Monitoring caspase-3 activation with a multimodality imaging sensor in living subjects

Antitumor Effects of Pegylated Zinc Protoporphyrin-Mediated Sonodynamic Therapy in Ovarian Cancer

Sonodynamic therapy (SDT) induces reactive oxygen species (ROS) to kill tumor cells. Heme oxygenase-1 (HO-1), as an important antioxidant enzyme, resists killing by scavenging ROS. Zinc protoporphyrin (ZnPP) not only effectively inhibits HO-1 activity, but also becomes a potential sonosensitizer. However, its poor water solubility limits its applications. Herein, we developed an improved water-soluble method. It was proved that pegylated zinc protoporphyrin-mediated SDT (PEG-ZnPP-SDT) could significantly enhance ROS production by destroying the HO-1 antioxidant system in ovarian cancer. Increased ROS could cause mitochondrial membrane potential collapse, release cytochrome c from mitochondria to the cytoplasm, and trigger the mitochondrial-caspase apoptotic pathway. In conclusion, our results demonstrated that PEG-ZnPP-SDT, as a novel sonosensitizer, could improve the antitumor effects by destroying the HO-1 antioxidant system. It provided a new therapeutic strategy for SDT to treat cancers, especially those with higher HO-1 expression.

Biosensors for Caspase-3: From chemical methodologies to biomedical applications

Caspase-3 plays irreplaceable roles in apoptosis and related diseases. An imbalance in the measured levels of Caspase-3 is implicated in irreversible apoptosis. Therefore, the detection of Caspase-3 is of great significance for apoptosis imaging and the evaluation effect of early tumor treatment and other diseases. Herein, advances in the recent innovations of Caspase-3 response fluorescence biosensors, including molecular probes and nanoprobes, are systematically summarized in sections corresponding. The performances of various luminescence probes in Caspase-3 detection are discussed intensively in the design strategy of chemical structure, response mechanism and biological application. Finally, the current challenges and prospects of the design of new Caspase-3 responsive fluorescence probes for apoptosis imaging, or similar molecular event are proposed.

Chemical Design of Activatable Photoacoustic Probes for Precise Biomedical Applications

Photoacoustic (PA) imaging technology, a three-dimensional hybrid imaging modality that integrates the advantage of optical and acoustic imaging, has great application prospects in molecular imaging due to its high imaging depth and resolution. To endow PA imaging with the ability for real-time molecular visualization and precise biomedical diagnosis, numerous activatable molecular PA probes which can specifically alter their PA intensities upon reacting with the targets or biological events of interest have been developed. This review highlights the recent developments of activatable PA probes for precise biomedical applications including molecular detection of the biotargets and imaging of the biological events. First, the generation mechanism of PA signals will be given, followed by a brief introduction to contrast agents used for PA probe design. Then we will particularly summarize the general design principles for the alteration of PA signals and activatable strategies for developing precise PA probes. Furthermore, we will give a detailed discussion of activatable PA probes in molecular detection and biomedical imaging applications in living systems. At last, the current challenges and outlooks of future PA probes will be discussed. We hope that this review will stimulate new ideas to explore the potentials of activatable PA probes for precise biomedical applications in the future.

Real-Time Visualization of Embryonic Apoptosis Using a Near-Infrared Fluorogenic Probe for Embryo Development Evaluation

Developing an accurate and reliable detection technique for early embryonic apoptosis is of great significance for real-time monitoring and evaluation of embryonic development in living systems. Herein, we have rationally designed and synthesized a novel near-infrared (NIR) fluorogenic probe CGK(QSY21)DEVD-Cy5.5 for real-time imaging of embryonic apoptosis. This probe is constructed with a NIR dye Cy5.5, a fluorescence quencher QSY21, and a peptide substrate Asp-Glu-Val-Asp (DEVD) of the caspase-3 enzyme that is a key executor of cell apoptosis. The probe was initially nonfluorescent in aqueous solution but emitted strong NIR fluorescence upon specific cleavage by activated caspase-3 in a concentration-dependent manner. Taking advantage of this unique feature, this fluorogenic probe was for the first time used for real-time imaging of caspase-3 activity in apoptotic embryos. More notably, significant fluorescence enhancement was solely determined from the apoptotic embryos with the treatment of the probe both in vitro and in vivo, highly suggesting that this probe has great potential to monitor the apoptosis of embryos. We thus envision that this probe would provide a very useful means for real-time visualization and accurate evaluation of embryonic development in the future.

A caspase-3 activatable photoacoustic probe for in vivo imaging of tumor apoptosis

Photoacoustic (PA) imaging is an emerging imaging technique, which combines high spatial resolution and deep tissue penetration of ultrasound imaging with high sensitivity of fluorescence imaging. In the past few years, PA has shown promise for noninvasive imaging of biomolecules in vivo. In this chapter, we present the synthesis and application of a tumor targeting and caspase-3 activatable PA probe (1-RGD) for real-time and noninvasive imaging of tumor apoptosis. 1-RGD can be efficiently delivered into tumor tissues and recognized by caspase-3, which triggered efficient proteolysis of DEVD substrate and subsequent intramolecular macrocyclization, followed by in situ self-assembly into nanoparticles, leading to prolonged retention in apoptotic tumors and enhanced PA signals. With 1-RGD, high-resolution 3D PA images of tumor tissues can be obtained, allowing to report on the activity and distribution of caspase-3 within DOX-treated tumors, which was helpful for early monitoring of tumor response to therapy. We provide detailed protocols for the synthesis, in vitro characterization and in vivo applications of 1-RGD.

