A collagen-like glycoprotein of the extracellular matrix is the undegraded form of type VI collagen

Differential interactions of decorin and decorin mutants with type I and type VI collagens

The small leucine-rich proteoglycan decorin can bind via its core protein to different types of collagens such as type I and type VI. To test whether decorin can act as a bridging molecule between these collagens, the binding properties of wild-type decorin, two full-length decorin species with single amino acid substitutions (DCN E180K, DCN E180Q), which previously showed reduced binding to collagen type I fibrils, and a truncated form of decorin (DCN Q153) to the these collagens were investigated. In a solid phase assay dissociation constants for wild-type decorin bound to methylated, therefore monomeric, triple helical type I collagen were in the order of 10(-10) m, while dissociation constants for fibrillar type I collagen were approximately 10(-9) m. The dissociation constant for type VI was approximately 10(-7) m. Using real-time analysis for a more detailed investigation DCN E180Q and DCN E180K exhibited lower association and higher dissociation constants to type I collagen, compared to wild-type decorin, deviating by at least one order of magnitude. In contrast, the affinities of these mutants to type VI collagen were 10 times higher than the affinity of wild-type decorin (K(D) approximately 10(-8) m). Further investigations verified that complexes of type VI collagen and decorin bound type I collagen and that the affinity of collagen type VI to type I was increased by the presence of decorin. These data show that decorin not only can regulate collagen fibril formation but that it also can act as an intermediary between type I and type VI collagen and that these two types of collagen interact via different binding sites.

Preparative procedures and purity assessment of collagen proteins

Collagens represent a large family (25 members identified so far) of closely related proteins. While the preparative procedures for the members that are ubiquitous and present in tissues in large quantities (typically fibre and network forming collagens types I, II, III, IV and V) are well established, the procedures for more recently discovered minor collagen types, namely those possessing large non-collagenous domain(s) in their molecule, are mostly micropreparative and for some collagenous proteins even do not exist. The reason is that the proof of their existence is based on immunochemical staining of tissue slices and nucleic database searching. Methods of preparation and identification of constituting alpha-polypeptide chains as well as collagenous and non-collagenous domains are also reviewed. Methods for revealing non-enzymatic posttranslational modifications (particularly of the fibre forming collagen types) are briefly described as well.

In the mammalian eye type VI collagen tetramers form three morphologically different aggregates

The organization of the aggregates occurring in the stroma: (1) of the murine and human cornea after incubation in an ATP acidic solution; (2) of surgically excised epiretinal membranes (ERM); and (3) of the trabecular meshwork of monkey eyes was investigated morphologically and immunocytochemically on thin section electron microscopy. Morphology. The aggregates in the cornea appeared as cross-banded fibrils. The bands were uniformly electron dense (single banded form); they were separated from each other by interbands consisting of a bundle of filaments emerging in cross section as small areas of randomly assembled dot-like structures. In the ERM, most of the aggregates stood out as heteromorphic cross-banded bodies showing dense bands with electron denser borders (double banded form) and interbands composed of longitudinally oriented, parallel sheets or laminae of amorphous material enclosing thin, similarly oriented filaments. These extended, thinner and double in number (since interlacing with similar components of the opposite sheet), into the pale central zone of the dense band. The aggregates of the trabecular meshwork were heteromorphic, had uniformly dense bands (single banded form as in the cornea), but their interbands displayed longitudinal sheets (as the ERM aggregates). Immunocytochemistry revealed type VI collagen in the three eye aggregates with gold particles preferentially localized at the interbands. The specificity of the antibodies used was tested by Western blot analysis of type VI collagen samples extracted from human placenta and on homogenates of human cornea. In conclusion, the results indicate that the tetramers of type VI collagen may aggregate differently into structures with distinct supramolecular arrangements. These are illustrated in schematic drawings.

Alternative splicing of VWFA modules generates variants of type VI collagen alpha 3 chain with a distinctive expression pattern in embryonic chicken tissues and potentially different adhesive function

Type VI collagen, a ubiquitous extracellular cell adhesion molecule, is formed by heterotrimeric monomers which associate into dimers and tetramers and assemble into larger oligomers constituting the 100 nm-long periodic microfilaments of connective tissues. One distinctive structural characteristic of type VI collagen is represented by an alpha 3 chain with a much larger molecular mass compared to the other two chains and with an extensive size heterogeneity, exemplified by the separation into up to five polypeptides in SDS-PAGE. There is evidence that the alpha 3(VI) mRNA can undergo alternative splicing of three VWFA modules at the 5'-end, potentially resulting in the expression of protein variants. Here we report that alternative splicing of alpha 3(VI) mRNA in chicken embryo did not result in the absolute predominance of a particular alpha 3(VI) form in any tissue; instead, the expression of variants including exons A9, A8 and A6 increased with age. In addition, these variants had a more restricted tissue distribution pattern compared to variants including only constitutive exons: A9+ were the rarest and were present almost exclusively in skin and skeletal muscle; A6+ were expressed in several of the examined tissues with local variations; A8+ had intermediate levels and were less widely distributed than A6+ variants. Quantitative densitometric scanning of immunoblots of type VI collagen purified from gizzard and stained with VWFA module-specific antibodies indicated that the polymorphic migration pattern of alpha 3(VI) polypeptides is contributed by concurrent or independent splicing of two exons (A8 and A6) and probably by processing and/or proteolysis at the N- and C-terminus. Three exon-specific recombinant polypeptides were examined in cell adhesion assays, and A6 appeared to be the most active, particularly at low substrate concentrations. The adhesion to the recombinant modules was not abrogated by EDTA nor by mAbs against the integrin beta 1 or alpha 2 subunits. Over all, these results suggest that the splicing of the alpha 3(VI) mRNA and the tissue distribution pattern of type VI collagen variants, apart from promoting cell adhesion to different extents, might also affect additional structural as well as functional properties of this molecule, including microfilament formation and interaction with other extracellular matrix molecules.

