D-periodic distribution of collagen type IX along cartilage fibrils

Osteoarthritic lesions: involvement of three different collagenases

OBJECTIVE

To assess the presence of fibroblast collagenase (MMP-1), neutrophil collagenase (MMP-8), and collagenase 3 (MMP-13) in osteoarthritic (OA) cartilage, with particular emphasis on areas of macroscopic cartilage erosion.

METHODS

Messenger RNA (mRNA) levels were assessed by reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization, and Northern blot analysis.

RESULTS

MMP-1 and MMP-13 were expressed at higher levels by OA chondrocytes than by normal chondrocytes. In addition, mRNA for MMP-8 was present in OA cartilage but not normal cartilage by PCR and Northern blot analyses. Chondrocytes from areas surrounding the OA lesion expressed greater quantities of MMP-1 and MMP-13 compared with normal chondrocytes, suggesting local modulation by mechanical and inflammatory factors. Tumor necrosis factor alpha stimulated the expression of all 3 collagenases. Retinoic acid, an agent which induces autodigestion of cartilage in vitro, stimulated only the expression of MMP-13.

CONCLUSION

These findings suggest a key role of MMP-13 and MMP-8, as well as MMP-1 in osteoarthritis.

Absence of the alpha1(IX) chain leads to a functional knock-out of the entire collagen IX protein in mice

Cartilage fibrils contain collagen II as well as smaller amounts of collagens IX and XI. The three collagens are thought to co-assemble into cartilage-specific arrays. The precise role of collagen IX in cartilage has been addressed previously by generating mice harboring an inactivated Col9a1 gene encoding the alpha1(IX) chain, i.e. one of the three constituent chains of collagen IX (Fässler, R., Schnegelsberg, P. N. J., Dausman, J., Shinya, T., Muragaki, Y., McCarthy, M. T., Olsen, B. R., and Jaenisch, R. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 5070-5074). The animals did not produce alpha1(IX) mRNA or polypeptides and were born with no conspicuous skeletal abnormality but post-natally developed early onset osteoarthritis. Here we show that the deficiency in alpha1(IX) chains leads to a functional knock-out of all polypeptides of collagen IX, whereas the Col9a2 and Col9a3 genes were normally transcribed. Therefore, synthesis of alpha1(IX) polypeptides is essential for the assembly of heterotrimeric collagen IX molecules. Surprisingly, cartilage fibrils of all shapes and banding patterns found in normal newborn, adolescent, or adult mice were formed in transgenic animals, although they lacked collagen IX. Therefore, collagen IX is not essential, and may be functionally redundant, in fibrillogenesis in cartilage in vivo. The protein is required, however, for long term tissue stability, presumably by mediating interactions between fibrillar and extrafibrillar macromolecules.

Ultrastructural organization of proteoglycans and fibrillar matrix of the tectorial membrane

The molecular and supramolecular structure of the tectorial membrane (TM) was studied by transmission electron microscopy (TEM). Collagen (type A) fibrils in the TM were found associated with proteoglycans (PGs) and type B fibrils. Most PGs were orthogonally oriented and attached D-periodically to collagen fibrils. Computer averaged projections of PG particles and linear aggregates of PGs in crystalline arrays, stained with Cuprolinic blue, showed an elongated, electron-dense structure 50-65 nm in length and 10 nm in width. Image analysis of type B fibrils showed that they are constructed of globular domains arranged with a periodicity of 12-14 nm. Each globular domain contains two thin 'arms', extended in opposite directions, which contact the 'arms' of adjacent fibrils. Numerous type B fibrils were found between collagen fibrils. They are attached to adjacent collagen fibrils by the 'arms' of their globular domains. An association of type B fibrils and PGs with collagen seems to result in the local ordered arrangement of the TM matrix. A hypothetical model of the TM matrix supramolecular structure is presented.

Developmental regulation of mRNA species for types II, IX and XI collagens during mouse embryogenesis

Several techniques were used to study the co-ordination of mRNA levels for five constituent chains of cartilage collagen fibrils during mouse development. Short cDNA clones were first constructed for mouse and human alpha3(IX) and for mouse proalpha1(XI) collagen mRNA species. Northern analysis of developing mouse embryos revealed that the mRNA species for alpha1, alpha2 and alpha3 chains of type IX collagen peaked earlier than those for proalpha1(II) and proalpha1(XI) collagen chains. Quantification of these mRNA species by slot-blot hybridization confirmed this developmental regulation: the mRNA ratios for type II/type IX/type XI collagens changed from 5.7:1:0.6 (at embryonic day 12.5) to 10.6:1:0.9 (in newborn mice). However, the genes coding for the three chains of type IX collagen seemed to be under more co-ordinated regulation during mouse development. In addition to high mRNA levels in cartilages and the eye, low levels of type IX collagen transcripts were identified in brain and skin of newborn mouse using RNase protection and reverse transcriptase-PCR assays. Finally, hybridization in situ revealed identical tissue distributions of the three type IX collagen mRNA species during early chondrogenesis but somewhat more widespread expression of the alpha1(IX) and alpha3(IX) mRNA species during endochondral ossification at day 16.5 of embryonic development. These results suggest a relatively tight co-ordination of the alpha1(IX), alpha2(IX), and alpha3(IX) collagen mRNA species in chondrocytes, but a lack of co-ordination in several non-cartilaginous tissues.

Immunolocalization of type IX collagen in normal and spontaneously osteoarthritic canine tibial cartilage and isolated chondrons

OBJECTIVE

The pericellular localization of type IX collagen in avian and mammalian hyaline cartilages remains controversial, while its distribution during osteoarthritic degeneration is poorly understood. This study aimed to compare and contrast the immunohistochemical distribution of type IX collagen in normal mature and spontaneously osteoarthritic canine tibial cartilage.

DESIGN

Thick vibratome sectioning techniques were evaluated and compared with isolated chondrons using a range of streptavidin-linked probes in combination with light, confocal and transmission electron microscopy.

