A comparative analysis of human and mouse islet G-protein coupled receptor expression

Adrenoceptor Expression and Function in the Endocrine Pancreas

The sympathetic nervous system plays an important role in the regulation of endocrine pancreatic function, most importantly insulin release. Among the nine adrenoceptor (AR) subtypes, the α2A-AR appears to be the subtype most abundantly expressed in the human pancreas. While α2- and β-AR have opposing effects, the net response to sympathetic stimulation is inhibition of insulin secretion mediated by α2-AR located in the plasma membrane of pancreatic β cells. This inhibition may be present physiologically as evidenced by increased insulin secretion in healthy and diabetic humans and animals in response to α2-AR antagonists, a finding that was confirmed in all studies. Based on such data and on an association of an α2A-AR polymorphism, that increases receptor expression levels, with an elevated risk for diabetes, increased α2A-AR signaling in the pancreatic β cells has been proposed as a risk factor for the development of type 2 diabetes. Thus, the α2A-AR was proposed as a drug target for the treatment of some forms of type 2 diabetes. Drug research and development programs leveraging this mechanism have reached the clinical stage, but none have resulted in an approved medicine due to a limited efficacy. While β-AR agonists can increase circulating insulin levels in vivo, it remains controversial whether this includes a direct effect on β cells or occurs secondary to general metabolic effects. Therefore, the regulation of endocrine pancreatic function is physiologically interesting but may be of limited therapeutic relevance.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

An shRNA screen in primary human beta cells identifies the serotonin 1F receptor as a negative regulator of survival during transplant

Islet transplantation can cure type 1 diabetes, but peri-transplant beta cell death limits this procedure to those with low insulin requirements. Improving human beta cell survival or proliferation may make islet transplantation a possibility for more type 1 patients. To identify novel regulators of beta cell survival and proliferation, we conducted a pooled small hairpin RNA (shRNA) screen in primary human beta cells transplanted into immunocompromised mice. shRNAs targeting several cyclin dependent kinase inhibitors were enriched after transplant. Here, we focused on the Gi/o-coupled GPCR, serotonin 1F receptor ( HTR1F, 5-HT 1F ) which our screen identified as a negative regulator of beta cell numbers after transplant. In vitro , 5-HT 1F knockdown induced human beta cell proliferation but only when combined with harmine and exendin-4. In vivo , knockdown of 5-HT 1F reduced beta cell death during transplant. To demonstrate the feasibility of targeting 5-HT 1F in islet transplant, we identified and validated a small molecule 5-HT 1F antagonist. This antagonist increased glucose stimulated insulin secretion from primary human islets and cAMP accumulation in primary human beta cells. Finally, the 5-HT 1F antagonist improved glycemia in marginal mass, human islet transplants into immunocompromised mice. We identify 5-HT 1F as a novel druggable target to improve human beta cell survival in the setting of islet transplantation.

One Sentence Summary

Serotonin 1F receptor (5-HT 1F ) negatively regulates insulin secretion and beta cell survival during transplant.

Transcriptomic profiling analysis of the effect of palmitic acid on 3D spheroids of β-like cells derived from induced pluripotent stem cells

Type 2 diabetes (T2D) is posing a serious public health concern with a considerable impact on human life and health expenditures worldwide. The disease develops when insulin plasma level is insufficient for coping insulin resistance, caused by the decline of pancreatic β-cell function and mass. In β-cells, the lipotoxicity exerted by saturated free fatty acids in particular palmitate (PA), which is chronically elevated in T2D, plays a major role in β-cell dysfunction and mass. However, there is a lack of human relevant in vitro model to identify the underlying mechanism through which palmitate induces β-cell failure. In this frame, we have previously developed a cutting-edge 3D spheroid model of β-like cells derived from human induced pluripotent stem cells. In the present work, we investigated the signaling pathways modified by palmitate in β-like cells derived spheroids. When compared to the 2D monolayer cultures, the transcriptome analysis (FDR set at  0.1) revealed that the 3D spheroids upregulated the pancreatic markers (such as GCG, IAPP genes), lipids metabolism and transporters (CD36, HMGSC2 genes), glucose transporter (SLC2A6). Then, the 3D spheroids are exposed to PA 0.5 mM for 72 h. The differential analysis demonstrated that 32 transcription factors and 135 target genes were mainly modulated (FDR set at  0.1) including the upregulation of lipid and carbohydrates metabolism (HMGSC2, LDHA, GLUT3), fibrin metabolism (FGG, FGB), apoptosis (CASP7). The pathway analysis using the 135 selected targets extracted the fibrin related biological process and wound healing in 3D PA treated conditions. An overall pathway gene set enrichment analysis, performed on the overall gene set (with pathway significance cutoff at 0.2), highlighted that PA perturbs the citrate cycle, FOXO signaling and Hippo signaling as observed in human islets studies. Additional RT-PCR confirmed induction of inflammatory (IGFBP1, IGFBP3) and cell growth (CCND1, Ki67) pathways by PA. All these changes were associated with unaffected glucose-stimulated insulin secretion (GSIS), suggesting that they precede the defect of insulin secretion and death induced by PA. Overall, we believe that our data demonstrate the potential of our spheroid 3D islet-like cells to investigate the pancreatic-like response to diabetogenic environment.

RFX6 Maintains Gene Expression and Function of Adult Human Islet α-Cells

Mutations in the gene encoding the transcription factor regulatory factor X-box binding 6 (RFX6) are associated with human diabetes. Within pancreatic islets, RFX6 expression is most abundant in islet α-cells, and α-cell RFX6 expression is altered in diabetes. However, the roles of RFX6 in regulating gene expression, glucagon output, and other crucial human adult α-cell functions are not yet understood. We developed a method for selective genetic targeting of human α-cells and assessed RFX6-dependent α-cell function. RFX6 suppression with RNA interference led to impaired α-cell exocytosis and dysregulated glucagon secretion in vitro and in vivo. By contrast, these phenotypes were not observed with RFX6 suppression across all islet cells. Transcriptomics in α-cells revealed RFX6-dependent expression of genes governing nutrient sensing, hormone processing, and secretion, with some of these exclusively expressed in human α-cells. Mapping of RFX6 DNA-binding sites in primary human islet cells identified a subset of direct RFX6 target genes. Together, these data unveil RFX6-dependent genetic targets and mechanisms crucial for regulating adult human α-cell function.

ARTICLE HIGHLIGHTS

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

Galanin 2 Receptor: A Novel Target for a Subset of Pancreatic Ductal Adenocarcinoma

Galanin is a 30 amino acid peptide that stimulates three subtype receptors (GAL_1-3R). M89b is a lanthionine-stabilized, C-terminally truncated galanin analog that specifically stimulates GAL₂R. We investigated the potential of M89b as a therapeutic for pancreatic ductal adenocarcinoma (PDAC) and assessed its safety. The anti-tumor activity of subcutaneously injected M89b on the growth of patient-derived xenografts of PDAC (PDAC-PDX) in mice was investigated. In addition, the safety of M89b was assessed in vitro using a multi-target panel to measure the off-target binding and modulation of enzyme activities. In a PDAC-PDX with a high GAL₂R expression, M89b completely inhibited the growth of the tumor (p < 0.001), while in two PDAC-PDXs with low GAL₂R expression, low or negligeable inhibition of tumor growth was measured, and in the PDX without GAL₂R expression no influence on the tumor growth was observed. The M89b treatment of the GAL₂R high-PDAC-PDX-bearing mice led to a reduction in the expression of RacGap1 (p < 0.05), PCNA (p < 0.01), and MMP13 (p < 0.05). In vitro studies involving a multi-target panel of pharmacologically relevant targets revealedexcellent safety of M89b. Our data indicated that GAL₂R is a safe and valuable target for treating PDACs with high GAL₂R expression.

