In vivo bioluminescence imaging of furin activity in breast cancer cells using bioluminogenic substrates

Bioluminescence Imaging of Potassium Ion Using a Sensory Luciferin and an Engineered Luciferase

Bioluminescent indicators are power tools for studying dynamic biological processes. In this study, we present the generation of novel bioluminescent indicators by modifying the luciferin molecule with an analyte-binding moiety. Specifically, we have successfully developed the first bioluminescent indicator for potassium ions (K+), which are critical electrolytes in biological systems. Our approach involved the design and synthesis of a K+-binding luciferin named potassiorin. Additionally, we engineered a luciferase enzyme called BRIPO (bioluminescent red indicator for potassium) to work synergistically with potassiorin, resulting in optimized K+-dependent bioluminescence responses. Through extensive validation in cell lines, primary neurons, and live mice, we demonstrated the efficacy of this new tool for detecting K+. Our research demonstrates an innovative concept of incorporating sensory moieties into luciferins to modulate luciferase activity. This approach has great potential for developing a wide range of bioluminescent indicators, advancing bioluminescence imaging (BLI), and enabling the study of various analytes in biological systems.

Recent advances in bioluminescent probes for neurobiology

Bioluminescence is a popular modality for imaging in living organisms. The platform relies on enzymatically (luciferase) generated light via the oxidation of small molecule luciferins. Since no external light is needed for photon production, there are no concerns with background autofluorescence or photobleaching over time-features that have historically limited other optical readouts. Bioluminescence is thus routinely used for longitudinal tracking across whole animals. Applications in the brain, though, have been more challenging due to a lack of sufficiently bioavailable, bright, and easily multiplexed probes. Recent years have seen the development of designer luciferase and luciferin pairs that address these issues, providing more sensitive and real-time readouts of biochemical features relevant to neurobiology. This review highlights many of the advances in bioluminescent probe design, with a focus on the small molecule light emitter, the luciferin. Specific efforts to improve luciferin pharmacokinetics and tissue-penetrant emission are covered, in addition to applications that such probes have enabled. The continued development of improved bioluminescent probes will aid in illuminating critical neurochemical processes in the brain.

Bioluminescence Imaging of Potassium Ion Using a Sensory Luciferin and an Engineered Luciferase

Bioluminescent indicators are power tools for studying dynamic biological processes. In this study, we present the generation of novel bioluminescent indicators by modifying the luciferin molecule with an analyte-binding moiety. Specifically, we have successfully developed the first bioluminescent indicator for potassium ions (K+), which are critical electrolytes in biological systems. Our approach involved the design and synthesis of a K+-binding luciferin named potassiorin. Additionally, we engineered a luciferase enzyme called BRIPO (bioluminescent red indicator for potassium) to work synergistically with potassiorin, resulting in optimized K+-dependent bioluminescence responses. Through extensive validation in cell lines, primary neurons, and live mice, we demonstrated the efficacy of this new tool for detecting K+. Our research demonstrates an innovative concept of incorporating sensory moieties into luciferins to modulate luciferase activity. This approach has great potential for developing a wide range of bioluminescent indicators, advancing bioluminescence imaging (BLI), and enabling the study of various analytes in biological systems.

Solid-Phase Synthesis of Caged Luminescent Peptides via Side Chain Anchoring

The synthesis of caged luminescent peptide substrates remains challenging, especially when libraries of the substrates are required. Most currently available synthetic methods rely on a solution-phase approach, which is less suited for parallel synthesis purposes. We herein present a solid-phase peptide synthesis (SPPS) method for the synthesis of caged aminoluciferin peptides via side chain anchoring of the P1 residue. After the synthesis of a preliminary test library consisting of 40 compounds, the synthetic method was validated and optimized for up to >100 g of resin. Subsequently, two separate larger peptide libraries were synthesized either having a P1 = lysine or arginine residue containing in total 719 novel peptide substrates. The use of a more stable caged nitrile precursor instead of caged aminoluciferin rendered our parallel synthetic approach completely suitable for SPPS and serine protease profiling was demonstrated using late-stage aminoluciferin generation.

Metal-enhanced bioluminescence by detergent stabilized Ag and Au nanoparticles

The assessment of microbial contamination is an important aspect of ensuring human food safety. One of the modern methods for the evaluation of microbial contamination is the estimation of the amount of ATP using firefly luciferase. In this case, the choice of an effective composition of the extraction buffer is crucial. In this study, we examined the influence of silver and gold nanoparticles on the firefly bioluminescent system during the ATP extraction process. It was found that gold nanoparticles stabilized with benzalkonium chloride and Triton X-100 enhanced bioluminescent system signal intensity due to metal-enhanced bioluminescence. Moreover, silver and gold nanoparticles could be used as extracting agents. So, using gold nanoparticles stabilized with BAC and Triton X-100 as ATP extraction agents with further detection by a bioluminescent system makes it possible to develop an ATP biosensor with higher sensitivity.

