Noninvasive real-time imaging of apoptosis

Harnessing luciferase chemistry in regulated cell death modalities and autophagy: overview and perspectives

Regulated cell death is a fate of cells in (patho)physiological conditions during which extrinsic or intrinsic signals or redox equilibrium pathways following infection, cellular stress or injury are coupled to cell death modalities like apoptosis, necroptosis, pyroptosis or ferroptosis. An immediate survival response to cellular stress is often induction of autophagy, a process that deals with removal of aggregated proteins and damaged organelles by a lysosomal recycling process. These cellular processes and their regulation are crucial in several human diseases. Exploiting high-throughput assays which discriminate distinct cell death modalities and autophagy are critical to identify potential therapeutic agents that modulate these cellular responses. In the past few years, luciferase-based assays have been widely developed for assessing regulated cell death and autophagy pathways due to their simplicity, sensitivity, known chemistry, different spectral properties and high-throughput potential. Here, we review basic principles of bioluminescent reactions from a mechanistic perspective, along with their implication in vitro and in vivo for probing cell death and autophagy pathways. These include applying luciferase-, luciferin-, and ATP-based biosensors for investigating regulated cell death modalities. We discuss multiplex bioluminescence platforms which simultaneously distinguish between the various cell death phenomena and cellular stress recovery processes such as autophagy. We also highlight the recent technological achievements of bioluminescent tools for the prediction of drug effectiveness in pathways associated with regulated cell death.

Detecting and monitoring tumors in orthotopic colorectal liver metastatic animal models with high-resolution ultrasound

The ability to noninvasively detect and monitor the growth of orthotopic liver transplantation tumors is critical for replicating advanced colorectal cancer liver metastases (CRLMs) in animal models. We assessed the use of high-resolution ultrasound (HRU) to monitor CRLMs transplanted using various cell concentrations. Sixty BALB/c female mice were randomly divided into 3 groups, and murine colonic CT26 cells were injected into the left liver lobe at concentrations of 1 × 10² (group 1), 1 × 10³ (group 2), or 1 × 10⁴ (group 3). Tumor presentation, location, number, size, shape, and echogenicity were assessed daily with 24-MHz center frequency HRU starting 6 days after injection. Animals were sacrificed when the largest tumor was ≥ 1 cm in diameter. Sensitivity, specificity, and area under curve (AUC) of CRLMs diagnosed with HRU were calculated using receiver operating characteristic curve analysis. In group 1, 94% of mice formed < 5 tumors, and 41% formed a single tumor. Tumors were first detected with HRU on day 12 in group 1, day 10 in group 2, and day 7 in group 3; tumor volume doubling times were 14-15 days, 11-12 days, and 7-8 days, respectively. With a long diameter threshold of 2.4 mm, diagnostic sensitivity and specificity of HRU were 94.1% and 88.7%, respectively, and the AUC was 0.962. These findings suggest that HRU can be used to accurately detect and monitor the growth of CRLMs in an orthotopic transplantation mouse model, especially when a lower concentration of cells is used.

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.

A highly selective iron oxide-based imaging nanoparticle for long-term monitoring of drug-induced tumor cell apoptosis

The ability to visualize and quantify apoptosis in vivo is critical to monitoring the disease response to treatment and providing prognostic information. However, the application of current apoptosis labeling probes faces significant challenges including nonspecific tissue uptake, inefficient apoptotic cell labeling and short monitoring windows. Here we report a highly specific apoptosis labeling nanoparticle (NP) probe with Pisum sativum agglutinin (PSA) as a tumor targeting ligand for prolonged in vivo apoptosis imaging. The NP (namely, IONP-Neu-PSA) consists of a magnetic iron oxide core (IONP) conjugated with PSA, and a reporter fluorophore. IONP-Neu-PSA demonstrated minimal cytotoxicity and high labeling specificity towards apoptotic cells in vitro. When applied in vivo, IONP-Neu-PSA tracks apoptotic tumors for a prolonged period of two weeks under near-IR imaging with low background noise. Moreover, IONP-Neu-PSA possesses T2 contrast enhancing properties that can potentially enable apoptosis detection by magnetic resonance imaging (MRI). The high specificity for apoptotic cells, sustained fluorescence signals, and non-invasive imaging capability exhibited by IONP-Neu-PSA make it a versatile tool for cancer treatment monitoring and pathological research.

