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

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Nanoscale imaging of morphological changes of umbilical cord in pre-eclampsia

It is known that pre-eclampsia affects the structure of the umbilical cord including changes in diameter and wall thickness. In this work, the morphological changes of umbilical cords associated with pre-eclampsia were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM images showed the overall structural changes in the umbilical cord, and the AFM imaged the surface of the cord in the nanometer range. The amount of Wharton's jelly was reduced in the cords of pre-eclampsia patients and it was holed along the boundary. Compared to a normal pregnancy, the surface of a pre-eclampsia cord was relatively smooth. In all components (Wharton's jelly, veins, and arteries), the values for surface roughness, Sa (average value of the roughness), Sq (root mean square), and Sz (peak to peak value), were smaller than those of the control (P < 0.05). Especially, the values for Sa of veins were ~fourfold less than those of the controls (P < 0.05). In pre-eclamptic cords, the amount of elastin in veins was increased while that of the artery was decreased.

Comparison of gene expression in individual preimplantation bovine embryos produced by in vitro fertilisation or somatic cell nuclear transfer

In vitro fertilisation (IVF) and somatic cell nuclear transfer (SCNT) have been implicated in a variety of developmental abnormalities. Aberrant gene expression is likely to account for much of the diminished viability and developmental abnormalities observed. In the present study, the expression of multiple genes in IVF and SCNT bovine blastocyst-stage embryos were evaluated and compared with in vivo-produced embryos. Eleven genes expressed at and following maternal-zygotic transcription transition were evaluated in individual blastocysts by real-time polymerase chain reaction following RNA amplification. A subset of those genes was also evaluated in individual IVF and SCNT eight-cell embryos. A fibroblast-specific gene, expressed by nuclear donor cells, was also evaluated in IVF and SCNT embryos. The observed gene expression pattern at the eight-cell stage was not different between IVF and SCNT embryos (P > 0.05). In vitro fertilisation and SCNT blastocyst expression was lower (P < 0.01) for all genes compared with their in vivo-produced counterparts, except for lactate dehydrogenase isoenzyme A (P < 0.001). The patterns of gene expression of the IVF and SCNT blastocysts were indistinguishable. Neither SCNT eight-cell nor blastocyst-stage embryos expressed the gene used as a fibroblast marker (collagen VIalpha1). For the genes evaluated, the level of expression was influenced more by the environment than by the method used to produce the embryos. These results support the notion that if developmental differences observed in IVF- and SCNT-produced fetuses and neonates are the result of aberrant gene expression during the preimplantation stage, those differences in expression are subtle.

Collagen VI deficiency affects the organization of fibronectin in the extracellular matrix of cultured fibroblasts

Fibronectin is one of the main components of the extracellular matrix and associates with a variety of other matrix molecules including collagens. We demonstrate that the absence of secreted type VI collagen in cultured primary fibroblasts affects the arrangement of fibronectin in the extracellular matrix. We observed a fine network of collagen VI filaments and fibronectin fibrils in the extracellular matrix of normal murine and human fibroblasts. The two microfibrillar systems did not colocalize, but were interconnected at some discrete sites which could be revealed by immunoelectron microscopy. Direct interaction between collagen VI and fibronectin was also demonstrated by far western assay. When primary fibroblasts from Col6a1 null mutant mice were cultured, collagen VI was not detected in the extracellular matrix and a different pattern of fibronectin organization was observed, with fibrils running parallel to the long axis of the cells. Similarly, an abnormal fibronectin deposition was observed in fibroblasts from a patient affected by Bethlem myopathy, where collagen VI secretion was drastically reduced. The same pattern was also observed in normal fibroblasts after in vivo perturbation of collagen VI-fibronectin interaction with the 3C4 anti-collagen VI monoclonal antibody. Competition experiments with soluble peptides indicated that the organization of fibronectin in the extracellular matrix was impaired by added soluble collagen VI, but not by its triple helical (pepsin-resistant) fragments. These results indicate that collagen VI mediates the three-dimensional organization of fibronectin in the extracellular matrix of cultured fibroblasts.

