Characterization of the major matrix degrading metalloproteinase of human corneal stroma. Evidence for an enzyme/inhibitor complex

How Modifications of Corneal Cross-Linking Protocols Influence Corneal Resistance to Enzymatic Digestion and Treatment Depth

Purpose

The purpose of this study was to determine the effects of the Photoactivated Chromophore for Keratitis Corneal Cross-Linking (PACK-CXL) protocol modifications on corneal resistance to enzymatic digestion and treatment depth.

Methods

Eight hundred one ex vivo porcine eyes were randomly divided into groups of 12 to 86 corneas, treated with various epi-off PACK-CXL modifications, including acceleration (30 > 2 minutes, 5.4 J/cm2), increased fluence (5.4 > 32.4 J/cm2), deuterium oxide (D2O) supplementation, different carrier types (dextran versus hydroxypropyl methylcellulose [HPMC]), increased riboflavin concentration (0.1 > 0.4%), and riboflavin replenishment during irradiation (yes/no). Control group eyes did not receive PACK-CXL. A pepsin digestion assay was used to determine corneal resistance to enzymatic digestion. A phalloidin fluorescent imaging assay was used to determine the PACK-CXL treatment effect depth. Differences between groups were evaluated using a linear model and a derivative method, respectively.

Results

PACK-CXL significantly increased corneal resistance to enzymatic digestion compared to no treatment (P < 0.03). When compared to a 10 minute, 5.4 J/cm2 PACK-CXL protocol, fluences of 16.2 J/cm2 and higher increased corneal resistance to enzymatic digestion by 1.5- to 2-fold (P < 0.001). Other protocol modifications did not significantly change corneal resistance. A 16.2 J/cm2 fluence also increased collagen compaction in the anterior stroma, whereas omitting riboflavin replenishment during irradiation increased PACK-CXL treatment depth.

Conclusions

Increasing fluence will likely optimize PACK-CXL treatment effectiveness. Treatment acceleration reduces treatment duration without compromising effectiveness.

Translational Relevance

The generated data help to optimize clinical PACK-CXL settings and direct future research efforts.

Corneal Response to Injury and Infection in the Horse

This article describes the natural responses of the immune system and the cornea to injury and infection. The process of reepithelialization and reformation of stromal collagen is discussed, as are the clinical signs and manifestations of the effects of the healing response when it is routine and when it is pathologic. Excessive inflammatory or immune responses by host tissues can cause further damage that may be present from the antecedent injury or the effect of a pathogen. The clinical signs and manifestations of wound healing as well as potential therapeutic interventions are described.

MMP regulation of corneal keratocyte motility and mechanics in 3-D collagen matrices

Previous studies have shown that platelet derived growth factor (PDGF) can stimulate corneal keratocyte spreading and migration within 3-D collagen matrices, without inducing transformation to a contractile, fibroblastic phenotype. The goal of this study was to investigate the role of matrix metalloproteinases (MMPs) in regulating PDGF-induced changes in keratocyte motility and mechanical differentiation. Rabbit corneal keratocytes were isolated and cultured in serum-free media (S-) to maintain their quiescent phenotype. A nested collagen matrix construct was used to assess 3-D cell migration, and a standard collagen matrix model was used to assess cell morphology and cell-mediated matrix contraction. In both cases constructs were cultured in S- supplemented with PDGF, with or without the broad spectrum MMP inhibitors GM6001 or BB-94. After 4 days, f-actin, nuclei and collagen fibrils were imaged using confocal microscopy. To assess sub-cellular mechanical activity (extension and retraction of cell processes), time-lapse DIC imaging was also performed. MT1-MMP expression and MMP-mediated collagen degradation were also examined. Results demonstrated that neither GM6001 nor BB-94 affected corneal keratocyte viability or proliferation in 3-D culture. PDGF stimulated elongation and migration of corneal keratocytes within type I collagen matrices, without causing a loss of their dendritic morphology or inducing formation of intracellular stress fibers. Treatment with GM6001 and BB-94 inhibited PDGF-induced keratocyte spreading and migration. Relatively low levels of keratocyte-induced matrix contraction were also maintained in PDGF, and the amount of PDGF-induced collagen degradation was similar to that observed in S- controls. The collagen degradation pattern was consistent with membrane-associated MMP activity, and keratocytes showed positive staining for MT1-MMP, albeit weak. Both matrix contraction and collagen degradation were reduced by MMP inhibition. For most outcome measures, the inhibitory effect of BB-94 was significantly greater than that of GM6001. Overall, the data demonstrate for the first time that even under conditions in which low levels of contractility and extracellular matrix proteolysis are maintained, MMPs still play an important role in mediating cell spreading and migration within 3-D collagen matrices. This appears to be mediated at least in part by membrane-tethered MMPs, such as MT1-MMP.

The keratoconus corneal proteome: loss of epithelial integrity and stromal degeneration

Keratoconus is a thinning corneal dystrophy that begins in the early teenage years and ultimately requires cornea transplantation to restore vision. Here we conducted a highly sensitive mass spectrometric analysis of the epithelium and the stroma from keratoconus and normal donor corneas. We identified a total of 932 and 1157 proteins in the consolidated data of the epithelium and stroma, respectively. Technical replicates showed strong correlations (≥0.88) in levels of all common proteins, indicating very low technical variations in the data. Analysis of the most increased (≥1.5 fold) and decreased (≤0.8 fold) proteins in the keratoconus corneal epithelial protein extracts identified proteins related to dermal diseases, inflammation, epithelial stratification and mesenchymal changes. Increased proteins included keratins 6A, 16 and vimentin, while the iron transporter lactotransferrin was decreased. The keratoconus stromal proteome suggests endoplasmic reticular stress, oxidative stress and widespread decreases in many extracellular matrix proteoglycan core proteins, lumican and keratocan, collagen types I, III, V and XII. Marked increase in apoptosis and endocytosis-related proteins suggest degenerative changes in keratocytes, the resident cells of the stroma. This is the most comprehensive proteome analysis of the cornea that highlights similarities of keratoconus with other neurodegenerative diseases.

