Protein release from highly charged peptide hydrogel networks

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.

Fucoidan/chitosan hydrogels as carrier for sustained delivery of platelet-rich fibrin containing bioactive molecules

Platelet-rich fibrin (PRF), derived from human blood, rich in wound healing components, has drawbacks in direct injections, such as rapid matrix degradation and growth factor release. Marine polysaccharides, mimicking the human extracellular matrix, show promising potential in tissue engineering. In this study, we impregnated the self-assembled fucoidan/chitosan (FU_CS) hydrogels with PRF obtaining PRF/FU_CS hydrogels. Our objective was to analyze the properties of a hydrogel and the sustained release of growth factors from the hydrogel that incorporates PRF. The results of SEM and BET-BJH demonstrated the relatively porous nature of the FU_CS hydrogels. ELISA data showed that combining FU_CS hydrogel with PRF led to a gradual 7-day sustained release of growth factors (VEGF, EGF, IL-8, PDGF-BB, TGF-β1), compared to pure PRF. Histology confirmed ELISA data, demonstrating uniform PRF fibrin network distribution within the FU_CS hydrogel matrix. Furthermore, the FU_CS hydrogels revealed excellent cell viability. The results revealed that the PRF/FU_CS hydrogel has the potential to promote wound healing and tissue regeneration. This would be the first step in the search for improved growth factor release.

Effect of molar ratio and concentration on the rheological properties of two-component supramolecular hydrogels: tuning of the morphological and drug releasing behaviour

Self-assembled supramolecular hydrogels offer great potential as biomaterials and drug delivery systems. Specifically, peptide-based multicomponent hydrogels are promising materials due to their advantage that their mechanical and physical properties can be tuned to enhance their functionalities and broaden their applications. Herein, we report two-component assembly and formation of hydrogels containing inexpensive complementary anionic, BUVV-OH (A), and cationic, KFFC12 (B), peptide amphiphiles. Individually, neither of these components formed a hydrogel, while mixtures with compositions 1 : 1, 1 : 2, and 2 : 1 (molar ratio) as A : B show hydrogel formation (Milli-Q water, at pH = 6.79). These hydrogels displayed a good shear-thinning behaviour with different mechanical stabilities and nano-fibrous network structures. The 1 : 1 hydrogel shows good cell viability for human embryonic kidney (HEK-293) cells and CHO cells indicating its non-cytotoxicity. The biocompatible, thixotropic 1 : 1 hydrogel with a nanofiber network structure shows the highest mechanical strength with a storage modulus of 3.4 × 103 Pa. The hydrogel is able to encapsulate drugs including antibiotics amoxicillin and rifampicin, and anticancer drug doxorubicin, and it exhibits sustainable release of 76%, 70%, and 81% respectively in vitro after 3 days. The other two mixtures (composition 1 : 2 and 2 : 1) are unable to form a hydrogel when they are loaded with these drugs. Interestingly, it is noticed that with an increase in concentration, the mechanical strength of a 1 : 1 hydrogel is significantly enhanced, showing potential that may act as a scaffold for tissue engineering. The two-component gel offers tunable mechanical properties, thixotropy, injectability, and biocompatibility and has great potential as a scaffold for sustained drug release and tissue engineering.

Design of Fmoc-Phenylalanine Nanofibrillar Hydrogel and Mechanistic Studies of Its Antimicrobial Action against Both Gram-Positive and Gram-Negative Bacteria

In pursuit of efficient antimicrobial agents, biomaterials such as hydrogels have drawn a considerable amount of attention due to their numerous advantages such as a high degree of hydration, biocompatibility, stability, and direct application at an infectious site. Particularly, biomaterials such as hydrogels based on Fmoc-protected peptides and amino acids have proven to be immensely advantageous. Such biomaterials can undergo gelation by simple pH modulation and can be used for various biological applications. Keeping this in mind, in this work, we reported the synthesis of Fmoc-phenylalanine (Fmoc-F)-based hydrogels using trisodium citrate as a pH modulator and compared them with the previously reported pH modulator glucono-δ-lactone. The gels were compared using various characterization techniques such as rheometry, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), small angle X-ray scattering (SAXS), FT-IR, thioflavin T (ThT) binding assay, and zeta potential studies. These studies highlighted the role of pH modulators in affecting various parameters such as the ability to alter the zeta potential of the nanofibrils, improve their bactericidal action, reduce the amyloidic characters, shift the lattice packing from amorphous to crystalline, and introduce fluorescence and thermoreversibility. Interestingly, this is the first report where the Fmoc-F-based hydrogel has been shown to be effective against Gram-negative bacteria along with Gram-positive bacteria as well. Additionally, the mechanism of antimicrobial action was investigated using docking and antioxidant studies.

