Recent advances in understanding GLP-1R (glucagon-like peptide-1 receptor) function. Biochem Soc Trans. 2013;41:172-179. [PMID: 23356279 DOI: 10.1042/bst20120236]

Binding sites and design strategies for small molecule GLP-1R agonists

Glucagon-like peptide-1 receptor (GLP-1R) is a pivotal receptor involved in blood glucose regulation and influencing feeding behavior. It has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. Peptide GLP-1 receptor agonists (GLP-1RAs) have achieved tremendous success in the market, driving the vigorous development of small molecule GLP-1RAs. Currently, several small molecules have entered the clinical research stage. Additionally, recent discoveries of GLP-1R positive allosteric modulators (PAMs) are also unveiling new regulatory patterns and treatment methods. This article reviews the structure and functional mechanisms of GLP-1R, recent reports on small molecule GLP-1RAs and PAMs, as well as the optimization process. Furthermore, it combines computer simulations to analyze structure-activity relationships (SAR) studies, providing a foundation for exploring new strategies for designing small molecule GLP-1RAs.

Chronic cold stress promotes inflammation and ER stress via inhibiting GLP-1R signaling, and exacerbates the risk of ferroptosis in the liver and pancreas

The cold climates in autumn and winter threatens human health. The aim of this study was to reveal the effects of prolonged cold exposure on the liver and pancreas based on GLP-1R signaling, oxidative stress, endoplasmic reticulum (ER) stress and ferroptosis by Yorkshire pig models. Yorkshire pigs were divided into the control group and chronic cold stress (CCS) group. The results showed that CCS induced oxidative stress injury, activated Nrf2 pathway and inhibited the expression of GLP-1R in the liver and pancreas (P < 0.05). The toll-like receptor 4 (TLR4) pathway was activated in the liver and pancreas, accompanied by the enrichment of IL-1β and TNF-α during CCS (P < 0.05). Moreover, the kinase RNA-like endoplasmic reticulum kinase (PERK), inositol requiring kinase 1 (IRE1), X-box-binding protein 1 (XBP1) and eukaryotic initiation factor 2α (eIF2α) expression in the liver and pancreas was up-regulated during CCS (P < 0.05). In addition, CCS promoted the prostaglandin-endoperoxide synthase 2 (PTGS2) expression and inhibited the ferritin H (FtH) expression in the liver. Summarily, CCS promotes inflammation, ER stress and apoptosis by inhibiting the GLP-1R signaling and inducing oxidative stress, and exacerbates the risk of ferroptosis in the liver and pancreas.

Molecular dynamics-guided optimization of BGM0504 enhances dual-target agonism for combating diabetes and obesity

The dual activation of glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) has emerged as a promising therapeutic strategy for managing type 2 diabetes and obesity. Tirzepatide, a dual agonist peptide, has exhibited superior clinical efficacy in glycemic and weight control compared to selective GLP-1R agonists. Nevertheless, the structural basis of Tirzepatide's extended half-life, attributed to an acylation side chain on the parent peptide, raises questions regarding its partial agonistic activity. Employing molecular dynamics simulations, we explored the dynamic processes of peptide-receptor interactions. We uncovered a crucial salt bridge between parent peptide and GLP-1R/GIPR at K20, a feature not discernible in cryo-electron microscopy structures. Building upon these insights, we developed an optimization strategy based on the parent peptide which involved repositioning the acylation side chain. The results of both in vitro and in vivo experiments demonstrated that the optimized peptide has twofold to threefold increase in agonistic activity compared to Tirzepatide while maintaining its extended half-life in plasma. This led to the design of BGM0504, which proved to be more effective than its predecessor, Tirzepatide, in both laboratory and animal studies.

Palmitoylation of the Glucagon-like Peptide-1 Receptor Modulates Cholesterol Interactions at the Receptor-Lipid Microenvironment

The G protein-coupled receptor (GPCR) superfamily of cell surface receptors has been shown to be functionally modulated by post-translational modifications. The glucagon-like peptide receptor-1 (GLP-1R), which is a drug target in diabetes and obesity, undergoes agonist-dependent palmitoyl tail conjugation. The palmitoylation in the C-terminal domain of GLP-1R has been suggested to modulate the receptor-lipid microenvironment. In this work, we have performed coarse-grain molecular dynamics simulations of palmitoylated and nonpalmitoylated GLP-1R to analyze the differential receptor-lipid interactions. Interestingly, the placement and dynamics of the C-terminal domain of GLP-1R are found to be directly dependent on the palmitoyl tail. We observe that both cholesterol and phospholipids interact with the receptor but display differential interactions in the presence and absence of the palmitoyl tail. We characterize important cholesterol-binding sites and validate sites that have been previously reported in experimentally resolved structures of the receptor. We show that the receptor acts like a conduit for cholesterol flip-flop by stabilizing cholesterol in the membrane core. Taken together, our work represents an important step in understanding the molecular effects of lipid modifications in GPCRs.

Conserved class B GPCR activation by a biased intracellular agonist

Class B G-protein-coupled receptors (GPCRs), including glucagon-like peptide 1 receptor (GLP1R) and parathyroid hormone 1 receptor (PTH1R), are important drug targets1-5. Injectable peptide drugs targeting these receptors have been developed, but orally available small-molecule drugs remain under development6,7. Here we report the high-resolution structure of human PTH1R in complex with the stimulatory G protein (Gs) and a small-molecule agonist, PCO371, which reveals an unexpected binding mode of PCO371 at the cytoplasmic interface of PTH1R with Gs. The PCO371-binding site is totally different from all binding sites previously reported for small molecules or peptide ligands in GPCRs. The residues that make up the PCO371-binding pocket are conserved in class B GPCRs, and a single alteration in PTH2R and two residue alterations in GLP1R convert these receptors to respond to PCO371. Functional assays reveal that PCO371 is a G-protein-biased agonist that is defective in promoting PTH1R-mediated arrestin signalling. Together, these results uncover a distinct binding site for designing small-molecule agonists for PTH1R and possibly other members of the class B GPCRs and define a receptor conformation that is specific only for G-protein activation but not arrestin signalling. These insights should facilitate the design of distinct types of class B GPCR small-molecule agonist for various therapeutic indications.

Protective role of IGF-1 and GLP-1 signaling activation in neurological dysfunctions

Insulin-like growth factor-1 (IGF-1), a pleiotropic polypeptide, plays an essential role in CNS development and maturation. Glucagon-like peptide-1 (GLP-1) is an endogenous incretin hormone that regulates blood glucose levels and fatty acid oxidation in the brain. GLP-1 also exhibits similar functions and growth factor-like properties to IGF-1, which is likely how it exerts its neuroprotective effects. Recent preclinical and clinical evidence indicate that IGF-1 and GLP-1, apart from regulating growth and development, prevent neuronal death mediated by amyloidogenesis, cerebral glucose deprivation, neuroinflammation and apoptosis through modulation of PI3/Akt kinase, mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK/ERK). IGF-1 resistance and GLP-1 deficiency impair protective cellular signaling mechanisms, contributing to the progression of neurodegenerative diseases. Over the past decades, IGF-1 and GLP-1 have emerged as an essential component of the neuronal system and as potential therapeutic targets for several neurodegenerative and neuropsychiatric dysfunctions. There is substantial evidence that IGF-1 and GLP-1 analogues penetrate the blood-brain barrier (BBB) and exhibit neuroprotective functions, including synaptic formation, neuronal plasticity, protein synthesis, and autophagy. Conclusively, this review represents the therapeutic potential of IGF-1 and GLP-1 signaling target activators in ameliorating neurological disorders.

