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9ß1 Integrin Implicated in Progression of the Disease
1From the Departments of Anatomy and Cell Biology and 2Biological Sciences, The George Washington University, Washington, DC; and the 3Department of Ophthalmology, The George Washington University Medical Center, Washington, DC.
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Abstract |
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9 integrin expression during wound healing.
METHODS. Corneal epithelial debridement wounds of two sizes (1.5 and 2.5 mm) were made on the ocular surface of BALB/c mice and were evaluated at various times after wounding. Corneas were processed either as whole mounts and stained with propidium iodide and an antibody against 9 integrin or for bromodeoxyuridine analyses of cell proliferation. A separate study involved analyses of corneal wound healing over time in individual mice with large and small debridement wounds. Mice were anesthetized once per week and their corneas stained with fluorescein to assess the quality of the corneal epithelium. After 6 weeks, mice were killed and eyes processed for study by immunofluorescence in either whole mounts or frozen sections.
RESULTS. Whole mount confocal microscopy showed open wounds on the ocular surface of mice at 1 and 2 weeks after large wounds were created, but not after small wounds. In addition, 9 integrin was upregulated during healing, and changes were observed in 9 integrin localization at the limbus with large wounds but not with small wounds. Although only 1 of 16 corneas with small wounds had erosions at 1 and 2 weeks, 11 of 16 with large wounds had erosions. However, by 6 weeks, 13 of 16 eyes showed signs of erosion whether wounds were small or large. With large wounds, RCES corneas frequently showed numerous goblet cells adjacent to a limbus lacking 9 integrin. Corneas from mice with documented RCES showed both retention of 9 integrin and tenascin-C expression at the anterior stromal–epithelial interface as well as impaired relocalization of 3ß1 integrin to the basement membrane zone.
CONCLUSIONS. These data show that spontaneous recurrent corneal erosions occur in a mouse model after manual creation of a single wound by debridement. Differences between the healing of small (1.5 mm) and large (2.5 mm) wounds were observed. Large wounds often resulted in the presence of goblet cells on the central cornea and a loss of 9 integrin at the limbus. Small wounds never showed differences in the localization of 9 integrin at the limbus, and no goblet cells were observed in the central cornea. More studies are needed to understand the causes of erosions in these mice.
The apparent cause of RCES is failure of the epithelial cells to regain tight adhesive contacts with the underlying corneal stroma after trauma. In epithelial basement membrane disease, it is the failure of the epithelial cells to maintain tight adhesion to a defective basement membrane. Cell adhesion molecules of the integrin family are in large part responsible for the adhesion of the corneal epithelium to the stroma. Integrins function as ß heterodimers and are involved in mediating actin-based cell adhesion and migration and intermediate filament-based cell–substrate adhesion through hemidesmosomes (HDs). HDs are the adhesion complexes that mediate tight adhesion of skin and corneal epithelium to their underlying basement membranes.10 11 They are composed—from inside to outside the cell—of keratin intermediate filaments, plectin, BP230, BP180, 6ß4 integrin, laminin-5 anchoring filaments, and type VII collagen anchoring fibrils.12 13 Plectin and BP230 form the intracellular plaque of the HD and mediate interaction with keratins. BP180 and 6ß4 integrin are both integral membrane components of HDs, laminin-5 is a specialized component of the basement membrane, and type VII collagen fibrils penetrate deep into the corneal stroma to ensure firm adhesion of the epithelium to the underlying connective tissue. In the cornea the optical clarity essential to allow light to focus on the retina requires an optically smooth surface. This anatomic necessity results in an exposed, sheer surface vulnerable to debridement by friction and rubbing. During migration after an injury, HDs disassemble, and epithelial sheet migration allows open wounds to be covered quickly and efficiently.11 14 Once migration is complete, the HDs must reassemble to restore tight epithelial adhesion.
Efforts to determine the molecular origin of and to develop more effective treatments for RCES have been slowed by the lack of rodent animal models. Studies of spontaneous erosions have typically used various purebred dogs known to be susceptible to RCES.15 16 Other studies have looked at chronic corneal erosion in animals by repetitive mechanical debridement of the cornea.17 18 For a number of years, we have studied the role of cell adhesion molecules in regulating reepithelialization after mechanical debridement of rat and mouse corneas,11 19 20 21 22 using a model in which a circular area of epithelial cells was removed with a dulled scalpel. The sizes of the wounds varied, depending on the study; either a 1.5- or a 2.5-mm diameter region of corneal epithelial cells was routinely removed from the central corneas of 8-week-old mice. The epithelial basement membrane was left intact at the time of injury and was not denatured by chemical or thermal means after wounding.
