Using Co-Cultures Expressing Fluorescence Resonance Energy Transfer Based Protein Biosensors to Simultaneously Image Caspase-3 and Ca2+ Signaling

Development and optimization of heavy metal lead biosensors in biomedical and environmental applications

The detrimental impact of the heavy metal lead (Pb) on human health has been studied for years. The fact that Pb impairs human body has been established from countless painful and sad historical events. Nowadays, World Health Organization and many developmental countries have established regulations concerning the use of Pb. Measuring the blood lead level (BLL) is so far the only way to officially evaluate the degree of Pb exposure, but the so-called safety value (10 μg/dL in adults and 5 μg/dL in children) seems unreliable to represent the security checkpoint for children through daily intake of drinking water or physical contact with a lower contaminated level of Pb contents. In general, unsolved mysteries about the Pb toxicological mechanisms still remain. In this review article, we report on the methods to prevent Pb poison for further Pb toxicological research. We establish high-sensitivity Pb monitoring, and also report on the use of fluorescent biosensors such as genetically-encoded fluorescence resonance energy transfer-based biosensors built for various large demands such as the detection of severe acute respiratory syndrome coronavirus 2. We also contribute to the development and optimization of the FRET-based Pb biosensors. Our well-performed version of Met-lead 1.44 M1 has achieved a limit of detection of 10 nM (2 ppb; 0.2 μg/dL) and almost 5-fold in dynamic range (DR) supported for the real practical applications-that is, the in-cell Pb sensing device for blood and blood-related samples, and the Pb environmental detections in vitro. The perspective of our powerful Pb biosensor incorporated with a highly sensitive bio-chip of the portable device for quick Pb measurements will be addressed for further manipulation.

Precise control of apoptosis via gold nanostars for dose dependent photothermal therapy of melanoma

Precise induction and monitoring of cell apoptosis are significant for cancer treatment.

Long-term fluorescence lifetime imaging of a genetically encoded sensor for caspase-3 activity in mouse tumor xenografts

Caspase-3 is known for its role in apoptosis and programmed cell death regulation. We detected caspase-3 activation in vivo in tumor xenografts via shift of mean fluorescence lifetimes of a caspase-3 sensor. We used the genetically encoded sensor TR23K based on the red fluorescent protein TagRFP and chromoprotein KFP linked by 23 amino acid residues (TagRFP-23-KFP) containing a specific caspase cleavage DEVD motif to monitor the activity of caspase-3 in tumor xenografts by means of fluorescence lifetime imaging-Forster resonance energy transfer. Apoptosis was induced by injection of paclitaxel for A549 lung adenocarcinoma and etoposide and cisplatin for HEp-2 pharynx adenocarcinoma. We observed a shift in lifetime distribution from 1.6 to 1.9 ns to 2.1 to 2.4 ns, which indicated the activation of caspase-3. Even within the same tumor, the lifetime varied presumably due to the tumor heterogeneity and the different depth of tumor invasion. Thus, processing time-resolved fluorescence images allows detection of both the cleaved and noncleaved states of the TR23K sensor in real-time mode during the course of several weeks noninvasively. This approach can be used in drug screening, facilitating the development of new anticancer agents as well as improvement of chemotherapy efficiency and its adaptation for personal treatment.

Quantum dots-based fluorescence resonance energy transfer biosensor for monitoring cell apoptosis

