Electron microscope studies on the structure of collagen fibrils by negative staining

Heterogeneous Structure and Dynamics of Water in a Hydrated Collagen Microfibril

Collagen type I is well-known for its outstanding mechanical properties which it inherits from its hierarchical structure. Collagen type I fibrils may be viewed as a heterogeneous material made of protein, macromolecules (such as glycosaminoglycans and proteoglycans) and water. Water content modulates the properties of these fibrils. Yet, the properties of water and the fine interactions of water with the protein constituent of these heterofibrils have only received limited attention. Here, we propose to model collagen type I fibrils as a hydrated structure made of tropocollagen molecules assembled in a microfibril crystal. We perform large-scale all-atom molecular dynamics simulations of the hydration of collagen fibrils beyond the onset of disassembly. We found that the structural and dynamic properties of water vary strongly with the level of hydration of the microfibril. More importantly, we found that the properties vary spatially within the 67 nm D-spacing periodic structure. Alteration of the structural and dynamical properties of the collagen microfibril occur first in the gap region. Overall, we identify that the change in the role of water molecules from glue to lubricant between tropocollagen molecules arises around 100% hydration while the microfibril begins to disassemble beyond 130% water content. Our findings are supported by a decrease in hydrogen bonding, recovery of bulk water properties and amorphization of the tropocollagen molecules packing. Our simulations reveal the structure and dynamics of hydrated collagen fibrils with unprecedented spatial resolution from physiological conditions to disassembly. Beyond the process of self-assembly and the emergence of mechanical properties of collagen type I fibrils, our results may also provide new insights into mineralization of collagen fibrils.

Designing collagens to shed light on the multi-scale structure-function mapping of matrix disorders

Collagens are the most abundant structural proteins in the extracellular matrix of animals and play crucial roles in maintaining the structural integrity and mechanical properties of tissues and organs while mediating important biological processes. Fibrillar collagens have a unique triple helix structure with a characteristic repeating sequence of (Gly-X-Y)n. Variations within the repetitive sequence can cause misfolding of the triple helix, resulting in heritable connective tissue disorders. The most common variations are single-point missense mutations that lead to the substitution of a glycine residue with a bulkier amino acid (Gly → X). In this review, we will first discuss the importance of collagen's triple helix structure and how single Gly substitutions can impact its folding, structure, secretion, assembly into higher-order structures, and biological functions. We will review the role of "designer collagens," i.e., synthetic collagen-mimetic peptides and recombinant bacterial collagen as model systems to include Gly → X substitutions observed in collagen disorders and investigate their impact on structure and function utilizing in vitro studies. Lastly, we will explore how computational modeling of collagen peptides, especially molecular and steered molecular dynamics, has been instrumental in probing the effects of Gly substitutions on structure, receptor binding, and mechanical stability across multiple length scales.

Structural polymorphisms in fibrillar aggregates associated with exfoliation syndrome

Exfoliation syndrome is largely considered an age-related disease that presents with fibrillar aggregates in the front part of the eye. A growing body of literature has investigated structural diversity of amyloids and fibrillar aggregates associated with neurodegenerative disease. However, in case of exfoliation syndrome, there is a dearth of information on the biophysical characteristics of these fibrils and structural polymorphisms. Herein, structural diversity of fibrils isolated from the anterior lens capsule of patients was evaluated using transmission electron microscopy techniques. It was apparent that, despite having a low sample number of different patients, there exists a wide range of fibril morphologies. As it is not precisely understood how these fibrils form, or what they are composed of, it is difficult to postulate a mechanism responsible for these differences in fibril structure. However, it is apparent that there is a wider range of fibril structure than initially appreciated. Moreover, these data may suggest the variance in fibril structure arises from patient-specific fibril composition and/or formation mechanisms.

Manipulation of in vitro collagen matrix architecture for scaffolds of improved physiological relevance

Type I collagen is a versatile biomaterial that is widely used in medical applications due to its weak antigenicity, robust biocompatibility, and its ability to be modified for a wide array of applications. As such, collagen has become a major component of many tissue engineering scaffolds, drug delivery platforms, and substrates for in vitro cell culture. In these applications, collagen constructs are fabricated to recapitulate a diverse set of conditions. Collagen fibrils can be aligned during or post-fabrication, cross-linked via numerous techniques, polymerized to create various fibril sizes and densities, and copolymerized into a wide array of composite scaffolds. Here, we review approaches that have been used to tune collagen to better recapitulate physiological environments for use in tissue engineering applications and studies of basic cell behavior. We discuss techniques to control fibril alignment, methods for cross-linking collagen constructs to modulate stiffness, and composite collagen constructs to better mimic physiological extracellular matrix.

