Development of a Human Estrogen Receptor Dimerization Assay for the Estrogenic Endocrine-Disrupting Chemicals Using Bioluminescence Resonance Energy Transfer

Endocrine-disrupting chemicals (EDCs) are found in food and various other substances, including pesticides and plastics. EDCs are easily absorbed into the body and have the ability to mimic or block hormone function. The radioligand binding assay based on the estrogen receptors binding affinity is widely used to detect estrogenic EDCs but is limited to radioactive substances and requires specific conditions. As an alternative, we developed a human cell-based dimerization assay for detecting EDC-mediated ER-alpha (ERα) dimerization using bioluminescence resonance energy transfer (BRET). The resultant novel BRET-based on the ERα dimerization assay was used to identify the binding affinity of 17β-estradiol (E2), 17α-estradiol, corticosterone, diethylhexyl phthalate, bisphenol A, and 4-nonylphenol with ERα by measuring the corresponding BRET signals. Consequently, the BRET signals from five chemicals except corticosterone showed a dose-dependent sigmoidal curve for ERα, and these chemicals were suggested as positive chemicals for ERα. In contrast, corticosterone, which induced a BRET signal comparable to that of the vehicle control, was suggested as a negative chemical for ERα. Therefore, these results were consistent with the results of the existing binding assay for ERα and suggested that a novel BRET system can provide information about EDCs-mediated dimerization to ERα.

Enhanced brightness of bacterial luciferase by bioluminescence resonance energy transfer

Using the lux operon (luxCDABE) of bacterial bioluminescence system as an autonomous luminous reporter has been demonstrated in bacteria, plant and mammalian cells. However, applications of bacterial bioluminescence-based imaging have been limited because of its low brightness. Here, we engineered the bacterial luciferase (heterodimer of luxA and luxB) by fusion with Venus, a bright variant of yellow fluorescent protein, to induce bioluminescence resonance energy transfer (BRET). By using decanal as an externally added substrate, color change and ten-times enhancement of brightness was achieved in Escherichia coli when circularly permuted Venus was fused to the C-terminus of luxB. Expression of the Venus-fused luciferase in human embryonic kidney cell lines (HEK293T) or in Nicotiana benthamiana leaves together with the substrate biosynthesis-related genes (luxC, luxD and luxE) enhanced the autonomous bioluminescence. We believe the improved luciferase will forge the way towards the potential development of autobioluminescent reporter system allowing spatiotemporal imaging in live cells.

BRED: bioluminescence energy transfer to dye for monitoring ceramide trafficking in cell

Bioluminescence resonance energy transfer (BRET) is a genetically encoded proximity-based tool to study biomolecular interactions. However, conventional BRET is usually restricted to only a few types of interactions like protein-protein or protein-ligand interactions. We here developed a spatially unbiased resonance energy transfer system, so-called BRED - bioluminescence resonance energy transfer to dye. BRED allows transferring energy from a genetically encoded bright human optimized luciferase to a fluorophore-labelled small molecule. The high efficiency of the system allows RET without specific interaction of donor and acceptor. Here, we applied BRED to monitor the trafficking of the signalling lipid ceramide, to the Golgi. This was enabled by an engineered Golgi-resident luciferase, which was used to sense the influx of BODIPY-labeled ceramide into the surrounding membrane. We demonstrated the implementation of the method via flow cytometry, thereby combining the sensitivity of bulk cell methods with the advantages of single-cell analysis. This toolbox enables simple and robust live-cell analysis of inhibitors of CERT-mediated ceramide transport. The design principle of our optogenetic tool can be applied to study intracellular trafficking of metabolites and screen for inhibitors of their key enzymes.