von Willebrand factor binds to native collagen VI primarily via its A1 domain

Collagen VI is abundant in the arterial subendothelium. To investigate its mechanism of interaction with von Willebrand factor (vWF), collagen VI was isolated from human placenta and from the extracellular matrix of the human lung fibroblast cell line MRC-5. Purified vWF bound to non-digested collagen VI with moderately high affinity (EC50 approximately 5 nM) and could be inhibited by the Hirudo medicinalis collagen inhibitor calin. The anti-(human vWF A1 domain) monoclonal antibody (AJvW-2), as well as aurin tricarboxylic acid (ATA), at concentrations that saturate the vWF A1 domain, also inhibited this binding. In contrast, the monoclonal anti-(human vWF A3 domain) antibody (82D6A3) inhibited vWF binding to collagens I, III and IV, but had no effect on vWF binding to collagen VI. Likewise, vWF binding to collagen VI was not inhibited by the recombinant vWF domain D4. Polyclonal anti-(collagen VI) antibodies, specifically neutralizing the binding of vWF to collagen VI, confirmed that in the intact endothelial cell extracellular matrix, collagen VI was accessible for interaction with vWF. This binding was only marginally affected by 82D6A3 but was dose-dependently inhibited by AJvW-2, ATA and the A1 domain analogue VCL (recombinant A1 domain of vWF), with IC50 values comparable to those found for the inhibition of vWF binding to isolated collagen VI. The weak interaction of isolated human platelets with collagen VI was mediated via the platelet collagen receptor (GPIa/IIa) and was competitively inhibited by vWF but not by VCL, suggesting that vWF and GPIa/IIa bind to neighbouring but distinct sites on collagen VI. We conclude that vWF binds to collagen VI primarily via its A1 domain, which distinguishes it from the vWF A3 domain-mediated binding to fibrillar collagens.

The 1.6 A structure of Kunitz-type domain from the alpha 3 chain of human type VI collagen

The C-terminal Kunitz-type domain from the alpha 3 chain of human type VI collagen (C5), a single 58 amino acid residue chain with three disulfide bridges, was cloned, expressed and crystallized in a monoclonic form, space group P2(1), with a = 25.7 A, b = 38.2 A, c = 28.8 A and beta = 109 degrees. The structure was resolved by molecular replacement, using Alzheimer's protein precursor inhibitor and bovine pancreatic trypsin inhibitor three-dimensional structures as search models. The molecule with one sulfate ion and 43 associated water molecules was refined by XPLOR to an R-factor of 18.9% at 1.6 A. The molecule was not degraded by trypsin and did not inhibit trypsin or tested serine proteases. As opposed to the other Kunitz family members, C5 demonstrates left-handed chirality of the Cys14-Cys38 disulfide bond. Inversion of the Thr13 carbonyl and bulky side-chains at the interface with trypsin in a model of the C5-trypsin complex may explain the lack of inhibition of trypsin.

Collagen type VI in neural crest development: distribution in situ and interaction with cells in vitro

We have examined the spatio-temporal distribution of collagen type VI (Col VI) during neural crest development in vivo and its ability to promote neural crest cell attachment and migration in vitro. An affinity purified antiserum and chain-specific monoclonal antibodies against chicken Col VI were employed to immunolocalize the collagen in tissue sections and by immunoblotting. At stages of initial neural crest cell migration, the alpha 1(VI) and alpha 2(VI) chains were immunolocalized in apposition with basement membranes of the neural tube, somites, notochord and ectoderm, whereas no immunoreactivity was seen for the alpha 3(VI) chain. Immunoblotting analysis confirmed the expression of alpha 1(VI) and alpha 2(VI) chains and the lack of detectable immunoreactivity for the alpha 3(VI) chain at these early phases of neural crest development. Conversely, at advanced phases of migration and following gangliogenesis, expression of alpha 3(VI) chain coincided with that of alpha 1(VI) and alpha 2(VI) chains in apposition with basement membranes, around the dorsal root ganglia, and in fibrillar arrangements within the developing dermis and ventral sclerotome. The ability of Col VI to promote neural crest cell attachment and migration was tested in vitro using quantitative assays for these processes. Both native microfilaments and isolated tetramers of Col VI strongly promoted neural crest cell attachment and migration. Optimal stimulation of neural crest cell adhesion and migration was dependent upon structural integrity of Col VI since unfolded and disassembled alpha chains only weakly promoted cell attachment and were virtually inactive in supporting cell movement. The importance of a native macromolecular organization of Col VI further was analyzed in experiments in which dissociated tetramers were reassociated by Ca(2+)- and temperature-dependent self-aggregation. In contrast to native microfilaments, these oligomeric complexes were less effective in promoting neural crest cell movement, but still retained the ability to stimulate maximal cell attachment. The results indicate that Col VI is a primary component of the extracellular matrix deposited along neural crest migratory pathways, where it may participate in the regulation of cell movement by functioning as a migratory substrate. The ability of Col VI to promote neural crest cell adhesion and motility is highly dependent upon maintainance of a native macromolecular arrangement.

