Identification of novel peptide agonists from a random peptide library for a 5-oxo-ETE receptor, a receptor for bioactive lipids

GPR31 and GPR151 are activated under acidic conditions

Recent studies have revealed that not only proton-sensing channels, but also one family of G protein-coupled receptors (GPCRs) comprising OGR1, GPR4, G2A and TDAG8 are responsible for the sensing of extracellular protons, or pH. Here, we report that two other GPCRs, GPR31 and GPR151, were also activated in acidic condition. Elevated pH of assay mixtures resulted in a remarkable increase in [35S]GTPγS binding by GPR31-Giα and GPR151-Giα fusion proteins in a narrow range between pH 6 and 5. Our reporter gene assays with CHO cells expressing recombinant GPR31 or GPR151 also showed that activation was maximal at pH ∼5.8. Although these results from in vitro and cellular assays revealed slightly different pH sensitivities, all of our results indicated that GPR31 and GPR151 sensed extracellular protons equally well as other proton-sensing GPCRs.

A novel partial agonist of GPBA reduces blood glucose level in a murine glucose tolerance test

GPBA is a G protein-coupled receptor that is activated by bile acids. Because activation of GPBA leads to increased cAMP levels and secretion of incretins and insulin, GPBA has been proposed as a promising drug target for the treatment of metabolic syndrome. Previously, we have developed a ligand-screening system to identify novel agonists of GPBA by means of a fusion protein of GPBA with G protein stimulatory α subunit (Gsα) and by a [³⁵S]GTPγS-binding assay. To express the GPBA-Gsα fusion protein, transgenic silkworms were employed in this study, and cell membrane fractions were prepared from their fat body or pupae. We applied them to the screening of a chemical library that contains 10,625 compounds from the RIKEN Natural Products Depository (NPDepo). Eventually, a unique partial agonist, GUM2, was successfully identified. Our results indicated that the GPCR-Gα fusion proteins were beneficial for ligand identification and that the transgenic silkworms were useful for large-scale production of GPCRs. In HEK293 cells transiently expressing GPBA, GUM2 showed 50% effective concentration (EC₅₀) of 3.5 ± 2.4μM and induced GPBA internalization as effectively as did an endogenous agonist, TLC. We also confirmed that GUM2 stimulates insulin secretion in MIN6 cells. Moreover, a single 2mg/kg dose of GUM2 significantly reduced blood glucose levels in mice during an intraperitoneal glucose tolerance test even though GUM2 is only a partial agonist with a low intrinsic activity. We concluded that GUM2 is a good candidate for research on GPBA signaling under physiological conditions and for the development of GPBA-targeting therapeutic compounds.

Identification and molecular docking studies for novel inverse agonists of SREB, super conserved receptor expressed in brain

The identification of novel synthetic ligands for G protein-coupled receptors (GPCRs) is important not only for understanding human physiology, but also for the development of novel drugs, especially for orphan GPCRs for which endogenous ligands are unknown. One of the orphan GPCR subfamilies, Super conserved Receptor Expressed in Brain (SREB), consists of GPR27, GPR85 and GPR173 and is expressed in the central nervous system. We report herein the identification of inverse agonists for the SREB family without their agonists. We carried out an in vitro screening of 5472 chemical compounds from the RIKEN NPDepo chemical library. The binding of [(35) S]GTPγS to the GPR173-Gsα fusion protein expressed in Sf9 cells was measured and resulted in the identification of 8 novel GPR173 inverse agonists. The most potent compound showed an IC50 of approximately 8 μm. The identified compounds were also antagonists for other SREB members, GPR27 and GPR85. These results indicated that the SREB family could couple Gs-type G proteins, and SREB-Gsα fusion proteins showed significant constitutive activities. Moreover, a molecular model of GPR173 was constructed using the screening results. The combination of computational and biological methods will provide a unique approach to ligand identification for orphan GPCRs and brain research.

In vivo and in vitro evaluation of novel μ-opioid receptor agonist compounds

Opioids are the most effective and widely used drugs for pain treatment. Morphine is an archetypal opioid and is an opioid receptor agonist. Unfortunately, the clinical usefulness of morphine is limited by adverse effects such as analgesic tolerance and addiction. Therefore, it is important to study the development of novel opioid agonists as part of pain control. The analgesic effects of opioids are mediated by three opioid receptors, namely opioid μ-, δ-, and κ-receptors. They belong to the G protein-coupled receptor superfamily and are coupled to Gi proteins. In the present study, we developed a ligand screening system to identify novel opioid μ-receptor agonists that measures [(35)S]GTPγS binding to cell membrane fractions prepared from the fat body of transgenic silkworms expressing μ-receptor-Gi1α fusion protein. We screened the RIKEN Natural Products Depository (NPDepo) chemical library, which contains 5848 compounds, and analogs of hit compounds. We successfully identified a novel, structurally unique compound, that we named GUM1, with agonist activity for the opioid μ-receptor (EC50 of 1.2 µM). The Plantar Test (Hargreaves' Method) demonstrated that subcutaneous injection of 3mg/kg of GUM1 into wild-type rats significantly extended latency time. This extension was also observed in a rat model of morphine tolerance and was inhibited by pre-treatment of naloxone. The unique molecular skeleton of GUM1 makes it an attractive molecule for further ligand-opioid receptor binding studies.