Among the proteins whose localizations change after wounding is 9 integrin.22 In the normal, unwounded adult mouse limbus 9 integrin localizes to the apical aspect of the basal cells where it is found both in the cytoplasm and in the cell membrane.23 Studies of the developing mouse eye have shown that this limbal basal cell localization is a relatively late event in the development of the ocular surface. When evaluated at 1 week after birth, mice have a uniform distribution of 9 integrin over the entire ocular surface. By 2 weeks, this distribution pattern changes dramatically, and the central corneal epithelial cells express less 9 integrin and the limbal basal cells more. The adult pattern is established by between 6 and 8 weeks.23
Because the focus of our past work has been reepithelialization, our efforts have focused primarily on the study of healing over short time spans—generally, from hours to several days. However, to provide strain-specific mouse control corneas for studies of knockout mice, we began to assess corneas at time points from 1 to 12 weeks after wounding. During the course of those studies, we observed that, after a single, initial manual debridement wound to an 8-week-old BALB/c mouse cornea, corneal epithelial erosions occurred spontaneously, at times ranging from 1 to 8 weeks after wounding. The results that follow—from studies of wild-type healthy BALB/c mice—are an extension of initial observations after studies of genetically altered mice.24
In the current studies, we characterize, for the first time, an in vivo mouse model of erosions that spontaneously developed in response to a single, initial corneal debridement wound. We show that the RCES epithelium was characterized by both a higher cell proliferation rate than that of unwounded corneal epithelium and by altered localization of 9 integrin. Goblet cells were observed on the corneal surface with large wounds but not with small wounds. We then assessed the healing history of the individual eyes over time (1–6 weeks after wounding), using fluorescein to demonstrate that we were describing RCES rather than chronic open wounds. We were thus able to observe recurrent erosions in the BALB/c mouse after creating both small and large wounds. With the healing history documented for individual eyes, we demonstrated that after the large wounds were made, most RCES eyes had less 9 integrin at the limbus and numerous goblet cells present over the corneal surface. Finally, we show that, in RCES eyes, 3ß1 integrin localization was disrupted, but 6ß4 was localized primarily at the basal aspect of the basal cells. These data will improve our understanding of RCES in both animals and humans. Furthermore, having an experimental model to test hypotheses about causes and possible treatments for RCES should, in time, reduce the pain and improve the quality of life of people with this condition.
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3 and 9, we used polyclonal antibodies we had previously characterized.25 Propidium iodide (PI; P-1304), Alexa Fluor 488 anti-rabbit (A-11,006), anti-mouse (A-11,001), and Alexa Fluor 594 anti-rat (A-11,007) secondary antibodies were obtained from Molecular Probes (Eugene, OR).
Manual Corneal Debridement
All experiments described in this article were conducted in voluntary compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, and all procedures were approved by the George Washington University Animal Care and Use Committee. Eight to 10-week-old male and female BALB/c mice (22 to 25 g) were anesthetized with 250 µL of a 1:10 dilution of a 1:1 mixture of ketamine (100 mg/mL; Aveco Co., Inc., Fort Dodge, IA) and xylazine (20 mg/mL; Miles Inc., Shawnee Mission, KS). Once the animals were anesthetized, a topical anesthetic (proparacaine ophthalmic solution) was applied to their ocular surfaces until the blink sensation was lost, and their corneas were scraped with a dull scalpel to remove the corneal epithelium. For small wounds, a 1.5-mm central corneal area was demarcated with a trephine and removed manually by gentle scraping with a dulled blade. For large wounds, a 2.5-mm area of epithelial tissue was removed, with care taken to avoid limbal blood vessels. After they were wounded, all eyes were treated with erythromycin ophthalmic ointment to minimize inflammation and to keep the ocular surface moist while the mice were under anesthesia. Corneas with large wounds were allowed to heal in vivo for 1, 1.5, 2, and 3 days, and 1, 2, and 4 weeks and those with small wounds for 6, 12, 18, 24, 48, and 72 hours, and 1 and 2 weeks. At least three eyes were assessed for each time point for the data presented in Figures 1 2 3 4 5 . After mice were killed by lethal injection using sodium pentobarbital, eyes were enucleated and either frozen in OCT compound for immunofluorescence (IF) or fixed as whole mounts.
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Immunofluorescence Microscopy on Tissue Sections
For sections, adult BALB/c mice were killed with a lethal injection, and eyes were frozen in OCT compound. Eight-micrometer cryostat sections were cut from the eyes and baked overnight at 37°C. The sections were then rehydrated in PBS for 15 minutes, blocked with blocking buffer, and incubated with primary antibodies (3, 9, and ß4 integrin and tenascin-C) for 1 hour in a moist chamber at room temperature. After they were washed with PBS, the sections were treated with blocking buffer for 15 minutes and were incubated with the appropriate Alexa secondary antibodies for 45 minutes in a moist chamber. They were then washed with blocking buffer for 15 minutes, covered in mounting medium, and coverslipped. The images were then captured with a confocal microscope.
