Intracellular Self-Assembly of Cyclic d-Luciferin Nanoparticles for Persistent Bioluminescence Imaging of Fatty Acid Amide Hydrolase

Amino-Acid-Encoded Supramolecular Nanostructures for Persistent Bioluminescence Imaging of Tumor

Bioluminescence imaging (BLI) is a powerful technique for noninvasive monitoring of biological processes and cell transplantation. Nonetheless, the application of D-luciferin, which is widely employed as a bioluminescent probe, is restricted in long-term in vivo tracking due to its short half-life. This study presents a novel approach using amino acid-encoded building blocks to accumulate and preserve luciferin within tumor cells, through a supramolecular self-assembly strategy. The building block platform called Cys(SEt)-X-CBT (CXCBT, with X representing any amino acid) utilizes a covalent-noncovalent hybrid self-assembly mechanism to generate diverse luciferin-containing nanostructures in tumor cells after glutathione reduction. These nanostructures exhibit efficient tumor-targeted delivery as well as sequence-dependent well-designed morphologies and prolonged bioluminescence performance. Among the selected amino acids (X = Glu, Lys, Leu, Phe), Cys(SEt)-Lys-CBT (CKCBT) exhibits the superior long-lasting bioluminescence signal (up to 72 h) and good biocompatibility. This study demonstrates the potential of amino-acid-encoded supramolecular self-assembly as a convenient and effective method for developing BLI probes for long-term biological tracking and disease imaging.

Luciferase-Loaded Calcium Phosphate Nanoparticles for Persistent Bioluminescence Imaging of Orthotopic Breast Tumors

Bioluminescence imaging (BLI) is an important noninvasive optical imaging technique that has been widely used to monitor many biological processes due to its high sensitivity, resolution, and signal-to-noise ratio. However, the BLI technique based on the firefly luciferin-luciferase system is limited by the expression of exogenous luciferase and the short half-life of firefly luciferin, which pose challenges for long-term tracking in vivo. To solve the problems, here we rationally designed an intelligent strategy for persistent BLI in tumors by combining luciferase-loaded calcium phosphate nanoparticles (Luc@CaP NPs) to provide luciferase and the probe Cys(SEt)-Lys-CBT (CKCBT) to slowly produce the luciferase substrate amino luciferin (Am-luciferin). Luc@CaP NPs constructed with CaP as a carrier could enable luciferase activity to be maintained in vivo for at least 12 h. And compared to the conventional substrate luciferin, CKCBT apparently prolonged the BL time by up to 2 h through GSH-induced intracellular self-assembly and subsequent protease degradation-induced release of Am-luciferin. We anticipate that this strategy could be applied for clinical translation in more disease diagnosis and treatment in the near future.

Amino-Acid-Encoded Bioinspired Supramolecular Self-Assembly of Multimorphological Nanocarriers

Supramolecular self-assembly has emerged as an efficient tool to construct well-organized nanostructures for biomedical applications by small organic molecules. However, the physicochemical properties of self-assembled nanoarchitectures are greatly influenced by their morphologies, mechanical properties, and working mechanisms, making it challenging to design and screen ideal building blocks. Herein, using a biocompatible firefly-sourced click reaction between the cyano group of 2-cyano-benzothiazole (CBT) and the 1,2-aminothiol group of cysteine (Cys), an amino-acid-encoded supramolecular self-assembly platform Cys(SEt)-X-CBT (X represents any amino acid) is developed to incorporate both covalent and noncovalent interactions for building diverse morphologies of nanostructures with bioinspired response mechanism, providing a convenient and rapid strategy to construct site-specific nanocarriers for drug delivery, cell imaging, and enzyme encapsulation. Additionally, it is worth noting that the biodegradation of Cys(SEt)-X-CBT generated nanocarriers can be easily tracked via bioluminescence imaging. By caging either the thiol or amino groups in Cys with other stimulus-responsive sites and modifying X with probes or drugs, a variety of multi-morphological and multifunctional nanomedicines can be readily prepared for a wide range of biomedical applications.

Monofluorophore-based Two-Photon Ratiometric Fluorescent Probe for the Quantitative Imaging of Fatty Acid Amide Hydrolase in Live Neurons and Mouse Brain Tissues

