S-Series Coelenterazine-Driven Combinatorial Bioluminescence Imaging Systems for Mammalian Cells

A unique combinatorial bioluminescence (BL) imaging system was developed for determining molecular events in mammalian cells with various colors and BL intensity patterns. This imaging system consists of one or multiple reporter luciferases and a series of novel coelenterazine (CTZ) analogues named "S-series". For this study, ten kinds of novel S-series CTZ analogues were synthesized and characterized concerning the BL intensities, spectra, colors, and specificity of various marine luciferases. The characterization revealed that the S-series CTZ analogues luminesce with blue-to-orange-colored BL spectra with marine luciferases, where the most red-shifted BL spectrum peaked at 583 nm. The colors completed a visible light color palette with those of our precedent C-series CTZ analogues. The synthesized substrates S1, S5, S6, and S7 were found to have a unique specificity with marine luciferases, such as R86SG, NanoLuc (shortly, NLuc), and ALuc16. They collectively showed unique BL intensity patterns to identify the marine luciferases together with colors. The marine luciferases, R86SG, NLuc, and ALuc16, were multiplexed into multi-reporter systems, the signals of which were quantitatively unmixed with the specific substrates. When the utility was applied to a single-chain molecular strain probe, the imaging system simultaneously reported three different optical indexes for a ligand, i.e., unique BL intensity and color patterns for identifying the reporters, together with the ligand-specific fold intensities in mammalian cells. This study directs a new combinatorial BL imaging system to specific image molecular events in mammalian cells with multiple optical indexes.

ATP Bioluminescence Assay To Evaluate Antibiotic Combinations against Extensively Drug-Resistant (XDR) Pseudomonas aeruginosa

Time-kill curves are used to study antibiotic combinations, but the colony count method to obtain the results is time-consuming. The aim of the study was to validate an ATP assay as an alternative to the conventional colony count method in studies of antibiotic combinations. The cutoff point for synergy and bactericidal effect to categorize the results using this alternative method were determined in Pseudomonas aeruginosa. The ATP assay was performed using the GloMax 96 microplate luminometer (Promega), which measures bioluminescence in relative light units (RLU). To standardize this assay, background, linearity, and the detection limit were determined with one strain each of multidrug-resistant P. aeruginosa and Klebsiella pneumoniae. Twenty-four-hour time-kill curves were performed in parallel by both methods with 12 strains of P. aeruginosa. The conventional method was used as a “gold” standard to establish the pharmacodynamic cutoff points in the ATP method. Normal saline solution was established as washing/dilution medium. RLU signal correlated with CFU when the assay was performed within the linear range. The categorization of the pharmacodynamic parameters using the ATP assay was equivalent to that of the colony count method. The bactericidal effect and synergy cutoff points were 1.348 (93% sensitivity, 81% specificity) and 1.065 (95% sensitivity, 89% specificity) log RLU/mL, respectively. The ATP assay was useful to determine the effectiveness of antibiotic combinations in time-kill curves. This method, less laborious and faster than the colony count method, could be implemented in the clinical laboratory workflow.

IMPORTANCE Combining antibiotics is one of the few strategies available to overcome infections caused by multidrug-resistant bacteria. Time-kill curves are usually performed to evaluate antibiotic combinations, but obtaining results is too laborious to be routinely performed in a clinical laboratory. Our results support the utility of an ATP measurement assay using bioluminescence to determine the effectiveness of antibiotic combinations in time-kill curves. This method may be implemented in the clinical laboratory workflow as it is less laborious and faster than the conventional colony count method. Shortening the obtention of results to 24 h would also allow an earlier guided combined antibiotic treatment.

Reporter gene comparison demonstrates interference of complex body fluids with secreted luciferase activity

Reporter gene assays are widely used to study cellular signaling and transcriptional activity. Few studies describe the use of reporter genes for studying cellular responses on complex body fluids, such as urine and blood. Selection of the optimal reporter gene is crucial for study outcome. Here, we compared the characteristics of five reporter genes (Firefly luciferase, stable- and unstable Nano luciferase, secretable Gaussia luciferase and Red Fluorescent Protein) to study complex body fluids. For this comparison, the NFκB Response Element (NFκB-RE) and Smad Binding Element (SBE) were identically cloned into the five different reporter vectors. Reporter characteristics were evaluated by kinetic and concentration–response measurements in SW1353 and HeLa cell lines. Finally, reporter compatibility with complex body fluids (fetal calf serum, knee joint synovial fluid and human serum) and inter-donor variation were evaluated. Red Fluorescent Protein demonstrated poor inducibility as a reporter gene and slow kinetics compared to luciferases. Intracellularly measured luciferases, such as Firefly luciferase and Nano luciferase, revealed good compatibility with complex body fluids. Secreted Gaussia luciferase appeared to be incompatible with complex body fluids, due to variability in inter-donor signal interference. Unstable Nano luciferase demonstrated clear inducibility, high sensitivity and compatibility with complex body fluids and therefore can be recommended for cellular signaling studies using complex body fluids.