BIOLOGICAL SIGNIFICANCE

This study provides, to our knowledge, the most comprehensive proteomic analysis of the vision threatening disease keratoconus, which affects a significant portion of the US and global populations. Using iTRAQ and LC/MS/MS, we have identified significant changes in the human corneal epithelium and stromal proteome that correlate to in vivo clinical findings. The protein changes identified will lead to molecular insights into disease pathogenesis and provide candidate genes for genetic studies of keratoconus.

The relation between keratoconus and plasma levels of MMP-2, zinc, and SOD

PURPOSE

To evaluate the levels of matrix metalloproteinase-2 (MMP-2), superoxide dismutase (SOD), and zinc in plasma taken from patients with keratoconus and to investigate the likely association between these factors and keratoconus.

METHODS

A total of 36 patients with keratoconus and 40 control subjects at the Department of Ophthalmology, Faculty of Medicine, Gaziosmanpaşa University, were included in the study. Plasma levels of zinc were determined with atomic absorption spectrometry for all the subjects. Measurements of plasma MMP-2 levels were performed by the enzyme-linked immunosorbent assay method. Total plasma (Cu/Zn and Mn) SOD activity was also determined photometrically.

RESULTS

The plasma concentrations of zinc and MMP-2 were significantly lower in patients with keratoconus than in the healthy controls (P < 0.001). Total plasma SOD levels were significantly higher in patients with keratoconus than in the healthy controls (P < 0.001).

CONCLUSIONS

We detected reduced plasma levels of zinc and MMP-2, and enhanced plasma levels of SOD in patients with keratoconus compared with the healthy subjects. The data presented provide insight into the potential role these molecules may play in the etiopathogenesis of this disease.

Cloning and characterization of cell strains derived from human corneal stroma and sclera

PURPOSE

To establish human corneal stroma- and sclera-derived cells as models for studying diseases of the anterior segment of the eye.

METHODS

Using a recombinant retrovirus system, we transfected human papilloma virus 16 E6 and E7 (HPV16 E6/E7) into human corneal stroma- and sclera-derived cells. The primary cells and established cell strains were characterized by assessing the mRNA expression of collagen, matrix metalloproteinase, and tissue inhibitor of metalloproteinase by reverse transcription-polymerase chain reaction. We also examined the effects of inflammatory cytokines on hyaluronan synthase expression and hyaluronan products.

RESULTS

Both a corneal stroma-derived cell strain, Cs3, and a sclera-derived cell strain, Sc1, were obtained, and both cell strains could be passaged up to 25 times. The mRNA expression pattern observed in the primary cells was identical to that observed in the cell strains. Hyaluronan synthase 1 and 2 mRNAs were increased by transforming growth factor beta and platelet-derived growth factor BB. Significant differences were observed between the hyaluronan products with and without cytokine treatment.

CONCLUSION

Cell strains derived from corneal stroma and sclera fibroblast cells can be established using HPV16 E6/E7 immortalized genes of the same origin. The phenotypic cell characteristics did not change after transfection, immortalization, or successive passages in culture.

Effect of serum and insulin modulation on the organization and morphology of matrix synthesized by bovine corneal stromal cells

The in vitro production of highly organized collagen fibrils by corneal keratocytes in a three-dimensional scaffold-free culture system presents a unique opportunity for the direct observation of organized matrix formation. The objective of this investigation was to develop such a culture system in a glass substrate (for optical accessibility) and to directly examine the effect of reducing serum and/or increasing insulin on the stratification and secretion of aligned matrix by fourth- to fifth-passage bovine corneal stromal keratocytes. Medium concentrations of 0%, 1%, or 10% fetal bovine serum and 0% or 1% insulin-transferrin-selenium were investigated. High-resolution differential interference contrast microscopy, quick-freeze/deep-etch, and conventional transmission electron microscopy were used to monitor the evolution, morphology, and ultrastructure of the cell-matrix constructs. In a medium containing 1% each of serum and insulin-transferrin-selenium, stromal cells stratified and secreted abundant and locally aligned matrix, generating the thickest cell-matrix constructs (allowing handling with forceps). The results of this study have the potential to significantly advance the field of developmental functional engineering of load-bearing tissues by (i) elucidating cues that modulate in vitro cell secretion of organized matrix and (ii) establishing an optically accessible cell culture system for investigating the mechanism of cell secretion of aligned collagen fibrils.

Differences in the TGF-{beta}1-induced profibrotic response of anterior and posterior corneal keratocytes in vitro

Purpose. To characterize phenotypic differences between anterior and posterior corneal keratocytes after stimulation with the profibrotic agent transforming growth factor-beta1 (TGF-beta1) in vitro. Methods. Sixteen corneas from healthy felines were obtained immediately after death. Lamellar dissection was performed to separate the anterior and posterior stroma at approximately 50% depth either manually (n = 2) or with a Moria microkeratome (300-mum head; n = 14). Cells from the anterior and posterior stroma were cultured separately but under identical conditions. Using immunohistochemistry and Western blot techniques, Ki-67 staining and relative expression of Thy-1, alpha smooth muscle actin (alpha-SMA), and fibronectin were assessed after stimulation with different TGF-beta1 concentrations. In addition, anterior and posterior cells cultured in different concentrations of TGF-beta1 were wounded with a razor blade, and the wound area and time to closure were determined. Results. Stimulation by all concentrations of TGF-beta1 increased the proportion of Ki-67-positive cells in anterior and posterior cell cultures, but this increase was noted earlier in posterior cells than in anterior cells. Increasing TGF-beta1 concentration also increased the relative expression of Thy-1, alpha-SMA, and fibronectin in anterior and posterior fibroblasts. However, anterior cells expressed these fibrotic markers at lower TGF-beta1 concentrations than did posterior keratocytes. After mechanical wounding, posterior cells closed the wound area faster than did anterior cells at all concentrations of TGF-beta1. Conclusions. The present experiments show that anterior and posterior corneal keratocytes exhibit different sensitivities to the profibrotic growth factor TGF-beta1. This heterogeneity of keratocyte response may impact wound closure after mechanical wounding.

