functional activity and affinity of the ligands.

Interspecies polymorphisms confer constitutive activity to the Mastomys cholecystokinin-B/gastrin receptor

The enteroendocrine hormone, gastrin, exerts trophic effects on the gastric mucosa through the CCK-B/gastrin receptor (CCK-BR). To varying degrees in different species, excess circulating gastrin leads to proliferation of enterochromaffin-like cells and to the development of gastric carcinoid tumors. The African rodent, Mastomys natalensis, is distinguished from other mammals by its propensity toward CCK-BR-mediated growth even in the absence of hypergastrinemia. Here, we report that the Mastomys CCK-BR, when expressed in COS-7 cells, differs from the respective human, canine, and rat receptor homologs by its ability to trigger ligand-independent (i.e., constitutive) inositol phosphate formation. To define the molecular basis of this observation, a series of Mastomys-human chimeric receptors was investigated. Functional characterization of these constructs revealed that a limited segment of the Mastomys CCK-BR, transmembrane domain VI through the C-terminal end, is sufficient to confer constitutive activity to the human protein. Mutagenesis studies within this CCK-BR region defined a combination of three Mastomys amino acids that, when introduced into the human receptor, together conferred a level of ligand-independent signaling comparable with the Mastomys CCK-BR. Complementing prior observations that single point mutations can lead to ligand-independent signaling, our findings suggest that multiple naturally occurring amino acid polymorphisms and/or mutations may together result in an enhanced basal level of receptor activity.

Minor modifications of a cholecystokinin-B/gastrin receptor non-peptide antagonist confer a broad spectrum of functional properties

The development of non-peptide agonists for peptide hormone receptors would markedly expand the treatment options for a large number of diseases. However, difficulty in identifying non-peptide molecules which possess intrinsic activity has been a major obstacle in achieving this goal. At present, most of the known non-peptide ligands for peptide hormone receptors appear in standard functional assays to be antagonists. Here, we report that a constitutively active mutant of the human cholecystokinin-B/gastrin receptor, Leu325 --> Glu, offers the potential to detect even trace agonist activity of ligands which, at the wild type receptor isoform, appear to lack efficacy. The enhanced functional sensitivity of the mutant receptor enabled us to detect intrinsic activity of L-365,260, an established non-peptide antagonist for the cholecystokinin-B/gastrin receptor. Extending from this observation, we were able to demonstrate that minor structural modifications could convert L-365, 260 into either: (i) an agonist or (ii) an inverse agonist (attenuates ligand-independent signaling). The ability to confer functional activity to small non-peptide ligands suggests that the properties of endogenous peptide hormones can be mimicked, and even extended, by considerably less complex molecules.

Abnormal gastric morphology and function in CCK-B/gastrin receptor-deficient mice

Mice lacking the cholecystokinin (CCK)-B/gastrin receptor have been generated by targeted gene disruption. The roles of this receptor in controlling gastric acid secretion and gastric mucosal growth have been assessed. The analysis of homozygous mutant mice vs. wild type included measurement of basal gastric pH, plasma gastrin concentrations as well as quantification of gastric mucosal cell types by immunohistochemistry. Mutant mice exhibited a marked increase in basal gastric pH (from 3.2 to 5.2) and about a 10-fold elevation in circulating carboxyamidated gastrin compared with wild-type controls. Histologic analysis revealed a decrease in both parietal and enterochromaffin-like (ECL) cells, thus explaining the reduction in acid output. Consistent with the elevation in circulating gastrin, antral gastrin cells were increased in number while somatostatin cells were decreased. These data support the importance of the CCK-B/gastrin receptor in maintaining the normal cellular composition and function of the gastric mucosa.