Loss of RREB1 in pancreatic beta cells reduces cellular insulin content and affects endocrine cell gene expression

AIMS/HYPOTHESIS

Genome-wide studies have uncovered multiple independent signals at the RREB1 locus associated with altered type 2 diabetes risk and related glycaemic traits. However, little is known about the function of the zinc finger transcription factor Ras-responsive element binding protein 1 (RREB1) in glucose homeostasis or how changes in its expression and/or function influence diabetes risk.

METHODS

A zebrafish model lacking rreb1a and rreb1b was used to study the effect of RREB1 loss in vivo. Using transcriptomic and cellular phenotyping of a human beta cell model (EndoC-βH1) and human induced pluripotent stem cell (hiPSC)-derived beta-like cells, we investigated how loss of RREB1 expression and activity affects pancreatic endocrine cell development and function. Ex vivo measurements of human islet function were performed in donor islets from carriers of RREB1 type 2 diabetes risk alleles.

RESULTS

CRISPR/Cas9-mediated loss of rreb1a and rreb1b function in zebrafish supports an in vivo role for the transcription factor in beta cell mass, beta cell insulin expression and glucose levels. Loss of RREB1 also reduced insulin gene expression and cellular insulin content in EndoC-βH1 cells and impaired insulin secretion under prolonged stimulation. Transcriptomic analysis of RREB1 knockdown and knockout EndoC-βH1 cells supports RREB1 as a novel regulator of genes involved in insulin secretion. In vitro differentiation of RREB1KO/KO hiPSCs revealed dysregulation of pro-endocrine cell genes, including RFX family members, suggesting that RREB1 also regulates genes involved in endocrine cell development. Human donor islets from carriers of type 2 diabetes risk alleles in RREB1 have altered glucose-stimulated insulin secretion ex vivo, consistent with a role for RREB1 in regulating islet cell function.

CONCLUSIONS/INTERPRETATION

Together, our results indicate that RREB1 regulates beta cell function by transcriptionally regulating the expression of genes involved in beta cell development and function.

Ablation of GPR56 Causes β-Cell Dysfunction by ATP Loss through Mistargeting of Mitochondrial VDAC1 to the Plasma Membrane

The activation of G Protein-Coupled Receptor 56 (GPR56), also referred to as Adhesion G-Protein-Coupled Ceceptor G1 (ADGRG1), by Collagen Type III (Coll III) prompts cell growth, proliferation, and survival, among other attributes. We investigated the signaling cascades mediating this functional effect in relation to the mitochondrial outer membrane voltage-dependent anion Channel-1 (VDAC1) expression in pancreatic β-cells. GPR56KD attenuated the Coll III-induced suppression of P70S6K, JNK, AKT, NFκB, STAT3, and STAT5 phosphorylation/activity in INS-1 cells cultured at 20 mM glucose (glucotoxicity) for 72 h. GPR56-KD also increased Chrebp, Txnip, and Vdac1 while decreasing Vdac2 mRNA expression. In GPR56-KD islet β-cells, Vdac1 was co-localized with SNAP-25, demonstrating its plasma membrane translocation. This resulted in ATP loss, reduced cAMP production and impaired glucose-stimulated insulin secretion (GSIS) in INS-1 and human EndoC βH1 cells. The latter defects were reversed by an acute inhibition of VDAC1 with an antibody or the VDAC1 inhibitor VBIT-4. We demonstrate that Coll III potentiates GSIS by increasing cAMP and preserving β-cell functionality under glucotoxic conditions in a GPR56-dependent manner by attenuating the inflammatory response. These results emphasize GPR56 and VDAC1 as drug targets in conditions with impaired β-cell function.

Quantification of changes in human islet G protein-coupled receptor mRNA expression in obesity

BACKGROUND

G protein-coupled receptors (GPCRs) play crucial roles in regulating islet function, with Gαs- and Gαq-coupled receptors being linked to the stimulation of insulin secretion. We have quantified the mRNA expression of 384 non-olfactory GPCRs in islets isolated from lean and obese organ donors to determine alterations in islet GPCR mRNA expression in obesity.

METHODS

RT-qPCR was used to quantify GPCR mRNAs relative to five reference genes (ACTB, GAPDH, PPIA, TBP, and TFRC) in human islets isolated from lean (BMI = 22.6 ± 0.5) and obese (BMI = 32.0 ± 0.8) donors.

RESULTS

Overall, 197 and 256 GPCR mRNAs were detected above trace level in islets from lean and obese donors, respectively, with 191 GPCR mRNAs being common to the lean and obese groups. 40.9% (n = 157) and 27.1% (n = 104) of the mRNAs were expressed at trace level whilst 7.8% and 6.3% were absent in islets from lean and obese donors, respectively. Hundred and seventeen GPCR mRNAs were upregulated at least twofold in islets from obese donors, and there was >twofold downregulation of 21 GPCR mRNAs. Of particular interest, several receptors signalling via Gαs or Gαq showed significant mRNA upregulation in islets from obese donors (fold increase: PTH2R: 54.0 ± 14.6; MC2R: 34.3 ± 11.5; RXFP1: 8.5 ± 2.1; HTR2B: 6.0 ± 2.0; GPR110: 3.9 ± 1.2; PROKR2: 3.9 ± 0.7).

CONCLUSIONS

Under conditions of obesity, human islets showed significant alterations in mRNAs encoding numerous GPCRs. The increased expression of Gαs- and Gαq-coupled receptors that have not previously been investigated in β-cells opens up possibilities of novel therapeutic candidates that may lead to the potentiation of insulin secretion and/or β-cell mass to regulate glucose homeostasis.

Neuropeptide Neuromedin B does not alter body weight and glucose homeostasis nor does it act as an insulin-releasing peptide

Neuromedin B (NMB) is a member of the neuromedin family of neuropeptides with a high level of region-specific expression in the brain. Several GWAS studies on non-obese and obese patients suggested that polymorphisms in NMB predispose to obesity by affecting appetite control and feeding preference. Furthermore, several studies proposed that NMB can act as an insulin releasing peptide. Since the functional study has never been done, the in vivo role of NMB as modulator of weight gain or glucose metabolism remains unclear. Here, we generated Nmb conditional mice and nervous system deficient NmB mice. We then performed olfactory and food preference analysis, as well as metabolic analysis under standard and high fat diet. Additionally, in direct islet studies we evaluated the role of NMB on basal and glucose-stimulated insulin secretion in mouse and humans.

G protein-coupled receptors as regulators of pancreatic islet functionality

Glucose homeostasis is maintained by hormones secreted from different types of pancreatic islets and its dysregulation can result in diseases including diabetes mellitus. The secretion of hormones from pancreatic islets is highly complex and tightly controlled by G protein-coupled receptors (GPCRs). Moreover, GPCR signaling may play a role in enhancing islet cell replication and proliferation. Thus, targeting GPCRs offers a promising strategy for regulating the functionality of pancreatic islets. Here, available RNAseq datasets from human and mouse islets were used to identify the GPCR expression profile and the impact of GPCR signaling for normal islet functionality is discussed.

