Changes in crosslinking during aging in bovine tendon collagen

The role of the tendon ECM in mechanotransduction: disruption and repair following overuse

Purpose: Tendon overuse injuries are prevalent conditions with limited therapeutic options to halt disease progression. The specialized extracellular matrix (ECM) both enables joint function and mediates mechanical signals to tendon cells, driving biological responses to exercise or injury. With overuse, tendon ECM composition and structure changes at multiple scales, disrupting mechanotransduction and resulting in inadequate repair and disease progression. This review highlights the multiscale ECM changes that occur with tendon overuse and corresponding effects on cell-matrix interactions and cellular response to load.Results: Different functional joint requirements and tendon types experience a wide range of loading profiles, creating varied downstream mechanical stimuli. Distinct ECM structure and mechanical properties within the fascicle matrix, interfascicle matrix, and enthesis and their varied disruption with overuse are considered. The pericellular matrix (PCM) comprising the microscale tendon cell environment has a unique composition that changes with overuse injury and exercise, suggesting an important role in mechanotransduction and promoting repair. Cell-matrix interactions are mediated by structures including cilia, integrins, connexins and cytoskeleton that signal downstream homeostasis, adaptation, or repair. ECM disruption with tendon overuse may cause altered mechanical loading and cell-matrix interactions, resulting in mechanobiological understimulation, apoptosis, and ineffective repair. Current interventions to promote repair of tendon overuse injuries including exercise, targeting cell signaling, and modulating inflammation are considered.Conclusion: Future therapeutics should be assessed with regard of their effects on multiscale mechanotransduction in addition to joint function, with consideration of the central role of ECM.

Extracellular matrix-derived biomaterials in engineering cell function

Extracellular matrix (ECM) derived components are emerging sources for the engineering of biomaterials that are capable of inducing desirable cell-specific responses. This review explores the use of biomaterials derived from naturally occurring ECM proteins and their derivatives in approaches that aim to regulate cell function. Biomaterials addressed are grouped into six categories: purified single ECM proteins, combinations of purified ECM proteins, cell-derived ECM, tissue-derived ECM, diseased and modified ECM, and ECM-polymer coupled biomaterials. Purified ECM proteins serve as a material coating for enhanced cell adhesion and biocompatibility. Cell-derived and tissue-derived ECM, generated by cell isolation and decellularization technologies, can capture the native state of the ECM environment and guide cell migration and alignment patterns as well as stem cell differentiation. We focus primarily on recent advances in the fields of soft tissue, cardiac, and dermal repair, and explore the utilization of ECM proteins as biomaterials to engineer cell responses.

Evolutionary origins of C-terminal (GPP)n 3-hydroxyproline formation in vertebrate tendon collagen

Approximately half the proline residues in fibrillar collagen are hydroxylated. The predominant form is 4-hydroxyproline, which helps fold and stabilize the triple helix. A minor form, 3-hydroxyproline, still has no clear function. Using peptide mass spectrometry, we recently revealed several previously unknown molecular sites of 3-hydroxyproline in fibrillar collagen chains. In fibril-forming A-clade collagen chains, four new partially occupied 3-hydroxyproline sites were found (A2, A3, A4 and (GPP)_n) in addition to the fully occupied A1 site at Pro986. The C-terminal (GPP)_n motif has five consecutive GPP triplets in α1(I), four in α2(I) and three in α1(II), all subject to 3-hydroxylation. The evolutionary origins of this substrate sequence were investigated by surveying the pattern of its 3-hydroxyproline occupancy from early chordates through amphibians, birds and mammals. Different tissue sources of type I collagen (tendon, bone and skin) and type II collagen (cartilage and notochord) were examined by mass spectrometry. The (GPP)_n domain was found to be a major substrate for 3-hydroxylation only in vertebrate fibrillar collagens. In higher vertebrates (mouse, bovine and human), up to five 3-hydroxyproline residues per (GPP)_n motif were found in α1(I) and four in α2(I), with an average of two residues per chain. In vertebrate type I collagen the modification exhibited clear tissue specificity, with 3-hydroxyproline prominent only in tendon. The occupancy also showed developmental changes in Achilles tendon, with increasing 3-hydroxyproline levels with age. The biological significance is unclear but the level of 3-hydroxylation at the (GPP)_n site appears to have increased as tendons evolved and shows both tendon type and developmental variations within a species.

Solubility study of phytochemical cross-linking agents on dentin stiffness

OBJECTIVES

The effects of interactions between cross-linking proanthocyanidins (PA) in polar solvents and type-I collagen of demineralized dentin were investigated.

METHODS

Three PA-rich extracts, two from grape seed (GSEP and GSES) and one from cocoa (COE), were dissolved (water, ethanol:water and acetone:water) and analyzed for their ability to increase the modulus of elasticity of demineralized dentin. Sound dentin beams (0.5mmx1.7mmx7mm) were fully demineralized and divided into 12 groups according to the type of cross-linking agent and solvents used. Specimens were immersed in the respective solutions and tested at baseline, 10, 30, 60, 120 and 240min.

RESULTS

The elastic modulus (EM) of dentin was significantly increased by the PA treatment regardless of time (p<0.05 for all times). The extracts showed different solubility in different solvents. GSEP showed the highest increase in EM when diluted in distilled water and acetone at all exposure times. Both GSEs showed superior results when diluted in distilled water and after 4h of treatment, while COE produced strongest enhancement when dissolved in ethanol:water.

CONCLUSIONS

The results indicates that herbal extraction process and other pharmacognostic parameters have an important influence on extract solubility as well as constitution and, consequently, on the PA-dentin matrix interaction.

