Understanding interactions of gastric inhibitory polypeptide (GIP) with its G-protein coupled receptor through NMR and molecular modeling

GIP and GLP-2 together improve bone turnover in humans supporting GIPR-GLP-2R co-agonists as future osteoporosis treatment

The intestinal hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are key regulators of postprandial bone turnover in humans. We hypothesized that GIP and GLP-2 co-administration would provide stronger effect on bone turnover than administration of the hormones separately, and tested this using subcutaneous injections of GIP and GLP-2 alone or in combination in humans. Guided by these findings, we designed series of GIPR-GLP-2R co-agonists as template for new osteoporosis treatment. The clinical experiment was a randomized cross-over design including 10 healthy men administered subcutaneous injections of GIP and GLP-2 alone or in combination. The GIPR-GLP-2R co-agonists were characterized in terms of binding and activation profiles on human and rodent GIP and GLP-2 receptors, and their pharmacokinetic (PK) profiles were improved by dipeptidyl peptidase-4 protection and site-directed lipidation. Co-administration of GIP and GLP-2 in humans resulted in an additive reduction in bone resorption superior to each hormone individually. The GIPR-GLP-2R co-agonists, designed by combining regions of importance for cognate receptor activation, obtained similar efficacies as the two native hormones and nanomolar potencies on both human receptors. The PK-improved co-agonists maintained receptor activity along with their prolonged half-lives. Finally, we found that the GIPR-GLP-2R co-agonists optimized toward the human receptors for bone remodeling are not feasible for use in rodent models. The successful development of potent and efficacious GIPR-GLP-2R co-agonists, combined with the improved effect on bone metabolism in humans by co-administration, support these co-agonists as a future osteoporosis treatment.

The Location of Missense Variants in the Human GIP Gene Is Indicative for Natural Selection

The intestinal hormone, glucose-dependent insulinotropic polypeptide (GIP), is involved in important physiological functions, including postprandial blood glucose homeostasis, bone remodeling, and lipid metabolism. While mutations leading to physiological changes can be identified in large-scale sequencing, no systematic investigation of GIP missense variants has been performed. Here, we identified 168 naturally occurring missense variants in the human GIP genes from three independent cohorts comprising ~720,000 individuals. We examined amino acid changing variants scattered across the pre-pro-GIP peptide using in silico effect predictions, which revealed that the sequence of the fully processed GIP hormone is more protected against mutations than the rest of the precursor protein. Thus, we observed a highly species-orthologous and population-specific conservation of the GIP peptide sequence, suggestive of evolutionary constraints to preserve the GIP peptide sequence. Elucidating the mutational landscape of GIP variants and how they affect the structural and functional architecture of GIP can aid future biological characterization and clinical translation.

Molecular interactions of full-length and truncated GIP peptides with the GIP receptor - A comprehensive review

Enzymatic cleavage of endogenous peptides is a commonly used principle to initiate, modulate and terminate action for instance among cytokines and peptide hormones. The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and the related hormone glucagon-like peptide-2 (GLP-2) are all rapidly N-terminally truncated with severe loss of intrinsic activity. The most abundant circulating form of full length GIP(1-42) is GIP(3-42) (a dipeptidyl peptidase-4 (DPP-4) product). GIP(1-30)NH₂ is another active form resulting from prohormone convertase 2 (PC2) cleavage of proGIP. Like GIP(1-42), GIP(1-30)NH₂ is a substrate for DPP-4 generating GIP(3-30)NH₂ which, compared to GIP(3-42), binds with higher affinity and very efficiently inhibits GIP receptor (GIPR) activity with no intrinsic activity. Here, we review the action of these four and multiple other N- and C-terminally truncated forms of GIP with an emphasis on molecular pharmacology, i.e. ligand binding, subsequent receptor activation and desensitization. Our overall conclusion is that the N-terminus is essential for receptor activation as GIP N-terminal truncation leads to decreased/lost intrinsic activity and antagonism (similar to GLP-1 and GLP-2), whereas the C-terminal extension of GIP(1-42), as compared to GLP-1, GLP-2 and glucagon (29-33 amino acids), has no apparent impact on the GIPR in vitro, but may play a role for other properties such as stability and tissue distribution. A deeper understanding of the molecular interaction of naturally occurring and designed GIP-based peptides, and their impact in vivo, may contribute to a future therapeutic targeting of the GIP system - either with agonists or with antagonists, or both.

