Stabilization of Fibronectin by Random Copolymer Brushes Inhibits Macrophage Activation

Protein-polymer bioconjugation, immobilization, and encapsulation: a comparative review towards applicability, functionality, activity, and stability

Polymer-based biomaterials have received a lot of attention due to their biomedical, agricultural, and industrial potential. Soluble protein-polymer bioconjugates, immobilized proteins, and encapsulated proteins have been shown to tune enzymatic activity, improved pharmacokinetic ability, increased chemical and thermal stability, stimuli responsiveness, and introduced protein recovery. Controlled polymerization techniques, increased protein-polymer attachment techniques, improved polymer surface grafting techniques, controlled polymersome self-assembly, and sophisticated characterization methods have been utilized for the development of well-defined polymer-based biomaterials. In this review we aim to provide a brief account of the field, compare these methods for engineering biomaterials, provide future directions for the field, and highlight impacts of these forms of bioconjugation.

Supra-biological performance of immobilized enzymes enabled by chaperone-like specific non-covalent interactions

Designing complex synthetic materials for enzyme immobilization could unlock the utility of biocatalysis in extreme environments. Inspired by biology, we investigate the use of random copolymer brushes as dynamic immobilization supports that enable supra-biological catalytic performance of immobilized enzymes. This is demonstrated by immobilizing Bacillus subtilis Lipase A on brushes doped with aromatic moieties, which can interact with the lipase through multiple non-covalent interactions. Incorporation of aromatic groups leads to a 50 °C increase in the optimal temperature of lipase, as well as a 50-fold enhancement in enzyme activity. Single-molecule FRET studies reveal that these supports act as biomimetic chaperones by promoting enzyme refolding and stabilizing the enzyme's folded and catalytically active state. This effect is diminished when aromatic residues are mutated out, suggesting the importance of π-stacking and π-cation interactions for stabilization. Our results underscore how unexplored enzyme-support interactions may enable uncharted opportunities for using enzymes in industrial biotransformations.

Cell- and Serum-Derived Proteins Act as DAMPs to Activate RAW 264.7 Macrophage-like Cells on Silicone Implants

Protein adsorption after biomaterial implantation is the first stage of the foreign body response (FBR). However, the source(s) of the adsorbed proteins that lead to damaged associated molecular patterns (DAMPs) and induce inflammation have not been fully elucidated. This study examined the effects of different protein sources, cell-derived (from a NIH/3T3 fibroblast cell lysate) and serum-derived (from fetal bovine serum), which were compared to implant-derived proteins (after a 30 min subcutaneous implantation in mice) on activation of RAW 264.7 cells cultured in minimal (serum-free) medium. Both cell-derived and serum-derived protein sources when preadsorbed to either tissue culture polystyrene or medical-grade silicone induced RAW 264.7 cell activation. The combination led to an even higher expression of pro-inflammatory cytokine genes and proteins. Implant-derived proteins on silicone explants induced a rapid inflammatory response that then subsided more quickly and to a greater extent than the studies with in vitro cell-derived or serum-derived protein sources. Proteomic analysis of the implant-derived proteins identified proteins that included cell-derived and serum-derived, but also other proteinaceous sources (e.g., extracellular matrix), suggesting that the latter or nonproteinaceous sources may help to temper the inflammatory response in vivo. These findings indicate that both serum-derived and cell-derived proteins adsorbed to implants can act as DAMPs to drive inflammation in the FBR, but other protein sources may play an important role in controlling inflammation.

Protein Folding Stability and Kinetics in Alginate Hydrogels

Proteins are commonly encapsulated in alginate gels for drug delivery and tissue-engineering applications. However, there is limited knowledge of how encapsulation impacts intrinsic protein properties such as folding stability or unfolding kinetics. Here, we use fast relaxation imaging (FReI) to image protein unfolding in situ in alginate hydrogels after applying a temperature jump. Based on changes in the Förster resonance energy transfer (FRET) response of FRET-labeled phosphoglycerate kinase (PGK), we report the quantitative impact of multiple alginate hydrogel concentrations on protein stability and folding dynamics. The gels stabilize PGK by increasing its melting temperature up to 18.4 °C, and the stabilization follows a nonmonotonic dependence on the alginate density. In situ kinetic measurements also reveal that PGK deviates more from two-state folding behavior in denser gels and that the gel decreases the unfolding rate and accelerates the folding rate of PGK, compared to buffer. Phi-value analysis suggests that the folding transition state of an encapsulated protein is structurally similar to that of folded protein. This work reveals both beneficial and negative impacts of gel encapsulation on protein folding, as well as potential mechanisms contributing to altered stability.