BrdU Cell Proliferation Analyses
Cell proliferation was assessed using the BrdU labeling and detection kit, as recommended by the manufacturer. All incubations were at room temperature unless specified. We assessed time points at 1.5 and 3 days, and 1, 2, and 3 weeks for large wounds, and 4 and 8 weeks for small wounds. Corneas were wounded as described earlier. Control (unwounded) eyes were obtained from mice that had not been subjected to wounding of either eye. At the given time points, the mice were killed and their eyes enucleated and placed immediately into a BrdU-containing labeling solution with complete minimum essential medium (MEM).22 After 30 minutes in the labeling solution, the eyes were washed three times in MEM without BrdU and allowed to incubate for an additional 15 minutes in MEM. Whole eyes were embedded in OCT, and 8-µm sections cut and baked overnight at 37°C. Next, sections were rehydrated in PBS and fixed in ice-cold 70% methanol in 50 mM glycine buffer (pH 2) for 20 minutes at –20°C. After fixation, sections were incubated in PBS twice, for 2 minutes each, then with 2.5% trypsin in 0.1% CaCl2 in PBS, for 3 minutes. They were then briefly washed in PBS and transferred to a solution containing 4 M HCl, for 3 minutes. Sections were washed twice in PBS, for 5 minutes each, and then transferred to blocking buffer. Slides were processed for IF as recommended by the manufacturer (Roche Diagnostics). At least three corneas and two slides per time point were used—each slide containing four to six corneal cross sections. The number of labeled cells in at least three different sections from each slide were counted and the data expressed as the number of cells labeled per unit length of basal cell basal membrane, as measured with image-analysis software (ImagePro Plus ver. 4.1 software; Media Cybernetics, Silver Spring, MD). The data were then analyzed for significance on computer with the unpaired t-test (Instat 1.1 for Macintosh; GraphPad Software, San Diego, CA).
Recurrent Erosion Study
To determine whether erosions were chronic or recurrent, we assessed healing over time in individual animals. We wounded eyes bilaterally, with the small wound in the left eye and the large wound in the right. The purpose of wounding both eyes in the same animal was to correlate the quality of healing of contralateral eyes. This experimental design was critical, because it allowed us to determine whether individual animals were poor healers or whether one size of wound healed more poorly than the other. After initial wounding, the mice were allowed to heal for 1, 2, 4, and 6 weeks. At each time point, the mice were placed under general anesthesia and their eyes stained with fluorescein and viewed using a dissecting microscope modified with blue-light illumination. Wounds were scored as open or closed based on fluorescein staining. Care was taken to make sure that the eyes remained moist after assessment and, before anesthesia wore off; each eye was treated with a single application of erythromycin ophthalmic ointment. At the last time point (6 weeks) wounds were assessed (using fluorescein) to be open or closed. Mice were killed by lethal injection, their eyes restained with a vital dye, and photographs taken using the dissecting microscope, to assess scaring and vascularization. After enucleation, eyes were carefully labeled and either processed for whole mounts or frozen for tissue sectioning.
Confocal Microscopy
Confocal microscopy was performed at the Center for Microscopy and Image Analysis (CMIA) at the George Washington University Medical Center. A confocal laser scanning microscope (model MRC 1024; Bio-Rad, Hercules, CA) equipped with a krypton-argon laser and an inverted microscope (model IX-70; Olympus) was used to image the localization of Alexa 488 (488-nm laser line excitation; 522/35 emission filter) and PI (568 nm excitation; 605/32 emission filter). Optical sections (z = 0.5 µm) of confocal epifluorescence images were acquired sequentially at 1-µm intervals with a 20x objective lens (NA = 0.7) and/or a 10x objective lens (NA = 0.3) with image acquisition software (LaserSharp ver. 3.2; Bio-Rad). Typically, six to eight optical sections, each taken a 1-µm intervals, were merged and viewed en face. Image management software (Photoshop ver. 7.0; Adobe Systems, Mountain View, CA; with Bio-Rad plugins) was used both to convert images from pic (Bio-Rad) into tiff files and to create montages, and the images were presented.
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9 integrin expression is increased in cross sections taken from open wounds 2 days after large wounds,22 this time point occurred late in the reepithelialization process. Most of the corneal epithelium had been forced to disassemble its HDs to migrate, and the entire epithelial sheet was loosely adherent. We have found that it is technically difficult to cut unfixed frozen sections at time points earlier than 2 days after large wounds are made22 ; therefore, we did not know how soon after wounding the increase in 9 integrin occurred or how it was first manifested. It could have increased gradually over the entire corneal surface—first at the limbal region and later toward the leading edge—or it could have increased just at the leading edge.
To study the spatial and temporal differences in 9 integrin localization after imposing large wounds, we used whole mounts of corneal tissues for immunofluorescence. In whole mounts of unwounded control cornea, 9 integrin was observed as a patchy pattern among a subpopulation of cells in the limbal region and was absent from the cells at the central cornea (Fig. 2A) . By sacrificing mice immediately after large wounds were made, we confirmed that these wounds left the 9 integrin–positive limbal basal cells intact at the limbus (Fig. 2B) . One day after imposition of large wounds, 9 integrin was upregulated in the limbal region, as well as in cells behind the leading edge (Fig. 3A) . Upregulation of 9 integrin at the limbus at 1 day was not uniform around the circumference of the cornea, and some regions of the limbus were more positive than others. The segment of the limbus shown in Figure 3A shows an intermediate level of 9 integrin staining. At sites where there were more 9-positive cells in the limbus, there were fewer cells expressing 9 integrin toward the leading edge. Two days after wounding (Fig. 3B) , more 9 integrin was present toward the leading edge but less localized in the limbus. These results indicate that, as a function of time after wounding, 9 integrin expression increased toward the leading edge and decreased at the limbal region (Fig. 3B) . All three wounds assessed at 1 and 2 days were open (Fig. 3 , asterisks). By 3 days, all large wounds assayed were closed, accompanied by decreased 9 integrin expression over the entire corneal surface (Fig. 3D) . In the limbal region, variability was seen in the number of 9 integrin–positive cells. Some regions had none (Fig. 3D) , whereas others had many 9 integrin–positive cells (Fig. 3C) , similar to that observed in unwounded corneas.
