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Keywords:

  • basement membrane;
  • cell adhesion;
  • cornea;
  • extracellular matrix;
  • integrin;
  • wound healing

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

Integrins are cell adhesion molecules important in cell–cell and cell–extracellular matrix interactions. These interactions are vital to numerous physiological processes including corneal wound healing. This review discusses the structure of integrins as well as the various roles that integrins play in the corneal wound healing process. Integrin profile abnormalities identified in various corneal pathological conditions are also reviewed.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

The cornea is composed of three primary tissue layers: (i) the corneal epithelium, which rests upon the basement membrane, (ii) the stroma, which is composed of collagen fibers, proteoglycans and glycoproteins and is maintained by keratocytes, and (iii) a monolayer of endothelial cells, which sit upon the secreted Descemet’s membrane. The interaction of these cells with their environment, including the extracellular matrix (ECM) and surrounding cells, is vital to cell survival, wound healing, and maintenance of the normal corneal architecture. Cell adhesion molecules (CAM) are important in many cellular processes that require cell–cell or cell–ECM contact. Without these interactions, cellular processes such as migration, proliferation, differentiation, and activation could not occur.1 There are four main groups of CAM including: integrins, immunoglobulin supergene, selectin, and cadherins.1 This review will focus on integrins, a family of transmembrane heterodimeric glycoproteins widely expressed on most cell types that mediate cell–cell and cell–ECM adhesion important in many pathological and physiological cellular processes.1 The numerous functions attributed to integrins include wound repair, embryogenesis, tumor invasion, immunological responses, and platelet function.

Integrin structure

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

Integrins are composed of a non-covalently linked heterodimers of α and β chains, or subunits. Currently, 24 α and 9 β subunits have been identified that can combine to form unique integrins with individual ligand binding characteristics and resultant impact on cellular functions.2,3 Each subunit is characterized as having three domains: extracellular, transmembrane and intracellular (cytoplasmic).2 The α chain is characterized by having a long extracellular domain with a short intracellular domain. The β chain has both a long extracellular and intracellular domain. Expression of specific subunits within a cell determines the range of possible pairings of αβ heterodimers and affects how the cell interacts with the ECM.2 Ligand binding by the integrin is principally governed by the α chain but is also affected by the different α and β chain combinations.1 In a large subset of integrins, the head of the extracellular domain commonly recognizes a tripeptide sequence of specific amino acids, arginine–glycine–aspartic acid or RGD.1,4 This RGD sequence is present in fibronectin, vitronectin, collagen, and laminin, principal components of the ECM.2 Another subset interacts with the ECM via non-RGD mechanisms. One integrin can recognize one ligand or multiple ligands.

The primary corneal collagen receptors include integrins α1β1 and α2β1.5 Additionally, α11β1 present in the corneal epithelium and stroma has been shown to bind collagen.6 Primary corneal laminin receptors include α3β1 and α6β4. Principle fibronectin-binding integrins include αvβ3, αvβ5, αvβ6, and α9β1.5 However, αvβ6 and α9β1 are only expressed following corneal wounding.7,8

The transmembrane domain of the integrin has been reported to affect the affinity of the integrin for the recognized ligand.9 The interactions of the transmembrane segments are important in activating the integrin by converting it to a high-affinity receptor.9 Mutated β subunits without a transmembrane domain have been shown to be incapable of associating with membrane-bound α subunits.4,10 The resultant integrins are not inserted into the membrane and are unable to form functional heterodimers capable of binding ligands.10 Additionally, integrin transmembrane domains are important for signal transduction necessary for the activation of many intracellular signaling pathways.11