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.

The distribution of type VI collagen in the developing tissues of the bovine femoral head

Type VI collagen appears central to the maintenance of tissue integrity. In adult articular cartilage, type VI collagen is preferentially localised in the chondron where it may be involved in cell attachment. In actively remodelling developing cartilage, the distribution is less certain. We have used confocal immunohistochemistry and in situ hybridisation to investigate type VI collagen distribution in third trimester bovine proximal femoral epiphyses. In general, type VI collagen immunofluorescence was concentrated in the chondrocyte pericellular matrix, with staining intensity strongest in regions which persist to maturity and weakest in regions that remodel during development. Type VI collagen was also present in cartilage canals. In the growth plate and around the secondary centre of ossification, the intensity of type VI collagen stain rapidly decreased with chondrocyte maturation and was absent at hypertrophy, except where canal branches penetrated the growth plate and stain was retained around the adjacent chondrocytes. In situ hybridisation confirmed the presence of type VI collagen mRNA in cartilage canal mesenchymal cells but the signal was low in chondrocytes, suggesting minimal levels of synthesis and turnover. The results are consistent with a role for type VI collagen in stabilising the extracellular matrix during development.

The role of the alpha3(VI) chain in collagen VI assembly. Expression of an alpha3(VI) chain lacking N-terminal modules N10-N7 restores collagen VI assembly, secretion, and matrix deposition in an alpha3(VI)-deficient cell line

Collagen VI is a microfibrillar protein found in the extracellular matrix of virtually all connective tissues. Three genetically distinct subunits, the alpha1(VI), alpha2(VI), and alpha3(VI) chains, associate intracellularly to form triple-helical monomers, which then assemble into disulfide-bonded dimers and tetramers before secretion. Although sequence considerations suggest that collagen VI monomers composed of all three chains are the most stable isoform, the precise chain composition of collagen VI remains controversial and alternative assemblies containing only alpha1(VI) and alpha2(VI) chains have also been proposed. To address this question directly and study the role of the alpha3(VI) chain in assembly, we have characterized collagen VI biosynthesis and in vitro matrix formation by a human osteosarcoma cell line (SaOS-2) that is deficient in alpha3(VI) production. Northern analysis showed an abundance of alpha1(VI) and alpha2(VI) mRNAs, but no detectable alpha3(VI) mRNA was apparent in SaOS-2 cells. By day 30 of culture, however, small amounts of alpha3(VI) mRNA were detected, although the level of expression was still much less than alpha1(VI) and alpha2(VI). Collagen VI protein was not detected in SaOS-2 medium or cell layer samples until day 30 of culture, demonstrating that despite the abundant synthesis of alpha1(VI) and alpha2(VI), no stable collagen VI protein was produced without expression of alpha3(VI). The alpha1(VI) and alpha2(VI) chains produced in the absence of alpha3(VI) were non-helical and were largely retained intracellularly and degraded. The critical role of the alpha3(VI) chain in collagen VI assembly was directly demonstrated after stable transfection of SaOS-2 cells with an alpha3(VI) cDNA expression construct that lacked 4 of the 10 N-terminal type A subdomains. The transfected alpha3(VI) N6-C5 chains associated with endogenous alpha1(VI) and alpha2(VI) and formed collagen VI dimers and tetramers, which were secreted and deposited into an extensive network in the extracellular matrix. These data demonstrated that alpha3(VI) is essential for the formation of stable collagen VI molecules and subdomains N10-N7 are not required for molecular assembly.

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.

