Imaging cell surface glycosylation in vivo using "double click" chemistry

In Situ Glycan Analysis and Editing in Living Systems

Besides proteins and nucleic acids, carbohydrates are also ubiquitous building blocks of living systems. Approximately 70% of mammalian proteins are glycosylated. Glycans not only provide structural support for living systems but also act as crucial regulators of cellular functions. As a result, they are considered essential pieces of the life science puzzle. However, research on glycans has lagged far behind that on proteins and nucleic acids. The main reason is that glycans are not direct products of gene coding, and their synthesis is nontemplated. In addition, the diversity of monosaccharide species and their linkage patterns contribute to the complexity of the glycan structures, which is the molecular basis for their diverse functions. Research in glycobiology is extremely challenging, especially for the in situ elucidation of glycan structures and functions. There is an urgent need to develop highly specific glycan labeling tools and imaging methods and devise glycan editing strategies. This Perspective focuses on the challenges of in situ analysis of glycans in living systems at three spatial levels (i.e., cell, tissue, and in vivo) and highlights recent advances and directions in glycan labeling, imaging, and editing tools. We believe that examining the current development landscape and the existing bottlenecks can drive the evolution of in situ glycan analysis and intervention strategies and provide glycan-based insights for clinical diagnosis and therapeutics.

Tetrazine bioorthogonal chemistry derived in vivo imaging

Bioorthogonal chemistry represents plenty of highly efficient and biocompatible reactions that proceed selectively and rapidly in biological situations without unexpected side reactions towards miscellaneous endogenous functional groups. Arise from the strict demands of physiological reactions, bioorthogonal chemical reactions are natively selective transformations that are rarely found in biological environments. Bioorthogonal chemistry has long been applied to tracking and real-time imaging of biomolecules in their physiological environments. Thereinto, tetrazine bioorthogonal reactions are particularly important and have increasing applications in these fields owing to their unique properties of easily controlled fluorescence or radiation off-on mechanism, which greatly facilitate the tracking of real signals without been disturbed by background. In this mini review, tetrazine bioorthogonal chemistry for in vivo imaging applications will be attentively appraised to raise some guidelines for prior tetrazine bioorthogonal chemical studies.

Metabolic Glycan Labeling of Cancer Cells Using Variably Acetylated Monosaccharides

Methylcyclopropene (Cyoc)-tagged tetra-acetylated monosaccharides, and in particular mannosamine derivatives, are promising tools for medical imaging of cancer using metabolic oligosaccharide engineering and the extremely fast inverse electron-demand Diels-Alder bioorthogonal reaction. However, the in vivo potential of these monosaccharide derivatives has yet to be fully explored due to their low aqueous solubility. To address this issue, we sought to vary the extent of acetylation of Cyoc-tagged monosaccharides and probe its effect on the extent of glycan labeling in various cancer cell lines. We demonstrate that, in the case of AcxManNCyoc, tri- and diacetylated derivatives generated significantly enhanced cell labeling compared to the tetra-acetylated monosaccharide. In contrast, for the more readily soluble azide-tagged sugars, a decrease in acetylation led to decreased glycan labeling. Ac3ManNCyoc gave better labeling than the azido-tagged Ac4ManNAz and has significant potential for in vitro and in vivo imaging of glycosylated cancer biomarkers.

T-cell surface generation of dual bivalent, bispecific T-cell engaging, RNA duplex cross-linked antibodies (dbBiTERs) for re-directed tumor cell lysis

BACKGROUND

Genetic engineered Bispecific T-cell engagers (BiTEs) generate potent cytotoxic effects.

METHODS

Alternately, click chemistry engineered, dual specific bivalent Bispecific T-cell engaging antibodies (dbBiTEs) on T-cell surfaces can be generated from parent monoclonal antibodies.

RESULTS

We show the formation of dbBiTEs on the surface of T-cells along with the introduction of complementary 2'-OMe RNA 32-mer oligonucleotides allowing duplex formation between antibodies, designated as dbBiTERs. dbBiTERs generated in solution from anti-CEA and anti-CD3 OKT3 antibodies retained specific binding to CEA positive versus CEA negative cancer cells and to CD3 positive T-cells comparable to dbBiTEs. When T-cells were precoated with dbBiTEs or dbBiTERs and mixed with CEA positive versus CEA negative cancer cells, similar dose dependent and specific cytotoxicity were observed in redirected cell lysis assays. On-cell generated dbBiTERs exerted potent cytotoxic responses against CEA positive targets and were localized at the cell surface by immuno-gold EM. In addition, we demonstrate that target and T-cells, each coated separately with complementary 2'OMe-RNA-linked antibodies can be cross-linked by RNA duplex formation in vitro to generate redirected cell lysis.

