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

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

Modulation of smooth muscle cell behaviour by platelet-derived factors and the extracellular matrix

We have studied the combined effects of platelet-derived soluble factors and three types of macromolecular substrata on the proliferation and migration of smooth muscle cells in vitro. Bovine aortic smooth muscle cells were plated onto three-dimensional gels of type I collagen or onto cell-free extracellular matrices deposited on such gels by either bovine aortic endothelial cells or smooth muscle cells. The cells were cultured in the presence of whole-blood serum (WBS) or platelet-poor plasma (PPP). Smooth muscle cell proliferation on type I collagen gels was dependent on the presence of platelet-derived factors, i.e. the cells proliferated in the presence of WBS but not in PPP. In contrast, cell proliferation on the extracellular matrices occurred at the same rate in PPP and WBS. Smooth muscle cells plated onto collagen gels rapidly migrated down into the gel matrix; the percentage of cells migrating was inversely proportional to cell density. The presence of extracellular matrices did not alter the rate of cell migration into the underlying gel matrix. Irrespective of the substratum used, smooth muscle cell migration was independent of platelet-derived or plasma factors and occurred in the absence of proliferation. These results indicate that possible chemotactic, chemokinetic, and/or mitogenic factors produced by the vascular cells and deposited within the extracellular matrix may play an important role in modulating smooth muscle cell behaviour in the vascular wall.

The biosynthesis of extracellular-matrix components by bovine retinal endothelial cells displaying distinctive morphological phenotypes

Previous studies have indicated that the morphology and behaviour of bovine retinal microvessel endothelial cells are influenced by culture conditions in vitro. Data are presented here concerning the biosynthesis of matrix macromolecules by bovine retinal endothelial cells cultured under conditions in which the cells display either the 'cobblestone' or the 'sprouting' phenotype. Newly synthesized matrix proteins were identified by their characteristic electrophoretic mobilities, immunoprecipitation with specific antibodies, susceptibilities to enzymic digestions and chromatographic behaviour. Type IV procollagen was the major collagenous species synthesized by early-passage cells forming a 'cobblestone' monolayer. In contrast, cells displaying the 'sprouting' morphology switched to the predominant synthesis of interstitial fibrillar collagens (types I and III). Fibronectin was synthesized by retinal endothelial cells under all the experimental conditions studied. A non-collagenous glycoprotein of Mr approx. 47,000 was also a major biosynthetic product of these cells. The synthesis of thrombospondin was very much dependent on the nature of the substratum on which the cells were cultured. This glycoprotein was synthesized in large amounts by 'cobblestone' endothelial cells cultured on gelatin-coated dishes, whereas its synthesis was markedly decreased by culturing the cells on collagen gels, and the protein appeared to be absent when the cells were plated within collagen gels ('sprouting' cells). Late-passage retinal cells synthesized predominantly type I procollagen, variable amounts of type III procollagen and only traces of type IV procollagen, irrespective of whether the cells displayed a 'cobblestone' or 'sprouting' morphology.

Immunoperoxidase localization of type X collagen in chick tibiae

Type X collagen was prepared from medium of long-term cultures of embryonic chick tibiotarsal chondrocytes. Antibodies to type X collagen were raised and used in immunoperoxidase localization studies with embryonic and growing chick tibiotarsus. Strong anti-type X collagen reactivity was detected mainly in the region of hypertrophic chondrocytes, and to a lesser extent in the zone of calcified cartilage. No reactivity was detected in the proliferative zone nor the superficial layer of the cartilage growth plate. These results suggest that type X collagen may play a key role in matrix calcification during growth and development of the skeletal system.

