From the Orthopedic Research Laboratories, University of Washington, Seattle, Washington 98195
Received for publication, March 10, 2003 , and in revised form, April 22, 2003.
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ABSTRACT |
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INTRODUCTION |
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Type IX collagen is a structural matrix component that is most abundant in hyaline cartilages of the developing skeleton. It functions as an adhesion protein in the extracellular matrix where it becomes covalently cross-linked to the surface of type II collagen fibrils and most concentrated on thin fibrils of the pericellular domain (46). The molecule is a heterotrimer of genetically distinct 1(IX),
2(IX), and
3(IX) chains, which form three triple-helical segments, COL1,1 COL2, and COL3 and four non-helical domains, NC1NC4. The largest, NC4 on
1(IX), is basically charged, suggesting that it may serve to interact with proteoglycans (7).
Recent reports have linked genetic polymorphisms to risk of disc degeneration (8, 9). Specifically, tryptophan substitutions at position 326 of the 2(IX) chain, and position 103 of the
3(IX) chain, were associated with increased risk of human lumbar disc degeneration and chronic sciatica but not knee osteoarthritis (8, 9). Nucleus pulposus, the gel-like central zone of the young intervertebral disc, has a similar collagen phenotype to that of hyaline cartilage, with types II, IX, and XI collagens being the principal fibrillar components (10). The association of the tryptophan polymorphisms with clinical conditions involving discs but not synovial joint cartilages implies that the molecular character of collagen IX in intervertebral disc collagen may differ from that in hyaline cartilage. The properties of collagen IX from disc tissue have not been fully characterized.
The human 1(IX) gene gives rise to two mRNA transcripts, a long and a short form (11). The long form is expressed in hyaline cartilages and the product has a globular NH2-terminal domain, NC4. Short
1(IX), expressed in embryonic chick cornea (12), is transcribed from an alternative start site and lacks the NC4 domain. A second promoter and an alternative exon 1, located in the intron between exons 6 and 7 of COL9A1, are used to express the short form of
1(IX). In the present study, by NH2-terminal sequence analysis of COL3(IX) domains isolated from the matrix of bovine nucleus pulposes and articular cartilage, we show that nucleus pulposus contains exclusively the short form of
1(IX). The covalent cross-linking properties of nucleus pulposus type IX collagen were also characterized to determine any consequences of having short instead of long
1(IX).
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MATERIALS AND METHODS |
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Column ChromatographyPepsin-solubilized type IX collagen COL1 and COL2 trimers were resolved on a C8 reverse-phase column (Brownlee Aquapore RP-300; 4.6 mm x 25 cm) with a linear gradient (2338%) of solvent B in A over 45 min at a flow rate of 1 ml/min collecting 1-ml fractions. Solvent A was 0.1% trifluoroacetic acid (v/v) in water, and solvent B was 0.085% trifluoroacetic acid (v/v) in acetonitrile:n-propyl alcohol (3:1, v/v). Eluent was monitored for 220 nm absorbance, and aliquots of collected fractions (100 µl each) were dried and analyzed by SDS-PAGE. The disulfide-bounded COL2 trimer was reduced, and cysteines were carboxymethylated with iodoacetate before rechromatographing the individual chains on the same column with a linear gradient (1540%) of solvent B in A over 50 min.
The COL3 domains of type IX collagen were isolated from the 4.0 M NaCl precipitated fraction by molecular sieve chromatography on a Bio-Gel A5m column (1.5 x 170 cm, 200400 mesh, Bio-Rad) eluted with a 0.05 M Tris-HCl buffer, pH 7.5, containing 2 M guanidine HCl, at a flow rate of 4 ml/h, collecting 2.0-ml fractions. Aliquots of collected fractions were desalted and analyzed by SDS-PAGE. Fractions containing the type IX collagen COL3 domain were pooled. The individual COL3(IX) chains were then resolved by HPLC on a C8 reverse-phase column with a linear gradient (1540%) of solvent B in A over 50 min followed by SDS-PAGE.
Gel Electrophoresis and Electroblotting of ProteinsCollagen fractions recovered by differential salt precipitation and column chromatography were routinely examined by SDS-PAGE (14). For amino-terminal sequence analysis and Western blotting, resolved protein bands were transblotted to PVDF membrane (Bio-Rad) using a MilliBlot-SDE electroblotting apparatus (Millipore). The membrane was washed thoroughly in ultrapure water (Millipore Milli-Q) to remove salts, then stained with Coomassie Brilliant Blue to detect protein bands. Bands were excised for automated sequence analysis (5). For Western blotting, a monoclonal antibody, 10F2, one of several raised against protease-generated neoepitopes in the collagen 1(II)C-telopeptide (15), was used as the primary, and biotin-conjugated goat anti-mouse IgG as the secondary, antibody then followed by ExtrAvidin-alkaline phosphatase (Sigma). The blots were developed using 5-bromo-4-chloro-3-indolyl phosphate and nitro blue tetrazolium.