The development of advanced methods for accurately monitoring cell apoptosis has extensive significance in the diagnostic and pharmaceutical fields. In this study, we developed a rapid, sensitive and selective approach for the detection of cell apoptosis by combining the site-specific recognition and cleavage of the DEVD-peptide with quantum dots (QDs)-based fluorescence resonance energy transfer (FRET). Firstly, biotin-peptide was conjugated on the surface of AuNPs to form AuNPs-pep through the formation of an Au-S bond. Then, AuNPs-pep-QDs nanoprobe was obtained through the connection between AuNPs-pep and QDs. FRET is on and the fluorescence of QDs is quenched at this point. The evidence of UV-vis spectra, transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy revealed that the connection was successful. Upon the addition of apoptosis cell lysis solution, peptide was cleaved by caspase-3, and AuNPs was dissociated from the QDs. At this time, FRET is off, and thus the QDs fluorescence was recovered. The experimental conditions were optimized in terms of ratio of peptide to AuNPs, buffer solution, and the temperature of conjugation and enzyme reaction. The biosensor was successfully applied to distinguishing apoptosis cells and normal cells within 2 h. This study demonstrated that the biosensor could be utilized to evaluate anticancer drugs.

Engineering a photoactivated caspase-7 for rapid induction of apoptosis

Apoptosis is a cell death program involved in the development of multicellular organisms, immunity, and pathologies ranging from cancer to HIV/AIDS. We present an engineered protein that causes rapid apoptosis of targeted cells in monolayer culture after stimulation with blue light. Cells transfected with the protein switch L57V, a tandem fusion of the light-sensing LOV2 domain and the apoptosis-executing domain from caspase-7, rapidly undergo apoptosis within 60 min after light stimulation. Constant illumination of under 5 min or oscillating with 1 min exposure had no effect, suggesting that cells have natural tolerance to a short duration of caspase-7 activity. Furthermore, the overexpression of Bcl-2 prevented L57V-mediated apoptosis, suggesting that although caspase-7 activation is sufficient to start apoptosis, it requires mitochondrial contribution to fully commit.

Design of fluorescent fusion protein probes

Many fluorescent probes depend on the fluorescence resonance energy transfer (FRET) between fluorescent protein pairs. The efficiency of energy transfer becomes altered by conformational changes of a fused sensory protein in response to a cellular event. A structure-based approach can be taken to design probes better with improved dynamic ranges by computationally modeling conformational changes and predicting FRET efficiency changes of candidate biosensor constructs. FRET biosensors consist of at least three domains fused together: the donor protein, the sensory domain, and the acceptor protein. To more efficiently subclone fusion proteins containing multiple domains, a cassette-based system can be used. Generating a cassette library of commonly used domains facilitates the rapid subcloning of future fusion biosensor proteins. FRET biosensors can then be used with fluorescence microscopy for real-time monitoring of cellular events within live cells by tracking changes in FRET efficiency. Stimulants can be used to trigger a range of cellular events including Ca(2+) signaling, apoptosis, and subcellular translocations.

FRET evidence that an isoform of caspase-7 binds but does not cleave its substrate

A caspase-7 biosensor (vDEVDc) based on FRET (fluorescence resonance energy transfer) was used to study the proteolytic properties of caspase-7, an executioner protease in cellular apoptosis. An active isoform of caspase-7 with the 56 N-terminal residues truncated (57casp7) cleaved vDEVDc at the recognition sequence, resulting in a FRET efficiency decrease of 61%. In contrast, an isoform with the 23 N-terminal residues truncated (24casp7) bound to vDEVDc but did not cleave the substrate, resulting in a FRET increase of 15%. Kinetic results showed an exponential substrate cleavage and binding curve for the 57casp7 and 24casp7 isoforms, respectively. FRET changes of the vDEVDc biosensor were also monitored in cos-7 cells upon STS-induced apoptosis. Finally, we modeled caspase-7 binding to vDEVDc and estimated a FRET emission ratio increase of 31.7%, which agrees with the 15% experimental result. We showed that two differently truncated isoforms of caspase-7 exhibit different enzymatic properties, namely binding by 24casp7 and hydrolysis by 57casp7.