RESULTS

In normal intact samples, type IX collagen was concentrated in the pericellular microenvironment, while a weaker extracellular reaction around each chondron separated the territorial matrix from the unstained interterritorial matrix. Further differentiation was evident in isolated chondrons where the fibrous pericellular capsule stained more intensely than the tail and interconnecting segments between columnated chondrons. Two regions of type IX reactivity were identified in osteoarthritic tissue: an intensely stained superficial reactive region below the eroding margins, and normal deep layer cartilage where pericellular staining persists. The superficial reactive region was characterized by chondron swelling and chondrocyte cluster formation, a loss of pericellular type IX staining, and a significant increase in matrix staining between clusters. Disintegration and loss of fibrillar collagens was evident in both the swollen microenvironment and adjacent territorial matrices.

CONCLUSIONS

The results suggest that changes in type IX distribution, expansion of the pericellular microenvironment and chondrocyte proliferation represent key elements in the chondron remodeling and chondrocyte cluster formation associated with osteoarthritic degeneration.

Skeletal development in transgenic mice expressing a mutation at Gly574Ser of type II collagen

Skeletal development of transgenic mice with a type II collagen mutation was analyzed and compared with wild-type littermates. The single base substitution in Col2a1 resulted in a glycine to serine mutation within the helical domain and corresponded to one previously identified in a patient with the lethal human chondrodysplasia, hypochondrogenesis (Horton et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:4583-4587). Skeletal staining of embryos from 14.5 through 18.5 days of gestation demonstrated a dwarf phenotype in the transgenic embryos, most notably short limb bones and vertebral column that was first detected at 15.5 days post-coitus. In addition to the reduced length, the extent of ossification was less in the transgenic mice. The architecture of the long bone growth plate was abnormal in the transgenic tissue, in particular there was no discernible proliferative zone. There were few stacks of characteristically flattened cells and the overall length of the growth plate in the mutant embryos was reduced. At the ultrastructural level, there were fewer collagen fibrils present in the transgenic mouse cartilage compared to that of wild-type littermates. Ultrastructural localization of collagen types II, IX and XI revealed a similar pattern between the transgenic and wild-type pups, suggesting that the collagen fibrils present in the matrix of littermates with both phenotypes had a similar composition. Skeletal analysis and cartilage histochemistry indicated that effect of the type II collagen mutation was to reduce the density of the collagen fibrils within the cartilage matrix which was associated with delayed bone formation and resulted in a short-limbed phenotype.

Involvement of prolyl 4-hydroxylase in the assembly of trimeric minicollagen XII. Study in a baculovirus expression system

We have shown previously that hydroxylation played a critical role in the trimer assembly and disulfide bonding of the three constituent alpha chains of a minicollagen composed of the extreme C-terminal collagenous (COL1) and noncollagenous (NC1) domains of type XII collagen in HeLa cells (Mazzorana, M., Gruffat, H., Sergeant, A., and van der Rest, M. (1993) J. Biol. Chem. 268, 3029-3032). We have further characterized the involvement of prolyl 4-hydroxylase in the assembly of the three alpha chains to form trimeric disulfide-bonded type XII minicollagen in an insect cell expression system. For this purpose, type XII minicollagen was produced in insect cells from baculovirus vectors, alone or together with wild-type human prolyl 4-hydroxylase or with the human enzyme mutated in the catalytic site of its alpha or beta subunits or with the individual alpha or beta subunits. When type XII minicollagen was produced alone, negligible amounts of disulfide-bonded trimers were found to be produced by the cells. However, coproduction of the collagen with the two subunits of the wild-type human enzyme dramatically increased the amount of disulfide-bonded trimeric type XII minicollagen molecules. In contrast, coproduction of the collagen with alpha subunits that had a mutation completely inactivating the human enzyme failed to enhance the trimer assembly. These results directly show that an active prolyl 4-hydroxylase is required for the assembly of disulfide-bonded trimers of type XII minicollagen.

Separable cis-regulatory elements that contribute to tissue- and site-specific alpha 2(XI) collagen gene expression in the embryonic mouse cartilage

Type XI collagen is a structural component of the cartilage extracellular matrix and plays an important role in skeletal morphogenesis. As a step toward defining the molecular mechanisms responsible for the regulation of type XI collagen expression, we characterized the promoter region of the mouse alpha 2(XI) collagen gene (Coll1a2). We also generated transgenic mice harboring various fragments of the promoter and the first intron of Coll1a2 linked to the Escherichia coli beta-galactosidase gene to identify the cis-acting elements responsible for tissue- and site-specific expression during development. Cloning and sequence analysis of the 5' flanking region of Coll1a2 showed that the putative 3' end of the retinoid X receptor beta gene was located 742 bp upstream of the Coll1a2 start site. This suggested that the promoter region of Coll1a2 was localized within this 742-bp sequence, which contained multiple consensus regulatory elements. Examination of the transgenic mice revealed that the longest DNA construct (containing the entire promoter and first intron sequences) directed lacZ expression in the notochord as well as in the primordial cartilage throughout the body, with the pattern of expression mimicking that of endogenous Coll1a2 transcripts. On the other hand, deletion of the upstream approximately 290 bp resulted in the elimination of lacZ expression in the primordial cartilage of the carpals, tarsals, and vertebral bodies, whereas lacZ expression in the notochord and in the other primordial cartilage elsewhere was not affected. Deletion of the first intron sequence also resulted in the loss of lacZ expression in the primordial cartilage of the carpals, tarsals, and vertebral bodies, as well as in the notochord. These results demonstrate that the upstream 742-bp and first intron segments of the mouse Coll1a2 gene contain the necessary information to confer high level tissue-specific expression in mouse embryos. In addition, our observations suggest the presence of site-specific cis-acting elements that control Coll11a2 gene expression in different cartilaginous components of the skeleton.

