Mammalian cartilage synthesizes both proteoglycan and non-proteoglycan forms of type IX collagen

Bovine epiphysial cartilage synthesizes both proteoglycan (PG) and non-PG forms of type IX collagen in a ratio of approx. 2:1. The PG form with its attached glycosaminoglycan on the alpha 2(IX) chain is the major form in the medium, whereas both forms are found in the tissue. The results are discussed with regard to cartilage matrix organization.

Macromolecular organization of chicken type X collagen in vitro

The macromolecular structure of type X collagen in the matrices of primary cultures of chick hypertrophic chondrocytes was initially investigated using immunoelectron microscopy. Type X collagen was observed to assemble into a matlike structure with-in the matrix elaborated by hypertrophic chondrocytes. The process of self assembly was investigated at the molecular level using purified chick type X collagen and rotary-shadowing EM. It was shown that under neutral conditions at 34 degrees C, individual type X collagen molecules associate rapidly into multimeric clusters via their carboxy-terminal globular domains forming structures with a central nodule of carboxy-terminal domains and the triple helices radiating outwards. Prolonged incubation resulted in the formation of a regular hexagonal lattice by lateral association of the juxtaposed triple-helical domains from adjacent multimeric clusters. This extended lattice may play an important role in modifying the cartilage matrix for subsequent events occurring in endochondral bone formation.

The response of endothelial cells to TGF beta-1 is dependent upon cell shape, proliferative state and the nature of the substratum

Endothelial cells plated on two-dimensional (2-D) substrata proliferate until they form a tightly apposed confluent monolayer of quiescent cells that display a typical ‘cobblestone’ morphology. When added to proliferating cultures TGF beta-1 (transforming growth factor beta-1) inhibited cell growth and caused marked morphological changes, with the cells becoming enlarged and ragged. These effects were dose-dependent and reversible. TGF beta-1 also reduced the cloning efficiency and colony size of these cells, indicating that TGF beta-1 is cytotoxic and cytostatic for endothelial cells. By contrast, TGF beta-1 added to quiescent cobblestone cultures did not affect cell morphology or cell numbers. In the presence of 20% serum, the level of total protein synthesis per cell was significantly increased by TGF beta-1 in a dose-dependent manner when the cells were cultured on a 2-D substratum, regardless of whether the cells were proliferating or cobblestone quiescent. The level of plasminogen activator inhibitor type 1 was specifically increased in these cultures, as demonstrated by reverse fibrin zymography and immunoprecipitation. Endothelial cells embedded within a 3-D collagen gel display an elongated ‘sprouting’ morphology. Such cells self-associate to form three-dimensional cellular networks within the gel, but do not proliferate. The addition of TGF beta-1 to these quiescent sprouting cells initially induced rounding-up without altering protein synthesis, and cell death occurred later. The effects of TGF beta-1 on sprouting endothelial cells were also examined using two culture systems where both the cobblestone and the sprouting phenotypes were present. TGF beta-1 reduced the number of cells present and the extent of migration of sprouting cells embedded within a type I collagen gel, but had no effect upon sprouting cells embedded within a complex endothelial-produced extracellular matrix. Large vessel (aortic) and microvessel (retinal) endothelial cells responded in a similar way to TGF beta-1; the only difference being that an increased synthesis of PAI-1 was not observed with sub-confluent BREC cultures. Our results suggest that the effects of TGF beta-1 upon endothelial cells depend on the shape (cobblestone or sprouting), on the proliferative state of the cells, and on the nature of the matrix surrounding the cells. The response of these cells to TGF beta-1 in vivo may be similarly modulated during angiogenesis by changes in the cell phenotype and the composition of the surrounding matrix.

Isolation and ultrastructural analysis of microfibrillar structures from foetal bovine elastic tissues. Relative abundance and supramolecular architecture of type VI collagen assemblies and fibrillin

Extensive intact assemblies of matrix macromolecules have been solubilized from foetal calf skin, nuchal ligament and aorta by a new procedure that includes bacterial collagenase digestion under non-reducing, non-denaturing conditions and gel filtration chromatography. Type VI collagen was identified as the major microfibrillar element of these tissues by SDS-PAGE analysis and Western blotting. Rotary shadowing electron microscopy of these preparations revealed by far the most abundant and extensive arrays of intact collagen VI microfibrils isolated to date. The distinct microfibrillar species, fibrillin, which was identified on the basis of its periodicity and morphology, was also solubilized in abundance by this protocol. Analysis of these complex polymers has generated new information on their supramolecular architecture and relative abundance in these tissues. The protocol also demonstrates that the release of intact collagen VI microfibrils from these tissues is largely dependent on the removal of the major collagen fibrils.