Fatty acid amide hydrolase (FAAH) plays a crucial role in the metabolism of the endocannabinoid system by hydrolyzing a series of bioactive amides, whose abnormal levels are associated with neuronal disorders including Alzheimer's disease (AD). However, due to the lack of suitable quantitative sensing tools, real-time and accurate monitoring of the activity of FAAH in living systems remains unresolved. Herein, a novel enzyme-activated near-infrared two-photon ratiometric fluorescent probe (CANP) based on a naphthylvinylpyridine monofluorophore is successfully developed, in which the electron-withdrawing amide moiety is prone to be hydrolyzed to an electron-donating amine group under the catalysis of FAAH, leading to the activation of the intramolecular charge transfer process and the emergence of a new 80 nm red-shifted emission, thereby achieving a ratiometric luminescence response. Benefiting from the high selectivity, high sensitivity, and ratiometric response to FAAH, the probe CANP is successfully used to quantitatively monitor and image the FAAH levels in living neurons, by which an amyloid β (Aβ)-induced upregulation of endogenous FAAH activity is observed. Similar increases in FAAH activity are found in various brain regions of AD model mice, indicating a potential fatty acid amide metabolite-involved pathway for the pathological deterioration of AD. Moreover, our quantitative FAAH inhibition experiments further demonstrate the great value of CANP as an efficient visual probe for in situ and precise assessment of FAAH inhibitors in complex living systems, assisting the discovery of FAAH-related therapeutic agents.

Bioorthogonal In Situ Polymerization of Dendritic Agents for Hijacking Lysosomes and Enhancing Antigen Presentation in Cancer Cells

A low-generation lysine dendrimer, SPr-G2, responds to intracellular glutathione to initiate bioorthogonal in situ polymerization, resulting in the formation of large assemblies in mouse breast cancer cells. The intracellular large assemblies of SPr-G2 can interact with lysosomes to induce lysosome expansion and enhance lysosomal membrane permeabilization, leading to major histocompatibility complex class I upregulation on tumor cell surfaces and ultimately tumor cell death. Moreover, the use of the SPr-G2 dendrimer to conjugate the chemotherapeutic drug, camptothecin (CPT), can boost the therapeutic potency of CPT. Excellent antitumor effects in vitro and in vivo are obtained from the combinational treatment of the SPr-G2 dendrimer and CPT. This combinational effect also enhances antitumor immunity through promoting activation of cytotoxic T cells in tumor tissues and maturation of dendritic cells. This study can shed new light on the development of peptide dendritic agents for cancer therapy.

Acidity-activatable upconversion afterglow luminescence cocktail nanoparticles for ultrasensitive in vivo imaging

Activatable afterglow luminescence nanoprobes enabling switched "off-on" signals in response to biomarkers have recently emerged to achieve reduced unspecific signals and improved imaging fidelity. However, such nanoprobes always use a biomarker-interrupted energy transfer to obtain an activatable signal, which necessitates a strict distance requisition between a donor and an acceptor moiety (<10 nm) and hence induces low efficiency and non-feasibility. Herein, we report organic upconversion afterglow luminescence cocktail nanoparticles (ALCNs) that instead utilize acidity-manipulated singlet oxygen (1O2) transfer between a donor and an acceptor moiety with enlarged distance and thus possess more efficiency and flexibility to achieve an activatable afterglow signal. After in vitro validation of acidity-activated afterglow luminescence, ALCNs achieve in vivo imaging of 4T1-xenograft subcutaneous tumors in female mice and orthotopic liver tumors in male mice with a high signal-to-noise ratio (SNR). As a representative targeting trial, Bio-ALCNs with biotin modification prove the enhanced targeting ability, sensitivity, and specificity for pulmonary metastasis and subcutaneous tumor imaging via systemic administration of nanoparticles in female mice, which also implies the potential broad utility of ALCNs for tumor imaging with diverse design flexibility. Therefore, this study provides an innovative and general approach for activatable afterglow imaging with better imaging performance than fluorescence imaging.

Cell-Penetrating and Enzyme-Responsive Peptides for Targeted Cancer Therapy: Role of Arginine Residue Length on Cell Penetration and In Vivo Systemic Toxicity

For the improved delivery of cancer therapeutics and imaging agents, the conjugation of cell-penetrating peptides (CPPs) increases the cellular uptake and water solubility of agents. Among the various CPPs, arginine-rich peptides have been the most widely used. Combining CPPs with enzyme-responsive peptides presents an innovative strategy to target specific intracellular enzymes in cancer cells and when combined with the appropriate click chemistry can enhance theranostic drug delivery through the formation of intracellular self-assembled nanostructures. However, one drawback of CPPs is their high positive charge which can cause nonspecific binding, leading to off-target accumulation and potential toxicity. Hence, balancing cell-specific penetration, toxicity, and biocompatibility is essential for future clinical efficacy. We synthesized six cancer-specific, legumain-responsive RnAANCK peptides containing one to six arginine residues, with legumain being an asparaginyl endopeptidase that is overexpressed in aggressive prostate tumors. When conjugated to Alexa Fluor 488, R1-R6AANCK peptides exhibited a concentration- and time-dependent cell penetration in prostate cancer cells, which was higher for peptides with higher R values, reaching a plateau after approximately 120 min. Highly aggressive DU145 prostate tumor cells, but not less aggressive LNCaP cells, self-assembled nanoparticles in the cytosol after the cleavage of the legumain-specific peptide. The in vivo biocompatibility was assessed in mice after the intravenous injection of R1-R6AANCK peptides, with concentrations ranging from 0.0125 to 0.4 mmol/kg. The higher arginine content in R4-6 peptides showed blood and urine indicators for the impairment of bone marrow, liver, and kidney function in a dose-dependent manner, with instant hemolysis and morbidity in extreme cases. These findings underscore the importance of designing peptides with the optimal arginine residue length for a proper balance of cell-specific penetration, toxicity, and in vivo biocompatibility.