Nanoparticle protein corona: from structure and function to therapeutic targeting

Nanoparticle (NP)-based therapeutics have ushered in a new era in translational medicine. However, despite the clinical success of NP technology, it is not well-understood how NPs fundamentally change in biological environments. When introduced into physiological fluids, NPs are coated by proteins, forming a protein corona (PC). The PC has the potential to endow NPs with a new identity and alter their bioactivity, stability, and destination. Additionally, the conformation of proteins is sensitive to their physical and chemical surroundings. Therefore, biological factors and protein-NP-interactions can induce changes in the conformation and orientation of proteins in vivo. Since the function of a protein is closely connected to its folded structure, slight differences in the surrounding environment as well as the surface characteristics of the NP materials may cause proteins to lose or gain a function. As a result, this can alter the downstream functionality of the NPs. This review introduces the main biological factors affecting the conformation of proteins associated with the PC. Then, four types of NPs with extensive utility in biomedical applications are described in greater detail, focusing on the conformation and orientation of adsorbed proteins. This is followed by a discussion on the instances in which the conformation of adsorbed proteins can be leveraged for therapeutic purposes, such as controlling protein conformation in assembled matrices in tissue, as well as controlling the PC conformation for modulating immune responses. The review concludes with a perspective on the remaining challenges and unexplored areas at the interface of PC and NP research.

Blood Protein Exclusion from Polymer Brushes

We report interactions between adsorbed copolymers of poly(ethylene glycol) (PEG) in the presence of two abundant blood proteins, serum albumin and an immunoglobulin G, up to physiological blood concentrations. We directly and nonintrusively measure interactions between PEG triblock copolymers (PEG-PPO-PEG) adsorbed to hydrophobic colloids and surfaces using Total Internal Reflection Microscopy, which provides kT- and nanometer-scale resolution of interaction potentials (energy vs separation). In the absence of protein, adsorbed PEG copolymer repulsion is consistent with dimensions and architectures of PEG brushes on both colloids and surfaces. In the presence of proteins, we observe concentration dependent depletion attraction and no change to brush repulsion, indicating protein exclusion from PEG brushes. Because positive and negative protein adsorption are mutually exclusive, our observations of concentration dependent depletion attraction with no change to brush repulsion unambiguously indicate the absence of protein coronas at physiological protein concentrations. These findings demonstrate a direct sensitive approach to determine interactions between proteins and particle/surface coatings important to diverse biotechnology applications.

Tuning the surface charge of phospholipid bilayers inhibits insulin fibrilization

The interactions between proteins and materials, in particular lipid bilayers, have been studied extensively for their relevance in diseases and for the formulation of protein-based therapeutics and vaccines. However, the precise rules by which material properties induce favorable or unfavorable structural states in biomolecules are incompletely understood, and as a result, the rational design of materials remains challenging. Here, we investigated the influence of lipid bilayers (in the form of small unilamellar vesicles) on the formation of insulin amyloid fibrils using a fibril-specific assay (thioflavin T), polyacrylamide gel electrophoresis, and circular dichroism spectroscopy. Lipid bilayers composed of equal mixtures of cationic and anionic lipids effectively inhibited fibril formation and stabilized insulin in its native conformation. However, other lipid bilayer compositions failed to inhibit fibril formation or even destabilized insulin, exacerbating fibrilization and/or non-amyloid aggregation. Our findings suggest that electrostatic interactions with lipid bilayers can play a critical role in stabilizing or destabilizing insulin, and preventing the conversion of insulin to its amyloidogenic, disease-associated state.