Distinct Metabolism of Bone Marrow Adipocytes and their Role in Bone Metastasis

Bone marrow adipocytes (BMAs) represent 10% of the total fat mass of the human body and serve as an energy reservoir for the skeletal niche. They function as an endocrine organ by actively secreting fatty acids, cytokines, and adipokines. The volume of BMAs increases along with age, osteoporosis and/or obesity. With the rapid development of multi-omic analysis and the advance in in vivo imaging technology, further distinct characteristics and functions of BMAs have been revealed. There is accumulating evidence that BMAs are metabolically, biologically and functionally unique from white, brown, beige and pink adipocytes. Bone metastatic disease is an uncurable complication in cancer patients, where primary cancer cells spread from their original site into the bone marrow. Recent publications have highlighted those BMAs could also serve as a rich lipid source of fatty acids that can be utilized by the cancer cells during bone metastasis, particularly for breast, prostate, lung, ovarian and pancreatic cancer as well as melanoma. In this review, we summarize the novel progressions in BMAs metabolism, especially with multi-omic analysis and in vivo imaging technology. We also update the metabolic role of BMAs in bone metastasis, and their potential new avenues for diagnosis and therapies against metastatic cancers.

Fasting blood glucose as a predictor of mortality: Lost in translation

Aging leads to profound changes in glucose homeostasis, weight, and adiposity, which are considered good predictors of health and survival in humans. Direct evidence that these age-associated metabolic alterations are recapitulated in animal models is lacking, impeding progress to develop and test interventions that delay the onset of metabolic dysfunction and promote healthy aging and longevity. We compared longitudinal trajectories, rates of change, and mortality risks of fasting blood glucose, body weight, and fat mass in mice, nonhuman primates, and humans throughout their lifespans and found similar trajectories of body weight and fat in the three species. In contrast, fasting blood glucose decreased late in life in mice but increased over the lifespan of nonhuman primates and humans. Higher glucose was associated with lower mortality in mice but higher mortality in nonhuman primates and humans, providing a cautionary tale for translating age-associated metabolic changes from mice to humans.

High Coexpression of the Ghrelin and LEAP2 Receptor GHSR With Pancreatic Polypeptide in Mouse and Human Islets

Islets represent an important site of direct action of the hormone ghrelin, with expression of the ghrelin receptor (growth hormone secretagogue receptor; GHSR) having been localized variably to alpha cells, beta cells, and/or somatostatin (SST)-secreting delta cells. To our knowledge, GHSR expression by pancreatic polypeptide (PP)-expressing gamma cells has not been specifically investigated. Here, histochemical analyses of Ghsr-IRES-Cre × Cre-dependent ROSA26-yellow fluorescent protein (YFP) reporter mice showed 85% of GHSR-expressing islet cells coexpress PP, 50% coexpress SST, and 47% coexpress PP + SST. Analysis of single-cell transcriptomic data from mouse pancreas revealed 95% of Ghsr-expressing cells coexpress Ppy, 100% coexpress Sst, and 95% coexpress Ppy + Sst. This expression was restricted to gamma-cell and delta-cell clusters. Analysis of several single-cell human pancreatic transcriptome data sets revealed 59% of GHSR-expressing cells coexpress PPY, 95% coexpress SST, and 57% coexpress PPY + SST. This expression was prominent in delta-cell and beta-cell clusters, also occurring in other clusters including gamma cells and alpha cells. GHSR expression levels were upregulated by type 2 diabetes mellitus in beta cells. In mice, plasma PP positively correlated with fat mass and with plasma levels of the endogenous GHSR antagonist/inverse agonist LEAP2. Plasma PP also elevated on LEAP2 and synthetic GHSR antagonist administration. These data suggest that in addition to delta cells, beta cells, and alpha cells, PP-expressing pancreatic cells likely represent important direct targets for LEAP2 and/or ghrelin both in mice and humans.

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

The small molecule kobusone can stimulate islet β-cell replication in vivo

OBJECTIVE

To investigate the ability of kobusone to reduce high glucose levels and promote β-cell proliferation.

METHODS

Four-week-old female db/db mice were assigned to the kobusone (25 mg/kg body weight, intraperitoneally twice a day) or control group (same volume of PBS). Glucose levels and body weight were measured twice a week. After 6 weeks, intraperitoneal glucose tolerance tests and immunohistochemical studies were performed, and insulin levels were determined. The expression of mRNAs involved in cell proliferation, such as PI3K, Akt, cyclin D3 and p57^Kip2, was measured by quantitative reverse transcription polymerase chain reaction (RT-qPCR).

RESULTS

Kobusone reduced blood glucose levels after 3 weeks and more strongly increased serum insulin levels than the vehicle. Immunohistochemistry illustrated that kobusone increased 5-bromo-2'-deoxyuridine incorporation into islet β-cells, suggesting that it can stimulate islet β-cell replication in vivo. RT-qPCR indicated that kobusone upregulated the mRNA expression of PI3K, Akt, and cyclin D3 and downregulated that of p57^Kip2.

CONCLUSION

Our findings suggest that kobusone is a potent pancreatic islet β-cell inducer that has the potential to be developed as an anti-diabetic agent.

The relevance of adhesion G protein-coupled receptors in metabolic functions

G protein-coupled receptors (GPCRs) modulate a variety of physiological functions and have been proven to be outstanding drug targets. However, approximately one-third of all non-olfactory GPCRs are still orphans in respect to their signal transduction and physiological functions. Receptors of the class of Adhesion GPCRs (aGPCRs) are among these orphan receptors. They are characterized by unique features in their structure and tissue-specific expression, which yields them interesting candidates for deorphanization and testing as potential therapeutic targets. Capable of G-protein coupling and non-G protein-mediated function, aGPCRs may extend our repertoire of influencing physiological function. Besides their described significance in the immune and central nervous systems, growing evidence indicates a high importance of these receptors in metabolic tissue. RNAseq analyses revealed high expression of several aGPCRs in pancreatic islets, adipose tissue, liver, and intestine but also in neurons governing food intake. In this review, we focus on aGPCRs and their function in regulating metabolic pathways. Based on current knowledge, this receptor class represents high potential for future pharmacological approaches addressing obesity and other metabolic diseases.

SNAP-tag-enabled super-resolution imaging reveals constitutive and agonist-dependent trafficking of GPR56 in pancreatic β-cells

Objective

Members of the adhesion G protein-coupled receptor (aGPCR) subfamily are important actors in metabolic processes, with GPR56 (ADGRG1) emerging as a possible target for type 2 diabetes therapy. GPR56 can be activated by collagen III, its endogenous ligand, and by a synthetic seven amino-acid peptide (TYFAVLM; P7) contained within the GPR56 Stachel sequence. However, the mechanisms regulating GPR56 trafficking dynamics and agonist activities are not yet clear.

Methods

Here, we introduced SNAPf-tag into the N-terminal segment of GPR56 to monitor GPR56 cellular activity in situ. Confocal and super-resolution microscopy were used to investigate the trafficking pattern of GPR56 in native MIN6 β-cells and in MIN6 β-cells where GPR56 had been deleted by CRISPR-Cas9 gene editing. Insulin secretion, changes in intracellular calcium, and β-cell apoptosis were determined by radioimmunoassay, single-cell calcium microfluorimetry, and measuring caspase 3/7 activities, respectively, in MIN6 β-cells and human islets.

Results

SNAP-tag labelling indicated that GPR56 predominantly underwent constitutive internalisation in the absence of an exogenous agonist, unlike GLP-1R. Collagen III further stimulated GPR56 internalisation, whereas P7 was without significant effect. The overexpression of GPR56 in MIN6 β-cells did not affect insulin secretion. However, it was associated with reduced β-cell apoptosis, while the deletion of GPR56 made MIN6 β-cells more susceptible to cytokine-induced apoptosis. P7 induced a rapid increase in the intracellular calcium in MIN6 β-cells (in a GPR56-dependent manner) and human islets, and it also caused a sustained and reversible increase in insulin secretion from human islets. Collagen III protected human islets from cytokine-induced apoptosis, while P7 was without significant effect.