Hydrolysis-Resistant Ester-Based Linkers for Development of Activity-Based NIR Bioluminescence Probes

Activity-based sensing (ABS) probes equipped with a NIR bioluminescence readout are promising chemical tools to study cancer biomarkers owing to their high sensitivity and deep tissue compatibility. Despite the demand, there is a dearth of such probes because NIR substrates (e.g., BL660 (a NIR luciferin analog)) are not equipped with an appropriate attachment site for ABS trigger installation. For instance, our attempts to mask the carboxylic acid moiety with standard self-immolative benzyl linkers resulted in significant background signals owing to undesirable ester hydrolysis. In this study, we overcame this longstanding challenge by rationally designing a new hydrolysis-resistant ester-based linker featuring an isopropyl shielding arm. Compared to the parent, the new design is 140.5-fold and 67.8-fold more resistant toward spontaneous and esterase-mediated hydrolysis, respectively. Likewise, we observed minimal cleavage of the ester moiety when incubated with a panel of enzymes possessing ester-hydrolyzing activity. These impressive in vitro results were corroborated through a series of key experiments in live cells. Further, we showcased the utility of this technology by developing the first NIR bioluminescent probe for nitroreductase (NTR) activity and applied it to visualize elevated NTR expression in oxygen deficient lung cancer cells and in a murine model of non-small cell lung cancer. The ability to monitor the activity of this key biomarker in a deep tissue context is critical because it is associated with tumor hypoxia, which in turn is linked to drug resistance and aggressive cancer phenotypes.

Micro-PET imaging of hepatitis C virus NS3/4A protease activity using a protease-activatable retention probe

Hepatitis C virus (HCV) NS3/4A protease is an attractive target for direct-acting antiviral agents. Real-time tracking of the NS3/4A protease distribution and activity is useful for clinical diagnosis and disease management. However, no approach has been developed that can systemically detect NS3/4A protease activity or distribution. We designed a protease-activatable retention probe for tracking HCV NS3/4A protease activity via positron emission topography (PET) imaging. A cell-penetrating probe was designed that consisted of a cell-penetrating Tat peptide, HCV NS3/4A protease substrate, and a hydrophilic domain. The probe was labeled by fluorescein isothiocyanate (FITC) and 124I in the hydrophilic domain to form a TAT-ΔNS3/4A-124I-FITC probe. Upon cleavage at NS3/4A substrate, the non-penetrating hydrophilic domain is released and accumulated in the cytoplasm allowing PET or optical imaging. The TAT-ΔNS3/4A-FITC probe selectively accumulated in NS3/4A-expressing HCC36 (NS3/4A-HCC36) cells/tumors and HCV-infected HCC36 cells. PET imaging showed that the TAT-ΔNS3/4A-124I-FITC probe selectively accumulated in the NS3/4A-HCC36 xenograft tumors and liver-implanted NS3/4A-HCC36 tumors, but not in the control HCC36 tumors. The TAT-ΔNS3/4A-124I-FITC probe can be used to represent NS3/4 protease activity and distribution via a clinical PET imaging system allowing. This strategy may be extended to detect any cellular protease activity for optimization the protease-based therapies.

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.

Luminescent Assay for the Screening of SARS-CoV-2 MPro Inhibitors

Since the outbreak of SARS‐CoV‐2 in December 2019 millions of infections have been reported globally. The viral chymotrypsin‐like main protease (M^Pro) exhibits a crucial role in viral replication and represents a relevant target for antiviral drug development. In order to screen potential M^Pro inhibitors we developed a luminescent assay using a peptide based probe containing a cleavage site specific for M^Pro. This assay was validated showing IC₅₀ values similar to those reported in the literature for known M^Pro inhibitors and can be used to screen new inhibitors.

Peptide probes for proteases - innovations and applications for monitoring proteolytic activity

Proteases are excellent biomarkers for a variety of diseases, offer multiple opportunities for diagnostic applications and are valuable targets for therapy. From a chemistry-based perspective this review discusses and critiques the most recent advances in the field of substrate-based probes for the detection and analysis of proteolytic activity both in vitro and in vivo.

