Fibrous long spacing collagen ultrastructure elucidated by atomic force microscopy

In vitrobioprinted 3D model enhancing osteoblast-to-osteocyte differentiation

In vitrobone models are pivotal for understanding tissue behavior and cellular responses, particularly in unravelling certain pathologies' mechanisms and assessing the impact of new therapeutic interventions. A desirablein vitrobone model should incorporate primary human cells within a 3D environment that mimics the mechanical properties characteristics of osteoid and faithfully replicate all stages of osteogenic differentiation from osteoblasts to osteocytes. However, to date, no bio-printed model using primary osteoblasts has demonstrated the expression of osteocytic protein markers. This study aimed to develop bio-printedin vitromodel that accurately captures the differentiation process of human primary osteoblasts into osteocytes. Given the considerable impact of hydrogel stiffness and relaxation behavior on osteoblast activity, we employed three distinct cross-linking solutions to fabricate hydrogels. These hydrogels were designed to exhibit either similar elastic behavior with different elastic moduli, or similar elastic moduli with varying relaxation behavior. These hydrogels, composed of gelatin (5% w/v), alginate (1%w/v) and fibrinogen (2%w/v), were designed to be compatible with micro-extrusion bioprinting and proliferative. The modulation of their biomechanical properties, including stiffness and viscoelastic behavior, was achieved by applying various concentrations of cross-linkers targeting both gelatin covalent bonding (transglutaminase) and alginate chains' ionic cross-linking (calcium). Among the conditions tested, the hydrogel with a low elastic modulus of 8 kPa and a viscoelastic behavior over time exhibited promising outcomes regarding osteoblast-to-osteocyte differentiation. The cessation of cell proliferation coincided with a significant increase in alkaline phosphatase activity, the development of dendrites, and the expression of the osteocyte marker PHEX. Within this hydrogel, cells actively influenced their environment, as evidenced by hydrogel contraction and the secretion of collagen I. This bio-printed model, demonstrating primary human osteoblasts expressing an osteocyte-specific protein, marks a significant achievement. We envision its substantial utility in advancing research on bone pathologies, including osteoporosis and bone tumors.

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.

Defective chicken skin collagen molecules, hydrolyzed by actinidain protease, assemble to form loosely packed fibrils that promote cell spheroid formation

Cells recognize collagen fibrils as the first step in the process of adherence. Fibrils of chicken skin actinidain-hydrolyzed collagen (low adhesive scaffold collagen, LASCol), in which the telopeptide domains are almost completely removed, cause adhering cells to form spheroids instead of adopting a monolayer morphology. Our goal was to elucidate the ultrastructure of the LASCol fibrils compared with pepsin-hydrolyzed collagen (PepCol) fibrils. At low concentration of 0.2 mg/mL, the time to reach the maximum increasing rate of turbidity for LASCol was all slower than that for PepCol. Differential scanning calorimetry showed that the thermal stability of collagen self-assembly changed significantly between pH 5.5 and pH 6.6 with and without a small number of telopeptides. However, the calorimetric enthalpy change did not vary much in that pH range. The melting temperature of LASCol fibrils at pH 7.3 was 55.1 °C, whereas PepCol fibrils exhibited a peak around 56.9 °C. The D-periodicity of each fibril was the same at 67 nm. Nevertheless, the looseness of molecular packing in LASCol fibrils was demonstrated by circular dichroism measurements and immuno-scanning electron microscopy with a polyclonal antibody against type I collagen. As there is a close relationship between function and structure, loosely packed collagen fibrils would be one factor that promotes cell spheroid formation.

In vitro stem cell modelling demonstrates a proof-of-concept for excess functional mutant TIMP3 as the cause of Sorsby fundus dystrophy

Sorsby fundus dystrophy (SFD) is a rare autosomal dominant disease of the macula that leads to bilateral loss of central vision and is caused by mutations in the TIMP3 gene. However, the mechanisms by which TIMP3 mutations cause SFD are poorly understood. Here, we generated human induced pluripotent stem cell-derived retinal pigmented epithelial (hiPSC-RPE) cells from three SFD patients carrying TIMP3 p.(Ser204Cys) and three non-affected controls to study disease-related structural and functional differences in the RPE. SFD-hiPSC-RPE exhibited characteristic RPE structure and physiology but showed significantly reduced transepithelial electrical resistance associated with enriched expression of cytoskeletal remodelling proteins. SFD-hiPSC-RPE exhibited basolateral accumulation of TIMP3 monomers, despite no change in TIMP3 gene expression. TIMP3 dimers were observed in both SFD and control hiPSC-RPE, suggesting that mutant TIMP3 dimerisation does not drive SFD pathology. Furthermore, mutant TIMP3 retained matrix metalloproteinase activity. Proteomic profiling showed increased expression of ECM proteins, endothelial cell interactions and angiogenesis-related pathways in SFD-hiPSC-RPE. By contrast, there were no changes in VEGF secretion. However, SFD-hiPSC-RPE secreted higher levels of monocyte chemoattractant protein 1, PDGF and angiogenin. Our findings provide a proof-of-concept that SFD patient-derived hiPSC-RPE mimic mature RPE cells and support the hypothesis that excess accumulation of mutant TIMP3, rather than an absence or deficiency of functional TIMP3, drives ECM and angiogenesis-related changes in SFD. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Phagocytosis of Collagen Fibrils by Fibroblasts In Vivo Is Independent of the uPARAP/Endo180 Receptor