Supramolecular Phenylalanine-Derived Hydrogels for the Sustained Release of Functional Proteins

Protein-based therapeutics have emerged as next-generation pharmaceutical agents for oncology, bone regeneration, autoimmune disorders, viral infections, and other diseases. The clinical application of protein therapeutics has been impeded by pharmacokinetic and pharmacodynamic challenges including off-target toxicity, rapid clearance, and drug stability. Strategies for the localized and sustained delivery of protein therapeutics have shown promise in addressing these challenges. Hydrogels are critical materials that enable these delivery strategies. Supramolecular hydrogels composed of self-assembled materials have demonstrated biocompatibility advantages over polymer hydrogels, with peptide and protein-based gels showing strong potential. However, cost is a significant drawback of peptide-based supramolecular hydrogels. Supramolecular hydrogels composed of inexpensive low-molecular-weight (LMW) gelators, including modified amino acid derivatives, have been reported as viable alternatives to peptide-based materials. Herein, we report the encapsulation and release of proteins from supramolecular hydrogels composed of perfluorinated fluorenylmethyloxcarbonyl-modified phenylalanine (Fmoc-F₅-Phe-DAP). Specifically, we demonstrate release of four model proteins (ribonuclease A (RNase A), trypsin inhibitor (TI), bovine serum albumin (BSA), and human immunoglobulin G (IgG)) from these hydrogels. The emergent viscoelastic properties of these materials are characterized, and the functional and time-dependent release of proteins from the hydrogels is demonstrated. In addition, it is shown that the properties of the aqueous solution used for hydrogel formulation have a significant influence on the in vitro release profiles, as a function of the isoelectric point and molecular weight of the protein payloads. These studies collectively validate that this class of supramolecular LMW hydrogel possesses the requisite properties for the sustained and localized release of protein therapeutics.

Green Hydrogel Synthesis: Emphasis on Proteomics and Polymer Particle-Protein Interaction

The field of drug discovery has seen significant progress in recent years. These advances drive the development of new technologies for testing compound's effectiveness, as well as their adverse effects on organs and tissues. As an auxiliary tool for drug discovery, smart biomaterials and biopolymers produced from biodegradable monomers allow the manufacture of multifunctional polymeric devices capable of acting as biosensors, of incorporating bioactives and biomolecules, or even mimicking organs and tissues through self-association and organization between cells and biopolymers. This review discusses in detail the use of natural monomers for the synthesis of hydrogels via green routes. The physical, chemical and morphological characteristics of these polymers are described, in addition to emphasizing polymer-particle-protein interactions and their application in proteomics studies. To highlight the diversity of green synthesis methodologies and the properties of the final hydrogels, applications in the areas of drug delivery, antibody interactions, cancer therapy, imaging and biomarker analysis are also discussed, as well as the use of hydrogels for the discovery of antimicrobial and antiviral peptides with therapeutic potential.

Peptide hydrogels for affinity-controlled release of therapeutic cargo: Current and potential strategies

The development of devices for the precise and controlled delivery of therapeutics has grown rapidly over the last few decades. Drug delivery materials must provide a depot with delivery profiles that satisfy pharmacodynamic and pharmacokinetic requirements resulting in clinical benefit. Therapeutic efficacy can be limited due to short half-life and poor stability. Thus, to compensate for this, frequent administration and high doses are often required to achieve therapeutic effect, which in turn increases potential side effects and systemic toxicity. This can potentially be mitigated by using materials that can deliver drugs at controlled rates, and material design principles that allow this are continuously evolving. Affinity-based release strategies incorporate a myriad of reversible interactions into a gel network, which have affinities for the therapeutic of interest. Reversible binding to the gel network impacts the release profile of the drug. Such affinity-based interactions can be modulated to control the release profile to meet pharmacokinetic benchmarks. Much work has been done developing affinity-based control in the context of polymer-based materials. However, this strategy has not been widely implemented in peptide-based hydrogels. Herein, we present recent advances in the use of affinity-controlled peptide gel release systems and their associated mechanisms for applications in drug delivery.