Rational design of genetically encoded reporter genes for optical imaging of apoptosis

Apoptosis is a process in which cells are genetically regulated to cause a series of changes in morphology and metabolic activity, which ultimately lead to cell death. Apoptosis plays a vital role in the entire life cycle of an organism. Too much or too little apoptosis can cause a variety of diseases. Therefore, efficient and convenient methods for detecting apoptosis are necessary for clinical treatment and drug development. Traditional methods for detecting apoptosis may cause damage to the body during sample collection, such as for flow cytometry analysis. So it is necessary to monitor apoptosis without invasion in vivo. Optical imaging technique provides a more sensitive and economical way for apoptosis visualization. A subset of engineered reporter genes based on fluorescent proteins or luciferases are currently developed to monitor the dynamic changes in apoptotic markers, such as activation of caspases and exposure of phosphatidylserine on the surface of dying cells. These reporters detect apoptosis when cells have not undergone significant morphological changes, providing conditions for early diagnosis of tumors. In addition, these reporters show considerable value in high-throughput screening of apoptosis-related drugs and evaluation of their efficacy in treating tumors. In this review, we will discuss the recent research progress in the optical imaging of apoptosis based on the genetically encoded reporter genes.

Recent development of near-infrared photoacoustic probes based on small-molecule organic dye

Photoacoustic imaging (PAI), which integrates the higher spatial resolution of optical imaging and the deeper penetration depth of ultrasound imaging, has attracted great attention. Various photoacoustic probes including inorganic and organic agents have been well fabricated in last decades. Among them, small-molecule based agents are most promising candidates for preclinical/clinical applications due to their favorite in vivo features and facile functionalization. In recent years, PAI, in the near-infrared region (NIR, 700-1700 nm) has developed rapidly and has made remarkable achievements in the biomedical field. Compared with the visible light region (400-700 nm), it can significantly reduce light scattering and meanwhile provide deeper tissue penetration. In this review, we discuss the recent developments of near-infrared photoacoustic probes based on small molecule dyes, which focus on their "always on" and "activatable" form in biomedicine. Further, we also suggest current challenges and perspectives.

A Photoacoustic Probe for the Imaging of Tumor Apoptosis by Caspase-Mediated Macrocyclization and Self-Assembly

Photoacoustic (PA) imaging shows promise in the sensitive detection of caspase-3 activated in early tumor apoptosis in response to chemotherapy; smart PA probes are thus in high demand. Herein, we report the first smart PA probe (1-RGD) responsive to caspase-3, enabling real-time and high-resolution imaging of tumor apoptosis. 1-RGD is designed to leverage the synergetic effect of active delivery and caspase-3 activation. It is selectively recognized by active caspase-3 to trigger peptide substrate cleavage and biocompatible macrocyclization-mediated self-assembly, leading to an amplified PA imaging signal and prolonged retention in apoptotic tumor cells. Strong, high-resolution PA images are obtained in chemotherapy-induced apoptotic tumors in living mice after intravenous administration with 1-RGD, facilitating sensitive reporting of caspase-3 activity and distribution within tumor tissues.

Advances in nanomaterial based optical biosensing and bioimaging of apoptosis via caspase-3 activity: a review

Caspase-3 plays a vital role in intrinsic and extrinsic pathways of programed cell death and in cell proliferation. Its detection is an important tool for early detection of some cancers and apoptosis-related diseases, and for monitoring the efficacy of pharmaceuticals and of chemo- and radiotherapy of cancers. This review (with 72 references) summarizes nanomaterial based methods for signal amplification in optical methods for the determination of caspase-3 activity. Following an introduction into the field, a first large section covers optical assays, with subsections on luminescent and chemiluminescence, fluorometric (including FRET based), and colorimetric assays. Further section summarize methods for bioimaging of caspase-3. A concluding section covers current challenges and future perspectives. Graphical Abstract ᅟ.

Reporter-Based BRET Sensors for Measuring Biological Functions In Vivo

Genetic reporter systems provide a good alternative to monitor cellular functions in vitro and in vivo and are contributing immensely in experimental research. Reporters like fluorescence and bioluminescence genes, which support optical measurements, provide exquisite sensitivity to the assay systems. In recent years several activatable strategies have been developed, which can relay specialized molecular functions from inside the cells. The application of bioluminescence resonance energy transfer (BRET) is one such strategy that has been proved to be extremely valuable as an in vitro or in vivo assay to measure dynamic events such as protein-protein interactions (PPIs).The BRET assay using RLuc-YFP was introduced in biological research in the late 1990s and demonstrated the interaction of two proteins involved in circadian rhythm. Since then, BRET has become a popular genetic reporter-based assay for PPI studies due to several inherent attributes that facilitate high-throughput assay development such as rapid and fairly sensitive ratio-metric measurement, the assessment of PPI irrespective of protein location in cellular compartment and cost effectiveness. In BRET-based screening, within a defined proximity range of 10-100 Å, the excited energy state of the luminescent molecule excites the acceptor fluorophore in the form of resonance energy transfer, causing it to emit at its characteristic emission wavelength. Based on this principle, several such donor-acceptor pairs, using Renilla luciferase or its mutants as donor and either GFP2, YFP, mOrange, TagRFP or TurboFP as acceptor, have been reported for use.In recent years, the applicability of BRET has been greatly enhanced by the adaptation of the assay to multiple detection devices such as a luminescence plate reader, a bioluminescence microscope and a small animal optical imaging platform. Apart from quantitative measurement studies of PPIs and protein dimerization, molecular spectral imaging has expanded the scope for fast screening of pharmacological compounds that modulate PPIs by unifying in vitro, live cell and in vivo animal/plant measurement, all using one assay. Using examples from the literature, we will describe methods to perform in vitro and in vivo BRET imaging experiments and some of its applications.

