Type VI collagen represents a major fraction of connective tissue collagens

Native collagen VI delays early muscle stem cell differentiation

Adult muscle stem cells (MuSCs) are critical for muscle homeostasis and regeneration, and their behavior relies on a finely regulated niche made of specific extracellular matrix (ECM) components and soluble factors. Among ECM proteins, collagen VI (Col6) influences the mechanical properties of the niche and, in turn, MuSC self-renewal capabilities. Here, we investigated whether Col6 can exert a direct function as a biochemical signal for regulating the stemness and differentiation of murine MuSCs and myoblasts. Native Col6, but not its pepsin-resistant fragment, counteracts the early differentiation of myogenic cells by reducing the expression of differentiation marker genes and preserving stemness features, with inhibition of the canonical Wnt pathway. Our data indicate that extracellular Col6 acts as a soluble ligand in delaying early myogenic differentiation by regulating intracellular signals involved in adult myogenesis.

Collagen VI sustains cell stemness and chemotherapy resistance in glioblastoma

Microenvironmental factors are known fundamental regulators of the phenotype and aggressiveness of glioblastoma (GBM), the most lethal brain tumor, characterized by fast progression and marked resistance to treatments. In this context, the extracellular matrix (ECM) is known to heavily influence the behavior of cancer cells from several origins, contributing to stem cell niches, influencing tumor invasiveness and response to chemotherapy, mediating survival signaling cascades, and modulating inflammatory cell recruitment. Here, we show that collagen VI (COL6), an ECM protein widely expressed in both normal and pathological tissues, has a distinctive distribution within the GBM mass, strongly correlated with the most aggressive and phenotypically immature cells. Our data demonstrate that COL6 sustains the stem-like properties of GBM cells and supports the maintenance of an aggressive transcriptional program promoting cancer cell proliferation and survival. In particular, we identified a specific subset of COL6-transcriptionally co-regulated genes, required for the response of cells to replicative stress and DNA damage, supporting the concept that COL6 is an essential stimulus for the activation of GBM cell response and resistance to chemotherapy, through the ATM/ATR axis. Altogether, these findings indicate that COL6 plays a pivotal role in GBM tumor biology, exerting a pleiotropic action across different GBM hallmarks, including phenotypic identity and gene transcription, as well as response to treatments, thus providing valuable information for the understanding of the complex microenvironmental cues underlying GBM malignancy.

Rho-kinase inhibitors in the management of glaucoma

Introduction: Glaucoma is a group of progressive optic neuropathies in which elevated intraocular pressure (IOP) as a consequence of an increased aqueous humor (AH) outflow resistance, is the main and only clinically modifiable risk factor for its development and progression. Relaxing Trabecular meshwork (TM) tissue, Rho-Kinase (ROCK) inhibitors directly decrease resistance in the conventional AH outflow, thus resulting in a significant IOP-lowering effect. Areas covered: The progress made in the field of ROCK inhibitors for glaucoma treatment will be discussed, referring to the recent patent literature published mainly in the last 3 years. Development and last studies conducted on the recently approved ripasudil and netarsudil will be described, along with newly reported combinations with other antiglaucoma agents. New molecular entities as ROCK inhibitors will be reported as well as new biological approaches to affect the Rho/ROCK pathway. Expert opinion: With three drugs currently available on the market belonging to this class, ROCK inhibitors have been definitely validated as therapeutic agents for glaucoma treatment. The literature of the last 3 years confirmed the success of the soft-drug and bis-functional approaches in the design of ROCK inhibitors. However, few completely new molecular scaffolds have been reported.

Rho-associated protein kinase inhibitor induced morphological changes in type VI collagen in the human trabecular meshwork

Aims

To investigate morphological changes in type VI collagen in the human trabecular meshwork associated with the rho kinase inhibitor ripasudil.

Methods

This cross-sectional study evaluated the effects of ripasudil eye drop administration (RA) or no ripasudil eye drop administration (NRA) in patients with primary open-angle glaucoma (POAG; age range 60–80 years) who underwent conventional outflow reconstruction between December 2015 and September 2016 at Tokai University Hachioji Hospital. The juxtacanalicular tissue was removed and imaged using transmission electron microscopy. Type VI collagen comprises cross-banded aggregates with transverse bands 30 nm apart repeating every 105 nm. The transverse bands are called the outer rod-like region (ORR) and the intervals are called the inner rod-like region (IRR). The waveform intensity in the type VI collagen was analysed in electron micrographs using Fourier transformation to detect the IRR and ORR borders.

Results

Ten eyes of 10 patients were included (n=5/group). The baseline characteristics did not differ significantly between groups. ORR width was significantly smaller in the RA group (37.85±3.43 nm) than in the NRA group (50.62±5.23 nm, p<0.05), whereas IRR width was significantly greater in the RA group (70.68±10.84 nm) than in the NRA group (58.19±5.34 nm, p<0.05). Morphological changes in the type VI collagen total width tended to correlate with the duration of ripasudil administration (r=0.9, p=0.08).

Conclusions

Ripasudil administration in patients with POAG induced morphological changes in type VI collagen. Patients with POAG administered RA had a significantly smaller ORR width and a significantly greater IRR width than patients with POAG not administered RA.

Pathology, protein expression and signaling in myxomatous mitral valve degeneration: comparison of dogs and humans

Myxomatous degenerative mitral valve disease (MMVD) is a common heart disease in dogs. Although several morphological similarities occur between human and canine MMVD differences exist. However, in advanced stages the accumulation of proteoglycans is the main finding in both species. The extracellular matrix (ECM) in normal canine and human mitral valves is similar. In MMVD of both species proteoglycans is the major alteration, although specific changes in collagen distribution exists. The valvular expression pattern of matrix metalloproteinases (MMPs) and of their inhibitors (TIMPs) differs, in part, between dogs and humans. The MMPs and TIMPs expression patterns are similar in normal canine and human mitral valves, but they are quite different during degenerative progression. Valve endothelial cells (VEC) and interstitial cells (VIC) are phenotypically transformed in canine and human MMVD. Inflammation is an unlikely cause of valve degeneration in humans and dogs. There are several lines of evidence suggesting that transforming growth factor β1 (TGF β1) and serotonin signaling may mediate valve degeneration in humans and dogs. Although human and canine MMVD share structural similarities, there are some differences in ECM changes, enzyme expression and cell transformation, which may reflect a varied pathogenesis of these diseases.