Effect of tear size, corticosteroids and subacromial decompression surgery on the hierarchical structural properties of torn supraspinatus tendons

OBJECTIVES

The effects of disease progression and common tendinopathy treatments on the tissue characteristics of human rotator cuff tendons have not previously been evaluated in detail owing to a lack of suitable sampling techniques. This study evaluated the structural characteristics of torn human supraspinatus tendons across the full disease spectrum, and the short-term effects of subacromial corticosteroid injections (SCIs) and subacromial decompression (SAD) surgery on these structural characteristics.

METHODS

Samples were collected inter-operatively from supraspinatus tendons containing small, medium, large and massive full thickness tears (n = 33). Using a novel minimally invasive biopsy technique, paired samples were also collected from supraspinatus tendons containing partial thickness tears either before and seven weeks after subacromial SCI (n = 11), or before and seven weeks after SAD surgery (n = 14). Macroscopically normal subscapularis tendons of older patients (n = 5, mean age = 74.6 years) and supraspinatus tendons of younger patients (n = 16, mean age = 23.3) served as controls. Ultra- and micro-structural characteristics were assessed using atomic force microscopy and polarised light microscopy respectively.

RESULTS

Significant structural differences existed between torn and control groups. Differences were identifiable early in the disease spectrum, and increased with increasing tear size. Neither SCI nor SAD surgery altered the structural properties of partially torn tendons seven weeks after treatment.

CONCLUSIONS

These findings may suggest the need for early clinical intervention strategies for torn rotator cuff tendons in order to prevent further degeneration of the tissue as tear size increases. Further work is required to establish the long-term abilities of SCI and SAD to prevent, and even reverse, such degeneration. Cite this article: Bone Joint Res 2014;3:252-61.

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.

The collagen fibril--a model system for studying the staining and fixation of a protein

A collagen fibril is made up of long rod-like molecules regularly D-staggered with respect to one another. This means that (i) its axially projected fine structure, resolvable to approximately 2 nm in electron micrographs, repeats D-periodically (D = 67 nm), and (ii) the amino acid residues contributing to each element of the fine structure can be inferred from sequence data. Electron-optical data from a fibril D-period can can therefore be correlated directly with chemical data. Such correlations confirm the electrostatic nature of the staining reaction when a fibril is positively stained. After negative staining, the principal factor determining the small-scale distribution of stain is local exclusion by 'bulky' amino acid side-chains. ('Bulkiness' is the average cross-sectional area, or 'plumpness', of a side-chain.) A small superimposed positive staining contribution can also be detected. Fixation of collagen by aldehydes and diimidoesters occurs via an initial reaction with lysyl (and hydroxylsyl) side-chains and alpha-amino groups, followed by secondary cross-linking reactions that differ from fixative to fixative. These secondary reactions determine the nature and abundance of the cross-links and the extent to which they influence subsequent staining behaviour.

Comparison of axial banding patterns in fibrils of type V collagen and type I collagen

Type V collagen and type I collagen were obtained from human placenta, essentially by salt fractionation. Precipitates were formed from mixed solutions of type V collagen and type I collagen in various ratios. They were incubated at 37 degrees C for 1 hour and negatively stained with 0.5% uranyl acetate (pH 4.4) at 37 degrees C. The specimens, seen by electron microscopy, were fibrils with a D-periodic banding pattern. Axial electron density profiles of collagen fibrils were obtained from selected electron micrographs by densitometric tracing. The slit width corresponded to 1.5 nm. The relative electron densities of overlap region vs. hole region were lower than 20% in fine fibrils containing a significant amount of type V collagen. It is suggested that the overlap region of such collagen fibrils may be loosely packed, being accessible to uranyl acetate, or the hole region may be filled by larger non-collagenous portions of type V collagen, resulting in loss of the light and dark alternation. Six to 8 white transverse lines were discerned per period and labeled consecutively with Arabic numerals. White lines 2 and 5 tended to merge with lines 1 and 4, respectively, in collagen fibrils formed from a solution containing a significant amount of type I collagen or pure type I collagen. The eight white lines corresponded to c2, c1, b2, b1, a4, a1, e1 and d with reference to their locations in the D-period. The locations of white lines in collagen fibrils which contain significant amount of type V collagen were identical with those in type I collagen fibrils. This is consistent with the primary structure that the axial distribution of charged amino acids in type V collagen is quite similar to that in type I collagen.

The banding pattern of rat tail tendon freeze-etched collagen fibrils

Banding patterns of freeze-etched and replicated rat tail tendon collagen fibrils were studied. Better banding definition was obtained by freezing the samples without using a cryoprotectant and by prolonging etching. Under these conditions, the banding pattern was characterized by a sequence of elevated and depressed segments with a D-period of 65 nm, by two ridges at the margins of the elevations and by a third ridge at an intermediate point in the depressions. A comparison between replicas and isolated negatively stained collagen fibril micrographs as well as densitometric determinations have allowed interpretation of the elevations and depressions as overlap and gap zones and of the three ridges as the main bands with higher molecular density. Estimates, carried out on densitometric diagrams obtained from replicas, gave values which may represent a new parameter in the study of collagen banding and led to the calculation of a 0.55 D long gap zone and of a 0.45 D long overlap zone.