Optical imaging with a novel cathepsin-activatable probe for enhanced detection of colorectal cancer

We evaluated a cysteine cathepsin-activatable optical imaging probe (LUM015) with improved kinetics relative to larger macromolecules for detection and characterization of colorectal cancer (CRC), and thereby assessed its potential use in fluorescence-guided colonoscopy. We showed that LUM015 is stable in plasma. In-vitro studies demonstrated selectivity of LUM015 for targeting cathepsins; there was robust increase in emitted fluorescence signal from the cathepsin overexpressing HT-29 CRC cells within 1-5 minutes after incubation with LUM015 compared to the cells incubated with combination of LUM015 and a pan-protease inhibitor (as negative control). Biodistribution, differential accumulation of the probe in the tumor and tumor-to-background fluorescence signal ratio of LUM015 were compared to ProSense680, a commercially available protease-activatable optical imaging probe, over 24 hours after intravenous injection of the probes in nude mice with subcutaneously implanted HT-29 tumors. LUM015 showed distinct kinetics compared to ProSense680 with time to peak signal for subcutaneous tumor-to-colon ratio of 3.3±0.3 (mean ± SD) at 4-8 hours compared to 2.9±0.2 at 24 hours, respectively (n=8 for each group). Near-infrared fluorescence imaging and dual channel colonoscopy of the mice with orthotopic colon tumors showed tumor-to-colon ratio of 3.7±0.2 in HT-29 tumors (n=4), 2.8±0.1 in genetically engineered mice with APC^KOKras^LSL-G12Dp53^flox/flox mutation (n=4), and 4.1±0.1 in mice with APC^LoxP/LoxPMsh2^LoxP/LoxP mutation (n=4) at 6 hours after LUM015 administration. Immunohistochemistry and laser confocal microscopy of the extracted tumors confirmed high expression of cysteine cathepsins in all colon tumor types tested. Optical imaging with cathepsin-activatable LUM015 in multiple models of CRC highlights its potential for increasing the efficacy of CRC screening and therapeutic procedures.

Positron Emission Tomography Imaging of Tumor Apoptosis with a Caspase-Sensitive Nano-Aggregation Tracer [18F]C-SNAT

Cellular apoptosis is an important criterion for evaluating the efficacy of cancer therapies. We have developed a new small molecule probe ([¹⁸F]C-SNAT) for positron emission tomography (PET) imaging of apoptosis. [¹⁸F]C-SNAT, when activated by caspase-3 and glutathione reduction, undergoes intramolecular cyclization followed by self-assembly to form nano-aggregates in apoptotic cells. This unique mechanism creates preferential retention of gamma radiation signals in targeted cells and thus enables the detection of apoptosis using PET, a sensitive and clinically practical technique. This protocol describes the chemical synthesis, radiolabeling and PET imaging of apoptosis using this probe.

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.

Nanostructured Ti surfaces and retinoic acid/dexamethasone present a spatial framework for the maturation and amelogenesis of LS-8 cells

PURPOSE

To investigate the amelogenesis-inductive effects of surface structures at the nanoscale. For this purpose, variable nanostructured titanium dioxide (TiO2) surfaces were used as a framework to regulate the amelogenic behaviors of ameloblasts with the administration of retinoic acid (RA)/dexamethasone (DEX).

MATERIALS AND METHODS

TiO2 nanotubular (NT) surfaces were fabricated via anodization. Mouse ameloblast-like LS-8 cells were seeded and cultured on NT surfaces in the presence or absence of RA/DEX for 48 h. The amelogenic behaviors and extracellular matrix (ECM) mineralization of LS-8 cells on nanostructured Ti surfaces were characterized using field emission scanning electron microscope, laser scanning confocal microscope, quantitative polymerase chain reaction, MTT assay, and flow cytometry.