As a crucial step in ECM remodeling, collagen degradation occurs through different processes, including both extracellular and intracellular degradation. The extracellular pathways of collagen degradation require secretion of collagenolytic proteases, whereas intracellular collagen degradation occurs in the lysosomal compartment after uptake, involving either pre-cleaved or intact fibrillar collagen. The endocytic collagen receptor uPARAP/Endo180 plays an important role in internalization of large collagen degradation products, whereas its role in the phagocytosis of fibrillar collagen has been debated. In fact, the role of this receptor in regular collagen phagocytosis in vivo has not been established. In this study, we have studied the role of uPARAP in the phagocytosis of collagen fibrils in vivo by analyzing different connective tissues of mice lacking uPARAP. Using transmission electron microscopy (TEM), we found that fibroblasts in the periosteum of tibia and calvaria, as well as in the periodontal ligament of molar and incisor, phagocytosed collagen fibrils independently of uPARAP. Quantification of phagocytosed collagen in the periodontal ligament of uPARAP-deficient mice and controls revealed no difference in the amount of fibrillar collagen taken up by uPARAP-deficient mice. The findings show that under in vivo conditions uPARAP does not play a role in the phagocytic uptake of collagen fibrils by fibroblasts. Consequently, the cellular uptake of collagen fibrils and collagen cleavage products probably occurs through fundamentally different pathways. J. Cell. Biochem. 118: 1590-1595, 2017. © 2016 Wiley Periodicals, Inc.

Self-Assembly, Dynamics, and Polymorphism of hIAPP(20-29) Aggregates at Solid-Liquid Interfaces

The misfolding and subsequent assembly of proteins and peptides into insoluble amyloid structures play important roles in the development of numerous diseases. The dynamics of self-assembly and the morphology of the resulting aggregates critically depend on various environmental factors and especially on the presence of interfaces. Here, we show in detail how the presence of surfaces with different physicochemical properties influences the assembly dynamics and especially the aggregate morphology of hIAPP(20-29), an amyloidogenic fragment of the peptide hormone human islet amyloid polypeptide (hIAPP), which is involved in the development of type 2 diabetes. Time-lapse atomic force microscopy is employed to study the assembly dynamics of hIAPP(20-29) and the morphology of the resulting aggregates in bulk solution as well as at hydrophilic and hydrophobic model surfaces. We find that the presence of hydrophilic mica surfaces promotes fibrillation when compared with the assembly in bulk solution and results in a more pronounced polymorphism. Three fibrillar species are found to coexist on the mica surface, that is, straight, coiled, and ribbon-like fibrils, whereas only the straight and coiled fibrils are observed in bulk solution after comparable incubation times. In addition, the straight and coiled fibrils assembled at the mica surface have significantly different dimensions compared with those assembled in bulk solution. The three fibrillar species found on the mica surface most likely form independently by lateral association of arbitrary numbers of protofibrils with about 2 nm height. On hydrophobic hydrocarbon surfaces, fibrillation is retarded but not completely suppressed, in contrast to previous observations for full-length hIAPP(1-37). Our results show that peptide-surface interactions may induce diverse, peptide-specific alterations of amyloid assembly dynamics and fibrillar polymorphism. They may therefore contribute to a deeper understanding of the molecular processes that govern amyloid aggregation at different surfaces.

Collagen immobilization on ultraviolet light-treated poly(ethylene terephthalate)

The present article is focused on the studies regarding the effects of ultraviolet (UV) light on poly(ethylene terephthalate) (PET) films surfaces using scanning electron microscopy (SEM), Fourier transform infrared, X-ray photoelectron spectrometry (XPS), and atomic force microscopy (AFM) measurements, subsequent to collagen immobilization. UV treatment influences the surface energy of polymers as the result of the polymer chain breaking, followed by insertion of oxygen-containing functional groups. Accordingly, after UV light treatment, collagen was adsorbed on the PET surfaces in different proportions. Significant changes in the surface topography appeared after collagen immobilization on UV-treated PET films, and they were put in evidence by SEM and tapping-mode AFM experiments. XPS measurements demonstrated the adsorption of collagen on PET UV light-altered surfaces by increasing of nitrogen content. The cytocompatibility tests using stem cells have shown good results for all treated polymers.

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

Collagen fibrils are present in the extracellular matrix of animal tissue to provide structural scaffolding and mechanical strength. These native collagen fibrils have a characteristic banding periodicity of ~67 nm and are formed in vivo through the hierarchical assembly of Type I collagen monomers, which are 300 nm in length and 1.4 nm in diameter. In vitro, by varying the conditions to which the monomer building blocks are exposed, unique structures ranging in length scales up to 50 microns can be constructed, including not only native type fibrils, but also fibrous long spacing and segmental long spacing collagen. Herein, we present procedures for forming the three different collagen structures from a common commercially available collagen monomer. Using the protocols that we and others have published in the past to make these three types typically lead to mixtures of structures. In particular, unbanded fibrils were commonly found when making native collagen, and native fibrils were often present when making fibrous long spacing collagen. These new procedures have the advantage of producing the desired collagen fibril type almost exclusively. The formation of the desired structures is verified by imaging using an atomic force microscope.

The structural origin of second harmonic generation in fascia

Fascia tissue is rich in collagen type I proteins and can be imaged by second harmonic generation (SHG) microscopy. While identifying the overall alignment of the collagen fibrils is evident from those images, the tridimensional structural origin for the observation of SHG signal is more complex than it apparently seems. Those images reveal that the noncentrosymmetric (piezoelectric) structures are distributed heterogeneously on spatial dimensions inferior to the resolution provided by the nonlinear optical microscope (sub-micron). Using piezoresponse force microscopy (PFM), we show that an individual collagen fibril has a noncentrosymmetric structural organization. Fibrils are found to be arranged in nano-domains where the anisotropic axis is preserved along the fibrillar axis, while across the collagen sheets, the phase of the second order nonlinear susceptibility is changing by 180 degrees between adjacent nano-domains. This complex architecture of noncentrosymmetric nano-domains governs the coherent addition of 2ω light within the focal volume and the observed features in the SHG images taken in fascia.

