Characterization of fibrous forms of collagen

Chitosan, Gelatin, and Collagen Hydrogels for Bone Regeneration

Hydrogels are versatile biomaterials characterized by three-dimensional, cross-linked, highly hydrated polymeric networks. These polymers exhibit a great variety of biochemical and biophysical properties, which allow for the diffusion of diverse molecules, such as drugs, active ingredients, growth factors, and nanoparticles. Meanwhile, these polymers can control chemical and molecular interactions at the cellular level. The polymeric network can be molded into different structures, imitating the structural characteristics of surrounding tissues and bone defects. Interestingly, the application of hydrogels in bone tissue engineering (BTE) has been gathering significant attention due to the beneficial bone improvement results that have been achieved. Moreover, essential clinical and osteoblastic fate-controlling advances have been achieved with the use of synthetic polymers in the production of hydrogels. However, current trends look towards fabricating hydrogels from biological precursors, such as biopolymers, due to the high biocompatibility, degradability, and mechanical control that can be regulated. Therefore, this review analyzes the concept of hydrogels and the characteristics of chitosan, collagen, and gelatin as excellent candidates for fabricating BTE scaffolds. The changes and opportunities brought on by these biopolymers in bone regeneration are discussed, considering the integration, synergy, and biocompatibility features.

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Surface modification of hydroxyapatite nanoparticles for bone regeneration by controlling their surface hydration and protein adsorption states

Autogenous bone and metallic implant grafting has been used to repair and regenerate bone defects. However, there are still many unresolved problems. It is suggested that bioceramic nanoparticles should be developed and designed to promote effective bone regeneration. In addition, it is necessary to synthesize bioceramic nanoparticles that can support proteins related to bone repair and regeneration such as collagen and albumin. As the protein-interactive bioceramic, hydroxyapatite (HA) deserves to be mentioned and has several attractive properties that are useful in biomedical fields (e.g., biocompatibility, protein adsorption capacity and stability in the physiological environment). In order to prepare novel HA nanoparticles with high biocompatibility, it can be considered that human bones are mainly composed of HA and contain a small amount of silicate, and therefore, the design of coexistence of HA with silicate can be focused. Moreover, it is proposed that the state of the hydration layer on the nanoparticle surfaces can be controlled by introducing heteroelements and polymer chains, which have a great influence on the subsequent protein adsorption and cell adhesion. In this perspective, in order to develop novel bioceramic nanoparticles for the treatment of bone defect, the design of highly biocompatible HA nanoparticles and the control of the hydration layer and protein adsorption states on the surfaces were systematically discussed based on their surface modification techniques, which are very important for the proper understanding of the interface between cells and bioceramics, leading to the further application in biomedical fields.

Collagen fibril formation in vitro: From origin to opportunities

Sometimes, to move forward, it is necessary to look back. Collagen type I is one of the most commonly used biomaterials in tissue engineering and regenerative medicine. There are a variety of collagen scaffolds and biomedical products based on collagen have been made, and the development of new ones is still ongoing. Materials, where collagen is in the fibrillar form, have some advantages: they have superior mechanical properties, higher degradation time and, what is most important, mimic the structure of the native extracellular matrix. There are some standard protocols for the formation of collagen fibrils in vitro, but if we look more carefully at those methods, we can see some controversies. For example, why is the formation of collagen gel commonly carried out at 37 ​°C, when it was well investigated that the temperature higher than 35 ​°C results in a formation of not well-ordered fibrils? Biomimetic collagen materials can be obtained both using culture medium or neutralizing solution, but it requires a deep understanding of all of the crucial points. One of this point is collagen extraction method, since not every method retains the ability of collagen to reconstitute native banded fibrils. Collagen polymorphism is also often overlooked in spite of the appearance of different polymorphic forms during fibril formation is possible, especially when collagen blends are utilized. In this review, we will not only pay attention to these issues, but we will overview the most prominent works related to the formation of collagen fibrils in vitro starting from the first approaches and moving to the up-to-date recipes.

A collagen-based hydrogel containing tacrolimus for bone tissue engineering

Bone tissue engineering aims to develop bone graft structure that can heal bone defects without using autografts or allografts. The current study was conducted to promote bone regeneration using a collagen type I hydrogel containing tacrolimus. For this purpose, different amounts of tacrolimus (10 μg/ml, 100 μg/ml, and 1000 μg/ml) were loaded into the hydrogel. The resulting drug-loaded hydrogels were characterized for their porosity, swelling capacity, weight loss, drug release, blood compatibility, and cell proliferation (MTT). For functional analysis, the developed hydrogel surrounded by a film made of gelatin and polycaprolactone (PCL) was administrated in the calvarias defect of Wistar rats. The results indicated that the hydrogel has a porosity of 89.2 ± 12.5% and an appropriate swelling, drug release, and blood compatibility behavior. The in vitro results indicated that the collagen hydrogel containing 1000 μg tacrolimus was adequate in terms of cell proliferation. Finally, in vivo studies provided some evidence of the potential of the developed hydrogel for bone healing.

Development prospects of curable osteoplastic materials in dentistry and maxillofacial surgery

The article presents classification of the thermosetting materials for bone augmentation. The physical, mechanical, biological, and clinical properties of such materials are reviewed. There are two main types of curable osteoplastic materials: bone cements and hydrogels. Compared to hydrogels, bone cements have high strength features, but their biological properties are not ideal and must be improved. Hydrogels are biocompatible and closely mimic the extracellular matrix. They can be used as cytocompatible scaffolds for tissue engineering, as can protein- and nucleic acid-activated structures. Hydrogels may be impregnated with osteoinductors such as proteins and genetic vectors without conformational changes. However, the mechanical properties of hydrogels limit their use for load-bearing bone defects. Thus, improving the strength properties of hydrogels is one of the possible strategies to achieve the basis for an ideal osteoplastic material.

A novel splicing pathogenic variant in COL1A1 causing osteogenesis imperfecta (OI) type I in a Chinese family

Background

Osteogenesis imperfecta (OI), a rare autosomal inheritable disorder characterized by bone fragility and skeletal deformity, is caused by pathogenic variants in genes impairing the synthesis and processing of extracellular matrix protein collagen type I. With the use of next‐generation sequencing and panels approaches, an increasing number of OI patients can be confirmed and new pathogenic variants can be discovered. This study sought to identify pathogenic gene variants in a Chinese family with OI I.