The intracellular (cytoplasmic) domain of most integrins are short, composed of 50 amino acids or less. The exception is the β4 chain which is greater than 1000 amino acids, 20 times longer than any other known β subunit.2,4,12,13 This characteristic difference is likely related to the role that the β4 chain plays in hemidesmosome formation through its interaction with the basement membrane and cytoskeletal intermediate filaments.5 The interaction with intermediate filaments is a unique characteristic, as all other integrins are linked to the cytoskeleton via the actin microfilament system. Interactions with the actin cytoskeleton are required for cell adhesion and migration. Anchoring of the integrin to actin occurs indirectly via an assemblage of cytoskeletal proteins.1,14 The cytoskeletal linkage proteins α-actinin, vinculin, and talin associate with β subunit cytoplasmic domains resulting in a connection of the integrin to both the ECM and the actin cytoskeleton.15,16 Both the α and β subunits are required for this interaction to occur.17 Additionally, the intracellular domain is linked to multiple structural and signaling molecules.15 Signal transduction from the ECM occurs through the interaction of integrins with adaptor proteins, cytoplasmic kinases and transmembrane receptors (such as growth factors).3 The expression of RGD sequences, the presence of active high-affinity and inactive low-affinity integrin forms, state of the tissue (normal vs. wounded), and binding to cytoskeletal adaptor proteins all play a role in regulating the activity of integrins.1,18

Integrins can be classified into three major families: β1, β2, and αv, discussed separately below.

  • 1
     The β1 integrins are the largest family and have been characterized in all three developmental layers of the organism (endodermal, ectodermal, and mesenchymal).5 The β1 integrin family has been found to be important in embryonic implantation, organogenesis, and hematopoiesis.5 This group contains integrins known as very late activation antigens (VLA) for their association with subunits α1-6.1 The primary function of this group of integrins is to modulate cell–ECM interactions through the binding of ligands in collagen, laminin, fibronectin or vitronectin.1 The importance of β1 in cell–ECM interaction was demonstrated in β1 null mice that were found to develop defects in epidermal adhesion and assembly of the basement membrane,19 which led to severe skin blistering, massive failures in basement membrane assembly, and hemidesmosome instability.20α3β1 has been identified as a contributor to hemidesmosome stability and ECM assembly.3,20 A role for β1 in cell–cell adhesion has also been established.21 Integrins α2β1 and α3β1 have a pericellular distribution and colocalize at intercellular contact sites.5 These two integrins can bind to one another or α3β1 can bind to itself.22–24 These interactions are likely necessary to maintain the integrity of the epithelium.
  • 2
     The β2 family of integrins are primarily involved with immune cell activity/inflammatory responses with the IgG family acting as primary ligands. Through their interaction with leukocytes this family primarily modulates cell–cell interactions.1,5
  • 3
     The αv-containing heterodimers represent the only α chain that can bind to multiple integrin β chains. An important member of this family includes αvβ3, an integrin important in mediating angiogenesis. Therapies used to block this integrin have been used to reduce the blood flow to certain types of tumors.25 This family of integrins is also important in cell signaling under the influence of growth factors such as transforming growth factor (TGF-β).3

Other important groupings of integrins include αIIbβ3, a platelet integrin important for clotting and α6β4, an important component of the hemidesmosome participating in epithelial cell adhesion to the basement membrane.4,5 Mice that lack either α6 or β4 develop a lethal trait of separation of the skin at the dermal–epidermal junction due to a lack of hemidesmosome formation.26–28 Additionally, this heterodimer has been found to be important in carcinogenesis by improving cell survival.29 Changes in integrin expression facilitate tumor invasion through the effects on cell adhesion and signaling.29,30 Tumors such as squamous cell carcinoma have previously been shown to lack hemidesmosomes.31 In a study by Mariotti et al., disruption of hemidesmosomes through the activation of a tyrosine kinase (Fyn) resulted in increased cell migration and carcinoma invasion.29 Integrin α6β4 also participates in cell signaling as it is associated with tyrosine kinase and becomes phosphorylated when bound to laminin-5 or activated by endothelial growth factor (EGF).29,32