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.

Noninvasive bioluminescence imaging of the dynamics of sanguinarine induced apoptosis via activation of reactive oxygen species

Most chemotherapeutic drugs exert their anti-tumor effects primarily by triggering a final pathway leading to apoptosis. Noninvasive imaging of apoptotic events in preclinical models would greatly facilitate the development of apoptosis-inducing compounds and evaluation of their therapeutic efficacy. Here we employed a cyclic firefly luciferase (cFluc) reporter to screen potential pro-apoptotic compounds from a number of natural agents. We demonstrated that sanguinarine (SANG) could induce apoptosis in a dose- and time-dependent manner in UM-SCC-22B head and neck cancer cells. Moreover, SANG-induced apoptosis was associated with the generation of reactive oxygen species (ROS) and activation of c-Jun-N-terminal kinase (JNK) and nuclear factor-kappaB (NF-κB) signal pathways. After intravenous administration with SANG in 22B-cFluc xenograft models, a dramatic increase of luminescence signal can be detected as early as 48 h post-treatment, as revealed by longitudinal bioluminescence imaging in vivo. Remarkable apoptotic cells reflected from ex vivo TUNEL staining confirmed the imaging results. Importantly, SANG treatment caused distinct tumor growth retardation in mice compared with the vehicle-treated group. Taken together, our results showed that SANG is a candidate anti-tumor drug and noninvasive imaging of apoptosis using cFluc reporter could provide a valuable tool for drug development and therapeutic efficacy evaluation.

Small Molecule Active Site Directed Tools for Studying Human Caspases

Caspases are proteases of clan CD and were described for the first time more than two decades ago. They play critical roles in the control of regulated cell death pathways including apoptosis and inflammation. Due to their involvement in the development of various diseases like cancer, neurodegenerative diseases, or autoimmune disorders, caspases have been intensively investigated as potential drug targets, both in academic and industrial laboratories. This review presents a thorough, deep, and systematic assessment of all technologies developed over the years for the investigation of caspase activity and specificity using substrates and inhibitors, as well as activity based probes, which in recent years have attracted considerable interest due to their usefulness in the investigation of biological functions of this family of enzymes.

A reliable self-assembled peptide based electrochemical biosensor for detection of caspase 3 activity and apoptosis

A sensitive electrochemical self-assembled peptide based biosensor was developed for the detection of caspase 3 activity and apoptosis using a Asp-Glu-Val-Asp (DEVD) modified peptide and horseradish peroxidase (HRP) as cleaving and electron transfer agents, respectively.

Distinct optical and kinetic responses from E/Z isomers of caspase probes with aggregation-induced emission characteristics

A dual-labeled probe for monitoring caspase activity was designed and synthesized based on a tetraphenylethene (TPE) fluorogen with aggregation-induced emission characteristics and a caspase-specific Asp-Glu-Val-Asp (DEVD) peptide. Two stereoisomers were furnished and successfully separated by HPLC. We demonstrated for the first time the effect of isomerization on the reaction kinetics between the probes and caspase. It was revealed that caspase can produce a much higher light-up ratio for the Z-TPE-2DEVD probe, while its kinetics favor E-TPE-2DEVD due to enhanced probability of optimal binding between the two. Understanding the stereoisomers and their biological functions will open new opportunities for bioprobe design with optimized performance.

A cyclic HSV1-TK reporter for real-time PET imaging of apoptosis

The coordination of cell proliferation and programmed death (apoptosis) is essential for normal physiology, and imbalance in these two opposing processes is implicated in various diseases. Objective and quantitative noninvasive imaging of apoptosis would significantly facilitate rapid screening as well as validation of therapeutic chemicals. Herein, we molecularly engineered an apoptosis switch-on PET-based cyclic herpes simplex virus type 1-thymidine kinase reporter (cTK266) containing a caspase-3 recognition domain as the switch. Translation of the reporter and protein splicing in healthy mammalian cells produce an inactive cyclic chimera. Upon apoptosis, caspase-3-specific cleavage of the circular product occurs, resulting in the restoration of the thymidine kinase activity, which can be detected in living cells and animals by noninvasive PET imaging. Our results showed the high sensitivity of this reporter in dynamic and quantitative imaging of apoptosis in living subjects. This reporter could be applied as a valuable tool for high-throughput functional screening of proapoptotic and antiapoptotic compounds in preclinical models in drug development, and monitoring the destination of therapeutic cells in clinical settings.

Measuring caspase activity in vivo

Caspases are a family of integral proteases playing a role in apoptosis. The importance of apoptosis in disease has made these proteases not only an attractive drug target but also a focal point for measuring apoptosis in vivo. The critical role caspases play in determining cell death has led to the development of a wide array of technologies to measure caspase activity in vivo, ranging from small molecule PET imaging reagents to fluorescent and luminescent protein-based reporters used in whole animal and cell-based applications. This chapter reviews this wide range of technologies available as well as the most appropriate applications for each reagent and the mechanism of how it measures caspase activity in vivo.