2-Aminothiophene derivatives as a new class of positive allosteric modulators of glucagon-like peptide 1 receptor

We report the discovery of two new 2-aminothiophene based small molecule positive allosteric modulators (PAMs) of glucagon-like peptide 1 receptor (GLP-1R) for the treatment of type 2 diabetes. One of the chemotypes, (S-1), has a molecular weight of 239 g/mol, the smallest molecule among all reported GLP-1R PAMs. When combined with GLP-1 peptide, S-1 increased the GLP-1R activity in a dose-dependent manner in a cell-based assay. When combined with the peptide agonist of vasoactive intestinal polypeptide receptor 1 (VIPR1), S-1 showed no specific activity on VIPR1, another class B GPCR present in the same HEK293-CREB cell line. Insulin secretion studies found S-1 combined with GLP-1 increased insulin secretion by 1.5-fold at 5 μM. In a mechanistic study, evidence is provided that the synergistic effect of S-1 with GLP-1 may be partly due to the enhanced impact on CREB based phosphorylation. Given the favorable profile of these chemotypes, the work reported herein suggests that 2-aminothiophene derivatives are a new and promising class of GLP-1R PAMs.

The mechanism and efficacy of GLP-1 receptor agonists in the treatment of Alzheimer's disease

Since type 2 diabetes mellitus (T2DM) is a risk factor for Alzheimer's disease (AD) and both have the same pathogenesis (e.g., insulin resistance), drugs used to treat T2DM have been gradually found to reduce the progression of AD in AD models. Of these drugs, glucagon-like peptide 1 receptor (GLP-1R) agonists are more effective and have fewer side effects. GLP-1R agonists have reducing neuroinflammation and oxidative stress, neurotrophic effects, decreasing Aβ deposition and tau hyperphosphorylation in AD models, which may be a potential drug for the treatment of AD. However, this needs to be verified by further clinical trials. This study aims to summarize the current information on the mechanisms and effects of GLP-1R agonists in AD.

Allosteric Modulators Enhancing GLP-1 Binding to GLP-1R via a Transmembrane Site

The glucagon-like peptide-1 receptor (GLP-1R) is a well-established drug target for the treatment of type II diabetes. The development of small-molecule positive allosteric modulators (PAMs) of GLP-1R is a promising therapeutic strategy. Here, we report the discovery and characterization of PAMs with distinct chemotypes, binding to a cryptic pocket formed by the cytoplasmic half of TM3, TM5, and TM6. Molecular dynamic simulations and mutagenesis studies indicate that the PAM enlarges the orthosteric pocket to facilitate GLP-1 binding. Further signaling assays characterized their probe-dependent signaling profiles. Our findings provide mechanistic insights into fine-tuning GLP-1R via this allosteric pocket and open up new avenues to design small-molecule drugs for class B G-protein-coupled receptors.

Emodin Attenuated the Kidney Damage of High-Fat-Diet Mice via the Upregulation of Glucagon-Like Peptide-1 Receptor

Objective

Secretion of glucagon-like peptide 1 (GLP-1) and its effect on target organs were impaired in individuals with obesity. However, its mechanism needs to be further studied. We aim to explore the roles of the receptor of GLP-1 (GLP-1R) involved in high-fat-diet- (HFD-) induced kidney damage improved by emodin.

Methods

Male C57bl/6 mice were fed with HFD diet and therapied by emodin. NRK-52E cells were cultured and treated with palmitic acid or low-density lipoprotein cholesterol (LDL-C). Emodin was used to remedy the NRK-52E cell damage. GW9662 was administrated to block the function of peroxisome proliferator-activated receptor γ (PPAR-γ). GLP-1 in the plasma was measured by ELISA. PPAR-γ and GLP-1R in the kidney and NRK-52E cells were detected by western blotting. The interaction between PPAR-γ protein and GLP-1R promoter regions was observed by chromatin immunoprecipitation (ChIP).

Results

Postprandial GLP-1 levels in plasma, as well as PPAR-γ and GLP-1R, decreased in kidney tissue of HFD mice, while they were reserved by emodin treatment. Although PPAR-γ and GLP-1R were not downregulated by LDL-C, they were suppressed by palmitic acid. Interestingly, GLP-1R mRNA was detected by PCR in the mixture pulled down with PPAR-γ antibody. Additionally, downregulation of PPAR-γ and GLP-1R by palmitic acid was remanded by emodin. Moreover, GW9662, an inhibitor of PPAR-γ, abolished the protective effect of emodin.

Conclusion

The kidney damage of HFD mice seems to be alleviated by emodin via the upregulation of GLP-1R in kidney tissue.

Sepsis plasma-derived exosomal miR-1-3p induces endothelial cell dysfunction by targeting SERP1

Acute lung injury (ALI) is the leading cause of death in sepsis patients. Exosomes participate in the occurrence and development of ALI by regulating endothelial cell inflammatory response, oxidative stress and apoptosis, causing serious pulmonary vascular leakage and interstitial edema. The current study investigated the effect of exosomal miRNAs on endothelial cells during sepsis. We found a significant increase in miR-1-3p expression in cecal ligation and puncture (CLP) rats exosomes sequencing and sepsis patients' exosomes, and lipopolysaccharide (LPS)-stimulated human umbilical vein endothelial cells (HUVECs) in vitro. However, the specific biological function of miR-1-3p in ALI remains unknown. Therefore, mimics or inhibitors of miR-1-3p were transfected to modulate its expression in HUVECs. Cell proliferation, apoptosis, contraction, permeability, and membrane injury were examined via cell counting kit-8 (CCK-8), flow cytometry, phalloidin staining, Transwell assay, lactate dehydrogenase (LDH) activity, and Western blotting. The miR-1-3p target gene was predicted with miRNA-related databases and validated by luciferase reporter. Target gene expression was blocked by siRNA to explore the underlying mechanisms. The results illustrated increased miR-1-3p and decreased stress-associated endoplasmic reticulum protein 1 (SERP1) expression both in vivo and in vitro. SERP1 was a direct target gene of miR-1-3p. Up-regulated miR-1-3p inhibits cell proliferation, promotes apoptosis and cytoskeleton contraction, increases monolayer endothelial cell permeability and membrane injury by targeting SERP1, which leads to dysfunction of endothelial cells and weakens vascular barrier function involved in the development of ALI. MiR-1-3p and SERP1 may be promising therapeutic candidates for sepsis-induced lung injury.

