Advances in the design and higher-order assembly of collagen mimetic peptides for regenerative medicine

Versatile Self-Assembly of Triblock Peptides into Stable Collagen Mimetic Heterotrimers

The construction of peptides to mimic heterogeneous proteins such as type I collagen plays a pivotal role in deciphering their function and pathogenesis. However, progress in the field has been severely hampered by the lack of capability to create stable heterotrimers with desired functional sequences and without the effect of homotrimers. We have herein developed a set of triblock peptides that can assemble into collagen mimetic heterotrimers with desired amino acids and are free from the interference of homotrimers. The triblock peptides comprise a central collagen-like block and two oppositely charged N-/C-terminal blocks, which display inherent incompetency of homotrimer formation. The favorable electrostatic attraction between two paired triblock peptides with complementary terminal charged sequences promptly leads to stable heterotrimers with controlled chain composition. The independence of the collagen-like block from the two terminal blocks endows this system with the adaptability to incorporate desired amino acid sequences while maintaining the heterotrimer structure. The triblock peptides provide a versatile and robust tool to mimic the composition and function of heterotrimer collagen and may have great potential in the design of innovative peptides mimicking heterogeneous proteins.

Carbohydrate Co-Solutes Stabilize Collagen Triple Helices

Carbohydrates are common co-solutes for the stabilization of proteins. The effect of carbohydrate solutions on the stability of collagen, the most abundant protein in mammals, is, however, underexplored. In this work, we studied the thermal stability of collagen triple helices derived from a molecularly defined collagen model peptide (CMP), Ac-(Pro-Hyp-Gly)7 -NH2 , in solutions of six common mono- and disaccharides. We show that the carbohydrates stabilize the collagen triple helix in a concentration-dependent manner, with an increase of the melting temperature of up to 17 °C. In addition, we show that the stabilizing effect is similar for all studied sugars, including trehalose, which is otherwise considered a privileged bioprotectant. The results provided insight into the effects of sugar co-solutes on collagen triple helices and can aid the selection of storage environments for collagen-based materials and probes.

Tissue engineering applications of recombinant human collagen: a review of recent progress

With the rapid development of synthetic biology, recombinant human collagen has emerged as a cutting-edge biological material globally. Its innovative applications in the fields of material science and medicine have opened new horizons in biomedical research. Recombinant human collagen stands out as a highly promising biomaterial, playing a pivotal role in crucial areas such as wound healing, stroma regeneration, and orthopedics. However, realizing its full potential by efficiently delivering it for optimal therapeutic outcomes remains a formidable challenge. This review provides a comprehensive overview of the applications of recombinant human collagen in biomedical systems, focusing on resolving this crucial issue. Additionally, it encompasses the exploration of 3D printing technologies incorporating recombinant collagen to address some urgent clinical challenges in regenerative repair in the future. The primary aim of this review also is to spotlight the advancements in the realm of biomaterials utilizing recombinant collagen, with the intention of fostering additional innovation and making significant contributions to the enhancement of regenerative biomaterials, therapeutic methodologies, and overall patient outcomes.

Synthetic Collagen Hydrogels through Symmetric Self-Assembly of Small Peptides

Animal-sourced hydrogels, such as collagen, are widely used as extracellular-matrix (ECM) mimics in tissue engineering but are plagued with problems of reproducibility, immunogenicity, and contamination. Synthetic, chemically defined hydrogels can avoid such issues. Despite the abundance of collagen in the ECM, synthetic collagen hydrogels are extremely rare due to design challenges brought on by the triple-helical structure of collagen. Sticky-ended symmetric self-assembly (SESSA) overcomes these challenges by maximizing interactions between the strands of the triple helix, allowing the assembly of collagen-mimetic peptides (CMPs) into robust synthetic collagen nanofibers. This optimization, however, also minimizes interfiber contacts. In this work, symmetric association states for the SESSA of short CMPs to probe their increased propensity for interfiber association are modelled. It is found that 33-residue CMPs not only self-assemble through sticky ends, but also form hydrogels. These self-assemblies behave with remarkable consistency across multiple scales and present a clear link between their triple-helical architecture and the properties of their hydrogels. The results show that SESSA is an effective and robust design methodology that enables the rational design of synthetic collagen hydrogels.