Corneal wound healing – epithelial

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

The corneal epithelium is normally maintained by a combined process of basal epithelial cell proliferation, shedding of superficial epithelial cells, and renewal by the centripetal migration of basal epithelial cells originating from stem cells at the limbus. Cellular adhesions to one another (cell–cell contact) as well as adhesions to the basement membrane (cell–ECM contact) are vital to maintain the mechanical integrity of the epithelium to resist sheer stress at the corneal surface. Integrin subunits identified in unwounded human corneal epithelium include α2, α3, α5, α6, αv, β1, β4, and β5 (Table 1).21,33 Most integrins have a polarized localization in the corneal epithelium dependent on their underlying functions.21 Unlike the other subunits, β5 was not found to have a polarized distribution as it was found in all cell membranes of the epithelium.21α2, α3, α5, αv and β1 are localized to sites of cell–cell contact, along basal cell membranes and suprabasalar cells with gradual loss in the more superficial epithelium.21α5, α6, and β4 are localized in the basal membrane of basal epithelial cells.21 Basal epithelial cells rest upon the basement membrane, a dense sheet of ECM primarily composed of type IV collagen, type VII collagen, laminin, the proteoglycan perlecan, and fibronectin.34,35 Integrin subunits αv, β1, and β4 are involved in interactions with proteins of the ECM, including components of the normal basement membrane as well as provisional matrix found in regions of high cell movement typical of healing wounds.4

Table 1.   Corneal integrins. Integrins identified in the cornea along with their respective ligands and recognized locations are listed
α SubunitHeterodimers formedLigandLocation
  1. RGD = recognized three-amino acid sequence (arginine–glycine–aspartic acid); VCAM-1 = vascular cell adhesion molecule-1; VEGF-C = vascular endothelial growth factor C. *Denotes localization in wounded corneal tissue.

α1α1β1Laminin, collagenEndothelium
α2α2β1Laminin, collagen, tenascin-cEpithelium, stromal keratocytes, endothelium
α3α3β1Laminin, collagen, fibronectinEpithelium, limbus, stromal keratocytes, endothelium
α4α4β1Fibronectin (non-RGD), VCAM-1stromal keratocytes, endothelium
α5α5β1Laminin, collagen, fibronectinEpithelium, stromal keratocytes in culture or wounding,* endothelium
α6α6β1Laminin, collagen, fibronectinBasal/suprabasalar epithelium, stromal keratocytes, endothelium
α6β4Laminin-5, collagen, fibronectinBasal epithelium
α8α8β1Fibronectin, tenascin-cOnly in wounded cornea*
α9α9β1Fibronectin (non-RGD), tenascin-c, osteopontin, fibrinogen, VEGF-CLimbus (normal), epithelium (only wounded)*
α11α11β1CollagenEpithelium, stroma
αvαvβ1Laminin, fibronectin, vitronectinEpithelium, stromal keratocytes
αvβ3Fibronectin, vitronectin, fibrinogen, osteopontin, tenascin-cStromal keratocytes, endothelium
αvβ5Vitronectin, collagen, laminin, fibronectinEpithelium, endothelium
αvβ6Vitronectin, fibronectin, tenascin-cEpithelium (only wounded)*

Firm adhesions between the basal epithelial cells and the basement membrane are required to maintain cellular attachment. This adhesion is accomplished through anchoring complexes composed of hemidesmosomes, anchoring filaments, and anchoring plaques.36 Hemidesmosomes are protein complexes of the basal cell membrane composed of an inner and outer plaque.37 The inner plaque is composed of proteins BP 230 and HD1/plectin on the cytoplasmic side of the cell which attach to cytoskeletal intermediate filaments. The outer plaque is composed of two transmembrane proteins: integrin α6β4, a laminin receptor and BP 180, a member of the collagen family.38 Components of the outer plaque link to anchoring fibrils composed of type VII collagen that project into the anterior corneal stroma. Once in the stroma, anchoring fibrils combine with stromal type I collagen and end in anchoring plaques composed of laminin.34 Integrin α6β4 has been identified as an important component of the hemidesmosome as blockage of α6β4 has been shown to prevent hemidesmosome formation.39