Degradation of human collagen isoforms by Clostridium collagenase and the effects of degradation products on cell migration

Clostridium collagenase has been widely used in biomedical research to dissociate tissues and isolate cells; and, since 1965, as a therapeutic drug for the removal of necrotic wound tissues. Previous studies found that purified collagenase-treated extracellular matrix stimulated cellular response to injury and increased cell proliferation and migration. This article presents an in vitro study investigating the digestive ability of Clostridium collagenase on human collagen types I, III, IV, V and VI. Our results showed that Clostridium collagenase displays proteolytic power to digest all these types of human collagen, except type VI. The degradation products derived were tested in cell migration assays using human keratinocytes (gold surface migration assay) and fibroblasts (chemotaxis cell migration assay). Clostridium collagenase itself and the degradation products of type I and III collagens showed an increase in keratinocyte and fibroblast migration, type IV-induced fibroblast migration only, and the remainder showed no effects compared with the control. The data indicate that Clostridium collagenase can effectively digest collagen isoforms that are present in necrotic wound tissues and suggest that collagenase treatment provides several mechanisms to enhance cell migration: collagenase itself and collagen degradation products.

Immunohistochemical characterization of the extracellular matrix in normal mitral valves and in chronic valve disease (endocardiosis) in dogs

This study aimed to characterize the composition and distribution of the extracellular matrix (ECM) components in normal canine mitral valves (MV) and in chronic heart valve disease (CVD). MV of 50 dogs (normal (n=9), mild (n=13), moderate (n=17), severe (n=11) CVD) were investigated macroscopically, histologically (H.-E., picrosirius red) and immunohistochemically (collagen I, III, IV, V, VI, elastin, laminin, fibronectin, heparan sulphate). In normal MV, ECM components were expressed in a typical layered pattern. In mild CVD, basement membrane components (laminin, collagen IV, fibronectin) were increased. Advanced CVD was characterized by myxomatous nodular lesions displaying a marginal and a central region comprised mainly of collagen I, VI and fibronectin in the former and collagen I and III in the latter. Collagen IV and laminin appeared multifocally in marked CVD. In conclusion, not only an accumulation of proteoglycans, but also a distinctly altered expression of basement membrane components, and collagens characterizes CVD.

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.

Collagen type VI expression during cardiac development and in human fetuses with trisomy 21

The role played by specific extracellular matrix molecules in normal endocardial cushion differentiation into valves and septa remains to be established. In this respect, type collagen VI is of particular interest because genes encoding the alpha1 and alpha2 chains are located on chromosome 21, and defects involving the atrioventricular (AV) cushions are frequent in trisomy 21. Collagen VI expression was studied in normal human embryonic and fetal hearts (5-18 weeks of development) and compared by immunohistochemistry with results from fetuses (10-16 weeks of development) with trisomy 21. During normal endocardial cushion differentiation (5-8 weeks) there was marked collagen VI expression in the AV cushions, whereas only minor expression was seen in the outflow tract cushions. In the normal fetuses (10-18 weeks), collagen VI in the AV cushions had condensed into a marked zone on the atrial side of the leaflets, as well as subendocardially in other regions of high shear stress. Morphological defects involving the endocardial cushion-derived structures were present in all trisomy 21 cases. An abnormally large membranous septum was observed in three cases. An AV septal defect (AVSD) was present in two, while one had a ventricular septal defect (VSD). Two cases presented with a secondary atrial septal defect (ASDII), and one had an AVSD. Mild to moderate valve dysmorphia was found in all cases. Collagen VI staining in trisomy 21 was more intense than in the normal subjects; however, there were no differences in the spatial expression patterns. We conclude that collagen VI is expressed in the AV cushions and persists during valve differentiation. Collagen VI is more prominent in fetal trisomy 21 hearts than in normal hearts. We hypothesise that collagen VI has a role in the development of heart defects involving endocardial cushion differentiation-specifically in the AV canal, the most common site of malformations affecting children with trisomy 21.

Interleukin (IL)-6 and IL-10 induce decorin mRNA in endothelial cells, but interaction with fibrillar collagen is essential for its translation

Decorin, a small multifunctional proteoglycan, is expressed by sprouting endothelial cells (ECs) during inflammation-induced angiogenesis in vivo and by human ECs co-cultured with fibroblasts in a collagen lattice. To investigate how decorin is induced, human EA.hy 926 ECs and/or human umbilical vein ECs were treated with interleukin (IL)-10 and IL-6. Both treatments induced decorin mRNA in human ECs. IL-6 and IL-10 led to a dose-dependent mRNA increase with a maximum at 10 and 50 ng/ml, respectively. The combination of both interleukins together had a stronger effect than one alone. Immunostaining demonstrated that both interleukins caused decorin synthesis in ECs and the formation of capillary-like structures in a collagen lattice. However, immunoprecipitations of interleukin-treated ECs cultured on plastic were negative. Only interleukin-stimulated ECs grown on a collagen type I matrix or growth factor-reduced Matrigel were able to synthesize the proteoglycan. Acid-soluble collagen type I did not support decorin protein synthesis. The addition of antibodies to alpha(1) or alpha(2) integrins or the alpha(2) integrin inhibitor rhodocetin led to an inhibition of synthesis. These data show that IL-10 and IL-6 induce decorin mRNA transcription, but additional signals from the extracellular matrix are necessary for its translation.

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.