Purification of fibrillin-containing microfibrils and collagen VI microfibrils by density gradient centrifugation

A method is described for the purification of collagen VI microfibrils and fibrillin-containing microfibrils, respectively. High M(r) microfibril-rich preparations isolated from nuchal ligament by bacterial collagenase digestion and size fractionation were purified by CsCl density gradient centrifugation. Localization of collagen VI and fibrillin within the gradient was achieved by SDS-PAGE/Western blotting. Large collagen VI microfibrillar aggregates were present at the top of the gradient. Hyaluronidase pretreatment dissociated these aggregates and enabled purification of collagen VI microfibrils at a density of 1.33 g/ml. Fibrillin-containing microfibrils separated at 1.37 g/ml and copurified with MAGP1, but not LTBP1, LTBP2, or fibronectin. Confirmation of the intact status of the purified microfibrils was obtained by rotary shadowing. The ability to separate and purify these complex macromolecules provides a powerful means of addressing their molecular composition, organization, and structure:function relationships.

Structural colocalisation of type VI collagen and fibronectin in agarose cultured chondrocytes and isolated chondrons extracted from adult canine tibial cartilage

Cell-matrix and matrix-matrix interactions are of critical importance in regulating the development, maintenance and repair of articular cartilage. In this study, we examined the structural colocalisation of type VI collagen and fibronectin in isolated chondrons and long-term agarose cultured chondrocytes extracted from normal adult canine articular cartilage. Using double labelling immunohistochemistry in conjunction with dual channel confocal microscopy and digital image processing we demonstrate that type VI collagen and fibronectin are distributed in a similar staining pattern and are colocalised at the surface of cultured chondrocytes and isolated chondrons. The results suggest that type VI collagen and fibronectin may play a role in both cell-matrix adhesion and matrix-matrix cohesion in the pericellular microenvironment surrounding articular cartilage chondrocytes.

Type VIII collagen is a product of vascular smooth-muscle cells in development and disease

Type VIII collagen is a short-chain collagen with considerable similarity to type X collagen. We have generated chain-specific antibodies to the alpha 1 and alpha 2 chains of type VIII collagen, and used them as probes to examine the synthesis of type VIII collagen by vascular smooth muscle cells (VSMC). In addition, chain-specific oligonucleotides have been used in reverse transcriptase-PCR (RT-PCR) reactions with RNA extracted from cultured smooth muscle cells in culture and from freshly isolated vascular tissues. Radiolabelling of VSMC in culture and immunoprecipitation with chain-specific antibodies showed that both chains were expressed. Lower levels of type VIII collagen were found in adult VSMC than in neonatal VSMC. RT-PCR showed that both chains were expressed in tissues as well as cells in culture. The results indicate that type VIII collagen is a product of VSMC of normal adult vessels and is expressed at high levels by VSMC in vascular lesions.

Purification and structural analysis of extracellular matrix of a skin tumor from a patient with juvenile hyaline fibromatosis

Juvenile hyaline fibromatosis is a rare mesenchymal dysplasia that is inherited in an autosomal recessive fashion. The histological features of the tumor-like lesions are characterized by the deposition of amorphous hyaline material in the extracellular spaces of the dermis and soft tissues. We have analyzed the hyaline substance in a specimen of a skin tumor obtained from a 4-year-old Japanese girl with juvenile hyaline fibromatosis. It was found to consist mainly of type VI collagen; a small amount of type I collagen was also present. These components were separated by DEAE-cellulose ion-exchange chromatography under reducing conditions. The ratio of the dry weights of type I and type VI collagen was 1:4. Of the three chains of type VI collagen (alpha 1(VI), alpha 2(VI) and alpha 3(VI)), alpha 3(VI) was the most abundant. Glycosaminoglycans in the tumor tissue comprised dermatan sulfate, chondroitin sulfate and hyaluronan, with dermatan sulfate predominating. In contrast, hyaluronan is the most abundant in normal skin.