Conclusions

These data indicate that GPR56 exhibits both agonist-dependent and -independent trafficking in β-cells and suggest that while GPR56 undergoes constitutive signalling, it can also respond to its ligands when required. We have also identified that constitutive and agonist-dependent GPR56 activation is coupled to protect β-cells against apoptosis, offering a potential therapeutic target to maintain β-cell mass in type 2 diabetes.

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GPR56 predominantly underwent constitutive internalisation in β-cells in the absence of exogenous agonist.

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The GPR56 agonists, collagen III and P7, showed differential effects on GPR56 trafficking and islet functions.

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Constitutive and agonist-dependent GPR56 activation is coupled to protection of β-cells against apoptosis.

Obesity-induced changes in human islet G protein-coupled receptor expression: Implications for metabolic regulation

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that are the targets for many different classes of pharmacotherapy. The islets of Langerhans are central to appropriate glucose homeostasis through their secretion of insulin, and islet function can be modified by ligands acting at the large number of GPCRs that islets express. The human islet GPCRome is not a static entity, but one that is altered under pathophysiological conditions and, in this review, we have compared expression of GPCR mRNAs in human islets obtained from normal weight range donors, and those with a weight range classified as obese. We have also considered the likely outcomes on islet function that the altered GPCR expression status confers and the possible impact that adipokines, secreted from expanded fat depots, could have at those GPCRs showing altered expression in obesity.

GRK2 contributes to glucose mediated calcium responses and insulin secretion in pancreatic islet cells

Diabetes is a metabolic syndrome rooted in impaired insulin and/or glucagon secretory responses within the pancreatic islets of Langerhans (islets). Insulin secretion is primarily regulated by two key factors: glucose-mediated ATP production and G-protein coupled receptors (GPCRs) signaling. GPCR kinase 2 (GRK2), a key regulator of GPCRs, is reported to be downregulated in the pancreas of spontaneously obesogenic and diabetogenic mice (ob/ob). Moreover, recent studies have shown that GRK2 non-canonically localizes to the cardiac mitochondrion, where it can contribute to glucose metabolism. Thus, islet GRK2 may impact insulin secretion through either mechanism. Utilizing Min6 cells, a pancreatic ß-cell model, we knocked down GRK2 and measured glucose-mediated intracellular calcium responses and insulin secretion. Silencing of GRK2 attenuated calcium responses, which were rescued by pertussis toxin pre-treatment, suggesting a Gαi/o-dependent mechanism. Pancreatic deletion of GRK2 in mice resulted in glucose intolerance with diminished insulin secretion. These differences were due to diminished insulin release rather than decreased insulin content or gross differences in islet architecture. Furthermore, a high fat diet feeding regimen exacerbated the metabolic phenotype in this model. These results suggest a new role for pancreatic islet GRK2 in glucose-mediated insulin responses that is relevant to type 2 diabetes disease progression.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

Direct Stimulatory Effects of the CB2 Ligand JTE 907 in Human and Mouse Islets

AIMS

The endocannabinoid system is a complex cell-signaling network through which endogenous cannabinoid ligands regulate cell function by interaction with CB1 and CB2 cannabinoid receptors, and with the novel cannabinoid receptor GPR55. CB1, CB2, and GPR55 are expressed by islet β-cells where they modulate insulin secretion. We have previously shown that administration of the putative CB2 antagonist/inverse agonist JTE 907 to human islets did not affect the insulinotropic actions of CB2 agonists and it unexpectedly stimulated insulin secretion on its own. In this study, we evaluated whether the lack of antagonism could be related to the ability of JTE 907 to act as a GPR55 agonist.

MATERIALS AND METHODS

We used islets isolated from human donors and from Gpr55+/+ and Gpr55-/- mice and quantified the effects of incubation with 10 μM JTE 907 on dynamic insulin secretion, apoptosis, and β-cell proliferation by radioimmunoassay, luminescence caspase 3/7 activity, and immunofluorescence, respectively. We also measured islet IP1 and cAMP accumulation using fluorescence assays, and monitored [Ca2+]i elevations by Fura-2 single cell microfluorometry.

RESULTS

JTE 907 significantly stimulated insulin secretion from islets isolated from human donors and islets from Gpr55+/+ and Gpr55-/- mice. These stimulatory effects were accompanied by significant elevations of IP1 and [Ca2+]i, but there were no changes in cAMP generation. JTE 907 also significantly reduced cytokine-induced apoptosis in human and mouse islets and promoted human β-cell proliferation.

CONCLUSION

Our observations show for the first time that JTE 907 acts as a Gq-coupled agonist in islets to stimulate insulin secretion and maintain β-cell mass in a GPR55-independent fashion.

Glucose-induced insulin secretion in isolated human islets: Does it truly reflect β-cell function in vivo?

BACKGROUND

Diabetes always involves variable degrees of β-cell demise and malfunction leading to insufficient insulin secretion. Besides clinical investigations, many research projects used rodent islets to study various facets of β-cell pathophysiology. Their important contributions laid the foundations of steadily increasing numbers of experimental studies resorting to isolated human islets.

SCOPE OF REVIEW

This review, based on an analysis of data published over 60 years of clinical investigations and results of more recent studies in isolated islets, addresses a question of translational nature. Does the information obtained in vitro with human islets fit with our knowledge of insulin secretion in man? The aims are not to discuss specificities of pathways controlling secretion but to compare qualitative and quantitative features of glucose-induced insulin secretion in isolated human islets and in living human subjects.

MAJOR CONCLUSIONS

Much of the information gathered in vitro can reliably be translated to the in vivo situation. There is a fairly good, though not complete, qualitative and quantitative coherence between insulin secretion rates measured in vivo and in vitro during stimulation with physiological glucose concentrations, but the concordance fades out under extreme conditions. Perplexing discrepancies also exist between insulin secretion in subjects with Type 2 diabetes and their islets studied in vitro, in particular concerning the kinetics. Future projects should ascertain that the experimental conditions are close to physiological and do not alter the function of normal and diabetic islets.

Targeting lipid GPCRs to treat type 2 diabetes mellitus - progress and challenges

Therapeutic approaches to the treatment of type 2 diabetes mellitus that are designed to increase insulin secretion either directly target β-cells or indirectly target gastrointestinal enteroendocrine cells (EECs), which release hormones that modulate insulin secretion (for example, incretins). Given that β-cells and EECs both express a large array of G protein-coupled receptors (GPCRs) that modulate insulin secretion, considerable research and development efforts have been undertaken to design therapeutic drugs targeting these GPCRs. Among them are GPCRs specific for free fatty acid ligands (lipid GPCRs), including free fatty acid receptor 1 (FFA1, otherwise known as GPR40), FFA2 (GPR43), FFA3 (GPR41) and FFA4 (GPR120), as well as the lipid metabolite binding glucose-dependent insulinotropic receptor (GPR119). These lipid GPCRs have demonstrated important roles in the control of islet and gut hormone secretion. Advances in lipid GPCR pharmacology have led to the identification of a number of synthetic agonists that exert beneficial effects on glucose homeostasis in preclinical studies. Yet, translation of these promising results to the clinic has so far been disappointing. In this Review, we present the physiological roles, pharmacology and clinical studies of these lipid receptors and discuss the challenges associated with their clinical development for the treatment of type 2 diabetes mellitus.