A biocompatible in vivo ligation reaction and its application for noninvasive bioluminescent imaging of protease activity in living mice

The discovery of biocompatible reactions had a tremendous impact on chemical biology, allowing the study of numerous biological processes directly in complex systems. However, despite the fact that multiple biocompatible reactions have been developed in the past decade, very few work well in living mice. Here we report that D-cysteine and 2-cyanobenzothiazoles can selectively react with each other in vivo to generate a luciferin substrate for firefly luciferase. The success of this "split luciferin" ligation reaction has important implications for both in vivo imaging and biocompatible labeling strategies. First, the production of a luciferin substrate can be visualized in a live mouse by bioluminescence imaging (BLI) and furthermore allows interrogation of targeted tissues using a "caged" luciferin approach. We therefore applied this reaction to the real-time noninvasive imaging of apoptosis associated with caspase 3/7. Caspase-dependent release of free D-cysteine from the caspase 3/7 peptide substrate Asp-Glu-Val-Asp-D-Cys (DEVD-(D-Cys)) allowed selective reaction with 6-amino-2-cyanobenzothiazole (NH(2)-CBT) in vivo to form 6-amino-D-luciferin with subsequent light emission from luciferase. Importantly, this strategy was found to be superior to the commercially available DEVD-aminoluciferin substrate for imaging of caspase 3/7 activity. Moreover, the split luciferin approach enables the modular construction of bioluminogenic sensors, where either or both reaction partners could be caged to report on multiple biological events. Lastly, the luciferin ligation reaction is 3 orders of magnitude faster than Staudinger ligation, suggesting further applications for both bioluminescence and specific molecular targeting in vivo.

A tunable, modular approach to fluorescent protease-activated reporters

Proteases are one of the most important and historically utilized classes of drug targets. To effectively interrogate this class of proteins, which encodes nearly 2% of the human proteome, it is necessary to develop effective and cost-efficient methods that report on their activity both in vitro and in vivo. We have developed a robust reporter of caspase proteolytic activity, called caspase-activatable green fluorescent protein (CA-GFP). The caspases play central roles in homeostatic regulation, as they execute programmed cell death, and in drug design, as caspases are involved in diseases ranging from cancer to neurodegeneration. CA-GFP is a genetically encoded dark-to-bright fluorescent reporter of caspase activity in in vitro, cell-based, and animal systems. Based on the CA-GFP platform, we developed reporters that can discriminate the activities of caspase-6 and -7, two highly related proteases. A second series of reporters, activated by human rhinovirus 3C protease, demonstrated that we could alter the specificity of the reporter by reengineering the protease recognition sequence. Finally, we took advantage of the spectrum of known fluorescent proteins to generate green, yellow, cyan, and red reporters, paving the way for multiplex protease monitoring.

Longitudinal bioluminescence imaging of the dynamics of Doxorubicin induced apoptosis

Objectives: Most chemotherapy agents cause tumor cell death primarily by the induction of apoptosis. The ability to noninvasively image apoptosis in vivo could dramatically benefit pre-clinical and clinical evaluation of chemotherapeutics targeting the apoptotic pathway. This study aims to visualize the dynamics of apoptotic process with temporal bioluminescence imaging (BLI) using an apoptosis specific bioluminescence reporter gene. Methods: Both UM-SCC-22B human head and neck squamous carcinoma cells and 4T1 murine breast cancer cells were genetically modified with a caspase-3 specific cyclic firefly luciferase reporter gene (pcFluc-DEVD). Apoptosis induced by different concentrations of doxorubicin in the transfected cells was evaluated by both annexin V staining and BLI. Longitudinal BLI was performed in xenografted tumor models at different time points after doxorubicin or Doxil treatment, to evaluate apoptosis. After imaging, DNA fragmentation in apoptotic cells was assessed in frozen tumor sections using TUNEL staining. Results: Dose- and time-dependent apoptosis induced by doxorubicin in pcFluc-DEVD transfected UM-SCC-22B and 4T1 cells was visualized and quantified by BLI. Caspase-3 activation was confirmed by both caspase activity assay and Glo^TM luciferase assay. One dose of doxorubicin treatment induced a dramatic increase in BLI intensity as early as 24 h after treatment in 22B-pcFluc-DEVD xenografted tumors. Sustained signal increase was observed for the first 3 days and the fluorescent signal from ex vivo TUNEL staining was consistent with BLI imaging results. Long-term imaging revealed that BLI signal consistently increased and reached a maximum at around day 12 after the treatment with one dose of Doxil. Conclusions: BLI of apoptosis with pcFluc-DEVD as a reporter gene facilitates the determination of kinetics of the apoptotic process in a real-time manner, which provides a unique tool for drug development and therapy response monitoring.

Non-invasive imaging of phosphoinositide-3-kinase-catalytic-subunit-alpha (PIK3CA) promoter modulation in small animal models

Activation of the PI3K/Akt pathway, a critical step for survival in cancer cells is often associated with decreased sensitivity to several chemotherapeutic drugs. PIK3CA gene amplification is observed in 16–24% of epithelial ovarian cancer (EOC) patients in conjunction with p53 mutations. A 900 bp long PIK3CA promoter is shown to be negatively regulated by p53 in ovarian surface epithelial cells but the consequence of chemotherapeutic drug treatments on this promoter in ovarian cancer cells is largely unknown. We aim to study the modulation of this promoter by cisplatin using an improved fusion reporter in ovarian cancer cells and tumor xenografts by non-invasive imaging approach. A PIK3CA sensor was developed using a bi-fusion reporter from a newly constructed library of bi- and tri-fusion vectors comprising of two mutant far red fluorescent proteins (mcherry/mch and tdTomato/tdt), a mutant firefly luciferase (fluc2), and a PET reporter protein (ttk). In vivo imaging of mice implanted with 293T cells transiently expressing these bi- and tri-fusion reporters along with respective controls revealed comparable activity of each reporter in the fusion background and fluc2-tdt as the most sensitive one. Repression of the PIK3CA sensor by drugs was inversely proportional to cellular p53 level in a germline (PA1) and in an EOC (A2780) cell line but not in a p53 deficient EOC (SKOV3) cell line. Bioluminescence imaging of tumor xenografts stably expressing the PIK3CA sensor in PA1 and A2780 cells exhibited attenuating activity without any change in SKOV3 tumors expressing the PIK3CA sensor after cisplatin treatment. Sequential mutation at p53 binding sites showed gradual increase in promoter activity and decreased effects of the drugs. These newly developed PIK3CA-fluc2-tdt and the mutant reporter sensors thus would be extremely useful for screening new drugs and for functional assessment of PIK3CA expression from intact cells to living subjects.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