A Smart Intracellular Self-Assembling Bioorthogonal Raman Active Nanoprobe for Targeted Tumor Imaging

Inspired by the principle of in situ self-assembly, the development of enzyme-activated molecular nanoprobes can have a profound impact on targeted tumor detection. However, despite their intrinsic promise, obtaining an optical readout of enzyme activity with high specificity in native milieu has proven to be challenging. Here, a fundamentally new class of Raman-active self-assembling bioorthogonal enzyme recognition (nanoSABER) probes for targeted tumor imaging is reported. This class of Raman probes presents narrow spectral bands reflecting their vibrational fingerprints and offers an attractive solution for optical imaging at different bio-organization levels. The optical beacon harnesses an enzyme-responsive peptide sequence, unique tumor-penetrating properties, and vibrational tags with stretching frequencies in the cell-silent Raman window. The design of nanoSABER is tailored and engineered to transform into a supramolecular structure exhibiting distinct vibrational signatures in presence of target enzyme, creating a direct causality between enzyme activity and Raman signal. Through the integration of substrate-specific for tumor-associated enzyme legumain, unique capabilities of nanoSABER for imaging enzyme activity at molecular, cellular, and tissue levels in combination with machine learning models are shown. These results demonstrate that the nanoSABER probe may serve as a versatile platform for Raman-based recognition of tumor aggressiveness, drug accumulation, and therapeutic response.

A Low-Cost, Sensitive Reporter System Using Membrane-Tethered Horseradish Peroxidase for Efficient Gene Expression Analysis

Reporter gene assays are essential for high-throughput analysis, such as drug screening or determining downstream signaling activation/inhibition. However, use of this technology has been hampered by the high cost of the substrate (e.g., d-Luciferin (d-Luc)) in the most common firefly luciferase (FLuc) reporter gene assay. Although alternate luciferase is available worldwide, its substrate has remained expensive, and a more affordable option is still in demand. Here, we present a membrane-tethered horseradish peroxidase (mHRP), a new reporter system composed of a cell membrane expressing HRP that can preserve its enzymatic function on the cell surface, facilitates contact with HRP substrates (e.g., ABTS and TMB), and avoids the cell lysis process and the use of the high-priced luciferase substrate. An evaluation of the light signal sensitivity of mHRP compared to FLuc showed that both had comparable signal sensitivity. We also identified an extended substrate half-life of more than 5-fold that of d-Luc. Of note, this strategy provided a more stable detection signal, and the cell lysis process is not mandatory. Furthermore, with this strategy, we decreased the total amount of time taken for analysis and increased the time of detection limit of the reporter assay. Pricing analysis showed a one-third to one twenty-eighth price drop per single test of reporter assay. Given the convenience and stability of the mHRP reporter system, we believe that our strategy is suitable for use as an alternative to the luciferase reporter assay.

Targeted Enzyme Activity Imaging with Quantitative Phase Microscopy

Quantitative phase imaging (QPI) is a powerful optical imaging modality for label-free, rapid, and three-dimensional (3D) monitoring of cells and tissues. However, molecular imaging of important intracellular biomolecules such as enzymes remains a largely unexplored area for QPI. Herein, we introduce a fundamentally new approach by designing QPI contrast agents that allow sensitive detection of intracellular biomolecules. We report a new class of bio-orthogonal QPI-nanoprobes for in situ high-contrast refractive index (RI) imaging of enzyme activity. The nanoprobes feature silica nanoparticles (SiO2 NPs) having higher RI than endogenous cellular components and surface-anchored cyanobenzothiazole-cysteine (CBT-Cys) conjugated enzyme-responsive peptide sequences. The nanoprobes specifically aggregated in cells with target enzyme activity, increasing intracellular RI and enabling precise visualization of intracellular enzyme activity. We envision that this general design of QPI-nanoprobes could open doors for spatial-temporal mapping of enzyme activity with direct implications for disease diagnosis and evaluating the therapeutic efficacy.