RESULTS

TiO2 NT surfaces (tube size ~30 and ~80 nm) were constructed via anodization at 5 or 20 V and denoted as NT5 and NT20, respectively. LS-8 cells exhibited significantly increased spread and proliferation, and lower rates of apoptosis and necrosis on NT surfaces. The amelogenic gene expression and ECM mineralization differed significantly on the NT20 and the NT5 and polished Ti sample surfaces in standard medium. The amelogenic behaviors of LS-8 cells were further changed by RA/DEX pretreatment, which directly drove maturation of LS-8 cells.

CONCLUSION

Controlling the amelogenic behaviors of ameloblast-like LS-8 cells by manipulating the nanostructure of biomaterials surfaces represents an effective tool for the establishment of a systemic framework for supporting enamel regeneration. The administration of RA/DEX is an effective approach for driving the amelogenesis and maturation of ameloblasts.

Caspase-3/-7-Specific Metabolic Precursor for Bioorthogonal Tracking of Tumor Apoptosis

Apoptosis is one of the most important intracellular events in living cell, which is a programmed cell death interrelated with caspase enzyme activity for maintaining homeostasis in multicellular organisms. Therefore, direct apoptosis imaging of living cells can provide enormous advantages for diagnosis, drug discovery, and therapeutic monitoring in various diseases. However, a method of direct apoptosis imaging has not been fully validated, especially for live cells in in vitro and in vivo. Herein, we developed a new apoptosis imaging technology via a direct visualization of active caspase-3/-7 activity in living cells. For this, we synthesized a caspase-3/-7-specific cleavable peptide (KGDEVD) conjugated triacetylated N-azidoacetyl-D-mannosamine (Apo-S-Ac₃ManNAz), wherein the Apo-S-Ac₃ManNAz can be cleaved by the active caspase-3/-7 in live apoptotic cells and the cleaved Ac₃ManNAz molecules can further generate targetable azido groups (N₃) on the living cell surface. Importantly, the azido groups on the apoptotic tumor cells could be visualized with Cy5.5-conjugated dibenzylcyclooctyne (DBCO-Cy5.5) via bioorthogonal click chemistry in vitro cell culture condition and in vivo tumor-bearing mice. Therefore, our Apo-S-Ac₃ManNAz can be utilized for the further applications in tumor therapy as a monitoring tool for anticancer efficacy and optimization of anticancer new drugs in cell culture system and in tumor-bearing mice.

Bone mesenchymal stem cell secretion of sRANKL/OPG/M-CSF in response to macrophage-mediated inflammatory response influences osteogenesis on nanostructured Ti surfaces

Although it has been well established that osteogenic differentiation of bone mesenchymal stem cells (bMSCs) as well as osteoclastic differentiation of macrophages can be manipulated by the nanostructure of biomaterial surfaces, the interactions among the effects of the surface on immune cells and bMSCs remained unknown. Therefore, in this study, the osteogenic behaviors and secretion of osteoclastogenesis-related cytokines of human bMSCs on TiO₂ nanotubular (NT) surfaces in conditioned medium (CM) generated by macrophages cultured on the respective NT surfaces (NT-CM) were analyzed. Although bMSCs showed consistent osteogenic behaviors on the NT5 and NT20 surfaces in both standard culture medium and both types of NT-CM, collagen synthesis and extracellular matrix mineralization were partially impeded on the NT20 surface in NT20-CM and bMSC cytokine secretions on the NT20 surface in NT20-CM elicited remarkable multinuclear giant cell and osteoclast formation compared with that observed on the NT5 surface in NT5-CM. After implantation in vivo, mineralized bone formation was significantly delayed around the NT20 implant compared with the NT5 implant, but both surfaces contributed to good bone formation after 12 weeks. The results obtained in this study advance our understanding of the confounding influence of the implant surface nanostructure, macrophage inflammatory response, and osteogenic differentiation of bMSCs as well as the retro-regulative effects of bMSCs on the osteoclastic differentiation of macrophages, and the culture system based on different NT surfaces and CM generated on the respective surfaces may provide a systematic research model for evaluating the performance of endosseous implants as well as a prospective approach for improving implant osseointegration via immune-regulation.