In vitro and in vivo antimammary tumor activities and mechanisms of the apple total triterpenoids

Consumption of apples has been linked to the prevention of various chronic diseases, including tumors and cardiovascular diseases. The apple total triterpenoid content (ATT) was extracted and concentrated from apple peels. The in vitro and in vivo antitumor activities, related antitumor mechanisms, were investigated. In vitro, ATT showed potent antiproliferative activities against human breast cancer (MCF-7, MDA-MB-231), human colon cancer (Caco-2), and human liver cancer (HepG2) cell lines. In vivo antitumor experiments showed that ATT could substantially reduce the occurrence and growth of mammary tumor with a good dose-dependent manner in a rat model. During the apoptosis in MDA-MB-231 cells induced by ATT, the caspase-independent pathway was involved in the process of apoptosis, and the mitochondrial transmembrane potential was markedly reduced. Also the PI3K/Akt/NF-κB pathway was activated. These results indicated that ATT-induced apoptosis of MDA-MB-231 cells may involve a mitochondrial-related pathway.

Bioluminescence based in vivo screening technologies

Bioluminescence is the biologically active luminescence light producing event encountered in nature. In recent years several new screening methods utilizing bioluminescent cell-based biosensors have been designed demonstrating their utility towards dynamic monitoring of a variety of cellular functions. Because luciferase is unnatural to mammalian physiology, assays utilizing specific substrates to yield a luminescent signal are attractive and serve the purpose with high sensitivity and specificity. Often genetic or chemical modifications in different luciferase-substrate system in use have afforded new functionalities making these assays even more robust. Finally, in the evolving paradigm of molecular imaging, in vivo bioluminescence imaging (BLI) has evolved as a very attractive tool for interrogating human cellular biology in rodent models. In this short review we explore various bioluminescence screening strategies developed and analyze their scope in future drug screening processes.

Molecular imaging: challenges of bringing imaging of intracellular targets into common clinical use

Molecular imaging (MI) takes advantage of several new techniques to detect biomarkers or biochemical and cellular processes, with the goal of obtaining high sensitivity, specificity and signal-to-noise ratio imaging of disease. The imaging modalities bearing the most promise for MI are positron emission tomography (PET), single photon emission computer tomography (SPECT) and different optical imaging techniques with high sensitivity. Also magnetic resonance imaging (MRI) with contrast agents like ultra-small superparamagnetic iron oxide particles (USPIO), magnetic resonance spectroscopy and ultrasound imaging with contrast agents may be useful approaches. MI techniques have been used in the clinic for many years, i.e. PET imaging using (18)  F-labeled fluorodeoxyglucose. Animal studies have during the last years revealed great potential for MI also with several other agents. The focus of the present article is the challenges of clinical imaging of intracellular targets following intravenous injection of the agents. Thus, the great challenge of getting enough contrast agent into the cytosol and at the same time obtaining a low signal from tissue just outside the diseased area is discussed.

Molecular imaging with activatable reporter systems

Molecular imaging is a newly emerged multiple disciplinary field that aims to visualize, characterize and quantitatively measure biological processes at cellular and molecular levels in humans and other living systems. A reporter gene is a piece of DNA encoding reporter protein, which presents as a readily measurable phenotype that can be distinguished easily from the background of endogenous protein. After being transferred into cells of organ systems (transgenes), the reporter gene can be utilized to visualize transcriptional and posttranscriptional regulation of gene expression, protein-protein interactions, or trafficking of proteins or cells in living subjects. Herein, we review previous classification of reporter genes and regroup the reporter gene based imaging as basic, inducible and activatable, based on the regulation of reporter gene transcription and post-translational modification of reporter proteins. We then focus on activatable reporters, in which the signal can be activated at the posttranslational level for visualizing protein-protein interactions, protein phosphorylation or tertiary structure changes. The applications of several types of activatable reporters will also be summarized. We conclude that activatable reporter imaging can benefit both basic biomedical research and drug development.

In vivo monitoring of liver damage using caspase-3 probe

Real-time monitoring of cellular and organ conditions improves our understanding of various physiopathological phenomena. Such monitoring is expected to provide important alternatives for clinical diagnosis and therapy. We have sought to show physiopathological changes of organs as well as cells. Here, we present an example of in vivo imaging of liver states using the luciferase-based caspase-3 optical probe. We examined dynamic changes of apoptosis (caspase-3 activity) of a mouse liver as well as those of liver cells, proving that the emitted signals reflected the biochemically evaluated apoptotic cell death. In live liver cell (AML 12) experiments, the optical probe for caspase-3 activity emitted signals in response to Fas-ligand, staurosporine and hypoxia/reoxygenation, demonstrating that the probe can measure cellular apoptosis quantitatively. We therefore applied this probe for mouse liver ischemia/reperfusion (I/R) and drug-toxicity to liver. By expressing the probe in a mouse liver adenovirally, we imaged liver caspase-3 activity (i.e. apoptotic damage) non-invasively and chronologically in the hepatic I/R model of mice. The duration of liver ischemia affected the post-ischemic caspase-dependent damage. Ischemia (up to 60 min) enhanced liver damage after reperfusion, but prolonged ischemia (90 min of ischemia) induced not apoptotic cell death but necrotic cell death. Direct observations of the changes of organ conditions elucidated the dynamism of organ function and damage. These technologies clearly possess clinical relevance. They are expected to provide a new diagnostic tool for various clinical settings in the future.