Distribution and quantification of pyridinium cross-links of collagen within the different maturational zones of the chick growth plate

In order to assess alterations in the collagen network during endochondral ossification the pyridinium cross-links of collagen were quantified in sequential transverse sections through the chick growth plate. This was accomplished using both morphological (alkaline phosphatase (ALP) histochemistry and collagen type X immunostaining) and analytical (HPLC) analyses. In articular cartilage, pyridinoline concentrations were maximal in the deep mature zones. In contrast, the proliferating chondrocyte zone of the growth plate had approximately a 10-fold greater pyridinoline cross-link concentration than the mature hypertrophic zone. Deoxypyridinoline was first found in the prehypertrophic zone of the growth plate cartilage that reacted positively for ALP activity but before collagen type X was detected. However, deoxypyridinoline concentrations were highest in the most differentiated regions of the growth plate where it was the principal pyridinium cross-link. In tibial dyschondroplasia, where chondrocyte differentiation is arrested in the prehypertrophic zone, higher concentrations of both cross-links were found with increasing distance down the lesion. We conclude that the decrease in pyridinoline cross-link concentration down the growth plate may be an essential adaptation (via increased collagenase activity and collagen turnover) of the matrix for vascular invasion and osteoclastic resorption to occur.

Proteoglycans in articular cartilage revealed with a quick freezing and deep etching method

OBJECTIVES

To clarify the three dimensional ultrastructure of proteoglycans, and their relationship with other matrix components in articular cartilage.

METHODS

Specimens from rat femoral heads were examined using three techniques: (1) Histochemical staining with cationic polyethyleneimine (PEI), using a pre-embedding or a postembedding method. Some tissues were pretreated with chondroitinase ABC or hyaluronidase. (2) Quick freezing and deep etching (QF-DE). Some specimens were fixed with paraformaldehyde and washed in buffer solution before quick freezing; others were frozen directly. (3) Ultrathin sections were studied after conventional preparation.

RESULTS

Proteoglycans were observed as aggregated clumps with PEI staining by the pre-embedding method, but as fine filaments by the postembedding method. They were lost with enzyme digestion; this was also demonstrated by the QF-DE method. The ultrastructure was well preserved by the QF-DE method when fixation and washing procedures were included, but not without these procedures. A fine mesh-like structure was connected to the cell membrane in the pericellular matrix. Filamentous structures suggestive of aggrecans were observed among collagen fibrils. They had side chains, approximately 50 nm in length, which branched from the central filaments at intervals of 10-20 nm, and were occasionally linked to other structures. Many thin filaments were also attached to the collagen fibrils.

CONCLUSIONS

The QF-DE method incorporating paraformaldehyde fixation and buffer washing procedures revealed three dimensional, extended structures suggestive of proteoglycans.

Ultrastructural changes in primary guinea pig osteoarthritis with special reference to collagen

Collagen fibril distribution and surface and volume densities in proximal tibial articular cartilage were measured in 6- and 12-month-old Dunkin-Hartley guinea pigs developing primary osteoarthritis. At 12 months, gross fibrillation and ulceration of the articular cartilage were observed on the medial but not on the lateral condyle. Collagen volume density decreased with age in the interterritorial compartments in the superficial zone, medially by 16% and laterally by 8%. In the upper radial zone, collagen volume density decreased interterritorially by 10% on the medial condyle only. Despite gross osteoarthritic changes, only moderate and predominantly focal ultrastructural collagen changes were observed. Thus neither gross network disruption nor fibril thickening seems to be a general feature in early guinea pig osteoarthritis.

Tissue distribution and developmental expression of type XVI collagen in the mouse

The expression of a recently identified collagen, alpha 1 (XVI), in adult mouse tissue and developing mouse embryo was examined by immunohistochemistry and in situ hybridization. A polyclonal antiserum was raised against a recombinant fusion protein, which contained a segment of 161 amino acids in the N-terminal noncollagenous domain of the human alpha 1 (XVI) collagen. Immunoprecipitation of metabolically labelled human or mouse fibroblast cell lysates with this antibody revealed a major, bacterial collagenase sensitive polypeptide of approximately 210 kDa. The size agrees with the prediction from the full-length cDNA. Immunofluorescence examination of adult mouse tissues using the affinity purified antibody revealed a rather broad distribution of the protein. The heart, kidney, intestine, ovary, testis, eye, arterial walls and smooth muscles all exhibited significant levels of expression, while the skeletal muscle, lung and brain showed very restricted and low signals. During development, no significant expression of the mRNA or protein was observed in embryo of day 8 of gestation, but strong signals was detected in placental trophoblasts. Expression in embryos was detectable first after day 11 of gestation with weak positive signals appearing in the heart. In later stages of development, stronger RNA hybridizations were observed in a variety of tissues, particularly in atrial and ventricular walls of the developing heart, spinal root neural fibers and skin. These data demonstrate that type XVI collagen represents another collagenous component widely distributed in the extracellular matrix and may contribute to the structural integrity of various tissues.

Crystalline arrays of proteoglycan and collagen in the tectorial membrane

Ultrastucture of the tectorial membrane in the chinchilla cochlea was studied by transmission electron microscopy using different fixatives and staining procedures. It was shown that the tectorial membrane is a highly structured matrix composed of collagen type A fibrils, noncollagenous type B fibrils and proteoglycan. The localization of type B fibrils surrounding bundles of parallel type A fibrils was observed. Staining of the tectorial membranes with the cationic dye Cuprolinic blue in a "critical electrolyte concentration" method revealed proteoglycan, D-periodically associated with collagen type A fibrils and orthogonal to them. The appearance and size of the proteoglycan, and its binding to collagen, were similar to small proteoglycans observed in cartilage and other tissues. In many regions of the tectorial membrane the collagen-bound proteoglycan forms crystalline-like arrays. The images of these arrays processed by Fourier analysis show long linear aggregates of proteoglycan arranged parallel each other.