Methods

Whole‐exome sequencing was used to identify pathogenic variants in the proband, which is confirmed by Sanger sequencing and cosegregation analysis; MES, HSF, and Spliceman were used to analyze this splicing variant；qRT‐PCR was performed to identify the mRNA expression level of COL1A1 in patient peripheral blood samples; Minigene splicing assay was performed to mimic the splicing process of COL1A1 variants in vitro; Analysis of evolutionary conservation of amino acid residues and structure prediction of the mutant protein.

Results

A novel splicing pathogenic variant (c.3814+1G>T) was identified in this OI family by using whole‐exome sequencing, Sanger sequencing, and cosegregation analysis. Sequencing of RT‐PCR products from the COL1A1 minigene variant reveals a 132‐nucleotide (nt) insertion exists at the junction between exons 48 and exon 49 of the COL1A1 cDNA. Splicing assay indicates that the mutated minigene produces an alternatively spliced transcript which may cause a frameshift resulting in early termination of protein expression. The molecular analysis suggested that the altered amino acid is located at the C‐terminus of type I procollagen.

Conclusion

Our study reveals the pathogenesis of a novel COL1A1 splicing pathogenic variant c.3814+1G>T in a Chinese family with OI I.

Sources of Collagen for Biomaterials in Skin Wound Healing

Collagen is the most frequently used protein in the fields of biomaterials and regenerative medicine. Within the skin, collagen type I and III are the most abundant, while collagen type VII is associated with pathologies of the dermal-epidermal junction. The focus of this review is mainly collagens I and III, with a brief overview of collagen VII. Currently, the majority of collagen is extracted from animal sources; however, animal-derived collagen has a number of shortcomings, including immunogenicity, batch-to-batch variation, and pathogenic contamination. Recombinant collagen is a potential solution to the aforementioned issues, although production of correctly post-translationally modified recombinant human collagen has not yet been performed at industrial scale. This review provides an overview of current collagen sources, associated shortcomings, and potential resolutions. Recombinant expression systems are discussed, as well as the issues associated with each method of expression.

Molecular mechanisms and clinical manifestations of rare genetic disorders associated with type I collagen

Type I collagen is an important structural protein of bone, skin, tendon, ligament and other connective tissues. It is initially synthesized as a precursor form, procollagen, consisting of two identical pro-α1(I) and one proα2(I) chains, encoded by COL1A1 and COL1A2, respectively. The N- and C- terminal propeptides of procollagen are cleavage by N-proteinase and C-proteinase correspondingly, to form the central triple helix structure with Gly-X-Y repeat units. Mutations of COL1A1 and COL1A2 genes are associated with osteogenesis imperfecta, some types of Ehlers-Danlos syndrome, Caffey diseases, and osteogenesis imperfect/Ehlers- Danlos syndrome overlapping diseases. Clinical symptoms caused by different variations can be variable or similar, mild to lethal, and vice versa. We reviewed the relationship between clinical manifestations and type I collagen - related rare genetic disorders and their possible molecular mechanisms for different mutations and disorders.

Monte Carlo type Simulations of Mineralized Collagen Fibril based on Two Scale Asymptotic Homogenization

A multi-scale model for mineralized collagen fibril is proposed by taking into account the uncertainties associated with the geometrical properties of mineral phase and its distribution in the organic matrix. The asymptotic homogenization approach along with periodic boundary conditions has been used to derive the effective elastic moduli at two hierarchical length scales, namely: microfibril and mineralized collagen fibril. The uncertainties associated with the mineral plates have been directly included in the finite element mesh by randomly varying their sizes. A total 100 realizations for mineralized collagen fibril model with random distribution have been generated using an in-house MATLAB® code and Monte-Carlo type simulations have been performed under tension load to obtain the statistical equivalent modulus. The deformation response has been studied in both small (= 10%) and large (= 10%) strain regimes. The stress transformation mechanism has also been explored in microfibril which showed stress relaxation in the organic phase upon different stages of mineralization. The elastic moduli for microfibril under small and large strain have been obtained as 1.88 and 6.102 GPa, respectively, and have been used as input for upper scale homogenization procedure. Finally, the characteristic longitudinal moduli of the mineralized collagen fibril in the small and large strain regimes are obtained as 4.08 ± 0.062 and 12.93 ± 0.148 GPa, respectively. All the results are in good agreement to those obtained from previous experiments and molecular dynamics simulations in the literature with a significant reduction in the computational cost.

Distributions: The Importance of the Chemist's Molecular View for Biological Materials

Characterization of materials with biological applications and assessment of physiological effects of therapeutic interventions are critical for translating research to the clinic and preventing adverse reactions. Analytical techniques typically used to characterize targeted nanomaterials and tissues rely on bulk measurement. Therefore, the resulting data represent an average structure of the sample, masking stochastic (randomly generated) distributions that are commonly present. In this Perspective, we examine almost 20 years of work our group has done in different fields to characterize and control distributions. We discuss the analytical techniques and statistical methods we use and illustrate how we leverage them in tandem with other bulk techniques. We also discuss the challenges and time investment associated with taking such a detailed view of distributions as well as the risks of not fully appreciating the extent of heterogeneity present in many systems. Through three case studies showcasing our research on conjugated polymers for drug delivery, collagen in bone, and endogenous protein nanoparticles, we discuss how identification and characterization of distributions, i.e., a molecular view of the system, was critical for understanding the observed biological effects. In all three cases, data would have been misinterpreted and insights missed if we had only relied upon spatially averaged data. Finally, we discuss how new techniques are starting to bridge the gap between bulk and molecular level analysis, bringing more opportunity and capacity to the research community to address the challenges of distributions and their roles in biology, chemistry, and the translation of science and engineering to societal challenges.