Type VI collagen is phagocytosed by fibroblasts and digested in the lysosomal apparatus: involvement of collagenase, serine proteinases and lysosomal enzymes

Type VI collagen is present in most connective tissues, where it is considered to play a crucial role in the attachment of cells to the extracellular matrix and/or in the three-dimensional organization of the collagen meshwork. Although some information is available on its formation, the mechanisms involved in its degradation are not understood. Here, we present evidence for lysosomal digestion of type VI collagen by fibroblasts of periosteal explants. In the lysosomal apparatus of these cells, broad-banded filamentous aggregates characterized by 100-nm periodicity were found, which proved to consist of type VI collagen as indicated by their stainability with anti-type VI collagen antibodies. By interfering with synthesis (ascorbate or alpha, alpha-dipyridyl), intracellular translocation of collagen-containing vesicles (colchicine) as well as phagocytosis (cytochalasin B), it was shown that the intracellular broad-banded type VI collagen represented phagocytosed material. In the presence of acidotropic agents (NH4Cl and methylamine) the amount of intracellular type VI collagen increased significantly (5- to 10-fold), suggesting that a rise of pH in the endosomal/lysosomal apparatus causes inhibition of its degradation. By using a variety of proteinase inhibitors, it was found that inhibition of collagenase (when used in combination with NH4Cl), or inhibition of cysteine proteinases (both with and without NH4Cl), resulted in an increased amount of intracellular type VI collagen, whereas inhibition of serine proteinases significantly lowered the level of intracellular type VI collagen. The data presented are the first to indicate a pathway by which type VI collagen degradation may occur: fibroblasts phagocytose type VI collagen and subsequently digest this collagen in their lysosomal apparatus. Degradation depends on the activity of several enzymes, among them collagenase and serine proteinases, probably exerting their activity in the extracellular space just before the actual internalization. After uptake, digestion involves pH-sensitive lysosomal enzymes, including those belonging to the class of cysteine proteinases.

Genetic variation in the COL6A1 region is associated with congenital heart defects in trisomy 21 (Down's syndrome)

Genetic variation in the COL6A1-COL6A2 gene cluster on chromosome 21 was studied in 113 controls and 58 European families (including control and family subgroups of British/Irish origin) having a child with trisomy 21. There were statistically significant differences among subgroups of trisomic children with and without congenital heart defects (CHD) in distributions of definitive, 3-RFLP haplotype classes received from their nondisjoining and disjoining parents. Haplotypes received by trisomic children with CHD from their disjoining parents were not a random sample of controls' haplotypes. Analysis of parental single-RFLP genotypes and linkage disequilibrium patterns confirmed this parent subgroup differed from a random sample of controls. There were no significant differences in parent subgroup genotype distribution at any of nine control loci distributed along chromosome 21q. This sample showed an association between genetic variation in the COL6A1 gene region and congenital heart defects in trisomy 21.

Cysteine-to-arginine point mutation in a ‘hybrid’ eight-cysteine domain of FBN1: consequences for fibrillin aggregation and microfibril assembly

Mutations in the FBN1 gene encoding the microfibrillar glycoprotein fibrillin cause Marfan syndrome, a relatively common autosomal dominant connective tissue disease. Causative FBN1 mutations appear to be dispersed throughout the coding frame, and to date no predictable genotype: phenotype correlations have emerged. We have identified a point mutation within an eight-cysteine ‘hybrid’ motif of the fibrillin polypeptide which results in the substitution of an arginine for a cysteine, in a patient severely affected in the cardiovascular, skeletal and ocular systems. We have utilised cell cultures from various tissues of this patient to investigate the effects of this mutation on fibrillin expression and deposition, and the consequences in terms of microfibril assembly and organisation. We have established that there is no difference in the expression of normal and mutant alleles, and fibrillin synthesis, secretion and deposition are also normal. However, the rate of fibrillin aggregation is reduced and microfibrillar assemblies are both remarkably scarce and morphologically abnormal. These data clearly demonstrate that the mutated allele interferes with normal assembly, and strongly implicate this particular region of the fibrillin-1 molecule in stabilising microfibrillar assemblies.