Bioinspired short peptide hydrogel for versatile encapsulation and controlled release of growth factor therapeutics

A short bioinspired octapeptide, GV8, can self-assemble under mild conditions into biodegradable supramolecular physical hydrogels with high storage modulus and good biocompatibility. GV8 hydrogels can encapsulate both single or multiple macromolecular protein-based therapeutics in a simple one-pot formulation manner, making it a promising candidate to address challenges faced by existing synthetic polymer or peptide hydrogels with complex gelation and drug-encapsulation processes. Alongside its versatility, the hydrogel exhibits concentration-dependent storage modulus and controlled drug-release action. We demonstrate that GV8 hydrogels loaded with adipose-derived mesenchymal stem cells (ADMSC) secretome remain mechanically robust, and exhibit promising potential for wound healing applications by preserving secretome activity while maintaining a constant supply of ADMSC secretome to promote epithelial cell migration. Overall, our work highlights the potential of GV8 peptide hydrogel as a versatile and safe carrier for encapsulation and delivery of macromolecular therapeutics. STATEMENT OF SIGNIFICANCE: Supramolecular peptide hydrogels are a popular choice for protein-based macromolecular therapeutics delivery; however, despite the development of abundant hydrogel systems, several challenges limit their adaptability and practical applications. GV8 short peptide hydrogel circumvents these drawbacks and demonstrates the ability to function as a versatile growth factor (GF) encapsulant. It can encapsulate precise concentrations of complex adipose-derived mesenchymal stem cells secretome mixtures with a one-pot formulation approach and perform controlled release of GFs with preserved activity without compromising the self-assembly and mechanical properties of the hydrogel's supramolecular network. The significance of GV8 hydrogel lies in its gelation simplicity and versatility to encapsulate and deliver macromolecular therapeutics, thus representing a promising biomaterial for regenerative medicine applications.

Improvement of wound healing by the development of ECM-inspired biomaterial coatings and controlled protein release

Implant design has evolved from biochemically inert substrates, minimizing cell and protein interaction, towards sophisticated bioactive substrates, modulating the host response and supporting the regeneration of the injured tissue. Important aspects to consider are the control of cell adhesion, the discrimination of bacteria and non-local cells from the desired tissue cell type, and the stimulation of implant integration and wound healing. Here, the extracellular matrix acts as a role model providing us with inspiration for sophisticated designs. Within this scope, small bioactive peptides have proven to be miscellaneously deployable for the mediation of surface, cell and matrix interactions. Combinations of adhesion ligands, proteoglycans, and modulatory proteins should guide multiple aspects of the regeneration process and cooperativity between the different extracellular matrix components, which bears the chance to maximize the therapeutic efficiency and simultaneously lower the doses. Hence, efforts to include multiple of these factors in biomaterial design are well worth. In the following, multifunctional implant coatings based on bioactive peptides are reviewed and concepts to implement strong surface anchoring for stable cell adhesion and a dynamic delivery of modulator proteins are discussed.

Designed protein- and peptide-based hydrogels for biomedical sciences

Proteins are fundamentally the most important macromolecules for biochemical, mechanical, and structural functions in living organisms. Therefore, they provide us with diverse structural building blocks for constructing various types of biomaterials, including an important class of such materials, hydrogels. Since natural peptides and proteins are biocompatible and biodegradable, they have features advantageous for their use as the building blocks of hydrogels for biomedical applications. They display constitutional and mechanical similarities with the native extracellular matrix (ECM), and can be easily bio-functionalized via genetic and chemical engineering with features such as bio-recognition, specific stimulus-reactivity, and controlled degradation. This review aims to give an overview of hydrogels made up of recombinant proteins or synthetic peptides as the structural elements building the polymer network. A wide variety of hydrogels composed of protein or peptide building blocks with different origins and compositions - including β-hairpin peptides, α-helical coiled coil peptides, elastin-like peptides, silk fibroin, and resilin - have been designed to date. In this review, the structures and characteristics of these natural proteins and peptides, with each of their gelation mechanisms, and the physical, chemical, and mechanical properties as well as biocompatibility of the resulting hydrogels are described. In addition, this review discusses the potential of using protein- or peptide-based hydrogels in the field of biomedical sciences, especially tissue engineering.