Collagen Mimetic Peptide with a Coiled Coil Trimerization Domain Forms Fibrils Having D-Period-like Structures

Fibrillar collagen is the major protein in the extracellular matrix and regulates cell behavior via chemical and mechanical cues. The key structural element of collagen fibrils is the axially repeating D-period, formed by the lateral association of collagen triple helices. We have developed fibril-forming collagen mimetic peptides (FCMPs) with repeated amino acid sequences, which form fibrils having D-period-like structures. Containing over 100 amino acid residues, these peptides are produced by bacterial expression using designed genes. Here, we report the fibrillogenesis of a new FCMP containing an α-helix coiled coil domain. The latest findings highlight the importance of the amino acid sequence periodicity in FCMP fibril formation. Additionally, our results demonstrate the remarkable adaptability of collagen fibrils' molecular packing. Mirroring native collagen fibrils, in both the structure and the fibrillogenesis process, these FCMPs are useful molecular tools for advancing collagen research and developing novel biomaterials.

In situ photo-crosslinkable hyaluronic acid-based hydrogel embedded with GHK peptide nanofibers for bioactive wound healing

A versatile hydrogel was developed for enhancing bioactive wound healing by introducing the amphiphilic GHK peptide (GHK-C16) into a photo-crosslinkable tyramine-modified hyaluronic acid (HA-Ty). GHK-C16 self-assembled into GHK nanofibers (GHK NF) in HA-Ty solution, which underwent in situ gelation after the wound area was filled with precursor solution. Blue light irradiation (460-490 nm), with riboflavin phosphate as a photoinitiator, was used to trigger crosslinking, which enhanced the stability of the highly degradable hyaluronic acid and enabled sustained release of the nanostructured GHK derivatives. The hydrogels provided a microenvironment that promoted the proliferation of dermal fibroblasts and the activation of cytokines, leading to reduced inflammation and increased collagen expression during wound healing. The complexation of Cu2+ into GHK nanofibers resulted in superior wound healing capabilities compared with non-lipidated GHK peptide with a comparable level of growth factor (EGF). Additionally, nanostructured Cu-GHK improved angiogenesis through vascular endothelial growth factor (VEGF) activation, which exerted a synergistic therapeutic effect. Furthermore, in vivo wound healing experiments revealed that the Cu-GHK NF/HA-Ty hydrogel accelerated wound healing through densely packed remodeled collagen in the dermis and promoting the growth of denser fibroblasts. HA-Ty hydrogels incorporating GHK NF also possessed improved mechanical properties and a faster wound healing rate, making them suitable for advanced bioactive wound healing applications. STATEMENT OF SIGNIFICANCE: By combining photo-crosslinkable tyramine-modified hyaluronic acid with self-assembled Cu-GHK-C16 peptide nanofibers (Cu-GHK NF), the Cu-GHK NF/HA-Ty hydrogel offers remarkable advantages over conventional non-structured Cu-GHK for wound healing. It enhances cell proliferation, migration, and collagen remodeling-critical factors in tissue regeneration. The incorporation of GHK nanofibers complexed with copper ions imparts potent anti-inflammatory effects, promoting cytokine activation and angiogenesis during wound healing. The Cu-GHK NF/hydrogel's unique properties, including in situ photo-crosslinking, ensure high customization and potency in tissue regeneration, providing a cost-effective alternative to growth factors. In vivo experiments further validate its efficacy, demonstrating significant wound closure, collagen remodeling, and increased fibroblast density. Overall, the Cu-GHK NF/HA-Ty hydrogel represents an advanced therapeutic option for wound healing applications.

Isonitrile-Proline - A Versatile Handle for the Chemoselective Derivatization of Collagen Peptides

Functional groups that allow for chemoselective and bioorthogonal derivatization are valuable tools for the labelling of peptides and proteins. The isonitrile is such a group but synthetic methods for its incorporation into peptides by solid-phase peptide synthesis are not known. Here, we introduce (4S)- and (4R)-isonitrileproline (Inp) as building blocks for solid-phase peptide synthesis. Conformational studies of (4S)- and (4R)-Inp and thermal stability analysis of Inp-containing collagen triple helices revealed that the isonitrile group exerts a stereoelectronic gauche effect. We showcase the value of Inp for bioorthogonal labelling by derivatization of Inp-containing collagen model peptides (CMPs). Dual labelling with a pair of bioorthogonal reactions of a CMP containing Inp and azidoproline residues further highlights the versatility of the new isonitrile-containing amino acids.

The Yin and Yang of How N-Terminal Acyl Caps Affect Collagen Triple Helices

N-terminal acylation is a common tool for the installation of functional moieties (e.g., sensors or bioactive molecules) on collagen model peptides (CMPs). The N-acyl group and its length are generally assumed to have little or no influence on the properties of the collagen triple helix formed by the CMP. Here, we show that the length of short (C1-C4) acyl capping groups has different effects on the thermal stability of collagen triple helices in POG, OGP, and GPO frames. While the effect of different capping groups on the stability of triple helices in the GPO frame is negligible, longer acyl chains stabilize OGP triple helices but destabilize POG analogues. The observed trends arise from a combination of steric repulsion, the hydrophobic effect, and n → π* interactions. Our study provides a basis for the design of N-terminally functionalized CMPs with predictable effects on triple helix stability.

