Chemoselective ligation reactions with proteins, oligosaccharides and cells

Chemical proteomics to study metabolism, a reductionist approach applied at the systems level

Cellular metabolism encompasses a complex array of interconnected biochemical pathways that are required for cellular homeostasis. When dysregulated, metabolism underlies multiple human pathologies. At the heart of metabolic networks are enzymes that have been historically studied through a reductionist lens, and more recently, using high throughput approaches including genomics and proteomics. Merging these two divergent viewpoints are chemical proteomic technologies, including activity-based protein profiling, which combines chemical probes specific to distinct enzyme families or amino acid residues with proteomic analysis. This enables the study of metabolism at the network level with the precision of powerful biochemical approaches. Herein, we provide a primer on how chemical proteomic technologies custom-built for studying metabolism have unearthed fundamental principles in metabolic control. In parallel, these technologies have leap-frogged drug discovery through identification of novel targets and drug specificity. Collectively, chemical proteomics technologies appear to do the impossible: uniting systematic analysis with a reductionist approach.

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

Atomic force microscopy applied to interrogate nanoscale cellular chemistry and supramolecular bond dynamics for biomedical applications

Understanding biological interactions at a molecular level grants valuable information relevant to improving medical treatments and outcomes. Among the suite of technologies available, Atomic Force Microscopy (AFM) is unique in its ability to quantitatively probe forces and receptor-ligand interactions in real-time. The ability to assess the formation of supramolecular bonds and intermediates in real-time on surfaces and living cells generates important information relevant to understanding biological phenomena. Combining AFM with fluorescence-based techniques allows for an unprecedented level of insight not only concerning the formation and rupture of bonds, but understanding medically relevant interactions at a molecular level. As the ability of AFM to probe cells and more complex models improves, being able to assess binding kinetics, chemical topographies, and garner spectroscopic information will likely become key to developing further improvements in fields such as cancer, nanomaterials, and virology. The rapid response to the COVID-19 crisis, producing information regarding not just receptor affinities, but also strain-dependent efficacy of neutralizing nanobodies, demonstrates just how viable and integral to the pre-clinical development of information AFM techniques are in this era of medicine.

Functional pH-responsive polymers containing dynamic enaminone linkages for the release of active organic amines

Dynamic covalent bonds have attracted attention for the development of pH-responsive polymers, however, studies using acid-cleavable enaminone linkages as a means of controlled drug release have been limited. Herein, we...

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

Harnessing cancer cell metabolism for theranostic applications using metabolic glycoengineering of sialic acid in breast cancer as a pioneering example

Abnormal cell surface display of sialic acids - a family of unusual 9-carbon sugars - is widely recognized as distinguishing feature of many types of cancer. Sialoglycans, however, typically cannot be identified with sufficiently high reproducibility and sensitivity to serve as clinically accepted biomarkers and similarly, almost all efforts to exploit cancer-specific differences in sialylation signatures for therapy remain in early stage development. In this report we provide an overview of important facets of glycosylation that contribute to cancer in general with a focus on breast cancer as an example of malignant disease characterized by aberrant sialylation. We then describe how cancer cells experience nutrient deprivation during oncogenesis and discuss how the resulting metabolic reprogramming, which endows breast cancer cells with the ability to obtain nutrients during scarcity, constitutes an "Achilles' heel" that we believe can be exploited by metabolic glycoengineering (MGE) strategies to develop new diagnostic methods and therapeutic approaches. In particular, we hypothesize that adaptations made by breast cancer cells that allow them to efficiently scavenge sialic acid during times of nutrient deprivation renders them vulnerable to MGE, which refers to the use of exogenously-supplied, non-natural monosaccharide analogues to modulate targeted aspects of glycosylation in living cells and animals. In specific, once non-natural sialosides are incorporated into the cancer "sialome" they can be exploited as epitopes for immunotherapy or as chemical tags for targeted delivery of imaging or therapeutic agents selectively to tumors.

