Red-emitting luciferases for bioluminescence reporter and imaging applications

North American firefly Photinus pyralis luciferase, which emits yellow-green light (557 nm), has been adapted for a variety of applications, including gene reporter assays, whole-cell biosensor measurements, and in vivo imaging. Luciferase variants with red-shifted bioluminescence and high specific activity can be paired with green-emitting counterparts for use in dual-color reporter assays or can be used alone for in vivo imaging. Beginning with a previously reported red-emitting thermostable mutant and using mutagenesis techniques, we engineered two luciferases with redder emission maxima while maintaining satisfactory specific activities and thermostability. The novel enzymes were expressed in HEK293 cells, where they performed similarly to Promega’s codon-optimized click beetle red luciferase in model reporter assays. When the firefly luciferase variants were codon-optimized and retested using optimized substrate concentrations, they provided 50- to 100-fold greater integrated light intensities than the click beetle enzyme. These results suggest that the novel enzymes should provide superior performance in dual-color reporter and in vivo imaging applications, and they illustrate the importance of codon optimization for assays in mammalian cells.     

Thermostability of firefly luciferases affects efficiency of detection by in vivo bioluminescence

Luciferase from the North American firefly (Photinis pyralis) is a useful reporter gene in vivo, allowing noninvasive imaging of tumor growth, metastasis, gene transfer, drug treatment, and gene expression. Luciferase is heat labile with an in vitro halflife of approximately 3 min at 37 degrees C. We have characterized wild type and six thermostabilized mutant luciferases. In vitro, mutants showed half-lives between 2- and 25-fold higher than wild type. Luciferase transfected mammalian cells were used to determine in vivo half-lives following cycloheximide inhibition of de novo protein synthesis. This showed increased in vivo thermostability in both wild-type and mutant luciferases. This may be due to a variety of factors, including chaperone activity, as steady-state luciferase levels were reduced by geldanamycin, an Hsp90 inhibitor. Mice inoculated with tumor cells stably transfected with mutant or wild-type luciferases were imaged. Increased light production and sensitivity were observed in the tumors bearing thermostable luciferase. Thermostable proteins increase imaging sensitivity. Presumably, as more active protein accumulates, detection is possible from a smaller number of mutant transfected cells compared to wild-type transfected cells.     

Continuous delivery of D-luciferin by implanted micro-osmotic pumps enables true real-time bioluminescence imaging of luciferase activity in vivo

Bioluminescence imaging (BLI) of luciferase reporters in small animal models offers an attractive approach to monitor regulation of gene expression, signal transduction, and protein-protein interactions, as well as following tumor progression, cell engraftment, infectious pathogens, and target-specific drug action. Conventional BLI can be repeated within the same animal after bolus reinjections of a bioluminescent substrate. However, intervals between image acquisitions are governed by substrate pharmacokinetics and excretion, therefore restricting temporal resolution of reinjection protocols to the order of hours, limiting analyses of processes in vivo with short time constants. To eliminate these constraints, we examined use of implanted micro-osmotic pumps for continuous, long-term delivery of bioluminescent substrates. Pump-assisted d-luciferin delivery enabled BLI for > or = 7 days from a variety of luciferase reporters. Pumps allowed direct repetitive imaging at < 5-minute intervals of the pharmacodynamics of proteasome- and IKK-inhibiting drugs in mice bearing tumors stably expressing ubiquitin-firefly luciferase or IkappaBalpha-firefly luciferase fusion reporters. Circadian oscillations in the olfactory bulbs of transgenic rats expressing firefly luciferase under the control of the period1 promoter also were temporally resolved over the course of several days. We conclude that implanted pumps provide reliable, prolonged substrate delivery for high temporal resolution BLI, traversing complications of repetitive substrate injections.     

Site-directed mutagenesis of firefly luciferase active site amino acids: a proposed model for bioluminescence color.

