The non-covalent decoration of self-assembling protein fibers

Amphipathic peptide-phospholipid nanofibers: Kinetics of fiber formation and molecular transfer between assemblies

Understanding the kinetics of nano-assembly formation is important to elucidate the biological processes involved and develop novel nanomaterials with biological functions. In the present study, we report the kinetic mechanisms of nanofiber formation from a mixture of phospholipids and the amphipathic peptide 18A[A11C], carrying cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11. 18A[A11C] with acetylated N-terminus and amidated C-terminus can associate with phosphatidylcholine to form fibrous aggregates at neutral pH and lipid-to-peptide molar ratio of ∼1, although the reaction pathways of self-assembly remain unclear. Here, the peptide was added to giant 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles to monitor nanofiber formation under fluorescence microscopy. The peptide initially solubilized the lipid vesicles into particles smaller than the resolution of optical microscope, and fibrous aggregates appeared subsequently. Transmission electron microscopy and dynamic light scattering analyses revealed that the vesicle-solubilized particles were spherical or circular, measuring ∼10-20 nm in diameter. The rate of nanofiber formation of 18A with 1,2-dipalmitoyl phosphatidylcholine from the particles was proportional to the square of lipid-peptide concentration in the system, suggesting that the association of particles, accompanied by conformational changes, was the rate-limiting step. Moreover, molecules in the nanofibers could be transferred between aggregates faster than those in the lipid vesicles. These findings provide useful information for the development and control of nano-assembling structures using peptides and phospholipids.

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Amphipathic Peptide-Phospholipid Nanofibers: Phospholipid Specificity and Dependence on Concentration and Temperature

The design of nanoassemblies is an important part of the development of new materials for applications in nanomedicine and biosensors. In our previous study, cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11, 18A[A11C], bound to 1-palmitoyl-2-oleoylphosphatidylcholine to form fibrous aggregates at a lipid-to-peptide molar ratio of ≤2 and a fiber diameter of 10-20 nm. However, the mechanisms underlying the lipid-peptide interactions that enable nanofiber formation remain unclear. Here, we evaluated the phospholipid specificity, concentration dependence, and temperature dependence of the formation of 18A[A11C]-lipid nanofibers. Nanofibers were found to form in the presence of specific phospholipids and have a constant lipid/peptide stoichiometry of 1.2 ± 0.2. Moreover, an increase in the length of the acyl chain in phosphatidylcholines was found to increase the structural stability of the nanofibers. These results indicate that specific molecular interactions between peptides and both the headgroups and acyl chains of phospholipids are involved in nanofiber formation. Furthermore, the formation and disassembly of the nanofibers were reversibly controlled by changes in temperature and concentration. The results of the present study provide an insight into the creation of nanoassembling structures.

Designer aromatic peptide amphiphiles for self-assembly and enzymatic display of proteins with morphology control

We herein designed bi-functional aromatic peptide amphiphiles both self-assembling to fibrous nanomaterials and working as a substrate of microbial transglutaminase, leading to peptidyl scaffolds with different morphologies that can be enzymatically post-functionalized with proteins.

Collagen mimetic peptide discs promote assembly of a broad range of natural protein fibers through hydrophobic interactions

Collagen mimetic peptides that alone formed two-dimensional nanoscale discs driven by hydrophobic interactions were shown in electron microscopy studies to also co-assemble with natural fibrous proteins to produce discs-on-a-string (DoS) nanostructures. In most cases, peptide discs also facilitated bundling of the protein fibers. This provides insight into how synthetic and natural proteins may be combined to develop multicomponent, multi-dimensional architectures at the nanoscale.