Targeting collagen in tumor extracellular matrix as a novel targeted strategy in cancer immunotherapy

Collagen, the most abundant protein in mammal, is widely expressed in tissues and organs, as well as tumor extracellular matrix. Tumor collagen mainly accumulates in tumor stroma or beneath tumor blood vessel endothelium, and is exposed due to the fragmentary structure of tumor blood vessels. Through the blood vessels with enhanced permeability and retention (EPR) effect, collagen-binding macromolecules could easily bind to tumor collagen and accumulate within tumor, supporting tumor collagen to be a potential tumor-specific target. Recently, numerous studies have verified that targeting collagen within tumor extracellular matrix (TEM) would enhance the accumulation and retention of immunotherapy drugs at tumor, significantly improving their anti-tumor efficacy, as well as avoiding severe adverse effects. In this review, we would summarize the known collagen-binding domains (CBD) or proteins (CBP), their mechanism and application in tumor-targeting immunotherapy, and look forward to future development.

Staphylococcus aureus Behavior on Artificial Surfaces Mimicking Bone Environment

Infections, which interfere with bone regeneration, may be a critical issue to consider during the development of biomimetic material. Calcium phosphate (CaP) and type I collagen substrates, both suitable for bone-regeneration dedicated scaffolds, may favor bacterial adhesion. Staphylococcus aureus possesses adhesins that allow binding to CaP or collagen. After their adhesion, bacteria may develop structures highly tolerant to immune system attacks or antibiotic treatments: the biofilms. Thus, the choice of material used for scaffolds intended for bone sites is essential to provide devices with the ability to prevent bone and joint infections by limiting bacterial adhesion. In this study, we compared the adhesion of three different S. aureus strains (CIP 53.154, SH1000, and USA300) on collagen- and CaP-coating. Our objective was to evaluate the capacity of bacteria to adhere to these different bone-mimicking coated supports to better control the risk of infection. The three strains were able to adhere to CaP and collagen. The visible matrix components were more important on CaP- than on collagen-coating. However, this difference was not reflected in biofilm gene expression for which no change was observed between the two tested surfaces. Another objective was to evaluate these bone-mimicking coatings for the development of an in vitro model. Thus, CaP, collagen-coatings, and the titanium-mimicking prosthesis were simultaneously tested in the same bacterial culture. No significant differences were found compared to adhesion on surfaces independently tested. In conclusion, these coatings used as bone substitutes can easily be colonized by bacteria, especially CaP-coating, and must be used with an addition of antimicrobial molecules or strategies to avoid bacterial biofilm development.

Predicting Collagen Triple Helix Stability through Additive Effects of Terminal Residues and Caps

Collagen model peptides (CMPs) consisting of proline-(2S,4R)-hydroxyproline-glycine (POG) repeats have provided a breadth of knowledge of the triple helical structure of collagen, the most abundant protein in mammals. Predictive tools for triple helix stability have, however, lagged behind since the effect of CMPs with different frames ([POG]_n , [OGP]_n , or [GPO]_n ) and capped or uncapped termini have so far been underestimated. Here, we elucidated the impact of the frame, terminal functional group and its charge on the stability of collagen triple helices. Combined experimental and theoretical studies with frame-shifted, capped and uncapped CMPs revealed that electrostatic interactions, strand preorganization, interstrand H-bonding, and steric repulsion at the termini contribute to triple helix stability. We show that these individual contributions are additive and allow for the prediction of the melting temperatures of CMP trimers.

Biopolymer-Based Wound Dressings with Biochemical Cues for Cell-Instructive Wound Repair

Regenerative medicine is an active research sphere that focuses on the repair, regeneration, and replacement of damaged tissues and organs. A plethora of innovative wound dressings and skin substitutes have been developed to treat cutaneous wounds and are aimed at reducing the length or need for a hospital stay. The inception of biomaterials with the ability to interact with cells and direct them toward desired lineages has brought about innovative designs in wound healing and tissue engineering. This cellular engagement is achieved by cell cues that can be biochemical or biophysical in nature. In effect, these cues seep into innate repair pathways, cause downstream cell behaviours and, ultimately, lead to advantageous healing. This review will focus on biomolecules with encoded biomimetic, instructive prompts that elicit desired cellular domino effects to achieve advanced wound repair. The wound healing dressings covered in this review are based on functionalized biopolymeric materials. While both biophysical and biochemical cues are vital for advanced wound healing applications, focus will be placed on biochemical cues and in vivo or clinical trial applications. The biochemical cues aforementioned will include peptide therapy, collagen matrices, cell-based therapy, decellularized matrices, platelet-rich plasma, and biometals.

