Hierarchical assembly and the onset of banding in fibrous long spacing collagen revealed by atomic force microscopy

Natural Polymer-Based Micronanostructured Scaffolds for Bone Tissue Engineering

Although bone tissue allografts and autografts aremoften used as a regenerative tissue during the bone healing, their availability, donor site morbidity, and immune response to grafted tissue are limiting factors their more common usage. Tissue engineered implants, such as acellular or cellular polymeric structures, can be an alternative solution. A variety of scaffold fabrication techniques including electrospinning, particulate leaching, particle sintering, and more recently 3D printing have been used to create scaffolds with interconnected pores and mechanical properties for tissue regeneration. Simply combining particle sintering and molecular self-assembly to create porous microstructures with imbued nanofibers to produce micronanostructures for tissue regeneration applications. Natural polymers like polysaccharides, proteins and peptides of plant or animal origin have gained significant attention due to their assured biocompatibility in tissue regeneration. However, majority of these polymers are water soluble and structures derived from them are in the form of hydrogels and require additional stabilization via cross-linking. For bone healing applications scaffolds are required to be strong, and support attachment, proliferation and differentiation of osteoprogenitors into osteoblasts. Our ongoing work utilizes plant polysaccharide cellulose derivatives and collagen to create mechanically stable and bioactive micronanostructured scaffold for bone tissue engineering. Scaffold microstructure is essentially solvent sintered cellulose acetate (CA) microspheres in the form of a negative template for trabecular bone with defined pore and mechanical properties. Collagen nanostructures are imbued into the 3D environment of CA scaffolds using collagen molecular self-assembly principles. The resultant CA-collagen micronanostructures provide the benefits of combined polymers and serve as an alternative material platform to many FDA approved polyesters. Our ongoing studies and published work confirm improved osteoprogenitor adhesion, proliferation, migration, differentiation, extracellular matrix (ECM) secretion in promoting bone healing. In this chapter we will provide a detailed protocol on the creation of micronanostructured CA-collagen scaffolds and their characterization for bone tissue engineering using human mesenchymal stem cells.

In vitro fibrillogenesis of collagen type I in varying ionic and pH conditions

Collagen is the most abundant protein in the human body, and has primary roles in the formation of tendons, cartilage and bone, it provides mechanical strength to skin and indeed almost every organ and muscle is associated with a layer of collagen. It is thus a key component of the extracellular matrix. Here we have studied the in vitro fibrillogenesis of acetic acid-soluble collagen type I under physiological and varying non-physiological conditions by TEM from negatively stained specimens. At pH 2.5 the collagen heterotrimer remains soluble at increasing buffer concentrations and in the presence of increasing NaCl concentrations. At pH 4.5 molecular aggregates form at low NaCl concentrations, but at higher NaCl concentrations fibrils with a diffuse ~11 nm banding are formed. At pH 7.0, initial molecular aggregates form at low NaCl concentrations that progressively form characteristic ~67 nm D-banded collagen fibrils at intermediate NaCl concentrations that cluster to form thicker multi-fibril D-banded fibres in higher NaCl concentrations. By contrast, increasing concentrations of sodium phosphate at pH 7.0 leads to the formation of flexuous, unbanded fibrils at higher concentrations from the initial, loosely aggregated form of collagen. At higher pHs, the formation of D-banded fibrils is less efficient, particularly at pH 9.0. Thus at neutral pH, the presence of chloride anions, rather than sodium cations, is required for the production of D-banded collagen fibrils; higher than normal physiological chloride concentrations in the form of NaCl or Tris·HCl at neutral pH, but not phosphate buffer, can also lead to the efficient in vitro formation of D-banded collagen fibrils.

