Emerging tools for bioluminescence imaging

Optimized in vivo multispectral bioluminescent imaging of tumor biology using engineered BRET reporters

The ability to visualize and track multiple biological processes in vivo in real time is highly desirable. Bioluminescence imaging (BLI) has emerged as an attractive modality for non-invasive cell tracking, with various luciferase reporters enabling parallel monitoring of several processes. However, simultaneous multiplexed imaging in vivo is challenging due to suboptimal reporter intensities and the need to image one luciferase at a time. We report a multiplexed BLI approach using a single substrate that leverages bioluminescence resonance energy transfer (BRET)-based reporters with distinct spectral profiles for triple-color BLI. These luciferase-fluorophore fusion reporters address light transmission challenges and use optimized coelenterazine substrates. Comparing BRET reporters across two substrate analogs identified a green-yellow-orange combination that allows simultaneous imaging of three distinct cell populations in vitro and in vivo. These tools provide a template for imaging other biological processes in vivo during a single BLI session using a single reporter substrate.

Research Note: Evaluating the vertical transmission potential of Salmonella Reading in broiler breeders

Salmonella Reading (S. Reading) recently emerged as a foodborne pathogen causing extensive human outbreaks in North America from consuming contaminated poultry products, mostly from turkeys. Understanding the transmission dynamics of this pathogen is crucial for preventing future outbreaks. This study investigated the ability of S. Reading to colonize the tissues and contaminate eggs of broiler breeders. We utilized 2 S. Reading strains, marked with bioluminescence gene: the outbreak strain RS330 and a reference strain RS326. We used 32 commercially sourced broiler breeder hens, 34 wk of age, randomly assigned to the 2 treatments (16 hens per strain). Each hen was intravaginally inoculated with 108 CFU of the respective strain on d 1 and was rechallenged on d 4. Eggs were collected daily postchallenge to recover bioluminescent S. Reading strains from the external eggshell surface and internal egg contents. On d 7 postchallenge, 10 hens from each treatment group were euthanized. Ovaries, oviducts, and ceca were aseptically collected to detect S. Reading colonization. Results showed that 70.5% (36 of 51) and 34.5% (19 of 55) of external eggshell surfaces, and 4.0% (2 of 50) and 1.8% (1 of 54) of the internal egg contents tested positive for the outbreak and nonoutbreak strains. Additionally, 40.0% of ovaries, 70.0% of oviduct, and 70.0% of ceca samples from the outbreak strain group, and 20.0% of ovaries, 70.0% of oviduct, and 80.0% of ceca samples from nonoutbreak strain group were positive. No significant difference (P = 0.05) was observed in all the findings among the strains except for the eggshell surface contamination. These findings suggest that S. Reading can effectively colonize reproductive tissues, translocate to the ceca, and contaminate the eggs of hens. Future research is needed to determine whether S. Reading can remain viable within the eggs throughout incubation and until hatching.

KPC-luciferase-expressing cells elicit an anti-tumor immune response in a mouse model of pancreatic cancer

Mouse models for the study of pancreatic ductal adenocarcinoma (PDAC) are well-established and representative of many key features observed in human PDAC. To monitor tumor growth, cancer cells that are implanted in mice are often transfected with reporter genes, such as firefly luciferase (Luc), enabling in vivo optical imaging over time. Since Luc can induce an immune response, we aimed to evaluate whether the expression of Luc could affect the growth of KPC tumors in mice by inducing immunogenicity. Although both cell lines, KPC and Luc transduced KPC (KPC-Luc), had the same proliferation rate, KPC-Luc tumors had significantly smaller sizes or were absent 13 days after orthotopic cell implantation, compared to KPC tumors. This coincided with the loss of bioluminescence signal over the tumor region. Immunophenotyping of blood and spleen from KPC-Luc tumor-bearing mice showed a decreased number of macrophages and CD4+ T cells, and an increased accumulation of natural killer (NK) cells in comparison to KPC tumor mice. Higher infiltration of CD8+ T cells was found in KPC-Luc tumors than in their controls. Moreover, the immune response against Luc peptide was stronger in splenocytes from mice implanted with KPC-Luc cells compared to those isolated from KPC wild-type mice, indicating increased immunogenicity elicited by the presence of Luc in the PDAC tumor cells. These results must be considered when evaluating the efficacy of anti-cancer therapies including immunotherapies in immunocompetent PDAC or other cancer mouse models that use Luc as a reporter for bioluminescence imaging.