The role of the CCL25-CCR9 axis in beta-cell function: potential for therapeutic intervention in type 2 diabetes

BACKGROUND AND PURPOSE

Chemokines are known to play essential roles mediating immunity and inflammation in many physiological and pathophysiological processes, with reports linking their action to the development of obesity, insulin resistance and type 2 diabetes (T2D). Given our findings of highly upregulated mRNA expression of the chemokine receptor CCR9 in islets from obese human donors, we have determined the effects of CCR9 activation by CCL25 on islet function and viability.

BASIC PROCEDURES

RT-qPCR was used to measure expression of 384 GPCR mRNAs in human islets from organ donors with normal and elevated BMI. mRNA encoding CCR9, a receptor that was highly upregulated in islets from obese donors, was also quantified in islets from lean and high-fat diet (HFD) mice. The effects of CCR9 activation by exogenous CCL25 in human and mouse islets and its inhibition by the CCR9 antagonist vercirnon on insulin secretion, apoptosis and cAMP accumulation were examined using standard techniques.

MAIN FINDINGS

The qPCR analysis showed altered expression of several GPCRs in islets isolated from lean and obese donors. CCR9 displayed over 90-fold upregulation in islets from obese individuals, and it was also significantly upregulated in islets from obese mice. In isolated human and mouse islets exogenous CCL25 inhibited glucose-induced insulin secretion in a concentration-dependent manner, enhanced cytokine-induced apoptosis and significantly reduced forskolin-induced elevation in cAMP levels. These detrimental effects of CCL25 in islets were blocked by vercirnon, which had no effect on its own.

PRINCIPAL CONCLUSIONS

We have shown that CCL25 acts via the Gαi-coupled receptor CCR9 to impair beta-cell function by inhibiting insulin secretion and promoting cytokine-induced apoptosis. Upregulation of CCR9 in islets in obesity, possibly secondary to accumulation of passenger immune cells, may predispose to metabolic dysfunction and our data suggest that CCL25 downregulation or CCR9 inhibition could be explored to treat T2D.

Generation and characterisation of C-terminally stabilised PYY molecules with potential in vivo NPYR2 activity

BACKGROUND

Activation of neuropeptide Y2 receptors (NPYR2) by the N-terminally truncated, dipeptidyl peptidase-4 (DPP-4) generated, Peptide YY (PYY) metabolite, namely PYY(3-36), results in satiating actions. However, PYY(3-36) is also subject to C-terminal enzymatic cleavage, which annuls anorectic effects.

METHODS

Substitution of l-Arg³⁵ with d-Arg³⁵ in the DPP-4 stable sea lamprey PYY(1-36) peptide imparts full C-terminal stability. In the current study, we have taken this molecule and introduced DPP-4 susceptibility by Iso³ substitution.

RESULTS

As expected, [Iso³]sea lamprey PYY(1-36) and [Iso³](d-Arg³⁵)sea lamprey PYY(1-36) were N-terminally degraded to respective PYY(3-36) metabolites in plasma. Only [Iso³](d-Arg³⁵)sea lamprey PYY(1-36) was C-terminally stable. Both peptides possessed similar insulinostatic and anti-apoptotic biological actions to native PYY(1-36) in beta-cells. Unlike native PYY(1-36) and [Iso³](d-Arg³⁵)sea lamprey PYY(1-36), [Iso³]sea lamprey PYY(1-36) displayed some proliferative actions in Npyr1 knockout beta-cells. In addition, [Iso³]sea lamprey PYY(1-36) induced more rapid NPYR2-dependent appetite suppressive effects in mice than its C-terminally stable counterpart. Twice daily administration of either peptide to high fat fed (HFF) mice resulted in significant body weight reduction and improvements in circulating triglyceride levels. [Iso³]sea lamprey PYY(1-36) treatment also prevented elevations in glucagon. Both peptides, and especially [Iso³]sea lamprey PYY(1-36), improved glucose tolerance. The treatment interventions also partially reversed the deleterious effects of sustained high fat feeding on pancreatic islet morphology.

CONCLUSION

The present study confirms that sustained NPYR2 receptor activation by [Iso³](d-Arg³⁵)sea lamprey induced significant weight lowering actions. However, identifiable benefits of this peptide over [Iso³]sea lamprey PYY(1-36), which was not protected against C-terminal degradation, were not pronounced.

Inhibitory effect of UDP-glucose on cAMP generation and insulin secretion

Type-2 diabetes (T2D) is a global disease caused by the inability of pancreatic β-cells to secrete adequate insulin. However, the molecular mechanisms underlying the failure of β-cells to respond to glucose in T2D remains unknown. Here, we investigated the relative contribution of UDP-glucose (UDP-G), a P2Y14-specific agonist, in the regulation of insulin release using human isolated pancreatic islets and INS-1 cells. P2Y14 was expressed in both human and rodent pancreatic β-cells. Dose-dependent activation of P2Y14 by UDP-G suppressed glucose-stimulated insulin secretion (GSIS) and knockdown of P2Y14 abolished the UDP-G effect. 12-h pretreatment of human islets with pertussis-toxin (PTX) improved GSIS and prevented the inhibitory effect of UDP-G on GSIS. UDP-G on GSIS suppression was associated with suppression of cAMP in INS-1 cells. UDP-G decreased the reductive capacity of nondiabetic human islets cultured at 5 mm glucose for 72 h and exacerbated the negative effect of 20 mm glucose on the cell viability during culture period. T2D donor islets displayed a lower reductive capacity when cultured at 5 mm glucose for 72 h that was further decreased in the presence of 20 mm glucose and UDP-G. Presence of a nonmetabolizable cAMP analog during culture period counteracted the effect of glucose and UDP-G. Islet cultures at 20 mm glucose increased apoptosis, which was further amplified when UDP-G was present. UDP-G modulated glucose-induced proliferation of INS-1 cells. The data provide intriguing evidence for P2Y14 and UDP-G's role in the regulation of pancreatic β-cell function.

Involvement of the Adhesion GPCRs Latrophilins in the Regulation of Insulin Release

Insulin secretion from pancreatic β cells is a highly complex and tightly regulated process. Its dysregulation is one characteristic of type 2 diabetes, and thus, an in-depth understanding of the mechanisms controlling insulin secretion is essential for rational therapeutic intervention. G-protein-coupled receptors (GPCRs) have been established as major regulators of insulin exocytosis. Recent studies also suggest the involvement of adhesion GPCRs, a non-prototypical class of GPCRs. Here, we identify latrophilins, which belong to the class of adhesion GPCRs, to be highly expressed in different cell types of pancreatic islets. In vitro and ex vivo analyses show that distinct splice variants of the latrophilin LPHN3/ADGRL3 decrease insulin secretion from pancreatic β cells by reducing intracellular cyclic AMP levels via the G_i-mediated pathway. Our data highlight the key role of LPHN3 in modulating insulin secretion and its potential as therapeutic target for type 2 diabetes.

Assessing Mouse Islet Function

Islets of Langerhans are clusters of endocrine cells embedded within the exocrine pancreas. Islets constitute only approximately 1-2% of the total pancreas mass in all species, so methods have been developed to digest the pancreas and purify islets from the surrounding acinar cells. This chapter provides detailed protocols for isolation of mouse islets and their in vitro functional characterization in terms of assessments of islet viability, hormone content and secretion, second messenger generation and β-cell proliferation.