Effects of Combined Simultaneous and Sequential Endostar and Cisplatin Treatment in a Mice Model of Gastric Cancer Peritoneal Metastases

Objective. Aimed to study the effects of endostar and cisplatin using an in vivo imaging system (IVIS) in a model of peritoneal metastasis of gastric cancer. Methods. NUGC-4 gastric cancer cells transfected with luciferase gene (NUGC-4-Luc) were injected i.p. into nude mice. One week later, mice were randomly injected i.p.: group 1, cisplatin (d1-3) + endostar (d4-7); group 2, endostar (d1-4) + cisplatin (d5-7); group 3, endostar + cisplatin d1, 4, and 7; group 4, saline for two weeks. One week after the final administration, mice were sacrificed. Bioluminescent data, microvessel density (MVD), and lymphatic vessel density (LVD) were analyzed. Results. Among the four groups, there were no significant differences in the weights and in the number of cancer cell photons on days 1 and 8 (P > 0.05). On day 15, the numbers in groups 3 and 1 were less than that in group 2 (P < 0.05). On day 21, group 3 was significantly less than group 2 (P < 0.05). MVD of group 4 was less than that of groups 1 and 2 (P < 0.01). There was no significant difference between groups 2 and 3 (P > 0.05) or in LVD number among the four groups (P > 0.05). Conclusions. IVIS® was more useful than weight, volume of ascites, and number of peritoneal nodules. The simultaneous group was superior to sequential groups in killing cancer cells and inhibiting vascular endothelium. Cisplatin-endostar was superior to endostar-cisplatin in killing cancer cells, while the latter in inhibiting peritoneal vascular endothelium.

Molecular imaging with engineered physiology

In vivo imaging techniques are powerful tools for evaluating biological systems. Relating image signals to precise molecular phenomena can be challenging, however, due to limitations of the existing optical, magnetic and radioactive imaging probe mechanisms. Here we demonstrate a concept for molecular imaging which bypasses the need for conventional imaging agents by perturbing the endogenous multimodal contrast provided by the vasculature. Variants of the calcitonin gene-related peptide artificially activate vasodilation pathways in rat brain and induce contrast changes that are readily measured by optical and magnetic resonance imaging. CGRP-based agents induce effects at nanomolar concentrations in deep tissue and can be engineered into switchable analyte-dependent forms and genetically encoded reporters suitable for molecular imaging or cell tracking. Such artificially engineered physiological changes, therefore, provide a highly versatile means for sensitive analysis of molecular events in living organisms.

Fluorogenic Substrates for In Situ Monitoring of Caspase-3 Activity in Live Cells

The in situ detection of caspase-3 activity has applications in the imaging and monitoring of multiple pathologies, notably cancer. A series of cell penetrating FRET-based fluorogenic substrates were designed and synthesised for the detection of caspase-3 in live cells. A variety of modifications of the classical caspase-3 and caspase-7 substrate sequence Asp-Glu-Val-Asp were carried out in order to increase caspase-3 affinity and eliminate caspase-7 cross-reactivity. To allow cellular uptake and good solubility, the substrates were conjugated to a cationic peptoid. The most selective fluorogenic substrate 27, FAM-Ahx-Asp-Leu-Pro-Asp-Lys(MR)-Ahx, conjugated to the cell penetrating peptoid at the C-terminus, was able to detect and quantify caspase-3 activity in apoptotic cells without cross-reactivity by caspase-7.

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.