Corneal wound healing is carefully orchestrated to direct changes in the cell adhesion, migration, behavior, and signaling. After epithelial injury, mitosis of epithelial cells ceases and cells at the edge of the wound retract, thicken, and lose their hemidesmosomal attachments in preparation for migration. Following debridement, α6β4 is up-regulated and activation by EGF results in phosphorylation of the integrin cytoplasmic domain by tyrosine kinase to promote hemidesmosome disassembly.5 Components are internalized by endocytosis during disassembly and distribution becomes suprabasalar.40 Basal epithelial cells and stromal keratocytes produce fibronectin rapidly following epithelial injury. Fibronectin acts as a provisional matrix allowing for epithelial cell migration and attachment across the wound.41,42 Integrin α5β1 is a major receptor for fibronectin and is found at the surface of basal cells in normal corneal epithelium.42 Following corneal debridement, epithelial cells actively migrating across the wound express α5β1 at increased levels compared to unwounded or healed cornea.43 The ability of cells to bind fibronectin is increased through the up-regulation of β1integrin expression.42 The necessity of this integrin for cellular adhesion to the provisional matrix was verified by inhibition of attachment of cells to fibronectin matrix in the presence of β1 antibodies.44 As wound healing is completed, β1 is down-regulated in coordination with a reduction in fibronectin.43 Other integrins capable of binding to fibronectin include α4β1, αvβ1, α3β1, and α11β3 through the recognition of RGD sequences.45

Integrin-mediated attachment of the migrating cell to the ECM results in focal contacts or focal adhesions (Fig. 1). Focal adhesions are points of cell–substratum and cytoskeleton–membrane contact.46 These points of attachment are required for changes in cell shape needed for spreading and migration.22,47 Deletion of β1 cytoplasmic domains interferes with the formation of focal contacts resulting in unstable adhesions to the ECM and delayed wound healing.48–50 ECM ligand binding leads to integrin clustering and recruitment of actin filaments to the integrin cytoplasmic domains. This process is facilitated by the cytoskeletal proteins talin, vinculin, α-actinin, and filamin. Integrin-mediated signaling controls the production and organization of actin filaments through the activation and inactivation of Rho GTPase and downstream effectors.15 The activation of Rho GTPases leads to assembly of actin/myosin contractile filaments into focal adhesion complexes. This assembly promotes cell polarity and motility necessary for cell migration, as actin filaments are subsequently organized into bundles (stress fibers) forming lamellipodia and filopodia that invade the ECM. Lamellipodia are sheet-like protrusions and filopodia are long, thin projections of the cell.51 Cell motility occurs when the cell extends lamellipodia or filopodia which are anchored to the underlying substratum through focal adhesions. Subsequent contraction of actin stress fibers generates tension resulting in cell movement.51

image

Figure 1.  Integrin mediated focal adhesion complexes. Immunofluorescence image of a primary human corneal epithelial cell plated onto self-assembled monolayers of ethylene glycol (EG) where 0.1% of the EG molecules were terminated with a GRGDS integrin ligand peptide via a method developed by Clare et al. (2005).* Because EG is resistant to protein deposition, the cells can only interact with the surface via the RGD sequence. Orange projections from the cell= actin fibers; central bright foci =focal adhesions. Photo courtesy of: Dr. Christopher Murphy, Laura Luettjohann and John Foley. *Clare BH, Abbott NL. Orientations of nematic liquid crystals on surfaces presenting controlled densities of peptides: amplification of protein-peptide binding events. Langmuir 2005; 21: 6451–61.