Collagen type IX from human cartilage: a structural profile of intermolecular cross-linking sites

Type IX collagen, a quantitatively minor collagenous component of cartilage, is known to be associated with and covalently cross-linked to type II collagen fibrils in chick and bovine cartilage. Type IX collagen molecules have also been shown to form covalent cross-links with each other in bovine cartilage. In the present study we demonstrate by structural analysis and location of cross-linking sites that, in human cartilage, type IX collagen is covalently cross-linked to type II collagen and to other molecules of type IX collagen. We also present evidence that, if the proteoglycan form of type IX collagen is present in human cartilage, it can only be a minor component of the matrix, similar to findings with bovine cartilage.

The interactions of 'non-aggregating' proteoglycans

The small proteoglycans decorin, biglycan and fibromodulin were prepared as a mixture from bovine nasal cartilage. The proteoglycans in this mixture were shown to interact with hyaluronate immobilized on Sepharose beads under isotonic conditions. The interaction could be disrupted by increasing the ionic strength of the solvent by enhancing the concentration of NaCl. To further characterize the proteoglycans of this mixture, they were visualized with the glycerol spraying/rotary shadowing technique for electron microscopy. They were shown to have a globular core protein and one or more glycosaminoglycan chains. The molecules, moreover, were organized as multimeric complexes, and their association one with another appeared to be mediated by either core protein or glycosaminoglycan chain interactions. Complexes were shown by rotary shadowing microscopy to associate with hyaluronate in solution. The combined results necessarily as discrete monomers but rather as multimeric complexes. The observations made in this study also suggest that a similar interaction could occur in vivo, where the interaction between small proteoglycans and hyaluronate may have a functional significance in the maintenance of cartilage homeostasis.

Collagens in an adult bovine medial collateral ligament: immunofluorescence localization by confocal microscopy reveals that type XIV collagen predominates at the ligament-bone junction

To understand the structure and function of medial collateral ligament, collagens present in an adult bovine ligament were determined. The mid-section of the ligament was powdered and extracted with 4M guanidinium hydrochloride, and the residue was digested with pepsin to solubilize the collagens. Type I collagen was the major fibril collagen recovered in the pepsin solubilized fraction, with types III and V each representing about 5% and 2%, respectively. Type VI collagen was the major collagen present in the guanidinium hydrochloride extract, and it accounted for about 40% of the proteins in the extract or 4% of the tissue dry weight. Type XII and XIV collagens were also detected in the guanadinium hydrochloride extract as minor components. Immunofluorescence localization using confocal microscopy showed that type XII and XIV collagens are associated with the ligament fibrillar network and that type XIV collagen was prominent at the ligament-bone junction. These data reinforce the notion that these collagens are associated with the type I collagen fibrillar network in connective tissues. In view of high mechanical stresses that exist at the ligament-bone interface, presence of type XIV collagen in high concentration at this junction may contribute to the modulation of the biomechanical properties of this tissue.

Structural analysis of cross-linking domains in cartilage type XI collagen. Insights on polymeric assembly

The collagen framework of hyaline cartilage is based on copolymers of types II, IX, and XI collagens. Previous studies have established specific covalent interactions between types II and IX collagens. The present study examined cross-linking sites in type XI collagen to define better the full heteropolymeric assembly. Pepsinsolubilized type XI collagen was purified from fetal bovine cartilage. The cross-linking amino acids in the preparation were primarily divalent, borohydride-reducible structures; pyridinoline residues were essentially absent. Individual alpha 1(XI), alpha 2(XI), and alpha 3(XI) chains were resolved by high performance liquid chromatography. Telopeptides still attached by cross-links to helical sites were released by periodate oxidation and identified by microsequencing. Analysis of cross-linked peptides isolated from trypsin digest of each alpha-chain identified the attachment helical sites for the telopeptides. A high degree of interchain specificity was evident in the cross-linking between N-telopeptides and the COOH terminus of the triple-helix, consistent with a head-to-tail interaction of molecules staggered by 4D (D = 67 nm) periods. In addition, alpha 1(II) C-telopeptide was linked to the amino-terminal site of the alpha 1(XI) triple helix. In summary, the results show that type XI collagen molecules are primarily cross-linked to each other in cartilage, implying that a homopolymer is initially formed. Links to type II collagen are also indicated, consistent with an eventual cofibrillar assembly. Analysis of cartilage extracts showed that all three chains, alpha 1(XI), alpha 2(XI), and alpha 3(XI), had at least in part retained their N-propeptides in cartilage matrix and that the alpha 3 (XI) chain was the IIB splicing variant product of the COL2A1 gene. Of particular note was the finding that the N-telopeptide cross-linking site in both alpha 1(XI) and alpha 2(XI) is located amino-terminal to the putative N-propeptidase cleavage site. This structural feature provides a potential mechanism for the proteolytic depolymerization of type XI collagen by proteases that can cleave between the cross-link and the triple helix (e.g. stromelysin).

Large and small splice variants of collagen XII: differential expression and ligand binding

Collagen XII has a short collagenous tail and a very large, three-armed NC3 domains consisting primarily of fibronectin type III repeats. Differential splicing within this domain gives rise to a large (320 kD) and a small (220 kD) subunit; the large but not the small can carry glycosaminoglycan. To investigate whether collagen XII variants have distinct expression patterns and functions, we generated antibody and cDNA probes specific for the alternatively spliced domain. We report here that the large variant has a more restricted expression in embryonic tissue than the small. For example, whereas the small variant is widespread in the dermis, the large is limited to the base of feather buds. Distinct proportions of mRNA for the two variants were detected depending on the tissue. Monoclonal antibodies allowed us to separate collagen XII variants, and to show that homo- and heterotrimers exist. Collagen XII variants differ in ligand binding. Small subunits interact weakly with heparin via their COOH-terminal domain. Large subunits have additional, stronger heparin-binding site(s) in their NH2-terminal extra domain. In vivo, both large and small collagen XII are associated with interstitial collagen. Here we show biochemically and ultrastructurally that collagen XII can be incorporated into collagen I fibrils when it is present during, but not after, fibril formation. Removal of the collagenous domain of collagen XII reduces its coprecipitation with collagen I. Our results indicate that collagen XII is specifically associated with fibrillar collagen, and that the large variant has binding sites for extracellular ligands not present in the small variant.