Structure and dynamics of water in native and tanned collagen fibers: Effect of crosslinking

The influence of crosslinking on the hydration structure of collagen has been investigated. Nuclear magnetic resonance, dielectric relaxation and thermoporometry were used to investigate water structure in native and crosslinked collagen fibers on both wet and dried specimen. Measurements reveal the influence of different chemical treatments on the transverse relaxation time and polarization of the collagen fibers. The frequency dependence of dielectric constant of collagen fibers displays an induction behavior on low frequencies. Bound water constrained in collagen fibers seems to provide signatures for changes induced by crosslinking agents on the pore diameter and distribution in collagen fibers. A correlation of transverse relaxation time of water in dry and wet states presented in this study presents an experimental tool for examining the differences in efficacy of crosslinking agents. Changes in the dielectric relaxation, dynamics of water structure and hydroporometric structure of collagen are dependent on the nature of crosslinking material.

Two-dimensional nanoscale structural and functional imaging in individual collagen type I fibrils

The piezoelectric properties of single collagen type I fibrils in fascia were imaged with sub-20 nm spatial resolution using piezoresponse force microscopy. A detailed analysis of the piezoresponse force microscopy signal in controlled tip-fibril geometry revealed shear piezoelectricity parallel to the fibril axis. The direction of the displacement is preserved along the whole fiber length and is independent of the fiber conformation. It is shown that individual fibrils within bundles in skeletal muscle fascia can have opposite polar orientations and are organized into domains, i.e., groups of several fibers having the same polar orientation. We were also able to detect piezoelectric activity of collagen fibrils in the high-frequency range up to 200 kHz, suggesting that the mechanical response time of biomolecules to electrical stimuli can be approximately 5 micros.

Actinidain-hydrolyzed type I collagen reveals a crucial amino acid sequence in fibril formation

We investigated the ability of type I collagen telopeptides to bind neighboring collagen molecules, which is thought to be the initial event in fibrillogenesis. Limited hydrolysis by actinidain protease produced monomeric collagen, which consisted almost entirely of alpha1 and alpha2 chains. As seen with ultrahigh resolution scanning electron microscopy, actinidain-hydrolyzed collagen exhibited unique self-assembly, as if at an intermediate stage, and formed a novel suprastructure characterized by poor fibrillogenesis. Then, the N- and C-terminal sequences of chicken type I collagen hydrolyzed by actinidain or pepsin were determined by Edman degradation and de novo sequence analysis with matrix-assisted laser desorption ionization-tandem time-of-flight mass spectrometry, respectively. In the C-telopeptide region of the alpha1 chain, pepsin cleaved between Asp(1035) and Phe(1036), and actinidain between Gly(1032) and Gly(1033). Thus, the actinidain-hydrolyzed alpha1 chain is shorter at the C terminus by three residues, Gly(1033), Phe(1034), and Asp(1035). In the alpha2 chain, both proteases cleaved between Glu(1030) and Val(1031). We demonstrated that a synthetic nonapeptide mimicking the alpha1 C-terminal sequence including GFD weakly inhibited the self-assembly of pepsin-hydrolyzed collagen, whereas it remarkably accelerated that of actinidain-hydrolyzed collagen. We conclude that the specific GFD sequence of the C-telopeptide of the alpha1 chain plays a crucial role in stipulating collagen suprastructure and in subsequent fibril formation.

Observation of the ultrastructure of anterior cruciate ligament graft by atomic force microscopy

This study presented the fibril ultrastructure of retrieved grafts from the reconstruction of anterior cruciate ligament (ACL) using atomic force microscopy (AFM). The tapping mode images of the AFM were taken from different areas of the longitudinally cut grafts. Regular arrangement of collagen fibrils was found in certain areas of the graft. In many areas, however, the fibrils were not well arranged in a single direction, with some smaller fibrils oriented vertically to larger parallel fibrils. The crossing and tangling of fibrils in ACL grafts was well represented in the three-dimensional AFM image. This abnormality of graft ultrastructure might indicate the possible alteration of the mechanical environment after ACL reconstruction. This study demonstrated the suitability and importance of ultrastructure observation of ACL grafts by AFM.

Effect of oxazolidine E on collagen fibril formation and stabilization of the collagen matrix

Oxazolidine E, an aldehydic cross-linking agent, is used to impart hydrothermal stability to collagen. The purpose of this study was to investigate the exact nature of oxazolidine E induced cross-links with collagen by using synthetic peptides having sequence homology with collagen type I. Tandem mass spectrometry revealed the formation of methylol and Schiff-base adducts upon reaction of oxazolidine E with the peptides. This was confirmed by allowing the reaction to proceed under reducing conditions using cyanoborohydride. Mass spectrometry (MS)-MS analysis clearly showed interaction of tryptophan and lysine residues with oxazolidine E and demonstrated that arginine could be cross-linked with glycine in the presence of oxazolidine E through the formation of a methylene bridge. Collagen fibrils regenerated from monomers in the presence and absence of oxazolidine E were studied using atomic force microscopy to investigate morphological alterations. Regenerated fibrils showing the typical 65 nm D-banding pattern were obtained from those formed both in the presence and absence of oxazolidine E, and there was no evidence of a change in the D-periodicity of these fibrils. This indicated that oxazolidine E does not hinder collagen molecules from correctly aligning to form the quarter-stagger structure.

