Bruce R. Branchini

Red-emitting luciferases for bioluminescence reporter and imaging applications

North American firefly Photinus pyralis luciferase, which emits yellow-green light (557nm), 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 Promegas 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.

A portable bioluminescence engineered cell-based biosensor for on-site applications

We have developed a portable biosensing device based on genetically engineered bioluminescent (BL) cells. Cells were immobilized on a 4 × 3 multiwell cartridge using a new biocompatible matrix that preserved their vitality. Using a fiber optic taper, the cartridge was placed in direct contact with a cooled CCD sensor to image and quantify the BL signals. Yeast and bacterial cells were engineered to express recognition elements, whose interaction with the analyte led to luciferase expression, via reporter gene technology. Three different biosensors were developed. The first detects androgenic compounds using yeast cells carrying a green-emitting P. pyralis luciferase regulated by the human androgen receptor and a red mutant of the same species as internal vitality control. The second biosensor detects two classes of compounds (androgens and estrogens) using yeast strains engineered to express green-or red-emitting mutant firefly luciferases in response to androgens or estrogens, respectively. The third biosensor detects lactose analogue isopropyl β-d-1-thiogalactopyranoside using two E. coli strains. One strain exploits the lac operon as recognition element for the expression of P. pyralis luciferase. The other strain serves as a vitality control expressing Gaussia princeps luciferase, which requires a different luciferin substrate. The immobilized cells were stable for up to 1 month. The analytes could be detected at nanomolar levels with good precision and accuracy when the specific signal was corrected using the internal vitality control. This portable device can be used for on-site multiplexed bioassays for different compound classes.

Mutation of a Protease-sensitive Region in Firefly Luciferase Alters Light Emission Properties

Luciferase (EC 1.13.12.7) from the North American firefly, Photinus pyralis, is widely used as a reporter enzyme in cell biology. One of its distinctive properties is a pronounced susceptibility to proteolytic degradation that causes luciferase to have a very short intracellular half-life. To define the structural basis for this behavior and possibly facilitate the design of more stable forms of luciferase, limited proteolysis studies were undertaken using trypsin and chymotrypsin to identify regions of the protein whose accessible and flexible character rendered them especially sensitive to cleavage. Regions of amino acid sequence 206–220 and 329–341 were found to be sensitive, and because the region around 206–220 had high homology with other luciferases, CoA ligases, and peptidyl synthetases, this region was selected for mutagenesis experiments intended to determine which of its amino acids were essential for activity. Surprisingly, many highly conserved residues including Ser198, Ser201, Thr202, and Gly203 could be mutated with little effect on the luminescent activity of P. pyralisluciferase. One mutation, however, S198T, caused several alterations in enzymatic properties including shifting the pH optimum from 8.1 to 8.7, lowering the K m for Mg-ATP by a factor of 4 and increasing the half-time for light emission decay by a factor of up to 150. While the S198T luciferase was less active than wild type, activity could be restored by the introduction of the additional L194F and N197Y mutations. In addition to indicating the involvement of this region in ATP binding, these results provide a new form of the enzyme that affords a more versatile reporter system.

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.

In Vivo Bioluminescence Imaging for the Study of Intestinal Colonization by Escherichia coli in Mice

ABSTRACT Bioluminescence imaging (BLI) is emerging as a powerful tool for real-time monitoring of infections in living animals. However, since luciferases are oxygenases, it has been suggested that the requirement for oxygen may limit the use of BLI in anaerobic environments, such as the lumen of the gut. Strains of Escherichia coli harboring the genes for either the bacterial luciferase from Photorhabdus luminescens or the PpyRE-TS and PpyGR-TS firefly luciferase mutants of Photinus pyralis (red and green thermostable P. pyralis luciferase mutants, respectively) have been engineered and used to monitor intestinal colonization in the streptomycin-treated mouse model. There was excellent correlation between the bioluminescence signal measured in the feces (R2 = 0.98) or transcutaneously in the abdominal region of whole animals (R2 = 0.99) and the CFU counts in the feces of bacteria harboring the luxABCDE operon. Stability in vivo of the bioluminescence signal was achieved by constructing plasmid pAT881(pGB2ΩPamiluxABCDE), which allowed long-term monitoring of intestinal colonization without the need for antibiotic selection for plasmid maintenance. Levels of intestinal colonization by various strains of E. coli could be compared directly by simple recording of the bioluminescence signal in living animals. The difference in spectra of light emission of the PpyRE-TS and PpyGR-TS firefly luciferase mutants and dual bioluminescence detection allowed direct in vitro and in vivo quantification of two bacterial populations by measurement of red and green emitted signals and thus monitoring of the two populations simultaneously. This system offers a simple and direct method to study in vitro and in vivo competition between mutants and the parental strain. BLI is a useful tool to study intestinal colonization.