The localization of 9 integrin was expected to return to the pattern observed in the unwounded control cornea after 3 days. However, that was not the case. At 1 week after making the large wounds, we found that open wounds had reappeared on the ocular surface, accompanied by increased expression of 9 integrin in the peripheral cornea and toward the leading edge (Fig. 4A 4B 4C) . We hypothesized that by 1 week after wounding, small patches of epithelial cells had eroded, and the cells at the periphery of the new wound site were forced once more to migrate to cover the newly exposed wound. All three large wounds studied showed signs of erosion at 1 week. By 2 weeks after wounding, we also observed 9 integrin on cells near the center of the cornea and around the corneal periphery (Figs. 4D 4E 4F) . While the specimen in Figure 4C showed an erosion at 1 week, the one in Figure 4F did not show an eroded area but rather a depression on the corneal surface, which was covered with 9 integrin-negative epithelial cells. In addition to observing the erosions, we saw fewer cells staining positively for 9 integrin in the limbus (compare Figs. 4A and 4D with Fig. 3 and with the unwounded limbus in Fig. 2 ).
When we evaluated corneas 4 weeks after making the large wounds, all corneas assessed had intact closed epithelia (Figs. 5A 5B) ; however, the corneal epithelium did not appear to be normal. Although 9 integrin was no longer observed in the central cornea, the quality of the healed epithelium varied within and among eyes and the localization of 9 integrin at the limbus was reduced or absent. The images shown in Figures 5A and 5B are from different regions of the same eye. In Figure 5A , the number of 9 integrin–positive cells in the limbal region appeared to be almost back to normal levels (compare Fig. 5A to the limbal region shown in Fig. 2 ), and no 9-positive cells were seen in the central cornea. However, in a different region of the same eye (Fig. 5B) , no 9 integrin–positive cells were seen at the limbal region, and a diffuse low level of staining for 9 integrin was seen among clusters of epithelial cells in the central cornea. In addition, numerous gobletlike cells (some indicated by white arrows) were seen. To confirm that these gobletlike cells were in fact goblet cells, we stained similar corneas 4 weeks after large wounds with anti-mucin antibody MUC5AC (Fig. 5C) . We routinely observed more goblet cells on the corneal surface in regions without 9 integrin in the limbus.
Evidence of Erosion after Manual Debridement in Small Wounds
Because, as shown in Figure 1 , initial reepithelialization was complete at 3 days, we were surprised to see open wounds after 3 days. It is possible that these results, suggesting recurrent epithelial erosions, could be a side effect of removing more than 50% of the corneal epithelial cells during the initial wounding process. To determine whether similar erosions could be observed after small wounds were made, involving removal of less than 40% of the corneal epithelium, we assessed corneas for 9 integrin localization and PI staining, using whole mounts at 6, 12, 18, 24, 48, and 72 hours and at 1 and 2 weeks after the wounds were made (Fig. 6) . No fewer than three eyes were assessed at each time point studied, and the data presented reflect typical results. There was no change in the number or the intensity of staining of 9 integrin–positive cells in the limbal region, at any time point studied (data not shown). From 6 to 12 hours after wounding (Figs. 6A 6B) , there was no expression of 9 integrin at or near the leading edge; between 12 and 24 hours, integrin localization became abundant in and around the margins of the wound as it closed (Figs. 6B 6C 6D) . At 48 hours after wounding, 9 integrin localization disappeared from the central cornea (Fig. 6E) ; however, at 72 hours (Fig. 6F) and at 1 week (Fig. 6G) , some 9 integrin–positive cells were present on the central cornea. The 2-week sample showed low but diffuse staining for 9 integrin on the corneal surface, but no signs of erosions or depressions (Fig. 6H) . These data suggest that erosions of the corneal surface can occur even after small wounding, but appear to be less frequent initially. The small depression seen in Figure 6F at 72 hours was similar to the larger depression seen in Figure 4F , 2 weeks after large wounding. These localized depression sites indicate thinning areas of the corneal stroma.
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Taken together, the data presented thus far indicate that erosion accompanied by increased cell proliferation occur frequently after creation of both small and large manual corneal epithelial debridement wounds.
Similar Frequency of Recurrent Erosions Documented after Induction of Large and Small Wounds
Although the data presented thus far certainly suggest that recurrent erosions occurred after manual debridement wounding in the mice, they are not definitive, given that no individual animal with a corneal wound that first closed and then reopened was studied. It is possible that any one of the individual eyes with a corneal erosion and/or elevated corneal epithelial cell proliferation weeks after wounding had never completely closed after the initial wounding.
