Inhibition of firefly luciferase by general anesthetics: effect on in vitro and in vivo bioluminescence imaging

Bioluminescence imaging is routinely performed in anesthetized mice. Often isoflurane anesthesia is used because of its ease of use and fast induction/recovery. However, general anesthetics have been described as important inhibitors of the luciferase enzyme reaction.

Aim

To investigate frequently used mouse anesthetics for their direct effect on the luciferase reaction, both in vitro and in vivo.

Materials and Methods

isoflurane, sevoflurane, desflurane, ketamine, xylazine, medetomidine, pentobarbital and avertin were tested in vitro on luciferase-expressing intact cells, and for non-volatile anesthetics on intact cells and cell lysates. In vivo, isoflurane was compared to unanesthetized animals and different anesthetics. Differences in maximal photon emission and time-to-peak photon emission were analyzed.

Results

All volatile anesthetics showed a clear inhibitory effect on the luciferase activity of 50% at physiological concentrations. Avertin had a stronger inhibitory effect of 80%. For ketamine and xylazine, increased photon emission was observed in intact cells, but this was not present in cell lysate assays, and was most likely due to cell toxicity and increased cell membrane permeability. In vivo, the highest signal intensities were measured in unanesthetized mice and pentobarbital anesthetized mice, followed by avertin. Isoflurane and ketamine/medetomidine anesthetized mice showed the lowest photon emission (40% of unanesthetized), with significantly longer time-to-peak than unanesthetized, pentobarbital or avertin-anesthetized mice. We conclude that, although strong inhibitory effects of anesthetics are present in vitro, their effect on in vivo BLI quantification is mainly due to their hemodynamic effects on mice and only to a lesser extent due to the direct inhibitory effect.     

Sensitive dual color in vivo bioluminescence imaging using a new red codon optimized firefly luciferase and a green click beetle luciferase

Background

Despite a plethora of bioluminescent reporter genes being cloned and used for cell assays and molecular imaging purposes, the simultaneous monitoring of multiple events in small animals is still challenging. This is partly attributable to the lack of optimization of cell reporter gene expression as well as too much spectral overlap of the color-coupled reporter genes. A new red emitting codon-optimized luciferase reporter gene mutant of Photinus pyralis, Ppy RE8, has been developed and used in combination with the green click beetle luciferase, CBG99.

Principal Findings

Human embryonic kidney cells (HEK293) were transfected with vectors that expressed red Ppy RE8 and green CBG99 luciferases. Populations of red and green emitting cells were mixed in different ratios. After addition of the shared single substrate, D-luciferin, bioluminescent (BL) signals were imaged with an ultrasensitive cooled CCD camera using a series of band pass filters (20 nm). Spectral unmixing algorithms were applied to the images where good separation of signals was observed. Furthermore, HEK293 cells that expressed the two luciferases were injected at different depth in the animals. Spectrally-separate images and quantification of the dual BL signals in a mixed population of cells was achieved when cells were either injected subcutaneously or directly into the prostate.

Significance

We report here the re-engineering of different luciferase genes for in vitro and in vivo dual color imaging applications to address the technical issues of using dual luciferases for imaging. In respect to previously used dual assays, our study demonstrated enhanced sensitivity combined with spatially separate BL spectral emissions using a suitable spectral unmixing algorithm. This new D-luciferin-dependent reporter gene couplet opens up the possibility in the future for more accurate quantitative gene expression studies in vivo by simultaneously monitoring two events in real time.     

The expression level of luciferase within tumour cells can alter tumour growth upon in vivo bioluminescence imaging.

In vivo bioluminescence imaging is becoming a very important tool for the study of a variety of cellular and molecular events or disease processes in living systems. In vivo bioluminescence imaging is based on the detection of light emitted from within an animal. The light is generated as a product of the luciferase-luciferin reaction taking place in a cell. In this study, we implanted mice with tumour cells expressing either a high or a low level of luciferase. In vivo bioluminescence imaging was used to follow tumour progression. Repeated luciferin injection and imaging of high and low luciferase-expressing tumours was performed. While low luciferase-expressing tumours grew similarly to vector controls, growth of the high luciferase-expressing tumours was severely inhibited. The observation that a high level of luciferase expression will inhibit tumour cell growth when an animal is subjected to serial in vivo bioluminescence imaging is potentially an important factor in designing these types of studies.     

PET imaging and optical imaging with D-luciferin [11C]methyl ester and D-luciferin [11C]methyl ether of luciferase gene expression in tumor xenografts of living mice

New carbon-11 labeled D-luciferin analogs D-luciferin [(11)C]methyl ester ([(11)C]LMEster, [(11)C]1) and D-luciferin [(11)C]methyl ether ([(11)C]LMEther, [(11)C]2) were synthesized in 25-55% radiochemical yield. PET studies with [(11)C]LMEster and [(11)C]LMEther demonstrate a lower retention of the C-11 label at 45 min post-injection in luciferase expression tumor. Optical imaging with unlabeled substrate D-luciferin and radiotracers [(11)C]LMEster and [(11)C]LMEther gave tumor luciferase images within a few minutes of photon counting.     