Self-assembling peptide-based hydrogels: Fabrication, properties, and applications

The hierarchical formation of self-assembling peptide-based hydrogels (SAPHs) starts from peptide to nanofibers, following with the entanglement into hydrogels with nanofibrous network. Such characteristic structure and extraordinary biocompatibility, and the peptide components endow the SAPHs with diverse applications in biotechnological field. Therefore, the thorough comprehension of SAPHs is significant to broadening their application. In this review, fabrication, properties, and biological applications of the SAPHs are introduced, and the factors influencing the synthesis process as well as the properties of the SAPHs products are also systematically explained. Meanwhile, we conclude the problems to be solved and provide our perspective to the future development of SAPHs in the biotechnology.

De Novo Designed Peptide and Protein Hairpins Self-Assemble into Sheets and Nanoparticles

The design and assembly of peptide-based materials has advanced considerably, leading to a variety of fibrous, sheet, and nanoparticle structures. A remaining challenge is to account for and control different possible supramolecular outcomes accessible to the same or similar peptide building blocks. Here a de novo peptide system is presented that forms nanoparticles or sheets depending on the strategic placement of a "disulfide pin" between two elements of secondary structure that drive self-assembly. Specifically, homodimerizing and homotrimerizing de novo coiled-coil α-helices are joined with a flexible linker to generate a series of linear peptides. The helices are pinned back-to-back, constraining them as hairpins by a disulfide bond placed either proximal or distal to the linker. Computational modeling indicates, and advanced microscopy shows, that the proximally pinned hairpins self-assemble into nanoparticles, whereas the distally pinned constructs form sheets. These peptides can be made synthetically or recombinantly to allow both chemical modifications and the introduction of whole protein cargoes as required.

Enhanced proliferation and angiogenic phenotype of endothelial cells via negatively-charged alginate and chondroitin sulfate microsphere hydrogels

Sodium alginate-based hydrogel was the one of the most used polymers for cell delivery. However, the adsorption of extracellular matrix and proteins was inhibited due to the formation of a hydrated surface layer of these hydrogels. In this study, a novel cell delivery system, negatively-charged alginate and chondroitin sulfate microsphere hydrogel (nCACSMH), was fabricated with excellent permeability and biocompatibility in the action of a high voltage direct-current electric field. Negative charge was introduced to the surface of nCACSMH to obtain the expanded network and enhanced permeability. Additionally, the increasing content of chondroitin sulfate in nCACSMH could give rise to the charge density and its asymmetric structure, thus the uneven, plicate and expanded surface of nCACSMH which was favorable to cell proliferation was developed. Moreover, chondroitin sulfate was released with the degradation of nCACSMH, which played a crucial role in maintaining the normal physiological functions of cells. Thus the proliferation of human umbilical vein endothelial cells (HUVECs) was further accelerated and the angiogenesis related genes expression in endothelial cells was continuously and dramatically up-regulated. After 4 d, the proliferation and viability of HUVECs were significantly improved, the cells were distributed evenly in nCACSMH. The novel nCACSMH has the potential to be used as cell delivery, three-dimensional (3D) cell cultures for cell therapy, 3D bioprinting, high-throughput screening for drugs, and disease model for regeneration and constructing of tissue engineering.