Pan-Cancer Analysis and Drug Formulation for GPR139 and GPR142

GPR (G protein receptor) 139 and 142 are novel foundling GPCRs (G protein-coupled receptors) in the class "A" of the GPCRs family and are suitable targets for various biological conditions. To engage these targets, validated pharmacophores and 3D QSAR (Quantitative structure-activity relationship) models are widely used because of their direct fingerprinting capability of the target and an overall accuracy. The current work initially analyzes GPR139 and GPR142 for its genomic alteration via tumor samples. Next to that, the pharmacophore is developed to scan the 3D database for such compounds that can lead to potential agonists. As a result, several compounds have been considered, showing satisfactory performance and a strong association with the target. Additionally, it is gripping to know that the obtained compounds were observed to be responsible for triggering pan-cancer. This suggests the possible role of novel GPR139 and GPR142 as the substances for initiating a physiological response to handle the condition incurred as a result of cancer.

Orphan G-protein coupled receptor 183 (GPR183) potentiates insulin secretion and prevents glucotoxicity-induced β-cell dysfunction

The expression and functional impact of most orphan G-protein coupled receptors (GPCRs) in β-cell is not fully understood. Microarray expression indicated that 36 orphan GPCRs are restricted in human islets, while 55 receptors overlapped between human islets and INS-1 cells. GPR183 showed higher expression in diabetic compared to non-diabetic human islets. GPR183 expression co-localized with β-cells while it was lacking in α-cells in human islets. The GPR183 agonist (7α-25-DHC) potentiated insulin secretion and protected against glucotoxicity-induced β-cell damage in human islets. Silencing of GPR183 in INS-1 cells decreased the expression of proinsulin genes, Pdx1, Mafa and impaired insulin secretion with a concomitant decrease in cAMP generation. Cultured INS-1 cells with 7α-25-DHC were associated with increased proliferation and expression of GPR183, INS2, PDX1, NeuroD, and INSR. In conclusion, the beneficial impact of GPR183 activation on β-cell function makes it a potential therapeutic target to prevent or reverse β-cell dysfunction.

Integrating the inputs that shape pancreatic islet hormone release

The pancreatic islet is a complex mini organ composed of a variety of endocrine cells and their support cells, which together tightly control blood glucose homeostasis. Changes in glucose concentration are commonly regarded as the chief signal controlling insulin-secreting beta cells, glucagon-secreting alpha cells and somatostatin-secreting delta cells. However, each of these cell types is highly responsive to a multitude of endocrine, paracrine, nutritional and neural inputs, which collectively shape the final endocrine output of the islet. Here, we review the principal inputs for each islet-cell type and the physiological circumstances in which these signals arise, through the prism of the insights generated by the transcriptomes of each of the major endocrine-cell types. A comprehensive integration of the factors that influence blood glucose homeostasis is essential to successfully improve therapeutic strategies for better diabetes management.

Spatial regulation of GPR64/ADGRG2 signaling by β-arrestins and GPCR kinases

Mechanisms of activation, signaling, and trafficking of adhesion G protein-coupled receptors (aGPCRs) have remained largely unknown. Several aGPCRs, including GPR56/ADGRG1 and GPR64/ADGRG2, show increased activity in the absence of their N-terminal fragment (NTF). This constitutive signaling is plausibly caused by the binding of extracellular N-terminal 15-25 amino acid-long tethered agonist to extracellular domains of the cognate aGPCRs. To test the role of NTF and tethered agonist in GPR64 signaling and endocytosis, we generated mutants that lack either NTF alone (ΔNTF) or NTF and tethered agonist (P622). We discover that unlike full-length GPR64, ΔNTF and P622 mutants interact with β-arrestin1 and β-arrestins2 and are constitutively internalized in steady states. However, only ΔNTF shows exaggerated basal activation of the Gα_s -cAMP-CRE signaling cascade. Neither ΔNTF nor P622 shows constitutive activation of the Gα₁₃ -SRE pathway, but both mutants respond to exogenously added agonistic peptide via CRE and SRE. GPCR kinases and dynamin mediate the constitutive internalization of ΔNTF and P622 to early endosomes, where ΔNTF constantly induces CRE. These data suggest that NTF not only shields the tethered agonist to prevent G protein signaling but also confers a conformation that inhibits the interaction with β-arrestins and the consequent endocytosis and sustained signaling from endosomes.

GPCRomics: An Approach to Discover GPCR Drug Targets

G protein-coupled receptors (GPCRs) are targets for ∼35% of approved drugs but only ∼15% of the ∼800 human GPCRs are currently such targets. GPCRomics, the use of unbiased, hypothesis-generating methods [e.g., RNA-sequencing (RNA-seq)], with tissues and cell types to identify and quantify GPCR expression, has led to the discovery of previously unrecognized GPCRs that contribute to functional responses and pathophysiology and that may be therapeutic targets. The combination of GPCR expression data with validation studies (e.g., signaling and functional activities) provides opportunities for the discovery of disease-relevant GPCR targets and therapeutics. Here, we review insights from GPCRomic approaches, gaps in knowledge, and future directions by which GPCRomics can advance GPCR biology and the discovery of new GPCR-targeted drugs.

Constitutively active BRS3 is a genuinely orphan GPCR in placental mammals

G protein-coupled receptors (GPCRs) play an important role in physiology and disease and represent the most productive drug targets. Orphan GPCRs, with their endogenous ligands unknown, were considered a source of drug targets and consequently attract great interest to identify their endogenous cognate ligands for deorphanization. However, a contrary view to the ubiquitous existence of endogenous ligands for every GPCR is that there might be a significant overlooked fraction of orphan GPCRs that function constitutively in a ligand-independent manner only. Here, we investigated the evolution of the bombesin receptor-ligand family in vertebrates in which one member-bombesin receptor subtype-3 (BRS3)-is a potential orphan GPCR. With analysis of 17 vertebrate BRS3 structures and 10 vertebrate BRS3 functional data, our results demonstrated that nonplacental vertebrate BRS3 still connects to the original ligands-neuromedin B (NMB) and gastrin-releasing peptide (GRP)-because of adaptive evolution, with significantly changed protein structure, especially in three altered key residues (Q127R, P205S, and R294H) originally involved in ligand binding/activation, whereas the placental mammalian BRS3 lost the binding affinity to NMB/GRP and constitutively activates Gs/Gq/G12 signaling in a ligand-independent manner. Moreover, the N terminus of placental mammalian BRS3 underwent positive selection, exhibiting significant structural differences compared to nonplacental vertebrate BRS3, and this domain plays an important role in constitutive activity of placental mammalian BRS3. In conclusion, constitutively active BRS3 is a genuinely orphan GPCR in placental mammals, including human. To our knowledge, this study identified the first example that might represent a new group of genuinely orphan GPCRs that will never be deorphanized by the discovery of a natural ligand and provided new perspectives in addition to the current ligand-driven GPCR deorphanization.

Adhesion G-protein coupled receptors: Implications for metabolic function

Adhesion G-protein coupled receptors (aGPCRs) are emerging as important actors in energy homeostasis. Recent biochemical and functional studies using transgenic mice indicate that aGPCRs play important roles in endocrine and metabolic functions including β-cell differentiation, insulin secretion, adipogenesis and whole body fuel homeostasis. Most aGPCRs are orphans, for which endogenous ligands have not yet been identified, and many of the endogenous ligands of the already de-orphanised aGPCRs are components of the extracellular matrix (ECM). In this review we focus on aGPCR expression in metabolically active tissues, their activation by ECM proteins, and current knowledge of their potential roles in islet development, insulin secretion, adipogenesis and muscle function.