Total synthesis and absolute structure of N55, a positive modulator of GLP-1 signaling

Glucagon-like peptide-1 (GLP-1) signaling is an established therapeutic target for type 2 diabetes mellitus (T2DM). We developed a 7-step synthesis of N55, a positive modulator of GLP-1 signaling isolated from fenugreek (Trigonella foenum-graecum) seeds, with 29% overall yield, and we determined the absolute structure of N55 to be N-((3R,4R,5S)-4,5-dimethyl-2-oxotetrahydrofur-3-yl)linoleic amide.

Display of quintuple glucagon-like peptide 1 (28-36) nonapeptide on Bacillus subtilis spore for oral administration in the treatment of type 2 diabetes

AIMS

To develop an oral delivery system of glucagon-like peptide 1 (GLP-1) (28-36) for treating type-2 diabetes, B.S-GLP-1(28-36), a recombinant Bacillus subtilis spores transformed with a plasmid vector encoding five consecutive GLP-1 (28-36) nonapeptides with an enterokinase site was constructed.

METHODS AND RESULTS

GLP-1(28-36) nonapeptide was successfully expressed on the surface of B. subtilis spores and validated by Western blot and immunofluorescence. The therapeutic effect of oral administration of B.S-GLP-1(28-36) spores was evaluated in type 2 diabetic model mice. The efficacy of recombinant spores was examined for a period of 13 weeks after oral administration in diabetic mice. At the end of the sixth week, diabetic mice with oral administration of BS-GLP-1(28-36) spores showed decreased blood glucose levels from 2·4 × 10^{- 2}  mol l^-1 to 1·7 × 10^{- 2}  mol l^-1 . By the ninth week, the mean fasting blood glucose level in the experimental group was significantly lower than that in the control group 30 min after injection of pyruvate. At the end of the 10th week of oral administration, the blood glucose of the experimental group was significantly lower than that of the control group after intraperitoneal injection of glucose. By the 12th week, fasting blood glucose level and fasting insulin level were measured in all mice, the results showed that the recombinant spores increased the insulin sensitivity of mice.

CONCLUSIONS

The results of pathological observation showed that the recombinant spores also had a certain protective effect on the liver and islets of mice, and the content of GLP-1(28-36) in the pancreas of the experimental group was increased.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study revealed that GLP-1(28-36) nonapeptides can reduce blood glucose and play an important role in the treatment of type 2 diabetes.

Recent advances in understanding the role of glucagon-like peptide 1

The discovery that glucagon-like peptide 1 (GLP-1) mediates a significant proportion of the incretin effect during the postprandial period and the subsequent observation that GLP-1 bioactivity is retained in type 2 diabetes (T2D) led to new therapeutic strategies being developed for T2D treatment based on GLP-1 action. Although owing to its short half-life exogenous GLP-1 has no use therapeutically, GLP-1 mimetics, which have a much longer half-life than native GLP-1, have proven to be effective for T2D treatment since they prolong the incretin effect in patients. These GLP-1 mimetics are a desirable therapeutic option for T2D since they do not provoke hypoglycaemia or weight gain and have simple modes of administration and monitoring. Additionally, over more recent years, GLP-1 action has been found to mediate systemic physiological beneficial effects and this has high clinical relevance due to the post-diagnosis complications of T2D. Indeed, recent studies have found that certain GLP-1 analogue therapies improve the cardiovascular outcomes for people with diabetes. Furthermore, GLP-1-based therapies may enable new therapeutic strategies for diseases that can also arise independently of the clinical manifestation of T2D, such as dementia and Parkinson's disease. GLP-1 functions by binding to its receptor (GLP-1R), which expresses mainly in pancreatic islet beta cells. A better understanding of the mechanisms and signalling pathways by which acute and chronic GLP-1R activation alleviates disease phenotypes and induces desirable physiological responses during healthy conditions will likely lead to the development of new therapeutic GLP-1 mimetic-based therapies, which improve prognosis to a greater extent than current therapies for an array of diseases.

Discovery of a potential positive allosteric modulator of glucagon-like peptide 1 receptor through virtual screening and experimental study

The Glucagon-like peptide 1 receptor (GLP-1R) is a well-established target for the treatment of type 2 diabetes and GLP-1R agonist-based therapies represent an effective approach which results in several GLP-1 analog drugs. However, the development of nonpeptidic agonist drugs targeting GLP-1R remains unsuccessful. A promising strategy aims to develop orally bioavailable, small-molecule positive allosteric modulators of GLP1-1R. Taking advantage of the recently reported cryo-EM structure of GLP-1R at its active state, we have performed structure-based screening studies which include potential allosteric binding site prediction and in silico screening of drug-like compounds, and conducted in vitro testing and site-specific mutagenesis studies. One compound with low molecular weight was confirmed as a positive allosteric modulator of GLP-1R as it enhances GLP-1's affinity and efficacy to human GLP-1R in a dose dependent manner. This compound also stimulates insulin secretion synergistically with GLP-1. With the molecular weight of 399, this compound represents one of the smallest known GLP-1R PAMs, and demonstrates other favorable drug-like properties. Site-specific mutagenesis studies confirmed that the binding site of this compound partially overlaps with that of a known antagonist in the transmembrane domain. These results demonstrate that structure-based approach is useful for discovering nonpeptidic allosteric modulators of GLP-1R and the compound reported here is valuable for further drug development.

Resistance Exercise Training Attenuates the Loss of Endogenous GLP-1 Receptor in the Hypothalamus of Type 2 Diabetic Rats

The aim of this study was to investigate the effects of resistance exercise training on hypothalamic GLP-1R levels and its related signaling mechanisms in T2DM. The animals were separated into three groups: a non-diabetic control (CON), diabetic control (DM), and diabetic with resistance exercise (DM + EXE) group. The resistance exercise training group performed ladder climbing (eight repetitions, three days per week for 12 weeks). Body weight was slightly lower in the DM + EXE group than the DM group, but difference between the groups was not significant. Food intake and glucose were significantly lower in the DM + EXE group than in the DM group. The blood insulin concentration was significantly higher and glucagon was significantly lower in the DM + EXE group. The DM + EXE group in the hypothalamus showed significant increases in GLP-1R mRNA, protein kinase A (PKA), glucose transporter 2 (GLUT2), and protein kinase B (AKT) and significant decrease in protein kinase C-iota (PKC-iota). Antioxidant enzymes and apoptosis factors were significantly improved in the DM + EXE group compared with the DM group in the hypothalamus. The results suggest that resistance exercise contributes to improvements the overall health of the brain in diabetic conditions.

