Regulator of G-protein signaling Gβ5-R7 is a crucial activator of muscarinic M3 receptor-stimulated insulin secretion

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can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy.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.

The emerging roles of GPR158 in the regulation of the endocrine system

G protein-coupled receptor 158 (GPR158) is a member of class C G protein-coupled receptors (GPCRs) and is highly expressed in the central nervous system (CNS) while lowly expressed in peripheral tissues. Previous studies have mainly focused on its functions in the CNS, such as regulating emotions, memory, and cognitive functions, whereas studies on its role in the non-nervous system are limited. It has been recently reported that GPR158 is directly involved in adrenal regulation, suggesting its role in peripheral tissues. Moreover, GPR158 is a stable dimer coupled to the regulator of G protein signaling protein 7 (RGS7) that forms the GPR158-RGS7-Gβ5 complex. Given that the RGS7-Gβ5 complex is implicated in endocrine functions, we speculate that GPR158 might be an active component of the endocrine system. Herein, we reviewed the relevant literature on GPR158, including its molecular structure, regulatory molecules, expression, and functions, and highlighted its roles in endocrine regulation. These findings not only enhance our understanding of GPR158 from an endocrine perspective but also provide valuable insights into drug exploration targeting GPR158 and their applicability in endocrine disorders.

Transcriptomic analysis and transgenerational effects of ZnO nanoparticles on Daphnia magna: Endocrine-disrupting potential and energy metabolism

The widespread application of zinc oxide nanoparticles (ZnO NPs) has raised concerns over the adverse effects on aquatic species. In this study, transcriptomic analysis was applied to evaluate the chronic toxicity of ZnO NPs on the freshwater invertebrate Daphnia magna and the intergenerational effects were then further investigated. Parent daphnia (F0) were exposed to ZnO NPs at 3, 60, and 300 μg L^-1 for 21 days. ZnO NPs significantly inhibited the reproduction (first pregnancy and spawning time, total number of offspring) and growth (molting frequency and body length) of F0. Here, differentially expressed genes (DEGs) involved in lysosomal and phagosome, energy metabolism and endocrine disruption pathways were significantly downregulated. Furthermore, disruption on the transport and catabolic processes probably resulted in the particle accumulation. The inhibited pathways related to energy metabolism may partially account for the body length, molting and reproductive restriction. The suppression of growth and reproduction may attribute to the down-regulation of insulin secretion and ovarian steroidogenesis pathways, respectively. Partial recovery of growth and reproductive inhibition in F1 - F3 descended from the F0 generation exposure did not support constant transgenerational effects. This study unravels the molecular mechanisms and transgenerational consequences of the toxicity of nanoparticles on Daphnia.

RGS4-Deficiency Alters Intracellular Calcium and PKA-Mediated Control of Insulin Secretion in Glucose-Stimulated Beta Islets

A number of diverse G-protein signaling pathways have been shown to regulate insulin secretion from pancreatic β-cells. Accordingly, regulator of G-protein signaling (RGS) proteins have also been implicated in coordinating this process. One such protein, RGS4, is reported to show both positive and negative effects on insulin secretion from β-cells depending on the physiologic context under which it was studied. We here use an RGS4-deficient mouse model to characterize previously unknown G-protein signaling pathways that are regulated by RGS4 during glucose-stimulated insulin secretion from the pancreatic islets. Our data show that loss of RGS4 results in a marked deficiency in glucose-stimulated insulin secretion during both phase I and phase II of insulin release in intact mice and isolated islets. These deficiencies are associated with lower cAMP/PKA activity and a loss of normal calcium surge (phase I) and oscillatory (phase II) kinetics behavior in the RGS4-deficient β-cells, suggesting RGS4 may be important for regulation of both Gαi and Gαq signaling control during glucose-stimulated insulin secretion. Together, these studies add to the known list of G-protein coupled signaling events that are controlled by RGS4 during glucose-stimulated insulin secretion and highlight the importance of maintaining normal levels of RGS4 function in healthy pancreatic tissues.