Localization of collagen type VI in articular cartilage of young and adult mice

Collagen type VI was demonstrated immunomorphologically in articular cartilage (distal femur) of young (2-8 weeks) and adult mice by fluorescence and electron microscopy (gold-labelled second antibody--sandwich method) using pre- and post-embedding techniques. This collagen type was mainly seen in the vicinity of chondrocytes, and in larger amounts in adult cartilage. Electron-microscopic inspection (pre-embedding technique) revealed labelling above plaques that were 40-160 nm in size, and from which up to 7 fine filaments (< or = 10 nm) per unit sectional plane radiated. Using the post-embedding technique, only labelled plaques could be demonstrated; fine filaments were not perceptible. This was partly a result of the low contrast. It is assumed that the globular ends of up to 20 of the fine type VI filaments are anchored in one plaque and that the antibodies bind to the non-collagenous globular domains. Filaments radiated from the plaques and formed a three-dimensional network that stabilized the structures of the cartilaginous matrix. Antibodies against fibronectin also labelled similar plaques. The ends of the type VI filaments are possibly linked into the plaques by fibronectin.

Distribution of type VI collagen expression in synovial tissue and cultured synoviocytes: relation to fibronectin expression

Type VI collagen has recently been shown to be an important component of connective tissue. Double label immunofluorescence procedures were used to immunolocalize type VI collagen in normal and rheumatoid synovium and its distribution was compared with that of fibronectin. In normal synovium type VI collagen is expressed in the synovial membrane but not in the interstitium of the villus. In rheumatoid synovium, however, type VI collagen is extensively deposited in both the interstitial connective tissue and along the lining of the synovial membrane. Cultured rheumatoid and normal synoviocytes produce type VI collagen and fibronectin and incorporate them into their extracellular matrix. These data suggest that type VI collagen may play a part in matrix remodelling of the inflamed joint.

Multiple forms of chicken alpha 3(VI) collagen chain generated by alternative splicing in type A repeated domains

Type VI collagen is a structurally unique component widely distributed in connective tissues. Its molecular structure consists of monomers that have the potential to assemble intracellularly into dimers and tetramers which, once secreted, can form microfilaments by end-to-end association. Individual monomers are composed of chains of Mr = approximately 140,000 (alpha 1 and alpha 2) and greater than 300,000 (alpha 3). Type VI collagen molecules contain a short triple helix with large globular domains at both ends. These domains are made for their greatest part of repetitive units similar to type A repeats of von Willebrand Factor. The alpha 3(VI) chain, contributing most of the mass of the NH2-terminal globule, appeared heterogenous both at the mRNA and protein level. Several alpha 3(VI)-specific clones that lack the sequences corresponding to repeats A8 and A6 were isolated from a chicken aorta cDNA library. Northern blot hybridization of poly (A+)-enriched RNA from chicken gizzard with cDNA fragments corresponding to several individual type A repeats showed that A8- and A6-specific probes did not hybridize to the lower Mr transcripts. Clones spanning approximately 20 kb of the 5'-end of the alpha 3(VI) gene were isolated from a chicken genomic library and subjected to analysis by restriction mapping, Southern blotting, and selective sequencing of the intron-exon boundaries. At the most 5'-end of the gene an additional type A repeat (A9), previously undetected in cDNA clones, was identified. Furthermore, it was determined that the presumed signal peptide and repeats A9 through A6 are encoded within individual exons. Reverse transcription and polymerase chain reaction of aorta RNA suggested that a mechanism of alternative mRNA splicing by a phenomenon of exon skipping generates alpha 3(VI) isoform variants that contain different numbers of type A repeats. Immunohistochemistry of frozen sections of chicken embryo tissues with repeat-specific mAbs showed that an antibody directed against a conditional exon has a more restricted tissue distribution compared to an antibody against a constitutive exon.

Characteristics and in vivo occurrence of type VIII collagen

Type VIII collagen was isolated from bovine Descemet's membranes by pepsin treatment and salt fractionation, as described by Kapoor et al. [(1986) Biochemistry 25, 3930-3937]. Contaminating type IV collagen was removed by ion-exchange chromatography. Purified type VIII collagen consisted of two different polypeptide chains and, compared to the fiber forming collagens, showed a higher thermal stability. Corresponding fractions isolated from pepsinized human Ewing's sarcoma and fetal calf aorta reacted immunologically with a protein of similar molecular mass. After extraction of Descemet's membranes with guanidine hydrochloride, a peptide of about 60 kDa was obtained. This seems to be the tissue form of type VIII collagen.