Frame Shifts Affect the Stability of Collagen Triple Helices

Collagen model peptides (CMPs), composed of proline-(2S,4R)-hydroxyproline-glycine (POG) repeat units, have been extensively used to study the structure and stability of triple-helical collagen─the dominant structural protein in mammals─at the molecular level. Despite the more than 50-year history of CMPs and numerous studies on the relationship between the composition of single-stranded CMPs and the thermal stability of the assembled triple helices, little attention has been paid to the effects arising from their terminal residues. Here, we show that frame-shifted CMPs, which share POG repeat units but terminate with P, O, or G, form triple helices with vastly different thermal stabilities. A melting temperature difference as high as 16 °C was found for triple helices from 20-mers Ac-OG[POG]₆-NH₂ and Ac-[POG]₆PO-NH₂, and triple helices of the constitutional isomers Ac-[POG]₇-NH₂ and Ac-[GPO]₇-NH₂ melt 10 °C apart. A combination of thermal denaturation, circular dichroism and NMR spectroscopic studies, and molecular dynamics simulations revealed that the stability differences originate from the propensity of the peptide termini to preorganize into a polyproline-II helical structure. Our results advise that care must be taken when designing peptide mimics of structural proteins, as subtle changes in the terminal residues can significantly affect their properties. Our findings also provide a general and straightforward tool for tuning the stability of CMPs for applications as synthetic materials and biological probes.

A Comprehensive Review on Collagen Type I Development of Biomaterials for Tissue Engineering: From Biosynthesis to Bioscaffold

Collagen is the most abundant structural protein found in humans and mammals, particularly in the extracellular matrix (ECM). Its primary function is to hold the body together. The collagen superfamily of proteins includes over 20 types that have been identified. Yet, collagen type I is the major component in many tissues and can be extracted as a natural biomaterial for various medical and biological purposes. Collagen has multiple advantageous characteristics, including varied sources, biocompatibility, sustainability, low immunogenicity, porosity, and biodegradability. As such, collagen-type-I-based bioscaffolds have been widely used in tissue engineering. Biomaterials based on collagen type I can also be modified to improve their functions, such as by crosslinking to strengthen the mechanical property or adding biochemical factors to enhance their biological activity. This review discusses the complexities of collagen type I structure, biosynthesis, sources for collagen derivatives, methods of isolation and purification, physicochemical characteristics, and the current development of collagen-type-I-based scaffolds in tissue engineering applications. The advancement of additional novel tissue engineered bioproducts with refined techniques and continuous biomaterial augmentation is facilitated by understanding the conventional design and application of biomaterials based on collagen type I.

Biomaterial functionalization with triple-helical peptides for tissue engineering

In the growing field of tissue engineering, providing cells in biomaterials with the adequate biological cues represents an increasingly important challenge. Yet, biomaterials with excellent mechanical properties often are often biologically inert to many cell types. To address this issue, researchers resort to functionalization, i.e. the surface modification of a biomaterial with active molecules or substances. Functionalization notably aims to replicate the native cellular microenvironment provided by the extracellular matrix, and in particular by collagen, its major component. As our understanding of biological processes regulating cell behaviour increases, functionalization with biomolecules binding cell surface receptors constitutes a promising strategy. Amongst these, triple-helical peptides (THPs) that reproduce the architectural and biological properties of collagen are especially attractive. Indeed, THPs containing binding sites from the native collagen sequence have successfully been used to guide cell response by establishing cell-biomaterial interactions. Notably, the GFOGER motif recognising the collagen-binding integrins is extensively employed as a cell adhesive peptide. In biomaterials, THPs efficiently improved cell adhesion, differentiation and function on biomaterials designed for tissue repair (especially for bone, cartilage, tendon and heart), vascular graft fabrication, wound dressing, drug delivery or immunomodulation. This review describes the key characteristics of THPs, their effect on cells when combined to biomaterials and their strong potential as biomimetic tools for regenerative medicine. STATEMENT OF SIGNIFICANCE: This review article describes how triple-helical peptides constitute efficient tools to improve cell-biomaterial interactions in tissue engineering. Triple helical peptides are bioactive molecules that mimic the architectural and biological properties of collagen. They have been successfully used to specifically recognize cell-surface receptors and provide cells seeded on biomaterials with controlled biological cues. Functionalization with triple-helical peptides has enabled researchers to improve cell function for regenerative medicine applications, such as tissue repair. However, despite encouraging results, this approach remains limited and under-exploited, and most functionalization strategies reported in the literature rely on biomolecules that are unable to address collagen-binding receptors. This review will assist researchers in selecting the correct tools to functionalize biomaterials in efforts to guide cellular response.

