Near-Infrared Bioluminescence Imaging with a through-Bond Energy Transfer Cassette

Cy5 Dye Cassettes Exhibit Through-Bond Energy Transfer and Enable Ratiometric Fluorescence Sensing

The chemosensor literature contains many reports of fluorescence sensing using polyaromatic hydrocarbon fluorophores such as pyrene, tetraphenylethylene, or polyaryl(ethynylene), where the fluorophore is excited with ultraviolet light (<400 nm) and emits in the visible region of 400-500 nm. There is a need for general methods that convert these "turn-on" hydrocarbon fluorescent sensors into ratiometric sensing paradigms. One simple strategy is to mix the responsive hydrocarbon sensor with a second non-responsive dye that is excited by ultraviolet light but emits at a distinctly longer wavelength and thus acts as a reference signal. Five new cyanine dye cassettes were created by covalently attaching a pyrene, tetraphenylethylene, or biphenyl(ethynylene) component as the ultraviolet-absorbing energy donor directly to the pentamethine chain of a deep-red cyanine (Cy5) energy acceptor. Fluorescence emission studies showed that these Cy5-cassettes exhibited large pseudo-Stokes shifts and high through-bond energy transfer efficiencies upon excitation with ultraviolet light. Practical potential was demonstrated with two examples of ratiometric fluorescence sensing using a single ultraviolet excitation wavelength. One example mixed a Cy5-cassette with a pyrene-based fluorescent indicator that responded to changes in Cu2+ concentration, and the other example mixed a Cy5-cassette with the fluorescent pH sensing dye, pyranine.

Bioluminescent Genetically Encoded Glutamate Indicators for Molecular Imaging of Neuronal Activity

Genetically encoded optical sensors and advancements in microscopy instrumentation and techniques have revolutionized the scientific toolbox available for probing complex biological processes such as release of specific neurotransmitters. Most genetically encoded optical sensors currently used are based on fluorescence and have been highly successful tools for single-cell imaging in superficial brain regions. However, there remains a need to develop new tools for reporting neuronal activity in vivo within deeper structures without the need for hardware such as lenses or fibers to be implanted within the brain. Our approach to this problem is to replace the fluorescent elements of the existing biosensors with bioluminescent elements. This eliminates the need of external light sources to illuminate the sensor, thus allowing deeper brain regions to be imaged noninvasively. Here, we report the development of the first genetically encoded neurotransmitter indicators based on bioluminescent light emission. These probes were optimized by high-throughput screening of linker libraries. The selected probes exhibit robust changes in light output in response to the extracellular presence of the excitatory neurotransmitter glutamate. We expect this new approach to neurotransmitter indicator design to enable the engineering of specific bioluminescent probes for multiple additional neurotransmitters in the future, ultimately allowing neuroscientists to monitor activity associated with a specific neurotransmitter as it relates to behavior in a variety of neuronal and psychiatric disorders, among many other applications.

Bioluminescent imaging systems boosting near-infrared signals in mammalian cells

Bioluminescence (BL) is broadly used as an optical readout in bioassays and molecular imaging. In this study, the near-infrared (NIR) BL imaging systems were developed. The system was harnessed by prototype copepod luciferases, artificial luciferase 30 (ALuc30) and its miniaturized version picALuc, and were characterized with 17 kinds of coelenterazine (CTZ) analogues carrying bulky functional groups or cyanine 5 (Cy5). They were analyzed of BL spectral peaks and enzymatic kinetics, and explained with computational modeling. The results showed that (1) the picALuc-based system surprisingly boosts the BL intensities predominantly in the red and NIR region with its specific CTZ analogues; (2) both ALuc30- and picALuc-based systems develop unique through-bond energy transfer (TBET)-driven spectral bands in the NIR region with a Cy5-conjugated CTZ analogue (Cy5-CTZ); and (3) according to the computational modeling, the miniaturized version, picALuc, has a large binding pocket, which can accommodate CTZ analogues containing bulky functional groups and thus allowing NIR BL. This study is an important addition to the BL imaging toolbox with respect to the development of orthogonal NIR reporter systems applicable to physiological samples, together with the understanding of the BL-emitting chemistry of marine luciferases.