CONCLUSION

The facile generation of dbBiTERs with specific cytolytic activity from intact antibodies and their generation on-cell offers a new avenue for antigen specific T-cell therapy.

In vivo metal-catalyzed SeCT therapy by a proapoptotic peptide

Selective cell tagging (SeCT) therapy is a strategy for labeling a targeted cell with certain chemical moieties via a catalytic chemical transformation in order to elicit a therapeutic effect. Herein, we report a cancer therapy based on targeted cell surface tagging with proapoptotic peptides (Ac-GGKLFG-X; X = reactive group) that induce apoptosis when attached to the cell surface. Using either Au-catalyzed amidation or Ru-catalyzed alkylation, these proapoptotic peptides showed excellent therapeutic effects both in vitro and in vivo. In particular, co-treatment with proapoptotic peptide and the carrier-Ru complex significantly and synergistically inhibited tumor growth and prolonged survival rate of tumor-bearing mice after only a single injection. This is the first report of Ru catalyst application in vivo, and this approach could be used in SeCT for cancer therapy.

Pentafulvene-Maleimide Cycloaddition for Bioorthogonal Ligation

The applications of bioconjugation chemistry are rapidly expanding, and the addition of new strategies to the bioconjugation and ligation toolbox will further advance progress in this field. Herein, we present a detailed study of the Diels-Alder cycloaddition (DAC) reaction between pentafulvenes and maleimides in aqueous solutions and investigate the reaction as an emerging bioconjugation strategy. The DAC reactions were found to proceed efficiently, quantitatively yielding cycloadducts with reaction rates ranging up to ∼0.7 M^-1 s^-1 for a series of maleimides, including maleimide-derivatized peptides and proteins. The absence of cross-reactivity of the pentafulvene with a large panel of functional (bio)molecules and biological media further demonstrated the bioorthogonality of this approach. The utility of the DAC reaction for bioorthogonal bioconjugation applications was further demonstrated in the presence of biological media and proteins, as well as through protein derivatization and labeling, which was comparable to the widely employed sulfhydryl-maleimide coupling chemistry.

Cell surface-localized imaging and sensing

Systematically dissecting the molecular basis of the cell surface as well as its related biological activities is considered as one of the most cutting-edge fields in fundamental sciences. The advent of various advanced cell imaging techniques allows us to gain a glimpse of how the cell surface is structured and coordinated with other cellular components to respond to intracellular signals and environmental stimuli. Nowadays, cell surface-related studies have entered a new era featured by a redirected aim of not just understanding but artificially manipulating/remodeling the cell surface properties. To meet this goal, biologists and chemists are intensely engaged in developing more maneuverable cell surface labeling strategies by exploiting the cell's intrinsic biosynthetic machinery or direct chemical/physical binding methods for imaging, sensing, and biomedical applications. In this review, we summarize the recent advances that focus on the visualization of various cell surface structures/dynamics and accurate monitoring of the microenvironment of the cell surface. Future challenges and opportunities in these fields are discussed, and the importance of cell surface-based studies is highlighted.

Click Chemistry as a Tool for Cell Engineering and Drug Delivery

Click chemistry has great potential for use in binding between nucleic acids, lipids, proteins, and other molecules, and has been used in many research fields because of its beneficial characteristics, including high yield, high specificity, and simplicity. The recent development of copper-free and less cytotoxic click chemistry reactions has allowed for the application of click chemistry to the field of medicine. Moreover, metabolic glycoengineering allows for the direct modification of living cells with substrates for click chemistry either in vitro or in vivo. As such, click chemistry has become a powerful tool for cell transplantation and drug delivery. In this review, we describe some applications of click chemistry for cell engineering in cell transplantation and for drug delivery in the diagnosis and treatment of diseases.