Arginine vasopressin stimulates net fluid secretion in a polarized subculture of cyst-forming MDCK cells

Transepithelial fluid secretion is an important process in the progressive enlargement of certain types of renal cysts. Arginine vasopressin (AVP) increases the rate of cyst formation and expansion in an in vitro model of renal cysts that uses Madin-Darby canine kidney (MDCK) cells grown in a gelled matrix of Type 1 collagen. In this study, it was determined if AVP promoted net fluid secretion by MDCK cells. The rate of volumetric fluid secretion was determined from the net movement of water across epithelial layers of MDCK cells grown on permeable, collagen-coated membranes. AVP in the basolateral medium (but not in apical medium) at concentrations exceeding 10(-9) M caused sustained basolateral to apical transepithelial fluid secretion (approximately 0.6 microL/cm2/h). 1-Desamino-8-D-AVP, a V2 receptor agonist, had a similar effect. The secreted fluid was hyperosmotic compared with the bath (5.7 to 9.7 mosM). Chloride was consistently secreted, but the absolute level in the secreted fluid was variable. Intracellular cAMP content was increased 187% by a 2-h exposure to AVP and 10(-4) M methylisobutylxanthine. Net fluid secretion was augmented by methylisobutylxanthine and theophylline and was inhibited by ouabain, bumetanide, and a sodium-dependent Cl-/HCO3- exchange inhibitor (L-645,695) but was not altered by clonidine, guanabenz, or indomethacin. AVP-induced fluid secretion was not accompanied by a change in transepithelial hydraulic conductivity. It is suggested that AVP stimulates fluid secretion of MDCK epithelial monolayers by activating V2 receptor-mediated adenylate cyclase. The regulation of net fluid secretion by AVP would appear to depend on modulation of solute transport, rather than on water permeability.

The synthesis of type X collagen by bovine and human growth-plate chondrocytes

Chondrocytes were isolated from bovine growth-plate cartilage and cultured within type I collagen gels. A major collagen with chains of Mr 59,000, decreasing to 47,000 on pepsinization, was synthesized and identified as type X collagen. This collagen was cleaved at two sites by mammalian collagenase, resulting in a major triple-helical fragment with chains of Mr 32,000. The species of Mr 59,000, 47,000 and 32,000 were not detected by SDS-polyacrylamide gel electrophoresis before reduction, indicating the presence of disulphide bonds within the triple helix. In contrast, similar biosynthetic studies with human growth-plate cartilage in organ culture, indicated that human type X collagen does not contain disulphide bonds. A polyclonal antiserum was raised to bovine type X collagen and used in immunolocalization studies to provide direct evidence for the association of type X collagen with the hypertrophic chondrocytes in both bovine and human growth plates during development.

Single-dose subcutaneous iodine-131-iodohippurate for determination of renal plasma flow

Subcutaneous administration of a single dose of 131I-iodohippurate was used for determination of renal plasma flow (RPF) in 20 subjects during water diuresis. Slow release of tracer (200 microCi) permitted serial clearance measurements over 5 hr that were compared to standard, constant infusion, PAH clearance (mean 379.5 +/- 34.9 ml/min/1.73 m2, range 50.9 to 696.3 ml/min/1.73 m2). RPF(Isotope) was 424.9 +/- 30.3 ml/min/1.73 m2 (range 144.4 to 746.5 ml/min/1.73 m2) and highly correlated with RPFPAH (r = 0.883, p less than 0.0001). This technique permits prolonged studies of renal plasma flow under steady-state conditions without constant infusion.

Isolation of cDNAs encoding the complete sequence of bovine type X collagen. Evidence for the condensed nature of mammalian type X collagen genes