Comparative analysis of collagens solubilized from human foetal, and normal and osteoarthritic adult articular cartilage, with emphasis on type VI collagen

The different collagen types were extracted sequentially, by 4 M guanidinium chloride and pepsin, from human foetal and normal and osteoarthritic adult articular cartilage. They were characterized by electrophoresis and immunoblotting. Most of the collagenous proteins present in articular cartilage from young human foetuses were solubilized: almost 40% of the total collagen was extracted in the native form with 4 M guanidinium chloride. Type VI collagen was detected in this fraction as high-molecular-mass chains (185-220 kDa) and a low-molecular-mass chain (140 kDa). Type II, IX and XI collagens were also present, but were extracted more extensively by pepsin digestion. Comparative analysis of normal and osteoarthritic cartilage from adults reveals some major differences: an increase in the solubility of the collagen and modifications of soluble collagen types in osteoarthritic cartilage. Furthermore, type VI collagen was present at a higher concentration in guanidinium chloride extracts of osteoarthritic cartilage than those of normal tissue. This finding was corroborated by electron microscopic observations of the same samples: abundant (100 nm) periodic fibrils were observed in the disorganized pericellular capsule of cloned cells in osteoarthritic cartilage. In normal tissues the pericellular zone was more compact and contained only a few such banded fibrils. The differences in the collagen types solubilized from normal and osteoarthritic cartilage, although corresponding to a minor proportion of the total collagen, demonstrate that important modifications in chondrocyte metabolism and in the collagenous network do occur in degenerated cartilage.

Structural and functional features of the alpha 3 chain indicate a bridging role for chicken collagen VI in connective tissues

Type VI collagen is a component of 100 nm long periodic filaments with a widespread distribution around collagen fibers and on the surface of cells. It is an unusual collagen constituted by three distinct chains, one of which (alpha 3) is much larger than the others and is encoded by a 9-kb mRNA. The amino acid sequence of the alpha 3(VI) deduced from the present cDNA clones specifies for a multidomain protein of at least 2648 residues made of a short collagenous sequence (336 residues), flanked at the N-terminus by nine 200 residue long repeating motifs and at the C-terminus by two similar motifs that share extensive identities with the collagen-binding type A repeats of von Willebrand factor. Type VI collagen and alpha 3(VI) fusion proteins bound to insolubilized type I collagen in a specific, time-dependent, and saturable manner. The alpha 3(VI) chain has three Arg-Gly-Asp sequences in the collagenous domain, and cell attachment was stimulated by the triple helix of type VI collagen and by alpha 3(VI) fusion proteins containing Arg-Gly-Asp sequences. This function was specifically inhibited by the Arg-Gly-Asp-Ser synthetic peptide. The type I collagen-binding and the cell-attachment properties of the alpha 3(VI) chain provide direct information for the role of type VI collagen in connective tissues.

Type VI collagen: high yields of a molecule with multiple forms of alpha 3 chain from avian and human tissues

A differential extraction procedure followed by molecular sieve column chromatography for the isolation of large quantities of the tissue form of type VI collagen is described. Recovery of the protein was more than 60% from both chick gizzard and human placenta. On reduced NaDodSO4-gels chick type VI collagen migrated as two major bands at Mr = 140,000 and 150,000 that were present in a 1:1 ratio and five less intense bands between Mr = 230,000 and 180,000. By immunoblotting with a polyclonal antibody against the pepsinized form of chick type VI collagen, all these bands were stained. Furthermore, the amino acid composition of the five higher Mr polypeptides indicated that they all contained hydroxyproline and hydroxylysine. In the chick type VI collagen molecule the five bands of higher Mr belong to the alpha 3 chain since they were recognized by monoclonal antibodies specific for the chick Mr = 260,000 alpha 3 chain. On examination of antigenic activity by solid-phase radioimmunobinding, densitometry of stained NaDodSO4 polyacrylamide gels, and protein content type VI was found to be an abundant collagen since it accounted for up to 0.1% of the tissue wet weight. The yields per tissue wet weight and the migration pattern of human type VI collagen polypeptides were similar to those of the chick. Agarose/polyacrylamide composite gels indicated that the molecular size of the tissue form of type VI collagen molecules under non-reduced conditions corresponded to a basic type of tetrameric molecule.

A type VI collagen-related glycopolypeptide is the major concanavalin A-binding component in pig skin

The major concanavalin A-binding component in urea/deoxycholate/mercaptoethanol extracts of pig skin was a collagenous disulphide-cross-linked glycopolypeptide with an apparent molecular mass of 150 kDa and a pI of 5.5. Antiserum against the electrophoretically purified glycopolypeptide gave strong dermal staining similar to that seen with fluorescent concanavalin A. Immunocytochemical labelling showed prominent labelling of 3-4 nm dermal microfilaments, particularly those associated with dermal blood vessels and mast cells. Immunoblotting with authentic antiserum indicated that the major skin glycopolypeptide was probably identical with collagen-like glycoprotein, the tissue form of the alpha 1/alpha 2 subunits of type VI collagen. This was confirmed by immunoblotting of authentic type VI collagen from pepsin-treated pig skin. Immunoblotting, metabolic labelling with [3H]glucosamine and immune precipitation showed that an immunoreactive collagenous glycopolypeptide was synthesized and secreted by cultured pig skin fibroblasts. The results suggest that type VI collagen is the major concanavalin A-binding component in pig skin.