Changes in Collagen With Aging Maintain Molecular Stability After Overload: Evidence From an In Vitro Tendon Model

Soft tissue injuries are poorly understood at the molecular level. Previous work using differential scanning calorimetry (DSC) has shown that tendon collagen becomes less thermally stable with rupture. However, most soft tissue injuries do not result in complete tissue rupture but in damaging fiber overextension. Covalent crosslinking, which increases with animal maturity and age, plays an important role in collagenous fiber mechanics. It is also a determinant of tissue strength and is hypothesized to inhibit the loss of thermal stability of collagen due to mechanical damage. Controlled overextension without rupture was investigated to determine if overextension was sufficient to reduce the thermal stability of collagen in the bovine tail tendon (BTT) model and to examine the effects of aging on the phenomenon. Baseline data from DSC and hydrothermal isometric tension (HIT) techniques were compared between two groups: steers aged 24–30 months (young group), and skeletally mature bulls and oxen aged greater than five years (old group). Covalent crosslinks were quantified by ion exchange chromatography. Overextension resulted in reduced collagen thermal stability in the BTT model. The Young specimens, showing detectably lower tissue thermomechanical competence, lost more thermal stability with overextension than did the old specimens. The effect on old specimens, while smaller, was detectable. Multiple overextension cycles increased the loss of stability in the young group. Compositional differences in covalent crosslinking corresponded with tissue thermomechanical competence and therefore inversely with the loss of thermal stability. HIT testing gave thermal denaturation temperatures similar to those measured with DSC. The thermal stability of collagen was reduced by overextension of the tendon—without tissue rupture—and this effect was amplified by increased cycles of overextension. Increased tissue thermomechanical competence with aging seemed to mitigate the loss of collagen stability due to mechanical overextension. Surprisingly, the higher tissue thermomechanical competence did not directly correlate with the concentration of endogenous enzymatically derived covalent crosslinking on a mole per mole of collagen basis. It did, however, correlate with the percentage of mature and thermally stable crosslinks. Compositional changes in fibrous collagens that occur with aging affect fibrous collagen mechanics and partially determine the nature of mechanical damage at the intermolecular level. As techniques develop and improve, this new information may lead to important future studies concerning improved detection, prediction, and modeling of mechanical damage at much finer levels of tissue hierarchy than currently possible.

Is otosclerosis a generalized connective tissue disorder?

It has been suggested that otosclerosis might be caused by a generalized disorder in the connective tissue.

The biophysical and biochemical properties of skin biopsies from twelve patients with otosclerosis and twelve age- and sex-matched controls were investigated. No differences were found in skin strength and extensibility, skin thickness, collagen content, the relationship between collagen type I and type III, reducible collagen cross-links and molecular stability of collagen type I of samples from patients with otosclerosis as compared with those from the controls. The present study does not support the suggestion that otosclerosis might be a generalized connective tissue disorder.

Structure and function of tuna tail tendons

The caudal tendons in tunas and other scombrid fish link myotomal muscle directly to the caudal fin rays, and thus serve to transfer muscle power to the hydrofoil-like tail during swimming. These robust collagenous tendons have structural and mechanical similarity to tendons found in other vertebrates, notably the leg tendons of terrestrial mammals. Biochemical studies indicate that tuna tendon collagen is composed of the (alpha1)(2),alpha2 heterotrimer that is typical of vertebrate Type I collagen, while tuna skin collagen has the unusual alpha1,alpha2,alpha3 trimer previously described in the skin of some other teleost species. Tuna collagen, like that of other fish, has high solubility due to the presence of an acid-labile intermolecular cross-link. Unlike collagen in mammalian tendons, no differences related to cross-link maturation were detected among tendons in tuna ranging from 0.05 to 72 kg (approx. 0.25-6 years). Tendons excised post-mortem were subjected to load cycling to determine the modulus of elasticity and resilience (mean of 1.3 GPa and 90%, respectively). These material properties compare closely to those of leg tendons from adult mammals that can function as effective biological springs in terrestrial locomotion, but the breaking strength is substantially lower. Peak tendon forces recorded during steady swimming appear to impose strains of much less than 1% of tendon length, and no more than 1.5% during bursts. Thus, the caudal tendons in tunas do not appear to function as elastic storage elements, even at maximal swimming effort.

In vitro mechanical properties of the accessory ligament of the deep digital flexor tendon in horses in relation to age

The material properties of the accessory ligament of the deep digital flexor tendon (AL) of 21 forelimbs from horses between ages one day and 15 years were determined. The force (634-11416 N), failure stress (45-138 N/mm2), failure strain (7-24%) and tangent modulus (33-1639 MPa) are presented in relation to age. Tangent modulus did not indicate changes in elasticity due to age. The results demonstrate that complete ligament failures (CLF) of ALs of older horses (mean 7835 N) occur at lower forces than ALs of young adult horses (mean 8894 N). Sudden decreases, 'dips', in the force-time curves were noticed in ligaments from foals and yearlings and in ligaments from horses > 10 years. They were interpreted as the failure of a number of fibres which either fail at lower forces or are subject to higher forces than the rest. These differences in mechanical properties could be the result of age related differences in the material properties of ALs of older horses similar to alterations in collagenous tissue in other species. When analysing the data of the proximal, middle and distal regions of the ligaments separately, higher strain and elasticity were found in the distal compared to the proximal parts. It is suggested that the clinical occurrence of desmitis of the AL of older horses could be due to fibrillar failure caused by differences in the material properties of the ligaments.

