Characterization of basement membrane collagen of bovine anterior lens capsule via segment-long-spacing crystallites and the specific cleavage of the collagen by pepsin

Segment-Long-Spacing (SLS) and the Polymorphic Structures of Fibrillar Collagen

The diverse and complex functions of collagen during the development of an organism are closely related to the polymorphism of its supramolecular structures in the extracellular matrix. SLS (segment-long-spacing) is one of the best understood alternative structures of collagen. SLS played an instrumental role in the original studies of collagen more than half a century ago that laid the foundation of nearly everything we know about collagen today. Despite being used mostly under in vitro conditions, the natural occurrence of SLS in tissues has also been reported. Here we will provide a brief overview of the major findings of the SLS and other structures of collagen based on a wealth of work published starting from the 1940s. We will discuss the factors that determine the stability and the structural specificity of the different molecular assemblies of collagen in light of the new studies using designed fibril forming collagen peptides. At the end of the chapter, we will summarize some recent discoveries of the alternative structures of collagen in tissues, especially those involved in pathogenic states. A revisit of SLS will likely inspire new understandings concerning the range of critical roles of fibrillar collagen in terms of its organizational diversity in the extracellular matrix.

The collagen type I segment long spacing (SLS) and fibrillar forms: Formation by ATP and sulphonated diazo dyes

The collagen type I segment long spacing (SLS) crystallite is a well-ordered rod-like molecular aggregate, ∼300nm in length, which is produced in vitro under mildly acidic conditions (pH 2.5-3.5) in the presence of 1mM ATP. The formation of the SLS crystallite amplifies the inherent linear structural features of individual collagen heterotrimers, due to the punctate linear distribution and summation of the bulkier amino acid side chains along the length of individual collagen heterotrimers. This can be correlated structurally with the 67nm D-banded collagen fibril that is found in vivo, and formed in vitro. Although first described many years ago, the range of conditions required for ATP-induced SLS crystallite formation from acid-soluble collagen have not been explored extensively. Consequently, we have addressed biochemical parameters such as the ATP concentration, pH, speed of formation and stability so as to provide a more complete structural understanding of the SLS crystallite. Treatment of collagen type I with 1mM ATP at neutral and higher pH (6.0-9.0) also induced the formation of D-banded fibrils. Contrary to previous studies, we have shown that the polysulphonated diazo dyes Direct red (Sirius red) and Evans blue, but not Congo red and Methyl blue, can also induce the formation of SLS-like aggregates of collagen, but under markedly different ionic conditions to those employed in the presence of ATP. Specifically, pre-formed D-banded collagen fibrils, prepared in a higher than the usual physiological NaCl concentration (e.g. 500mM NaCl, 20mM Tris-HCl pH7.4 or x3 PBS), readily form SLS aggregates when treated with 0.1mM Direct red and Evans blue, but this did not occur at lower NaCl concentrations. These new data are discussed in relation to the anion (Cl(-)) and polyanion (phosphate and sulphonate) binding by the collagen heterotrimer and their likely role in collagen fibrillogenesis and SLS formation.

Lens capsule as a model to study type IV collagen

The study of collagen IV has benefited greatly from the seminal work conducted by Arthur Veis and colleagues over three decades ago. Through a series of electron microscopy studies focused on lens basement membrane, an appreciation was gained for the distinct network-forming properties of collagen IV. Veis correctly suggested that network assembly is a phenomenon of the non-collagenous termini of the molecule. This review seeks to document how the field advanced following these seminal conclusions, including recent discoveries regarding the molecular reinforcement of networks that support Veis' conclusions.

Reaggregation behavior of different types of collagen in vitro: variations in the occurrence and structure of dimeric segment long-spacing collagen

Segment long-spacing collagen (SLS) can be precipitated from solutions of collagen using ATP as the inducing agent. Dimeric SLS aggregates have been observed in addition to monomeric SLS. We have compared collagen types I, II, III, and V with respect to their ability to form dimeric SLS in vitro. These collagen types were isolated from bovine tissues and characterized by polyacrylamide slab gel electrophoresis of the respective alpha-chains. Only monomeric SLS can be detected in preparations of collagen types I and III. Dimeric SLS, on the other hand, accounts for the majority of the crystallites seen in preparations of collagen types II and V. Dimeric SLS from both collagen types II and V reveal overlap zones at the carboxy-terminal ends of the collagen molecules. However, dimeric SLS from collagen types II and V differ with respect to their overlap distances. Significant portions of the triple helical domains of collagen molecules are occupied by the overlap region of dimeric SLS from type II collagen. On the other hand, dimeric SLS from type V collagen is composed of molecules overlapping only at their short nonhelical telopeptides. It is concluded that the ability of collagen molecules to aggregate into dimeric SLS under defined experimental conditions is collagen type dependent.