Recent Advances in Collagen Mimetic Peptide Structure and Design

Collagen mimetic peptides (CMPs) fold into a polyproline type II triple helix, allowing the study of the structure and function (or misfunction) of the collagen family of proteins. This Perspective will focus on recent developments in the use of CMPs toward understanding the structure and controlling the stability of the triple helix. Triple helix assembly is influenced by various factors, including the single amino acid propensity for the triple helix fold, pairwise interactions between these amino acids, and long-range effects observed across the helix, such as bend, twist, and fraying. Important progress in creating a comprehensive and predictive understanding of these factors for peptides with exclusively natural amino acids has been made. In contrast, several groups have successfully developed unnatural amino acids that are engineered to stabilize the triple helical structure. A third approach to controlling the triple helical structure includes covalent cross-linking of the triple helix to stabilize the assembly, which eliminates the problematic equilibrium of unfolding into monomers and enforces compositional control. Advances in all these areas have resulted in significant improvements to our understanding and control of this important class of protein, allowing for the design and application of more chemically complex and well-controlled collagen mimetic biomaterials.

Formation of Microcages from a Collagen Mimetic Peptide via Metal-Ligand Interactions

Here, the hierarchical assembly of a collagen mimetic peptide (CMP) displaying four bipyridine moieties is described. The CMP was capable of forming triple helices followed by self-assembly into disks and domes. Treatment of these disks and domes with metal ions such as Fe(II), Cu(II), Zn(II), Co(II), and Ru(III) triggered the formation of microcages, and micron-sized cup-like structures. Mechanistic studies suggest that the formation of the microcages proceeds from the disks and domes in a metal-dependent fashion. Fluorescently-labeled dextrans were encapsulated within the cages and displayed a time-dependent release using thermal conditions.

In situ Tissue Regeneration in the Cornea from Bench to Bedside

Corneal blindness accounts for 5.1% of visual deficiency and is the fourth leading cause of blindness globally. An additional 1.5-2 million people develop corneal blindness each year, including many children born with or who later develop corneal infections. Over 90% of corneal blind people globally live in low- and middle-income regions (LMIRs), where corneal ulcers are approximately 10-fold higher compared to high-income countries. While corneal transplantation is an effective option for patients in high-income countries, there is a considerable global shortage of corneal graft tissue and limited corneal transplant programs in many LMIRs. In situ tissue regeneration aims to restore diseases or damaged tissues by inducing organ regeneration. This can be achieved in the cornea using biomaterials based on extracellular matrix (ECM) components like collagen, hyaluronic acid, and silk. Solid corneal implants based on recombinant human collagen type III were successfully implanted into patients resulting in regeneration of the corneal epithelium, stroma, and sub-basal nerve plexus. As ECM crosslinking and manufacturing methods improve, the focus of biomaterial development has shifted to injectable, in situ gelling formulations. Collagen, collagen-mimetic, and gelatin-based in situ gelling formulas have shown the ability to repair corneal wounds, surgical incisions, and perforations in in-vivo models. Biomaterial approaches may not be sufficient to treat inflammatory conditions, so other cell-free therapies such as treatment with tolerogenic exosomes and extracellular vesicles may improve treatment outcomes. Overall, many of the technologies described here show promise as future medical devices or combination products with cell or drug-based therapies. In situ tissue regeneration, particularly with liquid formulas, offers the ability to triage and treat corneal injuries and disease with a single regenerative solution, providing alternatives to organ transplantation and improving patient outcomes.

Metal-Coordinated Supramolecular Self-Assemblies for Cancer Theranostics

Metal-coordinated supramolecular nanoassemblies have recently attracted extensive attention as materials for cancer theranostics. Owing to their unique physicochemical properties, metal-coordinated supramolecular self-assemblies can bridge the boundary between traditional inorganic and organic materials. By tailoring the structural components of the metal ions and binding ligands, numerous multifunctional theranostic nanomedicines can be constructed. Metal-coordinated supramolecular nanoassemblies can modulate the tumor microenvironment (TME), thus facilitating the development of TME-responsive nanomedicines. More importantly, TME-responsive organic-inorganic hybrid nanomaterials can be constructed in vivo by exploiting the metal-coordinated self-assembly of a variety of functional ligands, which is a promising strategy for enhancing the tumor accumulation of theranostic molecules. In this review, recent advancements in the design and fabrication of metal-coordinated supramolecular nanomedicines for cancer theranostics are highlighted. These supramolecular compounds are classified according to the order in which the coordinated metal ions appear in the periodic table. Furthermore, the prospects and challenges of metal-coordinated supramolecular self-assemblies for both technical advances and clinical translation are discussed. In particular, the superiority of TME-responsive nanomedicines for in vivo coordinated self-assembly is elaborated, with an emphasis on strategies that enhance the accumulation of functional components in tumors for an ideal theranostic outcome.