Quantitative bioluminescence imaging of transgene expression in vivo

Bioluminescent imaging (BLI) is a widely used in vivo method to determine the location and relative intensity of luciferase expression in mice. Luciferase expression is observed following an i.p. dose of d-luciferin, resulting in bioluminescence that is detected in anesthetized mice by a charge-coupled device camera. To establish whether BLI could be used as a quantitative measurement of non-viral-mediated luciferase expression, precise quantities of plasmid DNA encoding the luciferase gene were hydrodynamically dosed in mice. The results established a linear correlation between the DNA dose and the BLI response measured in liver which spanned five orders of magnitude. The level of luciferase expression was found to be a direct function of d-luciferin dose. The time course of luciferase expression and the influence of multidosing of substrate were measured by BLI. The recovery of luciferase from the liver of hydrodynamically dosed mice allowed calibration of the BLI measurements. The results establish BLI's limit-of-detection at 20 pg of luciferase per liver following a hydrodynamic dose of 100 pg of plasmid DNA. These results demonstrate that BLI is both sensitive and linear and should allow for the direct comparison of the efficiency of gene transfer vectors that target the liver.     

Evaluation of effector cell fate and function by in vivo bioluminescence imaging

The effector functions of immune cells have typically been examined using assays that require sampling of tissues or cells to reveal specific aspects of an immune response (e.g., antigen-specificity, cytokine expression or killing of target cells). The outcome of an immune response in vivo, however, is not solely determined by a single effector function of a specific cell population, but is the result of numerous cellular and molecular interactions that occur in the complex environment of intact organ systems. These interactions influence survival, migration, and activation, as well as final effector function of a given population of cells. Efforts to reveal the cellular and molecular basis of biological processes have resulted in a number of technologies that combine molecular biology and imaging sciences that are collectively termed as Molecular Imaging. This emerging field has developed to reveal functional aspects of cells, genes, and proteins in real time in living animals and humans and embraces multiple modalities, including established clinical imaging methods such as magnetic resonance imaging, single photon emission computed tomography, and positron emission tomography, as well as novel methodologies specifically designed for research animals. Here, we highlight one of the newer modalities, in vivo bioluminescence imaging, as a method for evaluating effector T cell proliferation, migration, and function in model systems of malignant and non-malignant diseases.     

Real-time bioluminescence imaging of a protein secretory pathway in living mammalian cells using Gaussia luciferase

Using photon counting and charge-coupled device (CCD) cameras, we have applied the method of real-time bioluminescence imaging to investigate protein trafficking in mammalian cells. In the living cells of Chinese hamster ovary and PC12D cells, exocytotic secretion of protein and protein targeting on the cell surface were visualized using the secreted Gaussia luciferase (GLase) as a reporter protein in a minute. After incubation of the cells with luciferin (coelenterazine) for 10min, luciferin was imported into the cells and the vesicle transport network in the cells could be shown by luminescence images of GLase activity. Further, we demonstrate that GLase with a heterologous signal peptide sequence is targeted to the cell surface in neuronally differentiated PC12D cells and luminescence signals could be detected in a few seconds.     

A quantitative study of factors affecting in vivo bioluminescence imaging

In vivo bioluminescence imaging (BLI) has the advantages of high sensitivity and low background. By counting the number of photons emitted from a specimen, BLI can quantify biological events such as tumour growth, gene expression and drug response. The intensities and kinetics of the BL signal are affected by many factors and may confound the quantitative results acquired from consecutive imaging sessions or different specimens. We used three different mouse models of tumours to examine whether anaesthetics, positioning and tumour growth may affect the consistency of the BL signal. The results showed that BLI signal could be affected by different anaesthetics and repetitive positioning. Using the same anaesthetics produced consistent peak times, while other factors were held constant. However, as the tumours grew the peak times shifted and the time course of BL signals had different shapes, depending on the positioning of the mice. The data indicate that a carefully designed BLI experiment is required to generate optimal and consistent results. Copyright © 2008 John Wiley & Sons, Ltd.     