Imaging bioluminescence by detecting localized haemodynamic contrast from photosensitized vasculature

Bioluminescent probes are widely used to monitor biomedically relevant processes and cellular targets in living animals. However, the absorption and scattering of visible light by tissue drastically limit the depth and resolution of the detection of luminescence. Here we show that bioluminescent sources can be detected with magnetic resonance imaging by leveraging the light-mediated activation of vascular cells expressing a photosensitive bacterial enzyme that causes the conversion of bioluminescent emission into local changes in haemodynamic contrast. In the brains of rats with photosensitized vasculature, we used magnetic resonance imaging to volumetrically map bioluminescent xenografts and cell populations virally transduced to express luciferase. Detecting bioluminescence-induced haemodynamic signals from photosensitized vasculature will extend the applications of bioluminescent probes.

Development of an integrated dual-modality 3D bioluminescence tomography and ultrasound imaging system for small animal tumor imaging

Ultrasound (US) is a valuable tool for imaging soft tissue and visualizing tumor contours. Taking the benefits of US, we presented an integrated dual-modality imaging system in this paper that achieves three-dimensional (3D) bioluminescence tomography (BLT) with multi-view bioluminescence images and 3D US imaging. The purpose of this system is to perform non-invasive, long-term monitoring of tumor growth in 3D images. US images can enhance the accuracy of the 3D BLT reconstruction and the bioluminescence dose within an object. Furthermore, an integrated co-registered scanning geometry was used to capture the fused BLT and US images. We validated the system with an in vivo experiment involving tumor-bearing mice. The results demonstrated the feasibility of reconstructing 3D BLT images in the tumor region using 3D US images. We used the dice coefficient and locational error to evaluate the similarity between the reconstructed source region and the actual source region. The dice coefficient was 88.5%, and the locational error was 0.4 mm when comparing the BLT and 3D US images. The hybrid BLT/US system could provide significant benefits for reconstructing the source of tumor location and conducting quantitative analysis of tumor size.

Molecularly generated light and its biomedical applications

Molecularly generated light, referred to here as "molecular light", mainly includes bioluminescence, chemiluminescence, and Cerenkov luminescence. Molecular light possesses unique dual features of being both a molecule and a source of light. Its molecular nature enables it to be delivered as molecules to regions deep within the body, overcoming the limitations of natural sunlight and physically generated light sources like lasers and LEDs. Simultaneously, its light properties make it valuable for applications such as imaging, photodynamic therapy, photo-oxidative therapy, and photobiomodulation. In this review article, we provide an updated overview of the diverse applications of molecular light and discuss the strengths and weaknesses of molecular light across various domains. Lastly, we present forward-looking perspectives on the potential of molecular light in the realms of molecular imaging, photobiological mechanisms, therapeutic applications, and photobiomodulation. While some of these perspectives may be considered bold and contentious, our intent is to inspire further innovations in the field of molecular light applications.

A Comprehensive Exploration of Bioluminescence Systems, Mechanisms, and Advanced Assays for Versatile Applications

Bioluminescence has been a fascinating natural phenomenon of light emission from living creatures. It happens when the enzyme luciferase facilitates the oxidation of luciferin, resulting in the creation of an excited-state species that emits light. Although there are many bioluminescent systems, few have been identified. D-luciferin-dependent systems, coelenterazine-dependent systems, Cypridina luciferin-based systems, tetrapyrrole-based luciferins, bacterial bioluminescent systems, and fungal bioluminescent systems are natural bioluminescent systems. Since different bioluminescence systems, such as various combinations of luciferin-luciferase pair reactions, have different light emission wavelengths, they benefit industrial applications such as drug discovery, protein-protein interactions, in vivo imaging in small animals, and controlling neurons. Due to the expression of luciferase and easy permeation of luciferin into most cells and tissues, bioluminescence assays are applied nowadays with modern technologies in most cell and tissue types. It is a versatile technique in a variety of biomedical research. Furthermore, there are some investigated blue-sky research projects, such as bioluminescent plants and lamps. This review article is mainly based on the theory of diverse bioluminescence systems and their past, present, and future applications.

