Collagen-binding domain within bovine propolypeptide of von Willebrand factor

Two reduced/alkylated fragments of bovine propolypeptide of von Willebrand factor (pp-vWF) that inhibit pp-vWF binding to collagen were isolated. One is a tryptic fragment of molecular mass of about 30 kDa and inhibits the binding at a molar concentration about 20 times higher than the intact pp-vWF. Amino acid sequence of this fragment was determined almost completely, and it was revealed that this fragment corresponded to the carboxyl-terminal region of pp-vWF molecule beginning with Phe557. The other active fragment was obtained by lysyl endopeptidase digestion. This migrated as a 21.5/21-kDa doublet in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but deglycosylation of this doublet resulted in production of single species of 19 kDa. The difference between the doublet constituents, therefore, was of carbohydrate composition. The extent of inhibition of collagen-binding by this 21.5/21-kDa fragment was comparable to that of the 30-kDa fragment, and furthermore, location of this fragment in the molecule was established to be between Phe570 and Lys682. These were the only fragments among those obtained by proteolytic digestions that had significant competitive effect on the binding of intact pp-vWF to collagen. These results strongly suggest that at least one collagen-binding site should be present in the carboxyl-terminal region of bovine pp-vWF extending from residue 570 to 682.     

A collagen-binding glycoprotein from bovine platelets is identical to propolypeptide of von Willebrand factor

Several proteins from bovine platelet lysate bound to type I collagen immobilized to the beads of formyl derivatives of cellulose. Among these proteins, a protein of about 100,000 daltons was purified to homogeneity by two additional affinity chromatographies, an organomercurial-agarose and a lentil lectin-agarose. This protein consisted of a single polypeptide chain which contains carbohydrate moiety and many intrapolypeptide disulfide bridges. In addition to platelets, this protein was present in plasma and cultured endothelial cells but not in red blood cells, leukocytes, and smooth muscle cells. Furthermore, it was released from platelets upon stimulation by various agonists. The purified 100-kDa protein was labeled with 125I to quantitate its binding to fibrillar type I collagen. The protein specifically bound to fibrillar collagen with the apparent dissociation constant of 5.6 x 10(-8) M for the high affinity site and 5.5 x 10(-7) M for the low affinity site. Analyses of amino acid sequences of both intact and tryptic fragments of this protein revealed that it had strong homology to the propolypeptide of human von Willebrand factor, which is also known as von Willebrand antigen II. Various properties of this protein listed above also strongly suggest that it was indeed the propolypeptide of bovine von Willebrand factor.     

Inhibition of platelet-collagen interaction by propolypeptide of von Willebrand factor

A collagen-binding glycoprotein was isolated from human platelets using affinity chromatography of immobilized collagen. Based upon characterizations of this protein we confirmed that it was identical to the propolypeptide of von Willebrand factor (pp-vWF), which is also called von Willebrand antigen II. The characteristics we have investigated are molecular weight, existence of carbohydrate chains, and the NH2-terminal amino acid sequence. pp-vWF has strong affinity to collagen and inhibits collagen-induced aggregation of human platelets at a concentration as low as 2 micrograms/ml even in the presence of plasma. This inhibitory effect is specific for collagen-induced aggregation since it does not inhibit aggregation of platelets induced by other agonists such as ADP, arachidonic acid, platelet-activating factor, ionophore A23187, and ristocetin. As pp-vWF is quickly released from platelets upon activation by various agonists, it is possible that pp-vWF functions as a repressor for excess platelet aggregation induced by collagen and constitutes a negative feed-back mechanism. Considering the fact that mature vWF supports platelet adhesion to subendothelium, present observations suggest that the propeptide portion and the mature protein could have opposing effects on hemostasis.     

Immunological detection of propolypeptide of von Willebrand factor on platelet surface

Recently we have found that propolypeptide of von Willebrand factor (pp-vWF) obtained from platelets binds to type I collagen. It is known that pp-vWF is present in platelet alpha-granules and is secreted upon activation. In this paper, we demonstrate the two following evidences to show that it is also present on the surface of resting platelets. [1] The antibody against pp-vWF bound to the surface of platelets. [2] The antibody induced aggregation of platelets. The binding of the antibody and the antibody-induced aggregation of platelets were inhibited in a dose-dependent manner by Fab fragment of the antibody. Platelets from von Willebrand disease patients bound less of the antibody and responded weakly to the antibody.     

A collagen/gelatin-binding decapeptide derived from bovine propolypeptide of von Willebrand factor.

