Conformation-dependent side reactions in interstrand-disulfide bridging of trimeric collagenous peptides by regioselective cysteine chemistry

Hollow Octadecameric Self-Assembly of Collagen-like Peptides

The folding of collagen is a hierarchical process that starts with three peptides associating into the characteristic triple helical fold. Depending on the specific collagen in question, these triple helices then assemble into bundles reminiscent of α-helical coiled-coils. Unlike α-helices, however, the bundling of collagen triple helices is very poorly understood with almost no direct experimental data available. In order to shed light on this critical step of collagen hierarchical assembly, we have examined the collagenous region of complement component 1q. Thirteen synthetic peptides were prepared to dissect the critical regions allowing for its octadecameric self-assembly. We find that short peptides (under 40 amino acids) are able to self-assemble into specific (ABC)6 octadecamers. This requires the ABC heterotrimeric composition as the self-assembly subunit, but does not require disulfide bonds. Self-assembly into this octadecamer is aided by short noncollagenous sequences at the N-terminus, although they are not entirely required. The mechanism of self-assembly appears to begin with the very slow formation of the ABC heterotrimeric helix, followed by rapid bundling of triple helices into progressively larger oligomers, terminating in the formation of the (ABC)6 octadecamer. Cryo-electron microscopy reveals the (ABC)6 assembly as a remarkable, hollow, crown-like structure with an open channel approximately 18 Å at the narrow end and 30 Å at the wide end. This work helps to illuminate the structure and assembly mechanism of a critical protein in the innate immune system and lays the groundwork for the de novo design of higher order collagen mimetic peptide assemblies.

A collagen-mimetic triple helical supramolecule that evokes integrin-dependent cell responses

Collagen is an abundantly distributed extracellular matrix protein in mammalian bodies that maintains structural integrity of the organs and tissues. Besides its function as a structural protein, collagen has various biological functions which regulate cell adhesion, migration and differentiation. In order to develop totally synthetic collagen-surrogates, we recently reported a basic concept for preparing collagen-like triple helical supramolecules based on the self-assembly of staggered trimeric peptides with self-complementary shapes. In this paper, we add one of the specific cellular functions of the native collagen to the collagen-mimetic supramolecule. We synthesized a self-assembling peptide unit containing the integrin-binding sequence Gly-Phe-Hyp-Gly-Glu-Arg. The supramolecule carrying the sequence exhibited significant binding activity to human dermal fibroblasts. The supramolecular structure was found to be essential for function in in vitro cell culture. Cell adhesion was shown to be comparable to that of native collagen, and was further demonstrated to be mediated solely by integrin alpha 2 beta 1. Well-grown focal contacts and stress fibers were observed in cells spread on the supramolecular collagen-mimetic. The results demonstrate the potential of peptide-based artificial collagen as a biomaterial for regulating specific cellular function and fate.

Artificial collagen gels via self-assembly of de novo designed peptides

Development of artificial collagens to replace the animal-derived collagens presents a challenge in the formation of safer and functional biomaterials. We report here the development of collagen-like gels by means of the self-assembly of chemically synthesized peptides. The peptides are disulfide-linked trimers of collagenous Gly-X-Y triplet repeats with self-complementary shapes. Upon cooling the peptide solutions, hydrogels of peptide supramolecules are formed by spontaneous intermolecular triple helix formation. The thermal gel-sol transition appeared to be reversible, and the transition temperatures were found to be tunable by the design of the peptides. Our systems for the formation of artificial collagen-like gels will offer possibilities for novel types of biomaterials.

Natural and artificial cystine knots for assembly of homo- and heterotrimeric collagen models

Native collagens are molecules that are difficult to handle because of their high tendency towards aggregation and denaturation. It was discovered early on that synthetic collagenous peptides are more amenable to conformational characterization and thus can serve as useful models for structural and functional studies. Single-stranded collagenous peptides of high propensity to self-associate into triple-helical trimers were used for this purpose as well as interchain-crosslinked homotrimers assembled on synthetic scaffolds. With the growing knowledge of the biosynthetic pathways of natural collagens and the importance of their interchain disulfide crosslinks, which stabilize the triple-helical structure, native as well as de novo designed cystine knots have gained increasing attention in the assembly of triple-stranded collagen peptides. In addition, natural sequences of collagens were incorporated in order to biophysically characterize their functional epitopes. This review is focused on the methods developed over the years, and future perspectives for the production of collagen-mimicking synthetic and recombinant triple-helical homo- and heterotrimers.