Advances in the synthesis and application of self-assembling biomaterials

The present study scrutinized some of the crucial advancements in the synthesis and functionalisation of self-assembling biomaterials for application in biomedicine. The basic concept of self-organization was discussed along with the mechanisms and methods involved in its implementation with biomaterials. Further, several recent applications of this technology in the biological and medical domain, and the avenues for future research and development were presented. This study brought to focus the vast potential of basic and applied research involved, especially in the context of hybrids and composites, as well as the difference in pace of new developments for different types of biomolecular materials. As nanobiotechnology matures, the tools and techniques available for developing and controlling self-assembled biomaterials as well as studying their interaction with biological tissue, will grow exponentially. Presently, self-assembly remains a potent tool for the synthesis of functional biomaterials.

Collagen-based biomaterials for biomedical applications

Collagen is an insoluble fibrous protein that composes the extracellular matrix in animals. Although collagen has been used as a biomaterial since 1881, the properties and the complex structure of collagen are still extensive study subjects worldwide. In this article, several topics of importance for understanding collagen research are reviewed starting from its historical milestones, followed by the description of the collagen superfamily and its complex structures, with a focus on type I collagen. Subsequently, some of the superior properties of collagen-based biomaterials, such as biocompatibility, biodegradability, mechanical properties, and cell activities, are pinpointed. These properties make collagen applicable in biomedicine, such as wound healing, tissue engineering, surface coating of medical devices, and skin supplementation. Moreover, some antimicrobial strategies and the general host tissue responses regarding collagen as a biomaterial are presented. Finally, the current status and clinical application of the three-dimensional (3D) printing techniques for the fabrication of collagen-based scaffolds and the reconstruction of the human heart's constituents, such as capillary structures or even the entire organ, are discussed. Besides, an overall outlook for the future of this unique biomaterial is provided.

Enhanced Collagen-like Protein for Facile Biomaterial Fabrication

We present a collagen-mimetic protein of bacterial origin based upon a modified subdomain of the collagen-like Sc12 protein from Streptococcus pyogenes, as an alternative collagen-like biomaterial platform that is highly soluble, forms stable, homogeneous, fluid-like solutions at elevated concentrations, and that can be efficiently fabricated into hydrogel materials over a broad range of pH conditions. This extended bacterial collagen-like (eBCL) protein is expressed in a bacterial host and purified as a trimeric assembly exhibiting a triple helical secondary structure in its collagen-like subdomain that is stable near physiological solution conditions (neutral pH and 37 °C), as well as over a broad range of pH conditions. We also show how this sequence can be modified to include biofunctional attributes, in particular, the Arg-Gly-Asp (RGD) sequence to elicit integrin-specific cell binding, without loss of structural function. Furthermore, through the use of EDC-NHS chemistry, we demonstrate that members of this eBCL protein system can be covalently cross-linked to fabricate transparent hydrogels with high protein concentrations (at least to 20% w/w). These hydrogels are shown to possess material properties and resistance to enzymatic degradation that are comparable or superior to a type I collagen control. Moreover, such hydrogels containing the constructs with the RGD integrin-binding sequence are shown to promote the adhesion, spreading, and proliferation of C2C12 and 3T3 cells in vitro. Due to its enhanced solubility, structural stability, fluidity at elevated concentrations, ease of modification, and facility of cross-linking, this eBCL collagen-mimetic system has potential for numerous biomedical material applications, where the ease of processing and fabrication and the facility to tailor the sequence for specific biological functionality are desired.

Reversible crosslinked assembly of a trimeric coiled-coil peptide into a three-dimensional matrix for cell encapsulation and release

Mimicking the extracellular matrix (ECM) continues to be a goal in the field of regenerative medicine. Herein, we report a modified trimeric GCN4 coiled-coil sequence containing three ligands for metal ions specifically positioned for crosslinked assembly (TriCross). In the presence of metal ions, TriCross assembles into a three-dimensional (3D) matrix with significant cavities to accommodate cells. The matrix was found to be stable in media with serum, and mild removal of the metal leads to disassembly. By assembling TriCross with a suspension of cells in media, the matrix encapsulates cells during the assembly process leading to high cell viability. Further disassembly under mild conditions allows for the release of cells from the scaffold. As such, this peptide-based material displays many of the characteristics necessary for successful 3D cell culture.

