Multi-Förster Resonance Energy Transfer-Based Fluorescent Probe for Spatiotemporal Matrix Metalloproteinase-2 and Caspase-3 Imaging

Multifunctional fluorescent mesoporous carbon nanoprobe for MMP-2-activated cancer cell imaging and targeted photothermal therapy

The integration of cancer imaging with therapy in a simple system is warranted for precise cancer therapy. In this study, carboxyl-functionalized mesoporous carbon nanospheres (MCN) which are efficient photothermal agents and excellent fluorescence quenchers, were used for cancer cell imaging and selective photothermal therapy (PTT) applications. Using MCN, a matrix metalloproteinase-2 (MMP-2)- responsive theranostic nanoprobe was generated by functionalizing an MMP-2-specific fluorescent-labeled PLGVR sequence on the surface of MCN. The nanoprobe not only can be used to detect MMP-2 with a low detection limit of 0.3 pg mL^-1, but also can achieve the sensitive intracellular MMP-2 imaging in living cells, validating the differentiation of cancer cells from healthy cells based on the recovered fluorescence intensity. More importantly, selective cancer PTT was achieved using MMP-2-triggered cancer cell imaging. Our in vitro studies showed that by regulating the power density and irradiation time, the nanoprobe can effectively kill cancer cells via PTT. Our strategy opens new avenues for precision medicine, especially phototherapy.

Enzyme-Responsive Double-Locked Photodynamic Molecular Beacon for Targeted Photodynamic Anticancer Therapy

An advanced photodynamic molecular beacon (PMB) was designed and synthesized, in which a distyryl boron dipyrromethene (DSBDP)-based photosensitizer and a Black Hole Quencher 3 moiety were connected via two peptide segments containing the sequences PLGVR and GFLG, respectively, of a cyclic peptide. These two short peptide sequences are well-known substrates of matrix metalloproteinase-2 (MMP-2) and cathepsin B, respectively, both of which are overexpressed in a wide range of cancer cells either extracellularly (for MMP-2) or intracellularly (for cathepsin B). Owing to the efficient Förster resonance energy transfer between the two components, this PMB was fully quenched in the native form. Only upon interaction with both MMP-2 and cathepsin B, either in a buffer solution or in cancer cells, both of the segments were cleaved specifically, and the two components could be completely separated, thereby restoring the photodynamic activities of the DSBDP moiety. This PMB could also be activated in tumors, and it effectively suppressed the tumor growth in A549 tumor-bearing nude mice upon laser irradiation without causing notable side effects. In particular, it did not cause skin photosensitivity, which is a very common side effect of photodynamic therapy (PDT) using conventional "always-on" photosensitizers. The overall results showed that this "double-locked" PMB functioned as a biological AND logic gate that could only be unlocked by the coexistence of two tumor-associated enzymes, which could greatly enhance the tumor specificity in PDT.

Target-activated cascade transcription amplification lights up RNA aptamers for label-free detection of metalloproteinase-2 activity

We demonstrate that target-activated cascade transcription amplification lights up RNA aptamers for label-free detection of metalloproteinase-2 (MMP-2) activity with zero background. This assay exhibits good specificity and high sensitivity with a limit of detection (LOD) of 0.6 fM. Moreover, it can analyze enzyme kinetic parameters, screen inhibitors, and accurately quantify MMP-2 in cancer cells and clinical serums.

Triple-FRET multi-purpose fluorescent probe for three-protease detection

A new, robust and reliable methodology for three-protease screening in a single-enzyme mode has been developed and verified, employing a multi-purpose peptide probe with three selectively cleavable sites furnished with four fluorophores. A triple-FRET-based single-excitation quadruple-emission concept for unambiguous sensing of trypsin, chymotrypsin and caspase-8 in the lowest detectable concentrations of 0.5 ng mL^-1, 0.2 μg mL^-1, and 2 U mL^-1, respectively, has been applied and graphically depicted. Then the developed 4-dye probe has been also studied from the perspective of simultaneous two-protease screening, which was found only partially feasible, primarily due to unselective chymotrypsin cleavage.

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.

The Theoretical Model, Method, and Applications of Scattering Photon Burst Counting Based on an Objective Scanning Technique

Scattering photon burst counting (SPBC) is a single-particle detection method, which is based on measuring scattering photon bursting of single nanoparticles through a detection volume of <1 fL. Although SPBC has been used for bioassays and analysis of nanoparticles, it is necessary to establish its theoretical model and develop a new detection mode in order to further enhance its sensitivity and enlarge its application fields. In this paper, we proposed a theoretical model for the confocal SPBC method and developed a novel SPBC detection mode using the fast objective scanning technique. The computer simulations and experiments documented that this model well describes the relation between photon counts and experimental parameters (such as nanoparticle concentration and diameter, temperature, and viscosity). Based on this model, we developed a novel SPBC detection mode by using the fast objective scanning technique. Compared to the current confocal SPBC method, the sensitivity of this new method was significantly increased due to the significantly increased photon counts per sampling time, the linear detection range is from 0.9 to 90 pM, and the limit of detection is reduced to 40 fM for 30 nm gold nanoparticles. Furthermore, this new method was successfully applied to determine the enzyme activity of caspase-3 and evaluate the inhibition effectiveness of some inhibitors.