Structural Modeling and in Silico Screening of Potential Small-Molecule Allosteric Agonists of a Glucagon-like Peptide 1 Receptor

The glucagon-like peptide 1 receptor (GLP-1R) belongs to the pharmaceutically important class B family of G-protein-coupled receptors (GPCRs), and its incretin peptide ligand GLP-1 analogs are adopted drugs for the treatment of type 2 diabetes. Despite remarkable antidiabetic effects, GLP-1 peptide-based drugs are limited by the need of injection. On the other hand, developing nonpeptidic small-molecule drugs targeting GLP-1R remains elusive. Here, we first constructed a three-dimensional structure model of the transmembrane (TM) domain of human GLP-1R using homology modeling and conformational sampling techniques. Next, a potential allosteric binding site on the TM domain was predicted computationally. In silico screening of druglike compounds against this predicted allosteric site has identified nine compounds as potential GLP-1R agonists. The independent agonistic activity of two compounds was subsequently confirmed using a cAMP response element-based luciferase reporting system. One compound was also shown to stimulate insulin secretion through in vitro assay. In addition, this compound synergized with GLP-1 to activate human GLP-1R. These results demonstrated that allosteric regulation potentially exists in GLP-1R and can be exploited for developing small-molecule agonists. The success of this work will help pave the way for small-molecule drug discovery targeting other class B GPCRs through allosteric regulations.

An Overview, Advantages and Therapeutic Potential of Nonpeptide Positive Allosteric Modulators of Glucagon-Like Peptide-1 Receptor

Due to uncomfortable injection regimens of peptidic agonists of glucagon-like peptide-1 receptor (GLP-1R), orally available nonpeptide positive allosteric modulators (PAMs) of GLP-1Rs are foreseen as the possible future mainstream therapy for type 2 diabetes. Herein, current GLP-1R PAMs are reviewed. Based on the effectiveness and in silico predicted physicochemical properties, pharmacokinetics, and toxicity, possible candidates for further development as oral drugs were selected. The suggestion is that GLP-1R PAMs might be used orally alone or in combination with dipeptidyl peptidase-4 (DPP-4) inhibitors, which could offer an optimal treatment option next to metformin monotherapy in type 2 diabetes mellitus, or in a wider spectrum of indications. Quercetin acts as a GLP-1R PAM and DPP-4 inhibitor, and therefore, might be considered as a pioneering agent with a dual mechanism of action, in terms of GLP-1R positive allosteric modulation and DPP-4 inhibition for potentiating GLP-1 dependent effects.

GSK-3 inhibition through GLP-1R allosteric activation mediates the neurogenesis promoting effect of P7C3 after cerebral ischemic/reperfusional injury in mice

An aminopropyl carbazole compound, P7C3, has been shown to be a potent neurogenesis promoting agent; however, its fundamental signaling action has yet to be elucidated. A cerebral ischemic/reperfusional (CI/R) injury model in mice was implemented to elucidate the neuronal protective mechanism(s) of P7C3. Treating CI/R mice using P7C3 (50-100 μg/kg, i.v.) significantly improved tracking distance and walking behavior, and reduced brain damage. Specifically, P7C3 promoted the expression of neurogenesis-associated proteins, including doublecortin, beta tubulin III (β-tub3), adam11 and adamts20, near the peri-infarct cortex, accompanied by glycogen synthase kinase 3 (GSK-3) inhibition and β-catenin upregulation. The application of a specific inhibitor against glucagon-like peptide 1 receptor (GLP-1R), exendin(9-39), revealed that the beneficial effects of P7C3 involved triggering the activation of GLP-1R-associated PKA/Akt signaling. P7C3 elicited the GLP-1R-dependent intracellular cAMP increment and the insulin secretion in cellular models. Surface plasmon resonance assay of P7C3 showed a Kd value of 0.53 μM for GLP-1R binding, and the docking of P7C3 to the putative active site on GLP-1R was successfully predicted by molecular modeling. Our findings indicate that P7C3 promotes the expression of neurogenesis proteins by activation of the cAMP/PKA-dependent and Akt/GSK3-associated β-catenin through positive allosteric stimulation of GLP-1R. Within the P7C3 class of neuroprotective molecules, this mechanism appears to be unique to the prototypical P7C3 molecule, as other active derivatives such as P7C2-A20 and P7C3-S243 they do not engage this same pathway and have been shown to work by nicotinamide phosphoribosyltransferase (NAMPT) stimulation.

A high-performance liquid chromatography-tandem mass spectrometry method for simultaneous determination of imigliptin, its five metabolites and alogliptin in human plasma and urine and its application to a multiple-dose pharmacokinetic study

Imigliptin is a novel DPP-4 inhibitor, designed to treat type 2 diabetes mellitus (T2DM). A selective and sensitive method was developed using high performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) to simultaneously quantify imigliptin, its five metabolites, and alogliptin in human plasma and urine. Solid-phase extraction (SPE) and direct dilution were used to extract imigliptin, its five metabolites, alogliptin from plasma and urine, respectively. The extracts were injected onto a SymmetryShield RP₈ column with a gradient elution of methanol and water containing 10 mM ammonium formate (pH = 7). Ionization of all analytes was performed using an electrospray ionization (ESI) source in positive mode and detection was carried out with multiple reaction monitoring (MRM) mode. The results revealed that the method had excellent selectivity and linearity. Inter- and intra-batch precisions of all analytes were less than 15% and the accuracies were within 85%-115% for both plasma and urine. The sensitivity, matrix effect, extraction recovery, linearity, and stabilities were validated for all analytes in human plasma and urine. In conclusion, the validation results showed that this method was robust, specific, and sensitive and it can successfully applied to a pharmacokinetic study of Chinese T2DM subjects after oral dose of imigliptin and alogliptin.

Metabolites characterization of a novel DPP-4 inhibitor, imigliptin in humans and rats using ultra-high performance liquid chromatography coupled with synapt high-resolution mass spectrometry

Imigliptin has been reported as a novel dipeptidyl-peptidase-IV (DPP-4) inhibitor to treat type 2 Diabetes Mellitus (T2DM), and is currently being tested in clinical trials. In the first human clinical study, imigliptin was well tolerated and proved to be a potent DPP-4 inhibitor. Considering its potential therapeutic benefits and promising future, it is of great importance to study the metabolite profiles in the early stage of drug development. In the present study, a robust and reliable analytical method based on the ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/Q-TOF MS) method combined with MassLynx software was established to investigate the characterization of metabolites of imigliptin in human and rat plasma, urine and feces after oral administration. As a result, a total of 9 metabolites were identified in humans, including 6, 9 and 8 metabolites in human plasma, urine, and feces, respectively. A total of 11 metabolites were identified in rats, including 7, 10 and 8 metabolites in rat plasma, urine, and feces, respectively. In addition, 6 of the metabolites detected in humans and rats were phase I metabolites, including demethylation, carboxylation, hydroxylation and dehydrogenation metabolites, and 5 of the metabolites were phase II metabolites, including acetylation and glucuronidation. There was no human metabolite detected compared to those in rats. The major metabolites detected in human plasma (M1 and M2) were products resulting from acetylation, and hydroxylation followed by dehydrogenation. M1 was the major metabolite in rat plasma. M2 and the parent drug were the major drug-related substances in human urine. The parent drug was the major drug-related substances in rat urine. M2, M5 (hydroxylation product) and M6 (2 × hydroxylation and acetylation product) were the predominant metabolites in human feces. M2 and M5 were the major metabolites in rat feces. In addition, renal clearance was the major route of excretion for imigliptin.