Activatable Ferritin Nanocomplex for Real-Time Monitoring of Caspase-3 Activation during Photodynamic Therapy

One mechanism of photodynamic therapy (PDT) for the ablation of tumors is to induce apoptosis. Visualization of apoptosis during PDT in real-time is of great benefit for predicting and evaluating therapeutic outcomes. Herein, we engineered a highly stable and sensitive caspase-3 ferritin activatable probe (FABP/ZnPc) for simultaneous delivery of a photosensitizer (ZnPc) and real-time visualization of apoptosis during PDT. Upon near-infrared (NIR) light irradiation, ZnPc becomes active and initiates apoptosis, upon which the outer layer of the FABP/ZnPc is degraded by the apoptotic marker, caspase-3, to boost strong fluorescent signals, ultimately allowing real-time imaging of apoptosis. Our results demonstrate the utility of FABP/ZnPc as a tool for PDT and simultaneous imaging of caspase-3 activation in vitro and in vivo. Overall, the ability of FABP/ZnPc to image apoptosis during PDT will not only facilitate optimizing and personalizing the PDT strategy but is also important for understanding the mechanisms of PDT.

Design of a functional cyclic HSV1-TK reporter and its application to PET imaging of apoptosis

Positron emission tomography (PET) is a sensitive and noninvasive imaging method that is widely used to explore molecular events in living subjects. PET can precisely and quantitatively evaluate cellular apoptosis, which has a crucial role in various physiological and pathological processes. In this protocol, we describe the design and use of an engineered cyclic herpes simplex virus 1-thymidine kinase (HSV1-TK) PET reporter whose kinase activity is specifically switched on by apoptosis. The expression of cyclic TK (cTK) in healthy cells leads to inactive product, whereas the activation of apoptosis through the caspase-3 pathway cleaves cTK, thus restoring its activity and enabling PET imaging. In addition to detailing the design and construction of the cTK plasmid in this protocol, we include assays for evaluating the function and specificity of the cTK reporter in apoptotic cells, such as assays for measuring the cell uptake of PET tracer in apoptotic cells, correlating doxorubicin (Dox)-induced cell apoptosis to cTK function recovery, and in vivo PET imaging of cancer cell apoptosis, and we also include corresponding data acquisition methods. The time to build the entire cTK reporter is ∼2-3 weeks. The selection of a stable cancer cell line takes ∼4-6 weeks. The time to implement assays regarding cTK function in apoptotic cells and the in vivo imaging varies depending on the experiment. The cyclization strategy described in this protocol can also be adapted to create other reporter systems for broad biomedical applications.

Non-invasive molecular imaging for preclinical cancer therapeutic development

Molecular and non-invasive imaging are rapidly emerging fields in preclinical cancer drug discovery. This is driven by the need to develop more efficacious and safer treatments, the advent of molecular-targeted therapeutics, and the requirements to reduce and refine current preclinical in vivo models. Such bioimaging strategies include MRI, PET, single positron emission computed tomography, ultrasound, and optical approaches such as bioluminescence and fluorescence imaging. These molecular imaging modalities have several advantages over traditional screening methods, not least the ability to quantitatively monitor pharmacodynamic changes at the cellular and molecular level in living animals non-invasively in real time. This review aims to provide an overview of non-invasive molecular imaging techniques, highlighting the strengths, limitations and versatility of these approaches in preclinical cancer drug discovery and development.

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.

Biosensor imaging in brain slice preparations

Cyclic-AMP dependent protein kinase (PKA) is present in most branches of the animal kingdom, and is an example in the nervous system where a kinase effector integrates the cellular effects of various neuromodulators. The recent development of FRET-based biosensors, such as AKAR, now allows the direct measurement of PKA activation in living cells by simply measuring the ratio between the fluorescence emission at the CFP and YFP wavelengths upon CFP excitation. This novel approach provides data with a temporal resolution of a few seconds at the cellular and even subcellular level, opening a new avenue of understanding the integration processes in space and time. Our protocol has been optimized to study morphologically intact mature neurons and we describe how simple and cheap wide-field imaging, as well as more elaborate two-photon imaging, allows real-time monitoring of PKA activation in pyramidal cortical neurons in neonate rodent brain slices. In addition, many practical details presented here also pertain to image analysis in other cellular preparations, such as cultured cells. Finally, this protocol can also be applied to the various other CFP-YFP-based FRET biosensors that are available for other kinases or other intracellular signals. It is likely that this kind of approach will be generally applicable to a broad range of assays in the near future.

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.