Influence of organ donor attributes and preparation characteristics on the dynamics of insulin secretion in isolated human islets

In vitro studies of human pancreatic islets are critical for understanding normal insulin secretion and its perturbations in diabetic β-cells, but the influence of islet preparation characteristics and organ donor attributes in such experiments is poorly documented. Preparations from normal donors were tested with a standardized protocol evaluating dynamic insulin secretion induced by glucose, tolbutamide, and cAMP (forskolin). Secretion rates, normalized to insulin content (fractional insulin secretion), were analyzed as a function of preparation and donor characteristics. Low purity (25-45%) of the preparation (n = 8) blunted the first phase of insulin secretion induced by glucose or tolbutamide and increased basal secretion, resulting in threefold lower stimulation index than in more pure (55-95%) preparations (n = 43). In these more pure preparations, cold ischemia time (1-13 h) before pancreas digestion did not impact insulin secretion. Islet size (estimated by the islet size index) did not influence the dynamics of secretion, but fractional insulin secretion rates were greater in large than small islets, and positively correlated with islet size. Age of the donors (20-68 years) had no influence on islet size and insulin content or on dynamics and amplitude of insulin secretion, which were also similar in islets from male and female donors. In contrast, islet size and islet insulin content (normalized for size), and basal or stimulated insulin secretion positively correlated with Body-Mass Index (19-33). These results contradict previous reports on the impact of donor age and islet size and point to possible confounding effects of donor BMI in insulin secretion studies with isolated human islets.

The functional impact of G protein-coupled receptor 142 (Gpr142) on pancreatic β-cell in rodent

We have recently shown that the G protein-coupled receptor 142 (GPR142) is expressed in both rodent and human pancreatic β-cells. Herein, we investigated the cellular distribution of GPR142 within islets and the effects of selective agonists of GPR142 on glucose-stimulated insulin secretion (GSIS) in the mouse islets and INS-1832/13 cells. Double-immunostaining revealed that GPR142 immunoreactivity in islets mainly occurs in insulin-positive cells. Potentiation of GSIS by GPR142 activation was accompanied by increased cAMP content in INS-1832/13 cells. PKA/Epac inhibition markedly suppressed the effect of GPR142 activation on insulin release. Gpr142 knockdown (Gpr142-KD) in islets was accompanied by elevated release of MCP-1, IFNγ, and TNFα during culture period and abolished the modulatory effect of GPR142 activation on the GSIS. Gpr142-KD had no effect on Ffar1, Ffar2, or Ffar3 mRNA while reducing Gpr56 and increasing Tlr5 and Tlr7 mRNA expression. Gpr142-KD was associated with an increased expression of Chrebp, Txnip, RhoA, and mitochondrial Vdac1 concomitant with a reduced Pdx1, Pax6, and mitochondrial Vdac2 mRNA levels. Long-term exposure of INS-1832/13 cells to hyperglycemia reduced Gpr142 and Vdac2 while increased Chrebp, Txnip, and Vdac1 mRNA expression. GPR142 agonists or Bt₂-cAMP counteracted this effect. Glucotoxicity-induced decrease of cell viability in Gpr142-KD INS-1 cells was not affected by GPR142-agonists while Bt₂-cAMP prevented it. The results show the importance of Gpr142 in the maintenance of pancreatic β-cell function in rodents and that GPR142 agonists potentiate GSIS by an action, which most likely is due to increased cellular generation of second messenger molecule cAMP.

RORB and RORC associate with human islet dysfunction and inhibit insulin secretion in INS-1 cells

Little is known about the expression and function of Retinoic acid-related orphan receptors (RORA, B, and C) in pancreatic β cells. Here in, we utilized cDNA microarray and RNA sequencing approaches to investigate the expression pattern of ROR receptors in normal and diabetic human pancreatic islets. Possible correlations between RORs expression and HbA_1c levels as well as insulin secretory capacity in isolated human islets were evaluated. The impact of RORB and RORC expression on insulin secretion in INS-1 (832/13) cells was validated as well. While RORA was the highest expressed gene among the three RORs in human islet cells, RORC was the highest expressed in INS-1 cells (832/13) and while RORB was the lowest expressed gene in human islet cells, RORA was the highest expressed in INS-1 cells (832/13). The expression of RORB and RORC was significantly lower in diabetic/hyperglycemic donors as compared with non-diabetic counterparts. Furthermore, while the expression of RORB correlated positively with insulin secretion and negatively with HbA_1c, that of RORC correlated negatively with HbA_1c. The expression pattern of RORA did not correlate with either of the two parameters. siRNA silencing of RORB or RORC in INS-1 (832/13) cells resulted in a significant downregulation of insulin mRNA expression and insulin secretion. These findings suggest that RORB and RORC are part of the molecular cascade that regulates insulin secretion in pancreatic β cells; and insight that provides for further work on the potential therapeutic utility of RORB and RORC genes in β cell dysfunction in type 2 diabetes.

GPCRs in Autocrine and Paracrine Regulations

G protein-coupled receptors (GPCRs) constitute the largest superfamily of integral membrane protein receptors. As signal detectors, the several 100 known GPCRs are responsible for sensing the plethora of endogenous ligands that are critical for the functioning of our endocrine system. Although GPCRs are typically considered as detectors for first messengers in classical signal transduction pathways, they seldom operate in isolation in complex biological systems. Intercellular communication between identical or different cell types is often mediated by autocrine or paracrine signals that are generated upon activation of specific GPCRs. In the context of energy homeostasis, the distinct complement of GPCRs in each cell type bridges the autocrine and paracrine communication within an organ, and the various downstream signaling mechanisms regulated by GPCRs can be integrated in a cell to produce an ultimate output. GPCRs thus act as gatekeepers that coordinate and fine-tune a response. By examining the role of GPCRs in activating and receiving autocrine and paracrine signals, one may have a better understanding of endocrine diseases that are associated with GPCR mutations, thereby providing new insights for treatment regimes.

The adhesion receptor GPR56 is activated by extracellular matrix collagen III to improve β-cell function

AIMS

G-protein coupled receptor 56 (GPR56) is the most abundant islet-expressed G-protein coupled receptor, suggesting a potential role in islet function. This study evaluated islet expression of GPR56 and its endogenous ligand collagen III, and their effects on β-cell function.

METHODS

GPR56 and collagen III expression in mouse and human pancreas sections was determined by fluorescence immunohistochemistry. Effects of collagen III on β-cell proliferation, apoptosis, intracellular calcium ([Ca2+]i) and insulin secretion were determined by cellular BrdU incorporation, caspase 3/7 activities, microfluorimetry and radioimmunoassay, respectively. The role of GPR56 in islet vascularisation and innervation was evaluated by immunohistochemical staining for CD31 and TUJ1, respectively, in pancreases from wildtype (WT) and Gpr56-/- mice, and the requirement of GPR56 for normal glucose homeostasis was determined by glucose tolerance tests in WT and Gpr56-/- mice.

RESULTS

Immunostaining of mouse and human pancreases revealed that GPR56 was expressed by islet β-cells while collagen III was confined to the peri-islet basement membrane and islet capillaries. Collagen III protected β-cells from cytokine-induced apoptosis, triggered increases in [Ca2+]i and potentiated glucose-induced insulin secretion from WT islets but not from Gpr56-/- islets. Deletion of GPR56 did not affect glucose-induced insulin secretion in vitro and it did not impair glucose tolerance in adult mice. GPR56 was not required for normal islet vascularisation or innervation.