Under physiological conditions firefly luciferase catalyzes the highly efficient emission of yellow-green light from the substrates luciferin, Mg-ATP, and oxygen. In nature, bioluminescence emission by beetle luciferases is observed in colors ranging from green (approximately 530 nm) to red (approximately 635 nm), yet all known luciferases use the same luciferin substrate. In an earlier report [Branchini, B. R., Magyar, R. M., Murtiashaw, M. H., Anderson, S. M., and Zimmer, M. (1998) Biochemistry 37, 15311-15319], we described the effects of mutations at His245 on luciferase activity. In the context of molecular modeling results, we proposed that His245 is located at the luciferase active site. We noted too that the H245 mutants displayed red-shifted bioluminescent emission spectra. We report here the construction and purification of additional His245 mutants, as well as mutants at residues Lys529 and Thr343, all of which are stringently conserved in the beetle luciferase sequences. Analysis of specific activity and steady-state kinetic constants suggested that these residues are involved in luciferase catalysis and the productive binding of substrates. Bioluminescence emission spectroscopy studies indicated that point mutations at His245 and Thr343 produced luciferases that emitted light over the color range from green to red. The results of mutational and biochemical studies with luciferase reported here have enabled us to propose speculative mechanisms for color determination in firefly bioluminescence. An essential role for Thr343, the participation of His245 and Arg218, and the involvement of bound AMP are indicated.     

Use of bacterial and firefly luciferases as reporter genes in DEAE-dextran-mediated transfection of mammalian cells.

The aim of this study was to compare three different luciferase genes by placing them in a single reporter vector and expressing them in the same mammalian cell type. The luciferase genes investigated were the luc genes from the fireflies Photinus pyralis (PP) and Luciola mingrelica (LM) and the lux AB5 gene, a translational fusion of the two subunits of the bacterial luciferase from Vibrio harveyi (VH). The chloramphenicol acetyltransferase (CAT) gene was also included in this study for comparison. The performances of the assay methods of the corresponding enzymes were evaluated using reference materials and the results of the expressed enzymes following transfection were calculated using calibration curves. All of the bioluminescent assays possess high reproducibility both within and between the batches (less than 15%). The comparison of the assay methods shows that firefly luciferases have the highest detection sensitivity (0.05 and 0.08 amol for PP and LM, respectively) whereas the VH bacterial luciferase has 5 amol and CAT 100 amol. On the other hand, the transfection of the various plasmids shows that the content of the expressed enzyme within the cells is much higher for CAT than for the other luciferase genes. VH luciferase is expressed at very low levels in mammalian cells due to the relatively high temperature of growing of the mammalian cells that seems to impair the correct folding of the active enzyme. PP and LM luciferases are both expressed at picomolar level but usually 10 to 70 times less in content with respect to CAT within the transfected cells. On the basis of these results the overall improvement in sensitivity related to the use of firefly luciferases as reporter genes in mammalian cells is about 30 to 50 times with respect to that of CAT.     

Characterization of coelenterazine analogs for measurements of Renilla luciferase activity in live cells and living animals

In vivo imaging of bioluminescent reporters relies on expression of light-emitting enzymes, luciferases, and delivery of chemical substrates to expressing cells. Coelenterazine (CLZN) is the substrate for a group of bioluminescent enzymes obtained from marine organisms. At present, there are more than 10 commercially available CLZN analogs. To determine which analog is most suitable for activity measurements in live cells and living animals, we characterized 10 CLZN analogs using Renilla luciferase (Rluc) as the reporter enzyme. For each analog, we monitored enzyme activity, auto-oxidation, and efficiency of cellular uptake. All CLZN analogs tested showed higher auto-oxidation signals in serum than was observed in phosphate buffer or medium, mainly as a result of auto-oxidation by binding to albumin. CLZN-f, -h, and -e analogs showed 4- to 8-fold greater Rluc activity, relative to CLZN-native, in cells expressing the enzyme from a stable integrant. In studies using living mice expressing Rluc in hepatocytes, administration of CLZN-e and -native produced the highest signal. Furthermore, distinct temporal differences in signal for each analog were revealed following intravenous or intraperitoneal delivery. We conclude that the CLZN analogs that are presently available vary with respect to hRluc utilization in culture and in vivo, and that the effective use of CLZN-utilizing enzymes in living animals depends on the selection of an appropriate substrate.     

Non-destructive detection of firefly luciferase (LUC) activity in single plant cells using a cooled, slow-scan CCD camera and an optimized assay

A flexible, comparatively inexpensive system based on a liquid nitrogen-cooled slow-scan CCD (charge coupled device) camera is presented, which can be employed for quantitative low-light (bioluminescence, chemiluminescence or fluorescence) imaging. Using this camera system and the firefly luciferase (LUC) as a screenable marker, transgenic tobacco lines have been produced by direct gene transfer. Bioluminescence emitted from single tobacco cells transiently expressing the firefly luciferase gene (Luc) as well as from stably transformed calli, regenerated shoots, plantlets and T₁ seedlings could be monitored in vivo with no effect on the viability of the material analysed. The patterns of light emission from sections through Luc -expressing leaves and bioluminescent single protoplasts isolated from such leaves were also imaged microscopically. The assay used to detect in vivo LUC activity was optimized by quantifying bioluminescence emitted from Luc -expressing tobacco protoplasts and leaves incubated in different substrate solutions and determining the kinetics of light emission during incubation in the substrate solution.     