Rhodamine B inhibits collagen synthesis by human lip fibroblasts in culture

To investigate the effect of the cosmetic dye, rhodamine B, on the metabolism of collagen in fibroblasts, confluent KD cells, an established cell line of fibroblasts from human lip, were cultured for 6 h in a serum-free medium in the presence of the dye at 100 micrograms/ml and below. It was found that rhodamine B significantly decreased the content of procollagen type I C-terminal peptide antigen in both the cell layer and the medium with an only slight decrease in the cell number. Rhodamine B significantly decreased the incorporation of [3H]proline into either the collagen-digestible protein or the non-collagen protein in the cell layer. The incorporation of both [3H]thymidine and [14C]leucine into the acid-insoluble fraction of the cell layer was significantly decreased by rhodamine B; the activity of lactate dehydrogenase that leaked into the medium was not changed by the dye. From these results, it was suggested that rhodamine B has the capacity of decreasing the collagen content of the fibroblast cell layer of the human lip, which may result from a non-specific inhibition of protein synthesis without non-specific cell damage. Rhodamine B may impair the formation of extracellular matrix which is important for the maintenance of the lip tissue.

Cross-linkage sites in type I collagen fibrils studied by neutron diffraction

Cross-links in tendon collagen are essential for the biomechanical strength of healthy tissue. The nature and position of these cross-links has long been a subject for conjecture. We have approached this problem in a non-destructive manner, by studying neutron diffraction from collagen fibrils that have been specifically deuterated by reduction at keto-amine and Schiff base groups with sodium borodeuteride (NaB2H4). The intensities of the first 23 meridional reflections were recorded for both native and reduced tendons. These data were used to calculate the neutron-scattering density profile of the 67 nm (D) repeat of type I collagen fibrils in rat tail tendon. This approach not only succeeds in determining the location of the cross-linkage sites with respect to the fibril structure, as projected onto the fibre axis, but also presents a novel form of the isomorphous derivative solution to the phase problem.

Reduced strength of skin in Ehlers Danlos syndrome, type III

The biochemical properties, ratio collagen type I/type III and the pattern of reducible collagen crosslinks were studied in skin biopsy samples from 10 patients with Ehlers Danlos syndrome type III (ED III) and 10 age- and sex-matched controls. The ED III patients had marked reductions in skin strength and stiffness (42% and 22%, respectively). The ultimate extensibility of the skin, however, was not different from that of the controls. Consequently, a certain load applied to the skin of these patients results in greater deformation than in that of the controls, due to reduced stiffness. The bursting limit is no different from that of the controls. Thus the skin in the ED III patients is not hyperextensible, it merely has reduced strength and stiffness. The reduction in strength could not be explained by differences in skin thickness or collagen content. Skin thickness, collagen content and ratio collagen type I/type III were the same as in the controls, as also was the pattern of reducible collagen crosslinks. These alterations in the mechanical properties of the skin of patients with ED III may be attributable to the molecular structure of the mature collagen or the arrangement of the collagen fibres.

Pinpointing the sites of hydroxylysine glycosides in peptide alpha 1-CB7 of bovine corneal collagen, and their possible role in determining fibril diameter and thus transparency

Two cyanogen bromide fragments (alpha 1-CB7 and alpha 1-CB8) of bovine corneal stromal collagen have been isolated and characterized. These added to those characterized in our previous work account for 95% of the amino acid sequence of the alpha 1(1)-chain. The hydroxylysine glycoside content of each fragment was determined and in this way the general distribution of glycoside over the entire molecule was deduced accounting for all the galactosylhydroxylysine and most of the glucosylgalactosylhydroxylysine of this heavily glycosylated type I collagen. The characterization of fragments alpha 1-CB7 and alpha 1-CB8 has enabled us to resolve the controversy over the relative mobilities of these fragments on SDS gels. Fragment alpha 1-CB7 of bovine corneal collagen was digested by trypsin and by staphylococcal proteinase V8. The resultant peptides were isolated by gel and ion-exchange chromatography and identified in relation to the known amino acid sequence of type I collagen. The hydroxylysine glycosides were determined in the relevant peptides providing a complete account of their distribution along this part of the collagen molecule. Most of the glycoside was found in the gap region of collagen especially near the edges of the axial holes where it could act as a peg to facilitate fibre formation. In addition, some glycoside was found in the overlap region where, being unable to fit into axial holes, it might impede the growth of the fibre and, with other glycoside of the overlap region, might be responsible for the narrow fibres of corneal collagen that are essential for corneal transparency. This glycoside, with that previously found in the peptide alpha 1-CB3 is the only hydroxylysine glycoside identified in the overlap region of a type I collagen.

Increase in cross-linking of type I and type III collagens associated with volume-overload hypertrophy

Types I, III, IV, and V collagen were isolated and characterized from eight normal dog hearts and seven with volume-overload hypertrophy. Animals with volume-overload hypertrophy were killed at a time when left ventricular end-diastolic pressure and stiffness were increased. The collagens were characterized by solubility properties, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, and enzyme-linked immunosorbent assay. The percentage of collagen obtained from canine left ventricles was decreased from 32.4% in normal hearts to 15.0% in hypertrophied hearts. We attribute this to a diminution in the extractability of types I and III collagen, which fell from 199.5 mg type I/g collagen and 76.4 mg type III/g collagen in normal hearts to 83.5 mg type I/g collagen and 26.4 mg type III/g collagen in hypertrophied hearts. The amount of types IV and V collagen isolated remained constant in both the control and arteriovenous shunt hearts averaging 15.7 mg type IV/g collagen and 32.1 mg type V/g collagen in control hearts and 12.5 mg type IV/g collagen and 28.9 mg type V/g collagen in hypertrophied hearts. The reduction in quantity of types I and III collagen probably reflects a greater degree of cross-linking in these two types of collagen. Cyanogen bromide peptide analysis confirmed that there was an increase of high molecular weight cross-linked peptides from 3.96% in normal samples to 8.88% in hypertrophied samples. We conclude that cross-linking of types I and III collagen increases in volume-overload hypertrophy and that this is associated with a rise in diastolic stiffness.