To confirm that the erosions we documented were recurrent rather than chronic, we designed an experiment to allow us to assess wound closure over time in individual mice. We anesthetized mice at 1, 2, 4, and 6 weeks after imposing both large and small wounds, and assessed the wounds (using fluorescein eye drops) as either open or closed in individual eyes. After the final assessment at 6 weeks, the mice were killed. All assessments were made under the dissecting microscope, and care was taken to make these observations rapidly, so that a minimal amount of anesthesia could be used. The eyes were then treated with erythromycin ophthalmic ointment to keep them moist and to prevent ocular surface drying during anesthesia. One mouse (animal 14) died at 4 weeks due to anesthesia overdose, and another was killed at 4 weeks due to poor healing of the large wound (animal 5). Data are shown in Table 1 .
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Of the small wounds, 15 of 16 were closed at both 1 and 2 weeks. Of note, the open wound at 1 week was closed at 2 weeks, and the one that was open at 2 weeks had been closed at 1 week. Rather than the number of open wounds decreasing (as occurred with the large wounds), after small wounds were created, the number of open wounds observed increased. At 4 weeks, 11 (69%) of 16 small wounds were closed and at 6 weeks, 8 (57%) of 14 were closed. Only 3 of 16 mice had small wounds closed at every time point assessed. The mice with small wounds that were closed at all four time points (mice 1, 2, and 3) were not the same mice with large wounds that were closed at all four time points (mice 11, 14, and 15).
Despite the trend for the closure rate of large wounds to improve and that of the small wounds to worsen over time, the overall frequency of erosions after induction of small and large wounds was similar (13 of 16 eyes wounded). We took photographs of both eyes of each mouse at the time of death (6 weeks) and evaluated those images for evidence of scaring and neovascularization (data not shown). There were no differences in either the frequency of scaring or in neovascularization between eyes with large or small wounds. The majority of eyes (70% to 80%) had corneal scars, and 50 to 60% showed some evidence of neovascularization at 6 weeks.
Poor Healing Observed in Some Corneas Despite Evidence of Wound Closure at All Time Points
Based on results of our studies of 9 integrin expression after wounding, using whole mounts (Figs. 2 3 4 5 6) , we predicted that the eyes with more frequently open wounds would show more evidence of goblet cells and a lack of 9 integrin in the limbal region. To address this question, 10 corneas from the animals described in Table 1 were used for whole mount IF to detect 9 integrin. Tissues were counterstained with PI. The animals that received the five large and five small wounds, the wound history data taken from Table 1 , and a summary of the results of the IF/PI staining are shown in Table 2 .
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9 staining and overall healing of large wounds. Typical staining of large (animals 16 and 6) and small (animal 15) wounds is shown in Figures 8A 8B 8C . In animal 16, whose large-wound history indicated that it was open at 1 and 4 weeks but closed at 2 and 6 weeks, 9 integrin was absent at the limbus, and there was extensive infiltration of the cornea with goblet cells (Fig. 8A) . For animal 6, whose large wound was open at weeks 1 and 2 but closed at weeks 4 and 6, IF/PI staining showed only a few goblet cells on the corneal surface and very few 9 integrin–positive cells at the limbus. However, numerous blood vessels were observed beneath the epithelial surface (Fig. 8B) . Despite the fact that it was closed at all four time points, the eye with the large wound in mouse 15 had numerous goblet cells and a reduced expression level of 9 integrin (data not shown but summarized in Table 2 ). For the small wound in animal 15, IF/PI staining showed that the cornea had numerous 9 integrin–positive cells at the corneal periphery, adjacent to a region of central depression in the cornea (Fig. 8C) . Whether or not there was 9 staining at the central cornea that had received a small wound, there was never any evidence of goblet cells on the corneal surface. Additional analyses of the small wounds at 6 weeks showed that there was always abundant 9 integrin at the limbus (data not shown but summarized in Table 2 ).
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3ß1 Integrin in Corneas with Recurrent Erosions6ß4 integrin to reassemble into HDs at the basal aspect of basal cells after migration was complete. In previous studies, we showed, by transmission electron microscopy, that the disassembly of HDs occurs during migration after wounding, and that this disassembly is accompanied by relocalization of 6ß4 from the basal aspect of the basal cells at the basement membrane zone (BMZ), to the lateral and apical aspects of the corneal basal and suprabasal cells.21 26 Thus, if 6ß4 fails to form stable HDs in the corneas with recurrent erosions, then localization of 6ß4 would not be restricted exclusively to the BMZ.