Vertebrae length and ultra-structure measurements of collagen fibrils and mineral content in the vertebrae of lordotic gilthead seabreams (Sparus aurata)

Skeletal deformities of gilthead seabream (Sparus aurata) are a major factor affecting the production cost, the external morphology and survival and growth of the fish. Adult individuals of S. aurata were collected from a commercial fish farm in Greece and were divided into two groups: one with the presence of lordosis, a skeletal deformity, and one without any skeletal deformity. Fishes were X-rayed, and cervical, abdominal and caudal vertebrae lengths were measured. Vertebrae were taken from the site of the vertebral column where lordosis occurred. One part was decalcified and prepared for collagen examination with transmission electron microscopy, and the rest were incinerated, and the Ca and P contents were measured. The stoichiometries of the samples were obtained by EDS (Energy Dispersive Spectroscopy). The same procedure was followed for fish without skeletal deformities (vertebrae were taken from the middle region of the vertebral column). The decalcified vertebrae parts were examined with TEM, collagen micrographs were taken and the fibrils' periods and diameters were measured. There were no significant differences for both Ca and P or the collagen fibrils' periods between the two fish groups. The mean lengths of the cervical, abdominal and caudal vertebrae where lordosis occurred were similar to the lengths of the respective regions of the individuals without the skeletal deformity. The TEM examination showed a significantly smaller mean vertebrae collagen fibril diameter from the fishes with lordosis compared with those from the controls, revealing the significance of collagen to bone structure.

Morphological diversity and polymorphism of self-assembling collagen peptides controlled by length of hydrophobic domains

Synthetic collagen mimetic peptides are used to probe the role of hydrophobic forces in mediating protein self-assembly. Higher order association is an integral property of natural collagens, which assemble into fibers and meshes that comprise the extracellular matrix of connective tissues. The unique triple-helix fold fully exposes two-thirds of positions in the protein to solvent, providing ample opportunities for engineering interaction sites. Inclusion of just a few hydrophobic groups in a minimal peptide promotes a rich variety of self-assembly behaviors, resulting in hundred-nanometer to micron size nanodiscs and nanofibers. Morphology depends primarily on the length of hydrophobic domains. Peptide discs contain lipophilic domains capable of sequestering small hydrophobic dyes. Combining multiple peptide types result in composite structures of discs and fibers ranging from stars to plates-on-a-string. These systems provide valuable tools to shed insight into the fundamental principles underlying hydrophobicity-driven higher order protein association that will facilitate the design of self-assembling systems in biomaterials and nanomedical applications.

Nanoscale structure of type I collagen fibrils: quantitative measurement of D-spacing

This article details a quantitative method to measure the D-periodic spacing of type I collagen fibrils using atomic force microscopy coupled with analysis using a two-dimensional fast fourier transform approach. Instrument calibration, data sampling and data analysis are discussed and comparisons of the data to the complementary methods of electron microscopy and X-ray scattering are made. Examples of the application of this new approach to the analysis of type I collagen morphology in disease models of estrogen depletion and osteogenesis imperfecta (OI) are provided. We demonstrate that it is the D-spacing distribution, not the D-spacing mean, that showed statistically significant differences in estrogen depletion associated with early stage osteoporosis and OI. The ability to quantitatively characterize nanoscale morphological features of type I collagen fibrils will provide important structural information regarding type I collagen in many research areas, including tissue aging and disease, tissue engineering, and gene knockout studies. Furthermore, we also envision potential clinical applications including evaluation of tissue collagen integrity under the impact of diseases or drug treatments.

Collagen for bone tissue regeneration

In the last decades, increased knowledge about the organization, structure and properties of collagen (particularly concerning interactions between cells and collagen-based materials) has inspired scientists and engineers to design innovative collagen-based biomaterials and to develop novel tissue-engineering products. The design of resorbable collagen-based medical implants requires understanding the tissue/organ anatomy and biological function as well as the role of collagen's physicochemical properties and structure in tissue/organ regeneration. Bone is a complex tissue that plays a critical role in diverse metabolic processes mediated by calcium delivery as well as in hematopoiesis whilst maintaining skeleton strength. A wide variety of collagen-based scaffolds have been proposed for different tissue engineering applications. These scaffolds are designed to promote a biological response, such as cell interaction, and to work as artificial biomimetic extracellular matrices that guide tissue regeneration. This paper critically reviews the current understanding of the complex hierarchical structure and properties of native collagen molecules, and describes the scientific challenge of manufacturing collagen-based materials with suitable properties and shapes for specific biomedical applications, with special emphasis on bone tissue engineering. The analysis of the state of the art in the field reveals the presence of innovative techniques for scaffold and material manufacturing that are currently opening the way to the preparation of biomimetic substrates that modulate cell interaction for improved substitution, restoration, retention or enhancement of bone tissue function.

Biomimetic soluble collagen purified from bones

Type I collagen has been extensively exploited as a biomaterial for biomedical applications and drug delivery; however, small molecular alterations occurring during the isolation procedure and its interaction with residual bone extracellular matrix molecules or proteins might affect the overall material biocompatibility and performance. The aim of the current work is to study the potential alterations in collagen properties and organization associated with the absence of proteoglycans, which mimic pathological conditions associated with age-related diseases. A new approach for evaluating the effect of proteoglycans on the properties of isolated type I collagen from the bone matrix is described. Additional treatment with guanidine hydrochloride was introduced to remove residual proteoglycans from the collagen matrix. The properties of the isolated collagen with/without guanidine hydrochloride treatment were investigated and compared with a commercial rabbit collagen as control. We demonstrate that the absence of proteoglycans in the isolated type I collagen affects its thermal properties, the extraction into its native structure, and its ability to hydrate and self-assemble into fibers. The fine control and tuning of all these features, linked to the absence of non-collagenous proteins as proteoglycans, offer the possibility of designing new strategies and biomaterials with advanced biomimetic properties aimed at regenerating bone tissue in the case of fragility and/or defects.

Self-association of streptococcus pyogenes collagen-like constructs into higher order structures

A number of bacterial collagen-like proteins with Gly as every third residue and a high Pro content have been observed to form stable triple-helical structures despite the absence of hydroxyproline (Hyp). Here, the high yield cold-shock expression system is used to obtain purified recombinant collagen-like protein (V-CL) from Streptococcus pyogenes containing an N-terminal globular domain V followed by the collagen triple-helix domain CL and the modified construct with two tandem collagen domains V-CL-CL. Both constructs and their isolated collagenous domains form stable triple-helices characterized by very sharp thermal transitions at 35-37 degrees C and by high values of calorimetric enthalpy. Procedures for the formation of collagen SLS crystallites lead to parallel arrays of in register V-CL-CL molecules, as well as centrosymmetric arrays of dimers joined at their globular domains. At neutral pH and high concentrations, the bacterial constructs all show a tendency towards aggregation. The isolated collagen domains, CL and CL-CL, form units of diameter 4-5 nm which bundle together and twist to make larger fibrillar structures. Thus, although this S. pyogenes collagen-like protein is a cell surface protein with no indication of participation in higher order structure, the triple-helix domain has the potential of forming fibrillar structures even in the absence of hydroxyproline. The formation of fibrils suggests bacterial collagen proteins may be useful for biomaterials and tissue engineering applications.