Type VI collagen-specific messenger RNA is expressed constitutively by cultured human synovial fibroblasts and is suppressed by interleukin-1

OBJECTIVE

Type VI collagen is a prominent constituent of the synovial extracellular matrix. The cellular source of this matrix protein and the identity of local factor sin synovium that may regulate its expression have not been delineated, however. We examined the capacity of human fibroblast-like synovial cells to synthesize type VI collagen as well as the effect of interleukin-1 (IL-1) on this expression.

METHODS

RNA was extracted from cultured human synovial cells derived from patients with rheumatoid arthritis (RA) and osteoarthritis (OA). Northern blots were analyzed using sequence-specific probes, and steady-state messenger RNA (mRNA) levels of the 3 alpha (VI) procollagen chains were measured. The effect of IL-1 treatment on these levels was determined.

RESULTS

Abundant expression of 3 characteristic mRNA transcripts, corresponding to the alpha 1 (4.2-kb), alpha 2 (3.5-kb), and alpha 3 (8.5-kb) chains of type VI procollagen, was observed in untreated cells derived from RA and OA patients. IL-1 treatment consistently suppressed steady-state mRNA levels for all 3 alpha (VI) procollagen chains in a time- and dose-dependent manner. Tumor necrosis factor alpha induced a response similar to that of IL-1, while IL-2 was ineffective in this regard. Indomethacin partially restored alpha (VI) mRNA expression in IL-1--treated cells.

CONCLUSION

These studies provide novel data demonstrating abundant steady-state levels of mRNA transcripts coding for all 3 type VI procollagen polypeptides in human synovial fibroblast-like cells, as well as coordinated down-regulation of these transcripts by IL-1. Local production of IL-1 may thus constitute an important means in vivo of regulating the production of type VI collagen.

Abnormal fibrillin assembly by dermal fibroblasts from two patients with Marfan syndrome

The microfibrillar glycoprotein fibrillin is linked to the Marfan syndrome, an autosomal dominant connective tissue disorder. In this study, fibrillin synthesis, deposition and assembly has been investigated in Marfan dermal fibroblast lines from two unrelated patients for whom distinct mutations in the fibrillin gene FBN1 have been identified. In patient NB, a point mutation has occurred which causes an amino acid substitution and the other patient (GK) has a deletion in one allele. The two cell lines were broadly comparable with respect to de novo fibrillin synthesis and its distribution between medium and cell layer compartments. Electrophoresis of fibrillin immunoprecipitates confirmed the presence of fibrillin in medium and cell layers. GK cells secreted an additional higher relative molecular mass fibrillin-immunoreactive component. The time-course of fibrillin secretion was similar for the two lines, but differences in fibrillin aggregation were apparent. Rotary shadowing electron microscopy of extracted cell layers demonstrated the presence of abundant and extensive microfibrils in NB cell layers. These were abnormal in their gross morphology in comparison to microfibrils isolated from control cultures. No periodic microfibrillar structures were isolated from GK cell layers. These studies underline the need to classify fibrillin defects in terms of biochemical and ultrastructural criteria. Examination of the effects of individual mutations on microfibril organization will be particularly informative in elucidating the relationship between microfibril dysfunction and the complex clinical manifestations of Marfan patients.

Fibrillin secretion and microfibril assembly by Marfan dermal fibroblasts

The Marfan syndrome has been linked to the FBN1 gene encoding the microfibrillar glycoprotein fibrillin. To date, there have been no descriptions of microfibrillar abnormalities characteristic of this connective tissue disorder, although biochemical analyses have highlighted apparent abnormalities in fibrillin synthesis, secretion and processing. We have conducted a biochemical and ultrastructural investigation of fibrillin expression and assembly by a panel of dermal fibroblast lines from patients with Marfan syndrome and related diseases. The study has highlighted marked differences between cells in terms of secretion and aggregation of newly-synthesised fibrillin. In addition, electron microscopic visualization of fibrillin assemblies has clearly demonstrated for the first time the plethora of microfibrillar abnormalities that underlie this heterogeneous disorder. These data emphasize the molecular complexity that is a feature of the diverse clinical phenotypes exhibited by Marfan patients.