The whole mount procedure used for localization of 9 integrin is limited in its ability to show the relocalization of proteins within HDs, because the fixation method reduces the antigenicity of many antigens. Furthermore, it is easier to view the linear basement membrane in cross section than en face. Most of the tissues obtained from mice after the erosion experiment were processed for whole mounts. Six eyes of animals 7, 10, and 13 were frozen unfixed for IF. Typical results are shown in Figure 9 , after staining with antibodies against 9 integrin and tenascin-C—an 9 integrin ligand (Figs. 9A 9B 9C) —and with antibodies against 3 and ß4 integrins (Figs. 9D 9E 9F) . The data show that in age-matched, unwounded control corneas, 3 integrin localized to lateral and apical membranes of the basal and suprabasal cells, as well as to the basal aspect of the basal cells, where it is thought to participate somehow in maintaining the organization of the basement membrane. ß4 integrin was present exclusively at the basal aspect of the basal cells in unwounded corneas where it was an integral component of the HDs. After imposition of both large and small wounds, ß4 integrin had relocalized to the BMZ—even in corneas known to have recurrent erosions—and there were only minor disruptions in linear BMZ staining. In contrast, 3 integrin did not relocalize as well to the BMZ in recurrently eroded corneas with both small and large wounds (Fig. 9E 9F) . This result could be best observed in the merged images of 3 and ß4 integrin staining at sites where the red ß4 staining was visible at the BMZ in the recurrently eroded eyes, but not in the unwounded eyes where there was colocalization of the two proteins, and intense green staining for 3 blocks the red ß4 staining (compare Figs. 9E and 9F with Fig. 9D ).
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9 integrin or tenascin-C. The top panel in Figure 9A is a double-labeled limbal cross-section of a control cornea, where the two antibodies show the presence of the 9 integrin–positive cells in the limbus, with tenascin-C present both in front of the 9-positive epithelial cells and within the deep stroma at the limbus. However, after creation of both small and large wounds, tenascin-C expression in recurrently eroded corneas remained at the anterior aspect of the corneal stroma, and, although it correlated with the expression of 9 integrin at similar sites, there was no clear evidence for colocalization at these sites (compare Figs. 9B and 9C with Figs. 9E and 9F and note the lack of overlap, as indicated by yellow at the BMZ). |
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9 Integrin Upregulation during Wound Repair
After both large and small wounds, our data showed that 9 integrin was upregulated within corneal epithelial cells migrating to cover the wound. 9 Integrin was most often observed just behind the leading edge of the epithelial sheet. Just before closure, the site of the most abundant localization of 9 integrin was where opposing epithelial sheets merged, suggesting a roll in cell–cell adhesion.
9 Integrin is one of the many partners of ß1 integrin and is expressed in unwounded corneal limbus,20 22 skin, airway epithelium, muscle, hepatocytes,27 neutrophils, and intestine.28 It mediates cell adhesion and migration through interaction with its ligands, such as tenascin-C,29 vascular cell adhesion molecule (VCAM)-1,30 the EIIIA segment of fibronectin (FN),31 the polypeptide of von Willebrand factor (pp-vWF), blood coagulation factor XIII (FXIII), tissue transglutaminase (tTG),32 L1-CAM,33 and a disintegrin and metalloproteinase (ADAM) family members.34 9 integrin is known to share numerous functional properties with its closest integrin homologue, 4 integrin, because both integrins recognize the same repertoire of ligands.34 35 Studies by Cai et al.36 and Vitale et al.37 showed, by in situ hybridization, that corneal epithelial cells synthesize alternatively spliced FN mRNAs that include EIIIA+ isoforms during corneal wound healing. In a previous study, we found no evidence for the production of 4 integrin by corneal epithelial cells before or during healing,19 but the expression of 9 integrin we report here would provide a cellular receptor for EIIIA+ forms of FN. The timing of the changes in expression of the EIIIA+ isoforms of FN mRNAs in the cornea in response to injury36 37 corresponds closely to the timing of the changes we observed in 9 integrin localization and suggest that 9 integrin might serve as a receptor for EIIIA+FN during healing.
The Role of HDs in Maintaining Adhesion
The importance of HDs in maintaining epithelial adhesion has been demonstrated in studies of knockout mice.38 39 40 41 Knocking out 6 or ß4 integrin results in the death of a mouse at birth and completely detached skin. Knocking out the 3 integrin gene in mice results in a less severe blistering phenotype, but the mice still die at birth, probably of kidney failure.42 43 It is clear from these gene-deletion experiments that HDs are essential for epithelial–stromal interaction. Whereas 3ß1 is not an absolute component of HDs, it plays an essential role in basement membrane organization that impacts HD assembly.39
The wounds in these studies were created with a dulled scalpel that left the basement membrane intact and not denatured by temperature or chemical treatment. This feature of the model may be critical for the induction of spontaneous erosions after wounding. We have shown in a previous study26 the partial disassembly as a function of time after wounding of the basement membrane after manual debridement in the mouse. The sheer nature of the wound and the partial disassembly of the lamina densa may both contribute to a situation in which reinsertion of deeply embedded anchoring fibrils are inhibited.