Small synthetic ligands of the cholecystokinin-B/gastrin receptor can mimic the function of endogenous peptide hormones

The gastric cholecystokinin-B/gastrin receptor (CCK-BR) is a key regulator of enterochromaffin-like cell function and proliferation. Over the last decade, a number of small non-peptide CCK-BR "antagonists" have been discovered. Here, we demonstrate that some of these non-peptide ligands in fact possess significant ability to activate the human CCK-BR, and are, therefore, more properly categorized as partial agonists. When tested in COS-7 cells transiently expressing the recombinant human CCK-BR, saturating concentrations of the small "peptoid" ligands PD 135,158 and PD 136,450 stimulated inositol phosphate formation to 23 and 43 percent, respectively, of the maximum response induced by a considerably larger endogenous peptide agonist, cholecystokinin octapeptide. In contrast, the benzodiazepine-derived CCK-BR ligand, YM022, acted as a "true" high-affinity antagonist of cholecystokinin-induced inositol phosphate formation (pA2 = 9.69). Consistent with recent findings in animal experiments, our data reveal that small synthetic ligands have the potential to function as either CCK-BR agonists or antagonists. These dual properties of synthetic molecules must be considered when evaluating candidate drugs for human disease.

Inter- and intraspecies polymorphisms in the cholecystokinin-B/gastrin receptor alter drug efficacy

The brain cholecystokinin-B/gastrin receptor (CCK-BR) is a major target for drug development because of its postulated role in modulating anxiety, memory, and the perception of pain. Drug discovery efforts have resulted in the identification of small synthetic molecules that can selectively activate this receptor subtype. These drugs include the peptide-derived compound PD135,158 as well as the nonpeptide benzodiazepine-based ligand, L-740,093 (S enantiomer). We now report that the maximal level of receptor-mediated second messenger signaling that can be achieved by these compounds (drug efficacy) markedly differs among species homologs of the CCK-BR. Further analysis reveals that the observed differences in drug efficacy are in large part explained by single or double aliphatic amino acid substitutions between respective species homologs. This interspecies variability in ligand efficacy introduces the possibility of species differences in receptor-mediated function, an important consideration when selecting animal models for preclinical drug testing. The finding that even single amino acid substitutions can significantly affect drug efficacy prompted us to examine ligand-induced signaling by a known naturally occurring human CCK-BR variant (glutamic acid replaced by lysine in position 288; 288E --> K). When examined using the 288E --> K receptor, the efficacies of both PD135,158 and L-740, 093 (S) were markedly increased compared with values obtained with the wild-type human protein. These observations suggest that functional variability resulting from human receptor polymorphisms may contribute to interindividual differences in drug effects.

Abnormal gastric histology and decreased acid production in cholecystokinin-B/gastrin receptor-deficient mice

BACKGROUND & AIMS

The cholecystokinin (CCK)-B/gastrin receptor is one of several regulators of gastric acid secretion and mucosal growth. To elucidate the contribution of this receptor relative to other trophic and secretory factors, mice that lack the CCK-B/gastrin receptor have been generated and studied.

METHODS

Both alleles of the CCK-B/gastrin receptor were inactivated by targeted gene disruption. Analysis of the mice included measurement of basal gastric pH and plasma gastrin levels. In addition, multiple gastric mucosal cell types were identified by immunostaining and quantified.

RESULTS

Homozygous mutant mice were viable, fertile, and appeared grossly normal into adulthood. The receptor-deficient mice exhibited a marked increase in basal gastric pH (from 3.2 to 5.2) and an approximately 10-fold elevation in plasma gastrin concentration compared with wild-type controls. In the stomach of mutant animals, parietal and enterochromaffin-like cells were decreased, providing a likely explanation for the reduction in acid output. In the antrum, a decrease in somatostatin cell density and an increase in the gastrin cell number were observed, consistent with the concomitant elevation in circulating gastrin.

CONCLUSIONS

Together, these findings demonstrate the importance of the CCK-B/gastrin receptor in maintaining the normal cellular composition and function of the gastric mucosa.