Activity-based Photoacoustic Probes Reveal Elevated Intestinal MGL and FAAH Activity in a Murine Model of Obesity

Obesity is a chronic health condition characterized by the accumulation of excessive body fat which can lead to and exacerbate cardiovascular disease, type-II diabetes, high blood pressure, and cancer through systemic inflammation. Unfortunately, visualizing key mediators of the inflammatory response, such as monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH), in a selective manner is a profound challenge owing to an overlapping substrate scope that involves arachidonic acid (AA). Specifically, these enzymes work in concert to generate AA, which in the context of obesity, has been implicated to control appetite and energy metabolism. In this study, we developed the first selective activity-based sensing probes to detect MGL (PA-HD-MGL) and FAAH (PA-HD-FAAH) activity via photoacoustic imaging. Activation of PA-HD-MGL and PA-HD-FAAH by their target enzymes resulted in 1.74-fold and 1.59-fold signal enhancements, respectively. Due to their exceptional selectivity profiles and deep-tissue photoacoustic imaging capabilities, these probes were employed to measure MGL and FAAH activity in a murine model of obesity. Contrary to conflicting reports suggesting levels of MGL can be attenuated or elevated, our results support the latter. Indeed, we discovered a marked increase of both targets in the gastrointestinal tract. These key findings set the stage to uncover the role of the endocannabinoid pathway in obesity-mediated inflammation.

Recent applications of CBT-Cys click reaction in biological systems

Click chemistry is a hot topic in many research fields. A biocompatible reaction from fireflies has attracted increasing attention since 2009. Herein, we focus on the firefly-sourced click reaction between cysteine (Cys) and 2-cyanobenzothiazole (2-CBT). This reaction has many excellent properties, such as rapidity, simplicity and high selectivity, which make it successfully applied in protein labeling, molecular imaging, drug discovery and other fields. Meanwhile, its unique ability to form nanoparticles expands its applications in biological systems. We review its principle, development, and latest applications in the past 5 years and hope this review provides more profound and comprehensive insights to its further application.

Constructing turn-on bioluminescent probes for real-time imaging of reactive oxygen species during cisplatin chemotherapy

Real-time imaging of reactive oxygen species (ROS) during cisplatin chemotherapy of cancer is imperative to fully reveal their functions in the biological response to cisplatin. Currently, using a bioluminescent probe for real-time imaging of a specific ROS in vivo during cisplatin chemotherapy has not been achieved. Herein, three bioluminescent probes, F Probe, N Probe and P Probe were synthesized for real-time imaging of the primary ROS, O₂^•-. They all consisted of a bioluminescent emitter _D-luciferin (_D-LH₂) and an O₂^•--recognition group, and their bioluminescent signal could be turned on in response to O₂^•-. In vitro results indicated that P Probe was the most suitable one among the three probes for detection of O₂^•-, with high sensitivity, excellent selectivity and stability. P Probe was then successfully applied for real-time imaging of O₂^•- in both cancer cells and tumors during cisplatin chemotherapy. The imaging results demonstrated that O₂^•- amount in cancer cells increased with the increasing dose of cisplatin, and that cisplatin-induced upregulation of O₂^•- level in cancer cells was upstream of the cancer-killing pathway of cisplatin. We envision that P Probe may serve as an elucidative tool to further explore the role of O₂^•- in cisplatin chemotherapy.

Renal clearable polyfluorophore nanosensors for early diagnosis of cancer and allograft rejection

Optical nanoparticles are promising diagnostic tools; however, their shallow optical imaging depth and slow clearance from the body have impeded their use for in vivo disease detection. To address these limitations, we develop activatable polyfluorophore nanosensors with biomarker-triggered nanoparticle-to-molecule pharmacokinetic conversion and near-infrared fluorogenic turn-on response. Activatable polyfluorophore nanosensors can accumulate at the disease site and react with disease-associated proteases to undergo in situ enzyme-catalysed depolymerization. This disease-specific interaction liberates renal-clearable fluorogenic fragments from activatable polyfluorophore nanosensors for non-invasive longitudinal urinalysis and outperforms the gold standard blood and urine assays, providing a level of sensitivity and specificity comparable to those of invasive biopsy and flow cytometry analysis. In rodent models, activatable polyfluorophore nanosensors enable ultrasensitive detection of tumours (1.6 mm diameter) and early diagnosis of acute liver allograft rejection. We anticipate that our modular nanosensor platform may be applied for early diagnosis of a range of diseases via a simple urine test.

Assembly Transformation Jointly Driven by the LAP Enzyme and GSH Boosting Theranostic Capability for Effective Tumor Therapy