Nanodisc-to-Nanofiber Transition of Noncovalent Peptide-Phospholipid Assemblies

We report a novel molecular architecture of peptide-phospholipid coassemblies. The amphiphilic peptide Ac-18A-NH₂ (18A), which was designed to mimic apolipoprotein α-helices, has been shown to form nanodisc structures with phospholipid bilayers. We show that an 18A peptide cysteine substitution at residue 11, 18A[A11C], forms fibrous assemblies with 1-palmitoyl-2-oleoyl-phosphatidylcholine at a lipid-to-peptide (L/P) molar ratio of 1, a fiber diameter of 10-20 nm, and a length of more than 1 μm. Furthermore, 18A[A11C] can form nanodiscs with these lipid bilayers at L/P ratios of 4-6. The peptide adopts α-helical structures in both the nanodisc and nanofiber assemblies, although the α-helical bundle structures were evident only in the nanofibers, and the phospholipids of the nanofibers were not lamellar. Fluorescence spectroscopic analysis revealed that the peptide and lipid molecules in the nanofibers exhibited different solvent accessibility and hydrophobicity from those of the nanodiscs. Furthermore, the cysteine substitution at residue 11 did not result in disulfide bond formation, although it was responsible for the nanofiber formation, suggesting that this free sulfhydryl group has an important functional role. Alternatively, the disulfide dimer of 18A[A11C] preferentially formed nanodiscs, even at an L/P ratio of 1. Interconversions of these discoidal and fibrous assemblies were induced by the stepwise addition of free 18A[A11C] or liposomes into the solution. Furthermore, these structural transitions could also be induced by the introduction of oxidative and reductive stresses to the assemblies. Our results demonstrate that heteromolecular lipid-peptide complexes represent a novel approach to the construction of controllable and functional nanoscale assemblies.

The application of self-assembled nanostructures in peptide-based subunit vaccine development

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Smaller polymer-peptide conjugates-based nanoparticles are often more immunogenic.

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Lipid-antigen conjugates-based nanoparticles can interact with immune receptors.

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Peptides with β-sheet conformation usually form nanofibers.

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α-Helical and random coil peptides tend to self-assemble into nanoparticles.

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Peptide-based nanostructures are usually poorer inducers of immune responses.

Peptide based-vaccines are becoming one of the most widely investigated prophylactic and therapeutic health care interventions against a variety of diseases, including cancer. However, the lack of a safe and highly efficient adjuvant (immune stimulant) is regarded as the biggest obstacle to vaccine development. The incorporation of a peptide antigen in a nanostructure-based delivery system was recently shown to overcome this obstacle. Nanostructures are often formed from antigens conjugated to molecules such as polymers, lipids, and peptide, with the help of self-assembly phenomenon. This review describes the application of self-assembly process for the production of peptide-based vaccine candidates and the ability of these nanostructures to stimulate humoral and cellular immune responses.

α-Helical coiled-coil peptide materials for biomedical applications

Self-assembling coiled coils, which occur commonly in native proteins, have received significant interest for the design of new biomaterials-based medical therapies. Considerable effort over recent years has led to a detailed understanding of the self-assembly process of coiled coils, and a diverse collection of strategies have been developed for designing functional materials using this motif. The ability to engineer the interface between coiled coils allows one to achieve variously connected components, leading to precisely defined structures such as nanofibers, nanotubes, nanoparticles, networks, gels, and combinations of these. Currently these materials are being developed for a range of biotechnological and medical applications, including drug delivery systems for controlled release, targeted nanomaterials, 'drug-free' therapeutics, vaccine delivery systems, and others. WIREs Nanomed Nanobiotechnol 2017, 9:e1424. doi: 10.1002/wnan.1424 For further resources related to this article, please visit the WIREs website.

Role of positions e and g in the fibrous assembly formation of an amphipathic α-helix-forming polypeptide

We previously characterized α3, a polypeptide that has a three times repeated sequence of seven amino acids (abcdefg: LETLAKA) and forms fibrous assemblies composed of amphipathic α-helices. Upon comparison of the amino acid sequences of α3 with other α-helix forming polypeptides, we proposed that the fibrous assemblies were formed due to the alanine (Ala) residues at positions e and g. Here, we characterized seven α3 analog polypeptides with serine (Ser), glycine (Gly), or charged residues substituted for Ala at positions e and g. The α-helix forming abilities of the substituted polypeptides were less than that of α3. The polypeptides with amino acid substitutions at position g and the polypeptide KEα3, in which Ala was substituted with charged amino acids, formed few fibrous assemblies. In contrast, polypeptides with Ala replaced by Ser at position e formed β-sheets under several conditions. These results show that Ala residues at position e and particularly at position g are involved in the formation of fibrous assemblies.