Shedding light on ultrasound in action: Optical and optoacoustic monitoring of ultrasound brain interventions

Monitoring brain responses to ultrasonic interventions is becoming an important pillar of a growing number of applications employing acoustic waves to actuate and cure the brain. Optical interrogation of living tissues provides a unique means for retrieving functional and molecular information related to brain activity and disease-specific biomarkers. The hybrid optoacoustic imaging methods have further enabled deep-tissue imaging with optical contrast at high spatial and temporal resolution. The marriage between light and sound thus brings together the highly complementary advantages of both modalities toward high precision interrogation, stimulation, and therapy of the brain with strong impact in the fields of ultrasound neuromodulation, gene and drug delivery, or noninvasive treatments of neurological and neurodegenerative disorders. In this review, we elaborate on current advances in optical and optoacoustic monitoring of ultrasound interventions. We describe the main principles and mechanisms underlying each method before diving into the corresponding biomedical applications. We identify areas of improvement as well as promising approaches with clinical translation potential.

Recent Advances in the Development of Non-Invasive Imaging Probes for Cancer Immunotherapy

The last two decades have witnessed a major revolution in the field of tumor immunology including clinical progress using various immunotherapy strategies. These advances have highlighted the potential for approaches that harness the power of the immune system to fight against cancer. While cancer immunotherapies have shown significant clinical successes, patient responses vary widely due to the complex and heterogeneous nature of tumors and immune responses, calling for reliable biomarkers and therapeutic strategies to maximize the benefits of immunotherapy. Especially, stratifying responding individuals from non-responders during the early stages of treatment could help avoid long-term damage and tailor personalized treatments. In efforts to develop non-invasive means for accurately evaluating and predicting tumor response to immunotherapy, multiple affinity-based agents targeting immune cell markers and checkpoint molecules have been developed and advanced to clinical trials. In addition, researchers have recently turned their attention to substrate and activity-based imaging probes that can provide real-time, functional assessment of immune response to treatment. Here, we highlight some of those recently designed probes that image functional proteases as biomarkers of cancer immunotherapy with a focus on their chemical design and detection modalities and discuss challenges and opportunities for the development of imaging tools utilized in cancer immunotherapy.

Caged luciferins enable rapid multicomponent bioluminescence imaging

Bioluminescence is a sensitive technique for imaging biological features over time. Historically, though, the modality has been challenging to employ for multiplexed tracking due to a lack of resolvable luciferase-luciferin pairs. Recent years have seen the development of numerous orthogonal probes for multi-parameter imaging. While successful, generating such tools often requires complex syntheses and lengthy enzyme evolution campaigns. This work showcases an alternative strategy for multiplexed bioluminescence that takes advantage of already-orthogonal caged luciferins and established uncaging enzymes. These probes generate unique bioluminescent signals that can be distinguished via a linear unmixing algorithm. Caged luciferins enabled two- and three-component imaging on the minutes time scale. We further showed that the tools can be used in conjunction with endogenous enzymes for multiplexed studies. Collectively, this approach lowers the barrier to multicomponent bioluminescence imaging and can be readily adopted by the broader community.

Glow-type conversion and characterization of a minimal luciferase via mutational analyses

