Dual-Color Bioluminescence Analysis for Quantitatively Monitoring G-Protein-Coupled Receptor and β-Arrestin Interactions

Live Cell Bioluminescence Imaging in Temporal Reaction of G Protein-Coupled Receptor for High-Throughput Screening and Analysis

G protein-coupled receptors (GPCRs) are notable targets of basic therapeutics. Many screening methods have been established to identify novel agents for GPCR signaling in a high-throughput manner. However, information related to the temporal reaction of GPCR with specific ligands remains poor. We recently developed a bioluminescence method for the quantitative detection of the interaction between GPCR and β-arrestin using split luciferase complementation. To monitor time-course variation of the interactions, a new imaging system contributes to the accurate evaluation of drugs for GPCRs in a high-throughput manner.

Dual-color bioluminescence imaging assay using green- and red-emitting beetle luciferases at subcellular resolution

Bioluminescence imaging is widely used to monitor cellular events, including gene expression in vivo and in vitro. Moreover, recent advances in luciferase technology have made possible imaging at the single-cell level. To improve the bioluminescence imaging system, we have developed a dual-color imaging system in which the green-emitting luciferase from a Brazilian click beetle (Emerald Luc, ELuc) and the red-emitting luciferase from a railroad worm (Stable Luciferase Red, SLR) were used as reporters, which were localized to the peroxisome and the nucleus, respectively. We clearly captured simultaneously the subcellular localization of ELuc in the peroxisome and SLR in the nucleus of a single cell using a high-magnification objective lens with 3-min exposure time without binning using a combination of optical filters. Furthermore, to apply this system to quantitative time-lapse imaging, the activation of nuclear factor triggered by tumor necrosis factor α was measured using nuclear-targeted SLR and peroxisome-targeted ELuc as the test and internal control reporters, respectively. We successfully quantified the kinetics of activation of nuclear factor κB using nuclear-targeted SLR and the transcriptional change of the internal control promoter using peroxisome-targeted ELuc simultaneously in a single cell, and showed that the activation kinetics, including activation rate and amplitude, differed among cells. The results demonstrated that this imaging system can visualize the subcellular localization of reporters and track the expressions of two genes simultaneously at subcellular resolution.

Cell-based assays and animal models for GPCR drug screening

The family of G protein-coupled receptors (GPCRs) remains a central focus of basic pharmacology and drug discovery efforts. Convenient methods to assess the efficacy of potentially therapeutic reagents for GPCRs are strongly required for high-throughput screening (HTS) assay. We recently developed a rapid, sensitive, and quantitative method for detecting potential chemicals that act on GPCRs using split luciferase complementation. In principle, this is based on the detection of interactions of GPCR with β-arrestin, which translocates to the activated GPCRs. This method can facilitate the construction of HTS systems in a multi-well plate format. Particularly, the method is compatible with single-cell imaging and animal models and even deeper tissues such as organs, because of its high sensitivity, suggesting that promising candidates from HTS assay can be moved easily to the next phase for additional analysis. This system can contribute to the effective evaluation of potentially therapeutic reagents and expedite the development of new drugs for GPCRs.

Bioluminescent tools for the analysis of G-protein-coupled receptor and arrestin interactions

New protein-based bioluminescent probes for monitoring GPCR interaction with β-arrestin are presented.

Molecular determinants of orexin receptor-arrestin-ubiquitin complex formation

BACKGROUND AND PURPOSE

The orexin system regulates a multitude of key physiological processes, particularly involving maintenance of metabolic homeostasis. Consequently, there is considerable potential for pharmaceutical development for the treatment of disorders from narcolepsy to metabolic syndrome. It acts through the hormonal activity of two endogenous peptides, orexin A binding to orexin receptors 1 and 2 (OX₁ and OX₂) with similar affinity, and orexin B binding to OX₂ with higher affinity than OX₁ receptors. We have previously revealed data differentiating orexin receptor subtypes with respect to their relative stability in forming orexin receptor-arrestin-ubiquitin complexes measured by BRET. Recycling and cellular signalling distinctions were also observed. Here, we have investigated, using BRET, the molecular determinants involved in providing OX₂ receptors with greater β-arrestin-ubiquitin complex stability.

EXPERIMENTAL APPROACH

The contribution of the C-terminal tail of the OX receptors was investigated by bulk substitution and site-specific mutagenesis using BRET and inositol phosphate assays.

KEY RESULTS

Replacement of the OX₁ receptor C-terminus with that of the OX₂ receptor did not result in the expected gain of function, indicating a role for intracellular domain configuration in addition to primary structure. Furthermore, two out of the three putative serine/threonine clusters in the C-terminus were found to be involved in OX₂ receptor-β-arrestin-ubiquitin complex formation.

CONCLUSIONS AND IMPLICATIONS

This study provides fundamental insights into the molecular elements that influence receptor-arrestin-ubiquitin complex formation. Understanding how and why the orexin receptors can be functionally differentiated brings us closer to exploiting these receptors as drug targets.