An in vivo dual-reporter system of cyanobacteria using two railroad-worm luciferases with different color emissions

In vivo genetic reporter systems using luciferase enzymes enable the real-time monitoring of gene expression in living cells. We have challenged concurrent monitoring of two independent promoter activities within the same cells to precisely compare their characteristics in vivo. In this report, we describe a simple dual-reporter system capable of simultaneously monitoring two promoter activities in living cyanobacterial cells. Two railroad-worm luciferases catalyzing the bioluminescent emissions of different colors served as the dual reporters; each emission was successfully separated by interference filters to estimate the individual bioluminescence signals using photomultiplier tubes. Using this system, we clearly demonstrated the difference in the expression profiles between promoters in the same cells.     

Generation of thermostable monomeric luciferases from Photorhabdus luminescens

Bacterial luciferases and the genes encoding these light-emitting enzymes have an increasing number of applications in biological sciences. Temperature lability and the heterodimeric nature of these luciferases have been the major obstacles for their widespread use, for instance, as genetic reporters. Escherichia coli expressing wild-type Photorhabdus luminescens luciferase was found to produce eight times more light than the corresponding Vibrio harveyi luciferase clone in vivo at 37 degrees C. Three monomeric luciferases were created by translationally fusing the two genes encoding luxA and luxB proteins of P. luminescens. These clones were equally active in producing light in vivo when cultivated at 37 degrees C compared to cultivation at 30 degrees C. The fusion containing the longest linker showed the highest activity. In vitro, the monomeric luciferases were less active having at best 20% of activity of the wild-type enzyme due to the partial formation of insoluble aggregates. The results suggest that P. luminescens luciferase and monomeric derivatives thereof should be more suitable than the corresponding V. harveyi enzyme to be used as reporters in cell types which need cultivation at elevated temperatures.     

Time course of bioluminescent signal in orthotopic and heterotopic brain tumors in nude mice

In vivo bioluminescence imaging is becoming increasingly popular. Quantification of bioluminescence signals requires knowledge of the variability and reproducibility of this technique. The objective of this study was to analyze the time course of luminescent signal emitted from firefly luciferase-expressing tumors in two locations, following luciferin injection and at different times after tumor cell implantation. Knowledge of the kinetics of the bioluminescent signals is required for the reliable quantification and comparison of signal during longitudinal studies. The kinetics of bioluminescence was evaluated in orthotopic and heterotopic brain tumors in mice using a human brain tumor cell line constitutively expressing luciferase. Tumor cells were implanted in the brains and flanks of the animals, and whole-body images revealing tumor location were obtained. Tumor burden was monitored over time by the quantitation of photon emission. The magnitude of bioluminescence measured in vivo varied with time after the injection of luciferin, as well as with dose, which necessitated that the comparison of the quantitative results take into consideration the time after injection. Heterotopic and orthotopic tumors exhibited significantly different time courses; however, time after implantation as characterized by kinetic studies performed on days 4 and 14 after cell implantation revealed no significant differences in orthotopic tumors. Future quantitative longitudinal studies must take into account the differences in the kinetics of different models.     

Firefly luciferase genes from the subfamilies Psilocladinae and Ototretinae (Lampyridae, Coleoptera)

Firefly luciferase genes have been isolated from approximately 20 species of Lampyrinae, Luciolinae, and Photurinae. These are mostly nocturnal luminescent species that use light signals for sexual communication. In this study, we isolated three cDNAs for firefly luciferase from Psilocladinae (Cyphonocerus ruficollis) and Ototretinae (Drilaster axillaris and Stenocladius azumai), which are diurnal non-luminescent or weakly luminescent species that may use pheromones for communication. The amino acid sequences deduced from the three cDNAs showed 81-89% identities to each other and 60-81% identities with known firefly luciferases. The three purified recombinant proteins showed luminescence and fatty acyl-CoA synthetic activities, as observed in other firefly luciferases. The emission maxima by the three firefly luciferases (λmax, 545-546 nm) were shorter than those by known luciferases from the nocturnal fireflies (λmax, 550-568 nm). These results suggest that the primary structures and enzymatic properties of luciferases are conserved in Lampyridae, but the luminescence colors were red-shifted in nocturnal species compared to diurnal species.     

