Collagen IX: evidence for a structural association between NC4 domains in cartilage and a novel cleavage site in the alpha 1(IX) chain

Loss of matrilin 1 does not exacerbate the skeletal phenotype in a mouse model of multiple epiphyseal dysplasia caused by a Matn3 V194D mutation

Objective

Mutations in matrilin 3 can result in multiple epiphyseal dysplasia (MED), a disease characterized by delayed and irregular bone growth and early-onset osteoarthritis. Although intracellular retention of the majority of mutant matrilin 3 was previously observed in a murine model of MED caused by a Matn3 V194D mutation, some mutant protein was secreted into the extracellular matrix. Thus, it was proposed that secretion of mutant matrilin 3 may be dependent on the formation of hetero-oligomers with matrilin 1. The aim of this study was to investigate the hypothesis that deletion of matrilin 1 would abolish the formation of matrilin 1/matrilin 3 hetero-oligomers, eliminate the secretion of mutant matrilin 3, and influence disease severity.

Methods

Mice with a Matn3 V194D mutation were crossed with Matn1-null mice, generating mice that were homozygous for V194D and null for matrilin 1. This novel mouse was used for in-depth phenotyping, while cartilage and chondrocytes were studied both histochemically and biochemically.

Results

Endochondral ossification was not disrupted any further in mice with a double V194D mutation compared with mice with a single mutation. A similar proportion of mutant matrilin 3 was present in the extracellular matrix, and the amount of retained mutant matrilin 3 was not noticeably increased. Retained mutant matrilin 3 formed disulfide-bonded aggregates and caused the co-retention of matrilin 1.

Conclusion

We showed that secretion of matrilin 3 V194D mutant protein is not dependent on hetero-oligomerization with matrilin 1, and that the total ablation of matrilin 1 expression has no impact on disease severity in mice with MED. Mutant matrilin 3 oligomers form non-native disulfide-bonded aggregates through the misfolded A domain.

Modification of the composition of articular cartilage collagen fibrils with increasing age

Recent studies have identified a range of interactions between type IX collagen and other cartilage matrix components. To determine the extent to which these interactions are important in maintaining the integrity of ageing articular cartilage, we analyzed an age range of normal healthy articular cartilage samples by Western blotting, immunohistochemical, and PCR analyses. Reduced levels of type IX collagen were detected in post adolescence cartilage. Type IX collagen epitopes were evident throughout the matrix in all cartilage samples up to 19 years of age. Post adolescence, however, the pattern of immunoreactivity revealed territorial staining only. Type IX collagen expression at the transcriptional level is maintained at all ages. Type IX collagen fragments were extracted from young tissue, supporting the hypothesis that young cartilage is continually remodelled, while mature cartilage maintains relatively low levels of collagen turnover. Clearly the age changes we observed may have significant effects on the integrity of the tissue as the chondrocytes in ageing articular cartilage have limited capacity to turnover the interterritorial matrix. However, this study provides evidence that even in old age, the chondrocyte attempts to maintain its pericellular environment and hence its mechanical role. Therefore, the potential of type IX collagen to interact with other matrix components continues to be of importance in the territorial environment, and these interactions may have significant roles in mechanotransduction.

Collagen and proteoglycan turnover in focally damaged human ankle cartilage: evidence for a generalized response and active matrix remodeling across the entire joint surface

OBJECTIVE

Although cartilage lesions occur in the ankles, osteoarthritis rarely develops in the ankles, suggesting that ankle cartilage can up-regulate mechanisms to repair the damaged matrix. To define these processes, we compared cartilage samples obtained from normal tali and from lesional sites of damaged tali.

METHODS

Cartilage samples were obtained from the tali of normal ankles and from 3 sites on tali with lesions (the lesion, adjacent to the lesion, and far removed from the lesion). Cartilage was analyzed for type II collagen (CII) messenger RNA, C-terminal type II procollagen propeptide (CPII), the collagenase cleavage neoepitope (Col2-3/4C(short)), and the denaturation epitope (Col2-3/4m). For the assessment of type IX collagen, the COL2 and NC4 domains were evaluated. The cartilage samples were also assayed for glycosaminoglycans, epitope 846 of aggrecan, and DNA.

