Collagen fibril surface structures: freeze-etching data and computer modelling

Tissue section AFM: In situ ultrastructural imaging of native biomolecules

Conventional approaches for ultrastructural high-resolution imaging of biological specimens induce profound changes in bio-molecular structures. By combining tissue cryo-sectioning with non-destructive atomic force microscopy (AFM) imaging we have developed a methodology that may be applied by the non-specialist to both preserve and visualize bio-molecular structures (in particular extracellular matrix assemblies) in situ. This tissue section AFM technique is capable of: i) resolving nm–µm scale features of intra- and extracellular structures in tissue cryo-sections; ii) imaging the same tissue region before and after experimental interventions; iii) combining ultrastructural imaging with complimentary microscopical and micromechanical methods. Here, we employ this technique to: i) visualize the macro-molecular structures of unstained and unfixed fibrillar collagens (in skin, cartilage and intervertebral disc), elastic fibres (in aorta and lung), desmosomes (in nasal epithelium) and mitochondria (in heart); ii) quantify the ultrastructural effects of sequential collagenase digestion on a single elastic fibre; iii) correlate optical (auto fluorescent) with ultrastructural (AFM) images of aortic elastic lamellae.

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.

Surface ultrastructure of collagen fibrils and their association with proteoglycans in human cornea and sclera by atomic force microscopy and energy-filtering transmission electron microscopy

PURPOSE

We aimed to investigate the possible association of proteoglycans with D-periodic collagen fibrils in the human cornea and sclera, using energy-filtering transmission electron microscopy (EF-TEM) and atomic force microscopy (AFM).

METHODS

Human cornea and sclera were digested with keratanase to eliminate keratan sulfate proteoglycans (KSPGs). For EF-TEM observation, surface proteoglycans were detected by cupromeronic blue (CB) staining. For AFM observation, cornea and sclera were treated with sodium hydroxide before and after keratanase digestion, and the surface topology of collagen fibrils was analyzed.

RESULTS

With CB staining, numerous CB-positive short filaments of surface proteoglycans (proteoglycan filaments) were observed in the interfibrillar spaces of cornea and sclera associated with collagen fibrils. AFM imaging showed that the depth and periodicity of D-periodic collagen fibrils in keratanase-treated corneal collagens were deeper and more regular than in untreated ones. Moreover, the depth and periodicity of keratanase-untreated corneal collagens were shallow and irregular in comparison with keratanase-untreated scleral collagens. On the other hand, there was no difference in depth or regularity between keratanase-treated and -untreated scleral collagen fibrils. Using AFM imaging, additional thin grooves sub-bands were detected on the surface of keratanase-treated corneal collagen fibrils. The grooves were not detected in keratanase-untreated collagen fibrils nor in scleral collagen fibrils with or without keratanase digestion. Comparing densitometry waves, the grooves of D-periodic corneal collagen sub-bands corresponded to a and c bands.

CONCLUSION

Using AFM and EF-TEM to study corneal and scleral collagen fibrils and their association with proteoglycans, we conclude that KSPG is found in ample amounts in the human cornea in comparison with sclera. Moreover, we topologically detected KSPG attached to a and c bands of collagen fibrils.

A model for type II collagen fibrils: distinctive D-band patterns in native and reconstituted fibrils compared with sequence data for helix and telopeptide domains

