Bioorthogonal strategies for site-directed decoration of biomaterials with therapeutic proteins

An Introductory Guide to Protease Sensitive Linker Design Using Matrix Metalloproteinase 13 as an Example

Proteases play a crucial role, not only in physiological, but also in pathological processes, such as cancer, inflammation, arthritis, Alzheimer's, and infections, to name but a few. Their ability to cleave peptides can be harnessed for a broad range of biotechnological purposes. To do this efficiently, it is essential to find an amino acid sequence that meets the necessary requirements, including interdependent factors like specificity, selectivity, cleavage kinetics, or synthetic accessibility. Cleavage sequences from natural substrates of the protease may not be optimal in terms of specificity and selectivity, which is why these frequently require arduous and sometimes unsuccessful optimization such as by iterative exchange of single amino acids. Hence, here we describe the systematic design of protease sensitive linkers (PSLs)─peptide sequences specifically cleaved by a target protease─guided by the mass spectrometry based determination of target protease specific cleavage sites from a proteome-based peptide library. It includes a procedure for identifying bespoke PSL sequences, their optimization, synthesis, and validation and introduces a program that can indicate potential cleavage sites by hundreds of enzymes in any arbitrary amino acid sequence. Thereby, we provide an introduction to PSL design, illustrated by the example of matrix metalloproteinase 13 (MMP13). This introduction can serve as a guide and help to greatly accelerate the development and use of protease-sensitive linkers in diverse applications.

Essential elements for spatiotemporal delivery of growth factors within bio-scaffolds: A comprehensive strategy for enhanced tissue regeneration

The precise delivery of growth factors (GFs) in regenerative medicine is crucial for effective tissue regeneration and wound repair. However, challenges in achieving controlled release, such as limited half-life, potential overdosing risks, and delivery control complexities, currently hinder their clinical implementation. Despite the plethora of studies endeavoring to accomplish effective loading and gradual release of GFs through diverse delivery methods, the nuanced control of spatial and temporal delivery still needs to be elucidated. In response to this pressing clinical imperative, our review predominantly focuses on explaining the prevalent strategies employed for spatiotemporal delivery of GFs over the past five years. This review will systematically summarize critical aspects of spatiotemporal GFs delivery, including judicious bio-scaffold selection, innovative loading techniques, optimization of GFs activity retention, and stimulating responsive release mechanisms. It aims to identify the persisting challenges in spatiotemporal GFs delivery strategies and offer an insightful outlook on their future development. The ultimate objective is to provide an invaluable reference for advancing regenerative medicine and tissue engineering applications.

Matrix Metalloproteinase-Responsive Delivery of PEGylated Fibroblast Growth Factor 2

Attachment of polyethylene glycol (PEG) chains is a common, well-studied, and Food and Drug Administration-approved method to address the pharmacokinetic challenges of therapeutic proteins. Occasionally, PEGylation impairs the activity of pharmacodynamics (PD). To overcome this problem, disease-relevant cleavable linkers between the polymer and the therapeutic protein can unleash full PD by de-PEGylating the protein at its target site. In this study, we engineered a matrix metalloproteinase (MMP)-responsive fibroblast growth factor 2 (FGF-2) mutant that was site-specifically extended with a PEG polymer chain. Using bioinspired strategies, the bioconjugate was designed to release the native protein at the desired structure/environment with preservation of the proliferative capacity in vitro on NIH3T3 cells. In vivo, hepatic exposure was diminished but not its renal distribution over time compared to unconjugated FGF-2. By releasing the growth factor from the PEG polymer in response to MMP cleavage, restored FGF-2 may enter hard-to-reach tissues and activate cell surface receptors or nuclear targets.

