Covalent Capture of a Heterotrimeric Collagen Helix

The diazirine-mediated photo-crosslinking of collagen improves biomaterial mechanical properties and cellular interactions

In tissue engineering, crosslinking with carbodiimides such as EDC is omnipresent to improve the mechanical properties of biomaterials. However, in collagen biomaterials, EDC reacts with glutamate or aspartate residues, inactivating the binding sites for cellular receptors and rendering collagen inert to many cell types. In this work, we have developed a crosslinking method that ameliorates the rigidity, stability, and degradation rate of collagen biomaterials, whilst retaining key interactions between cells and the native collagen sequence. Our approach relies on the UV-triggered reaction of diazirine groups grafted on lysines, leaving critical amino acid residues intact. Notably, GxxGER recognition motifs for collagen-binding integrins, ablated by EDC crosslinking, were left unreacted, enabling cell attachment, spreading, and colonization on films and porous scaffolds. In addition, our procedure conserves the architecture of biomaterials, improves their resistance to collagenase and cellular contraction, and yields material stiffness akin to that obtained with EDC. Importantly, diazirine-crosslinked collagen can host mesenchymal stem cells, highlighting its strong potential as a substrate for tissue repair. We have therefore established a new crosslinking strategy to modulate the mechanical features of collagen porous scaffolds without altering its biological properties, thereby offering an advantageous alternative to carbodiimide treatment. STATEMENT OF SIGNIFICANCE: This article describes an approach to improve the mechanical properties of collagen porous scaffolds, without impacting collagen's natural interactions with cells. This is significant because collagen crosslinking is overwhelmingly performed using carbodiimides, which results in a critical loss of cellular affinity. By contrast, our method leaves key cellular binding sites in the collagen sequence intact, enabling cell-biomaterial interactions. It relies on the fast, UV-triggered reaction of diazirine with collagen, and does not produce toxic by-products. It also supports the culture of mesenchymal stem cells, a pivotal cell type in a wide range of tissue repair applications. Overall, our approach offers an attractive option for the crosslinking of collagen, a prominent material in the growing field of tissue engineering.

Proton Transport Across Collagen Fibrils and Scaffolds: The Role of Hydroxyproline

Collagen is one of the most studied proteins due to its fundamental role in creating fibrillar structures and supporting tissues in our bodies. Accordingly, collagen is also one of the most used proteins for making tissue-engineered scaffolds for various types of tissues. To date, the high abundance of hydroxyproline (Hyp) within collagen is commonly ascribed to the structure and stability of collagen. Here, we hypothesize a new role for the presence of Hyp within collagen, which is to support proton transport (PT) across collagen fibrils. For this purpose, we explore here three different collagen-based hydrogels: the first is prepared by the self-assembly of natural collagen fibrils, and the second and third are based on covalently linking between collagen via either a self-coupling method or with an additional cross-linker. Following the formation of the hydrogel, we introduce here a two-step reaction, involving (1) attaching methanesulfonyl to the -OH group of Hyp, followed by (2) removing the methanesulfonyl, thus reverting Hyp to proline (Pro). We explore the PT efficiency at each step of the reaction using electrical measurements and show that adding the methanesulfonyl group vastly enhances PT, while reverting Hyp to Pro significantly reduces PT efficiency (compared with the initial point) with different efficiencies for the various collagen-based hydrogels. The role of Hyp in supporting the PT can assist in our understanding of the physiological roles of collagen. Furthermore, the capacity to modulate conductivity across collagen is very important to the use of collagen in regenerative medicine.

Stabilization of Synthetic Collagen Triple Helices: Charge Pairs and Covalent Capture

Collagen mimetic peptides are composed of triple helices. Triple helical formation frequently utilizes charge pair interactions to direct protein assembly. The design of synthetic triple helices is challenging due to the large number of competing species and the overall fragile nature of collagen mimetics. A successfully designed triple helix incorporates both positive and negative criteria to achieve maximum specificity of the supramolecular assembly. Intrahelical charge pair interactions, particularly those involved in lysine-aspartate and lysine-glutamate pairs, have been especially successful both in driving helix specificity and for subsequent stabilization by covalent capture. Despite this progress, the important sequential and geometric relationships of charged residues in a triple helical context have not been fully explored for either supramolecular assembly or covalent capture stabilization. In this study, we compare the eight canonical axial and lateral charge pairs of lysine and arginine with glutamate and aspartate to their noncanonical, reversed charge pairs. These findings are put into the context of collagen triple helical design and synthesis.

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.

Co-assembled Coiled-Coil Peptide Nanotubes with Enhanced Stability and Metal-Dependent Cargo Loading

Peptide nanotube biomaterials are attractive for their range of applications. Herein, we disclose the co-assembly of coiled-coil peptides, one with ligands for metal ions that demonstrate hierarchical assembly into nanotubes, with spatial control of the metal-binding ligands. Enhanced stability of the nanotubes to phosphate-buffered saline was successfully accomplished in a metal-dependent fashion, depending on the levels and placement of the ligand-containing coiled-coil peptide. This spatial control also allowed for site-specific labeling of the nanotubes with His-tagged fluorophores through the length of the tubes or at the termini, in a metal-dependent manner.