Theranostic Imaging of the Kinases and Proteases that Modulate Cell Death and Survival

Several signaling cascades are involved in cell death, with a significant amount of crosstalk between them. Despite the complexity of these cascades several key pro-survival and pro-death players have been identified. These include PI3-kinase, AKT and caspase-3. Here we review the approaches used to date to perform molecular imaging of these important targets. We focus in particular on approaches that include the possibility of modulating the activity of these kinases and proteases in a theranostic approach.

Multimodality molecular imaging of disease progression in living subjects

The enormous advances in our understanding of the progression of diseases at the molecular level have been supplemented by the new field of 'molecular imaging', which provides for in vivo visualization of molecular events at the cellular level in living organisms. Molecular imaging is a noninvasive assessment of gene and protein function, protein-protein interaction and or signal transduction pathways in animal models of human disease and in patients to provide insights into molecular pathogenesis. Five major imaging techniques are currently available to assess the structural and functional alterations in vivo in small animals. These are (i) optical bioluminescence and fluorescence imaging techniques, (ii) radionuclide-based positron emission tomography (PET) and single photon emitted computed tomography (SPECT), (iii) X-ray-based computed tomography (CT), (iv) magnetic resonance imaging (MRI) and (v) ultrasound imaging (US). Functional molecular imaging requires an imaging probe that is specific for a given molecular event. In preclinical imaging, involving small animal models, the imaging probe could be an element of a direct ('direct imaging') or an indirect ('indirect imaging') event. Reporter genes are essential for indirect imaging and provide a general integrated platform for many different applications. Applications of multimodality imaging using combinations of bioluminescent, fluorescent and PET reporter genes in unified fusion vectors developed by us for recording events from single live cells to whole animals with high sensitivity and accurate quantification are discussed. Such approaches have immense potential to track progression of metastasis, immune cell trafficking, stem cell therapy, transgenic animals and even molecular interactions in living subjects.

In vivo imaging of early stage apoptosis by measuring real-time caspase-3/7 activation

In vivo imaging of apoptosis in a preclinical setting in anticancer drug development could provide remarkable advantages in terms of translational medicine. So far, several imaging technologies with different probes have been used to achieve this goal. Here we describe a bioluminescence imaging approach that uses a new formulation of Z-DEVD-aminoluciferin, a caspase 3/7 substrate, to monitor in vivo apoptosis in tumor cells engineered to express luciferase. Upon apoptosis induction, Z-DEVD-aminoluciferin is cleaved by caspase 3/7 releasing aminoluciferin that is now free to react with luciferase generating measurable light. Thus, the activation of caspase 3/7 can be measured by quantifying the bioluminescent signal. Using this approach, we have been able to monitor caspase-3 activation and subsequent apoptosis induction after camptothecin and temozolomide treatment on xenograft mouse models of colon cancer and glioblastoma, respectively. Treated mice showed more than 2-fold induction of Z-DEVD-aminoluciferin luminescent signal when compared to the untreated group. Combining D: -luciferin that measures the total tumor burden, with Z-DEVD-aminoluciferin that assesses apoptosis induction via caspase activation, we confirmed that it is possible to follow non-invasively tumor growth inhibition and induction of apoptosis after treatment in the same animal over time. Moreover, here we have proved that following early apoptosis induction by caspase 3 activation is a good biomarker that accurately predicts tumor growth inhibition by anti-cancer drugs in engineered colon cancer and glioblastoma cell lines and in their respective mouse xenograft models.

Design of "smart" probes for optical imaging of apoptosis

Apoptosis is a mode of programmed cell death in multicellular organisms and plays a central role in controlling embryonic development, growth and differentiation and monitoring the induction of tumor cell death through anticancer therapy. Since the most effective chemotherapeutics rely on apoptosis, imaging apoptotic processes can be an invaluable tool to monitor therapeutic intervention and discover new drugs modulating apoptosis. The most attractive target for developing specific apoptosis imaging probes is caspases, crucial mediators of apoptosis. Up to now, various optical imaging strategies for apoptosis have been developed as an easy and economical modality. However, current optical applications are limited by poor sensitivity and specificity. A subset of molecular imaging contrast agents known as "activatable" or "smart" molecular probes allow for very high signal-to-background ratios compared to conventional targeted contrast agents and open up the possibility of imaging intracellular targets. In this review, we will discuss the unique design strategies and applications of activatable probes recently developed for fluorescence and bioluminescence imaging of caspase activity.

Multimodality imaging of tumor response to doxil

Purpose: Early assessment of tumor responses to chemotherapy could enhance treatment outcomes by ensuring that, from the beginning, treatments meet the individualized needs of patients. In this study, we applied multiple modality molecular imaging techniques to pre-clinical monitoring of early tumor responses to Doxil, focusing on imaging of apoptosis.

Methods: Mice bearing UM-SCC-22B human head and neck squamous cancer tumors received either PBS or 1 to 2 doses of Doxil® (doxorubicin HCl liposome injection) (10 mg/kg/dose). Bioluminescence signals from an apoptosis-responsive reporter gene were captured for apoptosis evaluation. Tumor metabolism and proliferation were assessed by ¹⁸F-FDG and 3'-¹⁸F-fluoro-3'-deoxythymidine (¹⁸F-FLT) positron emission tomography. Diffusion-weighted magnetic resonance imaging (DW-MRI) was performed to calculate averaged apparent diffusion coefficients (ADCs) for the whole tumor volume. After imaging, tumor samples were collected for histological evaluation, including terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), anti-CD31, and Ki-67 immunostaining.