Near-Infrared Imaging of Steroid Hormone Activities Using Bright BRET Templates

Bioluminescence (BL) is an excellent optical readout for bioassays and molecular imaging. Herein, we accomplished new near infrared bioluminescence resonance energy transfer (NIR-BRET) templates for monitoring molecular events in cells with higher sensitivity. We first identified the best resonance energy donor for the NIR-BRET templates through the characterization of many coelenterazine (CTZ)-marine luciferase combinations. As a result, we found that NLuc-DBlueC and ALuc47-nCTZ combinations showed luminescence in the blue emission wavelength with excellent BL intensity and stability, for example, the NLuc-DBlueC and ALuc47-nCTZ combinations were 17-fold and 22-fold brighter than their second highest combinations, respectively, and were stably bright in living mammalian cells for at least 10 min. To harness the excellent BL properties to the NIR-BRET systems, NLuc and ALuc47 were genetically fused to fluorescent proteins (FPs), allowing large "blue-to-red" shifts, such as LSSmChe, LSSmKate2, and LSSmNep (where LSS means Large Stokes Shift). The excellent LSSmNep-NLuc combination showed approximately 170 nm large resonance energy shift from blue to red. The established templates were further utilized in the development of new NIR-BRET systems for imaging steroid hormone activities by sandwiching the ligand-binding domain of a nuclear receptor (NR-LBD) between the luciferase and the FP of the template. The NIR-BRET systems showed a specific luminescence signal upon exposure to steroid hormones, such as androgen, estrogen, and cortisol. The present NIR-BRET templates are important additions for utilizing their advantageous imaging of various molecular events with high efficiency and brightness in physiological samples.

Systematic Comparison of Beetle Luciferase-Luciferin Pairs as Sources of Near-Infrared Light for In Vitro and In Vivo Applications

Luciferases catalyze light-emitting reactions that produce a rainbow of colors from their substrates (luciferins), molecular oxygen, and often additional cofactors. These bioluminescence (BL) systems have afforded an incredible variety of basic research and medical applications. Driven by the importance of BL-based non-invasive animal imaging (BLI) applications, especially in support of cancer research, new BL systems have been developed by engineering beetle luciferase (Luc) variants and synthetic substrate combinations to produce red to near-infrared (nIR) light to improve imaging sensitivity and resolution. To stimulate the application of BLI research and advance the development of improved reagents for BLI, we undertook a systematic comparison of the spectroscopic and BL properties of seven beetle Lucs with LH₂ and nine substrates, which included two new quinoline ring-containing analogs. The results of these experiments with purified Luc enzymes in vitro and in live HEK293T cells transfected with luc genes have enabled us to identify Luc/analog combinations with improved properties compared to those previously reported and to provide live cell BL data that may be relevant to in vivo imaging applications. Additionally, we found strong candidate enzyme/substrate pairs for in vitro biomarker applications requiring nIR sources with minimal visible light components. Notably, one of our new substrates paired with a previously developed Luc variant was demonstrated to be an excellent in vitro source of nIR and a potentially useful BL system for improved resolution in BLI.

Visible Light Bioluminescence Imaging Platform for Animal Cell Imaging

The present protocol introduces a visible light bioluminescence imaging (BLI) platform together with 12 novel coelenterazine (CTZ) analogues and luciferase sets. We exemplify to create diverse hues of bioluminescence (BL) ranging from blue to far red with the combination of marine luciferases and the three groups of CTZ analogues. We also show how to characterize the new CTZ analogues in detail such as the kinetic parameters, dose dependency, and luciferase specificity. The 2-series CTZ analogues interestingly have specificity to artificial luciferases and are completely dark with Renilla luciferase derivatives in contrast. The 3d is highly specific to only NanoLuc. This BL imaging system covering the visible region provides a useful multicolor imaging portfolio that efficiently images molecular events in mammalian cells.

Near-Infrared Bioluminescence Imaging of Animal Cells with Through-Bond Energy Transfer Cassette

Coelenterazine (CTZ) is the most general substrate for marine luciferases. The present protocol introduces a near-infrared (NIR ) bioluminescence (BL) imaging of mammalian cells with a cyanine-5 (Cy5) dye-conjugated CTZ . This unique Cy5-conjugated CTZ, named Cy5-CTZ , can act as a dual optical readout emitting both fluorescence (FL) and BL. The Cy5-CTZ exerts through-bond energy transfer (TBET)-based imaging modalities for mammalian cells. This novel derivative, Cy5-CTZ , is intrinsically fluorescent and emits NIR-shifted BL when reacting with an appropriate luciferase , such as Renilla luciferase (RLuc). The protocol exemplifies a unique live-cell imaging with Cy5-CTZ that is optically stable in physiological samples and rapidly permeabilize through plasma membrane and emit NIR-BL in live mammalian cells.