Isolation from bovine elastic tissues of collagen type VI and characterization of its form in vivo

Foetal-bovine nuchal ligament and aorta, together with adult-bovine aorta and pregnant uterus, were extracted under dissociative conditions in the absence and in the presence of a reducing agent. A collagenous glycoprotein of Mr 140000 [designated component 140K(VI)], identified in these extracts as the major periodate/Schiff-positive component, was shown to be related to collagen type VI. Digestion of non-reduced extracts with pepsin yielded periodate/Schiff-positive peptides that, on the basis of their electrophoretic mobilities, amino acid analyses and peptide 'maps', were identical with type VI collagen fragments prepared by standard procedures. It is concluded that collagen type VI occurs in vivo as molecule comprising three chains of Mr 140000 in which the helical domains account for about one-third of each polypeptide. Biosynthetic experiments with nuchal-ligament fibroblasts in culture demonstrated that a bacterial-collagenase-sensitive [3H]fucose-labelled glycoprotein, Mr 140000, was immunoprecipitated from culture medium by a specific antibody to the pepsin-derived form of collagen type VI. This result suggests that the collagenous polypeptides [140K(VI) components] represent the biosynthetic precursors of type VI collagen that do not undergo processing to smaller species before deposition in the extracellular matrix. Analyses of 5M-guanidinium chloride extracts of tissues with markedly different elastin contents and at different stages of development suggested that there was no relationship between collagen type VI and elastic-fibre microfibrils, a conclusion supported by the observation that the immunoprecipitated glycoprotein, Mr 140000, was distinct from the glycoprotein MFPI, Mr 150000, believed to be a constituent of these microfibrils [Sear, Grant & Jackson (1981) Biochem. J. 194, 587-598].

The secretion of tropoelastin by chick-embryo artery cells

Chymotryptic fingerprint analyses of tropoelastin a and tropoelastin b demonstrated a very close relationship between these two polypeptides synthesized in a cell-free system under the direction of chick-embryo polyribosomal mRNA. A similar study on tropoelastin polypeptides extracted in their hydroxylated and under-hydroxylated forms from artery cells incubated with [3H]valine in the absence and presence of alpha alpha'-bipyridine or 3,4-dehydroproline confirmed this close relationship and suggested that tropoelastins a and b are likely to be the products of a single gene. Pulse-chase experiments in which the synthesis and secretion of tropoelastin by artery cells were monitored demonstrated that, after a pulse with [3H]proline, the polypeptides rapidly appeared in the medium and the half-time of tropoelastin secretion was approx. 30 min. Further pulse-chase studies, in which [3H]tropoelastin contents of subcellular fractions were determined, showed that rough and smooth microsomal fractions contained maximal amounts of tropoelastin at different times. The quantity of tropoelastin in the smooth-microsomal fraction was always only a small proportion of that in the rough-microsomal fraction, suggesting rapid translocation of the polypeptides to the plasma membrane. Incubation of the cells with 0.1 mM-colchicine did not markedly alter the rate of secretion or the distribution of tropoelastin between the subcellular fractions, whereas when 1 microM-monensin was included in the incubations the polypeptides were retained in the rough microsomal fraction. The results are consistent with the proposal that tropoelastin may follow a pathway of secretion from rough endoplasmic reticulum to the plasma membrane via secretory vesicles.

Type X collagen, a product of hypertrophic chondrocytes

The synthesis of collagen types IX and X by explants of chick-embryo cartilages was investigated. When sternal cartilage labelled for 24h with [3H]proline was extracted with 4M-guanidinium chloride, up to 20% of the 3H-labelled collagen laid down in the tissue could be accounted for by the low-Mr collagenous polypeptides (H and J chains) of type IX collagen; but no type X collagen could be detected. Explants of tibiotarsal and femoral cartilages were found to synthesize type IX collagen mainly in zones 1 and 2 of chondrocyte proliferation and elongation, whereas type X collagen was shown to be a product of the hypertrophic chondrocytes in zone 3. Pulse-chase experiments with tibiotarsal (zone-3) explants demonstrated a time-dependent conversion of type X procollagen into a smaller species whose polypeptides were of Mr 49 000. The processed chains [alpha 1(X) chains] were shown by peptide mapping techniques to share a common identity with the pro alpha 1(X) chains of Mr 59 000. No evidence for processing of type IX collagen was obtained in analogous pulse-chase experiments with sternal tissue. When chondrocytes from tibiotarsal cartilage (zone 3) were cultured on plastic under standard conditions for 4-10 weeks they released large amounts of type X procollagen into the medium. However, 2M-MgCl2 extracts of the cell layer were found to contain mainly the processed collagen comprising alpha 1(X) chains. The native type X procollagen purified from culture medium was shown by rotary shadowing to occur as a short rod-like molecule 148 nm in length with a terminal globular extension, whereas the processed species comprising alpha 1(X) chains of Mr 49 000 was detected by electron microscopy as the linear 148 nm segment.