Carbohydrate-modified magnetic nanoparticles for radical scavenging

Maghemite (gamma-Fe2O3) nanoparticles, 12 nm in size, were prepared by co-precipitation of Fe(II) and Fe(III) chlorides with ammonium hydroxide and oxidation with hydrogen peroxide. To achieve stability and biocompatibility, obtained particles were coated with silica, to which glucose and ascorbic acid were bound by different mechanisms. The composite particles were thoroughly characterized by transmission electron microscopy, dynamic light scattering, elemental analysis, and FT-Raman and fluorescence spectroscopy to determine composition, morphology, size and its distribution, zeta-potential, and scavenging of peroxyl and hydroxyl radicals. As the particles showed promising antioxidative properties, they may have a possible application as a stable magnetically controlled scavenger of reactive oxygen species.

Glycocalyx Engineering with a Recycling Glycopolymer that Increases Cell Survival In Vivo

Synthetic glycopolymers that emulate cell-surface mucins have been used to elucidate the role of mucin overexpression in cancer. However, because they are internalized within hours, these glycopolymers could not be employed to probe processes that occur on longer time scales. In this work, we tested a panel of glycopolymers bearing a variety of lipids to identify those that persist on cell membranes. Strikingly, we found that cholesterylamine (CholA) anchored glycopolymers are internalized into vesicles that serve as depots for delivery back to the cell surface, allowing for the display of cell-surface glycopolymers for at least ten days, even while the cells are dividing. As with native mucins, the cell-surface display of CholA-anchored glycopolymers influenced the focal adhesion distribution. Furthermore, we show that these mimetics enhance the survival of nonmalignant cells in a zebrafish model of metastasis. CholA-anchored glycopolymers therefore expand the application of glycocalyx engineering in glycobiology.

Doxorubicin-loaded aromatic imine-contained amphiphilic branched star polymer micelles: synthesis, self-assembly, and drug delivery

Redox-and pH-sensitive branched star polymers (BSPs), BP(DMAEMA-co-MAEBA-co-DTDMA)(PMAIGP)_ns, have been successively prepared by two steps of reversible addition–fragmentation chain transfer (RAFT) polymerization. The first step is RAFT polymerization of 2-(N,N-dimethylaminoethyl)methacrylate (DMAEMA) and p-(methacryloxyethoxy) benzaldehyde (MAEBA) in the presence of divinyl monomer, 2,2′-dithiodiethoxyl dimethacrylate (DTDMA). The resultant branched polymers were used as a macro-RAFT agent in the subsequent RAFT polymerization. After hydrolysis of the BSPs to form BP(DMAEMA-co-MAEBA-co-DTDMA)(PMAGP)_ns (BSP-H), the anticancer drug doxorubicin (DOX) was covalently linked to branched polymer chains by reaction of primary amine of DOX and aldehyde groups in the polymer chains. Their compositions, structures, molecular weights, and molecular weight distributions were respectively characterized by nuclear magnetic resonance spectra and gel permeation chromatography measurements. The DOX-loaded micelles were fabricated by self-assembly of DOX-containing BSPs in water, which were characterized by transmission electron microscopy and dynamic light scattering. Aromatic imine linkage is stable in neutral water, but is acid-labile; controlled release of DOX from the BSP-H-DOX micelles was realized at pH values of 5 and 6, and at higher acidic solution, fast release of DOX was observed. In vitro cytotoxicity experiment results revealed low cytotoxicity of the BSPs and release of DOX from micelles in HepG2 and HeLa cells. Confocal laser fluorescence microscopy observations showed that DOX-loaded micelles have specific interaction with HepG2 cells. Thus, this type of BSP micelle is an efficient drug delivery system.

Smart hybrid materials by conjugation of responsive polymers to biomacromolecules

The chemical structure and function of biomacromolecules has evolved to fill many essential roles in biological systems. More specifically, proteins, peptides, nucleic acids and polysaccharides serve as vital structural components, and mediate chemical transformations and energy/information storage processes required to sustain life. In many cases, the properties and applications of biological macromolecules can be further expanded by attaching synthetic macromolecules. The modification of biomacromolecules by attaching a polymer that changes its properties in response to environmental variations, thus affecting the properties of the biomacromolecule, has led to the emergence of a new family of polymeric biomaterials. Here, we summarize techniques for conjugating responsive polymers to biomacromolecules and highlight applications of these bioconjugates reported so far. In doing so, we aim to show how advances in synthetic tools could lead to rapid expansion in the variety and uses of responsive bioconjugates.