Results: Two doses of Doxil significantly inhibited tumor growth. Bioluminescence imaging (BLI) indicated apoptosis of tumor cells after just 1 dose of Doxil treatment, before apparent tumor shrinkage. ¹⁸F-FDG and ¹⁸F-FLT PET imaging identified decreased tumor metabolism and proliferation at later time points than those at which BLI indicated apoptosis. MRI measurements of ADC altered in response to Doxil, but only after tumors were treated with 2 doses. Decreased tumor proliferation and increased apoptotic cells were confirmed by changes of Ki-67 index and apoptotic ratio.

Conclusion: Our study of tumor responses to different doses of Doxil demonstrated that it is essential to combine apoptosis imaging strategies with imaging of other critical biological or pathological pathways, such as metabolism and proliferation, to improve clinical decision making in apoptosis-related diseases and interventions.

Mechanism of a genetically encoded dark-to-bright reporter for caspase activity

Fluorescent proteins have revolutionized modern biology with their ability to report the presence of tagged proteins in living systems. Although several fluorescent proteins have been described in which the excitation and emission properties can be modulated by external triggers, no fluorescent proteins have been described that can be activated from a silent dark state to a bright fluorescent state directly by the activity of an enzyme. We have developed a version of GFP in which fluorescence is completely quenched by appendage of a hydrophobic quenching peptide that tetramerizes GFP and prevents maturation of the chromophore. The fluorescence can be fully restored by catalytic removal of the quenching peptide, making it a robust reporter of proteolysis. We have demonstrated the utility of this uniquely dark state of GFP as a genetically encoded apoptosis reporter that monitors the function of caspases, which catalyze the fate-determining step in programmed cell death. Caspase Activatable-GFP (CA-GFP) can be activated both in vitro and in vivo, resulting in up to a 45-fold increase in fluorescent signal in bacteria and a 3-fold increase in mammalian cells. We used CA-GFP successfully to monitor real-time apoptosis in mammalian cells. This dark state of GFP may ultimately serve as a useful platform for probes of other enzymatic processes.

Multimodality imaging of β-cells in mouse models of type 1 and 2 diabetes

OBJECTIVE

β-Cells that express an imaging reporter have provided powerful tools for studying β-cell development, islet transplantation, and β-cell autoimmunity. To further expedite diabetes research, we generated transgenic C57BL/6 "MIP-TF" mice that have a mouse insulin promoter (MIP) driving the expression of a trifusion (TF) protein of three imaging reporters (luciferase/enhanced green fluorescent protein/HSV1-sr39 thymidine kinase) in their β-cells. This should enable the noninvasive imaging of β-cells by charge-coupled device (CCD) and micro-positron emission tomography (PET), as well as the identification of β-cells at the cellular level by fluorescent microscopy.

RESEARCH DESIGN AND METHODS

MIP-TF mouse β-cells were multimodality imaged in models of type 1 and type 2 diabetes.

RESULTS

MIP-TF mouse β-cells were readily identified in pancreatic tissue sections using fluorescent microscopy. We show that MIP-TF β-cells can be noninvasively imaged using microPET. There was a correlation between CCD and microPET signals from the pancreas region of individual mice. After low-dose streptozotocin administration to induce type 1 diabetes, we observed a progressive reduction in bioluminescence from the pancreas region before the appearance of hyperglycemia. Although there have been reports of hyperglycemia inducing proinsulin expression in extrapancreatic tissues, we did not observe bioluminescent signals from extrapancreatic tissues of diabetic MIP-TF mice. Because MIP-TF mouse β-cells express a viral thymidine kinase, ganciclovir treatment induced hyperglycemia, providing a new experimental model of type 1 diabetes. Mice fed a high-fat diet to model early type 2 diabetes displayed a progressive increase in their pancreatic bioluminescent signals, which were positively correlated with area under the curve-intraperitoneal glucose tolerance test (AUC-IPGTT).

CONCLUSIONS

MIP-TF mice provide a new tool for monitoring β-cells from the single cell level to noninvasive assessments of β-cells in models of type 1 diabetes and type 2 diabetes.

Multimodal in vivo imaging and blood monitoring of intrinsic and extrinsic apoptosis

Noninvasive detection and in vivo imaging of apoptosis plays a critical role in the development of therapeutics in many different fields including cancer. We have developed an apoptosis biosensor by fusing green fluorescent protein (GFP) to the N-terminus of the naturally secreted Gaussia luciferase separated by a caspase-3 cleavage peptide consisting of aspartic acid (D), glutamic acid (E), valine (V), and aspartic acid (D) or DEVD. We showed that this fusion is retained in the cytoplasm of cells in an inactive form. Upon apoptosis, the DEVD peptide is cleaved in response to caspase-3 activation, freeing ssGluc, which can now enter the secretory pathway where it is folded properly and is released from the cells and can be detected in the conditioned medium in culture or in blood of live animals ex vivo over time. Because Gluc is secreted from cells via conventional pathway through the endoplasmic reticulum (ER), Golgi and vesicles, we showed that the presence of Gluc in these compartments in response to apoptosis can be visualized in vivo using bioluminescence imaging. This reporter provides a valuable tool for imaging and real-time monitoring of apoptosis and is compatible with high-throughput functional screening application in cultured cells and animal models.

Optical probes and the applications in multimodality imaging

Optical imaging essentially refers to in vivo fluorescence imaging and bioluminescence imaging. These types of imaging are widely used visualization methods in biomedical research and are important in molecular imaging. A new generation of imaging agents called multimodal probes have emerged in the past few years. These probes can be detected by two or more imaging modalities, which harnesses the strengths of the different modalities and enables researchers to obtain more information than can be achieved using only one modality. Owing to its low cost and the large number of probes available, the optical method plays an important role in multimodality imaging. In this mini-review, we describe the available multimodal imaging probes for in vivo imaging that combine optical imaging with other modalities.

