Exploring the Condensation Reaction between Aromatic Nitriles and Amino Thiols To Optimize In Situ Nanoparticle Formation for the Imaging of Proteases and Glycosidases in Cells

Ultrasensitive two-photon probes for rapid detection of β-galactosidase during fruit softening and cellular senescence

Senescence is happening in every corner of the living organisms. β-galactosidase (β-gal) is one of the most important biomarkers during senescence in both plant and mammalian cells. Most β-gal fluorescent probes were focused on bio-imaging, only a few probes were developed for the detection of β-gal in fruit, and the probes that could detect β-gal in both fruits and living cells were even less. Here, two β-gal probes (TNap-βGal and TBNap-βGal) were synthesized, which can not only image the increase of β-gal during both fruits softening and cellular senescence, but also prove that bananas are not suitable for storage in refrigerator and the subsequent accumulation of β-gal still in lysosome of mammalian cells. In addition, TNap-βGal was successfully applied to two-photon imaging of endogenous β-gal in both hDPMSCs and tissues of human dental pulp for the first time.

The Nitrile Bis-Thiol Bioconjugation Reaction

Electron-poor aryl nitriles are promising reagents for bioconjugation due to their high electrophilicity and selectivity for reaction with thiols, albeit generally in a reversible manner. A transient species has previously been observed in such reactions, involving the addition of two thiols to the nitrile functional group, forming a tetrahedral amino dithioacetal (ADTA). In this work, the reaction of heteroaryl nitriles with bis-thiols is explored in an attempt to generate stable ADTAs, which could facilitate new bioconjugation protocols. By use of a 1,2-dithiol, or the incorporation of an electrophilic trap into the aryl nitrile design, the formation of stable products is achieved. The resultant "nitrile bis-thiol" (NBT) reaction is then explored in the context of protein modification, specifically to carry out antibody conjugation. By addition of these nitriles to the reduced disulfide bond of an antibody fragment, it is shown that, depending on the reagent design, cysteine-to-lysine transfer or disulfide bridged NBT products can be generated. Both represent site-selective conjugates and are shown to be stable when challenged with glutathione under physiological conditions and upon incubation in serum. Furthermore, the NBT reaction is tested in the more challenging context of a full antibody, and all four disulfide bonds are effectively modified by these new one-carbon bridging reagents. Overall, this reaction of heteroaryl-nitriles with bis-thiols is shown to be highly efficient and versatile, of tunable reversibility, and offers enticing prospects as a new addition to the toolbox of biocompatible "click"-type reactions.

Recent Advances in the Development of Non-Invasive Imaging Probes for Cancer Immunotherapy

The last two decades have witnessed a major revolution in the field of tumor immunology including clinical progress using various immunotherapy strategies. These advances have highlighted the potential for approaches that harness the power of the immune system to fight against cancer. While cancer immunotherapies have shown significant clinical successes, patient responses vary widely due to the complex and heterogeneous nature of tumors and immune responses, calling for reliable biomarkers and therapeutic strategies to maximize the benefits of immunotherapy. Especially, stratifying responding individuals from non-responders during the early stages of treatment could help avoid long-term damage and tailor personalized treatments. In efforts to develop non-invasive means for accurately evaluating and predicting tumor response to immunotherapy, multiple affinity-based agents targeting immune cell markers and checkpoint molecules have been developed and advanced to clinical trials. In addition, researchers have recently turned their attention to substrate and activity-based imaging probes that can provide real-time, functional assessment of immune response to treatment. Here, we highlight some of those recently designed probes that image functional proteases as biomarkers of cancer immunotherapy with a focus on their chemical design and detection modalities and discuss challenges and opportunities for the development of imaging tools utilized in cancer immunotherapy.

DNA Nanoribbon-Assisted Intracellular Biosynthesis of Fluorescent Gold Nanoclusters for Cancer Cell Imaging

Metal nanoclusters (NCs) have emerged as a promising class of fluorescent probes for cellular imaging due to their high resistance to photobleaching and low toxicity. Nevertheless, their widespread use in clinical diagnosis is limited by their unstable intracellular fluorescence. In this study, we develop an intracellularly biosynthesized fluorescent probe, DNA nanoribbon-gold NCs (DNR/AuNCs), for long-term cellular tracking. Our results show that DNR/AuNCs exhibit a 4-fold enhancement of intracellular fluorescence intensity compared to free AuNCs. We also investigated the mechanism underlying the fluorescence enhancement of AuNCs by DNRs. Our findings suggest that the higher synthesis efficiency and stability of AuNCs in the lysosome may contribute to their fluorescence enhancement, which enables long-term (up to 15 days) fluorescence imaging of cancer cells (enhancement of ∼60 times compared to free AuNCs). Furthermore, we observe similar results with other metal NCs, confirming the generality of the DNR-assisted biosynthesis approach for preparing highly bright and stable fluorescent metal NCs for cancer cell imaging.