Luciferases are widely used as reporter proteins in various fields. Recently, we developed a minimal bright luciferase, picALuc, via partial deletion of the artificial luciferase (ALuc) derived from copepods luciferases. However, the structures of copepod luciferases in the substrate-bound state remain unknown. Moreover, as suggested by structural modeling, picALuc has a larger active site cavity, unlike that in other copepod luciferases. Here, to explore the bioluminescence mechanism of picALuc and its luminescence properties, we conducted multiple mutational analyses, and identified residues and regions important for catalysis and bioluminescence. Mutations of residues likely involved in catalysis (S33, H34, and D55) markedly reduced bioluminescence, whereas that of residue (E50) (near the substrate in the structural model) enhanced luminescence intensity. Furthermore, deletion mutants (Δ70-Δ78) in the loop region (around I73) exhibited longer luminescence lifetimes (~ 30 min) and were reactivated multiple times upon re-addition of the substrate. Due to the high thermostability of picALuc, one of its representative mutant (Δ74), was able to be reused, that is, luminescence recycling, for day-scale time at room temperature. These findings provide important insights into picALuc bioluminescence mechanism and copepod luciferases and may help with sustained observations in a variety of applications.

Optimizing a high-sensitivity NanoLuc-based bioluminescence system for in vivo evaluation of antimicrobial treatment

Focal and systemic infections are serious threats to human health. Preclinical models enable the development of new drugs and therapeutic regimens. In vivo, animal bioluminescence (BL) imaging has been used with bacterial reporter strains to evaluate antimicrobial treatment effects. However, high-sensitivity bioluminescent systems are required because of the limited tissue penetration and low brightness of the BL signals of existing approaches. Here, we report that NanoLuc (Nluc) showed better performance than LuxCDABE in bacteria. However, the retention rate of plasmid constructs in bacteria was low. To construct stable Staphylococcus aureus reporter strains, a partner protein enolase (Eno) was identified by screening of S. aureus strain USA300 for fusion expression of Nluc-based luciferases, including Nluc, Teluc, and Antares2. Different substrates, such as hydrofurimazine (HFZ), furimazine (FUR), and diphenylterazine (DTZ), were used to optimize a stable reporter strain/substrate pair for BL imaging. S. aureus USA300/Eno-Antares2/HFZ produced the highest number of photons of orange-red light in vitro and enabled sensitive BL tracking of S. aureus in vivo, with sensitivities of approximately 10 CFU from mouse skin and 750 CFU from mouse kidneys. USA300/Eno-Antares2/HFZ was a powerful combination based on the longitudinal evaluation of the therapeutic efficacy of antibiotics. The optimized S. aureus Eno-Antares2/HFZ pair provides a technological advancement for the in vivo evaluation of antimicrobial treatment.

A Photolabile Curcumin-Diazirine Analogue Enables Phototherapy with Physically and Molecularly Produced Light for Alzheimer's Disease Treatment

The development of Alzheimer's disease (AD) drugs has recently witnessed substantial achievement. To further enhance the pool of drug candidates, it is crucial to explore non-traditional therapeutic avenues. In this study, we present the use of a photolabile curcumin-diazirine analogue, CRANAD-147, to induce changes in properties, structures (sequences), and neurotoxicity of amyloid beta (Aβ) species both in cells and in vivo. This manipulation was achieved through irradiation with LED light or molecularly generated light, dubbed as "molecular light", emitted by the chemiluminescence probe ADLumin-4. Next, aided by molecular chemiluminescence imaging, we demonstrated that the combination of CRANAD-147/LED or CRANAD-147/ADLumin-4 (molecular light) could effectively slow down the accumulation of Aβs in transgenic 5xFAD mice in vivo. Leveraging the remarkable tissue penetration capacity of molecular light, phototherapy employing the synergistic effect of a photolabile Aβ ligand and molecular light emerges as a promising alternative to conventional AD treatment interventions.

Illuminating the oral microbiome and its host interactions: tools and approaches for molecular microbiology studies

Advancements in DNA sequencing technologies within the last decade have stimulated an unprecedented interest in the human microbiome, largely due the broad diversity of human diseases found to correlate with microbiome dysbiosis. As a direct consequence of these studies, a vast number of understudied and uncharacterized microbes have been identified as potential drivers of mucosal health and disease. The looming challenge in the field is to transition these observations into defined molecular mechanistic studies of symbiosis and dysbiosis. In order to meet this challenge, many of these newly identified microbes will need to be adapted for use in experimental models. Consequently, this review presents a comprehensive overview of the molecular microbiology tools and techniques that have played crucial roles in genetic studies of the bacteria found within the human oral microbiota. Here, we will use specific examples from the oral microbiome literature to illustrate the biology supporting these techniques, why they are needed in the field, and how such technologies have been implemented. It is hoped that this information can serve as a useful reference guide to help catalyze molecular microbiology studies of the many new understudied and uncharacterized species identified at different mucosal sites in the body.