Supramolecular Hydrogels for Protein Delivery in Tissue Engineering

Therapeutic proteins, such as growth factors (GFs), have been used in tissue engineering (TE) approaches for their ability to provide signals to cells and orchestrate the formation of functional tissue. However, to be effective and minimize off-target effects, GFs should be delivered at the target site with temporal control. In addition, protein drugs are typically sensitive water soluble macromolecules with delicate structure. As such, hydrogels, containing large amounts of water, provide a compatible environment for the direct incorporation of proteins within the hydrogel network, while their release rate can be tuned by engineering the network chemistry and density. Being formed by transient crosslinks, afforded by non-covalent interactions, supramolecular hydrogels offer important advantages for protein delivery applications. This review describes various types of supramolecular hydrogels using a repertoire of diverse building blocks, their use for protein delivery and their further application in TE contexts. By reviewing the recent literature on this topic, the merits of supramolecular hydrogels are highlighted as well as their limitations, with high expectations for new advances they will provide for TE in the near future.

Electrostatic interactions regulate the release of small molecules from supramolecular hydrogels

Supramolecular hydrogels have great potential as biomaterials for sustained delivery of therapeutics. While peptide-based supramolecular hydrogels have been developed that show promise for drug delivery applications, the high cost of production has limited their widespread adoption. Low molecular weight (LMW) supramolecular hydrogels are emerging as attractive and inexpensive alternatives to peptide-based hydrogels. We recently reported novel cationic fluorenylmethyloxycarbonyl-modified phenylalanine (Fmoc-Phe) hydrogels for localized and sustained in vivo release of an anti-inflammatory agent for functional pain remediation. In an effort to further elucidate design principles to optimize these materials for delivery of a variety of molecular agents, we herein report a systematic examination of electrostatic effects on the release of cargo molecules from Fmoc-Phe derived hydrogels. Specifically, we interrogate the release of cationic, anionic, and neutral cargo molecules from a series of cationic and anionic Fmoc-Phe derived hydrogels. We observed that cargo was readily released from the hydrogels except when the cargo and hydrogel network had complementary charges, in which case the cargo was highly retained in the network. These results demonstrate that the electrostatic characteristics of both the hydrogel network and the specific cargo are critical design parameters in the formulation of LMW supramolecular hydrogel systems in the development of next-generation materials for drug delivery applications.

A review on recent advances in polymer and peptide hydrogels

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Electrostatically Driven Guanidinium Interaction Domains that Control Hydrogel-Mediated Protein Delivery In Vivo

Protein biologics are an important class of drugs, but the necessity for frequent parenteral administration is a major limitation. Drug-delivery materials offer a potential solution, but protein-material adsorption can cause denaturation, which reduces their effectiveness. Here, we describe a new protein delivery platform that limits direct contact between globular protein domains and material matrix, yet from a single subcutaneous administration can be tuned for long-term drug release. The strategy utilizes complementary electrostatic interactions made between a suite of designed interaction domains (IDs), installed onto the terminus of a protein of interest, and a negatively charged self-assembled fibrillar hydrogel. These intermolecular interactions can be easily modulated by choice of ID to control material interaction and desorption energies, which allows regulation of protein release kinetics to fit desired release profiles. Molecular dynamics studies provided a molecular-level understanding of the mechanisms that govern release and identified optimal binding zones on the gel fibrils that facilitate strong ID-material interactions, which are crucial for sustained release of protein. This delivery platform can be easily loaded with cargo, is shear-thin syringe implantable, provides improved protein stability, is capable of a diverse range of in vitro release rates, and most importantly, can accomplish long-term control over in vivo protein delivery.

Drug-Mimicking Nanofibrous Peptide Hydrogel for Inhibition of Inducible Nitric Oxide Synthase

In this work, we develop a drug-mimicking nanofibrous peptide hydrogel that shows long-term bioactivity comparable to a small-molecule inhibitor of inducible nitric oxide synthase (iNOS). The iNOS inhibitor, N ⁶-(1-iminoethyl)-l-lysine (l-NIL), is a positively charged amino acid whose structure could be readily integrated into the framework of a positively charged multidomain peptide (MDP) through the modification of lysine side chains. This new l-NIL-MDP maintains the self-assembling properties of the base peptide, forming β-sheet nanofibers, which entangle into a thixotropic hydrogel. The l-NIL-MDP hydrogel supports cell growth in vitro and allows syringe-directed delivery that persists in a targeted location in vivo for several weeks. Multiple characterization assays demonstrate the bioactivity of the l-NIL-MDP hydrogel to be comparable to the l-NIL small molecule. This includes iNOS inhibition of macrophages in vitro, reduced nitrotyrosine immunostaining in murine subcutaneous histology, and reduced serum levels of vascular endothelial growth factor in vivo. This study expands the toolbox of available peptide hydrogel scaffold designs that can modify biological activity without the need for any additional small-molecule drugs, proteins, or cells.