Influence of cadmium on the chain dynamics of collagen in rat tail tendon

The molecular mobility of the chain dynamics of collagen was investigated by the thermally stimulated creep method on rat tail tendon after oral administration of cadmium (8 mg.kg-1.day-1) for six weeks. The high resolving power of the technique shows two manifestations of the pseudolathyrogen effect of cadmium: the polar side-chains of collagen, mobile in the immature specimen, which are cross-linked and so immobile in the mature specimen, remain mobile in the cadmium-treated mature specimen. There is also a subsequent decrease in the number of water molecules linked by two hydrogen atoms bound to the tropocollagen molecules. Probably these molecular modifications inhibit mineralization of the organic matrix and so osteogenesis.

Collagen type III deficiency in patients with rupture of intracranial saccular aneurysms

Samples of the middle cerebral artery (MCA) and the brachial artery were obtained post mortem from 14 patients who died following rupture of intracranial saccular aneurysms and from a control group of 14 age- and sex-matched patients who died of causes unrelated to aneurysm rupture. The biomechanical properties of ring-shaped arterial specimens were investigated by loading the specimens at a constant deformation rate until rupture. The relative amounts of collagen type I and type III were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies of cyanogen bromide peptides of collagen prepared from the arterial samples. A deficiency of collagen type III was demonstrated in specimens of the MCA in six of 14 patients with a ruptured intracranial saccular aneurysm. This deficiency was not accompanied by alterations in the mechanical arterial strength but resulted in a significant increase in the extensibility at stress values corresponding to blood pressures between 100 and 200 mm Hg. No difference was found between aneurysm patients and the control group in regard to the biomechanical properties of the brachial artery, despite the presence of a significant deficiency of collagen type III. The increase in vascular extensibility of the MCA may represent alterations in the fibrous structure and functional integrity of the cerebral arteries of aneurysm patients with collagen type III deficiency. Together with aggravating hemodynamic stresses, this deficiency may be an important factor in the pathogenesis of saccular aneurysms.

Mechanical properties of native and reconstituted rat tail tendon collagen upon maturation in vitro

Native and reconstituted rat tail tendon collagen were tested mechanically after in vitro maturation by incubation. The mechanical strength of the native tendons increased upon incubation and attained maximum strength values similar to those of tendons matured and aged in vivo. This finding indicates that the same stabilizing process occurs both in vivo and in vitro. However, the mechanical strength values similar to those of tendons matured and aged in vivo. This finding indicates that the same stabilizing process occurs both in vivo and in vitro. However, the mechanical strength increased at an initial higher rate in vitro than in vivo. The mechanical strength of fibrils reconstituted from purified tail tendon collagen increased during incubation in air as previously reported for fibrils prepared from skin collagen. Fibrils prepared from tail tendon and skin collagen shared common mechanical and thermal stability characteristics upon the incubation. However, distinct qualitative mechanical characteristics for fibrils of the two collagens were found. These characteristics showed a resemblance to those of the respective source tissues. The results indicate that the same process is responsible for the gain in mechanical strength of native tissues and reconstituted collagen fibrils. Thus, reconstituted collagen fibrils seem a useful model for studying mechanical stability changes during maturation of collagen.

Chemistry of the collagen cross-links. Origin and partial characterization of a putative mature cross-link of collagen

The conversion of the reducible divalent cross-links in collagen to non-reducible multivalent cross-links in mature collagen has resulted in the identification of several new amino acids as the putative mature cross-link. None of these compounds has completely satisfied the necessary criteria. We have now isolated an amino acid of high Mr, derived from lysine, that is only present in high-Mr peptides derived from mature collagen. Its increase with age of the tissue correlates with the decrease in the reducible cross-links, and it is present both in mature skin and bone, which are initially cross-linked through the aldimine and oxo-imine divalent cross-link respectively. We propose that this amino acid, as yet incompletely characterized and designated compound M, is a major cross-link of mature collagen.

Studies on EDTA extracts and collagenase digests from osteoporotic cancellous bone of the femoral head

Using EDTA extraction and collagenase digestion, cancellous bone of the femoral heads from 10 normal and 9 osteoporotic subjects were analyzed for their contents of collagen, sialoprotein, proteoglycan and carbohydrate. The percentage of extracted matrix proteins of the osteoporotic bone in EDTA was significantly decreased, as was the collagenase-resistant fraction (p less than 0.05). The sialic acid level in osteoporotic bone matrix was lower than in controls (p less than 0.05). The alterations found in bone matrix constituents in osteoporotic bone relative to controls suggest that in osteoporosis and fractures, not only bone mass changes, but also bone quality changes play a role in bone strength.