Molecular Dynamics Simulations for Rationalizing Polymer Bioconjugation Strategies: Challenges, Recent Developments, and Future Opportunities

The covalent modification of proteins with polymers is a well-established method for improving the pharmacokinetic properties of therapeutically valuable biologics. The conjugated polymer chains of the resulting hybrid represent highly flexible macromolecular structures. As the dynamics of such systems remain rather elusive for established experimental techniques from the field of protein structure elucidation, molecular dynamics simulations have proven as a valuable tool for studying such conjugates at an atomistic level, thereby complementing experimental studies. With a focus on new developments, this review aims to provide researchers from the polymer bioconjugation field with a concise and up to date overview of such approaches. After introducing basic principles of molecular dynamics simulations, as well as methods for and potential pitfalls in modeling bioconjugates, the review illustrates how these computational techniques have contributed to the understanding of bioconjugates and bioconjugation strategies in the recent past and how they may lead to a more rational design of novel bioconjugates in the future.

Regulating Chemokine-Receptor Interactions through the Site-Specific Bioorthogonal Conjugation of Photoresponsive DNA Strands

Oligonucleotide conjugation has emerged as a versatile molecular tool for regulating protein activity. A state-of-the-art labeling strategy includes the site-specific conjugation of DNA, by employing bioorthogonal groups genetically incorporated in proteins through unnatural amino acids (UAAs). The incorporation of UAAs in chemokines has to date, however, remained underexplored, probably due to their sometimes poor stability following recombinant expression. In this work, we designed a fluorescent stromal-derived factor-1β (SDF-1β) chemokine fusion protein with a bioorthogonal functionality amenable for click reactions. Using amber stop codon suppression, p-azido-L-phenylalanine was site-specifically incorporated in the fluorescent N-terminal fusion partner, superfolder green fluorescent protein (sfGFP). Conjugation to single-stranded DNAs (ssDNA), modified with a photocleavable spacer and a reactive bicyclononyne moiety, was performed to create a DNA-caged species that blocked the receptor binding ability. This inhibition was completely reversible by means of photocleavage of the ssDNA strands. The results described herein provide a versatile new direction for spatiotemporally regulating chemokine-receptor interactions, which is promising for tissue engineering purposes.

Site-specific PEGylation of recombinant tissue-type plasminogen activator

Tissue-type plasminogen activator (tPA) is the gold standard for emergency treatment of ischemic stroke, which is the third leading cause of death worldwide. Major challenges of tPA therapy are its rapid elimination by plasminogen activator inhibitor-1 (PAI-1) and hepatic clearance, leading to the use of high doses and consequent serious side effects, including internal bleeding, swelling and low blood pressure. In this regard, we developed three polyethylene glycol (PEG)ylated tPA bioconjugates based on the recombinant human tPA drug Alteplase using site-specific conjugation strategies. The first bioconjugate with PEGylation at the N-terminus of tPA performed by reductive alkylation showed a reduced proteolytic activity of 68 % compared to wild type tPA. PEGylation at the single-free cysteine of tPA with linear and branched PEG revealed similar proteolytic activities as the wild-type protein. Moreover, both bioconjugates with PEG-cysteine-modification showed 2-fold slower inhibition kinetics by PAI-1. All bioconjugates increased in hydrodynamic size as a critical requirement for half-life extension.

Precise assembly of inside-out cell membrane camouflaged nanoparticles via bioorthogonal reactions for improving drug leads capturing

Cell membrane camouflaged nanoparticles have been widely used in the field of drug leads discovery attribute to their unique biointerface targeting function. However, random orientation of cell membrane coating does not guarantee effective and appropriate binding of drugs to specific sites, especially when applied to intracellular regions of transmembrane proteins. Bioorthogonal reactions have been rapidly developed as a specific and reliable method for cell membrane functionalization without disturbing living biosystem. Herein, inside-out cell membrane camouflaged magnetic nanoparticles (IOCMMNPs) were accurately constructed via bioorthogonal reactions to screen small molecule inhibitors targeting intracellular tyrosine kinase domain of vascular endothelial growth factor recptor-2. Azide functionalized cell membrane acted as a platform for specific covalently coupling with alkynyl functionalized magnetic Fe₃O₄ nanoparticles to prepare IOCMMNPs. The inside-out orientation of cell membrane was successfully verified by immunogold staining and sialic acid quantification assay. Ultimately, two compounds, senkyunolide A and ligustilidel, were successfully captured, and their potential antiproliferative activities were further testified by pharmacological experiments. It is anticipated that the proposed inside-out cell membrane coating strategy endows tremendous versatility for engineering cell membrane camouflaged nanoparticles and promotes the development of drug leads discovery platforms.

