Atg4B and Cathepsin B-Triggered in Situ Luciferin Formation for Precise Cancer Autophagy Bioluminescence Imaging

In vivo monitoring of endogenous hydrogen sulfide and evaluation of natural protectants in liver injury mice using a highly selective bioluminescent probe

Hydrogen sulfide (H2S) is an essential endogenous gasotransmitter that can regulate a wide range of physiological processes. However, overproduction of H2S is toxic to humans and causes liver injury, cardiovascular diseases, central nervous system-related disease, diabetes, even cancer. Hence, designing efficient imaging probes for real-time monitoring of the alterations in endogenous H2S is a viable tactic for accurate diagnosis of these diseases. In this work, a bioluminescence (BL) probe, namely Luc-H2S, has been developed to achieve H2S detection in vitro and in vivo. This sensing probe enables a selective BL turn-on response to H2S, and showcases excellent sensitivity with a low detection limit (LOD = 0.337 μM). Furthermore, Luc-H2S has been successfully applied in BL imaging of endogenous H2S in cells, tumor-bearing mice and drug-induced liver injury mice. More importantly, Luc-H2S is utilized to accurately evaluate the protective effects of natural products against alcohol-induced liver injury through monitoring of the H2S fluctuations. We envision that Luc-H2S holds promise as a powerful imaging tool for diagnosis of H2S-mediated diseases and evaluation of drug therapy.

Alkaline Phosphatase and ATG4B Sequentially Activated Fluorescent Probe for Cancer Cell-Specific Live Imaging of Autophagy

Tracking autophagy in cancer cells is crucial for enhancing cancer therapies. Existing methods are often inefficient and cannot distinguish cancer from normal cells during autophagy. Herein, a sequentially activated peptide probe, NBD-1p-Dabcyl, was developed for achieving cancer cell-specific imaging of autophagy. The probe self-assembled and fluoresced brightly upon sequential processing by alkaline phosphatase (ALP) and autophagy-related protease (ATG4B), where NBD-1p-Dabcyl was dephosphorylated by ALP to give NBD-1-Dabcyl, which was then processed by ATG4B into nanofibers emitting strong fluorescence. Notably, the bright fluorescence of NBD was observed in cancer cells MDA-MB-231 and HeLa, while normal cells NIH3T3 exhibited weaker fluorescence, allowing differentiation between cancer and normal cells using a rapamycin (Rap)-induced autophagy cell model. The enhanced fluorescence in cancer cells was attributed to the higher activities of intracellular ALP and ATG4B. Next, NBD-1p-Dabcyl was used to assess the inhibition efficiency of an autophagy inhibitor NSC 185058 in MDA-MB-231 cells, where a strong correlation between fluorescence intensity and inhibitor concentration suggested that NBD-1p-Dabcyl could predict the activity of autophagy inhibitors. Finally, animal experiments revealed that NBD-1p-Dabcyl effectively facilitated in situ fluorescence imaging of autophagy in tumor tissues. The design of this sequentially activated peptide probe offers a practical approach for monitoring autophagy in cancer cells, enabling high-throughput screening of autophagy inhibitors for cancer therapy.

Visualization of mitochondrial molecular dynamics during mitophagy process by label-free surface-enhanced Raman scattering spectroscopy

BACKGROUND

Mitophagy is a selective way to eliminate dysfunctional mitochondria and recycle their constituents, which plays an important role in regulating and maintaining intracellular homeostasis. Real-time monitoring mitophagy process is of great importance for cellular physiological and pathological processes related to mitochondria. Howbeit, most of the current methods only focus on single-parameter detection of mitochondrial microenvironmental changes such as pH, viscosity and polarity. The mitochondrial molecular responses under mitophagy are not clear. Therefore, developing a new and simple method for molecular profiling is of great importance for accurately and comprehensively visualizing mitophagy.

RESULTS

In this work, Au NPs-based mitochondria-targeting nanoprobe was developed and the nanoprobe-based label-free surface enhanced Raman spectroscopy (SERS) method was proposed to track starvation induced mitophagy process at molecular level. The nanoprobe displayed good SERS performance and low cytotoxicity. Based on the developed strategy, the molecular response within mitochondria under mitophagy was validated. Meanwhile, the protein denaturation, conformational change, lipid degradation and DNA fragmentation within mitochondria under mitophagy were revealed for the first time, which provides molecular evidence for mitophagy. The changes in reactive oxygen species level and mitochondrial membrane potential further confirmed the damage of mitochondria. Moreover, the developed label-free SERS strategy was used to detect mitophagy in drug (cisplatin)-induced liver injury (DILI) cell model, and obvious mitophagy in DILI cells was observed.

SIGNIFICANCE

The molecular biochemical signature dynamic changes within mitochondria during mitophagy process were revealed by SERS for the first time. Moreover, compared with the current research, our study can provide new insights into mitophagy and mitophagy-involved diseases at molecular level. This study will provide new insights into the molecular mechanism of mitophagy and offer a simple and effective method for mitochondrial molecular event monitoring in mitophagy-involved cellular processes.