Destabilization of collagen structure by amides and detergents in solution

The effects of amides and detergents on collagen to gelatin transition have been studied at neutral pH. Simple amides denature the protein. The substitution of H-atoms by the alkyl groups at the nonpolar end of amide increases the effectiveness of the compounds in destabilizing the collagen structure whereas substitution of the H-atom at the polar amide end shows marginal effects on the collagen transition. The capabilities of these reagents to denature collagen are much less pronounced than their effects on denaturing globular proteins. Anionic detergents are found to destabilize collagen at very low concentrations (below their cmc values). In this respect, the effects of the detergents on collagen are comparable to the denaturing effects of the detergents on globular proteins. The effect of detergents increases with the increase in the length of the alkyl chain. The structure of the anion in the detergent is also important as seen from the lower potency of the sulfonate containing detergent compared to the sulfate containing detergent in denaturing collagen. Cationic and nonionic detergents do not denature collagen.

Determination of minoxidil in bulk drug and pharmaceutical formulations by ion-pairing high-performance liquid chromatography

An ion-pairing liquid chromatographic method with UV detection is described for the determination of minoxidil in bulk drug, compressed tablet, and topical solution formulations. The chromatographic system consists of a microparticulate octadecylsilica column and a mobile phase composed of sodium dioctylsulfosuccinate in aqueous methanol (pH 3). The bulk drug and the topical solution samples are prepared by the dissolution of the drug in internal standard solution. Sample preparation for the compressed tablet formulation involves dissolving the drug from an aliquot of pulverized sample and centrifuging to remove insoluble excipients. Quantitative recovery of minoxidil from formulation excipients was demonstrated; assay precision was less than 1% CV.

Elastin biosynthesis in chick-embryo arteries. Studies on the intracellular site of synthesis of tropoelastin

Electrophoretic analyses of the products of cell-free translation of elastin mRNA isolated from 17-day chick-embryo thoracic arteries have demonstrated that the elastin mRNA codes for polypeptides that are slightly larger than the cellular tropoelastin polypeptides synthesized and secreted by matrix-free artery cells. Pulse-chase experiments with cells labelled with [3H]proline established that newly synthesized tropoelastin polypeptides were associated solely with membrane-bound particulate fractions. Cell-free translation of membrane-bound and free polyribosomes isolated from artery cells revealed that the tropoelastin mRNA was associated predominantly with the membrane-bound fraction. When rough-microsomal fractions, isolated from cells labelled with [3H]proline for 10 min, were treated with proteinases in the presence and in the absence of detergent, the nascent tropoelastin polypeptides were shown to be susceptible to proteolysis only when the integrity of the membranes was destroyed by detergent treatment. In similar experiments tropoelastin polypeptides synthesized by membrane-bound polyribosomes in the nuclease-treated reticulocyte lysate were also resistant to the proteolytic-enzyme treatment. The results suggest that tropoelastin polypeptides are synthesized on membrane-bound polyribosomes and discharged into the lumen of the endoplasmic reticulum with co-translational removal of a signal peptide.

A collagenous glycoprotein found in dissociative extracts of foetal bovine nuchal ligament. Evidence for a relationship with type VI collagen

A collagenous glycoprotein (Mr 140000) was isolated from dissociative extracts of foetal bovine nuchal ligament and purified by a combination of ion-exchange and gel-filtration chromatography. This glycoprotein (designated MFPI) exists as a large-Mr disulphide-bonded aggregate in the absence of a reducing agent. The purified glycoprotein was shown to contain about 6% (w/w) carbohydrate, mostly as galactose, glucose and mannose. Amino acid analysis showed the presence of hydroxyproline and hydroxylysine, indicative of its collagenous nature. The collagenous nature of this glycoprotein was further investigated by enzyme digestion. Pepsin digestion produced three major fragments, which were identical with peptides of type VI collagen. Bacterial-collagenase digestion of the unreduced glycoprotein also produced several discrete peptides. However, reduction of the glycoprotein before bacterial-collagenase digestion resulted in the degradation of these discrete peptides. Glycoprotein MFPI extracted in dissociative conditions appears to be a larger-Mr form of type VI collagen, believed to originate from microfibrillar components in the intact tissue.