Instability Challenges and Stabilization Strategies of Pharmaceutical Proteins

Maintaining the structure of protein and peptide drugs has become one of the most important goals of scientists in recent decades. Cold and thermal denaturation conditions, lyophilization and freeze drying, different pH conditions, concentrations, ionic strength, environmental agitation, the interaction between the surface of liquid and air as well as liquid and solid, and even the architectural structure of storage containers are among the factors that affect the stability of these therapeutic biomacromolecules. The use of genetic engineering, side-directed mutagenesis, fusion strategies, solvent engineering, the addition of various preservatives, surfactants, and additives are some of the solutions to overcome these problems. This article will discuss the types of stress that lead to instabilities of different proteins used in pharmaceutics including regulatory proteins, antibodies, and antibody-drug conjugates, and then all the methods for fighting these stresses will be reviewed. New and existing analytical methods that are used to detect the instabilities, mainly changes in their primary and higher order structures, are briefly summarized.

Fabrication of 2D nanosheet sorbents through metastable emulsion droplets and subsequent two-step grafting polymerization for efficient blood lead removal in vitro

Hemoperfusion is a powerful and yet simple method for lead poisoning treatment, but creation of safe and effective sorbents with excellent selectivity remains a real challenge. To address this, we here construct 2D nanosheet sorbents (BM-SH) through metastable emulsion droplets and subsequent two-step grafting polymerization for efficient blood lead removal in vitro. Metastable emulsion droplets endow typical nanosized sheet-like structure (thickness of 30 nm) and relatively round shape. The consecutive two grafting processes using hydroxyethyl methacrylate (HEMA) and L-cysteine monomer (D-SH) provide BM-SH with a high density of accessible binding sites towards lead ions (Pb²⁺). A high adsorption capacity of 390.5 mg g^-1 and quick capture 97.35 % of Pb²⁺ within initial 10 min are obtained, surpassing most of the reported sorbents for lead removal. Besides, adsorption distribution coefficient (K_d) of BM-SH among four coexisting metal ions achieved 7792 mL g^-1, showing outstanding selectivity toward Pb²⁺. Importantly, a possible adsorption mechanism is recognized as coordination with carboxyl, sulfydryl and imino groups from L-cysteine, and mercapto ligand as the key chelating agent may be the reason for high Pb²⁺ affinity. And what's more, BM-SH displays good hemocompatibility and high efficiency of blood lead removal rate (above 86 % in vitro).

Clickable Biomaterials for Modulating Neuroinflammation

Crosstalk between the nervous and immune systems in the context of trauma or disease can lead to a state of neuroinflammation or excessive recruitment and activation of peripheral and central immune cells. Neuroinflammation is an underlying and contributing factor to myriad neuropathologies including neurodegenerative diseases like Alzheimer's disease and Parkinson's disease; autoimmune diseases like multiple sclerosis; peripheral and central nervous system infections; and ischemic and traumatic neural injuries. Therapeutic modulation of immune cell function is an emerging strategy to quell neuroinflammation and promote tissue homeostasis and/or repair. One such branch of 'immunomodulation' leverages the versatility of biomaterials to regulate immune cell phenotypes through direct cell-material interactions or targeted release of therapeutic payloads. In this regard, a growing trend in biomaterial science is the functionalization of materials using chemistries that do not interfere with biological processes, so-called 'click' or bioorthogonal reactions. Bioorthogonal chemistries such as Michael-type additions, thiol-ene reactions, and Diels-Alder reactions are highly specific and can be used in the presence of live cells for material crosslinking, decoration, protein or cell targeting, and spatiotemporal modification. Hence, click-based biomaterials can be highly bioactive and instruct a variety of cellular functions, even within the context of neuroinflammation. This manuscript will review recent advances in the application of click-based biomaterials for treating neuroinflammation and promoting neural tissue repair.