A Dual-Targeted Molecule for Disease-Activatable Proteolysis Targeting Chimeras and Targeted Radionuclide Therapy of Cancer

Proteolysis targeting chimeras (PROTACs) represent a cutting-edge approach for targeted protein degradation in cancer therapy, yet they face challenges such as poor pharmacokinetics and specificity issues, leading to undesirable off-target effects and limited antitumor potency. To address these issues, we introduce dual-targeted unimolecular theranostic probes (e.g., radioactive 177Lu-P-A and its cold counterpart natLu-P-A) for disease-activatable PROTACs in combination with targeted radionuclide therapy (TRT) against prostate cancer with high specificity and effectiveness. The probes achieve a cathepsin B (CTSB)-activatable pro-PROTAC moiety for precise degradation of bromodomain-containing protein 4 (BRD4) and a prostate-specific membrane antigen (PSMA)-targeted 177Lu-based TRT. Owing to the favorable pharmacokinetics and PSMA-mediated excellent targeting efficiency, the probe possesses high tumor imaging specificity and accumulation capacity of therapeutic units for highly effective PROTACs and TRT. In contrast, the free PROTACs unit (e.g., ARV-771) shows no observable therapeutic effect due to its poor targeting ability. Importantly, the BRD4 proteolysis by PROTAC activation can downregulate radiosensitivity-associated RAD51AP1 expression, synergistically enhancing the TRT effect and promoting apoptosis after combined therapy compared to individual treatment regimes. Additionally, the probe demonstrates high renal clearance, underscoring its biosafety for potential clinical translation. This study presents a potential approach for precise PROTACs combined with TRT for effective tumor therapy.

Enzyme-Activated Orthogonal Proteolysis Chimeras for Tumor Microenvironment-Responsive Immunomodulation

Precise modulation of dynamic and complex tumor microenvironment (TME) to disrupt tumorigenesis and reshape intratumoral immune infiltration has emerged as promising approaches for enhanced cancer therapy. Among recent innovations, proteolysis-targeting chimeras (PROTACs) represent a burgeoning chemical knockdown technology capable of degrading oncogenic protein homeostasis and inducing dynamic alternations within carcinoma settings, offering potential for antitumor manipulation. However, achieving selectivity in PROTACs that respond to disease environmental stimulation and precisely perturb on-target proteins remains challenging. The multi-step synthesis and limited permeability, attributed to high-molecular-weight and heterobifunctional structures, further hinder their in vivo efficacy. Herein, we present a unique TME-responsive enzyme-activated clickable PROTACs, which features a short peptide-tagged pomalidomide derivative to undergo tumor-specific cleavage by cathepsin protease to induce orthogonal crosslinking of the exposed cysteine with 2-cyanobenzothiazole-labeled epigenetic protein-ligand JQ1, facilitating in situ degrader formation within tumor regions only. Systematic protein profiling and proteomic analysis revealed that such TME-specific clickable-PROTACs not only selectively eliminate epigenetic proteins without tedious pre-synthesis to bridge disparate small-molecule bi-warhead fragments, but also demonstrated superior tumor penetration compared to conventional high-molecular-weight PROTACs. Importantly, these clickable-PROTACs efficiently downregulated immune checkpoint programmed death-ligand 1 (PD-L1) both in vitro and in vivo, remodeling TME for enhanced therapeutics, especially in anti-tumoral immunomodulation.

Dual-Locked Enzyme-Activatable Fluorescence Probes for Precise Bioimaging

Real-time visualization of endogenous enzymes not only helps reveal the underlying biological principles but also provides pathological information for cancer/disease diagnosis and even treatment guidance. To this end, enzyme-activatable fluorescence probes are frequently fabricated that turn their fluorescence signals "on" exclusively at the enzyme-rich region, thus enabling noninvasive and real-time imaging of enzymes of interest at the molecular level with superior sensitivity and selectivity. However, in a complex biological context, commonly used single enzyme-activatable (i.e., single-locked) probes may suffer from "false positive" signals at healthy tissues and be insufficient to accurately indicate the occurrence of certain diseases. Therefore, dual-locked fluorescence probes have been promoted to address these issues. Using dual enzymes (or an enzyme with another stimulus) as "keys", they permit simultaneous detection of distinct biomarkers, offering significantly enhanced imaging precision toward certain biological events. Considering that recent reviews on these probes remain scarce, we thus provide this review. We summarize the recent progress, particularly highlighting the breakthroughs in the last three years, and discuss the challenges in this field.