The role of the C1 and C2 a-domains in type VI collagen assembly

Constructs of each of the three chains of type VI collagen were generated and examined in an in vitro transcription/translation assay supplemented with semipermeabilized cells. Each of the constructs when used in the in vitro system was shown to be glycosylated and to undergo intracellular assembly, the extent of which was determined by the nature of the C-terminal globular domains. All three chains containing the C1 domain formed monomers; however, the C2 domain was required for dimer and tetramer formation. In the case of the full-length alpha2(VI) chain, monomers, dimers, and tetramers formed in a time-dependent manner. Although the splice variant alpha2(VI)C2a could form monomers, it was unable to form dimers and tetramers. Similar results to the alpha2(VI) chain were found for the full-length alpha1(VI) chain, although assembly was at a slower rate. In the case of the alpha3(VI) chain containing both C1 and C2 domains only monomers were observed. Addition of the C3, C4, and C5 did not change this pattern. Homology modeling suggested that a 10-amino acid insertion in the C2 domain of the alpha3(VI) chain may interfere with dimer formation. A near full-length construct of the alpha3(VI) chain only formed monomers but was shown to facilitate tetramer formation in cotranslation experiments.

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.

Type VI collagen expression during growth of human ovarian follicles

OBJECTIVE

To identify type VI collagen expression in human ovarian follicles during follicular growth.

DESIGN

In vitro experiment.

SETTING

Department of Obstetrics and Gynecology, Wakayama Medical College, Japan.

PATIENT(S)

Regularly cycling women who underwent adnexectomy.

INTERVENTION(S)

Immunohistochemistry and in situ hybridization for human type VI collagen.

MAIN OUTCOME MEASURE(S)

Expression of type VI collagen.

RESULT(S)

Expression of type VI collagen was observed in the theca cell layers during folliculogenesis, whereas no expression of type VI collagen was observed in the granulosa cell layers at the mRNA and protein levels. As the follicles grew, immunostaining for type VI collagen became intense in the theca cell layers, especially the theca externa. In preovulatory follicles, however, weak, fragmented, or discontinuous immunostaining of the theca cell layers was observed. This fragmented or discontinuous immunostaining was evident predominantly in the apical area of preovulatory follicles rather than in the basal area.

CONCLUSION(S)

Type VI collagen is present in the theca cell layers of follicles during folliculogenesis and plays an important role in interactions between the theca cells and extracellular matrix. These interactions may lead to changes in the shape, proliferation, migration, or differentiation of follicular cells during follicular development, maturation, and ovulation.

Loss of type VI collagen in experimental and most spontaneous human fibrosarcomas

Expression of type VI collagen, an adhesive protein of mesenchymal tissues, is significantly down-regulated upon viral transformation of fibroblasts. Likewise, most cell lines derived from spontaneous mesenchymal tumors, including fibrosarcomas, rhabdomyosarcomas, leiomyosarcomas, chondrosarcomas and liposarcomas, do not synthesize type VI collagen because they are not capable of expressing all 3 of the polypeptide chains required for the assembly of a functional heterotrimeric molecule. When injected into nude mice, neither fibrosarcoma cells (HT1080) nor rhabdomyosarcoma cells (A204) initiate the synthesis of type VI collagen, suggesting that the inhibition is not caused by deficiency of a paracrine factor. Immuno-histochemical studies further illustrate that 15 of 17 spontaneous adult fibrosarcomas lack type VI collagen in the tumor stroma. The absence of this important adhesion protein may contribute to tumorigenicity, invasiveness and/or metastasis of mesenchymal tumor cells.

CxGELSIX: a novel preparation of type VI collagen with possible use as a biomaterial

PURPOSE

This study was initiated to evaluate tissue acceptance and stability of a novel type VI collagen preparation (CxGelsix) as a biomaterial in the rabbit corneal stroma. We hypothesized that CxGelsix, embedded intrastromally, does not have any adverse affect on surrounding corneal tissues, and remains intact in the presence of an acute inflammatory reaction during corneal wound healing.

METHODS

Type VI collagen was extracted and purified from rabbit corneal stroma under nondenaturing conditions. This preparation, Gelsix, was concentrated and cross-linked with polyethylene glycol to produce a transparent film (CxGelsix). Discs of CxGelsix, 4.0-mm diameter, 9- to 35-microm thick were implanted intrastromally and clinically examined periodically for 4 months. In another experiment, implantation of CxGelsix, 2.0-mm-diameter, was followed by corneal wounding adjacent to the implant and examined clinically for 30 weeks. At the end of these periods, the tissues from these experiments were processed for light and transmission electron microscopy.

RESULTS

An intralamellar 4.0-mm-diameter disc of CxGelsix does not alter the structure of corneal epithelium above the implant, suggesting normal transport of nutrients through CxGelsix. Moreover, no structural abnormalities were seen in the rest of the cornea, and the cornea remains transparent. Although the cornea accepts the presence of CxGelsix disc as judged by clinical criteria, gradual degradation of the implant is seen ultrastructurally. CxGelsix is remarkably stable despite its exposure to endogenous enzymes during inflammation and wound healing. Partial degradation of the implant occurs only after many months, and it is gradually replaced with bundles of fine collagen fibrils reminiscent of normal cornea.

CONCLUSION

The results of this study suggest that CxGelsix is potentially useful as a biomaterial.