Trimeric assembly of collagen XII: effect of deletion of the C-terminal part of the molecule

The fibril-associated-collagens-with-interrupted-triple-helices (FACITs) are devoid of large C-propeptides like those involved in the trimeric assembly of the fibrillar collagens. Under these conditions, the C-terminal non triple-helical domain (NC1) and the adjacent triple-helical domain (COL1) are likely to be responsible for the trimeric assembly of these collagen molecules. Using a recombinant minigene of one of the FACITs, collagen XII, we show that a deletion covering most of the NC1 domain, except the first seven residues containing a cysteine and constituting the main part of the conserved junction between the COL1 and NC1 domains, does not prevent the formation of trimeric disulfide-bonded assembly of truncated alpha chains. These results suggest that if the non triple-helical NC1 domain is involved in the initial events governing the trimeric assembly, it must be through its amino acid residues participating in the junction. Our data confirm also the results obtained in a previous paper (Mazzorana et al.: J. Biol. Chem. 268:3029-3032, 1993) showing that the formation of disulfide bonds is dependent on hydroxylation and suggesting that the folding of the triple helix (or a part of it) precedes the formation of the disulfide bonds.

Another look at collagen V and XI molecules

The fibrillar collagens are the most abundant proteins of extracellular matrices. Among them, collagens V and XI are quantitatively minor components which participate in the formation of the fibrillar collagen network. Since these collagens were discovered, studies have demonstrated that they may play a fundamental role in the control of fibrillogenesis, probably by forming a core within the fibrils. Another characteristic of these collagens is the partial retention of their N-propeptide extensions in tissue forms, an unusual observation in comparison to the other known fibrillar collagens. The tissue locations of collagens V and XI are different, but their structural and biological properties seem to be closely related. It has been shown that their primary structures are highly conserved at both the gene and protein levels, and that these conserved features are the bases of their similar biological properties. In particular, they are both resistant to mammalian collagenases, and surprisingly sensitive to trypsin treatment. Collagens V and XI are usually buried within the major collagen fibrils, although they have both cell adhesion and heparin binding sites which could be of crucial importance in physiological processes such as development and wound healing. It has became evident that several molecules are in fact heterotypic associations of chains from both collagens V and XI, demonstrating that these two collagens are not distinct types but a single type which can be called collagen V/XI.

Quantitative analysis of chondroitin sulphate retention by tannic acid during preparation of specimens for electron microscopy

The ability of tannic acid to enhance binding of glycosaminoglycans to purified collagen was analysed in an in vitro system using amino sugar analysis on an amino acid analyser, transmission electron microscopy, and scanning electron microscopy. Collagen was purified by digestion with trypsin, papain, and hyaluronidase. Purified collagen was incubated with hyaluronic acid or with chondroitin sulphate glycosaminoglycan and then treated with tannic acid. Tannic acid was found to enhance retention during preparation for electron microscopy of either of the glycosaminoglycans onto collagen fibres. The ability of tannic acid to enhance binding of collagen and glycosaminoglycans might explain, at least in part, its structural reinforcement effect on resected synovial joint-apposing surfaces during preparation for scanning electron microscopy.

Differential expression of an acidic domain in the amino-terminal propeptide of mouse pro-alpha 2(XI) collagen by complex alternative splicing

We isolated and sequenced genomic and cDNA clones encoding the complete amino-terminal portion and the 5'-untranslated region of mouse pro-alpha 2(XI) collagen mRNA. Fourteen exons encoded the amino-terminal propeptide, which was divided into three consecutive domains (a long globular domain, an amino-terminal triple helical domain, and a telopeptide domain). The long globular domain was further divided into an upstream basic subdomain and a downstream highly acidic subdomain, as is the case for the amino-terminal propeptides of pro-alpha 1(V) and pro alpha 1(XI) collagens. We also demonstrated that the primary transcript undergoes complex alternative splicing. Three consecutive exons (exons 6, 7, and 8) encoding most of the acidic subdomain showed alternative splicing which dramatically affected the structure of the amino-terminal propeptide of pro-alpha 2(XI) collagen. Using the reverse transcription-polymerase chain reaction, we analyzed the expression of these exons in various tissues and in developing limb buds of mice. The pro-alpha 2(XI) transcripts were abundant in cartilage, but most of them lacked the 3-exon sequences encoding the acidic domain. Most of other tissues also contained mRNAs that corresponded to longer splice variants, including exons 6-8. The differential expression of specific domains of pro-alpha 2(XI) collagen may be important in modulating interactions between various components of the extracellular matrix and/or may influence heterotypic collagen assembly.

Identification of heterogeneous cell populations in normal human intervertebral disc

There is clear evidence that tissues related to the intervertebral disc, such as articular cartilage, contain several phenotypically different chondrocytic cell populations. Histological data for the disc suggest the same may be true for the annulus fibrosus and nucleus pulposus, but this has not been shown directly. For the first time, cells from adult human nondegenerative nucleus pulposus and annulus fibrosus were recovered after enzymatic digestion and maintained in an alginate bead culture system for up to 6 wk. The cells remained viable and produced matrix, but did not divide. Cultured cells were stained simultaneously for the presence of chondroitin sulphate and keratan sulphate, or types I and II collagen. The majority of the cells from both the annulus fibrosus and the nucleus pulposus produced both keratan sulphate and chondroitin sulphate (> 60%), a few only detectable levels of one or the other, but a significant population produced neither. This is an indication of a population of cells with a nonchondrocytic phenotype. In nondegenerative discs, the majority of the annulus fibrosus cells produced both types I and II collagen but the majority of nucleus pulposus cells produced only type II collagen. These observations are consistent with the presence of at least 2 phenotypically stable populations of cells in the adult human intervertebral disc and with the view that the phenotype of the major population of the annulus is different from that of the nucleus.

