Bovine tendons. Aging and collagen cross-linking

Mechanical and functional properties of the equine superficial digital flexor tendon

The in vitro and in vivo mechanical properties of the superficial digital flexor tendon have been described. To date the focus has been on single load to failure testing, however refined in vivo methods may prove useful to evaluate the effects of treatment and exercise on tendons. During maximal exercise, the adult superficial digital flexor tendon operates close to its functional limits with a narrow biomechanical safety margin. This combined with exercise and age associated microdamage, and a limited adaptive ability may increase the risk of fatigue failure. Studies evaluating treatment regimens for tendonitis have focused on repair and regeneration and yielded varying results. It would appear that the superficial digital flexor tendon has a limited ability if any to adapt positively to exercise after maturity. In contrast, the foal's superficial digital flexor tendon may have a greater adaptive ability and may respond to an appropriate exercise regimen to produce a more functionally adapted tendon. Recent studies have shown that foals allowed free pasture exercise develop a larger, stronger, more elastic tendon compared to foals that were confined or subjected to a training program. Effects on the non-collagenous matrix appear to be responsible for these differences. In contrast, training or excess exercise may have permanent detrimental effects on the biomechanical and functional properties of the superficial digital flexor tendon in the foal. The implication is that the determination of optimum exercise intensity and timing, and the role of the non-collagenous matrix in tendon physiology in the young horse may hold the key to developing tendons more capable of resisting injury.

Progressive adult-onset emphysema in transgenic mice expressing human MMP-1 in the lung

Mice with lung-specific expression of human matrix metalloproteinase-1 (MMP-1) develop emphysematous changes similar to those seen in smoking-induced emphysema in humans. Morphometric analyses of three transgenic lines [homozygous colony (Col) 34, Col 50, and Col 64] with varying temporal expression of MMP-1 were undertaken to determine the validity of this animal as a model of adult-onset emphysema. Line 50 mice, which have early expression of MMP-1 (14 days postconception), exhibited morphometric changes by 5 days of age. In contrast, homozygous line 34 and 64 with delayed expression (birth and 2 wk of age) were normal up until 4 wk of age when progressive changes in their mean linear intercept were first noted. In contrast, heterozygous mice from line 34 with lower transgene expression did not develop emphysema until 1 yr of age. The changes in mean linear intercept coincided with an increase in lung compliance. Emphysema in these mice was associated with decreased immunostaining for type III collagen within the alveolar septa. This study provides evidence that MMP-1 induces progressive adult-onset emphysema by the selective degradation of type III collagen within the alveolar wall.

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.

A potential mechanism for age-related declines in patellar tendon biomechanics

Injuries to soft tissues such as tendons are becoming ever more frequent among the elderly. While increasing levels of activity likely contribute to these injuries, age-related declines in tendon strength may also be important. Whether these declines in biomechanical properties are associated with changes in fibril diameter or collagen type remains in question. In this study, age-related changes were investigated in patellar tendons from young adult rabbits (1-year old, n = 17) and from rabbits at the onset of senescence (4-year old, n = 33). Patellar tendon biomechanics was correlated with both collagen fibril diameter and with the presence of type V collagen, a known regulator of collagen fibril diameter. We hypothesize that (a) aging from I to 4 years results in significant reductions in patellar tendon biomechanical properties, and (b) these age-related declines are associated with smaller fibril diameters and with the presence of type V collagen. Maximum stress declined 25% between I and 4 years of age (100.7 +/- 5.6 MPa and 74.3 +/- 3.4 MPa, respectively, p < 0.0003) (mean +/- SEM) and strain energy density declined 40% (p < 0.001). The distribution of collagen fibrils from 4-year old rabbits was skewed significantly towards smaller diameters compared to fibrils from 1-year old rabbits (p < 0.001). Type V collagen was observed only in the 4-year old rabbit tendons. These correlations suggest that with increasing age after skeletal maturity, type V collagen may help to regulate the assembly and thus diameter of collagen fibrils and thereby adversely affect patellar tendon strength.

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.