N-terminally and C-terminally truncated forms of glucose-dependent insulinotropic polypeptide are high-affinity competitive antagonists of the human GIP receptor

Background and Purpose

Glucose‐dependent insulinotropic polypeptide (GIP) affects lipid, bone and glucose homeostasis. High‐affinity ligands for the GIP receptor are needed to elucidate the physiological functions and pharmacological potential of GIP in vivo. GIP(1–30)NH₂ is a naturally occurring truncation of GIP(1–42). Here, we have characterized eight N‐terminal truncations of human GIP(1–30)NH₂.

Experimental Approach

COS‐7 cells were transiently transfected with human GIP receptors and assessed for cAMP accumulation upon ligand stimulation or competition binding with ¹²⁵I‐labelled GIP(1–42), GIP(1–30)NH₂, GIP(2–30)NH₂ or GIP(3–30)NH₂.

Key Results

GIP(1–30)NH₂ displaced ¹²⁵I‐GIP(1–42) as effectively as GIP(1–42) (K_i 0.75 nM), whereas the eight truncations displayed lower affinities (K_i 2.3–347 nM) with highest affinities for GIP(3–30)NH₂ and GIP(5–30)NH₂ (5–30)NH₂. Only GIP(1–30)NH₂ (E_max 100% of GIP(1–42)) and GIP(2–30)NH₂ (E_max 20%) were agonists. GIP(2‐ to 9–30)NH₂ displayed antagonism (IC₅₀ 12–450 nM) and Schild plot analyses identified GIP(3–30)NH₂ and GIP(5–30)NH₂ as competitive antagonists (K_i 15 nM). GIP(3–30) NH₂ was a 26‐fold more potent antagonist than GIP(3–42). Binding studies with agonist (¹²⁵I‐GIP(1–30)NH₂), partial agonist (¹²⁵I‐GIP(2–30)NH₂) and competitive antagonist (¹²⁵I‐GIP(3–30)NH₂) revealed distinct receptor conformations for these three ligand classes.

Conclusions and Implications

The N‐terminus is crucial for GIP agonist activity. Removal of the C‐terminus of the endogenous GIP(3–42) creates another naturally occurring, more potent, antagonist GIP(3–30)NH₂, which like GIP(5–30)NH₂, was a high‐affinity competitive antagonist. These peptides may be suitable tools for basic GIP research and future pharmacological interventions.

Species-specific action of (Pro3)GIP - a full agonist at human GIP receptors, but a partial agonist and competitive antagonist at rat and mouse GIP receptors

Background and Purpose

Specific, high potency receptor antagonists are valuable tools when evaluating animal and human physiology. Within the glucose‐dependent, insulinotropic polypeptide (GIP) system, considerable attention has been given to the presumed GIP receptor antagonist, (Pro3)GIP, and its effect in murine studies. We conducted a pharmacological analysis of this ligand including interspecies differences between the rodent and human GIP system.

Experimental Approach

Transiently transfected COS‐7 cells were assessed for cAMP accumulation upon ligand stimulation and assayed in competition binding using ¹²⁵I‐human GIP. Using isolated perfused pancreata both from wild type and GIP receptor‐deficient rodents, insulin‐releasing, glucagon‐releasing and somatostatin‐releasing properties in response to species‐specific GIP and (Pro3)GIP analogues were evaluated.

Key Results

Human (Pro3)GIP is a full agonist at human GIP receptors with similar efficacy (E_max) for cAMP production as human GIP, while both rat and mouse(Pro3)GIP were partial agonists on their corresponding receptors. Rodent GIPs are more potent and efficacious at their receptors than human GIP. In perfused pancreata in the presence of 7 mM glucose, both rodent (Pro3)GIP analogues induced modest insulin, glucagon and somatostatin secretion, corresponding to the partial agonist activities observed in cAMP production.

Conclusions and Implications

When evaluating new compounds, it is important to consider interspecies differences both at the receptor and ligand level. Thus, in rodent models, human GIP is a comparatively weak partial agonist. Human (Pro3)GIP was not an antagonist at human GIP receptors, so there is still a need for a potent antagonist in order to elucidate the physiology of human GIP.

Extracting structural requirements for activity of GPR119 agonists: a hologram quantitative structure activity relationship (HQSAR) study

GPR119 is a potential target for the treatment of diabetes mellitus. GPR119 agonists minimize the side-effects observed with sulphonyl ureas and glucagon-like peptide 1 analogs. Various reported GPR119 agonists from various patents were selected for the study and a 2D-QSAR study (HQSAR) was carried out. Fifty-five molecules were selected for the study. The study was performed on a training set of 40 structurally diverse molecules with reported biological activity. The most significant HQSAR model (q2 = 0.87, r2 = 0.99) was obtained using atoms, bond, connection, and acceptor and donor as fragment distinction. The fragment size was kept at 4–7. The predictive ability of the model was evaluated by an external test set containing 15 molecules not included in the training set, and the predicted values were in good agreement with the experimental values. The important fragments determined by the study were used to design new drug candidates having increased biological activity and comparable physicochemical properties.