Nanoprobe-based molecular imaging for tumor stratification

The responses of patients to tumor therapies vary due to tumor heterogeneity. Tumor stratification has been attracting increasing attention for accurately distinguishing between responders to treatment and non-responders. Nanoprobes with unique physical and chemical properties have great potential for patient stratification. This review begins by describing the features and design principles of nanoprobes that can visualize specific cell types and biomarkers and release inflammatory factors during or before tumor treatment. Then, we focus on the recent advancements in using nanoprobes to stratify various therapeutic modalities, including chemotherapy, radiotherapy (RT), photothermal therapy (PTT), photodynamic therapy (PDT), chemodynamic therapy (CDT), ferroptosis, and immunotherapy. The main challenges and perspectives of nanoprobes in cancer stratification are also discussed to facilitate probe development and clinical applications.

Orderly Self-Assembly of Organic Fluorophores for Sensing and Imaging

Fluorescence imaging utilizing traditional organic fluorophores is extensively applied in both cellular and in vivo studies. However, it faces significant obstacles, such as low signal-to-background ratio (SBR) and spurious positive/negative signals, primarily due to the facile diffusion of these fluorophores. To cope with this challenge, orderly self-assembled functionalized organic fluorophores have gained significant attention in the past decades. These fluorophores can create nanoaggregates via a well-ordered self-assembly process, thus prolonging their residency time within cells and in vivo settings. The development of self-assembled-based fluorophores is an emerging field, and as such, in this review, we present a summary of the progress and challenges of self-assembly fluorophores, focusing on their development history, self-assembly mechanisms, and biomedical applications. We hope that the insights provided herein will assist scientists in further developing functionalized organic fluorophores for in situ imaging, sensing, and therapy.

Enzyme-Instructed Aggregation/Dispersion of Fluorophores for Near-Infrared Fluorescence Imaging In Vivo

Near-infrared (NIR) fluorescence is a noninvasive, highly sensitive, and high-resolution modality with great potential for in vivo imaging. Compared with "Always-On" probes, activatable NIR fluorescent probes with "Turn-Off/On" or "Ratiometric" fluorescent signals at target sites exhibit better signal-to-noise ratio (SNR), wherein enzymes are one of the ideal triggers for probe activation, which play vital roles in a variety of biological processes. In this review, we provide an overview of enzyme-activatable NIR fluorescent probes and concentrate on the design strategies and sensing mechanisms. We focus on the aggregation/dispersion state of fluorophores after the interaction of probes and enzymes and finally discuss the current challenges and provide some perspective ideas for the construction of enzyme-activatable NIR fluorescent probes.

Tunable Heteroaromatic Nitriles for Selective Bioorthogonal Click Reaction with Cysteine

The binucleophilic properties of 1,2-aminothiol and its rare occurrence in nature make it a useful reporter for tracking molecules in living systems. The 1,2-aminothiol moiety is present in cysteine, which is a substrate for a biocompatible click reaction with heteroaromatic nitriles. Despite the wide range of applications for this reaction, the scope of nitrile substrates has been explored only to a limited extent. In this study, we expand the chemical space of heteroaromatic nitriles for bioconjugation under physiologically relevant conditions. We systematically assembled a library of 116 2-cyanobenzimidazoles, 1-methyl-2-cyanobenzimidazoles, 2-cyanobenzothiazoles, and 2-cyanobenzoxazoles containing electron-donating and electron-withdrawing substituents at all positions of the benzene ring. The compounds were evaluated for their stability, reactivity, and selectivity toward the N-terminal cysteine of model oligopeptides. In comparison to the benchmark 6-hydroxy-2-cyanobenzothiazole or 6-amino-2-cyanobenzothiazole, we provide highly selective and moderately reactive nitriles as well as highly reactive yet less selective analogs with a variety of enabling attachment chemistries to aid future applications in bioconjugation, chemical biology, and nanomaterial science.

Stimulus-Responsive Amino Acids Behind In Situ Assembled Bioactive Peptide Materials

In situ self-assembly of peptides into well-defined nanostructures represents one of versatile strategies for creation of bioactive materials within living cells with great potential in disease diagnosis and treatment. The intimate relationship between amino acid sequences and the assembling propensity of peptides has been thoroughly elucidated over the past few decades. This has inspired development of various controllable self-assembling peptide systems based on stimuli-responsive naturally occurring or non-canonical amino acids, including redox-, pH-, photo-, enzyme-responsive amino acids. This review attempts to summarize the recent progress achieved in manipulating in situ self-assembly of peptides by controllable reactions occurring to amino acids. We will highlight the systems containing non-canonical amino acids developed in our laboratory during the past few years, primarily including acid/enzyme-responsive 4-aminoproline, redox-responsive (seleno)methionine, and enzyme-responsive 2-nitroimidazolyl alanine. Utilization of the stimuli-responsive assembling systems in creation of bioactive materials will be specifically introduced to emphasize their advantages for addressing the concerns lying in disease theranostics. Eventually, we will provide the perspectives for the further development of stimulus-responsive amino acids and thereby demonstrating their great potential in development of next-generation biomaterials.