Role of in vivo imaging in Head and Neck cancer management

Intravital microscopy (IVM) and optical coherency tomography (OCT) are powerful optical imaging tools that allow visualization of dynamic biological activities in living subjects with subcellular resolutions. They have been used in preclinical and clinical cancer imaging, providing insights into the complex physiological, cellular, and molecular behaviors of tumors. They have revolutionized cancer diagnosis and therapies, allowing for real-time observation of biologic processes in vivo, including angiogenesis and immune cell interactions. Recent developments in techniques for observing deep tissues of living animals have improved bioluminescent proteins, fluorescent proteins, fluorescent dyes, and detection technologies like two-photon excitation microscopy. These technologies have become indispensable tools in basic sciences, preclinical research, and modern drug development. In Vivo imaging can detect subcellular signaling or metabolic events in living animals, but depth-dependent signal attenuation limits the depth from which significant data can be obtained. Cancer cell motility and invasion are key features of metastatic tumors, but only a small portion of tumor cells are motile and metastasize due to genetic, epigenetic, and microenvironmental heterogeneities.

Imagining the future of optical microscopy: everything, everywhere, all at once

The optical microscope has revolutionized biology since at least the 17th Century. Since then, it has progressed from a largely observational tool to a powerful bioanalytical platform. However, realizing its full potential to study live specimens is hindered by a daunting array of technical challenges. Here, we delve into the current state of live imaging to explore the barriers that must be overcome and the possibilities that lie ahead. We venture to envision a future where we can visualize and study everything, everywhere, all at once - from the intricate inner workings of a single cell to the dynamic interplay across entire organisms, and a world where scientists could access the necessary microscopy technologies anywhere.

Multispectral optoacoustic tomography is more sensitive than micro-computed tomography for tracking gold nanorod labelled mesenchymal stromal cells

Tracking the fate of therapeutic cell types is important for assessing their safety and efficacy. Bioluminescence imaging (BLI) is an effective cell tracking technique, but poor spatial resolution means it has limited ability to precisely map cells in vivo in 3D. This can be overcome by using a bimodal imaging approach that combines BLI with a technique capable of generating high-resolution images. Here we compared the effectiveness of combining either multispectral optoacoustic tomography (MSOT) or micro-computed tomography (micro-CT) with BLI for tracking the fate of luciferase+ human mesenchymal stromal cells (MSCs) labelled with gold nanorods. Following subcutaneous administration in mice, the MSCs could be readily detected with MSOT but not with micro-CT. We conclude that MSOT is more sensitive than micro-CT for tracking gold nanorod-labelled cells in vivo and depending on the route of administration, can be used effectively with BLI to track MSC fate in mice.

Activity-based bioluminescence probes for in vivo sensing applications

Bioluminescence imaging is a highly sensitive technique commonly used for various in vivo applications. Recent efforts to expand the utility of this modality have led to the development of a suite of activity-based sensing (ABS) probes for bioluminescence imaging by 'caging' of luciferin and its structural analogs. The ability to selectively detect a given biomarker has presented researchers with many exciting opportunities to study both health and disease states in animal models. Here, we highlight recent (2021-2023) bioluminescence-based ABS probes with an emphasis on probe design and in vivo validation experiments.

Towards an optimal model for gastric cancer peritoneal metastasis: current challenges and future directions

Peritoneal metastasis is a challenging aspect of clinical practice for gastric cancer. Animal models are crucial in understanding molecular mechanisms, assessing drug efficacy, and conducting clinical intervention studies, including those related to gastric cancer peritoneal metastasis. Unlike other xenograft models, peritoneal metastasis models should not only present tumor growth at the transplant site, but also recapitulate tumor cell metastasis in the abdominal cavity. Developing a reliable model of gastric cancer peritoneal metastasis involves several technical aspects, such as the selection of model animals, source of xenograft tumors, technology of transplantation, and dynamic monitoring of the tumor progression. To date, challenges remain in developing a reliable model that can completely recapitulate peritoneal metastasis. Thus, this review aims to summarize the techniques and strategies used to establish animal models of gastric cancer peritoneal metastasis, providing a reference for future model establishment.