Forces between mica and end-grafted statistical copolymers of sulfobetaine and oligoethylene glycol in aqueous electrolyte solutions

This study quantified the interfacial forces associated with end-grafted, statistical (AB) co-polymers of sulfobetaine methacrylate (SBMA) and oligoethylene glycol methacrylate (OEGMA) (poly(SBMA-co-OEGMA)). Surface force apparatus measurements compared forces between mica and end-grafted copolymers containing 0, 40, or 80 mol% SBMA. Studies compared forces measured at low grafting density (weakly overlapping chains) and at high density (brushes). At high density, the range of repulsive forces did not change significantly with increasing SBMA content. By contrast, at low density, both the range and the amplitude of the repulsion increased with the percentage of SBMA in the chains. The ionic strength dependence of the film thickness and repulsive forces increased similarly with SBMA content, reflecting the increasing influence of charged monomers and their interactions with ions in solution. The forces could be described by models of simple polymers in good solvent. However, the forces and fitted model parameters change continuously with the SBMA content. The latter behavior suggests that ethyene glycol and sulfobetaine behave as non-interacting, miscible monomers that contribute independently to the interfacial forces. The results suggest that molecular scale properties of statistical poly (SBMA-co-OEGMA) films can be readily tuned by simple variation of the monomer ratios.

Understanding Design Rules for Optimizing the Interface between Immobilized Enzymes and Random Copolymer Brushes

A long-standing goal in the field of biotechnology is to develop and understand design rules for the stabilization of enzymes upon immobilization to materials. While immobilization has sometimes been successful as a strategy to stabilize enzymes, the design of synthetic materials that stabilize enzymes remains largely empirical. We sought to overcome this challenge by investigating the mechanistic basis for the stabilization of immobilized lipases on random copolymer brush surfaces comprised of poly(ethylene glycol) methacrylate (PEGMA) and sulfobetaine methacrylate (SBMA), which represent novel heterogeneous supports for immobilized enzymes. Using several related but structurally diverse lipases, including Bacillus subtilis lipase A (LipA), Rhizomucor miehei lipase, Candida rugosa lipase, and Candida antarctica lipase B (CALB), we showed that the stability of each lipase at elevated temperatures was strongly dependent on the fraction of PEGMA in the brush layer. This dependence was explained by developing and applying a new algorithm to quantify protein surface hydrophobicity, which involved using unsupervised cluster analysis to identify clusters of hydrophobic atoms. Characterization of the lipases showed that the optimal brush composition correlated with the free energy of solvation per enzyme surface area, which ranged from -17.1 kJ/mol·nm² for LipA to -11.8 kJ/mol·nm² for CALB. Additionally, using this algorithm, we found that hydrophobic patches consisting of aliphatic residues had a higher free energy than patches consisting of aromatic residues. By providing the basis for rationally tuning the interface between enzymes and materials, this understanding will transform the use of materials to reliably ruggedize enzymes under extreme conditions.

Faster Surface Ligation Reactions Improve Immobilized Enzyme Structure and Activity

During integration into materials, the inactivation of enzymes as a result of their interaction with nanometer size denaturing "hotspots" on surfaces represents a critical challenge. This challenge, which has received far less attention than improving the long-term stability of enzymes, may be overcome by limiting the exploration of surfaces by enzymes. One way this may be accomplished is through increasing the rate constant of the surface ligation reaction and thus the probability of immobilization with reactive surface sites (i.e., ligation efficiency). Here, the connection between ligation reaction efficiency and the retention of enzyme structure and activity was investigated by leveraging the extremely fast reaction of strained trans-cyclooctene (sTCOs) and tetrazines (Tet). Remarkably, upon immobilization via Tet-sTCO chemistry, carbonic anhydrase (CA) retained 77% of its solution-phase activity, while immobilization via less efficient reaction chemistries, such as thiol-maleimide and azide-dibenzocyclooctyne, led to activity retention of only 46% and 27%, respectively. Dynamic single-molecule fluorescence tracking methods further revealed that longer surface search distances prior to immobilization (>0.5 μm) dramatically increased the probability of CA unfolding. Notably, the CA distance to immobilization was significantly reduced through the use of Tet-sTCO chemistry, which correlated with the increased retention of structure and activity of immobilized CA compared to the use of slower ligation chemistries. These findings provide an unprecedented insight into the role of ligation reaction efficiency in mediating the exploration of denaturing hotspots on surfaces by enzymes, which, in turn, may have major ramifications in the creation of functional biohybrid materials.