The bovine corneal SGP-complex is related to the tissue form of type VI collagen

A polyclonal antiserum was prepared in rabbits against the structural glycoprotein (SGP) complex previously isolated from a bacterial collagenase digest of bovine corneal stroma (R. Alper, Curr. Eye Res. 2:479, 1983). Direct and indirect enzyme-linked immunosorbent assays indicated that the antiserum was specific for the SGP-complex and did not react with Types I, III and IV collagen, fibronectin, laminin or actin. Immunoblot experiments indicated that the antiserum reacted with all of the components of the SGP-complex as well as with the cell matrix laid down by bovine keratocytes in culture. An attempt was made to isolate individual antibodies from the antiserum by selective elution from immunoblots of the components of the SGP-complex separated by SDS-PAGE. It was found that regardless of the protein band from which the antibody was eluted, every antibody isolated reacted with every protein component of the SGP-complex suggesting that the SGP-complex may have been derived from a single precursor protein and that the observed heterogeneity of the SGP-complex may have been the result of proteolytic breakdown of the protein held together by disulfide bonds. When the anti-SGP antiserum was used to immunoprecipitate 14C-proline labeled proteins from the media of bovine keratocytes in culture, the major protein observed had a Mr of about 140,000 daltons, similar to that of GP-140 also known as CL-glycoprotein. These proteins have been shown to represent the tissue form of Type VI collagen. To test the hypothesis that the SGP-complex may be related to the GP-140 (CL-glycoprotein), ELISA and immunoblotting studies were performed comparing the properties of the anti-SGP serum with those of a polyclonal antibody specific for Type VI collagen. The SGP-complex reacted positively by ELISA with the anti-human Type VI collagen antiserum and, conversely, human Type VI collagen gave a positive ELISA reaction with an antiserum against the SGP-complex. The anti-human Type VI collagen antiserum reacted with most of the major components of the SGP-complex on immunoblots of SDS-PAGE gels. These data indicate that the SGP-complex is related to and probably is derived from the tissue form of Type VI collagen.

Isolation of cDNA clones corresponding to the Mr = 150,000 subunit of chick type VI collagen

Type VI collagen is a disulfide-bonded protein with an unusual structure in that the molecule contains three short triple-helical domains and very extended non-collagenous regions. The molecule is a heterotrimer composed in the chick of two polypeptides of similar apparent size in SDS-PAGE (Mr = 140- and 150,000) but different structure, and a third component that is much larger (Mr = 260,000) than the other two chains. We report here on the isolation of several overlapping cDNA clones from a chicken aorta mRNA expression library in the plasmid vector pEX1. Antibodies affinity purified onto the fusion proteins recognized the chick type VI collagen Mr = 150,000 subunit. Northern blots using the cDNA inserts from the above clones revealed a single RNA species of about 4,600 nucleotides sufficient to code for a protein with the size of the Mr = 150,000 subunit.

Type VI collagen of the intervertebral disc. Biochemical and electron-microscopic characterization of the native protein

The collagen framework of the intervertebral disc contains two major fibril-forming collagens, types I and II. Smaller amounts of other types of collagen are also present. On examination of the nature and distribution of these minor collagens within bovine disc tissue, type VI collagen was found to be unusually abundant. It accounted for about 20% of the total collagen in calf nucleus pulposus, and about 5% in the annulus fibrosus. It was discovered by serially digesting disc tissue with chondroitin ABC lyase and Streptomyces hyaluronidase that native covalent polymers of type VI collagen could be extracted. Electron micrographs of this material prepared by rotary shadowing revealed the characteristic dimensions of tetramers and double tetramers of type VI molecules, with their central rods and terminal globular domains. Molecular-sieve column chromatography on agarose under non-reducing non-denaturing conditions gave a series of protein peaks with molecular sizes equivalent to the tetramer, double tetramer and higher multimers. On SDS/polyacrylamide-gel electrophoresis after disulphide cleavage, these fractions of type VI collagen all showed a main band at Mr 140,000 and four lesser bands between Mr 180,000 and 240,000. On electrophoresis without disulphide cleavage in agarose/2.4% polyacrylamide only dimeric (six chains) and tetrameric (12 chains) forms of type VI molecules were present. The ability to extract all the type VI collagen of the tissue in 4 M-guanidinium chloride, and absence of aldehyde-mediated cross-linking residues on direct analysis, showed that, in contrast with most matrix collagens, type VI collagen does not function as a covalently cross-linked structural polymer.

Characterization of three constituent chains of collagen type VI by peptide sequences and cDNA clones

Pepsin-solubilized collagen VI was prepared from human placenta and used to separate three constituent chains for determining partial amino acid sequences. Antibodies raised against the chains assisted in the identification and purification of several cDNA clones from three expression lambda gt11 libraries. Most of the clones hybridized to either a 3.5-kb or 4.2-kb mRNA species which by matching peptide and nucleotide sequences could be identified as coding for the alpha 2(VI) or alpha 1(VI) chain, respectively. Other clones hybridized to either an 8.5-kb mRNA which very likely encoded the alpha 3(VI) chain or to an unknown 2.0-kb mRNA. Northern blots revealed a considerable variation in the mRNA levels for each collagen VI chain in both skin and cornea fibroblasts and in several tumor cell lines. Limited sequence data generated from peptides and cDNA clones demonstrated a characteristic cysteine pattern at the junction between N-terminal globular domain and triple helix in all three chains. In addition, the data showed occasional interruptions of triplet sequences within the triple-helical domain and the presence of two Arg-Gly-Asp sequences which are potential cell-binding structures.