Local retention of antibodies in vivo with an injectable film embedded with a fluorogen-activating protein

Herein we report an injectable film by which antibodies can be localized in vivo. The system builds upon a bifunctional polypeptide consisting of a fluorogen-activating protein (FAP) and a β-fibrillizing peptide (βFP). The FAP domain generates fluorescence that reflects IgG binding sites conferred by Protein A/G (pAG) conjugated with the fluorogen malachite green (MG). A film is generated by mixing these proteins with molar excess of EAK16-II, a βFP that forms β-sheet fibrils at high salt concentrations. The IgG-binding, fluorogenic film can be injected in vivo through conventional needled syringes. Confocal microscopic images and dose-response titration experiments showed that loading of IgG into the film was mediated by pAG(MG) bound to the FAP. Release of IgG in vitro was significantly delayed by the bioaffinity mechanism; 26% of the IgG were released from films embedded with pAG(MG) after five days, compared to close to 90% in films without pAG(MG). Computational simulations indicated that the release rate of IgG is governed by positive cooperativity due to pAG(MG). When injected into the subcutaneous space of mouse footpads, film-embedded IgG were retained locally, with distribution through the lymphatics impeded. The ability to track IgG binding sites and distribution simultaneously will aid the optimization of local antibody delivery systems.

Synthesis and Self-Assembly of Bundle-Forming α-Helical Peptide-Dendron Hybrids

Dendronized helix bundle assemblies combine the sequence diversity and folding properties of proteins with the tailored physical properties of dendrimers. Assembly of peptide-dendron hybrids into α-helical bundles encapsulates the helix bundle motif in a dendritic sheath that will allow the functional, protein-like domain to be transplanted to nonbiological environments. A bioorthogonal graft-to synthetic strategy for preparing helix bundle-forming peptide-dendron hybrids is described herein for hybrids 1a, 1b, and 2. Titration experiments monitored by circular dichroism spectroscopy support our self-assembly model for how the peptide-dendron hybrids self-assemble into α-helical bundles with the dendrons on outside of the bundle.

Self-Assembled Proteins and Peptides as Scaffolds for Tissue Regeneration

Self-assembling proteins and peptides are increasingly gaining interest for potential use as scaffolds in tissue engineering applications. They self-organize from basic building blocks under mild conditions into supramolecular structures, mimicking the native extracellular matrix. Their properties can be easily tuned through changes at the sequence level. Moreover, they can be produced in sufficient quantities with chemical synthesis or recombinant technologies to allow them to address homogeneity and standardization issues required for applications. Here. recent advances in self-assembling proteins, peptides, and peptide amphiphiles that form scaffolds suitable for tissue engineering are reviewed. The focus is on a variety of motifs, ranging from minimalistic dipeptides, simplistic ultrashort aliphatic peptides, and peptide amphiphiles to large "recombinamer" proteins. Special emphasis is placed on the rational design of self-assembling motifs and biofunctionalization strategies to influence cell behavior and modulate scaffold stability. Perspectives for combination of these "bottom-up" designer strategies with traditional "top-down" biofabrication techniques for new generations of tissue engineering scaffolds are highlighted.

pH responsiveness of fibrous assemblies of repeat-sequence amphipathic α-helix polypeptides

We reported previously that our designed polypeptide α3 (21 residues), which has three repeats of a seven-amino-acid sequence (LETLAKA)3, forms not only an amphipathic α-helix structure but also long fibrous assemblies in aqueous solution. To address the relationship between the electrical states of the polypeptide and its α-helix and fibrous assembly formation, we characterized mutated polypeptides in which charged amino acid residues of α3 were replaced with Ser. We prepared the following polypeptides: 2Sα3 (LSTLAKA)3, in which all Glu residues were replaced with Ser residues; 6Sα3 (LETLASA)3, in which all Lys residues were replaced with Ser; and 2S6Sα3 (LSTLASA)3; in which all Glu and Lys residues were replaced with Ser. In 0.1M KCl, 2Sα3 formed an α-helix under basic conditions and 6Sα3 formed an α-helix under acid conditions. In 1M KCl, they both formed α-helices under a wide pH range. In addition, 2Sα3 and 6Sα3 formed fibrous assemblies under the same buffer conditions in which they formed α-helices. α-Helix and fibrous assembly formation by these polypeptides was reversible in a pH-dependent manner. In contrast, 2S6Sα3 formed an α-helix under basic conditions in 1M KCl. Taken together, these findings reveal that the charge states of the charged amino acid residues and the charge state of the Leu residue located at the terminus play an important role in α-helix formation.

A robust route to enzymatically functional, hierarchically self-assembled peptide frameworks

The addition of enzyme biofunctionality to self-assembling peptide nanofibers is challenging since such additions can inhibit functionality or self-assembly. We introduce a method for peptide nanofiber enzyme functionalization, demonstrated by the attachment of a polymerization synthase to peptide nanofibers. The enzyme generates a biocompatible, biodegradable biopolyester coat on the fibers with applicablity in medical engineering. This approach provides a template for generation of functional bionanomaterials.