In situ generation of redox active peptides driven by selenoester mediated native chemical ligation

Redox active peptides synthesized via selenoester mediated native chemical ligation with a propensity to self-assemble in aqueous medium. A gel–sol transition of self-assembled peptide in a reducing environment makes it a versatile candidate for the development of functional biomaterials.

Enhanced catalysis of oxime-based bioconjugations by substituted anilines

The conjugation of biomolecules by chemoselective oxime ligation is of great interest for the site-specific modification of proteins, peptides, nucleic acids, and carbohydrates. These conjugations proceed optimally at a reaction pH of 4-5, but some biomolecules are not soluble or stable under these conditions. Aniline can be used as a nucleophilic catalyst to enhance the rate of oxime formation, but even in its presence, the reaction rate at neutral pH can be slower than desired, particularly at low reagent concentrations and/or temperature. Recently, alternative catalysts with improved properties were reported, including anthranilic acid derivatives for small molecule ligations, as well as m-phenylenediamine at high concentrations for protein conjugations. Here, we report that p-substituted anilines containing an electron-donating ring substituent are superior catalysts of oxime-based conjugations at pH 7. One such catalyst, p-phenylenediamine, was studied in greater detail. This catalyst was highly effective at neutral pH, even at the low concentration of 2 mM. In a model oxime ligation using aminooxy-functionalized PEG, catalysis at pH 7 resulted in a 120-fold faster rate of protein PEGylation as compared to an uncatalyzed reaction, and 19-fold faster than the equivalent aniline-catalyzed reaction. p-Phenylenediamine (10 mM) was also an effective catalyst under acidic conditions and was more efficient than aniline throughout the pH range 4-7. This catalyst allows efficient oxime bioconjugations to proceed under mild conditions and low micromolar concentrations, as demonstrated by the PEGylation of a small protein.

Chemoselective immobilization of proteins by microcontact printing and bio-orthogonal click reactions

Herein, a combination of microcontact printing of functionalized alkanethiols and site-specific modification of proteins is utilized to chemoselectively immobilize proteins onto gold surfaces, either by oxime- or copper-catalyzed alkyne-azide click chemistry. Two molecules capable of click reactions were synthesized, an aminooxy-functionalized alkanethiol and an azide-functionalized alkanethiol, and self-assembled monolayer (SAM) formation on gold was confirmed by IR spectroscopy. The alkanethiols were then individually patterned onto gold surfaces by microcontact printing. Site-specifically modified proteins-horse heart myoglobin (HHMb) containing an N-terminal α-oxoamide and a red fluorescent protein (mCherry-CVIA) with a C-terminal alkyne-were immobilized by incubation onto respective stamped functionalized alkanethiol patterns. Pattern formation was confirmed by fluorescence microscopy.

Experimental Design Considerations for In Vitro Non-Natural Glycan Display via Metabolic Oligosaccharide Engineering

Metabolic oligosaccharide engineering (MOE) refers to a technique where non-natural monosaccharide analogs are introduced into living biological systems. Once inside a cell, these compounds intercept a targeted biosynthetic glycosylation pathway and in turn are metabolically incorporated into cell-surface-displayed oligosaccharides where they can modulate a host of biological activities or be exploited as "tags" for bio-orthogonal and chemoselective ligation reactions. Undertaking a MOE experiment can be a daunting task based on the growing repertoire of analogs now available and the ever increasing number of metabolic pathways that can be targeted; therefore, a major emphasis of this article is to describe a general approach for analog design and selection and then provide protocols to ensure safe and efficacious analog usage by cells. Once cell-surface glycans have been successfully remodeled by MOE methodology, the stage is set for probing changes to the myriad cellular responses modulated by these versatile molecules. Curr. Protoc. Chem. Biol. 2:171-194 © 2010 by John Wiley & Sons, Inc.