Segregation of type I collagen homo- and heterotrimers in fibrils

Normal type I collagen is a heterotrimer of two alpha1(I) and one alpha2(I) chains, but various genetic and environmental factors result in synthesis of homotrimers that consist of three alpha1(I) chains. The homotrimers completely replace the heterotrimers only in rare recessive disorders. In the general population, they may compose just a small fraction of type I collagen. Nevertheless, they may play a significant role in pathology; for example, synthesis of 10-15% homotrimers due to a polymorphism in the alpha1(I) gene may contribute to osteoporosis. Homotrimer triple helices have different stability and less efficient fibrillogenesis than heterotrimers. Their fibrils have different mechanical properties. However, very little is known about their molecular interactions and fibrillogenesis in mixtures with normal heterotrimers. Here we studied the kinetics and thermodynamics of fibril formation in such mixtures by combining traditional approaches with 3D confocal imaging of fibrils, in which homo- and heterotrimers were labeled with different fluorescent colors. In a mixture, following a temperature jump from 4 to 32 degrees C, we observed a rapid increase in turbidity most likely caused by formation of homotrimer aggregates. The aggregates promoted nucleation of homotrimer fibrils that served as seeds for mixed and heterotrimer fibrils. The separation of colors in confocal images indicated segregation of homo- and heterotrimers at a subfibrillar level throughout the process. The fibril color patterns continued to change slowly after the fibrillogenesis appeared to be complete, due to dissociation and reassociation of the pepsin-treated homo- and heterotrimers, but this remixing did not significantly reduce the segregation even after several days. Independent homo- and heterotrimer solubility measurements in mixtures confirmed that the subfibrillar segregation was an equilibrium property of intermolecular interactions and not just a kinetic phenomenon. We argue that the subfibrillar segregation may exacerbate effects of a small fraction of alpha1(I) homotrimers on formation, properties, and remodeling of collagen fibers.

The variability in type I collagen helical pitch is reflected in the D periodic fibrillar structure

The variability in amino acid axial rise per residue of the collagen helix is a potentially important parameter that is missing in many structural models of fibrillar collagen to date. The significance of this variability has been supported by evidence from collagen axial structures determined by electron microscopy and X-ray diffraction, as well as studies of the local sequence-dependent conformation of the collagen helix. Here, sequence-dependent variation of the axial rise per residue was used to improve the fit between simulated diffraction patterns derived from model structures of the axially projected microfibrillar structure and the observed X-ray diffraction pattern from hydrated rat tail tendon. Structural models were adjusted using a genetic algorithm that allowed a wide range of structures to be tested efficiently. The results show that variation of the axial rise per residue could reduce the difference metric between model and observed data by up to 50%, indicating that such a variable is a necessary part of fibril model structure building. The variation in amino acid translation was also found to be influenced by the number of proline and hydroxyproline residues in the triple helix structure.

Structure, stability and interactions of type I collagen with GLY349-CYS substitution in alpha 1(I) chain in a murine Osteogenesis Imperfecta model

Here we report the structural and functional studies of collagen from the Brtl mouse, a heterozygous knock-in model for Osteogenesis Imperfecta, which has a G349C substitution introduced in one col1a1 allele. We observed that 25+/-5% of alpha 1(I) chains in different tissues and in different extracts from matrix deposited by cultured cells were S-S-linked mutant dimers. Apparently mutant and normal molecules are equally well incorporated into the matrix and they form mature covalent crosslinks with the same efficiency. We found different extents of post-translational overmodification of mutant molecules in different tissues, but we found no consistent differences between lethal and non-lethal animals. We did not detect any changes in the thermal stability or rate of thermal denaturation of mutant collagen. We also did not detect any changes in collagen-collagen recognition and interactions except for disruption of quasi-crystalline lateral packing of molecules in tendons from some, mostly prepubertal, mutant animals. In contrast, alpha 1(I)(3) collagen from the oim mouse--the only other non-lethal murine OI model studied by similar techniques--has altered stability, fibrillogenesis, collagen-collagen interactions and produces a more consistent and more pronounced disruption of tendon crystallinity. Nevertheless, while the G349C substitution causes moderate or lethal OI, heterozygous oim mice are much less affected. Overall, our results suggest that OI symptoms and phenotype variation in G349C animals are related to abnormal interactions of mutant collagen helices with other matrix molecules or abnormal function of osteoblasts rather than to abnormal structure, physical properties or interactions between mutant collagen helices.

Avian fissura prima: differential accumulation of extracellular matrix at a fold

Extracellular matrix components that flank the fissura prima, a primary surface infolding of the cerebellum in birds and mammals, were examined in the embryonic chick using light and transmission electron microscopy. Cerebella dissected from Day 10 embryos were perfused with a paraformaldehyde-glutaraldehyde-tannic acid primary fixative and sectioned in the sagittal plane through the mid-vermis. Ultrastructural analysis revealed a distinct, continuous basal lamina separating the organ parenchyma (epithelia) from pia mater (mesenchyme) at the fissure surface (arbitrarily labeled; fissure floor, folia wall, and folia apex). The basal lamina was significantly thicker (P < 0.001) at the fissure floor compared to that found at the folia wall, which was significantly thicker (P < 0.001) than that observed at the folia apex. Folds in the basal lamina were observed exclusively at the fissure floor. Surface-associated collagen fibrils were distributed in an aligned, relatively dense manner at the fissure floor, compared with fibrils observed in various orientations and widely separated or absent at the folia wall and folia apex. Metachromasia was more pronounced in the fissure floor than in either the folia wall or folia apex in methylene blue-stained tissue sections. Together, the thicker, folded basal lamina and densely aligned collagen fibrils at the fissure floor provide a chemical rationale for this color change. These findings suggest that the differential accumulation of extracellular matrix at the fissura prima is positioned to play a structural and/or biochemical role in the maintenance of this fold.