Activity-Based Bioluminescent Logic-Gate Probe Reveals Crosstalk Between the Inflammatory Tumor Microenvironment and ALDH1A1 in Cancer Cells

Cancer cells with high expression of aldehyde dehydrogenase 1A1 (ALDH1A1) are more resistant to chemotherapy, contribute to tumor progression, and are associated with poor clinical outcomes. ALDH1A1 plays a critical role in protecting cells from reactive aldehydes and, in the case of stem cells, regulates their differentiation through the retinoic acid signaling pathway. Despite the importance of this enzyme, methods to study ALDH1A1 high-expressing cancer cells in vivo remain limited. In this work, we developed AlDeLuc, the first logic-gated bioluminescence probe designed to selectively evaluate ALDH1A1 activity in tumor cells. The probe is sequentially activated by acidic intracellular compartments (i.e., endosomes) and ALDH1A1, ensuring precise detection of ALDH1A1 high-expressing cells and minimizing off-target detection of non-ALDH1A1 cells. Beyond demonstrating efficacy in multiple cancer cell lines and a murine model of breast cancer, we employed AlDeLuc to investigate how the population of ALDH1A1 high-expressing cells is influenced by the inflammatory status of a tumor in the context of a high-fat diet. These findings establish a molecular link between obesity, inflammation, and tumor progression.

Light-Controlled Intracellular Synthesis of Poly(luciferin) Polymers Induces Cell Paraptosis

Accumulation of misfolded proteins challenges cellular proteostasis and is implicated in aging and chronic disorders. Cancer cells, moreover, face an elevated level of basal proteotoxic stress; hence, exacerbating endoplasmic reticulum (ER) stress has been shown to induce programmed cell death while enhancing anticancer immunogenicity. We hypothesize that hydrophobic abiotic macromolecules can trigger a similar stress response. Most polymers and nanoparticles, however, are sequestered in endo/lysosomes after endocytosis, which prevents their interaction with the proteostasis machinery. We adopted an in situ polymerization approach to synthesize polymers in cells with cell-permeable monomers. Specifically, we developed a biocompatible polycondensation between l-cysteine and 2-cyanobenzothiazole (CBT) with photochemical control to form insoluble poly(luciferin) aggregates. We identified that in situ polymerization activates the BiP-PERK-CHOP pathway of the unfolded protein response and that the unresolved ER stress initiates a form of regulated cell death consistent with paraptosis. In addition, the dying cells emit damage-associated molecular patterns (DAMPs), indicating an immunogenic cell death that could potentiate antitumor immunity. Our results show that in situ polymerization mimics misfolded protein aggregates to induce proteotoxic stress and cancer cell death, offering a novel therapeutic strategy to exploit cancer vulnerability.

Inspired by nature: Bioluminescent systems for bioimaging applications

Bioluminescence is a natural process where biological organisms produce light through chemical reactions. These reactions predominantly occur between small-molecule substrates and luciferase within bioluminescent organisms. Bioluminescence imaging (BLI) has shown significant potential in biomedical research owing to its non-invasive, real-time observation and quantification. In this review, we introduced the chemical mechanism of bioluminescent systems and categorized several strategies that successfully addressed the native limitations, including improvements on the chemical structures of luciferase-luciferin bioluminescence system and bioluminescence resonance energy transfer (BRET) methods. In addition, we also reviewed and summarized recent advances in bioimaging applications. We hope that this review can provide effective guidance for the development and application of bioluminescent systems in the field of bioimaging.

Spatially Quantitative Imaging of Enzyme Activity in a Living Cell

Enzyme activity plays a key role in cell heterogeneity. Its spatially quantitative imaging in a living cell not only directly displays but also helps people to understand cell heterogeneity. Current methods are hard to achieve due to the short intracellular retention or lack of internal reference of the imaging probes. Herein, we rationally designed a self-referenced Raman probe Val-Cit-Cys(StBu)-Pra-Gly-CBT (Yne-CBT) which takes an intracellular cathepsin B (CTSB)-initiated CBT-Cys click reaction to yield a long-retained cyclic dimer in cell. In the meantime, Raman signal changes of its two chemical bonds (C≡C and C≡N) after the reaction are used for self-referencing and quantitative Raman imaging of CTSB activity. In vitro experiments demonstrated that, with shell-isolated nanoparticle-enhanced Raman spectroscopy technique, 20 μM Yne-CBT was able to quantitatively detect CTSB activity with a limit of detection of 61.4 U L-1. Under a homemade microfluidic channel, Yne-CBT was successfully applied for spatially quantitative imaging CTSB activity in a living cell. Our strategy provides people with a facile method to directly and quantitatively display cell heterogeneity.

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.

Cathepsin B: The dawn of tumor therapy

Cathepsin B (CTSB) is a key lysosomal protease that plays a crucial role in the development of cancer. This article elucidates the relationship between CTSB and cancer from the perspectives of its structure, function, and role in tumor growth, migration, invasion, metastasis, angiogenesis and autophagy. Further, we summarized the research progress of cancer treatment related drugs targeting CTSB, as well as the potential and advantages of Traditional Chinese medicine in treating tumors by regulating the expression of CTSB.