Analysis of transcriptional isoforms of collagen types IX, II, and I in the developing avian cornea by competitive polymerase chain reaction

The genes for the alpha 1(IX), alpha 1(II), and alpha 2(I) collagen chains can give rise to different isoforms of mRNA, generated by alternative promotor usage [for alpha 1(IX) and alpha 2(I)] or alternative splicing [for alpha 1(II)]. In this study, we employed competitive reverse transcriptase PCR to quantitate the amounts of transcriptional isoforms for these genes in the embryonic avian cornea from its inception (about 3 1/2 days of development) to 11 days. In order to compare values at different time points, the results were normalized to those obtained for the "housekeeping" enzyme, glycerol-3-phosphate dehydrogenase (G3PDH). These values were compared to those obtained from other tissues (anterior optic cup and cartilage) that synthesize different combinations of the collagen isoforms. We found that, in the cornea, transcripts from the upstream promotor of alpha 1(IX) collagen (termed "long IX") were predominant at stage 18-20 (about 3 1/2 days), but then fell rapidly, and remained at a low level. By 5 days (just before stromal swelling) the major mRNA isoform of alpha 1(IX) was from the downstream promoter (termed "short IX"). The relative amount of transcript for the short form of type IX collagen rose to a peak at about 6 days of development, and then declined. Throughout this period, the predominant transcriptional isoform of the collagen type II gene was IIA (i.e., containing the alternatively spliced exon 2). This indicates that the molecules of type II collagen that are assembled into heterotypic fibrils with type I collagen possess, at least transiently, an amino-terminal globular domain similar to that found in collagen types I, III, and V. For type I, the "bone/tendon" mRNA isoform of the alpha 2(I) collagen gene was predominant; transcripts from the downstream promotor were at basal levels. In other tissues expressing collagen types IX and II, long IX was expressed predominantly with the IIA form in the anterior optic cup at stage 22/23; in 14 1/2 day cartilage, long IX was expressed predominantly along with the IIB form of alpha 1(II). The downstream transcript of the alpha 2(I) gene (Icart) was found at high levels only in cartilage.

Fibre reinforcing by collagen in cartilage and soft connective tissues

Mechanical support in animals is performed by connective tissues. The soft tissues consist of collagen fibres embedded in a highly hydrated proteoglycan gel. By considering them as fibrous composite materials, a unifying theme can be found to explain their mechanical behaviour in terms of their structure and composition. Interactions between fibres and matrix are essential to their functioning in this way. Calculations are made of the maximum stress transfer per D-period required to enable collagen fibres of a given axial ratio to provide effective reinforcing. Weak non-specific interactions are shown to be sufficient. A mechanical function is proposed for type X collagen in the epiphyseal growth plate on the basis that it modifies and supplements the properties of the type II fibrils. This provides extra reinforcing and, hence, a greater stiffness to the cartilage to compensate for the reduced amount of extracellular matrix.

Cellular invasion and collagen type IX in the primary corneal stroma in vitro

During different stages in the development of the avian cornea, various collagen types have been shown to participate in matrix formation and have been implicated in morphogenesis. One of these is the fibril-associated collagen type IX. This molecule is present when the primary corneal stroma is in a compact state, but rapidly disappears just prior to stromal swelling and its invasion by mesenchymal cells. The temporospatial pattern of the disappearance of type IX collagen in the developing cornea suggests that this molecule may be involved in stabilizing the primary corneal stromal matrix by interacting either with other type IX collagen molecules or with other matrix components. To explore further whether the removal of type IX collagen is involved in stromal swelling, we have employed an in vitro culture system in which swelling of the primary stroma and mesenchymal cell invasion can be experimentally manipulated by culturing chick corneal explants on a Nuclepore filter support in the presence or absence of an associated lens. We have also examined the effect of exogenously added human recombinant tissue inhibitor of metalloproteinases (TIMP-1) on the presence of type IX collagen and cellular invasion. When stage 25-26+ corneal explants were cultured with an associated lens, the primary stroma did not swell; immunohistochemically detectable type IX collagen was still present, and mesenchymal cell invasion failed to occur. Conversely, when the same stages of corneal explants were cultured without an associated lens, the primary stroma swelled; type IX collagen disappeared, and mesenchymal cell migration occurred. Under both conditions, however, the type II collagen of the stroma, which is known to be a component of the striated fibrils, remained clearly detectable and with time even seemed to increase in amount. This result is consistent with the proposition that type IX collagen is one factor involved in maintaining the primary stroma as a compact matrix, possibly by functioning as a bridging/stabilizing factor. When TIMP was added to cultures of corneal explants, type IX collagen remained detectable in focal regions, suggesting that one or more metalloproteinases are involved in the removal of the type IX collagen. In addition, some of these type IX-containing regions contained mesenchymal cells, suggesting that in addition to type IX collagen other factors are likely to be involved in regulating mesenchymal cell migration.

Ultrastructure of hypertrophic cartilage: histochemical procedures compared with high pressure freezing and freeze substitution

The effect of cationic dyes on the ultrastructure of hypertrophic cartilage was compared with results obtained with modern cryotechniques in studies on rat epiphyseal growth plate. Addition of alcian blue, acridine orange, cupromeronic blue, ruthenium hexamine trichloride, ruthenium red, or safranin O to conventional glutaraldehyde/osmium tetroxide fixatives to a large extent resulted in prevention of chondrocyte shrinkage except for alcian blue which showed poor tissue penetration. The fine structure of the matrix in pericellular and territorial compartments appeared very coarse with areas of high contrast in tissue exposed to fixatives containing cationic dyes. This indicates structural collapse and precipitation of electron-dense material, a pattern clearly differing from that observed in specimens prepared by the cryotechniques. The dyes giving a pattern most similar to that seen after high pressure freezing, freeze substitution, and low temperature embedding were acridine orange and safranin O. It is concluded that studies of matrix ultrastructure down to the molecular level necessitate the application of cryotechniques.

