How to Select Firefly Luciferin Analogues for In Vivo Imaging.

Bioluminescence reactions are widely applied in optical in vivo imaging in the life science and medical fields. Such reactions produce light upon the oxidation of a luciferin (substrate) catalyzed by a luciferase (enzyme), and this bioluminescence enables the quantification of tumor cells and gene expression in animal models. Many researchers have developed single-color or multicolor bioluminescence systems based on artificial luciferin analogues and/or luciferase mutants, for application in vivo bioluminescence imaging (BLI). In the current review, we focus on the characteristics of firefly BLI technology and discuss the development of luciferin analogues for high-resolution in vivo BLI. In addition, we discuss the novel luciferin analogues TokeOni and seMpai, which show potential as high-sensitivity in vivo BLI reagents.

Bioluminescence imaging using d-luciferin and its analogs for visualizing Bdnf expression in living mice; different patterns of bioluminescence signals using distinct luciferase substrates.

Brain-derived neurotrophic factor (BDNF) plays a crucial role in numerous brain functions, including memory consolidation. Previously, we generated a Bdnf-Luciferase transgenic (Bdnf-Luc) mouse strain to visualize changes in Bdnf expression using in vivo bioluminescence imaging. We successfully visualized activity-dependent Bdnf induction in living mouse brains using a d-luciferin analog, TokeOni, which distributes to the brain and produces near-infrared bioluminescence. In this study, we compared the patterns of bioluminescence signals within the whole body of the Bdnf-Luc mice produced by d-luciferin, TokeOni and seMpai, another d-luciferin analog that produces a near-infrared light. As recently reported, hepatic background signals were observed in wild-type mice when using TokeOni. Bioluminescence signals were strongly observed from the region containing the liver when using d-luciferin and TokeOni. Additionally, we detected signals from the brain when using TokeOni. Compared with d-luciferin and TokeOni, signals were widely detected in the whole body of Bdnf-Luc mice by seMpai. The signals produced by seMpai were strong in the regions containing skeletal muscles in particular. Taken together, the patterns of bioluminescence signals in Bdnf-Luc mice vary when using different luciferase substrates. Therefore, the expression of Bdnf in tissues and organs of interest could be visualized by selecting an appropriate substrate.

Absorption Spectra for Firefly Bioluminescence Substrate Analog: TokeOni in Various pH Solutions.

AkaLumine hydrochloride, named TokeOni, is one of the firefly luciferin analogs, and its reaction with firefly luciferase produces near-infrared (NIR) bioluminescence. Prior to studying the bioluminescence mechanism, basic knowledge about the chemical structures, electronic states, and absorption properties of TokeOni at various pH values of solution has to be acquired. In this paper, the absorption spectra for TokeOni and AkaLumine at pH 2-10 were measured. Density functional theory (DFT) calculations, time-dependent DFT calculations, and the vibrational analyses were carried out. The absorption spectra indicate that the chemical forms of TokeOni in solutions are same as those of AkaLumine. The peaks at pH 7-10 in the absorption spectra correspond to the excitation from the ground state of a carboxylate anion of AkaLumine, the peak at pH 2 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring and N-protonated dimethylamino group of AkaLumine, and the peak at pH 4 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring of AkaLumine.