Colorimetric and fluorometric monitoring of the helix composition of collagen-like peptides at the nM level

A Single-Chain Peptide Probe Targeting Pathological Collagen for Precise Staging of Hepatic Fibrosis by MR Imaging

Hepatic fibrosis, a chronic liver response to injury with potential severe outcomes like cirrhosis and liver cancer, necessitates urgent noninvasive diagnostic techniques to halt disease progression. We herein for the first time developed a single-chain peptide probe targeting pathological collagen for in vivo magnetic resonance imaging (MRI) of hepatic fibrosis. The novel (GhypO)10 probe, distinguished by its unique monomeric conformation achieved through Pro to (2S,4S)-hydroxyproline (hyp) substitution and subsequent disruption of hydrogen bonding, exhibits selectivity for pathological collagen over its intact counterpart in connective tissues. Fluorescence imaging of liver specimens from fibrotic models displayed a discernible relationship between pathological collagen levels and fibrosis stage. Moreover, T1-weighted MR images post Gd-GhypO administration revealed progressive signal enhancement congruent with fibrosis severity, corroborated by a corresponding increase in the contrast-to-noise ratio (ΔCNR). Biodistribution analysis indicates that Gd-GhypO has low Gd retention in the main organs 24 h postinjection, ensuring the probe's safety for molecular imaging. The Gd-GhypO probe therefore emerges as a potent tool for the precise, noninvasive delineation of hepatic fibrosis stages, offering significant implications for the diagnosis and management of liver fibrosis.

Analysis of Homo- and Heterotriple Helix Formation of Collagen Model Peptides and Evaluation of Their Stability in a Biological Environment

Self-assembled materials have attracted attention and have been extensively studied because the reversibility of noncovalent interactions allows them to possess various properties, such as stimulus responsiveness and self-healing. Collagen model peptides have an amino acid sequence characteristic of the triple helix region of collagen and exhibit repeatable triple helix formation. Many studies of their applications have used homotrimers, and although some studies on heterotrimers have been reported, few have clarified the details. If the characteristics of heterotrimers can be revealed, they are expected to be applied as new self-assembled materials. In this study, we analyzed the detailed self-assembling properties of hetero- and homohelices formed by (proline-proline-glycine)10 (PPG)10 and (proline-hydroxyproline-glycine)10 (POG)10 to evaluate the potential of the helices for biomedical application. Fluorescein isothiocyanate-labeled (PPG)10 (F(PPG)10) and (POG)10 (F(POG)10) were synthesized to analyze the heterotriple helix formation using concentration quenching based on triple helix formation. When (PPG)10 was added to F(POG)10, the fluorescence intensity did not reach a plateau, while the fluorescence intensity reached about 100% in the other pairs such as (POG)10-F(POG)10, (PPG)10-F(PPG)10, and (POG)10-F(PPG)10. The critical triple helix formation concentration was 7 μM for the heterotrimer prepared under 1:2 mixing conditions of (PPG)10 and (POG)10, 320 μM for [(PPG)10]3, and 4 μM for [(POG)10]3, indicating that the triple helix formation concentration of the heterotrimer is almost half that of [(POG)10]3 but 45 times higher than [(PPG)10]3. Furthermore, the heterotrimer formed at 37 °C was stable after 5 days, which was the same as [(POG)10]3. These results suggest that heterotrimers have different association properties from homotrimers and are expected to be applied in nanotechnology and biomaterials as new self-assembled materials.

Association between N-Terminal Pyrenes Stabilizes the Collagen Triple Helix

Collagen model peptides featuring the fluorophore pyrene at their N-termini have been synthesized, and their thermal denaturation has been examined using circular dichroism (CD) and fluorescence spectroscopies. Flanking the (Pro-Hyp-Gly)7 core of the peptide monomers at positions 1 and/or 23 in the primary sequence, Lys residues were introduced to ensure water solubility. Triple helices derived from such peptides show a broad excimer emission at ∼480 nm, indicative of interaction between the pyrene units. CD experiments show that the fluorophores enhance helix stability primarily through entropic effects. Unfolding temperatures (Tm) increase by up to 7 °C for systems with N-terminal lysine residues and by up to 21 °C for systems in which the first-position Lys is replaced by Ala. Tm values derived from fluorescence measurements (at 50 μM) typically lie within ∼1 °C of those obtained using CD (at 200 μM). Computational modeling in a water continuum using B3LYP-GD3 and M06-2X functionals predicts that face-to-face association of fluorophores can occur while H-bonding within the [(POG)n]3 assembly is retained. Such parallel stacking is consistent with hydrophobically driven stabilization. Labeling collagen peptides with pyrene is a synthetically simple way to promote triple helicity while providing a means to obtain Tm data on relatively dilute samples.