Production and activity of matrix metalloproteinase-9 on the ocular surface increase in dysfunctional tear syndrome

PURPOSE

To evaluate production and activity of metalloproteinase (MMP)-9 on the ocular surface of patients with dysfunctional tear syndrome (DTS) and determine any correlation between MMP-9 activity and clinical parameters.

METHODS

Forty-six patients with newly diagnosed DTS and 18 control subjects were recruited. Complete ocular surface examinations were performed. Tear MMP-9 activity was assessed with an MMP-9 activity assay in 1 microL of unstimulated tear fluid. Using conjunctival epithelial cells from 19 patients with DTS and 16 controls, levels of MMP-9 and its regulating cytokine mRNA transcripts were evaluated by semiquantitative real-time PCR.

RESULTS

Each of four DTS severity-based groups had significantly higher mean MMP-9 activities than did the control group, which was 8.39 +/- 4.70 ng/mL. The DTS4 group had the highest MMP-9 activity (381.24 +/- 142.83 ng/mL), for which the mean was significantly higher than that of other DTS groups. In addition, patients with DTS had significantly higher levels of IL-1beta, IL-6, TNF-alpha, and TGF-beta1 mRNA transcripts in their conjunctival epithelia than did the control subjects. Tear MMP-9 activities showed significant correlation with symptom severity scores, decreased low-contrast visual acuity, fluorescein tear break-up time, corneal and conjunctival fluorescein staining, topographic surface regularity index (SRI), and percentage area of abnormal superficial corneal epithelia by confocal microscopy.

CONCLUSIONS

Tear MMP-9 activity was significantly higher in patients with DTS. This activity was associated with increased mRNA expression of MMP-9 and its regulating genes and correlated strongly with clinical parameters. MMP-9 appears to be a potentially useful biomarker for diagnosing, classifying, and monitoring DTS.

Keratoconus: Matrix metalloproteinase-2 activation and TIMP modulation

Keratoconus is an ocular condition that causes corneal thinning, cone formation and scarring. In view of a hypothesis that activated MMP-2 may initiate or facilitate disease progression, the MMP-2/TIMP systems of stromal cells derived from normal and keratoconic corneas have been compared. To achieve this, stromal cell cultures were established from normal, clear keratoconic (KCS-1) and scarred keratoconic (KCS-2) corneas. The secreted MMP-2 was assayed using [(3)H]Type IV collagen and analysed by zymography. Optimally maintained and nutrient deprived cells were subsequently incubated with [(3)H]lysine. The secreted radiolabelled macromolecules were separated and quantified. The results obtained indicated that optimally maintained KCS-1 stromal cells produced more MMP-2 than normal stromal cells but not TIMP. Nutrient deprivation induced MMP-2 activation and cell death. Surviving cells upregulated TIMP-1 synthesis and in this respect became similar to the KCS-2 stromal cells that did not excessively generate activated MMP-2 or die as a consequence of nutrient deprivation. From these results, it was concluded that KCS-1 stromal cells over-expressed MMP-2 without increasing TIMP production. This may facilitate MMP-2 activation in vivo and hence advance the keratoconic condition. KCS-2 cultures over-expressed both MMP-2 and TIMP-1. Because TIMP-1 inhibits MMP-2 activity and protects against cell death it may be of significance in initiating repair processes and curtailing keratoconus.

How the cornea heals: cornea-specific repair mechanisms affecting surgical outcomes

In mammals, penetrating injuries typically heal by deposition of fibrotic "repair tissue" that fills and seals wounds but does not restore normal function. Excessive deposition of fibrotic repair tissue can lead to pathologies involving excessive scarring and contracture. In the cornea, fibrotic repair presents special challenges affecting both clarity and shape of the cornea. With the increasing popularity of surgical techniques that alter corneal refractive errors, understanding of cornea repair mechanisms has acquired new significance. The cornea has unique anatomic, cellular, molecular, and functional features that lead to important mechanistic differences in the process of repair in comparison with what occurs in skin and other organs. Moreover, corneal function calls for special outcomes. This review addresses these features from the viewpoint of the authors' research on factors of importance to understanding and improving surgical outcomes.

Thy-1 distinguishes human corneal fibroblasts and myofibroblasts from keratocytes

After corneal injury, keratocytes become activated and transform into repair phenotypes-corneal fibroblasts or myofibroblasts, however, these important cells are difficult to identify histologically, compromising studies of stromal wound healing. Recent studies indicate that expression of the cell surface protein, Thy-1, is induced in fibroblast populations associated with wound healing and fibrosis in other tissues. We investigated whether keratocyte transformation to either repair-associated phenotype induced Thy-1 expression. Human corneal keratocytes were isolated by collagenase digestion. The cells were either processed immediately (i.e. freshly isolated keratocytes) or were cultured in the presence of 10% fetal bovine serum or transforming growth factor-beta to induce transformation to the corneal fibroblast and myofibroblast phenotypes, respectively. Thy-1 mRNA and protein expression by freshly isolated keratocytes and corneal fibroblasts were assessed by RT-PCR and Western blotting. mRNA also was extracted from the whole intact stroma and assessed by RT-PCR. Thy-1 was localised immunocytochemically in cultured human corneal fibroblasts, myofibroblasts, and in keratocytes in normal human corneal tissue sections. Thy-1 mRNA and protein were detectable in cultured human corneal fibroblasts, but not freshly isolated keratocytes. Whole uninjured stroma showed no detectable Thy-1 mRNA expression. Cultured human corneal fibroblasts and myofibroblasts both labelled for Thy-1, but keratocytes in the stroma of normal human cornea did not. We conclude that Thy-1 expression is induced by transformation of keratocytes to corneal fibroblasts and myofibroblasts, suggesting a potential functional role for Thy-1 in stromal wound healing and providing a surface marker to distinguish the normal keratocyte from its repair phenotypes.