Red-Shifted Coumarin Luciferins for Improved Bioluminescence Imaging

Multicomponent bioluminescence imaging in vivo requires an expanded collection of tissue-penetrant probes. Toward this end, we generated a new class of near-infrared (NIR) emitting coumarin luciferin analogues (CouLuc-3s). The scaffolds were easily accessed from commercially available dyes. Complementary mutant luciferases for the CouLuc-3 analogues were also identified. The brightest probes enabled sensitive imaging in vivo. The CouLuc-3 scaffolds are also orthogonal to popular bioluminescent reporters and can be used for multicomponent imaging applications. Collectively, this work showcases a new set of bioluminescent tools that can be readily implemented for multiplexed imaging in a variety of biological settings.

Screening Multidrug Resistance Reversal Agents in Traditional Chinese Medicines by Efflux Kinetics of D-Luciferin in MCF-7/DOXFluc Cells

In this study, we established a simple and rapid in vitro method for screening multidrug resistance (MDR) reversal agents in traditional Chinese medicines (TCMs), which could better correspond to the MDR reversing effect in vivo. Here, D-luciferin, a substrate for the enzyme firefly luciferase and also a substrate for ATP-binding cassette transporters (ABC transporters), was used as the probe to detect its efflux kinetics caused by ABC transporters. First, we established a stable doxorubicin (DOX)-resistant cell line (MCF-7/DOX^Fluc) that overexpressed luciferase. Then, some kinds of TCMs were chosen for the MDR reversal agents to measure its effect on inhibiting the D-luciferin outflow from MCF-7/DOX^Fluc, and the ideal reversal agent with the least D-luciferin efflux from MCF-7/DOX^Fluc was selected to further investigate its effect combined with DOX on MCF-7/DOX^Fluc tumor-bearing mice. The results indicated that quercetin (Qu) could remarkably increase the retention of D-luciferin in MCF-7/DOX^Fluc in vitro and in vivo. Also, the combination of Qu and DOX could exceedingly inhibit the tumor growth, which proved the feasibility of this in vitro screening method. The study proposed a feasible method for mass screening of MDR agents from TCMs in vitro.

De novo design of luciferases using deep learning

De novo enzyme design has sought to introduce active sites and substrate-binding pockets that are predicted to catalyse a reaction of interest into geometrically compatible native scaffolds1,2, but has been limited by a lack of suitable protein structures and the complexity of native protein sequence-structure relationships. Here we describe a deep-learning-based 'family-wide hallucination' approach that generates large numbers of idealized protein structures containing diverse pocket shapes and designed sequences that encode them. We use these scaffolds to design artificial luciferases that selectively catalyse the oxidative chemiluminescence of the synthetic luciferin substrates diphenylterazine3 and 2-deoxycoelenterazine. The designed active sites position an arginine guanidinium group adjacent to an anion that develops during the reaction in a binding pocket with high shape complementarity. For both luciferin substrates, we obtain designed luciferases with high selectivity; the most active of these is a small (13.9 kDa) and thermostable (with a melting temperature higher than 95 °C) enzyme that has a catalytic efficiency on diphenylterazine (kcat/Km = 106 M-1 s-1) comparable to that of native luciferases, but a much higher substrate specificity. The creation of highly active and specific biocatalysts from scratch with broad applications in biomedicine is a key milestone for computational enzyme design, and our approach should enable generation of a wide range of luciferases and other enzymes.