The tissue form of chicken type VI collagen

We have purified intact type VI collagen from chicken gizzard. The protein was found to consist of a 130 kDa, a 140 kDa and a 180-200 kDa subunit. The 130 kDa and 140 kDa subunits were obtained in equimolar amounts and identified as the alpha 2 (VI) and the alpha 1 (VI) chains, respectively. The third subunit was usually obtained in the form of 3-4 closely related polypeptides, which may represent different processing or modification products of the alpha 3 (VI) chain.

Type VI collagen: immunohistochemical identification as a filamentous component of the extracellular matrix of the developing avian corneal stroma

Selected stages of the developing chicken cornea have been examined for type VI collagen, employing monoclonal antibodies specific for this molecule. By immunofluorescence, the molecule is not detectable in 5 1/2 day corneas, a time at which the epithelial-derived, acellular primary stroma is the only corneal matrix present. One day later, the presumptive stromal fibroblasts have invaded this stroma and have initiated synthesis of the secondary (mature) stroma. By that time, a strong fluorescent signal for the type VI collagen molecule is detectable throughout the stroma. It is present in all subsequent ages examined. The molecule is not restricted to the cornea, and is present in most stromal matrices examined, including those of the sclera, eyelid, and nictitating membrane. Immunoelectron microscopy was also performed, utilizing a colloidal gold-labeled secondary antibody. These data show that the type VI collagen is not a component of the striated collagen fibrils, but instead is assembled in the form of thin filaments. The monoclonal antibody bound to the filaments at periodic intervals of about 100 nm.

Fibrillin, a new 350-kD glycoprotein, is a component of extracellular microfibrils

A new connective tissue protein, which we call fibrillin, has been isolated from the medium of human fibroblast cell cultures. Electrophoresis of the disulfide bond-reduced protein gave a single band with an estimated molecular mass of 350,000 D. This 350-kD protein appeared to possess intrachain disulfide bonds. It could be stained with periodic acid-Schiff reagent, and after metabolic labeling, it contained [3H]glucosamine. It could not be labeled with [35S]sulfate. It was resistant to digestion by bacterial collagenase. Using mAbs specific for fibrillin, we demonstrated its widespread distribution in the connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule. Electron microscopic immunolocalization with colloidal gold conjugates specified its location to a class of extracellular structural elements described as microfibrils. These microfibrils possessed a characteristic appearance and averaged 10 nm in diameter. Microfibrils around the amorphous cores of the elastic fiber system as well as bundles of microfibrils without elastin cores were labeled equally well with antibody. Immunolocalization suggested that fibrillin is arrayed periodically along the individual microfibril and that individual microfibrils may be aligned within bundles. The periodicity of the epitope appeared to match the interstitial collagen band periodicity. In contrast, type VI collagen, which has been proposed as a possible microfibrillar component, was immunolocalized with a specific mAb to small diameter microfilaments that interweave among the large, banded collagen fibers; it was not associated with the system of microfibrils identified by the presence of fibrillin.

Avian type VI collagen. Monoclonal antibody production and immunohistochemical identification as a major connective tissue component of cornea and skeletal muscle

Two monoclonal antibodies have been characterized as being against avian type VI collagen. By competition ELISA, the antibodies bound to the native type VI collagen molecule but not to its separated chains or to any of the other native collagen types tested. By rotary shadowing analysis of complexes of antibody-type VI collagen monomers, one of the antibodies (VI-EC6) has been shown to bind to a site in the triple helical domain of the molecule. The site at which this antibody binds to the dimeric form of type VI collagen is consistent with the previously proposed model for a supramolecular organization of the molecule (Furthmayr et al., Biochem j 211 (1983) 303) in which the monomers are arranged in an antiparallel, slightly staggered overlap. Immunofluorescence analyses of sections of chicken eyes and skeletal muscle demonstrate that type VI collagen is a major component of most stromal matrices.

Type VI collagen in extracellular, 100-nm periodic filaments and fibrils: identification by immunoelectron microscopy