Collagen fibril structure is affected by collagen concentration and decorin

Collagen fibrils were obtained in vitro by aggregation from acid-soluble type I collagen at different initial concentrations and with the addition of decorin core or intact decorin. All specimens were observed by scanning electron microscopy and atomic force microscopy. In line with the findings of other authors, lacking decorin, collagen fibrils undergo an extensive lateral association leading to the formation of a continuous three-dimensional network. The addition of intact decorin or decorin core was equally effective in preventing lateral fusion and restoring the normal fibril appearance. In addition, the fibril diameter was clearly dependent on the initial collagen concentration but not on the presence/absence of proteoglycans. An unusual fibril structure was observed as a result of a very low initial collagen concentration, leading to the formation of huge, irregular superfibrils apparently formed by the lateral coalescence of lesser fibrils, and with a distinctive coil-structured surface. Spots of incomplete fibrillogenesis were occasionally found, where all fibrils appeared made of individual, interwined subfibrils, confirming the presence of a hierarchical association mechanism.

Analysis of the healthy rabbit lens surface using MAC Mode atomic force microscopy

In this investigation healthy rabbit crystalline lenses were characterized by atomic force microscopy (AFM). The lenses were cut in slices with thickness with 1mm and thus, put after cortex distinct regions of nucleus and cortex for AFM examination. AFM analysis were carried out using a PicoSPM I operating in Mac Mode. We obtained topographic images of rabbit lenses and a quantitative analysis of the width and height of fibers for nucleus and cortex regions. The longitudinal section analysis of fibers in the nucleus region indicated structures with an average width of 200nm and average height of 200nm. The intershells distance was determined as 4microm. Fiber cell cross-section dimensions, longitudinal and transverse widths, could be estimated in these regions from the AFM images. Structures with average widths as small as 1.0microm are observed in the nucleus; the intershell distance is 4.0microm. In cortical regions, hexagonal structures with average longitudinal and transverse widths of 5.0mum and 3.0mum, respectively, were identified. Three-dimensional images of tissue sections with resolution on a nanometer scale were obtained. The potential of AFM analysis for characterizing healthy and pathologic lens tissues is discussed.

Characterization of changes to the cell surface during the life cycle of Streptomyces coelicolor: atomic force microscopy of living cells

Cell surface changes that accompany the complex life cycle of Streptomyces coelicolor were monitored by atomic force microscopy (AFM) of living cells. Images were obtained using tapping mode to reveal that young, branching vegetative hyphae have a relatively smooth surface and are attached to an inert silica surface by means of a secreted extracellular matrix. Older hyphae, representing a transition between substrate and aerial growth, are sparsely decorated with fibers. Previously, a well-organized stable mosaic of fibers, called the rodlet layer, coating the surface of spores has been observed using electron microscopy. AFM revealed that aerial hyphae, prior to sporulation, possess a relatively unstable dense heterogeneous fibrous layer. Material from this layer is shed as the hyphae mature, revealing a more tightly organized fibrous mosaic layer typical of spores. The aerial hyphae are also characterized by the absence of the secreted extracellular matrix. The formation of sporulation septa is accompanied by modification to the surface layer, which undergoes localized temporary disruption at the sites of cell division. The characteristics of the hyphal surfaces of mutants show how various chaplin and rodlin proteins contribute to the formation of fibrous layers of differing stabilities. Finally, older spores with a compact rodlet layer develop surface concavities that are attributed to a reduction of intracellular turgor pressure as metabolic activity slows.

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.

Functional characterization of cell-wall-associated protein WapA in Streptococcus mutans

Streptococcus mutans is known as a primary pathogen responsible for dental caries. One of the virulence factors of S. mutans in cariogenicity is its ability to attach to the tooth surface and form a biofilm. Several surface proteins have been shown to be involved in this process. A 29 kDa surface protein named wall-associated protein A (WapA, antigen A or antigen III), was previously used as a vaccine in animal studies for immunization against dental caries. However, the function of WapA in S. mutans is still not clear. This study characterized the function of WapA in cell surface structure and biofilm formation. Compared to the wild-type, the wapA mutant had much-reduced cell chain length, diminished cell-cell aggregation, altered cell surface ultrastructure, and unstructured biofilm architecture. Furthermore, in vivo force spectroscopy revealed that the cell surface of the wapA mutant was less sticky than that of the wild-type cells. More interestingly, these phenotypic differences diminished as sucrose concentration in the medium was increased to 0.5 %. Real-time RT-PCR analysis demonstrated that sucrose strongly repressed wapA gene expression in both planktonic and biofilm cells. These results suggest that the WapA protein plays an important structural role on the cell surface, which ultimately affects sucrose-independent cell-cell aggregation and biofilm architecture.

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.

Nanometer-scale heat-conductivity measurements on biological samples

With semiconductor structures reaching the nanometer scale, heat conductivity measurements on the mesoscopic range of some tens of nanometers become an increasingly important aspect for the further improvement in digital processing and storage. Also the attempt to use atomic-force microscopy (AFM) technology for high-density data storage by writing information bits as nanometer-sized indentations into a polymer substrate with a heated cantilever tip asks for a careful investigation of the nano-scale heat-conductivity properties of polymers. Furthermore, in many AFM imaging applications, heat conductivity can provide additional information about the material the imaged structures consist of. In this respect, heat conductivity can also become very interesting in studies of usually quite heterogeneous biological samples, if the resolution can attain the nanometer range. In standard scanning thermal microscopy application, the tip forms a thermocouple, which precludes high-resolution imaging, as thermocouples cannot be made sufficiently small. In this paper, which focuses on biological applications, we demonstrate that by using an ultra sharp AFM cantilever with a Joule heating element above the tip structure different molecular components can be distinguished thanks to their different heat-conductivity properties. In this case, the resolution is determined by the actual tip size, and it can reach 10nm.