Design of a Peptide-Based Electronegative Hydrogel for the Direct Encapsulation, 3D Culturing, in Vivo Syringe-Based Delivery, and Long-Term Tissue Engraftment of Cells

Soft materials that facilitate the three-dimensional (3D) encapsulation, proliferation, and facile local delivery of cells to targeted tissues will aid cell-based therapies, especially those that depend on the local engraftment of implanted cells. Herein, we develop a negatively charged fibrillar hydrogel based on the de novo-designed self-assembling peptide AcVES3-RGDV. Cells are easily encapsulated during the triggered self-assembly of the peptide leading to gel formation. Self-assembly is induced by adjusting the ionic strength and/or temperature of the solution, while avoiding large changes in pH. The AcVES3-RGDV gel allows cell-material attachment enabling both two-dimensional and 3D cell culture of adherent cells. Gel-cell constructs display shear-thin/recovery rheological properties enabling their syringe-based delivery. In vivo cellular fluorescence as well as tissue resection experiments show that the gel supports the long-term engraftment of cells delivered subcutaneously into mice.

Supramolecular Nanofibrous Peptide/Polymer Hydrogels for the Multiplexing of Bioactive Signals

The ability to provide multiple functions within a single scaffold biomaterial is a major goal in tissue engineering. Self-assembling peptide-based hydrogels are gaining significant attention as three-dimensional biomaterials because they provide a network of nanofibers similar to the native extracellular matrix while allowing the presentation of multiple biochemical cues for cell signaling. Herein, we combine a positively charged peptide amphiphile (PA) and the negatively charged synthetic polymer poly(sodium 4-styrenesulfonate) (PSS) to fabricate hybrid hydrogels through supramolecular self-assembly. PSS/PA hydrogels show rather high mechanical stiffness while being stable in buffered environment. The sulfonate functionality in PSS promotes hydrogel mineralization which can be controlled if undertaken in standard osteogenic medium. Loading proteins with different charges in the hydrogels reveals their ability to retain and sustain their release and indicates their potential for the controlled delivery of growth factors. Human mesenchymal stem cells encapsulated in PSS/PA hydrogels remain viable. The biomimetic nanofibrous structure of the hydrogels, together with multiplexing of bioactive signals, can provide a suitable environment for stem cell differentiation.

Peptide Tectonics: Encoded Structural Complementarity Dictates Programmable Self-Assembly

Programmable self-assembly of peptides into well-defined nanostructures represents one promising approach for bioinspired and biomimetic synthesis of artificial complex systems and functional materials. Despite the progress made over the past two decades in the development of strategies for precise manipulation of the self-assembly of peptides, there is a remarkable gap between current peptide assemblies and biological systems in terms of structural complexity and functions. Here, the concept of peptide tectonics for the creation of well-defined nanostructures predominately driven by the complementary association at the interacting interfaces of tectons is introduced. Peptide tectons are defined as peptide building blocks exhibiting structural complementarity at the interacting interfaces of commensurate domains and undergoing programmable self-assembly into defined supramolecular structures promoted by complementary interactions. Peptide tectons are categorized based on their conformational entropy and the underlying mechanism for the programmable self-assembly of peptide tectons is highlighted focusing on the approaches for incorporating the structural complementarity within tectons. Peptide tectonics not only provides an alternative perspective to understand the self-assembly of peptides, but also allows for precise manipulation of peptide interactions, thus leading to artificial systems with advanced complexity and functions and paves the way toward peptide-related functional materials resembling natural systems.