Accelerated Blood Clearance of Antibodies by Nanosized Click Antidotes

Long blood half-life is one of the advantages of antibodies over small molecule drugs. At the same time, prolonged half-life is a problem for imaging applications or in the case of antibody-induced toxicities. There is a substantial need for antidotes that can quickly clear antibodies from systemic circulation and peripheral tissues. Engineered nanoparticles exhibit intrinsic affinity for clearance organs (mainly liver and spleen). trans-Cyclooctene (TCO) and methyltetrazine (MTZ) are versatile copper-free click chemistry components that are extensively being used for in vivo bioorthogonal couplings. To test the ability of nanoparticles to eliminate antibodies, we prepared a set of click-modified, clinically relevant antidotes based on several classes of drug carriers: phospholipid-PEG micelles, bovine serum albumin (BSA), and cross-linked dextran iron oxide (CLIO) nanoparticles. Mice were injected with IRDye 800CW-labeled, click-modified IgG followed by a click-modified antidote or PBS (control), and the levels of the IgG were monitored up to 72 h postinjection. Long-circulating lipid micelles produced a spike in IgG levels at 1 h, decreased IgG levels at 24 h, and did not decrease the area under the curve (AUC) and IgG accumulation in main organs. Long-circulating BSA decreased IgG levels at 1 and 24 h, decreased the AUC, but did not significantly decrease organ accumulation. Long-circulating CLIO nanoworms increased IgG levels at 1 h, decreased IgG levels at 24 h, did not decrease the AUC, and did not decrease the organ accumulation. On the other hand, short-circulating CLIO nanoparticles decreased IgG levels at 1 and 24 h, significantly decreasing the AUC and accumulation in the main organs. Multiple doses of CLIO and BSA were not able to completely eliminate the antibody from blood, despite the click reactivity of the residual IgG, likely due to exchange of IgG between blood and tissue compartments. Pharmacokinetic modeling suggests that short antidote half-life and fast click reaction rate should result in higher IgG depletion efficiency. Short-circulating click-modified nanocarriers are the most effective antidotes for elimination of antibodies from blood. This study sets a stage for future development of antidotes based on nanomedicine.

Emerging Applications of Fluorogenic and Non-fluorogenic Bifunctional Linkers

Homo- and hetero-bifunctional linkers play vital roles in constructing a variety of functional systems, ranging from protein bioconjugates with drugs and functional agents, to surface modification of nanoparticles and living cells, and to the cyclization/dimerization of synthetic polymers and biomolecules. Conventional approaches for assaying conjugation extents typically rely on ex situ techniques, such as mass spectrometry, gel electrophoresis, and size-exclusion chromatography. If the conjugation process involving bifunctional linkers was rendered fluorogenic, then in situ monitoring, quantification, and optical tracking/visualization of relevant processes would be achieved. In this review, conventional non-fluorogenic linkers are first discussed. Then the focus is on the evolution and emerging applications of fluorogenic bifunctional linkers, which are categorized into hetero-bifunctional single-caging fluorogenic linkers, homo-bifunctional double-caging fluorogenic linkers, and hetero-bifunctional double-caging fluorogenic linkers. In addition, stimuli-cleavable bifunctional linkers designed for both conjugation and subsequent site-specific triggered release are also summarized.

Inverse electron demand Diels-Alder reactions in chemical biology

The emerging inverse electron demand Diels-Alder (IEDDA) reaction stands out from other bioorthogonal reactions by virtue of its unmatchable kinetics, excellent orthogonality and biocompatibility. With the recent discovery of novel dienophiles and optimal tetrazine coupling partners, attention has now been turned to the use of IEDDA approaches in basic biology, imaging and therapeutics. Here we review this bioorthogonal reaction and its promising applications for live cell and animal studies. We first discuss the key factors that contribute to the fast IEDDA kinetics and describe the most recent advances in the synthesis of tetrazine and dienophile coupling partners. Both coupling partners have been incorporated into proteins for tracking and imaging by use of fluorogenic tetrazines that become strongly fluorescent upon reaction. Selected notable examples of such applications are presented. The exceptional fast kinetics of this catalyst-free reaction, even using low concentrations of coupling partners, make it amenable for in vivo radiolabelling using pretargeting methodologies, which are also discussed. Finally, IEDDA reactions have recently found use in bioorthogonal decaging to activate proteins or drugs in gain-of-function strategies. We conclude by showing applications of the IEDDA reaction in the construction of biomaterials that are used for drug delivery and multimodal imaging, among others. The use and utility of the IEDDA reaction is interdisciplinary and promises to revolutionize chemical biology, radiochemistry and materials science.