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.

Atomic force microscopy study of the structure-function relationships of the biofilm-forming bacterium Streptococcus mutans

Atomic force microscopy (AFM) has garnered much interest in recent years for its ability to probe the structure, function and cellular nanomechanics inherent to specific biological cells. In particular, we have used AFM to probe the important structure-function relationships of the bacterium Streptococcus mutans. S. mutans is the primary aetiological agent in human dental caries (tooth decay), and is of medical importance due to the virulence properties of these cells in biofilm initiation and formation, leading to increased tolerance to antibiotics. We have used AFM to characterize the unique surface structures of distinct mutants of S. mutans. These mutations are located in specific genes that encode surface proteins, thus using AFM we have resolved characteristic surface features for mutant strains compared to the wild type. Ultimately, our characterization of surface morphology has shown distinct differences in the local properties displayed by various S. mutans strains on the nanoscale, which is imperative for understanding the collective properties of these cells in biofilm formation.

Characterization of Superparamagnetic Nanoparticle Interactions with Extracellular Matrix in an in Vitro System

Controlled dispersion of therapeutic agents within liquid- and gel-filled cavities represents a barrier to treatment of some cancers and other pathological states. Interstitial delivery is compromised by the poor mobility of macromolecules and larger nanoscale structures. We developed an in vitro system to quantify the suitability of superparamagnetic nanoparticles (SPM NPs) as a site-specific therapeutic vehicle for delivery through fluid- and gel-based systems. SPM NP motion was induced by an external magnetic field. NP migration was modulated by NP concentration and surface coating. 135 nanometer radius PEGylated NPs moved through the extracellular matrix with an average velocity of 1.5 mm h(-1), suitable for some clinical applications. Increasing the SPM NP radius to 400 nm while maintaining the same per NP magnetic susceptibility resulted in a greater than 1,000-fold reduction in magnetic mobility, to less than 0.01 mm h(-1). The critical influence of NP size on gel permeation was also observed in silica-coated 135 nm SPM NPs that aggregated under the experimental conditions. Aggregation played a critical role in determining the behavior of the nanoparticles. SPM NPs allow significant free-solution mobility to specific sites within a cavity and generate sufficient force to penetrate common in vivo gels.

A study of the influence of polysaccharides on collagen self-assembly: Nanostructure and kinetics

Collagen, a critical part of the extra-cellular matrix of tissues, is a popular native material for building scaffolding for tissue-engineering applications. To mimic the structural and functional profiles of materials found in the native extra-cellular matrix, numerous efforts have been made toward developing a novel scaffold combining collagen with other biomacromolecules. All of these works have been focused on improving the mechanical or biochemical properties of the collagen-based matrix. Unfortunately, most of these studies have failed to consider the nanostructure of collagen in the complex matrix. The aim of our study was to investigate the aggregation pattern of collagen after addition of polysaccharides with positive or negative charge, the dose-response relationship, and the effect on reconstitution kinetics. Generally, collagen self-assembles into fibrils with a diameter of around 95 nm but, in the presence of various polysaccharides in varying amounts, collagen self-assembles into different shapes with larger diameters compared with collagen alone. Although the morphology and diameter of the collagen fibrils varies with reconstitution conditions, the D-periods of the fibrils all remained the same regardless of the species or concentration of polysaccharides. The kinetics of fibril formation was determined from turbidity-time curves. All turbidity curves demonstrated that polysaccharides only alter the lag time and time frame of reconstitution, but have no significant effect on the mechanism of reconstitution. Together our data indicate that the presence of biomacromolecules can alter the kinetics and the 3D fibril ultrastructure of assembled collagen and that the consequent structural changes may affect cellular responses in medical applications.

Atomic force microscopy of collagen structure in bone and dentine revealed by osteoclastic resorption

Mineralised tissues such as bone consist of two material phases: collagen protein fibrils, secreted by osteoblasts, form model structures for subsequent deposition of mineral, calcium hydroxyapatite. Collagen and mineral are removed in a three-dimensional manner by osteoclasts during bone turnover in skeletal growth or repair. Bone active drugs have recently been developed for skeletal diseases, and there is revived interest in changes in the structure of mineralised tissues seen in disease and upon treatment. The resolution of atomic force microscopy and use of unmodified samples has enabled us to image bone and dentine collagen exposed by the natural process of cellular dissolution of mineralised matrix. The morphology of bone and dentine has been analysed when fully mineralised and after osteoclast-mediated bone resorption, and compared with results from other microscopy techniques. Banded type I collagen, with 66.5+/-1.4 nm axial D-periodicity and 62.2+/-7.0 nm diameter, has been identified within resorption lacunae in bone and 69.4+/-4.3 nm axial D-periodicity and 140.6+/-12.4 nm diameter in dentine substrates formed by human and rabbit osteoclasts, respectively. This observation suggests a route by which the material and morphological properties of bone collagen can be analysed in situ, compared with collagen from non-skeletal sites, and contrasted in diseases of medical importance, such as osteoporosis, where skeletal tissue is mechanically weakened.