Filaments and fibrils that exhibit a 100-nm axial periodicity and occur in the medium and in the deposited extracellular matrix of chicken embryo and human fibroblast cultures have been tentatively identified with type VI collagen on the basis of their similar structural characteristics (Bruns, R. R., 1984, J. Ultrastruct. Res., 89:136-145). Using indirect immunoelectron microscopy and specific monoclonal and polyclonal antibodies, we now report their positive identification with collagen VI and their distribution in fibroblast cultures and in tendon. Primary human foreskin fibroblast cultures, labeled with anti-type VI antibody and studied by fluorescence microscopy, showed a progressive increase in labeling and changes in distribution with time up to 8 d in culture. With immunoelectron microscopy and monoclonal antibodies to human type VI collagen followed by goat anti-mouse IgG coupled to colloidal gold, they showed in thin sections specific 100-nm periodic labeling on extracellular filaments and fibrils: one monoclonal antibody (3C4) attached to the band region and another (4B10) to the interband region of the filaments and fibrils. Rabbit antiserum to type VI collagen also localized on the band region, but the staining was less well defined. Control experiments with antibodies to fibronectin and to procollagen types I and III labeled other filaments and fibrils, but not those with a 100-nm period. Heavy metal-stained fibrils with the same periodic and structural characteristics also have been found in both adult rat tail tendon and embryonic chicken tendon subjected to prolonged incubation in culture medium or treatment with adenosine 5'-triphosphate at pH 4.6. We conclude that the 100-nm periodic filaments and fibrils represent the native aggregate form of type VI collagen. It is likely that banded fibrils of the same periodicity and appearance, reported by many observers over the years in a wide range of normal and pathological tissues, are at least in part, type VI collagen.

Type VI collagen and glycoprotein MFPI are distinct components of the extracellular matrix

Two collagenous glycoproteins, Mr 140,000 and Mr 150,000, are synthesized and secreted into the medium of cultured fibroblasts. The glycoprotein of Mr 140,000 is identical with the 140K(VI) component of type VI collagen by both immunological and physicochemical criteria. The glycoprotein of Mr 150,000 is immunologically distinct and exhibits the physicochemical characteristics of the putative elastic microfibrillar glycoprotein MFPI.

Molecular assembly, secretion, and matrix deposition of type VI collagen

Monoclonal antibodies reactive with the tissue form of type VI collagen were used to isolate the type VI collagen polypeptides from cultured fibroblasts and muscle cells. Two [35S]methionine-labeled polypeptides of 260 and 140 kD were found intracellularly, in the medium, and in the extracellular matrix of metabolically labeled cells. These polypeptides were disulfide cross-linked into very large complexes. The 260- and 140-kD polypeptides were intimately associated and could not be separated from each other by reduction without denaturation. In the absence of ascorbic acid, both polypeptides accumulated inside the cell, and their amounts in the medium and in the matrix were decreased. These results suggest that both the 260- and the 140-kD polypeptides are integral parts of the type VI collagen molecule. Examination of type VI collagen isolated from the intracellular pool by electron microscopy after rotary shadowing revealed structures corresponding to different stages of assembly of type VI collagen. Based on these images, a sequence for the intracellular assembly of type VI collagen could be discerned. Type VI collagen monomers are approximately 125 nm long and are composed of two globules separated by a thin strand. The monomers assemble into dimers and tetramers by lateral association. Only tetramers were present in culture media, whereas both tetramers and multimers were found in extracellular matrix extracts. The multimers appeared to have assembled from tetramers by end-to-end association into filaments that had prominent knobs and a periodicity of approximately 110 nm. These results show that, unlike other collagens, type VI collagen is assembled into tetramers before it is secreted from the cells, and they also suggest an extracellular aggregation mechanism that appears to be unique to this collagen.

The immunohistochemical distribution of collagens type IV, V, VI and of laminin in the human oral mucosa

The distribution of collagens type V (form AB2) and VI was investigated on cryostat sections of normal human oral mucosa by indirect immunofluorescence. For comparison, antibodies to fragments of type IV collagen and laminin were also used to delineate basement membrane containing structures. All antibodies used were raised against human proteins. Type V collagen appeared as a microfibrillar structure throughout the interstitium, apparently touching but not being present within epithelial or vascular basement membranes. Microfibrils in blood vessel walls were limited to the intimal layer. Pericellular areas were not specifically stained. Type VI collagen appeared as an almost amorphous stromal structure becoming more prominent and more fibrillar in the upper connective tissue papillae. Intense staining was observed in the media of blood vessels and around smooth muscle cells. A possible role of type VI collagen in tissue stabilization may be expected from this ubiquitous and abundant distribution. The findings identify types V and VI collagen as important structures in the oral mucosa and serve as a basis for understanding morbid changes.

The platelet reactivity of collagen type VI

Collagen type VI in native (undenatured or triple-helical) form has been shown, like collagen types I-V, alpha 1(I) trimer and alpha 2(I) trimer, to possess platelet reactivity provided that essential quaternary structural needs are first satisfied. Thus platelet aggregation was induced by the collagenous domain of collagen type VI, isolated free of the non-collagenous elements, when this entity was presented to platelets in fibrillar form. This implies that platelet recognition sites in collagen type VI are located in the collagenous sequence of the molecule. Aggregation of platelets was also induced, although a higher concentration was required, by the intact, "parent" collagen following its polymerisation by random association of molecules with the aid of a cross-linking agent (glutaraldehyde) to yield an amorphous polymer. This permits the suggestion that the more ordered molecular assembly of collagen type VI thought to occur in vivo, to yield a microfibrillar form, is likely to be associated with significant platelet reactivity. Our results support the notion that any collagenous species may be reactive towards platelets provided that essential tertiary and quaternary structural requirements are met and in this sense, therefore, they favour more the idea of multiple platelet-reactive sites in collagen of relatively low structural specificity and low affinity.