The complete primary structure of the bovine alpha 1(X) collagen chain was determined by nucleotide sequencing of cDNA clones. The overlapping cDNA clones encode 3144 bp with a 5'-terminal untranslated region of 148 bp, a 2025 bp reading frame and a 3'-terminal untranslated region of 971 bp. This represents the first complete sequence of a mammalian type X collagen cDNA and has allowed a number of informative comparisons to be made with the previously published chick alpha 1(X) sequence. The primary translation products of both bovine and chick type X collagen are 674 amino acid residues in length and there is a 73.3% identity at the amino acid level (67.8% at the base level). Sequence analyses reveal that the greatest degree of identity between the two species occurs within the triple-helical domain and the C-terminal non-collagenous domain, whereas the identity within the N-terminal non-collagenous domain is markedly lower. The interchain disulphide-bonding observed previously within the triple helix of bovine type X collagen is explained by the presence of two cysteine residues within an imperfection of the triple-helical domain encoded by -Gly-Xaa-Cys-Xaa-Yaa-Cys-Xaa-Yaa-Gly-. Southern blot analyses of bovine genomic DNA demonstrate that the bovine type X collagen gene is likely to have a condensed structure, similar to that of the chick, with at least 1.3 kb of the coding sequence being contained within one exon.

Molecular composition of type VI collagen. Evidence for chain heterogeneity in mammalian tissues and cultured cells

The chain composition and relative abundance of type VI collagen synthesized by cells cultured from foetal bovine nuchal ligament and skin were compared with those of the type VI collagen present in these foetal tissues. Immunoprecipitation of intact collagen VI from medium and cell layers of nuchal ligament fibroblasts and skin fibroblasts at confluence revealed collagen type VI molecules with a chain composition consistent with an [alpha 1(VI)alpha 2(VI)alpha 3(VI)] monomeric assembly. Maintenance of cells in a post-confluent quiescent state promoted a marked phenotypic change in these ratios, with increased concentrations of assemblies composed of equimolar ratios of alpha 1(VI) and alpha 2(VI) chains detected in the medium of these cultures. Analysis of steady-state concentrations of mRNA for alpha 1(VI) and alpha 2(VI) chains revealed these species to be present in increased abundance at post-confluence in all the cultures, but no corresponding increase was observed in the alpha 3(VI) mRNA. In order to assess the physiological significance of these observations, the chain composition of the collagen VI content of the corresponding foetal tissues was assessed by Western blotting after extraction in guanidinium isothiocyanate under reducing conditions. Extracts of nuchal ligament revealed a collagen VI chain composition consistent with a heterotrimeric chain assembly. In contrast, the skin extracts revealed an abundance of alpha 1(VI) and alpha 2(VI) chains with only traces of the alpha 3(VI) chain detected. Increased equimolar concentrations of the alpha 1(VI)-chain and alpha 2(VI)-chain mRNAs in skin again reflected the increased concentrations of these polypeptide chains. Type VI collagen was present in greater abundance both in the nuchal ligament and in the corresponding nuchal-ligament fibroblast cultures. The results indicate that the chain composition of type VI collagen is subject to modulation at the level of transcription as a result of variations in the proliferative state of the cells, and demonstrate that different isoforms of collagen VI occur in foetal development.

Modulation of extracellular matrix biosynthesis by bovine retinal pericytes in vitro: effects of the substratum and cell density

Bovine retinal pericytes plated on a two-dimensional substratum display a characteristic stellate morphology. In post-confluent cultures these cells aggregate spontaneously to form multicellular nodules. The same cells plated within a three-dimensional collagen matrix display an elongated sprouting morphology. Sprouting pericytes may be embedded within a gel either as individual cells or as multicellular aggregates. We have compared the nature of the matrix proteins synthesised by pericytes displaying these different phenotypes. Stellate pericytes cultured on plastic dishes synthesised predominantly type I collagen, some type III collagen and only traces of type IV collagen. The same collagen types were secreted when nodules had formed in postconfluent cultures on plastic, and by sprouting cells plated as single cells within the collagen gel. By contrast, sprouting pericytes plated as aggregates within the collagen gel secreted increased levels of type IV collagen and reduced amounts of type I collagen. Fibronectin was synthesized by pericytes under all experimental conditions examined; thrombospondin was produced in relatively large amounts by cells grown on plastic dishes, whereas only trace amounts could be detected in the medium when the cells were cultured within a collagen gel matrix. Transmission electron microscopy revealed that pericyte aggregates within a collagen gel contained cells in close apposition surrounded by a dense extracellular matrix. In contrast, cells in the centre of a nodule on plastic appeared to be separated from each other by loose extracellular material. These results suggest that the morphological and biosynthetic phenotypes of retinal pericytes are modulated by cell-matrix and/or cell-cell interactions.