Fluorescence and ratiometric photoacoustic imaging of endogenous furin activity via peptide functionalized MoS2 nanosheets

Furin is an important cellular endoprotease, which is expressed at high levels in various cancer cells. Accurate and real-time detection of endogenous furin with high sensitivity and selectivity is significant for the diagnosis of cancer. Herein an activatable nanoprobe (MoS₂@PDA-PEG/peptide, MPPF) with dual-mode near-infrared fluorescence (NIRF)/ratiometric photoacoustic (PA) imaging of endogenous furin activity has been developed. The MPPF nanoprobes were constructed by the covalent functionalization of polydopamine (PDA) coated MoS₂ nanosheets (NSs) with Cy7-labeled furin substrate peptides. Upon cleavage of the peptides by furin, Cy7 molecules are released from MPPF nanoprobes and recover their fluorescence, realizing furin activity detection with the limit of detection (LOD) down to 3.73 × 10^-4 U mL^-1. Meanwhile, the ratio of the PA signal at 768 nm to that at 900 nm (PA₇₆₈/PA₉₀₀) decreases over time due to the destruction of fluorescence resonance energy transfer effect from Cy7 to MoS₂ NSs and the rapid clearance of small Cy7 molecules from tissues. Thus, the simultaneous change in NIRF and ratiometric PA signals enables the imaging of endogenous furin activity in real time, and with high sensitivity, and high selectivity in both tumor cells and tumor-bearing mice.

Transformable Helical Self-Assembly for Cancerous Golgi Apparatus Disruption

Golgi apparatus is a major subcellular organelle responsible for drug resistance. Golgi apparatus-targeted nanomechanical disruption provides an attractive approach for killing cancer cells by multimodal mechanism and avoiding drug resistance. Inspired by the poisonous twisted fibrils in Alzheimer's brain tissue and enhanced rigidity of helical structure in nature, we designed transformable peptide C₆RVRRF₄KY that can self-assemble into nontoxic nanoparticles in aqueous medium but transformed into left-handed helical fibrils (L-HFs) after targeting and furin cleavage in the Golgi apparatus of cancer cells. The L-HFs can mechanically disrupt the Golgi apparatus membrane, resulting in inhibition of cytokine secretion, collapse of the cellular structure, and eventually death of cancer cells. Repeated stimulation of the cancers by the precursors causes no acquired drug resistance, showing that mechanical disruption of subcellular organelle is an excellent strategy for cancer therapy without drug resistance. This nanomechanical disruption concept should also be applicable to multidrug-resistant bacteria and viruses.

Portable bioluminescent platform for in vivo monitoring of biological processes in non-transgenic animals

Bioluminescent imaging (BLI) is one of the most powerful and widely used preclinical imaging modalities. However, the current technology relies on the use of transgenic luciferase-expressing cells and animals and therefore can only be applied to a limited number of existing animal models of human disease. Here, we report the development of a “portable bioluminescent” (PBL) technology that overcomes most of the major limitations of traditional BLI. We demonstrate that the PBL method is capable of noninvasive measuring the activity of both extracellular (e.g., dipeptidyl peptidase 4) and intracellular (e.g., cytochrome P450) enzymes in vivo in non-luciferase-expressing mice. Moreover, we successfully utilize PBL technology in dogs and human cadaver, paving the way for the translation of functional BLI to the noninvasive quantification of biological processes in large animals. The PBL methodology can be easily adapted for the noninvasive monitoring of a plethora of diseases across multiple species.

Bioluminescence imaging tends to rely on transgenic luciferase-expressing cells and animals. Here the authors report a portable bioluminescent system to non-invasively measure intra- and extracellular enzymes in vivo in non-transgenic animals which do not express luciferase.

Molecular Design of d-Luciferin-Based Bioluminescence and 1,2-Dioxetane-Based Chemiluminescence Substrates for Altered Output Wavelength and Detecting Various Molecules

Optical imaging including fluorescence and luminescence is the most popular method for the in vivo imaging in mice. Luminescence imaging is considered to be superior to fluorescence imaging due to the lack of both autofluorescence and the scattering of excitation light. To date, various luciferin analogs and bioluminescence probes have been developed for deep tissue and molecular imaging. Recently, chemiluminescence probes have been developed based on a 1,2-dioxetane scaffold. In this review, the accumulated findings of numerous studies and the design strategies of bioluminescence and chemiluminescence imaging reagents are summarized.

Scratching the Surface of Unventured Possibilities with In Situ Self-Assembly: Protease-Activated Developments for Imaging and Therapy

In situ self-assembly has attracted increasing research interest for applications in imaging and therapy in recent years. Particularly for protease-activated developments, inspiration is drawn from the innate specificity of their catalytic activities, rapid discovery of the various roles they play in the proliferation of certain diseases, and inherent susceptibility of small molecule peptide conjugates to proteolytic digestion in vivo. The overexpression of a disease-related protease of interest can be exploited as an endogenous stimulus for site-specific self-assembly to largely amplify a molecular event happening at the cellular level. This holds great potential for applications in early stage disease detection, long-term disease monitoring, and sustained therapeutic effects. This review summarizes the recent developments in protease-activated self-assemblies for imaging and therapeutic applications toward the manifestation of tumors, bacterial infections, neurodegenerative disorders, and wound recovery.