Structure and macromolecular organization of type VI collagen

Collagen VI is a large, disulfide-bonded protein complex which is widely distributed in connective tissue. The constituent polypeptide chains (Mr = 110,000-140,000) consist of collagenous and noncollagenous segments, are degraded to chains of about half the size when collagen VI is solubilized by pepsin, and assemble to a unique pattern of oligomers. As revealed by electron microscopy, the triple-stranded protomer consists of a triple helix 105 nm in length flanked on each side by globular domains of similar size (diameter about 7 nm). Protomers are assembled to dimers by an antiparallel staggered alignment of triple-helical segments. This leads to inner regions, 75 nm in length, of two slightly supercoiled triple helices flanked by globular domains. At both sides 30-nm-long outer triple-helical segments emerge that are terminated by globules. Tetramers are formed from laterally aligned dimers that cross with their outer triple-helical segments in a scissors-like fashion. The same structures, except with much smaller globular domains, are found in pepsin-treated collagen VI. Disulfide-linked collagen VI produced by cultured fibroblasts has a size similar to that of genuine collagen VI found in tissue extracts. Larger forms of collagen VI are assembled from tetramers by end-to-end aggregation which because of an overlap of the outer segments brings all globular domains close together. This arrangement predicts microfibrillar structures in tissues with a periodicity of 100-110 nm and a diameter of 5-10 nm. Structures consistent with this proposal were indeed found by immunoelectron microscopy of placenta and aorta using the ferritin technique. Large, lateral aggregates of collagen VI microfibrils may in addition exist in cell cultures and tissues ("zebra collagen," "Luse bodies") and are presumably maintained by contacts between globular domains.

Radioimmunoassay for human type VI collagen and its application to tissue and body fluids

A liquid phase radioimmunoassay (RIA) was developed for pepsin-solubilized human type VI collagen, allowing quantitative analysis of this protein down to a concentration of 3 ng/ml. No cross-reactivity was observed with human collagens type I, III, IV (triple helical portion and 7-S domain), and V, nor with laminin fragment Pl and plasma fibronectin. Significant amounts of closely related antigenic material were detected in serum, bile, ascites, and mesenchymal cell culture media. Type VI collagen could be completely solubilized from several tissues by a repeated pepsin digest, and its content as determined by RIA was found to be less than 0.1% of total collagen (55-70 micrograms/g protein). In fibrotic liver tissue type VI collagen was elevated up to 10-fold (620 micrograms/g protein) when compared to normal liver. Sera of patients with fibrotic liver disease, however, revealed antigen levels usually below the narrow normal range of 22 +/- 7.8 ng/ml (mean +/- 2.5 SD). We conclude that, although type VI collagen represents a minor fraction of the interstitial collagens, its comparatively high serum levels point to a considerable turnover in the normal individual. Our data suggest that in fibrosis as exemplified in fibrotic liver disease, the metabolism of this collagen is down-regulated, while at the same time, it accumulates in the interstitial matrix.

Characterization of a type VI collagen-related Mr-140 000 protein from cutis-laxa fibroblasts in culture

The precise biochemical defects in connective-tissue metabolism that are responsible for the laxity of skin seen in the syndrome of cutis laxa are largely unknown. We have studied fibroblasts cultured from skin explants of a 2-year-old male with the syndrome. Electron-microscopic examination of this skin revealed decreased amounts of amorphous elastin and an increase in elastin-associated microfibrils. Although the cultured fibroblasts were similar to control skin fibroblasts in morphology, growth rate and total protein synthesis, there was a 4-6-fold increase in accumulation of a collagenous protein of Mr 140 000 in both the culture medium and in the cell layer. This protein was structurally distinct from collagen types I, III, IV, V and VIII. It was found to be related to a cell-surface-associated glycoprotein, GP140, by both antigenic cross-reactivity and peptide mapping. Our data support observations that GP140 is a precursor of at least one form of pepsin-extracted type VI collagen.

Size and domain structure of collagen VI produced by cultured fibroblasts

Disulfide-bonded forms of collagen VI were analyzed by immunoblotting of fibroblast culture medium and cell extracts. The protein consists of pepsin and collagenase-resistant domains of about equal size indicating a molecular mass of 340 kDa for collagen VI monomers.

Translatable mRNA for GP140 (a subunit of type VI collagen) is absent in SV40 transformed fibroblasts

Production of GP140, a major component of the extracellular matrix of cultured fibroblasts, is markedly decreased in SV40 transformed cells as compared with normal cells (Carter, W. G., 1982, J. Biol. Chem., 257:13805-13815). To determine at what step the biosynthesis is inhibited, we compared the levels of functional mRNA for GP140 in normal and transformed fibroblasts. Translation of total RNA from W138 cells in a reticulocyte lysate, followed by immunoprecipitation with affinity-purified antibodies to GP140, yielded a single polypeptide with an Mr of 125,000. This polypeptide was identified as GP140 based on its immunoreactivity, collagenase sensitivity, and comigration on polyacrylamide gels with GP140 synthesized by cells in the presence of tunicamycin and 2,2'-bipyridyl. No cell-free synthesis of GP140 was observed with total RNA from SV40 transformed W138 cells, indicating that these cells contain very low levels of GP140-specific mRNA. The biosynthesis of GP140 might therefore be blocked at the transcriptional level.