Fluorescent probes for visualizing ROS-associated proteins in disease

Abnormal expression of proteins, including catalytic and expression dysfunction, is directly related to the development of various diseases in living organisms. Reactive oxygen species (ROS) could regulate protein expression by redox modification or cellular signal pathway and thus influence the development of disease. Determining the expression level and activity of these ROS-associated proteins is of considerable importance in early-stage disease diagnosis and the identification of new drug targets. Fluorescence imaging technology has emerged as a powerful tool for specific in situ imaging of target proteins by virtue of its non-invasiveness, high sensitivity and good spatiotemporal resolution. In this review, we summarize advances made in the past decade for the design of fluorescent probes that have contributed to tracking ROS-associated proteins in disease. We envision that this review will attract significant attention from a wide range of researchers in their utilization of fluorescent probes for in situ investigation of pathological processes synergistically regulated by both ROS and proteins.

Near-Infrared Fluorescent Probes for the Detection of Cancer-Associated Proteases

Proteases are enzymes capable of catalyzing protein breakdown, which is critical across many biological processes. There are several families of proteases, each of which perform key functions through the degradation of specific proteins. As our understanding of cancer improves, it has been demonstrated that several proteases can be overactivated during the progression of cancer and contribute to malignancy. Optical imaging systems that employ near-infrared (NIR) fluorescent probes to detect protease activity offer clinical promise, both for early detection of cancer as well as for the assessment of personalized therapy. In this Review, we review the design of NIR probes and their successful application for the detection of different cancer-associated proteases.

A single-cell analytical approach to quantify activated caspase-3/7 during osteoblast proliferation, differentiation, and apoptosis

The protein heterogeneity at the single-cell level has been recognized to be vital for an understanding of various life processes during animal development. In addition, the knowledge of accurate quantity of relevant proteins at cellular level is essential for appropriate interpretation of diagnostic and therapeutic results. Some low-copy-number proteins are known to play a crucial role during cell proliferation, differentiation, and also in apoptosis. The fate decision is often based on the concentration of these proteins in the individual cells. This is likely to apply also for caspases, cysteine proteases traditionally associated with cell death via apoptosis but recently being discovered also as important factors in cell proliferation and differentiation. The hypothesis was tested in bone-related cells, where modulation of fate from apoptosis to proliferation/differentiation and vice versa is particularly challenging, e.g., towards anti-osteoporotic treatments and anti-cancer strategies. An ultrasensitive and highly selective method based on bioluminescence photon counting was used to quantify activated caspase-3/7 in order to demonstrate protein-level heterogeneity in individual cells within one population and to associate quantitative measurements with different cell fates (proliferation, differentiation, apoptosis). The results indicate a gradual increase of caspase-3/7 activation from the proliferative status to differentiation (more than three times) and towards apoptosis (more than six times). The findings clearly support one of the putative key mechanisms of non-apoptotic functions of pro-apoptotic caspases based on fine-tuning of their activation levels.

In Situ Protease Secretion Visualization and Metastatic Lymph Nodes Imaging via a Cell Membrane-Anchored Upconversion Nanoprobe

Matrix metalloproteinase (MMP) secretion is highly associated with tumor invasion and metastasis; therefore, monitoring MMP secretion is important for disease progression study and therapy choosing. Though working well for intracellular MMP imaging, the performance of current MMP detection probes is impaired in secretion monitoring due to the diffusion of MMP in an extracellular environment after secretion and low secreted amount. Here, we design a cell membrane-anchored ratiometric upconversion nanoprobe (UCNPs-Cy3/Pep-QSY7/Ab) for in situ MMP secretion visualization. Anti-EGFR is functionalized on the nanoprobe to provide specific recognition to tumor cells and guarantee fast response to MMP2 in the local place of secretion. MMP-responsive cleavage of Pep-QSY7 results in Cy3 luminescence recovery at 580 nm, which is ratioed over an internal standard of UCNP emission at 654 nm for MMP2 detection. The presented cell membrane-anchored ratiometric upconversion nanoprobe demonstrated that satisfactory results for in situ monitoring of MMP2 secretion from MDA-MB-231 cells and MCF-7 cells, as well as in vivo imaging of metastatic lymph nodes, would provide a universal platform for protease secretion study and contribute to tumor invasiveness assessment.

Biosensors and bioassays for determination of matrix metalloproteinases: state of the art and recent advances

Matrix metalloproteinases (MMPs) are closely associated with various physiological and pathological processes, and have been regarded as potential biomarkers for severe diseases including cancer. Accurate determination of MMPs would advance our understanding of their roles in disease progression, and is of great significance for disease diagnosis, treatment and prognosis. In this review, we present a comprehensive overview of the developed bioassays/biosensors for detection of MMPs, and highlight the recent advancement in nanomaterial-based immunoassays for MMP abundance measurements and nanomaterial-based biosensors for MMP activity determination. Enzyme-linked immunosorbent assay (ELISA)-based immunoassays provide information about total levels of MMPs with high specificity and sensitivity, while target-based biosensors measure the amounts of active MMPs, and allow imaging of MMP activities in vivo. For multiplex and high-throughput analysis of MMPs, microfluidics and microarray-based assays are described. Additionally, we put forward the existing challenges and future prospects from our perspective.