RESULTS

The DNA content, epitope 846, COL2(IX), and the denaturation epitope were significantly increased in lesional cartilage. Although there was a tendency toward an increase in CII content and CPII, the increase did not reach significance. Neither the NC4(IX) domain nor Col2-3/4C was elevated. Surprisingly, changes in cartilage both adjacent to and remote from the lesion were similar to those in the lesion.

CONCLUSION

The changes observed in cartilage obtained from the lesion and from sites adjacent to the lesion were not surprising; however, the changes in cartilage obtained from sites remote from the lesion were unexpected. This up-regulation of matrix turnover in ankles with degenerative lesions may indicate a physiologic response of the entire articular surface to repair the damaged matrix, which is not restricted to the lesion site. This suggests that there may be some mechanism of communication across the cartilage. The response by ankle cartilage obtained from a site remote from the lesion has not been observed in the knee.

Cellular invasion of the chicken corneal stroma during development: regulation by multiple matrix metalloproteases and the lens

Avian corneal development requires cellular invasion into the acellular matrix of the primary stroma. Previous results show that this invasion is preceded by the removal of the fibril-associated type IX collagen, which possibly stabilizes matrices through interfibrillar cross-bridges secured by covalent crosslinks. In the present study, we provide evidence for the expression of three matrix metalloproteinases (MMPs) in early corneas, two of which act cooperatively to selectively remove type IX collagen in situ. In organ cultures, MMP inhibitors (either TIMP-2 or a synthetic inhibitor) resulted in arrested development, in which collagen IX persisted, and the stroma remained compact and acellular. We also show that blocking covalent crosslinking of collagen allows for cellular invasion to occur, even when the removal of type IX collagen is prevented. Thus, one factor regulating corneal invasion is the physical structure of the matrix, which can be modified by either selective proteolysis or reducing interfibrillar cross-bridges. We also detected another level of regulation of cellular invasion involving inhibition by the underlying lens. This block, which seems to influence invasive behavior independently of matrix modification, is a transient event that is released in ovo just before invasion proceeds.

Characterization of recombinant amino-terminal NC4 domain of human collagen IX: interaction with glycosaminoglycans and cartilage oligomeric matrix protein

The N-terminal NC4 domain of collagen IX is a globular structure projecting away from the surface of the cartilage collagen fibril. Several interactions have been suggested for this domain, reflecting its location and its characteristic high isoelectric point. In an attempt to characterize the NC4 domain in more detail, we set up a prokaryotic expression system to produce the domain. The purified 27.5-kDa product was analyzed for its glycosaminoglycan-binding potential by surface plasmon resonance and solid-state assays. The results show that the NC4 domain of collagen IX specifically binds heparin with a K(d) of 0.6 microm, and the full-length recombinant collagen IX has an even stronger interaction with heparin, with an apparent K(d) of 3.6 nm. The heparin-binding site of the NC4 domain was located in the extreme N terminus, containing a heparin-binding consensus sequence, whereas electron microscopy suggested the presence of at least three additional heparin-binding sites on full-length collagen IX. The NC4 domain was also shown to bind cartilage oligomeric matrix protein. This interaction and the association of cartilage oligomeric matrix protein with other regions of collagen IX were found to be heparin-competitive. Circular dichroism analyses of the NC4 domain indicated the presence of stabilizing disulfide bonds and a thermal denaturation point of about 80 degrees C. The pattern of disulfide bond formation within the NC4 domain was identified by tryptic peptide mass mapping of the NC4 in native and reduced states. A similar pattern was demonstrated for the NC4 domain of full-length recombinant collagen IX.