Propolypeptide of von Willebrand factor (pp-vWF) binds to type I collagen, and we have reported that a binding domain exists in a 21.5/21-kDa fragment originated from the C-terminal portion [Takagi, J., Fujisawa, T., Sekiya, F., & Saito, Y. (1991) J. Biol. Chem. 266, 5575-5579]. The collagen-binding property of the 21.5/21-kDa fragment was compared with that of the intact pp-vWF. Although pp-v WF preferentially binds to native type I collagen fibrils, the isolated fragment no longer has this specificity and binds well to collagen of other types in the native and heat-denatured states. In order to determine the critical site that mediates this collagen/gelatin binding, several peptides were synthesized based on the primary structure of the 21.5/21-kDa fragment. Among these, a 25-residue peptide strongly inhibited the binding of the 125I-labeled 21.5/21-kDa fragment to collagen. Using this inhibitory effect as an index, the binding site was defined to the sequence as follows: WREPSFCALS. Furthermore, a decapeptide of this sequence bound to collagen and gelatin, indicating that this sequence is responsible for the binding of the 21.5/21-kDa fragment to collagen/gelatin.     

Purification and characterization of proteins,including procollagen type III,in salt extracts of fetal bovine skin

The nature of high-molecular-weight proteins in salt extracts of fetal bovine skin was investigated. A series of DEAE cellulose ion-exchange columns separated the mature collagen from the high molecular weight proteins and also separated the high molecular weight proteins from each other. The following proteins were isolated: (a) a very high molecular weight protein which appears to be aggregated mature collagen; (b) two high molecular weight proteins of slightly faster mobility on SDS polyacrylamide gels, one of which is collagen-like and one of which is not; and (c) a type III procollagen, purer than those previously reported in the literature. These latter three proteins were characterized by amino acid analysis, SDS polyacrylamide gel electrophoretic mobility, collagenase sensitivity, and CNBr peptide patterns from SDS-PAGE.     

Immunological identification of types I and III collagen in bovine lens epithelium and its anterior lens capsule

To define the molecular structure of bovine lens epithelium and its anterior lens capsule, we investigated the composition of lens capsule basement membrane proteins. Immunofluorescence and immunogold techniques were used to demonstrate the presence of type I and type III collagen in the lens capsule and in primary explant epithelial cultures grown on protein-binding membranes. Immunofluorescence staining with specific antibodies indicated that type I and type III collagen were constituents of lens basement membrane. We observed that deposition of type III collagen was more than type I collagen. The synthesis of fibrillar collagen by lens epithelium and its deposition in the lens capsule was established by localization of fibrillar collagen by transmission immunoelectron microscopy. These results demonstrate for the first time that normal lens epithelium synthesize fibrillar collagen which is an intrinsic component of the anterior lens capsule basement membrane.     

Interactions between bovine cornea proteoglycans and collagen.

Two types of proteoglycan subunits were obtained from bovine cornea, the first mainly composed of proteochondroitin sulphate and the second of proteokeratan sulphate. These two fractions can be obtained from the tissue as an aggregate, and are able to recombine each other after separation, to re-form the original structure. In order to investigate collagen-proteoglycan interactions, type-I collagen was isolated from bovine cornea by pepsin digestion followed by 3.5% (w/v) NaCl precipitation, and was then linked to CNBr-activated Sepharose 4B. Two identical columns were prepared, the first filled with collagen coupled to Sepharose 4B, the second with free Sepharose 4B. The two proteoglycan subunits and the aggregate were chromatographed on the two gels under the same conditions; the elution profiles showed that both the aggregate and the proteochondroitin sulphate subunit are retarded by the collagen coupled to Sepharose. No interaction, however, occurred when proteokeratan sulphate subunit was run through the columns. Chondroitinase digestion of the proteoglycan samples confirmed that chondroitin sulphate chains are mainly responsible for the interaction with collagen; their removal, in fact, completely abolishes any differences between the chromatographic behaviour on the collagen-Sepharose and the control columns.     

Morphological differentiation of endothelial cells co-cultured with astrocytes on type-I or type-IV collagen

Summary In this study bovine aortic endothelial cells were co-cultured with astrocytes from fetal Wistar Kyoto rats. Endothelial cells growing on type-I collagen, development. Although some cells appeared to be mature, horseradish peroxidase penetrated within 1 min of incubation through the intercellular junctions of these endothelial elements maintained on type-I collagen. In contrast, endothelial cells on type-IV collagen, co-cultured with astrocytes, were well developed; their intercellular junctions were well established, and plasmalemmal vesicles reduced in number. As a result, horseradish peroxidase was unable to penetrate through the endothelial cells grown on type-IV collagen and co-cultured with astrocytes because of the reduced extent of the junctional and vesicular transport. These findings reveal that (1) type-IV collagen is essential for the differentiation of endothelial cells, (2) endothelial cell-astrocyte interactions occur during co-culture, and (3) endothelial permeability depends on astrocyte-produced factors, in addition to type-IV collagen.     