In vivo bioluminescence imaging of granzyme B activity in tumor response to cancer immunotherapy

Cancer immunotherapy has revolutionized the treatment of cancer, but only a small subset of patients benefits from this new treatment regime. Imaging tools are useful for early detection of tumor response to immunotherapy and probing the dynamic and complex immune system. Here, we report a bioluminescence probe (GBLI-2) for non-invasive, real-time, longitudinal imaging of granzyme B activity in tumors receiving immune checkpoint inhibitors. GBLI-2 is made of the mouse granzyme B tetrapeptide IEFD substrate conjugated to D-luciferin through a self-immolative group. GBLI-2 was evaluated for imaging the dynamics of the granzyme B activity and predicting therapeutic efficacy in a syngeneic mouse model of CT26 murine colorectal carcinoma. The GBLI-2 signal correlated with the change in the population of PD-1- and granzyme B-expressing CD8+ T cells in tumors.

Intrinsic fluorescence from firefly oxyluciferin monoanions isolated in vacuo

Fireflies, click beetles, and railroad worms glow in the dark. The color varies from green to red among the insects and is associated with an electronically excited oxyluciferin formed catalytically by the luciferase enzyme. The actual color tuning mechanism has been, and still is, up for much debate. One complication is that oxyluciferin can occur in different charge states and isomeric forms. We present here emission spectra of oxyluciferin monoanions in vacuo at both room temperature and at 100 K recorded with a newly developed and unique mass-spectroscopy setup specially designed for gas-phase ion fluorescence spectroscopy. Ions are limited to the phenolate-keto and phenolate-enol forms that account for natural bioluminescence. At 100 K, fluorescence band maxima are at 599 ± 2 nm and 563 ± 2 nm for the keto and enol forms, respectively, and at 300 K about 5 nm further to the red. The bare-ion spectra, free from solvent effects, serve as important references as they reveal whether a protein microenvironment redshifts or blueshifts the emission, and they serve as important benchmarks for nontrivial excited-state calculations.

M. de Rosales R. Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques─known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

Near-Infrared Bioluminescence Imaging of Macrophage Sensors for Cancer Detection In Vivo

Melanoma is an aggressive type of skin cancer with a poor prognosis after it gets metastasized. The early detection of malignant melanoma is critical for effective therapy. Because melanoma often resembles moles, routine skin check-up may help for timely identification of suspicious areas. Recently, it has been shown that the interplay of melanoma cells with the immune system can help develop efficient therapeutic strategies. Here, we leveraged engineered macrophages (BMC2) as cell-based sensors for metastatic melanoma. To perform dual-color bioluminescence imaging (BLI) in vivo, macrophages were engineered to express a green click beetle luciferase (CBG2) and a near-infrared fluorescent dye (DiR), and B16F10 melanoma cells were instead engineered to express a near-infrared click beetle luciferase (CBR2). Using real-time in vivo dual-color BLI and near-infrared fluorescence (FL) imaging, we could demonstrate that macrophages were able to sense and substantially accumulate in subcutaneous and metastatic melanoma tissues at 72 h after systemic injections. Together, we showed the potentiality to use optical imaging technologies to track circulating macrophages for the non-invasive detection of metastatic melanoma.

Monitoring Immune Cell Function Through Optical Imaging: a Review Highlighting Transgenic Mouse Models

Transgenic mouse models have facilitated research of human diseases and validation of therapeutic approaches. Inclusion of optical reporter genes (fluorescent or bioluminescent genes) in the targeting vectors used to develop such models makes in vivo imaging of cellular and molecular events possible, from the microscale to the macroscale. In particular, transgenic mouse models expressing optical reporter genes allowed accurately distinguishing immune cell types from trafficking in vivo using intravital microscopy or whole-body optical imaging. Besides lineage tracing and trafficking of different subsets of immune cells, the ability to monitor the function of immune cells is of pivotal importance for investigating the effects of immunotherapies against cancer. Here, we introduce the reader to state-of-the-art approaches to develop transgenics, optical imaging techniques, and several notable examples of transgenic mouse models developed for immunology research by critically highlighting the models that allow the following of immune cell function.