The structure of secretin family GPCR peptide ligands: implications for receptor pharmacology and drug development

The secretin family G protein-coupled receptors, characterized by a large N-terminal extracellular domain and seven transmembrane helices, are drug targets in many diseases, including migraine, cardiovascular disease, diabetes, osteoporosis and inflammatory disorders. Their activating ligands are peptides with an average length of 30 amino acids. In this article we review the available structural data for these peptides and how this explains their activity. We emphasize how this information may be used to accelerate the development of new drugs against these receptors.

Conformational, receptor interaction and alanine scan studies of glucose-dependent insulinotropic polypeptide

Glucose-dependent insulinotropic polypeptide (GIP) is an insulinotropic incretin hormone that stimulates insulin secretion during a meal. GIP has glucose lowering abilities and hence is considered as a potential target molecule for type 2 diabetes therapy. In this article, we present the solution structure of GIP in membrane-mimicking environments by proton NMR spectroscopy and molecular modelling. GIP adopts an α-helical conformation between residues Phe(6)-Gly(31) and Ala(13)-Gln(29) for micellar and bicellar media, respectively. Previously we examined the effect of N-terminal Ala substitution in GIP, but here eight GIP analogues were synthesised by replacing individual residues within the central 8-18 region with alanine. These studies showed relatively minor changes in biological activity as assessed by insulin releasing potency. However, at higher concentration, GIP(Ala(16)), and GIP(Ala(18)) showed insulin secreting activity higher than the native GIP (P<0.01 to P<0.001) in cultured pancreatic BRIN-BD11 cells. Receptor interaction studies of the native GIP with the extracellular domain of its receptor were performed by using two different docking algorithms. At the optimised docking conformation, the complex was stabilised by the presence of hydrophobic interactions and intermolecular hydrogen bonding. Further, we have identified some potentially important additional C-terminal interactions of GIP with its N-terminal extracellular receptor domain.

Characterization and biological actions of N-terminal truncated forms of glucose-dependent insulinotropic polypeptide

The N-terminal domain of glucose-dependent insulinotropic polypeptide (GIP) plays an important role in regulating biological activity. This study examined biological properties of several N-terminal truncated forms of GIP and two novel forms with substitutions at Phe position-6 with Arg or Val. GIP(6-42), GIP(R6-42), GIP(V6-42), GIP(7-42) and GIP(9-42) stimulated cAMP production in BRIN-BD11 cells similar to native GIP, whereas responses to GIP(3-42), GIP(4-42), GIP(5-42) and GIP(8-42) were reduced (P<0.01 to P<0.001). GIP-induced cyclic AMP production was significantly inhibited by GIP(3-42), GIP(4-42), GIP(5-42), GIP(6-42), GIP(R6-42), GIP(7-42) and GIP(8-42) (P<0.001). Compared with native GIP, in vitro insulinotropic activity of GIP(3-42), GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) was reduced (P<0.05 to P<0.001), with GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) also potently inhibiting GIP-stimulated insulin secretion (P<0.001). In ob/ob mice, GIP(4-42) and GIP(8-42) increased (P<0.05 to P<0.01) plasma glucose concentrations compared to the glucose-lowering action of native GIP. When GIP(8-42) was co-administered with native GIP it countered the ability of the native peptide to lower plasma glucose and increase circulating insulin concentrations. These data confirm the importance of the N-terminal region of GIP in regulating bioactivity and reveal that sequential truncation of the peptide yields novel GIP receptor antagonists which may have functional significance.

Mapping interactions of gastric inhibitory polypeptide with GIPR N-terminus using NMR and molecular dynamics simulations