Binding of bovine lactoferrin to Streptococcus agalactiae

Bovine lactoferrin is an iron-binding protein present in mammary gland secretions. The exposure of Streptococcus agalactiae to bovine lactoferrin resulted in the binding of this protein to all the 12 strains of bovine origin tested, and also, although to a lesser degree, to the five tested strains of human origin. The interaction of lactoferrin with one high-binding bovine strain (24/60, the prototype NT/X strain) was studied. Binding was time-dependent, dose-dependent, and saturable. The binding of lactoferrin was slightly affected by cultivation conditions, and appeared to be heat-stable. The binding of biotinylated lactoferrin was inhibited by unlabelled lactoferrin but not by bovine serum albumin.     

Isolation and chemical characterization of collagen in bovine pulmonary tissues.

1. The contents of the fibrous proteins collagen and elastin in the pleural and parenchymal regions of bovine lungs were determined. The collagen content was approx. 70% (g/100g of salt-extracted defatted powder) in each tissue, and the elastin content was 28% in pleura and 13.5% in parenchyma. 2. Purification of the insoluble collagen from the pleura and parenchyma of bovine lungs by various methods was attempted. The collagen fractions isolated after incubation of the pulmonary tissues with the proteolytic enzymes collagenase ("collagenase-soluble" fraction) or pancreatic elastase ("elastase-insoluble" fraction) each contained approx. 87% of the total collagen initially present. 3. Both collagen fractions were chemically analysed for their amino acid and carbohydrate contents and were found to be similar to those of the intact interstitial collagens isolated from skin, bone and tendon. 4. The contents of the two aldimine cross-linking compounds, dehydrohydroxylysinonorleucine and dehydrodihydroxylysinonorleucine, were determined in the bovine pulmonary collagen fractions, and were found to decrease with increasing age of the animals, and were similar to the values found in intact collagens from bone and tendon.     

Electrostatic and non-electrostatic contributions of proteoglycans to the compressive equilibrium modulus of bovine articular cartilage

This study presents direct experimental evidence for assessing the electrostatic and non-electrostatic contributions of proteoglycans to the compressive equilibrium modulus of bovine articular cartilage. Immature and mature bovine cartilage samples were tested in unconfined compression and their depth-dependent equilibrium compressive modulus was determined using strain measurements with digital image correlation analysis. The electrostatic contribution was assessed by testing samples in isotonic and hypertonic saline; the combined contribution was assessed by testing untreated and proteoglycan-depleted samples.Though it is well recognized that proteoglycans contribute significantly to the compressive stiffness of cartilage, results demonstrate that the combined electrostatic and non-electrostatic contributions may add up to more than 98% of the modulus, a magnitude not previously appreciated. Of this contribution, about two thirds arises from electrostatic effects. The compressive modulus of the proteoglycan-depleted cartilage matrix may be as low as 3 kPa, representing less than 2% of the normal tissue modulus; experimental evidence also confirms that the collagen matrix in digested cartilage may buckle under compressive strains, resulting in crimping patterns. Thus, it is reasonable to model the collagen as a fibrillar matrix that can sustain only tension. This study also demonstrates that residual stresses in cartilage do not arise exclusively from proteoglycans, since cartilage remains curled relative to its in situ geometry even after proteoglycan depletion. These increased insights on the structure–function relationships of cartilage can lead to improved constitutive models and a better understanding of the response of cartilage to physiological loading conditions.     