Changes in thermal stability and microunfolding pattern of collagen helix resulting from the loss of alpha2(I) chain in osteogenesis imperfecta murine

Homozygous mutations resulting in formation of alpha1(I)(3) homotrimers instead of normal type I collagen cause mild to severe osteogenesis imperfecta (OI) in humans and mice. Limited studies of changes in thermal stability of type I homotrimers were reported previously, but the results were not fully consistent. We revisited this question in more detail using purified tendon collagen from wild-type (alpha1(I)(2)alpha2(I) heterotrimers) and oim (alpha1(I)(3)) mice as well as artificial alpha1(I)(3) homotrimers obtained by refolding of rat-tail-tendon collagen. We found that at the same heating rate oim homotrimers completely denature at approximately 2.5deg.C higher temperature than wild-type heterotrimers, as determined by differential scanning calorimetry. At the same, constant temperature, homotrimers denature approximately 100 times slower than heterotrimers, as determined by circular dichroism. Detailed analysis of proteolytic cleavage at different temperatures revealed that microunfolding of oim homotrimers and wild-type heterotrimers occurs at similar rate but within a number of different sites. In particular, the weakest spot on the oim triple helix is located approximately 100 amino acid residues from the C-terminal end within the cyanogen bromide peptide CB6. The same microunfolding site is also present in wild-type collagen, but the weakest spot of the latter is located close to the N-terminal end of CB8. Amino acid analysis and differential gel electrophoresis showed virtually no posttranslational overmodification of oim mouse tendon collagen. Moreover, thermal stability and microunfolding of artificial rat-tail-tendon homotrimers were similar to oim homotrimers. Thus, the observed changes are associated with difference in the amino acid composition of alpha1(I) and alpha2(I) chains rather than posttranslational overmodification.

Mineralization of type I collagen

It was previously found that the lateral spacing of the collagen molecules in wet mineralized tissues is exactly proportional to the inverse wet density. Several properties were investigated and the same type of relationship was observed each time. A possible explanation is offered. It is hypothesized that mineral is deposited initially in the extrafibrillar space so as to isolate the fibrils. Further deposition reduces the net free fibril volume thereby decreasing the spacing between collagen molecules. The linear relationship is derived from density considerations together with limitations on the collagen packing structure described as the generalized packing model. Three experimental situations were studied: lateral spacing wet tissue versus density; lateral spacing dry tissue versus density; and lateral spacing versus water content. The observed variations of the spacing can be attributed to a structure where the mass of the tissue remains constant but the volume decreases linearly with increasing mineral content.

Collagen fibers, reticular fibers and elastic fibers. A comprehensive understanding from a morphological viewpoint

Fibrous components of the extracellular matrix are light-microscopically classified into three types of fibers: collagen, reticular and elastic. The present study reviews the ultrastructure of these fibrous components as based on our previous studies by light, electron, and atomic force microscopy. Collagen fibers present a cord- or tape-shape 1-20 microm wide and run a wavy course in tissues. These fibers consist of closely packed thin collagen fibrils (30-100 nm thick in ordinary tissues of mammals), and exhibit splitting and joining in altering the number of the fibrils to form a three-dimensional network as a whole. Individual collagen fibrils (i.e., unit fibrils) in collagen fibers have a characteristic D-banding pattern whose length is ranges from 64 to 67 nm, depending on tissues and organs. During fibrogenesis, collagen fibrils are considered to be produced by fusing short and thin fibrils with tapered ends. Reticular fibers are usually observed as a delicate meshwork of fine fibrils stained black by the silver impregnation method. They usually underlie the epithelium and cover the surface of such cells of muscle cells, adipose cells and Schwann cells. Electronmicroscopically, reticular fibers are observed as individual collagen fibrils or a small bundle of the fibrils, although the diameter of the fibrils is thin (about 30 nm) and uniform. Reticular fibers are continuous with collagen fibers through the exchange of these collagen fibrils. In silver-impregnated specimens, individual fibrils in reticular fibers are densely coated with coarse metal particles, probably due to the high content of glycoproteins around the fibrils. Elastic fibers and laminae are composed of microfibrils and elastin components. Observations of the extracted elastin have revealed that elastin components are comprised of elastin fibrils about 0.1-0.2 microm thick. Elastic fibers and laminae are continuous with networks and/or bundles of microfibrils (or oxytalan fibers), and form an elastic network specific to individual tissues. The fibrous components of the extracellular matrix are thereby morphologically categorized into two systems: the collagen fibrillar system as a supporting framework of tissues and cells, and the microfibrilelastin system for uniformly distributing stress to maintain the resilience adapted to local tissue requirements.

Structure of type I and type III heterotypic collagen fibrils: an X-ray diffraction study

The molecular packing arrangement within collagen fibrils has a significant effect on the tensile properties of tissues. To date, most studies have focused on homotypic fibrils composed of type I collagen. This study investigates the packing of type I/III collagen molecules in heterotypic fibrils of colonic submucosa using a combination of X-ray diffraction data, molecular model building, and simulated X-ray diffraction fibre diagrams. A model comprising a 70-nm-diameter D- (approximately 65 nm) axial periodic structure containing type I and type III collagen chains was constructed from amino acid scattering factors organised in a liquid-like lateral packing arrangement simulated using a classical Lennard-Jones potential. The models that gave the most accurate correspondence with diffraction data revealed that the structure of the fibril involves liquid-like lateral packing combined with a constant helical inclination angle for molecules throughout the fibril. Combinations of type I:type III scattering factors in a ratio of 4:1 gave a reasonable correspondence with the meridional diffraction series. The attenuation of the meridional intensities may be explained by a blurring of the electron density profile of the D period caused by nonspecific or random interactions between collagen types I and III in the heterotypic fibril.