Molecular imaging based on metabolic glycoengineering and bioorthogonal click chemistry

Metabolic glycoengineering is a powerful technique that can introduce various chemical groups to cellular glycan by treatment of unnatural monosaccharide. Particularly, this technique has enabled many challenging trials for molecular imaging in combination with click chemistry, which provides fast and specific chemical conjugation reaction of imaging probes to metabolically-modified live cells. This review introduces recent progress in molecular imaging based on the combination of these two cutting-edge techniques. First, these techniques showed promising results in specific tumor cell imaging for cancer diagnosis and therapy. The related researches showed the surface of tumor cells could be labeled with bioorthogonal chemical groups by metabolic glycoengineering, which can be further conjugated with fluorescence dyes or nanoparticles with imaging probes by click chemistry, in vitro and in vivo. This method can be applied to heterogeneous tumor cells regardless of genetic properties of different tumor cells. Furthermore, the amount of targeting moieties on tumor cells can be freely controlled externally by treatment of unnatural monosaccharide. Second, this sequential use of metabolic glycoengineering and click chemistry is also useful in cell tracking to monitor the localization of the inoculated therapeutic cells including chondrocytes and stem cells. This therapeutic cell-labeling technique provided excellent viability of chondrocytes and stem cells during the whole process in vitro and in vivo. It can provide long-term and safe therapeutic cell imaging compared to traditional methods. These overall studies demonstrate the great potential of metabolic glycoengineering and click chemistry in live cell imaging.

Coupling of Immunostimulants to Live Cells through Metabolic Glycoengineering and Bioorthogonal Click Chemistry

The present study investigated the potential of metabolic glycoengineering followed by bioorthogonal click chemistry for introducing into cell-surface glycans different immunomodulating molecules. Mouse tumor models EG7 and MC38-OVA were treated with Ac4GalNAz and Ac4ManNAz followed by ligation of immunostimulants to modified cell-surface glycans of the living cells through bioorthogonal click chemistry. The presence of covalently bound oligosaccharide and oligonucleotide immunostimulants could be clearly established. The activation of a reporter macrophage cell line was determined. Depending on the tumor cell line, covalently and noncovalently bound CpG activated the macrophages by between 67 and 100% over controls. EG7 cells with covalently attached immunostimulants and controls were injected subcutaneously into C57BL/6 mice. All tumor cells subjected to the complete treatment with control molecules formed tumors like nontreated cells confirming cell viability. However, when CpG oligonucleotide was linked to cell-surface glycans, tumor growth was slowed significantly (60% reduction, n = 10, by covalently bound CpG compared to noncovalently bound CpG, n = 10). When mice that had not developed large tumors were challenged with unmodified EG7 cells, no new tumors developed, suggesting protection through the immune system.

CYP450 Enzymes Effect Oxygen-Dependent Reduction of Azide-Based Fluorogenic Dyes

Azide-containing compounds have broad utility in organic synthesis and chemical biology. Their use as powerful tools for the labeling of biological systems in vitro has enabled insights into complex cellular functions. To date, fluorogenic azide-containing compounds have primarily been employed in the context of click chemistry and as sensitive functionalities for hydrogen sulfide detection. Here, we report an alternative use of this functionality: as fluorogenic probes for the detection of depleted oxygen levels (hypoxia). Oxygen is imperative to all life forms, and probes that enable quantification of oxygen tension are of high utility in many areas of biology. Here we demonstrate the ability of an azide-based dye to image hypoxia in a range of human cancer cell lines. We have found that cytochrome P450 enzymes are able to reduce these probes in an oxygen-dependent manner, while hydrogen sulfide does not play an important role in their reduction. These data indicate that the azide group is a new bioreductive functionality that can be employed in prodrugs and dyes. We have uncovered a novel mechanism for the cellular reduction of azides, which has implications for the use of click chemistry in hypoxia.

Imaging Tumor Metabolism to Assess Disease Progression and Treatment Response

Changes in tumor metabolism may accompany disease progression and can occur following treatment, often before there are changes in tumor size. We focus here on imaging methods that can be used to image various aspects of tumor metabolism, with an emphasis on methods that can be used for tumor grading, assessing disease progression, and monitoring treatment response. Clin Cancer Res; 22(21); 5196-203. ©2016 AACR.