Apoptosis imaging: beyond annexin V

Induction of apoptosis is the primary mechanism through which most chemotherapies cause tumor cell death. Early assessment of tumor response is required to manage patients in terms of quality of life versus intensive chemotherapy. Although imaging with radiolabeled annexin V has been intensively investigated, it is still not sufficiently mature for clinical application. This article will summarize various alternative imaging techniques for visualization of phosphatidylserine externalization, activity of caspases, and mitochondrial membrane potential. Such imaging studies will promote the identification of novel molecular targets and the development of highly specific apoptosis-detecting imaging probes with potential clinical applications. It is highly possible that quantitative imaging of apoptosis will greatly improve clinical decision making in apoptosis-related diseases.

Mouse models in neurological disorders: applications of non-invasive imaging

Neuroimaging techniques represent powerful tools to assess disease-specific cellular, biochemical and molecular processes non-invasively in vivo. Besides providing precise anatomical localisation and quantification, the most exciting advantage of non-invasive imaging techniques is the opportunity to investigate the spatial and temporal dynamics of disease-specific functional and molecular events longitudinally in intact living organisms, so called molecular imaging (MI). Combining neuroimaging technologies with in vivo models of neurological disorders provides unique opportunities to understand the aetiology and pathophysiology of human neurological disorders. In this way, neuroimaging in mouse models of neurological disorders not only can be used for phenotyping specific diseases and monitoring disease progression but also plays an essential role in the development and evaluation of disease-specific treatment approaches. In this way MI is a key technology in translational research, helping to design improved disease models as well as experimental treatment protocols that may afterwards be implemented into clinical routine. The most widely used imaging modalities in animal models to assess in vivo anatomical, functional and molecular events are positron emission tomography (PET), magnetic resonance imaging (MRI) and optical imaging (OI). Here, we review the application of neuroimaging in mouse models of neurodegeneration (Parkinson's disease, PD, and Alzheimer's disease, AD) and brain cancer (glioma).

Noninvasive molecular imaging of apoptosis in vivo using a modified firefly luciferase substrate, Z-DEVD-aminoluciferin

Apoptosis is a highly regulated process of programmed cell death essential for normal physiology. Dysregulation of apoptosis contributes to the development and progression of various diseases, including cancer, neurodegenerative disorders, and chronic heart failure. Quantitative noninvasive imaging of apoptosis in preclinical models would allow for dynamic longitudinal screening of compounds and facilitates a more rapid determination of therapeutic efficacy. In this study, we report the in vivo characterization of Z-DEVD-aminoluciferin, a modified firefly luciferase substrate that in apoptotic cells is cleaved by caspase-3 to liberate aminoluciferin, which can be consumed by luciferase to generate a luminescent signal. In two oncology models, namely SKOV3-luc and MDA-MB-231-luc-LN, at 24, 48, and 72 h after treatment with docetaxel, animals were injected with Z-DEVD-aminoluciferin and bioluminescent images were acquired. Significantly more light was detected at 24 (P<0.05), 48 (P<0.01), and 72 h (P<0.01) in the docetaxel-treated group compared with the vehicle-treated group, with caspase-3 activation at these time points confirmed using immunohistochemistry. Importantly, whereas significant differences between groups were detected as early as 24 h after treatment by molecular imaging, caliper measurements were unable to detect a difference for 4-5 additional days. Taken together, these data show that in vivo imaging of apoptosis using Z-DEVD-aminoluciferin could provide a sensitive and rapid method for early detection of drug efficacy, which could potentially be used by numerous therapeutic programs.

In vivo investigation of breast cancer progression by use of an internal control

Optical imaging of breast cancer has been considered for detecting functional and molecular characteristics of diseases in clinical and preclinical settings. Applied to laboratory research, photonic investigations offer a highly versatile tool for preclinical imaging and drug discovery. A particular advantage of the optical method is the availability of multiple spectral bands for performing imaging. Herein, we capitalize on this feature to demonstrate how it is possible to use different wavelengths to offer internal controls and significantly improve the observation accuracy in molecular imaging applications. In particular, we show the independent in vivo detection of cysteine proteases along with tumor permeability and interstitial volume measurements using a dual-wavelength approach. To generate results with a view toward clinically geared studies, a transgenic Her2/neu mouse model that spontaneously developed mammary tumors was used. In vivo findings were validated against conventional ex vivo tests such as histology and Western blot analyses. By correcting for biodistribution parameters, the dual-wavelength method increases the accuracy of molecular observations by separating true molecular target from probe biodistribution. As such, the method is highly appropriate for molecular imaging studies where often probe delivery and target presence are not independently assessed. On the basis of these findings, we propose the dual-wavelength/normalization approach as an essential method for drug discovery and preclinical imaging studies.

Methods to monitor gene therapy with molecular imaging

Recent progress in scientific and clinical research has made gene therapy a promising option for efficient and targeted treatment of several inherited and acquired disorders. One of the most critical issues for ensuring success of gene-based therapies is the development of technologies for non-invasive monitoring of the distribution and kinetics of vector-mediated gene expression. In recent years many molecular imaging techniques for safe, repeated and high-resolution in vivo imaging of gene expression have been developed and successfully used in animals and humans. In this review molecular imaging techniques for monitoring of gene therapy are described and specific use of these methods in the different steps of a gene therapy protocol from gene delivery to assessment of therapy response is illustrated. Linking molecular imaging (MI) to gene therapy will eventually help to improve the efficacy and safety of current gene therapy protocols for human application and support future individualized patient treatment.