Osteogenesis imperfecta murine: interaction between type I collagen homotrimers

Types I, II, and III collagens are believed to have evolved from the same homotrimer ancestor and they have substantial sequence homology, but type I molecules are alpha1(I)(2)alpha2(I) heterotrimers, unlike homotrimeric types II and III. It is believed that the alpha2(I) chain first appeared in lower vertebrates and that it plays a particularly important role in bone formation. For instance, spontaneous mutations resulting in non- functional alpha2 chains and formation of type I homotrimers cause severe bone pathology (osteogenesis imperfecta) in humans and in animals. However, the exact role of the alpha2 chain is not known. Here, we report measurements of intermolecular forces between collagen helices in native and reconstituted fibers composed of type I homotrimers, heterotrimers and their mix. For comparison, we report forces between type II homotrimers in reconstituted fibers. In agreement with previous studies, we find that the absence of the alpha2 chain reduces temperature-favored attraction between collagen helices, either because of the difference in amino acid sequence of the alpha1 and alpha2 chains or because of more extensive post-translational modification of homotrimers. We find that forces between helices in fibers from type I (as well as type II) homotrimers are not sensitive to pH between pH 6 and 7.5, in contrast to type I heterotrimers. Apparently, the effect of pH is related to extra histidine residues present on alpha2 chains but not on alpha1 chains. Finally, our measurements indicate that the alpha2 chain is responsible for binding some soluble compound(s), possibly glycosaminoglycans, whose displacement results, e.g., in the loss of tendon crystallinity. The ability of the alpha2 chain to bind non-collagen matrix components may be particularly important for bone matrix formation and mineralization.

Measurement of the axial periodicity of collagen fibrils using an image processing method

In this paper, a method is described based on a computer-aided analysis of electron-optical images of collagen fibrils from various tissues, in order to determine the axial periodicity of such fibrils. The method gives information at a level of 2-3nm.

Collagen structure and functional implications

The bio-mechanical requirements to which the connective tissue is subjected suggest that a causal correlation exist between the substructure and the collagen fibril function. We discuss the relationship between the inner structure of collagen fibrils, their diameter, their spatial layout and the functional requirements they have to withstand, and suggest that collagen fibrils may belong to two different forms indicated as "T-type" and "C-type". The first class, consisting of large, heterogeneous fibrils, parallely tightly packed, subjected to tensile stress along their axis is found in highly tensile structures such as tendons, ligaments and bone. The other class, consisting of small, homogeneous fibrils, helically arranged, resisting multidirectional stresses, is mostly present within highly compliant tissues such as blood vessel walls, skin and nerve sheaths. What causes these architectures to appear is discussed in detail in this review.

Chlorination of pyridinium compounds. Possible role of hypochlorite, N-chloramines, and chlorine in the oxidation of pyridinoline cross-links of articular cartilage collagen type II during acute inflammation

Reactive oxygen species produced by activated neutrophils and monocytes are thought to be involved in mediating the loss of collagen and other matrix proteins at sites of inflammation. To evaluate their potential to oxidize the pyridinoline (Pyd) cross-links found in collagen types I and II, we reacted hydrogen peroxide (H(2)O(2)), hypochlorous acid/hypochlorite (HOCl/OCl(-)), and singlet oxygen (O(2)((1)delta g)) with the Pyd substitutes, pyridoxamine dihydrochloride and vitamin B(6), which share the same chemical structure and spectral properties of Pyd cross-links. Neither H(2)O(2) (125-500 microm) nor O(2)((1)delta g) (10-25 microm) significantly changed the spectral properties of pyridoxamine or vitamin B(6). Reaction of HOCl/OCl(-) (12.5-50 microm) with pyridoxamine at pH 7.2 resulted in a concentration-dependent appearance of two new absorbance peaks and a decrease in fluorescence at 400 nm (excitation 325 nm). The new absorbance peaks correlated with the formation of an N-chloramine and the product of its subsequent reaction with pyridoxamine. In contrast, the extent to which HOCl reacted with vitamin B(6), which lacks a primary amine group, was variable at this pH. At lysosomal pH 5.5, Cl(2)/HOCl/OCl(-) reacted with both pyridoxamine and vitamin B(6). Four of the chlorinated products of this reaction were identified by gas chromatography-mass spectrometry and included 3-chloropyridinium, an aldehyde, and several chlorinated products with disrupted rings. To evaluate the effects of Cl(2)/HOCl/OCl(-) on Pyd cross-links in collagen, we exposed bone collagen type I and articular cartilage type II to HOCl. Treatment of either collagen type with HOCl at pH 5. 0 or 7.2 resulted in the oxidation of amine groups and, for collagen type II, the specific decrease in Pyd cross-link fluorescence, suggesting that during inflammation both oxidations may be used by neutrophils and monocytes to promote the loss of matrix integrity.

Does the triple helical domain of type I collagen encode molecular recognition and fiber assembly while telopeptides serve as catalytic domains? Effect of proteolytic cleavage on fibrillogenesis and on collagen-collagen interaction in fibers

Over the last several decades, it has been established that proteolytic removal of short, non-helical terminal peptides (telopeptides) from type I collagen significantly alters the kinetics of in vitro fibrillogenesis. However, it has also been observed that the protein is still capable of forming fibers even after complete removal of telopeptides. This study focuses on the characterization of this fibrillogenesis competency of collagen. We have combined traditional kinetic and thermodynamic assays of fibrillogenesis efficacy with direct measurements of interaction between collagen molecules in fibers by osmotic stress and x-ray diffraction. We found that telopeptide cleavage by pepsin or by up to 20 h of Pronase treatment altered fiber assembly kinetics, but the same fraction of the protein still assembled into fibers. Small-angle x-ray diffraction showed that these fibers have normal, native-like D-stagger. Force measurements indicated that collagen-collagen interactions in fibers were not affected by either pepsin or Pronase treatment. In contrast, prolonged (>20 h) Pronase treatment resulted in cleavage of the triple helical domain as indicated by SDS-polyacrylamide gel electrophoresis. The triple-helix cleavage correlated with the observed decrease in the fraction of protein capable of forming fibers and with the measured loss of attraction between helices in fibers. These data suggest that telopeptides play a catalytic role, whereas the information necessary for proper molecular recognition and fiber assembly is encoded in the triple helical domain of collagen.

Fourier analysis of electron micrographs of positively stained collagen fibrils: application to type I and II collagen typing

Type I and II collagen (native-type) fibrils, positively stained with uranyl acetate, present typical periodic (D = 67 nm) cross-striation patterns. Although the two patterns are similar, the distributions of charged amino acids along the type I and II collagen molecules are different. After optical diffraction analysis or computer image processing of electron micrographs, different Fourier transforms were obtained from type I and II collagen fibrils, either as native fibrils or after in vitro reconstitution from purified molecules. With tissues such as tendon and cartilage, better results were obtained after mild trypsin treatment, which allowed better isolation and staining of the collagen fibrils. The main difference observed in the Fourier transforms was the presence in type II collagen fibrils of a strong tenth-order peak (corresponding to the tenth harmonic of the fundamental frequency). In order to discriminate between the two collagens, we measured the ratio (R) of the areas under the ninth- and tenth-order peaks. In trypsin treated tissues, the distributions of these ratios were clearly separated: below 1.0 for type II collagen fibrils and above 1.5 for type I collagen fibrils. This method appears to be suitable for rapid typing of type I and II collagen fibrils and might be useful for determining the exact composition of fibrils in tissues, such as intervertebral discs, that contain these both types of collagen.