Protein SequencingAmino-terminal sequence analysis was performed on a Porton 2090E gas-phase microsequencer equipped with on-line high performance liquid chromatography analysis of phenylthiohydantoin derivatives.
To remove pyroglutamate from the NH2-terminal end of blocked collagen chains, protein was solubilized in 0.1 M sodium phosphate buffer, 0.01 M EDTA, 5 mM dithiothreitol, 5% (v/v) glycerol, pH 8.0, and treated with the enzyme pyroglutamate aminopeptidase (Roche Applied Science) at 37 °C for 5 h before SDS-PAGE and transblotting (16).
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RESULTS |
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When the 1(IX) NH2-terminal sequence derived from bovine nucleus pulposus was compared with the cDNA-predicted human sequence, nucleus pulposus
1(IX) was identical to the predicted short-form of human
1(IX) beginning with an NH2-terminal glutamate encoded by alternative exon 1 and continuing with the sequence encoded by exon 8 (Fig. 3a). This result establishes that bovine nucleus pulposus contains the short form of
1(IX) that lacks the NC4 protein domain. In contrast, NH2-terminal sequence analysis of pepsin-derived
1(IX)COL3 from calf articular cartilage showed that the amino terminus was not blocked and the sequence was identical to that of the long form of
1(IX) predicted from the human cDNA corresponding to the COOH-terminal portion of the NC4 domain (Fig. 3b). The results, however, show that the 7-amino acid sequence encoded by a predicted exon 7 is missing. The present findings, therefore, suggest a bovine splicing variant (or genomic difference) that results in the long
1(IX) form of bovine hyaline cartilage lacking the exon 7 sequence of the NC4 domain.
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Previous studies have shown that all four triple-helical cross-linking sites of type IX collagen are located in the COL2 domain (5, 17). To determine whether the apparent exclusive usage of the short form of 1(IX) in the nucleus pulposus affects the cross-linking properties of type IX collagen, the 2 M NaCl-precipitated fraction of a pepsin digest of bovine nucleus pulposus was run on a C8 reverse-phase column to separate the COL2(IX) trimer from the COL1(IX) trimer (Fig. 4). Individual COL2(IX) chains were resolved from each other by reverse-phase HPLC (Fig. 5a) and SDS-PAGE (Fig. 5b) after reduction and alkylation of cysteine residues. Direct NH2-terminal sequence analysis of each protein band was performed after electroblotting to a PVDF membrane. Fig. 5d shows the sequences identified. Each type IX band gave multiple running sequences, one of which was the NH2 terminus of the NC3/COL2 domain. The others were the N- and C-telopeptides of type II collagen and
3(IX) NC1. All the results were consistent with previous findings on cartilage type IX collagen (5, 17), with an N-telopeptide cross-linked to all three type IX chains at a COL2 site,
3(IX) NC1 linked to
1(IX) and
3(IX) at this same COL2 site and a C-telopeptide of type II collagen linked to a second
3(IX)COL2 site.
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Cross-linking between the type II collagen C-telopeptide and 3(IX)COL2 was further confirmed using mAb 10F2 (Fig. 5c), which recognizes a proteolytic neoepitope in the type II collagen C-telopeptide sequence, KGPDPLQ, in which the COOH-terminal glutamine is essential for immunoreactivity.
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DISCUSSION |
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The short transcriptional form of 1(IX) has been identified in the developing chick cornea, vitreous humor, and notochord and in mouse eye and heart during embryonic development (12, 2328). In the young disc, nucleus pulposus is known to contain both chondrocytic and notochordal cells (29). During growth, the notochordal cells die while chondrocytic cells persist. Since only one transcription product, the short form of
1(IX), was detected in the young bovine nucleus pulposus, this suggests that either the chondrocytic cells of the nucleus differ from those of hyaline cartilage in their promoter usage of Col9a1 or, perhaps, that the notochordal cells produce the collagen IX. It is notable in this regard that in animals in which the nucleus pulposus retains a gelatinous, youthful texture throughout life (e.g. non-chondrodystrophoid breeds of dog), notochordal cells also persist (29).