Polymer Chemistry in Living Cells

The polymerization of biomolecules is a central operation in biology that connects molecular signals with proliferative and information-rich events in cells. As molecules arrange precisely across 3-D space, they create new functional capabilities such as catalysis and transport highways and exhibit new phase separation phenomena that fuel nonequilibrium dynamics in cells. Hence, the observed polymer chemistry manifests itself as a molecular basis leading to cellular phenotypes, expressed as a multitude of hierarchical structures found in cell biology. Although many milestone discoveries had accompanied the rise of the synthetic polymer era, fundamental studies were realized within a closed, pristine environment and that their behavior in a complex multicomponent system remains challenging and thus unexplored. From this perspective, there is a rich trove of undiscovered knowledge that awaits the polymer science community that can revolutionize understanding in the interactive nanoscale world of the living cell.In this Account, we discuss the strategies that have enabled synthetic polymer chemistry to be conducted within the cells (membrane inclusive) and to establish monomer design principles that offer spatiotemporal control of the polymerization. As reaction considerations such as monomer concentration, polymer growth dynamics, and reactivities are intertwined with the subcellular environment and transport processes, we first provide a chemical narrative of each major cellular compartment. The conditions within each compartment will therefore set the boundaries on the type of polymer chemistry that can be conducted. Both covalent and supramolecular polymerization concepts are explored separately in the context of scaffold design, polymerization mechanism, and activation. To facilitate transport into a localized subcellular space, we show that monomers can be reversibly modified by targeting groups or stimulus-responsive motifs that react within the specific compartment. Upon polymerization, we discuss the characterization of the resultant polymeric structures and how these phase-separated structures would impact biological processes such as cell cycle, metabolism, and apoptosis. As we begin to integrate cellular biochemistry with in situ polymer science, we identify landmark challenges and technological hurdles that, when overcome, would lead to invaluable discoveries in macromolecular therapeutics and biology.

Enzyme-mediated intratumoral self-assembly of nanotheranostics for enhanced imaging and tumor therapy

Enzyme-mediated intratumoral self-assembled (EMISA) nanotheranostics represent a new class of smart agents for combined imaging and therapy of cancer. Cancer cells overexpress various enzymes that are essential for high metabolism, fast proliferation, and tissue invasion and metastasis. By conjugating small molecules that contain an enzyme-specific cleavage site to appropriate chemical linkers, it is possible to induce self-assembly of nanostructures in tumor cells having the target enzyme. This approach of injecting small theranostic molecules that eventually become larger nanotheranostics in situ avoids some of the major limitations that are encountered when injecting larger, pre-assembled nanotheranostics. The advantage of EMISA nanotheranostics include the avoidance of nonspecific uptake and rapid clearance by phagocytic cells, increased cellular accumulation, reduced drug efflux and prolonged cellular exposure time, all of which lead to an amplified imaging signal and therapeutic efficacy. We review here the different approaches that can be used for preparing EMISA-based organic, inorganic, or organic/inorganic hybrid nanotheranostics based on noncovalent interactions and/or covalent bonding. Imaging examples are shown for fluorescence imaging, nuclear imaging, photoacoustic imaging, Raman imaging, computed tomography imaging, bioluminescent imaging, and magnetic resonance imaging. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Biology-Inspired Nanomaterials > Peptide-Based Structures.

Multiparameter Longitudinal Imaging of Immune Cell Activity in Chimeric Antigen Receptor T Cell and Checkpoint Blockade Therapies

Longitudinal multimodal imaging presents unique opportunities for noninvasive surveillance and prediction of treatment response to cancer immunotherapy. In this work we first designed a novel granzyme B activated self-assembly small molecule, G-SNAT, for the assessment of cytotoxic T lymphocyte mediated cancer cell killing. G-SNAT was found to specifically detect the activity of granzyme B within the cytotoxic granules of activated T cells and engaged cancer cells in vitro. In lymphoma tumor-bearing mice, the retention of cyanine 5 labeled G-SNAT-Cy5 correlated to CAR T cell mediated granzyme B exocytosis and tumor eradication. In colorectal tumor-bearing transgenic mice with hematopoietic cells expressing firefly luciferase, longitudinal bioluminescence and fluorescence imaging revealed that after combination treatment of anti-PD-1 and anti-CTLA-4, the dynamics of immune cell trafficking, tumor infiltration, and cytotoxic activity predicted the therapeutic outcome before tumor shrinkage was evident. These results support further development of G-SNAT for imaging early immune response to checkpoint blockade and CAR T-cell therapy in patients and highlight the utility of multimodality imaging for improved mechanistic insights into cancer immunotherapy.