PEtOxylated Interferon-α2a Bioconjugates Addressing H1N1 Influenza A Virus Infection

Influenza A viruses (IAV), including the pandemic 2009 (pdm09) H1N1 or avian influenza H5N1 virus, may advance into more pathogenic, potentially antiviral drug-resistant strains (including loss of susceptibility against oseltamivir). Such IAV strains fuel the risk of future global outbreaks, to which this study responds by re-engineering Interferon-α2a (IFN-α2a) bioconjugates into influenza therapeutics. Type-I interferons such as IFN-α2a play an essential role in influenza infection and may prevent serious disease courses. We site-specifically conjugated a genetically engineered IFN-α2a mutant to poly(2-ethyl-2-oxazoline)s (PEtOx) of different molecular weights by strain-promoted azide-alkyne cyclo-addition. The promising pharmacokinetic profile of the 25 kDa PEtOx bioconjugate in mice echoed an efficacy in IAV-infected ferrets. One intraperitoneal administration of this bioconjugate, but not the marketed IFN-α2a bioconjugate, changed the disease course similar to oseltamivir, given orally twice every study day. PEtOxylated IFN-α2a bioconjugates may expand our therapeutic arsenal against future influenza pandemics, particularly in light of rising first-line antiviral drug resistance to IAV.

Polymer selection impacts the pharmaceutical profile of site-specifically conjugated Interferon-α2a

Conjugation of poly(ethylene glycol) (PEG) to biologics is a successful strategy to favorably impact the pharmacokinetics and efficacy of the resulting bioconjugate. We compare bioconjugates synthesized by strain-promoted azide-alkyne cycloaddition (SPAAC) using PEG and linear polyglycerol (LPG) of about 20 kDa or 40 kDa, respectively, with an azido functionalized human Interferon-α2a (IFN-α2a) mutant. Site-specific PEGylation and LPGylation resulted in IFN-α2a bioconjugates with improved in vitro potency compared to commercial Pegasys. LPGylated bioconjugates had faster disposition kinetics despite comparable hydrodynamic radii to their PEGylated analogues. Overall exposure of the PEGylated IFN-α2a with a 40 kDa polymer exceeded Pegasys, which, in return, was similar to the 40 kDa LPGylated conjugates. The study points to an expanded polymer design space through which the selected polymer class may result in a different distribution of the studied bioconjugates.

Research Progress and Application of Bioorthogonal Reactions in Biomolecular Analysis and Disease Diagnosis

Bioorthogonal reactions are rapid, specific and high yield reactions that can be performed in in vivo microenvironments or simulated microenvironments. At present, the main biorthogonal reactions include Staudinger ligation, copper-catalyzed azide alkyne cycloaddition, strain-promoted [3 + 2] reaction, tetrazine ligation, metal-catalyzed coupling reaction and photo-induced biorthogonal reactions. To date, many reviews have reported that bioorthogonal reactions have been used widely as a powerful tool in the field of life sciences, such as in target recognition, drug discovery, drug activation, omics research, visualization of life processes or exogenous bacterial infection processes, signal transduction pathway research, chemical reaction dynamics analysis, disease diagnosis and treatment. In contrast, to date, few studies have investigated the application of bioorthogonal reactions in the analysis of biomacromolecules in vivo. Therefore, the application of bioorthogonal reactions in the analysis of proteins, nucleic acids, metabolites, enzyme activities and other endogenous molecules, and the determination of disease-related targets is reviewed. In addition, this review discusses the future development opportunities and challenges of biorthogonal reactions. This review presents an overview of recent advances for application in biomolecular analysis and disease diagnosis, with a focus on proteins, metabolites and RNA detection.

Biopharmaceutical quality control with mass spectrometry

Mass spectrometry (MS) is a powerful technique for protein identification, quantification and characterization that is widely applied in biochemical studies, and which can provide data on the quantity, structural integrity and post-translational modifications of proteins. It is therefore a versatile and widely used analytic tool for quality control of biopharmaceuticals, especially in quantifying host-cell protein impurities, identifying post-translation modifications and structural characterization of biopharmaceutical proteins. Here, we summarize recent advances in MS-based analyses of these key quality attributes of the biopharmaceutical development and manufacturing processes.

