Collagen fibrillogenesis in vitro: evidence for pre-nucleation and nucleation steps

Self-Assembly Behavior of Collagen and Its Composite Materials: Preparation, Characterizations, and Biomedical Engineering and Allied Applications

Collagen is the oldest and most abundant extracellular matrix protein and has many applications in biomedical, food, cosmetic, and other industries. Previous reviews have already introduced collagen's sources, structures, and biosynthesis. The biological and mechanical properties of collagen-based composite materials, their modification and application forms, and their interactions with host tissues are pinpointed. It is worth noting that self-assembly behavior is the main characteristic of collagen molecules. However, there is currently relatively little review on collagen-based composite materials based on self-assembly. Herein, we briefly reviewed the biosynthesis, extraction, structure, and properties of collagen, systematically presented an overview of the various factors and corresponding characterization techniques that affect the collagen self-assembly process, and summarize and discuss the preparation methods and application progress of collagen-based composite materials in different fields. By combining the self-assembly behavior of collagen with preparation methods of collagen-based composite materials, collagen-based composite materials with various functional reactions can be selectively prepared, and these experiences and outcomes can provide inspiration and practical techniques for the future development directions and challenges of collagen-based composite biomaterials in related applications fields.

A Bottom-Up Approach Grafts Collagen Fibrils Perpendicularly to Titanium Surfaces

Currently, titanium dental implant apposition to bone is achieved via osseointegration leading to ankylosis. A biomimetic Sharpey's fiber-type interface could be constructed around collagen fibrils robustly attached and projecting perpendicularly from the titanium surface. We present a proof-of-concept for a method to create upright-standing collagen nanofibrils covalently bonded to a titanium surface. The method involves activation of the titanium surface using a plasma discharge treatment followed by functionalization with an oxyamine-terminated silane coupling molecule. Using Rapoport's salt, the N-termini of individual type I collagen monomers are converted to ketones. When presented to the functionalized titanium surface, these ketones form oxime linkages with the silanes thus immobilizing the collagen. In a two-step process, these covalently bonded monomers act as sites for the formation of fibrils. Many fibril-surface junctions were observed by scanning electron microscopy on three different surfaces. These findings set the stage for working toward a high surface density of such features which might act as a platform from which to build a synthetic ligament.

Cell-instructive starPEG-heparin-collagen composite matrices

Polymer hydrogels can be readily modulated with regard to their physical properties and functionalized to recapitulate molecular cues of the extracellular matrix (ECM). However, they remain structurally different from the hierarchical supramolecular assemblies of natural ECM. Accordingly, we herein report a set of hydrogel composite materials made from starPEG-peptide conjugates, maleimide-functionalized heparin and collagen type I that combine semisynthetic and ECM-derived components. Collagen fibrillogenesis was controlled by temperature and collagen concentration to form collagen microstructures which were then homogeneously distributed within the 3D composite matrix during hydrogel formation. The collagen-laden hydrogel materials showed a heterogeneous local variation of the stiffness and adhesion ligand density. Composite gels functionalized with growth factors and cell adhesive peptides (RGDSP) supported the growth of embedded human umbilical cord vein endothelial cells (HUVECs) and induced the alignment of embedded bone marrow-derived human mesenchymal stem cells (MSCs) to the collagen microstructures in vitro. The introduced composite hydrogel material is concluded to faithfully mimic cell-instructive features of the ECM.

STATEMENT OF SIGNIFICANCE

Cell-instructive materials play an important role in the generation of both regenerative therapies and advanced tissue and disease models. For that purpose, biofunctional polymer hydrogels recapitulating molecular cues of the extracellular matrix (ECM) were successfully applied in various different studies. However, hydrogels generally lack the hierarchical supramolecular structure of natural ECM. We have therefore developed a hydrogel composite material made from starPEG-peptide conjugates, maleimide-functionalized heparin and collagen type I fibrils. The collagen-laden scaffolds showed a heterogeneous local variation in the stiffness of the material. The composite gels were successfully tested in culture experiments with human umbilical cord vein endothelial cells and bone marrow-derived human mesenchymal stem cells. It was concluded that the composite scaffold was able to faithfully mimic important cell-instructive features of the ECM.