Developing intelligent and morphology-transformable nanomaterials that can spatiotemporally undergo stimulus-responsive size transformation holds great promise for improving the tumor delivery efficiency of drugs in vivo. Here, we report a smart size-transformable theranostic probe Ce6-Leu consisting of a leucine amino peptidase (LAP) and glutathione (GSH) dual-responsive moiety, an 1,2-aminothiol group, and a clinically used photosensitizer Ce6. This probe tends to self-assemble into uniform nanoparticles with an initial size of ∼80 nm in aqueous solution owing to the amphiphilic feature. Surprisingly, taking advantage of the biocompatible CBT-Cys condensation reaction, the large nanoprobes can be transformed into tiny nanoparticles (∼23 nm) under the joint action of LAP and GSH in a tumor microenvironment, endowing them with great tumor accumulation and deep tissue penetration. Concomitantly, this LAP/GSH-driven disassembly and size shrinkage of Ce6-Leu can also activate the fluorescence/magnetic resonance signals and the photodynamic effect for enhanced multimodal imaging-guided photodynamic therapy of human liver HepG2 tumors in vivo. More excitingly, the Mn²⁺-chelating probe (Ce6-Leu@Mn²⁺) was demonstrated to have the capability to catalyze endogenous H₂O₂ to persistently release O₂ at the hypoxic tumor site, as a consequence improving the oxygen supply to boost the radiotherapy effect. We thus believe that this LAP/GSH-driven size-transformable nanosystem would offer a novel advanced technology to improve the drug delivery efficiency for achieving precise tumor diagnosis and treatment.

Molecular Probes for Autofluorescence-Free Optical Imaging

Optical imaging is an indispensable tool in clinical diagnostics and fundamental biomedical research. Autofluorescence-free optical imaging, which eliminates real-time optical excitation to minimize background noise, enables clear visualization of biological architecture and physiopathological events deep within living subjects. Molecular probes especially developed for autofluorescence-free optical imaging have been proven to remarkably improve the imaging sensitivity, penetration depth, target specificity, and multiplexing capability. In this Review, we focus on the advancements of autofluorescence-free molecular probes through the lens of particular molecular or photophysical mechanisms that produce long-lasting luminescence after the cessation of light excitation. The versatile design strategies of these molecular probes are discussed along with a broad range of biological applications. Finally, challenges and perspectives are discussed to further advance the next-generation autofluorescence-free molecular probes for in vivo imaging and in vitro biosensors.

Self-Assembled Hybrid Nanogel as a Multifunctional Theranostic Probe for Enzyme-Regulated Ultrasound Imaging and Tumor Therapy

Multifunctional theranostic nanoprobes integrated with stimuli-responsive imaging and therapeutic capabilities have shown great potential to enhance the early cancer diagnostic efficacy and therapeutic efficiency. Elevated levels of lactate and hydrogen peroxide have been considered as the characteristic feature of the tumor microenvironment and can thus be exploited for developing promising theranostic strategies. We demonstrate here that the biocompatible and responsive enzyme-based nanogel probe has been designed as a promising theranostic tool to target high lactate and hydrogen peroxide for ultrasound imaging (US) and cancer treatment. We encapsulate the dual enzyme lactate oxidase (LOD) and catalase (CAT) into the self-assembled nanogels to fabricate responsive nanoprobe LOD/CAT-loaded nanogels (LCNGs). The nanoprobes can respond to the lactate and H₂O₂ rich tumor microenvironment to generate abundant oxygen, which further accumulates into microbubbles for enhanced US imaging. Besides, LCNGs@DOX has been further created by integrating the nanoprobes with doxorubicin (DOX) for cancer therapy. Both in vitro and in vivo results demonstrate enhanced US imaging and effective cell proliferation inhibition of LCNGs@DOX, allowing the preparation of safe and efficient theranostic nanoprobes capable of responsive US imaging and treating tumors.

Furin-Mediated Self-Assembly of Olsalazine Nanoparticles for Targeted Raman Imaging of Tumors

Olsalazine (Olsa) is a broad-spectrum anti-cancer agent acting as a DNA-methylation inhibitor. When conjugated to 2-cyano-6-aminobenzothiazole and a peptide substrate specific for the tumor-overexpressed enzyme furin, it can self-assemble into nanoparticles that can be detected by chemical-exchange saturation-transfer magnetic-resonance imaging (CEST MRI). We report here that these nano-assemblies can also be detected with high specificity in furin-overexpressing tumor cells by Raman spectroscopy with a distinct scattering signature and demonstrate the utility of this sensing mechanism in vitro and in vivo. Our findings suggest that Raman spectroscopy could be used for high-resolution image-guided surgery to precisely delineate tumor margins during and after resection in real-time as well as to determine microscopic tumor invasion and multifocal locoregional tumor spread, which are currently impossible to visualize with available imaging technologies, including CEST MRI.

Directly observing intracellular nanoparticle formation with nanocomputed tomography

Directly observing intracellular nanostructure formation remains challenging. In this work, using a rationally designed small-molecule 4-nitrobenzyl carbamate-Cys(SEt)-Asp-Asp-Phe(iodine)-2-cyano-benzothiazole (NBC-Iod-CBT), we directly observed intracellular nanoparticle formation with nanocomputed tomography (nano-CT). In vitro, upon glutathione reduction and nitroreductase (NTR) cleavage, NBC-Iod-CBT undergoes a CBT-Cys click condensation reaction to self-assemble nanoparticles Iod-CBT-NPs with an average linear absorption coefficient (LAC) value of 0.182 ± 0.078 μm^-1 to x-ray. Nano-CT imaging of the NBC-Iod-CBT-treated, NTR-overexpressing HeLa cells showed the existence of Iod-CBT-NPs in their cytoplasm with an average LAC value of 0.172 ± 0.032 μm^-1 We anticipate that our strategy could help people to deeply understand the formation mechanism of intracellular nanostructures in the near future.