Collagen heterogeneity and quantification in developing bovine nuchal ligament

The collagenous components were investigated in peptic digests of developing bovine nuchal ligament. Types I and III collagen were the major species isolated, but the presence of types IV, V and VI was also shown. Changes in the pepsin-susceptibility of nuchal ligament during foetal development were observed. CNBr-cleavage peptide analysis indicated that type I collagen became cross-linked rapidly, as evidenced by the lack of alpha 1(I)CB6. At present it is not clear if this decrease in pepsin-susceptibility is due to cross-linking of collagen, to increased deposition of elastin, or to both. Quantification of collagen types I and III was shown to depend on the method used. When pepsin-solubilized material was examined an apparent increase in type III collagen with respect to foetal age was observed, whereas when CNBr digests of intact ligament were examined a relatively constant amount of type III collagen (approx. 24%) was found. The constant amount of type III collagen observed during foetal development changed at birth and increased in mature nuchal ligament to represent approx. 45% of the total collagen.

Embryonic chick cartilage collagens. Differences in the low-Mr species present in sternal cartilage and tibiotarsal articular cartilage

The collagenous polypeptides present in embryonic chick sternal and tibiotarsal cartilages have been solubilised by digestion with pepsin and separated by salt fractionation. Type II collagen, 1 alpha 2 alpha 3 alpha collagen, and two polypeptides (apparent molecular mass 150 and 42 kDa), which were reducible to a number of smaller peptides, were extracted from both tissues. However, also present in the peptic digests of tibiotarsal cartilages was a major non-reducible highly-soluble polypeptide of 45 kDa. This short-chain collagen is apparently identical to the pepsinized product of G collagen (Mr 59 000), a major low-Mr procollagen-like species previously detected in chick chondrocyte cultures.

Nine-year experience during 1973-1982 with 1,060 pacemakers in 805 patients

Our experience with over one-thousand pacemaker implantations resulting in almost 28,000 patient pacing months is reviewed. Data are presented regarding changing indications for pacing, experience with numerous pacemaker types, patient survival, comparative mortality of sick sinus syndrome and complete heart block, and telephone surveillance. Consistent with some previous reports, there were high one- and two-year post-implant mortality rates, and advanced age at implant was associated with decreased survival. Males had a significantly higher mortality rate. The period reported covers the transition from utilization of mercury-zinc batteries to lithium power cells and other new technology; significant differences were observed in device reliability and patient mortality when comparing these eras. Certain patients evidenced disparate survivals, the most notable difference being seen in those who received their primary implants prior to 1976 and subsequent implants of more modern units.

Synthesis and secretion of sulphated glycosaminoglycans by bovine peridontal ligament fibroblast cultures

The cultures were allowed to incorporate 35SO2-4 for various periods of time. 35S-labelled macromolecules were isolated from the medium, a trypsin digest of the cells and the cell residue. Ion-exchange chromatography separated the radioactive polysaccharides into heparan sulphate and a galactosaminoglycan population. Most heparan sulphate was in the trypsin digest and cell residue fractions. The galactosaminoglycan fractions were investigated by differential degradations with chondroitinase ABC and AC and ethanol fractionation. The medium galactosaminoglycans contained both glucuronic and iduronic acid residues and existed in copolymeric structures as chondroitin sulphate/dermatan sulphate hybrid molecules. Dermatan sulphate was also detected. In contrast, the trypsin-digest fraction contained mainly chondroitin sulphate-like molecules.