Download figure to PowerPoint

The leading edge of the migrating sheet is one cell layer thick. Presence of cell–cell interactions must be strong to withstand sheer forces and forces generated by dramatic changes in cell shape.21 Integrins α2β1 and α3β1, which modulate cell–cell adhesion, are likely important in the migration of an intact sheet of epithelial cells.4In vitro studies using antibodies to β1 resulted in epithelial cell dissociation from one another.52 Coordinated development of focal adhesions in the front of the cell and disassembly of focal adhesions at the rear of the cell are required to complete migration.51 This is accomplished by a breakage of the link between actin and integrins. α9, which is normally only identified in unwounded limbal basal cells, is found to increase expression surrounding the leading edge of the migrating epithelium and at sites where the epithelial sheets merge.53 This suggests that α9 functions to seal off the edges of advancing wound margins through the encouragement of cell–cell adhesions.53 Additionally, tenascin is found to accumulate under cells expressing α9.7 Tenascin, the ligand for α9β1, is an ECM protein important in epithelial–mesenchymal interactions, cell migration, and adhesion via integrin-dependent mechanisms.7 The appearance of tenascin during cell restratification correlates with remodeling of the basement membrane and re-insertion of hemidesmosomes.7 Upon cellular re-attachment, adhesion complexes are recycled to reform hemidesmosomes.38 More rapid healing occurs when the basement membrane is not disrupted and components of the adhesion complex can be reused, compared to deeper corneal wounds such as keratectomy.36 Reassembly of the anchoring complex begins at the periphery of the wound. It can take 2–4 weeks to reform a continuous basement membrane and 4 weeks to form new anchoring fibrils.36 After healing, the development of normal adhesion complexes can take longer than 12 months to be completed.36 The epithelial wound is eventually covered by a layered sheet composed of both basal and squamous epithelial cells. Once the wound is closed, mitosis is resumed to restore normal epithelial thickness.

Corneal wound healing – stromal

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

Stromal keratocytes normally express αvβ3, α2β1, α3β1, α6β1, and α4β1 (Table 1). There is no expression of α5β1 in the normal corneal stroma. However, feline keratocytes were found to express α5β1 upon cell culture.54 Stromal wound healing involves the transformation of keratocytes to fibroblasts, which proliferate to synthesize collagen and ECM components. Fibronectin is produced by fibroblasts as a provisional matrix to stimulate cell adhesion, migration, and additional protein synthesis.34 Stromal collagen becomes cross-linked and proteoglycan synthesis occurs to result in gradual wound remodeling.34 As the fibroblastic proliferation continues, the overlying epithelium is gradually displaced anteriorly by new collagen fibers and lamellae, which are often disorganized and may result in scar formation.55 Extensive work has evaluated the changes that occur in corneal wound healing as it relates to corneal scarring. The conversion of keratocytes to myofibroblasts (activated fibroblasts with contractile properties that express smooth muscle actin) is regulated by TGF-β.5,56 The addition of TGF-β leads to changes in the localization of integrins, function, and signal transduction that involve platelet-derived growth factor (PDGF), TGF-β, and RGD-sensitive integrins (αv family or α5β1).5 The expression of α4 and α5 subunits was detected in regions of scar formation in human corneal tissue.57 As these are primarily fibronectin receptors, it is likely that they are up-regulated in coordination with normal wound healing. Collagen binding integrins α2β1 and α11β1 mediate wound contraction by myofibroblasts.5 Integrins also regulate the ability of keratocytes and myofibroblasts to assemble and maintain their collagen/proteoglycan-rich matrix to limit corneal scarring.5

Corneal wound healing – endothelial

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

Endothelial healing is completed by the spreading of adjacent cells to cover defects as well as by cell enlargement. Endothelial pumping mechanisms and barrier function are re-established once a confluent monolayer is present. All members of the VLA integrins have been identified in the corneal endothelium (Table 1).57 Integrins mediate strong adhesions present between the endothelium and Descemet’s membrane composed of collagen, laminin, and fibronectin.57 Additionally, cell–cell interactions through the interaction of integrins are important in maintaining corneal deturgescence. Through research evaluating adenoviral-mediated gene transfer, members of the αv family have also been identified.58 Studies evaluating endothelial integrins are limited. Further work is needed to determine the potential impact that alterations in endothelial integrins may have on corneal health and age-related endothelial cell loss.