Identification of cholecystokinin-B/gastrin receptor domains that confer high gastrin affinity: utilization of a novel Xenopus laevis cholecystokinin receptor

A hallmark of the mammalian brain cholecystokinin (CCK) receptor, CCK-B/gastrin (CCK-BR), is its high affinity for two structurally related peptides, CCK and gastrin. Previous radioligand binding experiments suggested that the predominant CCK receptor from Xenopus laevis brain shares high affinity for sulfated cholecystokinin octapeptide but has > or = 1000-fold lower affinity for gastrin. To determine the molecular basis for this pharmacological divergence between mammalian and lower vertebrate receptors, we isolated a cDNA encoding the X. laevis brain CCK receptor (CCK-XLR). CCK-XLR shares approximately 50% homology at the amino acid level with both the human CCK-BR and the peripheral CCK-A receptor subtypes. The recombinant X. laevis receptor has a distinct pharmacological profile of agonist and antagonist affinities and as such offers a useful tool for structure-function studies. We used CCK-XLR to map the human CCK-BR domains that confer high affinity for gastrin. A series of chimeric CCK-BR/CCK-XLR constructs was generated and pharmacologically characterized. While maintaining wild-type affinity for sulfated cholecystokinin octapeptide, receptors with increasing amino-terminal contributions from CCK-BR demonstrated a stepwise increase in gastrin affinity. Further dissection of the amino-terminal third of the human receptor, a domain that confers a > 250-fold increase in gastrin affinity, revealed the importance of interactions among at least three subdomains. Additional structural requirements for gastrin affinity mapped to a segment spanning transmembrane domains IV and V.

Characterization of CCKA receptor mediated pepsinogen secretion in porcine chief cells

Cholecystokinin octapeptide (CCK-8s) is an endogenous stimulus of gastric pepsinogen secretion. Previous studies with isolated guinea pig chief cells indicated that this process is mediated through the CCKA receptor subtype, with an additional contribution from CCKB receptors. For comparison, we examined the mechanism of CCK-8s stimulated pepsinogen secretion in a larger nonrodent species, using highly enriched porcine chief cells as a functional in vitro model. Porcine chief cells responded weakly to stimulation by CCK-8s alone, but the efficacy was markedly enhanced in the presence of 10 mumol l-1 forskolin. Under these conditions, pepsinogen secretion was potently stimulated by CCK-8s and the CCKA receptor selective heptapeptide, A-71,378 (EC50 = 4.7 and 33 nmol l-1), but not by CCKB receptor selective agonists. The prototype CCKA receptor selective antagonist L-364,718 blocked pepsinogen secretion with approximately 2,000-fold higher affinity than the CCKB receptor selective analogue, L-365,260. This functional profile was consistent with the affinity rank order of all tested compounds at CCKA-receptor-like [125I]-BH-CCK-8s binding sites in the porcine gastric mucosa. Comparison with cloned CCKA receptors from other species revealed that the receptors mediating pepsinogen secretion in the pig have similar pharmacology, possibly with slight differences in agonist potencies. In contrast to the guinea pig, porcine CCKB receptors appear to have no direct role in pepsinogen secretion.

Purinoceptor P2U identification and function in human intrahepatic biliary epithelial cell lines

The mechanisms that regulate ion and fluid transport by the human intrahepatic bile duct have not been well defined. Human intrahepatic biliary cell lines that we have developed were used to identify and characterize purinoceptors based on increases in intracellular calcium in response to ATP and other nucleotides. Intracellular free calcium was measured in cell suspensions using the fluorescent probe Fura-2 and a fluorescence spectrophotometer. Halide efflux was measured in single cells using fluorescence microscopy and the fluorescent probe SPQ. Intracellular calcium increases equivalently in response to ATP and UTP, peaking, then diminishing to a new, elevated baseline. The peak elevation of calcium is the result of both the release of intracellular stores of calcium and the influx of extracellular calcium. The purinoceptor P2U-subtype was identified based on the potency rank order of ATP-analogues. Halide efflux increases with P2U-purinoceptor stimulation which is consistent with the opening of a Ca(2+)-sensitive Cl- channel. The physiological significance of P2U-purinoceptor activation and its effect on the ionic content and flow rate of bile remains to be determined.