Biomimetic self-assembling peptides as scaffolds for soft tissue engineering

Tissue engineered therapies are emerging as solutions to several of the medical challenges facing aging societies. To this end, a fundamental research goal is the development of novel biocompatible materials and scaffolds. Self-assembling peptides are materials that have undergone rapid development in the last two decades and they hold promise in meeting some of these challenges. Using amino acids as building blocks enables a great versatility to be incorporated into the structures that peptides form, their physical properties and their interactions with biological systems. This review discusses several classes of short self-assembling sequences, explaining the principles that drive their self-assembly into structures with nanoscale ordering, and highlighting in vitro and in vivo studies that demonstrate the potential of these materials as novel soft tissue engineering scaffolds.

Effects of chain length of an amphipathic polypeptide carrying the repeated amino acid sequence (LETLAKA)(n) on α-helix and fibrous assembly formation

Polypeptide α3 (21 residues), with three repeats of a seven-amino-acid sequence (LETLAKA)(3), forms an amphipathic α-helix and a long fibrous assembly. Here, we investigated the ability of α3-series polypeptides (with 14-42 residues) of various chain lengths to form α-helices and fibrous assemblies. Polypeptide α2 (14 residues), with two same-sequence repeats, did not form an α-helix, but polypeptide α2L (15 residues; α2 with one additional leucine residue on its carboxyl terminal) did form an α-helix and fibrous assembly. Fibrous assembly formation was associated with polypeptides at least as long as polypeptide α2L and with five leucine residues, indicating that the C-terminal leucine has a critical element for stabilization of α-helix and fibril formation. In contrast, polypeptides α5 (35 residues) and α6 (42 residues) aggregated easily, although they formed α-helices. A 15-35-residue chain was required for fibrous assembly formation. Electron microscopy and X-ray fiber diffraction showed that the thinnest fibrous assemblies of polypeptides were about 20 Å and had periodicities coincident with the length of the α-helix in a longitudinal direction. These results indicated that the α-helix structures were orientated along the fibrous axis and assembled into a bundle. Furthermore, the width and length of fibrous assemblies changed with changes in the pH value, resulting in variations in the charged states of the residues. Our results suggest that the formation of fibrous assemblies of amphipathic α-helices is due to the assembly of bundles via the hydrophobic faces of the helices and extension with hydrophobic noncovalent bonds containing a leucine.

Smart self-assembled hybrid hydrogel biomaterials

Hybrid biomaterials are systems created from components of at least two distinct classes of molecules, for example, synthetic macromolecules and proteins or peptide domains. The synergistic combination of two types of structures may produce new materials that possess unprecedented levels of structural organization and novel properties. This Review focuses on biorecognition-driven self-assembly of hybrid macromolecules into functional hydrogel biomaterials. First, basic rules that govern the secondary structure of peptides are discussed, and then approaches to the specific design of hybrid systems with tailor-made properties are evaluated, followed by a discussion on the similarity of design principles of biomaterials and macromolecular therapeutics. Finally, the future of the field is briefly outlined.

From short peptides to nanofibers to macromolecular assemblies in biomedicine

In the last few years, a variety of self-assembling short peptides that consist exclusively of simple amino acids have been designed and modified. These peptides exhibit self-assembling dynamic behaviors. At the molecular structural level, they form α-helical, β-sheet and β-hairpins structures in water. These structures further undergo spontaneous assembly to form nanofibers which aggregate into supramolecular scaffolds that entrap large volumes of water. Furthermore, nanostructures and supramolecular structures that self-organized from these short peptides also have a broad spectrum of biotechnological applications. They are useful as biological materials for 2D and 3D tissue cell cultures, regenerative and reparative medicine, tissue engineering as well as injectable drug delivery matrices that gel in situ. We have endeavored to do a comprehensive review of short peptides that form nanofibrous hydrogels. In particular, we have focused on recent advances in peptide assembly motifs and applications.