Oxygen sensitivity of reporter genes: implications for preclinical imaging of tumor hypoxia

Reporter gene techniques have been applied toward studying the physiologic phenomena associated with tumor hypoxia, a negative prognostic indicator. The purpose of this study was to assess the potential adverse effects of hypoxic conditions on the effectiveness of four commonly used reporter genes: Renilla luciferase, monomeric red fluorescent protein, thymidine kinase, and lacZ. Tumor-forming A375 cells expressing a trifusion reporter consisting of Renilla luciferase, monomeric red fluorescent protein, and thymidine kinase were subjected to decreasing oxygen tensions and assayed for reporter expression and activity. A375 cells expressing beta-galactosidase were similarly exposed to hypoxia, with activity of the reporter monitored by cleavage of the fluorescent substrate 7-hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one)-beta-galactoside (DDAOG). Generation of signal in in vivo tumor models expressing bioluminescent or beta-galactosidase reporters were also examined over the course of hypoxic stresses, either by tumor clamping or the antivascular agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA). Our findings indicate that bioluminescent and fluorescent reporter activity are decreased under hypoxia despite minimal variations in protein production, whereas beta-galactosidase reporter activity per unit protein was unchanged. These results demonstrate that combining beta-galactosidase with the DDAOG optical probe may be a robust reporter system for the in vivo study of tumor hypoxia.     

The influence of insertion of a critical residue (Arg356) in structure and bioluminescence spectra of firefly luciferase

The firefly bioluminescence reaction, which uses luciferin, Mg-ATP, and molecular oxygen to yield an electronically excited oxyluciferin, is carried out by luciferase and visible light is emitted. The bioluminescence color of firefly luciferases is determined by the luciferase structure and assay conditions. Among different beetle luciferases, those from Phrixothrix railroad worm emit either yellow or red bioluminescence colors. Sequence alignment analysis shows that the red-emitter luciferase from Phrixothrix hirtus has an additional Arg residue at 353, which is absent in firefly luciferases. We report here the construction and purification of a mutant at residue Arg(356), which is not conserved in beetle luciferases. By insertion of an additional residue (Arg(356)) using site-specific insertion mutagenesis in a green-emitter luciferase (Lampyris turkestanicus) the color of emitted light was changed to red and the optimum temperature of activity was also increased. Insertion of this Arg in an important flexible loop showed changes of the bioluminescence color and the luciferase reaction took place with relatively retention of its basic kinetic properties such as Km and relative activity. Comparison of native and mutant luciferases using homology modeling reveals a significant conformational change of the flexible loop in the red mutant. Movement of flexible loop brought about a new ionic interaction concomitant with a change in polarity of the emitter site, thereby leading to red emission. It is worthwhile to note that the increased optimum temperature and emission of red light might make mutant luciferase a suitable reporter for the study of gene expression and bioluminescence imaging.     

Evaluation of substrate and inhibitor binding to yeast and human isoprenylcysteine carboxyl methyltransferases (Icmts) using biotinylated benzophenone-containing photoaffinity probes

The technology of real-time imaging in living cells is crucial for understanding of intracellular events. For this purpose, bioluminescent reporters have been introduced as sensitive and convenient tools. Metridia luciferase (MLuc) from the copepod Metridia longa is a coelenterazine-dependent luciferase containing a natural signal peptide for secretion. We report the high-active MLuc mutants with deletion of the N-terminal variable part of amino acid sequence. The MLuc variants were produced in Escherichia coli cells, converted to an active protein, and characterized. We demonstrate that the truncated MLucs have significantly increased bioluminescent activity as against the wild type enzyme but substantially retain other properties. One of the truncated variants of MLuc was transiently expressed in HEK 293 cells. The results clearly suggest that the truncated Metridia luciferase is well suited as a secreted reporter ensuring higher detection sensitivity in comparison with a wild type enzyme.     