Collagen defects in lethal perinatal osteogenesis imperfecta

Quantitative and qualitative abnormalities of collagen were observed in tissues and fibroblast cultures from 17 consecutive cases of lethal perinatal osteogenesis imperfecta (OI). The content of type I collagen was reduced in OI dermis and bone and the content of type III collagen was also reduced in the dermis. Normal bone contained 99.3% type I and 0.7% type V collagen whereas OI bone contained a lower proportion of type I, a greater proportion of type V and a significant amount of type III collagen. The type III and V collagens appeared to be structurally normal. In contrast, abnormal type I collagen chains, which migrated slowly on electrophoresis, were observed in all babies with OI. Cultured fibroblasts from five babies produced a mixture of normal and abnormal type I collagens; the abnormal collagen was not secreted in two cases and was slowly secreted in the others. Fibroblasts from 12 babies produced only abnormal type I collagens and they were also secreted slowly. The slower electrophoretic migration of the abnormal chains was due to enzymic overmodification of the lysine residues. The distribution of the cyanogen bromide peptides containing the overmodified residues was used to localize the underlying structural abnormalities to three regions of the type I procollagen chains. These regions included the carboxy-propeptide of the pro alpha 1(I)-chain, the helical alpha 1(I) CB7 peptide and the helical alpha 1(I) CB8 and CB3 peptides. In one baby a basic charge mutation was observed in the alpha 1(I) CB7 peptide and in another baby a basic charge mutation was observed in the alpha 1(I) CB8 peptide. The primary defects in lethal perinatal OI appear to reside in the type I collagen chains. Type III and V collagens did not appear to compensate for the deficiency of type I collagen in the tissues.

Intermolecular cross-linking and stereospecific molecular packing in type I collagen fibrils of the periodontal ligament

A trypsin digest of denatured NaB3H4-reduced native bovine periodontal ligament was prepared and fractionated by gel filtration and cellulose ion-exchange column chromatography. Prior to trypsin digestion, a complete acid hydrolysate was subjected to analyses for nonreducible stable and reducible intermolecular cross-links. Minute amounts of the former and significant amounts of the reduced cross-links dihydroxylysinonorleucine (1.1 mol/mol of collagen), hydroxylysinonorleucine (0.9 mol/mol of collagen), and histidinohydroxymerodesmosine (0.6 mol/mol of collagen) were found. The covalent intermolecular cross-linked two-chained peptides that were isolated were subjected to amino acid and sequence analyses. The structures for the different two-chained linked peptides were alpha 1CB4-5(76-90)[Hyl-87] X alpha 1CB6-(993-22c)[Lysald-16c], alpha 1CB4-5(76-90)[Hyl-87] X alpha 1CB6(993-22c)[Hylald-16c], alpha 2CB4(76-90)[Hyl-87] X alpha 1CB6(993-22c)[Lysald-16c], and alpha 2CB4(76-90)[Hyl-87] X alpha 1CB6(993-22c)[Hylald-16c]. The cross-link in each peptide was glycosylated. This is the first characterization by sequence analysis of a cross-link involving Hyl-87 in an alpha 2 chain in collagen. A stoichiometric conversion of residue 16c aldehyde to an intermolecular cross-link in each of the COOH-terminal nonhelical peptide regions of both alpha 1 chains in a molecule of type I collagen was found. The ratio of alpha 1 to alpha 2 intermolecularly cross-linked chains involved was 3.3:1, indicating a stereospecific three-dimensional molecular packing of type I collagen molecules in bovine periodontal ligament.

Collagen and elastin changes in D-penicillamine-induced pseudoxanthoma elasticum-like skin

A patient with cystinuria who was treated with large doses of D-penicillamine for 19 years developed skin abnormalities resembling those seen in pseudoxanthoma elasticum. Biochemical and histological examination of the dermis showed that the collagen content, the ratio of the major genetic forms of collagen and the distribution of collagen types was normal. Light microscopy demonstrated the presence of vastly increased amounts of elastin in the dermis, and the individual elastin fibres were shown by electron microscopy to be abnormal; chemical analysis showed the elastin to be poorly cross-linked. Some of the collagen also appeared structurally abnormal, and biochemically resembled that seen in the dermis of a young child with respect to cross-linking and hexosyl-lysine content. The therapy led to an increased deposition of collagen and elastin fibres which appeared abnormal, and resulted in an increase in total skin surface area. These data indicate that D-penicillamine was not fully effective in inhibiting collagen and elastin cross-linking, and appeared to prevent or inhibit the natural maturation process of the collagen.

Use of galactose oxidase in labelling hydroxylysine glycosides of collagen

Commercial preparations of galactose oxidase were shown to contain a proteolytic component active against collagen and collagen CNBr peptides which could not be inhibited by reagents such as EDTA, PMSF, PCMB and TLCK nor activated by Ca++ or Mn++. 30% of the total galactose oxidase activity could be recovered free of proteolytic activity in a simple one-step combined gel filtration/affinity chromatography procedure. The biological activity of such preparations was tested with reference to collagen hydroxylysine glycosides. It was shown that over 70% of the covalently linked tritium label introduced by KB3H4 reduction after galactose oxidase treatment was not associated with galactose but was present in the same non-specifically labelled components in control experiments. The enzyme could be used in a controlled manner to label hydroxylysine glycosides in collagen by immobilising it on agarose beads. In this form it was easy to store, re-usable and retained 95% of its original activity even after prolonged (18 h) incubation with substrates.

Reduced strength of skin in osteogenesis imperfecta

The biochemical properties and ratio collagen type I/type III of skin biopsies from nine patients with osteogenesis imperfecta and nine age- and sex-matched controls were studied. Four of six patients with osteogenesis imperfecta Sillence type I had pronounced reductions in skin tensile strength, decreased ratios of collagen type I/type III, primarily accomplished by reduced amounts of collagen type I, moderate or no disability. The three patients with osteogenesis imperfecta Sillence type III had severe skeletal deformities, but normal skin tensile strength, and ratios of collagen type I/type III within the normal range. These observations may be explained as resulting from various structural defects in the type I collagen of patients with osteogenesis imperfecta.