Azoramide, a novel regulator, favors adipogenesis against osteogenesis through inhibiting the GLP-1 receptor-PKA-β-catenin pathway

BACKGROUND

The reciprocal fate decision of mesenchymal stem cells (MSCs) to either bone or adipocytes is determined by Wnt-related signaling and the glucagon-like peptide-1 receptor (GLP-1R). Azoramide, an ER stress alleviator, was reported to have an antidiabetic effect. In this study, we investigated the function of azoramide in regulating the lineage determination of MSCs for either adipogenic or osteogenic differentiation.

METHODS

In this study, microcomputed tomography and histological analysis on bone morphogenetic protein (BMP)2-induced parietal periosteum bone formation assays, C3H10T1/2 and mouse bone marrow MSC-derived bone formation and adipogenesis assays, and specific staining for bone tissue and lipid droplets were used to evaluate the role of azoramide on the lineage determination of MSC differentiation. Cells were harvested for Western blot and quantitative real-time polymerase chain reaction (PCR), and immunofluorescence staining was used to explore the potential mechanism of azoramide for regulating MSC differentiation.

RESULTS

Based on MSC-derived bone formation assays both in vivo and in vitro, azoramide treatment displayed a cell fate determining ability in favor of adipogenesis over osteogenesis. Further mechanistic characterizations disclosed that both the GLP-1R agonist peptide exendin-4 (Ex-4) and GLP-1R small interfering (si)RNA abrogated azoramide dual effects. Moreover, cAMP-protein kinase A (PKA)-mediated nuclear β-catenin activity was responsible for the negative function of azoramide on bone formation in favor of adipogenesis.

CONCLUSIONS

These data provide the first evidence to show that azoramide may serve as an antagonist against GLP-1R in MSC lineage determination.

Therapeutic potential of spinal GLP-1 receptor signaling

GLP-1 signaling pathway has been well studied for its role in regulating glucose homeostasis, as well as its beneficial effects in energy and nutrient metabolism. A number of drugs based on GLP-1 have been used to treat type 2 diabetes mellitus. GLP-1R is expressed in multiple organs and numerous experimental studies have demonstrated that GLP-1 signaling pathway exhibits pro-survival functions in various disorders. In the central nervous system, stimulation of GLP-1R produces neuroprotective effects in specific neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. The preproglucagon neurons located in the brainstem can also produce GLP-1. GLP-1 analogs have a long-acting effect and are able to pass the blood-brain barrier, which probably extends the therapeutic efficacy of GLP-1R activation. Neurodegenerative or traumatic conditions can damage the spinal cord and result in motor and sensory dysfunction. Evidence supports that GLP-1R activation in the spinal cord possesses beneficial effects and significant therapeutic potential. Herein, we review studies that have focused on GLP-1 and the spinal cord, and summarize the expression of GLP-1R and the innervation of PPG neurons in the spinal cord, as well as the potential therapeutic benefits of GLP-1R activation.

Glucagon-Like Peptide-1 (GLP-1)-Based Therapeutics: Current Status and Future Opportunities beyond Type 2 Diabetes

Glucagon-like peptide-1 (GLP-1) is secreted by intestinal L-cells following food intake, and plays an important role in glucose homeostasis due to its stimulation of glucose-dependent insulin secretion. Further, GLP-1 is also associated with protective effects on pancreatic β-cells and the cardiovascular system, decreased appetite, and weight loss, making GLP-1 derivatives an exciting treatment for type 2 diabetes and obesity. Despite these benefits, wild-type GLP-1 exhibits a short circulation time due to its poor metabolic stability and rapid renal clearance, and must be administered by injection, making it a poor therapeutic agent. Many strategies have been used to improve the circulation time of GLP-1 (e.g., mutations, unnatural amino acids, depot formulations, use of exendin-4 sequences, and fusions with high-molecular-weight proteins or polymers), with its therapeutic utility further improved by adding agonist activity for gastric inhibitory peptide and glucagon receptors. This minireview focuses on strategies that have been used to improve the pharmacokinetics of GLP-1 and provides an overview of GLP-1-based therapeutics in the pipeline.

Glucagon-Like Peptide-1 Mediates the Protective Effect of the Dipeptidyl Peptidase IV Inhibitor on Renal Fibrosis via Reducing the Phenotypic Conversion of Renal Microvascular Cells in Monocrotaline-Treated Rats

Chronic kidney diseases are characterized by renal fibrosis with excessive matrix deposition, leading to a progressive loss of functional renal parenchyma and, eventually, renal failure. Renal microcirculation lesions, including the phenotypic conversion of vascular cells, contribute to renal fibrosis. Here, renal microcirculation lesions were established with monocrotaline (MCT, 60 mg/kg). Sitagliptin (40 mg/kg/d), a classical dipeptidyl peptidase-4 (DPP-4) inhibitor, attenuated the renal microcirculation lesions by inhibiting glomerular tuft hypertrophy, glomerular mesangial expansion, and microvascular thrombosis. These effects of sitagliptin were mediated by glucagon-like peptide-1 receptor (GLP-1R), since they were blocked by the GLP-1R antagonist exendin-3 (Ex-3, 40 ug/kg/d). The GLP-1R agonist liraglutide showed a similar renal protective effect in a dose-independent manner. In addition, sitagliptin, as well as liraglutide, alleviated the MCT-induced apoptosis of renal cells by increasing the expression of survival factor glucose-regulated protein 78 (GRP78), which was abolished by the GLP-1R antagonist Ex-3. Sitagliptin and liraglutide also effectively ameliorated the conversion of vascular smooth muscle cells (SMCs) from a synthetic phenotype to contractile phenotype. Moreover, sitagliptin and liraglutide inhibited endothelial-mesenchymal transition (EndMT) via downregulating transforming growth factor-β1 (TGF-β1). Collectively, these findings suggest that DPP-4 inhibition can reduce microcirculation lesion-induced renal fibrosis in a GLP-1-dependent manner.

Liraglutide, a glucagon-like peptide-1 receptor agonist, facilitates osteogenic proliferation and differentiation in MC3T3-E1 cells through phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), extracellular signal-related kinase (ERK)1/2, and cAMP/protein kinase A (PKA) signaling pathways involving β-catenin

Previous studies have proven that glucagon-like peptide-1 (GLP-1) and its receptor agonist exert favorable anabolic effects on skeletal metabolism. However, whether GLP-1 could directly impact osteoblast-mediated bone formation is still controversial, and the underlying molecular mechanism remains to be elucidated. Thus in this paper, we investigated the effects of liraglutide, a glucagon-like peptide-1 (GLP-1) receptor agonist, on murine MC3T3-E1 preosteoblasts proliferation and differentiation and explored the potential cellular basis. Our study confirmed the presence of GLP-1R in MC3T3-E1, and demonstrated that liraglutide promotes osteoblasts proliferation at an intermediate concentration (100nM) and time (48h), upregulated the expression of osteoblastogenic biomarkers at various stages, and stimulated osteoblastic mineralization. Liraglutide also elevated the intracellular cAMP level and phosphorylation of AKT, ERK and β-catenin simultaneously with increased nuclear β-catenin content and transcriptional activity. Pretreatment of cells with the inhibitors LY294002, PD98059, H89 and GLP-1R and β-catenin siRNA partially blocked the liraglutide-induced signaling activation and attenuated the facilitating effect of liraglutide on MC3T3-E1 cells. Collectively, liraglutide was capable of acting upon osteoblasts directly through GLP-1R by activating PI3K/AKT, ERK1/2, cAMP/PKA/β-cat-Ser675 signaling to promote bone formation via GLP-1R. Thus, GLP-1 analogues may be potential therapeutic strategy for the treatment of osteoporosis in diabetics.