Designing quantum rods for optimized energy transfer with firefly luciferase enzymes

The bioluminescence resonance energy transfer (BRET) between firefly luciferase from Photinus pyralis (Ppy) with core/shell semiconductive quantum rods (QRs) has been studied as a function of QR aspect ratio and internal microstructure. The QRs were found to be ideal energy acceptors, and Ppy-to-core distances were optimized using rod-in-rod microstructures that were achieved by the synthetic control of rod morphology, surface chemistry, and Ppy:QR loading. The BRET ratios of >44 measured are the highest efficiencies to date.

Sequential bioluminescence resonance energy transfer–fluorescence resonance energy transfer-based ratiometric protease assays with fusion proteins of firefly luciferase and red fluorescent protein

We report here the preparation of ratiometric luminescent probes that contain two well-separated emission peaks produced by a sequential bioluminescence resonance energy transfer (BRET)-fluorescence resonance energy transfer (FRET) process. The probes are single soluble fusion proteins consisting of a thermostable firefly luciferase variant that catalyze yellow-green (560nm maximum) bioluminescence and a red fluorescent protein covalently labeled with a near-infrared fluorescent dye. The two proteins are connected by a decapeptide containing a protease recognition site specific for factor Xa, thrombin, or caspase 3. The rates of protease cleavage of the fusion protein substrates were monitored by recording emission spectra and plotting the change in peak ratios over time. Detection limits of 0.41nM for caspase 3, 1.0nM for thrombin, and 58nM for factor Xa were realized with a scanning fluorometer. Our results demonstrate for the first time that an efficient sequential BRET-FRET energy transfer process based on firefly luciferase bioluminescence can be employed to assay physiologically important protease activities.

Novel multistep BRET-FRET energy transfer using nanoconjugates of firefly proteins, quantum dots, and red fluorescent proteins

Sequential bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) from firefly luciferase to red fluorescent proteins using quantum dot or rod acceptor/donor linkers is described. The effect of morphology and tuned optical properties on the efficiency of this unique BRET-FRET system was evaluated.

Novel Heterocyclic Analogues of Firefly Luciferin

Five novel firefly luciferin analogues in which the benzothiazole ring system of the natural substrate was replaced with benzimidazole, benzofuran, benzothiophene, benzoxazole, and indole were synthesized. The fluorescence, bioluminescence, and kinetic properties of the compounds were evaluated with recombinant Photinus pyralis wild type luciferase. With the exception of indole, all of the substrates containing heterocycle substitutions produced readily measurable flashes of light with luciferase. Compared to that of luciferin, the intensities ranged from 0.3 to 4.4% in reactions with varying pH optima and times to reach maximal intensity. The heteroatom changes influenced both the fluorescence and bioluminescence emission spectra, which displayed maxima of 479-528 and 518-574 nm, respectively. While there were some interesting trends in the spectroscopic and bioluminescence properties of this group of structurally similar substrate analogues, the most significant findings were associated with the benzothiophene-containing compound. This synthetic substrate produced slow decay glow kinetics that increased the total light-based specific activity of luciferase more than 4-fold versus the luciferin value. Moreover, over the pH range of 6.2-9.4, the emission maximum is 523 nm, an unusual 37 nm blue shift compared to that of the natural substrate. The extraordinary bioluminescence properties of the benzothiophene luciferin should translate into greater sensitivity for analyte detection in a wide variety of luciferase-based applications.

Bioluminescence is produced from a trapped firefly luciferase conformation predicted by the domain alternation mechanism.

According to the domain alternation mechanism and crystal structure evidence, the acyl-CoA synthetases, one of three subgroups of a superfamily of adenylating enzymes, catalyze adenylate- and thioester-forming half-reactions in two different conformations. The enzymes accomplish this by presenting two active sites through an ~140° rotation of the C-domain. The second half-reaction catalyzed by another subgroup, the beetle luciferases, is a mechanistically dissimilar oxidative process that produces bioluminescence. We have demonstrated that a firefly luciferase variant containing cysteine residues at positions 108 and 447 can be intramolecularly cross-linked by 1,2-bis(maleimido)ethane, trapping the enzyme in a C-domain-rotated conformation previously undocumented in the available luciferase crystal structures. The cross-linked luciferase cannot adenylate luciferin but is nearly fully capable of bioluminescence with synthetic luciferyl adenylate because it retains the ability to carry out the oxidative half-reaction. The cross-linked luciferase is apparently trapped in a conformation similar to those adopted by acyl-CoA synthetases as they convert acyl adenylates into the corresponding CoA thioesters.