Applications of bioluminescence in biotechnology and beyond

Bioluminescent probes have hugely benefited from the input of synthetic chemistry and protein engineering. Here we review the latest applications of these probes in biotechnology and beyond, with an eye on current limitations and future directions.

Recent achievements of bioluminescence imaging based on firefly luciferin-luciferase system

Bioluminescence imaging (BLI) is a newly developed noninvasive visual approach which facilitates the understanding of a plethora of biological processes in vitro and in vivo due to the high sensitivity, resolution and selectivity, low background signal, and the lack of external light excitation. BLI based on firefly luciferin-luciferase system has been widely used for the activity evaluation of tumor-specific enzymes, for the detection of diseases-related bioactive small molecules and metal ions, and for the diagnosis and therapy of diseases including the studies of drug transport, the research of immune response, and the evaluation of drug potency and tissue distribution. In this review, we highlight the recent achievements in luciferin derivatives with red-shifted emission spectra, mutant luciferase-luciferin pairs, and the diagnostic and therapeutic application of BLI based on firefly luciferin-luciferase system. The development and application of BLI will expand our knowledge of the occurrence and development of diseases and shed light on the diagnosis and treatment of various diseases.

A caged 2-hydroxyethyl luciferin for bioluminescence imaging of nitroxyl in living cells

Nitroxyl (HNO), a one-electron reduction product of nitric oxide, demonstrates distinct biological and pharmacological activities. Here we designed a bioluminescent turn-on probe, HNO-8, that could be used to visualize HNO without the need for excitation light. HNO-8 was prepared by caging 2-hydroxyethyl luciferin with a triphenylphosphine unit, in which 2-hydroxyethyl luciferin as a novel substrate of firefly luciferase was characterized by stronger and more sustained bioluminescent signals than the most popular substrates of d-luciferin and 6'-aminoluciferin. In vitro experiments showed that HNO-8 could selectively respond to HNO generated from Angeli's salt(AS) in the range 1-50 μM, with a limit of detection of 0.196 μM. The probe was successfully applied for visualizing HNO in luciferase-transfected Huh7 cancer cells. We envision that HNO-8 could be used as a powerful bioluminescent sensor for researching HNO biological roles.

One-step 18F-fluorination of smart positron emission tomography tracer for sensing furin activity in tumors

INTRODUCTION

Peptide analogues have attracted considerable attention in the field of developing novel positron emission tomography (PET) imaging agents due to their unique properties. Nevertheless, the complicated radiolabeling process and fast metabolism usually pose challenges to the clinical applications of peptide-based molecular probes. Herein a novel PET tracer containing a specific peptide sequence Arg-Val-Arg-Arg (RVRR), Acetyl-Arg-Val-Arg-Arg-Cys(StBu)-Gly(AMB[¹⁸F]F₃)-CBT ([¹⁸F]1), was designed and radiosynthesized using a simple and convenient one-step ¹⁸F-fluorination procedure. The smart tracer can be activated by the protease furin and then undergoes an intermolecular cyclization reaction in tumor cells, leading to improved PET imaging efficiency of tumor.

METHODS

The radiosynthesis of the target tracer [¹⁸F]1 and the control tracer [¹⁸F]1-ctrl was performed under facile conditions in pyridazine-HCl buffer (pH~2.5) at 80 °C within 30 min. The enzyme-controlled condensation was studied for non-radioactive compound 1 in the human breast cancer cell lysates (MDA-MB-468). The cellular uptake of [¹⁸F]1 and [¹⁸F]1-ctrl was studied and compared by measuring the activity in MDA-MB-468 cells using a γ-counter after incubation with 37 kBq of [¹⁸F]1 or [¹⁸F]1-ctrl, respectively. In vivo behavior of [¹⁸F]1 was examined through PET imaging of MDA-MB-468 tumor-bearing mice and compared with that of [¹⁸F]1-ctrl as well as that of [¹⁸F]1 co-injected with non-radioactive compound 1.