Noncovalently Caged Firefly Luciferins Enable Amplifiable Bioluminescence Sensing of Hyaluronidase-1 Activity in Vivo

Hyaluronidase 1 (Hyal-1) is an important enzyme involved in intracellular hyaluronic acid (HA) catabolism for performing various physiological functions, and its aberrant level is closely associated with many malignant diseases. Bioluminescence imaging is advantageous for monitoring Hyal-1 activity in vivo, but it remains challenging to design an available probe for differentiating Hyal-1 from other isoforms by a traditional strategy that covalently masks the firefly luciferase substrate. Herein, we, for the first time, present a noncovalently caging approach to construct a Hyal-1-specific bioluminogenic nanosensor by entrapping d-luciferin (d-Luc) inside the cholesterylamine-modified HA (CHA) nanoassembly to inhibit the bioluminescence production. When encountered with intracellular Hyal-1, CHA could be fully dissembled to liberate multiple copies of the loaded d-Luc, thereby emitting light by the luciferase-catalyzed bioluminescence reaction. Because of its cascade signal amplification feature, d-Luc@CHA displayed a remarkable "turn-on" response (248-fold) to 5 μg/mL Hyal-1 with a detection limit of 0.07 ng/mL. Importantly, bioluminescence imaging results validated that d-Luc@CHA could be competent for dynamically visualizing endogenous Hyal-1 changes in living cells and animals and possessed the capability of discriminating between normal and cancer cells, thus offering a promising toolbox to evaluate Hyal-1 roles in biological processes as well as to diagnose Hyal-1-related diseases.

A redox-sensitive core-crosslinked nanosystem combined with ultrasound for enhanced deep penetration of nanodiamonds into tumors

Nanodiamonds (NDs) as drug delivery vehicles are of great significance in anticancer therapy through enhancing drug retention. However, the major barrier to clinical application of NDs is insufficient tumor penetration owing to their strong aggregation and low passive penetration efficiency. Herein, the core-crosslinked pullulan carrier, assembled using the visible light-induced diselenide (Se-Se) bond crosslinking method for encapsulating nanodiamonds-doxorubicin (NDX), is proposed to improve monodispersity. Furthermore, the core-crosslinked diselenide bond provides the nanosystem with redox-responsive capability and improved structural stability in a physiological environment, which prevents premature drug leakage and achieves tumor site-specific controlled release. What's more, ultrasound (US) is utilized to promote nanosystem intratumoral penetration via enlarged tumor vascular endothelium cell gaps. As expected, the nanosystem combined with ultrasound can enhance anti-tumor efficacy with deep penetration and excellent retention performance in a HepG2 xenograft mouse model. This study highlights the ability of the integrated therapeutic paradigm to overcome the limitation of nanodiamonds and the potential for further application in cancer therapy.

Furin-Guided Intracellular 68Ga Nanoparticle Formation Enhancing Tumor MicroPET Imaging

Positron-emission tomography (PET) is routinely used in the clinic for tumor imaging with ultrahigh sensitivity, but tumor-targeted PET imaging probes are quite few. In this work, we rationally designed a furin-responsive radiotracer Acetyl-Arg-Val-Arg-Arg-Cys(StBu)-Lys(DOTA-⁶⁸Ga)-CBT (CBT-⁶⁸Ga) and demonstrated that coinjection of the radiotracer with its cold analogue CBT-Ga instructed the formation of ⁶⁸Ga nanoparticles in furin-overexpressing MDA-MB-468 cancer cells, which significantly enhanced microPET imaging of the tumor in vivo. In vitro results showed that CBT-Ga subjected to furin-initiated CBT-Cys condensation reaction and self-assembly to form the nanoparticles CBT-Ga-NPs with an average diameter of 258.3 nm. In vivo microPET imaging results indicate that the mice coinjected with CBT-⁶⁸Ga and CBT-Ga, which warrants ⁶⁸Ga nanoparticle formation in their MDA-MB-468 tumors, had a tumor/liver ratio 9.1-fold of that of the mice only injected with CBT-⁶⁸Ga. We envisioned that, by replacing the RVRR substrate of CBT-⁶⁸Ga with other enzyme-specific ones and using the strategy of intracellular nanoparticle formation, a series of radioactive probes could be developed for more sensitive and precise tumor microPET imaging in the near future.