Covalent Capture of a Collagen Mimetic Peptide with an Integrin-Binding Motif

Collagen mimetic peptides (CMPs) are an excellent model to study the structural and biological properties of the extracellular matrix (ECM) due to ease of synthesis and variability in sequence. To ensure that synthetic materials accurately mimic the structure and function of natural collagen in the ECM, it is necessary to conserve the triple helix. However, CMP folding is subject to equilibrium, and frequently peptides exist in solution as both monomer and triple helix. Additionally, the stability of CMPs is highly dependent on peptide length and amino acid composition, leading to suboptimal performance. Here, we report the utility of covalent capture, a method to (a) direct the folding of a supramolecular triple helix and (b) form isopeptide bonds between the helix strands, in the design of an integrin-binding peptide with a GFOGER motif. Covalent capture effectively locked the triple helix and yielded a peptide with high thermal stability and a rapid folding rate. Compared to supramolecular triple helices bearing the same GFOGER-binding site, cell adhesion was substantially increased. In vitro assays using EDTA/Mg²⁺ and an anti-α2β1 antibody demonstrated the preservation of the high specificity of the binding event. This covalently captured integrin-binding peptide provides a template for the future design of bioactive ECM mimics, which can overcome limitations of supramolecular approaches for potential drug and biomaterial designs.

Impact of collagen-like peptide (CLP) heterotrimeric triple helix design on helical thermal stability and hierarchical assembly: a coarse-grained molecular dynamics simulation study

Collagen-like peptides (CLP) are multifunctional materials garnering a lot of recent interest from the biomaterials community due to their hierarchical assembly and tunable physicochemical properties. In this work, we present a computational study that links the design of CLP heterotrimers to the thermal stability of the triple helix and their self-assembly into fibrillar aggregates and percolated networks. Unlike homotrimeric helices, the CLP heterotrimeric triple helices in this study are made of CLP strands of different chain lengths that result in 'sticky' ends with available hydrogen bonding groups. These 'sticky' ends at one end or both ends of the CLP heterotrimer then facilitate inter-helix hydrogen bonding leading to self-assembly into fibrils (clusters) and percolated networks. We consider the cases of three sticky end lengths - two, four, and six repeat units - present entirely on one end or split between two ends of the CLP heterotrimer. We observe in CLP heterotrimer melting curves generated using coarse grained Langevin dynamics simulations at low CLP concentration that increasing sticky end length results in lower melting temperatures for both one and two sticky ended CLP designs. At higher CLP concentrations, we observe non-monotonic trends in cluster sizes with increasing sticky end length with one sticky end but not for two sticky ends with the same number of available hydrogen bonding groups as the one sticky end; this nonmonotonicity stems from the formation of turn structures stabilized by hydrogen bonds at the single, sticky end for sticky end lengths greater than four repeat units. With increasing CLP concentration, heterotrimers also form percolated networks with increasing sticky end length with a minimum sticky end length of four repeat units required to observe percolation. Overall, this work informs the design of thermoresponsive, peptide-based biomaterials with desired morphologies using strand length and dispersity as a handle for tuning thermal stability and formation of supramolecular structures.

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.

Bioinspired photo-crosslinkable self-assembling peptides with pH-switchable "on-off" luminescence

Significant progress has been made in peptide self-assembly over the past two decades; however, the in situ cross-linking of self-assembling peptides yielding better performing nanomaterials is still in its infancy. Indeed, self-assembling peptides (SAPs), relying only on non-covalent interactions, are mechanically unstable and susceptible to solvent erosion, greatly hindering their practical application. Herein, drawing inspiration from the biological functions of tyrosine, we present a photo-cross-linking approach for the in situ cross-linking of a tyrosine-containing LDLK12 SAP. This method is based on the ruthenium-complex-catalyzed conversion of tyrosine to dityrosine upon light irradiation. We observed a stable formation of dityrosine cross-linking starting from 5 minutes, with a maximum peak after 1 hour of UV irradiation. Furthermore, the presence of a ruthenium complex among the assembled peptide bundles bestows unusual fluorescence intensity stability up to as high as 42 °C, compared to the bare ruthenium complex. Also, due to a direct deprotonation-protonation process between the ruthenium complex and SAP molecules, the fluorescence of the photo-cross-linked SAP is capable of exhibiting "off-on-off-on" luminescence switchable from acid to basic pH. Lastly, we showed that the photo-cross-linked hydrogel exhibited enhanced mechanical stability with a storage modulus of ∼26 kPa, due to the formation of a densely entangled fibrous network of SAP molecules through dityrosine linkages. As such, this ruthenium-mediated photo-cross-linked SAP hydrogel could be useful in the design of novel tyrosine containing SAP materials with intriguing potential for biomedical imaging, pH sensing, photonics, soft electronics, and bioprinting.

Selective covalent capture of collagen triple helices with a minimal protecting group strategy

A minimal protecting group strategy is developed to allow selective covalent capture of collagen-like triple helices. This allows stabilization of this critical fold while preserving charge–pair interactions critical for biological applications.