RESULTS

The tracer [¹⁸F]1 was obtained with a high radiochemical yield (RCY) of 42.5 ± 1.47% and an excellent radiochemical purity (RCP > 99%). Under the activation of furin and GSH, the tracer suffered a condensation reaction to form dimers and then self-assembled into nanoparticles to produce enduring signal. The cellular uptake of [¹⁸F]1 and [¹⁸F]1-ctrl was determined to be 10.2 ± 0.37 and 1.19 ± 0.25%ID at 120 min, respectively. For in vivo PET imaging, [¹⁸F]1 exhibited the optimum tumor uptake of 2.39 ± 0.31%ID/g and the tumor-to-muscle uptake ratio of 2.93 ± 0.92 at 10 min post injection. Co-injection of [¹⁸F]1 and non-radioactive compound 1 produced a high tumor uptake ranging from 2.83 ± 0.23%ID/g to 3.40 ± 0.18%ID/g at 10 min and 60 min post injection, respectively.

CONCLUSIONS

The one-step labeling method of tracer [¹⁸F]1 showed advantage in simplifying the radiolabeling process with high RCY, which could enable a real kit process for the synthesis of ¹⁸F-radiopharmaceuticals and was significant for the large-scale production of tracers for clinical applications. PET imaging results suggested that the tracer [¹⁸F]1 had good tumor uptake and the co-injection strategy of [¹⁸F]1 with 1 could enhance the imaging signal in tumor.

Cell-specific image-guided transcriptomics identifies complex injuries caused by ischemic acute kidney injury in mice

The kidney's inherent complexity has made identifying cell-specific pathways challenging, particularly when temporally associating them with the dynamic pathophysiology of acute kidney injury (AKI). Here, we combine renal cell-specific luciferase reporter mice using a chemoselective luciferin to guide the acquisition of cell-specific transcriptional changes in C57BL/6 background mice. Hydrogen peroxide generation, a common mechanism of tissue damage, was tracked using a peroxy-caged-luciferin to identify optimum time points for immunoprecipitation of labeled ribosomes for RNA-sequencing. Together, these tools revealed a profound impact of AKI on mitochondrial pathways in the collecting duct. In fact, targeting the mitochondria with an antioxidant, ameliorated not only hydrogen peroxide generation, but also significantly reduced oxidative stress and the expression of the AKI biomarker, LCN2. This integrative approach of coupling physiological imaging with transcriptomics and drug testing revealed how the collecting duct responds to AKI and opens new venues for cell-specific predictive monitoring and treatment.

Development and Applications of Bioluminescent and Chemiluminescent Reporters and Biosensors

Although fluorescent reporters and biosensors have become indispensable tools in biological and biomedical fields, fluorescence measurements require external excitation light, thereby limiting their use in thick tissues and live animals. Bioluminescent reporters and biosensors may potentially overcome this hurdle because they use enzyme-catalyzed exothermic biochemical reactions to generate excited-state emitters. This review first introduces the development of bioluminescent reporters, and next, their applications in sensing biological changes in vitro and in vivo as biosensors. Lastly, we discuss chemiluminescent sensors that produce photons in the absence of luciferases. This review aims to explore fundamentals and experimental insights and to emphasize the yet-to-be-reached potential of next-generation luminescent reporters and biosensors.

Caged luciferins for bioluminescent activity-based sensing

Bioluminescence imaging is a powerful modality for in vivo imaging owing to its low background and high signal-to-noise ratio. Because bioluminescent emission occurs only upon the catalytic reaction between the luciferase enzyme and its luciferin substrate, caging luciferins with analyte-reactive triggers offers a general approach for activity-based sensing of specific biochemical processes in living systems across cell, tissue, and animal models. In this review, we summarize recent efforts in the development of synthetic caged luciferins for tracking enzyme, small molecule, and metal ion activity and their contributions to physiological and pathological processes.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

Biotinylated Bioluminescent Probe for Long Lasting Targeted in Vivo Imaging of Xenografted Brain Tumors in Mice

Bioluminescence is a useful tool for imaging of cancer in in vivo animal models that endogenously express luciferase, an enzyme that requires a substrate for visual readout. Current bioluminescence imaging, using commonly available luciferin substrates, only lasts a short time (15-20 min). To avoid repeated administration of luciferase substrate during cancer detection and surgery, a long lasting bioluminescence imaging substrate or system is needed. A novel water-soluble biotinylated luciferase probe, B-YL (1), was synthesized. A receptor-targeted complex of B-YL with streptavidin (SA) together with a biotinylated epidermal growth factor short peptide (B-EGF) (SA/B-YL/B-EGF = 1:3:1, molar ratio) was then prepared to demonstrate selective targeting. The complex was incubated with brain cancer cell lines overexpressing the EGF receptor (EGFR) and transfected with the luciferase gene. Results show that the complex specifically detects cancer cells by bioluminescence. The complex was further used to image xenograft brain tumors transfected with a luciferase gene in mice. The complex detects the tumor immediately, and bioluminescence lasts for 5 days. Thus, the complex generates a long lasting bioluminescence for cancer detection in mice. The complex with selective targeting may be used in noninvasive cancer diagnosis and accurate surgery in cancer treatment in clinics in the future.