CONCLUSION

We have demonstrated that collagen III improves islet function by increasing insulin secretion and protecting against apoptosis. Our data suggest that collagen III may be effective in optimising islet function to improve islet transplantation outcomes, and GPR56 may be a target for the treatment of type 2 diabetes.

Mesenchymal stromal cell secretory factors induce sustained improvements in islet function pre- and post-transplantation

BACKGROUND AIMS

Mesenchymal stromal cells (MSCs) enhance islet function both in vitro and in vivo, at least in part by secreting ligands that activate islet G-protein coupled receptors (GPCRs). We assessed whether pre-treatment with a defined "cocktail" of MSC-secreted GPCR ligands enhances islet functional survival in vitro and improves the outcomes of islet transplantation in an experimental model of diabetes.

METHODS

Isolated islets were cultured for 48 h with ANXA1, SDF-1 or C3a, alone or in combination. Glucose-stimulated insulin secretion (GSIS) and cytokine-induced apoptosis were measured immediately after the 48 h culture period and at 24 h or 72 h following removal of the ligands from the culture media. Islets were syngeneically transplanted underneath the kidney capsule of streptozotocin-induced diabetic C57BL/6 mice and blood glucose levels monitored for 28 days.

RESULTS

Pre-culturing islets with a cocktail of ANXA1/SDF-1/C3a potentiated GSIS and protected islet cells from cytokine-induced apoptosis in vitro. These effects were maintained for up to 72 h after the removal of the factors from the culture medium, suggesting a sustained protection of islet graft functional survival during the immediate post-transplantation period. Islets pre-treated with the cocktail of MSC secretory factors were more effective in reducing blood glucose in diabetic mice, consistent with their improved functional survival in vivo.

DISCUSSION

Pre-culturing islets with a cocktail of MSC secretory products offers a well-defined, cell-free approach to improve clinical islet transplantation outcomes while avoiding many of the safety, regulatory and logistical hurdles of incorporating MSCs into transplantation protocols.

Activation of imidazoline receptor I2, and improved pancreatic β-cell function in human islets

AIM

The impact of BL11282, an imidazoline receptor (NISCH) agonist, on potentiation of glucose-stimulated insulin secretion (GSIS) from isolated human non-diabetic (ND) and type 2 diabetic (T2D) islets was investigated.

METHODS

Analysis of mRNA was performed by RNA-sequencing and qPCR. Insulin and cAMP by RIA and ELISA respectively.

RESULTS

RNA-sequencing data revealed that NISCH is highly expressed in fat tissues, islets, liver and muscles, with eight detectable splice variants of transcripts in islets. NISCH had a positive correlation with GLP-1 (GLP1R) and GIP (GIPR) receptor transcripts. The expression of NISCH was confirmed by qPCR in human islets. NISCH and GLP1R were comparably higher expressed in mouse islets compared to human islets. GSIS was dose-dependently potentiated by BL11282 from incubated islets of ND and T2D human islet donors. The insulinotropic action of BL11282 was associated with increased cAMP. While the harmful effect of high glucose on reductive capacity of islet cells was enhanced by glibenclamide during long-term culture, it was counteracted by BL11282 or Bt2-cAMP. BL11282 also increased proliferation of INS-1 cells during long-time culture.

CONCLUSION

Our data suggest that BL11282 potentiates GSIS by an action involving cAMP/PKA system and BL11282 could be an attractive insulinotropic and β-cell protective agent.

Potential of mesenchymal stromal cells for improving islet transplantation outcomes

Allogeneic islet transplantation as a therapy for Type 1 Diabetes (T1D) is restricted by the limited availability of donor islets, loss of functional islets during pre-transplantation culture in vitro and further extensive loss during the immediate post-transplantation period when islet function and survival is compromised by the hypoxic, inflammatory host environment. In the longer term pathogenic T cell responses drive autoimmunity and chronic allograft rejection. Experimental studies have demonstrated that mesenchymal stromal cells (MSCs) have significant potential to improve the outcomes of clinical islet transplantation. This review explores the potential for MSCs and their 'secretome' to influence donor islet cell function and survival, as well as the host niche. We discuss the possibility of harnessing the therapeutic benefits of MSCs in a cell-free strategy to offer a well-defined, cell-free approach to improve the outcomes of clinical islet transplantation.

Defining G protein-coupled receptor peptide ligand expressomes and signalomes in human and mouse islets

INTRODUCTION

Islets synthesise and secrete numerous peptides, some of which are known to be important regulators of islet function and glucose homeostasis. In this study, we quantified mRNAs encoding all peptide ligands of islet G protein-coupled receptors (GPCRs) in isolated human and mouse islets and carried out in vitro islet hormone secretion studies to provide functional confirmation for the species-specific role of peptide YY (PYY) in mouse islets.

MATERIALS AND METHODS

GPCR peptide ligand mRNAs in human and mouse islets were quantified by quantitative real-time PCR relative to the reference genes ACTB, GAPDH, PPIA, TBP and TFRC. The pathways connecting GPCR peptide ligands with their receptors were identified by manual searches in the PubMed, IUPHAR and Ingenuity databases. Distribution of PYY protein in mouse and human islets was determined by immunohistochemistry. Insulin, glucagon and somatostatin secretion from islets was measured by radioimmunoassay.

RESULTS

We have quantified GPCR peptide ligand mRNA expression in human and mouse islets and created specific signalomes mapping the pathways by which islet peptide ligands regulate human and mouse GPCR signalling. We also identified species-specific islet expression of several GPCR ligands. In particular, PYY mRNA levels were ~ 40,000-fold higher in mouse than human islets, suggesting a more important role of locally secreted Pyy in mouse islets. This was confirmed by IHC and functional experiments measuring insulin, glucagon and somatostatin secretion.

DISCUSSION

The detailed human and mouse islet GPCR peptide ligand atlases will allow accurate translation of mouse islet functional studies for the identification of GPCR/peptide signalling pathways relevant for human physiology, which may lead to novel treatment modalities of diabetes and metabolic disease.

Animal research for type 2 diabetes mellitus, its limited translation for clinical benefit, and the way forward

Obesity and type 2 diabetes mellitus (T2DM) have reached pandemic proportions worldwide, and considerable research efforts have been dedicated to investigating disease pathology and therapeutic options. The two hallmark features of T2DM, insulin resistance and pancreatic dysfunction, have been studied extensively by using various animal models. Despite the knowledge acquired from such models, particularly mechanistic discoveries that sometimes mimic human T2DM mechanisms or pathways, many details of human T2DM pathogenesis remain unknown, therapeutic options remain limited, and a cure has eluded research. Emerging human data have raised concern regarding inter-species differences at many levels (e.g. in gene regulation, pancreatic cytoarchitecture, glucose transport, and insulin secretion regulation), and the subsequent impact of these differences on the clinical translation of animal research findings. Therefore, it is important to recognise and address the translational gap between basic animal-based research and the clinical advances needed to prevent and treat T2DM. The purpose of this report is to identify some limitations of T2DM animal research, and to propose how greater human relevance and applicability of hypothesis-driven basic T2DM research could be achieved through the use of human-based data acquisition at various biological levels. This report addresses how in vitro, in vivo and in silico technologies could be used to investigate particular aspects of human glucose regulation. We do not propose that T2DM animal research has been without value in the identification of mechanisms, pathways, or potential targets for therapies, nor do we claim that human-based methods can provide all the answers. We recognise that the ultimate goal of T2DM animal research is to identify ways to advance the prevention, recognition and treatment of T2DM in humans, but postulate that this is where the use of animal models falls short, despite decades of effort. The best way to achieve this goal is by prioritising human-centred research.