Genetically encoded photocross-linkers determine the biological binding site of exendin-4 peptide in the N-terminal domain of the intact human glucagon-like peptide-1 receptor (GLP-1R)

The glucagon-like peptide-1 receptor (GLP-1R) is a key therapeutic target in the management of type II diabetes mellitus, with actions including regulation of insulin biosynthesis and secretion, promotion of satiety, and preservation of β-cell mass. Like most class B G protein-coupled receptors (GPCRs), there is limited knowledge linking biological activity of the GLP-1R with the molecular structure of an intact, full-length, and functional receptor·ligand complex. In this study, we have utilized genetic code expansion to site-specifically incorporate the photoactive amino acid p-azido-l-phenylalanine (azF) into N-terminal residues of a full-length functional human GLP-1R in mammalian cells. UV-mediated photolysis of azF was then carried out to induce targeted photocross-linking to determine the proximity of the azido group in the mutant receptor with the peptide exendin-4. Cross-linking data were compared directly with the crystal structure of the isolated N-terminal extracellular domain of the GLP-1R in complex with exendin(9-39), revealing both similarities as well as distinct differences in the mode of interaction. Generation of a molecular model to accommodate the photocross-linking constraints highlights the potential influence of environmental conditions on the conformation of the receptor·peptide complex, including folding dynamics of the peptide and formation of dimeric and higher order oligomeric receptor multimers. These data demonstrate that crystal structures of isolated receptor regions may not give a complete reflection of peptide/receptor interactions and should be combined with additional experimental constraints to reveal peptide/receptor interactions occurring in the dynamic, native, and full-length receptor state.

Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes

The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.

Morroniside, a secoiridoid glycoside from Cornus officinalis, attenuates neuropathic pain by activation of spinal glucagon-like peptide-1 receptors

BACKGROUND AND PURPOSE

Iridoid glycosides containing the double bond scaffold of cyclopentapyran are reversible and orthosteric agonists of glucagon-like peptide-1 (GLP-1) receptors and exert anti-nociceptive and neuroprotective actions. Morroniside, derived from the medicinal herb Cornus officinalis, is an atypical secoiridoid containing a six-membered cyclic inner ether fragment. Here we investigated whether morroniside was an orthosteric GLP-1 receptor agonist and had anti-hypersensitivity activities in a model of neuropathic pain.

EXPERIMENTAL APPROACH

We used a model of neuropathic pain, induced by tight ligation of L5/L6 spinal nerves in rats. Hydrogen peroxide-induced oxidative damage was also assayed in N9 microglial cells and human HEK293 cells stably expressing GLP-1 receptors.

KEY RESULTS

Morroniside protected against hydrogen peroxide-induced oxidative damage in N9 microglial and HEK293 cells that expressed mouse or human GLP-1 receptors, but not in HEK293T cells without GLP-1 receptors. The GLP-1 receptor orthosteric antagonist exendin(9-39) also concentration-dependently shifted the concentration-protective response curves of morroniside and exenatide to the right without affecting maximal protection, with similar pA₂ values. Furthermore, morroniside given by oral gavage or intrathecally in neuropathic rats dose-dependently attenuated mechanical allodynia, with comparable E_max values and ED₅₀ s of 335 mg·kg^-1 and 7.1 μg and completely blocked thermal hyperalgesia. Daily intrathecal injections of morroniside over 7 days did not induce anti-allodynic tolerance. Pretreatment with intrathecal exendin(9-39) completely blocked systemic and intrathecal morroniside-induced mechanical anti-allodynia.

CONCLUSION AND IMPLICATIONS

Our data demonstrated that morroniside was an orthosteric agonist of GLP-1 receptors and produced antihypersensitivity in a neuropathic pain model by activation of spinal GLP-1 receptors.

p38β Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial β-Endorphin Expression

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1Rs) mediates neuroprotection and antinociception through microglial β-endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial β-endorphin. GLP-1Rs and β-endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1R agonist exenatide concentration-dependently stimulated microglial expression of the β-endorphin precursor gene proopiomelanocortin (POMC) and peptides, with EC₅₀ values of 4.1 and 7.5 nM, respectively. Exenatide also significantly increased intracellular cAMP levels and expression of p-protein kinase A (PKA), p-p38, and p-cAMP response element binding protein (CREB) in cultured primary microglia. Furthermore, exenatide-induced microglial expression of POMC was completely blocked by reagents that specifically inhibit adenylyl cyclase and activation of PKA, p38, and CREB. In addition, knockdown of p38β (but not p38α) using short interfering RNA (siRNA) eliminated exenatide-induced microglial p38 phosphorylation and POMC expression. In contrast, lipopolysaccharide increased microglial activation of p38, and knockdown of p38α (but not p38β) partially suppressed expression of proinflammatory factors (including tumor necrosis factor-α, interleukin-1β, and interleukin-6). Exenatide-induced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38β, but not by siRNA/p38α Seven-day intrathecal injections of siRNA/p38β (but not siRNA/p38α) completely blocked exenatide-induced spinal p38 activation, β-endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38β and siRNA/p38α were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38β mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism-mediated microglial β-endorphin expression. The differential role of p38α and p38β activation in inflammation and nociception was also highlighted.

Discovery of a Novel Series of Orally Bioavailable and CNS Penetrant Glucagon-like Peptide-1 Receptor (GLP-1R) Noncompetitive Antagonists Based on a 1,3-Disubstituted-7-aryl-5,5-bis(trifluoromethyl)-5,8-dihydropyrimido[4,5-d]pyrimidine-2,4(1H,3H)-dione Core

A duplexed, functional multiaddition high throughput screen and subsequent optimization effort identified the first orally bioavailable and CNS penetrant glucagon-like peptide-1 receptor (GLP-1R) noncompetitive antagonist. Antagonist 5d not only blocked exendin-4-stimulated insulin release in islets but also lowered insulin levels while increasing blood glucose in vivo.