Identification of two further collagenous fractions from articular cartilage

Salt fractionation of pepsin-solubilized human and porcine articular cartilage has revealed the presence of two further collagenous fractions, CF1 and CF2, at high salt concentration following the precipitation of Type-II, 1 alpha, 2 alpha, 3 alpha, and Type-M collagens. Both fractions precipitate at 2.0 M NaCl, but higher yields of CF1 are obtained at 3.0 M NaCl. CF1 and CF2 can be separated in the native form using carboxymethyl-cellulose chromatography. Under denaturing conditions, CF1 has an apparent molecular weight of 25 000 and is unaffected by mercaptoethanol, whereas CF2 has a molecular weight of 35 000 before and 12 000 after reduction by mercaptoethanol. These fractions are probably fragments derived from larger collagen molecules, although the cyanogen bromide digest studies suggest that they are derived from a collagenous type other than one of those previously identified in cartilage.

Electron microscopy shows periodic structure in collagen fibril cross sections

X-ray diffraction was used to monitor the effects of electron microscope fixation, staining, and embedding procedures on the preservation of the three-dimensional crystalline order in collagen fibrils of rat tail tendon. A procedure is described in which the characteristic 3.8-nm lateral spacing is preserved, with increased contrast, in the diffraction pattern of the embedded fiber. This spacing is correlated with the separation between the tangentially oriented equally spaced lines of density observed in electron microscope ultrathin fibril cross sections of the same material. Optical diffraction of electron micrographs gives an objective measure of the periodicity and suggests that the fibril is composed of concentrically oriented crystalline domains. These observations, when combined with a recent interpretation of the native x-ray diffraction data [Hulmes, D. J. S. & Miller, A. (1979) Nature (London) 282, 878-880] suggest a tentative model for the three-dimensional structure of collagen fibrils.

The Ehlers-Danlos syndrome: an analysis of the structure of the collagen fibres of the skin

The structure of the collagen fibres of the skin in the Ehlers-Danlos syndrome (ED-S) was studied in eight patients with ED-S Type I, three patients with ED-S Type II and three patients with the X-linked Type V. The results show that the reducible cross-links are present and undergo the same maturation process to non-reducible cross-links as in normal skin. Transmission electron microscopy revealed a normal ultrastructure of the collagen fibrils. At a higher morphological level of organization scanning electron microscopy demonstrated progressive increase in fibre bundle disorder from the X-linked to mitis, to gravis, in which the fibres making up the large fibre bundles demonstrated a considerable inability to aggregate.

Locust collagen: morphological and biochemical characterization

Segment-long-spacing crystallites and reconstituted fibrils have been made from collagen extracted from the ejaculatory duct of the adult male locust, Locusta migratoria. These show the same banding pattern after positive staining as segment-long-spacing crystallites and fibrils made from mammalian type I collagen. Native fibrils show the same periodic pattern as type I fibrils, but it is not so distinct. Biochemical analysis of pepsin-digested locust collagen shows that there are two collagenous components. The alpha chains of the major component are similar to mammalian alpha 1 (I) chains, except that the number of hydroxylysine residues is elevated and the CNBr peptides differ. There are no alpha 2 chains; hence this locust collagen molecule is an alpha 1 trimer. The second component, which is present in only minute quantities, may be a type IV basement membrane collagen. It is concluded that the fibrous collagen molecule of the locust is very similar to that of mammalian type I trimers and to those of other invertebrates which have been examined.

An historical account of the development and applications of the negative staining technique to the electron microscopy of viruses

A brief historical account of the development and applications of the negative staining techniques to the study of the structure of viruses and their components as observed in the electron microscope is presented. Although the basic method of surrounding or embedding specimens in opaque dyes was used in light microscopy dating from about 1884, the equivalent preparative techniques applied to electron microscopy were comparatively recent. The combination of experiments on a sophisticated bacterial virus and the installation of a high resolution electron microscope in the Cavendish Laboratory, Cambridge, during 1954, subsequently led to the analysis of several important morphological features of animal, plant and bacterial viruses. The implications of the results from these early experiments on viruses and recent developments in negative staining methods for high resolution image analysis of electron micrographs are also discussed.

Negative staining of rat tail tendon collagen fibrils with uranyl formate

Negative staining of rat tail tendon collagen fibrils with uranyl formate appears to reveal more detail in the axial banding pattern than any other positive or negative staining method hitherto employed. In addition, uranyl formate and other uranyl solutions appear to reveal fine, closely spaced, longitudinal filaments which may represent the individual tropocollagen molecules.