An Advanced 'clickECM' That Can be Modified by the Inverse-Electron-Demand Diels-Alder Reaction

The extracellular matrix (ECM) represents the natural environment of cells in tissue and therefore is a promising biomaterial in a variety of applications. Depending on the purpose, it is necessary to equip the ECM with specific addressable functional groups for further modification with bioactive molecules, for controllable cross-linking and/or covalent binding to surfaces. Metabolic glycoengineering (MGE) enables the specific modification of the ECM with such functional groups without affecting the native structure of the ECM. In a previous approach (S. M. Ruff, S. Keller, D. E. Wieland, V. Wittmann, G. E. M. Tovar, M. Bach, P. J. Kluger, Acta Biomater. 2017, 52, 159-170), we demonstrated the modification of an ECM with azido groups, which can be addressed by bioorthogonal copper-catalyzed azide-alkyne cycloaddition (CuAAC). Here, we demonstrate the modification of an ECM with dienophiles (terminal alkenes, cyclopropene), which can be addressed by an inverse-electron-demand Diels-Alder (IEDDA) reaction. This reaction is cell friendly as there are no cytotoxic catalysts needed. We show the equipment of the ECM with a bioactive molecule (enzyme) and prove that the functional groups do not influence cellular behavior. Thus, this new material has great potential for use as a biomaterial, which can be individually modified in a wide range of applications.

From Thermogelling Hydrogels toward Functional Bioinks: Controlled Modification and Cytocompatible Crosslinking

Hydrogels are key components in bioink formulations to ensure printability and stability in biofabrication. In this study, a well-known Diels-Alder two-step post-polymerization modification approach is introduced into thermogelling diblock copolymers, comprising poly(2-methyl-2-oxazoline) and thermoresponsive poly(2-n-propyl-2-oxazine). The diblock copolymers are partially hydrolyzed and subsequently modified by acid/amine coupling with furan and maleimide moieties. While the thermogelling and shear-thinning properties allow excellent printability, trigger-less cell-friendly Diels-Alder click-chemistry yields long-term shape-fidelity. The introduced platform enables easy incorporation of cell-binding moieties (RGD-peptide) for cellular interaction. The hydrogel is functionalized with RGD-peptides using thiol-maleimide chemistry and cell proliferation as well as morphology of fibroblasts seeded on top of the hydrogels confirm the cell adhesion facilitated by the peptides. Finally, bioink formulations are tested for biocompatibility by incorporating fibroblasts homogenously inside the polymer solution pre-printing. After the printing and crosslinking process good cytocompatibility is confirmed. The established bioink system combines a two-step approach by physical precursor gelation followed by an additional chemical stabilization, offering a broad versatility for further biomechanical adaptation or bioresponsive peptide modification.

Enzyme-mediated bioorthogonal technologies: catalysts, chemoselective reactions and recent methyltransferase applications

Transferases have emerged as among the best catalysts for enzyme-mediated bioorthogonal functional group installation to advance innovative in vitro, cell-based and in vivo chemical biology applications. This review introduces the key considerations for selecting enzyme catalysts and chemoselective reactions most amenable to bioorthogonal platform development and highlights relevant key technology development and applications for one ubiquitous transferase subclass - methyltransferases (MTs). Within this context, recent advances in MT-enabled bioorthogonal labeling/conjugation relevant to DNA, RNA, protein, and natural products (i.e. complex small molecule metabolites) are highlighted.