Modulating the Stability of Collagen Triple Helices by Terminal Charged Residues

Cationic or anionic residues are frequently located at the termini of proteins because their charged side chain can form electrostatic interactions with a terminal carboxylate or ammonium group to stabilize the structure under physiological conditions. Here, we used collagen-mimetic peptides (CMPs) to examine how the terminal charge-charge interactions affect the collagen triple helix stability. We designed a series of CMPs with either a Lys or Glu incorporated into the terminus and measured their pH-dependent stability. The results showed that the terminal electrostatic attractions stabilized the triple helix, while the terminal electrostatic repulsions destabilized the trimer. The data also revealed that the repulsions imposed a greater effect than did the attractions on the triple helix. An amino acid with a shorter side chain, such as aspartate and ornithine, was also installed to investigate the length effect on electrostatic interactions, which was found to be insignificant. Meanwhile, simultaneously incorporating cationic and anionic residues into the termini showed slight additive stabilization effects but pronounced additive destabilization consequences. We have demonstrated that the collagen triple helix stability can be modulated by introducing a cationic or anionic residue into the terminus of a peptide, giving useful information for the design of collagen-associated materials.

Charge-Free, Stabilizing Amide-π Interactions Can Be Used to Control Collagen Triple-Helix Self-Assembly

There is a noted lack of understood, controllable interactions for directing the organization of collagen triple helices. While the field has had success using charge-pair interactions and cation-π interactions in helix design, these alone are not adequate for achieving the degree of specificity desirable for these supramolecular structures. Furthermore, because of the reliance on electrostatic interactions, designed heterotrimeric systems have been heavily charged, a property undesirable in some applications. Amide-π interactions are a comparatively understudied class of charge-free interactions, which could potentially be harnessed for triple-helix design. Herein, we propose, validate, and utilize pairwise amino acid amide-π interactions in collagen triple-helix design. Glutamine-phenylalanine pairs, when arranged in an axial geometry, are found to exhibit a moderately stabilizing effect, while in the lateral geometry, this pair is destabilizing. Together this allows glutamine-phenylalanine pairs to effectively set the register of triple helices. In contrast, interactions between asparagine and phenylalanine appear to have little effect on triple-helical stability. After deconvoluting the contributions of these amino acids to triple-helix stability, we demonstrate these new glutamine-phenylalanine interactions in the successful design of a heterotrimeric triple helix. The results of all of these analyses are used to update our collagen triple-helix thermal stability prediction algorithm, Scoring function for Collagen Emulating Peptides' Temperature of Transition (SCEPTTr).

Cross-Linked Self-Assembling Peptides and Their Post-Assembly Functionalization via One-Pot and In Situ Gelation System

Supramolecular nanostructures formed through peptide self-assembly can have a wide range of applications in the biomedical landscape. However, they often lose biomechanical properties at low mechanical stress due to the non-covalent interactions working in the self-assembling process. Herein, we report the design of cross-linked self-assembling peptide hydrogels using a one-pot in situ gelation system, based on 1-ethyl-3-[3-dimethylaminopropyl] carbodiimide/N-hydroxysulfosuccinimide (EDC/sulfo–NHS) coupling, to tune its biomechanics. EDC/sulfo–NHS coupling led to limited changes in storage modulus (from 0.9 to 2 kPa), but it significantly increased both the strain (from 6% to 60%) and failure stress (from 19 to 35 Pa) of peptide hydrogel without impairing the spontaneous formation of β-sheet-containing nano-filaments. Furthermore, EDC/sulfo–NHS cross-linking bestowed self-healing and thixotropic properties to the peptide hydrogel. Lastly, we demonstrated that this strategy can be used to incorporate bioactive functional motifs after self-assembly on pre-formed nanostructures by functionalizing an Ac-LDLKLDLKLDLK-CONH₂ (LDLK12) self-assembling peptide with the phage display-derived KLPGWSG peptide involved in the modulation of neural stem cell proliferation and differentiation. The incorporation of a functional motif did not alter the peptide’s secondary structure and its mechanical properties. The work reported here offers new tools to both fine tune the mechanical properties of and tailor the biomimetic properties of self-assembling peptide hydrogels while retaining their nanostructures, which is useful for tissue engineering and regenerative medicine applications.

Chain alignment of collagen I deciphered using computationally designed heterotrimers

The most abundant member of the collagen protein family, collagen I (COL1), is composed of two similar (chain A) and one unique (chain B) polypeptides that self-assemble with one amino acid offset into a heterotrimeric triple helix. Given the offset, chain B can occupy either the leading (BAA), middle (ABA) or trailing (AAB) position of the triple helix, yielding three isomeric biomacromolecules with different protein recognition properties. Despite five decades of intensive research, there is no consensus on the position of chain B in COL1. Here, three triple-helical heterotrimers that each contain a putative Von Willebrand Factor (VWF) and discoidin domain receptor (DDR) recognition sequence from COL1 were designed with chain B permutated in all three positions. AAB demonstrated a strong preference for both VWF and DDR and also induced higher levels of cellular DDR phosphorylation. Thus, we resolve this long-standing mystery and show that COL1 adopts an AAB register.

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.