Characterization of collagenolytic activity in the ulcerating cornea

The Relationship between Corneal Nerve Morphology and Inflammatory Mediators and Neuropeptides in Healthy Individuals

SIGNIFICANCE

This study set out to explore the relationship between the ocular surface immune and nervous systems by exploring corneal nerve structure and the presence of inflammatory mediators and neuropeptides in the tear film.

PURPOSE

The purpose of this study was to determine the association between corneal nerve morphology and tear film inflammatory mediators and a neuropeptide in healthy individuals.

METHODS

Flush tears were collected from both eyes of 21 healthy participants aged 39.7 ± 9.9 years (10 females, 11 males) and analyzed for substance P, matrix metalloproteinase-9, tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), tumor necrosis factor α, and interleukin 6. In vivo central corneal confocal microscopy was performed on the right eye, and eight images were captured. Variables measured were corneal nerve fiber length (CNFL), corneal nerve density (CNFD), corneal nerve branch density, fiber total branch density, corneal nerve fiber area, corneal nerve fiber width (CNFW), and corneal nerve fractal dimension (CNFrac). For each eye, the average across the images and the maximum and minimum values were determined for each variable. Pearson correlation analysis was performed to test for associations.

RESULTS

Substance P correlated with CNFrac (max) (r = -0.48, P = .03) and CNFW (min) (r = -0.52, P = .02). TIMP-1 correlated with CNFD (average) (r = -0.53, P = .03), CNFL (average) (r = -0.49, P = .05), CNFrac (max) (r = -0.49, P = .05), and CNFD (min) (r = -0.55, P = .02). Interleukin 6 correlated with CNFW (average) (r = -0.49, P = .05), the standard deviation of CNFL (r = -0.51, P = .04), CNFL (max) (r = -0.50, P = .04), CNFrac (max) (r = -0.50, P = .04), and CNFW (min) (r = -0.55, P = .02). Tumor necrosis factor α, matrix metalloproteinase-9, and its ratio with TIMP-1 did not correlate with any corneal nerve parameters.

CONCLUSIONS

Both inflammatory mediators and neuropeptides correlated with measures of corneal nerve morphology, supporting the link between the inflammatory and nervous systems.

Effect of topical antiinflammatory drugs on mechanical behavior of rabbit cornea

Background

A variety of antiinflammatory therapies are employed to promote corneal wound healing. The effects of steroidal and nonsteroidal antiinflammatory drugs on the biomechanical properties of rabbit cornea were investigated over time using tensile tests.

Methods

Full-thickness incisions were made and used to analyze the effects of dexamethasone sodium phosphate 0.1% and diclofenac sodium 0.1% on corneal biomechanical properties during wound healing at 7, 14 and 21 days after surgery.

Results

The full-thickness incision deeply modified all of the mechanical properties. At 3 weeks after incision, regardless of the drug therapy, the tensile modulus was about 70% of the value for the intact cornea.

Conclusions

Topical treatment with dexamethasone was particularly effective during the first week after surgery; the second week after surgery, a similar result was observed in the corneas treated with diclofenac. Low doses of steroidal and nonsteroidal antiinflammatory drugs would seem to have the potential to improve biomechanical properties only during the early stage of the healing process of the cornea.

Distribution of collagenase and cell types in sterile ulceration of human corneal grafts

Extensive clinical and experimental evidence has suggested a role for corneal epithelium, keratocytes, and acute inflammatory cells in sterile corneal ulceration, although the precise role of these cell types in stromal matrix degradation has not been elucidated. We studied two corneal buttons and two whole corneas from 4 patients with corneal grafts and sterile stromal ulceration. Each cornea was divided into several sections and examined morphologically, while adjacent sections were placed in tissue culture and assayed for collagenase activity against type I collagen. In each case, collagenase activity was present in tissue from ulcerating and nonulcerating areas and, in most regions assayed, exceeded collagenase activity in normal control peripheral donor corneas. Collagenase levels were not always greatest in areas of most advanced ulceration. Although polymorphonuclear leukocytes were extensively present histologically in two cases, one case had a mixed inflammatory cell population at the site of ulceration, and another had only keratocytes (fibroblasts) present in the entire cornea. We conclude that regional differences in collagen degradation are due not only to the presence of collagenase, but also to cellular and humoral activators and inhibitors that determine collagenolytic activity.