Fibrillation properties of human α₁-acid glycoprotein

Human α(1)-acid glycoprotein (AGP) is a positive acute phase plasma protein containing two disulfide bridges. Structural studies have shown that under specific conditions AGP undergoes aggregation. In this study, we analysed the nature of AGP's aggregates formed under reducing and non-reducing conditions at pH 5.5 and at relatively low temperatures. Thioflavin T and Congo red spectroscopic analyses indicated the presence of cross-β structures in both unreduced and reduced AGP aggregates. In these samples amyloid-like fibrils were detected by transmission electron microscopy. The fibrils are branched and bent and present in very large amount in reduced AGP. Kinetics of AGP fibrillation proceeds without a lag phase and the rate constants of cross-β formation are linearly dependent on AGP concentration and result higher under reducing conditions. The data suggest a possible downhill mechanism of polymerization with a first-order monomer concentration dependence. Bioinformatics tools highlighted an extended region that sheathes one side of the molecule containing aggregation-prone regions. Reducing conditions make the extended region less constricted, allowing greater exposure of aggregation-prone regions, thus explaining the higher propensity of AGP to aggregate and fibrillate.

Structural and mechanical differences between collagen homo- and heterotrimers: relevance for the molecular origin of brittle bone disease

Collagen constitutes one-third of the human proteome, providing mechanical stability, elasticity, and strength to organisms. Normal type I collagen is a heterotrimer triple-helical molecule consisting of two α-1 chains and one α-2 chain. The homotrimeric isoform of type I collagen, which consists of three α-1 chains, is only found in fetal tissues, fibrosis, and cancer in humans. A mouse model of the genetic brittle bone disease, osteogenesis imperfect, oim, is characterized by a replacement of the α-2 chain by an α-1 chain, resulting also in a homotrimer collagen molecule. Experimental studies of oim mice tendon and bone have shown reduced mechanical strength compared to normal mice. The relationship between the molecular content and the decrease in strength is, however, still unknown. Here, fully atomistic simulations of a section of mouse type I heterotrimer and homotrimer collagen molecules are developed to explore the effect of the substitution of the α-2 chain. We calculate the persistence length and carry out a detailed analysis of the structure to determine differences in structural and mechanical behavior between hetero- and homotrimers. The results show that homotrimer persistence length is half of that of the heterotrimer (96 Å vs. 215 Å), indicating it is more flexible and confirmed by direct mechanical testing. Our structural analyses reveal that in contrast to the heterotrimer, the homotrimer easily forms kinks and freely rotates with angles much larger than heterotrimer. These local kinks may explain the larger lateral distance between collagen molecules seen in the fibrils of oim mice tendon and could have implications for reducing the intermolecular cross-linking, which is known to reduce the mechanical strength.

Deformation and failure of protein materials in physiologically extreme conditions and disease

Biological protein materials feature hierarchical structures that make up a diverse range of physiological materials. The analysis of protein materials is an emerging field that uses the relationships between biological structures, processes and properties to probe deformation and failure phenomena at the molecular and microscopic level. Here we discuss how advanced experimental, computational and theoretical methods can be used to assess structure-process-property relations and to monitor and predict mechanisms associated with failure of protein materials. Case studies are presented to examine failure phenomena in the progression of disease. From this materials science perspective, a de novo basis for understanding biological processes can be used to develop new approaches for treating medical disorders. We highlight opportunities to use knowledge gained from the integration of multiple scales with physical, biological and chemical concepts for potential applications in materials design and nanotechnology.

Stress-strain experiments on individual collagen fibrils

Collagen, a molecule consisting of three braided protein helices, is the primary building block of many biological tissues including bone, tendon, cartilage, and skin. Staggered arrays of collagen molecules form fibrils, which arrange into higher-ordered structures such as fibers and fascicles. Because collagen plays a crucial role in determining the mechanical properties of these tissues, significant theoretical research is directed toward developing models of the stiffness, strength, and toughness of collagen molecules and fibrils. Experimental data to guide the development of these models, however, are sparse and limited to small strain response. Using a microelectromechanical systems platform to test partially hydrated collagen fibrils under uniaxial tension, we obtained quantitative, reproducible mechanical measurements of the stress-strain curve of type I collagen fibrils, with diameters ranging from 150-470 nm. The fibrils showed a small strain (epsilon < 0.09) modulus of 0.86 +/- 0.45 GPa. Fibrils tested to strains as high as 100% demonstrated strain softening (sigma(yield) = 0.22 +/- 0.14 GPa; epsilon(yield) = 0.21 +/- 0.13) and strain hardening, time-dependent recoverable residual strain, dehydration-induced embrittlement, and susceptibility to cyclic fatigue. The results suggest that the stress-strain behavior of collagen fibrils is dictated by global characteristic dimensions as well as internal structure.