Monotreme glucagon-like peptide-1 in venom and gut: one gene - two very different functions

The importance of Glucagon like peptide 1 (GLP-1) for metabolic control and insulin release sparked the evolution of genes mimicking GLP-1 action in venomous species (e.g. Exendin-4 in Heloderma suspectum (gila monster)). We discovered that platypus and echidna express a single GLP-1 peptide in both intestine and venom. Specific changes in GLP-1 of monotreme mammals result in resistance to DPP-4 cleavage which is also observed in the GLP-1 like Exendin-4 expressed in Heloderma venom. Remarkably we discovered that monotremes evolved an alternative mechanism to degrade GLP-1. We also show that monotreme GLP-1 stimulates insulin release in cultured rodent islets, but surprisingly shows low receptor affinity and bias toward Erk signaling. We propose that these changes in monotreme GLP-1 are the result of conflicting function of this peptide in metabolic control and venom. This evolutionary path is fundamentally different from the generally accepted idea that conflicting functions in a single gene favour duplication and diversification, as is the case for Exendin-4 in gila monster. This provides novel insight into the remarkably different metabolic control mechanism and venom function in monotremes and an unique example of how different selective pressures act upon a single gene in the absence of gene duplication.

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

Puerarin Protects Pancreatic β-Cells in Obese Diabetic Mice via Activation of GLP-1R Signaling

Diabetes is characterized by a loss and dysfunction of the β-cell. Glucagon-like peptide 1 receptor (GLP-1R) signaling plays an important role in β-cell survival and function. It is meaningful to identify promising agents from natural products which might activate GLP-1R signaling. In this study, puerarin, a diet isoflavone, was evaluated its beneficial effects on β-cell survival and GLP-1R pathway. We showed that puerarin reduced the body weight gain, normalized blood glucose, and improved glucose tolerance in high-fat diet-induced and db/db diabetic mice. Most importantly, increased β-cell mass and β-cell proliferation but decreased β-cell apoptosis were observed in puerarin-treated diabetic mice as examined by immunostaining of mice pancreatic sections. The protective effect of puerarin on β-cell survival was confirmed in isolated mouse islets treated with high glucose. Further mechanism studies showed that the circulating level of GLP-1 in mice was unaffected by puerarin. However, puerarin enhanced GLP-1R signaling by up-regulating expressions of GLP-1R and pancreatic and duodenal homeobox 1, which subsequently led to protein kinase B (Akt) activation but forkhead box O1 inactivation, and promoted β-cell survival. The protective effect of puerarin was remarkably suppressed by Exendin(9-39), an antagonist of GLP-1R. Our study demonstrated puerarin improved glucose homeostasis in obese diabetic mice and identified a novel role of puerarin in protecting β-cell survival by mechanisms involving activation of GLP-1R signaling and downstream targets.

A Hydrogen-Bonded Polar Network in the Core of the Glucagon-Like Peptide-1 Receptor Is a Fulcrum for Biased Agonism: Lessons from Class B Crystal Structures

The glucagon-like peptide 1 (GLP-1) receptor is a class B G protein-coupled receptor (GPCR) that is a key target for treatments for type II diabetes and obesity. This receptor, like other class B GPCRs, displays biased agonism, though the physiologic significance of this is yet to be elucidated. Previous work has implicated R2.60(190), N3.43(240), Q7.49(394), and H6.52(363) as key residues involved in peptide-mediated biased agonism, with R2.60(190), N3.43(240), and Q7.49(394) predicted to form a polar interaction network. In this study, we used novel insight gained from recent crystal structures of the transmembrane domains of the glucagon and corticotropin releasing factor 1 (CRF1) receptors to develop improved models of the GLP-1 receptor that predict additional key molecular interactions with these amino acids. We have introduced E6.53(364)A, N3.43(240)Q, Q7.49(394)N, and N3.43(240)Q/Q7.49(394)N mutations to probe the role of predicted H-bonding and charge-charge interactions in driving cAMP, calcium, or extracellular signal-regulated kinase (ERK) signaling. A polar interaction between E6.53(364) and R2.60(190) was predicted to be important for GLP-1- and exendin-4-, but not oxyntomodulin-mediated cAMP formation and also ERK1/2 phosphorylation. In contrast, Q7.49(394), but not R2.60(190)/E6.53(364) was critical for calcium mobilization for all three peptides. Mutation of N3.43(240) and Q7.49(394) had differential effects on individual peptides, providing evidence for molecular differences in activation transition. Collectively, this work expands our understanding of peptide-mediated signaling from the GLP-1 receptor and the key role that the central polar network plays in these events.

International Union of Basic and Clinical Pharmacology. XCIII. The parathyroid hormone receptors--family B G protein-coupled receptors

The type-1 parathyroid hormone receptor (PTHR1) is a family B G protein-coupled receptor (GPCR) that mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues. The type-2 parathyroid hormone receptor (PTHR2) binds a peptide ligand, called tuberoinfundibular peptide-39 (TIP39), and while the biologic role of the PTHR2/TIP39 system is not as defined as that of the PTHR1, it likely plays a role in the central nervous system as well as in spermatogenesis. Mechanisms of action at these receptors have been explored through a variety of pharmacological and biochemical approaches, and the data obtained support a basic "two-site" mode of ligand binding now thought to be used by each of the family B peptide hormone GPCRs. Recent crystallographic studies on the family B GPCRs are providing new insights that help to further refine the specifics of the overall receptor architecture and modes of ligand docking. One intriguing pharmacological finding for the PTHR1 is that it can form surprisingly stable complexes with certain PTH/PTHrP ligand analogs and thereby mediate markedly prolonged cell signaling responses that persist even when the bulk of the complexes are found in internalized vesicles. The PTHR1 thus appears to be able to activate the Gα(s)/cAMP pathway not only from the plasma membrane but also from the endosomal domain. The cumulative findings could have an impact on efforts to develop new drug therapies for the PTH receptors.

New screening strategy and analysis for identification of allosteric modulators for glucagon-like peptide-1 receptor using GLP-1 (9-36) amide

The glucagon-like peptide-1 receptor (GLP-1R) is an important physiologic regulator of insulin secretion and a major therapeutic target for diabetes mellitus. GLP-1 (7-36) amide (active form of GLP-1) is truncated to GLP-1 (9-36) amide, which has been described as a weak agonist of GLP-1R and the major form of GLP-1 in the circulation. New classes of positive allosteric modulators (PAMs) for GLP-1R may offer improved therapeutic profiles. To identify these new classes, we developed novel and robust primary and secondary high-throughput screening (HTS) systems in which PAMs were identified to enhance the GLP-1R signaling induced by GLP-1 (9-36) amide. Screening enabled identification of two compounds, HIT-465 and HIT-736, which possessed new patterns of modulation of GLP-1R. We investigated the ability of these compounds to modify GLP-1R signaling enhanced GLP-1 (9-36) amide- and/or GLP-1 (7-36) amide-mediated cyclic adenosine monophosphate (cAMP) accumulation. These compounds also had unique profiles with regard to allosteric modulation of multiple downstream signaling (PathHunter β-arrestin signaling, PathHunter internalization signaling, microscopy-based internalization assay). We found allosteric modulation patterns to be obviously different among HIT-465, HIT-736, and Novo Nordisk compound 2. This work may enable the design of new classes of drug candidates by targeting modulation of GLP-1 (7-36) amide and GLP-1 (9-36) amide.

