Collagen cross-linking in corneal scar formation

Collagen cross-linking mediated by lysyl hydroxylase 2: an enzymatic battlefield to combat fibrosis

The hallmark of fibrosis is an excessive accumulation of collagen, ultimately leading to organ failure. It has become evident that the deposited collagen also exhibits qualitative modifications. A marked modification is the increased cross-linking, leading to a stabilization of the collagen network and limiting fibrosis reversibility. Not only the level of cross-linking is increased, but also the composition of cross-linking is altered: an increase is seen in hydroxyallysine-derived cross-links at the expense of allysine cross-links. This results in irreversible fibrosis, as collagen cross-linked by hydroxyallysine is more difficult to degrade. Hydroxyallysine is derived from a hydroxylysine in the telopeptides of collagen. The expression of lysyl hydroxylase (LH) 2 (LH2), the enzyme responsible for the formation of telopeptidyl hydroxylysine, is universally up-regulated in fibrosis. It is expected that inhibition of this enzyme will lead to reversible fibrosis without interfering with the normal repair process. In this review, we discuss the molecular basis of collagen modifications and cross-linking, with an emphasis on LH2-mediated hydroxyallysine cross-links, and their implications for the pathogenesis and treatment of fibrosis.

Corneal light scattering with stromal reformation after laser in situ keratomileusis and photorefractive keratectomy

OBJECTIVE

To correlate corneal light scattering with keratocyte and extracellular matrix reformation after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).

SETTING

Wilmer Ophthalmological Institute, Johns Hopkins University, Baltimore, Maryland, USA.

METHODS

Sixteen pigmented rabbit eyes were randomly divided into 2 groups. Group 1 (n = 8) had a 5.0 mm, -10.0 diopter (D) LASIK treatment and Group 2 (n = 8), a 5.0 mm, -10.0 D surface PRK treatment after mechanical epithelial debridement. The stromal surface exposed at surgery was stained with dichlorotriazinylaminofluorescein (DTAF) solution. Slitlamp biomicroscopic and objective measurement of corneal light scattering using a scatterometer were performed 1 and 2 days and 1, 2, 3, 4, 8, and 12 weeks after surgery. In each group, 2 corneas were harvested at 1 week and 1 month and 4 corneas were harvested at 12 weeks. Tissue sections were examined by light and fluorescence microscopy. The percentage of newly formed stromal tissue was calculated and correlated with the scatterometry index.

RESULTS

In Group 1, corneas remained clear and healed without significant scarring throughout the study. In Group 2, subepithelial scarring was noted. Extracellular matrix reformation peaked at 1 month and showed a slight regression thereafter. The percentage of extracellular matrix reformation was strongly correlated with the scatterometry index (r = .86, P < .001).

CONCLUSIONS

In this study, significant subepithelial stromal tissue reformation followed PRK. The percentage of extracellular matrix reformation correlated well with the objective corneal light scattering measurements.

Structure of corneal scar tissue: an X-ray diffraction study

Full-thickness corneal wounds (2 mm diameter) were produced in rabbits at the Schepens Eye Research Institute, Boston. These wounds were allowed to heal for periods ranging from 3 weeks to 21 months. The scar tissue was examined using low- and wide-angle x-ray diffraction from which average values were calculated for 1) the center-to-center collagen fibril spacing, 2) the fibril diameter, 3) the collagen axial periodicity D, and 4) the intermolecular spacing within the collagen fibrils. Selected samples were processed for transmission electron microscopy. The results showed that the average spacing between collagen fibrils within the healing tissue remained slightly elevated after 21 months and there was a small increase in the fibril diameter. The collagen D-periodicity was unchanged. There was a significant drop in the intermolecular spacing in the scar tissues up to 6 weeks, but thereafter the spacing returned to normal. The first-order equatorial reflection in the low-angle pattern was visible after 3 weeks and became sharper and more intense with time, suggesting that, as healing progressed, the number of nearest neighbor fibrils increased and the distribution of nearest neighbor spacings reduced. This corresponded to the fibrils becoming more ordered although, even after 21 months, normal packing was not achieved. Ultrastructural changes in collagen fibril density measured from electron micrographs were consistent with the increased order of fibril packing measured by x-ray diffraction. The results suggest that collagen molecules have a normal axial and lateral arrangement within the fibrils of scar tissue. The gradual reduction in the spread of interfibrillar spacings may be related to the progressive decrease in the light scattered from the tissue as the wound heals.

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.

Morphogenesis of rabbit corneal endothelium

We studied ultrastructurally the development of rabbit corneal endothelium from the 13th day of gestation to 3 days after birth. Precursor corneal endothelial cells, stromal cells, and a vascular network migrate in close association with each other between the developing corneal and lens epithelia. During development, newly deposited extracellular fibrous matrices separate the prospective endothelium from the capillaries and corneal stroma. The extracellular matrix between the apical endothelial surface and the vascular network loses its fibrous appearance early in development. Simultaneously, randomly organized fibrils are deposited on the basal endothelial surface facing the stroma. These fibrils, gradually obscured by the deposition of a nonfibrous component, eventually become part of Descemet's membrane. Early in development, prospective endothelial cells cannot be distinguished morphologically from the overlying corneal stromal cells. Morphologic differentiation of the endothelial cell is characterized by the formation of sinuous lateral borders that interdigitate with those of adjacent cells to form a continuous single-cell layer of tissue. The basal endothelial membrane forms a pitted surface, distinguishing it from the apical cell membrane. Intercellular junctions between lateral membranes, a cilium projecting into the anterior chamber, and deposition of Descemet's membrane on the basal endothelial surface contribute to the polarization of the endothelium. Throughout most of corneal development the vascular pupillary membrane maintains a close association with the apical surface of the differentiating endothelium. We conclude that fetal corneal endothelium develops within a complex extracellular matrix environment and in proximity to the underlying vascular network. These structures play an important role in the morphogenesis of corneal endothelium.