Development of a Promising 18F-Radiotracer for PET Imaging Legumain Activity In Vivo

Legumain has been found overexpressed in several cancers, which serves as an important biomarker for cancer diagnosis. In this research, a novel fluorine-18 labeled radioactive tracer [18F]SF-AAN targeting legumain was designed and synthesized for positron emission tomography (PET) imaging. Nonradioactive probe [19F]SF-AAN was obtained through chemical and solid phase peptide synthesis. After a simple one-step 18F labeling, the radiotracer [18F]SF-AAN was obtained with a high radiochemical conversion rate (>85%) and radiochemical purity (99%) as well as high molar activity (12.77 ± 0.50 MBq/nmol). The targeting specificity of [18F]SF-AAN for detecting legumain activity was investigated systematically in vitro and in vivo. In vitro cellular uptake assay showed that the uptake of [18F]SF-AAN in legumain-positive MDA-MB-468 cells was twice as much as that in legumain-negative PC-3 cells at 4 h. In vivo PET imaging revealed that the tumor uptake of [18F]SF-AAN in MDA-MB-468 tumor-bearing mice was about 2.7 times of that in PC-3 tumor-bearing mice at 10 min post injection. The experimental results indicated that [18F]SF-AAN could serve as a promising PET tracer for detecting the legumain expression sensitively and specifically, which would be beneficial for the diagnosis of legumain-related diseases.

Stimuli-Responsive Macrocyclization Scaffold Allows In Situ Self-Assembly of Radioactive Tracers for Positron Emission Tomography Imaging of Enzyme Activity

Target-enabled bioorthogonal reaction and self-assembly of a small-molecule probe into supramolecules have shown promise for molecular imaging. In this paper, we report a new stimuli-responsive bioorthogonal reaction scaffold (SF) for controlling in situ self-assembly by engineering the condensation reaction between 2-cyanobenzothiazole and cysteine. For probes with the SF scaffold, intramolecular cyclization took place soon after activation, which could efficiently outcompete free cysteine even at a low concentration and result in efficient aggregation in the target. Through integration with different enzyme-responsive substrates and an ammoniomethyl-trifluoroborate moiety (AmBF₃), two radioactive positron emission tomography (PET) tracers, [¹⁸F]SF-DEVD and [¹⁸F]SF-Glu, were designed, which showed high stability under physiological conditions and could produce clear PET signal in tumors to detect enzyme activity (e.g., caspase-3, γ-glutamyltranspeptidase) timely and accurately. Our results demonstrated that the scaffold SF could serve as a general molecular scaffold in the development of smart PET tracers for noninvasive imaging of enzyme activity, which could contribute to tumor detection and treatment efficacy evaluation.

Designing bioresponsive nanomaterials for intracellular self-assembly

Supramolecular assemblies are essential components of living organisms. Cellular scaffolds, such as the cytoskeleton or the cell membrane, are formed via secondary interactions between proteins or lipids and direct biological processes such as metabolism, proliferation and transport. Inspired by nature’s evolution of function through structure formation, a range of synthetic nanomaterials has been developed in the past decade, with the goal of creating non-natural supramolecular assemblies inside living mammalian cells. Given the intricacy of biological pathways and the compartmentalization of the cell, different strategies can be employed to control the assembly formation within the highly crowded, dynamic cellular environment. In this Review, we highlight emerging molecular design concepts aimed at creating precursors that respond to endogenous stimuli to build nanostructures within the cell. We describe the underlying reaction mechanisms that can provide spatial and temporal control over the subcellular formation of synthetic nanostructures. Showcasing recent advances in the development of bioresponsive nanomaterials for intracellular self-assembly, we also discuss their impact on cellular function and the challenges associated with establishing structure–bioactivity relationships, as well as their relevance for the discovery of novel drugs and imaging agents, to address the shortfall of current solutions to pressing health issues.

Creating artificial nanostructures inside living cells requires the careful design of molecules that can transform into active monomers within a complex cellular environment. This Review explores the recent development of bioresponsive precursors for the controlled formation of intracellular supramolecular assemblies.

Luciferin-Bioinspired Click Ligation Enables Hydrogel Platforms with Fine-Tunable Properties for 3D Cell Culture

There is an increasing interest in coupling reactions for cross-linking of cell-encapsulating hydrogels under biocompatible, chemoselective, and tunable conditions. Inspired by the biosynthesis of luciferins in fireflies, here we exploit the cyanobenzothiazole-cysteine (CBT-Cys) click ligation to develop polyethylene glycol hydrogels as tunable scaffolds for cell encapsulation. Taking advantage of the chemoselectivity and versatility of CBT-Cys ligation, a highly flexible gel platform is reported here. We demonstrate luciferin-inspired hydrogels with important advantages for cell encapsulation applications: (i) gel precursors derived from inexpensive reagents and with good stability in aqueous solution (>4 weeks), (ii) adjustable gel mechanics within physiological ranges (E = 180-6240 Pa), (iii) easy tunability of the gelation rate (seconds to minutes) by external means, (iv) high microscale homogeneity, (v) good cytocompatibility, and (iv) regulable biological properties. These flexible and robust CBT-Cys hydrogels are proved as supportive matrices for 3D culture of different cell types, namely, fibroblasts and human mesenchymal stem cells. Our findings expand the toolkit of click chemistries for the fabrication of tunable biomaterials.