Profiles of matrix metalloproteinase activity in equine tear fluid during corneal healing in 10 horses with ulcerative keratitis

OBJECTIVE

Levels of tear film matrix metalloproteinases (MMPs) activity are significantly elevated in horses with ulcerative keratitis and contribute to the excessive breakdown of stromal collagen. Changes in the amount of proteolytic activity in horse tear film during corneal healing and stromal remodeling have not yet been reported, but we hypothesize they should decrease. In the present study we analyzed serial tear fluid from horses with ulcerative keratitis to identify any changes in MMP activity during corneal healing and stromal remodeling.

PROCEDURES

Samples of tear fluid were obtained from both eyes of 10 horses with ulcerative keratitis on the day of admission (day 1) at the hospital and then at various time points until complete healing of the cornea. Tear film MMP2 and MMP9 activity was determined by quantitative gelatin zymography. In all cases medical treatment included topical applications of equine serum, antibiotics, atropine and systemic administration of anti-inflammatory drugs. Surgical procedures were performed in several cases on day 2 in addition to the medical treatment.

RESULTS

The mean total MMP activity (+/- SD) measured in relative standard units (RSU) in the tear fluid of the ulcerated eye (2.44 +/- 1.44) of the 10 horses was significantly higher than the mean in the contralateral eye (0.81 +/- 0.68) (P = 0.006), on the day of admission at the VMTH. The mean MMP activity in these ulcerated eyes significantly decreased (-82.4%) between the first day of admission and the day when the ulcer had completely healed (P = 0.0002). The activity level in the healed eye (0.43 +/- 0.17) was not significantly different to the one in the contralateral eye (0.36 +/- 0.18) on the day of complete corneal healing (P = 0.374). The level of MMP activity in the contralateral eye also decreased from 0.81 +/- 0.68-0.36 +/- 0.18 but this decrease (56%) was not significant (P = 0.069).

CONCLUSIONS

Ulcerative keratitis in horses is associated with initially high levels of tear film proteolytic activity that decrease as the ulcers heal. The success of medical and surgical treatment of the corneal ulcers is reflected by the enzyme activity in tears. In horses successful treatment does lead to a rapid reduction in tear film proteolytic activity that corresponded with the improvement in the clinical signs of corneal ulceration. Measurement of MMP activity in the tear film might represent a way to monitor the progression of corneal healing in horses with ulcerative keratitis.

Evaluation of various compounds to inhibit activity of matrix metalloproteinases in the tear film of horses with ulcerative keratitis

OBJECTIVE

To examine in vitro effects of various antiproteolytic compounds on activity of matrix metalloproteinase (MMP)-2 and -9 in the tear film of horses with active corneal ulcers.

SAMPLE POPULATION

Samples of tear film obtained from the eyes of 34 horses with active ulcerative keratitis.

PROCEDURE

Horses were sedated, and tear samples were collected from the lower fornix of 34 ulcerated eyes by use of capillary tubes. The protease inhibitors 0.2% EDTA, 0.1% doxycycline, 10% N-acetylcysteine (NAC), 0.1% solution of a modified dipeptide that contains hydroxamic acid (ie, ilomostat), 0.1% alpha1-proteinase inhibitor (PI), 0.5% alpha1-PI, and 100% fresh equine serum (ES) were used to treat pooled samples. Amount of latent and active MMP-2 and -9 was measured by optical density scanning of gelatin zymograms of treated and untreated tear samples.

RESULTS

Pooled tear samples obtained from ulcerated eyes contained the latent and active forms of MMP-2 and -9. Compared with MMP activity in untreated samples, total MMP activity (sum of all bands detected) observed on the gelatin zymogram gels was reduced by 99.4% by EDTA, 96.3% by doxycycline, 98.8% by NAC, 98.9% by ilomostat, 52.4% by 0.1% alpha1-PI, 93.6% by 0.5% alpha1-PI, and 90.0% by ES.