Interaction between amino propeptides of type XI procollagen alpha1 chains

Type XI collagen is a quantitatively minor yet essential constituent of the cartilage extracellular matrix. The amino propeptide of the alpha1 chain remains attached to the rest of the molecule for a longer period of time after synthesis than the other amino propeptides of type XI collagen and has been localized to the surface of thin collagen fibrils. Yeast two-hybrid system was used to demonstrate that a homodimer of alpha1(XI) amino propeptide (alpha1(XI)Npp) could form in vivo. Interaction was also confirmed using multi-angle laser light scattering, detecting an absolute weight average molar mass ranging from the size of a monomer to the size of a dimer (25,000-50,000 g/mol), respectively. Binding was shown to be saturable by ELISA. An interaction between recombinant alpha1(XI)Npp and the endogenous alpha1(XI)Npp was observed, and specificity for alpha1(XI)Npp but not alpha2(XI)Npp was demonstrated by co-precipitation. The interaction between the recombinant form of alpha1(XI)Npp and the endogenous alpha1(XI)Npp resulted in a stable association during the regeneration of cartilage extracellular matrix by fetal bovine chondrocytes maintained in pellet culture, generating a protein that migrated with an apparent molecular mass of 50-60 kDa on an SDS-polyacrylamide gel.

The assembly and remodeling of the extracellular matrix in the growth plate in relationship to mineral deposition and cellular hypertrophy: an in situ study of collagens II and IX and proteoglycan

The recent development of new specific immunoassays has provided an opportunity to study the assembly and resorption of type II and IX collagens of the extracellular matrix in relationship to endochondral calcification in situ. Here, we describe how in the bovine fetal physis prehypertrophic chondrocytes deposit an extensive extracellular matrix that, initially, is rich in both type II and type IX collagens and proteoglycan (PG; principally, aggrecan). The majority of the alpha1(IX)-chains lack the NC4 domain consistent with our previous studies with cultured chondrocytes. During assembly, the molar ratio of type II/COL2 domain of the alpha1(IX)-chain varied from 8:1 to 25:1. An increase in the content of Ca2+ and inorganic phosphate (Pi) was initiated in the prehypertrophic zone when the NC4 domain was removed selectively from the alpha1(IX)-chain. This was followed by the progressive loss of the alpha1(IX) COL2 domain and type II collagen. In the hypertrophic zone, the Ca2+/Pi molar ratio ranged from 1.56 to a maximum of 1.74, closely corresponding to that of mature hydroxyapatite (1.67). The prehypertrophic zone had an average ratio Ca2+/Pi ranging from 0.25 to 1, suggesting a phase transformation. At hypertrophy, when mineral content was maximal, type II collagen was reduced maximally in content coincident with a peak of cleavage of this molecule by collagenase when matrix metalloproteinase 13 (MMP-13) expression was maximal. In contrast, PG (principally aggrecan) was retained when hydroxyapatite was formed consistent with the view that this PG does not inhibit and might promote calcification in vivo. Taken together with earlier studies, these findings show that matrix remodeling after assembly is linked closely to initial changes in Ca2+ and Pi to subsequent cellular hypertrophy and mineralization. These changes involve a progressive and selective removal of types II and IX collagens with the retention of the PG aggrecan.

Cartilage oligomeric matrix protein interacts with type IX collagen, and disruptions to these interactions identify a pathogenetic mechanism in a bone dysplasia family

Cartilage oligomeric matrix protein (COMP) and type IX collagen are key structural components of the cartilage extracellular matrix and have important roles in tissue development and homeostasis. Mutations in the genes encoding these glycoproteins result in two related human bone dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia, which together comprise a "bone dysplasia family." It has been proposed that these diseases have a similar pathophysiology, which is highlighted by the fact that mutations in either the COMP or the type IX collagen genes produce multiple epiphyseal dysplasia, suggesting that their gene products interact. To investigate the interactions between COMP and type IX collagen, we have used rotary shadowing electron microscopy and real time biomolecular (BIAcore) analysis. Analysis of COMP-type IX collagen complexes demonstrated that COMP interacts with type IX collagen through the noncollagenous domains of type IX collagen and the C-terminal domain of COMP. Furthermore, peptide mapping identified a putative collagen-binding site that is associated with known human mutations. These data provide evidence that disruptions to COMP-type IX collagen interactions define a pathogenetic mechanism in a bone dysplasia family.