Identification of collagens isolated from bovine Descemet's membrane

In this study collagens were isolated and identified from a morphologically pure preparation of bovine Descemet's membrane (DM) which was obtained by gentle scraping of the cornea, sieving and subsequent treatment with detergents. An alternative procedure of DM isolation with forceps resulted in stromal contamination of the preparation as verified by light and transmission electron microscopy, and gel electrophoresis. The amino acid profile of collagens isolated by pepsinization and salt precipitation from the pure sample was similar to the analysis of the intact bovine DM. Polyacrylamide gel electrophoresis of this collagen under non-reducing conditions resulted in five major bands: 300 000 daltons (300 K), 200 000 daltons (200 K), 100 000 daltons (100 K) and lower molecular weight fractions (50 K1 and 50 K2). Individual collagen chains were isolated from preparative polyacrylamide gels and characterized by 125I two dimensional peptide mapping patterns. This data suggests that (1) the majority of collagen fragments seen in bovine DM pepsin supernatant are derived from a single genetically distinct collagen molecule, and (2) that type I and V collagens are not major components of bovine DM.

Production of collagen by cells isolated from a retrocorneal fibrous membrane rabbit model

The morphological and biosynthetic characteristics of cells from an experimentally induced rabbit retrocorneal fibrous membrane (RCFM) model were investigated. By transmission electron microscopy the cells within the RCFM demonstrated overlapping cytoplasmic processes and intercellular junctions, neither of which are fibroblast-like characteristics. The extracellular matrix within the RCFM had a fibrillar and amorphous component. Collagenous biosynthetic products of primary cultures of RCFM cells were compared to normal corneal endothelial cells, which produce mainly type IV collagen and a small amount of type V collagen, and fibroblasts, which produce types I, III and V collagens. The collagenous components produced by the RCFM cells were a combination of types I, V and low molecular weight fragments of type IV collagen. Therefore, these morphological and biochemical data suggest that RCFM cells are a type of modified corneal endothelial cell that produce collagens distinctly different from normal corneal endothelial cells.

Determination of radioactive proline and hydroxyproline by a simple paper-chromatographic procedure

A simple method to separate and determine radioactive proline and hydroxyproline by paper chromatography is described. The localization of the imino acids after separation is achieved by direct nondestructive staining with 1-fluoro-2,4-dinitrobenzene dye. The imino acids are quantitated directly by liquid scintillation counting in the presence of paper strips. The method was applied to bone cultures with good reproducibility, sensitivity and linearity over a wide range of radioactivity. The procedure was also tested in fibroblast cultures. The results for hydroxyproline were in good correlation with the widely used method of Juva and Prockop (Juva, K. and Prockop, D.J. (1966) Anal. Biochem. 15, 77-83), in which hydroxyproline is oxidized to pyrrole, and then extracted and purified by column chromatography before counting radioactivity.

Partial purification of collagenase and gelatinase from human polymorphonuclear leucocytes. Analysis of their actions on soluble and insoluble collagens

The separation and further purification of human polymorphonuclear-leucocyte collagenase and gelatinase, using modifications of the method of Cawston & Tyler [(1979) Biochem J. 183, 647-656], are described. The final preparations yielded collagenase of specific activity 260 units/mg and gelatinase of specific activity 13 000 units/mg. Gelatinase was purified to apparent homogeneity in a latent form, and analysis of the activation of 125I-labelled latent enzyme by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and gel-filtration techniques suggested that no peptide material was lost on conversion into the active form. The purified natural inhibitors alpha 2-macroglobulin, tissue inhibitor of metalloproteinases ('TIMP') and amniotic-fluid inhibitor of metalloproteinases all inhibited the two polymorphonuclear-leucocyte metalloproteinases, but the last two inhibitors were slow to act and complete inhibition was difficult to attain. Collagenase degraded soluble types I and III collagen equally efficiently, but soluble type II collagen less well. Gelatinase alone had little activity on these substrates, although it enhanced the action of collagenase. Gelatinase was capable of degrading soluble types IV and V collagen at 25 degrees C, whereas collagenase was only active at higher temperatures when the collagens were susceptible to trypsin activity. By using tissue preparations of insoluble collagens (type I, II or IV) the activity of leucocyte collagenase was low and gelatinase activity was negligible, as measured by the solubilization of hydroxyproline-containing material. The two enzymes together were two or three times more effective in the degradation of these insoluble collagens.