CONCLUSIONS AND CLINICAL RELEVANCE

We documented that EDTA, doxycycline, NAC, ilomostat, alpha1PI, and ES inhibited MMP activity in vitro. Because these compounds use different mechanisms to inhibit various families of proteases in the tear film of horses, a combination of these protease inhibitors may be beneficial for treatment of corneal ulcers in horses.

PAF antagonists as possible inhibitors of corneal epithelial defects and ulceration

The extracellular matrix (ECM) plays a crucial role in cell adhesion, differentiation and wound-healing. Its stability is tightly controlled by enzymes that regulate the metabolism of its components (e.g collagen, fibronectin, laminin). We have found that in the cornea, a potent lipid inflammatory mediator platelet-activating factor (PAF) activates the expression of two metalloproteinases (MMP-1 and MMP-9) as well as urokinase-plasminogen activator (uPA). uPA is of particular interest because, as a serine protease, it is at the top of the protease cascade. PAF may contribute to the destruction of the ECM and the formation of epithelial defects and corneal ulcers by activating uPA and then proteases. We also investigated how several PAF antagonists with different binding affinities can block the expression of the uPA gene. Our results suggest that PAF antagonists with affinities for intracellular binding sites and/or specific structures derived from triazolobenzodiazepine could be of therapeutic use to limit the breakdown of the ECM and the development of ulcer formation.

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.

Stromal fibroblasts synthesize collagenase and stromelysin during long-term tissue remodeling

The process of connective tissue remodeling is an important mechanism contributing to tissue morphogenesis in development and homeostasis. Although it has long been known that remodeling tissues actively mediate collagenolysis, little is understood about the molecular mechanisms controlling this cell-regulated process. In this study, we examined the biosynthesis of collagenase and the related metalloproteinase, stromelysin, during remodeling of repair tissue deposited after mechanical injury to the rabbit cornea. Neither enzyme was synthesized by uninjured corneas; however, synthesis and secretion was detectable within one day after injury. Collagenase accumulated in its latent form while stromelysin appeared to be partially activated. Enzymes were synthesized by cells having a fibroblast phenotype. These cells were found within the stroma. New synthesis was correlated with accumulation of enzyme-specific mRNA. Highest levels of enzyme synthesis were observed in the repair tissue. However, stromal cells outside of the repairing area also synthesized both enzymes. The level of synthesis decreased in a gradient radiating from the repair tissue. Total synthetic levels in a given area of cornea were dependent on both the number of cells expressing enzyme and the rate of enzyme synthesis. Synthesis of collagenase was detected in repair tissue as long as nine months after injury. Our findings provide direct support for the hypothesis that new collagenase synthesis by cells in repair tissue is the first step in collagen degradation during long-term tissue remodeling.

Inflammatory mediators in alkali-burned corneas: preliminary characterization

Leukocyte chemotactic factors (LCF) are important inflammatory mediators which activate and recruit leukocytes from the circulation into sites of tissue damage. These factors were recently detected in the tear fluid of inflamed eyes induced by alkali burn. It remains unclear, however, whether the detected LCF are released from injured corneal tissues or leaked from the circulation. Using a corneal cup model developed in our laboratory, we began examining the capability of corneal tissues to produce LCF in response to alkali injury. We also evaluated the influence of citric acid on the production of LCF from corneas preinjured by the alkali sodium hydroxide (NaOH). For these studies, the epithelial surfaces of corneas isolated from bovine and human eyes were exposed to 1N NaOH for 35 seconds at room temperature. The NaOH was then removed and the epithelial surfaces washed once with buffer and incubated with culture medium for 1, 2, 4, and 6 hours at 37 degrees C/5% CO2 atmosphere. Our results showed that (1) NaOH2 induced corneal epithelial cell injury ranging from cell discoloration and moderate damage of the upper half of the epithelium (1-4 hrs) to total destruction of the epithelium (6 hrs); (2) NaOH-injured corneas (2 hr incubation post injury) produced significant levels of chemotactic activity (via checkerboard analysis) specific for neutrophils (115% maximum chemotactic response [MCR]) and mononuclear cells (94% MCR); (3) preliminary characterization of these factors revealed that they are protease and heat sensitive, extractable by organic solvents, and possess molecular weight values greater than 100,000 daltons; and (4) incubation of NaOH-pretreated corneas with 0.01% citric acid for 2 hours markedly inhibited the production of LCF for both neutrophils (98% inhibition) and mononuclear cells (91% inhibition). Results of these studies indicate that alkali-burned bovine and human corneas generate leukocyte chemoattractants which differ in their biochemical characteristics from previously known low molecular weight chemotactic factors such as C5a, interleukin-1, or leukotriene B4.