Corneal wound healing – influence of growth factors

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

An increase in the synthesis of growth factors receptors is seen following corneal wounding.34 Growth factors have numerous roles in promoting cell migration, proliferation, and cell survival.59 Growth factors activate transcription of genes involved in protein synthesis, cellular proliferation, and differentiation.34 Many cellular responses to soluble growth factors (EGF, PDGF) are dependent on the cell being adhered to the substrate via integrins. Ligation of integrins results in bidirectional signal transduction events across the plasma membrane.18,33 These pathways are intimately coupled with those stimulated by growth factors allowing for cellular responses to soluble growth factors and cytokines.3,33 For example, receptor activation by EGF induces proliferation and differentiation of corneal epithelial cells. This is accomplished through the activation of intracellular signaling proteins Ras, MAP kinase, phospholipase C and phosphatidylinotol 3-kinase.34 Additionally, inflammatory cytokines such as IL-1, IL-6, IL-10 and tumor necrosis factor-α (TNF-α) are increased following wounding.34 IL-6 stimulates integrin α5β1, important in cell migration due to its presence in adhesion plaques to fibronectin.34 Integrins αvβ5, αvβ6, and αvβ8 have been shown to mediate TGF-β activation.33,60 This results in increases in ECM expression, matrix metalloproteinase expression and differentiation.61,62 Increased expression of TGF-β, IL-1β, γ-interferon and TNF-α have also been shown to modify the expression of integrins resulting in increased adhesion to ECM components.1

Pathobiology – role of integrins in corneal disease

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References

Bullous keratopathy

Integrins have been implicated in playing a pathological role in multiple corneal disorders. Aphakic and pseudophakic bullous keratopathy (BK) is characterized by the development of endothelial cell dysfunction and cell loss, progressive corneal edema, development of subepithelial bullae, subepithelial fibrosis, and formation of a collagenous retrocorneal membrane.5,63 An excessive deposition of ECM components has been described.63 Abnormal expression of tenascin-C (TN-C) has been implicated in this disorder and may result in abnormal expression of binding proteins.64 The presence of TN-C is associated with inflammation and can weaken cell-substrate attachment.64 Normally TN-C is present in central fetal corneas and only found at the limbus in adults. However, in BK, TN-C is deposited in the stroma and basement membrane.64,65 TN-C has multiple binding sites for integrins, including α2β1, α8β1, αvβ3, and αvβ6.66 In normal cornea, there is no expression of TN-C receptors α8β1, α9β1, and αvβ6. However, in BK, expression of these integrins in the central cornea has been reported.64 Presumably these changes are seen based on the ability of these integrins to bind tenascin and contribute to the fibrotic process that occurs.64,67 Additionally, increased α2β1 and α3β1 have been reported in BK.68 Conversely, α2β1, a collagen/laminin receptor, with a role in cell–cell and cell–ECM interactions, was found to be decreased in the epithelium of patients with BK.63 This finding may be related to alterations in laminin and/or collagen in the epithelial basement membrane.63