Ectopically expressed olfactory receptors OR51E1 and OR51E2 suppress proliferation and promote cell death in a prostate cancer cell line

Olfactory receptors (ORs), the largest family of G protein–coupled receptors, are expressed in the nasal epithelium where they mediate the sense of smell. However, ORs are also found in other non-nasal tissues, but the role of these ectopic ORs in cell signaling, proliferation, and survival is not well understood. Here, using an inducible expression system in the lymph node carcinoma of the prostate (LNCaP) cell line, we investigated two ectopic ORs, OR51E1 and OR51E2, which have been shown to be upregulated in prostate cancer. We found that, consistent with previous studies, OR51E1 stimulated adenylyl cyclase in response to treatment by short-chain to medium-chain organic acids (C3–C9) but not by acetate. OR51E2 responded to acetate and propionate but not to the longer chain organic acids. Stimulation of LNCaP cells with butyrate inhibited their growth, and the knockdown of the endogenous OR51E1 negated this cytostatic effect. Most significantly, overexpression of OR51E1 or OR51E2 suppressed LNCaP cell proliferation. Overexpression of another ectopic OR OR2AT4, β2-adrenergic receptor, or treatment of cells with forskolin did not suppress cell proliferation, indicating that a rise in cAMP is not sufficient to induce cytostasis. Overexpression of OR51E1 caused an upregulation of cytostatic and cell death markers including p27, p21, and p53, strongly increased annexin V staining, and stimulated extracellular signal–regulated protein kinases 1 and 2. Overexpression and/or activation of OR51E1 did not affect human embryonic kidney 293 cell proliferation, indicating that cytotoxicity of OR51E1/OR51E2 is specific for LNCaP cells. Together, our results further our understanding of prostate cancer etiology and suggest that ectopic ORs may be useful therapeutic targets.

The regulatory G protein signaling complex, Gβ5-R7, promotes glucose- and extracellular signal-stimulated insulin secretion

G protein-coupled receptors (GPCRs) are important modulators of glucose-stimulated insulin secretion, essential for maintaining energy homeostasis. Here we investigated the role of Gβ5-R7, a protein complex consisting of the atypical G protein β subunit Gβ5 and a regulator of G protein signaling of the R7 family. Using the mouse insulinoma MIN6 cell line and pancreatic islets, we investigated the effects of G protein subunit β 5 (Gnb5) knockout on insulin secretion. Consistent with previous work, Gnb5 knockout diminished insulin secretion evoked by the muscarinic cholinergic agonist Oxo-M. We found that the Gnb5 knockout also attenuated the activity of other GPCR agonists, including ADP, arginine vasopressin, glucagon-like peptide 1, and forskolin, and, surprisingly, the response to high glucose. Experiments with MIN6 cells cultured at different densities provided evidence that Gnb5 knockout eliminated the stimulatory effect of cell adhesion on Oxo-M-stimulated glucose-stimulated insulin secretion; this effect likely involved the adhesion GPCR GPR56. Gnb5 knockout did not influence cortical actin depolymerization but affected protein kinase C activity and the 14-3-3ϵ substrate. Importantly, Gnb5-/- islets or MIN6 cells had normal total insulin content and released normal insulin amounts in response to K+-evoked membrane depolarization. These results indicate that Gβ5-R7 plays a role in the insulin secretory pathway downstream of signaling via all GPCRs and glucose. We propose that the Gβ5-R7 complex regulates a phosphorylation event participating in the vesicular trafficking pathway downstream of G protein signaling and actin depolymerization but upstream of insulin granule release.

Multiomic Profiling Identifies cis-Regulatory Networks Underlying Human Pancreatic β Cell Identity and Function

EndoC-βH1 is emerging as a critical human β cell model to study the genetic and environmental etiologies of β cell (dys)function and diabetes. Comprehensive knowledge of its molecular landscape is lacking, yet required, for effective use of this model. Here, we report chromosomal (spectral karyotyping), genetic (genotyping), epigenomic (ChIP-seq and ATAC-seq), chromatin interaction (Hi-C and Pol2 ChIA-PET), and transcriptomic (RNA-seq and miRNA-seq) maps of EndoC-βH1. Analyses of these maps define known (e.g., PDX1 and ISL1) and putative (e.g., PCSK1 and mir-375) β cell-specific transcriptional cis-regulatory networks and identify allelic effects on cis-regulatory element use. Importantly, comparison with maps generated in primary human islets and/or β cells indicates preservation of chromatin looping but also highlights chromosomal aberrations and fetal genomic signatures in EndoC-βH1. Together, these maps, and a web application we created for their exploration, provide important tools for the design of experiments to probe and manipulate the genetic programs governing β cell identity and (dys)function in diabetes.