MMP2 activation by collagen I and concanavalin A in cultured human hepatic stellate cells

Fibrosis occurs in most chronic liver injuries and results from changes in the balance between synthesis and degradation of extracellular matrix components. In fibrotic livers, there is a markedly increased activity of matrix metalloproteinase 2 (MMP2), a major enzyme involved in extracellular matrix remodeling. We have previously shown that hepatic stellate cells secrete latent MMP2 and that MMP2 activation occurs in coculture of hepatic stellate cells and hepatocytes concomitantly with matrix deposition. In the present work we investigated the effects of various extracellular matrix components and concanavalin A, an inducer of immune-mediated liver injuries, on MMP2 activation in cultured human hepatic stellate cells. Collagen I induced a dose-dependent MMP2 activation, which was not blocked by both actinomycin and cycloheximide. Collagen VI, laminin, and a reconstituted basement membrane (matrigel) were ineffective in inducing activation. Specific antibodies against the subunits of alpha2beta1 integrins, the major collagen I receptor, induced partial inhibition of MMP2 activation. Treatment of cells with concanavalin A resulted in a marked activation of MMP2 that correlated with the proteolytic processing of MT1-MMP, the MMP2 activator, from a Mr=60 kd toward a Mr=43 kd polypeptide. Actinomycin and cycloheximide inhibited the MMP2 activation induced by concanavalin A. Recombinant tissue inhibitor of metalloproteinase-2 and the MMP inhibitor BB-3103, but not PMSF, blocked MMP2 activation induced by collagen I or concanavalin A, and MT1-MMP processing to its Mr-43 kd form. These results suggest that the accumulation of collagen I may specifically contribute to the remodeling of extracellular matrix in fibrotic livers by inducing MMP2 activation.

Sequential changes in the localization of the type IV collagen alpha chain in the infarct zone: immunohistochemical study of experimental myocardial infarction in the rat

Collagen, as a component of the extracellular matrix, have a role in the healing process after myocardial infarction (MI). For type IV collagen, a major structural protein present in the basal membrane of myocytes, six alpha chains [alpha 1 (IV)-alpha 6(IV)] have been identified. We examined the sequential changes in the appearance and localization of the alpha 1 (IV)-alpha 5(IV) after experimental MI in rats. Hearts were excised from 1 day to 8 weeks after permanent left coronary artery ligation. Immunohistochemical staining with monoclonal antibodies was performed. On day 3, staining for both alpha 1(IV) and alpha 2(IV) first appeared, forming a wavy pattern in the infarct peripheral zone, and the staining was not restricted to the cell membrane. The staining intensity and distribution for both alpha 1(IV) and alpha 2(IV) in the peripheral zone then gradually increased, reaching a maximum around day 7. The distribution progressed from the peripheral to the central zone of the infarct for 1-2 days, reaching the center point after 2 weeks. The staining distribution gradually decreased after reaching the maximum, but the staining had not completely disappeared at 8 weeks. In contrast, no positive staining for alpha 3(IV), alpha 4(IV) or alpha 5(IV) was observed at any time during the 8-week observation period. Thus, the present results demonstrated that in rats, type IV collagen consisting of alpha 1 and alpha 2 chains appears in the infarct zone at a relatively early phase after MI, indicating that type IV collagen composed of alpha 1 and alpha 2 chains contributes to infarct healing.

Adipose tissue extracellular matrix: newly organized by adipocytes during differentiation

The distribution of eight types of extracellular matrix (ECM) proteins (type I-VI) collagen, laminin and fibronectin) in the skeletal muscle of Japanese Black cattle was determined by indirect immunofluorescence using specific antibodies against each protein. ECM proteins were well organized in the intramuscular connective tissue: type I, II, III collagen and fibronectin were localized primarily in the perimysium, type V and VI collagen in both the perimysium and endomysium, and type IV collagen and laminin were virtually confined to the endomysium. In the loose connective tissue holding the adipocytes together to form a tissue mass between the muscular bundles, seven of the ECM proteins not type II collagen were relatively abundant in a disordered arrangement. Further analysis by in vitro immunocytochemical staining also demonstrated that a stromal-vascular preadipocyte cell line (BIP cell), derived from Japanese Black cattle, synthesized various ECMs in much the same way as fibroblasts. Exponentially growing BIP cells with a fibroblastic phenotype were found to produce type II, V, and VI collagens, in addition to the other previously identified connective tissue glycoproteins of mouse 3T3 preadipocytes. When confluent preadipocyte cultures were stimulated with adipogenic medium, a fibrillar network of ECM was observed to bridge the intercellular space and connect adjacent cell surfaces. During adipocyte differentiation, type III collagen and laminin were arranged in a non-fibrous structure, and type-II collagen was only barely detected. These results are supported by the staining of the adipose tissue, where all ECM proteins studied except type II collagen were stained intensely. These data indicate that in vivo under conditions permissive for adipose conversion, the production and organization of ECM, accompanied by hyperplasia and hypertrophy of precursor cells, gives rise to adipose tissue in skeletal muscle with its own ECM products. These data further suggest that each ECM protein might have some role for the adipocytes in forming tissue.

Microfibrillar elements of the dermal matrix

Connective tissue microfibrils are key structural elements of the dermal matrix which play major roles in establishing and maintaining the structural and mechanical integrity of this complex tissue. Type VI collagen microfibrils form extensive microfibrillar networks which intercalate between the major collagen fibrils and are juxtaposed to cellular basement membranes, blood vessels and other interstitial structures. Fibrillin microfibrils define the continuous elastic network of skin, and are present in dermis as microfibril bundles devoid of measureable elastin extending from the dermal-epithelial junction and as components of the thick elastic fibres present in the deep reticular dermis. Electron microscopic analyses have revealed both classes of microfibrils to have complex ultrastructures. The ability to isolate intact native microfibrils from skin has enabled a combination of high resolution and biochemical techniques to be applied to elucidate their structure:function relationships. These approaches have generated new information about their molecular organisation and physiological interactions in health and disease.

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.

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.