Synthesis of single- and multiple-stranded cystine-rich peptides

The large abundance of bioactive single- and multiple-stranded cystine-rich peptides in nature has fostered the development of orthogonal thiol-protection schemes and of efficient chemistries for regioselective disulfide formation in synthetic replica for decades. In parallel to these entirely synthetic strategies, an increased knowledge of oxidative refolding mechanisms of proteins has been accumulated, and the collective experience with air oxidation of cysteine-rich peptides into their native disulfide frameworks have largely confirmed Anfinsen's principle of the self-assembly of polypeptide chains. In fact, a continuously growing number of cysteine-rich bioactive peptides from the most diverse sources and with differing cysteine patterns were found to retain the critical sequence-encoded structural information for correct oxidative folding into the native structures as dominant isomers, although in the biosynthetic pathways the mature peptide forms are mostly generated by posttranslational processing of folded precursors. Such self-assembly processes can be optimized by opportune manipulation of the experimental conditions or by induction of productive intermediates. But there are also numerous cases where folding and disulfide formation are thermodynamically not coupled and where the application of a defined succession of regioselective cysteine pairings still represents the method of choice to install the desired native or non-native cystine frameworks. Among our contributions to the state of the art in the synthesis of cystine-rich peptides, we have mainly addressed the induction of correct oxidative refolding of single-stranded cysteine-rich peptides into their native structures by the use of selenocysteine and suitable strategies for disulfide-mediated assembly of monomers into defined oligomers as mimics of homo- and heterotrimeric collagens as a synthetic approach for the development of new biomaterials.

Synthetic heterotrimeric collagen peptides as mimics of cell adhesion sites of the basement membrane

Collagen type IV forms a network in the basement membrane into which other constituents of the tissue are incorporated. It also provides cell-adhesion sites that are specifically recognized by cell-surface receptors, i.e., the integrins. Different from the ubiquitous sequential RGD adhesion motif found in most of the matrix proteins, in collagen type IV, the responsible binding sites for alpha1beta1 integrin have been identified as Asp461 of the two alpha1 chains and Arg461 of the alpha2 chain. Because of the heterotrimeric character of this collagen, the spatial geometry of the binding epitope depends not only on the triple-helical fold, but decisively even on the stagger of the chains. To investigate the effects of chain registration on the conformational properties and binding affinities of this adhesion epitope, two synthetic heterotrimeric collagen peptides consisting of the identical three chains were assembled by an artificial cystine knot in two different registers, i.e., in the most plausible alpha2alpha1alpha1' and less probable alpha1alpha2alpha1' chain alignment. A detailed conformational characterization of both trimers allowed to correlate their different binding affinities for alpha1beta1 integrin with the degree of local plasticity of the two different triple helices. Optimal local breathing of the rod-shaped collagens is apparently crucial for selective recognition by proteins interacting with these main components of the extracellular matrix.

Synthesis of heterotrimeric collagen models containing Arg residues in Y-positions and analysis of their conformational stability

An Arg residue incorporated into the Y-position of collagenous host-guest peptide Ac-(Gly-Pro-Hyp)(3)-Gly-Pro-Y-(Gly-Pro-Hyp)(4)-Gly-Gly-NH(2) is reported to stabilize the triple helical structure as well as a 4(R)-hydroxyproline (Hyp) residue. Here, we synthesized heterotrimeric collagen models containing Arg in Y-positions utilizing the cystine knot strategy. Analysis of their thermal transition temperatures using circular dichroism spectrometry demonstrated unexpected decrease in the triple helical stability as the number of Arg increased. The obtained results indicated that an Arg residue in a Y-position is not always an equivalent of a Hyp residue, and that it possesses a potential helix destabilizing effect.

The chain register in heterotrimeric collagen peptides affects triple helix stability and folding kinetics

Collagen type IV is a highly specialized form of collagen found only in basement membranes, where it provides mechanical stability and structural integrity to tissues and organs, and binding sites for cell adhesion. In its ubiquitous form, collagen type IV consists of two alpha1 chains and one alpha2 chain, whose internal alignment within the triple helix seems to exert a strong influence on the binding affinity to alpha1beta1 integrin receptor. This has been assessed recently using two synthetic collagen peptides that contain the cell adhesion epitope of collagen type IV and are assembled into the most plausible alpha1alpha2alpha1' and alpha2alpha1alpha1' registers. In the present study, the effects of the chain register on the stability of the triple helix and the folding kinetics of these collagen peptides were investigated by CD spectroscopy and microcalorimetry. The results revealed a multi-domain structural organization for both trimers, with an unexpected strong effect of the chain alignment on the conformational stability. Molecular dynamics simulations served to rationalize more properly the experimental results.