Diamines and aminoalcohols: neutral solvents for native collagen

A substantial fraction of interstitial collagens (types I, III, V and VI) can be dissolved from finely divided, connective tissues by homogenization in 0.5-1 M ethylene diamine and certain other organic amines neutralized by hydrochloric or other acids to pH 7-8.5. The amount of collagen solubilized from the tendons and other tissues of young animals is similar to that extracted by dilute acetic acid. In ethylene diamine hydrochloride solutions, native type I collagen denatures at 38 degrees and it has a sedimentation coefficient similar to that measured in acidic citrate solutions. From these amine hydrochloride solutions native collagen fibrils can be regenerated simply by tenfold dilution with water or by dialysis. The form of these precipitates is determined by temperature, pH, enzyme pretreatment of the collagen, and the dialysate salt concentrations and composition. Weak gels can be formed with less than 0.1 mg collagen per ml. Regenerated matrices containing types V and VI collagens, and with different fibril sizes can be used to support cell growth or to form sheets or hydrated gels possibly suitable for prosthetic uses.

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.

Purification and characterization of an antibacterial factor from snail mucus

The antibacterial factor from the body surface of the African giant snail, Achatina fulica Férussac, was isolated by DEAE-Toyopearl 650M ion exchange chromatography. The isolated preparation exhibited highly positive antibacterial activity both for the Gram-positive bacteria, Bacillus subtilis and Staphylococcus aureus and for the Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, but it lost such activity when heated at 75 degrees C for 5 min. The antibacterial factor of the snail mucus was a glycoprotein whose molecular weight (MW) was about 160,000. It was composed of two subunits of MW 70,000-80,000.

The collagens of the developing bovine cornea

The morphology of the developing bovine eye has been examined and the collagens in fetal bovine eyes from three months' gestation to maturity have been solubilized by pepsin treatment and analyzed to determine the ratios of the predominant types of collagen. The type I collagen decreased, while the type V collagen increased with age. Type III collagen comprised less than 1% of all the corneas, except for the three-month fetal calf. The anterior to posterior thickness of the paraffin-embedded fetal calf cornea increased from the third to the seventh month, decreased from the seventh month to birth, and then increased after birth. Descemet's membrane increased in thickness with age. Analysis of dissected regions of the calf cornea showed a uniform distribution of the collagen populations from the center to the limbus (89% type I, 10% type V and less than 1% type III collagen) and uniformity through the depth of the stroma, except that type III was concentrated around Bowman's layer, and type IV in Descement's membrane. The localization of the different collagens was consistent with the immunofluorescent staining studies with anticollagen antibodies, but the ratios of the intensities of the fluorescence did not correspond to the quantitative analyses. These results are concordant with other studies that have shown that antibody binding may be masked or diminished in certain tissues and therefore immunofluorescence cannot be used reliably for quantitative measurements.

Quantitation of types I and III collagens in human tissue samples and cell culture by cyanogen bromide peptide analysis

A procedure for the quantitation of types I and III collagens by cyanogen bromide peptide analysis was developed with the aim of eliminating certain problems associated with this method. Ion-exchange chromatography reduced high background levels on gel scans used to quantitate the peptides; reduction with beta-mercaptoethanol substantially increased the efficiency of the cyanogen bromide cleavage; use of a concave gradient in acrylamide from 8 to 20% improved the resolution of cyanogen bromide peptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and a normalization procedure eliminated variations due to differences in the amount of material loaded on the gel system. This method of quantitation was applied to human aorta samples and to collagen secreted by human skin fibroblasts. Metachromasy of type I and type III collagen cyanogen bromide peptides stained with Coomassie blue R-250 was established and this was used as an index of the purity of the cyanogen bromide peptide preparations. Type I and III collagens were prepared from human placental tissue, and these purified collagens were used to construct calibration curves to determine the relationship between the quantity of diagnostic cyanogen bromide peptides present and the composition of the sample in terms of types I and III collagens.

A simple vertebrate collagenase assay using soluble radioactive collagen substrate

A highly sensitive assay for vertebrate collagenase has been developed using [14C]proline- or [3H]proline-labeled collagen as soluble substrate. The substrate was easy to prepare, gave high specific activity (1.4 X 10(6) cpm/mg collagen), and was stable at -20 degrees C for a long period. The digestion reaction for the assay was done at 21 degrees C to minimize the cleavage of collagen by proteases other than collagenase and to protect the 3/4 and 1/4 cleavage fragments of collagen from being further attacked by proteases. The cleaved products were denatured and then separated from undigested native collagen by precipitation with 1 M NaCl at pH 3.5. The conditions selected for denaturation and separation gave better discrimination between the cleaved products and uncleaved substrate than did conditions used in some other assays. The digestion products can be examined further by gel electrophoresis at the end of the assay to confirm the activity of vertebrate collagenase. This assay can also be adapted to assess telopeptidase activity independently of collagenase activity.