Identification of biomarkers for apoptosis in cancer cell lines using metabolomics: tools for individualized medicine

BACKGROUND

Metabolomics is a versatile unbiased method to search for biomarkers of human disease. In particular, one approach in cancer therapy is to promote apoptosis in tumour cells; this could be improved with specific biomarkers of apoptosis for monitoring treatment. We recently observed specific metabolic patterns in apoptotic cell lines; however, in that study, apoptosis was only induced with one pro-apoptotic agent, staurosporine.

OBJECTIVE

The aim of this study was to find novel biomarkers of apoptosis by verifying our previous findings using two further pro-apoptotic agents, 5-fluorouracil and etoposide, that are commonly used in anticancer treatment.

METHODS

Metabolic parameters were assessed in HepG2 and HEK293 cells using the newborn screening assay adapted for cell culture approaches, quantifying the levels of amino acids and acylcarnitines with mass spectrometry.

RESULTS

We were able to identify apoptosis-specific changes in the metabolite profile. Moreover, the amino acids alanine and glutamate were both significantly up-regulated in apoptotic HepG2 and HEK293 cells irrespective of the apoptosis inducer.

CONCLUSION

Our observations clearly indicate the potential of metabolomics in detecting metabolic biomarkers applicable in theranostics and for monitoring drug efficacy.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

A novel reporter system for molecular imaging and high-throughput screening of anticancer drugs

Apoptosis is irreversible programmed cell death, characterized by a cellular cascade activation of caspase 3, which subsequently degrades proteins and other components of cells with a motif sequence. Here we report a novel reporter system to detect apoptosis, growth arrest, and cell death based on controlled and self-amplified protein degradation. The key element of the reporter system is an apoptotic sensor chimerical protein which consists of three components: procaspase 3, ubiquitin (Ub), and a strong consensus sequence of N-degron. Between each of these units is a DEVD (Asp-Glu-Val-Asp) sequence, which acts as the cleavage target of caspase 3. This non-conventional signal loss approach is much more sensitive than other native methods that are based on signal gain. The superior sensitivity is demonstrated by its effective application in 386-well high-throughput screening (HTS) with low drug concentrations and a short incubation time. The HTS selection process using this reporter system is very simple and economic. The simplicity eliminates potential errors introduced by multiple steps; there is no need for any substrate. Furthermore, the cells in the assay need not be disrupted, and the morphology of the cells can provide additional information on mechanisms. After HTS, the intact cells can also be used for other analytic analysis. This system thus has a potentially important role in the discovery and development of new anticancer drugs. It also appears to be very versatile, can be used both in vitro and in vivo with different linked reporter genes, and can be used for a variety of imaging applications.

Continuous, real-time bioimaging of chemical bioavailability and toxicology using autonomously bioluminescent human cell lines

Bioluminescent imaging is an emerging biomedical surveillance strategy that uses external cameras to detect in vivo light generated in small animal models of human physiology or in vitro light generated in tissue culture or tissue scaffold mimics of human anatomy. The most widely utilized of reporters is the firefly luciferase (luc) gene; however, it generates light only upon addition of a chemical substrate, thus only generating intermittent single time point data snapshots. To overcome this disadvantage, we have demonstrated substrate-independent bioluminescent imaging using an optimized bacterial bioluminescence (lux) system. The lux reporter produces bioluminescence autonomously using components found naturally within the cell, thereby allowing imaging to occur continuously and in real-time over the lifetime of the host. We have validated this technology in human cells with demonstrated chemical toxicological profiling against exotoxin exposures at signal strengths comparable to existing luc systems (~1.33 × 10⁷ photons/second). As a proof-in-principle demonstration, we have engineered breast carcinoma cells to express bioluminescence for real-time screening of endocrine disrupting chemicals and validated detection of 17β-estradiol (EC₅₀ = ~ 10 pM). These and other applications of this new reporter technology will be discussed as potential new pathways towards improved models of target chemical bioavailability, toxicology, efficacy, and human safety.

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.

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.