Imaging Cancer Metabolism

It is widely accepted that altered metabolism contributes to cancer growth and has been described as a hallmark of cancer. Our view and understanding of cancer metabolism has expanded at a rapid pace, however, there remains a need to study metabolic dependencies of human cancer in vivo. Recent studies have sought to utilize multi-modality imaging (MMI) techniques in order to build a more detailed and comprehensive understanding of cancer metabolism. MMI combines several in vivo techniques that can provide complementary information related to cancer metabolism. We describe several non-invasive imaging techniques that provide both anatomical and functional information related to tumor metabolism. These imaging modalities include: positron emission tomography (PET), computed tomography (CT), magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS) that uses hyperpolarized probes and optical imaging utilizing bioluminescence and quantification of light emitted. We describe how these imaging modalities can be combined with mass spectrometry and quantitative immunochemistry to obtain more complete picture of cancer metabolism. In vivo studies of tumor metabolism are emerging in the field and represent an important component to our understanding of how metabolism shapes and defines cancer initiation, progression and response to treatment. In this review we describe in vivo based studies of cancer metabolism that have taken advantage of MMI in both pre-clinical and clinical studies. MMI promises to advance our understanding of cancer metabolism in both basic research and clinical settings with the ultimate goal of improving detection, diagnosis and treatment of cancer patients.

Firefly Luciferin-Inspired Biocompatible Chemistry for Protein Labeling and In Vivo Imaging

Biocompatible reactions have emerged as versatile tools to build various molecular imaging probes that hold great promise for the detection of biological processes in vitro and/or in vivo. In this Minireview, we describe the recent advances in the development of a firefly luciferin-inspired biocompatible reaction between cyanobenzothiazole (CBT) and cysteine (Cys), and highlight its versatility to label proteins and build multimodality molecular imaging probes. The review starts from the general introduction of biocompatible reactions, which is followed by briefly describing the development of the firefly luciferin-inspired biocompatible chemistry. We then discuss its applications for the specific protein labeling and for the development of multimodality imaging probes (fluorescence, bioluminescence, MRI, PET, photoacoustic, etc.) that enable high sensitivity and spatial resolution imaging of redox environment, furin and caspase-3/7 activity in living cells and mice. Finally, we offer the conclusions and our perspective on the various and potential applications of this reaction. We hope that this review will contribute to the research of biocompatible reactions for their versatile applications in protein labeling and molecular imaging.

In vivo bioluminescence imaging of labile iron accumulation in a murine model of Acinetobacter baumannii infection

Iron is an essential metal for all organisms, yet disruption of its homeostasis, particularly in labile forms that can contribute to oxidative stress, is connected to diseases ranging from infection to cancer to neurodegeneration. Iron deficiency is also among the most common nutritional deficiencies worldwide. To advance studies of iron in healthy and disease states, we now report the synthesis and characterization of iron-caged luciferin-1 (ICL-1), a bioluminescent probe that enables longitudinal monitoring of labile iron pools (LIPs) in living animals. ICL-1 utilizes a bioinspired endoperoxide trigger to release d-aminoluciferin for selective reactivity-based detection of Fe2+ with metal and oxidation state specificity. The probe can detect physiological changes in labile Fe2+ levels in live cells and mice experiencing iron deficiency or overload. Application of ICL-1 in a model of systemic bacterial infection reveals increased iron accumulation in infected tissues that accompany transcriptional changes consistent with elevations in both iron acquisition and retention. The ability to assess iron status in living animals provides a powerful technology for studying the contributions of iron metabolism to physiology and pathology.

Wenshen Zhuanggu formula effectively suppresses breast cancer bone metastases in a mouse Xenograft model

Wenshen Zhuanggu formula (WSZG) is a traditional Chinese medicine used as an adjuvant for the prevention of bone metastases in breast cancer patients. In this study we investigated the efficacy of WSZG in preventing bone metastases and the potential mechanisms in a mouse xenograft model of breast cancer bone metastases. This model was established by injection of human MDA-MB-231BO-Luc breast cancer cells alone or a mixture of the cancer cells with bone marrow-derived mesenchymal stem cells (BMSCs) into left ventricle of the heart in female nude mice. Then the mice were treated with WSZG (3.25, 6.5 or 13.0 mg·kg^-1·d^-1, ig) for four weeks, whereas zoledronic acid (100 μg/kg per week, ig) was used as a positive control. The occurrence and development of bone metastases were monitored via bioluminescent imaging, and bone lesions were assessed using micro-CT. Intracardiac injection of the mixture of MDA-MB-231BO-Luc breast cancer cells with BMSCs significantly facilitated the bone metastatic capacity of the breast cancer cells, and aggravated bone lesions in the mouse xenograft model of breast cancer bone metastases. Administration of WSZG dose-dependently inhibited the incidence and intensity of bone metastases and protected against bone lesions by suppressing osteoclast formation and tumor cell infiltration. Furthermore, administration of WSZG caused a marked reduction in the expression of CCL5/CCR5 and IL-17B/IL-17BR in bone metastatic tissues. The results demonstrate that WSZG exerts potential therapeutic effects in a mouse xenograft model of breast cancer bone metastases, which are partially mediated by weakening the interaction between BMSCs and breast cancer cells in the tumor microenvironment.