Insulin-like growth factor-I (IGF-I) and transforming growth factor-beta (TGF-beta) modulate tenascin-C and fibrillin-1 in bullous keratopathy stromal cells in vitro

PURPOSE

Pseudophakic bullous keratopathy (PBK) is a major indication for corneal transplantation. Previous studies showed that PBK corneas had increased levels of insulin-like growth factor-I (IGF-I), bone morphogenetic protein-4 (BMP-4), transforming growth factor-beta (TGF-beta), interleukin-1alpha (IL-1alpha) and IL-8. The PBK corneas also had accumulations of tenascin-C (TN-C), fibrillin-1 (Fib-1), matrix metalloproteinase-2 (MMP-2), inflammatory cells but not myofibroblasts. Our goal is to determine if the growth factors/cytokines that are elevated in PBK corneas alter the expression of extracellular matrix (ECM) and/or degradative enzymes in vitro.

METHODS

Stromal cell cultures from normal and PBK human corneas were established and treated for 6 days with IGF-I, BMP-4, IL-1alpha, IL-8 or TGF-beta1/beta2. Immunostaining, Western blot and dot blot analyses for TN-C, Fib-1, alpha-smooth muscle actin (alpha-SMA, a marker for myofibroblasts) or tissue inhibitor of metalloproteinase-1 (TIMP-1) were performed. RNAs were collected and analyzed with Northern blots for TN-C, Fib-1 and beta(2)-microglobulin. Culture media were analyzed using gelatin zymography with or without ethylenediaminetetraacetic acid (EDTA). Some samples were activated with p-aminophenylmercuric acetate (APMA) and reduction/alkylation, and the degradative activities were measured by the MMP-gelatinase activity assay kit.

RESULTS

The IGF-I and TGF-beta1/TGF-beta2 increased (a) TN-C protein deposition, and (b) Fib-1 protein and RNA levels, but (c) had no significant affect on TIMP-1, matrix metalloproteinase-2 (MMP-2) or gelatinase activities. TGF-beta1/TGF-beta2 induced alpha-SMA protein (myofibroblasts) while IGF-I did not. BMP-4, IL-1alpha and IL-8 had little affect on the cells.

CONCLUSIONS

Based upon our data, the fibrotic markers, TN-C and Fib-1, found in PBK corneas may be accounted for by IGF-I and TGF-beta. These growth factors promote fibrosis and ECM deposition without promoting proteolysis. While the other growth factors/cytokines are elevated in PBK corneas, their role(s) in PBK pathogenesis are not clear. In addition, exogenous IGF-I most closely elicited a response that was most similar to the characteristics of the PBK/ABK corneas, i.e. accumulation of TN-C and Fib-1 proteins in the absence of myofibroblasts.

Possible role of the vitamin E solubilizer in topical diclofenac on matrix metalloproteinase expression in corneal melting: an analysis of postoperative keratolysis

OBJECTIVE

To analyze tissue matrix metalloproteinase (MMP) expression in three patients who developed postoperative corneal melts after treatment with topical diclofenac sodium 0.1% (Falcon; Fort Worth, TX) ophthalmic solution.

DESIGN

Retrospective noncomparative interventional case series with tissue analysis.

MAIN OUTCOME MEASURES

Three patients were examined in this study. We report two patients from the same center with acute corneal melts after uncomplicated photorefractive keratectomy (PRK). Prior to these cases, 1500 patients were treated at the Zale Lipshy University Laser Center for Vision with no adverse effects. All 1500 patients were treated with the same postoperative regimen of ciprofloxacin, rimexolone, and suprofen ([Profenal, (CIBA, Duluth, GA]). The next 27 cases were treated postoperatively with ciprofloxacin and rimexolone. However, diclofenac sodium 0.1% was used instead of Profenal. A third case was also discussed. This melt occurred at another center in a postoperative cataract patient who developed cystoid macular edema after cataract extraction with intraocular lens placement. He was initially treated with diclofenac sodium 0.1% (Ciba Vision, Duluth, GA) then with diclofenac sodium 0.1%. He subsequently developed a corneal perforation requiring penetrating keratoplasty. All tissue specimens were examined by light microscopy. Microbiologic cultures and stains were also performed. Immunolocalization and in situ hybridization were performed on all keratoplasty specimens to detect expression and localization of MMPs. All patients had a complete diagnostic evaluation for systemic autoimmune diseases.

RESULTS

Postoperatively, all patients developed corneal perforations requiring surgical intervention while being treated with diclofenac sodium 0.1%. Microbiologic cultures and special stains were negative for microorganisms. Induced expression of specific tissue degrading enzymes of the matrix metalloproteinase family was demonstrated within corneal epithelial cells, stromal keratocytes, and at the level of Descemet's membrane. The uniform distribution pattern of expression was not consistent with the localization expected of a repair response, suggesting the involvement of some outside agent.

CONCLUSIONS

Whereas MMP expression is a normal component of repair, excessive or inappropriate MMP activity is associated with corneal keratolysis. Our study provides preliminary evidence that topical application of diclofenac sodium 0.1% may be associated with aberrant MMP expression in the cornea.

Is the corneal degradation in keratoconus caused by matrix-metalloproteinases?

The thinning of the cornea that occurs in keratoconus has been well described; however, the mechanism of tissue degradation remains unknown. Elevated proteinase activity is one possibility and approximately 20 publications over the last 20 years have addressed this hypothesis. Early studies reported increased collagenase and gelatinase activities in the medium of keratoconus corneal cultures. After the characterization of the matrix metalloproteinase (MMP) enzymes, studies focused on the expression of specific MMPs, in particular the gelatinases, MMP-2 and MMP-9. Matrix metalloproteinase-2 was found to be the major MMP of the cornea and was constitutively produced in normal tissue, whereas MMP-9 expression was induced by various stimuli, including phorbol esters and even tissue culturing. These studies suggested that there were no differences in the amounts or states of activation of MMP between normal and keratoconus corneas, although the amounts of some proteinase inhibitors, including tissue inhibitors of metalloproteinases, alpha-1-proteinase inhibitor and alpha-2-macroglobulin, were decreased in keratoconus. Most recently, the lysosomal proteinases, cathepsin B and cathepsin G were reported to be elevated in keratoconus corneas, and it is possible that it was cathepsin activity, not MMP activity, that was measured in some early studies. Nevertheless, there are now about 20 human MMPs identified and it is possible that some of these, other than the well known collagenase (MMP-1) and gelatinases (MMP-2 and MMP-9), could be implicated in the pathology of keratoconus. Studies have begun to address more recently described MMPs and it has been reported that the membrane-bound MT1-MMP (MMP-14), which activates latent MMP-2, was found to have increased expression in keratoconus corneas, whereas the stromelysins, MMP-3 and MMP-10, were not.