Mass-Encoded Reporters Reporting Proteolytic Activity from within the Extracellular Matrix

Reporting matrix metalloproteinase (MMP) activity directly from the extracellular matrix (ECM) may provide critical insights to better characterize 2D and 3D cell culture model systems of inflammatory diseases and potentially leverage in vivo diagnosis. In this proof-of-concept study, we designed MMP-sensors, which were covalently linked onto the ECM by co-administration of the activated transglutaminase factor XIIIa (FXIIIa). Elements of the featured MMP-sensors are the D-domain of insulin-like growth factor I (IGF-I) through which co-administered FXIIIa covalently links the sensor to the ECM followed by an MMP sensitive peptide sequence and locally reporting on MMP activity, an isotopically labeled mass tag encoding for protease activity, and an affinity tag facilitating purification from fluids. All sensors come in identical pairs, other than the MMP sensitive peptide sequence, which is synthesized with l-amino acids or d-amino acids, the latter serving as internal standard. As a proof of concept for multiplexing, we successfully profiled two MMP-sensors with different MMP sensitive peptide sequences reporting MMP activity directly from an engineered 3D ECM. Future use may include covalently ECM bound diagnostic depots reporting MMP activity from inflamed tissues.

Gelatin Matrices for Growth Factor Sequestration

Gelatin is used in a broad range of tissue engineering applications because of its bioactivity, mild processing conditions, and ease of modification, which have increased interest in its use as a growth factor delivery vehicle. Traditional methods to control growth factor sequestration and delivery have relied on controlling hydrogel mesh size via chemical crosslinking with corollary changes to the physical properties of the hydrogel. To decouple growth factor release from scaffold properties, affinity sequestration modalities have been developed to preserve the bioactivity of the growth factor through interactions with the modified gelatin. This review provides a summary of these mechanisms, highlights current gelatin growth factor delivery systems, and addresses the future perspective of gelatin matrices for growth factor delivery in tissue engineering.

A bioorthogonally activatable photosensitiser for site-specific photodynamic therapy

Inverse-electron-demand Diels–Alder reaction of a 1,2,4,5-tetrazine-substituted boron dipyrromethene with a biotin-conjugated trans-cyclooctene results in site-specific activation of the photoactivity of the former photosensitiser.

Biodistribution of Site-Specific PEGylated Fibroblast Growth Factor-2

Fibroblast growth factor 2 (FGF-2) is a small 18 kDa protein with clinical potential for ischemic heart disease, wound healing, and spinal cord injury. However, the therapeutic potential of systemic FGF-2 administration is challenged by its fast elimination. Therefore, we deployed genetic codon expansion to integrate an azide functionality to the FGF-2 N-terminus, which was site-directly decorated with poly(ethylene glycol) (PEG) through bioorthogonal strain-promoted azide-alkyne cycloaddition (SPAAC). PEGylated FGF-2 was as bioactive as wild-type FGF-2 as demonstrated by cell proliferation and Erk phosphorylation of fibroblasts. The PEGylated FGF-2 conjugate was radiolabeled with [¹¹¹In] Indium cation ([¹¹¹In]In³⁺) to study its biodistribution through noninvasive imaging by single-photon emission computed tomography (SPECT) and by quantitative activity analysis of the respective organs in healthy mice. This study details the biodistribution pattern of site-specific PEGylated FGF-2 in tissues after intravenous (iv) administration compared to the unconjugated protein. Low accumulation of the PEGylated FGF-2 variant in the kidney and the liver was demonstrated, whereas specific uptake of PEGylated FGF-2 into the retina was significantly diminished. In conclusion, site-specific PEGylation of FGF-2 by SPAAC resulted in a superior outcome for the synthesis yield and in conjugates with excellent biological performances with a gain of half-life but reduced tissue access in vivo.

β-Hydroxy-Stabilized Boron-Nitrogen Heterocycles Enable Rapid and Efficient C-Terminal Protein Modification