Optimized molecular design of PET probe for the visualization of γ-glutamyltranspeptidase activity in tumors

A fluorine-18 labeled probe [18F]JM-10 was rationally designed and optimized, which can self-assemble to generate more rigid and hydrophobic dimers upon response to γ-glutamyltranspeptidase (GGT) and glutathione (GSH) in tumor cells.

Tumor Microenvironment Responsive "Head-to-Foot" Self-Assembly Nanoplatform for Positron Emission Tomography Imaging in Living Subjects

Sensitivity and specificity of molecular probes are two important factors in determining the accuracy of cancer diagnosis or the efficacy of cancer treatment. However, the development of probes with high sensitivity and strong specificity still poses many challenges. Herein, we report an ¹⁸F-labeled smart tracer ([¹⁸F]1) targeting cancer-associated biotin receptor (BR) and self-assembling into nanoparticles in response to intracellular glutathione. The tracer [¹⁸F]1 selectively targeted BR-positive cancer cells A549 and Hela and formed nanoparticles through self-assembly with an average diameter of 138.2 ± 16.3 nm. The character of self-assembly into nanoparticles enhanced the uptake and extended the retention of probe [¹⁸F]1 in the target tissue and hence improved the quality of positron emission tomography (PET) images. Thus, [¹⁸F]1 is a promising PET tracer for accurately detecting BR-positive cancers. Moreover, the tumor microenvironment responsive "head-to-foot" self-assembly nanoplatform is particularly attractive for development of other smart molecular probes.

Enzymatically Forming Intranuclear Peptide Assemblies for Selectively Killing Human Induced Pluripotent Stem Cells

Tumorigenic risk of undifferentiated human induced pluripotent stem cells (iPSCs), being a major obstacle for clinical application of iPSCs, requires novel approaches for selectively eliminating undifferentiated iPSCs. Here, we show that an l-phosphopentapeptide, upon the dephosphorylation catalyzed by alkaline phosphatase (ALP) overexpressed by iPSCs, rapidly forms intranuclear peptide assemblies made of α-helices to selectively kill iPSCs. The phosphopentapeptide, consisting of four l-leucine residues and a C-terminal l-phosphotyrosine, self-assembles to form micelles/nanoparticles, which transform into peptide nanofibers/nanoribbons after enzymatic dephosphorylation removes the phosphate group from the l-phosphotyrosine. The concentration of ALP and incubation time dictates the morphology of the peptide assemblies. Circular dichroism and FTIR indicate that the l-pentapeptide in the assemblies contains a mixture of an α-helix and aggregated strands. Incubating the l-phosphopentapeptide with human iPSCs results in rapid killing of the iPSCs (=<2 h) due to the significant accumulation of the peptide assemblies in the nuclei of iPSCs. The phosphopentapeptide is innocuous to normal cells (e.g., HEK293 and hematopoietic progenitor cell (HPC)) because normal cells hardly overexpress ALP. Inhibiting ALP, mutating the l-phosphotyrosine from the C-terminal to the middle of the phosphopentapeptides, or replacing l-leucine to d-leucine in the phosphopentapeptide abolishes the intranuclear assemblies of the pentapeptides. Treating the l-phosphopentapeptide with cell lysate of normal cells (e.g., HS-5) confirms the proteolysis of the l-pentapeptide. This work, as the first case of intranuclear assemblies of peptides, not only illustrates the application of enzymatic noncovalent synthesis for selectively targeting nuclei of cells but also may lead to a new way to eliminate other pathological cells that express a high level of certain enzymes.

Supramolecular Bioimaging through Signal Amplification by Combining Indicator Displacement Assay with Förster Resonance Energy Transfer

Fluorescent chemosensors are powerful imaging tools in the fields of life sciences and engineering. Based on the principle of supramolecular chemistry, indicator displacement assay (IDA) provides an alternative approach for constructing and optimizing chemosensors, which has the advantages of simplicity, tunability, and modularity. However, the application of IDA in bioimaging continues to face a series of challenges, including interfering signals, background noise, and inconsistent spatial location. Accordingly, we herein report a supramolecular bioimaging strategy of Förster resonance energy transfer (FRET)-assisted IDA by employing macrocyclic amphiphiles as the operating platform. By merging FRET with IDA, the limitations of IDA in bioimaging were addressed. As a proof of concept, the study achieved mitochondria-targeted imaging of adenosine triphosphate in live cells with signal amplification. This study opens a non-covalent avenue for bioimaging with advancements in tunability, generality, and simplicity, apart from the covalent approach.