The control of matrix metalloproteinase-2 expression in normal and keratoconic corneal keratocyte cultures

PURPOSE

Early phase keratoconic corneas and their cultured keratocytes abnormally produce the Mr 62,000 form of the matrix metalloproteinase-2 (MMP-2). It is known that platelet derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta) are involved in the regulation of MMP activity and tissue inhibitor of metalloproteinase (TIMP) production in non-ocular tissues. The purpose of this enquiry was to determine whether these growth factors also play a role in the activity and/or production of corneal MMP-2 and TIMP, and whether their activity could account for the existence of the Mr 62,000 form of MMP-2 in keratoconic corneas.

METHODS

Confluent cultures of normal and early-phase keratoconic corneal keratocytes were established and incubated in serum-free media in the presence or absence of PDGF and TGF-beta. The proteins secreted by these cells over periods of 7 days were harvested for analysis. The total protein produced was determined spectrophotometrically. MMP-2 was visualised by SDS-gelatin polyacrylamide gel electrophoresis and assayed using radiolabelled type IV collagen as substrate. The enzyme inhibitors, TIMP-1 and TIMP-2, were quantified by dot blot immunoassay.

RESULTS

The addition of PDGF or TGF-beta to the culture medium of keratoconic corneal keratocytes had no significant effect on overall protein production, MMP-2 activity or on the amounts of TIMP- 1 and TIMP-2 secreted. These observations also applied to normal corneal keratocytes, with the exception that PDGF induced expression of the Mr 62,000 species of MMP-2.

CONCLUSIONS

PDGF may be involved in the production of the Mr 62,000 species of MMP-2 that is abnormally produced by early-phase keratoconic corneal keratocytes.

Matrix metalloproteinase 2: involvement in keratoconus

PURPOSE

The activation of matrix metalloproteinase-2 (MMP-2) is postulated to be a crucial pathogenic factor behind progressive and chronic diseases in which basement membranes are disrupted. An ocular example is keratoconus. The purpose of the present enquiry was therefore to investigate and compare the activities of the MMP-2 secreted by keratocytes of normal and keratoconic corneas.

METHODS

The spectrum of MMP-2 activities secreted by cultures of keratocytes derived from normal and keratoconic corneas was analysed by zymography. Subsequently, selected preparations were assayed for peptidase activity, using Type I, Type III, Type IV and Type V collagen as substrate, under native conditions and after treatment with a variety of putative activating reagents.

RESULTS

Although MMP-2 of Mr 65,000 on SDS gelatin polyacrylamide gels is the major protease secreted by keratocytes of normal corneas, the keratocytes of early-phase keratoconic corneas secrete an additional zymographic activity of Mr 61,000. From their N-terminal amino acid sequences, both these proteins were shown to be conformers of proMMP-2. Treatment with SDS followed by protein fractionation was required to achieve in vitro activation of the MMP-2 secreted by normal corneal keratocytes. Treatment with SDS alone partially activated the enzyme produced by early-phase keratoconic corneal keratocytes. This procedure and autocatalysis, yielded an enzyme of Mr 43,000 that selectively hydrolysed Type IV and denatured Type 1 collagen.

CONCLUSIONS

The zymographic gelatinase activities of apparent Mr 65,000 and 61,000 are conformers of corneal proMMP-2. Activated enzyme, of Mr 43,000, is more readily generated from protein preparations of the culture media of early phase keratoconic corneal keratocytes than from protein preparations of the culture media of normal corneal keratocytes.

Role of ocular matrix metalloproteinases in peripheral ulcerative keratitis

AIM

Peripheral ulcerative keratitis (PUK) is an ocular manifestation of rheumatoid arthritis and other similar systemic diseases. The purpose of this inquiry was to investigate the involvement of matrix metalloproteinases (MMPs) in the induction and/or maintenance of PUK.

METHODS

Substrate gel electrophoresis was used to characterise the MMP activities secreted by primary cultures of keratocytes derived from normal and perforated pathological corneal specimens, and those present in tears of normal subjects and patients with PUK. Substrate specificity and the in vivo activity status of the secreted MMPs was assessed by SDS-polyacrylamide gel electrophoresis of standard collagens incubated in the presence or absence of the various enzyme preparations.

RESULTS

In addition to MMP-2 of M(r) 66,000, cultured keratocytes derived from perforated corneas of patients with PUK abnormally produce the MMP-2 of apparent M(r) 62,000. Other MMPs and in particular MMP-9 of M(r) 92,000, also occur in the tears of these patients. Their visualisation on substrate polyacrylamide gels correlated with clinical manifestations of disease activity; during periods of disease quiescence they were barely detectable. The steroid prednisolone, frequently used in systemic therapy, had no effect on the in vitro activity of MMP-2, or on its production by cultured corneal keratocytes. Although the in vitro activity of MMP-2 was inhibited by both Cu(2+) and Zn(2+), Cu(2+) apparently induced the keratocytes to produce activated enzyme and Zn(2+) irreversibly inhibited their production of MMP-2.

CONCLUSION

Overexpression of corneal MMP-2 and tear film MMP-9 are characteristic features of patients with PUK and their activation may be a crucial facet of disease initiation or progression. Although effective in systemic therapy for PUK, prednisolone had no direct control over corneal MMP-2 production or activity. Zn(2+) on the other hand inhibited both MMP-2 production and MMP-2 activity and may, therefore, be of therapeutic value if suitably formulated and used in conjunction with systemic steroid treatment.