Modulation of type X collagen gene expression by calcium beta-glycerophosphate and levamisole: implications for a possible role for type X collagen in endochondral bone formation

Chondrocytes from the cephalic region of 18-day chick embryo sterna were cultured within type I collagen gels in the presence of increasing concentrations of calcium beta-glycerophosphate (Ca beta GP) (2.5 mM, 5 mM and 10 mM) or 2 mM levamisole. Addition of Ca beta GP produced an increase in type X collagen synthesis and when 10 mM Ca beta GP was used, mineral deposits were observed in the matrix elaborated by the chondrocytes. Measurement of mRNA levels of type II and type X collagen microgram-1 total RNA revealed that the level of alpha 1(II) mRNA decreased whereas the level of alpha 1(X) mRNA remained relatively constant in the presence of Ca beta GP. Thus, Ca beta GP appears to increase type X collagen synthesis by enhancing the rate of collagen type X mRNA translation. In levamisole-treated cultures the synthesis of type X collagen and its deposition within the matrix were inhibited. Measurement of mRNA levels micrograms-1 total RNA revealed that the level of alpha 1(II) mRNA increased whereas the level of alpha 1(X) mRNA was markedly decreased in the presence of levamisole. Consequently, it must be concluded that levamisole inhibits type X collagen synthesis by inhibiting type X collagen gene transcription. These results add support for type X collagen having an important role in the mineralization process and have demonstrated that type X collagen synthesis is probably regulated at the mRNA translational stage during chondrocyte hypertrophy.

Differences between adult and foetal fibroblasts in the regulation of hyaluronate synthesis: correlation with migratory activity

We have previously reported that confluent foetal fibroblasts migrate into three-dimensional collagen gel matrices to a significantly greater extent than do adult cells. Hyaluronic acid (HA) is a major constituent of the extracellular matrix deposited by fibroblasts and has been demonstrated to stimulate the migration of a number of different cell types. Previous studies have indicated that the synthesis of HA by normal adult skin fibroblasts declines significantly when the cells achieve confluence. Data presented in this paper indicate that foetal fibroblasts differ from adult cells in this respect, in that they do not show an inverse relationship between cell density and HA synthesis, i.e. confluent foetal fibroblasts continue to produce approximately the same amount of HA as do subconfluent cells. These data suggest that the synthesis of relatively high levels of HA by foetal fibroblasts at confluence may be causally related to the elevated migration displayed by these cells. In this context, a close correlation was observed between the level of HA synthesized by confluent foetal and adult fibroblasts and the differential migratory activity displayed by these cells. Such differences in HA synthesis and migratory behaviour were only apparent at cell confluence, with subconfluent foetal and adult fibroblasts being indistinguishable in terms of these two criteria. Our data further reveal that: (1) cell density affects the size class of HA synthesized by both foetal and adult cells; and that (2) there is a considerable degree of heterogeneity amongst the nine different fibroblast lines examined in this study in terms of the size class of HA that they produce.

Comparative studies of type X collagen expression in normal and rachitic chicken epiphyseal cartilage

The levels of type X collagen in mineralizing normal chicken epiphyses and nonmineralizing rachitic chicken tibial epiphyses were measured and compared. Qualitative immunoperoxidase studies with anti-chick type X collagen monoclonal antibodies on sections from normal and rachitic cartilage demonstrated that the type X collagen levels in rachitic growth plates are reduced. Northern hybridization of mRNA and biosynthetic studies have confirmed that type X collagen synthesis in rickets is also decreased. In hypocalcemic rickets, the level of type X collagen mRNA is reduced by 80% whereas the level of type X collagen mRNA is only reduced by 50% in normocalcemic rickets. These observations provide additional evidence that type X collagen is involved in the process of cartilage mineralization and also suggest that the partial recovery of type X collagen synthesis in normocalcemic rickets may be related to the elevated plasma concentration of calcium. Calcium concentration may therefore play an important role in the control of type X collagen synthesis.

Bovine cartilage types VI and IX collagens. Characterization of their forms in vivo

1. Collagens were extracted from bovine cartilage by 4 M-guanidinium chloride in the presence of proteinase inhibitors and identified by immunoblotting with specific anti-collagen sera. 2. The collagens retained their native conformations (shown by the resistance of their triple-helical domains to pepsin digestion), and the molecular masses of their component alpha-chains indicated that the chains were intact. 3. Type VI collagen was extracted as a large-molecular-mass disulphide-bonded aggregate composed of components of molecular mass 140 kDa and 200-240 kDa, and was therefore similar to type VI collagen identified in noncartilaginous tissues. Immunoblotting established the 200-240 kDa components as intact forms of the alpha 3(VI) chain. 4. Type IX collagen consisted of three clearly separable components of molecular mass 84 kDa, 72 kDa and 66 kDa, which were assigned to the alpha 1(IX)-, alpha 3(IX)- and alpha 2(IX)-chains respectively, and a large proportion of this collagen had no covalently bound glycosaminoglycan attached to the alpha 2(IX)-chain. 5. Differences between the type IX collagen extracted from bovine cartilage and that identified in biosynthetic studies on chick cartilage are discussed.