The collagen fibril: the almost crystalline structure

The structure of collagen fibrils has intrigued many investigators over the years. A crystal structure has been available for some time, but the crystal structure has been difficult to reconcile with other observations about collagen fibrils such as their roundness and their growth from paraboloidal tips. Several alternative models recently have been suggested, but none of them fully account for all the data. One recent approach to solving the fibrillar structure is to define specific binding sites on the collagen monomer that direct self-assembly of monomers into fibrils.

The potential of collagenase as a new therapy for separation of human retained placenta: hydrolytic potency on human, equine and bovine placentae

The purpose of this study was to determine to what degree bacterial collagenase may digest human placentae compared to equine and bovine placentae. Placenta samples from human, equine and bovine were incubated with bacterial collagenase solution at various concentrations. The degree of hydrolysis and collagen breakdown was measured by the release of total proteins and hydroxyproline into the incubation media. Also, whole placentae were injected via umbilical cord arteries with collagenase solution (200 U/ml, 200 ml total volume in human and 1000 ml in equine) and hydrolysis determined chemically and subjectively. Human and equine placental collagens were the most sensitive to collagenase digestion. Overall mean collagenase activity determined by the release of hydroxyproline from human placenta was 1.6 times and in equine placenta three times greater than in bovine placenta, while the breakdown of non-collagenous proteins remained negligible. When injected into whole placenta, the collagenase digested placentae evenly within 6-12 h. At 24 h, placentae were liquefied, although, umbilical blood vessels resisted collagenase digestion. Bacterial collagenase was highly effective in breaking down human placenta collagen. Intraplacental injections of collagenase via umbilical cord arteries may help to detach retained placenta in women as it does in mares and cows.

Inhibition of the self-assembly of collagen I into fibrils with synthetic peptides. Demonstration that assembly is driven by specific binding sites on the monomers

A series of experiments were carried out to test the hypothesis that the self-assembly of collagen I monomers into fibrils depends on the interactions of specific binding sites in different regions of the monomer. Six synthetic peptides were prepared with sequences found either in the collagen triple helix or in the N- or C-telopeptides of collagen I. The four peptides with sequences found in the telopeptides were found to inhibit self-assembly of collagen I in a purified in vitro system. At concentrations of 2.5 mM, peptides with sequences in the C-telopeptides of the alpha1(I) and alpha2(I) chain inhibited assembly at about 95%. The addition of the peptide with the alpha2-telopeptide sequence was effective in inhibiting assembly if added during the lag phase and early propagation phase but not later in the assembly process. Experiments with biotinylated peptides indicated that both the N- and C-telopeptides bound to a region between amino acid 776 and 822 of the alpha(I) chain. A fragment of nine amino acids with sequences in the alpha2-telopeptide was effective in inhibiting fibril assembly. Mutating two aspartates in the 9-mer peptide to serine had no effect on inhibition of fibril assembly, but mutating two tyrosine residues and one phenylalanine residue abolished the inhibitory action. Molecular modeling of the binding sites demonstrated favorable hydrophobic and electrostatic interactions between the alpha2telopeptide and residues 781-794 of the alpha(I) chain.

The effect of lithium treatment on collagenous tissues: an electron microscope study

Structural alterations of mouse skin collagen fibrils brought about by treatment with lithium chloride at various doses, as studied by electron microscopy, are discussed. The effect of the duration of treatment on such fibrils is also investigated. An analysis of positive staining patterns from fibrils from mice treated with lithium is included. In addition, this review describes how computer analyses of electron-optical images from collagen fibrils can lead to information about the changes produced by lithium on the molecular architecture of the fibril.

Altered collagen structure in mouse tail tendon lacking the alpha 2(I) chain

Type I collagen is the most prevalent member of the fibril forming family of collagens in higher vertebrates and its heterotrimeric form is comprised of two alpha 1(I) chains and one alpha2(I) polypeptide chain. The functional importance of having two distinct chain types in type I collagen is largely undefined. The existence of a mouse model with a Cola-2 gene mutation (termed oim) that results in non-functional pro alpha 2(I) chains presents a unique opportunity to explore changes in collagen structure resulting from the complete (oim/oim mice) and partial (oim/+ mice) loss of functional alpha 2(I) chains. Tail tendon is a tissue with a well-defined, hierarchical organization of type I collagen. X-ray diffraction studies on oim/oim versus control tendons indicate that the total absence of alpha 2(I) chains results in a decrease in the order of axial packing and a loss of crystalline lateral packing. This suggests that the non-equivalence of three chains is an important determinant of lateral interactions between adjacent molecules and may be involved in the long-range axial order in type I collagen-containing tissues. Both homotrimeric and heterotrimeric type I collagen molecules are found in heterozygous oim mice and these appear to be present in the same co-polymeric fibrils, preventing crystalline lateral packing. In addition to these changes at a fibrillar level, the absence of the alpha 2(I) chain results in an increased enzymatic susceptibility at one site. The oim/oim mice are observed to have reduced body size and smaller tendon bundles, which may be a consequence of these molecular and fibrillar changes in collagen. Furthermore, it is likely that a similar alteration in the molecular packing of collagen in bone fibrils contributes to the osteopenia and decreased bone strength in mice with the oim mutation that are also characteristic of human osteogenesis imperfecta.