The short form of the type IX collagen molecule in chick vitreous also bears a long glycosaminoglycan chain on the NC3 domain of the 2(IX) chain (23). The functional significance of the long glycosaminoglycan chain is still uncertain, although it has been proposed that it might prevent lateral fibril aggregation in the gel-like vitreous. Collagen fibrils in the vitreous are particularly thin and widely dispersed from each other compared with cartilage (30), in which in the mammal, the
2(IX) chain lacks the glycosaminoglycan chain (17). Thin fibrils, and a gelatinous texture, are also features of the young nucleus pulposus. It is not known whether the
2(IX) chain of nucleus pulposus has a glycosaminoglycan chain attached to its NC3 domain. The present findings do show that despite lacking an NC4 domain on the
1(IX) chain, the cross-linking properties of nucleus pulposus type IX collagen are essentially the same as in cartilage. As in cartilage, the disc-type IX molecules are cross-linked to type II collagen and to other type IX collagen molecules, so that a recently proposed model of the molecular interactions (31), which accommodates all known cross-linking sites, is applicable to disc and cartilage.
The results also show that the 7-amino acid sequence encoded by human exon 7 is absent from bovine 1(IX). It has been established for chick cartilage that the equivalent exon 7 sequence is present in the cDNA from the long mRNA transcript of Col9a1 and by sequence analysis of tryptic peptides that the protein sequence is expressed in tissue
1(IX) (7). The present findings, therefore, indicate either a gene sequence difference in bovine Col9a1 or a spliced-out version of
1(IX) lacking the exon 7 sequence in our bovine cartilage samples. No evidence emerged for an
1(IX) chain that included the exon 7 domain, so it is not known whether the alternative version can be expressed in any tissue in this species. Bovine genomic sequence is not available to help resolve this issue. The
1(IX) chain NC4 domain is encoded by exons 27. It is not known whether any exons other than 7 can be variably spliced. Transcripts of the other cartilage-specific collagens, types II and XI, are known to be variably spliced (3235).
Tryptophan substitutions at position 326 of the 2(IX) chain, and position 103 of the
3(IX) chain, have been linked to increased risk of human lumbar disc degeneration and chronic sciatica (8, 9). It has been proposed that the tryptophan residue in the
2(IX)COL2 domain near the known cross-linking site at residue 330 in
3(IX)COL2 might interfere with the interaction between collagens II and IX (8). This site cross-links the C-telopeptide of type II collagen to
3(IX)COL2. Western blot analysis using monoclonal antibody 10F2, which is specific to the C-telopeptide of type II collagen, confirms that the type II collagen C-telopeptide is attached selectively to
3(IX)COL2 (Fig. 5c). Antibody 10F2 recognizes a cleavage site in the type II C-telopeptide generated by pepsin.2 The yields on NH2-terminal sequence analysis also indicate that essentially every molecule has an attached C-telopeptide. Future work with this antibody might, therefore, provide a sensitive method for detecting any changes in cross-linking between
3(IX)COL2 and type II collagen C-telopeptide caused by the tryptophan-containing polymorphic variants (38). Peptide mapping using a multi-epitope recognizing antiserum to type IX collagen has proved to be useful for displaying the cross-linking properties in general of type IX collagen in fetal human cartilage (3638).
In conclusion, we have found that nucleus pulposus of the intervertebral disc contains the short form of the 1(IX) chain, a product of an alternative promoter, which lacks the NC4 domain. The cross-linking properties of nucleus pulposus type IX collagen are, however, similar to those of hyaline cartilage. We also have identified in bovine cartilage a previously undescribed splicing variant of the long form of
1(IX) that lacks the sequence encoded by exon 7. Its pattern of expression (in development, tissue type, species, etc.), and any functional significance remains unknown.
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FOOTNOTES |
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To whom correspondence should be addressed. Dept. of Orthopedics and Sports Medicine, University of Washington, Box 356500, Seattle, WA 98195-6500. Tel.: 206-543-4700; Fax: 206-685-4700; E-mail: wujj{at}u.washington.edu.
1 The abbreviations used are: COL, collagenous; NC, non-collagenous; HPLC, high performance liquid chromatography; PVDF, polyvinylidene difluoride; N- and C-telopeptides, short sequences that lack the (Gly-XY)n repeat and from the amino and carboxyl ends of various collagen chains.
2 J.-J. Wu and D. R. Eyre, unpublished observations.
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REFERENCES |
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