Synergizing nucleic acid aptamers with 1-dimensional nanostructures as label-free field-effect transistor biosensors

Since the introduction by Gold et al. in 1990, nucleic acid aptamers had evolved to become a true contender in biosensors for protein and cell detections. Aptamers are short strands of synthetically designed DNA or RNA oligonucleotides that can be self-assembled into unique 3-dimensional structures and can bind to different proteins, cells or even small molecules at a high level of specificity and affinity. In recent years, there had been many reports in literature in using aptamers in place of conventional antibodies as capture biomolecules on the surface. This is mainly due to the better thermal stability properties and ease in production. Consequently, also these characteristics allowed the aptamers to find use in field effect transistors (FETs) based upon 1D nanostructured (1D-NS) as label-free biosensing. In terms of designing label-free platforms for biosensors applications, 1D-NS FET had been an attractive option due to reported high sensitivities toward protein targets arising from the large surface area for detection as well as to their label-free nature. Since the first aptamer-based 1D-NS FET biosensor had surfaced in 2005, there had been many more improvements in the overall design and sensitivity in recent years. In this review, the latest developments in synergizing these two interesting areas of research (aptamers and 1D-NS FET) will be discussed for a range of different nanowire types as well as for the detection results.

An efficient method for selectively imaging and quantifying in situ the expression of sialylated glycoproteins on living cells

A simple and efficient method for selectively imaging and monitoring in situ the expression of sialylated glycoproteins on living cells has been developed. Treating living cells by mild periodate oxidation to selectively generate aldehydes on sialylated glycoproteins, followed by direct labeling of aldehydes with a commercially available fluorescent tag, fluorescein-5-thiosemicarbazide (FTSC), allows in situ imaging and quantification of sialylated glycoproteins on living cells. Under optimum reaction conditions, the periodate oxidation-based FTSC ligation (PF) strategy could be completed within 40 min. The cells undergoing the PF assay revealed a 91% viability and a fairly high-level of metabolic activity. Compared with current labeling methods, the PF assay proved to be a simpler and faster means of imaging sialylated glycoproteins on living cells. The PF assay has been successfully applied to imaging the location and quantification of the abundance of sialylated glycoproteins on tumor and normal cells. Our results demonstrated the methodological significance in clinical diagnosis and functional elucidation studies.

Combination of integrin-binding peptide and growth factor promotes cell adhesion on electron-beam-fabricated patterns

Understanding and controlling cell adhesion on engineered scaffolds is important in biomaterials and tissue engineering. In this report we used an electron-beam (e-beam) lithography technique to fabricate patterns of a cell adhesive integrin ligand combined with a growth factor. Specifically, micron-sized poly(ethylene glycol) (PEG) hydrogels with aminooxy- and styrene sulfonate-functional groups were fabricated. Cell adhesion moieties were introduced using a ketone-functionalized arginine-glycine-aspartic acid (RGD) peptide to modify the O-hydroxylamines by oxime bond formation. Basic fibroblast growth factor (bFGF) was immobilized by electrostatic interaction with the sulfonate groups. Human umbilical vein endothelial cells (HUVECs) formed focal adhesion complexes on RGD- and RGD and bFGF-immobilized patterns as shown by immunostaining of vinculin and actin. In the presence of both bFGF and RGD, cell areas were larger. The data demonstrate confinement of cellular focal adhesions to chemically and physically well-controlled microenvironments created by a combination of e-beam lithography and "click" chemistry techniques. The results also suggest positive implications for addition of growth factors into adhesive patterns for cell-material interactions.

Copper-Catalyzed Azide-Alkyne Click Chemistry for Bioconjugation

The copper-catalyzed azide-alkyne cycloaddition reaction is widely used for the connection of molecular entities of all sizes. A protocol is provided here for the process with biomolecules. Ascorbate is used as reducing agent to maintain the required cuprous oxidation state. Since these convenient conditions produce reactive oxygen species, five equivalents of a copper-binding ligand is used with respect to metal. The ligand both accelerates the reaction and serves as a sacrificial reductant, protecting the biomolecules from oxidation. A procedure is also described for testing the efficiency of the reaction under desired conditions for purposes of optimization, before expensive biological reagents are used.