Bioorthogonal chemistry has enabled the development of bioconjugates in physiological environments while averting interference from endogenous biomolecules. Reactions between carbonyl-containing molecules and alkoxyamines or hydrazines have experienced a resurgence in popularity in bioorthogonal chemistry owing to advances that allow the reactions to occur under physiological conditions. In particular, ortho-carbonyl-substituted phenylboronic acids (CO-PBAs) exhibit greatly accelerated rates of hydrazone and oxime formation via intramolecular Lewis acid catalysis. Unfortunately, the rate of the reverse reaction is also increased, yielding a kinetically less stable bioconjugate. When the substrate is a hydrazine derivative, an intramolecular reaction between the boronic acid and the hydrazone can lead to the formation of a heterocycle containing a boron-nitrogen bond. We have shown previously that α-amino hydrazides undergo rapid reaction with CO-PBAs to form highly stable, tricyclic products, and that this reaction is orthogonal to the popular azide-alkyne and tetrazine-alkene reactions. In this work, we explore a series of heteroatom-substituted hydrazides for their ability to form tricyclic products with two CO-PBAs, 2-formylphenylboronic acid (2fPBA), and 2-acetylphenylboronic acid (AcPBA). In particular, highly stable products were formed using β-hydroxy hydrazides and 2fPBA. C-Terminal β-hydroxy hydrazide proteins are available using conventional biochemical methods, which alleviates one of the difficulties with applications of bioorthogonal chemical reactions: site-specific incorporation of a reactive group into the biomolecular target. Using sortase-mediated ligation (SML), C-terminal threonine and serine hydrazides were appended to a model eGFP protein in high yield. Subsequent labeling with 2fPBA functionalized probes could be performed quickly and quantitatively at neutral pH using micromolar concentrations of reactants. The SML process was applied directly to an expressed protein in cellular extract, and the C-terminal modified target protein was selectively immobilized using 2fPBA-agarose. Elution from the agarose yielded a highly pure protein that retained the hydrazide functionality. This strategy should be generally applicable for rapid, efficient site-specific protein labeling, protein immobilization, and preparation of highly pure functionalized proteins.

Redox-Sensitive Gelatin/Silica-Aptamer Nanogels for Targeted siRNA Delivery

RNA interference (RNAi) has potential advantages over other gene therapy approaches due to its high specificity and the ability to inhibit target gene expression. However, the stability and tissue-specific delivery of siRNA remain as the biggest obstacles for RNAi therapeutics. Here, we developed such a system by conjugating gelatin-based nanogels with the nucleolin-targeted AS1411 aptamer and deoxynucleotide-substituted siRNA together (Apt-GS/siRNA) via a disulfide linker to achieve transient docking of siRNA. These Apt-GS/siRNA nanogels demonstrated favorable release of siRNA under reducing conditions owing to disulfide cleavage. Furthermore, this smart system could electively release siRNA into the cytosol in nucleolin-positive cells (A549) by a glutathione-triggered disassembly and subsequently efficient RNAi for luciferase. Besides, disulfide-equipped Apt-GS nanogels showed good biocompatibility in vitro. Taken together, this redox-responsive, tumor-targeting smart nanogels display great potential in exploiting functionalized siRNA delivery and tumor therapy.

Semisynthesis of an Active Enzyme by Quantitative Click Ligation

The incorporation of clickable noncanonical amino acids (ncAAs) has proven to an invaluable tool in chemical biology and protein science research. Nevertheless, the number of examples in which the method is used for preparative purposes is extremely limited. We report the synthesis of an active enzyme by quantitative, Cu(I)-catalyzed ligation of two inactive protein halves, expressed and equipped with an azide and alkyne moiety, respectively, through ncAA incorporation. The reported quantitative conversion is exceptional given the large size of the protein fragments and the fact that no linker or excess of either of the polypeptides was used. The triazole bridge formed between the ncAA side chains was shown to effectively mimic a natural protein loop, providing an enzyme with the same activity as its natural counterpart. We envision that this strategy, termed split-click protein chemistry, can be used for the production of proteins that are difficult to express as full-length entities. It also paves the way for the design of new proteins with tailor-made functionalities.

Quantification of azide groups on a material surface and a biomolecule using a clickable and cleavable fluorescent compound

We propose a novel method for quantifying azide groups on a solid surface and a protein.