Collagen cross-links: location of pyridinoline in type I collagen

Collagen from bone, dentine and tendon (type I), all of which contain the pyridinoline cross-link at varying levels, were each digested with CNBr. The resulting peptide mixtures were resolved by gel filtration on A1.5m agarose and assayed for pyridinoline. The polymeric cross-linked peptide complex, poly alpha 1CB6 [(1980) Biochem. J. 189, 111] isolated from each of these tissues did not contain pyridinoline. Only one peptide fraction contained the pyridinoline cross-link; that identified as alpha 2CB3,5. However, this peptide showed only a small increase in Mr in its cross-linked form (approx. 2000-5000) demonstrating that pyridinoline is not involved in the formation of polymeric structures like poly alpha 1CB6. These data, considered in the light of the recent finding that pyridinoline is present in type I collagens from different sources in widely varying amounts, cast doubt on its role in collagen maturation.

Skeletal muscle injury--molecular changes in the collagen during healing

Changes in the collagen types and cross-linking of granulation and scar tissue in the injured site of partially ruptured gastrocnemius muscle were studied after a reproducible contusion injury to the left calf of a rat. In normal i.m. collagen the proportion of Type I collagen was considerably higher than Type III. Following injury there was a rapid increase in the proportion of Type III collagen reaching a maximum at 5 days after injury. After a further 2 days the proportion of Type I had increased significantly resulting in a decrease of the Type III/I ratio to below that of the control. However, as healing progressed there was a gradual shift back to the Type III/I ratio for normal i.m. collagen. The collagen produced in response to an injury was initially stabilized by the stable keto-imine cross-link hydroxylysino-5-keto-norleucine, characteristic of embryonic collagenous tissues. The proportion of the stable keto-imine cross-link gradually decreased, and a reversion to the cross-link pattern of normal uninjured i.m. collagenous connective tissue occurred towards the end of the 42-day follow-up period. The present biochemical study demonstrates that during the early phases of the repair process there is a reversion to the collagens typically present in high proportions in embryonic dermal connective tissue. This suggests that the fibroblasts have the ability to modify their product expression under varying circumstances. The study also demonstrates the importance of collagen cross-linking in determining the tensile strength of collagen fibre during the repair process.

Evidence for glucose-mediated covalent cross-linking of collagen after glycosylation in vitro

Rabbit forelimb tendons incubated for 15 or 21 days at 35 degrees C in the presence of 8 or 24 mg of glucose/ml were shown to change their chemical, biochemical and mechanical characteristics. The tendons treated with glucose contained up to three times as much hexosyl-lysine and hexosylhydroxylysine as did control tendons as judged by assay of NaB3H4-reduced samples. Measurement of the force generated on thermal contraction showed significant increases in glycosylated tendons compared with controls, indicating the formation of new covalent stabilizing bonds. This conclusion was supported by the decreased solubility of intact tendons and re-formed fibres glycosylated in vitro, and by the evidence from peptide maps of CNBr-digested glucose-incubated tendons. The latter, when compared with peptide maps of control tendons, revealed the presence of additional high-Mr peptide material. These peptides appear to be cross-linked by a new type of covalent bond stable to mild thermal and chemical treatment. This system in vitro provides a readily controlled model for the study of the chemistry of changes brought about in collagen by non-enzymic glycosylation in diabetes.

Interferometric evaluation of collagen concentration in tendon fibers

The dry mass concentration of collagen in native rat tail tendon fibers was studied by interferometric techniques. It could be shown that the collagen concentration of rat tail tendon fibers is not constant nor does it vary uniformly with the fiber diameter. Evidence is presented that the collagen concentration shows significant oscillations as a function of diameter. In order to explain the experimental findings, a layered model is proposed for the fiber structure; age dependent changes were also studied. It seems that the fiber structure is unaltered by ageing at the supramolecular level but its functional capacities are affected and swelling is inhibited.

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.

Collagen composition of normal and myxomatous human mitral heart valves

The collagens were studied in 13 normal and 19 myxomatous human mitral valves. The collagens of the valve were completely solubilized by using a method consisting of guanidinium chloride extraction, limited pepsin digestions and CNBr cleavage of the residue. The normal valves contained 74% type I, 24% type III and 2% type V collagen. The type I and type III collagens had similar solubility patterns, although only type I collagen was detected in the guanidinium chloride extract. Type V collagen was only detected in the first pepsin extract. The type I and III collagens had higher contents of hydroxylysine than did the same collagens from age-matched dermis. The two-dimensional electrophoretic 'maps' of CNBr-cleavage peptides showed low recoveries of the C-terminal alpha 1(I) CB6 and alpha 1(III) CB9 peptides, which are involved in forming intermolecular cross-linkages. Most of the reducible cross-linkages were present in large-Mr peptide complexes, and these complexes were shown by labelling with 125I to include the tyrosine-containing alpha 1(I) CB6 peptide. The myxomatous valves contained 67% type I, 31% type III and 2% type V collagens. There was a significant increase in the concentration of each type of collagen, which consisted of a 9% increase of type I collagen, a 53% increase of type III collagen and a 25% increase of type V collagen. The contents of hydroxylysine in type I and III collagens and the electrophoretic 'maps' of the CNBr-cleavage peptides involved in cross-linkages did not differ significantly from the results obtained from the normal valves. The biochemical findings suggest that there is an increased production of collagen, in particular type III collagen, and glycosaminoglycan as well as a proliferation of cells as part of a repair process in the myxomatous valves.