Intelligent Nanoprobe: Acid-Responsive Drug Release and In Situ Evaluation of Its Own Therapeutic Effect

The design of an intelligent nanoprobe capable of intracellular controlled release of apoptosis inducers and subsequent high-fidelity imaging of the drug-induced apoptosis is highly desirable for precise cancer treatment. Herein, we report an intelligent nanoprobe that combined therapeutic and imaging functions in one agent. Briefly, a gold nanoparticle is designed to be conjugated with acid-responsive DNA duplexes (Dox intercalates in this region) and caspase-3-specific cleavable peptides (labeled with fluorophore). We demonstrated that the nanoprobe could efficiently deliver an anticancer drug (Dox) into cancer cells and achieve acid-responsive drug release. Furthermore, the apoptotic process was in situ-monitored by detection of fluorescence through the cleavage of the peptide linker by caspase-3, which is one of the executioner caspases involved in apoptosis. This newly developed nanoprobe could serve as a theranostic agent for targeted responsive chemotherapy and also provide feedback apoptosis imaging of the self-therapeutic effect.

Electrochemical impedance spectroscopy for quantization of matrix Metalloproteinase-14 based on peptides inhibiting its homodimerization and heterodimerization

We reported here two novel electrochemical impedance spectroscopy biosensors were developed for the first time for highly sensitive quantification of matrix metalloproteinase-14 (MMP-14) based on binding interaction between hemopexin-like domain (PEX) of MMP-14 (PEX-14) and its inhibitory peptides. Specific inhibitory peptides (IVSC or ISC) inhibiting homodimerization or heterodimerization of MMP-14 was first self assembled on the surface of gold electrode and blocked with 6-mercapto-1-hexanol on a gold electrode surface used as IVSC or ISC modified biosensor, respectively. IVSC modified biosensor can be used for detection of MMP-14 by using the direct IVSC-MMP-14 interaction inhibiting MMP-14 homodimerization as well as ISC modified biosensor for indirect detection of MMP-14 via PEX-14 mediated peptide-MMP-14 binding. The electron transfer resistance (R_et) of biosensor was monitored to measure MMP-14 using Fe(CN)₆^3-/4- as probe. The increase of the R_et of the biosensors are linear with the concentration of MMP-14 in the range from 1 μg L^-1 to 10 μg L^-1 with detection limit of 0.19 μg L^-1 for IVSC modified biosensor and 0.1 ng L^-1 to 50 ng L^-1 with detection limit of 7 ng L^-1 for ISC modified biosensor. This work demonstrates that probing the interaction between peptide inhibitor and PEX of MMPs represents a novel approach to assess MMPs-mediated cancer dissemination.

Fluorogenic probes for disease-relevant enzymes

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

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.

An upconversion nanoparticle-based fluorescence resonance energy transfer system for effectively sensing caspase-3 activity

We report a new fluorescence resonance energy transfer (FRET) sensing platform for the sensitive detection of caspase-3 activity in vitro and in cells using NaGdF₄:Yb³⁺,Er³⁺@NaGdF₄ upconversion nanoparticles (UCNPs) as the energy donor and Rhodamine B (RB) as the energy acceptor. The phosphorylated RB-modified peptide containing a caspase-3 cleavage site and cell-penetrating peptide (CPP) motif (sequence, (RB)-DEVDGGS(p)GCGT(p)GRKKRRQRRRPQ) is immobilized on the UCNP surface via the strong coordination interaction between Gd³⁺ ions with phosphate. After the cleavage of DEVD by caspase-3, the RB is released from the UCNP surface and the reduced upconversion luminescence (UCL) is recovered. Under the optimum conditions, the recovery ratio of the UCL is linearly dependent on the caspase-3 concentration within the range of 0.01 to 1000 pg mL^-1 and with a limit of detection (LOD) of 0.01 pg mL^-1 (S/N = 3). In particular, the as-proposed UCNP-based FRET sensing platform has reasonable selectivity which is successfully employed to monitor caspase-3 activity in drug-induced apoptosis of HeLa cells.

A novel three-fluorophore system as a ratiometric sensor for multiple protease detection

A ratiometric fluorescent system for the detection of two proteases solely or in their mixture.

A label-free and blocker-free photoelectrochemical strategy for highly sensitive caspase-3 assay

Based on CC-DEVD-peptide modified nitrogen-doped porous carbon-ZnO nanopolyhedra/CdS hybrids, a label-free and blocker-free photoelectrochemical strategy was developed for caspase-3 assay.

Novel oligopeptide nanoprobe for targeted cancer cell imaging

Co-assembly of WFFW tetrapeptide and RGDWFFW heptapeptide generated the photostable and fluorescence-tunable nanoprobe, which could selectively image the cancer cells.

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.