Collagen metabolism following corneal laceration in rabbits

We investigated the synthesis and degradation of collagen in lacerated rabbit corneas. The rate of collagen synthesis was measured by the incorporation and hydroxylation of [14C]proline up to 5 weeks after a penetrating laceration. Our results indicate that protein and collagen synthesis is initially reduced during the 24 h period after laceration and then increases in two phases. A moderate increase in protein and collagen synthesis occurs up to day 5 after laceration. Synthesis then decreases to a low level through the 10th day after injury. A second wave of increase in protein and collagen synthesis takes place reaching a new peak of approximately twice the activity as found in control corneas after 5 weeks of healing. Afterwards, the rate of protein and collagen synthesis declines and reaches the basal level after 7 weeks of healing. In a separate set of experiments, rabbit corneas were lacerated and allowed to heal for one or three weeks at which point 100 microCi of [14C]proline was injected into each anterior chamber. The corneas were excised 1 to 17 days later and subjected to collagen analysis. Our data indicates that the degradation of collagen synthesized by the cornea 1 week after laceration followed biphasic kinetics. In the first phase, the half-life of newly-synthesized collagen is 20 days, whereas it is approximately 4 days in the second phase. It is of particular interest that the accelerated degradation of the newly-synthesized [14C]collagen is concomitant with the increased rate of collagen synthesis during the wound healing process. The degradation of collagen synthesized 3 weeks after injury was slower and followed monophasic kinetics having a half-life of 14 days. The degradation of non-collagenous 14C-proteins follow a monophasic kinetics having a half-life of 6-7 days. Polyacrylamide gel electrophoresis of newly-synthesized collagen indicated that the collagen(I) and collagen(V) were the main collagenous components synthesized by the lacerated corneas.

Comparative collagen biochemistry of bovine periodontium, gingiva, and dental pulp

Cross-linking patterns of the collagen from bovine gingiva, periodontium, and dental pulp were analyzed chromatographically. The ratios of two main cross-links, dihydroxylysinonorleucine to hydroxylysinonorlecuine, were 0.18, 0.31, and 0.49 for the bovine gingiva, periodontium, and dental pulp collagen, respectively. These ratios are similar to that of skin collagen rather than that of bone and dentin collagen.

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.

Collagen heterogeneity in normal rabbit cornea. I. Isolation and biochemical characterization of the genetically-distinct collagens

Limited proteolysis of preparations of mature rabbit cornea with pepsin allows the recovery of approximately 90% of the tissue collagen in soluble form. Using recently described selective precipitation techniques, the soluble cornea collagen can be resolved into two major fractions. The predominant fraction, which accounts for about 95% of the solubilized collagen, was identified by means of solubility properties, electrophoretic and chromatographic properties, as well as amino acid analyses of its constituent chains, as Type I collagen. The alternate fraction, which accounts for virtually all of the remainder of the solubilized cornea collagen, was identified by means of the same criteria as collagen comprised of the A and B chains. In addition, the stoichiometry of the latter chains in preparations of cornea collagen indicate that in the cornea these chains are likely to participate in the formation of only one type of collagen molecule with the chain composition, AB2. And finally, the demonstration that Type I and the AB2 collagens are the predominant forms of collagen recoverable from the mature cornea strongly suggests that molecules comprised of the A and B chains are not necessarily confined to basement membrane structures.

Isolation and hydroxylysine glycoside content of some cyanogen bromide-cleaved fragments of collagen from bovine corneal stroma

Six CNBr-cleaved fragments of insoluble collagen from bovine cornea were isolated, characterized and examined for hydroxylysine glycosides. Thus the general distribution of most of the glycoside along the collagen molecule was determined. Collagen from bovine stroma is almost entirely type I. This work forms a basis for the pinpointing of glycoside-attachment sites along the collagen molecules of bovine cornea.

Aging, and crosslinking in mammlian collagen

The amount and type of borohydride-reducible crosslinks in collagen have been examined as a function of animal age. In a variety of bovine, canine and human tissues the level of redicible crosslinks decreases with time and the ratios of individual compounds change. There is both tissue and species specificity in the extent of these changes. A decrease in the level of reducible crosslinks correlates with the cessation of growth. Loss of reducible crosslinks does not imply a small total number of crosslinks since physical changes with age imply the opposite. We conclude that reducible crosslinks are converted to a stable nonreducible state and the persistence of low levels of reducible crosslinks may be indicative of a low level of turnover in the tissue. Changes in ratios of reducible crosslinks are of doubtful functioal significance and may simply reflect variation in post-translational modification of lysine residues.