Bioluminescence Sensing of γ-Glutamyltranspeptidase Activity In Vitro and In Vivo

γ-Glutamyltranspeptidase (GGT) is an important tumor biomarker but using a bioluminescence (BL) probe to real time monitor its activity has not been reported. Herein, we rationally designed two GGT-cleavable BL probes Glu-AmLH₂ (1) and Glu-p-aminobenzyloxycarbonyl-AmLH₂ (2), and successfully applied them for sensing GGT activity with high sensitivity and excellent selectivity both in vitro and in vivo. The results indicated that, although 2 had lower background BL signal than 1, GGT had higher catalytic efficiency for 1 than 2, and 1 was superior to 2 for sensing GGT activity in living cells and tumors. We envision that our probe 1 could be widely applied for the diagnosis of important GGT-related diseases in animal models in the near future.

The proprotein convertase furin in tumour progression

Proprotein convertases are proteases that have been implicated in the activation of a wide variety of proteins. These proteins are generally synthesised as precursor proteins and require limited proteolysis for conversion into their mature bioactive counterparts. Many of these proteins, including metalloproteases, growth factors and their receptors or adhesion molecules, have been shown to facilitate tumour formation and progression. Hence, this review will focus on the proprotein convertase furin and its role in cancer. The expression of furin has been confirmed in a large spectrum of cancers such as head and neck squamous cell carcinoma, breast cancer and rhabdomyosarcoma. Functional studies modulating furin activity uncovered its importance for the processing of many cancer-related substrates and strongly indicate that high furin activity promotes the malignant phenotype of cancer cells. In this review, we summarise the expression and function of furin in different cancer types, discuss its role in processing cancer-related proproteins and give examples of potential therapeutic approaches that take advantage of the proteolytic activity of furin in cancer cells.

In Vivo Molecular Bioluminescence Imaging: New Tools and Applications

in vivo bioluminescence imaging (BLi) is an optical molecular imaging technique used to visualize molecular and cellular processes in health and diseases and to follow the fate of cells with high sensitivity using luciferase-based gene reporters. The high sensitivity of this technique arises from efficient photon production, followed by the reaction between luciferase enzymes and luciferin substrates. Novel discoveries and developments of luciferase reporters, substrates, and gene-editing techniques, and emerging fields of applications, promise a new era of deeper and more sensitive molecular imaging.

Bioluminescent Turn-On Probe for Sensing Hypochlorite in Vitro and in Tumors

Hypochlorite (ClO^-) is one of the most important reactive oxygen species but using a BL probe for its selective detection (or imaging) still remains challenging. Herein, we report a latent BL probe benzoylhydrazine luciferin (1) for highly selective detection of ClO^- in vitro and imaging ClO^- in living cells and tumors. In vitro tests indicated that 1 could be applied for highly selective detection of ClO^- within the range of 0-62.5 μM with a limit of detection of 0.705 μM. Using these unique features of 1, we successfully applied it to image ClO^- in living cells and tumors. We envision that probe 1 might be applied to elucidate the biological roles of ClO^- in wider physiological and pathological processes in the near future.

Dioxetane formation and chemiluminescent emission upon the combination of a vinylphenol derivative with naphthalene endoperoxide

We designed a chemiluminescent system using nanoparticles.

Dual aggregation-induced emission for enhanced fluorescence sensing of furin activity in vitro and in living cells

A dual aggregation-induced emission probe was designed for effectively sensing furin activity in vitro and in living cells.

Intracellular synthesis of d-aminoluciferin for bioluminescence generation

Chemically stable precursors were employed to intracellularly synthesize d-aminoluciferin for bioluminescence generation.