Tuning the architecture of three-dimensional collagen hydrogels by physiological macromolecular crowding

Macromolecular crowding is an optimal physiological feature in intracellular and extracellular spaces, and results from a variety of macromolecules occupying space and contributing to a fractional volume occupancy. Here, we show that soft collagen hydrogels assembled in nature-inspired crowded conditions feature enhanced biophysical properties. We demonstrate that crowding tunes the rate of collagen nucleation and fiber growth, affecting fiber diameter and organization. Adjustments of crowding levels during collagen assembly tune the gel pore size, protein permeability, transparency and resistance to enzymatic degradation. Furthermore, gels assembled in crowded conditions are twice as resistant to mechanical stress as the controls, inducing a 70% boost of proliferation of stem cells cultured on tuned hydrogels. Emulating the crowdedness of interstitial fluids therefore represents a way to optimize the properties of soft collagen gels, with promising applications in soft biomaterials design.

Effect of nonablative laser energy on the joint capsule: an in vivo rabbit study using a holmium:YAG laser

BACKGROUND AND OBJECTIVE

The nonablative application of holmium:yttrium-aluminum-garnet (Ho:YAG) laser energy to the joint capsule of patients with glenohumeral instability has been found to shrink capsular tissue and to help stabilize the joint. The purpose of this study was to evaluate the effect of nonablative laser energy on the short-term histological properties of joint capsular tissue in an in vivo rabbit model.

STUDY DESIGN/MATERIALS AND METHODS

Eighteen mature New Zealand white rabbits were used in this study. One randomly selected stifle was treated with laser energy, and the contralateral stifle was sham-operated. Animals were euthanized immediately after surgery (day 0), at 7 days postsurgery and 30 days postsurgery. Specimens were processed for histology and transmission electron microscopy.

RESULTS

Laser-treated samples at day 0 showed diffuse hyalinization of collagen with nuclear karyorrhexis of fibroblasts. Laser-treated tissue at 7 days postsurgery revealed fibroblast proliferation around and into acellular hyalinized regions of collagen. At 30 days postlaser treatment, areas of fused collagen were greatly reduced as large reactive fibroblasts migrated and secreted matrix.

CONCLUSION

This study illustrates the short-term in vivo tissue response to nonablative laser treatment, where acellular hyalinized regions of collagen are infiltrated by fibroblasts that have used the treated collagen as the framework for migration and secretion of new collagen matrix in order for tissue repair to proceed.

Viscosity and elasticity during collagen assembly in vitro: relevance to matrix-driven translocation

In order to better understand the gelation process associated with collagen assembly, and the mechanism of the in vitro morphogenetic phenomenon of "matrix-driven translocation" [S.A. Newman et al. (1985) Science, 228, 885-889], the viscosity and elastic modulus of assembling collagen matrices in the presence and absence of polystyrene latex beads was investigated. Viscosity measurements at very low shear rates (0.016-0.0549 s(-1)) were performed over a range of temperatures (6.9-11.5 degrees C) in a Couette viscometer. A magnetic levitation sphere rheometer was used to measure the shear elastic modulus of the assembling matrices during the late phase of the gelation process. Gelation was detected by the rapid increase in viscosity that occurred after a lag time tL that varied between O and approximately 500 s. After a rise in viscosity that occurred over an additional approximately 500 s, the collagen matrix was characterized by an elastic modulus of the order of several Pa. The lag time of the assembly process was relatively insensitive to differences in shear rate within the variability of the sample preparation, but was inversely proportional to the time the sample spent on ice before being raised to the test temperature, for test temperatures > 9 degrees C. This suggests that structures important for fibrillogenesis are capable of forming at 0 degrees C. The time dependence of the gelation process is well-described by an exponential law with a rate constant K approximately 0.1 s(-1). Significantly, K was consistently larger in collagen preparations that contained cell-sized polystyrene beads. From these results, along with prior information on effective surface tension differences of bead-containing and bead-lacking collagen matrices, we conclude that changes in matrix organization contributing to matrix-driven translocation are initiated during the lag phase of fibrillogenesis when the viscosity is < or = 0.1 Poise. The phenomenon may make use of small differentials in viscosity and/or elasticity, resulting from the interaction of the beads with the assembling matrix. These properties are well described by standard models of concentrated solutions.