Identification and partial characterization of three low-molecular-weight collagenous polypeptides synthesized by chondrocytes cultured within collagen gels in the absence and in the presence of fibronectin

Culture of chick-embryo sternal-cartilage chondrocytes within three-dimensional collagen gels promotes the synthesis of three low-molecular-weight collagenous polypeptides. The proportions of these novel collagens synthesized and released into the medium are markedly influenced by the presence or the absence of fibronectin in the serum supplement. Chondrocytes cultured on plastic dishes appear to synthesize only small amounts of these low-molecular-weight species. The three species (designated G, H and J) were characterized with respect to the proportion of [14C]proline incorporated into each polypeptide occurring as hydroxy[14C]proline and with respect to their susceptibilities to bacterial collagenase. On the basis of their electrophoretic mobilities under reducing conditions, the G, H and J polypeptides were calculated to have Mr 59 000, 69 000 and 84 000 respectively. Chymotrypsin digestion converted the G collagen into a species containing polypeptides of Mr 45 000, whereas the H and J polypeptides yielded a single band of Mr 53 000. The H and J polypeptides were found to occur as disulphide-linked aggregates, as was the chymotrypsin-digestion product. Peptide 'mapping' has shown that G, H and J polypeptides show no common identity and are distinct from the known interstitial collagens. Native G collagen was digested by human collagenase to discrete products, whereas H and J chains were not cleaved under identical conditions.

Interaction of blood platelets with a microfibrillar extract from adult bovine aorta: requirement for von Willebrand factor

Adult bovine aortic tissue was treated with 6 M guanidinium chloride in the presence of proteinase inhibitors to obtain an extract that was essentially devoid of collagenous components and appeared homogeneous by electron microscopy. When this extract was dispersed by sonication it was found to be a very potent inducer of human platelet aggregation. This interaction required the presence of von Willebrand factor and of its receptor (glycoprotein Ib) on platelet membrane. This was demonstrated by the fact that the aggregation of normal blood platelets resuspended in plasmas deficient in von Willebrand factor was significantly diminished as compared to aggregation in control plasma. Moreover, this aggregation was inhibited by a monoclonal antibody, IgG AN51, to platelet glycoprotein Ib. These studies provide direct biochemical evidence for the existence of a thrombogenic constituent of the vessel wall that is noncollagenous and von Willebrand factor-dependent.

Effects of matrix macromolecules on chondrocyte gene expression: synthesis of a low molecular weight collagen species by cells cultured within collagen gels

Chick-embryo sternal chondrocytes have been cultured within three-dimensional collagen gels as part of a study concerned with the effects of extracellular matrix macromolecules on chondrocyte gene expression. Data are presented indicating that chondrocytes cultured within such a collagenous environment synthesize significantly more of an hitherto unidentified, low molecular weight collagen species than do cells grown on plastic tissue-culture dishes in the conventional manner. This low molecular weight collagen species contains noncollagenous domains (as indicated by its decreased molecular size after mild pepsin digestion), is distinct from the known collagen types (as judged by CNBr peptide analysis), and forms part of the insoluble collagenous matrix produced by the chondrocytes. Cells growing within the gel tend to form colonies consisting of a linear array of cells reminiscent of the cellular organization in growth cartilage.

The biosynthesis of glycoproteins by cultured bovine tendon fibroblasts

Confluent bovine fetal tendon fibroblasts maintained in a chemically defined medium incorporated L-[6-3H]fucose and L-[5-3H]proline in a linear manner into non-diffusible macromolecules for up to 48 hrs. Equilibrium CsCl density gradient centrifugation indicated that [3H]fucose-labelled macromolecules released into the medium were predominantly glycoproteins. The [3H]fucose-labelled glycoproteins in the culture medium were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. This technique demonstrated the presence of a number of high mol. wt. fucosylated components, the most notable of which was a glycoprotein of apparent mol. wt. 150,000. Immunological procedures allowed the tentative identification of four glycoproteins including fibronectin which was found in the cell medium and in extracts of the cell layer. Two of the glycoproteins (mol. wts. 150,000 and 270,000) released into the incubation medium were shown to be related to the microfibrillar components of elastic tissue. One or more of the newly synthesized [3H]fucose labelled molecules was shown to be immunologically related to a glycoprotein (mol. wt. 60,000) extracted from bovine Achilles tendon. These studies represent the first demonstration of the synthesis of microfibril-related and tendon glycoprotein-related macromolecules by tendon fibroblasts in culture.