Ln3+-Triggered self-assembly of a heterotrimer collagen mimetic peptide into luminescent nanofibers

Type I collagen, the most abundant and arguably the most complex molecule in the human body, is an ABB heterotrimer that self-assembles to form well-defined nanofibers. We herein for the first time report the construction of peptides that could simultaneously mimic the heterotrimer composition and the self-assembly features of Type I collagen.

Synthetic biology for protein-based materials

Recombinant protein polymers that mimic the structures and functions of natural proteins and those tailor-designed with new properties provide a family of uniquely tunable and functional materials. However, the diversity of genetically engineered protein polymers is still limited. As a powerful engine for the creation of new biological devices and systems, synthetic biology is promising to tackle the challenges that exist in conventional studies on protein polymers. Here we review the advances in design and biosynthesis of advanced protein materials by synthetic biology approaches. In particular, we highlight their roles in expanding the variety of designer protein polymers and creating programmable materials with live cells.

Studying biomolecular folding and binding using temperature-jump mass spectrometry

Characterizing folding and complex formation of biomolecules provides a view into their thermodynamics, kinetics and folding pathways. Deciphering kinetic intermediates is particularly important because they can often be targeted by drugs. The key advantage of native mass spectrometry over conventional methods that monitor a single observable is its ability to identify and quantify coexisting species. Here, we show the design of a temperature-jump electrospray source for mass spectrometry that allows one to perform fast kinetics experiments (0.16-32 s) at different temperatures (10-90 °C). The setup allows recording of both folding and unfolding kinetics by using temperature jumps from high to low, and low to high, temperatures. Six biological systems, ranging from peptides to proteins to DNA complexes, exemplify the use of this device. Using temperature-dependent experiments, the folding and unfolding of a DNA triplex are studied, providing detailed information on its thermodynamics and kinetics.

Templated Collagen "Double Helices" Maintain Their Structure

The self-assembly of collagen-mimetic peptides (CMPs) that form sticky-ended triple helices has allowed the production of surprisingly stable artificial collagen fibers and hydrogels. Assembly through sticky ends requires the recognition of a single strand by a templated strand dimer. Although CMPs and their triple helices have been studied extensively, the structure of a strand dimer is unknown. Here, we evaluate the physical characteristics of such dimers, using disulfide-templated (PPG)10 dimers as a model. Such "linked-dimers" retain their collagen-like structure even in the absence of a third strand, but only when their strands are capable of adopting a triple-helical fold. The intrinsic collagen-like structure of templated CMP pairs helps to explain the success of sticky-ended CMP association and changes the conception of new synthetic collagen designs.

Recent trends in protein and peptide-based biomaterials for advanced drug delivery

Engineering protein and peptide-based materials for drug delivery applications has gained momentum due to their biochemical and biophysical properties over synthetic materials, including biocompatibility, ease of synthesis and purification, tunability, scalability, and lack of toxicity. These biomolecules have been used to develop a host of drug delivery platforms, such as peptide- and protein-drug conjugates, injectable particles, and drug depots to deliver small molecule drugs, therapeutic proteins, and nucleic acids. In this review, we discuss progress in engineering the architecture and biological functions of peptide-based biomaterials -naturally derived, chemically synthesized and recombinant- with a focus on the molecular features that modulate their structure-function relationships for drug delivery.

Modification of Collagen Derivatives with Water-Soluble Polymers for the Development of Cross-Linked Hydrogels for Controlled Release

Novel cross-linked hydrogels were synthesized as potential materials for the development of smart biofertilizers. For this purpose, hydrogels were prepared using collagen hydrolysate recovered from tannery waste. The water-soluble polymer poly(sodium 4-styrenesulfonate-co-glycidyl methacrylate) (P(SSNa-co-GMAx)) was among others used for the cross-linking reaction that combined hydrophilic nature with epoxide groups. The synthetic procedure was thoroughly investigated in order to ensure high percentage of epoxide groups in combination with water-soluble behavior. The copolymer did not show cytotoxicity against normal lung, skin fibroblasts, or nasal polyps fibroblasts. Through the present work, we also present the ability to control the properties of cross-linked hydrogels by altering copolymer's composition and cross-linking parameters (curing temperature and time). Hydrogels were then studied in terms of water-uptake capacity for a period up to six days. The techniques Proton Nuclear Magnetic Resonance (1H NMR), Thermogravimetric Analysis (TGA), Size Exclusion Chromatography (SEC), and Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) were applied for the characterization of the synthesized copolymers and the cross-linked hydrogels. Three samples of biofertilizers based on collagen hydrolysate functionalized with P(SSNa-co-GMAx) and starch and having nutrients encapsulated (N, P, K) were prepared and characterized by physical-chemical analysis and Energy Dispersive X-ray analysis-Scanning Electron Microscope (EDAX-SEM) in terms of microstructure. Preliminary tests for application as fertilizers were performed including the release degree of oxidable organic compounds.