Effects of synthetic inhibitor of metalloproteinase and cyclosporin A on corneal haze after excimer laser photorefractive keratectomy in rabbits

To evaluate the effects of synthetic inhibitor of metalloproteinase (SIMP) and cyclosporin A (CsA) on corneal haze after excimer laser photorefractive keratectomy (PRK) in rabbits, PRK was performed on 60 rabbits. They were randomized to one of four groups: group A which received topical SIMP, group B which received topical CsA, group C which received both SIMP and CsA, and group D which received vehicles. Another 16 rabbits did not undergo PRK and were randomized to one of four groups: group E which received topical SIMP, group F which received topical CsA, group G which received both SIMP and CsA, and group H which received vehicles. SIMP solution (1 mm) was instilled every two hours and 2% cyclosporin was instilled four times a day, this was carried out for as long as 6 weeks after surgery. At one, two, four, and six weeks after surgery, slit lamp examination was performed with haze gradings recorded, and corneal specimens were obtained from groups A, B, C, and D. In groups E-H, all rabbits were killed after six weeks of eyedrops instillation. Light microscopy and immunohistochemistry for collagen types III, IV, and VI were performed on the specimens obtained. Slit lamp examination and light microscopy revealed that SIMP significantly reduced corneal haze after PRK, but CsA did not. Immunohistochemistry revealed that deposition of types III and IV collagen was detected in ablated area in groups A-D, and SIMP reduced the frequency of positive staining for type III collagen. In groups E-F, corneas were normal. These findings suggest that SIMP significantly reduced corneal haze and the synthesis of type III collagen after excimer laser PRK in rabbits.

Localization of TIMP-1, TIMP-2, TIMP-3, gelatinase A and gelatinase B in pathological human corneas

PURPOSE

Determine the tissue distribution patterns for tissue inhibitors of metalloproteinases (TIMP-1, TIMP-2, TIMP-3), gelatinase A and gelatinase B in normal and pathologic corneas.

METHODS

Corneas were examined by immunohistochemistry, using antibodies to TIMP-1, TIMP-2, TIMP-3, gelatinase A or gelatinase B.

RESULTS

In normal corneas, TIMP-1 antibody stained the epithelium and endothelium. TIMP-2 and TIMP-3 stained the epithelium, keratocytes and endothelium. Gelatinase A staining was weak and restricted to the epithelial cells. Radial keratotomy scars showed increased staining for TIMP-1 and TIMP-2 around the epithelial cell plug and along the incision. Bullous keratopathy corneas showed TIMP staining patterns similar to normal corneas and increased gelatinase A staining in regions of subepithelial fibrosis. Stromal scars of keratoconus corneas also had increased staining with TIMP-1 and TIMP-2 antibodies. In many keratoconus corneas, the TIMP-3 staining pattern was similar to normal corneas. However, in some keratoconus corneas, when Bowman's layer was missing, the stroma beneath was completely devoid of TIMP-3 antibody staining. No gelatinase B was seen in either the normal or diseased corneas.

CONCLUSION

These data suggest that TIMP-1 and TIMP-2 are important for scar formation and corneal remodeling, since they were found in increased amounts at radial keratotomy incision sites and keratoconus scars. The significance of the focal stromal defects in TIMP-3 staining, associated with absence of Bowman's layer on keratoconus corneas, needs to be elucidated. At the stages of disease examined in this study, gelatinase B may not play a significant role in these pathological processes, since it was not seen in any of the corneas examined.

In vivo characterization of host and bacterial protease expression during Pseudomonas aeruginosa corneal infections in naive and immunized mice

PURPOSE

The corneal destruction resulting from P. aeruginosa keratitis is thought to be due, in part, to the actions of host and bacterial proteases on corneal tissue. The intent of the current study was to compare host and bacterial protease expression in the ocular tissues of mice that have been shown to be susceptible to corneal infection with mice that have been protected from such infection as a result of prior immunization.

METHODS

Supernatants of infected corneal homogenates from naive mice or from mice immunized as a result of prior corneal infection or that had been immunized against bacterial protease were analyzed by gelatin zymography for the presence of host gelatinases and bacterial proteases at various times after infection. In addition, studies were carried out to examine the effect of immunization with bacterial protease on numbers of ocular bacteria.

RESULTS

Mice that were protected against the tissue destruction during infection were found to have reduced proteolytic activity in corneal tissues when compared with susceptible mice. More specifically, the resistant mice displayed lower levels of bacterial alkaline protease, latent MMP-2, and both the active and latent forms of MMP-9. In addition, immunization against either bacterial alkaline protease or elastase failed to reduce bacterial numbers in ocular tissues from 6 to 72 hours after infection.

CONCLUSIONS

These results suggest that increased proteolytic activity in ocular tissues during P. aeruginosa infection may contribute to the irreversible corneal damage observed during the infection.

Chemical injuries of the eye: current concepts in pathophysiology and therapy

Chemical injuries of the eye may produce extensive damage to the ocular surface epithelium, cornea, and anterior segment, resulting in permanent unilateral or bilateral visual impairment. Pathophysiological events which may influence the final visual prognosis and which are amenable to therapeutic modulation include 1) ocular surface injury, repair, and differentiation, 2) corneal stromal matrix injury, repair and/or ulceration, and 3) corneal and stromal inflammation. Immediately following chemical injury, it is important to estimate and clinically grade the severity of limbal stem cell injury (by assessing the degree of limbal, conjunctival, and scleral ischemia and necrosis) and intraocular penetration of the noxious agent (by assessing clarity of the corneal stroma and anterior segment abnormalities). Immediate therapy is directed toward prompt irrigation and removal of any remaining reservoir of chemical contact with the eye. Initial medical therapy is directed promoting re-epithelialization and transdifferentiation of the ocular surface, augmenting corneal repair by supporting keratocyte collagen production and minimizing ulceration related to collagenase activity, and controlling inflammation. Early surgical therapy if indicated, is directed toward removal of necrotic corneal epithelium and conjunctiva, prompt re-establishment of an adequate limbal vascularity, and re-establishment of limbal stem cell population early in the clinical course, if sufficient evidence exists of complete limbal stem cell loss. Re-establishment of limbal stem cells by limbal autograft or allograft transplantation, or by transfer in conjunction with large diameter penetrating keratoplasty, may facilitate development of an intact, phenotypically correct corneal epithelium. Limbal stem cell transplantation may prevent the development of fibrovascular pannus or sterile corneal corneal ulceration, simplify visual rehabilitation, and improve the visual prognosis. Advances in ocular surface transplantation techniques which allow late attempts at visual rehabilitation of a scarred and vascularized cornea include limbal stem cell transplantation for incomplete transdifferentiation and persistent corneal epithelial dysfunction, and conjunctival and/or mucosal membrane transplantation for ocular surface mechanical dysfunction. Rehabilitation of the ocular surface may be followed, if necessary, by standard penetrating keratoplasty if all aspects of ocular surface rehabilitation are complete, or by large diameter penetrating keratoplasty if successful limbal stem cell transplantation cannot be achieved but other ocular surface rehabilitation is complete.