Immunohistochemical distribution of type VI collagen in developing human kidney

The distribution of type VI collagen was investigated immunohistochemically in the developing human kidney from 15 to 32 weeks gestational age and it was compared with that observed in the normal infantile and adult human kidney. In fetal kidney, type VI collagen was widely distributed as a fibrillar network in the subcapsularly undifferentiated mesenchyme and intertubular interstitium, and as a basement membrane-like structure around the ureteral bud branches, tubules, and collecting ducts. During nephrogenesis, type VI collagen disappeared from the induced mesenchyme close to the tips of ureteral branches, while it formed a distinct basement membrane-like structure around the early stages of nephron differentiation (comma-shaped and S-shaped bodies) and later along Bowman's capsule of capillary loop and maturing glomeruli A strong immureactivity for type VI collagen was also found in the glomerular basement membrane and mesangial areas of capillary loop and maturing glomeruli. In infantile kidney, type VI collagen showed a distribution pattern similar to that observed during the fetal period. In adult human kidney, glomerular basement membrane showed a weak positivity for type VI collagen and the basement membrane-like staining around Bowman's capsule, tubules, and collecting ducts was less evident than in fetal and infantile kidney. Our immunohistochemical findings suggest that type VI collagen is a normal component of the glomerular and extraglomerular extracellular matrix of developing human kidney and that it undergoes changes in the expression during maturation.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

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.

Immunohistochemical expression of type VI collagen in superficial fibromatoses

The expression of type VI collagen was studied immunohistochemically in 26 cases of superficial fibromatoses (palmar, plantar and penile) using an immunoperoxidase method for light microscopic visualization. The polyclonal antibody against type VI collagen used in this study was isolated from human placenta and its specifity was tested by immunoblotting assay. All cases consisted of multiple nodules showing a variable degree of cellularity and fibrosis. Depending on the predominant histological appearance of these nodules, each case was assigned to the three following phases: proliferative, involutional and residual. Morphologically normal palmar and plantar aponeuroses were included as controls. Immunohistochemical findings showed that type VI collagen was present as longitudinal thin fibers in normal palmar and plantar aponeuroses. A differential expression of this collagen was found in the different stages of superficial fibromatoses. Type VI collagen was markedly expressed as a distinct fibrillar network in the extracellular matrix (ECM) surrounding proliferating stromal cells in proliferative and involutional phases. Its expression completely disappeared from the connective tissue undergoing fibrotic transformation during involutional and residual phases. The results of the present study suggest that type VI collagen is an extracellular marker of stromal tissue proliferation and is involved in the early phases of tissue remodelling occurring in the superficial fibromatoses.

Human type VI collagen: purification from human subcutaneous fat tissue and an immunohistochemical study of morphea and systemic sclerosis

Collagen was isolated from human placenta by pepsin digestion and salt precipitation. This collagen was similar in its electrophoretic mobility and immunological reactivity with monoclonal antibody to form B of type VI collagen in the literature (Trueb B, Schreier T, Bruckner P and Winterhalter K. 1987. Eur. J. Biochem. 166: 699-703). We prepared polyclonal rabbit antiserum against alpha 2 chain of type VI collagen and performed an immunohistochemical study using this polyclonal antibody. It reacted in fat tissue and around vessels and peripheral nerves in normal human skin. To confirm the presence of type VI collagen in fat tissue, we isolated collagen from human subcutaneous tissue. This collagen showed a similar pattern in polyacrylamide gel electrophoresis with that from human placenta and cross-reacted with monoclonal or polyclonal antibody against type VI collagen. By immunohistochemical staining, abundant type VI collagen was observed in the septum of subcutaneous fat tissue in morphea or systemic sclerosis. In the mild hyalinizing areas or after treatment with 6M urea or hyaluronidase in highly hyalinized areas, the staining of type VI collagen increased. These data suggest that the amount of type VI collagen in subcutaneous tissue is involved in the early phases of these fibrosing disorders and that type VI collagen accumulates even more in hyalinizing tissue in late phases of these diseases.

Distribution of extracellular matrix glycoproteins in the human mesonephros

We analyzed the expression and distribution of collagen types IV and VI, laminin and fibronectin during the development and regression of the mesonephros in human embryos and fetuses ranging from 6 to 12 weeks of gestation by indirect immunoperoxidase methods. Type IV collagen, laminin and fibronectin were detected along the glomerular, tubular and capsular basement membranes of developing and mature nephrons. Only type IV collagen and fibronectin were found in the mesangium. Type VI collagen formed a delicate interstitial fibrillar network and a continuous basement membrane-like structure along the mesonephric nephrons. Basement membranes (GBM) of developing and mature glomeruli showed a distinct continuous staining for this collagen. The mesangial matrix was rich in type VI collagen. Mesonephric involution started during the 8th week of gestation and coincided with a moderate expansion of mesangial matrix and progressive collapse of the capillary walls, while the tubules became thinner and shorter. Staining for all extracellular matrix glycoproteins studied showed GBM wrinkling, gradual disintegration of some capillary loops and glomerulosclerosis. The sclerotic glomeruli were strongly positive for type IV collagen and less positive for type VI collagen and fibronectin. Laminin was absent. Our results indicate that collagen types IV, VI, laminin and fibronectin may be involved in the development and regression of the human mesonephros.

Secretion and matrix assembly of recombinant type VI collagen

A monomer of type VI collagen is composed of three different chains of 140 (alpha 1), 130 (alpha 2), and 250-350 kDa (alpha 3). Monomers assemble into dimers (6 chains) and tetramers (12 chains) that are stabilized by disulfide bonds and, once associated one to another, give rise to a microfilamentous network in close apposition with cell surfaces and banded collagen fibers. We have derived murine NIH/3T3 cell lines that were transfected with the cDNAs for the three chains and that constitutively expressed chicken type VI collagen. Cotransfection was efficient because, in three out of six isolated cell lines, all chicken chains were expressed. Southern blotting demonstrated that several copies of each cDNA were integrated approximately in equal number. Expression of the three polypeptide chains was consistent with the levels of the respective mRNAs. The three chicken chains assembled by disulfide bonding to form correctly folded triple helical aggregated composites with sizes corresponding to type VI collagen monomers, dimers, and tetramers. These functional recombinant assemblies were secreted and became incorporated into the extracellular matrix, where they formed an extensive fibrillar network.

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.