EndoC-βH1 is becoming an important cellular model to study genes and pathways governing human β cell identity and function, but its (epi)genomic similarity to primary human islets is unknown. Lawlor et al. complete and compare extensive EndoC and primary human islet multiomic maps to identify shared and distinct genomic circuitry.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Glycemic Index, Starch, and Protein Digestibility in Tempeh Gembus Cookies

Diabetes mellitus is directly related to diet and lifestyle. Control of blood glucose levels is needed to reduce the risk of complications, and one way is to choose foods with a low glycemic index. Cookies made from tempeh gembus/tempeh gembus flour are expected to be eaten as a snack and are safe for people with diabetes. The aim of this research was to analyze glycemic index (GI), glycemic load (GL), dietary fiber, in vitro starch, and protein digestibility of cookies with tempeh gembus flour substitution. Completely randomized design research with one primary factor used cookies with variations of 0%, 25%, and 50% tempeh gembus flour substitution. GI was calculated using the Incremental Area Under the Blood Glucose Response Curve (IAUC) method. Dietary fiber concentration analysis was done by enzymatic methods. The starch and protein digestion rates were calculated using the in vitro method. GI, GL, dietary fiber, starch digestion rate, and protein digestion rate data were analyzed with descriptive methods. Cookies with lowest GI (47.01 ± 11.08%) and GL (6.90 ± 1.63) were found in cookies with 50% tempeh gembus flour substitution. The highest dietary fiber content (24.61 ± 0.41%), digestibility of starch (48.07 ± 0.01%), and protein (20.27 ± 0.43%) cookies were found in cookies with 50% tempeh gembus flour substitution. The higher tempeh gembus flour substitution produced low GI and GL while its dietary fiber, in vitro starch, and protein digestibility were highest.

JONES P. Determining the insulin secretion potential for certain specific G-protein coupled receptors in MIN6 pancreatic beta cells

Background/aim

The polypeptide hormone insulin is essential for the maintenance of whole-body fuel homeostasis, and defects in insulin secretion and/or action are associated with the development of type 1 and type 2 diabetes. The aim of this study was to assess the role of some G-protein coupled receptors (GPCRs), GPR54, GPR56, and GPR75, and cannabinoid receptors CB1R and CB2R, in the regulation of pancreatic β-cell function.

Materials and methods

Insulin secretion from mouse insulinoma β-cell line (MIN6) monolayers was assessed via insulin radioimmunoassay (RIA). Reverse transcription-polymerase chain reaction (RT-PCR) was used to assess the expression of some specific GPCRs and the other receptors by MIN6 pancreatic β-cells.

Results

The agonists were not found to be toxic for the MIN6 pancreatic β-cells within the range of the doses used in this study, whereas insulin secretion altered depending on the ligands and receptors. In addition, arachidonyl-2’-chloroethylamide (ACEA), carbachol, chemokine (C-C motif) ligand-5 (CCL5), and exendin as well as phorbol myristate acetate (PMA) ligands showed significant increases in the insulin secretion of MIN6 pancreatic β-cells.

Conclusion

Understanding the normal β-cell function and identifying the defects in β-cell function that lead to the development of diabetes will generate new therapeutic targets.

Structural organization of a major neuronal G protein regulator, the RGS7-Gβ5-R7BP complex

Signaling by the G-protein-coupled receptors (GPCRs) plays fundamental role in a vast number of essential physiological functions. Precise control of GPCR signaling requires action of regulators of G protein signaling (RGS) proteins that deactivate heterotrimeric G proteins. RGS proteins are elaborately regulated and comprise multiple domains and subunits, yet structural organization of these assemblies is poorly understood. Here, we report a crystal structure and dynamics analyses of the multisubunit complex of RGS7, a major regulator of neuronal signaling with key roles in controlling a number of drug target GPCRs and links to neuropsychiatric disease, metabolism, and cancer. The crystal structure in combination with molecular dynamics and mass spectrometry analyses reveals unique organizational features of the complex and long-range conformational changes imposed by its constituent subunits during allosteric modulation. Notably, several intermolecular interfaces in the complex work in synergy to provide coordinated modulation of this key GPCR regulator.