Cross-linking in type IV collagen

Type IV collagen could not be extracted from human placenta using 6M-urea containing 10mM-dithiothreitol, indicating that the type IV molecule is stabilized within the basement membrane by covalent cross-links. However, various forms of type IV collagen molecule were extractable by means of increasingly severe proteolytic conditions. Type IV collagen tetramers ('spiders') lacking only the C-terminal globular region (NC1) were further digested to the 'long-form' 7S fragment and an assortment of helical rod-like molecules derived from the 'leg' region. These molecules were separated by salt fractionation and examined by rotary-shadowing electron microscopy. Isolation of these fractions from placenta which had been reduced with NaB3H4 revealed that both the 7S (N-terminal) and C-terminal regions contained significant proportions of reducible lysine-derived cross-links. These cross-links were shown to be exclusively the stable oxo-imine hydroxylysino-5-oxonorleucine. The number of the cross-links per molecule was significantly lower than might be expected in order to fully stabilize the molecule. These results suggest that the keto-imine cross-links in type IV collagen have been stabilized in part by conversion into an unknown non-reducible form, although a sensitive immunoassay failed to show the presence of any pyridinoline. Comparison with the fibrous collagen from placenta suggested that the rate of this conversion is much greater in basement-membrane collagen.

Metabolism of rat skin collagen: the effect of arrested development and subsequent catch-up growth on the stability of the connective tissue fibers

The growth of 33 day-old rats was temporarily arrested by feeding a low-protein diet for 6 weeks. The skin collagen hydroxyproline was labelled previously at 31 days by giving a single intraperitoneally injection of L-[5-3H] proline. This label was rapidly incorporated during the subsequent 2 days of normal growth and then remained at a steady level in the skin collagen when growth was halted. Control animals continued to incorporate label for 2 weeks and then the level of [3H] hydroxyproline declined as this label was subsequently lost from the skin collagen. During arrested development, the skin collagen became increasingly insoluble and the analysis of reducible crosslinks revealed a pattern similar to the collagen of an older animal. In addition the isometric tension developed per unit mass of collagen during thermal denaturation was three-fold greater than that from the age-matched control animals. Refeeding the normal diet gave an immediate burst of rapid growth. The pre-labelled collagen did not turnover during this period since there was no significant change in the level of labelled hydroxyproline in the skin. The newly synthesized collagen, which diluted out these older fibers, appeared to be immature, since it was initially rich in type III, it was very poorly crosslinked, and it caused a fall in the isometric tension value. It was concluded that there need not be limited degradation of the older collagen fibers in skin as an a priori requirement for growth.

Connective tissue composition of the equine sarcoid

The connective tissue composition and organisation of the "equine sarcoid" was compared with that of normal adult equine skin to determine whether the cells which produce their respective connective tissue matrices show similar biosynthetic characteristics. No major qualitative difference could be found between the collagen compositions of skin and sarcoid material, although the organisation into fibres of Type III collagen in the sarcoid was markedly greater than that of skin.

Age-related changes in rat glomerular basement membrane

1. The yield of GBM from rat kidney increased with rat age and the membrane became more collagen-like, as judged by the glycine, hydroxyproline, hydroxylysine content and the degree of glycosylation. 2. GBM became progressively less soluble in sodium dodecyl sulphate (SDS) and 2-mercaptoethanol with age. An increase in the lower molecular weight subunits of membranes prepared from older rats was demonstrated by polyacrylamide gel electrophoresis.

Composition, cross-linking and thermal stability of bone and skin collagens in patients with osteogenesis imperfecta

The composition, cross-linking, and thermal stability of the collagens were determined in bone and skin biopsies from 4 patients with moderate to severe forms of osteogenesis imperfecta (OI). The major modification observed with respect to control subjects was an overhydroxylation of lysine in type I bone collagen (hydroxylysine content doubled in three patients and increased by 50% in the last patient). This overhydroxylation is confirmed by a similar increase in the dihydroxylated cross-link of bone collagen: the dehydrodihydroxylysinonorleucine. The type II collagen from cartilage and the pepsin-soluble collagens from the skin of these patients contained the normal amount of hydroxylysine. A small amount of type III collagen has been found in three patients, while only in one patient a slight increase in the type III/type I collagen ratio was observed in skin. In all patients the thermal stability of the collagen triple helix, measured by differential scanning calorimetry, was normal in both bone and skin. Although in at least three patients the clinical features allowed us to classify our patients into two different groups (Sillence et al., 1979 classification--groups I and III), the biochemical results are similar, suggesting that the overhydroxylation of the lysine in type I bone collagen is a common feature of severe forms of osteogenesis imperfecta.

Analysis of the heterogeneity of human collagens by two-dimensional polyacrylamide-gel electrophoresis

The heterogeneity of the CNBr-cleavage peptides of human types I, II, III and V collagens were studied by using two-dimensional electrophoresis combining non-equilibrium pH-gradient-gel electrophoresis and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Specific 'maps' were produced by the peptides obtained from the chains of each type of collagen, and most peptides had at least three charged forms of the same molecular weight. Specific 'maps' were also produced by the peptides of types I, III and V collagens from insoluble dermis and the peptides of types I and V collagens from decalcified bone. The alpha 1(I) CB7 and alpha 1(I) CB8 and the alpha 2 CB4 peptides obtained from the type I collagens of these tissues contained the same number of charged components, but there was a relative increase in the more basic components in bone. Some aspects of the involvement of the alpha 1(I) CB6 and the alpha 1(III) CB9 peptides in cross-linkages were also studied. The recovery of the alpha 1(I) CB6 peptide from bone and dermis was decreased and the alpha 1(III) CB9 peptide was not detected in dermis. Additional peptides, which were probably cross-linked peptides involving the alpha 1(I) CB6 peptide, were also observed.