Bioluminescent method for the rapid screening of toxic heayy metals in environmental samples using Photobacterium leiognathi strain AK-MIE

Assessment of eco-toxicant using bioluminescent bacterial assay is a widely used and globally accepted method. In this work, a new luminescent bacterium was isolated from squid (Loligo duvauceli) and identified as Photobacterium leiognathi strain AK-MIE using 16S rRNA, phylogeny analysis. The predicted optimum conditions by RSM were 2.76% (w/v) NaCl, 2.28% (w/v) peptone, 0.34% (w/v) yeast extract, and pH 6.83 with 541,211.80 RLU of luminescent production whereas the predicted optimum conditions by ANN were 2.21% (w/v) NaCl, 2.27% (w/v) peptone, 0.39% (w/v) yeast extract, and pH 6.94 which produced 541,986.20 RLU. The validation analysis of both RSM and ANN show 0.60% and 0.69% deviation from the predicted results indicating that both models provided good quality predictions with ANN showing a superior data fitting capability for non-linear regression analysis. Toxicity tests show strain AK-MIE was sensitive to mercury (concentration causing 50% inhibition or IC₅₀ of 0.00978 mgL^-1), followed by cadmium (IC₅₀ of 0.5288 mgL^-1), copper IC₅₀ of (0.8117 mgL^-1), silver (IC₅₀ of 1.109 mgL^-1), and lead (IC₅₀ of 10.71 mgL^-1) which are more sensitive than previously isolated luminescent bacteria, suggesting that strain AK-MIE has the potential to be used in toxicity assessment of heavy metals in the environment. Based on the field trial results, several sediment samples from industrial areas in Bangi, Selangor managed to inhibit the bioluminescence of strain AK-MIE. Validation method carried out using ICP-MS proved the presence of several toxic heavy metal elements.

Continuous long-term cytotoxicity monitoring in 3D spheroids of beetle luciferase-expressing hepatocytes by nondestructive bioluminescence measurement

Background

Three-dimensional (3D) spheroids are frequently used in toxicological study because their morphology and function closely resemble those of tissue. As these properties are maintained over a long term, repeated treatment of the spheroids with a test object is possible. Generally, in the repeated treatment test to assess cytotoxicity in the spheroids, ATP assay, colorimetric measurement using pigments or high-content imaging analysis is performed. However, continuous assessment of cytotoxicity in the same spheroids using the above assays or analysis is impossible because the spheroids must be disrupted or killed. To overcome this technical limitation, we constructed a simple monitoring system in which cytotoxicity in the spheroids can be continuously monitored by nondestructive bioluminescence measurement.

Results

Mouse primary hepatocytes were isolated from transchromosomic (Tc) mice harboring a mouse artificial chromosome (MAC) vector expressing beetle luciferase Emerald Luc (ELuc) under the control of cytomegalovirus immediate early enhancer/chicken β-actin promoter/rabbit β-globin intron II (CAG) promoter, and used in 3D cultures. We confirmed that both luminescence and albumin secretion from the spheroids seeded in the 96-well format Cell-able^TM were maintained for approximately 1 month. Finally, we repetitively treated the luminescent 3D spheroids with representative hepatotoxicants for approximately 1 month, and continuously and nondestructively measured bioluminescence every day. We successfully obtained daily changes of the dose-response bioluminescence curves for the respective toxicants.

Conclusions

In this study, we constructed a monitoring system in which cytotoxicity in the same 3D spheroids was continuously and sensitively monitored over a long term. Because this system can be easily applied to other cells, such as human primary cells or stem cells, it is expected to serve as the preferred platform for simple and cost-effective long-term monitoring of cellular events, including cytotoxicity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0374-1) contains supplementary material, which is available to authorized users.

Correlation between luminescence intensity and cytotoxicity in cell-based cytotoxicity assay using luciferase

The luciferase reporter assay has become one of the conventional methods for cytotoxicity evaluation. Typically, the decrease of luminescence expressed by a constitutive promoter is used as an index of cytotoxicity. However, to our knowledge, there have been no reports of the correlation between cytotoxicity and luminescence intensity. In this study, to accurately verify the correlation between them, beetle luciferase was stably expressed in human hepatoma HepG2 cells harboring the multi-integrase mouse artificial chromosome vector. We showed that the cytotoxicity assay using luciferase does not depend on the stability of luciferase protein and the kind of constitutive promoter. Next, HepG2 cells in which green-emitting beetle luciferase was expressed under the control of CAG promoter were exposed to 58 compounds. The luminescence intensity and cytotoxicity curves of cells exposed to 48 compounds showed similar tendencies, whereas those of cells exposed to 10 compounds did not do so, although the curves gradually approached each other with increasing exposure time. Finally, we demonstrated that luciferase expressed under the control of a constitutive promoter can be utilized both as an internal control reporter for normalizing a test reporter and for monitoring cytotoxicity when two kinds of luciferases are simultaneously used in the cytotoxicity assay.