Design of a Multicompartment Hydrogel that Facilitates Time-Resolved Delivery of Combination Therapy and Synergized Killing of Glioblastoma

There is significant current interest in identifying new combination therapies that synergize to treat disease, and it is becoming increasingly clear that the temporal resolution of their administration greatly impacts efficacy. To facilitate effective delivery, a multicompartment hydrogel material was developed that is composed of spherical vesicles interlaced within a self-assembled peptide-based network of physically crosslinked fibrils that allows time-resolved independent co-delivery of small molecules. This material architecture effectively delivers the EGFR kinase inhibitor Erlotinib (ERL) and Doxorubicin (DOX, DNA intercalator) in an ERL→DOX sequential manner to synergistically kill glioblastoma, the most aggressive form of brain cancer.

Macromolecule-Network Electrostatics Controlling Delivery of the Biotherapeutic Cell Modulator TIMP-2

Tissue inhibitor of metalloproteinase 2 (TIMP-2) is an endogenous 22 kDa proteinase inhibitor, demonstrating antitumorigenic, antimetastatic and antiangiogenic activities in vitro and in vivo. Recombinant TIMP-2 is currently undergoing preclinical testing in multiple, murine tumor models. Here we report the development of an inert, injectable peptide hydrogel matrix enabling encapsulation and sustained release of TIMP-2. We studied the TIMP-2 release profile from four β-hairpin peptide gels of varying net electrostatic charge. A negatively charged peptide gel (designated AcVES3) enabling encapsulation of 4 mg/mL of TIMP-2, without effects on rheological properties, facilitated the slow sustained release (0.9%/d) of TIMP-2 over 28 d. Released TIMP-2 is structurally intact and maintains the ability to inhibit MMP activity, as well as suppress lung cancer cell proliferation in vitro. These findings suggest that the AcVES3 hydrogel will be useful as an injectable vehicle for systemic delivery of TIMP-2 in vivo for ongoing preclinical development.

Balancing hydrophobicity and sequence pattern to influence self-assembly of amphipathic peptides

Amphipathic peptides with alternating polar and nonpolar amino acid sequences efficiently self-assemble into functional β-sheet fibrils as long as the nonpolar residues have sufficient hydrophobicity. For example, the Ac-(FKFE)2 -NH2 peptide rapidly self-assembles into β-sheet bilayer nanoribbons, while Ac-(AKAE)2 -NH2 fails to self-assemble under similar conditions due to the significantly reduced hydrophobicity and β-sheet propensity of Ala relative to Phe. Herein, we systematically explore the effect of substituting only two of the four Ala residues at various positions in the Ac-(AKAE)2 -NH2 peptide with amino acids of increasing hydrophobicity, β-sheet potential, and surface area (including Phe, 1-naphthylalanine (1-Nal), 2-naphthylalanine (2-Nal), cyclohexylalanine (Cha), and pentafluorophenylalanine (F5 -Phe)) on the self-assembly propensity of the resulting sequences. It was found that double Phe variants, regardless of the position of substitution, failed to self-assemble under the conditions used in this study. In contrast, all double 1-Nal and 2-Nal variants readily self-assembled, albeit at differing rates depending on the substitution patterns. To determine whether this was due to hydrophobicity or side chain surface area, we also prepared double Cha and F5 -Phe variant peptides (both side chain groups are more hydrophobic than Phe). Each of these variants also underwent effective self-assembly, with the aromatic F5 -Phe peptides doing so with greater efficiency. These findings provide insight into the role of amino acid hydrophobicity and sequence pattern on self-assembly proclivity of amphipathic peptides and on how targeted substitutions of nonpolar residues in these sequences can be exploited to tune the characteristics of the resulting self-assembled materials.