Expression and distribution of type VI collagen in gynecomastia

We investigated the distribution of type VI collagen in 36 cases of routinely fixed and paraffin-embedded gynecomastia using an immunoperoxidase method for light microscopic visualization. Four samples of normal male mammary gland tissue were also included as controls. A protease predigestion was essential for the visualization of this extracellular matrix (ECM) glycoprotein. In normal male breast, no immunoreaction for type VI collagen was detected in the stroma surrounding the ducts. Gynecomastia was classified into three histological types: florid (type I), fibrous (type II), and intermediate (type III). Type VI collagen was differentially expressed in the periductal stroma of all types. This collagen was markedly expressed at the early disease stage (type I) when the periductal stroma is highly cellular and vascular. Its expression decreased when periductal stroma undergoing fibrotic transformation (type III) and completely disappeared from the dense periductal stroma of fibrous stage (type II). These findings suggest that type VI collagen is involved in the ECM remodelling occurring in gynecomastia.

Patterns of type VI collagen compared to types I, III and V collagen in human embryonic and fetal skin and in fetal skin-derived cell cultures

The distribution of type VI collagen was examined in human embryonic and fetal skin and in cultured cells and matrix from this tissue. Frozen sections were immunolabeled with primary antibodies against type VI collagen and types I, III or V collagen, and processed further for fluorescence microscopy and immunoelectron microscopy. At 6 weeks estimated gestational age (EGA), type VI collagen was identified by positive fluorescence and by immunogold staining of filaments and fibers beneath the dermal-epidermal junction (DEJ), weaker fluorescence in the fine matrix of the dermis, and stronger fluorescence in the subcutis. At progressive stages of gestation, immunolabeling for type VI collagen increased in the dermis in parallel with increased deposition of types I and III collagen. By 15 weeks EGA, type VI collagen stained intensely throughout the dermis. At 13 weeks EGA, type VI collagen appeared diminished from the growing tips of invaginating hair buds, but as the hair peg developed, type VI collagen accumulated in adnexal sheaths. Cell cultures were derived from fetal skin at 7.5 to 12 weeks EGA. In primary explant cultures containing both keratinocytes and fibroblasts, mats of type V collagen were present beneath keratinocytes and associated with dense spots that co-labeled for both type VI and type V collagen. In passaged cultures of fibroblasts, individual cells with or without pretreatment with monensin were positive for type VI and/or types I, III or V collagen. Fibrous matrix that was labeled for type VI collagen was also immunopositive for type I or III collagen, while filamentous matrix that was type VI collagen positive tended to exclude types I and III collagen but in some areas to overlap with type V collagen. These findings support the hypothesis that type VI collagen present in both filamentous and fibrous matrix and networks of type VI collagen may serve as a fine scaffolding that facilitates the integration of types I and III collagen into developing fibrous matrix.

Ultrastructural immunolocalization of type-VI collagen and chondroitin sulphate in ligament

Immunological methods were used to determine the identity of the major components comprising a network of electron-dense seams (described by the authors in a previous work) within the extracellular matrix of medial collateral ligament (MCL) from humans and rabbits. Tissue obtained from MCL midsubstance was subjected to pre-embedding labelling with colloidal gold at the electron microscopic level with monoclonal antibodies (MAbs) against type-VI collagen and chondroitin sulphate (CS), before and after digestion with chondroitinase ABC and testicular hyaluronidase. Tissue labelled with anti-type-VI MAbs showed gold conjugates attached to the microfilamentous component of the seams both before and after enzyme digestion, which confirmed the identity of the beaded microfilaments as type-VI collagen. Treatment of the tissue with anti-CS MAbs resulted in labelling of undigested tissue only. In these treatments, gold particles were found attached to granules that were interspersed throughout the network of type-VI microfilaments. Both the granules and gold labels were absent from the network following enzyme digestion. Thin nonbeaded microfilaments that did not label with anti-type-VI MAbs also were present within the seams. The loss of these nonbeaded microfilaments following enzyme digestion suggested that they might represent strands of hyaluronan. The codistribution and sequestering of type-VI collagen and CS within discrete seams or channels suggests that these regions of the MCL midsubstance may contain higher concentrations of water than the surrounding dense fibrillar matrix.

Increased rat myocardial type VI collagen in diabetes mellitus and hypertension

Diabetic cardiomyopathy, a condition characterized by the accumulation of carbohydrate-containing material surrounding the myocardial small blood vessels, has been studied in alloxan-diabetic normotensive and hypertensive rats. Immunochemical techniques were used to monitor several extracellular matrix constituents present in extracts of cardiac tissue, namely types I, IV and VI collagen, laminin and fibronectin, as well as myosin. These studies have indicated that after induction of diabetes, type VI collagen but none of the other matrix components studied, was significantly increased (from 2.29 +/- 0.04 mg/g in normal to 2.85 +/- 0.18 mg/g in diabetic ventricles, p < 0.01). Hypertension, whether induced by the clipping of one renal artery or genetically determined (spontaneously hypertensive rats), resulted in a similar elevation in type VI collagen (2.71 +/- 0.12 mg/g, p < 0.005 compared to normal rats). In the presence of diabetes plus hypertension the effect was not additive, the type VI collagen level being 2.93 +/- 0.15 (p < 0.001 compared to normal rats). Basement membrane collagen (type IV) in the myocardium appeared to be unaffected by diabetes or hypertension and the myosin contents of the hearts of the four experimental groups were similar. Quantitative determinations indicate that compared to type IV collagen, laminin or fibronectin, type VI collagen represents the major periodic acid-Schiff-reactive extracellular constituent of the rat ventricle. Its preferential increase in the heart in diabetes may provide insight into the molecular mechanisms of the diabetic microvascular disease.