Peptide-triggered self-assembly of collagen mimetic peptides into nanospheres by electrostatic interaction and π-π stacking

Collagen is the most abundant protein in various connective tissues, providing mechanical integrity as well as regulating cellular activities. Self-assembled peptides have been extensively explored to develop collagen mimetic materials, due to their attractive features such as easy synthesis, selective sequences and low immunogenicity. Metal ion-triggered self-assembly of collagen mimetic peptides has recently received increasing interests, since the addition of external stimuli offers programmable control of the self-assembly process. We have for the first time reported a peptide-stimulated self-assembly of collagen mimetic peptides into nanospheres by electrostatic interaction and π-π stacking. We have accidentally discovered that FAM-modified positively-charged triple helical peptide FAM-PRG was highly soluble, while the addition of a single-stranded negatively-charged peptide EOG-10 efficiently drove its self-assembly into well-ordered spherical nanomaterials. Peptide EOG-10 has been shown to mediate similar self-assembly of TPE-modified triple-helical peptide TPE-PRG into luminescent exquisite nanospheres, consistently demonstrating the robustness of this peptide-triggered strategy. Fluorescence monitoring of the interaction of EOG-10 and TPE-PRG at different ratios indicated that EOG-10 specifically binds to TPE-PRG to form a 3 : 1 complex. High salt concentration was shown to inhibit the self-assembly of TPE-PRG with EOG-10, suggesting that their self-assembly was controlled by electrostatic interaction. The self-assembly of TPE-PRG with EOG-10 has been further revealed to require the exact lengths of both peptides as well as complementary sequences without mutations, indicating a pairwise "side-by-side" binding mode. Notably, the identity of the N-terminal residues of X-PRG has been found to play a determinant role in the self-assembly, while non-aromatic residues lost the self-assembling capability, suggesting that π-π stacking and electrostatic interactions collectively modulate the self-assembly of X-PRG and EOG-10. To conclude, we have developed a highly biocompatible and programmably controlled peptide-triggered self-assembly approach to create novel collagen mimetic nanomaterials, which may have great potential in advanced functional materials.

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.

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.

Highly specific recognition of denatured collagen by fluorescent peptide probes with the repetitive Gly-Pro-Pro and Gly-Hyp-Hyp sequences

Denatured collagen is a key biomarker for various critical diseases such as cancer. Peptide probes with the repetitive (Gly-Pro-Hyp)n sequences have recently been found to selectively target denatured collagen; however, thermal or UV pretreatment is required to drive the peptides into the monomer conformation, which poses a substantial challenge for clinical applications. We herein construct two peptide probes, FAM-GOO and FAM-GPP, consisting of the repetitive (Gly-Hyp-Hyp)8 and (Gly-Pro-Pro)8 sequences, respectively. The CD, fluorescence and colorimetric studies have consistently revealed that FAM-GOO showed strong capability of forming the triple helical structure, while FAM-GPP pronouncedly displayed the single stranded conformation at temperatures as low as 4 °C. The binding experiments have indicated that both peptide probes could recognize denatured collagen with high specificity, and FAM-GPP remarkably did not need the preheating treatment. The tissue staining results have shown that preheated FAM-GOO and unheated FAM-GPP could target denatured collagen in a wide variety of rat frozen and human FFPE tissue sections. Compared with antibodies specific for a certain type of collagen, both FAM-GOO and FAM-GPP act as broad-spectrum probes for the selective detection of denatured collagen of different types and from different species. Importantly, FAM-GPP possessed the unique capability of maintaining the monomer conformation by itself, thus avoiding the potential risks of the thermal or UV pretreatment. This novel peptide probe provides a handy and versatile biosensor for specifically targeting denatured collagen, which has attractive potential in the diagnosis and therapeutics of collagen-involved diseases.

Detection of target collagen peptides with single amino acid mutation using two fluorescent peptide probes

Collagen with a single amino acid substitution is the main cause of a plethora of heritable disorders such as Osteogenesis Imperfecta and Ehlers-Danlos syndrome. Though significant advances have been achieved in the development of protein assays, it remains very challenging to distinguish a protein with a single amino acid mutation from the wild-type protein. A novel fluorescent self-quenching assay has been constructed to detect target collagen peptides with a single amino acid mutation using two probe peptides. The hybridization of the probe peptide and the natural target collagen peptide results in a complete heterotrimer and strong fluorescence, whereas the mixture of the probe peptide and the mutation collagen sequences leads to a partial homotrimer and pronounced fluorescence self-quenching. The extent of fluorescence quenching is dependent on the identity of the residue replacing Gly following the order of Ala < Ser < Arg, while the Gly-Ala mutation causes the mildest fluorescence loss. The probe peptide-based fluorescence self-quenching assay facilitates specific detection of the target collagen sequence with a single Gly mutation at the nM level. The simultaneous utilization of both probe peptides enables efficient discrimination between different mutation peptides. To our knowledge, our work may be the first report of a robust analytical assay that can identify collagen fragments with single amino acid mutation, which will greatly contribute to deciphering the molecular mechanism of Osteogenesis Imperfecta as well as developing novel diagnostic strategies.

A single stranded fluorescent peptide probe for targeting collagen in connective tissues

We herein report the construction of a novel single stranded fluorescent collagen mimetic peptide by introducing a bulky FAM dye in the central region rather than the N terminus. Without the need for any prior thermal or ultraviolet treatment, the peptide probe can be conveniently applied to specifically target collagen in connective tissues for fluorescence imaging.