Evidence for Cell Surface Extracellular Matrix Binding Proteins in Hydra vulgaris

The present study was designed to identify and functionally characterize potential cell surface extracellular matrix binding proteins in Hydra vulgaris. Using [³H]-laminin as a probe, radioreceptor analysis of a dissociated mixed hydra cell preparation indicated that the average number of laminin binding sites per cell was about 10,000 with a dissociation constant of 1.49 nM. These binding sites could be displaced with unlabelled laminin in a dose-dependent manner and with high concentrations (500 nM) of unlabelled fibronectin. No displacement with type-IV collagen and type-I collagen was observed. Immunoscreerting studies with a battery of antibodies raised to mammalian extracellular matrix (ECM) binding proteins indicated potential cell surface binding sites for the anti-β₁ integrin monoclonal antibody, mAb JG22. Cell adhesion studies indicated that mAb JG22 blocked binding of hydra cells to laminin, but did not affect their binding to fibronectin, type-IV collagen, or type-I collagen. Light and electron microscopic immunocytochemical studies indicated that mAb JG22 localized to the basal plasma membrane of ectodermal and endodermal epithelial cells. Immunoprecipitation studies identified two major bands with masses of about 196 kDa and 150 kDa under reducing conditions, and two bands with masses of >200 kDa under non-reducing conditions. Functional studies indicated that mAb JG22 could reversibly block morphogenesis of hydra cell aggregates, and could block in vivo interstitial cell migration in hydra grafts. These observations indicate that hydra has cell surface binding sites for ECM components which are functionally important during development of this simple Cnidarian     

Distinct maturations of N-propeptide domains in fibrillar procollagen molecules involved in the formation of heterotypic fibrils in adult sea urchin collagenous tissues

We have characterized the primary structure of a new sea urchin fibrillar collagen, the 5alpha chain, including nine repeats of the sea urchin fibrillar module in its N-propeptide. By Western blot and immunofluorescence analyses, we have shown that 5alpha is co-localized in adult collagenous ligaments with the 2alpha fibrillar collagen chain and fibrosurfin, two other extracellular matrix proteins possessing sea urchin fibrillar modules. At the ultrastructural level, the 5alpha N-propeptide is detected at the surface of fibrils, suggesting the retention of this domain in mature collagen molecules. Biochemical characterization of pepsinized collagen molecules extracted from the test tissue (the endoskeleton) together with a matrix-assisted laser desorption ionization time-of-flight analysis allowed us to determine that 5alpha is a quantitatively minor fibrillar collagen chain in comparison with the 1alpha and 2alpha chains. Moreover, 5alpha forms heterotrimeric molecules with two 1alpha chains. Hence, as in vertebrates, sea urchin collagen fibrils are made up of quantitatively major and minor fibrillar molecules undergoing distinct maturation of their N-propeptide regions and participating in the formation of heterotypic fibrils.     

Protein L-isoaspartyl methyltransferase repairs abnormal aspartyl residues accumulated in vivo in type-I collagen and restores cell migration

Abnormal aspartyl residue formation such as L-isoaspartates occurs frequently during aging in long-lived proteins, resulting in the alteration of their structures and biological functions. In this study, we investigated the alteration of aspartyl residues in extracellular matrix (ECM) proteins, type-I collagen and fibronectin, and in integrin- and ECM-binding motifs during aging, as well as the resulting effects on cell biological functions such as migration and attachment. Using protein L-isoaspartyl methyltransferase (PIMT) to monitor the presence of L-isoaspartyl residues, we showed their accumulation during in vivo aging in type-I collagen from rats. In vitro aging of fibronectin as well as of peptides containing an integrin- or ECM-binding motif such as RGDSR, KDGEA and KDDL also resulted in the formation of L-isoaspartyl residues. While aged fibronectin does not alter cell adhesion and migration, type-I collagen aged 20 months reduced by 65% cell motility, but not adhesion, when compared to 3-month-aged type-I collagen. Finally, by repairing 20-month-old type-I collagen with recombinant PIMT (rPIMT), cell migration was recovered by 72%. These results strongly suggest that L-isoaspartyl residue formation in ECM proteins such as type-I collagen could play an important role in reducing cell migration and that PIMT could be a therapeutic tool to restore normal cell migration in pathological conditions where cell motility is crucial.     

Cellular topoisomerase I modulates origin binding by bovine papillomavirus type 1 E1

In addition to viral proteins E1 and E2, bovine papillomavirus type 1 (BPV1) depends heavily on host replication machinery for genome duplication. It was previously shown that E1 binds to and recruits cellular replication proteins to the BPV1 origin of replication, including DNA polymerase alpha-primase, replication protein A (RPA), and more recently, human topoisomerase I (Topo I). Here, we show that Topo I specifically stimulates the origin binding of E1 severalfold but has no effect on nonorigin DNA binding. This is highly specific, as binding to nonorigin DNA is not stimulated, and other cellular proteins that bind E1, such as RPA and polymerase alpha-primase, show no such effect. The stimulation of E1's origin binding by Topo I is not synergistic with the stimulation by E2. Although the enhanced origin binding of E1 by Topo I requires ATP and Mg2+ for optimal efficiency, ATP hydrolysis is not required. Using an enzyme-linked immunosorbent assay, we showed that the interaction between E1 and Topo I is decreased in the presence of DNA. Our results suggest that Topo I participates in the initiation of papillomavirus DNA replication by enhancing E1 binding to the BPV1 origin.     