Recent development of highly sensitive protease assay methods: Signal amplification through enzyme cascades

Proteases are involved in almost all biological processes, and therefore, aberrant activity of many of these enzymes is an important indicator of disease. Various methods have been developed to analyze protease activity, among which, protease assays based on resonance energy transfer are currently used most widely. However, quantitative methods with relatively higher sensitivity are needed, especially for disease diagnosis at early stages. One of the strategies to achieve higher sensitivity is to implement signal amplification of the protease activity. In this review, we briefly summarize the protease assay methods based on resonance energy transfer, and then elaborate the efforts to develop sensitive protease assays through signal amplification by using enzyme cascades.

Optical imaging

Non-invasive optical imaging techniques, such as fluorescence imaging (FI) or bioluminescence imaging (BLI) have emerged as important tools in biomedical research. As demonstrated in different animal disease models, they enable visualization of physiological and pathophysiological processes at the cellular and molecular level in vivo with high specificity. Optical techniques are easy to use, fast, and affordable. Furthermore, they are characterized by their high sensitivity. In FI, very low amounts of the imaging agent (nano- to femtomol or even less) can be detected. Due to the absorption and scattering of light in tissue, optical techniques exhibit a comparably low spatial resolution in the millimeter range and a depth limit of a few centimeters. However, non-invasive imaging of biological processes in small animals and in outer or inner surfaces as well as during surgery even in humans is feasible. Currently two agents for fluorescence imaging are clinically approved, namely indocyanine green (ICG) and 5-aminolevulinic acid (5-ALA). In the past years, a number of new optical imaging agents for FI and reporter systems for BLI have been developed and successfully tested in animal models. Some of the FI agents might promise the application in clinical oncology. In this chapter, we describe the basic principles of non-invasive optical imaging techniques, give examples for the visualization of biological processes in animal models of cancer, and discuss potential clinical applications in oncology.

Specific biomarkers of receptors, pathways of inhibition and targeted therapies: pre-clinical developments

A deeper understanding of the role of specific genes, proteins, pathways and networks in health and disease, coupled with the development of technologies to assay these molecules and pathways in patients, promises to revolutionise the practice of clinical medicine. Especially the discovery and development of novel drugs targeted to disease-specific alterations could benefit significantly from non-invasive imaging techniques assessing the dynamics of specific disease-related parameters. Here we review the application of imaging biomarkers in the management of patients with brain tumours, especially malignant glioma. In our other review we focused on imaging biomarkers of general biochemical and physiological processes related with tumour growth such as energy, protein, DNA and membrane metabolism, vascular function, hypoxia and cell death. In this part of the review, we will discuss the use of imaging biomarkers of specific disease-related molecular genetic alterations such as apoptosis, angiogenesis, cell membrane receptors and signalling pathways and their application in targeted therapies.

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.

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.

Imaging inflammation: molecular strategies to visualize key components of the inflammatory cascade, from initiation to resolution

Dysregulation of inflammation is central to the pathogenesis of innumerable human diseases. Understanding and tracking the critical events in inflammation are crucial for disease monitoring and pharmacological drug discovery and development. Recent progress in molecular imaging has provided novel insights into spatial associations, molecular events and temporal sequelae in the inflammatory process. While remaining a burgeoning field in pre-clinical research, increasing application in man affords researchers the opportunity to study disease pathogenesis in humans in situ thereby revolutionizing conventional understanding of pathophysiology and potential therapeutic targets. This review provides a description of commonly used molecular imaging modalities, including optical, radionuclide and magnetic resonance imaging, and details key advances and translational opportunities in imaging inflammation from initiation to resolution.

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.

Imaging molecular pathways: reporter genes

Molecular imaging is a rapidly advancing field that allows cancer biologists to look deeper into the complex inner workings of tumor cells, or whole tumors, in a non-invasive manner. In this review, we will summarize some recent advances that enable investigators to study various important biological processes in tumors in vivo. We will discuss novel imaging approaches that allow investigators to visualize and quantify molecular pathways, such as receptor tyrosine kinase activation, hypoxia signal transduction, apoptosis, and DNA double-strand breaks. Select examples of these applications will be discussed. Because of the limited scope of this review, we will only focus on natural reporters, such as bioluminescence and fluorescent proteins.