Carboxylesterase-Cleavable Biotinylated Nanoparticle for Tumor-Dual Targeted Imaging

Near-infrared (NIR) nanoprobes with fluorescence "Turn-On" property are advantageous in cancer diagnosis but, to the best of our knowledge, "smart" nanoprobe that simultaneously targets both biotin receptor and carboxylesterase (CES) for HepG2 tumor-dual targeted imaging has not been reported. Methods: Using CBT-Cys click condensation reaction, we rationally designed a "smart" NIR fluorescence probe H2N-Cys(StBu)-Lys(Biotin)-Ser(Cy5.5)-CBT (NIR-CBT) and used it to facilely prepare the fluorescence-quenched nanoparticle NIR-CBT-NP. Results: In vitro results indicated that, after NIR-CBT-NP was incubated with CES for 6 h, its fluorescence was turned "On" by 69 folds. Cell experiments verified that NIR-CBT-NP was uptaken by HepG2 cells via biotin receptor-assisted endocytosis and its fluorescence was turned "On" by intracellular CES hydrolysis. Moreover, NIR-CBT-NP was successfully applied to image both biotin receptor- and CES-overexpressing HepG2 tumors. Conclusion: Fluorescence-quenched nanoparticle NIR-CBT-NP was facilely prepared to actively target biotin receptor-overexpressing HepG2 cancer cells and turn the fluorescence "On" by intracellular CES hydrolysis for tumor-dual targeted imaging. We anticipate that our fluorescence "Turn-On" nanoparticle could be applied for liver cancer diagnosis in clinic in the near future.

Chemiluminescence and Bioluminescence Imaging for Biosensing and Therapy: In Vitro and In Vivo Perspectives

Chemiluminescence (CL) and bioluminescence (BL) imaging technologies, which require no external light source so as to avoid the photobleaching, background interference and autoluminescence, have become powerful tools in biochemical analysis and biomedical science with the development of advanced imaging equipment. CL imaging technology has been widely applied to high-throughput detection of a variety of analytes because of its high sensitivity, high efficiency and high signal-to-noise ratio (SNR). Using luciferase and fluorescent proteins as reporters, various BL imaging systems have been developed innovatively for real-time monitoring of diverse molecules in vivo based on the reaction between luciferin and the substrate. Meanwhile, the kinetics of protein interactions even in deep tissues has been studied by BL imaging. In this review, we summarize in vitro and in vivo applications of CL and BL imaging for biosensing and therapy. We first focus on in vitro CL imaging from the view of improving the sensitivity. Then, in vivo CL applications in cells and tissues based on different CL systems are demonstrated. Subsequently, the recent in vitro and in vivo applications of BL imaging are summarized. Finally, we provide the insight into the development trends and future perspectives of CL and BL imaging technologies.

Enzymatic Assemblies Disrupt the Membrane and Target Endoplasmic Reticulum for Selective Cancer Cell Death

The endoplasmic reticulum (ER) is responsible for the synthesis and folding of a large number of proteins, as well as intracellular calcium regulation, lipid synthesis, and lipid transfer to other organelles, and is emerging as a target for cancer therapy. However, strategies for selectively targeting the ER of cancer cells are limited. Here we show that enzymatically generated crescent-shaped supramolecular assemblies of short peptides disrupt cell membranes and target ER for selective cancer cell death. As revealed by sedimentation assay, the assemblies interact with synthetic lipid membranes. Live cell imaging confirms that the assemblies impair membrane integrity, which is further supported by lactate dehydrogenase (LDH) assays. According to transmission electron microscopy (TEM), static light scattering (SLS), and critical micelle concentration (CMC), attaching an l-amino acid at the C-terminal of a d-tripeptide results in the crescent-shaped supramolecular assemblies. Structure-activity relationship suggests that the crescent-shaped morphology is critical for interacting with membranes and for controlling cell fate. Moreover, fluorescent imaging indicates that the assemblies accumulate on the ER. Time-dependent Western blot and ELISA indicate that the accumulation causes ER stress and subsequently activates the caspase signaling cascade for cell death. As an approach for in situ generating membrane binding scaffolds (i.e., the crescent-shaped supramolecular assemblies), this work promises a new way to disrupt the membrane and to target the ER for developing anticancer therapeutics.

Self-assembly of Fluorescent Dehydroberberine Enhances Mitochondria-Dependent Antitumor Efficacy

Selective imaging and inducing mitochondrial dysfunction in tumor cells using mitochondria-targeting probes has become as a promising approach for cancer diagnosis and therapy. Here, we report the design of a fluorescent berberine analog, dehydroberberine (DH-BBR), as a new mitochondria-targeting probe capable of self-assembling into monodisperse organic nanoparticles (DTNPs) upon integration with a lipophilic counter anion, allowing for enhanced fluorescence imaging and treatment of tumors in living mice. X-ray crystallography revealed that the self-assembly process was attributed to a synergy of different molecular interactions, including π-π stacking, O⋅⋅⋅π interaction and electrostatic interaction between DH-BBR and counter anions. We demonstrated that DTNPs could efficiently enter tumor tissue following intravenous injection and enhance mitochondrial delivery of DH-BBR via an electrostatic interaction driven anion exchange process. Selective accumulation in the mitochondria capable of emitting strong fluorescence and causing mitochondrial dysfunction was achieved, enabling efficient inhibition of tumor growth in living mice. This study demonstrates promise for applying lipophilic anions to control molecular self-assembly and tune antitumor activity of mitochondria-targeting probes, which can facilitate to improve cancer treatment in vivo.