Binding-Induced Fibrillogenesis Peptides Recognize and Block Intracellular Vimentin Skeletonization against Breast Cancer

Life is recognized as a sophisticated self-assembling material system. Cancer involves the overexpression and improper self-assembly of proteins, such as cytoskeleton protein vimentin, an emerging target related to tumor metastasis. Herein, we design a binding-induced fibrillogenesis (BIF) peptide that in situ forms fibrous networks, blocking the improper self-assembly of vimentin against cancer. The BIF peptide can bind to vimentin and subsequently perform fibrillogenesis to form fibers on vimentin. The resultant peptide fibrous network blocks vimentin skeletonization and inhibits the migration and invasion of tumor cells. In mouse models of tumor metastasis, the volume of tumor and the number of lung metastases are markedly decreased. Moreover, the efficacy of BIF peptide (5 mg/kg) is much higher than small molecular antimetastasis drug withaferin A (5 mg/kg) as a standard, indicating that the BIF peptide shows advantages over small molecular inhibitors in blocking the intracellular protein self-assembly.

Structure-Based Programming of Supramolecular Assemblies in Living Cells for Selective Cancer Cell Inhibition

Here we report on the design, synthesis, and assembly of an enzymatic programmable peptide system inspired by endocytic processes to induce molecular assemblies formation spatiotemporally in living cancer cells, resulting in glioblastoma cell death mainly in necroptosis. Our results indicate the stability and glycosylation of molecules play an essential role in determining the final bioactivity. Detailed mechanistic studies by CLSM, Flow cytometry, western blot, and Bio-EM suggest the site-specific formation of assemblies, which could induce the LMP and activate the downstream cell death pathway. Moreover, we also demonstrate that our strategy can boost the activity of commercial chemotherapy drug by escaping lysosome sequestration. We expected this work would be expanded towards artificial intelligent biomaterials for cancer therapy and imaging precisely.

Enhancing Rechargeable Persistent Luminescence via Organic Dye Sensitization

Owing to their unique afterglow ability, long-wavelength light activatable persistent luminescence (PersL) nanoparticles (PLNPs) have been emerging as an important category of imaging probes. Long-wavelength LED light has been shown to be effective in recharging these nanoparticles. However, finding a simple and effective method to amplify such renewable PersL signals under long-wavelength light is still a key challenge. Herein, we discovered that a dye-sensitization strategy was able to effectively boost the renewable PersL signals of the NIR emitting ZnGa₂ O₄ :Cr³⁺ (ZGC)) under long-wavelength LED light. Moreover, as a proof-of-principle tumorectomy demonstration, this new class of dye sensitized ZGC enabled simultaneous intraoperative anatomic tumor navigation and effective microscopic detection of tumor cells in pathological diagnosis.

[18F]-C-SNAT4: an improved caspase-3-sensitive nanoaggregation PET tracer for imaging of tumor responses to chemo- and immunotherapies

Positron emission tomography (PET) imaging of apoptosis can noninvasively detect cell death in vivo and assist in monitoring tumor response to treatment in patients. While extensive efforts have been devoted to addressing this important need, no apoptosis PET imaging agents have yet been approved for clinical use. This study reports an improved ¹⁸F-labeled caspase-sensitive nanoaggregation tracer ([¹⁸F]-C-SNAT4) for PET imaging of tumor response to chemo- and immunotherapies in preclinical mouse models.

METHODS

We rationally designed and synthesized a new PET tracer [¹⁸F]-C-SNAT4 to detect cell death both in vitro and in vivo. In vitro radiotracer uptake studies were performed on drug-sensitive and -resistant NSCLC cell lines (NCI-H460 and NCI-H1299, respectively) treated with cisplatin at different doses. In vivo therapy response monitoring by [¹⁸F]-C-SNAT4 PET imaging was evaluated with two treatment modalities-chemotherapy and immunotherapy in two tumor xenografts in mice. Radiotracer uptake in the tumors was validated ex vivo using γ-counting and cleaved caspase-3 immunofluorescence.

RESULTS

This [¹⁸F]-C-SNAT4 PET tracer was facilely synthesized and displayed improved serum stability profiles. [¹⁸F]-C-SNAT4 cellular update was elevated in NCI-H460 cells in a time- and dose-dependent manner, which correlated well with cell death. A significant increase in [¹⁸F]-C-SNAT4 uptake was measured in NCI-H460 tumor xenografts in mice. In contrast, a rapid clearance of [¹⁸F]-C-SNAT4 was observed in drug-resistant NCI-H1299 in vitro and in tumor xenografts. Moreover, in BALB/C mice bearing murine colon cancer CT26 tumor xenografts receiving checkpoint inhibitors, [¹⁸F]-C-SNAT4 showed its ability for monitoring immunotherapy-induced apoptosis and reporting treatment-responding mice from non-responding.

CONCLUSION

The uptake of [¹⁸F]-C-SNAT4 in tumors received chemotherapy and immunotherapy is positively correlated with the tumor apoptotic level and the treatment efficacy. [¹⁸F]-C-SNAT4 PET imaging can monitor tumor response to two different treatment modalities and predict the therapeutic efficacy in preclinical mouse models.