Increased expression of type VI collagen genes in drug-induced gingival enlargement

Fibrotic gingival enlargements induced by phenytoin or nifedipine were examined with special reference to type VI collagen expression. Immunolocalization studies showed abnormal accumulation of type VI collagen around the collagen fiber bundles in the fibrotic gingival enlargements. Examination of total RNA extracted from fibroblasts and tissues of enlarged gingivae demonstrated increased type VI collagen steady-state mRNA levels. These results suggest that excessive deposition of type VI collagen in drug-induced gingival enlargement is attributed to increased expression of the collagen genes.

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.

Synthesis and assembly of fibrillin by fibroblasts and smooth muscle cells

The expression and assembly of the microfibrillar glycoprotein fibrillin has been investigated in cultures of nuchal ligament fibroblasts, skin fibroblasts and vascular smooth muscle cells. The level of fibrillin expression varied with the cell type and growth conditions. Higher levels of synthesis were recorded in quiescent post-confluent cells than in actively dividing subconfluent cultures. Nuchal ligament fibroblasts consistently synthesized the highest levels of fibrillin. Growth of cells in the presence of ascorbate resulted in an increased proportion of newly synthesized fibrillin retained within cell layers. Fibrillin was immunoprecipitated from medium and cell layer extracts in the form of monomers and high-M(r) disulphide-bonded aggregates. Rotary shadowing electron microscopy of cell layer extracts and collagen gels provided direct evidence for the assembly of extensive intact microfibrils by smooth muscle cells and fibroblast cultures. Gel filtration chromatography of medium and cell layer extracts, in combination with immunoprecipitation of column fractions, provided a means of analysing the size distribution and assembly of newly synthesized fibrillin. This cell culture approach provides an opportunity to evaluate normal and aberrant synthesis and assembly of fibrillin in a wide range of cell types.

Inverse relationship between hyaluronan and collagens in development and angiogenesis

The extracellular matrix plays a vital role in regulating normal tissue development and function--largely via the specific arrangement of macromolecules such as collagens, proteoglycans, glycosaminoglycans and glycoproteins. Previous reports have concentrated on associations between combinations of collagens/proteoglycans, collagens/glycoproteins and proteoglycans/glycosaminoglycans whilst little information is available on associations between collagens and free glycosaminoglycans. In this review, we discuss possible associations between collagens and the glycosaminoglycan hyaluronan; macromolecules which are known to exhibit changes in amount and composition during development and under pathological conditions. We demonstrate two types of collagen/hyaluronan association in vivo: the first, during the formation of extracellular matrix structures where neither collagens nor hyaluronan are degraded, resulting in the regulation of collagen fibrillogenesis, and the second, involving an inverse correlation between collagen synthesis and hyaluronan degradation and vice versa. We suggest that associations between collagens and hyaluronan play an important role in the initiation and maintenance of angiogenesis and put forward a model of cartilage vascularisation which relies on these associations.

Hyperglycemic glucose concentrations up-regulate the expression of type VI collagen in vitro. Relevance to alterations of peripheral nerves in diabetes mellitus

Electron microscopy of peripheral nerves obtained from two diabetic patients revealed large deposits of microfibrils and the presence of Luse bodies in the vicinity of perineurial cells. Microfibrils were found to accumulate also in the sciatic nerves of diabetic BB rats; these microfibrillar deposits were shown to contain type VI collagen by immunoelectron microscopy. Connective tissue cells cultured from rat sciatic nerves were exposed to high glucose concentrations. High glucose concentrations up-regulated the mRNA steady-state levels of alpha 1(VI), alpha 2(VI), and alpha 3(VI) chains of type VI collagen and caused accumulation of type VI collagen-containing fibrils in the cultures. Immunostaining and in situ hybridizations demonstrated that perineurial cells, Schwann cells, and fibroblasts expressed type VI collagen at the mRNA and protein levels. The results suggest that the turnover and supramolecular assembly of type VI collagen are perturbed in diabetic nerves and that glucose per se increases the expression of type VI collagen in vitro.