Recent progress in enzymatic protein labelling techniques and their applications

Protein-based conjugates are valuable constructs for a variety of applications. Conjugation of proteins to fluorophores is commonly used to study their cellular localization and the protein-protein interactions. Modification of therapeutic proteins with either polymers or cytotoxic moieties greatly enhances their pharmacokinetics or potency. To label a protein of interest, conventional direct chemical reaction with the side-chains of native amino acids often yields heterogeneously modified products. This renders their characterization complicated, requires difficult separation steps and may impact protein function. Although modification can also be achieved via the insertion of unnatural amino acids bearing bioorthogonal functional groups, these methods can have lower protein expression yields, limiting large scale production. As a site-specific modification method, enzymatic protein labelling is highly efficient and robust under mild reaction conditions. Significant progress has been made over the last five years in modifying proteins using enzymatic methods for numerous applications, including the creation of clinically relevant conjugates with polymers, cytotoxins or imaging agents, fluorescent or affinity probes to study complex protein interaction networks, and protein-linked materials for biosensing. This review summarizes developments in enzymatic protein labelling over the last five years for a panel of ten enzymes, including sortase A, subtiligase, microbial transglutaminase, farnesyltransferase, N-myristoyltransferase, phosphopantetheinyl transferases, tubulin tyrosin ligase, lipoic acid ligase, biotin ligase and formylglycine generating enzyme.

Adequate Reducing Conditions Enable Conjugation of Oxidized Peptides to Polymers by One-Pot Thiol Click Chemistry

Thiol(-click) chemistry has been extensively investigated to conjugate (bio)molecules to polymers. Handling of cysteine-containing molecules may however be cumbersome, especially in the case of fast-oxidizing coiled-coil-forming peptides. In the present study, we investigated the practicality of a one-pot process to concomitantly reduce and conjugate an oxidized peptide to a polymer. Three thiol-based conjugation chemistries (vinyl sulfone (VS), maleimide, and pyridyldithiol) were assayed along with three reducing agents (tris(2-carboxyethyl)phosphine (TCEP), dithiothreitol, and β-mercaptoethanol). Seven out of the nine possible combinations significantly enhanced the conjugation yield, provided that an adequate concentration of reductant was used. Among them, the coincubation of an oxidized peptide with TCEP and a VS-modified polymer displayed the highest level of conjugation. Our results also provide insights into two topics that currently lack consensus: TCEP is stable in 10 mM phosphate buffered saline and it reacts with thiol-alkylating agents at submillimolar concentrations, and thus should be carefully used in order to avoid interference with thiol-based conjugation reactions.

Click Chemistry-Based Injectable Hydrogels and Bioprinting Inks for Tissue Engineering Applications

BACKGROUND

The tissue engineering and regenerative medicine approach require biomaterials which are biocompatible, easily reproducible in less time, biodegradable and should be able to generate complex three-dimensional (3D) structures to mimic the native tissue structures. Click chemistry offers the much-needed multifunctional hydrogel materials which are interesting biomaterials for the tissue engineering and bioprinting inks applications owing to their excellent ability to form hydrogels with printability instantly and to retain the live cells in their 3D network without losing the mechanical integrity even under swollen state.

METHODS

In this review, we present the recent developments of in situ hydrogel in the field of click chemistry reported for the tissue engineering and 3D bioinks applications, by mainly covering the diverse types of click chemistry methods such as Diels-Alder reaction, strain-promoted azide-alkyne cycloaddition reactions, thiol-ene reactions, oxime reactions and other interrelated reactions, excluding enzyme-based reactions.

RESULTS

The click chemistry-based hydrogels are formed spontaneously on mixing of reactive compounds and can encapsulate live cells with high viability for a long time. The recent works reported by combining the advantages of click chemistry and 3D bioprinting technology have shown to produce 3D tissue constructs with high resolution using biocompatible hydrogels as bioinks and in situ injectable forms.

CONCLUSION

Interestingly, the emergence of click chemistry reactions in bioink synthesis for 3D bioprinting have shown the massive potential of these reaction methods in creating 3D tissue constructs. However, the limitations and challenges involved in the click chemistry reactions should be analyzed and bettered to be applied to tissue engineering and 3D bioinks. The future scope of these materials is promising, including their applications in in situ 3D bioprinting for tissue or organ regeneration.