Tuning and Predicting Mesh Size and Protein Release from Step Growth Hydrogels

Hydrogel-based depots are of growing interest for release of biopharmaceuticals; however, a priori selection of hydrogel compositions that will retain proteins of interest and provide desired release profiles remains elusive. Toward addressing this, in this work, we have established a new tool for the facile assessment of protein release from hydrogels and applied it to evaluate the effectiveness of mesh size estimations on predicting protein retention or release. Poly(ethylene glycol) (PEG)-based hydrogel depots were formed by photoinitiated step growth polymerization of four-arm PEG functionalized with norbornene (PEG-norbornene, 4% w/w to 20% w/w, Mn ∼ 5 to 20 kDa) and different dithiol cross-linkers (PEG Mn ∼ 1.5 kDa or enzymatically degradable peptide), creating well-defined, robust materials with a range of mesh sizes estimated with Flory-Rehner or rubber elasticity theory (∼5 to 15 nm). A cocktail of different model proteins was released from compositions of interest, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was used to facilely and quantitatively analyze temporal release profiles. Mesh size was predictive of retention of relatively large proteins and release of relatively small proteins. Proteins with diameters comparable to the mesh size, which is often the case for growth factors, were released by hindered diffusion and required experimental assessment of retention and release. With this knowledge, hydrogels were designed for the controlled release of a therapeutically relevant growth factor, PDGF-BB.

Partially Acetylated or Benzoylated Arabinose Derivatives as Structurally Simple Organogelators: Effect of the Ester Protecting Group on Gel Properties

Sugar-based low-molecular-weight gelators (LMWGs) have been used for various applications for a long time. Herein, structurally simple, ester-protected arabinosides are reported as low-molecular-weight organogelators (LMOGs) that are able to gel aromatic solvents, as well as petrol and diesel. Studies on the mechanical strength of the gels, through detailed rheological experiments, indicate that gels from the 1,2-dibenzoylated arabinose gelator possess better mechanical properties than those from the 1,2-diacetylated gelator. These results are interpreted in terms of the tendency of the former to form fibers with comparatively lower diameter than those of the latter, based on detailed field-emission SEM and AFM studies. Investigations of the interactions responsible for the self-assembly of gelators through IR spectroscopy and wide-angle X-ray scattering reveal that the primary interactions responsible are hydrogen bonds between the hydroxyl groups and ester C=O, which is absent in the solid state of the gelators. In addition, π interactions present in the 1,2-dibenzoylated derivative result in a more regular arrangement, which, in turn, leads to better mechanical properties of the gels compared with those of the 1,2-diacetylated gelator.

Probing the surface chemistry of self-assembled peptide hydrogels using solution-state NMR spectroscopy

The surface chemistry of self-assembled hydrogel fibres - their charge, hydrophobicity and ion-binding dynamics - is recognised to play an important role in determining how the gels develop as well as their suitability for different applications. However, to date there are no established methodologies for the study of this surface chemistry. Here, we demonstrate how solution-state NMR spectroscopy can be employed to measure the surface chemical properties of the fibres in a range of hydrogels formed from N-functionalised dipeptides, an effective and versatile class of gelator that has attracted much attention. By studying the interactions with the gel fibres of a diverse range of probe molecules and ions, we can simultaneously study a number of surface chemical properties of the NMR invisible fibres in an essentially non-invasive manner. Our results yield fresh insights into the materials. Most notably, gel fibres assembled using different tiggering methods bear differing amounts of negative charge as a result of a partial deprotonation of the carboxylic acid groups of the gelators. We also demonstrate how chemical shift imaging (CSI) techniques can be applied to follow the formation of hydrogels along chemical gradients. We apply CSI to study the binding of Ca²⁺ and subsequent gelation of peptide assemblies at alkaline pH. Using metal ion-binding molecules as probes, we are able to detect the presence of bound Ca²⁺ ions on the surface of the gel fibres. We briefly explore how knowledge of the surface chemical properties of hydrogels could be used to inform their practical application in fields such as drug delivery and environmental remediation.

Peptide-Based Supramolecular Hydrogels for Delivery of Biologics

The demand for therapeutic biologics has rapidly grown over recent decades, creating a dramatic shift in the pharmaceutical industry from small molecule drugs to biological macromolecular therapeutics. As a result of their large size and innate instability, the systemic, topical, and local delivery of biologic drugs remains a highly challenging task. Although there exist many types of delivery vehicles, peptides and peptide conjugates have received continuously increasing interest as molecular blocks to create a great diversity of supramolecular nanostructures and hydrogels for the effective delivery of biologics, due to their inherent biocompatibility, tunable biodegradability, and responsiveness to various biological stimuli. In this context, we discuss the design principles of supramolecular hydrogels using small molecule peptides and peptide conjugates as molecular building units, and review the recent effort in using these materials for protein delivery and gene delivery.