Micro-processor synthesis and analysis of the polarcardiogram

A low-cost micro-processor combined with a standard 3-channel electrocardiographic recorder was adapted to polar coordinate conversion of Frank orthogonal lead signals and display of the resultant polarcardiogram. The processor also functions as a computer of average transients for the ECG complexes and provides automatic recorder function at specified intervals during electrocardiographic exercise stress testing. Polarcardiographic variables such as magnitude, longitude and latitude at particular temporal points are calculated and values displayed by the micro-processor. The configuration provides an economical, accurate and reliable alternative to larger systems and may easily be adapted to other applications.

Two major non-collagenous glycoproteins in embryonic chick arteries

Glycoprotein-containing extracts were obtained from thoracic arteries of embryonic chicks by sequential treatment involving 6 M guanidinium chloride, purified bacterial collagenase, and 6 M guanidinium chloride plus 50 mM dithiothreitol. Two major glycopolypeptides, designated G1 and G2, having apparent mol. wts. of 140,000 and 130,000 respectively were detected by SDS/polyacrylamide gel electrophoresis. Equilibrium density gradient ultracentrifugation demonstrated G1 and G2 to be glycoproteins and not proteoglycans or glycosaminoglycans. Amino acid analysis of a glycoprotein-enriched fraction confirmed the non-collagenous nature of G1 and G2. The highly insoluble nature of these glycoproteins suggests that these species are intimately associated with the extracellular matrix. Glycoproteins of similar size were also extracted from wing tendons indicating that G1 and G2 may be common to the elastic tissues of the chick.

Current concepts of basement-membrane structure and function

Conclusion

In this brief review we have attempted to describe the known components of basement membranes in relation to the morphology and function of these matrices. Further details of the molecular structures and biosynthesis of these components may be found in original papers and in various reviews (Kefalides, 1973; Spiro, 1976; Kefalides et al., 1979; Heathcote & Grant, 1981).

Although basement membranes appear to contain essentially similar protein and carbohydrate moieties, the proportions and organization of these may differ and, in the opinion of the authors, the key to an understanding of basement membranes lies in the recognition of this heterogeneity. At present, structural models of basement membrane are far from satisfactory and should be regarded with reservation.

The nature of the microfibrillar glycoproteins of elastic fibres. A biosynthetic study

1. Cell cultures propagated from foetal bovine ligamentum nuchae synthesized and secreted two glycoproteins, designated MFP I and MFP II, that are closely related to elastic-fibre microfibrils. Glycoproteins MFP I (apparent mol.wt. 150 000) and MFP II (apparent mol.wt. 300 000) were metabolically labelled, separated from other culture-medium components by immunoprecipitation with a specific anti-(microfibrillar protein) serum, and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and sodium dodecyl sulphate/gel-filtration chromatography. 2. Ligament cells also synthesized and secreted fibronectin, but salt-fractionation and immunoprecipitation studies with a specific anti-(cold-insoluble globulin) serum established that neither glycoprotein MFP I nor glycoprotein MFP II was related to fibronectin. 3. The secretion of glycoprotein MFP I, but not that of glycoprotein MFP II, was enhanced by the addition of ascorbate to the culture medium. 4. Ascorbate-supplemented ligament cells incorporated [3H]proline into glycoprotein MFP I, and 36% of the nondiffusible proline residues were hydroxylated, exclusively as 4-hydroxy[3H]proline. Less than 1% of the total proline residues in [3H]proline-labelled glycoprotein MFP II were hydroxylated. 5. Ascorbate-supplemented cells incorporated [14C]lysine into glycoprotein MFP I and 30% of the non-diffusible lysine residues were hydroxylated. 6. Newly secreted glycoprotein MFP I was digested by highly purified bacterial collagenase to yield polypeptide fragments of apparent mol.wts. 50 000 and 30 000. Glycoprotein MFP II was not digested by bacterial collagenase. 7. The results suggest that elastic-fibre microfibrils are composed of a novel collagenous glycoprotein MFP I in association, as yet undefined, with a non-collagenous glycoprotein MFP II.