Color-tunable bioluminescence imaging portfolio for cell imaging

The present study describes a color-tunable imaging portfolio together with twelve novel coelenterazine (CTZ) analogues. The three groups of CTZ analogues create diverse hues of bioluminescence (BL) ranging from blue to far red with marine luciferases. We found that the hue completes the whole color palette in the visible region and shows red-shifted BL with a marine luciferase: for example, Renilla luciferase 8 (RLuc8) and Artificial Luciferase 16 (ALuc16) show 187 nm- and 105 nm-redshifted spectra, respectively, by simply replacing the substrate CTZ with 1d. The optical properties of the new CTZ analogues were investigated such as the kinetic parameters, dose dependency, and luciferase specificity. The 2-series CTZ analogues interestingly have specificity to ALucs and are completely dark with RLuc derivatives, and 3d is highly specific to only NanoLuc. We further determined the theoretical background of the red-shifted BL maximum wavelengths (λ_BL) values according to the extended π conjugation of the CTZ backbone using Density Functional Theory (DFT) calculations. This color-tunable BL imaging system provides a useful multicolor imaging portfolio that efficiently images molecular events in mammalian cells.

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1',2':1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald-Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were "dressed" with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.

Coelenterazine Analogue with Human Serum Albumin-Specific Bioluminescence

A synthetic luciferin comprising an imidazopyrazinone core, named HuLumino1, was designed to generate specific bioluminescence with human serum albumin (HSA) in real serum samples. HuLumino1 was developed by attaching a methoxy-terminated alkyl chain to C-6 of coelenterazine and by eliminating a benzyl group at C-8. HSA levels were quantified within 5% error margins of an enzyme-linked immunosorbent assay without the need for any sample pretreatments because of the high specificity of HuLumino1.

Bioluminescent Imaging Systems for Assay Developments

Bioluminescence (BL) is an excellent optical readout platform that has great potential to be utilized in various bioassays and molecular imaging. The advantages of BL-based bioassays include the long dynamic range, minimal background, high signal-to-noise ratios, biocompatibility for use in cell-based assays, no need of external light source for excitation, simplicity in the measurement system, and versatility in the assay design. The recent intensive research in BL has greatly diversified the available luciferase-luciferin systems in the bioassay toolbox. However, the wide variety does not promise their successful utilization in various bioassays as new tools. This is mainly due to complexity and confusion with the diversity, and the unavailability of defined standards. This review is intended to provide an overview of recent basic developments and applications in BL studies, and showcases the bioanalytical utilities. We hope that this review can be used as an instant reference on BL and provides useful guidance for readers in narrowing down their potential options in their own assay designs.

Palette of Luciferases: Natural Biotools for New Applications in Biomedicine

Optoanalytical methods based on using genetically encoded bioluminescent enzymes, luciferases, allow one to obtain highly sensitive signals, are non-invasive, and require no external irradiation. Bioluminescence is based on the chemical reaction of oxidation of a low-molecular-weight substrate (luciferin) by atmospheric oxygen, which is catalyzed by an enzyme (luciferase). Relaxation of the luciferin oxidation product from its excited state is accompanied by a release of a quantum of light, which can be detected as an analytical signal. The ability to express luciferase genes in various heterological systems and high quantum yields of luminescence reactions have made these tools rather popular in biology and medicine. Among several naturally available luciferases, a few have been found to be useful for practical application. Luciferase size, the wavelength of its luminescence maximum, enzyme thermostability, optimal pH of the reaction, and the need for cofactors are parameters that may differ for luciferases from different groups of organisms, and this fact directly affects the choice of the application area for each enzyme. It is quite important to overview the whole range of currently available luciferases based on their biochemical properties before choosing one bioluminescent probe suitable for a specific application.

Bioluminescence Profiling of NanoKAZ/NanoLuc Luciferase Using a Chemical Library of Coelenterazine Analogues

We describe here an extensive structure-bioluminescence relationship study of a chemical library of analogues of coelenterazine, using nanoKAZ/NanoLuc, a mutated luciferase originated from the catalytic subunit of the deep-sea shrimp Oplophorus gracilirostris. Out of the 135 O-acetylated precursors that were prepared by using our recently reported synthesis and following their hydrolysis to give solutions of the corresponding luciferins, notable bioluminescence improvements were achieved in comparison with furimazine, which is currently amongst the best substrates of nanoKAZ/NanoLuc. For instance, the rather more lipophilic analogue 8-(2,3-difluorobenzyl)-2-((5-methylfuran-2-yl)methyl)-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one provided a 1.5-fold improvement of the total light output over a 2 h period, a close to threefold increase of the initial signal intensity and a signal-to-background ratio five times greater than furimazine. The kinetic parameters for the enzymatic reaction were obtained for a selection of luciferin analogues and provided unexpected insights into the luciferase activity. Most prominently, along with a general substrate-dependent and irreversible inactivation of this enzyme, in the case of the optimized luciferin mentioned above, the consumption of 2664 molecules was found to be required for the detection of a single Relative Light Unit (RLU; a luminometer-dependent fraction of a photon).