Collagen of umbilical cord arteries and its alterations in EPH-gestosis

We decided to investigate the EPH-gestosis--connected alterations in collagen of the umbilical cord arteries. The samples of arterial walls were submitted to histological and biochemical studies. It was found that umbilical cord arteries taken from newborns of mothers with EPH-gestosis contain more than twice the amount of collagen in comparison to corresponding arteries of newborns from normal pregnancies. An increase of collagen content in these vessels in accompanied by a slight decrease of its solubility. Types I, III, IV, V and VI collagens were found both in normal umbilical cord arteries and in those of newborns delivered by mothers with EPH-gestosis but their proportional relationships were different. EPH-gestosis is accompanied by an increase of a proportional amount of type III-collagen and a decrease of type I collagen in umbilical cord arteries. It seems that these changes in the umbilical cord arteries may be responsible for the decrease of blood flow in fetus of woman with EPH-gestosis.

Tendon collagens: extracellular matrix composition in shear stress and tensile components of flexor tendons

Outer synovial tissues were separated from the remainder of avian flexor tendon and the collagens characterized biochemically and compared with those of the internal portion of tendon and sheath. The collagen content of tendon synovium was 23%, whereas that of tendon and sheath were 78% and 73%, respectively, based on dry weight. Four genetic types of collagen were found in the pepsin solubilized matrices: in the synovium, types I (78%) and III (19%) predominated; types V and possibly VI were present as minor collagens. Purified synovial type V collagen was a heterotrimer, with chain composition [alpha 1(V)]2 alpha 2(V). In contrast, the internal portion of tendon and sheath were comprised of only type I collagen. There was a large amount (41%) of ethanol extractable, noncollagenous material present in synovium, a part of which was proteoglycans. In addition, collagen cross-links of these tissues were quantified: the internal tendon had an abundant concentration of pyridinoline; synovium exhibited high amounts of labile, reducible cross-links, particularly dihydroxylysinonorleucine. In the case of sheath, lysine aldehyde-derived cross-links appeared to be predominant. These results indicate that each tissue has its own collagen type distribution as well as cross-linking pattern reflecting their maturational and functional differences.

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.

Light microscopic and ultrastructural distribution of type VI collagen in human liver: alterations in chronic biliary disease

We have investigated the distribution of type VI collagen in normal human liver obtained from cadaveric renal transplant donors, using a peroxidase-antiperoxidase method for light microscopic visualization, and an immunogold labelling method for ultrastructural localization. The distribution was compared with that of the more abundant interstitial collagen type III, using antibodies to amino terminal procollagen type III. Staining for type VI collagen was identified in Glisson's capsule, in portal tract stroma and within the space of Disse. Perisinusoidal staining showed intra-acinar heterogeneity with the intensity in acinar zones 2 and 3 being greater than in zone 1. Type III collagen was also found in the space of Disse although no significant intra-acinar variation in staining intensity was noted. Immuno-gold labelling for type VI collagen was demonstrated on amorphous or microfilamentous material lying between, and occasionally appearing to interconnect, cross-striated collagen fibrils, whereas labelling for amino terminal procollagen type III was exclusively on fibrils. Intracellular staining for type VI collagen was noted in perisinusoidal (lto) cells. These results confirm that type VI collagen is a ubiquitous constituent of the normal hepatic extracellular matrix and suggest that it may be synthesized by perisinusoidal (lto) cells. The distribution of type VI collagen was also studied in biopsy material from patients with different histological stages of primary biliary cirrhosis. Intense staining was noted around proliferating bile ductules within developing fibrous septa and in established septa of cirrhotic liver. These observations indicate that this 'minor' matrix component may play an important role in hepatic fibrogenesis.

Neuritogenesis on collagen substrates. Involvement of integrin-like matrix receptors in retinal fibre outgrowth on collagen

Extracellular matrix molecules such as laminin, fibronectin and collagen promote neurite outgrowth in vitro. We have investigated the capacity of hydrated gels of collagen types I-III and monomeric collagen types I-VI on plastic surfaces to support neuritogenesis. The attachment and survival of explants from the day 6 chick embryo were studied and neurite outgrowth measured as mean elongation rate and maximal neurite length. Collagen types I and III, both as three-dimensional gels or as native monomers supported neuritogenesis equal to or better than laminin. Collagen type V also supported neurite out-growth although less effectively. Collagen types II, IV and VI, as well as denatured collagens of all types tested, did not support outgrowth. The monoclonal anti-beta 1 integrin antibody (CSAT), as well as rabbit polyclonal antibodies directed to the integrin beta 1-chain, effectively inhibited neurite outgrowth on permissive collagenous substrata, indicating that collagen-binding integrins were involved in the neuritogenesis. These beta 1-integrins were independent of Arg-Gly-Asp (RGD) since neurite formation proceeded in the presence of synthetic RGD-containing peptides. Fluorescence immunohistochemistry revealed the presence of the integrin beta 1-chain on the outgrowing neurites. The results suggest a possible function of collagen and collagen-binding integrins in the development of the visual system.

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.

Isolation, characterization, and localization of cardiac collagen type VI. Associations with other extracellular matrix components

We have isolated and characterized collagen type VI from murine, canine, and nonhuman primate hearts. In the three species studied, collagen type I was the major collagenous component of the cardiac interstitium (80% of total collagen), whereas collagen type VI represented approximately 5% of total collagen. To define the exact distribution of collagen type VI and its possible interactions with other components of the cardiac extracellular matrix, collagen types I, III, IV, and VI, laminin, and fibronectin were localized in the rat myocardium by immunohistochemistry, using monospecific antibodies. In the rat myocardium, collagen type VI was prevalent in the media and adventitia of muscular arteries, in fine connective tissue septa, in the area surrounding capillaries, and in the delicate endomysium in proximity to myocardial cells. When compared with the immunohistochemical localization of collagen types I, III, and IV, laminin, and fibronectin, the continuity and hierarchical organization of the cardiac extracellular matrix became apparent. The matrix forms a continuous network extending from the pericardium to the endocardium. Furthermore, there is an arborescent hierarchy in the system such that collagen type I is more prevalent in the wider septa, collagen type III being more obvious in medium-sized branches, and fibronectin and collagen type VI prevailing in the terminal (pericellular) aspects of the network. In this pericellular location, fibronectin and collagen type VI, by means of specific interactions, may act as anchor components linking the myocardial cell basement membranes not only to the extracellular matrix but also to the cardiac interstitial cells. This continuity, organization, and coupling of the cardiac extracellular matrix appears well suited to integrate and distribute the physical stress generated by the continuous contraction and relaxation of the myocardium.