In Vivo Imaging of Methionine Aminopeptidase II for Prostate Cancer Risk Stratification

Prostate cancer is one of the most common malignancies worldwide, yet limited tools exist for prognostic risk stratification of the disease. Identification of new biomarkers representing intrinsic features of malignant transformation and development of prognostic imaging technologies are critical for improving treatment decisions and patient survival. In this study, we analyzed radical prostatectomy specimens from 422 patients with localized disease to define the expression pattern of methionine aminopeptidase II (MetAP2), a cytosolic metalloprotease that has been identified as a druggable target in cancer. MetAP2 was highly expressed in 54% of low-grade and 59% of high-grade cancers. Elevated levels of MetAP2 at diagnosis were associated with shorter time to recurrence. Controlled self-assembly of a synthetic small molecule enabled design of the first MetAP2-activated PET imaging tracer for monitoring MetAP2 activity in vivo. The nanoparticles assembled upon MetAP2 activation were imaged in single prostate cancer cells with post-click fluorescence labeling. The fluorine-18-labeled tracers successfully differentiated MetAP2 activity in both MetAP2-knockdown and inhibitor-treated human prostate cancer xenografts by micro-PET/CT scanning. This highly sensitive imaging technology may provide a new tool for noninvasive early-risk stratification of prostate cancer and monitoring the therapeutic effect of MetAP2 inhibitors as anticancer drugs. SIGNIFICANCE: This study defines MetAP2 as an early-risk stratifier for molecular imaging of aggressive prostate cancer and describes a MetAP2-activated self-assembly small-molecule PET tracer for imaging MetAP2 activity in vivo.

Nanoparticle-Based Activatable Probes for Bioimaging

Molecular imaging can provide functional and molecular information at the cellular or subcellular level in vivo in a noninvasive manner. Activatable nanoprobes that can react to the surrounding physiological environment or biomarkers are appealing agents to improve the efficacy, specificity, and sensitivity of molecular imaging. The physiological parameters, including redox status, pH, presence of enzymes, and hypoxia, can be designed as the stimuli of the activatable probes. However, the success rate of imaging nanoprobes for clinical translation is low. Herein, the recent advances in nanoparticle-based activatable imaging probes are critically reviewed. In addition, the challenges for clinical translation of these nanoprobes are also discussed in this review.

Supramolecular ensembles modified by near-infrared dyes and their biological applications

Near-infrared dyes possess the qualities of lower interference with biological autofluorescence, low photon scattering, and deep tissue penetration, and are being increasingly involved in the development of biomaterials for sensing and precision medicine. However, dyes usually suffer from the disadvantages of poor water solubility and photobleaching, factors that limit their application in vivo. The introduction of supramolecular ensembles can provide an ideal solution. This review presents recently developed supramolecular ensembles modified by near-infrared dyes. Compared with small-molecule fluorophores, the specific size of a supramolecular-based fluorophore endows it with longer circulation time in the bloodstream, increasing its chances of reaching a specific target. In addition, the construction of supramolecule-based fluorophores with versatile functions can be achieved by simple encapsulation or doping, instead of by complicated chemical synthesis. Thus, supramolecular-complex-based fluorophores offer high potential in diagnosis and therapy. This review outlines four different species of near-infrared dye based ensembles in terms of their method of formation, including simple encapsulation or doping and copolymerisation. Recently, a new technology has employed modified fluorophores for in situ self-assembly that form supramolecular ensembles at a specific position, thus solving the problem of poor uptake of nanoparticles by cells, and is included in this review. Finally, the future of this field is considered.

Prebiotic synthesis of cysteine peptides that catalyze peptide ligation in neutral water

Peptide biosynthesis is performed by ribosomes and several other classes of enzymes, but a simple chemical synthesis may have created the first peptides at the origins of life. α-Aminonitriles-prebiotic α-amino acid precursors-are generally produced by Strecker reactions. However, cysteine's aminothiol is incompatible with nitriles. Consequently, cysteine nitrile is not stable, and cysteine has been proposed to be a product of evolution, not prebiotic chemistry. We now report a high-yielding, prebiotic synthesis of cysteine peptides. Our biomimetic pathway converts serine to cysteine by nitrile-activated dehydroalanine synthesis. We also demonstrate that N-acylcysteines catalyze peptide ligation, directly coupling kinetically stable-but energy-rich-α-amidonitriles to proteinogenic amines. This rare example of selective and efficient organocatalysis in water implicates cysteine as both catalyst and precursor in prebiotic peptide synthesis.