Diabetic patients often have corneal abnormalities, including epithelial defects associated with recurrent erosions, and reduced corneal sensitivity. In experiments performed on animal and human diabetic corneas, reduced numbers of HDs and reduced insertion of anchoring fibrils have been reported.44 45 46 47 Both factors have been hypothesized to contribute to the loose epithelial adhesion observed in the corneas of diabetic persons. In a detailed study of basement membrane, integrin, and related protein expression in healthy and diabetic human corneas, Saghizadeh et al.48 found that diabetic corneas upregulated mRNAs for matrix metalloproteinase (MMP)-3 and -10. MMPs degrade extracellular matrix proteins during wound healing. They also play roles in remodeling the basement membrane when migration is complete. In addition to increased MMP-3 and -10 in diabetic patients, increased 3 and ß1 integrin mRNA and protein were observed in extracts of total diabetic corneas, which included endothelial cells and fibroblasts as well as corneal epithelium. Surprisingly, the corneal epithelial cells alone showed no increase in 3 and ß1 integrin. 3ß1 has been shown to induce MMP expression.49 It is possible that increased 3ß1 integrin expression in the stroma by keratocytes in diabetic persons could lead to increased MMP expression, which in turn could alter the structure of the stroma and basement membrane making it difficult to maintain the assembly of HDs. The idea that elevation of MMP levels may play a role in the development of RCES is supported by the work of Garrana et al.50 who showed elevated MMP levels in the epithelial tissues of patients with recurrent erosions. Whether the etiology of recurrent erosions in diabetic persons is similar to the etiology of erosions after trauma to normal eyes is unclear.
Involvement of Both 6ß4 and 3ß1 Integrins in the Stability of HDs
We also found that, compared with 3ß1 in unwounded control corneas, more 3ß1 was localized away from the basal cell BMZ in corneas of RCES eyes. Although it is still not clear what role 3ß1 integrin plays in the reassembly of HDs, data from other studies suggest that remodeling of laminin-5 in the matrix is required to make it competent for the assembly of 6ß4 into HDs, and that this remodeling is regulated by 3ß1 integrin.51 52
In quiescent cells, 3ß1 is found both in the basolateral and basal membranes of the basal cells, whereas 6ß4 is present virtually exclusively at the basal surface of the basal cells within HDs. The interaction of 3ß1 and 6ß4 with one another during migration has been shown to activate Rho-dependent cell adhesion.53 As cells migrate, they secrete laminin-5 with an 3 chain that has a molecular weight of 190 kDa (LN3-190).51 LN3-190 supports 3ß1-mediated migration and upregulation of MMP-2 and -9 expression.49 These MMPs cleave the LN3-190 into the LN3-160 form and play roles in mediating the cleavage of other matrix molecules as migration proceeds. LN3-160 supports migration through 6ß4, and as cells migrate over their underlying wound bed, a matrix containing LN3-160 accumulates beneath them.51
We did not directly measure the number of HDs or the depth of insertion of the anchoring fibrils that develop in the RCES corneas in mice. However, we looked at the localization of the HD component 6ß4 at the BMZ and found little difference between the unwounded corneas and those from RCES mice. Our data do not tell us whether HDs were present but suggest that the initial stages of their reassembly takes place even at sites where erosions occur. 6ß4 is acted on during migration by various tyrosine kinases, including Fyn—phosphorylation of several residues on its cytoplasmic tail induces recruitment of Shc and Ras to the 6ß4 integrin and leads to activation of both the ERK and JNK pathways.54 Migration is regulated by actinomycin contraction, and it remains unclear whether 6ß4 can directly bind to the actin filament network.55 If 3ß1 and 6ß4 were complexed together during migration, 6ß4 could function primarily by mediating those signaling events necessary to sustain migration and 3ß1 could mediate actin cytoskeletal rearrangements. Once migration stopped, laminin-5 synthesis would be downregulated at the transcriptional level53 . Most of the laminin-5 present beneath cells after migration is complete would be LN3-160. In the absence of abundant de novo synthesis of LN3-190, Rho-dependent activation of 3ß1 and 6ß4 would cease, and 3ß1 would no longer support migration but would begin reorganizing the underlying matrix, making it competent for the nucleation of HDs.14 52
Tenascin-C Modulation of Cell Adhesion
In addition to accumulating laminin-5 in the underlying matrix, migrating keratinocytes also secrete molecules such as FN, fibrin, and tenascin-C. Tenascin-C is a large molecule that functions as a hexamer. It can bind to several different integrins, including 9ß1, and to heparin and FN in the cell matrix.56 57 58 Several studies have focused attention on the role that tenascin-C plays in interfering with adhesion, primarily to FN, during development and wound healing.57 59 Studies of skin and corneal wound healing in tenascin-C knockout mice have shown that, in the absence of tenascin-C, less FN accumulates around cells at the wound site.60 61 Our own studies of corneal debridement wounding in tenascin-C null mice failed to show any evidence of recurrent erosions—even when we looked as late as 8 weeks after creation of small wounds.25 The data presented in this study on wild-type mice fit with the hypothesis that retention of tenascin-C in the anterior stroma could interfere with HD reassembly. The tenascin-C knockout mice were created on a genetic background very different from BALB/c mice. They were pigmented and on a GRS/A genetic background.25 It will be important to confirm whether erosions develop after manual debridement of mice on different genetic backgrounds other than BALB/c.