Mechanism of action of the migration stimulating factor produced by fetal and cancer patient fibroblasts: effect on hyaluronic and synthesis

We have previously demonstrated that confluent fetal fibroblasts migrate into three-dimensional collagen gels to a significantly greater extent than their normal adult counterparts. Recent studies have revealed that this behavioral difference results from the secretion by fetal fibroblasts of a soluble migration-stimulating factor (MSF) which acts on these cells in an autocrine fashion. Adult fibroblasts do not produce MSF but remain responsive to it. Skin fibroblasts from cancer patients resemble fetal fibroblasts (rather than normal adult cells) with respect to their migratory behavior on collagen gels and continued production of MSF. This communication is concerned with elucidating the biochemical basis of MSF activity. Data are presented indicating that a) hyaluronic acid is required for the elevated migratory activity displayed by confluent fetal and breast cancer patient skin fibroblast; b) adult fibroblasts exhibit a bell-shaped dose-response to MSF, with maximal stimulation of migration observed at a concentration of 10 ng/ml; c) the migratory activity of adult fibroblasts pre-incubated with MSF remains high in the absence of additional factor: and d) MSF affects both the quantity and size class distribution of hyaluronic acid synthesized by adult fibroblasts. We have previously speculated that the persistent fetal-like fibroblasts of breast cancer patients play a direct role in disease pathogenesis by perturbing normal epithelial-mesenchymal interactions. The observations reported here suggest that MSF-induced alterations in hyaluronic acid synthesis may contribute to the molecular basis of such perturbations.

Plasminogen activator inhibitor-type I is a major biosynthetic product of retinal microvascular endothelial cells and pericytes in culture

Previous studies have shown that a glycoprotein of Mr 47,000 (designated Gp47) is a major biosynthetic product of retinal endothelial cells in vitro (Canfield, Schor, West, Schor & Grant (1987) Biochem. J. 246, 121-129). We now present data indicating that (a) an identical protein is secreted by bovine retinal pericytes, (b) this protein is plasminogen activator inhibitor-type I (PAI-1), as revealed by immunoprecipitation with specific antibodies and reverse fibrin zymography, and (c) retinal endothelial cells and pericytes synthesize different species of matrix macromolecules, that is: type IV collagen is the major collagen secreted by endothelial cells, whereas pericytes produce predominantly type I collagen; fibronectin and thrombospondin are synthesized by both cell types. Our studies also indicate that PAI-1 is produced, albeit at considerably lower levels, by large vessel vascular cells (aortic endothelial and smooth muscle cells) and human skin fibroblasts. PAI-1 produced by human skin fibroblasts appears to be a distinct molecular species compared to its bovine counterpart as assessed by its slower mobility on SDS/polyacrylamide-gel electrophoresis. The potential significance of elevated PAI-1 production by retinal endothelial cells and pericytes, as well as their distinctive patterns of matrix biosynthesis, is discussed in terms of the involvement of these cells in the maintenance and remodelling of microvessel basement membrane.

Cartilage proteoglycan aggregate and fibronectin can modulate the expression of type X collagen by embryonic chick chondrocytes cultured in collagen gels

Chick embryo sternal chondrocytes from the caudal and cephalic regions were cultured within type I collagen gels and type I collagen/proteoglycan aggregate composite gels in normal serum. Caudal region chondrocytes were also cultured within type I collagen gels in the presence of fibronectin-depleted serum. There was a marked stimulation of type X collagen synthesis by the caudal region chondrocytes after 9 days in the presence of fibronectin-depleted serum and after 14 days in the presence of proteoglycan aggregate. These results provide evidence for the ability of chondrocytes from a zone of permanent cartilage to synthesise type X collagen and for the involvement of extracellular matrix components in the control of type X collagen gene expression.