Introduction to beetle luciferases and their applications

All beetle luciferases have evolved from a common ancestor: they all use ATP, O₂, and a common luciferin as substrates. The most studied of these luciferases is that derived from the firefly Photinus pyralis, a beetle in the superfamily of Cantharoidea. The sensitivity with which the activity of this enzyme can be assayed has made it useful in the measurement of minute concentrations of ATP. With the cloning of the cDNA coding this luciferase, it has also found wide application in molecular biology as a reporter gene. We have recently cloned other cDNAs that code for luciferases from the bioluminescent click beetle, Pyrophorus plagiophthalamus, in the superfamily Elateroidea. These newly acquired luciferases are of at least four different types, distinguishable by their ability to emit different colours of bioluminescence ranging from green to orange. Unique properties of these luciferases, especially their emission of multiple colours, may make them additionally useful in applications.     

利用萤光素酶标记的肿瘤细胞建立肝癌原位移植模型及其活体成像

目的：利用生物发光成像技术非侵入性地监测活体裸鼠原位肝癌发展过程。方法：将包含有萤火虫萤光素酶基因的pCI-neo-Luc载体转染人肝癌HepG2细胞系,筛选获得具有高萤光素酶活性的细胞克隆;利用流式细胞仪对萤光素酶表达的稳定性进行初步研究,并分析细胞的生物发光情况;持续表达萤光素酶的肿瘤细胞培养扩增后被植入裸鼠皮下,2周后以形成的异体瘤作为供体瘤,进行肝脏原位移植手术;对建立的肝癌原位移植模型,用影像学资料显示肿瘤部位,用IVIS成像系统动态监测肿瘤生长情况。结果：体外影像的结果显示,表达萤光素酶细胞的数量与发光强度呈正相关;活体成像的结果显示。成功地建立了萤光素酶标记的原位肝癌动物模型。结论：生物发光成像可以监测活体内肝癌演进过程,为抗肿瘤药物的筛选和评价提供了新的手段和工具。     

Improved practical usefulness of firefly luciferase by gene chimerization and random mutagenesis

To improve the practical usefulness of the firefly luciferase, we performed gene chimerization between Photinus pyralis luciferase and a thermostable variant of Luciola cruciata luciferase. One chimeric luciferase showed low K(m) value for substrate ATP and similar stability to thermostable L. cruciata luciferase. We then introduced random mutations in the corresponding gene and screened for increased catalytic efficiency. Amino acid replacement of Thr219, Val239 and Val290 affected the kinetic parameters. Therefore, we combined these three mutations. One mutant, ABcT219I,V239I, showed high catalytic efficiency comparable to P. pyralis luciferase and high stability similar to thermostable L. cruciata luciferase. The pH-dependence of the bioluminescence emission spectra was also examined. In contrast to wild-type firefly luciferases characterized to date, the mutant did not show the pH-dependent red spectrum shift.     

Toxin–antitoxin-stabilized reporter plasmids for biophotonic imaging of Group A streptococcus

Bioluminescence is a rapid and cost-efficient optical imaging technology that allows the detection of bacteria in real-time during disease development. Here, we report a novel strategy to generate a wide range of bioluminescent group A streptococcus (GAS) strains by using a toxin–antitoxin-stabilized plasmid. The bacterial luciferin–luciferase operon (lux) or the firefly luciferase gene (ffluc) was introduced into GAS via a stabilized plasmid. The FFluc reporter gave significantly stronger bioluminescent signals than the Lux reporter, and was generally more stable. Plasmid-based luciferase reporters could easily be introduced into a variety of GAS strains and the signals correlated linearly with viable cell counts. Co-expression of the streptococcal ω–ε–ζ toxin–antitoxin operon provided segregational stability in the absence of antibiotics for at least 17 passages in vitro and up to 7 days in a mouse infection model. In addition, genome-integrated reporter constructs were also generated by site-specific recombination, but were found to be technically more challenging. The quick and efficient generation of various M-type GAS strains expressing plasmid-based luciferase reporters with comparable and quantifiable bioluminescence signals allows for comparative analysis of different GAS strains in vitro and in vivo.     

Long-term ex vivo and in vivo monitoring of tumor progression by using dual luciferases

We propose a new concept of tumor progression monitoring using dual luciferases in living animals to reduce stress for small animals and the cost of luciferin. The secreted Cypridina luciferase (CLuc) was used as an ex vivo indicator to continuously monitor tumor progression. On the other hand, the non-secreted firefly luciferase was used as an in vivo indicator to analyze the spatial distribution of the tumor at suitable time points indicated by CLuc. Thus, the new monitoring systems that use dual luciferases are available, allowing long-term bioluminescence imaging under minimal stress for the experimental animals.