Assembly in vitro of thin and thick fibrils of collagen II from recombinant procollagen II. The monomers in the tips of thick fibrils have the opposite orientation from monomers in the growing tips of collagen I fibrils

Human type II procollagen was prepared in a recombinant system and cleaved to pC-collagen II by procollagen N-proteinase. The pC-collagen II was then used as a substrate to generate collagen II fibrils by cleavage with procollagen C-proteinase at 37 degrees C. Electron microscopy of the fibrils demonstrated that, at the early stages of fibril assembly, very thin fibrils were formed. As the system approached equilibrium over 7-12 h, however, the thin fibrils were largely but not completely replaced by thick fibrils that had diameters of about 240 nm and a distinct D-period banding pattern. One typical fibril was photographed and analyzed in its entirety. The fibril was 776 D-periods (52 microM) long. It had a central shaft with a uniform diameter that was about 516 D-periods long and two tips of about 100 D-periods each. Most of the central shaft had a symmetrical banding pattern flanked by two transition regions of about 30 D-periods each. Measurements by scanning transmission electron microscopy demonstrated that the mass per unit length from the tips to the shafts increased linearly over approximately 100 D-periods from the fibril end. The linear increase in mass per unit length was consistent with previous observations for collagen I fibrils and established that the tips of collagen II also had a near paraboloidal shape. However, the orientation of monomers in the tips differed from the tips of collagen I fibrils in that the C termini instead of the N termini were directed toward the tips. The thin fibrils that were present at early stages of assembly and at equilibrium were comparable to the collagen II fibrils seen in embryonic tissues and probably represented intermediates on the pathway of thick fibrils formation. The results indicated that the molecular events in the self-assembly of collagen II fibrils are apparently similar to those in self-assembly of collagen I fibrils, but that there are also important differences in the structural information contained in collagen I and collagen II monomers.

Radial packing, order, and disorder in collagen fibrils

Collagen fibrils resemble smectic, liquid crystals in being highly ordered axially but relatively disordered laterally. In some connective tissues, x-ray diffraction reveals three-dimensional crystallinity in the molecular packing within fibrils, although the continued presence of diffuse scatter indicates significant underlying disorder. In addition, several observations from electron microscopy suggest that the molecular packing is organized concentrically about the fibril core. In the present work, theoretical equatorial x-ray diffraction patterns for a number of models for collagen molecular packing are calculated and compared with the experimental data from tendon fibrils. None of the models suggested previously can account for both the crystalline Bragg peaks and the underlying diffuse scatter. In addition, models in which any of the nearest-neighbor, intermolecular vectors are perpendicular to the radial direction are inconsistent with the observed radial orientation of the principal approximately 4 nm Bragg spacing. Both multiple-start spiral and concentric ring models are devised in which one of the nearest-neighbor vectors is along the radial direction. These models are consistent with the radial orientation of the approximately 4 nm spacing, and energy minimization results in radially oriented crystalline domains separated by disordered grain boundaries. Theoretical x-ray diffraction patterns show a combination of sharp Bragg peaks and underlying diffuse scatter. Close agreement with the observed equatorial diffraction pattern is obtained. The concentric ring model is consistent with the observation that the diameters of collagen fibrils are restricted to discrete values.

Mapping the heparin-binding sites on type I collagen monomers and fibrils

The glycosaminoglycan chains of cell surface heparan sulfate proteoglycans are believed to regulate cell adhesion, proliferation, and extracellular matrix assembly, through their interactions with heparin-binding proteins (for review see Ruoslahti, E. 1988. Annu. Rev. Cell Biol. 4:229-255; and Bernfield, M., R. Kokenyesi, M. Kato, M. T. Hinkes, J. Spring, R. L. Gallo, and E. J. Lose. 1992. Annu. Rev. Cell Biol. 8:365-393). Heparin-binding sites on many extracellular matrix proteins have been described; however, the heparin-binding site on type I collagen, a ubiquitous heparin-binding protein of the extracellular matrix, remains undescribed. Here we used heparin, a structural and functional analogue of heparan sulfate, as a probe to study the nature of the heparan sulfate proteoglycan-binding site on type I collagen. We used affinity coelectrophoresis to study the binding of heparin to various forms of type I collagen, and electron microscopy to visualize the site(s) of interaction of heparin with type I collagen monomers and fibrils. Using affinity coelectrophoresis it was found that heparin has similar affinities for both procollagen and collagen fibrils (Kd's approximately 60-80 nM), suggesting that functionally similar heparin-binding sites exist in type I collagen independent of its aggregation state. Complexes of heparin-albumin-gold particles and procollagen were visualized by rotary shadowing and electron microscopy, and a preferred site of heparin binding was observed near the NH2 terminus of procollagen. Native or reconstituted type I collagen fibrils showed one region of significant heparin-gold binding within each 67-nm period, present near the division between the overlap and gap zones, within the "a" bands region. According to an accepted model of collagen fibril structure, our data are consistent with the presence of a single preferred heparin-binding site near the NH2 terminus of the collagen monomer. Correlating these data with known type I collagen sequences, we suggest that the heparin-binding site in type I collagen may consist of a highly basic triple helical domain, including several amino acids known sometimes to function as disaccharide acceptor sites. We propose that the heparin-binding site of type I collagen may play a key role in cell adhesion and migration within connective tissues, or in the cell-directed assembly or restructuring of the collagenous extracellular matrix.

Collagen organization in an oriented fibrous capsule

X-ray diffraction and polarized light microscopy studies were carried out on a fibrous capsule to examine the collagen organization produced in a granuloma response. The capsule had been formed around a silicone tube covered with polyester mesh following implantation in a sheep. The fibrous capsule has previously been shown to include a substantial amount of type III collagen, in addition to type I collagen. Whereas granulomas generally show little order, the sample in the present study has a high degree of orientation of the collagen fibrils along the tube direction. X-ray diffraction patterns of the capsule showed an axial D period near 65 nm, a value similar to that of other type III collagen-containing tissues. This axial period is less than the 67 nm period seen in tendon and bone, which contain only type I collagen. The decreased D period is consistent with an increase in the tilting of the collagen molecules with respect to the fibril axis.

Direct measurement of forces between self-assembled proteins: temperature-dependent exponential forces between collagen triple helices

We report direct measurements of force vs. separation between self-assembled proteins. These forces are observed between collagen triple helices in native and reconstituted fibers. They are a combination of a short-range repulsion, which varies exponentially over at least five decay lengths, and an inferred, longer-ranged attraction responsible for spontaneous assembly. From 5 degrees C to 35 degrees C the relative contribution of the attraction to the net force increases with temperature. These forces are strikingly similar to the "hydration" forces measured between several other linear macromolecules (DNA, polysaccharides) and between lipid bilayer membranes. The decay length of the repulsive force agrees well with a theoretical estimate based on axial periodicity of the triple helix, suggesting another connection between molecular architecture and protein-protein interaction.