Studies on the underhydroxylated basement membrane procollagen synthesized by rat parietal yolk sacs in the presence of alpha,alpha'-dipydridyl

Rat embryo parietal yolk sacs synthesized and secreted underhydroxylated [14C]proline-labeled basement membrane procollagen when incubated in the presence of the iron chelator alpha,alpha'-dipyridyl and [14C]proline. Upon reduction and sodium dodecyl sulfate-agarose gel filtration, this procollagen eluted as a discrete peak of labeled material close to the position of the beta components of type I collagen. Secreted underhydroxylated procollagen contained interchain disulfide bonds and was completely degraded by a 6 hr treatment with alpha-chymotrypsin at 4 degrees C. Results from 4 hr incubations indicated that the distribution of 14C-procollagen between tissue and medium was unaffected by the presence of alpha,alpha'-dipyridyl, but analysis of results from pulse-label and chase experiments revealed that underhydroxylated procollagen was secreted at less than half the rate of control procollagen.

Metalloproteinases from rabbit bone culture medium degrade types IV and V collagens, laminin and fibronectin

Gel-filtration chromatography of culture medium from rabbit bone explants separates three latent metalloproteinases with activities against collagen, proteoglycan and gelatin respectively. The fractions degrading proteoglycan also degrade laminin, fibronectin and the polymeric products of pepsin-solubilized type IV collagen and can also solubilize insoluble type IV collagen. The fractions degrading gelatin are capable of degrading solubilized type V and 1 alpha,2 alpha,3 alpha (cartilage) collagens, as well as the lower-molecular-weight products of pepsin-solubilized type IV collagen. All activities can be inhibited by 1,10-phenanthroline and occur in either partially or totally latent forms that can be activated by 4-aminophenylmercuric acetate.

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.

Type IV collagens' isolation and characterization of 7S collagen from human kidney, liver and lung

7S collagens were isolated after bacterial collagenase treatment of basement membrane material prepared from the pepsin digest of human kidney, liver and lung. The 7S collagens were purified by combination of molecular sieve and ion exchange chromatography. 7S collagen from each of these sources showed similar amino acid composition, electrophoretic patterns of reduced and unreduced samples. Antibodies raised against human placental 7S collagen in rabbits completely cross-reacted with the 7S collagen preparation obtained from kidney, liver and lung in enzyme-immunoassays. Since 7S collagen in known to be a major cross-linked segment of type IV collagens, the data indicate that collagenous proteins of basement membranes are organized in similar networks in a variety of organs.

The structure of acid-soluble basement membrane collagen from bovine anterior lens capsule: molecular parameters and thermal gelation properties

Basement membrane collagen was extracted from bovine anterior lens capsules with acetic acid in the presence of protease and proteinase inhibitors. This undegraded soluble basement membrane has a sedimentation coefficient S0 20,w=7.8 X 10-13 sec-1, a diffusion coefficient D020,w=5.82 X 10-8/ cm2/sec and a zero-shear intrinsic viscosity [eta] = 2200 cm3/g. These data lead to a molecular weight, Mr of 1.1 X 10(6). Electron microscopic observation indicates that the molecules are rod-like in character with a length of 300 nm. Theoretical models describing the intrinsic viscosity in terms of the distribution of mass in such an elongated molecule show that a dumbbell-like structure is the only tenable model. In this model a single 300 nm rod-like region joins two more densely packed globular regions. The globular domains contain hydroxyproline and may has collagenase sensitive regions since digestion with purified bacterial collagenase at 37 degree C for 18 h degrades these domains as well as the central triple-helical domain. The intact acid-soluble basement membrane collagen precipitates from neutral pH solutions upon warming 4 degree C stable solutions. The pH, ionic strength and concentration dependence parameters of this thermal precipitation have been examined. The basement membrane collagen precipitation appears to be dominated by end-region domain hydrophobic interactions. The end-regions are required for precipitation in contrast to the Type I procollagen situation wherein the intact end-regions inhibit thermal gelation. Electron microscopic examination of the basement membrane thermal gels shows networks of very thin filaments which may be related directly to the structure within intact basement membrane.

Epithelial invasion and secretion of banded collagen in the regenerating lens capsule of the rat

The invasion of large areas of the anterior lens capsule by epithelial cells occurs during regeneration of this basement membrane, in the 3 to 4 months after the capsule has been thinned by collagenase enzyme. The regenerated capsule in these areas is grossly abnormal; it contains up to 15 alternating layers of banded collagen and apparently normal capsular material. The epithelial cells within the laminated capsule, which may secrete the banded collagen, are fibroblast-like and may migrate from the lens epithelium. Banded collagen in the lens capsule has a much smaller periodicity (approximately 5.3nm) than that found in other parts of the eye and may be composed of the same type of filament as normal lens capsule.