FPMOD: a modeling tool for sampling the conformational space of fusion proteins

Fusion proteins are an important class of proteins with diverse applications in biotechnology. They consist of 2 or more rigid domains joined by a flexible linker. Understanding the conformational space of fusion proteins conferred by the flexible linkers is important to predicting its behavior. In this paper, we introduce a modeling tool called FPMOD (Fusion Protein MODeller) which samples the conformational space of fusion proteins by treating all domains as rigid bodies and rotating each of them around their flexible linkers. As a demonstration, FPMOD was used to predict the fluorescence resonance energy transfer (FRET) efficiency of three different fusion protein biosensors. The simulation results of the FRET efficiency prediction were consistent with the in vitro experimental data, which verified that FPMOD is a valid tool to predicting the behavior of fusion proteins.

FRET evidence that an isoform of caspase-7 binds but does not cleave its substrate

Caspase-7 is one of the executioner proteases in cellular apoptosis. Its kinetics has been monitored using biosensors based on the principle of fluorescence resonance energy transfer (FRET). Here, a caspase-7 biosensor (named vDEVDc) using fluorescent proteins as the donor and acceptor of FRET was used to study the biochemical properties of caspase-7. An active isoform of caspase-7 with the 56 N-terminal residues truncated (named 57casp7) cleaved the vDEVDc biosensor at the recognition sequence, resulting in a FRET efficiency decrease of 61%. In contrast, another caspase-7 isoform with the 23 N-terminal residues truncated (named 24casp7) bound the vDEVDc biosensor without cleaving the substrate, resulting in a FRET increase of 15%. The kinetics of the two caspase-7 isoforms were studied by monitoring the FRET change of the vDEVDc biosensor over time, which showed an exponential substrate cleavage and binding curve for the 57casp7 and 24casp7 isoform, respectively. Lastly, we modeled caspase-7 binding to the vDEVDc biosensor and estimated a FRET emission ratio increase of 16.2% after binding to caspase-7, which agrees with the 15% experimental result. We showed that two isoforms of caspase-7 with differently truncated prodomain exhibit different enzymatic properties, namely binding by the 24casp7 isoform and hydrolysis by 57casp7. We also demonstrated that our FRET biosensor (vDEVDc) can be used to detect not only the substrate cleavage event, but also the substrate binding event.

Molecular optical imaging of therapeutic targets of cancer

Recent progress in discerning the molecular events that accompany carcinogenesis has led to development of new cancer therapies directly targeted against the molecular changes of neoplasia. Molecular-targeted therapeutics have shown significant improvements in response rates and decreased toxicity as compared to conventional cytotoxic therapies which lack specificity for tumor cells. In order to fully explore the potential of molecular-targeted therapy, a new set of tools is required to dynamically and quantitatively image and monitor the heterogeneous molecular profiles of tumors in vivo. Currently, molecular markers can only be visualized in vitro using complex immunohistochemical staining protocols. In this chapter, we discuss emerging optical tools to image in vivo a molecular profile of risk-based hallmarks of cancer for selecting and monitoring therapy. We present the combination of optically active, targeted nanoparticles for molecular imaging with advances in minimally invasive optical imaging systems, which can be used to dynamically image both a molecular and phenotypic profile of risk and to monitor changes in this profile during therapy.

Protein biosensors based on the principle of fluorescence resonance energy transfer for monitoring cellular dynamics

Genetically-coded, fluorescence resonance energy transfer (FRET) biosensors are widely used to study molecular events from single cells to whole organisms. They are unique among biosensors because of their spontaneous fluorescence and targeting specificity to both organelles and tissues. In this review, we discuss the theoretical basis of FRET with a focus on key parameters responsible for designing FRET biosensors that have the highest sensitivity. Next, we discuss recent applications that are grouped into four common biosensor design patterns--intermolecular FRET, intramolecular FRET, FRET from substrate cleavage and FRET using multiple colour fluorescent proteins. Lastly, we discuss recent progress in creating fluorescent proteins suitable for FRET purposes. Together these advances in the development of FRET biosensors are beginning to unravel the interconnected and intricate signalling processes as they are occurring in living cells and organisms.