Genetic mapping of a locus for multiple epiphyseal dysplasia (EDM2) to a region of chromosome 1 containing a type IX collagen gene

Multiple epiphyseal dysplasia (MED) is a dominantly inherited chondrodysplasia characterized by mild short stature and early-onset osteoarthrosis. Some forms of MED clinically resemble another chondrodysplasia phenotype, the mild form of pseudoachondroplasia (PSACH). On the basis of their clinical similarities as well as similar ultrastructural and biochemical features in cartilage from some patients, it has been proposed that MED and PSACH belong to a single bone-dysplasia family. Recently, both mild and severe PSACH as well as a form of MED have been linked to the same interval on chromosome 19, suggesting that they may be allelic disorders. Linkage studies with the chromosome 19 markers were carried out in a large family with MED and excluded the previously identified interval. Using this family, we have identified an MED locus on the short arm of chromosome 1, in a region containing the gene (COL9A2) that encodes the alpha 2 chain of type IX collagen, a structural component of the cartilage extracellular matrix.

Cartilage proteoglycans: structure and potential functions

Hyaline cartilage contains five well-characterized proteoglycans in its extracellular matrix, and it is likely that others exist. The largest in size and most abundant by weight is aggrecan, a proteoglycan that possesses over 100 chondroitin sulfate and keratan sulfate chains. Aggrecan is also characterized by its ability to interact with hyaluronic acid to form large proteoglycan aggregates. Both the high anionic charge on the individual aggrecan molecules endowed by the sulfated glycosaminoglycan chains and the localization within the matrix endowed by aggregate formation are essential for aggrecan function. The molecule provides cartilage with its osmotic properties, which give articular cartilage its ability to resist compressive loads. The other proteoglycans are characterized by their ability to interact with collagen. They are much smaller than aggrecan in size but may be present in similar molar amounts. Decorin, biglycan, and fibromodulin are closely related in protein structure but differ in glycosaminoglycan composition and function. Decorin and biglycan possess one and two dermatan sulfate chains, respectively, whereas fibromodulin bears several keratan sulfate chains. Decorin and fibromodulin both interact with the type II collagen fibrils in the matrix and may play a role in fibrillogenesis and interfibril interactions. Biglycan is preferentially localized in the pericellular matrix, where it may interact with type VI collagen. Finally, type IX collagen can also be considered as a proteoglycan, as its alpha 2(IX) chain may bear a glycosaminoglycan chain. It may serve as a bridge between the collagen fibrils or with the interspersed aggrecan network.

Structure and stability of the triple-helical domains of human collagen XIV

Two triple-helical domains, Col 1 and Col 2, were obtained from a pepsin digest of human placental collagen XIV and separated from each other under nondenaturing conditions. Edman degradation demonstrated 106 amino acids residues in the Col 1 and 149 residues in the Col 2 domain. All except one of the 37 prolines in the Yaa position of the Gly-Xaa-Yaa triplets were completely hydroxylated to 4-hydroxyproline, and there were three imperfections in the triplet repeat. Partial or complete hydroxylation and glycosylation were found for all seven lysines in the Yaa position. Domain Col 1 was joined by disulfide bonds into a trimer, while Col 2 appeared as a mixture of monomers and disulfide-linked dimers. Circular dichroic spectra were typical for the collagen triple helix and revealed relatively high melting temperatures for Col 1 (38 degrees C) and Col 2 (43 degrees C). An almost perfect refolding of the triple helix was observed for Col 1 but not for Col 2, emphasizing the importance of disulfide bonds for the folding kinetics and in part the stability of the triple helix. Circular dichroic spectra of the large nontriple helical domain, NC3, of collagen XIV indicated 11% alpha helix and 63% beta structure. Comparative melting profiles of NC3 and intact collagen XIV indicated that the triple helices in intact collagen XIV have a melting temperature of 44 degrees C.

Structure and function of cartilage collagens

Collagens are the major proteinaceous constituents of cartilage. Three collagen types participate in the formation of striated fibrils of cartilage, collagens II, IX, and XI. Collagen II and XI belong to the subgroup of fibrillar collagens and are structurally closely related, differing mainly in their N-propeptides. Collagen IX has a very different structure but is nevertheless an essential constituent of the striated fibrils. Two other collagen types are also found in cartilage but form distinct structures. Collagen VI, found mainly in the periphery of the chondrocytes, forms beaded filaments. These filaments are probably formed by interaction of collagen VI with hyaluronan. Collagen X is expressed by hypertrophic chondrocytes. It has been shown to form in vitro hexagonal lattices and in vivo to be associated either with striated fibrils or with mats which may correspond to the lattices. The functional role of the collagen diversity in cartilage is discussed.

Mice lacking alpha 1 (IX) collagen develop noninflammatory degenerative joint disease

Type IX collagen is a nonfibrillar collagen composed of three gene products, alpha 1(IX), alpha 2(IX), and alpha 3(IX). Type IX molecules are localized on the surface of type II-containing fibrils and consist of two arms, a long arm that is crosslinked to type II collagen and a short arm that projects into the perifibrillar space. In hyaline cartilage, the alpha 1(IX) collagen transcript encodes a polypeptide with a large N-terminal globular domain (NC4), whereas in many other tissues an alternative transcript encodes an alpha 1(IX) chain with a truncated NC4 domain. It has been proposed that type IX molecules are involved in the interaction of fibrils with each other or with other components of the extracellular matrix. To test this hypothesis, we have generated a mouse strain lacking both isoforms of the alpha 1(IX) chain. Homozygous mutant mice are viable and show no detectable abnormalities at birth but develop a severe degenerative joint disease resembling human osteoarthritis.