Electrophile-modified lipoic derivatives of PDC-E2 elicits anti-mitochondrial antibody reactivity

Our laboratory has hypothesized that xenobiotic modification of the native lipoyl moiety of the major mitochondrial autoantigen, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), may lead to loss of self-tolerance in primary biliary cirrhosis (PBC). This thesis is based on the finding of readily detectable levels of immunoreactivity of PBC sera against extensive panels of protein microarrays containing mimics of the inner lipoyl domain of PDC-E2 and subsequent quantitative structure-activity relationships (QSARs). Importantly, we have demonstrated that murine immunization with one such mimic, 2-octynoic acid coupled to bovine serum albumin (BSA), induces anti-mitochondrial antibodies (AMAs) and cholangitis. Based upon these data, we have focused on covalent modifications of the lipoic acid disulfide ring and subsequent analysis of such xenobiotics coupled to a 15mer of PDC-E2 for immunoreactivity against a broad panel of sera from patients with PBC and controls. Our results demonstrate that AMA-positive PBC sera demonstrate marked reactivity against 6,8-bis(acetylthio)octanoic acid, implying that chemical modification of the lipoyl ring, i.e. disruption of the S-S disulfide, renders lipoic acid to its reduced form that will promote xenobiotic modification. This observation is particularly significant in light of the function of the lipoyl moiety in electron transport of which the catalytic disulfide constantly opens and closes and, thus, raises the intriguing thesis that common electrophilic agents, i.e. acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDs), may lead to xenobiotic modification in genetically susceptible individuals that results in the generation of AMAs and ultimately clinical PBC.

Designing a binding interface for control of cancer cell adhesion via 3D topography and metabolic oligosaccharide engineering

This study combines metabolic oligosaccharide engineering (MOE), a technology where the glycocalyx of living cells is endowed with chemical features not normally found in sugars, with custom-designed three-dimensional biomaterial substrates to enhance the adhesion of cancer cells and control their morphology and gene expression. Specifically, Ac(5)ManNTGc, a thiol-bearing analog of N-acetyl-d-mannosamine (ManNAc) was used to introduce thiolated sialic acids into the glycocalyx of human Jurkat T-lymphoma derived cells. In parallel 2D films and 3D electrospun nanofibrous scaffolds were prepared from polyethersulfone (PES) and (as controls) left unmodified or aminated. Alternately, the materials were malemided or gold-coated to provide bio-orthogonal binding partners for the thiol groups newly expressed on the cell surface. Cell attachment was modulated by both the topography of the substrate surface and by the chemical compatibility of the binding interface between the cell and the substrate; a substantial increase in binding for normally non-adhesive Jurkat line for 3D scaffold compared to 2D surfaces with an added degree of adhesion resulting from chemoselective binding to malemidede-derivatived or gold-coated surfaces. In addition, the morphology of the cells attached to the 3D scaffolds via MOE-mediated adhesion was dramatically altered and the expression of genes involved in cell adhesion changed in a time-dependent manner. This study showed that cell adhesion could be enhanced, gene expression modulated, and cell fate controlled by introducing the 3D topograhical cues into the growth substrate and by creating a glycoengineered binding interface where the chemistry of both the cell surface and biomaterials scaffold was controlled to facilitate a new mode of carbohydrate-mediated adhesion.

Emerging synthetic approaches for protein-polymer conjugations

Protein-polymer conjugates are important in diverse fields including drug delivery, biotechnology, and nanotechnology. This feature article highlights recent advances in the synthesis and application of protein-polymer conjugates by controlled radical polymerization techniques. Special emphasis on new applications of the materials, particularly in biomedicine, is provided.

Protein nanopatterns by oxime bond formation

Patterning proteins on the nanoscale is important for applications in biology and medicine. As feature sizes are reduced, it is critical that immobilization strategies provide site-specific attachment of the biomolecules. In this study, oxime chemistry was exploited to conjugate proteins onto nanometer-sized features. Poly(Boc-aminooxy tetra(ethylene glycol) methacrylate) was synthesized by free radical polymerization. The polymer was patterned onto silicon wafers using an electron beam writer. Trifluoroacetic acid removal of the Boc groups provided the desired aminooxy functionality. In this manner, patterns of concentric squares and contiguous bowtie shapes were fabricated with 150-170-nm wide features. Ubiquitin modified at the N-terminus with an α-ketoamide group and N(ε)-levulinyl lysine-modified bovine serum albumin were subsequently conjugated to the polymer nanopatterns. Protein immobilization was confirmed by fluorescence microscopy. Control studies on protected surfaces and using proteins presaturated with O-methoxyamine indicated that attachment occurred via oxime bond formation.