The organization of cross-linking in collagen fibrils

The cross-linked peptides in cyanogen bromide digests from rabbit and calf fibrils of type I collagen have been compared by gel filtration and electrophoresis. Fibrils were prepared in vitro from acid-soluble collagen, or tendons were used; both were reduced with borohydride to stabilize cross-linking adducts. The cross-linked peptides were isolated and hydrolyzed, and the reducible cross-links were analyzed. We propose that a prominent pattern of cross-linking involves in-register palisades of molecules overlapping by 27 nm and joined through the condensation of amino-terminal aldol adducts on the carboxy-terminal helical regions of overlapping molecules. Lateral association probably occurs through the carboxy-terminal aldehydes from two molecules forming tri- or quadrivalent adducts with residues in the overlapped molecule. This model favors the recently proposed quasi-hexagonal organization of molecules in fibrils in which rows of molecules occur in lateral register.

Effects of age and diabetes mellitus on cyanogen bromide digestion of human dura mater collagen

To extend previous studies on the apparent acceleration of collagen aging in diabetes mellitus, dura collagen from human adults of different ages, with and without diabetes, was processed to yield soluble and insoluble fractions. Insoluble fractions were cleaved by cyanogen bromide. Release of peptides from insoluble collagen by cyanogen bromide decreased markedly with age and was much less from the collagen of diabetics than from nondiabetics of similar ages. The acrylamide gel profiles of peptides released were similar, but not identical, for samples of different ages and for samples from diabetics. It was concluded that age-dependent and diabetes-dependent cross-linking was widespread throughout helical regions of collagen molecules and that collagen throughout the body is altered in diabetes. Analyses of fragments of insoluble collagen are required to gain information on the chemistry of the cross-links that form with aging and diabetes.

Collagenase digestion demonstrates carboxy-terminal crosslinking in acid-soluble collagen

Among the products of the collagenase cleavage of Type I acid-soluble collagen from calf and rabbit tendons, there can be found fragments with the lengths of half alpha-chains. Because purified collagenase cleaves the alpha-chains three-quarters of the length from the amino-terminus, the presence of half-length chains is evidence for the occurrence of crosslinks between two carboxy-terminal, quarter-length fragments, The collagen preparations were reduced with [3H]borohydride, the collagenase-cleaved fragments were separated by gel electrophoresis, and their 3H-labeled crosslink derivatives were analyzed. The major labeled components in the half-length chains were the reduced aldol condensation product and hydroxylysinonorleucine. These experiments demonstrate that the carboxy-terminal telopeptides in monomer-enriched collagen samples form aldol crosslinks which are probably intramolecular, but some intermolecular aldol and aldimine crosslinks may also be formed.

Quantitative analysis of collagen from normal developing corneas and corneal scars

We measured the relative solubility of collagen in acetic acid after pepsin digestion and tentatively identified the types of collagen present in corneas of rabbits of various ages and in corneal scar tissue, using hydroxyproline assays and polyacrylamide gel electrophoretic analyses. More than 80% of the collagen in normal developing rabbit cornea was soluble after pepsin treatment; no more than 45% of that in two-week-old corneal scars was soluble. The predominant collagens in normal cornea and healing tissue were types I and AB. Type AB increased from 6% of the total collagen in fetal cornea to 11% in cornea from young adults. Collagen from two-week-old corneal wounds contained 16% type AB. Corneal type AB collagen was less soluble and more resistant to degradation by mammalian collagenase than was type I collagen. Unlike the normal cornea, in healing tissue the relative rate of synthesis of type I to type AB collagens did not correspond to their deposition. These results suggest a basic alteration in the molecular structure of the corneal scar, which may be instrumental in preventing the healing tissue from producing a normal, functioning organ.

Bovine type I collagen: A study of cross-linking in various mature tissues

The cyanogen bromide peptides from insoluble and pepsin solubilised type I collagen of bovine bone, dentine, meniscus, tendon, skin and cornea were compared by SDS-polyacrylamide gel electrophoresis. In each case alpha 1CB6 was shown to be the only peptide of molecular weight greater than 10 000 involved in cross-linking. The major helical peptides alpha 1CB3, alpha 1CB8, alpha 1CB7 and alpha 2CB4 were not implicated in cross-linking in any tissue either by end overlap or helix-helix interaction. The C-terminal alpha 2 chain peptide alpha 2CB3,5, which contains a large helical region, was not involved in cross-linking to any large peptides, although a slight increase in molecular weight in all tissues examined did suggest a possible interaction(s) with a very small peptide of molecular weight 4--5000.