Controlled Detachment of Chemically Glued Cells

We demonstrate a chemically detachable cell-glue system based on linkers containing disulfide bonds as well as functional groups for metabolic glycoengineering and bioorthogonal click chemistry. Azide groups are generated on the cell surface by metabolic glycoengineering, and they are further modified into tetrazine (Tz) or trans-cyclooctene (TCO) using rationally designed cross-linkers. When the Tz-modified and TCO-modified cells are mixed together, cell gluing between these two cell groups is established by Tz-TCO click chemistry. This artificial cell-cell adhesion can be broken by the administration of glutathione (5 mM), which triggers the degradation of disulfide bonds. Both the gluing and detachment processes are rapid (<10 min) and minimally cytotoxic.

Designing sugar mimetics: non-natural pyranosides as innovative chemical tools

The importance of oligosaccharides in myriad biological processes is becoming increasingly clear. However, these carbohydrate-mediated processes are often challenging to dissect due to the often poor affinity, stability and selectivity of the oligosaccharides involved. To circumvent these issues, non-natural carbohydrates-carbohydrate mimics-are being designed as innovative tools to modify biomolecules of interest or to understand biological pathways using fluorescence microscopy, X-ray or nuclear magnetic resonance spectroscopy (NMR). This review focuses on key examples of recently developed non-natural sugars to answer specific biological needs.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).

Imaging Glycosylation In Vivo by Metabolic Labeling and Magnetic Resonance Imaging

Glycosylation is a ubiquitous post-translational modification, present in over 50 % of the proteins in the human genome,1 with important roles in cell-cell communication and migration. Interest in glycome profiling has increased with the realization that glycans can be used as biomarkers of many diseases,2 including cancer.3 We report here the first tomographic imaging of glycosylated tissues in live mice by using metabolic labeling and a gadolinium-based bioorthogonal MRI probe. Significant N-azidoacetylgalactosamine dependent T1 contrast was observed in vivo two hours after probe administration. Tumor, kidney, and liver showed significant contrast, and several other tissues, including the pancreas, spleen, heart, and intestines, showed a very high contrast (>10-fold). This approach has the potential to enable the rapid and non-invasive magnetic resonance imaging of glycosylated tissues in vivo in preclinical models of disease.

Bioorthogonal Click Chemistry-Based Synthetic Cell Glue

Artificial methods of cell adhesion can be effective in building functional cell complexes in vitro, but methods for in vivo use are currently lacking. Here, a chemical cell glue based on bioorthogonal click chemistry with high stability and robustness is introduced. Tetrazine (Tz) and trans-cyclooctene (TCO) conjugated to the cell surface form covalent bonds between cells within 10 min in aqueous conditions. Glued, homogeneous, or heterogeneous cell pairs remain viable and stably attached in a microfluidic flow channel at a shear stress of 20 dyn cm(-2) . Upon intravenous injection of assembled Jurkat T cells into live mice, fluorescence microscopy shows the trafficking of cell pairs in circulation and their infiltration into lung tissues. These results demonstrate the promising potential of chemically glued cell pairs for various applications ranging from delivering therapeutic cells to studying cell-cell interactions in vivo.

Imaging Glycosylation In Vivo by Metabolic Labeling and Magnetic Resonance Imaging

Glycosylation is a ubiquitous post‐translational modification, present in over 50 % of the proteins in the human genome,1 with important roles in cell–cell communication and migration. Interest in glycome profiling has increased with the realization that glycans can be used as biomarkers of many diseases,2 including cancer.3 We report here the first tomographic imaging of glycosylated tissues in live mice by using metabolic labeling and a gadolinium‐based bioorthogonal MRI probe. Significant N‐azidoacetylgalactosamine dependent T₁ contrast was observed in vivo two hours after probe administration. Tumor, kidney, and liver showed significant contrast, and several other tissues, including the pancreas, spleen, heart, and intestines, showed a very high contrast (>10‐fold). This approach has the potential to enable the rapid and non‐invasive magnetic resonance imaging of glycosylated tissues in vivo in preclinical models of disease.