Synthetic Coelenterazine Derivatives and Their Application for Bioluminescence Imaging

Bioluminescence (BL), the emission light resulting from the enzyme-catalyzed oxidative reaction, is a powerful imaging modality for monitoring biological phenomena both in vitro and in vivo. Coelenterazine (CTZ), the known widespread luciferin found in bioluminescent organisms, develops bioluminescence imaging (BLI). Here, we describe an approach to synthesize a series of novel CTZ derivatives for diversifying the toolbox of the BL substrates. Furthermore, we exemplify some of them display excellent BL signals in vitro and in vivo, and thus should be noted as one of the ideal substrates for in vivo BLI compared with a well-known conventional substrate, DeepBlueC.

The NanoBiT-Based Homogenous Ligand-Receptor Binding Assay

NanoLuc Binary Technology (NanoBiT) was recently developed by Promega, based on a large NanoLuc fragment (LgBiT) and two small complementation tags, the low-affinity SmBiT tag and the high-affinity HiBiT tag. In recent studies, we applied NanoBiT to ligand-binding assays of some G protein-coupled receptors via genetic fusion of a secretory LgBiT (sLgBiT) to the extracellular N-terminus of the receptors and covalent attachment of the low-affinity SmBiT tag to an appropriate position of their peptide ligands. The NanoBiT-based homogenous ligand-receptor binding assay is convenient for use and suitable for both the wild-type and mutant receptors, representing a novel tool for interaction mechanism studies of these receptors with their ligands. In the present chapter, we provide detailed protocols for setting up the NanoBiT-based homogenous binding assay using growth hormone secretagogue receptor type 1a (GHSR1a) and its endogenous agonist and antagonist as a representative model system.

In Vivo Assessment of Protein-Protein Interactions Using BRET Assay

Proteins play an important part in almost all life activities and across all organisms. Proteins occasionally act on their own but rather fulfill most of their biological tasks by cooperating with other proteins or ligand molecules. The bioluminescence resonance energy transfer (BRET) assay serves to measure dynamic events such as protein-protein or protein-ligand interactions in vitro or in-vivo. With several inherent attributes such as rapid and fairly sensitive ratio-metric measurements, assessment of interactions irrespective of protein location within the cellular compartment, cost-effectiveness consenting to high-throughput screening compatibility, makes BRET a popular genetic reporter-based assay system for protein-protein interaction (PPI) studies. Based on the Förster principle, BRET allows to judge if the proximity has been achieved between the interacting partners. In recent years, the BRET application has emerged as a significantly versatile assay format by using multiple detection devices such as a plate reader or in-vivo optical imaging platform, or even a bioluminescence microscope has expanded its scope for advancing PPI studies. Beyond the scope of quantitative measurement of PPIs, molecular optical imaging applications based on BRET assay have expanded the scope for screening pharmacological compounds by unifying live cell and in-vivo animal-/plant-based experiments using the same platform technology. In this chapter, we have given intricate methodological details for performing in-vitro and in-vivo BRET experiments, primarily by using donor/acceptor reporter protein combinations.

Multicomponent Bioluminescence Imaging with Naphthylamino Luciferins

Bioluminescent tools have been used for decades to image processes in complex tissues and preclinical models. However, few distinct probes are available to probe multicellular interactions. We and others are addressing this limitation by engineering new luciferases that can selectively process synthetic luciferin analogues. In this work, we explored naphthylamino luciferins as orthogonal bioluminescent probes. Three analogues were prepared using an optimized synthetic route. The luciferins were found to be robust emitters with native luciferase in vitro and in cellulo. We further screened the analogues against libraries of luciferase mutants to identify unique enzyme-substrate pairs. The new probes can be used in conjunction with existing bioluminescent tools for multi-component imaging.

Near-infrared bioluminescence imaging of two cell populations in living mice

Multicolor bioluminescence imaging using near-infrared emitting luciferases is an attractive application to detect two cell populations within one animal model. Herein, we describe how to distinguish dual-color bioluminescent signals co-localized in the same compartment. We tested CBG2 click beetle (λ = 660 nm) and CBR2 click beetle (λ = 730 nm) luciferases paired with NH₂-NpLH2 luciferin. Following a spectral unmixing algorithm, single spectral contributions can be resolved and quantified, enabling the visualization of multiple cell types in deep tissue by injection of a single substrate. For complete details on the use and execution of this protocol, please refer to Zambito et al. (2020).