A comparison of the morphological, histochemical and biochemical features of embryonic chick sternal chondrocytes in vivo with chondrocytes cultured in three-dimensional collagen gels

Disaggregated chondrocytes from embryonic chick whole sterna proliferate in three-dimensional collagen gels forming mixtures of cartilage nodules (chondroids) and columns of cells. A typical matrix elaborated by the cells is composed of collagen type II and chondroitin and keratan sulphate proteoglycans. The cells of the chondroids are small and uniform whilst those in the columns are larger (hypertrophic) and have a vacuolated cytoplasm. Disaggregated cells from the caudal portion of the sternum (which in vivo contains small uniform cells and does not calcify) tend to form chondroids in culture whilst cells from the cephalic portion (which in vivo contains large hypertrophic cells and whose matrix calcifies) tend to form columns of hypertrophic cells. Only the cultures containing hypertrophic cells synthesise collagen type X and only the hypertrophic cells contain alkaline phosphatase. Collagen type X and alkaline phosphatase may therefore be coregulators of the calcification of hypertrophic cartilage matrix.

Identification and partial characterization of two major proteins of Mr 47,000 synthesized by bovine retinal endothelial cells in culture

Biosynthetic experiments with cultured bovine retinal endothelial cells have identified a glycoprotein of Mr 47,000 (Gp47) as a major component secreted into the medium. Gp47 is a non-collagenous glycoprotein with a pI of 4.6-5.5, which does not bind to either gelatin-Sepharose or heparin-Sepharose but is retained by concanavalin A-Sepharose. The Mr of this species decreases to approx. 42,000 in the presence of tunicamycin, indicating that it contains asparagine-linked oligosaccharides. A second protein of Mr 47,000 (P47) is present in the cell layer/matrix of these cultured cells. The electrophoretic mobility of P47 remains unaltered when synthesized in the presence of tunicamycin. Peptide-mapping experiments using N-chlorosuccinimide and Staphylococcus aureus V8 proteinase demonstrate that Gp47 and P47 are distinct proteins, and are not related to colligin, a membrane-bound collagen-receptor protein of similar size, or to SPARC, a major secreted product of parietal endodermal cells and sparse cultures of aortic endothelial cells.

Partial characterization of type X collagen from bovine growth-plate cartilage. Evidence that type X collagen is processed in vivo

Sequential extraction of bovine growth-plate cartilage with 4 M guanidinium chloride and pepsin was used to identify the intact and pepsinized forms respectively of type X collagen. This collagen occurs predominantly as the processed [alpha 1(X)]3 form in vivo, although the procollagen [pro alpha 1(X)]3 form can also be detected. The bovine pro alpha 1(X) and alpha 1(X) chains have Mr values identical to the corresponding chick species (Mr 59,000 and 49,000). However, the pepsinized alpha 1(X)p chains (Mr 47,000) are larger than those of the chick (Mr 45,000), and the bovine collagen type X is further distinguished by being disulphide-bonded within the triple-helical domain.

A comparison of the effects of different substrata on chondrocyte morphology and the synthesis of collagen types IX and X

Embryonic chick sternal chondrocytes were cultured either within three dimensional gels of type I collagen, type II collagen or agar, or as monolayers on plastic dishes coated with air-dried films of these matrix macromolecules. It was observed that cell shape and cell growth varied markedly between the different culture conditions. Flattened monolayers of cells on plastic or films of type I or type II collagen, proliferated more rapidly and reached a higher final cell density per culture than the more rounded cells found in the cultures on agar films or within three-dimensional gels. Biosynthetic studies demonstrated that in addition to the synthesis of type II collagen, all the cultures were producing collagen types IX and X. Chondrocytes cultured on plastic or films of the different matrix macromolecules all showed a similar expression of types IX and X collagen, independent of whether they displayed a flattened or round cell morphology. In contrast, marked variations in the proportions of the minor collagens, particularly type X collagen, were observed when the cells were cultured within three-dimensional gels. The data suggest that direct interaction of the cell surface with matrix constituents displaying a particular spatial array could be an important aspect in the control of type IX and X collagen expression by chondrocytes.

Identification of disulphide-bonded type X procollagen polypeptides in embryonic chick chondrocyte cultures

A high-Mr (Mr 120,000), disulphide-bonded collagenous polypeptide was observed to co-purify with the prox1(X) chain during isolation of cartilage collagens from culture medium of embryonic chick tibiotarsal chondrocytes. This high Mr polypeptide was subsequently shown by two-dimensions l SDS-PAGE and peptide mapping to represent a dimer of the prox1(X) chain of type X collagen linked by disulphide bonding in the non-collagenous domains.