Luminescent Biofunctional Collagen Mimetic Nanofibers

Collagen has long been one of the top targets for biomimetic design due to its superior structural and functional properties. Significant progress has been achieved to construct self-assembling peptides to mimic the fibrous nanostructure of native collagen, while it is still very demanding to fabricate peptide assemblies that can recapitulate both structural and biofunctional features of collagen. Herein, collagen-like peptides have been synthesized to contain negatively charged amino acids as the binding groups of lanthanide ions and the integrin-binding motif GFOGER. The simultaneous inclusion of negatively charged amino acids in the middle as well as at both terminals drives the peptides to self-assemble to form well-ordered nanofibers with distinct periodic banding patterns specifically mediated by lanthanide ions. The aggregation tendency and the morphology of the final assembled materials for the peptides are modulated in a pH-cooperative manner, which well mimics the pH-dependent fibrillogenesis of Type I collagen. The utilization of lanthanide ions in the system not only offers a convenient external stimulus but also functionalizes assembled materials with excellent luminescent features. Most notably, the lanthanide-triggered peptide assembled nanomaterials possess good cell adhesion properties, which resemble the biological function of collagen. This peptide-Ln³⁺ system provides a facile and potent strategy to generate nanofibers that mimic both the structural and functional properties of natural collagen. These novel pH-responsive, luminescent, and biofunctional collagen mimetic nanofibers open fascinating opportunities in the development of improved functional biomaterials in tissue engineering, drug delivery, and medical diagnostics.

Stabilization of the triple helix in collagen mimicking peptides

Collagen mimics are peptides designed to reproduce structural features of natural collagen. A triple helix is the first element in the hierarchy of collagen folding. It is an assembly of three parallel peptide chains stabilized by packing and interchain hydrogen bonds. In this review we summarize the existing chemical approaches towards stabilization of this structure including the most recent developments. Currently proposed methods include manipulation of the amino acid composition, application of unnatural amino acid analogues, stimuli-responsive modifications, chain tethering approaches, peptide amphiphiles, modifications that target interchain interactions and more. This ability to manipulate the triple helix as a supramolecular self-assembly contributes to our understanding of the collagen folding. It also provides essential information needed to design collagen-based biomaterials of the future.

Promotion of the collagen triple helix in a hydrophobic environment

The collagen triple helix is better suited for octanol than for water.

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.

Short peptide analogs as alternatives to collagen in pro-regenerative corneal implants

Short collagen-like peptides (CLPs) are being proposed as alternatives to full-length collagen for use in tissue engineering, on their own as soft hydrogels, or conjugated to synthetic polymer for mechanical strength. However, despite intended clinical use, little is known about their safety and efficacy, mechanism of action or degree of similarity to the full-length counterparts they mimic. Here, we show the functional equivalence of a CLP conjugated to polyethylene glycol (CLP-PEG) to full-length recombinant human collagen in vitro and in promoting stable regeneration of corneal tissue and nerves in a pre-clinical mini-pig model. We also show that these peptide analogs exerted their pro-regeneration effects through stimulating extracellular vesicle production by host cells. Our results support future use of CLP-PEG implants for corneal regeneration, suggesting the feasibility of these or similar peptide analogs in clinical application in the eye and other tissues.

Statement of significance

Although biomaterials comprising full-length recombinant human collagen and extracted animal collagen have been evaluated and used clinically, these macromolecules provide only a limited number of functional groups amenable to chemical modification or crosslinking and are demanding to process. Synthetic, customizable analogs that are functionally equivalent, and can be readily scaled-up are therefore very desirable for pre-clinical to clinical translation. Here, we demonstrate, using cornea regeneration as our test bed, that collagen-like-peptides conjugated to multifunctional polyethylene glycol (CLP-PEG) when grafted into mini-pigs as corneal implants were functionally equivalent to recombinant human collagen-based implants that were successfully tested in patients. We also show for the first time that these materials affected regeneration through stimulation of extracellular vesicle production by endogenous host cells that have migrated into the CLP-PEG scaffolds.