Influences of hyaluronan on type II collagen fibrillogenesis in vitro

The effect to the kinetics of type II collagen fibrillogenesis with the addition of hyaluronan (HA), (Mw of 1.8x10(6) Da), at various concentrations of HA (0.01, 0.05 and 0.1 wt.%) for a series of fibril formation systems was examined in this study. Evidences deduced from the turbidity-time curves revealed that the inclusion of HA had minor or no impact to the fibrillogenesis of type II collagen (collagen conc. at 0.2 mg/mL). The apparent rate constants, klag (lag phase) increased slightly but kg (growth phase) decreased not very significantly with addition of HA, as compared to the case of pure collagen. This leads us to believe tentatively that, with the addition of HA to collagen solutions, the nucleation process of the fibril formation might have been sped up slightly whereas the growth process slowed up slightly. However, data from TEM observations on the resulting fibrils indicated that the presence of HA did not significantly affect the diameters and the characteristic D-banding periods of the collagen fiber formed. And, from the statistical analyses, we found only insignificant difference (P>0.05) between the specimens from the various experimental groups. It seems to indicate that the ultimate packing of collagen monomers was probably not interfered or affected significantly by the presence of HA in vitro.

Structural investigations on native collagen type I fibrils using AFM

This study was carried out to determine the elastic properties of single collagen type I fibrils with the use of atomic force microscopy (AFM). Native collagen fibrils were formed by self-assembly in vitro characterized with the AFM. To confirm the inner assembly of the collagen fibrils, the AFM was used as a microdissection tool. Native collagen type I fibrils were dissected and the inner core uncovered. To determine the elastic properties of collagen fibrils the tip of the AFM was used as a nanoindentor by recording force-displacement curves. Measurements were done on the outer shell and in the core of the fibril. The structural investigations revealed the banding of the shell also in the core of native collagen fibrils. Nanoindentation experiments showed the same Young's modulus on the shell as well as in the core of the investigated native collagen fibrils. In addition, the measurements indicate a higher adhesion in the core of the collagen fibrils compared to the shell.

Nano measurements with micro-devices: mechanical properties of hydrated collagen fibrils

The mechanical response of a biological material to applied forces reflects deformation mechanisms occurring within a hierarchical architecture extending over several distinct length scales. Characterizing and in turn predicting the behaviour of such a material requires an understanding of the mechanical properties of the substructures within the hierarchy, the interaction between the substructures, and the relative influence of each substructure on the overall behaviour. While significant progress has been made in mechanical testing of micrometre to millimetre sized biological specimens, quantitative reproducible experimental techniques for making mechanical measurements on specimens with characteristic dimensions in the smaller range of 10-1000 nm are lacking. Filling this void in experimentation is a necessary step towards the development of realistic multiscale computational models useful to predict and mitigate the risk of bone fracture, design improved synthetic replacements for bones, tendons and ligaments, and engineer bioinspired efficient and environmentally friendly structures. Here, we describe a microelectromechanical systems device for directly measuring the tensile strength, stiffness and fatigue behaviour of nanoscale fibres. We used the device to obtain the first stress-strain curve of an isolated collagen fibril producing the modulus and some fatigue properties of this soft nanofibril.

Fibrous long spacing type collagen fibrils have a hierarchical internal structure

Nanodissection of single fibrous long spacing (FLS) type collagen fibrils by atomic force microscopy (AFM) reveals hierarchical internal structure: Fibrillar subcomponents with diameters of approximately 10 to 20 nm were observed to be running parallel to the long axis of the fibril in which they are found. The fibrillar subcomponent displayed protrusions with characteristic approximately 270 nm periodicity, such that protrusions on neighboring subfibrils were aligned in register. Hence, the banding pattern of mature FLS-type collagen fibrils arises from the in-register alignment of these fibrillar subcomponents. This hierarchical organization observed in FLS-type collagen fibrils is different from that previously reported for native-type collagen fibrils, displaying no supercoiling at the level of organization observed.