Recurrent epithelial erosions

Recurrent epithelial erosions (REE) are characterized by a lack of adherence of the migrating corneal epithelial cells to the underlying stroma. When epithelial cell migration is followed by sloughing of epithelial cells, repair of adhesions structures is at fault.36 Studies of human and canine keratectomy samples from patients affected with recurrent erosions have identified similar pathological features including an acellular, hyalanized zone in the superficial corneal stroma, decreased numbers or abnormalities of hemidesmosomes, and basement membrane and ECM components either absent or present only in discontinuous segments on the surface of the erosion.69–71 Following photorefractive keratectomy (PRK), focal discontinuities of hemidesmosomes or BM may correlate with epithelial wound healing problems.72 Therefore, abnormalities in the basement membrane may be responsible for the delay or inhibition of normal corneal wound healing. During normal wound healing, reassembly of the basement membrane occurs from the wound periphery to the wound center.36 The basement membrane is replaced in segments at the same time that anchoring fibril collagen and hemidesmosomes appear.36 Integrin α6β4 is important to the formation of the hemidesmosome and α3β1 is important in organizing the assembly of the basement membrane. Therefore, variations in the presence of integrins may contribute to the pathological features and the failure to reform adhesion complexes in these patients. Studies evaluating integrin expression in recurrent erosions are limited; however, α9 was found at increased levels REEs.73 Additional support for the potential role of integrins is provided by studies evaluating β1knockout mice which are unable to reassemble the basement membrane. The impact of age in this disease requires further investigation as aging human corneas show discontinuous localization of α6β4 at the basement membrane.44 Further study of the role of integrins in the pathophysiology of REE is warranted.

Diabetes

Diabetic patients have been characterized as being at risk for the development of epithelial defects, REE, decreased corneal sensitivity, decreased rates of re-epithelialization, and abnormal wound repair.74 A similar clinical observation of increased postoperative keratitis with development of REE or delayed healing times have been reported in diabetic dogs.75 In human studies, diabetic corneas were found to have an increase in basement membrane thickness, decreased numbers of hemidesmosomes, and impaired endothelial cell function.74 The epithelial basement membrane composition was altered as amounts of nidogen, laminin-1, and laminin-10 were markedly diminished.74 Additionally, levels of α3β1 were significantly reduced. This change in α3β1, a laminin binding integrin, may be secondary to growth factor modulation or increased levels of proteinases.74

The role of integrins has been evaluated in multiple additional corneal disorders. Angiogenesis is a highly regulated process and is the final common pathway for multiple ocular disorders. Integrins have been recognized as receptors involved in embryonic and pathologic angiogenic pathways. αvβ3 and α5β1 have been investigated for their roles in promoting corneal neovascularization. In the normal cornea of domestic animals, αvβ3 is not expressed.76 However, they are found in the vascularized cornea. Inhibition of both αvβ3 and α5β1 resulted in regression of corneal neovascularization.77,78 In corneal wounding models, vascularized corneas were also found to have up-regulation of α5. In keratoconjunctivitis sicca, integrins have been implicated in promoting local inflammatory reactions through the recruitment and activation of leukocytes. Application of α4β1 inhibitor led to a suppression of local inflammatory changes and to a decrease in dry eye symptoms.79 Currently, topical integrin antagonists are being used in clinical trials for the treatment of dry eye. Findings in keratoconus have been mixed but reductions in collagen XII and β4 have been reported.68,80 Additionally, limbal stem cell deficiencies have been associated with loss of α9. The presence of α9 appears to protect the central cornea from goblet cell invasion.5

Corneal integrins play a vital role in normal corneal maintenance and wound healing. Changes in integrin expression and localization have been investigated and identified in numerous pathological corneal disorders. Unfortunately, the characterization of integrin profiles in domestic species is minimal at this time. Currently, the evaluation of normal canine cornea for the presence and distribution of integrins is in progress. At the present time, subunits α3, α6, β1, and β4 have been identified.81 Further work is needed to characterize normal integrin profiles prior to investigation of the potential role that integrins play in acquired corneal disorders of domestic species.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Integrin structure
  5. Corneal wound healing – epithelial
  6. Corneal wound healing – stromal
  7. Corneal wound healing – endothelial
  8. Corneal wound healing – influence of growth factors
  9. Pathobiology – role of integrins in corneal disease
  10. References
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