Keratouveitis--two families with a dominantly inherited disorder

Herein we describe a relapsing acute keratouveitis without known aetiology. The disorder has been found in two families and shows an autosomal dominant inheritance. Acute anterior uveitis can be traced for five generations in one of these families. In the same family there is also an association to an autosomal dominantly inherited vascular disorder (Osler-Rendu-Weber disease). The anterior uveitis has an acute onset, a recurrent pattern and a mild activity. The associated keratitis is seen as a midstromal thin flat disc in the central cornea, sometimes with folds in Descemet's membrane as a sign of oedema. This causes a change in refraction and a mild reduction of the corrected visual acuity. There is a prompt response to topical steroid treatment, but the corneal changes have in one case become permanent. Clinical documentation and the mode of inheritance is presented.

Cleavage of structural components of mammalian vitreous by endogenous matrix metalloproteinase-2

Our goal was to determine if the major endogenous vitreous matrix metalloproteinase (MMP-2) could digest known collagenous components of the vitreous body. Matrix metalloproteinase-2 and its associated inhibitors were isolated from porcine vitreous by affinity column chromatography. The inhibitors were inactivated by chemical modification with dithiothreitol and iodoacetamide. The latent MMP-2 was then activated with the organomercurial, p-aminophenyl mercuric acetate (APMA). Bovine vitreous fibrillar collagens (types II, V/XI and IX) were isolated by pepsin extraction and differential salt precipitation. Intact type IX collagen was purified by selective salt precipitation followed by ion exchange and size exclusion chromatography. These isolated collagens were incubated for 6 to 24 h with different concentrations of activated MMP-2, and the extent of collagen degradation was analyzed. Activated MMP-2 was also introduced into freshly isolated vitreous gels and the degree of liquefaction was determined. Our results showed that the activated MMP-2 has no apparent effect upon type II collagen but can degrade type V/XI collagen and type IX collagen fragments (COL2 and COL2 + COL3). In addition, when the type IX collagen was in the intact helical form, MMP-2 appeared to selectively digest alpha 3 (IX) chains. This suggested that vitreous MMP-2 preferentially cleaved certain vitreous collagen chains into large fragments rather than small peptides. MMP-2 also disrupted the vitreous gel in vitro, releasing proteins but not hexuronic acid or sulfated glycosaminoglycans into the liquefied supernatant. We conclude that MMP-2 activity should be considered as a potential mechanism of vitreous liquefaction that is seen in aging and various pathological states.

Collagenase (MMP-1) and TIMP-1 in destructive corneal disease associated with rheumatoid arthritis

The aim of the study was to immunolocalise interstitial collagenase (MMP-1) and the tissue inhibitor of metalloproteinases (TIMP-1) in ulcerating corneas from patients with rheumatoid arthritis, to determine whether changes in expression are associated with destructive corneal disease. Collagenase was expressed by stromal cells in 8 of 8 ulcerating corneas but was not seen in normal tissue (n = 3). TIMP-1 was abundant throughout the normal stroma, but was much reduced or absent from diseased corneas. Collagenase staining was frequently more intense near the epithelial surface and associated with a cellular infiltrate consisting of activated antigen-presenting cells (HLA-DR+), many of which were macrophages (CD68+) and derived from the epithelium or limbus (S100+). Interstitial collagenase produced by infiltrating macrophages and/or stimulated corneal fibrocytes is probably a major mediator of collagen degradation in rheumatoid corneal ulceration. In addition, reduced levels of TIMP-1 expression are consistent with collagenase activity and tissue destruction. Epithelial-stromal cell interactions and the production of local inflammatory mediators are of major importance in the pathogenesis of corneal destruction, although the precise nature of the antigenic stimulation and/or cellular interactions remains to be elucidated.

Over-expression of a gelatinase A activity in keratoconus

Keratoconus is an ocular disorder in which the central cornea becomes thin, conical and frequently scarred. We are exploring the possibility that this condition is induced and maintained by proteases that exist in the corneal matrix in an activated form. In this study, the activities of the proteases secreted in vitro and in vivo by keratocytes of normal, clear keratoconic, scarred keratoconic and traumatically scarred corneas have been compared and partially characterised. Data obtained by assaying acyl transferase activity showed that the matrix metalloproteinases account for a minimum of 95% of the total protease secreted by cultured keratocytes. Their summated specific activity was consistently and significantly higher in the culture medium of keratoconic keratocytes than in the medium of other keratocyte cultures. Analysis of the individual protease activities secreted by these corneal keratocytes in vitro and in vivo by SDS-gelatin polyacrylamide gel electrophoresis showed that a gelatinase of molecular weight 65,000 is the major protease secreted by normal keratocytes. Whereas clear keratoconic and traumatically scarred corneal keratocytes secrete an additional activity of molecular weight 61,000, scarred keratoconic corneal keratocytes generally produced little or none of this gelatinase activity. Both activities may be ascribed to gelatinase A, and although the 61,000 molecular weight form may be a significant feature of keratoconus, neither appears to be active as secreted.

Characterization of an endogenous metalloproteinase in human vitreous

The vitreous is a gel-like connective tissue that undergoes liquefaction during aging and pathological processes. We isolated and characterized a degradative enzyme from the vitreous of different species and identified it to be matrix metalloproteinase-2 (MMP-2). This enzyme was found in a latent form and may be associated with endogenous inhibitors. Vitreous isolated from both non-diabetic and diabetic patients contained MMP-2 in the same concentrations. However, the diabetic samples had an additional gelatinase activity at 92 kDa which may be associated with a compromised vasculature. These results suggest that the normal human vitreous contains an endogenous MMP and the appearance of an additional activity is associated with pathologic conditions.