Gut-brain connection: The neuroprotective effects of the anti-diabetic drug liraglutide

Long-acting glucagon-like peptide-1 (GLP-1) analogues marketed for type 2 diabetes (T2D) treatment have been showing positive and protective effects in several different tissues, including pancreas, heart or even brain. This gut secreted hormone plays a potent insulinotropic activity and an important role in maintaining glucose homeostasis. Furthermore, growing evidences suggest the occurrence of several commonalities between T2D and neurodegenerative diseases, insulin resistance being pointed as a main cause for cognitive decline and increased risk to develop dementia. In this regard, it has also been suggested that stimulation of brain insulin signaling may have a protective role against cognitive deficits. As GLP-1 receptors (GLP-1R) are expressed throughout the central nervous system and GLP-1 may cross the blood-brain-barrier, an emerging hypothesis suggests that they may be promising therapeutic targets against brain dysfunctional insulin signaling-related pathologies. Importantly, GLP-1 actions depend not only on the direct effect mediated by its receptor activation, but also on the gut-brain axis involving an exchange of signals between both tissues via the vagal nerve, thereby regulating numerous physiological functions (e.g., energy homeostasis, glucose-dependent insulin secretion, as well as appetite and weight control). Amongst the incretin/GLP-1 mimetics class of anti-T2D drugs with an increasingly described neuroprotective potential, the already marketed liraglutide emerged as a GLP-1R agonist highly resistant to dipeptidyl peptidase-4 degradation (thereby having an increased half-life) and whose systemic GLP-1R activity is comparable to that of native GLP-1. Importantly, several preclinical studies showed anti-apoptotic, anti-inflammatory, anti-oxidant and neuroprotective effects of liraglutide against T2D, stroke and Alzheimer disease (AD), whereas several clinical trials, demonstrated some surprising benefits of liraglutide on weight loss, microglia inhibition, behavior and cognition, and in AD biomarkers. Herein, we discuss the GLP-1 action through the gut-brain axis, the hormone’s regulation of some autonomic functions and liraglutide’s neuroprotective potential.

Geniposide promotes beta-cell regeneration and survival through regulating β-catenin/TCF7L2 pathway

T-cell factor 7-like 2 (TCF7L2) is an important transcription factor of Wnt/β-catenin signaling, which has critical roles in β-cell survival and regeneration. In preliminary screening assay, we found geniposide, a naturally occurring compound, was able to increase TCF7L2 mRNA level in Min6 cells. Here we aimed to investigate the role of geniposide in β-cell and underlying mechanism involved. Geniposide was found to promote β-cell survival by increasing β-cell proliferation and decreasing β-cell apoptosis in cultured mouse islets after challenge with diabetic stimuli. Geniposide protected β-cell through activating Wnt signaling, enhanced expressions of TCF7L2 and GLP-1R, activated AKT, inhibited GSK3β activity, and promoted β-catenin nuclear translocation. The protective effect of geniposide was remarkably suppressed by siRNAs against β-catenin, or by ICG001 (β-catenin/TCF-mediated transcription inhibitor). Moreover, geniposide promoted β-cell regeneration in vivo to normalize blood glucose in high-fat diet and db/db mice. Increased β-cell proliferation was observed in pancreatic sections of geniposide-treated diabetic mice. Most importantly, geniposide triggered small islet-like cell clusters formation as a result of β-cell neogenesis from ductal epithelium, which was well correlated with the increase in TCF7L2 expression. In exocrine cells isolated from mouse pancreas, geniposide could induce duct cell differentiation through upregulating TCF7L2 expression and activating JAK2/STAT3 pathway. Taken together, we identified a novel role of geniposide in promoting β-cell survival and regeneration by mechanisms involving the activation of β-catenin/TCF7L2 signaling. Our finding highlights the potential value of geniposide as a possible treatment for type 2 diabetes.

GPCR structure, function, drug discovery and crystallography: report from Academia-Industry International Conference (UK Royal Society) Chicheley Hall, 1-2 September 2014

G-protein coupled receptors (GPCRs) are the targets of over half of all prescribed drugs today. The UniProt database has records for about 800 proteins classified as GPCRs, but drugs have only been developed against 50 of these. Thus, there is huge potential in terms of the number of targets for new therapies to be designed. Several breakthroughs in GPCRs biased pharmacology, structural biology, modelling and scoring have resulted in a resurgence of interest in GPCRs as drug targets. Therefore, an international conference, sponsored by the Royal Society, with world-renowned researchers from industry and academia was recently held to discuss recent progress and highlight key areas of future research needed to accelerate GPCR drug discovery. Several key points emerged. Firstly, structures for all three major classes of GPCRs have now been solved and there is increasing coverage across the GPCR phylogenetic tree. This is likely to be substantially enhanced with data from x-ray free electron sources as they move beyond proof of concept. Secondly, the concept of biased signalling or functional selectivity is likely to be prevalent in many GPCRs, and this presents exciting new opportunities for selectivity and the control of side effects, especially when combined with increasing data regarding allosteric modulation. Thirdly, there will almost certainly be some GPCRs that will remain difficult targets because they exhibit complex ligand dependencies and have many metastable states rendering them difficult to resolve by crystallographic methods. Subtle effects within the packing of the transmembrane helices are likely to mask and contribute to this aspect, which may play a role in species dependent behaviour. This is particularly important because it has ramifications for how we interpret pre-clinical data. In summary, collaborative efforts between industry and academia have delivered significant progress in terms of structure and understanding of GPCRs and will be essential for resolving problems associated with the more difficult targets in the future.

GLP-1(28-36)amide, a Long Ignored Peptide Revisited

Glucagon-like peptide-1 (GLP-1), which has been extensively applied for treating type 2 diabetes mellitus (T2DM), is an incretin hormone that regulates glucose homeostasis. GLP-1(28-36)amide, a C-terminal nonapeptide (FIAWLVKGRamide) of GLP-1, is a major product derived from the cleavage of GLP-1 by the neutral endopeptidase (NEP). GLP-1(28-36)amide has long been regarded as a metabolically inactive byproduct, however, recent findings reveal that GLP-1(28-36)amide plays multiple novel roles in ameliorating hepatic metabolism, protecting β cells, improving glucose disposal and inhibiting weight gain. Here, we summarize the latest progress on the effects of GLP-1(28-36)amide with a focus on its roles in regulating the Wnt and mitochondrial-mediated signaling pathways.

Discovery of (S)-2-cyclopentyl-N-((1-isopropylpyrrolidin2-yl)-9-methyl-1-oxo-2,9-dihydro-1H-pyrrido[3,4-b]indole-4-carboxamide (VU0453379): a novel, CNS penetrant glucagon-like peptide 1 receptor (GLP-1R) positive allosteric modulator (PAM)

A duplexed, functional multiaddition high throughput screen and subsequent iterative parallel synthesis effort identified the first highly selective and CNS penetrant glucagon-like peptide-1R (GLP-1R) positive allosteric modulator (PAM). PAM (S)-9b potentiated low-dose exenatide to augment insulin secretion in primary mouse pancreatic islets, and (S)-9b alone was effective in potentiating endogenous GLP-1R to reverse haloperidol-induced catalepsy.

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

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