Systemic Fluorescence Imaging of Zebrafish Glycans with Bioorthogonal Chemistry

Vertebrate glycans constitute a large, important, and dynamic set of post-translational modifications that are notoriously difficult to manipulate and image. Although the chemical reporter strategy has been used in conjunction with bioorthogonal chemistry to image the external glycosylation state of live zebrafish and detect tumor-associated glycans in mice, the ability to image glycans systemically within a live organism has remained elusive. Here, we report a method that combines the metabolic incorporation of a cyclooctyne-functionalized sialic acid derivative with a ligation reaction of a fluorogenic tetrazine, allowing for the imaging of sialylated glycoconjugates within live zebrafish embryos.

Site-Specific Glycoconjugation of Protein via Bioorthogonal Tetrazine Cycloaddition with a Genetically Encoded trans-Cyclooctene or Bicyclononyne

Efficient access to proteins modified site-specifically with glycans is important in glycobiology and for therapeutic applications. Herein, we report a biocompatible protein glycoconjugation by inverse demand Diels-Alder reaction between tetrazine and trans-cyclooctene. Tetrazine functionalized glycans were obtained in one step by CuAAC (Cu-catalyzed alkyne azide cycloaddition) between glycosyl azide and an alkyne-tetrazine adduct. Site-specific glycoconjugation was performed chemoselectively on a target protein in which a trans-cyclooctene derivatized lysine was genetically encoded. Glycoconjugation proceeded to completion on purified protein and was shown to be selective for the target protein in E. coli.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Live-cell MRI with xenon hyper-CEST biosensors targeted to metabolically labeled cell-surface glycans

The targeting of metabolically labeled glycans with conventional MRI contrast agents has proved elusive. In this work, which further expands the utility of xenon Hyper-CEST biosensors in cell experiments, we present the first successful molecular imaging of such glycans using MRI. Xenon Hyper-CEST biosensors are a novel class of MRI contrast agents with very high sensitivity. We designed a multimodal biosensor for both fluorescent and xenon MRI detection that is targeted to metabolically labeled sialic acid through bioorthogonal chemistry. Through the use of a state of the art live-cell bioreactor, it was demonstrated that xenon MRI biosensors can be used to image cell-surface glycans at nanomolar concentrations.

Design strategies for bioorthogonal smart probes

Bioorthogonal chemistry has enabled the selective labeling and detection of biomolecules in living systems. Bioorthogonal smart probes, which become fluorescent or deliver imaging or therapeutic agents upon reaction, allow for the visualization of biomolecules or targeted delivery even in the presence of excess unreacted probe. This review discusses the strategies used in the development of bioorthogonal smart probes and highlights the potential of these probes to further our understanding of biology.

P53 and cancer-associated sialylated glycans are surrogate markers of cancerization of the bladder associated with Schistosoma haematobium infection

BACKGROUND

Bladder cancer is a significant health problem in rural areas of Africa and the Middle East where Schistosoma haematobium is prevalent, supporting an association between malignant transformation and infection by this blood fluke. Nevertheless, the molecular mechanisms linking these events are poorly understood. Bladder cancers in infected populations are generally diagnosed at a late stage since there is a lack of non-invasive diagnostic tools, hence enforcing the need for early carcinogenesis markers.

METHODOLOGY/PRINCIPAL FINDINGS

Forty-three formalin-fixed paraffin-embedded bladder biopsies of S. haematobium-infected patients, consisting of bladder tumours, tumour adjacent mucosa and pre-malignant/malignant urothelial lesions, were screened for bladder cancer biomarkers. These included the oncoprotein p53, the tumour proliferation rate (Ki-67>17%), cell-surface cancer-associated glycan sialyl-Tn (sTn) and sialyl-Lewisa/x (sLea/sLex), involved in immune escape and metastasis. Bladder tumours of non-S. haematobium etiology and normal urothelium were used as controls. S. haematobium-associated benign/pre-malignant lesions present alterations in p53 and sLex that were also found in bladder tumors. Similar results were observed in non-S. haematobium associated tumours, irrespectively of their histological nature, denoting some common molecular pathways. In addition, most benign/pre-malignant lesions also expressed sLea. However, proliferative phenotypes were more prevalent in lesions adjacent to bladder tumors while sLea was characteristic of sole benign/pre-malignant lesions, suggesting it may be a biomarker of early carcionogenesis associated with the parasite. A correlation was observed between the frequency of the biomarkers in the tumor and adjacent mucosa, with the exception of Ki-67. Most S. haematobium eggs embedded in the urothelium were also positive for sLea and sLex. Reinforcing the pathologic nature of the studied biomarkers, none was observed in the healthy urothelium.