Alpha 1-antichymotrypsin is present in and synthesized by the cornea

The proteinase inhibitor alpha 1-antichymotrypsin is present in the epithelial, stromal and endothelial layers of the human cornea. This was determined by immunolocalization in corneal sections and by Western blot analysis of extracts from the three separated layers. The inhibitor was quantified in the extracts by immunodot blot analysis. The levels observed were 1.3 +/- 0.3 microgram/cornea for the epithelial layer, 22.8 +/- 3.8 micrograms/cornea for the stromal layer and an average of 0.17 micrograms/cornea for the endothelial layer. alpha 1-Antichymotrypsin is being synthesized by the cornea. Metabolically labeled inhibitor was immunoprecipitated from the three layers following organ culture of the intact cornea. Two major forms were detected. These were the native, mature 64 kDa form and a 50 kDa form which is either a degradation product or an incompletely glycosylated form. These results indicate that the cornea has the ability to locally control degradation through synthesis of this inhibitor. Local synthesis of this inhibitor releases the cornea from total dependance upon the vascular system for its supply of alpha 1-antichymotrypsin.

Characterization of a human corneal metalloproteinase inhibitor (TIMP-1)

The gradual corneal thinning seen in keratoconus may be due to altered degradation of the corneal extracellular matrix. Studies have shown that human keratocytes produce matrix metalloproteinase-2 (MMP-2) and two proteins (28 kDa and 21 kDa) that are capable of inhibiting the activity of MMP-2. In the present study, the 28 kDa inhibitor from keratoconus keratocyte cultures has been characterized as it may be important to the elevated MMP-2 activity seen in these cultures. Biochemical analyses indicated that this keratoconus corneal inhibitor was similar to TIMP-1 from other sources. Oligonucleotides to the reported sequence of human tumor cell TIMP-1 were used for reverse-transcriptase PCR to generate a 700 bp clone of the 28 kDa inhibitor from keratoconus keratocyte cytoplasmic RNA. Sequence analysis verified that the clone was nearly identical to the reported human TIMP-1 with a single base substitution that did not affect the predicted amino acid sequence. In addition, protein translated from the clone corresponded to the expected size. This data suggests that the elevated levels of gelatinolytic activity in these keratoconus keratocyte cultures is not due to a primary alteration of the TIMP-1 molecule. Protein expression studies of the TIMP-1 clone are currently underway.

Keratoconus corneas: increased gelatinolytic activity appears after modification of inhibitors

We examined the metalloproteinase activity from normal and keratoconus corneal extracts. No differences were detected in the total amount of the metalloproteinase or its physical form of activation. However, there was a significant elevation in enzymatic activity in the keratoconus extracts after chemical modification of inhibitory elements. This suggests either a difference in the enzymatic capabilities of keratoconus corneas or, as suggested previously, a decrease in the amount of TIMP (tissue inhibitor of metalloproteinase) present in the tissue.

Collagenolytic/gelatinolytic metalloproteinases in normal and keratoconus corneas

Cells of keratoconus corneas have been reported to produce higher levels of collagenolytic/gelatinolytic enzymatic activities than do cells of normal corneas. The current study investigates the contribution of 1) specific enzyme gene products, and 2) the degree to which these proteins are present in the activated forms, to the increased enzymatic activities. We demonstrate that two neutral gelatinolytic enzymes, a 66/59 kD form and a 92 kD form, can be directly extracted from both normal and keratoconus corneas. These enzymes are identified as the pro- and activated forms of MMP-2 and as the pro-form of MMP-9, specific members of the matrix metalloproteinase family. Normal and keratoconus corneas show no significant differences in amounts or types of extractable neutral gelatinases, nor in the amounts or types that they synthesize in culture. Furthermore, in both the normal and keratoconus corneas, gelatinases are found primarily in the inactive form. These studies suggest the possible importance of changes in proteinase inhibitor levels to the characteristic biochemical features of keratoconus corneas.

Wound healing of the ocular surface

Wound healing is a complex, long-lasting regulatory sequence that involves expression of a number of genes, which are active during the individual's development. Some of the phenomena differ from normal tissue turnover and growth only quantitatively. This article reviews the current data on corneal wound healing, with particular reference to mesenchymal matrix proteins and their integrin receptors, to growth factors and to proteolytic enzymes. Some inflammatory mediators are also discussed. The theoretical basis for therapeutic interventions is also discussed briefly, in the light of present knowledge.

The presence of neutral metalloproteolytic activity and metalloproteinase inhibitors in the interphotoreceptor matrix

Neutral proteolytic activity, having a pH optimum of about 7, was present in the high molecular weight fraction of bovine interphotoreceptor matrix (IPM) separated by gel filtration on Sephacryl S-500. The enzyme(s) was active toward a number of exogenous substrates, including albumin, Azocoll, and gelatin. However, it was inactive toward a synthetic substrate for bacterial collagenase. Proteolytic activity was proportional to protein; however, the time course of the reaction was nonlinear, suggesting that "activation" of a precursor form might be necessary. Of a number of specific inhibitors tested, those directed toward metalloproteinases (1,10-phenanthroline greater than EDTA greater than EGTA) proved most effective. While activity was also inhibited by sulfhydryl reagents and dithiothreitol, inhibitors specific for cysteine proteinases were ineffective. Higher specific activity was present in IPM obtained from retinal pigment epithelium (RPE) than from retina. An endogenous proteinase inhibitor(s) was also found in IPM from both RPE and retina. It was effective against the endogenous metalloproteolytic activity of IPM and also against thermolysin, but not against trypsin or papain. Fractionation of IPM on Sephacryl S-500 revealed a broad peak of inhibitory activity at molecular weights of less than 10(5) daltons. This is the first report of the presence of neutral proteolytic activity and metalloproteinase inhibitor(s) in bovine IPM. These materials may function in concert to maintain the proper level of various components within this matrix.