Stabilization of collagen using plant polyphenol: Role of catechin

Collagen, a unique connective tissue protein finds extensive application as biocompatible biomaterial in wound healing, as drug carriers, cosmetics, etc. A work has been undertaken to study the stabilization of type I collagen using the plant polyphenol catechin. Catechin treated collagen fibres showed a shrinkage temperature around 70 degrees C implying that catechin is able to impart thermal stability to collagen. Catechin treated collagen fibres has been found to be stable even after treatment with high concentration of the secondary structural destabilizer, urea. Circular dichroism studies revealed that there is no major alteration in the structure of collagen on treatment with catechin. The study has demonstrated the involvement of hydrogen bonding and hydrophobic interactions as the major forces involved in the stabilization of collagen by the plant polyphenol, catechin.

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.

Nanoscale visualization and characterization of Myxococcus xanthus cells with atomic force microscopy

Multicellular microbial communities are the predominant form of existence for microorganisms in nature. As one of the most primitive social organisms, Myxococcus xanthus has been an ideal model bacterium for studying intercellular interaction and multicellular organization. Through previous genetic and EM studies, various extracellular appendages and matrix components have been found to be involved in the social behavior of M. xanthus, but none of them was directly visualized and analyzed under native conditions. Here, we used atomic force microscopy (AFM) imaging and in vivo force spectroscopy to characterize these cellular structures under native conditions. AFM imaging revealed morphological details on the extracellular ultrastructures at an unprecedented resolution, and in vivo force spectroscopy of live cells in fluid allowed us to nanomechanically characterize extracellular polymeric substances. The findings provide the basis for AFM as a useful tool for investigating microbial-surface ultrastructures and nanomechanical properties under native conditions.

Assembly of collagen into microribbons: effects of pH and electrolytes

Collagen represents the major structural protein of the extracellular matrix. Elucidating the mechanism of its assembly is important for understanding many cell biological and medical processes as well as for tissue engineering and biotechnological approaches. In this work, conditions for the self-assembly of collagen type I molecules on a supporting surface were characterized. By applying hydrodynamic flow, collagen assembled into ultrathin ( approximately 3 nm) highly anisotropic ribbon-like structures coating the entire support. We call these novel collagen structures microribbons. High-resolution atomic force microscopy topographs show that subunits of these microribbons are built by fibrillar structures. The smallest units of these fibrillar structures have cross-sections of approximately 3 x 5nm, consistent with current models of collagen microfibril formation. By varying the pH and electrolyte of the buffer solution during the self-assembly process, the microfibril density and contacts formed within this network could be controlled. Under certain electrolyte compositions the microribbons and microfibers display the characteristic D-periodicity of approximately 65 nm observed for much thicker collagen fibrils. In addition to providing insight into the mechanism of collagen assembly, the ultraflat collagen matrices may also offer novel ways to bio-functionalize surfaces.

Structural changes in human type I collagen fibrils investigated by force spectroscopy

In the field of biomechanics, collagen fibrils are believed to be robust mechanical structures characterized by a low extensibility. Until very recently, information on the mechanical properties of collagen fibrils could only be derived from ensemble measurements performed on complete tissues such as bone, skin, and tendon. Here, we measure force-elongation/relaxation profiles of single collagen fibrils using atomic force microscopy (AFM)-based force spectroscopy (FS). The elongation profiles show that in vitro-assembled human type I collagen fibrils are characterized by a large extensibility. Numerous discontinuities and a plateau in the force profile indicate major reorganization occurring within the fibrils in the 1.5- to 4.5-nN range. Our study demonstrates that newly assembled collagen fibrils are robust structures with a significant reserve of elasticity that could play a determinant role in the extracellular matrix (ECM) remodeling associated with tissue growth and morphogenesis.

Interaction of aldehydes with collagen: effect on thermal, enzymatic and conformational stability

Stabilization of type I rat tail tendon (RTT) collagen by various aldehydes, viz. formaldehyde, gluteraldehyde, glyoxal and crotanaldehyde was studied to understand the effect of each on the thermal, enzymatic and conformational stability of collagen. The aldehydes have been found to increase the heat stability of rat tail tendon collagen fibres from 62 to 77-86 degrees C. The increase in thermal stability was found to be in a species dependent manner. The variation in the thermal stability of collagen brought about by aldehydes was in the order of formaldehyde > gluteraldehyde > glyoxal > crotanaldehdye. The aldehydes also impart a high degree of stability to collagen against the activity of the degrading enzyme, collagenase. The order of enzymatic stability brought about by aldehydes follows the same trend as the thermal stability brought about by them. This shows that the number of cross-links formed influence both the thermal and enzymatic stability in the similar manner. The effect of various aldehydes on the secondary structure of collagen was studied using circular dichroism and it was found that the aldehydes lead to changes in the amplitude of the circular dichroic (CD) spectrum but did not alter the triple helical conformation of collagen. The secondary structure of collagen is not significantly altered on interaction with different aldehydes.

Nanoscale organization of adsorbed collagen: Influence of substrate hydrophobicity and adsorption time

The adsorption of collagen on polystyrene (PS) and polystyrene oxidized by oxygen plasma discharge (PSox) was studied as a function of time using radiolabeling, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Radiolabeling and XPS indicated that the initial step of adsorption was faster on PS than on PSox. AFM imaging under water revealed very different supramolecular organization of the adsorbed films depending on time and on the nature of the substrate: PS showed patterns of collagen aggregates at all adsorption times (from 1 min to 24 h); PSox was covered with a smooth layer except at long adsorption times (24 h), for which a mesh of collagen structures was observed. After fast drying, the collagen layer remained continuous and showed a morphology which recalled that observed under water. The mechanical stability of the adsorbed films was assessed under water by scraping with the AFM probe at different loading forces: no perturbations were created on PSox; in contrast, the layer adsorbed on PS was sensitive to scraping, the minimum force required to alter the collagen layer morphology increasing with time. These differences in the film properties were correlated with force measurements upon retraction: multiple adhesion forces were observed with collagen adsorbed on PS samples, whereas such an effect was never observed on PSox. The results show that the amount adsorbed and the organization of the adsorbed film respond differently to the adsorption time and that this is influenced by surface hydrophobicity. The quick initial adsorption on PS, compared to PSox, is thought to leave dangling collagen segments that are responsible for the observed morphology, for adhesion forces, and for lower mechanical resistance of the adsorbed layer.