Thermal stability of reconstituted collagen fibrils. Shrinkage characteristics upon in vitro maturation

Thermal stability measured as area shrinkage without tension during heating was determined for membranes of collagen fibrils reconstituted from solutions of highly purified rat skin collagens. Shrinkage in per cent of area at 25 degrees C and shrinkage temperature were quantitated in a standardized way and determined as a function of in vitro maturation time for 11 to 104 days after aggregation for the collagen membranes. Similar to reports on intact rat skin, shrinkage temperature remained constant and shrinkage per cent declined with a rate decreasing with time during maturation. Solubility in water at 80 degrees C for 2 hours was 95-96% and remained unchanged for the maturation time (about 2 months) studied. The decreased shrinkage reflecting a lower degree of collapse is ascribed to an increasing thermal stability of the membranes during maturation. Development of heat-stable bonds in the reconstituted collagen fibrils is taken up to be amenable to this increased stability. Similarity in changes of shrinkage characteristics during in vivo and in vitro maturation indicates that maturation changes in reconstituted collagen fibrils reflect those occurring in intact collagen during in vivo aging.

Location of an intermolecular crosslink in bovine bone collagen

A peptide containing 59 amino acid residues with a stoichiometric amount of dihydroxylysinonorleucine (0.7 mole) and hydroxylysinonorleucine (0.2 mole) was isolated from reduced 3H labelled bovine bone collagen sequentially cleaved with CNBr and trypsin. Further cleavage of the isolated crosslinked peptide with periodate yielded a radioactive peptide of 45 residues and a non-radioactive peptide of 16 residues. From the characteristic amino acid composition of these peptides it was deduced that the peptide was derived from an intermolecularly crosslinked region between lysyl or hydroxylysl residues in the carboxy-terminal extension of alpha 1-CB6 (17C residue) and alpha 1-CB5 (87th residue). This finding supports the observation that the alpha 1-CB6 peak was prominent on carboxymethyl cellulose chromatography of the CNBr digest of bone collagen only after limited pepsin digestion, and is consistent with the results obtained from a smaller crosslinked peptide previously isolated from calf bone collagen.

Mechanical properties of reconstituted collagen fibrils. A study on reconstitution methodology and influence of in vitro maturation

Acid extracted collagen from rat skin was reconstituted into fibrils at physiological pH and ion concentration. The fibrils were indistinguishable from native collagen in ultrastructural appearance. This report describes a procedure for reconstituting collagen fibrils into thin membranes which allows the preparation of a set of collagen membranes with identical mechanical properties. Reconstituted collagen fibrils showed a gain in mechanical strength upon maturation in vitro similar to the increase in mechanical strength known to occur during maturation of collagenous tissues in vivo. It is suggested that the changes in the mechanical properties of reconstituted collagen fibrils during in vitro maturation may reflect the changes observed during in vivo maturation of collagenous tissues. The possible influence of the amounts of reducible intermolecular cross-links on the gain in strength during maturation is discussed.

Polymeric C-terminal cross-linked material from type-I collagen. A modified method for purification, anomalous behaviour on gel filtration, molecular weight estimation, carbohydrate content and lipid content

Polymeric cross-linked C-terminal peptide material (poly-alpha 1CB6) from mature bovine tendon type-I collagen was prepared and purified by a modification of the method previously described [Light & Bailey (1980) Biochem. J. 185, 373-381]. Poly-alpha 1CB6 was shown to exhibit concentration-dependent aggregation effects on gel filtration due to interaction with a filtration medium. The material had an amino acid content that was very similar to a mixture of alpha 1CB6 and alpha 1CB5. The material was shown to be polydisperse with a mol.wt. range of 50 000-350 000, but chromatographic fractions were relatively homogeneous over this molecular weight range with respect to amino-acid composition. The heterogeneity of the material was not due to incomplete CNBr peptide cleavage, as poly-alpha 1CB6 did not contain detectable quantities of methionine. The material showed no discrete bands on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis but gave a constant blue stain throughout the molecular weight range described above. Lipid analysis showed that the partially purified material contained elevated levels of stearate when compared to the crude CNBr-digested starting material. This may indicate the specific association of a stearic-acid-rich lipid with the peptide material. On carbohydrate analysis poly-alpha 1CB6 was shown to contain only galactose and glucose at levels of 0.72 and 0.28% respectively. The carbohydrate and amino acid analyses indicated that (alpha 1CB6)2-(alpha 1CB5)1 may be the basic cross-linked structural unit of poly-alpha 1CB6)2-(alpha 1CB5)1 units, although the carbohydrate analysis indicated that the higher molecular weight oligomers may be enriched in alpha 1CB6.

An investigation of pyridinoline, and putative collagen cross-link

A component, termed pyridinoline, has been reported to be derived from 'lysine aldehyde' (2,6-diaminohexanaldehyde) and designated as the stable cross-link of mature collagen. Commerically prepared collagen and freshly obtained mature bovine tendon collagen were both investigated with regard to their pyridinoline content. Both sources of material could be depleted of this component by mild washing procedures. Pepsin-solubilized collagen and peptides derived from CNBr cleavage of intact collagen did not contain the compound. Pure pyridinoline was isolated and shown to be hydrolysed by water, as previously reported, but neither hydroxylysine nor lysine could be ds not a cross-linking component of collagen.