Controlled Self-Assembly of Collagen Fibrils by an Automated Dialysis System

In vitro self-assembled collagen fibrils form a variety of different structures during dialysis. The self-assembly is dependent on several parameters, such as concentrations of collagen and α1-acid glycoprotein, temperature, dialysis time, and the acid concentration. For a detailed understanding of the assembly pathway and structural features like banding pattern or mechanical properties it is necessary to study single collagen fibrils. In this work we present a fully automated system to control the permeation of molecules through a membrane like a dialysis tubing. This allows us to ramp arbitrary diffusion rate profiles during the self-assembly process of macromolecules, such as collagen. The system combines a molecular sieving method with a computer assisted control system for measuring process variables. With the regulation of the diffusion rate it is possible to control and manipulate the collagen self-assembly process during the whole process time. Its performance is demonstrated by the preparation of various collagen type I fibrils and native collagen type II fibrils. The combination with the atomic force microscope (AFM) allows a high resolution characterization of the self-assembled fibrils. In principle, the represented system can be also applied for the production of other biomolecules, where a dialysis enhanced self-assembly process is used.

Fibrillar assembly and stability of collagen coating on titanium for improved osteoblast responses

Collagen, as a major constituent of human connective tissues, has been regarded as one of the most important biomaterials. As a coating moiety on Ti hard-tissue implants, the collagen has recently attracted a great deal of attention. This article reports the effects of fibrillar assembly and crosslinking of collagen on its chemical stability and the subsequent osteoblastic responses. The fibrillar self-assembly of collagen was carried out by incubating acid-dissolved collagen in an ionic-buffered medium at 37 degrees C. The degree of assembly was varied with the incubation time and monitored by the turbidity change. The differently assembled collagen was coated on the Ti and crosslinked with a carbodiimide derivative. The partially assembled collagen contained fibrils with varying diameters as well as nonfibrillar aggregates. On the other hand, the fully assembled collagen showed the complete formation of fibrils with uniform diameters of approximately 100-200 nm with periodic stain patterns within the fibrils, which are typical of native collagen fibers. Through this fibrillar assembly, the collagen coating had significantly improved chemical stability in both the saline and collagenase media. The subsequent crosslinking step also improved the stability of the collagen coating, particularly in the unassembled collagen. The fibrillar assembly and the crosslinking of collagen significantly influenced the osteoblastic cell responses. Without the assembly, the collagen layer on Ti adversely affected the cell attachment and proliferation. However, those cellular responses were improved significantly when the collagen was assembled to fibrils and the assembly degree was increased. After crosslinking the collagen coating, these cellular responses were significantly enhanced in the case of the unassembled collagen but were not altered much in the assembled collagen. Based on these observations, it is suggested that the fibrillar assembly and the crosslinking of collagen require careful considerations in the collagen administration as a coating moiety.

Collagen adsorption and structure on polymer surfaces observed by atomic force microscopy

The structure and adsorption patterns of type I and type III collagen were imaged on various polymer substrates with atomic force microscopy. Type I collagen had higher adsorption on polystyrene than on a series of polymethacrylates and formed a network of tightly, interwoven strands. Upon adsorption to different polymethacrylates, with varying side chain lengths, the collagen molecules formed long, branching fibrils. Types I and III collagen had different adsorption patterns, in some cases, on the identical substrate material. For example, instead of forming a tightly packed network, type III forms long, branching fibers on the polystyrene surface. On other materials, such as poly(n-butyl methacrylate), the two types of collagen showed similar adsorption pattern and structure. Adsorbed collagen was also imaged on various blends of polystyrene and polymethacrylates to determine how the polymer surface chemical structure and surface topography mediates protein adsorption.