Directed intermixing in multicomponent self-assembling biomaterials

The noncovalent coassembly of multiple different peptides can be a useful route for producing multifunctional biomaterials. However, to date, such materials have almost exclusively been investigated as homogeneous self-assemblies, having functional components uniformly distributed throughout their supramolecular structures. Here we illustrate control over the intermixing of multiple different self-assembling peptides, in turn providing a simple but powerful means for modulating these materials' mechanical and biological properties. In β-sheet fibrillizing hydrogels, significant increases in stiffening could be achieved using heterobifunctional cross-linkers by sequestering peptides bearing different reactive groups into distinct populations of fibrils, thus favoring interfibril cross-linking. Further, by specifying the intermixing of RGD-bearing peptides in 2-D and 3-D self-assemblies, the growth of HUVECs and NIH 3T3 cells could be significantly modulated. This approach may be immediately applicable toward a wide variety of self-assembling systems that form stable supramolecular structures.

Bioorthogonal dual functionalization of self-assembling peptide fibers

The ability to modify peptide- and protein-based biomaterials selectively under mild conditions and in aqueous buffers is essential to the development of certain areas of bionanotechnology, tissue engineering and synthetic biology. Here we show that Self-Assembling peptide Fibers (SAFs) can incorporate multiple modified peptides non-covalently, stoichiometrically and without disrupting their structure or stability. The modified peptides contain groups suitable for post-assembly click reactions in water, namely azides and alkenes. Labeling of these groups is achieved using the orthogonal Cu(I)-catalyzed azide-alkyne and photoinitiated thiol-ene reactions, respectively. Functionalization is demonstrated through the conjugation of biotin followed by streptavidin-nanogold particles, or rhodamine, and visualized by electron and light microscopy, respectively. This has been shown for fibers harboring either or both of the modified peptides. Furthermore, the amounts of each modified peptide in the fibers can be varied with concomitant changes in decoration. This approach allows the design and assembly of fibers with multiple functional components, paving the way for the development of multi-component functionalized systems.

Peptide synthesis and self-assembly

Peptides and proteins are the most diverse building blocks in biomolecular self-assembly in terms of chemistry, nanostructure formation and functionality. Self-assembly is an intrinsic property of peptides. In this chapter, we attempt to address the following issues: How can we synthesize a self-assembling peptide? What are the fundamental physical and chemical principles that underpin peptide self-assembly? How can we learn to finely control peptide self-assembly? The merits of answering these questions are inspiring both for biology and medicine in terms of new opportunities for understanding, preventing and curing of diseases, and for nanotechnology in terms of new prescribed routes to achieving peptide-based nanostructures with a range of properties appropriate for specific applications.

Alpha-helical peptide assemblies giving new function to designed structures

The design of alpha-helical tectons for self-assembly is maturing as a science. We have now reached the point where many different coiled-coil topologies can be reliably produced and validated in synthetic systems and the field is now moving on towards more complex, discrete structures and applications. Similarly the design of infinite or fiber assemblies has also matured, with the creation fibers that have been modified or functionalized in a variety of ways. This chapter discusses the progress made in both of these areas as well as outlining the challenges still to come.

More than just bare scaffolds: towards multi-component and decorated fibrous biomaterials

We are entering a new phase in biomaterials research in which rational design is being used to produce functionalised materials tailored to specific applications. As is evident from this Themed Issue, there are now a number of distinct types of designed, self-assembling, fibrous biomaterials. Many of these are ripe for development and application for example as scaffolds for 3D cell culture and tissue engineering, and in templating inorganic materials. Whilst a number of groups are making headway towards such applications, there is a general challenge to translate a wealth of excellent basic research into materials with a genuine future in real-life applications. Amongst other contemporary aspects of this evolving research area, a key issue is that of decorating or functionalising what are mostly bare scaffolds. There are a number of hurdles to overcome to achieve effective and controlled labelling of the scaffolds, for instance: maintaining biocompatibility, i.e., by minimising covalent chemistry, or using milder bioconjugation methods; attaining specified levels of decoration, and, in particular, high and stoichiometric labelling; introducing orthogonality, such that two or more functions can be appended to the same scaffold; and, in relevant cases, maintaining the possibility for recombinant peptide/protein production. In this critical review, we present an overview of the different approaches to tackling these challenges largely for self-assembled, peptide-based fibrous systems. We review the field as it stands by placing work within general routes to fibre functionalisation; give worked examples on our own specific system, the SAFs; and explore the potential for future developments in the area. Our feeling is that by tackling the challenges of designing multi-component and functional biomaterials, as a community we stand to learn a great deal about self-assembling biomolecular systems more broadly, as well as, hopefully, delivering new materials that will be truly useful in biotechnology and biomedical applications (107 references).