Stable expression of chicken type-VI collagen alpha 1, alpha 2 and alpha 3 cDNAs in murine NIH/3T3 cells

As a component of an extensive network of microfibrils interwoven with large collagen fibers and in close contact with cell surfaces, type VI collagen plays an important role in cell-matrix interactions. To investigate the behaviour of chicken type VI collagen chains in heterologous host cells as a means to understanding the pattern of assembly of this collagen, we transfected murine NIH/3T3 cells with cDNAs encoding chicken alpha 1(VI), alpha 2(VI) and alpha 3(VI) chains. Cell lines that constitutively expressed the individual chains were analyzed by metabolic labeling and immunoprecipitation with specific antibodies. No self-association was observed for either alpha 1(VI) or alpha 2(VI) chains which were secreted as monomeric polypeptides. Furthermore, neither the chicken alpha 1(VI) nor alpha 2(VI) chains associated with the endogenous murine chains to form chimeric chicken/murine heterotrimers. In contrast, chimeric chicken/murine heterotrimers were detected in cell lines transfected with chicken alpha 3(VI) cDNA. These chimeric forms appeared to be properly aligned since their triple helices were stable to pepsin digestion. In addition, the chimeric heterotrimers coassembled and gave rise to disulfide-linked type VI collagen molecules.

Type VI collagen microfibrils: evidence for a structural association with hyaluronan

Type VI collagen, a widespread structural component of connective tissues, has been isolated in abundance from fetal bovine skin by a procedure involving bacterial collagenase digestion under nonreducing, nondenaturing conditions and gel filtration chromatography. Rotary shadowing electron microscopic analysis revealed that the collagen VI was predominantly in the form of extensive intact microfibrillar arrays. These microfibrils were seen in association with hyaluronan, which was identified by its ability to bind the G1 fragment of cartilage proteoglycan. Treatment with highly purified hyaluronidase largely disrupted the collagen VI microfibrils into component tetramers, double tetramers, and short microfibrillar sections. Subsequent incubation of disrupted collagen VI in the presence of hyaluronan facilitated a partial repolymerization of the microfibrils. In vitro binding studies have also demonstrated that type VI collagen binds hyaluronan with a relatively high affinity. These studies demonstrate that a specific structural relationship exists between type VI collagen and hyaluronan. This association is likely to be of primary importance in the growth and remodeling processes of connective tissues.

Corneal cell-matrix interactions: type VI collagen promotes adhesion and spreading of corneal fibroblasts

Type VI collagen is a nonfibrillar collagen present as a network throughout the chick secondary stroma. Immunolocalization of type VI collagen both in the chick corneal stroma and in other systems demonstrates that type VI collagen is present associated with cells and between striated fibrils. We hypothesize that type VI collagen may function in cell-matrix interactions important in corneal development. To examine this possibility, we have isolated and characterized bovine corneal type VI collagen and determined that the chain composition and morphology of type VI collagen isolated from cornea is similar to that isolated from other sources. The tissue form of type VI collagen was localized to filaments forming a network around fibrils and close to corneal fibroblasts. We then analyzed relative attachment and spreading on type VI collagen as compared to the other collagens present in the secondary stroma, and found that although corneal fibroblasts attach equally well to type VI and type I collagen, cells spread to a much greater extent on type VI collagen. Although corneal fibroblasts do have an RGD-dependent receptor which functions during adhesion to fibronectin, attachment to type VI collagen is RGD-independent unless the molecule is denatured. Blocking of the RGD-dependent receptor with soluble RGD peptides results in no change in attachment or spreading. These data imply a role for type VI collagen in cell-matrix interactions during corneal stroma development.