Dentin matrix protein 1 immobilized on type I collagen fibrils facilitates apatite deposition in vitro

During bone and dentin mineralization, the crystal nucleation and growth processes are considered to be matrix regulated. Osteoblasts and odontoblasts synthesize a polymeric collagenous matrix, which forms a template for apatite initiation and elongation. Coordinated and controlled reaction between type I collagen and bone/dentin-specific noncollagenous proteins are necessary for well defined biogenic crystal formation. However, the process by which collagen surfaces become mineralized is not understood. Dentin matrix protein 1 (DMP1) is an acidic noncollagenous protein expressed during the initial stages of mineralized matrix formation in bone and dentin. Here we show that DMP1 bound specifically to type I collagen, with the binding region located at the N-telopeptide region of type I collagen. Peptide mapping identified two acidic clusters in DMP1 responsible for interacting with type I collagen. The collagen binding property of these domains was further confirmed by site-directed mutagenesis. Transmission electron microscopy analyses have localized DMP1 in the gap region of the collagen fibrils. Fibrillogenesis assays further demonstrated that DMP1 accelerated the assembly of the collagen fibrils in vitro and also increased the diameter of the reconstituted collagen fibrils. In vitro mineralization studies in the presence of calcium and phosphate ions demonstrated apatite deposition only at the collagen-bound DMP1 sites. Thus specific binding of DMP1 and possibly other noncollagenous proteins on the collagen fibril might be a key step in collagen matrix organization and mineralization.

Vascular smooth muscle cells orchestrate the assembly of type I collagen via alpha2beta1 integrin, RhoA, and fibronectin polymerization

Assembly of collagen into fibrils is widely studied as a spontaneous and entropy-driven process. To determine whether vascular smooth muscle cells (SMCs) impact the formation of collagen fibrils, we microscopically tracked the conversion of soluble to insoluble collagen in human SMC cultures, using fluorescent type I collagen at concentrations less than that which supported self-assembly. Collagen microaggregates were found to form on the cell surface, initially as punctate collections and then as an increasingly intricate network of fibrils. These fibrils displayed 67-nm periodicity and were found in membrane-delimited cellular invaginations. Fibril assembly was inhibited by an anti-alpha2beta1 integrin antibody and accelerated by an alpha2beta1 integrin antibody that stimulates a high-affinity binding state. Newly assembled collagen fibrils were also found to co-localize with newly assembled fibronectin fibrils. Moreover, inhibition of fibronectin assembly with an anti-alpha5beta1 integrin antibody completely inhibited collagen assembly. Collagen fibril formation was also linked to the cytoskeleton. Fibrils formed on the stretched tails of SMCs, ran parallel to actin microfilament bundles, and formed poorly on SMCs transduced with retrovirus containing cDNA for dominant-negative RhoA and robustly on SMCs expressing constitutively active RhoA. Lysophosphatidic acid, which activates RhoA and stimulates fibronectin assembly, stimulated collagen fibril formation, establishing for the first time that collagen polymerization can be regulated by soluble agonists of cell function. Thus, collagen fibril formation is under close cellular control and is dynamically integrated with fibronectin assembly, opening new possibilities for modifying collagen deposition.

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

The mechanism of formation of fibrillar collagen with a banding periodicity much greater than the 67 nm of native collagen, i.e. the so-called fibrous long spacing (FLS) collagen, has been speculated upon, but has not been previously studied experimentally from a detailed structural perspective. In vitro, such fibrils, with banding periodicity of approximately 270 nm, may be produced by dialysis of an acidic solution of type I collagen and alpha(1)-acid glycoprotein against deionized water. FLS collagen assembly was investigated by visualization of assembly intermediates that were formed during the course of dialysis using atomic force microscopy. Below pH 4, thin, curly nonbanded fibrils were formed. When the dialysis solution reached approximately pH 4, thin, filamentous structures that showed protrusions spaced at approximately 270 nm were seen. As the pH increased, these protofibrils appeared to associate loosely into larger fibrils with clear approximately 270 nm banding which increased in diameter and compactness, such that by approximately pH 4.6, mature FLS collagen fibrils begin to be observed with increasing frequency. These results suggest that there are aspects of a stepwise process in the formation of FLS collagen, and that the banding pattern arises quite early and very specifically in this process. It is proposed that typical 4D-period staggered microfibril subunits assemble laterally with minimal stagger between adjacent fibrils. alpha(1)-Acid glycoprotein presumably promotes this otherwise abnormal lateral assembly over native-type self-assembly. Cocoon-like fibrils, which are hundreds of nanometers in diameter and 10-20 microm in length, were found to coexist with mature FLS fibrils.

