In vitro synthesis of native, fibrous long spacing and segmental long spacing collagen

Assessing Collagen D-Band Periodicity with Atomic Force Microscopy

The collagen superfamily includes more than fifty collagen and/or collagen-like proteins with fibril-forming collagen type I being the most abundant protein within the extracellular matrix. Collagen type I plays a crucial role in a variety of functions, it has been associated with many pathological conditions and it is widely used due to its unique properties. One unique nano-scale characteristic of natural occurring collagen type I fibers is the so-called D-band periodicity, which has been associated with collagen natural structure and properties, while it seems to play a crucial role in the interactions between cells and collagen and in various pathological conditions. An accurate characterization of the surface and structure of collagen fibers, including D-band periodicity, on collagen-based tissues and/or (nano-)biomaterials can be achieved by Atomic Force Microscopy (AFM). AFM is a scanning probe microscope and is among the few techniques that can assess D-band periodicity. This review covers issues related to collagen and collagen D-band periodicity and the use of AFM for studying them. Through a systematic search in databases (PubMed and Scopus) relevant articles were identified. The study of these articles demonstrated that AFM can offer novel information concerning D-band periodicity. This study highlights the importance of studying collagen D-band periodicity and proves that AFM is a powerful tool for investigating a number of different properties related to collagen D-band periodicity.

Structural and Biochemical Changes in Pericardium upon Genipin Cross-Linking Investigated Using Nondestructive and Label-Free Imaging Techniques

Tissue cross-linking represents an important and often used technique to enhance the mechanical properties of biomaterials. For the first time, we investigated biochemical and structural properties of genipin (GE) cross-linked equine pericardium (EP) using optical imaging techniques in tandem with quantitative atomic force microscopy (AFM). EP was cross-linked with GE at 37 °C, and its biochemical and biomechanical properties were observed at various time points up to 24 h. GE cross-linked EP was monitored by the normalized ratio between its second-harmonic generation (SHG) and two-photon autofluorescence emissions and remained unchanged for untreated EP; however, a decreasing ratio due to depleted SHG and elevated autofluorescence and a fluorescence band at 625 nm were found for GE cross-linked EP. The mean autofluorescence lifetime of GE cross-linked EP also decreased. The biochemical signature of GE cross-linker and shift in collagen bands were detected and quantified using shifted excitation Raman difference spectroscopy as an innovative approach for tackling artifacts with high fluorescence backgrounds. AFM images indicated a higher and increasing Young's modulus correlated with cross-linking, as well as collagen structural changes in GE cross-linked EP, qualitatively explaining the observed decrease in the second-harmonic signal. In conclusion, we obtained detailed information about the biochemical, structural, and biomechanical effects of GE cross-linked EP using a unique combination of optical and force microscopy techniques in a nondestructive and label-free manner.

Surface-Directed Mineralization of Fibrous Collagen Scaffolds in Simulated Body Fluid for Tissue Engineering Applications

The use of polymer additives that stabilize fluidic amorphous calcium phosphate is key to obtaining intrafibrillar mineralization of collagen in vitro. On the other hand, this biomimetic approach inhibits the nucleation of mineral crystals in unconfined extrafibrillar spaces, that is, extrafibrillar mineralization. The extrafibrillar mineral content is a significant feature to replicate from hard connective tissues such as bone and dentin as it contributes to the final microarchitecture and mechanical stiffness of the biomineral composite. Herein, we report a straightforward route to produce densely mineralized collagenous composites via a surface-directed process devoid of the aid of polymer additives. Simulated body fluid (1×) is employed as a biomimetic crystallizing medium, following a preloading procedure on the collagen surface to quickly generate the amorphous precursor species required to initiate matrix mineralization. This approach consistently leads to the formation of extrafibrillar bioactive minerals in bulk collagen scaffolds, which may offer an advantage in the production of osteoconductive collagen-apatite materials for tissue engineering and repair purposes.

Intact Telopeptides Enhance Interactions between Collagens

Collagen is the fundamental structural component of a wide range of connective tissues and of the extracellular matrix. It undergoes self-assembly from individual triple-helical proteins into well-ordered fibrils, a process that is key to tissue development and homeostasis, and to processes such as wound healing. Nucleation of this assembly is known to be slowed considerably by pepsin removal of short nonhelical regions that flank collagen's triple helix, known as telopeptides. Using optical tweezers to perform microrheology measurements, we explored the changes in viscoelasticity of solutions of collagen with and without intact telopeptides. Our experiments reveal that intact telopeptides contribute a significant frequency-dependent enhancement of the complex shear modulus. An analytical model of polymers associating to establish chemical equilibrium among higher-order species shows trends in G' and G″ consistent with our experimental observations, including a concentration-dependent crossover in G″/c around 300 Hz. This work suggests that telopeptides facilitate transient intermolecular interactions between collagen proteins, even in the acidic conditions used here.

In vitro fibrillogenesis of tropocollagen type III in collagen type I affects its relative fibrillar topology and mechanics

Tropocollagen types I and III were simultaneously fibrilized in vitro, and the differences between the geometric and mechanical properties of the heterotypic fibrils with different mixing ratios of tropocollagen III to I were investigated. Transmission electron microscopy was used to confirm the simultaneous presence of both tropocollagen types within the heterotypic fibrils. The incorporation of collagen III in I caused the fibrils to be thinner with a shorter D-banding than pure collagen I. Hertzian contact model was used to obtain the elastic moduli from atomic force microscope indentation testing using a force volume analysis. The results indicated that an increase in the percentage of tropocollagen III reduced the mechanical stiffness of the obtained fibrils. The mechanical stiffness of the collagen fibrils was found to be greater at higher loading frequencies. This observation might explain the dominance of collagen III over I in soft distensible organs such as human vocal folds.

Visualizing In Vitro Type I Collagen Fibrillogenesis by Transmission Electron Microscopy

Techniques and protocols for the in vitro formation of collagen type I fibrils and the extensive biochemical variation of the fibrillogenesis conditions are presented. In all cases, the incubation and fibrillogenesis product can be readily monitored by transmission electron microscopic study of negatively stained specimens. Representative TEM data is presented and discussed within the context of the products of the fibrillogenesis protocols, from which the extensive biochemical and structural possibilities of this integrated approach can be appreciated.