Type IV collagen from chicken muscular tissues. Isolation and characterization of the pepsin-resistant fragments

Type IV collagen has been isolated from adult chicken gizzard after limited pepsin digestion and subsequent differential salt fractionation in acidic and neutral conditions. After denaturation, three fragments (called F1, F2, and F3) were isolated by agarose gel filtration and carboxymethylcellulose chromatography. F1 and F2 possessed apparent molecular weights of 53 000 and 50 000, respectively, and were consistently isolated in a 2:1 proportion. F3 was larger and after reduction of disulfide bonds gave rise to three fragments (called F3A, F3B, and F3C) of apparent molecular weights 68 000, 40 000, and 29 000. No alpha-chain-sized components of Type IV collagen were observed. A native fraction containing F1 and F2, but no F3, was isolated after extraction using less pepsin and an additional salt fractionation in acidic conditions. F1 and F2 in the native form were not separated by carboxymethylcellulose or diethylaminoethylcellulose chromatography performed in nondenaturating conditions or by differential salt precipitation in acidic or neutral conditions; these results suggest that F1 and F2 arise as a single native component of structure (F1)2F2. The fraction containing F1 and F2 also gave rise to a single segment long spacing crystallite pattern and to a circular dichroism spectrum which was typical for a native collagen. F1 and F2 were also isolated from chicken heart, blood vessels, and skeletal muscle, whereas from bovine aorta, using the same isolation procedures, two alpha-chain-sized components were obtained, which appeared to be similar to the two Type IV chains recently described by other groups. The data suggest that (i) pepsin fragmentation of type IV collagen from chicken tissues occurs in a different manner compared to Type IV collagen from mammalian tissues and (ii) for the chicken there must be at least two Type IV chains which are assembled into a single native molecule.

A rapid assay to measure collagen synthesis in cell cultures

Limited pepsin digestion and precipitation of resistant parts of proteins with perchloric acid on glass-fiber filters has been used as a rapid way to determine the radioactive collagen secreted into fibroblast culture media. The specificity of the pepsin cleavage was tested by digesting [14C]- or [3H]proline- and [3H]tyrosine-labeled procollagens. Radioactivities obtained with this method were comparable with those obtained with collagenase digestions or hydroxyproline determinations. Dialysis of the samples is avoided and the radioactive collagen can thus be determined from the small medium samples obtained from microtest plates. The method was used to localize a collagen synthesis-increasing factor in preparative isoelectric focusing of microphage culture media.

Type-IV collagens. Isolation and characterization of two structurally distinct collagen chains from bovine kidney cortices

Two sets of structurally distinct collagen components resembling type IV collagen have been isolated and purified from the pepsin digest of kidney cortex homogenates. C and D chains with the size of alpha chains were separated from disulfide-linked material on molecular sieves and were further purified by ion-exchange chromatography. The molecular gel filtration of the starting material also gave two high-molecular-weight collageneous fractions. The reduction of these fractions with dithiothreitol followed by carboxymethyl-cellulose chromatography resulted in the separation of two major peaks, one corresponding to C-1 and the second peak corresponding to D-1 and a 75 000-Mr components. These latter two components were further separated from each other by molecular sieve chromatography. In addition some smaller peptides (two 50 000-Mr and one 15 000-Mr peptides) were purified and analkyzed. By amino acid composition, chromatographic properties and cyanogen bromide peptide patterns on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the C-1 and C chains and one of the 50 000-Mr components appear to be derived from the same chain by pepsin cleavage at different sites. Similarly, the D1 and D chains, and the 75 000-Mr, the second 50 000-Mr and the 15 000-Mr components are related to each other but are structurally distinct from the C-1 and C components. The presented data are consistent with the presence of two structurally distinct chains in basement membrane collageneous molecules.