The periodical D-band pattern is generally considered a unique ultrastructural feature shared by all fibril-forming collagens, which correlates with the intrafibril, paracrystalline array of tropocollagen monomers. Distinct band patterns have been reported, however, for collagen stained long-spacing (SLS) crystallites of genetic types I, II, and III. Moreover, D-band patterns of negatively stained, native type II collagen fibrils were found to be not identical to those of type I in our previous research. Because of (a) these distinctive features, (b) tropocollagen heterotrimeric conditions (type I) vs homotrimeric conditions (type II), and (c) different lengths and poor homology between extrahelical telopeptides, the molecular array or telopeptide conformation within the extensively studied type I collagen fibrils could be not the same as those in the very much less intensively studied type II collagen fibrils. In this investigation, a distinctive positive-staining D-band pattern was found for type II collagen fibrils obtained from human cartilages. A fibril model was developed by analyzing actual D-band patterns, and matching them against simulated patterns based on the primary structure of extrahelical and helical domains in human type II tropocollagen. In particular, a more prominent b(1) band was apparent in native type II collagen fibrils than in type I. This distinctive feature was also observed for native-type collagen fibrils reconstituted from purified type II collagen, i.e., free from associated minor type XI collagen. On modeling possible monomer arrays, the best fit between microdensitograms and simulation traces was found for 234 amino acid staggering, as is also the case for type I collagen fibrils. On comparing this model with an analogous one for type I collagen fibrils, there was a higher intraband distribution of charged residues for band b(1), consistent with the higher electrondensity observed for this band in type II collagen fibrils. N- and C-telopeptide displacement in the model corresponded to D-locations of a c(2) subband, which we named c(2.0), and band a(3), respectively. In simulation profiles, c(2.0) -like and a(3) -like peaks mimicked the corresponding peaks in microdensitograms when molecular reversals were adopted at positions 10N-12N, 12C-14C, and 17C-19C for N- and C-telopeptides. Hydrophobic interactions and algorithmic predictions of protein secondary structure, according to Chou and Fasman and Rost and Sander criteria, were consistent with these conformational models, and suggest that an additional molecular reversal may occur at positions 3N-5N. These telopeptide "S-fold" conformations, interpreted as axial projections of tridimensional conformation, may represent starting points for further investigation into the still unresolved tridimensional conformation of telopeptides in monomers arrayed within type II collagen fibrils.

Direct visualization of collagen-bound proteoglycans by tapping-mode atomic force microscopy

Most studies on the interaction of collagen with proteoglycans, two universal components of connective tissues, use technical approaches which substantially modify the shape and size of the proteoglycans themselves. In the present study unfixed, untreated collagen fibrils from rat tail tendon were dehydrated and observed by tapping-mode atomic force microscopy. The surface of collagen fibrils immediately reveals a periodic alternation of gap and overlap zones. A thin, transverse ridge decorates the gap zone, while other filamentous structures run on the fibril surface, either parallel or perpendicular to the fibril axis. These surface structures are much enhanced by Cupromeronic Blue preincubation, while pretreatment with chondroitinase ABC removes them completely, leaving barely detectable transverse ridges. The ridge and filaments are likely to represent, respectively, the core protein and the glycosaminoglycan side chains of proteoglycans, displayed with a far better resolution than with conventional histochemical or immunohistochemical techniques. Our data suggest that proteoglycan molecules are capable of different, multiple interactions with the collagen fibril surface as well as with each other.

Collagen fibril surface: TMAFM, FEG-SEM and freeze-etching observations

Native, unfixed collagen fibrils from rat tail tendon were dehydrated following different procedures and observed under a FEG-SEM and an AFM operated in Tapping Mode (TMAFM). Freeze-etched, untreated fibrils from the same tissue were also observed for comparison. The most notable features of the fibril surface, i.e., the gap/overlap alternation and three prominent intraperiod ridges, were simultaneously visible only in freeze-etched specimens, while under the SEM and the TMAFM their appearance was dependent on both the dehydration procedure and the visualization technique. The different susceptibility of the collagen fibril surface structures to various treatments clearly implies the existence of domains of different composition. Moreover, identical specimens were imaged differently by SEM and TMAFM, highlighting instrument-specific advantages and limitations. The onset of dehydration-dependent, procedure-specific artifacts should be considered in high-resolution studies of connective tissues. As for any biological specimen, the final aspect of collagen fibrils is determined no less by the preliminary treatments than by the visualization approach.