The Aggregation Regularity Effect of Multiarylpyrroles on Their Near-Infrared Aggregation-Enhanced Emission Property

Increasing the quantum yield of near-infrared (NIR) emissive dyes is critical for biological applications because these fluorescent dyes generally show decreased emission efficiency under aqueous conditions. In this work, we designed and synthesized several multiarylpyrrole (MAP) derivatives, in which a furanylidene (FE) group at the 3-position of the pyrrole forms donor-π-acceptor molecules, MAP-FE, with a NIR emissive wavelength and aggregation-enhanced emission (AEE) features. Different alkyl chains of MAP-FEs linked to phenyl groups at the 2,5-position of the pyrrole ring resulted in different emissive wavelengths and quantum yields in aggregated states, such as powders or single crystals. Powder XRD data and single crystal analysis elucidated that the different lengths of alkyl chains had a significant impact on the regularity of MAP-FEs when they were forced to aggregate or precipitate, which affected the intermolecular interaction and the restriction degree of the rotating parts, which are essential components. Therefore, an increasing number of NIR dyes could be developed by this design strategy to produce efficient NIR dyes with AEE. Moreover, this method can provide general guidance for other related fields, such as organic solar cells and organic light-emitting materials, because they are all applied in the aggregated state.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Reduction Triggered In Situ Polymerization in Living Mice

"Smart" biomaterials that are responsive to physiological or biochemical stimuli have found many biomedical applications for tissue engineering, therapeutics, and molecular imaging. In this work, we describe in situ polymerization of activatable biorthogonal small molecules in response to a reducing environment change in vivo. We designed a carbohydrate linker- and cyanobenzothiazole-cysteine condensation reaction-based small molecule scaffold that can undergo rapid condensation reaction upon physiochemical changes (such as a reducing environment) to form polymers (pseudopolysaccharide). The fluorescent and photoacoustic properties of a fluorophore-tagged condensation scaffold before and after the transformation have been examined with a dual-modality optical imaging method. These results confirmed the in situ polymerization of this probe after both local and systemic administration in living mice.

Single-Molecule Observation of Intermediates in Bioorthogonal 2-Cyanobenzothiazole Chemistry

We report a single-molecule mechanistic investigation into 2-cyanobenzothiazole (CBT) chemistry within a protein nanoreactor. When simple thiols reacted reversibly with CBT, the thioimidate monoadduct was approximately 80-fold longer-lived than the tetrahedral bisadduct, with important implications for the design of molecular walkers. Irreversible condensation between CBT derivatives and N-terminal cysteine residues has been established as a biocompatible reaction for site-selective biomolecular labeling and imaging. During the reaction between CBT and aminothiols, we resolved two transient intermediates, the thioimidate and the cyclic precursor of the thiazoline product, and determined the rate constants associated with the stepwise condensation, thereby providing critical information for a variety of applications, including the covalent inhibition of protein targets and dynamic combinatorial chemistry.

Fluorogenic Enzyme-Triggered Domino Reactions Producing Quinoxalin-2(1H)-one-based Heterocycles

A simple and effective biocompatible domino reaction triggered by a model protease and leading to the formation of strongly fluorescent quinoxalin-2(1H)-one N-heterocycles is described. Some positive attributes including versatility and the ability to provide outstanding fluorescence "OFF-ON" responses were revealed by this work. They open the way for practical applications of this novel type of "covalent-assembly"-based fluorescent probe in the fields of sensing and bioimaging.

Recent advances in stimuli-responsive in situ self-assembly of small molecule probes for in vivo imaging of enzymatic activity

Stimuli-responsive in situ self-assembly of small molecule probes into nanostructures has been promising for the construction of molecular probes for in vivo imaging. In the past few years, a number of intelligent molecular imaging probes with fluorescence, magnetic resonance imaging (MRI), positron electron tomography (PET) or photoacoustic imaging (PA) modality have been developed based on the in situ self-assembly strategy. In this minireview, we summarize the recent advances in the development of different modality imaging probes through controlling in situ self-assembly for in vivo imaging of enzymatic activity. This review starts from the brief introduction of two different chemical approaches amenable for in situ self-assembly, including (1) stimuli-mediated proteolysis and (2) stimuli-triggered biocompatible reaction. We then discuss their applications in the design of fluorescence, MRI, PET, PA, and bimodality imaging probes for in vivo imaging of different enzymes, such as caspase-3, furin, gelatinase and phosphatase. Finally, we discuss the current and prospective challenges in the stimuli-responsive in situ self-assembly strategy for in vivo imaging.

Pre-targeted Imaging of Protease Activity through In Situ Assembly of Nanoparticles

The pre-targeted imaging of enzyme activity has not been reported, likely owing to the lack of a mechanism to retain the injected substrate in the first step for subsequent labeling. Herein, we report the use of two bioorthogonal reactions-the condensation reaction of aromatic nitriles and aminothiols and the inverse-electron demand Diels-Alder reaction between tetrazine and trans-cyclooctene (TCO)-to develop a novel strategy for pre-targeted imaging of the activity of proteases. The substrate probe (TCO-C-SNAT4) can be selectively activated by an enzyme target (e.g. caspase-3/7), which triggers macrocyclization and subsequent in situ self-assembly into nanoaggregates retained at the target site. The tetrazine-imaging tag conjugate labels TCO in the nanoaggregates to generate selective signal retention for imaging in vitro, in cells, and in mice. Owing to the decoupling of enzyme activation and imaging tag immobilization, TCO-C-SNAT4 can be repeatedly injected to generate and accumulate more TCO-nanoaggregates for click labeling.