Distribution and cellular origin of collagen VI during development and in cirrhosis

Collagen VI is a ubiquitous microfibrillar collagen that forms a network in most interstitial connective tissues, including soft organs and cartilage. The extracellular and intracellular distribution of collagen VI in human liver was studied by light and electron microscopy using the indirect immunoperoxidase method. In normal adult liver, collagen VI was seen mainly in portal spaces and formed a continuous layer in the sinusoids. Fetal liver contained more of collagen VI in the sinusoid than newborn and adult livers. In alcoholic fibrotic and cirrhotic livers, collagen VI antibodies intensely stained fibrous septa that invaded the lobule. Immunoelectron microscopy on normal liver showed that collagen VI antibodies labeled microfibrillar material and occasionally the surface of cells including hepatocytes. In both perinatal and fibrotic livers, electron-dense deposits were abundant in the space of Disse, intensely staining fibrils located around bundles of banded collagen. In both normal and fibrotic adult livers, collagen VI was abundant in the rough endoplasmic reticulum of Ito cells, while hepatocytes were constantly negative. In fetal livers, hepatocytes also contained collagen VI. These results suggest that collagen VI is a major constituent of the hepatic extracellular matrix. Furthermore, the cellular sources of collagen VI appear to be different in adult and developing livers.

Activation of collagen gene expression in keloids: co-localization of type I and VI collagen and transforming growth factor-beta 1 mRNA

Untreated, clinically active keloids were examined as model system to study the spatial expression of extracellular matrix and transforming growth factor-beta 1 (TGF-beta 1) genes in fibrotic skin diseases. In situ hybridizations localized active expression of type I and VI collagen genes to the areas containing an abundance of fibroblasts and apparently representing the expanding border of the lesions. Within this zone, microvascular endothelial cells also expressed the type I collagen genes, as evaluated by simultaneous use of in situ hybridization for collagen gene expression and immunolocalization for factor VIII-related antigen, a marker for endothelial cell differentiation. Slot-blot hybridizations of RNA isolated from this zone suggested that the expression of type I and IV collagen genes was selectively enhanced, as compared to type III collagen gene expression. TGF-beta 1 protein and mRNA were also detected in areas active in type I and type VI collagen gene expression, indicating that TGF-beta 1 gene is transcribed and the corresponding protein is deposited in areas of elevated collagen gene expression, including microvascular endothelial cells. We conclude that the initial step in the development of fibrotic reaction in keloids involves the expression of the TGF-beta 1 gene by the neovascular endothelial cells, thus activating the adjacent fibroblasts to express markedly elevated levels of TGF-beta 1, as well as type I and VI collagen genes.

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.

Extracellular matrix molecules and their receptors: an overview with special emphasis on periodontal tissues

Knowledge of extracellular matrix molecules and their cell receptors has increased exponentially during the last 2 decades. It is now known that the structure and function of each tissue is based on specific combinations of matrix molecules. The major constituents of the extracellular matrix are collagens, proteoglycans, and adhesive glycoproteins. The rapid development of biochemical, molecular biological, and immunological research has revealed a lot of interesting details pertaining to these molecules. Several new collagen types have been discovered. In addition to being responsible for the strength and form of tissues, each collagen type has specific sequences providing them with special features such as flexibility and the ability to interact with other matrix molecules and cells. Proteoglycans are another large group of matrix molecules with a variety of functions. Proteoglycans play an important role in tissue resilience and filtering. Some proteoglycans have a capacity to specifically bind other matrix molecules and growth factors, while others act as matrix receptors on the cell surface. An important part of regulation of the cell behavior is played by adhesive glycoproteins belonging to the fibronectin and laminin families. Several isoforms of fibronectin and laminin that result from alternative RNA splicing serve specific functions such as controlling the attachment, migration, and synthetic activity of cells. A major group of cell receptors for cell-matrix and cell-cell interactions is termed integrins. The integrins are cell surface proteins composed of two polypeptides whose structure dictates the specificity of each receptor. The cytoplasmic domain of the integrins interacts with cytoskeletal elements within the cell, and thereby relays the information from the extracellular space into the protein synthesis machinery. The expression of the integrins is controlled by the extracellular matrix and growth factors, most notably TGF beta. During periodontal diseases several aspects of the cell-matrix interactions may be disturbed. Therefore, an understanding of the special features of the extracellular matrix and their receptors in periodontal tissues is a prerequisite for developing new approaches to the prevention and treatment of periodontal diseases.

Biochemical mechanisms of laser vascular tissue fusion

This study examines the biochemical changes that occur in argon laser-fused canine veins compared with control segments of vein. Laser fusions were formed using 0.5 W argon laser energy (1100-1500 J/cm2). Immediately following tissue fusion, blood flow was reestablished to test the integrity of the welds. 1-mm3 sections of the anastomoses and control sections were minced and protein extraction was performed by solubilizing the tissue in hot SDS Laemmli gel sample buffer. The proteins were separated electrophoretically on 5 and 10% polyacylamide SDS gels and silver stained. The analysis demonstrated significant biochemical differences between control and lased veins. We noted increases in several proteins after laser welding: the putative beta chain of type V collagen (5/5 gels), the putative gamma chain of type I collagen (4/5 gels), a 156-kDa protein (based on collagen molecular weight standards) 7/7 gels), an 82-kDa protein (8/9 gels), and several proteins of lower molecular weight (3/8 gels). The increases may be due to crosslinking of lower molecular weight proteins, degradation of higher molecular weight proteins, or increased solubility of certain proteins. These findings suggest that laser welding may occur by formation of crosslinks or by denaturation and reannealment of structural proteins.