In vivo bioluminescence imaging of bone marrow-derived cells in brain inflammation

It has been accepted that bone marrow cells infiltrate the brain and play important roles in neuroinflammation. However, there is no good tool for the visualization of these cells in living animals. In this study, we generated mice that were transplanted with GFP- or luciferase-expressing bone marrow cells, and performed in vivo fluorescence imaging (FLI) and in vivo bioluminescence imaging (BLI) to visualize the infiltrated cells. Brain inflammation was induced by intrahippocampal injection of lipopolysaccharide (LPS). Immunohistochemical investigation demonstrated an increase in the infiltration of bone marrow cells into the hippocampus because of the LPS injection and differentiation of the infiltrated cells into microglia, but not into neurons or astrocytes. BLI, but not FLI, successfully detected an increase in signal intensity with the LPS injection, and the increase of BLI coincided with that of luciferase activity in hippocampus. BLI could quantitatively and continuously monitor bone marrow-derived cells in vivo.     

Increase in bioluminescence intensity of firefly luciferase using genetic modification

Firefly luciferase is widely used for enzymatic measurement of ATP, and its gene is used as a reporter for gene expression experiments. From our mutant library, we selected novel mutations in Photinus pyralis luciferase with higher luminescence intensity. These included mutations at Ile423, Asp436, and Leu530. Luciferase is structurally composed of a large N-terminal active site domain (residues 1-436), a flexible linker (residues 436-440) peptide, and a small C-terminal domain (residues 440-550) facing the N domain. Thus, the mutations are located at the junction of the N-terminal domain and the flexible linker, in the flexible linker peptide, and in the tip of the C-terminal domain, respectively. Substitution of Asp436 with a nonbulky amino acid such as Gly remarkably increased the substrate affinity for ATP and d-luciferin. Substitution of Ile423 with a hydrophobic amino acid such as Leu and that of Leu530 with a positively charged amino acid such as Arg increased the substrate affinity and the turnover rate. Combining these mutations, we obtained luciferases that generate more than 10-fold higher luminescence intensity than the wild-type enzyme.     

In vivo imaging of retinoic acid receptor activity using a sodium/iodide symporter and luciferase dual imaging reporter gene

Retinoic acids are natural derivatives of vitamin A, and play important roles in modulating tumor cell growth by regulating differentiation, thus suggesting the potential use of these derivatives in cancer therapy and prevention. To visualize the intranuclear responses of functional retinoic acid receptors, we have developed a dual-imaging reporter gene system based on the use of sodium/iodide symporter (NIS) and luciferase in cancer cell lines. NIS and luciferase genes were linked with an internal ribosome entry site, and placed under the control of an artificial cis-acting retinoic acid responsive element (pRARE/NL). After retinoic acid treatment, I-125 uptake by pRARE/NL transfected cells was found to have increased by up to about five times that of nontreated cells. The bioluminescence intensity of pRARE/NL transfected cells showed dose-dependency. In vivo luciferase images showed higher intensity in retinoic acid treated SK-RARE/NL tumors, and scintigraphic images of SK-RARE/NL tumors showed increased Tc-99m uptake after retinoic acid treatment. The NIS/luciferase imaging reporter system was sufficiently sensitive to allow the visualization of intranuclear retinoic acid receptor activity. This cis-enhancer imaging reporter system may be useful in vitro and in vivo for the evaluation of retinoic acid responses in such areas as cellular differentiation and chemoprevention.     

An alternative mechanism of bioluminescence color determination in firefly luciferase

Beetle luciferases (including those of the firefly) use the same luciferin substrate to naturally display light ranging in color from green (lambda(max) approximately 530 nm) to red (lambda(max) approximately 635 nm). In a recent communication, we reported (Branchini, B. R., Murtiashaw, M. H., Magyar, R. A., Portier, N. C., Ruggiero, M. C., and Stroh, J. G. (2002) J. Am. Chem. Soc. 124, 2112-2113) that the synthetic adenylate of firefly luciferin analogue D-5,5-dimethylluciferin was transformed into the emitter 5,5-dimethyloxyluciferin in bioluminescence reactions catalyzed by luciferases from Photinus pyralis and the click beetle Pyrophorus plagiophthalamus. 5,5-Dimethyloxyluciferin is constrained to exist in the keto form and fluoresces mainly in the red. However, bioluminescence spectra revealed that green light emission was produced by the firefly enzyme, and red light was observed with the click beetle protein. These results, augmented with steady-state kinetic studies, were taken as experimental support for mechanisms of firefly bioluminescence color that require only a single keto form of oxyluciferin. We report here the results of mutagenesis studies designed to determine the basis of the observed differences in bioluminescence color with the analogue adenylate. Mutants of P. pyralis luciferase putative active site residues Gly246 and Phe250, as well as corresponding click beetle residues Ala243 and Ser247 were constructed and characterized using bioluminescence emission spectroscopy and steady state kinetics with adenylate substrates. Based on an analysis of these and recently reported (Branchini, B. R., Southworth, T. L., Murtiashaw, M. H., Boije, H., and Fleet, S. E. (2003) Biochemistry 42, 10429-10436) data, we have developed an alternative mechanism of bioluminescence color. The basis of the mechanism is that luciferase modulates emission color by controlling the resonance-based charge delocalization of the anionic keto form of the oxyluciferin excited state.     