CONCLUSION/SIGNIFICANCE

This preliminary study suggests that p53 and sialylated glycans are surrogate biomarkers of bladder cancerization associated with S. haematobium, highlighting a missing link between infection and cancer development. Eggs of S. haematobium express sLea and sLex antigens in mimicry of human leukocytes glycosylation, which may play a role in the colonization and disease dissemination. These observations may help the early identification of infected patients at a higher risk of developing bladder cancer and guide the future development of non-invasive diagnostic tests.

Dopamine-Based Copper-Free Click Kit for Efficient Surface Functionalization

Strain-promoted azide-alkyne cycloaddition reactions are combined with a dopamine functional species to generate a highly efficient method for surface modification. The resulting conjugate containing 4-dibenzocyclooctynol (DIBO) and dopamine results in a versatile surface labeling technology that can replicate patterns generated from photolithography and microcontact printing techniques.

Imaging bacterial peptidoglycan with near-infrared fluorogenic azide probes

Fluorescent probes designed for activation by bioorthogonal chemistry have enabled the visualization of biomolecules in living systems. Such activatable probes with near-infrared (NIR) emission would be ideal for in vivo imaging but have proven difficult to engineer. We present the development of NIR fluorogenic azide probes based on the Si-rhodamine scaffold that undergo a fluorescence enhancement of up to 48-fold upon reaction with terminal or strained alkynes. We used the probes for mammalian cell surface imaging and, in conjunction with a new class of cyclooctyne D-amino acids, for visualization of bacterial peptidoglycan without the need to wash away unreacted probe.

Sequential "click" - "photo-click" cross-linker for catalyst-free ligation of azide-tagged substrates

Heterobifunctional linker allows for selective catalyst-free ligation of two different azide-tagged substrates via strained-promoted azide-alkyne cycloaddition (SPAAC). The linker contains an azadibenzocyclooctyne (ADIBO) moiety on one end and a cyclopropenone-masked dibenzocyclooctyne (photo-DIBO) group on the other. The first azide-derivatized substrate reacts only at the ADIBO end of the linker as the photo-DIBO moiety is azide-inert. After the completion of the first SPAAC step, photo-DIBO is activated by brief exposure to 350 nm light from a fluorescent UV lamp. The unmasked DIBO group then reacts with the second azide-tagged substrate. Both click reactions are fast (k = 0.4 and 0.07 M(-1) s(-1), respectively) and produce quantitative yield of ligation in organic solvents or aqueous solutions. The utility of the new cross-linker has been demonstrated by conjugation of azide functionalized bovine serum albumin (azido-BSA) with azido-fluorescein and by the immobilization of the latter protein on azide-derivatized silica beads. The BSA-bead linker was designed to incorporate hydrolytically labile fragment, which permits release of protein under the action of dilute acid. UV activation of the second click reaction permits spatiotemporal control of the ligation process.

Cell labeling and tracking method without distorted signals by phagocytosis of macrophages

Cell labeling and tracking are important processes in understanding biologic mechanisms and the therapeutic effect of inoculated cells in vivo. Numerous attempts have been made to label and track inoculated cells in vivo; however, these methods have limitations as a result of their biological effects, including secondary phagocytosis of macrophages and genetic modification. Here, we investigated a new cell labeling and tracking strategy based on metabolic glycoengineering and bioorthogonal click chemistry. We first treated cells with tetra-acetylated N-azidoacetyl-D-mannosamine to generate unnatural sialic acids with azide groups on the surface of the target cells. The azide-labeled cells were then transplanted to mouse liver, and dibenzyl cyclooctyne-conjugated Cy5 (DBCO-Cy5) was intravenously injected into mice to chemically bind with the azide groups on the surface of the target cells in vivo for target cell visualization. Unnatural sialic acids with azide groups could be artificially induced on the surface of target cells by glycoengineering. We then tracked the azide groups on the surface of the cells by DBCO-Cy5 in vivo using bioorthogonal click chemistry. Importantly, labeling efficacy was enhanced and false signals by phagocytosis of macrophages were reduced. This strategy will be highly useful for cell labeling and tracking.