Interspecies comparison of peptide substrate reporter metabolism using compartment-based modeling

Peptide substrate reporters are fluorescently labeled peptides that can be acted upon by one or more enzymes of interest. Peptide substrates are readily synthesized and more easily separated than full-length protein substrates; however, they are often more rapidly degraded by peptidases. As a result, peptide reporters must be made resistant to proteolysis in order to study enzymes in intact cells and lysates. This is typically achieved by optimizing the reporter sequence in a single cell type or model organism, but studies of reporter stability in a variety of organisms are needed to establish the robustness and broader utility of these molecular tools. We measured peptidase activity toward a peptide substrate reporter for protein kinase B (Akt) in E. coli, D. discoideum, and S. cerevisiae using capillary electrophoresis with laser-induced fluorescence (CE-LIF). Using compartment-based modeling, we determined individual rate constants for all potential peptidase reactions and explored how these rate constants differed between species. We found the reporter to be stable in D. discoideum (t _1/2 = 82-103 min) and S. cerevisiae (t _1/2 = 279-314 min), but less stable in E. coli (t _1/2 = 21-44 min). These data suggest that the reporter is sufficiently stable to be used for kinase assays in eukaryotic cell types while also demonstrating the potential utility of compartment-based models in peptide substrate reporter design. Graphical abstract Cell lysates from several evolutionarily divergent species were incubated with a peptide substrate reporter, and compartment-based modeling was used to determine key steps in the metabolism of the reporter in each cell type.

Hydrazide d-luciferin for in vitro selective detection and intratumoral imaging of Cu(2.)

Copper is an essential micronutrient involved in fundamental life processes but using a bioluminescence (BL) probe to selectively sense Cu(2+)in vitro or image Cu(2+)in vivo is still unavailable. Herein, a latent BL probe hydrazide d-luciferin (1) was rationally designed and successfully applied it for selective detection of Cu(2+)in vitro and imaging Cu(2+) in living cells and in tumors. Upon the catalysis of Cu(2+), 1 was converted to d-luciferin and turned on the BL in the presence of firefly luciferase (fLuc). In vitro tests indicated that 1 could be applied for highly selective sensing Cu(2+) within the range of 0-80μM with a limit of detection (LOD) of 39.0nM. Cell and animal experiments indicated that 1 could be applied for specific BL imaging of Cu(2+) in living cells and tumors and the BL signal of 1 was more stable and longer than that of d-luciferin. We envision that this unique probe 1 might serve as an elucidative tool for further exploration of the biological roles of Cu(2+) in physiological and pathological processes in the near future.

Furin as proprotein convertase and its role in normal and pathological biological processes

Furin belongs to serine intracellular Ca2+-dependent endopeptidases of the subtilisin family, also known as proprotein convertase (PC). Human furin is synthesized as zymogen with a molecular weight of 104 kDa, which is then activated by autocatalytic in two stages. This process can occur when zymogen migrates from the endoplasmic reticulum to the Golgi apparatus, where a large part of furin is accumulated. The molecular weigh t of the active furin is 98 kDa. Furin relates to enzymes with a narrow substrate specificity: it hydrolyzes peptide bonds at the site of paired basic amino acids and furin activity exhibits in a wide pH range 5-8. Its main biological function is activation of the functionally important protein precursors. It is accompanied by the launch of a cascade of reactions, which lead to appearance of biologically active molecules involved in realization of specific biological functions both in normal and in some patologicheskih processes. Furin substrates are biologically important proteins such as enzymes, hormones, growth factors and differentiation, receptors, adhesion proteins, proteins of blood plasma. Furin plays an important role in the development of processes such as proliferation, invasion, cell migration, survival, maintenance of homeostasis, embryogenesis, as well as the development of a number of pathologies, including cardiovascular, oncologic and neurodegenerative diseases. Furin and furin-like proprotein convertases participate as key factors in the realization of the regulatory functions of proteolytic enzymes, the value of which is currently being evaluated as most important in comparison with the degradative function of proteases.

Development of a Bioluminescent Nitroreductase Probe for Preclinical Imaging

Bacterial nitroreductases (NTRs) have been widely utilized in the development of novel antibiotics, degradation of pollutants, and gene-directed enzyme prodrug therapy (GDEPT) of cancer that reached clinical trials. In case of GDEPT, since NTR is not naturally present in mammalian cells, the prodrug is activated selectively in NTR-transformed cancer cells, allowing high efficiency treatment of tumors. Currently, no bioluminescent probes exist for sensitive, non-invasive imaging of NTR expression. We therefore developed a "NTR caged luciferin" (NCL) probe that is selectively reduced by NTR, producing light proportional to the NTR activity. Here we report successful application of this probe for imaging of NTR in vitro, in bacteria and cancer cells, as well as in vivo in mouse models of bacterial infection and NTR-expressing tumor xenografts. This novel tool should significantly accelerate the development of cancer therapy approaches based on GDEPT and other fields where NTR expression is important.