The effect of nonablative laser energy on the ultrastructure of joint capsular collagen

This study was designed to evaluate the effect of laser energy at nonablative levels on the ultrastructure of joint capsular collagen. The femoropatellar joint capsules of six mature New Zealand white rabbits were harvested immediately after death. Specimens were divided into three treatment groups (5, 10, and 15 watts) and one control group. Laser energy was applied using a holmium: YAG laser. Transmission electron microscopy showed significant ultrastructural alterations in collagenous architecture for all laser treatment groups, with increased fibril cross-sectional diameter for each of the treated groups. The fibrils began to lose their distinct edges and their periodical cross-striations at subsequently higher energy densities. A morphometric analysis showed that each subsequently higher laser energy caused a significant increase in collagen fibril diameter. Ultrastructural alteration of collagen fibril architecture caused by the thermal effect of laser energy is probably the dominant mechanism of laser-induced tissue shrinkage.

In vitro formation and aggregation of heterotypic collagen I and III fibrils

In vitro fibrillogenesis of solutions containing pepsin digested and acid soluble collagens I and III from human and bovine skin were investigated by turbidity-time measurements, dark-field and electron microscopy. The maximum turbidity of these solutions exhibited inversely proportional dependence on the collagen III content. Self-assembly was accelerated by collagen III. As a measure of mass per unit length, the maximum turbidity shows a mean decrease of 88% when comparing the absorbance at 313 nm for 0% and 50% collagen III in a composite solution of acid extracted collagen. In contrast to these findings, the diameter of fibrils from acid extracted fetal calf skin with 50% collagen III, determined from electron micrographs, was only 23% smaller than for pure collagen I. Correspondence with investigations on in vitro fibrillogenesis with dark-field microscopy and electron microscopy, this phenomenon apparently derives from the bundling of fibrils. This may be interpreted to mean that bundling of fibrils is already suppressed at low collagen III concentrations. A comparison of acid and pepsin extracted fetal calf skin yielded similar behaviour of collagen I and III mixtures, even though the pepsin extract displayed a turbidity reduction that was about 25% less than the acid extract. For pepsin digested collagen from human and bovine skin, differences were found for maximum turbidity and the ability to form bundles decreasing with the biological age of the donor.

Collagenolytic enzymes assayed by spectrophotometry with suspensions of reconstituted collagen fibrils

Collagenolytic enzymes were quantitated by a method based on spectrophotometry of suspended reconstituted collagen fibrils. To obtain optically stable suspensions it was necessary to perform a short sonication of the aggregated fibrils at 10 degrees C. When fibrils were cleaved with mammalian fibroblast collagenase at 35 degrees C the triple helical collagen fragments (TCA and TCB) would uncoil spontaneously and the decreasing turbidity was used as an estimate of enzyme activity. The method is a specific collagenase assay since a possible cleavage in the non-helical parts of the collagen molecule with contaminating proteinases is without effect on the turbidity of the suspension and the collagen substrate is not converted to gelatin at 35 degrees C. After 1 h of incubation 0.2 U (equivalent to 0.2 micrograms) of fibroblast collagenase could be detected. In purification procedures with microbial collagenases many fractions were tested by overnight incubations in disposable cuvettes. Sealing of cuvettes with square silicone stoppers allowed rotation of enzyme-substrate mixtures directly in the cuvettes. Only standard laboratory equipment is required for this assay, which is not dependent on radiolabeling or preparation of specific immunologic reagents.

Effect of proteoglycans on type I collagen fibre formation

Collagen fibrillogenesis is a multistep process involving assembly of molecules into fibrils and bundles of fibrils. The exact role of proteoglycans in collagen fibrillogenesis is unclear. The purpose of these studies is to study the effect of proteoglycans on collagen fibrillogenesis in vitro. Results of these studies suggest that the proteoglycans dermatan sulphate and chondroitin sulphate do not change the final turbidity and hence the diameter of fibrils formed during the early stages of fibrillogenesis. This suggests that proteoglycans may not influence the early phases of collagen assembly, such as nucleation. However, proteoglycans added during the final stages of collagen fibre formation in vitro cause changes in ultimate tensile strength. In the presence of the high-molecular-weight proteoglycan, the ultimate tensile strength is increased by a factor of 1.5 above that of the control, whilst in the presence of low-molecular weight chondroitin sulphate proteoglycan the tensile strength is significantly decreased. It is concluded that proteoglycans influence the later stages of fibre formation. The presence of high-molecular-weight chondroitin sulphate proteoglycan leads to efficient stress transfer between collagen fibrils, altering the ultimate tensile strength. The results of these studies will be useful in optimizing the design of collagen tendon-ligament prostheses.