The role of the cholecystokinin-B/gastrin receptor transmembrane domains in determining affinity for subtype-selective ligands

We have examined the role of transmembrane domain amino acids in conferring subtype-selective ligand affinity to the human cholecystokinin-B (CCK-B)/gastrin receptor. Fifty-eight residues were sequentially replaced by the corresponding amino acids from the pharmacologically distinct CCK-A receptor subtype. 125I-CCK-8 competition binding experiments were performed to compare all mutant CCK-B/gastrin receptor constructs with the wild type control. Affinities for the nonselective agonist, CCK-8, as well as the subtype-selective peptide (gastrin), peptide-derived (PD135,158), and nonpeptide (L365,260) and L364,718) ligands were assessed. All of the mutants retained relatively high affinity for CCK-8, suggesting that the tertiary structure of these receptors was well maintained. Only eight of the amino acid substitutions had a significant effect on subtype selective binding. When compared with the wild type, single point mutations in the CCK-B/gastrin receptor decreased affinity for gastrin, L365,260, and PD135,158 up to 17-,23-, and 61-fold, respectively. In contrast, the affinity for L364,718 increased up to 63-fold. None of the single amino acid substitutions, however, was sufficient to fully account for the subtype selectivity of any tested compound. Rather, CCK-B/gastrin receptor affinity appears to be influenced by multiple residues acting in concert. The 8 pharmacologically important amino acids cluster in the portion of the transmembrane domains adjacent to the cell surface. The spatial orientation of these residues was analyzed with a rhodopsin-based three-dimensional model of G-protein coupled receptor structure (Baldwin, J.M. (1993) EMBO J. 12, 1693-1703). This model predicts that the 8 crucial residues project into a putative ligand pocket, similar to the one which is well established for biogenic amine receptors (Caron, M. G., and Lefkowitz, R.J. (1993) Recent Prog. Horm. Res. 48, 277-290; Strader, C.D., Sigal, I.S., and Dixon, R.A. (1989) Trends Pharmacol. Sci. 10, Dec. Suppl., 26-30).

The effect of recombinant cytokines on [14C]-aminopyrine accumulation by isolated canine parietal cells

Interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) have been shown to inhibit basal and pentagastrin-stimulated gastric secretion in rats and histamine-stimulated secretion in dogs. IL-1 also reduces the severity of ethanol and stress-induced gastroduodenal damage. The aim of this study was to examine the effects of human recombinant IL-1 alpha and TNF-alpha on enzymatically dispersed and enriched (> 90%) parietal cells stimulated with histamine, histamine plus 3-isobutyl-1-methylxanthine (IMX) or carbachol (all 10(-5) M). Acid secretion was assessed indirectly by quantitating [14C]-aminopyrine (AP) accumulation. IL-1 alpha (500 and 1000 ng/ml) inhibited histamine-stimulated AP uptake by 53% and 60% respectively, and it inhibited IL-1 alpha (1500 ng/ml) by 69%. IL-1 alpha (500 and 1000 ng/ml) inhibited histamine plus IMX-stimulated AP uptake by 36% and 34%, respectively. IL-1 alpha (500 ng/ml) also inhibited carbachol-stimulated AP accumulation. TNF-alpha (100 and 250 ng/ml) inhibited histamine-stimulated AP accumulation by 38% and 36%, respectively. TNF-alpha also significantly inhibited histamine/IMX- and carbachol-stimulated AP uptake (P < or = .01). Indomethacin did not affect IL-1 alpha-induced inhibition. These results show that IL-1 alpha and TNF-alpha inhibit histamine- and carbachol-stimulated isolated parietal cell secretion and that, for IL-1 alpha, this effect does not depend on mucosal prostaglandin synthesis.