Characterization of collagen synthesized by normal and chemically transformed rat liver epithelial cell lines

A rat liver epithelial cell line (K16) and a 2-[N-(acetoxyacetyl)amino]fluorine-transformed K16 cell line (W8) [Weinstein, I.B., Yamaguchi, R., Gerbert, R., & Kaighn, M.E. (1975) In Vitro 11, 130-141] synthesize and secrete similar quantities of collagenous proteins. The transformed cells produce more total protein so that the percent collagen produced is decreased compared to that of the parent cell line. The type of collagen produced by the transformants differed from that of the parent cell line. After limited pepsin digestion, the pepsin-resistant collagen molecules synthesized by K16 cells precipitated at 2.6 M NaCl and contained both alpha 1 (I) and alpha 2 chains in a 2.5:1 ratio. The collagen synthesized by the W8 cells precipitated at higher salt concentrations, and no pepsin-resistant alpha 2 chains could be demonstrated by CM-cellulose chromatography or gel electrophoresis. The cyanogen bromide peptides of the W8 alpha 1 chains did not cochromatograph with type I rat skin collagen cyanogen bromide peptides on CM-cellulose chromatography, whereas the alpha 1 chains of the K16 cells did cochromatograph. The conversion of procollagen to collagen was also decreased in the transformant cells. Therefore, the chemical transformation of K16 produced transformants with increased protein synthesis and altered collagen metabolism.

Degradation of basement-membrane collagen by neutral proteases from human leukocytes

A neutral extract from human leukocytes was shown to have proteolytic activity which could degrade triple-helical basement membrane collagen from bovine lens capsules into specific triple-helical sub-fragments. The enzymes responsible is not identical to the leukocyte collagenase active against interstitial collagen types I, II and III. After denaturation, the neutral protease reaction products showed three major peptides with molecular weights of 70,000, 50,000 and 30,000, by dodecylsulphate gel electrophoresis analysis. Electron microscopic observation and viscosity measurements showed the initial twice-pepsinized collagen to be comprised of intact rod-like molecules about 300 nm long. The fragments produced by the protease were also triple-helical and were resistant to the action of trypsin at 20 degrees C. The fragments were completely degraded by purified bacterial collagenase or by raising the reaction temperature above the melting temperature of the basement membrane collagen during incubation with leukocyte extract. Enzyme activity has its pH optimum between 7 and 9, EDTA, EGTA and 1,10-phenanthroline (metal-chelating agents) completely inhibited enzyme activity, as did serum. Partial inhibition of the activity was obtained with phenylmethylsulfonyl fluoride and soybean trypsin inhibitor.

Structural basis for apparent heterogeneity of collagens in human basement membranes: type IV procollagen contains two distinct chains

Fetal cells isolated from human amniotic fluid synthesize type IV procollagen when grown in monolayer culture. The procollagen, which contains two biochemically distinct chains, was found to be structurally and immunologically related to type IV collagen chains and collagenous fragments isolated from human placenta. Limited pepsin digestion of the intact procollagen that was deposited in the cell layer during culture produced a heterogeneous population of collagenous peptides comparable to that obtained during isolation of type IV collagens from human tissues. These studies support the hypothesis that basement membranes contain at least two genetically distinct type IV procollagen chains and suggest that the heterogeneity of collagenous components obtained after pepsin digestion of tissues and isolated basement membranes can result from degradative cleavage of the procollagen at a limited number of protease-sensitive sites.

Characterization of bovine anterior-lens-capsule basement-membrane collagen. 2. Segment-long-spacing precipitates: further evidence for large N-terminal and C-terminal extensions

Collagen extracted from bovine anterior lens capsule basement membrane under minimally degradative conditions can be precipitated in the form of segment-long-spacing crystallites. Positive and negative stain preparations of these precipitates, using uranyl acetate, show the crystallites to have an asymmetric band pattern, indicating unidirectional orientation of all molecules in each crystallite. Non-banded globular masses appear at both ends of most of the crystallites, giving the segment-long-spacing aggregates the form of dumbells, with a total length of 300 nm. Repeated pepsin digestion removes first one then both globular regions leaving the helical region intact but with a length still equal to 300 nm. It thus appears that the pepsin-susceptibile protein is folded back over and covers the ends of the helical regions. Preparations with the most intact end regions tend to aggregate in fibrous segment-long-spacing form, that is, in end-interacted strings or nets. The very prominent molecular end-region interaction suggests a similar filamentous but non-periodic organization within native basement membrane.

Isolation and characterization of a native placental basement-membrane collagen and its component alpha chains

Native type IV collagen was isolated from human placenta using pepsin solubilisation followed by fractional salt precipitation and chromatogarphic purification. The native preparation was characterised using amino acid analyses, disc gel electrophoresis, segment-long-spacing crystallites and immunological methods. Two component alpha chains were isolated with molecular weights of approximately 95000 and 70000. Cyanogen bromide digests of these chains indicated that they are not related to any of the known alpha chains of interstitial collagens or to the recently described collagen containing alphaA and alphaB chains. They are also not related to one another and are therefore probably fragments of two genetically distinct type IV collagen alpha chains.