Integrin activation can induce Rho-mediated signaling; Rho-mediated signaling can also activate integrins.53 This amplification of RhoGTP signaling through integrins is downregulated by mechanisms not well characterized. Tenascin-C has been shown to inhibit the activation of RhoGTP.62 Thus, a model emerges, as proposed by Ridley63 and Murphy-Ullrich,64 in which cells respond to accumulated tenascin-C by inhibiting RhoGTP. Inhibition of RhoGTP stops the activation of 3ß1 and 6ß4 integrins and mediates the conversion of migrating cells from a migratory to a quiescent phenotype.
Differences in the Healing of Small and Large Wounds
Important differences in the way the cornea responds to large and small wounds were observed in our studies. The limbal region in eyes after large wounds was left intact and the localization of 9 integrin–positive cells at this site resembles the limbus in unwounded eyes. Yet, goblet cells were observed on the ocular surface in most corneas by 4 weeks after large wounds. We did not observe goblet cells on the central cornea after small wounds. Further, depletion of 9 integrin–positive cells at the limbal region adjacent to sites containing numerous goblet cells on the cornea surface was seen in most of the large wounds but not in the small wounds. The presence of goblet cells on the corneal surface is sufficient evidence for many studies to conclude that a limbal stem cell deficiency (LSCD) condition exists.
Our observations suggest that the LSCD resulting from large wounds may be caused by the depletion of 9 integrin–positive cells at the limbus. At both 1 and 2 weeks after large wounds were created, fewer 9 integrin–positive cells were observed at the limbus. 9 integrin was also absent at regions adjacent to sites containing numerous goblet cells at 4 and 6 weeks after imposition of large wounds. Conjunctival cells could also have displaced the 9 integrin–positive cells at the limbus. If stem cell proliferation could not keep up with the demand for additional corneal epithelial cells, then conjunctival cell proliferation and migration could contribute to healing.
It has been known for several years that conjunctival epithelial cells can participate in the healing of wounds to the corneal surface when the limbus is removed surgically or damaged chemically or thermally.65 66 67 Initial excitement about the hypothesis that conjunctival cells transdifferentiate into corneal epithelial cells turned out to be premature. Later experiments showed that conjunctival cells can resemble corneal cells morphologically in organ culture and yet not express cornea-specific keratins68 and that healing from the conjunctiva in the absence of the limbus in vivo in mice does not represent transdifferentiation, since the cells on the corneal surface continue to express conjunctival-specific keratins.69 The cause of erosions after large wounds occur is not clear; further studies are needed to clarify this question.
The number of 9 integrin–positive cells in the limbal region remained unchanged with the small wounds and there were no goblet cells on the ocular surface. Therefore, the erosions that developed at later time points after the small wounds were created were not the result of an LSCD but were due to other causes. Finding those causes also necessitates additional experiments but our data implicate altered integrin-mediated cell signaling and/or adhesion involving 9ß1-tenascin and/or 3ß1-6ß4-laminin-5.
Limitations of the Mouse Model for RCES
Although development of animal models for the study of human disease is important, it is also important to keep in mind that mouse and human corneas are very different. In addition to the lack of a Bowman’s layer, the vast differences in stromal thickness and proximity of mouse central corneal epithelial cells to their blood supply probably gave rise during evolution to differences in the biology of cells within mouse and human corneas. Because little is known about recurrent erosion in people, it is not clear how closely this mouse model mimics human RCES. In a study of patients with RCES and epithelial basement membrane dystrophy, using in vivo confocal microscopy, Rosenberg et al.70 detected the presence of microfolds in the subbasal nerve plexus of the eyes of patients with RCES. These folds and depressions appear to be similar to the depressed sites we saw in several of the mouse RCES eyes. Much more study should be undertaken, but the availability of numerous transgenic and knockout mice now make it possible to ask questions about what causes recurrent erosion.
In summary, we have described the characterization of an in vivo mouse model that spontaneously develops RCES in response to a single initial corneal debridement wound. We also show that RCES, after large (2.5 mm) wounding, is accompanied by a decrease in 9 integrin–positive cells at the limbal region and by the presence of goblet cells on the ocular surface. In addition, we demonstrate that tenascin-C accumulates at the epithelial stromal interface in RCES eyes and that 3ß1 integrin mislocalization is observed after wounding. To improve our understanding of the etiology of RCES in animals and people, an experimental model is now available and can be used to test hypotheses about the causes and possible treatments for RCES. Our hope is to eliminate pain and suffering from corneal erosions through a better understanding of the cornea and how it responds to injury.
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Submitted for publication October 31, 2003; revised January 21, 2004; accepted February 4, 2004.
Disclosure: S. Pal-Ghosh, None; A. Pajoohesh-Ganji, None; M. Brown, None; M.A. Stepp, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Mary Ann Stepp, 2300 I Street NW, Washington DC, 20037; mastepp{at}gwu.edu.
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) Leading edge of migration. Note that the initial reepithelialization was complete by 3 days. Bars, 50 µm.