In vivo bioluminescence imaging of Ca signalling in the brain of Drosophila

Many different cells' signalling pathways are universally regulated by Ca(2+) concentration [Ca(2+)] rises that have highly variable amplitudes and kinetic properties. Optical imaging can provide the means to characterise both the temporal and spatial aspects of Ca(2+) signals involved in neurophysiological functions. New methods for in vivo imaging of Ca(2+) signalling in the brain of Drosophila are required for probing the different dynamic aspects of this system. In studies here, whole brain Ca(2+) imaging was performed on transgenic flies with targeted expression of the bioluminescent Ca(2+) reporter GFP-aequorin (GA) in different neural structures. A photon counting based technique was used to undertake continuous recordings of cytosolic [Ca(2+)] over hours. Time integrals for reconstructing images and analysis of the data were selected offline according to the signal intensity. This approach allowed a unique Ca(2+) response associated with cholinergic transmission to be identified by whole brain imaging of specific neural structures. Notably, [Ca(2+)] transients in the Mushroom Bodies (MBs) following nicotine stimulation were accompanied by a delayed secondary [Ca(2+)] rise (up to 15 min. later) in the MB lobes. The delayed response was sensitive to thapsigargin, suggesting a role for intra-cellular Ca(2+) stores. Moreover, it was reduced in dunce mutant flies, which are impaired in learning and memory. Bioluminescence imaging is therefore useful for studying Ca(2+) signalling pathways and for functional mapping of neurophysiological processes in the fly brain.     

Addition of glucose enables determination of luciferase activity in carbon-starved, stationary phase Lactococcus lactis cells

We describe a simple method for measuring luciferase activity in the stationary phase of Lactococcus lactis. Due to large fluctuations in the energy and redox pools of stationary phase bacterial cells, measurement of luciferase activities does not yield reliable results. Upon addition of relatively small amounts of glucose, the pools are restored and measurement of luciferase becomes possible. Since luciferase activities are easily measured, our method allows to apply this simple analytical tool in stationary phase cells.     

In vivo imaging of ligand receptor binding with Gaussia luciferase complementation

Studies of ligand-receptor binding and the development of receptor antagonists would benefit greatly from imaging techniques that translate directly from cell-based assays to living animals. We used Gaussia luciferase protein fragment complementation to quantify the binding of chemokine (C-X-C motif) ligand 12 (CXCL12) to chemokine (C-X-C motif) receptor 4 (CXCR4) and CXCR7. Studies established that small-molecule inhibitors of CXCR4 or CXCR7 specifically blocked CXCL12 binding in cell-based assays and revealed differences in kinetics of inhibiting chemokine binding to each receptor. Bioluminescence imaging showed CXCL12-CXCR7 binding in primary and metastatic tumors in a mouse model of breast cancer. We used this imaging technique to quantify drug-mediated inhibition of CXCL12-CXCR4 binding in living mice. We expect this imaging technology to advance research in areas such as ligand-receptor interactions and the development of new therapeutic agents in cell-based assays and small animals.     

Real-time in vivo bioluminescence imaging of lentiviral vector-mediated gene transfer in mouse testis

Although much research has focused on transferring exogenous genes into living mouse testis to investigate specific gene functions in spermatogenic, Sertoli, and Leydig cells, relatively little is known regarding real-time gene expression in vivo. In this study, we constructed a bicistronic lentiviral vector (LV) encoding firefly luciferase and enhanced green fluorescence protein (EGFP); this was a highly efficient in vivo gene transfer tool. After microinjecting LV into the seminiferous tubules the ICR mouse testis, we detected luciferase and EGFP expression in vivo and ex vivo in the injected tubules using bioluminescence imaging (BLI) with the IVIS-200 system and fibered confocal fluorescence microscopy (CellViZio), respectively. In addition, with an in vivo BLI system, luciferase expression in the testis was detected for ∼3 mo. Furthermore, EGFP expression in seminiferous tubules was confirmed in excised testes via three-dimensional fluorescent imaging with a confocal laser-scanning microscope. With immunostaining, EGFP expression was confirmed in several male germ cell types in the seminiferous tubules, as well as in Sertoli and Leydig cells. In conclusion, we demonstrated that real-time in vivo BLI analysis can be used to noninvasively (in vivo) monitor long-term luciferase expression in mouse testis, and we verified that EGFP expression is localized in seminiferous tubules after bicistronic LV-mediated gene transfer into mouse testes. Furthermore, we anticipate the future use of in vivo BLI technology for real-time study of specific genes involved in spermatogenesis.