NaYbF4@NaYF4 Nanoparticles: Controlled Shell Growth and Shape-Dependent Cellular Uptake

This study presents a controlled synthesis of NaYbF₄@NaYF₄ core-shell upconversion nanoparticles using the hot-injection technique. NaYF₄ shells with tunable morphologies including long-rod, short-rod, and quasi-sphere are grown on identical NaYbF₄ core nanoparticles by controlled injection of acetate or trifluoroacetate precursors. Mechanistic investigations reveal that anisotropic interfacial strain accounts for the preferential growth of shell layers along the c-axis. However, the strain effect can be offset by the fast injection of shell precursors, leading to nearly isotropic growth of NaYF₄ shells over the NaYbF₄ core nanoparticles. The core-shell nanoparticles are further modified with DNA molecules and incubated with adenocarcinomic human alveolar basal epithelial cells. Based on a combination of characterizations by flow cytometry and confocal microscopy, favorable cellular uptake and DNA delivery are observed for the quasi-sphere nanoparticles, owing to the high dispersibility and easy membrane wrapping. The method described here could be extended to synthesize other types of functional nanostructures for the study of morphology-dependent properties.

Controlling Experimental Parameters to Improve Characterization of Biomaterial Fouling

Uncontrolled protein adsorption and cell binding to biomaterial surfaces may lead to degradation, implant failure, infection, and deleterious inflammatory and immune responses. The accurate characterization of biofouling is therefore crucial for the optimization of biomaterials and devices that interface with complex biological environments composed of macromolecules, fluids, and cells. Currently, a diverse array of experimental conditions and characterization techniques are utilized, making it difficult to compare reported fouling values between similar or different biomaterials. This review aims to help scientists and engineers appreciate current limitations and conduct fouling experiments to facilitate the comparison of reported values and expedite the development of low-fouling materials. Recent advancements in the understanding of protein-interface interactions and fouling variability due to experiment conditions will be highlighted to discuss protein adsorption and cell adhesion and activation on biomaterial surfaces.

Prostaglandin E2 and Its Receptor EP2 Modulate Macrophage Activation and Fusion in Vitro

The foreign body response (FBR) has impaired progress of new implantable medical devices through its hallmark of chronic inflammation and foreign body giant cell (FBGC) formation leading to fibrous encapsulation. Macrophages are known to drive the FBR, but efforts to control macrophage polarization remain challenging. The goal for this study was to investigate whether prostaglandin E2 (PGE2), and specifically its receptors EP2 and/or EP4, attenuate classically activated (i.e., inflammatory) macrophages and macrophage fusion into FBGCs in vitro. Lipopolysaccharide (LPS)-stimulated macrophages exhibited a dose-dependent decrease in gene expression and protein production of tumor necrosis factor alpha (TNF-α) when treated with PGE2. This attenuation was primarily by the EP4 receptor, as the addition of the EP2 antagonist PF 04418948 to PGE2-treated LPS-stimulated cells did not recover TNF-α production while the EP4 antagonist ONO AE3 208 did. However, direct stimulation of EP2 with the agonist butaprost to LPS-stimulated macrophages resulted in a ∼60% decrease in TNF-α secretion after 4 h and corresponded with an increase in gene expression for Cebpb and Il10, suggesting a polarization shift toward alternative activation through EP2 alone. Further, fusion of macrophages into FBGCs induced by interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) was inhibited by PGE2 via EP2 signaling and by an EP2 agonist, but not an EP4 agonist. The attenuation by PGE2 was confirmed to be primarily by the EP2 receptor. Mrc1, Dcstamp, and Retlna expressions increased upon IL-4/GM-CSF stimulation, but only Retnla expression with the EP2 agonist returned to levels that were not different from controls. This study identified that PGE2 attenuates classically activated macrophages and macrophage fusion through distinct EP receptors, while targeting EP2 is able to attenuate both. In summary, this study identified EP2 as a potential therapeutic target for reducing the FBR to biomaterials.