Investigating the ultrastructure of fibrous long spacing collagen by parallel atomic force and transmission electron microscopy

The ultrastructure of fibrous long spacing (FLS) collagen fibrils has been investigated by performing both atomic force microscopy (AFM) and transmission electron microscopy (TEM) on exactly the same area of FLS collagen fibril samples. These FLS collagen fibrils were formed in vitro from type I collagen and alpha1-acid glycoprotein (AAG) solutions. On the basis of the correlated AFM and TEM images obtained before and after negative staining, the periodic dark bands observed in TEM images along the longitudinal axis of the FLS collagen fibril correspond directly to periodic protrusions seen by AFM. This observation is in agreement with the original surmise made by Gross, Highberger, and Schmitt (Gross J, Highberger JH, Schmitt FO, Proc Natl Acad Sci USA 1954;40:679-688) that the major repeating dark bands of FLS collagen fibrils observed under TEM are thick relative to the interband region. Although these results do not refute the idea of negative stain penetration into gap regions proposed by Hodge and Petruska (Petruska JA, Hodge AJ. Aspects of protein structure. Ramachandran GN, editor. New York: Academic Press; 1963. p. 289-300), there is no need to invoke the presence of gap regions to explain the periodic dark bands observed in TEM images of FLS collagen fibrils.

A study of fibrous long spacing collagen ultrastructure and assembly by atomic force microscopy

Fibrous long spacing collagen (FLS) fibrils are collagen fibrils that display a banding with periodicity greater than the 67nm periodicity of native collagen. FLS fibrils can be formed in vitro by addition of alpha(1)-acid glycoprotein to an acidified solution of monomeric collagen, followed by dialysis of the resulting mixture. We have investigated the ultrastructure of FLS fibrils formed in vitro using the atomic force microscope (AFM). The majority of the fibrils imaged showed typical diameters of approximately 150nm and had a distinct banding pattern with a approximately 250nm periodicity. However, we have also observed an additional type of FLS fibril, which is characterized by a secondary banding pattern surrounding the primary bands. These results are compared with those obtained in past investigations of FLS ultrastructure carried out using the transmission electron microscope (TEM). The importance of the fibril's surface topography in TEM staining patterns is discussed. Images of FLS fibrils in various stages of assembly have also been collected, and the implications of these images in determining the mechanism of assembly and the formation of the characteristic banding pattern of the fibrils is discussed.

Ultrastructure and assembly of segmental long spacing collagen studied by atomic force microscopy

The in vitro formation of segmental long spacing (SLS) collagen as induced by the addition of ATP to acidified Type I collagen solutions has been examined with the atomic force microscope (AFM). AFM images obtained suggest that the assembly proceeds in a stepwise manner, through an intermediate stage of oligomers, which then associate laterally to form the so-called "SLS crystallites". Attempts to induce SLS formation by the addition of other polyanionic species to monomeric collagen solutions met with mixed success; ATP-gamma-S and GTP produced SLS crystallites, whereas inorganic phosphate and other polyanionic dyes did not. This indicates that the formation of SLS cannot simply be attributed to the negation of positive charges believed to be located on the end of the collagen monomer, but rather it is a complex function of the structure and charge of both the collagen monomer and polyanion.

Ultrastructural organization of amyloid fibrils by atomic force microscopy

Atomic force microscopy has been employed to investigate the structural organization of amyloid fibrils produced in vitro from three very different polypeptide sequences. The systems investigated are a 10-residue peptide derived from the sequence of transthyretin, the 90-residue SH3 domain of bovine phosphatidylinositol-3'-kinase, and human wild-type lysozyme, a 130-residue protein containing four disulfide bridges. The results demonstrate distinct similarities between the structures formed by the different classes of fibrils despite the contrasting nature of the polypeptide species involved. SH3 and lysozyme fibrils consist typically of four protofilaments, exhibiting a left-handed twist along the fibril axis. The substructure of TTR(10-19) fibrils is not resolved by atomic force microscopy and their uniform appearance is suggestive of a regular self-association of very thin filaments. We propose that the exact number and orientation of protofilaments within amyloid fibrils is dictated by packing of the regions of the polypeptide chains that are not directly involved in formation of the cross-beta core of the fibrils. The results obtained for these proteins, none of which is directly associated with any human disease, are closely similar to those of disease-related amyloid fibrils, supporting the concept that amyloid is a generic structure of polypeptide chains. The detailed architecture of an individual fibril, however, depends on the manner in which the protofilaments assemble into the fibrillar structure, which in turn is dependent on the sequence of the polypeptide and the conditions under which the fibril is formed.

Assembly of type I collagen: fusion of fibril subunits and the influence of fibril diameter on mechanical properties

Structural stability of the extracellular matrix is primarily a consequence of fibrillar collagen and the extent of cross-linking. The relationship between collagen self-assembly, consequent fibrillar shape and mechanical properties remains unclear. Our laboratory developed a model system for the preparation of self-assembled type I collagen fibers with fibrillar substructure mimicking the hierarchical structures of tendon. The present study evaluates the effects of pH and temperature during self-assembly on fibrillar structure, and relates the structural effects of these treatments on the uniaxial tensile mechanical properties of self-assembled collagen fibers. Results of the analysis of fibril diameter distributions and mechanical properties of the fibers formed under the different incubation conditions indicate that fibril diameters grow via the lateral fusion of discrete approximately 4 nm subunits, and that fibril diameter correlates positively with the low strain modulus. Fibril diameter did not correlate with either the ultimate tensile strength or the high strain elastic modulus, which suggests that lateral aggregation and consequently fibril diameter influences mechanical properties during small strain mechanical deformation. We hypothesize that self-assembly is mediated by the formation of fibrillar subunits that laterally and linearly fuse resulting in fibrillar growth. Lateral fusion appears important in generating resistance to deformation at low strain, while linear fusion leading to longer fibrils appears important in the ultimate mechanical properties at high strain.