Selective assembly and remodelling of collagens II and IX associated with expression of the chondrocyte hypertrophic phenotype

The assembly and resorption of the extracellular matrix in the physis of the growth plate are poorly understood. By examining isolated fetal growth plate chondrocytes in culture and using immunochemical methods we show that type II collagen, proteoglycan aggrecan, and type IX collagen are assembled into a matrix that is initially enriched in type II collagen over proteoglycan and type IX collagen. When compared to the content of the COL2 domain in the alpha(1)(IX) chain it is evident that the majority ( 90%) of type IX molecules lack the NC4 domain unlike in articular cartilage. During matrix assembly the molar ratio of type II/COL2 of alpha(1)(IX) varied from 25:1 to 2.5:1. Following expression of the hypertrophic phenotype (initiation of type X collagen synthesis) there are parallel changes in both collagen and proteoglycan contents (inversely related to collagenase cleavage of type II collagen). The NC4 domain is then selectively, rapidly and irreversibly removed as mineralization is initiated, leaving the alpha(1)(IX) chain COL2 domain. Subsequently as mineralization progresses type II and type IX collagen (COL2 domain), but not the proteoglycan aggrecan, are resorbed coincident with a markedly increased cleavage of type II collagen by collagenase as mineral is deposited in the matrix. This study, therefore reveals a carefully orchestrated series of events in matrix assembly and resorption that prepares the extracellular matrix for mineralization.

Structural macromolecules and supramolecular organisation of the vitreous gel

The vitreous gel is a transparent extracellular matrix that fills the cavity behind the lens of the eye and is surrounded by and attached to the retina. This gel liquefies during ageing and in 25-30% of the oppulation the residual gel structure eventually collapses away from the posterior retina in a process called posterior retina in a process called posterior vitreous detachment. This process plays a pivotal role in a number of common blinding conditions including rhegmatogenous retinal detachment, proliferative diabetic retinopathy and macular hole formation. In order to understand the molecular events underlying vitreous liquefaction and posterior vitreous detachment and to develop new therapies it is important to understand the molecular basis of normal vitreous gel structure and how this is altered during ageing. It has previously been established that a dilute dispersion of thin (heterotypic) collagen fibrils is essential to the gel structure and that age-related vitreous liquefaction is intimately related to a process whereby these collagen fibrils aggregate. Collagen fibrils have a natural tendency to aggregate so a key question that has to be addressed is: what normally maintains the spacing of the collagen fibrils? In mammalian vitreous a network of hyaluronan normally fills the spaces between these collagen fibrils. This hyaluronan network can be removed without destroying the gel structure, so the hyaluronan is not essential for maintaining the spacing of the collagen fibrils although it probably does increase the mechanical resilience of the gel. The thin heterotypic collagen fibrils have a coating of non-covalently bound macromolecules which, along with the surface features of the collagen fibrils themselves, probably play a fundamental role in maintaining gel stability. They are likely to both maintain the short-range spacing of vitreous collagen fibrils and to link the fibrils together to form a contiguous network. A collagen fibril-associated macromolecule that may contribute to the maintenance of short-range spacing is opticin, a newly discovered extracellular matrix leucine-rich repeat protein. In addition, surface features of the collagen fibrils such as the chondroitin sulphate glycosaminoglycan chains of type IX collagen proteoglycan may also play an important role in maintaining fibril spacing. Furthering our knowledge of these and other components related to the surface of the heterotypic collagen fibrils will allow us to make important strides in understanding the macromolecular organisation of this unique and fascinating tissue. In addition, it will open up new therapeutic opportunities as it will allow the development of therapeutic reagents that can be used to modulate vitreous gel structure and thus treat a number of common, potentially blinding, ocular conditions.

Specific glycanforms of type IX collagen accumulate in embryonic chick sterna after 17 days of development

Type IX collagen is a key component of the extracellular matrix of cartilage where it occurs at the surfaces of type II collagen fibrils as a glycanated molecule. The function of the glycosaminoglycan (GAG) side chain of the molecule is, however, unknown. We have shown that type IX collagen in chicken sternal cartilage is synthesized with a unimodal distribution of GAG chain size, but at post 17 days of development three predominant glycanforms of type IX collagen accumulate. Such accumulation did not occur in sterna from day 15 embryos. In day 17 embryos predominant glycanforms were found in the caudal region of the sternum. By day 19 of development the three predominant glycanforms are widespread throughout the caudal and cephalic regions. The results indicate that developmental and anatomical changes occur to type IX collagen that depend on the size of the GAG chain attached to the alpha2(IX) chain of the molecule.