Inhibition of collagenase activity by extracts of bovine ocular tissues

Bovine eyes were dissected and separate pools of lens, lens capsule, cornea and vitreous were extracted in guanidine, subjected to ultrafiltration, and examined for their effects on collagenolytic activity. Although lens extract was not inhibitory, the cornea and vitreous both contained inhibitors of collagenase. More inhibition was present in the filtrate of the vitreous than in the retentate, whereas the total amount of inhibition in the cornea was distributed almost equally between the two fractions. The inhibition observed was dose dependent. The partially purified inhibitors from cornea and vitreous blocked the activity of human skin and tadpole back skin collagenases, but they failed to inhibit the bacterial (Clostridium histolyticum) collagenase. The inhibitor was stable to heating to 60 degrees for 30 minutes and to trypsinization.

Enzymatic activity of beta-N-acetylglucosaminidase in the alkali-burned rabbit cornea

Rabbit eyes burned with 1 n NaOH were studied for enzymatic activity by biochemical methods. The activity of beta-N-acetylglucosaminidase (E.C. 3.2.1.50), one of the hydrolases, was found to be significantly increased in corneal epithelium and stroma 1, 2, 3, and 4 weeks after alkali burns. The increase in enzyme activity seemed to be in correlation with the polymorphonuclear leukocytes (PMNs) infiltration of the stroma, as found by histopathologic studies. Therefore the PMNs might be the source of the increasing activity of this enzyme after alkali burns.

Decision-making in the therapy of external eye disease: noninfected corneal ulcers

Despite diverse and multiple clinical etiologies, the pathogenesis of noninfected corneal ulcers consistently relates to persistent or extensive epithelial defects, concomitant with stromal inflammatory cell infiltration and enzymatic degradation of collagen and ground substance. The therapeutic approaches to these disorders may be considered on three levels: (1) the determination of etiology and initiation of primary therapy; (2) promotion of epithelial healing; and (3) limitation of ulceration and support of repair, the latter aspects currently featuring pharmacologic management and limited surgical modalities, particularly the use of tissue adhesive.

Peripheral corneal melting syndrome in association with psoriasis: a report of two cases

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Ulceration of the cornea in rheumatoid arthritis

Five patients with melting of the cornea in association with rheumatoid arthritis are described. The arthritis was often inactive and without systemic manifestations, in contrast to that seen in association with scleritis. In 3 there was evidence of reduced tear formation, but in none was tear production absent. In 3 patients the lesions healed during treatment with azathioprine or penicillamine.

Collagenolysis of rat tail tendons by crude corneal collagenase and clostridiopeptidase A

An experimental apparatus, which uses freshly collected, nondenatured rat tail tendons as substrate against crude corneal collagenase from alkali-burned rabbit corneas and clostridiopeptidase A, is introduced. The apparatus makes it possible to compare the collagenolytic activity of the two enzymes directly on intact connective tissue similar to intact corneal tissue. It was found that the two enzymes were able to reduce the tensile strength of rat tail tendons to less than 100 g in less than 5 h. The two enzymes attacked the tendons in structurally the same manner, estimated from a statistical model. The conclusion drawn is that crude corneal collagenase can degradate intact connective tissue indicating that it can attack the intact cornea.