Kinetic Analysis of Nanostructures Formed by Enzyme-Instructed Intracellular Assemblies against Cancer Cells

Recent studies have demonstrated that enzyme-instructed self-assembly (EISA) in extra- or intracellular environments can serve as a multistep process for controlling cell fate. There is little knowledge, however, about the kinetics of EISA in the complex environments in or around cells. Here, we design and synthesize three dipeptidic precursors (ld-1-SO₃, dl-1-SO₃, dd-1-SO₃), consisting of diphenylalanine (l-Phe-d-Phe, d-Phe-l-Phe, d-Phe-d-Phe, respectively) as the backbone, which are capped by 2-(naphthalen-2-yl)acetic acid at the N-terminal and by 2-(4-(2-aminoethoxy)-4-oxobutanamido)ethane-1-sulfonic acid at the C-terminal. On hydrolysis by carboxylesterases (CES), these precursors result in hydrogelators, which self-assemble in water at different rates. Whereas all three precursors selectively kill cancer cells, especially high-grade serous ovarian carcinoma cells, by undergoing intracellular EISA, dl-1-SO₃ and dd-1-SO₃ exhibit the lowest and the highest activities, respectively, against the cancer cells. This trend inversely correlates with the rates of converting the precursors to the hydrogelators in phosphate-buffered saline. Because CES exists both extra- and intracellularly, we use kinetic modeling to analyze the kinetics of EISA inside cells and to calculate the cytotoxicity of each precursor for killing cancer cells. Our results indicate that (i) the stereochemistry of the precursors affects the morphology of the nanostructures formed by the hydrogelators, as well as the rate of enzymatic conversion; (ii) decreased extracellular hydrolysis of precursors favors intracellular EISA inside the cells; (iii) the inherent features ( e.g., self-assembling ability and morphology) of the EISA molecules largely dictate the cytotoxicity of intracellular EISA. As the kinetic analysis of intracellular EISA, this work elucidates how the stereochemistry modulates EISA in the complex extra- and/or intracellular environment for developing anticancer molecular processes. Moreover, it provides insights for understanding the kinetics and cytotoxicity of aggregates of aberrant proteins or peptides formed inside and outside cells.

Nanoparticle Regrowth Enhances Photoacoustic Signals of Semiconducting Macromolecular Probe for In Vivo Imaging

Smart molecular probes that emit deep-tissue penetrating photoacoustic (PA) signals responsive to the target of interest are imperative to understand disease pathology and develop innovative therapeutics. This study reports a self-assembly approach to develop semiconducting macromolecular activatable probe for in vivo imaging of reactive oxygen species (ROS). This probe comprises a near-infrared absorbing phthalocyanine core and four poly(ethylene glycol) (PEG) arms linked by ROS-responsive self-immolative segments. Such an amphiphilic macromolecular structure allows it to undergo an ROS-specific cleavage process to release hydrophilic PEG and enhance the hydrophobicity of the nanosystem. Consequently, the residual phthalocyanine component self-assembles and regrows into large nanoparticles, leading to ROS-enhanced PA signals. The small size of the intact macromolecular probe is beneficial to penetrate into the tumor tissue of living mice, while the ROS-activated regrowth of nanoparticles prolongs the retention along with enhanced PA signals, permitting imaging of ROS during chemotherapy. This study thus capitalizes on stimuli-controlled self-assembly of macromolecules in conjunction with enhanced heat transfer in large nanoparticles for the development of smart molecular probes for PA imaging.

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

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

Metabolism of the Endocannabinoid Anandamide: Open Questions after 25 Years

Cannabis extracts have been used for centuries, but its main active principle ∆⁹-tetrahydrocannabinol (THC) was identified about 50 years ago. Yet, it is only 25 years ago that the first endogenous ligand of the same receptors engaged by the cannabis agents was discovered. This “endocannabinoid (eCB)” was identified as N-arachidonoylethanolamine (or anandamide (AEA)), and was shown to have several receptors, metabolic enzymes and transporters that altogether drive its biological activity. Here I report on the latest advances about AEA metabolism, with the aim of focusing open questions still awaiting an answer for a deeper understanding of AEA activity, and for translating AEA-based drugs into novel therapeutics for human diseases.

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

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

Prolonged bioluminescence imaging in living cells and mice using novel pro-substrates forRenillaluciferase

The prodrug or caged-luciferin strategy affords an excellent platform for persistent bioluminescence imaging.

Intracellular synthesis of d-aminoluciferin for bioluminescence generation

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