In vitro multimerization of von Willebrand factor is triggered by low pH. Importance of the propolypeptide and free sulfhydryls

Large von Willebrand factor (vWf) multimers are assembled by the formation of disulfide bonds between dimers in trans Golgi and post-Golgi compartments. We were able to reproduce this process in a cell-free system using purified vWf dimers. The multimers formed in vitro extended to 5 x 10(6) Da and were similar in size distribution to those secreted constitutively by endothelial cells in culture. Multimerization occurred only at acidic pH with an optimum at pH 5.8 and needed the continued presence of an acidic pH for it to proceed. Pro-vWf dimers multimerized, whereas mature dimers failed to assemble into large multimers. Multimerization required the propolypeptide to be a contiguous part of pro-vWf subunits since free propolypeptide did not promote multimerization of mature dimers. In addition, multimers formed in the presence of both pro-vWf and mature vWf dimers incorporated only pro-vWf dimers. Two out of six available monoclonal antibodies to the prosequence inhibited multimerization. Multimerization was also inhibited by chemical blocking of free sulfhydryl(s). The free sulfhydryl(s) were localized to the mature region of the pro-vWf by examination of the mature subunit and the propolypeptide derived by proteolytic cleavage of pro-vWf subunits.     

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.     

Characterization of pepsin-solubilized bovine heart-valve collagen.

Collagens extracted from heart valves by using limited pepsin digestion were fractionated by differential salt precipitation. Collagen types were identified by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, amino acid analysis and cleavage with CNBr. Heart-valve collagen was heterogeneous in nature, consisting of a mixture of type-I and type-III collagens. The identity of type-III collagen was established on the basis of (a) insolubility in 1.7 M-NaC1 at neutral pH, (b) behaviour of this collagen fraction on gel electrophoresis under reducing and non-reducing conditions, (c) amino acid analysis showing a hydroxyproline/proline ratio greater than 1, and (d) profile of CNBr peptides on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis showing a peak characteristic for type-III collagen containing peptides alpha1(III)CB8 and alpha1(III)CB3. In addition to types-I and -III collagen, a collagen polypeptide not previously described in heart valves was identified. This polypeptide represented approx. 30% of the collagen fraction precipitated at 4.0 M-NaCl, it migrated between beta- and alpha1-collagen chains on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and its electrophoretic behaviour was not affected by disulphide-bond reduction. All collagen fractions from the heart valves contained increased amounts of hydroxylysine when compared with type-I and -III collagens from other tissues. The presence of beta- and gamma-chains and higher aggregates in pepsin-solubilized collagen indicated that these collagens were highly cross-linked and suggested that some of these cross-links involved the triple-helical regions of the molecule. It is likely that the higher hydroxylysine content of heart-valve collagen is responsible for the high degree of intermolecular cross-linking and may be the result of an adaptive mechanism for the specialized function of these tissues.     

Molecular composition and function of integrin-based collagen glues—Introducing COLINBRIs

Background

Despite detailed knowledge about the structure and signaling properties of individual collagen receptors, much remains to be learned about how these receptors participate in linking cells to fibrillar collagen matrices in tissues. In addition to collagen-binding integrins, a group of proteins with affinity both for fibrillar collagens and integrins link these two protein families together. We have introduced the name COLINBRI (COLlagen INtegrin BRIdging) for this set of molecules. Whereas collagens are the major building blocks in tissues and defects in these structural proteins have severe consequences for tissue integrity, the mild phenotypes of the integrin type of collagen receptors have raised questions about their importance in tissue biology and pathology.

Scope of review

We will discuss the two types of cell linkages to fibrillar collagen (direct- versus indirect COLINBRI-mediated) and discuss how the parallel existence of direct and indirect linkages to collagens may ensure tissue integrity.

Major conclusions

The observed mild phenotypes of mice deficient in collagen-binding integrins and the relatively restricted availability of integrin-binding sequences in mature fibrillar collagen matrices support the existence of indirect collagen-binding mechanisms in parallel with direct collagen binding in vivo.

General significance

A continued focus on understanding the molecular details of cell adhesion mechanisms to collagens will be important and will benefit our understanding of diseases like tissue- and tumor fibrosis where collagen dynamics are disturbed. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.