Glucose-dependent insulinotropic polypeptide (gastric inhibitory polypeptide, or GIP), a 42-amino acid incretin hormone, modulates insulin secretion in a glucose-concentration-dependent manner. Its insulinotropic action is highly dependent on glucose concentration that surmounts the hypoglycemia side effects associated with current therapy. In order to develop a GIP-based anti-diabetic therapy, it is essential to establish the 3D structure of the peptide and study its interaction with the GIP receptor (GIPR) in detail. This will give an insight into the GIP-mediated insulin release process. In this article, we report the solution structure of GIP(1-42, human)NH(2) deduced by NMR and the interaction of the peptide with the N-terminus of GIPR using molecular modelling methods. The structure of GIP(1-42, human)NH(2) in H(2)O has been investigated using 2D-NMR (DQF-COSY, TOCSY, NOESY, (1)H-(13)C HSQC) experiments, and its conformation was built by constrained MD simulations with the NMR data as constraints. The peptide in H(2)O exhibits an alpha-helical structure between residues Ser8 and Asn39 with some discontinuity at residues Gln29 to Asp35; the helix is bent at Gln29. This bent gives the peptide an 'L' shape that becomes more pronounced upon binding to the receptor. The interaction of GIP with the N-terminus of GIPR was modelled by allowing GIP to interact with the N-terminus of GIPR under a series of decreasing constraints in a molecular dynamics simulation, culminating with energy minimization without application of any constraints on the system. The canonical ensemble obtained from the simulation was subjected to a detailed energy analysis to identify the peptide-protein interaction patterns at the individual residue level. These interaction energies shed some light on the binding of GIP with the GIPR N-terminus in a quantitative manner.

Identification of determinants of glucose-dependent insulinotropic polypeptide receptor that interact with N-terminal biologically active region of the natural ligand

Glucose-dependent insulinotropic polypeptide receptor (GIPR), a member of family B of the G-protein coupled receptors, is a potential therapeutic target for which discovery of nonpeptide ligands is highly desirable. Structure-activity relationship studies indicated that the N-terminal part of glucose-dependent insulinotropic polypeptide (GIP) is crucial for biological activity. Here, we aimed at identification of residues in the GIPR involved in functional interaction with N-terminal moiety of GIP. A homology model of the transmembrane core of GIPR was constructed, whereas a three-dimensional model of the complex formed between GIP and the N-terminal extracellular domain of GIPR was taken from the crystal structure. The latter complex was docked to the transmembrane domains of GIPR, allowing in silico identification of putative residues of the agonist binding/activation site. All mutants were expressed at the surface of human embryonic kidney 293 cells as indicated by flow cytometry and confocal microscopy analysis of fluorescent GIP binding. Mutation of residues Arg183, Arg190, Arg300, and Phe357 caused shifts of 76-, 71-, 42-, and 16-fold in the potency to induce cAMP formation, respectively. Further characterization of these mutants, including tests with alanine-substituted GIP analogs, were in agreement with interaction of Glu3 in GIP with Arg183 in GIPR. Furthermore, they strongly supported a binding mode of GIP to GIPR in which the N-terminal moiety of GIP was sited within transmembrane helices (TMH) 2, 3, 5, and 6 with biologically crucial Tyr1 interacting with Gln224 (TMH3), Arg300 (TMH5), and Phe357 (TMH6). These data represent an important step toward understanding activation of GIPR by GIP, which should facilitate the rational design of therapeutic agents.

C-terminal mini-PEGylation of glucose-dependent insulinotropic polypeptide exhibits metabolic stability and improved glucose homeostasis in dietary-induced diabetes

Glucose-dependent insulinotropic polypeptide has been proposed as a potential therapeutic for type 2 diabetes, however, efforts to bring forward this drug have been hindered due to its short circulating half-life. We have adopted a novel strategy to increase potency and prolong GIP action through C-terminal mini-PEGylation (GIP[mPEG]). In contrast to GIP, GIP[mPEG] was resistant to dipeptidylpeptidase-IV (DPP-IV) up to and including 24h. Both GIP[mPEG] and GIP concentration-dependently stimulated cAMP production (EC50 6.6 and 0.7 nM, respectively) and insulin secretion (p < 0.01 to p < 0.001) in pancreatic BRIN-BD11 cells. Acute injection of GIP[mPEG] together with glucose to high fat fed mice significantly lowered plasma glucose (p < 0.05) and increased plasma insulin responses (p < 0.05). Furthermore, GIP[mPEG] markedly lowered plasma glucose when administered 4-24h prior to a glucose load (p < 0.05). Daily administration of GIP[mPEG] for 20 days in high fat mice did not alter body weight, food intake or non-fasting plasma insulin, however, non-fasting plasma glucose concentrations were significantly lowered (p < 0.05). Moreover, glucose tolerance was significantly improved (p < 0.05) together with glucose-mediated plasma insulin responses (p < 0.05). Insulin sensitivity, pancreatic insulin content, triglyceride and adiponectin levels were not changed. In summary, these data demonstrate that C-terminal mini-PEGylation of GIP is a useful strategy to prolong metabolic stability and improve biological action thus representing a novel therapeutic option for type 2 diabetes.