Extraction and characterization of the tissue forms of collagen types II and IX from bovine vitreous

We report for the first time that, after centrifugation of adult bovine vitreous, the hyaluronan-rich supernatant contains collagens which can be isolated in their intact forms by precipitation with 4.5 M NaCl. This precipitate constituted approx. 4% of the total vitreous collagen and comprised collagen types IX and II (in the approximate ratio of 4:1) with negligible amounts of type-V/XI collagen. Type-II collagen was present partly in a pro-alpha 1(II) form, suggesting that there is active synthesis of type-II collagen into the matrix of adult bovine vitreous. Type-IX collagen was purified (2-2.5 mg/l of vitreous) and its glycosaminoglycan chain composition was analysed. Bovine vitreous type-IX collagen always possessed a glycosaminoglycan chain of comparatively low M(r) that was predominantly 4-sulphated, with chondroitin 6-sulphate representing a more minor component. By contrast, chick vitreous has been shown to contain type-IX collagen which always possesses a high-M(r) chondroitin sulphate chain that is predominantly 6-sulphated. The functional significance of these different glycosaminoglycan chain lengths and sulphation patterns is discussed.

The complete primary structure of the long form of mouse alpha 1(IX) collagen chain and its expression during limb development

Type IX collagen is a newly discovered collagen molecule that is associated with Type II-containing collagen fibrils in cartilage, vitreous and embryonic cornea. It consists of three distinct chains: alpha 1(IX), alpha 2(IX) and alpha 3(IX). The alpha 1(IX) chain has been to be synthesized in two different forms, which are generated by alternative transcription and splicing. In this manuscript we describe the isolation and sequencing of a cDNA coding for the entire coding region of the long form of mouse alpha 1(IX) chain. Nucleotide sequence analysis of this cDNA determined for the first time the primary structure of the entire long form of the mouse alpha 1(IX) chain. RT-PCR was used to examine collagen gene expression during limb development from day 10 to 18 in mouse embryos. Collagen I and II mRNA levels gradually increased all through the developmental stages. Collagen X expression increased further after day 16 in limb development, whereas the alpha 1(IX)mRNA level dropped at this time. This could be due to active bone formation relative to cartilage synthesis in the embryonic limb bud around day 16 in mouse development.

Assembly and sequencing of a cDNA covering the entire mouse alpha 1(IX) collagen chain

Type IX collagen, a member of the FACIT family of collagenous proteins, contains heterotrimeric molecules of distinct alpha 1(IX), alpha 2(IX) and alpha 3(IX) chains. In this paper we describe the assembly and nucleotide sequence of a cDNA encoding the entire mouse alpha 1(IX) collagen chain. The nucleotide sequence provides for the first time the complete primary structure of the mouse chain. Knowledge of the complete structure of mouse alpha 1(IX) collagen will be useful for investigations of type IX collagen expression during normal mouse development and for the generation of transgenic mice with specific defects in this collagen.

Characterisation of articular and growth plate cartilage collagens in porcine osteochondrosis

The articular and growth plate cartilages of osteochondrotic pigs were examined and compared with those from clinically normal animals. Both types of osteochondrotic cartilage showed considerable localised thickening apparently due to a lack of ossification. Histological examination of cartilage lesions demonstrated a breakdown in the normal pattern of chondrocyte maturation. Articular cartilage lesions lacked mature clones of chondrocytes in the calcifying region. Growth plate cartilage showed an accumulation of disorganised hypertrophic chondrocytes rather than the well-defined columns seen in normal tissue. The overall percentages of collagen in osteochondrotic lesions from both articular and growth plate cartilage were significantly reduced compared with levels in unaffected cartilage. There were substantial increases in the proportion of type I collagen in lesions from both osteochondrotic articular and growth plate cartilages and a reduction in the proportion of type II collagen. Type X collagen was detected in osteochondrotic but not normal articular cartilage. The proportion of type X collagen was unchanged in osteochondrotic growth plate cartilage. The levels of the collagen cross-links, hydroxylysylpyridinoline, hydroxylysyl-ketonorleucine and dehydrohydroxylysinonorleucine were radically reduced in samples from osteochondrotic growth-plate cartilage lesions when compared with normal tissue. Less dramatic changes were observed in articular cartilage although there was a significant decrease in the level of hydroxylysylketonorleucine in osteochondrotic lesions. Immunofluorescence examination of osteochondrotic lesions showed a considerable disruption of the organisation of the collagenous components within both articular and growth-plate cartilages. Normal patterns of staining of types I and VI collagen seen at the articular surface in unaffected tissue were replaced by a disorganised, uneven stain in osteochondrotic articular cartilage lesions. Incomplete removal of cartilage at the ossification front of osteochondrotic growth plate was demonstrated by immunofluorescence staining of type IX collagen. Type X collagen was produced in the matrix of the calcifying region of osteochondrotic articular cartilage by small groups of hypertrophic chondrocytes, but was not detected in normal articular cartilage. The distribution of type X collagen was unchanged in osteochondrotic growth plate cartilage.

Collagen fibrillogenesis in a three-dimensional fibroblast cell culture system

The purpose of this study was to follow collagen fibril formation in a newly developed three dimensional cell culture system. Human neonatal foreskin fibroblasts were grown on a nylon mesh in Dulbecco's Modified Eagles Medium (DMEM) supplemented with 10% fetal calf serum and antibiotics. Fibrillogenesis was initiated by the addition of 50 micrograms/ml ascorbate to confluent cultures. Sample meshes were processed for electron microscopy or immuno-electron microscopy. Fibrils approximately 20-30 nm in diameter, with 67 nm periodicity, were first detected five days after the addition of ascorbate. As cultures progressed, cells organized into parallel layers between which collagen fibers continued to form and increase in diameter. By day 50, fiber diameter ranged from 30 to 80 nm and large bundles were seen. No collagen fibril formation occurred in control cultures to which no ascorbate was added. However, large amounts of microfibrils were observed. Antibodies against the aminopropeptide of type I procollagen were found to bind to fibrils with diameters less than 34 nm while antibodies against the aminopropeptide of type III collagen bound primarily to fibers which ranged from 35-54 nm in diameter. We believe that this system, which morphologically resembles a normal dermis, will serve as an excellent model for the study of collagen fibrillogenesis.